Rebalancing cell system for redox flow batteries

Abstract

A system and method for rebalancing cells in a redox flow battery is provided. In one embodiment, the rebalancing cell system includes a first rebalancing cell in series fluid communication with a second rebalancing cell and a hydrogen source, the first rebalancing cell including a first electrode assembly stack having a hydrogen flow path, the hydrogen flow path extending through the first electrode assembly stack and having a higher pressure than an electrolyte within the first electrode assembly stack. Further, the second rebalancing cell includes a second electrode assembly stack having a hydrogen flow path, the hydrogen flow path extending through the second electrode assembly stack and having a higher pressure than an electrolyte within the second electrode assembly stack.

Description

[Technical Field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Utility Application No. 17 / 821,400, entitled "REBALANCING CELL SYSTEM FOR REDOX FLOW BATTERY," filed August 22, 2022. The entire contents of the above-identified application are incorporated herein by reference for all purposes.

[0002] The present specification relates generally to a system for rebalancing cells for use in redox flow batteries, and a method for operating such a rebalancing cell system. [Background technology]

[0003] Redox flow batteries are well suited for grid-scale energy storage applications due to their ability to scale power and capacity independently, and are suitable for thousands of charge and discharge 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 hazardous chemicals, and reduce their environmental footprint.

[0004] IFB consists of a positive electrode (redox electrode) where the redox reaction occurs and ferrous iron (Fe 2+ and a negative (plating) electrode where the positive electrode (positive electrode) can be reduced and plated. The IFB may further include a rebalancing cell to maintain electrolyte health and battery capacity. In previous IFBs, hydrogen flow through the rebalancing cells was performed in a parallel configuration. Summary of the Invention [Problem to be solved by the invention]

[0005] The inventors have recognized that passing hydrogen flow through rebalancing cells in parallel has several drawbacks. For example, a parallel flow arrangement can reduce the flow rate through the rebalancing cells and increase the likelihood of hydrogen maldistribution due to uneven pressure drops across each of the parallel-coupled rebalancing cells. Furthermore, previous IFBs often required multiple devices to drive hydrogen flow, thereby increasing the manufacturing costs and maintenance demands of the system. [Means for solving the problem]

[0006] In one embodiment, the above-mentioned problems may be addressed by a rebalancing cell system for a redox flow battery. The rebalancing cell system includes, in one embodiment, a first rebalancing cell in series fluid communication with a second rebalancing cell and a hydrogen source. In such an example, the first rebalancing cell includes a first electrode stack having hydrogen flow channels, the hydrogen flow channels extending through the first electrode stack and having a higher pressure than the electrolyte within the first electrode stack. The second rebalancing cell similarly includes a second electrode stack having hydrogen flow channels, the hydrogen flow channels extending through the second electrode stack and having a higher pressure than the electrolyte within the second electrode stack. To drive hydrogen flow within the system, the hydrogen source may include a hydrogen tank fluidly connected in series with a hydrogen flow generator (e.g., a Venturi injector or hydrogen injector). For example, the outlet of the hydrogen flow generator may be in fluid communication with a hydrogen gas inlet within the first rebalancing cell. Alternatively, the hydrogen flow generator may be positioned upstream of the hydrogen tank and include an inlet fluidly connected to the hydrogen gas outlet port of the second rebalancing cell. Connecting rebalancing cells in series can increase the flow rate of hydrogen gas through each cell compared to cells in a parallel flow configuration, resulting in a more even distribution of the hydrogen gas. Furthermore, connecting rebalancing cells in series can allow a system to use fewer flow generators (e.g., a single flow generator) than a parallel rebalancing cell flow configuration. As a result, manufacturing and maintenance of the rebalancing cells can be simplified.

[0007] In one embodiment, the first rebalancing cell includes a first hydrogen gas inlet port in fluid communication with a hydrogen source. The first rebalancing cell further includes a first hydrogen gas outlet port and / or a first hydrogen gas relief port. Similarly, in such an example, the second rebalancing cell includes a second hydrogen gas inlet port in fluid communication with one of the first hydrogen gas outlet port and the first hydrogen gas relief port. The second rebalancing cell further includes a second hydrogen gas outlet port and / or a second hydrogen gas relief port in fluid communication with a hydrogen source. In this manner, hydrogen gas can flow out of one or both of the outlet port and the relief port. As a result, the hydrogen flow path can be tailored based on the desired reaction rate within the rebalancing cell, thereby improving the operation of the rebalancing cell.

[0008] It should be understood that the foregoing summary is provided to introduce a selection of concepts in a simplified form 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 uniquely 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 anywhere in this disclosure. [Brief explanation of the drawings]

[0009] [Figure 1] FIG. 1 shows a schematic diagram of an exemplary redox flow battery system including battery cells having redox and plating electrodes fluidly coupled to respective rebalancing reactors. [Figure 2A] FIG. 1 shows a perspective view of a rebalancing cell including a stack of internal short-circuit electrode assemblies. [Figure 2B] FIG. 1 shows a perspective view of a rebalancing cell including a stack of internal short-circuit electrode assemblies. [Figure 3] FIG. 3 shows an exploded view of the electrode assembly for the rebalancing cell of FIGS. 2A and 2B. [Figure 4A]FIG. 2C shows a cross-sectional view of a first H2 gas flow pattern within the rebalancing cell of FIGS. 2A and 2B. [Figure 4B] FIG. 2C shows a magnified inset of a first H2 gas flow pattern within the rebalancing cell of FIGS. 2A and 2B. [Figure 5A] FIG. 2C shows a cross-sectional view of a second H2 gas flow pattern within the rebalancing cell of FIGS. 2A and 2B. [Figure 5B] FIG. 2C shows a magnified inset of a second H2 gas flow pattern within the rebalancing cell of FIGS. 2A and 2B. [Figure 6] 1 shows an example of a rebalancing cell system with cells in a serial flow arrangement. [Figure 7] 1 shows an example of a rebalancing cell system with cells in different series flow arrangements. [Figure 8] 1 shows an example of a rebalancing cell system with cells in different series flow arrangements. [Figure 9] 1 shows an example of a rebalancing cell system with cells in different series flow arrangements. [Figure 10] 1 shows a flowchart of a method of operating a rebalancing cell system including a plurality of serially connected cells. DETAILED DESCRIPTION OF THE INVENTION

[0010] The following description relates to a system and method for distributing hydrogen through rebalancing cells in a series flow arrangement. The rebalancing cells maintain the electrolyte health and capacity of a redox flow battery. To perform this function, the rebalancing cells require sufficient hydrogen flow into the unit to support the desired reaction rate. Using a series flow arrangement increases the hydrogen flow rate through the cells, achieving a more balanced hydrogen distribution compared to a parallel flow arrangement.

[0011] A redox flow battery is depicted schematically in FIG. 1 as having an integrated multi-chamber tank with separate positive and negative electrolyte chambers. In some embodiments, the redox flow battery can be 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 chambers can be coupled to one or more battery cells, each cell containing a positive and negative electrode. From there, electrolyte can be pumped through the positive and negative electrode compartments, which house the positive and negative electrodes, respectively.

[0012] In some examples, the redox flow battery may be a hybrid redox flow battery. A hybrid redox flow battery 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, therefore, may depend on the efficiency of the plating system and the volume and surface area available for plating.

[0013] In some instances, electrolytic imbalance in redox flow batteries can result from numerous side reactions that compete with the desired redox chemistry, including hydrogen (H) gas production reactions such as proton reduction and iron corrosion: H + +e - ←→1 / 2H2 (proton reduction) (1) Fe 0 +2H + ←→Fe 2+ +H2 (iron corrosion) (2) and excess ferric iron (Fe) produced during the oxidation of iron plating. 3+ ) charge imbalance from: 2Fe 3+ +Fe 0 ←→3Fe 2+ (iron plating oxidation) (3) Includes: The reactions in equations (1)-(3) can limit iron plating, thereby reducing overall battery capacity. To address this imbalance, electrolyte rebalancing can be utilized to produce Fe in a single redox reaction. 3+ This can both reduce H and eliminate excess H. Fe 3+ +1 / 2H2 →Fe 2+ +H + (Electrolyte Rebalancing) (4)

[0014] As illustrated by the embodiments herein, sufficient Fe for relatively high performance applications 3+ The reduction rate can be reliably achieved through a rebalancing cell, such as the exemplary rebalancing cell of FIGS. 2A and 2B, including a stack of internally shorted electrode assemblies, such as the exemplary electrode assembly of FIG. 3. FIGS. 4A and 4B depict a first H2 gas flow pattern within the rebalancing cell. H2 gas flows across the negative electrode and into the hydrogen gas outlet port. FIGS. 5A and 5B depict a second H2 gas flow pattern within the rebalancing cell. The hydrogen gas outlet port is closed, and H2 gas flows across the positive electrode and into the hydrogen gas relief port. FIGS. 6-9 depict rebalancing cell systems in which different ports of serially connected rebalancing cells are fluidly connected to achieve different flow patterns. An exemplary method of operating a rebalancing cell system is depicted in FIG. 10. As described herein, serial flow connection refers to the connection of the inlet of a first device and the outlet of a second device. Conversely, a parallel flow connection indicates that the inlets of two devices are connected to one another and the outlets of two devices are likewise connected to one another.

[0015] 1, in a redox flow battery system 10 having a redox flow battery 11, 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.

