A SERS study of the electrochemical reduction of carbon dioxide on copper electrodes

Abstract

A Surface Enhanced Raman Scattering (SERS) study of the intermediates formed on copper during the electrochemical reduction of CO2 and of the species which poisoned this reaction was conducted. The spectroelectrochemical cell designed for this study allowed SERS spectra from a copper surface in non-halide media to be observed. An electrode pretreatment method using fifteen 1 V/s and two 10 mV/s Oxidation Reduction Cycles (ORCs) was developed to produce clean, SERS active surfaces. The ORC process which created the SERS active surface was explored. The SERS active surface was found to have been formed by the decomposition of an oxide/hydroxide/carbonate layer on the copper electrode surface.

The results from voltammetry, potential step, and time dependence studies were used to assign the observed bands. Potential step experiments were used to explore the origin of the voltammetric features and thus aid in the identification of interfacial species at the copper surface before, during, and after CO2 reduction. Time dependence studies were performed at -1.06 V, as CO2 reduction occurred at this potential.

Adsorbed CO was shown to be an intermediate in the reduction of CO2 and band assignments were made for the CO stretch, the Cu-CO stretch and Cu-CO frustrated rotation. The intensity of all these bands decreased with time while the intensity of the band attributed to the poisoning of the reaction increased. This demonstrated that the changes in band intensity were related to the electrochemical reduction process and not the loss of SERS active sites.

Band assignments for a Cflx species were made. The relationship between the Cflx species and graphite was explored and it was found that a possible interchange existed between these substances. Neither of these substances was found to be associated with the poisoning of CO2 reduction.

SERS bands of a large number of carbonate/bicarbonate modes were observed. These were from the interfacial region, but did not appear to be involved in the reduction of CO2 as they varied only slightly in the time dependence studies. Bands for these species were assigned to v11(Cu OCO2), V1(CO/·), V2(CO/·), V4(CO/·), V5(HCO3·).

Bands attributable to Cu2O were assigned and compared favourably with spectra from oxide films of exposed copper. An oxide/hydroxide patina is proposed as the poisoning species in the reaction, since graphite and carbide were ruled out.