Electrolyte Management for Rebalancing Cells

Abstract

A system and method for a redox flow battery system is provided. In one embodiment, the redox flow battery system includes one or more redox flow battery cells, a rebalancing cell that receives electrolyte from the one or more redox flow battery cells and catalyzes hydrogen reduction of metal cations in the electrolyte, and a conductive coupler that connects the rebalancing cell to an external voltage source. The conductive coupler enables application of an ionic repulsion potential to a stack of electrode assemblies of the rebalancing cell during operation of the redox flow battery system.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 267,522, entitled “ELECTRODE MANAGEMENT FOR A REBALANCING CELL,” filed February 3, 2022. The above-listed applications are hereby incorporated by reference in their entireties for all purposes. [Background technology]

[0002] The present description relates generally to systems and methods for storing electrolytes for use in redox flow battery systems, and for flowing and rebalancing the electrolytes during operation of the redox flow battery systems. Summary of the Invention

[0003] Redox flow batteries are suitable for grid-scale energy storage applications due to their ability to scale power and capacity independently, and are suitable for charging and discharging over thousands of cycles with little performance loss compared to conventional battery technologies. All-iron hybrid redox flow batteries are particularly attractive because they incorporate low-cost, earth-abundant materials. In general, iron redox flow batteries (IFBs) rely on iron, salt, and water as the electrolyte, and are therefore simple, contain earth-abundant, and inexpensive materials, eliminate the incorporation of harsh chemicals, and reduce their environmental footprint.

[0004] The IFB is a positive electrode where the redox reaction occurs, and a ferrous metal (Fe 2+ ) can be reduced and plated. Various side reactions, including proton reduction, iron corrosion, and iron plating oxidation, can occur at the negative (plating) electrode where Fe 2+ May conflict with reduction:

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[0005] In some embodiments, the electrolyte rebalancing of equation (4) can be achieved via rebalancing cells configured as fuel cells, where H2 gas and electrolyte can contact each other at the catalyst surface while applying a direct current (DC) between a pair of positive and negative electrodes. However, reliability issues can arise in the fuel cell as a result of an inadvertent reverse current spike that interrupts the DC flow. In other embodiments, the rebalancing cells can have a trickle bed or jelly roll reactor setup, either of which can similarly contact H2 gas and electrolyte at the catalyst surface of each rebalancing cell.

[0006] In some embodiments, to achieve a relatively high rebalancing performance with a relatively small amount of H2 gas, the rebalancing cell may include a stack of electrode assemblies, each of which may include a positive electrode and a negative electrode in surface-to-surface contact with one another, such that the positive and negative electrodes may be continuously conductive (e.g., at the surfaces of the positive and negative electrodes in surface-to-surface contact). In additional or alternative embodiments, current may not be directed away from the rebalancing cell. In this manner, electrolyte rebalancing within the rebalancing cell may be driven via an internal electrical short of the bonded pair of internal positive and negative electrodes. Furthermore, in some embodiments, the rebalancing cell may be configured to draw out each of the liquid electrolyte and H2 gas (e.g., via forced convection, gravity feed, capillary action, etc.). By managing the flow of electrolyte and H2 gas in this manner, in combination with the internal electrical short, the Fe of the rebalancing cell may be reduced. 3+ The reduction rate can be increased (eg, 20-fold or more) compared to conventional rebalancing cell installations.

[0007] The surface of the negative electrode in the rebalancing cell requires the catalyst of equation (4) to be energetically favorable. Over time, the catalyst surface may adsorb anions from the electrolyte, which in turn attract cations creating a diffuse double layer at the catalyst surface. The diffuse double layer may prevent desired reactants from reaching the catalyst surface, thus poisoning the catalyst and hindering the performance of the rebalancing cell.

[0008] In one embodiment, the above problem can be addressed by a redox flow battery system comprising one or more redox flow battery cells, a rebalancing cell configured to catalyze hydrogen reduction of metal cations and receiving electrolyte from the one or more redox flow battery cells, and a conductive coupler connecting the rebalancing cell to an external voltage source. The conductive coupler allows application of an ionic repulsion potential to the stack of electrode assemblies of the rebalancing cell during operation of the redox flow battery system. In this manner, the applied ionic repulsion potential can interrupt the formation of a diffuse double layer, thus maintaining the catalytic performance of the rebalancing cell.

[0009] As an example, the rebalancing cell may be fitted with a conductive rod inserted between each of the multiple electrode assemblies collectively forming the rebalancing cell. The conductive rod may establish a conductive path through the rebalancing cell. A conductive coupler may be connected to a base plate of the rebalancing cell and may extend between the rebalancing cell and an external voltage source, which is coupled to a counter electrode located in the electrolyte tank. By connecting the conductive coupler to the rebalancing cell, a potential may be applied to each of the multiple electrode assemblies. The potential may repel ions from the surface of a catalyst disposed on the negative electrode of the multiple electrode assemblies, thus preventing the formation of a diffuse double layer at the catalyst surface. Thus, the degradation of catalyst performance is mitigated.

[0010] It should be appreciated that the foregoing summary is provided to introduce in a simplified form a selection of concepts that are further described in the Detailed Description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined solely by the claims that follow the Detailed Description. Moreover, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. [Brief description of the drawings]

[0011] [Figure 1]FIG. 1 shows a schematic diagram of an exemplary redox flow battery system including a battery cell having a redox electrode and a plating electrode fluidly coupled to an electrolyte management system. [Figure 2A] FIG. 2 shows a first perspective view of a first embodiment of a rebalancing cell including a stack of internal shorting electrode assemblies. [Figure 2B] FIG. 2B shows a second perspective view of the rebalancing cell of FIG. 2A. [Diagram 3] FIG. 3 shows an exploded view of the electrode assembly for the rebalancing cell of FIGS. 2A and 2B. [Figure 4A] FIG. 2 shows an isometric view of a second embodiment of a rebalancing cell electrically coupled to an external voltage source. [Figure 4B] 4B shows a cross-sectional view of the rebalancing cell of FIG. 4A. [Figure 4C] FIG. 4C shows a detailed view of the cross-sectional area of ​​FIG. 4B. [Diagram 5] 1 shows a schematic diagram of an electrolyte management system. [Figure 6] 1 shows a flow chart of an example of a method for maintaining performance of a rebalancing cell by applying an ion repulsion potential. [Figure 7] 4B shows a graph illustrating the performance of the rebalancing cell of FIG. 4A operated with and without an applied ion repulsion potential. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0012] The following description relates to systems and methods for maintaining catalytic performance of a rebalancing cell, the rebalancing cell being powered via an internal electrical short of an electrode assembly contained therein. In an exemplary embodiment, the rebalancing cell may be fluidly coupled to an electrolyte subsystem, each contained within the electrolyte subsystem of a redox flow battery system. A redox flow battery system is depicted in FIG. 1 as having an integrated multi-chamber tank with separate positive and negative electrolyte chambers (e.g., for storing positive and negative electrolytes, respectively) and respective gas headspaces (e.g., for storing H2 gas). In some examples, the redox flow battery system may include an all-iron flow battery (IFB), which utilizes iron redox chemistry at both the positive (redox) and negative (plating) electrodes of the IFB. The electrolyte chamber may be coupled to one or more battery cells, each cell including a positive electrode and a negative electrode. The electrolyte may be pumped through the cell's positive and negative electrode compartments, which house the positive and negative electrodes, respectively.

[0013] In some examples, the redox flow battery system may include a hybrid redox flow battery. A hybrid redox flow battery is a redox flow battery that may be characterized by the deposition of one or more electroactive materials as a solid layer on an electrode (e.g., a negative electrode). A hybrid redox flow battery may include, for example, chemical species that may plate via electrochemical reactions as solids on a substrate throughout the battery charging process. During battery discharge, the plated species may ionize and become soluble in the electrolyte via further electrochemical reactions. In a hybrid redox flow battery system, the charge capacity (e.g., maximum amount of energy stored) of the redox flow battery may be limited by the amount of metal plated during battery charging and may therefore depend on the efficiency of the plating system as well as the volume and surface area available for plating.

[0014] In some embodiments, the electrolytic imbalance in a redox flow battery can be mediated by hydrogen (H) gas production reactions such as proton reduction and iron corrosion:

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[0015] As described by the embodiments herein, sufficient Fe for higher performance applications 3+ The reduction rate can be reliably achieved at lower H2 gas partial pressures through a rebalancing cell of an electrolyte management system, such as the first exemplary rebalancing cell depicted in Figures 2A and 2B, which includes a stack of internally shorted electrode assemblies. An example of one of the electrode assemblies is shown in Figure 3. The performance of the rebalancing cell, e.g., the reduction rate, can be maintained by applying a voltage, e.g., an ion-repulsion voltage or potential, across the electrode assemblies. Figures 4A-4C show a second example of a rebalancing cell of an electrolyte management system configured to connect a stack of electrode assemblies to an external voltage source. A schematic diagram of the electrolyte management system is shown in Figure 5. An example of a method used to protect the catalyst of the rebalancing cell is shown in Figure 6. Experimental results comparing the performance of a rebalancing cell with an ion-repulsion voltage applied to it and a rebalancing cell without an ion-repulsion voltage applied are illustrated in Figure 7.

[0016] 1, in the redox flow battery system 10, the negative electrode 26 may be referred to as the plating electrode and the positive electrode 28 may be referred to as the redox electrode. The negative electrolyte in the plating side (e.g., negative electrode compartment 20) of the redox flow battery cell 18 may be referred to as the plating electrolyte, and the positive electrolyte on the redox side (e.g., positive electrode compartment 22) of the redox flow battery cell 18 may be referred to as the redox electrolyte.

[0017] "Anode" refers to an electrode at which an electroactive material loses electrons, and "cathode" refers to an electrode at which an electroactive material gains electrons. During battery charging, the negative electrolyte gains electrons at the negative electrode 26, which is the cathode of the electrochemical reaction. During battery discharging, the negative electrolyte loses electrons, and the negative electrode 26 is the anode of the electrochemical reaction. Alternatively, during battery discharging, the negative electrolyte and the negative electrode 26 may be referred to as the anolyte and anode, respectively, of the electrochemical reaction, while the positive electrolyte and the positive electrode 28 may be referred to as the catholyte and cathode, respectively, of the electrochemical reaction. During battery charging, the negative electrolyte and the negative electrode 26 may be referred to as the catholyte and cathode, respectively, of the electrochemical reaction, while the positive electrolyte and the positive electrode 28 may be referred to as the anolyte and anode, respectively, of the electrochemical reaction. For simplicity, the terms "positive" and "negative" are used herein to refer to the electrodes, electrolytes, and electrode compartments in a redox flow battery system.

[0018] One example of a hybrid redox flow battery is an all-iron redox flow battery (IFB), in which the electrolyte includes iron ions in the form of iron salts (e.g., FeCl2, FeCl3, etc.) and the negative electrode 26 includes metallic iron. For example, ferrous iron (Fe 2+ ) gains two electrons during battery charging and becomes iron metal (Fe 0 ) on the negative electrode 26, and Fe 0 During battery discharge, it loses two electrons to become Fe 2+ At the positive electrode 28, Fe 2+ During battery charging, ferric iron (Fe 3+ ) and Fe 3+During battery discharge, it gains electrons to form Fe 2+ The electrochemical reactions are summarized in equations (5) and (6), where the forward reaction (left to right) represents the electrochemical reaction during battery charging, while the reverse reaction (right to left) represents the electrochemical reaction during battery discharging.

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[0019] As mentioned above, the negative electrolyte used in the IFB is Fe 2+ accepts two electrons from the negative electrode 26 to form Fe 0 and plated onto the substrate. 2+ During battery discharge, the plated Fe 0 loses two electrons to become Fe 2+ and can dissolve back into the electrolyte. The equilibrium potential of the above reaction is -0.44 V, so this reaction provides the negative terminal for the desired system. On the positive side of the IFB, the electrolyte loses electrons and becomes Fe 3+ During battery charging, Fe is oxidized to 2+ During battery discharge, Fe provided by the electrolyte 3+ by absorbing electrons provided by the positive electrode 28. 2+ The equilibrium potential for this reaction is +0.77V, creating a positive terminal in the desired system.

[0020] The IFB may provide the ability to charge and recharge the electrolyte therein, in contrast to other battery types that utilize non-regenerative electrolytes. Charging may be accomplished by applying a current across electrodes 26 and 28 via terminals 40 and 42, respectively. The negative electrode 26 contains (e.g., Fe 2+ But Fe 3+ The negative side of a voltage source may be electrically coupled via terminal 40 such that electrons may be delivered to the negative electrolyte via positive electrode 28 (as Fe is oxidized to Fe 2+ is reduced to form Fe on the (plating) substrate.0 and Fe 2+ is plated onto the negative electrode 26.

[0021] Discharge is Fe 0 remains available in the negative electrolyte for oxidation, and Fe 3+ As an example, Fe can persist for as long as Fe remains available in the positive electrolyte for reduction. 3+ Availability may be increased by adding additional Fe via an external source such as the external positive electrolyte chamber 52. 3+ This can be maintained by increasing the concentration or volume of the positive electrolyte in the positive electrode compartment 22 of the redox flow battery cell 18 to provide ions. More generally, during discharge, 0 The availability of Fe can be an issue in IFB systems, where the Fe available for discharge 0 can be proportional to the surface area and volume of the negative electrode substrate and the plating efficiency. The charging capacity is 2+ As an example, Fe 2+ Availability is via an external source, such as the external negative electrolyte chamber 50, for additional Fe 2+ This can be maintained by providing ions to increase the concentration or volume of negative electrolyte to the negative electrode compartment 20 side of the redox flow battery cell 18.

[0022] In an IFB, the positive electrolyte may include ferrous, ferric, ferric complexes, or any combination thereof, while the negative electrolyte may include ferrous or ferrous complexes, depending on the state of charge (SOC) of the IFB system. As previously mentioned, the utilization of iron ions in both the negative and positive electrolytes may enable utilization of the same electrolyte species on both sides of the redox flow battery cell 18, which may reduce electrolyte cross-contamination and increase the efficiency of the IFB system, resulting in less electrolyte exchange compared to other redox flow battery systems.

