WSe₂ Nanoflakes on Graphite Sheets for Flexible Symmetric Supercapacitors

In this study, we have investigated the electrochemical performances of a flexible symmetric supercapacitor (FSS) device based on WSe2 nanoflakes thin film electrodes. The nanoflakes thin film electrodes were fabricated using environmentally friendly and cost-effective DC magnetron sputtering. The as-prepared FSS device was tested in four different aqueous electrolytes containing group 1 alkali metal cations (HCl, LiCl, NaCl, and KCl), using several techniques, including cyclic voltammetry, galvanostatic charge/discharge, and impedance spectroscopy. The obtained results revealed that the FSS devices based on WSe2 with nanoflake-type morphology in various electrolytes exhibit distinctive double-layer characteristics. The device tested in HCl electrolyte demonstrated the highest electrochemical performance, including an impressive areal capacitance of 88 mF cm–2 with corresponding energy and power density of 27.5 μWh cm–2 and 3000 μW cm–2, respectively, at a constant current density of 4 mA cm–2, along with a solution resistance of 0.5 Ω, within an operating voltage range of 1.5 V. The cationic mobility (Li+ < Na+ < K+ < H+) affects ionic conductivity, efficient ion/charge transition, as well as relaxation time and is found to be directly correlated with capacitance value. On the other hand, the order of the long-term cycle stability (LiCl < NaCl < HCl < KCl) is caused by the intricate interactions between ion size, charge density, mechanical stresses, and chemical interactions with the electrode material during the charging and discharging cycles. Furthermore, the use of 1 M HCl as an electrolyte yields superior capacitance performance (88 mF cm–2) and high cycling stability (75.36% capacitance retention after 5000 cycles) at 4 mA cm–2. As a result, the electrolyte cations exert a significant impact on the electrochemical performance of the FSS.