WO2014014503A1

WO2014014503A1 - Increased durability in hydrogen bromide cell stacks by inhibiting corrosion of hydrogen electrodes

Info

Publication number: WO2014014503A1
Application number: PCT/US2013/030405
Authority: WO
Inventors: Paravastu Badrinarayanan; Timothy Banks GREJTAK; Arthur J. Esswein; Thomas H. Madden; Oleg Grebenyuk
Original assignee: Sun Catalytix Corporation
Priority date: 2012-07-17
Filing date: 2013-03-12
Publication date: 2014-01-23

Abstract

This disclosure relates to hydrogen bromide flow batteries and methods of operating the same, including methods of inhibiting corrosion of hydrogen electrodes during full cycle operations of the cells.

Description

INCREASED DURABILITY IN HYDROGEN BROMIDE CELL STACKS BY INHIBITING CORROSION OF HYDROGEN ELECTRODES

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims priority to U.S. Application No. 61/672,434, filed July 17, 2012, which is incorporated by reference in its entirety.

TECHNICAL FIELD

[0002] This disclosure is in the field of hydrogen bromide flow batteries and methods of operating the same, including methods of inhibiting corrosion of hydrogen electrodes during full cycle operations of the cells.

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 with chemistries such as zinc / bromine, all vanadium, iron-chrome, and others which may be combinations of the prior.

[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.

[0006] The present application addresses several problems associated with the use of hydrogen bromide flow batteries.

SUMMARY

[0007] Various embodiments of the present invention provide methods of inhibiting corrosion of a hydrogen electrode caused during the charging operation, discharging operation, startup, shutdown and non-operating cycle of an HBr flow battery cell, each method comprising maintaining a hydrogen pressure over the hydrogen electrode during one or more of the startup, shutdown, or non-operating cycles, said hydrogen pressure sufficient to inhibit bromide/bromine corrosion of the hydrogen electrode. In certain instances, the corrosion is otherwise caused by crossover bromide or bromine interacting with the hydrogen electrode.

[0008] Still other embodiments further reduce the risk of hydrogen electrode corrosion by further comprising, providing, or maintaining a hydrogen pressure, and/or an inerting fluid, over the bromine electrode during at least one of these cycles while maintaining hydrogen over the hydrogen electrode(s).

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 / Figures exemplary embodiments of the subject matter; however, the presently disclosed subject matter is not limited to the specific methods, devices, and systems disclosed. The Figures are not necessarily limited to the scales presented. The drawings are:

[0010] FIG. 1 is a block diagram of an exemplary energy storage and generation system.

[0011] FIG. 2 is an electron micro-probe platinum map of a pristine membrane- electrode assembly with platinum on both sides. Brackets to the right of the map highlight the position of the platinum on the membrane.

[0012] FIG. 3 is an electron micro-probe platinum map of a used membrane-electrode assembly with platinum only on the hydrogen side. The bromine electrode side is shown along the bottom. Brackets to the right of the map highlight the presence of the platinum on the hydrogen side and the absence of platinum on the hydrogen bromide side of the membrane- electrode assembly.

[0013] FIG. 4 illustrates the mechanism whereby a Pt catalyst is (or Pt catalysts are) dissolved under conditions of Br₂ crossover in an HBr flow battery.

DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS

[0014] 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.

[0015] 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.

[0016] When a value is expressed as an approximation 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.

[0017] 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. That is, unless obviously incompatible or specifically excluded, each individual embodiment is deemed to be combinable with any other embodiment(s) and such a combination is considered to be another 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 sub-combination. Finally, while an embodiment may be described as part of a series of steps or part of a more general structure, each said step may also be considered an independent embodiment in itself.

[0018] Certain embodiments of the present invention provide methods of inhibiting corrosion of a hydrogen electrode during a charging operation, discharging operation, startup, shutdown and non-operating cycle of an HBr flow battery cell. In some of these, the methods comprise maintaining a hydrogen pressure over the hydrogen electrode during one or more of the charging operation, discharging operation, startup, shutdown, or non-operating cycles, said hydrogen pressure sufficient to inhibit corrosion of the hydrogen electrode, typically by bromide / tribromide / bromine, said (tri)bromide/bromide arising from crossover during the charging or discharging of the flow battery. Descriptions of the construction and full cycle operation of typical flow batteries are provided next to assist the reader in placing these embodiments in proper context. The readers should appreciate that use of the word "typical" in this context means that variations are both possible and within the scope of the present invention(s).

