MATHEMATICAL-MODELING OF OXYGEN-EXCHANGE AND TRANSPORT IN AIR-PEROVSKITE-YTTRIA-STABILIZED ZIRCONIA INTERFACE REGIONS - II - DIRECT EXCHANGE OF OXYGEN VACANCIES

The transport of oxygen in a porous perovskite solid oxide fuel cell c athode with a relatively high oxygen ion conductivity is modeled by ta king into account exchange kinetics at the gas/electrode interface, bu lk diffusion of oxygen vacancies, surface diffusion of adsorbed oxygen atoms, and electrochemical kinetics at the cathode/electrolyte interf ace. The electrochemical mechanism is assumed to be controlled by dire ct exchange of oxygen vacancies between the cathode and electrolyte ph ases. Simulated polarization curves typically exhibit Tafel-like behav ior in the cathodic direction, which, however, is caused by concentrat ion rather than activation polarization. In the anodic direction, a li miting current behavior is predicted, due to occupation of oxygen latt ice sites on the cathode side of the interface. The effective polariza tion resistance either decreases or remains constant upon reduction of the oxygen partial pressures depending on prevailing kinetic and mate rial 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 densi ty, 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 m aterials with high oxygen nonstoichiometries. An overall assessment of the two parts of this study indicates that the catalytic properties o r the perovskite surface, which enhances adsorption and surface diffus ion of oxygen, is more significant than processes involving the bulk m aterial, such as fast oxygen exchange with the bulk and vacancy diffus ion, in determining cathode performance.