A new amphoteric membrane was prepared by blending long-side-chain sulfonated poly(2

6-dimethyl-1

4-phenylene oxide) (S-L-PPO) and polybenzimidazole (PBI) for vanadium redox flow battery (VRFB) application. An acid-base pair structure formed between the imidazole of PBI and sulfonic acid of S-L-PPO resulted in lowered swelling ratio. It favors to reduce the vanadium permeation. While

the increased sulfonic acid concentration ensured that proton conductivity was still at a high level. As a result

a better balance between the vanadium ion permeation (6.1×10

-9

cm

2

·s

-1

) and proton conductivity (50.8 mS·cm

-1

) in the S-L-PPO/PBI-10% membrane was achieved. The VRFB performance with S-L-PPO/PBI-10% membrane exhibited an EE of 82.7%

which was higher than those of pristine S-L-PPO (81.8%) and Nafion 212 (78.0%) at 120 mA·cm

-2

. In addition

the S-LPPO/PBI-10% membrane had a much longer self-discharge duration time (142 h) than that of Nafion 212 (23 h).