Prof. Dr. Harry Hoster

Energy Technology
University of Duisburg-Essen

Contact

Hub
  • Sodiation Of Hard Carbon: How Separating Enthalpy And Entropy Contributions Can Find Transitions Hidden In The Voltage Profile
    Mercer, M.P. and Affleck, S. and Gavilán-Arriazu, E.M. and Zülke, A.A. and Maughan, P.A. and Trivedi, S. and Fichtner, M. and Reddy Munnangi, A. and Leiva, E.P.M. and Hoster, H.E.
    ChemPhysChem 23 (2022)
    Sodium-ion batteries (NIBs) utilize cheaper materials than lithium-ion batteries (LIBs) and can thus be used in larger scale applications. The preferred anode material is hard carbon, because sodium cannot be inserted into graphite. We apply experimental entropy profiling (EP), where the cell temperature is changed under open circuit conditions. EP has been used to characterize LIBs; here, we demonstrate the first application of EP to any NIB material. The voltage versus sodiation fraction curves (voltage profiles) of hard carbon lack clear features, consisting only of a slope and a plateau, making it difficult to clarify the structural features of hard carbon that could optimize cell performance. We find additional features through EP that are masked in the voltage profiles. We fit lattice gas models of hard carbon sodiation to experimental EP and system enthalpy, obtaining: 1. a theoretical maximum capacity, 2. interlayer versus pore filled sodium with state of charge. © 2021 Wiley-VCH GmbH
    view abstract10.1002/cphc.202100748
  • Voltammetric Behaviour of LMO at the Nanoscale: A Map of Reversibility and Diffusional Limitations
    Gavilán-Arriazu, E.M. and Mercer, M.P. and Barraco, D.E. and Hoster, H.E. and Leiva, E.P.M.
    ChemPhysChem 23 (2022)
    Understanding and optimizing single particle rate behaviour is normally challenging in composite commercial lithium-ion electrode materials. In this regard, recent experimental research has addressed the electrochemical Li-ion intercalation in individual nanosized particles. Here, we present a thorough theoretical analysis of the Li+ intercalation voltammetric behaviour in single nano/micro-scale LiMn2O4 (LMO) particles, incorporating realistic interactions between inserted ions. A transparent 2-dimensional zone diagram representation of kinetic-diffusional behaviour is provided that allows rapid diagnosis of the reversibility and diffusion length of the system depending on the particle geometry. We provide an Excel file where the boundary lines of the zone diagram can be rapidly recalculated by setting input values of the rate constant, (Formula presented.) and diffusion coefficient, (Formula presented.). The model framework elucidates the heterogeneous behaviour of nanosized particles with similar sizes but different shapes. Hence, we present here an outlook for realistic multiscale modelling of real materials. © 2021 Wiley-VCH GmbH.
    view abstract10.1002/cphc.202100700
  • Engendering Unprecedented Activation of Oxygen Evolution via Rational Pinning of Ni Oxidation State in Prototypical Perovskite: Close Juxtaposition of Synthetic Approach and Theoretical Conception
    Pittkowski, R. and Divanis, S. and Klementová, M. and Nebel, R. and Nikman, S. and Hoster, H. and Mukerjee, S. and Rossmeisl, J. and Krtil, P.
    ACS Catalysis 11 (2021)
    Rational optimization of the OER activity of catalysts based on LaNiO3 oxide is achieved by maximizing the presence of trivalent Ni in the surface structure. DFT investigations of the LaNiO3 catalyst and surface structures related to it predict an improvement in the OER activity for these materials to levels comparable with the top of the OER volcano if the La content is minimized while the oxidation state of Ni is maintained. These theoretically predicted structures of high intrinsic OER activity can be prepared by a templated spray-freeze freeze-drying synthesis followed by a simple postsynthesis exfoliation-like treatment in acidic media. These nanocrystalline LaNiO3-related materials confirm the theoretical predictions, showing a dramatic improvement in OER activity. The exfoliated surfaces remain stable in OER catalysis, as shown by an in-operando ICP-OES study. The unprecedented OER activation of the synthesized LaNiO3-based materials is related to a close juxtaposition of the theoretical conception of ideal structural motifs and the ability to engender such motifs using a unique synthetic procedure, both principally related to stabilization and pinning of the Ni oxidation state within the local coordination environment of the perovskite structure. © 2020 American Chemical Society.
    view abstract10.1021/acscatal.0c04733
  • High-Energy Nickel-Cobalt-Aluminium Oxide (NCA) Cells on Idle: Anode- versus Cathode-Driven Side Reactions
    Zülke, A. and Li, Y. and Keil, P. and Burrell, R. and Belaisch, S. and Nagarathinam, M. and Mercer, M.P. and Hoster, H.E.
    Batteries and Supercaps 4 (2021)
    We report on the first year of calendar ageing of commercial high-energy 21700 lithium-ion cells, varying over eight state of charge (SoC) and three temperature values. Lithium-nickel-cobalt-aluminium oxide (NCA) and graphite with silicon suboxide (Gr-SiOx) form cathodes and anodes of those cells, respectively. Degradation is fastest for cells at 70–80 % SoC according to monthly electrochemical check-up tests. Cells kept at 100 % SoC do not show the fastest capacity fade but develop internal short circuits for temperatures T≥40 °C. Degradation is slowest for cells stored close to 0 % SoC at all temperatures. Rates of capacity fade and their temperature dependencies are distinctly different for SoC values below and above 60 %, respectively. Differential voltage analyses, apparent activation energy analysis, and endpoint slippage tracking provide useful insights into the degradation mechanisms and the respective roles of anode and cathode potential. We discuss how reversible losses of lithium might play a role in alleviating the rate of irreversible losses on commercial cells. © 2021 The Authors. Batteries & Supercaps published by Wiley-VCH GmbH
    view abstract10.1002/batt.202100046
  • Interface Phenomena
    Engstfeld, A.K. and Hoster, H. and Magnussen, O.M.
    ChemPhysChem 22 (2021)
    view abstract10.1002/cphc.202100712
  • Parametrisation and Use of a Predictive DFN Model for a High-Energy NCA/Gr-SiOx Battery
    Zülke, A. and Korotkin, I. and Foster, J.M. and Nagarathinam, M. and Hoster, H. and Richardson, G.
    Journal of the Electrochemical Society 168 (2021)
    We demonstrate the predictive power of a parametrised Doyle-Fuller-Newman (DFN) model of a commercial cylindrical (21700) lithium-ion cell with NCA/Gr-SiOx chemistry. Model parameters result from the deconstruction of a fresh commercial cell to determine/confirm chemistry and micro-structure, and also from electrochemical experiments with half-cells built from electrode samples. The simulations predict voltage profiles for (i) galvanostatic discharge and (ii) drive-cycles. Predicted voltage responses deviate from measured ones by <1% throughout at least ∼95% of a full galvanostatic discharge, whilst the drive cycle discharge is matched to a ∼1%-3% error throughout. All simulations are performed using the online computational tool DandeLiion, which rapidly solves the DFN model using only modest computational resources. The DFN results are used to quantify the irreversible energy losses occurring in the cell and deduce their location. In addition to demonstrating the predictive power of a properly validated DFN model, this work provides a novel simplified parametrisation workflow that can be used to accurately calibrate an electrochemical model of a cell. © 2021 The Electrochemical Society ("ECS"). Published on behalf of ECS by IOP Publishing Limited.
    view abstract10.1149/1945-7111/ac3e4a
  • Voltage hysteresis during lithiation/delithiation of graphite associated with meta-stable carbon stackings
    Mercer, M.P. and Peng, C. and Soares, C. and Hoster, H.E. and Kramer, D.
    Journal of Materials Chemistry A 9 (2021)
    Cell voltage is a fundamental quantity used to monitor and control Li-ion batteries. The open circuit voltage (OCV) is of particular interest as it is believed to be a thermodynamic quantity, free of kinetic effects and history and, therefore, "simple"to interpret. Here we show that the OCV characteristics of graphite show hysteresis between charge and discharge that do not solely originate from Li dynamics and that the OCV is in fact history dependent. Combining first-principles calculations with temperature-controlled electrochemical measurements, we identify a residual hysteresis that persists even at elevated temperatures of greater than 50 °C due to differences in the phase succession between charge and discharge. Experimental entropy profiling, as well as energies and volume changes determined from first-principles calculations, suggest that the residual hysteresis is associated with different host lattice stackings of carbon and is related to Li disorder across planes in stage II configurations. © The Royal Society of Chemistry.
    view abstract10.1039/d0ta10403e
  • A Python Package to Preprocess the Data Produced by Novonix High-Precision Battery-Testers
    Gonzalez-Perez, V. and Keil, P. and Li, Y. and Zülke, A. and Burrel, R. and Csala, D. and Hoster, H.
    Journal of Open Research Software 8 (2020)
    We present preparenovonix, a Python package that handles common issues encountered in data fles generated with a range of software versions from the Novonix battery-testers.1 This package can also add extra information that makes easier coulombic counting and relating a measurement to the experimental protocol. The package provides a master function that can run at once the cleaning and adding derived information, with fexibility to choose only some features. There is a separate function to simply read a column by its given name. The usage of all the functions is documented in the code including examples. The code presented here can be installed either as a python package2 or from a GitHub repository.3 © 2020. All Rights Reserved.
    view abstract10.5334/jors.281
  • Communication⇔identifying and managing reversible capacity losses that falsify cycle ageing tests of lithium-ion cells
    Burrell, R. and Zulke, A. and Keil, P. and Hoster, H.
    Journal of the Electrochemical Society 167 (2020)
    We report on a cycle ageing study of commercial NCA/Gr+Si cells, in which reversible capacity fluctuations turn a central experimental finding upside down: an upper voltage limit of 4.1 V seems to cause faster degradation than going all the way to 4.2 V. The underlying effect is the reversible loss of lithium inventory into passive anode overhang areas. We demonstrate how the resulting artefact arises from a combination of slow transport processes and the related time periods spent in specific state-of-charge regions. We propose an alternative visualisation tool to identify and manage such artefacts, often neglected in typical ageing studies. © 2020 The Electrochemical Society (“ECS”).
    view abstract10.1149/1945-7111/abbce1
  • Effect of temperature on the kinetics of electrochemical insertion of li-ions into a graphite electrode studied by kinetic Monte Carlo
    Gavilán-Arriazu, E.M. and Mercer, M.P. and Pinto, O.A. and Oviedo, O.A. and Barraco, D.E. and Hoster, H.E. and Leiva, E.P.M.
    Journal of the Electrochemical Society 167 (2020)
    The effect of temperature on the kinetics of electrochemical insertion/removal of lithium in graphite is analyzed by kinetic Monte Carlo methods. Different electrochemical techniques are simulated at different temperatures and responses are compared with experimental results. Simulated voltammograms show, similarly to experiment, how the behavior of the system becomes closer to equilibrium as temperature increases. Calculated chronoamperometric profiles show a different qualitative behavior in the current at different temperatures, especially in the Cottrell representation peaks, explained in terms of the relative importance of diffusive versus charge transfer processes at different temperatures. Results at room temperature are in good agreement with experiment, and we further evaluate trends at elevated temperature that have not yet been described in experimental or theoretical works. Exchange current densities for different degrees of lithium intercalation at different temperatures are predicted using potentiostatic simulations, showing an Arrhenius-type relationship. The dependence of the exchange current on electrolyte composition is simulated by investigating the effect of different activation energy barriers at different temperatures. The influence of temperature on diffusion coefficients as a function of lithiation fraction in graphite is simulated and related to Arrhenius plots, explaining the experimentally observed changes in diffusion phenomena with lithium composition and temperature. © The Author(s) 2019. Published by ECS.
    view abstract10.1149/2.0332001JES
  • Numerical simulations of cyclic voltammetry for lithium-ion intercalation in nanosized systems: finiteness of diffusion versus electrode kinetics
    Gavilán-Arriazu, E.M. and Mercer, M.P. and Pinto, O.A. and Oviedo, O.A. and Barraco, D.E. and Hoster, H.E. and Leiva, E.P.M.
