System and process for rebalancing the state of charge of a flow battery

Abstract

Disclosed herein are improvements to flow battery systems that maintain the state of charge of such batteries while maintaining osmotic pressure within the battery itself. Disclosed herein are flow batteries and methods for maintaining their state of charge that do not require flammable hydrogen reservoirs or complex power supplies.

Description

[Technical field]

[0001] This application was filed as a PCT international patent application on November 16, 2022, and claims the benefit of priority to U.S. Provisional Patent Application No. 63 / 279,928, filed on November 16, 2022, the entire disclosure of which is incorporated herein by reference in its entirety.

[0002] The present invention relates generally to energy storage. More specifically, embodiments relate to electrochemical rebalancing systems, devices, and methods for regulating the state of charge of redox flow battery reactants. [Background technology]

[0003] A flow battery reacts two liquid or solution-phase chemicals, a negative electrolyte (also called anolyte or negolyte) and a positive electrolyte (also called catholyte or polyte), at the negative and positive electrodes, respectively, of a device called a flow battery stack, which consists of one or more flow battery cells. These flow battery cells are electrochemical cells. The battery stack extracts electrical energy from the chemical reaction. Spent chemicals are stored in separate tanks and can be recharged with electricity from a source such as a solar panel, allowing the chemical reaction to proceed in reverse.

[0004] Because the same stack operates as a galvanic cell during discharge of the flow battery and as an electrolytic cell during charging of the flow battery, for clarity, the remainder of this document will not use the terms anode, cathode, anolyte, or catholyte, since the same electrode may be called an anode or cathode depending on whether the cell is being charged or discharged. Instead, the terms negative electrode, positive electrode, negolite, or posolite will be used.

[0005] Flow batteries have an advantage over solid-electrode batteries such as lithium-ion in that they have a long energy storage period. Rather than spending money on unnecessarily large flow battery stacks, a longer duration battery can be created simply by using larger tanks of posolite or negolite. One practical consideration for flow batteries operating under realistic conditions is the effect that atmospheric oxygen has on the reactants over time. Conceptually, flow batteries are closed systems that are not open to the outside air, but in reality, oxygen from the air can diffuse through the walls of the plastic electrolyte tank or enter through microscopic leaks or cracks that may exist throughout the flow battery system, including the electrolyte tank, tubing or piping, tube fittings, pumps, and the flow battery cells or stacks.

[0006] Many examples of flow batteries use at least one reactant that reacts with oxygen in at least one redox state. For example, the negolites of flow batteries, including viologens, quinones, chromium, etc., are easily converted from the reduced form to the oxidized form as a result of reaction with oxygen. Furthermore, the dissolved oxygen in the negolites can be reduced to hydroxides at the negative electrode of the battery. Summary of the Invention

[0007] The detrimental effect of oxygen on the long-term operation of a flow battery is illustrated in FIG. 1. For simplicity, in this diagram of a flow battery, sufficient amounts of negolite and posolite are provided in each electrolyte to store 100 ampere-hours (Ah) of charge. Negolite and posolite are circulated in and out of the flow battery stack (rectangular section between the negolite and posolite tanks, associated pumps not shown) and contact the negative and positive electrodes (not shown), respectively. An ion-selective membrane (not shown) separates the negative and positive electrodes, preventing the negolite and posolite from mixing while allowing the charge to flow in the form of small ions. These ions include protons (or hydrogen ions), alkali metal ions, halide ions, sulfate ions, ammonium ions, perchlorate ions, etc. These ions can have the same charge sign (positive or negative) as the redox-active species in the posolite and negolite, or they can have a different charge sign. The exact ion or combination of ions that pass through the membrane will vary depending on the type of membrane (such as anion-selective or cation-selective membrane), the pH of the negolite and posolite, and the composition of the negolite and posolite solutions themselves.

[0008] At the fully discharged state (Figure 1a), negolite exists in a fully oxidized state and posolite exists in a fully reduced state (represented here as 0 / 100 Ah). When an external potential is applied to the negative and positive electrodes of the flow battery stack, electrons are extracted from posolite (i.e., posolite is oxidized) and transferred to negolite (i.e., negolite is reduced), charging the flow battery. Thus, electrons flow from the positive electrode to the negative electrode through the external circuit, and ions flow from posolite to negolite through the ion-selective membrane, or in the opposite direction depending on the sign of the ion, maintaining charge neutrality. At the fully charged state (represented as 100 / 100 Ah, Figure 1b), negolite exists in a fully reduced state and posolite exists in a fully oxidized state. Negolite or posolite may each have more than one accessible redox state, so the term "fully" may not be accurate in that case, but in this example each has two accessible redox states. Finally, the charged flow battery is discharged by the flow of electrons and ions in the opposite direction to the charging process, returning it to its original discharged state (0 / 100Ah, Figure 1c). Thus, the capacity of this example flow battery is 100Ah.

[0009] Complications arise when oxygen reacts with negolite and / or posolite. In the most common case, as mentioned above, negolite in the reduced state can be reoxidized and released by oxygen. In this case, during the process of charging (Figure 1d), storage (not shown), and discharging (Figure 1e), the effect of oxygen on the reduced negolite causes a portion of the negolite to be discharged. This change is reversible, but creates an imbalance in the state of charge (SOC) of each electrolyte, reducing the net capacity of the flow battery to 98 Ah. The SOC of an electrolyte solution is the percentage of the total redox-active material in the solution that is in a "charged" or more energized state. In the case of negolite, it is the percentage of redox-active negolite that is reduced, and in the case of posolite, it is the percentage of redox-active posolite that is oxidized. For example, if the negolite solution contains 20% reduced active material and 80% oxidized active material, the negolite SOC will be 20%. If 25% of the active material in the posolite solution is in a reduced state and 75% is in an oxidized state, then the posolite SOC is 75%. The SOC of a redox flow battery is the charge stored in the redox flow battery that can be removed by discharging the flow battery, expressed as a percentage of the maximum charge that can be stored in the flow battery. Thus, a redox flow battery with a negolite capacity of 20 / 100Ah and a posolite capacity of 60 / 100Ah will have a negolite capacity of 0 / 100Ah and a posolite capacity of 40 / 100Ah when fully discharged, and a negolite capacity of 60 / 100Ah and a posolite capacity of 100 / 100Ah when fully charged. Thus, the capacity of a flow battery containing these negolite and posolite solutions will be 60Ah, and the SOC of the flow battery will be 20 / 60=33.3%. One feature of flow batteries is that the amounts, volumes, or charge capacities of the posolites and negolites do not have to be equal. One can be greater than the other. In Figure 1, equal capacities are shown for clarity.

