Preparation And Characterization Of Bifunctional Membranes For Vanadium Redox Flow Battery Application

Ion exchange membrane is one of the most important parts of all vanadiumredox flow battery （VRB）. It determines the life and performances of VRB to agreat extent. At present, Nafion （Dupont） and sPEEK （sulfonated poly ether etherketone） are the most widely used membranes in VRB. These two membranes havegood stability and electrochemical properties, but they also suffer from highvanadium and water permeability when they are used in VRB. These problems leadto imbalance of electrolyte and affect cycling performance of VRB. Singlefunctional modifier is normally used to prepare Nafion and sPEEK compositemembrane in traditional method. The single functional modifier can reduce thevanadium permeability, and it also reduces the electrical conductivity of the singleVRB cell and lead to the decrease of the votage efficiency of the VRB.Therefore, bifunctional s-MPMDMS was firstly prepared by sulfonationreaction using MPMDMS （（3-mercaptopropyl） triethoxysilane）. Then thebifunctional composite membrane （Nafion/s-MPMDMS and sPEEK/s-MPMDMSmembrane） with lower vanadium permeability and higher electrical conductivity forVRB application were prepared by solution recasting method. The effect of modifercontent and sulfonation on the performance of Nafion and sPEEK compositemembrane was studied particularly. Finally, the relationship between themechanisem and physical-chemical parameters of VRB membrane wasinvestigated.FT-IR, SEM and EDS test show that the additive has been added into thecomposite membrane and distributed evenly. The XRD test shows that theamorphous additive affects little to the crystallization state of composite membrane.For Nafion and sPEEK composite membrane, further tests prove that they can blockthe vanadium permeation and keep the conductivity of the membrane at the sametime. For N/sM composite membrane, when the content of modifier is4wt.%, thevanadium permeability （P） is2.60×10^-7cm²·min^-1, and electrical conductivity is93.10mS·cm^-1. Battery test shows that at the current density of40mA·cm^-2,4wt.%N/sM membrane has the highest energy efficiency （82.1%）. For the4wt.%N/Mmembrane, P is1.19×10^-7cm²·min^-1and conductivity is57.30mS·cm^-1, and thehighest energy efficiency is75.0%at the current density of40mA·cm^-2. ForsPEEK composite membrane, the10wt.%sP/sM membrane shows the higherenergy efficiency and conductivity than that of sP/M membrane, which is87.80%and97.99mS·cm^-1, respectively. Further tests show that the composite membraneswith additive of s-MPMDMS have longer self-discharge time, longer cycle life and better chemical stability than that of pure Nafion and sPEEK membrane. Thecomposite membrane can reach up to the long time utility requirement when it isused in VRB.Finally, the relationship between the performances and its physical-chemicalparameters of Nafion has been studied by establishing a proton infiltration model.The results show that protons penetrated through membrane has great influence onconductivity and vanadium permeability in Nafion based composite membrane.Vanadium permeability and conductivity increase with the increasemnt of protontransportation through membrane.