[0023] Efficiency losses in the IFB can result from electrolyte crossover through the separator 24 (e.g., ion exchange membrane barrier, microporous membrane, etc.). For example, Fe in a positive electrolyte 3+ The ion is Fe 3+ The negative electrolyte can then be driven toward the negative electrolyte by the ion concentration gradient and the electrophoretic force across the separator 24. The Fe ions then penetrate the separator 24 and cross over into the negative electrode compartment 20. 3+ Fe ions crossing over from the low pH redox side (e.g., the more acidic positive electrode compartment 22) to the high pH plating side (e.g., the less acidic negative electrode compartment 20) can result in coulombic efficiency losses. 3+ The ions may result in the precipitation of Fe(OH)3. The precipitation of Fe(OH)3 may degrade the separator 24 and cause permanent battery performance and efficiency loss. For example, Fe(OH)3 precipitates may chemically foul the organic functional groups of the ion exchange membrane or physically clog the micropores of the ion exchange membrane. In either case, the Fe(OH)3 precipitates may cause the membrane's ohmic resistance to increase over time and reduce battery performance. The precipitates may be removed by washing the IFB with acid, but the constant maintenance and downtime may be disadvantageous for commercial battery applications. Furthermore, washing may depend on periodic preparation of the electrolyte, contributing to additional processing costs and complexity. Alternatively, the addition of certain organic acids to the positive and negative electrolytes in response to electrolyte pH changes may mitigate precipitate formation during battery charge and discharge cycling without driving up overall costs. Additionally, Fe 3+ Implementing a membrane barrier that inhibits ionic crossover may also mitigate fouling.

[0024] Additional coulombic efficiency loss, H + (e.g., protons) and subsequent formation of H2 gas, and the reaction of protons in the negative electrode compartment 20 with electrons supplied to the plated iron metal of the negative electrode 26, may cause the formation of H2 gas.

[0025] IFB electrolytes (e.g., FeCl2, FeCl3, FeSO4, Fe2(SO4)3, etc.) may be readily available and may be produced at low cost. In one example, the IFB electrolyte may be formed from ferrous chloride (FeCl2), potassium chloride (KCl), manganese(II) chloride (MnCl2), and boric acid (H3BO3). The IFB electrolyte may provide a higher reuse value since the same electrolyte may be used for the negative and positive electrolytes, resulting in reduced cross-contamination issues compared to other systems. Furthermore, due to the electron configuration of iron, it may solidify into a generally uniform solid structure during plating onto the negative electrode substrate. In the case of zinc and other metals commonly used in hybrid redox batteries, solid dendritic structures may form during plating. The stable electrode morphology of the IFB system may increase the efficiency of the battery compared to other redox flow batteries. Still further, the iron redox flow battery may reduce the use of toxic raw materials and may operate at a relatively neutral pH compared to other redox flow battery electrolytes. Therefore, the IFB system may pose less environmental hazard than all other current advanced redox flow battery systems in production.

[0026] Continuing with FIG. 1, a schematic diagram of a redox flow battery system 10 is shown. The redox flow battery system 10 may include a redox flow battery cell 18 fluidly coupled to an integrated multi-chamber electrolyte storage tank 110 via an electrolyte flow path 124. The redox flow battery cell 18 may include a negative electrode compartment 20, a separator 24, and a positive electrode compartment 22. The separator 24 may include an electrically insulating ionically conductive barrier that prevents bulk mixing of the positive and negative electrolytes while allowing conduction of certain ions. For example, as discussed above, the separator 24 may include an ion exchange membrane and / or a microporous membrane.

[0027] The negative electrode compartment 20 may include a negative electrode 26, and the negative electrolyte may include an electroactive material. The positive electrode compartment 22 may include a positive electrode 28, and the positive electrolyte may include an electroactive material. In some embodiments, multiple redox flow battery cells 18 may be combined in series or parallel to generate higher voltages or currents in the redox flow battery system 10.

[0028] 1 are negative and positive electrolyte pumps 30 and 32, both of which are used to pump electrolyte solution through the redox flow battery system 10 via the electrolyte flow path 124. Electrolyte is stored in one or more tanks external to the cell and is pumped through the negative electrode compartment 20 and positive electrode compartment 22 sides of the redox flow battery cell 18 via the negative and positive electrolyte pumps 30 and 32, respectively.

[0029] The redox flow battery system 10 may also include a first bipolar plate 36 and a second bipolar plate 38, each positioned along a back side of the negative electrode 26 and the positive electrode 28, respectively, e.g., opposite the side facing the separator 24. The first bipolar plate 36 may be in contact with the negative electrode 26 and the second bipolar plate 38 may be in contact with the positive electrode 28. However, in other embodiments, the bipolar plates 36 and 38 may be arranged in close proximity to the electrodes 26 and 28, but spaced apart from the electrodes 26 and 28 and housed within the respective electrode compartments 20 and 22. In either case, the bipolar plates 36 and 38 may be electrically coupled to terminals 40 and 42, respectively, via direct contact with the negative and positive electrodes 26 and 28, respectively, or via the negative and positive electrodes 26 and 28, respectively. The IFB electrolyte may be transported to the reaction sites of the negative and positive electrodes 26 and 28 by the first and second bipolar plates 36 and 38 due to the conductive properties of the materials of the bipolar plates 36 and 38. The flow of electrolyte may also be assisted by the negative and positive electrolyte pumps 30 and 32 to facilitate forced convection through the redox flow battery cell 18. Reacted electrochemical species may also be directed away from the reaction sites by a combination of forced convection and the presence of the first and second bipolar plates 36 and 38.

[0030] As illustrated in FIG. 1 , the redox flow battery cell 18 may further include a negative battery terminal 40 and a positive battery terminal 42. When a charging current is applied to the battery terminals 40 and 42, the positive electrolyte may be oxidized (lose one or more electrons) at the positive electrode 28 and the negative electrolyte may be reduced (gain one or more electrons) at the negative electrode 26. During battery discharge, reverse redox reactions may occur on the electrodes 26 and 28. In other words, the positive electrolyte may be reduced (gain one or more electrons) at the positive electrode 28 and the negative electrolyte may be oxidized (lose one or more electrons) at the negative electrode 26. A potential difference across the battery may be maintained by electrochemical redox reactions in the positive electrode compartment 22 and the negative electrode compartment 20, which may induce a current through the current collectors while the reactions persist. The amount of energy stored by a redox battery may be limited by the amount of electroactive material available in the electrolyte for discharge, depending on the total amount of electrolyte and the solubility of the electroactive material.

[0031] The redox flow battery system 10 may further include an integrated multi-chamber electrolyte storage tank 110. The multi-chamber electrolyte storage tank 110 may be included in the electrolyte management system 140 of the redox flow battery system 10, and the interior volume of the multi-chamber electrolyte storage tank 110 may be divided by a bulkhead 98. The bulkhead 98 may create multiple chambers in the multi-chamber electrolyte storage tank 110 such that both positive and negative electrolytes may be included in a single tank. The negative electrolyte chamber 50 holds a negative electrolyte including an electroactive material, and the positive electrolyte chamber 52 holds a positive electrolyte including an electroactive material. The bulkhead 98 may be positioned in the multi-chamber electrolyte storage tank 110 to provide a desired volume ratio between the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In one embodiment, the bulkhead 98 may be positioned to set the volume ratio of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 according to a stoichiometric ratio between the negative redox reaction and the positive redox reaction. FIG. 1 further illustrates the fill height 112 of the multi-chamber electrolyte storage tank 110, which may indicate the liquid level in each tank compartment. FIG. 1 also illustrates a gas headspace 90 located above the fill height 112 of the negative electrolyte chamber 50, and a gas headspace 92 located above the fill height 112 of the positive electrolyte chamber 52. The gas headspace 92 may be utilized to store H2 gas that is generated during the operation of the redox flow battery (e.g., by the side reaction of proton reduction and iron corrosion) and is carried to the multi-chamber electrolyte storage tank 110 with the electrolyte returning from the redox flow battery cell 18. The H2 gas may naturally separate at the gas-liquid interface (e.g., the fill height 112) in the multi-chamber electrolyte storage tank 110, thereby eliminating the need to have an additional gas-liquid separator as part of the redox flow battery system 10. Once separated from the electrolyte, the H2 gas may fill the gas headspaces 90 and 92.In this manner, the stored H2 gas may serve to purge other gases from the multi-chamber electrolyte storage tank 110, thereby acting as an inert gas blanket to reduce oxidation of electrolyte species, which may help to reduce redox flow battery capacity loss. In this manner, utilizing an integrated multi-chamber electrolyte storage tank 110 may eliminate the need to have separate negative and positive electrolyte storage tanks, hydrogen storage tanks, and gas-liquid separators common to conventional redox flow battery systems, thereby simplifying the system design, reducing the physical footprint of the redox flow battery system 10, and reducing system costs.

[0032] FIG. 1 also shows a spillover hole 96, which may create an opening in a bulkhead 98 between the gas headspaces 90 and 92 and provide a means of equalizing gas pressure between the chambers 50 and 52. The spillover hole 96 may be located at a threshold height above the fill height 112. The spillover hole 96 may further enable the ability to self-balance the electrolyte in each of the negative and positive electrolyte chambers 50 and 52 in the event of a cell crossover. In the case of an all-iron redox flow battery system, the same electrolyte (Fe 2+) may be used in both the negative and positive electrode compartments 20 and 22, so that although electrolyte spillover between the negative electrolyte chamber 50 and the positive electrolyte chamber 52 may reduce overall system efficiency, overall electrolyte composition, battery module performance, and battery module capacity may be maintained. Flange joints may be utilized for all plumbing connections between the inlet and outlet multi-chamber electrolyte storage tank 110 to maintain a leak-free, continuously pressurized condition. The multi-chamber electrolyte storage tank 110 may include at least one outlet from each of the negative and positive electrolyte chambers 50 and 52, and at least one inlet to each of the negative and positive electrolyte chambers 50 and 52. Additionally, one or more outlet connections may be provided from the gas headspaces 90 and 92 to direct H2 gas to the rebalancing reactors or cells 80 and 82, such that the rebalancing reactors or cells 80 and 82 may be fluidly coupled to the gas headspaces 90 and 92, respectively. The rebalancing reactors / cells 80 and 82 may also be included within an electrolyte management system 140, in addition to components that fluidly couple the rebalancing reactors / cells 80, 82 to the multi-chamber electrolyte storage tank 110 and devices that drive the flow of electrolyte therethrough.

[0033] As shown, the electrolyte flow path 124 may fluidly couple the integrated multi-chamber electrolyte storage tank 110 to the redox flow battery cell 18 and each of the rebalancing reactors / cells 80 and 82. In some embodiments, the electrolyte flow path 124 may be a closed flow path in the sense that during operation of the redox flow battery system 10, the negative and positive electrolytes may be circulated through only one redox flow battery cell 18 along the electrolyte flow path without entering any other redox flow battery cell or otherwise being discharged from the redox flow battery system 10. In such embodiments, if the redox flow battery system 10 includes multiple redox flow battery cells 18, multiple (closed) electrolyte flow paths 124 may be included in the redox flow battery system 10, with the number of the multiple electrolyte flow paths 124 being equal to the number of the multiple redox flow battery cells 18. In this manner, each of the redox flow battery cells 18 may be fluidly isolated from one another, thereby eliminating inter-stack short circuits via the negative and positive electrolytes. In other embodiments, the redox flow battery system 10 may include multiple redox flow battery cells 18 fluidly coupled to one another via electrolyte flow paths 124 .

[0034] The electrolyte flow path 124 may include a negative electrolyte flow loop 120 and a positive electrolyte flow loop 122, where the negative and positive electrolytes may be cycled through the negative electrolyte flow loop 120 and the positive electrolyte flow loop 122, respectively. The negative and positive electrolyte flow loops 120 and 122 may be completely or nearly completely fluidly isolated from one another (e.g., fluid coupling may occur only via spillover hole 96 when included in bulkhead 98 of integrated multi-chamber electrolyte storage tank 110).

[0035] As shown, the negative electrolyte flow loop 120 may cycle the negative electrolyte sequentially through the negative electrolyte chamber 50 of the integrated multi-chamber electrolyte storage tank 110, the negative electrolyte pump 30, the negative electrode compartment 20 of the redox flow battery cell 18, and the (negative) rebalancing reactor / cell 80, and from there back to the negative electrolyte chamber 50 of the integrated multi-chamber electrolyte storage tank 110. As further shown, the positive electrolyte flow loop 122 may cycle the positive electrolyte sequentially through the positive electrolyte chamber 52 of the integrated multi-chamber electrolyte storage tank 110, the positive electrolyte pump 32, the positive electrode compartment 22 of the redox flow battery cell 18, and the (positive) rebalancing reactor / cell 82, and from there back to the positive electrolyte chamber 52 of the integrated multi-chamber electrolyte storage tank 110. Thus, the electrolyte solution stored primarily in the multi-chamber electrolyte storage tank 110 may be pumped throughout the redox flow battery system 10 via the negative and positive electrolyte pumps 30 and 32, the electrolyte stored in the negative electrolyte chamber 50 may be pumped through the negative electrode compartment 20 side of the redox flow battery cell 18 via the negative electrolyte pump 30, and the electrolyte stored in the positive electrolyte chamber 52 may be pumped through the positive electrode compartment 22 side of the redox flow battery cell 18 via the positive electrolyte pump 32. In this manner, during operation of the redox flow battery system 10, the negative and positive electrolytes may be cycled through the redox flow battery cell 18 generally independently of one another via the negative and positive electrolyte flow loops 120 and 122, respectively.

[0036] Although not shown in FIG. 1 , the integrated multi-chamber electrolyte storage tank 110 may further include one or more heaters thermally coupled to each of the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In an alternative embodiment, only one of the negative and positive electrolyte chambers 50 and 52 may include one or more heaters. If only the positive electrolyte chamber 52 includes one or more heaters, the negative electrolyte may be heated by transferring heat generated in the redox flow battery cell 18 to the negative electrolyte. In this manner, the redox flow battery cell 18 may heat the negative electrolyte and facilitate temperature regulation of the negative electrolyte. The one or more heaters may be operated by the controller 88 to regulate the temperature of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 independently or together.