    Journal of Solid State Electrochemistry 24 (2020)
    The voltammetric behavior of Li+ intercalation/deintercalation in/from LiMn2O4 thin films and single particles is simulated, supporting very recent experimental results. Experiments and calculations both show that particle size and geometry are crucial for the electrochemical response. A remarkable outcome of this research is that higher potential sweep rates, of the order of several millivolts per second, may be used to characterize nanoparticles by voltammetry sweeps, as compared with macroscopic systems. This is in line with previous conclusions drawn for related single particle systems using kinetic Monte Carlo simulations. The impact of electrode kinetics and finite space diffusion on the reversibility of the process and the finiteness of the diffusion in ion Li / LiMn2O4 (de)intercalation is also discussed in terms of preexisting modeling. © 2020, Springer-Verlag GmbH Germany, part of Springer Nature.
    view abstract10.1007/s10008-020-04717-9
  • Communication—why high-precision coulometry and lithium plating studies on commercial lithium-ion cells require thermal baths
    Zülke, A. and Li, Y. and Keil, P. and Hoster, H.
    Journal of the Electrochemical Society 166 (2019)
    We demonstrate how insufficient heat transport in environmental chambers compromises the meaningfulness of high-precision charge counting and leads to an underestimation of lithium plating in fast-charged lithium-ion batteries. Direct-contact liquid cooling of cylindrical cells excludes temperature fluctuations observed in thermal chambers and restricts self-heating to 2 K in comparison to the 14 K in thermal chambers, under 1.5 C-rate cycling. Our thermal-electrochemical model replicated well the experimental results. For high-precision coulometric studies to be meaningful and more comparable across different laboratories, especially for large-format and high-power cells, direct-contact cooling in thermal baths must become the new standard. © 2019 The Electrochemical Society.
    view abstract10.1149/2.0841913jes
  • Data-driven health estimation and lifetime prediction of lithium-ion batteries: A review
    Li, Y. and Liu, K. and Foley, A.M. and Zülke, A. and Berecibar, M. and Nanini-Maury, E. and Van Mierlo, J. and Hoster, H.E.
    Renewable and Sustainable Energy Reviews 113 (2019)
    Accurate health estimation and lifetime prediction of lithium-ion batteries are crucial for durable electric vehicles. Early detection of inadequate performance facilitates timely maintenance of battery systems. This reduces operational costs and prevents accidents and malfunctions. Recent advancements in “Big Data” analytics and related statistical/computational tools raised interest in data-driven battery health estimation. Here, we will review these in view of their feasibility and cost-effectiveness in dealing with battery health in real-world applications. We categorise these methods according to their underlying models/algorithms and discuss their advantages and limitations. In the final section we focus on challenges of real-time battery health management and discuss potential next-generation techniques. We are confident that this review will inform commercial technology choices and academic research agendas alike, thus boosting progress in data-driven battery health estimation and prediction on all technology readiness levels. © 2019 Elsevier Ltd
    view abstract10.1016/j.rser.2019.109254
  • Transitions of lithium occupation in graphite: A physically informed model in the dilute lithium occupation limit supported by electrochemical and thermodynamic measurements
    Mercer, M.P. and Otero, M. and Ferrer-Huerta, M. and Sigal, A. and Barraco, D.E. and Hoster, H.E. and Leiva, E.P.M.
    Electrochimica Acta 324 (2019)
    Understanding the role of the phase transitions during lithiation and delithiation of graphite remains a problem of fundamental importance, but also practical relevance owing to its widespread use as the anode material in most commercial lithium-ion cells. Previously performed density functional theory (DFT) calculations show a rapid change in the lithium-carbon interaction at low occupation, due to partial charge transfer from Li to C. We integrate this effect in our previously developed two level mean field model, which describes the Stage I – Stage II transition in graphite. The modified model additionally describes the most predominant transition that occurs at low Li content in graphite, which results in a previously unexplained feature in voltage and dQ/dV profiles, and thermodynamic measurements of partial molar enthalpy. In contrast with the Stage I-Stage II transition, this extra feature is not associated with observable features in the partial molar entropy and our model demonstrates why. There is a sharp change in the open circuit voltage at very low Li occupation, followed by a transition to a voltage plateau (peak in dQ/dV). The behaviour arises due to the contrasting effects of the partial molar entropy and enthalpy terms on the partial molar Gibbs energy and hence cell voltage. Hence the voltage profile and phase transitions can be approximated for all lithium occupations, potentially allowing a predictive capability in cell level models. © 2019
    view abstract10.1016/j.electacta.2019.134774
  • Formation of the Solid Electrolyte Interphase at Constant Potentials: A Model Study on Highly Oriented Pyrolytic Graphite
    Antonopoulos, B.K. and Maglia, F. and Schmidt-Stein, F. and Schmidt, J.P. and Hoster, H.E.
    Batteries and Supercaps 1 (2018)
    The solid electrolyte interphase (SEI) on graphite anodes is a key enabler for rechargeable lithium-ion batteries (LIBs). It ensures that only Li+ ions and no damaging electrolyte components enter the anode and hinders electrolyte decomposition. Its growth should be confined to the initial SEI formation process and stop once the battery is in operation to avoid capacity/power loss. In technical LIB cells, the SEI is formed at constant current, with the potential of the graphite anode slowly drifting from higher to lower voltages. SEI formation rate, composition, and structure depend on the potential and on the chemical properties of the anode surface. Here, we characterize SEIs formed at constant potentials on the chemically inactive basal plane of highly oriented pyrolytic graphite (HOPG). X-ray photoemission spectroscopy (XPS) detects carbonate species only at lower formation potentials. Cyclic voltammetry (CV) and Electrochemical Impedance Spectroscopy (EIS) with Fc/Fc+ as an electrochemical probe demonstrate how the formation potential influences ion transport and electrochemical kinetics to and at the anode surface, respectively. Breaking the EIS data down to a Distribution of Relaxation Times (DRT) reveals distinct kinetics and transport related peaks with varying Arrhenius-type temperature dependencies. We discuss our findings in the context of previous electrochemical studies and existing SEI models and of SEI formation protocols suitable for industry. © 2018 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/batt.201800029
  • Quantifying structure dependent responses in Li-ion cells with excess Li spinel cathodes: Matching voltage and entropy profiles through mean field models
    Schlueter, S. and Genieser, R. and Richards, D. and Hoster, H.E. and Mercer, M.P.
    Physical Chemistry Chemical Physics 20 (2018)
    Measurements of the open circuit voltage of Li-ion cells have been extensively used as a non-destructive characterisation tool. Another technique based on entropy change measurements has also been applied for this purpose. More recently, both techniques have been used to make qualitative statements about aging in Li-ion cells. One proposed cause of cell failure is point defect formation in the electrode materials. The steps in voltage profiles, and the peaks in entropy profiles are sensitive to order/disorder transitions arising from Li/vacancy configurations, which are affected by the host lattice structures. We compare the entropy change results, voltage profiles and incremental capacity (dQ/dV) obtained from coin cells with spinel lithium manganese oxide (LMO) cathodes, Li1+yMn2-yO4, where excess Li y was added in the range 0 ≤ y ≤ 0.2. A clear trend of entropy and dQ/dV peak amplitude decrease with excess Li amount was determined. The effect arises, in part, from the presence of pinned Li sites, which disturb the formation of the ordered phase. We modelled the voltage, dQ/dV and entropy results as a function of the interaction parameters and the excess Li amount, using a mean field approach. For a given pinning population, we demonstrated that the asymmetries observed in the dQ/dV peaks can be modelled by a single linear correction term. To replicate the observed peak separations, widths and magnitudes, we had to account for variation in the energy interaction parameters as a function of the excess Li amount, y. All Li-Li repulsion parameters in the model increased in value as the defect fraction, y, increased. Our paper shows how far a computational mean field approximation can replicate experimentally observed voltage, incremental capacity and entropy profiles in the presence of phase transitions. © 2018 the Owner Societies.
    view abstract10.1039/c8cp02989j
  • Solid electrolyte interphase: Can faster formation at lower potentials yield better performance?
    Antonopoulos, B.K. and Stock, C. and Maglia, F. and Hoster, H.E.
    Electrochimica Acta 269 (2018)
    To make a Lithium Ion Battery (LIB) reliably rechargeable over many cycles, its graphite-based negative electrode requires the solid electrolyte interphase (SEI) as a protection layer. The SEI is formed through chemical and particularly electrochemical side reactions of electrolyte components in the first charging cycle(s) after manufacturing of a LIB. The SEI ideally serves two purposes: (i) act as a sieve permeable to Li ions but not to other electrolyte components and (ii) passivate the electrode against further electrolyte decomposition. Core element of conventional SEI formation is a lengthy, low-current galvanostatic charging step, which due to its time consumption contributes heavily to cell manufacturing costs. Here, we report on some non-conventional SEI formation protocols for composite carbon electrodes, inspired by recent experimental findings at smooth model electrodes. Acknowledging that the SEI forms in two main steps, taking place in a high-potential and a low-potential region, respectively, we demonstrate that less time spent in the high-potential region not only makes the process faster but even yields SEIs with superior kinetic properties. We tentatively explain this via basic rules of thin film growth and the role of grain boundaries for ion transport. We also report on the positive influence of multi-frequency potential modulations applied between high-potential and low-potential formation. Given that any new cell chemistry in principle requires its own tailor-made formation process, technologic success of future LIB cells will benefit from a systematic, well-understood toolbox of formation protocols. This paper is meant as a first step, highlighting potentially low-hanging fruits, but also flagging the demand for further systematic studies on model systems and on commercially manufactured cells. © 2018 Elsevier Ltd
    view abstract10.1016/j.electacta.2018.03.007
  • Electrified Interfaces (ICEI 2016): 14th International Conference, 3-8 July 2016, Singapore
    Hoster, H. and Toghill, K. and Xu, Z.J.
    Electrochimica Acta 249 (2017)
    The special issue of Electrochimica Acta celebrates the 14th International Conference on Electrified Interfaces (ICEI 2016) that was held in Singapore, from 3rd to 8th July 2016. Lancaster University (UK), in collaboration with Nanyang Technological University (Singapore). Five companies presented their lab technologies at booths in the coffee and lunch break area: SPECS, TriTech, Ametek, Princeton/Solartron, and Metrohm. Metrohm also kindly helped us sponsoring the refresh- ments at the Poster Session through an extra contribution. The oral presentations were divided into 10 consecutive sessions, In-situ spectroscopy, Atomic Scale Imaging and Diffraction, Fundamentals of Electrocatalysis, Applied Electrochemistry, Electrocatalysis towards OER and ORR, New Electrocatalytic Materials, Fundamental Electrochemistry, Nanostructures and Nanomaterials, Batteries. The Poster Session on day 2 was initiated by 90-minutes of a poster-pitch session in the afternoon. All submitted manuscripts underwent a rigorous standard review process, which was overseen by Professor Sergio Trasatti.
    view abstract10.1016/j.electacta.2017.08.008
  • Electrochemical Kinetics: a Surface Science-Supported Picture of Hydrogen Electrochemistry on Ru(0001) and Pt/Ru(0001)
    Mercer, M.P. and Hoster, H.E.
