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.