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 en-sured that proton conductivity was still at a high level. As a result, a better balance between the vanadium ion permeation (6.1 × 10?9 cm2·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/PBI-10％membrane had a much longer self-discharge duration time (142 h) than that of Nafion 212 (23 h). ? 2020 The Chemical Industry and Engineering Society of China, and Chemical Industry Press Co., Ltd. All rights