Silicotungstic acid/organically modified silane proton-conducting membranes

Proton-conducting gels and membranes were made from an organically modified silane incorporating a heteropoly acid, i.e. silicotungstic acid. The ORMOSIL host was generated from a bis end-capped triethoxysilane chemically bonded via the urea bridges to (1) a long poly(propylene glycol) chain or (2) a long poly(dimethylsiloxane) chain. The heteropolyacid acted simultaneously as a source of mobile protons and as a catalyst, initiating the hydrolysis/condensation reactions of the sol–gel composite. In addition to the bis end-capped triethoxysilane network former, different alkoxysilanes were tested as network modifiers to optimize the gelation time, mechanical properties, ionic conductivity and the retention of the heteropolyacid during soaking of the membranes in water. Among alkoxysilane modifiers the most promising results were given by perfluorooctyltriethoxysilane and phenyltriethoxysilane. The conductivity of the membranes was up to 10^–2–10^–1 S cm^–1 at elevated temperatures and saturated humidity conditions. Fourier transform IR attenuated total reflection spectroscopy was used to follow the gelation of the samples and, in parallel to thermogravimetric/differential scanning calorimetry measurements, also the thermal stability of the membranes.