WO2013090269A2

WO2013090269A2 - Enhanced current efficiencies in reversible hydrogen bromide fuel cells using in-line bromine sequestering devices

Info

Publication number: WO2013090269A2
Application number: PCT/US2012/068957
Authority: WO
Inventors: Arthur J. Esswein; Ned Cipollini; Oleg Grebenyuk; Paravastu Badrinarayanan; Timothy Banks GREJTAK; Thomas M. MADDEN; John GOELTZ
Original assignee: Sun Catalytix Corporation
Priority date: 2011-12-13
Filing date: 2012-12-11
Publication date: 2013-06-20
Also published as: WO2013090267A1; WO2013090269A3

Abstract

This disclosure relates to flow battery components, cells, and systems and methods of operating the same, hydrogen bromide flow batteries using in-line bromine sequestering devices.

Description

ENHANCED CURRENT EFFICIENCIES IN REVERSIBLE HYDROGEN BROMIDE FUEL CELLS USING IN-LINE BROMINE SEQUESTERING DEVICES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] The present application claims priority to U.S. Patent Application No.

61/569,861 filed December 13, 201 1, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] This disclosure relates to flow battery components, cells, and systems and methods of operating the same, including configurations and methods of operating hydrogen bromide flow batteries.

BACKGROUND

[0003] Efficient and cost-effective energy storage is critical to avoid the high costs of providing backup electricity in areas where the electrical grid is highly unreliable. In particular, the needs for base transceiver station applications are especially pressing, due to the high uptime required and the high costs of deploying traditional diesel power generator set technologies.

[0004] In addition to power generator sets, alternatives for backup power include traditional secondary batteries (lead acid, lithium ion, among others) and advanced secondary batteries involving technologies with higher cycle life and thus potentially lower life-cycle costs. These technologies include water electrolysis / fuel cells and flow batteries.

[0005] Successful electrical energy storage schemes demand that the system have a high round trip energy efficiency and also be inexpensive to purchase and maintain. Hydrogen bromide is among the flow battery technologies that seem to have significant merit. Reversible HBr fuel cells are useful for electrical energy storage applications because the forward and reverse chemical reactions operate with high efficiency and high reaction rates. This combination makes HBr fuel cells particularly amenable to flow battery configurations.

Advantages of this system include the high degree of reversibility of the reactions at both electrodes, the gas-liquid phases of the reactants, and the potential for high power densities. However, to date, practical constraints as to the use of hydrogen bromide systems have limited their widespread use. SUMMARY

[0006] The present invention concerns a series of strategies and embodiments to improve current efficiency in a reversible HBr fuel cell by reducing crossover from the halogen side of the membrane separator to the hydrogen side.

[0007] Various embodiments of the present invention provide flow batteries, each battery comprising (a) a bromine half-cell, comprising an aqueous electrolyte comprising hydrogen bromide, bromine, and polybromide anions; and (b) a bromine sequestering device in fluid communication with the bromine half-cell; wherein bromine sequestering device, contains a poorly soluble or insoluble material capable of reversibly complexing bromine. In other embodiments, these batteries also comprise a hydrogen half-cell and a polymer electrolyte membrane positioned between and separating the hydrogen half-cell and bromine half-cell;

[0008] Other embodiments provide methods of operating a hydrogen bromide flow battery, each method comprising circulating an electrolyte comprising aqueous solutions of hydrogen bromide, bromine, and (poly)bromide anions into and out of the flow battery, respectively, such that at least one of the circulating streams contacts a poorly soluble or insoluble material capable of reversibly complexing bromine. In some of these embodiments, when the flow battery is operating in a charging mode, the material capable of reversibly complexing bromine is removing bromine/(poly)bromide from the stream. In other of these embodiments, when the flow battery is operating in a discharging mode, the material capable of reversibly complexing bromine is releasing bromine/(poly)bromide into the stream.

BRIEF DESCRIPTION OF THE DRAWINGS

[0009] The present application is further understood when read in conjunction with the appended drawings. For the purpose of illustrating the subject matter, there are shown in the drawings exemplary embodiments of the subject matter; however, the presently disclosed subject matter is not limited to the specific methods, devices, and systems disclosed. In addition, the drawings are not necessarily drawn to scale. In the drawings:

[0010] FIG. 1 is a schematic diagram of an exemplary HBr flow battery system.

[0011] FIG. 2 illustrates one process flow diagram showing a reversible HBr fuel cell employing an insoluble bromine-complexing agent in both charge and discharge modes. In some embodiments, the uptake or release of bromine from the complexing agent may be induced by temperature. When charging, the bromine (Br₂) rich stream flows from the stack through or past the complexing agent where bromine is scrubbed from the solution. In discharge, bromine release is induced providing a bromine rich stream to the stack.

[0012] FIG. 3 shows the uptake of Br₂ equivalents for Amberlite IRA-67 resin in units of mmol/g of dry beads across a range of equilibrium values of solution concentrations of Br₂ equivalents and at several relevant concentrations of HBr.

