Electrochemical CO₂ Reduction in Deep Eutectic Solvent: Effect of Water and Organic Diluents

Deep eutectic solvents (DESs) are promising media for CO2 electroreduction because of their high CO2 solubility, low cost, and eco-friendliness. However, the high viscosity and poor ionic conductivity severely limit their applications. In this study, low-viscosity polar aprotic solvents, propylene carbonate (PC) and sulfolane (SL), were used as diluents in Ethaline (ChCl:2EG) to prepare mixed electrolytes for electrochemical CO2 reduction. Raman and infrared spectra indicate that PC and SL can participate in the eutectic network of Ethaline through intermolecular hydrogen bonding, thereby decreasing the viscosity. The current density of the Ag cathode increases from 1.7 to 8.0 and 5.3 mA/cm2 with the addition of PC and SL, respectively. The two diluents also result in improved CO faradaic efficiency (FECO) and suppressed HER because the C═O and S═O bonds in the molecular frameworks of PC and SL can disrupt the hydrogen bonds of accumulated water molecules during electrolysis. At −2.0 V, FECO increases from 63% to over 90% upon the addition of PC and SL. This study provides new insights into the design of high-performance DES electrolytes for CO2 electroreduction.