In Situ Monitoring Using ATR-SEIRAS of the Electrocatalytic Reduction of CO₂ on Au in an Ionic Liquid/Water Mixture

The electrochemical reduction of CO2 to CO in ionic liquids and ionic-liquid / water mixtures has received considerable attention due to recent claims of extraordinarily high energy efficiencies. We report here a study of CO2 electroreduction on Au in a [EMIM]BF4 / H2O mixture (18% mol / mol) combining cyclic voltammetry and surface-enhanced infrared absorption spectroscopy in the attenuated total reflection mode (ATR-SEIRAS). The onset of the reduction current in the CV coincides with a decrease of the interfacial CO2 concentration, but the appearance of adsorbed CO (COad) is slightly delayed, as CO must probably first reach a minimum concentration at the interface. Comparisons with spectra collected in the absence of CO2 and in CO-saturated electrolyte reveal that the structure of the double layer at negative potentials is different when CO2 is present (probably due to the formation of COad) and allow us to assign the main band in the spectra to CO adsorbed linearly on Au (COL), with a smaller band corresponding to bridge-bonded CO (COB). The CO bands show a large inhomogeneous broadening and are considerably broader than those typically observed in aqueous electrolytes. While both COL and COB can be observed in the CO adlayer generated by the electroreduction of CO2, only a single, even broader band, at a frequency characteristic of COL is seen in CO-saturated solutions. We attribute this to the lower coverage of the adlayer formed upon reduction of CO2, which leads to a lower degree of dipole-dipole coupling. Upon reversing the direction of the sweep in the CV, the intensity of the CO bands continues increasing for as long as a reduction current flows, but starts decreasing at more positive potentials due to CO desorption from the surface.