A Review of Precious-Metal-Free Bifunctional Oxygen Electrocatalysts: Rational Design and Applications in Zn-Air Batteries

The bifunctional electrocatalysis of oxygen reduction reactions (ORRs) and oxygen evolution reactions (OERs) is critical for the development of rechargeable Zn-air batteries. Both ORRs and OERs suffer from sluggish kinetics and high overpotentials, and the bifunctional reactivity is limited by the scaling relationship. Therefore, smart designs on electrocatalysts are requested to achieve high bifunctional ORR/OER activity and excellent performance in Zn-air batteries. Herein, the requirements for bifunctional oxygen electrocatalysts are first introduced. Then the recent advances of precious-metal-free active materials and regulation methods of their intrinsic activity are introduced. The structural design principles to improve the accessibility of the active sites are further presented. Finally, a brief overview of the applications of bifunctional oxygen electrocatalysts in Zn-air batteries, including routine liquid batteries and flexible solid-state batteries, is presented. This review affords rational design principles and strategies for nonprecious bifunctional ORR/OER electrocatalysts, which are expected to guide the targeted optimization of electrocatalysts and further exploration of emerging candidates.