Competing Effect of Co3+ Reducibility and Oxygen-Deficient Defects Toward High Oxygen Evolution Activity in Co3O4 Systems in Alkaline Medium

In Co3O4 systems, the oxygen vacancy is reported to improve the oxygen evolution reaction (OER) activity because of higher Co2+/Co3+ surface ratio. In situ studies have revealed Co3+-site reducibility as the key factor for OER activity of cobalt oxide-based systems. In this context, we have synthesized and analyzed OER activity of two Co3O4 systems; c-Co3O4 with higher oxygen defects or Co2+/Co3+ ratio and n-Co3O4 with relatively less Co2+/Co3+ ratio but more Co3+ reducibility. The systems, n- and c-Co3O4 show overpotential of 380 and 440 mV at 10 mA/cm(2) and Tafel slope of 153 and 53 mV/dec, respectively, for OER. Electrochemical characterization reveals that the lowering of OER onset potential is influenced by Co3+ reducibility rather than defects in Co3O4 systems while adsorption capacitance arising from surface irregularities, pores and their geometry, and Co3+-O-h sites cause an increase in the Tafel slope values or decrease in OER kinetics. The correlation of the key factors such as Co3+ reducibility and oxygen defects of two different Co3O4 systems toward OER activity can aid the designing of highly efficient cobalt oxide-based OER catalysts.