Thermal Stability of Concentrated V(V) Electrolytes in the Vanadium Redox Cell

Abstract

The vanadium redox battery currently employs solutions of up to 2 M V(II)/V(III) and 2 M V(IV)/V(V) as the negative and positive half‐cell electrolytes. This concentration is limited by the solubility of the different vanadium ions in the temperature range of 10 to 40°C. Generally, the solubility of V(II), V(III), and V(IV) increases with an increase in temperature; however, the V(V) electrolyte suffers from the effect of thermal precipitation at temperatures of 40°C and above. While thermal precipitation is a serious problem in solutions of V(V) concentrations between 1.5 and 2.0 M, a surprising result was observed at concentrations above 3.0 M. As the results presented here show, at higher vanadium concentrations the V(V) solution demonstrated increased stability and there was no evidence of thermal precipitation over a 30 day period at temperatures above 40°C.

Communication—Observation of Arrhenius Behavior of Catholyte Stability in Vanadium Flow Batteries

“…This indicates that more concentrated V V solutions are less stable with respect to precipitation, and that stability improves with increasing sulfate concentration, as previously reported. 17 Series of experiments were carried out in which the induction time τ for precipitation was measured as described above for a range of temperatures. Typical results are shown in Table I for two different electrolyte solutions.…”

“…Thus, there have been several studies 7,[15][16][17][18][19] of the stability of V V in the catholyte of VFBs, and several mechanisms of precipitation have been proposed. 7,18 However, there is an absence in the literature of detailed kinetic studies of the precipitation process and the variation with temperature has never been quantitatively analyzed.…”

“…the fraction of vanadium present as V V ) of the catholyte increases. 16 Stability improves with increasing concentration of sulfate 17 and in the presence of certain additives 1 such as H 3 PO 4 . Thus, there have been several studies 7,[15][16][17][18][19] of the stability of V V in the catholyte of VFBs, and several mechanisms of precipitation have been proposed.…”

Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods

“…Transient behavior in the flow cell, spanning several hours and tens of cycles after the resumption of cycling, hindered the measurement of the SOC-dependence of the fade rate, but the highest capacity fade rate was in the reduced form, at roughly 0.2%/day. Chemical decomposition was observed of the reduced form but not of the oxidized form of AQDS with NMR spectrometry of samples held for two weeks at 45 • C. c. Cycling of K 4 [Fe(CN) 6 ] /K 3 [Fe(CN) 6 ]in 1 M KOH is characterized initially by a low capacity fade rate that becomes high when a SOC imbalance between the two sides grows to the extent that both sides become capacity-limiting, as illustrated in Fig. 6b In all cases reviewed to date of organic or organometallic redoxactive species, capacity fade is independent of the number of charge-discharge cycles imposed, suggesting that dissolved molecular reactants are unaffected by changes in oxidation state and that a cycle-based metric is unsuitable for predicting flow battery lifetime.…”

Towards understanding the poor thermal stability of V5+ electrolyte solution in Vanadium Redox Flow Batteries

“…In reality, the fully charged V 5+ electrolyte solution displays poor stability at elevated temperatures (>310 K) and also at high vanadium concentrations (>2 M) [5][6][7]. This poor stability is witnessed as hydrated V 2 O 5 precipitation, which leads to energy loss and failure of the battery [8,9]. The incapacity to increase the vanadium concentration (>2 M) in the electrolyte solution without increasing the sulfuric acid concentration (>5 M) puts a limit on the maximum energy density of the VRFB (≤25 Wh kg −1 ).…”