Environmentally benign non-fluoro deep eutectic solvent and free-standing rice husk-derived bio-carbon based high-temperature supercapacitors

Deep eutectic solvents (DESs) have been emerging as newer environmentally benign electrolytes for energy storage devices with the high-temperature operational capability. For the first-time, we propose ethylene glycol and tetrapropylammonium bromide based non-fluoro DESs as electrolytes for the supercapacitor. Different compositions of DESs were also evaluated. Also, the high-temperature operational capability of the proposed electrolytes was evaluated with the free-standing microporous commercial activated carbon (AC) and mesoporous rice husk-derived bio-activated carbon (Bio-AC). The maximum cell voltages of 1.3 V and 1.0 V are identified for AC and Bio-AC based supercapacitors, respectively at 25 °C. The specific capacitances of both AC and Bio-AC electrodes are ∼100 F g−1 at 0.1 A g−1. The proposed DES electrolyte can be operated at the maximum temperature of 115 °C, which is higher than the previously reported DESs (80 °C). The specific capacitance increases with increasing temperature, and the maximum specific capacitance of an electrode is 172 F g−1 at 115 °C for Bio-AC supercapacitor. Excellent long-term stability is noted after 10,000 cycles at 25 °C and 115 °C for both AC and Bio-AC supercapacitors. The supercapacitors here may be practically used in high-temperature applications.