[0037] For example, in response to an electrolyte temperature falling below a threshold temperature, the controller 88 may increase the power provided to one or more heaters so that heat flux to the electrolyte may increase. The electrolyte temperature may be indicated by one or more temperature sensors attached to the multi-chamber electrolyte storage tank 110, such as sensors 60 and 62. By way of example, the one or more heaters may include a coil-type heater or other immersion heater immersed in the electrolyte fluid, or a surface mantle-type heater that conductively transfers heat through the walls of the negative and positive electrolyte chambers 50 and 52 to heat the fluid therein. Other known types of tank heaters may be used without departing from the scope of the present disclosure. Additionally, the controller 88 may deactivate one or more heaters in the negative and positive electrolyte chambers 50 and 52 in response to a liquid level falling below a solids fill threshold level. Stated another way, in some embodiments, the controller 88 may activate one or more heaters in the negative and positive electrolyte chambers 50 and 52 only in response to a liquid level above a solids fill threshold level. In this way, operating one or more heaters without sufficient liquid in the negative and / or positive electrolyte chambers 50, 52 can be avoided, thereby reducing the risk of the heater(s) overheating or burning out.

[0038] Still further, one or more inlet connections may be provided to each of the negative and positive electrolyte chambers 50 and 52 from a field hydration system (not shown). In this manner, the field hydration system may facilitate the start-up of the redox flow battery system 10, including installation, charging, and hydration of the redox flow battery system 10, at the end-use location. Furthermore, prior to the start-up of the redox flow battery system 10 at the end-use location, the redox flow battery system 10 may be dry assembled at a battery manufacturing facility different from the end-use location, without the need to charge and hydrate the redox flow battery system 10 before shipping it to the end-use location. In one embodiment, the end-use location may correspond to a location where the redox flow battery system 10 is installed and utilized for on-site energy storage. In other words, the redox flow battery system 10 may be designed such that once installed and hydrated at the end-use location, the location of the redox flow battery system 10 is fixed and the redox flow battery system 10 may no longer be considered a portable dry system. Thus, from an end user's perspective, the dry portable redox flow battery system 10 may be delivered to a site, after which the redox flow battery system 10 may be installed, hydrated, and commissioned. Prior to hydration, the redox flow battery system 10 may be referred to as a dry portable system, in which the redox flow battery system 10 does not include or is free of water and wet electrolyte. Once hydrated, the redox flow battery system 10 may be referred to as a wet non-portable system, in which the redox flow battery system 10 includes wet electrolyte.

[0039] The electrolyte rebalancing reactors / cells 80 and 82 may be connected in-line or in parallel with the electrolyte recirculation flow paths at the negative and positive sides of the redox flow battery cells 18 in the redox flow battery system 10, respectively. One or more rebalancing reactors may be connected in-line with the electrolyte recirculation flow paths at the negative and positive sides of the battery, and other rebalancing reactors may be connected in parallel for redundancy (e.g., the rebalancing reactors may be maintained without interrupting the battery and rebalancing operation) and for increased rebalancing capacity. In one embodiment, the electrolyte rebalancing reactors / cells 80 and 82 may be disposed in the return flow paths from the negative and positive electrode compartments 20 and 22, respectively, to the negative and positive electrolyte chambers 50 and 52. The electrolyte rebalancing reactors / cells 80 and 82 may help rebalance electrolyte charge imbalances in the redox flow battery system 10 that occur due to side reactions, ionic crossover, etc., as described herein.

[0040] Rebalancing reactors / cells 80 and 82 without sacrificing their overall reliability 3+ To increase the reduction rate, embodiments of the present disclosure provide a rebalancing cell including a stack of internally shorted electrode assemblies, examples of which are shown in FIGS. 2A and 2B and 4A-4C, with an example of an internal electrode assembly shown in FIG. 3. The rebalancing cell may be configured to drive H2 gas and electrolyte to react at the catalyst surface via a combination of internal electrical current, convection, gravity feed, and capillary action. In embodiments described herein, the electrode assemblies of the stack of internally shorted electrode assemblies may be referred to as "internally shorted" in that no electrical current is directed away from the stack of internally shorted electrode assemblies during operation of the rebalancing cell. Such an internal electrical short may result in the formation of a large amount of Fe 3+ Dramatically increase the reduction rate (e.g., about 50-70 mol / m 2time (or more than 20 times as compared to a rebalancing reactor configuration without internal electrical shorting) and concomitantly reduce side reaction rates (e.g., the rates of the reactions of Equations (1)-(3)), while reducing or eliminating reverse current spikes. Furthermore, each electrode assembly of a stack of internally shorted electrode assemblies can be electrically isolated from the other electrode assemblies of the stack of internally shorted electrode assemblies, such that degradation to the stack of internally shorted electrode assemblies during a current spike at one electrode assembly can be limited thereto (e.g., reverse current cannot be driven from one electrode assembly through the other electrode assemblies). In such a case, a single depleted electrode assembly can be easily removed from the stack of internally shorted electrode assemblies and replaced with a non-depleted electrode assembly.

[0041] To achieve an internal short circuit, each electrode assembly of a stack of internal short-circuiting electrode assemblies may include a jointed pair of positive and negative electrodes (e.g., configured in continuous, electrically conductive, surface-sharing contact with each other). As used herein, a pair of first and second components (e.g., a positive electrode and a negative electrode of an electrode assembly) may be described as "joined" with each other when the first component is disposed adjacent to the second component such that the first and second components are in surface-sharing contact with each other ("adjacent" is used herein to refer to any two components with no intervening components). Furthermore, as used herein, "continuously" when describing the electrical conductivity of a plurality of electrodes may refer to an electrical path through the interior that has substantially or practically zero resistance at any surface-sharing contact of the plurality of electrodes.

[0042] In an exemplary embodiment, the (positive) rebalancing reactor / cell 82 of FIG. 1 may be a rebalancing cell that includes a stack of internal short-circuiting electrode assemblies. 3+ can be generated, resulting in higher Fe 3+The reduction rate may be desirable for rebalancing the positive electrolyte (see equation (6)). In additional or alternative embodiments, the (negative) rebalancing reactor / cell 80 may be of similar construction [Fe 3+ may be generated at the negative electrode 26 during iron plating oxidation (see equation (3))].

[0043] Fe 3+ The reduction rate may decrease over time due to a decrease in catalytic activity at the negative electrode of the rebalancing cell. Such a decrease is due to the formation of a diffuse double layer on the catalyst at the negative electrode of the rebalancing cell during operation of the redox flow battery system, leading to adsorption of anions at the catalyst surface, which then causes an additional layer of cations to surround the catalyst. Thus, the diffuse double layer may inactivate the catalyst by blocking the interaction between the catalyst and the iron cations. Application of a voltage (e.g., an ion-repulsion potential) to the rebalancing cell from an external voltage source may suppress the generation of the diffuse double layer. An example of a rebalancing cell coupled to an external voltage source is shown in Figures 4A-4C and described further below.

[0044] During operation of the redox flow battery system 10, the sensors and probes may monitor and control electrolyte chemical properties such as electrolyte pH, concentration, SOC, etc. For example, as illustrated in FIG. 1, sensors 62 and 60 may be positioned to monitor the state of the positive and negative electrolytes in the positive and negative electrolyte chambers 52 and 50, respectively. In another embodiment, the sensors 62 and 60 may each include one or more electrolyte level sensors to indicate the level of electrolyte in the positive and negative electrolyte chambers 52 and 50, respectively. As another example, the sensors 72 and 70, also illustrated in FIG. 1, may monitor the state of the positive and negative electrolytes in the positive and negative electrode compartments 22 and 20, respectively. The sensors 72 and 70 may be pH probes, optical probes, pressure sensors, voltage sensors, etc. It will be understood that sensors may be positioned elsewhere throughout the redox flow battery system 10 to monitor electrolyte chemical properties and other properties.

[0045] For example, sensors may be positioned in an external acid tank (not shown) to monitor the amount of acid or pH in the external acid tank, and acid from the external acid tank may be supplied to the redox flow battery system 10 via an external pump (not shown) to reduce precipitate formation in the electrolyte. Additional external tanks and sensors may be installed to supply other additives to the redox flow battery system 10. For example, various sensors, including temperature, conductivity, and level sensors of the field hydration system, may send signals to the controller 88. Additionally, the controller 88 may send signals to actuators, such as valves and pumps of the field hydration system, during hydration of the redox flow battery system 10. The sensor information may be transmitted to the controller 88, which may then, by way of example, operate the pumps 30 and 32 to control electrolyte flow through the redox flow battery cell 18, or perform other control functions. In this manner, the controller 88 may respond to one or a combination of sensors and probes.

[0046] The redox flow battery system 10 may further include a source of H2 gas. In one embodiment, the source of H2 gas may include a separate dedicated hydrogen gas storage tank. In the embodiment of FIG. 1, H2 gas may be stored in and supplied from an integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supply additional H2 gas to the positive electrolyte chamber 52 and the negative electrolyte chamber 50. The integrated multi-chamber electrolyte storage tank 110 may alternately supply additional H2 gas to the inlets of the electrolyte rebalancing reactors / cells 80 and 82.

[0047] As an example, a mass flow meter or other flow control device (which may be controlled by the controller 88) may regulate the flow of H2 gas from the integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supplement the H2 gas produced within the redox flow battery system 10. For example, when a gas leak is detected in the redox flow battery system 10 or when a reduction reaction rate (e.g., Fe 3+ If the reduction rate (SOC) is too low, H2 gas can be provided from the integrated multi-chamber electrolyte storage tank 110 to rebalance the SOC of the electroactive materials in the positive and negative electrolytes. As an example, the controller 88 can provide H2 gas from the integrated multi-chamber electrolyte storage tank 110 in response to a measured change in pH or in response to a measured change in the SOC of the electrolyte or electroactive materials.

[0048] For example, an increase in pH in the negative electrolyte chamber 50 or the negative electrode compartment 20 may indicate that H2 is leaking from the redox flow battery system 10 and / or that the reaction rate is too slow at the available hydrogen partial pressure, and the controller 88 may increase the supply of H2 gas from the integrated multi-chamber electrolyte storage tank 110 to the redox flow battery system 10 in response to the increase in pH. As a further example, the controller 88 may supply H2 gas from the integrated multi-chamber electrolyte storage tank 110 in response to a pH change that increases above a first threshold pH or decreases above a second threshold pH. In the case of an IFB, the controller 88 may supply additional H2 to increase the pH of the redox flow battery system 10. 3+ This can increase the rate of reduction of ions and the rate of production of protons, thereby decreasing the pH of the positive electrolyte. Furthermore, the pH of the negative electrolyte can be increased by increasing the rate of Fe crossover from the positive electrolyte to the negative electrolyte. 3+ The pH of the negative electrolyte can be lowered by hydrogen reduction of the ion or by protons generated on the positive side crossing over into the negative electrolyte due to the proton concentration gradient and electrophoretic forces. In this way, the pH of the negative electrolyte is reduced by Fe(OH)3. 3+The risk of precipitation of ions (crossing over from the positive electrode compartment 22) can be kept within a stable region.

[0049] Other control schemes may be implemented to control the rate of H2 gas supply from the integrated multi-chamber electrolyte storage tank 110 in response to changes in electrolyte pH or changes in electrolyte SOC detected by other sensors, such as an oxygen reduction potential (ORP) meter or optical sensor. Still further, the change in pH or SOC trigger action of the controller 88 may be based on a rate of change or a change measured over a period of time. The period of the rate of change may be predetermined or adjusted based on the time constant of the redox flow battery system 10. For example, if the recirculation rate is high, the period may be shortened and the time constant may be small so that local changes in concentration (e.g., due to side reactions or gas leaks) may be measured quickly.

[0050] The controller 88 may further execute a control scheme based on the operating mode of the redox flow battery system 10. For example, as discussed in detail below with reference to FIG. 6, the controller may control the ion repulsion voltage applied to the rebalancing cell. In doing so, the formation of a diffuse double layer in the catalyst of the rebalancing cell may be avoided, thereby maintaining the performance of the rebalancing cell. As another example, the controller 88 may control the charging and discharging of the redox flow battery cell 18 to cause pre-formation of iron at the negative electrode 26 during system conditioning (system conditioning may include an operating mode used to optimize the electrochemical performance of the redox flow battery system 10 outside of battery cycling). That is, during system conditioning, the controller 88 may adjust one or more operating conditions of the redox flow battery system 10 to plate iron metal on the negative electrode 26 to improve the battery charging capacity during subsequent battery cycling (so that iron metal may be pre-formed for battery cycling). The controller 88 further performs electrolyte rebalancing, as discussed above, to remove excess hydrogen gas from the redox flow battery system 10 and to rebalance the Fe3+ In this manner, preforming iron at the negative electrode 26 and performing electrolyte rebalancing during system conditioning may increase the overall capacity of the redox flow battery cell 18 during battery cycling by mitigating iron plating losses. As used herein, battery cycling (also referred to as "charge cycling") may include alternating between charge and discharge modes of the redox flow battery system 10.

[0051] It will be appreciated that all components except the sensors 60 and 62 and the integrated multi-chamber electrolyte storage tank 110 (and components included therein) may be considered to be included in the power module 130. Thus, the redox flow battery system 10 may be described as including a power module 130 fluidly coupled to the integrated multi-chamber electrolyte storage tank 110 and communicatively coupled to the sensors 60 and 62. In some embodiments, each of the power module 130 and the multi-chamber electrolyte storage tank 110 may be included in a single housing or packaging (not shown), such that the redox flow battery system 10 may be included as a single unit at a single location. It will be further appreciated that the positive electrolyte, the negative electrolyte, the sensors 60 and 62, the electrolyte rebalancing reactors / cells 80 and 82, and the integrated multi-chamber electrolyte storage tank 110 (and components included therein) may be considered to be included in the electrolyte management system 140. Thus, the electrolyte management system 140 may provide one or more electrolytes to the redox flow battery cells 18 (and components included therein).

