Vanadium Electrolyte Densification and Gel Formation Processes

This paper describes the processes used to transform liquid vanadium electrolytes into solid form for shipment from the manufacturing site to the vanadium redox flow battery application site. The processes involve removing a large amount of water from the vanadium electrolytes by forward osmosis and transforming the concentrated electrolytes into a low-crystallinity, gel-like solid that can be quickly reconstituted to its original liquid form by the addition of water. The osmosis cell consists of two liquid compartments separated by a Nafion 211 membrane supported by a porous carbon layer on each side. Concentrated sulfuric acid was used as the draw solution and fed in the counterdirection from the vanadium electrolyte feed. This study identifies the optimal conditions for full-gel formation, requiring the following: electrolyte densification level >= 50%, nucleation material density >= 10 mg/mL, and mixing time >= 1 h. The precipitation process was successfully demonstrated for volumes up to 200 mL. This study also found that phosphoric acid, often added to improve the stability of these vanadium sulfate electrolytes, has an adverse impact on the precipitation process but can be overcome with a higher electrolyte densification level or higher nucleation material density.