Solid-state gel polymer electrolytes based on ionic liquids containing imidazolium cations and tetrafluoroborate anions for electrochemical double layer capacitors: Influence of cations size and viscosity of ionic liquids

In this study, we report physicochemical properties of poly(vinylidene fluoride-hexafluoropropylene) based gel polymer electrolytes embedded with different ionic li quids such as 1-butyl-2,3-dimethylimidazolium tetrafluoroborate, 1-butyl-3-methylimidazolium tetrafluoroborate and 1-ethyl-3-methylixnidazolium tetrafluoroborate with a view to using them as electrochemical devices such as electrochemical capacitors. We also study the influence of cationic size and viscosity, dielectric constant of the ionic liquids on the physicochemical properties and electrochemical stability of the gels and also on electrochemical performance of the electrochemical capacitors fabricated with these gel electrolytes. The electrochemical performance of the electrochemical capacitors is studied by cyclic voltammerty, galvanostatic charge-discharge and electrochemical impedance spectroscopy. It is observed that the polymer electrolyte embedded with 1-ethyl-3-methylimidazolium tetrafluoroborate ionic liquid having smallest size of cations and highest ionic conductivity displays a promising set of physicochemical properties and performs well as an electrolyte/separator in device in terms of specific capacitance, energy density, and cycle life comparatively. The values of specific capacitance obtained from galvanostatic charge-discharge, equivalent series resistance, energy density, response time, and pulse power for the electrochemical capacitor assembled with 1-ethyl-3-methylimida-zolium tetrafluoroborate doped gel electrolytes are 113.6 Fg(-1), 20 Omega-cm(2), 17.48 Wh kg(-)(1), 5.46 s and 7.19 kW kg(-1) at 1.0 mA cm(-2) current respectively.