Research On The Cycle Stability Of Vanadium Redox Flow Batteries

Due to the unique advantages,such as environmental benign,high energy efficiency,long cycle life,independent power and capacity,etc,all vanadium redox flow battery（VRB）has a sound application prospect in the fields of renewable energy power storage,power peaking and military power storage and so on.Researches have achieved some achievements on VRB,but VRB application development is still in its infancy,further research is needed in the electrode material,the electrolyte,the membrane and cycling stability,etc.At present,on the long-term operation of VRB,a serious problem is the poor battery cycle stability,especially capacity fading fast during cycling.Therefore,on the one hand,In order to find ways to improve the cycle performance of VRB,it is necessary to explore the causes of cycle fading.On the other hand,there are so many factors that influent the cycle performance,furthermore,the cycle test spend long time and high cost.it is convenient to simulate the performance of VRB through mathematical model in a variety of situations,so as to optimize design and control system of VRB and promote the industrialization development of VRB.Based on the comprehensive literature review of domestic and foreign researchers on VRB,this paper used analytical testing methods such as automatic potentiometric titrator and battery performance tester,as well as mathematical modeling tool of Matlab7.0 to make a systematic study in cycling performances of VRB on experimental tests and mathematical simulation.In order to measure the changes of vanadium ion concentration during charge-discharge cycling,the third chapter of the paper proposes a potentiometric titration method for the analyzing the concentrations of vanadium ions with different valance in the electrolytes of VRB.The low valence vanadium ions in solution were oxidized to VO₂⁺using Ce（SO₄）₂as an oxidizing agent,followed by（NH₄）₂Fe（SO₄）₂ as titrant,potentiometric titration analysis by potentiometric titrator.According to potentiometric titration curve obtained two titration end point,corresponding to Ce⁴⁺reduced to Ce³⁺and VO₂⁺is reduced to VO²⁺.For any vanadium electrolyte system,this same method could be used to determine the concentration of vanadium ions with different valence state.The relative standard deviations（N=5）of the concentrations of vanadium ions determined by this method were no more than 2.55%.The method of operation is simple,fast and can be used to accurately analyze the concentrations of vanadium ions with different valance in the electrolytes of VRB.According to the above analysis methods of vanadium ion concentration,the fourth chapter of the paper discusses the performance changes of vanadium battery cycle.Firstly,the chapter studies volume variation of the positive and negative electrolyte during charge-discharge cycling,the results showed that:the volume change of the positive and negative,especially after 4-5 cycles,mainly due to migration of vanadium ions between the positive and negative.Secondly,change of the vanadium ions during charge-discharge cycling is studied,the results showed that:during the cycle,the total amount of vanadium ions in the positive electrode is gradually increased.Finally,the chapter studies the change of the electrical properties of VRB during cycling process.The results show demonstrate that positive VO₂⁺ions was gradually accumulate,V²⁺gradually reduce in the negative half-cell,at the same time the volume of the positive electrode was increased gradually,the volume of the negative electrode decreased.This is in accordance with the electrolyte of the negative electrode is less and less,resulting in the charge and discharge time is reduced,the battery capacity to decay.The fourth chapter also examines the carbon felt,separator,electrolyte additive,the negative charge state of reserves and the different positive and negative volume ratio on the cell cycle stability.The results showed:carbon felt and membrane have good stability in the electrolyte,no obvious deterioration of performance after the cycle.The negative electrode added Al₂（SO₄）₃ additive has better performance,the cell capacity is 79.6%of initial capacity,68.6%of the maximum capacity after 200 circulation,and the cycling performance of the cell can be improved to a certain extent.In addition,the charge state reserve capacity of the negative electrode is advantageous to improve the capacity of the VRB,positive and negative electrode electrolyte volume ratio 1:1.3 with the higher average discharge capacity,but there is no significant improvement in the cycle stability of the battery.In the fifth chapter,based on the reference of the original mathematical model,a new dynamic mathematical model is established by introducing the volume variables and the hydrogen sulfate ion variables.First,this model is used to simulate various ions during cycle through the membrane as well as changes in volume,capacity changes.The simulated result show that it is accordance with the data of literature just slightly different in the hydrogen ion variation from the literature.Secondly,the effects of different concentration diffusion data on the simulation results of the two groups were compared in detail,one group of data simulation results is consistent with experimental data of the literature.In addition,the present model is also simulate the effects of different K₂ values（two levels of dissociation of sulfuric acid）??on the performance of battery,the results show that the different K₂ values nearly have no influence on the shape of the cycle diagram,just a little change on the value.Finally,some experiments of the fourth chapter were simulated by the present model,simulation results compared with experimental results,indicated that the model simulation results and the experimental results are in good agreement,and verified the reliability of the present model.