Nafion equivalent weight

A considerable decrease in platinum consumption without performance loss was attained when a certain amount (30 to 40% by mass) of the proton-conducting polymer was introduced into the catalytically active layer of the electrode. To this end a mixture of platinized carbon black and a solution of (low-equivalent-weight ionomeric ) Nafion is homogenized by ultrasonic treatment, applied to the diffusion layer, and freed of its solvent by exposure to a temperature of about 100°C. The part of the catalyst s surface area that is in contact with the electrolyte (which in the case of solid electrolytes is always quite small) increases considerably, due to the ionomer present in the active layer. 

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... 

Based on this new model for the morphology of Nafion, the dimensional variations of the scattering entities with water content were used in simple space filling calculations to estimate the cluster diameter, the number of sulfonate sites per cluster, and the number of water molecules per cluster. The results of these model calculations showed that, for a given equivalent weight, the cluster diameter. 

Tant et al. reported a dynamic mechanical transition of around 100 °C (maximum in G") for acid form Nafion having 1140 EW. o Since this transition also appeared for the sulfonyl fluoride precursor, but at a much lower temperature ( 0 °C), they concluded that it involved main chain motions that are restricted by the conversion to the acid form. These motions were further restricted by the conversion to the Na " sulfonate form owing to strong ionic associations between the side chains. In contrast with the work of Kyu and Eisenberg, no transition appeared at 0 °C in addition to that at 100 °C. While the equivalent weights of the samples utilized by Eisenberg and Kyu and Tant et al. were not quite the same, the notable difference in matrix Tg assignment is cause for confusion. 

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. 

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. 

Differences in conductivity data of modified perfluorosulfonate membranes can be related to structural differences on the basis of the pore structure models. Smaller equivalent weights (e.g., for Nafion 105, Dow, Membrane C), that is, higher specific ion content, lead to superior performance compared to Nafion 117 due to narrower psds and, thus, more homogeneous water distributions. 

Figure 3. Small-angle x-ray scattering scans of hydrolyzed Nafion-Na with various equivalent weights. Samples are conditioned by boiling in 0.2Z NaOH for 1 h. Reproduced with permission from Ref. 25. Copyright 1981 John Wiley Sons, Inc.

Partially Neutralized Nafion-Cs. Dynamic mechanical studies of partially neutralized Nafion-Cs from the Nafion acid was performed by Kyu et al (58). The starting material used in their study was Nafion acid of equivalent weight 1155. The acid samples were first boiled in distilled water to ensure extensive swelling, and then Immersed in... 

The channels, which had catalyzed electrodes on the surfaces, were covered with Nafion 112 (thickness 50 pm, equivalent weight 1,100 gmoF, ionic conductivity 0.083 S cm" ) to provide ionic conductivity between the anode and the cathode. The Nafion membrane was pressed with a glass plate to avoid solution leakage (Fig. 3.4a). Voltage-current measurements were performed at room temperature with a mass flow control system of fuel and oxidant as shown in Fig. 3.4b. The fuel and oxidant solutions were supplied to the electrodes with the micro-syringe pumps from the outlet of each channel. The flow rate of both the fuel and oxidant solutions was 80 pL miu". Composition of the fuel solution was 2M methanol solution... 

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