High-Energy-Density 3.5 V Carbon Supercapacitor Fabricated with Ionic-Liquid-Incorporated Redox-Active Gel Polymer Electrolyte

Abstract

Introducing redox-active species in the electrolyte component is an effective approach to improving the energy density of carbon supercapacitors via additional pseudocapacitive redox activities at the electrode–electrolyte interfaces. Herein, we report a quasisolid-state supercapacitor fabricated with symmetrical activated carbon electrodes and an ionic liquid (IL, 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)imide, BMPTFSI)-incorporated nonaqueous, redox-active gel polymer electrolyte (R-GPE) added with a redox-additive IL (BMPBr), entrapped in a polymer matrix of poly(vinylidene fluoride-co-hexafluoropropylene). The R-GPE film with an optimum composition of an additive (BMPBr) showing a high mechanical stability (tensile strength ∼0.32 MPa and elongation at break ∼154%), a wide thermal stability range (up to ∼385 °C), and excellent electrochemical properties (an ionic conductivity of ∼1.2 × 10–3 S cm–1 at room temperature and an electrochemical stability window of ∼6.5 V) is found as an excellent substitute of liquid electrolytes in supercapacitors. The quasisolid-state supercapacitor is fabricated from biomass (pollen-cone)-derived activated carbon electrodes separated by the R-GPE film and characterized via electrochemical techniques, namely, electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge–discharge tests. The Br–-related redox activities at the interfaces lead to a significant improvement in specific capacitance (from ∼164 to ∼248 F g–1), specific energy (from ∼65 to ∼105 W h kg–1), and maximum power (from ∼15 to 31 kW kg–1). With a moderate rate capability, the supercapacitor demonstrates a good cycling performance with an initial ∼23% fading in the specific capacitance and a ∼100% Coulombic efficiency for ∼10 000 charge–discharge cycles.