V(V) at carbon-based electrodes: a computational study
M.Bon1,2, D. Polino3, T.Laino1 and M.Parrinello2,3
1.IBM Research – Zurich, Saumerstrasse 4, 8803, Rueschlikon, Switzerland
2.Department of Chemistry and Applied Biosciences, Eidgenossische Technische Hochschule Zurich,
8093 Zurich, Switzerland
3.Facoltà di Informatica, Istituto di Scienze Computazionali, Università della Svizzera Italiana, 6900 Lugano, Switzerland
In the last decades we observed a growing interest in energy storage systems based on the redox flow battery (RFB).  Among all the RFBs presently under study, the all-vanadium RFB shows good properties such as good electrochemical activity, reversibility, and low maintenance costs.  Despite the various advantages, this device presents also some drawbacks. In particular, the slow kinetics of V(V) near the electrode can be an issue. [3, 4] It has been observed that the reaction at the cathode highly depends on the type of electrode: both the intrinsic structure of the surface and the presence of chelating agents can influence the redox rate and the reversibility of the process. For this reason carbon surfaces are commonly subject to both thermal and chemical treatments,  whose major effect is the formation of oxygen functional groups on the carbon material. These functional groups have two opposite effects: they are able to facilitate the electron transfer step in the V(V)/V(IV) reaction due to the formation of C-O-V-O intermediates, but if in excess, they can diminish the ET due to an increase in charge transfer resistance. Despite the large number of experimental studies conducted, the correlation between surface treatment and electrochemical activity [6, 7] is still unclear. For instance, some studies [3, 5-7] suggest that these groups are important while others  argue that they only play a secondary role and emphasize instead a direct correlation between surface sp2 states and the oxidation peak potential. A comprehensive understanding of the factors that influence the kinetics of the reactions is a key step in the design of new and more efficient devices. Unfortunately, at present there is a limited number of studies on the basis steps of the kinetics of the VRFB reactions at the electrodes because more attention has been paid to the applied technologies and device development. Here we propose to study the interaction between the V(V) with the surface passivated with different functional groups by means of ab-initio Molecular Dynamics simulations  boosted by Well-Tempered Metadynamics . In particular, we determine the structure and energetics of the V(V) in the five functionalized model systems, differing for the structural and chemical properties of the surface in contact with the solution.
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