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+ )

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.

In this study, a simple, low-cost, and powerful titanium niobium oxidereduced graphene oxide (TiNb2O7–rGO) nanocomposite electrocatalyst was synthesized through dispersion and blending in aqueous solution followed by freeze-drying and annealing for all-vanadium redox flow batteries (VRFBs).

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