Scientific Output

Over 10.000 scientific papers have been published by members of the Materials Chain since the foundation of the University Alliance Ruhr in 2010. This tremendous output is proof of the excellent environment the Ruhr Area provides for research in the field of materials science and technology.

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  • 2020 • 232 Prospects of Value-Added Chemicals and Hydrogen via Electrolysis
    Garlyyev, B. and Xue, S. and Fichtner, J. and Bandarenka, A.S. and Andronescu, C.
    ChemSusChem 13 2513-2521 (2020)
    Cost is a major drawback that limits the industrial-scale hydrogen production through water electrolysis. The overall cost of this technology can be decreased by coupling the electrosynthesis of value-added chemicals at the anode side with electrolytic hydrogen generation at the cathode. This Minireview provides a directory of anodic oxidation reactions that can be combined with cathodic hydrogen generation. The important parameters for selecting the anodic reactions, such as choice of catalyst material and its selectivity towards specific products are elaborated in detail. Furthermore, various novel electrolysis cell architectures for effortless separation of value-added products from hydrogen gas are described. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstractdoi: 10.1002/cssc.202000339
  • 2020 • 231 Electrochemical CO2 Reduction-The Effect of Chalcogenide Exchange in Ni-Isocyclam Complexes
    Gerschel, P. and Battistella, B. and Siegmund, D. and Ray, K. and Apfel, U.-P.
    Organometallics 39 1497-1510 (2020)
    Among the numerous homogeneous electrochemical CO2 reduction catalysts, [Ni(cyclam)]2+ is known as one of the most potent catalysts. Likewise, [Ni(isocyclam)]2+ was reported to enable electrochemical CO2 conversion but has received significantly less attention. However, for both catalysts, a purposeful substitution of a single nitrogen donor group by chalcogen atoms was never reported. In this work, we report a series of isocyclam-based Ni complexes with {ON3}, {SN3}, {SeN3}, and {N4} moieties and investigated the influence of nitrogen/chalcogen substitution on electrochemical CO2 reduction. While [Ni(isocyclam)]2+ showed the highest selectivity toward CO2 reduction within this series with a Faradaic efficiency of 86% for the generation of CO at an overpotential of-1.20 V and acts as a homogeneous catalyst, the O-and S-containing Ni complexes revealed comparable catalytic activities at ca. 0.3 V milder overpotential but tend to form deposits on the electrode, acting as precursors for a heterogeneous catalysis. Moreover, the heterogeneous species generated from the O-and S-containing complexes enable a catalytic hydride transfer to acetonitrile, resulting in the generation of acetaldehyde. The incorporation of selenium, however, resulted in loss of CO2 reduction activity, mainly leading to hydrogen generation that is also catalyzed by a heterogeneous electrodeposit. Copyright © 2020 American Chemical Society.
    view abstractdoi: 10.1021/acs.organomet.0c00129
  • 2020 • 230 Polymer/enzyme-modified HF-etched carbon nanoelectrodes for single-cell analysis
    Marquitan, M. and Ruff, A. and Bramini, M. and Herlitze, S. and Mark, M.D. and Schuhmann, W.
    Bioelectrochemistry 133 (2020)
    Carbon-based nanoelectrodes fabricated by means of pyrolysis of an alkane precursor gas purged through a glass capillary and subsequently etched with HF were modified with redox polymer/enzyme films for the detection of glucose at the single-cell level. Glucose oxidase (GOx) was immobilized and electrically wired by means of an Os-complex-modified redox polymer in a sequential dip coating process. For the synthesis of the redox polymer matrix, a poly(1-vinylimidazole-co-acrylamide)-based backbone was used that was first modified with the electron transfer mediator [Os(bpy)2Cl]+ (bpy = 2,2′-bipyridine) followed by the conversion of the amide groups within the acrylamide monomer into hydrazide groups in a polymer-analogue reaction. The hydrazide groups react readily with bifunctional epoxide-based crosslinkers ensuring high film stability. Insertion of the nanometre-sized polymer/enzyme modified electrodes into adherently growing single NG108-15 cells resulted in a positive current response correlating with the intracellular glucose concentration. Moreover, the nanosensors showed a stable current output without significant loss in performance after intracellular measurements. © 2020
    view abstractdoi: 10.1016/j.bioelechem.2020.107487
  • 2020 • 229 Trimetallic Mn-Fe-Ni Oxide Nanoparticles Supported on Multi-Walled Carbon Nanotubes as High-Performance Bifunctional ORR/OER Electrocatalyst in Alkaline Media
    Morales, D.M. and Kazakova, M.A. and Dieckhöfer, S. and Selyutin, A.G. and Golubtsov, G.V. and Schuhmann, W. and Masa, J.
    Advanced Functional Materials 30 (2020)
    Discovering precious metal-free electrocatalysts exhibiting high activity and stability toward both the oxygen reduction (ORR) and the oxygen evolution (OER) reactions remains one of the main challenges for the development of reversible oxygen electrodes in rechargeable metal–air batteries and reversible electrolyzer/fuel cell systems. Herein, a highly active OER catalyst, Fe0.3Ni0.7OX supported on oxygen-functionalized multi-walled carbon nanotubes, is substantially activated into a bifunctional ORR/OER catalyst by means of additional incorporation of MnOX. The carbon nanotube-supported trimetallic (Mn-Ni-Fe) oxide catalyst achieves remarkably low ORR and OER overpotentials with a low reversible ORR/OER overvoltage of only 0.73 V, as well as selective reduction of O2 predominantly to OH−. It is shown by means of rotating disk electrode and rotating ring disk electrode voltammetry that the combination of earth-abundant transition metal oxides leads to strong synergistic interactions modulating catalytic activity. The applicability of the prepared catalyst for reversible ORR/OER electrocatalysis is evaluated by means of a four-electrode configuration cell assembly comprising an integrated two-layer bifunctional ORR/OER electrode system with the individual layers dedicated for the ORR and the OER to prevent deactivation of the ORR activity as commonly observed in single-layer bifunctional ORR/OER electrodes after OER polarization. © 2019 The Authors. Published by WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201905992
  • 2020 • 228 Electroenzymatic CO2 Fixation Using Redox Polymer/Enzyme-Modified Gas Diffusion Electrodes
    Szczesny, J. and Ruff, A. and Oliveira, A.R. and Pita, M. and Pereira, I.A.C. and De Lacey, A.L. and Schuhmann, W.
    ACS Energy Letters 5 321-327 (2020)
    We describe the fabrication of gas diffusion electrodes modified with polymer/enzyme layers for electroenzymatic CO2 fixation. For this, a metal-free organic low-potential viologen-modified polymer has been synthesized that reveals a redox potential of around-0.39 V vs SHE and is thus able to electrically wire W-dependent formate dehydrogenase from Desulfovibrio vulgaris Hildenborough, which reversibly catalyzes the conversion of CO2 to formate. The use of gas diffusion electrodes eliminates limitations arising from slow mass transport when solid carbonate is used as CO2 source. The electrodes showed satisfactory stability that allowed for their long-term electrolysis application for electroenzymatic formate production. Copyright © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsenergylett.9b02436
  • 2020 • 227 Fabrication and optical characterization of photonic crystal nanocavities with electrodes for gate-defined quantum dots
    Tajiri, T. and Sakai, Y. and Kuruma, K. and Ji, S.M. and Kiyama, H. and Oiwa, A. and Ritzmann, J. and Ludwig, Ar. and Wieck, A.D. and Ota, Y. and Arakawa, Y. and Iwamoto, S.
    Japanese Journal of Applied Physics 59 (2020)
    Among various solid-state systems, gate-defined quantum dots (QD) with high scalability and controllability for single electron spin qubits are promising candidates to realize quantum spin-photon interface. The efficiency of the spin-photon interface is expected to be significantly enhanced by optical coupling of gate-defined QDs with photonic crystal (PhC) nanocavities. As the first step towards this optical coupling, we designed and experimentally demonstrated a PhC nanocavity with electrodes. The electrodes, which can form a single QD, were introduced on the top surfaces of two-dimensional PhC nanocavities with a position accuracy of a few tens of nanometers. Despite the electrodes, a resonant mode was confirmed for the PhC nanocavities through micro-photoluminescence spectroscopy. This work marks a crucial step towards optical coupling between gate-defined QDs and PhC nanocavities. © 2020 The Japan Society of Applied Physics.
    view abstractdoi: 10.7567/1347-4065/ab5b62
  • 2020 • 226 Surface Properties of Battery Materials Elucidated Using Scanning Electrochemical Microscopy: The Case of Type I Silicon Clathrate
    Tarnev, T. and Wilde, P. and Dopilka, A. and Schuhmann, W. and Chan, C.K. and Ventosa, E.
    ChemElectroChem 7 665-671 (2020)
    Silicon clathrates have attracted interest as potential anodes for lithium-ion batteries with unique framework structures. However, very little is known about the surface reactivity and solid electrolyte interphase (SEI) properties of clathrates. In this study, operando scanning electrochemical microscopy (SECM) is used to investigate the effect of pre-treatment on the formation dynamics and intrinsic properties of the SEI in electrodes prepared from type I Ba8Al16Si30 silicon clathrates. Although X-ray photoelectron spectroscopy (XPS) analysis does not reveal large changes in SEI composition, it is found through SECM measurements that ball-milling combined with chemical acid/base etching of the clathrates lead to a more stable and rapidly formed SEI as compared to purely ball-milled samples, resulting in enhanced coulombic efficiency. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201901688
  • 2020 • 225 Reassessing the rationale behind herbicide biosensors: The case of a photosystem II/redox polymer-based bioelectrodefs
    Wang, P. and Zhao, F. and Hartmann, V. and Nowaczyk, M.M. and Ruff, A. and Schuhmann, W. and Conzuelo, F.
    Bioelectrochemistry 136 (2020)
    Interfacing photosynthetic protein complexes with electrodes is frequently used for the identification of electron transfer mechanisms and the fabrication of biosensors. Binding of herbicide compounds to the terminal plastoquinone QB at photosystem II (PSII) causes disruption of electron flow that is associated with a diminished performance of the associated biodevice. Thus, the principle of electron transport inhibition at PSII can be used for herbicide detection and has inspired the fabrication of several biosensors for this purpose. However, the biosensor performance may reveal a more complex behavior than generally expected. As we present here for a photobioelectrode constituted by PSII embedded in a redox polymer matrix, the effect caused by inhibitors does not only impact the electron transfer from PSII but also the properties of the polymer film used for immobilization and electrical wiring of the protein complexes. Incorporation of phenolic inhibitors into the polymer film surprisingly translates into enhanced photocurrents and, in particular cases, in a higher stability of the overall electrode architecture. The achieved results stress the importance to evaluate first the possible influence of analytes of interest on the biosensor architecture as a whole and provide important insights for consideration in future design of bioelectrochemical devices. © 2020
    view abstractdoi: 10.1016/j.bioelechem.2020.107597
  • 2019 • 224 Spray-Flame-Synthesized LaCo1−xFexO3 Perovskite Nanoparticles as Electrocatalysts for Water and Ethanol Oxidation
    Alkan, B. and Cychy, S. and Varhade, S. and Muhler, M. and Schulz, C. and Schuhmann, W. and Wiggers, H. and Andronescu, C.
    ChemElectroChem 6 4266-4274 (2019)
    Coupling electrochemical generation of hydrogen with the concomitant formation of an industrially valuable product at the anode instead of oxygen can balance the high costs usually associated with water electrolysis. We report the synthesis of a variety of nanoparticulate LaCo1−xFexO3 perovskite materials through a specifically optimized spray-flame nanoparticle synthesis method, using different ratios of La, Co, and Fe precursor compounds. Structural characterization of the resulting materials by XRD, TEM, FTIR, and XPS analysis revealed the formation of mainly perovskite-type materials. The electrocatalytic performance of the formed perovskite-type materials towards the oxygen evolution reaction and the ethanol oxidation reaction was investigated by using rotating disk electrode voltammetry. An increased Fe content in the precursor mixture leads to a decrease in the electrocatalytic activity of the nanoparticles. The selectivity towards alcohol oxidation in alkaline media was assessed by using the ethanol oxidation reaction as a model reaction. Operando electrochemistry/ATR-IR spectroscopy results reveal that acetate and acetaldehyde are the final products, depending on the catalyst composition as well as on the applied potential. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201900168
  • 2019 • 223 Introducing Pseudocapacitive Bioelectrodes into a Biofuel Cell/Biosupercapacitor Hybrid Device for Optimized Open Circuit Voltage
    Alsaoub, S. and Conzuelo, F. and Gounel, S. and Mano, N. and Schuhmann, W. and Ruff, A.
    ChemElectroChem 6 2080-2087 (2019)
    We report the fabrication of a polymer/enzyme-based biosupercapacitor (BSC)/biofuel cell (BFC) hybrid device with an optimized cell voltage that can be switched on demand from energy conversion to energy storage mode. The redox polymer matrices used for the immobilization of the biocatalyst at the bioanode and biocathode act simultaneously as electron relays between the integrated redox enzymes and the electrode surface (BFC) and as pseudocapacitive charge storing elements (BSC). Moreover, owing to the self-charging effect based on the continuously proceeding enzymatic reaction, a Nernstian shift in the pseudocapacitive elements, that is, in the redox polymers, at the individual bioelectrodes leads to a maximized open circuit voltage of the device in both operating modes. Comparison with a conventional fuel cell design, that is, using redox mediators with redox potentials that are close to the potentials of the used redox proteins, indicates that the novel hybrid device shows a similar voltage output. Moreover, our results demonstrate that the conventional design criteria commonly used for the development of redox polymers for the use in biofuel cells have to be extended by considering the effect of a Nernstian shift towards the potentials of the used biocatalysts in those pseudocapacitive elements. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201900256
  • 2019 • 222 Cobalt metalloid and polybenzoxazine derived composites for bifunctional oxygen electrocatalysis
    Barwe, S. and Andronescu, C. and Engels, R. and Conzuelo, F. and Seisel, S. and Wilde, P. and Chen, Y.-T. and Masa, J. and Schuhmann, W.
    Electrochimica Acta 297 1042-1051 (2019)
    The development of bifunctional oxygen electrodes is a key factor for the envisaged application of rechargeable metal-air batteries. In this work, we present a simple procedure based on pyrolysis of polybenzoxazine/metal metalloid nanoparticles composites into efficient bifunctional oxygen reduction and oxygen evolution electrocatalysts. This procedure generates nitrogen-doped carbon with embedded metal metalloid nanoparticles exhibiting high activity towards both, oxygen reduction and oxygen evolution, in 0.1 M KOH with a roundtrip voltage of as low as 0.81 V. Koutecký-Levich analysis coupled with scanning electrochemical microscopy reveals that oxygen is preferentially reduced in a 4e− transfer pathway to hydroxide rather than to hydrogen peroxide. Furthermore, the polybenzoxazine derived carbon matrix allows for stable catalyst fixation on the electrode surface, resulting in unattenuated activity during continuous alternate polarisation between oxygen evolution at 10 mA cm−2 and oxygen reduction at −1.0 mA cm−2. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2018.12.047
  • 2019 • 221 Piece by Piece—Electrochemical Synthesis of Individual Nanoparticles and their Performance in ORR Electrocatalysis
    Evers, M.V. and Bernal, M. and Roldan Cuenya, B. and Tschulik, K.
    Angewandte Chemie - International Edition 58 8221-8225 (2019)
    The impact of individual HAuCl4 nanoreactors is measured electrochemically, which provides operando insights and precise control over the modification of electrodes with functional nanoparticles of well-defined size. Uniformly sized micelles are loaded with a dissolved metal salt. These solution-phase precursor entities are then reduced electrochemically—one by one—to form nanoparticles (NPs). The charge transferred during the reduction of each micelle is measured individually and allows operando sizing of each of the formed nanoparticles. Thus, particles of known number and sizes can be deposited homogenously even on nonplanar electrodes. This is demonstrated for the decoration of cylindrical carbon fibre electrodes with 25±7 nm sized Au particles from HAuCl4-filled micelles. These Au NP-decorated electrodes show great catalyst performance for ORR (oxygen reduction reaction) already at low catalyst loadings. Hence, collisions of individual precursor-filled nanocontainers are presented as a new route to nanoparticle-modified electrodes with high catalyst utilization. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201813993
  • 2019 • 220 Carbon-templated conductive oxide supports for oxygen evolution catalysis
    Hufnagel, A.G. and Häringer, S. and Beetz, M. and Böller, B. and Fattakhova-Rohlfing, D. and Bein, T.
    Nanoscale 11 14285-14293 (2019)
    We present a novel route for the preparation of supported IrO2 catalysts for the oxygen evolution reaction in proton exchange membrane electrolyzers. It uses carbon soot as a nanostructure template, which is sequentially coated with a conductive niobium-doped titanium oxide (NTO) layer and an ultrathin, highly pure IrO2 catalyst layer by atomic layer deposition (ALD). The NTO acts as an oxidation-stable conductor between the metal current distributor and the catalyst. The highly controlled film growth by ALD enables the fabrication of electrodes with a very low noble metal loading. Nonetheless, these electrodes exhibit very high catalytic activity and good stability under cyclic and constant load conditions. At an IrO2 content of less than 10 percent by mass of the oxide material and an area-based Ir content of 153 μg cm-2, the nanostructured NTO/IrO2 electrode achieves an oxygen evolution current density of 1 mA cm-2 at an overpotential of ∼250 mV, which is significantly lower than the reported values for particulate NTO/IrO2 catalysts. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c9nr03013a
  • 2019 • 219 Synthesis of Mixed AuZn Nanoparticles by Spark Discharge Technique
    Kala, S. and Kruis, F.E.
    MRS Advances 4 1621-1629 (2019)
    In this study, feasibility of spark discharge technique to generate mixed metal nanoparticles is demonstrated. Two immiscible metals Au and Zn are selected to prepare AuZn mixed nanoparticles. Ignition of spark between Au and Zn electrodes under normal pressure, in the presence of carrier gas, leads to formation of mixed nanoparticles by condensation and nucleation. Online particle size-distribution is monitored by a scanning mobility particle sizer on changing carrier gas flow rate and capacitor charging current during co-sparking between Au and Zn electrodes. The technique provides flexibility to generate binary mixture of AuZn nanoparticles in the size range of 10-80 nm. Distribution of Au and Zn in the prepared mixed nanoparticles is mapped by scanning electron microscopy and high resolution electron microscopy. © Materials Research Society 2019.
    view abstractdoi: 10.1557/adv.2019.221
  • 2019 • 218 Epitaxial and contamination-free Co(0001) electrodes on insulating substrates for molecular spintronic devices
    Königshofen, S. and Matthes, F. and Bürgler, D.E. and Schneider, C.M. and Dirksen, E. and Müller, T.J.J.
    Thin Solid Films 680 67-74 (2019)
    The growing field of molecular spintronics is an auspicious route to future concepts of data storage and processing. It has been reported that the hybridization of the electronic structures of non-magnetic organic molecules and ferromagnetic transition-metal (FM) surfaces can form new magnetic units, so-called hybrid molecular magnets, with distinct magnetic properties, which promise molecular spintronic devices with extremely high information density and low energy consumption. The investigation and profound understanding of these device concepts require the formation of clean and epitaxial interfaces between the surface of a FM bottom electrode and molecular thin films. This can only be realized under ultra-high vacuum conditions. In addition, the FM electrodes must be grown on an insulating substrate to electrically separate neighboring devices. Here, we report on procedures to realize an entirely in-situ preparation of mesoscopic test devices featuring structurally and chemically well-defined interfaces. Au(111)-buffered Co(0001) electrodes are deposited by molecular-beam epitaxy onto sapphire or mica substrates using a shadow-mask to define the geometry. The surface quality is subsequently characterized by scanning tunneling microscopy (STM) and other surface science analysis tools. 2,7-dibenzyl 1,4,5,8-naphthalenetetracarboxylic diimide (BNTCDI), which serves as an exemplary molecule, is sublimed through another shadow-mask, and the interface formation in the monolayer regime is also studied by STM. Finally, we deposit a Cu top electrode through yet another shadow-mask to complete a mesoscopic (200 × 200 μm2) test device, which reveals in ex-situ transport measurements for the Co/BNTCDI/Cu junction non-metallic behavior and a resistance-area product of 24 MΩ·μm2 at 10 K. © 2019
    view abstractdoi: 10.1016/j.tsf.2019.04.021
  • 2019 • 217 Block copolymer-directed synthesis of porous anatase for lithium-ion battery electrodes
    McRae, O.F. and Xia, Q. and Tjaberings, S. and Gröschel, A.H. and Ling, C.D. and Müllner, M.
    Journal of Polymer Science, Part A: Polymer Chemistry 57 1890-1896 (2019)
    A templating method is developed to produce porous nanocrystalline anatase materials for negative electrodes in lithium-ion batteries (LIBs). Amphiphilic diblock copolymers are used to generate template films with phase-separated internal structure. Subsequent swelling with acidified titanium(IV) bis(ammonium lactato) dihydroxide (TALH) solution yielded structured hybrid films. Upon heating, the formation of TiO2 nanocrystals is induced, resulting in a three-dimensional mesoporous structure directed by the bulk morphology of the polymer template. In comparison to commercial nanosized anatase, the structured anatase shows significant performance improvements in lithium-ion coin cell batteries in terms of capacity, stability, and rate capability. © 2018 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2019, 57, 1890–1896. © 2018 Wiley Periodicals, Inc.
    view abstractdoi: 10.1002/pola.29312
  • 2019 • 216 Magnetic control of nonlinear electron resonance heating in a capacitively coupled radio frequency discharge
    Oberberg, M. and Engel, D. and Berger, B. and Wölfel, C. and Eremin, D. and Lunze, J. and Brinkmann, R.P. and Awakowicz, P. and Schulze, J.
    Plasma Sources Science and Technology 28 (2019)
    In magnetized capacitively coupled radio frequency (RF) plasmas operated at low pressure, the magnetic asymmetry effect (MAE) provides the opportunity to control the discharge symmetry, the DC self-bias, and the ion energy distribution functions at boundary surfaces by adjusting a magnetic field, that is oriented parallel to the electrodes, at one electrode, while leaving it constant at the opposite electrode. This effect is caused by the presence of different plasma densities in regions of different magnetic field strength. Here, based on a balanced magnetron magnetic field configuration at the powered electrode, we demonstrate that the magnetic control of the plasma symmetry allows to tailor the generation of high frequency oscillations in the discharge current induced by the self-excitation of the plasma series resonance (PSR) through adjusting the magnetic field adjacent to the powered electrode. Experimental current measurements performed in an argon discharge at 1 Pa as well as results of an equivalent circuit model show that nonlinear electron resonance heating can be switched on and off in this way. Moreover, the self-excitation of the PSR can be shifted in time (within the RF period) and in space (from one electrode to the other) by controlling the discharge symmetry via adjusting the magnetic field. © 2019 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/ab53a0
  • 2019 • 215 Bio-inspired design: Bulk iron-nickel sulfide allows for efficient solvent-dependent CO 2 reduction
    Piontek, S. and Junge Puring, K. and Siegmund, D. and Smialkowski, M. and Sinev, I. and Tetzlaff, D. and Roldan Cuenya, B. and Apfel, U.-P.
    Chemical Science 10 1075-1081 (2019)
    The electrocatalytic reduction of carbon dioxide (CO 2 RR) to valuable bulk chemicals is set to become a vital factor in the prevention of environmental pollution and the selective storage of sustainable energy. Inspired by structural analogues to the active site of the enzyme CODH Ni , we envisioned that bulk Fe/Ni sulfides would enable the efficient reduction of CO 2 . By careful adjustment of the process conditions, we demonstrate that pentlandite (Fe 4.5 Ni 4.5 S 8 ) electrodes, in addition to HER, also support the CO 2 RR reaching a peak faradaic efficiency of 87% and 13% for the formation of CO and methane, respectively at 3 mA cm -2 . The choice of solvent, the presence of water/protons and CO 2 solubility are identified as key-properties to adjust the balance between HER and CO 2 RR in favour of the latter. Such experiments can thus serve as model reactions to elucidate a potential catalyst within gas diffusion electrodes. © 2019 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8sc03555e
  • 2019 • 214 Enhancing Light Emission in Interface Engineered Spin-OLEDs through Spin-Polarized Injection at High Voltages
    Prieto-Ruiz, J.P. and Miralles, S.G. and Prima-García, H. and López-Muñoz, A. and Riminucci, A. and Graziosi, P. and Aeschlimann, M. and Cinchetti, M. and Dediu, V.A. and Coronado, E.
    Advanced Materials 31 (2019)
    The quest for a spin-polarized organic light-emitting diode (spin-OLED) is a common goal in the emerging fields of molecular electronics and spintronics. In this device, two ferromagnetic (FM) electrodes are used to enhance the electroluminescence intensity of the OLED through a magnetic control of the spin polarization of the injected carriers. The major difficulty is that the driving voltage of an OLED device exceeds a few volts, while spin injection in organic materials is only efficient at low voltages. The fabrication of a spin-OLED that uses a conjugated polymer as bipolar spin collector layer and ferromagnetic electrodes is reported here. Through a careful engineering of the organic/inorganic interfaces, it is succeeded in obtaining a light-emitting device showing spin-valve effects at high voltages (up to 14 V). This allows the detection of a magneto-electroluminescence (MEL) enhancement on the order of a 2.4% at 9 V for the antiparallel (AP) configuration of the magnetic electrodes. This observation provides evidence for the long-standing fundamental issue of injecting spins from magnetic electrodes into the frontier levels of a molecular semiconductor. The finding opens the way for the design of multifunctional devices coupling the light and the spin degrees of freedom. © 2019 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adma.201806817
  • 2019 • 213 A Z-Scheme-Inspired Photobioelectrochemical H2O/O2 Cell with a 1 V Open-Circuit Voltage Combining Photosystem II and PbS Quantum Dots
    Riedel, M. and Wersig, J. and Ruff, A. and Schuhmann, W. and Zouni, A. and Lisdat, F.
    Angewandte Chemie - International Edition 58 801-805 (2019)
    A biohybrid photobioanode mimicking the Z-scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO2 architecture giving rise to a rather negative water oxidation potential of about −0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light-sensitive entities has been established through an Os-complex-modified redox polymer (POs), which allows the formation of a multi-step electron-transfer chain under illumination starting with the photo-activated water oxidation at PSII followed by an electron transfer from PSII through POs to the photo-excited QDs and finally to the TiO2 electrode. The photobioanode was coupled to a novel, transparent, inverse-opal ATO cathode modified with an O2-reducing bilirubin oxidase for the construction of a H2O/O2 photobioelectrochemical cell reaching a high open-circuit voltage of about 1 V under illumination. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201811172
  • 2019 • 212 A Z-Scheme-Inspired Photobioelectrochemical H 2 O/O 2 Cell with a 1 V Open-Circuit Voltage Combining Photosystem II and PbS Quantum Dots
    Riedel, M. and Wersig, J. and Ruff, A. and Schuhmann, W. and Zouni, A. and Lisdat, F.
    Angewandte Chemie - International Edition 58 801-805 (2019)
    A biohybrid photobioanode mimicking the Z-scheme has been developed by functional integration of photosystem II (PSII) and PbS quantum dots (QDs) within an inverse opal TiO 2 architecture giving rise to a rather negative water oxidation potential of about −0.55 V vs. Ag/AgCl, 1 m KCl at neutral pH. The electrical linkage between both light-sensitive entities has been established through an Os-complex-modified redox polymer (P Os ), which allows the formation of a multi-step electron-transfer chain under illumination starting with the photo-activated water oxidation at PSII followed by an electron transfer from PSII through P Os to the photo-excited QDs and finally to the TiO 2 electrode. The photobioanode was coupled to a novel, transparent, inverse-opal ATO cathode modified with an O 2 -reducing bilirubin oxidase for the construction of a H 2 O/O 2 photobioelectrochemical cell reaching a high open-circuit voltage of about 1 V under illumination. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201811172
  • 2019 • 211 Polymer-Bound DuBois-Type Molecular H 2 Oxidation Ni Catalysts Are Protected by Redox Polymer Matrices
    Ruff, A. and Janke, S. and Szczesny, J. and Alsaoub, S. and Ruff, I. and Lubitz, W. and Schuhmann, W.
    ACS Applied Energy Materials 2 2921-2929 (2019)
    The immobilization, protection, and electrical wiring of sensitive catalysts by specifically designed supporting matrixes are of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H 2 oxidation catalyst, which was covalently bound to an inert polymer matrix, against molecular O 2 by forming blends together with an O 2 -reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b00269
  • 2019 • 210 Polymer-Bound DuBois-Type Molecular H2 Oxidation Ni Catalysts Are Protected by Redox Polymer Matrices
    Ruff, A. and Janke, S. and Szczesny, J. and Alsaoub, S. and Ruff, I. and Lubitz, W. and Schuhmann, W.
    ACS Applied Energy Materials 2 2921-2929 (2019)
    The immobilization, protection, and electrical wiring of sensitive catalysts by specifically designed supporting matrixes are of particular importance for technological relevant applications. Here, we describe the protection of a DuBois-type H2 oxidation catalyst, which was covalently bound to an inert polymer matrix, against molecular O2 by forming blends together with an O2-reducing redox polymer matrix. This matrix simultaneously acts as an electron relay for shuttling electrons between the catalyst and the electrode. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acsaem.9b00269
  • 2019 • 209 Facile Protocol for Alkaline Electrolyte Purification and Its Influence on a Ni-Co Oxide Catalyst for the Oxygen Evolution Reaction
    Spanos, I. and Tesch, M.F. and Yu, M. and Tüysüz, H. and Zhang, J. and Feng, X. and Müllen, K. and Schlögl, R. and Mechler, A.K.
    ACS Catalysis 9 8165-8170 (2019)
    We report a simple and effective electrochemical method to remove Fe impurities from commercial KOH electrolyte. We therefore utilize a MoS2 catalyst deposited on porous Ni foam as both the anode and cathode in a two-electrode electrolysis setup. After 12 h of constant galvanostatic electrolysis at 100 mA, the Fe impurities from the KOH electrolyte were successfully removed, as confirmed by means of inductively coupled plasma optical emission spectroscopy analysis. In the purified KOH, a Ni-Co3O4 composite oxide catalyst showed no Fe-induced activation. In contrast, we directly observed the uptake of Fe on the Ni-Co3O4 catalyst from the nontreated electrolyte during catalyst operation using a coupled spectroelectrochemical setup. Interestingly, we further identified an influence on the dissolution behavior of Ni and Co in the presence of Fe impurities. Whereas hitherto mainly the activation effect of Fe impurities has been discussed, we hereby show that they additionally suppress corrosion under reaction conditions. Using our fast and low-cost method for the purification of large amounts of electrolyte, catalyst materials can be widely studied without these additional effects induced by Fe impurities in commercial KOH. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.9b01940
  • 2019 • 208 A garnet structure-based all-solid-state Li battery without interface modification: Resolving incompatibility issues on positive electrodes
    Tsai, C.-L. and Ma, Q. and Dellen, C. and Lobe, S. and Vondahlen, F. and Windmüller, A. and Grüner, D. and Zheng, H. and Uhlenbruck, S. and Finsterbusch, M. and Tietz, F. and Fattakhova-Rohlfing, D. and Buchkremer, H.P. and Guillon, O.
    Sustainable Energy and Fuels 3 280-291 (2019)
    The development of high-performance Li 7 La 3 Zr 2 O 12 (LLZO)-based all-solid-state lithium batteries (SSLB) is usually hampered by highly resistive interfaces due to the need for sintering at elevated temperatures to form ionic diffusion paths through the grains. Many strategies have been proposed to solve the problem but the achievements have been limited. Herein, a new design principle is introduced, based on co-sintering crystalline LCO and Ta-substituted LLZO instead of using the more reactive Li-Co-O precursors and Al-substituted LLZO, which allows the fabrication of high specific areal density and low cell area resistance without the interface modification of LLZO-based SSLB. Detailed studies using micro-Raman and EDS mapping revealed that the well-sintered interfaces are free from detrimental secondary phases. To demonstrate that a true bulk-type SSLB can be constructed by this straightforward strategy, the material loading for a composite positive electrode was increased to about 10 times that in previous reports, which resulted in a high areal capacity of 1.63 mA h cm -2 (i.e. 110 mA h g -1 ) when discharged with a current density of 50 μA cm -2 . It also allows one to discharge the fabricated SSLB at a very high current density of 500 μA cm -2 at 50 °C due to the minimized cell areal resistance. The new fabrication strategy for the LLZO-based SSLB paves the way for achieving SSLB with high safety and energy density. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8se00436f
  • 2019 • 207 Nanosized Lithium-Rich Cobalt Oxide Particles and Their Transformation to Lithium Cobalt Oxide Cathodes with Optimized High-Rate Morphology
    Zehetmaier, P.M. and Cornélis, A. and Zoller, F. and Böller, B. and Wisnet, A. and Döblinger, M. and Böhm, D. and Bein, T. and Fattakhova-Rohlfing, D.
    Chemistry of Materials (2019)
    We report the formation of crystalline dispersible LixCo1-xOy (with y 1) nanoparticles with an unusual rock-salt phase containing â¼15 at. % Li in the crystalline structure. This is the first time that this composition was formed at temperatures as low as 150 °C under conditions of a solvothermal process, although it is referred to as a higherature metastable phase in a very limited number of known publications. The Li0.15Co0.85Oy nanoparticles of 2-3 nm size completely transform to higherature LiCoO2 (HT-LCO) nanoparticles at 560 °C in the presence of slightly overstoichiometric amounts of Li source. The presence of lithium in the CoO lattice slows down the kinetics of its phase transformation, enabling to obtain very small HT-LCO nanocrystals during the subsequent calcination. The HT-LCO particles formed after this transformation have an elongated shape with a mean size of about 17 × 60 nm, which is targeted as an optimum size for battery applications. An attractive feature of the Li0.15Co0.85Oy nanoparticles is their high dispersibility enabling their assembly into different nanostructures with optimized morphology. Open porous HT-LCO electrodes prepared via self-assembly of Li0.15Co0.85Oy nanoparticles and Pluronic F127 as a structure-directing agent demonstrate very good performances at high current densities representing short charge/discharge times below 10 min. Even at a charge/discharge time of 72 s (50C), 50% of the theoretical capacity has been preserved. After 250 cycles at a charge/discharge time of 6 min (10C), over 60% of the initial discharge capacity was retained. © 2019 American Chemical Society.
    view abstractdoi: 10.1021/acs.chemmater.9b02231
  • 2019 • 206 A light-driven Nernstian biosupercapacitor
    Zhao, F. and Bobrowski, T. and Ruff, A. and Hartmann, V. and Nowaczyk, M.M. and Rögner, M. and Conzuelo, F. and Schuhmann, W.
    Electrochimica Acta 306 660-666 (2019)
    Following inspiration by natural photosynthesis, the design and fabrication of semi-artificial biophotoelectrochemical devices able to harvest solar energy and aiming on the implementation of green and sustainable energy conversion systems is presently an important field of research. Here we present the development of a fully light-driven biosupercapacitor fabricated by incorporation of isolated photosystem 2 and photosystem 1 protein complexes embedded within the same Os-complex modified redox polymer. By this, light energy is stored at both electrodes within the polymer-based pseudocapacitive matrix in the form of Os 3+ centers at the photosystem1-based biocathode and in the form of Os 2+ centers at the photosystem 2-based bioanode. The stored energy can be released on demand into bursts of electricity. Due to the purely light-driven self-charging process, the biosupercapacitor provided a power output of 1.0 μW cm −2 after 200 s charging time. Moreover, the use of different electrode materials and their implication on the performance of the implemented biodevice is evaluated. © 2019 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2019.03.168
  • 2018 • 205 Antibacterial Efficacy of Sacrifical Anode Thin Films Combining Silver with Platinum Group Elements within a Bacteria-Containing Human Plasma Clot
    Abuayyash, A. and Ziegler, N. and Gessmann, J. and Sengstock, C. and Schildhauer, T.A. and Ludwig, Al. and Köller, M.
    Advanced Engineering Materials 20 (2018)
    Silver (Ag) dots arrays (64 and 400 dots per mm2) are fabricated on a continuous platinum (Pt), palladium (Pd), or iridium (Ir) thin film (sacrifical anode systems for Ag) and for comparison on titanium (Ti) film (non-sacrifical anode system for Ag) by sputter deposition and photolithographic patterning. The samples are embedded within a tissue-like plasma clot matrix containing Staphylococcus aureus (S. aureus), cultivated for 24 h. Bacterial growth is analyzed by fluorescence microscopy. Among platinum group sacrifical anode elements and a dense Ag sample, only the high Ag ion releasing Ag–Ir system is able to inhibit the bacterial growth within the adjacent plasma clot matrix. This study demonstrates that the antibacterial efficiency of Ag coatings is reduced under tissue-like conditions. However, the new sacrificial anode based Ag–Ir system can overcome this limitation. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adem.201700493
  • 2018 • 204 Rechargeable, flexible and mediator-free biosupercapacitor based on transparent ITO nanoparticle modified electrodes acting in µM glucose containing buffers
    Bobrowski, T. and González Arribas, E. and Ludwig, R. and Toscano, M.D. and Shleev, S. and Schuhmann, W.
