WO2013086100A1

WO2013086100A1 - Hydrogen bromide flow battery system for distributed scale applications using pressure balanced cells

Info

Publication number: WO2013086100A1
Application number: PCT/US2012/068120
Authority: WO
Inventors: Jonathan Andrew HAMEL; Thomas M. MADDEN; Oleg Grebenyuk; Curtis S. WARRINGTON; Timothy Banks GREJTAK; Matthew Dorson
Original assignee: Sun Catalytix Corporation
Priority date: 2011-12-06
Filing date: 2012-12-06
Publication date: 2013-06-13

Abstract

Various embodiments of the present invention provide methods of charging a flow battery having at least one cell, said cell having: (a) a hydrogen chamber further having a hydrogen electrode and comprising an outlet coupled to a control device capable of maintaining a predetermined pressure within the hydrogen chamber; (b) an electrolyte chamber having a bromine electrode, a volume of working electrolyte, an inlet for addition of fresh electrolyte into said electrolyte chamber, and an outlet for removal of reacted electrolyte out of said electrolyte chamber, said working, fresh, and reacted electrolytes comprising hydrogen bromide; and (c) a solid electrolyte membrane disposed between said chambers.

Description

HYDROGEN BROMIDE FLOW BATTERY SYSTEM FOR DISTRIBUTED SCALE APPLICATIONS USING PRESSURE BALANCED CELLS

CROSS REFERENCE TO RELATED APPLICATIONS

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

61/567,314 filed December 6, 201 1, which is incorporated herein by reference in its entirety.

TECHNICAL FIELD

[0002] This disclosure relates to flow batteries and methods of operating the same, especially those flow batteries comprising hydrogen bromide.

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] Hydrogen bromide is among the flow battery technologies that seem to have significant merit. 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] Various embodiments of the present invention provide methods of charging a flow battery having at least one cell, said cell having: (a) a hydrogen chamber further having a hydrogen electrode and comprising an outlet coupled to a control device capable of maintaining a predetermined pressure within the hydrogen chamber; (b) an electrolyte chamber having a bromine electrode, a volume of working electrolyte, an inlet for addition of fresh electrolyte into said electrolyte chamber, and an outlet for removal of reacted electrolyte out of said electrolyte chamber, said working, fresh, and reacted electrolytes comprising hydrogen bromide; and (c) a solid electrolyte membrane disposed between said chambers; each method comprising passing current through the cell during charging to produce a partial pressure of hydrogen in the hydrogen chamber in a range of about 50 psig to about 250 psig, and controlling the total pressure in the electrolyte chamber so as to be substantially similar to or less than the total pressure of the hydrogen chamber.

[0007] Other embodiments provide a method or methods as described above, wherein the inlet and outlet of the electrolyte chamber are fluidicly coupled to at least one storage tank containing stored electrolyte, such that the electrolyte chamber, the inlet, the outlet, and the at least one storage tank fluidicly coupled thereto comprise a fluidic or electrolyte loop. In some embodiments, the entire fluidic loop is maintained to be at the same or substantially similar pressure as, or at a pressure differential to, the pressure of the electrolyte chamber. In other embodiments, only a portion of the fluidic loop is maintained to be at the same or substantially similar pressure as, or at a predetermined pressure differential to, the pressure of the electrolyte chamber. In these latter embodiments, for example, the electrolyte chamber may be maintained to be at a specific pressure differential, and the pressure in an associated loop tank is not so controlled.

[0008] In certain embodiments, the pressure in the electrolyte chamber is maintained so as to be substantially similar to the generated pressure in the hydrogen chamber. In other embodiments, the pressure of the electrolyte chamber is maintained at a pressure less than the pressure of the hydrogen chamber. The pressure of the electrolyte chamber may be maintained at a predetermined pressure differential or pressure differential range, relative to the total pressure the hydrogen chamber. In each case, the pressure differential may be achieved through a passive pressure transmitting device - for example, through the use of a free floating piston, a flexible diaphragm, or a combination thereof between the hydrogen and electrolyte chambers, or pipes, tanks, etc. associated therewith - or may be achieved through an active pressure controlling device - e.g., use of hydraulics or addition of an inert gas to the electrolyte chamber or loop - or a combination of active controlling and passive pressure transmitting devices.

