Water vapor sorption in a short-side-chain perfluorosulphonic acid ionomer membrane, suitable foruse as electrolyte in Proton Exchange Membranes Fuel Cells, was studied in detail by using FourierTransform Infrared-Attenuated Total Reflectance (FTIR-ATR) spectroscopy. The analysis of the IRspectra of the membrane at different values of relative humidity (0.00-0.50) and at 35oc allowed torecognize four types of water molecules, characterized by decreasing strength of interaction withthe sulphonate sites of the membrane. The concentration of each water population inside themembrane was then determined through calibration with independent measurements of water vaporuptake in the membrane. Two different sorption mechanisms were identified: a Langmuir-likeadsorption (1st, 2nd, 3rd water population) and a Henry-like absorption (4th water population). Inparticular, the 1st population is strongly bound directly to sulphonate sites, while the 2nd and the 3rdones form subsequently adsorbed layers; the 4th population is formed by mobile water molecules,which are not directly interacting with the acid sites. Only three adjustable parameters are requiredto describe the sorption behavior of the different populations, whose values are consistent withthose reported in the literature, confirming the validity of the physical description presented.
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