Optimization Of Electrolyte For Iron-Chromium Flow Battery

Flow battery is recognized as one of the most suitable energy storage module for building large-scale energy storage system due to its outstanding advantages such as high safety,long cycle life and flexible design,and has made remarkable progress in commercial applications.Among them,the iron-chromium flow battery(ICFB),as the earliest flow battery,uses low-cost Cr3+/Cr2+and Fe3+/Fe2+redox couple as active materials,which is expected to become a cost-effective energy storage equipment.However,in the practical application,the commercialized development of ICFB is limited by the problems including the slow kinetics of the Cr3+/Cr2+couple,and hydrogen evolution reaction(HER).It is well kown that electrolyte is the critical material for energy storage of flow battery,and the property of the interface between electrolyte and electrode directly affects and determines the performance of battery.Therefore,it is beneficial to improve the power and stability of ICFB by revealing the influence of electrolyte properties on the reaction characteristics of electrode interface and cell performance,optimizing the composition of electrolyte and selecting hydrogen evolution inhibitors..In view of the problems existing in ICFB,the influence of the changes of Pb2+and Bi3+ ion concentrations in the electrolyte on the electrode reaction at different temperatures,especially at 65? is systematically analyzed and described,in order to reveal how to select the suitable hydrogen evolution inhibition additives for ICFB operated at high temperature.It can be predicted that,based on the optimization of electrolyte,the selection of hydrogen evolution inhibitors more suitable for ICFB will greatly improve the stability of the battery,and as a result,provide new supporting data for the further development and application of ICFB.The main research results are as follows:First of all,the influence of temperature on the reaction kinetics of Cr3+/Cr2+ and hydrogen evolution reaction in the electrolyte is studied by means of cyclic voltammetry and linear sweep voltammetry,and the influence of the concentration of Pb2+ ion and Bi3+ion on the negative reaction with electrolyte on the glassy carbon electrode is studied at 25?and 65?,respectively.The results show that increasing temperature is beneficial to the electrochemical reaction of Cr3+/Cr2+and the reactions related with hydrogen evolution.Moreover,Pb2+ion and Bi3+ion both are in favor of the electrode reactions.At the sale time,Pb2+ ion has inhibitory effect on hydrogen evolution,while Bi3+ ion has promotion effect on hydrogen evolution reaction to certain extent,especially is more conducive to the occurrence of hydrogen evolution reaction at higher temperature.Secondly,the effect of the variation concentrations of Fe2+ and Cr3+ ions as well as hydrochloric on the the physic-chemical properties,electrochemical behaviors of the electrolytes and corresponding charge-discharge performance of ICFB are systemically investigated so that to determine the optimal electrolyte formula.That is,the viscosity of electrolyte increases along with the increasing concentration of FeCl2,CrCl3 and HCl,however,the corresponding conductivity decreases with the increasing concentration of FeCl2 and CrCl3,but increases with the increasing HCl concentration.It is worth noting that the electrolyte with 1.0 M FeCl2,1.0 M CrCl3 and 3.0 M HCl presents the best electrochemical performance based on a comprehensive consideration of the viscosity,conductivity and electrochemical activity of the electrolyte.Most importantly,ICFB with the optimized electrolyte presents higher battery efficiency and lower capacity decay,and the energy efficiency could reache to 81.5%and 73.5%when the operating current density is high up to 120 mA cm-2 and 200 mA cm-2,respectively.Last but not the least,the effect of InCl3 as the electrolyte additive on the electrochemical reactions as well as the performance and stability of ICFB is investigated by means of cyclic voltammetry test,electrochemical impedance spectroscopy test and single cell test.It is found that the In3+ can not only effectively inhibit the serious HER,but also improve the electrochemical activity of both reactions of Cr3+/Cr2+and Fe3+/Fe2+to some extent.Moreover,ICFB made of the anolyte with additive of 0.01 M In3+in an optimal amout exhibits higher energy efficiency of 77.0%at 200 mA cm-2.More importantly,after 140 cycles at 160 mAcm-2,the signle cell battery shows discharge capacity retention of 36.3%,higher than that without additive of 0.01 M In3+ ion,and which exhibits and an average capacity fade rate of 0.16%per cycle,which is much lower than that in the previous studies.