Amorphous-quantized WO₃·H₂O films as novel flexible electrode for advanced electrochromic energy storage devices

The existing synthetic approaches for high-performance WO₃-based electrodes require energy-intensive instrumentation and complex processing, which hinder the development of flexible electrochromic (EC) energy storage devices. Herein, new low-temperature-synthesized amorphous (a)-quantized WO₃·H₂O films for application in EC energy storage devices are proposed. The WO₃·H₂O films are fabricated by the spontaneous hydrolysis of a spin-coated WCl₆ solution by the water molecules in the surrounding atmosphere followed by annealing at 80 °C. This is an original and unique concept in that the induced quantization of a-WO₃ increases the number of electroactive sites, provides abundant oxygen vacancies, and widens the band gap, while the intercalated water molecules stabilize the structure, resulting in efficient charge transfer and stress alleviation during electrochemical reactions. Consequently, the transmittance (53.8% at 633 nm) and specific capacitance (78.5 F g⁻¹ at 1 A g⁻¹) of the flexible electrodes improves. Additionally, the carrier concentration and mobility increase due to a-WO₃ quantization, thereby increasing the electrical conductivity, resulting in the rapid switchability (3.7 s for coloration and 2.9 s for bleaching) and high rate capability of the flexible electrodes. The flexible solid-state devices light a 1.5 V white-light-emitting diode and maintain their good EC energy storage performance (76.1% transmittance modulation and 73.1% specific capacitance) even after 300 bending cycles at a bending radius of 1.3 cm. Therefore, the proposed a-quantized WO₃·H₂O films are promising active electrodes for flexible EC energy storage devices.