## 5 Research Progress on Membranes for Redox Flow Batteries
### Non Fluorinated Membranes  
Non fluorinated membranes have received broad attention in VRB due to their low cost, excellent mechanical and chemical stability and high ion selectivity [13]. Polyether ketone (PEK), Polyether sulfones (PES), polybenzimidazoles (PBI), polyphenyl quinoxalines (PPQ) are several families of thermoplastic polymers that are known for their excellent mechanical and chemical resistance properties at high temperatures. All of these polymers consist of 1,4-disubstituted phenyl groups separated by a number of linkages [92] which can be shown as -[X-(phenyl)-Y-(phenyl)-Z-(phenyl)]\({}_{\text{h}}\), where X, Y, and Z represent -O-, -C (O)-, -C (CH\({}_{3}\))\({}_{2}\)-, -SO\({}_{2}\)-, -S- groups. For PEK, X and Y could denote -O- whereas Z could be -C (O)-. For PES, X could be -SO\({}_{2}\)-, Y could denote -O-, _etc._  
Due to having benzene ring on their structures, -SO\({}_{3}\)H functional group can be introduced while fabricating an ion exchange membrane subsequently enhancing its ion exchange capacity and inducing good proton conductivity. However, the concentrated sulphuric acid used in sulfonation may cause decomposition of the polymers. Furthermore, due to high solubility of some sulfonated polymers in water, only a few of them can be converted into ionomers via sulfonation and fashioned into membranes that can be substituted for high cost commercially available perfluorocarbon sulfonates (e.g., Nafion). Sulfonated PES could only be prepared with a degree of sulfonation (DS) of 29.5% using concentrated sulphuric acid alone as the sulfonating agent. Attempts to sulfonate to a higher degree resulted in water soluble polymers that are unsuitable for membrane fabrication [92]. Polyether ether ketone (PEEK) is the most thoroughly studied, because it could easily be sulfonated into sulfonated PEEK (SPEEK) and could be produced with a wide range of sulphuric acid content [93, 94]. The time and temperature of the reaction can be varied in order to achieve the desired level of sulfonation. Also, membranes based on sulfonated PEEK are promising materials for direct methanol fuel cell applications, exhibiting lower methanol permeability compared to Nafion membrane [95, 96, 97, 98]. It has been reported that this was probably due to the difference in microstructures of Nafion and SPEEK. In Nafion membrane, the microstructures of polymers are composed of two parts: a highly hydrophobic fluorocarbon backbone and the highly hydrophilic sulfonic functional groups. This couldgive rise to hydrophobic/hydrophilic nano-separation, especially in the presence of water. The hydrophilic sulfonic groups cluster to form hydrophilic domains, which are well interconnected because of the high flexibility of the fluorocarbon of Nafion membrane. In SPEEK membrane, the backbone is less hydrophobic and the sulfonic acid group is less acidic which results in smaller hydrophobic/hydrophilic separation. Thus, the less pronounced hydrophobic/hydrophilic separation of SPEEK membrane produces narrow channels and a highly branched structure. A novel sandwich-type SPEEK/tungstophosphoric acid (TPA)/polyppropylene (PP) composite membrane was tested in VRB by [PERSON] and co-workers [PERSON] and co-workers [PERSON] and [PERSON] (2012). The sandwich membrane comprised of a layer of polypropylene (PP) (135 \(\upmu\)m) between two layers of SPEEK/TPA membrane (total thickness: 100 \(\upmu\)m). The Polypropylene membrane layer was immersed in a solution of SPEEK/TPA during the casting process. An extended stability of these membranes in VRB is based on the premise that even if the outer layer breaks, the other layers would still be able to perform the required function. The area resistance of the sandwich membrane was found to be higher than that of Nafion 212. The V(IV) permeability of SPEEK+TPA+PP is lower than Nafion because of the difference in the microstructures of SPEEK and Nafion, as mentioned in [PERSON] and [PERSON] (2012).