A Thin-Film-Free Electrochromic System Based on Dual Conductive Glass Electrodes and Sodium Tungstate Electrolyte

Abstract Electrochromic devices (ECDs) are widely used in smart windows, displays, and mirrors for dynamic light control, but conventional thin-film electrodes are costly and complex to fabricate. This study presents a film-free electrochromic device that eliminates the need for electrochromic coatings on transparent conductive oxide substrates. The device consists of an acidified sodium tungstate (Na₂WO₄·2H₂O) electrolyte sandwiched between two bare FTO glass electrodes. Upon applying a low voltage of 3 V, a reversible blue coloration forms in situ near the working electrode and disappears when the voltage is removed, restoring transparency. At pH 1.5, when applying 3 V, transmittance measurements reveal an optical modulation of 87.45% and an optical density of 0.96 at 648 nm wavelength, confirming the device's excellent optical performance. The response times for coloration (14 s) and bleaching (35 s) indicate good switching dynamics. The devices show a high coloration efficiency of 66.7 cm 2 /C at 648 nm wavelength. These findings highlight the excellent performance of the device and demonstrate that eliminating the thin-film coating process not only simplifies fabrication but also enables the development of film-free electrochromic devices as high-performance alternatives for next-generation electrochromic applications.