A High Energy Density Vanadium Redox Flow Battery with 3 M Vanadium Electrolyte

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 ...

Section: Resultsmentioning

confidence: 99%

mentioning

confidence: 99%

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

Oboroceanu

Quill

Lenihan

et al. 2017

“…Because samples were relatively small (∼10 cm 3 ), volumes were measured by weighing and converting to volume by accurately measured densities. Vanadium concentrations were determined volumetrically against standard 0.1 N KMnO 4 (Fisher Scientific) and H 2 SO 4 concentrations were determined against standard 0.1 N KOH (Sigma-Aldrich).…”

Section: Methodsmentioning

confidence: 99%

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

Oboroceanu

Quill

Lenihan

et al. 2016

The stability of typical vanadium flow battery (VFB) catholytes with respect to precipitation of V 2 O 5 was investigated at temperatures in the range 30-60 • C. In all cases a precipitate formed after an induction time, which decreased with increasing temperature and concentration of V V and increased with concentration of sulfate. Arrhenius-type plots are shown for two typical solutions. These have excellent linearity and have similar slopes which yield an apparent activation energy of 1.79 eV (172 kJ mol −1 ). The variation of induction time with temperature for various concentrations of V V was simulated, and stability diagrams for additive-free VFB catholytes were generated. Vanadium flow batteries 1,2 (VFBs), also known as vanadium redox flow batteries (VRFBs), are currently the subject of much interest and recent research 3-7 because they are attractive for a variety of largescale energy storage applications. 8 An important advantage of a flow battery is that its energy storage capacity and its power capability can be scaled independently.2 VFBs have the additional 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.9 Also, since aqueous vanadium species are highly colored, the state-of-charge may be precisely monitored using ultraviolet-visible spectroscopy. 10,11The energy density of VFBs is limited by the solubility of V II , V III , V IV and V V in the electrolyte. In the anolyte, the solubility of V 2+ and V 3+ generally increases with temperature and decreases with increasing concentration of H 2 SO 4 and this is also true for the solubility of the V IV species, VO 2+ , in the catholyte. 12 The predominant V V species 13 present in strongly acidic solutions such as typical VFB catholytes is the pervanadyl ion VO 2 + . The solubility of vanadium (V) oxide, V 2 O 5 , in this region of pH is ∼0.1 mol dm −3 or less. 14 Thus, at the concentrations typically encountered in VFB catholytes, V V is expected to be thermodynamically unstable in solution with respect to precipitation as V 2 O 5 . However, precipitation is usually found to be very slow and, in practice, supersaturated solutions of V V 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 V V increases. 15 This is reflected in a lowering of stability at a particular vanadium concentration as the state-of-charge (i.e. the fraction of vanadium present as V V ) of the catholyte increases. 16Stability 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. 7,18 However, there is an absence in the literature of detailed kinetic studies of the precipitation process and the variation w...

“…9 All the capacity fade mechanisms mentioned above can also occur in RFBs with molecular active species. In addition to those mechanisms, however, these compounds can also decompose or convert into redox-inactive molecules, leading to irreversible capacity loss over time.…”

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confidence: 99%

Flow Battery Molecular Reactant Stability Determined by Symmetric Cell Cycling Methods

Goulet

Aziz

2018

207

334

We present an unbalanced compositionally-symmetric flow cell method for revealing and quantifying different mechanisms for capacity fade in redox flow batteries that are based on molecular energy storage. We utilize it, accompanied in some cases by a corresponding static-cell cycling method, to study capacity fade in cells comprising anthraquinone di-sulfonate, di-hydroxy anthraquinone, iron hexacyanide, methyl viologen, and bis-trimethylammoniopropyl viologen. In all cases the cycling capacity decay is reasonably consistent with exponential in time and is independent of the number of charge-discharge cycles imposed. By introducing pauses at various states of charge of the capacity-limiting side during cycling, we show that in some cases the temporal fade time constant is dependent on the state of charge. These observations suggest that molecular lifetime is dominated by chemical rather than electrochemical mechanisms. Growing interest in redox flow batteries (RFBs) for grid-scale energy storage has prompted development of new redox-active organic and organometallic molecules synthesized from commonly available raw materials containing earth-abundant elements such as carbon, nitrogen and oxygen. Flow batteries based upon these synthetic compounds might achieve lower electrolyte capital costs than current vanadium-based technologies without the scarcity risks associated with finite resources.1 To become a cost-effective solution for matching of intermittent renewable energy supply to demand, these flow batteries should retain most of their capacity over decadal time scales. As opposed to the elements of the periodic table, e.g. transition metals, traditionally used for the redox-active components of flow batteries, these new organic and organometallic compounds may be susceptible to molecular decomposition, leading to an additional mechanism for flow battery charge storage capacity fade.2 Typically, capacity fade in conventional flow batteries can occur through various mechanisms such as precipitation of reactant species or crossover of these species through the membrane.3,4 Some of these sources of capacity loss are theoretically reversible while others are not. Some are related to device engineering, such as electrolyte leakage, whereas others are intrinsic to the electrolyte chemistry. RFBs that rely upon electrolytes with different dissolved redox-active species on either side, such as the chromium/iron cell, suffer from irreversible crossover of these species unless their electrolytes are balanced by putting both species on both sides, thereby doubling the cost of active materials.5 a The all-vanadium RFB was developed to avoid these problems by having a single common redox-active species on both sides of the cell such that any asymmetric crossover of active species could be reversed by rebalancing the electrolytes by simple remixing. 6 Other examples of capacity fade that can be recovered by electrolyte rebalancing methods include those due to heterogeneous side reactions, such as hydrogen evolution, or fro...