Single-atom sites regulation by the second-shell doping for efficient electrochemical CO2 reduction

Nitrogen-doped carbon loaded single-atom catalysts (SACs) are promising candidates for electrocatalytic conversion of CO2 into high-valuable chemicals, and the modification of catalysts by heteroatom-doping strategy is an effective approach to enhance the CO2 reduction performance. However, the large difference exists in atomic radius between nitrogen atoms and the doped heteroatoms may lead to the poor stability of active sites. In this study, we have synthesized a Ni single atom catalyst with S doping at the second-shell on the ultrathin carbon nanosheets support (Ni-N4-SC) by solid-phase pyrolysis. The S atom in the second-shell contributes to the higher efficiency of CO2 conversion at lower potentials while the Ni-N4-SC can be more stable. The experimental results and theoretical calculations indicate that the S atom in second-shell breaks the uniform charge distribution and reduces the free energy of hydrogenation, which can increase the adsorption of CO2, accelerate charge transfer, and reduce the reaction energy barrier. This work reveals the close relationship between the second-shell and the electrocatalytic activity of single atom sites, which also provides a new perspective to design efficient single atom catalysts.