Heterogenized Copper(II) Phenanthroline Catalysts for Electroreduction of CO2 to C2 Compounds: Substitution on the Ligand Causes Structural Changes to the Molecular Framework and Stability Enhancement

Abstract

Molecular Cu catalysts have shown promise for electrochemical CO₂ reduction (eCO₂RR) 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 eCO₂RR 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 µ₂- and µ₃-OH groups. When immobilized on a gas diffusion electrode, stable operation with a Faradaic efficiency of >70% for C₂ 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 Cu²⁺ 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 eCO₂RR processes at industrial current densities.

Keywords: CO2 reduction; copper; electrocatalysis; molecular catalyst; phenanthroline.