    Biosensors and Bioelectronics 101 84-89 (2018)
    We present a transparent and flexible self-charging biosupercapacitor based on an optimised mediator- and membrane-free enzymatic glucose/oxygen biofuel cell. Indium tin oxide (ITO) nanoparticles were spray-coated on transparent conducting ITO supports resulting in a flocculent, porous and nanostructured electrode surface. By this, high capacitive currents caused by an increased electrochemical double layer as well as enhanced catalytic currents due to a higher number of immobilised enzyme molecules were obtained. After a chemical pre-treatment with a silane derivative, bilirubin oxidase from Myrothecium verrucaria was immobilized onto the ITO nanostructured electrode surface under formation of a biocathode, while bioanodes were obtained by either immobilisation of cellobiose dehydrogenase from Corynascus thermophilus or soluble PQQ-dependent glucose dehydrogenase from Acinetobacter calcoaceticus. The latter showed a lower apparent KM value for glucose conversion and higher catalytic currents at µM glucose concentrations. Applying the optimised device as a biosupercapacitor in a discontinuous charge/discharge mode led to a generated power output of 0.030 mW/cm2 at 50 µM glucose, simulating the glucose concentration in human tears. This represents an enhancement by a factor of 350 compared to the power density obtained from the continuously operating biofuel cell with a maximum power output of 0.086 µW/cm2 under the same conditions. After 17 h of charging/discharging cycles a remarkable current enhancement was still measured. The entire device was transferred to flexible materials and applied for powering a flexible display showing its potential applicability as an intermittent power source in smart contact lenses. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2017.10.016
  • 2018 • 203 Efficient p-n junction-based thermoelectric generator that can operate at extreme temperature conditions
    Chavez, R. and Angst, S. and Hall, J. and Maculewicz, F. and Stoetzel, J. and Wiggers, H. and Thanh Hung, L. and Van Nong, N. and Pryds, N. and Span, G. and Wolf, D.E. and Schmechel, R. and Schierning, G.
    Journal of Physics D: Applied Physics 51 (2018)
    In many industrial processes, a large proportion of energy is lost in the form of heat. Thermoelectric generators can convert this waste heat into electricity by means of the Seebeck effect. However, the use of thermoelectric generators in practical applications on an industrial scale is limited in part because electrical, thermal, and mechanical bonding contacts between the semiconductor materials and the metal electrodes in current designs are not capable of withstanding thermal-mechanical stress and alloying of the metal-semiconductor interface when exposed to the high temperatures occurring in many real-world applications. Here we demonstrate a concept for thermoelectric generators that can address this issue by replacing the metallization and electrode bonding on the hot side of the device by a p-n junction between the two semiconductor materials, making the device robust against temperature induced failure. In our proof-of-principle demonstration, a p-n junction device made from nanocrystalline silicon is at least comparable in its efficiency and power output to conventional devices of the same material and fabrication process, but with the advantage of sustaining high hot side temperatures and oxidative atmosphere. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6463/aa9b6a
  • 2018 • 202 The Open Circuit Voltage in Biofuel Cells: Nernstian Shift in Pseudocapacitive Electrodes
    Conzuelo, F. and Marković, N. and Ruff, A. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 13681-13685 (2018)
    In the development of biofuel cells great effort is dedicated to achieving outstanding figures of merit, such as high stability, maximum power output, and a large open circuit voltage. Biofuel cells with immobilized redox mediators, such as redox polymers with integrated enzymes, show experimentally a substantially higher open circuit voltage than the thermodynamically expected value. Although this phenomenon is widely reported in the literature, there is no comprehensive understanding of the potential shift, the high open circuit voltages have not been discussed in detail, and hence they are only accepted as an inherent property of the investigated systems. We demonstrate that this effect is the result of a Nernstian shift of the electrode potential when catalytic conversion takes place in the absence or at very low current flow. Experimental evidence confirms that the immobilization of redox centers on the electrode surface results in the assembled biofuel cell delivering a higher power output because of charge storage upon catalytic conversion. Our findings have direct implications for the design and evaluation of (bio)fuel cells with pseudocapacitive elements. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201808450
  • 2018 • 201 Exceeding 6500 cycles for LiFePO4/Li metal batteries through understanding pulsed charging protocols
    García, G. and Dieckhöfer, S. and Schuhmann, W. and Ventosa, E.
    Journal of Materials Chemistry A 6 4746-4751 (2018)
    Improving the performance of Li metal anodes is of key importance for the next generation high energy-density batteries. Here, we study an easily implementable strategy for prolonging the cycle stability of Li metal anodes that is based on the application of pulsed charging protocols. Introducing short periods of relaxation without current flow allows the concentration of Li+ ions to be replenished in front of the electrode surface promoting a uniform and efficient plating of Li metal. We demonstrate that the cycle life of LiFePO4/Li metal batteries is prolonged from 700 to more than 6500 cycles at high charge-rates. In contrast to the assumed failure due to Li dendrite formation, we show that the proposed potential pulse protocols mitigate the growth of a porous film within the Li metal electrode which appears to be responsible for the battery failure. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c8ta00962g
  • 2018 • 200 Electrochemical C-H Cyanation of Electron-Rich (Hetero)Arenes
    Hayrapetyan, D. and Rit, R.K. and Kratz, M. and Tschulik, K. and Gooßen, L.J.
    Chemistry - A European Journal (2018)
    A straightforward method for the electrochemical C-H cyanation of arenes and heteroarenes that proceeds at room temperature in MeOH, with NaCN as the reagent in a simple, open, undivided electrochemical cell is reported. The platinum electrodes are passivated by adsorbed cyanide, which allows conversion of an exceptionally broad range of electron-rich substrates all the way down to dialkyl arenes. The cyanide electrolyte can be replenished with HCN, opening opportunities for salt-free industrial C-H cyanation. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201802247
  • 2018 • 199 Robotic microplate voltammetry for real-time hydrogel drug release testing
    Jaikaew, W. and Ruff, A. and Khunkaewla, P. and Erichsen, T. and Schuhmann, W. and Schulte, A.
    Analytica Chimica Acta 1041 33-39 (2018)
    Robotic square wave voltammetry (SVW) in 24-well microtiter plates has been developed as a reliable non-manual procedure for quantifying drug release from pharmaceutical hydrogels. The assay was established using 1% agarose disks containing Paracetamol® (PCT) as a model preparation. Computerized buffer delivery and SVW in calibration and hydrogel sample wells were performed by a three-electrode arrangement combined with a thin plastic tube. For the glassy carbon working electrode of the assembly the upper limit of the linear response and the lower detection limit of sequential ‘in-well’ PCT-SVW were 1000 and 0.5 μM, respectively. During non-stop runs through plate wells with equal drug titers the voltammetric PCT signal was stable for at least 6 h. For the construction of drug-release curves with triplicate data points PCT-SVW was performed sequentially on three identical hydrogel samples in neighboring plate wells, preceded and followed by sensor calibrations for response validation. The results showed bi-phasic PCT release profiles exhibiting an initial rapid loss of the drug near the surface of the gel, followed by slowly decelerating release of more deeply buried drug and the dissipation of the concentration gradient that drives diffusion. The proposed automation of voltammetric testing generates reliable hydrogel drug release profiles without the need for operator intervention, avoiding human errors from monotonous manual electroanalysis and releasing skilled staff for other work. This approach is therefore suggested as an economic option for hydrogel dissolution testing in academic or industrial R&D, particularly when the required multi-parameter optimization creates many samples. © 2018 Elsevier B.V.
    view abstractdoi: 10.1016/j.aca.2018.08.033
  • 2018 • 198 Valence change detection in memristive oxide based heterostructure cells by hard X-ray photoelectron emission spectroscopy
    Kindsmüller, A. and Schmitz, C. and Wiemann, C. and Skaja, K. and Wouters, D.J. and Waser, R. and Schneider, C.M. and Dittmann, R.
    APL Materials 6 (2018)
    The switching mechanism of valence change resistive memory devices is widely accepted to be an ionic movement of oxygen vacancies resulting in a valence change of the metal cations. However, direct experimental proofs of valence changes in memristive devices are scarce. In this work, we have employed hard X-ray photoelectron emission microscopy (PEEM) to probe local valence changes in Pt/ZrOx/Ta memristive devices. The use of hard X-ray radiation increases the information depth, thus providing chemical information from buried layers. By extracting X-ray photoelectron spectra from different locations in the PEEM images, we show that zirconia in the active device area is reduced compared to a neighbouring region, confirming the valence change in the ZrOx film during electroforming. Furthermore, we succeeded in measuring the Ta 4f spectrum for two different resistance states on the same device. In both states, as well as outside the device region, the Ta electrode is composed of different suboxides without any metallic contribution, hinting to the formation of TaOx during the deposition of the Ta thin film. We observed a reduction of the Ta oxidation state in the low resistance state with respect to the high resistive state. This observation is contradictory to the established model, as the internal redistribution of oxygen between ZrOx and the Ta electrode during switching would lead to an oxidation of the Ta layer in the low resistance state. Instead, we have to conclude that the Ta electrode takes an active part in the switching process in our devices and that oxygen is released and reincorporated in the ZrOx/TaOx bilayer during switching. This is confirmed by the degradation of the high resistance state during endurance measurements under vacuum. © 2018 Author(s).
    view abstractdoi: 10.1063/1.5026063
  • 2018 • 197 Potential pulse-assisted immobilization of Myrothecium verrucaria bilirubin oxidase at planar and nanoporous gold electrodes
    Lopez, F. and Siepenkoetter, T. and Xiao, X. and Magner, E. and Schuhmann, W. and Salaj-Kosla, U.
    Journal of Electroanalytical Chemistry 812 194-198 (2018)
    A potential pulse-assisted approach was used to immobilize Myrothecium verrucaria bilirubin oxidase at planar and nanoporous gold electrodes (NPG) containing pores of ca. 20 nm and ca. 40 nm in diameter. An increase in the current due to the bioelectrocatalytic reduction of oxygen by MvBOD-modified gold electrodes obtained from a 20 μL drop by the proposed pulse-assisted approach was observed when compared to the response obtained with electrodes modified by drop-casting. This increase likely arises from a preferential orientation of MvBOD molecules at the planar gold surface obtained by fast switching of the potential pulses between opposite charges. The concomitant ion stirring effect induces the attraction of the enzymes to the charged gold surface and forces access to the internal pore volume of the NPG. Immobilization of MvBOD using the potential pulse-assisted approach significantly increases current densities by facilitating the electron transfer between the enzyme and the electrode surface. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jelechem.2017.12.023
  • 2018 • 196 Experimental investigations of the magnetic asymmetry effect in capacitively coupled radio frequency plasmas
    Oberberg, M. and Kallahn, J. and Awakowicz, P. and Schulze, J.
    Plasma Sources Science and Technology 27 (2018)
    The electrical asymmetry effect allows control of the discharge symmetry, the DC self-bias, and charged particle energy distribution functions electrically by driving a capacitive radio frequency discharge with multiple consecutive harmonics with fixed, but adjustable relative phases. Recently, Trieschmann et al (2013 J. Phys. D: Appl. Phys. 46 084016) and Yang et al (2017 Plasma Process. Polym. 14 1700087; 2018 Plasma Sources Sci. Technol. 27 035008) computationally predicted that the discharge symmetry can also be controlled magnetically via the magnetic asymmetry effect (MAE). By particle-in-cell simulations they demonstrated that a magnetic field, that is parallel to the electrodes and inhomogeneous in the direction perpendicular to the electrodes, induces a discharge asymmetry due to different ion densities adjacent to both electrodes. This, in turn, is predicted to lead to the generation of a DC self-bias as a function of the difference of the magnetic field at both electrodes. In this way the MAE should allow control of the mean ion energy at both electrodes as a function of the magnetic field configuration. Here, we present the first experimental investigation of the MAE. In a low pressure discharge operated in argon at 13.56 MHz, we use a magnetron-like magnetic field configuration at the powered electrode, which leads to an inhomogeneous profile of the magnetic field perpendicular to the electrodes. By measuring the DC self-bias and the ion flux-energy distribution function at the grounded electrode as a function of the magnetic field strength at the powered electrode, the driving voltage amplitude and the neutral gas pressure we experimentally verify the concept of the MAE and demonstrate this technology to be a powerful method to control the discharge symmetry and process relevant energy distribution functions. © 2018 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6595/aae199
  • 2018 • 195 Influence of the Fe:Ni Ratio and Reaction Temperature on the Efficiency of (FexNi1-x)9S8 Electrocatalysts Applied in the Hydrogen Evolution Reaction
    Piontek, S. and Andronescu, C. and Zaichenko, A. and Konkena, B. and Junge Puring, K. and Marler, B. and Antoni, H. and Sinev, I. and Muhler, M. and Mollenhauer, D. and Roldan Cuenya, B. and Schuhmann, W. and Apfel, U.-P.
    ACS Catalysis 8 987-996 (2018)
    Inspired by our recent finding that Fe4.5Ni4.5S8 rock is a highly active electrocatalyst for HER, we set out to explore the influence of the Fe:Ni ratio on the performance of the catalyst. We herein describe the synthesis of (FexNi1-x)9S8 (x = 0-1) along with a detailed elemental composition analysis. Furthermore, using linear sweep voltammetry, we show that the increase in the iron or nickel content, respectively, lowers the activity of the electrocatalyst toward HER. Electrochemical surface area analysis (ECSA) clearly indicates the highest amount of active sites for a Fe:Ni ratio of 1:1 on the electrode surface pointing at an altered surface composition of iron and nickel for the other materials. Specific metal-metal interactions seem to be of key importance for the high electrocatalytic HER activity, which is supported by DFT calculations of several surface structures using the surface energy as a descriptor of catalytic activity. In addition, we show that a temperature increase leads to a significant decrease of the overpotential and gain in HER activity. Thus, we showcase the necessity to investigate the material structure, composition and reaction conditions when evaluating electrocatalysts. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.7b02617
  • 2018 • 194 Balancing electrical and optical losses for efficient 4-terminal Si-perovskite solar cells with solution processed percolation electrodes
    Ramírez Quiroz, C.O. and Shen, Y. and Salvador, M. and Forberich, K. and Schrenker, N. and Spyropoulos, G.D. and Heumüller, T. and Wilkinson, B. and Kirchartz, T. and Spiecker, E. and Verlinden, P.J. and Zhang, X. and Green, M.A...
    Journal of Materials Chemistry A 6 3583-3592 (2018)
    The unprecedented rise in efficiency of perovskite-based photovoltaics has sparked interest in semi-transparent devices, particularly for tandem structures. Despite promising reports regarding efficiency and reduced parasitic absorption, many devices still rely on processes from the gas phase, compromising both applicability and cost factors. Here, we report all-solution perovskite solar cells with improved infrared transparency ideally suited as top-cells for efficient multi-junction device configurations. We demonstrate the functionality of copper(i) thiocyanate as antireflective layer and as selective contact between the transparent conductive oxide and the perovskite. This concept allows us to fabricate an opaque device with steady state efficiency as high as 20.1%. By employing silver nanowires with robust environmental stability as the bottom electrode, we demonstrate different regimes of device performance that can be described through a classical percolation model, leading to semi-transparent solar cells with efficiencies of up to 17.1%. In conjunction with the implementation of an infrared-tuned transparent conductive oxide contact deposited on UV-fused silica, we show a full device average transmittance surpassing 84% between 800 and 1100 nm (as opposed to 77% with PEDOT:PSS as the selective contact). Finally, we mechanically stacked optimized perovskite devices on top of high performing PERL and IBC silicon architectures. The measured imputed output efficiency of the 4-terminal perovskite-silicon solar cell was 26.7% and 25.2% for the PERL-perovskite and IBC-perovskite, respectively. © 2018 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ta10945h
  • 2018 • 193 Atomic Layer Deposition of Nickel on ZnO Nanowire Arrays for High-Performance Supercapacitors
    Ren, Q.-H. and Zhang, Y. and Lu, H.-L. and Wang, Y.-P. and Liu, W.-J. and Ji, X.-M. and Devi, A. and Jiang, A.-Q. and Zhang, D.W.
    ACS Applied Materials and Interfaces 10 468-476 (2018)
    A novel hybrid core-shell structure of ZnO nanowires (NWs)/Ni as a pseudocapacitor electrode was successfully fabricated by atomic layer deposition of a nickel shell, and its capacitive performance was systemically investigated. Transmission electron microscopy and X-ray photoelectron spectroscopy results indicated that the NiO was formed at the interface between ZnO and Ni where the Ni was oxidized by ZnO during the ALD of the Ni layer. Electrochemical measurement results revealed that the Ti/ZnO NWs/Ni (1500 cycles) electrode with a 30 nm thick Ni-NiO shell layer had the best supercapacitor properties including ultrahigh specific capacitance (∼2440 F g-1), good rate capability (80.5%) under high current charge-discharge conditions, and a relatively better cycling stability (86.7% of the initial value remained after 750 cycles at 10 A g-1). These attractive capacitive behaviors are mainly attributed to the unique core-shell structure and the combined effect of ZnO NW arrays as short charge transfer pathways for ion diffusion and electron transfer as well as conductive Ni serving as channel for the fast electron transport to Ti substrate. This high-performance Ti/ZnO NWs/Ni hybrid structure is expected to be one of a promising electrodes for high-performance supercapacitor applications. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b13392
  • 2018 • 192 Light as Trigger for Biocatalysis: Photonic Wiring of Flavin Adenine Dinucleotide-Dependent Glucose Dehydrogenase to Quantum Dot-Sensitized Inverse Opal TiO2 Architectures via Redox Polymers
    Riedel, M. and Parak, W.J. and Ruff, A. and Schuhmann, W. and Lisdat, F.
    ACS Catalysis 8 5212-5220 (2018)
    The functional coupling of photoactive nanostructures with enzymes creates a strategy for the design of light-triggered biocatalysts. This study highlights the efficient wiring of flavin adenine dinucleotide (FAD)-dependent glucose dehydrogenase (FAD-GDH) to PbS quantum dot (QD)-sensitized inverse opal TiO2 electrodes (IO-TiO2) by means of an Os-complex-containing redox polymer for the light-driven glucose oxidation. For the construction of IO-TiO2 scaffolds, a template approach has been developed, enabling the tunability of the surface area and a high loading capacity for the integration of QDs, redox polymer, and enzyme. The biohybrid signal chain can be switched on with light, generating charge carriers within the QDs, triggering a multistep electron-transfer cascade from the enzyme toward the redox polymer via the QDs and finally to the IO-TiO2 electrode. The resulting anodic photocurrent can be modulated by the potential, the excitation intensity, and the glucose concentration, providing a new degree of freedom for the control of biocatalyic reactions at electrode interfaces. Maximum photocurrents of 207 μA cm-2 have been achieved in the presence of glucose, and a first gain of electrons from the biocatalytic reaction is found at -540 mV vs Ag/AgCl, 1 M KCl, which lowers the working potential by >500 mV as compared to light-insensitive electrodes. The biohybrid system combines the advantages of a high surface area of IO films, an efficient charge-carrier generation and separation at the QDs/TiO2 interface, and an efficient wiring of FAD-GDH to the QDs via a redox polymer, resulting in photo(bio)anodes of high performance for sensing and power supply. © 2018 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.8b00951
  • 2018 • 191 A fully protected hydrogenase/polymer-based bioanode for high-performance hydrogen/glucose biofuel cells
    Ruff, A. and Szczesny, J. and Marković, N. and Conzuelo, F. and Zacarias, S. and Pereira, I.A.C. and Lubitz, W. and Schuhmann, W.
    Nature Communications 9 (2018)
    Hydrogenases with Ni- and/or Fe-based active sites are highly active hydrogen oxidation catalysts with activities similar to those of noble metal catalysts. However, the activity is connected to a sensitivity towards high-potential deactivation and oxygen damage. Here we report a fully protected polymer multilayer/hydrogenase-based bioanode in which the sensitive hydrogen oxidation catalyst is protected from high-potential deactivation and from oxygen damage by using a polymer multilayer architecture. The active catalyst is embedded in a low-potential polymer (protection from high-potential deactivation) and covered with a polymer-supported bienzymatic oxygen removal system. In contrast to previously reported polymer-based protection systems, the proposed strategy fully decouples the hydrogenase reaction form the protection process. Incorporation of the bioanode into a hydrogen/glucose biofuel cell provides a benchmark open circuit voltage of 1.15 V and power densities of up to 530 µW cm−2 at 0.85 V. © 2018, The Author(s).
    view abstractdoi: 10.1038/s41467-018-06106-3
  • 2018 • 190 Time-resolved impact electrochemistry - A new method to determine diffusion coefficients of ions in solution
    Saw, E.N. and Blanc, N. and Kanokkanchana, K. and Tschulik, K.
    Electrochimica Acta 282 317-323 (2018)
    Diffusion is often the rate-limiting factor of reactions in condensed phase. Thus, knowing the diffusion coefficient is key in numerous aspects ranging from drug release to steering of reactions in both homogeneous liquid phase and electrochemical reactions. Cyclic voltammetry at macro electrodes and chronoamperometry at micro electrodes are well-established methods to determine the diffusion coefficients of redox-active species dissolved in a solution. However, if the formal potentials of the redox species are outside of the potential window of the solvent, then these methods cannot be readily applied. Here we demonstrate a new concept to determine the diffusion coefficient of ions to overcome this limitation. We use their reaction with a well-defined amount of a redox-active indicator substance, which is confined in a nanoparticle suspended in a solution containing the species of interest. Employing transformative nanoparticle impact analysis, the diffusion-limited reaction of an indicator nanoparticle with the species of interest is initiated and followed by chronoamperometry. Measuring the time it takes to fully convert the indicator particle enables the determination of the diffusion coefficient of interest. This concept is demonstrated for variety of (pseudo-)halides in aqueous solution using Ag nanoparticles as redox indicator. Using chloride as an example, is further shown that this new methodology can be applied to study effects of temperature and viscosity on the diffusion coefficients. Given the multitude of nanoparticles that may serve as electrochemical redox indicator, this approach can be used to determine the diffusion coefficients for a large variety of species in different liquid environments. © 2018 Elsevier Ltd
    view abstractdoi: 10.1016/j.electacta.2018.06.013
  • 2018 • 189 Effect of Pt and Au current collector in LiMn2O4 thin film for micro-batteries
    Trócoli, R. and Dushina, A. and Borhani-Haghighi, S. and Ludwig, Al. and La Mantia, F.
    Nanotechnology 29 (2018)
    The crystal orientation and morphology of sputtered LiMn2O4 thin films is strongly affected by the current collector. By substituting Pt with Au, it is possible to observe in the x-ray diffraction pattern of LiMn2O4 a change in the preferential orientation of the grains from (111) to (400). In addition, LiMn2O4 thin films deposited on Au show a higher porosity than films deposited on Pt. These structural differences cause an improvement in the electrochemical performances of the thin films deposited on Au, with up to 50% more specific charge. Aqueous cells using thin film based on LiMn2O4 sputtered on Au or Pt as the cathode electrode present a similar retention of specific charge, delivering 85% and 100%, respectively, of the initial values after 100 cycles. The critical role of the nature of the substrate used in the morphology and electrochemical behaviour observed could permit the exploration of similar effects for other lithium intercalation electrodes. © 2017 IOP Publishing Ltd.
    view abstractdoi: 10.1088/1361-6528/aa9e33
  • 2018 • 188 Electrostatic Self-Assembly Enabling Integrated Bulk and Interfacial Sodium Storage in 3D Titania-Graphene Hybrid
    Xu, G.-L. and Xiao, L. and Sheng, T. and Liu, J. and Hu, Y.-X. and Ma, T. and Amine, R. and Xie, Y. and Zhang, X. and Liu, Y. and Ren, Y. and Sun, C.-J. and Heald, S.M. and Kovacevic, J. and Sehlleier, Y.H. and Schulz, C. and Matt...
    Nano Letters 18 336-346 (2018)
    Room-temperature sodium-ion batteries have attracted increased attention for energy storage due to the natural abundance of sodium. However, it remains a huge challenge to develop versatile electrode materials with favorable properties, which requires smart structure design and good mechanistic understanding. Herein, we reported a general and scalable approach to synthesize three-dimensional (3D) titania-graphene hybrid via electrostatic-interaction-induced self-assembly. Synchrotron X-ray probe, transmission electron microscopy, and computational modeling revealed that the strong interaction between titania and graphene through comparably strong van der Waals forces not only facilitates bulk Na+ intercalation but also enhances the interfacial sodium storage. As a result, the titania-graphene hybrid exhibits exceptional long-term cycle stability up to 5000 cycles, and ultrahigh rate capability up to 20 C for sodium storage. Furthermore, density function theory calculation indicated that the interfacial Li+, K+, Mg2+, and Al3+ storage can be enhanced as well. The proposed general strategy opens up new avenues to create versatile materials for advanced battery systems. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.7b04193
  • 2017 • 187 Tapering of nanoelectrodes for an intracellular contact via a double hard mask technique
    Allani, S. and Jupe, A. and Figge, M. and Goehlich, A. and Vogt, H.
    PRIME 2017 - 13th Conference on PhD Research in Microelectronics and Electronics, Proceedings 305-308 (2017)
    To realize an intracellular contact between nanoelectrodes and cells, a sufficient small electrode diameter is needed [1]. A sacrificial layer process developed by the Fraunhofer IMS using deep reactive ion etching and atomic layer deposition [2] is varied. A double hard mask technique is used to taper structures in a sacrificial layer and thereby the nanoelectrodes' diameter. The principles and evaluation of the spacing technique, which allows the fabrication of sub-lithographic structures, are presented here. © 2017 IEEE.
    view abstractdoi: 10.1109/PRIME.2017.7974168
  • 2017 • 186 Overcoming the Instability of Nanoparticle-Based Catalyst Films in Alkaline Electrolyzers by using Self-Assembling and Self-Healing Films
    Barwe, S. and Masa, J. and Andronescu, C. and Mei, B. and Schuhmann, W. and Ventosa, E.
    Angewandte Chemie - International Edition 56 8573-8577 (2017)
    Engineering stable electrodes using highly active catalyst nanopowders for electrochemical water splitting remains a challenge. We report an innovative and general approach for attaining highly stable catalyst films with self-healing capability based on the in situ self-assembly of catalyst particles during electrolysis. The catalyst particles are added to the electrolyte forming a suspension that is pumped through the electrolyzer. Particles with negatively charged surfaces stick onto the anode, while particles with positively charged surfaces stick to the cathode. The self-assembled catalyst films have self-healing properties as long as sufficient catalyst particles are present in the electrolyte. The proof-of-concept was demonstrated in a non-zero gap alkaline electrolyzer using NiFe-LDH and NixB catalyst nanopowders for anode and cathode, respectively. Steady cell voltages were maintained for at least three weeks during continuous electrolysis at 50–100 mA cm−2. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/anie.201703963
  • 2017 • 185 Reforming results of a novel radial reactor for a solid oxide fuel cell system with anode off-gas recirculation
    Bosch, T. and Carré, M. and Heinzel, A. and Steffen, M. and Lapicque, F.
    Journal of Power Sources 371 197-208 (2017)
    A novel reactor of a natural gas (NG) fueled, 1 kW net power solid oxide fuel cell (SOFC) system with anode off-gas recirculation (AOGR) is experimentally investigated. The reactor operates as pre-reformer, is of the type radial reactor with centrifugal z-flow, has the shape of a hollow cylinder with a volume of approximately 1 L and is equipped with two different precious metal wire-mesh catalyst packages as well as with an internal electric heater. Reforming investigations of the reactor are done stand-alone but as if the reactor would operate within the total SOFC system with AOGR. For the tests presented here it is assumed that the SOFC system runs on pure CH4 instead of NG. The manuscript focuses on the various phases of reactor operation during the startup process of the SOFC system. Startup process reforming experiments cover reactor operation points at which it runs on an oxygen to carbon ratio at the reactor inlet (ϕRI) of 1.2 with air supplied, up to a ϕRI of 2.4 without air supplied. As confirmed by a Monte Carlo simulation, most of the measured outlet gas concentrations are in or close to equilibrium. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2017.10.025
  • 2017 • 184 High-resolution analysis of photoanodes for water splitting by means of scanning photoelectrochemical microscopy
    Conzuelo, F. and Sliozberg, K. and Gutkowski, R. and Grutzke, S. and Nebe, M. and Schuhmann, W.
    Analytical Chemistry 89 1222-1228 (2017)
    In pursuance of efficient tools for the local analysis and characterization of novel photoelectrocatalytic materials, several SECM-based techniques have been developed, aiming on the combined benefit of a local irradiation of the analyzed sample and a microelectrode probe for the localized electrochemical analysis of the surface. We present the development and application of scanning photoelectrochemical microscopy (SPECM) for the laterally resolved characterization of photoelectrocatalytic materials. Particularly, the system was developed for the photoelectrochemical characterization of n-type semiconductor- based photoanodes for water splitting. By using the tip microelectrode simultaneously for local irradiation and as an electrochemical probe, SPECM was capable to simultaneously provide information about the local photocurrent generated at the sample under irradiation and to detect the photoelectrocatalytically evolved oxygen at the microelectrode. In combination with a novel means of irradiation of the interrogated sample, local analysis of semiconductor materials for light-induced water splitting with improved lateral resolution is achieved. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b03706
  • 2017 • 183 The effect of realistic heavy particle induced secondary electron emission coefficients on the electron power absorption dynamics in single- and dual-frequency capacitively coupled plasmas
    Daksha, M. and Derzsi, A. and Wilczek, S. and Trieschmann, J. and Mussenbrock, T. and Awakowicz, P. and Donkó, Z. and Schulze, J.
    Plasma Sources Science and Technology 26 (2017)
    doi: 10.1088/1361-6595/aa7c88
  • 2017 • 182 A Unified Interdisciplinary Approach to Design Antibacterial Coatings for Fast Silver Release
    El Arrassi, A. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Köller, M. and Ludwig, Al. and Tschulik, K.
    ChemElectroChem (2017)
    The increasing number of surgical treatments performed per year requires novel approaches to inhibit implant-associated infections, caused by multi-antibiotic resistant bacteria. Silver ions (Ag+) are known for their effective antimicrobial activity. Therefore, a system that efficiently and locally releases the minimum required amount of Ag+ directly after the surgical treatment is in high demand. Herein we study electrochemically, microbiologically, microscopically and spectroscopically sacrificial Ag anode coatings for antibacterial implant applications. It is found that Ag dot arrays deposited on noble metals (Pd, Ir) release Ag+ much faster than continuous Ag thin films. The Ag+ release qualitatively scales with the difference of standard potentials between Ag and the noble metal. Furthermore, with higher numbers of Ag dots, the total amount of released Ag+ increases, while the release efficiency declines. Notably, an efficient killing of Staphylococcus aureus bacteria was seen for coatings containing as little as 23ng of Ag per mm2. Thus, the use of sacrificial Ag anodes as highly efficient antibacterial coating materials is evaluated. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201700247
  • 2017 • 181 Cobalt boride modified with N-doped carbon nanotubes as a high-performance bifunctional oxygen electrocatalyst
    Elumeeva, K. and Masa, J. and Medina, D. and Ventosa, E. and Seisel, S. and Kayran, Y.U. and Genç, A. and Bobrowski, T. and Weide, P. and Arbiol, J. and Muhler, M. and Schuhmann, W.
    Journal of Materials Chemistry A 5 21122-21129 (2017)
    The development of reversible oxygen electrodes, able to drive both the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR), is still a great challenge. We describe a very efficient and stable bifunctional electrocatalytic system for reversible oxygen electrodes obtained by direct CVD growth of nitrogen-doped carbon nanotubes (NCNTs) on the surface of cobalt boride (CoB) nanoparticles. A detailed investigation of the crystalline structure and elemental distribution of CoB before and after NCNT growth reveals that the NCNTs grow on small CoB nanoparticles formed in the CVD process. The resultant CoB/NCNT system exhibited outstanding activity in catalyzing both the OER and the ORR in 0.1 M KOH with an overvoltage difference of only 0.73 V between the ORR at -1 mA cm-2 and the OER at +10 mA cm-2. The proposed CoB/NCNT catalyst showed stable performance during 50 h of OER stability assessment in 0.1 M KOH. Moreover, CoB/NCNT spray-coated on a gas diffusion layer as an air-breathing electrode proved its high durability during 170 galvanostatic charge-discharge (OER/ORR) test cycles (around 30 h) at ±10 mA cm-2 in 6 M KOH, making it an excellent bifunctional catalyst for potential Zn-air battery application. © 2017 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c7ta06995b
  • 2017 • 180 Alternating current-bipolar electrochemistry
    Eßmann, V. and Clausmeyer, J. and Schuhmann, W.
    Electrochemistry Communications 75 82-85 (2017)
    Rotation of a bipolar electrode in a constant electric field between feeder electrodes causes an alternating bipolar current at an AC frequency that depends on the rotation rate. The corresponding oscillation of the feeder current is evaluated by means of a lock-in amplifier. This innovative approach allows the current flowing through the non-wired bipolar electrode in an open bipolar system to be extracted without relying on electrochemical reporter reactions. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2017.01.006
  • 2017 • 179 Parasitic Reactions in Nanosized Silicon Anodes for Lithium-Ion Batteries
    Gao, H. and Xiao, L. and Plümel, I. and Xu, G.-L. and Ren, Y. and Zuo, X. and Liu, Y. and Schulz, C. and Wiggers, H. and Amine, K. and Chen, Z.
    Nano Letters 17 1512-1519 (2017)
    When designing nano-Si electrodes for lithium-ion batteries, the detrimental effect of the c-Li15Si4 phase formed upon full lithiation is often a concern. In this study, Si nanoparticles with controlled particle sizes and morphology were synthesized, and parasitic reactions of the metastable c-Li15Si4 phase with the nonaqueous electrolyte was investigated. The use of smaller Si nanoparticles (∼60 nm) and the addition of fluoroethylene carbonate additive played decisive roles in the parasitic reactions such that the c-Li15Si4 phase could disappear at the end of lithiation. This suppression of c-Li15Si4 improved the cycle life of the nano-Si electrodes but with a little loss of specific capacity. In addition, the characteristic c-Li15Si4 peak in the differential capacity (dQ/dV) plots can be used as an early-stage indicator of cell capacity fade during cycling. Our findings can contribute to the design guidelines of Si electrodes and allow us to quantify another factor to the performance of the Si electrodes. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b04551
  • 2017 • 178 Complete Prevention of Dendrite Formation in Zn Metal Anodes by Means of Pulsed Charging Protocols
    Garcia, G. and Ventosa, E. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 18691-18698 (2017)
    Zn metal as anode in rechargeable batteries, such as Zn/air or Zn/Ni, suffers from poor cyclability. The formation of Zn dendrites upon cycling is the key limiting step. We report a systematic study of the influence of pulsed electroplating protocols on the formation of Zn dendrites and in turn on strategies to completely prevent Zn dendrite formation. Because of the large number of variables in electroplating protocols, a scanning droplet cell technique was adapted as a high-throughput methodology in which a descriptor of the surface roughness can be in situ derived by means of electrochemical impedance spectroscopy. Upon optimizing the electroplating protocol by controlling nucleation, zincate ion depletion, and zincate ion diffusion, scanning electron microscopy and atomic force microscopy confirmed the growth of uniform and homogenous Zn deposits with a complete prevention of dendrite growth. The implementation of pulsed electroplating as the charging protocol for commercially available Ni-Zn batteries leads to substantially prolonged cyclability demonstrating the benefits of pulsed charging in Zn metal-based batteries. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.7b01705
  • 2017 • 177 Solar biosupercapacitor
    González-Arribas, E. and Aleksejeva, O. and Bobrowski, T. and Toscano, M.D. and Gorton, L. and Schuhmann, W. and Shleev, S.
    Electrochemistry Communications 74 9-13 (2017)
    Here we report on an entirely new kind of bioelectronic device – a solar biosupercapacitor, which is built from a dual-feature photobioanode combined with a double-function enzymatic cathode. The self-charging biodevice, based on transparent nanostructured indium tin oxide electrodes modified with biological catalysts, i.e. thylakoid membranes and bilirubin oxidase, is able to capacitively store electricity produced by direct conversion of radiant energy into electric energy. When self-charged during 10 min, using ambient light only, the biosupercapacitor provided a maximum of 6 mW m− 2 at 0.20 V. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2016.11.009
  • 2017 • 176 Transparent, mediator- and membrane-free enzymatic fuel cell based on nanostructured chemically modified indium tin oxide electrodes
    González-Arribas, E. and Bobrowski, T. and Di Bari, C. and Sliozberg, K. and Ludwig, R. and Toscano, M.D. and De Lacey, A.L. and Pita, M. and Schuhmann, W. and Shleev, S.
    Biosensors and Bioelectronics 97 46-52 (2017)
    We detail a mediator- and membrane-free enzymatic glucose/oxygen biofuel cell based on transparent and nanostructured conducting supports. Chemically modified indium tin oxide nanoparticle modified electrodes were used to substantially increase the active surface area without significantly compromising transparency. Two different procedures for surface nanostructuring were employed, viz. spray-coating and drop-coating. The spray-coated biodevice showed superior characteristics as compared to the drop-coated enzymatic fuel cell, as a result of the higher nanostructured surface area as confirmed by electrochemical characterisation, as well as scanning electron and atomic force microscopy. Subsequent chemical modification with silanes, followed by the immobilisation of either cellobiose dehydrogenase from Corynascus thermophiles or bilirubin oxidase from Myrothecium verrucaria, were performed to obtain the bioanodes and biocathodes, respectively. The optimised biodevice exhibited an OCV of 0.67 V and power output of up to 1.4 µW/cm2 at an operating voltage of 0.35 V. This is considered a significant step forward in the field of glucose/oxygen membrane- and mediator-free, transparent enzymatic fuel cells. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2017.05.040
  • 2017 • 175 Micrometer-Precise Determination of the Thin Electrolyte Layer of a Spectroelectrochemical Cell by Microelectrode Approach Curves
    Hiltrop, D. and Masa, J. and Botz, A.J.R. and Lindner, A. and Schuhmann, W. and Muhler, M.