    Electrocatalysis 8 (2017)
    In this short review, we compare the kinetics of hydrogen desorption in vacuum to those involved in the electrochemical hydrogen evolution/oxidation reactions (HER/HOR) at two types of atomically smooth model surfaces: bare Ru(0001) and the same surface covered by a 1.1 atomic layer thick Pt film. Low/high H2 (D2) desorption rates at room temperature in vacuum quantitatively correspond to low/high exchange current densities for the HOR/HER in electrochemistry. In view of the “volcano plot” concept, these represent two surfaces that adsorb hydrogen atoms, Had, too strongly and too weakly, respectively. Atomically smooth, vacuum annealed model surfaces are the closest approximation to the idealized slab geometries typically studied by density functional theory (DFT). A predictive volcano plot based on DFT-based adsorption energies for the Had intermediates agrees well with the experiments if two things are considered: (i) the steady-state coverage of Had intermediates and (ii) local variations in film thickness. The sluggish HER/HOR kinetics of Ru(0001) allows for excellent visibility of cyclic voltammetry (CV) features even in H2-saturated solution. The CV switches between a Had- and a OHad-/Oad-dominated regime, but the presence of H2 in the electrolyte increases the Had-dominated potential window by a factor of two. Whereas in plain electrolyte two electrochemical adsorption processes compete in forming adlayers, it is one electrochemical and one chemical one in the case of H2-saturated electrolyte. We demonstrate and quantitatively explain that dissociative H2 adsorption is more important than H+ discharge for Had formation in the low potential regime on Ru(0001). [Figure not available: see fulltext.]. © 2017, The Author(s).
    view abstract10.1007/s12678-017-0381-y
  • Emissions: Step on the natural gas for German cars
    Csala, D. and Hoster, H.
    Nature 541 (2017)
    view abstract10.1038/541157b
  • Influence of carbon microstructure on the Li–O2 battery first-discharge kinetics
    Wijaya, O. and Hoster, H.E. and Rinaldi, A.
    International Journal of Energy Research 41 (2017)
    Defects in the carbon microstructure have been reported to enhance the discharge performance of Li–O2 battery. However, systematic studies correlating the presence of defects with the discharge kinetics have not addressed the variation of carbon electrode surface areas. In this work, carbon blacks and carbon nanofibers with different defect densities were investigated for their discharge properties. The electrolyte-accessible areas of the carbon electrodes were obtained from Cyclic voltammetry measurements. The microstructure and surface areas of the carbons were characterized by Raman spectroscopy, electron microscopy, and N2 isotherm. Linear sweep voltammetry and galvanostatic discharge experiments consistently demonstrated that graphitic carbons have more negative onset potentials and more negative discharge potentials at the same current density than defective carbons. The linear sweep voltammetry data were normalized to the carbon masses, Brunauer–Emmet–Teller surface areas, and double layer capacitance-derived areas for comparison. Plot of inverse charge transfer resistance and double layer capacitance from electrochemical impedance spectroscopy measurements were used to extract current density values without knowledge of electrode areas. The current densities from impedance measurements exhibited good agreement with the data from linear sweep experiments. The electrochemical experiments conclusively showed that defects on the graphitic microstructure increase the discharge kinetics of the Li–O2 battery. Copyright © 2016 John Wiley &amp; Sons, Ltd. Copyright © 2016 John Wiley & Sons, Ltd.
    view abstract10.1002/er.3690
  • The influence of point defects on the entropy profiles of Lithium Ion Battery cathodes: a lattice-gas Monte Carlo study
    Mercer, M.P. and Finnigan, S. and Kramer, D. and Richards, D. and Hoster, H.E.
    Electrochimica Acta 241 (2017)
    In-situ diagnostic tools have become established to as a means to understanding the aging processes that occur during charge/discharge cycles in Li-ion batteries (LIBs). One electrochemical thermodynamic technique that can be applied to this problem is known as entropy profiling. Entropy profiles are obtained by monitoring the variation in the open circuit potential as a function of temperature. The peaks in these profiles are related to phase transitions, such as order/disorder transitions, in the lattice. In battery aging studies of cathode materials, the peaks become suppressed but the mechanism by which this occurs is currently poorly understood. One suggested mechanism is the formation of point defects. Intentional modifications of LIB electrodes may also lead to the introduction of point defects. To gain quantitative understanding of the entropy profile changes that could be caused by point defects, we have performed Monte Carlo simulations on lattices of variable defect content. As a model cathode, we have chosen manganese spinel, which has a well-described order-disorder transition when it is half filled with Li. We assume, in the case of trivalent defect substitution (M = Cr,Co) that each defect M permanently pins one Li atom. This assumption is supported by Density Functional Theory (DFT) calculations. Assuming that the distribution of the pinned Li sites is completely random, we observe the same trend in the change in partial molar entropy with defect content as observed in experiment: the peak amplitudes become increasing suppressed as the defect fraction is increased. We also examine changes in the configurational entropy itself, rather than the entropy change, as a function of the defect fraction and analyse these results with respect to the ones expected for an ideal solid solution. We discuss the implications of the quantitative differences between some of the results obtained from the model and the experimentally observed ones. © 2017 Elsevier Ltd
    view abstract10.1016/j.electacta.2017.04.115
  • Current density distribution in cylindrical Li-Ion cells during impedance measurements
    Osswald, P.J. and Erhard, S.V. and Noel, A. and Keil, P. and Kindermann, F.M. and Hoster, H. and Jossen, A.
    Journal of Power Sources 314 (2016)
    In this work, modified commercial cylindrical lithium-ion cells with multiple separate current tabs are used to analyze the influence of tab pattern, frequency and temperature on electrochemical impedance spectroscopy. In a first step, the effect of different current tab arrangements on the impedance spectra is analyzed and possible electrochemical causes are discussed. In a second step, one terminal is used to apply a sinusoidal current while the other terminals are used to monitor the local potential distribution at different positions along the electrodes of the cell. It is observed that the characteristic decay of the voltage amplitude along the electrode changes non-linearly with frequency, where high-frequent currents experience a stronger attenuation along the current collector than low-frequent currents. In further experiments, the decay characteristic is controlled by the cell temperature, driven by the increasing resistance of the current collector and the enhanced kinetic and transport properties of the active material and electrolyte. Measurements indicate that the ac current distribution depends strongly on the frequency and the temperature. In this context, the challenges for electrochemical impedance spectroscopy as cell diagnostic technique for commercial cells are discussed. © 2016 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2016.02.070
  • Lithium–Oxygen Cells: An Analytical Model to Explain Key Features in the Discharge Voltage Profiles
    Rinaldi, A. and Wijaya, O. and Hoster, H.
    ChemElectroChem 3 (2016)
    Lithium–oxygen (Li–O2) cells are popular because of their potentially high energy density. A characteristic fingerprint of a given cell is the voltage profile during constant-current discharge. We suggest that the typical initial dip and the following increase of the voltage result from a temporary increase and slow decrease in the concentration of dissolved superoxide, respectively, feeding into the Nernst equation. The steady-state superoxide concentration decreases as the surface area of the solid precipitation product (Li2O2) increases. Importantly, these products bury the electrochemically active carbon surface. Assuming that the electrochemical step only occurs on bare carbon, the Tafel equation provides an expression for the increasing overpotential as a result of the shrinking effective electrode area. This boils the discharge voltage profile down to the sum of two logarithms, grasping all relevant features in the recorded discharge voltage profiles. © 2016 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim
    view abstract10.1002/celc.201600184
  • On battery recovery effect in wireless sensor nodes
    Narayanaswamy, S. and Schlueter, S. and Steinhorst, S. and Lukasiewycz, M. and Chakraborty, S. and Hoster, H.E.
    ACM Transactions on Design Automation of Electronic Systems 21 (2016)
    With the perennial demand for longer runtime of battery-powered Wireless Sensor Nodes (WSNs), several techniques have been proposed to increase the battery runtime. One such class of techniques exploiting the battery recovery effect phenomenon claims that performing an intermittent discharge instead of a continuous discharge will increase the usable battery capacity. Several works in the areas of embedded systems and wireless sensor networks have assumed the existence of this recovery effect and proposed different power management techniques in the form of power supply architectures (multiple battery setup) and communication protocols (burst mode transmission) in order to exploit it. However, until now, a systematic experimental evaluation of the recovery effect has not been performed with real battery cells, using high-accuracy battery testers to confirm the existence of this recovery phenomenon. In this article, a systematic evaluation procedure is developed to verify the existence of this battery recovery effect. Using our evaluation procedure, we investigated Alkaline, Nickel-Metal Hydride (NiMH), and Lithium-Ion (Li-Ion) battery chemistries, which are commonly used as power supplies for Wireless Sensor Node (WSN) applications. Our experimental results do not show any evidence of the aforementioned recovery effect in these battery chemistries. In particular, our results show a significant deviation from the stochastic battery models, which were used by many power management techniques. Therefore, the existing power management approaches that rely on this recovery effect do not hold in practice. Instead of a battery recovery effect, our experimental results show the existence of the rate capacity effect, which is the reduction of usable battery capacity with higher discharge power, to be the dominant electrochemical phenomenon that should be considered for maximizing the runtime of WSN applications. We outline power management techniques that minimize the rate capacity effect in order to obtain a higher energy output from the battery. © 2016 ACM.
    view abstract10.1145/2890501
  • Temperature dependency of state of charge inhomogeneities and their equalization in cylindrical lithium-ion cells
    Osswald, P.J. and Erhard, S.V. and Rheinfeld, A. and Rieger, B. and Hoster, H.E. and Jossen, A.
