Low methanol permeable composite Nafion/silica/PWA membranes for low temperature direct methanol fuel cells

Nafion/silica/phosphotungstic acid (PWA) composite membranes were studied for low temperature (<100 °C) direct methanol fuel cells (DMFCs). The composite membranes were prepared by sol–gel reaction of tetraethoxysilane (TEOS) within the nanophase of hydrated perfluorosulfonic acid membranes (Nafion 117) and the subsequent treatment in phosphotungstic acid (PWA) solution. X-ray diffraction (XRD) indicates that the silica can decrease the crystallinity of Nafion. The silica in membranes can greatly decrease the methanol diffusion coefficient (D) within appropriate silica content range. The proton conductivities (σ) of these composite membranes are higher than those of commercial Nafion 117. The result of σ/D shows the optimal silica content in composite membrane is 38.2 μg/g. Thermogravimetric analysis (TGA) shows that the incorporation of silica and PWA does not affect the thermostability of membrane. The data from single direct methanol fuel cell using these two membranes of commercial Nafion without treatment and Nafion/silica/PWA composite membranes with optimal silica content as polymer–electrolyte show that the cell with composite membrane has higher open circuit voltage (OCV = 0.75 V) and maximal power density (Pmax = 70 mW/cm2) than those of commercial Nafion without treatment (OCV = 0.68 V, Pmax = 62 mW/cm2) at 80 °C.