Nafion water diffusion coefficients

Nafion absorbs MeOH more selectively than water, and the MeOH diffusion flow is higher than the osmotic water flow in Nafion membranes. Diffusion coefficients of Nafion 117 determined by different techniques have been reported. Ren et al. measured MeOH diffusion coefficients in Nafion 117 membranes exposed to 1.0 M MeOH solutions using pulsed field gradient (PPG) NMR techniques. The MeOH self-diffusion coefficient was 6 x 10 cm S and roughly independent of concentration over the range of 0.5-8.0 M at 30°C. A similar diffusion coefficient was obtained for Nafion 117 at 22°C by Hietala, Maunu, and Sundholm with the same technique. Kauranen and Skou determined the MeOH diffusion coefficient of 4.9 x 10 cm for Nafion... 

Figure 12. Water self-diffusion coefficient of Nafion 117 (EW =1100 g/equiv), as a function of the water volume fraction Xy and the water diffusion coefficient obtained from a Monte Carlo (MC) simulation (see text). The proton conductivity diffusion coefficient (mobility) is given for comparison. The corresponding data points are displayed in Figure 14.

For comparison purposes, the proton mobility. Do (for Nafion solvated with water), which is closely related to the self-diffusion coefficient of water, is also plotted. At low degrees of hydration, where only hydrated protons (e.g., H3O+) are mobile, it has a tendency to fall below the water diffusion coefficient (this effect is even more pronounced in other polymers), which may be due to the stiffening of the water structure within the regions that contain excess protons, as discussed in Section 3.1.1. . Interestingly, the proton mobility in Nafion solvated with methanol (Da(MeOH) in Figure 14a) is even lower than the methanol self-diffusion (Z ieon). This may... 

Yeager et al. [00] reported diffusion coefficients of water in Nafion 120 membranes containing various alkali-metal cations, determined by radiotracer measurements. In these studies, the diffusion coefficient of water was measured for fully hydrated (i.e., immersed) membranes. The water diffusion coefficient was found to be only slightly dependent on the cation present in the immersed membrane, with a... 

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]. 

Figure 23.5 Proton conductivity and water diffusion coefficient (D and Dh2o) of hydrated Nafion as a function of its water volume fraction. Data are taken from Refs. [12, 43, 77, 78, 108-116], unpublished data from the author s...

The first application of PFGSE NMR for the measurement of water diffusion coefficients in NAFION was carried out by Zawodzinski et al. [67,68]. They determined the diffusion coefficients increased from 0.6 X 10 to 5.8 X 10 cm s as the hydration level of the membrane increased from 2 to 14. At a hydration level of about 20, the diffusion coefficients are 2 X 10 cm s for proton and 6 x 10 cm s for water in NAFION 117. Ochi et al, investigated the proton dynamics in NAFION 117 by comparison of the diffusion coefficient D mr estimated from PFGSE NMR and estimated from electrical conductivity, and concluded that... 

Recendy, Song et al. measured the local water diffusion coefficient and proton diffusion coefficient Dp within 5—10 A of spin probes that are partitioned into selectively different local environments of the swollen NAFION using Overhauser dynamic nuclear polarization relaxometry (ODNP) for nuclei of water at 9.8 GHz. This experiment concluded that... 

Estimate the water content, proton conductivity, water diffusion coefficient, and electro-osmotic drag coefficient for a Nafion membrane under humidity conditions with water activity a = 0.9 and an operating temperature of 80°C. 

Bicontimous network of ionic clusters Elliott and co-workers demonstrated a unified morphological description of PFSAs based on both statistical (MaxEnt) and thermodynamic (DPD) descriptions, which broadly favours a bicontinuous network of ionic clusters embedded in a matrix of fluorocarbon chains. Elliott, 201 The existence of a continuous network of water-filled channels explains the high water diffusion coefficient of water in Nafion. 

