Methods for short-term battery idle

Abstract

A system and method for a redox flow battery is provided. In one embodiment, the method for a redox flow battery includes operating the redox flow battery in a short idle mode by discharging the redox flow battery at a constant current density for the duration of the short idle mode. The discharging current density can maintain a plating surface at the negative electrode of the redox flow battery.

Description

[Technical field]

[0001] CROSS-REFERENCE TO RELATED APPLICATIONS This application claims priority to U.S. Provisional Application No. 63 / 267,533, entitled METHODS FOR SHORT-TERM BATTERY IDLE, filed February 3, 2022, and U.S. Provisional Application No. 63 / 267,534, entitled METHODS FOR SHORT-TERM BATTERY IDLE, filed February 3, 2022. The entire contents of the above-identified applications are hereby incorporated by reference for all purposes.

[0002] The present description relates generally to redox flow battery systems. [Background technology]

[0003] Redox flow batteries are well suited for grid-scale energy storage applications due to their ability to scale power and capacity independently, and for charging and discharging for thousands of cycles with minimal loss of performance. While idle and not actively charging or discharging, redox flow battery systems typically maintain electrolyte flow at a certain level to maintain the system's readiness to provide power in response to a charge or discharge command. Summary of the Invention [Problem to be solved by the invention]

[0004] However, the inventors herein have recognized potential problems that may occur while the redox flow battery system is maintained in an idle state. During idling, the bipolar plates (bipolar plates coupled to plating electrodes) of the redox flow battery system may come into contact with the electrolyte for periods of time during which charge is not supplied to or drawn from the redox flow battery system. That is, the metal plated on the bipolar plates may crack and flake when exposed to the electrolyte without active current flow. Loss of metal from the bipolar plates may cause a drop in the charging potential of the redox flow battery system. Additionally, metal flakes lost from the bipolar plates may persist and circulate throughout the redox flow battery system if they do not dissolve in the electrolyte. The metal flakes may contact and degrade the membranes that separate the positive and negative chambers of the redox flow battery cells. Furthermore, circulation of the metal flakes may cause electrical shorts, promote dendrite formation, and block electrolyte flow in the redox flow battery system. [Means for solving the problem]

[0005] In one embodiment, the above-mentioned problems may be addressed by a method for operating a redox flow battery system, comprising operating the redox flow battery system in a short-term idle mode by discharging the battery at a constant current density for the duration of the short-term idle mode, thereby maintaining a plating surface at the negative electrode of the redox flow battery system. In this manner, the plating layer on the surface of the bipolar plate coupled to the plating electrode may be preserved, thereby allowing redox flow battery performance to be maintained and restored after idling the battery for a relatively short period of time.

[0006] The inventors herein provide a method for managing the surface of a bipolar plate during short idle periods when the bipolar plate is in contact with an electrolyte. The amount of charge stored in the redox flow battery system can be dissipated during the idle period to minimize cracking and flaking of the plating layer on the bipolar plate surface after the idle period. In one embodiment, the redox flow battery can be discharged and current can be continuously flowed at a low current density (e.g., compared to the current density used during operation of the redox flow battery system in a discharge mode) for the duration of the short idle period.

[0007] In this way, the technical effect of inhibiting and mitigating cracking and flaking of metal plating on the bipolar plate bonded to the negative electrode of the redox flow battery after idle periods can be achieved while minimizing the amount of current sacrificed due to discharge.

[0008] It should be appreciated that the foregoing summary is provided to introduce in a simplified form a selection of concepts that are further described in the Detailed Description. It is not intended to identify key or essential features of the claimed subject matter, the scope of which is defined solely by the claims that follow the Detailed Description. Moreover, the claimed subject matter is not limited to implementations that solve any disadvantages noted above or in any part of this disclosure. [Brief description of the drawings]

[0009] [Figure 1] FIG. 1 shows a schematic diagram of an exemplary redox flow battery system including a bipolar plate in contact with a negative electrode. [Diagram 2] FIG. 2 illustrates a side view of an exemplary layout of the redox flow battery system of FIG. [Diagram 3] 2 shows a high level flow chart of an example of a method for operating the redox flow battery system of FIG. [Figure 4] 1 shows a flowchart of an example of a method for determining an idle mode of a redox flow battery system. [Diagram 5] 1 illustrates a flow chart of an example of a method for operating a redox flow battery system in a trickle discharge short idle mode. [Figure 6A] 1 shows an image of a bipolar plate exhibiting cracks due to an electrolyte flow of 24 mL / min. [Figure 6B] FIG. 6B shows an enlarged view of the crack at the inlet of the bipolar plate of FIG. 6A. [Figure 6C] 1 shows an image of a bipolar plate exhibiting cracks due to electrolyte flow of 120 mL / min. [Figure 6D] 6A and 6B show plots of voltage as a function of time for the bipolar plate shown in FIG. [Figure 7A] 1 shows an image of a bipolar plate exhibiting cracks due to an electrolyte temperature of 48° C. [Figure 7B] An example image of a bipolar plate exhibiting cracks due to an electrolyte temperature of 60° C. is shown. [Figure 7C] 7A and 7B show plots of voltage as a function of time for the bipolar plate shown in FIGS. 7A and 7B. [Figure 8] Shown is an image of a bipolar plate that was kept idle for 2 hours without the trickle discharge applied. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0010] The following description relates to methods and systems for redox flow batteries. In one embodiment, a method is provided for operating a redox flow battery in an idle state to allow for surface management of plated iron. For example, the method may include techniques for controlling the condition of the plated surface to reduce the presence of defects that may degrade the battery's performance. The techniques may include discharging the battery at a constant current density during idling to maintain the surface quality of the plated surface of the bipolar plate connected to the negative electrode of the battery. Figures 1-2 show schematics of a redox flow battery and a system of two or more redox flow batteries, respectively. Figure 3 shows an example of a method for adjusting battery operation between a charge mode, a discharge mode, and an idle state. An example of a method for operating a redox flow battery in an idle state is shown in Figures 4-5. Figures 6A-8 show experimental examples of the idle state and the effect of discharge during the idle state, including voltage vs. time profiles and images of an example bipolar plate surface coating.

[0011] 1, in the redox flow battery system 10, the negative electrode 26 may be referred to as the plating electrode and the positive electrode 28 may be referred to as the redox electrode. The negative electrolyte in the plating side (e.g., negative electrode compartment 20) of the redox flow battery cell 18 may be referred to as the plating electrolyte, and the positive electrolyte on the redox side (e.g., positive electrode compartment 22) of the redox flow battery cell 18 may be referred to as the redox electrolyte.

[0012] "Anode" refers to an electrode at which an electroactive material loses electrons, and "cathode" refers to an electrode at which an electroactive material gains electrons. During battery charging, the negative electrolyte gains electrons at the negative electrode 26, which is the cathode of the electrochemical reaction. During battery discharging, the negative electrolyte loses electrons, and the negative electrode 26 is the anode of the electrochemical reaction. Alternatively, during battery discharging, the negative electrolyte and the negative electrode 26 may be referred to as the anolyte and anode, respectively, of the electrochemical reaction, while the positive electrolyte and the positive electrode 28 may be referred to as the catholyte and cathode, respectively, of the electrochemical reaction. During battery charging, the negative electrolyte and the negative electrode 26 may be referred to as the catholyte and cathode, respectively, of the electrochemical reaction, while the positive electrolyte and the positive electrode 28 may be referred to as the anolyte and anode, respectively, of the electrochemical reaction. For simplicity, the terms "positive" and "negative" are used herein to refer to the electrodes, electrolytes, and electrode compartments in a redox flow battery system.

[0013] One example of a hybrid redox flow battery is an all-iron redox flow battery (IFB), in which the electrolyte includes iron ions in the form of iron salts (e.g., FeCl2, FeCl3, etc.) and the negative electrode 26 includes metallic iron. For example, ferrous iron (Fe 2+ ) gains two electrons during battery charging and becomes iron metal (Fe 0 ) on the negative electrode 26, and Fe 0 During battery discharge, it loses two electrons to become Fe 2+ At the positive electrode 28, Fe 2+ During battery charging, ferric iron (Fe 3+ ) and Fe 3+ During battery discharge, it gains electrons to form Fe 2+ The electrochemical reactions are summarized in equations (1) and (2), where the forward reaction (left to right) represents the electrochemical reaction during battery charging, while the reverse reaction (right to left) represents the electrochemical reaction during battery discharging.

number

[0014] As mentioned above, the negative electrolyte used in the IFB is Fe 2+ accepts two electrons from the negative electrode 26 to form Fe 0 and plated onto the substrate. 2+ During battery discharge, the plated Fe 0 loses two electrons to become Fe 2+ and can dissolve back into the electrolyte. The equilibrium potential of the above reaction is -0.44 V, so this reaction provides the negative terminal for the desired system. On the positive side of the IFB, the electrolyte loses electrons and becomes Fe 3+ During battery charging, Fe is oxidized to 2+ During battery discharge, Fe provided by the electrolyte 3+ by absorbing electrons provided by the positive electrode 28. 2+ The equilibrium potential for this reaction is +0.77V, creating a positive terminal in the desired system.

