CuxCo3-xO4 used as bifunctional electrocatalyst II. Electrochemical characterization for the oxygen reduction reaction

Composite film electrodes containing mechanically mixed Co3O4 or CuCo2O4 particles, carbon-black Vulcan XC-72R, and poly(vinylidene fluoride-co-hexafluoropropylene) (PVdF-HFP) were formed on the glassy carbon disk surface of a rotating ring-disk electrode (RRDE) and studied for the oxygen reduction reaction (ORR) in O-2-saturated 1 M KOH solution. The highest current densities were observed with CuCo2O4 and they increased with the oxide content in the film, hence clearly demonstrating the excellent intrinsic electrocatalytic activity of CuCo2O4 for this reaction. The results also showed that CuCo2O4 is a better electrocatalyst than Co3O4 with higher current densities and a greater number of electrons exchanged per O-2 molecule. It was found that the copper-cobalt spinel oxide component favors a total of 4e(-) in the oxygen reduction process. At the CuCo2O4-based composite electrode, direct reduction of O-2 into OH- ions (rate constant k(1)) and the peroxide pathway (formation of HO2- ions and their reduction into OH- ions, rate constants k(2) and k(3)) are proceeding in parallel, with a k(1)/k(2) ratio that increases with the overpotential when the oxide content is greater than 23.5%. At the Co3O4-based composite electrode, k(1) is very weak with a k(1)/k(2) ratio that decreases rapidly with the overpotential.