Perfluorosulfonate ion exchange

All work has been accomplished using perfluorosulfonate ion exchange membranes manufactured by E.l. duPont and sold under the trade name Nafion. Nafion is a copolymer of polytetrafluoroethy-lene (PTFE) and polysulfonylfluoride vinyl ether containing pendent sulfonic acid groups. The sulfonic acid groups are chemically bound to the perfluorocarbon backbone. 

Diffusion coefficients for methanol (concentration range 1.0-5.0 M) in ion-exchange membranes of UV-crosslinked sulfonated poly(bis-3-methoxyphenoxyphosphazene) (134) have been reported to be much smaller than those in Nafion perfluorosulfonic ion-exchange membranes. Application of polyphosphazene-based membranes in methanol-based fuel cells has been reported. ... 

Z. Ogumi, K. Toyama, Z. Takehara and S. Inuta, Diffusion of aniline through perfluorosulfonate ion exchange membrane, J. Membr. Sci., 1992, 65, 205-212. 

Nation is a perfluorosulfonated ion-exchange polymer and has received a great deal of attention as a modifier for polymer-coated electrodes mainly because of its outstanding chemical stability and excellent ionic conductivity. Nafion-coated electrodes have been proved to be effective for the determination of electroactive compounds " . The attractive features of Nafion which are useful for electroanalytical purposes are its preconcentrating ability sof cations and its ionic selectivity. 

Perfluorosulfonic acid polymers, for example, Nafion, or ionic and cross-linked polystyrene derivatives, are the best known examples of ion-exchange membrane materials (see also Section 2.6.4). 

Figure 21. Dependence of membrane conductivity on ion-exchange capacity for perfluorosulfonate and carboxylate membranes, in 35% NaOH, at 90°C. (Ref. 146 reprinted by permission of the publisher, The Electrochemical Society, Inc.)...

In such a form this material is melt fabricable and after hydrolysis is converted to a ion exchange membrane with a perfluorosulfonate group, -S03Na. The sodium counter ion can be exchanged by other metal ion or hydrogen ion. 

Water content in Figure 4 can be expressed as a function of ion exchange capacity and external solution concentration by the following empirical equation, which is similar to that proposed for perfluorosulfonic acid membranes by W.G.F. Grot in 1972 (44). The water content of perfluorocarboxylic acid membrane is much lower than that of perfluorosulfonic acid membrane. 

Figure 7. Electric resistance, ion exchange capacity (meq/g dry resin) and concentration of NaOH, for perfluorosulfonic acid membrane.

Cation, anion, and water transport in ion-exchange membranes have been described by several phenomenological solution-diffusion models and electrokinetic pore-flow theories. Phenomenological models based on irreversible thermodynamics have been applied to cation-exchange membranes, including DuPont s Nafion perfluorosulfonic acid membranes [147, 148]. These models view the membrane as a black box and membrane properties such as ionic fluxes, water transport, and electric potential are related to one another without specifying the membrane structure and molecular-level mechanism for ion and solvent permeation. For a four-component system (one mobile cation, one mobile anion, water, and membrane fixed-charge sites), there are three independent flux equations (for cations, anions, and solvent species) of the form... 

R. Schlogl and F. Helfferich, Theory of exchange membrane potentials, Z. Elektro-chem, 1952, 56, 644-647 N. Ishibashi, T. Seiyama and W Sakai, Electrochemical studies on ion exchangers (Part 10) Mobilities of Ca+ and Cl- in the cation exchange membrane, Denki Kagaku (J. Electrochem. Soc. Jpn.), 1955, 23, 182-186 E.E. Boakye and H.L. Yeager, Water sorption and ionic diffusion in short side chain perfluorosulfonate ionomer membranes, J. Membr. Sci., 1992, 69, 155-167. 

I, 1980, 76, 2558-2574 L.Y. Levy, A. Jenard and H.D. Hurwitz, Hydration and ion-exchange process in carboxylic membranes. Part 1. Infrared spectroscopic investigation of the acid membranes, J. Chem. Soc. Trans. 1, 1982, 78, 29-36 M. Falk, Infrared spectra of perfluorosulfonated polymer and water in perfluorosulfonate polymer, Perfluorinated Ionomer Membranes, ed. A. Eisenberg, H.L. Yeager, ACS Symposium Series, American Chemical Society, Washington DC, 1982, p. 139 C. Heitner-Wirguin and D. Hall, An infrared study of an anion exchange membrane,... 

Naflon (a perfluorosulfonic acid type ion exchange resin, Du Pont) have also been synthesized from CH3Br and CF3Br, respectively, with Nafion-Br in a similar way102 (equation 77). 

An alternative method for the preparation of facilitated transport membranes is the subject of the first paper in this section. Way and Noble (113) report a study of H,S facilitated transport in reactive ion exchange membranes. The use of a perfluorosulfonic acid lEM as a support for organic amine counterions avoids problems of solvent and carrier loss often encountered with ILMs. High carrier loadings of greater than 8 M in the lEMs were attained which helped to account for the high facilitation factors of 26.4 which are observed at low partial pressures. An analytical model predicted facilitation factors in excellent agreement with the experimental data. Separation factors for HjS over CH., of 792 to 1200 are reported. Implications of the mathematical model for industrial applications are also discussed. 

Du Font s Nafione membranes are perfluorinated ion exchange resins fabricated into reinforced and unreinforced membranes. Nafione Membrane 425, which we used in some of our tests, is a homogeneous film 5 mils thick of 1200 equivalent weight perfluorosulfonic acid resin laminated with T-12 fabric of Teflone TFE resin to make a 50 mil membrane. Properties and characteristics of Nafione membranes are given in reference 2. 

A characteristic feature of alternative membranes is that they nearly always exhibit a lower proton conductivity compared to Nafion for a similar ion content. The ionic conductivity can be improved by increasing the ion-exchange capacity (lEC) of the constituent polymer(s) but mechanical strength is frequently sacrificed firstly, in the dehydrated state because of the high ionic content, and secondly in the hydrated state due to excessive swelling [43]. Moreover, virtually all alternatives to perfluorosulfonic acid... 

Comparison of the conductivity of two perfluorosulfonic acid membranes in Fig. 4.8.11 shows that the conductivity increases [59] with increasing water content and also increasing ion-exchange capacity or concentration of fixed ionic groups. The Dow membrane has a higher EC and a shorter side-chain length than Nafion . 

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