Developments and perspectives of transition metal–nitrogen–carbon catalysts with a regulated coordination environment for enhanced oxygen reduction reaction performance

Owing to the sluggish kinetics of the oxygen reduction reaction (ORR) at the cathode in proton exchange membrane fuel cells (PEMFCs) and metal–air batteries, high-performance catalysts are usually required to reduce the reaction overpotential in these devices for practical applications. Among the various electrocatalysts, the most effective are platinum group metal (PGM) catalysts; however, they suffer from the drawbacks of high cost, scarcity, and poor cycling stability. Accordingly, platinum group metal-free (PGM-free) catalysts, especially transition metal and nitrogen co-doped carbon (TM-N–C) catalysts, including single-atom catalysts and single-atom and cluster/nanoparticle catalysts, have recently received increasing attention due to their low-cost, high atom-utilization and remarkable ORR performance. However, TM-N–C catalysts with different local coordination environments typically exhibit completely different ORR catalytic activity and selectivity in both alkaline and acidic media. Therefore, the research progress on TM-N–C catalysts with a regulated coordination environment for enhanced ORR performance are systematically summarized in this review. Specially, the strategies for regulating the coordination environment of TM-N–C catalysts are emphasized, including coordination number regulation, types of N regulation, heteroatom coordination or doping in M–N_x, and synergies of clusters or nanoparticles in M–N_x. Finally, the key challenges and prospects regarding the future development of catalysts with a regulated coordination environment for ORR in this emerging field are discussed.