Rational design of spinel oxides as bifunctional oxygen electrocatalysts for rechargeable Zn-air batteries

Rechargeable (secondary) zinc-air batteries (ZABs) are a highly attractive type of electrochemical energy storage device with high theoretical energy density, an outstanding safety record, and low cost. The performance of ZABs strongly depends on the development of bifunctional electrocatalysts toward oxygen reduction reaction (ORR) and oxygen evolution reaction (OER). Spinel oxides (AB₂O₄) have received particular attention because of their high chemical stability under operation conditions, rich raw materials, and compositional flexibility, which provides ample room for the design of spinel-structured oxides for ORR/OER catalysis. Many spinel oxides have been successfully applied as air cathodes in ZABs, but the related research is rather scattered. In this review paper, we will provide an in-time comprehensive review of the recent advances in the development of spinel oxides as ORR/OER electrocatalysts and their particular application as air electrodes in rechargeable ZABs. First, we will introduce the electrochemical fundamentals of OER and ORR on spinel oxides and the key factors determining their activity. Then, we will present the activity regulation strategies of spinel oxides and the performance of rechargeable ZABs using spinel-based air cathodes. Finally, we will highlight the current challenges and future developing directions of spinel-based air cathodes. This review could inspire further mechanistic study of the ORR/OER catalysis on spinel oxides and the development of high-performance air cathodes.