W. Münch

Bio: W. Münch is an academic researcher from Max Planck Society. The author has contributed to research in topics: Proton & Perovskite (structure). The author has an hindex of 16, co-authored 16 publications receiving 1748 citations.

Proton conducting alkaline earth zirconates and titanates for high drain electrochemical applications

Abstract: The mobility and stability of protonic defects in acceptor-doped perovskite-type oxides (ABO 3 ) in the system SrTiO 3 –SrZrO 3 –BaZrO 3 –BaTiO 3 have been examined experimentally and by computational simulations. These materials have the potential to combine high proton conductivity and thermodynamic stability. While any structural and chemical perturbation originating from the B-site occupation (poor chemical matching of the acceptor-dopant or Zr/Ti-mixing) leads to a significant reduction of the mobility of protonic defects, Sr/Ba-mixing on the A-site appears to be less critical. The stability of protonic defects is found to essentially scale with the basicity of the lattice oxygen, which is influenced by both A- and B-site occupations. The highest proton conductivities are observed for acceptor-doped BaZrO 3 . Despite its significantly higher ionic radius compared to Zr 4+ , Y 3+ is found to be optimal as an acceptor dopant for BaZrO 3 . Mulliken population analysis shows that Y does not change the oxide's basicity (i.e. it chemically matches on the Zr-site of BaZrO 3 ). The highest proton conductivities have been observed for high Y-dopant concentrations (15–20 mol%). For temperatures below about 700°C, the observed proton conductivities clearly exceed the oxide ion conductivities of the best oxide ion conductors. The high conductivity and thermodynamic stability make these materials interesting alternatives for oxide ion conductors such as Y-stabilized zirconia, which are currently used as separator material for high drain electrochemical applications, such as solid oxide fuel cells.

The diffusion mechanism of an excess proton in imidazole molecule chains: first results of an ab initio molecular dynamics study

Abstract: The diffusion mechanism of an excess proton in imidazole molecule chains is studied by Car–Parrinello-type ab initio molecular dynamics simulations. The diffusion process is described by a Grotthuss mechanism (structure diffusion) involving proton transfer and local rather than long-range cooperative reorientation of the imidazole chain. At T =390 K, the proton transfer step is found to be fast with a time scale of 0.3 ps. The reorientation step is found to be rate-determining. According to our model, the time scale for the reorientation step is estimated to be approximately 30 ps in this temperature range.

Partial conductivities in SrTiO3 : bulk polarization experiments, oxygen concentration cell measurements, and defect-chemical modeling

Abstract: Knowledge of the exchange kinetics of O2 in SrTiO3 allows us to design appropriate strategies to separate the ionic and the electronic conductivity. In the low-temperature range, where the overall surface reaction is very slow compared to bulk diffusion and measuring time, electrochemical cells of the type Pt|SrTiO3|Pt are self-blocking and self-sealing and a Wagner–Hebb-type polarization succeeds without the necessity of using selectively blocking electrodes. In the present study the ionic conductivity data obtained for Feand Ni-doped SrTiO3 in this way are compared to data obtained from the analysis of the oxygen partial pressure dependence of the total conductivity as well as to defect chemical calculations. In complete contrast to the low temperature situation, at high temperatures, where the surface reaction is fast, the emf technique is conveniently applicable. Results are presented for Pt, O2|SrTiO3|O2, Pt cells. The conductivity behavior of SrTi(Fe)O3 as a function of temperature (20°–1000°C) is complex, due to partially frozen-in equilibria, but even details can be quantitatively understood in terms of a simple defect chemistry. The turnover of the diffusion-controlled regime to the surface reaction-controlled regime can be shifted to significantly lower temperatures by using YBa2Cu3O7–8 electrodes.

Proton diffusion in perovskites: comparison between BaCeO3, BaZrO3, SrTiO3, and CaTiO3 using quantum molecular dynamics

Abstract: Quantum molecular dynamics simulations have been carried out to calculate the diffusion coefficients and the activation energies of protonic defects in BaCeO3, BaZrO3, SrTiO3 and CaTiO3. The calculated activation energies are in agreement with experimental data within statistical uncertainty. The activation energy for proton transfer is found to be significantly affected by the repulsive interaction of the proton with the B-cation (B=Ce, Zr, Ti). A physical interpretation for the measured infra-red spectra can also be obtained from the numerical results.

