Non-precious metal-based single-atom catalysts for oxygen reduction reaction: fundamentals and applications

Currently, due to their high atom-utilization efficiency, tunable chemical structure, excellent catalytic properties, as well as the expectational cost effectiveness, more and more efforts have been put persistently into the development of M-X-C (M = transition metal; X = N, O, S, P, etc.; C = carbon) -based single-atom catalysts (SACs) for boosting oxygen reduction reaction (ORR), which is critically important for the advances of fuel cells, metal-air batteries, and on-site hydrogen peroxide (H₂O₂) production. Conceptionally, the ORR behaviors fundamentally rely on 2-electron or 4-electron transfers, which could be manipulated by modulating the central M and coordinated X atoms. In current review, we first outline the fundamentals between the 2-electron and 4-electron transfer pathways based on the underlying mechanisms. After that, the main approaches for catalyst design and performance evaluation are overviewed. Subsequently, we focus on the strategies and progresses to regulate the ORR pathways for target high-performance M-X-C SACs. Finally, the challenges and perspectives in terms of the future development of M-X-C-based SACs have been discussed.