Nafion membranes, water uptake

Nicotera et al. investigated the behavior of water confined in recast NAFION and in NAFION-clay hybrids membranes using PFGSE NMR and spin—lattice relaxation time Tj, and concluded that the transport mechanism is influenced from the dimensions of the dispersed platelets and mainly from the type of nanocomposites formed upon mixing the clay particles with the polymer matrix. Compared to pure NAFION, the water uptake and the water diffusion of the hybrid membranes are increased, with the exception of the Kunipia-NAFION composite [83]. 

Broka and Ekdunge investigated water uptake from the vapor phase by Nafion 117 membrane and recast film. The results show water vapor uptake by both Nafion 117 membrane and recast film decreased with increasing temperature. The lower water uptake at higher temperatures has also been reported by other researchers. Broka and Ekdunge also found PFSA membrane water uptake from the liquid water is higher than those for water vapor. This phenomenon is known as Schroeder s paradox. Zawodzinski et al. explored the isopiestic sorption curve for Nafion 117-H". ... 

For ETFE- -PSSA membranes with the same lEC, water uptake is higher than MeOH uptake of the membrane, but for Nation and S-SEBS membranes, MeOH uptake of membrane is always higher than water uptake. Chemical structure and morphology of membranes affect the solvent absorption. Nafion is considered to consist of ionic clusters that are separated from the polymer phase. For grafted polymers, heterogeneity exists to some extent due to the hydrophobic base polymer however, a regular clustered structure, as in the case of Nafion, has not been proposed for these materials. 

Water uptake and swelling of reinforced PEMs have also been studied. It has been found that water uptake and water flux of Nafion/porous PTFE PEMs increases with increased ionomer loading and that the rate of water uptake as a function of temperature is actually higher for the reinforced membrane in comparison to Nation 112.2 However, it was also seen that the actual water uptake of a similar reinforced membrane was about half as much as a Nation 1135 sample with a commensurate lower degree of swelling.2 ... 

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Figure 4. Equilibrium water uptake curve for Nafion membrane (a) measurement at 30 (b) measurement...

Most recently, Gallagher et al.21 measured the water uptake of Nafion membrane under subfreezing temperatures, which showed a significant reduction in the maximum water content corresponding to membrane full hydration. The Nafion membrane with 1,100 equivalent weight, for example, uptakes A 8 of water at -25°C when it equilibrates with vapor over ice because of the low vapor pressure of ice compared to supercooled liquid water. They also found the electro-osmotic drag coefficient to be 1 for Nafion membrane under sub freezing temperatures. 

Ren X, Springer TE, Gottesfeld S (2000) Water and methanol uptakes in Nafion membranes and membrane effects on direct methanol cell performance. J Electrochem Soc 147 92-8... 

FIGURE 27.17 Water uptake from vapor phase (100% relative humidity [RH]) by Nafion 117 membrane and recast Nafion film at different equilibration temperatures (O) Nafion 117 membrane, (A) recast Nafion film, (o) heat-treated Nafion 117 membrane. Fitted curves second order polynomial X is the number of water molecules per sulfonate site. (Reproduced from Broka, K. and Ekdunge, P., J. Appl. Electrochem., 27, 117, 1997. With permission from Springer Science and Business Media.)... 

K. Broka and P. Ekdunge. Oxygen and hydrogen permeation properties and water uptake of Nafion(R) 117 membrane and recast film for PEM fuel cell. Journal of Applied Electrochemistry 27, 117-123 1997. 

T.A. Zawodzinski, C. Derouin, S. Radzinski, R.J. Sherman, V.T. Smith, T.E. Springer, and S. Gottesfeld. Water-uptake by and transport through Nafion(R) 117 membranes. Journal of the Electrochemical Society 140, 1041-1047 1993. 

