Carbon-nitrogen bond formation on Cu electrodes during CO2 reduction in NO3- solution

We demonstrate by Raman Spectroscopy that simultaneous reduction of NO₃^– and CO₂ on Cu surfaces leads to formation of Cu-C ≡ N–like species, showing Raman bands at 2080 and 2150 cm⁻¹ when associated with reduced or oxidized Cu surfaces, respectively. Furthermore Cu-C ≡ N–like species are soluble, explaining vast restructuring of the Cu surface observed after co-electrolysis of CO₂ and nitrate. Oxidation of deposited Cu-C ≡ N–like species results in the formation of NO. Cu-C ≡ N–like species do not form in electrolytes containing i) NH₄⁺ and CO₂, or ii) NO₃^– and HCOO^–, suggesting these likely originate from Cu-CO, the commonly accepted intermediate in electrochemical reduction of CO₂, and Cu-NH_x species, previously identified in the literature as intermediate towards C-N bond formation. The implications of the previously unresolved formation of Cu-C ≡ N–like species for the development of electrodes and processes for electrochemical formation of carbon-nitrogen bonds, including urea, amines or amides, are briefly discussed.