REBALANCING CELL SYSTEM FOR REDOX FLOW BATTERY

Systems and methods are provided for rebalancing cells in a redox flow battery. In one example, a method of operating a rebalancing cell in a redox flow battery system includes measuring a state-of-charge (SOC) of a redox flow battery in the redox flow battery system, calculating an electrolyte concentration from the measured SOC of the redox flow battery, and maintaining a rebalancing reaction rate at the rebalancing cell. In one example, maintaining the rebalancing includes, responsive to a change in the electrolyte concentration, adjusting an electrolyte flow rate to the rebalancing cell and adjusting a hydrogen flow rate to the rebalancing cell.

FIELD

The present description relates generally to systems for rebalancing cells for use in redox flow batteries and methods for operating such rebalancing cell systems.

BACKGROUND AND SUMMARY

Redox flow batteries are suitable for grid-scale storage applications due to their capability for scaling power and capacity independently, as well as for charging and discharging over thousands of cycles with reduced performance losses in comparison to conventional battery technologies. An all-iron hybrid redox flow battery is particularly attractive due to incorporation of low-cost, earth-abundant materials. In general, iron redox flow batteries (IFBs) rely on iron, salt, and water for electrolyte, thus including simple, earth-abundant, and inexpensive materials, and eliminating incorporation of harsh chemicals and reducing an environmental footprint thereof.

The IFB may include a positive (redox) electrode where a redox reaction occurs and a negative (plating) electrode where ferrous iron (Fe2+) in the electrolyte may be reduced and plated. The IFB may further include a plurality of rebalancing cells to maintain electrolyte health and battery capacity. In one example, electrolyte rebalancing occurs by reducing electrolyte with hydrogen gas at the rebalancing cell. Specifically, hydrogen gas supplied at a negative side of the rebalancing cell is contacted with liquid electrolyte supplied at a positive side of the rebalancing cell. Electrolyte flooding at the negative side of the rebalancing cell can occur when hydrogen gas pressure at the negative side is insufficient. In one example, the hydrogen gas pressure may be insufficient when the electrolyte rebalancing reaction rate at the rebalancing cell is higher and the hydrogen supplied to the negative side of the rebalancing cell is less than a stoichiometric amount of hydrogen. In order to mitigate electrolyte flooding, the size or volume of the rebalancing cell may be reduced to aid in maintaining a higher hydrogen gas pressure. Furthermore, to reduce electrolyte flooding, the rebalancing cell may be operated by supplying a constant excess amount of hydrogen gas to the rebalancing cell and/or maintaining a constant electrolyte flow while operating the rebalancing cell, independent of redox flow battery system operating conditions.

The inventors have recognized several drawbacks of the above-mentioned rebalancing cell operation. In particular, smaller rebalancing cells exhibit lower rebalancing reaction rates, especially during conditions such as lower redox flow battery state-of-charge conditions when hydrogen gas at the rebalancing cell may be in excess. Similarly, maintaining a constant electrolyte flow at the rebalancing cell while operating the rebalancing cell necessitates maintaining an electrolyte flow lower than a stoichiometric amount of electrolyte, which lowers the rebalancing reaction rate at the rebalancing cell, thereby reducing performance of the rebalancing cell. Furthermore, supplying a constant excess amount of hydrogen gas to the rebalancing cell while operating the rebalancing cell increases operational costs. Moreover, the presence of excess hydrogen gas in the rebalancing cell increases the rate of parasitic side reactions, leading to reduced performance of the redox flow battery system. Further still, permitting electrolyte flooding of the rebalancing cell reduces the performance of the redox flow battery system, and can shorten the useful life of the rebalancing cell.

In one example, the issues described above may be at least partially addressed by a method of operating a rebalancing cell system. The method includes, in one example, measuring a state-of-charge (SOC) of a redox flow battery in the redox flow battery system, calculating an electrolyte concentration from the measured SOC of the redox flow battery, and maintaining a rebalancing reaction rate at the rebalancing cell. In such an example, maintaining the rebalancing reaction rate at the rebalancing cell includes, responsive to a change in the electrolyte concentration, adjusting an electrolyte flow rate to the rebalancing cell and adjusting a hydrogen flow rate to the rebalancing cell. In one example, adjusting the electrolyte flow rate to the rebalancing cell may include reducing the electrolyte flow rate responsive to an increase in the electrolyte concentration when the electrolyte concentration is above a threshold electrolyte concentration. In another example, adjusting the hydrogen flow rate may include increasing the hydrogen flow rate responsive to the increase in the electrolyte concentration when the electrolyte concentration is below the threshold electrolyte concentration. In this way, flooding of the rebalancing cell can be reduced, while reducing excess reactant flow, thereby lowering operational costs. Furthermore, power losses arising from parasitic side reactions are decreased, thereby increasing overall redox flow battery system efficiency.

DETAILED DESCRIPTION

The following description relates to systems and methods for distributing hydrogen through rebalancing cells in a series flow arrangement. The rebalancing cells maintain electrolyte health and capacity in the redox flow battery. To perform this function, the rebalancing cells demand enough hydrogen flow into the unit to support a desired reaction rate. Using the series flow arrangement increase the hydrogen flowrate through the cells and achieve a more balanced hydrogen distribution when compared to parallel flow arrangements.

The redox flow battery is depicted schematically inFIG.1with an integrated multi-chambered tank having separate positive and negative electrolyte chambers. In some examples, the redox flow battery may be an all-iron flow battery (IFB) utilizing iron redox chemistry at both a positive (redox) electrode and the negative (plating) electrode of the IFB. The electrolyte chambers may be coupled to one or more battery cells, each cell including the positive and negative electrodes. Therefrom, electrolyte may be pumped through positive and negative electrode compartments respectively housing the positive and negative electrodes.

In some examples, the redox flow battery may be a hybrid redox flow battery. Hybrid redox flow batteries are redox flow batteries which may be characterized by deposition of one or more electroactive materials as a solid layer on an electrode (e.g., the negative electrode). Hybrid redox flow batteries may, for instance, include a chemical species which may plate via an electrochemical reaction as a solid on a substrate throughout a battery charge process. During battery discharge, the plated species may ionize via a further electrochemical reaction, becoming soluble in the electrolyte. In hybrid redox flow battery systems, a charge capacity (e.g., a maximum amount of energy stored) of the redox flow battery may be limited by an amount of metal plated during battery charge and may accordingly depend on an efficiency of the plating system as well as volume and surface area available for plating.

In some examples, electrolytic imbalances in the redox flow battery may result from numerous side reactions competing with desired redox chemistry, including hydrogen (H2) gas generating reactions such as proton reduction and iron corrosion:

and charge imbalances from excess ferric iron (Fe3+) generated during oxidation of iron plating:

The reactions of equations (1) to (3) may limit iron plating and thereby decrease overall battery capacity. To address such imbalances, electrolyte rebalancing may be leveraged to both reduce Fe3+and eliminate excess H2gas via a single redox reaction:

As described by embodiments herein, Fe3+reduction rates sufficient for relatively high performance applications may be reliably achieved via a rebalancing cell, such as the exemplary rebalancing cell ofFIGS.2A and2B, including a stack of internally shorted electrode assemblies, such as the exemplary electrode assembly ofFIG.3.FIGS.4A and4Bdepict a first of H2gas flow pattern in the rebalancing cell, where the H2gas flows across negative electrodes and to a hydrogen gas outlet port.FIGS.5A and5Bdepict a second of H2gas flow pattern in the rebalancing cell, where the hydrogen gas outlet port is closed and the H2gas flows across positive electrodes and to a hydrogen gas relief port.FIGS.6A and6Bdepict aspects of electrolyte flow in the rebalancing cell, where the electrolyte may be distributed across positive electrodes of the internally shorted electrode assemblies via a combination of gravity feeding and capillary action (additionally or alternatively, and similar to convection of the H2gas across the negative electrodes.FIGS.7A and7Bdepict partial flow configurations of the redox flow battery system ofFIG.1, including electrolyte and hydrogen flow configurations to and from the rebalancing cells ofFIGS.2A and2B.FIGS.8A and8Bdepict plots of rebalancing cell and redox flow battery system operating conditions. An exemplary method of operating a rebalancing cell system is depicted atFIGS.9and10. The redox flow battery state of charge may be determined based on a redox flow battery plating efficiency, a shunt current, and an open circuit voltage state of charge. The plating efficiency may be correlated to pH and the open circuit voltage state of charge may be correlated to the open circuit voltage of the redox flow battery, as illustrated by the plots ofFIGS.11A and11B.

As shown inFIG.1, in a redox flow battery system10with redox flow battery11, a negative electrode26may be referred to as a plating electrode and a positive electrode28may be referred to as a redox electrode. A negative electrolyte within a plating side (e.g., a negative electrode compartment20) of a redox flow battery cell18may be referred to as a plating electrolyte, and a positive electrolyte on a redox side (e.g., a positive electrode compartment22) of the redox flow battery cell18may be referred to as a redox electrolyte.

“Anode” refers to an electrode where electroactive material loses electrons and “cathode” refers to an electrode where electroactive material gains electrons. During battery charge, the negative electrolyte gains electrons at the negative electrode26, and the negative electrode26is the cathode of the electrochemical reaction. During battery discharge, the negative electrolyte loses electrons, and the negative electrode26is the anode of the electrochemical reaction. Alternatively, during battery discharge, the negative electrolyte and the negative electrode26may be respectively referred to as an anolyte and the anode of the electrochemical reaction, while the positive electrolyte and the positive electrode28may be respectively referred to as a catholyte and the cathode of the electrochemical reaction. During battery charge, the negative electrolyte and the negative electrode26may be respectively referred to as the catholyte and the cathode of the electrochemical reaction, while the positive electrolyte and the positive electrode28may be respectively referred to as the anolyte and the anode of the electrochemical reaction. For simplicity, the terms “positive” and “negative” are used herein to refer to the electrodes, electrolytes, and electrode compartments in redox flow battery systems.

One example of a hybrid redox flow battery is an all-iron redox flow battery (IFB), in which the electrolyte includes iron ions in the form of iron salts (e.g., FeCl2, FeCl3, and the like), where the negative electrode26includes metal iron. For example, at the negative electrode26, ferrous iron (Fe2+) gains two electrons and plates as iron metal (Fe0) onto the negative electrode26during battery charge, and Fe0loses two electrons and re-dissolves as Fe2+during battery discharge. At the positive electrode28, Fe2+loses an electron to form ferric iron (Fe3+) during battery charge, and Fe3+gains an electron to form Fe2+during battery discharge. The electrochemical reaction is summarized in equations (5) and (6), where the forward reactions (left to right) indicate electrochemical reactions during battery charge, while the reverse reactions (right to left) indicate electrochemical reactions during battery discharge:

As discussed above, the negative electrolyte used in the IFB may provide a sufficient amount of Fe2+so that, during battery charge, Fe2+may accept two electrons from the negative electrode26to form Fe0and plate onto a substrate. During battery discharge, the plated Fe0may lose two electrons, ionizing into Fe2+and dissolving back into the electrolyte. An equilibrium potential of the above reaction is −0.44 V and this reaction therefore provides a negative terminal for the desired system. On the positive side of the IFB, the electrolyte may provide Fe2+during battery charge which loses an electron and oxidizes to Fe3+. During battery discharge, Fe3+provided by the electrolyte becomes Fe2+by absorbing an electron provided by the positive electrode28. An equilibrium potential of this reaction is +0.77 V, creating a positive terminal for the desired system.

The IFB may provide the ability to charge and recharge electrolytes therein in contrast to other battery types utilizing non-regenerating electrolytes. Charge may be achieved by respectively applying an electric current across the electrodes26and28via terminals40and42. The negative electrode26may be electrically coupled via the terminal40to a negative side of a voltage source so that electrons may be delivered to the negative electrolyte via the positive electrode28(e.g., as Fe2+is oxidized to Fe3+in the positive electrolyte in the positive electrode compartment22). The electrons provided to the negative electrode26may reduce the Fe2+in the negative electrolyte to form Fe0at the (plating) substrate, causing the Fe2+to plate onto the negative electrode26.

Discharge may be sustained while Fe0remains available to the negative electrolyte for oxidation and while Fe3+remains available in the positive electrolyte for reduction. As an example, Fe3+availability may be maintained by increasing a concentration or a volume of the positive electrolyte in the positive electrode compartment22side of the redox flow battery cell18to provide additional Fe3+ions via an external source, such as an external positive electrolyte chamber52. More commonly, availability of Fe0during discharge may be an issue in IFB systems, wherein the Fe0available for discharge may be proportional to a surface area and a volume of the negative electrode substrate, as well as to a plating efficiency. Charge capacity may be dependent on the availability of Fe2+in the negative electrode compartment20. As an example, Fe2+availability may be maintained by providing additional Fe2+ions via an external source, such as an external negative electrolyte chamber50to increase a concentration or a volume of the negative electrolyte to the negative electrode compartment20side of the redox flow battery cell18.

In an IFB, the positive electrolyte may include ferrous iron, ferric iron, ferric complexes, or any combination thereof, while the negative electrolyte may include ferrous iron or ferrous complexes, depending on a state of charge (SOC) of the IFB system. As previously mentioned, utilization of iron ions in both the negative electrolyte and the positive electrolyte may allow for utilization of the same electrolytic species on both sides of the redox flow battery cell18, which may reduce electrolyte cross-contamination and may increase the efficiency of the IFB system, resulting in less electrolyte replacement as compared to other redox flow battery systems.

Efficiency losses in an IFB may result from electrolyte crossover through a separator24(e.g., ion-exchange membrane barrier, microporous membrane, and the like). For example, Fe3+ions in the positive electrolyte may be driven toward the negative electrolyte by a Fe3+ion concentration gradient and an electrophoretic force across the separator24. Subsequently, Fe3+ions penetrating the separator24and crossing over to the negative electrode compartment20may result in coulombic efficiency losses. Fe3+ions crossing over from the low pH redox side (e.g., more acidic positive electrode compartment22) to the high pH plating side (e.g., less acidic negative electrode compartment20) may result in precipitation of Fe(OH)3. Precipitation of Fe(OH)3may degrade the separator24and cause permanent battery performance and efficiency losses. For example, Fe(OH)3precipitate may chemically foul an organic functional group of an ion-exchange membrane or physically clog micropores of the ion-exchange membrane. In either case, due to the Fe(OH)3precipitate, membrane ohmic resistance may rise over time and battery performance may degrade. Precipitate may be removed by washing the IFB with acid, but constant maintenance and downtime may be disadvantageous for commercial battery applications. Furthermore, washing may be dependent on regular preparation of electrolyte, contributing to additional processing costs and complexity. Alternatively, adding specific organic acids to the positive electrolyte and the negative electrolyte in response to electrolyte pH changes may mitigate precipitate formation during battery charge and discharge cycling without driving up overall costs. Additionally, implementing a membrane barrier that inhibits Fe3+ion crossover may also mitigate fouling.

Additional coulombic efficiency losses may be caused by reduction of H+(e.g., protons) and subsequent formation of H2gas, and a reaction of protons in the negative electrode compartment20with electrons supplied at the plated iron metal of the negative electrode26to form H2gas.

The IFB electrolyte (e.g., FeCl2, FeCl3, FeSO4, Fe2(SO4)3, and the like) may be readily available and may be produced at low costs. In one example, the IFB electrolyte may be formed from ferrous chloride (FeCl2), potassium chloride (KCl), manganese(II) chloride (MnCl2), and boric acid (H3BO3). The IFB electrolyte may offer higher reclamation value because the same electrolyte may be used for the negative electrolyte and the positive electrolyte, consequently reducing cross-contamination issues as compared to other systems. Furthermore, because of iron's electron configuration, iron may solidify into a generally uniform solid structure during plating thereof on the negative electrode substrate. For zinc and other metals commonly used in hybrid redox batteries, solid dendritic structures may form during plating. A stable electrode morphology of the IFB system may increase the efficiency of the battery in comparison to other redox flow batteries. Further still, iron redox flow batteries may reduce the use of toxic raw materials and may operate at a relatively neutral pH as compared to other redox flow battery electrolytes. Accordingly, IFB systems may reduce environmental hazards as compared with all other current advanced redox flow battery systems in production.

Continuing withFIG.1, a schematic illustration of the redox flow battery system10is shown. The redox flow battery system10may include the redox flow battery cell18fluidly coupled to an integrated multi-chambered electrolyte storage tank110. The redox flow battery cell18may include the negative electrode compartment20, separator24, and positive electrode compartment22. The separator24may include an electrically insulating ionic conducting barrier which prevents bulk mixing of the positive electrolyte and the negative electrolyte while allowing conductance of specific ions therethrough. For example, and as discussed above, the separator24may include an ion-exchange membrane and/or a microporous membrane.

The negative electrode compartment20may include the negative electrode26, and the negative electrolyte may include electroactive materials. The positive electrode compartment22may include the positive electrode28, and the positive electrolyte may include electroactive materials. In some examples, multiple redox flow battery cells18may be combined in series or in parallel to generate a higher voltage or electric current in the redox flow battery system10.

Further illustrated inFIG.1are negative and positive electrolyte pumps30and32, both used to pump electrolyte solution through the redox flow battery system10. Electrolytes are stored in one or more tanks external to the cell, and are pumped via the negative and positive electrolyte pumps30and32through the negative electrode compartment20side and the positive electrode compartment22side of the redox flow battery cell18, respectively.

