Unlocking Na⁺-Based Electrochromic Capacity in Hexagonal Tungsten Oxide Nanorods via Thermally Removable Dopants

Hexagonal tungsten oxide nanorods are promising materials for Na⁺-based near-infrared electrochromic windows due to their large hexagonal tunnels. However, conventional dopants such as Cs⁺ within these tunnels hinder Na⁺ insertion, thereby limiting charge capacity and overall modulation. Here, we address this challenge by developing NH₄⁺/NH₃-doped hexagonal tungsten oxide nanorods and progressively removing dopants through simple thermal annealing. Films annealed at 400 °C exhibit a more than 4-fold increase in charge capacity compared to Cs⁺-doped counterparts. Combining this enhanced capacity with the high coloration efficiency of nanorods, a 150 nm-thick film achieved 74% optical modulation at 1200 nm, comparable to that of Li⁺-based systems. Full-cell devices using sodium electrolytes demonstrated effective temperature regulation of 20 °C between bleached and colored states. Our work highlights the critical role of dopant engineering in electrochromic performance and suggests that sodium electrolytes offer a viable pathway for the development of next-generation electrochromic windows.