A quantum molecular dynamics study of the cubic phase of BaTiO3 and BaZrO3

Abstract: High proton mobility in perovskite-type oxides of composition ABO3 strongly depends on the dynamics of the proton environment, especially on the fluctuations of the oxide ion separations. The dynamics of the oxide host lattices of the model materials BaTiO3, and BaZrO3 have been studied using quantum molecular dynamics simulations. The simulation method has already been shown to yield numerical results in agreement with experimental findings for the cubic phase of BaCeO3. At elevated temperatures, rotational diffusion of the protons around the oxygen atoms in the plane perpendicular to the B-O-B axis is found. The free energy of the oxygen lattice vibrations is evaluated and the activation energy for proton transfer is estimated to be 0.45 eV for BaTiO3, 0.69 eV for BaZrO3, and 0.64 eV for BaCeO3.

Proton Conductivity: Materials and Applications

Abstract: In this review the phenomenon of proton conductivity in materials and the elements of proton conduction mechanismsproton transfer, structural reorganization and diffusional motion of extended moietiesare discussed with special emphasis on proton chemistry. This is characterized by a strong proton localization within the valence electron density of electronegative species (e.g., oxygen, nitrogen) and self-localization effects due to solvent interactions which allows for significant proton diffusivities only when assisted by the dynamics of the proton environment in Grotthuss and vehicle type mechanisms. In systems with high proton density, proton/proton interactions lead to proton ordering below first-order phase transition rather than to coherent proton transfers along extended hydrogen-bond chains as is frequently suggested in textbooks of physical chemistry. There is no indication for significant proton tunneling in fast proton conduction phenomena for which almost barrierless proton transfer is suggest...

Proton-conducting oxides

Abstract: ▪ Abstract The structural and chemical parameters determining the formation and mobility of protonic defects in oxides are discussed, and the paramount role of high-molar volume, coordination numbers, and symmetry are emphasized. Symmetry also relates to the structural and chemical matching of the acceptor dopant. Y-doped BaZrO3-based oxides are demonstrated to combine high stability with high proton conductivity that exceeds the conductivity of the best oxide ion conductors at temperatures below about 700°C. The unfavorably high grain boundary impedances and brittleness of ceramics have been reduced by forming solid solutions with small amounts of BaCeO3, and an initial fuel cell test has demonstrated that proton-conducting electrolytes based on Y-doped BaZrO3 provide alternatives for separator materials in solid oxide fuel cells (SOFCs). These materials have the potential to operate at lower temperatures compared with those of conventional SOFCs, and the appearance of chemical water diffusion across the...

Transport in proton conductors for fuel-cell applications: simulations, elementary reactions, and phenomenology.

Abstract: 1. Introduction 46372. Theoretical Methodologies and Simulation Tools 46402.1. Ab Initio Quantum Chemistry 46412.2. Molecular Dynamics 46422.2.1. Classical Molecular Dynamics and MonteCarlo Simulations46432.2.2. Empirical Valence Bond Models 46442.2.3. Ab Initio Molecular Dynamics (AIMD) 46452.3. Poisson−Boltzmann Theory 46452.4. Nonequilibrium Statistical Mechanical IonTransport Modeling46462.5. Dielectric Saturation 46473. Transport Mechanisms 46483.1. Proton Conduction Mechanisms 46483.1.1. Homogeneous Media 46483.1.2. Heterogeneous Systems (ConfinementEffects)46553.2. Mechanisms of Parasitic Transport 46613.2.1. Solvated Acidic Polymers 46613.2.2. Oxides 46654. Phenomenology of Transport inProton-Conducting Materials for Fuel-CellApplications46664.1. Hydrated Acidic Polymers 46664.2. PBI−H

Fuel cell materials and components

Abstract: Fuel cells offer the possibility of zero-emissions electricity generation and increased energy security. We review here the current status of solid oxide (SOFC) and polymer electrolyte membrane (PEMFC) fuel cells. Such solid electrolyte systems obviate the need to contain corrosive liquids and are thus preferred by many developers over alkali, phosphoric acid or molten carbonate fuel cells. Dramatic improvements in power densities have been achieved in both SOFC and PEMFC systems through reduction of the electrolyte thickness and architectural control of the composite electrodes. Current efforts are aimed at reducing SOFC costs by lowering operating temperatures to 500–800 °C, and reducing PEMFC system complexity be developing ‘water-free’ membranes which can also be operated at temperatures slightly above 100 °C.

On the development of proton conducting materials for technological applications

Abstract: Some aspects of the simultaneous optimisation of material properties of proton conductors which are relevant for their use in electrochemical cells such as fuel cells, electrochemical reactors and sensors (high proton conductivity, chemical, electrochemical and morphological stability) are discussed. Suggestions are made for the further development of proton conducting perovskite type oxides, proton conducting polymer membranes and medium temperature proton conducting materials.