WO2019246538A1 - All-iron flow battery and systems - Google Patents

Abstract

This invention relates to aqueous, reduction-oxidation (redox) flow cells and batteries. Cells, batteries, and compositions containing them find use in industrial, governmental, and commercial settings including, for example, grid-scale storage applications such as peak-shaving, load-levelling, demand charge management and backup power.

Description

ALL-IRON FLOW BATTERY AND SYSTEMS

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. Provisional Application No. 62/688,064, filed June 21, 2018, and U.S. Provisional Application No. 62/788,492, filed January 4, 2019 which are hereby incorporated by reference in their entireties.

FIELD OF THE APPLICATION

[0002] This invention relates to aqueous, reduction-oxidation (redox) flow cells and batteries. Cells, batteries, and compositions containing them find use in industrial, governmental, and commercial settings including, for example, grid-scale storage applications such as peak shaving, load-levelling, demand charge management and backup power.

BACKGROUND OF THE INVENTION

[0003] Flow batteries are promising for grid-scale storage applications such as peak-shaving, load-levelling, demand charge management and backup power.

[0004] Benefits of using aqueous flow batteries have been well documented (see, e.g., M. Park et al, Nature Reviews Materials, vol. 2, no. November, p. 16080, 2016; L. F. Arenas et al, Journal of Energy Storage, vol. 11, pp. 119-153, 2017; J. Winsberg, T et al, Angewandte Chemie - International Edition, vol. 56, no. 3, pp. 686-711, 2017). The most-developed type of flow battery is the all-vanadium flow battery, which operates in highly acidic chemical environment (> 2 molar) of sulfuric acid or mixed sulfuric-hydrochloric acids (see, e.g., M. Skyllas-Kazacos, Journal of The Electrochemical Society, vol. 134, no. 12, p. 2950, 1987; M. Skyllas-Kazacos,

M. et al, U. S. Patent 4,786,567).

[0005] Another type of flow battery that has undergone significant development and scale-up is the iron-chromium flow battery developed by the National Aeronautics and Space

Administration (NASA). Both the iron-chromium and all-vanadium flow batteries are known as “true” flow batteries in the sense that there is no solid phase formed. These types of flow batteries are also referred to as“classical,”“traditional,”“full,”“all-solution,”“all-soluble” or “double-redox” flow batteries to indicate there is no solid phase formation during battery charging.

[0006] Another true flow battery, which operates in alkaline environment, using titanium catecholate as the negative electrode and ferricyanide as the positive electrode, has been described (see, e.g., Goeltz et al., US 8,753,761 B2). There are multiple drawbacks of using cyanide (e.g., ferrocyanide/ferri cyanide) as the iron complexing ligand including low solubility of approximately 0.2-0.4 molar, light-sensitivity (see, e.g., O. Scialdone, A et al., Journal of Electroanalytical Chemistry, vol. 704, pp. 1-9, 2013) as well as significant safety concerns. Although ferricyanide is generally regarded as safe, certain conditions, such as mixing with strong acid or overheating can lead to the release of highly toxic cyanide vapors. It is not possible to use the ferricyanide redox couple in acidic media (pH < 7) because the cyanide ions dissociate.

[0007] Previously described flow batteries have sought to use a single metal for the negative and positive electrode reactions, for example, to reduce problems associated with cross

contamination and to enable longer battery lifetime. For example, all- vanadium batteries use only vanadium redox couples (see, e.g., M. Skyllas-Kazacos, Journal of The Electrochemical Society, vol. 134, no. 12, p. 2950, 1987), all-copper batteries (see, e.g., L. Sanz et al., Journal of Power Sources, vol. 268, pp. 121-128, 2014; P. Leung et al., Journal of Power Sources, vol. 310, pp. 1-11, 2016) use only copper redox couples, and all-iron batteries use only iron-based redox couples (see, e.g., L. W. Hruska, Journal of The Electrochemical Society, vol. 128, no. 1, p. 18, 1981). All-iron batteries can be divided between hybrid all-iron batteries, which involve metal plating (electrodeposition), and true all-iron flow batteries, which do not.

[0008]

[0009] All-iron hybrid flow batteries have been described in patents and peer-reviewed publications (see, e.g., L. W. Hruska, Journal of The Electrochemical Society, vol. 128, no. 1, p. 18, 1981; R. Zito, U.S. Patent 4,069,371, 1978; J. Mellentine, Skemman.Is, p. 139, 2011; K. L. Hawthorne et al, ECS Transactions, vol. 50, no. 1, pp. 49-56, 2013; J. Escudero-Gonzalez et al, International Journal of Electrical Power and Energy Systems, vol. 61, pp. 421-428, 2014; K.

L. Hawthorne et al, Journal of the Electrochemical Society, vol. 161, no. 10, pp. A1662-A1671, 2014; K. L. Hawthorne et al., Journal of Power Sources, vol. 269, pp. 216-224, Dec 2014; K. L. Hawthorne et al., Journal of the Electrochemical Society, vol. 162, no. 1, pp. A108-A113, 2014;

M. C. Tucker et al., ChemSusChem, vol. 8, no. 23, pp. 3996-4004, 2015; A. K. Manohar et al., Journal of The Electrochemical Society, vol. 163, no. 1, pp. A5118-A5125, 2016; S. Selverston et al., Journal of Power Sources, vol. 324, pp. 674-678, 2016; R. Savinell and J. Wainright,“Iron Flow Batteries,” ET.S. Patent 9559375, 2017; A. Dinesh et al., Environmental Chemistry Letters, no. February, pp. 1-12, 2018). The seminal publication describing an all-iron hybrid flow battery was authored by Hruska and Savinell in 1981 (see L. W. Hruska, Journal of The Electrochemical Society, vol. 128, no. 1, p. 18, 1981). An all-iron chloride hybrid flow battery has been described by Energy Storage Systems, Inc. (Portland, Oregon).

[0010] Hybrid flow batteries present several fundamental challenges and limitations. For example, the energy storage capacity per battery stack is limited by the spacing between the electrodes (typically, 0.5-3.0 mm). This spacing limits the amount of metal that can be electroplated and consequently limits the amount of energy stored in each battery stack. In some hybrid flow batteries, effort has been made to mitigate this problem by using slurry electrodes, wherein conductive particles are circulated and the metal is electroplated onto the particles as they flow by the electrode (see, e.g., R. Savinell and J. Wainright,“Iron Flow Batteries,” ET.S. Patent 9559375; A. Marshall et al., Journal of Electroanalytical Chemistry and Interfacial Electrochemistry, vol. 59, no. 1, pp. 33-40, 1975; F. R. McLamon, Journal of The

Electrochemical Society, vol. 138, no. 2, p. 645, 1991). Slurry batteries are challenging in practice, and they still have challenges involving metal corrosion, as well as possible pumping and flow-related issues. In all-iron hybrid flow batteries, iron complexing agents (ligands) may be added as“stabilizing agents” in order to help prevent precipitation of solid iron hydroxides such as Fe(OH)₂ and Fe(OH)₃, but the ligands themselves form no part of and are not involved in the battery half-cell reactions (see, e.g., R. Savinell and J. Wainright,“Iron Flow Batteries,” ET.S. Patent 9559375).

[0011] The idea of using iron-based solution redox couples originated in the early 1980’s (see, e.g., Y.-W. D. Chen, Journal of The Electro- chemical Society, vol. 128, no. 7, p. 1460, 1981; J. G. Ibanez, Journal of The Electrochemical Society, vol. 134, no. 12, p. 3083, 1987; A. S. N. Murthy et al., Journal of Power Sources, vol. 27, no. 2, pp. 119-126, 1989). Examples of iron complexing agents (ligands) that have been investigated are listed in Table 2.

[0012] Table 2: Examples of iron complexing agents (ligands) studied for use in battery applications

[0013] For example, Chen et al. investigated phenanthroline-based complexes of iron and acid sulfate media for positive electrodes. Chen considered ligands that shifted the potential more positive, but did not present or describe any battery testing data. Murthy and Srivastava studied possible iron complexes for negative electrodes, also in acid sulfate media (see, e.g., [35]). The complexing agents considered were EDTA, DTPA and NT A, and it was postulated that any one of them could work in conjunction with one of the iron complexes described by Chen in 1981 (see, e.g., Y.-W. D. Chen, Journal of The Electro- chemical Society, vol. 128, no. 7, p. 1460, 1981), however no battery testing data was presented or described.