[0013] FIG. 4 shows the uptake of Br₂ equivalents for Dowex 1X8-100 resin in units of mmol/g of dry beads across a range of equilibrium values of solution concentrations of Br₂ equivalents and at several relevant concentrations of HBr.

[0014] FIG. 5 shows the developing concentration of bromine as a function of state of charge in the presence of an in-line quantity of Amberlite IRA-67 resin beads.

[0015] FIG. 6 illustrates the current efficiency as a function of state of charge in the presence and absence of an in-line quantity of Amberlite IRA-67 resin beads.

DETAILED DESCRIPTION

[0016] The present invention may be understood more readily by reference to the following description taken in connection with the accompanying Figures and Examples, all of which form a part of this disclosure. It is to be understood that this invention is not limited to the specific products, methods, conditions or parameters described and / or shown herein, and that the terminology used herein is for the purpose of describing particular embodiments by way of example only and is not intended to be limiting of any claimed invention. Similarly, unless specifically otherwise stated, any description as to a possible mechanism or mode of action or reason for improvement is meant to be illustrative only, and the invention herein is not to be constrained by the correctness or incorrectness of any such suggested mechanism or mode of action or reason for improvement. Throughout this text, it is recognized that the descriptions refer both to methods of operating a device and systems and to the devices and systems providing said methods. That is, where the disclosure describes and/or claims a method or methods for operating a flow battery, it is appreciated that these descriptions and/or claims also describe and/or claim the devices, equipment, or systems for accomplishing these methods.

[0017] In the present disclosure the singular forms "a," "an," and "the" include the plural reference, and reference to a particular numerical value includes at least that particular value, unless the context clearly indicates otherwise. Thus, for example, a reference to "a material" is a reference to at least one of such materials and equivalents thereof known to those skilled in the art, and so forth. [0018] When values are expressed as approximations by use of the descriptor "about," it will be understood that the particular value forms another embodiment. In general, use of the term "about" indicates approximations that can vary depending on the desired properties sought to be obtained by the disclosed subject matter and is to be interpreted in the specific context in which it is used, based on its function. The person skilled in the art will be able to interpret this as a matter of routine. In some cases, the number of significant figures used for a particular value may be one non-limiting method of determining the extent of the word "about." In other cases, the gradations used in a series of values may be used to determine the intended range available to the term "about" for each value. Where present, all ranges are inclusive and combinable. That is, references to values stated in ranges include every value within that range.

[0019] It is to be appreciated that certain features of the invention which are, for clarity, described herein in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention that are, for brevity, described in the context of a single embodiment, may also be provided separately or in any subcombination. Finally, while an embodiment may be described as part of a series of steps or part of a more general structure, each said step or part may also be considered an independent embodiment in itself.

[0020] HBr Flow Battery Systems.

[0021] This disclosure relates to flow battery systems, especially HBr flow battery systems, that utilize the same cell(s) for both energy storage and energy generation, during the respective charging and discharging operations. As used herein, the terms "regenerative fuel cell" or "reversible fuel cell" or "flow battery" or "flow energy device" generally connote a type of device that utilize the same cell for both energy storage and energy generation, and each are considered within the scope of the present invention. It should also be appreciated that, while the various embodiments described herein are described in terms of flow systems, the same strategies and design / operating embodiments may also be employed with stationary (i.e., non- flow) electrochemical cells and systems, and that each of these embodiments are considered within the scope of the present invention.

[0022] The general operation principle of flow battery systems, including hydrogen- bromide systems, can be described with respect to the charging (energy storage) and discharging (energy generation) stages. The relevant charging / discharging reactions for an HBr system are described by equation 1 : Charging

2HBr _^ H₂ + Br₂

Discharging [1 ].

These reactions can also be described in terms of the half reactions:

Charging [Red'n]

Discharging [Ox'n]

Charging [Ox'n]

2 Br^" - Br₂ + 2e^"

Discharging [Red'n] [ib]

[0023] In hydrogen bromide flow batteries, the electrolyte comprises aqueous hydrogen bromide / bromine. The formation of tribromide ion in the presence of bromine and bromide is given by equation 2:

Br₂ + Br^" - Br₃ ^" _[2]

Given the favorable formation of tribromide (and higher polybromide species) under most operating conditions, an electrolyte described herein as comprising aqueous HBr or HBr/Br2 necessarily comprises a mixture of HBr, Br₃ ^~ (and higher polybromide anions), and Br₂.

