Non-Fickian Diffusion of Water In Nafion

Understanding the dynamics of water in solid-state polymer electrolytes (e.g., Nafion) is important for a variety of applications ranging from membrane-based water purification to hydrogen fuel cells. In this study, the dynamics of water in Nafion was investigated at both low and high humidities with time-resolved Fourier transform infrared−attenuated total reflectance (FTIR−ATR) spectroscopy; a technique that provides a molecular fingerprint of both the diffusant and the polymer simultaneously in real time. At low humidities (0−22% RH), an extended initial time lag resulted in non-Fickian behavior, where dynamic infrared data provided evidence for a reaction between water and sulfonic acid. A diffusion−reaction model was developed and predicted this anomalous behavior, where the time lag was a function of water content. At high humidities (0−100% RH), a slow approach to steady state resulted in non-Fickian behavior, where dynamic infrared data provided evidence of water-induced relaxation in the polymer ...

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