[0014] In a 2006 study, Wen et al. investigated the use of Fe-Cit as a negative redox couple for an iron-bromine battery application (see, e.g., Y. H. Wen et al., Journal of The Electrochemical Society, vol. 153, no. 5, p. A929, 2006). However, significant safety challenges exist regarding the use of bromine which is highly volatile and considered highly toxic. [0015] More recently, Gong et al. described an alkaline all-iron chemistry that used iron- triethanolamine (Fe-TEA) for the negative redox couple and ferri cyanide, Fe(CN)6 , for the positive redox couple (see, e.g., K. Gong, ACS Energy Letters, vol. 1, no. 1, pp. 89-93, 2016). Triethanolamine (TEA) was also used as a complexing agent for iron in alkaline solutions (see, e.g., J. G. Ibanez, Journal of The Electrochemical Society, vol. 134, no. 12, p. 3083, 1987; Y. H. Wen et al., Electrochimica Acta, vol. 51, no. 18, pp. 3769-3775, 2006; N. Arroyo-Curras et al., Journal of the Electrochemical Society, vol. 162, no. 3, pp. A378-A383, 2014). These batteries required use of an ion-exchange membranes (IEM) such as Nafion™ (DuPont™). The battery described by Gong also suffered from low energy density because of limited solubility of ferricyanide, which is approximately 0.2-0.4 molar. Also, it was reported that the TEA could, after diffusing through the membrane, be irreversibly oxidized at the positive electrode. Another major reported drawback was that the TEA could react directly with the ferricyanide. These undesirable side reactions involving TEA pose serious problems in terms of long-term stability.

SUMMARY OF INVENTION

[0016] The present invention provides true all-iron aqueous, reduction-oxidation (redox) flow cells and batteries.

[0017] Accordingly, in some embodiments, the invention provides an all-iron redox flow cell designed to operate in acidic media (pH < 7) (e.g., a flow-cell comprising electrolytes with a pH < 7).

[0018] In some embodiments, the invention provides an all-iron redox flow cell comprising:a first half-cell comprising a positive electrolyte and a non-metal plating electrode; a second half cell comprising a negative electrolyte and a non-metal plating electrode; and a membrane that is not electrically conductive. In some embodiments, a first storage tank external to the first half cell for circulating the positive electrolyte to and from the first half-cell; and a second storage tank external to the second half-cell for circulating the negative electrolyte to and from the second half-cell, the half-cells conducting an oxidation reduction reaction to charge and discharge the battery. In some embodiments, the positive electrolyte comprises ammonium chloride, iron(II) chloride, iron(III) chloride and a citrate source. In some embodiments, the negative electrolyte comprises ammonium chloride, iron(II) chloride, iron(III) chloride and a citrate source. The invention is not limited by the citrate source. Indeed, a variety of citrate sources may be used in an electrolyte of the invention including, but not limited to, citric acid, ferric ammonium citrate, sodium citrate, trisodium citrate, ferric citrate hydrate, and/or a combination thereof. Similarly, the invention is not limited by the type of electrolyte used for the positive and negative electrolytes. Indeed, any electrolyte that is useful in the flow cells described herein may be used. Specific examples of electrolytes are detailed herein and include, but are not limited to, FeCl2 + FeCl3 + Citric Acid + KC1; FeCl2 + FeCl3 + Citric Acid + NH4C1; FeCl2 + Ferric Ammonium Citrate + KC1; FeCl2 + Trisodium Citrate + FeCl3 + NaCl; FeCl2 + Ferric Citrate Hydrate + NH4C1; and/or any combination thereof. In some embodiments, ammonium ferric citrate is used in the starting electrolyte composition for the positive and/or negative electrolyte. In some embodiments, the pH of the positive electrolyte is different from the pH of the negative electrolyte. In some embodiments, no solid phase is formed during charging of the cell. In some embodiments the positive electrolyte and the negative electrolyte are the same electrolyte, except that the pH of the two electrolytes is different. In some embodiments, the positive electrolyte has a pH between 0.0 - 2.0 and the negative electrolyte has a pH between 4.0 - 7.0. In some embodiments, the positive electrolyte has a pH of about 1 and the negative electrolyte has a pH of about 5. In some embodiments, a pH gradient is maintained between the positive electrolyte and the negative electrolyte. In some embodiments, the pH gradient is between 1-5 pH units (e.g., 1, 1.5, 2, 2.5, 3, 3.5, 4, 4.5, 5, or any pH unit range there between). In some embodiments, the flow rate to and from the half cells is between 0.2 mL/min/cm² and about 5 mL/min/cm². In some embodiments, the flow rate to from the half cells is 120 mL/min. In some embodiments, a flow cell of the invention has a current efficiency that is greater than 90% (e.g., greater than 91, 92, 93, 94, 95, 96, 97, 98, or 99 % efficient). The invention is not limited by the type of membrane used. Exemplary membranes include, but are not limited to, membranes comprising polypropylene (PP), polyethersulfone (PES),

polybenzimidazole (PBI), and/or a combination thereof. In some embodiment, the membrane is a microporous membrane. In some embodiments, a plurality of cells are combined in series (“stacked”) using bipolar plates. The invention is not limited by the number of cells stacked. Indeed, any number of cells may be stacked including, but not limited to, 10, 20, 30 , 40, 50, 60 , 70, 80, 90, 100, 110, 120, 130, 140, 150, 160, 170, 180, 190, 200 or more cells). In some embodiments, the stack the of cells combined in series produces an output of 12-48 volts and/or a current up to 100 amperes. In some embodiments, a surfactant is added to the first and/or second electrolyte to reduce surface tension. The invention is not limited by the type of surfactant. Indeed, any surfactant known in the art may be used including, but not limited to, those described herein. In some embodiments, the surfactant is sodium dodecyl sulfate (SDS) or polyethylene glycol (PEG). In some embodiments, the temperature of the electrolytes within a flow cell of the invention is maintained between 20-60 C (e.g., between 30-45 C) when in operation. In some embodiments, the electrodes in each of the half cells are non-metal plating electrodes. The invention is not limited by the type of non-metal plating electrodes used.

Exemplary non-metal plating electrodes include, but are not limited to, carbon felt, carbon paper, or carbon cloth electrodes. In some embodiments, the carbon paper is AvCarb P75 untreated carbon paper. In some embodiments, a the carbon cloth is ELAT hydrophilic carbon cloth. In some embodiments, the electrodes each have a geometric between 400-5000 cm2.

[0019] The invention also provides a battery comprising one or more flow cells described herein.

BRIEF DESCRIPTION OF DRAWINGS

[0020] FIG.1 depicts (A) an iron-hybrid flow battery of the prior art and (B) a non-limiting example of an all-iron citrate (Fe-Cit) flow battery of the present invention.

[0021] FIG. 2 shows (A) the cell potential during battery charge-discharge cycling of a battery in one embodiment of the invention with a cell area of 50 cm², charging at 7 mAcm ² and discharging at 4 mA cm ², and (B) that the average current efficiency was approximately 100 %. During each cycle, the battery was charged until reaching a voltage cutoff of 1.3 volts, and then discharged until reaching a cutoff of 0.0 volts.

[0022] FIG. 3 illustrates the measured effect of temperature on the Fe-Cit redox couple in unstirred solution containing 0.5 M FeCl2, 0.5 M FeCl3, 0.5 M Na3Cit and 3 M NH4C1 at pH=5 on graphite rod in unstirred solution.

[0023] FIG. 4 provides a schematic of in-line pH rebalancing devices in some embodiments of the invention (a) Galvanic (discharge) and (b) supergalvanic (discharge) configuration for batteries with negative electrolyte pH greater than positive electrolyte pH, and (c) electrolytic (charge) configuration for batteries with negative pH less than positive electrolyte pH. [0024] FIG. 5 shows a comparison of pH gradient change between Vanadion and BPM in iron chloride electrolytes (a) pH changes when using a bipolar membrane (BPM) (Fumatech FBM, GmbH), and (b) pH gradient (difference) between the oxidation compartment and the reduction compartment, comparing effects when a VANADION™ membrane (Ion Power, Inc. New Castle, DE, USA) was used.