    Analytical Chemistry 89 4367-4372 (2017)
    A spectroelectrochemical cell is presented that allows investigations of electrochemical reactions by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The electrode holder for the working (WE), counter and reference electrode as mounted in the IR spectrometer cause the formation of a thin electrolyte layer between the internal reflection element (IRE) and the surface of the WE. The thickness of this thin electrolyte layer (dTL) was estimated by performing a scanning electrochemical microscopy-(SECM) like approach of a Pt microelectrode (ME), which was leveled with the WE toward the IRE surface. The precise lowering of the ME/WE plane toward the IRE was enabled by a micrometer screw. The approach curve was recorded in negative feedback mode of SECM and revealed the contact point of the ME and WE on the IRE, which was used as reference point to perform the electro-oxidation of ethanol over a drop-casted Pd/NCNT catalyst on the WE at different thin-layer thicknesses by cyclic voltammetry. The reaction products were detected in the liquid electrolyte by IR spectroscopy, and the effect of variations in dTL on the current densities and IR spectra were analyzed and discussed. The obtained data identify dTL as an important variable in thin-layer experiments with electrochemical reactions and FTIR readout. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b03732
  • 2017 • 174 Prussian Blue Analogues: A Versatile Framework for Solid-Contact Ion-Selective Electrodes with Tunable Potentials
    Klink, S. and Ishige, Y. and Schuhmann, W.
    ChemElectroChem 4 490-494 (2017)
    The development of solid-contact ion-selective electrodes (SC-ISEs) (e.g. for point-of-care sensors) requires simple inner reference electrodes (iREs) with predictable and reproducible potentials. Intercalation compounds fulfill these requirements, as they respond to target ions present in the ion-selective membrane. Their applicability, however, is limited by the availability of intercalation frameworks capable to intercalate the target ion of interest. We report that Prussian Blue analogues (PBAs) can serve as versatile iREs for a range of target ions of clinical interest, such as Na+, K+, or Ca2+. Combining target-ion intercalated PBAs with ion-selective membranes results in a family of all-solid SC-ISEs, which are capable as ISEs with an inner filling, yet cheap and suitable for mass-production. The SC-ISEs′ standard potential is predictable and can be tuned by altering the PBAs′ redox-active transition metal or by changing its state of charge. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201700091
  • 2017 • 173 Antibacterial activity of microstructured sacrificial anode thin films by combination of silver with platinum group elements (platinum, palladium, iridium)
    Köller, M. and Bellova, P. and Javid, S.M. and Motemani, Y. and Khare, C. and Sengstock, C. and Tschulik, K. and Schildhauer, T.A. and Ludwig, Al.
    Materials Science and Engineering C 74 536-541 (2017)
    Five different Ag dots arrays (16 to 400dots/mm2) were fabricated on a continuous platinum, palladium, or iridium thin film and for comparison also on titanium film by sputter deposition and photolithographic patterning. To analyze the antibacterial activity of these microstructured films Staphylococcus aureus (S. aureus) were placed onto the array surfaces and cultivated overnight. To analyze the viability of planktonic as well as surface adherent bacteria, the applied bacterial fluid was subsequently aspirated, plated on blood agar plates and adherent bacteria were detected by fluorescence microscopy. A particular antibacterial effect towards . S. aureus was induced by Ag dot arrays on each of the platinum group thin film (sacrificial anode system for Ag) in contrast to Ag dot arrays fabricated on the Ti thin films (non-sacrificial anode system for Ag). Among platinum group elements the Ir-Ag system exerted the highest antibacterial activity which was accompanied by most advanced dissolution of the Ag dots and Ag ion release compared to Ag dots on Pt or Pd. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.msec.2016.12.075
  • 2017 • 172 Optimizing inVitro Impedance and Physico-Chemical Properties of Neural Electrodes by Electrophoretic Deposition of Pt Nanoparticles
    Koenen, S. and Rehbock, C. and Heissler, H.E. and Angelov, S.D. and Schwabe, K. and Krauss, J.K. and Barcikowski, S.
    ChemPhysChem 18 1108-1117 (2017)
    Neural electrodes suffer from an undesired incline in impedance when in permanent contact with human tissue. Nanostructures, induced by electrophoretic deposition (EPD) of ligand-free laser-generated nanoparticles (NPs) on the electrodes are known to stabilize impedance invivo. Hence, Pt surfaces were systematically EPD-coated with Pt NPs and evaluated for impedance as well as surface coverage, contact angle, electrochemically active surface area (ECSA) and surface oxidation. The aim was to establish a systematic correlation between EPD process parameters and physical surface properties. The findings clearly reveal a linear decrease in impedance with increasing surface coverage, which goes along with a proportional reduction of the contact angle and an increase in ECSA and surface oxidation. EPD process parameters, prone to yield surface coatings with low impedance, are long deposition times (40-60min), while high colloid concentrations (>250μgmL-1) and electric field strengths (>25Vcm-1) should be avoided due to detrimental NP assemblage effects. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201601180
  • 2017 • 171 Optimizing in Vitro Impedance and Physico-Chemical Properties of Neural Electrodes by Electrophoretic Deposition of Pt Nanoparticles
    Koenen, S. and Rehbock, C. and Heissler, H.E. and Angelov, S.D. and Schwabe, K. and Krauss, J.K. and Barcikowski, S.
    ChemPhysChem 18 1108-1117 (2017)
    Neural electrodes suffer from an undesired incline in impedance when in permanent contact with human tissue. Nanostructures, induced by electrophoretic deposition (EPD) of ligand-free laser-generated nanoparticles (NPs) on the electrodes are known to stabilize impedance in vivo. Hence, Pt surfaces were systematically EPD-coated with Pt NPs and evaluated for impedance as well as surface coverage, contact angle, electrochemically active surface area (ECSA) and surface oxidation. The aim was to establish a systematic correlation between EPD process parameters and physical surface properties. The findings clearly reveal a linear decrease in impedance with increasing surface coverage, which goes along with a proportional reduction of the contact angle and an increase in ECSA and surface oxidation. EPD process parameters, prone to yield surface coatings with low impedance, are long deposition times (40–60 min), while high colloid concentrations (>250 μg mL−1) and electric field strengths (>25 V cm−1) should be avoided due to detrimental NP assemblage effects. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cphc.201601180
  • 2017 • 170 Metallic NiPS3@NiOOH Core-Shell Heterostructures as Highly Efficient and Stable Electrocatalyst for the Oxygen Evolution Reaction
    Konkena, B. and Masa, J. and Botz, A. J. R. and Sinev, I. and Xia, W. and Kossmann, J. and Drautz, R. and Muhler, M. and Schuhmann, W.
    ACS Catalysis 7 229--237 (2017)
    We report metallic NiPS3@NiOOH core shell heterostructures as an efficient and durable electrocatalyst for the oxygen evolution reaction, exhibiting a low onset potential of 1.48 V (vs RHE) and stable performance for over 160 h. The atomically thin NiPS3 nanosheets are obtained by exfoliation of bulk NiPS3 in the presence of an ionic surfactant. The OER mechanism was studied by a combination of SECM, in situ Raman spectroscopy, SEM, and XPS measurements, which enabled direct observation of the formation of a NiPS3@NiOOH core shell heterostructure at the electrode interface. Hence, the active form of the catalyst is represented as NiPS3@NiOOH core shell structure. Moreover, DFT calculations indicate an intrinsic metallic character of the NiPS3 nanosheets with densities of states (DOS) similar to the bulk material. The high OER activity of the NiPS3 nanosheets is attributed to a high density of accessible active metallic-edge and defect sites due to structural disorder, a unique NiPS3@NiOOH core shell heterostructure, where the presence of P and S modulates the rface electronic structure of Ni in NiPS3, thus providing excellent conductive pathway for efficient electron-transport to the NiOOH shell. These findings suggest that good size control during liquid exfoliation may be advantageously used for the formation of electrically conductive NiPS3@ NiOOH core shell electrode materials for the electrochemical water oxidation.
    view abstractdoi: 10.1021/acscatal.6b02203
  • 2017 • 169 Manufacturing of high performance solid oxide fuel cells (SOFCs) with atmospheric plasma spraying (APS) and plasma spray-physical vapor deposition (PS-PVD)
    Marcano, D. and Mauer, G. and Vaßen, R. and Weber, A.
    Surface and Coatings Technology 318 170-177 (2017)
    In the present work, a metal supported SOFC half-cell was fabricated by means of plasma spray. As support, a Fe-Cr alloy with a porous structure was used. The anode and electrolyte were applied using atmospheric plasma spray (APS) and plasma spray-physical vapor deposition (PS-PVD), respectively. A standard Ni/YSZ (coat mix) powder was used for the anode and the cathode layer consisted of a screen-printed La0.58Sr0.4Co0.2Fe0.8O3-δ (LSCF) non-sintered paste. The development of a thin, dense, gas-tight 8YSZ electrolyte was the key issue of this work. Analysis of microstructure, phases, and gas-tightness were carried out for various processing conditions. Different parameters were varied, such as: powder feed rate and carrier gas flow rate, robot speed, spraying distance and plasma gas composition. A partially reduced anode coating with 9% porosity and a gas-tight 26μm electrolyte layer were obtained. Such an assembly was air-tight and delivered a cell with an acceptable open circuit voltage (OCV) and an excellent performance of 1A/cm2 at 800C and 0.7V. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.10.088
  • 2017 • 168 Soluble metal oxo alkoxide inks with advanced rheological properties for inkjet-printed thin-film transistors
    Meyer, S. and Pham, D.V. and Merkulov, S. and Weber, D. and Merkulov, A. and Benson, N. and Schmechel, R.
    ACS Applied Materials and Interfaces 9 2625-2633 (2017)
    Semiconductor inks containing an indium-based oxo alkoxide precursor material were optimized regarding rheology requirements for a commercial 10 pL inkjet printhead. The rheological stability is evaluated by measuring the dynamic viscosity of the formulations for 12 h with a constant shear rate stress under ambient conditions. It is believed that the observed superior stability of the inks is the result of effectively suppressing the hydrolysis and condensation reaction between the metal oxo alkoxide precursor complex and atmospheric water. This can be attributed to a strong precursor coordination and the resulting reduction in ligand exchange dynamics of the solvent tetrahydrofurfuryl alcohol which is used as the main solvent in the formulations. It is also shown that with a proper selection of cosolvents, having high polar Hansen solubility parameter values, the inks drop formation properties and wettability can be finetuned by maintaining the inks rheological stability. Good drop jetting performance without satellite formation and high drop velocities of 8.25 m/s were found with the support of dimensionless numbers and printability windows. By printing single 10 pL ink dots onto short channel indium-tin-oxide electrodes, In2O3 calcination at 350 °C and a solution-processed back-channel protection, high average saturation mobility of approximately 10 cm2/(V s) are demonstrated in a bottom-contact coplanar thinfilm transistor device structure. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b12586
  • 2017 • 167 Spatially resolved measurements of the physical plasma parameters and the chemical modifications in a twin surface dielectric barrier discharge for gas flow purification
    Offerhaus, B. and Lackmann, J.-W. and Kogelheide, F. and Bracht, V. and Smith, R. and Bibinov, N. and Stapelmann, K. and Awakowicz, P.
    Plasma Processes and Polymers 14 (2017)
    A twin surface dielectric barrier discharge consisting of an aluminium oxide plate with grid-structured copper traces on both sides is presented. Due to the size of the electrode configuration spatially resolved optical emission spectroscopy for characterisation of the discharge is performed on two different length scales in order to show its homogeneous behaviour. A broadband echelle spectrometer is employed for a comparison of the plasma parameters at different sites along the copper traces with a spatial resolution on a scale of millimetres. In addition, an ICCD camera with bandpass filters yields homogeneity of the plasma parameters on a scale of micrometres at a given node of the grid-structured copper traces. The discharge is shown to be homogeneous all along the electrode. However, due to the changing composition of the gas stream, it cannot be concluded that the gas phase chemistry follows the same trend. Therefore, FTIR spectroscopy of cysteine is used to monitor the spatial dependence of the gas phase chemistry, showing a transition from purely oxygen-related modifications at the front of the electrode to a mixture of oxygen-related and nitrogen-related modifications at the rear. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/ppap.201600255
  • 2017 • 166 Frost induced damages within porous materials - from concrete technology to fuel cells technique
    Palecki, S. and Gorelkov, S. and Wartmann, J. and Heinzel, A.
    Journal of Power Sources 372 204-211 (2017)
    Porous media like concrete or layers of membrane electrode assemblies (MEA) within fuel cells are affected by a cyclic frost exposure due to different damage mechanisms which could lead to essential degradation of the material. In general, frost damages can only occur in case of a specific material moisture content. In fuel cells, residual water is generally available after shut down inside the membrane i.e. the gas diffusion layer (GDL). During subsequent freezing, this could cause various damage phenomena such as frost heaves and delamination effects of the membrane electrode assembly, which depends on the location of pore water and on the pore structure itself. Porous materials possess a pore structure that could range over several orders of magnitudes with different properties and freezing behaviour of the pore water. Latter can be divided into macroscopic, structured and pre-structured water, influenced by surface interactions. Therefore below 0 °C different water modifications can coexist in a wide temperature range, so that during frost exposure a high amount of unfrozen and moveable water inside the pore system is still available. This induces transport mechanisms and shrinkage effects. The physical basics are similar for porous media. While the freezing behaviour of concrete has been studied over decades of years, in order to enhance the durability, the know-how about the influence of a frost attack on fuel cell systems is not fully understood to date. On the basis of frost damage models for concrete structures, an approach to describe the impact of cyclic freezing and thawing on membrane electrode assemblies has been developed within this research work. Major aim is beyond a better understanding of the frost induced mechanisms, the standardization of a suitable test procedure for the assessment of different MEA materials under such kind of attack. Within this contribution first results will be introduced. © 2017 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2017.10.065
  • 2017 • 165 Choosing the right nanoparticle size-designing novel ZnO electrode architectures for efficient dye-sensitized solar cells
    Pfau, M.W. and Kunzmann, A. and Segets, D. and Peukert, W. and Wallace, G.G. and Officer, D.L. and Clark, T. and Costa, R.D. and Guldi, D.M.
    Journal of Materials Chemistry A 5 7516-7522 (2017)
    A novel concept for constructing optimized ZnO-based photoanodes as integrative components of dye-sensitized solar cells (DSSCs) is realized by deploying differently sized nanoparticles, ranging from 2 to 10 nm, together with commercially available 20 nm nanoparticles. The 2 nm nanoparticles were used to construct an efficient buffer layer for transparent electrodes based on 10 nm nanoparticles, resulting in a relative increase of device efficiency from 1.8 to 3.0% for devices without and with a buffer layer, respectively. A mixture of 10 and 20 nm nanoparticles was optimized to maximize the diffuse reflection and to minimize the charge transport resistance in a light-scattering layer. This optimization resulted in a homogenous layer of more than 15 μm that provided a device efficiency of 3.3%. The buffer layer, transparent electrode, and light-scattering electrode, were then combined to give an overall efficiency of around 5%. Thus, this work demonstrates that varying the electrode architecture with nanoparticles of different diameters is a powerful strategy for improving the overall efficiency of ZnO-based DSSCs. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6ta11012f
  • 2017 • 164 A Self-Powered Ethanol Biosensor
    Ruff, A. and Pinyou, P. and Nolten, M. and Conzuelo, F. and Schuhmann, W.
    ChemElectroChem 4 890-897 (2017)
    We describe the fabrication of a self-powered ethanol biosensor comprising a β-NAD+-dependent alcohol dehydrogenase (ADH) bioanode and a bienzymatic alcohol oxidase (AOx) and horseradish peroxidase (HRP) biocathode. β-NAD+ is regenerated by means of a specifically designed phenothiazine dye (i.e. toluidine blue, TB) modified redox polymer in which TB was covalently anchored to a hexanoic acid tethered poly(4-vinylpyridine) backbone. The redox polymer acts as an immobilization matrix for ADH. Using a carefully chosen anchoring strategy through the formation of amide bonds, the potential of the TB-based mediator is shifted to more positive potentials, thus preventing undesired O2 reduction. To counterbalance the rather high potential of the TB-modified polymer, and thus the bioanode, a high-potential AOx/HRP-based biocathode is suggested. HRP is immobilized in a direct-electron-transfer regime on screen-printed graphite electrodes functionalized with multi-walled carbon nanotubes. The nanostructured cathode ensures the wiring of the iron-oxo complex within oxidized HRP, and thus a high potential for the reduction of H2O2 of about +550mV versus Ag/AgCl/3M KCl. The proposed biofuel cell exhibits an open-circuit voltage (OCV) of approximately 660mV and was used as self-powered device for the determination of the ethanol content in liquor. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201600864
  • 2017 • 163 Micrometer-sized nano-structured silicon/carbon composites for lithium-ion battery anodes synthesized based on a three-step Hansen solubility parameter (HSP) concept
    Sehlleier, Y.H. and Dobrowolny, S. and Xiao, L. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Industrial and Engineering Chemistry 52 305-313 (2017)
    The processing towards Si/C composites, components and synthesis parameters were selected based on the concept of Hansen solubility parameters (HSP). Si/polymer composites were generated through modified bulk polymerization and subsequent pyrolysis transformed the polymer into the desired porous carbon matrix. Coulombic efficiencies (CE) in excess of 76% after the first cycle and 99.95% after solid electrolyte interphase (SEI) formation have been achieved. A notably high specific delithiation capacity of around 1600 mAh/g with an extremely stable cycling performance even after 400 cycles is obtained. This scalable and economical synthesis approach is readily applicable to the commercial production of anode materials. © 2017 The Korean Society of Industrial and Engineering Chemistry
    view abstractdoi: 10.1016/j.jiec.2017.04.001
  • 2017 • 162 Co3O4@Co/NCNT Nanostructure Derived from a Dicyanamide-Based Metal-Organic Framework as an Efficient Bi-functional Electrocatalyst for Oxygen Reduction and Evolution Reactions
    Sikdar, N. and Konkena, B. and Masa, J. and Schuhmann, W. and Maji, T.K.
    Chemistry - A European Journal 23 18049-18056 (2017)
    There has been growing interest in the synthesis of efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reactions (OER), for their potential use in a variety of renewable energy technologies, such as regenerative fuel cells and metal-air batteries. Here, a bi-functional electrocatalyst, derived from a novel dicyanamide based nitrogen rich MOF {[Co(bpe)2(N(CN)2)]⋅(N(CN)2)⋅(5 H2O)}n [Co-MOF-1, bpe=1,2-bis(4-pyridyl)ethane, N(CN)2 −=dicyanamide] under different pyrolysis conditions is reported. Pyrolysis of the Co-MOF-1 under Ar atmosphere (at 800 °C) yielded a Co nanoparticle-embedded N-doped carbon nanotube matrix (Co/NCNT-Ar) while pyrolysis under a reductive H2/Ar atmosphere (at 800 °C) and further mild calcination yielded Co3O4@Co core–shell nanoparticle-encapsulated N-doped carbon nanotubes (Co3O4@Co/NCNT). Both catalysts show bi-functional activity towards ORR and OER, however, the core–shell Co3O4@Co/NCNT nanostructure exhibited superior electrocatalytic activity for both the ORR with a potential of 0.88 V at a current density of −1 mA cm−2 and the OER with a potential of 1.61 V at 10 mA cm−2, which is competitive with the most active bi-functional catalysts reported previously. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/chem.201704211
  • 2017 • 161 Alkaline fuel cell with nitride membrane
    Sun, S.-H. and Pilaski, M. and Wartmann, J. and Letzkus, F. and Funke, B. and Dura, G. and Heinzel, A.
    Proceedings of SPIE - The International Society for Optical Engineering 10246 (2017)
    The aim of this work is to fabricate patterned nitride membranes with Si-MEMS-technology as a platform to build up new membrane-electrode-assemblies (MEA) for alkaline fuel cell applications. Two 6-inch wafer processes based on chemical vapor deposition (CVD) were developed for the fabrication of separated nitride membranes with a nitride thickness up to 1 μm. The mechanical stability of the perforated nitride membrane has been adjusted in both processes either by embedding of subsequent ion implantation step or by optimizing the deposition process parameters. A nearly 100% yield of separated membranes of each deposition process was achieved with layer thickness from 150 nm to 1 μm and micro-channel pattern width of 1μm at a pitch of 3 μm. The process for membrane coating with electrolyte materials could be verified to build up MEA. Uniform membrane coating with channel filling was achieved after the optimization of speed controlled dip-coating method and the selection of dimethylsulfoxide (DMSO) as electrolyte solvent. Finally, silver as conductive material was defined for printing a conductive layer onto the MEA by Ink-Technology. With the established IR-thermography setup, characterizations of MEAs in terms of catalytic conversion were performed successfully. The results of this work show promise for build up a platform on wafer-level for high throughput experiments. © 2017 SPIE.
    view abstractdoi: 10.1117/12.2265689
  • 2017 • 160 Solid electrolyte interphase (SEI) at TiO2 electrodes in li-ion batteries: Defining apparent and effective SEI based on evidence from X-ay photoemission spectroscopy and scanning electrochemical microscopy
    Ventosa, E. and Madej, E. and Zampardi, G. and Mei, B. and Weide, P. and Antoni, H. and La Mantia, F. and Muhler, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 9 3123-3130 (2017)
    The high (de)lithiation potential of TiO2 (ca. 1.7 V vs Li/ Li+ in 1 M Li+) decreases the voltage and, thus, the energy density of a corresponding Li-ion battery. On the other hand, it offers several advantages such as the (de)lithiation potential far from lithium deposition or absence of a solid electrolyte interphase (SEI). The latter is currently under controversial debate as several studies reported the presence of a SEI when operating TiO2 electrodes at potentials above 1.0 V vs Li/Li+. We investigate the formation of a SEI at anatase TiO2 electrodes by means of X-ray photoemission spectroscopy (XPS) and scanning electrochemical microscopy (SECM). The investigations were performed in different potential ranges, namely, during storage (without external polarization), between 3.0-2.0 V and 3.0-1.0 V vs Li/Li+, respectively. No SEI is formed when a completely dried and residues-free TiO2 electrode is cycled between 3.0 and 2.0 V vs Li/Li+. A SEI is detected by XPS in the case of samples stored for 6 weeks or cycled between 3.0 and 1.0 V vs Li/Li+. With use of SECM, it is verified that this SEI does not possess the electrically insulating character as expected for a "classic" SEI. Therefore, we propose the term apparent SEI for TiO2 electrodes to differentiate it from the protecting and ef fective SEI formed at graphite electrodes. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsami.6b13306
  • 2017 • 159 High-Performance Energy Storage and Conversion Materials Derived from a Single Metal-Organic Framework/Graphene Aerogel Composite
    Xia, W. and Qu, C. and Liang, Z. and Zhao, B. and Dai, S. and Qiu, B. and Jiao, Y. and Zhang, Q. and Huang, X. and Guo, W. and Dang, D. and Zou, R. and Xia, D. and Xu, Q. and Liu, M.
    Nano Letters 17 2788-2795 (2017)
    Metal oxides and carbon-based materials are the most promising electrode materials for a wide range of low-cost and highly efficient energy storage and conversion devices. Creating unique nanostructures of metal oxides and carbon materials is imperative to the development of a new generation of electrodes with high energy and power density. Here we report our findings in the development of a novel graphene aerogel assisted method for preparation of metal oxide nanoparticles (NPs) derived from bulk MOFs (Co-based MOF, Co(mIM)2 (mIM = 2-methylimidazole). The presence of cobalt oxide (CoOx) hollow NPs with a uniform size of 35 nm monodispersed in N-doped graphene aerogels (NG-A) was confirmed by microscopic analyses. The evolved structure (denoted as CoOx/NG-A) served as a robust Pt-free electrocatalyst with excellent activity for the oxygen reduction reaction (ORR) in an alkaline electrolyte solution. In addition, when Co was removed, the resulting nitrogen-rich porous carbon-graphene composite electrode (denoted as C/NG-A) displayed exceptional capacitance and rate capability in a supercapacitor. Further, this method is readily applicable to creation of functional metal oxide hollow nanoparticles on the surface of other carbon materials such as graphene and carbon nanotubes, providing a good opportunity to tune their physical or chemical activities. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.nanolett.6b05004
  • 2017 • 158 Synergistic Effect of Cobalt and Iron in Layered Double Hydroxide Catalysts for the Oxygen Evolution Reaction
    Yang, F. and Sliozberg, K. and Sinev, I. and Antoni, H. and Bähr, A. and Ollegott, K. and Xia, W. and Masa, J. and Grünert, W. and Cuenya, B.R. and Schuhmann, W. and Muhler, M.
    ChemSusChem 10 156-165 (2017)
    Co-based layered double hydroxide (LDH) catalysts with Fe and Al contents in the range of 15 to 45 at % were synthesized by an efficient coprecipitation method. In these catalysts, Fe3+ or Al3+ ions play an essential role as trivalent species to stabilize the LDH structure. The obtained catalysts were characterized by a comprehensive combination of surface- and bulk-sensitive techniques and were evaluated for the oxygen evolution reaction (OER) on rotating disk electrodes. The OER activity decreased upon increasing the Al content for the Co- and Al-based LDH catalysts, whereas a synergistic effect in Co- and Fe-based LDHs was observed, which resulted in an optimal Fe content of 35 at %. This catalyst was spray-coated on Ni foam electrodes and showed very good stability in a flow-through cell with a potential of approximately 1.53 V at 10 mA cm−2 in 1 m KOH for at least 48 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/cssc.201601272
  • 2017 • 157 High-Density Droplet Microarray of Individually Addressable Electrochemical Cells
    Zhang, H. and Oellers, T. and Feng, W. and Abdulazim, T. and Saw, E.N. and Ludwig, Al. and Levkin, P.A. and Plumeré, N.
    Analytical Chemistry 89 5832-5839 (2017)
    Microarray technology has shown great potential for various types of high-throughput screening applications. The main read-out methods of most microarray platforms, however, are based on optical techniques, limiting the scope of potential applications of such powerful screening technology. Electrochemical methods possess numerous complementary advantages over optical detection methods, including its label-free nature, capability of quantitative monitoring of various reporter molecules, and the ability to not only detect but also address compositions of individual compartments. However, application of electrochemical methods for the purpose of high-throughput screening remains very limited. In this work, we develop a high-density individually addressable electrochemical droplet microarray (eDMA). The eDMA allows for the detection of redox-active reporter molecules irrespective of their electrochemical reversibility in individual nanoliter-sized droplets. Orthogonal band microelectrodes are arranged to form at their intersections an array of three-electrode systems for precise control of the applied potential, which enables direct read-out of the current related to analyte detection. The band microelectrode array is covered with a layer of permeable porous polymethacrylate functionalized with a highly hydrophobic-hydrophilic pattern, forming spatially separated nanoliter-sized droplets on top of each electrochemical cell. Electrochemical characterization of single droplets demonstrates that the underlying electrode system is accessible to redox-active molecules through the hydrophilic polymeric pattern and that the nonwettable hydrophobic boundaries can spatially separate neighboring cells effectively. The eDMA technology opens the possibility to combine the high-throughput biochemical or living cell screenings using the droplet microarray platform with the sequential electrochemical read-out of individual droplets. © 2017 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.7b00008
  • 2017 • 156 Interrogation of a PS1-Based Photocathode by Means of Scanning Photoelectrochemical Microscopy
    Zhao, F. and Plumeré, N. and Nowaczyk, M.M. and Ruff, A. and Schuhmann, W. and Conzuelo, F.
    Small 13 (2017)
    In the development of photosystem-based energy conversion devices, the in-depth understanding of electron transfer processes involved in photocurrent generation and possible charge recombination is essential as a basis for the development of photo-bioelectrochemical architectures with increased efficiency. The evaluation of a bio-photocathode based on photosystem 1 (PS1) integrated within a redox hydrogel by means of scanning photoelectrochemical microscopy (SPECM) is reported. The redox polymer acts as a conducting matrix for the transfer of electrons from the electrode surface to the photo-oxidized P700 centers within PS1, while methyl viologen is used as charge carrier for the collection of electrons at the reduced FB site of PS1. The analysis of the modified surfaces by SPECM enables the evaluation of electron-transfer processes by simultaneously monitoring photocurrent generation at the bio-photoelectrode and the associated generation of reduced charge carriers. The possibility to visualize charge recombination processes is illustrated by using two different electrode materials, namely Au and p-doped Si, exhibiting substantially different electron transfer kinetics for the reoxidation of the methyl viologen radical cation used as freely diffusing charge carrier. In the case of p-doped Si, a slower recombination kinetics allows visualization of methyl viologen radical cation concentration profiles from SPECM approach curves. © 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/smll.201604093
  • 2016 • 155 Co@Co3O4 Encapsulated in Carbon Nanotube-Grafted Nitrogen-Doped Carbon Polyhedra as an Advanced Bifunctional Oxygen Electrode
    Aijaz, A. and Masa, J. and Rösler, C. and Xia, W. and Weide, P. and Botz, A.J.R. and Fischer, R.A. and Schuhmann, W. and Muhler, M.
    Angewandte Chemie - International Edition 55 4087-4091 (2016)
    Efficient reversible oxygen electrodes for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) are vitally important for various energy conversion devices, such as regenerative fuel cells and metal-air batteries. However, realization of such electrodes is impeded by insufficient activity and instability of electrocatalysts for both water splitting and oxygen reduction. We report highly active bifunctional electrocatalysts for oxygen electrodes comprising core-shell Co@Co3O4 nanoparticles embedded in CNT-grafted N-doped carbon-polyhedra obtained by the pyrolysis of cobalt metal-organic framework (ZIF-67) in a reductive H2 atmosphere and subsequent controlled oxidative calcination. The catalysts afford 0.85 V reversible overvoltage in 0.1 m KOH, surpassing Pt/C, IrO2, and RuO2 and thus ranking them among one of the best non-precious-metal electrocatalysts for reversible oxygen electrodes. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201509382
  • 2016 • 154 Change of the arc attachment mode and its effect on the lifetime in automotive high intensity discharge lamps
    Alexejev, A. and Flesch, P. and Mentel, J. and Awakowicz, P.
    Journal of Applied Physics 120 (2016)
    In modern cars, the new generation Hg-free high intensity discharge (HID) lamps, the so called xenon lamps, take an important role. The long lifetime of these lamps is achieved by doping the tungsten electrodes with thorium. Thorium forms a dipole layer on the electrode surface, thus reducing the work function of tungsten. However, thoriating the electrodes is also an issue of trade and transport regulation, so a substitute is looked into. This work shows the influence of the arc attachment mode on the lifetime of the lamps. The mode of the arc attachment changes during the run-up phase of automotive HID lamps after a characteristic time period depending, i.e., on the filling of the lamps, which is dominated by scandium. It will be shown that this characteristic time period for the change of the attachment mode determines the long term performance of Hg-free xenon lamps. Measurements attributing the mode change to the scandium density in the filling are presented. The emitter effect of scandium will be suggested to be the reason of the mode change. © 2016 Author(s).
    view abstractdoi: 10.1063/1.4963280
  • 2016 • 153 Electrophoretic deposition of ligand-free platinum nanoparticles on neural electrodes affects their impedance in vitro and in vivo with no negative effect on reactive gliosis
    Angelov, S.D. and Koenen, S. and Jakobi, J. and Heissler, H.E. and Alam, M. and Schwabe, K. and Barcikowski, S. and Krauss, J.K.
    Journal of Nanobiotechnology 14 (2016)
    Background: Electrodes for neural stimulation and recording are used for the treatment of neurological disorders. Their features critically depend on impedance and interaction with brain tissue. The effect of surface modification on electrode impedance was examined in vitro and in vivo after intracranial implantation in rats. Electrodes coated by electrophoretic deposition with platinum nanoparticles (NP; <10 and 50 nm) as well as uncoated references were implanted into the rat's subthalamic nucleus. After postoperative recovery, rats were electrostimulated for 3 weeks. Impedance was measured before implantation, after recovery and then weekly during stimulation. Finally, local field potential was recorded and tissue-to-implant reaction was immunohistochemically studied. Results: Coating with NP significantly increased electrode's impedance in vitro. Postoperatively, the impedance of all electrodes was temporarily further increased. This effect was lowest for the electrodes coated with particles <10 nm, which also showed the most stable impedance dynamics during stimulation for 3 weeks and the lowest total power of local field potential during neuronal activity recording. Histological analysis revealed that NP-coating did not affect glial reactions or neural cell-count. Conclusions: Coating with NP <10 nm may improve electrode's impedance stability without affecting biocompatibility. Increased impedance after NP-coating may improve neural recording due to better signal-to-noise ratio. © 2016 Angelov et al.
    view abstractdoi: 10.1186/s12951-015-0154-9
  • 2016 • 152 Detection of individual nanoparticle impacts using etched carbon nanoelectrodes
    Clausmeyer, J. and Wilde, P. and Löffler, T. and Ventosa, E. and Tschulik, K. and Schuhmann, W.
    Electrochemistry Communications 73 67-70 (2016)
    A rapid and reliable nanofabrication route produces electrodes with beneficial properties for electrochemistry based on stochastic nanoparticle collision events. Carbon nanoelectrodes are etched to expose conical carbon tips which present an increased surface area for the detection of nanoparticle impacts. The tuneable electrode size as well as the conical geometry allow to increase the observed particle impact frequency while maintaining low background noise. Moreover, anodic particle coulometry for the sizing of silver nanoparticles shows that the detected impacts are representative of the polydisperse particle population. © 2016
    view abstractdoi: 10.1016/j.elecom.2016.11.003
  • 2016 • 151 Nanoelectrodes: Applications in electrocatalysis, single-cell analysis and high-resolution electrochemical imaging
    Clausmeyer, J. and Schuhmann, W.
    TrAC - Trends in Analytical Chemistry 79 46-59 (2016)
    High sensitivity and high spatial resolution in localized electrochemical measurements are the key advantages of electroanalysis using nanometer-sized electrodes. Due to recent progress in nanoelectrode fabrication and electrochemical instrument development, nanoelectrochemical methods are becoming more widespread. We summarize different protocols for the fabrication of needle-type nanoelectrodes and discuss their properties with regard to various applications. We discuss the limits of conventional theory to describe electrochemistry at the nanoscale and point out technical aspects for characterization and handling of nanometric electrodes. Different applications are highlighted: i) Nanoelectrodes are powerful tools for non-ensemble studies of electrocatalysis at single nanoparticles at high mass transport rates. ii) Electrochemical nanosensors are employed for highly localized non-invasive analysis of single living cells and intracellular detection of neurotransmitters and metabolites. iii) Used in scanning electrochemical probe techniques, nanoprobes afford topographical and truly chemical imaging of samples with high spatial resolution. © 2016 Published by Elsevier B.V.
    view abstractdoi: 10.1016/j.trac.2016.01.018
  • 2016 • 150 Perovskite-based bifunctional electrocatalysts for oxygen evolution and oxygen reduction in alkaline electrolytes
    Elumeeva, K. and Masa, J. and Sierau, J. and Tietz, F. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 208 25-32 (2016)
    Due to the high cost of precious metal-based electrocatalysts for oxygen reduction and oxygen evolution, the development of alternative low cost and efficient catalysts is of high importance for energy storage and conversion technologies. Although non-precious catalysts that can efficiently catalyze oxygen reduction and oxygen evolution have been developed, electrocatalysts with high bifunctional activity for both oxygen evolution and reduction are needed. Perovskites based on modified lanthanum cobaltite possess significant activity for the oxygen evolution reaction. We describe the synthesis of a bifunctional oxygen electrode with simultaneous activity for the oxygen evolution reaction (OER) and the oxygen reduction reaction (ORR) in alkaline media by direct growth of nitrogen-doped carbon nanotubes on the surface of a perovskite containing Co and Fe by means of chemical vapor deposition. The difference in the overvoltage between ORR (at 1 mA/cm2) and OER (at 10 mA/cm2) was below 880 mV in 0.1 M KOH. The formation of H2O2 during the ORR was reduced by at least three fold when using the bifunctional catalyst as compared to the non-modified perovskite. Long-term durability tests indicate stable performance for at least 37 h during the OER and 23 h during the ORR. © 2016 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2016.05.010
  • 2016 • 149 A Simple Approach towards High-Performance Perovskite-Based Bifunctional Oxygen Electrocatalysts
    Elumeeva, K. and Masa, J. and Tietz, F. and Yang, F. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 3 138-143 (2016)
    To accelerate the large-scale commercialization of electrochemical energy storage and conversion technologies through water splitting and regeneration in reversible fuel cells, cost-effective, highly efficient, and durable reversible oxygen electrodes are required. We report a comparatively simple approach to modify a group of oxygen-evolving perovskites based on lanthanum cobaltite into effective bifunctional systems through partial atom substitution, which, upon intermixing with nitrogen-doped carbon nanotubes, achieve remarkably low round-trip overvoltage of <850mV in the electrocatalysis of oxygen reduction and oxygen evolution in an alkaline electrolyte, KOH (0.1m). Besides the bifunctional electrocatalytic performance, the composite systems with a low Fe content possessed promising long-term stability. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500353
  • 2016 • 148 Bipolar Electrochemistry for Concurrently Evaluating the Stability of Anode and Cathode Electrocatalysts and the Overall Cell Performance during Long-Term Water Electrolysis
    Eßmann, V. and Barwe, S. and Masa, J. and Schuhmann, W.