Recent SAXS work by Wu [56] has demonstrated that the ionic domains in Nafion membranes are most probably spherical and exhibit a size distribution and spacing that does not vary much with equivalent weight (EW). The latter work suggested that strings of smaller spherical aggregates could be sufficiently close to coalesce upon swelling with water uptake by the membrane, and thus could provide percolation pathways for ionic transport. This picture could replace the nanoscale (1.2 nm-wide) channels suggested in earlier models for Nafion to explain transport phenomena. 

The dependence of water uptake from the liquid phase on membrane thermal pretreatment was mentioned in the earliest descriptions of Nafion properties. As Grot et al. have pointed out [57], Nafion membranes take up dramatically more water from liquid water at very high temperatures-up to 100% of the polymer diy weight in the case of Nafion 120 when the membrane is in contact with liquid water at 180 °C. Such expanded ( E-form ) membranes maintain a constant, high water content when in contact with liquid water at temperatures at or below the pretreatment temperature. In contrast, S -form (S for shrunken) membranes are prepared by drying the membranes completely at elevated temperatures. Such membranes imbibe less water than as-received ( normal , N-form) membranes. 

Zawodzinski et al. [58, 59] have reported on the amount of water taken up by immersed protonic PFSA membranes after different thermal treatments. After complete drying at 105 °C, the water uptake upon immersion of membranes is relatively small and increases with the temperature of the water bath in which the membrane is immersed. In contrast, the water content of well-swollen membranes dried at room temperature and then re-immersed in liquid water is independent of the temperature of the re-immersion bath between room temperature and boiling water. This phenomenon was referred to earlier qualitatively by LaConti et al. [49]. The results of Zawodzinski et al. are summarized in Table 6 for three membranes, Nafion (EW= 1100), Membrane C (Chlorine Engineers, EW= 900), and Dow XUS (EW= 800). As seen in Table 6, the uptake expressed as a percentage by weight... 

Fig. 28. Water uptake by a Nafion membrane immersed in liquid water at 28 °C as a function of the temperature of membrane pretreatment in a hot glycerol bath.

Diffusion coefficients for sorbed solvent and ions in Nafion have been estimated using several techniques. Yeo and Eisenberg [53] studied the sorption of water by a dry slab of Nafion (EW 1155) and estimated the interdiffusion coefficient of water in the membrane over the temperature range 0-99 °C from the water uptake dynamics. Diffusion coefficients from these measurements increased with increasing temperature over the range (1-10) x 10 cm /s with a reported activation energy of 4.5 kcal/ mol (18.8 kJ/mol). The method used to estimate the diffusion coefficients by Yeo and Eisenberg [53] was based on the dependence of the uptake in the initial portion of the uptake curve and is probably not fully appropriate [82]). 

Zelsmann and co-workers [88] have reported tracer diffusion coefficients for water in Nafion membranes exposed to water vapor of controlled activity. These were determined by various techniques, including isotopic exchange across the membrane. They reported apparent self-diffiision coefficients of water much lower than those determined by Zawodzinski et al. [64], with a weaker dependence on water content, varying from 0.5 x 10 cm to 3 x 10 cm /s as the relative humidity is varied from 20 to 100%. It is likely that a different measurement method generates these large differences. In the experiments of Zelsmaim et al., water must permeate into and through the membrane from vapor phase on one side to vapor phase on the other. Since the membrane surface in contact with water vapor is extremely hydrophobic (see Table 7), there is apparently a surface barrier to water uptake from the vapor which dominates the overall rate of water transport in this type of experiment. 

An adequate structure of polymer molecules promotes the advantageous phase separation into hydrophobic and hydrophilic domains upon water uptake. The most notable class of membranes based on this principle are the perfluorosulfonic acid ionomers (PFSI), Nafion [26] and similar membranes [27]. In these membranes, perfluorosulfonate side chains, terminated with hydrophilic —SO3H groups, are attached to a hydrophobic fluorocarbon backbone. The tendency of ionic groups to aggregate into ion clusters due to the amphiphilic nature of the ionomer leads to the formation of basic aqueous units. At sufficient humidity these units first get connected by narrow channels and then may even fuse to provide continuous aqueous pathways [28]. 

---