Nafion/polytetrafluoroethylene

Xing, D., He, G., Hou, Z., Ming, P, and Song, S. (2013) Properties and morphology of Nafion/polytetrafluoroethylene composite membrane fabricated by a solution-spray process, Int. J. Hydrogen Energ., 38, 8400-8408. 

Nafion (17) is a perfluorinated polymer related to teflon (polytetrafluoroethylene). An electrode is conveniently coated by allowing an ethanolic solution of the polymer to evaporate. The film produced is stable, rather more so in fact than other polymer films, e.g. polyvinylpyridine (see Section 57.3.2.2). At the microscopic level the polymer separates into two phases, the bulk polymer and the lower density ionic cluster phase. Diffusion of ions can occur quite freely for example, the diffusion coefficient of Na+ in Nafion (MW 1200) is only slightly less than in water.44... 

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. 

The mechanical Y peak occurs at approximately -100 C at about IHz for the acid which corresponds to the Y peak found in PTFE (polytetrafluoroethylene) and the Nafion salts in the same temperature region. According to Yeo and Eisenberg (46), the y peak position remains unaffected by the variations in counterion size and degree of neutralization. 

The most diffused material for membranes is based on co-polymers of tetrafluoroethylene (TEE) with perfluorosulfonate monomers. The resulting co-polymer is constituted by polytetrafluoroethylene polymeric chain (PTFE, commercially known as Teflon) in which some fluorine atoms are substituted by sulfonated side chains. The monomer perfluoro-sulfonyfluoride ethyl-propyl-vinyl ether is used in membranes commercialized by Dupont with the registered trademark Nafion (Fig. 3.2), which is the most well-known material used as electrolyte in PEM fuel cells. 

In Table I are summarized the results obtained for different sample Nafion perfluoroethylene membrane (NAF), acrylic acid irradiation grafted polytetrafluoroethylene (PTFE), sulfonated styrene irradiation grafted fluorinated ethylene propylene copolymer (RAI), and sulfonated polysulfone (SPS). 

The most intense absorptions in the spectrum are those due to the fluorocarbon main chain. The spectrum of Nafion therefore strongly resembles that of polytetrafluoroethylene (PTFE ... 

Polymers with a polytetrafluoroethylene (PTFE) backbone and pendant perfluorosulfonate or perfluor-ocarboxylate groups have become commercially important materials, although they are expensive. The sulfonates were introduced as Nafion by Du Pont in the early 1970s and the carboxylates as Flemion by Asahi Glass in 1978. They are made by free-radical copolymerization of tetrafluoroethylene and perfluorovinyl monomers giving precursor copolymers, (XXII) and (XXIII), which can be post-functionalized by hydrolysis to generate sulfonic and carboxylic acid groups. The perfluorovinyl... 

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. 

Figure 10.3.18 illustrates the structure of the electrode/membrane system where the membrane (e.g., Nafion 900 Series) is reinforced by a nonconductive polytetrafluoroethylene (PTFE) fabric. Note that the current flux deforms around the PTFE reinforcement, leading to an increase in the voltage drop through the membrane. Therefore, the arrangement of the PTFE reinforcement must be optimized for mechanical strength and voltage drop. 

Basically, there are two methods to form the MEA of a PEM fuel cell. One alternative is using appropriate techniques to add the carbon-supported catalyst to a porous and conductive material, such as carbon cloth or carbon paper, called a gas diffusion layer (GDL). Normally, polytetrafluoroethylene (PTFE) and Nafion... 

Inappropriate matching of the physicochemical properties of the binder with the carbon material may influence dramatically upon the electroactive area via blocking of the SPE film or simply decreasing the electroactive surface area. Another point to be considered is the cost of the binders. For example, fluoropolymers such Nafion, polyvinylidene fluoride (PVDF) and polytetrafluoroethylene (PTFE) are commonly used as binder in electrode preparation in the field of lithium-ion batteries or fuel cells. However, the curing process has to be soft in most cases... 

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. 

In the first step, the F.-C. reaction is catalyzed by Nafion H, a broadly used industrial catalyst based on sulfonated polytetrafluoroethylene, a strong protic acid bound to polymeric support. Selective acylation in the para-position is controlled by the bulky ferf-butyl group. 

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). 

ME A series 2-7 were prepared to test SPE performance as a function of hydrophobic properties of the MEA. GDL with different polytetrafluoroethylene (P IPE) loading were used with MEA 2-A, and hydrophilic GDL were used with MEA 6 and 7.While pressing the first series of MEA, the carbon cloth adhered to the PTEE liner on the hot press. This was attributed to the extra layer of Nafion placed on the carbon cloth before pressing. To avoid this problem, the Nafion content in subsequent inks was increased to 0.8g. 

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