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To increase the electrocatalytic activity of graphite felt (GF) electrodes in vanadium redox flow batteries (VRFBs) toward the VO 2 + /VO 2+ redox couple, we prepared a stable, high catalytic activity and uniformly distributed hexagonal Ta 2 O 5 nanoparticles on the surface of GF by varying the Ta 2 O 5 content. Scanning electron microscopy (SEM) revealed the amount and distribution uniformity of the electrocatalyst on the surface of GF. It was found that the optimum amount and uniformly immobilized Ta 2 O 5 nanoparticles on the GF surface provided the active sites, enhanced hydrophilicity, and electrolyte accessibility, thus remarkably improved electrochemical performance of GF. In particular, cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) results showed that the Ta 2 O 5 -GF nanocomposite electrode with a weight percentage of 0.75 wt % of Ta 2 O 5 to GF exhibited the best electrochemical activity and reversibility toward the VO 2 + /VO 2+ redox reaction, when compared with the other electrodes. The corresponding energy efficiency was enhanced by ∼9% at a current density of 80 mA cm −2 , as compared with untreated GF. Furthermore, the charge−discharge stability test with a 0.75 wt % Ta 2 O 5 -GF electrode at 80 mA cm −2 showed that, after 100 cycles, there was no obvious attenuation of efficiencies signifying the best stability of Ta 2 O 5 nanoparticles, which strongly adhered on the GF surface. KEYWORDS: Vanadium redox flow batteries, Ta 2 O 5 nanoparticles, Redox couple (VO 2 + /VO 2+ )

In this paper, we report a facile hydrothermal method to synthesize low-cost, high-catalytic-activity, and stable niobiumdoped hexagonal tungsten trioxide nanowires (Nb-doped h-WO3 NWs); these NWs were employed as catalysts to improve the electrocatalytic activity of graphite felt (GF) electrodes for use as positive electrodes in an all-vanadium redox flow battery (VRFB). The effect of Nb doping and its composition on the electrochemical performance of GF electrodes for a VRFB was investigated. Cyclic voltammetry and electrochemical impedance spectroscopy results showed that Nb-doped h-WO3 NWs with a Nb/W atomic ratio of 0.03 exhibited the highest electrocatalytic activities for VO 2+ /VO2 + couples among all the tested electrodes. This observation was attributed to the optimal Nb-doping concentration producing moderate defect states, thereby creating structural disorders, such as oxygen vacancies, in WO3 and leading to the generation of more active sites for the VO 2+ /VO2 + redox reaction on the electrode. Moreover, in charge-discharge tests, a VRFB single cell using the Nb-doped h-WO3 NWs (Nb/W = 0.03) catalyst demonstrated an excellent energy efficiency of 78.10% with a current density of 80 mA cm −2 . This efficiency is much higher than that demonstrated by VRFB cells with untreated GF (67.12%) and heat-treated GF obtained through conventional method (72.01%). Furthermore, in the stability test of a VRFB single cell with the Nb-doped h-WO3 NWs (Nb/W = 0.03) catalyst, almost no decay of the cell was observed even after 30 cycles. This observation indicates the outstanding stability of the cell during the redox reaction of vanadium ions under highly acidic conditions.

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.