A Systematic Study of Modified Carbon Electrodes for the All-Vanadium Redox Flow Cell  from Model Electrodes to Application
Authors: Taylor Susan Margaret , Patru Alexandra , Nibel Olga , El Kazzi Mario , Gubler Lorenz , Schmidt Thomas J. 
Published: February 2020

Research into all-vanadium redox flow battery technologies is on the rise, with a large focus being placed on the understanding of reaction kinetics involved in the system i.e. V(II) oxidation at the anode and V(V) reduction at the cathode. Carbon based electrodes materials appear to be the most promising catalysts for both reactions. However, it is not clear which properties of carbon are desirable for improving reaction kinetics. Some groups have suggested that the introduction of oxygen groups onto the carbon electrode surface facilitate the electron transfer step acting as active sites for the redox reactions and improve reaction kinetics [1] [2] [3]. On the other hand, other authors claim that the activity improvements are due to changes in the carbon properties such as microstructure, roughness and wettability [4] [5] [6] [7]. Furthermore, when carbon electrodes are used in a redox flow cell system, there is no consensus in the literature on which of the reactions has the slower kinetics and limits the overall cell performance.
In order to gain a better understanding of these reactions, a systematic approach is proposed here. In the first part of this study, the effect of oxygen functional groups and carbon structural modification on the activity of both half reactions is investigated using a model glassy carbon (GC) electrode surface. The GC surface was exposed to oxidative treatments (i.e. introduction of oxygen surface groups) and also to non-oxidative treatments (i.e. mechanical treatments) and finally a mixed treatment (combination of mechanical treatment followed by an oxidative process). Correlations between activity, oxygen surface content and carbon microstructure were established. X-Ray photoelectron spectroscopy (XPS) was used to quantify the surface oxygen content, while Raman spectroscopy and atomic force microscopy (AFM) were used to investigate the structural changes of the CG surfaces.
In a second step of this study the V(II) oxidation and V(V) reduction reactions were studied in a symmetric flow cell system using carbon paper electrodes (SGL39AA®). A reference electrode was implemented in the redox flow cell test system. This specific experimental set-up allowed the decoupling of the contributions of anode and cathode to the overall cell performance. Significant insight into the limiting half-cell reaction was gained on pristine and oxygen heat treated carbon paper electrodes. Cell performance and stability issues were identified and correlated with the half-cell reaction studies.
Acknowledgements: The authors thank The Swiss National Science Foundation for their financial support within the REPCOOL Project (Grant No. 147 661)
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