Understanding Mechanisms of Non-Fickian Water Diffusion in Polymer Electrolyte Membranes

Polymer electrolyte membranes (PEMs) have the potential to advance technologies such as fuel cells. Water diffusion in PEMs, such as Nafion, is important for ion conduction, a key requirement for high power density fuel cells. Maintaining sufficient conductivity requires high water content, which conflicts with the goal of high temperature and low humidity operation. Therefore, understanding water diffusion in Nafion is critical. Although many investigators have reproduced sorption isotherms of water in Nafion, reported diffusion coefficients vary considerably from one another.

Using time-resolved Fourier transform infrared - attenuated total reflectance (FTIR-ATR) spectroscopy, water diffusion in Nafion was measured at a molecular level. Several causes of variation in reported diffusion coefficients were identified, specifically vapor-phase mass transfer resistance and non-Fickian diffusion. Vapor-phase mass transfer resistance was found to be significant at low vapor flow rates. At high vapor flow rates (without mass transfer resistance) there was a discontinuity between water vapor and liquid dynamics. This was due to non-Fickian behavior at high vapor activity, where relaxation of the polymer was significant. Another type of non-Fickian diffusion was observed at low humidity, where a time lag was caused by the hydrolysis reaction between water and sulfonic acid in the PEM. As the water content of the PEM decreased the effect of reaction increased with respect to diffusion. Models were developed for each non-Fickian regime. Both diffusion-reaction and diffusion-relaxation models will be presented and the results explained in terms of fuel cell applications.