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 comprise membrane electrode assemblies (MEAs), wherein each MEA comprises a polymer electrolyte membrane (PEM) having first and second surfaces; a bromine electrode comprising a bromine catalyst layer, said bromine catalyst layer disposed adjacent to the second surface of the PEM, the bromine catalyst layer and the second surface of the PEM separated by a spacing; and optionally, a hydrogen disposed adjacent to the first surface of the PEM. In certain embodiments, the MEA is part of a flow cell, a flow battery or a flow battery system.

[0008] Other embodiments provide flow battery cells, each cell comprising: a polymer electrolyte membrane (PEM) having first and second surfaces; a hydrogen electrode disposed adjacent to the first surface of the PEM; and a bromine electrode comprising a bromine catalyst layer, said bromine catalyst layer disposed adjacent to the second surface of the PEM, the bromine catalyst layer and the second surface of the PEM separated by a spacing; wherein the spacing between the PEM and the bromine electrode layer comprises a hydrogen bromide- containing electrolyte. In certain other embodiments, the flow battery cell is operating and the spacing between the PEM and the bromine electrode layer comprises a hydrogen bromide- containing electrolyte; wherein electricity passes between the hydrogen and bromine electrodes, such the hydrogen is generated at the hydrogen electrode and bromine is generated at the bromine catalyst layer, and wherein the bromine immediately adjacent to the bromine catalyst layer may comprise a bromine equivalent formed by the reaction of bromine and bromide. In other embodiments of the operating flow battery cell, a concentration gradient exists between the bromine catalyst layer and the second surface of the PEM, such that the combined concentration of bromine and bromine equivalent immediately adjacent to the second surface of the PEM is independently at least about 10 mol%, 20 mol%, 30 mol%, 40 mol%, 50 mol%, 95 mol% lower than the concentration of bromine or bromine equivalent immediately adjacent to the bromine catalyst layer.

[0009] Other embodiments provide membrane electrode assemblies (MEAs), each MEA comprising: a polymer electrolyte membrane (PEM) having first and second surfaces; a bromine electrode comprising a bromine catalyst layer disposed adjacent to and separated from the second surface of the PEM; and optionally, a hydrogen electrode disposed adjacent to the first surface of the PEM; wherein the MEA is configured such that an electrolyte may pass between the second surface of the PEM and the bromine catalyst layer. In certain embodiments, the MEA is part of a flow battery cell, flow battery, or a flow battery system.

BRIEF DESCRIPTION OF THE DRAWINGS

[0010] 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:

[0011] FIG. 1 is a schematic diagram of an exemplary HBr flow battery system.

[0012] FIG. 2 illustrates electrode reactions and direction of ion transport for (FIG. 2A) charging and (FIG. 2B) discharging cycles of a reversible HBr flow battery under normal operation conditions.

[0013] FIG. 3 illustrates some of the reactions that lead to reduced current efficiency in a reversible HBr fuel cell, including carbon electrode corrosion, H ₂ oxidation at the halogen (bromine / bromide) electrode, and Br ₂ /Br ₃ ^~ reduction at the hydrogen electrode.

[0014] FIG. 4A shows a simplified schematic of a cell comprising a flow battery cell containing a membrane electrode assembly in which a catalytically active portion of the electrode 42 is positioned between the bipolar plate 41 and the membrane 44, providing a spacing 43. FIG. 4B illustrates a bromine/tribromide (Br ₂ /Br ₃ ^~ ) concentration gradient resulting from increasing the separation between the catalytically active zone away from the membrane surface.

DETAILED DESCRIPTION

[0015] 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.

[0016] 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.

[0017] 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.

[0018] 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. [0019] HBr Flow Battery Systems.

[0020] 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.

[0021] 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 ^ H ₂ + Br ₂

Discharging [1]·

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

Discharging [Ox'n] [ la]

Charging [Ox'n]

2 Br ^" - Br ₂ + 2e ^"

Discharging [Red'n] [ib]

[0022] 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:

B ^r 2 ⁺ Br — ΒΓ3 | ^" 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/Br ₂ necessarily comprises a mixture of HBr, Br ₃ ^~ (and higher polybromide anions), and Br ₂ .

[0023] FIG. 1 illustrates a schematic diagram of an exemplary HBr flow battery system. The system comprises two circulation loops - one for the aqueous HBr/Br ₂ 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/Br ₂ 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.

[0024] 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.

[0025] 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 (Br ₂ ) 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.

[0026] During charging, the HBr/Br ₂ (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.

[0027] 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.

[0028] During such a discharge, the HBr/Br ₂ (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.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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 FS0 ₂ — CF ₂ CF ₂ CF ₂ CF ₂ -0-CF=CF ₂ .

