Alexei A. Kornyshev

Modelling the performance of the cathode catalyst layer of polymer electrolyte fuel cells

Abstract A known macrohomogeneous model for the cathode catalyst layer of a low temperature fuel cell, which includes the kinetics of oxygen reduction at the catalystxa0∣xa0electrolyte interface, proton transport through the polymer electrolyte network, and oxygen diffusion through hydrophobized voids, is considered. Analytical expressions in the relevant ranges of parameter values are obtained. These are the limits of (i) small currents, (ii) fast oxygen diffusion, (iii) fast proton transport, (iv) high current densities (when the layer is depleted in oxygen). They help to trace the main system properties and operation modes, and the routes for the structure-function optimization. Namely, they rationalize the explicit effect of the kinetic parameters of the oxygen reduction on the performance, the effect of the catalyst layer thickness, the width and the position of the active fraction of the layer, and the dependence of the performance on the oxygen partial pressure. In current–voltage plots, three regions are distinguished, each dominated by one of the three main physico-chemical processes in the cathode. Ranges of the optimum cathode thickness are revealed depending on the current density regimes. A bridge between the structural composition of the cathode and its performance is built with the help of an effective medium theory; the model reproduces the experimentally observed trends.

Statistical geometry of reaction space in porous cermet anodes based on ion-conducting electrolytes: Patterns of degradation

Abstract The statistical geometry of the electrochemically active triple-phase boundary in solid oxide fuel cell (SOFC) anodes with oxygen-ion-conducting (zyrconya type) electrolyte is analyzed by means of an ‘effective-medium’ theory and verified by Monte Carlo simulations. Variation of the triple-phase boundary with time due to spontaneous sintering of metal particles is described by kinetic effective-medium equations. Their solution reveals possible degradation scenarios, as well as the factors that impede degradation, or even cause a rise of the active triple-phase boundary in the course of SOFC operation. The cermet composition, i.e. the relative portion of electrolyte, metal and pores, is among these factors. It is shown that the ‘best’ composition before degradation may not be the one that provides the best performance after degradation. The latter depends on the probability of pore opening in sintering of two metal grains. Rough estimates of this probability (and determination of the ‘optimum’ composition) would be possible from a comparison of the calculated porosity before and after degradation with experimental data, which are not available so far.

Double layer capacitance on a rough metal surface: Surface roughness measured by “Debye ruler”

The properties of the double layer capacity on rough electrodes are discussed in terms of the recently developed linear Poisson-Boltzmann theory (L. I. Daikhin, A. A. Kornyshev, M. Urbakh, Phys. Rev. E 53, 6192 (1996)). This theory offers a concept of a Debye-length K dependent roughness factor, ie roughness function, which determines the deviation of capacitance from the Gouy-Chapman result for a flat interface. Analytical expression for the roughness function is available for the case of weak Euclidean roughness. The two parameters-mean square height and correlation length-appear there. The way how the result changes in the case of moderate and strong roughness is analyzed on the basis of a numerical solution obtained for a model roughness profile; an efficient interpolation formula which covers the limits of weak and strong roughness is suggested. The role of anisotropy of the roughness profile is investigated. The predicted effects could be screened by the crystallographic inhomogeneity of a rough surface not taken into account here. Tentatively ignoring such (often important) complications we discuss the results in the context of a possible method for in situ characterization of surface roughness of metal electrodes, based on the double layer capacity measurements in solutions of variable concentration. 0 1997 Published by Elsevier Science Ltd

Phenomenological Theory of Solid Oxide Fuel Cell Anode

We develop a phenomenological theory of oxygen-ion-conducting porous cermet anode for solid oxide fuel cells utilizing hydrogen, based on a simple picture of macro- and microkinetics of charge and gas transport in the cermet. Its basic equations account for the transport of hydrogen molecules and oxygen anions to the reaction spots, the hydrogen oxidation reaction (whose various mechanisms, including different adsorption stages, are considered) and the water-product removal. Simple analytical results are obtained for a linear current-voltage regime, which demonstrate the interplay of these three processes. The nonlinear behavior is analyzed and classified. Various mechanisms of reaction kinetics are considered, subject to three possible mechanisms of water adsorption, in order to specify the law of conversion of ionic current into electronic one. Revealed is the nature of the intermediate quasi-Tafel regime, in which the anode is usually employed, and of two possible large current regimes: the saturation regime and the blocking regime (due to oxidation of the anode). The study rationalizes principles of anode functioning and builds a basis for a systematic analysis of the effects due to composite structure, that enter through the basic parameters of the theory.