[0016] "Anode" refers to the electrode where the electroactive material loses electrons, and "cathode" refers to the electrode where the 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 discharge, the negative electrolyte loses electrons, and the negative electrode 26 is the anode of the electrochemical reaction. Alternatively, during battery discharge, the negative electrolyte and negative electrode 26 may be referred to as the anolyte and anode of the electrochemical reaction, respectively, while the positive electrolyte and positive electrode 28 may be referred to as the catholyte and cathode of the electrochemical reaction, respectively. During battery charging, the negative electrolyte and negative electrode 26 may be referred to as the catholyte and cathode of the electrochemical reaction, respectively, while the positive electrolyte and positive electrode 28 may be referred to as the anolyte and anode of the electrochemical reaction, respectively. For simplicity, the terms "positive" and "negative" are used herein to refer to the electrodes, electrolyte, and electrode compartments in a redox flow battery system.

[0017] 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., FeCl, FeCl, etc.), and the negative electrode 26 includes metallic iron. For example, at the negative electrode 26, ferrous iron (Fe 2+ ) gains two electrons during battery charging and becomes iron metal (Fe 0 ) on the negative electrode 26, and Fe 0 loses two electrons during battery discharge, becoming Fe 2+ At the positive electrode 28, Fe 2+ During battery charging, ferric iron (Fe 3+) and Fe 3+ During battery discharge, 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 discharge. Fe 2+ +2e - ←→Fe 0 -0.44V (negative electrode) (5) Fe 2+ ←→2Fe 3+ +2e - +0.77V (positive electrode) (6)

[0018] As mentioned above, the negative electrolyte used in the IFB is a volatile electrolyte that dissolves Fe during battery charging. 2+ accepts two electrons from the negative electrode 26 and 0 and plated onto the substrate. 2+ During battery discharge, the plated Fe 0 loses two electrons to form Fe 2+ 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 is charged with Fe 2+ which loses an electron to provide Fe 3+ During battery discharge, Fe provided by the electrolyte is oxidized to 3+ by absorbing electrons provided by the positive electrode 28. 2+ The equilibrium potential for this reaction is +0.77 V, creating a positive terminal in the desired system.

[0019] IFBs may offer the ability to charge and recharge the electrolyte therein, as opposed to other battery types that utilize non-regenerating electrolytes. Charging may be accomplished by applying current across electrodes 26 and 28 via terminals 40 and 42, respectively. Negative electrode 26 contains (e.g., Fe 2 O 3 ) in a positive electrolyte in positive electrode compartment 22. 2+ Fe 3+The negative side of a voltage source can be electrically coupled via terminal 40 so that electrons can be delivered to the negative electrolyte via positive electrode 28 (when Fe is oxidized to Fe). The electrons provided to negative electrode 26 are transferred to the negative side of the negative electrolyte via terminal 40. 2+ is reduced to form Fe on the (plating) substrate. 0 can form Fe 2+ is plated onto the negative electrode 26.

[0020] The discharge is Fe 0 remains available to the negative electrolyte for oxidation, and Fe 3+ can persist while Fe remains available in the positive electrolyte for reduction. 3+ Availability can be achieved by providing additional Fe via an external source, such as an 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, Fe 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. 2+ As an example, Fe 2+ Availability is via an external source, such as the external negative electrolyte chamber 50, by adding 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.

[0021] In an IFB, the positive electrolyte may include ferrous iron, ferric iron, a ferric iron complex, or any combination thereof, while the negative electrolyte may include ferrous iron or a ferrous iron complex, 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 the 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.

[0022] Efficiency losses in IFBs can result from electrolyte crossover through the separator 24 (e.g., ion exchange membrane barrier, microporous membrane, etc.). For example, Fe in the positive electrolyte 3+ The ion is Fe 3+ The Fe ions can be driven towards the negative electrolyte by the ion concentration gradient and electrophoretic force across the separator 24. The Fe ions then penetrate the separator 24 and cross over into the negative electrode compartment 20. 3+ ions can result in coulombic efficiency losses. 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) 3+ The ions can lead to the precipitation of Fe(OH)3. The precipitation of Fe(OH)3 can degrade the separator 24 and cause permanent loss of battery performance and efficiency. For example, Fe(OH)3 precipitates can chemically foul the organic functional groups of the ion-exchange membrane or physically clog the pores of the ion-exchange membrane. In either case, the Fe(OH)3 precipitates can increase the membrane's ohmic resistance over time and reduce battery performance. While the precipitates can be removed by washing the IFB with acid, the constant maintenance and downtime can be disadvantageous for commercial battery applications. Furthermore, washing can depend on periodic electrolyte preparation, contributing to additional processing costs and complexity. Alternatively, adding certain organic acids to the positive and negative electrolytes in response to changes in the electrolyte pH can mitigate precipitate formation during battery charge and discharge cycling without increasing overall costs. Furthermore, Fe3+ Fouling may also be mitigated by implementing membrane barriers that inhibit ionic crossover.

[0023] Additional coulombic efficiency loss occurs due to 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, can cause the formation of H2 gas.

[0024] IFB electrolytes (e.g., FeCl, FeCl, FeSO, Fe(SO), etc.) may be readily available and can be produced at low cost. In one example, an IFB electrolyte may be formed from ferrous chloride (FeCl), potassium chloride (KCl), manganese(II) chloride (MnCl), and boric acid (HBO). Because the same electrolyte can be used for the negative and positive electrolytes, an IFB electrolyte may offer higher reusability, resulting in reduced cross-contamination issues compared to other systems. Furthermore, due to the electron configuration of iron, iron 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 an IFB system may enhance the battery's efficiency compared to other redox flow batteries. Furthermore, an iron redox flow battery may reduce the use of toxic raw materials and operate at a relatively neutral pH compared to other redox flow battery electrolytes. Therefore, the IFB system may pose a reduced environmental hazard compared to all other current advanced redox flow battery systems in production.

[0025] 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. 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 described above, the separator 24 may include an ion exchange membrane and / or a microporous membrane.

[0026] 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 produce higher voltages or currents in the redox flow battery system 10.

[0027] 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. The 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.

[0028] The redox flow battery system 10 may also include a first bipolar plate 36 and a second bipolar plate 38, each positioned along the 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 or through the negative and positive electrodes 26 and 28, respectively.

[0029] The IFB electrolyte may be transported to the reaction sites on 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, which 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 at 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 divided by a bulkhead 98. The bulkhead 98 may create multiple chambers within the multi-chamber electrolyte storage tank 110 so that both positive and negative electrolytes may be contained within a single tank. The negative electrolyte chamber 50 holds a negative electrolyte containing an electroactive material, and the positive electrolyte chamber 52 holds a positive electrolyte containing an electroactive material. The bulkhead 98 may be positioned within 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 the stoichiometric ratio between the negative and positive redox reactions. FIG. 1 further illustrates a fill height 112 of the multi-chamber electrolyte storage tank 110, which may indicate the liquid level within each tank compartment.

[0032] FIG. 1 also shows 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 can be utilized to store H gas generated during operation of the redox flow battery (e.g., by side reactions of proton reduction and iron corrosion) and carried to the multi-chamber electrolyte storage tank 110 with electrolyte returning from the redox flow battery cells 18. The H gas can naturally separate at the gas-liquid interface (e.g., fill height 112) within the multi-chamber electrolyte storage tank 110, thereby eliminating the need for an additional gas-liquid separator as part of the redox flow battery system 10. Once separated from the electrolyte, the H gas can fill the gas headspaces 90 and 92. The stored H gas can thus 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 can help reduce redox flow battery capacity loss. In this manner, utilizing the integrated multi-chamber electrolyte storage tank 110 may eliminate the need for separate negative and positive electrolyte storage tanks, hydrogen storage tanks, and gas-liquid separators common to conventional redox flow battery systems, thereby simplifying system design, reducing the physical footprint of the redox flow battery system 10, and reducing system costs.

[0033] 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 for equalizing gas pressure between the chambers 50 and 52. The spillover hole 96 may be positioned 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+) is 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, the overall electrolyte composition, battery module performance, and battery module capacity may be maintained.

[0034] Flange joints may be utilized for all plumbing connections between the inlet and outlet of the multi-chamber electrolyte storage tank 110 to maintain a leak-free, continuously pressurized state. 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 H gas to the rebalancing cells 80 and 82, such that the rebalancing cells 80 and 82 may be fluidly coupled to the gas headspaces 90 and 92, respectively. Hydrogen gas may circulate through the rebalancing cells 80 and 82 to maintain electrolyte health and battery capacity. The hydrogen gas flow architecture is described herein with reference to FIGS. 2A-9.

[0035] 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 example, 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 temperatures of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 independently or together. For example, in response to the electrolyte temperature falling below a threshold temperature, the controller 88 may increase the power supplied to the 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.

[0036] By way of example, the one or more heaters may include coil-type heaters or other immersion heaters immersed in the electrolyte fluid, or surface mantle-type heaters that conductively transfer 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 this disclosure. Additionally, the controller 88 may deactivate one or more heaters in the negative and positive electrolyte chambers 50 and 52 in response to the liquid level dropping below a solids fill threshold level. Alternatively, 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 the liquid level rising above a solids fill threshold level. In this manner, activation of one or more heaters without sufficient liquid in the negative and / or positive electrolyte chambers 50, 52 may be avoided, thereby reducing the risk of overheating or burning out of the heater(s).

[0037] Furthermore, 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. In this manner, the field hydration system may facilitate start-up of the redox flow battery system 10 at the end-use location, including installation, filling, and hydration of the redox flow battery system 10. Furthermore, prior to 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 having to fill 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 will be 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 can no longer be considered a portable dry system. Thus, from an end user's perspective, a 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.