[0010] Ultimately, after many cycles in the presence of oxygen, the SOC imbalance between posolite and negolite will grow and significantly reduce the capacity of the flow battery, even if no redox-active material is lost due to leakage, decomposition, etc. (Figure 1f). Certain passive methods, such as improving system seals to exclude oxygen or pressurizing the headspace of the posolite and negolite reservoirs with inert gases, can slow but not reverse the effects of the SOC imbalance. Thus, a system or process that removes the SOC imbalance between posolite and negolite (or maintains the SOC balance) is needed for the sustained long-term operation of flow batteries under real-world conditions.

[0011] There are other mechanisms that can cause SOC imbalances in flow battery systems through parasitic reactions, such as hydrogen evolution at the negative electrode during charging or oxygen evolution at the positive electrode during charging. This is common in flow batteries that use metal reactants that act as electrocatalysts for side reactions, such as iron / iron chemistries (e.g., Fe(0) / Fe(II) negolite, Fe(II) / Fe(III) posolite), or reactants that may contain certain metal contaminants that are electrocatalysts for these side reactions. Some flow battery chemistries are designed to plate metals such as zinc or iron at the negative electrode during charging, and the plated metal is corroded by acidic or basic electrolytes, resulting in the evolution of hydrogen gas. Several types of rebalancing cells have been reported to date. Some utilize hydrogen gas as a chemical or electrochemical rebalancing reactant, others use iron ions or metallic iron in an acidic medium as a rebalancing reactant, and still others use a photochemical cell in addition to the chemical or electrochemical cell. Other rebalancing cells focus on restoring the proper pH of the negolite and posolite solutions by incorporating a bipolar membrane into a three-chamber rebalancing cell and flushing either negolite or posolite (but not both) through the electrode compartments of the rebalancing cell, without significantly altering the SOC of either electrolyte solution.

[0012] One of the main advantages of aqueous-organic flow batteries is the non-flammability of the system. Thus, using a rebalancing system that uses flammable hydrogen gas as a rebalancing reactant defeats the purpose of such a system and limits its use in environments where non-flammability is a requirement or advantage. It is similarly inconvenient to provide an external supply of rebalancing reactants, flammable or not. In addition, photochemical rebalancing cells also require a light source, which is not always practical and costly.

[0013] It is therefore desirable to provide a system that can maintain the SOC of aqueous-organic flow batteries without the added flammability, complexity, or cost as mentioned above. Recent advances in aqueous-organic battery technology include methods that do not require an external rebalancing reagent. Both methods couple oxygen evolution from the positive electrode with the reduction of negolite or negolite. As one such method, Poli et al. discovered a rebalancing technique for vanadium redox flow batteries (see Poli et al., “A new electrolyte rebalancing method for vanadium redox flow batteries,” 405Chem. Eng. J 126583 (2021)). In this method, a portion of posolite is fed into a rebalancing cell. The posolite contacts the positive and negative electrodes of the rebalancing cell, and an electric current reduces the SOC of vanadium species in the negative electrode of the rebalancing cell, while oxygen evolution occurs at the positive electrode of the rebalancing cell, which contains iridium(IV) oxide (IrO2) as an oxygen evolution catalyst.

[0014] However, this method also has drawbacks. The Coulombic efficiency of rebalancing is low at about 80%; in comparison, the average Coulombic efficiency of flow batteries is typically above 99%. The low Coulombic efficiency of this process is believed to be due to two causes: (a) over-reduction at the negative electrode of the rebalancing cell, and (b) the posolite active material in contact with the positive electrode of the rebalancing cell. When the posolite SOC is less than 100%, part of the current at the positive electrode of the rebalancing cell flows in the direction of reducing the posolite rather than oxidizing water to generate oxygen. Secondly, the rebalancing system requires monitoring the posolite SOC using UV / Vis spectrophotometers and numerical models to determine the optimal end point of rebalancing, which is laborious and costly. Finally, the IrO2 oxygen-evolving electrocatalyst is also very expensive, so the cost of the rebalancing cell is comparable to the cost of the parent flow battery system itself.

[0015] In the second method reported by Paez et al., the redox flow battery cell itself is used as the rebalancing cell, rather than an external system (see Paez et al., “Mitigating Capacity Fading in Aqueous Organic Redox Flow Batteries by a Simple Electrochemical Charge-Balancing Protocol,” J. Power Sources. 2021, 512, 230516). There, a quinone or phenazine negolite pairs with a ferrocyanide / ferricyanide posolite (Fe(CN)) at a strongly alkaline pH. The absorbed oxygen is released at the positive electrode of the flow battery cell by applying a charging voltage much higher than the typical charging voltages that are usually encountered when charging a flow battery. This voltage is high enough to trigger oxygen evolution at the positive electrode. The authors of this report used graphite felt as the positive electrode, but also raised the possibility of including an oxygen evolution reaction (OER) catalyst, such as Ni(OH)2, at the positive electrode to promote oxygen evolution.

[0016] This method has several further drawbacks. First, the same electrodes are used for charging and discharging the redox flow battery, as well as the rebalancing process, so the rebalancing cannot be performed simultaneously with cycling the redox flow battery. Second, the positive electrode material in this report (carbon) is generally not compatible with the high voltages required for oxygen evolution and tends to oxidize itself in the process.

[0017] In both above examples combining the rebalancing process with oxygen evolution at one electrode, the rebalancing process indeed restores the electrochemical balance between the negolite and posolite reservoirs, but at the cost of creating an osmotic imbalance between the two reservoirs. According to the half-cell chemical equation below, oxygen reduction at the negative or cathode introduces hydroxide ions into the negolite solution, while oxygen evolution at the positive electrode depletes hydroxide ions from the positive electrode solution. ·Negative electrode: O2+2H2O+4e - ⇒ 4OH - (OH - ions are produced) Positive electrode: 4OH - ⇒ O2+2H2O+4e - (OH - ions are consumed)

[0018] The net result is an increase in the osmotic pressure of negolite and a decrease in the osmotic pressure of posolite, which results in water movement from posolite to negolite, diluting negolite and concentrating posolite, and ultimately leading to cell damage. Indeed, Paez et al. identify undesirable water transport due to this osmotic imbalance as a major unsolved problem.

[0019] Finally, it has been reported that some quinone reactants, when used as negolites in redox flow batteries, can restore lost redox flow battery capacity through a decomposition process that can be reversed by aeration or electrochemical oxidation. Rebalancing cells capable of restoring negolite capacity may also be useful, but have not yet been reported.

