Perfluorinated PEM

Perfluorinated PEMs are based on random copolymers derived from tefra-fluoroethylene and a PESA-based monomer. A range of different structures has been generated and is shown in Eigure 3.16. 

Nation membranes (EW 1,100 g/mol) (la) are the most widely used and studied of all the perfluorinated PEMs. By comparison, little information is publicly available on structure-property relationships for non-Nation... 

Perfluorinated PEMs with sulfonic acid (la = Nafion, Flemion lb = Aciplex 2a = DOW, Hyflon-ion 2b = 3M, 2c = Asahi Kasei 3 = Asahi Glass) and b)s[(perfluoroalkyl sulfonyl] (4) groups for proton conduction. 

Schiraldi, D., Zhou, C. and Zawodzinski, T. (2006) Model Studies of Perfluorinated PEM Membrane Degradation, 232nd ACS Meeting, San Francisco, CA. 

Since perfluorinated PEM, Nation, was developed by the Dupont Company in the 1960s, it has an over 50 years of history. The tremendous fundamental and applied research results built up over these decades on the membrane, which contribute to much deeper understanding of the membrane and the wider application of the membrane. In the foreseeable future, perfluorinated PEM continues to be the most widely studied and employed membrane for PEMECs due to its excellent oxidative stability and superior proton conductivity. However, there are some drawbacks for perfluorinated PEM such as poor mechanical and chemical stability and poor performance at elevated temperature, insufficient resistance to methanol crossover, and high cost. Both chemical and physical modifications on the membrane are the current and future hot points of researcher works. At the same time, a better understanding of the membrane nanostructures and their relationship to the proton transport mechanism is needed to enhance the performance of the membrane. 

An alternative approach to the use of partially fluorinated systems to reduce the cost of fluorinated PEMs has been developed by DeSimone et al. a perfluo-rinated vinyl ether is copolymerized with a hydrocarbon monomer (styrene), sulfonated, and then subsequently fluorinated to replace existing C-H bonds with C-E bonds (Eigure 3.18). Thus yields the perfluorinated, cross-linked sul-fonyl fluoride membrane that can then be hydrolyzed to give the PEM (7). Because the membranes are cross-linked, considerably higher acid contents (up to 1.82 meq/g) are possible for these materials in comparison to Nafion, leading also to higher proton conductivity values. 

Radiation-grafted PEMs based on perfluorinated substrates (32) have been developed by Rouilly et al., who used FEP as the base polymer... 

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. 

Among the proton-conducting membranes Nation or Nafion-like sulfonated perfluorinated polymers should also be mentioned. These materials are used for polymer electrolyte membrane (PEM) fuel cells, and in addition to being chemically very stable, they exhibit high proton conductivity at temperatures lower than 100°C. It is believed that permeability and thermal stability may be increased if tailor-made lamellar nanoparticles are added to a proton conducting polymer. 

FCs must cost around 45/kW to be competitive with the internal combustion engine. Perfluorinated sulfonic acid membranes currently cost 100-200/KW resulting in PEM FC prices estimated to be of the order of 1000/KW. 

Nafion, a sulfonated perfluorinated polymer developed by Dupont, which triggered the development of PEM fuel cells fed with hydrogen was also the PEM used in all the first generation of DMFC and DEFC. Very early in the development of DAFC it was recognized that alcohol crossover due to the relatively large permeability of Nafion to alcohols was a severe limitation to be overcome. In the case of methanol the high permeability can be understood by the preference of Nafion for sorbing methanol instead of water in the binary mixtures. 

In this sectiOTi the Protmi Exchange Membranes (PEM) used or with potential to be used in DAFC will be introduced. The ample spectra of PEM chemical structures, including perfluoiinated sulfraiic membranes, and non-perfluorinated membranes and their corresponding inorganic and organic composites or blends, will be addressed. 

At present, the most widely used commercial PEM is Naflon produced by DuPont since 1992. Naflon is a plain perfluorosulfonic membrane that is thermally stable and is excellent for PEMFC because of its high proton conductivity. However, Naflon is not suitable for DMFC applications, partly due to its cost. This type of membrane has high permeability toward methanol even at low temperatures, which drastically reduces the DMFC performance (Neburchilov et al. 2007). This is worsened by high water permeability in perfluorinated membranes that can cause cathode flooding and thus lower cathode performance, which also contributes to lower DMFC performance. 

This is also called the Solid Polymer Fuel Cell (SPFQ and the Direct Methanol Fuel Cell (DMFC) is included in this classification. These cells use a solid perfluorinated sulfonated polymer ion exchange membrane (e.g. DuPont Nafion) [65] in the form of a thin plastic film, which serves as the electrolyte in the PEM fuel cell operating at 50-100°C. 

It is mainly the PEM that distinguishes a PEM fuei ceii from aii other types of fuel cells. As its name implies, a PEM has the capability of transporting protons. It is typicaiiy made of a solid ionomer with acidic groups such as sulfonic acid (-SO3H) at the end of the polymer side chains. Polystyrene sulfonic acid is one such ionomer, and it was used as the PEM in the early days of the PEM fuel cell development around the 1960s. However, since the PEM fuel cell environment is warm, corrosive, and oxidative (at cathode), an ionomer with higher chemical and electrochemical stability is required. State-of-the-art PEMs are made of perfluorinated polysulfonic acids, and include DuPont s Nafion . 

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. 

Proton exchange membranes (PEMs) are used in fuel cells to ensure transport of protons between the anode and the cathode. The perfluorinated polymer Nation (from DuPont) is extensively used as a PEM. This copolymer possesses a Pi EE backbone and randomly distributed fluorinated ether side-chains each terminated with... 

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