[0038] 1 , the electrolyte solution initially stored in the multi-chamber electrolyte storage tank 110 can be pumped throughout the redox flow battery system 10 via negative and positive electrolyte pumps 30 and 32. The electrolyte stored in the negative electrolyte chamber 50 can be pumped through the negative electrode compartment of the redox flow battery cell 18 via the negative electrolyte pump 30, and the electrolyte stored in the positive electrolyte chamber 52 can be pumped through the positive electrode compartment 22 of the redox flow battery cell 18 via the positive electrolyte pump 32.

[0039] Electrolyte rebalancing cells 80 and 82 (e.g., reactors) may be connected in series or parallel with the electrolyte recirculation flow paths on the negative and positive sides of the redox flow battery cells 18, respectively, within the redox flow battery system 10. One or more rebalancing reactors may be connected in series with the electrolyte recirculation flow paths on the negative and positive sides of the battery, and other rebalancing reactors may be connected in parallel for redundancy (e.g., a rebalancing reactor may be maintained without interrupting the battery and rebalancing operation) and increased rebalancing capacity. In one example, the electrolyte rebalancing 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 cells 80 and 82 may help rebalance electrolyte charge imbalances in the redox flow battery system 10 that occur due to side reactions, ion crossover, etc., as described herein.

[0040] In one example, one or both of the rebalancing cells 80 and 82 may comprise a trickle-bed reactor, where H gas and (liquid) electrolyte may be contacted at a catalyst surface within a packed bed to carry out the electrolyte rebalancing reaction. Additionally or alternatively, one or both of the rebalancing cells 80 and 82 may have a catalyst bed configured within a jelly roll. In an additional or alternative example, one or both of the rebalancing cells 80 and 82 may comprise a flow-through reactor, which may contact H gas with the electrolyte and carry out the electrolyte rebalancing reaction without a packed catalyst bed. However, a lower Fe concentration during electrolyte rebalancing may be required. 3+ Reduction rate (e.g., about 1 to 3 mol / m 2 The long time required for the rebalancing reactor to reach the desired temperature (approximately 1000°C) may prevent the implementation of such a rebalancing reactor configuration in higher performance applications.

[0041] In other embodiments, one or both of rebalancing cells 80 and 82 may include a fuel cell, where H gas and an electrolyte may be contacted at a catalytic surface to carry out an electrolyte rebalancing reaction, and a closed circuit may be formed by directing current from the fuel cell through an external load. However, reverse current spikes in such fuel cells (e.g., a temporary increase in reverse current, where "reverse current" may be used herein to refer to any current traveling along an electrical path in a direction opposite to that expected (i.e., opposite to the "forward" direction)) may be unavoidable in certain circumstances and may undermine the reliability of such a rebalancing reaction configuration.

[0042] Fe without sacrificing the overall reliability of the rebalancing cells 80 and 82 3+To increase the reduction rate, embodiments of the present disclosure provide a rebalancing cell, such as the rebalancing cell of FIGS. 2A and 2B, that includes a stack of internally shorted electrode assemblies, such as the electrode assembly of FIG. 3, configured to drive H gas and electrolyte to react at the catalyst surface via a combination of internal 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 electrical current is not directed away from the stack of internally shorted electrode assemblies during operation of the rebalancing cell. Such an internal electrical short can result in the formation of a large amount of Fe 3+ Dramatically increase the reduction rate (e.g., about 50-70 mol / m 2 This can reduce or eliminate reverse current spikes while concomitantly reducing side reaction rates (e.g., the rates of the reactions of Equations (1)-(3)) and reducing reaction times. Furthermore, each electrode assembly in a stack of internal short-circuiting electrode assemblies can be electrically isolated from each other electrode assembly in the stack of internal short-circuiting electrode assemblies, so that degradation to the stack of internal short-circuiting electrode assemblies during a current spike in 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 cases, a single degraded electrode assembly can be easily removed from the stack of internal short-circuiting electrode assemblies and replaced with a non-degraded electrode assembly.

[0043] To achieve an internal short circuit, each electrode assembly of a stack of internal short-circuiting electrode assemblies may include a bonded pair of positive and negative electrodes (e.g., configured in continuous, surface-to-surface contact with each other for continuous electrical conductivity). 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 "bonded" to each other if the first component is positioned adjacent to the second component such that the first and second components are in surface-to-surface 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 multiple electrodes may refer to an electrical path therethrough that has substantially or practically zero resistance at any surface-to-surface contact of the multiple electrodes.

[0044] In an exemplary embodiment, the (positive) rebalancing cell 82 may be a rebalancing cell that includes a stack of internal short-circuit electrode assemblies. 3+ can be produced, 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 cell 80 may be similarly configured [Fe 3+ can be generated at the negative electrode 26 during iron plating oxidation (see equation (3))].

[0045] During operation of the redox flow battery system 10, sensors and probes can monitor and control electrolyte chemical properties, such as electrolyte pH, concentration, SOC, etc. For example, as illustrated in FIG. 1 , sensors 62 and 60 can be positioned to monitor the status of the positive and negative electrolytes in the positive electrolyte chamber 52 and the negative electrolyte chamber 50, respectively. In another example, sensors 62 and 60 can each include one or more electrolyte level sensors to indicate the level of electrolyte in the positive electrolyte chamber 52 and the negative electrolyte chamber 50, respectively. As another example, sensors 72 and 70, also illustrated in FIG. 1 , can monitor the status of the positive and negative electrolytes in the positive electrode compartment 22 and the negative electrode compartment 20, respectively. Sensors 72 and 70 can be pH probes, optical probes, pressure sensors, voltage sensors, etc. It will be understood that sensors can be positioned elsewhere throughout the redox flow battery system 10 to monitor electrolyte chemical and other properties.

[0046] For example, sensors may be positioned in the external acid tank to monitor the acid level or pH of 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 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. Furthermore, 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. Sensor information may then be sent to the controller 88, which may, for example, operate 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.

[0047] The redox flow battery system 10 may further include a source of H gas. In one embodiment, the source of H gas may include a separate, dedicated hydrogen gas storage tank. In the embodiment of FIG. 1 , H 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 H 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 H gas to the inlets of the electrolyte rebalancing cells 80 and 82. As one example, a mass flow meter or other flow control device (which may be controlled by the controller 88) may regulate the flow of H gas from the integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supplement the H gas generated within the redox flow battery system 10. For example, if a gas leak is detected in the redox flow battery system 10, or if a reduction reaction rate (e.g., Fe 3+ If the reduction rate (SOC) is too low, H gas can be supplied from the integrated multi-chamber electrolyte storage tank 110 to rebalance the SOC of the electroactive materials in the positive and negative electrolytes. As one example, the controller 88 can supply H 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 H gas 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 H 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 H gas from the integrated multi-chamber electrolyte storage tank 110 in response to a pH change where the pH 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 H to increase the pH of the redox flow battery system 10. 3+ This can increase the rate of ion reduction and proton production, thereby decreasing the pH of the positive electrolyte. Furthermore, the pH of the negative electrolyte can be increased by the amount of Fe crossing over from the positive electrolyte to the negative electrolyte. 3+ The pH of the negative electrolyte can be lowered by hydrogen reduction of the ions 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 the release of Fe(OH)3 as Fe(OH). 3+ This can be maintained within a stable region while reducing the risk of precipitation of ions (crossing over from the positive electrode compartment 22).

[0049] Other control schemes can be implemented to control the supply rate of H gas 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. Furthermore, the change in pH or SOC trigger action of the controller 88 can be based on a rate of change or a change measured over a period of time. The rate of change period can 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 can be shortened, and the time constant can be small so that local changes in concentration (e.g., due to side reactions or gas leaks) can be measured quickly.

[0050] The controller 88 may further implement 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. 10, in parallel with controlling the flow of H gas to the rebalancing cells 80 and 82 as described above, the controller 88 may simultaneously remove excess H gas from the redox flow battery system 10 and control the Fe 3+ To reduce the ion concentrations, the controller 88 may control the flow of negative and positive electrolytes to the rebalancing cells 80 and 82, respectively, during charging and discharging of the redox flow battery cell 18. After electrolyte rebalancing, the controller 88 may control the flow of rebalanced negative and positive electrolytes (e.g., Fe 3+ and reduced concentrations of Fe 2+ The flow of any excess or unreacted H, along with the increased concentration of H, may be directed back to the respective electrolyte chambers 50 and 52 of the multi-chamber electrolyte storage tank 110. Additionally or alternatively, the unreacted H gas may be returned to a separate, dedicated hydrogen gas storage tank.

[0051] As another example, the controller 88 may further control the charging and discharging of the redox flow battery cell 18 to cause pre-forming 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 increase battery charge capacity during subsequent battery cycling (thus, iron metal may be pre-formed for battery cycling). In this manner, pre-forming 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 loss. 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.

[0052] It will be understood that all components except the sensors 60 and 62 and the integrated multi-chamber electrolyte storage tank 110 (and components contained therein) can be considered to be included in the power module 120. As such, the redox flow battery system 10 can be described as including the power module 120 fluidly coupled to the integrated multi-chamber electrolyte storage tank 110 and communicatively coupled to the sensors 60 and 62. In some embodiments, the power module 120 and the multi-chamber electrolyte storage tank 110 can each be included in a single housing or packaging, such that the redox flow battery system 10 can be included as a single unit in a single location. It will further be understood that the positive electrolyte, the negative electrolyte, the sensors 60 and 62, the electrolyte rebalancing cells 80 and 82, and the integrated multi-chamber electrolyte storage tank 110 (and components contained therein) can be considered to be included in the electrolyte subsystem 130. As such, the electrolyte subsystem 130 can supply one or more electrolytes to the redox flow battery cells 18 (and components contained therein).

