Development of zinc nitrate hexahydrate salt (ZNH)/ethylene glycol (EG) binary composite deep eutectic electrolytes

Abstract

In this study, zinc nitrate hexahydrated (ZNH)/ethylene glycol (EG) binary composite deep eutectic solvent (DES) were developed by a combination of hydrogen bond acceptor (HBA) (i.e. ZNH) and hydrogen bond donor (HBD) (i.e. EG) at various molar ratios (1:3, 1:4, 1:5 and 1:6) using heating method. The electrochemical and physicochemical properties DES were analyzed using Ionic conductivity (σ), refractive index, density, viscosity, thermogravimetric (TGA), Fourier transform infrared spectroscopy (FTIR), cyclic voltammetry (CV), and electrochemical impedance (EIS). The results revealed that DES 1:6 exhibited the lowest viscosity values of 579, 381, 119, 110, and 58 mPa s at shear rate values of 20, 30, 60, 100, and 200 (s-1) respectively. Similarly, the lowest values of density (1.392 g/cm3) and lowest values refractive index (1.441) and highest value of σ (7.51 mS/cm) were obtained. The cyclic voltammetry (CV) result also presented a wider range value of electrochemical potential window (EPW) (i.e. 1.06 V) showing the reversibility of electrochemical electrolytes due to the delocalization of charged ions that creates a bigger void volume inside the solution. Hence, these findings show that DES with the molar ratio of 1:6 can be a potential solvent for different electrochemical storage applications.