Defective impacts on amorphous WO3·H2O films using accelerated hydrolysis effects for flexible electrochromic energy-storage devices

Highlights: • Amorphous WO₃·H₂O films with porosity and oxygen vacancy via hydrolysis effects. • Porous morphology provides high electrochemical activity by extra active sites. • Abundant oxygen vacancies provide extra electrons and favored electron pathway. • Novel strategy to fabricate flexible WO₃ films for flexible EC energy-storage devices. We newly developed amorphous WO₃·H₂O (a-WO₃·H₂O) films with porosity and oxygen vacancy (V_O) defects through a humidity adjustment causing the accelerated hydrolysis of WOCl₄ with H₂O during spin-coating and during low-temperature annealing for flexible electrochromic (EC) energy-storage devices. Optimizing the hydrolysis effect in all a-WO₃·H₂O films, we adjusted the humidity to 25, 35, and 45% in a humid chamber. Specifically, the a-WO₃·H₂O film fabricated at 35% exhibited a developed porous morphology and an increased number of V_O defects, providing increased electrochemically active sites and enhanced electrical conductivity, respectively, due to the accelerated hydrolysis of WOCl₄ and the increased intercalation of water molecules. Such behaviors of the a-WO₃·H₂O film bring about superior flexible EC energy-storage performances of widened transmittance modulation (60.0% at 633 nm), fast switching speeds (3.4 s for coloration speed and 4.2 s for bleaching speed), a high CE (62.7 cm²/C), good specific capacitance (94.2 F/g at 2 A/g), and rate capability (74.3%). Specifically, the increased transmittance modulation and specific capacitance stem from the increased electrochemical activity caused by the enriched electrochemically active sites. Moreover, the fast switching speeds and good rate capability are generated by electrochemical kinetics improved with the porous morphology and the increased V_O.