Bifunctional 2D structured catalysts for air electrodes in rechargeable metal–air batteries

The inherent technical challenges of metal–air batteries (MABs), arising from the sluggish redox electrochemical reactions on the air electrode, significantly affect the efficiency and life cycle of these batteries. Two-dimensional (2D) nanomaterials with near-atomic thickness have potential as bifunctional catalysts in MABs, because of their distinct structures, exceptional physical properties, and tunable surface chemistries. In this study, the chemistry of representative 2D materials was elucidated and the comprehensive analysis of the primary modification techniques including geometric structure manipulation, defect engineering, crystal facet selection, heteroatom doping, single-atom catalyst construction, and composite material synthesis was conducted. The correlation between material structure and activity is illustrated by example, with the aim of leading the development of advanced catalysts in MABs. We also focus on the future of MABs from the perspective of bifunctional catalysts, definite mechanisms, and standard measurement. We expect this work to serve as a guide for the design of air electrode materials that can be used in MABs.