Precipitation Recovery of VO(OH)₂from Sodium Metavanadate Solution and Its Application in a Vanadium Redox Flow Battery

The vanadium redox flow battery has received intensive attention because of the increasing demand for safe energy storage systems. Generally, V₂O₅is used for the preparation of a V electrolyte, but the synthesis of V₂O₅from V leaching solution leads to significant environmental impact and requires complicated sequential reduction processes for the preparation of the V electrolyte. For sustainable V electrolyte production, we directly recovered VO(OH)₂from V leaching solution via a reduction-precipitation process using hydrazine (N₂H₄) as a reducing agent and used the VO(OH)₂to prepare a V electrolyte. When the V concentration was 1 M, the optimal conditions for the V recovery efficiency were a N₂H₄/V ratio of 1, pH 4, and a reaction time of 3 h. The effect of co-existing ions (Na, Mg, K, Fe, and Al) in the NaVO₃solution was also investigated. K exhibited the strongest adverse effect on the V recovery efficiency, and Fe and Al impurities were not removed from the recovered V compound by the washing process. A V electrolyte was prepared using the recovered VO(OH)₂, and its charge-discharge performance was evaluated, revealing a high voltage efficiency (∼90.15% on average) and energy efficiency (∼86.79% on average) at a constant current density of 50 mA/cm². These results demonstrate that directly using VO(OH)₂to prepare a V electrolyte is a simple and cost-effective process.