In situ Raman and UV–visible study of hybrid electrochromic devices with bis end-capped designed trialkoxysilyl-functionalized ionic liquid based electrolytes

Abstract

Stable function of hybrid electrochromic devices (ECDs) is achieved by the application of quasi-solid-state redox electrolytes with high ionic conductivity and electrochemical stability. For this purpose, a polymeric gel electrolyte based on an imidazolium ionic liquid (IL)-functionalized siloxane network is prepared. First, a sol-gel processable, bis end-capped trialkoxysilyl-functionalized imidazolium-based IL is synthesized. The characteristics of the IL enable solvolysis and condensation on both ends of the molecule. In this way, the crosslinked IL forms a firm and brittle gel, as confirmed by IR and 29Si NMR analyses. The addition of iodine for I−/I3− redox pair formation does not produce a gel with sufficiently high ionic conductivity for ECD assembly. To obtain a more conductive gel electrolyte with an ionic conductivity of the order of magnitude 10−3 S cm−1, a nonreactive, 1-methyl-3-propyl imidazolium iodide IL is added as the cosolvent. These redox electrolytes with different concentrations of iodine are tested in hybrid EC devices for ~11,000 cycles. All the devices reveal an increase in lightness with cycling, either for the bleached or colored states, which occur due to gradual transformation of I5− to I3− and I− species. In situ Raman spectroelectrochemistry is applied to gain insight into the ECDs during 1250 cycles of functioning and enables the various iodide species to be differentiated.