The oxygen evolution on La0.5Ba0.5CoO3: Overpotential decay behaviour : Theory and experimental results

Abstract

In section (I) the overpotential decay behaviour of an electrode during oxygen evolution is described on the basis of calculations for two mechanisms with a chemical rate-determining step (rds). Quasi-equilibrium in the steps preceding the rds and Frumkin-type adsorption of intermediates is assumed. In section (II) decay experiments are reported for oxygen evolution on La0.5Ba0.5CoO3 in 6M KOH solution at room temperature. Both aged and renewed oxide electrodes were used. Measured decay rates at constant overpotential were found to vary as a function of starting overpotential, while the theory requires equal rates. Also, an increased capacitance for the aged electrode with respect to the renewed one was not accompanied by an expected increase in current density. Neither phenomenon can be explained by assuming non-equilibrium in the mechanism, but can be understood on the basis of a growing passive multilayer with a presumably enhanced CoIV/CoIII-ratio. The peroxide mechanism with a chemical rds and low coverage of the second intermediate is the most likely of the alternatives considered. Capacitances calculated from the initial overpotential decay were approximately equal to capacitances derived from ac impedances measured prior to the interruption.