Identification of Single-Atom Ni Site Active toward Electrochemical CO₂ Conversion to CO

Electrocatalytic conversion of CO₂ into value-added products offers a new paradigm for a sustainable carbon economy. For active CO₂ electrolysis, the single-atom Ni catalyst has been proposed as promising from experiments, but an idealized Ni-N₄ site shows an unfavorable energetics from theory, leading to many debates on the chemical nature responsible for high activity. To resolve this conundrum, here we investigated CO₂ electrolysis of Ni sites with well-defined coordination, tetraphenylporphyrin (N₄-TPP) and 21-oxatetraphenylporphyrin (N₃O-TPP). Advanced spectroscopic and computational studies revealed that the broken ligand-field symmetry is the key for active CO₂ electrolysis, which subordinates an increase in the Ni redox potential yielding Ni^I. Along with their importance in activity, ligand-field symmetry and strength are directly related to the stability of the Ni center. This suggests the next quest for an activity-stability map in the domain of ligand-field strength, toward a rational ligand-field engineering of single-atom Ni catalysts for efficient CO₂ electrolysis.