[0009] During the charging cycle, various methods can be used to provide for the manipulation of the pressure in the electrolyte chamber, so as to substantially match or maintain a defined pressure differential with the pressure in the hydrogen chamber.

[0010] In some embodiments, using methods previously described, the flow battery is operated such that the electrolyte chamber is substantially full of liquid working electrolyte. In other cases, the electrolyte chamber may be operated with a small or appreciable gaseous or vapor headspace above the liquid working electrolyte at any temperature within the operating or contemplated temperature range of operation.

BRIEF DESCRIPTION OF THE DRAWINGS

[0011] 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:

[0012] FIG. 1 is a schematic diagram of an exemplary HBr flow battery system.

[0013] FIG. 2 illustrates an exemplary balanced pressure stack embodiment comprising a fully-hydraulic pressurization system.

[0014] FIG. 3 illustrates another exemplary balanced pressure stack embodiment using a differential head pump, with system configuration in charging (energy storage) mode.

[0015] FIG. 4 illustrates another exemplary balanced pressure stack embodiment using external compression with an inert gas, with system configuration in charging (energy storage) mode.

[0016] FIG. 5 illustrates another exemplary balanced pressure stack embodiment using pressure communication between storage tanks, with the system configured in charging (energy storage) mode.

DETAILED DESCRIPTION

[0017] 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 (charging or discharging) 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.

[0018] 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.

[0019] 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.

[0020] 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.

[0021] HBr Flow Battery Systems.

[0022] 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.

[0023] 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]

2H⁺ + 2e _^ H₂

Discharging [Ox'n] [ i_a]

Charging [Ox'n]

2 Br^" - Br₂ + 2e^"

Discharging [Red'n] [ib]

[0024] 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.

[0025] 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. [0026] Also as shown in FIG. 1, the hydrogen loop 15 comprises of 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.

[0027] 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.

[0028] 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. However, compressing the hydrogen to pressures which are commercially useful is expensive, and contributes significantly to the cost of operating such systems.

[0029] 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.

[0030] 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.

[0031] 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.

[0032] 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.

[0033] 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.

[0034] 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.

[0035] 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 DuPont Chemicals, Wilmington Del, as well as co-polymers of tetrafluoroethylene (TFE) and FSO₂— CF₂CF₂CF₂CF₂-0-CF=CF₂.

[0036] 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.

[0037] 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 or other carbide matrix materials, titanium dioxide, silicon dioxide, among others.

[0038] 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 acid.

[0039] 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).

[0040] 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).

[0041] 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.

[0042] Charging Hydrogen Bromide Flow Batteries so as to Produce High Hydrogen Pressures

[0043] One of the challenges faced by traditional hydrogen bromide (or generally, hydrogen halide) systems is that the hydrogen generated during charging must be stored at reasonable pressures to be cost effective. As described above, achieving reasonable pressures by conventional means usually involves the use of mechanical compression. Such compression is costly as both a capital expense and an operating expense. While large-scale (~1 MW) systems may be able to cost effectively accommodate a compressor, small-scale systems for distributed applications (<10 kW) cannot. It is therefore desired to have a system whereby compression and storage are achieved without the use of mechanical compression. The present inventions contemplate charging operations without pumps or other mechanical devices to compress the hydrogen during charging.

