Metal-Coordinated Phthalocyanines as Platform Molecules for Understanding Isolated Metal Sites in the Electrochemical Reduction of CO.

Single-atom catalysts (SACs) of non-precious transition metals (TMs) often show unique electrochemical performance, including the electrochemical carbon dioxide reduction reaction (CORR). However, the inhomogeneity in their structures makes it difficult to directly compare SACs of different TM for their CORR activity, selectivity, and reaction mechanisms. In this study, the comparison of isolated TMs (Fe, Co, Ni, Cu, and Zn) is systematically investigated using a series of crystalline molecular catalysts, namely TM-coordinated phthalocyanines (TM-Pcs), to directly compare the intrinsic role of the TMs with identical local coordination environments on the CORR performance. The combined experimental measurements, characterization, and density functional theory calculations of TM-Pc catalysts reveal a TM-dependent CORR activity and selectivity, with the free energy difference of Δ(*HOCO) - Δ(*CO) being identified as a descriptor for predicting the CORR performance.