[0052] In some embodiments, as discussed above, the redox flow battery system 10 may be configured as a redox flow battery pack including a plurality of fluidically isolated redox flow battery subsystems 150. In such embodiments, each of the plurality of fluidically isolated redox flow battery subsystems 150 may include all components of the redox flow battery system 10 apart from the controller 88. More specifically, each of the plurality of fluidically isolated redox flow battery subsystems 150 may include, in one embodiment, a separate integrated multi-chamber electrolyte storage tank 110, separate electrolyte pumps 30 and 32, separate redox flow battery cells 18, separate rebalancing reactors / cells 80 and 82, etc. Thus, each of the plurality of fluidically isolated redox flow battery subsystems 150 may be referred to herein as a redox flow battery, and each redox flow battery may be independently configured to output power during discharge. In such a configuration, the controller 88 may be communicatively coupled to each of the redox flow batteries and thus may control the operating state of each of the redox flow batteries in parallel or individually, as determined based on a given application.

[0053] Referring now to FIGS. 2A and 2B, a first perspective view 200 and a second perspective view 250 of a balancing cell 202 for a redox flow battery system, such as the redox flow battery system 10 of FIG. 1, are shown respectively. In an exemplary embodiment, the balancing cell 202 may include a stack of internal short-circuit electrode assemblies, such as an electrode assembly described in detail below with reference to FIG. 3. An example of a stack of internal short-circuit electrode assemblies of the balancing cell is illustrated in FIGS. 4A-4C. The balancing cell 202 may be capable of causing an electrolyte balancing reaction by allowing H2 gas to contact an electrolyte from a positive electrode compartment or a negative electrode compartment of a redox flow battery, such as the redox flow battery cell 18 of FIG. 1, at a catalytic surface of a negative electrode of the stack of internal short-circuit electrode assemblies. Thus, the balancing cell 202 may be one or both of the balancing reactors / cells 80 and 82 of FIG. 1. A set of reference axes 201 is provided for explaining the relative positioning of the components shown and for comparison between the figures of FIGS. 2A-4C, and the axes 201 indicate the x-axis, the y-axis, and the z-axis. As further shown in dashed lines in FIGS. 2A and 2B, an additional axis g may be parallel to the direction of gravity (e.g., the positive direction along the axis g) and the perpendicular direction (e.g., the negative direction along the axis g, opposite to the direction of gravity).

[0054] The number of balancing cells 202 included in the redox flow battery system and the number of electrode assemblies included in the stack of internal short-circuit electrode assemblies are not particularly limited and may correspondingly be increased to be adapted for higher performance applications. For example, a 75 kW redox flow battery system may include two balancing cells 202, each including a stack of 20 electrode assemblies (e.g., a stack of 19 bipolar assemblies having two end plates positioned at opposite ends of the stack).

[0055] As shown, the stack of internal shorting electrode assemblies may be removably enclosed within a housing or external cell enclosure 204. Thus, in some embodiments, the cell enclosure 204 may include a top cover removably attached to an enclosure base such that the top cover may be temporarily removed to replace or diagnose one or more electrode assemblies of the stack of internal shorting electrode assemblies. In additional or alternative embodiments, the cell enclosure 204, depicted in FIGS. 2A and 2B as a rectangular prism, may be molded to be a clearance fit relative to other components of the redox flow battery system such that the rebalancing cell 202 may be in flush contact with such components. In some embodiments, the cell enclosure 204 may be constructed of a material having low electrical conductivity, such as a plastic or other polymer, to reduce undesirable shorting events.

[0056] The cell enclosure 204 may be further configured to include openings or cavities for mating components of the rebalancing cell 202. For example, the cell enclosure 204 may include a number of inlet and outlet ports configured to fluidly couple to other components of the redox flow battery system. In one example, as shown, the multiple inlet and outlet ports may include polypropylene (PP) flange fittings welded to PP piping.

[0057] In an exemplary embodiment, the multiple inlet and outlet ports may include an electrolyte inlet port 206 for flowing electrolyte into the cell enclosure 204 and an electrolyte outlet port 208 for draining electrolyte from the cell enclosure 204. In one example, the electrolyte inlet port 206 may be located in an upper half of the cell enclosure 204 and the electrolyte outlet port 208 may be located in a lower half of the cell enclosure 204 (the upper and lower halves of the cell enclosure 204 are separated by a plane along the z-axis and parallel to each of the x-axis and y-axis). Thus, the electrolyte outlet port 208 may be located lower with respect to the direction of gravity (e.g., along axis g) than the electrolyte inlet port 206.

[0058] Specifically, as the electrolyte enters the cell enclosure 204 via the electrolyte inlet port 206, it may be distributed across the stack of internally shorting electrode assemblies, gravity fed through the stack of electrode assemblies, wicked up (e.g., relative to the direction of gravity) through the positive electrode of the stack of internally shorting electrode assemblies to react at the catalytic surface of the negative electrode in a cathodic half-reaction, and drained from the cell enclosure 204 via the electrolyte outlet port 208. To aid in the gravity feed of the electrolyte and reduce its pressure drop, the rebalancing cell 202 may be further tilted or tilted relative to the direction of gravity via a graded support 220 coupled to the cell enclosure 204. In some embodiments, tilting the cell enclosure 204 in this manner may further aid in the electrolyte drainage of the rebalancing cell 202 (e.g., during an idle mode of the redox flow battery system) and keep the catalytic surface relatively dry (as the catalytic surface may corrode in some embodiments after being immersed in the electrolyte for a sufficient period of time).

[0059] As shown, the gradient support 220 can tilt the cell enclosure 204 at an angle 222 such that the plane of the electrode sheets of the stack of the internal shorting electrode assembly is tilted at the angle 222 relative to the bottom surface (not shown) on which the gradient support 220 rests. In some examples, the angle 222 (e.g., of the cell enclosure 204 relative to the bottom surface) can be between 0° and 30° (in embodiments where the angle 222 is substantially 0°, the rebalancing cell 202 can still function, but the pressure drop can be greater and electrolyte crossover to the negative electrode can be reduced when the cell enclosure 204 is tilted). In some examples, the angle 222 can be between 2° and 30°. In some examples, the angle 222 can be between 2° and 20°. In one example, the angle 222 can be about 8°. Thus, the electrolyte pressure drop can be increased by increasing the angle 222 and decreased by decreasing the angle 222. Additionally or alternatively, one or more support rails 224 may be coupled to an upper half of the cell enclosure 204 (e.g., opposite the sloped support 220). In some embodiments, as shown in the perspective view 200 of FIG. 2A, the one or more support rails 224 may be tilted relative to the cell enclosure 204 at an angle 222 such that the one or more support rails 224 may removably secure the rebalancing cell 202 to an upper surface above and parallel to the lower surface. In this manner, and based on geometric considerations, the z-axis may be offset from the axis g at an angle 222 as well (e.g., the cell enclosure 204 may be tilted relative to a vertical direction opposite the direction of gravity by an angle 222 as shown in FIGS. 2A and 2B). In some embodiments, gravity feed of electrolyte through the rebalancing cell 202 can be further assisted by positioning the rebalancing cell 202 above an electrolyte storage tank of a redox flow battery system (e.g., multi-chamber electrolyte storage tank 110 of FIG. 1 ) in a vertical direction opposite the direction of gravity.

[0060] As further shown, the electrolyte outlet port 208 may include a plurality of openings in the cell enclosure 204 (each of the plurality of openings including a PP flange fitting welded to a PP tubing) configured to drain at least a portion of the electrolyte. For example, in FIGS. 2A and 2B, the electrolyte outlet port 208 is shown to include five openings. In this manner, the electrolyte may be evenly distributed across the stack of internal shorting electrode assemblies and may drain from the cell enclosure 204 in a substantially smooth flow. In other embodiments, the electrolyte outlet port 208 may include more than five openings or fewer than five openings. In one embodiment, the electrolyte outlet port 208 may include only one opening. In additional or alternative embodiments, the electrolyte outlet port 208 may be positioned below the cell enclosure 204 with respect to the z-axis (e.g., on a face of the cell enclosure 204 facing in the negative direction of the z-axis).

[0061] The electrolyte inlet port 206 and the electrolyte outlet port 208 may be positioned on the cell enclosure 204 based on the flow path of the electrolyte through the stack of internal shorting electrode assemblies (e.g., including channels, passages, manifolds, plenums, wells, etc., within the cell enclosure 204 fluidly coupled to the electrolyte inlet port 206 and the electrolyte outlet port 208, as well as from the electrolyte inlet port 206 to the electrolyte outlet port 208). In some embodiments, as shown, the electrolyte inlet port 206 and the electrolyte outlet port 208 may be positioned on adjacent sides of the cell enclosure 204 (e.g., faces of the cell enclosure 204 that share a common edge). In other embodiments, the electrolyte inlet port 206 and the electrolyte outlet port 208 may be positioned on opposite sides of the cell enclosure 204. In other embodiments, the electrolyte inlet port 206 and the electrolyte outlet port 208 may be positioned on the same side of the cell enclosure 204.

[0062] In some embodiments, the electrolyte inlet port 206 may be positioned on a side of the cell enclosure 204 that faces in the negative direction of the x-axis. In additional or alternative embodiments, the electrolyte inlet port 206 may be positioned on a side of the cell enclosure 204 that faces in the positive direction of the x-axis. In one embodiment, as shown, one opening of the electrolyte inlet port 206 may be positioned on a side of the cell enclosure 204 that faces in the negative direction of the x-axis and another opening of the electrolyte inlet port 206 may be positioned on a side of the cell enclosure 204 that faces in the positive direction of the x-axis.

[0063] In some embodiments, the plurality of inlet and outlet ports may further include a hydrogen gas inlet port 210 for flowing H2 gas into the cell enclosure 204 and a hydrogen gas outlet port 212 (shown in FIG. 2B ) for exhausting H2 gas from the cell enclosure 204. In one embodiment, as shown, each of the hydrogen gas inlet port 210 and the hydrogen gas outlet port 212 may be positioned in the lower half of the cell enclosure 204 (e.g., at the bottom electrode assembly of a stack of internal shorting electrode assemblies along the z-axis). In another embodiment, each of the hydrogen gas inlet port 210 and the hydrogen gas outlet port 212 may be positioned in the upper half of the cell enclosure 204 (e.g., at the top electrode assembly of a stack of internal shorting electrode assemblies along the z-axis). In yet another embodiment, the hydrogen gas inlet port 210 may be positioned in the lower half of the cell enclosure 204 and the hydrogen gas outlet port 212 may be positioned in the upper half of the cell enclosure 204. In such an embodiment, the hydrogen gas inlet port 210 may be positioned lower relative to the direction of gravity (eg, along axis g) than the hydrogen gas outlet port 212.

[0064] Specifically, when H2 gas enters the cell enclosure 204 via the hydrogen gas inlet port 210, the H2 gas may be distributed across and through the stack of internal shorting electrode assemblies via forced convection (e.g., induced by the flow field configuration of the respective flow field plates) and decomposed in an anodic half-reaction at the catalytic surface of the negative electrode. However, in some embodiments, excess unreacted H2 gas may remain in the rebalancing cell 202 after contact with the catalytic surface. In some embodiments, at least a portion of the H2 gas that has not reacted at the catalytic surface may pass to the electrolyte. In such embodiments, to avoid undesirable pressure buildup, thereby preventing electrolyte pooling on the positive electrode and concomitant electrolyte flooding of the negative electrode, the multiple inlet and outlet ports may further include a pressure release outlet port 214 to vent unreacted H2 gas from the electrolyte. Furthermore, in some embodiments, the hydrogen gas outlet port 212 may be configured to vent at least a portion of the H2 gas that has not reacted at the catalytic surface and has not flowed through the negative electrode to the electrolyte.

[0065] The hydrogen gas inlet port 210 and the hydrogen gas outlet port 212 may be positioned on the cell enclosure 204 based on a flow path of H2 gas through the stack of internal short circuit electrode assemblies (e.g., including from the hydrogen gas inlet port 210 to the hydrogen gas outlet port 212 (if included), as well as channels, passages, plenums, etc. within the cell enclosure 204 fluidly coupled to the hydrogen gas inlet port 210 and the hydrogen gas outlet port 212). In some embodiments, the hydrogen gas inlet port 210 and the hydrogen gas outlet port 212 may be positioned on opposite sides of the cell enclosure 204, as shown. In other embodiments, the hydrogen gas inlet port 210 and the hydrogen gas outlet port 212 may be positioned on adjacent sides of the cell enclosure 204. In other embodiments, the hydrogen gas inlet port 210 and the hydrogen gas outlet port 212 may be positioned on the same side of the cell enclosure 204. Further, although hydrogen gas inlet port 210 is shown in FIGS. 2A and 2B as being positioned on the side of cell enclosure 204 facing in the negative direction of the x-axis and hydrogen gas outlet port 212 is shown in FIGS. 2A and 2B as being positioned on the side of cell enclosure 204 facing in the positive direction of the x-axis, in other embodiments, hydrogen gas inlet port 210 may be positioned on the side of cell enclosure 204 facing in the positive direction of the x-axis and hydrogen gas outlet port 212 may be positioned on the side of cell enclosure 204 facing in the negative direction of the x-axis.

[0066] In one example, the hydrogen gas inlet port 210, the hydrogen gas outlet port 212, the electrolyte inlet port 206, and the electrolyte outlet port 208 may be positioned on the cell enclosure 204 in a lateral configuration. Specifically, the lateral configuration may include the hydrogen gas outlet port 212 and the electrolyte inlet port 206 positioned on different sides (e.g., faces) of an upper half of the cell enclosure 204, and the hydrogen gas inlet port 210 and the electrolyte outlet port 208 positioned on different sides of a lower half of the cell enclosure 204.

[0067] In other embodiments, there may be no hydrogen gas outlet port 212 for exhausting H2 gas that has not reacted at the catalytic surface of the negative electrode and that has not flowed through the negative electrode to the electrolyte. However, in such embodiments, there may still be a pressure release outlet port 214 for exhausting unreacted H2 gas from the electrolyte, and the unreacted H2 gas may be exhausted from the cell enclosure 204 only after it flows through the negative electrode to the electrolyte and through the pressure release outlet port 214. An exemplary rebalancing cell configuration that lacks a hydrogen gas outlet port 212 may be referred to as a "dead-end configuration" regardless of whether it includes a pressure release outlet port 214. In a dead-end configuration, substantially all of the H2 gas may be brought into contact with the catalytic surface of the negative electrode, where the H2 gas may decompose via the anode half-reaction, and / or the H2 gas may enter the electrolyte after passing through the negative electrode (e.g., without reacting on its catalytic surface).

