Nafion different water contents

Snapshots of the final microstructure in hydrated Nafion membrane at different water contents. Hydrophilic domains (water, hydronium, and side chains) are shown in gray, while hydrophobic domains are shown in black. 

Figure 5. ATR spectra of the Li form of Nafion 113 in the region of 1100 to 1000 cm 1 for different water contents (3).

Figure 6. Transmittance spectra of Nafion 142 membranes in the range of 1100 to 1000 cm 7 with different counterions and at different water contents, expressed as H20/—SOs mol ratio.

Fig. 2 Replotted high temperature data of Cappadonia et al. [49,50] on activation free energy of similarly prepared Nafion 117 samples with different water content...

It was possible to rationalize the family of Arrhenius plots measured for Nafion 117 at different water contents [46]. Under an assumption that the surface conductivity has higher activation energy, supported by microscopic considerations in Refs. 40, 43, the Arrhenius slope should become steeper with the decreasing amount of water in the membrane [39], that is, the smaller the amount of the bulk water that we have in pores. Activation energies obtained from these plots are 0.1 eV for the largest possible water contents (Activation energies of proton transfer in water, estimated from nuclear magnetic resonance relaxation times, are 0.1 eV [47].) and 0.3-0.4eV at small water contents. How to rationalize this variation. ... 

The situation for hydrated Nafion in the acid form, or as containing aqueous acids or strong bases, is more complex because protons and defect protons (i.e., OH ions), migrate according to a somewhat different mechanism. Proton transfer in either case occurs throughout and between clusters of hydrogen bonded water molecules to a degree that depends on the relative water content. 

The morphology of the water clusters and their connectivity can be better understood with a visual aid as shown in Fig. 5 and 6 where the snapshots of hydrated Nafion (Fig. 5) and SSC (Fig. 6) PFSA membrane are presented with the ionomer rendered invisible at ). = 4.4 and 9.6 respectively. The snapshots for both the membranes clearly indicate the presence of small clusters whose connectivity is poor at low water content and the clusters appears to have grown in size and more densely packed with better chWiel networks when the water content is increased. However, it is difficult, based solely on snapshots, to discern subtle differences in the size, shape and connectivity of the aqueous phase as a function of side chain length. In order to analyze these differences, we must invoke a more statistical characterization of the stmcture. 

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... 

Zawodzinski et al. [64] have reported self-diffusion coefficients of water in Nafion 117 (EW 1100), Membrane C (EW 900), and Dow membranes (EW 800) equilibrated with water vapor at 303 K, and obtained results summarized in Fig. 36. The self-diffusion coefficients were deterinined by pulsed field gradient NMR methods. These studies probe water motion over a distance scale on the order of microns. The general conclusion was the PFSA membranes with similar water contents. A, had similar water self-diffusion coefficients. The measured self-diffusion coefficients in Nafion 117 equilibrated with water vapor decreased by more than an order of magnitude, from roughly 8 x 10 cm /s down to 5 x 10 cm /s as water content in the membrane decreased from A = 14 to A = 2. For a Nafion membrane equilibrated with water vapor at unit activity, the water self-diffusion coefficient drops to a level roughly four times lower than that in bulk liquid water whereas a difference of only a factor of two in local mobility is deduced from NMR relaxation measurements. This is reasonably ascribed to the additional effect of tortuosity of the diffusion path on the value of the macrodiffusion coefficient. For immersed Nafion membranes, NMR diffusion imaging studies showed that water diffusion coefficients similar to those measured in liquid water (2.2 x 10 cm /s) could be attained in a highly hydrated membrane (1.7 x 10 cm /s) [69]. 

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. 

Nafion (DuPont) has a fluorocarbon polymer backbone with fixed sulfonic groups. Nafion membranes are produced as films with thicknesses between 0.05 and 0.18 mm and different degrees of sulfona-tion. They can be used as support for the porous electrodes, which can be applied by hot isostatic pressing onto the membrane [9]. A disadvantage of Nafion membranes is that their conductivity depends strongly on the water content of the membrane, which is given by the ratio of number of water molecules per available number of sulfonic acid groups (see Fig. 9) [10]. The... 

Yeo et al have reported an analysis of the conductivity of Nafion in different alkaline electrolytes, based on the correlation of membrane conductivity with water content. The analysis reveals that larger conductivities arise when the membranes are equilibrated with NaOH solutions than with KOH solutions of equal molarity. Also, it is shown that better conductivity can be realized with thinner and lower-EW membranes. These effects have been proven in an alkaline-water electrolyzer and in relation to the conductivity... 

Another important conclusion is obtained from the coupled DSC/NMR experiments. The water content of the Nafion membranes strongly depends on the temperature. Therefore the analysis of a possible water phase separation cannot be done with experiments involving temperature changes like DSC. This is pretty different from what is obtained with y-alumina which represent a relative fixed and non temperature dependent hydrophobic matrix. The endothermic and exothermic peaks observed during heating and cooling runs of the water-Nafion systems may be interpreted in two ways... 

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