Electrochemical CO2 Reduction in Ionic Liquid Using Two Compartment Cell Separated with Proton-Conducting Membrane

Abstract

Reduction of CO2 gas using natural energy resource to produce useful chemicals has been an important issue to reduce CO2 emission and to establish the sustainable society. Some of ionic liquid such as 1-ethylimidazoliumbis (trifluoromethylsulfonyl) imide (EMIM-TSFI) ionic liquid is known to absorb CO2 gas and thus considered as a medium for CO2 gas separation and transportation. In this study EMIM-TSFI was used for electrolyte solution in which CO2 gas is absorbed and electrochemically reduced to form, for example, methane. EMIM-TSFI is also an hydrophobic liquid and thus water content can be limited to be low level. This means that the sub-reaction of electrochemical CO2 reduction such as water decomposition can be suppressed. In our previous work, it was found that a proper water content suitable for efficient CO2 reduction to form methane was ca. 5000 ppm. In a traditional single cell, however, reaction of H2O decomposition to form proton on a counter electrode (CE) requires large overpotential due to low water content resulting in decomposition of ionic liquid. To solve this problem, we have developed a two-compartment electrochemical cell composed of two rooms one of which contains working electrode (WE) and ionic liquid and an another room contains CE and KCl aqueous solution. These tow rooms were separated with a Nafion membrane to allow proton transportation between two rooms. In a room of aqueous solution, water decomposition (oxygen evolution) reaction was proceeded on a Pt-CE. In a room of ionic liquid, CO2 gas was bubbled to be absorbed and cathodically reduced on Cu-WE to form methane with protons supplied via membrane. Voltammogram of Cu-WE in the ionic liquid was measured as a function of CO2 gas supply and water content in ionic liquid. The result confirmed that supplying CO2 gas and 5000 ppm water increased cathodic polarization current in a potential lower than –1.4 V vs Ag. Cathodic current also responded rapidly to changing the flow gas between Ar and CO2. Analysis of gas composition using a gas chromatography showed that the production ratio of CH4 and H2 depends on polarization potential and a major product was CH4 in the potential range of –1.4 ~ –1.5 V while H2 in the potential range lower than –1.6 V. Spatial arrangement of electrodes and membrane was an another important factor to affect the efficiency, i.e., the efficiency of methane formation increased with decreasing distance between WE and Nafion membrane, indicating that transportation of proton from membrane to WE via ionic liquid medium limited the reaction rate. Contact electrode assembly of Cu-mesh CE/Nafion membrane/ Ti-mesh CE was therefore constructed and revealed better conversion efficiency of CO2 reduction to methane without any sign of ionic liquid decomposition. On the Cu-CE and membrane, however, byproduct of black color was deposited after long time operation. The deposits was supposed to be originated from the silicone sealant used to assemble the electrochemical cell because the deposits contained Si but not carbon. Currently we are working on improvement and refining the electrolysis system for long and efficient operation.