WSe<sub>2</sub> Nanoflakes\non Graphite Sheets for\nFlexible Symmetric Supercapacitors

In this study, we have investigated the electrochemical\nperformances\nof a flexible symmetric supercapacitor (FSS) device based on WSe<sub>2</sub> nanoflakes thin film electrodes. The nanoflakes thin film\nelectrodes were fabricated using environmentally friendly and cost-effective\nDC magnetron sputtering. The as-prepared FSS device was tested in\nfour different aqueous electrolytes containing group 1 alkali metal\ncations (HCl, LiCl, NaCl, and KCl), using several techniques, including\ncyclic voltammetry, galvanostatic charge/discharge, and impedance\nspectroscopy. The obtained results revealed that the FSS devices based\non WSe<sub>2</sub> with nanoflake-type morphology in various electrolytes\nexhibit distinctive double-layer characteristics. The device tested\nin HCl electrolyte demonstrated the highest electrochemical performance,\nincluding an impressive areal capacitance of 88 mF cm<sup>–2</sup> with corresponding energy and power density of 27.5 μWh cm<sup>–2</sup> and 3000 μW cm<sup>–2</sup>, respectively,\nat a constant current density of 4 mA cm<sup>–2</sup>, along\nwith a solution resistance of 0.5 Ω, within an operating voltage\nrange of 1.5 V. The cationic mobility (Li<sup>+</sup> < Na<sup>+</sup> < K<sup>+</sup> < H<sup>+</sup>) affects ionic conductivity,\nefficient ion/charge transition, as well as relaxation time and is\nfound to be directly correlated with capacitance value. On the other\nhand, the order of the long-term cycle stability (LiCl < NaCl <\nHCl < KCl) is caused by the intricate interactions between ion\nsize, charge density, mechanical stresses, and chemical interactions\nwith the electrode material during the charging and discharging cycles.\nFurthermore, the use of 1 M HCl as an electrolyte yields superior\ncapacitance performance (88 mF cm<sup>–2</sup>) and high cycling\nstability (75.36% capacitance retention after 5000 cycles) at 4 mA\ncm<sup>–2</sup>. As a result, the electrolyte cations exert\na significant impact on the electrochemical performance of the FSS.