High energy density and low self-discharge of a quasi-solid-state supercapacitor with carbon nanotubes incorporated redox-active ionic liquid-based gel polymer electrolyte

To achieve a quasi-solid-state supercapacitor (SC) with high energy density and low self-discharge, a redox-active poly(vinyl alcohol) (PVA)-based gel polymer electrolyte (GPE) is synthesized by the addition of 1-butyl-3-methylimidazolium bromide (BMIMBr) ionic liquid (IL) and carbon nanotubes (CNTs) into a PVA aqueous solution, followed by the addition of Li2SO4 and then the removal of excess water. The influences of BMIMBr-to-PVA and CNTs-to-PVA mass ratios on the ionic conductivity of GPE are discussed. The maximum ionic conductivity of GPE can reach 61.1 mS cm−1. A SC consisting of the optimized GPE and a pair of activated carbon (AC) electrodes exhibits a high energy density of 43.1 Wh kg−1, which can be assigned to the wide working voltage of 1.8 V as a result of the strong solvation effect of Li+ cations and SO42− anions, pseudocapacitance contribution from the redox reaction of Br−/Br3− at the electrolyte/electrode interface, and the three-dimensional (3D) conductive network of CNTs throughout the PVA matrix providing transport channel to facilitate ion transport. And the self-discharge is greatly suppressed due to the presence of 3D network of CNTs in the GPE. Additionally, the SC presents excellent cyclic stability.