Preparation And Properties Of Sulfonated Polyetheretherketone-based Composite Membranes For All Vanadium Redox Flow Batteries

The vanadium redox flow battery(VRB)has received considerable attention due to its long cycle life,flexible design,fast response time,deep-discharge capability,and low pollution emissions in large-scale energy storage.The key component of VRB is an ion exchange membrane that prevents cross mixing of the positive and negative electrolytes by separating two electrolyte solutions,while allowing the conduction of ions.The membranes commonly used in VRB are perfluorosulfonic acid(PFSA)membranes such as Dupont's Nafion due to its high chemical and mechanical stability and high proton conductivity.However,there are serious drawbacks in these membranes,which are the extremely high cost and high vanadium crossover.Also,the high vanadium ion permeability leads to the decrease of CE and EE of VRB.Thus,further experiments to find an alternative membrane for VRB system are carried out.Series of 3-aminobenzene sulfonic acid-functionalized graphene oxide/sulfonated poly(ether ether ketone)(SPEEK/SGO)hybrid membrane were prepared through a post-grafting technique.The addition of SGO enhanced the proton conductivity of SPEEK/SGO hybrid membrane.The physicochemical properties such as swelling ratio,ion exchange capacity,proton conductivity,vanadium ion permeability,etc.change with the content of SGO.Compared with Nafion 117 membrane(CE:90.2%,VE:73.8%),SPEEK/SGO-3 membrane exhibits the better coulombic efficiency(95.6%)and energy efficiency(81%)at 30 mA cm-2.After 300 charge-discharge cycles,SPEEK/SGO-3 membrane gives a better chemical stability at the circumstance of oxidizing and acid condition.Hybrid membranes(SPEEK/g-C3N4)composed of sulfonated poly(ether ether ketone)(SPEEK)and graphitic carbon nitride(g-C3N4)were fabricated via a solution-casting method.SPEEK/g-C3N4-1.5 hybrid membrane has the lowest vanadium permeability about 3.7×10-7 cm2 min-1 because of the interaction between the acid and amino groups.SPEEK/g-C3N4-1.5 hybrid membrane exhibits a higher coulombic efficiency(CE:97%)and energy efficiency(EE:83.6%)at 30 mA cm-2,as compared with Nafion 117(CE:90%and EE:73.8%)and SPEEK membrane(CE:89.9%and EE:76.1%).The self-discharge time of VRB with SPEEK/g-C3N4-1.5(68 h)is longer than that of Nafion 117 membrane(26 h).A good structure stability against strong oxidizing and acid condition is proved under 300 charge-discharge cycles.The formed acid-base pairs between-NH?-NH2 groups of g-C3N4 and sulfonic acid groups of SPEEK contribute the comprehensive properties of vanadium ion permeability and its selectivity,accompanied with an enhanced proton transport channel.These SPEEK/g-C3N4 hybrid membranes provide the acceptable candidate for VRB application because of their high structure stability and ion selectivity.