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

Abstract

The charge of adsorbed oxygen species such as O⁻_ad, O⁻_2,ad or O²⁻_2,ad 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 θ 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 θ–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 k^δ, k* in oxygen stoichiometry change or ¹⁸O tracer exchange experiments, respectively.