[0025] FIG. 6 shows (a) a comparison of pH gradient between Vanadion and BPM in iron chloride citrate electrolytes; and (b) measured pH as a function of time in iron chloride citrate electrolytes.

[0026] FIG. 7 shows an example of a charge discharge cycle as well as pH stability during repeated charge-discharge cycling when using the rebalance cell. Battery charge-discharge efficiency values and measured pH during continuous charge-discharge cycling at +/- 15 mA/cm², where each charge was carried out for approximately 3.3 hours. The battery was operated using the rebalancing configuration shown in Figure 1 (b).

DETAILED DESCRIPTION

[0027] The present invention provides aqueous, reduction-oxidation (redox) flow cells and batteries.

[0028] In one embodiment, the invention provides an all-iron flow battery. In a further embodiment, the invention provides an all-iron- flow cell that does not incorporate or utilize a plating reaction. In another embodiment, the all-iron flow cell is configured to operate in acidic media (pH < 7).

[0029] A non-limiting example of a cell/battery of the invention is shown in FIG. 1B. In one embodiment, the redox flow cell 100 comprises a first half-cell 110 containing a source (e.g., electrolyte) of iron(III)-ligand complex 111 and an electrode 112 and a second half-cell 113 containing a source (e.g., electrolyte) of ferrous ions, Fe2+ 114, and an electrode 115; and a separator 116 between the first and second half-cells, where the electrolyte pH is less than 7. In a further embodiment, the cell/battery comprises a first storage tank external to the first half-cell for circulating the source (e.g., electrolyte) of iron(III)-ligand complex 111 to and from the first half-cell; and a second storage tank external to the second half-cell for circulating the source (e.g., electrolyte) of ferrous ions, Fe2+ 114 to and from the second half-cell, the half-cells conducting an oxidation reduction reaction to charge and discharge the battery.

[0030] An all iron flow cell of the invention provides significant improvements and advantages over other flow batteries available in the art. For example, an all-iron flow cell of the invention, in one embodiment, utilizes a microporous separator that is not electrically conductive (e.g., that is incapable of transporting dissolved ions). That is, in some embodiments, in contrast to requiring the use of an ion exchange membrane (e.g., Nafion™, described as an essential and required component of a flow cell of Gong et al., ACS Energy Letters, vol. 1, no. 1, pp. 89-93, 2016), a flow cell of the invention utilizes a non-electrically conductive membrane. The use of a non-electrically conductive membrane provides several advantages including that, unlike ion- selective polymers, they can be made from inexpensive polymers such as polyethylene or polypropylene. For comparison, the most common ion-exchange membrane (Nafion) is a sulfonated fluoropolymer, which is approximately one order of magnitude more expensive. In addition to being relatively inexpensive, use of a non-electrically conductive membrane (e.g., a microporous membraned) in a flow cell of the present invention provides, in some embodiments, superior current efficiencies (e.g., even at a low current density) that are not achievable with conventional flow cells. The invention is not limited by the efficiency attained. In some embodiments, a flow cell of the invention is greater than 90%, greater than 91%, greater than 92%, greater than 93%, greater than 94%, greater than 95%, greater than 96%, greater than 97%, greater than 98%, or greater than 99% efficient. Similarly, the invention is not limited by the type of non-electrically conductive membrane (e.g., microporous separator). Non-limiting examples of non-electrically conductive membrane that find use in the compositions and methods of the invention include, but are not limited to 175 / Daramic™ LLC / Polyethylene- Silica, Vanadion™ / Ion Power / Polyethylene-Nafion™ Composite, Dialysis Membranes, Polyethersulfone (PES), Porous Polybenzimidazole (PBI). In one embodiment, a polyethylene- based non-electrically conductive membrane is used.

[0031] ETse of a non-electrically conductive membrane in a flow cell of the present invention provides, in some embodiments, further improvements and advantages, including relatively high solubility of active materials contained in a flow cell of the invention (e.g., compared to the solubility of ferricyanide or other active material of the art); elimination of side reactions that occur upon mixing of positive and negative electrolytes; and/or the ability to operate the flow cell in the absence of harmful reagents or products (e.g., cyanide).

[0032] In one embodiment, the invention provides an aqueous flow cell that combines Fe-citrate (anion pair) with Fe^2+/3+ (cation pair) and that does not contain an ion-exchange membrane. For example, in one embodiment, the invention provides an aqueous flow cell wherein the negative redox couple comprises Fe-citrate (Fe-Cit) and the positive couple comprises Fe^2+/3+.

[0033] For example, in one embodiment of an all iron flow cell of the invention, the half-cell reactions are as follows:

[0034] Fe(III) - Cit + e- ^ Fe(II) - Cit (1) (negative electrode)

[0035] Fe3+ + e- ^ Fe2+ (2) (positive electrode)

[0036] Fe(III) - Cit + Fe2+ ^ Fe(II) - Cit + Fe3+ (3) (overall reaction)

[0037] In some embodiments, and as described herein, a flow cell and/or battery containing electrolyte/reactants participating in the reactions described in Equations 1-3 and that possesses a non-electrically conductive membrane (e.g., does not contain an ion-exchange membrane) displays a standard potential of approximately 0.77 volts when operated in acidic conditions (e.g., acidic electrolyte). The flow cell and/or battery can further comprise electrodes coupled to either supply electrical energy or receive electrical energy from a load or source. In like manner, the flow cell or battery may comprise other monitoring and/or control electronics (e.g., included in the load) that control the flow of electrolyte through the half cells. A plurality of flow cells can be electrically coupled (“stacked”) in series to achieve higher voltage or in parallel in order to achieve higher current.

[0038] The invention is not limited by the standard potential of a flow cell of the invention. In some embodiments, the standard potential is greater than 0.3 volts, greater than 0.4 volts, greater than 0.5 volts, greater than 0.6 volts, greater than 0.7 volts, greater than 0.8 volts, greater than 0.9 volts, or greater than 1.0 volts (e.g., 1.1 volts, 1.2 volts, 1.3 volts, or higher). In some

embodiments, a cell potential of at least 0.77 volts is sufficient for grid applications including, but not limited to, peak-shaving, load-levelling, demand charge management and backup power.

[0039] Although citrate has long been known to function as an iron complexing (or“chelating”) agent, the details of the iron complexes and their oxidation-reduction reactions have remained elusive and not understood (see, e.g., L. Battistini et al., Analytical Chemistry, vol. 66, no. 13, pp. 2005-2009, 1994; P. Vukosav et al., Analytica Chimica Acta, vol. 745, pp. 85-91, 2012). Furthermore, Fe-citrate is widely known to display slow reaction kinetics (see, e.g., K. L.

Hawthorne et al., Journal of the Electrochemical Society, vol. 161, no. 10, pp. A1662-A1671, 2014). As such, other than its use as a stabilizing agent to prevent formation of solid precipitates Fe-citrate has been discouraged for use in battery applications.

[0040] It has been widely observed in most aqueous flow batteries that faradaic efficiency (also called current efficiency or coulombic efficiency) is lowest at low current densities of 1-10 mA/cm² (due to the increased relative effects of crossover or self-discharge), and that faradaic efficiency increases with increasing current density (mA/cm² ), up to a certain point (see, e.g., M C. Tucker et al., ChemSusChem, vol. 8, no. 23, pp. 3996-4004, 2015; A. K. Manohar et al., Journal of The Electrochemical Society, vol. 163, no. 1, pp. A5118-A5125, 2016; M. C. Tucker et al., Journal of Power Sources, vol. 245, pp. 691-697, 2014; X. Wei et al., Journal of Power Sources, vol. 218, pp. 39-45, 2012; D. Lloyd, E. Magdalena et al., Journal of Power Sources, vol. 292, pp. 87-94, 2015). For this reason, many batteries are required to operate at relatively higher current density, but at the expense of voltaic efficiency, which has the opposite trend. In other words, it is widely known that voltaic efficiency tends to decrease with increasing current density (see, e.g., M. C. Tucker et al., Journal of Power Sources, vol. 245, pp. 691-697, 2014).