    Journal of Power Sources 329 (2016)
    The influence of cell temperature on the current density distribution and accompanying inhomogeneities in state of charge (SOC) during cycling is analyzed in this work. To allow for a detailed insight in the electrochemical behavior of the cell, commercially available 26650 cells were modified to allow for measuring local potentials at four different, nearly equidistant positions along the electrodes. As a follow-up to our previous work investigating local potentials within a cell, we apply this method for studying SOC deviations and their sensitivity to cell temperature. The local potential distribution was studied during constant current discharge operations for various current rates and discharge pulses in order to evoke local inhomogeneities for temperatures ranging from 10 °C to 40 °C. Differences in local potentials were considered for estimating local SOC variations within the electrodes. It could be observed that even low currents such as 0.1C can lead to significant inhomogeneities, whereas a higher cell temperature generally results in more pronounced inhomogeneities. A rapid SOC equilibration can be observed if the variation in the SOC distribution corresponds to a considerable potential difference defined by the open circuit voltage of either the positive or negative electrode. With increasing temperature, accelerated equalization effects can be observed. © 2016 Elsevier B.V.
    view abstract10.1016/j.jpowsour.2016.08.120
  • Ultrahigh vacuum and electrocatalysis – The powers of quantitative surface imaging
    P. Mercer, M. and E. Hoster, H.
    Nano Energy 29 (2016)
    We highlight the impact of Ultrahigh Vacuum (UHV)-born surface science on modern electrocatalysis. The microscopic, atomic level picture of surface adsorption and reaction, which was developed in the surface science community in decades of systematic research on single crystals in UHV, has meanwhile become state-of-the-art also in electrochemistry. For the example of CO on Pt(111) single crystals, which has been extensively studied at the solid/gas and the solid/liquid interface using atomic resolution scanning tunnelling microscopy (STM), we highlight how both interfaces may have even more in common than often assumed. We then illustrate how planar model surfaces such as mono- and bimetallic single crystals and surface alloys, prepared and thoroughly analysed in UHV, enabled a systematic search for improved electrocatalysts. Surface alloys, thermodynamically more stable than foreign metal islands, are a particularly important sub-group of model surfaces, which so far have only been fabricated in UHV. We also flag that model surfaces may not always assume the structure anticipated for the respective experiment, regardless of their preparation in UHV or by electrochemical methods. “Accidental” surface alloying may be more common than often assumed, leading to misinterpretations of the structure-property relationships targeted in many model studies. We highlight that controlled surface alloy formation should be a key step in any model study looking at bimetallic systems in order to get an idea what the effect of unintended alloying could possibly be, and to cross-check whether alloyed surfaces may potentially be the better electrocatalysts in the first place. © 2016
    view abstract10.1016/j.nanoen.2016.04.015
  • Fast and Accurate Measurement of Entropy Profiles of Commercial Lithium-Ion Cells
    Osswald, P.J. and Del Rosario, M. and Garche, J. and Jossen, A. and Hoster, H.E.
    Electrochimica Acta 177 (2015)
    We report on an effective approach to speed up the measurement of thermodynamic characterization curves (entropy of reaction ΔrS(x)) of rechargeable batteries, in particular commercial 18650 lithium ion cells. We propose and demonstrate a measurement and data processing protocol that reduces the time required to record entropy profiles from time scales of weeks to time scales of hours - without loss in accuracy. For time consuming studies such as investigations on ageing of battery cells, entropy profile measurements thus become as feasible as conventional electrochemical characterisation techniques like dV/dQ or cyclic voltammetry. We demonstrate this at the examples of two ageing protocols applied to a commercial high power and a commercial high energy cell, respectively: (i) accelerated calendric aging by storing cells at 100% state of charge at 60 °C and (ii) continuous cycling with a 1C current at 25 °C. © 2015 Published by Elsevier Ltd.
    view abstract10.1016/j.electacta.2015.01.191
  • Simulation and measurement of local potentials of modified commercial cylindrical cells: I. Cell preparation and measurements
    Osswald, P.J. and Erhard, S.V. and Wilhelm, J. and Hoster, H.E. and Jossen, A.
    Journal of the Electrochemical Society 162 (2015)
    This work presents a modification approach and first measurements of commercial cylindrical Li-ion cells with multiple local potential probes and an internal temperature sensor. Local potential measurements at low currents show a non-uniform potential distribution along the electrode, dominated by the open circuit voltage (OCV) of the negative electrode. For higher currents, the overpotential along the current collector becomes dominant and instead of a corrugated potential distribution, a significant current dependent voltage gradient can be detected, indicating a highly non-uniform state of charge (SOC) distribution with increasing distance to the current collecting tab. After the discharge operation, a quick potential equalization can be witnessed which results in a non-detectable potential difference between the single electrode sections after 12 min, even though the overall cell voltage relaxation has not reached an equilibrium state yet. The presented modification approach combines the advantages of high quality industrial manufactured cells showing uniform coating thicknesses and packing density with the advantages of special tailor made cells for in situ measurements. Due to the low impact of the modification and its long-term stability, highly reproducible measurements can be conducted at different locations of the electrodes. © 2015 The Electrochemical Society.
    view abstract10.1149/2.0561510jes
  • Suppressing Vertical Displacement of Lithiated Silicon Particles in High Volumetric Capacity Battery Electrodes
    Yu, D.Y.W. and Zhao, M. and Hoster, H.E.
    ChemElectroChem 2 (2015)
    Silicon is a potential high-capacity anode material for lithium-ion batteries. However, large volume changes in the material remains a bottleneck to its commercialization. Many works have been devoted to nanostructured composites with voids to accommodate the volume expansion. Yet, the full capability of silicon cannot be utilized, because these nanostructured electrodes have low volumetric capacities. Herein, we redesign dense silicon electrodes with three times the volumetric capacity of graphite. Insitu electrochemical dilatometry reveals that the electrode thickness change is nonlinear as a function of state of charge and highly affected by the electrode composition. One key problem is the large vertical displacement of the silicon particles during lithiation, which leads to irreversible particle detachment and electrode porosity increase. Better reversibility in electrode thickness changes can be achieved by using polyimide, with a higher modulus and larger ultimate elongation, as the binder, leading to better cycle stability. On the move: Vertical displacement of silicon particles, owing to volume expansion and contraction during charge and discharge in a high volumetric capacity battery electrode, is monitored by using electrochemical dilatometry and suppressed by the use of a polyimide binder. © 2015 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/celc.201500133
  • Vanadium-based polyoxometalate as new material for sodium-ion battery anodes
    Hartung, S. and Bucher, N. and Chen, H.-Y. and Al-Oweini, R. and Sreejith, S. and Borah, P. and Yanli, Z. and Kortz, U. and Stimming, U. and Hoster, H.E. and Srinivasan, M.
    Journal of Power Sources 288 (2015)
    Affordable energy storage is crucial for a variety of technologies. One option is sodium-ion batteries (NIBs) for which, however, suitable anode materials are still a problem. We report on the application of a promising new class of materials, polyoxometalates (POMs), as an anode in NIBs. Specifically, Na6[V10O28]·16H2O is being synthesized and characterized. Galvanostatic tests reveal a reversible capacity of approximately 276 mA h g-1 with an average discharge potential of 0.4 V vs. Na/Na+, as well as a high cycling stability. The underlying mechanism is rationalized to be an insertion of Na+ in between the [V10O28]6- anions rather than an intercalation into a crystal structure; the accompanying reduction of V+V to V+IV is confirmed by X-ray Photoelectron Spectroscopy. Finally, a working full-cell set-up is presented with the POM as the anode, substantiating the claim that Na6[V10O28]·16H2O is a promising option for future high-performing sodium-ion batteries. © 2015 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2015.04.009
  • Vehicle emissions: Volkswagen and the road to Paris
    Whiteman, G. and Hoster, H.
    Nature 527 (2015)
    view abstract10.1038/527038a
  • A novel ionic liquid for Li ion batteries-uniting the advantages of guanidinium and piperidinium cations
    Bucher, N. and Hartung, S. and Arkhipova, M. and Yu, D. and Kratzer, P. and Maas, G. and Srinivasan, M. and Hoster, H.E.
    RSC Advances 4 (2014)
    We report on the synthesis and the properties of N,N,N′,N′- tetramethyl-N′′,N′′-pentamethyleneguanidinium bis(trifluoromethylsulfonyl)imide (PipGuan-TFSI). The cation of this novel ionic liquid combines guanidinium and piperidinium structural elements. We tested it for its viscosity, ion conductivity, and also for its thermal and electrochemical stability. Furthermore, a 0.5 M solution of lithium TFSI in PipGuan-TFSI was tested as an electrolyte for Li-ion batteries. These experiments included cycles of Li deposition/dissolution on stainless steel and (de)intercalation into/from LiFePO4 electrodes. The tests involving LiFePO4 cathodes were performed at various C-rates and temperatures for a better quantitative comparison to other electrolyte systems. We discuss in how far PipGuan-TFSI successfully combines the advantages of guanidinium and piperidinium ionic liquids for battery electrolyte applications. © 2014 The Royal Society of Chemistry.
    view abstract10.1039/c3ra46118a
  • Bulk antimony sulfide with excellent cycle stability as next-generation anode for lithium-ion batteries
    Yu, D.Y.W. and Hoster, H.E. and Batabyal, S.K.
    Scientific Reports 4 (2014)
    Nanomaterials as anode for lithium-ion batteries (LIB) have gained widespread interest in the research community. However, scaling up and processibility are bottlenecks to further commercialization of these materials. Here, we report that bulk antimony sulfide with a size of 10-20 μm exhibits a high capacity and stable cycling of 800 mAh g-1. Mechanical and chemical stabilities of the electrodes are ensured by an optimal electrode-electrolyte system design, with a polyimide-based binder together with fluoroethylene carbonate in the electrolyte. The polyimide binder accommodates the volume expansion during alloying process and fluoroethylene carbonate suppresses the increase in charge transfer resistance of the electrodes. We observed that particle size is not a major factor affecting the charge-discharge capacities, rate capability and stability of the material. Despite the large particle size, bulk antimony sulfide shows excellent rate performance with a capacity of 580 mAh g-1 at a rate of 2000 mA g-1.
    view abstract10.1038/srep04562
  • Enhanced cycling stability of o-LiMnO2 cathode modified by lithium boron oxide coating for lithium-ion batteries
    Nagasubramanian, A. and Yu, D.Y.W. and Hoster, H. and Srinivasan, M.
    Journal of Solid State Electrochemistry 18 (2014)
    The effect of lithium boron oxide (LBO) coating on the electrochemical performance of orthorhombic LiMnO2 (o-LiMnO2) cathode for lithium-ion batteries is investigated. o-LiMnO2 synthesized via solid state synthesis technique is modified with LBO addition. The presence of LBO is identified via Fourier transform infrared spectroscopy analysis. o-LiMnO 2 is observed to transform to a spinel-like phase during cycling which undergoes capacity fading. Studies indicate that the presence of 1-2 wt% LBO results in an improved capacity and better capacity retention with cycling. The pristine sample reveals a maximum specific capacity of 172 mAhg -1, whereas the LBO-modified samples display about 189.1 mAhg -1 in the cycling tests conducted at a rate of 50 mAg-1 in the voltage range of 2-4.5 V. After 70 cycles, the LBO-modified LiMnO 2 displayed higher capacity retention of 175 mAhg-1 as compared to the pristine sample that exhibited 130 mAhg-1. By analyzing the charge-discharge behavior, it is observed that the capacity obtained from lithium insertion into the tetrahedral sites of the spinel structure is more or less constant throughout the cycling and that the bulk of the capacity loss is resulting when lithium is inserted into the octahedral sites of the spinel structure. Impedance measurement reveals a reduced charge-transfer resistance for the LBO-modified samples suggesting that the presence of LBO is countering capacity loss arising from insertion of lithium into the octahedral sites thus contributing to the overall cycling stability. © 2014 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s10008-014-2421-3
  • Hierarchically porous three-dimensional electrodes of CoMoO4 and ZnCo2O4 and their high anode performance for lithium ion batteries
    Yu, H. and Guan, C. and Rui, X. and Ouyang, B. and Yadian, B. and Huang, Y. and Zhang, H. and Hoster, H.E. and Fan, H.J. and Yan, Q.
