Recent Advances in Fe-Free M-N-C Catalysts for Oxygen Reduction Reaction

Atomically dispersed metal-nitrogen-carbon (M-N-C) materials, characterized by well-defined coordination structures, have emerged as promising candidates to supersede costly platinum-based catalysts for the oxygen reduction reaction (ORR). Although Fe-N-C catalysts exhibit the highest ORR activity among Pt-free systems, their practical application is hindered by durability challenges stemming from Fenton reaction-induced degradation. Fe-free M-N-C catalysts (MCo, Mn, Ni, etc.), which mitigate Fenton reactivity, have attracted significant attention; however, their catalytic activities remain markedly lower than those of Fe-based counterparts. Critical uncertainties persist in establishing structure-activity-stability relationships, particularly regarding the interplay between coordination architecture engineering, single-atom site density, and overall catalytic performance. This review critically examines ORR mechanisms underpinning M-N-C systems, evaluates innovative strategies to decouple activity and stability in Fe-free catalysts, and proposes frameworks to translate atomic-scale materials into enhanced device-level performance. Finally, a multidisciplinary roadmap integrating advanced synthesis, operando diagnostics, and machine learning to speed up the development of durable, high-performance ORR electrocatalysts is proposed.