Stable WO3 electrochromic system based on NH4+ hydrogen bond chemistry

Abstract

As the best-known and extensively-studied electrochromic material, tungsten trioxide (WO3) has drawn tremendous interest due to its low material cost, environmental benignity, dual band regulation, high optical modulation, and high chemical stability. However, these films always suffer from ion-trapping-induced degradation in electrochromic performance when cycled in alkali cation (such as Li+, Na+, K+, etc.) electrolytes. Herein, we report a stable WO3 electrochromic system enabled by a NH4CF3SO3-TEP (Triethyl phosphate) organic electrolyte. Based on comparative experiments and systematic characterizations, the prolonged cycling stability is mainly attributed to the formation of hydrogen bonds between the NH4+ ions and the WO3 lattice, which are much weaker than metallic coordination bonds. Furthermore, the bulky CF3SO3− anion and large TEP solvent molecule are also favorable to stabilize WO3, since they are more resistive to be co-inserted into the WO3 lattice along with the shuttling NH4+ cations. Thanks to these merits, this NH4CF3SO3-TEP electrolyte simultaneously enables great electrochromic activity (76.1 % transmittance modulating ability at 633 nm) and impressive cyclic color-switching stability (1000 times color change without any detectable electrochromic performance or mechanical degradation), overwhelmingly outperforming the conventional Li+/K+ electrolytes. Furthermore, as a demonstration of its application, a WO3/Zn electrochromic device is fabricated based on a NH4+/Zn2+-CF3SO3 dual-cation TEP electrolyte. These findings may shed light on the in-depth understanding of WO3′s “ion trapping” effect, as well as the electrolyte design of high-performance WO3-based electrochromic devices.