High current density, long duration cycling of soluble organic active species for non-aqueous redox flow batteries

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Nonaqueous redox flow batteries (NAqRFBs) are a promising, but nascent, concept for cost-effective grid-scale energy storage. While most studies report new active molecules and proof-of-concept prototypes, few discuss cell design. The direct translation of aqueous RFB design principles to nonaqueous systems is hampered by a lack of materials-specific knowledge, especially concerning the increased viscosities and decreased conductivities associated with nonaqueous electrolytes. To guide NAqRFB reactor design, recent techno-economic analyses have established an area specific resistance (ASR) target of <5 cm 2 . Here, we employ a state-of-the-art vanadium flow cell architecture, modified for compatibility with nonaqueous electrolytes, and a model ferrocene-based redox couple to investigate the feasibility of achieving this target ASR. We identify and minimize sources of resistive loss for various active species concentrations, electrolyte compositions, flow rates, separators, and electrode thicknesses via polarization and impedance spectroscopy, culminating in the demonstration of a cell ASR of ca. 1.7 cm 2 . Further, we validate performance scalability using dynamically similar cells with a ten-fold difference in active areas. This work demonstrates that, with appropriate cell engineering, low resistance nonaqueous reactors can be realized, providing promise for the cost-competitiveness of future NAqRFBs. Grid-scale energy storage has emerged as a critical technology for alleviating the intermittency of renewables, 1 improving the efficiency of the existing grid infrastructure, and offering frequency or voltage regulation.2 Redox flow batteries (RFBs) are particularly attractive storage devices for energy-intensive grid applications.2,3 In a typical RFB, charge-storing active species are dissolved in liquid electrolytes, which are housed in large and inexpensive tanks. The electrolytes are pumped through a power-generating electrochemical reactor, where the active species undergo reduction or oxidation on the surfaces of porous electrodes to charge or discharge the battery. [3][4][5][6] This unique architecture offers several advantages over conventional battery systems, including decoupled power and energy scaling, long operational lifetimes, easy maintenance, simplified manufacturing, and high active-to-inactive materials ratio (particularly at long storage durations). Despite many attractive features, state-of-the-art RFBs are currently too expensive for widespread adoption.7 While the majority of RFB electrolytes are aqueous, 6 transitioning to nonaqueous chemistries could enable higher cell potentials via wider electrolyte stability windows, subsequently increasing the electrolyte energy density and reducing chemical costs. 6,[8][9][10][11] Furthermore, the use of nonaqueous solvents enables a broader palette of potentially inexpensive active materials, which could significantly lower RFB costs. 7,12 In contrast to their aqueous counterparts, nonaqueous redox flow batteries (NAqRFBs) are a nascent technology,...

Section: Vomentioning

confidence: 99%

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confidence: 99%

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confidence: 99%

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Towards Low Resistance Nonaqueous Redox Flow Batteries

Milshtein

Barton

Carney

et al. 2017

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“…[43][44][45] Backing plates were machined from propylene. Inter-digitated flow fields were machined from 3.18 mm thick impregnated graphite (G347B graphite, MWI, Inc.) with 7 channels (1 mm wide × 0.5 mm deep), 4 inlets and 3 outlets.…”

Section: A2040mentioning

confidence: 99%

Evaluation of Electrospun Fibrous Mats Targeted for Use as Flow Battery Electrodes

Liu

Kok

Kim

et al. 2017

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Electrospinning was used to create custom-made fibrous electrode materials for redox flow batteries with targeted structural properties. The aim was to increase the available surface area for electrochemical reaction without diminishing the transport properties of the electrode. Electrospinning conditions were identified that could produce fibers several times larger than those typically yielded by the technique, yet much smaller than in commercially available electrodes. These materials were subsequently carbonized using widely reported protocols. The resultant materials were subjected to a range of characterization tests to confirm that the feasibility of the target material, including surface area, pore and fiber sizes, porosity, conductivity, and permeability. The most promising material to emerge from this selection processes was then tested for electrochemical performance in a flow cell. The produced material performed markedly better than a commercially available material. Further optimizations such as improved consistency in the production and some surface activation treatments could provide significant advancements.

“…species. 15,16 In the present study, we further develop the method with experimental variations that provide more specific information about possible causes of capacity fade.…”

mentioning

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...