Prof. Dr. Corina Andronescu

Technical Chemistry III
University of Duisburg-Essen

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  • B-Cu-Zn Gas Diffusion Electrodes for CO2 Electroreduction to C2+ Products at High Current Densities
    Song, Y. and Junqueira, J.R.C. and Sikdar, N. and Öhl, D. and Dieckhöfer, S. and Quast, T. and Seisel, S. and Masa, J. and Andronescu, C. and Schuhmann, W.
    Angewandte Chemie - International Edition 60 (2021)
    Electroreduction of CO2 to multi-carbon products has attracted considerable attention as it provides an avenue to high-density renewable energy storage. However, the selectivity and stability under high current densities are rarely reported. Herein, B-doped Cu (B-Cu) and B-Cu-Zn gas diffusion electrodes (GDE) were developed for highly selective and stable CO2 conversion to C2+ products at industrially relevant current densities. The B-Cu GDE exhibited a high Faradaic efficiency of 79 % for C2+ products formation at a current density of −200 mA cm−2 and a potential of −0.45 V vs. RHE. The long-term stability for C2+ formation was substantially improved by incorporating an optimal amount of Zn. Operando Raman spectra confirm the retained Cu+ species under CO2 reduction conditions and the lower overpotential for *OCO formation upon incorporation of Zn, which lead to the excellent conversion of CO2 to C2+ products on B-Cu-Zn GDEs. © 2021 The Authors. Angewandte Chemie International Edition published by Wiley-VCH GmbH
    view abstract10.1002/anie.202016898
  • Electrocatalysis in confined space
    Andronescu, C. and Masa, J. and Tilley, R.D. and Gooding, J.J. and Schuhmann, W.
    Current Opinion in Electrochemistry 25 (2021)
    The complex interplay of restricted mass transport leading to local accumulation or depletion of educts, intermediates, products, counterions and co-ions influences the reactions at the active sites of electrocatalysts when electrodes are rough, three-dimensionally mesoporous or nanoporous. This influence is important with regard to activity, and even more to selectivity, of electrocatalytic reactions. The underlying principles are discussed based on the growing awareness of these considerations over recent years. © 2020 Elsevier B.V.
    view abstract10.1016/j.coelec.2020.100644
  • Is Cu instability during the CO2reduction reaction governed by the applied potential or the local CO concentration?
    Wilde, P. and O'Mara, P.B. and Junqueira, J.R.C. and Tarnev, T. and Benedetti, T.M. and Andronescu, C. and Chen, Y.-T. and Tilley, R.D. and Schuhmann, W. and Gooding, J.J.
    Chemical Science 12 (2021)
    Cu-based catalysts have shown structural instability during the electrochemical CO2reduction reaction (CO2RR). However, studies on monometallic Cu catalysts do not allow a nuanced differentiation between the contribution of the applied potential and the local concentration of CO as the reaction intermediate since both are inevitably linked. We first use bimetallic Ag-core/porous Cu-shell nanoparticles, which utilise nanoconfinement to generate high local CO concentrations at the Ag core at potentials at which the Cu shell is still inactive for the CO2RR. Usingoperandoliquid cell TEM in combination withex situTEM, we can unequivocally confirm that the local CO concentration is the main source for the Cu instability. The local CO concentration is then modulated by replacing the Ag-core with a Pd-core which further confirms the role of high local CO concentrations. Product quantification during CO2RR reveals an inherent trade-off between stability, selectivity and activity in both systems. © The Royal Society of Chemistry 2021.
    view abstract10.1039/d0sc05990k
  • Recovering activity of anodically challenged oxygen reduction electrocatalysts by means of reductive potential pulses
    Medina, D. and Löffler, T. and Morales, D.M. and Masa, J. and Bobrowski, T. and Barwe, S. and Andronescu, C. and Schuhmann, W.
    Electrochemistry Communications 124 (2021)
    The stability of electrocatalysts is of great importance to ensure their applicability, but stability is generally only considered for catalysts polarised to a constant potential or current density. This excludes stability evaluation under start/stop conditions in a fuel cell or in reversible batteries in which the catalyst is alternately polarised to high opposite potentials. For example, the poor cyclability of metal-air batteries is mainly due to the decrease in the oxygen reduction activity of electrocatalysts during the high applied potentials for the oxygen evolution reaction during battery charging. To investigate and at least partially mitigate the loss of electrocatalytic activity for the oxygen reduction reaction, we employed reductive pulses with the aim of restoring the catalytic activity of the active sites for the oxygen reduction reaction. Optimisation of the reductive pulse parameters makes it possible to substantially prolong the oxygen reduction activity of a Fe-Nx-doped carbon-based oxygen reduction electrocatalyst. © 2021 The Author(s)
    view abstract10.1016/j.elecom.2021.106960
  • Searching novel complex solid solution electrocatalysts in unconventional element combinations
    Krysiak, O.A. and Schumacher, S. and Savan, A. and Schuhmann, W. and Ludwig, A. and Andronescu, C.
    Nano Research (2021)
    Despite outstanding accomplishments in catalyst discovery, finding new, more efficient, environmentally neutral, and noble metal-free catalysts remains challenging and unsolved. Recently, complex solid solutions consisting of at least five different elements and often named as high-entropy alloys have emerged as a new class of electrocatalysts for a variety of reactions. The multicomponent combinations of elements facilitate tuning of active sites and catalytic properties. Predicting optimal catalyst composition remains difficult, making testing of a very high number of them indispensable. We present the high-throughput screening of the electrochemical activity of thin film material libraries prepared by combinatorial co-sputtering of metals which are commonly used in catalysis (Pd, Cu, Ni) combined with metals which are not commonly used in catalysis (Ti, Hf, Zr). Introducing unusual elements in the search space allows discovery of catalytic activity for hitherto unknown compositions. Material libraries with very similar composition spreads can show different activities vs. composition trends for different reactions. In order to address the inherent challenge of the huge combinatorial material space and the inability to predict active electrocatalyst compositions, we developed a high-throughput process based on co-sputtered material libraries, and performed high-throughput characterization using energy dispersive X-ray spectroscopy (EDS), scanning transmission electron microscopy (SEM), X-ray diffraction (XRD) and conductivity measurements followed by electrochemical screening by means of a scanning droplet cell. The results show surprising material compositions with increased activity for the oxygen reduction reaction and the hydrogen evolution reaction. Such data are important input data for future data-driven materials prediction. [Figure not available: see fulltext.] © 2021, The Author(s).
    view abstract10.1007/s12274-021-3637-z
  • The Effect of Interlayer Anion Grafting on Water Oxidation Electrocatalysis: A Comparative Study of Ni- and Co-Based Brucite-Type Layered Hydroxides, Layered Double Hydroxides and Hydroxynitrate Salts
    Radha, A.V. and Weiß, S. and Sanjuán, I. and Ertl, M. and Andronescu, C. and Breu, J.
    Chemistry - A European Journal (2021)
    The urge for carbon-neutral green energy conversion and storage technologies has invoked the resurgence of interest in applying brucite-type materials as low-cost oxygen evolution reaction (OER) electrocatalysts in basic media. Transition metal layered hydroxides belonging to the brucite-type structure family have been shown to display remarkable electrochemical activity. Recent studies on the earth-abundant Fe3+ containing mössbauerite and Fe3+ rich Co−Fe layered oxyhydroxide carbonates have suggested that grafted interlayer anions might play a key role in OER catalysis. To probe the effect of such interlayer anion grafting in brucite-like layered hydroxides, we report here a systematic study on the electrocatalytic performance of three distinct Ni and Co brucite-type layered structures, namely, (i) brucite-type M(OH)2 without any interlayer anions, (ii) LDHs with free interlayer anions, and (iii) hydroxynitrate salts with grafted interlayer anions. The electrochemical results indeed show that grafting has an evident impact on the electronic structure and the observed OER activity. Ni- and Co-hydroxynitrate salts with grafted anions display notably earlier formations of the electrocatalytically active species. Particularly Co-hydroxynitrate salts exhibit lower overpotentials at 10 mA cm−2 (η=0.34 V) and medium current densities of 100 mA cm−2 (η=0.40 V) compared to the corresponding brucite-type hydroxides and LDH materials. © 2021 The Authors. Chemistry - A European Journal published by Wiley-VCH GmbH
    view abstract10.1002/chem.202100452
  • A Universal Nano-capillary Based Method of Catalyst Immobilization for Liquid-Cell Transmission Electron Microscopy
    Tarnev, T. and Cychy, S. and Andronescu, C. and Muhler, M. and Schuhmann, W. and Chen, Y.-T.
