Pre-treated biomass waste melon peels for high energy density semi solid-state supercapacitors

Semi-solid-state supercapacitors employing highly porous activated carbon (AC) electrodes are promising, cost-effective, and environmentally friendly energy storage devices with exceptional power performance. In this work, for the first time in literature, we report a comparative study on different pre-treatments made on melon waste starting precursor to produce hierarchical large surface area porous AC. Also, for the first time in our knowledge, a gel polymer electrolyte (GPE) consisting in lithium trifluoromethanesulfonate in ethylmethylimidazolium trifluoromethanesulfonate, with a high ionic conductivity (similar to 3.3 x 10(-3) S cm(-1)) and a working voltage window of similar to 3.9 V vs Ag/Ag+, was used. The supercapacitors were electrochemically characterized with electrochemical impedance spectroscopy, cyclic voltammetry, and galvanostatic charge-discharge tests. The working voltage window of the device was optimized in the range of 0-2.3V. Hydrothermally pre-treated AC-based supercapacitor is characterized by the best performance in terms of capacitance (similar to 161-170 F g(-1)), specific energy (29-31.34 Wh kg(-1)), and power density (839-860 W kg(-1)) at 1 A g(-1). Supercapacitors based on ACs pre-treated via hydrothermal and ethanol soaking outperform devices based on simple chemically/physically ACs in rate performance, while hydrothermally pre-treated AC demonstrates superior stability over 8000 cycles, exhibits initial 15 % capacitance fading and coulombic efficiency close to 99-100 %.