Surface studies of CuxCo3-xO4 electrodes for the electrocatalysis of oxygen evolution

The surface properties of the spinel oxide CuxCo3-xO4, used as an electrocatalyst for oxygen evolution, were investigated. The oxide was obtained by thermal decomposition of the nitrate precursors, in air, on a nickel substrate.X-ray photoelectron spectroscopy (XPS) analyses have shown that Cu+, CU2+, Co2+, and Co3+ cations are present at the-Surface of a freshly prepared electrode, octahedral Cu2+ and Co3+ species being predominant when the oxide is prepared at 300 degrees C. Quantitative analysis has evidenced depletion of the Cu2+ cations at the surface, following a strong anodic treatment (1.8 A cm-(2)) in KOH solution, suggesting a preferential dissolution of these species. The resistivity value of 0.047 Omega cm, measured at 25 degrees C on a homogeneous nonporous film, is indicative of semiconductor material; The charge carrier density is 5.83 x 10(17) cm(-3). Capacitance measurements reveal a p-type behavior at the surface with a flatband potential of 0.45 V vs. NHE (normal hydrogen electrode), suggesting that the oxygen evolution takes place on a metallized surface. The coverage of the oxide surface by the adsorbed OH- species is significant at high pH values, but the bond strength is relatively weak. The surface redox transition observed at the electrode prior to the oxygen evolution is ascribed to the oxidation of Co3+ species to Co4+.