Restructuring of Benzimidazole-Based Copper Complexes during Electrochemical CO ₂ Reduction

Copper-based molecular catalysts have been widely used for the electrochemical reduction of carbon dioxide (ERCO2). However, establishing a correlation between structure and catalytic performance has been challenging due to the dynamic nature of these materials under ERCO2 working conditions. Here, we report on the restructuring of the copper complexes [Cu(bzimpy)Cl2] and [Cu(pyrben)2(NO3)]NO3 during ERCO2 in an acidic medium, evidencing its effects on the observed activity, selectivity, and stability. In situ X-ray absorption spectroscopy (XAS) suggested that, at sufficiently negative potentials, the Cu2+ centers of both complexes are reduced to undercoordinated Cu0 species. Pb underpotential deposition of post-ERCO2 samples indicated that such Cu0 species also present a dominant contribution of the (100) domain, which is in line with the high Faradaic efficiency toward C2 products (∼45%) shown by both complexes at −1.24 V vs RHE. By combining online electrochemical mass spectrometry, in situ XAS, electron paramagnetic resonance (EPR), and quantitative product analysis, we found that [Cu(bzimpy)Cl2] exhibits a partially reversible restructuring, regenerating the main complex moiety, [Cu(bzimpy)]2+, and its geometry.