Evaluation of additive formulations to inhibit precipitation of positive electrolyte in vanadium battery

A comprehensive study has been performed to develop blended additive formulations based on organic and inorganic compounds to prevent the precipitation of supersaturated V(V) species in the Vanadium Flow Battery at high temperatures around 50 °C. It was found that organic formulations are oxidized by the strong oxidizing effect of V(V) species and are hence ineffective. The inorganic additive formulation KS11 which consists of 1 wt% of K₃PO₄⁺ 1 wt% of SHMP appears to be very effective with a vanadium solution of composition 3.5 M V(V) in total sulphate/bisulphate concentration of 5.7 M up to a temperature of 40 °C. The phosphate ions (PO₄³⁻) released from K₃PO₄ and PO₃⁻ ions from SHMP (NaPO₃)₆ are believed to adsorb onto the nucleating ions, thus inhibiting the precipitation of scale forming species, or adsorption onto the growing crystals, deforming and/or inhibiting further formation of vanadium crystals. Although the electrochemical activity of 3.5 M V solutions was unaffected in the presence of the KS11, increasing vanadium concentration above 3.5 M and total sulphate/bisulphate concentration above 6 M is probably increasing the formation of electrochemically inactive complexes of vanadium-sulphate and polyvanadic species. This results in increased solution viscosity and subsequently reduces the electrochemical activity exponentially.