[0034] 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.

[0035] 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 (PVDF), or

polytetrafluoroethylene (PTFE). Preferred inorganic fillers include silicon carbide matrix material, titanium dioxide, silicon dioxide, among others.

[0036] 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. [0037] 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).

[0038] 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 (FTL) or gas diffusion layers (GDL's).

[0039] 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.

[0040] Spatially Separating the Active Catalytic Zone Away from the Membrane Surface

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

RT _DC -DC ⁼ EDC,out*EDC,in (4)

[0042] The E _D c,in and E _D c,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 H ₂ production and oxidation using highly dispersed Pt catalysts on carbon supports are typically small (less than 5 mV). Overpotentials for Br7Br ₂ 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. Typical cell operation in both charge and discharge modes are shown in FIGs. 2A and 2B.

[0045] Additionally, alternative redox reactions and reactive species crossover can lead to reductions in current efficiency in a reversible HBr fuel cell including Br ₂ /Br ₃ ^~ crossover, H ₂ crossover, and carbon corrosion, FIG. 3.

[0046] Typical strategies employed in the prior art to improve current efficiency in reversible HBr fuel cells attempt to reduce Br ₂ /Br ₃ ^~ and H ₂ 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 a series of embodiments which have the effect of reducing the crossover of H ₂ , Br ₂ or Br ₃ ^" 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 Br ₂ at the halogen electrode and H ⁺ ions move across the membrane to the hydrogen side where they are reduced to H ₂ . In discharge, the H ₂ 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 Br ₂ /Br ₃ ^~ 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 Br ₂ /Br ₃ ^~ at the membrane surface.

[0049] Various embodiments of the present invention comprise membrane electrode assemblies (MEAs), wherein each MEA comprises a polymer electrolyte membrane (PEM) having first and second surfaces; a bromine electrode comprising a bromine catalyst layer, said bromine catalyst layer disposed adjacent to the second surface of the PEM, the bromine catalyst layer and the second surface of the PEM separated by a finite, non-zero spacing; and optionally, a hydrogen electrode disposed adjacent to the first surface of the PEM. Independent

embodiments provide that this finite, non-zero spacing is at least about 1, about 2, about 3, about 4, about 5, about 10, about 15, about 20, about 25, about 50, about 75, about 100, about 500, or about 1000 microns.

[0050] As described in certain of these embodiments, an electrode may comprise a support material comprising a porous, electrically conductive current collector (e.g., graphite) for the electrode onto which an appropriate catalyst is deposited. In the case of the hydrogen electrode, the electrode/catalyst is a material suitable for reducing H ⁺ to ¾ during charge and oxidizing H ₂ to H ⁺ during discharge, typically including platinum and/or other precious or non- precious metal or metals. Similarly, the bromine electrode / catalyst / catalyst layer comprises a material suitable for oxidizing bromide (or polybromide) to bromine during charge and reducing bromine to bromide or polybromide during discharge. The catalyst can include, for example, neat high surface area carbon such as a Vulcan carbon, acetylene black carbon, Black Pearls carbon, Ketjenblack carbon or other high surface area catalytic carbon, etc., or such a surface further comprising at least one non-precious metal catalyst such as Co, Cr, Fe, Mn, Ni, Ti, or Zr, and/or at least one precious metal catalyst such as Rh, Ir, Ru, Os, Pd, Pt, Mo, Re, or an alloy or mixture thereof.

[0051] Catalysts may be deposited onto a suitable current collector or other support medium by sputtering, or other chemical or physical vapor deposition method, by solution deposition, or by other suitable means. As used herein, the term "catalyst layer" refers to a layer within or upon the electrode having the highest concentration of catalyst, the highest catalytic activity, or both highest concentration and activity of catalyst, in which the catalyst is capable of improving the efficiency of the respective electrochemical conversion, under the appropriate electrochemical conditions. This layer is typically, but not necessarily, positioned at one surface of the electrode, resulting, for example, as a consequence of sputtering or vapor depositing the catalyst onto the surface of the electrode. The presence of a catalyst layer on an electrode does not preclude the presence of catalyst elsewhere within the electrode, for example as a gradient within the electrode. In some cases, where the catalyst layer is distributed substantially evenly throughout the electrode support, the entire electrode may be deemed to be the catalyst layer. [0052] Throughout this specification, words are to be afforded their normal meaning, as would be understood by those skilled the relevant art. However, so as to avoid

misunderstanding, the meanings of certain terms will be specifically defined or clarified.

