Effects of solvent on microstructure and proton conductivity of organic-inorganic hybrid membranes

Effects of solvent on the microscopic structure and proton conductivity of organic-inorganic hybrids containing heteropolyacid are reported. The organic-inorganic hybrids were prepared by sol-gel synthesis of 1,8-bis(triethoxysilyl)octane (TES-Oct) in the presence of phosphotungstic acid (PWA). The proton conductivity of the membranes prepared under different solvents exhibited greater values in the order methanol < 1-butanol < 2-propanol < ethanol < 1-propanol < 2-butanol. The conductivity strongly correlated to the amount of incorporated PWA in the membranes, which was determined by titration of the solutions containing PWA leaked from the membrane. Small-angle X-ray scattering (SAXS), atomic force microscopy (AFM) and thermogravimetric analysis (TGA) were employed to reveal the relationship between the proton conductivity and microscopic structure of the TES-Oct/PWA membranes. It was found that there is an optimal condition for the formation of the condensed domain structure, indicating the important role of the structures in the membrane conductivity.