Rearranging spin electrons by axial-ligand-induced hybridization state transition to boost the activity of nickel single-atom-catalysts for electrochemical CO ₂ 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 <i>in situ</i> 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.