Stability of highly supersaturated vanadium electrolyte solution and characterization of precipitated phases for vanadium redox flow battery

The vanadium redox flow battery (VRFB) has been receiving great attention in recent years as one of the most viable energy storage technologies for large-scale applications. However, higher concentrations of vanadium species are required in the H2O-H2SO4 electrolyte in order to improve the VRFB energy density. This might lead to unwanted precipitation of vanadium compounds, whose nature has not been accurately characterized yet. For this purpose, this study reports the preparation of V(II), V(III), V(IV) and V(V) supersaturated solutions in a 5 M H2SO4-H2O electrolyte by an electrolytic method, from the only vanadium sulfate compound commercially available (VOSO4). The precipitates obtained by ageing of the stirred solutions are representative of the solids that may form in a VRFB operated with such supersaturated solutions. The solid phases are identified using thermogravimetric analysis, X-ray diffraction and SEM. We report that dissolved V(II), V(III) and V(IV) species precipitate as crystals of VSO4, V2(SO4)3 and VOSO4 hydrates and not in their anhydrous form; conversely V(V) precipitates as an amorphous V2O5 oxide partially hydrated. The measured hydration degrees (respectively 1.5, 9, 3 and 0.26 mol of H2O per mol of compound) might significantly affect the overall engineering of VRFB operating with high vanadium concentrations.