FTIR-ATR Study of Water Distribution in Hyflon-Ion® H Membranes

Hyflon-Ion® H is a short-side-chain perfluorosulphonic acid ionomer (PFSI) membrane that can be used as electrolyte in Proton Exchange Membranes Fuel Cells (PEM-FCs). This material consists of a hydrophobic poly(tetrafluoroethylene) backbone with perfluorovinyl ether short side chains terminated by sulphonic acid (-SO3H) groups and has a rather low equivalent weight (800 g(pol)/mol(-SO3H)). The water sorption isotherm at 35°C and up to 50% relative humidity, was measured for an extruded membrane through the use of a FTIR-ATR technique that has been already applied to the study of water transport in Nafion® 117. The technique can provide a deeper insight into the mechanism of water diffusion and absorption into PFSI membranes since it allows to differentiate among different classes of water molecules, based on the extent of their hydrogen bond interactions with the sulphonic groups, and also to monitor simultaneously the time evolution of the concentration of the different populations of water molecules. The calibration was performed by comparing sorption isotherm of water, collected in Hyflon-Ion® H at 35°C in a pressure decay apparatus, with the FTIR-ATR results obtained by measuring the change in time of the integrated absorbance of the peak, usually related to stretching of -OH groups, in the range from 3000 to 3800 cm-1. The water solubility isotherm was decomposed into the contributions of the different water populations based on the polymer spectra, taken at different relative humidities, by considering the large –OH stretching band as the sum of several Gaussian peaks. In particular, four different peaks, and thus four types of water molecules, were identified in the portion of IR spectrum of the polymer considered and their concentration was monitored as a function of time and activity. The experiments show that the first two species, at the lower wavenumbers, quickly reach their equilibrium value with increasing water content, while the species at the highest wavenumber appears only at high hydration levels and its concentration increases monotonously with activity. The results suggest that the water populations detected are characterized by a different degree of interaction with the acid –SO3H group. One observes water populations characterized by sorption isotherms with an early saturation, which show a behavior similar to an adsorption isotherm: the sorption of these molecules appears limited by the finite ability of sulphonic groups to coordinate water molecules, and therefore these families are clearly related to these hydrophilic sites. That supports the idea that the first water molecules entering the system react with sulphonic groups to produce hydronium ions, while the following ones substantially solvate with successive layers the SO3--H3O+ complexes and, consequently, have lower interaction energy with sulphonic groups and, consistently, appear in the FTIR spectra as peaks centred at higher wavenumbers. The FTIR-ATR technique allows to detect, also, the water molecules weakly bonded that show a more linear isotherm, likely associated to "free" water in the sample. In view of these experimental findings, a simulation diffusion/reaction model was proposed that considers the first three water populations as directly interacting with the sulphonic groups, through Langmuir type adsorption mechanism, and the fourth as physically absorbed, with parameters taken from Nafion® literature. The final agreement with the experimental data is indeed satisfactory and further supports the physical picture obtained from the FTIR-ATR experimental results.