Vanadium flow batteries (VFBs) are an attractive technology for a variety of energy storage applications1-5. They offer the 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. Since aqueous vanadium species are highly colored, the vanadium concentrations and state-of-charge of both sides of a VFB may be precisely monitored using UV-visible spectroscopy3.
The energy density of VFBs is limited by the solubility of VII, VIII, VIV and VV in the electrolyte. The solubility of V3+ and V2+ generally increases with temperature and decreases with increasing concentration of H2SO4 and this is also true of the solubility of the VIV species, vanadyl ion (VO2+). However, the VV species in the catholyte, pervanadyl ion (VO2+), can precipitate as V2O5. This reaction is usually found to be very slow and, in practice, supersaturated solutions of VV in sulphuric acid can persist for very long periods of time4. There have been several studies1,2,4,5 of the stability of VV in the catholyte of VFBs. In this paper we discuss our recent work in measuring and modelling the kinetics of precipitation of VV from H2SO4 solutions in the absence and in the presence of additives.
We investigated the stability of typical vanadium flow battery (VFB) catholytes at temperatures in the range 30–60°C for VV concentrations of 1.4–2.2 mol dm-3 and sulfate concentrations of 3.6–5.4 mol dm-3. In all cases, V2O5 precipitates after an induction time, which decreases with increasing temperature. The logarithm of induction time for precipitation increases linearly with inverse temperature and with sulfate concentration and decreases linearly with VV concentration. The slopes of these plots give values of activation energy and concentration coefficients which we used to generate a quantitative model of catholyte stability.
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 behavior 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 at 70°C using other phosphate additives (sodium triphosphate, Na5P3O10, and sodium hexametaphosphate, (NaPO3)6) showed similar results to H3PO4. Other additives were similarly investigated: the results will be presented and discussed.
1. D. Oboroceanu et al., J. Electrochem. Soc., 164, A2101 (2017).
2. D. Oboroceanu et al., J. Electrochem. Soc., 163, A2919 (2016).
3. C. Petchsingh et al., J. Electrochem. Soc. 163, A5068 (2016)
4. S. Roe et al., J. Electrochem. Soc., 163, A5023 (2016)
5. M. Skyllas-Kazacos et al., J. Electrochem. Soc., 158, R55 (2011).