[0053] For example, as used herein, the term "disposed adjacent to" connotes a spatial relationship between two objects or two surfaces positioned such that the first object or surface overlaps and is positioned next to or adjacent to a second object or surface. The terms includes those situations where the two objects or surfaces are near, but not in physical contact with one another, as well as those situations where the two objects or surfaces are in physical contact with one another. The term "disposed directly adjacent to" connotes the special circumstance where the two bodies or surfaces are in physical contact with one another. Additionally, unless otherwise specified, a description that a second surface is disposed adjacent to a first surface connotes that the two surfaces are substantially parallel to or coplanar with one another.

However, such a description may also connote those embodiments wherein a small planar angle between the two surfaces, for example that this planar angle is less than about 20°, less than about 10°, less than about 5°, less than about 4°, less than about 3°, less than about 2°, less than or less than about 1°.

[0054] Similarly, within the context of this invention, unless otherwise specified, the term "spacing" is intended to represent a finite, non-zero distance between two objects, typically representing the closest approach between two objects. In the case of two planar objects, and unless otherwise specified, the spacing is intended to connote the mean spacing between the two planar surfaces. For example, a spacing between the catalyst layer and the PEM should be read as representing the mean spacing distance between the two surfaces, so as to refer to the average spacing across the overlapping areas of the catalyst layer and PEM. Also, throughout this specification, unless otherwise specifically indicated, the term "spacing" connotes independent embodiments wherein this mean spacing distance is at least about 1, about 2, about 3, about 4, about 5, about 10, about 15, about 20, about 25, about 50, about 75, about 100, about 500, or about 1000 microns, even where other functional descriptions are provided. Practical considerations of operating electrochemical cells define the upper limits of these spacings.

Further, the term spacing does not necessarily connote that any particular composition fills this spacing, such that, for example, the spacing between the catalyst layer and the PEM may comprise or consist of solid, liquid, or gas, including porous or non-porous solids, electrolyte, and air and may be electrically conducting, semi-conducting, or non-conducting. Porous solid spacer materials may comprise, for example, conductive porous materials such as carbon felts or carbon paper (e.g. TORAY® paper available from Toray Industries, Japan or PYROFIL® paper available from Mitsubishi Rayon Co., Ltd., Japan), or metal foams, or non-conductive materials such as woven and non-woven meshes of inert polymers (e.g. polyethylene, polypropylene, PTFE, etc.).

[0055] In certain embodiments, the MEA is part of a flow battery cell, a flow battery, or a flow battery system.

[0056] Other embodiments provide flow battery cells, each cell comprising: a polymer electrolyte membrane (PEM) having first and second surfaces; a hydrogen electrode disposed adjacent to the first surface of the PEM; and a bromine electrode comprising a bromine catalyst layer, said bromine catalyst layer disposed adjacent to the second surface of the PEM, the bromine catalyst layer and the second surface of the PEM separated by a spacing of a finite nonzero distance; wherein the spacing between the PEM and the bromine electrode layer comprises a hydrogen bromide-containing electrolyte. Independent embodiments provide that this finite non-zero distance be at least about 1, about 2, about 3, about 4, about 5, about 10, about 15, about 20, about 25, about 50, about 75, about 100, about 500, or about 1000 microns. As described above, the hydrogen bromide-containing electrolyte may contain bromide, tribromide, pentabromide, or higher polybromide anions, or a combination thereof. In certain of these embodiments, the flow battery cell is configured for the passage of electricity between the hydrogen and bromine electrodes, so as to generate hydrogen at the hydrogen electrode and bromine at the bromine catalyst layer. These flow battery cells may also be incorporated into larger flow battery systems.

[0057] In still other embodiments, the flow cell battery is an operating flow battery cell comprising: a polymer electrolyte membrane (PEM) having first and second surfaces; a hydrogen electrode disposed adjacent to the first surface of the PEM; and a bromine electrode comprising a bromine catalyst layer, said bromine catalyst layer disposed adjacent to the second surface of the PEM, the bromine catalyst layer and the second surface of the PEM separated by a spacing of a finite non-zero distance; wherein the spacing between the PEM and the bromine electrode layer comprises a hydrogen bromide-containing electrolyte; wherein electricity passes between the hydrogen and bromine electrodes, such the hydrogen is generated at the hydrogen electrode and bromine is generated at the bromine catalyst layer, and wherein the bromine immediately adjacent to the bromine catalyst layer may comprise a bromine equivalent formed by the reaction of bromine and bromide. As above, independent embodiments provide that this finite non-zero distance be at least about 1, about 2, about 3, about 4, about 5, about 10, about 15, about 20, about 25, about 50, about 75, about 100, about 500, or about 1000 microns. As used here, the term "bromine equivalent" refers to a species or complex comprising bromine, such as tribromide, pentabromide, or other complex anion containing bromine, wherein said species or complex has the potential of crossing through the PEM.