[0020] It is therefore desirable to increase the SOC of aqueous organic batteries without introducing an osmotic imbalance or causing degradation, thereby preserving the battery's life and utility. [Brief description of the drawings]

[0021] In the following description, reference is made to the following figures, in which the same reference numbers may be used to identify similar / identical components in multiple figures. However, when a number is used to refer to a component in a particular figure, it is not intended to limit the component in another figure that is labeled with the same number. Additionally, the figures are not necessarily drawn to scale.

[0022] [Figure 1] We demonstrate the charging and discharging of an idealized flow battery and how the influence of oxygen builds up an SOC imbalance between negolite (the anode active material) and posolite (the cathode active material), which has the effect of reducing the capacity of the flow battery. [Diagram 2] FIG. 1 is a schematic diagram of a redox flow battery and a rebalancing cell, where the rebalancing cell is configured to act on a negolite solution and another supporting electrolyte solution, according to certain embodiments. [Diagram 3] FIG. 1 is a schematic diagram of a redox flow battery and rebalancing cell configured to operate on a posolite solution and another supporting electrolyte solution, in accordance with certain embodiments. [Figure 4] FIG. 1 is a schematic diagram of a redox flow battery and rebalancing cell configured to operate on a negolite solution according to certain embodiments. [Diagram 5] FIG. 1 is a schematic diagram of a redox flow battery and rebalancing cell configured to operate on a posolite solution in accordance with certain embodiments. [Figure 6] FIG. 1 is a schematic diagram of a redox flow battery with a rebalancing cell integrated into a negolite tank and configured to act on a negolite solution, according to certain embodiments. [Figure 7] FIG. 1 is a schematic diagram of a redox flow battery with a rebalancing cell integrated into a potholite tank and configured to act on a potholite solution, according to certain embodiments. [Figure 8] FIG. 1 is a schematic diagram of a redox flow battery and a rebalancing cell, where the rebalancing cell is configured to operate on negolite and posolite solutions, according to certain embodiments. [Figure 9] FIG. 1 is a schematic diagram of a redox flow battery and a rebalancing cell, where the rebalancing cell is configured to operate on negolite and posolite solutions, according to certain embodiments. [Figure 10] FIG. 1 is a schematic diagram of a redox flow battery and a rebalancing cell, where the rebalancing cell is configured to operate on negolite and posolite solutions, according to certain embodiments. [Figure 11] FIG. 1 is a schematic diagram of a redox flow battery and a rebalancing cell, where the rebalancing cell is configured to operate on negolite and posolite solutions, according to certain embodiments. [Figure 12] 1 is a flowchart of a method according to certain embodiments. [Figure 13] 1 is a flowchart of a method according to certain embodiments. [Figure 14] 1 is a flowchart of a method according to certain embodiments. [Figure 15] 1 shows the capacity and coulombic efficiency of a cycling cell with intermittent rebalancing. DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0023] Considering the previously reported redox flow battery rebalancing cells and systems, a relatively inexpensive and easy-to-operate rebalancing cell is highly desirable for the commercial deployment of redox flow battery systems.

[0024] Described herein is a first electrochemical system including a redox flow battery and a second electrochemical system including a rebalancing cell. Redox flow batteries can include a wide variety of chemistries of negolites and posolites, including vanadium-vanadium, zinc-bromine, chromium-iron, iron-iron, metal complexes of metal complexes, metal complexes of ferrocyanide / ferricyanide (Fe(CN)), quinones of Fe(CN), viologen derivatives of ferrocene derivatives, and many other examples in the prior art. Either or both of the negolite and posolite solutions may include one or more supporting electrolytes, which are typically added to ensure the correct pH of the solutions and to improve the electrical conductivity of the negolite and posolite solutions. Examples of supporting electrolytes include strong acids such as sulfuric acid and hydrochloric acid, strong bases such as sodium hydroxide and potassium hydroxide, neutral salts such as sodium sulfate and potassium chloride, and pH buffers such as potassium dihydrogen phosphate, dipotassium hydrogen phosphate, and sodium bicarbonate. Negolite and posolite solutions may also contain additives that improve the solubility of the active substance in the solution. The solvent for negolite and posolite solutions is water.

[0025] The amounts, volumes, or charge capacities of the negolite solution and the posolite solution do not have to be equal or substantially equal to each other. They can be any amount. In some embodiments, it is advantageous to oversupply the redox flow battery with either negolite or posolite. In this way, the redox flow battery can have more parasitic reactions before rebalancing is required. This allows rebalancing to be performed less frequently. In the above example, going from Figure 1a → Figure 1d → Figure 1e → Figure 1f, posolite quickly becomes the capacity limiting side of the redox flow battery. If the posolite capacity was 110Ah instead of 100Ah, the system would have tried to maintain the accessible capacity at the full 100Ah until the posolite reached a 10 / 110Ah SOC with the flow battery fully discharged, rather than losing the accessible capacity from the beginning.

[0026] A redox flow battery can consist of just a single electrochemical cell or can have multiple cells of the same or different sizes arranged in series or parallel to form an electrochemical stack, and hereafter any use of the terms "cell" or "stack" when referring to this or any other electrochemical system should be understood to mean one or more cells.

[0027] The rebalancing cell is composed of a rebalancing cell negative electrode and a rebalancing cell positive electrode, separated by zero, one, or two separators to define one, two, or three separate chambers, respectively. The separators may each be a microporous separator (e.g., Celgard), an anion exchange membrane, a cation exchange membrane, a bipolar membrane, or any combination thereof. The rebalancing cell is configured to accept a fluid input from a negolite or posolite reservoir into at least one chamber of the system (including the negative electrode of the rebalancing cell) and return the fluid to the same negolite or posolite reservoir. Thus, fluid moving from the negolite reservoir passes through the negative electrode of the rebalancing cell and is returned to the negolite reservoir, or fluid moving from the posolite reservoir passes through the negative electrode of the rebalancing cell and is returned to the posolite reservoir. In some embodiments, the rebalancing cell may be fluidly reversibly disconnected and reconnected from the negolite or posolite reservoir by a valve or similar flow controller as needed. In some embodiments, the rebalancing cell is not separate from either the negolite or posolite reservoir, but is integrated into one of the negolite or posolite reservoirs. In other embodiments, the rebalancing cell is integrated into one of the negolite or posolite reservoirs as before, but the electrodes may be withdrawn, surrounded by air or an inert gas, or a valve may be incorporated into the negolite or posolite reservoir to prevent the electrodes from contacting the negolite or posolite solution. In some embodiments where the rebalancing cell is integrated into the negolite or posolite reservoir, negolite circulates around the negative electrode of the rebalancing cell, and rebalancing of the cell positive electrode may be performed by an additional pump, the existing pump of the redox flow battery, a magnetic stirrer, a mechanical stirrer, agitator, etc. The chamber containing the rebalancing cell positive electrode is separate from the chamber containing the rebalancing cell negative electrode and is configured to receive the same fluid streams as the chamber containing the rebalancing cell negative electrode, or the solution contains only supporting electrolyte and does not contain negolite or posolite active material.

