Broadband Dielectric Spectroscopy and Conductivity Mechanism of Nafion 117 and Nafion/[ZrO2] Hybrid Inorganic-Organic Membranes

In this report is described the dielectric characteriation by Broadband Dielectric Spectroscopy (BDS) of a Nafion 117 reference membrane and of a hybrid inorganic-organic membrane based on Nafion and ZrO2 nanopowders with formula Nafion/[ZrO2]. The measurements on the two systems were collected both in completely dry and in fully-humidified (wet) conditions. Results allowed the identification of two polarization phenomena and of several dielectric relaxation modes, which were assigned taking into account: a) the primary and secondary structure of the Nafion polymer; and b) the interactions Nafion-water and Nafion-ZrO2 nanofiller. The dependence on temperature of conductivity (sigma(dc,i)), relaxation frequencies (f(k)), dielectric strength (Delta epsilon(k)) and shape parameters (m(k) and n(k)) for the investigated systems in both dry and wet conditions is studied in detail. It is shown that the ZrO2 nanofiller: a) stabilizes both the hydrophobic and the hydrophilic domains of the Nafion/[ZrO2] membrane; and b) promotes the coupling between the various relaxation modes of Nafion. In fully-humidified conditions, the stability range of conductivity and the conductivity of the Nafion/[ZrO2] membrane at 125 degrees C are respectively 5 degrees C < T < 135 degrees C and 9.51.10(-2) S/cm. For the pristine Nafion 117 membrane, the corresponding values are 5 degrees C < T < 95 degrees C and 1.22.10(-2) S/cm. Results allowed to propose a coherent model describing the conductivity mechanism in Nafion-based systems.