    Angewandte Chemie - International Edition 59 (2020)
    A universal nano-capillary based method for sample deposition on the silicon nitride membrane of liquid-cell transmission electron microscopy (LCTEM) chips is demonstrated. It is applicable to all substances which can be dispersed in a solvent and are suitable for drop casting, including catalysts, biological samples, and polymers. Most importantly, this method overcomes limitations concerning sample immobilization due to the fragility of the ultra-thin silicon nitride membrane required for electron transmission. Thus, a straightforward way is presented to widen the research area of LCTEM to encompass any sample which can be externally deposited beforehand. Using this method, NixB nanoparticles are deposited on the μm-scale working electrode of the LCTEM chip and in situ observation of single catalyst particles during ethanol oxidation is for the first time successfully monitored by means of TEM movies. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201916419
  • Differentiation between Carbon Corrosion and Oxygen Evolution Catalyzed by NixB/C Hybrid Electrocatalysts in Alkaline Solution using Differential Electrochemical Mass Spectrometry
    Möller, S. and Barwe, S. and Dieckhöfer, S. and Masa, J. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 7 (2020)
    Carbon is a frequently used electrode material and an important additive in catalyst films. Its corrosion is often reported during electrocatalysis at high anodic potentials, especially in acidic electrolyte. Investigation of the carbon corrosion in alkaline environment is difficult due to the CO2/CO32− equilibrium. We report the on-line determination of electrolysis products generated on NixB/C hybrid electrocatalysts in alkaline electrolyte at anodic potentials using differential electrochemical mass spectrometry (DEMS). NixB/C catalyst films were obtained from mixtures containing different ratios of NiXB and benzoxazine monomers followed by polymerization and pyrolysis. The impact of the composition of the electrocatalyst on the dominant electrolysis process allows to distinguish between the oxygen evolution reaction and carbon corrosion using DEMS results as well as the catalyst surface composition evaluated from X-ray photoelectron spectra. At the imposed highly oxidative conditions, an increasing amount of NixB in the electrocatalyst leads to a suppression of carbon corrosion. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.202000697
  • Electrocatalysis as the Nexus for Sustainable Renewable Energy: The Gordian Knot of Activity, Stability, and Selectivity
    Masa, J. and Andronescu, C. and Schuhmann, W.
    Angewandte Chemie - International Edition 59 (2020)
    The use of renewable energy by means of electrochemical techniques by converting H2O, CO2 and N2 into chemical energy sources and raw materials, is the basis for securing a future sustainable “green” energy supply. Some weaknesses and inconsistencies in the practice of determining the electrocatalytic performance, which prevents a rational bottom-up catalyst design, are discussed. Large discrepancies in material properties as well as in electrocatalytic activity and stability become obvious when materials are tested under the conditions of their intended use as opposed to the usual laboratory conditions. They advocate for uniform activity/stability correlations under application-relevant conditions, and the need for a clear representation of electrocatalytic performance by contextualization in terms of functional investigation or progress towards application is emphasized. © 2020 The Authors. Published by Wiley-VCH GmbH
    view abstract10.1002/anie.202007672
  • Insights into the Formation, Chemical Stability, and Activity of Transient NiyP@NiO x Core-Shell Heterostructures for the Oxygen Evolution Reaction
    Wilde, P. and Dieckhöfer, S. and Quast, T. and Xiang, W. and Bhatt, A. and Chen, Y.-T. and Seisel, S. and Barwe, S. and Andronescu, C. and Li, T. and Schuhmann, W. and Masa, J.
    ACS Applied Energy Materials 3 (2020)
    NiyP emerged as a highly active precatalyst for the alkaline oxygen evolution reaction where structural changes play a crucial role for its catalytic performance. We probed the chemical stability of NiyP in 1 M KOH at 80 °C and examined how exposure up to 168 h affects its structure and catalytic performance. We observed selective P-leaching and formation of NiyP/NiOx core-shell heterostructures, where shell thickness increases with ageing time, which is detrimental for the activity. By tuning the particle size, we demonstrate that prevention of complete catalyst oxidation is essential to preserve the outstanding electrochemical performance of NiyP in alkaline media. © 2020 American Chemical Society.
    view abstract10.1021/acsaem.9b02481
  • Needle-type organic electrochemical transistor for spatially resolved detection of dopamine
    Mariani, F. and Quast, T. and Andronescu, C. and Gualandi, I. and Fraboni, B. and Tonelli, D. and Scavetta, E. and Schuhmann, W.
    Microchimica Acta 187 (2020)
    In this work, the advantages of carbon nanoelectrodes (CNEs) and orgonic electrochemical transistors (OECTs) were merged to realise nanometre-sized, spearhead OECTs based on single- and double-barrel CNEs functionalised with a conducting polymer film. The needle-type OECT shows a high aspect ratio that allows its precise positioning by means of a macroscopic handle and its size is compatible with single-cell analysis. The device was characterised with respect to its electrolyte-gated behaviour and was employed as electrochemical sensor for the proof-of-concept detection of dopamine (DA) over a wide concentration range (10−12—10−6 M). Upon application of fixed drain and gate voltages (Vd = − 0.3 V, Vg = − 0.9 V, respectively), the nano-sized needle-type OECT sensor exhibited a linear response in the low pM range and from 0.002 to 7 μM DA, with a detection limit of 1 × 10−12 M. [Figure not available: see fulltext.]. © 2020, The Author(s).
    view abstract10.1007/s00604-020-04352-1
  • 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 (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 abstract10.1002/cssc.202000339
  • Reduced-graphene-oxide-based needle-type field-effect transistor for dopamine sensing
    Quast, T. and Mariani, F. and Scavetta, E. and Schuhmann, W. and Andronescu, C.
    ChemElectroChem 7 (2020)
    Owing to their intrinsic amplifying effect together with their temporal resolution, field-effect transistors (FETs) are gaining momentum for the detection of different biomolecules at ultralow concentration levels such as, for example, neurotransmitters, particularly if the concentration level of the analyte is below the detection limit of commonly used electrochemical sensing methods. We demonstrate the fabrication of a spearhead reduced graphene oxide (rGO)-based FET. The fabrication of the rGO-based FET by means of an electrochemical pulse deposition technique enables a controllable process including both the deposition and reduction of the deposited graphene oxide between two carbon nanoelectrodes to form the channel of the rGO-based FET. While using double-barrel carbon nanoelectrodes, the as-produced FETs offer new possibilities in terms of their applicability in very small volumes as well as the option of being positioned close to the desired measurement region. The fabrication process was evaluated and optimized to obtain rGO-based FETs with high performance. The as-fabricated devices were evaluated in terms of sensitivity and selectivity towards dopamine. The tested devices not only showed high sensitivity towards dopamine with a linear response ranging from 1nM to 1 μM, but also maintained a similar sensing performance in the presence of 500 μM ascorbic acid. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.202000162
  • Spray-Flame-Prepared LaCo1–xFexO3 Perovskite Nanoparticles as Active OER Catalysts: Influence of Fe Content and Low-Temperature Heating
    Alkan, B. and Medina, D. and Landers, J. and Heidelmann, M. and Hagemann, U. and Salamon, S. and Andronescu, C. and Wende, H. and Schulz, C. and Schuhmann, W. and Wiggers, H.
