Ionic-Liquid-Based Deep Eutectic Solvents as Novel Electrolytes for Supercapacitors: COSMO-SAC Predictions, Synthesis, and Characterization

The current work investigated two ionic-liquid (IL)-based deep eutectic solvents (DESs) composed of ethylene glycol (EG) and N -methylacetamide (NMAc) as hydrogen bond donors (HBD) and high-melting IL, namely, 1-butyl-3-methylimidazolium methanesulfonate ([BMIM][MeSO 3 ]), as the hydrogen bond acceptor (HBA). Initially, the COSMO-SAC model was employed for prediction of the eutectic points of the DESs. The computed melting points of the formulated DESs were found to be 70–100 °C lower than that of HBA. The viscosity of the newly developed DESs (∼15 cp) was significantly lower than that of neat room temperature IL electrolytes, and their ionic conductivity was found to be comparable to that of ILs. TGA study revealed no mass loss up to 90 °C, favoring the high temperature application of supercapacitors. To assess electrolytic performance in supercapacitors, electrochemical characterization was done using linear scan voltammetry (LSV), cyclic voltammetry (CV), and galvanostatic charge–discharge (GCD) techniques. LSV provided electrochemical stability up to 3.8 V against a glassy carbon electrode. [BMIM][MeSO 3 ] + EG and [BMIM][MeSO 3 ] + NMAc resulted in operating potential windows (OPWs) of 2 and 3 V, respectively, with a carbon electrode. Moderate values of specific capacitance (55–67 F g –1 ) and power (0.56–1.3 kW kg –1 ) were observed due to higher internal resistance. However, [BMIM][MeSO 3 ] + NMAc resulted in noteworthy specific energy (∼84 Wh kg –1 ) due to its wider OPW.