The redox flow battery system10may also include a first bipolar plate36and a second bipolar plate38, each positioned along a rear-facing side, e.g., opposite of a side facing the separator24, of the negative electrode26and the positive electrode28, respectively. The first bipolar plate36may be in contact with the negative electrode26and the second bipolar plate38may be in contact with the positive electrode28. In other examples, however, the bipolar plates36and38may be arranged proximate but spaced away from the electrodes26and28and housed within the respective electrode compartments20and22. In either case, the bipolar plates36and38may be electrically coupled to the terminals40and42, respectively, either via direct contact therewith or through the negative and positive electrodes26and28, respectively.

The IFB electrolytes may be transported to reaction sites at the negative and positive electrodes26and28by the first and second bipolar plates36and38, resulting from conductive properties of a material of the bipolar plates36and38. Electrolyte flow may also be assisted by the negative and positive electrolyte pumps30and32, facilitating forced convection through the redox flow battery cell18. Reacted electrochemical species may also be directed away from the reaction sites by a combination of forced convection and a presence of the first and second bipolar plates36and38.

As illustrated inFIG.1, the redox flow battery cell18may further include the negative battery terminal40and the positive battery terminal42. When a charge current is applied to the battery terminals40and42, the positive electrolyte may be oxidized (loses one or more electrons) at the positive electrode28, and the negative electrolyte may be reduced (gains one or more electrons) at the negative electrode26. During battery discharge, reverse redox reactions may occur on the electrodes26and28. In other words, the positive electrolyte may be reduced (gains one or more electrons) at the positive electrode28, and the negative electrolyte may be oxidized (loses one or more electrons) at the negative electrode26. An electrical potential difference across the battery may be maintained by the electrochemical redox reactions in the positive electrode compartment22and the negative electrode compartment20, and may induce an electric current through a current collector while the reactions are sustained. An amount of energy stored by a redox battery may be limited by an amount of electroactive material available in electrolytes for discharge, depending on a total volume of electrolytes and a solubility of the electroactive materials.

The redox flow battery system10may further include the integrated multi-chambered electrolyte storage tank110. The multi-chambered electrolyte storage tank110may be divided by a bulkhead98. The bulkhead98may create multiple chambers within the multi-chambered electrolyte storage tank110so that both the positive and negative electrolytes may be included within a single tank. The negative electrolyte chamber50holds negative electrolyte including the electroactive materials, and the positive electrolyte chamber52holds positive electrolyte including the electroactive materials. The bulkhead98may be positioned within the multi-chambered electrolyte storage tank110to yield a desired volume ratio between the negative electrolyte chamber50and the positive electrolyte chamber52. In one example, the bulkhead98may be positioned to set a volume ratio of the negative and positive electrolyte chambers50and52according to a stoichiometric ratio between the negative and positive redox reactions.FIG.1further illustrates a fill height112of the multi-chambered electrolyte storage tank110, which may indicate a liquid level in each tank compartment.

FIG.1also shows a gas head space90located above the fill height112of the negative electrolyte chamber50, and a gas head space92located above the fill height112of the positive electrolyte chamber52. The gas head space92may be utilized to store H2gas generated through operation of the redox flow battery (e.g., due to proton reduction and iron corrosion side reactions) and conveyed to the multi-chambered electrolyte storage tank110with returning electrolyte from the redox flow battery cell18. The H2gas may be separated spontaneously at a gas-liquid interface (e.g., the fill height112) within the multi-chambered electrolyte storage tank110, thereby precluding having additional gas-liquid separators as part of the redox flow battery system10. Once separated from the electrolyte, the H2gas may fill the gas head spaces90and92. As such, the stored H2gas may aid in purging other gases from the multi-chambered electrolyte storage tank110, thereby acting as an inert gas blanket for reducing oxidation of electrolyte species, which may help to reduce redox flow battery capacity losses. In this way, utilizing the integrated multi-chambered electrolyte storage tank110may forego having separate negative and positive electrolyte storage tanks, hydrogen storage tanks, and gas-liquid separators common to conventional redox flow battery systems, thereby simplifying a system design, reducing a physical footprint of the redox flow battery system10, and reducing system costs.

FIG.1also shows a spillover hole96, which may create an opening in the bulkhead98between the gas head spaces90and92, and may provide a means of equalizing gas pressure between the chambers50and52. The spillover hole96may be positioned at a threshold height above the fill height112. The spillover hole96may further enable a capability to self-balance the electrolytes in each of the negative and positive electrolyte chambers50and52in the event of a battery crossover. In the case of an all-iron redox flow battery system, the same electrolyte (Fe2+) is used in both negative and positive electrode compartments20and22, so spilling over of electrolyte between the negative and positive electrolyte chambers50and52may reduce overall system efficiency, but overall electrolyte composition, battery module performance, and battery module capacity may be maintained.

Flange fittings may be utilized for all piping connections for inlets and outlets to and from the multi-chambered electrolyte storage tank110to maintain a continuously pressurized state without leaks. The multi-chambered electrolyte storage tank110may include at least one outlet from each of the negative and positive electrolyte chambers50and52, and at least one inlet to each of the negative and positive electrolyte chambers50and52. Furthermore, one or more outlet connections may be provided from the gas head spaces90and92for directing H2gas to rebalancing cells80and82, such that the rebalancing cells80and82may be respectively fluidically coupled to the gas head spaces90and92. Electrolyte and H2gas may be circulating through the rebalancing cells80and82to maintain electrolyte health and battery capacity. The hydrogen gas flow architecture is expanded upon herein with regard toFIGS.2A-2B,4A-4B, and5A-5B.

Although not shown inFIG.1, the integrated multi-chambered electrolyte storage tank110may further include one or more heaters thermally coupled to each of the negative electrolyte chamber50and the positive electrolyte chamber52. In alternate examples, only one of the negative and positive electrolyte chambers50and52may include one or more heaters. In the case where only the positive electrolyte chamber52includes one or more heaters, the negative electrolyte may be heated by transferring heat generated at the redox flow battery cell18to the negative electrolyte. In this way, the redox flow battery cell18may heat and facilitate temperature regulation of the negative electrolyte. The one or more heaters may be actuated by a controller88to regulate a temperature of the negative electrolyte chamber50and the positive electrolyte chamber52independently or together. For example, in response to an electrolyte temperature decreasing below a threshold temperature, the controller88may increase a power supplied to one or more heaters so that a heat flux to the electrolyte may be increased. The electrolyte temperature may be indicated by one or more temperature sensors mounted at the multi-chambered electrolyte storage tank110, such as sensors60and62.

As examples, 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 transfer heat conductively through the walls of the negative and positive electrolyte chambers50and52to heat the fluid therein. Other known types of tank heaters may be employed without departing from the scope of the present disclosure. Furthermore, the controller88may deactivate the one or more heaters in the negative and positive electrolyte chambers50and52in response to a liquid level decreasing below a solids fill threshold level. Said in another way, in some examples, the controller88may activate the one or more heaters in the negative and positive electrolyte chambers50and52only in response to a liquid level increasing above the solids fill threshold level. In this way, activating the one or more heaters without sufficient liquid in the negative and/or positive electrolyte chambers50,52may be averted, thereby reducing a risk of overheating or burning out the heater(s).

Further still, one or more inlet connections may be provided to each of the negative and positive electrolyte chambers50and52from a field hydration system. In this way, the field hydration system may facilitate commissioning of the redox flow battery system10, including installing, filling, and hydrating the redox flow battery system10, at an end-use location. Furthermore, prior to commissioning the redox flow battery system10at the end-use location, the redox flow battery system10may be dry-assembled at a battery manufacturing facility different from the end-use location without filling and hydrating the redox flow battery system10, before delivering the redox flow battery system10to the end-use location. In one example, the end-use location may correspond to a location where the redox flow battery system10is to be installed and utilized for on-site energy storage. Said another way, the redox flow battery system10may be designed such that, once installed and hydrated at the end-use location, a position of the redox flow battery system10may become fixed, and the redox flow battery system10may no longer be deemed a portable, dry system. Thus, from a perspective of an end-user, the dry, portable redox flow battery system10may be delivered on-site, after which the redox flow battery system10may be installed, hydrated, and commissioned. Prior to hydration, the redox flow battery system10may be referred to as a dry, portable system, the redox flow battery system10being free of or without water and wet electrolyte. Once hydrated, the redox flow battery system10may be referred to as a wet, non-portable system, the redox flow battery system10including wet electrolyte.

Further illustrated inFIG.1, electrolyte solutions primarily stored in the multi-chambered electrolyte storage tank110may be pumped via the negative and positive electrolyte pumps30and32throughout the redox flow battery system10. Electrolyte stored in the negative electrolyte chamber50may be pumped via the negative electrolyte pump30through the negative electrode compartment20side of the redox flow battery cell18, and electrolyte stored in the positive electrolyte chamber52may be pumped via the positive electrolyte pump32through the positive electrode compartment22side of the redox flow battery cell18.

The electrolyte rebalancing cells80and82(e.g., reactors) may be connected in line or in parallel with the recirculating flow paths of the electrolyte at the negative and positive sides of the redox flow battery cell18, respectively, in the redox flow battery system10. One or more rebalancing reactors may be connected in-line with the recirculating flow paths of the electrolyte at 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 serviced without disrupting battery and rebalancing operations) and for increased rebalancing capacity. In one example, the electrolyte rebalancing cells80and82may be placed in a return flow path from the negative and positive electrode compartments20and22to the negative and positive electrolyte chambers50and52, respectively. The electrolyte rebalancing cells80and82may serve to rebalance electrolyte charge imbalances in the redox flow battery system10occurring due to side reactions, ion crossover, and the like, as described herein.

As shown inFIG.1(and as further described with reference toFIGS.7A and7B), at least a portion of the liquid electrolyte pumped from the negative electrolyte chamber by the negative electrolyte pump30may be diverted upstream of the negative electrode compartment20to the rebalancing cell80. An electrolyte flow control device76may be positioned between the negative electrolyte pump30and the rebalancing cell80to regulate the flow of electrolyte diverted to the rebalancing cell80. Analogously, a portion of the liquid electrolyte pumped from the positive electrolyte chamber by the positive electrolyte pump32may be diverted upstream of the positive electrode compartment22to the rebalancing cell82. An electrolyte flow control device78may be positioned between the positive electrolyte pump32and the rebalancing cell82to regulate the flow of electrolyte diverted to the rebalancing cell82. Electrolyte flow control devices76and78are communicatively coupled to the controller88. As such, the controller88may regulate a flow of electrolyte diverted by way of the electrolyte flow control devices76and78to the rebalancing cells80and82, respectively. As further described with reference toFIGS.9and10, the controller88may modulate the electrolyte flow control devices76and78to maintain a rebalancing reaction rate at the rebalancing cells80and82.

In some examples, one or both of the rebalancing cells80and82may include trickle bed reactors, where the H2gas and the (liquid) electrolyte may be contacted at catalyst surfaces in a packed bed for carrying out the electrolyte rebalancing reaction. Additionally or alternatively, one or both of the rebalancing cells80and82may have catalyst beds configured in a jelly roll. In additional or alternative examples, one or both of the rebalancing cells80and82may include flow-through type reactors that are capable of contacting the H2gas and the electrolyte and carrying out the electrolyte rebalancing reactions absent a packed catalyst bed. However, lower Fe3+reduction rates (e.g., on the order of ˜1-3 mol/m2hr) during electrolyte rebalancing may preclude implementation of such rebalancing reactor configurations in higher performance applications.

In other examples, one or both of the rebalancing cells80and82may include fuel cells, where the H2gas and the electrolyte may be contacted at catalyst surfaces for carrying out the electrolyte rebalancing reaction and where a closed circuit may be formed by directing electric current from the fuel cells through an external load. However, reverse current spikes (e.g., transient increases in reverse electric current, where “reverse electric current” may be used herein to refer to any electric current traveling along an electrical pathway in a direction opposite from expected (that is, opposite from a “forward” direction)) in such fuel cells may be unavoidable in certain circumstances, undermining a reliability of such rebalancing reaction configurations.

To increase the Fe3+reduction rate without sacrificing an overall reliability of the rebalancing cells80and82, embodiments of the present disclosure provide a rebalancing cell, such as the rebalancing cell ofFIGS.2A and2B, including a stack of internally shorted electrode assemblies, such as the electrode assembly ofFIG.3, configured to drive the H2gas and the electrolyte to react at catalyst surfaces via a combination of internal electric current, convection, gravity feeding, and capillary action. In embodiments described herein, the electrode assemblies of the stack of internally shorted electrode assemblies may be referred to as “internally shorted,” in that no electric current may be directed away from the stack of internally shorted electrode assemblies during operation of the rebalancing cell. Such internal electrical shorting may reduce or obviate reverse current spikes while drastically increasing the Fe3+reduction rate (e.g., to as high as ˜50-70 mol/m2hr) and concomitantly decreasing side reaction rates (e.g., rates of the reactions of equation (1)-(3)). Further, each electrode assembly of the stack of internally shorted electrode assemblies may be electrically decoupled from each other electrode assembly of the stack of internally shorted electrode assemblies, such that degradation to the stack of internally shorted electrode assemblies during current spikes at one electrode assembly may be limited thereto (e.g., reverse electric current may not be driven from one electrode assembly through the other electrode assemblies). In such cases, the single, degraded electrode assembly may be easily removed from the stack of internally shorted electrode assemblies and replaced with a non-degraded electrode assembly.

To realize the internally shorted circuit, each electrode assembly of the stack of internally shorted electrode assemblies may include an interfacing pair of positive and negative electrodes (e.g., configured in face-sharing contact with one another so as to be continuously electrically conductive). As used herein, a pair of first and second components (e.g., positive and negative electrodes of an electrode assembly) may be described as “interfacing” with one another when the first component is arranged adjacent to the second component such that the first and second components are in face-sharing contact with one another (where “adjacent” is used herein to refer to any two components having no intervening components therebetween). Further, as used herein, “continuously” when describing electrical conductivity of multiple electrodes may refer to an electrical pathway therethrough having effectively or practically zero resistance at any face-sharing interfaces of the multiple electrodes.

In an exemplary embodiment, the (positive) rebalancing cell82may be the rebalancing cell including the stack of internally shorted electrode assemblies. Higher Fe3+reduction rates may be desirable to rebalance the positive electrolyte, as significant amounts of Fe3+may be generated at the positive electrode28during battery charging (see equation (6)). In additional or alternative embodiments, the (negative) rebalancing cell80may be of like configuration [Fe3+may be generated at the negative electrode26during iron plating oxidation (see equation (3))].

During operation of the redox flow battery system10, sensors and probes may monitor and control chemical properties of the electrolyte such as electrolyte pH, concentration, SOC, and the like. For example, as illustrated inFIG.1, sensors62and60maybe be positioned to monitor positive electrolyte and negative electrolyte conditions at the positive electrolyte chamber52and the negative electrolyte chamber50, respectively. In another example, sensors62and60may each include one or more electrolyte level sensors to indicate a level of electrolyte in the positive electrolyte chamber52and the negative electrolyte chamber50, respectively. As another example, sensors72and70, also illustrated inFIG.1, may monitor positive electrolyte and negative electrolyte conditions at the positive electrode compartment22and the negative electrode compartment20, respectively. The sensors72and70may be pH probes, optical probes, pressure sensors, voltage sensors, etc. It will be appreciated that sensors may be positioned at other locations throughout the redox flow battery system10to monitor electrolyte chemical properties and other properties.

For example, a sensor may be positioned in an external acid tank to monitor acid volume or pH of the external acid tank, wherein acid from the external acid tank may be supplied via an external pump to the redox flow battery system10in order to reduce precipitate formation in the electrolytes. Additional external tanks and sensors may be installed for supplying other additives to the redox flow battery system10. For example, various sensors including, temperature, conductivity, and level sensors of a field hydration system may transmit signals to the controller88. Furthermore, the controller88may send signals to actuators such as valves and pumps of the field hydration system during hydration of the redox flow battery system10. Sensor information may be transmitted to the controller88which may in turn actuate the pumps30and32to control electrolyte flow through the redox flow battery cell18, or to perform other control functions, as an example. In this manner, the controller88may be responsive to one or a combination of sensors and probes.

The redox flow battery system10may further include a source of H2gas. In one example, the source of H2gas may include a separate dedicated hydrogen gas storage tank. In the example ofFIG.1, H2gas may be stored in and supplied from the integrated multi-chambered electrolyte storage tank110. The integrated multi-chambered electrolyte storage tank110may supply additional H2gas to the positive electrolyte chamber52and the negative electrolyte chamber50. The integrated multi-chambered electrolyte storage tank110may alternately and/or additionally supply additional H2gas to an inlet of the electrolyte rebalancing cells80and82. As an example, hydrogen gas flow control devices66and68, which are communicatively coupled to and controlled by the controller88, may regulate flow of the H2gas from the integrated multi-chambered electrolyte storage tank110to the rebalancing cells80and82, respectively. As shown inFIG.1(and further described with reference toFIGS.7A and7B), the hydrogen gas flow control devices66and68may be fluidly coupled between the gas head spaces90and92of the multi-chambered electrolyte storage tank110and the rebalancing cells80and82, respectively. As further described with reference toFIGS.9and10, the controller88may modulate the hydrogen gas flow control devices66and68to maintain a rebalancing reaction rate at the rebalancing reactors80and82.

