Imidazolium-based ionic liquid derivatives for application in electrochromic devices

Novel proton-conducting electrolytes were prepared from the sol–gel precursor 1-3-(trimethoxy-λ⁴-silyl)propyl]imidazole with the addition of either trifluoroacetic or acetic acid. The presence of trimethoxysilyl groups enabled the solvolysis and condensation reactions of silsesquioxane species. IR spectroscopy revealed that more cube-like species formed in the electrolyte prepared from trifluoroacetic acid, while cube- and ladder-like silsesquioxanes were present in the electrolyte with acetic acid. This assignation was independently confirmed by ²⁹Si NMR analyses revealing the T³ signals of trisiloxane bonding. IR spectroscopy also pointed to the formation of hydrogen bonding in the latter electrolyte, since the frequencies of the observed bands at 1710, 1409, and 1272 cm⁻¹ approached those of acetic acid. In contrast, the IR bands at 1662, 1204, and 1130 cm⁻¹ confirmed the existence of trifluoroacetate anions in the case when the electrolyte was prepared from trifluoroacetic acid. The presence of free trifluoroacetate anions contributed to the moderately higher specific conductivity of this electrolyte (4.6×10⁻⁵ S/cm) compared to that of acetic acid (1.6×10⁻⁵ S/cm). The specific conductivity of the electrolytes could be further increased by the addition of a lithium salt. All electrolytes were employed in electrochromic devices with optically active WO₃ and various inorganic counter-electrodes (CeVO₄, V₂O₅, Ti/V-oxide). Photopic transmittance changes from 30% to 40% were achieved.