A review on the modification strategies of transition metal based bifunctional electrocatalysts for air-cathode in zinc-air batteries

The shift from fossil fuels to alternative renewable energy sources requires storing the sporadically available renewable energy for later use. For this purpose, efficient energy storage systems are crucial, and metal-air batteries are significant among them. Rechargeable zinc-air batteries are drawing attention due to their high energy density and clean nature. However, one of the major challenges is the development of low-cost and efficient bifunctional electrocatalysts for air electrodes. A bifunctional catalyst can effectively catalyze oxygen evolution reaction (OER) during charging and oxygen reduction reaction (ORR) during discharging. Since OER and ORR have different and complicated mechanisms, they cannot be catalyzed by one type of active site. Various strategies have proved effective in solving this problem, and transition metal-based electrocatalysts have shown excellent results. This review discusses modification strategies for air electrodes of zinc-air batteries concerning the bifunctional electrocatalysts based on transition metals.