Insight into the structure–activity relationship in electrochromism of WO3 with rational internal cavities for broadband tunable smart windows

Tungsten trioxide (WO3)-based electrochromic devices have attracted great interest in smart windows, low-power displays, and other cutting-edge fields. The electrochromic behavior of WO3 is strongly dominated by both electron conduction and ion diffusion process. Nevertheless, the structure–activity relationship of the WO3 in the electrochromism has not been well elucidated so far. Herein, we systematically studied the relationship between the crystal structure composed of the fundamental units of typical WO3 and the electron conduction/ion diffusion processes at a sub-nanoscale level. Our theoretical results unveil that the h-WO3 with appropriate band gap by piling up the WO6 octahedral molecular units along c axis profit electron conduction, and the continuous three-, six-ring tunnels along ab plane built by the units are conducive to ion diffusion. Therefore, the h-WO3 film exhibits large optical modulation (up to 86% at 633 nm and 90% at 1000 nm), excellent cycling stability (86.4% of retention over 3000 cycles), and fast switching speed (1.9 s of bleaching time at 633 nm). Moreover, we provide unique insights between H+ adsorption/desorption and the electrochromic response in the near-infrared region. We believe that this study lays a foundation for the design and construction of high-performance electrochromic nanomaterials and broadband tunable smart windows.