PBI/ILs Composite Membrane Preparation, Mechanism Research And Application In All Vanadium Redox Flow Batteries

As a large-scale energy storage device,vanadium redox flow battery(VRFB)has received extensive attention from researchers with the development and utilization of renewable energy.The proton exchange membrane(PEM)is one of the most key components in the VRFB,which determines the battery efficiency and cycle life of the entire battery system.So far,the most widely used PEM is DuPont’s Nafion membrane,but there are disadvantageous in high cost and vanadium ion permeability severely limit their large-scale commercial application in VRFB.Polybenzimidazole(PBI)is a kind of polymer with high stability and relatively low cost.Dense PBI membranes exhibit excellent vanadium resistance in VRFB,but their proton conductivity still needs to be improved.In order to solve this problem,a series of PBI/ILs proton exchange membranes were prepared by blending various ionic liquids(ILs)with Poly(oxyphenylene benzimidazole)(OPBI),and the function mechanism of ILs was studied in the composite membrane.The main research contents are as follows;1.In order to improve the proton conductivity,OPBI was selected as the film-forming substrate,l-butyl-3-methylimidazolium tetrafluoroborate([BMIm]BF4)as the active unit.The hydrogen bond formed between the anion of[BMIm]BF4 and the-NH group in the OPBI imidazole ring,this reduces the loss of[BMIm]BF4.In addition,due to the Donnan repulsion effect between vanadium ions and cations of ILs,as the IL content increased,the vanadium resistance and proton conductivity of the composite membranes increased.The proton selectivity parameter of OPBI/BF4-20 composite membrane was as high as 1.41×106 S min cm-3,which was much higher than that of pristine OPBI membrane and Nafion115 membrane.What’s more,the OPBI/BF4-20 composite membrane exhibited stable battery efficiency after 1000-time cycles test,showing excellent cycle stability.In conclusion,the OPBI/BF4-20 composite membrane exhibits excellent Comprehensive performance in vanadium batteries.2.The function mechanism of ILs in PEM for VRFB is not clear.In this paper,three composite membranes were prepared by blending OPBI with[BMIm]BF4,1-ethyl-3-methylimidazolium tetrafluoroborate([EMIm]BF4)and l-vinyl-3-methylimidazolium tetrafluoroborate([VEIm]BF4),respectively,these three ILs contain the same anion.Under the premise of keeping the temperature and the content of ILs constant,this article discusses several factors that affect membrane proton conductivity,such as the viscosity,size,self-conductivity of ILs,and the affect of different ILs on the structure and molecular chain flexibility of composite membranes.Inaddition,according the two mechanisms of proton transfer,the function mechanism of ILs in composite membranes was investigated.It was found that the addition of ILs has an effect on the proton transport capacity of the composite membrane from the Grotthuss mechanism.