Perfluorosulfonic membranes

Feldheim DL, Lawson DR, Martin CR (1993) Influence of the sulfonate countercation on the thermal stability of Nation perfluorosulfonate membranes. J Polym Sci Part B Polym Phys 31(8) 953—7... 

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. 

Barnes et al. (112) considered the use of suifonate-ionomers, SPS and perfluorosulfonate membranes, as catalysts for carrying out chemical reactions. In a variation on this theme, Maurltz et al. have prepared unique mlcrocomposlte membranes by the In situ growth of silicon oxide in solvent swollen Nafion membranes. This work is described later in this book. 

To this end, a number of infrared (IR) studies have addressed the issue of water in ioncxners either in passing or moi-e substantially (28,52-59). In perfluorosulfonated membranes, for instance, at least two types of loonding environments for water have been detected (52,54). It is also clear that the extent and effect of water alasoiption depends on the salt involved (54,58). 

Further anomalies are seen in the diffusional behavior of cesium ion for the perfluorosulfonate membrane. Figure 10 represents the plot of the logarithm of the diffusion coefficient vs. the function Vp/(l-Vp), where Vp is the volume fraction of polymer in the water-swollen material and 1 - Vp is the volume fraction of water, as calculated from sorption measurements. This plot corresponds to a test of the equation... 

The results of ion and water sorption measurements for the two polymers under these solution conditions help to explain this difference. Table 5 " lists the concentrations of various sorbed species and the mole ratio of water to cation/anion in the polymer phase for NaCl and NaOH solution environments. This ratio decreases both in the polymers and in solution with increasing concentration. In solution, the ratio varies from 10.8 to 4.0 over the concentration range of 5-12.5 M NaOH, so that ions in the polymer phase exist in a significantly less aqueous environment compared to the solution phase. As noted by Mauritz and co-workers for perfluorosulfonate membranes, these water contents are insufficient to provide even primary hydration spheres for sodium ions, sorbed anions, and exchange sites, and the likelihood... 

Some of these features are illustrated in Figures 14-18. A rather typical literature plot of current efficiency vs, sodium hydroxide concentration for perfluorosulfonate membranes is shown in Fig. 14. Nation 427 is a 1200-EW sulfonate membrane with fabric reinforcement. Poor hydroxide rejection occurs at catholyte concentrations above 10 wt % but a minimum is seen at higher concentrations, wtih increasing current efficiency from 28 to 40% caustic (9-14 M). The current efficiency of a 1200-EW homogeneous perfluorosulfonate film is shown in more detail over this concentration region in Fig. 15. Sodium ion transport number niol F ), which is equivalent to caustic current efficiency, is plotted vs. both brine anolyte and caustic catholyte concentration. These values were determined using radiotracer techniques, which have proven to be rapid and accurate methods for the determination of membrane performance. " " " A rather sharp maximum is seen at 14 M NaOH, and the influence of brine con-... 

Figure 14. Current efficiency vs. catholyte concentration for Nafion 427 perfluorosulfonate membrane. (Ref. 62 reprinted by permission of the publisher. The Electrochemical Society, Inc.)...

Mauritz and co-workersand Hopfinger have reported spectroscopic and sorption studies of perfluorosulfonate membranes. Several conclusions drawn in these studies are useful in the interpretation of current efficiencies as a function of caustic solution concentration. Sorption measurements of an 1100-EW film in equilibrium with NaOH solutions from 7.5 to 18 M show that the... 

The cost imperative comes into play greatly with fuel cell membrane assemblies. It is up to the automotive chemist to design ways to improve the cost structure of these assemblies. For instance, the perfluorosulfonic membrane (Nafion) by DuPont is a very expensive component [13], Other companies, such as Arkema and PolyFuel, are proposing other films as a cost alternative to the Nafion product [14,15], The precious metal catalyst used in membrane assemblies is applied to carbon using batch methods if other deposition methods (such as vapor deposition) can be developed, cost can be reduced [12],... 

Experimental results which yield insight into the nature of these processes in perfluorosulfonate membranes are emphasized in this chapter. This information, when correlated with structural studies and results of membrane performance in practical applications, should help to produce a unified understanding of this important new type of polymer membrane. 

Most of the current applications of perfluorosulfonate membranes involve electrochemical cells in which concentrated electrolyte solutions are employed, often at elevated temperatures. Relatively little diffusion data are available under these conditions, although a larger amount of membrane resistance and other operating data have been published. Sodium ion self-diffusion coefficients have been measured in various Nafion membranes in concentrated NaOH solutions at elevated temperatures (23). This... 

Applications of perfluorosulfonate membranes commonly involve their use as separation materials in electrolytic cells, in which concentrated solutions are employed. A primary consideration in such applications is the conductivity of the membrane, because the ohmic loss due to membrane resistance can significantly increase energy consumption of the cell. The conductivities of common Nafion membranes have been investigated for several... 

The Cluster-Network Model of Ion Clustering in Perfluorosulfonated Membranes... 

N. Kawasaki, T. Okada and K. Ogawara, Water swelling behavior of perfluorosulfonic membrane, Soda Enso (Soda Chlorine), 1986, 37, 374-383. 

T.D. Gierke, G.E. Munn and EC. Wilson, Morphology of Nafion perfluorosulfonated membrane products, as determined by wide- and small-angle X-ray studies, J. Polym. Sci., Polym. Phys. Ed., 1981, 19, 1687-1704 W.Y. Hsu and T.D. Gierke, Elastic theory for ionic clustering in perfluorinated ionomers, Macromolecules, 1982, 15, 101-105. 

T.D. Gierke and W.Y. Hsu, The cluster-network model of ion clustering in perfluorosulfonated membranes, Perfluorinated Ionomer Membranes, ACS Symposium Series, ed. A. Eisenberg and H.L. Yearger, American Chemical Society, Washington D.C., 1982, Vol. 180, pp. 283-307. 

Recently, Dahr [1], Stonehart [2] and Watanabe [3] have made an attempt to reduce the humidification constraints in solid polymer electrolyte fuel cells (SPEFCs) by using modified perfluorosulfonic membranes. A recast Nafion film sandwiched between the two electrodes was first proposed by Dahr [1] for the realization of an internally humidified SPEFC. Stonehart [2] suggested the inclusion of small amounts of silica powder into the recast film in order to retain the electrochemically produeed water inside the membrane. Watanabe et al [3] have tried to exploit the H2/O2 crossover through the membrane to produce a chemical recombination to water on small Pt clusters inside the membrane. All of these membranes were operated with H2/O2 at 80°C and allowed the development of systems without assisted humidification or with near ambient humidification. 

Gierke, T.D., G.E. Munn, and F.C. Wilson, Morphology of perfluorosulfonated membrane products - wide-angle and small-angle X-Ray studies. Acs S nnposium Series, 1982. 180 pp. 195-216. 

The significant contribution of Nafion or perfluorosulfonic membranes to the cost of the fuel cells stacks and the high alcohol crossover levels that affect the fuel efficiency, prompted the development of radiation grafted proton exchange membranes based on poly(ethylene-tetrafluoroethylene) (ETFE) [172-178], PVdF [175], andPTFE [179]. The peroxy radicals produced on the base polymer by y-ray, electron- or proton-beam, react with styrene to form a co-polymer that is then sulphonated. 

Tiicoli V (1998) Proton and methanol transport in poly(perfluorosulfonate) membranes containing Cs and H cations. J Electrochem Soc 145 3798-3801... 

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