[0024] FIG. 1 illustrates a schematic diagram of an exemplary HBr flow battery system. The system comprises two circulation loops - one for the aqueous HBr/Br₂ electrolyte 10 and one for the hydrogen 15 - which are separated by a solid electrolyte membrane, said electrolyte membrane contained within an electrochemical cell comprising separate electrolyte and hydrogen chambers. Multiple cells may be configured into a cell stack, as is known in the art. As shown in FIG. 1, the electrolyte circulation loop comprises an electrolyte tank 25, the electrolyte chamber(s), and one or more electrolyte-compatible circulation pumps 30, for circulating the aqueous HBr/Br₂ electrolyte through the electrolyte chamber during both charge and discharge stages. This electrolyte circulation loop (also called a fluidic loop) may also comprise one or more valves, additional tanks, sensors, monitors, pressure regulators, looped feedback control devices, a pressure equalizing line, or any combination thereof.

[0025] Also as shown in FIG. 1, the hydrogen loop 15 comprises a hydrogen tank 35, the hydrogen chamber(s), an optional hydrogen purifier 45, an optional liquid absorber 50, and an optional recycle blower 60. The hydrogen loop may also comprise additional pumps, tanks, one or more valves, sensors, monitors, pressure regulators, looped feedback control devices, a gas circulation ejector, or any combination thereof. In some configurations, the hydrogen loop also comprises a gas compressor. In other embodiments it does not. It should be appreciated that the specific positioning of the various optional elements are illustrative of a single configured embodiment and may be positioned differently in other embodiments as desired.

[0026] In a flow battery, electrical energy is used to charge the battery by running the thermodynamically "uphill" chemical reaction of splitting HBr into ¾ and B¾ by the forward reaction of Equation 1, 1a, and lb. On charging, bromine (Br₂) forms at the positive bromine electrode (the bromine electrode is always at a potential more positive than the hydrogen electrode), which is converted to tri- and polybromide complex ions form, as described above.

[0027] During charging, the HBr/Br₂ (typically bromide-rich) electrolyte is pumped or otherwise flows from the electrolyte tank into the electrolyte chamber(s) through an electrolyte chamber inlet and the bromide (or polybromide) is therein oxidized to bromine. Charged electrolyte is then removed from the electrolyte chamber(s) through an electrolyte chamber outlet and returned to the electrolyte tank 25, or may be transferred to a separate storage tank While shown in FIG. 1 as a single tank, it should be appreciated that multiple tanks, including separate tanks for charged and discharged electrolytes, may be used. The electrolyte may be moved through the electrolyte chamber(s) in continuous or batch-wise fashion. During the same charging stage, hydrogen is produced at the hydrogen electrode in the hydrogen chamber(s) of the cell(s) or cell stack. Once produced, and after optional purification steps, hydrogen may then be captured within a hydrogen pressure vessel 35. Typically this latter operation - i.e., capturing the hydrogen gas at pressure in a hydrogen pressure vessel - requires the use of compression pumps, in order to provide the necessary pressure lift for practical gas storage.

[0028] The flow battery is designed also to operate in a discharge mode wherein the thermodynamically "downhill" recombination of ¾ and B¾ to give HBr (in the reverse reactions of Equation 1, 1a, and lb) generates electrical power for external use as needed.

[0029] During such a discharge, the HBr/Br₂ (typically bromine-rich) electrolyte flows from the electrolyte tank 25 into the cell(s) or stacks and the bromine is therein reduced to bromide (reverse reaction of Equations 1 and lb). In certain configurations, electrolyte from the fuel cell stacks is returned to the electrolyte tank 25, or into separate tanks holding discharged electrolyte.

[0030] During the same discharge stage, hydrogen from the tank 35 is provided from pressurized storage tanks to the hydrogen chamber(s), where it is oxidized (reverse reactions of Equation 1 and la). Any unreacted hydrogen may then be recirculated by the recycle blower 60 for re-use in the hydrogen chamber(s). While the internal utilization (defined herein as the rate of flow through the stack divided by the rate of hydrogen consumption) may be substantially higher than the stoichiometric amount required by the operating current, the external utilization (defined as the rate of hydrogen consumption divided by the net flow of hydrogen to the stack and recycle system) is unity or close to unity.

[0031] The various active, sensing, and feedback elements within each loop of the system need to be controlled and coordinated for the system to operate as required. This is

accomplished using one or more suitable programmable devices (including logic circuits and memory) within an overall process management system which operates within and between the hydrogen and electrolyte (fluidic) loops.

[0032] In certain embodiments, during discharge, pressure may be maintained on the hydrogen side to a predetermined pressure using a passive regulator attached to a pressurized hydrogen container or through use of a hydrogen injector, in each case the regulator or injector providing fresh (non-recycled) hydrogen. Alternatively or additionally, some portion of the excess hydrogen may be captured in a separate accumulation tank, where it is held until required, at which point it may be returned to the hydrogen cell.

[0033] As described above, the hydrogen and electrolyte chambers of each cell are separated by a membranes which are generally categorized as either solid (non-porous) or porous membranes / separators. In HBr flow battery systems, the membranes / separators form durable, electrically non-conductive mechanical barriers between the hydrogen and electrolyte chambers and facilitate the transport of protons therethrough. As should be readily apparent, all of the cell components must be capable of resisting the system chemistries associated with the electrolyte systems employed therein, and in the case of HBr flow batteries or cells must be capable of resisting corrosion associated with aqueous hydrobromic acid / bromine systems.

