Interface Engineering of Hydrated Tungsten Oxide via Carbon Nitride for Durable Electrochromic Energy Storage

Electrochromic smart windows demand high optical contrast, fast switching, long-term durability, and charge-storage functionality, yet hydrated tungsten oxides suffer from restricted ion transport and structural degradation during repeated ion insertion/extraction. Here, we present a carbon nitride-driven phase transformation strategy for a durable energy-storing electrochromic hydrated tungsten oxide. Under mild hydrogen annealing, orthorhombic WO3·H2O nanoplates containing an optimized 10 wt % carbon nitride (CNx) selectively transform into predominantly hexagonal WO3·0.33H2O nanorods (∼80%), accompanied by a relatively minor orthorhombic fraction (∼20%). The CNx promotes structural rearrangement while providing nitrogen-rich Li+-interactive sites that regulate interfacial ion transport and improve electrochemical reversibility. Supported by density functional theory, impedance, diffusion, and postcycling analyses, the optimized WHX-CNx10% film shows favorable Li+ insertion, improved charge-transfer kinetics, and excellent structural retention. The air spray-coated film in 3-electrode system delivers 57% optical modulation at 650 nm, coloration/bleaching times of 2.9/0.6 s, stable cycling over 20,000 cycles, and an areal capacitance of 16.9 mF/cm2. A 5 × 5 cm2 device further exhibits optical modulation of 52% at 650 nm with a coloration efficiency of ∼156 cm2/C. These results establish CNx-mediated phase and interface engineering as an effective route toward scalable, durable, and energy-storing electrochromic smart windows.