    Nanoscale 6 (2014)
    Ternary metal oxides have been receiving wide attention in electrochemical energy storage due to their rich redox reactions and tuneable conductivity. We present a simple solution-based method to prepare a 3D interconnected porous network of ternary metal oxide (CoMoO4 and ZnCo2O 4) nanostructures on macroporous nickel foam. The open-structured networks with different degrees of porosity endow them with high surface areas of electro-active sites. The Li ion storage properties of both anodes are investigated. High rate capability and long term cycling stability are achieved for both systems. © the Partner Organisations 2014.
    view abstract10.1039/c4nr03631j
  • History effects in lithium-oxygen batteries: How initial seeding influences the discharge capacity
    Rinaldi, A. and Wijaya, O. and Hoster, H.E. and Yu, D.Y.W.
    ChemSusChem 7 (2014)
    In laboratory experiments, Li-O2 systems show "sudden death" at capacities far below the theoretical value. Identifying how discharge products limit the total capacity is crucial in Li-O2 system. We investigated the effect of Li2O2 seed layer deposited on carbon cathode under potentiostatic conditions at increasing overpotentials to the subsequent slow discharge at galvanostatic condition. The discharge capacity attainable in the second step is found to vary by more than a factor of 3 depending on the history, i.e., the seed layer. These results provide evidence that the battery history is decisive for the total discharge capacities. History lesson: The discharge product will at some point form the surface of the ongoing electrochemical reaction in Li-O2 battery. The nature of Li2O2 deposits are crucial for a battery's capacity performance. The discharge profiles of carbon cathodes that are precovered by Li2O2 seed layers are compared. The layers are Coulometrically equal but are deposited at varying deposition rates, and demonstrate how faster initial seeding leads to lower total discharge capacities. © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/cssc.201300986
  • Impact of active material surface area on thermal stability of LiCoO 2 cathode
    Geder, J. and Hoster, H.E. and Jossen, A. and Garche, J. and Yu, D.Y.W.
    Journal of Power Sources 257 (2014)
    Thermal stability of charged LiCoO2 cathodes with various surface areas of active material is investigated in order to quantify the effect of LiCoO2 surface area on thermal stability of cathode. Thermogravimetric analyses and calorimetry have been conducted on charged cathodes with different active material surface areas. Besides reduced thermal stability, high surface area also changes the active material decomposition reaction and induces side reactions with additives. Thermal analyses of LiCoO2 delithiated chemically without any additives or with a single additive have been conducted to elaborate the effect of particle size on side reactions. Stability of cathode-electrolyte system has been investigated by accelerating rate calorimetry (ARC). Arrhenius activation energy of cathode decomposition has been calculated as function of conversion at different surface area of active material. © 2014 Elsevier B.V. All rights reserved.
    view abstract10.1016/j.jpowsour.2014.01.116
  • Iron(III) sulfate: A stable, cost effective electrode material for sodium ion batteries
    Mason, C.W. and Gocheva, I. and Hoster, H.E. and Yu, D.Y.W.
    Chemical Communications 50 (2014)
    Iron(III) sulfate, a rhombohedral NASICON compound, has been demonstrated as a sodium intercalation host. This cost-effective material is attractive, as it can be slurry processed in bulk with ball-milling, while utilizing the iron 2+/3+ redox couple, offering stable 3.2 V performance for over 400 cycles. © 2014 The Royal Society of Chemistry.
    view abstract10.1039/c3cc47557c
  • Layered NaxMnO2+z in sodium ion batteries-influence of morphology on cycle performance
    Bucher, N. and Hartung, S. and Nagasubramanian, A. and Cheah, Y.L. and Hoster, H.E. and Madhavi, S.
    ACS Applied Materials and Interfaces 6 (2014)
    Due to its potential cost advantage, sodium ion batteries could become a commercial alternative to lithium ion batteries. One promising cathode material for this type of battery is layered sodium manganese oxide. In this investigation we report on the influence of morphology on cycle performance for the layered NaxMnO2+z. Hollow spheres of Na xMnO2+z with a diameter of ∼5 μm were compared to flake-like NaxMnO2+z. It was found that the electrochemical behavior of both materials as measured by cyclic voltammetry is comparable. However, the cycle stability of the spheres is significantly higher, with 94 mA h g-1 discharge capacity after 100 cycles, as opposed to 73 mA h g-1 for the flakes (50 mA g-1). The better stability can potentially be attributed to better accommodation of volume changes of the material due to its spherical morphology, better contact with the added conductive carbon, and higher electrode/electrolyte interface owing to better wetting of the active material with the electrolyte. © 2014 American Chemical Society.
    view abstract10.1021/am406009t
  • Oxygen reduction on structurally well defined, bimetallic PtRu surfaces: Monolayer PtxRu1-x/Ru(0001) surface alloys versus Pt film covered Ru(0001)
    Brimaud, S. and Engstfeld, A.K. and Alves, O.B. and Hoster, H.E. and Behm, R.J.
    Topics in Catalysis 57 (2014)
    The electrocatalytic activity of different, structurally well defined bimetallic PtRu surfaces in the oxygen reduction reaction was investigated by a combination of scanning tunnelling microscopy and electrochemical measurements performed under controlled mass transport conditions in a flow cell. We compare the effect of pseudomorphic Pt cover layers, mimicking the situation in a core-shell Pt/Ru nanoparticle, and of mixed PtxRu1-x monolayer surface alloys, reflecting the situation in an alloyed nanoparticle. The results unambiguously demonstrate that these bimetallic surfaces can reach activities well in excess of that of Pt(111), both for the film surfaces and the surface alloys, by optimizing the Pt surface content (surface alloys) or the Pt film thickness (film surfaces). The results are compared with simulated kinetic current-potential profiles based on existent density functional theory calculations (Greeley and Nørskov, J Phys Chem C 113:4932, 2009; Lischka et al., Electrochim Acta 52:2219, 2007) revealing very good agreement in trends. Potential and limits of this approach are discussed. © 2013 Springer Science+Business Media New York.
    view abstract10.1007/s11244-013-0177-0
  • Sodium vanadium oxide: A new material for high-performance symmetric sodium-ion batteries
    Hartung, S. and Bucher, N. and Nair, V.S. and Ling, C.Y. and Wang, Y. and Hoster, H.E. and Srinivasan, M.
    ChemPhysChem 15 (2014)
    Room-temperature sodium-ion batteries have the potential to become the technology of choice for large-scale electrochemical energy storage because of the high sodium abundance and low costs. However, not many materials meet the performance requirements for practical applications. Here, we report a novel sodium-ion battery electrode material, Na2.55V6O 16·0.6H2O, that shows significant capacities and stabilities at high current rates up to 800 mAg-1. X-ray photoelectron spectroscopy measurements are carried out to better understand the underlying reactions. Moreover, due to the different oxidation states of vanadium, this material can also be employed in a symmetric full cell, which would decrease production costs even further. For these full cells, capacity and stability tests are conducted using various cathode:anode mass ratios. © 2014 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201402020
  • Strongly coupled carbon nanofiber-metal oxide coaxial nanocables with enhanced lithium storage properties
    Zhang, G. and Wu, H.B. and Hoster, H.E. and Lou, X.W.
    Energy and Environmental Science 7 (2014)
    A facile two-step strategy involving a polyol method and subsequent thermal annealing treatment is successfully developed for the general synthesis of metal oxide/carbon coaxial nanocables. Benefitting from the strong coupling effect, these hybrid nanocables exhibit remarkable lithium storage properties with high capacity, long cycle life and excellent rate capability. © 2014 The Royal Society of Chemistry.
    view abstract10.1039/c3ee43123a
  • Activating vanadium's highest oxidation state in the NASICON structure
    Mason, C.W. and Gocheva, I. and Hoster, H.E. and Yu, D.Y.W.
    ECS Transactions 58 (2013)
    The capacity and energy density of Na3V2(PO 4)3 cathode material, for sodium ion batteries, was enhanced by partial substitution of iron in the octahedral vanadium site. Through this process, we show for the first time that the V4+/V 5+ redox couple can be utilized in rhombohedral NASICON at above 4.0V vs. Na/Na+. At this juncture, we discovered that iron is unique in its ability to easily activate vanadium's highest oxidation state. Furthermore, an additional 12% capacity is realized due to the increased sodium concentration in the synthesized compound. © The Electrochemical Society.
    view abstract10.1149/05812.0041ecst
  • Building 3D structures of vanadium pentoxide nanosheets and application as electrodes in supercapacitors
    Zhu, J. and Cao, L. and Wu, Y. and Gong, Y. and Liu, Z. and Hoster, H.E. and Zhang, Y. and Zhang, S. and Yang, S. and Yan, Q. and Ajayan, P.M. and Vajtai, R.
    Nano Letters 13 (2013)
    Various two-dimensional (2D) materials have recently attracted great attention owing to their unique properties and wide application potential in electronics, catalysis, energy storage, and conversion. However, large-scale production of ultrathin sheets and functional nanosheets remains a scientific and engineering challenge. Here we demonstrate an efficient approach for large-scale production of V2O5 nanosheets having a thickness of 4 nm and utilization as building blocks for constructing 3D architectures via a freeze-drying process. The resulting highly flexible V 2O5 structures possess a surface area of 133 m2 g-1, ultrathin walls, and multilevel pores. Such unique features are favorable for providing easy access of the electrolyte to the structure when they are used as a supercapacitor electrode, and they also provide a large electroactive surface that advantageous in energy storage applications. As a consequence, a high specific capacitance of 451 F g-1 is achieved in a neutral aqueous Na2SO4 electrolyte as the 3D architectures are utilized for energy storage. Remarkably, the capacitance retention after 4000 cycles is more than 90%, and the energy density is up to 107 W·h·kg-1 at a high power density of 9.4 kW kg -1. © 2013 American Chemical Society.
    view abstract10.1021/nl402969r
  • Combustion-synthesized sodium manganese (cobalt) oxides as cathodes for sodium ion batteries
    Bucher, N. and Hartung, S. and Gocheva, I. and Cheah, Y.L. and Srinivasan, M. and Hoster, H.E.
    Journal of Solid State Electrochemistry 17 (2013)
    We report on the electrochemical properties of layered manganese oxides, with and without cobalt substituents, as cathodes in sodium ion batteries. We fabricated sub-micrometre-sized particles of Na0.7MnO2 + z and Na0.7Co0.11Mn0.89O2 + z via combustion synthesis. X-ray diffraction revealed the same layered hexagonal P2-type bronze structure with high crystallinity for both materials. Potentiostatic and galvanostatic charge/discharge cycles in the range 1.5-3.8 V vs. Na | Na+ were performed to identify potential-dependent phase transitions, capacity, and capacity retention. After charging to 3.8 V, both materials had an initial discharge capacity of 138 mA h g-1 at a rate of 0.3 C. For the 20th cycle, those values reduced to 75 and 92 mA h g -1 for Co-free and Co-doped samples, respectively. Our findings indicate that earlier works probably underestimated the potential of (doped) P2-type Na0.7MnO2 + z as cathode material for sodium ion batteries in terms of capacity and cycle stability. Apart from doping, a simple optimization parameter seems to be the particle size of the active material. © 2013 Springer-Verlag Berlin Heidelberg.
    view abstract10.1007/s10008-013-2047-x
  • Controlled synthesis of manganese oxyhydroxide nanotubes: Implications for high-efficiency supercapacitors
    Tan, H.T. and Rui, X. and Shi, W. and Xu, C. and Yu, H. and Hoster, H.E. and Yan, Q.