The integrated multi-chambered electrolyte storage tank110may supplement the H2gas generated in the redox flow battery system10. For example, when gas leaks are detected in the redox flow battery system10or when a reduction reaction rate (e.g., the Fe3+reduction rate) is too low at low hydrogen partial pressure, the H2gas may be supplied from the integrated multi-chambered electrolyte storage tank110in order to rebalance the SOC of the electroactive materials in the positive electrolyte and the negative electrolyte. As an example, the controller88may supply the H2gas from the integrated multi-chambered electrolyte storage tank110in response to a measured change in pH or in response to a measured change in SOC of an electrolyte or an electroactive material.

For example, an increase in pH of the negative electrolyte chamber50, or the negative electrode compartment20, may indicate that H2gas is leaking from the redox flow battery system10and/or that the reaction rate is too slow with the available hydrogen partial pressure, and the controller88, in response to the pH increase, may increase a supply of H2gas from the integrated multi-chambered electrolyte storage tank110to the redox flow battery system10. As a further example, the controller88may supply H2gas from the integrated multi-chambered electrolyte storage tank110in response to a pH change, wherein the pH increases beyond a first threshold pH or decreases beyond a second threshold pH. In the case of an IFB, the controller88may supply additional H2gas to increase a rate of reduction of Fe3+ions and a rate of production of protons, thereby reducing the pH of the positive electrolyte. Furthermore, the pH of the negative electrolyte may be lowered by hydrogen reduction of Fe3+ions crossing over from the positive electrolyte to the negative electrolyte or by protons, generated at the positive side, crossing over to the negative electrolyte due to a proton concentration gradient and electrophoretic forces. In this manner, the pH of the negative electrolyte may be maintained within a stable region, while reducing the risk of precipitation of Fe3+ions (crossing over from the positive electrode compartment22) as Fe(OH)3.

Other control schemes for controlling a supply rate of H2gas from the integrated multi-chambered electrolyte storage tank110responsive to a change in an electrolyte pH or to a change in an electrolyte SOC, detected by other sensors such as an oxygen-reduction potential (ORP) meter or an optical sensor, may be implemented. Further still, the change in pH or SOC triggering action of the controller88may be based on a rate of change or a change measured over a time period. The time period for the rate of change may be predetermined or adjusted based on time constants for the redox flow battery system10. For example, the time period may be reduced if a recirculation rate is high, and local changes in concentration (e.g., due to side reactions or gas leaks) may quickly be measured since the time constants may be small.

The controller88may further execute control schemes based on an operating mode of the redox flow battery system10. For example, and as discussed in detail below with reference toFIGS.9and10, in tandem with controlling flow of the H2gas to the rebalancing cells80and82as described above, the controller88may control flows of the negative and positive electrolytes to the rebalancing cells80and82, respectively, during charging and discharging of the redox flow battery cell18so as to simultaneously reduce excess concentrations of H2gas and Fe3+ion in the redox flow battery system. After electrolyte rebalancing, the controller88may direct flow of any excess or unreacted H2along with the rebalanced negative and positive electrolytes (e.g., including a decreased concentration of Fe3+and an increased concentration of Fe2+) from the rebalancing cells80and82back into the respective electrolyte chambers50and52of the multi-chambered electrolyte storage tank110. Additionally or alternatively, the unreacted H2gas may be returned to the separate dedicated hydrogen gas storage tank.

As another example, the controller88may further control charging and discharging of the redox flow battery cell18so as to cause iron preformation at the negative electrode26during system conditioning (where system conditioning may include an operating mode employed to optimize electrochemical performance of the redox flow battery system10outside of battery cycling). That is, during system conditioning, the controller88may adjust one or more operating conditions of the redox flow battery system10to plate iron metal on the negative electrode26to increase a battery charge capacity during subsequent battery cycling (thus, the iron metal may be preformed for battery cycling). In this way, preforming iron at the negative electrode26and running electrolyte rebalancing during the system conditioning may increase an overall capacity of the redox flow battery cell18during battery cycling by mitigating iron plating loss. As used herein, battery cycling (also referred to as “charge cycling”) may include alternating between a charging mode and a discharging mode of the redox flow battery system10.

It will be appreciated that all components apart from the sensors60and62and the integrated multi-chambered electrolyte storage tank110(and components included therein) may be considered as being included in a power module120. As such, the redox flow battery system10may be described as including the power module120fluidly coupled to the integrated multi-chambered electrolyte storage tank110and communicably coupled to the sensors60and62. In some examples, each of the power module120and the multi-chambered electrolyte storage tank110may be included in a single housing or packaging, such that the redox flow battery system10may be contained as a single unit in a single location. It will further be appreciated the positive electrolyte, the negative electrolyte, the sensors60and62, the electrolyte rebalancing cells80and82, and the integrated multi-chambered electrolyte storage tank110(and components included therein) may be considered as being included in an electrolyte subsystem130. As such, the electrolyte subsystem130may supply one or more electrolytes to the redox flow battery cell18(and components included therein).

Referring now toFIGS.2A and2B, perspective views are respectively shown, each of the perspective views depicting a rebalancing cell202for a redox flow battery system, such as redox flow battery system10ofFIG.1. In an exemplary embodiment, the rebalancing cell202may include a stack of internally shorted electrode assemblies, such as the electrode assembly described in detail below with reference toFIG.3, which may drive an electrolyte rebalancing reaction by promoting contact between H2gas and an electrolyte from positive or negative electrode compartments of a redox flow battery, such as the redox flow battery cell18ofFIG.1, at catalytic surfaces of negative electrodes of the stack of internally shorted electrode assemblies. Accordingly, the rebalancing cell202may be one or both of the rebalancing cells80and82ofFIG.1.

A set of reference axes250is provided for describing relative positioning of the components shown and for comparison between the views ofFIGS.2A-5B, the axes250indicating an x-axis, a y-axis, and a z-axis. As further shown in dashing inFIGS.2A and2B, an additional axis g may be parallel with a direction of gravity (e.g., in a positive direction along the axis g) and a vertical direction (e.g., in a negative direction along the axis g and opposite to the direction of gravity).

A number of rebalancing cells202included in the redox flow battery system and a number of electrode assemblies included in the stack of internally shorted electrode assemblies are not particularly limited and may increase to accommodate correspondingly higher performance applications. For example, a 75 KW redox flow battery system may include two rebalancing cells202including a stack of 20 electrode assemblies (e.g., a stack of 19 bipolar assemblies with 2 end plates positioned at opposite ends of the stack).

As shown, the stack of internally shorted electrode assemblies may be removably enclosed within an external cell enclosure (e.g., housing)204. Accordingly, in some examples, the cell enclosure204may include a top cover removably affixed 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 internally shorted electrode assemblies. In additional or alternative examples, the cell enclosure204, depicted inFIGS.2A and2Bas a rectangular prism, may be molded to be clearance fit against other components of the redox flow battery system such that the rebalancing cell202may be in face-sharing contact with such components. In some examples, the cell enclosure204may be composed of a material having a low electrical conductivity, such as a plastic or other polymer, so as to reduce undesirable shorting events.

The cell enclosure204may further be configured to include openings or cavities for interfacial components of the rebalancing cell202. For example, the cell enclosure204may include a plurality of inlet and outlet ports configured to fluidically couple to other components of the redox flow battery system. In one example, and as shown, the plurality of inlet and outlet ports may include polypropylene (PP) flange fittings fusion welded to PP plumbing.

In an exemplary embodiment, the plurality of inlet and outlet ports may include an electrolyte inlet port206for flowing the electrolyte into the cell enclosure204and electrolyte outlet ports208for expelling the electrolyte from the cell enclosure204. The electrolyte inlet and outlet ports may be referred to as positive inlet and outlet ports due to the cell chemistry.

In one example, the electrolyte inlet port206may be positioned on an upper half of the cell enclosure204and the electrolyte outlet ports208may be positioned on a lower half of the cell enclosure204(where the upper half and the lower half of the cell enclosure204are separated along the z-axis by a plane parallel with each of the x- and y-axes). Accordingly, the electrolyte outlet ports208may be positioned lower than the electrolyte inlet port206with respect to the direction of gravity (e.g., along the axis g).

Specifically, upon the electrolyte entering the cell enclosure204via the electrolyte inlet port206, the electrolyte may be distributed across the stack of internally shorted electrode assemblies, gravity fed through the stack of electrode assemblies, wicked up (e.g., against the direction of gravity) through positive electrodes of the stack of internally shorted electrode assemblies to react at the catalytic surfaces of the negative electrodes in a cathodic half reaction, and expelled out of the cell enclosure204via the electrolyte outlet ports208. To assist in the gravity feeding of the electrolyte and increase a pressure drop thereof, the rebalancing cell202may further be tilted or inclined with respect to the direction of gravity via a sloped support220coupled to the cell enclosure204. In some examples, tilting of the cell enclosure204in this way may further assist in electrolyte draining of the rebalancing cell202(e.g., during an idle mode of the redox flow battery system) and keep the catalytic surfaces relatively dry (as the catalytic surfaces may corrode after being soaked in the electrolyte for a sufficient duration, in some examples).

As shown, the sloped support220may tilt the cell enclosure204at an angle222such that planes of electrode sheets of the stack of internally shorted electrode assemblies are inclined with respect to a lower surface on which the sloped support220rests, at the angle222. In some examples, the angle222(e.g., of the cell enclosure204with respect to the lower surface) may be between 0° and 30°. In embodiments wherein the angle222is substantially 0°, the rebalancing cell202may still function, though tilting the cell enclosure204by an angle greater than 0° may allow the pressure drop to be greater and for electrolyte crossover to the negative electrodes to be reduced. In some examples, the angle222may be between 2° and 30°. In some examples, the angle222may be between 2° and 20°. In one example, the angle222may be about 8°. Accordingly, the pressure drop of the electrolyte upon entering the cell enclosure204, e.g., through the electrolyte inlet port206, may be increased by increasing the angle222and decreased by decreasing the angle222.

Additionally or alternatively, one or more support rails224may be coupled to the upper half of the cell enclosure204(e.g., opposite from the sloped support220). In some examples, and as shown in the perspective view ofFIG.2A, the one or more support rails224may be tilted with respect to the cell enclosure204at the angle222such that the one or more support rails224may removably fasten the rebalancing cell202to an upper surface above and parallel with the lower surface. In this way, and based on geometric considerations, the z-axis may likewise be offset from the axis g at the angle222(e.g., the cell enclosure204may be tilted with respect to a vertical direction opposite the direction of gravity by the angle222, as shown inFIGS.2A and2B). In some examples, gravity feeding of the electrolyte through the rebalancing cell202may further be assisted by positioning the rebalancing cell202above an electrolyte storage tank (e.g., the multi-chambered electrolyte storage tank110ofFIG.1) of the redox flow battery system with respect to the vertical direction opposite to the direction of gravity.

As further shown, the electrolyte outlet ports208may include a plurality of openings in the cell enclosure204configured to expel at least a portion of the electrolyte (each of the plurality of openings including the PP flange fitting fusion welded to PP plumbing). For instance, inFIGS.2A and2B, the electrolyte outlet ports208is shown including five openings. In this way, the electrolyte may be evenly distributed across the stack of internally shorted electrode assemblies and may be expelled from the cell enclosure204with substantially unimpeded flow (“substantially” may be used herein as a qualifier meaning “effectively”). In other examples, the electrolyte outlet ports208may include more than five openings or less than five openings. In one example, the electrolyte outlet ports208may include a single opening. In additional or alternative examples, the electrolyte outlet ports208may be positioned beneath the cell enclosure204with respect to the z-axis (e.g., on a face of the cell enclosure204facing a negative direction of the z-axis).

The electrolyte inlet port206and the electrolyte outlet ports208may be positioned on the cell enclosure204based on a flow path of the electrolyte through the stack of internally shorted electrode assemblies (e.g., from the electrolyte inlet port206to the electrolyte outlet ports208and inclusive of channels, passages, manifolds, plenums, wells, etc. within the cell enclosure204fluidically coupled to the electrolyte inlet port206and the electrolyte outlet ports208). In some examples, and as shown, the electrolyte inlet port206and the electrolyte outlet ports208may be positioned on adjacent sides of the cell enclosure204(e.g., faces of the cell enclosure204sharing a common edge). In other examples, the electrolyte inlet port206and the electrolyte outlet ports208may be positioned on opposite sides of the cell enclosure204. In other examples, the electrolyte inlet port206and the electrolyte outlet ports208may be positioned on the same side of the cell enclosure204.

In some examples, the electrolyte inlet port206may be positioned on a face of the cell enclosure204facing a negative direction of the x-axis. In additional or alternative examples, the electrolyte inlet port206may be positioned on a face of the cell enclosure204facing a positive direction of the x-axis. In one example, and as shown, one opening of the electrolyte inlet port206may be positioned on the face of the cell enclosure204facing the negative direction of the x-axis and another opening of the electrolyte inlet port206may be positioned on the face of the cell enclosure204facing the positive direction of the x-axis.

In some examples, the plurality of inlet and outlet ports may further include a hydrogen gas inlet port210for flowing the H2gas into the cell enclosure204and a hydrogen gas outlet port212(as shown inFIG.2B) for expelling the H2gas from the cell enclosure204. The hydrogen gas inlet and outlet ports may be referred to as negative inlet and outlet ports due to cell chemistry.

In one example, and as shown, each of the hydrogen gas inlet port210and the hydrogen gas outlet port212may be positioned on the lower half of the cell enclosure204(e.g., at a lowermost electrode assembly of the stack of internally shorted electrode assemblies along the z-axis). In another example, each of the hydrogen gas inlet port210and the hydrogen gas outlet port212may be positioned on the upper half of the cell enclosure204(e.g., at an uppermost electrode assembly of the stack of internally shorted electrode assemblies along the z-axis). In yet another example, the hydrogen gas inlet port210may be positioned on the lower half of the cell enclosure204and the hydrogen gas outlet port212may be positioned on the upper half of the cell enclosure204. In such an example, the hydrogen gas inlet port210may be positioned lower than the hydrogen gas outlet port212with respect to the direction of gravity (e.g., along the axis g).

Specifically, upon the H2gas entering the cell enclosure204via the hydrogen gas inlet port210, the H2gas may be distributed across and through the stack of internally shorted electrode assemblies via forced convection (e.g., induced by flow field configurations of respective flow field plates) and decomposed at the catalytic surfaces of the negative electrodes in an anodic half reaction. However, in some examples, excess, unreacted H2gas may remain in the rebalancing cell202following contact with the catalytic surfaces. In some examples, at least a portion of the H2gas which has not reacted at the catalytic surfaces may pass into the electrolyte. To avoid undesirable pressure buildup and thereby prevent electrolyte pooling on the positive electrodes and concomitant electrolyte flooding of the negative electrodes in such examples, the plurality of inlet and outlet ports may further include a hydrogen gas relief port214(as shown inFIG.2A) to expel unreacted H2gas from the electrolyte. The hydrogen gas relief port214may specifically receive hydrogen that has travel through the electrolyte and therefore may be referred to as a positive side hydrogen relief port214may specifically receive hydrogen that has travel through the electrolyte and therefore may be referred to as a positive side hydrogen relief port.

Further, in some examples, the hydrogen gas outlet port212may be configured to expel at least a portion of the H2gas which has not reacted at the catalytic surfaces and that has not flowed through the negative electrodes into the electrolyte. Further aspects of the H2gas flow will be discussed in greater detail below with reference toFIGS.4A-5B.

The hydrogen gas inlet port210and the hydrogen gas outlet port212may be positioned on the cell enclosure204based on a flow path of the H2gas through the stack of internally shorted electrode assemblies. For example, the flow path may be from the hydrogen gas inlet port210to the hydrogen gas outlet port212(when included) and inclusive of channels, passages, manifolds, plenums, etc., within the cell enclosure204and fluidically coupled to the hydrogen gas inlet port210and the hydrogen gas outlet port212(when included). In some examples, and as shown, the hydrogen gas inlet port210and the hydrogen gas outlet port212may be positioned on opposite sides of the cell enclosure204. In other examples, the hydrogen gas inlet port210and the hydrogen gas outlet port212may be positioned on adjacent sides of the cell enclosure204. In other examples, the hydrogen gas inlet port210and the hydrogen gas outlet port212may be positioned on the same side of the cell enclosure204. Further, though the hydrogen gas inlet port210is shown inFIGS.2A and2Bas being positioned on the face of the cell enclosure204facing the negative direction of the x-axis and the hydrogen gas outlet port212is shown inFIGS.2A and2Bas being positioned on the face of the cell enclosure204facing the positive direction of the x-axis, in other examples, the hydrogen gas inlet port210may be positioned on the face of the cell enclosure204facing the positive direction of the x-axis and the hydrogen gas outlet port212may be positioned on the face of the cell enclosure204facing the negative direction of the x-axis.

In one example, the hydrogen gas inlet port210, the hydrogen gas outlet port212, the electrolyte inlet port206, and the electrolyte outlet ports208may be positioned on the cell enclosure204in a crosswise configuration. The crosswise configuration may include the hydrogen gas outlet port212and the electrolyte inlet port206being positioned on different sides (e.g., faces) of the upper half of the cell enclosure204and the hydrogen gas inlet port210and the electrolyte outlet ports208being positioned on different sides of the lower half of the cell enclosure204.

