Highly selective core-shell structural membrane with cage-shaped pores for flow battery

A blend membrane with the core-shell structure was designed and prepared for vanadium flow batteries (VFBs). The core is the membrane manufactured by poly(vinylidene fluoride-hexafluoropropylene) (PVDF-HFP) and polyvinylpyrrolidone (PVP) anion exchange resin, while the shell is composed of cage-shaped pores induced by VO2+ treatment. The high ion selectivity of PVDF-HFP/PVP blend membranes with the core-shell structure is assured by the ion sieving effect of cage-shaped pores and Donnan exclusion effect of anion exchange resin. The convection mechanism of charge carriers across pores on the surface and surface layers, together with the Grotthuss mechanism across the deep-seated anion exchange membrane jointly contributes to high ion conductivity of blend membranes. A great balance between high ion selectivity and high ion conductivity can thus be achieved. Moreover, the formation of cage-shaped pores can remedy the loss of chemical stability caused by anion exchange groups in PVP. A VFB with the resultant membrane could achieve a columbic efficiency of 98.16%, and an energy efficiency of 88.01% at 80 mA cm−2, much higher than those of Nafion 115. This core-shell structural blend membrane is an entirely novel concept to combine advantages of porous membranes and ion exchange membranes, while avoid their drawbacks simultaneously.