Perfluorinated Nafion ’ membranes

Among the various types of fuel cells listed in the preceding, the PEMFC, which uses a sheet of polymer membrane as the solid electrolyte, is the subject of this book. The solid polymer electrolyte membrane is the most important constituent of these fuel cells that allows protons, but not electrons, to pass through. These fuel cells currently use perfluorinated Nafion membranes (Du Pont), which exhibit a number of shortcomings to optimum efficiency. Also, the polymer membranes represent approximately 30% of the material cost of the fuel cell. The membrane in... 

Du Font s Nafion membranes have been the first truly successful membranes in these applications. They are made of a perfluorinated and sulfonated polyal-kylene ionomer (-CFj- or >CF- throughout). Such a composition leads to... 

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

Nafion membrane materials were originally developed for electrochemical applications.155 The acid strength (—Ho) is estimated to be between 11 and 13156 that is, the acidity of Nafion is comparable to that of concentrated sulfuric acid. In these systems, the sulfonic acid group is attached to a CF2 or CF group in a perfluorinated backbone. 

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. 

Figure 3.3.5 (A) Chemical structure of sulfonated perfluorinated polyethylene (Nafion ). (B) Schematic illustration of the microscopic structure of hydrated Nafion membrane perfluorinated polyethylene backbone chains form spherical hydrophobic clusters. Sulfonic end groups interface with water-filled channels and mediate the migration and diffusion of protons. The channels are filled with water and hydronium ions. Figure adapted from [4].

The most successful solid protonic membrane for fuel cell use is NAFION membrane (saturated with aqueous acid solution) used in kW-size fuel cells in the US Gemini space program up to 150 °C. NAFION is a perfluorinated polymer with sulfonic acid groups. 

Nafion-117, a perfluorinated polymer membrane, has been studied for use in a direct separation of H2O from HI. Its ability to remove H2O from a flow stream of HI, at 125°C has been successfully demonstrated (figure 4.22). The permeability or flux through the Naflon-117 membrane is temperature dependent and inversely correlated to the water concentration. ... 

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.)...

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. 

Wide-Angle X-Ray Diffraction Studies. An interesting feature of the Nafion membranes is its ability to crystallize in spite of the long side-chain groups. The presence of crystallites in perfluorinated... 

The effect of NaOH concentration on the ion transport and rheological properties of the Nafion ion exchange membranes may be attributable to some variation in the ionic domain structure in the presence of NaOH. Therefore, it is extremely Important to understand the ionic domain structure under these conditions. The anomalous behavior of Na" " ion transport as a function of NaOH concentration is seen more frequently in bilayer Nafion membranes in which one layer is treated with diamine and also in perfluorinated carboxylic ion exchange membranes. Several mechanisms have been proposed to explain their ion transport results including water absorption, transport of hydroxide ion tunneling, ion pairing mechanisms, etc. (54-56). As the ion transport properties are beyond the scope of this review, no detailed discussion will be presented. 

The preparation of the supported catalytic Fe-clusters have been recently reported by our laboratory for Fe/Nafion membranes [1,2,3] for Fe/Nafion/glass-mats [4,5] micro-encapsulated Fe-alginate beads [6] Fe-amorphous polycrystalline thin film fused copolymers [7] and finally Fe/silica woven fabrics [8]. Experiments were conducted with Nafion perfluorinated cation transfer membrane. Photolysis experiments were carried out by means of a Hanau Suntest Lamp with tunable light intensity equipped with an IR filter to remove infrared radiation. Light... 

Perfluorinated carboxylate membranes were introduced about seven years ago. These membranes can be synthesized by a variety of methods or by various chemical conversions from the Nafion polymer.Composite membranes which contain both sulfonate and carboxylate functional groups have also been produced (see Section IV.l for more details). These carboxylate membranes have been widely employed in the advanced membrane chlor-alkali cells. This major chemical technology is in the process of being revolutionized by the use of these materials, a remarkable accomplishment for such a small group of polymers. ... 

The perfluorinated sulfonic acid (Nafion) membranes have found a great variety of electrochemical applications. These include the SPE water electrolyzers, alkaline water elec-trolyzers, " hydrochloric acid electrolyzers, Na2S04... 

Hopfinger and Mauritz and Hopfinger also presented a general formalism to describe the structural organization of Nafion membranes under different physicochemical conditions. It was assumed that ionic clustering does not exist in the dry polymer. This assumption is applicable to the perfluorinated carboxylic acid polymer" but not the perfluorosulfonate polymers." They consider the balance in energy between the elastic deformation of the matrix and the various molecular interactions that exist in the polymer. 

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. 

The perfluorinated carboxylic acid membrane exhibits a higher resistance than Nafion membranes in SPE water electrolyzers. This is primarily due to small membrane swelling and slight dissociation of the carboxylic acid group in water or acid electrolyte with a pH <... 

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. 

A model for ionic clustering in "Nafion" (registered trademark of E. I. du Pont de Nemours and Co.) perfluorinated membranes is proposed. This "cluster-network" model suggests that the solvent and ion exchange sites phase separate from the fluorocarbon matrix into inverted micellar structures which are connected by short narrow channels. This model is used to describe ion transport and hydroxyl rejection in "Nafion" membrane products. We also demonstrate that transport processes occurring in "Nafion" are well described by percolation theory. 

The Nafion membrane has been used as a separator in both the hydrogen-halogen cell (45,50) and in the zinc-bromine cell (38) because Nafion is a highly stable perfluorinated material which is not affected by strong acids and halogen. 

FIGURE 9. Chromatographic separation of alkali metal ions using 1200 EW Nafion at 25 °C. Reprinted with permission from H. L. Yeager, in Perfluorinated Ionomer Membranes (Eds. A. Eisenberg and H. L. Yeager), Chap. 3,... 

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. 

MAJOR APPLICATIONS Nafion is the DuPont trademark for its family of perfluorinated ionomers, that is, resins and membranes. Asahi Chemical Industry Company produces Aciplex and Asahi Glass Company, Ltd., Japan, produces Flemion both are competitive products to Nafion in form and function. These perfluorinated ionomers are used in a variety of applications, the largest of which are as an ion exchange resin and in membrane separators in the commercial electrolysis of brine to produce caustic and chlorine. Nafion membranes are also being used in the development of fuel cells and as heterogeneous super acid catalysts in supported, cubed, or powdered form. 

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