Electron transfer kinetics of the VO ^{2 +} / VO 2 + - Reaction on multi-walled carbon nanotubes

Multi-walled carbon nanotubes (MWCNTs) are suitable electrode materials for the all-vanadium redox flow battery. In addition to their high specific surface area, catalytic properties for the VO²⁺/VO2+ redox reaction have been reported in literature. Electrochemical impedance spectroscopy was employed to study the VO²⁺/VO2+- and the Fe²⁺/Fe ³⁺-reaction on MWCNTs with varying amounts of surface functional groups. Our analysis method is based on taking the large electrochemical interface area of the MWCNTs into account to obtain a truly comparable value for the exchange current density. When evaluating the results for Fe ²⁺/Fe³⁺ it was found that the exchange current density on MWCNTs decorated with a large amount of functional groups is more than 10 times larger than for thermally defunctionalized MWCNTs. For the VO²⁺/VO2+ reaction, however, a decrease in activity for an increase in amount of functional groups was observed. A possible reaction mechanism and the influence of defects on MWCNTs are discussed. This work distinguishes itself from previous publications, by showing the absence of a catalytic effect of functional groups for the VO²⁺/VO2+ reaction. Therefore, a new discourse in understanding the catalytic effect of MWCNTs and specifically of the surface functional groups of carbon materials in electrochemical reactions is necessary.