[0053] 2A and 2B, perspective views are shown, each depicting a rebalancing cell 202 for a redox flow battery system, such as redox flow battery system 10 of FIG. 1. In an exemplary embodiment, rebalancing cell 202 may include a stack of internally shorted electrode assemblies, such as the electrode assemblies described in detail below with reference to FIG. 3, which may drive an electrolyte rebalancing reaction by facilitating contact of H gas from the positive or negative electrode compartment of a redox flow battery, such as redox flow battery cell 18 of FIG. 1, with an electrolyte at a catalytic surface of the negative electrode of the stack of internally shorted electrode assemblies. Thus, rebalancing cell 202 may be one or both of rebalancing cells 80 and 82 of FIG. 1.

[0054] 2A-5B, a set of reference axes 250 is provided, which indicate the x-axis, y-axis, and z-axis. As further shown by dashed lines in FIGS. 2A and 2B, an additional axis g may be parallel to the direction of gravity (e.g., a positive direction along axis g) and the vertical direction (e.g., a negative direction along axis g, opposite the direction of gravity).

[0055] The number of rebalancing cells 202 included in a redox flow battery system and the number of electrode assemblies included in a stack of internally shorted electrode assemblies are not particularly limited and may likewise be increased to accommodate higher performance applications. For example, a 75 kW redox flow battery system may include two rebalancing cells 202, each including a stack of 20 electrode assemblies (e.g., a stack of 19 bipolar assemblies with two end plates positioned at opposite ends of the stack).

[0056] As shown, the stack of internal shorting electrode assemblies may be removably enclosed within an external cell enclosure (e.g., housing) 204. Accordingly, 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 shaped 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 with low electrical conductivity, such as a plastic or other polymer, to reduce undesired shorting events.

[0057] 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 plurality of inlet and outlet ports configured to fluidly couple to other components of the redox flow battery system. In one example, as shown, the plurality of inlet and outlet ports may include polypropylene (PP) flange fittings welded to PP piping.

[0058] In an exemplary embodiment, the plurality of 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. The electrolyte inlet and outlet ports may be referred to as positive inlet and outlet ports for purposes of the cell chemistry.

[0059] In one example, the electrolyte inlet port 206 may be positioned in the upper half of the cell enclosure 204, and the electrolyte outlet port 208 may be positioned in the lower half of the cell enclosure 204 (the upper and lower halves of the cell enclosure 204 are separated along the z-axis by a plane parallel to each of the x-axis and y-axis). Thus, the electrolyte outlet port 208 may be positioned lower than the electrolyte inlet port 206 with respect to the direction of gravity (e.g., along axis g).

[0060] Specifically, as electrolyte enters cell enclosure 204 via 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, react at the catalytic surface of the negative electrode in a cathodic half-reaction, and discharged from cell enclosure 204 via electrolyte outlet port 208. To assist in the gravity supply of electrolyte and increase its pressure drop, rebalancing cell 202 may be further tilted or inclined relative to the direction of gravity via sloped supports 220 coupled to cell enclosure 204. In some embodiments, tilting cell enclosure 204 in this manner may further assist in the drainage of electrolyte from rebalancing cell 202 (e.g., during an idle mode of the redox flow battery system) and keep the catalytic surface relatively dry (because, in some embodiments, the catalytic surface may corrode after being immersed in electrolyte for a sufficient period of time).

[0061] As shown, the graded support 220 can tilt the cell enclosure 204 at an angle 222 such that the plane of the electrode sheets of the stack of internal shorting electrode assemblies is tilted at the angle 222 relative to the bottom surface on which the graded 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; however, tilting the cell enclosure 204 at an angle greater than 0° can increase the pressure drop and reduce electrolyte crossover to the negative electrode. 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 approximately 8°. Thus, for example, the pressure drop of the electrolyte as it enters the cell enclosure 204 through the electrolyte inlet port 206 can be increased by increasing the angle 222 and can be decreased by decreasing the angle 222 .

[0062] Additionally or alternatively, one or more support rails 224 may be coupled to an upper half of the cell enclosure 204 (e.g., the side opposite the sloped support 220). In some embodiments, as shown in the perspective view 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 parallel to the upper surface of the lower surface. In this manner, and based on geometric considerations, the z-axis may similarly be offset from the axis g at an angle 222 (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 feeding of electrolyte through rebalancing cell 202 can be further assisted by positioning rebalancing cell 202 above an electrolyte storage tank (e.g., multi-chamber electrolyte storage tank 110 of FIG. 1 ) of a redox flow battery system, in a vertical direction opposite the direction of gravity.

[0063] As further shown, the electrolyte outlet port 208 may include multiple openings in the cell enclosure 204 (each of which may include a PP flange joint fixed to the PP piping) configured to discharge 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 short-circuiting electrode assemblies and may be discharged from the cell enclosure 204 in a substantially smooth flow ("substantially" may be used herein as a modifier meaning "effectively"). In other embodiments, the electrolyte outlet port 208 may include more or less than five openings. In one embodiment, the electrolyte outlet port 208 may include a single 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 the negative direction of the z-axis).

[0064] 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 short-circuiting 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). 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.

[0065] 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.

[0066] In some embodiments, the plurality of inlet and outlet ports may further include a hydrogen gas inlet port 210 for flowing H gas into the cell enclosure 204, and a hydrogen gas outlet port 212 (shown in FIG. 2B) for exhausting H gas from the cell enclosure 204. The hydrogen gas inlet and outlet ports may be referred to as negative inlet and outlet ports for cell chemistry purposes.

[0067] In one example, 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 example, 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 example, 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 example, the hydrogen gas inlet port 210 may be positioned lower than the hydrogen gas outlet port 212 with respect to the direction of gravity (e.g., along axis g).

[0068] Specifically, when H gas enters the cell enclosure 204 through the hydrogen gas inlet port 210, the H gas is distributed across and through the stack of internal short-circuiting electrode assemblies via forced convection (e.g., induced by the flow field configuration of each flow field plate) and may be decomposed in an anodic half-reaction at the catalytic surface of the negative electrode. However, in some embodiments, excess unreacted H gas may remain within the rebalancing cell 202 after contact with the catalytic surface. In some embodiments, at least a portion of the H gas that does not react 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 at the negative electrode, the multiple inlet and outlet ports may further include a hydrogen gas relief port 214 (as shown in FIG. 2A) to vent unreacted H gas from the electrolyte. The hydrogen gas relief port 214 may specifically receive hydrogen that has traveled through the electrolyte and therefore may be referred to as the positive side hydrogen relief port 214, and may specifically receive hydrogen that has traveled through the electrolyte and therefore may be referred to as the positive side hydrogen relief port.

[0069] Additionally, in some embodiments, the hydrogen gas outlet port 212 can be configured to exhaust at least a portion of the H gas that has not reacted at the catalyst surface and has not flowed through the negative electrode to the electrolyte. Further aspects of H gas flow are discussed in more detail below with reference to Figures 4A-5B.

[0070] 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 H gas through the stack of internal short-circuiting electrode assemblies. For example, the flow path may be from the hydrogen gas inlet port 210 to the hydrogen gas outlet port 212 (if included) and may include channels, passages, manifolds, plenums, etc. within the cell enclosure 204 and be fluidly coupled to the hydrogen gas inlet port 210 and the hydrogen gas outlet port 212 (if included). 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. Furthermore, although hydrogen gas inlet port 210 is shown in Figures 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 Figures 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.

[0071] In one example, hydrogen gas inlet port 210, hydrogen gas outlet port 212, electrolyte inlet port 206, and electrolyte outlet port 208 may be positioned on cell enclosure 204 in a lateral configuration. The lateral configuration may include hydrogen gas outlet port 212 and electrolyte inlet port 206 positioned on different sides (e.g., faces) of an upper half of cell enclosure 204, and hydrogen gas inlet port 210 and electrolyte outlet port 208 positioned on different sides of a lower half of cell enclosure 204.

[0072] In other embodiments, the hydrogen gas outlet port 212 may be closed, omitted, or otherwise inhibited from venting H gas that does not react at the catalytic surface of the negative electrode and flow through the negative electrode into the electrolyte. However, in such embodiments, the hydrogen gas relief port 214 remains present for venting unreacted H gas from the electrolyte, and unreacted H gas may be vented from the cell enclosure 204 only after flowing through the negative electrode into the electrolyte and through the hydrogen gas relief port 214. An exemplary rebalancing cell configuration lacking the hydrogen gas outlet port 212 may be referred to as a "dead-end configuration," regardless of whether it includes the hydrogen gas relief port 214. In a dead-end configuration, substantially all of the H gas may be forced into contact with the catalytic surface of the negative electrode, where it may decompose via the anodic half-reaction, and / or the H gas may enter the electrolyte after passing through the negative electrode (e.g., without reacting at its catalytic surface).

[0073] 2A and 2B can operate with a relatively small pressure gradient between the negative (hydrogen, high pressure) and positive (electrolyte, low pressure) sides of the cell. Because the cell is internally shorted, gravity and the higher hydrogen gas pressure prevent electrolyte from flooding the negative channel in the cell.