In other examples, the hydrogen gas outlet port212may be closed, omitted, or otherwise inhibited from expelling H2gas which has not reacted at the catalytic surfaces of the negative electrodes and which has not flowed through the negative electrodes into the electrolyte. In such examples, however, the hydrogen gas relief port214for expelling unreacted H2gas from the electrolyte may still be present, and the unreacted H2gas may only be expelled from the cell enclosure204after flowing through the negative electrodes into the electrolyte and through the hydrogen gas relief port214. Exemplary rebalancing cell configurations lacking the hydrogen gas outlet port212, whether or not including the hydrogen gas relief port214, may be referred to as “dead ended configurations.” In dead ended configurations, substantially all of the H2gas may be forced into contact with the catalytic surfaces of the negative electrodes, whereat the H2gas may either decompose via the anodic half reaction and/or the H2gas may enter the electrolyte after passing through the negative electrodes (e.g., without reacting at catalytic surfaces thereof).

The rebalancing cell202depicted inFIGS.2A and2Bmay operate with a comparatively small pressure gradient between the negative (hydrogen, higher pressure) and the positive (electrolyte, lower pressure) sides of the cell. As the cell is internally shorted, gravity and a higher hydrogen gas pressure keeps the electrolyte from flooding the negative channels in the cells.

Referring now toFIG.3, an exploded view of an electrode assembly302for a rebalancing cell, such as the rebalancing cell202ofFIGS.2A and2B, is shown. Accordingly, the electrode assembly302may be internally shorted (e.g., electric current flowing through the electrode assembly302is not channeled through an external load). In an exemplary embodiment, the electrode assembly302may be included in a stack of electrode assemblies of like configuration in a cell enclosure so as to form the rebalancing cell. The electrode assembly302may include a plate304with an activated carbon foam306, a positive electrode308(also referred to herein as a “cathode” in certain examples), and a negative electrode310(also referred to herein as an “anode” in certain examples) sequentially stacked thereon. The electrode assembly302may be positioned within the rebalancing cell so as to receive an electrolyte through the carbon foam306, wherefrom the electrolyte may enter pores of the positive electrode308via capillary action and come into contact with the negative electrode310. The electrode assembly302may further be positioned within the rebalancing cell so as to receive H2gas across a catalytic surface of the negative electrode310opposite to the positive electrode308via convection. The convection of the H2gas across the catalytic surface may be assisted by a flow field plate interfacing with the catalytic surface. Upon decomposition of the H2gas at the catalytic surface via an anodic half reaction, protons and electrons may flow to an interface of the negative electrode310and the positive electrode308, whereat ions in the electrolyte may be reduced via a cathodic half reaction (e.g., Fe3+may be reduced to Fe2+). In this way, the electrode assembly302may be configured for electrolyte rebalancing for a redox flow battery, such as the redox flow battery cell18ofFIG.1, fluidically coupled to the rebalancing cell including the electrode assembly302.

In some examples, the plate304may be composed of a material having a low electrical conductivity, such as a plastic or other polymer, so as to reduce undesirable shorting events. Accordingly, in one example, the plate304may be formed from the same material as the cell enclosure204ofFIGS.2A and2B.

As shown, the plate304may include a plurality of inlets and outlets therethrough. For example, the plurality of inlets and outlets may include an electrolyte outlet channel section316, a hydrogen gas inlet channel section318a, and a hydrogen gas outlet channel section318b. Specifically, the plate304may include the electrolyte outlet channel section316for directing the electrolyte out of the rebalancing cell, the hydrogen gas inlet channel section318afor directing the H2gas into the rebalancing cell and across the negative electrode310, and the hydrogen gas outlet channel section318bfor directing the H2gas out of the rebalancing cell. The plate304may further include an electrolyte inlet well312for receiving the electrolyte at the electrode assembly302, the electrolyte inlet well312fluidically coupled to a plurality of electrolyte inlet passages314aset into a berm314bpositioned adjacent to the carbon foam306for distributing the received electrolyte across the carbon foam306. In some examples, the electrolyte inlet well312may receive the electrolyte from an electrolyte inlet port (e.g., the electrolyte inlet port206ofFIGS.2A and2B) fluidically coupled thereto (e.g., via an electrolyte inlet channel), the electrolyte outlet channel section316may expel the electrolyte through an electrolyte outlet port (e.g., the electrolyte outlet ports208ofFIGS.2A and2B) fluidically coupled thereto, the hydrogen gas inlet channel section318amay receive the H2gas from a hydrogen gas inlet port (e.g., the hydrogen gas inlet port210ofFIGS.2A and2B) fluidically coupled thereto, and the hydrogen gas outlet channel section318bmay expel the H2gas through a hydrogen gas outlet port (e.g., the hydrogen gas outlet port212ofFIGS.2A and2B) fluidically coupled thereto.

It will be appreciated that, though the hydrogen gas inlet channel section318ais described herein as a section of a hydrogen gas inlet channel and the hydrogen gas outlet channel section318bis described herein as a section of a hydrogen gas outlet channel, in other examples, the channel section318bmay be a section of a hydrogen gas inlet channel (e.g., for directing the H2gas into the rebalancing cell and across the negative electrode310after receiving the H2gas from the hydrogen gas inlet port) and the gas inlet channel section318amay be a section of a hydrogen gas outlet channel (e.g., for directing the H2gas out of the rebalancing cell by expelling the H2gas through the hydrogen gas outlet port). In other examples, the rebalancing cell may be configured as a dead ended configuration and no hydrogen gas outlet port may be fluidically coupled to the hydrogen gas outlet channel section318b. In such examples, the hydrogen gas outlet channel section318bmay direct the H2gas back across the negative electrode310or the hydrogen gas outlet channel section318bmay instead be configured as another hydrogen gas inlet channel section (e.g., for directing a portion of the H2gas into the rebalancing cell and across the negative electrode310after receiving the portion of the H2gas from the hydrogen gas inlet port).

The plurality of inlets and outlets may be configured to case electrolyte and H2gas flow throughout the rebalancing cell. As an example, a size of each of the hydrogen gas inlet channel section318aand the hydrogen gas outlet channel section318bmay be selected to minimize a pressure drop therethrough, thereby aiding in flow distribution into each electrode assembly302of the stack of internally shorted electrode assemblies. As another example, a size of each electrolyte inlet passage314aand a total number of the plurality of electrolyte inlet passages314arelative to the berm314bmay be selected to induce a relatively small pressure drop to substantially evenly distribute electrolyte flow. In such an example, the selection of the size of each electrolyte inlet passage314aand the total number of the plurality of electrolyte inlet passages314amay be dependent on a number of factors specific to a given configuration of the rebalancing cell, such as a size of an electrolyte flow field and a desired electrolyte flow rate.

In additional or alternative examples, the electrolyte outlet channel section316may further be configured for distributing the electrolyte through multiple openings included in the electrolyte outlet port. For instance, in the exploded view ofFIG.3, the electrolyte outlet channel section316is shown including two openings. In some examples, a number of openings included in the electrolyte outlet channel section316may be equal to a number of openings included in the electrolyte outlet port, such that the openings of the electrolyte outlet channel section316may respectively correspond to the openings of the electrolyte outlet port. In this way, the electrolyte may be evenly distributed across the electrode assembly302and may be expelled from the rebalancing cell with substantially unimpeded flow. In other examples, the electrolyte outlet channel section316may include more than two openings or less than two openings (e.g., a single opening).

Further, when the electrode assembly302is included in a stack of electrode assemblies, electrolyte outlet channel sections316, hydrogen gas inlet channel sections318a, and hydrogen gas outlet channel sections318bmay align to form a continuous electrolyte outlet channel, a continuous hydrogen gas inlet channel, and a continuous hydrogen gas outlet channel, respectively (as variously shown inFIGS.4A,4B,5A, and5B, described below). In this way, the stack of electrode assemblies may be formed in a modular fashion, whereby any practical number of electrode assemblies302may be stacked and included in a rebalancing cell.

As further shown, a plurality of sealing inserts may be affixed (as used herein, “affix,” “affixed,” or “affixing” includes, but is not limited to, gluing, attaching, connecting, fastening, joining, linking, or securing one component to another component through a direct or indirect relationship) or otherwise coupled to the plate304. As an example, the plurality of sealing inserts may include a hydrogen gas inlet channel seal insert320aand a hydrogen gas outlet channel seal insert320bfor inducing flow of the H2gas across the negative electrode310by mitigating H2gas bypass. Specifically, the hydrogen gas inlet channel seal insert320aand the hydrogen gas outlet channel seal insert320bmay be affixed or otherwise coupled adjacent to the hydrogen gas inlet channel section318aand the hydrogen gas outlet channel section318b, respectively, on a side of the plate304including the carbon foam306, the positive electrode308, and the negative electrode310. In some examples, and as discussed in greater detail with reference toFIGS.4A and4B, the hydrogen gas inlet channel seal insert320aand the hydrogen gas outlet channel seal insert320bmay be coincident with an x-y plane of the negative electrode310such that the hydrogen gas inlet channel seal insert320aand the hydrogen gas outlet channel seal insert320bmay extend from a locus of affixation or coupling with the plate304and partially overlap the positive electrode308.

As another example, the plurality of sealing inserts may further include each of a hydrogen gas inlet channel O-ring322aand a hydrogen gas outlet channel O-ring322bfor respectively sealing an interface of the hydrogen gas inlet channel section318awith a hydrogen gas inlet channel section of another electrode assembly and an interface of the hydrogen gas outlet channel section318bwith a hydrogen gas outlet channel section of another electrode assembly. Specifically, the hydrogen gas inlet channel O-ring322aand the hydrogen gas outlet channel O-ring322bmay be affixed or otherwise coupled to the plate304so as to respectively circumscribe the hydrogen gas inlet channel section318aand the hydrogen gas outlet channel section318b.

As another example, the plurality of sealing inserts may further include an overboard O-ring324for sealing an interface of the electrode assembly302with another electrode assembly at outer edges thereof. Specifically, the overboard O-ring324may be affixed or otherwise coupled to the plate304so as to circumscribe each of the electrolyte inlet well312, the electrolyte inlet passages314a, the berm314b, the electrolyte outlet channel section316, the hydrogen gas inlet channel section318a, and the hydrogen gas outlet channel section318b.

The carbon foam306may be positioned in a cavity326of the plate304between the berm314band the electrolyte outlet channel section316along the y-axis and between the hydrogen gas inlet channel section318aand the hydrogen gas outlet channel section318balong the x-axis. Specifically, the carbon foam306may be positioned in face-sharing contact with a side of the plate304forming a base of the cavity326. In some examples, the carbon foam306may be formed as a continuous monolithic piece, while in other examples, the carbon foam306may be formed as two or more carbon foam sections. In an exemplary embodiment, the carbon foam306may be conductive, permeable, and porous, providing a distribution field for the electrolyte being gravity fed therethrough from the plurality of electrolyte inlet passages314a. In some examples, a pore distribution of the carbon foam306may be between 10 and 100 PPI. In one example, the pore distribution may be 30 PPI. In additional or alternative examples, a permeability of the carbon foam306may be between 0.02 and 0.5 mm2. As such, each of the pore distribution and the permeability, in addition to an overall size, of the carbon foam306may be selected to target a relatively small pressure drop and thereby induce convection of the electrolyte from the carbon foam306into the positive electrode308. For example, the pressure drop may be targeted to between 2 to 3 mm of electrolyte head rise.

In some examples, the carbon foam306may be replaced with a flow field plate configured to transport the electrolyte into the positive electrode308via convection induced by a flow field configuration of the flow field plate. Specifically, the flow field plate may be fluidically coupled to each of the plurality of electrolyte inlet passages314aand the electrolyte outlet channel section316. In one example, the flow field plate may be integrally formed in the plate304of the electrode assembly302, positioned beneath the positive electrode308with respect to the z-axis. In other examples, the flow field plate may be a separate, removable component.

In some examples, the flow field configuration may be an interdigitated flow field configuration, a partially interdigitated flow field configuration, or a serpentine flow field configuration. In some examples, each electrode assembly302may interface with a flow field configuration of like configuration (e.g., interdigitated, partially interdigitated, serpentine, and the like) as each other electrode assembly302. In other examples, a number of different flow field configurations may be provided among the electrode assemblies302in the stack of electrode assemblies dependent upon a location of a given electrode assembly302in the rebalancing cell202ofFIGS.2A and2B. In this way, the electrolyte may be directed from the electrolyte inlet port (e.g., the electrolyte inlet port206ofFIGS.2A and2B) to the flow field plates respectively interfacing with the positive electrodes308in the stack of electrode assemblies, the flow field plates being configured in interdigitated flow field configurations, partially interdigitated flow field configurations, serpentine flow field configurations, or a combination thereof.

In certain examples, and as discussed in greater detail below with reference toFIGS.4A and4B, in addition to the carbon foam306being replaced with the 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 interface with the negative electrode310opposite from the positive electrode308with respect to the z-axis. However, in other examples, the electrolyte flow field plate may be included (e.g., replacing the carbon foam306) and no hydrogen gas flow field plate may be present. In still other examples, the hydrogen gas flow field plate may be included (e.g., interfacing with the negative electrode310) and no electrolyte flow field plate may be present.

The positive electrode308may be positioned in the cavity326in face-sharing contact with a side of the carbon foam306opposite from the plate304along the z-axis. In an exemplary embodiment, the positive electrode308may be a wicking conductive carbon felt, sponge, or mesh which may bring the electrolyte flowing through the carbon foam306into contact with the negative electrode310via capillary action. Accordingly, in some examples, the positive electrode308may be conductive and porous (though less porous than the carbon foam306in such examples). In one example, the electrolyte may be wicked into the positive electrode308when the porosity of the carbon foam306may be within a predefined range (e.g., below an upper threshold porosity so as to retain enough solid material to promote wicking up and into the positive electrode308and above a lower threshold porosity so as to not impede electrolyte flow through the carbon foam306). In an additional or alternative example, each of a sorptivity of the positive electrode308may decrease and a permeability of the positive electrode308may increase with an increasing porosity of the positive electrode308(e.g., at least until too little solid material of the positive electrode308remains to promote wicking of the electrolyte, such as when a threshold porosity of the positive electrode308is reached). In some examples, surfaces of the positive electrode308may be sufficiently hydrophilic for desirable rebalancing cell operation (e.g., by facilitating thorough electrolyte wetting and thereby forming an ionically conductive medium). In such examples, an overall hydrophilicity of the positive electrode308may be increased by coating or treating the surfaces thereof. Further, though at least some of the H2gas may pass into the positive electrode308in addition to a portion of the electrolyte wicked into the positive electrode308, the positive electrode308may be considered a separator between a bulk of the H2gas thereabove and a bulk of the electrolyte therebelow.

In some examples, each of the positive electrode308and the negative electrode310may be formed as a continuous monolithic piece (e.g., as opposed to discrete particles or a plurality of pieces), such that interphase mass-transport losses across boundary layer films may be reduced when bringing the electrolyte into contact with the H2gas at the catalytic surfaces of the negative electrode310, thereby promoting ion and proton movement. In contrast, a packed bed configuration including discretely packed catalyst particles may include mass-transport limiting boundary layer films surrounding each individual particle, thereby reducing a rate of mass-transport of the electrolyte from a bulk thereof to surfaces of the particles.

The negative electrode310may be positioned in the cavity326in face-sharing contact with a side of the positive electrode308opposite from the carbon foam306along the z-axis, such that a three-phase contact interface between the (wicked) electrolyte, the catalytic surfaces of the negative electrode310, and the H2gas may be formed for proton (e.g., H+) and ion movement (H3O+) therethrough. In tandem, the positive electrode308may reduce an overall electronic resistance by providing a conductive path for electrons to move into the electrolyte front and reduce Fe3+ions thereat.

In an exemplary embodiment, the negative electrode310may be a porous non-conductive material or a conductive carbon substrate with a metal catalyst coated thereon. 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 precious metal catalyst. In some examples, the precious metal catalyst may include Pt. In additional or alternative examples, the precious metal catalyst may include Pd, Rh, Ru, Ir, Ta, or alloys thereof. In some examples, a relatively small amount (e.g., 0.2 to 0.5 wt. %) of the precious metal catalyst supported on the conductive carbon substrate may be employed for cost considerations. In practice, however, the amount of the precious metal catalyst may not be particularly limited and may be selected based on one or more of a desired rate of reaction for the rebalancing cell and an expected lifetime of the rebalancing cell. Furthermore, alloys included in the precious metal catalyst may be utilized to reduce cost and increase a corrosion stability of the precious metal catalyst. For example, 10% addition of Rh to Pt may reduce corrosion of Pt by Fe3+by over 98%. In other examples, the metal catalyst may include a non-precious metal catalyst selected for stability in ferric solution and other such acidic environments (e.g., molybdenum sulfide). In one example, the negative electrode310may include carbon cloth coated with 1.0 mg/cm2Pt and may include a microporous layer bound with a polytetrafluoroethylene (PTFE) binder (e.g., for hydrophobicity). Indeed, inclusion of the PTFE binder may increase a durability of rebalancing cell performance over extended durations relative to electrode assemblies formed using other binders.

In some examples, such as when the precious metal catalyst includes Pt, soaking of the negative electrode310may eventually result in corrosion of the precious metal catalyst. In other examples, and as discussed in greater detail above with reference toFIGS.2A and2B, the electrode assembly302(along with the stack of electrode assemblies and the entire rebalancing cell) may be tilted or inclined with respect to a surface on which the rebalancing cell rests (e.g., the z-axis may be non-parallel with a direction of gravity) such that the precious metal catalyst may remain relatively dry as flow of the electrolyte is drawn through the carbon foam306toward the electrolyte outlet channel section316via gravity feeding. Thus, in some examples, the electrode assembly302may either be horizontal or inclined with respect to the surface on which the rebalancing cell rests at an angle of between 0° and 30°.

