Role of Ionic Solvents in the Electrocatalytic CO₂ Conversion and H₂ Evolution Suppression: From Ionic Liquids to Deep Eutectic Solvents

Abstract

The ionic solvents, including both ionic liquids (ILs) and deep eutectic solvents (DES), are deeply studied for their potential in the carbon dioxide (CO2) capture and its further electrochemical conversion using different electrocatalysts. The aim of this review is to present and critically compare the role of ILs and DES in the activation of the electrochemical CO2 reduction reaction (CO2RR) and suppression of the hydrogen evolution reaction (HER). Therefore, the most relevant advances in the use of these ionic solvents in CO2RR, either as neat reaction medium or as electrolyte in molecular solvents, have been summarized and discussed. A special focus has been made on comparing the current density, overpotential, faradaic efficiency and products selectivity of the CO2RR in the presence of the ionic solvents and relaying those results with their chemical composition. Herein, the most recent strategies reported in the literature based on the use of either DES or ILs for enhancing the electrocatalytic CO2 conversion are reviewed, and some new perspectives based on immobilized ILs at the electrode surface are discussed.

Electrocatalysis in deep eutectic solvents: from fundamental properties to applications

“…DESs as electrolytes play pivotal roles in modulating the electron-transfer kinetics, facilitating mass-transfer kinetics, tuning the EDL interface structure, broadening ESW, changing the ion solvation environment, and suppressing side reactions in electrocatalytic reactions. 40,155–157…”

“…In contrast, the CO 2 solubility is 0.03 mol CO 2 L −1 in aqueous solutions and 0.16–0.27 mol CO 2 L −1 in organic solvents under the same conditions. 157 The excellent capture ability for CO 2 increases the CO 2 availability at the electrode surface, thereby benefiting CO 2 RR performance. However, despite the unique advantages of DESs in terms of electron transfer as well as improving the concentration of CO 2 near the electrode, the current density obtained in the CO 2 RR using pure DESs as electrolytes is currently quite low.…”

“…Notably, the solubility of CO 2 in organic solvents such as acetonitrile can reach levels up to eight times higher than in aqueous solutions, further contributing to the performance enhancements. 157 …”