Electrochromic devices (ECDs) control light transmittance via electrochemical redox reactions, finding applications in displays and energy-efficient buildings. Recent advancements have integrated energy storage functionality, creating electrochromic supercapacitors (ECSs) that change optical properties and store energy, with color intensity reflecting the stored energy level. Transition metal oxides, particularly tungsten trioxide (WO₃), are attractive EC chromophores due to their electrochemical properties, optical modulation, and coloration efficiency. Previous WO₃-based ECSs, however, suffered from slow switching speeds and low optical modulation due to the dense structure of the WO₃ films. To address this, various WO₃ nanostructures have been explored, but ultrafast dynamics required for practical applications remain a challenge. Mesoporous structures, with their fully interconnected small pores, are ideal for achieving ultrafast response. While hard templating methods exist for creating mesoporous structures, they involve multiple steps and limited pore size tunability. Evaporation-induced self-assembly (EISA), combining sol-gel chemistry and self-assembly of amphiphilic molecules, offers a powerful alternative for producing mesoporous metal oxides with easily controllable pore sizes and compatibility with various solution processes. This study utilizes EISA to fabricate mesoporous amorphous WO₃ for ultrafast response ECSs.

The literature review section discusses existing research on electrochromic devices (ECDs) and electrochromic supercapacitors (ECSs), highlighting the advantages and limitations of using tungsten trioxide (WO3) as an electrochromic material. It compares different WO3 nanostructures (nanosheets, nanoparticles, macroporous films) used in previous ECSs, emphasizing their shortcomings in achieving ultrafast switching speeds and sufficient optical modulation. The review then explains the advantages of mesoporous structures for improved performance and contrasts the EISA method with traditional hard templating methods for creating mesoporous materials.

Mesoporous WO₃ films were fabricated using evaporation-induced self-assembly (EISA). A mixed solution of tetrahydrofuran (THF), polystyrene-block-poly-ethylene oxide (PS19k-b-PEO6.5k), and ethanol-containing WCl₆ was spin-coated onto FTO-coated glass. THF evaporation led to the formation of PS19k-b-PEO6.5k micelles, encapsulating tungsten species. Calcination at 350 °C partially removed the organic components, leaving a mesoporous amorphous carbon/WO₃ structure. Subsequent O₂ plasma treatment eliminated the amorphous carbon, resulting in a mesoporous WO₃ film. The structure and composition were characterized using Raman spectroscopy, SEM, XPS, XRD, HR-TEM, and SAED. ECS devices were fabricated using the mesoporous WO₃ film as the electrochromic layer and a NiO film as the ion-storage layer, with 1 M LiClO₄ in propylene carbonate as the electrolyte. Compact-WO₃ ECS devices were fabricated for comparison using WO₃ nanoparticles. Electrochromic performance was evaluated by measuring UV-vis transmittance spectra at various applied voltages, determining coloration/bleaching times, and calculating optical modulation and coloration efficiency. Nitrogen adsorption-desorption experiments were performed to characterize the surface area and pore size distribution. Electrochemical performance was assessed using electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV), and galvanostatic charging/discharging (GCD) measurements. The long-term stability was evaluated through cycling tests. Printing-assisted EISA (PEISA) was used to fabricate patterned mesoporous WO₃ for ECS displays (ECSDs).

The mesoporous WO₃-based ECSs exhibited significantly superior performance compared to compact-WO₃ ECSs. Key findings include: 1) A large optical modulation (ΔT = 76%) and ultrafast switching speeds (tc = 0.8 s, tb = 0.4 s), much faster than previously reported WO₃-based ECDs. 2) High areal capacitance (2.57 mF/cm² at 1.0 mA/cm²) with good rate capability, maintaining 68% capacitance retention when increasing the current density from 0.02 to 1.0 mA/cm². 3) High coloration efficiency (682 cm²/C), significantly higher than that of compact-WO₃ ECSs (188 cm²/C). 4) Excellent cycling stability, retaining 85.5% of initial optical modulation after 1000 cycles under fast switching conditions. 5) Successful fabrication of patterned ECS displays (ECSDs) using PEISA, directly visualizing stored energy levels.

The superior performance of the mesoporous WO₃ ECSs is attributed to the combination of its high surface area, amorphous nature, and interconnected pore structure. The large surface area facilitates faster ion intercalation and extraction, leading to ultrafast switching speeds and high areal capacitance. The amorphous structure promotes enhanced Li⁺ ion diffusion, further contributing to the fast response. The interconnected pores ensure efficient ion transport throughout the film. The results demonstrate the potential of EISA as a powerful technique for fabricating high-performance ECSs with applications in energy storage and electrochromic displays. The significant improvement in performance compared to compact WO₃ devices highlights the importance of mesoporous structure design for next-generation electrochromic energy storage components.

This study successfully demonstrated ultrafast electrochromic supercapacitors based on mesoporous WO₃ prepared by EISA. The devices exhibited superior performance compared to compact-WO₃ counterparts, showcasing significant potential for applications in smart windows and portable energy-storage displays. Future research could focus on exploring other mesoporous metal oxides and optimizing the EISA process for even faster switching speeds and higher energy densities.

The study primarily focused on the performance of the mesoporous WO3 ECSs under laboratory conditions. Further research is needed to evaluate the long-term durability and stability under real-world operating conditions, including variations in temperature and humidity. Scalability and cost-effectiveness of the PEISA method for large-scale production of ECS displays also need to be addressed.