Sulfonated poly(ether ether ketone) membranes for vanadium redox flow battery enabled by the incorporation of ionic liquid-covalent organic framework complex

In vanadium redox flow battery (VRFB). Sulfonated poly(ether ether ketone) (SPEEK) is viewed as a promising alternative to Nafion as a material for vanadium liquid flow battery membrane, due to its low cost and high proton conductivity. However, SPEEK membranes have a Trade-Off effect of proton conduction and ion selectivity, which limits the performance of VRFB. Here, we exploited the Donnan effect of imidazole cations in ionic liquids (ILs) to improve the ion selectivity of SPEEK membrane. In addition, used porous covalent organic framework materials (COF) with sulfonic acid functional groups to immobilize ionic liquids, which reduced the loss of ILs. The ion selectivity of the SPEEK membrane was further improved. As a result, the cell performance of the composite membrane is improved. At a constant current density of 40 mA cm(-2), SPEEK/IL-5@TpPa-SO3H-3 membrane exhibits high coulombic efficiencies (CE: 98.43%) and energy efficiencies (EE: 91.7%). Furthermore, the energy efficiency of the SPEEK/IL-5@TpPa-SO3H-3 membrane decreases only 2.1% after 60 charge/discharge cycles at 100 mA cm(-2). Our work proposed a cost-effective, high-performance membrane for VRFB.