Low-cost wire-electrospun sulfonated poly(ether ether ketone)/poly(vinylidene fluoride) blend membranes for hydrogen-bromine flow batteries

Abstract Cost-effective dense membranes were developed by blending proton-conductive sulfonated poly(ether ether ketone) (SPEEK) with inert, mechanically stable poly(vinylidene fluoride) (PVDF) for hydrogen-bromine flow batteries (HBFBs). Wire-electrospinning followed by hot-pressing was employed to prepare dense membranes. Their properties and performance were compared to solution-cast membranes of similar composition and thickness. Electrospinning improved the ionic conductivity and bromine diffusion properties by providing interconnected ion-conductive SPEEK nanofiber pathways through a PVDF matrix. Relatively thin (∼50–60 μm) electrospun membranes with a SPEEK/PVDF ratio (wt%/wt%) of 90/10 and 80/20 showed comparable Br3− diffusion rates as the relatively thick and commercially available perfluorosulfonic acid (PFSA) membrane (∼100 μm) at a 35%–42% lower proton conductivity. The latter can be attributed to the poorer ion conductivity of SPEEK compared to PFSA and the presence of PVDF. The HBFB single cell featured the best polarization behavior and ohmic area resistance with the electrospun membrane containing 80/20 (wt%/wt%) SPEEK/PVDF. However, the low thickness and insufficient chemical/mechanical stability of the ES 80/20 causes a rapid decay in the HBFB cycling performance. This study promotes a life-time comparison study between the low-cost wire-electrospun SPEEK/PVDF blend membranes (∼€100 m−2) and the typically used PFSA membranes (∼€400 m−2) for a long-term HBFB performance.