Effect Of Metal Ion Modified Ion Exchange Membrane On Cycling Stability Of Vanadium Redox Flow Batteries

The vanadium flow battery?VRFB?is considered to be an excellent green large-scale energy storage technology due to its advantages of high energy conversion efficiency,long cycle life,simple maintenance and environmental friendly.It can be widely used in peak load leveling of power plant,renewable energy generation and smart grid energy storage,as well as uninterruptible power supply or emergency power systems.In recent years,several sets of megawatt-level VRFB energy storage demonstration systems for solar and wind power generation have been established at home and abroad,indicating that VRFB-related technology has gradually matured.However,large-scale applications of VRFB still require breakthroughs in some key technologies.The rapid attenuation of VRFB capacity during charge-discharge cycles is one of the major technical obstacles to be solved.For this reason,this article first screened the influencing factors that led to the rapid decay of VRFB capacity.On this base,modification measures of ion exchange membranes were studied,and the feasibility of alleviating the rapid attenuation of VRFB capacity was explored.In the third chapter,a VRFB battery was prepared using a Nafion ion-exchange membrane as the separator,and the heat-treated graphite felts as the positive and negative electrodes respectively.The performance of as-prepared battery was tested using galvanostatic charge-discharge method.It was found that the morphology of the graphite felt and the ion exchange membrane before and after the charge-discharge cycle did not change significantly,and then the capacity decay of the battery can be ascribed to the electrolyte.In addition,during the charge-discharge cycle,the electrolyte volume of the positive electrode continuously increases,and the electrolyte at the negative electrode side decreases.According to the analysis of vanadium ions,the concentration of VO²⁺in the electrolyte at the positive electrode gradually decreases as the number of cycle's increases.The VO²⁺concentration gradually increases and the total vanadium ion concentration also increases.Changes in electrolyte volume and vanadium ion concentration destroy the charge balance of the positive and negative electrolytes,causing a rapid decay in the charge and discharge capacity of the battery.Further analysis shows that the unbalanced permeation of vanadium ions with different amounts of bound water across the ion membrane is the main reason for the net migration of vanadium ions and water from the negative electrode to the positive electrode.This is mainly due to the fact that commercial Nafion ion exchange membranes prevent the inability of vanadium ions to diffuse.Based on the above findings,in the fourth chapter of the thesis,the liquid phase impregnation method was used to react the sulfonic acid group of Nafion membrane with Pb²⁺in solution,and the H⁺ion in Nafion membrane sulfonic acid group was replaced with Pb²⁺ion to make Pb-Nafion117?Pb-N?ion membrane.The experimental results show that the water absorption,swelling ratio,ion exchange capacity and proton conductivity of Pb-N film decrease with the increase of the concentration of lead nitrate solution used in the impregnation process.This can be attributed to the fact that some of the sulfonic groups in the ionic membrane are converted to sparingly soluble lead sulfonates.In addition,it was also found that the vanadium ion permeability of the Pb-N film also decreased significantly with the increase of the concentration of the lead nitrate solution during the impregnation.On the one hand,due to the decrease of ion exchange capacity,the amount of vanadium ions passing through the membrane through ion exchange is decreased;on the other hand,it can be attributed to the fact that the ion membrane channel is blocked by partially insoluble salt of lead sulfonate,and the pores become smaller.Hydrated vanadium ions are difficult to diffuse through the membrane.This indicates that the use of Pb²⁺modified Nafion membranes to inhibit the penetration of vanadium ions is feasible.With different Pb-N ion membranes assembled into a simulated battery,galvanostatic charge and discharge experiments were performed with a current density of 50 mA·cm^-2.The capacity decay of batteries assembled with Pb-N membranes is slower than that of the original Nafion117 membranes.The effect of Nafion membranes modified by metal ions such as Ag⁺,Ca²⁺and Ba²⁺which can form insoluble salts with sulfonic acid groups on the diffusion of vanadium ions through the membranes was further investigated.The experimental results show that the amount of metal elements in the modified ion membrane is not the same due to the influence of Ag⁺,Ca²⁺,Ba²⁺ion radius and the solubility of corresponding salts,and the water absorption and swelling rate of the modified ion membrane and The capacity of ion exchange has decreased to some extent,and the capacity of vanadium resistance has also been improved to a certain extent.Cells with Ag,Ca,Ba,and Pb-N membranes as the separators exhibited good charge-discharge stability.Compared with the unmodified Nafion membrane,the battery capacity decay phenomenon has been suppressed to a certain extent,and the battery with the Pb-N membrane as the separator shows the best charge and discharge stability.