Investigation Of Nafion Membrane With Ionic Sieving Effect For Vanadium Redox Flow Battery

All-vanadium redox flow battery is an important large-scale energy storage device.Nafion membranes are widely used as separators for all-vanadium redox flow batteries because of their high proton conductivity and excellent chemical stability.But at the same time,because of its severe penetration of vanadium ions,the problem of low coulombic efficiency of the battery and rapid rate of capacity decay limits its wider application.Therefore,modification of Nafion membrane to resistance vanadium and increase ion selectivity have drawn much attention.In this paper,Nafion membrane is selected as the matrix material,and ZSM-35 with strong sieving effect and graphene oxide?GO?with two-dimensional lamellar structure are selected to modify Nafion membrane by solution casting method and spin-coating method to enhance the vanadium-resistance of the membrane,increase the ion selectivity of the membrane,and achieve ideal cell performance.Firstly,Zeolite/Nafion blend membranes were prepared using a solution casting method,ZSM-35 was introduced into Nafion membrane to limit the swelling of Nafion,and the pore sieving effect of ZSM-35 was used to enhance vanadium resistance of the membrane while ensuring the proton conductivity.The results show that M-10%has the best overall performance.The vanadium permeability coefficient is reduced by 28.8%and the ion selectivity is increased by 57.0%compared with P.N.At 80 mA cm^-2,the coulombic efficiency increased by 2.6%and the energy efficiency increased by 2.9%.The number of long-cycle laps increased from 28 to42 laps,and its capacity decay was only 0.49%/cycle,which was far less than the 2.10%/cycle of P.N.In order to prepare a composite membrane with stronger vanadium resistance,a ZSM-35zeolite layer was spin-coated on the surface of Nafion212 membrane,and the vanadium ions were blocked by the sieving effect of the zeolite layer,which could simultaneously conduct hydrogen ions.Ultrathin spin-coating layer also reduced the effect of zeolite on proton conductivity,thus increasing ion selectivity.Among them,the spin-coating layer thickness of M-10 composite membrane is 1.43?m,and vanadium ion penetration is significantly weakened.Compared with Nafion212,the penetration coefficient of vanadium ion of M-10 is reduced by93.6%,and ion selectivity is improved by an order of magnitude.At 100 mA cm^-2,the coulombic efficiency of M-10 is increased by 5.8%,and the energy efficiency is increased by4.9%.In the long-cycle test,M-10 showed excellent cycle performance,and its capacity decay rate was only 0.29%/cycle at 100 cycles,which was far less than that of Nafion212 with 0.47%/cycle?50 cycles?.Finally,a sieving GO/N212 composite membrane with strong interfacial binding force was prepared.By pretreating the Nafion212 soaked in APTES solution,the inorganic nanoparticles containing amino groups on the Nafion212 surface can produce molecular bonds with the epoxy groups in GO and strengthen the interfacial binding force.Different diamine cross-linkers were used to control the spacing of the GO layers of the composite membranes,and to enhance the sieving effect and vanadium resistance of the membrane.Among them,the permeability coefficient of M-mXDA is reduced by 84.1%and the ion selectivity is increased by 3.54 times compared with Nafion212.At 20 mA cm^-2,the coulombic efficiency is increased by 36.8%and the energy efficiency is increased by 35.3%.The membrane was stable at 180laps in long-cycle test,and its capacity decay rate was only 0.25%per cycle,which was far less than that of Nafion212 with 0.47%/cycle.