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 x 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-L-PPO/PB1-10% membrane had a much longer self-discharge duration time (142 h) than that of Nafion 212 (23 h). (C) 2020 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights reserved.