Achieving high proton conductivity, low vanadium ion permeability, and high chemical stability using a single material remains a key challenge for hydrocarbon-based membranes for use in vanadium redox flow batteries (VRFBs). Herein, we report amorphous poly(2,5-benzimidazole) (ABPBI) membranes with alkyl spacers which can meet these requirements. Spacer-grafted poly(2,5-benzimidazole)s (ABPBIs) with different grafting ratios were synthesized via an N-substitution reaction and denoted as ABPBI-0, -10, -25, -40, and -50, respectively. The structure to battery performance relationship was investigated through spectroscopic and electrochemical analysis. As the grafting ratio increased, the microstructure of the ABPBI derivative membranes was transformed from a semicrystalline to an amorphous structure because of reduced chain packing, increasing H2SO4-absorption capability and consequently reducing area-specific resistances. Also, they exhibited lower vanadium permeabilities compared with Nafion 115 owing to Donnan exclusion effect. As a combined effect of these characteristics, the membranes outperformed Nafion 115 membrane with notably high coulomb efficiencies and energy efficiencies. Furthermore, a grafted ABPBI membrane showed stable battery cycling performance more than 500 cycles at 200 mAcm(-2), and 1000 h continuous operation at 1C-rate, demonstrating excellent chemical stability against highly oxidizing VO2+ solution.