[0068] Referring now to FIG. 3, an exploded view 300 of an electrode assembly 302 for a rebalancing cell, such as the rebalancing cell 202 of FIGS. 2A-2B and 4A-4C, is shown. Thus, the electrode assembly 302 may be internally shorted (e.g., current flowing through the electrode assembly 302 is not channeled through an external load). In an exemplary embodiment, the electrode assembly 302 may be included within a stack of similarly configured electrode assemblies within a cell enclosure to form a rebalancing cell. The electrode assembly 302 may include a plate 304 having an activated carbon foam 306, a positive electrode 308 (also referred to herein as a "cathode" in some embodiments), and a negative electrode 310 (also referred to herein as an "anode" in some embodiments) stacked in sequence. In one embodiment, the plate 304 may be a bipolar plate. The electrode assembly 302 may be positioned within the rebalancing cell to receive electrolyte through the carbon foam 306, from which the electrolyte may enter the pores of the positive electrode 308 via capillary action and contact the negative electrode 310. The electrode assembly 302 may further be positioned within the rebalancing cell to receive H2 gas across the catalytic surface of the negative electrode 310 opposite the positive electrode 308 via convection. The convection of H2 gas across the catalytic surface may be assisted by a flow field plate (not shown in FIG. 3) in contact with the catalytic surface. As H2 gas is decomposed at the catalytic surface via the anodic half-reaction, protons and electrons may flow to the junction of the negative electrode 310 and the positive electrode 308, where ions in the electrolyte may be reduced via a cathodic half-reaction (e.g., Fe 3+ is Fe 2+ In this manner, the electrode assembly 302 can be configured for electrolyte rebalancing for a redox flow battery, such as the redox flow battery cell 18 of FIG. 1 , fluidly coupled to a rebalancing cell that includes the electrode assembly 302.

[0069] In some embodiments, the plate 304 may be constructed of a material having low electrical conductivity, such as a plastic or other polymer, to reduce undesirable short circuit events. Thus, in one embodiment, the plate 304 may be formed from the same material as the cell enclosure 204 of Figures 2A and 2B.

[0070] As shown, the plate 304 may include a plurality of inlets and outlets therethrough. For example, the plurality of inlets and outlets may include an electrolyte outlet channel section 316, a hydrogen gas inlet channel section 318a, and a hydrogen gas outlet channel section 318b. Specifically, the plate 304 may include an electrolyte outlet channel section 316 for directing electrolyte from the rebalancing cell, a hydrogen gas inlet channel section 318a for directing H2 gas to the rebalancing cell across the negative electrode 310, and a hydrogen gas outlet channel section 318b for directing H2 gas from the rebalancing cell. The plate 304 may further include an electrolyte inlet well 312 for receiving electrolyte at the electrode assembly 302, the electrolyte inlet well 312 being fluidly coupled to a plurality of electrolyte inlet passages 314a set in a berm 314b positioned adjacent to the carbon foam 306 for distributing the received electrolyte across the carbon foam 306. In some examples, the electrolyte inlet well 312 may receive electrolyte from an electrolyte inlet port (e.g., electrolyte inlet port 206 in FIGS. 2A and 2B ) fluidly coupled thereto (e.g., via an electrolyte inlet channel, not shown in FIG. 3 ), the electrolyte outlet channel section 316 may exhaust electrolyte through an electrolyte outlet port (e.g., electrolyte outlet port 208 in FIGS. 2A and 2B ) fluidly coupled thereto, the hydrogen gas inlet channel section 318 a may receive H gas from a hydrogen gas inlet port (e.g., hydrogen gas inlet port 210 in FIGS. 2A and 2B ) fluidly coupled thereto, and the hydrogen gas outlet channel section 318 b may exhaust H gas through a hydrogen gas outlet port (e.g., hydrogen gas outlet port 212 in FIGS. 2A and 2B ) fluidly coupled thereto.

[0071] Although hydrogen gas inlet channel section 318a is described herein as a section of a hydrogen gas inlet channel and hydrogen gas outlet channel section 318b is described herein as a section of a hydrogen gas outlet channel, it will be understood that in other embodiments, channel section 318b may be a section of a hydrogen gas inlet channel (e.g., for receiving H2 gas from a hydrogen gas inlet port and then directing the H2 gas across negative electrode 310 to the rebalancing cell) and channel section 318a may be a section of a hydrogen gas outlet channel (e.g., for directing H2 gas from the rebalancing cell by exhausting H2 gas through a hydrogen gas outlet port). In other embodiments, the rebalancing cell may have a dead-end configuration and no hydrogen gas outlet port may be fluidly coupled to hydrogen gas outlet channel section 318b. In such embodiments, the hydrogen gas outlet channel section 318b may direct the H2 gas back across the negative electrode 310, or the hydrogen gas outlet channel section 318b may instead be configured as a separate hydrogen gas inlet channel section (e.g., to receive a portion of the H2 gas from the hydrogen gas inlet port and then direct a portion of the H2 gas to the rebalancing cell and across the negative electrode 310).

[0072] The multiple inlets and outlets may be configured to control electrolyte and H2 gas flow throughout the rebalancing cell. As an example, the size of each of the hydrogen gas inlet channel section 318a and the hydrogen gas outlet channel section 318b may be selected to minimize the pressure drop therethrough, thereby aiding in flow distribution to each electrode assembly 302 of the stack of internal short circuit electrode assemblies. As another example, the size of each electrolyte inlet passage 314a and the total number of electrolyte inlet passages 314a relative to the berm 314b may be selected to induce a relatively small pressure drop to distribute the electrolyte flow substantially evenly. In such an example, the selection of the size of each electrolyte inlet passage 314a and the total number of electrolyte inlet passages 314a may depend on several factors specific to a given configuration of the rebalancing cell, such as the size of the electrolyte flow field and the desired electrolyte flow rate.

[0073] In additional or alternative embodiments, the electrolyte outlet channel section 316 may be further configured to distribute electrolyte through a plurality of openings included in the electrolyte outlet port. For example, in the exploded view 300 of FIG. 3, the electrolyte outlet channel section 316 is shown to include two openings. In some embodiments, the number of openings included in the electrolyte outlet channel section 316 may be equal to the number of openings included in the electrolyte outlet port, such that the openings in the electrolyte outlet channel section 316 may each correspond to an opening in the electrolyte outlet port. In this manner, the electrolyte may be evenly distributed across the electrode assembly 302 and may exit the rebalancing cell with a substantially smooth flow. In other embodiments, the electrolyte outlet channel section 316 may include more than two openings or less than two openings (e.g., only one opening).

[0074] Furthermore, when the electrode assembly 302 is included within a stack of electrode assemblies, the electrolyte outlet channel section 316, the hydrogen gas inlet channel section 318a, and the hydrogen gas outlet channel section 318b may be aligned to form a continuous electrolyte outlet channel, a continuous hydrogen gas inlet channel, and a continuous hydrogen gas outlet channel, respectively. In this manner, the stack of electrode assemblies may be formed in a modular manner such that any practical number of electrode assemblies 302 may be stacked and included within a rebalancing cell.

[0075] As further shown, the sealing inserts may be affixed (as used herein, "affix", "affixed" or "affixing" includes, but is not limited to, adhering, attaching, connecting, fastening, joining, coupling, or fastening one component to another component through a direct or indirect relationship) or otherwise coupled to the plate 304. As an example, the sealing inserts may include a hydrogen gas inlet channel sealing insert 320a and a hydrogen gas outlet channel sealing insert 320b for directing the flow of H2 gas across the negative electrode 310 by mitigating H2 gas bypass. Specifically, the hydrogen gas inlet channel sealing insert 320a and the hydrogen gas outlet channel sealing insert 320b may be affixed or otherwise coupled adjacent to the hydrogen gas inlet channel section 318a and the hydrogen gas outlet channel section 318b, respectively, on the side of the plate 304 that includes the carbon foam 306, the positive electrode 308, and the negative electrode 310. In some embodiments, the hydrogen gas inlet channel sealing insert 320a and the hydrogen gas outlet channel sealing insert 320b may coincide with the xy plane of the negative electrode 310 such that the hydrogen gas inlet channel sealing insert 320a and the hydrogen gas outlet channel sealing insert 320b extend from the point of attachment or bonding to the plate 304 and may partially overlap the positive electrode 308.

[0076] As another example, the plurality of sealing inserts may further include a hydrogen gas inlet channel O-ring 322a and a hydrogen gas outlet channel O-ring 322b for sealing a junction between the hydrogen gas inlet channel section 318a and a hydrogen gas inlet channel section of another electrode assembly, and a junction between the hydrogen gas outlet channel section 318b and a hydrogen gas outlet channel section of another electrode assembly, respectively. Specifically, the hydrogen gas inlet channel O-ring 322a and the hydrogen gas outlet channel O-ring 322b may be affixed or otherwise coupled to the plate 304 to circumscribe the hydrogen gas inlet channel section 318a and the hydrogen gas outlet channel section 318b, respectively.

[0077] As another example, the plurality of sealing inserts may further include an overboard O-ring 324 for sealing an outer periphery of a joint between the electrode assembly 302 and another electrode assembly. Specifically, the overboard O-ring 324 may be affixed or otherwise coupled to the plate 304 so as to circumscribe each of the electrolyte inlet well 312, the plurality of electrolyte inlet passages 314a, the berm 314b, the electrolyte outlet channel section 316, the hydrogen gas inlet channel section 318a, and the hydrogen gas outlet channel section 318b.

[0078] The carbon foam 306 can be positioned within the cavity 326 of the plate 304 between the berm 314b and the electrolyte outlet channel section 316 along the y-axis, and between the hydrogen gas inlet channel section 318a and the hydrogen gas outlet channel section 318b along the x-axis. Specifically, the carbon foam 306 can be positioned in flush contact with a side of the plate 304 that forms the base of the cavity 326. In some examples, the carbon foam 306 can be formed as a continuous monolithic piece, while in other examples, the carbon foam 306 can be formed as two or more carbon foam sections. In an exemplary embodiment, the carbon foam 306 can be conductive, permeable, and porous to provide a distribution field for electrolyte gravity-fed therethrough from the multiple electrolyte inlet passages 314a. In some examples, the pore distribution of the carbon foam 306 can be 10-100 PPI. In one example, the pore distribution can be 30 PPI. In additional or alternative embodiments, the permeability of the carbon foam 306 is between 0.02 and 0.5 mm. 2 Thus, the overall size of the carbon foam 306 as well as the pore distribution and permeability can each be selected to target a relatively small pressure drop, thereby inducing convection of electrolyte from the carbon foam 306 to the positive electrode 308. For example, the pressure drop can be targeted to provide an electrolyte head rise of 2-3 mm.

[0079] In some embodiments, the carbon foam 306 may be replaced with a flow field plate configured to transport electrolyte to the positive electrode 308 via convection induced by the flow field configuration of the flow field plate. Specifically, the flow field plate may be fluidly coupled to each of the plurality of electrolyte inlet passages 314a and electrolyte outlet channel sections 316. In one embodiment, the flow field plate may be integrally formed within a plate 304 of the electrode assembly 302 positioned below the positive electrode 308 relative to the z-axis. In other embodiments, the flow field plate may be a separate, removable component.

[0080] In some embodiments, the flow field configuration may be a crossed flow field configuration, a partially crossed flow field configuration, or a serpentine flow field configuration. In some embodiments, each electrode assembly 302 may interface with a similar (e.g., staggered, partially staggered, serpentine, etc.) flow field configuration with each other electrode assembly 302. In other embodiments, several different flow field configurations may be provided between the electrode assemblies 302 within a stack of electrode assemblies (e.g., depending on the location of a given electrode assembly 302 within the rebalancing cell 202 of FIGS. 2A and 2B). In this manner, electrolyte may be directed from an electrolyte inlet port (e.g., electrolyte inlet port 206 of FIGS. 2A and 2B) to flow field plates each interfaced with a positive electrode 308 within a stack of electrode assemblies, the flow field plates configured with a staggered flow field configuration, a partially staggered flow field configuration, a serpentine flow field configuration, or a combination thereof.

[0081] In some embodiments, a separate flow field plate (also referred to herein as a "hydrogen gas flow field plate") may be bonded to the negative electrode 310 on the opposite side of the z-axis from the positive electrode 308. However, in other embodiments, only an electrolyte flow field plate may be included (e.g., replacing the carbon foam 306) and a hydrogen gas flow field plate may not be present. In yet other embodiments, only a hydrogen gas flow field plate may be included (e.g., bonded to the negative electrode 310) and an electrolyte flow field plate may not be present.

[0082] The positive electrode 308 may be positioned within the cavity 326 in face-to-face contact with the side of the carbon foam 306 opposite the plate 304 along the z-axis. In an exemplary embodiment, the positive electrode 308 may be a wicking conductive carbon felt, sponge, or mesh that may allow electrolyte flowing through the carbon foam 306 to contact the negative electrode 310 via capillary action. Thus, in some examples, the positive electrode 308 may be conductive and porous (although in such examples, not as porous as the carbon foam 306). In one example, electrolyte may be wicked to the positive electrode 308 if the porosity of the carbon foam 306 may be within a predetermined range (e.g., below an upper threshold porosity to retain enough solid material to facilitate wicking to the positive electrode 308 and above a lower threshold porosity to not impede the flow of electrolyte through the carbon foam 306). In additional or alternative embodiments, as the porosity of the positive electrode 308 increases, the adsorptivity of the positive electrode 308 may decrease and the permeability of the positive electrode 308 may increase (e.g., at least until only a small amount of solid material of the positive electrode 308 remains to facilitate electrolyte wicking, such as when a critical porosity of the positive electrode 308 is reached). In some embodiments, the surface of the positive electrode 308 may be sufficiently hydrophilic for desired rebalancing cell operation (e.g., by promoting thorough electrolyte wetting, thereby forming an ionically conductive medium). In such embodiments, the overall hydrophilicity of the positive electrode 308 may be increased by coating or treating its surface. Additionally, although at least a portion of the H2 gas may pass to the positive electrode 308 in addition to the portion of the electrolyte wicked to the positive electrode 308, the positive electrode 308 may be considered a separator between the bulk of H2 gas above it and the bulk of the electrolyte below it.