Choi et al. proposed a pore transport model to describe proton diffusion within Nafion." The diffusion coefficients are predicted. The surface diffusion coefficient is 1.01 X10 cm /s at room temperature the vehicular diffusion coefficient is 1.71x10 cm /s and the Grotthuss diffusion coefficient is 7x10 cm /s. The Grotthuss diffusion is the fastest proton transport mechanism within Nafion. The surface diffusion coefficient is much lower than the other two diffusion coefficients. The surface diffusion does not contribute significantly to the overall conductivity of protons except at low water levels. 

Proton mobility (D J and water self-diffusion coefficient (D q) as a function of the water volume fraction (X ) in Nafion and SPEEKK, where X, = volume of water in membrane divided by volume of wet membrane. (From Kreuer, K. D. 2001. Journal of Membrane Science 185 29-39.)... 

The effects of equivalent weight (FW = g polymer/mol SO3H) and water content on diffusion coefficient, solubility, and permeability of oxygen for fully hydrated BAM, S-SEBS, ETFE- -PSSA, Nafion 117, and BPSH membranes have been studied. It has been found that the diffusion coefficients of all the studied membranes decrease with increasing EW, while the solubility correspondingly increases. These trends are the same as found in... 

The reduction of the long-range diffusivity, Di by a factor of four with respect to bulk water can be attributed to the random morphology of the nanoporous network (i.e., effects of connectivity and tortuosity of nanopores). For comparison, the water self-diffusion coefficient in Nafion measured by PFG-NMR is = 0.58 x 10 cm s at T = 15. Notice that PFG-NMR probes mobilities over length scales > 0.1 /rm. Comparison of QENS and PFG-NMR studies thus reveals that the local mobility of water in Nafion is almost bulk-like within the confined domains at the nanometer scale and that the effective water diffusivity decreases due to the channeling of water molecules through the network of randomly interconnected and tortuous water-filled domains. ... 

Yeager and Steck derived diffusion coefficients for water in totally hydrated Nafion 120 membranes that were exchanged with alkali metal cations, using a radiotracer technique. At 25 °C, 77for the Na+ form was 2.65 X 10 cm /s and the values for the K+ and Cs+ forms were somewhat smaller, which would seem to reflect the lower maximal degree of hydration of these forms. 

The self-diffusion coefficient of water in Nafion, as measured by PFG is shown in... 

Figure 14. Solvent (water, methanol) diffusion coefficients of (a) Nafion 117 (EW =1100 g/equiv) and (b) sulfonated poly(arylene ether ketone)s, as a function of the solvent volume fraction. Self-diffusion data (AiaO. T eOi-i) are taken from refs 197, 224, 226, 255—263 and unpublished data from the laboratory of one of the authors) chemical diffusion coefficients (Z>h2o) are calculated from self-diffu-sion coefficients (see text), and permeation diffusion coefficients are determined from permeation coefficients. ...

Figure 5. Electro-osmotic drag coefficient and water diffusivity as functions of water content in Nafion membranes.

Nafion (17) is a perfluorinated polymer related to teflon (polytetrafluoroethylene). An electrode is conveniently coated by allowing an ethanolic solution of the polymer to evaporate. The film produced is stable, rather more so in fact than other polymer films, e.g. polyvinylpyridine (see Section 57.3.2.2). At the microscopic level the polymer separates into two phases, the bulk polymer and the lower density ionic cluster phase. Diffusion of ions can occur quite freely for example, the diffusion coefficient of Na+ in Nafion (MW 1200) is only slightly less than in water.44... 

The ionic resistance of a polymer electrolyte membrane is an important parameter in determining the mobility of protons through the membrane and the corresponding voltage loss across the membrane. Currently, the most commonly used membranes in PEM fuel cells are Nafion membranes produced by DuPont. However, these membranes are limited to low-temperature uses (usually below 80°C) because membrane dehydration at high temperatures can lead to reduced water content and then a lower proton transfer rate, resulting in a significant decrease in conductivity. The relationship between conductivity and the diffusion coefficient of protons can be expressed by the Nemst-Einstein equation ... 

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