[0028] When a rebalancing cell has two separators, the middle chamber defined by the two separators on either side is configured to receive the same fluid stream as the chamber containing the rebalancing cell negative electrode, or to receive the same fluid stream as the chamber containing the rebalancing cell positive electrode, or the solution contains only supporting electrolyte and no negolite or posolite active material. The central chamber of the rebalancing cell does not contain either a positive or negative electrode. Instead, these two electrodes are located in two different chambers or side chambers on either side of the central chamber of the rebalancing cell. When a three-chamber, two-separator rebalancing cell is used, in each case the negolite solution flows into one of the side chambers and the posolite solution flows into the other side chamber.

[0029] The negative electrode of the rebalancing cell is configured to provide electrons to the active material in the stream of posolite or negolite in contact with the electrode, thereby performing an electrochemical reduction on the active material. This has the effect of raising the SOC of the negolite stream or lowering the SOC of the posolite stream. The rebalancing cell negative electrode may be composed of a metallic material such as gold, platinum, stainless steel, titanium, nickel, or a conductive carbon material such as carbon felt, carbon foam, carbon paper, glassy carbon, graphite felt, carbon black, carbon nanotubes, etc. Optionally, an electrocatalyst may be present on the negative electrode of the rebalancing cell. In some embodiments, the rebalancing cell negative electrode comprises the same material as the redox flow battery negative electrode when configured to receive negolite, or the same material as the redox flow battery positive electrode when configured to receive posolite.

[0030] The positive electrode of the rebalancing cell is configured to accept electrons from the solvent (water) in the solution in contact with the electrode, thereby performing electrochemical oxidation. This has the effect of generating gaseous oxygen at the positive electrode of the rebalancing cell. Depending on other species present in the solution in contact with the positive electrode of the rebalancing cell, other electrochemical reactions may occur simultaneously with oxygen generation, such as oxidation of active substances in the stream of posolite or negolite. The rebalancing cell positive electrode may be composed of metallic materials such as gold, platinum, stainless steel, titanium, nickel, or conductive carbon materials such as carbon felt, carbon cloth, carbon foam, carbon paper, glassy carbon, graphite felt, carbon black, carbon nanotubes, etc. Optionally, an electrocatalyst may be present on the rebalancing cell positive electrode. In some embodiments, when the stream in contact with the rebalancing cell positive electrode is at an alkaline pH, the electrode comprises a nickel-iron alloy that includes nickel-iron oxyhydroxide, which functions as a hydroxide electrocatalyst. In embodiments where the rebalancing cell positive electrode is integrated into the same reservoir as the negolite or posolite, a conduit, tube, guide, etc. may be provided to capture, collect, and direct the evolved oxygen gas bubbles out of the reservoir through an outlet. Regardless of the exact configuration, this outlet is ideally located close to the positive electrode of the rebalancing cell to minimize the distance the bubbles must travel before escaping the system.

[0031] When a sufficiently high potential is applied to the electrodes of the rebalancing cell, the net effect is to raise the SOC of the input negolite stream or lower the SOC of the input posolite stream, and oxygen is produced as another gas output from the rebalancing cell and the redox flow battery. The potential may be applied galvanostatically (i.e., constant current), potentiostatically (i.e., constant voltage), as a current or voltage pulse, as a series of constant current or voltage steps, or any combination thereof. The oxygen outlet may be provided with a one-way valve, such as a check valve. The outlet may be completely passive or may be optionally equipped with a pump to extract the generated oxygen gas. The rebalancing cell can be operated continuously (at constant or variable current or constant or variable voltage), at scheduled times (e.g., once a day, once a week, once a month, once a year, once per charge / discharge cycle of the redox flow battery, once per 10 cycles of the redox flow battery, once per 100 cycles of the redox flow battery, once per 1000 cycles of the redox flow battery, etc.), or intermittently as needed, or combinations thereof, to maximize the accessible capacity of the connected redox flow battery. In certain embodiments, the average rate of oxygen production from the rebalancing cell positive electrode is approximately equal to the rate of oxygen absorption and reduction occurring in the redox flow battery, so that the SOC balance between negolite and posolite is significantly extended or preserved indefinitely.

[0032] In some embodiments where the rebalancing cell is configured to perform oxygen evolution (water oxidation) on the negolite solution, the rebalancing cell may also be used simultaneously to decompose degraded negolite reactants back to the original negolite reactants by electrochemical oxidation. The rebalancing cell may also be optionally operated at a lower voltage (e.g., below the water splitting potential of 1.23 V) to recover lost negolite capacity without reforming the negolite SOC by oxygen evolution as described above.

[0033] FIG. 2 shows a redox flow battery 200 and a rebalance cell 210 according to some embodiments of the present invention. The flow battery 200 is composed of a negolite tank 201 filled with a negolite solution 202, a posolite tank 203 filled with a posolite solution 204, and a redox flow battery stack 205 connected to an external load or power source 206. The negolite solution 202 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 204 can be a negatively charged Fe(CN) solution dissolved in water. The negolite solution 202 and the posolite solution 204 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The rebalance cell 210 includes a first chamber 211 and a second chamber 212 separated by an ion selective membrane 213. Those skilled in the art will appreciate that the ion selective membrane can include multiple subcategories, including but not limited to cation exchange membranes and bipolar membranes. In one embodiment, the ion selective membrane 213 can be a cation exchange membrane that allows potassium or sodium ions to pass freely. The first chamber 211 contains a first electrode 214 and the second chamber 212 contains a second electrode 215. The first electrode 214 can include a conductive carbon material such as carbon cloth without other electrocatalysts and the second electrode 215 can include nickel / iron oxyhydroxide. The two electrodes 214, 215 are connected to an external power source 216.

[0034] The negolite solution 202 is circulated from the negolite tank 201 to the first chamber 211 and back to the negolite tank 201. The solution of supporting electrolyte 221 is circulated from the supporting electrolyte tank 220 to the second chamber 212 and back to the supporting electrolyte tank 220.

