CO2 electroreduction on Cu Electrodes in Ionic Liquid-Containing Electrolytes

Abstract

The CO2 level is increasing continuously in the atmosphere and causing environmental problems. Among the several methods which have been studied to mitigate the CO2 amount in the atmosphere, CO2 electroreduction (CO2ER) has attracted much attention. However, CO2 electroreduction is not efficient due to poor selectivity, high overpotential requirement and the presence of the parasitic hydrogen evolution reaction (HER). As a method to address these challenges, using ionic liquids (ILs) has been recently proposed due to their unique properties such as high CO2 absorption capacity. ILs can enhance CO2ER by making a complex with CO2 and stabilizing the intermediates on the surface. The properties of ILs are affected by several factors such as the nature of anion, cation, and functional groups. In this study, we have investigated the effect of anion in diluted IL/water mixtures on the product selectivity and activity of the copper catalysts.CO2ER was performed on electropolished Cu foils in electrolytes containing 0.1 M KHCO3 and 10 mM of an IL. A range of 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. These ILs were chosen to cover different size, hydrophilicity and CO2 absorption ability. The catalytic activity was significantly impacted by adding ILs to the electrolyte. The cyclic voltammograms (CVs) in CO2-saturated electrolytes showed that by adding ILs (except for [BMIM][DCA]) to the buffer electrolyte, the onset potential shifted toward positive potentialswhich indicates the enhanced activity in the presence of ILs. Moreover, electrochemical impedance spectroscopy (EIS) showed that all ILs (except for [BMIM][DCA]) had alower charge transfer resistance compared to IL-free electrolyte at -0.92 V. Results also showed that ILs significantly affected the product selectivity. Faradaic efficiency (FE%) toward formate for all ILs increased (except for [BMIM][DCA]) compared to IL-free electrolyte. [BMIM][NTF2] showed the maximum FE% for formate (39%) at -0.92 V. This observation can be attributed to its high CO2 absorption capacity and high hydrophobicity which could attract more CO2 molecules to the surface. The results showed that adding ILs decreased the FE% toward the C2 products compared to IL-free electrolyte likely due to the presence of adsorbed [BMIM]+ cations on the surface which prevent CO2 molecules approach to each other and do dimerization. [BMIM][Ac] had the maximum FE% for CO (a 211% increase in FECO% compared to IL-free electrolyte at -1.02V) and C2 products (a 27% increase in FEC2% compared to IL-free electrolyte at -1.12V) compared to other ILs studied. It has been reported that [BMIM][Ac] can chemically react with CO2, where other ILs in this study were expected to have physical interaction with CO2. A completely different behavior was observed for [BMIM][DCA] in our study. [BMIM][DCA] had a very high activity and low charge transfer resistance in both N2- and CO2-saturate electrolytes. However, this activity was due to the enhanced HER not CO2ER. [BMIM][DCA] had the highest FE% for hydrogen and the lowest FEs for hydrocarbons.This is likely attributed to the high hydrophilicity and low CO2 absorption capacity of [BMIM][DCA] which can cause more water molecules to be attracted to the surface and enhance HER. X-ray photoelectron spectroscopy (XPS) for the Cu electrodes after CO2ER showed that ILs had a strong interaction with the electrode surface. This study showed how anions of imidazolium-based ILs affect the selectivity and activity in CO2 electroreduction.