Atomically Dispersed\nMetal–Nitrogen–Carbon\nCatalysts with <i>d</i>‑Orbital Electronic Configuration-Dependent\nSelectivity for Electrochemical CO₂‑to-CO Reduction

Abstract

A variety of atomically dispersed transition-metal-anchored\nnitrogen-doped\ncarbon (M–N–C) electrocatalysts have shown encouraging\nelectrochemical CO₂ reduction reaction (CO₂RR)\nperformance, with the underlying fundamentals of central transition-metal\natom determined CO₂RR activity and selectivity yet remaining\nunclear. Herein, a universal impregnation-acid leaching method was\nexploited to synthesize various M–N–C (M: Fe, Co, Ni,\nand Cu) single-atom catalysts (SACs), which revealed d-orbital electronic\nconfiguration-dependent activity and selectivity toward CO₂RR for CO production. Notably, Ni–N–C exhibits a very\nhigh CO Faradaic efficiency (FE) of 97% at −0.65 V versus RHE\nand above 90% CO selectivity in the potential range from −0.5\nto −0.9 V versus RHE, much superior to other M–N–C\n(M: Fe, Co, and Cu). With the d-orbital electronic configurations\nof central metals in M–N–C SACs well elucidated by crystal-field\ntheory, Dewar–Chatt–Duncanson (DCD) and differential\ncharge density analysis reveal that the vacant outermost d-orbital\nof Ni²⁺ in a Ni–N–C SAC would benefit the\nelectron transfer from the C atoms in CO₂ molecules to\nthe Ni atoms and thus effectively activate the surface-adsorbed CO₂ molecules. However, the outermost d-orbital of Fe³⁺, Co²⁺, and Cu²⁺ occupied by unpaired electrons\nwould weaken the electron-transfer process and then impede CO₂ activation. In situ spectral investigations demonstrate that\nthe generation of *COOH intermediates is favored over Ni–N–C\nSAC at relatively low applied potentials, supporting its high CO₂-to-CO conversion performance. Gibbs free energy difference\nanalysis in the rate-limiting step in CO₂RR and hydrogen\nevolution reaction (HER) reveals that CO₂RR is thermodynamically\nfavored for Ni–N–C SAC, explaining its superior CO₂RR performance as compared to other SACs. This work presents\na facile and general strategy to effectively modulate the CO₂-to-CO selectivity from the perspective of electronic configuration\nof central metals in M–N–C SACs.