The Influence Of Impurities On The Performance Of Electrolyte And Mechanism Of Impurities Regulation For Vanadium Redox Flow Battery

The vanadium redox flow battery（VRFB）have many advantages,such as long lifetime,no cross-reaction,fast response,deep discharge ability and flexible module design.These advantages make VRFB widely commercial applied and researched.The electrolyte is not only the core part of the VRFB systems,but also the active materials for energy storage and conversion,so the performance of electrolyte directly determines the overall performance of VRFB.Currently,the influence and mechanism of impurity ions on the performance of electrolyte for VRFB are not clear,but impurity ions have important effects on the preparation and application of electrolyte.Therefore,this paper have a systematic research on the effects of impurities on the performance of electrolyte and the mechanism of impurities regulation from electrolyte for VRFB.The main research contents and conclusions are as follows:（1）Effects of V₂O₅purity on the performance of electrolyte for VRFBWhen the purity of V₂O₅decreased,the electrolyte conductivity,the electrode reaction activity and reversibility decreased,while the viscosity,the solution resistance and charge transfer resistance of electrolytes increased,which resulted in the decrease of charge-discharge efficiency and capacity of the electrolyte.In addition,the performance of the electrolytes are related to the total concentration of impurity ions in the electrolyte,and there were higher concentration of impurities such as Fe,K and Na in the electrolyte prepared with lower purity V₂O₅,and Na,K impurity ions were the main factors that adversely affect the electrolyte performance.Due to the negative influence of the impurity ions on the charge-discharge efficiency and capacity of the electrolyte,the preparation costs and the investment recovery life of the electrolyte with the lower purity V₂O₅ were increased.（2）Effects and mechanism of impurity ions on the performance of electrolyteComparing the difference of different concentrations of Fe,K,Na impurity ions on the performance of electrolyte,the ultimate concentration of Fe,K and Na impurities in electrolyte were 1.96×10^-2 mol/L,8.0×10^-3 mol/L and 4.8×10^-3 mol/L,respectively.When the concentration of Fe impurities in electrolyte exceed its ultimate concentration,the V（V）ions accelerated to precipitation,and the Fe impurity ions decreased the diffusion coefficient of V（IV）species.Furthermore,Fe impurity ions is competitive with vanadium ions for the adsorption on the electrode surface and redox reaction,which increased the interface resistance,the electrolyte film resistance and the charge transfer resistance,resulting the charge transfer process was difficult,and then reduce the electrochemical performance and charge-discharge capacity of electrolyte.When the concentration of K,Na impurities in electrolyte exceed its ultimate concentration,the stability of V（III）,V^3.5+and V（V）electrolytes were decreased,and the electrolyte conductivity decreased while the viscosity increased.In addition,the Na/V and K/V precipitation physical adhered to the electrode surface,so as to affect the performance of graphite electrode and hinder the contact of electrolyte with electrode,which increased the oxygen evolution side reaction and reduced battery efficiency.（3）Technology of capacitive deionization remove the impurities from electrolyteThe solution chemical research of electrolyte had found that the existence form of Fe,Mg,Al,K,Na impurity ions in electrolyte are mainly in the form of sulphate complex cation,and a small amounts are in the form of sulfate complex anion.Based on the solution chemical research of electrolyte and the influence mechanism of impurities,the capacitive deionization method is adopted to remove the impurity ions in the multiple impurity electrolyte,and reduce the concentration of impurities to within its ultimate concentration.When the anode and cathode composite electrode use the D354/AC and D001/AC,at the operating voltage of 0.5 V and treatment time of 2 h,the remove rate of Fe,Mg,Al,K,Na were 18.84%,12.06%,11.92%,5.25%,8.13%,respectively.The FTIR experiments illustrated that the electrode adsorption of impurity ions in electrolyte with both physical and chemical adsorption.The coulombic efficiency,voltage efficiency and energy efficiency of removed impurities electrolyte were 89.43%,93.72%and 83.81%.Compared with the pristine electrolyte,the charge-discharge performance of electrolyte improved by 7.09%,5.13%and 10.87%,respectively.（4）（NH₄）₂HPO₄ additive promotes the electrochemical properties of the electrolyteThe（NH₄）₂HPO₄ additive could effectively inhibit electrode side reaction and oxygen evolution side reaction caused by impurities.Furthermore,the performance of the procprocessed electrolyte was improved by 0.5 mol%（NH₄）₂HPO₄ additive,and the coulombic efficiency,voltage efficiency and energy efficiency were increased by 0.84%,1.23%and 1.90%,respectively.After the combined treatment of multiple impurity electrolyte by capacitive deionization-（NH₄）₂HPO₄additive,the energy efficiency of the electrolyte reached 98.38%of the standard electrolyte during the charge-discharge process.The mechanisms of the（NH₄）₂HPO₄additive for improving the electrochemical properties of the electrolyte is that the PO₄^3-can form complex clathrate with metal impurities in electrolyte,which could effectively inhibit the negative influence of impurities on the elecrolyte.Moreover,the N atom is introduced by（NH₄）₂HPO₄additive on the electrode surface and it can provide more active sites for the electrode reaction,which can accelerate the charge transfer process,therefore the electrochemical performance of electrolyte can be obviously improved.