Rearranging spin electrons by axial-ligand-induced hybridization state transition to boost the activity of nickel single-atom-catalysts for electrochemical CO2 reduction†

Single-atom catalysts (SACs) with M–N₄ active sites show great potential to catalyze the electrochemical CO₂ reduction reaction (eCO₂RR) toward CO. The activity and selectivity of SACs are determined by the local coordination configuration of central metal atoms in M–N₄ sites, which is readily tuned by axial ligands. In this work, we construct axial ligands in situ on two Ni–N₄-type model SACs, NiPc and Ni–N–C, by adding Cl⁻ into the electrolyte taking advantage of the strong chemisorption of Cl⁻ over Ni–N₄. Cl axial ligand lowers the energy barrier of the potential-determining step for the eCO₂RR due to a hybridization state transition of Ni orbitals and the resulting rearrangement of spin electrons. Consequently, both NiPc and Ni–N–C with axial Cl exhibit superior activity for the eCO₂RR toward CO. Finally, we propose the magnetic moment of Ni as a universal descriptor for the eCO₂RR toward CO on Ni–N₄ with various axial ligands.