    Analytical Chemistry 88 8835-8840 (2016)
    Electrochemical efficiency and stability are among the most important characteristics of electrocatalysts. These parameters are usually evaluated separately for the anodic and cathodic half-cell reactions in a three-electrode system or by measuring the overall cell voltage between the anode and cathode as a function of current or time. Here, we demonstrate how bipolar electrochemistry can be exploited to evaluate the efficiency of electrocatalysts for full electrochemical water splitting while simultaneously and independently monitoring the individual performance and stability of the half-cell electrocatalysts. Using a closed bipolar electrochemistry setup, all important parameters such as overvoltage, half-cell potential, and catalyst stability can be derived from a single galvanostatic experiment. In the proposed experiment, none of the half-reactions is limiting on the other, making it possible to precisely monitor the contribution of the individual half-cell reactions on the durability of the cell performance. The proposed approach was successfully employed to investigate the long-term performance of a bifunctional water splitting catalyst, specifically amorphous cobalt boride (Co2B), and the durability of the electrocatalyst at the anode and cathode during water electrolysis. Additionally, by periodically alternating the polarization applied to the bipolar electrode (BE) modified with a bifunctional oxygen electrocatalyst, it was possible to explicitly follow the contributions of the oxygen reduction (ORR) and the oxygen evolution (OER) half-reactions on the overall long-term durability of the bifunctional OER/ORR electrocatalyst. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b02393
  • 2016 • 147 Benchmarking the Performance of Thin-Film Oxide Electrocatalysts for Gas Evolution Reactions at High Current Densities
    Ganassin, A. and Maljusch, A. and Colic, V. and Spanier, L. and Brandl, K. and Schuhmann, W. and Bandarenka, A.
    ACS Catalysis 6 3017-3024 (2016)
    Oxide materials are among the state-of-the-art heterogeneous electrocatalysts for many important large-scale industrial processes, including O2 and Cl2 evolution reactions. However, benchmarking their performance is challenging in many cases, especially at high current densities, which are relevant for industrial applications. Serious complications arise particularly due to (i) the formation of a nonconducting gas phase which blocks the surface during the reactions, (ii) problems in determination of the real electroactive electrode area, and (iii) the large influence of surface morphology alterations (stability issues) under reaction conditions, among others. In this work, an approach overcoming many of these challenges is presented, with a focus on electrochemically formed thin-film oxide electrocatalysts. The approach is based on benefits provided by the use of microelectrodes, and it gives comprehensive information about the surface roughness, catalyst activity, and stability. The key advantages of the proposed method are the possibility of characterization of the whole microelectrode surface by means of atomic force microscopy and an accurate assessment of the specific activity (and subsequently stability) of the catalyst, even at very high current densities. Electrochemically deposited CoOx thin films have been used in this study as model catalysts. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b00455
  • 2016 • 146 A Three-Electrode, Battery-Type Swagelok Cell for the Evaluation of Secondary Alkaline Batteries: The Case of the Ni-Zn Battery
    Garcia, G. and Schuhmann, W. and Ventosa, E.
    ChemElectroChem 3 592-597 (2016)
    Three-electrode cells are essential in understanding battery materials under operando conditions. A three-electrode, battery-type Swagelok cell for electrochemical studies of secondary alkaline batteries, in particular Ni-Zn batteries, is presented. The relevance of the three-electrode battery-type cell (i.e. sealed and non-flooded) configuration is demonstrated as analytical tool with three observations: 1)The Ni electrode is shown to limit the system in the first cycles, while the Zn electrode becomes limiting in subsequent cycles. 2)Non-woven separators (NWSs) clearly improve the performance of the battery. Besides the known fact of hindering the dendritic growth of Zn, NWSs inhibit the evolution of oxygen and hydrogen at the positive and negative electrodes. 3)The kinetics of the Ni electrode is much slower than that of the Zn electrode, as derived from the charge-transfer resistance of the Ni electrode, which is substantially larger than that of the Zn electrode. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500474
  • 2016 • 145 Analysis of liquid water formation in polymer electrolyte membrane (PEM) fuel cell flow fields with a dry cathode supply
    Gößling, S. and Klages, M. and Haußmann, J. and Beckhaus, P. and Messerschmidt, M. and Arlt, T. and Kardjilov, N. and Manke, I. and Scholta, J. and Heinzel, A.
    Journal of Power Sources 306 658-665 (2016)
    PEM fuel cells can be operated within a wide range of different operating conditions. In this paper, the special case of operating a PEM fuel cell with a dry cathode supply and without external humidification of the cathode, is considered. A deeper understanding of the water management in the cells is essential for choosing the optimal operation strategy for a specific system. In this study a theoretical model is presented which aims to predict the location in the flow field at which liquid water forms at the cathode. It is validated with neutron images of a PEM fuel cell visualizing the locations at which liquid water forms in the fuel cell flow field channels. It is shown that the inclusion of the GDL diffusion resistance in the model is essential to describe the liquid water formation process inside the fuel cell. Good agreement of model predictions and measurement results has been achieved. While the model has been developed and validated especially for the operation with a dry cathode supply, the model is also applicable to fuel cells with a humidified cathode stream. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2015.12.060
  • 2016 • 144 Improved photoelectrochemical performance of electrodeposited metal-doped BiVO4 on Pt-nanoparticle modified FTO surfaces
    Gutkowski, R. and Peeters, D. and Schuhmann, W.
    Journal of Materials Chemistry A 4 7875-7882 (2016)
    The recombination of photogenerated electron-hole pairs is one of the main limiting factors of photoelectrocatalysts absorbing in the visible part of the solar spectrum. Especially for BiVO4 the slow electron transport to the back contact facilitates charge recombination. Hence, thin layers have to be used to obtain higher photocurrents which are concomitantly only allow low absorption of the incident light. To address this limitation we have modified FTO substrates with Pt-nanoparticles before electrodepositing BiVO4. The Pt-nanoparticles decrease the overpotential for the electrodeposition of BiVO4, but more importantly they provide the basis for decreased charge recombination. Electrodeposited Mo-doped BiVO4 on Pt-nanoparticle modified FTO exhibits a substantially decreased recombination of photogenerated charge carriers during frontside illumination. Simultaneous co-doping of BiVO4 with two different metals leads to a substantial enhancement of the incident-photon-to-current efficiency (IPCE) during light driven oxygen evolution reaction. Highest IPCE (&gt;30% at 1.2 V vs. RHE) values were obtained for Mo/Zn- and Mo/B-doped BiVO4. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c6ta01340f
  • 2016 • 143 Electrochemical bromination of organosulfur containing species for the determination of the strength of garlic (A. sativum)
    Hall, E.M. and Tschulik, K. and Batchelor-McAuley, C. and Compton, R.G.
    Food Chemistry 199 817-821 (2016)
    The extraction by ethyl acetate and subsequent electrochemical detection of organosulfur containing molecules from garlic is demonstrated. The electrochemical results first evidence the high sensitivity of the process towards the model compound propyl disulfide. Through the in situ formation of bromine at a platinum electrode the propyl disulfide can be readily detected at concentrations as low as 12.5 μM. Second, the work focuses on the detection of organosulfur from fresh garlic samples. Extraction of the organosulfur 'flavour' molecules is achieved with ethyl acetate. Addition of this extract to the electrochemical cell results in an analytically useful signal allowing the voltammetric peak height to be successfully correlated with the garlic strength, as measured using an organoleptic tasting panel. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.foodchem.2015.12.086
  • 2016 • 142 Antenna induced hot restrike of a ceramic metal halide lamp recorded by high-speed photography
    Hermanns, P. and Hoebing, T. and Bergner, A. and Ruhrmann, C. and Awakowicz, P. and Mentel, J.
    Journal of Applied Physics 119 (2016)
    The hot restrike is one of the biggest challenges in operating ceramic metal halide lamps with mercury as buffer gas. Compared to a cold lamp, the pressure within a ceramic burner is two orders of magnitude higher during steady state operation due to the high temperature of the ceramic tube and the resulting high mercury vapour pressure. Room temperature conditions are achieved after 300 s of cooling down in a commercial burner, enclosed in an evacuated outer bulb. At the beginning of the cooling down, ignition voltage rises up to more than 14 kV. A significant reduction of the hot-restrike voltage can be achieved by using a so called active antenna. It is realized by a conductive sleeve surrounding the burner at the capillary of the upper electrode. The antenna is connected to the lower electrode of the lamp, so that its potential is extended to the vicinity of the upper electrode. An increased electric field in front of the upper electrode is induced, when an ignition pulse is applied to the lamp electrodes. A symmetrically shaped ignition pulse is applied with an amplitude, which is just sufficient to re-ignite the hot lamp. The re-ignition, 60 s after switching off the lamp, when the mercury pressure starts to be saturated, is recorded for both polarities of the ignition pulse with a high-speed camera, which records four pictures within the symmetrically shaped ignition pulse with exposure times of 100 ns and throws of 100 ns. The pictures show that the high electric field and its temporal variation establish a local dielectric barrier discharge in front of the upper electrode inside the burner, which covers the inner wall of the burner with a surface charge. It forms a starting point of streamers, which may induce the lamp ignition predominantly within the second half cycle of the ignition pulse. It is found out that an active antenna is more effective when the starting point of the surface streamer in front of the sleeve is a negative surface charge on the inner tube wall. The high-speed photos show that the ignition process is very similar in lamps with Hg or Xe as buffer gas. © 2016 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4943621
  • 2016 • 141 Pd deposited on functionalized carbon nanotubes for the electrooxidation of ethanol in alkaline media
    Hiltrop, D. and Masa, J. and Maljusch, A. and Xia, W. and Schuhmann, W. and Muhler, M.
    Electrochemistry Communications 63 30-33 (2016)
    Large scale commercialization of direct ethanol fuel cells is hampered by the high cost and scarcity of noble metal electrocatalysts employed at both the anode and cathode. We demonstrate improved utilization of palladium as anode catalyst for ethanol oxidation by exploiting the strong interaction between Pd nanoparticles and nitrogen-doped carbon nanotubes (NCNTs) as support. 0.85 wt% Pd supported on NCNTs achieved a specific current density of 517 A gPd - 1 compared with 421 A gPd - 1 for 0.86 wt% Pd on oxygen-functionalized carbon nanotubes. The electrocatalytic performance deteriorated only gradually and catalysis was sustained for at least 80 h. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2015.11.010
  • 2016 • 140 The anodic emitter effect and its inversion demonstrated by temperature measurements at doped and undoped tungsten electrodes
    Hoebing, T. and Bergner, A. and Hermanns, P. and Mentel, J. and Awakowicz, P.
    Journal of Physics D: Applied Physics 49 (2016)
    The admixture of a small amount of emitter oxides, e.g. ThO2, La2O3 or Ce2O3 to tungsten generates the so-called emitter effect. t reduces the work function of tungsten cathodes that are applied in high intensity discharge (HID) lamps. After leaving the electrode ulk and moving to the surface, a monolayer of Th, La, or Ce atoms is formed on the surface which reduces the effective work function. Depending on the coverage of the electrode, the effective reduction in is subjected to the thermal desorption of the monolayer rom the hot electrode surface. The thermal desorption of emitter atoms from the cathode is compensated not only by the supply from he interior of the electrode and by surface diffusion of the emitter material to its tip, but also to a large extent by a repatriation f the emitter ions from the plasma by the strong electric field in front of the cathode. Yet, an emitter ion current from the arc ischarge to the anode may only be present, if the anode is cold enough to refrain from thermionic emission. Therefore, the ability of mitter oxides to reduce the temperature of tungsten anodes is only given for a moderate temperature so that the thermal desorption is ow and an additional ion current is present in front of the anode. A higher electrode temperature leads to their evaporation and to an nversion of the emitter effect, which increases the temperature of the respective anodes in comparison with pure tungsten anodes. ithin this article, the emitter effect of doped tungsten anodes and the transition to its inversion is investigated for thoriated, anthanated, and ceriated tungsten electrodes by measurements of the electrode temperature in dependence on the discharge current. It s shown for a lanthanated and a ceriated anode that the emitter effect is sustained by an ion current at anode temperatures at which he thermal evaporation of emitter material is completed. © 2016 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/49/15/155504
  • 2016 • 139 Intercalation Compounds as Inner Reference Electrodes for Reproducible and Robust Solid-Contact Ion-Selective Electrodes
    Ishige, Y. and Klink, S. and Schuhmann, W.
    Angewandte Chemie - International Edition 55 4831-4835 (2016)
    With billions of assays performed every year, ion-selective electrodes (ISEs) provide a simple and fast technique for clinical analysis of blood electrolytes. The development of cheap, miniaturized solid-contact (SC-)ISEs for integrated systems, however, remains a difficult balancing act between size, robustness, and reproducibility, because the defined interface potentials between the ion-selective membrane and the inner reference electrode (iRE) are often compromised. We demonstrate that target cation-sensitive intercalation compounds, such as partially charged lithium iron phosphate (LFP), can be applied as iREs of the quasi-first kind for ISEs. The symmetrical response of the interface potentials towards target cations ultimately results in ISEs with high robustness towards the inner filling (ca. 5 mV dec-1 conc.) as well as robust and miniaturized SC-ISEs. They have a predictable and stable potential derived from the LiFePO4/FePO4 redox couple (97.0±1.5 mV after 42 days). © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201600111
  • 2016 • 138 Potential-Pulse-Assisted Formation of Thiol Monolayers within Minutes for Fast and Controlled Electrode Surface Modification
    Jambrec, D. and Conzuelo, F. and Estrada-Vargas, A. and Schuhmann, W.
    ChemElectroChem 3 1484-1489 (2016)
    We propose a potential-pulse-assisted method for the formation of highly compact thiol self-assembled monolayers (SAMs), ensuring fully covered surfaces within minutes. By pulsing between potentials that are more positive and more negative with respect to the potential of zero charge, kinetics of SAM formation is substantially enhanced. The formation of the SAM is followed by using real-time impedance measurements by superimposing the applied potential-pulse profile with a high-frequency AC signal that allows for calculation of the interfacial capacitance and provides information about the compactness of the formed layers. A systematic study of the influence of the pulse potential intensity, the pulse duration, and the nature of the thiol derivative on the potential-pulse-assisted SAM formation is performed. We show that compact thiol monolayers are obtained much faster with the suggested technique, as compared to SAM formation performed at the open-circuit potential or by applying a constant potential. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/celc.201600308
  • 2016 • 137 Mesoporous nitrogen containing carbon materials for the simultaneous detection of ascorbic acid, dopamine and uric acid
    Joshi, A. and Schuhmann, W. and Nagaiah, T.C.
    Sensors and Actuators, B: Chemical 230 544-555 (2016)
    Mesoporous nitrogen rich carbonaceous (MNC) materials have been synthesized by pyrolyzing the polymerized ethylenediamine nanocasted into a SBA-15 hard template at 600 and 800 °C and explored for simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The electrocatalytic activity of these materials for the oxidation of analyte molecules was examined by means of redox-competition mode of scanning electrochemical microscopy (SECM), voltammetric, chronoamperometric and rotating disc electrode (RDE) measurements. MNC material exhibits a superior sensitivity towards the oxidation of AA, DA, and UA with a lowest detection limit of 0.01 μM, 0.001 μM and 0.01 μM respectively without any substantial interferences including glucose at physiologically relevant concentrations. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.snb.2016.02.050
  • 2016 • 136 Interplay of the Open Circuit Potential-Relaxation and the Dissolution Behavior of a Single H2 Bubble Generated at a Pt Microelectrode
    Karnbach, F. and Yang, X. and Mutschke, G. and Fröhlich, J. and Eckert, J. and Gebert, A. and Tschulik, K. and Eckert, K. and Uhlemann, M.
    Journal of Physical Chemistry C 120 15137-15146 (2016)
    The dissolution behavior of a single H2 bubble electrochemically generated at a Pt microelectrode in 1 M H2SO4 was studied. The open circuit potential (OCP) relaxation after the polarization end was recorded and correlated with the dissolved H2 concentration at the interface electrode/electrolyte/gas. Simultaneously, the shrinking of the bubble was followed optically by means of a high speed camera. In addition, analytical modeling and numerical simulations for the bubble dissolution were performed. Three characteristic regions are identified in the OCP and the bubble radius transients: (i) slow relaxation and shrinking, (ii) transition region, and (iii) a long-term slowed down dissolution process. The high supersaturation after polarization remains longer than theoretically predicted and feeds the bubble in region (i). This reduces the dissolution rate of the bubble which differs significantly from that of nonelectrochemically produced bubbles. Numerical multispecies simulations prove that oxygen and nitrogen dissolved in the electrolyte additionally influence the bubble dissolution and slow down its shrinkage compared to pure hydrogen diffusion. In region (iii), a complete exchange of hydrogen gas with nitrogen and oxygen has occurred in the gas bubble. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.6b02305
  • 2016 • 135 Selection of Highly SERS-Active Nanostructures from a Size Gradient of Au Nanovoids on a Single Bipolar Electrode
    Kayran, Y.U. and Eßmann, V. and Grützke, S. and Schuhmann, W.
    ChemElectroChem 3 399-403 (2016)
    As surface-enhanced Raman scattering (SERS) crucially depends on the morphology of nanostructured metal surfaces, we developed a convenient approach to produce a size gradient of truncated spherical Au nanovoids on a single bipolar electrode. The continuous potential drop in solution implies a linearly changing interfacial potential difference at the wireless electrode, leading to a linearly changing rate of Au electrodeposition. Such a structural gradient enables fast and reproducible screening for those structures, evoking high SERS intensity in a particular experiment. The optimal Au deposition potential with respect to the highest SERS amplification was determined and applied for the fabrication of highly active SERS substrates. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500423
  • 2016 • 134 Nano fabricated silicon nanorod array with titanium nitride coating for on-chip supercapacitors
    Lu, P. and Ohlckers, P. and Müller, L. and Leopold, S. and Hoffmann, M. and Grigoras, K. and Ahopelto, J. and Prunnila, M. and Chen, X.
    Electrochemistry Communications 70 51-55 (2016)
    We demonstrate high aspect ratio silicon nanorod arrays by cyclic deep reactive ion etching (DRIE) process as a scaffold to enhance the energy density of a Si-based supercapacitor. By unique atomic layer deposition (ALD) technology, a conformal nanolayer of TiN was deposited on the silicon nanorod arrays as the active material. The TiN coated silicon nanorods as a supercapacitor electrode lead to a 6 times improvement in capacitance compared to flat TiN film electrode. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2016.07.002
  • 2016 • 133 Nitrogen-Doped Hollow Amorphous Carbon Spheres@Graphitic Shells Derived from Pitch: New Structure Leads to Robust Lithium Storage
    Ma, Q. and Wang, L. and Xia, W. and Jia, D. and Zhao, Z.
    Chemistry - A European Journal 22 2339-2344 (2016)
    Nitrogen-doped mesoporous hollow carbon spheres (NHCS) consisting of hybridized amorphous and graphitic carbon were synthesized by chemical vapor deposition with pitch as raw material. Treatment with HNO3 vapor was performed to incorporate oxygen-containing groups on NHCS, and the resulting NHCS-O showed excellent rate capacity, high reversible capacity, and excellent cycling stability when tested as the anode material in lithium-ion batteries. The NHCS-O electrode maintained a reversible specific capacity of 616 mAh g-1 after 250 cycles at a current rate of 500 mA g-1, which is an increase of 113 % compared to the pristine hollow carbon spheres. In addition, the NHCS-O electrode exhibited a reversible capacity of 503 mAh g-1 at a high current density of 1.5 A g-1. The superior electrochemical performance of NHCS-O can be attributed to the hybrid structure, high N and O contents, and rich surface defects. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201503462
  • 2016 • 132 Advanced Evaluation of the Long-Term Stability of Oxygen Evolution Electrocatalysts
    Maljusch, A. and Conradi, O. and Hoch, S. and Blug, M. and Schuhmann, W.
    Analytical Chemistry 88 7597-7602 (2016)
    Evaluation of the long-term stability of electrocatalysts is typically performed using galvanostatic polarization at a predefined current density. A stable or insignificant increase in the applied potential is usually interpreted as high long-term stability of the tested catalyst. However, effects such as (i) electrochemical degradation of a catalyst due to its oxidation, (ii) blocking of the catalyst surface by evolved gas bubbles, and (iii) detachment of the catalyst from the electrode surface may lead to a decrease of the catalyst's active surface area being exposed to the electrolyte. In order to separate these effects and to evaluate the true electrochemical degradation of electrocatalysts, an advanced evaluation protocol based on subsequently performed electrochemical impedance, double layer capacitance, cyclic voltammetry, and galvanostatic polarization measurements was developed and used to evaluate the degradation of IrO2 particles drop-coated on glassy carbon rotating disk electrode using Nafion as a binder. A flow-through electrochemical cell was developed enabling circulation of the electrolyte leading to an efficient removal of evolved oxygen bubbles even at high current densities of up to 250 mA/cm2. The degradation rate of IrO2 was evaluated over 225 test cycles (0.733 ± 0.022 mV/h) with a total duration of galvanostatic polarization measurements of over 55 h. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.6b01289
  • 2016 • 131 Sensitivity Analyses on the Impact of Air Contaminants on Automotive Fuel Cells
    Misz, U. and Talke, A. and Heinzel, A. and Konrad, G.
    Fuel Cells 16 444-462 (2016)
    It is known that traffic related air contaminants cause power loss, decreasing lifetime or a complete failure of proton exchange membrane fuel cell (PEMFC). Therefore, the present study aims for a better understanding and the development of a data basis for further decisions in dealing with air contaminants for automobile applications. The first section provides an overview of scientific literature about the influence of important air contaminants on proton exchange membrane fuel cells (PEMFC). The second section describes an extensive study of air contaminants at possible automotive operating conditions using a full factorial matrix test. The specific variation of temperature, cell potential and harmful gas concentration resulted in 27 operating points for each used air contaminant. The gases NO, NO2, SO2, NH3, toluene and ethane were used. The results indicate significant degradation but as well the possibility of regeneration. The degradation caused by different harmful gases is both, dependent on temperature and potential. Furthermore, a clear difference of the influence of NO and NO2 at low concentrations could be shown. The experiments give an overview of the cathode harming potential of relevant air contaminants. Hence, the work provides a basis for the development of cathode air filter and regeneration techniques for automotive applications. Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/fuce.201500159
  • 2016 • 130 Nanostructured Antimony-Doped Tin Oxide Layers with Tunable Pore Architectures as Versatile Transparent Current Collectors for Biophotovoltaics
    Peters, K. and Lokupitiya, H.N. and Sarauli, D. and Labs, M. and Pribil, M. and Rathouský, J. and Kuhn, A. and Leister, D. and Stefik, M. and Fattakhova-Rohlfing, D.
    Advanced Functional Materials 26 6682-6692 (2016)
    Nanostructured transparent conducting oxide (TCO) layers gain increasing importance as high surface area electrodes enabling incorporation of functional redox species with high loading. The fabrication of porous TCO films, namely, antimony-doped tin oxide (ATO), is reported using the self-assembly of preformed ATO nanocrystals with poly(ethylene oxide-b-hexyl acrylate) (PEO-b-PHA) block copolymer. The high molar mass of the polymer and tunable solution processing conditions enable the fabrication of TCO electrodes with pore sizes ranging from mesopores to macropores. Particularly notable is access to uniform macroporous films with a nominal pore size of around 80 nm, which is difficult to obtain by other techniques. The combination of tunable porosity with a large conducting interface makes the obtained layers versatile current collectors with adjustable performance. While all the obtained electrodes incorporate a large amount of small redox molecules such as molybdenum polyoxometalate, only the electrodes with sufficiently large macropores are able to accommodate high amounts of bulky photoactive photosystem I (PSI) protein complexes. The 11-fold enhancement of the current response of PSI modified macroporous ATO electrodes compared to PSI on planar indium tin oxide (ITO), makes this type of electrodes promising candidates for the development of biohybrid devices. © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstractdoi: 10.1002/adfm.201602148
  • 2016 • 129 Wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces via entrapment in low potential phenothiazine-modified redox polymers
    Pinyou, P. and Ruff, A. and Pöller, S. and Alsaoub, S. and Leimkühler, S. and Wollenberger, U. and Schuhmann, W.
    Bioelectrochemistry 109 24-30 (2016)
    Phenothiazine-modified redox hydrogels were synthesized and used for the wiring of the aldehyde oxidoreductase PaoABC to electrode surfaces. The effects of the pH value and electrode surface modification on the biocatalytic activity of the layers were studied in the presence of vanillin as the substrate. The enzyme electrodes were successfully employed as bioanodes in vanillin/O2 biofuel cells in combination with a high potential bilirubin oxidase biocathode. Open circuit voltages of around 700mV could be obtained in a two compartment biofuel cell setup. Moreover, the use of a rather hydrophobic polymer with a high degree of crosslinking sites ensures the formation of stable polymer/enzyme films which were successfully used as bioanode in membrane-less biofuel cells. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2015.12.005
  • 2016 • 128 Investigation of the pH-Dependent Impact of Sulfonated Polyaniline on Bioelectrocatalytic Activity of Xanthine Dehydrogenase
    Sarauli, D. and Borowski, A. and Peters, K. and Schulz, B. and Fattakhova-Rohlfing, D. and Leimkühler, S. and Lisdat, F.
    ACS Catalysis 6 7152-7159 (2016)
    We report on the pH-dependent bioelectrocatalytic activity of the redox enzyme xanthine dehydrogenase (XDH) in the presence of sulfonated polyaniline PMSA1 (poly(2-methoxyaniline-5-sulfonic acid)-co-aniline). Ultraviolet-visible (UV-vis) spectroscopic measurements with both components in solution reveal electron transfer from the hypoxanthine (HX)-reduced enzyme to the polymer. The enzyme shows bioelectrocatalytic activity on indium tin oxide (ITO) electrodes, when the polymer is present. Depending on solution pH, different processes can be identified. It can be demonstrated that not only product-based communication with the electrode but also efficient polymer-supported bioelectrocatalysis occur. Interestingly, substrate-dependent catalytic currents can be obtained in acidic and neutral solutions, although the highest activity of XDH with natural reaction partners is in the alkaline region. Furthermore, operation of the enzyme electrode without addition of the natural cofactor of XDH is feasible. Finally, macroporous ITO electrodes have been used as an immobilization platform for the fabrication of HX-sensitive electrodes. The study shows that the efficient polymer/enzyme interaction can be advantageously combined with the open structure of an electrode material of controlled pore size, resulting in good processability, stability, and defined signal transfer in the presence of a substrate. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.6b02011
  • 2016 • 127 High-yield and scalable synthesis of a Silicon/Aminosilane-functionalized Carbon NanoTubes/Carbon (Si/A-CNT/C) composite as a high-capacity anode for lithium-ion batteries
    Sehlleier, Y.H. and Dobrowolny, S. and Plümel, I. and Xiao, L. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    Journal of Applied Electrochemistry 46 229-239 (2016)
    In this study, we present a novel anode architecture for high-performance lithium-ion batteries based on a Silicon/3-aminosilane-functionalized CNT/Carbon (Si/A-CNT/C) composite. A high-yield, low-cost approach has been developed to stabilize and support silicon as an active anode material. Silicon (Si) nanoparticles synthesized in a hot-wall reactor and aminosilane-functionalized carbon nanotubes (A-CNT) were dispersed in styrene and divinylbenzene (DVB) and subsequently polymerized forming a porous Si/A-CNT/C composite. Transmission electron microscopy showed that this method enables the interconnection and a uniform encapsulation of Si nanoparticles within a porous carbon matrix especially using aminosilane-functionalized CNT (A-CNT). Electrochemical characterization shows that this material can deliver a delithiation capacity of 2293 mAh g−1 with a capacity retention of more than 90 % after 200 cycles at lithiation and delithiation rate of 0.5 C. We conclude that the porous Si/A-CNT/C composite material can accommodate sufficient space for Si volume expansion and extraction and improve the electronic and ionic conduction. Excellent electrochemical performance during repeated cycling can thus be achieved. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10800-015-0897-x
  • 2016 • 126 Exploring the mineral-water interface: Reduction and reaction kinetics of single hematite (α-Fe2O3) nanoparticles
    Shimizu, K. and Tschulik, K. and Compton, R.G.
    Chemical Science 7 1408-1414 (2016)
    In spite of their natural and technological importance, the intrinsic electrochemical properties of hematite (α-Fe2O3) nanoparticles are not well understood. In particular, particle agglomeration, the presence of surface impurities, and/or inadequate proton concentrations are major obstacles to uncover the fundamental redox activities of minerals in solution. These are particularly problematic when samples are characterized in common electrochemical analyses such as cyclic voltammetry in which nanoparticles are immobilized on a stationary electrode. In this work, the intrinsic reaction kinetics and thermodynamics of individual hematite nanoparticles are investigated by particle impact chronoamperometry. The particle radius derived from the integrated area of spikes recorded in a chronoamperogram is in excellent agreement with electron microscopy results, indicating that the method provides a quantitative analysis of the reduction of the nanoparticles to the ferrous ion. A key finding is that the suspended individual nanoparticles undergo electrochemical reduction at potentials much more positive than those immobilized on a stationary electrode. The critical importance of the solid/water interface on nanoparticle activity is further illustrated by a kinetic model. It is found that the first electron transfer process is the rate determining step of the reductive dissolution of hematite nanoparticles, while the overall process is strongly affected by the interfacial proton concentration. This article highlights the effects of the interfacial proton and ferrous ion concentrations on the reductive dissolution of hematite nanoparticles and provides a highly effective method that can be readily applied to study a wide range of other mineral nanoparticles. © 2016 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5sc03678j
  • 2016 • 125 Optimization of a transferred arc reactor for metal nanoparticle synthesis
    Stein, M. and Kruis, F.E.
    Journal of Nanoparticle Research 18 (2016)
    The demand for metal nanoparticles is increasing strongly. Transferred arc synthesis is a promising process in this respect, as it shows high production rates, good quality particles and the ability of up-scaling. The influence of several process parameters on the performance of the process in terms of production rate and particle size is investigated. These parameters are the electrode design and adjustment, the gas flow rate and power input. A novel feeding mechanism allows process operation over an extended time period. It is shown that the process is capable of producing pure metal nanoparticles with variable primary particle sizes and comparatively high production rates. Optimal process conditions for a single transferred arc electrode pair are found, which allow further scale-up by numbering up. © 2016, The Author(s).
    view abstractdoi: 10.1007/s11051-016-3559-y
  • 2016 • 124 TiO2 quantum dots embedded in bamboo-like porous carbon nanotubes as ultra high power and long life anodes for lithium ion batteries
    Tang, Y. and Liu, L. and Wang, X. and Jia, D. and Xia, W. and Zhao, Z. and Qiu, J.
    Journal of Power Sources 319 227-234 (2016)
    TiO2 quantum dots embedded in bamboo-like porous carbon nanotubes have been constructed through the pyrolysis of sulfonated polymer nanotubes and TiO2 hybrids. The TiO2 quantum dots are formed during the pyrolysis, due to the space confinement within the highly cross-linked copolymer networks. The sulfonation degree of the polymer nanotubes is a critical factor to ensure the formation of the unique interpenetrating structure. The nanocomposites exhibit high reversible capacity of 523 mAh g-1 at 100 mA g-1 after 200 cycles, excellent rate capability and superior long-term cycling stability at high current density, which could attain a high discharge capacity of 189 mAh g-1 at 2000 mA g-1 for up to 2000 cycles. The enhanced electrochemical performance of the nanocomposites benefit from the uniform distribution of TiO2 quantum dots, high electronic conductivity of porous carbons and unique interpenetrating structure, which simultaneously solved the major problems of TiO2 anode facing the pulverization, loss of electrical contact and particle aggregation. © 2016 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2016.04.033
  • 2016 • 123 Investigation and optimization of the tribo-mechanical properties of CrAlCN coatings using Design of Experiments
    Tillmann, W. and Stangier, D. and Schröder, P.
    Surface and Coatings Technology 308 147-157 (2016)
    The control of friction as well as its adaption is essential for forming operations. Thin hard coatings have a significant influence on the performance of production processes and the service life of tools, especially for Sheet-Bulk Metal Forming (SBMF) processes with high contact normal stresses and issues concerned with the filling of filigree functional elements. To handle these challenges, the CrAlCN coating system is generated by means of bipolar-pulsed reactive magnetron sputtering, using Design of Experiments. A Central Composite Design is selected to investigate the cathode power, bias voltage, as well as the reactive gas flow composition (nitrogen and acetylene). The aim is to evaluate the correlations and the interaction of the investigated process parameters on the mechanical as well as the tribological behavior of the CrAlCN coating, and to develop models to obtain the desired coating properties. The generated coatings show a clear dependency on the selected process parameters. An increased acetylene flow leads to a reduction of the mechanical properties (hardness and Young's modulus) as well as a decreased adhesion of the CrAlCN coating. In contrast to the influence of the acetylene flow, a higher negative bias voltage leads to improved mechanical properties in the context of wear resistant thin films. The tribological properties revealed that the coefficient of friction is related to the chemical composition of the coating which can, on the one hand, be adjusted by the acetylene flow and, on the other hand, by the cathode power. The optimized CrAlCN coating was deposited onto forming punches and the friction was evaluated using DC04 and DP600 specimens in an adapted ring-compression test. In comparison to polished and heat-treated ASP®2023 steel (62 HRC) and a CrAlN reference coating, the developed coating shows a significant reduction of friction due to the carbon incorporation. © 2016 Elsevier B.V.
    view abstractdoi: 10.1016/j.surfcoat.2016.07.110
  • 2016 • 122 Making the hydrogen evolution reaction in polymer electrolyte membrane electrolysers even faster
    Tymoczko, J. and Calle-Vallejo, F. and Schuhmann, W. and Bandarenka, A.S.
    Nature Communications 7 (2016)
    Although the hydrogen evolution reaction (HER) is one of the fastest electrocatalytic reactions, modern polymer electrolyte membrane (PEM) electrolysers require larger platinum loadings (∼0.5-1.0 mg cm-2) than those in PEM fuel cell anodes and cathodes altogether (∼0.5 mg cm-2). Thus, catalyst optimization would help in substantially reducing the costs for hydrogen production using this technology. Here we show that the activity of platinum(111) electrodes towards HER is significantly enhanced with just monolayer amounts of copper. Positioning copper atoms into the subsurface layer of platinum weakens the surface binding of adsorbed H-intermediates and provides a twofold activity increase, surpassing the highest specific HER activities reported for acidic media under similar conditions, to the best of our knowledge. These improvements are rationalized using a simple model based on structure-sensitive hydrogen adsorption at platinum and copper-modified platinum surfaces. This model also solves a long-lasting puzzle in electrocatalysis, namely why polycrystalline platinum electrodes are more active than platinum(111) for the HER.
    view abstractdoi: 10.1038/ncomms10990
  • 2016 • 121 Spearhead Nanometric Field-Effect Transistor Sensors for Single-Cell Analysis
    Zhang, Y. and Clausmeyer, J. and Babakinejad, B. and López Córdoba, A. and Ali, T. and Shevchuk, A. and Takahashi, Y. and Novak, P. and Edwards, C. and Lab, M. and Gopal, S. and Chiappini, C. and Anand, U. and Magnani, L. and Co...
    ACS Nano 10 3214-3221 (2016)
    Nanometric field-effect-transistor (FET) sensors are made on the tip of spear-shaped dual carbon nanoelectrodes derived from carbon deposition inside double-barrel nanopipettes. The easy fabrication route allows deposition of semiconductors or conducting polymers to comprise the transistor channel. A channel from electrodeposited poly pyrrole (PPy) exhibits high sensitivity toward pH changes. This property is exploited by immobilizing hexokinase on PPy nano-FETs to give rise to a selective ATP biosensor. Extracellular pH and ATP gradients are key biochemical constituents in the microenvironment of living cells; we monitor their real-time changes in relation to cancer cells and cardiomyocytes. The highly localized detection is possible because of the high aspect ratio and the spear-like design of the nano-FET probes. The accurately positioned nano-FET sensors can detect concentration gradients in three-dimensional space, identify biochemical properties of a single living cell, and after cell membrane penetration perform intracellular measurements. © 2016 American Chemical Society.
    view abstractdoi: 10.1021/acsnano.5b05211
  • 2015 • 120 Atomic oxygen dynamics in an air dielectric barrier discharge: A combined diagnostic and modeling approach
    Baldus, S. and Schröder, D. and Bibinov, N. and Schulz-Von Der Gathen, V. and Awakowicz, P.
    Journal of Physics D: Applied Physics 48 (2015)
    Cold atmospheric pressure plasmas are a promising alternative therapy for treatment of chronic wounds, as they have already shown in clinical trials. In this study an air dielectric barrier discharge (DBD) developed for therapeutic use in dermatology is characterized with respect to the plasma produced reactive oxygen species, namely atomic oxygen and ozone, which are known to be of great importance to wound healing. To understand the plasma chemistry of the applied DBD, xenon-calibrated two-photon laser-induced fluorescence spectroscopy and optical absorption spectroscopy are applied. The measured spatial distributions are shown and compared to each other. A model of the afterglow chemistry based on optical emission spectroscopy is developed to cross-check the measurement results and obtain insight into the dynamics of the considered reactive oxygen species. The atomic oxygen density is found to be located mostly between the electrodes with a maximum density of n<inf>O<inf>3</inf></inf> = 6 x 10^16 cm. Time resolved measurements reveal a constant atomic oxygen density between two high voltage pulses. The ozone is measured up to 3 mm outside the active plasma volume, reaching a maximum value of nO = 3 x 1016 cm-3 between the electrodes.
    view abstractdoi: 10.1088/0022-3727/48/27/275203
  • 2015 • 119 Metal-halide Nanoparticle Formation: Electrolytic and Chemical Synthesis of Mercury(I) Chloride Nanoparticles
    Bartlett, T.R. and Batchelor-Mcauley, C. and Tschulik, K. and Jurkschat, K. and Compton, R.G.
