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

Electrocatalytic conversion of CO into value-added products offers a new paradigm for a sustainable carbon economy. For active CO electrolysis, the single-atom Ni catalyst has been proposed as promising from experiments, but an idealized Ni-N 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 CO electrolysis of Ni sites with well-defined coordination, tetraphenylporphyrin (N-TPP) and 21-oxatetraphenylporphyrin (NO-TPP). Advanced spectroscopic and computational studies revealed that the broken ligand-field symmetry is the key for active CO electrolysis, which subordinates an increase in the Ni redox potential yielding Ni. 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 CO electrolysis.