[0034] Non-porous membranes (alternatively called polymer electrolyte membranes (PEM) or proton exchange membranes (PEM) or ion-conducting membranes) typically, but not exclusively, comprise highly fluorinated and most typically perfluorinated polymer backbones, for example copolymers of tetrafluoroethylene and one or more fluorinated, acid- functional co- monomers, containing pendant functional groups, such as sulfonate groups, carboxylate groups, or other functional groups that form acids when protonated. Such polymer electrolytes include those commercially available as NAFION™ perfluorinated polymer electrolytes from E.I. du Pont de Nemours and Company, Wilmington Del, as well as co-polymers of tetrafluoroethylene

(TFE) and FS0₂— CF₂CF2CF₂CF2-0-CF=CF2. [0035] Non-fluorinated non-porous membranes may also be used. These membranes comprise polymers with substantially aromatic backbones— e.g., poly-styrene, polyphenylene, bi-phenyl sulfone, or thermoplastics such as polyetherketones or polyethersulfones— that are modified with sulfonic acid or similar acid groups.

[0036] Battery-separator style porous membranes may also be used. Porous membranes are non-conductive membranes which allow charge transfer between two electrodes via open channels filled with conductive electrolyte. Because these contain no inherent proton conduction capability, they must be impregnated with acid in order to function. These membranes are typically comprised of a mixture of a polymer, and inorganic filler, and open porosity. Preferred polymers include those chemically compatible with hydrogen bromide and/or bromine, including high density polyethylene, polypropylene, polyvinylidene difluoride, or polytetrafluoroethylene. Preferred inorganic fillers include silicon carbide matrix material, titanium dioxide, silicon dioxide, among others.

[0037] Layers of refractory ceramic powders may also be used into which an acid can be imbibed. These powders form very small, hydrophilic pores that retain acid by virtue of very high capillary forces, and exhibit high corrosion resistance. Preferred embodiments include silicon carbide and nanoporous carbon powders that be imbibed with a variety of acids, including hydrogen bromide, hydrogen chloride, sulfuric acid, and phosphoric acid.

[0038] Porous membranes are easily permeable to liquid or gaseous chemicals. This permeability increases probability of chemicals passing through porous membrane from one electrode to another causing cross-contamination and/or reduction in cell energy efficiency. The degree of this cross-contamination depends on, among other features, the size (the effective diameter and channel length), and character (hydrophobicity / hydrophilicity) of the pores, the nature of the electrolyte, and the degree of wetting between the pores and the electrolyte).

[0039] Also, whichever the type (porous, non-porous, or a combination of both), such membranes are generally incorporated into structures such as membrane electrode assemblies (MEAs). Typical MEAs used in HBr systems comprise a polymer electrolyte membrane (PEM) within a four or five layer structure, said structure also including hydrogen and bromine catalysts layers positioned on opposite sides of the PEM, and one or more fluid transport layers or gas diffusion layers.

[0040] Each catalyst layer may include at least one electrochemical catalyst, typically including platinum and/or other precious or non-precious metal or metals. As used herein, the terms "catalyst layer," "hydrogen catalyst layer" and "bromine catalyst layer" refer to layers of such a catalyst material capable of improving the efficiency of the respective electrochemical conversion, under the appropriate electrochemical conditions. Such catalysts are known by those skilled in the art.

[0041] High round trip energy efficiencies are important for commercial applications of HBr reversible fuel cells. The round trip direct current energy efficiency (RT_DC-DC) is determined by multiplying the DC energy efficiency for both the energy input (E_Dc,in) and output (EDC_OU from the storage system (Equation 4).

RTDC-DC ⁼ EDC,out*EDC,in (4)

[0042] The E_Dc,in and E_Dc,out are products of both the voltaic and coulombic efficiency for battery charging and discharging respectively (Equation 5).

EDC ⁼ IlVoltaic * IlCoulombic (5)

[0043] Typical sources of cell voltaic loss are activation overpotentials for the redox reactions as well as resistance losses encountered from ion transport. Activation overpotentials for H₂ production and oxidation using highly dispersed Pt catalysts on carbon supports are typically small (less than 5 mV). Overpotentials for Br7Br₂ redox chemistry have also been found to be low at when using high surface area carbon electrodes. The majority of voltaic losses in reversible HBr fuels cells stems from ion transport resistance encountered from the membrane separator.

[0044] Coulombic efficiency losses typically stem from undesirable side reactions and crossover of reactive species through the membrane separator.

[0045] Additionally, alternative redox reactions and reactive species crossover can lead to reductions in current efficiency in a reversible HBr fuel cell including Br₂/Br₃ ^~ crossover, ¾ crossover, and carbon corrosion.

