Perfluorinated membranes application

Dolye, M. and Rajendran, G. 2003. Perfluorinated membranes. In Handbook of fuel cells—Fundamentals, technology and applications, part 3, ed. W. Vielstich, A. Lamm and H. A. Gasteiger, 351. Colchester, England John Wiley Sons. 

Eisman, G. A. 1990. The applications of Dow GhemicaTs perfluorinated membranes in proton exchange membrane fuel cells. Journal of Power Sources 29 389-398. 

S. Tsushima, S. Hirai, K. Kitamura, M. Yamashita, S. Takasel, MRI application for clarifying fuel cell performance with variation of polymer electrolyte membranes Comparison of water content of a hydrocarbon membrane and a perfluorinated membrane. Appl. Magn. Reson. 32, 233-241 (2007)... 

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. 

Doyle, M. and Rajendran, G., Perfluorinated membranes, in Elandbook of Fuel Cells Fundamentals, Technology, and Applications, 1st ed., Vielstich, W., Lamm, A., and Gasteiger, H.A., Eds., John Wiley Sons, West Sussex, England, 2003, p. 351. 

Industrial production of perfluorinated ionomers, Nafion membranes, and all perfluorinated membranes is costly due to several factors first, the monomers used are expensive to manufacture, since the synthesis requires a large number of steps and the monomers are dangerous to handle. The precautions for safe handling are considerable and costly. Secondly, the PSEPVE monomer is not used for other applications, which limits the volume of production. The most significant cost driver is the scale of production. Today, the volume of the Nafion market for chlor-aUcali electrolysis (150,000 m year ) and fuel cells (150,000 m year ) is about 300,000 m year resulting in a production capacity of 65,000 kg year. When compared to large-scale production of polymers like Nylon (1.2 x 10 m year ), the perfluorinated ionomer membrane is a specialty polymer produced in small volumes. 

Perm selectivity and ionic conductivity are the two important parameters, besides the mechanical and chemical stability, to be considered in applications for ion exchange membranes no connection is usually done with the microstructure. The aim of this paper is to present results on the physical structure of perfluorinated membranes. Crystallinity and distribution of the ionic sites across the thickness will be first considered. 

Studies of these perfluorinated membranes in dilute and in concentrated solution environments still leave many unanswered questions about the nature of membrane transport properties. However, the obvious importance of these polymers in membrane separation applications, coupled with the fundamental significance of their ion clustered morphology, makes the continued study of these materials a fruitful area of research for the future. 

The thermal stability of perfluorinated material is excellent as evidenced by the higher glass transition temperature over their respective non-fluorinated analogues (11). Accordingly, these perfluorinated materials can be used in electrochemical cells at an elevated temperature for better cell efficiency because of high conductivity and fast kinetics (12) The relatively high cost of the perfluorinated membrane limits its application in many electrochemical cells when cost-effectiveness is a major concern. 

G.A. Eisman, The application of Dow Chemical s perfluorinated membrane in proton-exchange fuel cell, J. Power Sources, 1990, 29, 389-398. 

Different from PIPE and ETFE, cost reduction of the PSVE polymer is one of the problems for prevailing perfluorinated membranes. Recently, a new direct fluorination process with elementary fluorine was reported to produce perfluorinated fluorosulfonyl polymers, as shown in Fig. 8 [ 19). This new process consists of the following features and is free from explosion in the gas phase and the difficulty in finding solvents with stability to fluorine gas and solubihty for hydrocarbon compoimds, these problems having limited the application of the conventional direct fluorination methods ... 

Doyle M, Rajendran G (2003) Perfluorinated membranes. In Vielstich W, Gasteiger HA, Lamm A (eds) Handbook of fuel cells fimdamentals, technology and applications. WUey, Chichester, pp 351-395... 

In industrial electrolysers, the cell operates with NaCl anolyte concentration greater than 3moll and the sodium hydroxide produced concentration is 20-40%. The current density is about 2000 A m". In this application, the perfluorinated membrane acts as a sodium ion conductor working in a medium of high pH. The counter transport of hydroxyl ion must be minimized for achieving high current efficiency. 

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. 

Tokuyama, a Japanese company specializing in membrane technology for electrodialysis and desalination, has undertaken development of AEMs in OH form, targeting fuel-cell applications. Tokuyama s 901 membrane anion conductivity, 30 mScm, at roughly half that of the proton conductivity of the perfluorinated membranes, is at an acceptable level for fuel-ceU development. Other material properties, such as dimensional stability due to the swelling as a result of the uptake of water, are also reasonable and are, in fact, better than those of typical PFSA membranes [36]. 

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. 

Research has been actively conducted for the past 20 years in search of alternatives to perfluorinated membranes for PEMFC and DMFC applications. Partially fluorinated ionomers are of great interest among other various candidates in such efforts. Like perfluorinated membranes, partially fluorinated ionomers also have a PTFE-like polymer backbone as a main part to resist chemical attacks. However, ionic groups are attached to styrenic moieties instead of perfluorinated side chains. Due to its availability and easy sulfonation, styrene is a reasonable choice for ionomeric materials. In the late 1990s, Ballard Power Systems introduced a partially fluorinated low-cost membrane for fuel cell applications [24,47,48]. Ballard Advanced Materials (BAM) membrane is a family of sulfonated styrenic co-polymers of a,p,p-trifluorostyrene and substituted a,p,p-trifluorostyrene co-monomers by emulsion polymerization. The chemical structure of BAM membrane is shown in Fig. 13.9. 

The characteristics of fluoropolymers are summarized in Table 1. Thermal and chemical resistance is in general with most of plastics, elastomers and perfluorinated membranes. Weather resistance with the outdoor durability for more than 20 years is specific for fluorinated paint resins. Surface properties such as water and oil repellency are provided by acrylic polymer-based textile finishes and coatings with long-chain per-fluoroalkyl groups. Electrical properties as well as a low refractive index are important for optoelectronics applications like optical fibers. 

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