Preparation And Performance Of Sulfonated Polysulfone-based Composite Membrane For All-vanadium Redox Flow Battery

All-vanadium flow battery(VRB)has received extensive attention as a high-energy conversion rate and cost-effective large-scale energy storage system.Proton exchange membrane is the core component of VRB,which plays the role of isolating electrolyte and transporting protons.Therefore,its performance seriously affects the commercialization prospects of batteries.Nafion series proton exchange membranes are commercial products of DuPont;however,their high cost and high vanadium ion permeability affects the large-scale application.Therefore,the development of high proton conductivity,low vanadium ion permeability,good chemical stability,and low cost proton membranes is highly desirable as an alternative for Nafion series membrane.A sulfonated polysulfone having a sulfonation degree(DS)of 62%was prepared by sulfonation reaction,and then a series of SPSF/SPEEK composite proton exchange membranes were prepared by adjusting the mass ratio of sulfonated polysulfone(SPSF)and sulfonated polyetheretherketone(SPEEK).The addition of SPEEK improves the proton conductivity of the composite membrane.The composite membrane at the ratio of 5:5 for SPSF/SPEEK exhibits the highest ion selectivity(16.33 S min cm-3)for SPSF/SPEEK-50,about 4 times that of Nafion 117 membrane(3.99 S min cm-3).The vanadium battery performance showed that the energy efficiency(EE)of the composite membrane reached 87.55%at 50 mA cm-2 current density,which was higher than Nafion 117(EE:83.5%).The self-discharge time of the composite membrane was 54 h,which is 2.3 times higher than Nafion 117(23.4 h).The SPSF/SPEEK-50 composite membrane maintains good stability after 100 cycles.The graphite-like phase carbonitride carbon(g-C3N4)was prepared by thermal polycondensation method,and the SPSF/SPEEK/g-C3N4 composite membrane with different g-C3N4 content was prepared by selecting SPSF/SPEEK-50 as the base membrane.Studies have shown that the g-C3N4 effectively reduces the vanadium ion permeability of SPSF and improves the proton conductivity of the composite membrane.The SPSF/SPEEK/g-C3N4-1 composite membrane presented the highest ion selectivity,and the assembled vanadium redox flow battery was tested.At 50 mA cm-2 current density,the coulombic efficiency and energy efficiency of the composite membrane(CE:94.2%,EE:88.24%)were significantly better than Nafion 117 membrane(CE:94%,EE:83.5%).After 100 cycles of charge and discharge,the SPSF/SPEEK/g-C3N4-1 composite membrane exhibited excellent cycle stability.