Evaluation of Structural, Thermal, and Electrochemical Properties of PEO/Ionic Liquid Based Quasi-Solid-State Electrolytes for Electrical Double Layer Capacitor Devices

In this paper, quasi-solid electrolytes (QSEs) “PEO + xwt.% BMIMPF₆” for x = 0–20 were prepared by the immobilization of ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate (BMIMPF₆) to the PEO polymer matrix by solution casting technique. These quasi-solid electrolytes (QSEs) are in the thin film form of good mechanical integrity. The QSEs are characterized by X-ray diffraction (XRD), Attenuated total reflectance Infrared (ATR-IR) spectroscopy, differential scanning calorimetry (DSC)/thermogravimetric analysis (TGA), field emission scanning electron microscopy (FESEM), impedance spectroscopy, and electrochemical techniques. XRD/DSC results confirm an increase in the flexibility (and hence polymer chain mobility) with the increasing amount of IL in the QSEs, as confirmed by the analysis of degree of crystallinity (X_c). The maximum room temperature ionic conductivity ~1.32 × 10⁻⁵ S. cm⁻¹ is obtained for the 20 wt.% IL (BMIMPF₆) added QSEs. The interaction/complexation between the dopant IL-cation BMIM⁺ with the ether oxygen (i.e., COC bond of PEO) has been confirmed by FTIR spectroscopic analysis. FESEM results confirm the appearance of crystalline spherical grains (spherulites), whose size decreases with the increasing amount of IL in the membranes and shows overall semicrystalline microstructures. The TGA analysis confirmed that the onset decomposition temperature of the QSEs is found to be ~175°C, which is the sufficient temperature range of operation for the solid-state electrochemical devices. The electrochemical performances of the QSEs were examined by fabricating the symmetrical electrical double-layer capacitor (EDLC) device. The fabricated EDLC cell with optimized QSE “PEO + 20 wt.% BMIMPF₆” with biomass-based honeycomb activated carbon (HCAC) electrodes offers specific energy ~5.8 Wh kg⁻¹ at power density ~ 79.9 W kg⁻¹. It also displays excellent cycling stability with 81.3% of the initial specific capacitance after 2500 charge–discharge cycles.