The Role of Imidazolium 2-Cyanopyrrolide Ionic Liquid in Reduction of CO2 to CO, Formate, and Ethylene on Copper Electrode Revealed By SERS

Abstract

Electrochemical conversion of CO2 over copper catalyst is known to produce higher value products such as methane, ethylene, formate and alcohols in aqueous electrolytes. Since CO2 is a stable molecule, the electrochemical reduction reaction requires significant overpotential (-1.9 V vs SHE) for the first electron transfer to form CO2 -. Ionic liquids (ILs) have high CO2 solubility and can be further functionalized to bind with CO2 such that CO2 can be brought to the electrode surface in a more reactive state. In this study, we studied electroreduction of CO2 over copper in non-aqueous electrolyte with the reactive IL 1-ethyl-3-methylimidazolium 2-cyanopyrolide ([EMIM][2-CNpyr]) in the concentration range of 0.1 to 1 M. The linear sweep voltammetry experiments show the absence of a noticeable Faradic current within the potential window of -1.8 to -2.2 V vs Ag/Ag+ under N2 demonstrating the stability of the imidazolium cation against electrochemical conversion, which is also confirmed by NMR analysis. In the presence of CO2, the onset potential of reduction is at -1.9 V vs Ag/Ag+ (-1.4 V vs. SHE) and does not change with IL concentration. By employing surface enhanced Raman spectroscopy (SERS), potential dependent changes in the interfacial microenvironment during CO2 reduction was revealed. Specifically, imidazolium enrichment and surface adsorbed CO formation were captured. These results confirmed the catalytic role of the IL by facilitating the transport of the reactive CO2 to the electrode surface via imidazolium in particular. On the other hand, surface coverage by imidazolium cations on the electrode seems to prohibit higher level of C-C couplings, thus limiting reaction products to C1 and C2 products. During 3 hrs of bulk electrolysis at -2.1 V (vs. Ag/Ag+) in an H-cell with non-aqueous negolyte containing the IL and an aqueous posolyte (proton source), gaseous products of CO and H2, liquid product of HCOO- and dissolved C2H4were determined by GC and NMR analysis, respectively. Oxidation of Cu electrode was separately captured by ex-situ XPS measurements after 3 hrs of bulk electrolysis. Even though high CO2 concentrations are achieved in ILs and they are stable at CO2 reduction conditions even after 3 hours on Cu surface, IL structure should be optimized to allow C-C coupling to obtain C2+ products.