Role of Cations in Ionic Liquid-Based Polymer Composite Electrolyte Organic Supercapacitors

Solid-state supercapacitors utilizing ionic liquid-based polymer composite electrolytes (IL-<i>b</i>-PCEs) have gained attention as promising candidates for next-generation energy storage systems due to their high ionic conductivity, wide electrochemical stability, and nonvolatility. However, the specific influence of cation species on the electrochemical performance of IL-<i>b</i>-PCEs has remained insufficiently explored. This study aims to analyze the impact of three distinct cations 1-ethyl-3-methylimidazolium (EMIM⁺), 1-butyl-3-methylimidazolium (BMIM⁺), and proton (H⁺), on the physicochemical characteristics and charge storage behavior in PVA-based IL-<i>b</i>-PCEs systems. The Fourier-transform infrared (FTIR) spectroscopy elucidates a notable shift in the O-H stretching band from 3268.96 cm^-1 in HCl/PVA to 3358.69 cm^-1 for BMIMCl/PVA, and 3351.01 cm^-1 for EMIMCl/PVA, indicating a reduced hydrogen bonding strength due to the bulky nature of the imidazolium cations. The electrochemical analysis reveals that HCl/PVA exhibits the highest specific capacitance (12.67 F/g) and the lowest equivalent series resistance (7.7 Ω), a phenomenon attributed to the small size and high mobility of protons (H⁺), which enhance the efficiency of charge storage and transfer. Conversely, BMIMCl/PVA and EMIMCl/PVA demonstrate lower capacitance and higher resistance as a result of their larger size, which hinders ion mobility and causes steric effects. These findings emphasize the critical role of cation selection in tailoring the electrochemical performance of IL-based solid-state supercapacitors, offering valuable insight for future design of high-performance energy storage systems.