In this paper, we report a facile hydrothermal method to synthesize low-cost, high-catalytic-activity, and stable niobium-doped 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 VO2+/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 VO2+/VO2+ redox reaction on the electrode. Moreover, in charge-discharge tests, a VRFB single cell using the Nb-doped h-WO3 NW (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 the conventional method (72.01%). Furthermore, in the stability test of a VRFB single cell with the Nb-doped h-WO3 NW (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. Copyright 2016 The Royal Society of Chemistry.