[0019] HBr Flow Battery Systems.

[0020] This disclosure relates to flow battery systems, especially HBr flow battery systems, that utilize the same electrochemical cell(s) or cell stacks 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 ^ H₂ + Br₂

Discharging [1]·

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

Discharging [Ox'n] [ la] Charging [Ox'n]

2 Br Br₂ + 2e^"

Discharging [Red'n] [lb]

[0022] In hydrogen bromide flow batteries, the electrolyte comprises aqueous hydrog bromide / bromine. The formation of tribromide ion in the presence of bromine and bromide 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.

[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/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 20 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 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 H₂ and Br₂ 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.

[0026] 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.

[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/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.

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

[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, or polytetrafluoroethylene. Preferred inorganic fillers include silicon carbide or other carbide matrix materials, 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 hydrobromic acid.

[0037] Porous membranes are easily permeable to liquid or gaseous chemicals. This permeability increases the 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 membrane 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).

[0039] Typical MEAs comprise a polymer electrolyte membrane (PEM), which functions as a solid electrolyte. In the present context, one face of the PEM is in contact with the hydrogen electrode layer and the opposite face is in contact with the bromine electrode layer. As used herein, the term "hydrogen electrode" refers to the working electrode on which hydrogen, ¾ is formed (by a reduction reaction) during the charging operation or 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 electrode potential is always negative with respect to the bromine electrode regardless of whether the cell is charging or discharging. Similarly, the term "bromine electrode" refers to the working electrode on bromine is formed (oxidizing a bromide or polybromide anion) during the charging operation, or on which bromine 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 electrode potential is always positive with respect to the hydrogen electrode regardless of whether the cell is charging or discharging.

[0040] An electrode may comprise a support material comprising a porous 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 ¾ to H⁺ during discharge. Similarly, the bromine electrode / catalyst comprises a material suitable for oxidizing bromide (or polybromide) to bromine during charge and reducing bromine to bromide or polybromide during discharge

[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. The catalyst can include, for example, neat high surface area carbon such as a Vulcan carbon, acetylene black carbon, Black Pearls carbon, Ketjen black carbon or other high surface area catalytic carbon, etc., 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. Other such catalysts are known by those skilled in the art.

Providing / Maintaining a Low Potential Environment on the Hydrogen Electrode So As to Prevent Corrosion / Dissolution of This Electrode

[0042] As described above, each electrode layer includes at least one electrochemical catalyst, typically including platinum and/or other precious or non-precious metal or alloy thereof. Platinum is particularly attractive because it is an excellent catalyst for hydrogen oxidation and evolution under acidic conditions. The use of platinum as the hydrogen electrode

(negative electrode) for an HBr cell has been taught previously. See, e.g., Yeo and Chin, J.

Electrochem. Soc_, 127 ^', (3), pp 549 - 555, (1980). Dissolution (or corrosion) of these metals, including platinum, under certain electrochemical operating conditions occurs and has been previously studied. See, e.g., Darling and Meyers, J. Electrochem. Soc, 2003, 150 (1 1) A1523- 1527 (2003). For example, the present inventors have shown that even platinum initially present on the HBr electrode (positive electrode) dissolves and moves into solution under operating conditions. Compare FIG. 2 and FIG. 3. FIG. 3 clearly indicates that the platinum can dissolve away when exposed to bromine, whereas the platinum on the hydrogen electrode appears stable if operated under certain conditions. However, if the platinum on the hydrogen electrode is allowed to dissolve, a significant performance penalty for an HBr flow battery is observed, particularly when operated in discharge mode.

[0043] When the potential of a platinum or platinum containing electrode vs. NHE exceeds about 0.98V at pH of 0, the platinum is susceptible to corrosion. Additionally, when in the presence of significant bromide ions, platinum is susceptible to dissolution when the potential of the electrode vs. NHE exceeds about 0.63 V at pH of 0. As long as hydrogen is present at the negative electrode, a low interfacial potential difference is maintained (less than 0.1 V) and this prevents the corrosion of the hydrogen electrode (including platinum electrodes / catalysts). During normal operation of a hydrogen bromide cell stack, corrosion of a platinum hydrogen electrode is unlikely as hydrogen is sufficiently present at this electrode. Nevertheless, at shutdown of the power plant, if hydrogen flow to the stack is stopped, the residual hydrogen in the stack can slowly be removed due to leaks,reaction with B¾ that may cross over the membrane seperator, and electrical shorts in the stack. This may lead to a depletion of hydrogen at the negative electrode, and the electrode potential of the negative electrode may begin to rise could reach the potential of the bromine / bromide / polybromides that cross over. This may result in conditions that can cause the platinum in the negative electrode to corrode.