[0046] Typical strategies employed in the prior art to improve current efficiency in reversible HBr fuel cells attempt to reduce Br₂/Br₃ ^~ and ¾ crossover by increasing the thickness of the membrane separator. This strategy is effective, but incurs significant resistance losses, leading to reduced voltage efficiency of the cell, leading to a tradeoff between voltage efficiency and current efficiency.

[0047] The invention describes embodiments which have the effect of reducing the crossover of H₂, Br₂ or Br₃ ^" in HBr reversible fuel cells reactions to varying degrees, without increasing the membrane thickness or ion selectivity. [0048] Device testing by the present inventors has revealed that the coulombic efficiency for charging (typical -85%) was much lower than when discharging the cell (typically 96+%). In charge mode, HBr is oxidized to B¾ at the halogen electrode and H⁺ ions move across the membrane to the hydrogen side where they are reduced to H₂. In discharge, the H₂ is oxidized to H⁺ and the ions move through the membrane to the halogen side where HBr is reformed. Without being necessarily bound by any specific theory, these results suggested that coulombic efficiency may be a function of the direction of ion transport. Additionally, the inventors noticed that the coulombic efficiency decreased as the concentration of Br₂/Br₃ ^~ in the electrolyte increased (as the battery went to a higher state of charge). In both cases the coulombic efficiency losses could be correlated with increased concentration of Br₂/Br₃ ^~ at the membrane surface.

[0049] Insoluble Bromine Complexing Agents

[0050] Certain embodiments include flow batteries, each battery comprising: (a) a hydrogen half-cell; (b) a bromine half-cell, comprising an aqueous electrolyte comprising hydrogen bromide, bromine, and polybromide anions; (c) a polymer electrolyte membrane positioned between and separating the hydrogen half-cell and bromine half-cell; and (d) a bromine sequestering device in fluid communication with the bromine half-cell; wherein bromine sequestering device, contains a poorly soluble or insoluble material capable of reversibly complexng bromine. See, e.g., FIG. 2. Such sequestering devices may be connected in series between the bromine / bromide cell or battery and the HBr/Br₂ storage tank(s) as shown in either arrangement of FIG. 2, and may comprise substantially water insoluble materials, such as graphite, zeolites, or ion (preferably anion) exchange resins containing a tethered or immobilized bromine / polybromide complexing agent. As used herein, the terms "poorly soluble or insoluble" refer to those materials which do not measurably circulate within the electrolyte loop, as would be understood by the skilled artisan. The terms "tethered" or "immobilized" complexing agent are intended to connote a moiety capable of reversibly complexing bromine / (poly)bromide which is either linked covalently or electrostatically to an organic or inorganic polymer, or held immobilized by physical entrapment or size exclusion mechanism (i.e., embedded within), under the conditions of operation. The insoluble materials comprising the tethered or immobilized complexing agents may be polymer resin beads or other forms known in the art. The chemistries of the tethered or immobilized complexing agents may comprise at least one type of tethered or immobilized pyrrolidinium or morpholinium salt (for example, including N-alkyl-N-ethylpyrrolidinium bromide, N-alkyl-N-methylpyrrolidinium bromide, N-alkyl-N- chloroethylpyrrolidinium bromide, N-alkyl-N-chloromethylpyrrolidinium bromide, N-alkyl-N- ethylmorpholinium bromide, N-alkyl-N-methylmorpholinium bromide), or other type of immobilized quaternized ammonium salt. In certain embodiments, the insoluble complexing agent comprises a styrene-divinylbenzene copolymer, for example with quaternary ammonium groups attached to the polystyrene portion of the copolymer. In some embodiments, the ion exchange resins comprise quaternary ammonium anion exchange resins. One such example is Dowex 1X8, a styrene-divenylbenzene copolymer with benzyl trimethyl ammonium groups attached to the polystyrene polymer chains. One such non-limiting arrangement is shown in FIG. 2. In this example, bromine complexation is shown to be done in a tank or other in-line device that is distinct from the stack or the HBr storage tank. In some embodiments the bromine uptake or release from the complexing agent may be induced by changing the temperature of the agent. When charging, a bromine-rich stream flows from the stack through or past the complexing agent where bromine is removed (sequestered) from the solution, probably as a trior polybromide anion. Alternatively or in addition, a sequestering device may be positioned between the feed tank and the inlet stream to the cell, so as to reduce the level of any residual bromine before entering the flow battery cell. Either case results in a minimal steady state concentration of bromine in the flow battery stack, and therefore a significant enhancement in current efficiency. In discharge, bromine release is induced, providing a bromine-rich stream to the stack.

[0051] In use, these sequestering devices provide for methods of operating a hydrogen bromide flow battery, each method comprising circulating an electrolyte comprising inlet and outlet streams of aqueous solutions of hydrogen bromide, bromine, and polybromide anions into and out of the flow battery, respectively, such that at least one of the inlet or outlet streams contacts a poorly soluble or insoluble material capable of reversibly complexing bromine. When the flow battery is operating in a charging mode, the material capable of reversibly complexing bromine is configured to remove bromine from the stream, and when the flow battery is operating in a discharging mode, and the material capable of reversibly complexing bromine is configured to release bromine into the stream.

