Diffusion of water in Nafion using time-resolved Fourier transform infrared-attenuated total reflectance spectroscopy.
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abstract
Hydrogen fuel cells are attractive alternative power sources for applications such as transportation; however, fuel cell performance is a strong function of water equilibrium content and water sorption and desorption kinetics in polymer electrolyte membranes (e.g., Nafion). Although similar water sorption isotherms for Nafion have been reproduced in many laboratories, reported diffusion coefficients of water in Nafion vary by 4 orders of magnitude. In this study, sorption and desorption dynamics of water vapor in Nafion were measured as a function of water vapor activity and flow rate using time-resolved Fourier transform infrared-attenuated total reflectance (FTIR-ATR) spectroscopy. Both integral and differential experiments were performed, where integral experiments consisted of increasing the vapor activity from 0% RH to one of five values (22, 43, 56, 80, or 100% RH), while in differential experiments the activity was sequentially increased in smaller steps from 0 to 22 to 43 to 56 to 80 to 100% RH. For integral experiments, non-Fickian behavior was observed at both low and high vapor activities, while Fickian behavior was observed at moderate vapor activities. For differential experiments, Fickian behavior was observed at all vapor activities except at low vapor activities (0-22% RH). Sorption kinetics was found to be a function of flow rate, where mass transfer resistance at the vapor/polymer interface was significant at low flow rates but was insignificant at high flow rates. Accurate sorption and desorption diffusion coefficients were calculated in this study (measured at high flow rates with no mass transfer resistance) and were similar, on the order of 10(-7) cm(2)/s, and weak functions of water vapor activity.