[0044] The present inventors have discovered that such dissolution of the hydrogen electrodes may be avoided by maintaining the hydrogen electrode under reducing conditions, even during those portions of the full operating cycle in which the cell is not actively charging or discharging. Such portions of the operating cycle in which the cell is not actively charging or discharging include startup and shutdown of the cell and those times of the cycle during which the system is non-operating or de-energized. The startup and shutdown conditions may be considered transition or transitory conditions, and may be further distinguished in terms of charging startup, charging shutdown, discharging startup, and discharging shutdown. Charging and discharging transitions may further be defined in terms of the relative speed of the transition - e.g., "rapid charge-discharge transition," and "rapid discharge-charge transition," which lie along a continuum of such transitions. Various embodiments of the present invention include methods involving each of these transitions and/or de-energized or non-operating states. [0045] The present inventors also realized that another benefit of controlling the potential on the hydrogen electrode is to limit the development of high potentials on the corresponding bromine electrode when mixed reactants are present on the hydrogen electrode. For example, in certain situations bromine / bromide may crossover to the hydrogen during charge, which results in coulombic efficiency loss. If enough bromine is present on part of the hydrogen electrode, the opposing bromine electrode can be driven to damaging high potentials. This occurs since the overpotential for bromine reduction in these regions is very high due to the hydrogen present in other regions, which drives a demand for protons from the opposing bromine electrode which will be served by corrosion of carbon materials that are present with water. This mechanism was highlighted for hydrogen air systems by Reiser et. al, in

Electrochem. Solid-State Lett. 8,6 pp. A273-A276 (2005). Depending on the materials in use, the source of carbon may include one or all of the following: high surface carbon as bromine reduction / bromide oxidation catalyst, graphitized carbon in the GDL, and/or

graphitized/amorphous carbon in the bipolar plate / current collector of the cell. Hence, the carbon in these "hydrogen-starved" regions of the bromine electrode will be rapidly corroded, leading to loss of active surface area, performance, and eventually structural integrity.

[0046] Various embodiments of the present invention provide methods of inhibiting corrosion of a hydrogen electrode caused during a charging, discharging, startup, shutdown and non-operating cycle of a hydrogen bromide flow battery cell (described above), each method comprising maintaining a hydrogen pressure over the hydrogen electrode during one or more of the charging, discharging, startup, shutdown, or non-operating cycles, such that said hydrogen pressure is sufficient to inhibit (poly)bromide / bromine corrosion of the hydrogen electrode. In certain embodiments, the corrosion is otherwise caused by crossover (poly)bromide or bromine, or bromine generated at the hydrogen electrode. Hydrobromic acid may be generated at the hydrogen electrode by the reduction of crossover (poly)bromide during the charging or discharging cycle of the operating cell.

[0047] While described in terms of a hydrogen bromide flow battery cell, the invention is not necessarily limited only to individual cells. That is, it should be appreciated that the embodiments directing the use of these principles to multiple cells arranged in one or more stack arrangements are also within the scope of the present invention. The skilled artisan will understand the concept of a stack arrangement of cells, and their associated configurations.

[0048] The methods which are described as "inhibiting corrosion" include those methods wherein the degree of corrosion / dissolution of the hydrogen electrode is at least reduced, if not completely eliminated, relative to the degree of corrosion which would otherwise occur in the absence of hydrogen. In certain individual embodiments, a method may be quantified as reducing the degree of corrosivity by at least about 25%, 50%, 60%, 70%, 80%, 90% or 95%, as measured on a weight % basis by loss of electrode mass, relative to the initial mass of the electrode, or increase in dissolved metal.

[0049] Still other embodiments include those methods further comprising monitoring the hydrogen pressure during at least one of the charging, discharging, startup, shutdown, or non- operating cycles. It is reiterated here that the term "during at least one of the charging, discharging, startup, shutdown, or non-operating cycles" includes one or more of the transitory conditions described in this context or a non-operating or de-energized condition and any combination of transitory or non-operating conditions, and each set of conditions is considered an individual method embodiment.

