Nitrogen-Doped Tungsten Carbide-Modified Graphite Felt as a Bifunctional Electrocatalyst for an All-Vanadium Redox Flow Battery

To enhance the electrochemical capability of graphite felt (GF) electrodes in vanadium redox flow batteries (VRFBs), we prepare nitrogen-doped tungsten carbide nanowires (N-WC NWs), which are grown on the GF surface via a two-step route. In the first step, we use a hydrothermal process using tungstic acid (H2WO4) as a precursor, and the WO3 NWs are deposited on the GF surface. Subsequently, the grown WO3 NWs are simultaneously carbonized and N-doped using melamine as the source of nitrogen and carbon. Compared to pristine GF, the prepared N-WC NWs-GF electrode exhibits an apparent electrocatalytic effect on the VO2+/VO2+ and V3+/V2+ redox reaction. The presence of a negatively charged density on the nitrogen atom encourages the vanadium ions' absorption. The nanowire structure exposes more active sites for the vanadium ions' redox reactions. Hence, the prepared N-WC NWs-GF electrode demonstrates the optimal electrochemical activity with energy and voltage efficiencies of 74.8% and 78.9%, respectively, which are much greater than those of the 59.8% and 63.2% attained from pristine GF at 100 mA cm–2. Moreover, the N-WC NWs-GF presented good stability and durability in acidic electrolyte during long-term operations for 100 cycles at a higher current density of 100 mA cm–2.