The p(O2) dependence of oxygen surface coverage and exchange current density of mixed conducting oxide electrodes: model considerations.

The charge of adsorbed oxygen species such as O, O or O electrostatically affects the kinetics of the oxygen exchange reaction (1/2O2+2e-<-->O2-) taking place on mixed conducting oxides. For a model assuming a homogeneous double layer of adsorbed ions and counter charges in the mixed conducting electrode it is calculated how the surface coverage theta of the different species depends on the oxygen partial pressure p(O2). Mixed conducting "electron rich" oxides with high electronic carrier concentrations are considered. Models with p(O2) independent hole concentration or p(O2) independent vacancy concentration are discussed as limiting cases. It is quantified how strongly the electrostatic repulsion of adsorbed ions flattens the theta-p(O2) relationships compared to Langmuir's case; even situations can occur in which the surface coverage of some oxygen species decreases with increasing p(O2). In a second step the p(O2)-dependence of the equilibrium exchange rate of the surface reaction 1/2O2+2e-<-->O2- is deduced for several possible rate limiting steps. These relations may serve as a basis for future mechanistic interpretations of the p(O2) dependence of SOFC electrode polarization and of effective surface rate constants kdelta, k* in oxygen stoichiometry change or 18O tracer exchange experiments, respectively.