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[Display omitted] • A comprehensive overview of atomically dispersed M−N−C catalysts for ORR. • Focus on active site engineering to improve activity and stability in fuel cells and Zn-air batteries. • Future directions of developing advanced M−N−C catalysts for ORR are outlined. 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−N x 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. [ABSTRACT FROM AUTHOR]

The Zn-air battery is a promising next-generation battery because of the higher energy density and safety compared with the Li-ion battery. However, the Zn-air battery suffers from huge energy loss and poor cycling performance, which are due to the sluggish kinetics of oxygen reduction/evolution reactions (ORR/OER) and the instability of bifunctional catalysts. The state-of-the-art bifunctional catalyst is the composite of Pt and Ir/Ru oxides, which is hindered by the scarce resource and high cost. Therefore, developing low-cost and high-efficient bifunctional non-noble metal-based catalysts is essential to the deployment of Zn-air batteries. Herein, we report a dual-nitrogen-source mediated pyrolysis route to rationally design and synthesize the hybrid Co/Co–N–C bifunctional catalyst. The tailored catalyst consisting of carbon confined Co nanoparticles and atomically dispersed Co–N–C moiety is identified. Remarkably, the dual-nitrogen-source derived Co/Co–N–C catalyst demonstrates excellent electrochemical performances with a half-wave potential of 0.882 V for ORR and a potential of 1.640 V at 10 mA cm−2 for OER; reasonably, the bifunctionality of Co/Co–N–C, which is supported by the excellent self-breathing Zn-air battery performance (the maximum energy density of 897.1 Wh kg Zn −1; no obvious degradation within 167h cycling). The abundant Co-Nx sites and metallic Co nanoparticles synergistically boost the ORR and OER. Image 1 • A dual-nitrogen-source method is proposed to synthesize porous Co-based catalysts. • The dual nitrogen sources play different roles in morphology and structure control. • The hybrid carbon-confined Co nanoparticles and Co-N x sites are well tailored. • The good bifunctionality is due to the hybrid active site configuration. • Excellent performance and lifetime are obtained in a Zn-air battery. [ABSTRACT FROM AUTHOR]