Dual-Phase Electrolyte for Supercapacitors via Non-Aqueous Emulsion Templating

Abstract

At present, liquid and solid/quasi-solid electrolytes are the primary types used in the fabrication of supercapacitors (SCs). Liquid electrolytes typically exhibit higher ionic conductivity (10^-2 to 1 S/cm [1, 2]) but face challenges such as leakage. On the other hand, solid electrolytes address the issue of leakage but generally have lower ionic conductivity (10^-5 to 10^-7 S/cm [3]) compared to liquid electrolytes. Quasi-solid electrolytes offer a balance by achieving higher ionic conductivity than solid electrolytes; however, they lack the ability to withstand mechanical pressure, and the ionic conductivity (10^-4 to 10^-3 S/cm [3]) is still lower than liquid electrolytes. In this research, a dual-phase electrolyte (DPE) has been fabricated, consisting of a solid phase with a free-flowing liquid phase within its porous structure. The DPE combines good mechanical properties (Young’s modulus > 500 kPa [4]) provided by the solid phase and high ionic conductivity (> 1 mS/cm [5]) offered by the liquid phase. This thesis presents a DPE prepared using an emulsion-templating method. This method involves creating an emulsion with dispersed internal phase (liquid phase) droplets within the external phase, followed by the solidification of the external phase (solid phase). The ionic conductivity improves with an increase in the internal phase content; however, this often leads to a decline in mechanical properties. To achieve a balance between desired mechanical properties and ionic conductivity, a polymerised medium internal phase emulsion (polyMIPE) is used. Bisphenol A diglycidyl ether (DGEBA) is selected as the main component of the external phase to provide good mechanical properties, act as a separator, and to ensure the bonding between the DPE and the electrode. Deep Eutectic Solvent (DES) is selected for the internal phase due to its high ionic conductivity (0.02 to 7.61 mS/cm [6]). In the scope of this research, optimal mechanical performance is achieved with a 15 vol.% surfactant content and a 6 ml/min internal phase addition rate. To improve the electrochemical performance of the SC, the polyMIPE is fabricated into a film. The addition of trimethylolpropane triglycidyl ether (TMPTGE) reduces the viscosity of the emulsion and facilitates film spreading. In polyMIPE films, 5 wt.% TMPTGE content achieves the best combination of ionic conductivity and mechanical performance. Various materials and methods are explored to fabricate SCs based on the polyMIPE films. The maximum specific capacitance of the SC is achieved when carbon (C)-spray is employed to form the electrodes and electrodag is used as the adhesive between the current collector and the electrode, reaching a value of 171.68 ± 6.37 mF/g.