[0074] Referring now to FIG. 3, an exploded view of an electrode assembly 302 for a rebalancing cell, such as rebalancing cell 202 of FIGS. 2A and 2B, is shown. Thus, electrode assembly 302 can be internally shorted (e.g., current flowing through electrode assembly 302 is not channeled through an external load). In an exemplary embodiment, electrode assembly 302 can be included within a stack of similarly configured electrode assemblies within a cell enclosure to form a rebalancing cell. Electrode assembly 302 can include a plate 304 sequentially stacked with activated carbon foam 306, a positive electrode 308 (also referred to herein as the “cathode” in certain embodiments), and a negative electrode 310 (also referred to herein as the “anode” in certain embodiments). Electrode assembly 302 can be positioned within the rebalancing cell to receive electrolyte through carbon foam 306, from which the electrolyte can enter the pores of positive electrode 308 via capillary action and contact negative electrode 310. The electrode assembly 302 may further be positioned within the rebalancing cell to receive H gas across the catalytic surface of the negative electrode 310 opposite the positive electrode 308 via convection. The convection of H gas across the catalytic surface may be assisted by a flow field plate in contact with the catalytic surface. As H gas decomposes 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 the 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 the rebalancing cell, including the electrode assembly 302.

[0075] In some embodiments, plate 304 may be constructed of a material with low electrical conductivity, such as plastic or other polymer, to reduce undesired short circuit events. Thus, in one embodiment, plate 304 may be formed from the same material as cell enclosure 204 of Figures 2A and 2B.

[0076] As shown, the plate 304 may include multiple inlets and outlets therethrough. For example, the multiple 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 the electrolyte outlet channel section 316 for directing electrolyte from the rebalancing cell, the hydrogen gas inlet channel section 318a for directing H gas to the rebalancing cell across the negative electrode 310, and the hydrogen gas outlet channel section 318b for directing H 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 fluidly coupled to multiple electrolyte inlet passages 314a defined 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 to the electrolyte inlet well 312 (e.g., via an electrolyte inlet channel), the electrolyte outlet channel section 316 may discharge electrolyte through an electrolyte outlet port (e.g., electrolyte outlet port 208 in FIGS. 2A and 2B ) fluidly coupled to the electrolyte outlet channel section 316, 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 to the hydrogen gas inlet channel section 318 a, and the hydrogen gas outlet channel section 318 b may discharge H gas through a hydrogen gas outlet port (e.g., hydrogen gas outlet port 212 in FIGS. 2A and 2B ) fluidly coupled to the hydrogen gas outlet channel section 318 b.

[0077] 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., to receive H gas from a hydrogen gas inlet port and then direct the H gas across negative electrode 310 to the rebalancing cell) and gas inlet channel section 318a may be a section of a hydrogen gas outlet channel (e.g., to direct H gas from the rebalancing cell by exhausting the H gas through a hydrogen gas outlet port). In other embodiments, the rebalancing cell may be configured as a dead-end configuration, and no hydrogen gas outlet port may be fluidly coupled to hydrogen gas outlet channel section 318b. In such an embodiment, 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 the portion of the H2 gas to the rebalancing cell and across the negative electrode 310).

[0078] The multiple inlets and outlets may be configured to facilitate electrolyte and H gas flow throughout the rebalancing cell. As one 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 in the stack of internally shorted 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 for substantially even distribution of electrolyte flow. 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.

[0079] In additional or alternative embodiments, the electrolyte outlet channel section 316 may be further configured to distribute electrolyte through multiple openings included in the electrolyte outlet ports. For example, in the exploded view 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 ports, such that the openings in the electrolyte outlet channel section 316 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 fewer than two openings (e.g., a single opening).

[0080] Furthermore, when the electrode assembly 302 is included in 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 (as variously shown in Figures 4A, 4B, 5A, and 5B, described below). In this manner, the stack of electrode assemblies may be formed in a modular manner, whereby any practical number of electrode assemblies 302 may be stacked and included in a rebalancing cell.

[0081] As further shown, the multiple 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 one example, the multiple sealing inserts may include a hydrogen gas inlet channel sealing insert 320a and a hydrogen gas outlet channel sealing insert 320b to direct the flow of H gas across the negative electrode 310 by mitigating H gas bypass. Specifically, the hydrogen gas inlet channel sealing insert 320a and the hydrogen gas outlet channel sealing insert 320b may be affixed to 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, as discussed in more detail with reference to Figures 4A and 4B, 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 with the plate 304 and may partially overlap the positive electrode 308.

[0082] 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 the junction between the hydrogen gas inlet channel section 318a and the hydrogen gas inlet channel section of another electrode assembly, and the junction between the hydrogen gas outlet channel section 318b and the 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 fixedly attached to or otherwise adjacently coupled to the plate 304 so as to circumscribe the hydrogen gas inlet channel section 318a and the hydrogen gas outlet channel section 318b, respectively.

[0083] As another example, the multiple sealing inserts may further include an overboard O-ring 324 for sealing the junction of the electrode assembly 302 with another electrode assembly at its outer edge. 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 electrolyte inlet passage 314 a, the berm 314 b, the electrolyte outlet channel section 316, the hydrogen gas inlet channel section 318 a, and the hydrogen gas outlet channel section 318 b.

[0084] Carbon foam 306 may be positioned within cavity 326 of plate 304 between berm 314b and electrolyte outlet channel section 316 along the y-axis and between hydrogen gas inlet channel section 318a and hydrogen gas outlet channel section 318b along the x-axis. Specifically, carbon foam 306 may be positioned in flush contact with the side of plate 304 that forms the base of cavity 326. In some embodiments, carbon foam 306 may be formed as a continuous monolithic piece, while in other embodiments, carbon foam 306 may be formed as two or more carbon foam sections. In exemplary embodiments, carbon foam 306 may be conductive, permeable, and porous, providing a distribution field for electrolyte gravity-fed therethrough from multiple electrolyte inlet passages 314a. In some embodiments, the pore distribution of carbon foam 306 may be 10-100 PPI. In one embodiment, the pore distribution may 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 an electrolyte head rise of 2-3 mm.

[0085] 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 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.

[0086] In some embodiments, the flow field configuration can 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 can interface with each other electrode assembly 302 in a similar configuration (e.g., crossed, partially crossed, serpentine, etc.). In other embodiments, the location of a given electrode assembly 302 within the rebalancing cell 202 of FIGS. 2A and 2B can provide several different flow field configurations between the electrode assemblies 302 within the stack of electrode assemblies. In this manner, electrolyte can be directed from electrolyte inlet ports (e.g., electrolyte inlet ports 206 of FIGS. 2A and 2B) to flow field plates each interfaced with a positive electrode 308 within the stack of electrode assemblies, with the flow field plates configured in a staggered flow field configuration, a partially crossed flow field configuration, a serpentine flow field configuration, or a combination thereof.

[0087] 4A and 4B, in addition to replacing the carbon foam 306 with a flow field plate (also referred to herein as an "electrolyte flow field plate"), another 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, 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 still other embodiments, 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.

[0088] The positive electrode 308 can be positioned within the cavity 326 in flush contact with the side of the carbon foam 306 opposite from the plate 304 along the z-axis. In an exemplary embodiment, the positive electrode 308 can be a wicking conductive carbon felt, sponge, or mesh that can 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 can be conductive and porous (although in such examples, not as porous as the carbon foam 306). In one example, electrolyte can be wicked to the positive electrode 308 if the porosity of the carbon foam 306 can 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 threshold 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. Furthermore, although at least a portion of the H gas may pass through 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 H gas above it and the bulk of the electrolyte below it.

[0089] In some examples, each of the positive electrode 308 and the negative electrode 310 can be formed as a continuous monolithic piece (e.g., as opposed to discrete particles or multiple pieces), such that interphase mass transport losses across the boundary layer film can be reduced, thereby facilitating ion and proton transfer, when electrolyte is contacted with H gas at the catalytic surface of the negative electrode 310. In contrast, a packed bed configuration including discretely packed catalyst particles can include a mass transport-limiting 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.

[0090] 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 from 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 allows for the transfer of protons (e.g., H + ) and ion transport through the three-phase contact boundary (H3O + 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.

[0091] 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. In some examples, the porous non-conductive material may include polytetrafluoroethylene (PTFE), polypropylene, or 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 noble metal catalyst. In some examples, the noble metal catalyst may include Pt. In additional or alternative examples, the noble 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 noble metal catalyst supported on the conductive carbon substrate may be used due to cost considerations. However, in practice, the amount of the noble metal catalyst may not be 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. Furthermore, alloys included within the noble metal catalyst may be utilized to reduce cost and increase the corrosion stability of the noble metal catalyst. For example, adding 10% Rh to Pt reduces the Fe content of Pt. 3+ In other examples, the metal catalyst may include a non-noble 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 The electrode assembly 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.

[0092] In some embodiments, immersion of the negative electrode 310 can ultimately result in corrosion of the precious metal catalyst, such as when the precious metal catalyst comprises Pt. In other embodiments, as discussed in more detail above with reference to FIGS. 2A and 2B , the electrode assembly 302 (along with the electrode assembly stack and the entire rebalancing cell) can be tilted or inclined (e.g., the z-axis may not be parallel to the direction of gravity) relative to the surface on which the rebalancing cell rests, so that the precious metal catalyst can remain relatively dry as electrolyte flow 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 can be horizontal or inclined at an angle between 0° and 30° relative to the surface on which the rebalancing cell rests.

[0093] In an exemplary embodiment, the electrode assembly 302, including each of the carbon foam 306, the positive electrode 308, and the negative electrode 310, can be compressed along the z-axis, with the positive electrode 308 having a greater deflection under a given compression pressure than the carbon foam 306 and the negative electrode 310. Thus, the depth of the cavity 326 can 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.

