## 3 Types of Ion Exchange Membranes  
Traditionally, ion exchange membranes are classified into cation exchange membranes and anion exchange membranes depending on the type of ionic functional groups attached to the membrane matrix [19]. Cation exchange membranes contain negatively charged groups such as -SO3, -COO-, -PO3-, -PO3H-, -C6H4O-. They allow the passage of cations, such as the Na+ shown in Figure 2, but are non-permeable to anions such as Cl-. Anion exchange membranes have positive functional groups such as -NH3+, -NRH2+, -NR2H+, -NR3+, -SR2+, and thus allow the passage of anions. Composite membranes, called bipolar membranes, can be produced to contain a cation selective layer (with  
Figure 2: Schematic representation of an ion exchange process. negative fixed ionic groups) and an anionic layer (with positive ionic groups). Apart from polymeric ion exchange membranes, ion exchange membranes are also prepared from inorganic material like zeolites, bentonite or phosphate salts. However, they have certain disadvantages because of their high cost and relatively bad electrochemical properties [19]. In general, membranes can be classified based on a lot of factors like nature of the material, morphology, pore size, the driving force and their configuration. When identified by morphology, membranes are often classified by their structures, being homogenous, heterogeneous, interpolymeric, symmetric or asymmetric [27]. In homogenous membranes, the ion exchange groups are attached directly to the base polymer structure so that the ionic charges are distributed over the membrane material [28]. Slight irregularities in local cross-linking or in densities of fixed charges may exist, however, they are considered to be chemically uniform [20]. Homogeneous membranes are the most desirable for many applications. Their properties will be most reproducible and they are often characterised by good electrochemical properties [28]. Heterogeneous membrane sheets are prepared by compressing a mixture of fine ion exchange granules and an inert elastic binder. The resins are embedded in the matrix irregularly [28]. The water-filled pores are formed by swelling the membrane in water. Ion exchange is a function of the exchanger particles; the binder only provides flexibility and mechanical strength to the membrane. The proportion of the binder used, depends on the ion exchange capacity (IEC), strength and selectivity of the membrane. Compared to homogenous membranes, the heterogeneous membranes have better mechanical strength, are easier to prepare but lack in good electrochemical properties [28]. Interpolymeric membranes are obtained by casting a film from a homogenous solution of two polymers, one of which represents the polyelectrolyte and the other being a water soluble filmogenic material [23]. Both components are strongly bound together so that immersion in water does not influence the polyelectrolyte. The interpolymeric membranes produced, possess properties of both the homogenous and heterogeneous membranes. Asymmetric membranes are commonly used in separation processes, often having high mass transport rates for certain components and good mechanical stability [27]. Asymmetric membranes are most commonly produced by the process of phase inversion (described further in Section 3). Phase inversion asymmetric membranes are composed of a thin, dense active layer with a porous support layer underneath. The combination of two or more membranes in series results in a composite membrane [29]. Development of membranes for redox flow cells has tended to mirror the work carried out in the area of fuel cell membranes. Membrane materials used for fuel cell applications can be separated into five categories [30]: (i) Perfluorinated ionomers; (ii) Partially fluorinated polymers; (iii) Non-fluorinated hydrocarbons; (iv) Non-fluorinated membranes with aromatic backbone; and (v) Acid-base blends. Most conventional hydrocarbon ion exchange membranes degrade in the presence of oxidising agents.