    ChemElectroChem 7 (2020)
    Spray-flame synthesis was used to produce high-surface-area perovskite electrocatalysts with high phase purity, minimum surface contamination, and high electrochemical stability. In this study, as-prepared LaCo1–xFexO3 perovskite nanoparticles (x=0.2, 0.3, and 0.4) were found to contain a high degree of combustion residuals, and mostly consist of both, stoichiometric and oxygen-deficient perovskite phases. Heating them at moderate temperature (250 °C) in oxygen could remove combustion residuals and increases the content of stoichiometric perovskite while preventing particle growth. A higher surface crystallinity was observed with increasing iron content coming along with a rise in oxygen deficient phases. With heat treatment, OER activity and stability of perovskites improved at 30 and 40 at.% Fe while deteriorating at 20 at.% Fe. This study highlights spray-flame synthesis as a promising technique to synthesize highly active nanoscale perovskite catalysts with improved OER activity. © 2020 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/celc.201902051
  • Trivalent iron rich CoFe layered oxyhydroxides for electrochemical water oxidation
    Weiß, S. and Ertl, M. and Varhade, S.D. and Radha, A.V. and Schuhmann, W. and Breu, J. and Andronescu, C.
    Electrochimica Acta 350 (2020)
    Layered double hydroxides (LDHs) are presently among the best-performing oxygen evolution reaction (OER) electrocatalysts in alkaline media. The high activity of LDHs is due to synergistic effects between two transition metals as well as the layered structure which facilitates electron transfer. Because of a perfect match with the size of interlayer carbonate a ratio of 2:1 for the di- and tri-valent octahedral cations is energetically preferred. Here we present a strategy, where first mixed valent (Co2+ 1-zFe2+ z)4 Fe3+ 2 - LDHs, with z values between 0 and 0.75 are synthesized, which are subsequently oxidized to Co2+Fe3+ LDH-type layered (oxy)hydroxides with an unusual high trivalent Fe content. Characterization of the chemically oxidized materials using bulk and surface techniques demonstrated the successful synthesis of LDH-like trivalent iron rich (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides with a final Fe content ranging from 33.3 to 83.3%. Current densities of up to 200 mA cm−2 were obtained at potentials lower than 1.7 V vs. RHE for (Co2+)4-4z (Fe3+)2+4z (oxy)hydroxides containing a maximum of 80% Fe. © 2020 Elsevier Ltd
    view abstract10.1016/j.electacta.2020.136256
  • A sensitive and selective graphene/cobalt tetrasulfonated phthalocyanine sensor for detection of dopamine
    Diab, N. and Morales, D.M. and Andronescu, C. and Masoud, M. and Schuhmann, W.
    Sensors and Actuators, B: Chemical 285 (2019)
    Electrochemical dopamine-sensing surfaces were fabricated by deposition of graphene sheets modified with cobalt tetrasulfonated phthalocyanine on glassy carbon electrodes (CoTSPc/Gr-GC) using differential pulse amperometry. Differential pulse stripping voltammetry was used to detect dopamine (DA) and the influence of pH value, scan rate, accumulation potential and time as well as dopamine concentration on the performance of CoTSPc/Gr-GC electrodes was investigated. The modified electrodes were successfully used as sensors for the selective and high sensitive determination of DA in presence of high concentrations of ascorbic acid (AA) and uric acid (UA) with a detection limit of 0.87 nM over the dynamic linear range of 20 nM to 220 nM. © 2019 Elsevier B.V.
    view abstract10.1016/j.snb.2019.01.022
  • Cascade Reactions in Nanozymes: Spatially Separated Active Sites inside Ag-Core-Porous-Cu-Shell Nanoparticles for Multistep Carbon Dioxide Reduction to Higher Organic Molecules
    O'Mara, P.B. and Wilde, P. and Benedetti, T.M. and Andronescu, C. and Cheong, S. and Gooding, J.J. and Tilley, R.D. and Schuhmann, W.
    Journal of the American Chemical Society 141 (2019)
    Enzymes can perform complex multistep cascade reactions by linking multiple distinct catalytic sites via substrate channeling. We mimic this feature in a generalized approach with an electrocatalytic nanoparticle for the carbon dioxide reduction reaction comprising a Ag core surrounded by a porous Cu shell, providing different active sites in nanoconfined volumes. The architecture of the nanozyme provides the basis for a cascade reaction, which promotes C-C coupling reactions. The first step occurs on the Ag core, and the subsequent steps on the porous copper shell, where a sufficiently high CO concentration due to the nanoconfinement facilitates C-C bond formation. The architecture yields the formation of n-propanol and propionaldehyde at potentials as low as-0.6 V vs RHE. Copyright © 2019 American Chemical Society.
    view abstract10.1021/jacs.9b07310
  • Catalytic Reactivation of Industrial Oxygen Depolarized Cathodes by in situ Generation of Atomic Hydrogen
    Öhl, D. and Franzen, D. and Paulisch, M. and Dieckhöfer, S. and Barwe, S. and Andronescu, C. and Manke, I. and Turek, T. and Schuhmann, W.
    ChemSusChem 12 (2019)
    Electrocatalytically active materials on the industrial as well as on the laboratory scale may suffer from chemical instability during operation, air exposure, or storage in the electrolyte. A strategy to recover the loss of electrocatalytic activity is presented. Oxygen-depolarized cathodes (ODC), analogous to those that are utilized in industrial brine electrolysis, are analyzed: the catalytic activity of the electrodes upon storage (4 weeks) under industrial process conditions (30 wt % NaOH, without operation) diminishes. This phenomenon occurs as a consequence of surface oxidation and pore blockage, as revealed by scanning electron microscopy, focused ion beam milling, X-ray photoelectron spectroscopy, and Raman spectroscopy. Potentiodynamic cycling of the oxidized electrodes to highly reductive potentials and the formation of “nascent” hydrogen re-reduces the electrode material, ultimately recovering the former catalytic activity. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201900628
  • Cellulose acetate/layered double hydroxide adsorptive membranes for efficient removal of pharmaceutical environmental contaminants
    Raicopol, M.D. and Andronescu, C. and Voicu, S.I. and Vasile, E. and Pandele, A.M.
    Carbohydrate Polymers 214 (2019)
    The increasing amount of residual pharmaceutical contaminants in wastewater has a negative impact on both the environment and human health. In the present study, we developed new cellulose acetate/Mg-Al layered double hydroxide (Mg-Al LDH) nanocomposite membranes as an efficient method to remove pharmaceutical substances from wastewater. The morphology, porosity, surface properties and thermal stability of nanocomposite membranes containing various amounts of nanofiller were evaluated by scanning electron microscopy (SEM), X-ray microtomography (μCT), contact angle measurements and thermogravimetric analysis (TGA). The Mg-Al LDH nanofiller showed a high degree of exfoliation in the polymer matrix, evidenced by X-ray diffraction (XRD) and transmission electron microscopy (TEM). The hydrodynamic properties and adsorption capacity were evaluated with pure water and aqueous solutions of two common drugs, diclofenac sodium (DS) and tetracycline (TC), and the nanocomposite membranes showed an improved permeability compared with neat cellulose acetate. The membrane prepared with 4 wt.% Mg-Al LDH loading exhibited the highest water flux compared with the pure polymer one (529 vs 36 L·m −2 ·h -1 ) and a tenfold increase in adsorption capacity for DS. This enhancement is attributed to electrostatic interactions between the negatively charged drug molecule and positively charged Mg-Al LDH layers. Conversely, in the case of TC, the increase in adsorption capacity was smaller and was assigned to hydrogen bonding interactions between the drug molecule and the nanofiller. © 2019 Elsevier Ltd
    view abstract10.1016/j.carbpol.2019.03.042
  • 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 (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 abstract10.1016/j.electacta.2018.12.047
  • Enhancing Electrocatalytic Activity through Liquid-Phase Exfoliation of NiFe Layered Double Hydroxide Intercalated with Metal Phthalocyanines in the Presence of Graphene
    Morales, D.M. and Barwe, S. and Vasile, E. and Andronescu, C. and Schuhmann, W.
