In Situ Electrochemistry of Formate on Cu Thin Films Using ATR-FTIR Spectroscopy and X-ray Photoelectron Spectroscopy

Formate (HCOO^-) is the most dominant intermediate identified during carbon dioxide electrochemical reduction (CO₂ER). While previous studies showed that copper (Cu)-based materials that include Cu(0), Cu₂O, and CuO are ideal catalysts for CO₂ER, challenges to scalability stem from low selectivity and undesirable products in the -1.0-1.0 V range. There are few studies on the binding mechanism of intermediates and products for these systems as well as on changes to surface sites upon applying potential. Here, we use an in situ approach to study the redox surface chemistry of formate on Cu thin films deposited on Si wafers using a VeeMAX III spectroelectrochemical (SEC) cell compatible with attenuated total reflectance Fourier transform infrared spectroscopy (ATR-FTIR). Spectra for surface species were collected in real time as a function of applied potential during cyclic voltammetry (CV) experiments. Results showed the reproducibility of CV curves on freshly prepared Cu/Si wafers with relatively high signal-to-noise ATR-FTIR absorbance features of surface species during these electrochemical experiments. The oxidation reaction of HCOO^- to bicarbonate (HCO₃^-) was observed using ATR-FTIR at a voltage of 0.27 V. Samples were then subjected to reduction in the CV, and the aqueous phase products below the detection limit of the SEC-ATR-FTIR were identified using ion chromatography (IC). We report the formation of glycolate (H₃C₂O₃^-) and glyoxylate (HC₂O₃^-) with trace amounts of oxalate (C₂O₄^2-), indicating that C-C coupling reactions proceed in these systems. Changes to the oxidation state of surface Cu were measured using X-ray photoelectron spectroscopy, which showed a reduction in Cu(0) and an increase in Cu(OH)₂, indicating surface oxidation.