Water transport in a short side chain PFSI membrane: Differences between cast and extruded membranes subject to different thermal treatments

Short side chain PFSI membranes, of low equivalent weight (860 gpol/mol(SO3H)), suitable to be used in high temperature (80-120°C) PEM fuel cells, have been studied, considering in particular the effect of different thermal treatments and preparation protocols on vapor transport. Two types of membranes have been considered, obtained via solution casting and via extrusion, respectively, and characterized by rather different thickness values. The water vapor sorption has been measured with both a quartz crystal microbalance and a quartz spring balance. No significant dependence of the water vapor uptake on the membrane preparation procedure has been observed, the main differences in the transport behavior being attributed to the different thermal treatment. The samples pre-treated under vacuum at the highest temperature (160°C) are characterized by a lower water uptake value, indicating possible permanent modifications of the membrane structure, due to the decreased availability of hydrophilic groups in the membrane. A qualitative analysis of the membrane structure before and after the treatment has been performed with a FTIR-ATR spectrometer. The weight loss of the membrane as a function of temperature under vacuum up to 160°C has also been measured with a magnetic suspension balance, in order to evaluate the residual water content in the membrane as a function of water pressure, temperature and thermal history.