In an exemplary embodiment, the electrode assembly302, including each of the carbon foam306, the positive electrode308, and the negative electrode310, may be under compression along the z-axis, with the positive electrode308having a greater deflection than the carbon foam306and the negative electrode310under a given compressive pressure. Accordingly, a depth of the cavity326may be selected based on a thickness of the carbon foam306, a thickness of the positive electrode308, a desired compression of the positive electrode308, and a thickness of the negative electrode310.

Specifically, the depth of the cavity326may be selected to be greater than a lower threshold depth of a sum of the thickness of the carbon foam306after substantially complete compression thereof and the thickness of the positive electrode308after substantially complete compression thereof (to avoid overstressing and crushing of the carbon foam306, which may impede electrolyte flow) and less than an upper threshold depth of a sum of the thickness of the carbon foam306and the thickness of the positive electrode308(to avoid zero compression of the positive electrode308and possibly a gap, which may result in insufficient contact of the H2gas and the electrolyte). For instance, in an example wherein the thickness of the carbon foam306is 6 mm, the thickness of the positive electrode308is 3. 4 mm, the desired compression of the positive electrode308is 0. 4 mm (so as to achieve a desired compressive pressure of 0. 01 MPa), and the thickness of the negative electrode310is 0. 2 mm, the depth of the cavity326may be 9. 2 mm (=3. 4 mm+6 mm+0. 2 mm−0. 4 mm). As another example, the thickness of the carbon foam306may be between 2 and 10 mm, the thickness of the positive electrode308may be between 1 and 10 mm, the desired compression of the positive electrode308may be between 0 and 2. 34 mm (so as to achieve the desired compressive pressure of 0 to 0. 09 MPa), and the thickness of the negative electrode310may be between 0. 2 and 1 mm, such that the depth of the cavity326may be between 0. 86 and 21 mm.

In additional or alternative examples, the thickness of the positive electrode308may be 20% to 120% of the thickness of the carbon foam306. In one example, the thickness of the positive electrode308may be 100% to 110% of the thickness of the carbon foam306. In one example, the depth of the cavity326may further depend upon a crush strength of the carbon foam306(e.g., the depth of the cavity326may be increased with decreasing crush strength). For instance, a foam crush factor of safety (FOS) may be 5. 78 when the depth of the cavity326is 9. 2 mm (e.g., when the desired compression of the positive electrode is 0. 4 mm). The foam crush FOS may have a minimum value of 0. 34 in some examples, where foam crush FOS values less than 1 may indicate that at least some crushing is expected. In some examples, the crush strength of the carbon foam306may be reduced by heat treatment of the carbon foam306during manufacturing thereof (from 0. 08 MPa to 0. 03 MPa, in one example).

It will be appreciated that the electrode assembly302may be configured such that the depth of the cavity326is as low as possible (e.g., within the above constraints), as generally thinner electrode assemblies302may result in a reduced overall size of the rebalancing cell and a reduced electrical resistance across the electrode assembly302(e.g., as the electrolyte flow may be closer to the negative electrode310).

In this way, the electrode assembly302may include a sequential stacking of the carbon foam306and an interfacing pair of the positive electrode308and the negative electrode310being in face-sharing contact with one another and being continuously electrically conductive. Specifically, a first interface may be formed between the positive electrode308and the carbon foam306and a second interface may be formed between the positive electrode308and the negative electrode310, the second interface being opposite to the first interface across the positive electrode308, and each of the carbon foam306, the positive electrode308, and the negative electrode310may be electrically conductive. Accordingly, the electrode assembly302may be internally shorted, such that electric current flowing through the electrode assembly302may not be channeled through an external load.

In an exemplary embodiment, and as discussed above, forced convection may induce flow of the H2gas into the electrode assembly302and across the negative electrode310(e.g., via a flow field plate interfacing with the negative electrode310). Thereat, the H2gas may react with the catalytic surface of the negative electrode310via equation (4a) (e.g., the reverse reaction of equation (1)):

The proton (H+) and the electron (e−) may be conducted across the negative electrode310and into the positive electrode308. The electrolyte, directed through the electrode assembly302via the carbon foam306, may be wicked into the positive electrode308. At and near the second interface between the positive electrode308and the negative electrode310, Fe3+in the electrolyte may be reduced via equation (4b):

Summing equations (4a) and (4b), the electrolyte rebalancing reaction may be obtained as equation (4):

Since the electrode assembly302is internally shorted, a cell potential of the electrode assembly302may be driven to zero as:

where Eposis a potential of the positive electrode308, Enegis a potential of the negative electrode310, ηactis an activation overpotential, ηmtis a mass transport overpotential, and ηohmis an ohmic overpotential. For the electrode assembly302as configured inFIG.3, ηmtand ηactmay be assumed to be negligible. Further, ηohmmay depend on an overpotential ηelectrolyteof the electrolyte and an overpotential ηfeltof the carbon felt forming the positive electrode308as:

Accordingly, performance of the electrode assembly302may be limited at least by an electrical resistivity σelectrolyteof the electrolyte and an electrical resistivity σfeltof the carbon felt. The electrical conductivity of the electrolyte and the electrical conductivity of the carbon felt may further depend on a resistance Relectrolyteof the electrolyte and a resistance Rfeltof the carbon felt, respectively, which may be given as:

where telectrolyteis a thickness of the electrolyte (e.g., a height of the electrolyte front), tfeltis a thickness of the carbon felt (e.g., the thickness of the positive electrode308), Aelectrolyteis an active area of the electrolyte (front), and Afeltis an active area of the carbon felt. Accordingly, the performance of the electrode assembly302may further be limited based on a front location of the electrolyte within the carbon felt and therefore the distribution of the electrolyte across the carbon foam306and an amount of the electrolyte wicked into the carbon felt forming the positive electrode308.

After determining Relectrolyteand Rfelt, an electric current Iassemblyof the electrode assembly302may be determined as:

and a rate vrebalancingof the electrolyte rebalancing reaction (e.g., the rate of reduction of Fe3+) may further be determined as:

where n is a number of electrons flowing through the negative electrode310, F is Faraday's constant, and Arebalancingis an active area of the electrolyte rebalancing reaction (e.g., an area of an interface between the electrolyte front and the negative electrode310). As a use-case example, for an uncompressed carbon felt having tfelt=3 mm, vrebalancingmay have a maximum value of 113 mol/m2hr. However, other characteristics of the uncompressed carbon felt may be used, in alternate examples.

The hydrogen gas flow through the rebalancing cell202, shown inFIGS.2A and2Bmay be implemented via at least two patterns. A first flow pattern is shown inFIGS.4A and4Bin which the hydrogen gas outlet port212, shown inFIG.2B, is open. Conversely, a second flow pattern is shown inFIGS.5A and5Bin which the hydrogen gas outlet port212, shown inFIG.2B, is closed. Closing the hydrogen gas outlet port provides a dead-ended flow configuration. The cutting planes for the cross-sectional views ofFIGS.4A-5Bare parallel to the z-x plane and extend through the central axis of the hydrogen gas inlet port210.

Referring now specifically toFIGS.4A and4B, a cross-sectional view and a magnified inset view450of the rebalancing cell202are respectively shown. Each of the cross-sectional view and the magnified inset view450depict exemplary aspects of H2gas flow within the rebalancing cell202. Specifically, the magnified inset view450magnifies a portion of the cross-sectional view delimited by a dashed ellipse410.

As shown inFIGS.4A and4B, the rebalancing cell202may include an electrode assembly stack402formed as a stack of individual electrode assemblies302aligned such that the hydrogen gas inlet channel section318aof each electrode assembly302forms a continuous hydrogen gas inlet channel404with the hydrogen gas inlet channel section318aof each electrode assembly302. A hydrogen gas inlet manifold406may further be included in the hydrogen gas inlet channel404, the hydrogen gas inlet manifold406fluidically coupling the hydrogen gas inlet channel404to the hydrogen gas inlet port210.

Respective hydrogen gas inlet channel O-rings322aand overboard O-rings324may seal the hydrogen gas inlet channel404at interfaces between pairs of the electrode assemblies302. It will be appreciated that cut portions of the rebalancing cell202are depicted in the cross-sectional view ofFIG.4Aand the magnified inset view450ofFIG.4Bfor detail, and that additional features of the rebalancing cell202(e.g., shown inFIGS.2A and2B) may not be depicted. Further, it will be appreciated that greater or fewer electrode assemblies302may be included in the electrode assembly stack402than shown in the cross-sectional view for a given application (however, in one use-case example, scale-up performance may be substantially insensitive to H2gas flow at or below 50% H2gas utilization). Further, though structural features of the hydrogen gas inlet channel404and adjacent components are described in detail with reference toFIGS.4A and4B, it will be appreciated that structural features of a corresponding hydrogen gas outlet channel (e.g., formed by aligning a hydrogen gas outlet channel section318b(seeFIG.3) of each electrode assembly302) and adjacent components may be similarly configured (excepting that a hydrogen gas outlet manifold in fluidic communication with the hydrogen gas outlet channel may be positioned opposite to the hydrogen gas inlet manifold406along the x and z axes).

As shown, and as indicated by arrows408a, the H2gas enters the hydrogen gas inlet port210. The H2gas may be delivered from a hydrogen source such as a hydrogen flow generator or storage tank. Next, as indicated via arrow408b, the hydrogen gas flows from the hydrogen gas inlet port210to the hydrogen gas inlet manifold406. Next, as indicated via arrows408c, the hydrogen gas flows through the hydrogen gas inlet channel404from the hydrogen gas inlet manifold406. From the hydrogen gas inlet channel404, gas flows into hydrogen gas inlet passages452between the electrodes, as indicated via arrows408d.

In the electrode assembly stack flow configuration shown inFIGS.4A and4B, the hydrogen gas outlet port212, shown inFIG.2B, is open. Hydrogen gas therefore flows across the electrode assembly stack402(generally in directions parallel to the x-axis) through hydrogen gas channels420between the plates304and the negative electrodes310as indicated via arrows408c. Arrows408fdepict the flow of hydrogen gas from the hydrogen gas channels420to the hydrogen gas outlet manifold and then to the hydrogen gas outlet port212, shown inFIG.2B. In this way, hydrogen may be effectively flowed through the cell.

Referring now toFIGS.5A and5B, a cross-sectional view and a magnified inset view450of the rebalancing cell202are respectively shown. Each of the cross-sectional view and the magnified inset view550depict exemplary aspects of H2gas flow within the rebalancing cell202. Specifically, the magnified inset view550magnifies a portion of the cross-sectional view delimited by a dashed ellipse510.

In the rebalancing cell flow configuration shown inFIGS.5A and5B, the hydrogen gas outlet port212, shown inFIG.2B, is closed, thereby dead-ending the hydrogen flow, as noted above.

The electrode assembly stack402in the rebalancing cell202is again shown inFIGS.5A and5Bwith sequentially arranged electrode assemblies302. Each electrode assembly302again includes the negative electrode310, the positive electrode308, the activated carbon foam306, the plate304, and the hydrogen gas inlet channel seal insert320a, in the illustrated example. However, other electrode assembly configurations may be used, in other examples.

As shown, and as indicated by arrows508a, the H2gas enters the hydrogen gas inlet port210. Next, as indicated via arrow508b, the hydrogen gas flows from the hydrogen gas inlet port210to the hydrogen gas inlet manifold406. Next, as indicated via arrows508c, the hydrogen gas flows through the hydrogen gas inlet channel404from the hydrogen gas inlet manifold406. From the hydrogen gas inlet channel404, hydrogen gas flows into hydrogen gas inlet passages452between the electrodes, as indicated via arrows508d. This initial stage of the hydrogen flow is similar to the initial portion of the hydrogen flow pattern depicted inFIGS.4A and4B.

However, as illustrated inFIGS.5A and5B, as indicated via arrows508e, hydrogen gas flows from the hydrogen gas channels420and through the negative electrode310and then the positive electrode308. In this way, due to the closure of the hydrogen gas outlet, hydrogen gas is forced through the electrolyte side of the electrode assemblies. Once the hydrogen gas is on the positive sides of the electrode assemblies, the gas flows into the electrolyte outlet manifold and then exits through the hydrogen gas relief port214shown inFIG.2A. To elaborate, the hydrogen gas flows into the page and to the hydrogen gas relief port214shown inFIG.2A. Further, some of the hydrogen gas flow may be entrained in the electrolyte and exit out of the electrolyte outlet ports208, shown inFIG.2A.

The two flow patterns in the rebalancing cell202depicted inFIGS.4A-5Ballow for four different series plumbing orientations to connect multiple rebalancing cells to increase (e.g., maximize) the hydrogen gas flowrate through each rebalancing cell unit. The cells can therefore achieve a desired reaction rate, thereby increasing battery efficiency.FIGS.6-9depict different flow arrangements in a rebalancing cell system600for a redox flow battery such as the redox flow battery11shown inFIG.1or other suitable redox flow battery.

Referring now toFIGS.6A and6B, a cross-sectional view600and a magnified inset view650are respectively shown, each of the cross-sectional view600and the magnified inset view650depicting exemplary aspects of electrolyte flow within the rebalancing cell202. Specifically, the magnified inset view650magnifies a portion of the cross-sectional view600delimited by a dashed ellipse610. As shown inFIGS.6A and6B, the rebalancing cell202may include one or more electrolyte inlet channels614fluidically coupled to the electrolyte inlet wells312included in the individual electrode assemblies302of the electrode assembly stack402. Each of the one or more electrolyte inlet channels614may be fluidically coupled to an electrolyte inlet plenum606alocated above the electrode assembly stack402with respect to the z-axis via a respective nozzle or orifice612modulating, restricting, or otherwise controlling flow of the electrolyte into the respective electrolyte inlet channel614. The electrolyte inlet plenum606amay further be fluidically coupled to the electrolyte inlet port (e.g., the electrolyte inlet port206ofFIGS.2A and2B; not shown atFIGS.6A and6B). The electrode assembly stack402may further be formed as a stack of the individual electrode assemblies302aligned such that the electrolyte outlet channel section316of each electrode assembly forms a continuous electrolyte outlet channel604with the electrolyte outlet channel section316of each electrode assembly302, the electrolyte outlet channel604being parallel to the one or more electrolyte inlet channels614and to the z-axis and on an opposite end of the rebalancing cell202from the one or more electrolyte inlet channels614with respect to the y-axis. An electrolyte outlet plenum606bmay further be included in the electrolyte outlet channel604, the electrolyte outlet plenum606bfluidically coupling the electrolyte outlet channel604to the electrolyte outlet port208. Respective overboard O-rings324may seal the electrolyte outlet channel604at interfaces between pairs of electrode assemblies302. It will be appreciated that cut portions of the rebalancing cell202are depicted in the cross-sectional view600and the magnified inset view650for detail, and that additional features of the rebalancing cell202(e.g., shown inFIGS.2A and2B) may not be depicted. Further, it will be appreciated that greater or fewer electrode assemblies302may be included in the electrode assembly stack402than shown in the cross-sectional view600for a given application.

The electrolyte may enter the electrolyte inlet plenum606avia the electrolyte inlet port, wherefrom the electrolyte may be directed into the one or more electrolyte inlet channels614via the one or more orifices612, respectively. In some examples, a cross-sectional shape of the electrolyte inlet plenum606amay be selected for ease of machining. As an example, the cross-sectional shape of the electrolyte inlet plenum606amay be rectangular. As another example, the cross-sectional shape of the electrolyte inlet plenum606amay be circular. A size of the electrolyte inlet plenum606amay be selected to realize a relatively low pressure drop upon entry of the electrolyte into the rebalancing cell202.

In some examples, a size of each of the one or more orifices612may be between 3 and 10 mm, as dependent on a total number of electrode assemblies302in the electrode assembly stack402, an overall size of the rebalancing cell202, and an electrolyte flow path design. The size and overall configuration of each of the one or more orifices612may be selected to maintain substantially even electrolyte flow throughout each electrode assembly302of the electrode assembly stack402.

In some examples, each of the one or more electrolyte inlet channels614may be a continuous and unbroken channel configured adjacent to the electrode assembly stack402. In other examples, each electrode assembly302of the electrode assembly stack402may include one or more electrolyte inlet channel sections corresponding to the one or more electrolyte inlet channels614, respectively. In such examples, the electrode assemblies302of the electrode assembly stack402may be aligned such that the one or more electrolyte inlet channel sections of each electrode assembly302respectively form the one or more electrolyte inlet channels614with the one or more electrolyte inlet channel sections of each other electrode assembly302.

In some examples, the one or more electrolyte inlet channels614may include a plurality of electrolyte inlet channels614and the one or more orifices612may include a plurality of orifices612respectively fluidically coupled to the plurality of electrolyte inlet channels614, such that an electrolyte inlet manifold may be formed. In the cross-sectional view600ofFIG.6A, a single nearest electrolyte inlet channel614of the plurality of electrolyte inlet channels614is visible, obscuring each other electrolyte inlet channel614of the plurality of electrolyte inlet channels614aligned therewith parallel to the x-axis. In some examples, each of the plurality of electrolyte inlet channels614forming the electrolyte inlet manifold may be respectively fluidically coupled to a single electrode assembly302of the electrode assembly stack402so as to evenly flow the electrolyte across the electrode assemblies302of the electrode assembly stack (e.g., at an electrolyte flow rate of ˜10-40 L/min per m2of the catalytic surfaces of the negative electrode310).

