(277b) Diffusion and Interfacial Transport of Water in Nafion

A modified Fick's diffusion model was developed to model the transport process of water through 1100 equivalent weight Nafion membranes as functions of temperature and water activity. The three major features of this model are the inclusions of variable transport volume, water diffusivity, and interfacial mass transport coefficient as functions of temperature and water activity. Firstly, since water only enters the hydrophilic phase of Nafion, by measuring the swelling strain of the membrane upon water uptake in an environmentally controlled chamber, the fractional volume change of the hydrophilic phase was demonstrated to be an order of magnitude larger than the overall volume change of the membrane. Therefore, an areal correction factor was incorporated into the model to account for the actual volume available for transport. Then, PG-SE NMR was used to determine the asymptotic self-diffusivity of water in the membrane for a range of water activities at 23oC, 50oC and 70oC. The data were fit to an Arrhenius form to obtain an empirical equation for water diffusivity as functions of activity and temperature that can be integrated into the diffusion model. Additionally, the interfacial mass transport coefficient was modeled by Langmuir kinetics and quantified as a function of temperature using steady state water permeation data. The complete model was used to resolve the water activity profile in the membrane at both steady state and transient conditions. The model was able to predict literature results from gravimetric water absorption, desorption and MRI imaging.