[0094] Specifically, the depth of cavity 326 may be selected to be greater than a lower threshold depth of the sum of the thickness of carbon foam 306 after substantially full compression (to avoid overstressing and fracturing of carbon foam 306, which could impede electrolyte flow), and less than an upper threshold depth of the sum of the thickness of carbon foam 306 and the thickness of positive electrode 308 (to avoid zero compression of positive electrode 308, or possibly gaps, which could result in insufficient contact between H gas and electrolyte). For example, in an example where the carbon foam 306 is 6 mm thick, the positive electrode 308 is 3.4 mm thick, the desired compression of positive electrode 308 is 0.4 mm (to achieve a desired compression pressure of 0.01 MPa), and the negative electrode 310 is 0.2 mm thick, the depth of cavity 326 may be 9.2 mm (= 3.4 mm + 6 mm + 0.2 mm - 0.4 mm). As another example, the thickness of carbon foam 306 may be 2-10 mm, the thickness of positive electrode 308 may be 1-10 mm, the desired compression of positive electrode 308 may be 0-2.34 mm (to achieve a desired compression pressure of 0-0.09 MPa), the thickness of negative electrode 310 may be 0.2-1 mm, and the resulting depth of cavity 326 may be 0.86-21 mm.

[0095] In additional or alternative embodiments, the thickness of positive electrode 308 can be 20% to 120% of the thickness of carbon foam 306. In one embodiment, the thickness of positive electrode 308 can be 100% to 110% of the thickness of carbon foam 306. In one embodiment, the depth of cavity 326 can further depend on the crush strength of carbon foam 306 (e.g., the depth of cavity 326 can increase with decreasing crush strength). For example, if the depth of 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) can be 5.78. The foam crush FOS can have a minimum value of 0.34 in some embodiments, and a foam crush FOS value of less than 1 can indicate that at least some crushing is expected. In some embodiments, the crush strength of carbon foam 306 can be reduced (in one embodiment, to 0.08 MPa to 0.03 MPa) by heat treating carbon foam 306 during its manufacture.

[0096] 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 above constraints) 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.

[0097] 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 in face-to-face contact with one another and 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, the second joint being 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.

[0098] In an exemplary embodiment, as discussed above, forced convection can induce a flow of H gas across the negative electrode 310 (e.g., via a flow field plate interfaced with the negative electrode 310) and into the electrode assembly 302, where the H gas can 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 - ) can 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 can 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 in the electrolyte 3+ can be reduced via equation (4b). Fe 3+ +e - →Fe 2+ (Cathodic half-reaction) (4b) By summing equations (4a) and (4b), the electrolyte rebalancing reaction can be obtained as equation (4). Fe 3+ +1 / 2H2 →Fe 2+ +H + (Electrolyte Rebalancing) (4)

[0099] Because 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 of the electrolyte. 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 electrolyte thickness (e.g., 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 front position of the electrolyte within the carbon felt, and therefore the distribution of the electrolyte across the carbon foam 306, and the amount of electrolyte wicked into the carbon felt that forms the positive electrode 308.

[0100] R 電解質 and R フェルト After determining the current I of the electrode assembly 302 アセンブリ is the following: I アセンブリ =(E 正 -E 負 ) / (R 電解質 +R フェルト ) (11) It can be determined as follows: Rate of the electrolyte rebalancing reaction v リバランシング (e.g., Fe 3+ The reduction rate of v リバランシング =I アセンブリ / (nFA リバランシング ) (12) It can be further determined as where n is the number of electrons flowing through the negative electrode 310, F is Faraday's 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 junction 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 / m2 · time maximum. However, in alternative embodiments, other properties of the uncompressed carbon felt may be used.

[0101] The hydrogen gas flow through the rebalancing cell 202 shown in Figures 2A and 2B can be implemented via at least two patterns. A first flow pattern is shown in Figures 4A and 4B, in which the hydrogen gas outlet port 212 shown in Figure 2B is open. Conversely, a second flow pattern is shown in Figures 5A and 5B, in which the hydrogen gas outlet port 212 shown in Figure 2B is closed. Closing the hydrogen gas outlet port provides a dead-end flow configuration. The cutting plane of the cross-sectional views of Figures 4A-5B is parallel to the zx plane and extends through the central axis of the hydrogen gas inlet port 210.

[0102] 4A and 4B, a cross-sectional view and a close-up inset 450, respectively, of rebalancing cell 202 are shown. Each of the cross-sectional view and close-up inset 450 depicts an exemplary aspect of H gas flow within rebalancing cell 202. Specifically, close-up inset 450 enlarges a portion of the cross-sectional view bounded by dashed ellipse 410.

[0103] 4A and 4B, the rebalancing cell 202 may include an electrode assembly stack 402 formed as a stack of individual electrode assemblies 302 aligned such that the hydrogen gas inlet channel section 318a of each electrode assembly 302 forms a continuous hydrogen gas inlet channel 404 with the hydrogen gas inlet channel section 318a of each electrode assembly 302. A hydrogen gas inlet manifold 406 may be further included within the hydrogen gas inlet channel 404, the hydrogen gas inlet manifold 406 fluidly coupling the hydrogen gas inlet channel 404 to the hydrogen gas inlet port 210.

[0104] Respective hydrogen gas inlet channel O-rings 322a and overboard O-rings 324 can seal hydrogen gas inlet channels 404 at the junctions between pairs of electrode assemblies 302. It will be understood that a cut-away portion of the rebalancing cell 202 is depicted in detail in the cross-sectional view of FIG. 4A and the enlarged inset 450 of FIG. 4B , and that additional features of the rebalancing cell 202 (e.g., shown in FIGS. 2A and 2B ) may not be depicted. Furthermore, it will be understood that more or fewer electrode assemblies 302 than shown in the cross-sectional view for a given application may be included in the electrode assembly stack 402 (however, in one use case, scale-up performance may be substantially insensitive to H gas flow at or below 50% H gas utilization). Furthermore, although the structural features of the hydrogen gas inlet channel 404 and adjacent components have been described in detail with reference to Figures 4A and 4B, it will be understood that the corresponding hydrogen gas outlet channel (e.g., formed by aligning the hydrogen gas outlet channel section 318b (see Figure 3) of each electrode assembly 302) and the structural features of the adjacent components may be similarly configured (except that the hydrogen gas outlet manifold in fluid communication with the hydrogen gas outlet channel may be positioned on the opposite side of the hydrogen gas inlet manifold 406 along the x-axis and z-axis).

[0105] As shown, and as indicated by arrow 408a, H gas enters hydrogen gas inlet port 210. The H gas may be delivered from a hydrogen source, such as a hydrogen flow generator or storage tank. The hydrogen gas then flows from hydrogen gas inlet port 210 to hydrogen gas inlet manifold 406, as indicated via arrow 408b. The hydrogen gas then flows from hydrogen gas inlet manifold 406 through hydrogen gas inlet channel 404, as indicated via arrow 408c. From hydrogen gas inlet channel 404, gas flows into hydrogen gas inlet passage 452 between the electrodes, as indicated via arrow 408d.

[0106] 4A and 4B, the hydrogen gas outlet port 212 shown in FIG. 2B is open. Thus, hydrogen gas flows across the electrode assembly stack 402 (generally in a direction parallel to the x-axis) through the hydrogen gas channels 420 between the plate 304 and the negative electrode 310, as shown via arrows 408e. Arrows 408f depict the flow of hydrogen gas from the hydrogen gas channels 420 to the hydrogen gas outlet manifold and then to the hydrogen gas outlet port 212, as shown in FIG. 2B. In this manner, hydrogen can effectively flow through the cell.

[0107] 5A and 5B, a cross-sectional view and a close-up inset 450, respectively, of rebalancing cell 202 are shown. Each of the cross-sectional view and close-up inset 550 depicts an exemplary aspect of H gas flow within rebalancing cell 202. Specifically, close-up inset 550 enlarges a portion of the cross-sectional view bounded by dashed ellipse 510.

[0108] In the rebalancing cell flow configuration shown in Figures 5A and 5B, the hydrogen gas outlet port 212 shown in Figure 2B is closed, thereby dead-ending the hydrogen flow as described above.

[0109] 5A and 5B with the electrode assemblies 302 arranged in series. Each electrode assembly 302 again includes, in the illustrated embodiment, a negative electrode 310, a positive electrode 308, an activated carbon foam 306, a plate 304, and a hydrogen gas inlet channel sealing insert 320a. However, in other embodiments, other electrode assembly configurations may be used.

[0110] As shown, and as indicated by arrow 508a, H gas enters hydrogen gas inlet port 210. Hydrogen gas then flows from hydrogen gas inlet port 210 to hydrogen gas inlet manifold 406, as indicated via arrow 508b. Hydrogen gas then flows from hydrogen gas inlet manifold 406 through hydrogen gas inlet channel 404, as indicated via arrow 508c. From hydrogen gas inlet channel 404, hydrogen gas flows into hydrogen gas inlet passage 452 between the electrodes, as indicated via arrow 508d. This initial stage of hydrogen flow is similar to the initial portion of the hydrogen flow pattern depicted in FIGS. 4A and 4B.

[0111] However, as illustrated in FIGS. 5A and 5B, hydrogen gas flows from the hydrogen gas channel 420 through the negative electrode 310 and then the positive electrode 308, as shown via arrow 508e. Thus, closure of the hydrogen gas outlet 508 forces hydrogen gas through the electrolyte side of the electrode assembly. Once hydrogen gas is on the positive side of the electrode assembly, the gas flows to the electrolyte outlet manifold and then exits through the hydrogen gas relief port 214 shown in FIG. 2A. Specifically, hydrogen gas flows to the electrolyte outlet manifold and the hydrogen gas relief port 214 shown in FIG. 2A. Additionally, as shown in FIG. 2A, some of the hydrogen gas flow may be entrained in the electrolyte and exit through the electrolyte outlet port 208.

