Perfluorosulfonic acid ion-exchange membranes

For facilitated transport of ethylene through silver ion-containing perfluorosulfonic acid ion-exchange membranes, high ethylene/ethane separation factors are obtained by Way and coworkers in Chapter 19. Ethylene of greater than 99 percent purity is obtained from a 50 50 mixture of ethylene and ethane at ambient temperature with feed and permeate pressures of one atmosphere. 

Separation of Ethylene from Ethane Using Perfluorosulfonic Acid Ion-Exchange Membranes... 

Facilitated transport of ethylene through Ag -containing perfluorosulfonic add ion-exchange membranes results in high separation factors for ethylene over ethane. Ethylene of higher than 99 percent purity was obtained from a 50 50 mbcture of ethylene and ethane at 25 T and feed and permeate pressures of 1 atmosphere. An ethylene permeability of over 2000 Barrer was obtained. Two different types of perfluorosulfonic acid membranes were studied to determine the importance of ionic site dendty and water content on membrane performance. The effect of temperature on the transport mechanism was also studied over the range of 5-35 ""C. In addition, the transport data obtained at high pressure show carrier-saturation and membrane compaction phenomena. 

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. 

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). 

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... 

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. 

Some vinyl fluoride-based polymers with side chains of perfluorosulfonic acid (the Nation family) are important ion-exchange membrane materials used in practice for electrolysis of NaCl and in certain fuel cells. They show a proton conductivity of 0.01 S cm- at room temperature. However, such fast ionic transport occurs only when they are swollen with water. It is therefore not appropriate to call them solid electrolytes in the tme sense of the word. It was in 1970 that anionic conductivity, though not high, was reported for crown ether complexes such as dibenzo-18-crown-6 KSCN, in which cations are trapped by the ligand. " A few years later much higher cationic (instead of anionic) conduction was found in complexes of a chain-like polyether such as PEO or PPO with alkaline salts here, PEO stands for poly(ethyleneoxide), (CHjCHj-O), and PPO for poly(propyleneoxide)."2>"3 These were the flrst examples of tme polymer solid electrolytes and were followed by a great number of studies. Polymeric electrolytes are advantageous in practice because they are easily processed and formed into flexible Aims. 

This review will outline the materials requirements for advanced alternative proton exchange membranes for fuel cells, assess recent progress in this area, and provide directions for the development of next-generation materials. The focus will be on the synthesis of polymeric materials that have attached ion conducting groups. State-of-the-art Nation and its commercially available perfluorosulfonic acid relatives will initially be discussed. Other chain-growth co-... 

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.

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,... 

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 . 

The diaphragm may be replaced by a cation exchange membrane which is suitable for high concentrated hydrochloric acid like Nafion (DuPont, perfluorosulfonic acid polymer, PFSA, see entry Chlorine and Caustic Technology, Membrane Cell Process ). This membrane has almost no usual porosity and is nearly exclusively permeable for ions including a hydration shell of some water molecules. Thus, product quality is significantly increased, process operation can be simplified, and cell voltage is reduced by about 0.3 V [1, 6]. However, the mechanical durability... 

The standard electrolyte material in PEFCs belongs to the fully fluorinated Teflon -based family similar to that produced by E.I. DuPont de Nemours for space application in the mid-1960s. The membrane is characterized by its equivalent weight (inversely proportional to the ion exchange capacity). A typical equivalent weight range is 800 to 1100 milliequivalents per dry gram of polymer. The type used most often in the past was a melt-extruded membrane manufactured by DuPont and sold under the label Nafion No. 117. The perfluorosulfonic acid family of... 

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