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Swelling-induced morpholine functionalized adamantane-containing poly(aryl ether ketone) (MAPEK) membranes were prepared for vanadium flow batteries. MAPEK membranes were prepared from chloromethylated polymer and morpholine and further swelling-induced with hot phosphoric acid to obtain membranes with enhanced ionic conductivity. The swelling, selectivity, and ionic conductivity of MAPEK membranes were regulated by varying the swelling temperature. Selective swelling-induced microphase separation in MAPEK membranes, forming wider ion transport pathways and resulting in low area resistance. The unique rigid adamantane-containing backbone limited the swelling of membranes. Consequently, MAPEK membranes showed excellent selectivity and conductivity (vanadium ion permeability coefficient of MAPEK membranes was lower than 3.82 × 0−7 cm2min−1) (Nafion212 membrane, 42.5 × 0−7 cm2min−1), and MAPEK-150 membrane exhibited low area resistance (0.17  Ωcm2). The vanadium flow batteries (VFB) with MAPEK-150 membrane exhibited high energy efficiency (91.1% at 80 mAcm−2, 81.4% at 200 mAcm−2). Furthermore, MAPEK membranes showed good stability in VFB and oxidative electrolytes. The swelling-induced method utilized in this work is a versatile and facile method to enhance the conductivity of ion-exchange membranes.