Preparation And Properties Of Sulfonated Polysulfone Proton Exchange Membranes For All Vanadium Redox Flow Batteries

The vanadium redox flow battery(VRFB)is a kind of green,safe and efficient energy storage battery.It has the advantages of long service life,fast charging and discharging response,recyclable and through the four valence states of vanadium ions to store and release electric energy.Therefore,it is suitable for the application of large-scale energy storage and backup power supply.Proton exchange membrane as an important part of VRFB,its role is to isolate the positive and negative electrolyte,avoiding the cross-contamination of vanadium ions and realizing the proton transfer.Commercialized Nafion series membranes have good proton conductivity and mechanical properties their high price and high vanadium ion permeability seriously affect the development of VRFB.So,a membrane with low cost,low vanadium ion permeability and high proton conductivity is the key to push the development of VRFB.In this paper,based on the preparation of polysulfone-based proton exchange membrane with different sulfonation degree,the structure and performance of sulfonated polysulfone membrane doped with graphitic carbon nitride(g-C3N4)are investigated.The application and battery performance of VRFB are studied.A series of sulfonated polysulfones(SPSF)with different sulfonated degree are prepared by controlling the molar ratio of trimethylchlorosilane-chlorosulfonic acid to polysulfone.The effect of sulfonated degree on the physical and chemical properties of SPSF membrane and the performance of VRFB is investigated.The results show that the water absorption,swelling degree,vanadium ion permeability and proton conductivity of SPSF membrane increases with the increased sulfonated degree,followed by a decreased mechanical property.The SPSF membrane(SPSF-62 membrane)with the highest ionic selectivity is selected to test the performance of vanadium redox flow battery.The results show that the cell efficiency of SPSF-62 membrane is better than that of the Nafion 117 membrane,and it has a good chemical stability.By incorporation of graphitic carbon nitride(g-C3N4)into SPSF-62 membrane,SPSF hybrid membrane is prepared.The addition of g-C3N4 nanosheets effectively reduces the permeability of vanadium ions,and the sulfonic group of the membrane formed an acid-base interaction with the amino group in g-C3N4,forming a stable proton transport channel,thereby enhancing the proton conductivity of the composite membrane.The SPSF-62/g-C3N4-1 hybrid membrane with the highest ion selectivity is tested for VRFB performance.The coulombic efficiency and energy efficiency of the composite membranes are 99.1%and 74.8%,respectively,which are significantly better than those of Nafion 117 membrane(94.3%and 58.9%)at 60 mA cm-2 current density.After 300 charging-discharging cycles,the SPSF-62/g-C3N4-1 hybrid membrane demonstrates a excellent cyclic stability.