[0015] The IFB may provide the ability to charge and recharge the electrolyte therein, in contrast to other battery types that utilize non-regenerative electrolytes. Charging may be accomplished by applying a current across electrodes 26 and 28 via terminals 40 and 42, respectively. The negative electrode 26 contains (e.g., Fe 2+ But Fe 3+ The negative side of a voltage source may be electrically coupled via terminal 40 such that electrons may be delivered to the negative electrolyte via positive electrode 28 (as Fe is oxidized to Fe 2+ is reduced to form Fe on the (plating) substrate. 0 and Fe 2+ is plated onto the negative electrode 26.

[0016] Discharge is Fe 0 remains available in the negative electrolyte for oxidation, and Fe 3+ remains available in the positive electrolyte for reduction. 3+ Availability may be increased by adding additional Fe via an external source such as the external positive electrolyte chamber 52.3+ This can be maintained by increasing the concentration or volume of the positive electrolyte in the positive electrode compartment 22 of the redox flow battery cell 18 to provide ions. More generally, during discharge, 0 The availability of Fe can be an issue in IFB systems, where the Fe available for discharge 0 can be proportional to the surface area and volume of the negative electrode substrate and the plating efficiency. The charging capacity is 2+ As an example, Fe 2+ Availability is via an external source, such as the external negative electrolyte chamber 50, for additional Fe 2+ This can be maintained by providing ions to increase the concentration or volume of negative electrolyte to the negative electrode compartment 20 side of the redox flow battery cell 18.

[0017] In an IFB, the positive electrolyte may include ferrous, ferric, ferric complexes, or any combination thereof, while the negative electrolyte may include ferrous or ferrous complexes, depending on the state of charge (SOC) of the IFB system. As previously mentioned, the utilization of iron ions in both the negative and positive electrolytes may enable utilization of the same electrolyte species on both sides of the redox flow battery cell 18, which may reduce electrolyte cross-contamination and increase the efficiency of the IFB system, resulting in less electrolyte exchange compared to other redox flow battery systems.

[0018] Efficiency losses in the IFB can result from electrolyte crossover through the separator 24 (e.g., ion exchange membrane barrier, microporous membrane, etc.). For example, Fe in a positive electrolyte 3+ The ion is Fe 3+ The negative electrolyte can then be driven toward the negative electrolyte by the ion concentration gradient and the electrophoretic force across the separator 24. The Fe ions then penetrate the separator 24 and cross over into the negative electrode compartment 20. 3+ Fe ions crossing over from the low pH redox side (e.g., the more acidic positive electrode compartment 22) to the high pH plating side (e.g., the less acidic negative electrode compartment 20) can result in coulombic efficiency losses.3+ The ions may result in the precipitation of Fe(OH)3. The precipitation of Fe(OH)3 may degrade the separator 24 and cause permanent battery performance and efficiency loss. For example, Fe(OH)3 precipitates may chemically foul the organic functional groups of the ion exchange membrane or physically clog the micropores of the ion exchange membrane. In either case, the Fe(OH)3 precipitates may cause the membrane's ohmic resistance to increase over time and reduce battery performance. The precipitates may be removed by washing the IFB with acid, but the constant maintenance and downtime may be disadvantageous for commercial battery applications. Furthermore, washing may depend on periodic preparation of the electrolyte, contributing to additional processing costs and complexity. Alternatively, the addition of certain organic acids to the positive and negative electrolytes in response to electrolyte pH changes may mitigate precipitate formation during battery charge and discharge cycling without driving up overall costs. Additionally, Fe 3+ Implementing a membrane barrier that inhibits ionic crossover may also mitigate fouling.

[0019] Additional coulombic efficiency loss, H + (e.g., protons) and subsequent formation of H2 gas, and the reaction of protons in the negative electrode compartment 20 with electrons supplied to the plated iron metal of the negative electrode 26, may cause the formation of H2 gas.

[0020] IFB electrolytes (e.g., FeCl2, FeCl3, FeSO4, Fe2(SO4)3, etc.) may be readily available and may be produced at low cost. In one example, the IFB electrolyte may be formed from ferrous chloride (FeCl2), potassium chloride (KCl), manganese(II) chloride (MnCl2), and boric acid (H3BO3). The IFB electrolyte may provide a higher reuse value since the same electrolyte may be used for the negative and positive electrolytes, resulting in reduced cross-contamination issues compared to other systems. Furthermore, due to the electron configuration of iron, it may solidify into a generally uniform solid structure during plating onto the negative electrode substrate. In the case of zinc and other metals commonly used in hybrid redox batteries, solid dendritic structures may form during plating. The stable electrode morphology of the IFB system may increase the efficiency of the battery compared to other redox flow batteries. Still further, the iron redox flow battery may reduce the use of toxic raw materials and may operate at a relatively neutral pH compared to other redox flow battery electrolytes. Therefore, the IFB system may pose less environmental hazard than all other current advanced redox flow battery systems in production.

[0021] Continuing with FIG. 1, a schematic diagram of a redox flow battery system 10 is shown. The redox flow battery system 10 may include a redox flow battery cell 18 fluidly coupled to an integrated multi-chamber electrolyte storage tank 110. The redox flow battery cell 18 may include a negative electrode compartment 20, a separator 24, and a positive electrode compartment 22. The separator 24 may include an electrically insulating ionically conductive barrier that prevents bulk mixing of the positive and negative electrolytes while allowing conduction of certain ions. For example, as discussed above, the separator 24 may include an ion exchange membrane and / or a microporous membrane.

[0022] The negative electrode compartment 20 may include a negative electrode 26, and the negative electrolyte may include an electroactive material. The positive electrode compartment 22 may include a positive electrode 28, and the positive electrolyte may include an electroactive material. In some embodiments, multiple redox flow battery cells 18 may be combined in series or parallel to generate higher voltages or currents in the redox flow battery system 10.

[0023] 1 are negative and positive electrolyte pumps 30 and 32, both of which are used to pump electrolyte solution through the redox flow battery system 10. Electrolyte is stored in one or more tanks external to the cell and is pumped through the negative electrode compartment 20 and positive electrode compartment 22 sides of the redox flow battery cell 18 via the negative and positive electrolyte pumps 30 and 32, respectively.

[0024] The redox flow battery system 10 may also include a first bipolar plate 36 and a second bipolar plate 38, each positioned along a back side of the negative electrode 26 and the positive electrode 28, respectively, e.g., opposite the side facing the separator 24. The first bipolar plate 36 may be in contact with the negative electrode 26 and the second bipolar plate 38 may be in contact with the positive electrode 28. However, in other embodiments, the bipolar plates 36 and 38 may be arranged in close proximity to the electrodes 26 and 28, but spaced apart from the electrodes 26 and 28 and housed within the respective electrode compartments 20 and 22. In either case, the bipolar plates 36 and 38 may be electrically coupled to terminals 40 and 42, respectively, via direct contact with the negative and positive electrodes 26 and 28, respectively, or via the negative and positive electrodes 26 and 28, respectively. The IFB electrolyte may be transported to the reaction sites of the negative and positive electrodes 26 and 28 by the first and second bipolar plates 36 and 38 due to the conductive properties of the materials of the bipolar plates 36 and 38. The flow of electrolyte may also be assisted by the negative and positive electrolyte pumps 30 and 32 to facilitate forced convection through the redox flow battery cell 18. Reacted electrochemical species may also be directed away from the reaction sites by a combination of forced convection and the presence of the first and second bipolar plates 36 and 38.

[0025] As illustrated in FIG. 1 , the redox flow battery cell 18 may further include a negative battery terminal 40 and a positive battery terminal 42. When a charging current is applied to the battery terminals 40 and 42, the positive electrolyte may be oxidized (lose one or more electrons) at the positive electrode 28 and the negative electrolyte may be reduced (gain one or more electrons) at the negative electrode 26. During battery discharge, reverse redox reactions may occur on the electrodes 26 and 28. In other words, the positive electrolyte may be reduced (gain one or more electrons) at the positive electrode 28 and the negative electrolyte may be oxidized (lose one or more electrons) at the negative electrode 26. A potential difference across the battery may be maintained by electrochemical redox reactions in the positive electrode compartment 22 and the negative electrode compartment 20, which may induce a current through the current collectors while the reactions persist. The amount of energy stored by a redox battery may be limited by the amount of electroactive material available in the electrolyte for discharge, depending on the total amount of electrolyte and the solubility of the electroactive material.

[0026] The redox flow battery system 10 may further include an integrated multi-chamber electrolyte storage tank 110. The multi-chamber electrolyte storage tank 110 may be divided by a bulkhead 98. The bulkhead 98 may create multiple chambers within the multi-chamber electrolyte storage tank 110 such that both positive and negative electrolytes may be contained within a single tank. The negative electrolyte chamber 50 holds a negative electrolyte containing an electroactive material, and the positive electrolyte chamber 52 holds a positive electrolyte containing an electroactive material. The bulkhead 98 may be positioned within the multi-chamber electrolyte storage tank 110 to provide a desired volume ratio between the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In one embodiment, the bulkhead 98 may be positioned to set the volume ratio of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 according to a stoichiometric ratio between the negative redox reaction and the positive redox reaction. FIG. 1 further illustrates a fill height 112 of the multi-chamber electrolyte storage tank 110, which may indicate the liquid level within each tank compartment.