[0044] Various embodiments of the present invention provide methods of charging a flow battery having at least one cell, said cell having: (a) a hydrogen chamber further having a hydrogen electrode and comprising an outlet coupled to a control device capable of maintaining a predetermined pressure within the hydrogen chamber; (b) an electrolyte chamber having a bromine electrode, a volume of working electrolyte, an inlet for addition of fresh (pre-charged) electrolyte into said electrolyte chamber, and an outlet for removal of reacted (charged) electrolyte out of said electrolyte chamber, said working, fresh, and reacted electrolytes comprising hydrogen bromide; and (c) a solid electrolyte membrane disposed between said chambers; each method comprising passing current through the cell during charging to produce a partial pressure of hydrogen in the hydrogen chamber in a range of about 50 psig to about 250 psig, and controlling the total pressure in the electrolyte chamber so as to be substantially similar to or less than the total pressure of the hydrogen chamber. It should be appreciated that the flow batteries may also comprise a plurality of such cells, configured into cell stacks, connected to multiply the effect of each cell.

[0045] Other embodiments provide a method or methods as described above, wherein the inlet and outlet of the electrolyte chamber are each fluidicly coupled to at least one storage tank containing stored electrolyte, such that the electrolyte chamber, the inlet, the outlet, and the at least one storage tank fluidicly coupled thereto comprise a fluidic or electrolyte loop. In some embodiments, the entire fluidic loop is maintained to be at the same or substantially similar pressure as, or at a pressure differential to, the pressure of the electrolyte chamber. In other embodiments, only a portion of the fluidic loop is maintain to be at the same or substantially similar pressure as, or at a predetermined pressure differential to as the pressure of the electrolyte chamber. In these latter embodiments, for example, the electrolyte chamber may be maintained to be at a specific pressure differential, while the pressure in an associated loop tank(s) is/are not so controlled.

[0046] Other embodiments provide methods as described above, wherein the outlet of the hydrogen chamber is fluidicly coupled to a gas tank suitable for storing hydrogen gas at the elevated pressure produced in the hydrogen chamber.

[0047] While terms used within this disclosure typically are given their plain meanings to those skilled in the art to which it pertains, for clarity, certain terms used herein are specifically defined. For example, as used herein, the term "hydrogen electrode" refers to the working electrode on which hydrogen, H₂ is formed (by a reduction reaction) during the charging operation or on which ¾ is consumed (oxidized) during the discharge operation. The term

"hydrogen electrode" is sometimes also called the "negative electrode" in these systems, since the hydrogen electrode potential is always negative relative to the potential of the bromine electrode regardless of whether the cell is charging or discharging. Similarly, the term "bromine electrode" refers to the working electrode on which bromine is formed (oxidizing a bromide or polybromide anion) during the charging operation, or on which Br₂ is reduced to a bromide (or polybromide) anion during the discharging operation. The term "bromine electrode" is sometimes also called the "positive electrode" in these systems, since the bromine electrode potential is always positive relative to the potential of the hydrogen electrode regardless of whether the cell is charging or discharging. Additionally, in the context of the present descriptions, which are described in terms of a charging operation, the term "fresh electrolyte" refers to the "pre-charged" electrolyte composition entering the electrolyte chamber, the term

"working electrolyte" refers to the electrolyte occupying the electrolyte chamber during the passage of electrical current during charging, and the term "reacted electrolyte" refers to the "charged" electrolyte leaving the electrolyte chamber, having been already subject to the passage of electric current while within the electrolyte chamber. It should be appreciated that, in the context of a typical charging operation, bromide ion is converted to bromine while in the electrolyte chamber, such that the concentration of bromide ion (or polybromide equivalent) in the electrolyte entering the electrolyte chamber (the fresh electrolyte) is higher than that leaving the chamber (the reacted electrolyte).

