Activity Origin and Catalytic Mechanism of the M–N–C Catalysts for the Oxygen Reduction Reaction

Oxygen reduction reaction (ORR), involving either a two-electron (2e^–) pathway or a four-electron (4e^–) pathway, is an important reaction in energy conversion and storage systems. It is well-known that metal–nitrogen–carbon (M–N–C) catalysts, as emerging state-of-the-art electrocatalysts, are applied to fuel cells via the 4e^– pathway (e.g., Fe–N–C) while generating hydrogen peroxide via the 2e^– pathway (e.g., Co–N–C). However, the effects of the MN_x and C–N species on the catalytic activity of ORR require thorough clarification. Especially, the real active sites of the M–N–C configuration are a long-standing conundrum. In this review, the latest advanced M–N–C catalysts were categorized according to the ORR pathways and MN_x moieties. Then, the effects of coordination atoms, N-coordinated structures, and pH on the activity of the M–N–C catalysts were discussed. The detection and quantification of the active sites of M–N–C catalysts by in situ Raman spectroscopy and electrochemical techniques were summarized. Finally, the opportunities and challenges for the M–N–C catalysts with efficient activity were highlighted.