Biphase Nanocrystalline WO₃ Thin Films for Dual-Functional Electrochromic Smart Windows with Long-Term Stability

Electrochromic energy storage devices (EESDs) integrating optical modulation and energy storage are gaining attention for smart building applications. The WO₃ thin films with a biphasic nanocrystalline structure were prepared using a facile solution-based method and a low-temperature scalable blade-coating technology. Combined with zinc as the counter electrode, the WO₃–Zn EESDs utilizes the redox reactions of WO₃ and Zn to achieve dual functionality of energy storage and electrochromism. The structure with biphasic nanocrystals embedded in the amorphous WO₃ matrix strengthens the electrochemical activity and structural stability of the film, thereby improving the optical performance and cycling stability of the device. The device exhibits a 76% optical modulation at 633 nm, a high areal capacity of 106.8 mAh m^–2 at 0.1 A m^–2, and exceptional cycling stability, retaining 90.8% of optical modulation after 5000 cycles. These findings highlight the potential of WO₃–Zn EESDs for sustainable energy systems and multifunctional smart window applications.