Polytetrafluoroethylene ionomers

Polytetrafluoroethylene ionomers, properties of, 14 475 76 Poly(tetrafluoroethylene-co-hexafluoropropylene) films, 23 720... 

See also Teflon PTFE-based ionomers, 14 481 PTFE dispersion, 18 288. See also Polytetrafluoroethylene (PTFE)... 

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. 

As shown in Figure 1.6, the optimized cathode and anode structures in PEMFCs include carbon paper or carbon cloth coated with a carbon-PTFE (polytetrafluoroethylene) sub-layer (or diffusion layer) and a catalyst layer containing carbon-supported catalyst and Nafion ionomer. The two electrodes are hot pressed with the Nafion membrane in between to form a membrane electrode assembly (MEA), which is the core of the PEMFC. Other methods, such as catalyst coated membranes, have also been used in the preparation of MEAs. 

Although there have been various membranes used, none is more researched or seen as the standard than the Nafion family by E. I. du Pont de Nemours and Company. Like the other membranes used, the general structure of Nafion is a copolymer between polytetrafluoroethylene and polysulfonyl fluoride vinyl ether. These perfluorinated sulfonic acid (PFSA) ionomers exhibit many interesting properties such as a high conductivity, prodigious water uptake, and high anion exclusion to name a few. Nafion is the main membrane studied in this chapter. 

The sol-gel synthesis of titanimn-silicon nanocomposites based on perfluorosulfonate ionomers was reported. Also the preparation of nanosized particles (10-50nm) derived from Ti(OR)4 and Zr(OR)4 within matrices of styrene/ 4-vinylphenol copolymer was performed via the sol-gel method. These approaches can also be used for the surface activation of polymers. For example, hydrofobic perfluorinated polymers such as polytetrafluoroethylene (PTFE) or copolymers of... 

In this chapter, we describe a variety of methodologies for applying multidimensional NMR (mostly 2D- and some 3D-NMR) for the characterization of fluoropolymers. Space limitations preclude a comprehensive survey of the literature. Instead, a few of the primary methodologies are described involving combined use of multidimensional NMR methods for structure elucidation. Then, a selected group of papers were reviewed to illustrate the applications of these methodologies to the characterization of some of the most common classes of fluoropolymers, including homo- and copolymers with poly(vinylidene fluoride), fluorinated polyethers, fluori-nated ionomers, poly(vinyl fluoride) and its copolymers, and polytetrafluoroethylene (PTFE) and its copolymers. 

The catalyst ink usually includes catalyst, carbon powder, binder, and solvent. Sometimes, other additives are added to improve the dispersion of the components and stabilize the catalyst ink. The catalyst either covers the surface of the GDL or directly coats the surface of the membrane (catalyst coated membrane, CCM). The CL usually consists of (1) an ionic conductor such as perfluorosulfonate acid (PFSA) ionomer to provide a passage for protons to be transported in or out, (2) metal catalysts supported on a conducting matrix like carbon, to provide a means for electron conduction, and (3) a water-repelling agent such as polytetrafluoroethylene (PTFE) to provide sufficient porosity for the gaseous reactants to be transferred to catalyzed sites [5, 6]. Every individual factor must be optimized to provide the best overall performance of a CL. 

Nafion ionomer is not the only perfluorosulfonic membrane material to be considered for PEMFCs. There are many other commercial perfluorinated ionomers, such as Asahi Glass (Flemion ), Asahi Kasei (Aciplex ), 3M (3M polymer), and Solvay Solexis (Hyflon ), all of which share some structural similarities with the polytetrafluoroethylene-based Nafion but use different perfluorinated vinyl ethers, as shown in Table 4.1. 

The biggest application for fiuorinated ionomers currently is as unreinforced membranes for fuel cells, or polytetrafluoroethylene (PTFE) fiber-reinforced composite membranes for electrolytic baths. The membranes can be fabricated by the extrusion method or the solution-cast method. 

Sulfonated polystyrene grafted onto polytetrafluoroethylene. Perfluorosulfonic acid ionomer. 

The anode and cathode CLs are the key components in PEM fuel cells because both the ORR and the HOR take place within them to yield fuel cell performance. They are thin layers (-10-100 (im, usually <50 (im), mainly composed of catalyst powders, proton conductive ionomer (normally Nafion ionomer), and polytetrafluoroethylene (PTFE). 

Polymer electrolyte fuel cells (PEFCs) are unique in that they are the only variety of low-temperature fuel cell to utilize a solid electrolyte. The most common polymer electrolyte used in PEFCs is Nafion , produced by DuPont, a perfluorosulfonic ionomer that is commercially available in films of thicknesses varying from 25 to 175 pm. This material has a fluorocarbon polytetrafluoroethylene (PTFE)-kbone with side chains ending in pendant sulfonic acid moieties. The presence of sulfonic acid promotes water uptake, enabling the membrane to be a good protonic conductor, and thereby facilitating proton transport through the cell. This chapter reviews PEFC development, structure, and properties and presents an overview of PEM technology to date. 

In a solid polymer electrolyte, such as used in the PEFC, ion mobility is a result of an electrolyte solution integrated into an inert polymer matrix. Early electrolyte membranes developed for the United States space program consisted of treated hydrocarbons, which resulted in poor longevity due to the relatively weaker hydrocarbon bonds [3]. Most modem solid electrolytes are perflourinated ionomers with a fixed side chain of sulphonic acid bonded covalently to the inert, but chemically stable, polymer polytetrafluoroethylene (FIFE) stracture. As a result, the membrane consists of two very different sub-stractures 1) a hydrophilic and ionically conductive phase related to the bonded sulphonic acid groups... 

Yang, L., Li, H., Ai, F., Chen, X., Tang, J., Zhu, Y, Wang, C., Yuan, W.Z., Zhang, Y, and Zhang, Y. (2013) A new method to prepare high performance fluorinated sulfonic acid ionomer/porous expanded polytetrafluoroethylene composite membranes based on perfluorinated suHonyl fluoride polymer solution, J. Power Sourc., 243, 392-396. 

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