Cation-exchange membranes, perfluorinated

Their availability has greatly expanded the potential for electrolytic processes in synthesis and fuel cells as well as in environmental control. Perfluorinated cation exchange membranes such as Nafion outlast the material that preceeded them by up to four and a half years [48], Unfortunately very little has been published on their behaviour outside their use in chlor-alkali electrolysis. 

In addition to the various perfluorinated cation-exchange membranes an anion-exchange membrane has also been developed. The anion-exchange membrane has similar chemical and thermal properties to the perfluorinated cation-exchange membrane. 

Membranes can be characterized by their structure and function, that is how they form and how they perform. It is essential that the cation exchange membranes used in chlor-alkali cells have very good chemical stability and good structural properties. The combination of unusual ionic conductivity, high ionic selectivity and resistance to oxidative hydrolysis, make the perfluorinated ionomer materials prime candidates for chlor-alkali membrane cell separators. 

Perfluorinated ionomers such as Nafion are of significant commercial importance as cation exchange membranes in brine electrolysis cells ( 1). Outstanding chemical and thermal stability make this class of polymers uniquely suited for use in such harsh oxidizing environments. The Nafion polymer consists of a perfluorinated backbone and perfluoroalkylether sidechains which are terminated with sulfonic acid and/or carboxylic acid functionality. 

Other perfluorinated cation exchange membrane materials have also been produced for chlor-alkali cell and other applications. These are the Flemion membrane products (Asahi Glass Co. 

The ratio of the permeabilities of two cations in a cation exchange membrane is equal to the product of the ion exchange equilibrium constant and their mobility ratio (1). Therefore it is important to characterize the equilibrium ion exchange selectivity of ion exchange polymers in order to understand their dynamic properties when used in membrane form. Nafion (E.I. du Pont de Nemours and Co.) perfluorinated sulfonate membranes have found wide use in a variety of applications, many of which involve exchange of cations across membranes that separate solutions of different ionic composition. The inherent cationic selectivity of the polymer is an important consideration for such applications. Results of ion exchange selectivity studies of Nafion polymers are reviewed in this chapter, and are compared to those of other sulfonate ion exchange polymers. 

G. Zundel, Hydrate structures, intermolecular interactions and proton conducting mechanism in polyelectrolyte membranes - infrared results, J. Membr. Sci., 1982, 11, 249-274 C. Heitner-Wirguin, Infra-red spectra of perfluorinated cation-exchange membranes, Polym., 1979, 20, 371-374 L.Y. Levy, A. Jenard and H.D. Hurwitz, Infrared investigation of ionic hydration in ion-exchange membranes. Part 1. Alkaline salts of grafted polystyrene sulphonic acid membranes, J. Chem. Soc., Faraday Trans. 

The catalytic activity of H and perfluorinated cation-exchange membrane salts, F-4SK and Nafion, was analyzed during olefin carboxymethylation [196] and condensation of epichlorohydrin with cyclic ketones [197]. The possibility of using these membranes instead of Friedel-Krafts catalysts for synthesizing ethylbenzene and then styrene was cited. The activity of membrane salt species decreased in the sequence Cu >Sn >A1 >Co >Zn. p-Toluic acid was obtained from toluene by the action of the rhodium salt. 

Zouahri, A. and Elmidaoui, A. 1996. Synthesis of perfluorinated cation exchange membranes by preirradiation grafting of acrylic acid onto ethylene-tetrafluoroethylene films. 7 PoZym Sci. 34 1793-1798. 

Typically, the tip of the perfluorinated ionic polymer strip bends toward the anode (in case of cation exchange membranes) under the influence of an electric potential. Also, the appearance of water on the surface of the expansion side and the disappearance of water on the contraction side occurs near the electrodes. 

Y. M. Volfkovich, N. A. Dreiman, O. N. Belyaeva, and I. A. Blinov, Standard-Porosimetry Study of Perfluorinated Cation-Exchange Membranes, Soviet Electrochemistry, 24, 324 (1988). 

The 3-compartment cell is satisfactory for acids that do not decompose at the anode, but it would be unsatisfactory for splitting NaCl, because CI2 would form at the anode. To alleviate this problem a cation-exchange membrane could be added next to the anode, and H2SO4 could be circulated as the anolyte. The additional membrane could be a perfluorinated membrane that resists oxidation at the anode, and that addition would improve the chemical stability of the cell. 

Water Uptake. The physical and transport properties of perfluorinated polymeric cation-exchange membranes are greatly influenced by the amount of water and electrolyte uptake, which depends on the polymer EW, the nature of the electrolyte, and the pretreatment procedures. The water uptake increases [41] with increasing temperature, as shown in Fig. 4.8.3. 

Holze, R. and Ahn, J. (1992) Advances in the use of perfluorinated cation exchange membranes in integrated water electrolysis and hydrogen/oxygen fuel cell systems./. Membr. Sd., 73, 87-97. 

The upper part of Fig. 2 shows the molecular structure of the perfluorinated Nafion cation exchange membranes with sulfonic acid as well as with carboxylic acid groups as fixed ions. These are covalently bonded at the end of side chains of the PTFE (polytetrafiuoroethylene) polymer backbone. The polymer has excellent chemical and thermal stability, similar to PTFE [9]. 

Surowiec J, Bogoczek R. Studies on the thermal stability of the perfluorinated cation-exchange membrane nafion-417. J Therm Anal Calorim. 1998 33 1097-102. 

DLFCs can use a cation exchange membrane (CEM) or an anion exchange membrane as an electrolyte, the principles of which are shown in Fig. 6.1a and b. DLFCs that use acidic and alkaline reaction media correspond to the CEM and AEM types, respectively. Perfluorinated sulfonic acid membranes, that is, Nafion , are typical CEMs, in which a countercation of sulfonate can be exchanged for another... 

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