Unveiling the Proton-Feeding Effect in Sulfur-Doped Fe-N-C Single-Atom Catalyst for Enhanced CO 2 Electroreduction.

Heteroatom-doping in metal-nitrogen-carbon single-atom catalysts (SACs) is considered a powerful strategy to promote the electrocatalytic CO 2 reduction reaction (CO 2 RR), but the origin of enhanced catalytic activity is still elusive. Here, we disclose that sulfur doping induces an obvious proton-feeding effect for CO 2 RR. The model SAC catalyst with sulfur doping in the second-shell of FeN 4 (Fe 1 -NSC) was verified by X-ray absorption spectroscopy and aberration-corrected scanning transmission electron microscopy. Fe 1 -NSC exhibits superior CO 2 RR performance compared to sulfur-free FeN 4 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 H 2 O activation and feeds sufficient protons for promoting CO 2 conversion to *COOH, which is also corroborated by the theoretical results. This work deepens the understanding of the CO 2 RR mechanism based on SAC catalysts.