Phosphonic acid-grafted mesostructured silica/Nafion hybrid membranes for fuel cell applications

Highly conductive and hydration retentive mesoporous silica/Nafion and organically modified mesoporous silica/Nafion membranes are prepared by a surfactant templated sol–gel process involving Nafion solution and silica precursors. Spectroscopic analyses reveal that the in situ generation of a well-condensed silica network and organic (–PO3H2) functionalization of the inorganic segment are effectively achieved in the prepared membranes. The homogeneous dispersion of silica nanoparticles in the polymer matrix is apparent in electron micrographs. Structural analyses using small-angle X-ray scattering confirms the periodic short range structural order associated with these mesostructured hybrid membranes. These hybrid membranes exhibit an increased water uptake and an associated conductivity enhancement at 100% RH, compared to unmodified Nafion. More significantly, the functionalized silica/Nafion membranes show high proton conductivities at 80 °C and 50% RH, which is more than 6 times higher than that of Nafion. Thermogravimetric analysis, low temperature DSC studies and a comparison of activation energies (Ea) obtained from temperature-dependent conductivity plots of the membranes at different humidities, provide evidence for the better retention of water in the hybrid membranes compared to Nafion; thereby demonstrating the promising potential of these membranes to tolerate the variations in humidity at elevated temperatures.