Modified multiwalled carbon nanotubes as an electrode reaction catalyst for an all vanadium redox flow battery

Different modified multiwalled carbon nanotubes (MWCNTs) are prepared by heat treatments in the air and in the H₂SO₄?+?HNO₃ (1:1) mixed acids which are investigated by transmission electron microscopy, X-ray photoelectron spectroscopy, X-ray diffraction, Brunaur–Emmett–Teller, and cyclic voltammetry measurements. The results show the physicochemical properties of MWCNTs change significantly after these different modification processes, especially the electrochemical catalytic activity towards the VO₂ ⁺/VO²⁺ and V³⁺/V²⁺ redox pairs. The MWCNTs treated in the air at 600 °C for 30 min shows better electrochemical performances for the VO₂ ⁺/VO²⁺ redox reactions (58.8 and ?32.4 μA for the oxidation and reduction peaks at 10 mV?s^?1, respectively) than any other samples. Compared with the V³⁺/V²⁺ redox couple, the VO₂ ^+/VO²⁺ redox reactions are more easily affected by the physicochemical property changes of the MWCNTs. The enhanced electrochemical catalytic activity of the modified MWCNTs is not only related to the surface oxygen content, but also to the specific surface area, conductivity and the unique structure variations of the MWCNTs. The investigation demonstrated that the modified MWCNTs have a promising future application in the vanadium redox flow battery.