Amphoteric ion exchange membrane synthesized by radiation-induced graft copolymerization of styrene and dimethylaminoethyl methacrylate into PVDF film for vanadium redox flow battery applications

Poly(vinylidene difluoride) (PVDF) film was grafted with styrene (St) and dimethylaminoethyl methacrylate (DMAEMA) using γ-irradiation techniques. Through subsequent sulfonation and protonation processes, a new kind of amphoteric ion exchange membrane (AIEM) was synthesized. The grafting yield (GY) increased with absorbed dose and leveled off at about 60 kGy. The composition of poly(St-co-DMAEMA) grafts was correlated to the ratio of St to DMAEMA monomer in the feed. Micro-FTIR and XPS analyses testified that the grafting and sulfonation of St unit in poly(St-co-DMAEMA) grafts had been carried out as designed. Further characterizations showed that the properties of the AIEM strongly depended on the composition and GY of the film, i.e. higher content of DMAEMA brought lower permeability of vanadium ions and conductivity, while higher GY led to higher water uptake, ion exchange capacity (IEC) and conductivity. Finally, an AIEM with a GY of 26.1% was assembled and tested in the vanadium redox flow battery (VRFB) system. It was found that the VRFB assembled with the AIEM maintained an open circuit voltage (OCV) higher than 1.2 V after placed for 68 h, which was much longer than that with the Nafion117 membrane. Therefore, this work provides a novel method to develop potential substitute of Nafion membranes to be applied in VRFB system.