Preparation and properties of sulfonated poly(fluorenyl ether ketone)proton exchange membranes for vanadium redox flow battery
Poly(fluorenyl ether ketone)s(PFEK-x)were firstly synthesized from high-temperature polycondensation of 9,9-bis(3,5-dimethyl-4-hydroxyphenyl)fluorene(DMBHF),9,9-bis(4-hydroxyphenyl)fluorene(BHF)and 4,4'-difluorophenone(DFB),herein,x=30,40,50,and refers to the content of DMBHF(based on the amount of substance of DFB,the same below).The PFEK-x obtained were then brominated by bromination of the methyl groups and then reacted with sodium 4-hydroxybenzenesulfonate to yield sulfonated poly(fluorenyl ether ketone)s(SPFEK-x)with different ion exchange capacities.Finally,novel and low-cost proton exchange membranes(PEMs)were prepared by solution-casting SPFEK-x into membranes and acidification.The samples were characterized by 1HNMR,FTIR and TGA,followed by analysis on their properties.The results showed that SPFEK-40 membrane exhibited excellent comprehensive performances,such as high proton conductivity and icon selectivity,low VO2+ permeability,and low surface resistance.The energy efficiency of vanadium redox flow battery(VRFB)assembled with SPFEK-40 membrane reached 88.2%at a current density of 80 mA/cm2,higher than that of the VRFB assembled with Nafion 212(84.8%).In addition,the discharge capacity retention ratio of the VRFB assembled with SPFEK-40 membrane after 30 cycles was 84.3%,which was much higher than that of the VRFB assembled with Nafion 212(66.1%).Moreover,the synthetic route could be scaled-up easily since it was based on inexpensive raw materials without dangerous sulfonation reactions.The SPFEK-x membranes also displayed good mechanical properties and oxidation stability.