[0041] Nonetheless, as described in detail herein, it was unexpectedly observed that the current efficiency of a flow cell of the invention was very high even at low current densities of 4-7 mA/cm². This remarkable result indicated that, in some embodiments, a flow cell/battery of the invention (e.g., operating with the all iron chemistry of Equations 1-3) is able to operate at both high faradaic and voltaic efficiencies for long periods of time (e.g., hours days, weeks, months, years). In some embodiments, a flow cell / battery of the invention operates for about a year, about two, three, four, five, six, seven, eight, nine or more years (e.g., with limited maintenance). Accordingly, in some embodiments, an all iron flow cell/battery of the invention finds us in long-duration flow battery applications (e.g., including, but not limited to, demand-charge management, backup power, micro-grids, peak shaving and frequency regulation).

[0042] Experiments conducted during development of embodiments of the invention identified another unexpected finding regarding a marked effect of temperature on the performance of the iron citrate complex. Although temperature generally has important effects on chemical reactions, the effect of temperature on the electrochemical performance of iron-ligand complexes is not well understood. In particular, whereas Fe-citrate is widely known to display slow reaction kinetics, this was not true at elevated temperatures where there was a marked improvement in reaction kinetics of the iron citrate electrode at elevated temperatures (see, e.g., the

voltammogram shown in FIG. 3).

[0043] In some embodiments, a flow cell of the invention operates utilizing a common starting electrolyte in both half cells (e.g., the same electrolyte composition is used both for the negative and positive electrolytes). The invention is not limited to any particular common starting electrolyte (e.g., used in both half cells). Exemplary electrolytes include, but are not limited to,

[0044] FeCl2 + FeCl3 + Citric Acid + KC1;

[0045] FeCl2 + FeCl3 + Citric Acid + NH4C1;

[0046] FeCl2 + Ferric Ammonium Citrate + KC1;

[0047] FeCl2 + Trisodium Citrate + FeCl3 + NaCl;

[0048] FeCl2 + Ferric Citrate Hydrate + NH4C1; and/or any combination of the above.

[0049] In some embodiments, relatively low-purity electrolytes (e.g., 97 % or 98 % purity) are used. Thus, in some embodiments, electrolytes useful in the cells and batteries of the invention advantageously provide a significant cost savings over the use of elecrtolytes required in other cell systems (e.g., vanadium flow batteries that require electrolytes of at least 99 % purity).

[0050] In some embodiments, the electrolytes shown in FIG. 1B and equations 1-3 are used. In other embodiments, citric acid (H3Cit) is used as a citrate source in an electrolyte (e.g., rather than trisodium citrate Na3Cit as the source of citrate).

[0051] Experiments conducted during development of embodiments of the invention identified and characterized all-iron citrate flow cells/batteries with good cycling stability. For example, a battery was repeatedly charged and discharged without any apparent degradation (see Examples 1-4).

[0052] In one embodiment, the invention provides an all-iron flow cell system (e.g., suitable for use in, for example, electrical grid-scale storage applications such as peak-shaving, load- levelling, demand charge management, and backup power, or with other aspects of the invention). The flow cell may contain two half-cells separated by a separator. The half-cells may contain electrodes in contact with an electrolyte present in each half-cell such that an anodic reaction occurs at the surface of one of the electrodes and a cathodic reaction occurs at the other electrode. The electrodes may be configured (e.g., coupled) to supply electrical energy or to receive electrical energy from a load or source. Electrolyte flows through each half-cell as the oxidation and reduction reactions take place. A cathodic reaction takes place at an electrode in the half-cell referred to as the positive electrode or the cathode, and an anodic reaction takes at an electrode in the half-cell referred to as the negative electrode or the anode.

[0053] In some embodiments, electrolyte in the half-cells flows through the system (e.g., via pipes) to storage tanks and/or holding tanks, and fresh/regenerated electrolyte flows from the tanks back into the half-cells. Systems of the invention can be configured to control the flow of electrolyte through the system and may include, for example, any suitable pumps or valve systems.

[0054] As described herein, the present invention provides electrolytes for the half-cells that provide a suitable source of ions required to carry out the reactions in each half-cell (e.g., described in Equations 1-3 above). The electrolyte used for the redox reactions at the positive electrode is a suitable salt solution comprising a source of ferrous (Fe²⁺) and ferric (Fe³⁺) ions. This electrolyte is also referred to herein as the positive electrolyte. The electrolyte used for the reactions at the negative electrode comprises a source of Fe²⁺ ions. This electrolyte is also referred to herein as the negative electrolyte. In some embodiments, the positive electrolyte and negative electrolyte compositions are the same except for the pH. In some embodiments, the positive electrolyte has a low pH (ca. pH = 1), and therefore has free Fe2+ and Fe3+ ions (standard potential ~ 0.77 V vs standard hydrogen electrode). The negative electrolyte, on the other hand, in some embodiments has a high pH (ca. pH = 5), and therefore has complexed Fe(II)-Cit and Fe(III)-Cit (standard potential ~0.0 V vs standard hydrogen electrode). The invention is not limited by the type of electrolyte used for the positive and negative electrolytes. In some embodiments, the electrolyte is:

[0055] FeCl2 + FeCl3 + Citric Acid + KC1;

[0056] FeCl2 + FeCl3 + Citric Acid + NH4C1; [0057] FeQ2 + Ferric Ammonium Citrate + KC1;

[0058] FeCl2 + Trisodium Citrate + FeCl3 + NaCl;

[0059] FeCl2 + Ferric Citrate Hydrate + NH4C1; and/or any combination thereof.

[0060] The invention is not limited to a particular concentration of electrolytes. Indeed, in some embodiments, electrolytes may be used at a molar concentration of 0.1-5.0 molar for any of the components. Indeed, any electrolyte that is useful in a flow cell of the invention (e.g., in an acidic chemical environment) may be used.

[0061] In some embodiments, a flow cell of the invention comprises FeCl2 at a concentration between 0.1 and 5 M. In one embodiment, a flow cell of the invention comprises 0.5 M FeCl2. In some embodiments, a flow cell of the invention comprises FeCl3 at a concentration between 0.1 and 5 M. In one embodiment, a flow cell of the invention comprises 0.5 M FeCl3. In some embodiments, a flow cell of the invention comprises Na3Cit at a concentration between 0.1 and 5 M. In one embodiment, a flow cell of the invention comprises 0.5 M Na3Cit. In some embodiments, a flow cell of the invention comprises NH4C1 at a concentration between 1 and 5 M. In one embodiment, a flow cell of the invention comprises 3 M NH4C1. In some

embodiments, a flow cell of the invention comprises KC1 at a concentration between 1 and 5 M. In one embodiment, a flow cell of the invention comprises 3 M KC1. In some embodiments, a flow cell of the invention comprises citric acid at a concentration between 1 and 5 M. In one embodiment, a flow cell of the invention comprises 1 M citric acid. In some embodiments, a flow cell of the invention comprises ferric citrate at a concentration between 1 and 5 M. In one embodiment, a flow cell of the invention comprises 1 M ferric citrate.

[0062] In some embodiments, additives are added to the cell (e.g., in order to assist the chemical reactions or to improve solubility (e.g., in order to hinder precipitation). The invention is not limited to any particular additive. Indeed, a variety of additives may be used including, but not limited to, phosphate salts, nitrate salts, bismuth salts, indium salts, and surfactants (e.g., SDS, CTAB). In some embodiments, an additive is added to the positive electrolyte. In other embodiments, an additive is added to the negative electrolyte. In some embodiments, an additive is added to both the positive and negative electrolytes. In some embodiments, different additives are added to the positive and negative electrolytes. [0063] A flow cell of the invention can be configured, in some embodiments, to operate with a flow rate of between about 0.2 mL/min/cm² (e.g., a low flow rate) and about 5 mL/min/cm² (a high flow rate), although lower and higher rates may be used.