[0112] The two flow patterns within the rebalancing cell 202 depicted in Figures 4A-5B allow for four different series piping orientations to connect multiple rebalancing cells to increase (e.g., maximize) the hydrogen gas flow rate through each rebalancing cell unit. Thus, the cells can achieve desired reaction rates, thereby increasing cell efficiency. Figures 6-9 depict various flow arrangements in a rebalancing cell system 600 for a redox flow battery, such as the redox flow battery 11 shown in Figure 1 or other suitable redox flow batteries.

[0113] 6-9, the rebalancing cell system 600 includes similar components, including a first rebalancing cell 602 and a second rebalancing cell 604. These rebalancing cells may include similarly structured electrode assembly stacks to enhance manufacturing efficiency. The rebalancing cell system 600 further includes a hydrogen source 606 including a hydrogen storage tank 608 and a hydrogen flow generator 610 (e.g., a venturi injector or hydrogen injector) that may be in electronic communication with a controller, such as the controller 88 shown in FIG. 1, or other suitable controller. The venturi injector may include a constriction that introduces hydrogen gas into the gas flow at the restriction.

[0114] 2A and 2B . Specifically, the first rebalancing cell 602 includes a hydrogen gas inlet port 612, a hydrogen gas outlet port 614, and a hydrogen gas relief port 616. Similarly, the second rebalancing cell 604 includes a hydrogen gas inlet port 618, a hydrogen gas outlet port 620, and a hydrogen gas relief port 622. The first and second rebalancing cells 602 and 604 may further include positive inlets and outlets through which the electrolyte flows.

[0115] The hydrogen storage tank 608 includes an inlet 624 and an outlet 626. The flow generator 610 similarly includes an inlet 628 and an outlet 630. Figures 6-9 depict various flow arrangements 650, 750, 850, 950 of the components of the rebalancing cell system 600. To achieve the variations in flow arrangements, different ports in the first and second rebalancing cells are fluidly connected via conduits, lines, etc. via various schemes.

[0116] 6 illustrates a flow configuration 650 that uses the hydrogen (negative) inlet and outlet ports to establish positive pressure relative to a hydrogen storage tank. In this flow configuration, the high-pressure side (outlet 630) of the hydrogen flow generator 610 is fluidly connected to the hydrogen gas inlet port 612 of the first rebalancing cell 602. Furthermore, the hydrogen gas outlet port 614 of the first rebalancing cell 602 is fluidly connected to the hydrogen gas inlet port 618 of the second rebalancing cell 604. Furthermore, the hydrogen gas outlet port 620 of the second rebalancing cell 604 is fluidly connected to the inlet 624 of the hydrogen storage tank 608, and the outlet 626 of the hydrogen storage tank is fluidly connected to the inlet 628 of the hydrogen flow generator 610.

[0117] 7 illustrates a flow configuration 750 that uses the hydrogen gas inlet port 612 and hydrogen relief port 616 of the first rebalancing cell 602 to establish a positive pressure relative to the hydrogen storage tank 608. Specifically, in flow configuration 750, the high-pressure side (outlet 630) of the hydrogen flow generator 610 is fluidly connected to the hydrogen gas inlet port 612 of the first rebalancing cell 602. Higher-pressure hydrogen then enters the hydrogen channel (negative channel) and is forced into the electrolyte section (positive side) by closing the hydrogen gas outlet. The hydrogen gas relief port 616 of the first rebalancing cell 602 is then fluidly connected to the hydrogen gas inlet port 618 of the second rebalancing cell 604. The hydrogen gas relief port 622 of the second rebalancing cell 604 is then fluidly connected to the inlet 624 of the hydrogen storage tank 608.

[0118] 8 is a combination of the flow configurations 650 and 750 shown in Figures 6 and 7. Specifically, the hydrogen gas relief port 616 and the hydrogen gas outlet port 614 of the first rebalancing cell 602 are connected to the hydrogen gas inlet port 618 of the second rebalancing cell 604. Furthermore, the hydrogen gas relief port 622 and the hydrogen gas outlet port 620 of the second rebalancing cell 604 are connected to the hydrogen storage tank 608.

[0119] 9 is a configuration for establishing a negative pressure relative to the hydrogen storage tank 608. In this flow configuration, the low-pressure side (inlet 628) of the hydrogen flow generator 610 is connected to the hydrogen gas relief port 622 of the second rebalancing cell 604. Additionally, the hydrogen gas relief port 618 of the first rebalancing cell 602 is connected to the hydrogen gas inlet port 618 of the second rebalancing cell 604. Additionally, in this flow configuration, the hydrogen gas inlet port 612 in the first rebalancing cell is connected to the outlet 626 of the hydrogen storage tank 608, and the outlet 630 of the flow generator 610 is fluidly connected to the inlet 624 of the hydrogen storage tank 608. When the flow generator 610 is turned on, it creates a negative pressure at the hydrogen gas relief port 622 of the second rebalancing cell 604. This creates a pressure differential between the hydrogen gas inlet port 618 of the second rebalancing cell 604 and the hydrogen relief port 616 of the first rebalancing cell 602, inducing hydrogen flow from the high-pressure storage tank 608 through the low-pressure series-connected rebalancing cells.

[0120] Although two rebalancing cells are depicted in Figures 6-9, additional rebalancing cells may be connected in series with the other cells as needed. When adding cells to a system, the last cell in the series may return unused hydrogen gas through a hydrogen gas outlet port to a hydrogen storage tank or flow generator. In this manner, the number of cells in a system may be adjusted based on the design requirements of the end application.

[0121] Referring to FIG. 10 , a method 1000 for operating a rebalancing cell system is shown. Specifically, the rebalancing cell system can be implemented in a redox flow battery to increase H gas flow rate through cells arranged in series, reduce excess H gas, and rebalance charge imbalances in the electrolyte therein. In an exemplary embodiment, the redox flow battery is the redox flow battery 11 of FIG. 1 , and the rebalancing cell system can be one or more of the rebalancing cell systems shown in FIGS. 6-9 , which can include multiple rebalancing cells arranged in series, such as the rebalancing cell 202 of FIGS. 2A and 2B . Thus, the method 1000 can be discussed with reference to the embodiments of FIGS. 1-9 alone or in combination (although it will be understood that similar methods can be applied to other systems without departing from the scope of this disclosure). For example, in method 1000, at least some steps or portions of steps (e.g., involving dispensing H gas) may be performed via controller 88 of FIG. 1 and may be stored as executable instructions in a non-transitory storage medium (e.g., memory) communicatively coupled to controller 88. In particular, the H gas flow may be driven by sending commands from a controller to a hydrogen flow generator. Additional components described with reference to FIG. 10 may be embodiments of corresponding components in FIGS. 1-9.

[0122] At 1002, the method includes receiving H gas from a hydrogen source at a first rebalancing cell via a hydrogen gas inlet port. As previously described, the hydrogen source may include a hydrogen storage tank and a hydrogen flow generator. Thus, in one embodiment, the hydrogen flow generator may flow hydrogen gas directly to the inlet port of the first rebalancing cell through conduits, lines, etc. In other embodiments, hydrogen may flow directly from a storage tank to the inlet port of the first rebalancing cell through conduits, lines, etc.

[0123] Next, at 1004, the method includes distributing hydrogen gas through the electrode assembly stack in the first rebalancing cell. The hydrogen flow pattern in the first cell can be the flow paths depicted in Figures 4A and 4B or 5A and 5B.

[0124] In 1006, the method includes venting hydrogen from the first rebalancing cell and directing the hydrogen to a second hydrogen gas inlet port in the second rebalancing cell. Step 1006 may include sub-steps 1010 and / or 1012. In sub-step 1010, the method includes venting hydrogen gas through a first hydrogen gas outlet port of the first rebalancing cell and directing the gas to a second hydrogen gas inlet port. In sub-step 1012, the method includes venting hydrogen in the electrolyte through a first hydrogen gas relief port in the first rebalancing cell and directing the hydrogen to a second hydrogen gas inlet port. In either sub-step 1010 or 1012, a conduit, line, or the like may be used to fluidly connect a port in the first rebalancing cell to a port in the second rebalancing cell.

[0125] At 1014, the method includes distributing H throughout the electrode assembly stack in a second rebalancing cell. The hydrogen flow pattern in the second cell can be within the flow paths depicted in Figures 4A and 4B or 5A and 5B.

[0126] At 1016, the method includes venting hydrogen from the second rebalancing cell and directing the hydrogen to a hydrogen source. Step 1016 may include sub-steps 1018 and / or 1020. At sub-step 1018, the method includes venting hydrogen gas through a second hydrogen gas outlet port and directing the hydrogen to a hydrogen tank or hydrogen flow generator. At sub-step 1020, the method includes venting hydrogen in the electrolyte through a second hydrogen gas relief port and directing the hydrogen to a hydrogen tank or hydrogen flow generator. Next, at 1022, the method includes flowing hydrogen between the hydrogen storage tank and the hydrogen flow generator, or vice versa. For example, hydrogen can flow between the outlet of the hydrogen storage tank and the inlet of the flow generator. Method 1000 allows hydrogen gas to be efficiently distributed among the rebalancing cells in series, thereby increasing hydrogen flow rate and providing more equal hydrogen distribution within the cells.

[0127] The technical effect of the rebalancing cell system method described herein is to improve battery efficiency and reduce uneven distribution of hydrogen between cells when compared to parallel cell architectures by increasing the hydrogen flow rate through the rebalancing cells.

