Constructing WO₃-Iodine Complementary Electrochromic Windows Through NH₄⁺ Electrolyte and Counter Electrode Design

Although WO₃ electrochromism exhibits remarkable optical modulation ability, its practical application is largely constrained by poor cycling durability in alkaline metal electrolytes. Herein, an NH₄⁺ aqueous electrolyte with optimized concentration and acidity is proposed to improve the lifetime and electrochromic performance of WO₃ films by leveraging the cation's unique hydrogen-bond intercalation chemistry. To construct full electrochromic devices, the electrolyte and counter electrode are further modified with NH₄I and P10/ITO (polyquaternium-10/indium tin oxide nanoparticle) composite coating, respectively. The NH₄I additive provides a reversible I⁰/I^- couple for charge compensation, while the P10/ITO composite coating enhances the I⁰/I^- conversion reversibility by adsorbing and immobilizing the polyiodide intermediates. More interestingly, the I⁰/I^- conversion introduces an additional electrochromic ability on the counter electrode, and can therefore further improve the optical modulation amplitude in the 400-600 nm band. Thanks to these merits, the resulting WO₃-iodine complementary device achieves an impressive optical modulation amplitude (45.5% at 633 nm) and impressive cycling durability (94.4% of the optical modulation ability remaining after 200 cycles).