In some examples, the electrolyte entering the electrolyte inlet plenum606amay have an adjustable flow rate (e.g., by a controller of the redox flow battery system, such as the controller88ofFIG.1, executing instructions stored in non-transitory memory thereof) such that even distribution of the electrolyte into and within the rebalancing cell202may be controllably adjusted based on a given application. In certain examples, electrolyte flow distribution between individual electrode assemblies302of the electrode assembly stack402may be correspondingly adjusted based on adjustments to the electrolyte flow rate of the electrolyte entering the electrolyte inlet plenum606a.

In other examples, each of the plurality of electrolyte inlet channels614may be fluidically coupled to each electrode assembly302of the electrode assembly stack402so as to evenly distribute the electrolyte across the electrode assembly stack402with respect to both the x- and y-axes. In alternative examples, the one or more electrolyte inlet channels614may include one electrolyte inlet channel614which may be fluidically coupled to each electrode assembly302of the electrode assembly stack402.

In some examples, a cross-sectional shape of each of the one or more electrolyte inlet channels614may be a circle. However, the cross-sectional shape of each of the one or more electrolyte inlet channels614is not particularly limited and other geometric shapes may be employed. A size of each of the one or more electrolyte inlet channels614may be selected to realize a relatively low pressure drop for the electrolyte flow rate of ˜10-40 L/min per m2of the catalytic surfaces of the negative electrode310(e.g., relatively small sizes may result in poor distribution of the electrolyte) while maintaining practical size considerations of the rebalancing cell202as a whole (e.g., relatively large sizes may result in an undesirably large rebalancing cell202). In one example, the cross-sectional shape of each of the one or more electrolyte inlet channels614may be a circle having a diameter of between 10 and 30 mm.

Upon entering the one or more electrolyte inlet channels614, a pressure therein may be substantially similar to a pressure of an electrolyte source (e.g., the negative and positive electrode compartments22and20and/or the integrated multi-chambered electrolyte storage tank110ofFIG.1), such that gravity may substantially exclusively drive electrolyte flow through the one or more electrolyte inlet channels614. Specifically, and as indicated by arrows608a, the electrolyte may flow through the one or more electrolyte inlet channels614in a negative direction along the z-axis and into the electrolyte inlet wells312of the electrode assembly stack402. The sloped support220may tilt the rebalancing cell202such that the z-axis is offset from the axis g coincident with the direction of gravity, and the electrolyte may flow through the carbon foams306of the electrode assembly stack402via gravity feeding (as indicated by arrows608b).

As further shown, and as indicated by arrows608c, while flowing through the carbon foams306of the electrode assembly stack402, at least some of the electrolyte may be induced into the positive electrodes308of the electrode assembly stack402towards the negative electrodes310of the electrode assembly stack402via capillary action. Fe3+ions in the electrolyte may be reduced by electrons flowing through the negative electrodes310of the electrode assembly stack402in a cathodic half reaction (see equation (4b)) to generate Fe2+ions. For each electrode assembly302of the electrode assembly stack402, to ensure that no gap is present between the positive electrode308and the negative electrode310(which may result in a decreased Fe3+reduction rate), a depth652of the cavity (e.g., the cavity326ofFIG.3) may be selected such that the positive electrode308is at least partially compressed without excessively compressing the carbon foam306(which may buckle and degrade a foam structure thereof). Accordingly, to minimize compression of the carbon foam306in each electrode assembly302of the electrode assembly stack402, a thickness654of the adjacent positive electrode308may be decreased (e.g., by about 10%) relative to when the positive electrode308is fully uncompressed. In some examples, for each electrode assembly302of the electrode assembly stack402, the thickness654of the positive electrode308may be 20% to 120% of the thickness656of the carbon foam306, where each of the thickness654of the positive electrode308and the thickness656of the carbon foam306may be selected based on structural considerations such as the permeability of the carbon foam306, an overall size of the positive electrode308, etc. In one example, for each electrode assembly302of the electrode assembly stack402, the thickness654of the positive electrode308may be 100% to 110% of the thickness656of the carbon foam306.

As further shown, and as indicated by arrows608d, after flowing through the carbon foams306of the electrode assembly stack402, the electrolyte may be directed through electrolyte outlet passages658of the electrode assembly stack402, into the electrolyte outlet channel604, and out through the electrolyte outlet port208therefrom. Specifically, for each given electrode assembly302of the electrode assembly stack402, the electrolyte may flow from the carbon foam306through the electrolyte outlet passage658and into the electrolyte outlet channel section316, wherefrom the electrolyte may flow with the direction of gravity (e.g., along the positive direction of the axis g) into the electrolyte outlet plenum606b(after passing through any further electrolyte outlet channel sections316interposed between the given electrode assembly302and the electrolyte outlet plenum606b). The electrolyte may then pass through the electrolyte outlet plenum606band into the electrolyte outlet port208, wherefrom the electrolyte may be expelled from the rebalancing cell202. In this way, the electrolyte may be directed from the electrolyte inlet port (e.g., the electrolyte inlet port206ofFIGS.2A and2B; not shown atFIGS.6A and6B) through the carbon foams306of the electrode assembly stack402to the electrolyte outlet port208.

In some examples, an overall size of each of the electrolyte outlet passages658may be selected so as to be sufficiently large to generate a suitable pressure drop and to not overfill the electrolyte outlet plenum606b(which may flood the electrode assemblies302at a bottom of the electrode assembly stack402with respect to the z-axis). Accordingly, in such examples, the overall size of each of the electrolyte outlet passages658may depend on an overall size of the electrolyte outlet plenum606band an overall number of openings corresponding to the electrolyte outlet port208. In other examples, dimensions of the electrolyte outlet plenum606bmay be larger to accommodate an electrolyte outlet port208having fewer, larger openings. In examples wherein the electrolyte outlet port208is positioned on the face of the cell enclosure204facing the negative direction of the z-axis, larger openings may be accommodated while maintaining a thickness of a lowest electrode assembly302along the z-axis and the pressure drop may be further reduced (e. g., as the electrolyte would not flow at a ˜90° angle from the electrolyte outlet plenum606bto the electrolyte outlet port208).

As further shown, flow field plates626may respectively interface with the electrode assemblies302of the electrode assembly stack402. In some examples, the flow field plate626may interface (e.g., be in face-sharing contact) with the negative electrode310of a given electrode assembly302and may be integrally formed in the plate304of an adjacent electrode assembly302, positioned beneath the carbon foam306of the adjacent electrode assembly302with respect to the z-axis. In other examples, the flow field plate626interfacing with the negative electrode310of the given electrode assembly302may be a separate, removable component. Additionally, and as further shown, a topmost flow field plate626with respect to the z-axis may not be integrally formed with any electrode assembly302and may instead be included in the rebalancing cell202as either a separate, removable component or an integral feature of another component (e.g., the cell enclosure204ofFIGS.2A and2B).

In an exemplary embodiment, the one or more hydrogen gas inlet passages452, configured to flow the H2gas across a given electrode assembly302, may be formed from the flow field plate626interfacing with the negative electrode310of the given electrode assembly302. For instance, the one or more hydrogen gas inlet passages452may be configured as either a plurality of hydrogen gas inlet passages452parallel to one another and the x-axis (e.g., in the interdigitated flow field configuration or the partially interdigitated flow field configuration) or a single, coiled hydrogen gas inlet passage452into which the H2gas may enter parallel to the x-axis (e.g., in the serpentine flow field configuration). In some examples, the one or more hydrogen gas inlet passages452may extend parallel to the x-axis while the electrolyte may flow through the carbon foam306of the given electrode assembly302parallel to the y-axis (as indicated by the arrows608b). Accordingly, in such examples, the H2gas may be directed into the electrode assembly stack402at a 90° angle from which the electrolyte may be directed into the electrode assembly stack402.

In additional or alternative examples, the carbon foam306of a given electrode assembly302may be replaced with a flow field plate of substantially similar flow field configuration to the flow field plate626. In one such example, the flow field configuration of the flow field plate replacing the carbon foam306of the given electrode assembly302may be oriented in the same direction as the flow field configuration of the flow field plate626with respect to the x- and y-axes. In another such example, the flow field configuration of the flow field plate replacing the carbon foam306of the given electrode assembly302may be oriented in a different direction as the flow field configuration of the flow field plate626(e.g., at a 90° angle, a 180° angle, or a 270° angle) with respect to the x- and y-axes.

Referring now toFIGS.7A and7B, partial flow configurations700and750for the redox flow battery system10ofFIG.1are shown. In particular, the partial flow configurations700and750illustrate flow paths of electrolyte and hydrogen gas in the redox flow battery system10between the multi-chambered electrolyte storage tank110and RBC780. RBC780corresponds to either RBC80or RBC82ofFIG.1. In particular, the partial flow configurations700and750illustrate flow paths of electrolyte and hydrogen gas diverted upstream of the redox flow battery18. Partial flow configurations700and750may thus represent flow paths for the negative electrolyte and hydrogen gas between the multi-chambered electrolyte storage tank110and the rebalancing cell80, or the positive electrolyte and hydrogen gas between the multi-chambered electrolyte storage tank110and the rebalancing cell82. As such main electrolyte pump730may correspond to electrolyte pumps30and32, and control valve774and auxiliary electrolyte pump776may correspond to electrolyte flow control device76and electrolyte flow control device78, and H2flow generator770may correspond to hydrogen gas flow control device66and hydrogen gas flow control device68.

As described with reference toFIG.1, the control valve774ofFIG.7Aand the auxiliary electrolyte pump776ofFIG.7Bmay be communicatively coupled to the controller88. Accordingly, the controller88may modulate the control valve774and the auxiliary electrolyte pump776in order to regulate the flow of electrolyte supplied to the RBC780. In one example, the control valve774may include a proportional control valve. In one example, modulating the control valve774may include adjusting the control valve774to a more open position or to a more closed position to increase or decrease, respectively, the electrolyte flow to the RBC780. In another example, modulating the auxiliary electrolyte pump776may include increasing or decreasing a speed of the auxiliary electrolyte pump776in order to increase or decrease, respectively, an electrolyte flow to the RBC780. In one example both the electrolyte flow control devices76and78may include the control valve774; in another example, both the electrolyte flow control devices76and78may include the auxiliary electrolyte pump776. In another example, one of the electrolyte flow control devices76and78may include a control valve774and the other of the electrolyte flow control devices76and78may include an auxiliary electrolyte pump776. An electrolyte flow measurement device784(not shown inFIG.1for clarity), such as a flow meter, may be positioned in the electrolyte supply line upstream of RBC780. Downstream of the RBC780, liquid electrolyte is returned by way of an electrolyte return line to the multi-chambered electrolyte storage tank110.

The H2flow generator770may correspond to hydrogen gas flow control devices66and68, and may include one or more of a hydrogen gas injector, venturi injector, mass flow controller, mass flow meter, a gas blower, and a gas pressure regulator. The H2flow generator may be communicatively coupled to the controller88. Accordingly, the controller88may modulate the H2flow generator770in order to regulate the flow of hydrogen gas supplied to the RBC780. As described herein, H2gas may be supplied from one or more of the head spaces90and92of the multi-chambered electrolyte storage tank110and a hydrogen gas source760. As non-limiting examples, the hydrogen gas source760can include a hydrogen storage tank or a hydrogen gas cylinder external to the multi-chambered electrolyte storage tank110.

In one example, modulating the H2flow generator770may include adjusting a gas pressure regulator to increase or decrease a hydrogen gas pressure, thereby increasing or decreasing, respectively, a flow rate of the hydrogen gas to the RBC780. In another example, modulating the H2flow generator770may include increasing or decreasing a magnitude of an input signal (e.g., voltage, current, and the like) transmitted to a mass flow controller in order to increase or decrease, respectively, a flow rate of the hydrogen gas to the RBC780. In another example, modulating the H2flow generator770may include increasing or decreasing a magnitude of an input signal (e.g., voltage, current, and the like) transmitted to a hydrogen gas injector in order to increase or decrease, respectively, a flow rate of the hydrogen gas to the RBC780. In another example, modulating the H2flow generator770may include increasing or decreasing a motor speed of a gas blower in order to increase or decrease, respectively, a flow rate of the hydrogen gas to the RBC780. A hydrogen gas flow measurement device786(not shown inFIG.1for clarity), such as a gas flow meter, may be positioned in the H2supply line upstream of RBC780. Downstream of the RBC780, hydrogen gas is returned by way of an H2return line to one or more of the multi-chambered electrolyte storage tank110and the hydrogen gas source760. For the case where H2is supplied from H2source760, hydrogen gas is returned by way of the H2return line to the H2source.

As described above with reference to rebalancing reaction equation (3), electrolyte and hydrogen gas are supplied to the RBC780to maintain electrolyte health and battery capacity. In particular, excess ferric ion (e.g., in the case where the redox flow battery system includes an IFB) and hydrogen in the redox flow battery system are reduced by way of the rebalancing reaction (represented by equation (4)), thereby limiting parasitic side reactions (e.g., as represented by equations (1) and (2) for the case of an IFB). In the case where the RBC780corresponds to the rebalancing cell ofFIGS.2A and2Bdescribed herein, a small pressure gradient may be maintained within the RBC780where the negative (hydrogen gas) side pressure is slightly higher than the positive (electrolyte) side pressure. Higher gas pressure and gravity aid in reducing electrolyte flooding at the negative side during operation of the RBC780. However, electrolyte flooding may occur when hydrogen flow at the RBC780is insufficient to support the rebalancing reaction rate. In one example, when the hydrogen gas flow rate is below a lower threshold hydrogen gas flow rate, hydrogen gas at the negative side of the rebalancing reactor may be consumed and depleted such that a pressure at the negative side of the RBC780substantially decreases below a threshold hydrogen gas pressure, thereby leading to electrolyte flooding thereat.

Referring toFIGS.4A and4B, the negative side of the RBC780may refer to the internal volumes of the electrode assemblies302and electrode assembly stack402where the hydrogen gas flows during conditions where the hydrogen gas outlet port212is open and during conditions when the RBC780is not flooded. Accordingly, the negative side of the RBC780may include the hydrogen gas inlet manifold406, hydrogen gas inlet channel404(including the hydrogen gas inlet channel sections318aof each electrode assembly302), hydrogen gas outlet channel (including the hydrogen gas outlet channel sections318bof each electrode assembly302), hydrogen gas inlet passages452, and the hydrogen gas channels420between the plates304and the negative electrodes310. The negative side of the RBC780may further include the hydrogen gas flow field plate, when present in the electrode assembly302.

Referring toFIGS.6A and6B, the positive side of the RBC780may refer to the internal volumes of the electrode assemblies302and electrode assembly stack402where the liquid electrolyte flows during conditions when the RBC780is not flooded. Accordingly, the positive side of the RBC780may include an electrolyte inlet manifold (including the electrolyte inlet channels614), electrolyte inlet wells312, electrolyte inlet plenum606a, orifices612, carbon foams306, positive electrodes308, electrolyte outlet channel sections316, electrolyte outlet channel604, and electrolyte outlet plenum606b. The positive side of the RBC780may further include the electrolyte flow field plate, when present in the electrode assembly302.

Referring now toFIGS.8A and8B, data plots800and850illustrate operating conditions for rebalancing cells in a redox flow battery system. The electrolyte flow rate810and the hydrogen gas flow rate820supplied to the RBC780are plotted as a function of SOC (data plot800) and electrolyte concentration (data plot850). Both the electrolyte flow rate810and the hydrogen gas flow rate820correspond to volumetric flow rates.

The SOC may be described as a ratio of an amount of electric charge stored in a redox flow battery to the full or total theoretical amount of electric charge that may be stored in the redox flow battery. Accordingly 100% SOC may correspond to when the redox flow battery system is in a fully charged state (e.g., fully charged to capacity), wherein additional metal cannot be plated at the plating electrode; whereas 0% SOC may correspond to when the redox flow battery system is in a fully discharged state, wherein no more metal can be oxidized and de-plated at the plating electrode. SOC values between 0% and 100% may correspond to when a redox flow battery SOC is intermediate between the fully charged and fully discharged states. Furthermore, the SOC may correspond to a charging SOC, wherein the SOC is measured during conditions when a redox flow battery is operating in a charging mode; and a discharging SOC, wherein the SOC is measured during conditions when a redox flow battery is operating in a discharging mode.

The SOC may be measured indirectly by one or more of sensors60,62,70, and72by various means, including chemical means, such as measuring a pH or specific gravity of the liquid electrolyte. Furthermore, one or more of sensors60,62,70, and72may include an oxygen-reduction potential (ORP) meter or an optical sensor for detecting a change in an electrolyte pH or a change in an electrolyte SOC. In the case of an IFB, the positive electrolyte ORP provides an indication of [Fe3+] in the positive electrolyte as a measure of SOC. As such, the SOC may be monitored during operation of the redox flow battery system10at the positive electrolyte chamber52, the negative electrolyte chamber50, the positive electrode compartment22, and the negative electrode compartment20.

