On the origin of the elusive first intermediate of CO2 electroreduction.

We resolve the long-standing controversy about the first step of the CO2 electroreduction to fuels in aqueous electrolytes by providing direct spectroscopic evidence that the first intermediate of the CO2 conversion to formate on copper is a carboxylate anion *CO2 - coordinated to the surface through one of its C-O bonds. We identify this intermediate and gain insight into its formation, its chemical and electronic properties, as well as its dependence on the electrode potential by taking advantage of a cutting-edge methodology that includes operando surface-enhanced Raman scattering (SERS) empowered by isotope exchange and electrochemical Stark effects, reaction kinetics (Tafel) analysis, and density functional theory (DFT) simulations. The SERS spectra are measured on an operating Cu surface. These results advance the mechanistic understanding of CO2 electroreduction and its selectivity to carbon monoxide and formate.