Insight into the active sites of M–N–C single-atom catalysts for electrochemical CO2 reduction

Electrochemical carbon dioxide reduction (CO₂RR) to chemicals and fuels is a promising way to alleviate global environmental problems and energy issues. Among the various catalysts, metal-nitrogen-carbon (M–N–C) single-atom catalysts (SACs) have intrigued great excitement in catalysis due to their low cost and high efficiency. However, precisely identifying the active site structure at an atomic level and disclosing the structure-performance relationship remains a grand challenge. In this review, the active structures of the M–N–C catalysts in CO₂RR are first summarized, including isolated metal-N_x (x = 2, 3, 4, 5) sites, dual-metal centers, and the crucial role of substrates. Subsequently, the role of active structure in changing the adsorption properties of reactants toward CO₂RR is discussed. In particular, the structure-performance relationship and constructive strategies to optimize the CO₂RR pathway are highlighted. Finally, challenges and potential outlooks for the development of M–N–C SACs toward CO₂RR are presented.