[0035] When an appropriate potential is applied between the electrodes 214, 215 of the rebalancing cell 210 using an external power source 216, the negolite solution 202 is reduced (its SOC increases) at the first electrode 214. At the same time, water (or hydroxide ions) in the supporting electrolyte solution 221 is oxidized at the second electrode 215 to form oxygen gas, which escapes the rebalancing cell 210 to the outside of the system 230. If the ion selective membrane 213 is a cation or anion exchange membrane, the cations or anions, respectively, flow through the ion selective membrane, balancing the charge of both reservoirs. If the ion selective membrane 213 is a bipolar membrane, protons and hydroxide ions are generated on the opposite faces of the bipolar membrane depending on how the bipolar membrane is arranged in the rebalancing cell 210.

[0036] This operation is preferably performed when the SOC of the negolite solution 202 is less than 100% (e.g., <99%, <90%, <80%, <60%, <40%, <20%, <10%, or 0%) so that the first electrode 214 can continue to receive electrons. When the SOC imbalance between the negolite solution 202 and the posolite solution 204 falls below a threshold, the electrical potential provided by the external power source 216 may be altered, reduced, or turned off.

[0037] In the rebalancing process, water tends to be transported from the supporting electrolyte 221 solution to the negolite solution 202 by osmosis, since hydroxide ions are formed in the negolite solution 202 by reaction with oxygen and consumed in the supporting electrolyte 221 solution during the rebalancing process. This can be addressed by equipping the supporting electrolyte tank 220 with a concentrated solution of the supporting electrolyte 221 (e.g., 2 molar or more) and having a higher osmotic pressure than the negolite solution 202. Then, when the negolite solution 202 and the supporting electrolyte solution 221 are flowed through the rebalancing cell 210 in the absence of a potential applied by the external power source 216, water moves from the negolite solution 202 and the supporting electrolyte solution 221 by osmosis, thereby preventing water absorption in the opposite direction expected from the rebalancing process.

[0038] FIG. 3 shows a redox flow battery 300 and a rebalance cell 310 according to some embodiments of the present invention. The flow battery 300 includes a negolite tank 301 filled with a negolite solution 302, a posolite tank 303 filled with a posolite solution 304, and a redox flow battery stack 305 connected to an external load or power source 306. The negolite solution 302 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 304 can be a negatively charged Fe(CN) solution dissolved in water. The negolite solution 302 and the posolite solution 304 can use potassium hydroxide, sodium hydroxide, or a mixture of both as the supporting electrolyte. The rebalance cell 310 includes a first chamber 311 and a second chamber 312 separated by an ion selective membrane 313. One skilled in the art will recognize that ion selective membranes can include multiple subcategories, including but not limited to cation exchange membranes, anion exchange membranes, proton exchange membranes, and bipolar membranes. In one embodiment, the ion selective membrane 313 can be a cation exchange membrane that freely passes potassium or sodium ions. The first chamber 311 contains a first electrode 314 and the second chamber 312 contains a second electrode 315. The first electrode 314 can include a conductive carbon material, such as carbon cloth, without other electrocatalysts, and the second electrode 315 can include nickel / iron oxyhydroxide. The two electrodes 314, 315 are connected to an external power source 316.

[0039] The posolite solution 304 is circulated from the posolite tank 303 to the first chamber 311 and back to the posolite tank 303. The solution of supporting electrolyte 321 is circulated from the supporting electrolyte tank 320 to the second chamber 312 and back to the supporting electrolyte tank 320.

[0040] When an appropriate potential is applied across the electrodes 314, 315 of the rebalancing cell 310 using an external power source 316, the posolite solution 304 is reduced (its SOC decreases) at the first electrode 314. At the same time, water (or hydroxide ions) in the supporting electrolyte solution 321 is oxidized at the second electrode 315 to form oxygen gas, which escapes the rebalancing cell 310 to the outside of the system 330.

[0041] This operation is preferably performed when the SOC of the posolite solution 304 is greater than 0% (e.g., >1%, >10%, >20%, >40%, >60%, >80%, >90%, or 100%) so that the first electrode 314 can continue to receive electrons. When the SOC imbalance between the posolite solution 302 and the posolite solution 304 falls below a threshold, the electrical potential provided by the external power source 316 can be changed, reduced, or turned off.

[0042] FIG. 4 shows a redox flow battery 400 and a rebalance cell 410 according to some embodiments of the present invention. The flow battery 400 includes a negolite tank 401 filled with a negolite solution 402, a posolite tank 403 filled with a posolite solution 404, and a redox flow battery stack 405 connected to an external load or power source 406. The negolite solution 402 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 404 can be a negatively charged Fe(CN) solution dissolved in water. The negolite solution 402 and the posolite solution 404 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The rebalance cell 410 includes a first chamber 411 and a second chamber 412 separated by an ion selective membrane 413. The ion selective membrane 413 can be a cation exchange membrane that allows potassium ions or sodium ions to pass freely. The first chamber 411 contains a first electrode 414 and the second chamber 412 contains a second electrode 415. The first electrode 414 may include a conductive carbon material such as carbon cloth without other electrocatalysts, and the second electrode 415 may include nickel / iron oxyhydroxide. The two electrodes 414, 415 are connected to an external power source 416.

[0043] The Negolite solution 402 is circulated from the Negolite tank 401 to both the first and second chambers 411, 412 and from both chambers 411, 412 back to the Negolite tank 201 again.

[0044] When an appropriate potential is applied across the electrodes 414, 415 of the rebalancing cell 410 using an external power source 416, the negolite solution 402 is depleted (its SOC is increased) at the first electrode 414. At the same time, the water (or hydroxide ions) in the negolite solution 402 is oxidized at the second electrode 415 to form oxygen gas, which escapes from the rebalancing cell 410 to 430 outside the system.

[0045] This operation is preferably performed when the SOC of the negolite solution 402 is close to or at 0% (e.g., <40%, <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%) so that the first electrode 414 can continue to receive electrons, so that the proportion of the negolite solution 402 that is reoxidized at the second electrode 415 is small compared to the amount of oxygen that is generated. This also has the advantage of minimizing the reaction of the oxygen generated from the second electrode 415 with the negolite solution 402 before it escapes from the system to 430. When the SOC imbalance between the negolite solution 402 and the posolite solution 404 falls below a threshold, the potential provided by the external power source 416 can be changed, reduced, or turned off.

