Side-Chain Grafting-Modified Sulfonated Poly(ether ether ketone) with Significantly Improved Selectivity for a Vanadium Redox Flow Battery

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 x 105 S min cm-3), and its selectivity is significantly improved compared with SPEEK (4.9 x 104 S min cm-3) and the Nafion 212 membrane (4.8 x 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%).