Systematic Analysis of Oxygen Functional Groups and Graphitic Defects on Vanadium Flow Battery Electrodes

Abstract

For many electrochemical applications in the energy sector such as batteries, electrolyzers, or supercapacitors, surface-active oxygen functional groups (OFGs) are considered essential. This has sparked a lot of scientific as well as industrial activities for instance in the domain of vanadium redox flow batteries, focusing on the optimization of graphite electrodes by various oxidative treatments. The introduced OFGs are considered to catalyze the vanadium redox reactions effectively. However, these studies often disregard that the harsh attack of the surface also leads to an increase of graphitic edge sites which may also contribute to the observed catalytic activity. In a previous study, we could demonstrate that the relative amount of OFGs is less important for the catalysis than the electrochemical stability of structural defects. Further, the initial surface composition and defect structure was altered significantly, which complicates a prediction of the performance based on starting OFGs. Therefore, a more systematic approach is taken to analyze the influence of OFGs and defects on the electrocatalytic activity. To separate the effect of OFGs and defects we took the reverse approach by thermally deoxygenating two different kinds of graphitic electrodes; a pristine one with a low quantity of defects and oxygen species, and a thermally activated one with a high amount, respectively. By varying the stripping temperature and saturating resulting dangling bonds with hydrogen, specific OFGs were removed prior to electrochemical cycling experiments. Subsequently, the electronic and defect structure of the samples was analyzed by X-Ray photoelectron and Raman spectroscopy. Cyclic voltammetry and impedance spectroscopy were used to study the electrochemical performance. Our results suggest that the initial amount or kind of OFG has no beneficial effect on the catalysis of the vanadium redox reactions. The pristine graphite felt exhibited even higher half-cell activity after the thermal treatment, which is attributed to an increase of graphitic edge sites by the removal of carbon atoms and OFGs. For the activated felt no remarkable change in activity was observed after stripping due to its already initially quite high amount of defects. However, the deoxygenated samples showed a decrease of the charge-transfer resistance during reductive, and an increase during oxidative cycling. The evolution of OFGs and defects after electrolytic immersion and polarization experiments was investigated. Similarly, a pristine and a deoxygenated electrode have been subjected to long-term cycling, studying the evolution of the charge-transfer resistance, the chemical composition, and the microstructure. The results of our study disprove the universal opinion about the role of carbonaceous OFGs for the catalysis of the vanadium redox reactions. It is much rather the degree of disorder that is responsible for the activity of the electrode. Figure 1