Heterogenized Copper(II) Phenanthroline Catalysts for Electroreduction of CO ₂ to C ₂ Compounds: Substitution on the Ligand Causes Structural Changes to the Molecular Framework and Stability Enhancement

Abstract Molecular Cu catalysts have shown promise for electrochemical CO 2 reduction (eCO 2 RR) to multi‐carbon products. Unlike metallic Cu facets, they offer precise control over the active site's electronic and steric configuration. However, prior studies identified critical challenges related to irreversible potential‐induced formation of Cu particles, which participate in the eCO 2 RR and obscure the role of molecular motifs. Based on a previously reported binuclear Cu(II) phenanthroline catalyst, a structurally modified second‐generation system with enhanced stability is developed. By introducing methoxy groups to the phenanthroline ligand, the molecular framework changes from a binuclear complex to an oligonuclear step‐like structure consisting of Cu(II) ions linked by µ 2 ‐ and µ 3 ‐OH groups. When immobilized on a gas diffusion electrode, stable operation with a Faradaic efficiency of >70% for C 2 products is achieved at elevated current densities. In situ XAS spectroscopy shows only negligible changes of the Cu coordination environment up to 50 mA cm −2 . When approaching 250 mA cm −2 , partial and reversible phase evolution occurs under Cu 2+ valence state reduction, followed by phase recovery upon bias removal. This system combines structural robustness with adaptive redox behavior, demonstrating a route for implementing molecular electrocatalysts in eCO 2 RR processes at industrial current densities.