Axial coordination regulation of MOF-based single-atom Ni catalysts by halogen atoms for enhanced CO₂ 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 = Cl, 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 (Cl) catalyst, decorated with the most electronegative Cl atoms, exhibits Faradaic efficiency of CO up to 94.7% in electrocatalytic CO₂ reduction, outperforming Ni₁-N-C (Br) and Ni₁-N-C (I) catalysts. Moreover, Ni₁-N-C (Cl) also presents superb performance in Zn-CO₂ 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 CO₂ reduction performance of Ni₁-N-C (Cl). 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.