Redox Flow Batteries (RFBs), unlike traditional batteries that store energy in electrode materials and can convert the chemical energy in the incoming fuels into electricity at the electrodes.
The power (kW) of an RFB system is determined by the size of the electrodes and the number of cells in a stack, whereas the energy storage capacity (kWh) is determined by the concentration and volume of the electrolyte. Vanadium Redox Flow Batteries VRBs exploit the capability of vanadium to exist in solution in four different oxidation states and use this property to make a flow battery that has only one active element in both anolyte and catholyte, which significantly diminishes cross-contamination.
In a VRF, the anolyte is a solution of V(III)/V(II), and the catholyte is a solution of V(V)/V(IV). H₂SO₄ has mainly been used as a supporting electrolyte in both the anolyte and catholyte.

The concentration of vanadium in the electrolyte is currently limited because of the stability of vanadium species and the solubility of VOSO₄ (as starting electrolytes). However, the energy density of VRBs depends on the concentration of vanadium species: the higher the concentration, the higher the energy capacity. Thus, it is important to optimize the operating conditions to improve the stability of both positive and negative solutions. Some studies reported that the stability of vanadium electrolyte with sulfuric acid could be improved to some extent by adding some organic or inorganic chemicals as stabilizing agents. However, even with the positive effects of the additives, currently the vanadium concentration is still limited to under 2 M for most practical VRB systems in the limited temperature range of 10-40 °C. Further current limitations are the thermal and long-term stability of the electrolytes. This project aims to significantly improve the solubility of vanadium species and the liquid temperature range of the electrolyte, which will have a significant impact on increasing the energy density by estimated >25 % with at the same time prolonged thermal and long-term stability of the VRB.

Current research in load-levelling applications based on the ‘value-stacking’ support of energy storage solutions requires BESS units which can deliver rapid and high-density energy for dynamic loading applications; thus VRBs deliver technologies which can attain the critical research ambition of adequate grid-level storage support for >100 kV transmission voltages and BESS capacities which exceed 250 MW.

Figure 1: Schematic view of a vanadium redox flow battery (© P.N.).