[0048] It is generally preferred that current be passed through the cell during charging to produce a partial pressure of hydrogen in the hydrogen chamber of at least 150 psig, though higher pressures, for example at least about 200 psig, may be more preferred. The upper limits of the hydrogen partial pressures are generally defined by the practical limits of available pressure vessels or piping. Having said this, additional individual embodiments include those methods which provides that current is passed through the cell during charging of the flow battery so as to produce a partial pressure of hydrogen in the hydrogen cell in a range having a lower limit of about 50 psig, about 75 psig, about 100 psig, about 125 psig, about 150 psig, about 175 psig, and about 200 psig, and an upper limit in a range of about range of about 250 psig, about 225 psig, about 200 psig, about 175 psig, and about 150 psig. Exemplary, non-limiting embodiments, then, provide that the current is passed through the cell during charging of the flow battery to produce a partial pressure of hydrogen in a range of about 50 psig to about 250 psig, about 150 psig to about 250 psig, about 175 psig to about 250 psig, about 175 psig to about 225 psig, or about 200 psig to about 250 psig. As used herein, the term "psig" refers to pounds per square inch (psi) gauge, or psi above ambient external pressure (i.e., 0 psig is ambient external pressure), as is commonly understood by the engineering community.

[0049] It should be appreciated that the total pressure of the hydrogen chamber may be higher than the partial pressure of the hydrogen generated by the charging of the flow battery. For example, the presence of an additional gas or vapor pressure of a liquid within the hydrogen chamber may provide for this incrementally higher pressure. Given the reducing character of high pressure hydrogen, it is preferred that any additional gas or vapor be either chemically inert or practically chemically inert toward high pressure hydrogen.

[0050] The invention also provides embodiments wherein the pressure in the electrolyte chamber is maintained so as to be substantially similar to the generated pressure in the hydrogen chamber. In this context, the term "substantially similar" is intended to mean equal to or as nearly equal to as practically controllable. Several exemplary methods for maintaining the pressure of the electrolyte chamber are described below. [0051] In other embodiments, the pressure of the electrolyte chamber is maintained at a pressure less than the pressure of the hydrogen chamber. In certain of these embodiments, the pressure of the electrolyte chamber is maintained at a predetermined pressure differential or pressure differential range, relative to the total pressure the hydrogen chamber. The maintenance of a substantially similar or deliberate pressure differential may be achieved through a passive pressure transmitting device - for example, through the use of a free floating piston, a flexible diaphragm, or a combination thereof between the hydrogen and electrolyte chambers, or pipes, tanks, etc. associated therewith— or may be achieved through an active pressure controlling device - e.g., use of hydraulics or addition of an inert gas to the electrolyte chamber or loop - or a combination of active controlling and passive pressure transmitting devices.

[0052] In certain embodiments, the pressure differential between the hydrogen and electrolyte chambers (or pipes, tanks, etc. associated therewith) may be practically zero (i.e., when "substantially similar"), small (e.g., when the pressures are within about 50 psi, within about 25 psi, within about 10, or within about 5 psi or less of one another), or large (e.g., when the pressure differential is greater than about 50 psi, greater than about 75 psi, greater than about 100 psi, greater than about 125 psi, greater than about 150 psi, greater than about 175 psi, or greater than about 200 psi), such that the pressure of the electrolyte chamber (or pipes, tanks, etc. associated therewith) is at least 0 psig. Small or substantially similar pressure differentials between the hydrogen side and the electrolyte side minimize the physical stresses experienced by the interposed membrane or membrane assembly.

[0053] During the charging cycle, various methods can be used to provide for the manipulation of the pressure in the electrolyte chamber, so as to substantially match or maintain a defined pressure differential with the pressure in the hydrogen chamber.

[0054] In some embodiments, using methods previously described, the flow battery is operated such that the electrolyte chamber is substantially full of liquid working electrolyte - i.e., there is no appreciable gaseous or vapor headspace above the working electrolyte. By carefully controlling the amount or rate of fresh electrolyte added to the electrolyte chamber, relative to the amount of reacted electrolyte being removed, while the flow battery is charging (i.e., during the passing of current through the cell), the pressure in the electrolyte chamber can be maintained hydraulically. These embodiments may be encompassed by at least two strategies.

[0055] In certain of these embodiments, separate pump systems may be used decouple the flow of electrolyte through the electrolyte chamber from the maintenance of the pressure within that chamber. That is, a first pump or pump system may maintain circulation of the electrolyte through the electrolyte chamber at a controlled rate of feed, while a second pump or pump system provides incremental changes in the overall (hydraulic) pressure within the electrolyte chamber. The operation of the second pump or pump system may be coupled and controlled relative to the pressure of the hydrogen chamber. Such configurations may provide reduced power requirements of each pump relative to other control mechanisms. See, e.g., FIG. 2.

