Hydrogen-Treated Defect-Rich W18O49 Nanowire-Modified Graphite Felt as High-Performance Electrode for Vanadium Redox Flow Battery

To improve the electrochemical performance of graphite felt (GF) electrodes in vanadium redox flow batteries (VRFBs), we synthesized simple, inexpensive, and conductive W18O49 nanowires (W18O49NWs) as electrocatalysts on the surface of GF through the one-step solvothermal process. Cyclic voltammetry and electrochemical impedance spectroscopy studies revealed that W18O49NWs exhibit electrocatalytic effects on a VO2+/VO2+ redox couple on the positive side, which enhance the electrochemical kinetics of the redox reactions. To further improve the electrochemical performance of the W18O49NWs, we thermally annealed the sample with a controlled amount of H2/Ar atmosphere to form oxygen-vacancy-rich hydrogen-treated W18O49NWs (H-W18O49NWs). When used as an electrode in a VRFB single cell, this material demonstrated outstanding performance with 9.1 and 12.5% higher energy efficiency than cells assembled with W18O49NWs and treated GF, respectively, at a high current density of 80 mA cm-2. The superior performance of the H-W18O49NW electrocatalyst-based electrode can be attributed to the presence of numerous oxygen vacancies, which were proven to act as active sites for the VO2+/VO2+ redox reaction. Moreover, the uniformly immobilized and 1D nature of the W18O49NWs facilitated the charge-transport process, enhanced hydrophilicity, and electrolyte accessibility, and thus remarkably reduced electrochemical polarization during the mass transfer of active species. The long-term cycling performance confirmed the outstanding durability of the as-prepared H-W18O49NW-based electrode with negligible activity decay after 100 cycles.