Cleaner production of 3.5 valent vanadium electrolyte from ammonium metavanadate by ammonia reduction-sulfuric acid dissolution method

The vanadium redox flow battery (VRFB) has been widely used in large-scale energy storage areas due to the advantages of long lifespan and high safety. However, the high preparation cost of vanadium electrolyte limits the large-scale commercial application of VRFB. In this work, a new efficient cleaner short process for preparing V^3.5+ vanadium electrolyte was proposed. With ammonium metavanadate (NH₄VO₃) and ammonia as raw materials, a mixture of V(IV) and V(III) vanadium oxides was produced by a roasting process directly, which could be further dissolved to obtain the V^3.5+ vanadium electrolyte product. The reaction mechanism of ammonia reduction of NH₄VO₃ was studied. It was found that the valence of vanadium in the reduction product decreased with the increase of the reaction temperature. The influence of roasting and dissolving parameters was also investigated, and it was found that increasing the reaction temperature and the ammonia flow rate would promote the reduction of NH₄VO₃, and keeping the reaction temperature and ammonia flow rate constant, extending the reaction time would also increase the reduction degree of NH₄VO₃. And, the solubility of the reduction product showed a trend of increase first and then decrease with raising sulfuric acid concentration in the solvent. Under the reaction temperature of 480 °C, ammonia flow rate of 100 mL·min⁻¹, reaction time of 50 min, and dissolved solution of 20% sulfuric acid, a V^3.5+ electrolyte can be obtained.