Synthesis and Characterization of Proton Conducting Inorganic-Organic Hybrid Nanocomposite Membranes Based on mixed PWA-PMA-TEOS-GPTMS-H3PO4-APTES for H2/O2 Fuel Cells

A series of novel fast proton conductive inorganic-organic nanocomposite hybrid membranes doped with a mixture of phosphotungstic acid (PWA) and phosphomolybdic acid (PMA) have been prepared by sol-gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), 3-aminopropyltriethoxysilane (APTES), phosphoric acid (H3PO4), and tetraethoxysilane (TEOS) as precursors. These hybrid membranes were studied with respect to their structural, thermal, elastic moduli, and proton conductivity properties. The X-ray diffraction measurement revealed the amorphous nature of the hybrid membranes. The Fourier transform infrared spectroscopy has shown a good complexation of H3PO4 in the membrane matrix and the both characteristic Keggin anions PW12O403- and PMo12O403- were present in the nanocomposite membranes. Thermal analysis including thermogravimetric and differential thermal analysis confirmed that the membranes were thermally stable up to 300 degrees C. Thermal stability of the membranes was significantly enhanced by the presence of inorganic SiO2 framework. The effect of mixed heteropolyacid (HPA) concentration on the microstructure of the membranes was studied by scanning electron and transmission electron micrographs and no phase separation at the surfaces of the TEOS-GPTMS-H3PO4-APTES-HPA membranes was observed, indicating that these membranes are homogeneous in nature. High proton conductivity of 3 x 10(-2) S/cm with composition of 50TEOS-25GPTMS-20H(3)PO(4)-5APTES-3PMA-6PWA was obtained (6.35 x 10(-3)S/cm at 150 degrees C, 50% RH) at 120 degrees C under 90% relative humidity. The high proton conductivity of the nanocomposite membranes is due to the proton-conducting path through the GPTMS-derived "pseudopolyethylene oxide" (pseudo-PEO) network in which the trapped solid acids (PWA and PMA) as proton donors are 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 an increasing relative humidity. These results indicate that the inorganic-organic hybrids synthesized in this work are promising electrolytes for proton exchange membrane fuel cells and also for future fuel-cell design and development.