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

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