The transport of oxygen in a porous perovskite solid oxide fuel cell cathode with a relatively high oxygen ion conductivity is modeled by taking into account exchange kinetics at the gas/electrode interface, bulk diffusion of oxygen vacancies, surface diffusion of adsorbed oxygen atoms, and electrochemical kinetics at the cathode/electrolyte interface. The electrochemical mechanism is assumed to be controlled by direct exchange of oxygen vacancies between the cathode and electrolyte phases. Simulated polarization curves typically exhibit Tafel-like behavior in the cathodic direction, which, however, is caused by concentration rather than activation polarization. In the anodic direction, a limiting current behavior is predicted, due to occupation of oxygen lattice sites on the cathode side of the interface. The effective polarization resistance either decreases or remains constant upon reduction of the oxygen partial pressures depending on prevailing kinetic and material parameters. Analytical expressions valid for the asymptotic case of a fast oxygen adsorption process at the gas/electrode interface are derived for the apparent Tafel slope, apparent exchange current density, anodic limiting current, and the effective polarization resistance. The theoretical results are consistent with experimental data in the literature for dense perovskite electrodes and for porous electrode materials with high oxygen nonstoichiometries. An overall assessment of the two parts of this study indicates that the catalytic properties of the perovskite surface, which enhances adsorption and surface diffusion of oxygen, is more significant than processes involving the bulk material, such as fast oxygen exchange with the bulk and vacancy diffusion, in determining cathode performance. IntroductianRecent development of solid oxide fuel cell (SOFC) cath-* Electrochemical Society Active Member.

The solid-state potentiostatic technique is a convenient and versatile tool for studying oxygen transport in electronically conducting perovskites. Furthermore, a systematic analysis of the relaxation data helps elucidate the mechanistic nature of the prevalent oxygen rate process. As a case in point, we describe potential step measurements on 90% dense samples of SrCo1_Fe03_, focusing per se on the assumptions underlying the technique and the interpretation of results. Careful analysis of the current-relaxation data pointed to the presence of high-diffusivity paths in the samples. It also indicated that the overall transport rate is controlled by an oxygen exchange reaction at the grain boundaries. This complication prevented unambiguous assessment of the chemical diffusion coefficient in this material. In addition, the analysis showed that wide disagreements in the oxygen chemical diffusion coefficients reported in the literature for doped perovskites may be attributed to differences in the sample quality (e.g., density, grain size, and distribution) and the measurement technique employed.

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