    ChemPhysChem 20 (2019)
    Earth-abundant transition-metal-based catalysts are attractive for alkaline water electrolysis. However, their catalytic properties are often limited by their poor electrical conductivity. Here, we present a strategy for enhancing the electrical conductivity of NiFe layered double hydroxide (LDH) in order to further improve its properties as an electrocatalyst for the oxygen evolution reaction (OER) in alkaline media. We show that NiFe LDH containing metal tetrasulfonate phthalocyanine in the interlayers between the NiFe oxide galleries can be coupled with graphene during liquid-phase exfoliation by taking advantage of their π-π stacking capabilities. A substantial enhancement in the electrocatalytic activity of NiFe LDH with respect to the OER was observed. Moreover, the activity and selectivity of the catalyst materials towards the oxygen reduction reaction were investigated, demonstrating that both the metal hydroxide layer and the interlayer species contribute to the electrocatalytic performance of the composite material. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cphc.201900577
  • Fe/Co/Ni mixed oxide nanoparticles supported on oxidized multi-walled carbon nanotubes as electrocatalysts for the oxygen reduction and the oxygen evolution reactions in alkaline media
    Kazakova, M.A. and Morales, D.M. and Andronescu, C. and Elumeeva, K. and Selyutin, A.G. and Ishchenko, A.V. and Golubtsov, G.V. and Dieckhöfer, S. and Schuhmann, W. and Masa, J.
    Catalysis Today (2019)
    Fabrication of efficient and cost-effective bifunctional oxygen electrocatalysts for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) remains a challenge for the development of rechargeable metal-air batteries and unitized regenerative fuel cells technologies. Herein, we report high-performance bifunctional ORR/OER electrocatalysts consisting of mixed transition metal (Fe, Co, Ni) oxide nanoparticles supported on oxidized multi-walled carbon nanotubes (MWCNT). Investigation of the ORR and OER activity of samples with different metal compositions showed that trimetallic/MWCNT composites having Fe:Ni:Co = x:x:(1-2x) ratios, with 0.25 ≤ x ≤ 0.4, exhibit highest bifunctional activity in terms of the reversible ORR/OER overvoltage at a given current density. Moreover, the trimetallic catalysts exhibited improved selectivity with respect to the reduction of O 2 to OH − compared to the bimetallic Fe-Ni, Fe-Co and Co-Ni catalysts, thus revealing synergistic interactions among the metal oxide components. Correlation of the electrocatalytic activity with the structure of the composites is discussed for the most representative cases. © 2019 Elsevier B.V.
    view abstract10.1016/j.cattod.2019.02.047
  • Hema-Functionalized Graphene Oxide: a Versatile Nanofiller for Poly(Propylene Fumarate)-Based Hybrid Materials
    Vasile, E. and Pandele, A.M. and Andronescu, C. and Selaru, A. and Dinescu, S. and Costache, M. and Hanganu, A. and Raicopol, M.D. and Teodorescu, M.
    Scientific Reports 9 (2019)
    Poly(propylene fumarate) (PPF) is a linear unsaturated polyester which has been widely investigated for tissue engineering due to its good biocompatibility and biodegradability. In order to extend the range of possible applications and enhance its mechanical properties, current approaches consist in the incorporation of various fillers or obtaining blends with other polymers. In the current study we designed a reinforcing agent based on carboxylated graphene oxide (GO-COOH) grafted with 2-hydroxyethyl methacrylate (GO@HEMA) for poly(propylene fumarate)/poly(ethylene glycol) dimethacrylate (PPF/PEGDMA), in order to enhance the nanofiller adhesion and compatibility with the polymer matrix, and in the same time to increase the crosslinking density. The covalent modification of GO-COOH was proved by Fourier-transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), thermogravimetric analysis (TGA) and Raman spectroscopy. The mechanical properties, water uptake capacity, morphology, biodegradability, mineralization and in vitro cytotoxicity of PPF/PEGDMA hybrid materials containing GO@HEMA were investigated. A 14-fold increase of the compressive modulus and a 2-fold improvement in compressive strength were observed after introduction of the nanofiller. Moreover, the decrease in sol fraction and solvent swelling in case of the hybrid materials containing GO@HEMA suggests an increase of the crosslinking density. SEM images illustrate an exfoliated structure at lower nanofiller content and a tendency for agglomeration at higher concentrations. Finally, the synthesized hybrid materials proved non-cytotoxic to murine pre-osteoblast cells and induced the formation of hydroxyapatite crystals under mineralization conditions. © 2019, The Author(s).
    view abstract10.1038/s41598-019-55081-2
  • MOFs for Electrocatalysis: From Serendipity to Design Strategies
    Aiyappa, H.B. and Masa, J. and Andronescu, C. and Muhler, M. and Fischer, R.A. and Schuhmann, W.
    Small Methods 3 (2019)
    The rapid upsurge of metal–organic frameworks (MOFs) as well as MOF-derived materials has stimulated profound interest to capitalize on their many potential untapped benefits in electrocatalysis for energy applications. The possibility of tuning the metal–ligand junctions of the MOF architecture opens new avenues to design robust, extended heterostructures for addressing the present-day energy challenges. Interestingly, despite having detailed crystallographic information, it is often difficult to envisage the interplay of charge transport (electrons and ions), mass transport (pore system) together with the specific effects of the molecularly defined reaction center of MOFs for a given electrocatalytic reaction. Here, guidelines are offered for judiciously engineering the electronic structure of MOFs to deliver targeted electrocatalytic function. Some of the pivotal works on MOF-based materials for electrocatalysis are discussed, which can be correlated to the biological models in terms of their structural resemblance and an instructive insight is provided about the “new chemistry” that can be explored based on the lessons learned from nature in combination with the theoretical understanding of the energetics of the reactions. © 2018 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/smtd.201800415
  • Nitrogen-Doped Metal-Free Carbon Materials Derived from Cellulose as Electrocatalysts for the Oxygen Reduction Reaction
    Wütscher, A. and Eckhard, T. and Hiltrop, D. and Lotz, K. and Schuhmann, W. and Andronescu, C. and Muhler, M.
    ChemElectroChem 6 (2019)
    Development of metal-free carbon-based electrocatalysts for reducing oxygen to water (ORR), preferentially following a 4 electron transfer pathway, is of high importance. We present a two-step synthesis of N-doped carbon-based ORR electrocatalysts by using an efficient thermal treatment of hydrothermally carbonized cellulose in ammonia combining devolatilization, reduction and nitrogen doping. The influence of the synthesis temperature as well as of the ammonia concentration used during the synthesis on the electrocatalytic ORR activity was analyzed using bulk- and surface-sensitive techniques. Correlation of electrocatalytic activity with structural features of the catalysts provided deeper mechanistic understanding and enabled us to optimize the synthesis conditions. The nitrogen-doped metal-free catalyst originating from the treatment in 100 % NH3 at 800 °C achieved a current density of −1 mA cm−2 at 0.83 V vs. RHE positioning it among the most active noble-metal free and biomass-based ORR catalysts reported so far. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201801217
  • On the Theory of Electrolytic Dissociation, the Greenhouse Effect, and Activation Energy in (Electro)Catalysis: A Tribute to Svante Augustus Arrhenius
    Masa, J. and Barwe, S. and Andronescu, C. and Schuhmann, W.
    Chemistry - A European Journal 25 (2019)
    Svante Augustus Arrhenius (1859, Vik - 1927, Stockholm) received the Nobel Prize for Chemistry in 1903 “in recognition of the extraordinary services he rendered to the advancement of chemistry by his electrolytic theory of dissociation”. Arrhenius was a physicist, and he received his PhD from the University of Uppsala, where he later became a professor for phyiscal chemistry, the first in the country for this subject. He was offered several positions as professor abroad, but decided to remain in Sweden and to build a Nobel Institute for physical chemistry using the Nobel funds. He remained director of the Institute until his death. There are powerful lessons to take from Svante August Arrhenius’ journey leading to a Nobel laureate as there are from his tremendous contributions to chemistry and science in general, including climate science, immunochemistry and cosmology. The theory of electrolytic dissociation for which Arrhenius received the 1903 Nobel Prize in Chemistry has had a profound impact on our understanding of the chemistry of solutions, chemical reactivity, mechanisms underlying chemical transformations as well as physiological processes. As a tribute to Arrhenius, we present a brief historical perspective and present status of the theory of electrolytic dissociation, its relevance and role to the development of electrochemistry, as well as some perspectives on the possible role of the theory to future advancements in electroanalysis, electrocatalysis and electrochemical energy storage. The review briefly highlights Arrhenius’ contribution to climate science owing to his studies on the potential effects of increased anthropogenic CO2 emissions on the global climate. These studies were far ahead of their time and revealed a daunting global dilemma, global warming, that we are faced with today. Efforts to abate or reverse CO2 accumulation constitute one of the most pressing scientific problems of our time, “man's urgent strive to save self from the adverse effects of his self-orchestrated change on the climate”. Finally, we review the application of the Arrhenius equation that correlates reaction rate constants (k) and temperature (T); (Formula presented.), in determining reaction barriers in catalysis with a particular focus on recent modifications of the equation to account for reactions exhibiting non-linear Arrhenius behavior with concave curvature due to prevalence of quantum mechanical tunneling, as well as infrequent convexity of Arrhenius plots due to decrease of the microcanonical rate coefficient with energy as observed for some enzyme catalyzed reactions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201805264
  • Operando Thin-Layer ATR-FTIR Spectroelectrochemical Radial Flow Cell with Tilt Correction and Borehole Electrode
    Cychy, S. and Hiltrop, D. and Andronescu, C. and Muhler, M. and Schuhmann, W.
