Diluted Ionic Liquid/Water Mixtures As Electrolytes for Electrochemical Reduction of CO2 on Cu Catalysts

Abstract

The electrochemical reduction of CO2 on copper catalysts to produce high energy density products such as hydrocarbons and alcohols has attracted many researchers’ attention. CO2 reduction can serve as an alternative solution to the environmental issues and energy crisis. However, a wide verity of the products is formed on copper catalysts in aqueous electrolytes and also a significant fraction of the total current goes to undesired products such as hydrogen. Moreover, large overpotentials are needed to produce energy-intensive products. Many approaches have been proposed to address these challenges so far. Recently, ionic liquids (ILs) as electrolytes have been reported to control the product selectivity in CO2 reduction on many catalysts such as Ag, Au, and Cu1-3. ILs can also make a complex with CO2 and stabilize the intermediates on the surface3, 4. The properties of the ILs are affected by the length of alkyl chain, anion, cation, and functional groups5. The concentration of water in the IL solution is also an important factor in CO2 electroreduction. Most of the studies in this area have used the solutions with a high concentration of ILs3, 4, 6. However, the concentrated solutions can cause mass transfer limitations because of the high viscosity of the ILs. Also, in order to produce hydrocarbons, formate and alcohols, a hydrogen source is needed. In this work, we have studied the effect of using diluted IL/water mixtures as electrolytes on the product selectivity and activity in CO2 reduction over Cu catalysts.CO2 electroreduction was performed on the electropolished Cu catalysts in a solution containing 0.1 M KHCO3 and 10 mM of an IL. ILs with different hydrophobicity containing the 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 results showed that for all ILs, the faradaic efficiency (FE%) for formate increased (except for [BMIM] [DCA]) compared to the electrolyte without any IL. The electrolyte containing 10 mM [BMIM][NTF2] showed the maximum FE% for fomate (40%) at -0.92 V vs RHE. The partial current density for formate increased by adding the ILs to the electrolyte. Also, using 10 mM [BMIM][NTF2], [BMIM][Ac], and [BMIM][Cl] enhanced the FE% for methane at -1.12V vs RHE. Although adding [BMIM][DCA] to the electrolyte significantly increased the total current density, it decreased the FE% for CO2 electroreduction reaction products. This can be attributed to its hydrophilicity which adsorbs water to the surface and enhances the hydrogen evolution reaction. Moreover, the results showed that increasing the concentration of both [BMIM][Ac] and [BMIM][OTF] to 50 mM was not favorable for CO2 reduction and increased hydrogen evolution reaction compared to 10 mM of these ILs.