[0050] Unless otherwise indicated, the term hydrogen or hydrogen pressure refers to a gas or an atmosphere which is substantially (>95 mol%) molecular hydrogen. However, in certain embodiments, the term hydrogen pressure may also refer to an atmosphere which contains hydrogen admixed with at least one non-oxidizing fluid, preferably a non-reactive liquid or an inert gas, e.g., nitrogen or argon. Individual embodiments include those wherein the admixture comprises at least about 10, 20, 30, 40, 50, 60, 70, 80, or 90 mol% hydrogen, the balance comprising the non-oxidizing fluid / inert gas. As used herein, the term "inert" refers to a substance which does not chemically react deleteriously with the system. In various embodiments, adequate hydrogen pressure is present during a non-operating state of the flow battery, such that a stack of at least one cell may be de-energized to maintain an average cell voltage of < 0.2 V/cell as measured by the total stack voltage divided by the number of cells.

[0051] In certain further embodiments, the hydrogen pressure is maintained at a predetermined pressure above ambient pressure. In exemplary cases, the hydrogen pressure is maintained at least 2 psi, preferably at least 5 psi, above ambient pressure. Such pressure may be maintained by monitoring the system pressure through use of a pressure gauge connected to a valved or regulated source of hydrogen and which, on indication of a pressure drop below a certain threshold (defined with respect to the predetermined pressure), the valve or regulator can be actuated to introduce sufficient hydrogen to maintain the desired or predetermined pressure.

[0052] Such monitoring systems may also be used to avoid or minimize the effects of cross-over bromine / bromide on the hydrogen electrode, accompanying hydrogen depletion, for example during discharging or non-operating conditions. In such cases, the system may alternatively be configured to enter a charging cycle - where hydrogen is actually generated at the hydrogen catalyst - immediately when a monitor "senses" that the hydrogen pressure over the hydrogen electrode falls below a predetermined level (and possible resulting in a rise in potential at the hydrogen electrode), for a time and with current sufficient at least to consume any cross-over bromine. Such a charging may be maintained, for example, until another source of hydrogen (e.g., from a piped source) is provided. The system is then returned to the discharge or shutdown condition. Such a triggering mechanism ensures that hydrogen is present throughout the hydrogen catalyst and ensures low potential.

[0053] In certain circumstances (e.g., when a cell is one of a plurality of cells in a stack arrangement; as described above), it may be useful to monitor the cell voltages (reflecting the "effective hydrogen pressure") of each individual hydrogen electrode within the stack for hydrogen starvation. While the hydrogen pressure above the stack as a whole may still be above the predetermined pressure, a given cell within the stack may be compromised and be subject to hydrogen starvation; for example, if the cell is liquid flooded and the hydrogen electrode material is coated with liquid and subject to corrosion. Accordingly, in certain embodiments, the "effective hydrogen pressure" above a given hydrogen electrode may be monitored by measuring the individual and/or average cell voltage (from a portion of a stack, for example). In such cases, a change in the measured voltage of an individual electrode may be used to trigger the onset of the charging condition.

[0054] In general, and as described above, the hydrogen electrode contains a catalyst for the oxidation of hydrogen gas or reduction of protons which comprises at least one of the platinum group metals, which includes ruthenium, rhodium, palladium, osmium, iridium, and platinum, and/or any alloy or admixture thereof. The metals of the hydrogen electrode catalyst may be in the form of nanoparticles or a sputtered or sprayed film or array distributed on a conductive support (e.g., carbon), preferably a high surface area conductive support. Platinum and iridium are preferred electrode catalyst materials. Platinum is most preferred.

[0055] To this point, the embodiments have been described only in terms of the conditions affecting the hydrogen electrode. However, additional embodiments also further provide for conditions which affect the bromide electrode. For example, while maintaining a hydrogen pressure over the hydrogen electrode during a transitory or de-energized state, hydrogen or an inerting media may be introduced or made to envelop the bromine electrode. In certain embodiments, then, methods for inhibiting corrosion of the hydrogen electrode comprises maintaining a hydrogen pressure over the hydrogen electrode during one or more of the charging operation, discharging operation, startup, shutdown, or non-operating cycles and further comprises providing or maintaining a hydrogen pressure over the bromine electrode during at least one of these cycles. Again, the methods providing and/or maintaining a hydrogen pressure over at least a portion of the bromine electrode during the shutdown and during the non- operating cycle are considered to be individual embodiments. When hydrogen pressure is provided or maintained over each of the hydrogen and bromine electrodes, the rate of addition or maintained pressure may be the same or different to each half cells containing each electrode.

