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

The study addresses the challenge of improving electrochromic (EC) energy storage performance in devices that combine optical modulation with charge storage. While EC devices (e.g., smart windows) can significantly reduce energy consumption by modulating light and heat flux, integrating robust energy storage remains difficult. Tungsten oxide (WO3), particularly in amorphous form (a-WO3), offers favorable ion transport and EC characteristics but suffers from limitations including modest coloration efficiency, relatively slow switching, and low specific capacitance due to low ion diffusion coefficients and long diffusion pathways. Prior approaches—structural tuning, embedding crystalline WO3 nanoparticles into a-WO3, and forming composites with conductive polymers—have improved certain metrics but often compromise long-term electrochemical activity, kinetics, or film integrity due to physical mixing and interfacial scattering. The research hypothesizes that a chemically cross-linked hybrid of chitosan and WO3·H2O nanoparticles on a-WO3 can synergistically enhance ion/electron transport, increase electroactive sites, and improve stability, thereby advancing multifunctional EC energy storage device performance.

- EC devices can save over 40% of building energy by voltage-controlled modulation of transparency/reflectance and are relevant for smart windows, displays, and mirrors. Incorporating energy storage allows simultaneous optical modulation and charge storage via pseudocapacitive redox at electrode surfaces. - WO3 is a leading EC/pseudocapacitive material; crystalline WO3 offers cycling stability but hinders ion movement due to dense packing, whereas amorphous WO3 enables superior ion kinetics from loosely packed structures with large tunnels. - a-WO3 devices typically show limited CE (20–38 cm²/C), slow large-area switching, and modest capacitance (60–100 F/g), attributed to low ion diffusivity and long diffusion paths. - Strategies explored include: (i) heterostructures with crystalline WO3 nanoparticles in an a-WO3 matrix to increase electroactive sites and ion kinetics; (ii) composites with conducting polymers (polyaniline, polypyrrole) that improve switching speed but suffer from degraded long-term electrochemical activity/kinetics and poor adhesion due to physical mixing and boundary scattering. - Chitosan is a renewable polymer rich in –NH2 and –OH groups, known for cross-linking, chelation, and membrane functions; however, it is electrochemically inactive alone, motivating its use as a functional medium to enhance a-WO3 electrochemistry via chemical interactions and cross-linking.

- Film fabrication: a-WO3 films were prepared by spin-coating a 10 wt% tungsten(VI) chloride (WCl6) solution in isopropanol onto F-doped SnO2 (FTO, 8.0 Ω/□) substrates, followed by annealing at 300 °C in air for 1 h (bare a-WO3). - WO3·H2O nanoparticle (WHNP) synthesis: Hydrothermal route using 0.05 M H2WO4 and 6 M HCl dissolved in deionized water; the solution was placed in a Teflon-lined autoclave and heated at 180 °C for 1 h, cooled, and washed to yield WHNPs. - Hybrid layer deposition: WHNPs (0.01 wt%) were dispersed in a chitosan solution (0.5 wt% chitosan in DI water with 1 vol% acetic acid). The mixture was spin-coated onto a-WO3 films at 2000 rpm for 30 s and dried at 80 °C for 1 h to form a-WO3/CH@WHNP. For comparison, a-WO3/CH films (without WHNPs) were also prepared. - Characterization: XRD (Cu Kα) for phase; XPS and FTIR for chemical states/bonding; FESEM and AFM for morphology; EDS for elemental distribution; Hall-effect measurements for electrical properties; UV–vis for optical properties; electrochemical testing (CV, galvanostatic charge–discharge, EIS) in a three-electrode setup with 1 M LiClO4 in propylene carbonate electrolyte, Pt counter, and Ag reference; contact angle for electrolyte wettability; in situ EC-energy storage performance using coupled potentiostat/UV–vis.

- The a-WO3/CH@WHNP hybrid exhibited markedly improved EC performance versus bare a-WO3, including: • Fast switching: coloration 4.0 s and bleaching 0.8 s. • High coloration efficiency: 62.4 cm²/C. • Long cycling stability: 91.5% retention after 1000 cycles. - Enhanced energy storage behavior with specific capacitance of 154.0 F/g at 2 A/g and stable rate capability. The improved energy density enabled brighter illumination of a 1.5-V white LED compared to bare a-WO3. - Structural/chemical evidence: • XRD confirmed amorphous WO3 and an added WHNP peak at 16.5° (WO3·H2O (020), orthorhombic). • UV–vis showed additional absorption features from chitosan (~400 nm) and WHNPs in the visible region. • XPS W 4f revealed W6+ for all films and W5+ components for a-WO3/CH@WHNP, consistent with ~2.3% oxygen vacancies in WHNPs. O 1s showed V_O-related peak at 529.2 eV for the hybrid. N 1s displayed amine/protonated amine in a-WO3/CH and additional peaks (399.0, 401.5 eV) in the hybrid indicative of chemical interactions/cross-linking. • Electrical conductivity increased from 4.59 × 10^-8 S/cm (bare a-WO3) to 5.25 × 10^-8 S/cm (with chitosan), with further improvements attributed to WHNP-induced carriers and cross-linking. - Mechanistic insights: Chitosan’s –NH2/–OH groups facilitate ion transport and form protonated –NH3+ under acidic conditions, enabling electrostatic cross-linking. WHNPs provide oxygen vacancies (W5+), acting as color/charge-storage centers and enhancing carrier density and pathways, collectively boosting ion diffusivity and electron transport.

The hybridization of a-WO3 with a WHNP-embedded chitosan thin film addresses the core limitations of amorphous WO3 by simultaneously enhancing electroactive site density, ion diffusivity, and electronic conductivity. Chitosan contributes abundant functional groups that promote cation interaction and, when protonated, promote chemical cross-linking both within chitosan and with WHNPs, improving film robustness and interfacial charge transport. The WHNPs introduce oxygen vacancies (W5+) that act as centers for charge storage and facilitate additional electronic carriers, narrowing the bandgap and improving conductivity. Together, these effects yield faster EC switching, higher coloration efficiency, superior cycling stability, and increased specific capacitance with good rate capability. The observed brighter LED operation directly reflects improved energy density and charge delivery. Overall, the findings validate the hypothesis that chemically cross-linked inorganic–organic hybrids can advance multifunctional EC energy storage devices beyond the performance of bare a-WO3 or physically mixed composites.

The work demonstrates a chemically cross-linked hybrid architecture—WO3·H2O nanoparticle-embedded chitosan thin film on amorphous WO3—that delivers fast and stable electrochromic behavior alongside enhanced electrochemical energy storage. Key advances include sub-5 s switching, high coloration efficiency (62.4 cm²/C), robust cycling (91.5% after 1000 cycles), and elevated specific capacitance (154.0 F/g at 2 A/g), culminating in improved device-level performance (brighter 1.5-V LED). These results establish a viable design strategy leveraging chitosan’s functional chemistry and WHNP-induced oxygen vacancies to synergistically improve ion/electron transport and stability in multifunctional devices.