Ionomer membranes, perfluorinated

The term chlor-alkali refers to those products obtained from the commercial electrolysis of aqueous sodium chloride. These are chlorine, sodium hydroxide, and sodium carbonate. The first two are produced simultaneously during the electrolysis while the latter is included because it is also produced in small quantities and shares many of the end uses of sodium hydroxide. Perfluorinated ionomer membranes are permeable to sodium ions but not the chloride ions, and hence they are useful for these electrolytic cells. The arrangement of a typical membrane cell is shown in Figure 10.2. 

Fujimura M., Hashimoto T., and Kawai H., Small-angle x-ray scattering study of perfluorinated ionomer membranes. 2. Models for ionic scattering maximum. Macromolecules, 15, 136, 1982. 

M. Seko, S. Ogawa, and K. Kimoto, in Perfluorinated Ionomer Membranes, ACS Symposium Series 180, American Chemical Society., Washington, D.C. (1982), pp. 365 110. 

Gebel, G. and Lambard, J. 1997. Small-angle scattering study of water-swollen perfluorinated ionomer membranes. Macromolecules 30 7914—7920. 

Scheme 1. Structure of the perfluorinated ionomer membrane (Nafion ).

Other perfluorinated ionomer membranes, chemically very similar to Nafion, are also available commercially. Aciplex, manufactured by the Asahi Chemical Company, is very similar to Nafion, except that it has perfluoropropanesulfonic acid side chains. Flemion (Asahi Glass Company), in contrast, possesses perfluorobutanoic acid functions. 

A. Eisenberg, and H. L. Yeager (eds.), Perfluorinated Ionomer Membranes, The American Chemical Society, Washington, DC, 1982. 

Heitner-Wirguin, C. (1996). Recent advances in perfluorinated ionomer membranes Structure, properties and applications, J. Membrane Sci. 120, 1. 

Figure 3.43. Structure of Nafion-115 at ambient humidity derived from small-angle X-ray spectroscopy. The lighter areas are cluster structures in the material. (Reprinted with permission from J. Elliott, S. Hanna, A. Elliott, G. Cooley (2000). Interpretation of the small-angle X-ray scattering from swollen and oriented perfluorinated ionomer membranes, Macromolecules 33, 4161-4171. Copyright American Chemical Society.)...

Elliott, J., Haima, S., Elliott, A., Cooley, G. (2000). Interpretation of the small-angle X-ray scattering from swollen and oriented perfluorinated ionomer membranes. 

Industrial production of perfluorinated ionomers, Nafion membranes, and all perfluorinated membranes is costly due to several factors first, the monomers used are expensive to manufacture, since the synthesis requires a large number of steps and the monomers are dangerous to handle. The precautions for safe handling are considerable and costly. Secondly, the PSEPVE monomer is not used for other applications, which limits the volume of production. The most significant cost driver is the scale of production. Today, the volume of the Nafion market for chlor-aUcali electrolysis (150,000 m year ) and fuel cells (150,000 m year ) is about 300,000 m year resulting in a production capacity of 65,000 kg year. When compared to large-scale production of polymers like Nylon (1.2 x 10 m year ), the perfluorinated ionomer membrane is a specialty polymer produced in small volumes. 

Pinerii and coworkers, and a few other groups, have used ESR and Mossbauer spectroscopy as well as SANS, extended x-ray absorption fine structure (EX.AFS), and magnetization and susceptibility data to analyze local. struct.ure in perfluorinated ionomer membranes and the distribution of water within them isee, for inst,ance, (61-65) 1. The application of the KNDOR (electron nuclear double resonance) technique to deuteriated methanol-swollen Scunples of these membranes has been reportesd i-ecentiy (66). Photophysical methods have also tef n applied in hydration. si.udies of these membranes (67-69). Finally, some NMR results on the same hydrated perfluorinat,ed ionomer.s well as on hydrated... 

Perfluorinated ionomer membranes show considerable promise with respect to their performance characteristics, low resistivity, high permselectivity, and long-term stability. However, the present cost of these membranes is more than 300 per square meter. The relatively high cost limits their application in many electrochemical cells when cost effectiveness is a major concern. 

Studies of the diffusional properties of perfluorinated ionomer membranes provide the necessary basic information which is needed for their application in various configurations. Large ionic... 

Self-diffusion coefficients of polyvalent cations in these perfluorinated ionomer membranes have not been reported. It can be inferred from the use of the sulfonate membranes as Donnan dialysis devices that transport of cations such as CuflT), Mg(II), and Al(III) under a concentration gradient is rapid. Also, column chromatographic separation of the alkaline-earth ion is readily accomplished with a powdered Nafion perfluorosulfonate polymer, which is again an indication of facile diffusion of these cations within the polymer phase. 

Industrial applications of perfluorinated ionomer membranes such as the electrolysis of sodium chloride solution to produce chlorine and sodium hydroxide often involve the use of highly concentrated solutions at elevated temperatures. The optimization of these systems depends upon a sound characterization of membrane transport processes under such conditions. Sodium ion is the major current-carrying species through the membrane in a chlor-alkali cell, and... 

Figure 12. Activation energy of Na diffusion in perfluorinated ionomer membranes vs. NaOH solution concentration, 70-90°C temperature interval. (Ref. 168 reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

The most important commercial application of perfluorinated ionomer membranes is currently in the chlor-alkali industry. These materials are used as permselective separators in brine electrolysis cells for the production of chlorine and sodium hydroxide. This... 

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