Proton and cesium conductivity in perfluorosulfonate ionomers at low and high relative humidity

Abstract

Nafion exhibits one of the highest proton conductivity at room temperature and it is the standard electrolyte of proton exchange membrane fuel cells (PEMFC). However, the temperature dependence of ionic conductivity of Nation is highly dependent on the measuring conditions and it is still a matter of debate. In the present study, detailed dielectric spectroscopy (DS) measurements in both dry (under N2 flow) and water-saturated conditions were carried out in a broad range of temperature and frequency. Such DS results were correlated to differential scanning calorimetry (DSC) and dynamic mechanical analysis (DMA) data taken in similar conditions. The main results revealed that in samples conditioned in N2 flow (RH ~ 0%) the transport of both proton and cesium ions is coordinated with the dynamics of Nafion relaxations. In hydrated Nafion (proton form), conductivity measurements at different frequencies revealed two regimes: one at high-frequency, in which the Vogel-Tamman-Fulcher (VTF) law indicates a close relation between the polymer glass transition temperature Tg; and, a second one at low frequency, bearing great similarity to the transport observed in nearly dry samples. The reported experimental results contribute to disentangle the intricate transport properties of Nafion.