## 5 Research Progress on Membranes for Redox Flow Batteries
### Other Hydrocarbon Based Cation Exchange Membranes  
Poly (arylene ether ketone), poly (arylene ether sulfone) and polyimide can be sulfonated and therefore can potentially be used as cation exchange membranes in VRB. A series of sulfonated poly (tetramethyldiphenyl ether ether ketone) membranes with different degrees of sulfonation was prepared by direct aromatic nucleophilic substitution step polymerization. In this work that was carried out by [PERSON] and co-workers [PERSON] and co-workers [PERSON] and [PERSON] (2012), the performance of four different membranes namely, Nafion 115, SPEEK 40, SPEEK 50 and SPEEK 60 was studied in comparison to each other. It was observed that the ion exchange capacity increased with the degree of sulfonation. The area resistance of SPEEK was found to be higher than Nafion. This is probably because the conductivity is enhanced by the presence of SO\({}_{4}\)\({}^{2}\)-/HSO\({}_{4}\)\({}^{-}\) ions in the pores of the membrane The V\({}^{4+}\) and H\({}^{+}\) ion selectivity was highest in SPEEK 40 [102].  
Sulfonated poly (fluorenyl ether ketone) membranes also known as SPFEK membranes, were synthesized directly via aromatic nucleophilic polycondensation of bisphenol fluorene with 60% sulfonated difluorobenzene and 40% difluorobenzene. [PERSON] and co-workers studied and compared the properties and performance of Nafion 117 and SPFEK membranes [103]. Water uptake and swelling ratio was found to be higher for SPFEK when compared to Nafion. Because of a robust aromatic backbone, SPFEK has a higher tensile strength than Nafion 117. The permeability of V (IV) ions increased with temperature and time, and at a fixed temperature, the permeability was higher in SPFEK than Nafion. Also, the energy efficiency of SPFEK was found to be higher than Nafion [103]. In an attempt to improve proton selectivity while maintaining low vanadium ions permeability, a small amount of silica was introduced into SPFEK membrane by a simple sol-gel method [104]. The addition of silica enhanced the mechanical properties and thermal stability of the membrane.  
Sulfonated poly(sulfone) membrane was prepared by [PERSON] _et al._ by sulfonating commercially available poly(sulfone) (Radel) with trimethylsilyl chlorosulfonate in tetrachloroethane, followed by casting the sulfonated polymer solution on a glass plate [105]. Sulfonated Radel showed lower permeability of vanadium ions than both Nafion 117 and pre-treated Nafion 117. The better ion selectivity of this membrane resulted in longer charge retention than Nafion 117. However, a decrease in Coulombic and energy efficiency after 40 cycles was observed, indicating problems in terms of stability [105]. Further study on stability of these membranes revealed that mechanical degradation during cycling might be due to local precipitation of solid particles inside the membrane. Chemical degradation was not significant when investigated by Raman spectroscopy [106].  
Two other membranes, sulfonated poly (arylene ether sulfone) and sulfonated poly (arylene thioether) were synthesized via one-pot polymerization followed by sulfonation using chlorosulfonic acid as the sulfonating agent. Both membranes showed lower permeability of vanadium ions and a higher Coulombic efficiency than Nafion [107,108].