Accelerated Testing of Vanadium Catholyte Stability
Authors: Buckley D. Noel , Oboroceanu Daniela , Quill Nathan , Lenihan Catherine , Lynch Robert P. 
Published: May 2020

There is great interest in redox flow batteriesfor large-scale energy storage1-8. The energy storage capability and power output of a flow battery, unlike conventional batteries, can be scaled independently to suit the desired application.4,8 Other advantages9 include a high degree of safety, long lifetime, potentially low capital costs, high reliability and relatively high energy efficiency. Among the numerous systems that have been studied, the vanadium flow battery (VFB) is commonly regarded as one of the most promising.2,3,10-12 Compared to other flow battery systems, VFBs have the additional advantage 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.13

The thermal stability of the catholyte in the charged condition can be a limiting factor for the performance of VFBs. At the pH of typical VFB catholytes, the solubility of vanadium (V) oxide, V2O5, is ~0.1 mol dm-3 or less14 and so VO2
+ is expected to be thermodynamically unstable in solution with respect to precipitation as V2O5. Nevertheless, catholytes with high concentrations of VV (> 1.5 M) in sulphuric acid can persist for very long periods of time.15-20 The stability of these metastable solutions (VFB catholytes) decreases, as expected, as the concentration of VV increases. Stability improves with increasing concentration of sulphate and in the presence of certain additives.2,16-18

We have developed a technique16,19,20 for evaluating the stability of VFB catholytes using a standardised methodology for measuring the induction time for precipitation over a range of temperature. We have quantified the observed variations in terms of concentration coefficients for sulphate and VV and developed a model to predict the stability of VFB catholytes as a function of temperature and composition20.
This model is based primarily on data obtained at higher temperatures or with less stable catholytes (i.e., high VV and low sulphate concentration). In this presentation, we extend our range of measurements to much longer times using electrolyte compositions close to those used in practical flow batteries. These results are incorporated into our new, improved model allowing accelerated testing of catholyte stability.

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