Influence and Electrochemical Stability of Oxygen Groups and Edge Sites in Vanadium Redox Reactions

Abstract

It is widely accepted that surface-active oxygen functional groups (OFGs) effectively catalyze the vanadium redox reactions. Initial graphitic edge sites, OFGs and their electrochemical stability were examined using graphite felts, which were modified with multi-walled carbon nanotubes and activated with KOH. It is demonstrated that OFGs cannot exclusively be responsible for the electrocatalysis since they did not correlate to the electrochemical activity. The surface composition after electrochemical cycling in the positive half-cell was still different for all samples but did not reflect the performance either. However, a correlation was found between the activity and stable edge site defects. There was neither a correlation between the electrocatalytic activity and the amount of oxygen, nor for the kind of OFG in the negative half-cell. The oxygen concentration after electrochemistry was very similar, even more highlighting the importance of edge sites in the VIII/VII redox reaction. The results of this work indicate that the major electrocatalytic effect for both half-cell redox reactions is related to stable graphitic edge sites in sufficient quantity.