Oxygen-Electrode Catalysis on Oxoperovskites at 700 °C versus 20 °C

The oxygen-reduction and oxygen-evolution reactions (ORR and OER) at 700 degrees C on the perovskites ABO(3-delta) (A = La, Sr and B = Co, Fe, Mn, Cr with undoped versus Nb- or Ta-doped SrCoO3-delta) have been evaluated experimentally with a reversible solid oxide fuel cell (R-SOFC). The predictor for active ORR catalysis at 20 degrees C in alkaline solution is not applicable at 700 degrees C; the adsorbed water on the oxide catalyst surface is lost. In a SOFC, the ORR is split between the fuel and the oxygen electrode; in an alkaline air battery, the entire ORR occurs at the oxygen electrode. On the other hand, the OER reaction occurs by a similar process at the oxygen electrode in a R-SOFC at 700 degrees C and in an air battery or room-temperature fuel cell in an alkaline solution. A proposed condition for both the ORR and the OER occurring at the same oxoperovskite surface at 700 degrees C is a transition-metal cation of the perovskite at its equilibrium oxidation state at the operating temperature and pO(2).