    Analytical Chemistry (2019)
    A novel spectroelectrochemical ATR-FTIR thin-film cell was designed and applied to elucidate the intermediates during electrocatalytic alcohol oxidation. In the novel cell design, the working electrode is positioned coplanar above the internal reflection element (IRE) to ensure uniform electrolyte film thickness at reaction conditions. The depletion of the reactant (i.e., ethanol or ethylene glycol in the case of electrocatalytic alcohol oxidation) is decreased by a specifically designed flow-through glassy carbon borehole electrode embedded in PEEK. The electrolyte can be pumped through the disk-shaped gap between the ring working electrode and the IRE into the borehole via an external peristaltic pump. To ensure a radially uniform electrolyte flow, the working electrode and the internal reflection element need to be aligned in parallel at a well-controlled distance, which was achieved by a three-microelectrode-assisted tilt correction. Tilt correction of this four-electrode ensemble and the IRE was performed by three step-motor-driven micrometer screws that allow adjustment of the electrode orientation. The effect of electrolyte pumping through the borehole electrode was analyzed by performing anodic ethanol oxidation using nickel boride as electrocatalyst. The applicability, reliability, and functionality of the cell was further assessed by oxidizing ethylene glycol and determining the reaction products as a function of the electrolyte flow rate. It is found to be essential to induce forced electrolyte convection into the thin electrolyte layer to achieve well-defined steady-state conditions, as mass transport by diffusion is by far insufficient, resulting in reactant depletion, product accumulation, and local pH changes. © 2019 American Chemical Society.
    view abstract10.1021/acs.analchem.9b02734
  • Optimizing the synthesis of Co/Co–Fe nanoparticles/N-doped carbon composite materials as bifunctional oxygen electrocatalysts
    Medina, D. and Barwe, S. and Masa, J. and Seisel, S. and Schuhmann, W. and Andronescu, C.
    Electrochimica Acta 318 (2019)
    A future widespread application of electrochemical energy conversion and storage technologies strongly depends on the substitution of precious metal-based electrocatalysts for the high-overpotential oxygen reduction and oxygen evolution reactions. We report a novel Co/Co–Fe nanoparticles/N-doped carbon composite electrocatalyst (Co/CoxFey/NC) obtained by pyrolysis of CoFe layered double hydroxide (CoFe LDH) embedded in a film of a bisphenol A and tetraethylenepentamine-based polybenzoxazine poly(BA-tepa). During pyrolysis poly(BA-tepa) forms a highly conductive nitrogen-doped carbon matrix encapsulating Co/Co–Fe nanoparticles, thereby circumventing the need of any additional binder material and conductive additives. Optimization with respect to pyrolysis temperature, the CoFe LDH/BA-tepa ratio, as well as of the gas atmosphere used during the thermal treatment was performed. The optimized Co/CoxFey/NC composite material catalyst exhibits remarkable bifunctional activity towards oxygen reduction (ORR) and oxygen evolution (OER) reactions in 0.1 M KOH represented by a potential difference of only 0.77 V between the potentials at which current densities of −1 mA cm−2 for the ORR and 10 mA cm−2 for the OER were recorded. Moreover, the Co/CoxFey/NC composite material pyrolyzed in ammonia atmosphere exhibits promising stability during both the ORR and the OER. © 2019 Elsevier Ltd
    view abstract10.1016/j.electacta.2019.06.048
  • Oxygen Evolution Electrocatalysis of a Single MOF-Derived Composite Nanoparticle on the Tip of a Nanoelectrode
    Aiyappa, H.B. and Wilde, P. and Quast, T. and Masa, J. and Andronescu, C. and Chen, Y.-T. and Muhler, M. and Fischer, R.A. and Schuhmann, W.
    Angewandte Chemie - International Edition 58 (2019)
    Determination of the intrinsic electrocatalytic activity of nanomaterials by means of macroelectrode techniques is compromised by ensemble and film effects. Here, a unique “particle on a stick” approach is used to grow a single metal–organic framework (MOF; ZIF-67) nanoparticle on a nanoelectrode surface which is pyrolyzed to generate a cobalt/nitrogen-doped carbon (CoN/C) composite nanoparticle that exhibits very high catalytic activity towards the oxygen evolution reaction (OER) with a current density of up to 230 mA cm−2 at 1.77 V (vs. RHE), and a high turnover frequency (TOF) of 29.7 s−1 at 540 mV overpotential. Identical location transmission electron microscopy (IL-TEM) analysis substantiates the “self-sacrificial” template nature of the MOF, while post-electrocatalysis studies reveal agglomeration of Co centers within the CoN/C composite during the OER. “Single-entity” electrochemical analysis allows for deriving the intrinsic electrocatalytic activity and furnishes insight into the transient behavior of the electrocatalyst under reaction conditions. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201903283
  • Role of Boron and Phosphorus in Enhanced Electrocatalytic Oxygen Evolution by Nickel Borides and Nickel Phosphides
    Masa, J. and Andronescu, C. and Antoni, H. and Sinev, I. and Seisel, S. and Elumeeva, K. and Barwe, S. and Marti-Sanchez, S. and Arbiol, J. and Roldan Cuenya, B. and Muhler, M. and Schuhmann, W.
    ChemElectroChem 6 (2019)
    The modification of nickel with boron or phosphorus leads to significant enhancement of its electrocatalytic activity for the oxygen evolution reaction (OER). However, the precise role of the guest elements, B and P, in enhancing the OER of the host element (Ni) remains unclear. Herein, we present insight into the role of B and P in enhancing electrocatalysis of oxygen evolution by nickel borides and nickel phosphides. The apparent activation energy, Ea*, of electrocatalytic oxygen evolution on Ni2P was 78.4 kJ/mol, on Ni2B 65.4 kJ/mol, and on Ni nanoparticles 94.0 kJ/mol, thus revealing that both B and P affect the intrinsic activity of nickel. XPS data revealed shifts of −0.30 and 0.40 eV in the binding energy of the Ni 2p3/2 peak of Ni2B and Ni2P, respectively, with respect to that of pure Ni at 852.60 eV, thus indicating that B and P induce opposite electronic effects on the surface electronic structure of Ni. The origin of enhanced activity for oxygen evolution cannot, therefore, be attributed to such electronic modification or ligand effect. Severe changes induced on the nickel lattice, specifically, the Ni-Ni atomic order and interatomic distances (strain effect), by the presence of the guest atoms seem to be the dominant factors responsible for enhanced activity of oxygen evolution in nickel borides and nickel phosphides. © 2019 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800669
  • Scanning Electrochemical Cell Microscopy Investigation of Single ZIF-Derived Nanocomposite Particles as Electrocatalysts for Oxygen Evolution in Alkaline Media
    Tarnev, T. and Aiyappa, H.B. and Botz, A. and Erichsen, T. and Ernst, A. and Andronescu, C. and Schuhmann, W.
