Membrane perfluorosulphonic

By electropolymerization of pyrrole in solvents containing polyelectrolytes such as potassium polyvinylsulfate, it is possible to prepare films of polypyrrole with polymeric counterions which have good conductivity (1-10 S cm-1) and strength (49 MPa) 303 304,305). Such a material could be used reversibly to absorb cations in an ion exchange system. Pyrrole has also been electrochemically polymerized in microporous polytetrafluoroethylene membranes (Gore-tex), impregnated with a perfluorosulphonate ionomer 3061. 

Yeo, R.S., Dual cohesive energy densities of perfluorosulphonic acid (Nafion) membrane, Polymer, 21, 432, 1980. 

Gebel, G., Aldebert, P., and Pineri, M., Swelling study of perfluorosulphonated ionomer membranes, Polymer, 34, 333, 1993. 

Dupont de Nemours) or analogous perfluorosulphonic acid membranes have been the dominating choice. The structure of the repeat structure of the polymer fluorocarbon backbone and a side chain with sulphonic acid ends upon which Nafion is based is shown in Fig. 3.42 (the commercial product is sold with various thicknesses and dimensions denoted by a number code such as "Nafion-117", related to non-SI units). The membranes should have high protonic conductivity, low gas permeability and, of course, a suitable mechanical strength and low temperature sensitivity. 

T. A. Zawodzinski Jr., S. Gottesfeld, S. Shoichet, and T. J. McCarthy, The Contact Angle between Water and the Surface of Perfluorosulphonic Acid Membranes, Journal of Applied Electrochemistry, 23, 86 (1993). 

For chlor-alkali electrolysers, a biiayer perfluorinated membrane is produced by Du Pont de Nemours. One layer is perfluorosulphonic, the other is perfl uorocarboxylic. 

In the presence of protons, perfluoroacetic membranes are not ionized. Therefore, only perfluorosulphonic membranes are proton conductors having high electrical conductivity combined with a good mechanical behaviour, and a strong chemical resistance even in contact with oxidizing agents. 

Perfluorosulphonic Nation membrane separators are used in direct contact with electrodes as solid polymer electrolytes (SPE) in fuel cells . In this case, the membrane is both the electrolyte and the separator. The use of perfluorosulphonic membranes as SPE started 30 years ago with the US space program Gemini and the realization of low temperature H2/O2 SPE fuel cells. Since then, the feasibility of operating the SPE fiiel cells on hydrogen/halogen couples has been demonstrated. In addition, the introduction of perfluorinated membranes for use in water and brine electrolysis and more recently in organic synthesis has taken place . 

Among the cation permeable perfluorinated membranes previously mentioned, only the perfluorosulphonic membranes are proton conductors. Their synthesis is described in this chapter . ... 

Fig. 19.1. Cluster network model for Nation perfluorosulphonic membrane reprinted by permission of Elsevier Science Publishers, B.V.

The transport properties of perfluorosulphonic membranes are largely influenced by the water content of the membrane, particularly when the membrane is in the acid form. In the dry state, the Nafion membrane behaves like an insulator but, when hydrated, the membrane becomes conductive as a function of the water content. Yeo established that the minimum threshold corresponds to about six molecules of water per sulphonic site, whereas Pourcelly et al. estimated about seven molecules. Randin has shown that membranes with six molecules of water per sulphonic site have suflicient conductivity for use as a semi-solid proton... 

The mobility of protons in Nafion perfluorosulphonic membranes is strongly assoeiated with the water content of the membrane phase. This mobility is directly related to the molality of fixed charged sulphonic sites. By IR measurements, Jenard has observed the protonated form of sulphonic sites for low water contents in these membranes. Excepting these extreme conditions, the proton transfer process in perfluorosulphonic membranes must take account of the ionic composition of the membrane phase. Two cases must be examined on the one hand, the membrane eontains counter-ions, lone protons or protons with other eations, and on the other hand, the membrane contains sorbed acids. In the first case, the conduction is due only to the counter-ions and proton motion is considered to involve the jump from one site located in the... 

Nafion is the leader of perfluorosulphonic proton conductors. Produced by Dupont de Nemours, it is commercially available in different forms homogeneous or reinforced membranes, powders, tubes and solutions. From solutions of perfluorosulphonic acid or salt in organic solvents, gels and films can also be made. From Nafion , many kinds of composite material may also be prepared either by including small, dispersed particles inside the polymeric phase or by using it to coat solid electronic conductors. The adhesive power of this material is important when it is maintained in contact with a wide variety of supports. 

Perfluorosulphonic membranes have also been produced by grafting, under irradiation, fluorinated monomers onto perfluorinated, preformed films . A composite perfluorinated proton conductor can also be prepared from porous polyethylene sheets impregnated with a Nafion organic solution . 

The first applications of Nafion membranes in SPE technology were in fuel cells for space applications, which have been developed since the end of the 1960s. The perfluorosulphonic polymer is used as a proton conductor which provides to the cathode the protons that have been generated electrochemically at the anode. Perfluorosulphonic SPE is particularly well-suited to this application ". The principle of the H2"C>2 fuel cell is shown schematically in Fig. 32.3. At the anode, gaseous hydrogen is reduced following the electrochemical reaction... 

The transniembrane transport of sodium is coupled with co-electro-osmotic transport of water which dilutes the caustic soda formed. Perfluorocarboxylic membranes can be substituted by perfluorosulphonic membranes for reducing the water transport. 

Ion exchange membranes are widely used in electrodialysis and other membrane separation techniques. Perfluorosulphonic membranes are much less used in this area because of their high cost. For separation techniques, less expensive membranes may be used because the physical conditions and the chemical environment are less drastic than in the chlor-alkali electrolysis or the various SPE applications. 

Electrodes coated with a perfluorosulphonic acid Nafion membrane can be used to inject protons into non-aqueous media and enable one to exercise additional control on the course of an electro-organic process in which the proton transfer reaction plays an important role . 

In the protonic form, perfluorosulphonic membranes are proton conductors which can be used under very drastic chemical conditions. Their chemical inertness makes them suitable for electrochemical applications where they play a role which cannot be obtained with other proton conducting materials. The thermal and chemical stabilities are combined with the ability to bond to electronic conductors or to be doped with dispersed electrocatalytic materials. 

Possibly one of the most complex deliberately designed of all polymer products is the family of membranes described by Seko for the selective passage of sodium (Na+) ions and repulsion of hydroxide ions in a chlorine cell. These perfluorocarboxylic acid membranes are claimed to represent an improvement on the Dupont Nafion class of perfluorosulphonic membranes designed for the same purpose, in that CF/OH" segregation is practically complete. Industrially, the implications for chlorine and caustic soda technology are profound. 

Zawodzinski TA, Gottesfeld S, Shoichet S, McCarthy TJ. The contact angle between water and the surface of perfluorosulphonic acid membranes. J Appl Electrochem 1993 23 86-8. 

Pourcelly, G., Oikonomou, A., Gavach, C., Hurwitz, H.D. 1990. Influence of the water content on the kinetics of counter-ion transport in perfluorosulphonic membranes. Journal of Electroanalytical Chemistry, 287,43-59. 

Ren X. and Gottesfeld S. (2001) Electro-osmotic drag of water in a poly(perfluorosulphonic acid) membrane . Journal of the Electrochemical Society, 148(1), A87-A93. 

The cell design employed (Fig. 9.13) is based on a simple parallel-plate design. The anodes and cathodes are divided by a perfluorosulphonic acid exchange membrane which prevents an otherwise drastic loss of current efficiency due to the parasitic redox shuttle ... 

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