Electrodialysis fuel cell membranes

This chapter, in addition to surveying membrane types and production, overviews applications of gas and liquid membrane separation and polymer films as banier layers. Water purification for reuse and in desalination using reverse osmosis and nano-, ultra-, and microfiltration are discussed. Electrodialysis, dialysis, and hemodialysis are also covered. Membranes in emerging technologies are described including fuel cell membranes, membranes in lithium batteries, conducting polymer membranes, and thin film membranes used in LED and photovoltaic applications. 

Sulfonated EPDMs are formulated to form a number of rubbery products including adhesives for footwear, garden hoses, and in the formation of calendered sheets. Perfluori-nated ionomers marketed as Nation (DuPont) are used for membrane applications including chemical-processing separations, spent-acid regeneration, electrochemical fuel cells, ion-selective separations, electrodialysis, and in the production of chlorine. It is also employed as a solid -state catalyst in chemical synthesis and processing. lonomers are also used in blends with other polymers. 

Electrodialysis is by far the largest use of ion exchange membranes, principally to desalt brackish water or (in Japan) to produce concentrated brine. These two processes are both well established, and major technical innovations that will change the competitive position of the industry do not appear likely. Some new applications of electrodialysis exist in the treatment of industrial process streams, food processing and wastewater treatment systems but the total market is small. Long-term major applications for ion exchange membranes may be in the nonseparation areas such as fuel cells, electrochemical reactions and production of acids and alkalis with bipolar membranes. 

Membranes for electrodialysis and polymer electrolyte membrane fuel cell (PEMFC) have electric charges. Most of the nanofiltration membranes also carry negative charges. The content of electric charge in a polymer is given by ion-exchange capacity (meq (milliequivalent)/g of dry polymer). 

Ion exchange membranes have been used in various industrial fields, and have great potential for use in new fields due to their adaptable polymer membrane. As mentioned in the Introduction, membranes are characterized mainly by ion conductivity, hydrophilicity and the existence of carriers, which originate from the ion exchange groups of the membrane. Table 6.1 shows reported examples of applications of ion exchange membranes and the membrane species used in various fields. Various driving forces are usable for separation electrochemical potential, chemical potential, hydraulic pressure such as piezodialysis and pervaporation, temperature difference (thermo-osmosis), etc. Of these, the main applications of the membrane are to electrodialysis, diffusion dialysis, as a separator for electrolysis and a solid polymer electrolyte such as in fuel cells. 

The desalination of brackish water by electrodialysis and the electrolytic production of chlorine and caustic soda are the two most popular processes using ion-exchange membranes. There are, however, many other processes such as diffusion dialysis, Donnan dialysis, electrodialytic water dissociation, etc. which are rapidly gaining commercial and technical relevance. Furthermore ion-exchange membranes are vital elements in many energy storage and conversion systems such as batteries and fuel cells. 

Despite great efforts, this has not been achieved until now. Existing types of anion-exchange membranes (used for electrodialysis) are far inferior to Nafion, both in their conductivity and in the chemical and thermal stability. Therefore, one cannot so far meaningfully discuss their potential as substitutes for the circulating or matrix electrolyte in alkaline hydrogen-oxygen fuel cells. 

Electrochemical cells (electrolysers, batteries and fuel cells) require separators, which allow a flow of specific ionic charges but prevent the transfer of chemical species which remain located either in the cathodic or in the anodic compartment. Among the various separators of electrochemical cells, the ion permeable organic membranes are also used for separation processes such as dialysis and electrodialysis. 

Kariduraganavar, M.Y., Nagarale, R.K., Kittur, A.A. and Kulkarni, S.S. 2006. Ion-exchange membranes Preparative methods for electrodialysis and fuel cell applications. Desalination 197 225-246. 

The ion exchange membranes are used in systems where ions of specific charge must be transported. They are used as separators in fuel cells and for electrodialysis where ions must move between phases. 

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]. 

It should be noted that although anion exchange membranes were initially developed for electrodialysis, it was not until last decade that initial work on anion exchange membranes for alkaline membrane fuel cells begun. In the last 4 years, research on anion exchange membranes is proliferating, mainly after first polymers with high anion conductivity were presented. 

Cornet et al. [141] used ammonium ions with different sizes in order to check the occurrence of a critical size for transport restriction. The ionic conductivity first decreases as the ammonium size increases (from 5 to 30 A ) due to a lower mobility compared to protons. For larger coimterion sizes, the conductivity is roughly constant until 1000 A where a cutoff is observed. This result suggests a radius of 10 A for the conductive pathways, at least between two ionic domains. Despite SPI membranes being designed for fuel cell applications, these membranes can be used efficiently as the separator in electrodialysis experiments. For example, SPI membranes appear to be promising materials for separating copper or chromium ions from acidic solutions [172]. 

A good AEM should fulfill stringent mechanical, thermal, and chemical properties as mentioned in Section 11.2. Historically, the first AEM material was developed by researchers from the Toknyama Soda Company. They introduced quaternary ammonium groups to the divinylbenzene-cross-linked polychloropropene polymer matrix via trimethylamine. Since then, several membrane-associated companies explored various kinds of AEMs and pushed them to commercial market most of them were based on cross-linked polystyrene, polyvinyl alcohol, low (or high)-density polyethylene, and other aliphatic polymers through irradiation-grafting method. The primary objective of developing these materials was for applications in the fields such as electrodialysis, desalination, selective electrode, and waste acid recovery. However, they showed performance in AEM fuel cells far below practical... 

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