Perfluorosulphonic acid membranes

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). 

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. 

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. 

Key words computational chemistry, molecular dynamics, poly(ethylene oxide), Li mobility mechanism, perfluorosulphonic acid membranes. Nation, proton transport, battery, fuel cell. 

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

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. 

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 . 

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. 

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 ... 

Abstract This chapter describes the use of molecular dynamics (MD) simulations to understand structure and transport processes in polymer electrolytes for energy storage and conversion applications. For batteries, the polymer electrolytes studied with MD techniques have generally been of poly(ethylene oxide) (PEO)-type, while the fuel cell polymer electrolytes have been perfluorosulphonic acid (PFSA) membrane materials. The MD methodology, its benefits and its limitations are explained in the chapter, together with a review of some significant MD studies of polymer electrolytes. 

The polymer electrolytes used for low-temperature proton exchange membrane fuel cells (PEMFCs) are fundamentally different from the polymer electrolytes used in batteries. Here, the polymer is a medium for a solvent, normally water, and it is mainly in the solvent that ion transport occurs. The polymer serves several functions, of which the most important is to provide mechanical stability and electrode separation in the fuel cell application. Since the fuel cell needs proton transport from the anode to the cathode, the polymer also contains proton donating groups, often sul-phonic acid (-SO3H). The prototype PEMFC membrane materials have been perfluorosulphonic acids (PFSAs), of which the most established membrane material is Nafion (Fig. 8.8). These consist of hydrophobic teflon -CF2-CF2- backbones, with fluorinated hydrophilic and acidic side-chains for Nafion -0CF2CF(CF3)0CF2Cp2S03H. 

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... 

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