    ChemPlusChem 78 (2013)
    Successful attempts have been made to control the synthesis of tubular MnOOH with nanodimensions on high electronic conductivity graphite felt (GF) to be used as a flexible supercapacitor electrode. As a fundamental study, the time-dependent kinetics was investigated to interpret its formation mechanism, which can be depicted as the curling of a two-dimensional precursor into a one-dimensional structure with a hollow interior. As a result of the nanotube structure, the active surface area of MnOOH is completely accessible to electrolyte ions and has a shorter charge-transport length and greater ability to withstand structural deformation. Hence, hollow-structured MnOOH shows great promise as an electrochemical system, which is reflected in its high specific capacitance of 1156 F g-1 at 1 A g-1. Furthermore, the high energy density of 1125 W h kg-1 and power density of 5.05 kW kg-1 reveal the outstanding energy-storage behavior of the MnOOH/GF composites as flexible supercapacitor electrodes. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/cplu.201300095
  • Cu doped V2O5 flowers as cathode material for high-performance lithium ion batteries
    Yu, H. and Rui, X. and Tan, H. and Chen, J. and Huang, X. and Xu, C. and Liu, W. and Yu, D.Y.W. and Hng, H.H. and Hoster, H.E. and Yan, Q.
    Nanoscale 5 (2013)
    Hierarchical Cu doped vanadium pentoxide (V2O5) flowers were prepared via a simple hydrothermal approach followed by an annealing process. The flower precursors are self-assembled with 1D nanobelts surrounding a central core. The morphological evolution is investigated and a plausible mechanism is proposed. As the cathode material for lithium ion batteries, the Cu doped V2O5 samples exhibit improved electrochemical performance compared to the un-doped ones. Among them Cu 0.02V1.98O5 delivered higher reversible specific capacities, better cycling stabilities and excellent rate capabilities, e.g. 97 mA h g-1 at 20.0 C. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c3nr00548h
  • Facile preparation of ordered porous graphene-metal Oxide@C binder-free electrodes with high Li storage performance
    Zhu, J. and Yang, D. and Rui, X. and Sim, D. and Yu, H. and Hoster, H.E. and Ajayan, P.M. and Yan, Q.
    Small 9 (2013)
    A facile and general method is reported to prepare ordered porous graphene-based binder-free electrodes on a large scale. This preparation process allows the easy adjustment of the selected components, weight ratio of componets, and the thickness of the electrodes. Such ordered porous electrodes demonstrate superior Li storage properties; for example, graphene-Fe 3O4@C depicts high capacities of 1123.8 and 505 mAh g -1 at current densities of 0.5 and 10 A g-1, respectively. Copyright © 2013 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/smll.201300755
  • Hierarchical hollow spheres composed of ultrathin Fe2O 3 nanosheets for lithium storage and photocatalytic water oxidation
    Zhu, J. and Yin, Z. and Yang, D. and Sun, T. and Yu, H. and Hoster, H.E. and Hng, H.H. and Zhang, H. and Yan, Q.
    Energy and Environmental Science 6 (2013)
    Hollow hierarchical spheres self-organized from the ultrathin nanosheets of α-Fe2O3 were prepared by a simple process. These ultrathin nanosheet subunits possess an average thickness of around 3.5 nm and show preferential exposure of (110) facets. Their Li ion storage and visible-light photocatalytic water oxidation performance are tested. Such hierarchical nanostructures show high Li storage properties with good cycling stability and excellent rate capabilities. The water oxidation catalytic activity is 70 μmol h-1 g-1 for O2 evolution under visible light irradiation and can be maintained for 15 hours. The structural features of these α-Fe2O3 nanocrystals are considered to be important to lead to the attractive properties in both Li storage and photocatalytic water oxidation, e.g. hollow interior, ultrathin thickness and largely exposed active facets. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c2ee24148j
  • Stabilization of large adsorbates by rotational entropy: A time-resolved variable-temperature STM study
    Waldmann, T. and Klein, J. and Hoster, H.E. and Behm, R.J.
    ChemPhysChem 14 (2013)
    Investigating the dynamics in an adlayer of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4-BTP) on Ag(111) by fast scanning tunneling microscopy (video-STM), we found that rotating 2,4′-BTP adsorbates coexist in a two-dimensional (2D) liquid phase (β-phase) in a dynamic equilibrium with static adsorbate molecules. Furthermore, exchange between an ordered phase (α-phase) and β-phase leads to fluctuations of the domain boundary on a time scale of seconds. Quantitative evaluation of the temperature-dependent equilibrium between rotating and static adsorbates, evaluated from a large number of STM images, gains insight into energetic and entropic stabilization and underlines that the rotating adsorbate molecules are stabilized by an entropy contribution, which is compatible with that derived by using statistical mechanics. The general validity of the concept of entropic stabilization of rotating admolecules, favoring rotation already at room temperature, is tested for other typical small, mid-size and large adsorbates. Big is best: Bis(terpyridine) adsorbates on a Ag(111) surface in combination with large data sets from scanning tunneling microscopy and statistical mechanics calculations are used as a model system for entropic stabilization of large rotating adsorbates (see picture). The general validity of this concept is tested for other typical small, mid-sized and large adsorbates. Copyright © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201200531
  • Synthesis of cobalt phosphides and their application as anodes for lithium ion batteries
    Yang, D. and Zhu, J. and Rui, X. and Tan, H. and Cai, R. and Hoster, H.E. and Yu, D.Y.W. and Hng, H.H. and Yan, Q.
    ACS Applied Materials and Interfaces 5 (2013)
    A facile thermal decomposing method has been developed for the fabrication of CoxP nanostructures with controlled size, phase, and shape (e.g., Co2P rod and spheres, CoP hollow and solid particles). An amorphous carbon layer could be introduced by the carbonization of organic surfactants from the precursors. The electrochemical performance of typical CoP and Co 2P samples as anode materials has been investigated and the CoP hollow nanoparticle with carbon coating layer depicts good capacity retention and high rate capability (e.g., specific capacity of 630 mA h g-1 at 0.2 C after 100 cycles, and a reversible capacity of 256 mA h g-1 can be achieved at a high current rate of 5 C). © 2013 American Chemical Society.
    view abstract10.1021/am302877q
  • Synthesis of one-dimensional hierarchical NiO hollow nanostructures with enhanced supercapacitive performance
    Zhang, G. and Yu, L. and Hoster, H.E. and Lou, X.W.
    Nanoscale 5 (2013)
    One-dimensional hierarchical hollow nanostructures composed of NiO nanosheets are successfully synthesized through a facile carbon nanofiber directed solution method followed by a simple thermal annealing treatment. With the advantages of high electro-active surface area, carbon nanofiber supported robust structure and short ion and electron transport pathways, the hierarchical hybrid nanostructures deliver largely enhanced capacitance with excellent cycling stability when evaluated as electrode materials for supercapacitors. More specifically, a high capacitance of 642 F g-1 is achieved when the charge-discharge current density is 3 A g-1 and the total capacitance loss is only 5.6% after 1000 cycles. © 2013 The Royal Society of Chemistry.
    view abstract10.1039/c2nr33326k
  • Directed assembly of Ru nanoclusters on Ru(0001)-supported graphene: STM studies and atomistic modeling
    Engstfeld, A.K. and Hoster, H.E. and Behm, R.J. and Roelofs, L.D. and Liu, X. and Wang, C.-Z. and Han, Y. and Evans, J.W.
    Physical Review B - Condensed Matter and Materials Physics 86 (2012)
    Directed assembly of an array of Ru nanoclusters (NCs) is achieved by deposition of Ru at around room temperature on a single layer of graphene supported on Ru(0001). In this system, directed assembly is guided by the periodic moiré structure of the buckled graphene sheet. Behavior is analyzed utilizing both scanning tunneling microscopy and atomistic lattice-gas modeling together with kinetic Monte Carlo simulation. We elucidate the kinetics of NC nucleation and growth, specifically assessing the coverage dependence of the NC density and height distribution. In addition, we provide a detailed characterization of the development of short-range spatial order within the NC array, identifying a tendency for row formation. © 2012 American Physical Society.
    view abstract10.1103/PhysRevB.86.085442085442
  • Fe2O3 nanocluster-decorated graphene as O2 electrode for high energy Li-O2 batteries
    Zhang, W. and Zeng, Y. and Xu, C. and Tan, H. and Liu, W. and Zhu, J. and Xiao, N. and Hng, H.H. and Ma, J. and Hoster, H.E. and Yazami, R. and Yan, Q.
    RSC Advances 2 (2012)
    Fe2O3 nanocluster-decorated graphene (Fe 2O3/graphene) hybrids with controlled contents of Fe 2O3 were prepared by a facile electrochemical process. These Fe2O3/graphene hybrids were tested as O2 electrodes for Li-O2 batteries, which exhibited enhanced discharge capacities as compared to that of a pure graphene based O2 electrode, e.g. the Fe2O3/graphene electrode with 29.0 wt% of Fe2O3 delivered a discharge capacity of 8290 mA h g -1 and a round-trip efficiency of 65.9% as compared to 5100 mA h g-1 and 57.5% for a pure graphene electrode. The excellent electrochemical properties of Fe2O3/graphene as an O 2 electrode is ascribed to the combination of the fast kinetics of electron transport provided by the graphene sheets and the high electrocatalytic activity for O2 reduction provided by the Fe2O 3. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2ra20757e
  • Formation of ZnMn 2O 4 ball-in-ball hollow microspheres as a high-performance anode for lithium-ion batteries
    Zhang, G. and Yu, L. and Wu, H.B. and Hoster, H.E. and Lou, X.W.
    Advanced Materials 24 (2012)
    Novel ZnMn 2O 4 ball-in-ball hollow microspheres are fabricated by a facile two-step method involving the solution synthesis of ZnMn-glycolate hollow microspheres and subsequent thermal annealing in air. When evaluated as an anode material for lithium-ion batteries, these ZnMn 2O 4 ball-in-ball hollow microspheres show significantly enhanced electrochemical performance with high capacity, excellent cycling stability and good rate capability. Copyright © 2012 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/adma.201201779
  • Formation, atomic distribution and mixing energy in two-dimensional Pd xAg 1-x surface alloys on Pd(111)
    Engstfeld, A.K. and Hoster, H.E. and Behm, R.J.
    Physical Chemistry Chemical Physics 14 (2012)
    The formation and atom distribution in two-dimensional Pd xAg 1-x/Pd(111) monolayer surface alloys were studied by high resolution scanning tunnelling microscopy (STM) with chemical contrast. From short-range order (SRO) parameters, we calculate preferences for like or unlike nearest neighbours to elucidate the mixing behaviour of the two components for various sub monolayer Ag surface contents. In the regime of low Ag surface contents (&lt;40% Ag), the system shows a weak tendency towards phase separation, high Ag coverages (&gt;60% Ag) result in a disperse distribution of the atoms in the surface. Effective pair interactions (EPIs) were derived by comparing the measured distribution with distributions obtained using Monte Carlo (MC) simulations. From the EPIs, we derived a function for the mixing energy, which can describe the change from clustering to a disperse distribution. The effects of the resulting surface atom distributions and of the Ag coverage dependent surface mixing/demixing on catalytic reactions are discussed. © 2012 the Owner Societies.
    view abstract10.1039/c2cp41104k
  • Oxidation of an organic adlayer: A bird?s eye view
    Waldmann, T. and Künzel, D. and Hoster, H.E. and Groß, A. and Behm, R.J.