[0027] FIG. 1 also shows a gas headspace 90 located above the fill height 112 of the negative electrolyte chamber 50, and a gas headspace 92 located above the fill height 112 of the positive electrolyte chamber 52. The gas headspace 92 may be utilized to store H2 gas that is generated during the operation of the redox flow battery (e.g., by the side reactions of proton reduction and iron corrosion) and is carried to the multi-chamber electrolyte storage tank 110 with electrolyte returning from the redox flow battery cell 18. The H2 gas may be naturally separated at the gas-liquid interface (e.g., fill height 112) in the multi-chamber electrolyte storage tank 110, thereby eliminating the need to have an additional gas-liquid separator as part of the redox flow battery system 10. Once separated from the electrolyte, the H2 gas may fill the gas headspaces 90 and 92. Thus, the stored H2 gas may help purge other gases from the multi-chamber electrolyte storage tank 110, thereby acting as an inert gas blanket to reduce oxidation of electrolyte species, which may help reduce redox flow battery capacity loss. In this manner, utilizing an integrated multi-chamber electrolyte storage tank 110 may eliminate the need to have separate negative and positive electrolyte storage tanks, hydrogen storage tanks, and gas-liquid separators common to conventional redox flow battery systems, thereby simplifying the system design, reducing the physical footprint of the redox flow battery system 10, and reducing system costs.

[0028] FIG. 1 also shows a spillover hole 96, which may create an opening in a bulkhead 98 between the gas headspaces 90 and 92 and provide a means of equalizing gas pressure between the chambers 50 and 52. The spillover hole 96 may be located at a threshold height above the fill height 112. The spillover hole 96 may further enable the ability to self-balance the electrolyte in each of the negative and positive electrolyte chambers 50 and 52 in the event of a cell crossover. In the case of an all-iron redox flow battery system, the same electrolyte (Fe 2+) are used in both the negative and positive electrode compartments 20 and 22, so that electrolyte spillover between the negative electrolyte chamber 50 and the positive electrolyte chamber 52 may reduce overall system efficiency, but overall electrolyte composition, battery module performance, and battery module capacity may be maintained. Flange joints may be utilized for all plumbing connections between the inlet and outlet multi-chamber electrolyte storage tank 110 to maintain a leak-free, continuously pressurized condition. The multi-chamber electrolyte storage tank 110 may include at least one outlet from each of the negative and positive electrolyte chambers 50 and 52, and at least one inlet to each of the negative and positive electrolyte chambers 50 and 52. Additionally, one or more outlet connections may be provided from the gas headspaces 90 and 92 to direct H2 gas to the rebalancing reactors or cells 80 and 82.

[0029] Although not shown in FIG. 1 , the integrated multi-chamber electrolyte storage tank 110 may further include one or more heaters thermally coupled to each of the negative electrolyte chamber 50 and the positive electrolyte chamber 52. In an alternative example, only one of the negative and positive electrolyte chambers 50 and 52 may include one or more heaters. In the case where only the positive electrolyte chamber 52 includes one or more heaters, the negative electrolyte may be heated by transferring heat generated in the redox flow battery cell 18 to the negative electrolyte. In this manner, the redox flow battery cell 18 may heat the negative electrolyte and facilitate temperature regulation of the negative electrolyte. The one or more heaters may be operated by the controller 88 to regulate the temperature of the negative electrolyte chamber 50 and the positive electrolyte chamber 52 independently or together. For example, in response to the electrolyte temperature falling below a threshold temperature, the controller 88 may increase the power supplied to the one or more heaters such that the heat flux to the electrolyte may be increased. The electrolyte temperature may be indicated by one or more temperature sensors attached to the multi-chamber electrolyte storage tank 110, such as sensors 60 and 62. By way of example, the one or more heaters may include coil-type heaters or other immersion heaters immersed in the electrolyte fluid, or surface mantle-type heaters that conductively transfer heat through the walls of the negative and positive electrolyte chambers 50 and 52 to heat the fluid therein. Other known types of tank heaters may be used without departing from the scope of the present disclosure. Additionally, the controller 88 may deactivate one or more heaters in the negative and positive electrolyte chambers 50 and 52 in response to a liquid level below a solids fill threshold level. Stated differently, in some embodiments, the controller 88 may activate one or more heaters in the negative and positive electrolyte chambers 50 and 52 only in response to a liquid level above a solids fill threshold level. In this manner, activation of one or more heaters without sufficient liquid in the negative and / or positive electrolyte chambers 50, 52 may be avoided, thereby reducing the risk of overheating or burning out of the heater(s).

[0030] Still further, one or more inlet connections may be provided to each of the negative and positive electrolyte chambers 50 and 52 from a field hydration system (not shown). In this manner, the field hydration system may facilitate the start-up of the redox flow battery system 10, including installation, charging, and hydration of the redox flow battery system 10, at the end-use location. Furthermore, prior to the start-up of the redox flow battery system 10 at the end-use location, the redox flow battery system 10 may be dry assembled at a battery manufacturing facility different from the end-use location, without the need to charge and hydrate the redox flow battery system 10 before shipping it to the end-use location. In one embodiment, the end-use location may correspond to a location where the redox flow battery system 10 is installed and utilized for on-site energy storage. In other words, the redox flow battery system 10 may be designed such that once installed and hydrated at the end-use location, the location of the redox flow battery system 10 is fixed and the redox flow battery system 10 may no longer be considered a portable dry system. Thus, from an end user's perspective, the dry portable redox flow battery system 10 may be delivered to a site, after which the redox flow battery system 10 may be installed, hydrated, and commissioned. Prior to hydration, the redox flow battery system 10 may be referred to as a dry portable system, in which the redox flow battery system 10 does not include or is free of water and wet electrolyte. Once hydrated, the redox flow battery system 10 may be referred to as a wet non-portable system, in which the redox flow battery system 10 includes wet electrolyte.

[0031] 1, the electrolyte solution initially stored in the multi-chamber electrolyte storage tank 110 can be pumped throughout the redox flow battery system 10 via negative and positive electrolyte pumps 30 and 32. The electrolyte stored in the negative electrolyte chamber 50 can be pumped through the negative electrode compartment 20 side of the redox flow battery cell 18 via the negative electrolyte pump 30, and the electrolyte stored in the positive electrolyte chamber 52 can be pumped through the positive electrode compartment 22 side of the redox flow battery cell 18 via the positive electrolyte pump 32.

[0032] The electrolyte rebalancing reactors 80 and 82 may be connected in-line or in parallel with the electrolyte recirculation flow paths at the negative and positive sides of the redox flow battery cells 18 in the redox flow battery system 10, respectively. One or more rebalancing reactors may be connected in-line with the electrolyte recirculation flow paths at the negative and positive sides of the battery, and other rebalancing reactors may be connected in parallel for redundancy (e.g., the rebalancing reactors may be maintained without interrupting the battery and rebalancing operation) and for increased rebalancing capacity. In one embodiment, the electrolyte rebalancing reactors 80 and 82 may be disposed in the return flow paths from the negative and positive electrode compartments 20 and 22, respectively, to the negative and positive electrolyte chambers 50 and 52.

[0033] The electrolyte rebalancing reactors 80 and 82 can serve to rebalance electrolyte charge imbalances in the redox flow battery system 10 that occur due to side reactions, ionic crossover, etc., as described herein. In one embodiment, the rebalancing reactors 80 and 82 can include trickle bed reactors, where H2 gas and electrolyte can contact at a catalyst surface in a packed bed to perform the electrolyte rebalancing reaction. In another embodiment, the rebalancing reactors 80 and 82 can include flow-through reactors that can contact H2 gas and electrolyte liquid and perform the electrolyte rebalancing reaction without a packed catalyst bed.

[0034] During operation of the redox flow battery system 10, the sensors and probes may monitor and control electrolyte chemical properties such as electrolyte pH, concentration, SOC, etc. For example, as illustrated in FIG. 1, sensors 62 and 60 may be positioned to monitor the state of the positive and negative electrolytes in the positive and negative electrolyte chambers 52 and 50, respectively. In another example, the sensors 62 and 60 may each include one or more electrolyte level sensors to indicate the level of electrolyte in the positive and negative electrolyte chambers 52 and 50, respectively. As another example, the sensors 72 and 70, also illustrated in FIG. 1, may monitor the state of the positive and negative electrolytes in the positive and negative electrode compartments 22 and 20, respectively. The sensors 72 and 70 may be pH probes, optical probes, pressure sensors, voltage sensors, etc. It will be understood that sensors may be positioned elsewhere throughout the redox flow battery system 10 to monitor electrolyte chemical properties and other properties.

[0035] For example, sensors may be positioned in an external acid tank to monitor the amount of acid or pH of the external acid tank (not shown), and acid from the external acid tank may be supplied to the redox flow battery system 10 via an external pump (not shown) to reduce precipitate formation in the electrolyte. Additional external tanks and sensors may be installed to supply other additives to the redox flow battery system 10. For example, various sensors including temperature, conductivity, and level sensors of the field hydration system may send signals to the controller 88. Additionally, the controller 88 may send signals to actuators such as valves and pumps of the field hydration system during hydration of the redox flow battery system 10. The sensor information may be sent to the controller 88, which may then, by way of example, operate the pumps 30 and 32 to control electrolyte flow through the redox flow battery cells 18, or perform other control functions. In this manner, the controller 88 may respond to one or a combination of sensors and probes.

