Electron transfer kinetics of the image – Reaction on multi-walled carbon nanotubes Original Research Article

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 image redox reaction have been reported in literature. Electrochemical impedance spectroscopy was employed to study the image- and the Fe2+/Fe3+-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 Fe2+/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 image 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 image 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.