CO Adsorption Dynamics during CO₂ Electrolysis in Aprotic Organic Electrolytes

Carbon dioxide electrolysis (CO 2 E) in fully aprotic organic electrolytes primarily yields oxalate (C 2 O 4 2−), carbon monoxide (CO), and carbonate (CO 3 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/DMFbased electrolyte. A distinct vibrational feature at 2007 cm −1 , emerged after the CO 2 E 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/TBAPF 6 /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 CO 2 E reaction environment. When CsClO 4 is used in place of TBAPF 6 , 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.