[0046] Because hydroxide ions are formed in the negolite solution 402 by reaction with oxygen, but are consumed at the second electrode 415 in the same solution, there is no net change in the ionic strength (or osmotic potential) of the negolite solution 402 as a result of oxygen absorption and subsequent rebalancing to produce oxygen from the same solution. Thus, in this embodiment, no long-term net transport of water occurs from the negolite solution 402 to the posolite solution 404 or vice versa. This configuration of the rebalancing cell 410 may also be used to convert depleted negolite 402 back to active negolite and restore lost capacity.

[0047] FIG. 5 shows a redox flow battery 500 and a rebalance cell 510 according to some embodiments of the present invention. The flow battery 500 includes a negolite tank 501 filled with a negolite solution 502, a posolite tank 503 filled with a posolite solution 504, and a redox flow battery stack 505 connected to an external load or power source 506. The negolite solution 502 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 504 can be a negatively charged Fe(CN) solution dissolved in water. The negolite solution 502 and the posolite solution 504 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The rebalance cell 510 includes a first chamber 511 and a second chamber 512 separated by an ion selective membrane 513. The ion selective membrane 513 can be a cation exchange membrane that allows potassium ions or sodium ions to pass freely. The first chamber 511 contains a first electrode 514 and the second chamber 512 contains a second electrode 515. The first electrode 514 may contain a conductive carbon material such as carbon cloth without other electrocatalysts and the second electrode 515 may contain nickel / iron oxyhydroxide. The two electrodes 514, 515 are connected to an external power source 516.

[0048] The posolite solution 504 is circulated from the posolite tank 503 to both the first and second chambers 511, 512 and from both chambers 511, 512 back to the posolite tank 503 again.

[0049] When an appropriate potential is applied across the electrodes 514, 515 of the rebalancing cell 510 using an external power source 516, the posolite solution 504 is depleted (its SOC decreases) at the first electrode 514. At the same time, the water (or hydroxide ions) in the posolite solution 504 is oxidized at the second electrode 515 to form oxygen gas, which escapes the rebalancing cell 510 to the outside of the system at 530.

[0050] This operation is preferably performed when the SOC of the posolite solution 504 is close to or at 100% (e.g., >60%, >80%, >90%, >95%, >98%, >99%, >99.5%, >99.8%, >99.9%, or 100%) so that the first electrode 514 can continue to receive electrons, thereby reducing the percentage of the posolite solution 504 that is reoxidized at the second electrode 515 compared to the amount of oxygen that is produced. This also has the advantage of minimizing the opportunity for oxygen evolved from the second electrode 515 to redissolve in the posolite solution 504 and eventually diffuse back into the negolite solution 502, rather than escaping the system. When the SOC imbalance between the negolite solution 502 and the posolite solution 504 falls below a threshold, the potential provided by the external power source 516 can be changed, reduced, or turned off.

[0051] FIG. 6 shows a redox flow battery including an integrated rebalancing cell 600 according to some embodiments of the present invention. The flow battery including the integrated rebalancing cell 600 includes a negolite tank 601 filled with a negolite solution 602 and further equipped with a first electrode 614 and a second electrode 615, a posolite tank 603 filled with a posolite solution 604, and a redox flow battery stack 605 connected to an external load or power source 606. The negolite solution 602 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 604 can be a negatively charged Fe(CN) solution dissolved in water. The negolite solution 602 and the posolite solution 604 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The first electrode 614 can include a conductive carbon material such as carbon cloth without other electrocatalysts, and the second electrode 615 can include nickel / iron oxyhydroxide. The conduit 620 is positioned to efficiently collect and direct oxygen gas bubbles generated from the second electrode 615 to the exterior 630 of the flow battery including the integrated rebalancing cell 600.

[0052] When an appropriate potential is applied between the electrodes 614, 615 using an external power source 616, the negolite solution 602 is depleted (i.e., its SOC increases) at the first electrode 614. At the same time, water (or hydroxide ions) in the negolite solution 602 is oxidized at the second electrode 615 to form oxygen gas, which is guided by the conduit 620 and vented to 630 outside the system.

[0053] This operation is preferably performed when the SOC of the negolite solution 602 is close to or at 0% (e.g., <40%, <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%) so that the first electrode 614 can continue to receive electrons, resulting in a smaller proportion of the negolite solution 602 being reoxidized at the second electrode 615 compared to the amount of oxygen produced. This also has the advantage of minimizing the reaction between the oxygen generated from the second electrode 615 and the negolite solution 602 before it escapes from the system to 630. When the SOC imbalance between the negolite solution 602 and the posolite solution 604 falls below a threshold, the potential provided by the external power source 616 can be changed, reduced, or turned off.

[0054] Because hydroxide ions are formed in the negolite solution 602 by reaction with oxygen, but are consumed at the second electrode 615 in the same solution, there is no net change in the ionic strength (or osmotic potential) of the negolite solution 602 as a result of the absorption of oxygen and the subsequent rebalancing to produce oxygen from the same solution. Thus, in this embodiment, no long-term net transport of water occurs from the negolite solution 602 to the posolite solution 604 or vice versa. This redox flow battery configuration including the integrated rebalancing cell 600 may also be used to convert depleted negolite 602 back to active negolite, thereby restoring lost capacity.

[0055] FIG. 7 shows a redox flow battery including an integrated rebalancing cell 700 according to some embodiments of the present invention. The flow battery including the integrated rebalancing cell 700 includes a negolite tank 701 filled with a negolite solution 702, a posolite tank 703 filled with a posolite solution 704 and further equipped with a first electrode 714 and a second electrode 715, and a redox flow battery stack 705 connected to an external load or power source 706. The negolite solution 702 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 704 can be a negatively charged Fe(CN) solution dissolved in water. Both the negolite solution 702 and the posolite solution 704 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The first electrode 714 can include a conductive carbon material such as carbon cloth without other electrocatalysts, and the second electrode 715 can include nickel / iron oxyhydroxide. The conduit 720 is positioned to efficiently collect and direct oxygen gas bubbles generated from the second electrode 715 to the exterior 730 of the flow battery including the integrated rebalancing cell 700.

[0056] When an appropriate potential is applied between the electrodes 714, 715 using an external power source 716, the posolite solution 704 is reduced (its SOC decreases) at the first electrode 714. At the same time, water (or hydroxide ions) in the posolite solution 704 is oxidized at the second electrode 715 to form oxygen gas, which is conducted by conduit 720 and escapes to 730 outside the system.

