CuxCo3-xO4 used as bifunctional electrocatalyst. physicochemical properties and electrochemical characterization for the oxygen evolution reaction

CuxCo3-xO4 (x = 0 and 1) powders were prepared by a sol-gel method which favors high oxide specific surface areas with a larger value for Co3O4, ascribed to a larger powder mesopore volume. X-ray diffraction measurements reveal that CuCo2O4 particles are less crystalline than Co3O4 with crystallite size 10 times smaller. The sol-gel method allows formation of spinel oxide particles that do not contain any resistive CuO phase. X-ray photoelectron spectroscopy analyses have shown that Co3O4 contains Co2+ and Co3+ species at the surface, tetrahedral Co2+ cations being predominant. In the case of CuCo2O4, Cu+, Cu2+, and possibly Cu3+ cations are also detected, octahedral Cu2+ showing the highest concentration among the copper species. Composite film electrodes, based on mechanically mixed Co3O4 or CuCo2O4 particles, carbon black Vulcan XC-72R, and poly(vinylidene fluoride-co-hexafluoropropylene), were formed on a glassy carbon disk surface. The highest intrinsic electrocatalytic activity for the oxygen evolution reaction is obtained for the CuCo2O4 composite electrode containing the larger amount of oxide particles. Cyclic voltammetry experiments suggest that the surface Co2+/Co3+ ratio is decreased when the electrodes are immersed into the KOH electrolyte, which may be associated to the formation of a superficial CoOOH layer.