Atomically Dispersed Metal-Nitrogen-Carbon Catalysts with d-Orbital Electronic Configuration-Dependent Selectivity for Electrochemical CO₂-to-CO Reduction

A variety of atomically dispersed transition-metal-anchored nitrogen-doped carbon (M-N-C) electrocatalysts have shown encouraging electrochemical CO₂ reduction reaction (CO₂RR) performance, with the underlying fundamentals of central transition-metal atom determined CO₂RR activity and selectivity yet remaining unclear. Herein, a universal impregnation-acid leaching method was exploited to synthesize various M-N-C (M: Fe, Co, Ni, and Cu) single-atom catalysts (SACs), which revealed d-orbital electronic configuration-dependent activity and selectivity toward CO₂RR for CO production. Notably, Ni-N-C exhibits a very high CO Faradaic efficiency (FE) of 97% at −0.65 V versus RHE and above 90% CO selectivity in the potential range from −0.5 to −0.9 V versus RHE, much superior to other M-N-C (M: Fe, Co, and Cu). With the d-orbital electronic configurations of central metals in M-N-C SACs well elucidated by crystal-field theory, Dewar-Chatt-Duncanson (DCD) and differential charge density analysis reveal that the vacant outermost d-orbital of Ni²⁺ in a Ni-N-C SAC would benefit the electron transfer from the C atoms in CO₂ molecules to the 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 would weaken the electron-transfer process and then impede CO₂ activation. In situ spectral investigations demonstrate that the generation of *COOH intermediates is favored over Ni-N-C SAC at relatively low applied potentials, supporting its high CO₂-to-CO conversion performance. Gibbs free energy difference analysis in the rate-limiting step in CO₂RR and hydrogen evolution reaction (HER) reveals that CO₂RR is thermodynamically favored for Ni-N-C SAC, explaining its superior CO₂RR performance as compared to other SACs. This work presents a facile and general strategy to effectively modulate the CO₂-to-CO selectivity from the perspective of electronic configuration of central metals in M-N-C SACs.