Effect of Tungsten Oxide Structure on Enhancing the V(II)/(VIII) Reaction and Inhibiting H2 Evolution for All-Vanadium Redox Flow Battery

Abstract

Renewable energy sources require high-efficiency storage systems to allow for its integration within the electrical grid and to supply that energy when needed. All-vanadium redox flow batteries (VRFBs) are very promising for this application because they can store high amounts of energy, are designed to work for long periods, and their solutions can be used indefinitely.1 VRFBs have various advantages when combined with renewable energy sources. The relatively high cost of VRFBs limits their widespread deployment. Enhancing the kinetics of the electrochemical reactions is needed to increase the power density and energy efficiency of the VRFB, and hence decrease the kWh cost of VRFBs. Different carbon materials are used as electrodes in VRFBs (carbon felt, paper and cloth), and each of these materials affects the performance of the battery differently either through sluggish kinetics or a high chance for parasitic reactions catalyzation.2 Modification of these electrodes by carbon-based3 or metal oxides2 nanomaterials can increase the power density of the battery and suppress the parasitic reactions. Some metal oxides like tungsten oxide4 can enhance vanadium reactions alone or when used as composites with nanocarbon materials.This work aims at enhancing the VRFBs negative half-cell reaction (V2+/V3+) kinetics and inhibiting the hydrogen evolution parasitic reaction. Carbon cloth electrodes were modified, using different tungsten oxide nanostructures such as nanowires, nanoflakes, and nanospheres, and tested at different loadings. The results suggest that the change in tungsten oxide structure would lead to a change in the electrode overall performance as a result of changing the electrical conductivity, the wettability of the electrodes, the coverage of the carbon cloth, and the number and nature of the active sites available to catalyze the V2+/V3+ reaction, with the tungsten oxide nanowires showing to be the best electrode modifier.