A novel kind of composite proton exchange membrane was prepared by doping the nano-SiO₂ into poly(vinylidene fluoride) grafted polystyrene sulfonated acid (PVDF/xSiO₂-g-PSSA). The structure and stability of the membranes were characterized with the IR spectra and thermal gravity analysis (TGA). The morphology of the surface and the dispersion of elements in the cross section of the membrane were observed by the scanning electron microscope (SEM). The water-uptake, proton conductivity and methanol permeability of the membranes were measured. The results showed that the nano-SiO₂ could slow the methanol permeation; the permeability coefficient of the composite membrane with SiO₂ (w＝10%) was up to 1.0×10^－7 cm²/s, which is lower than that of PVDF-g-PSSA and just one twentieth of that of Nafion-117. This kind of composite membranes has high selectivity and shows a promising practical value in direct methanol fuel cells (DMFC).

Key words: direct methanol fuel cell (DMFC), proton exchange membrane, poly(vinylidene fluoride) grafted styrene sulfonic acid, SiO₂, methanol permeability