FTIR-ATR study of water distribution in Hyflon-Ion® H membranes

Fourier Transform Infrared-Attenuated Total Reflectance (FTIR-ATR) spectroscopy was used to study in detail the water vapor sorption in a short-side-chain perfluorosulphonic acid ionomer membrane (Hyflon® Ion H) suitable for use as electrolyte in Proton Exchange Membranes Fuel Cells. From the analysis of the IR spectra of the membrane at different values of relative humidity (0–0.50) and at 35°C, four types of water molecules, characterized by decreasing strength of interaction with the sulphonate sites of the membrane, were identified. The actual concentration of the various water populations in the membrane was determined through calibration with independent measurements of water sorption: each family exhibits a different shape of the solubility isotherm, corresponding to a specific sorption mechanism. The sorption of the first two populations can be represented with Langmuir isotherms, with each layer adsorbed onto the previous one in a shell-like structure. The other population, are characterized by less strong hydrogen bonding and can thus related to more "free" water absorbed into the matrix. The experimental trends, related to different water populations, were also modeled invoking a simple scheme of equilibrium acid dissociation reactions and obtaining a fairly good agreement