[0056] The means of achieving the same or different pressures over the individual half cells of a given cell or stack of cells containing each electrode is considered to be within the skillset of the ordinary artisan. For the sake of completeness, though, as used herein, such hydrogen pressure management may be realized by the use of separate individual piping or other distributing means of conveying the hydrogen into each half cell (for example the use of manifolds or flow paths to fluidicly connect the hydrogen and bromine half cells) or through use of regulated gas shunts between the hydrogen and bromine/bromide half cells. Also, as described above, for example, during discharge or non-operating cycles, hydrogen may be generated by an intermittent, temporary return to a charging state.

[0057] In additional embodiments, a shutdown cycle further comprises (a) separating the bromine electrode from the bulk electrolyte, for example by draining the electrolyte from the bromine electrode and optionally replacing the electrolyte with a non-oxidizing fluid; and (b) operating the cell and / or contacting the bromine electrode with hydrogen for a time sufficient to consume any residual bromine remaining in contact with the bromine electrode after separating from the bulk electrolyte.

[0058] In certain of these embodiments, a method comprises maintaining hydrogen pressure over the hydrogen electrode during the shutdown cycle while also introducing hydrogen into the bromine electrode during shutdown after the HBr/Br₃ ^~ liquid has been drained to consume the residual Br₂/ Br₃ ^". An excess of hydrogen can be directed to this side to set the bromine electrode at hydrogen potential as well. The flow of hydrogen can be stopped as soon as the bromine electrode comes down to hydrogen potentials or the cell voltage gets close to zero. This, in addition to maintaining hydrogen at the negative electrode will further ensure that the hydrogen electrode will not see a rise in potential to the levels that could cause platinum dissolution.

[0059] The non-oxidizing fluid may comprise water, hydrogen, an inert gas, or a combination thereof. The term "inert gas" is described above; demonstrative examples include nitrogen or argon, as contrasted to air or oxygen. When used, water may also contain electrolytes or surfactants, provided their use does not otherwise compromise the startup and operation cycles of the system.

[0060] While it is preferred that both the hydrogen and bromine electrodes be maintained under hydrogen pressure during shutdown and non-operating cycles, a strategy of maintaining the hydrogen electrode under hydrogen and the bromine electrodes under inert conditions during these cycles also operates to inhibit the undesirable corrosion.

[0061] Another embodiment provides monitoring the average stack potential during shutdown, and then subsequently controlling the feed of hydrogen to the bromine electrode, and/or the pressure of the hydrogen on the hydrogen electrode until the potential is maintained below a desired level. This desired level is substantially close to 0.0 V which indicates a fully chemically shorted cell.

[0062] The present invention also encompasses the use of a low-resistance contact between the hydrogen and bromine electrodes in combination with one or more of the preceding embodiments, said low-resistance contact capable of maintaining a potential difference of less than 0.1 V between the hydrogen and bromine electrodes at least during the non-operating cycle. Similarly, still another method comprises maintaining adequate pressure of the hydrogen on the hydrogen electrode during the process and duration of shutdown. If at any time a non-zero voltage starts to develop in the stack as measured by the stack voltage sensor, then a low- resistance contactor shorts the positive and negative terminals of the stack to rapidly consume any remaining bromine in the stack. During this process the maintenance of hydrogen pressure on the hydrogen electrode ensures that adequate hydrogen is available to facilitate this consumption.

[0063] Alternatively, the individual cell(s) can be constructed so as to provide for a small, albeit constant, amount of hydrogen remaining in contact with at least a portion of the bromine electrode. As used herein, the term "at least a portion" refers to any amount of the bromide electrode (e.g., greater than about 5 area%) up to and including the entire electrode, especially when the bromide electrode is not in contact with the HBr / bromine electrolyte. This may be achieved, for example, by providing a piped source which delivers hydrogen across a portion of the bromide electrode. In providing access to the bromide by the hydrogen, the bromide electrode remains electrochemically shorted within each cell, such that when the cell is deprived of bromine / poly -bromide species this electrochemical short provides a means for rapid inerting and de-energizing of the cell-stack in the shutdown and non-operating cycles. [0064] Combining several of these embodiments, another method involves the use of an inert gas purge to remove bromine from the system, thereby avoiding the efficiency loss that may result from the use of hydrogen. This method, implemented during the transition from charging or discharging into an non-operating or de-energized state, comprises (a) maintaining hydrogen pressure on the hydrogen electrode throughout; (b) replacing the bromine reactant with some diluent, which may be hydrogen, deionized water, nitrogen, or some other non-oxidizing species; (c) when the cell stack potential has dropped below a pre-determined level (e.g. 0.2 V/cell) ceasing the replacement on the bromine electrode; (d) closing a low resistance contactor across the positive and negative terminals of the cell or stack for the duration of the idle period, with the possible inclusion of maintaining finite hydrogen pressure on the hydrogen electrode; (e) when start-up is required, removing the contactor and feeding the hydrogen electrode with the appropriate flow and pressure of hydrogen; and then (f) initiating the flow of reactant to the bromine electrode.

