Enhancing Carbon Dioxide Reduction Performance on Copper via Surface Reconstruction Induced by Spontaneous Diazonium Salt Grafting

The electrochemical reduction of CO₂ (eCO₂RR) is a promising strategy for sustainable energy storage and the production of carbon-neutral chemical feedstocks. Among available catalysts, Cu-based materials uniquely enable the formation of valuable multicarbon (C₂₊) products; however, achieving high selectivity remains a major challenge. In this study, we present a straightforward and effective strategy to enhance C₂₊ product formation by spontaneously grafting a benzenediazonium salt onto polycrystalline Cu electrodes. The resulting covalently bound polyphenylene-like film yields a 6-fold increase in Faradaic efficiency for C₂₊ products compared to bare Cu, while concurrently suppressing the hydrogen evolution reaction. Surface characterization reveals that the grafting process induces significant Cu surface reconstruction, alters the Cu valence state, and promotes the formation of high-index facets and defect sites, features associated with improved C-C coupling. Additionally, the organic layer increases the surface hydrophobicity, which may further contribute to product selectivity by modifying the local reaction environment. To isolate the role of surface restructuring and oxidation, control experiments using electrografted films and oxide-derived Cu electrodes were performed. These controls exhibited inferior performance, confirming that the improved selectivity primarily arises from the structural and chemical changes induced by spontaneous grafting. This work highlights the critical role of molecular-film-induced surface reconstruction and hydrophobic tuning in steering product distribution during CO₂ electroreduction.