[0036] The redox flow battery system 10 may further include a source of H2 gas. In one embodiment, the source of H2 gas may include a separate dedicated hydrogen gas storage tank. In the embodiment of FIG. 1, H2 gas may be stored in and supplied from an integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supply additional H2 gas to the positive electrolyte chamber 52 and the negative electrolyte chamber 50. The integrated multi-chamber electrolyte storage tank 110 may alternately supply additional H2 gas to the inlets of the electrolyte rebalancing reactors 80 and 82. As an example, a mass flow meter or other flow control device (which may be controlled by the controller 88) may regulate the flow of H2 gas from the integrated multi-chamber electrolyte storage tank 110. The integrated multi-chamber electrolyte storage tank 110 may supplement the H2 gas generated within the redox flow battery system 10. For example, if a gas leak is detected in the redox flow battery system 10 or if the reduction reaction rate is too slow at low hydrogen partial pressure, H2 gas can be provided from the integrated multi-chamber electrolyte storage tank 110 to rebalance the SOC of the electroactive materials in the positive and negative electrolytes. As an example, the controller 88 can provide H2 gas from the integrated multi-chamber electrolyte storage tank 110 in response to a measured change in pH or in response to a measured change in the SOC of the electrolyte or electroactive materials.

[0037] For example, an increase in pH in the negative electrolyte chamber 50 or the negative electrode compartment 20 may indicate that H2 is leaking from the redox flow battery system 10 and / or that the reaction rate is too slow at the available hydrogen partial pressure, and the controller 88 may increase the supply of H2 gas from the integrated multi-chamber electrolyte storage tank 110 to the redox flow battery system 10 in response to the increase in pH. As a further example, the controller 88 may supply H2 gas from the integrated multi-chamber electrolyte storage tank 110 in response to a pH change that increases above a first threshold pH or decreases above a second threshold pH. In the case of an IFB, the controller 88 may supply additional H2 to increase the pH of the redox flow battery system 10.3+ This can increase the rate of reduction of ions and the rate of production of protons, thereby decreasing the pH of the positive electrolyte. Furthermore, the pH of the negative electrolyte can be increased by increasing the rate of Fe crossover from the positive electrolyte to the negative electrolyte. 3+ The pH of the negative electrolyte can be lowered by hydrogen reduction of the ion or by protons generated on the positive side crossing over into the negative electrolyte due to the proton concentration gradient and electrophoretic forces. In this way, the pH of the negative electrolyte is reduced by Fe(OH)3. 3+ The risk of precipitation of ions (crossing over from the positive electrode compartment 22) can be kept within a stable region.

[0038] Other control schemes may be implemented to control the rate of H2 gas supply from the integrated multi-chamber electrolyte storage tank 110 in response to changes in electrolyte pH or changes in electrolyte SOC detected by other sensors, such as an oxygen reduction potential (ORP) meter or optical sensor. Still further, the change in pH or SOC trigger action of the controller 88 may be based on a rate of change or a change measured over a period of time. The period of the rate of change may be predetermined or adjusted based on the time constant of the redox flow battery system 10. For example, if the recirculation rate is high, the period may be shortened and the time constant may be small so that local changes in concentration (e.g., due to side reactions or gas leaks) may be measured quickly.

[0039] The controller 88 may further execute a control scheme based on the operation mode of the redox flow battery system 10. For example, as discussed in detail with reference to Figs. 3-5, the controller 88 may adjust the operation mode of the redox flow battery system 10 according to demand or based on the operating conditions, e.g., based on the SOC of the redox flow battery system 10, the load on the system, etc. As an example, the controller 88 may command the redox flow battery system 10 to be placed in an idle state. The idle state may include a long-term idle mode and at least one short-term idle mode, which may be selected, for example, based on an operator input. When placed in the short-term idle mode, a strategy for surface management at the bipolar plate 36 in contact with the negative electrode 26 may be applied, as will be further described below.

[0040] It will be appreciated that all components except the sensors 60 and 62 and the integrated multi-chamber electrolyte storage tank 110 (and components included therein) may be considered to be included in the power module 120. Thus, the redox flow battery system 10 may be described as including the power module 120 fluidly coupled to the integrated multi-chamber electrolyte storage tank 110 and communicatively coupled to the sensors 60 and 62. In some embodiments, each of the power module 120 and the multi-chamber electrolyte storage tank 110 may be included in a single housing (not shown), such that the redox flow battery system 10 may be included as a single unit in a single location. It will be further appreciated that the positive electrolyte, the negative electrolyte, the sensors 60 and 62, the electrolyte rebalancing reactors 80 and 82, and the integrated multi-chamber electrolyte storage tank 110 (and components included therein) may be considered to be included in the electrolyte subsystem 130. Thus, the electrolyte subsystem 130 may provide one or more electrolytes to the redox flow battery cells 18 (and components included therein).

[0041] 2, a side view of an exemplary redox flow battery system layout 200 for the redox flow battery system 10 is shown. The redox flow battery system layout 200 may be housed within a housing 202 that facilitates long-distance transportation and delivery of the redox flow battery system 10. In some embodiments, the housing 202 may comprise a standard steel cargo container or cargo trailer that may be transported via rail, truck, or ship. The redox flow battery system layout 200 may include an integrated multi-chamber electrolyte storage tank 110, one or more rebalancing reactors (e.g., rebalancing reactor 80) positioned on a first side of the housing 202, a power module 210, and a power control system (PCS) 288 on a second side of the housing 202. Supports 206 and auxiliary components such as various tubing 204, pumps 230, valves (not shown in FIG. 2) may be included within the housing 202 (as further described above with reference to FIG. 1) to stabilize and fluidly connect the various components positioned therein. For example, one or more pumps 230 may be utilized to transport electrolyte from the integrated multi-chamber electrolyte storage tank 110 to one or more redox flow battery cell stacks 214 within the power module 210. Additionally, additional pumps 230 may be utilized to return electrolyte from the power module 210 to the negative electrolyte chamber 50 or the positive electrolyte chamber 52 of the integrated multi-chamber electrolyte storage tank 110.

[0042] The power module 210 may include one or more redox flow battery cell stacks 214 electrically connected in parallel and / or series. Each of the one or more redox flow battery cell stacks 214 may further include a plurality of redox flow battery cells, such as the redox flow battery cell 18 of FIG. 1, connected in parallel and / or series. In this manner, the power module 210 may be capable of supplying a range of currents and / or voltages to an external load. The PCS 288 includes the controller 88 of FIG. 1 and other electronics for controlling and monitoring the operation of the redox flow battery system 10. Furthermore, the PCS 288 may regulate and monitor the voltage supplied to the external load, as well as supply current and / or voltage from an external source for charging the power module 210. The PCS 288 may further regulate and control the operation of the redox flow battery system 10 during an idle state or mode. The redox flow battery system 10 in an idle state may include when the power module 210 is not in a charging mode or a discharging mode. As an example, the power module 210 may be in a charging mode when an external voltage or current is provided to one or more redox flow battery cells 18 of the power module 210, resulting in reduction of electrolyte at the bipolar plate 36 connected to the negative electrode(s) of the one or more redox flow battery cells 18 and plating of the reduced electrolyte. In the case of an IFB, ferrous ions may be reduced at the plating electrode(s) of the one or more redox flow battery cells 18, thereby plating iron therein during charging of the power module 210. As another example, the power module 210 may be in a discharging mode when a voltage or current is provided from one or more redox flow battery cells 18 of the power module 210, resulting in oxidation of the plated metal at the negative electrode, resulting in de-plating (e.g., loss of metal) and dissolution of the oxidized metal ions. In the case of an IFB, iron may be oxidized at the plating electrode(s) of the one or more redox flow battery cells 18, thereby dissolving ferrous ions therein during discharging of the power module 210.Further details regarding the conditions for entering and terminating battery charging and discharging, as well as the idle mode of the redox flow battery system 10, are described with reference to FIGS.

[0043] As mentioned above, a redox flow battery system may be placed in an idle state when it is not actively charging or discharging. A method for determining whether a redox flow battery system is in a charging mode, a discharging mode, or an idle state is detailed in FIG. 3 below. A redox flow battery system may be compromised in terms of charge capacity, cycling performance, and overall electrochemical performance when the system is left in an idle state in a non-zero state of charge. For example, in an idle state, the redox flow battery system is not charging or discharging, but electrolyte may continue to flow to allow the system to easily switch to one of the charge / discharge modes. However, the quality of the metal plated on at least one surface of a bipolar plate, e.g., the bipolar plate in contact with the negative electrode of the redox flow battery system, may be adversely affected when electrolyte is flowing but no current is flowing to or from the bipolar plate. For example, the plated metal (e.g., the plating layer) may crack and, in some embodiments, flake off into the electrolyte of the redox flow battery system, which may be distributed throughout the redox flow battery system, increasing the risk of contact between the metal flakes and a membrane separator, such as separator 24 of FIG. 1. In some embodiments, such contact may degrade the membrane separator. Additionally, cracks in the plated metal may increase the risk of dendrite formation, which increases the risk of shorting the cell.

[0044] The above problems may worsen with longer idling periods. For example, if the charging cycle of the redox flow battery cell is interrupted and the system goes through a period where the electrolyte continues to flow, e.g., short idle mode, when charging of the redox flow battery is resumed, cracks in the plated metal may become noticeable. In one embodiment, when the redox flow battery system operation is adjusted to a short idle mode, as shown in FIG. 4, the quality of the plating on the bipolar plates may be preserved by discharging the battery at a constant current density, while other redox flow battery system components (e.g., electrolyte, pump, etc.) may also be maintained in a state where they can be easily switched to a charge or discharge mode. A method of maintaining a constant discharge current (hereinafter referred to as trickle discharge) during the idle period to discharge the battery throughout the idle period is shown in FIG. 5. The trickle discharge current may be less than the discharge current during the discharge cycle of the redox flow battery. The conditions and operating parameters for implementing the short idle mode are further described below.