[0064] A flow cell of the invention can be configured, in some embodiments, to operate in a variety of temperatures. In some embodiments, a flow cell operates (e.g., chemical reactions including the electrolytes described herein) at a temperature between about 15-70 C. In some embodiments, a flow cell operates at between 30-60C. In some embodiments, a flow cell operates at about 55C.

[0065] A flow cell of the invention may be configured, in some embodiments, such that the electrolytes present in the cells are further present in an environment in which the pH is less than 7. As described herein, in some embodiments, the same electrolyte is used for both the positive electrolyte and the negative electrolyte, except that the pH of the electrolyte is different. In some embodiments, the pH gradient between the positive electrolyte and negative electrolyte is between about 2-7 pH units. For example, in some embodiments the negative electrolyte is pH = 4.0-7.0, and the positive electrolyte is pH = 0.0-2.0. In some embodiments, the pH gradient between the catholye and anolyte is greater than 0.5 (e.g., greater than 1, greater than 1.5, greater than 2, greater than 2.5, greater than 3, greater than 3.5, greater than 4, greater than 4.5, greater than 5, greater than 5.5, greater than 6, or greater) pH units.

[0066] In some embodiments, a pH control system is used in conjunction with the cell/battery to maintain a pH gradient of between about 1-5 pH units. The pH control system may be realized, for example, by addition of NH4 OH to the negative electrolyte and the addition of HC1 to the positive electrolyte, or by other means known to those skilled in the art. For example, in some embodiments, a pH control system described in U.S. Pat. Pub. No. 2014/02742493A1 (hereby incorporated by reference in its entirety) is used.

[0067] In some embodiments, the invention provides devices and methods for maintaining chemical balance in flow battery electrolytes (e.g., for maintaining the concentration of active species in a flow cell/battery).

[0068] Aqueous flow batteries often require chemical rebalancing in order to maintain battery chemistry and to ensure long lifetime (low degradation rates) of the battery. A wide variety of rebalancing methods exist, and they are often specific to each type of battery and battery chemistry.

[0069] For example, in lead-acid and nickel metal hydride batteries, there are hydrogen and oxygen side reactions, and the rebalancing can be accomplished by combining hydrogen and oxygen gas over a catalyst to form water as shown in Equation I. Recombination in lead acid batteries has been described (see, e.g., B. K. Mahato et al., Journal of The Electrochemical Society, vol. 121, no. 1, p. 13, 1974; C. S. C. Bose et al., Journal of Power Sources, vol. 19, no.

4, pp. 261-267, 1987; J. S. Symanski, Journal of The Electrochemical Society, vol. 135, no. 3, p. 548, 1988; D. Pavlov et al., U.S. Pat., No. 4925746):

[0070] 2H₂ + O2 -> 2H2O (I)

[0071] Rebalancing is also needed in many kinds of flow batteries (see, e.g., P. J. Morrissey et al., EiS Pat. No. 6841294; S. Corcuera et al., European Chemical Bulletin, vol. 1, no. 12, pp. 511-519, 2012; A. R. Winter, US Patent App. Pub. No. 20120202095; S. Rudolph et al., Journal of Electroanalytical Chemistry, vol. 703, pp. 29-37, 2013; A. H. Whitehead et al., International Flow Battery Forum (IFBF), p. 56, 2013; A. H. Whitehead et al., Journal of Power Sources, vol. 230, pp. 271-276, 2013; A. Q. Pham et al., U.S. Pat. No. 8980454.

[0072] In some iron-based flow batteries such as iron-chromium and all-iron hybrid batteries, a hydrogen-ferric ion rebalancing scheme (see Equation II) has been described (see, e.g., L. H. Thaller, U.S. Pat. No. 4159366; S. Selverston et al., Journal of Power Sources, vol. 324, pp. 674- 678, 2016; Y. K. Zeng et al., Journal of Power Sources, vol. 352, pp. 77-82, 2017..

[0073] H₂ + Fe³⁺ -> 2H⁺ + Fe²⁺ (II)

[0074] In some cases, rather than balancing the concentration of active species, the rebalancer adjusts electrolyte pH. It is widely known that bipolar membranes can be used to split water into protons and hydroxide ions (see Equation III), and that this phenomenon is useful in a wide variety of chemical and engineering applications (see, e.g., A. Tanioka et al., Bull. Soc. Sea Water Sci. Jpn., vol. 51, no. 4, p. 205, 1997; G. Pourcelly, Russian Journal of Electrochemistry, vol. 38, no. 8, pp. 919-926, 2002; T. Xu, Desalination, vol. 140, no. 3, pp. 247-258, 2001; A. T. Heijne et al., Environmental Science & Technology, vol. 40, no. 17, pp. 5200-5205, 2006; L. J. Cheng Biomicrofluidics, vol. 5, no. 4, 2011; M. B. McDonald et al., ChemSusChem, vol. 7, no. 11, pp. 3021-3027, 2014; R. S. Reiter et al., Journal of The Electrochemical Society, vol. 163, no. 4, pp. H3132-H3134, 2016). However, each application uses them in unique, specific ways depending on the needs. The primary use of bipolar membranes has been for production of acids and bases from their respective salts (see G. Pourcelly, Russian Journal of Electrochemistry, vol. 38, no. 8, pp. 919-926, 2002; F. G. Wilhelm et al., Journal of Membrane Science, vol. 182, no. 1-2, pp. 13-28, 2001). However, they have found recent applications including the food industry (A. Bazinet et al., Trends Trends in food science & Technology, vol. 9, pp. 107-113, 1998), hydrogen generation (Z. Wen, Journal of Materials Chemistry A, vol. 6, no. 12, p. 4948, 2018), solar fuels (M. B. McDonald et al., ChemSusChem, vol. 7, no. 11, pp. 3021-3027, 2014; R. S. Reiter et al., Journal of The Electrochemical Society, vol. 163, no. 4, pp. H3132-H3134, 2016), microfluidics (L. J. Cheng Biomicrofluidics, vol. 5, no. 4, 2011), regenerative fuel cells (X. Lu et al., Journal of Power Sources, vol. 314, pp. 76-84, 2016) and flow batteries (S. Kniajanski et al., US Pat. Pub. No. 20170098851; S. Y. Reece US Pat. Pub. No. 20160308235; J. J. H. Pijpers, US Pat. Pub. No. 20170317363).

[0075] H2O -> H⁺ + OH (III)

[0076] For example, a rebalancing system based on using three compartments, one bipolar membrane and one ion-exchange membrane per cell has been described (see, e.g., S. Y. Reece US Pat. Pub. No. 20160308235). However, the design described by Reece has several notable disadvantages such as the complexity of three-compartment cells, the high cost of using two membranes per cell, and the considerable energy losses associated with the operation thereof. Another three-compartment rebalancing cell has been described (see, e.g., J. J. H. Pijpers, US Pat. Pub. No. 20170317363) and it shares similar disadvantages. An additional disadvantage of these systems is that due to the design, operation is inefficient because the cation-exchange membrane allows all cations, including H⁺, to migrate into the same electrolyte whose pH is supposed to increase by adding hydroxide ions. However, any H⁺ allowed to enter the electrolyte whose pH is supposed to increase will counter-act the effect of any added hydroxide (OH ) ions.

[0077] In some embodiments, the invention provides systems and method for overcoming the limitations of Reece and/or Pijpers. For example, in some embodiments the invention provides a pH rebalancing system described in Example 6 or FIG. 4. In some embodiments, the pH rebalancing system contains only two compartments/chambers per cell (e.g., it contains less than three compartments/chambers as required by Reece). Using two compartments per cell decreases the physical footprint, reduces the possibility of fluid leakage, and reduces pumping energy losses. In some embodiments, a rebalancing system of the present invention utilizes a single bipolar membrane per cell (e.g., is does not contain two separate membranes (e.g., a bipolar membrane and additional ion exchange membrane) as required by Reece. In some embodiments, a rebalancing system of the present invention operates with significantly greater energy efficiency (e.g., because it has fewer ohmic and transport losses associated with a third compartment and/or additional membrane per cell). Furthermore, in some embodiments, a rebalancing system of the present invention, when operated in electrolytic mode, allows for the energy used in the rebalancing device to participate in battery charging. In contrast, a device of Reece and/or Pijpers cannot recover energy used for rebalancing under any described

configuration. In some embodiments, a rebalancing system of the present invention is operable in supergalvanic mode (e.g., for batteries with negative electrolyte pH greater than positive electrolyte pH). In some embodiments, when operated in supergalvanic mode, potential is applied to the galvanic cell in order to supply current greater than that provided in the short- circuit condition. Accordingly, a rebalancing system of the present invention provides a significantly more efficient system than conventional systems.

