Development of a halide-free, stabilised vanadium redox flow battery electrolyte
Nguyen, Duy Tam
Date of Issue2018-01-25
Interdisciplinary Graduate School (IGS)
Energetics Research Institute
NTU, Gildemeister, SGL Group
The large-scale implementation of renewable energy technologies, e.g. solar photovoltaic and wind turbine, requires effective and stable electric energy storage systems, due to the inconstancy of such renewable energies. Among some potential candidates, the vanadium redox flow battery (VRFB) is promising to play such a role, thanks to its advantages: power and energy capacity can be independently designed; the simple structure of cell and stack; quick response and long cycle life. One of the challenges in VRFB development is to improve the low solubility of V(V) species in sulfuric acid. The positive electrolyte of the VRFB consists of a mixture of vanadium salts in oxidation states 4+ (IV) and 5+ (V), dissolved in sulfuric acid, with a total vanadium concentration of typically 1.5 – 2 mol.dm‒3. As the battery is charged, the ratio of V(V) to V(IV) increases. Therefore, the VRFB is generally operating with the positive electrolyte in a metastable state. Consequently, there exists the risk that V(V) ions may condense to form V2O5 and then followed by the precipitation at high temperatures, which is almost irreversible in the charged electrolyte. In practice, the kinetics of precipitation is strongly dependent on temperature and state-of-charge (SOC). By careful control of temperature and state-of-charge, VRFB manufacturers avoid precipitation. However, these measures add to the cost and can reduce performance. Therefore, much effort has been made to find electrolyte additives that would allow wider operational temperature windows. The most common stabilizers to date are H3PO4, (NH4)2SO4, and HCl. It has been suggested that halide ions can form complexes such as [V2O3Cl2.6H2O]2+ with the hydrate pentacoordinated [VO2(H2O)3]+ cation, in which the chloride hinders the polymerization to V2O5. However, halide in the electrolyte gives the risk of forming poisonous halogen vapors, while H3PO4 and (NH4)2SO4 are rather limited in their effectiveness. A large number of organic stabilizers have also been suggested, but little has been reported as to their long-term stability. Therefore, in this thesis, a systematic development strategy (precipitation test, electrochemical test, material characteristics, and etc…) was conducted for finding out an electrolyte formulation (especially new additives) for a vanadium-based electrolyte, which is halide-free and stable to high temperatures. The electrolyte is also expected to demonstrate the longer-term stability and suitability for laboratory-scale VRFB and eventually for a 1 kW system. In this Ph.D. thesis, we summarize the progress and achievement in the development of novel electrolyte formulation for the VRFB. It is demonstrated that the novel thermally stable additives have been successfully developed for the VRFB electrolyte, through the combination of one inorganic component and one organic component, which could provide the co-stabilizing effect to prevent the precipitation of positive vanadium electrolyte at high temperatures. The thermal stability of pristine vanadium electrolyte has been improved nearly 4 times by the addition of combined additives. In addition, as-developed combined inorganic-organic additives also could maintain the rate of V2O5 precipitate below the value of ~10 mol.% of V(V). By contrast, more than 60 mol.% of V(V) in the pristine electrolyte was precipitated within 7 days at 50 ºC. Since the recipe is halide-free, the risk of toxic halogen vapor formation has been completely eliminated. Besides that, no other harmful contaminants were released under the operation of new electrolyte formula. So-developed halogen-free thermally stable electrolytes also satisfied the longer-term stability and suitability in a practical VRFB cell at high temperatures. The electrolyte formulas with the presence of combined inorganic-organic additives could preserve the performance of a laboratory-scale VRFB at 50 ºC for over 100 cycling number. It also preserved the performance of a large-scale 3-stack VRFB system. Although there was a minimal drop in the efficiency of the cell, the drop could be diminished by varying the addition amount of additives or using suitable ion exchange membrane. Moreover, owing to its high thermal durability, the VRFB could be operated at relatively high temperatures without the necessity to install a cooling system, resulting in higher efficiency and lower cost. The innovative vanadium electrolyte developed in this Ph.D. study is highly promising to be used in commercial VRFB system.