The Research On The Electrolyte Of All Vanadium Redox Flow Battery

Due to the unique merits, such as environmental benign, high energy conversion efficiency, long service life, independent power capacity, etc, vanadium redox flow battery(VRB) has a sound application prospect in the fields of renewable energy power storage, power peaking and military power storage and so on. In general, VRB uses sulfuric acid as supporting electrolyte, but vanadium ions, especially pentavalent vanadium ions are prone to precipitate in sulfuric acid system, which exerts a negative impact on the stability of the battery. To circumvent this problem, the concentration of vanadium ions in the electrolyte is often reduced, which endows VRB with a lower specific energy. Therefore, developing the high concentration vanadium electrolyte has become an important direction in the research of VRB with a high-performance. Based on the comprehensive literature reviews of researches on VRB and its electrolyte at home and abroad, this paper adopted analytical testing methods such as potentiometric titration and ICP-MS, as well as electrochemical research methods such as CV and EIS to make a systematic study on the electrochemical stability of different electrolyte system and their response system dynamics. Meanwhile, battery tests have been carried out to make a comprehensive evaluation of the relationship between the electrolyte and the battery performance.Firstly, a systematic research was made on the mixed system of sulfuric acid and methyl, concentrating on the impact of mixed acid on the solubility and stability of vanadium ion, and the battery performance. ICP-MS results show that the solubility of vanadyl sulfate increases with the augment of methyl sulfonic acid in electrolyte. It is also found that the adding of methanesulfonic acid has a significant influence on electrochemical behavior of V(IV)/V(V). Electrolyte displays the best comprehensive performance when added 0.25 mol/L methanesulfonic acid and 2.75 mol/L sulfuric acid. Battery test results further reveal that under such a condition, all the coulombic efficiency, voltage efficiency, energy efficiency and cycle stability of VRB are significantly better than those in the battery system taking sulfuric acid as the supporting electrolyte. In addition, a stable electrolyte with 2 mol/L vanadyl sulfate can be made under such a condition, and the average coulombic efficiency and the energy efficiency of the battery made from this kind of electrolyte are as high as 95.80% and 77.95% respectively, exceeding the conventional sulfuric acid electrolyte system(vanadium ion concentration is about 1.5 mol/L). It is further proved that the mixed acid(methane sulfonic acid- sulfuric acid) could provide a solution to resolve the low specific energy of VRB.Despite taking the mixture of methanesulfonic acid and sulfuric acid as the supporting electrolyte by replacing the sulfuric acid can significantly improve the overall performance of the battery, but it pushes up the manufacturing cost of the battery with the introduction of methyl sulfate. Therefore in this paper, the fourth chapter systematically studies the impact of some sulfate on the electrolyte of VRB and battery performance. Results reveal that the addition of magnesium sulfate inhibits the electrochemical reaction between vanadium ions, and sodium sulfate, potassium sulfate, ammonium sulfate was added to improve the electrochemical reversibility and kinetics of vanadium ions, and the addition of 3% Ammonium sulfate could produce the most significant effect. Battery tests show that 3% ammonium sulfate average coulombic efficiency and energy efficiency of the battery with 3% ammonium sulfate can be as high as 97.45% and 78.18% respectively, 2.69% and 3.99% higher than 94.90% and 75.18% in the blank cells. Furthermore, 2.5 mol/L vanadyl sulfate may be dissolved by 3 mol/L sulfuric acid solution with 3% ammonium sulfate at room temperature. And the battery performance test proves that a battery with such a concentration has good cycling stability and its coulombic efficiency keeps over 97%, and energy efficiency increases from 73.35% to 75.58%, which further explains the battery with a high concentration of vanadium ion not only has good stability, but also has excellent charge and discharge performance.In addition to the sulfate additive, the amino acid as an additive has also been widely used in electrochemical systems. The fifth chapter focuses on the influences of acidic and alkaline amino acids on the vanadium battery electrolyte and battery performance. Electrochemical tests show that the vanadium ions have a better electrochemical reversibility and kinetics in the amino acid system. In addition, the experimental results show that the addition of asparagine amino acid can most improve the electrochemical properties of vanadium ions, and 3% is the best amount. Battery performance test reveals that the energy efficiency of the battery with the 3% asparagine amino acid is up to 81.69%, 5.67% higher than that of sulfuric acid battery which is 77.31%, indicating that the vanadium ions reflect better electrochemical performance in the system.The sixth chapter mainly studies the impact of amino sulfonic acid and taurine as mixed acid additives of VRB on the system, and the electrochemical performance tests show that mixed acid reflects a smaller peak potential difference and higher anodic and cathodic peak currents, indicating that the vanadium ions have better electrochemical reversibility and kinetics in mixed acid. In addition, the electrochemical performance tests show that the mixed acid with 0.15 mol/L amino sulfonic acid and 2.85 mol/L sulfuric and the mixed acid with 0.25 mol/L taurine and 2.75 mol/L sulfuric acid are best mixed. Battery tests show that the energy efficiency of taurine system is the highest, reaching 86.35%, 2.85% higher than that of the sulfuric acid system battery(83.96%). The most prominent result is that 2.6 mol/L vanadyl sulfate can be dissolved in the mixture of 0.25 mol/L taurine and 2.75 mol/L sulfuric acid, which will significantly improve the specific capacity and specific energy of vanadium battery and the coulombic efficiency of this battery with high concentration is up to 95.14%, but with the increase in the concentration of vanadyl sulfate, voltage efficiency of the battery is reduced, resulting in a slight decrease in energy efficiency(maintained at about 81.25%). Nevertheless, this is a breakthrough of improving the specific capacity and specific energy of vanadium battery, therefore, taurine is a good mixed acid additive to vanadium battery electrolyte.