Optimizing Phosphonium-Based Ionic Liquid Electrolytes for Enhanced Supercapacitor Performance

To meet the growing demand for sustainable energy, this study explores phosphonium-based ionic liquids (ILs) as efficient electrolytes for supercapacitors. Two ILs, [P4444][TFSI] and [P44414][TFSI], were dissolved in dimethyl sulfoxide (DMSO) and dimethylformamide (DMF), and their electrochemical performance was evaluated using graphene oxide (GO) and reduced graphene oxide–nickel oxide (rGO–NiO) electrodes. Electrochemical analyses using cyclic voltammetry (CV), galvanostatic charge–discharge (GCD), and electrochemical impedance spectroscopy (EIS) revealed that [P4444][TFSI] in DMSO with rGO–NiO electrodes achieved superior performance, including a high specific capacitance of 213.85 F·g–1, energy density of 200.8 W·h·kg–1, and power density of 1300 W·kg–1. Density Functional Theory (DFT) supported these findings, showing enhanced ionic transport with the smaller [P4444]+ cation. The study underscores the critical interplay among IL structure, solvent choice, and electrode composition in optimizing supercapacitor efficiency.