A Novel Triethylammonium Tetrafluoroborate Electrolyte for Enhanced Supercapacitor Performance over a Wide Temperature Range

The wide operating temperature and voltage window are favourable properties that increase the practical applications of supercapacitors. Ionic liquids (IL) are suitable electrolytes that allow supercapacitors to be used in wide operating ranges. In this study, triethylammonium tetrafluoroborate (Et3NHBF4) is tested as a new IL to operate supercapacitors in a wide temperature range (−40 °C, 25 °C, and 80 °C) in the presence of commercial activated carbon. The performance of Et3NHBF4 is compared to two different commercial ILs. This study also investigates the application of heat treatment to determine suitable activated carbon surface characteristics for ILs. The results indicate that heat treatment enhances the electrode–electrolyte interaction, and the electrochemical performances of the supercapacitors prepared from the heat-treated activated carbon are significantly higher than the original commercial activated carbon. Electrochemical tests show that the synthesised Et3NHBF4 (with propylene carbonate) can be used over a wide temperature range and has a better energy storage performance, especially at −40 °C (specific capacitance of 42.12 F/g at 2 A/g), compared to the other two commercial ionic liquids.