Abstract A novel long-side-chain sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) (S-L-PPO) membrane was successfully prepared for VRFB applications. The long side chains were introduced onto the PPO backbones by a simple and controllable acylation with 4-fluorobenzoyl chloride and a subsequent condensation with sodium 4-hydroxybenzenesulfonate. The introduction of long side chains drives the formation of a good hydrophilic/hydrophobic micro-phase separation structure, which is evidenced by AFM. The S-L-PPO membrane with a degree of sulfonation (DS) of 51% showed an ultralow vanadium permeability (4.3 × 10−9 cm2 s−1) and a decent proton conductivity (44 mS cm−1). As a result, the energy efficiency of VRFB with S-L-PPO-51% membrane was up to 81.8% that was higher than that with Nafion 212 (78.0%) at a current density of 120 mA cm−2. In addition, the self-discharge duration of the cell with S-L-PPO-51% membrane was 222 h that was nine times longer than that of Nafion 212 (23 h). All these indicate that the membrane prepared here is promising for the application in VRFB.