The increasing global energy demand necessitates innovative energy-saving technologies. Electrochromic (EC) devices, particularly in smart windows, offer significant potential for energy efficiency by dynamically controlling sunlight penetration, resulting in energy savings exceeding 40%. Beyond smart windows, EC devices find applications in displays, electronic paper, and mirrors due to their ability to display multiple colors through voltage adjustment. Integrating electrochemical energy storage into EC devices further expands their capabilities. This dual functionality—optical modulation and energy storage—allows for self-monitoring of stored energy through optical property changes. The performance of such multifunctional devices hinges on the electrochemical double-layer behavior of electrons and cations within the active materials. Tungsten oxide (WO3) stands out as a promising pseudocapacitive material for EC applications, offering reversible color change, high contrast, good stability, and low cost. Amorphous WO3 (a-WO3) displays superior electrochemical kinetics than crystalline WO3 due to its loosely packed structure facilitating ion movement. However, a-WO3-based devices suffer from limitations in coloration efficiency (CE), switching speed, and specific capacitance due to low ion diffusion coefficients. Previous attempts to improve performance using a-WO3 films have involved tuning the crystal structure and morphology or incorporating polymer materials. While polymer-based composites often achieve fast switching speeds, they suffer from electrochemical degradation due to physically mixed structures, hindering charge transport and adhesion. This study proposes a novel design using chitosan, a biocompatible and biodegradable natural polymer with abundant hydroxyl and amine groups, as a linking medium to enhance the electrochemical behavior of a-WO3 films and improve EC energy storage performance. Chitosan's functional groups act as accelerators for electrochemical behavior with cations and facilitate chemical cross-linking with WHNPs, improving kinetics and stability.

The literature review extensively covers existing research on electrochromic devices and their applications in energy-efficient smart windows and other optoelectronic applications. It highlights the advantages and disadvantages of using various materials, such as tungsten oxide (WO3) in its amorphous and crystalline forms, and the use of polymer composites. It points out the limitations of previously reported multifunctional devices, specifically addressing issues related to coloration efficiency, switching speed, and specific capacitance due to low ion diffusion and electron conductivity. The review emphasizes the need for novel hybrid film designs that leverage the unique properties of both inorganic and organic materials to overcome these limitations. The properties of chitosan as a functional polymer are also reviewed, noting its potential for cross-linking and its ability to act as an accelerator for electrochemical behavior.

a-WO3/WHNP-embedded chitosan hybrid films were fabricated using a spin-coating method. First, a-WO3 films were prepared by spin-coating a tungsten(VI) chloride solution in 2-propanol onto FTO substrates, followed by annealing at 300 °C. WHNPs were synthesized hydrothermally using tungstic acid and hydrochloric acid at 180 °C. The WHNPs (0.01 wt%) were then mixed with a chitosan solution (0.5 wt% in DI water with 1 vol% acetic acid) and spin-coated onto the a-WO3 films at 2000 rpm. Films were dried at 80 °C. Control films (bare a-WO3 and a-WO3/CH) were also prepared. Material characterization involved X-ray diffraction (XRD) to analyze crystal structure, X-ray photoelectron spectroscopy (XPS) to determine chemical binding states, Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), and energy dispersive spectroscopy (EDS). Electrical properties were measured using a Hall-effect measurement system, and optical properties using UV-vis spectroscopy. Electrochemical analysis used a three-electrode system (1 M LiClO4 in propylene carbonate electrolyte, Pt counter electrode, Ag reference electrode) with a potentiostat/galvanostat and electrochemical impedance spectroscopy (EIS). Contact angle measurements determined electrolyte wettability. EC energy storage performance was assessed in situ using a combination of potentiostat/galvanostat and UV-vis spectroscopy.

XRD analysis confirmed the amorphous nature of a-WO3 films and the presence of orthorhombic WO3·H2O in a-WO3/CH@WHNP films. XPS revealed the presence of W5+ and oxygen vacancies (VO) in a-WO3/CH@WHNP, indicating improved electrochemical activity. The incorporation of chitosan increased the -OH/W-O peak area ratio, suggesting enhanced interaction with hydroxyl groups. N 1s XPS spectra showed the presence of amine (-NH2) and protonated amine (-NH3+) groups in chitosan films, contributing to improved electrical conductivity. In a-WO3/CH@WHNP, changes in N 1s peaks suggested chemical cross-linking between chitosan and WHNPs. The hybrid films exhibited significantly faster switching speeds (4.0 s for coloration and 0.8 s for bleaching) than bare a-WO3 films. This was attributed to enhanced electrical conductivity and Li-ion diffusivity. The coloration efficiency was substantially improved (62.4 cm²/C) due to increased electrochemical activity. Long-cycling stability was also enhanced (91.5% retention after 1000 cycles). Furthermore, the hybrid films displayed superior energy storage performance with a high specific capacitance (154.0 F/g at 2 A/g) and a stable rate capability resulting from enhanced electrochemical activity and fast electrical conductivity. The improved energy density led to brighter illumination intensity for a 1.5-V white-light-emitting diode.

The improved EC energy storage performance of the a-WO3/CH@WHNP hybrid films is attributed to the synergistic effect of chitosan and WHNPs. Chitosan's abundant functional groups enhance Li-ion diffusivity, while the chemical cross-linking between chitosan and WHNPs provides improved electron transport pathways and structural stability, suppressing degradation during cycling. The presence of oxygen vacancies in WHNPs contributes to increased electrochemical activity and serves as color/energy storage centers. The results demonstrate a successful strategy for creating multifunctional devices by combining the electrochemical properties of inorganic materials with the cross-linking capabilities of organic polymers. The enhanced performance surpasses that of previous a-WO3-based devices, addressing the limitations of low CE, slow switching speeds, and poor cycling stability.

This study successfully demonstrates a new design strategy for multifunctional electrochromic energy storage devices using a-WO3/CH@WHNP hybrid films. The chemical cross-linking between chitosan and WHNPs significantly improves EC performance (switching speed, coloration efficiency, cycling stability) and energy storage capacity (specific capacitance, rate capability). The brighter LED illumination highlights the enhanced energy density. Future research could explore other organic polymers or incorporate different inorganic nanoparticles to further optimize device performance and investigate the long-term stability under various environmental conditions.

The study focuses on a specific type of chitosan and WHNP concentration. Further investigation is needed to explore the effects of varying these parameters and other organic materials. The long-term stability of the devices under real-world conditions remains to be thoroughly investigated. The study primarily uses laboratory-scale measurements, and larger-scale fabrication and testing are needed for practical applications.