[0052] Several examples of the beneficial effect of this latter strategy (i.e., using immobilized bromine complexing agents) are shown in the Examples, beginning with Example

2. FIGs. 3 and 4 provide a quantitative measure of bromine adsorption onto Amberlite IRA-67

(FIG. 3) and Dowex 1X8-100 (FIG. 4) resin beads as a function of bromine concentrations, when the resin beads are contacted with bromine solutions comprising various concentrations of

HBr under the experimental conditions described in Example 2. Without being bound by the correctness of any particular theory, these resins may be operating by reversibly complexing bromine as a polybromide (e.g., Since charging an HBr flow battery system results in the transformation of (hydrogen) bromide to bromine/polybromides, a higher HBr concentration may correspond to a lower charge state, and a lower HBr concentration corresponds to a higher charge state.

[0053] FIG. 5 illustrates one example of the effect of in-line Amberlite IRA-67 beads on the development of bromine passing through an operating flow battery cell. Again, during charging, bromide is transformed into bromine, reflected in the increasing concentrations of bromine at increasing states of charge. As shown in FIG. 5, the presence of the in-line sequestering devices comprising the bromine/polybromides complexing resins results significantly lower levels of circulating bromine/polybromides, relative to the amounts bromine/polybromides circulating in the absence of such in-line sequestering devices.

[0054] The importance of this difference is shown in FIG. 6, which plots current efficiency as a function of battery state-of-charge. In the absence of the in-line complexing agents (described in FIG. 6 as circles associated with "Typical HBr Cell"), there is a significant drop in current efficiency as a function of the battery state-of-charge corresponding to an increase concentration of circulating bromine in the former. By comparison, operating the same cell with an in-line complexing agents results in a relatively higher current efficiency at higher states-of-charge (described in FIG. 6 as inverted triangle of "HBr cell with Beads"). Again, without being bound by the correctness of any particular theory, this drop in efficiency may be attributable to losses associated with increased bromine crossover at the higher circulating bromine concentrations. By comparison, where an in-line complexing agent is present, the current efficiency remains high, even at high state of charge, possibly because of the lower concentration of circulating bromine and associated lower bromine crossover rates, under these conditions. The resins achieve equilibrium between complexed Br₂/Br₃ ^~ on the resin and Br₂/Br₃ ^~ in solution. If the solution concentration dips below the equilibrium point, some Br₂/Br₃ ^~release from the resin, and vice versa. The equilibrium value for this Br₂/Br₃ ^~ uptake/release process generally appear to be a function of temperature - i.e., in some cases, higher temperatures favor the dissociation / release of Br₂/Br₃ ^~ from the resin.

[0055] To this point, the descriptions have been largely directed to flow battery cells, and flow battery cell systems, but it should be appreciated that the invention also includes those methods for operating said assemblies, cells, or systems, wherein the methods generally comprise the passage of electricity through the assemblies, cells, or systems in charging or discharging operations, as would be understood by the skilled artisan. [0056] Additional embodiments provide that these cells, cell stacks, or batteries are incorporated into larger energy storage systems, with all of the piping and controls necessary for operation of these large units. The types of equipment for such systems are known in the art, and include, for example, piping and pumps in fluid communication with the respective

electrochemical reaction chambers for moving electrolytes into and out of the respective chambers and storage tanks for holding charged and discharged electrolytes.

[0057] In certain embodiments, then, a flow battery system may comprise a flow battery (including a cell or cell stack); storage tanks and piping for containing and transporting the electrolytes; control hardware and software (which may include safety systems); and a power conditioning unit. The flow battery cell stack accomplishes the conversion of charging and discharging cycles and determines the peak power of energy storage system. The storage tanks contain the positive and negative active materials where the tank volume determines the quantity of energy stored in the system. The control software, hardware, and optional safety systems include all sensors, mitigation equipment and electronic/hardware controls and safeguards to ensure safe, autonomous, and efficient operation of the flow battery energy storage system. Such systems are known in the art. A power conditioning unit is used at the front end of the energy storage system to convert incoming and outgoing power to a voltage and current that is optimal for the energy storage system or the application. For the example of an energy storage system connected to an electrical grid, in a charging cycle the power conditioning unit would convert incoming AC electricity into DC electricity at an appropriate voltage and current for the electrochemical stack. In a discharging cycle the stack produces DC electrical power and the power conditioning unit converts to AC electrical power at the appropriate voltage and frequency for grid applications.

