Hydrocarbon　proton　exchange　membranes　(PEMs)　have　been　extensively　employed　in　redox　flow　batteries　(RFBs)　due　to　their　environmental　benefits　and　ion　selectivity　towards　redox　species.　However,　high　cost　of　polymer　monomers　and　complex　synthesis　processes　continue　to　hinder　the　widespread　adoption　of　such　PEMs　in　RFBs.　In　this　work,　a　series　of　sulfonated　polybenzimidazole　membranes　(SNPBI-x)　are　simply　designed　through　direct　sulfonation　and　the　corresponding　application　in　iron-chromium　redox　flow　battery　(ICRFB)　has　been　verified.　The　SNPBI-x　membrane　is　composed　of　rigid　polybenzimidazole　and　naphthalene　rings,　which　not　only　mitigate　the　reduction　in　mechanical　properties　caused　by　sulfonic　acid　groups　but　also　ensure　the　chemical　stability　of　the　membrane　during　operation.　Also,　SNPBI-x　membrane　exhibits　relatively　high　ion　exchange　capacity　(IEC),　high　water　uptake　and　low　swelling　ratio　compared　to　Nafion　212.　Meanwhile,　the　permeability　of　SNPBI-1.42　is　notably　low,　with　values　of　only　0.762　x　10(-7)cm　s(-1)　for　Fe3+　and　0.438　x　10(-7)cm　s(-1　)for　Cr3+　over　96　h.　The　high　ion　selectivity　is　attributed　to　the　Donnan　effect　introduced　by　the　protonation　of　imidazole　groups,　which　enhances　the　ion　barrier　properties　of　the　membrane　and　battery　performance.　Moreover,　optimized　SNPBI-1.42　demonstrates　the　best　battery　performance　among　all　the　membranes　tested,　achieving　a　coulombic　efficiency　(CE)　of　97.55　%,　an　energy　efficiency　(EE)　of　86.88　%　and　a　voltage　efficiency　(VE)　of　89.06　%　at　100　mA　cm(-　2)　.　Furthermore,　SNPBI-1.42　maintains　an　EE　of　over　80　%　for　more　than　1600　cycles　in　ICRFB,　further　confirming　its　superior　chemical　stability　in　ICRFB.