Additionally or alternatively, the SOC of the redox flow battery may be determined from a plating efficiency of the redox flow battery and a shunt current flow of the redox flow battery, as described further below with reference toFIGS.11A and11B. The plating efficiency at the negative electrode may be indicative of a redox flow battery's capacity to source and sink current, and the plating efficiency values may correlate to the side reactions (e.g., as represented by equations (1), (2), and (3)) occurring at the negative electrode. In other words, the plating efficiency may be used to count coulombs entering and exiting an oxidation-reduction flow battery and to determine a SOC estimate for the oxidation-reduction flow battery. As such, determining SOC from the plating efficiency may be more accurate than other indirect measures of SOC such as ORP because the plating efficiency measurement may account for the deleterious effects of the negative side reactions on the apparent SOC.

Electrolyte concentration may be determined from the redox flow battery SOC, according to the electrochemical reactions taking place at the redox flow battery electrodes. In data plot850, the electrolyte concentration varies from a lower electrolyte concentration A (corresponding to when the redox flow battery SOC is 0% or fully discharged) to a higher electrolyte concentration B (corresponding to when the redox flow battery SOC is 100% or fully charged). For the case of an IFB, the electrolyte concentration may correspond to the ferric ion concentration, [Fe3+]. [Fe3+] may be correlated to the redox flow battery SOC as represented by equations (5) and (6). Ferric ion is produced at the positive electrode during charging of the redox flow battery; conversely, ferric ion is consumed at the positive electrode during discharging of the redox flow battery. Accordingly, as the redox flow battery SOC increases during battery charging, the ferric ion concentration in the redox flow battery system may increase; and as the redox flow battery SOC decreases during battery discharging, the ferric ion concentration in the redox flow battery system may decrease. The ferric concentration may be measured at the positive electrode compartment22and the positive electrolyte chamber52by sensors72and62, respectively.

The electrolyte flow rate810may be determined from one or more of a speed of the auxiliary electrolyte pump776, a speed of the main electrolyte pump730, and a % open position of the control valve774. Additionally or alternatively, the electrolyte flow rate810may be measured with the electrolyte flow measurement device784. The hydrogen gas flow rate820may be determined from one or more of a pressure at a hydrogen gas source760, a pressure at head spaces90and92of the multi-chambered electrolyte storage tank110, an output signal from the H2flow generator770, an input signal supplied to the H2flow generator770, or a motor speed for the case when the H2flow generator includes a blower. Additionally or alternatively, the hydrogen gas flow rate820may be measured with the hydrogen gas flow measurement device786.

The upper threshold hydrogen gas flow rate824may correspond to the hydrogen gas flow rate820during conditions when the H2flow generator770is configured to deliver hydrogen gas at an upper threshold pressure. The upper threshold pressure may correspond to one or more of a pressure above which integrity of the seals in the RBC780may be compromised and a highest available delivery pressure from a hydrogen gas source760and/or the head spaces90and92of the multi-chambered electrolyte storage tank110. Additionally or alternatively, the upper threshold hydrogen gas flow rate824may correspond to when an input signal is transmitted to a hydrogen mass flow controller or a hydrogen gas injector is at its highest value; or when a hydrogen gas blower is operating at full blower speed.

Both the electrolyte flow rate810and hydrogen gas flow rate820are shown as a percentage of a respective upper threshold electrolyte flow rate814and upper threshold hydrogen gas flow rate824supplied to the RBC780. In one example, the upper threshold electrolyte flow rate814may correspond to the electrolyte flow rate810when a control valve774is fully open and a pump speed of the main electrolyte pump730is at its highest capable pump speed, or when an auxiliary electrolyte pump is at its highest pump speed and a pump speed of the main electrolyte pump730is at its highest pump speed. In another example, the upper threshold electrolyte flow rate814may correspond to an electrolyte flow rate810above which a probability of electrolyte flooding at the negative side of the RBC780is increased, including when operating at the upper threshold hydrogen gas flow rate824. Furthermore, the electrolyte flow rate810may be maintained above a lower threshold electrolyte flow rate812and the hydrogen flow rate820may be maintained above a lower threshold hydrogen flow rate822. The lower threshold electrolyte flow rate812may correspond to an electrolyte flow rate below which a rebalancing reaction rate decreases such that a performance of the redox flow battery system is substantially reduced. The lower threshold hydrogen gas flow rate822may correspond to a hydrogen gas flow rate below which flooding of the RBC780is increased, even at or near a redox flow battery SOC of 0%.

From rebalancing reaction equation (4), the electrolyte rebalancing rate, Rrebalancing, for an IFB can be expressed as:

In equation (13), k represents a rebalancing reaction rate constant; k may be relatively constant with electrolyte and hydrogen gas concentration but may vary with temperature according to an Arrhenius relationship. The rebalancing reaction, as expressed by Rrebalancing, consumes ferric ion at a rate proportional to [Fe3+], and consumes hydrogen gas proportional to the square root of the hydrogen concentration, [H2]1/2. During operation of the redox flow battery system10, the electrolyte concentration may vary depending on the redox flow battery SOC. In the case of an IFB. [Fe3+] increases as the redox flow battery SOC increases during battery charging, and [Fe3+] decreases as the redox flow battery SOC decreases during battery discharging. Accordingly, the controller88may modulate the flow rates of the electrolyte and the hydrogen gas to the RBC780in order to sustain the rebalancing reaction, whereby a constant Rrebalancingis maintained independent of the redox flow battery SOC. Sustaining the rebalancing reaction and maintaining a constant Rrebalancingincludes mitigating electrolyte flooding at the RBC780, since electrolyte flooding reduces Rrebalancing. In the example data plots800and850, the changes in electrolyte flow rate810and hydrogen gas flow rate820modulated by the controller88are depicted as linear with respect to the changes in SOC (or electrolyte concentration). However, in other examples, the relationship between electrolyte flow rate810and hydrogen gas flow rate820and redox flow battery SOC (or electrolyte concentration) may be non-linear.

In order to maintain a higher performance of the redox flow battery system, the controller88may modulate the electrolyte and hydrogen gas flow rates supplied to the RBC780in order to achieve the maximum Rrebalancinggiven the amount of hydrogen available for the rebalancing reaction. Further still, maintaining a constant Rrebalancing(at the maximum Rrebalancing) and mitigating electrolyte flooding at the RBC780may include modulating the electrolyte and hydrogen gas flow rates to the RBC780while maintaining a mass flow ratio of hydrogen to electrolyte (Qmass,H2/Qmass,electrolyte) supplied to the RBC780constant. The mass flow ratio of hydrogen to electrolyte may be represented by equation (14):

Qmass,H2and Qmass,electrolyterepresent the mass flow rates of hydrogen gas and electrolyte, respectively; and QH2and Qelectrolyterepresent the volumetric flow rates of hydrogen gas and electrolyte, respectively. In the case of an IFB redox flow battery system, [Electrolyte] may correspond to [Fe3+]. Que and Qelectrolytecorrespond to the hydrogen gas flow rate820and the electrolyte flow rate810, as plotted in data plots800and850.

In one example, Qmass,H2/Qmass,electrolytemay be maintained above a threshold mass flow ratio, (Qmass,H2/Qmass,electrolyte)TH. (Qmass,H2/Qmass,electrolyte)THmay correspond to a mass flow ratio where the amount of hydrogen gas supplied to the RBC780includes a small excess hydrogen gas relative to the stoichiometric amount of hydrogen as given by the rebalancing reaction equation (4). Maintaining Qmass,H2/Qmass,electrolytegreater than (Qmass,H2/Qmass,electrolyte)THmay aid in ensuring that sufficient hydrogen gas flow is supplied to the RBC780to support Rrebalancingfor the given Qmass,electrolyte. Said another way, modulating the electrolyte and hydrogen gas flow rates to the RBC780to maintain Qmass,H2/Qmass,electrolytegreater than (Qmass,H2/Qmass,electrolyte)THindependent of redox flow battery SOC aids in preserving a small pressure gradient within the RBC780where a pressure at the negative side of the RBC780is slightly higher than the pressure at the positive side of the RBC780. The higher pressure at the negative side relative to the positive side of the RBC780combined with induced gravity flow of the electrolyte at the positive side away from the negative side of the RBC780may aid in mitigating electrolyte flooding at the negative side of the RBC780.

Maintaining Qmass,H2/Qmass,electrolytegreater than (Qmass,H2/Qmass,electrolyte)THmay further include limiting the excess hydrogen gas flow rate820at the RBC780to a small excess hydrogen gas flow rate. Limiting the excess hydrogen gas flow rate820to a small excess hydrogen gas flow rate aids in reducing operating costs while reducing parasitic power losses due to side reactions (e.g., equations (1) and (2)). In one example, (Qmass,H2/Qmass,electrolyte)THmay include,

In equation (15), QH2,TH,upperand Qelectrolyte,TH,upperrepresent upper threshold hydrogen gas flow rate824and the upper threshold electrolyte flow rate814, respectively; and [Electrolyte]THrepresents the threshold electrolyte concentration858. For the case of an IFB. [Electrolyte]THmay correspond to [Fe3+]TH.

The dependence of Rrebalancingwith SOC (and [Fe3+]) is reflected in how the controller88may modulate electrolyte and hydrogen flow rates to the RBC780, as shown in the redox flow battery system operating condition data plots800and850. The data plots800and850may be divided into three operating regions: a first condition may include when a redox flow battery SOC is less than a threshold redox flow battery SOC808, and may further include when an electrolyte concentration is less than a threshold electrolyte concentration858; a second condition may include when a redox flow battery SOC is equivalent to a threshold redox flow battery SOC808, and may further include when an electrolyte concentration is equivalent to a threshold electrolyte concentration858; a third condition may include when a redox flow battery SOC is greater than a threshold redox flow battery SOC808, and may further include when an electrolyte concentration is greater than a threshold electrolyte concentration858.

Although the data plots800and850represent the threshold redox flow battery SOC808and the threshold electrolyte concentration858as discrete values, in some embodiments, the threshold redox flow battery SOC808and the threshold concentration858may include a threshold range of SOC values and a threshold range of electrolyte concentrations, respectively. As an example, the threshold range of SOC values may include the threshold redox flow battery SOC808plus or minus a threshold SOC deviation, and the threshold range of electrolyte concentrations may include the threshold electrolyte concentration858plus or minus a threshold electrolyte concentration deviation. Accordingly, the first condition may include when the redox flow battery SOC is less than the threshold SOC minus the threshold SOC deviation and the third condition may include when the redox flow battery SOC is greater than the threshold SOC plus the threshold SOC deviation. Additionally or alternatively, the first condition may include when the electrolyte concentration is less than the threshold electrolyte concentration minus the threshold electrolyte concentration deviation and the third condition may include when the electrolyte concentration is greater than the threshold electrolyte concentration plus the threshold electrolyte concentration deviation.

As described above, in the case of an IFB, the electrolyte concentration may correspond to the ferric ion concentration. During the first condition, the redox flow battery SOC is lower than the threshold redox flow battery SOC (and the electrolyte concentration is lower than the threshold electrolyte concentration), and the electrolyte concentration at the RBC780may be a limiting reagent (relative to H2) in the rebalancing reaction. Thus, responsive to the redox flow battery system operating under the first condition, electrolyte may be supplied to and maintained at the RBC780at a flow rate equivalent to the upper threshold electrolyte flow rate814to maximize Rrebalancingfor the given hydrogen gas flow rate820. Furthermore, as the redox flow battery SOC increases from 0% to the threshold redox flow battery SOC808(and as the electrolyte concentration increases from A to the threshold electrolyte concentration858), the hydrogen gas flow rate820supplied to the RBC780is responsively increased, in order to supply enough hydrogen gas to sustain Rrebalancing. As the redox flow battery SOC decreases from the threshold redox flow battery SOC808to 0% (and as the electrolyte concentration decreases from the threshold electrolyte concentration858to A), the hydrogen gas flow rate820supplied to the RBC780is responsively decreased, in order to supply enough hydrogen gas to sustain Rrebalancing. In this way, modulating the hydrogen gas flow rate responsive to a change in the redox flow battery SOC during the first condition may include maintaining Qmass,H2/Qmass,electrolytegreater than (Qmass,H2/Qmass,electrolyte)THto preserve a slight stoichiometric excess in H2relative to the electrolyte (e.g., ferric ion for an IFB) as given by the rebalancing reaction equation (4) at the RBC780in order to mitigate flooding of the RBC780.

During the third condition, the redox flow battery SOC is higher than the threshold redox flow battery SOC (and the electrolyte concentration is higher than the threshold electrolyte concentration), and the hydrogen gas concentration at the RBC780may be a limiting reagent (relative to the electrolyte) in the rebalancing reaction. Thus, responsive to the redox flow battery system operating under the third condition, hydrogen gas may be supplied to and maintained at the RBC780at a flow rate equivalent to the upper threshold hydrogen gas flow rate824to maximize Rrebalancingfor the given electrolyte flow rate810. Furthermore, as the redox flow battery SOC increases from the threshold redox flow battery SOC808to 100% (and as the electrolyte concentration increases from the threshold electrolyte concentration858to B), the electrolyte flow rate810supplied to the RBC780is responsively decreased to avoid operating the RBC780with a large excess of electrolyte, which can lead to flooding of the RBC780. Reducing the electrolyte flow rate as the redox flow battery SOC increases helps to maintain enough hydrogen gas at the RBC780to avoid flooding, while maintaining a constant Rrebalancing. As the redox flow battery SOC decreases from 100% to the threshold redox flow battery SOC808(and as the electrolyte concentration decreases from B to the threshold electrolyte concentration858), the electrolyte flow rate810supplied to the RBC780is responsively increased, in order to supply enough electrolyte to maintain a constant Rrebalancing. In this way, modulating the electrolyte flow rate810responsive to a change in the redox flow battery SOC during the third condition may include maintaining Qmass,H2/Qmass,electrolytegreater than (Qmass,H2/Qmass,electrolyte)THto preserve a slight stoichiometric excess in H2relative to the electrolyte (e.g., ferric ion for an IFB) at the RBC780as given by the rebalancing reaction equation (4) in order to mitigate flooding of the RBC780.

During the second condition, the redox flow battery SOC is equivalent to the threshold redox flow battery SOC808(and the electrolyte concentration is equivalent to the threshold electrolyte concentration858). Thus, responsive to the redox flow battery system operating under the second condition, electrolyte may be supplied to and maintained at the RBC780at a flow rate equivalent to the upper threshold electrolyte flow rate814while hydrogen gas may be supplied to and maintained at the RBC780at a flow rate equivalent to the upper threshold hydrogen gas flow rate824, to maximize Rrebalancing. Thus, during the second condition, Qmass,H2=QH2,TH,upper*[H2] and Qmass,electrolyte=Qelectrolyte,TH,upper*[Electrolyte]TH, and (Qmass,H2/Qmass,electrolyte)TH=(QH2,TH,upper*[H2])/(Qelectrolyte,TH,upper*[Electrolyte]TH), as represented by equation (15). The second condition represents a transitional operating region from the first condition to the third condition where the limiting reagent in the rebalancing reaction transitions from being the electrolyte (e.g., ferric ion for an IFB) to being hydrogen gas. Conversely, the second condition also represents a transitional operating region from the third condition to the first condition where the limiting reagent in the rebalancing reaction transitions from being hydrogen gas to being the electrolyte (e.g., ferric ion for an IFB).

Referring now toFIGS.9and10, methods900and1000for operating a redox flow battery system10including a rebalancing cell780is shown. Specifically, the rebalancing cell780may be implemented in a redox flow battery system10for maintaining a rebalancing reaction rate and a mass flow ratio of hydrogen gas to electrolyte at a threshold mass flow ratio, while reducing a chance for electrolyte flooding at the RBC780. In an exemplary embodiment, the redox flow battery system may be the redox flow battery system10ofFIG.1and the rebalancing cell may be the rebalancing cell202ofFIGS.2A and2B. Accordingly, methods900and1000may be considered with reference to the embodiments ofFIGS.1.2A,2B,3,4A,4B,5A,5B,6A,6B,7A, and7B, alone or in combination, (though it may be understood that similar methods may be applied to other systems without departing from the scope of the present disclosure). For example, with methods900and1000, at least some steps or portions of steps (e.g., involving modulating the electrolyte flow rate and the H2gas flow rate) may be carried out via the controller88ofFIG.1, and may be stored as executable instructions at a non-transitory storage medium (e.g., memory) communicably coupled to the controller88. To elaborate, the H2gas flow rate to the RBC780may be driven by sending commands from a controller to the hydrogen flow generator770and the electrolyte flow rate to the RBC780may be driven by sending commands from the controller88to one or more of the main electrolyte pump730, auxiliary electrolyte pump776, and the control valve774. Further components described with reference toFIGS.9and10may be examples of corresponding components ofFIGS.1,2A,2B,3,4A,4B,5A,5B,6A,6B,7A, and7B.

Method900includes step910where the controller88may estimate and/or measure redox flow battery system operating conditions such as redox flow battery SOC, hydrogen gas flow rate, electrolyte flow rate, the operating mode of the redox flow battery (e.g., charging mode, discharging mode, idle mode), and the like.

At920, the method900includes calculating the electrolyte concentration from the measured SOC. In one example, the controller88may infer the electrolyte concentration from a pre-determined calibration look-up table of electrolyte concentration data versus redox flow battery SOC.

Next, at930, the method900includes maintaining the rebalancing reaction rate, Rrebalancing, and maintaining (Qmass,H2/Qmass,electrolyte)>(Qmass,H2/Qmass,electrolyte)TH. In particular, at934the controller88may adjust the electrolyte flow rate and/or may adjust the hydrogen gas flow rate supplied to the RBC780, according to method1000ofFIG.10.

