Recovery of High-Purity V(IV) and V(III) Compounds from V(V) Sources by Reduction–Precipitation

We propose a simple chemical reduction method for the preparation of electrolytes for vanadium redox flow batteries (VRFBs). Our process, a reduction–precipitation method, uses strong reducing agents such as hydrazine (N2H4) and sodium dithionite (Na2S2O4), which were carefully chosen with the aid of thermodynamic analysis. The reduction experiments resulted in V precipitates in the form of a mixture compound of V2O4·xH2O and V2O3·xH2O. A recovery efficiency greater than 96% was achieved both in the V–N2H4 system at pH 4 and in the V–Na2S2O4 system at pH 3. When the reduction–precipitation method was used to purify commercial V2O5, the concentrations of metal impurities such as Na, Fe, and Al were greatly reduced to 16.5, 4.0, and 6.6 mg/L in the VRFB electrolyte thus prepared, which exhibited a purity of 99.95%.