Unveiling the Proton-Feeding Effect in Sulfur-Doped Fe-N-C Single-Atom Catalyst for Enhanced CO₂ Electroreduction

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.