Recent advances in bifunctional catalysts for zinc-air batteries: Synthesis and potential mechanisms

Zinc-air batteries (ZABs) are considered promising candidates for next-generation clean and sustainable energy storage devices because of their low cost, safety, environment-friendliness, and high specific energy density. However, owing to its poor charge-discharge capacity and low efficiency, its practical application remains challenging. The main obstacles are the intrinsic slow reaction kinetics on the air cathode, including the oxygen reduction reaction (ORR) during discharging and oxygen evolution reaction (OER) during recharging. Therefore, a reasonable design of bifunctional ORR/OER electrocatalysts with high activity and stability is the key to the development of ZABs. In this review, the recent advances in bifunctional ORR/OER electrocatalysts as air cathodes in ZABs are discussed from three perspectives: metal-organic framework-based catalysts, metal-free carbon catalysts, and metal-based catalysts. In particular, the synthesis, electrocatalytic activity, and potential mechanism of bifunctional catalysts in ZABs are discussed. In recent years, research on bifunctional catalysts has intensified, and the performance of these catalysts has significantly improved. However, most of the experimental products have complex preparation processes and high costs; therefore, the industrialization of these experimental products remains difficult. This review offers prospects for the optimal design of high-activity rechargeable ZAB bifunctional air cathodes.