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

Using attenuated total reflection (ATR) infrared spectroscopy and ∼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 D2 is dominant, resulting in a relatively high concentration of dissolved carbonate, with a maximum IR intensity at ∼1410 cm-1. When the potential is less negative than -1.2 V, spectroscopically resolved interconversion of carbonate (CO32-) to bicarbonate (D)CO3- 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 ∼1510 to 1570 cm-1. In the potential range of -1.2 to -0.5 V (vs RHE), using D2O, the recently proposed CO2-dimer-radical-anion was observed, adsorbed on the polycrystalline copper film. We also assign a previously unresolved band at ∼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 ∼1590 cm-1 appears at ∼-0.5 V. We discuss the spectra and chemistry in detail, based on the reference spectra of carbonate, bicarbonate, and formate and using 13CO2 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.