Synthesis and characterization of proton conducting inorganic-organic hybrid nanocomposite films from mixed phosphotungstic acid/phosphomolybdic acid/tetramethoxysilane/3-glycidoxypropyltrimethoxysilane/phosphoric acid for H2/O2 fuel cells

Abstract

Novel proton conductive inorganic-organic hybrid films doped with a mixture of phosphotungstic acid and phosphomolybdic acid were prepared by sol-gel process with 3-glycidoxypropyltrimethoxysilane (GPTMS), tetramethoxysilane (TMOS), and phosphoric acid as precursors. These hybrid membranes were studied with respect to their structural, thermal, elastic modulus, proton conductivity, and hydrogen permeability properties. The Fourier transform infrared spectroscopy and P31 nuclear magnetic resonance measurements have shown good chemical stability and complexation of H3PO4 with both PW12O403− and PMo12O403− Keggin anions in the studied hybrid films. The thermogravimetric analysis and differential thermal analysis measurements confirmed that the membranes were thermally stable up to 350 °C. The thermal stability of the membranes was enhanced by the presence of SiO2 framework. The effect of mixed heteropoly acid (HPA) concentration on the microstructure of the membranes was studied by scanning electron microscopy, energy dispersive x-ray analysis, and transmission electron microscopy micrographs and no phase separation at the surfaces of the TMOS-GPTMS-H3PO4-HPA membranes was observed, and also homogeneous distribution of all elements was confirmed, indicating that these membranes are homogeneous at nanoscale. High proton conductivity of 9.8×10−2 S/cm with composition of 20TMOS-50GPTMS-30H3PO4-3PMA-6PWA (mol %) was obtained at 130 °C and 90% relative humidity (RH). The hydrogen permeability was found to decrease from 1.12×10−10 to 8.5×10−11 cm2/s in the temperature range of 30–130 °C. For 3PMA/6PWA (mol %) doped hybrid film, membrane electrode assemblies were prepared and a maximum power density of 68 mW/cm2 at 140 mA/cm2 as well as a current density of 262 mA/cm2 were obtained at 130 °C and 50% RH when utilized in a H2/O2 fuel cell.