Turning now toFIG.10, the method1000includes step1010, where the controller88determines if a first condition is satisfied. As described above with reference toFIGS.8A and8B, the first condition being satisfied includes when a redox flow battery SOC is less than a threshold SOC808. The first condition being satisfied may further include when an electrolyte concentration is less than a threshold electrolyte concentration858. In the case of an IFB, the electrolyte concentration being less than a threshold electrolyte concentration858may correspond to a ferric ion concentration being less than a threshold ferric ion concentration. Responsive to the first condition being satisfied, method1000continues to1014, where the controller88sets the electrolyte volumetric flow rate, Qelectrolyte=Qelectrolyte,TH,upper. Furthermore, responsive to an increase in SOC and/or electrolyte concentration, the controller88may increase the hydrogen gas flow rate while maintaining Qelectrolyte=Qelectrolyte,TH,upper. Further still, responsive to a decrease in SOC and/or electrolyte concentration, the controller88may decrease the hydrogen gas flow rate while maintaining Qelectrolyte=Qelectrolyte,TH,upper. Accordingly, the controller88may adjust the hydrogen gas flow rate responsive to changes in the SOC while maintaining Qelectrolyte=Qelectrolyte,TH,upperduring the first condition to supply enough hydrogen to the RBC780to sustain the desired rebalancing reaction rate while mitigating flooding. Further still, at1014, while modulating the hydrogen gas flow rate responsive to the changes in the redox flow battery SOC, the controller88may maintain the hydrogen flow rate820above a lower threshold hydrogen flow rate822, QH2,TH,lower. QH2,TH,lowermay correspond to a hydrogen gas flow rate below which flooding of the RBC780is increased, even at or near a redox flow battery SOC of 0% and an electrolyte concentration of A (seeFIGS.8A and8B).

Returning to1010, for the case where the first condition is not satisfied, method1000continues at1020where the controller determines if a third condition is satisfied. As described above with reference toFIGS.8A and8B, the third condition being satisfied includes when a redox flow battery SOC is greater than a threshold SOC808. The third condition being satisfied may further include when an electrolyte concentration is greater than a threshold electrolyte concentration858. In the case of an IFB, the electrolyte concentration being greater than a threshold electrolyte concentration858may correspond to a ferric ion concentration being greater than a threshold ferric ion concentration. Responsive to the third condition being satisfied, method1000continues to1024, where the controller88sets the hydrogen volumetric gas flow rate, QH2=QH2,TH,upper. Furthermore, responsive to an increase in SOC and/or electrolyte concentration, the controller88may decrease the electrolyte volumetric flow rate while maintaining QH2=QH2,TH,upper. Further still, responsive to a decrease in SOC and/or electrolyte concentration, the controller88may increase the electrolyte volumetric flow rate while maintaining QH2=QH2,TH,upper. Accordingly, the controller88may adjust the electrolyte volumetric flow rate responsive to changes in the SOC while maintaining QH2=QH2,TH,upperduring the third condition to supply enough electrolyte to the RBC780to sustain the desired rebalancing reaction rate while mitigating flooding. Further still, at1024, while modulating the electrolyte flow rate responsive to changes in the redox flow battery state of charge, the controller88may maintain the electrolyte flow rate810above a lower threshold electrolyte flow rate812, Qelectrolyte,TH,lower. Qelectrolyte,TH,lowermay correspond to an electrolyte flow rate below which a rebalancing reaction rate decreases such that a performance of the redox flow battery system is substantially reduced.

Returning to1020, for the case where the third condition is not satisfied, method1000continues at1030where the controller88determines if a second condition is satisfied. As described above with reference toFIGS.8A and8B, the second condition being satisfied includes when a redox flow battery SOC is equivalent to the threshold SOC808. The second condition being satisfied may further include when an electrolyte concentration is equivalent to the threshold electrolyte concentration858. Responsive to the second condition being satisfied, method1000continues to1034, where the controller88sets the hydrogen volumetric gas flow rate, QH2=QH2,TH,upperand sets the volumetric electrolyte volumetric flow rate Qelectrolyte=Qelectrolyte,TH,upper. Accordingly, responsive to either an increase or decrease in SOC and/or electrolyte concentration, the controller88maintains Qelectrolyte=Qelectrolyte,TH,upperwhile maintaining QH2=QH2,TH,upperduring the third condition to sustain the desired rebalancing reaction rate while mitigating flooding.

In this manner, a method of operating a rebalancing cell in a redox flow battery system includes, measuring a state-of-charge (SOC) of a redox flow battery in the redox flow battery system, calculating an electrolyte concentration from the measured SOC of the redox flow battery, and maintaining a rebalancing reaction rate at the rebalancing cell, including, responsive to a change in the electrolyte concentration, adjusting an electrolyte flow rate to the rebalancing cell and adjusting a hydrogen flow rate to the rebalancing cell. In a first example, the method further includes, wherein adjusting the electrolyte flow rate responsive to the change in the electrolyte concentration includes reducing the electrolyte flow rate responsive to an increase in the electrolyte concentration during a third condition, including when the electrolyte concentration is above a threshold electrolyte concentration. In a second example, optionally including the first example, the method further includes, wherein adjusting the hydrogen flow rate responsive to the change in the electrolyte concentration includes increasing the hydrogen flow rate responsive to the increase in the electrolyte concentration during a first condition, including when the electrolyte concentration is below the threshold electrolyte concentration. In a third example, optionally including one or more of the first and second examples, the method further includes, wherein adjusting the electrolyte flow rate responsive to the change in the electrolyte concentration includes setting the electrolyte flow rate to an upper threshold electrolyte flow rate during the first condition. In a fourth example, optionally including one or more of the first through third examples, the method further includes, wherein adjusting the hydrogen flow rate responsive to the change in the electrolyte concentration includes setting the hydrogen flow rate to an upper threshold hydrogen flow rate during the third condition. In a fifth example, optionally including one or more of the first through fourth examples, the method further includes, wherein maintaining the rebalancing reaction rate at the rebalancing cell includes maintaining the electrolyte flow rate below an upper threshold electrolyte flow rate. In a sixth example, optionally including one or more of the first through fifth examples, the method further includes, wherein maintaining the rebalancing reaction rate at the rebalancing cell includes maintaining the hydrogen flow rate above a lower threshold hydrogen flow rate. In a seventh example, optionally including one or more of the first through sixth examples, the method further includes, wherein adjusting the electrolyte flow rate responsive to the change in the electrolyte concentration includes setting the electrolyte flow rate to the upper threshold electrolyte flow rate during a second condition, including when the electrolyte concentration is at the threshold electrolyte concentration. In an eighth example, optionally including one or more of the first through seventh examples, the method further includes, wherein adjusting the hydrogen flow rate responsive to the change in the electrolyte concentration includes setting the hydrogen flow rate to an upper threshold hydrogen flow rate during the second condition.

In this manner, a method of operating a redox flow battery system includes, recirculating an electrolyte between a redox flow battery and a rebalancing cell, supplying a hydrogen gas to the rebalancing cell, measuring a state-of-charge (SOC) of the redox flow battery, and maintaining a rebalancing reaction rate at the rebalancing cell, including, adjusting a flow rate of the electrolyte to the rebalancing cell and adjusting a flow rate of the hydrogen gas to the rebalancing cell based on the measured SOC of the redox flow battery. In a first example, the method further includes, wherein adjusting the flow rate of the electrolyte to the rebalancing cell and adjusting the flow rate of the hydrogen gas to the rebalancing cell based on the measured SOC of the redox flow battery includes reducing the flow rate of the electrolyte responsive to an increase in the measured SOC of the redox flow battery while maintaining a flow rate of the hydrogen gas at an upper threshold hydrogen gas flow rate during a condition when the measured SOC of the redox flow battery is above a threshold redox flow battery SOC. In a second example, optionally including the first example, the method further includes, wherein adjusting the flow rate of the electrolyte to the rebalancing cell and adjusting the flow rate of the hydrogen gas to the rebalancing cell based on the measured SOC of the redox flow battery includes increasing the flow rate of the hydrogen gas responsive to an increase in the measured SOC of the redox flow battery while maintaining a flow rate of the electrolyte at an upper threshold electrolyte flow rate during a condition when the measured SOC of the redox flow battery is below a threshold redox flow battery SOC. In a third example, optionally including one or more of the first and second examples, the method further includes, wherein adjusting the flow rate of the electrolyte to the rebalancing cell and adjusting the flow rate of the hydrogen gas to the rebalancing cell based on the measured SOC of the redox flow battery includes maintaining the flow rate of the hydrogen gas at the upper threshold hydrogen gas flow rate and maintaining the flow rate of the electrolyte at the upper threshold electrolyte flow rate during a condition when the measured SOC of the redox flow battery is at the threshold redox flow battery SOC. In a fourth example, optionally including onc or more of the first through third examples, the method further includes, wherein maintaining a rebalancing reaction rate at the rebalancing cell includes maintaining a ratio of a mass flow rate of the hydrogen gas to the rebalancing cell to a mass flow rate of the electrolyte to the rebalancing cell above a threshold mass flow ratio. In a fifth example, optionally including one or more of the first through fourth examples, the method further includes, wherein maintaining the ratio of the mass flow rate of the hydrogen gas to the rebalancing cell to the mass flow rate of the electrolyte to the rebalancing cell above the threshold mass flow ratio includes maintaining a stoichiometric excess in hydrogen gas relative to the electrolyte at the rebalancing cell.

As described herein with reference toFIGS.11A and11B, in one example, the redox flow battery SOC may be determined from the redox flow battery plating efficiency and shunt current flow. In particular, the plating efficiency and shunt current flow are used to calculate positive and negative electrolyte SOC values; in addition, the redox flow battery open circuit voltage (OCV) is used to calculate an OCV SOC value, SOCOCV. Subsequently, an estimate for the redox flow battery SOC may be determined from the combination of the positive and negative electrolyte SOC values, and the estimated redox flow battery SOC may be adjusted based on a comparison of the estimated redox flow battery SOC to the SOCOCV.

Referring now toFIG.11A, a plot1100that illustrates an example relationship between plating electrolyte pH and negative electrode plating efficiency is shown. The plot represents a function that outputs a negative electrode plating efficiency. The function may be referenced or indexed via plating electrolyte pH. The vertical axis represents negative electrolyte plating efficiency and negative electrolyte plating efficiency increases in the direction of the vertical axis arrow. The horizontal axis represents plating electrolyte pH and plating electrolyte pH increases in the direction of the horizontal axis arrow.

Curve1102represents the relationship between plating electrolyte pH and negative electrode plating efficiency, which may be referred to as the coulombic efficiency for the negative plating reaction. In one example, curve302may be expressed in equation (15) as:

Plateeff=0.138*ln(pH)+0.8514  (15)where Plateeffis the plating efficiency of the negative electrode, In is the natural logarithm, and pH is the pH value of the plating electrolyte.

In one example, the plating efficiency may be empirically determined via adjusting the pH of the plating electrolyte and determining the plating efficiency for each pH value during charging of the iron flow electric energy storage cell. The plating efficiency may be determined via dividing the actual weight of metal deposited to the negative electrode during charging of the iron flow electric energy storage cell by the theoretical weight of metal that would be deposited to the negative electrode during charging of the iron flow electric energy storage cell according to Faraday's law.

Referring now toFIG.11B, a plot1152that illustrates an example relationship between battery SOC and open circuit voltage (OCV) is shown. The plot represents a function that outputs SOC for a battery. The function may be referenced or indexed by OCV (e.g., voltage of the IFB cell or cell stack when the IFB cell or cell stack is disconnected from external electric loads). The vertical axis represents plating OCV and OCV increases in the direction of the vertical axis arrow. The horizontal axis represents SOC % and SOC % increases in the direction of the horizontal axis arrow.

Curve1152represents the relationship between SOC % and OCV. In one example, curve402may be expressed in equation (16) as:

SOCOCV=−0.518*(OCV)2+67.098*(OCV)−2010.7  (16)where SOCOCVis the battery SOC determined from OCV and OCV is the open circuit voltage of the selected battery.

In one example, the SOC and OCV relationship may be empirically determined by measuring the OCV and then fully discharging the battery while measuring the amount of charge that leaves the battery during the discharge process. The amount of charge that exits the battery during the discharge process divided by theoretical amount of charge the battery may store indicates the SOC for the particular OCV.

The change in operation from charging to discharging, or vice-versa, provides an opportunity to determine an OCV of the redox flow battery without interrupting operation of the redox flow battery system. As examples, the controller may switch from charging redox flow battery to discharging the redox flow battery when a SOC increases above an upper threshold SOC during charging. Similarly, the controller may switch from discharging redox flow battery to charging the redox flow battery when a SOC decreases below a lower threshold SOC while discharging. Alternatively, a controller88may demand that the redox flow battery switch operation from discharging mode to charging mode, or vice-versa. The upper threshold SOC and lower threshold SOC may be predetermined quantities, and may correspond to when the redox flow battery is charged to capacity and fully discharged, respectively. Controller88may electrically decouple the redox flow battery from the voltage source or load, other redox flow batteries, and external electric power supplies and consumers for a threshold decoupling duration, for example, at least 30 seconds. The controller88then determines the SOCOCV(state of charge based on open circuit voltage) by way of equation (16).

SOC values for the redox flow battery positive and negative electrolytes, SOCposand SOCnegrespectively, are determined from equations (17) and (18):

where SOCposis SOC based on positive electrolyte, n is the number of steps in the summation during the time interval in which SOC is estimated, IT100 is the total current flow through the electric energy cell stack during the time interval in which SOC is estimated, F is Faraday's number, Is,pos,iis the total shunt current for the positive electrolyte during the time interval in which SOC is estimated, Au is the iron flow battery system active area, NFe3+is the flux density for ferric ions from the positive electrolyte to the negative electrolyte, Δtiis the time interval between steps, Vpos,0is the initial volume of the positive electrolyte, [Fe2+]0is the initial concentration of ferrous ions in the positive and negative electrolytes, NFe2+is the flux density for ferrous ions from the positive electrolyte to the negative electrolyte, SOCnegis SOC based on negative electrolyte, η is the coulombic efficiency (e.g., the plating efficiency given by equation (14)) for the negative plating reaction, Is,neg,iis the total shunt current for the negative electrolyte during the time interval in which SOC is estimated, and Vneg,0is the initial volume of the negative electrolyte.

The shunt current may be empirically determined. In one example, the shunt current in the redox flow battery may be empirically determined via measuring battery capacity loss over idling conditions where no external current is applied, but all redox flow battery cells are connected via an electrolyte shunt path. Ionic movement within the electric energy storage device cell stack. The ionic movement (e.g., flux density per redox flow battery system active area) may be empirically determined. In one example, the ionic movement in the redox flow battery may be empirically determined via ex-situ measurement of electrolyte ionic concentrations via ion chromatography.

During charging, an estimate for the redox flow battery SOC is given by equation (19):

where max is a function that returns the greater value of arguments SOCposand SOCneg. During discharging, an estimate for the redox flow battery SOC is given by equation (20):

where min is a function that returns the lesser value of arguments SOCposand SOCneg.

When the estimate for the redox flow battery SOC, given by equation (19) or (20) is within 10% of SOCOCV. SOC may be adjusted according to equation (21):

When the estimate for the redox flow battery SOC, given by equation (19) or (20) is not within 10% of SOCOCV, electrolyte rebalancing and or cleansing may be indicated.

In this manner, a redox flow battery system includes, a redox flow battery, a rebalancing cell fluidly coupled to the redox flow battery an electrolyte flow control device fluidly coupled between the redox flow battery and the rebalancing cell, a hydrogen flow control device fluidly coupled between the redox flow battery and the rebalancing cell, and a controller, including executable instructions stored in non-transitory memory thereon to, maintain an electrolyte rebalancing reaction rate at the rebalancing cell, including, responsive to a change in a concentration of an electrolyte, modulating the electrolyte flow control device to adjust a flow rate of the electrolyte supplied to the rebalancing cell, and modulating the hydrogen flow control device to adjust a flow rate of hydrogen supplied to the rebalancing cell. In a first example, the redox flow battery system further includes, wherein the redox flow battery includes an all iron redox flow battery (IFB), and the electrolyte includes ferric ion. In a second example, optionally including the first example, the redox flow battery system further includes, wherein the rebalancing cell includes a stack of electrode assemblies, each electrode assembly of the stack of electrode assemblies including an interfacing pair of positive and negative electrodes, wherein the electrolyte rebalancing reaction rate is driven via internal electrical shorting of interfacing pairs of the positive and negative electrodes of the rebalancing cell and the electrolyte and the hydrogen being convected through the electrolyte rebalancing cell. In a third example, optionally including one or more of the first and second examples, the redox flow battery system further includes, wherein the electrolyte flow control device includes one of a pump and a flow control valve. In a fourth example, optionally including one or more of the first through third examples, the redox flow battery system further includes, wherein the hydrogen flow control device includes a speed modulation device coupled to one of a venturi injector and a blower.

The technical effect of the method for the redox flow battery system described herein is to maximize the rebalancing reaction rate and reduce electrolyte flooding at a rebalancing cell while decreasing operating costs by modulating hydrogen gas flow and electrolyte flow to the rebalancing cell based on the redox flow battery SOC and/or electrolyte concentration.