State of Water in Perfluorosulfonic Ionomer (Nafion 117) Proton Exchange Membranes

The nature of water in acid-form Nafion 117 was quantified at several hydration levels by dielectric relaxation spectroscopy. Two independent experimental setups were used to collect complex dielectric permittivity spectra at low frequencies ( at to ) and in the microwave region ( at ). We directly observed the states of water, manifested through three population averages with distinctly resolved dynamical behaviors, and their changes with temperature and hydration level. The fastest process observed was identified as the cooperative picosecond relaxation of free (isotropic, bulklike) water, whereas the slowest process (microsecond relaxation times) corresponded to water molecules strongly bound to the charged sulfonic groups. A third type of water was also observed, also characterized by picosecond relaxation times, close to and about three times slower than those of bulk water, which was attributed to loosely bound water and may contain substantial dynamical heterogeneities.