[0056] In certain other of these embodiments involving hydraulic control, a single pump or pump system may be employed to control both the circulation of the electrolyte through the electrolyte chamber as well as the pressure within the electrolyte chamber. As shown in FIG. 3, for example, the pressure balance may be maintained using a gear pump or other similar pump, whose outlet pressure is matched to the increasing electrolyte pressure during charge by means of a throttle valve. In these embodiments, the pressure of the electrolyte chamber is controlled relative to the pressure of the hydrogen chamber, while the pressure in the HBr/Br₂ tank is not, and may even be separately vented. This mode of operation has the advantage of simple system operation, but may require using a relatively expensive pump that is compatible with the HBr electrolyte and Br₂ reactants. In FIG. 3, pressurized parts of the system are depicted by the dotted lines / outlines.

[0057] In other embodiments, using methods previously described, the flow battery may be operated such that the electrolyte chamber is not substantially full of liquid working electrolyte - i.e., there is a small or appreciable gaseous or vapor headspace above the liquid working electrolyte at any temperature within the operating or contemplated temperature range of operation. The presence of vapor headspace is a concept well understood by the skilled engineer. In such cases, the pressure of the electrolyte chamber may be controlled by adding an inert gas to said electrolyte chamber or any portion of the fluidic loop associated with the electrolyte chamber, including the electrolyte storage tanks. As used herein, the term "inert" refers to a gas which does not chemically participate in the operation of the cell. As shown in

FIG. 3, for example, the pressure balance is shown to be maintained by pressurizing the headspace of both the electrolyte chamber and the connected storage tank with an inert gas.

Since Br₂ gas or vapor has a high density, using an inert gas such as Ar or N₂, each of which is significantly less dense than Br₂ gas or vapor, with a suitably designed vessel would allow the selective release of the inert during gas depressurization. In FIG. 4 pressurized parts of the system are depicted by the dotted lines / outlines. Compared to the method shown in FIG. 3, the use of an inert gas allows the use of a less expensive pump, owing to the lower pressure differential associated with pumping the electrolyte solution.

[0058] In still other embodiments, and again using methods previously described, the pressure of the electrolyte tank may be maintained through the use of devices capable of mechanically transmitting the pressures of the hydrogen chamber to the electrolyte chamber. Such devices, for example, may include a movable piston or flexible membrane or a combination or both. An example of this embodiment is shown schematically in FIG. 5, where the pressure associated with the hydrogen storage vessel (and the hydrogen chamber) is mechanically transmitted to the HBr/Br₂ storage vessel via a moving piston or flexible membrane. In FIG. 5, pressurized parts of the system are depicted by the dotted lines / outlines.

[0059] It should be appreciated that controlling the pressure of the second, electrolyte chamber so as to be either substantially similar or at a defined pressure differential relative to the hydrogen chamber requires feedback monitoring, if control is to be maintained. Such control may be provided using the process management system(s) described above.

[0060] It should also be appreciated that, in those cases where a pressure differential is maintained between the hydrogen and electrolyte chamber, unless physically supported, such pressure differential will stress the separator membrane or membrane electrode assembly interposed between the two chambers, both in terms of the physical integrity of the membranes as well as the membrane's ability to withstand the diffusion of hydrogen to the second, electrolyte chamber. This may limit the choice of separator options to those having very low liquid permeability and/or porosity. Separators comprising solid non-porous polymer electrolyte membranes may be the preferred mode of separator for this system. For example, NAFION- based membranes are expected to have hydrogen crossover currents of about 2.5 mA/cm² at a hydrogen pressure of 200 psig on the hydrogen and ambient pressure on the bromine side. If the operating current density is 500 mA/cm², then this represents a very tolerable -0.5% impact on current efficiency. By contrast, porous membranes will have substantially less ability to withstand pressure differentials. For example, if the electrolyte was highly wetting (contact angle of about 0 degrees) within 1-10 micron pores then the theoretical cross-pressure difference that could be sustained without gas breakthrough is in a range of about 2.5 to about 25 psi. This cross pressure will go down to the extent that the fluid is not completely wetting. Hence, the use of porous membranes will require that the degree of hydrogen over-pressure be only that which is dictated by bubble pressures with the porous membrane of interest with the liquid electrolyte of interest. This cross-pressure can be roughly approximated by that given by the Young- Laplace equation using the appropriate wetting angle and fluid surface tension.

