Self-seeded growth of hexagonal-phase WO3 film by a one-step hydrothermal method for high-performance electrochromic energy storage devices

Electrochromic devices have garnered significant interest due to their great potential in intelligent multifunctional electrochromic energy storage devices (EESDs) and smart electronics. Tungsten oxide (WO₃) exhibits both electrochromic and pseudocapacitive characteristics, and combining these two functionalities into a single device with enhanced overall performance is a popular research topic. However, synthesizing high-performance WO₃ electrochromic films with an efficient and simple method remains a challenge. Herein, a one-step fabrication strategy is developed for the in-situ growth of hexagonal WO₃ films on FTO substrates. This strategy employs a low-temperature hydrothermal method, involving xylitol as a self-seeding agent and cesium sulfate as a capping agent. The synergistic effect of the self-seeding and capping agents results in highly efficient WO₃ films with strong substrate adhesion and a large optical modulation (86.8 %). An EESD is assembled using the WO₃ film as the working electrode and a PB film as the counter electrode. The EESD demonstrates fast bleaching/coloration switching times and a high coloration efficiency of 104.8 cm 2  C⁻ 1 . Additionally, the EESD shows a distinct and reversible color transition from transparent to dark blue, accompanied by a change in optical transmittance of 74.45 %.