    Journal of the American Chemical Society 134 (2012)
    The reaction of O 2 with an adlayer of the oligopyridine 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl)-pyridine-2-yl)pyrimidine (2,4′-BTP), adsorbed on the (111) surfaces of silver and gold and on HOPG - which can be considered as a model system for inorganic|organic contacts - was investigated by fast scanning tunneling microscopy (video STM) and dispersion corrected density functional theory (DFT-D) calculations. Only on Ag(111), oxidation of the 2,4′-BTP adlayer was observed, which is related to the fact that under the experimental conditions O 2 adsorbs dissociatively on this surface leading to reactive O adatoms, but not on Au(111) or HOPG. There is a distinct regiospecifity of the oxidation reaction caused by intermolecular interactions. In addition, the oxidation leads to a chiral ordering. The relevance of these findings for reactions involving organic monolayers is discussed. © 2012 American Chemical Society.
    view abstract10.1021/ja302593v
  • Single-crystalline NiCo 2O 4 nanoneedle arrays grown on conductive substrates as binder-free electrodes for high-performance supercapacitors
    Zhang, G.Q. and Wu, H.B. and Hoster, H.E. and Chan-Park, M.B. and Lou, X.W.
    Energy and Environmental Science 5 (2012)
    In this work, we have successfully grown single-crystalline nanoneedle arrays of NiCo 2O 4 on conductive substrates such as Ni foam and Ti foil through a simple solution method together with a post-annealing treatment. Remarkably, the NiCo 2O 4-Ni foam binder-free electrode exhibits greatly improved electrochemical performance with very high capacitance and excellent cycling stability. © 2012 The Royal Society of Chemistry.
    view abstract10.1039/c2ee22572g
  • The role of surface defects in large organic molecule adsorption: Substrate configuration effects
    Waldmann, T. and Nenon, C. and Tonigold, K. and Hoster, H.E. and Groß, A. and Behm, R.J.
    Physical Chemistry Chemical Physics 14 (2012)
    The role of the configuration of metal surface atoms in the interaction between individual large, planar organic molecules and a metal substrate was investigated by low-temperature scanning tunneling microscopy and density functional theory calculations, including a semi-empirical correction scheme to account for dispersion effects. As test case, we used the adsorption of the oligopyridine derivative 2-phenyl-4,6-bis(6-(pyridine-2-yl)-4-(pyridine-4-yl) pyridine-2-yl)pyrimidine (2,4′-BTP) on a stepped Ag(100) surface. Both experiment, via statistical evaluation of the adsorption site and orientation of 2,4′-BTP admolecules, and theory indicate distinct structural effects. The results are compared with the adsorption behavior of pyridine derivatives and benzene on metal surfaces. Consequences on the understanding of the interaction between heteroatoms or functional groups in large organic adsorbates and metal atoms in typical nano-scaled surface defects and hence of the interaction with more realistic metal surfaces are discussed. © 2012 the Owner Societies.
    view abstract10.1039/c2cp40800g
  • Adsorption of supramolecular building blocks on graphite: A force field and density functional theory study
    Künzel, D. and Tonigold, K. and Kučera, J. and Roos, M. and Hoster, H.E. and Behm, R.J. and Groß, A.
    ChemPhysChem 12 (2011)
    Supramolecular building blocks: The adsorption of the oligopyridine isomers 2,4′-BTP and 3,3′-BTP on graphite (see picture) is studied with force field and dispersion-corrected density functional theory (DFT-D) methods. Whereas the used force fields yield different adsorption geometries and strongly varying adsorption energies, the adsorption energy obtained with DFT-D is in rather good agreement with experiment. © 2011 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201100240
  • Anodic hydrogen oxidation at bare and Pt-modified Ru(0001) in flowing electrolyte - Theory versus experiment
    Hoster, H.E.
    Materials Research Society Symposium Proceedings 1388 (2011)
    This paper reports on electrochemical hydrogen oxidation at atomically smooth single crystal surfaces. These surfaces are considered as planar models for (bi)metallic nanoparticles that are commonly used as catalytically active electrode materials in low-temperature fuel cells. These samples are prepared in ultrahigh vacuum but are characterized under conditions of enhanced mass transport in hydrogen saturated electrolyte. The two examples shown in this paper are Ru(0001) with or without an atomically thin layer of Pt. The Pt thin layer turns out to be more active than pure Ru(0001) by three orders of magnitude and also more active than bulk Pt electrodes. We show that those findings agree very well with predictions based on density functional theory in combination with a simple kinetic model. © 2012 Materials Research Society.
    view abstract10.1557/opl.2012.820
  • Direct observation of local chemical surface properties by scanning tunneling microscopy
    Hoster, H.E.
    AIP Conference Proceedings 1454 (2011)
    Vapor deposited Pt films with a thickness locally varying between one and two atomic layers on Ru(0001) are studied as model surfaces with a well-defined lateral variation of the chemical properties. These are probed by reversible CO adsorption at 300 K and with CO pressures in the range pCO=10-10...10-5 mbar under real-time STM observation. Upon exposure to 10-5 mbar CO, no densely packed adlayer is formed on the areas with a local thickness of one atomic layer, whereas a c(4×2) adlayer containing 0.5 CO molecules per surface atom is reversibly formed on areas with locally two atomic layers of Pt. Based on experimental and theoretical data points and parameters from the literature we calculate adsorption isotherms for Pt(111) and the two Ru(0001) supported Pt thin films at 300 K. For all three surfaces, STM observations on CO adsorption are found to be fully consistent with the respective isotherms. © 2012 American Institute of Physics.
    view abstract10.1063/1.4730676
  • Electrochemistry at Ru(0001) in a flowing CO-saturated electrolyte - Reactive and inert adlayer phases
    Alves, O.B. and Hoster, H.E. and Behm, R.J.
    Physical Chemistry Chemical Physics 13 (2011)
    We investigated the electrochemical oxidation and reduction processes on ultrahigh vacuum prepared, smooth and structurally well-characterized Ru(0001) electrodes in a CO-saturated and, for comparison, in a CO-free flowing perchloric acid electrolyte by electrochemical methods and by comparison with previous structural data. Structure and reactivity of the adsorbed layers are largely governed by a critical potential of E = 0.57 V, which determines the onset of Oad formation on the COad saturated surface in the positive-going scan and of Oad reduction in the negative-going scan. Oad formation proceeds via nucleation and 2D growth of high-coverage Oad islands in a surrounding COad phase, and it is connected with COad oxidation at the interface between the two phases. In the negative-going scan, mixed (COad + Oad) phases, most likely a (2 × 2)-(CO + 2O) and a (2×2)-(2CO + O), are proposed to form at E &lt; 0.57 V by reduction of the Oad-rich islands and CO adsorption into the resulting lower-density Oad structures. CO bulk oxidation rates in the potential range E &gt; 0.57 V are low, but significantly higher than those observed during oxidation of pre-adsorbed CO in the CO-free electrolyte. We relate this to high local CO ad coverages due to CO adsorption in the CO-saturated electrolyte, which lowers the CO adsorption energy and thus the barrier for COad oxidation during CO bulk oxidation. © the Owner Societies. 2011.
    view abstract10.1039/c0cp01001d
  • Growth of an oligopyridine adlayer on Ag(100) - A scanning tunnelling microscopy study
    Waldmann, T. and Nenon, C. and Hoster, H.E. and Behm, R.J.
    Physical Chemistry Chemical Physics 13 (2011)
    The growth behaviour of the oligopyridine derivative 2-phenyl-4,6-bis(6- (pyridine-2-yl)-4-(pyridine-4-yl)pyridine-2-yl)pyrimidine (2,4′-BTP) on Ag(100) in the sub-monolayer regime was investigated by variable temperature scanning tunneling microscopy under ultra-high vacuum conditions. Over the entire coverage range, the molecules are adsorbed in a flat lying configuration, with preferential orientations with respect to the 〈110〉 direction of the surface. The azimuth angles are derived using a previously introduced algorithm that fits the positions of the intramolecular N atoms geometrically to the underlying surface lattice ("points-to-lattice fit") [H.E. Hoster et al., Langmuir 2007, 23, 11570], indicating that the orientation of the admolecules and thus of the adllayer structure with respect to the Ag(100) surface lattice is determined by the 2,4′-BTP-Ag(100) interaction, while intermolecular interactions are decisive for the structure of the adlayer. The results will be compared to other adsorption systems. © the Owner Societies 2011.
    view abstract10.1039/c1cp22546d
  • Hierarchical interactions and their influence upon the adsorption of organic molecules on a graphene film
    Roos, M. and Künzel, D. and Uhl, B. and Huang, H.-H. and Brandao Alves, O. and Hoster, H.E. and Gross, A. and Behm, R.J.
    Journal of the American Chemical Society 133 (2011)
    The competition between intermolecular interactions and lateral variations in the molecule-substrate interactions has been studied by scanning tunneling microscopy (STM), comparing the phase formation of (sub)monolayers of the organic molecule 2,4′-BTP on buckled graphene/Ru(0001) and Ag(111) oriented thin films on Ru(0001). On the Ag films, the molecules form a densely packed 2D structure, while on graphene/Ru(0001), only the areas between the maxima are populated. The findings are rationalized by a high corrugation in the adsorption potential for 2,4′-BTP molecules on graphene/Ru(0001). These findings are supported by temperature programmed desorption (TPD) experiments and theoretical results. © 2011 American Chemical Society.
    view abstract10.1021/ja2025855
  • High fidelity self-recognition of isomeric oligopyridines in binary 2D self-assembly and its application for separation
    Caterbow, D. and Roos, M. and Hoster, H.E. and Behm, R.J. and Landfester, K. and Ziener, U.
    Chemistry - A European Journal 17 (2011)
    Self-assembly in two binary mixtures based on three isomeric oligopyridines at the liquid/HOPG (highly oriented pyrolytic graphite) interface is presented. Despite their structural similarity the molecules display exclusive phase separation, which is attributed to the highly specific intermolecular hydrogen bonding interactions. Variation of the mole fractions in solution reveal strongly preferred adsorption of the major compound, which underlines the importance of self-recognition for self-assembly. Those findings at the molecular level can be applied to separation issues on a macroscopic scale, leading to a completely new concept of separation, which could have a strong impact on various chromatographic processes. Binary SAMs put to work: Binary mixtures of structurally closely related isomeric oligopyridine molecules show exclusive phase separation in two-dimensional self-assembly at the solid/liquid interface (see figure). The presence of only a slight imbalance of the oligopyridines in the supernatant causes the adsorption of only one isomer, in a process driven by the energy penalty of the hetero phase boundaries. Such S-shape behaviour can be exploited for separation purposes. Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/chem.201003319
  • Imaging an ionic liquid adlayer by scanning tunneling microscopy at the solid|vacuum interface
    Waldmann, T. and Huang, H.-H. and Hoster, H.E. and Höfft, O. and Endres, F. and Behm, R.J.
