Competing Effect of Co³⁺Reducibility and Oxygen-Deficient Defects Toward High Oxygen Evolution Activity in Co₃O₄Systems 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/cm2 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+-Oh 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.