Gas Permeation through Nafion. Part 2: Resistor Network Model

In the first part of this study, the hydrogen and oxygen permeabilities of Nafion were measured. The aim of the second part of this study presented here is to physically characterize the influence of the aqueous phase, the solid phase, and the intermediate phase in Nafion on the macroscopic hydrogen and oxygen permeabilities. Hereto, a resistor network model morphologically representative for Nafion based on structural investigations reported in the literature is presented in which the different phases are described by individual permeabilities. As a result of the simulations, an enlarged permeability of the solid phase in comparison to that of dry Nafion had to be assumed in order to reproduce the measured influence of temperature and relative humidity on the permeability. This increase of the permeability of the solid phase toward greater water uptake was explained by the effect of water as a plasticizer and the resulting softening of the polymeric matrix. On the basis of the identified mechanisms, approaches to reduce the gas permeability of polymer electrolyte membranes are identified and discussed.