A sulfonated poly(ether ether ketone) (SPEEK) proton exchange membrane with side-chain grafted sulfamic acid (SA) was designed. The introduction of SA with an amphoteric functional group achieves the purpose of preventing the migration of vanadium ions in the case of a lossless proton conductive group (−SO3H), which is caused by the Donnan repulsion of a protonated −NH– group to a vanadium ion. At the same time, acid–base pairs and hydrogen bonds are formed between the sulfonated side-chain grafted secondary amine (−NH−) group and the sulfonic (−SO3H) group, which hinders the penetration of vanadium ions. The results show that the prepared SPEEK-SA-50 membrane has high proton conductivity (0.099 S cm–1) and vanadium ion selectivity (1.8 × 105 S min cm–3), and its selectivity is significantly improved compared with SPEEK (4.9 × 104 S min cm–3) and the Nafion 212 membrane (4.8 × 104 S min cm–3). Under the condition of 80 mA cm–2, the Coulombic efficiency (CE) of the vanadium redox flow battery with the SPEEK-SA-50 membrane is 95.45%, and the energy efficiency (EE) is 76.11%, which is better than that of the Nafion 212 membrane (CE: 86.39%; EE: 68.26%).

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