Multifunctional electrochromic energy storage devices by chemical cross-linking: impact of a WO3·H2O nanoparticle-embedded chitosan thin film on amorphous WO3 films

With the advent of multifunctional devices with electrochromic (EC) behavior and electrochemical energy storage, complementary design of film structures using inorganic-organic materials has shown great potential for developing EC energy storage devices. Herein, hybrid films consisting of WO3·H2O nanoparticle (WHNP)-embedded chitosan thin films on amorphous WO3 (a-WO3) films were designed. By exploiting the hybrid effect of chitosan and WHNPs to generate unique chemical cross-linking between them, the designed films exhibited attractive EC behaviors compared to bare a-WO3 films. These included fast switching speeds (4.0 s for coloration and 0.8 s for bleaching) due to enhanced electrical conductivity and Li-ion diffusivity, high coloration efficiency (62.4 cm²/C) as a result of increased electrochemical activity, and superb long-cycling retention (91.5%) after 1000 cycles due to improved electrochemical stability. In addition, hybrid films exhibited a noticeable energy storage performance with a high specific capacitance (154.0 F/g at a current density of 2 A/g) and a stable rate capability as a result of improved electrochemical activity and fast electrical conductivity, respectively. This resulted in brighter illumination intensity for the 1.5-V white-light-emitting diode due to improved energy density compared to a bare a-WO3 film. Therefore, the results suggest a new design strategy for materials to realize the coincident application of multifunctional devices with EC energy storage performance.