[0057] This operation is preferably performed when the SOC of the posolite solution 704 is close to or at 100% (e.g., >60%, >80%, >90%, >95%, >98%, >99%, >99.5%, >99.8%, >99.9%, or 100%) so that the first electrode 714 can continue to receive electrons, resulting in a smaller percentage of the posolite solution 704 being reoxidized at the second electrode 715 compared to the amount of oxygen produced. This also has the advantage of minimizing the opportunity for oxygen evolved from the second electrode 715 to redissolve in the posolite solution 704 and eventually diffuse back into the negolite solution 702, rather than escaping the system. When the SOC imbalance between the negolite solution 702 and the posolite solution 704 falls below a threshold, the potential provided by the external power source 716 can be altered, reduced, or turned off.

[0058] FIG. 8 shows a redox flow battery 800 and a rebalance cell 810 according to some embodiments of the present invention. The flow battery 800 includes a negolite tank 801 filled with a negolite solution 802, a posolite tank 803 filled with a posolite solution 804, and a redox flow battery stack 805 connected to an external load or power source 806. The negolite solution 802 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 804 can be a negatively charged Fe(CN) solution dissolved in water. Both the negolite solution 802 and the posolite solution 804 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The rebalance cell 810 includes a first chamber 811 and a second chamber 812 separated by an ion selective membrane 813. The ion selective membrane 813 can be a cation exchange membrane that allows potassium ions or sodium ions to pass freely. The first chamber 811 contains a first electrode 814 and the second chamber 812 contains a second electrode 815. The two electrodes 814, 815 are connected to an external power source 816.

[0059] The negolite solution 802 is circulated from the negolite tank 801 to the first chamber 811 and then returned from the first chamber 811 back to the negolite tank 801. Similarly, the posolite solution 804 is circulated from the posolite tank 803 to the second chamber 812 and then returned from the second chamber 812 back to the posolite tank 803.

[0060] When an appropriate potential is applied across the electrodes 814, 815 of the rebalancing cell 810 using an external power source 816, the posolite solution 804 is reduced (its SOC is reduced) at the second electrode 815. At the same time, the water (or hydroxide ions) in the negolite solution 802 is oxidized at the first electrode 814 to form oxygen gas, which escapes from the rebalancing cell 810 to 820 outside the system.

[0061] This operation is preferably performed when the SOC of the posolite solution 804 is greater than 0% (e.g., >1%, >10%, >20%, >40%, >60%, >80%, >90%, or 100%) so that the second electrode 815 can continue to receive electrons. It is also preferably performed when the SOC of the negolite solution 802 is close to or at 0% (e.g., <40%, <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%), so that the proportion of the negolite solution 802 that is reoxidized at the first electrode 814 is small compared to the amount of oxygen that is generated.

[0062] This also has the advantage of minimizing reaction between the oxygen evolved from the first electrode 814 and the negolite solution 802 before it escapes the system 820. When the SOC imbalance between the negolite solution 802 and the posolite solution 804 falls below a threshold, the electrical potential provided by the external power source 816 can be altered, reduced, or turned off.

[0063] Because hydroxide ions are formed in the negolite solution 802 by reaction with oxygen, but are consumed at the first electrode 814 in the same solution, there is no net change in the ionic strength (or osmotic potential) of the negolite solution 802 as a result of the absorption of oxygen and the subsequent rebalancing to produce oxygen from the same solution. Thus, in this embodiment, no long-term net transport of water occurs from the negolite solution 802 to the posolite solution 804 or vice versa. This rebalancing cell 810 configuration may also be used to convert depleted negolite 802 back to active negolite and restore lost capacity.

[0064] FIG. 9 shows a redox flow battery 900 and a rebalance cell 910 according to some embodiments of the present invention. The flow battery 900 includes a negolite tank 901 filled with a negolite solution 902, a posolite tank 903 filled with a posolite solution 904, and a redox flow battery stack 905 connected to an external load or power source 906. The negolite solution 902 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 904 can be a negatively charged Fe(CN) solution dissolved in water. The negolite solution 902 and the posolite solution 904 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The rebalance cell 910 includes a first chamber 911 and a second chamber 912 separated by a bipolar membrane 913.

[0065] The bipolar membrane 913 is positioned such that application of an appropriate potential across the bipolar membrane 913 produces protons (or hydronium ions) which enter the first chamber 911 and hydroxide ions which enter the second chamber 912.

[0066] The first chamber 911 contains a first electrode 914 and the second chamber 912 contains a second electrode 915. The first electrode 914 may contain a conductive carbon material such as carbon cloth without other electrocatalysts and the second electrode 915 may contain nickel / iron oxyhydroxide. The two electrodes 914, 915 are connected to an external power source 916.

[0067] The negolite solution 902 is circulated from the negolite tank 901 to the first chamber 911 and then returned from the first chamber 911 back to the negolite tank 901. Similarly, the posolite solution 904 is circulated from the posolite tank 903 to the second chamber 912 and then returned from the second chamber 912 back to the posolite tank 903.

[0068] When an appropriate potential is applied across the electrodes 914, 915 of the rebalancing cell 910 using an external power source 916, the negolite solution 902 is depleted (its SOC decreases) at the first electrode 914. At the same time, the water (or hydroxide ions) in the posolite solution 904 is oxidized at the second electrode 915 to form oxygen gas, which escapes from the rebalancing cell 910 to 920 outside the system.

[0069] This operation is preferably performed (a) when the SOC of the negolite solution 902 is less than 100% (e.g., <99%, <90%, <80%, <60%, <40%, <20%, <10%, or 0%) so that the first electrode 914 can continue to accept electrons, and (b) when the SOC of the posolite solution 904 is close to or at 100% (e.g., >60%, >80%, >90%, >95%, >98%, >99%, >99.5%, >99.8%, >99.9%, or 100%) so that the proportion of the posolite solution 904 that is reoxidized at the second electrode 915 is small compared to the amount of oxygen produced. This also has the advantage of minimizing the opportunity for oxygen evolved from the second electrode 915 to redissolve in the posolite solution 904 and eventually diffuse back into the negolite solution 902, rather than escaping the system 920. When the SOC imbalance between the negolite solution 902 and the posolite solution 904 falls below a threshold, the potential provided by the external power source 916 can be altered, reduced, or turned off.

[0070] Hydroxide ions are formed in the negolite solution 902 by reaction with oxygen, but these are consumed by the formation of protons (or hydronium ions) at the bipolar membrane 913 and conducted to the negolite solution 902 in the first chamber 911, so that there is no net change in the ionic strength (or osmotic potential) of the negolite solution 902 as a result of oxygen absorption and subsequent rebalancing. Hydroxide ions generated at the bipolar membrane 913 are conducted to the posolite solution 904 in the second chamber 912, and an equal amount is consumed at the second electrode 915. Note that water molecules are generated at the negolite solution 902 and at the second electrode 915, but an equal amount is consumed within the bipolar membrane 913. Thus, in this embodiment, no net long-term transport of water occurs from the negolite solution 902 to the posolite solution 904, or vice versa. The use of a bipolar membrane allows osmotic neutrality to be maintained even when oxygen evolution from the second electrode 915 is occurring in the posolite solution 904. This has the added benefit of not exposing the oxygen evolution electrocatalyst on the second electrode 915 to a reducing chemical environment (e.g., high SOC posolite solution 902) in which it may become unstable.

