Designing a new process to prepare amphoteric ion exchange membrane with well-distributed grafted chains for vanadium redox flow battery

A combination of radiation grafting technique and solution phase inversion method has been successfully utilized to prepare amphoteric ion exchange membranes (AIEMs) from poly(vinylidene fluoride) (PVDF) powder. First, styrene (St) and dimethylaminoethyl methacrylate (DMAEMA) were grafted into PVDF powder via radiation-induced graft copolymerization. Subsequently, the grafted powder was transferred into film by casting method. Finally the AIEM was obtained by sulfonation and protonation of the grafted film. Micro-FTIR, XPS and TG analyses testified that the grafting and sulfonation of St and DMAEMA units in poly(St-co-DMAEMA) grafts had been carried out as designed. SEM-EDX image of the AIEM indicates the well distribution of the grafted chains across the AIEM. The physicochemical properties of the resulting AIEMs and the conventional AIEMs based PVDF film grafting were comparatively investigated. The AIEMs obtained by this novel process were found to have superior conductivity compared with the AIEMs based PVDF film grafting. Open circuit voltage measurements showed that the vanadium redox flow battery (VRFB) assembled with this new AIEM maintained the value above 1.3 V after a period of 30 h, which was much longer than that with the Nafion117 membrane, suggesting this approach is effective for imparting preferable structure-properties to the membrane for VRFB.