Spectroscopic evidences of the presence of hydrogenated species on the surface of copper during CO2 electroreduction at low cathodic potentials

Carbon dioxide electroreduction on copper electrode was studied by surface enhanced Raman scattering (SERS) in K2SO4 aqueous solutions with different pH values. CO2 was bubbled into the solution at 0 V vs. Ag/AgCl, i.e., on an oxidized copper surface. In acidic solutions (pH around 2.5), at −0.2 V, bands indicative of the presence of ethylene on the electrode surface were detected. Although ethylene is knowledgably a product of CO2 electroreduction on copper, it was not experimentally identified on the electrode’s surface at such a low cathodic potential in prior works. In solutions with pH around 2.5, CO bands were not observed, suggesting that hydrocarbons could be formed by a pathway that does not occur via adsorbed CO. In solutions with higher pHs, a complex spectral pattern, between 800 and 1700 cm−1, was observed at approximately −0.4 V. The observed spectrum closely resembles those reported in the literature for adsorption of monocarboxylic acids with small chains. The spectral features indicate the presence of a structure containing a double CC bond, a carboxyl group, and C–H bonds on the electrode’s surface. SERS spectra obtained in CO-saturated solution are also presented. However, in this case, no SERS bands were observed in the region between 800 and 1700 cm−1 at low cathodic potentials.