A critical investigation of the effect of hygrothermal cycling on hydration and in-plane/through-plane proton conductivity of Nafion 117 at medium temperature (70–130 °C)

Abstract

Hydration, in-plane and through-plane conductivity of Nafion 117 membranes are investigated in the temperature range 70–130 °C and in the relative humidity (RH) range 50–90% upon cycling RH, at constant temperature, and cycling temperature at constant RH. Both temperature and RH cycling result in hysteresis of conductivity and hydration. During the RH cycle, conductivity changes at decreasing RH are faster than hydration changes, thus indicating the presence of water molecules contributing weakly to conductivity. During the temperature cycle, the in-plane conductivity shows a hysteresis loop where, as expected, the more hydrated state is the more conductive state as well. However, under the same conditions of temperature and RH, the through-plane conductivity exhibits an anomalous behaviour where the lower conductivity is associated with the higher hydration level. Upon temperature cycling, through-plane and in-plane conductivity show different temperature dependence during heating but the same dependence during cooling. This behaviour is attributed to irreversible structural changes occurring during heating when the membrane is pressed between the electrodes in the through-plane conductivity cell. The possible influence of the through-plane conductivity hysteresis on the performance of a PEM fuel cell in the range 70–130 °C is also discussed.