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 V III /V II 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.

Origin of the catalytic activity at graphite electrodes in vanadium flow batteries

“…The degree of oxidation at this material is high as it was studied by our group previously. 37 The oxygen-free electrodes further decrease R CT during cycling. In the positive half-cell, R CT steadily decreases with higher deoxygenation temperatures.…”

“…In a previous study, we have already shown that OFGs are electrochemically unstable, and predictions on the activity cannot be made based alone on a previous physicochemical characterization. 37 To study the inuence and electrochemical performance of OFGs on graphite felt electrodes, we took a different approach compared to previously discussed studies. While usually the oxygen concentration is increased by an oxidative pretreatment, we thermally deoxygenated felt electrodes (Fig.…”

2021: A Surface Odyssey. Role of Oxygen Functional Groups on Activated Carbon‐Based Electrodes in Vanadium Flow Batteries

“…edge sites 2020 [6] ited several oxygen-related functional groups after activation, which were held responsible for the enhanced electron transfer properties.…”

“…It was recently demonstrated that they are a poor activity descriptor because there was no correlation between half‐cell activity and concentration of surface oxygen moieties before or after electrochemistry. [6] Oxidative activation creates pores and corrosion‐like pits on the otherwise smooth pristine GF, as it is observed by scanning electron microscopy (SEM). If the damaging exceeds the mechanical tolerance of the material, its structural properties change and the long‐range order is lost, resulting in an amorphous material with low electrical conductivity.…”

Origin of the catalytic activity at graphite electrodes in vanadium flow batteries

“…In a previous study, we have already shown that OFGs are electrochemically unstable, and predictions on the activity cannot be made based alone on a previous physicochemical characterization. 37 Figure 1. Schematic representation of the graphite structure and the experimental procedure.…”