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-N-4 structure. The coordination-unsaturated M-N-2 structure is more sterically favorable for the insertion of alien atoms to optimize the electronic structure. Herein, a Ni-N-2 catalyst with out-of-plane coordinated chlorine (Cl) atoms (Ni-N2Cl/C) is successfully constructed on chlorine-functionalized carbon supports (C-Cl) for an efficient carbon dioxide reduction reaction (CO2RR). Density functional theory calculations demonstrate that the prepared Ni-N2Cl/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-N2Cl/C facilitates CO escape, leading to excellent CO2RR performance. A high CO Faradaic efficiency (FECO) of more than 80% is achieved from -0.6 to -1.2 V versus reversible hydrogen electrode on the Ni-N2Cl/C and it exhibits negligible FECO 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-N-2/C (1476 h(-1)) without coordinated Cl atoms.