Insights into the Solvation of Vanadium Ions in the Vanadium Redox Flow Battery Electrolyte Using Molecular Dynamics and Metadynamics

Insights into the solvation of vanadium ions in the vanadium redox flow battery electrolyte using molecular dynamics and metadynamics

Sukriti Gupta,^a  Nyunt Wai,^a  Tuti M. Lim,^b and Samir H. Mushrif ^a,c*

Email: shmushrif@ntu.edu.sg

^aEnergy Research Institute and Interdisciplinary Graduate School, Nanyang Technological University (NTU), Singapore ^bSchool of Civil and Environmental Engineering, NTU, Singapore ^cSchool of Chemical and Biomedical Engineering, NTU, Singapore.­­

Abstract

Vanadium Redox Flow Batteries (VRFB) can be integrated with renewable energy technologies like Solar or Wind energy to store the energy generated by these systems and to supply it according to requirements. The energy storage capacity of VRFB depends on the volume of the electrolyte and the amount of ions dissolved in it, whereas its power depends on the electrode surface area and cell stack. [1] Thus, the main advantage of using a redox flow battery over any other rechargeable battery system is that the energy storage capacity can be increased to any extent by increasing the size of the electrolyte storage tanks. In an attempt to commercialize VRFB, efforts are being made to reduce the size of electrolyte storage tanks, by increasing the solubility of vanadium ions in these electrolytes. Additionally, these electrolytes are stable only in the temperature range of 10 to 40°C. Different approaches, like changing the sulphuric acid concentration, using a mixture of hydrochloric and sulphuric acids, or using various chemicals as additives have been tried. [2] Increasing the sulphuric acid concentration stabilizes VO₂⁺ ions better, but the remaining V²⁺, V³⁺ and VO²⁺ ions become relatively unstable. [3] Different additives alter the solubility of vanadium ions to different extents.  However, fundamental insights into the interactions that govern the solubility and stability of vanadium ions in the electrolyte system are lacking and hence systematic screening and selection of additives is not feasible. Hence, classical molecular dynamics simulations, coupled with metadynamics were performed to investigate the local molecular solvation structure of vanadium ions in the electrolyte system. Free energy landscapes corresponding to the migration of chloride, sulphate and bi-sulphate ions were computed. Force-field parameters for vanadium ions were recently developed in our group and were implemented in these simulations [4]. These simulations allow us to estimate (i) the relative stability of counter ions within the local solvation shell of vanadium ions and (ii) the kinetics associated with the migration of counter ions and with the reorganization of water solvation shell around vanadium.

We observed that it is kinetically possible for sulphate, bisulphate and chloride ions to enter the 1st solvation shell of all four vanadium ions at room temperature and they are thermodynamically stable there. Sulphate ions coordinate with the vanadium ion via two or three of its oxygens by replacing the first solvation shell water molecules. This distorts the octahedral structure of VO₂⁺, thus increasing chances for interaction with other vanadium ions, possibly leading to the formation polymers and dimers. Bisulphate ion however could only share one of its oxygen with vanadium ion. As the chloride ion is small and can only replace one water molecule from the 1^st solvation shell of vanadium ions, it does not disturb the octahedral structure of vanadium ions. Chloride ions in the electrolyte can compete with sulphate ions, to enter the 1st solvation shell.

References

 [1]     F. Grossmith, Efficient Vanadium Redox Flow Cell, Journal of Electrochemical Society. (1987) 2950–2953.

[2]   A. Parasuraman, T.M. Lim, C. Menictas, M. Skyllas-Kazacos, Review of material research and development for vanadium redox flow battery applications, Electrochimica Acta. 101 (2013) 27–40.

[3]        F. Rahman, M. Skyllas-Kazacos, Vanadium redox battery: Positive half-cell  electrolyte studies, Journal of Power Sources. 189 (2009) 1212–1219.

 [4]     S. Gupta, N. Wai, T.M. Lim, S.H. Mushrif, Force-field parameters for vanadium ions (+2, +3, +4, +5) to investigate their interactions within the vanadium redox flow battery electrolyte solution, Journal of Molecular Liquids. 215 (2016) 596–602.