Effects of Temperature and Composition on Catholyte Stability in Vanadium Flow Batteries: Measurement and Modeling

Abstract

The stability of typical vanadium flow battery (VFB) catholytes was investigated at temperatures in the range 30-60 • C for V V concentrations of 1.4-2.2 mol dm −3 and sulfate concentrations of 3.6-5.4 mol dm −3 . In all cases, V 2 O 5 precipitates after an induction time, which decreases with increasing temperature. Plots of the logarithm of induction time versus the inverse of temperature (equivalent to Arrhenius plots) show excellent linearity and all have similar slopes. The logarithm of induction time also increases linearly with sulfate concentration and decreases linearly with V V concentration. The slopes of these plots give values of concentration coefficients β S and β V5 which were used to normalize induction times to reference concentrations of sulfate and V V . An Arrhenius plot of the normalized induction times gives a good straight line, the slope of which yields a value of 1.791 ± 0.020 eV for the activation energy. Combining the Arrhenius equation with the observed variation with sulfate and V V concentrations, an equation was derived for the induction time for any catholyte at any temperature in the range investigated. Although the mechanism of precipitation of V V from catholytes is not yet well understood, a precise activation energy can now be assigned to the induction process. The rapid growth of renewable electricity generation from intermittent sources such as solar photovoltaic and wind is driving a need for advanced, cost-effective, electrical energy storage (EES) technologies.1-3 Redox flow batteries 4-11 (RFBs) have attracted much interest for large-scale energy storage due to advantages over other EES technologies, and research activities in this area have grown exponentially in recent years.12,13 The energy storage capability and power output of a flow battery, unlike conventional batteries, can be scaled independently to suit the desired application. 7 Other advantages 14 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), also known as the vanadium redox flow battery (VRFB), is commonly regarded as one of the most promising. [5][6][7][15][16][17] The chemistry of this system is perhaps the most thoroughly characterized and the cell design has been considerably optimized. 2,18,19 It has seen the widest commercial deployment 17 and systems as large as 250-1000 kWh have been demonstrated. 20 Compared to other flow battery systems, VFBs have the additional advantage that crosscontamination due to transport through the separating membrane is effectively eliminated because the anolyte and catholyte differ only in the oxidation state of the vanadium. 21 As a result, electrolyte maintenance issues are reduced; in theory, the electrolyte is indefinitely reuseable. Furthermore, if rebalancing of the system is required the electrolytes in the two reservoirs can be mixed with each other. Since aqueous vanadium species are highly ...

Measurement and Computer Simulation of Catholyte Stability in Vanadium Flow Batteries (VFBs)

“…In a typical experiment, 0.8 cm 3 samples of solutions with selected concentrations of V V and sulfate a were placed in small glass vials (7 cm in length × 4.6 mm internal diameter). Vials were then immersed in a thermostatic water bath which had been equilibrated at a selected temperature and the time of immersion was recorded: the solution temperature reached a value within 0.1 K of the bath temperature in ∼60 s. As described previously, 35 the time at which precipitation was observed in each of the separate vials was subsequently recorded. The water-bath reservoir was made of transparent glass and was fitted with a lamp so that the solution in the vial was very clearly visible and the a The term sulfate concentration in this paper is used to include HSO 4 -(which predominates), SO first signs of precipitation could be observed.…”

“…The water-bath reservoir was made of transparent glass and was fitted with a lamp so that the solution in the vial was very clearly visible and the a The term sulfate concentration in this paper is used to include HSO 4 -(which predominates), SO first signs of precipitation could be observed. [34][35][36][37] Since the measured induction times ranged from 2.7 × 10 3 s to 5.9 × 10 5 s, the worst-case uncertainty due to temperature ramp-up was less than ∼2%, and much less in most cases. We also carried out a small number of experiments in larger volume vials to verify that volume had no significant effect.…”

“…Similar behavior was observed for all electrolyte compositions examined. We have shown elsewhere [34][35][36][37] that plots such as that in Fig. 1 are effectively Arrhenius plots and can be used to derive values of the Arrhenius slope m which quantifies the variation with temperature.…”

“…Apart from its implications for the mechanism, knowledge of the precise activation energy governing the induction time for precipitation has great practical utility in assessing catholyte stability. We have demonstrated [34][35][36][37] that the induction time also decreases exponentially with V V concentration and increases exponentially with sulfate concentration and derived an equation governing its behavior. In this paper, we compare the predictions of this equation with experiment Table I.…”

“…The molecules and complexes of V V present in solution are dependent not only on solution acidity and V V concentration but also on sulfate concentration and therefore the thermal stability of V V in a sulfate solution is dependent on both the V V and sulfate concentrations. 87 We have reported [34][35][36][37] that precipitation occurs after an induction time that shows Arrhenius behavior -decreasing exponentially with temperature -and have derived a value of 1.79 eV for the activation energy (E # ) of the induction process. Based on DFT calculations Vijakumar and coworkers 33,80 estimated that an energy barrier of ∼1.25 eV needs to be overcome to convert [VO 2 (H 2 O) 3 ] + to VO(OH) 3 , the first step in their proposed mechanism.…”