    ChemElectroChem 2 522-528 (2015)
    Mercury(I) chloride (Hg<inf>2</inf>Cl<inf>2</inf>) nanoparticles (NPs) are synthesised for the first time by using two different techniques. First, particles are formed by implosion of a calomel nanolayer, induced by partial electrolysis at a mercury hemisphere microelectrode. The resulting NPs are then characterised by the nanoimpact method, demonstrating the first time metal chloride NPs have been sized by this technique and showing the ability to form and study NPs insitu. Second, Hg<inf>2</inf>Cl<inf>2</inf> NPs are synthesised by using the precipitation reaction of Hg<inf>2</inf>(NO<inf>3</inf>)<inf>2</inf> with KCl. The NPs are characterised on both mercury and carbon microelectrodes and their size is found to agree with TEM results. Sizable studies: Mercury(I) chloride (Hg<inf>2</inf>Cl<inf>2</inf>) nanoparticles (NPs) are synthesised for the first time by using two different techniques. First, particles are formed by implosion of a calomel nanolayer, induced by partial electrolysis at a mercury hemisphere microelectrode. Second, Hg<inf>2</inf>Cl<inf>2</inf> NPs are synthesised by the precipitation reaction between Hg<inf>2</inf>(NO<inf>3</inf>)<inf>2</inf> and KCl. The NPs are characterised on both mercury and carbon microelectrodes by using the nanoimpact method and their size is found to agree with TEM results. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201402401
  • 2015 • 118 Onset potential determination at gas-evolving catalysts by means of constant-distance mode positioning of nanoelectrodes
    Botz, A.J.R. and Nebel, M. and Rincón, R.A. and Ventosa, E. and Schuhmann, W.
    Electrochimica Acta 179 38-44 (2015)
    The onset potential of an electrocatalytic reaction is frequently used as an indicator to compare the catalytic performance of electrocatalysts. However, in addition to the fact that the onset potential is an undefined physico-chemical value which is dependent on the sensitivity of the used potentiostat its determination using voltammetry at the catalyst-modified electrode surface may be superimposed by additional Faradaic reactions e.g. from redox conversions of the catalyst material or corrosion processes. Gas-evolving electrodes suffer additionally from the dynamics of gas bubble formation and departure leading to inherent limitations of voltammetric studies directly performed at the catalyst-modified electrode. Nanometer-sized electrodes accurately positioned by means of shearforce-based constant-distance mode SECM are proposed for the highly sensitive determination of the onset potential of microcavity electrodes filled with different perovskites as oxygen evolution catalysts. Double barrel microcavity electrodes are additionally suggested for the simultaneous investigation of two catalysts. They enable direct referencing of a catalyst with a benchmark catalyst material in a single experiment. © 2015 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2015.04.145
  • 2015 • 117 Experimental Aspects in Benchmarking of the Electrocatalytic Activity
    Čolić, V. and Tymoczko, J. and Maljusch, A. and Ganassin, A. and Schuhmann, W., Prof. and Bandarenka, A.S.
    ChemElectroChem 2 143-149 (2015)
    With the high interest in improving the performance of electrocatalysts for technologically significant reactions, great efforts are directed at the assessment of the activities of various catalytic materials. For this purpose, it is important to compare the catalytic activities measured using different methods and under different conditions. To achieve this, it is of utmost importance to avoid certain methodological and instrumental issues that can severely affect the obtained experimental results. Using well-defined systems, we demonstrate the importance of experimental conditions in the assessment and benchmarking of the activity of catalytic processes for various reactions. Particularly, we demonstrate that the correction of the uncompensated ohmic resistance using impedance spectroscopy measurements requires particular attention and additional procedures which are normally ignored. Additionally, we demonstrate how the uncompensated resistance changes with the potential if a non-conducting gas phase is accumulated in the system, hence influencing the activity measurement. It is further shown that a correct choice for surface-limited reactions for the determination of the real surface area of catalytic electrodes plays a key role in ensuring more meaningful activity assessment. Not as easy as it seems: Benchmarking of the electrocatalytic activity can be unexpectedly very demanding due to experimental issues which are often underestimated. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201402295
  • 2015 • 116 Controlling the charge of pH-responsive redox hydrogels by means of redox-silent biocatalytic processes. A biocatalytic off/on switch
    Contin, A. and Plumeré, N. and Schuhmann, W.
    Electrochemistry Communications 51 50-53 (2015)
    Coupling of redox-silent biocatalytic processes for analyte detection with enzyme-catalyzed redox reactions for signal generation is proposed by the modulation of electrostatic interactions between a pH-responsive polymer and a redox enzyme to control the off-on transition for electrochemical signal generation. Glassy carbon electrodes are modified with a poly(vinyl)imidazole Os(bipyridine)2Cl redox hydrogel film entrapping urease and PQQ-dependent glucose dehydrogenase, while glucose is present in the solution. The off-on transition is based on the detection of urea as model analyte which is hydrolyzed to ammonia by urease within the hydrogel film concomitantly increasing the local pH value thus invoking deprotonation of the imidazole groups at the polymer backbone. The decrease of positive charges at the polymer decreases electrostatic repulsion between the polymer and the positively charged PQQ-dependent glucose dehydrogenase. Hence, electron transfer rates between polymer-bound Os complexes and PQQ inside the enzyme are enhanced activating electrocatalytic oxidation of glucose. This process generates the electrochemical signal for urea detection. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.elecom.2014.12.001
  • 2015 • 115 Silicon/carbon nano-composite based anodes for advanced lithium-ion batteries
    Dobrowolny, S. and Mahlendorf, F. and Heinzel, A.
    ECS Transactions 66 29-36 (2015)
    In this study, the investigation of high capacity and high efficiency graphene coated silicon composite (Si/C composite) based electrodes prepared by using a wet chemical manufacturing process is presented. The active material provides a capacity of >2000 mAh g-1 with a coulombic efficiency >99% for more than 500 cycles. The focus is set to the investigation of the electrode structure during cycling progression by using galvanostatic cycling, electrochemical impedance spectroscopy, scanning electron microscopy, confocal microscopy and the measurement of the coating adhesion strength. Results show the applicability of improved Si/C composite electrodes for future lithium-ion batteries, both in half cells as well as in full cells in combination with a commercially available cathode material. © The Electrochemical Society.
    view abstractdoi: 10.1149/06609.0029ecst
  • 2015 • 114 Three-dimensional Cu foam-supported single crystalline mesoporous Cu2O nanothorn arrays for ultra-highly sensitive and efficient nonenzymatic detection of glucose
    Dong, C. and Zhong, H. and Kou, T. and Frenzel, J. and Eggeler, G. and Zhang, Z.
    ACS Applied Materials and Interfaces 7 20215-20223 (2015)
    Highly sensitive and efficient biosensors play a crucial role in clinical, environmental, industrial, and agricultural applications, and tremendous efforts have been dedicated to advanced electrode materials with superior electrochemical activities and low cost. Here, we report a three-dimensional binder-free Cu foam-supported Cu<inf>2</inf>O nanothorn array electrode developed via facile electrochemistry. The nanothorns growing in situ along the specific direction of <011> have single crystalline features and a mesoporous surface. When being used as a potential biosensor for nonenzyme glucose detection, the hybrid electrode exhibits multistage linear detection ranges with ultrahigh sensitivities (maximum of 97.9 mA mM-1 cm-2) and an ultralow detection limit of 5 nM. Furthermore, the electrode presents outstanding selectivity and stability toward glucose detection. The distinguished performances endow this novel electrode with powerful reliability for analyzing human serum samples. These unprecedented sensing characteristics could be ascribed to the synergistic action of superior electrochemical catalytic activity of nanothorn arrays with dramatically enhanced surface area and intimate contact between the active material (Cu<inf>2</inf>O) and current collector (Cu foam), concurrently supplying good conductivity for electron/ion transport during glucose biosensing. Significantly, our findings could guide the fabrication of new metal oxide nanostructures with well-organized morphologies and unique properties as well as low materials cost. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b05738
  • 2015 • 113 The effect of insulator nano-sheath thickness on the steady state current at a micro-disc electrode
    Ellison, J. and Eloul, S. and Batchelor-Mcauley, C. and Tschulik, K. and Salter, C. and Compton, R.G.
    Journal of Electroanalytical Chemistry 745 66-71 (2015)
    Abstract The relative size of the insulating sheath to electrode area at a micro-disc electrode can lead to significant perturbations in the steady state current observed. A minimum, constant steady state current value is realised once the sheath thickness is greater than twice the radius Δl&gt;2rd. However, as the sheath thickness decreases below this value, the observed current increases. In this paper a theoretical model is presented, allowing for the accurate determination of the outer sheath thickness. The effects of an ultra-thin sheath on steady state currents are demonstrated experimentally and these results are shown to accurately fit with the simulated model developed. Therefore, the model presented here can be used to determine the size of a sheath of unknown thickness. Furthermore, it allows these size effects on the steady state current to be explored. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jelechem.2015.02.027
  • 2015 • 112 Diffusional impacts of nanoparticles on microdisc and microwire electrodes: The limit of detection and first passage statistics
    Eloul, S. and Kätelhön, E. and Batchelor-McAuley, C. and Tschulik, K. and Compton, R.G.
    Journal of Electroanalytical Chemistry 755 136-142 (2015)
    We derive approximate expressions for the average number of diffusive impacts/hits of nanoparticles on microdisc and microwire electrodes for the case where the impact leads to the loss of the nanoparticles from solution either via irreversible adsorption or complete electro-dissolution. The theory can also be applied to sub-micrometre size electrodes (nano-electrodes). The resulting equations can be utilised to analyse the number of impacts and its variance in the 'nano-impact' experiment. We also provide analytical expressions for the first passage time of an impact for dilute nanoparticle solutions in the continuum limit of Fickian diffusion. The expressions for the first passage times are used to estimate the lower limit of detection in ultra-dilute nanoparticle solutions for typical nano-impact experiments, and show the advantage of using microwire electrodes in ultra-dilute solutions or solutions containing larger nano-particles. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jelechem.2015.07.042
  • 2015 • 111 Diffusional nanoimpacts: The stochastic limit
    Eloul, S. and Kätelhön, E. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
    Journal of Physical Chemistry C 119 14400-14410 (2015)
    The probability expressions for the average number of diffusional impact events on a surface are established using Fick's diffusion in the limit of a continuum flux. The number and the corresponding variance are calculated for the case of nanoparticles impacting on an electrode at which they are annihilated. The calculations show the dependency on concentration in the limit of noncontinuous media and small electrode sizes for the cases of linear diffusion to a macroelectrode and of convergent diffusion to a small sphere. Using random walk simulations, we confirm that the variance follows a Poisson distribution for ultradilute and dilute solutions. We also present an average "first passage time" for the ultradilute solutions expression that directly relates to the lower limit of detection in ultradilute solutions as a function of the electrode size. The analytical expressions provide a straightforward way to predict the stochastics of impacts in a "nanoimpact" experiment by using Fick's second law and assuming a continuum dilute flux. Therefore, the study's results are applicable to practical electrochemical systems where the number of particles is very small but much larger than one. Moreover, the presented analytical expression for the variance can be utilized to identify effects of particle inhomogeneity in the solution and is of general interest in all studies of diffusion processes toward an absorbing wall in the stochastic limit. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b03210
  • 2015 • 110 Phenothiazine-functionalized redox polymers for a new cathode-active material
    Golriz, A.A. and Suga, T. and Nishide, H. and Berger, R. and Gutmann, J.S.
    RSC Advances 5 22947-22950 (2015)
    Redox-active, phenothiazine-functionalized polymers were synthesized and employed as a promising cathode-active material (∼3.7 V vs. Li, 77 Ah kg-1) in a rechargeable battery. The longer spacer between phenothiazine and the polymer backbone contributed to the stability of the formed radical cations, resulting in decelerated self-discharge and improved cycle performance. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra17107a
  • 2015 • 109 Controllable Synthesis of Mesoporous Peapod-like Co3O4@Carbon Nanotube Arrays for High-Performance Lithium-Ion Batteries
    Gu, D. and Li, W. and Wang, F. and Bongard, H. and Spliethoff, B. and Schmidt, W. and Weidenthaler, C. and Xia, Y. and Zhao, D. and Schüth, F.
    Angewandte Chemie - International Edition 54 7060-7064 (2015)
    Abstract Transition metal oxides are regarded as promising anode materials for lithium-ion batteries because of their high theoretical capacities compared with commercial graphite. Unfortunately, the implementation of such novel anodes is hampered by their large volume changes during the Li+ insertion and extraction process and their low electric conductivities. Herein, we report a specifically designed anode architecture to overcome such problems, that is, mesoporous peapod-like Co<inf>3</inf>O<inf>4</inf>@carbon nanotube arrays, which are constructed through a controllable nanocasting process. Co<inf>3</inf>O<inf>4</inf> nanoparticles are confined exclusively in the intratubular pores of the nanotube arrays. The pores between the nanotubes are open, and thus render the Co<inf>3</inf>O<inf>4</inf> nanoparticles accessible for effective electrolyte diffusion. Moreover, the carbon nanotubes act as a conductive network. As a result, the peapod-like Co<inf>3</inf>O<inf>4</inf>@carbon nanotube electrode shows a high specific capacity, excellent rate capacity, and very good cycling performance. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201501475
  • 2015 • 108 Electrochemical communication between electrodes and rhodobacter capsulatus grown in different metabolic modes
    Hasan, K. and Reddy, K.V.R. and Eßmann, V. and Górecki, K. and Conghaile, P.O. and Schuhmann, W. and Leech, D. and Hägerhäll, C. and Gorton, L.
    Electroanalysis 27 118-127 (2015)
    The majority of efforts on microbial and photosynthetic microbial fuel cells are both curiosity driven and made to possibly meet the future growing demand for sustainable energy. The most metabolically versatile purple bacteria Rhodobacter capsulatus is a potential candidate for this purpose. However, utilizing bacteria in such systems requires efficient electronic transfer communication between the microbial cells and the electrodes, which is one of the greatest challenges. Previous studies demonstrated that osmium redox polymers (ORPs) could be used for extracellular electron transfer between the cells and electrodes. Recently, heterotrophically grown R. capsulatus has been wired with ORP modified electrodes. Here in this communication, we report electron transfer from R. capsulatus grown under heterotrophic as well as under photoheterotrophic conditions to electrodes. The cells, immobilized on bare graphite and ORP modified graphite electrodes, were excited with visible light and subsequent photosynthetic electron transfer was recorded using cyclic voltammetric and chronoamperometric measurements. Photoheterotrophically grown R. capsulatus cells on bare graphite generate a significant photocurrent density of 3.46μAcm-2, whereas on an ORP modified electrode the current density increases to 8.46μAcm-2. Furthermore, when 1mM p-benzoquinone is added to the electrolyte the photocurrent density reaches 12.25μAcm-2. Our results could have significant implications in photosynthetic energy conversion and in development of photobioelectrochemical devices. © 2015 WILEY-VCH Verlag GmbH and Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/elan.201400456
  • 2015 • 107 The subtleties of the reversible hydrogen evolution reaction arising from the nonunity stoichiometry
    Jiao, X. and Batchelor-Mcauley, C. and Kätelhön, E. and Ellison, J. and Tschulik, K. and Compton, R.G.
    Journal of Physical Chemistry C 119 9402-9410 (2015)
    The proton/hydrogen redox couple underpins the electrochemical sciences; however, the nonunity stoichiometry of the reaction leads to distinct voltammetric complications. This Article provides a joint analytical, numerical, and experimental investigation into the reversible hydrogen evolution reaction at a platinum microelectrode. Literature obscurities and nuances are highlighted and corrected, allowing the presentation of an holistic overview of the electrochemical reaction at the reversible limit. Under such conditions, it is demonstrated, first, how the reaction may be misinterpreted as being irreversible and, second, that the transfer coefficient for the reversible (Nernstian) hydrogen evolution reaction is equal to 2. Importantly, the use of the reversible hydrogen electrode (RHE) as a reference potential in voltammetric experiments is critically evaluated. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcc.5b01864
  • 2015 • 106 Electron collection in host-guest nanostructured hematite photoanodes for water splitting: The influence of scaffold doping density
    Kondofersky, I. and Dunn, H.K. and Müller, A. and Mandlmeier, B. and Feckl, J.M. and Fattakhova-Rohlfing, D. and Scheu, C. and Peter, L.M. and Bein, T.
    ACS Applied Materials and Interfaces 7 4623-4630 (2015)
    Nanostructuring has proven to be a successful strategy in overcoming the trade-off between light absorption and hole transport to the solid/electrolyte interface in hematite photoanodes for water splitting. The suggestion that poor electron (majority carrier) collection hinders the performance of nanostructured hematite electrodes has led to the emergence of host-guest architectures in which the absorber layer is deposited onto a transparent high-surface-area electron collector. To date, however, state of the art nanostructured hematite electrodes still outperform their host-guest counterparts, and a quantitative evaluation of the benefits of the host-guest architecture is still lacking. In this paper, we examine the impact of host-guest architectures by comparing nanostructured tin-doped hematite electrodes with hematite nanoparticle layers coated onto two types of conducting macroporous SnO2 scaffolds. Analysis of the external quantum efficiency spectra for substrate (SI) and electrolyte side (EI) illumination reveals that the electron diffusion length in the host-guest electrodes based on an undoped SnO2 scaffold is increased substantially relative to the nanostructured hematite electrode without a supporting scaffold. Nevertheless, electron collection is still incomplete for EI illumination. By contrast, an electron collection efficiency of 100% is achieved by fabricating the scaffold using antimony-doped SnO2, showing that the scaffold conductivity is crucial for the device performance. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/am5078667
  • 2015 • 105 Wet Nanoindentation of the Solid Electrolyte Interphase on Thin Film Si Electrodes
    Kuznetsov, V. and Zinn, A.-H. and Zampardi, G. and Borhani-Haghighi, S. and La Mantia, F. and Ludwig, Al. and Schuhmann, W. and Ventosa, E.
    ACS Applied Materials and Interfaces 7 23554-23563 (2015)
    The solid electrolyte interphase (SEI) film formed at the surface of negative electrodes strongly affects the performance of a Li-ion battery. The mechanical properties of the SEI are of special importance for Si electrodes due to the large volumetric changes of Si upon (de)insertion of Li ions. This manuscript reports the careful determination of the Young's modulus of the SEI formed on a sputtered Si electrode using wet atomic force microscopy (AFM)-nanoindentation. Several key parameters in the determination of the Young's modulus are considered and discussed, e.g., wetness and roughness-thickness ratio of the film and the shape of a nanoindenter. The values of the Young's modulus were determined to be 0.5-10 MPa under the investigated conditions which are in the lower range of those previously reported, i.e., 1 MPa to 10 GPa, pointing out the importance of the conditions of its determination. After multiple electrochemical cycles, the polymeric deposits formed on the surface of the SEI are revealed, by force-volume mapping in liquid using colloidal probes, to extend up to 300 nm into bulk solution. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b06700
  • 2015 • 104 Formation and Movement of Cationic Defects during Forming and Resistive Switching in SrTiO3 Thin Film Devices
    Lenser, C. and Koehl, A. and Slipukhina, I. and Du, H. and Patt, M. and Feyer, V. and Schneider, C.M. and Lezaic, M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 6360-6368 (2015)
    The resistance switching phenomenon in many transition metal oxides is described by ion motion leading to the formation of oxygen-deficient, highly electron-doped filaments. In this paper, the interface and subinterface region of electroformed and switched metal-insulator-metal structures fabricated from a thin Fe-doped SrTiO3 (STO) film on n-conducting Nb-doped SrTiO3 crystals are investigated by photoemission electron microscopy, transmission electron microscopy, and hard X-ray photoelectron spectroscopy in order to gain a deeper understanding of cation movement in this specific system. During electroforming, the segregation of Sr to the top interface and the generation of defect-rich cones in the film are observed, apparently growing from the anode toward the cathode during electroforming. An unusual binding energy component of the Sr 3d emission line is observed which can be assigned to Sr Ti-VO∗ defect complexes by performing ab initio calculations. Since this Sr component can be reversibly affected by an external electrical bias, the movement of both oxygen and Sr point defects and the formation of defect complexes Sr Ti-VO∗ during resistive switching are suggested. These findings are discussed with regard to the point defect structure of the film and the local oxidation of the donor-doped substrate. In particular, the apparent dichotomy between the observation of acceptor-type defects and increased electronic conductivity in STO is addressed. A low binding energy component of the Sr 3d photoemission line is observed in Fe-doped SrTiO3 memristive devices and assigned to Sr′Ti-V∗O defect complexes by ab initio calculations. Since this Sr component can be reversibly affected by an electrical bias, the movement of both oxygen and Sr vacancies and the formation of Sr′Ti-V∗O defect complexes during resistive switching are suggested. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201500851
  • 2015 • 103 Note: Ion-induced secondary electron emission from oxidized metal surfaces measured in a particle beam reactor
    Marcak, A. and Corbella, C. and de los Arcos, T. and von Keudell, A.
    Review of Scientific Instruments 86 106102 (2015)
    The secondary electron emission of metals induced by slow ions is characterized in a beam chamber by means of two coaxial semi-cylindrical electrodes with different apertures. The voltages of the outer electrode (screening), inner electrode (collector), and sample holder (target) were set independently in order to measure the effective yield of potential and kinetic electron emissions during ion bombardment. Aluminum samples were exposed to quantified beams of argon ions up to 2000 eV and to oxygen atoms and molecules in order to mimic the plasma-surface interactions on metallic targets during reactive sputtering. The variation of electron emission yield was correlated to the ion energy and to the oxidation state of Al surfaces. This system provides reliable measurements of the electron yields in real time and is of great utility to explore the fundamental surface processes during target poisoning occurring in reactive magnetron sputtering applications. (C) 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4932309
  • 2015 • 102 Enhanced stability of multilayer graphene-supported catalysts for polymer electrolyte membrane fuel cell cathodes
    Marinkas, A. and Hempelmann, R. and Heinzel, A. and Peinecke, V. and Radev, I. and Natter, H.
    Journal of Power Sources 295 79-91 (2015)
    Abstract One of the biggest challenges in the field of polymer electrolyte membrane fuel cells (PEMFC) is to enhance the lifetime and the long-term stability of PEMFC electrodes, especially of cathodes, furthermore, to reduce their platinum loading, which could lead to a cost reduction for efficient PEMFCs. These demands could be achieved with a new catalyst support architecture consisting of a composite of carbon structures with significant different morphologies. A highly porous cathode catalyst support layer is prepared by addition of various carbon types (carbon black particles, multi-walled carbon nanotubes (MWCNT)) to multilayer graphene (MLG). The reported optimized cathodes shows extremely high durability and similar performance to commercial standard cathodes but with 89% lower Pt loading. The accelerated aging protocol (AAP) on the membrane electrode assemblies (MEA) shows that the presence of MLG increases drastically the durability and the Pt-extended electrochemical surface area (ECSA). In fact, after the AAP slightly enhanced performance can be observed for the MLG-containing cathodes instead of a performance loss, which is typical for the commercial carbon-based cathodes. Furthermore, the presence of MLG drastically decreases the ECSA loss rate. The MLG-containing cathodes show up to 6.8 times higher mass-normalized Pt-extended ECSA compared to the commercial standard systems. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.jpowsour.2015.06.126
  • 2015 • 101 Assembling Paramagnetic Ceruloplasmin at Electrode Surfaces Covered with Ferromagnetic Nanoparticles. Scanning Electrochemical Microscopy in the Presence of a Magnetic Field
    Matysiak, E. and Botz, A.J.R. and Clausmeyer, J. and Wagner, B. and Schuhmann, W. and Stojek, Z. and Nowicka, A.M.
    Langmuir 31 8176-8183 (2015)
    Adsorption of ceruloplasmin (Cp) at a gold electrode modified with ferromagnetic iron nanoparticles encapsulated in carbon (Fe@C Nps) leads to a successful immobilization of the enzyme in its electroactive form. The proper placement of Cp at the electrode surface on top of the nanocapsules containing an iron core allowed a preorientation of the enzyme, hence allowing direct electron transfer between the electrode and the enzyme. Laser ablation coupled with inductively coupled plasma mass spectrometry indicated that Cp was predominantly located at the paramagnetic nanoparticles. Scanning electrochemical microscopy measurements in the sample-generation/tip-collection mode proved that Cp was ferrooxidative inactive if it was immobilized on the bare gold surface and reached the highest activity if it was adsorbed on Fe@C Nps in the presence of a magnetic field. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.langmuir.5b01155
  • 2015 • 100 Advancing from Rules of Thumb: Quantifying the Effects of Small Density Changes in Mass Transport to Electrodes. Understanding Natural Convection
    Ngamchuea, K. and Eloul, S. and Tschulik, K. and Compton, R.G.
    Analytical Chemistry 87 7226-7234 (2015)
    Understanding mass transport is prerequisite to all quantitative analysis of electrochemical experiments. While the contribution of diffusion is well understood, the influence of density gradient-driven natural convection on the mass transport in electrochemical systems is not. To date, it has been assumed to be relevant only for high concentrations of redox-active species and at long experimental time scales. If unjustified, this assumption risks misinterpretation of analytical data obtained from scanning electrochemical microscopy (SECM) and generator-collector experiments, as well as analytical sensors utilizing macroelectrodes/microelectrode arrays. It also affects the results expected from electrodeposition. On the basis of numerical simulation, herein it is demonstrated that even at less than 10 mM concentrations and short experimental times of tens of seconds, density gradient-driven natural convection significantly affects mass transport. This is evident from in-depth numerical simulation for the oxidation of hexacyanoferrate (II) at various electrode sizes and electrode orientations. In each case, the induced convection and its influence on the diffusion layer established near the electrode are illustrated by maps of the velocity fields and concentration distributions evolving with time. The effects of natural convection on mass transport and chronoamperometric currents are thus quantified and discussed for the different cases studied. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.analchem.5b01293
  • 2015 • 99 Water-dispersible small monodisperse electrically conducting antimony doped tin oxide nanoparticles
    Peters, K. and Zeller, P. and Stefanic, G. and Skoromets, V. and Němec, H. and Kužel, P. and Fattakhova-Rohlfing, D.
    Chemistry of Materials 27 1090-1099 (2015)
    We describe the fabrication of crystalline electrically conducting antimony-doped tin oxide (ATO) nanoparticles highly dispersible in polar solvents such as water and ethanol without any stabilizing agents. Nonagglomerated monodisperse ATO nanoparticles with different doping levels are obtained by a facile solvothermal reaction in tert-butanol, leading to the formation of monodisperse nanocrystals with a size of about 3 nm directly after synthesis. Electrical conductivity of ATO nanoparticles strongly increases due to the substitutional doping with antimony, reaching 6.8 × 10-2 S cm-1 for the as-synthesized nanoparticles prepared with 3-5 mol % Sb. This increase stems from transition from hopping in the undoped samples to band-like conduction in the doped samples as revealed by terahertz (THz) spectroscopy measurements describing transport on nanometer distances. The dc conductivity of the doped nanoparticles increases by about 3 orders of magnitude up to 62 S cm-1 after annealing in air at 500 °C. The electrical conductivity, crystallinity, small size, and high dispersibility in polar solvents make the obtained ATO nanoparticles promising building blocks for the direct assembly of more complex conducting architectures using polymer templates that could be damaged in organic solvents. We illustrate the benefits of the water-dispersible ATO nanoparticles by their assembly to periodic macroporous electrodes using poly(methyl methacrylate) (PMMA) beads as the porosity templates. Aqueous dispersion of ATO nanoparticles can be directly combined with PMMA beads that are easily removed by calcination, enabling a facile deposition of 3D-macroporous ATO electrodes featuring optical transparency and a large periodically ordered conducting interface. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/cm504409k
  • 2015 • 98 Coupling of an enzymatic biofuel cell to an electrochemical cell for self-powered glucose sensing with optical readout
    Pinyou, P. and Conzuelo, F. and Sliozberg, K. and Vivekananthan, J. and Contin, A. and Pöller, S. and Plumeré, N. and Schuhmann, W.
    Bioelectrochemistry 106 22-27 (2015)
    A miniaturized biofuel cell (BFC) is powering an electrolyser invoking a glucose concentration dependent formation of a dye which can be determined spectrophotometrically. This strategy enables instrument free analyte detection using the analyte-dependent BFC current for triggering an optical read-out system. A screen-printed electrode (SPE) was used for the immobilization of the enzymes glucose dehydrogenase (GDH) and bilirubin oxidase (BOD) for the biocatalytic oxidation of glucose and reduction of molecular oxygen, respectively. The miniaturized BFC was switched-on using small sample volumes (ca. 60μL) leading to an open-circuit voltage of 567mV and a maximal power density of (6.8±0.6) μWcm-2. The BFC power was proportional to the glucose concentration in a range from 0.1 to 1.0mM (R2=0.991). In order to verify the potential instrument-free analyte detection the BFC was directly connected to an electrochemical cell comprised of an optically-transparent SPE modified with methylene green (MG). The reduction of the electrochromic reporter compound invoked by the voltage and current flow applied by the BFC let to MG discoloration, thus allowing the detection of glucose. © 2015 Elsevier B.V..
    view abstractdoi: 10.1016/j.bioelechem.2015.04.003
  • 2015 • 97 The fate of nano-silver in aqueous media
    Plowman, B.J. and Tschulik, K. and Walport, E. and Young, N.P. and Compton, R.G.
    Nanoscale 7 12361-12364 (2015)
    Silver nanoparticles offer highly attractive properties for many applications, however concern has been raised over the possible toxicity of this material in environmental systems. While it is thought that the release of Ag+ can play a crucial role in this toxicity, the mechanism by which the oxidative dissolution of nano-silver occurs is not yet understood. Here we address this through the electrochemical analysis of gold-core silver-shell nanoparticles in various solutions. This novel method allows the direct quantification of silver dissolution by normalisation to the gold core signal. This is shown to be highly effective at discriminating between silver dissolution and the loss of nanoparticles from the electrode surface. We evidence through this rigorous approach that the reduction of O<inf>2</inf> drives the dissolution of nano-silver, while in the presence of Cl- this dissolution is greatly inhibited. This work is extended to the single nanoparticle level using nano-impact experiments. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5nr02995c
  • 2015 • 96 Using cavity microelectrodes for electrochemical noise studies of oxygen-evolving catalysts
    Rincón, R.A. and Battistel, A. and Ventosa, E. and Chen, X. and Nebel, M. and Schuhmann, W.
    ChemSusChem 8 560-566 (2015)
    Cavity microelectrodes were used as a binder-free platform to evaluate oxygen evolution reaction (OER) electrocatalysts with respect to gas bubble formation and departure. Electrochemical noise measurements were performed by using RuO2 as a benchmark catalyst and the perovskite La0.58Sr0.4Fe0.8Co0.2O3 as a non-noble metal OER catalyst with lower intrinsic conductivity. Changes in the current during the OER originate from variations in electrolyte resistance during the formation of the gas phase and partial coverage of the active area. Fluctuations observed in current and conductance transients were used to establish the contribution from the ohmic overpotential and to determine the characteristic frequency of oxygen evolution. The proposed quantitative determination of gas bubble growth and departure opens up the route for a rational interface design by considering gas bubble growth and departure as a main contributing factor to the overall electrocatalytic activity at high current densities. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201402855
  • 2015 • 95 The gas phase emitter effect of lanthanum within ceramic metal halide lamps and its dependence on the la vapor pressure and operating frequency
    Ruhrmann, C. and Hoebing, T. and Bergner, A. and Groeger, S. and Denissen, C. and Suijker, J. and Awakowicz, P. and Mentel, J.
    Journal of Applied Physics 118 (2015)
    The gas phase emitter effect increases the lamp lifetime by lowering the work function and, with it, the temperature of the tungsten electrodes of metal halide lamps especially for lamps in ceramic vessels due to their high rare earth pressures. It is generated by a monolayer on the electrode surface of electropositive atoms of certain emitter elements, which are inserted into the lamp bulb by metal iodide salts. They are vaporized, dissociated, ionized, and deposited by an emitter ion current onto the electrode surface within the cathodic phase of lamp operation with a switched-dc or ac-current. The gas phase emitter effect of La and the influence of Na on the emitter effect of La are studied by spatially and phase-resolved pyrometric measurements of the electrode tip temperature, La atom, and ion densities by optical emission spectroscopy as well as optical broadband absorption spectroscopy and arc attachment images by short time photography. An addition of Na to the lamp filling increases the La vapor pressure within the lamp considerably, resulting in an improved gas phase emitter effect of La. Furthermore, the La vapor pressure is raised by a heating of the cold spot. In this way, conditions depending on the La vapor pressure and operating frequency are identified, at which the temperature of the electrodes becomes a minimum. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4927734
  • 2015 • 94 Interaction of Fructose Dehydrogenase with a Sulfonated Polyaniline: Application for Enhanced Bioelectrocatalysis
    Sarauli, D. and Wettstein, C. and Peters, K. and Schulz, B. and Fattakhova-Rohlfing, D. and Lisdat, F.
    ACS Catalysis 5 2081-2087 (2015)
    We report on efficient bioelectrocatalysis of the redox enzyme fructose dehydrogenase (FDH) upon its interaction with the sulfonated polyaniline PMSA1 (poly(2-methoxyaniline-5-sulfonic acid)-co-aniline). This interaction has been monitored in solution and on the surface of planar and macroporous indium tin oxide (ITO) electrodes by UV - vis and cyclic voltammetric measurements. Moreover, an enhancement of the catalytic activity for fructose conversion induced by a structural change of sulfonated polyaniline PMSA1 caused by the presence of Ca2+ ions is observed. An entrapment of the Ca2+-bound polymer and enzyme inside the pores of macroporous ITO electrodes leads to a significantly increased (∼35-fold) bioelectrocatalytic signal in comparison to that of a flat ITO and allows the fabrication of highly efficient electrodes with good stability. (Figure Presented) © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acscatal.5b00136
  • 2015 • 93 Electrochemical detection of single E. coli bacteria labeled with silver nanoparticles
    Sepunaru, L. and Tschulik, K. and Batchelor-McAuley, C. and Gavish, R. and Compton, R.G.
    Biomaterials Science 3 816-820 (2015)
    A proof-of-concept for the electrochemical detection of single Escherichia coli bacteria decorated with silver nanoparticles is reported. Impacts of bacteria with an electrode - held at a suitably oxidizing potential - lead to an accompanying burst of current with each collision event. The frequency of impacts scales with the concentration of bacteria and the charge indicates the extent of decoration. © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5bm00114e
  • 2015 • 92 Avalanche-Discharge-Induced Electrical Forming in Tantalum Oxide-Based Metal-Insulator-Metal Structures
    Skaja, K. and Baumer, C. and Peters, O. and Menzel, S. and Moors, M. and Du, H. C. and Bornhofft, M. and Schmitz, C. and Feyer, V. and Jia, C. L. and Schneider, C. M. and Mayer, J. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 25 7154--7162 (2015)
    Oxide-based metal-insulator-metal structures are of special interest for future resistive random-access memories. In such cells, redox processes on the nanoscale occur during resistive switching, which are initiated by the reversible movement of native donors, such as oxygen vacancies. The formation of these filaments is mainly attributed to an enhanced oxygen diffusion due to Joule heating in an electric field or due to electrical breakdown. Here, the development of a dendrite-like structure, which is induced by an avalanche discharge between the top electrode and the Ta2O5- x layer, is presented, which occurs instead of a local breakdown between top and bottom electrode. The dendrite-like structure evolves primarily at structures with a pronounced interface adsorbate layer. Furthermore, local conductive atomic force microscopy reveals that the entire dendrite region becomes conductive. Via spectromicroscopy it is demonstrated that the subsequent switching is caused by a valence change between Ta4+ and Ta5+, which takes place over the entire former Pt/Ta2O5- x interface of the dendrite-like structure.
    view abstractdoi: 10.1002/adfm.201502767
  • 2015 • 91 Non-destructive Patterning of Carbon Electrodes by Using the Direct Mode of Scanning Electrochemical Microscopy
    Stratmann, L. and Clausmeyer, J. and Schuhmann, W.
    ChemPhysChem 16 3477-3482 (2015)
    Patterning of glassy carbon surfaces grafted with a layer of nitrophenyl moieties was achieved by using the direct mode of scanning electrochemical microscopy (SECM) to locally reduce the nitro groups to hydroxylamine and amino functionalities. SECM and atomic force microscopy (AFM) revealed that potentiostatic pulses applied to the working electrode lead to local destruction of the glassy carbon surface, most likely caused by etchants generated at the positioned SECM tip used as the counter electrode. By applying galvanostatic pulses, and thus, limiting the current during structuring, corrosion of the carbon surface was substantially suppressed. After galvanostatic patterning, unambiguous proof of the formation of the anticipated amino moieties was possible by modulation of the pH value during the feedback mode of SECM imaging. This patterning strategy is suitable for the further bio-modification of microstructured surfaces. Alkaline phosphatase, as a model enzyme, was locally bound to the modified areas, thus showing that the technique can be used for the development of protein microarrays. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201500585
  • 2015 • 90 One-Pot Synthesis of Carbon-Coated Nanostructured Iron Oxide on Few-Layer Graphene for Lithium-Ion Batteries
    Sun, Z. and Madej, E. and Wiktor, C. and Sinev, I. and Fischer, R.A. and Van Tendeloo, G. and Muhler, M. and Schuhmann, W. and Ventosa, E.