[0083] In some examples, each of the positive electrode 308 and the negative electrode 310 may be formed as a continuous monolithic piece (e.g., as opposed to discrete particles or multiple pieces) such that when electrolyte is contacted with H2 gas at the catalytic surface of the negative electrode 310, interphase mass transport losses across the boundary layer film may be reduced, thereby facilitating ion and proton transfer. In contrast, a packed bed configuration including discretely packed catalyst particles may include a mass transport-limited boundary layer film surrounding each individual particle, thereby reducing the rate of mass transport of electrolyte from the bulk of the electrolyte to the surface of the particle.

[0084] The negative electrode 310 may be positioned within the cavity 326 in face-to-face contact with the side of the positive electrode 308 opposite the carbon foam 306 along the z-axis, such that a three-phase contact interface between the (wicked) electrolyte, the catalytic surface of the negative electrode 310, and the H gas is formed to allow for the transfer of protons (e.g., H + ) and ion transfer through the three-phase contact boundary (HO + In parallel, the positive electrode 308 provides a conductive path for electrons to travel to the electrolyte front, where Fe 3+ By reducing the ions, the overall electronic resistance can be reduced.

[0085] In an exemplary embodiment, the negative electrode 310 may be a porous non-conductive material or a conductive carbon substrate coated with a metal catalyst thereon. In some examples, the porous non-conductive material may include polytetrafluoroethylene (PTFE), polypropylene, and the like. In some examples, the conductive carbon substrate may include carbon cloth or carbon paper. In some examples, the metal catalyst may include a precious metal catalyst. In some examples, the precious metal catalyst may include Pt. In additional or alternative examples, the precious metal catalyst may include Pd, Rh, Ru, Ir, Ta, or alloys thereof. In some examples, a relatively small amount (e.g., 0.2-0.5 wt %) of the precious metal catalyst supported on the conductive carbon substrate may be used due to cost considerations. However, in practice, the amount of the precious metal catalyst is not particularly limited and may be selected based on one or more of the desired reaction rate for the rebalancing cell and the expected life of the rebalancing cell. Additionally, alloys included within the precious metal catalyst may be utilized to reduce costs and increase the corrosion stability of the precious metal catalyst. For example, the addition of 10% Rh to Pt reduces the Fe content of Pt. 3+ In other examples, the metal catalyst may include a non-precious metal catalyst selected for its stability in ferric solutions and other such acidic environments (e.g., molybdenum sulfide). In one example, the negative electrode 310 has a 1.0 mg / cm 2 It may include a Pt-coated carbon cloth and a microporous layer bonded with a polytetrafluoroethylene (PTFE) binder (e.g., for hydrophobicity). Indeed, the inclusion of a PTFE binder may increase the durability of rebalancing cell performance over time relative to electrode assemblies formed using other binders.

[0086] In other examples, activity of the catalyst in the negative electrode 310 may be maintained by applying an ion-repulsion potential across the electrode assembly from an external voltage source. Applying an ion-repulsion potential may inhibit the formation of a diffuse double layer at the catalyst surface that may otherwise reduce catalytic activity. Modifications to the rebalancing cell configuration may be implemented to allow for application of an ion-repulsion potential to the electrode assembly of the rebalancing cell. Such modifications are discussed in detail with respect to FIGS. 4A-4C.

[0087] In some embodiments, immersion of the negative electrode 310 in the electrolyte may eventually result in corrosion of the precious metal catalyst, such as when the precious metal catalyst comprises Pt. In other embodiments, as discussed above with reference to Figures 2A and 2B, the rebalancing cell may be tilted or inclined (e.g., the z-axis may not be parallel to the direction of gravity) with respect to the surface on which the rebalancing cell is placed, so that the precious metal catalyst may remain relatively dry as the flow of electrolyte is drawn through the carbon foam 306 via gravity feed toward the electrolyte outlet channel section 316. Thus, in some embodiments, the electrode assembly 302 may be horizontal or inclined at an angle between 0° and 30° with respect to the surface on which the rebalancing cell is placed.

[0088] In an exemplary embodiment, the electrode assembly 302 including each of the carbon foam 306, the positive electrode 308, and the negative electrode 310 may be compressed along the z-axis, with the positive electrode 308 having a greater deflection than the carbon foam 306 and the negative electrode 310 under a given compression pressure. Thus, the depth of the cavity 326 may be selected based on the thickness of the carbon foam 306, the thickness of the positive electrode 308, the desired compression of the positive electrode 308, and the thickness of the negative electrode 310. Specifically, the depth of the cavity 326 may be selected to be greater than a lower threshold depth of the sum of the thickness of the carbon foam 306 after being substantially fully compressed (to avoid overstressing and fracturing the carbon foam 306, which may impede electrolyte flow) and the thickness of the positive electrode 308 after being substantially fully compressed (to avoid zero compression and possibly gaps in the positive electrode 308, which may result in insufficient contact between the H2 gas and the electrolyte).

[0089] For example, in an embodiment where the carbon foam 306 has a thickness of 6 mm, the positive electrode 308 has a thickness of 3.4 mm, the desired compression of the positive electrode 308 is 0.4 mm (to achieve a desired compression pressure of 0.01 MPa), and the negative electrode 310 has a thickness of 0.2 mm, the depth of the cavity 326 may be 9.2 mm (=3.4 mm+6 mm+0.2 mm-0.4 mm). As another embodiment, the carbon foam 306 may have a thickness of 2-10 mm, the positive electrode 308 may have a thickness of 1-10 mm, the desired compression of the positive electrode 308 may be 0-2.34 mm (to achieve a desired compression pressure of 0-0.09 MPa), and the negative electrode 310 may have a thickness of 0.2-1 mm, such that the depth of the cavity 326 may be 0.86-21 mm. In additional or alternative embodiments, the thickness of the positive electrode 308 may be 20%-120% of the thickness of the carbon foam 306. In one embodiment, the thickness of the positive electrode 308 may be 100% to 110% of the thickness of the carbon foam 306 .

[0090] In one embodiment, the depth of the cavity 326 may further depend on the crush strength of the carbon foam 306 (e.g., the depth of the cavity 326 may increase with decreasing crush strength). For example, if the depth of the cavity 326 is 9.2 mm (e.g., if the desired compression of the positive electrode is 0.4 mm), the foam crush factor of safety (FOS) may be 5.78. The foam crush FOS may have a minimum value of 0.34 in some embodiments, and a foam crush FOS value less than 1 may indicate that at least some fracture is expected. In some embodiments, the crush strength of the carbon foam 306 may be reduced by heat treatment of the carbon foam 306 during its manufacture (in one embodiment, 0.08 MPa to 0.03 MPa). It will be appreciated that the electrode assembly 302 may be configured such that the depth of the cavity 326 is as low as possible (e.g., within the constraints set forth above) because a generally thinner electrode assembly 302 (e.g., when electrolyte flow may be closer to the negative electrode 310) may result in a reduced overall size of the rebalancing cell and reduced electrical resistance across the electrode assembly 302.

[0091] In this manner, the electrode assembly 302 may include a sequential stack of carbon foam 306 and a jointed pair of positive and negative electrodes 308, 310 that are in face-to-face contact with one another and are continuously conductive. Specifically, a first joint may be formed between the positive electrode 308 and the carbon foam 306, and a second joint may be formed between the positive electrode 308 and the negative electrode 310, where the second joint is opposite the first joint across the positive electrode 308, and each of the carbon foam 306, positive electrode 308, and negative electrode 310 may be conductive. Thus, the electrode assembly 302 may be internally shorted such that current flowing through the electrode assembly 302 cannot be channeled through an external load.

[0092] In an exemplary embodiment, as discussed above, forced convection can induce a flow of H2 gas across the negative electrode 310 to the electrode assembly 302 (e.g., via a flow field plate interfaced with the negative electrode 310, not shown in FIG. 3). The H2 gas can then react with the catalytic surface of the negative electrode 310 via equation (4a) (e.g., the reverse reaction of equation (1)): 1 / 2H2→H + +e - (anodic half reaction) (4a) Proton (H + ) and electrons (e - ) may be conducted across the negative electrode 310 to the positive electrode 308. The electrolyte conducted through the electrode assembly 302 via the carbon foam 306 may be wicked to the positive electrode 308. At and near the second junction between the positive electrode 308 and the negative electrode 310, the Fe 3+ can be reduced via equation (4b): Fe 3+ +e - →Fe 2+ (Cathodic half-reaction) (4b) Adding together equations (4a) and (4b), the electrolyte rebalancing reaction can be obtained as equation (4): Fe 3+ +1 / 2H2→Fe 2+ +H + (Electrolyte Rebalancing) (4)

[0093] Since the electrode assembly 302 is internally shorted, the cell potential of the electrode assembly 302 can be driven to zero as follows: 0=(E 正 -E 負 )-(η act +η mt +η オーム ) (7) In the formula, E 正 is the potential of the positive electrode 308, and E 負 is the potential of the negative electrode 310, and η act is the activation overpotential, and η mt is the mass transport overpotential, and η オームis the ohmic overvoltage. For the electrode assembly 302 configured in FIG. mt and η act can be assumed to be negligible. Furthermore, η オーム is the electrolyte overpotential η 電解質 and the overvoltage η of the carbon felt forming the positive electrode 308 フェルト may depend on. η オーム = η 電解質 +η フェルト (8) Therefore, the performance of the electrode assembly 302 depends on at least the electrical resistivity σ 電解質 and the electrical resistivity σ of carbon felt フェルト The electrical conductivity of the electrolyte and the electrical conductivity of the carbon felt can be limited by the resistance of the electrolyte R 電解質 and the resistance of the carbon felt R フェルト and may be given as follows: R 電解質 = σ 電解質 ×t 電解質 / A 電解質 (9) R フェルト = σ フェルト ×t フェルト / A フェルト (10) In the formula, t 電解質 is the thickness of the electrolyte (i.e., the height of the electrolyte front) and t フェルト is the thickness of the carbon felt (e.g., the thickness of the positive electrode 308), and A 電解質 is the active area (front) of the electrolyte, and A フェルト is the active area of ​​the carbon felt. Thus, the performance of the electrode assembly 302 may be further limited based on the electrolyte front location within the carbon felt and therefore the distribution of electrolyte across the carbon foam 306 and the amount of electrolyte wicked into the carbon felt that forms the positive electrode 308.

[0094] R 電解質 and R フェルト After determining, the current I アセンブリ may be determined as follows: Iアセンブリ =(E 正 -E 負 ) / (R 電解質 +R フェルト ) (11) Rate of the electrolyte rebalancing reaction v リバランシング (For example, Fe 3+ The reduction rate of the cation exchanger (C) can be further determined as follows. v リバランシング =I アセンブリ / (nFA リバランシング ) (12) where n is the number of electrons flowing through the negative electrode 310, F is the Faraday constant, and A リバランシング is the active area of ​​the electrolyte rebalancing reaction (e.g., the area of ​​the interface between the electrolyte front and the negative electrode 310). フェルト For uncompressed carbon felt with v = 3 mm, リバランシング is 113 mol / m 2 It may have a maximum value of time.

[0095] 4A, an isometric view 400 of a rebalancing cell 402 for a redox flow battery system, such as the redox flow battery system 10 of FIG. 1, is shown. It will be understood that the rebalancing cell 402 represents another embodiment of one or more of the rebalancing reactors / cells 80 and 82 of FIG. 1. Additionally, the rebalancing cell 402 may include components and an overall shape similar to the rebalancing cell 202 of FIG. 2A-2B. Thus, the rebalancing cell 402 includes a stack of electrode assemblies, each of which is configured similarly to the electrode assembly 302 of FIG. 3, stacked between an upper end plate 432 and a lower end plate 436. The rebalancing cell 402 may drive an electrolyte rebalancing reaction between H2 gas and a positive or negative electrolyte from a compartment of a redox flow battery, such as the redox flow battery cell 18 of FIG. 1.

[0096] The rebalancing cell 402 may be configured to receive electrolyte at an electrolyte inlet port 404. The electrolyte inlet port 404 may be similar to the electrolyte inlet port 206 of FIGS. 2A-2B. The electrolyte may be pumped from the negative and positive electrode compartments, e.g., the negative and positive electrode compartments 20 and 22 of FIG. 1, to the inlet port 404 of the rebalancing cell 402. Additionally or alternatively, the electrolyte may be pumped from an electrolyte storage tank, as shown in FIG. 5 and described below. The electrolyte may pass through the rebalancing cell 402 and may undergo reduction with hydrogen gas according to equation (4), after which the electrolyte may exit the rebalancing cell 402 through the electrolyte outlet port 410. The electrolyte outlet port 410 may be similar to the electrolyte outlet port 208 of FIGS. 2A-2B. The electrolyte exiting the rebalancing cell 402 may be pumped to an electrolyte storage tank before refilling the electrode compartments of the redox flow battery system. Hydrogen gas that reacts with the electrolyte in the rebalancing cell 402 can enter through hydrogen inlet port 412, which can be similar to hydrogen gas inlet port 210 of FIGS. 2A-2B. Any unreacted hydrogen gas can pass through the rebalancing cell 402 and exit through hydrogen gas outlet port 406, which can be similar to hydrogen gas outlet port 212 of FIGS. 2A-2B. As discussed with reference to FIGS. 2A-2B, other embodiments of the rebalancing cell 402 can include more or less of the electrolyte inlet port 404 and electrolyte outlet port 410, and more or less of the hydrogen inlet port 412 and hydrogen outlet port 406. Additionally, the electrolyte inlet port 404 and electrolyte outlet port 410, and the hydrogen inlet port 412 and hydrogen outlet port 406 can be located at different locations on the rebalancing cell 402 compared to those shown in FIGS. 4A-4B, as discussed above with reference to FIGS. 2A-2B.