[0045] Referring now to Figure 3, a method 300 for operating a redox flow battery system is depicted. For example, the redox flow battery system may operate in a charging mode, a discharging mode, or an idle state. Instructions for carrying out the method 300, and the remaining methods contained herein, may be executed by a controller, such as the controller 88 of Figure 1, based on executable instructions stored in a non-transitory memory of the controller in conjunction with signals received from sensors of the redox flow battery system, such as the sensors described above with reference to Figure 1. The controller may use actuators of the redox flow battery system to regulate battery operation according to methods described below.

[0046] Method 300 may begin at 302, where the method includes determining, estimating, and / or measuring current battery operating parameters. The current battery operating parameters may include, but are not limited to, one or more of the battery's state of charge (SOC), power module voltage, DC current, pump activity (e.g., electrolyte pump on / off status, electrolyte pump flow rate, pump timer, etc.), electrolyte temperature, power supplied to the power module (including current and voltage), power supplied by the power module (including current and voltage), internal power demand set point, and external power demand set point, etc.

[0047] At 304, the method 300 includes determining whether a request for charging the redox flow battery system has been received. Receiving the request may indicate that the redox flow battery system is currently charging or in a charging mode, or that adjustment of the redox flow battery system from a different operating mode to the charging mode is desired. The redox flow battery system in a charging mode may include one or more redox flow battery cells of the redox flow battery system in a charging mode. The charging mode may be indicated by a set point or desired SOC for one or more redox flow battery cells being greater than an actual SOC for one or more redox flow battery cells. In another embodiment, the charging mode may be indicated by a desired SOC being greater than the actual SOC by more than a charging threshold SOC difference. Additionally or alternatively, the redox flow battery system may be charged when the DC current from the power module is positive. In one embodiment, the DC current may be positive when current is flowing into the power module from an external power source. The magnitude and direction of the DC current can be measured by determining the voltage drop across a shunt resistor electrically connected to the power module.

[0048] In an alternative embodiment, a redox flow battery system in a charging mode may be indicated by a power supply (including a supply of current and / or voltage) to the power module being greater than a charging threshold supply of power. The charging threshold supply of power may refer to a rate of power supply to the redox flow battery system that is greater than a secondary or auxiliary rate of power supply to the power module that is used to power sensors, lighting, and other auxiliary devices associated with the power module. In this manner, supplying power greater than the threshold supply of power indicates that current is being supplied directly to one or more plating electrodes, resulting in reduction of metal ions at the plating electrode surface and plating of the reduced metal at the plating electrode surface. Similarly, a charging mode may be indicated by the SOC of one or more redox flow battery cells increasing at a rate greater than a threshold rate of SOC increase as a result of supplying power to the redox flow battery system during charging.

[0049] In another example, the determination of the redox flow battery system in a charging mode may be based on the flow rate of electrolyte delivered to one or both of the negative and positive electrolyte chambers of the redox flow battery cell being greater than a charging threshold flow rate. The first threshold (negative or positive electrolyte) flow rate may refer to a flow rate greater than the flow rate of a pump used to recirculate the electrolyte during idle conditions. Pumping the electrolyte at a flow rate greater than the charging threshold flow rate may allow the flow rate of electrolyte delivered to the negative and positive electrolyte chambers to be high enough to maintain a desired charging rate. Thus, the electrolyte delivery rate may be related to the stoichiometry of the redox reactions occurring at the redox flow battery cell electrodes. As an idealized simple example, for an IFB, for every two electrons delivered during charging at the negative electrode, one ferrous ion is delivered to the negative electrolyte chamber for reduction in the negative electrolyte chamber and two ferric ions are delivered to the positive electrolyte chamber for oxidation in the positive electrolyte chamber. In this way, the electrolyte flow rates to each of the positive and negative electrolyte chambers and the charging threshold electrolyte flow rates corresponding to operation in a charging mode may not be equal. Additionally, an electrolyte flow rate in excess of the ideal stoichiometric flow rate may be provided to the redox flow battery cell to accommodate a desired charge rate to account for non-ideal mixing and losses in the system.

[0050] As a further example, during charging, the ionic species in the positive electrode compartment 22 and the negative electrode compartment 20 may change at a rate that is characteristic of being in a charging mode or may achieve a steady state value (relative to the electrolyte pump flow rate) associated with the charging mode. For example, during charging, plating of ferrous ions may result in a characteristic decrease (or a characteristic rate of decrease) in the ferrous ion concentration in the negative electrolyte compartment. Similarly, the concentrations (or concentration rate of change) of other ionic species, such as ferric ions, chloride ions, hydrogen protons (e.g., pH), and other species, may be characteristic of operating a redox flow battery cell in a charging mode. Furthermore, other electrolyte properties, such as ionic strength, pH, etc., may have characteristic steady state values ​​or rates of change that may be used to indicate that the redox flow battery system is in a charging mode. In other words, the controller may estimate and / or measure one or more species concentrations (or their rates of change), including measuring pH and / or ionic strength, and determine whether the redox flow battery system is in a charging mode based on one or a combination of those measurements exceeding a charging threshold characteristic of operating in a charging mode. Additionally, the controller may determine that the redox flow battery system is in a charging mode based on the rate of power supply to the power module, the measured DC current due to the voltage drop across a shunt resistor electrically connected to the power module, the rate of increase in SOC, the difference between the desired SOC and the actual SOC, and / or the flow rate of electrolyte to one or more redox flow battery cells, as described above.

[0051] In a further embodiment, the operation of the redox flow battery system may be adjusted to a charging mode when the SOC of one or more of the redox flow battery cells is discharged below a lower threshold SOC. The lower threshold state of charge may include when the charge of the redox flow battery cells is completely depleted. In another embodiment, the lower threshold state of charge may correspond to a SOC below which the risk of degradation of the redox flow battery cells may increase. Other conditions for entering or initiating the charging mode may include when the desired power from the external load is greater than the available power from the redox flow battery system by more than a threshold power difference. Thus, the controller may also determine that the redox flow battery system is in the charging mode when the conditions for entering or initiating the charging mode are met.

[0052] Upon determining that a request for the charging mode has been received, the method 300 proceeds to 306 to start or resume / continue charging the redox flow battery system. As described above, charging the redox flow battery system may include operating an electrolyte pump (e.g., one or more of the negative and positive electrolyte pumps 30 and 32 of FIG. 1 ) to flow electrolyte into the negative and positive electrolyte chambers of the redox flow battery at a negative and positive threshold flow rate of charging, respectively. In another example, the controller may command power to be supplied to the power module at a level greater than a charging threshold supply of power to raise the actual SOC of one or more of the redox flow battery cells to a desired SOC. Raising the SOC of one or more of the redox flow battery cells to a desired SOC may include increasing the SOC at a rate of SOC increase greater than a charging threshold rate of SOC increase. Additionally, the controller may direct actuation of one or more actuators to maintain one or more of a combination of electrolyte species concentration, pH, ionic strength, and other electrolyte properties at desired values ​​that may correspond to the redox flow battery system being in a charging mode. In one example, the desired values ​​may include exceeding thresholds specific to operation of the redox flow battery system in a charging mode.

[0053] If the redox flow battery system is not charging, the method 300 proceeds from 304 to 308 to determine whether a request to discharge the redox flow battery system has been received. Receiving a request to discharge may indicate that the redox flow battery system is in a discharge mode or that adjustment of the redox flow battery system to a discharge mode is desired. A redox flow battery system in a discharge mode may include one or more redox flow battery cells of the redox flow battery system in a discharge mode. The discharge mode may be indicated by a set point or desired SOC for one or more redox flow battery cells being less than an actual SOC for one or more redox flow battery cells. In another example, the discharge mode may be indicated by a desired SOC being less than the actual SOC by more than a threshold difference.

[0054] In one embodiment, a redox flow battery system in a discharge mode may be indicated by a supply of power (including a supply of current and / or voltage) from the power module to an external load being greater than a charging threshold supply of power. The charging threshold supply of power may refer to a rate of power supply from the redox flow battery system to an external load that is greater than a secondary or auxiliary rate of power supply to the power module used to power sensors, lighting, and other auxiliary devices associated with the power module. In this manner, supplying power from the power module that is greater than the charging threshold supply of power indicates that current is being supplied directly to the external load, resulting in oxidation of the plated metal at the plating electrode surface to metal ions and solubilization of the metal ions in the negative electrolyte compartment. Similarly, a discharge mode may be indicated by the SOC of one or more redox flow battery cells decreasing at a rate greater than a threshold rate of SOC decrease as a result of supplying power from the redox flow battery system during discharge.

[0055] Additionally or alternatively, the redox flow battery system may be in a discharge mode when the DC current from the power module is negative. In one embodiment, the DC current may be negative when current is flowing out of the power module to an external load. As described above, the magnitude and direction of the DC current may be determined by measuring the voltage drop across a shunt resistor electrically connected to the power module.

