Unraveling Structure-Dependent Performance of Cu-N-C Molecular Catalysts for Electrocatalytic CO₂ Methanation

High-rate CH₄ electrosynthesis offers a promising strategy for converting CO₂ into energy-dense fuels compatible with existing natural gas infrastructure. Cu-N-C molecular catalysts with well-defined Cu-N₄ sites show strong potential for CO₂-to-CH₄ production, yet the role of the macrocyclic backbone remains unclear. Herein, we compare two representative Cu-N-C molecular catalysts, copper phthalocyanine (CuPc) and copper tetraphenylporphyrin (CuPr), to clarify how the macrocyclic backbone influences CO₂-to-CH₄ electrocatalysis. When supported on conductive carbon and integrated into gas-diffusion electrodes, CuPc markedly outperforms CuPr, achieving a CH₄ Faradaic efficiency of 79.5%, a high partial current density of -575 mA cm^-2, a mass activity of 19 166.7 A g^-1, and stable operation for over 80 h at -150 mA cm^-2. Characterization results suggest that the Pc-macrocyclic backbone modulates the electronic structure of the Cu center and interfacial charge-transfer behavior, possibly through its extended π-conjugated Pc-ring and bridge-N environment. Theoretical calculations reveal a favorable CH₄-forming pathway on the CuPc/C model, with a lower free-energy barrier for the potential-determining step than that reported for CuPr, suggesting facilitated CH₄ formation. These findings indicate that the macrocyclic backbone can regulate reaction pathways and intermediate adsorption energetics, providing mechanistic insight and design guidance for molecular catalysts for CO₂-to-CH₄ electroreduction.