[0058] In certain embodiments, an operating flow battery cell has a concentration gradient between the bromine catalyst layer and the second surface of the PEM, such that the combined concentration of bromine and bromine equivalent immediately adjacent to the second surface of the PEM is at least about 10 mol% lower than the concentration of bromine or bromine equivalent immediately adjacent to the bromine catalyst layer. Additional independent embodiments provide that this concentration gradient provides that the combined concentration of bromine and bromine equivalent immediately adjacent to the second surface of the PEM is at least about is at least 20 mol%, 30 mol%, 40 mol%, 50 mol%, 60 mol%, 70 mol%, 80 mol%, 90 mol%, or even 95 mol% lower than the concentration of bromine or bromine equivalent immediately adjacent to the bromine catalyst layer.

[0059] In certain embodiments, the MEA, cell, battery, or system is configured such that an electrolyte is capable of flowing or directed to flow between the second surface of the PEM and the bromine electrode or bromine catalyst layer or both.

[0060] Without being bound to the correctness of the theory, it is believed that the presence of convective fluid flow between the catalytic layer and the PEM induces mixing such that the higher concentration of Br ₂ /Br ₃ ^~ developed at the catalytic layer during charging is reduced to that of the bulk electrolyte more quickly. In this application the bulk electrolyte concentration of Br ₂ /Br ₃ ^~ can be thought of as being at a minimum value in the absence of more advanced mitigations.

[0061] In certain of these embodiments, the bromine electrode has a first electrode surface facing the PEM and a second electrode surface facing opposite the first (i.e., away from the PEM), said bromine electrode further comprising a bromine electrode catalyst, wherein the concentration of the bromine electrode catalyst is distributed as a gradient between the first and second surfaces of said bromine electrode such that the concentration of the bromine electrode catalyst is higher at the second electrode surface than at the first electrode surface. Further embodiments provide that the first electrode surface (i.e., the surface facing the PEM) is substantially free of bromine electrode catalyst.

[0062] Additional embodiments provide those operating flow battery cells, wherein the coulombic efficiency of the cell is at least about 5% higher, on an absolute basis, than an otherwise equivalent cell where the bromine catalyst layer physically contacts the second surface of the PEM. In other independent embodiments, the cell delivers a coulombic efficiency of at least about 2%, 3%, 4%, 6%, 7%, 8%, 9%, 10%, 15%, or 20% higher, on an absolute basis, than an otherwise equivalent cell where the bromine catalyst layer physically contacts the second surface of the PEM. Methods for determining coulombic efficiency are described below

[0063] Certain of these embodiments are exemplified in FIG. 4, which shows one embodiment of the catalytic zone spaced from the PEM, in which a catalytically active portion of the electrode 42 is positioned between a bipolar plate 41 and the PEM membrane 44, providing a spacing 43 (FIG. 4A). FIG. 4B illustrates a bromine/tribromide (Br ₂ /Br ₃ ^" ) concentration gradient resulting from increasing the separation between the catalytically active zone away from the membrane surface.

[0064] Conventional wisdom associated with membrane electrode assemblies (e.g., those designed for PEM fuel cells or electrolyzers) teaches that increasing voltaic efficiency requires reducing the distance between the catalytic zone and the membrane surface to a minimum. By contrast, and without being necessarily bound by the correctness of the theory, the embodiments of the present invention where there is separation between the catalytic zone and the membrane surface, are believed to provide enhanced current efficiencies resulting from the lowering of the concentration of bromine result at the surface of the membrane, relative to the levels which would develop without separation.

[0065] Depending on the configuration and operating conditions of the cell(s), the increased separation of the catalytic zone from the membrane may result in slightly increased resistive (iR) loss, but the concentrated HBr/Br ₂ media has sufficiently high conductivity (>250 mS/cm) that the voltage loss incurred should be small. Similarly, depending on the particular configuration and operating system of the cell(s), there may exist a performance maximum, in terms of round trip efficiency, resulting from the optimal balance of bromine surface

concentration and resistive loss.

[0066] To this point, the descriptions have been largely directed to membrane electrode assemblies, 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] EXAMPLES

[0072] Experimental Determination of Current Efficiency

[0073] 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/Br ₂ 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.

[0074] As an example, consider a system in which a hydrogen/bromine flow battery has an electrolyte tank filled with 2L 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 Br ₂ , 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%

[0075] 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.

[0076] 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.