[0078] In some embodiments, a rebalancing system of the present invention finds use with an aqueous flow battery (e.g., any battery that operates with a pH gradient (See, e.g., Example 6). In some embodiments, a rebalancing system of the present invention works with an aqueous flow battery comprising a negative electrolyte that operates at a lower pH than the positive electrolyte, or, in other embodiments with a flow battery wherein the negative electrolyte operates at a higher pH than the positive electrolyte. The invention is not limited by the type of flow battery with which a rebalancing system of the invention is used. Indeed, there are many batteries that could benefit from such operation, but which have had limited application in the field due to the unavailability of an appropriate pH rebalancing device and method for its operation. A non limiting example of an aqueous flow battery that benefits from a pH rebalancing device described herein is an all-iron citrate battery (e.g., wherein the negative electrolyte pH operates in the approximate pH range of 4-6 and the positive electrolyte pH operates in the range of 0-2). Accordingly, in some embodiments, the invention provides an all-iron citrate battery utilizing a pH rebalancing system of the invention (e.g., with pH regulation using galvanic and supergalvanic modes of operation). For example, in some embodiments, for batteries wherein the negative electrolyte pH must be less than the positive electrolyte pH, a cell of the invention operates in an electrolytic mode and the energy provided contributes to charging the battery. In other embodiments, for batteries wherein the negative electrolyte pH must be greater than the positive electrolyte pH, the cell operates in a galvanic or supergalvanic mode. Accordingly, in some embodiments, the invention provides a two-compartment, single membrane cell for rebalancing pH in flow batteries using positive and negative electrolytes at different pH values.

A rebalancing system of the invention may be employed in flow batteries with a pH gradient that operates in either direction; the negative electrolyte pH may be either less than or greater than the positive electrolyte pH.

[0079] The invention is not limited by the type of electrode utilized in a flow cell of the invention. In some embodiments, the electrode is a non-metal plating electrode. Examples of non-metal plating electrodes include, but are not limited to, electrodes comprising carbon felt, carbon cloth, carbon paper, graphite felt, graphite paper, porous titanium, or a combination thereof.

[0080] In some embodiments, one or more surfactants may be added to a cell of the invention. In some embodiments, a surfactant is utilized that reduces surface tension within the cell (e.g., thereby increasing pumping efficiency). In some embodiments, a surfactant is utilized that alters (e.g., increases) ion diffusion through the microporous membrane. The invention is not limited to any particular surfactant. Exemplary surfactants include, but are not limited to, any one or more anionic surfactants, cationic surfactants, zwitterionic surfactants, nonionic surfactants, ethoxylates, fatty alcohol ethoxylates, alkylphenol ethoxylates, fatty acid ethoxylates, special ethoxylated fatty esters and oils, ethoxylated amines and/or fatty acid amides, terminally blocked ethoxylates (e.g., poloxamers), fatty acid esters of polyhydroxy compounds, fatty acid esters of glycerol (e,g, glycerol monostearate, glycerol monolaurate), fatty acid esters of sorbitol, spans, tweens, fatty acid esters of sucrose, alkyl polyglucosides, amine oxides, and/or sulfoxides known in the art. In some embodiments, the surfactant is sodium dodecyl sulfate (SDS),

hexadecyltrimethylammonium bromide (CTAB), dodecyltrimethylammonium bromide (DTAB), or polyethylene glycol (PEG). [0081] One advantage is that the flow uniformity is much less critical than for a battery that has plating. When a battery has plating, the flow needs to be perfectly uniform so that the plate deposits smoothly, and any channeling or eddies or the like can cause major problems. With a true (traditional) flow battery without plating, there is much more forgiveness with the flow pattern. Another big advantage is the symmetry of the cells- the same porous electrodes are used on both sides. This can be contrasted with most plating batteries, where there is a porous electrode on the positive side of the cell but a“flat plate” electrode on the other side, and this asymmetry adds complexity and engineering challenges to the battery, and makes it harder to balance the pressure (it is ideal to have zero pressure drop across the membrane/separator).

[0082] The invention is not limited to a specific geometric structure or size of the flow cell of the invention. In some embodiments, each of the cells of a battery of the invention comprises a geometric cell area of between about 400-5,000 cm2.

[0083] In some embodiments, a plurality of cells (a stack) is combined in series using bipolar plates in order to increase the output voltage. The invention is not limited by the number of cells stacked. A battery stack may have between 2-300 (e.g., between 25-200, 25-150, 100-150 or other configuration of) cells comprising bipolar electrodes and provide an output of 12-48 volts, and current up to 100 amperes. Battery stacks may be connected in series, parallel, and series- parallel combinations in order to provide greater voltage and current. Larger electrolyte reservoirs can be used to increase the energy storage capacity in watt-hours (Wh) to the desired value. A non-limiting example of a grid-scale installation may be configured to operate with a total power of 500 kW and a total energy of 2-4 MWh. In some embodiments, stacking allows battery voltage to increase from about 0.77 volts (single-cell) to 24-48 volts (stacked). In some embodiments, a stack is generated and/or designed that produces a voltage of 48 volts. In some embodiments, voltage out from a stack of cells is regulated and/or adjusted using a converter (e.g., a DC-DC converter).

[0084] In some embodiments, a supporting electrolyte is added to one or both half cells. In some embodiments, the supporting electrolyte is added to alter (e.g., increase) ionic conductivity (mS/cm) of the electrolyte, thereby increasing the energy efficiency of the battery. In some embodiments, the supporting electrolyte alters (e.g., increases) solubility of the active species and/or electrode kinetics. The invention is not limited by the type of supporting electrolyte. Supporting electrolytes include, but are not limited to, K+, Na+, C1-, and/or S0₄ ² .

[0085] An all-iron flow battery in accordance with the invention can be charged and discharged repeatedly and is suitable as a battery for storage (e.g., temporary storage) of electric power in a variety of applications described herein.

[0086] Certain exemplary embodiments will now be described to provide an overall

understanding of the principles of the structure, function, manufacture, and use of the

therapeutics and methods disclosed herein. One or more examples of these embodiments are illustrated in the accompanying drawings. Those skilled in the art will understand that the therapeutics and methods specifically described herein and illustrated in the accompanying drawings are non-limiting exemplary embodiments and that the scope of the present invention is defined solely by the claims. The features illustrated or described in connection with one exemplary embodiment may be combined with the features of other embodiments. Such modifications and variations are intended to be included within the scope of the present invention.

[0087] All publications, patents and patent applications cited herein are hereby incorporated by reference in their entirety. As used in this specification and the appended claims, the singular forms a,”“an,” and“the” include plural references unless the content clearly dictates otherwise. The terms used in this invention adhere to standard definitions generally accepted by those having ordinary skill in the art.

I. Definitions

[0088] As used herein,“about” will be understood by persons of ordinary skill in the art and will vary to some extent depending upon the context in which it is used. If there are uses of the term which are not clear to persons of ordinary skill in the art given the context in which it is used, “about” will mean up to plus or minus 10% of the particular term.

[0089] As used herein, the term“comprising” is intended to mean that the systems, devices, and methods include the recited elements, but not excluding others. Accordingly, it is intended that the systems, devices, and methods can include additional steps and features/elements

(comprising). [0090] The systems, devices, and methods of using the same of the present invention are further described by reference to the following examples, which are provided for illustration only. The invention is not limited to the examples, but rather includes all variations that are evident from the teachings provided herein. All publicly available documents referenced herein, including but not limited to U.S. patents, are specifically incorporated by reference.

