Study On The Optimization Strategy Of Transfer Process For Vanadium Redox Flow Battery

All vanadium redox flow（VRFB）is one green kind of high-efficient and environmental protection electrochemical storage equipment.Its main components are porous graphite felt electrode,current collector,electrolyte storage tank,and different valences of vanadium electrolyte with sulfuric acid.The positive and negative poles of battery are separated from ion exchange membrane,and the pros and cons of electronic in charge and discharge process are provided by oxidation and reduction reaction.The all vanadium redox flow has been widely used for the storage and distribution of wind energy,water energy and so on,which do well in cutting peak and filling valley on power system,with its characteristics of the output power and energy storage are independent,high charge and discharge efficiency,flexible and convenient design,long service life and low cost.Based on the mass transfer equation,charge transfer equation and electrochemical kinetics equation,a two-dimensional model of all vanadium redox flow battery and concentration polarization coefficient（CPC）were established and introduced,respectively.The polarization process in different electrolyte concentration,temperature,flow rate was studied quantitatively through the research of VFB polarization process and CPC with the finite element method.We also studied the mass transfer process of vanadium ion in proton exchange membrane.we have focused on the synergistic effect of concentration field and electric field in different operating conditions and the influence of electric field on the mass transfer process across membrane of vanadium ion was observed.Finally,we integrated the impact of the transfer process of battery performance in the proton exchange membrane area and electrode area,proposed the selection strategies of battery operating conditions.The results show:（1）Increasing electrolyte concentration,the cell activation polarization over potential and concentration polarization over potential was decreased obviously,the battery performance improves.In the middle region of the porous electrode,the electrode surface concentration is more close to the electrolyte bulk concentration;the difference between the surface concentration and the bulk concentration will be greater in the interface of electrode and the current collector.Increasing electrolyte concentration,CPC decreased,namely the influence of the factors that concentration lead to battery polarization reduced,electrochemical polarization tends to affect the electrochemical system,the affection of electron transfer steps on electrochemical process are more and more great.Based on this,It can increase electrolyte concentration with elevating the cell operating temperature appropriately to reduce the activation polarization,which is more conducive to further improve the performance of battery,in the real operation electric field environment with high concentration of electrolyte,S_t is obviously small,which helps to reduce the polarity cross contamination.（2）Elevated temperature can effectively reduce the cell activation over potential,energy efficiency increased gradually,which lead to improve the performance of the battery.Increasing electrolyte temperature,the concentration polarization coefficient CPC increases,concentration polarization become factors restricting the performance of battery,if further increase the concentration of electrolyte,battery performance will increase significantly.But the increase of temperature also makes the field factor S_t increased,exacerbated the mass transfer process of vanadium ions in the membrane,and with the increase of time,the electric field intensification of mass transfer through membrane degree is more and more serious,the higher the temperature is,the more obvious the trends is.In addition,the high temperature easy to cause the precipitation of pentavalent vanadium ion VO²⁺in cathode electrolyte with crystal,which leads to plug the porous graphite electrode and the proton exchange membrane channel,seriously affected the performance of battery.So for many long time cycles of vanadium battery operation situation,we need to reduce the operating temperature appropriately to slow the vanadium ion membrane permeability which could cause performance degradation.（3）Increasing the electrolyte flow rate,the concentration polarization over potential decreased significantly,the battery energy efficiency improved.The increasing electrolyte flow rate made the concentration polarization coefficient CPC decreased,the main factor restricting the cell performance is the activation polarization caused by electron transfer,by increasing cell operating temperature can reduce the activation polarization.Increase the pump speed,electrolyte turbulence intensified in the electrode,the electric field factor S_t value increases,the phenomenon of electric field affecting mass transfer process will be more obvious,the appearance of vanadium ions penetrating across the membrane will be more serious,after running for a longer period of time,there is a possibility of performance deterioration.（4）From the inlet to the outlet in electrode region with the flow of electrolyte,the concentration polarization coefficient CPC increases gradually,the mass transfer effect is more and more strong.Compared to the middle region of the electrode,CPC is relatively large in the interface of current collector and porous electrode,the affecting of the mass transfer process is greater.