Chlorine-Coordinated Unsaturated Ni-N₂ Sites for Efficient Electrochemical Carbon Dioxide Reduction

Heteroatom-doping is an effective method for modifying the geometric symmetry of metal-nitrogen-carbon (M-N-C) single-atom catalysts and thereby tuning the electronic structure. Up to now, most of the current reports have concentrated on introducing heteroatoms into the highly symmetrical M-N₄ structure. The coordination-unsaturated M-N₂ structure is more sterically favorable for the insertion of alien atoms to optimize the electronic structure. Herein, a Ni-N₂ catalyst with out-of-plane coordinated chlorine (Cl) atoms (Ni-N₂ Cl/C) is successfully constructed on chlorine-functionalized carbon supports (C-Cl) for an efficient carbon dioxide reduction reaction (CO₂ RR). Density functional theory calculations demonstrate that the prepared Ni-N₂ Cl/C catalyst exhibits a higher capability in balancing COOH* formation and CO* desorption. In addition, in situ Raman spectra confirm that the lower CO binding energy on the Ni-N₂ Cl/C facilitates CO escape, leading to excellent CO₂ RR performance. A high CO Faradaic efficiency (FE_CO ) of more than 80% is achieved from -0.6 to -1.2 V versus reversible hydrogen electrode on the Ni-N₂ Cl/C and it exhibits negligible FE_CO and current declination over a 40-h stability test. Furthermore, a high turnover frequency (TOF) value of 15 808 h^-1 is obtained, which is more than ten times that of Ni-N₂ /C (1476 h^-1 ) without coordinated Cl atoms.