A graphene oxide nanosheets/multi-walled carbon nanotubes (GO/MWCNTs) hybrid with excellent electrocatalytic redox reversibility towards VO 2+ /VO 2 + redox couples for vanadium redox flow batteries (VRFB) has been prepared by an electrostatic spray technique after efficient ultrasonic treatment. The structures and electrochemical properties of GO/MWCNTs are investigated by transmission electron microscopy, scanning electron microscopy, X-ray diffraction, X-ray photoelectron spectroscopy, Raman spectroscopy and cyclic voltammetry. GO/MWCNTs are shown to be cross-linked and form an electrocatalytic hybrid with an effective mixed conducting network, leading to efficiently fast ion and electron transport characteristics. Compared with the pure GO nanosheets and MWCNTs, GO/ MWCNTs deliver a much better electrocatalytic redox reversibility towards the positive VO 2+ /VO 2 + couple, especially for the reduction from VO 2 + to VO 2+ . The excellent experimental results demonstrate that the newly developed hybrid material holds great promise in the application of VRFB. IntroductionThe vanadium redox flow battery (VRFB) proposed by M. Skyllas-Kazacos et al. [1][2][3] in 1985 has been investigated extensively. [4][5][6][7] It uses VO 2+ /VO 2 + and V 2+ /V 3+ redox couples in sulphuric acid as the positive and the negative half-cell electrolytes, respectively, and exhibits an open circuit voltage of approximately 1.26 V at 100% state of charge. 8 In contrast to common secondary batteries utilizing solid active materials, charge/discharge reactions in the VRFB are based entirely on the redox reactions between soluble ionic species. Accordingly, a long cycle life can be expected for the rechargeable cell. 9Commonly, the typical electrodes of VRFB are carbon based materials because of their wide operating potential range, stability as both an anode and a cathode, and availability in high surface area at reasonable cost. VRFB electrodes mainly use graphite felt, 10 carbon cloth, 11 and carbon fiber, 12 and so on. However, the above electrode materials were proved to show poor kinetic reversibility. Therefore, much attention has been

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