Effect of silane hydrophilicity on membrane transport properties of in-situ hybrid Nafion/organically modified silicate proton conductors for DMFC applications

In order to build a better understanding of in-situ hybrid Nafion/ORMOSIL (Organically MOdified SILicates) membranes for DMFC (direct methanol fuel cells) applications, we investigate the effect of silane hydrophilicity on membrane transport properties of the hybrid proton conductors in terms of water-channel structure and state of water. The following three contrast silanes are introduced in the decreasing order of hydrophilicity: tetraethoxy silane (TEOS), methyl triethoxy silane (MTES), and diethoxy dimethyl silane (DEDMS). To focus exclusively on the silane hydrophilicity, a pre-requisite condition assuring that the same concentration of silanes should be immersed into Nafion115 is fulfilled by examining the silane diffusion behavior. As the silane hydrophilicity increases, the water-channel size and water uptake of the hybrid membranes become larger but the number of freezable water molecules decreases. Meanwhile, the hybrid membranes prepared from the hydrophobic silanes show the higher proton conductivity, leading to the superior selectivity. This interesting transport behavior is explained by considering the morphological change and state of water in the hybrid membranes.