Stability of Catholytes in Vanadium Redox Flow Batteries and the Kinetics of Precipitation of VV
Authors: Oboroceanu Daniela , Quill Nathan , Lenihan Catherine , Ní Eidhin Deirdre , Albu Sergiu Petru , Lynch Robert Patrick , Buckley D. Noel 
Published: February 2020

There is considerable interest in vanadium flow batteries (VFBs) for storage of electrical energy, particularly in conjunction with renewable energy sources such as wind and solar1. Like other flow battery systems the VFB is an electrochemical device that converts electrical energy to chemical energy which is stored in the electrolyte2. Typical cells have carbon felt electrodes separated by a proton exchange membrane. The catholyte and the anolyte are circulated through the electrodes from reservoirs. Both electrolytes are highly acidic, typically 3 mol dm-3 H2SO4. The active species are V2+/V3+ (i.e. VII/VIII) in the anolyte and VO2+/VO2
+ (i.e. VIV/VV) in the catholyte.
The energy density of VFBs is limited by the solubility of the vanadium species in the electrolyte. On the negative side, the solubility of V3+ and V2+ generally increases with temperature at any given acid concentration and decreases with increasing concentration of the common anion (bisulphate). On the positive side, the solubility of the VV species, VO2+, increases with temperature and decreases with increasing concentration of bisulphate due to the common ion effect.
The VV species in the catholyte, VO2
+, can precipitate according to the reaction3

2 VO2
+ + H2SO4 → V2O5 + 2H+. (1)
This reaction is usually found to be very slow and, in practice, supersaturated solutions in sulphuric acid where the concentration of VO2
+ exceeds the thermodynamic limit set by Equation (1) can persist for very long periods of time3. The stability of such metastable solutions decreases, as expected, as the concentration of VO2
+ is increased. However, the stability (time to precipitation) of solutions increases with increasing concentration of bisulphate at a given concentration of VO2
+. Although there have been several studies3,4 of the stability of VVin the catholyte of VFBs, there is an absence of detailed kinetic studies of the precipitation process. In this paper we report a quantitative study of the kinetics of precipitation as a function of composition and temperature.
In agreement with earlier studies, when a solution of VV in H2SO4was held above room temperature an induction period was observed (which depended on the solution composition and the temperature) after which precipitation occurred. Light scattering measurements were carried out to investigate the precipitation process in detail. In a typical measurement, a sample of solution was held at a constant temperature for a predetermined period of time. It was then allowed to cool to room temperature and placed in the light-scattering apparatus where the scattered light was measured. This was repeated for other heating times.
Typical results are shown in Fig. 1. For shorter heating times, very little light scattering is observed. However, after an induction period, the light scattering increases sharply indicating the onset of precipitation. Results on the effects of composition temperature and on the kinetics of precipitation will be presented and discussed and a simulation method will be described to predict the stability of catholytes as a function of vanadium and bisulphate concentrations.

References

1. M. J. Leahy, D. Connolly and D. N. Buckley, Wind Energy Storage Technologies, Wind Power Generation and Wind Turbine Design, ed. W. Tong, WIT Press, Southhampton, pp. 661 (2010).
2. M. Skyllas-Kazacos, M. H. Chakrabarti, S. A. Hajimolana, F. S. Mjalli, and M. Saleem, Journal of The Electrochemical Society, 158, R55 (2011).
3. Sarah Roe, Chris Menictas, and Maria Skyllas-Kazacosa, J. Electrochem. Soc., 163, A5023-A5028 (2016)

Figure 1