Investigation On The Preparation And Performances Of Metal-organic Framework Composite Membrane For Vanadium Redox Flow Battery

All vanadium redox flow batteries（VRB）are receiving more and more attention due to their long life and low cost.At present,the high price and high vanadium ion permeability of commercial Nafion membrane limits the promotion and application of VRB.Therefore,the development of low-cost,high-performance vanadium battery separators is of great practical significance.Vanadium battery membrane can be divided into porous membranes and ion exchange membranes.Whether it is a porous membrane with a relatively wide pore size distribution prepared by phase inversion method or a dense ion exchange membrane prepared by solvent evaporation method,it is necessary to further improve their ion selectivity.Metal organic framework（MOF）is a type of microporous material assembled by metal-containing units and organic linking groups.Among all the MOFs,a zirconium-based MOF-UiO-66 and its derivatives are expected to improve the vanadium barrier performance of the membrane due to the appropriate pore size（0.24nm<d<0.6 nm）and strong hydrophilicity which can improve the ability of the membrane to conduct H⁺.Therefore,this paper aims to apply MOF to VRB membrane and obtain highly selective membrane by optimizing the ratio of MOF and their functional derivatives in porous and ion exchange membranes.The porous membrane formed by the phase inversion method has a relatively wide pore size distribution.Nanoparticles are introduced into skin layer of porous membrane by interfacial polymerization to obtain a TFN（thin film nanocomposite membrane）membrane with a smaller average pore size.In this paper,UiO-66 and its functionalized derivatives are introduced into polyamine TFC（thin film composite membrane）through interfacial polymerization to prepare four different series of TFN membranes.The best ratio of the UiO-66 and its functionalized derivatives required for preparing TFN membranes are obtained through ion selectivity tests.Single cell test was performed on optimized membrane of four different MOFs.The results show that when the current density is 80 mA cm^-2,the energy efficiency（EE）of TU/0.1%is 72.4%,which is higher than polyamine TFC（63.3%）;The EE of T-U-N/0.15%、T-U-2C/0.15%and T-U-S/0.1%are all higher than 75%.Therefore,all four TFN films have better battery performance than polyamine TFC films.Solvent casting is a commonly used method for preparing dense membranes.In this paper,UiO-66 and its functionalized derivatives are blended with polyvinylidene fluoride（PVDF）and polypyrrolidone（PVP）to prepare blended membranes by solvent casting.The conductivity,area resistance of the blend membranes with different addition amounts in each series were tested.The results show that the conductivity of the blend membrane with UiO-66-SO₃H is the highest,which is in accordance with the area resistance test.The vanadium permeation and the ion selectivity of PVDF-PVP/U-S membranes with different doping levels were further tested.The results show that the ion selectivity of M_U-S/12%（3000）is the highest.The membrane M_U-S/12%were soaked in the VO₂⁺/H₂SO₄solution for 10 days,and its mass loss rate is 0.56%,which proves that the membrane can withstand the strong acid and strong oxidizing environment of the vanadium battery.The single cell test show that when the current density is 80 mA cm^-2,the EE of M_U-S/12%is 81.4%,which is 5.0%higher than Nafion212.Obviously,M_U-S/12%has better battery performance than N212.The EE of M_U-S/12%can be maintained at about 79%in 100 long cycles.In a word,UiO-66 and its functionalized derivatives play a positive role in the barrier of vanadium ions and help to tramsportation of H⁺both in porous and ion exchange membranes.