Electroreduction of CO2 in Ionic Liquid-Aqueous Mixtures on Cu Catalysts

Abstract

Electrochemical reduction of CO2 (CO2ER) has attracted increasing attention due to its potential to mitigate the CO2 level in the atmosphere as well as generating valuable chemicals and fuels. Among many catalysts used for CO2 reduction, Cu is the only metal catalyst able to produce hydrocarbons and oxygenates. However, CO2 reduction on Cu in aqueous electrolytes suffers from poor selectivity. Recently, ionic liquids (ILs) have been reported to enhance the product selectivity. ILs can stabilize the intermediates on the surface and enhance CO2 reduction. Many factors such as basicity, hydrophilicity, size, and CO2 absorption capacity of the ILs affect their performance in the electrochemical systems. In this work, a range of ILs were used to study the effect of anion in diluted IL/buffer mixtures on product selectivity and activity in CO2ER over Cu.Electrolysis experiments were performed on the Cu electrodes in a mixture of 0.1 M KHCO3 and 10 mM of an IL. 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 for this study. A positive shift for the onset potential in cyclic voltammetry (CVs) was observed after addingILs (except for [BMIM][DCA]) to the buffer electrolyte. Moreover, alower charge transfer resistance for all ILs (except for [BMIM][DCA])compared to IL-free electrolyte at -0.92 V was observed in electrochemical impedance spectroscopy (EIS). The presence of ILs in the electrolyte also influenced 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 probably due to its high CO2 absorption capacity and high hydrophobicity which could attract more CO2 molecules to the surface. The results also showed that adding ILs decreased the FE% toward the C2 products compared to IL-free electrolyte. This observation can be attributed 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. [BMIM][Ac] has been reported to chemically react with CO2, where other ILs in this study were expected to have physical interaction with CO2. The maximum FE% for hydrogen and the lowest FEs for hydrocarbons were observed for [BMIM][DCA] likely due 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. The results in this study showed that the nature of anion significantly influences the CO2 electroreduction in IL-containing electrolytes.