Atomically dispersed nickel-nitrogen-carbon (Ni-N-C) moieties is promising for efficient electrochemical CO 2 -to-CO conversion. To improve the intrinsic electrocatalytic activity, it is essential but still challenging to steer the coordination environment of Ni centers for promoting the CO formation kinetics. Here, we show a strategy by introducing alien sulfur atoms to tune the local electronic density of unsaturated NiN 2 species. A coordinated structure evolution is detected and S vacancies are generated at high overpotentials, as confirmed by X-ray absorption spectroscopy. The sulfur-dopants enhance CO selectivity and activity over normal unsaturated NiN 2 structure, reaching a high CO Faradaic efficiency of 97% and a large CO current density of 40.3 mA cm -2 in a H-cell at -0.8 V and -0.9 V (vs. RHE), respectively. Density functional theory (DFT) calculations reveal both doped S atoms and evolved S vacancies in the NiN 2 coordination environment contribute to the reduced energy barriers for CO 2 electroreduction to CO.