[0097] An upper end plate 432 may form an upper outer surface of the rebalancing cell 402. Hydrogen gas and electrolyte may pass through a plurality of electrode assemblies 434 that form the body of the rebalancing cell 402, each of which may be configured similarly to the electrode assembly 302 of FIG. 3. The plurality of electrode assemblies (i.e., electrode assembly stack) 434 may each include a negative electrode and a positive electrode in face-to-face contact with each other, in addition to other components described in detail above with reference to FIG. 3. A lower end plate 436 may form a lower outer surface (e.g., relative to the negative z-axis) of the rebalancing cell 402. An electrolyte outlet 410 and a hydrogen inlet 412 may be positioned such that hydrogen gas and electrolyte may pass through the lower end plate 436.

[0098] The conductive coupler 408 may extend from the lower end plate 436 of the rebalancing cell 402 to conductively couple the electrode assembly to an external voltage source, as shown in FIG. 4B. In an exemplary embodiment, the conductive coupler 408 may be a conductive wire that protrudes from a face of the rebalancing cell 402 along which the electrolyte outlet 410 and the hydrogen inlet 412 are located. For example, the conductive coupler 408 may protrude from an outer surface of the lower end plate 436. The conductive wire may be formed from a rigid, conductive, ductile material. As an example, the material of the conductive wire may be titanium or carbon.

[0099] 4B, the rebalancing cell 402 is shown in a cross-sectional view 430. Similar to the isometric view 400 of FIG. 4A, the gas outlet port 406 and the electrolyte inlet port 404 are located at the top of the rebalancing cell 402 (relative to the positive z-axis), allowing fluids and gases to pass, for example, across the inner surface of the top end plate 432. The end plate may be coupled to the outermost (i.e., at either end of the rebalancing cell 402 along the z-axis) electrode assembly of the multiple electrode assemblies 434, which may be coupled to each other in a manner that seals the fluids and gases within the rebalancing cell 402. As a result, fluids and gases can only enter and exit the rebalancing cell 402 through the appropriate inlets and outlets. The conductive coupler 408 may extend from one of a number of electrode assemblies 434 at the bottom of the rebalancing cell 402 and may be coupled to a lower end plate 436 and may protrude from the rebalancing cell 402 through the lower end plate 436 via a compression joint 440. The compression joint 440 is described in more detail below with respect to FIG.

[0100] 4C, the region 438 shown in FIG. 4B is shown in more detail in FIG. 4C, showing the arrangement of conductive rods 462 that may be disposed between the positive electrode of one electrode cell of the plurality of electrode assemblies 434 and the negative electrode of an adjacent electrode cell. Thus, some conductive rods 462 included in the rebalancing cell 402 may correspond to some electrode assemblies of the plurality of electrode assemblies 434, and the conductive rods may be aligned vertically, e.g., parallel to the z-axis, along the height of the rebalancing cell 402 and along each stack of the plurality of electrode assemblies 434. The conductive rods 462 may be graphite rods in one embodiment. The diameter of the conductive rods 462 may be selected to match the inner diameter of the bores 464 that protrude from each bipolar plate 450 of the plurality of electrode assemblies 434 along the z-axis. The bores 464 of each bipolar plate 450 may contact the face of the adjacent bipolar plate 450. The length (eg, as defined along the z-axis) of the conductive rods 462 can be equal to or greater than the distance between adjacent bipolar plates 450 .

[0101] The length of the conductive rod 462 may cause a positive electrode 452, formed from a flexible material such as felt, of each of the multiple electrode assemblies 434 to be compressed between the end of the conductive rod 462 and the bipolar plate 450. Compression of the positive electrode 452 increases contact between the positive electrode 452 and a negative electrode, such as the negative electrode 310 of FIG. 3, coupled to the bipolar plate 450. Thus, the negative electrode may be electrically shorted to the positive electrode 452, creating a conductive path through each of the multiple electrode assemblies 434 of the rebalancing cell 402.

[0102] The inner diameter of the bore 464 may be larger than the diameter of the conductive rod 462. The inner diameter of the bore 464 may allow the conductive rod 462 to have an interference fit that maintains the position of the conductive rod 462 within the bore 464 but allows the conductive rod 462 to be repositioned, e.g., shifted, during assembly of the rebalancing cell 402. The bore 464 may be configured as a sleeve that circumferentially surrounds the conductive rod 462 along the space between the bipolar plates 450. In one embodiment, the bore 464 may be connected to, e.g., integrated into, the bipolar plate 450, for example, as shown in FIG. 4C. However, in other embodiments, the bore 464 may be a separate piece that may be attached to the bipolar plate 450, for example, by welding.

[0103] The conductive coupler 408 may electrically couple the first electrode cell 434a of the plurality of electrode assemblies 434 to an external voltage source. When the first electrode cell 434a is electrically coupled to an adjacent electrode cell, e.g., the electrode cell directly above the first electrode cell 434a along the z-axis, via the conductive rod 462, the conductive coupler 408 may be electrically coupled to each electrode cell of the plurality of electrode assemblies 434. In an exemplary embodiment, the conductive coupler 408 may contact and be coupled to the positive electrode 452 of the first electrode cell 434a. The conductive coupler 408 may extend through a compression fitting 440, which may be an opening in the sleeve-reinforced lower end plate 436. The compression fitting 440 may allow the conductive coupler 408 to extend from outside the rebalancing cell 402 to an external voltage source while maintaining the electrolyte and hydrogen gas sealed within the rebalancing cell.

[0104] An implementation of conductive wires (e.g., conductive couplers) for applying an ion-repulsive potential to the rebalancing cell is shown in FIG. 5. Therein, a schematic diagram of an electrolyte management system 500 is illustrated, showing electrical and fluid connectivity between a rebalancing cell 508 (which may be configured similarly to the rebalancing cell 402 of FIGS. 4A-4C) and an electrolyte tank 504 (which may be configured similarly to the multi-chamber electrolyte storage tank 110 of FIG. 1). Fluid pathways are represented as arrows. Electrolyte may flow from the electrolyte tank 504 to the rebalancing cell 508 through a conduit 502 as driven by a pump 518. Gravity may also assist in draining the electrolyte from the rebalancing cell 508 to the electrolyte tank 504, as described above with reference to FIGS. 2A-2B. One terminal of an external voltage source 510 may be connected to the rebalancing cell 508 via a conductive wire 514, which may be similar to the conductive coupler 408 of FIGS. 4A-4C. The external voltage source 510 may be a battery, such as, for example, a lithium ion battery or a nickel metal hydride battery. Although a negative terminal is coupled to the conductive wire 514 in FIG. 5, in other embodiments, the positive terminal of the external voltage source 510 may instead be coupled to the conductive wire 514. A terminal of the external voltage source 510, e.g., the terminal not coupled to the conductive wire 514, may be connected to a counter electrode 506 disposed within the electrolyte tank 504, the counter electrode 506 being electrically coupled to the external voltage source 510 via an electrical wire 516. Additionally or alternatively, the counter electrode 506 may be disposed within a separate compartment fluidly coupled to the conduit 502.

[0105] The counter electrode 506 can be a conductive material that is physically and chemically capable of withstanding prolonged exposure to the electrolyte. For example, the counter electrode can be formed from titanium mesh, conductive carbon rod, or graphite felt. In one embodiment, the counter electrode 506 can have a surface area that corresponds to a target ratio of the total surface area of ​​the negative electrode of the rebalancing cell 508. For example, the surface area of ​​the counter electrode 506 can be one-fifth or less of the total surface area of ​​the negative electrode. Maintaining the surface area of ​​the counter electrode 506 less than the total surface area of ​​the negative electrode according to the target ratio can reduce the possibility of catalyst contamination at the negative electrode due to interactions with electrolyte anions.

[0106] Further, the external voltage source 510 may be grounded via an electrical ground 512, which may be a part of the redox flow battery system coupled to the rebalancing cell 508, as an example. For example, the housing of the redox flow battery system may be used to ground the external voltage source 510. The external voltage source 510 may be used to deliver a constant potential in the range of +2V to -2V to the electrode cell or assembly of the rebalancing cell 508. In one example, the constant potential may be a negative potential, such as -50mV to -800mV, and may be applied continuously during operation of the rebalancing cell 508. In another example, the external voltage source 510 may be used to selectively apply a potential to the rebalancing cell 508 during charging of the redox flow battery system.

[0107] By applying an ion-repulsive potential to the electrode cells of the rebalancing cell during operation of the rebalancing cell, ions can be repelled from the catalytic surface at the negative electrode of the electrode assembly. For example, when a negative potential is applied to the rebalancing cell via a conductive wire, a negative charge is placed on the electrode assembly, which can repel anions at the negative electrode and catalytic surface. Similarly, when a positive potential is applied, cations can be repelled from the negative electrode and catalytic surface. As a result, the formation of a double diffusion layer at the catalytic surface is suppressed.

[0108] In some examples, the magnitude of the applied potential may be varied according to an estimated deterioration in catalyst performance: for example, if the rate of ferric iron reduction is determined to be slowing, the magnitude of the applied potential may be increased accordingly to increase the repulsive forces that prevent ion adsorption at the catalyst surface.

[0109] An example of a method 600 for operating a rebalancing cell of an electrolyte management system shows a rebalancing cell including a stack of internal shorting electrode assemblies (e.g., current flowing through the stack of internal shorting electrode assemblies is not channeled to an external load). Furthermore, the rebalancing cell is operated at an ion repulsion potential applied from an external voltage source via a system such as the electrolyte management system 500 depicted in FIG. 5. The rebalancing cell can be implemented in a redox flow battery system to manage excess H2 gas and rebalance charge imbalance in the electrolyte of the redox flow battery system, so that the redox flow battery system can operate at a low internal pressure (e.g., the maximum pressure of the electrolyte and H2 gas in the redox flow battery system can be kept low, such as below a relatively low threshold pressure). For example, the low internal pressure can be at or near ambient pressure. In an exemplary embodiment, the redox flow battery system can be the redox flow battery system 10 of FIG. 1 and the rebalancing cell can be the rebalancing cell 402 of FIGS. 4A-4C. At least some steps or portions of steps (e.g., involving application of an ionic repulsion potential and receiving H2 gas and electrolyte for distribution in the rebalancing cell) can be performed via a controller, such as the controller 88 of FIG. 1, and can be stored as executable instructions in a non-transitory storage medium (e.g., memory) communicatively coupled to the controller. The method 600 can be performed during operation of the redox flow battery system. For example, electrolyte flow can be driven by one or more active pumps, and the redox flow battery system can be operated in a charging, discharging, or idle mode.

[0110] At 602, the method includes applying an ion-repulsive potential to the rebalancing cell from an external voltage source. As described above, the external voltage source may be a lithium-ion or nickel-metal hydride battery, and the electrical coupling of the external voltage source to the rebalancing cell may be controlled, for example, by a switch. When the switch is adjusted to a closed position, current may flow from the external voltage source to the rebalancing cell through conductive wires extending between the terminals of the external voltage source and the electrode assemblies of the stack of electrode assemblies of the rebalancing cell. In one embodiment, the switch may be continuously closed during operation of the redox flow battery system, thereby maintaining a constant potential on the stack of electrode assemblies. Alternatively, the switch may be selectively closed during charging of the redox flow battery system.

[0111] At 604, the method includes receiving H2 gas and electrolyte in the rebalancing cell through respective inlet ports of the rebalancing cell. Specifically, the electrolyte can be received in the rebalancing cell through a first inlet port, and H2 gas can be received in the rebalancing cell through a second inlet port. In one embodiment, the first inlet port can be positioned above the second inlet port with respect to the direction of gravity. The H2 gas can be delivered from a headspace of an electrolyte storage tank, and the electrolyte can be delivered from an electrode compartment of the redox flow battery system. The electrolyte can be continuously received in the rebalancing cell during operation of the redox flow battery system, while the H2 gas can be received in the rebalancing cell when the pressure in the electrolyte storage tank reaches a threshold pressure due to accumulation of H2 gas. However, in other embodiments, when H gas is detected to reach a threshold pressure and is selectively delivered to the rebalancing cell along with electrolyte from the electrode compartment, electrolyte can be selectively flowed to the rebalancing cell, for example, by opening and closing a valve and / or by operating a hydrogen pump.

[0112] At 606, the method includes distributing the H2 gas and electrolyte throughout the stack of internal shorting electrode assemblies. Specifically, the electrolyte may be distributed through an inlet manifold including a plurality of first inlet channels each coupled to an electrode assembly of the stack of internal shorting electrode assemblies, and the H2 gas may be distributed through a second inlet channel formed by the stack of internal shorting electrode assemblies and fluidly coupled to each electrode assembly of the stack of internal shorting electrode assemblies. In some embodiments, after distribution through the inlet manifold, the electrolyte may flow through a first flow field plate that is in contact with each positive electrode of the stack of internal shorting electrode assemblies. In other embodiments, after distribution through the inlet manifold, the electrolyte may flow through an activated carbon foam in contact with the positive electrode. In some embodiments, after distribution through the second inlet channel, the H2 gas may be distributed through a second flow field plate that is in contact with a negative electrode of the stack of internal shorting electrode assemblies.

[0113] At 608, the method includes inducing a flow (e.g., lateral, parallel, or opposing flow) of H2 gas and electrolyte to facilitate an electrolyte rebalancing reaction at the negative and positive electrodes of the stack of internal shorting electrode assemblies. The negative and positive electrodes may be disposed between the stack of internal shorting electrode assemblies in a mated pair of negative and positive electrodes. As discussed above, each positive electrode of the mated pair of negative and positive electrodes may further be mated with a respective activated carbon foam or a respective first flow field plate. In one example, the negative electrode may be a conductive carbon substrate coated with Pt catalyst thereon, and the positive electrode may be a carbon felt.

[0114] In some examples, inducing a flow of H2 gas and electrolyte may include (i) inducing a flow of H2 gas across the negative electrode of the internal shorting electrode assembly stack via convection (e.g., forced convection through a second flow field plate mating with the negative electrode of the internal shorting electrode assembly stack) at 610, and (ii) inducing a flow of electrolyte across the positive electrode of the internal shorting electrode assembly stack via one or more of gravity feed (e.g., by tilting the rebalancing cell against the direction of gravity), capillary action (e.g., wicking the electrolyte to the positive electrode of the internal shorting electrode assembly stack), and convection (e.g., forced convection through a first flow field plate mating with the positive electrode of the internal shorting electrode assembly stack) at 612. In one example, the flow of H2 gas may be induced across the negative electrode by convection, and the flow of electrolyte may be induced across the positive electrode at low internal pressure by each of gravity feed and capillary action. Flowing H2 gas and electrolyte across the negative and positive electrodes of the internal shorting electrode assembly stack can result in an electrolyte rebalancing reaction at 614 that can include reacting H2 gas with positively charged ions in the electrolyte to reduce the positively charged ions (see equation (4)).

