Equivalent weight membrane

Membrane Type Membrane Equivalent Weight (EW) (g equiv ) Thickness (mm)... 

Nation /Silica System Jungetal. [174] have reported the synthesis of a Nafion /silica composite membrane obtained via the sol-gel acid catalysis of tetraethoxysilane (TEOS) in a Nafion 115 membrane (equivalent weight = 1100). The amount of TEOS impregnated in the membrane varied with the reaction time. These authors noted that the water absorption of the composite membrane increased... 

Stable performance was demonstrated to 4,000 hours with Nafion membrane cells having 0.13 mg Pt/cm and cell conditions of 2.4/5.1 atmospheres, H2/air, and 80°C (4000 hour performance was 0.5 V at 600 mA/cm ). These results mean that the previous problem of water management is not severe, particularly after thinner membranes of somewhat lower equivalent weight have become available. Some losses may be caused by slow anode catalyst deactivation, but it has been concluded that the platinum catalyst "ripening" phenomenon does not contribute significantly to the long-term performance losses observed in PEFCs (5). 

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... 

Biichi, F. N., Wakizoe, M. and Srinivasan, S. 1996. Microelectrode investigation of oxygen permeation in perfluorinated proton exchange membranes with different equivalent weights. Journal of the Electrochemical Society 143 927-932. 

Basura, V. I., Chuy, C., Beattie, P. D. and Holdcroft, S. 2001. Effect of equivalent weight on electrochemical mass transport properties of oxygen in proton exchange membranes based on sulfonated a,j3,j3-trifluorostyrene (BAM) and sulfonated-styrene-(ethylene-butylene)-styrene triblock (DAIS-analytical) copolymers. Journal ofElectroanalytical Chemistry 501 77-88. 

Figure 17. Room-temperature proton conductivity of two Dow membranes of different EW values, Nation, two varieties of sulfonated poly (ary lene ether ketone) s (S— PEK and S—PEEKK, unpublished data from the laboratory of one of the authors), and sulfonated poly(phenoxyphos-phazene)s (S—POPs °9 of different equivalent weights (685 and 833 g/equiv), as a function of the degree of hydration n = [H20]/I—SO3H] (number below the compound acronym/ name indicates the EW value).

Figure 18 shows the temperature dependence of the proton conductivity of Nafion and one variety of a sulfonated poly(arylene ether ketone) (unpublished data from the laboratory of one of the authors). The transport properties of the two materials are typical for these classes of membrane materials, based on perfluorinated and hydrocarbon polymers. This is clear from a compilation of Do, Ch 20, and q data for a variety of membrane materials, including Dow membranes of different equivalent weights, Nafion/Si02 composites ° ° (including unpublished data from the laboratory of one of the authors), cross-linked poly ary lenes, and sulfonated poly-(phenoxyphosphazenes) (Figure 19). The data points all center around the curves for Nafion and S—PEK, indicating essentially universal transport behavior for the two classes of membrane materials (only for S—POP are the transport coefficients somewhat lower, suggesting a more reduced percolation in this particular material). This correlation is also true for the electro-osmotic drag coefficients 7 20 and Amcoh...

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. 

The validity of MD simulation is impacted by the choice of system, interaction potential and algorithm implemented. We first discuss the choice of system. In this work we chose to simulate Nafion with an equivalent weight (EW) of 1144 g/mol, which is a practically reasonable EW. In order to have a direct comparison of the effect of side chain length, SSC PFSA polymer electrolyte was simulated with an EW of 978 g/mol. (Commonly used SSC iono-mer has an EW 800 g/mol). The repeat units of these PFSA membranes are shown in Fig. 3. These two materials have the same backbone separating side chains thus the only differentiating feature is side chain length. 

FIGURE 27.20 Structure of Nation. The characteristic value of proton-conducting polymer membranes is the EW, which is defined as the weight of polymer that will neutralize one equivalent of base, and is inversely proportional to the lEC. The values n, m, n can he varied to produce materials with different equivalent weights. 

Membrane Typical Basis Equivalent Weight of Polymer That Will Neutralize One... 

Acid capacity, mequiv g 0.89 A base titration procedure measures the equivalents of sulfonic acid in the polymer, and uses the measurement to calculate the acid capacity or equivalent weight of the membrane. 

Another finding reported by DuPont is that the equivalent weight (EW) exhibits a pronounced influence on performance and methanol permeation. Whereas the highest performances were found with low EW membranes, the membranes with high EW had the lowest relative methanol permeation (Figure 27.24). According to DuPont, a 2 mil experimental membrane is in development which exhibits better performance for DMFCs compared to the 7 mil commercial membrane. 

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