EXAMPLES

Example 1 - True all iron flow cell

[0091] A battery was charged at 7 mA/cm2 and discharged at 4 mA/cm2 until reaching respective voltage cutoffs of 1.3 V and 0.0 V. The cell contained porous carbon felt electrodes and a microporous polyethylene-based separator. The geometric cell area was 50 cm2 . The flow rate was 120 mL/min. The negative electrolyte contained ammonium chloride, iron(II) chloride, iron(III) chloride and sodium citrate, and had pH = 5. The positive electrolyte contained ammonium chloride, iron(II) chloride, iron(III) chloride and sodium citrate and had pH = 1. Each charge lasted for approximately 1.5 hours and each discharge lasted for approximately 2.5 hours. Battery voltages are shown as a function of time in FIG. 2A, and the calculated current efficiency is shown in FIG. 2B. Surprisingly, the current efficiency was approximately 100 %

Example 2 - True all iron flow cell

[0092] A battery was charged at 8 mA/cm2 and discharged at 8 mA/cm2 until reaching respective voltage cutoffs of 1.3 V and 0.2 V. The cell contained porous carbon felt electrodes and a microporous polyethylene-based separator. The geometric cell area was 50 cm2 . The flow rate was 120 mL/min. The negative electrolyte contained ammonium chloride, iron(II) chloride, iron(III) chloride and sodium citrate, and had pH = 1. The positive electrolyte contained ammonium chloride, iron(II) chloride, iron(III) chloride and sodium citrate and had a pH = 1

Example 3 - Cyclic voltammetry

[0093] Cyclic voltammetry was carried out in a solution containing ammonium chloride, iron(II) chloride, iron(III) chloride and sodium citrate at pH=5. A marked enhancement in performance was observed at elevated temperature of 38 degrees Celsius. The experiment was conducted in unstirred solution using a graphite rod as the working electrode, a graphite rod as the counter electrode, and silver-silver chloride as the reference electrode. A nitrogen atmosphere was maintained. The current is shown as a function of potential in FIG. 3. Reaction kinetics were greatly improved at the higher temperature.

Example 4 - True all iron flow cell

[0094] A battery was charged at 8 mA/cm2 and discharged at 4 mA/cm2 at ambient temperature (approximately 22C) until reaching respective voltage cutoffs of 1.3 V and 0.1 V. The cell contained untreated porous carbon felt electrodes and an untreated microporous polyethylene- based separator (175 mh\ thickness). The geometric cell area was 50 cm2 . The flow rate was 30 mL/min. The electrolyte contained 1 M ferric ammonium citrate, 1 M iron(II) chloride and 3 M ammonium chloride. Each electrolyte volume was 150 mL (300 mL total). The battery was charged for approximately six hours and discharge for approximately 10 hours.

Example 5 - True all iron flow cell

[0095] A single-cell flow battery was tested at ambient temperature (approximately 22 C).

Untreated carbon felt electrodes (1/8" thick, Cera Materials Pan-Based Carbon Felt) were used in a flow-through configuration, where the electrolytes flowed through them at approximately 60 mL/min using 1/4" polyethylene tubing. Electrolyte flow was provided by using two diaphragm pumps (KNF NFB 25 Series). An initial common electrolyte was made using 300 mL of solution that contained 2.5 mol/L KC1, 0.6 mol/L citric acid, 0.4 mol/L iron chloride hydrate, and 0.3 mol/L iron chloride hexahydrate. This was divided into two equal portions of 150 mL each, and then one portion was treated with ammonium hydroxide (NH40H) until reaching pH = 6.0, and this was used for the negative electrolyte. The other 150 mL portion had a natural pH = 0.5 and it was used for the positive electrolyte. The membrane used was untreated Vanadion 20 (Ion Power), and the geometric area was approximately 50 cm². Using a programmable potentiostat (Admiral Instruments SquidStat Plus), the battery was charged and discharged at +/- 8 mA/cm2, where each charge duration was one hour or 1.3 V (whichever was reached first) and each discharge duration was one hour or 0.0 V (whichever was reached first). The measured coulombic efficiency was approximately 100 %, and the voltaic efficiency was varied from approximately 60-75 %.

Example 6 - pH rebalancing device [0096] A device was constructed and tested for its ability to maintain chemical balance in flow battery electrolytes (e.g., for maintaining the concentration of active species in a flow

cell/battery). As described in detail below, a device and system of using the device was tested in flow batteries that operates with a pH gradient. The device was constructed/configured and tested for operation in both galvanic (discharge) and supergalvanic (discharge) modes for batteries with negative electrolyte pH greater than positive electrolyte pH. The device was also tested for operation for electrolytic (charge) configuration for batteries with negative pH less than positive electrolyte pH.

[0097] As shown in FIG. 4, the pH rebalancing device comprises a two-compartment, single membrane cell for rebalancing pH in flow batteries using positive and negative electrolytes at different pH values. The device was tested with flow batteries with a pH gradient that operates in either direction; the negative electrolyte pH may be either less than or greater than the positive electrolyte pH.

[0098] The ability to split water in iron-based electrolytes relevant to flow battery applications was tested. The oxidation compartment carried out the reaction shown in Equation IV, and the reduction compartment carried out the reaction shown in Equation V.

[0099] Fe2+ - e- -> Fe3+ (IV)

[0100] Fe3+ + e- -> Fe2+ (V)

[0101] As shown in FIG. 5(a), the electrolyte of the reduction compartment decreased. This result indicates that the bipolar membrane successfully split water to provide protons into the positive electrolyte. In contrast, the pH of the oxidation compartment did not change

considerably even though OH- ions were generated. This is because the current was relatively high and the OH- concentration was so high that solid iron hydroxides formed in the oxidation compartment near the membrane. In practice, when using a device disclosed herein, it a smaller current density value may be used (e.g., in order to prevent severe pH gradients from

developing). When carrying out the same experiment except with a Vanadion composite membrane instead of the bipolar membrane, very different results were obtained. Specifically, there was little change in pH. Indeed, the pH change was in the opposite direction, and this can be explained because Fe3+ acts as a strong acid through various hydrolysis reactions. For example, Equations VI and VII show two possible successive hydrolysis reactions that generate protons. There are many possible such reactions, and the invention is not limited to any particular reaction. Therefore, in the oxidation compartment, Fe3+ was generated, causing the pH to decrease. In the reduction compartment, Fe3+ was consumed, causing the pH to increase.

[0102] Fe3+ + H20 -> H+ + Fe(OH)2+ (VI)

[0103] Fe(OH)2+ + H20 -> H+ + Fe(OH)2+ (VII)

[0104] When citrate was present in the electrolytes, it was observed that ferric (Fe3+) ions acted as considerably stronger acids, and their generation or removal dominated the measured pH changes in solution (See FIG. 6). Therefore, the electrolyte in the oxidation compartment experienced pH decrease and the reduction compartment experienced pH increase, regardless of the type of membrane employed. Still, a significant difference was observed that demonstrated the effects of H+ and OH- generation in the respective compartments, which counter-acted the pH changes generated by ferric ion generation or removal, which affected pH in the opposite direction.

[0105] Batteries with Negative Electrolyte pH Greater than Positive Electrolyte pH

[0106] In configurations where the negative electrolyte pH is greater than the positive electrolyte pH, the rebalancing device operates in a galvanic (discharge) mode that does not require an applied potential. In this mode of operation, the cation-exchange portion of the bipolar membrane faces the positive electrode and the anion-exchange portion of the bipolar membrane faces the negative electrode. This configuration is illustrated in FIG. 4(a).

