Effect of different acid and lithium salt used in polyethylene glycol–titanium oxide based solvent-free electrolytes on electrochromic performance of WO3 thin films

Polymeric nanocomposites of polyethylene glycol (PEG) with titanium oxide compound, PEG–Ti, are solvent-free viscous materials obtained by sol–gel method in acidic medium. Lithium salt (LiX) has been added into PEG–Ti to form PEG–Ti–LiX polymeric electrolytes. Electrochromic devices based on tungsten oxide thin films and PEG–Ti–LiX electrolyte may show excellent optical contrast in the wavelength range from 300 to 2500 nm, however it depends on the type of lithium salt as well as the acid source used during the sol–gel process. With LiI, the color change speed of the devices is very fast, but they show a yellow color at bleaching state. The use of LiClO4 makes the devices totally transparent in visible region, and the optical contrast is small. On the other hand, hydrochloric acid is good catalyst for hydrolysis of the titanium precursor during the sol–gel process, but it also originates the yellowish appearance of the devices. The substitution of HCl by acetic acid makes the devices more transparent in 250–500 nm, but the optical contrast as well as color change speed were deteriorated. Possible chemical reaction mechanisms during the formation of these polymeric electrolytes have been proposed to explain the relation between electrochromic performance of tungsten oxide and chemical composition of the electrolytes.

► Polyethylene glycol (PEG)–TiO2 complex was prepared by sol–gel in acidic condition. ► Strong acid promotes the crosslinking between PEG and titanium oxide complex. ► Low solubility of LiClO4 in PEG deteriorates WO3 electrochromic performance. ► HCl or LiI gives larger color change speed (ν) and optical contrast (ΔT) in devices. ► Acetic acid or LiClO4 increases the visible transparency but reduces ν and ΔT.