Synthesis and characterization of proton conducting inorganic–organic hybrid nanocomposite membranes based on tetraethoxysilane/trimethylphosphate/3-glycidoxypropyltrimethoxysilane/heteropoly acids

A series of novel proton conductive inorganic–organic nanocomposite hybrid membranes doped with phosphotungstic acid (PWA)/phosphomolybdic acid (PMA) and trimethylphosphate PO(OCH3)3 have been prepared by sol–gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), and tetraethoxysilane (TEOS) as precursors. The hybrid membranes were studied with respect to their structural and thermal properties, elastic moduli and proton conductivity. Thermal analysis including TG and DTA confirmed that the membranes were thermally stable up to 200 °C. Thermal stability of membranes was significantly enhanced by the presence of SiO2 framework. Proton conductivity of 1.59 × 10−2 S/cm with composition of 50TEOS-5PO(OCH3)3-35GPTMS-10PWA was obtained (1.15 × 10−2 S/cm for 10 mol% PMA) at 90 °C under 90% relative humidity. The proton conductivity of the nanocomposite membranes is due to the proton-conducting path through the GPTMS-derived “pseudopolyethylene oxide (pseudo-PEO)” networks in which the trapped solid acid (PWA/PMA) as a proton donor is contained. The molecular water absorbed in the polymer matrix is also presumed to provide high proton mobility, resulting in an increase of proton conductivity with increasing relative humidity.