Competing effect of Co³⁺reducibility and oxygen-deficient defects toward high oxygen evolution activity in Co₃O₄Systems in alkaline medium

Abstract

In Co₃O₄ systems, the oxygen vacancy is reported to improve the oxygen evolution reaction (OER) activity because of higher Co³⁺/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.