Promising Additives for the Stability of Vanadium Flow Battery Catholytes
Authors: Oboroceanu Daniela , Quill Nathan , Buckley D. Noel , Lynch Robert P. 
Published: February 2020

The redox flow battery (RFB) has seen an explosion in research activity in recent years due to its attractiveness for large scale energy storage applications (e.g. grid level storage).1-5 RFBs offer many advantages over conventional batteries including their relatively high reliability and long cycle life, as well as the ability to scale their energy and power output independently. The vanadium flow battery (VFB) (also known as the vanadium redox flow battery or all-vanadium flow battery) is currently the most promising of these technologies, with a number of large scale applications and demonstration projects already underway.6 The primary advantage of VFBs is that cross-contamination due to transport through the separating membrane is effectively eliminated because the anolyte and catholyte differ only in the oxidation state of the vanadium.
One of the primary disadvantages of VFBs (and RFBs in general) is their relatively low energy density which is limited by the solubility of the different vanadium (reduced/oxidized) species in the electrolyte. In the anolyte, the solubility of V2+ and V3+ generally increases with temperature and decreases with increasing concentration of H2SO4 and this is also true for the solubility of the VIV species, vanadyl (typically in the form of VO2+) ions, in the catholyte.7 However, the solubility characteristics of the VV species, pervanadyl (typically in the form of VO2
+) ions, are considerably more complex.
For example, the solubility of vanadium(V) oxide, V2O5, in this region of pH is ~0.1 mol dm-3 or less. Thus, at the concentrations typically encountered in VFB catholytes (>1.5 mol dm-3), VV is expected to be thermodynamically unstable in solution. However, precipitation is usually found to be very slow and, in practice, supersaturated solutions of VV in sulfuric acid can persist for very long periods of time. The stability of these metastable solutions (VFB catholytes) decreases, as expected, as the concentration of VV increases.8, 9 Stability improves with increasing concentration of sulfate and in the presence of certain additives such as H3PO4.9, 10

We have previously reported quantitative studies of the kinetics of VV precipitation from VFB catholytes and demonstrated that the induction time for this process (i.e. the time until precipitation is observed) has an Arrhenius-type dependence on temperature.4 Comprehensive experiments on the variation of the rate of this process with vanadium concentration, sulfate concentration and temperature have allowed us to develop a model which can predict the induction time (and hence, the stability) of VFB catholytes over a wide range of compositions and temperatures.9 We have also published the first quantitative analysis of the effect of H3PO4 and other phosphate based additives on the induction time of VFB catholytes.10 In this talk, we will extend this research to other additives and show the resulting very promising effects on the stability of VFB catholytes.

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