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-ad(-), O-2,ad(-) or O-2,ad(2-) electrostatically affects the kinetics of the oxygen exchange reaction (1/2O(2) + 2e(-) reversible arrow 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 0 of the different species depends on the oxygen partial pressure p(O-2). Mixed conducting "electron rich" oxides with high electronic carrier concentrations are considered. Models with p(O-2) independent hole concentration or p(O-2) independent vacancy concentration are discussed as limiting cases. It is quantified how strongly the electrostatic repulsion of adsorbed ions flattens the theta-p(O-2) relationships compared to Langmuir's case; even situations can occur in which the surface coverage of some oxygen species decreases with increasing p(O-2). In a second step the p(O-2)-dependence of the equilibrium exchange rate of the surface reaction 1/2O(2) + 2e(-) reversible arrow O2- is deduced for several possible rate limiting steps. These relations may serve as a basis for future mechanistic interpretations of the p(O-2) dependence of SOFC electrode polarization and of effective surface rate constants k(delta), k* in oxygen stoichiometry change or O-18 tracer exchange experiments, respectively.