Perfluorinated sulfonic acid PFSA

Nafion, a perfluorinated sulfonic acid (PFSA) polymer electrolyte developed and produced by the E. I. Dupont Company, has been extensively studied as a fuel cell membrane. Despite its age, it remains the industry standard membrane because of its relatively high proton conductivity, toughness and quick start capabilities. Attempts to build upon the strengths of Nafion have resulted in a class of PFSA polymer electrolytes, including the short-side-chain (SSC) PFSA polymer electrolyte, originally synthesized by Dow and now produced by Solvay Solexis. Stracturally, PFSA polymer... 

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 most frequently used polymer membranes are proton conducting polymers of the perfluorinated sulfonic acid (PFSA) type, the most well known being Nafion from DuPont. A review on the state of understanding has been given by Mauritz and Moore [10]. A section of Nations chemical formula is given in Fig. 14.4. 

Of all of the potential materials which can meet these requirements, only perfluorinated sulfonic acid (PFSA) membranes have so far found widespread acceptance in low-temperature (65-80 °C) hydrogen fuel-cell applications. The most widely used form of these PFSA-type membranes is produced by DuPont under the trade name Nation. Other PFSA types of PEMs are sold by Asahi Chemical (Flemion and Aciplex) and Solvay Solexis (Hyflon). Gore sells its reinforced PFSA membrane as an integral part of an MEA as its PRIMEA series. Likewise, 3M also sells its own PFSA-type membrane as part of its MEA. One of the best known and basic properties of the current PEMs is that their proton conductivity is a strong function of their level of hydration. 

It has been suggested that instead of membranes made from perfluorinated sulfonic acids (PFSAs), membranes be used made from other, heat-resistant polymers containing the sulfonic acid group. One could think here of sulfonated polyimides (SPIs) and sulfonated polyether ether ketones (SPEEKs). Membranes made from such polymers have a sufficiently high protonic conductivity at elevated temperatures but are less sensitive to lower humidities and water loss. 

At present, in almost all work on fuel cells of the PEMFC and DMFC type, Nafion-type proton-conducting membranes based on perfluorinated sulfonic acids (PFSAs) have been used (Figure 13.1). Some properties of these membranes had been reported in Section 3.1.1. They have a high chemical stability and fully adequate protonic conductivity. Fuel cells built with such membranes offer rather good performance and relatively long life. It is not an exaggeration to say that the broad development of fuel cells and the general interest displayed in fuel cells today would have been impossible without the development and availability of these membranes. 

Before discussing membrane chemical degradation in detail, the factors governing the degradation mechanism must be identified. Among three major types of membrane materials, hydrocarbon, partially fluorinated, and perfluorinated ionomers, perfluorinated sulfonic acid (PFSA) is the most widely used membrane material owing to its high chemical stability (Schiraldi 2006). 

Fig. 3 Impact of RH on the chemical degradation rate of perfluorinated sulfonic acid (PFSA) membranes. All tests employed Nafion 112 with 0.4 mg Pt cm per electrode and were conducted with HyO at 95 C, 300kPa, and 525 seem gas flow (Liu et al. 2006)...

Importantly, for elevated temperature PEM fuel cell operation, the HPAs may be structurally stable to >600°C, although under anhydrous conditions their stability may be limited to 200°C, and they incorporate some water molecules and protons up to >300°C depending on the system [15]. Because of their structural diversity, these materials are particularly suitable for incorporation into a wide variety of membrane materials for which they can be specifically tailored. They have been studied in four composite systems HPAs infused into perfluorinated sulfonic acid (PFSA) polymers such as Nafion [16,17], HPA cast in inert matrices such as poly (vinyl alcohol) (PVA) [18], HPA immobilized in polymer/silicate nanocomposites via sol-gel methods [19], and HPA directly incorporated into polymer films via functionalization to monomers [20]. Here after a discussion of fundamental studies, we review the various HPA-based materials used for fuel cells. 

Perfluorinated sulfonic acid containing polymer (PFSA)... 

Fig. 17.2 Structures of some perfluorinated sulfonic acid containing polymers (PFSAs). Polymer 1 is available from DuPont (Nafion ), Asahi Glass (Flemion ), and others Polymer 2 is the short-side-chain ionomer developed at Dow, currently available from Solvacore and Polymer 3 is the ionomer available from 3M Company...

For instance, the Dow experimental membrane and the recently introduced Hyflon Ion E83 membrane by Solvay-Solexis are "short side chain" (SSC) fluoropolymers, which exhibit increased water uptake, significantly enhanced proton conductivity, and better stability at T > 100°C due to higher glass transition temperatures in comparison to Nafion. The membrane morphology and the basic mechanisms of proton transport are, however, similar for all PFSA ionomers mentioned. The base polymer of Nation, depicted schematically in Figure 6.3, consists of a copolymer of tetrafluoro-ethylene, forming the backbone, and randomly attached pendant side chains of perfluorinated vinyl ethers, terminated by sulfonic acid head groups. °... 

Perfluorinated membranes are still regarded as the best in the class for PEM fuel cell applications. - These materials are commercially available in various forms from companies such as DuPont, Asahi Glass, Asahi Chemical, 3M, Gore, and Sol-vay. Perfluorosulfonic acid (PFSA) polymers all consist of a perfluorocarbon backbone that has side chains terminated with sulfonated groups. 

Perfluorinated ionomers functionalized with sulfonic acid groups (i.e., Nafion and Dow s PFSA) have been extensively studied and are used in a variety of applications [9]. The hydrophobic fluorocarbon backbone... 

Figure 1 describes the chemical structure of PFSA polymers, which was widely used as perfluorinated electrolytes for PEFCs. The structure is based on copolymers of tetrafluor-oethylene (TFE) and perfluorovinyl ether monomers with a side chain of sulfonic acid group. While many acid groups have been examined in... 

PFSA-based membranes have a combination structure built up from hydrophilic segments associated with the sulfonic acid groups and a backbone of hydrophobic, perfluorinated hydrocarbon within which the sulfonic acid groups are distributed uniformly. The branched network of interconnected hydrophilic segments provides the protonic conduction path of the membrane but at the same time offers methanol molecules a chance to diffuse from the anodic to the cathodic side of a DMFC. Therefore, anything done to reduce methanol crossover will cause some decrease in protonic conductivity. 

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