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Heteroatom-doping in metal-nitrogen-carbon single-atom catalysts (SACs) is considered a powerful strategy to promote the electrocatalytic CO2 reduction reaction (CO2 RR), but the origin of enhanced catalytic activity is still elusive. Here, we disclose that sulfur doping induces an obvious proton-feeding effect for CO2 RR. The model SAC catalyst with sulfur doping in the second-shell of FeN4 (Fe1 -NSC) was verified by X-ray absorption spectroscopy and aberration-corrected scanning transmission electron microscopy. Fe1 -NSC exhibits superior CO2 RR performance compared to sulfur-free FeN4 and most reported Fe-based SACs, with a maximum CO Faradaic efficiency of 98.6 % and turnover frequency of 1197 h-1 . Kinetic analysis and in situ characterizations confirm that sulfur doping accelerates H2 O activation and feeds sufficient protons for promoting CO2 conversion to *COOH, which is also corroborated by the theoretical results. This work deepens the understanding of the CO2 RR mechanism based on SAC catalysts.
Chen et al. (Wed,) studied this question.