All-Vanadium Redox Flow Battery Electrodes of Carbon Cloth Modified with Tungsten-Bismuth-Based Oxide Nanostructures for an Enhanced V(II)/(VIII) Reaction Kinetics and Inhibited H2 Evolution

Abstract

All-vanadium redox flow batteries (VRFBs) are the most promising and highly efficient storage systems for renewable energy power plants. VRFBs can store high amounts of energy and are designed to work for long periods and their solutions can be used indefinitely.1 VRFBs are mainly carbon-based electrodes as they have high conductivity, high surface area, and high chemical stability, are relatively cheap and available,2 and can be tailored as desired for specific properties. However, these electrodes' performance is limited in their pristine form due to their resistance, hydrophobicity, and surface structure among other factors, resulting in VRFBs with a low power density. The VRFBs' low power density is usually related to the electrodes’ poor performance. Modifying the carbon electrodes using nanomaterials was proven to increase the power density of the battery. Metal oxides like tungsten oxide3 are known to enhance vanadium reactions when used as composites with carbon nanomaterials, while bismuth is known to inhabit the parasitic hydrogen evolution reaction.4 The material of the electrode and the metal molar ratio and the types of metal oxide modifiers highly affect the final performance of the carbon-based electrodes in VRFBs.In this work, different tungsten-bismuth-based oxide nanostructures were synthesized using the hydrothermal method. It is suggested that using different metal molar ratios, pH, and metal precursors significantly affects the electrode's physical and chemical structure and hence its electrochemical performance. The synthesized BiWOx nanostructures are evaluated using XRD, SEM, FT-IR, EDS, XPS, and water contact angle measurements. The effect of using BiWOx nanostructures as a modifier for carbon cloth electrodes on the kinetics of the VRFBs negative half-cell reaction (V2+/V3+) was evaluated using cyclic voltammetry (CV) and electrochemical impedance spectroscopy. Results showed that using BiWOx as an electrode modifier led to an increase in electrode activity and stability, with BiWOx showing the best stability and BiW2Ox showing the best activity. The enhancement effect of BiWOx nanostructures is related to the interaction between the two metal oxides and the electrode surface, the enhanced hydrophilicity of the modified electrode, and the increase of the catalytic active sites.