Axial coordination regulation of MOF-based single-atom Ni catalysts by halogen atoms for enhanced CO_(2) electroreduction

Single-atom catalysts(SACs),with the utmost atom utilization,have attracted extensive interests for various catalytic applications.The coordination environment of SACs has been recognized to play a vital role in catalysis while their precise regulation at atomic level remains an immense challenge.Herein,a post metal halide modification(PMHM)strategy has been developed to construct Ni-N4 sites with axially coordinated halogen atoms,named Ni1-N-C(X)(X=CI,Br,and I),on pre-synthetic nitrogen-doped carbon derived from metal-organic frameworks.The axial halogen atoms with distinct electronegativity can break the symmetric charge distribution of planar Ni-N4 sites and regulate the electronic states of central Ni atoms in Ni1-N-C(X)(X=Cl,Br,and I).Significantly,the Ni1-N-C(CI)catalyst,decorated with the most electronegative Cl atoms,exhibits Faradaic efficiency of CO up to 94.7%in electrocatalytic CO_(2) reduction,outperforming Ni1-N-C(Br)and Ni1-N-C(I)catalysts.Moreover,Ni1-N-C(CI)also presents superb performance in Zn-CO_(2) battery with ultrahigh CO selectivity and great durability.Theoretical calculations reveal that the axially coordinated Cl atom remarkably facilitates*COOH intermediate formation on single-atom Ni sites,thereby boosting the CO_(2) reduction performance of Ni1-N-C(CI).This work offers a facile strategy to tailor the axial coordination environments of SACs at atomic level and manifests the crucial role of axial coordination microenvironments in catalysis.