Role of the glass transition temperature of Nafion 117 membrane in the preparation of the membrane electrode assembly in a direct methanol fuel cell (DMFC)

The glass transition temperature (Tg) of the Nafion 117 membrane was traced by DSC step by step during the preparation of the membrane electrode assembly (MEA). Wide-angle x-ray diffraction and frequency response analysis were used for the determination of the crystallinity and proton conductivity of the membrane. As-received Nafion 117 membrane showed two glass transition temperatures in the DSC thermogram. The first Tg, caused by the mobility of the main chain in the polymer matrix, was 125 °C; the second Tg, derived from the side chain due to the strong interaction between the sulfonic acid functional groups, was 195 °C. During the pretreatment of the membrane, the Tg of the Nafion 117 membrane drastically decreased because of the plasticizer effect of water. In the hot-pressing process, the Tg of the Nafion 117 membrane gradually increased due to the loss of water. When the Nafion 117 was completely dried, the Tg of the membrane finally reached 132 °C. Thermal heat treatment was then applied to the MEA to obtain high interfacial stability; however, the membrane developed a crystalline morphology that led to reduced water uptake and proton conductivity. Therefore, the thermal heat treatment of the MEA should be carefully controlled in the region of the glass transition temperature (120–140 °C) of the Nafion 117 membrane to ensure the high performance of the MEA.