Single-atom sites regulation by the second-shell doping for efficient electrochemical CO_(2)reduction

Nitrogen-doped carbon loaded single-atom catalysts(SACs)are promising candidates for electrocatalytic conversion of CO_(2)into high-valuable chemicals,and the modification of catalysts by heteroatom-doping strategy is an effective approach to enhance the CO_(2)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 secondshell on the ultrathin carbon nanosheets support(Ni-N_(4)-SC)by solid-phase pyrolysis.The S atom in the second-shell contributes to the higher efficiency of CO_(2)conversion at lower potentials while the Ni-N_(4)-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 CO_(2),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.