Active site engineering toward atomically dispersed M−N−C catalysts for oxygen reduction reaction

Atomically dispersed metal-nitrogen-carbon (M−N−C) oxygen reduction reaction (ORR) catalysts have been proven as one of the most promising non-precious metal catalysts. In this review, we summarize recent advances in the active site engineering toward M−N−C catalysts for ORR, which focuses on modulating coordination environment of active sites to enhance intrinsic activity, such as coordination structures, heteroatomic doping M−N−C, dual-atom sites, coupled catalytic sites, and defective M−N−C. Moreover, the strategies for increasing the density and utilization of active sites are discussed, including chelation, stepwise atom doping, spatial confinement, surface exposed M−Nx sites, and porous structure optimization. Thereafter, the performance decline mechanisms are introduced and the strategies for improving stability are systematically analyzed by regulating the structure of the active center and carbon support. Finally, we also proposed the future perspectives of M−N−C catalysts. This review aims to provide some guidelines towards synthesis of M−N−C catalysts for fuel cells and Zn-air batteries.