    Chemistry - A European Journal 21 16154-16161 (2015)
    Nanostructure engineering has been demonstrated to improve the electrochemical performance of iron oxide based electrodes in Li-ion batteries (LIBs). However, the synthesis of advanced functional materials often requires multiple steps. Herein, we present a facile one-pot synthesis of carbon-coated nanostructured iron oxide on few-layer graphene through high-pressure pyrolysis of ferrocene in the presence of pristine graphene. The ferrocene precursor supplies both iron and carbon to form the carbon-coated iron oxide, while the graphene acts as a high-surface-area anchor to achieve small metal oxide nanoparticles. When evaluated as a negative-electrode material for LIBs, our composite showed improved electrochemical performance compared to commercial iron oxide nanopowders, especially at fast charge/discharge rates. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201501935
  • 2015 • 89 Doping Level of Boron-Doped Diamond Electrodes Controls the Grafting Density of Functional Groups for DNA Assays
    Švorc, L. and Jambrec, D. and Vojs, M. and Barwe, S. and Clausmeyer, J. and Michniak, P. and Marton, M. and Schuhmann, W.
    ACS Applied Materials and Interfaces 7 18949-18956 (2015)
    The impact of different doping levels of boron-doped diamond on the surface functionalization was investigated by means of electrochemical reduction of aryldiazonium salts. The grafting efficiency of 4-nitrophenyl groups increased with the boron levels (B/C ratio from 0 to 20 000 ppm). Controlled grafting of nitrophenyldiazonium was used to adjust the amount of immobilized single-stranded DNA strands at the surface and further on the hybridization yield in dependence on the boron doping level. The grafted nitro functions were electrochemically reduced to the amine moieties. Subsequent functionalization with a succinic acid introduced carboxyl groups for subsequent binding of an amino-terminated DNA probe. DNA hybridization significantly depends on the probe density which is in turn dependent on the boron doping level. The proposed approach opens new insights for the design and control of doped diamond surface functionalization for the construction of DNA hybridization assays. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acsami.5b06394
  • 2015 • 88 Charging effect reduction in electron beam lithography and observation of single nanopillars on highly insulating substrates
    Tirpanci, Ş. and Bürgler, D.E. and Schneider, C.M. and Rameev, B. and Aktaş, B.
    Microelectronic Engineering 140 33-37 (2015)
    Electron beam writing and imaging of nanoscale structures on highly insulating substrates severely suffer from charging effects, which cause reduction in pattern resolution, positioning precision, and imaging quality. Conductive layers deposited above or below the resist layer can effectively reduce charge accumulation, but often give rise to contamination impairing the physical and chemical properties of functional nanostructures. Here we deal with top and bottom contacted, sub-micron-sized nanopillars made from multilayer stacks comprising ferromagnetic and non-magnetic materials for the study of current-induced magnetization dynamics. We show how the charging effects in a previously established fabrication process for single-crystalline nanopillars by H. Dassow et al. (2006) [1] can be significantly reduced by using the bottom electrode layer as charge dissipater and only isolating and disconnecting the bottom electrodes from ground after the fabrication of the delicate nanopillar structure by electron beam lithography. The modified process is successfully applied to Co<inf>2</inf>MnSi/Ag/Co<inf>2</inf>MnSi(001) multilayer stacks grown on highly insulating MgO substrates. Ellipsoidal nanopillars with a cross-section of 75 × 120 nm2 reveal 2% giant magnetoresistance and angular dependent magnetization behavior due to the magnetic anisotropy of the elliptical nanomagnets. © 2015 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.mee.2015.05.007
  • 2015 • 87 Non-Invasive Probing of Nanoparticle Electrostatics
    Tschulik, K. and Cheng, W. and Batchelor-Mcauley, C. and Murphy, S. and Omanović, D. and Compton, R.G.
    ChemElectroChem 2 112-118 (2015)
    Electrostatic interactions between surface-charged nanoparticles (NPs) and electrodes studied using existing techniques unavoidably and significantly alter the system being analyzed. Here we present a methodology that allows the probing of unperturbed electrostatic interactions between individual NPs and charged surfaces. The uniqueness of this approach is that stochastic NP impact events are used as the probe. During a single impact, only an attomole of the redox species reacts and is released at the interface during each sensing event. As an example, the effect of electrostatic screening on the reduction of negatively charged indigo NPs at a mercury microelectrode is explored at potentials positive and negative of the potential of zero charge. At suitable overpotentials fully driven electron transfer is seen for all but very low (<0.005M) ionic strengths. The loss of charge transfer in such dilute electrolytes is unambiguously shown to arise from a reduced driving force for the reaction rather than a reduced population of NPs near the electrode, contradicting popular perceptions. Electrostatics were found not to significantly affect the reactivity of the studied NPs. Importantly, the presented technique is general and can be applied to a wide variety of NPs, including metals, metal oxides and organic compounds. Not what you might think: A new and non-invasive technique to probe the electrostatic interaction between surface-charged nanoparticles and a charged metal/solution interface shows that electrostatic effects are insignificant in all but very dilute electrolytes. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201402285
  • 2015 • 86 Influence of the alkali metal cations on the activity of Pt(1 1 1) towards model electrocatalytic reactions in acidic sulfuric media
    Tymoczko, J. and Colic, V. and Ganassin, A. and Schuhmann, W. and Bandarenka, A.S.
    Catalysis Today 244 96-102 (2015)
    The impact of the alkali metal cations (Li+, Na+, K+, Rb+, Cs+) on the catalytic activity of Pt(1 1 1) electrodes towards model reactions (oxygen reduction, oxygen evolution, hydrogen evolution and hydrogen oxidation) in sulfuric acid has been evaluated. In contrast to essentially monotonic activity trends (i.e. from Li+ to Cs+) reported in the literature for alkaline media, the nature of the cations influences the activity of the Pt electrodes largely non-monotonously in the presence of SO4 2- ions. This is in certain cases due to the specifically adsorbing (bi)sulfate anions which make interactions between electrolyte components and reaction intermediates very complex. Surprisingly, the activity of the Pt(1 1 1) electrodes towards all investigated electrocatalytic reactions was substantially higher in Rb+ ions containing electrolytes. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.cattod.2014.07.007
  • 2015 • 85 Nitrogen-doped carbon cloth as a stable self-supported cathode catalyst for air/H2-breathing alkaline fuel cells
    Vivekananthan, J. and Masa, J. and Chen, P. and Xie, K. and Muhler, M. and Schuhmann, W.
    Electrochimica Acta 182 312-319 (2015)
    The power output of a fuel cell is limited by among others, the intrinsic activity of the active matrix and the mass transport of the products and reactants. Of equally crucial importance is the long-term durability of the cell components including the electrocatalysts. Herein, carbon cloth (CC) was functionalized with nitrogen-containing groups by treatment with NH<inf>3</inf> at 400 °C or by pyrolysis of a composite of polypyrrole on CC at 800 °C. The resulting N-doped CC (NCC) was employed as an air-breathing cathode in a custom-made air/H<inf>2</inf> alkaline fuel cell, serving as the current collector as well as catalytic matrix with enhanced oxygen transport. The cell exhibited high operational durability with only 2% loss in activity after 25 days and delivered a maximum power density of 120 mW m-2 at a voltage of 0.35 V. The concept of a self-supported highly stable metal-free catalyst and the breathing H<inf>2</inf>/air cell design provide platforms for the design and investigation of catalysts. Moreover, a higher cell voltage can be realized if the cell is operated under pressurized conditions or by replacing air with O<inf>2.</inf> © 2015 Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2015.09.064
  • 2015 • 84 The influence of different pre-treatments of current collectors and variation of the binders on the performance of Li4Ti5O12 anodes for lithium ion batteries
    Wennig, S. and Langklotz, U. and Prinz, G.M. and Schmidt, A. and Oberschachtsiek, B. and Lorke, A. and Heinzel, A.
    Journal of Applied Electrochemistry 45 1043-1055 (2015)
    In order to optimize the electron transfer between the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf>-based active mass and the current collector, the surface of aluminum foil was modified either by alkaline etching or by a carbon coating. The as-modified aluminum foils were coated with an active mass of Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> mixed with polyvinylidene fluoride, sodium carboxymethyl cellulose, or polyacrylic acid as binders. Untreated aluminum and copper foils served as reference current collectors. The corrosion reactions of aluminum foil with the applied binder solutions were studied and the electrode structure has been analyzed, depending on the binder. Finally, the electrochemical performance of the prepared electrodes was investigated. Based on these measurements, conclusions concerning the electrical contact between the different current collectors and the active masses were drawn. The energy density of the Li<inf>4</inf>Ti<inf>5</inf>O<inf>12</inf> electrodes cast on carbon-coated aluminum foils was significantly increased, compared to the corresponding electrodes with a copper current collector. © 2015, Springer Science+Business Media Dordrecht.
    view abstractdoi: 10.1007/s10800-015-0878-0
  • 2015 • 83 Electrolyte effects in a model of proton discharge on charged electrodes
    Wiebe, J. and Kravchenko, K. and Spohr, E.
    Surface Science 631 35-41 (2015)
    We report results on the influence of NaCl electrolyte dissolved in water on proton discharge reactions from aqueous solution to charged platinum electrodes. We have extended a recently developed combined proton transfer/proton discharge model on the basis of empirical valence bond theory to include NaCl solutions with several different concentrations of cations and anions, both stoichiometric (1:1) compositions and non-stoichiometric ones with an excess of cations. The latter solutions partially screen the electrostatic potential from the surface charge of the negatively charged electrode. 500-1000 trajectories of a discharging proton were integrated by molecular dynamics simulations until discharge occurred, or for at most 1.5 ns. The results show a strong dependence on ionic strength, but only a weak dependence on the screening behavior, when comparing stoichiometric and non-stoichiometric solutions. Overall, the Na+ cations exert a more dominant effect on the discharge reaction, which we argue is likely due to the very rigid arrangements of the cations on the negatively polarized electrode surface. Thus, our model predicts, for the given and very high negative surface charge densities, the fastest discharge reaction for pure water, but obviously cannot take into account the fact that such high charge densities are even more out of reach experimentally than for higher electrolyte concentrations. © 2014 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.susc.2014.06.016
  • 2015 • 82 Implementation of graphene multilayer electrodes in quantum dot light-emitting devices
    Wolff, S. and Jansen, D. and Terlinden, H. and Kelestemur, Y. and Mertin, W. and Demir, H.V. and Bacher, G. and Nannen, E.
    Applied Physics A: Materials Science and Processing 120 1197-1203 (2015)
    Graphene is a highly attractive candidate for implementation as electrodes in next-generation large-area optoelectronic devices thanks to its high electrical conductivity and high optical transparency. In this study, we show all-solution-processed quantum dot-based light-emitting devices (QD-LEDs) using graphene mono- and multilayers as transparent electrodes. Here, the effect of the number of graphene layers (up to three) on the QD-LEDs performance was studied. While the implementation of a second graphene layer was found to reduce the turn-on voltage from 2.6 to 1.8 V, a third graphene layer was observed to increase the turn-on voltage again, which is attributed to an increased roughness of the graphene layer stack. © 2015, Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s00339-015-9304-z
  • 2015 • 81 Si-CNT/rGO Nanoheterostructures as High-Performance Lithium-Ion-Battery Anodes
    Xiao, L. and Sehlleier, Y.H. and Dobrowolny, S. and Orthner, H. and Mahlendorf, F. and Heinzel, A. and Schulz, C. and Wiggers, H.
    ChemElectroChem 2 1983-1990 (2015)
    A robust and electrochemically stable 3D nanoheterostructure consisting of Si nanoparticles (NPs), carbon nanotubes (CNTs) and reduced graphene oxide (rGO) is developed as an anode material (Si-CNT/rGO) for lithium-ion batteries (LIBs). It integrates the benefits from its three building blocks of Si NPs, CNTs, and rGO; Si NPs offer high capacity, CNTs act as a mechanical, electrically conductive support to connect Si NPs, and highly electrically conductive and flexible rGO provides a robust matrix with enough void space to accommodate the volume changes of Si NPs upon lithiation/delithiation and to simultaneously assure good electric contact. The composite material shows a high reversible capacity of 1665mAhg-1 with good capacity retention of 88.6% over 500 cycles when cycled at 0.5C, that is, a 0.02% capacity decay per cycle. The high-power capability is demonstrated at 10C (16.2Ag-1) where 755mAhg-1 are delivered, thus indicating promising characteristics of this material for high-performance LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500323
  • 2015 • 80 Atomic layer-by-layer construction of Pd on nanoporous gold via underpotential deposition and displacement reaction
    Yan, X. and Xiong, H. and Bai, Q. and Frenzel, J. and Si, C. and Chen, X. and Eggeler, G. and Zhang, Z.
    RSC Advances 5 19409-19417 (2015)
    Atomic layer-by-layer construction of Pd on nanoporous gold (NPG) has been investigated through the combination of underpotential deposition (UPD) with displacement reaction. It has been found that the UPD of Cu on NPG is sensitive to the applied potential and the deposition time. The optimum deposition potential and time were determined through potential- and time-sensitive stripping experiments. The NPG-Pd electrode shows a different voltammetric behavior in comparison to the bare NPG electrode, and the deposition potential was determined through the integrated charge control for the monolayer UPD of Cu on the NPG-Pd electrode. Five layers of Pd were constructed on NPG through the layer-by-layer deposition. In addition, the microstructure of the NPG-Pdx (x = 1, 2, 3, 4 and 5) films was probed by scanning electron microscopy (SEM), transmission electron microscopy (TEM) and scanning transmission electron microscopy (STEM). The microstructural observation demonstrates that the atomic layers of Pd form on the ligament surface of NPG through epitaxial growth, and have no effect on the nanoporous structure of NPG. In addition, the hydrogen storage properties of the NPG-Pdx electrodes have also been addressed. This journal is © The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c4ra17014h
  • 2015 • 79 [001] Preferentially-oriented 2D tungsten disulfide nanosheets as anode materials for superior lithium storage
    Yang, W. and Wang, J. and Si, C. and Peng, Z. and Frenzel, J. and Eggeler, G. and Zhang, Z.
    Journal of Materials Chemistry A 3 17811-17819 (2015)
    Rechargeable lithium ion batteries (LIBs) have transformed portable electronics and will play a crucial role in transportation, such as electric vehicles. For higher energy storage in LIBs, two issues should be addressed, that is, the fundamental understanding of the chemistry taking place in LIBs and the discovery of new materials. Here we design and fabricate two-dimensional (2D) WS<inf>2</inf> nanosheets with preferential [001] orientation and perfect single crystalline structures. Being used as an anode for LIBs, the WS<inf>2</inf>-nanosheet electrode exhibits a high specific capacity, good cycling performance and excellent rate capability. Considering the controversy in the lithium storage mechanism of WS<inf>2</inf>, ex-situ X-ray diffraction (XRD), Raman and X-ray photoelectron spectroscopy (XPS) analyses clearly verify that the recharge product (3.0 V vs. Li+/Li) of the WS<inf>2</inf> electrode after fully discharging to 0.01 V (vs. Li+/Li) tends to reverse to WS<inf>2</inf>. More remarkably, the [001] preferentially-oriented 2D WS<inf>2</inf> nanosheets are also promising candidates for applications in photocatalysis, water splitting, and so forth. © The Royal Society of Chemistry 2015.
    view abstractdoi: 10.1039/c5ta04176g
  • 2015 • 78 Enhanced absorption in tandem solar cells by applying hydrogenated In2O3 as electrode
    Yin, G. and Steigert, A. and Manley, P. and Klenk, R. and Schmid, M.
    Applied Physics Letters 107 (2015)
    To realize the high efficiency potential of perovskite/chalcopyrite tandem solar cells in modules, hydrogenated In2O3 (IO:H) as electrode is investigated. IO:H with an electron mobility of 100 cm2 V-1 s-1 is demonstrated. Compared to the conventional Sn doped In2O3 (ITO), IO:H exhibits a decreased electron concentration and leads to almost no sub-bandgap absorption up to the wavelength of 1200 nm. Without a trade-off between transparency and lateral resistance in the IO:H electrode, the tandem cell keeps increasing in efficiency as the IO:H thickness increases and efficiencies above 22% are calculated. In contrast, the cells with ITO as electrode perform much worse due to the severe parasitic absorption in ITO. This indicates that IO:H has the potential to lead to high efficiencies, which is otherwise constrained by the parasitic absorption in conventional transparent conductive oxide electrode for tandem solar cells in modules. © 2015 AIP Publishing LLC.
    view abstractdoi: 10.1063/1.4936328
  • 2015 • 77 Combined AFM/SECM Investigation of the Solid Electrolyte Interphase in Li-Ion Batteries
    Zampardi, G. and Klink, S. and Kuznetsov, V. and Erichsen, T. and Maljusch, A. and LaMantia, F. and Schuhmann, W. and Ventosa, E.
    ChemElectroChem 2 1607-1611 (2015)
    The solid electrolyte interphase (SEI) is an electronically insulating film formed from the decomposition of the organic electrolyte at the surface of the negative electrodes in Li-ion batteries (LIBs). This film is of vital importance in the performance and safety of LIBs. Atomic force microscopy (AFM) and scanning electrochemical microscopy (SECM) are combined in one platform for the consecutive insitu investigation of surface reactions in LIBs inside an Ar-filled glovebox. As proof of concept, the formation and the electrochemical properties of the SEI formed on glassy carbon electrodes are investigated. Changes in topography during film formation of the SEI are studied via AFM. The AFM tip is then used to partially remove a small area (50×50μm2) of the SEI, which is subsequently probed using SECM in feedback mode. The AFM-scratched spot is clearly visualized in the SECM image, demonstrating the strength of the AFM/SECM combination for the investigation in the field of LIBs. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201500085
  • 2015 • 76 Determination of the formation and range of stability of the SEI on glassy carbon by local electrochemistry
    Zampardi, G. and La Mantia, F. and Schuhmann, W.
    RSC Advances 5 31166-31171 (2015)
    The solid electrolyte interphase (SEI) is an electronic insulating and ionic conducting layer that is of main importance in lithium-ions batteries, since it critically affects the final performance of the battery system. The formation of this electronic insulating layer was determined in operando on a glassy carbon electrode by means of a microelectrode positioned in close proximity to its surface using scanning electrochemical microscopy (SECM). Glassy carbon was chosen as an ideal model system for carbonaceous materials, since it forms a SEI similar in composition to the one on graphite but concomitantly shows negligible intercalation of lithium ions. Moreover, the stability of the SEI was analysed depending on different potential ranges and the role of the cations on the insulating character of the SEI was investigated. © 2015 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c5ra02940f
  • 2015 • 75 Light Induced H2 Evolution from a Biophotocathode Based on Photosystem 1 - Pt Nanoparticles Complexes Integrated in Solvated Redox Polymers Films
    Zhao, F. and Conzuelo, F. and Hartmann, V. and Li, H. and Nowaczyk, M.M. and Plumeré, N. and Rögner, M. and Schuhmann, W.
    Journal of Physical Chemistry B 119 13726-13731 (2015)
    We report on a biophotocathode based on photosystem 1 (PS1)-Pt nanoparticle complexes integrated in a redox hydrogel for photoelectrocatalytic H2 evolution at low overpotential. A poly(vinyl)imidazole Os(bispyridine)2Cl polymer serves as conducting matrix to shuttle the electrons from the electrode to the PS1-Pt complexes embedded within the hydrogel. Light induced charge separation at the PS1-Pt complexes results in the generation of photocurrents (4.8 ± 0.4 μA cm-2) when the biophotocathodes are exposed to anaerobic buffer solutions. Under these conditions, the protons are the sole possible electron acceptors, suggesting that the photocurrent generation is associated with H2 evolution. Direct evidence for the latter process is provided by monitoring the H2 production with a Pt microelectrode in scanning electrochemical microscopy configuration over the redox hydrogel film containing the PS1-Pt complexes under illumination. © 2015 American Chemical Society.
    view abstractdoi: 10.1021/acs.jpcb.5b03511
  • 2015 • 74 Electrochemical detection of synthetic DNA and native 16S rRNA fragments on a microarray using a biotinylated intercalator as coupling site for an enzyme label
    Zimdars, A. and Gebala, M. and Hartwich, G. and Neugebauer, S. and Schuhmann, W.
    Talanta 143 19-26 (2015)
    Abstract The direct electrochemical detection of synthetic DNA and native 16S rRNA fragments isolated from Escherichia coli is described. Oligonucleotides are detected via selective post-labeling of double stranded DNA and DNA-RNA duplexes with a biotinylated intercalator that enables high-specific binding of a streptavidin/alkaline phosphatase conjugate. The alkaline phosphatase catalyzes formation of p-aminophenol that is subsequently oxidized at the underlying gold electrode and hence enables the detection of complementary hybridization of the DNA capture strands due to the enzymatic signal amplification. The hybridization assay was performed on microarrays consisting of 32 individually addressable gold microelectrodes. Synthetic DNA strands with sequences representing six different pathogens which are important for the diagnosis of urinary tract infections could be detected at concentrations of 60 nM. Native 16S rRNA isolated from the different pathogens could be detected at a concentration of 30 fM. Optimization of the sensing surface is described and influences on the assay performance are discussed. © 2015 Elsevier B.V.
    view abstractdoi: 10.1016/j.talanta.2015.04.041
  • 2014 • 73 Electrochemical nanoprobes for single-cell analysis
    Actis, P. and Tokar, S. and Clausmeyer, J. and Babakinejad, B. and Mikhaleva, S. and Cornut, R. and Takahashi, Y. and López Córdoba, A. and Novak, P. and Shevchuck, A.I. and Dougan, J.A. and Kazarian, S.G. and Gorelkin, P.V. and...
    ACS Nano 8 875-884 (2014)
    The measurement of key molecules in individual cells with minimal disruption to the biological milieu is the next frontier in single-cell analyses. Nanoscale devices are ideal analytical tools because of their small size and their potential for high spatial and temporal resolution recordings. Here, we report the fabrication of disk-shaped carbon nanoelectrodes whose radius can be precisely tuned within the range 5-200 nm. The functionalization of the nanoelectrode with platinum allowed the monitoring of oxygen consumption outside and inside a brain slice. Furthermore, we show that nanoelectrodes of this type can be used to impale individual cells to perform electrochemical measurements within the cell with minimal disruption to cell function. These nanoelectrodes can be fabricated combined with scanning ion conductance microscopy probes, which should allow high resolution electrochemical mapping of species on or in living cells. © 2013 American Chemical Society.
    view abstractdoi: 10.1021/nn405612q
  • 2014 • 72 CrN/AlN nanolaminate coatings deposited via high power pulsed and middle frequency pulsed magnetron sputtering
    Bagcivan, N. and Bobzin, K. and Ludwig, Al. and Grochla, D. and Brugnara, R.H.
    Thin Solid Films 572 153-160 (2014)
    Nanolaminate coatings based on transition metal nitrides such as CrN, AlN and TiN deposited via physical vapor deposition (PVD) have shown great advantage as protective coatings on tools and components subject to high loads in tribological applications. By varying the individual layer materials and their thicknesses it is possible to optimize the coating properties, e.g. hardness, Young's modulus and thermal stability. One way for further improvement of coating properties is the use of advanced PVD technologies. High power pulsed magnetron sputtering (HPPMS) is an advancement of pulsed magnetron sputtering (MS). The use of HPPMS allows a better control of the energetic bombardment of the substrate due to the higher ionization degree of metallic species. It provides an opportunity to influence chemical and mechanical properties by varying the process parameters. The present work deals with the development of CrN/AlN nanolaminate coatings in an industrial scale unit by using two different PVD technologies. Therefore, HPPMS and mfMS (middle frequency magnetron sputtering) technologies were used. The bilayer period Λ, i.e. the thickness of a CrN/AlN double layer, was varied between 6.2nm and 47.8 nm by varying the rotational speed of the substrate holders. In a second step the highest rotational speed was chosen and further HPPMS CrN/AlN coatings were deposited applying different HPPMS pulse lengths (40, 80, 200 μs) at the same mean cathode power and frequency. Thickness, morphology, roughness and phase composition of the coatings were analyzed by means of scanning electron microscopy (SEM), confocal laser microscopy, and X-ray diffraction (XRD), respectively. The chemical composition was determined using glow discharge optical emission spectroscopy (GDOES). Detailed characterization of the nanolaminate was conducted by transmission electron microscopy (TEM). The hardness and the Young's modulus were analyzed by nanoindentation measurements. The residual stress was determined via Si microcantilever curvature measurements. The phase analysis revealed the formation of h-Cr2N, c-CrN and c-AlN mixed phases for the mfMS CrN/AlN coatings, whereas the HPPMS coatings exhibited only cubic phases (c-CrN, c-AlN). A hardness of 31.0 GPa was measured for the HPPMS coating with a bilayer period of 6.2 nm. The decrease of the HPPMS pulse length at constant mean power leads to a considerable increase of the cathode current on the Cr and Al target associated with an increased ion flux towards the substrate. Furthermore, it was observed that the deposition rate of HPPMS CrN/AlN decreases with shorter pulse lengths, so that a CrN/AlN coating with a bilayer period of 2.9 nm, a high hardness of 40.8 GPa and a high compressive stress (- 4.37 GPa) was achieved using a short pulse length of 40 μs. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.tsf.2014.06.058
  • 2014 • 71 Immunologically Controlled Biofuel Cell as a Self-Powered Biosensor for Antibiotic Residue Determination
    Conzuelo, F. and Vivekananthan, J. and Pöller, S. and Pingarrón, J.M. and Schuhmann, W.
    ChemElectroChem 1 1854-1858 (2014)
    A biofuel cell consisting of a self-powered sulfonamide immunosensor as biocathode and a cellobiose dehydrogenase (CDH)-based bioanode was developed for the determination of sulfonamide antibiotics in milk. A graphite-rod electrode was modified with proteinG for the immobilization of selective capture antibodies. A direct competitive immunoassay with a horseradish-peroxidase-labeled analog of the antibiotic and the 2,2′-azino-bis(3-ethyl benzothiazoline-6-sulfonic acid) diammonium salt-mediated reduction of H2O2 allows quantification of antibiotic residues. CDH was co-immobilized with a toluidine-blue-modified redox polymer on a graphite electrode for the biocatalytic oxidation of lactose in milk. An open-circuit voltage of 676mV and a maximal power density of 6.9μWcm-2 were obtained. The power densities measured at 550mV (vs. the anode) as a function of antibiotic concentration in milk samples allowed the construction of a calibration curve with a detection limit for sulfapyridine as low as 2.4ngmL-1. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201402098
  • 2014 • 70 Multi-layer thin-film electrolytes for metal supported solid oxide fuel cells
    Haydn, M. and Ortner, K. and Franco, T. and Uhlenbruck, S. and Menzler, N.H. and Stöver, D. and Bräuer, G. and Venskutonis, A. and Sigl, L.S. and Buchkremer, H.-P. and Vaßen, R.
    Journal of Power Sources 256 52-60 (2014)
    A key to the development of metal-supported solid oxide fuel cells (MSCs) is the manufacturing of gas-tight thin-film electrolytes, which separate the cathode from the anode. This paper focuses the electrolyte manufacturing on the basis of 8YSZ (8 mol.-% Y2O3 stabilized ZrO2). The electrolyte layers are applied by a physical vapor deposition (PVD) gas flow sputtering (GFS) process. The gas-tightness of the electrolyte is significantly improved when sequential oxidic and metallic thin-film multi-layers are deposited, which interrupt the columnar grain structure of single-layer electrolytes. Such electrolytes with two or eight oxide/metal layers and a total thickness of about 4 μm obtain leakage rates of less than 3 × 10 -4 hPa dm3 s-1 cm-2 (Δp: 100 hPa) at room temperature and therefore fulfill the gas tightness requirements. They are also highly tolerant with respect to surface flaws and particulate impurities which can be present on the graded anode underground. MSC cell tests with double-layer and multilayer electrolytes feature high power densities more than 1.4 W cm-2 at 850 C and underline the high potential of MSC cells. © 2014 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.jpowsour.2014.01.043
  • 2014 • 69 Wireless tear glucose sensor
    Hennig, A. and Lauko, J. and Grabmaier, A. and Wilson, C.
    Procedia Engineering 87 66-69 (2014)
    This paper presents a novel wireless tear glucose level sensor for diabetes patients. The miniaturized sensor can be worn noninvasively under the eye lid. It is composed of a chronoamperometric glucose sensor and an ASIC set with integrated potentiostat and transponder circuits. Wireless energy and data transmission according to the passive transponder standard ISO18000-3 is used to power and readout the sensor. A special coil shape enables high comfort for the patient. High integration level is achieved by a combination of antenna and sensor electrode wires. A complete demonstrator system including ASIC and sensor fabrication as well as assembly was manufactured and the function is demonstrated. © 2014 The Authors. Published by Elsevier Ltd.
    view abstractdoi: 10.1016/j.proeng.2014.11.267
  • 2014 • 68 Assessment of a cylindrical and a rectangular plate differential mobility analyzer for size fractionation of nanoparticles at high-aerosol flow rates
    Hontañón, E. and Rouenhoff, M. and Azabal, A. and Ramiro, E. and Kruis, F.E.
    Aerosol Science and Technology 48 333-339 (2014)
    An existing differential mobility analyzer (DMA) of cylindrical electrodes and a novel DMA of rectangular plate electrodes are demonstrated for size fractionation of nanoparticles at high-aerosol flow rates in this work. The two DMAs are capable of delivering monodisperse size selected nanoparticles (SMPS σg < 1.1) at gas flow rates ranging from 200 slm to 500 slm. At an aerosol flow rate of 200 slm, the maximum attainable particle mean size is of about 20 nm for the cylindrical DMA and of nearly 50 nm for the rectangular plate DMA. The number concentration of the monodisperse nanoparticles delivered by the high-flow DMAs spans from 104 cm-3 to 10 6 cm-3 depending upon the particle mean size and particle size dispersion. Copyright © 2014 American Association for Aerosol Research.
    view abstractdoi: 10.1080/02786826.2013.875116
  • 2014 • 67 Nanoparticle-Impact Experiments are Highly Sensitive to the Presence of Adsorbed Species on Electrode Surfaces
    Kätelhön, E. and Cheng, W. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
    ChemElectroChem 1 1057-1062 (2014)
    We theoretically and experimentally investigate the influence of partial surface blocking on the electrochemistry of nanoparticles impacting at an electrode. To this end, we introduce an analytical model for the adsorption of single blocking molecules on the electrode and calculate the resulting fractional electrode coverage. We find that even small amounts of adsorbed molecules can fully suppress detection of impacts of nanoparticles while the electrode characteristics in the detection of electroactive molecules hardly change. Our findings are supported by experimental data on the indigo nanoparticle electroreduction at a carbon microelectrode (radius 5.5μm) in aqueous solution. We find that nanoimpacts are fully suppressed in the presence of acetone at concentrations of 250nm, which have a negligible effect on the electrode kinetics of the Fe(CN)3-/4- 6 couple. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201402014
  • 2014 • 66 New insights into SEI formation in lithium ion batteries: Inhomogeneous distribution of irreversible charge losses across graphite electrodes
    Klink, S. and Weide, P. and Ventosa, E. and Muhler, M. and Schuhmann, W. and La Mantia, F.
    ECS Transactions 62 265-271 (2014)
    A vertical split electrode (VSE) with three layers was developed to investigate the formation of the solid electrolyte interphase (SEI) during first charge of graphite electrodes in lithium ion batteries. Ex-situ X-ray photoelectron spectroscopy (XPS) on each layer revealed that the first layer showed distinctively different signal patterns in the O 1s and C 1s regions. It was concluded that the SEI formed in the first layer closest to the counter electrode is thicker as well as different in chemical nature as compared to the SEI in the electrode bulk. © The Electrochemical Society.
    view abstractdoi: 10.1149/06201.0265ecst
  • 2014 • 65 Vertical distribution of overpotentials and irreversible charge losses in lithium ion battery electrodes
    Klink, S. and Schuhmann, W. and La Mantia, F.
    ChemSusChem 7 2159-2166 (2014)
    Porous lithium ion battery electrodes are characterized using a vertical distribution of cross-currents. In an appropriate simplification, this distribution can be described by a transmission line model (TLM) consisting of infinitely thin electrode layers. To investigate the vertical distribution of currents, overpotentials, and irreversible charge losses in a porous graphite electrode in situ, a multi-layered working electrode (MWE) was developed as the experimental analogue of a TLM. In this MWE, each layer is in ionic contact but electrically insulated from the other layers by a porous separator. It was found that the negative graphite electrodes get lithiated and delithiated stage-by-stage and layer-by-layer. Several mass-transport- as well as non-mass-transport-limited processes could be identified. Local current densities can reach double the average, especially on the outermost layer at the beginning of each intercalation stage. Furthermore, graphite particles close to the counter electrode act as "electrochemical sieve" reducing the impurities present in the electrolyte such as water. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201400056
  • 2014 • 64 Insights into nanoscale electrochemical reduction in a memristive oxide: The role of three-phase boundaries
    Lenser, C. and Patt, M. and Menzel, S. and Köhl, A. and Wiemann, C. and Schneider, C.M. and Waser, R. and Dittmann, R.
    Advanced Functional Materials 24 4466-4472 (2014)
    The nanoscale electro-reduction in a memristive oxide is a highly relevant field for future non-volatile memory materials. Photoemission electron microscopy is used to identify the conducting filaments and correlate them to structural features of the top electrode that indicate a critical role of the three phase boundary (electrode-oxide-ambient) for the electro-chemical reduction. Based on simulated temperature profiles, the essential role of Joule heating through localized currents for electro-reduction and morphology changes is demonstrated. The three-phase boundary between electrode, oxide and ambient is shown to play a crucial role for the electroreduction in resistively switchable devices fabricated from Fe-doped SrTiO3. Nanoscale chemical mapping by X-ray photoemission electron microscopy, combined with simulated temperature profiles of the filaments, provide evidence that localized Joule heating at the electrode edge is essential for the formation of conducting filaments. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/adfm.201304233
  • 2014 • 63 A critical evaluation of the interpretation of electrocatalytic nanoimpacts
    Ly, L.S.Y. and Batchelor-Mcauley, C. and Tschulik, K. and Kätelhön, E. and Compton, R.G.
    Journal of Physical Chemistry C 118 17756-17763 (2014)
    The kinetics of the proton reduction reaction is studied on a variety of gold surfaces including both macro (r0 = 1.0 mm) and micro (r 0 = 4.6 μm) electrodes, as well as gold nanoparticles (r NP = ∼10 nm). For the gold nanoparticles, two complementary methodologies of study are used. First the particles are investigated as part of an ensemble response in an array (k0 ∼ 7 × 10-8 m s-1). Second, the rate is recorded stochastically at individually impacting nanoparticles (k0 ∼2 × 10-9 m s -1). This apparent decrease in reaction rates on transitioning from the ensemble to individual nanoparticles is understood in terms of the differing connectivity of the nanoparticles to the electrode surface. During the course of the individual catalytic impacts, or "pulses", the recorded current is found to be highly variable; this variability is interpreted as originating from the nanoscopic motion of the particle above the electrode interface. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/jp504968j
  • 2014 • 62 MnxOy/NC and CoxOy/NC nanoparticles embedded in a nitrogen-doped carbon matrix for high-performance bifunctional oxygen electrodes
    Masa, J. and Xia, W. and Sinev, I. and Zhao, A. and Sun, Z. and Grützke, S. and Weide, P. and Muhler, M. and Schuhmann, W.
    Angewandte Chemie - International Edition 53 8508-8512 (2014)
    Reversible interconversion of water into H2 and O2, and the recombination of H2 and O2 to H2O thereby harnessing the energy of the reaction provides a completely green cycle for sustainable energy conversion and storage. The realization of this goal is however hampered by the lack of efficient catalysts for water splitting and oxygen reduction. We report exceptionally active bifunctional catalysts for oxygen electrodes comprising Mn3O4 and Co 3O4 nanoparticles embedded in nitrogen-doped carbon, obtained by selective pyrolysis and subsequent mild calcination of manganese and cobalt N4 macrocyclic complexes. Intimate interaction was observed between the metals and nitrogen suggesting residual M-Nx coordination in the catalysts. The catalysts afford remarkably lower reversible overpotentials in KOH (0.1M) than those for RuO2, IrO2, Pt, NiO, Mn3O4, and Co3O4, thus placing them among the best non-precious-metal catalysts for reversible oxygen electrodes reported to date. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/anie.201402710
  • 2014 • 61 Molecular-Scale Imaging of Water Near Charged Surfaces
    Mehlhorn, M. and Schnur, S. and Groß, A. and Morgenstern, K.
    ChemElectroChem 1 431-435 (2014)
    The orientation of water molecules on water bilayers is investigated on Cu(111) by a combination of scanning tunneling microscopy and density functional theory. Theory predicts that the application of a field reorients the adsorbed water molecules at a distance of close to a nanometer from the surface. Experimental evidence is presented for this prediction. Furthermore, the process differs strongly between adsorption on two and on three ordered layers. We propose that these results give insight into the behavior of the diffusive layer close to electrodes. So simple? Since the basic idea of ultrahigh-vacuum (UHV) electrochemical modeling emerged, it has been claimed that UHV model experiments are too simple because they do not include the electrode potential. This combined scanning tunneling microscopy and density functional theory study gives insight into the influence of the electric field on single molecules in the diffusive layer. A field reorients adsorbed water molecules on water bilayers on Cu(111) at a distance of about 1nm from the surface. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300063
  • 2014 • 60 Covalent immobilization of redox protein within the mesopores of transparent conducting electrodes
    Müller, V. and Rathousky, J. and Fattakhova-Rohlfing, D.
