Constructed TiO₂/WO₃ heterojunction with strengthened nano-trees structure for highly stable electrochromic energy storage device

Tungsten trioxide (WO₃) has been widely regarded as a prospective bifunctional material due to its electrochromic and pseudocapacitive properties, while still facing the dilemma of inadequate cycle stability and trapping-induced degradation. Here, inspired by the trees-strengthening approach, a unique titanium dioxide (TiO₂) nanorod arrays strengthened WO₃ nano-trees (TWNTs) heterojunction was rationally designed and constructed. In sharp contrast to the transmittance modulation (ΔT) attenuation of primary WO₃ nano-trees during cycling, the TWNTs film showed not only excellent electrochromic performance but also fascinating cycle stability (77.35% retention of the initial ΔT after 10,000 cycles). Besides, the trapping-induced degradation could be easily rejuvenated by a potentiostatic de-trapping process. An electrochromic energy storage device (EESD) was further assembled based on the TWNTs film to deliver excellent ΔT (up to 79.5% at 633 nm), fast switching speed (t_c/t_b =1.9 s/14.8 s), extremely high coloration efficiency value (443.4 cm²·C⁻¹), and long-term cycle stability (over 10,000 charge/discharge cycles). This innovative study provided in-depth insights into the electrochromism nature and a significant step in the realization of stable electrochromic-energy storage application, paving the way for multifunctional smart windows as well as next-generation optoelectronic devices.