Electrochromic (EC) devices that reversibly modulate solar transmittance are promising for energy-efficient smart windows. However, aqueous EC systems still suffer from limited cycling stability due to irreversible electrochemical reactions during repeated switching. In this study, we developed a lithium iodide (LiI)-based aqueous electrolyte for a complementary EC system coupling tungsten trioxide (WO₃) with reversible iodine redox chemistry. The electrolyte was prepared using a poly(vinyl alcohol) (PVA)-based aqueous matrix containing 0.8 M LiI with its composition and acidity adjusted to regulate both WO₃ coloration and iodine redox reactions. During coloration, the reduction of W⁶⁺ in WO₃ is considered to be predominantly associated with proton (H⁺) insertion, resulting in blue coloration. Simultaneously, oxidation of iodine species is considered to occur at the counter electrode. These coupled reactions enhance the overall coloration while enabling operation at a relatively low voltage. The optimized device exhibited an optical modulation of 61.7 % at 700 nm, with coloring and bleaching times of 4.0 and 6.0 s, respectively. Reversible EC switching was achieved over a narrow voltage range of -0.6 to 0.2 V, and the device retained 83.1% of its initial optical modulation after 6,000 cycles. Electrochemical and spectroscopic analyses revealed that the iodine redox state and electrolyte acidity played critical roles in determining the reversibility and long-term stability of the WO₃-based EC system. These results indicate that coupling the proton-insertion EC behavior of WO₃ with reversible iodine redox chemistry is an effective strategy for enhancing the optical modulation, reducing the operating voltage, and improving the cycling stability of aqueous EC devices. This approach provides a promising pathway toward durable and energy-efficient smart-window systems.

Electrochromic device,tungsten trioxide,lithium iodide,iodide redox chemistry,PEDOT:PSS counter electrode