Charge storage mechanism and supercapacitive behavior of transparent vanadium pentoxide thin films in various aqueous electrolytes

• Vanadium pentoxide (V 2 O 5 ) thin films were synthesized via spin-coating on FTO substrates and annealed at 573 K. • Optical analysis revealed a high transmittance (∼75 %) and a direct band gap of 2.78 eV. • Electrochemical characterization identified 2 M KCl and 1.5 M Na 2 SO 4 as optimal electrolytes. • Maximum areal capacitance of 13.38 mF.cm -2 was achieved 2 M KCl at a scan rate of 10 mV.s -1 . • Long-term cycling in 1.5 M Na 2 SO 4 showed 55 % capacitance retention after 400 cycles. In this study, V 2 O 5 /FTO were synthesized using spin-coating deposition, followed by thermal annealing at 573 K. The formation of an orthorhombic V 2 O 5 crystalline phase was confirmed by X-ray diffraction and Raman spectroscopy. Optical analysis in the visible light spectrum revealed an average transmittance of approximately 75 % and a band gap of 2.78 eV. Scanning electron microscopy images showed uniform nanograins distributed across the entire film surface, Energy-dispersive X-ray spectroscopy confirmed the elemental composition and homogeneous distribution of elements within the film. The electrochemical performances of the films were evaluated in different aqueous electrolytes at different concentrations, using cyclic voltammetry, galvanostatic charge–discharge, and electrochemical impedance spectroscopy measurements. Among the various tested electrolytes, neutral aqueous media, specifically 2 M KCl and 1.5 M Na 2 SO 4 demonstrated the most favorable supercapacitive performance. V 2 O 5 /FTO exhibited its highest areal capacitance of 13.38 mF·cm -2 in 2 M KCl at 10 mV.s -1 , while 1.5 M Na 2 SO 4 yielded a slightly lower value of 12.45 mF·cm -2 . Notably, at higher charge-discharge rates, the films demonstrated superior capacitive performance in Na 2 SO 4 compared to KCl. Moreover, after 400 cycles the electrodes showed a capacitance retention of 55 % in Na 2 SO 4 , indicating their superior electrochemical durability.