Oxygen reduction reactions in the SOFC cathode of Ag/CeO ₂

The interactions between oxygen molecules and a silver surface or a CeO ₂ (111) supported atomic layer of silver are predicted using first-principles calculations based on spin polarized DFT with PAW method. The juncture between the CeO ₂ (111), the atomic layer of silver, and O ₂ represents a triple-phase boundary (TPB) whereas the interface between silver surfaces and O ₂ corresponds to a 2-phase boundary (2PB) in a solid oxide fuel cell (SOFC). Results suggest that the O ₂ dissociation process on a monolayer of silver supported by CeO ₂ (111) surfaces (or TPB) with oxygen vacancies has lower reaction barrier than on silver surfaces (or 2PB), and the dissociated oxygen ions can quickly bond with subsurface Ce atom via a barrierless and highly exothermic reaction. The oxygen vacancies at TPB are found to be responsible for the lower energy barrier and high exothermicity because of the strong interaction between subsurface Ce and adspecies, implying that oxygen molecules prefer being reduced at TPB than on silver surfaces (2PB). The results suggest that, for a silver-based cathode in a SOFC, the adsorption and dissociation of oxygen occur rapidly and the most stable surface oxygen species would be the dissociated oxygen ion with - 0.78|e| Bader charges; the rate of oxygen reduction is most likely limited by subsequent processes such as diffusion or incorporation of the oxygen ions into the electrolyte.