    ChemPhysChem 12 (2011)
    Structure formation in an ionic liquid adlayer: First molecularly resolved scanning tunneling microscopy images of an ionic liquid adlayer ([Py 1,4]+ [FAP]- (see image)), evaporated on a Au(111) surface, resolve a molecular pattern at 210 K with a distinct short range order, indicating a 2D solid, while at room temperature, the mobility of the adlayer is too high to resolve molecular features, as expected for a 2D liquid. Copyright © 2011 WILEY-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201100413
  • Intermolecular vs molecule-substrate interactions: A combined STM and theoretical study of supramolecular phases on graphene/Ru(0001)
    Roos, M. and Uhl, B. and Künzel, D. and Hoster, H.E. and Groß, A. and Behm, R.J.
    Beilstein Journal of Nanotechnology 2 (2011)
    The competition between intermolecular interactions and long-range lateral variations in the substrate-adsorbate interaction was studied by scanning tunnelling microscopy (STM) and force field based calculations, by comparing the phase formation of (sub-) monolayers of the organic molecules (i) 2-phenyl-4,6-bis(6-(pyridin-3-yl)-4-(pyridin-3-yl)pyridin-2-yl)pyrimidine (3,3'-BTP) and (ii) 3,4,9,10-perylene tetracarboxylic-dianhydride (PTCDA) on graphene/Ru(0001). For PTCDA adsorption, a 2D adlayer phase was formed, which extended over large areas, while for 3,3'-BTP adsorption linear or ring like structures were formed, which exclusively populated the areas between the maxima of the moiré structure of the buckled graphene layer. The consequences for the competing intermolecular interactions and corrugation in the adsorption potential are discussed and compared with the theoretical results. © 2011 Roos et al.
    view abstract10.3762/bjnano.2.42
  • Segregation and stability in surface alloys: PdxRu 1-x/Ru(0001) and PtxRu1-x/Ru(0001)
    Bergbreiter, A. and Hoster, H.E. and Behm, R.J.
    ChemPhysChem 12 (2011)
    The stability of PdRu/Ru(0001) and PtRu/Ru(0001) surface alloys and the tendency for surface segregation of Pd and Pt subsurface guest metals in these surface alloys is studied by scanning tunneling microscopy (STM) and Auger electron spectroscopy (AES). Atomic resolution STM imaging and AES measurements reveal that upon overgrowing the surface alloys with a 1-2 monolayer Ru film and subsequent annealing to the temperatures required for initial surface alloy formation, the Ru-covered Pd (Pt) atoms float back to the outermost layer. The lateral distribution of these species is also essentially identical to that of the initial surface alloys, before overgrowth by Ru. In combination, this clearly demonstrates that the surface alloys represent stable surface configurations, metastable only towards entropically favored bulk dissolution, and that there is a distinct driving force for surface segregation of these species. Consequences of these data on the mechanism for surface alloy formation are discussed. Floating in PtRu (PdRu) surface alloys on Ru(0001): The PtRu (PdRu)monolayer surface alloy layer is covered with the substrate metal Ru by means of physical vapour depositon. Subsequent annealing to temperatures necessary for surface alloy formation reconstitutes the original Pt (Pd) amount as well as the original atom distribution of the initial equilibrated alloy layer (see picture). Copyright © 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201001087
  • Concentration and coverage dependent adlayer structures: From two-dimensional networks to rotation in a bearing
    Meier, C. and Roos, M. and Kü Nzel, D. and Breitruck, A. and Hoster, H.E. and Landfester, K. and Gross, A. and Behm, R.J. and Ziener, U.
    Journal of Physical Chemistry C 114 (2010)
    A bisterpyridine based molecule, 3,3′-BTP, shows a variety of adlayer structures at the interface between highly oriented pyrolytic graphite (HOPG) and the liquid depending on the concentration in solution. Three closely related linear and one hexagonal 2D patterns are found. Comparison with the self-assembly at the HOPG|gas interface shows that in the absence of the solvent one of the linear and the hexagonal structures can be found. The concentration dependent order of appearance of the different surface structures is rationalized by a thermodynamic model. In the adlayer unit cell, the hexagonal phase offers a central void which is mostly filled with a seventh 3,3′-BTP molecule. In the presence of the solvent, those molecules are presumably rotating, whereas at the HOPG|gas interface no clear rotation can be observed. © 2010 American Chemical Society.
    view abstract10.1021/jp910029z
  • Entropic stabilization of large adsorbates on weakly binding substrates - A thermal desorption and scanning tunneling microscopy study
    Roos, M. and Breitruck, A. and Hoster, H.E. and Behm, R.J.
    Physical Chemistry Chemical Physics 12 (2010)
    Thermal desorption (TD) of oligopyridine from HOPG shifts from ∼700 to ∼500 K when going from (sub-)mono- to multilayers. Stabilization of low coverages results from a continuous shift of the frequency factor ν, from 1015 s-1 for submonolayers to 1024 s -1 for multilayers, whereas the desorption barrier is virtually constant. Applying transition state theory (TST), we can explain this by a change from rotationally/translationally mobile, flat-lying molecules (submonolayers) to immobile, upright molecules (multilayers). At room temperature, (time resolved) scanning tunneling microscopy (STM) gives evidence for the existence and the stability of the mobile phase. © 2010 the Owner Societies.
    view abstract10.1039/b920481d
  • Entropy effects in atom distribution and electrochemical properties of AuxPt1-x/Pt(111) surface alloys
    Bergbreiter, A. and Alves, O.B. and Hoster, H.E.
    ChemPhysChem 11 (2010)
    We report on the structural and electrochemical properties of AuxPt1-x surface alloys prepared by Au vapour deposition onto Pt(111) followed by annealing to 1000 K. Driven by configurational entropy, Pt and Au atoms are distributed homogeneously over the surface. On the nm scale, however, atomically resolved scanning tunnelling microscopy images with chemical contrast reveal the formation of nm-sized Pt-rich and Au-rich aggregates, similar to the behaviour recently reported for PdxRu1-x/Ru(0001) [H. Hartmann, T. Diemant, A. Bergbreiter, J. Bansmann, H. E. Hoster, R. J. Behm, Surf. Sci. 2009, 603, 1439]. Based on the STM data, we determine the abundance of specific adsorption sites for different Au contents, and we derive effective pair interaction parameters that allow reproducing the lateral distribution in Monte Carlo simulations. Cyclic voltammograms of the surface alloys have many similarities with Pt(111). Had and OHad related features both decrease with increasing amount of Au. Both seem to adsorb only on Pt sites, but Had requires smaller ensembles of Pt atoms than OHad. The onset potential for Had-formation decreases with increasing Au content. This is can be explained by an effect of the Au atoms on the entropy of adsorption. © 2010 Wiley-VCH Verlag GmbH&amp; Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201000105
  • From adlayer islands to surface alloy: Structural and chemical changes on bimetallic PtRu/Ru(0001) surfaces
    Diemant, T. and Bergbreiter, A. and Bansmann, J. and Hoster, H.E. and Behm, R.J.
    ChemPhysChem 11 (2010)
    The correlation between structural and chemical properties of bimetallic PtRu/Ru(0001) model catalysts and their modification upon stepwise annealing of a submonolayer Pt-covered Ru(0001) surface up to the formation of an equilibrated PtxRu1-x/Ru(0001) monolayer surface alloy was investigated by scanning tunneling microscopy and by the adsorption of CO and D2 probe molecules. Both temperature-programmed desorption and IR measurements demonstrate the influence of the surface structure on the adsorption properties of the bimetallic surface, which can be explained by changes of the composition of the adsorption ensembles (ensemble effects) for D adsorption and by changes in the electronic interaction (ligand effects, strain effects) of the metallic constituents for CO and D adsorption upon alloy formation. © 2010 Wiley-VCH Verlag GmbH &amp; Co. KGaA, Weinheim.
    view abstract10.1002/cphc.201000391
  • Microscopic views on interface phenomena
    Hoster, H.E.
    ChemPhysChem 11 (2010)
    view abstract10.1002/cphc.201000203
  • Pt promotion and spill-over processes during deposition and desorption of upd-Had and OHad on PtxRu1-x/Ru(0001) surface alloys
    Hoster, H.E. and Janik, M.J. and Neurock, M. and Behm, R.J.
    Physical Chemistry Chemical Physics 12 (2010)
    The electrochemical adsorption of underpotential deposited hydrogen (upd-Had) and OHad on structurally well-defined PtxRu1-x/Ru(0001) surface alloys was investigated by cyclic voltammetry and density functional theory (DFT) calculations. The adsorption energies of both upd-Had and OHad decrease with increasing Pt content in the adsorption ensemble, shifting the onset of upd-Had and OHad formation to increasingly cathodic and anodic potentials, respectively. For bare Ru(0001) and for Ru3 sites in the surface alloy, the stability regions of these two species overlap or almost overlap, respectively. Similar to previous findings for upd-Had adsorption/desorption on partly Pt monolayer island covered Ru(0001) surfaces (J. Phys. Chem. B 2004, 108, 14780), we find a sharp peak at ∼100 mV vs. RHE in each scan direction, which is attributed to a Pt catalyzed OHad↔ upd-Had replacement on Ru3 sites, via adsorption on Pt rich sites and spill-over to Ru3 sites. The decrease of the integrated charge in these peaks with the third power of the Ru surface concentration, which for a random distribution of surface atoms reflects the availability of Ru3 sites, supports the above assignment. © the Owner Societies.
    view abstract10.1039/c003635h
  • Substrate registry in disordered layers of large molecules
    Waldmann, T. and Reichert, R. and Hoster, H.E.
    ChemPhysChem 11 (2010)
    We present a statistic evaluation of the azimuth orientations of flat-adsorbed oligopyridine molecules in disordered adlayers on Au(111) and (111) oriented Ag-adlayers on Ru(0001). On both surfaces, we find a strong preference for a set of twelve angles, which belong to one specific, unsymmetrical alignment and its symmetry equivalents. These angles are also those that exclusively occur in more densely packed, ordered structures on the same surfaces. We describe a geometric fitting algorithm, which correctly predicts these angles, and which only requires the substrate lattice and the positions of the nitrogen atoms within the flat-adsorbed molecule as input parameters. Such predictions are particularly valuable to reduce the parameter space in structure simulations [C. Rohr, M. Balbäs Gambra, K. Gruber, E. C. Constable, E. Frey, T. Franosch, B. A. Hermann, Nano Lett. 2009, 10, 833]. © 2010 Wiley-VCH Verlag GmbH& Co. KGaA, Weinheim.
    view abstract10.1002/cphc.200901028
  • Tuning adsorption via strain and vertical ligand effects
    Hoster, H.E. and Alves, O.B. and Koper, M.T.M.
    ChemPhysChem 11 (2010)
    We report on the structure and electrochemical adsorption properties of well-defined pseudomorphic Pt mono-and multilayers on Ru(0001). These act as model surfaces for Pt(111) with slightly decreased affinity to adsorbed hydrogen (Had) and hydroxyl (OHad). In cyclic voltammograms, this is reflected in more negative/positive potential regions for the reversible adsorption of upd-Had/OHad, respectively, compared to Pt(111). For upd-Had, we show that the corresponding trends can be predicted with high accuracy by density functional theory (DFT). In particular, the upd-Had onset regions can be precisely simulated using the Had adsorption energies from DFT, the layer thickness distribution from STM, and the base voltammogram of Pt(111) as reference.© 2010 Wiley-VCH Verlag GmbH&amp; Co. KGaA, Weinheim.
    view abstract10.1002/cphc.200900500
  • batteries

  • electrolytes

  • scanning tunneling microscopy

« back