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

Abstract We newly developed amorphous WO3·H2O (a-WO3·H2O) films with porosity and oxygen vacancy (VO) 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 VO 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 VO.