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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+ )

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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).

This paper presents a three-dimensional annealed tungsten trioxide nanowire/graphene sheet (3D annealed WO3 NWs/GS) foam as an excellent and low-cost electrocatalyst.