(697c) Gas Permeation in Perflurosulphonated Membranes: Influence of Temperature and Relative Humidity

Marco Giacinti Baschetti, Matteo Minelli, Jacopo Catalano[1], Giulio C. Sarti

Dipartimento di Ing. Chimica, Mineraria e delle Tecnologie Ambientali (DICMA)

Alma Mater Studiorum - Università di Bologna, via Terracini 28, 40131, Bologna, Italy.

Perfluorosulfonated ionomer (PFSI) are of high interest in energy application due to their ability to be used as polyelectrolyte membranes (PEM) inside hydrogen or methanol based fuel cells (FC).[1,2] In fuel cells (FC) applications, the polyelectrolyte membranes (PEM) are exposed to different gases such as hydrogen, oxygen, nitrogen and carbon dioxide, depending on the type of FC considered, at varying operative conditions, both in terms of temperature and relative humidity. For these reasons, the knowledge of gas permeability of different gases in such materials as a function of these parameters is of interest for a proper description and modeling of the system. Different studies appeared in the literature addressing these issues [3-10], however systematic investigation of these phenomena are relative scarce and a thorough characterization is still missing for many of the PSFI now available.

In the present study, gas permeability in two different PFSI, namely Nafion^® and Aquivion^®, have been investigated as a function of temperature and relative humidity. In particular, permeation of gases such as O₂, N₂ and CO₂ and also Helium, used to mimic H₂, have been studied from 25 to 50°C and at relative humidity up to 80%.

The results showed that both materials posses very similar permeability values and a common behavior with respect to temperature and relative humidity. The permeability of the different gases in the two ionomers increases as the temperature or the humidity increase, the latter parameter  having a really dramatic effect on the gas flow rate across the membrane. The measured permeability indeed showed increases of up to two orders of magnitude when humidified gas were flown in the system.

The effect of water on the membrane transport properties is rather complex since the permeability has a sharp increase already at very low humidity, and then shows a more gentle, exponential rise with RH that  seems to lead towards values comparable with those of gas permeability in pure water.

The comparison between the two materials does not show a clear trend; in general, however, Aquivion seems to have higher permeability than Nafion at low relative humidity and at the lower temperatures inspected, while Nafion shows higher permeation rates at larger R.H. and high temperatures.

These findings can be related to the fact that at a given activity Nafion in general sorbs less water than Aquivion, due to the higher equivalent weight. Such difference seems to be significant at low activity, where the water content is still low. It becomes then less important at higher activity where interconnected hydrophilic domains are formed in both membranes. 

The temperature dependence follows an Arrhenius type of behavior for all gases in the two polymers, as expected for permeation processes substantially related to a solution diffusion mechanism. The activation energies were calculated for both materials as a function of R.H, and showed higher values in Nafion than in Aquivion, in agreement with the permeability behavior described above.

 In all cases, for both materials the activation energy ranges between the value of the dry polymer and that obtained for the gas permeation in pure polymer. In the case of Nafion, a well defined trend of permeability with activation energy can be found with water activity, at least in the case of nitrogen and oxygen. In the case of Aquivion, on the other hand, no clear trend was detected.

All the experimental evidences suggest that gas permeation in humid PFSI is driven by gas sorption and diffusion in the hydrophilic domains of the materials and ultimately in the water channels which are usually considered to form in the hydrated matrix. This hydrated phase indeed has a largely higher gas permeability than that of the hydrophobic phase and controls the overall permeation process.

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