[0128] The invention is further described in the following paragraphs: In one aspect, a rebalancing cell system for a redox flow battery is provided, the rebalancing cell system comprising: a first rebalancing cell in series fluid communication with a second rebalancing cell and a hydrogen source, the first rebalancing cell comprising a first electrode stack having hydrogen flow channels, the hydrogen flow channels extending through the first electrode stack and having a higher pressure than an electrolyte in the first electrode stack; and a second rebalancing cell comprising a second electrode stack having hydrogen flow channels, the hydrogen flow channels extending through the second electrode stack and having a higher pressure than an electrolyte in the second electrode stack.

[0129] In another aspect, a method of a redox flow battery system is provided, the method including flowing hydrogen gas from a hydrogen source to a first hydrogen gas inlet port of a first rebalancing cell, and flowing hydrogen gas from a first hydrogen gas outlet port or a first hydrogen gas relief port to a second negative hydrogen gas inlet port. In one embodiment, the method can further include flowing hydrogen gas in series between the hydrogen storage tank and the hydrogen flow generator. In another embodiment, the method can further include flowing hydrogen gas from one of a second hydrogen gas outlet port and a second hydrogen gas relief port to a hydrogen source.

[0130] In yet another aspect, a rebalancing cell system for a redox flow battery is provided, the rebalancing cell system comprising: a first rebalancing cell and a second rebalancing cell, the first rebalancing cell including a first hydrogen gas inlet port in fluid communication with one of a hydrogen flow generator and a hydrogen storage tank, and a first hydrogen gas outlet port and / or a first hydrogen gas relief port, and the second rebalancing cell including a second hydrogen gas inlet port in fluid communication with one of the first hydrogen gas outlet port and the first hydrogen gas relief port, and a second hydrogen gas outlet port and / or a second hydrogen gas relief port in fluid communication with one of the hydrogen flow generator and the hydrogen storage tank.

[0131] In any aspect or combination of aspects, the first rebalancing cell may include a first hydrogen gas inlet port in fluid communication with the hydrogen source, a first hydrogen gas outlet port, and / or a first hydrogen gas relief port, and the second rebalancing cell may include a second hydrogen gas inlet port in fluid communication with one of the first hydrogen gas outlet port and the first hydrogen gas relief port, and a second hydrogen gas outlet port and / or a second hydrogen gas relief port in fluid communication with the hydrogen source.

[0132] In any embodiment or combination of embodiments, the second hydrogen gas inlet port may be in fluid communication with the first hydrogen gas outlet port.

[0133] In any embodiment or combination of embodiments, the first hydrogen gas relief port may be in fluid communication with the second hydrogen gas inlet port.

[0134] In any embodiment or combination of embodiments, the first hydrogen gas relief port and the first hydrogen gas outlet port may both be in fluid communication with the second hydrogen gas inlet port.

[0135] In any embodiment or combination of embodiments, the hydrogen source may include a hydrogen tank fluidly coupled in series with a hydrogen flow generator.

[0136] In any aspect or combination of aspects, the hydrogen flow generator may be in fluid communication with the first hydrogen gas inlet port.

[0137] In any embodiment or combination of embodiments, the hydrogen flow generator may be in fluid communication with the second hydrogen gas relief port.

[0138] In any aspect or combination of aspects, the hydrogen flow generator may be a venturi injector or a hydrogen injector.

[0139] In any embodiment or combination of embodiments, the hydrogen source may include a hydrogen flow generator.

[0140] In any embodiment or combination of embodiments, the hydrogen source may include a hydrogen storage tank.

[0141] In any embodiment or combination of embodiments, the outlet of the hydrogen flow generator may be in series fluid communication with the inlet of the hydrogen storage tank.

[0142] In any embodiment or combination of embodiments, the outlet of the hydrogen storage tank may be in series fluid communication with the inlet of the hydrogen flow generator.

[0143] In any embodiment or combination of embodiments, the hydrogen flow generator may be a venturi injector.

[0144] In any aspect or combination of aspects, the hydrogen flow generator may be a hydrogen injector.

[0145] In any aspect or combination of aspects, the first rebalancing cell and the second rebalancing cell may have the same layout.

[0146] In any aspect or combination of aspects, the first and second rebalancing cells may include stacks of electrode assemblies having the same layout.

[0147] 1-9 illustrate exemplary configurations with the relative positioning of various components. When elements are shown to be in direct contact or directly coupled to one another, then such elements may, in at least one embodiment, be referred to as being in direct contact or directly coupled, respectively. Similarly, elements shown contiguous or adjacent to one another may, in at least one embodiment, be contiguous or adjacent to one another, respectively. As one example, components placed in surface-sharing contact with one another may be referred to as being in surface-sharing contact. As another example, in at least one embodiment, elements positioned apart from one another have only space between them, and other components may not be referred to as such. As yet another example, elements shown above / below one another, opposite one another, or to the left / right of one another may be referred to as such relative to one another. Furthermore, as shown in the figures, the topmost element or point of an element may, in at least one embodiment, be referred to as the "top" of the component, and the bottommost element or point of an element may, in at least one embodiment, be referred to as the "bottom" of the component. As used herein, top / bottom, upper / lower, and up / down may be relative to the vertical axis of the drawing and are used to describe the positioning of elements in the drawing relative to one another. Thus, in one embodiment, elements shown above other elements are positioned perpendicularly 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, in at least one embodiment, be referred to as intersecting elements or intersecting elements. Still further, in one embodiment, elements shown within or outside of another element may be so referred to. While Figures 2A-5B are drawn approximately to scale, other dimensions or relative dimensions may be used.

[0148] 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 from the original claims, are also deemed to be encompassed within the subject matter of this disclosure.

Claims

1. 1. A rebalancing cell system for a redox flow battery, comprising: a first rebalancing cell in series fluid communication with a second rebalancing cell and a hydrogen source; the first rebalancing cell includes a first electrode assembly stack having a hydrogen flow path, the hydrogen flow path extending through the first electrode assembly stack and having a higher pressure than an electrolyte within the first electrode assembly stack; the second rebalancing cell includes a second electrode assembly stack having a hydrogen flow path extending through the second electrode assembly stack and having a higher pressure than an electrolyte within the second electrode assembly stack; Rebalancing cell system.

2. The first rebalancing cell a first hydrogen gas inlet port in fluid communication with the hydrogen source; a first hydrogen gas outlet port and / or a first hydrogen gas relief port; The second rebalancing cell a second hydrogen gas inlet port in fluid communication with one of the first hydrogen gas outlet port and the first hydrogen gas relief port; 10. The rebalancing cell system of claim 1, further comprising: a second hydrogen gas outlet port and / or a second hydrogen gas relief port in fluid communication with the hydrogen source.

3. The rebalancing cell system of claim 2 , wherein the second hydrogen gas inlet port is in fluid communication with the first hydrogen gas outlet port.

4. The rebalancing cell system of claim 2 , wherein the first hydrogen gas relief port is in fluid communication with the second hydrogen gas inlet port.

5. 3. The rebalancing cell system of claim 2, wherein the first hydrogen gas relief port and the first hydrogen gas outlet port are both in fluid communication with the second hydrogen gas inlet port.

6. The rebalancing cell system of claim 2 , wherein the hydrogen source comprises a hydrogen tank fluidly coupled in series with a hydrogen flow generator.

7. The rebalancing cell system of claim 6 , wherein the hydrogen flow generator is in fluid communication with the first hydrogen gas inlet port.

8. The rebalancing cell system of claim 6 , wherein the hydrogen flow generator is in fluid communication with the second hydrogen gas relief port.

9. The rebalancing cell system of claim 6 , wherein the hydrogen flow generator is a venturi injector or a hydrogen injector.

10. flowing hydrogen gas from a hydrogen source into a first hydrogen gas inlet port of a first rebalancing cell; flowing hydrogen gas from the first hydrogen gas outlet port or the first hydrogen gas relief port of the first rebalancing cell to the second hydrogen gas inlet port of the second rebalancing cell; A method for a rebalancing cell system.

11. The method of claim 10 , wherein the hydrogen source comprises a hydrogen flow generator.

12. The method of claim 11 , wherein the hydrogen source comprises a hydrogen storage tank.

13. The method of claim 12 further comprising flowing hydrogen gas in series between the hydrogen storage tank and the hydrogen flow generator.

14. 11. The method of claim 10, further comprising flowing hydrogen gas from one of a second hydrogen gas outlet port and a second hydrogen gas relief port to the hydrogen source.

15. A rebalancing cell system for a redox flow battery, the rebalancing cell system comprising: a first rebalancing cell and a second rebalancing cell; The first rebalancing cell a first hydrogen gas inlet port in fluid communication with one of the hydrogen flow generator and the hydrogen storage tank; a first hydrogen gas outlet port and / or a first hydrogen gas relief port; The second rebalancing cell a second hydrogen gas inlet port in fluid communication with one of the first hydrogen gas outlet port and the first hydrogen gas relief port; a second hydrogen gas outlet port and / or a second hydrogen gas relief port in fluid communication with one of the hydrogen flow generator and the hydrogen storage tank.

16. 16. The rebalancing cell system of claim 15, wherein the outlet of the hydrogen flow generator is in series fluid communication with the inlet of the hydrogen storage tank.

17. 16. The rebalancing cell system of claim 15, wherein the outlet of the hydrogen storage tank is in series fluid communication with the inlet of the hydrogen flow generator.

18. 16. The rebalancing cell system of claim 15, wherein the hydrogen flow generator is a venturi injector.

19. The rebalancing cell system of claim 15 , wherein the hydrogen flow generator is a hydrogen injector.

20. 16. The rebalancing cell system of claim 15, wherein the first and second rebalancing cells comprise stacks of electrode assemblies having identical layouts.

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