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.