[0061] 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.

[0062] 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 method of charging a flow battery having at least one cell, said cell having:

a hydrogen chamber further having a hydrogen electrode and comprising an outlet coupled to a control device capable of maintaining a preselected pressure within the hydrogen chamber;

an electrolyte chamber having a bromine electrode, a volume of working electrolyte, an inlet for addition of fresh electrolyte into said electrolyte chamber, and an outlet for removal of reacted electrolyte out of said electrolyte chamber, said working, fresh, and reacted electrolytes comprising hydrogen bromide; and

a solid electrolyte membrane assembly disposed between said hydrogen and electrolyte chambers;

said method comprising passing current through the cell to produce a partial pressure of hydrogen in the hydrogen chamber in a range of about 150 psig to about 250 psig, and controlling the total pressure in the electrolyte chamber so as to be (i) substantially similar or (ii) at a defined pressure differential relative to the total pressure of the hydrogen chamber.

2. The method of claim 1, wherein the inlet and outlet of the electrolyte chamber are fluidicly coupled to at least one storage tank containing stored electrolyte.

3. The method of claim 2, wherein at least one storage tank is maintained at a pressure that is substantially similar to the pressure of the electrolyte chamber.

4. The method of any one of claims 1-3, wherein the outlet of the hydrogen chamber is fluidicly coupled to a gas tank suitable for storing hydrogen gas at the pressure produced in the hydrogen chamber.

5. The method of any one of claims 1-4, wherein the partial pressure of hydrogen produced in the hydrogen chamber is in a range of about 200 psig to about 250 psig.

6. The method of any one of claims 1-5, wherein the pressure in the electrolyte chamber is maintained at a pressure which is substantially similar to the pressure in the hydrogen chamber

7. The method of any one of claims 1-5, wherein the pressure in the electrolyte chamber is maintained to be within about 50 psi of the pressure in the hydrogen chamber.

8. The method of any one of claims 1-7, wherein an amount of fresh electrolyte is added and an amount of reacted electrolyte is removed during the passing of current through the cell, the pressure in the electrolyte chamber being controlled by adjusting the respective rates of addition of the fresh electrolyte to and the removal of the reacted electrolyte from said electrolyte chamber.

9. The method of any one of claims 1-8, wherein the electrolyte chamber is substantially full of the working electrolyte and the pressure in the electrolyte chamber is maintained hydraulically.

10. The method of claim 9, wherein the addition of the fresh electrolyte to and the removal of the reacted electrolyte from said electrolyte chamber is maintained by use of a first pump and the pressure within the electrolyte chamber is maintained by use of a second pump.

1 1. The method of any one of claims 1-3, wherein the pressure of the hydrogen chamber or gas tank fluidicly coupled thereto is mechanically transmitted to the electrolyte chamber or at least one storage tank fluidicly coupled thereto.

12. The method of claim 1 1, wherein the pressure is mechanically transmitted by a movable piston or flexible membrane.

13. The method of any one of claims 1-7, wherein the electrolyte chamber has an internal volume which provides for a headspace volume over the volume of the working electrolyte and the total pressure of the electrolyte chamber is controlled by adding an inert gas to said electrolyte chamber, where inert gas refers to a gas which does not chemically participate in the operation of the cell.

14. The method of claim 13, wherein the inert gas is nitrogen or argon.