Stable WO3 electrochromic system based on NH4+ hydrogen bond chemistry

As the best-known and extensively-studied electrochromic material, tungsten trioxide (WO 3 ) 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 WO 3 electrochromic system enabled by a NH 4 CF 3 SO 3 -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 NH 4 + ions and the WO 3 lattice, which are much weaker than metallic coordination bonds. Furthermore, the bulky CF 3 SO 3 − anion and large TEP solvent molecule are also favorable to stabilize WO 3 , since they are more resistive to be co-inserted into the WO 3 lattice along with the shuttling NH 4 + cations. Thanks to these merits, this NH 4 CF 3 SO 3 -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 WO 3 /Zn electrochromic device is fabricated based on a NH 4 + /Zn 2+ -CF 3 SO 3 dual-cation TEP electrolyte. These findings may shed light on the in-depth understanding of WO 3 ′s “ion trapping” effect, as well as the electrolyte design of high-performance WO 3 -based electrochromic devices.