Rational Design of Axially Halogen-Coordinated Ni–N–C Single-Atom Catalysts for CO ₂ Electroreduction: A Density Functional Theory Study

Transition-metal single-atom catalysts based on M–N–C motifs (M = Fe, Ni, Co) are promising electrocatalysts for the CO2 reduction reaction (CO2RR) because of their well-defined active sites, high metal utilization, and tunable electronic structures. In particular, Ni–N4 sites with axial ligands have recently shown markedly enhanced activity and selectivity by modifying the electronic environment of the Ni center. However, the microscopic role of axial halogen coordination in Ni-based single-atom catalysts remains poorly understood. Herein, we employ density functional theory (DFT) calculations to elucidate how halogen ligands (F, Cl, Br, and I) modulate the electronic structure of Ni–N4–C and thereby govern its CO2RR performance. We reveal that (i) the ligand electronegativity systematically shifts the Ni 3d energy levels and thus regulates CO2 adsorption; (ii) halogen-induced electron transfer precisely tunes the binding strength of the key *COOH intermediate, following the trend F > Cl > Br > I; and (iii) among the series, the NiN4–F catalyst exhibits the lowest limiting potential and the highest predicted selectivity for CO formation over the competing hydrogen evolution reaction. This work clarifies the atomic-scale mechanism of halogen coordination and establishes a quantitative design principle linking coordination chemistry, electronic structure, and catalytic performance, providing a theoretical basis for high-throughput screening and rational design of high-performance CO2RR electrocatalysts.