All solid-state electrochromic device consisting of a water soluble viologen dissolved in gelatin-based ionogel

An all solid-state electrochromic system made of a viologen dissolved in a gelatin-based ionogel called Ion Jelly is presented. The obtained Ion Jelly possesses higher ionic conductivity than the constituent ionic liquid, which is ascribed to the broken ionic aggregates caused by the presence of water. Spectro-electrochemical studies carried out with the solid-state cell show that the kinetics of the reduction process is slower than that of the oxidation process, as denoted by the calculated time constants obtained from current vs. time plots. This may be owing to formation of an insoluble reduced species, which remains at the surface of the current collector, facilitating its oxidation process. The kinetics studies also show that the color change in response to the charge transfer process is faster during the reduction step. Both reversibility and durability of the device, upon successive oxidation/reduction steps, are related with the employed reduction potential, being the best compromise with the color contrast achieved at −0.25 V vs. Ag/Ag+, where 30% of transmittance change is observed up to 75 switching cycles.