In situ formation of polyethylene glycol–titanium complexes as solvent-free electrolytes for electrochromic device application

Transparent and ionic conductive polymeric electrolytes have been prepared through sol–gel method by adding titanium isopropoxide into an acidic polyethylene glycol (PEG) solution. After hydrolysis and condensation processes, new associations between titanium cations and ether oxygen atoms of PEG have been formed according to Fourier-transform infrared spectroscopy. Thermogravimetric analysis results of these hybrid materials indicate a better thermal stability with a less polydispersion of the molecular mass distribution in comparison with PEG. For the purpose of electrochromic or photoelectrochromic device applications, LiI was added into the hybrid materials to form solvent-free polymeric electrolytes. Optical transmittance spectra of these electrolytes show a red shift of the cutoff wavelength as a function of titanium isopropoxide percentage in the original sol–gel solutions. It is also observed that the amount of hydroxyl groups in the hybrid materials was reduced in comparison with the PEG one. This makes electrical conductivity of the hybrid electrolytes with LiI salt insensitive to humidity and solvents, which was about 2 × 10-4 Ω−1 cm−1 at room temperature. A solid WO3-based electrochromic device with the hybrid electrolyte keeps the same optical transmittance value after 1,000 cycles of switching polarization potentials between −1 and +1 V.