Clarifying and controlling the surface catalytic active sites is at the heart of developing low-cost effective bifunctional oxygen catalysts to replace precious metals for metal-air batteries. Herein, a shape-control of hexagon nickel cobalt oxide spinel nanosheets was reported to engineer the surface metal active sites for enhanced electrocatalysis of oxygen evolution and oxygen reduction reactions (OER/ORR). Specifically, through simply tuning annealing temperature, different Ni3+/Ni2+ and Co3+/Co2+ atomic configurations on the nickel cobalt oxide surface were controllably synthesized. Electrochemical results show that the oxide treated at 250 °C (NCO-250) with the highest value of Ni3+/Ni2+ sites and the lowest value of Co3+/Co2+ sites exhibits superior OER/ORR activity in alkaline electrolytes and better discharge/charge performance in Zn-air batteries among all the samples. The optimized surface active site configuration of the NCO-250 sample leads to the optimal energy of adsorption, activation, and desorption for water molecules and oxygen species, thus promoting a high electrocatalytic activity. This work provides a strategy to design cost-effective, highly active, and durable electrocatalysts through regulating active sites on transition-metal surface for Zn-air battery and other advanced energy devices.
Keywords: active sites; metal−air battery; nickel cobalt oxide; oxygen electrocatalysis; spinel.