Bifunctional Ionic Deep Eutectic Electrolytes for CO₂ Electroreduction

CO₂ is a low-cost monomer capable of promoting industrially scalable carboxylation reactions. Sustainable activation of CO₂ through electroreduction process (ECO₂R) can be achieved in stable electrolyte media. This study synthesized and characterized novel diethyl ammonium chloride-diethanolamine bifunctional ionic deep eutectic electrolyte (DEACl-DEA), using diethanolamine (DEA) as hydrogen bond donors (HBD) and diethyl ammonium chloride (DEACl) as hydrogen bond acceptors (HBA). The DEACl-DEA has -69.78 °C deep eutectic point and cathodic electrochemical stability limit of -1.7 V versus Ag/AgCl. In the DEACl-DEA (1:3) electrolyte, electroreduction of CO₂ to CO₂ ^•- was achieved at -1.5 V versus Ag/AgCl, recording a faradaic efficiency (FE) of 94%. After 350 s of continuous CO₂ sparging, an asymptotic current response is reached, and DEACl-DEA (1:3) has an ambient CO₂ capture capacity of 52.71 mol/L. However, DEACl-DEA has a low faradaic efficiency <94% and behaves like a regular amine during the CO₂ electroreduction process when mole ratios of HBA-HBD are greater than 1:3. The electrochemical impedance spectroscopy (EIS) and COSMO-RS analyses confirmed that the bifunctional CO₂ sorption by the DEACl-DEA (1:3) electrolyte promote the ECO₂R process. According to the EIS, high CO₂ coverage on the DEACl-DEA/Ag-electrode surface induces an electrochemical double layer capacitance (EDCL) of 3.15 × 10^-9 F, which is lower than the 8.76 × 10^-9 F for the ordinary DEACl-DEA/Ag-electrode. COSMO-RS analysis shows that the decrease in EDCL arises due to the interaction of CO₂ non-polar sites (0.314, 0.097, and 0.779 e/nm²) with that of DEACl (0.013, 0.567 e/nm²) and DEA (0.115, 0.396 e/nm²). These results establish for the first time that a higher cathodic limit beyond the typical CO₂ reduction potential is a criterion for using any deep eutectic electrolytes for sustainable CO₂ electroreduction process.