S doping can promote H₂O activation and adjust Co active site. As a result, Co₁-SNC catalyst exhibits a greatly enhanced CO₂RR to CO performance compared to Co₁-NC.

Comprehensive Summary

Electrocatalytic reduction of CO₂ to fuels and chemicals possesses huge potential to alleviate current environmental crisis. Heteroatom doping in metal-nitrogen-carbon (M-N-C) single-atom catalysts (SACs) has been found to be capable to promote the electrocatalytic CO₂ reduction reaction (CO₂RR). However, the origin of the enhanced activity is still elusive. Here, we report that sulfur-doped cobalt-nitrogen-carbon single-atom catalyst (Co₁-SNC) exhibits superior CO₂RR performance compared to sulfur-free counterpart (Co₁-NC). On the basis of in situ attenuated total reflectance surface-enhanced infrared absorption spectroscopy (ATR-SEIRAS), kinetic isotope effect (KIE) and theoretical calculation, it is demonstrated that sulfur doping can promote water activation, elevate the d-band center of Co active site, and reduce the free energy of *COOH intermediate formation. This work deepens the understanding of the CO₂RR chemistry over heteroatom-doped SACs for designing efficient CO₂RR processes.