[0056] In another example, the determination of the redox flow battery system in a discharge mode may be based on the flow rate of electrolyte delivered to one or both of the negative and positive electrolyte chambers of the redox flow battery cell being greater than a discharge threshold flow rate. The discharge threshold (negative or positive electrolyte) flow rate may refer to a flow rate greater than the flow rate of the pump used to recirculate the electrolyte during idle mode. Pumping the electrolyte at a flow rate greater than the discharge threshold flow rate may allow the flow rate of electrolyte delivered to the negative or positive electrolyte chamber to be high enough to maintain a desired redox flow battery system discharge rate. Thus, the electrolyte delivery rate may be related to the stoichiometry of the redox reactions occurring at the redox flow battery cell electrodes. As an idealized simple example, in the case of an IFB, for every two electrons delivered from the redox flow battery system during discharge at the negative electrode, one ferrous ion is oxidized and two ferrous ions are delivered to the positive electrolyte chamber for reduction at the positive electrolyte chamber. In this way, the electrolyte flow rate and the discharge threshold electrolyte flow rate to each of the positive and negative electrolyte chambers corresponding to operation in a charge mode may not be equal. Additionally, an electrolyte flow rate above the ideal stoichiometric flow rate may be provided to the redox flow battery cell to accommodate a desired discharge rate to account for non-ideal mixing and losses in the system.

[0057] As a further example, while in the discharge mode, the ionic species in the positive electrode compartment 22 and the negative electrode compartment 20 may change at a rate that is characteristic of being in the discharge mode or may achieve a steady value (relative to the electrolyte pump flow rate) associated with the discharge mode. For example, during discharge, plating of ferrous ions may result in a characteristic decrease (or a characteristic rate of decrease) in the ferrous ion concentration in the negative electrolyte compartment. Similarly, the concentrations (or concentration rates) of other ionic species, such as ferric ions, chloride ions, hydrogen protons (e.g., pH), and other species, may be characteristic of operating the redox flow battery cell in the discharge mode. Furthermore, other electrolyte properties, such as ionic strength, pH, etc., may have characteristic values ​​or rates of change that may be used to indicate that the redox flow battery system is in the discharge mode. In other words, the controller may estimate and / or measure one or more species concentrations (or their rates of change), including measuring pH and / or ionic strength, and determine whether the redox flow battery system is in the discharge mode based on one or a combination of those measurements exceeding a threshold characteristic of operating in the discharge mode. Additionally, the controller may determine the discharge mode based on the rate of power supply to the power module, the rate of increase of the SOC, the difference between the desired SOC and the actual SOC, and / or the flow rate of electrolyte to one or more redox flow battery cells, as described above.

[0058] In a further embodiment, the controller can enter the discharge mode when the SOC of one or more of the redox flow battery cells is charged above an upper threshold SOC. The upper threshold state of charge can include when the redox flow battery cells are fully charged to their maximum capacity. In another embodiment, the upper threshold state of charge can correspond to an SOC above which the risk of overcharging and degradation of the redox flow battery cells may increase. Other conditions for entering or starting the discharge mode can include when the actual power delivered from the redox flow battery system to the external load is less than the desired power by more than a discharge threshold power difference. Thus, the controller can also determine that the redox flow battery system is in the discharge mode when the conditions for entering or starting the discharge mode are met.

[0059] Upon determining that a request for a discharge mode has been received, the method 300 continues from 308 to 310 to initiate or resume / continue a discharge mode of the redox flow battery system. As described above, discharging the redox flow battery system may include operating an electrolyte pump (e.g., one or more of the negative and positive electrolyte pumps 30 and 32 of FIG. 1 ) to flow electrolyte into the negative and positive electrolyte chambers of the redox flow battery at a negative and positive threshold flow rate of discharge, respectively. In another example, the controller may command a power supply from the power module to the external load that is greater than a discharge threshold supply of power to reduce the actual SOC of one or more of the redox flow battery cells to a desired SOC. Reducing the SOC of one or more of the redox flow battery cells to a desired SOC may include reducing the SOC at a SOC reduction rate that is greater than a threshold rate of SOC reduction. Additionally, the controller may direct one or more actuators to operate to maintain one or more of a combination of electrolyte species concentration, pH, ionic strength, and other electrolyte properties at desired values ​​that may correspond to the redox flow battery system being in a discharge mode. In one example, the desired values ​​may include exceeding thresholds specific to operation of the redox flow battery system in a discharge mode.

[0060] If a request to operate the redox flow battery system in a discharge mode is not received, the method 300 proceeds to 312 to place the redox flow battery system in an idle state if the system is already in an idle state or to maintain the operation mode of the redox flow battery system. In one embodiment, the redox flow battery may be in an idle state if the DC current from the power module is less than or substantially equal to the idle threshold current. In one embodiment, the idle threshold current may be zero. In another embodiment, the idle threshold current may be a non-zero value, as further described below. The method 300 proceeds to method 400 to determine an idle mode of the redox flow battery system, which may be a sub-state of the idle state.

[0061]

[0033] Referring now to Figure 4, a method 400 is shown for selecting an idle mode when the redox flow battery system is adjusted to an idle state, as described above for the method 300 of Figure 3. The idle state of the redox flow battery may also be referred to as a standby state. The method 400 may start after the method 312 of the method 300 of Figure 3 when the redox flow battery system enters the idle state.

[0062] Method 400 begins at 402, where a controller estimates and / or measures the state of charge (SOC) of the battery. The SOC may be determined based on signals from one or more sensors of the redox flow battery system, such as a signal from a hydrometer, voltage and / or current measurements, modeling of the battery system with a Kalman filter applied, etc. At 403, method 400 includes determining whether the duration of the idle period, e.g., a target period for which the redox flow battery system is idle, is known. For example, information regarding the desired length of time for the idle period may be provided by a user input at a user interface of the controller. The user may input an idle time that is less than or equal to a maximum idle time that may be determined based on the current SOC, as described below with reference to FIG. 5. The known idle time may be referred to as a set idle duration.

[0063] If the idle time is not known (in other words, the set idle duration is unknown), the method proceeds to 404 to enter a long-term idle mode. Entering a long-term idle mode may include adjusting the redox flow battery system operation to preserve the electrolyte and battery components without regard for the need to quickly enter a charge or discharge mode. Additionally, the electrodes may be preserved by purging the chamber with either argon or hydrogen gas. Additionally, the long-term idle mode may include powering down the power components of the redox flow battery, such as the electrolyte heater and electrolyte pump.

[0064] In yet other embodiments, the long idle mode may include adjusting various settings of the redox flow battery system in addition to or in place of the long idle mode strategies described above. For example, electrolyte may be completely drained from the redox flow battery when a long idle mode is anticipated.

[0065] If the duration of the idle period is known, the method proceeds to 405 to determine whether the known idle period is less than a threshold value. For example, the threshold value may be a time associated with an amount of charge (e.g., coulombs) representing the amount of iron plated on the negative electrode and bipolar plate of the redox flow battery system, such as the negative electrode 26 and bipolar plate 36 of FIG. 1. The amount of charge from plating may be predicted based on the total amount of charge (coulombs) plated on the negative electrode and the known plating efficiency of the redox flow battery system, which is determined based on the surface area of ​​the negative electrode and bipolar plate. The known idle time (in seconds) is multiplied by the idle period setting discharge current density (mA / cm) according to Equation 3 below: 2 (unit: cm) and active plating area (cm 2 ) to estimate the amount of charge (e.g., C) that can be discharged from the negative electrode and bipolar plate. idle ) (Coulombs). (known idle time)*(trickle discharge current)*(active area)=C idle (3)

[0066] Idle period set discharge constant current density (e.g., 4.0 mA / cm), also referred to herein as trickle discharge current 2 ) may be selected based on the proven ability of the discharge current magnitude to mitigate poor plating quality after an idling period, as further described below with reference to FIG. 5. The trickle discharge allows a small amount of current to flow from the negative plating electrode to the positive electrode of the redox flow battery system. In the IFB, during the application of the trickle discharge, Fe 0 is deposited on the bipolar plate connected to the plating electrode. 2+ The trickle discharge current density is, for example, 1 mA / cm 2 ~10mA / cm 2 , 2mA / cm 2 ~6mA / cm 2 , or 3.5mA / cm 2 ~5.5mA / cm 2 In one embodiment, the preferred range for trickle discharge is 3.5 mA / cm 2~5.5mA / cm 2 It could be.

[0067] C idle The estimate of C can be compared to the available charge from plating. If the known idle time is greater than or equal to the available charge from plating, idle If the idle time corresponds to a value that is less than the charge from plating, it is determined that the estimated plating amount at the bipolar plates is not sufficient to sustain a trickle discharge for the known duration of the idle period and the idle time is greater than the threshold value, and method 400 proceeds to 404 to enter a long idle mode, as described above. If the idle time corresponds to a value that is less than the charge from plating, it is determined that the estimated plating amount is sufficient to sustain a trickle discharge for the known duration of the idle period and the idle time is less than the threshold value, and method 400 proceeds to 406 to enter a short idle mode, as described below with reference to FIG.

[0068] 5, a method 500 for operating a redox flow battery system in a short idle mode is shown. Method 500 continues from 406 of FIG.

[0069] The method 500 describes a surface management strategy for the plating surface in a bipolar plate corresponding to a negative electrode based on application of a trickle discharge current for the duration of a short idle period. During trickle discharge, the plated metal in the bipolar plate connected to the (e.g., negative) plating electrode is oxidized. The plated metal is Fe when the redox flow battery is an IFB. 0 The trickle discharge current density may be set to ensure that cracking of the metal plated on the bipolar plates is mitigated upon resumption of the charge state. The trickle discharge current density may further be selected to minimize the capacity lost through discharge during idle periods while still preserving the quality of the plated surface of the bipolar plates. A possible range of trickle discharge current densities is provided above with reference to FIG. 4. In one example, the trickle discharge current density is 4 mA / cm 2In an alternative embodiment, when the idle time is less than 2 hours, the 2 For example, an example of a bipolar plate that has been left idle for an hour and a half without trickle discharge is shown in Figure 8, which exhibits uniform and continuous plating across the surface of the bipolar plate.