    Electrochimica Acta 116 1-8 (2014)
    Redox protein cytochrome c was immobilized at high electrochemically accessible loading on mesoporous films of antimony doped tin oxide (ATO) exhibiting high conductivity, transparency and a large surface area. The grafting was achieved by covalent attachment of the protein to the electrode surface. Alternatively, cytochrome c was effectively adsorbed on the ATO electrode surface due to strong electrostatic interaction between the positively charged cytochrome c and the negatively charged ATO surface. The amount of electrochemically addressable cytochrome c is proportional to the specific surface area, reaching up to 440 pmol/cm2 and 600 pmol/cm2 for covalently attached and adsorbed protein, respectively, for the 370 nm thick films. The covalently attached protein exhibits substantially higher stability towards leaching than the adsorbed one. The combination of transparent conducting porous electrode matrix with the electroactive proteins is promising for the development of efficient bio-optoelectronic devices and for in situ spectroelectrochemical studies of biomolecules. © 2013 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2013.10.136
  • 2014 • 59 3D-electrode architectures for enhanced direct bioelectrocatalysis of pyrroloquinoline quinone-dependent glucose dehydrogenase
    Sarauli, D. and Peters, K. and Xu, C. and Schulz, B. and Fattakhova-Rohlfing, D. and Lisdat, F.
    ACS Applied Materials and Interfaces 6 17887-17893 (2014)
    We report on the fabrication of a complex electrode architecture for efficient direct bioelectrocatalysis. In the developed procedure, the redox enzyme pyrroloquinoline quinone-dependent glucose dehydrogenase entrapped in a sulfonated polyaniline [poly(2-methoxyaniline-5-sulfonic acid)-co-aniline] was immobilized on macroporous indium tin oxide (macroITO) electrodes. The use of the 3D-conducting scaffold with a large surface area in combination with the conductive polymer enables immobilization of large amounts of enzyme and its efficient communication with the electrode, leading to enhanced direct bioelectrocatalysis. In the presence of glucose, the fabricated bioelectrodes show an exceptionally high direct bioelectrocatalytical response without any additional mediator. The catalytic current is increased more than 200-fold compared to planar ITO electrodes. Together with a high long-term stability (the current response is maintained for >90% of the initial value even after 2 weeks of storage), the transparent 3D macroITO structure with a conductive polymer represents a valuable basis for the construction of highly efficient bioelectronic units, which are useful as indicators for processes liberating glucose and allowing optical and electrochemical transduction. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/am5046026
  • 2014 • 58 Control of molecular orientation and morphology in organic bilayer solar cells: Copper phthalocyanine on gold nanodots
    Sasaki, T. and Tabata, K. and Tsukagoshi, K. and Beckel, A. and Lorke, A. and Yamamoto, Y.
    Thin Solid Films 562 467-470 (2014)
    Molecular orientation, morphology of donor (D)/acceptor (A) interface and photoabsorptivity in organic bilayer solar cells were controlled using Au nanodots with an ∼ 20 nm diameter inserted between the bottom electrode and the organic layer. Copper phthalocyanine (CuPc) molecules deposited onto the Au nanodot-coated electrode were mostly oriented face-on with large surface roughness, which is beneficial for photoabsorption, charge separation and transport. Furthermore, Au nanodots exhibit blue-shifted plasmon bands so that CuPc absorbs light more efficiently than that on thin Au layer. Bilayer C 60/CuPc solar cells containing Au nanodots exhibited 1.4 times higher photoelectric conversion efficiency than those without Au nanodots. Factors for the enhanced efficiency are (i) improvement of the optical absorption characteristics by face-on orientation of CuPc and (ii) increase of the D/A heterointerface area. In addition, the shift of the plasmon absorption band of Au by the formation of nanodots makes absorption of the CuPc layer much more efficiently, resulting in better photovoltaic output. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.tsf.2014.03.090
  • 2014 • 57 D-lactate-selective amperometric biosensor based on the cell debris of the recombinant yeast Hansenula polymorpha
    Smutok, O.V. and Dmytruk, K.V. and Karkovska, M.I. and Schuhmann, W. and Gonchar, M.V. and Sibirny, A.A.
    Talanta 125 227-232 (2014)
    A d-lactate-selective biosensor has been developed using cellsdebris of recombinant thermotolerant methylotrophic yeast Hansenula polymorpha, overproducing d-lactate: cytochrome c-oxidoreductase (EC 1.1.2.4, d-lactate dehydrogenase (cytochrome), DlDH). The H. polymorpha DlDH-producer was constructed in two steps. First, the gene CYB2 was deleted on the background of the C-105 (gcr1 catX) strain of H. polymorpha impaired in glucose repression and devoid of catalase activity to avoid specific l-lactate-cytochrome c oxidoreductase activity. Second, the homologous gene DLD1 coding for DlDH was overexpressed under the control of the strong H. polymorpha alcohol oxidase promoter in the frame of a plasmid for multicopy integration in the Δcyb2 strain. The selected recombinant strain possesses 6-fold increased DlDH activity as compared to the initial strain. The cellsdebris was used as a biorecognition element of a biosensor, since DlDH is strongly bound to mitochondrial membranes. The cellsdebris, prepared by mechanic disintegration of recombinant cells, was immobilized on a graphite working electrode in an electrochemically generated layer using an Os-complex modified cathodic electrodeposition polymer. Cytochrome c was used as additional native electron mediator to improve electron transfer from reduced DlDH to the working electrode. The constructed d-lactate-selective biosensors are characterized by a high sensitivity (46.3-61.6 A M-1 m-2), high selectivity and sufficient storage stability. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.talanta.2014.02.041
  • 2014 • 56 Gold electrodes from recordable CDs for the sensitive, semi-quantitative detection of commercial silver nanoparticles in seawater media
    Stuart, E.J.E. and Tschulik, K. and Lowinsohn, D. and Cullen, J.T. and Compton, R.G.
    Sensors and Actuators, B: Chemical 195 223-229 (2014)
    We report the use of homemade disposable gold electrodes fabricated from commercial recordable CDs for the detection and quantification of silver nanoparticles from a consumer product in a seawater sample. The "CDtrode" is immersed in a seawater sample containing silver nanoparticles for a certain amount of time during which the silver nanoparticles adsorb onto the CDtrode surface under open circuit conditions. The CDtrode is then transferred to an aqueous electrolyte and oxidative stripping is used to determine the amount of silver nanoparticles that have become stuck to the electrode surface. Depending on immersion time and silver nanoparticle concentration, up to a full monolayer coverage of silver nanoparticles on the CDtrode surface has been achieved. © 2014 Elsevier B.V.
    view abstractdoi: 10.1016/j.snb.2014.01.040
  • 2014 • 55 Electrochemical observation of single collision events: Fullerene nanoparticles
    Stuart, E.J.E. and Tschulik, K. and Batchelor-Mcauley, C. and Compton, R.G.
    ACS Nano 8 7648-7654 (2014)
    Individual fullerene nanoparticles are detected and sized in a non-aqueous solution via cathodic particle coulometry where the direct, quantitative reduction of single nanoparticles is achieved upon collision with a potentiostated gold electrode. This is the first time that the nanoparticle impact technique has been shown to work in a non-aqueous electrolyte and utilized to coulometrically size carbonaceous nanoparticles. Contrast is drawn between single-nanoparticle electrochemistry and that seen using nanoparticle ensembles via modified electrodes. © 2014 American Chemical Society.
    view abstractdoi: 10.1021/nn502634n
  • 2014 • 54 Chemical interactions between silver nanoparticles and thiols: A comparison of mercaptohexanol against cysteine
    Toh, H.S. and Batchelor-Mcauley, C. and Tschulik, K. and Compton, R.G.
    Science China Chemistry 57 1199-1210 (2014)
    The interaction between citrate capped silver nanoparticles and two different thiols, mercaptohexanol (MH) and cysteine, was investigated. The thiols interacted with silver nanoparticles in a significantly contrasting manner. With MH, a sparingly soluble silver(I) thiolate complex AgSRm (Rm = -(CH2)6OH) was formed on the silver nanoparticle surface. Cyclic voltammograms and UV-vis spectra were used to infer that the AgSRm complex on the nanoparticle surface undergoes a phase transition to give a mixture of AgSRm and Ag2S-like complexes. In contrast, when silver nanoparticles were exposed to cysteine, the citrate capping agent on the silver nanoparticles was replaced by cysteine to give cysteine capped nanoparticles. As cysteine capped nanoparticles form, the electrochemical data displayed a decrease in oxidative peak charge but the UV-vis spectra showed a constant signal. Therefore, cysteine capped nanoparticles were suggested to have either inactivated the silver surface or else promoted detachment from the electrode surface. © 2014 Science China Press and Springer-Verlag Berlin Heidelberg.
    view abstractdoi: 10.1007/s11426-014-5141-8
  • 2014 • 53 Position of Cu atoms at the Pt(111) electrode surfaces controls electrosorption of (H)SO4 (2)- from H2SO4 electrolytes
    Tymoczko, J. and Schuhmann, W. and Bandarenka, A.S.
    ChemElectroChem 1 (2014)
    Selective positioning of monolayer amounts of foreign atoms at the surface and subsurface regions of metal electrodes is a promising way to fine-tune the properties of the electrode/ electrolyte interface. The latter is critical as it largely governs the adsorption of electrolyte components and reaction intermediates and, therefore, controls many key electrocatalytic processes. Using model Pt(111) single-crystal electrodes, we demonstrate how the relative position of Cu atoms at the surface drastically changes the adsorption energies for (bi)sulfate anions. Our measurements involve pseudomorphic overlayers of Cu on Pt(111) as well as Cu-Pt(111) surface and sub-surface alloys, where Cu atoms were located either in the first or in the second atomic layers of Pt, respectively. In the case of Cu- Pt(111) surface alloys, specific adsorption of the anions starts earlier compared to the unmodified Pt(111) surface. In contrast, placing Cu atoms into the second atomic layer weakens the binding between the surface and the anions. Surprisingly, Cu pseudomorphic overlayers do not reveal any specific adsorption of (bi)sulfates (within the region of the overlayer stability). Taking into account that electrified interfaces between Pt(111) electrodes and sulfate-containing electrolytes often play the role of benchmark systems in fundamental physico-chemical and, particularly, electrocatalytic studies, our findings demonstrate a promising and relatively easy route of tuning the properties of these interfaces. © 2014 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201300107
  • 2014 • 52 Biofuel-Cell Cathodes Based on Bilirubin Oxidase Immobilized through Organic Linkers on 3D Hierarchically Structured Carbon Electrodes
    Vivekananthan, J. and Rincón, R.A. and Kuznetsov, V. and Pöller, S. and Schuhmann, W.
    ChemElectroChem 1 1901-1908 (2014)
    Different modification procedures to stabilize and control the orientation of Myrothecium verrucaria bilirubin oxidase (MvBOD) on 3D carbon nanotube/carbon microfiber-modified graphite electrode surfaces were evaluated for the development of biofuel-cell cathodes. The surface properties of different linkers for covalent binding of BOD were investigated by using atomic force microscopy-based techniques. For all immobilization strategies, the maximal current response was obtained at a pH value of 6.5 with temperatures between 20 and 35°C. The biocathode based on MvBOD immobilized through an imino bond to the electrode showed the highest current density (1600μAcm-2) and was resistant to the presence of chloride ions. A biofuel cell was constructed, and it exhibited a maximal power of 54μWcm-2 at 350mV with an open-circuit voltage of about 600mV by using a cellobiose dehydrogenase based bioanode and glucose as the fuel. © 2014 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/celc.201402099
  • 2014 • 51 Double layer effects in a model of proton discharge on charged electrodes
    Wiebe, J. and Spohr, E.
    Beilstein Journal of Nanotechnology 5 973-982 (2014)
    We report first results on double layer effects on proton discharge reactions from aqueous solutions to charged platinum electrodes. We have extended a recently developed combined proton transfer/proton discharge model on the basis of empirical valence bond theory to include specifically adsorbed sodium cations and chloride anions. For each of four studied systems 800-1000 trajectories of a discharging proton were integrated by molecular dynamics simulations until discharge occurred. The results show significant influences of ion presence on the average behavior of protons prior to the discharge event. Rationalization of the observed behavior cannot be based solely on the electrochemical potential (or surface charge) but needs to resort to the molecular details of the double layer structure. © 2014 Wiebe and Spohr; licensee Beilstein-Institut.
    view abstractdoi: 10.3762/bjnano.5.111
  • 2014 • 50 The role of hydrophobicity of Os-complex-modified polymers for photosystem 1 based photocathodes
    Zhao, F. and Sliozberg, K. and Rögner, M. and Plumeré, N. and Schuhmann, W.
    Journal of the Electrochemical Society 161 H3035-H3041 (2014)
    The integration of photosystem 1 in redox hydrogels based on Os-complexes modified redox polymers on electrodes yields efficient photocathodes. The generation of high photocurrent relies on high loading in PS1 and fast electron transfer rates from the electrode to PS1. The interaction between the redox polymer and PS1 influences both the loading in protein and the electron transfer rates. Since PS1 exhibits extended hydrophobic regions, polymers with similar properties may favor attractive interactions. Here we investigate three approaches to confer hydrophobicity to the redox polymer. We demonstrate that the pyridine functionality enables to switch, via basic pH values, the polymer properties from hydrophilic to hydrophobic. The transition triggers a hydrogel collapse which allows for efficient entrapment of PS1. In addition the hydrophobic-hydrophilic balance was tuned by the addition of hydrophobic group in i) the polymer backbone and ii) as substituents at the Os-complex. The increased hydrophobicity of the backbone results in higher photocurrents from PS1 integrated in the corresponding hydrogel. On the other hand, further increasing hydrophobicity of the redox relay decreases the photocurrent due to either lower mobility of the Os-complexes or poor interaction with the hydrophilic site where the redox center of PS1 is located. © The Author(s) 2014. Published by ECS.
    view abstractdoi: 10.1149/2.0081413jes
  • 2013 • 49 A disposable sticky electrode for the detection of commercial silver NPs in seawater
    Cheng, W. and Stuart, E.J.E. and Tschulik, K. and Cullen, J.T. and Compton, R.G.
    Nanotechnology 24 (2013)
    The ability to perform efficient and affordable field detection and quantification of nanoparticles in aquatic environmental systems remains a significant technical challenge. Recently we reported a proof of concept of using 'sticky' electrodes for the detection of silver nanoparticles (Tschulik et al 2013 Nanotechnology 29 295502). Now a disposable electrode for detection and quantification of commercial Ag nanoparticles in natural seawater is presented. A disposable screen printed electrode is modified with cysteine and characterized by sticking and stripping experiments, with silver nanoparticle immobilization on the electrode surface and subsequent oxidative stripping, yielding a quantitative determination of the amount of Ag nanoparticles adhering to the electrode surface. The modified electrode was applied to natural seawater to mimic field-based environmental monitoring of Ag NPs present in seawater. The results demonstrated that commercial Ag NPs in natural seawater can be immobilized, enriched and quantified within short time period using the disposable electrodes without any need for elaborate experiments. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/24/50/505501
  • 2013 • 48 Electrochemical formation and surface characterisation of Cu 2-xTe thin films with adjustable content of Cu
    Huang, M. and Maljusch, A. and Calle-Vallejo, F. and Henry, J.B. and Koper, M.T.M. and Schuhmann, W. and Bandarenka, A.S.
    RSC Advances 3 21648-21654 (2013)
    Electrochemically driven "intercalation" of Cu into Te was used to prepare Cu<inf>2-x</inf>Te (0.2 < x ≤ 2) thin films and accurately control the composition of the resulting samples. A thorough theoretical analysis of the system using density functional theory (DFT) calculations showed that in the absence of external electric fields the driving forces for Cu atoms to move into the subsurface layers of the Te electrodes depend on the surface coverage of copper atoms. The Cu atoms tend to preferentially occupy the subsurface layers in the telluride films. The effective electric charge on Cu atoms inside the Te-electrodes is positive. These effective charge differences with respect to pure Cu and pure Te are only 0.2 e-. Scanning Kelvin probe (SKP), atomic force microscopy (AFM) and electrochemical techniques were used to characterise the surface status of the obtained samples. Both, DFT-calculated work function differences and the SKP-measured contact potential differences (CPD) change non-linearly with the variation of the film composition. Interfacial (solid/liquid) properties evaluated using electrochemical impedance spectroscopy depend on the nominal composition of the samples and display an abrupt change that correlates with a large change in the work function and CPD. While the proposed electrochemical synthetic route can efficiently and accurately control the composition of the Cu<inf>2-x</inf>Te thin films, SKP-measurements performed under close to ambient conditions in combination with DFT calculations can provide a promising tool to link fundamental surface properties and parameters which define the interface between solids and liquids. © The Royal Society of Chemistry 2013.
    view abstractdoi: 10.1039/c3ra42504e
  • 2013 • 47 Coupling osmium complexes to epoxy-functionalised polymers to provide mediated enzyme electrodes for glucose oxidation
    Ó Conghaile, P. and Pöller, S. and MacAodha, D. and Schuhmann, W. and Leech, D.
    Biosensors and Bioelectronics 43 30-37 (2013)
    Newly synthesised osmium complex-modified redox polymers were tested for potential application as mediators in glucose oxidising enzyme electrodes for application to biosensors or biofuel cells. Coupling of osmium complexes containing amine functional groups to epoxy-functionalised polymers of variable composition provides a range of redox polymers with variation possible in redox potential and physicochemical properties. Properties of the redox polymers as mediators for glucose oxidation were investigated by co-immobilisation onto graphite with glucose oxidase or FAD-dependent glucose dehydrogenase using a range of crosslinkers and in the presence and absence of multiwalled carbon nanotubes. Electrodes prepared by immobilising [P20-Os(2,2'-bipyridine)2(4-aminomethylpyridine)Cl].PF6, carbon nanotubes and glucose oxidase exhibit glucose oxidation current densities as high as 560μAcm-2 for PBS containing 100mM glucose at 0.45V vs. Ag/AgCl. Films prepared by crosslinking [P20-Os(4,4'-dimethoxy-2,2'-bipyridine)2(4-aminomethylpyridin e)Cl].PF6, an FAD-dependent glucose dehydrogenase, and carbon nanotubes achieve current densities of 215μAcm-2 in 5mM glucose at 0.2V vs. Ag/AgCl, showing some promise for application to glucose oxidising biosensors or biofuel cells. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2012.11.036
  • 2013 • 46 Stabilizing redox polymer films by electrochemically induced crosslinking
    Pöller, S. and Koster, D. and Schuhmann, W.
    Electrochemistry Communications 34 327-330 (2013)
    Electrochemically induced crosslinking is suggested to stabilize electrodeposition polymer/enzyme films selectively on an electrode surface. 4 different protected diamine or dithiol based bi-functional crosslinkers have been synthesized, which can be activated by a pH-shift invoked by electrochemical water oxidation or proton reduction. Deprotection occurs either simultaneously or sequentially to the deposition of specifically designed redox electrodeposition polymers. The stability of the resulting polymer films was substantially enhanced as evaluated using continuous potentiodynamic cycling alternated by difference pulse voltammetry. Electrochemically induced crosslinking is compatible with biological recognition elements using Trametes hirsuta laccase or glucose oxidase entrapped within specifically adapted Os-complex modified or phenothiazine-modified redox polymers. © 2013 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2013.07.033
  • 2013 • 45 A permeation model for the electrochemical interface
    Preiss, U. and Borukhovich, E. and Alemayehu, N. and Steinbach, I. and LaMantia, F.
    Modelling and Simulation in Materials Science and Engineering 21 (2013)
    We show the transferability of the recently introduced concept of permeation from the context of finite dissipation in simple metallic interfaces to much more complicated electrochemical interfaces. The phenomenological bridge is formed by the exchange current, which can be measured by either impedance spectroscopy or by cyclic voltammetry. In a proof-of-concept phase field model, Nernst-Planck diffusion and transport of charged species in a potential gradient as the solution of the Poisson equation are considered. It is shown that charges build up on the outer electrode surface in a fashion resembling the electrochemical double layer. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0965-0393/21/7/074006
  • 2013 • 44 Direct electron transfer of Trametes hirsuta laccase adsorbed at unmodified nanoporous gold electrodes
    Salaj-Kosla, U. and Pöller, S. and Schuhmann, W. and Shleev, S. and Magner, E.
    Bioelectrochemistry 91 15-20 (2013)
    The enzyme Trametes hirsuta laccase undergoes direct electron transfer at unmodified nanoporous gold electrodes, displaying a current density of 28μA/cm2. The response indicates that ThLc was immobilised at the surface of the nanopores in a manner which promoted direct electron transfer, in contrast to the absence of a response at unmodified polycrystalline gold electrodes. The bioelectrocatalytic activity of ThLc modified nanoporous gold electrodes was strongly dependent on the presence of halide ions. Fluoride completely inhibited the enzymatic response, whereas in the presence of 150mM Cl-, the current was reduced to 50% of the response in the absence of Cl-. The current increased by 40% when the temperature was increased from 20°C to 37°C. The response is limited by enzymatic and/or enzyme electrode kinetics and is 30% of that observed for ThLc co-immobilised with an osmium redox polymer. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2012.11.001
  • 2013 • 43 Mutual enhancement of the current density and the coulombic efficiency for a bioanode by entrapping bi-enzymes with Os-complex modified electrodeposition paints
    Shao, M. and Nadeem Zafar, M. and Sygmund, C. and Guschin, D.A. and Ludwig, R. and Peterbauer, C.K. and Schuhmann, W. and Gorton, L.
    Biosensors and Bioelectronics 40 308-314 (2013)
    A bioanode with high current density and coulombic efficiency was developed by co-immobilization of pyranose dehydrogenase from Agaricus meleagris (AmPDH) with the dehydrogenase domain of cellobiose dehydrogenase from Corynascus thermophiles (recDH. CtCDH) expressed recombinantly in Escherichia coli. The two enzymes were entrapped in Os-complex modified electrodeposition polymers (Os-EDPs) with specifically adapted redox potential by means of chemical co-deposition. AmPDH oxidizes glucose at both the C2 and C3 positions whereas recDH. CtCDH oxidizes glucose only at the C1 position. Electrochemical measurements reveal that maximally 6 electrons can be harvested from one glucose molecule at the two-enzyme anode via a cascade reaction, as AmPDH oxidizes the products formed from of the recDH. CtCDH catalyzed substrate oxidation and vice versa. Furthermore, a significant increase in current density can be obtained by combining AmPDH and recDH. CtCDH in a single modified electrode. We propose the use of this bioanode in biofuel cells with increased current density and coulombic efficiency. © 2012 Elsevier B.V.
    view abstractdoi: 10.1016/j.bios.2012.07.069
  • 2013 • 42 The anodic stripping voltammetry of nanoparticles: Electrochemical evidence for the surface agglomeration of silver nanoparticles
    Toh, H.S. and Batchelor-McAuley, C. and Tschulik, K. and Uhlemann, M. and Crossley, A. and Compton, R.G.
    Nanoscale 5 4884-4893 (2013)
    Analytical expressions for the anodic stripping voltammetry of metallic nanoparticles from an electrode are provided. First, for reversible electron transfer, two limits are studied: that of diffusionally independent nanoparticles and the regime where the diffusion layers originating from each particle overlap strongly. Second, an analytical expression for the voltammetric response under conditions of irreversible electron transfer kinetics is also derived. These equations demonstrate how the peak potential for the stripping process is expected to occur at values negative of the formal potential for the redox process in which the surface immobilised nanoparticles are oxidised to the corresponding metal cation in the solution phase. This work is further developed by considering the surface energies of the nanoparticles and its effect on the formal potential for the oxidation. The change in the formal potential is modelled in accordance with the equations provided by Plieth [J. Phys. Chem., 1982, 86, 3166-3170]. The new analytical expressions are used to investigate the stripping of silver nanoparticles from a glassy carbon electrode. The relative invariance of the stripping peak potential at low surface coverages of silver is shown to be directly related to the surface agglomeration of the nanoparticles. © 2013 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c3nr00898c
  • 2013 • 41 'Sticky electrodes' for the detection of silver nanoparticles
    Tschulik, K. and Palgrave, R.G. and Batchelor-Mcauley, C. and Compton, R.G.
    Nanotechnology 24 (2013)
    Detection and quantification of nanoparticles in environmental systems is a task that requires reliable and affordable analytical methods. Here an approach using a cysteine-modified 'sticky' glassy carbon electrode is presented. The electrode is immersed in a silver nanoparticle containing electrolyte and left in this suspension without an applied potential, i.e. under open circuit condition, for a variable amount of time. The amount of silver nanoparticles immobilized on the electrode within this sticking time is then determined by oxidative stripping, yielding the anodic charge and thus the amount of Ag nanoparticles sticking to the electrode surface. When using a cysteine-modified glassy carbon electrode, significant and reproducible amounts of silver nanoparticles stick to the surface, which is not the case for unmodified glassy carbon surfaces. Additionally, proof-of-concept experiments are performed on real seawater samples. These demonstrate that also under simulated environmental conditions an increased immobilization and hence improved detection of silver nanoparticles on cysteine-modified glassy carbon electrodes is achieved, while no inhibitive interference with this complex matrix is observed. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/24/29/295502
  • 2013 • 40 TiO2(B)/anatase composites synthesized by spray drying as high performance negative electrode material in Li-ion batteries
    Ventosa, E. and Mei, B. and Xia, W. and Muhler, M. and Schuhmann, W.
    ChemSusChem 6 1312-1315 (2013)
    The power of spray-dried TiO2 in LIBs: TiO2(B)/ anatase is synthesized by spray drying and investigated as negative electrode material in Li-ion batteries. It exhibits excellent Li-ion storage performances, especially at high charge/discharge rates. The presence of the β phase of TiO2 improves Li-ion diffusivity. Additionally, the scalable synthesis method also allows for Nb-doping, which assists in the maintenance of the electronic conductivity as the thickness of film increases. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201300439
  • 2013 • 39 A study of electrode temperature lowering in Dy-containing ceramic metal halide lamps: II. An investigation of the converse effect of Tl and/or Na additives
    Westermeier, M. and Ruhrmann, C. and Bergner, A. and Denissen, C. and Suijker, J. and Awakowicz, P. and Mentel, J.
    Journal of Physics D: Applied Physics 46 (2013)
    The lowering of the gas phase emitter effect of Dy in ceramic metal halide lamps by the admixture of TlI and NaI to the rare earth iodide salt DyI 3 is investigated at lamps with different additives. The arcs are operated in an Hg buffer gas atmosphere of 2 MPa between rod-shaped pure tungsten electrodes within transparent YAG lamp tubes with a switched-dc current at operating frequencies from 1 Hz to 1 kHz. The atomic ground state density of Dy is measured phase resolved half way between the electrodes and in front of an electrode by broad band absorption spectroscopy, the Dy ion density in front of an electrode by emission spectroscopy and the electrode tip temperature pyrometrically within lamps seeded with differently composed fillings. The measurements confirm that a strong reduction in the electrode tip temperature is correlated with a high Dy ion density in front of the electrode within the cathodic half period. The Dy ion density is depressed predominantly and with it the reduction in the electrode tip temperature by a competing ionization of Tl, and in addition by a lowering of the Dy vapour pressure above the pool of molten salt by TlI. The influence of Na is of minor importance. © 2013 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/46/18/185202
  • 2013 • 38 Improving the current density and the coulombic efficiency by a cascade reaction of glucose oxidizing enzymes
    Zafar, M.N. and Shao, M. and Ludwig, R. and Leech, D. and Schuhmann, W. and Gorton, L.
    ECS Transactions 53 131-143 (2013)
    Improvements in current density and coulombic efficiency of a glucose oxidizing electrode were realized by a combination of pyranose dehydrogenase from Agaricus meleagris (AmPDH) with glucose dehydrogenase from Glomerella cingulata (GcGDH). The mixed enzyme electrode oxidizes glucose in several combinations at the C-1, C-2 and C-3 positions of the pyranose ring. This concerted action of enzymes increases (i) the coulombic efficiency by extracting more than 2e- per substrate molecule and (ii) the current density of the electrode when the mass-transfer of substrates becomes rate limiting. The electrodes were investigated with flow injection analysis (FIA) using different substrates under physiological conditions (pH 7.4). These investigations showed that the product of one enzyme can be used as substrate for the other enzyme and maximally 6e- can be gained from the oxidation of one glucose molecule using mixed enzyme electrode AmPDH/GcGDH/Os-polymer. We propose a bioanode for use in biofuel cells with an increased current density and coulombic efficiency obtained by a cascade reaction catalyzed by redox enzymes with a different site-specificity for glucose. © The Electrochemical Society.
    view abstractdoi: 10.1149/05302.0131ecst
  • 2012 • 37 Microstructural impact of anodic coatings on the electrochemical chlorine evolution reaction
    Chen, R. and Trieu, V. and Zeradjanin, A.R. and Natter, H. and Teschner, D. and Kintrup, J. and Bulan, A. and Schuhmann, W. and Hempelmann, R.
    Physical Chemistry Chemical Physics 14 7392-7399 (2012)
    Sol-gel Ru 0.3Sn 0.7O 2 electrode coatings with crack-free and mud-crack surface morphology deposited onto a Ti-substrate are prepared for a comparative investigation of the microstructural effect on the electrochemical activity for Cl 2 production and the Cl 2 bubble evolution behaviour. For comparison, a state-of-the-art mud-crack commercial Ru 0.3Ti 0.7O 2 coating is used. The compact coating is potentially durable over a long term compared to the mud-crack coating due to the reduced penetration of the electrolyte. Ti L-edge X-ray absorption spectroscopy confirms that a TiO x interlayer is formed between the mud-crack Ru 0.3Sn 0.7O 2 coating and the underlying Ti-substrate due to the attack of the electrolyte. Meanwhile, the compact coating shows enhanced activity in comparison to the commercial coating, benefiting from the nanoparticle-nanoporosity architecture. The dependence of the overall electrode polarization behaviour on the local activity and the bubble evolution behaviour for the Ru 0.3Sn 0.7O 2 coatings with different surface microstructure are evaluated by means of scanning electrochemical microscopy and microscopic bubble imaging. © 2012 the Owner Societies.
    view abstractdoi: 10.1039/c2cp41163f
  • 2012 • 36 Understanding properties of electrified interfaces as a prerequisite for label-free DNA hybridization detection
    Gebala, M. and Schuhmann, W.
    Physical Chemistry Chemical Physics 14 14933-14942 (2012)
    Label-free electrochemical detection of DNA hybridization with high selectivity and sensitivity is only achievable if the properties of DNA at an electrified interface are understood in depth. After a short summary of concepts of electrochemical DNA detection as well as initial attempts towards label-free DNA assays the review discusses the physico-chemical properties and differences between single-stranded and double-stranded DNA immobilized at electrode surfaces in the light of their persistence lengths, structural conformation, impact of the charge screening by ion condensation and the electric field generated upon polarization of the electrode. Electrochemical impedance spectroscopy as a tool for label-free elucidation of DNA hybridization is reviewed and the necessity for an in-depth understanding of the interfacial properties is highlighted. Our major aim is to demonstrate the advantageous application of specifically designed intercalating compounds for the design of label-free detection of DNA hybridization. This journal is © 2012 the Owner Societies.
    view abstractdoi: 10.1039/c2cp42382k
  • 2012 • 35 A modular and wireless exg signal acquisition system with a dense array of dry electrodes
    Ghoshdastider, U. and Lange, C. and Viga, R. and Grabmaier, A.
    Proceedings of IEEE Sensors (2012)
    A modular, configurable, wearable bio-potential signal measurement diagnostic system is developed. The system is wirelessly configurable for multiple numbers of channels from 1 to 256 channels according to their field of applications. EEG (Electroencephalography), ECG (Electrocardiography), EMG (Electromyography) can be measured with the system by encapsulating in a cap, in a vest and in a hand sleeves respectively. Active dry electrodes with 19 Au-Pins are utilized. It achieves a high signal quality due to high special resolution. An electrode-bus containing eight of such dry electrodes is designed utilizing the modern hard-flex PCB concept so that it can be molded like an S and be placed on the area of measurement. A Meridian-ADC digitalizes the amplified analog signal with a 24-bit Δ∑-ADC. The central component of the system is an Islandcontroller, incorporating an AVR-μC and a Wi-Fi-module with an external patch antenna. © 2012 IEEE.
    view abstractdoi: 10.1109/ICSENS.2012.6411473
  • 2012 • 34 Impact of single basepair mismatches on electron-transfer processes at Fc-PNA·DNA modified gold surfaces
    Hüsken, N. and Gȩbala, M. and Battistel, A. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    ChemPhysChem 13 131-139 (2012)
    Gold-surface grafted peptide nucleic acid (PNA) strands, which carry a redox-active ferrocene tag, present unique tools to electrochemically investigate their mechanical bending elasticity based on the kinetics of electron-transfer (ET) processes. A comparative study of the mechanical bending properties and the thermodynamic stability of a series of 12-mer Fc-PNA·DNA duplexes was carried out. A single basepair mismatch was integrated at all possible strand positions to provide nanoscopic insights into the physicochemical changes provoked by the presence of a single basepair mismatch with regard to its position within the strand. The ET processes at single mismatch Fc-PNA·DNA modified surfaces were found to proceed with increasing diffusion limitation and decreasing standard ET rate constants k 0 when the single basepair mismatch was dislocated along the strand towards its free-dangling Fc-modified end. The observed ET characteristics are considered to be due to a punctual increase in the strand elasticity at the mismatch position. The kinetic mismatch discrimination with respect to the fully-complementary duplex presents a basis for an electrochemical DNA sensing strategy based on the Fc-PNA·DNA bending dynamics for loosely packed monolayers. In a general sense, the strand elasticity presents a further physicochemical property which is affected by a single basepair mismatch which may possibly be used as a basis for future DNA sensing concepts for the specific detection of single basepair mismatches. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201100578
  • 2012 • 33 Glucose oxidase/horseradish peroxidase Co-immobilized at a CNT-modified graphite electrode: Towards potentially implantable biocathodes
    Jia, W. and Jin, C. and Xia, W. and Muhler, M. and Schuhmann, W. and Stoica, L.
    Chemistry - A European Journal 18 2783-2786 (2012)
    Concerted efforts: A high-potential biocathode based on co-immobilization of glucose oxidase (GOx) and horseradish peroxidase (HRP) onto a carbon nanotube/carbon microfiber modified graphite rod electrode (CNT/CMF/GR) is described (see figure). The GOx/HRP biocathode shows a remarkable biocatalytic activity in the presence of glucose and oxygen. Copyright © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201102921
  • 2012 • 32 Assembly of mesoporous indium tin oxide electrodes from nano-hydroxide building blocks
    Liu, Y. and Štefanić, G. and Rathouský, J. and Hayden, O. and Bein, T. and Fattakhova-Rohlfing, D.
    Chemical Science 3 2367-2374 (2012)
    We describe the elaboration of nanostructured transparent conducting indium tin oxide (ITO) materials that is based on controlled self-assembly of ultra-small indium tin hydroxide nanoparticles. We developed a strategy for preparing nanosized, nearly spherical and highly dispersible nanoparticles of indium tin hydroxide ("nano-hydroxides"), which can be assembled into regular mesoporous architectures directed by a commercially available Pluronic polymer. The assembled structures are easily transformed into conducting crystalline mesoporous ITO films by a mild heat treatment at 300 °C. The resulting ITO layers feature a regular mesoporosity with a mesostructure periodicity of about 13 ± 2 nm, high surface area of 190 m2 cm-3, porosity of 44% and electrical conductivity up to 9.5 S cm -1. The ITO films can accommodate large amounts of redox-active molecules and serve as efficient conducting electrodes with a very high surface area. The perfect dispersibility of nano-hydroxides without any stabilizing agents, their preferential interaction with the hydrophilic part of amphiphilic molecules leading to their self-assembly, and a facile transformation of the assembled nano-hydroxides into crystalline ITO with similar morphology make the nano-hydroxides very attractive building blocks for the elaboration of nanostructured ITO materials. We believe that the nano-hydroxides can become universal building blocks for the fabrication of crystalline ITO materials with arbitrary nano-morphologies. © The Royal Society of Chemistry 2012.
    view abstractdoi: 10.1039/c2sc20042b
  • 2012 • 31 Electrochemical synthesis of metal-polypyrrole composites and their activation for electrocatalytic reduction of oxygen by thermal treatment
    Masa, J. and Schilling, T. and Bron, M. and Schuhmann, W.
    Electrochimica Acta 60 410-418 (2012)
    This work presents a new approach for synthesis of oxygen reduction catalysts constituted of a transition metal, nitrogen and carbon, by thermal treatment of electrochemically synthesized metal-polypyrrole (M-PPy) composites on glassy carbon electrodes. The synthesis procedure involves immobilization of PPy on glassy carbon followed by dosing of metal (M = Mn, Fe and Co) particles, alternately, by electropolymerization and electrochemical reduction respectively. Electrochemical characterization by cyclic voltammetry (CV) and hydrodynamic rotating disk electrode (RDE) measurements show that the M-PPy composites inherently catalyse the electroreduction of oxygen under acidic conditions. The activity of the composites is significantly augmented when they are heat treated at high temperatures (450-850 °C) under a continuous flow of nitrogen. The presence of metallic entities within the M-PPy composite structures and in the structures ensuing after heat treatment was confirmed by energy dispersive X-ray (EDX) analysis. © 2011 Elsevier Ltd. All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2011.11.076
  • 2012 • 30 Fabrication, laser structuring and field emission properties of carbon nanowalls
    Neubert, M. and Behrenberg, D. and Hartmann, N. and Buck, V. and Serbun, P. and Navitski, A. and Muller, G.
