Electrochemical Oxygen Reduction Properties of Perovskite-type Oxides La_1−xA_xMnO₃(A = Na, K, Rb) in Concentrated Alkaline Solution

Abstract

Oxygen reduction properties of gas diffusion-type oxygen electrodes loaded by 50 wt% with perovskite-type oxides, La_1−xA_xMnO₃(A = Na, K, Rb, 0.0 ≦ x ≦ 0.2), were investigated in 8 mol dm⁻³ KOH aqueous solution at 60°C under air flow. Among these oxides, La_0.8Rb_0.2MnO₃ gave the highest electrode performance, i.e., 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 La_0.8Rb_0.2MnO₃ 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.