MATHEMATICAL-MODELING OF OXYGEN-EXCHANGE AND TRANSPORT IN AIR-PEROVSKITE-YSZ INTERFACE REGIONS .1. REDUCTION OF INTERMEDIATELY ADSORBED OXYGEN

The transport of oxygen in a porous perovskite solid oxide fuel cell c athode is modeled by use of the principles of porous electrode modelin g, by taking into account exchange kinetics at the gas/electrode inter face, bulk diffusion of oxygen vacancies, surface diffusion of adsorbe d oxygen atoms, and electrochemical kinetics at the cathode/electrolyt e interface. The mechanism for the latter is based on the assumption t hat intermediately adsorbed oxygen atoms are reduced at the cathode/el ectrolyte interface in favor of direct exchange of oxygen vacancies. T he significance of concentration polarization is demonstrated even at very low overpotentials, especially if the adsorption process is slow. Under such conditions, the empirical correlation R-p(eff) proportiona l to p(O2)(-m) claimed to exist between the measured potential resista nce and the partial pres sure of oxygen cannot be justified on fundame ntal grounds. A limiting current is obtained at high cathodic overpote ntials due to the depletion of intermediately adsorbed species at the cathode/electrolyte interface. The existence of a correlation i(lim) p roportional to p(O2)(n) is predicted, where the exponent n is determin ed by the kinetic and transport properties of the cathode for oxygen e xchange and transport.