High-Energy-Density Carbon Supercapacitors Incorporating a Plastic-Crystal-Based Nonaqueous Redox-Active Gel Polymer Electrolyte

Abstract

Addition of redox additives in electrolytes to enhance the electrochemical activity at electrode–electrolyte interfaces is one of the prime approaches these days to develop high-energy-density supercapacitors. Here, we report an investigation on a quasi-solid-state supercapacitor, fabricated with a nonaqueous, gel polymer electrolyte (GPE) based on a mixture of a plastic crystal succinonitrile and an ionic liquid 1-butyl-1-methylpyrrolidinium bis(trifluoromethyl sulfonyl) imide, added with a redox additive hydroquinone (HQ), immobilized in a polymer poly(vinylidine fluoride-co-hexafluoropropylene). The HQ-incorporated GPE is observed to be a freestanding, easily processible, and reusable film, showing excellent flexibility and thermal stability up to ∼100 °C. The high ionic conductivity (∼4.2 mS cm–1) and wide electrochemical stability window (∼5.0 V) through linear sweep voltammetry measurements make the optimum composition of GPE a potential electrolyte for high-energy-density supercapacitors. The symmetric supercapacitor coin cells have been fabricated with peanut shell-derived porous carbon electrodes separated by GPE films. The electrochemical activity due to the presence of HQ at carbon electrode–GPE interfaces introduces additional pseudocapacitance over the double-layer capacitance, leading to enhanced overall specific capacitance (289 F g–1), and hence corresponds to the high specific energy (∼40 Wh kg–1) and maximum power (∼20 kW kg–1). The capacitor cell shows prolonged cyclic profile up to ∼10 000 charge–discharge cycles with ca. 85–93% Coulombic efficiency.