Poly(vinylidene difluoride-co-hexafluoropropylene) Grafted with Polyether Amphoteric Ion-Exchange Membranes for Aqueous Redox Flow Batteries

Aqueous redox flow batteries (ARFBs) are promising candidates for large-scale energy storage due to their high energy efficiency. Nafion ion-exchange membranes have been used in ARFB applications due to their good ionic conductivity and excellent chemical and mechanical stability, but Nafion's high cost, excessive swelling, and low ion selectivity limit its use for commercialization. Amphoteric ion-exchange membranes (AIEMs) have potential for preventing unwanted ion penetration, thus increasing ion selectivity. In our current work, we have synthesized an economical cross-linked network to make AIEMs. Membranes were synthesized by grafting of an epichlorohydrin (ECH)- and propargyl glycidyl ether (PGE)-based, charged copolymer (S-PPGE-ECH) to the poly(vinylidene difluoride-co-hexafluoropropylene) membrane matrix. We varied the copolymer composition in the matrix to study its effect on the ionic conductivity. All membranes were characterized using NMR and Fourier transform infrared spectroscopy to confirm the presence of sulfonate and quaternary ammonium charged groups. A gradual increase of the copolymer content decreased the contact angle from 83 degrees (1% S-PPGE-ECH) to 73 degrees (10% S-PPGE-ECH), indicating improvement in the hydrophilicity of membrane surfaces, which resulted in a commensurate increase in the swelling ratio from 17.5% to 27.0%, comparable to 18.6% for Nafion117. We further saw a proportionate increase in the ionic conductivity with an increase in the copolymer content up to 4.00 mS/cm. The cation-exchange capacity also increased steadily with increasing copolymer content while the anion-exchange capacity increased only slightly. The increase in the exchange capacity was attributed to the presence of -NR3+ and -SO3H groups in the membranes. In this work, we demonstrate control over the ionic conductivities by tuning the copolymer content without compromising the thermal and mechanical stability of the membrane.