Preparation And Performance Study Of Electrolyte For All Vanadium Flow Battery

With the increasing consumption of traditional energy sources and environmental problems,it is urgent to develop and utilize renewable energy sources.Renewable energy sources such as wind and solar energy are unstable due to geography and climate and cannot be applied on a large scale.Therefore,there is an urgent need for a new long-time energy storage technology to make energy efficient and stable output.The vanadium redox flow battery（VRFB）is gaining more focus due to its no flexible power design,safe,long service life and high efficiency.Vanadium-containing electrolyte is an important energy storage medium for VRFB and its raw material is mainly vanadium pentoxide.However,the low solubility of vanadium pentoxide in the sulfuric acid system leads to low electrolyte production efficiency and the high production cost of the electrolyte limits the development of VRFB.In addition,the poor thermal stability of the positive electrolyte at high temperatures limits its operating temperature range to 10～40°C and the vanadium ion cross-membrane transport problems lead to self-discharge to reduce battery capacity,further hindering the application of VRFB.Therefore,the dissolution performance of vanadium pentoxide as raw material in sulfuric acid system was studied by activation,and the clean preparation of electrolyte by catalytic hydrogen reduction was attempted to reduce the electrolyte production cost.At the same time,the effect of sodium phosphate additives on the thermal stability and charge/discharge performance of the positive electrode electrolyte was studied.The main research content and progress of this thesis can be summarized as follows:（1）Concentrated sulfuric acid activation to improve vanadium pentoxide solubilityPentavalent vanadium electrolyte is directly prepared from vanadium pentoxide solid by activating vanadium pentoxide with sulfuric acid at elevated temperatures.The composition,structure,and dissolution processes of the activated solid mixture are analyzed by XRD,Raman,and FT-IR.The results showed that at an activation temperature of 180°C,an activation time of 3 h and a molar ratio of sulfuric acid to vanadium pentoxide of 4,the dissolved vanadium pentoxide mass percentage was up to98.5%and the dissolved vanadium ion concentration was up to 3 mol·L^-1.Activation of sulfuric acid with vanadium pentoxide produced V₂O₃（SO₄）₂,changing the structure of the original vanadium pentoxide,resulting in increased solubility of the activated material when water soluble,and the soluble vanadium ion valence in the form of V⁵⁺.（2）Preparation of V^3.5+electrolyte by catalytic hydrogen reductionIn this work,a simple,green and low-cost method is proposed to prepare the V^3.5+electrolyte for VRFB.The clean hydrogen is chosen as the reducing agent to obtain V³⁺from V⁴⁺.The Pt/C material is used as the catalyst to accelerate the reduction rate at atmospheric pressure.The cells prepared by the catalytic hydrogen reduction process exhibited excellent battery performance with CE of 93%and EE of 85%.Furthermore,a catalytic reactor using Pt/C decorated graphite felt is designed and used to continuously produce the mixed valent vanadium electrolyte.According to the result of simple cost analysis,the proposed catalytic hydrogen reduction process can reduce theoretically the manufacturing cost by approximately 22.6%compared with the present industrial electrolytic process.（3）Effect of sodium phosphate on battery performanceThe addition of sodium phosphate（Na₃PO₄）to the electrolyte not only improves the thermal stability of the electrolyte but also significantly increases the discharge capacity of the battery.The introduction of Na₃PO₄allowed the positive electrolyte containing 2mol·L^-1V⁵⁺to remain stable at 50°C for more than 7 days,while the electrolyte without the addition of Na₃PO₄showed significant precipitation on the first day.In addition,the battery containing 0.3 mol·L^-1Na₃PO₄has a capacity retention rate of 83.02%after 100charge/discharge cycles,which is 13.2%higher than that of the original battery.By examining the electrochemical performance of the battery containing 0.3 mol·L^-1Na₃PO₄at 50°C,it was found that a high energy efficiency of 80%could be maintained at 150m A·cm^-2and the capacity retention was improved by 28.47%over the original cell after100 charge/discharge cycles.