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).