[0065] 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, the various embodiments of the preceding disclosure are described in terms of methods of inhibiting corrosion of a hydrogen electrode. It should be appreciated that the physical devices needed to realize these methods are also embodiments of the present invention. Further, 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.

[0066] 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 inhibiting corrosion of a hydrogen electrode arising from crossover bromine or (poly)bromide ions during at least one of a charging, discharging, startup, shutdown and/or non-operating cycle of an HBr flow battery cell or stack of cells, said method comprising maintaining a hydrogen pressure over the hydrogen electrode during one or more of the charging, discharging, startup, shutdown, or non-operating cycles, said hydrogen pressure sufficient to inhibit bromide/bromine corrosion of the hydrogen electrode, wherein

the HBr flow battery cell comprises a hydrogen electrode, a bromine electrode, and a proton exchange membrane interposed therebetween;

said HBr flow battery cell functions through the charging operation, discharging operation, startup, shutdown, and non-operating cycles; and

the bromine electrode is in contact with a bromine/bromide electrolyte in a bromine half-cell during the operating cycle of the HBr flow battery cell or stack of cells.

2. The method of claim 1, further comprising monitoring the hydrogen pressure over the hydrogen electrode during at least one of the charging, discharging, startup, shutdown, or non-operating cycles.

3. The method of claim 1 or 2, further comprising maintaining the hydrogen pressure over the hydrogen electrode at a predetermined pressure above ambient pressure.

4. The method of any of the preceding claims, wherein the hydrogen electrode comprises platinum or iridium.

5. The method of any of the preceding claims, wherein the hydrogen electrode comprises platinum.

6. The method of any of the preceding claims, wherein the hydrogen pressure is sufficient to maintain a cell voltage of less than about 0.2 V per cell during at least one of the start-up, shutdown, or non-operating cycles

7. The method of any of the preceding claims, further comprising providing or maintaining a hydrogen pressure over the bromine electrode during at least one of the startup, shutdown, or non-operating cycles.

8. The method of any of the preceding claims, further comprising providing and/or

maintaining a hydrogen pressure over at least a portion of the bromine electrode during the shutdown cycle.

9. The method of any of the preceding claims, further comprising providing and/or

maintaining a hydrogen pressure over at least a portion of the bromine electrode during the non-operating cycle.

10. The method of any one of claims 7-9, wherein the hydrogen pressure over the bromine electrode is the same as the pressure as the pressure of hydrogen over the hydrogen electrode

11. The method of any of the preceding claims (including claim 1), wherein the shutdown cycle further comprises: a. separating the bromine electrode from the bulk electrolyte; and

b. contacting the bromine electrode with hydrogen for a time sufficient to consume any residual bromine remaining in contact with the bromine electrode after separating from the bulk electrolyte.

12. The method of claim 1 1, wherein separating the bromine electrode from the electrolyte comprises draining the electrolyte from the bromine half-cell and replacing it with a non- oxidizing fluid.

13. The method of claim 12, wherein the non-oxidizing fluid comprises water, hydrogen, an inert gas, or a combination thereof.

14. The method of claim 1, further comprising providing or maintaining a non-oxidizing environment over the bromine electrode during at least one of the shutdown or non- operating cycles.

15. The method of claim 14, wherein the non-oxidizing environment comprises water or an inert gas

16. The method of claim 1, further comprising providing a low-resistance contact between the hydrogen and bromine electrodes, said low-resistance contact capable of maintaining a potential difference of less than 0.1 V between the hydrogen and bromine electrodes at least during the non-operating cycle.

17. The method of claim 8 or 9, wherein the hydrogen pressure is maintained over the entire bromine electrode.

18. The method of claim 3, comprising monitoring the hydrogen pressure during the

discharging and non-operating cycles, wherein said monitoring is capable of triggering a change of operation to a charging cycle when the hydrogen pressure falls below the predetermined pressure.

19. The method of claim 18, further comprising operating the flow battery cell or stack of cells in a charging cycle for a time and with current sufficient at least to consume any cross-over bromine.