Thermal Stability of Catholyte during Operation of Vanadium Flow Batteries
Authors: Oboroceanu Daniela , Quill Nathan , Buckley D. Noel , Lynch Robert P. 
Published: February 2020

Vanadium flow batteries (VFBs) are an attractive technology for a variety of energy storage applications1-4. An important advantage of a flow battery is that its energy storage capacity and its power capability can be scaled independently. VFBs have the additional advantage that cross-contamination due to transport through the membrane is effectively eliminated because the anolyte and catholyte differ only in the oxidation state of the vanadium.

However, the thermal stability of VFB catholytes can impose limits on battery operating temperature. The VV species in the catholyte, VO2
+, can precipitate according to the reaction5

2 VO2
+ + H2O → V2O5 + 2 H+

(1)
But this reaction is usually found to be very slow. 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 time and several studies have been reported on the stability of VV in the catholyte of VFBs. Recently,3 we reported a quantitative study of catholyte stability as a function of composition and temperature. Our results showed that precipitation of VV from sulfuric acid solution occurs after an induction time which increases exponentially with the inverse of temperature, i.e. shows Arrhenius behaviour. Furthermore, the induction time increases exponentially with overall concentration of sulphate (which is mainly in the form of bisulphate in these catholytes) and decreases exponentially with concentration of VV.
The addition of H3PO4 has a strong stabilizing effect on VFB catholytes at higher temperatures. For example, at 50°C the induction time for precipitation for a typical catholyte is enhanced ~12.5-fold by 0.1 M added H3PO4. However, the behaviour is rather complex and at higher concentrations induction time begins to decrease with increasing concentration of H3PO4. Arrhenius plots for low concentrations of added H3PO4 show reasonable fits to straight lines. Experiments using other phosphate additives (sodium triphosphate, Na5P3O10, and sodium hexametaphosphate, (NaPO3)6) show similar results to H3PO4. Many other additives have been suggested but work remains to be done to demonstrate their effectiveness.
In this paper, we use the methodology for measurement of thermal stability, which we have developed, to extend our investigation and examine the stability of VFB catholytes under actual operating conditions. Results on the effect of state of charge, deterioration of stability under certain operating conditions, and recovery of stability will be presented and discussed.

References

Z. Yang, J. Zhang, M. C. W Kintner-Meyer, X. Lu, D. Choi, J.P Lemmon and J. Liu, Chem. Rev., 111, 3577 (2011) and references therein

D. Oboroceanu, N. Quill, C. Lenihan, D. Ní Eidhin, S. P. Albu, R. P. Lynch, and D. N. Buckley, J. Electrochem. Soc.
163, A2919 (2016) and references therein

C. Petchsingh, N. Quill, J. T. Joyce, D. Ní Eidhin, D. Oboroceanu, C. Lenihan, X. Gao, R. P. Lynch, and D. N. Buckley, J. Electrochem. Soc.
163, A5068 (2016) and references therein

M. A. Miller, A. Bourke, N. Quill, J. S. Wainright, R. P. Lynch, D. N. Buckley, and R. F. Savinell, J. Electrochem. Soc.
163, A2095 (2016)

M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, National Association of Corrosion Engineers, 2nd ed. (1974)