Low-temperature synthesis of perovskite-type oxides of lanthanum and cobalt and their electrocatalytic properties for oxygen evolution in alkaline solutions

Abstract

A new hydroxide-solid-solution precursor route has been employed to synthesize perovskite-type oxides of lanthanum and cobalt at 500 °C. The oxide powders were used to obtain thin films on nicket supports by painting with a slurry of the oxide followed by sintering at 400 °C. The films were satisfactorily adherent and porous. They have been observed to exhibit p-type semiconducting behaviour in the potential region –50 to +300 mV in 1 mol I^–1 KOH. Cyclic voltametric study indicated the formation of a diffusion-controlled quasi-reversible redox couple (E°= 417 mV) prior to the onset of O₂ evolution at the oxide surface. It was found that Sr (or Ca) substitution enhanced greatly both the electrochemically active area as well the apparent electrocatalytic activity. The oxygen evolution reaction followed approximately first-order kinetics in OH^– concentration, regardless of the nature of cobaltates. Values of the Tafel slope were 65 ± 5 mV decade^–1 up to a current density of ca. 100 mA cm^–2. Sr-(or Ca-) substituted cobaltates were more active for oxygen evolution than those prepared by other methods.