## 2 Ion Exchange Membranes  
The membrane is an important component in a redox flow cell. A great amount of effort is generally put into selecting a suitable membrane for redox flow systems. In vanadium redox systems, an ideal membrane should offer the following characteristics: good chemical stability under acidic conditions, resistance to the highly oxidising environment of the positive half cell electrolyte, low electrical resistance, low permeability to the vanadium or polyhalide ions, high permeability to the charge carrying hydrogen ions, good mechanical properties and low cost [14]. In addition to the above mentioned characteristics, an important property that an ideal membrane must possess, is the ability to prevent the preferential transfer of water from one half cell to the other, as this results in flooding of one half cell while diluting the other [15]. In redox flow cells, the function of the membrane is to prevent cross mixing of the positive and negative electrolytes and the short circuiting of the two half cell electrodes while allowing the transfer of ions to complete the circuit during the passage of current [15, 16, 17]. The membrane has been identified as one of the main obstacles in the commercialisation of many redox flow cells [18]. Ion exchange membrane dependent processes were first developed by [PERSON] in 1890, who studied semi permeable membranes and discovered that a membrane can be impermeable to any electrolyte if it is impermeable to either its cation or anion [19]. Ion exchange membranes are sheets, ribbons or tubes, which separate two fluids and are capable of ion exchange [20]. Ion exchange membranes are very similar to ion exchange resins, the difference arising from the mechanical requirements of the membranes. Ion exchange resins are dimensionally unstable, as cation exchange resins are often brittle, whilst anion exchange resins are soft [21]. These mechanical properties arise from the lack of backing material that gives membranes the necessary strength and dimensional stability [21]. The membranes are made up of cross-linked linear polymer chains, which form a three-dimensional network. Without the cross-linking, the membrane would dissolve in water forming a polyelectrolyte solution. Ion exchange membranes, like the resins, have fixed ion functional groups and oppositely charged counter ions, present in sufficient numbers to render the whole exchanger electrically neutral. The ionic functional groups are the exchange sites which are capable of forming an electrostatic bond with an ion of opposite charge [22]. The process of ion exchange occurs when the mobile counter ions are replaced by other ions with the same charge from solution. Ion exchange is a reversible and stoichiometry process, with the displacement of one ionic species by another on the exchanger [23]. The ease with which the ion may be replaced depends on the strength of the bond, which varies similar to the dissociation of strong and weak electrolytes [22]. Figure 2 depicts an ion exchange process. Research into the basic principles of ion exchange continued with the work of [PERSON] [24], [PERSON] and others. However, it was not until 1950 that commercial applications began to expand with the stable, highly selective and low electric resistance membrane of Juda and McRae (Ionics Inc., Cambridge, MA, USA) [25]. Today, ion exchange membranes are applied in desalination, in the treatment of industrial effluents, in the food, manufacturing and pharmaceutical industries. Their applications range from removal of heavy metals to the removal of salt in an advanced continuously operated upflow microbial desalination cell [26]. In recent years, research in membrane technology has greatly focussed on battery applications. In vanadium redox batteries, the most widely investigated membranes have been perfluorosulfonic acid polymers such as DuPont's Nafion(r) that have good stability to the highly oxidising V (V) solution in the charged positive half cell electrolyte [13].