[0107] Batteries with Negative Electrolyte pH less than Positive Electrolyte pH

[0108] In configurations where the negative electrolyte pH is less than the positive electrolyte pH, the rebalancing device operates in an electrolytic (charge) mode that requires an applied potential. In this mode, some of the energy used when operating the rebalancing cell effectively contributes to charging the battery. For example, in the case where the flow battery is being charged, it would be possible to use 95 % of the charging energy in the main flow battery cells (or stacks), and 5 % of the charging energy in the rebalancing cells (or stacks). In this mode of operation, the cation-exchange portion of the bipolar membrane faces the negative electrode and the anion-exhchange portion of the bipolar membrane faces the positive electrode. This configuration is illustrated in FIG. 4(b). [0109] In one experiment, iron chloride electrolytes were prepared by mixing iron (II) chloride, iron (III) chloride and potassium chloride (KC1) to form an aqueous solution. Equal volumes of each solution were placed in an El-Cell and separated by a membrane. Each compartment contained a graphite electrode. A potential was applied across the electrodes such that in one compartment, iron (II) was oxidized to form iron (III) and in the other compartment, iron (III) was reduced to form iron (II). This type of cell is a symmetric cell configuration. Current was passed through the cell and the pH was measured as a function of volumetric capacity (mAh/L) for each electrolyte. The process was carried out using both a bipolar membrane (BPM)

(Fumatech FBM, GmbH) and a VANADION™ membrane (See FIGS. 5A and 5B, respectively composite membrane as well as a bipolar membrane (Fumatech FBM).

[0110] In yet another experiment, a single-cell, all-iron citrate flow battery, as described in Examples 1-5 above, was repeatedly charged and discharged at +/-15 mA/cm² with an in-line pH rebalancing cell. The duration of each charge was approximately 2.5-3.0 h. The geometric battery area was 50 cm², and Vanadion was used as the separator. Carbon felt (Cera Materials pan-based carbon felt, 1/8" thick) was used for both the positive and negative battery electrodes. During the battery operation, pH was regulated via the rebalance cell (no chemicals, such as HC1 or NaOH, were added to either electrolyte). The rebalance cell used flat graphite plates (flow-by mode) and a bipolar membrane separator (Fumatech). A representative charge-discharge cycle is shown in FIG. 7(a). For each charge-discharge cycle, the battery was charged for 3.3 hours, and discharged until reaching a cutoff of 0.0 volts. The pH rebalancing cell was operated in supergalvanic (discharge) mode during battery operation. A potential of 1.5 volts was continuously applied to the rebalancing cell and the resulting current was measured as a function of time. The current increased when the battery approached high state-of-charge (SoC) and decreased when the battery approached low SoC. The current density in the rebalance cell ranged from approximately 5 mA/cm² (low SoC) to 20 mA/cm² high SoC. As shown in FIG. 7(b), an exceptionally stable pH gradient was maintained.

[0111] All publications and patents mentioned in the above specification are herein incorporated by reference. Various modifications and variations of the described method and system of the invention will be apparent to those skilled in the art without departing from the scope and spirit of the invention. Although the invention has been described in connection with specific preferred embodiments, it should be understood that the invention as claimed should not be unduly limited to such specific embodiments. Indeed, various modifications of the described modes for carrying out the invention that are obvious to those skilled in relevant fields are intended to be within the scope of the following claims.

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Claims

WHAT IS CLAIMED:

1. An all-iron redox flow cell designed to operate in acidic media (pH < 7).

2. An all-iron redox flow cell comprising:

a first half-cell comprising a positive electrolyte containing ammonium chloride, iron(II) chloride, iron(III) chloride and a citrate source and a non-metal plating electrode;

a second half-cell comprising a negative electrolyte containing ammonium chloride, iron(II) chloride, iron(III) chloride and a citrate source and a non-metal plating electrode; and a non-electrically conductive membrane;

wherein the pH of the positive electrolyte is different from the pH of the negative electrolyte, and wherein no solid phase is formed during charging of the cell.

3. The all-iron flow cell of claim 2, further comprising a first storage tank external to the first half-cell for circulating the positive electrolyte to and from the first half-cell; and a second storage tank external to the second half-cell for circulating the negative electrolyte to and from the second half-cell, the half-cells conducting an oxidation reduction reaction to charge and discharge the battery.

4. The all-iron flow cell of claim 3, wherein the positive electrolyte and the negative electrolyte are the same electrolyte.

5. The all-iron flow cell of claim 4, wherein the positive electrolyte has a pH between 0.0 - 2.0 and the negative electrolyte has a pH between 4.0 - 7.0.

6. The all-iron flow cell of claim 4, wherein the positive electrolyte has a pH of about 1 and the negative electrolyte has a pH of about 5.

7. The all-iron flow cell of claim 6, wherein the flow rate to from the half cells is 120 mL/min.

8 The all-iron flow cell of claim 2, wherein the current efficiency is greater than

90%.

9. The all-iron flow cell of claim 2, wherein the current efficiency is greater than

95%.

10. The all-iron flow cell of claim 2, wherein the current efficiency is greater than

98%.

11. The all-iron flow cell of claim 2, wherein a plurality of cells are combined in series using bipolar plates..

12. The all-iron flow cell of claim 11, wherein the plurality of cells combined in series produces an output of 12-48 volts and a current up to 100 amperes.

13. The all-iron flow cell of claim 2, wherein a surfactant is added to the first and/or second electrolyte to reduce surface tension.

14. The all-iron flow cell of claim 13, wherein the surfactant is sodium dodecyl sulfate (SDS) or polyethylene glycol (PEG).

15. The all-iron flow cell of claim 2, wherein the temperature of the electrolytes is maintained between 20-60 C.

16. The all-iron flow cell of claim 2, wherein the temperature of the electrolytes is maintained between 30-45 C.

17. The all-iron flow cell of claim 2, wherein a pH gradient is maintained between the positive electrolyte and the negative electrolyte.

18. The all-iron flow cell of claim 17, wherein the pH gradient is between 1-5 pH units.

19. The all-iron flow cell of claim 2, wherein electrodes in each of the half cells are carbon felt, carbon paper, or carbon cloth electrodes.

20. The all-iron flow cell of claim 19, wherein the carbon paper is AvCarb P75 untreated carbon paper.

21. The all-iron flow cell of claim 19, wherein the carbon cloth is ELAT hydrophilic carbon cloth.

22. The all-iron flow cell of claim 19, wherein the electrodes each have a geometric area of 400-5000 cm2.

23. The all-iron flow cell of claim 2, wherein ammonium ferric citrate is used in the starting electrolyte composition for the positive and/or negative electrolyte.

24. The all-iron flow cell of claim 2, wherein the non-electrically conductive membrane comprises a material selected from the group consisting of polypropylene (PP), polyethersulfone (PES), polybenzimidazole (PBI), or a combination thereof.

25. The all-iron flow cell of claim 2, wherein a monopolar stack of cells is used.

26. The all-iron flow cell of claim 2, wherein the citrate source is citric acid.

27. The all-iron flow cell of claim 2, wherein the citrate source is sodium citrate.

28. The all-iron flow cell of claim 2, wherein the citrate source is selected from the group consisting of citric acid, ferric ammonium citrate, sodium citrate, trisodium citrate, ferric citrate hydrate, and/or a combination thereof.

29. A battery comprising one or more of the all-iron flow cells of claim 2.

30. A pH rebalancing device comprising two compartments separated by one bipolar membrane, wherein each compartment contains an electrode.

31. The device of claim 30, wherein the electrodes are carbon felt, carbon cloth, carbon paper, or graphite.

32. The device of claim 30, wherein the cell operates in electrolytic (charge) mode for batteries with negative electrolyte pH lower than positive electrolyte pH.

33. The device of claim 30, wherein the cell operates in galvanic or supergalvanic discharge mode for batteries with negative electrolyte pH greater than positive electrolyte pH.

34. The device of claim 30, wherein the device contains a plurality of single- compartment cells.

35. The device of claim 30, wherein the bipolar membrane is prepared by combining two monopolar membranes

36. The device of claim 35, wherein one monopolar membrane is an anion-exchange membrane and the other monopolar membrane is a cation-exchange membrane.

37. The device of claim 30, wherein the bipolar membrane contains a water-splitting catalyst.

38. The device of claim 37, wherein the water-splitting catalyst is FeCb.

39. The device of claim 30, wherein the rebalancing device is active during battery charging and not active during battery discharging.

40. The device of claim 30, wherein the rebalancing device is active during battery discharging and not active during battery charging.

41. The device of claim 30, wherein the rebalance device is placed upstream or downstream of a battery device.