On the decay of Nafion proton conductivity at high temperature and relative humidity

The irreversible conductivity decay exhibited by Nafion 117 membranes above certain values of temperature and relative humidity (RH) has been investigated by two-probe impedance measurements carried out at 120 °C with the electric field normal to the membrane surface, under controlled applied pressure on the electrodes. The analysis of the evolution of both frequency response and normal conductivity during the decay has suggested that the decay arises from changes in the bulk transport properties of the Nafion membrane. This has been confirmed by determining, under stability conditions, the conductivity of membranes pre-treated under decay conditions. The results of these measurements have shown that the decay occurs only if the membrane undergoes an anisotropic deformation along the direction parallel to the electrode surface. Four-probe impedance measurements with the electric field parallel to the membrane surface have also been carried out to determine the membrane tangential conductivity before and after the decay. Comparison of normal and tangential conductivity has indicated that the decay is associated, to a certain extent, with an increase in the conduction anisotropy.