(405c) 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.