Thermal stability of proton exchange fuel-cell membranes based on crosslinked-polytetrafluoroethylene for membrane-electrode assembly preparation

We investigated thermal properties of proton exchange membranes (PEMs) prepared by the radiation-induced grafting of styrene into crosslinked-polytetrafluoroethylene films and the subsequent sulfonation for fuel-cell applications. A conventional thermogravimetric analysis was found to be unreliable because the resulting curve varied greatly with the heating rate. Thus, in order to obtain accurate information, we performed an ex-situ heat-treatment analysis, which involved heating of the PEMs at fixed temperatures of 200-350 °C and measurement of their remaining weight, ion exchange capacity (IEC) and proton conductivity (σ) after washing in pure water. The IEC and σ did not change at any temperature up to 200 °C, indicating high thermal stability. At 250 °C, however, the PEM properties deteriorated probably via radical cleavage of the C-S bond between a sulfonic acid group and an aromatic ring, and condensation of two sulfonic acid groups. Finally, the PEM was hot-pressed with two electrodes at 200 °C to produce a good membrane-electrode assembly for a fuel cell.