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.　©　2024　Fine　Chemicals.　All　rights　reserved.