    Angewandte Chemie - International Edition 58 (2019)
    “Single entity” measurements are central for an improved understanding of the function of nanoparticle-based electrocatalysts without interference arising from mass transfer limitations and local changes of educt concentration or the pH value. We report a scanning electrochemical cell microscopy (SECCM) investigation of zeolitic imidazolate framework (ZIF-67)-derived Co−N-doped C composite particles with respect to the oxygen evolution reaction (OER). Surmounting the surface wetting issues as well as the potential drift through the use of a non-interfering Os complex as free-diffusing internal redox potential standard, SECCM could be successfully applied in alkaline media. SECCM mapping reveals activity differences relative to the number of particles in the wetted area of the droplet landing zone. The turnover frequency (TOF) is 0.25 to 1.5 s−1 at potentials between 1.7 and 1.8 V vs. RHE, respectively, based on the number of Co atoms in each particle. Consistent values at locations with varying number of particles demonstrates OER performance devoid of macroscopic film effects. © 2019 The Authors. Published by Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/anie.201908021
  • 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 (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 abstract10.1002/celc.201900168
  • Time-resolved ATP measurements during vesicle respiration
    Lin, J. and Weixler, D. and Daboss, S. and Seibold, G.M. and Andronescu, C. and Schuhmann, W. and Kranz, C.
    Talanta 205 (2019)
    In vitro synthesis of ATP catalyzed by the ATP-synthase requires membrane vesicles, in which the ATP-synthase is present within the bilayer membrane. Inverted vesicle prepared from Gram negative cells (e.g., Escherichia coli or Pseudomonas putida) can be readily obtained and used for in vitro ATP-synthesis. Up to now, quantification of ATP synthesized by membrane vesicles has been mostly analyzed via bioluminescence-based assays. Alternatively, vesicle respiration and the associated ATP level can be determined using biosensors, which not only provide high selectivity, but allow ATP measurements without the sample being illuminated. Here, we present a microbiosensor for ATP in combination with scanning electrochemical microscopy (SECM) using an innovative two-compartment electrochemical cell for the determination of ATP levels at E.coli or P. putida inverted vesicles. For a protein concentration of 22 mg/ml, a total amount of 0.29 ± 0.03 μM/μl ATP per vesicle was determined in case of E.coli; in turn, P. putida derived vesicles yielded 0.48 ± 0.02 μM/μl ATP per vesicle at a total protein concentration of 25.2 mg/ml. Inhibition experiments with Venturicidin A clearly revealed that the respiratory chain enzyme complex responsible for ATP generation is effectively involved. © 2019 Elsevier B.V.
    view abstract10.1016/j.talanta.2019.06.083
  • Electrocatalytic Nanoparticles That Mimic the Three-Dimensional Geometric Architecture of Enzymes: Nanozymes
    Benedetti, T.M. and Andronescu, C. and Cheong, S. and Wilde, P. and Wordsworth, J. and Kientz, M. and Tilley, R.D. and Schuhmann, W. and Gooding, J.J.
    Journal of the American Chemical Society 140 (2018)
    Enzymes are characterized by an active site that is typically embedded deeply within the protein shell thus creating a nanoconfined reaction volume in which high turnover rates occur. We propose nanoparticles with etched substrate channels as a simplified enzyme mimic, denominated nanozymes, for electrocatalysis. We demonstrate increased electrocatalytic activity for the oxygen reduction reaction using PtNi nanoparticles with isolated substrate channels. The PtNi nanoparticles comprise an oleylamine capping layer that blocks the external surface of the nanoparticles participating in the catalytic reaction. Oxygen reduction mainly occurs within the etched channels providing a nanoconfined reaction volume different from the bulk electrolyte conditions. The oxygen reduction reaction activity normalized by the electrochemically active surface area is enhanced by a factor of 3.3 for the nanozymes compared to the unetched nanoparticles and a factor of 2.1 compared to mesoporous PtNi nanoparticles that possess interconnecting pores. © Copyright 2018 American Chemical Society.
    view abstract10.1021/jacs.8b08664
  • High resolution, binder-free investigation of the intrinsic activity of immobilized NiFe LDH nanoparticles on etched carbon nanoelectrodes
    Wilde, P. and Barwe, S. and Andronescu, C. and Schuhmann, W. and Ventosa, E.
    Nano Research 11 (2018)
    The determination of the intrinsic properties of nanomaterials is essential for their optimization as electrocatalysts, however it poses great challenges from the standpoint of analytical tools and methods. Herein, we report a novel methodology that allows for a binder-free investigation of electrocatalyst nanoparticles. The potential-assisted immobilization of a non-noble metal catalyst, i.e., nickel-iron layered double hydroxide (NiFe LDH) nanoparticles, was employed to directly attach small nanoparticle ensembles from a suspension to the surface of etched carbon nanoelectrodes. The dimensions of this type of electrodes allowed for the immobilization of the catalyst material below the picogram scale and resulted in a high resolution towards the faradaic current response. In addition the effect of the electrochemical aging on the intrinsic activity of the catalyst was investigated in alkaline media by means of continuous cyclic voltammetry. A change in the material properties could be observed, which was accompanied by a substantial decrease in its intrinsic activity. [Figure not available: see fulltext.] © 2018, Tsinghua University Press and Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s12274-018-2119-4
  • Influence of Temperature and Electrolyte Concentration on the Structure and Catalytic Oxygen Evolution Activity of Nickel–Iron Layered Double Hydroxide
    Andronescu, C. and Seisel, S. and Wilde, P. and Barwe, S. and Masa, J. and Chen, Y.-T. and Ventosa, E. and Schuhmann, W.
    Chemistry - A European Journal 24 (2018)
    NiFe layered double hydroxide (LDH) is inarguably the most active contemporary catalyst for the oxygen evolution reaction under alkaline conditions. However, the ability to sustain unattenuated performance under challenging industrial conditions entailing high corrosivity of the electrolyte (≈30 wt. % KOH), high temperature (>80 °C) and high current densities (>500 mA cm−2) is the ultimate criterion for practical viability. This work evaluates the chemical and structural stability of NiFe LDH at conditions akin to practical electrolysis, in 30 % KOH at 80 °C, however, without electrochemical polarization, and the resulting impact on the OER performance of the catalyst. Post-analysis of the catalyst by means of XRD, TEM, FT-IR, and Raman spectroscopy after its immersion into 7.5 m KOH at 80 °C for 60 h revealed a transformation of the structure from NiFe LDH to a mixture of crystalline β-Ni(OH)2 and discrete predominantly amorphous FeOOH containing minor non-homogeneously distributed crystalline domains. These structural and compositional changes led to a drastic loss of the OER activity. It is therefore recommended to study catalyst stability at industrially relevant conditions. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201803165
  • 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 (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 abstract10.1021/acscatal.7b02617
  • Local Surface Structure and Composition Control the Hydrogen Evolution Reaction on Iron Nickel Sulfides
    Bentley, C.L. and Andronescu, C. and Smialkowski, M. and Kang, M. and Tarnev, T. and Marler, B. and Unwin, P.R. and Apfel, U.-P. and Schuhmann, W.
    Angewandte Chemie - International Edition 57 (2018)
    In order to design more powerful electrocatalysts, developing our understanding of the role of the surface structure and composition of widely abundant bulk materials is crucial. This is particularly true in the search for alternative hydrogen evolution reaction (HER) catalysts to replace platinum. We report scanning electrochemical cell microscopy (SECCM) measurements of the (111)-crystal planes of Fe4.5Ni4.5S8, a highly active HER catalyst. In combination with structural characterization methods, we show that this technique can reveal differences in activity arising from even the slightest compositional changes. By probing electrochemical properties at the nanoscale, in conjunction with complementary structural information, novel design principles are revealed for application to rational material synthesis. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201712679
  • Oxygen Evolution Catalysis with Mössbauerite—A Trivalent Iron-Only Layered Double Hydroxide
    Ertl, M. and Andronescu, C. and Moir, J. and Zobel, M. and Wagner, F.E. and Barwe, S. and Ozin, G. and Schuhmann, W. and Breu, J.