    Technical Digest - 25th International Vacuum Nanoelectronics Conference, IVNC 2012 354-355 (2012)
    We have synthesized carbon nanowalls (CNWs) on Si substrate by inductively/capacitively coupled plasma enhanced chemical vapor deposition (ICP/CCP-PECVD). The shape and density of CNWs are controlled by adjusting the synthesis parameters. Local field emission measurements of Ø 30 μm spots reproducibly yielded stable current up to 1 μA. Integral pulsed measurements results on the unstructured cathodes showed fairly homogeneous emission and current density of at least 3 mA/cm 2 at 5.6 V/μm, limited by the power load on the phosphor screen. In order to get cathodes for gate controlled devices and to improve the emission homogeneity, structuring by laser was performed. © 2012 IEEE.
    view abstractdoi: 10.1109/IVNC.2012.6316966
  • 2012 • 29 A new synthesis route for Os-complex modified redox polymers for potential biofuel cell applications
    Pöller, S. and Beyl, Y. and Vivekananthan, J. and Guschin, D.A. and Schuhmann, W.
    Bioelectrochemistry 87 178-184 (2012)
    A new synthesis route for Os-complex modified redox polymers was developed. Instead of ligand exchange reactions for coordinative binding of suitable precursor Os-complexes at the polymer, Os-complexes already exhibiting the final ligand shell containing a suitable functional group were bound to the polymer via an epoxide opening reaction. By separation of the polymer synthesis from the ligand exchange reaction at the Os-complex, the modification of the same polymer backbone with different Os-complexes or the binding of the same Os-complex to a number of different polymer backbones becomes feasible. In addition, the Os-complex can be purified and characterized prior to its binding to the polymer. In order to further understand and optimize suitable enzyme/redox polymer systems concerning their potential application in biosensors or biofuel cells, a series of redox polymers was synthesized and used as immobilization matrix for Trametes hirsuta laccase. The properties of the obtained biofuel cell cathodes were compared with similar biocatalytic interfaces derived from redox polymers obtained via ligand exchange reaction of the parent Os-complex with a ligand integrated into the polymer backbone during the polymer synthesis. © 2011 Elsevier B.V.
    view abstractdoi: 10.1016/j.bioelechem.2011.11.015
  • 2012 • 28 Thermally induced reactions between lithiated nano-silicon electrode and electrolyte for lithium-ion batteries
    Profatilova, I.A. and Langer, T. and Badillo, J.P. and Schmitz, A. and Orthner, H. and Wiggers, H. and Passerini, S. and Winter, M.
    Journal of the Electrochemical Society 159 A657-A663 (2012)
    The thermal stability of nano-silicon electrodes before and after lithiation was studied by means of differential scanning calorimetry (DSC). It was found that pristine Si electrodes heated in presence of ECDEC 1M LiPF 6 electrolyte show exothermic reactions between sodium carboxymethylcellulose (Na CMC binder) and LiPF 6. The products of thermal decomposition of a lithiated nano-Si electrode with electrolyte at different temperatures were identified using attenuated total reflection Fourier transform infrared spectroscopy (ATR-FTIR). SEI layer was found to be responsible for the thermal reactions in the range between 77 and 107°C. Exothermic events between 107 and 140°C were caused by partial decomposition of LiPF 6 salt, which products initiated further transformations of SEI layer compounds and esterification of Na CMC. Interaction between nano-Li xSi and ECDEC 1M LiPF 6 was the reason for the main exothermic peaks at temperatures between 150 and 300°C. Nano-Li xSi heated with ECDEC solvent mixture without LiPF 6 resulted in electrolyte decomposition at much lower temperatures (&gt;105°C). Therefore, the important role of LiPF 6 in the thermal stabilization of nano-Li xSi with electrolyte at temperatures < 140°C was confirmed while LiTFSI salt added to ECDEC was ineffective in the prevention of the main exothermic reaction starting at 105°C. © 2012 The Electrochemical Society.
    view abstractdoi: 10.1149/2.095205jes
  • 2012 • 27 Direct electron transfer of bilirubin oxidase (Myrothecium verrucaria) at an unmodified nanoporous gold biocathode
    Salaj-Kosla, U. and Pöller, S. and Beyl, Y. and Scanlon, M.D. and Beloshapkin, S. and Shleev, S. and Schuhmann, W. and Magner, E.
    Electrochemistry Communications 16 92-95 (2012)
    Well defined mediatorless bioelectrocatalytic reduction of oxygen with high current densities of 0.8 mA cm - 2 was obtained on nanoporous gold electrodes modified with Myrothecium verrucaria bilirubin oxidase. A stable faradaic response was observed when the enzyme modified electrode was coated with a specifically designed electrodeposition polymer layer. The response of the enzyme electrode was only slightly inhibited by the addition of F -. © 2011 Elsevier B.V. All rights reserved.
    view abstractdoi: 10.1016/j.elecom.2011.12.007
  • 2012 • 26 Mass transport controlled oxygen reduction at anthraquinone modified 3D-CNT electrodes with immobilized Trametes hirsuta laccase
    Sosna, M. and Stoica, L. and Wright, E. and Kilburn, J.D. and Schuhmann, W. and Bartlett, P.N.
    Physical Chemistry Chemical Physics 14 11882-11885 (2012)
    Carbon nanotubes covalently modified with anthraquinone were used as an electrode for the immobilization of Trametes hirsuta laccase. The adsorbed laccase is capable of oxygen reduction at a mass transport controlled rate (up to 3.5 mA cm-2) in the absence of a soluble mediator. The storage and operational stability of the electrode are excellent. This journal is © 2012 the Owner Societies.
    view abstractdoi: 10.1039/c2cp41588g
  • 2012 • 25 Detection of filament formation in forming-free resistive switching SrTiO 3 devices with Ti top electrodes
    Stille, S. and Lenser, Ch. and Dittmann, R. and Koehl, A. and Krug, I. and Muenstermann, R. and Perlich, J. and Schneider, C.M. and Klemradt, U. and Waser, R.
    Applied Physics Letters 100 (2012)
    We investigated the influence of Ti top electrodes on the resistive switching properties of SrTiO 3 thin film devices. Above a Ti layer thickness of 5 nm, the initial resistance is strongly reduced, giving rise to forming-free devices. Hard x-ray photoemission experiments reveal the Ti layer to be composed of several oxide phases, induced by the redox-reaction at the Ti/SrTiO 3 interface. Grazing incidence small angle x-ray scattering measurements indicate that the reduction of the SrTiO 3 thin film occurs in a filamentary way. We attribute this behavior to the preferential reduction of SrTiO 3 thin films along highly defective areas. © 2012 American Institute of Physics.
    view abstractdoi: 10.1063/1.4724108
  • 2012 • 24 Evaluation of the catalytic performance of gas-evolving electrodes using local electrochemical noise measurements
    Zeradjanin, A.R. and Ventosa, E. and Bondarenko, A.S. and Schuhmann, W.
    ChemSusChem 5 1905-1911 (2012)
    Characterization of gas evolution reactions at the electrode/electrolyte boundary is often difficult due to the dynamic behavior of interfacial processes. Electrochemical noise measurements determined by scanning electrochemical microscopy were used to characterize Cl 2 evolution at gas-evolving electrodes (GEEs). Analysis of the electrochemical noise is a powerful method to evaluate the efficiency of the catalyst layer at a GEE. The high sensitivity of the developed measurement system enabled accurate monitoring of the current fluctuations caused by gas-bubble detachment from the electrode surface. Fourier transform analysis of the obtained current responses allows extraction of the characteristic frequency, which is the main parameter of the macrokinetics of GEEs. The characteristic frequency was used as part of a methodology to evaluate the catalyst performance and, in particular, to estimate the fraction of the catalyst layer that is active during the gas evolution reaction. Tip of the iceberg: Positioned scanning electrochemical microscopy tips are used to determine the characteristic frequency of gas-bubble detachment from ruthenium-based dimensionally stable anodes at different applied potentials (see picture). Geometrical factors and optimized microstructures of the electrode surface are essential for improving the overall catalytic activity for industrial applications. © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200262
  • 2012 • 23 Role of Water in the Chlorine Evolution Reaction at RuO 2-Based Electrodesa-Understanding Electrocatalysis as a Resonance Phenomenon
    Zeradjanin, A.R. and Menzel, N. and Strasser, P. and Schuhmann, W.
    ChemSusChem 5 1897-1904 (2012)
    The reaction path of the Cl 2 evolution reaction (CER) was investigated by combining electrochemical and spectroscopic methods. It is shown that oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. The state of the RuO 2 surface during the electrochemical reaction was analyzed in situ by using Raman spectroscopy to monitor vibrations of the crystal lattice of RuO 2 and changes in the surface concentration of the adsorbed species as a function of the electrode potential. The role of the solvent was recognized as being crucial in the formation of an oxygen-containing hydrophilic layer, which is a key prerequisite for electrocatalytic Cl 2 formation. Water (more precisely the OH adlayer) is understood not just as a medium that allows adsorption of intermediates, but also as an integral part of the intermediate formed during the electrochemical reaction. New insights into the general understanding of electrocatalysis were obtained by utilizing the vibration frequencies of the crystal lattice as a dynamic catalytic descriptor instead of thermodynamic descriptors, such as the adsorption energy of intermediates. Interpretation of the derived "volcano" curve suggests that electrocatalysis is governed by a resonance phenomenon. Water powered! The reaction path of the Cl 2 evolution reaction (CER) is investigated by combining electrochemical and spectroscopic methods. Oxidation and reconstruction of the catalyst surface during CER is a consequence of the interaction between RuO 2 and water. Interpretation of the derived volcano curve suggests that electrocatalysis is governed by a resonance phenomenon (see picture). © 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cssc.201200193
  • 2011 • 22 Construction of uricase-overproducing strains of Hansenula polymorpha and its application as biological recognition element in microbial urate biosensor
    Dmytruk, K.V. and Smutok, O.V. and Dmytruk, O.V. and Schuhmann, W. and Sibirny, A.A.
    BMC Biotechnology 11 (2011)
    Background: The detection and quantification of uric acid in human physiological fluids is of great importance in the diagnosis and therapy of patients suffering from a range of disorders associated with altered purine metabolism, most notably gout and hyperuricaemia. The fabrication of cheap and reliable urate-selective amperometric biosensors is a challenging task.Results: A urate-selective microbial biosensor was developed using cells of the recombinant thermotolerant methylotrophic yeast Hansenula polymorpha as biorecognition element. The construction of uricase (UOX) producing yeast by over-expression of the uricase gene of H. polymorpha is described. Following a preliminary screening of the transformants with increased UOX activity in permeabilized yeast cells the optimal cultivation conditions for maximal UOX yield namely a 40-fold increase in UOX activity were determined.The UOX producing cells were coupled to horseradish peroxidase and immobilized on graphite electrodes by physical entrapment behind a dialysis membrane. A high urate selectivity with a detection limit of about 8 μM was found.Conclusion: A strain of H. polymorpha overproducing UOX was constructed. A cheap urate selective microbial biosensor was developed. © 2011 Dmytruk et al; licensee BioMed Central Ltd.
    view abstractdoi: 10.1186/1472-6750-11-58
  • 2011 • 21 Mechanistic studies of Fc-PNA(·DNA) surface dynamics based on the kinetics of electron-transfer processes
    Hüsken, N. and Gȩbala, M. and La Mantia, F. and Schuhmann, W. and Metzler-Nolte, N.
    Chemistry - A European Journal 17 9678-9690 (2011)
    N-Terminally ferrocenylated and C-terminally gold-surface-grafted peptide nucleic acid (PNA) strands were exploited as unique tools for the electrochemical investigation of the strand dynamics of short PNA(·DNA) duplexes. On the basis of the quantitative analysis of the kinetics and the diffusional characteristics of the electron-transfer process, a nanoscopic view of the Fc-PNA(·DNA) surface dynamics was obtained. Loosely packed, surface-confined Fc-PNA single strands were found to render the charge-transfer process of the tethered Fc moiety diffusion-limited, whereas surfaces modified with Fc-PNA·DNA duplexes exhibited a charge-transfer process with characteristics between the two extremes of diffusion and surface limitation. The interplay between the inherent strand elasticity and effects exerted by the electric field are supposed to dictate the probability of a sufficient approach of the Fc head group to the electrode surface, as reflected in the measured values of the electron-transfer rate constant, k 0. An in-depth understanding of the dynamics of surface-bound PNA and PNA·DNA strands is of utmost importance for the development of DNA biosensors using (Fc-)PNA recognition layers. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/chem.201003764
  • 2011 • 20 A novel automated electrochemical ascorbic acid assay in the 24-well microtiter plate format
    Intarakamhang, S. and Leson, C. and Schuhmann, W. and Schulte, A.
    Analytica Chimica Acta 687 1-6 (2011)
    Automatic ascorbic acid (AA) voltammetry was established in 24-well microtiter plates. The assay used a movable assembly of a pencil rod working, an Ag/AgCl reference and a Pt counter electrode with differential pulse voltammetry (DPV) for concentration-dependent current generation. A computer was in command of electrode (z) and microtiter plate (x, y) positioning and timed potentiostat operation. Synchronization of these actions supported sequential approach of all wells and subsequent execution of electrode treatment procedures or AA voltammetry at defined intervals in a measuring cycle. DPV in well solutions offered a linear current/concentration range between 0.1 and 8.0mM, a sensitivity of about 1μAmM-1 AA, and a detection limit of 50μM. When used with a calibration curve or standard addition, automated voltammetry of samples with added known amounts of AA demonstrated good recovery rates. Also, the assay achieved the accurate determination of the AA content of vitamin C tablets, a fruit juice and an herbal tea extract. Robotic AA voltammetry has the advantage of conveniently handling multiple samples in a single measuring run without the continuous attention of laboratory personnel. It is a good option when the goal is cost-effective AA screening of sample libraries and has potential for applications in health care and the food processing, cosmetic and pharmaceutical industries. © 2010 Elsevier B.V.
    view abstractdoi: 10.1016/j.aca.2010.11.023
  • 2011 • 19 Stoichiometry of alloy nanoparticles from laser ablation of PtIr in acetone and their electrophoretic deposition on PtIr electrodes
    Jakobi, J. and Menéndez-Manjón, A. and Chakravadhanula, V.S.K. and Kienle, L. and Wagener, P. and Barcikowski, S.
    Nanotechnology 22 (2011)
    Charged Pt-Ir alloy nanoparticles are generated through femtosecond laser ablation of a Pt9Ir target in acetone without using chemical precursors or stabilizing agents. Preservation of the target's stoichiometry in the colloidal nanoparticles is confirmed by transmission electron microscopy (TEM)-energy-dispersive x-ray spectroscopy (EDX), high angle annular dark field (HAADF) scanning transmission electron microscopy (STEM)-EDX elemental maps, high resolution TEM and selected area electron diffraction (SAED) measurements. Results are discussed with reference to thermophysical properties and the phase diagram. The nanoparticles show a lognormal size distribution with a mean Feret particle size of 26nm. The zeta potential of - 45mV indicates high stability of the colloid with a hydrodynamic diameter of 63nm. The charge of the particles enables electrophoretic deposition of nanoparticles, creating nanoscale roughness on three-dimensional PtIr neural electrodes within a minute. In contrast to coating with Pt or Ir oxides, this method allows modification of the surface roughness without changing the chemical composition of PtIr. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0957-4484/22/14/145601
  • 2011 • 18 Radiofrequency driven and low cost fabricated microhollow cathode discharge for gaseous atomic emission spectrometry
    Meyer, C. and Heming, R. and Gurevich, E.L. and Marggraf, U. and Okruss, M. and Florek, S. and Franzke, J.
    Journal of Analytical Atomic Spectrometry 26 505-510 (2011)
    The current research presents a microhollow cathode discharge (MHCD) used as an analytical microplasma gas detector combining the advantages of a hollow cathode geometry in a miniaturized system offering atmospheric pressure operation. The plasma is driven by a homemade resonant radiofrequency generator (f = 1-10 MHz) reducing the electrode sputtering by a factor of 6.5 compared to common direct current operation leading to an extension of the lifetime of the microplasma chip of the same range. This paper aims further for the development of a novel low priced and therefore replaceable MHCD chip exchanging formerly used Pt electrodes by thin Cu electrodes. The analytical performance of the low cost Cu-MHCD with lifetime enhancing radiofrequency operation is demonstrated by atomic emission spectrometry with halogenated hydrocarbons with the Cl emission line at 912.114 nm. This leads to an excellent detection limit of 15 ppb v/v gaseous Cl in He making this microplasma chip suitable for lab on a chip application. © 2011 The Royal Society of Chemistry.
    view abstractdoi: 10.1039/c0ja00216j
  • 2011 • 17 Antimony doped tin oxide nanoparticles and their assembly in mesostructured film
    Müller, V. and Rasp, M. and Stefanic, G. and Günther, S. and Rathousky, J. and Niederberger, M. and Fattakhova-Rohlfing, D.
    Physica Status Solidi (C) Current Topics in Solid State Physics 8 1759-1763 (2011)
    Mesoporous transparent conducting films of antimony-doped tin oxide (ATO) were prepared by self-assembly of crystalline ATO nanoparticles, which enables to obtain a fully crystalline frameworks with a sufficient electric conductivity. Such frameworks are promising as transparent electrodes with a high surface area, as shown for ferrocene molecules covalently immobilized within a conducting mesoporous matrix. © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/pssc.201000129
  • 2011 • 16 Investigating the dependence of the temperature of high-intensity discharge (HID) lamp electrodes on the operating frequency by pyrometric measurements
    Reinelt, J. and Westermeier, M. and Ruhrmann, C. and Bergner, A. and Awakowicz, P. and Mentel, J.
    Journal of Physics D: Applied Physics 44 (2011)
    Phase-resolved temperature distributions are determined along a rod-shaped tungsten electrode, by which an ac arc is operated within a model lamp filled with argon. Switched dc and sinusoidal currents are applied with amplitudes of several amperes and operating frequencies being varied between 10 Hz and 10 kHz. The temperature is deduced from the grey body radiation of the electrode being recorded with a spectroscopic measuring system. Phase-resolved values of the electrode tip temperature Ttip and of the power input Pin are determined comparing the measured temperature distributions with the integral of the one-dimensional heat balance with these parameters as integration constants. They are supplemented by phase-resolved measurements of the sum of cathode and anode fall called the electrode sheath voltage. If a switched dc current is applied it is found that both quantities are within the cathodic phase only marginally higher than for a cathode being operated with a dc current. Ttip and Pin start to decrease for low currents and to increase for high currents at the beginning of the anodic phase. But with increasing operating frequency the deviations from the cathodic phase are reduced until they cannot be resolved for frequencies of several kHz. A more pronounced modulation, but the same tendencies, is observed with a sinusoidal current waveform. For 10 kHz a diffuse arc attachment with an almost phase-independent electrode tip temperature, which deviates only marginally from that of a dc cathode, and an electrode sheath voltage proportional to the arc current is established with both current waveforms. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/9/095204
  • 2011 • 15 Investigating the gas phase emitter effect of caesium and cerium in ceramic metal halide lamps in dependence on the operating frequency
    Ruhrmann, C. and Westermeier, M. and Bergner, A. and Luijks, G.M.J.F. and Awakowicz, P. and Mentel, J.
    Journal of Physics D: Applied Physics 44 (2011)
    The work function and with it the temperature of tungsten electrodes in HID lamps can be lowered and the lifetime of lamps increased by the gas phase emitter effect. A determination of the emitter effect of Cs and Ce is performed by phase resolved measurements of the electrode tip temperature T tip(φ), plasma temperature Tpl(φ) and particle densities N(φ) by means of pyrometric, optical emission and broadband absorption spectroscopy in dependence on the operating frequency. The investigated HID lamps are ceramic metal halide lamps with transparent discharge vessels made of YAG, filled with a buffer gas consisting of Ar, Kr and predominantly Hg and seeded with CsI or CeI3. In the YAG lamp seeded with CsI and CeI3 as well as in a YAG lamp seeded with DyI 3 (corresponding results can be found in a preceding paper) a gas phase emitter effect is observed in the cathodic phase due to a Cs, Ce or Dy ion current. In the YAG lamp seeded with CsI the phase averaged coverage of the electrode surface with emitter atoms decreases and the electrode temperature rises with increasing frequency, whereas the emitter effect of Ce and Dy is extended to the anodic phase, which leads to a decreased average temperature Ttip(φ) with increasing frequency. This different behaviour of the averaged values of Ttip(φ) for increasing frequency is caused by the differing adsorption energies Ea of the respective emitter materials. In spite of the influence of Ea on the coverage of the electrode with emitter atoms, the cathodic gas phase emitter effect produces in the YAG lamps seeded with CsI, CeI3 and DyI3 a general reduction in the electrode tip temperature Ttip(φ) in comparison with a YAG lamp with Hg filling only. © 2011 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0022-3727/44/35/355202
  • 2011 • 14 Electronic excitations induced by hydrogen surface chemical reactions on gold
    Schindler, B. and Diesing, D. and Hasselbrink, E.
    Journal of Chemical Physics 134 (2011)
    Associated with chemical reactions at surfaces energy may be dissipated exciting surface electronic degrees of freedom. These excitations are detected using metal-insulator-metal (MIM) heterostructures (Ta-TaOx-Au) and the reactions of H with and on a Au surface are probed. A current corresponding to 510-5 electrons per adsorbing H atom and a marked isotope effect are observed under steady-state conditions. Analysis of the current trace when the H atom flux is intermitted suggests that predominantly the recombination reaction creates electronic excitations. Biasing the front versus the back electrode of the MIM structure provides insights into the spectrum of electronic excitations. The observed spectra differ for the two isotopes H and D and are asymmetric when comparing negative and positive bias voltages. Modeling indicates that the excited electrons and the concurrently created holes differ in their energy distributions. © 2011 American Institute of Physics.
    view abstractdoi: 10.1063/1.3523647
  • 2011 • 13 Electron transfer between genetically modified Hansenula polymorpha yeast cells and electrode surfaces via os-complex modified redox polymers
    Shkil, H. and Schulte, A. and Guschin, D.A. and Schuhmann, W.
    ChemPhysChem 12 806-813 (2011)
    Graphite electrodes modified with redox-polymer-entrapped yeast cells were investigated with respect to possible electron-transfer pathways between cytosolic redox enzymes and the electrode surface. Either wild-type or genetically modified Hansenula polymorpha yeast cells over-expressing flavocytochrome b2 (FC b2) were integrated into Os-complex modified electrodeposition polymers. Upon increasing the L-lactate concentration, an increase in the current was only detected in the case of the genetically modified cells. The overexpression of FC b2 and the related amplification of the FC b2/L-lactate reaction cycle was found to be necessary to provide sufficient charge to the electron-exchange network in order to facilitate sufficient electrochemical coupling between the cells, via the redox polymer, to the electrode. The close contact of the Os-complex modified polymer to the cell wall appeared to be a prerequisite for electrically wiring the cytosolic FC b2/L-lactate redox activity and suggests the critical involvement of a plasma membrane redox system. Insights in the functioning of whole-cell-based bioelectrochemical systems have to be considered for the successful design of whole-cell biosensors or microbial biofuel cells. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201000889
  • 2010 • 12 Controlled orientation of DNA in a binary SAM as a key for the successful determination of DNA hybridization by means of electrochemical impedance spectroscopy
    Gebala, M. and Schuhmann, W.
    ChemPhysChem 11 2887-2895 (2010)
    Determination of DNA hybridization at electrode surfaces modified with thiol-tethered single-stranded DNA (ssDNA) capture probes and co-assembled with short-chain thiol derivatives using electrochemical impedance spectroscopy requires a careful design of the electrode/electrolyte interface as well as an in-depth understanding of the processes at the interface during DNA hybridization. The influence of the electrode potential, the ssDNA coverage, the ionic strength, the nature of the thiol derivative and especially the Debye length are shown to have a significant impact on the impedance spectra. A mixed monolayer comprising-in addition to the ssDNA capture probe-both mercaptohexanol (MCH) and mercaptopropionic acid (MPA) is suggested as an interface design which allows a high efficiency of the DNA hybridization concomitantly with a reliable modulation of the charge-transfer resistance of the electrode upon hybridization. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.201000210
  • 2010 • 11 Redox electrodeposition polymers: Adaptation of the redox potential of polymer-bound Os complexes for bioanalytical applications
    Guschin, D.A. and Castillo, J. and Dimcheva, N. and Schuhmann, W.
    Analytical and Bioanalytical Chemistry 398 1661-1673 (2010)
    The design of polymers carrying suitable ligands for coordinating Os complexes in ligand exchange reactions against labile chloro ligands is a strategy for the synthesis of redox polymers with bound Os centers which exhibit a wide variation in their redox potential. This strategy is applied to polymers with an additional variation of the properties of the polymer backbone with respect to pH-dependent solubility, monomer composition, hydrophilicity etc. A library of Os-complex-modified electrodeposition polymers was synthesized and initially tested with respect to their electron-transfer ability in combination with enzymes such as glucose oxidase, cellobiose dehydrogenase, and PQQ-dependent glucose dehydrogenase entrapped during the pH-induced deposition process. The different polymer-bound Os complexes in a library containing 50 different redox polymers allowed the statistical evaluation of the impact of an individual ligand to the overall redox potential of an Os complex. Using a simple linear regression algorithm prediction of the redox potential of Os complexes becomes feasible. Thus, a redox polymer can now be designed to optimally interact in electron-transfer reactions with a selected enzyme. © 2010 Springer-Verlag.
    view abstractdoi: 10.1007/s00216-010-3982-3
  • 2010 • 10 Shearforce-based constant-distance scanning electrochemical microscopy as fabrication tool for needle-type carbon-fiber nanoelectrodes
    Hussien, E.M. and Schuhmann, W. and Schulte, A.
    Analytical Chemistry 82 5900-5905 (2010)
    Carbon fiber nanoelectrodes with nanometer radii tip curvatures were fabricated using a shearforce-based constant-distance scanning electrochemical microscope and electrochemically induced polymer deposition. A simple DC etching procedure in alkaline solution provided conically sharpened single carbon fibers with well-formed nanocones at their bottom. Coating the stems but not the end of the tips of the tapered structures with anodic electrodeposition paint was the strategy for limiting the bare carbon to the foremost end and restricting a feasible voltammetry current response to exactly this section. The electrodeposition of the polymer was prevented at the foremost end of the tip using a shearforce-based tip-to-sample distance control that allowed approaching the etched tips carefully in just touching distance to a film of a silicone elastomer. Analysis of the steady-state cyclic voltammograms in presence of a reversible redox compound revealed effective radii for the obtained needle-type carbon-fiber nanoelectrodes down to as small as 46 nm. The method offers an alternative pathway toward the fabrication of highly miniaturized carbon electrodes. © 2010 American Chemical Society.
    view abstractdoi: 10.1021/ac100738b
  • 2010 • 9 Towards a high potential biocathode based on direct bioelectrochemistry between horseradish peroxidase and hierarchically structured carbon nanotubes
    Jia, W. and Schwamborn, S. and Jin, C. and Xia, W. and Muhler, M. and Schuhmann, W. and Stoica, L.
    Physical Chemistry Chemical Physics 12 10088-10092 (2010)
    Adsorption of horseradish peroxidase (HRP) on graphite rod electrodes sequentially modified with carbon microfibers (CMF) carrying carbon nanotubes in a hierarchically structured arrangement and finally pyrene hexanoic acid (PHA) for improving hydrophilicity of the electrode surface is the basis for the direct bioelectrocatalytic reduction of H 2O 2 at potentials as high as about +600 mV. The high-potential direct bioelectrocatalytic reduction of H 2O 2 is implying a direct bioelectrochemical communication between the Fe IVO,P + redox state known as compound I. The HRP loading was optimized leading to a current of 800 μA at a potential of 300 mV. © 2010 the Owner Societies.
    view abstractdoi: 10.1039/c0cp00349b
  • 2010 • 8 Electrochemical synthesis of core-shell catalysts for electrocatalytic applications
    Kulp, C. and Chen, X. and Puschhof, A. and Schwamborn, S. and Somsen, C. and Schuhmann, W. and Bron, M.
    ChemPhysChem 11 2854-2861 (2010)
    A novel electrochemical method to prepare platinum shells around carbon-supported metal nanoparticles (Ru and Au) by pulsed electrodeposition from solutions containing Pt ions is presented. Shell formation is confirmed by characteristic changes in the cyclic voltammograms, and is further evidenced by monitoring particle growth by transmission electron microscopy as well as by energy-dispersive analysis of X rays (EDX). Scanning electrochemical microscopy and EDX measurements indicate a selective Pt deposition on the metal/carbon catalyst, but not on the glassy carbon substrate. The thus prepared carbon-supported core-shell nanoparticles are investigated with regard to their activity in electrocatalytic oxygen reduction, which demonstrates the applicability of these materials in electrocatalysis or sensors. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/cphc.200900881
  • 2010 • 7 On the action of magnetic gradient forces in micro-structured copper deposition
    Mutschke, G. and Tschulik, K. and Weier, T. and Uhlemann, M. and Bund, A. and Fröhlich, J.
    Electrochimica Acta 55 9060-9066 (2010)
    In order to shed more light on the role of magnetic gradient forces and Lorentz forces on the deposition pattern found recently at copper electrodes, experiments and numerical simulations have been performed in a simple geometry that consists of a single small cylindrical permanent magnet which is placed behind the cathode. The cylinder axis coincides with the magnetization direction and points normal to the electrode surface. The electrode is oriented vertically which allows a separate discussion of the influence of both forces. Experiments and numerical simulations are found to give very good qualitative agreement with respect to the deposition pattern. Our analysis clearly shows that the major influence is due to the action of the magnetic gradient force. Numerical simulations prove that the separate action of the Lorentz force does not reproduce the deposition structure. A detailed analytical discussion of the motion forced by the different magnetic forces in superposition with natural convection is given. © 2010 Elsevier Ltd All rights reserved.
    view abstractdoi: 10.1016/j.electacta.2010.08.046
  • 2010 • 6 An in situ tensile tester for studying electrochemical repassivation behavior: Fabrication and challenges
    Neelakantan, L. and Schönberger, B. and Eggeler, G. and Hassel, A.W.
    Review of Scientific Instruments 81 (2010)
    An in situ tensile rig is proposed, which allows performing electrochemical (repassivation) experiments during dynamic mechanical testing of wires. Utilizing the basic components of a conventional tensile tester, a custom-made minitensile rig was designed and fabricated. The maximal force that can be measured by the force sensor is 80 N, with a sensitivity of 0.5 mV/V. The maximum travel range of the crosshead induced by the motor is 10 mm with a minimum step size of 0.5 nm. The functionality of the tensile test rig was validated by investigating Cu and shape memory NiTi wires. Wires of lengths between 40 and 50 mm with varying gauge lengths can be tested. An interface between wire and electrochemical setup (noncontact) with a smart arrangement of electrodes facilitated the electrochemical measurements during tensile loading. Preliminary results on the repassivation behavior of Al wire are reported. © 2010 American Institute of Physics.
    view abstractdoi: 10.1063/1.3292685
  • 2010 • 5 Filamentary and homogeneous modes of dielectric barrier discharge (DBD) in air: Investigation through plasma characterization and simulation of surface irradiation
    Rajasekaran, P. and Mertmann, P. and Bibinov, N. and Wandke, D. and Viöl, W. and Awakowicz, P.
    Plasma Processes and Polymers 7 665-675 (2010)
    The reported dielectric barrier discharge (DBD) source comprises of a ceramic-covered copper electrode, and plasma can be ignited in ambient air with grounded Opposite' electrodes or with objects of high capacitance (e.g., human body), when breakdown conditions are satisfied. Filamentary plasma mode is observed when the same source is operated using grounded opposite electrodes like aluminium plate and phosphate buffered saline solution, and a homogeneous plasma mode when operated on glass. When the source is applied on human body, both homogeneous and filamentary discharges occur simultaneously which cannot be resolved into two separate discharges. Here, we report the characterization of filamentary and homogeneous modes of DBD plasma source using the above mentioned grounded electrodes, by applying optical emission spectroscopy, microphotography and numerical simulation. Averaged plasma parameters like electron velocity distribution function and electron density are determined. Fluxes of nitric oxide, ozone and photons reaching the treated surface are simulated. These fluxes obtained in different discharge modes namely, single-filamentary discharge (discharge ignited in same position), stochastical filamentary discharge and homogeneous discharge are compared to identify their applications in human skin treatment. It is concluded that the fluxes of photons and chemicallyactive particles in the single filamentary mode are the highest but the treated surface area is very small. For treating larger area, the homogeneous DBD is more effective than stochastical filamentary discharge. (Figure Presented) © 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstractdoi: 10.1002/ppap.200900175
  • 2010 • 4 Carbon nanotubes modified with electrodeposited metal porphyrins and phenanthrolines for electrocatalytic applications
    Schilling, T. and Okunola, A. and Masa, J. and Schuhmann, W. and Bron, M.
    Electrochimica Acta 55 7597-7602 (2010)
    Composites consisting of multi-walled carbon nanotubes (MWCNTs) and iron-nitrogen containing compounds as catalysts for the electroreduction of oxygen in acidic media were directly prepared on a glassy carbon (GC) electrode in a bottom-up synthesis. In a first step, MWCNTs were drop-coated in form of an ink onto the electrode. Afterwards the nanotubes were modified with catalytically active films of iron porphyrin (FeTMPP-Cl) or iron phenanthroline (Fe(phen)3) through a pulsed potential deposition technique. Finally the prepared electrodes were heat-treated in an inert gas atmosphere. By employing cyclic voltammetry and rotating disc electrode measurements it is shown that the activity for the oxygen reduction reaction (ORR) at such composites increases progressively with every applied synthesis step showing the possibility for direct synthesis of a catalyst on an electrode. The activities of FeTMPP-Cl/MWCNT and Fe(phen)3/MWCNT composites prepared by this technique are higher than that of similar electrocatalysts prepared by wet impregnation and heat treatment. The presented approach opens possibilities for systematic tuning of electrode structures, for example by stepwise build-up of gas diffusion electrodes. © 2009 Elsevier Ltd.
    view abstractdoi: 10.1016/j.electacta.2009.11.092
  • 2010 • 3 Scanning electrochemical microscopy in neuroscience
    Schulte, A. and Nebel, M. and Schuhmann, W.
    Annual Review of Analytical Chemistry 3 299-318 (2010)
    This article reviews recent work involving the application of scanning electrochemical microscopy (SECM) to the study of individual cultured living cells, with an emphasis on topographical and functional imaging of neuronal and secretory cells of the nervous and endocrine system. The basic principles of biological SECM and associated negative amperometric-feedback and generator/collector-mode SECM imaging are discussed, and successful use of the methodology for screening soft and fragile membranous objects is outlined. The drawbacks of the constant-height mode of probe movement and the benefits of the constant-distance mode of SECM operation are described. Finally, representative examples of constant-height and constant-distance mode SECM on a variety of live cells are highlighted to demonstrate the current status of single-cell SECM in general and of SECM in neuroscience in particular. Copyright © 2010 by Annual Reviews. All rights reserved.
    view abstractdoi: 10.1146/annurev.anchem.111808.073651
  • 2010 • 2 The correlation between the coating quality and the moving direction of the twin wire arc spraying gun
    Tillmann, W. and Vogli, E. and Abdulgader, M.
    Journal of Thermal Spray Technology 19 409-421 (2010)
    Asymmetric melting behavior of the electrodes is a process-related feature of the twin wire arc spraying (TWAS) technique since the heating of the negative wire is different from that of the positive wire. The asymmetric melting behavior, particle crossover, irregular plume shape, and last but not least the arc voltage fluctuations affect the spraying jet on the whole and lead to an inhomogeneous plume. To investigate the effect of inhomogeneous spraying plume on coating characteristics, coatings were produced by moving the spraying gun in different directions, with respect to the electrodes. The porosity, micro-cracks, hardness, thickness, and adhesion strength of the sprayed coatings were measured and brought in correlation with the gun moving direction. In this study, two different wire types were investigated in order to find out the effect of the spraying gun moving direction on the coating quality. © 2009 ASM International.
    view abstractdoi: 10.1007/s11666-009-9452-9
  • 2010 • 1 Proton transfer to charged platinum electrodes. A molecular dynamics trajectory study
    Wilhelm, F. and Schmickler, W. and Spohr, E.
    Journal of Physics Condensed Matter 22 (2010)
    A recently developed empirical valence bond(EVB) model for proton transfer on Pt(111) electrodes (Wilhelm et al 2008 J. Phys. Chem. C 112 10814) has been applied in molecular dynamics(MD) simulations of a water film in contact with a charged Pt surface. A total of seven negative surface charge densities σ between - 7.5 and - 18.9νCcm-2 were investigated. For each value of σ, between 30 and 84 initial conditions of a solvated proton within a water slab were sampled, and the trajectories were integrated until discharge of a proton occurred on the charged surfaces. We have calculated the mean rates for discharge and for adsorption of solvated protons within the adsorbed water layer in contact with the metal electrode as a function of surface charge density. For the less negative values of σ we observe a Tafel-like exponential increase of discharge rate with decreasing σ. At the more negative values this exponential increase levels off and the discharge process is apparently transport limited. Mechanistically, the Tafel regime corresponds to a stepwise proton transfer: first, a proton is transferred from the bulk into the contact water layer, which is followed by transfer of a proton to the charged surface and concomitant discharge. At the more negative surface charge densities the proton transfer into the contact water layer and the transfer of another proton to the surface and its discharge occur almost simultaneously. © 2010 IOP Publishing Ltd.
    view abstractdoi: 10.1088/0953-8984/22/17/175001