Electrochemical oxygen reduction properties of perovskite-type oxides La1-xAxMnO3 (A = Na, K, Rb) in concentrated alkaline solution

Oxygen reduction properties of gas diffusion-type oxygen electrodes loaded by 50 wt% with perovskite-type oxides, La(1-x)A(x)MnO(3)(A = Na, K. Rb, 0.0 less than or equal to x less than or equal to 0.2), were investigated in 8 mol dm(-3) KOII aqueous solution at 60 degrees C under air flow. Among these oxides, La0.8Rb0.2MnO3 gave the highest electrode performance, i.c., current density of 341 mA cm(-2) at -150 mV vs. Hg/HgO. This oxide was found to be highly active for the direct 4-electron reduction of oxygen as revealed by a rotating ring-disk electrode (RRDE) analysis. Electrode performances changed with a change in A or x over the oxides, and tended to be higher with the oxide which exhibited a smaller amount of oxygen desorption in temperature-programmed desorption (TPD) experiments, On the basis of the iodometry and electron spin resonance (ESR) analysis, the 4-electron reduction was suggested to take place most favorably at the sites composed of a pair of Mn3+ and Mn4+ on the oxide surface. The electrode loaded with La0.8Rb0.2MnO3 was confirmed to be fairly stable over a continuous operation for 100 h under a galvanostatic condition of 300 mA cm(-2). The same electrode allowed to construct a zinc-air battery with a maximum power density as large as 293 mW cm(-2) at a cell voltage of 0.7 V.