CO Adsorption Dynamics during CO₂ Electrolysis in Aprotic Organic Electrolytes

Carbon dioxide electrolysis (CO₂E) in fully aprotic organic electrolytes primarily yields oxalate (C₂O₄^2-), carbon monoxide (CO), and carbonate (CO₃^2-). To understand this system, we investigated the adsorption dynamics in the CO adsorption region (COAR) using attenuated total reflectance surface-enhanced infrared absorption spectroscopy on polycrystalline Cu in a DMSO/DMF-based electrolyte. A distinct vibrational feature at 2007 cm^-1, emerged after the CO₂E onset potential (-1.56 V vs Ag/AgCl) and increased from -1.8 V to -3.0 V vs Ag/AgCl. This band does not show a clear Stark effect, and the non-Gaussian shape and inconsistent peak shift indicate that this may be a convolution of two CO vibrational modes associated with adsorption on higher-coordinated (∼2004 cm^-1) and lower-coordinated (∼2021 cm^-1) Cu surface atoms. Despite prior comparisons with aqueous and DMSO/TBAPF₆/CO systems, the relatively lower wavenumber of the band position and the dynamic shift of the peaks suggest a unique CO adsorption environment shaped by complex electrolyte-adsorbate interactions specific to the CO₂E reaction environment. When CsClO₄ is used in place of TBAPF₆, the COAR band around ∼2000 cm^-1 is no longer observed. Instead, a broad band around 1800 cm^-1 emerges, attributed to bridged CO adsorption.