[0070] At 502, the method 500 includes obtaining an idle discharge set point, e.g., a preset idle discharge set point stored in a memory of the controller. For example, the controller may point to a look-up table that indicates a relationship between the current battery SOC and an amount of coulombs to be discharged, where the amount may be a target percentage of the SOC. As another example, the charge density may be input by a user through a user interface.

[0071] At 503, the method 500 includes calculating a maximum idle duration. The maximum idle duration is determined by dividing the coulombs plated, as determined from the known plating efficiency, by the idle discharge current set at the idle discharge setting in 502. In other words, Equation 3 is solved for the idle time given the idle discharge current, where C idle is the plated coulombs. At 504, the method 500 includes comparing the longest idle duration to the known idle duration. If the known idle duration is greater than or equal to the longest idle duration, the method proceeds to 505 to enter a long idle mode, as described above with reference to FIG. 4. The method 500 ends.

[0072] If the known idle duration is less than the longest idle duration, the method proceeds to 506 to set operating parameters corresponding to a short idle mode, including setting a discharge current density at 507, setting an electrolyte flow rate at 508, and setting an electrolyte temperature at 509. The discharge current density may be set to the trickle discharge current density set point obtained at 502.

[0073] Setting the electrolyte flow rate at 508 and setting the electrolyte temperature at 509 may include repeatedly cycling the operation of the electrolyte pump between an active state and an inactive state. For example, the flow rate may be cycled from zero flow rate to a maximum possible flow rate. The temperature range of the electrolyte may include a low temperature set to ambient, e.g., room temperature, below the maximum temperature that maintains the integrity of the electrolyte. When in the active state, the electrolyte pump may drive electrolyte flow through the redox flow battery system at an idle threshold flow rate. When the electrolyte pump is in the inactive state, the electrolyte pump may be shut off and the electrolyte heater set point may be reduced. The electrolyte heater set point may be reduced to room temperature. By cycling the electrolyte pump, the redox flow battery system may remain responsive to charge and discharge commands while in an idle state, and parasitic power losses due to pumping, heating, shunt currents, etc. may be reduced.

[0074] Setting the electrolyte temperature may further include setting the electrolyte temperature to a set point that reduces power loss of the redox flow battery during the short idle mode. The minimum idle threshold temperature may be based on the solubility or stability of the electrolyte during the short idle mode. For example, below the idle threshold temperature, there may be an increased risk of destabilizing the electrolyte. Destabilization of the electrolyte may include precipitation of electrolyte salts, which reduces the capacity and performance of the redox flow battery system.

[0075] At 510, method 500 includes setting a timer to the known idle duration and starting the timer. Method 500 includes verifying that the time elapsed since starting the timer reaches or has passed the known idle time. If the elapsed time as monitored by the timer is less than the known idle time, method 500 proceeds to 512 to continue monitoring the timer and maintain the operating parameters set in step 506. A corresponding amount of metal is sacrificed from the negative electrode while maintaining the discharge current set in 507. Method 500 returns to 511. If at 511 the elapsed time is greater than or equal to the known idle time, method 500 proceeds to 513 and includes terminating the discharge current. Method 500 ends.

[0076] Degradation of plating quality during idle periods can occur over a wide range of operating parameters with respect to electrolyte flow rate and temperature. For example, a redox flow battery was tested to evaluate the effect of operating parameters on plating quality on the negative electrode bipolar plate of the cell. The type, volume, and pH of the battery's negative and positive electrolytes were kept constant, while other operating parameters were varied. The experimental setup of the redox battery cell included electrolyte temperatures ranging from 48 °C to 60 °C. The electrolyte flow rate was varied between 24 mL / min and 120 mL / min. The charging current density was 29 mA / cm. 2 and 45mA / cm 2 Regarding the idle period, the duration of the idle period was varied from 30 min to 6 h, and the discharge current density was 0 mA / cm 2 to 5.0mA / cm 2 The above parameters are given as possible examples, and it will be appreciated that many other combinations of operating parameters may be implemented.

[0077] The effect of idle conditions on plating quality at the negative electrode and overall flow battery health was determined by charging the battery to a pre-set SOC, then stopping charging, placing the battery in idle mode at target operating parameters, and then resuming charging to a pre-set final SOC. After charging the redox flow battery to the final SOC, the plating on the negative bipolar plate was examined for signs of degradation.

[0078] Exemplary experimental results for evaluating the effect of application of trickle discharge current density are shown in Figures 6A-8. The results shown in Figures 6A-6D and 7A-7C indicate the lack of sensitivity of the bipolar plate coating to other redox flow battery operating conditions other than the discharge current. Therefore, other operating conditions can be optimally set during short-term idling of the redox flow battery system, for example, to reduce parasitic current losses or increase the readiness of the battery for operation in other modes.

[0079] The plating electrodes were tested at two different electrolyte flow rates, including a lower flow rate and a higher flow rate. The electrolyte flow rate could be from 0 mL / min (e.g., no electrolyte flow) to the maximum recommended flow rate for the redox flow battery system. It was demonstrated that the degradation of plating quality after 2 hours of idle at zero discharge current density was similar regardless of the electrolyte flow rate.

[0080] The effect of electrolyte flow rate on plating quality is demonstrated in an image of a first bipolar plate 600 in FIG. 6A exposed to a lower electrolyte flow rate. A close-up view of the inlet 602 where most of the plate degradation was observed is shown in FIG. 6B. A second bipolar plate 620 is shown in FIG. 6C, exposed to a higher electrolyte flow rate. A graph 650 depicting the voltage change over time, e.g., the voltage profile, for the bipolar plate shown in FIGS. 6A-6C is shown in FIG. 6D. Arrow 656 corresponds to the direction of increasing potential on the y-axis of graph 650. Arrow 658 corresponds to the direction of increasing time on the x-axis of graph 650. A first plot 652 corresponds to the first bipolar plate 600 in FIG. 6A and a second plot 654 corresponds to the second bipolar plate 620 in FIG. 6B. The first plot 652 and the second plot 654 are similar and overlap, indicating similar electrochemical performance.

[0081] The bipolar plates were also tested at two different electrolyte temperatures, including a lower temperature and a higher temperature. As mentioned above, the electrolyte temperature can be in the range between room temperature and the maximum temperature that maintains electrolyte stability during short idle periods. The bipolar plates were kept idle for 2 hours with zero current discharge for the tests performed at each electrolyte temperature. Examples of bipolar plates after testing under the above conditions are shown in Figures 7A and 7B.

[0082] A first bipolar plate 700, tested at a lower temperature, is shown in FIG. 7A and shows early evidence of cracking at the bipolar plate channel inlet and cracking within the channels. A second bipolar plate 720, shown in FIG. 7B, was tested at a higher temperature and shows cracking throughout the second bipolar plate 720. A graph 750 is shown in FIG. 7C depicting voltage versus time (e.g., voltage profile) for each of the bipolar plates shown in FIG. 7A and FIG. 7B, each of which was tested in a redox flow battery cell according to the conditions described above. For example, plot 752 of graph 750 represents the voltage profile of the bipolar plate shown in FIG. 7A, and plot 754 shows the voltage profile of the bipolar plate shown in FIG. 7B. Arrow 756 corresponds to increasing potential on the y-axis of graph 750. Arrow 758 corresponds to increasing elapsed time on the x-axis of graph 6750. As shown in graph 750, the voltage profiles of the bipolar plates overlap, indicating similar performance at both electrolyte temperatures.

[0083] The degradation of plating on the bipolar plates was also evaluated over different times at idle. The plating quality was tested for idle times of 1 hour and 1.5 hours while varying other parameters as described above with reference to Figures 6A-7C. When the idle time was kept below 1.5 hours, there was no sign of plating degradation even when the trickle discharge current density was zero. However, all tests covering different operating conditions where the idle time was 2 hours or more resulted in degradation of the deposited metal on the bipolar plates when the trickle discharge current density was zero.

[0084] The relationship between the density of the trickle discharge current and the surface quality of the bipolar plate was also experimentally determined. 2 ~5mA / cm 2 The discharge current was tested at 1mA / cm 2At low current densities such as 4 mA / cm, some plating damage may still occur despite the applied trickle discharge current. 2 It has been shown that a discharge current of 100% or more can be more effective in preventing damage to the plated surface during idle periods. Referring now to FIG. 8, a third bipolar plate 800 is shown, which was kept idle for 2 hours without applying a trickle discharge current. The third bipolar plate 800 exhibits blistering and flaking. The deterioration observed in the third bipolar plate 800 confirms that without the trickle discharge current, cracking and / or flaking can occur with various levels of severity, as evidenced by FIG. 6B, FIG. 7A, and FIG. 8. The cracking and / or flaking can be mitigated using a sufficiently high trickle discharge current during idle periods.