Phase transitions at the electrochemical interface

Abstract Structural transformations at the electrochemical interface induced by variation of the electrode potential are overviewed, ranging from the transitions in adsorbed layers to surface reconstruction. Particular emphasis is given to the missing row reconstruction on noble metals, for which relatively simple statistical theory is developed. The phase diagram in the charge temperature plane is analyzed for systems with and without specific adsorption of ions. The role of water in reconstruction-deconstruction transitions is discussed, together with the effect of reconstruction on the double-layer capacitance.

Electron tunnelling in electrochemical processes and in situ scanning tunnel microscopy of structurally organized systems

Abstract The electronic tunnel factor in homogeneous long-range electron transfer (ET) systems has been characterized experimentally and theoretically in great detail. Outstanding questions, however, remain, for example interference in multiple ET routes and environmentally induced barrier fluctuations. Well characterized electrochemical long-range ET systems have also become available where the tunnel factor can be accurately deconvoluted from the total current. Intriguing new effects here are associated with the electrode charge or overpotential effects on the tunnel factor and with the feasibility of observation of critical current behaviour near phase transitions in thin films across which electron tunnelling occurs. In situ scanning tunnel microscopy (STM) offers a conceptual and technical new frame for mapping of molecular ET routes through large adsorbate molecules with low-lying local fluctuating levels. Such a configuration would be representative of large transition metal complexes or redox metalloproteins. High-resolution images and new theoretical approaches to STM mapping of large adsorbates are, moreover reported. The independent in situ electrode potential control provides new, non-monotonous features in the current-voltage relations which differ from electrochemical ET behaviour at a single interface.

Effect of capillary waves on the double layer capacitance of the interface between two immiscible electrolytes

Abstract The effect of capillary waves on the double layer capacitance of the interface between two immiscible electrolytes has been studied. We have found that a corrugation of the interface due to capillary waves leads to an increase in the capacitance, as compared with the Gouy–Chapman result for a flat interface. The theory offers a concept of a roughness function, R , which determines the deviation of capacitance from the Gouy–Chapman result. R is a function of Debye lengths of the two immiscible electrolytes. Analytical expression for the roughness function has been derived which shows a strong dependence of R on the interfacial tension and the dielectric constants of contacting solutions. Predictions amenable to experimental tests have been discussed.

Nonlocal nonlinear static dielectric response of polar liquids

Abstract A linear, phenomenological theory of the nonlocal dielectric response is extended to the non-linear case. For small external electric fields, we give solutions within a perturbation approach. Results, obtained by molecular dynamics computer simulations for the non-linear response of water are in very good agreement with the theory predictions. The results are applied to the calculation of the electrostatic potential of ions in solution. For the electric field strengths, for which the perturbation theory is still applicable, there are only minor differences with respect to the linear case. A fitting procedure to the simulation data, which covers a wider range of electric fields shows, however, that at larger fields the non-linear effects may change the results considerably, relative to those obtained on the basis of the linear response approximation. We discuss a generalization of the nonlocal electrostatic approach to the calculation of solvation energies, which takes into account non-linear effects.

Hydration force microscopy as a new option for studies of solid-liquid interfaces : some theoretical considerations

Abstract A rationale for in situ scanning force microscopy based on hydration forces is given. Such microscopy would permit the distribution of the hydrated species along a polar surface to be probed directly with another polar surface, preferably of small dimensions. Formulae are derived for the relationship between the lateral correlation functions of hydrated surface groups and the correlated values of the surface force measured by a specially designed and programmed “Gedanken” hydration force nanoscope. Some prospects and problems in the realization of this idea are discussed.