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Non-Fickian Diffusion of Water In Nafion

Daniel T. Hallinan Jr.1, Maria Grazia De Angelis2, Marco Giacinti Baschetti2, Giulio C. Sarti2, and Yossef A. Elabd1. (1) Chemical and Biological Engineering, Drexel University, 3141 Chestnut St., Philadelphia, PA 19104, (2) Chemical Engineering, Mining and Environmental technology, University of Bologna, Via Terracini 28, Bologna, Italy

Polymer electrolyte membranes (PEMs), such as Nafion, have been extensively explored in applications including fuel cells, actuators, and membrane-based separations. Fuel cells in particular require high PEM proton conductivities, which are strong functions of water content. Maintaining sufficient conductivity for fuel cell performance requires high water content. Therefore, a fundamental understanding of water sorption and dynamics in Nafion is critical. Although water sorption isotherms for Nafion have been reproduced in many laboratories, reported diffusion coefficients of water in Nafion vary by four orders of magnitude.

In this study, the diffusion of water vapor in Nafion was measured using time-resolved Fourier-transform infrared, attenuated total reflectance (FTIR-ATR) spectroscopy. At moderate vapor activities, Fickian behavior was observed. However, at both low and high vapor activities, two different forms of non-Fickian behavior were observed. Models have been developed for both non-Fickian regimes, where a reaction between water and immobile sulfonic acid sites within the polymer has been considered at low activities and polymer relaxation has been considered at high activities. Both diffusion-reaction and diffusion-relaxation models regress well to the experimental data. In addition to modeling non-Fickian behavior at low and high vapor activities in order to extract true diffusion coefficients, the determination of accurate diffusion coefficients over the entire activity range will be explained in terms of an appropriate assessment of the vapor-polymer boundary conditions. More specifically, water sorption dynamics were a strong function of the vapor flow rate. Inaccurately accounting for this boundary condition will result in diffusion coefficients an order of magnitude lower than the true values.