Fluorine-Free Ionic Liquid-Based Electrolyte for Supercapacitors Operating at Elevated Temperatures

We synthesized tetra(n-butyl)phosphonium furoate [P-4444][FuA] ionic liquid (IL) by the reaction of tetra(n-butyl)phosphonium hydroxide and 2-furoic acid using water as a solvent at room temperature. The thermal stability and phase behavior of the IL are investigated through thermogravimetric analysis (TGA) and differential scanning calorimetry (DSC), while the ionic conductivity measurement is carried out using impedance spectroscopy. Hybrid carbon-based material composed of multiwalled carbon nanotubes (MWCNTs) and activated charcoal is fabricated and used as electrodes. The effect of potential scan rate, temperature, and voltage on the electrochemical performance of the capacitor is thoroughly investigated through cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and electrochemical impedance spectroscopy (EIS). The results showed that the internal resistance and specific capacitance are highly dependent on the temperature and voltage and high specific capacitance of 141.4 F g(-1) (5 mV s(-1)) from CV and 182 F g(-1) (1 A g(-1)) from GCD at 100 degrees C is achieved, indicating excellent electrochemical performance. The capacitor demonstrated 29.0 Wh kg(-1) energy density and 13.3 kW kg(-1) power density at 20 degrees C and 3 V potential, while 177 Wh kg(-1) energy density and 82 kW kg(-1) power density are achieved at a higher temperature (100 degrees C). The FTIR analysis of the capacitor after electrochemical studies confirmed that no changes occurred in the structure of the IL, indicating high electrochemical stability of the IL for supercapacitor applications in an extended temperature (-20 to 100 degrees C) and a wide potential range (3 to 4.6 V).