Spectroscopic Evidence for the C2O4 − Dimer Radical Anion during Electrochemical CO2 Reduction on Cu Electrodes in Aqueous Conditions

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Understanding of reaction pathways and mechanism plays an important role in determining the kinetics of electrochemical reactions, and thus in the design of electrochemical processes. Attenuated Total Reflection - Surface Enhanced Infrared Absorption Spectroscopy (ATR-SEIRAS) provides a very sensitive tool for analysis of surface-electrochemical transformations. One of the applications of this technique is operando study of the electrochemical reduction of CO2. However, there is still much uncertainty concerning the nature of the intermediates towards CO in the conversion of CO2, and the interpretation of the measured peak positions in the IR spectra.In this study, ATR-SEIRAS allowed identification of notable intermediates of electrochemical CO2 reduction at moderately negative potential ranges using a surface enhanced ~12 nm thick sputtered Cu-film and isotopically labeled water (D2O) and CO2 (13CO2). By increasing the potential from -1.3 V (vs RHE) to 0 V, we could identify the transition in selectivity of surface-adsorbed species. At highly negative potential the formation of OH- and H2 is dominant, resulting in a relatively high concentration of carbonate in solution. When the potential is lowered, the rate of H2 formation decreases, which induces lowering of the concentration of carbonate and increased quantities of bicarbonates, a transition which is spectroscopically well resolved. To explain the formation of CO (clearly visible within the investigated potential range), alternatively or additionally to H+-assisted reduction of CO2, adsorbed CO2 - is shown to react with physisorbed CO2, forming the CO2 dimer radical anion. We could identify this CO2 dimer radical anion (C2O4 −) on polycrystalline copper electrodes, at moderately negative potentials. Decomposition of the dimer leads to CO and (surface) carbonate. Finally, at the lowest potentials investigated for reduction of CO2, the formation of (surface) formate is evident, which cannot be further reduced.