Preparation And Performance Improvement Study Of High-performance Electrolyte Based On Sulfuric Acid And Sulfate-chloride Mixed Acid For Vanadium Redox Flow Battery

For the dissipation of fossil fuels and strict environmental protectionrequirements,the use of sustainable energy sources such as solar and wind gradually becomes the main direction of energy industry development.Due to the intermittent,varied nature of such renewable resources,the energy-storage systems are need to be applied in the process of electric energy conversion.Among these energy-storage systems,the vanadium redox flow battery(VRFB)is regarded as the most promising candidate for its long cycle life,deep charge capacity and flexible design.In addition,electrolyte which is the energy storage medium of VRFB is mainly prepared from vanadium oxides,vanadium is a superior resource with enormous reserves in China.Therefore,the development of VRFB in China has natural advantages.At present,the low energy density of VRFB makes it still face many barriers in the application.The key for the improvement of energy density is improving the performance of electrolyte.Based on the investigations of existence for vanadium ions in electrolytes,this study has a systematic study on the preparation and performance improvement of sulfuric acid and sulfate-chloride mixed acid electrolyte,and has an in-depth analysis about the mechanism of performance improvement.The main research contents and conclusions are as follows:(1)Optimization of electrolyte preparation by chemical reduction methodThe thermodynamics and preparation efficiency analysis show that the reactions of preparing electrolytes with sulfurous acid,oxalic acid,formic acid,acetic acid,tartaric acid,citric acid,and hydrogen peroxide can happen spontaneously.The reducing ability can be sorted as oxalic acid,formic acid,tartaric acid,citric acid,acetic acid,sulfurous acid,and hydrogen peroxide in descending order.The prepartion of electrolyte by reduction with oxalic acid has higher reduction rate and conversion rate,it does not affect the stability of the electrolyte and the electrolyte has the best electrochemical activity.Thus oxalic acid is suitable for the reducing agent in preparation of electrolyte.The optimal process parameters for the preparation of electrolyte with oxalic acid reduction are as follows:n(H2C2O4):n(V2O4)is 1:1,reaction temperature is 90℃,reaction time is 100 min,n(H2SO4):n(V2O5)is 5:1,and reduction rate and conversion rate are 93.35%and 94.80%.The electrochemical polarization in the electrode reaction of electrolyte prepared by oxalic acid reduces,so that the charging voltage reduced and the discharge voltage increases,therefore the charge and discharge capacity is increases.The electrolyte prepared by oxalic acid does not have adversely affect on the battery performance which can be directly used in VRFB,and its battery performance improves compared to the standard electrolyte.(2)Solvent structure of hydrated vanadium ions in electrolyteThe thermodynamics analysis indicate that the basic existence forms of V(Ⅱ),V(Ⅲ),V(Ⅳ)and V(Ⅴ)ions in electrolyte are V2+,V3+,VO2+and VO2+ respectively.When the vanadium concentration increases,lowering the pH can effectively improve the stability of V(Ⅴ)ions.Increasing the temperature will promote the conversion of VO2+into H3VO4 intermediates,then dehydrates and condenses to form hydrated V2O5 precipitates which is the reason for the poor thermal stability of the V(Ⅴ)electrolyte.Controlling the appropriate pH is the key to the stable existence of VO2+.The simulated calculation and thermodynamics analysis show that The solvated structures of monomeric hydrates V2+,V3+,VO2+,and VO2+are[V(6H2O)]2+,[V(6H2O)]3+,[VO(5H2O)]2+,and[VO2(3H2O)]+.[VO(5H2O)]2+and[VO2(3H2O)]+in high-concentration V(Ⅳ)and V(Ⅴ)electrolytes will not self-associate to form a polymer structure,only presence as the form of a monomer structure.(3)Performance improvement and mechanism of electrolyte based on sulfuric acidWhen the vanadium concentration is 2.0 mol/L,sulfate concentration is 5.5 mol/L and SOC range is at the range of 0-90%,the electrolyte can operate stably in VRFB at-10-40℃ and the efficiency can be maintained in the range of 75%-80%.Reducing the SOC can significantly improve the thermal stability of the V(VⅤ)electrolyte.The spectrum tests and simulated calculation indicate that V(Ⅳ)and V(Ⅴ)ions can polymerize to form[V2O3(7H2O)]3+dimer,and the formation of this dimer structure is why the decrease of SOC can improve the thermal stability of the positive electrolyte.The best sulfonic acid additive is taurine,when the addition amount is 4 mol%,the SOC of electrolyte can reach 95%and can run stable at-10-40℃.The energy efficiency can reach more than 80%,and the energy density can reach 27.98 Wh/L which is 8.83%higher than the blank electrolyte(25.71 Wh/L).The taurine combines with V(Ⅴ)ion which forms the synergistic effect of coulomb repulsion and steric hindrance can increase the degree of dispersion between V(Ⅴ)ions.At the same time,it acts as carrier in the mass transfer process and bridge in the charge transfer process which improves the electrochemical performance of the positive electrode electrolyte.(4)Performance improvement and mechanism of electrolyte based on sulfate-chloride mixed acid electrolyteThe introduction of chloride ions is beneficial to improve the stability of the V(Ⅴ),V(Ⅳ)and V(Ⅱ)electrolytes but is harmful to the stability of theⅤ(Ⅲ)electrolytes.When the sulfate concentration is 2.0-3.0 mol/L and the chloride ion concentration does not exceed 6.4 mol/L,the volatilization of hydrogen chloride and the evolution of chlorine gas can be effectively avoid.The concentration of vanadium in the electrolyte can reach 2.4 mol/L,and the stable temperature range can reach-20-50℃.The spectrum analysis and simulated calculation indicate that Cl-can replace the water molecules in the first solvent layer of[VO2(3H2O)]+to coordinate with vanadium ions which can suppress the deprotonation reaction of[VO2(3H2O)]+and improve the high temperature stability of V(V)electrolyte.The stannous chloride additive can effectively improve the electrochemical activity and reversibility of the negative electrolyte.The energy density of electrolyte can reach 33.45 Wh/L which is 15.58%higher than the blank electrolyte(28.94 Wh/L).Sn2+can be reduced into Sn and deposited on the electrode surface before the reduction of V3+,the Sn deposited will not be oxidized and detached from the electrode surface before oxidation of V2+.The Sn on the electrode surface can provide more active sites for the electrode reaction and reduce the activation energy of the reaction which can accelerate the charge transfer process,therefore electrochemical performance of negative electrolyte can be observably improved.