Determination And Characterization Of Mobility Of Active Ion In Electrolyte Of All-vanadium Redox Flow Battery

In recent years,wind energy,solar energy and other renewable energy developed rapidly under the background of energy crisis and environmental pollution.However,these renewable energy sources are intermittent,and only effective,safe,inexpensive and reliable energy storage systems can be made to make renewable energy practical.Energy storage technology plays a key role in improving the stability of the grid,cutting the peak valley and regulating the load.The flow battery is considered to be the most promising energy storage technology because it has the characteristics of non-dependence on geographical conditions,separates the power density from the energy density and is suitable for a variety of applications.Among them,all-vanadium redox flow battery(VRFB)is constituted of electrode,exchange membrane and vanadium electrolyte.And VRFB is the most mature energy storage technology,which has so many advantages such as high energy efficiency,long life,no cross contamination,low price and so on.It can be used as large-scale energy storage devices for wind energy,solar energy and other renewable energy.The chemical energy of the electrolyte can be converted into electrical energy output at the time of use.However,all-vanadium redox flow battery is also faced with technical problems in the commercial application process,including ion exchange membranes without perfect selectivity as well as lower power density and lower energy density.Increasing the power density can reduce battery costs,improve energy efficiency,and make the battery more flexible for load changes.The power density is determined by kinetic polarization,ohmic polarization and mass transfer polarization.The ohmic polarization and mass transfer polarization are directly related to the coupling of mass transfer and charge transport.Therefore,it is of great significance to develop a porous media electrode with a sufficiently large area of activity and to deepen the understanding of the transmission process and properties in an all-vanadium redox flow battery.However,the all-vanadium redox flow battery is a complex multi-scale system and contains multiphase flow and electrochemical reactions in porous electrodes.So there is little research on the transmission characteristics of the porous electrode in the vanadium flow battery and the corresponding influence on the power density.The conception and basic idea of this paper are mainly to improve the methodof characterization of mass transfer characteristics in the porous electrode of all-vanadium redox flow battery.The current ion mobility expression used for all-vanadium redox flow cells does not contain the interaction between ions and ions and solvents.Based on the theoretical framework of electrode mass transfer of all-vanadium redox flow battery and the theory of electrochemical principle and porous media transfer,this paper presented a new method for the determination of active ion mobility in the electrolyte solution of vanadium.By the difference between the limit current,we indirectly obtained the true theoretical model of ion mobility.In this paper,a set of flexible and innovative experimental devices were designed and assembled by using the applied electric field and the newly designed battery structure.The influence of the electrolyte concentration and the electrolyte flow rate on the ion mobility was systematically studied.The size and direction of the electric field strength were changed by adjusting the spacing between the parallel plates which were 3 cm、7 cm respectively and the magnitude of the applied voltage which were 0 V、0.5 V、4 V、32 V respectively.The concentrations of vanadium ions in the mass transfer half-cell were 0.1 M,0.2 M and0.4 M,respectively,and the flow rates of the control electrolyte were 6 mL / min,12 mL / min and 18 m L / min,respectively.This thesis describes the experimentally encountered while effectively solved four problems,namely: the electrolyte is highly corrosive;The V2+ ions can easily be oxidized and can not guarantee the battery SOC consistent;The collector plate on the external electric field generated electrostatic shielding effect;And the defects of aluminum foil structure have an influence on the field intensity distribution.The experimental results show that the applied electric field has little effect on the limit current density of the all-vanadium redox flow battery,and three possible reasons are proposed for the explanation of this phenomenon.Finally,based on the experimental data,the ion mobility in a specific working range is fitted with respect to the concentration of the electrolyte,which reflects the effects of ion concentration and ion size.