Sulfur-doped unsaturated Ni-N₃ coordination for efficient electroreduction of CO₂

Tuning coordination environment of metal atoms in catalysts can improve intrinsic performance of CO2 reduction and accelerate kinetics of CO formation. However, constructing unsaturated coordination configuration of metal single-atom by heteroatom doping still remains a challenge. Herein, a novel electrocatalyst of S/N co-coordinated Ni single-atom (denoted as Ni-SNC) was synthesized by calcining SO42- doped Zn/Ni ZIF for electrocatalytic CO2 reduction. The existence of single-atom Ni was verified by HAADF-STEM, while formation of "unsaturated" Ni-N-3-S active sites was confirmed by XANES and EXAFS. The energy barrier of pivotal steps in CO2RR reaction process was reduced on "unsaturated" Ni-N-3-S active sites, and sulfur doping improved current density of electrocatalytic process. The energy barrier of CO2 -> COOH* over Ni-SNC catalyst was only +0.69 eV, which was much less than that (+2.02 eV) over Ni-NC catalyst. The Ni-SNC exhibited CO Faradaic efficiency (FECO) of >90% at -0.6 similar to -0.9 V vs. RHE with the highest FECO of 95% at -0.8 V.