Perfluorinated membranes, development

Through the intense research efforts of a small number of companies over the past 10 to 15 years, several types of high-performance perfluorinated membranes have been developed for chlor-alkali... 

Summary. Membrane cell processes have become important to modem technology to a great extent because of the development and utilization of perfluorinated membranes. The combination of metal anodes and the perfluorinated membranes has provided a modem revolution in the area of chlor-alkali production. 

In this work we propose a model for ionic clustering, which we have called the cluster-network model (2), to account for hydroxyl rejection in nNafionM perfluorinated membranes. In developing this model we have been guided by two requirements 1. the model should be consistent with the available data on the microscopic structure of the polymer (1-5) 2. the model should... 

While Nafion , a perfluorinated polymer developed by DuPont, is the most commonly used proton conductive polymer electrolyte membrane it is an insufficient solution in a number of areas. It has high cationic transport (approximately 9.56 5/cm) [8] but also has high levels of methanol fuel crossover, slow anode kinetics and very high cost [12]. Fuel cell membrane performance can be estimated from the ratio of proton conductivity (a) to methanol permeability (P). The higher the value of a/P, the better the membrane performance would be [13]. Chitosan has been shown to have a much lower methanol permeability than Nafion [14], and as such, a great deal of attention focused on developing chitosan membranes with high levels of ionic conduction and low methanol permeability as delineated in Table 3.1. 

Current membrane development focuses on perfluorinated ionomers, hydrocarbon and aromatic polymers and acid-base polymer complexes. Good recent reviews on membrane synthesis and experimental characterization can be found in this volume and for example in [23-29]. 

The development of perfluorinated membranes has been most important to the progress in membrane cells. The first perfluorinated membranes were made by DuPont, and they were followed by products from Asahi Glass [37] and Asahi... 

The preparation of perfluorinated membranes from these monomers is rather complex, and several research groups developed different routes to similar end points. Typically they utihzed the monomers TFE and HFPO and a variety of other reagents to synthesize a complex perfluorinated monomer (PFM) containing ether linkages. The general stracture of PFM is... 

Perfluorinated membranes (Dow membrane) were developed by Dow Chemical Co. (USA). Their equivalent masses are equal to 800-850 g, while dry state thickness is of 5 tm. Flemion membranes with equivalent masses of 1000 were developed by Asahi Glass Co. (Japan) [5]. Aciplex-S membranes were developed by Asahi Chemical Industry (Japan) and possess equivalent masses of 1000-1200 g. 

Among the cation permeable membranes, the perfluorinated membranes which have been developed as separators for fuel cells and chlor-alkali electrolysers show the characteristic features of superselectivity, very high thermal stability and chemical resistance, which are not obtained by the other classes of polymeric ion permeable membranes. Three commercial forms of cation permeable perfluorinated membranes have been proposed ... 

W.G. Grot, Discovery and Development of Nafion Perfluorinated Membranes The SCI. Castner Medal Lecture. In K. Wall (ed.). Modem Chlor-Alkali Technology, Ellis Horwood Ltd, Chichester, Vol. 3, (1986), p. 122. 

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]. 

The breakthrough in membrane development and the precondition for the membrane process as an alternative for the amalgam process was in 1975 the replacement of sulfonic acid groups by carboxylic acid groups in a perfluorinated membrane polymer, initially applied by the Asahi Glass Company [1,2]. 

In addition improvements in the sulfonated perfluorinated membranes, or development of new protonic membranes with reduced alcohol cross-over and better conductivity and stability at higher temperatures (up to 200 °C) are stiU challenging topics before any commercialization of DAFC systems. 

The electrode reactions in a membrane cell are the same as those in a diaphragm cell but the separator is now a cation-permeable membrane. The development of modern membrane cell technology dates only from about 1970 when it was recognized that the perfluorinated membranes (section 3.2.2) had the properties essential to the chemistry of a chtor-alkali cell. 

Organic components of the catholyte were found to decompose anodically and, hence, it was necessary to use a separator. After considerable difficulties, a cation-permeable membrane was identified as suitable in the electrolysis conditions. The process, however, predated the developments in modern perfluorinated membranes (Chapter 3) and for many years the performance of the membrane was a problem. Indeed, this factor determined the manage-... 

To date, many membranes have been developed [3-12] and research has been carried out to develop a high performance membrane to replace perfluorinated polymer electrolyte membranes, such as Nafion membranes. However, no membranes developed to date satisfy aU of the above criteria required for DMFC membranes [2-12]. 

Another option is to use partially perfluorinated membrane. Ballard Company develops such product called BAMS that has good thermal stability, chemical stability, and mechanical properties, low EW ( W=407), and high water content. The thickness of hydration BAM-407 membrane is 140 om where water content is 87 wt%. In contrast, Naflon 117 membrane has 190 om of hydration thickness and 19 wt% of water content. The solubility coefficient of oxygen in BAM-407 is 4 times less than that in Naflon 117, but the diffusion coefficient of oxygen in BAM-407 is 4 times larger than Naflon 117 [49,50]. 

To combine the advantages of the two types, bilayer membranes with the sulfonic acid side facing the anode and the carboxylic acid side facing the cathode were developed. Table 12.10 shows the comparison of the three types perfluorinated membranes. 

The development of perfluorinated membranes by DuPont during the 1960s has played a vital role in electrochemical system applications. The Nafion family or perfluorinated ionomer membranes meet the requirements for several electrochemical systems (chloroalkafine, fuel cells and some other non-fuel cell applications). A lifetime of over 60000 hours has... 

Perfluorinated membranes based on weak acid functions have been developed by Asahi Chemicals (Japan). Their EW is similar to that of the above membranes. The exact synthesis process of the membrane is not indicated. They have been developed for the chlor-alkali industry where they are used in bilayer membranes. In these membranes a strongly acidic membrane (-SO3H) (pK < 1) is coated on one side, and the second side is coated with a thin layer of a weak-acid membrane (-COOH) = 3). 

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