Perfluorosulfonic ionic polymer

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). 

Solid Polymer Electrolyte Fuel Cell Here, there is no apparent liquid solution, or high-temperature ionic conductor. The usual ionic solution between the electrodes is replaced by a well-humidified membrane made of a perfluorosulfonic acid polymer that conducts protons. 

As a proton-conducting polymer, Nafion (a perfluorosulfonic acid polymer) has high ionic conductivity and has also been investigated as an electrolyte for different types of solid-state ESs [852-858]. It has been found that high scan rates could be achieved for ESs with Nafion electrolytes [854,855,859]. 

Ionic polymers usually used for the IPMC are perfluorosulfonic acid or perfulorocar-boxylic acid polymers, of which the typical chemical structures are shown in Figure 5.2 [10]. Commercially available products of thin films made from perfluorosulfonie acid can be obtained from E.I. Dupont de Nemours Co. (Nafion). Several other companies supply similar compounds. Asahi Glass Co. produces perfluorocarboxylic acid type (Hemion). 

It must be noted that widespread electrochemical processes and deviees use poly (perfluorosulfonic acid) ionic polymers. These materials exhibit [1-20] good chemi-eal stability, remarkable mechanical strength, good thermal stability and high elec-trieal eonduetivity when sufficiently hydrated and made into a nanoeomposite with a eonduetive phase such as metals, conductive polymers or graphite. As described elsewhere [7], a number of physical models have been developed to understand the meehanisms of water and ion transport in ionic polymers and membranes. Morphologieal features influence transport of ions in ionie polymers. These features... 

An adequate structure of polymer molecules promotes the advantageous phase separation into hydrophobic and hydrophilic domains upon water uptake. The most notable class of membranes based on this principle are the perfluorosulfonic acid ionomers (PFSI), Nafion [26] and similar membranes [27]. In these membranes, perfluorosulfonate side chains, terminated with hydrophilic —SO3H groups, are attached to a hydrophobic fluorocarbon backbone. The tendency of ionic groups to aggregate into ion clusters due to the amphiphilic nature of the ionomer leads to the formation of basic aqueous units. At sufficient humidity these units first get connected by narrow channels and then may even fuse to provide continuous aqueous pathways [28]. 

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. 

There have been Mossbauer studies of Nafion membranes by several groups (J —6), besides some work on other ionomers (7,8) and a body of results on polymers where iron or tin is introduced into the polymeric matrix as a probe impurity (8). This chapter will concentrate on the information that has been extracted from the Mossbauer studies of Nafion membranes, and their variations as a function of parameters such as temperature, applied magnetic field, water content and chemical treatment. The results will be discussed in terms of the information they provide about the structure of the ionic phase in perfluorosulfonate materials. 

T.D. Gierke, G.E. Munn and EC. Wilson, Morphology of Nafion perfluorosulfonated membrane products, as determined by wide- and small-angle X-ray studies, J. Polym. Sci., Polym. Phys. Ed., 1981, 19, 1687-1704 W.Y. Hsu and T.D. Gierke, Elastic theory for ionic clustering in perfluorinated ionomers, Macromolecules, 1982, 15, 101-105. 

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... 

While a munber of alternative polymer membranes have been developed. Nation is still considered the benchmark of proton conducting polymer membranes, and has the largest body of research hterature devoted to its study. Alternative polymer membranes are almost invariably compared to Nation . Nation is a free radical initiated copolymer consisting of crystaUiz-able, hydrophobic tetrafluoroethylene and a perfluorinated vinyl ether terminated by perfluorosulfonic acid. Nation 117 possesses an equivalent weight of 1100 (EW = mass of dry ionized polymer (g) in the protonic acid form that would neutralize one equivalent of base). Thus, there are 13 perfluoro-methylene groups (-CF2-) ( = 6.5) between pendent ionic side chains. 

Many experimental techniques have been used to examine the detailed structure of perfluorinated polymeric membranes. These include transmission electron microscopy [23], small angle X-ray scattering [24], Infra Red spectroscopy [25,26], neutron diffraction [27], Nuclear Magnetic Resonance [26,28], mechanical and dielectric relaxation [25,29], X-ray diffraction, and transport measurements. All these methods show convincing evidence for the existence of two phases in the perfluorosulfonate and perfluorocarboxylate polymers. One phase has crystallinity and a structure close to that of polytetrafluoroethylene (PTFE), and the other is an aqueous phase containing ionic groups. 

In perfluorinated ionomers, a PTFE-based polymeric backbone offers chemical stability from the radical species or acid-base, which causes hydrolytic degradation of the polymer chain. Ionic conductivity is provided by pendant acidic moiety in carboxylate or sulfonate form. There are some reports on perfluorinated carboxylic acid (PFCA) materials, most of which are derived from Nafion [26-29]. However, PFCA is not suitable for fuel cell application due to its low proton conductivity. Perfluorosulfonic acid (PFSA) is the most favored choice among not only perfluorinated membranes but all other ionomers in fuel cell applications. Sulfonic acid form of Nafion is a representative PFSA and thus has been intensively studied since 1960s. Reported chemical structure of Nafion membrane is given in Fig. 13.8. 

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. 

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