Cu[sub x]Co[sub 3−x]O[sub 4] Used as Bifunctional Electrocatalyst

Abstract

Composite film electrodes containing mechanically mixed Co3O4Co3O4 or CuCo2O4CuCo2O4 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 O2O2 -saturated 1MKOH1MKOH solution. The highest current densities were observed with CuCo2O4CuCo2O4 and they increased with the oxide content in the film, hence clearly demonstrating the excellent intrinsic electrocatalytic activity of CuCo2O4CuCo2O4 for this reaction. The results also showed that CuCo2O4CuCo2O4 is a better electrocatalyst than Co3O4Co3O4 with higher current densities and a greater number of electrons exchanged per O2O2 molecule. It was found that the copper–cobalt spinel oxide component favors a total of 4e−4e− in the oxygen reduction process. At the CuCo2O4CuCo2O4 -based composite electrode, direct reduction of O2O2 into OH−OH− ions (rate constant k1k1 ) and the peroxide pathway (formation of HO2−HO2− ions and their reduction into OH−OH− ions, rate constants k2k2 and k3k3 ) are proceeding in parallel, with a k1∕k2k1∕k2 ratio that increases with the overpotential when the oxide content is greater than 23.5%. At the Co3O4Co3O4 -based composite electrode, k1k1 is very weak with a k1∕k2k1∕k2 ratio that decreases rapidly with the overpotential.