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 degrees C and measurement of their remaining weight, ion exchange capacity (IEC) and proton conductivity (sigma) after washing in pure water. The IEC and sigma did not change at any temperature up to 200 degrees C, indicating high thermal stability. At 250 degrees 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 degrees C to produce a good membrane-electrode assembly for a fuel cell. (C) 2008 Elsevier Ltd. All rights reserved.