[0085] The technical effect of the short-term idle methods described in methods 400 and 500 is to maintain the performance of a redox flow battery system after a short-term idle period by allowing the system to discharge current over the short-term idle period. Specifically, when the operation of the redox battery system in a charging mode is interrupted and the operation is adjusted to an idle state for a relatively short period (e.g., 2 hours), cracking and flaking of the metal plating layer in the bipolar plate is inhibited. At least one surface of the bipolar plate may be electrically coupled to the negative electrode or plating electrode of the redox flow battery system, and the metal plating layer may contribute to the overall charge capacity of the redox flow battery system after completion of a charging cycle. By discharging the battery during the short-term idle period, the plating layer may be kept intact, thereby mitigating problems associated with cracking in the plating layer and distribution of metal flakes through the redox flow battery system, such as short circuits, dendrite formation, membrane degradation, and electrolyte blockage. Discharging the current may involve sacrificing a relatively small amount of charge capacity corresponding to the amount of metal plated on the bipolar plate, which may occur continuously for the duration of the short idling period as a trickle discharge current. The present discharge method may provide a low-cost, efficient strategy for maintaining the performance of a redox flow battery system.

[0086] The present disclosure also provides support for a method for a redox flow battery, comprising operating the redox flow battery in a short idle mode by discharging the redox flow battery at a constant current density for a duration of the short idle mode, where discharging the redox flow battery maintains a plating surface at the negative electrode of the redox flow battery. In a first embodiment of the method, operating the redox flow battery in a short idle mode comprises adjusting an operation mode of the redox flow battery to a state where the redox flow battery is not charging or discharging. In a second embodiment of the method, optionally including the first embodiment, operating the redox flow battery in a short idle mode comprises adjusting the operation of the redox flow battery to an idle state for a set idle duration. In a third embodiment of the method, optionally including one or both of the first and second embodiments, adjusting the operation of the redox flow battery to an idle state for a set idle duration comprises operating the redox flow battery in an idle state for a duration of 6 hours or less. In a fourth embodiment of the method, optionally including one or more of the first to third embodiments, adjusting the operation of the redox flow battery to an idle state includes adjusting an electrolyte temperature and adjusting an electrolyte flow rate. In a fifth embodiment of the method, optionally including one or more of the first to fourth embodiments, discharging the redox flow battery at a constant current density includes discharging a current at a current density lower than a current density of a current discharge used during operation of the redox flow battery in a discharge mode. In a sixth embodiment of the method, optionally including one or more of the first to fifth embodiments, discharging the redox flow battery at a constant current density includes discharging at a current density between 0 mA / cm2 and 5 mA / cm2. In a seventh embodiment of the method, optionally including one or more of the first to sixth embodiments, discharging the redox flow battery at a constant current density includes sacrificing a corresponding amount of metal plated at the negative electrode.

[0087] The present disclosure also provides support for a redox flow battery system comprising a battery cell including a negative electrode, a positive electrode, and an electrolyte; a bipolar plate electrically coupled to the negative electrode, the bipolar plate having a plated metal layer over at least one surface of the bipolar plate; and a controller including executable instructions stored in a non-transitory memory, the executable instructions, when executed, causing the controller to continuously discharge a current density in response to operation of the redox flow battery system in a short-term idle mode. In a first embodiment of the system, the bipolar plate is configured to receive a metal that forms the plated metal layer during operation of the redox flow battery system in a charge mode, and the metal is oxidized to dissolve in the electrolyte during operation of the redox flow battery system in a discharge mode. In a second embodiment of the system, optionally including the first embodiment, the metal is oxidized and dissolves in the electrolyte during operation of the redox flow battery system in a short-term idle mode. In a third embodiment of the system, optionally including one or both of the first and second embodiments, the amount of metal that dissolves during operation of the redox flow battery system in a short idle mode is less than the amount of metal that dissolves during operation of the redox flow battery system in a discharge mode. In a fourth embodiment of the system, optionally including one or more or each of the first through third embodiments, the amount of metal that dissolves in the electrolyte during operation of the redox flow battery system in a short idle mode is less than the amount of metal that is plated onto the bipolar plates during operation of the redox flow battery system in a charge mode.

[0088] The present disclosure also provides support for a method for operating a redox flow battery system in an idle state, comprising discharging a constant current density for a set idle duration to mitigate cracking and / or flaking of a plating layer at a negative electrode of the redox flow battery system in response to a set idle duration less than the longest idle duration. In a first embodiment of the method, discharging the longest idle duration is calculated based on a state of charge of the redox flow battery system and a constant current density. In a second embodiment of the method, optionally including the first embodiment, discharging the constant current density comprises comparing the set idle duration to the longest idle duration. In a third embodiment of the method, optionally including one or both of the first and second embodiments, comparing the set idle duration to the longest idle duration comprises adjusting operation of the redox flow battery system to a long idle mode when the set idle duration is longer than the longest idle duration. In a fourth embodiment of the method, optionally including one or more or each of the first through third embodiments, discharging the constant current density comprises monitoring the set idle duration using a timer. In a fifth embodiment of the method, optionally including one or more of the first through fourth embodiments, discharging the constant current density includes terminating the discharge when the set idle duration expires. In a sixth embodiment of the method, optionally including one or more of the first through fifth embodiments, the method further includes adjusting operation of the redox flow battery system to a long idle mode in response to the unknown set idle duration.

[0089] The following claims particularly point out certain combinations and subcombinations that are deemed novel and unobvious. These claims may refer to "an" element or "first" element or equivalents. Such claims should be understood to include the incorporation of one or more such elements, and do not require or exclude two or more such elements. Other combinations and subcombinations of the disclosed features, functions, elements, and / or properties may be claimed through amendment of the claims or through the presentation of new claims in this or a related application. Such claims, whether broader, narrower, equal, or different in scope to the original claims, are also deemed to be included within the subject matter of this disclosure.

Claims

1. 1. A method for a redox flow battery comprising:

1. The method of claim 1, further comprising: operating the redox flow battery in a short idle mode by discharging the redox flow battery at a constant current density for a duration of the short idle mode, wherein discharging the redox flow battery maintains a plating surface at a negative electrode of the redox flow battery.

2. 2. The method of claim 1, wherein operating the redox flow battery in the short idle mode comprises adjusting an operation mode of the redox flow battery to a state where the redox flow battery is not charging or discharging.

3. 2. The method of claim 1, wherein operating the redox flow battery in the short idle mode comprises adjusting operation of the redox flow battery to an idle state for a set idle duration.

4. 4. The method of claim 3, wherein adjusting operation of the redox flow battery to the idle state for the set idle duration comprises operating the redox flow battery in the idle state for a duration of six hours or less.

5. 4. The method of claim 3, wherein adjusting operation of the redox flow battery to the idle state comprises adjusting an electrolyte temperature and adjusting an electrolyte flow rate.

6. 2. The method of claim 1, wherein discharging the redox flow battery at the constant current density comprises discharging a current at a current density lower than a current density of a current discharge used during operation of the redox flow battery in a discharge mode.

7. Discharging the redox flow battery at the constant current density is ２ ~5mA / cm ２ 2. The method of claim 1, comprising discharging at a current density of

8. 10. The method of claim 1, wherein discharging the redox flow battery at the constant current density comprises sacrificing a corresponding amount of metal plated at the negative electrode.

9. A redox flow battery system, a battery cell including a negative electrode, a positive electrode, and an electrolyte; a bipolar plate electrically coupled to the negative electrode, the bipolar plate having a plated metal layer over at least one surface of the bipolar plate; A controller comprising executable instructions stored in a non-transitory memory, the executable instructions, when executed, causing the controller to: a controller that continuously discharges a current density in response to operation of the redox flow battery system in a short idle mode.

10. 10. The redox flow battery system of claim 9, wherein the bipolar plates are configured to receive a metal that forms the plated metal layer during operation of the redox flow battery system in a charge mode, and the metal is oxidized to dissolve in the electrolyte during operation of the redox flow battery system in a discharge mode.

11. 11. The redox flow battery system of claim 10, wherein the metal is oxidized and dissolves in the electrolyte during operation of the redox flow battery system in the short idle mode.

12. 12. The redox flow battery system of claim 11, wherein an amount of said metal that dissolves during operation of said redox flow battery system in said short idle mode is less than an amount of said metal that dissolves during operation of said redox flow battery system in said discharge mode.

13. 11. The redox flow battery system of claim 10, wherein an amount of said metal that dissolves into said electrolyte during operation of said redox flow battery system in said short idle mode is less than an amount of said metal that plates onto said bipolar plates during operation of said redox flow battery system in said charging mode.

14. 1. A method for operating a redox flow battery system in an idle state, comprising: in response to a configured idle duration that is less than the maximum idle duration; discharging a constant current density for the configured idle duration to mitigate cracking and / or flaking of a plating layer on a negative electrode of the redox flow battery system.

15. 15. The method of claim 14, wherein discharging the longest idle duration is calculated based on a state of charge of the redox flow battery system and the constant current density.

16. The method of claim 15 , wherein discharging the constant current density comprises comparing the set idle duration to the longest idle duration.

17. 17. The method of claim 16, wherein comparing the set idle duration with the longest idle duration comprises adjusting operation of the redox flow battery system to a long idle mode when the set idle duration is longer than the longest idle duration.

18. The method of claim 14 , wherein discharging the constant current density includes monitoring the set idle duration using a timer.

19. The method of claim 14 , wherein discharging the constant current density comprises terminating the discharging when the set idle duration expires.

20. 15. The method of claim 14, further comprising adjusting operation of the redox flow battery system to a long idle mode in response to an unknown set idle duration.

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