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 cm2 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 %.