Developments and Perspectives of Transition Metal-Nitrogen-Carbon Catalysts with Regulated Coordination Environment for Enhanced Oxygen Reduction Reaction Performance

The sluggish kinetics of oxygen reduction reaction (ORR) at the cathode in those proton exchange membrane fuel cells (PEMFCs) and metal-air batteries usually require high-performance catalysts to reduce the reaction overpotential for practical applications. Among various electrocatalysts, the most effective platinum group metal (PGM) catalysts suffer from the drawbacks of high cost, scarcity, and poor cycling stability. Platinum group metal-free (PGM-free) catalysts, especially transition metal and nitrogen co-doped carbon (TM-N-C) catalysts, including single atom catalysts, single atom and clusters/nanoparticles catalysts have received increasing attention due to their low-cost, high atom-utilization and remarkable ORR performance recently. However, the 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 progresses of TM-N-C catalysts with 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-Nx, and synergies of clusters or nanoparticles to M-Nx. Finally, key challenges and prospects regarding the future development of catalysts with regulated coordination environment for ORR in the emerging field are discussed.