Study On Sulfonated Polyetheretherketone/Ionic Iiquid/Covalent Organic Framework Composite Membrane For Vanadium Redox Flow Battery

Under the trend of energy transformation,vanadium redox flow batteries are favored by researchers as a safe and efficient energy storage method.However,the high cost of ion-exchange membranes,as one of the core components of vanadium redox flow batteries,hinders the commercialization of vanadium redox flow batteries.Sulfonated polyetheretherketone（SPEEK）has the advantages of excellent physicochemical and electrochemical properties and low cost,and is regarded as a potential material to replace Nafion as a separator for vanadium flow batteries.However,the SPEEK membrane has a Trade-Off effect of proton conductivity and vanadium resistance.Doping imidazole-based ionic liquid（IL）is an effective solution.While IL has good ionic conductivity,imidazolium cations have a repulsive effect on vanadium ions,which is expected to solve the problem of SPEEK.Therefore,1-butyl-3-methylimidazolium tetrafluoroborate ionic liquid is selected to modify SPEEK in this paper.In addition,to prevent the loss of IL,IL is immobilized into covalent organic framework（COF）,and it is expected that the two would work together to enhance the performance of the composite membranes.The specific research contents are as follows:1.SPEEK/IL composite membranes with different IL contents are prepared by coating.The interaction between IL and SPEEK and the effect of IL content on the performance of SPEEK are explored.Due to the Donnan repulsion effect of imidazolium cations and vanadium ions,the barrier properties of the composite membranes are significantly improved.The VO²⁺permeability of SPEEK/IL-5 membrane is 3.92×10^-7 cm²·min^-1,which is significantly lower than that of SPEEK and Nafion115 membranes.Meanwhile,the SPEEK/IL-5 membrane has excellent battery performance.At a current density of 40m A·cm^-2,the coulombic efficiency and energy efficiency of the SPEEK/IL-5 membrane are 98.7%and 88.1%,respectively,which are higher than that of the SPEEK and Nafion115 membrane.At a current density of 100 m A·cm^-2,the energy efficiency decreases by 3.3%after 60 cycles.The decrease in energy efficiency may be caused by IL loss.2.With the aim of solving the problem of the loss of IL and further improving the battery performance of the composite membrane,IL is loaded into COF（Tp Pa-SO₃H）containing sulfonic acid groups by vacuum loading method,and SPEEK/IL@Tp Pa-SO₃H composite membrane is prepared.The interaction among Tp Pa-SO₃H,IL,SPEEK and the effect of Tp Pa-SO₃H content on membrane properties are explored.The results show that,based on the confinement of COF pores and the interaction between groups,the loss rate of IL is significantly reduced.The IL loss of the composite membrane is reduced by 41.7%after adding Tp Pa-SO₃H@IL.For battery performance,the energy efficiency of the SPEEK/IL@Tp Pa-SO₃H-3 composite membrane is 91.7%at a current density of 40m A·cm^-2,which is better than that of the SPEEK/IL-5 and Nafion115 membranes（88.1%,84.4%）.At the same time,the cycle stability is further enhanced.At a current density of100 m A·cm^-2,the energy efficiency drops by 2.1%after 60 charge-discharge cycles.