Atomic-level active sites of efficient imidazolate framework-derived nickel catalysts for CO₂ reduction

Nickel and nitrogen co-doped carbon (Ni-N-C) has emerged as a promising catalyst for the CO₂ reduction reaction (CO₂RR); however, the chemical nature of its active sites has remained elusive. Herein, we report the exploration of the reactivity and active sites of Ni-N-C for the CO₂RR. Single atom Ni coordinated with N confined in a carbon matrix was prepared through thermal activation of chemically Ni-doped zeolitic imidazolate frameworks (ZIFs) and directly visualized by aberration-corrected scanning transmission electron microscopy. Electrochemical results show the enhanced intrinsic reactivity and selectivity of Ni-N sites for the reduction of CO₂ to CO, delivering a maximum CO faradaic efficiency of 96% at a low overpotential of 570 mV. Density functional theory (DFT) calculations predict that the edge-located Ni-N₂₊₂ sites with dangling bond-containing carbon atoms are the active sites facilitating the dissociation of the C-O bond of the ∗COOH intermediate, while bulk-hosted Ni-N₄ is kinetically inactive. Furthermore, the high capability of edge-located Ni-N₄ being able to thermodynamically suppress the competitive hydrogen evolution is also explained. The proposal of edge-hosed Ni-N₂₊₂ sites provides new insight into designing high-efficiency Ni-N-C for CO₂ reduction.