[0115] At 616, the method continues by adding electrolyte (reduced positively charged ions, e.g., Fe at a lower concentration than when initially received at the first inlet port at 604). 3+ , and venting any unreacted H2 gas from the rebalancing cell through an H2 gas outlet port. Specifically, at 618, the electrolyte may be vented from the rebalancing cell through a first outlet port (e.g., an electrolyte outlet port), and at 620, the unreacted H2 gas may be vented from the rebalancing cell through a second outlet port (e.g., a gas relief port). The method ends.

[0116] The benefit of applying an ion-repulsion voltage to the rebalancing cell can be experimentally demonstrated by testing a sub-scale redox flow battery design. Figure 7 shows the ion-repulsion voltage in mol / m2 Ferric iron (Fe 3+ 7 shows a graph 700 depicting the normalized performance of the negative electrode of a rebalancing cell, plotted on a percentage scale against the rate of reduction of .) ions. Data set 702, plotted as solid circles, represents the rebalancing cell operation with an applied potential of +2V to -2V relative to the counter electrode from an external voltage source. Data set 704, plotted as diamonds, represents the rebalancing cell operation without an applied potential. The results illustrated in graph 700 show improved durability and long-term efficiency of the negative electrode (e.g., the catalyst at the negative electrode) when a potential is applied compared to when no potential is applied.

[0117] In this manner, the multiple electrode assemblies of the rebalancing cell can be conductively coupled to an external voltage source that can be used to apply a potential across each negative electrode of the multiple electrode assemblies, thus repelling ions from the catalyst at the surface of the negative electrode. By repelling ions, the formation of a diffuse double layer at the catalyst can be inhibited, which could otherwise reduce the activity of the catalyst and result in degradation of the rebalancing cell performance.

[0118] 2A-4C show example configurations with relative positioning of various components. Where elements are shown to be in direct contact or directly coupled to one another, then such elements may, at least in one embodiment, be referred to as being in direct contact or directly coupled, respectively. Similarly, elements shown contiguous or adjacent to one another may, at least in one embodiment, be contiguous or adjacent to one another, respectively. As one embodiment, components placed in surface-sharing contact with one another may be referred to as surface-sharing contact. As another embodiment, in at least one embodiment, elements positioned apart from one another have only space between them, and other components may not be so referred to. As yet another embodiment, elements shown above / below one another, opposite one another, or to the left / right of one another may be so referred to relative to one another. Additionally, as shown in the figures, the topmost element or point of an element may, at least in one embodiment, be referred to as the "top" of the element, and the bottommost element or point of an element may, at least in one embodiment, be referred to as the "bottom" of the element. As used herein, top / bottom, upper / lower, above / below may be relative to the vertical axis of the drawing and may be used to describe the positioning of elements of the drawing relative to one another. Thus, in one embodiment, elements shown above other elements are positioned vertically above the other elements. As yet another example, the shapes of elements depicted in the drawings may be referred to as having those shapes (e.g., circular, rectilinear, flat, curved, rounded, chamfered, angled, etc.). Furthermore, elements shown intersecting one another may, at least in one embodiment, be referred to as intersecting elements or elements intersecting one another. Still further, in one embodiment, elements shown within or outside another element may be so referred to. Although FIGS. 2A-4C are drawn approximately to scale, other dimensions or relative dimensions may be used.

[0119] The present disclosure also provides support for a redox flow battery system comprising one or more redox flow battery cells, a rebalancing cell configured to catalyze hydrogen reduction of metal cations, receiving electrolyte from the one or more redox flow battery cells, and a conductive coupler connecting the rebalancing cell to an external voltage source to enable application of an ionic repulsion potential to the stack of electrode assemblies of the rebalancing cell during operation of the redox flow battery system. In a first embodiment of the system, the conductive coupler is a conductive wire electrically coupled to the external voltage source and to one electrode assembly of the stack of electrode assemblies, the external voltage source being one of a lithium ion battery or a nickel metal hydride battery. In a second embodiment of the system, optionally including the first embodiment, each electrode assembly of the stack of electrode assemblies includes a positive electrode in face-to-face contact with a negative electrode. In a third embodiment of the system, optionally including one or both of the first and second embodiments, each electrode assembly is electrically shorted by a surface-shared contact between the positive and negative electrodes. In a fourth embodiment of the system, optionally including one or more or each of the first through third embodiments, the negative electrode of each electrode assembly includes a catalyst on a surface of the negative electrode, the catalyst catalyzing hydrogen reduction of metal cations. In a fifth embodiment of the system, optionally including one or more or each of the first through fourth embodiments, the ion repelling potential is a negative potential between -50 mV and -800 mV, the ion repelling potential repelling anions from the catalytic surface of the rebalancing cell. In a sixth embodiment of the system, optionally including one or more or each of the first through fifth embodiments, the ion repelling potential is a potential between -2 mV and 2 mV. In a seventh embodiment of the system, optionally including one or more or each of the first through sixth embodiments, the system further comprises a counter electrode electrically coupled to an external voltage source and positioned within the electrolyte storage tank of the redox flow battery system.

[0120] The present disclosure also provides support for a method for a redox flow battery system, comprising: applying an ion-repulsive potential to a rebalancing cell of the redox flow battery system from an external voltage source to inhibit adsorption of ions on a catalytic surface of the rebalancing cell; and receiving hydrogen gas and an electrolyte in the rebalancing cell to facilitate a rebalancing reaction in an electrode assembly stack of the rebalancing reaction, the hydrogen gas being delivered from an electrolyte storage tank and the electrolyte being delivered from an electrode compartment of the redox flow battery system, the rebalancing reaction comprising reducing metal cations in the electrolyte via the hydrogen gas. In a first embodiment of the method, applying the ion-repulsive potential to the rebalancing cell comprises coupling a conductive wire to a first electrode assembly of the rebalancing cell, the conductive wire extending between the first electrode assembly and the external voltage source. In a second embodiment of the method, optionally including the first embodiment, coupling the conductive wire to the first electrode assembly comprises attaching the conductive wire to a positive electrode of the first electrode assembly. In a third embodiment of the method, optionally including one or both of the first and second embodiments, applying an ionic repulsion potential to the rebalancing cell includes conducting an ionic repulsion potential from a first electrode assembly to a plurality of electrode assemblies of an electrode assembly stack of the rebalancing cell, the electrode assembly stack including the first electrode assembly, the first electrode assembly and the plurality of electrode assemblies being electrically coupled. In a fourth embodiment of the method, optionally including one or more or each of the first through third embodiments, hydrogen gas is received from a headspace of an electrolyte storage tank, the hydrogen gas being generated by a side reaction occurring at a plating electrode of a redox flow battery cell of the redox flow battery system. In a fifth embodiment of the method, optionally including one or more or each of the first through fourth embodiments, the electrolyte is received from one or more of a negative electrolyte compartment and a positive electrolyte compartment of a redox flow battery cell of the redox flow battery system.In a sixth embodiment of the method, optionally including one or more of the first through fifth embodiments, applying an ion repulsive potential to the rebalancing cell includes applying a potential between +2V and -2V. In a seventh embodiment of the method, optionally including one or more of the first through sixth embodiments, the rebalancing reaction is catalyzed at a negative electrode of the electrode assembly stack. In an eighth embodiment of the method, optionally including one or more of the first through seventh embodiments, the rebalancing reaction is catalyzed at an internal shorting electrode assembly of the electrode assembly stack, the internal shorting electrode assembly being internally shorted by placing the positive and negative electrodes of each of the internal shorting electrode assemblies in direct contact with each other.

[0121] The present disclosure also provides support for an electrolyte management system for a redox flow battery system comprising a redox flow battery cell, an electrolyte storage tank fluidly coupled to the redox flow battery cell, and a rebalancing cell configured to receive electrolyte from the redox flow battery cell and hydrogen gas from the electrolyte storage tank, the rebalancing cell being electrically coupled to an external voltage source via a conductive coupler to receive an ionic repulsion potential from the external voltage source, the ionic repulsion potential mitigating the formation of a double diffusion layer on a catalytic surface of the rebalancing cell. In a first embodiment of the system, the conductive coupler is connected to an electrode assembly disposed proximate an end plate of the rebalancing cell, the conductive coupler protruding from the rebalancing cell through a compression fitting extending through the end plate. In a second embodiment of the system, which optionally includes the first embodiment, the rebalancing cell includes an internal shorting electrode assembly having a positive electrode in direct contact with a negative electrode of the rebalancing cell, the positive electrode being in direct contact with the negative electrode by pressing the positive electrode against the negative electrode using a conductive rod coupled to each bipolar plate of the internal shorting electrode assembly.

[0122] The following claims particularly point out certain combinations and subcombinations that are deemed novel and unobvious. These claims may refer to "an" element or "first" element or equivalents. Such claims should be understood to include the incorporation of one or more such elements, and do not require or exclude two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed through amendment of the claims or through the presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, are also deemed to be included within the subject matter of this disclosure.

Claims

1. A redox flow battery system, one or more redox flow battery cells; a rebalancing cell that receives electrolyte from the one or more redox flow battery cells, the rebalancing cell configured to catalyze hydrogen reduction of metal cations; a conductive coupler connecting the rebalancing cell to an external voltage source to enable application of an ionic repulsion potential to the stack of electrode assemblies of the rebalancing cell during operation of the redox flow battery system.

2. 2. The redox flow battery system of claim 1, wherein the conductive coupler is a conductive wire electrically coupled to the external voltage source and to one electrode assembly of the stack of electrode assemblies, and the external voltage source is one of a lithium ion battery or a nickel metal hydride battery.

3. 3. The redox flow battery system of claim 2, wherein each electrode assembly of said stack of electrode assemblies includes a positive electrode in face-to-face contact with a negative electrode.

4. 4. The redox flow battery system of claim 3, wherein each electrode assembly is electrically shorted by the shared surface contact between the positive electrode and the negative electrode.

5. 4. The redox flow battery system of claim 3, wherein the negative electrode of each electrode assembly includes a catalyst on a surface of the negative electrode, the catalyst catalyzing the hydrogen reduction of the metal cations.

6. 2. The redox flow battery system of claim 1, wherein the ion repelling potential is a negative potential between −50 mV and −800 mV, and the ion repelling potential repels anions from a catalytic surface of the rebalancing cell.

7. 2. The redox flow battery system according to claim 1, wherein the ion repulsion potential is a potential of -2 mV to 2 mV.

8. 10. The redox flow battery system of claim 1, further comprising a counter electrode electrically coupled to the external voltage source and positioned within an electrolyte storage tank of the redox flow battery system.

9. 1. A method for a redox flow battery system, comprising: applying an ion repulsion potential from an external voltage source to the rebalancing cell of the redox flow battery system to inhibit ion adsorption on a catalyst surface of the rebalancing cell; receiving hydrogen gas and an electrolyte in the rebalancing cell to facilitate a rebalancing reaction in an electrode assembly stack of a rebalancing reaction, the hydrogen gas being delivered from an electrolyte storage tank and the electrolyte being delivered from an electrode compartment of the redox flow battery system, the rebalancing reaction comprising reducing metal cations in the electrolyte via the hydrogen gas.

10. 10. The method of claim 9, wherein applying the ion repulsion potential to the rebalancing cell comprises coupling a conductive wire to a first electrode assembly of the rebalancing cell, the conductive wire extending between the first electrode assembly and the external voltage source.

11. The method of claim 10 , wherein coupling the conductive wire to the first electrode assembly comprises attaching the conductive wire to a positive electrode of the first electrode assembly.

12. 11. The method of claim 10, wherein applying the ionic repulsive potential to the rebalancing cell comprises conducting the ionic repulsive potential from the first electrode assembly to a plurality of electrode assemblies of the electrode assembly stack of the rebalancing cell, the electrode assembly stack including the first electrode assembly, and the first electrode assembly and the plurality of electrode assemblies are electrically coupled.

13. 10. The method of claim 9, wherein the hydrogen gas is received from a headspace of the electrolyte storage tank, and the hydrogen gas is produced by a side reaction occurring at a plating electrode of a redox flow battery cell of the redox flow battery system.

14. 10. The method of claim 9, wherein the electrolyte is received from one or more of a negative electrode compartment and a positive electrode compartment of a redox flow battery cell of the redox flow battery system.

15. 10. The method of claim 9, wherein applying the ion-repulsive potential to the rebalancing cell comprises applying a potential between +2V and -2V.

16. The method of claim 9 , wherein the rebalancing reaction is catalyzed at a negative electrode of the electrode assembly stack.

17. 10. The method of claim 9, wherein the rebalancing reaction is catalyzed in an internally shorting electrode assembly of the electrode assembly stack, the internally shorting electrode assemblies being internally shorted by placing the positive and negative electrodes of each of the internally shorting electrode assemblies in direct contact with one another.

18. 1. An electrolyte management system for a redox flow battery system, comprising: A redox flow battery cell; an electrolyte storage tank fluidly coupled to the redox flow battery cell; a rebalancing cell configured to receive electrolyte from the redox flow battery cell and hydrogen gas from the electrolyte storage tank, the rebalancing cell being electrically coupled to an external voltage source via a conductive coupler such that the rebalancing cell receives an ionic repulsion potential from the external voltage source, the ionic repulsion potential mitigating formation of a double diffusion layer on a catalyst surface of the rebalancing cell.

19. 20. The electrolyte management system of claim 18, wherein the conductive coupler is connected to an electrode assembly disposed proximate an end plate of the rebalancing cell, the conductive couple protruding from the rebalancing cell through a compression joint extending through the end plate.

20. 20. The electrolyte management system of claim 19, wherein the rebalancing cell includes an internal shorting electrode assembly having a positive electrode in direct contact with a negative electrode of the rebalancing cell, the positive electrode being in direct contact with the negative electrode by pressing the positive electrode against the negative electrode with a conductive rod coupled to each bipolar plate of the internal shorting electrode assembly.

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