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Pristine multi-walled carbon nanotubes (MWCNTs), and those functionalized with hydroxyl groups, or carboxyl groups were used as electrode reaction catalyst for VO(2)(+)/VO(2+) redox couples for vanadium redox flow battery. The structure, composition, electrochemical properties and battery performance were characterized with scanning electron microscopy, X-ray photoelectron spectroscopy, cyclic voltammetry and cell charge-discharge tests. The electrochemical activities of the redox couple are greatly increased over those of modified glassy carbon electrodes and the electrocatalytic kinetics of the redox reactions are in the order of carboxyl MWCNTs > hydroxyl MWCNTs > pristine MWCNTs. The peak currents of the redox reactions (63.8 and -51.1 mu A for oxidation and reduction processes, respectively) on the electrode modified by carboxyl MWCNTs are about three times those for the other electrodes. The battery exhibits excellent storage efficiency when the carboxyl MWCNTs were used as positive electrode reaction catalyst, suggesting the oxygen functional groups especially the carboxyl can significantly facilitate the VO(2)(+)/VO(2+) redox reactions. (C) 2011 Elsevier Ltd. All rights reserved.