    Chemistry - A European Journal 24 (2018)
    Mössbauerite is investigated for the first time as an “iron-only” mineral for the electrocatalytic oxygen evolution reaction in alkaline media. The synthesis proceeds via intermediate mixed-valence green rust that is rapidly oxidized in situ while conserving the layered double hydroxide structure. The material catalyzes the oxygen evolution reaction on a glassy carbon electrode with a current density of 10 mA cm−2 at 1.63 V versus the reversible hydrogen electrode. Stability measurements, as well as post-electrolysis characterization are presented. This work demonstrates the applicability of iron-only layered double hydroxides as earth-abundant oxygen evolution electrocatalysts. Mössbauerite is of fundamental importance since as an all Fe3+ material its performance has no contributions from unknown synergistic effects as encountered for mixed valence Co/Ni/Fe LDH. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/chem.201801938
  • Towards Reproducible Fabrication of Nanometre-Sized Carbon Electrodes: Optimisation of Automated Nanoelectrode Fabrication by Means of Transmission Electron Microscopy
    Wilde, P. and Quast, T. and Aiyappa, H.B. and Chen, Y.-T. and Botz, A. and Tarnev, T. and Marquitan, M. and Feldhege, S. and Lindner, A. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 5 (2018)
    The reproducible fabrication of nanometre-sized carbon electrodes poses great challenges. Especially, the field of single entity electrochemistry has strict requirements regarding the geometry of these electrochemical probes. Herein, an automated setup for the fabrication of carbon nanoelectrodes based on the pyrolysis of a propane/butane gas mixture within pulled quartz capillaries by means of a moving heating coil is presented. It is shown that mere electrochemical characterisation with conventional redox mediators does not allow for a reliable assessment of the electrode's geometry and quality. Therefore, high-throughput transmission electron microscopy is used in parallel to evaluate and optimise preparation parameters. Control of the latter gives access to three different electrode types: nanopipettes, nanosamplers and nanodisks. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201800600
  • Influence of Ni to Co ratio in mixed Co and Ni phosphides on their electrocatalytic oxygen evolution activity
    Barwe, S. and Andronescu, C. and Vasile, E. and Masa, J. and Schuhmann, W.
    Electrochemistry Communications 79 (2017)
    Prompted by the impact of Ni-based support materials on the intrinsic activity of electrocatalysts, we investigated the influence of partial Co substitution by Ni during the reductive thermal synthesis of cobalt-cobalt phosphide nanoparticles from triphenylphosphine complexes. The obtained catalysts were characterised by X-ray diffraction and electrochemistry. Increasing the amount of Ni in the precursor complexes leads to materials with lower overpotential for the OER at low current densities, and lower Tafel slopes. Co nanoparticles, which are only formed in materials with low Ni content, increase the intrinsic material conductivity and reduce the OER overpotential at high current densities. © 2017
    view abstract10.1016/j.elecom.2017.04.014
  • 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 (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 abstract10.1002/anie.201703963
  • Poly(benzoxazine) as an Immobilization Matrix for Miniaturized ATP and Glucose Biosensors
    Ziller, C. and Lin, J. and Knittel, P. and Friedrich, L. and Andronescu, C. and Pöller, S. and Schuhmann, W. and Kranz, C.
    ChemElectroChem 4 (2017)
    The reproducible immobilization of enzymes represents a key requirement in developing sensitive and fast-responding amperometric microbiosensors. A microbiosensor for the respective detection of glucose and adenosine-5′-triphosphate (ATP) is presented by using a poly(benzoxazine) derivative for the entrapment of the enzymes glucose oxidase (GOD) and hexokinase (HEX) at platinum (Pt) microelectrodes (MEs). For glucose, a sensitivity of 123.05±10.78 pA/mM (n=5) was obtained, which shows twice as high sensitivity compared to microbiosensors that use electrophoretic paints as the immobilization matrix for the same size ME (radius: 25 μm). For the determination of ATP, a sensitivity of 48.47±5.12 pA/μM and a signal-to-noise ratio of 40 at physiological pH values were obtained. Apart from their enhanced sensitivity, a significant improvement of these sensors is related to their improved mechanical stability. The applicability of these poly(benzoxine)-based microbiosensors for ATP detection was demonstrated with measurements at receptor protein tyrosine phosphatase zeta (PTPRζ) osteoblastic cells during mechanical stimulation. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600765
  • Polybenzoxazine-Derived N-doped Carbon as Matrix for Powder-Based Electrocatalysts
    Barwe, S. and Andronescu, C. and Masa, J. and Ventosa, E. and Klink, S. and Genç, A. and Arbiol, J. and Schuhmann, W.
    ChemSusChem 10 (2017)
    In addition to catalytic activity, intrinsic stability, tight immobilization on a suitable electrode surface, and sufficient electronic conductivity are fundamental prerequisites for the long-term operation of particle- and especially powder-based electrocatalysts. We present a novel approach to concurrently address these challenges by using the unique properties of polybenzoxazine (pBO) polymers, namely near-zero shrinkage and high residual-char yield even after pyrolysis at high temperatures. Pyrolysis of a nanocubic prussian blue analogue precursor (KmMnx[Co(CN)6]y⋅n H2O) embedded in a bisphenol A and aniline-based pBO led to the formation of a N-doped carbon matrix modified with MnxCoyOz nanocubes. The obtained electrocatalyst exhibits high efficiency toward the oxygen evolution reaction (OER) and more importantly a stable performance for at least 65 h. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/cssc.201700593
  • Powder Catalyst Fixation for Post-Electrolysis Structural Characterization of NiFe Layered Double Hydroxide Based Oxygen Evolution Reaction Electrocatalysts
    Andronescu, C. and Barwe, S. and Ventosa, E. and Masa, J. and Vasile, E. and Konkena, B. and Möller, S. and Schuhmann, W.
    Angewandte Chemie - International Edition 56 (2017)
    Highly active electrocatalysts for the oxygen evolution (OER) reaction are in most cases powder nanomaterials, which undergo substantial changes upon applying the high potentials required for high-current-density oxygen evolution. Owing to the vigorous gas evolution, the durability under OER conditions is disappointingly low for most powder electrocatalysts as there are no strategies to securely fix powder catalysts onto electrode surfaces. Thus reliable studies of catalysts during or after the OER are often impaired. Herein, we propose the use of composites made from precursors of polybenzoxazines and organophilically modified NiFe layered double hydroxides (LDHs) to form a stable and highly conducting catalyst layer, which allows the study of the catalyst before and after electrocatalysis. Characterization of the material by XRD, SEM, and TEM before and after 100 h electrolysis in 5 m KOH at 60 °C and a current density of 200 mA cm−2 revealed previously not observed structural changes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/anie.201705385
  • Promotional Effect of Fe Impurities in Graphene Precursors on the Activity of MnOX/Graphene Electrocatalysts for the Oxygen Evolution and Oxygen Reduction Reactions
    Morales, D.M. and Masa, J. and Andronescu, C. and Schuhmann, W.
    ChemElectroChem 4 (2017)
    Bifunctional oxygen electrocatalysts were fabricated following a three-step synthesis method, which consisted of i) liquid-phase exfoliation of graphite in the presence of nitrogen-containing manganese macrocyclic complexes, using DMF as the dispersion medium under formation of few-layer graphene sheets. Subsequently, ii) solvent removal by vacuum filtration and drying, and iii) pyrolysis of the resulting composites under an inert gas atmosphere with subsequent mild calcination yielded manganese oxides embedded within a graphitic carbon matrix (MnOX/G). We further demonstrate that traces of Fe impurities in the used graphite result in enhanced electrocatalytic activity of the MnOX/G towards both the oxygen reduction and the oxygen evolution reactions, owing to synergistic interaction of the iron impurities with the species formed upon thermal decomposition of Mn macrocyclic complexes. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201700496
  • Topotactic Synthesis of Porous Cobalt Ferrite Platelets from a Layered Double Hydroxide Precursor and Their Application in Oxidation Catalysis
    Ortega, K.F. and Anke, S. and Salamon, S. and Özcan, F. and Heese, J. and Andronescu, C. and Landers, J. and Wende, H. and Schuhmann, W. and Muhler, M. and Lunkenbein, T. and Behrens, M.
