Synthesis of Transition Metal Carbide Catalysts for Vanadium Redox Flow Batteries

Abstract

Vanadium redox flow batteries (VRFBs) proposed by Skyllas-Kazacos’ group in the 1980s [1] have recently attracted considerable attention for large-scale energy storage due to their desirable features such as flexible and safe design, high efficiency, and long cycle life. Graphite or carbon materials have been the most common catalysts for VRFBs [2]. However, they often show limited activity and reversibility. This issue may be circumvented with the development of new and efficient catalysts. Transition metal carbides are regarded as attractive candidates because they possess good electronic conductivity, low cost, high abundance, and outstanding thermal and chemical stabilities [3]. However, transition metal carbides fabricated by traditional carbothermal reaction methods exhibit a small specific surface area and inevitable aggregation at high reaction temperatures.A transition metal carbide fabrication process is under development at Hawaii Natural Energy Institute, which favors the formation of nano-sized particles that are expected to possess a large specific surface area. Figure 1a shows X-ray diffraction patterns for graphite as carbon source and the resulting vanadium carbide. The dominant phase is V8C7 and the particle size is approximately 50 nm. As shown in Figure 1b, cathodic currents for both electrodes in 3 M H2SO4 are due to the hydrogen evolution reaction. Although the cathodic current remarkably increases for graphite in 1 M (V2++V3+) + 3 M H2SO4 in Figure 1c, a significant anodic current is not detected indicating that the reversibility of the graphite toward the V2+/V3+ redox reactions is poor. In contrast, anodic and cathodic currents substantially increase on vanadium carbide, signifying an improvement in catalytic activity and reversibility toward the negative electrode reactions in comparison to graphite. The undesired hydrogen evolution reaction appears to have an important influence on the V3+ to V2+ reduction reaction. This influence was removed to more clearly show the vanadium V2+/V3+ redox reactions partial currents. Figure 1d shows the catalytic activity of graphite and vanadium carbide after background current subtraction. Vanadium carbide reveals more pronounced redox peaks than graphite signaling that vanadium carbide has a higher catalytic activity and enhanced reversibility toward V2+/V3+ redox reactions than graphite. The vanadium carbide composition, structure, morphology, and particle size will be tuned to improve activity.Figure 1. (a) X-ray diffraction patterns and (b, c, d) cyclic voltammograms (positive scan) for vanadium carbide and graphite. Acknowledgments Authors are grateful to the Office of Naval Research (award N00014-18-1-2127) and the Hawaiian Electric Company.