(187b) CO2 Electroreduction in Ionic Liquid/Buffer Electrolytes over Cu Electrodes

Electroreduction of CO₂ to valuable chemicals and fuels is a promising method to reduce CO₂ levels in the atmosphere. However, making an efficient system for CO₂ electroreduction (CO2ER) is challenging due to the high overpotential requirement and poor selectivity for desirable products. Ionic liquids (ILs) have been reported to enhance the product selectivity and activity by stabilizing the intermediates on the surface. The properties of the ILs are influenced by the nature of ions. In this study, we have investigated the effect of the anion in imidazolium- based ILs on the product selectivity and activity in CO₂ reduction over Cu.

CO₂ electroreduction was performed on Cu foils in 10 mM IL and 0.1 M KHCO₃. ILs with same cation (1-butyl-3-methylimidazolium ([BMIM]⁺)) and different anions (bis(trifluoromethylsulfonyl)imide ([NTF₂]^-), triflate ([OTF]^-), dicyanamide ([DCA]^-), acetate ([Ac]^-), and chloride ([Cl]^-)) were used. The cyclic voltammograms showed that all ionic liquids had more positive onset potentials compared to the IL-free electrolyte. ILs also significantly affected the product selectivity. Faradaic efficiency (FE%) of formate for all ILs increased (except for [BMIM][DCA]) compared to the electrolyte without any IL at all studied potentials. The maximum FE (38.7%) was observed for [BMIM][NTF₂] at -0.92 V. [BMIM][DCA] had the highest FE% for hydrogen and the lowest FEs for hydrocarbons. The X-ray photoelectron spectroscopy for the Cu electrodes after CO₂ electroreduction showed that ILs had a strong interaction with the Cu surface. The maximum amount of IL was observed for [BMIM][DCA] which has a high hydrophilicity. The adsorption of hydrophilic ILs on the surface can be a reason for enhancing HER. These observations indicate that although the adsorption of [BMIM]⁺ on the surface is important for making the IL-CO₂ complex and stabilizing the intermediates on the surface, the adsorption of hydrophilic anions on the surface can enhance HER and suppress CO2ER.