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

Abstract

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 (CO3 2–) 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 CO3 2–, 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.

Electrochemical reduction of CO2 to ethylene : spectroscopic evaluation of copper electrodes

“…Based on experimental evidence, the 1540 cm -1 band has been assigned to the vibration of adsorbed carbonate 108,119,122,177 or more precisely, the asymmetric C-O stretching mode of strongly adsorbed bidentate carbonate 184 . This assignment is also consistent with our previous findings measuring similar electrodes using SEIRAS 73 ,…”

“…Starting from higher frequencies, the band corresponding to adsorption of CO on atop sites (linearly adsorbed, COatop) appeared at 2070 cm -1 at -0.4 V and shifted to 2050 cm -1 at more negative potentials, which is consistent with the literature 73,108 .…”

“…Galvanostatic deposition was performed at 0.8 mA cm -2 using a potentiostat/galvanostat (PAR, Versastat 3) until a film thickness equivalent to 3 Coulomb was achieved. Cu thin-film electrodes were prepared by magnetron sputtering on a silicon ATR crystal, as described in our previous work 73 . The electrolyte solutions of KHCO3 (Sigma Aldrich, 99.99% metals basis), KClO4 (Alfa Aesar, 99-100.5%), KOH (Sigma Aldrich, 99.99%), K2HPO4 (Sigma Aldrich, ≥98%) and KH2PO4 (Sigma Aldrich, ≥98%) were prepared using deionized water (Millipore MilliQ, 18.2 MΩ cm).…”

“…The correlation can be easily noticed by looking at crossed arrows in figure 8b. Several reports have assigned the 1540 cm -1 band to adsorbed carbonate 73,99,110,165,177 . Based on these reports on peak position and observed potentials range, we assign this set of peaks to adsorbed carbonate.…”

“…Electrochemical surface-enhanced vibrational spectroscopy has already established itself as a powerful tool to examine the presence of adsorbed species and related surface phenomena. For instance, in our recent study, we explored surface adsorbed products and intermediates of CO2RR on Cu using in-situ surfaceenhanced infrared absorption spectroscopy (SEIRAS), and demonstrated the presence of a variety of species, including adsorbed carbonates, adsorbed CO, and possibly a carbon dioxide dimer radical anion 73 . However, due to limitations in the spectral range of the SEIRAS technique, the presence of surface (hydro)oxides during CO2RR is difficult to identify.…”