Electrochemical investigation of polyhalide ion oxidation–reduction on carbon nanotube electrodes for redox flow batteries

Polyhalide ions, Br−/BrCl2-(ClBr2-), are an important redox couple for redox flow batteries. The oxidation–reduction behavior of polyhalide ions on carbon nanotube (CNT) electrodes has been investigated with cyclic voltammetry and electrochemical impedance spectroscopy. The onset oxidation potential of Br−/BrCl2-(ClBr2-) is negatively shifted by >100 mV, and the redox current peaks are greatly enhanced on the CNT electrode compared with that on the most widely-used graphite electrode. The reaction resistance of the redox couple [Br−/BrCl2-(ClBr2-)] as determined from the electrochemical impedance spectroscopy, is decreased on the CNT electrode for example by one order of magnitude at 900 mV as compared with that on graphite electrode. The redox reversibility is increased on a CNT electrode even though it still needs further improvement. CNT is a promising electrode material for redox flow batteries.