Influence of the Interfacial Water Transfer on the Analysis of Dynamic Sorption and Desorption Experiments in Nafion® Membrane

Water sorption and desorption in Nafion^®117 membranes in acid form were studied by means of dynamic experiments as a response of relative humidity steps from 20% to 80% using a balance IGASORP (Hiden Isochema Ltd) which allows the measurement of very small mass variations with an accuracy of about 1 mg. The recorded mass variations with time of the membrane samples were analyzed by means of the least squares method based on a model that takes into account a transfer resistance at the interface between the membrane and the surrounding humid gas [1]. As the diffusion time is small (the diffusion coefficient being large), the diffusive internal resistance can be neglected compared to the interfacial resistance so that the water content of the membrane can be assumed uniform, which was confirmed using NMR imaging [2]. The model predicts than the kinetics of sorption is much faster than the kinetics of desorption, and the comparisons with the experimental results show a good agreement and give the value of the corresponding mass transfer coefficients (Figure 1). These results also agree with the results reported by Satterfield and Benziger [3] and they contribute to the debate concerning the existence of an interfacial resistance as reported in the recent review of Weber et al. [4]. For the experimental conditions considered in this study, the membrane is in equilibrium with the humid gas at the interface but the main resistance to water transfer is located in the humid gas phase.

REFERENCES

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