    Chemistry - A European Journal (2017)
    Monocrystalline, yet porous mosaic platelets of cobalt ferrite, CoFe2O4, can be synthesized from a layered double hydroxide (LDH) precursor by thermal decomposition. Using an equimolar mixture of Fe2+, Co2+, and Fe3+ during co-precipitation, a mixture of LDH, (FeIICoII)2/3FeIII 1/3(OH)2(CO3)1/6mH2O, and the target spinel CoFe2O4 can be obtained in the precursor. During calcination, the remaining FeII fraction of the LDH is oxidized to FeIII leading to an overall Co2+:Fe3+ ratio of 1:2 as required for spinel crystallization. This pre-adjustment of the spinel composition in the LDH precursor suggests a topotactic crystallization of cobalt ferrite and yields phase pure spinel in unusual anisotropic platelet morphology. The preferred topotactic relationship in most particles is [111]Spinel∥[001]LDH. Due to the anion decomposition, holes are formed throughout the quasi monocrystalline platelets. This synthesis approach can be used for different ferrites and the unique microstructure leads to unusual chemical properties as shown by the application of the ex-LDH cobalt ferrite as catalyst in the selective oxidation of 2-propanol. Compared to commercial cobalt ferrite, which mainly catalyzes the oxidative dehydrogenation to acetone, the main reaction over the novel ex-LDH cobalt is dehydration to propene. Moreover, the oxygen evolution reaction (OER) activity of the ex-LDH catalyst was markedly higher compared to the commercial material. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201702248
  • Few-layer graphene modified with nitrogen-rich metallo-macrocyclic complexes as precursor for bifunctional oxygen electrocatalysts
    Morales, D.M. and Masa, J. and Andronescu, C. and Kayran, Y.U. and Sun, Z. and Schuhmann, W.
    Electrochimica Acta 222 (2016)
    We propose a method for the formation of highly active bifunctional oxygen electrocatalysts, by exploiting the unique features of nitrogen-rich metallo-macrocyclic complexes and the structural and electronic properties of few-layer graphene. The precursors of the electrocatalysts were synthesized by sonication of graphite in DMF leading to exfoliation and the formation of few-layer graphene sheets in the presence of a suitable transition metal macrocyclic complex. After pyrolysis and subsequent mild calcination metal oxide nanoparticles as well as metal-nitrogen (MNx) moieties embedded within a N-doped graphitic carbon matrix are obtained. The formation, in-depth characterization and electrochemical performance of two different catalysts derived from Co and Ni containing precursor complexes are demonstrated. © 2016 Elsevier Ltd
    view abstract10.1016/j.electacta.2016.11.092
  • Low Overpotential Water Splitting Using Cobalt-Cobalt Phosphide Nanoparticles Supported on Nickel Foam
    Masa, J. and Barwe, S. and Andrpnescu, C. and Sinev, I. and Ruff, A. and Jayaramulu, K. and Elumeeva, K. and Konkena, B. and Cuenya, B. R. and Schuhmann, W.
    ACS Energy Letters 1 (2016)
    We report a simple, facile, and safe route for preparation of cobalt-cobalt phosphide (Co/Co2P) nanoparticles and demonstrate their application as efficient low-cost catalysts for electrochemical water splitting. The catalyst achieves good performance in catalyzing both the cathode and anode half-cell water-splitting reactions in 1.0 M KOH and the hydrogen evolution reaction in an acidic electrolyte, 0.5 M H2SO4. For the oxygen evolution reaction in 1.0 M KOH, a current of 10 mA cm(-2) was attained at 0.39 V overpotential on a glassy carbon electrode, while an overpotential of 0.19 V was attained at 50 mA cm(-2) when the catalyst was supported on nickel foam.
    view abstract10.1021/acsenergylett.6b00532
  • Codeposited Poly(benzoxazine) and Os-Complex Modified Polymethacrylate Layers as Immobilization Matrix for Glucose Biosensors
    Barwe, S. and Andronescu, C. and Pöller, S. and Schuhmann, W.
    Electroanalysis 27 (2015)
    Benzoxazine oligomers synthesized by a Mannich type reaction of bisphenol A, tetraethylenepentamine and formaldehyde were electrochemically crosslinked in presence of both an Os-complex modified poly(methacrylate) polymer and glucose oxidase. The crosslinking led to the formation of a biocatalytically active layer on an electrode surface exhibiting a swelling process after immersion in an electrolyte solution containing glucose most likely due to the local decrease of the pH value upon glucose oxidation. Optimization of the poly(benzoxazine) to Os-complex modified poly(methacrylate) ratio was performed leading to a reagentless glucose biosensor with improved stability. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/elan.201500131
  • Phenolated Oleic Acid Based Polybenzoxazine Derivatives as Corrosion Protection Layers
    Bələnucə, B. and Raicopol, M. and Maljusch, A. and Garea, S. and Hanganu, A. and Schuhmann, W. and Andronescu, C.
    ChemPlusChem 80 (2015)
    Benzoxazine derivatives were synthesized using phenolated methyl oleate and either aniline, 1,6-diaminohexane, or 4,4′-diaminodiphenylmethane, respectively, as amine components. Polymerization of the benzoxazine derivatives led to the formation of hydrophobic and dense coatings on Zn-Mg-Al alloy coated steel sheets for passive corrosion protection. The polybenzoxazine coatings which are formed by crosslinking during a heat-treatment step invoked a substantial anodic shift of the open-circuit potential as well as the breakthrough potential in potentiodynamic measurements. The proposed polybenzoxazine derivatives pave the way for a new type of passive polymer protection system based on sustainably obtained precursor components. Copyright © 2015 Wiley-VCH Verlag GmbH & Co. KGaA.
    view abstract10.1002/cplu.201500092
  • Poly(benzoxazine)s Modified with Osmium Complexes as a Class of Redox Polymers for Wiring of Enzymes to Electrode Surfaces
    Alsaoub, S. and Barwe, S. and Andronescu, C. and Pöller, S. and Ruff, A. and Schuhmann, W.
    ChemPlusChem 80 (2015)
    Benzoxazine-based redox polymers bearing Os complexes are synthesized and used as an immobilization matrix for glucose oxidase (GOx) as a model system for a reagentless biosensor. The polymers are formed by electrochemically induced anodic polymerization of the corresponding benzoxazine monomers modified with Os complexes. The precursors are synthesized in a Mannich-type reaction between bisphenol A, formaldehyde, and the corresponding Os complexes or ligands, which contain free amino groups. The Os complexes are redox active within the polymer films, and thus, can be used as redox relays for the electron transfer between the electrode surface and the prosthetic group within the enzyme. Entrapment of GOx within the poly(benzoxazine) film is achieved successfully, as shown by the biocatalytic activity of the poly(benzoxazine)/GOx films upon the addition of glucose. © 2015 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201500135
  • Vegetable oil-based polybenzoxazine derivatives coatings on Zn-Mg-Al alloy coated steel
    Raicopol, M. and Bălănucă, B. and Sliozberg, K. and Schlüter, B. and Gârea, S.A. and Chira, N. and Schuhmann, W. and Andronescu, C.
    Corrosion Science 100 (2015)
    Based on environmentally friendly (bio-based) precursor materials a new class of benzoxazine derivatives was synthesized using phenolated high oleic sunflower oil as phenol component and either aniline or 1,6-diaminohexane as amine components. Hydrophobic and dense poly(benzoxazine) coatings on Zn-Mg-Al alloy coated steel were obtained after spin-coating or air-brush type spray coating by crosslinking during a heat treatment step. The poly(benzoxazine)-coated ZM-steel samples showed an anodic shift of the open circuit potential as well as the break-through potential. Using an automatic scanning droplet cell the impact of the polymer film thickness on corrosion protection was evaluated. © 2015 Elsevier Ltd.
    view abstract10.1016/j.corsci.2015.08.018
  • Electrochemically induced deposition of poly(benzoxazine) precursors as immobilization matrix for enzymes
    Andronescu, C. and Pöller, S. and Schuhmann, W.
    Electrochemistry Communications 41 (2014)
    Water-soluble benzoxazine oligomers were synthesized by reacting bisphenol A, tetraethylenepentamine and formaldehyde. The pre-formed benzoxazine oligomers can be further electropolymerized from aqueous suspensions under formation of poly(benzoxazine)-based films on electrode surfaces. Integration of glucose oxidase in a poly(benzoxazine) film let to highly reproducible and stable biosensors. Poly(benzoxazines) are proposed as a new family of stable polymers for the design of enzyme electrodes. © 2014 Elsevier B.V.
    view abstract10.1016/j.elecom.2014.01.015
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