Optimization of porous polymer electrolyte for quasi-solid-state electrical double layer supercapacitor

Abstract We report the poly(vinylidene fluoride-co-hexfluoropropylene) (PVdF-HFP) based porous polymer electrolyte membranes, prepared via phase-inversion/solvent-nonsolvent methods, activated with an organic liquid electrolyte ethylene carbonate (EC):propylene carbonate (PC)-NaClO4 for the application in electric double layer capacitor (EDLC). The simple, quick and environment-friendly phase-inversion method, involving condensing steam as non-solvent, has been taken as the optimized process to obtain the porous PVdF-HFP film. The films of porous PVdF-HFP and the electrolyte (after soaking with liquid electrolyte) have been characterized for their morphological/structural aspects, porosity, liquid electrolyte retention, interaction with electrolyte, thermal properties, electrochemical stability and ionic conductivity. A pore-formation mechanism during phase-inversion at 100 °C has been proposed on the basis of thermal studies. The electrolyte film has been found to have excellent mechanical flexibility, porosity (∼80%), electrolyte retention (∼400%), ionic conductivity (∼2 mS cm−1 at room temperature), and electrochemical stability window (ESW) of ∼4.35 V. The EDLC, fabricated with activated carbon electrodes and porous polymer electrolyte, exhibits excellent performance characteristics in terms of the specific capacitance (∼150 F g−1, evaluated from EIS), specific energy (∼17.7 Wh kg−1) and specific power (14.3 kW kg−1). The device shows stable specific capacitance (after ∼17% initial fading) and high Coulombic efficiency (over 99%) for ∼10,000 charge-discharge cycles.