LOW-TEMPERATURE SYNTHESIS OF PEROVSKITE-TYPE OXIDES OF LANTHANUM AND COBALT AND THEIR ELECTROCATALYTIC PROPERTIES FOR OXYGEN EVOLUTION IN ALKALINE-SOLUTIONS

A new hydroxide-solid-solution precursor route has been employed to sy nthesize perovskite-type oxides of lanthanum and cobalt at 500 degrees C. The oxide powders were used to obtain thin films on nickel support s by painting with a slurry of the oxide followed by sintering at 400 degrees C. The films were satisfactorily adherent and porous. They hav e been observed to exhibit p-type semiconducting behaviour in the pote ntial region -50 to +300 mV in 1 mol l(-1) KOH. Cyclic voltammetric st udy indicated the formation of a diffusion-controlled quasi-reversible redox couple (E degrees = 417 mV) prior to the onset of O-2 evolution at the oxide surface. It was found that Sr (or Ca) substitution enhan ced greatly both the electrochemically active area as well the apparen t electrocatalytic activity. The oxygen evolution reaction followed ap proximately first-order kinetics in OH- concentration, regardless of t he nature of cobaltates. Values of the Tafel slope were 65 +/- 5 mV de cade(-1) up to a current density of ca. 100 mA cm(-2). Sr- (or Ca-)sub stituted cobaltates were more active for oxygen evolution than those p repared by other methods.