[0058] The energy storage systems of the present invention are well suited to sustained charge or discharge cycles of several hour durations. As such, the systems of the present invention are suited to smooth energy supply/demand profiles and provide a mechanism for stabilizing intermittent power generation assets (e.g. from renewable energy sources). It should be appreciated, then, that various embodiments of the present invention include those electrical energy storage applications where such long charge or discharge durations are valuable. For example, non-limiting examples of such applications include those where systems of the present invention are connected to an electrical grid include, so as to allow renewables integration, peak load shifting, grid firming, baseload power generation / consumption, energy arbitrage, transmission and distribution asset deferral, weak grid support, and/or frequency regulation.

Additionally the cells, stacks, or systems of the present invention can be used to provide stable power for applications that are not connected to a grid, or a micro-grid, for example as power sources for remote camps, forward operating bases, off-grid telecommunications, or remote sensors.

[0059] The present invention include those various embodiments include those related to the operation of the batteries or systems described herein. Such embodiments include, for example, the preparation, movement, and storage of the electrolytes, such as are known in flow battery systems. Embodiments also include those where appropriate voltage is applied and current is passed to effect the desired electrochemical transformations contemplated by the choice of a hydrogen bromide system, both in charging and discharging modes.

[0060] EXAMPLES

[0061] Example 1: Experimental Determination of Current Efficiency

[0062] The current efficiency of a hydrogen/bromine flow battery is determined by the following procedure. A solution sample of known volume is drawn from the HBr/Br₂ electrolyte to determine initial B¾ concentration. Potassium iodide and a starch indicator are added to the samples, which are then titrated with sodium thiosulfate to yield bromine concentration, using standard practices. Next, the flow battery is charged or discharged by an external circuit and the amount of charge passed is recorded. After charging or discharging, samples are again drawn from the electrolyte and titrated to determine the final bromine concentration. From the difference in bromine concentration, the total moles of bromine generated or consumed is calculated. The current efficiency is determined by the ratio of the amount of B¾ produced or consumed to the charge passed by the circuit.

[0063] As an example, consider a system in which a hydrogen/bromine flow battery has an electrolyte tank filled with 2 liters of HBr/ B¾ electrolyte, with an initial bromine

concentration of 0.10 M. The battery is then charged by an electrical circuit for 2.0 hours at 50 amps. A second sample is drawn and titrated to reveal a final B¾ concentration of 1.0 M. The charging has resulted in the generation of (1.0 M - 0.1 M) x 2L = 1.8 moles Br₂, requiring the use of 3.6 moles of electrons (2 electrons per mole bromine). The current efficiency is then determined by dividing this value by the number of electrons actually passed through the cell:

Efficiency = moles electrons actually consumed / moles electrons charged, or Efficiency = (moles bromine x 2 electrons per bromine) / moles electrons charged = 1.8 moles bromine x 2 electrons per bromine / (120 min x 50 A / 96,485 C/mole electrons)

= 3.6 moles electrons consumed / 3.73 moles electrons charged = 97%

[0064] Materials and Experimental Conditions for Resin Bead Experiments

[0065] Ion exchange resins may be used as insoluble bromine complexing agents. Two non-limiting commercially available examples include the polystyrene-based strong base Dowex 1X8-100 and the acrylic -based weak base Amberlite IRA-67. Dowex 1X8-100, a styrene- divinylbenzene copolymer with benzyl trimethyl ammonium groups attached to the polystyrene polymer chains, was obtained from Sigma-Aldrich. Amberlite IRA-67, a styrene-divinylbenzene copolymer with polyamine groups attached to the polystyrene polymer chains, was also obtained from Sigma-Aldrich.

[0066] The beads were received in the chloride and free base forms, respectively, and were converted to the Br^" form by washing sequentially with water, 2 M KOH, water until the eluent was neutral, 1 M HBr, and water again. The beads were then dried in an oven at 1 10 °C. The equilibrium uptake of Br₂ equivalents by the beads as shown in FIG. 3 and FIG. 4 were determined as follows. A glass vial was charged with 0.221 g dried Dowex 1X8-100, Br^" form. The beads were shaken and allowed to equilibrate with 20.0 mL 3.7 M HBr for 10 minutes. A quantity of the supernatant (10.0 mL) was removed, and 9.0 mL 3.7 M HBr and 1.0 mL 0.93 M Br2 in 3.7 M HBr were added. After several minutes of shaking, 10.0 mL of the supernatant was removed. Several more iterations of the above procedure, adding each time a known quantity of Br2 equivalents, allowed the generation of the curves seen in FIG. 3 and FIG. 4. Titration of the Br2 in each sample removed as described above and comparison of the quantity of B¾ added with the quantity in solution allowed direct inference of the quantity held by the resin for a given solution concentration. This procedure may be repeated for any combination of relevant ion exchange resins and ranges of solution [HBr] and [Br₂].

