## 3 Types of Ion Exchange Membranes
The perfluorinated membranes have been the most widely researched as they have the most desired properties of good chemical stability, high conductivity and mechanical strength. Perfluorinated ion exchange membranes are derived from copolymers of tetrafluoroethylene (TFE) and perfluorvinyl ether terminated by a sulfonyl fluoride group. The precursor of the perfluorinated membrane is shown in Scheme 1. The value of m may be 1 and n depends on the equivalent weight of the membrane. **Scheme 1. ** Structure of the precursor of perfluorinated ion exchange membranes [31]. \[-\left(\mathrm{CF_{2}CF_{2}}\right)_{n}-\mathrm{CF_{2}CF}-\] \[\left|\right.\] \[\left(\mathrm{OCF_{2}CF-}\right)_{n}\mathrm{OCF_{2}SO_{2}F}\] \[\left|\right.\] \[\mathrm{CF_{3}}\]  
The sulfonyl fluoride groups can be hydrolys to yield Nafion polymers in the form of sulphonic acid [31]. Here, the main chain is hydrophobic, whilst the acid or salt side chains are highly hydrophilic. Of particular importance is the perfluorosulfonic acid Nafion(r) membrane produced by DuPont. In another class of perfluorinated membranes, the sulfonic acid groups are replaced by carboxylic acid. These include the Flemion(r) (Asahi Glass Co. Ltd., Yokohama, Japan), NEOSEPTA-F(r) (Tokoyama Soda Co. Ltd., Yamaguchi, Japan), and other Asahi Chemical Industry Company membranes. The Gore Select(r) membranes (by W.L. Gore and Associates, Newark, DE, USA) [32] are micro-reinforced perfluorinated composites. They consist of an ultra thin microporous integral composite of a base material made of expanded PTFE whose pores are filled with the ion exchange material such as perfluorinated sulphonic acid resin or perfluorinated carboxylic acid resin [32]. A surfactant is usually applied to ensure impregnation. These membranes may also be reinforced with a woven or non-woven material bonded to one side of the base material. Although perfluorinated membranes often possess properties desired in the battery industry, their application has until recently, been limited by a number of factors, the most significant being their cost (>SUS800 for Nafion(r) 117 [33]). Other disadvantages of these include safety concerns and temperature related limitations. Safety concerns arise from the evolution of toxic intermediates and corrosive gasses liberated at temperatures above 150 \({}^{\circ}\)C [30]. This is a concern both during the manufacturing process as well as vehicle accidents and could limit fuel recycling options. Degradation of membrane performance has also been observed at elevated temperatures, above 80 \({}^{\circ}\)C.  
Partially fluorinated membranes have a fluorocarbon base, with a hydrocarbon or aromatic side chain. The side chain is often grafted onto the backbone (to be discussed later) and is thus easily modified [30]. These membranes are relatively strong when compared to the perfluorinated membranes, but are less durable and likely to degrade faster. Hydrocarbon membranes are non-fluorinated, giving them inferior chemical and thermal stability. They are typically modified using polar functional groups. They have advantages in some applications over the perfluorinated as they are less expensive and therefore commercially available [30]. Hydrocarbon membranes can be modified by incorporating aromatic groups directly into their backbone or onto polymer groups in the backbone. This can enhance the stability of the polymer at elevated temperatures, and provide greater chemical stability in oxidising, reducing and acidic environments [30]. Acid-base blend membranes involve the incorporation of an acid component onto an alkaline polymer base [30]. This aids in promoting proton conduction. Improved thermal and chemical stability has been demonstrated in some short-term studies. A detailed analysis of some of these membranes can be found in Section 4.