Swelling-induced ethylmorpholinium-functionalized adamantane-containing poly(aryl ether ketone) membranes with high conductivity and selectivity for vanadium redox flow batteries

Novel swelling-induced ethylmorpholinium-functionalized adamantane-containing poly(aryl ether ketone) (EMAPEK) membranes are prepared for vanadium redox flow batteries (VRFBs). Ethylmorpholinium groups are grafted onto the rigid adamantane-containing backbone, and then the membranes are swelling-induced with concentrated H3PO4 to generate microphase separation and enhance membrane conductivity. The EMAPEK membranes exhibit a low swelling ratio (<4.4%, in-plane) and lower areal resistance (e. g., EMAPEK-160, 0.12 Ω cm2) than the Nafion212 membrane (0.14 Ω cm2). EMAPEK membranes exhibit impressive battery performance in VRFB (e.g., EMAPEK-160 membrane, at 80 mAcm−2, coulombic efficiency = 98.3%, energy efficiency = 92.7%; and at 200 mAcm−2, coulombic efficiency = 98.5%, energy efficiency = 85.2%). These energy efficiencies are the highest reported values for dense ion-exchange membranes. Furthermore, the VRFB with EMAPEK membrane exhibits excellent long-term stability at high current density (≥200 mAcm−2) for 3000 cycles. This work provides a facile and practical method for high-performance membrane preparation and available promising membranes for VRFB.