Infrared Analysis of Interfacial Phenomena during Electrochemical Reduction of CO2 over Polycrystalline Copper Electrodes

Using attenuated total reflection (ATR) infrared spectroscopy and similar to 10 nm thick, sputtered Cu-films on single bounce Si-ATR-crystals, we have analyzed the electrochemical conversion of CO2 in 0.1 M NaOH/D2O solutions. By using cyclic voltammetry, transitions in the composition of dissolved and surface-adsorbed species could be identified. At a highly negative potential [more negative than -1.2 V (vs RHE)], the formation of OD- and D-2 is dominant, resulting in a relatively high concentration of dissolved carbonate, with a maximum IR intensity at , similar to 1410 cm(-1). When the potential is less negative than -1.2 V, spectroscopically resolved interconversion of carbonate (CO32-) to bicarbonate (D)CO(3)(- )is evident, explained by a decrease in the local pH. Furthermore, adsorbed carbonate can now be distinguished from dissolved carbonate due to the strongly potential-dependent peak position of adsorbed carbonate ranging from similar to 1510 to 1570 cm(-1). In the potential range of -1.2 to -0.5 V (vs RHE), using D2O, the recently proposed CO2-dimerradical-anion was observed, adsorbed on the polycrystalline copper film. We also assign a previously unresolved band at similar to 1610 cm(-1) to this species. The dimer disproportionates to adsorbed CO and CO32-, the latter being converted to bicarbonate by proton addition. Adsorbed CO is sensitive to a Stark shift, that is, a shift as a function of applied potential. Eventually, CO disappears, and the infrared signature of (dissolved) formate at similar to 1590 cm(-1) appears at similar to -0.5 V. We discuss the spectra and chemistry in detail, based on the reference spectra of carbonate, bicarbonate, and formate and using (CO2)-C-13 to substantiate the formation of the dimer intermediate. The results are discussed and compared to recent literature on infrared analysis of electrochemical reduction of CO2.