[0067] Example 2: Effect of Incorporating Resins Comprising Bromine

Immobilizing Groups on the Charging Efficiency of HBr Flow Battery Cells

[0068] The HBr cell used in the present work was constructed using hardware from Fuel Cell Technologies (Albuquerque, NM): stainless steel pressure plates, gold plated current collectors, and 5 cm² active are single serpentine flow fields machined in POCO blocks. A single-sided catalyst coated membrane (0.3 mg Pt/C on NR212) was obtained from Ion Power Inc. (New Castle, DE) and was boiled in DI water before use. Viton gaskets were used to obtain compression with a Toray 120 carbon paper on the hydrogen side of the membrane (i.e., with the catalyst coating) and a U105 carbon paper that had been spray-coated with a Vulcan-based ink on the bromine side. [0069] For all data contained in FIG. 5 a typical "HBr flow battery" configuration was employed. The HBr/Br₂ electrolyte was flowed via a peristaltic pump at a rate of 35 mL/min using a thick-walled glass vial as a reservoir. Hydrogen gas was flowed during the entire experiment at a rate of 100 mL/min via a mass flow controller. An Arbin Instruments (College Station, TX) battery tester was used to control the applied power and load. Typically, a -50 mL sample of HBr/Br₂ electrolyte was charged from 0.05 M Br₂ in 5.7 M HBr (0% SOC) to 1.85 M Br₂ in 2.1 M HBr (100% SOC) at 400 mA/cm² and was then correspondingly discharged. The hydrogen effluent was discharged through a series of two traps containing aqueous KOH of known concentration (titrated against an HC1 standard).

[0070] The number of equivalents of acid in the base traps can be determined by titration to measure the crossover of Br₂, which exits the hydrogen side as HBr. The Br₂ concentration in the electrolyte reservoir can be found by titrating an aliquot treated with excess KI and starch against a thiosulfate standard solution. In this manner the coulombic efficiency of the cell can be determined.

[0071] A 5 cm² active area HBr flow battery was run with Amberlite IRA-67 beads in the HBr reservoir. Dried Br^" form beads (10.0 g) and a solution of 170 mM Br₂ in 5.6 M HBr were held in a glass vial and flowed through the liquid side of a typical HBr flow cell with a peristaltic pump. The inlet tubing was plugged loosely with glass wool to prevent introduction of the beads into the pump tubing and cell. A typical current density was used to pass 5.36 Ah, which is sufficient to produce a solution concentration of 1.56 M Br₂. Titration of the solution revealed a concentration of 0.52 M Br₂, consistent with the observed increase in dark orange coloration of the beads. Taken together, these yield a current efficiency in excess of 90%, a significant improvement over the typical value of 70% observed for the same experiment performed without the ion exchange resin in the electrolyte reservoir. This is depicted graphically in FIG. 6.

[0072] As those skilled in the art will appreciate, numerous modifications and variations of the present invention are possible in light of these teachings, and all such are contemplated hereby. For example, in addition to the embodiments described herein, the present invention contemplates and claims those inventions resulting from the combination of features of the invention cited herein and those of the cited prior art references which complement the features of the present invention. Similarly, it will be appreciated that any described material, feature, or article may be used in combination with any other material, feature, or article, and such combinations are considered within the scope of this invention. [0073] The disclosures of each patent, patent application, and publication cited or described in this document are hereby incorporated herein by reference, each in its entirety.

Claims

What is Claimed:

1 A flow battery comprising

(a) a bromine half-cell, comprising an aqueous electrolyte comprising hydrogen bromide, bromine, and polybromide anions; and

(d) a bromine sequestering device in fluid communication with the bromine half-cell; wherein bromine sequestering device, contains a poorly soluble or insoluble material capable of reversibly complexing bromine.

2. The flow battery of claim 1 , wherein the material capable of reversibly complexing bromine comprises a graphite, zeolite, or ion exchange resin containing a tethered or immobilized bromine / polybromide complexing agent.

3. The flow battery of claim 2, wherein the ion exchange resin comprises a tethered or immobilized pyrrolidinium or morpholinium salt.

4. The flow battery of claim 2, wherein the ion exchange resin comprises a quaternary ammonium anion exchange resin.

5. The flow battery of claim 4, wherein the ion exchange resin comprises a styrene- divinylbenzene copolymer with quaternary ammonium groups attached to the polystyrene polymer chains.

6. The flow battery of claim 5, wherein the quaternary ammonium groups comprise benzyl trimethyl ammonium groups.

7. A method of operating a hydrogen bromide flow battery, said method comprising circulating an electrolyte comprising inlet and outlet streams of aqueous solutions of hydrogen bromide, bromine, and polybromide anions into and out of the flow battery, respectively, such that at least one of the inlet or outlet streams contacts a poorly soluble or insoluble material capable of reversibly complexing bromine.

8. The method of claim 7, wherein the flow battery is operating in a charging mode, and the material capable of reversibly complexing bromine is removing bromine from at least one inlet or outlet stream.

9. The method of claim 7, wherein the flow battery is operating in a discharging mode, and the material capable of reversibly complexing bromine is releasing bromine into at least one inlet or outlet stream.

10. The method of any one of claims 7-9, wherein the material capable of reversibly complexing bromine comprises a graphite, zeolite, or ion exchange resin.