Precipitation Mechanism of VOSO4 in Oversaturated Electrolytes of the Solid-Liquid Storage Method in Vanadium Redox Flow Batteries

Abstract

Redox flow batteries (RFBs)-based storage systems have the unique feature of separate power generating and energy storage units that can be individually sized. RFBs can be used for days-long energy storage, but because of the low solubility of most ions and molecules in both aqueous and non-aqueous solvents,[1,2] scaling these RFB systems for days-long applications requires a lot of storage tanks and floor area. Our team has developed a new storage method that can significantly increase energy storage density while still maintaining the traditional flow battery design.[3] This method involves storing the reactants in both soluble ions and undissolved solid form, and only the liquid containing the soluble ions is circulated through the batteries. A larger than 4X improvement in storage energy density may be achieved with this strategy. The feasibility of this method was demonstrated in a hydrogen-vanadium (VI/V) RFB.[4] From the study, a major factor in achieving high performance with this hybrid storage system is the rates of the precipitation of the saturated electrolyte and dissolution of the solid precipitate. It was discovered that not all vanadium sulfate solids have precipitation rates fast enough to generate the active ions at the rate needed in the electrode reaction. Only the low crystallinity solid generated by rapid precipitation by the use of a suitable nucleation material was found to have the dissolution rate needed. This presentation will discuss the role of the molecule structures and the mechanism of the precipitation process in the formation of solids that have slow and fast dissolution rates.