[0071] FIG. 10 shows a redox flow battery 1000 and a rebalance cell 1010 according to some embodiments of the present invention. The flow battery 1000 includes a negolite tank 1001 filled with a negolite solution 1002, a posolite tank 1003 filled with a posolite solution 1004, and a redox flow battery stack 1005 connected to an external load or power source 1006. The negolite solution 1002 can be a negatively charged quinone derivative dissolved in water, and the posolite solution 1004 can be a negatively charged Fe(CN) solution dissolved in water. Both the negolite solution 1002 and the posolite solution 1004 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The rebalancing cell 1010 comprises a first chamber 1011 bounded on one side by a bipolar membrane 1014 and a second chamber 1012, and on the other side by an ion selective membrane 1015 and a third chamber 1013. The bipolar membrane 1014 is arranged such that application of an appropriate electrical potential across the bipolar membrane 1014 produces hydroxide ions which enter the first chamber 1011 and protons (or hydronium ions) which enter the second chamber 1012. The ion selective membrane 1015 may be a cation exchange membrane that allows potassium or sodium ions to pass freely.

[0072] The second chamber 1012 contains a first electrode 1016 and the third chamber 1013 contains a second electrode 1017. The first electrode 1016 may include a conductive carbon material such as carbon cloth without other electrocatalysts, and the second electrode 1017 may include nickel / iron oxyhydroxide. The two electrodes 1016, 1017 are connected to an external power source 1018.

[0073] The negolite solution 1002 is circulated from the negolite tank 1001 to the second chamber 1012, passes through the first electrode 1016, and then returns from the second chamber 1012 to the negolite tank 1001.

[0074] The posolite solution 1004 is circulated from the posolite tank 1003 to both the first chamber 1011 and the third chamber 1013 and back again from both chambers 1011, 1013 to the posolite tank 1003. The posolite solution that has passed through the third chamber 1013 flows past the second electrode 1017.

[0075] When an appropriate potential is applied between the electrodes 1016, 1017 of the rebalancing cell 1010 using an external power source 1018, the negolite solution 1002 is depleted (SOC increases) at the first electrode 1016. At the same time, water (or hydroxide ions) in the posolite solution 1004 is oxidized at the second electrode 1017 to form oxygen gas, which escapes the rebalancing cell 1010 to the outside of the system 1020. At the same time, water splitting occurs within the bipolar membrane 1014, releasing hydroxide ions that enter the first chamber 1011 and protons (or hydronium ions) that enter the second chamber 1012.

[0076] This operation is preferably performed (a) when the SOC of the negolite solution 1002 is less than 100% (e.g., <99%, <90%, <80%, <60%, <40%, <20%, <10%, or 0%) so that the first electrode 1016 can continue to accept electrons, and (b) when the SOC of the posolite solution 1004 is close to or at 100% (e.g., >60%, >80%, >90%, >95%, >98%, >99%, >99.5%, >99.8%, >99.9%, or 100%) so that the proportion of the posolite solution 1004 that is reoxidized at the second electrode 1017 is small compared to the amount of oxygen produced. This also has the advantage of minimizing the opportunity for oxygen evolved from the second electrode 1017 to redissolve in the posolite solution 1004 rather than escaping 1020 the system, and eventually diffuse back into the negolite solution 1002. When the SOC imbalance between the negolite solution 1002 and the posolite solution 1004 falls below a threshold, the potential provided by the external power source 1018 can be altered, reduced, or turned off.

[0077] Other embodiments are possible in which the identities of the first electrode 1016 and the second electrode 1017 are swapped. In these embodiments, an external voltage provided by an external power source 1018 causes oxygen evolution at the first electrode 1016 in contact with the negolite solution and reduction of the posolite solution in the third chamber 1013, thereby lowering the SOC.

[0078] Thus, operation is preferably performed (a) when the SOC of the posolite solution 1004 is greater than 0% (e.g., >1%, >10%, >20%, >40%, >60%, >80%, >90%, or 100%) so that the second electrode 1017 can continue to receive electrons, and (b) when the SOC of the negolite solution 1002 is close to or at 0% (e.g., <40%, <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%) so that the proportion of the negolite solution 1002 that is reoxidized at the first electrode 1016 is small compared to the amount of oxygen that is generated. This also has the advantage of minimizing the opportunity for oxygen generated from the first electrode 1016 to react again with the negolite solution 1002 before escaping the system to 1020. When the SOC imbalance between the negolite solution 1002 and the posolite solution 1004 falls below a threshold, the potential provided by the external power source 1018 may again be changed, reduced, or turned off.

[0079] Hydroxide ions are formed in the negolite solution 1002 by reaction with oxygen, but are consumed by the formation of protons (or hydronium ions) at the bipolar membrane 1014, so there is no net change in the ionic strength (or osmotic potential) of the negolite solution 1002 as a result of oxygen absorption and subsequent rebalancing. Hydroxide ions generated at the bipolar membrane 1014 are conducted to the posolite solution 1004 in the first chamber 1011, flow to the third chamber 1013, and an equal amount is consumed at the second electrode 1017. Note that water molecules are generated at the negolite solution 1002 and the second electrode 1017, but an equal amount is also consumed within the bipolar membrane 1014. Thus, in this embodiment, no net long-term transport of water occurs from the negolite solution 1002 to the posolite solution 1004, or vice versa. The use of a bipolar membrane allows osmotic neutrality to be maintained even when oxygen evolution from the second electrode 1017 is occurring in the posolite solution 1004. This has the added benefit of not exposing the oxygen evolution electrocatalyst on the second electrode 1017 to a reducing chemical environment (e.g., high SOC posolite solution 1002) in which it may become unstable.

[0080] FIG. 11 shows a redox flow battery 1100 and a rebalancing cell 1110 according to some embodiments of the present invention. The flow battery 1100 includes a negolite tank 1101 filled with a negolite solution 1102, a posolite tank 1103 filled with a posolite solution 1104, and a redox flow battery stack 1105 connected to an external load or power source 1106. The negolite solution 1102 is a negatively charged quinone derivative dissolved in water, and the posolite solution 1104 can be a negatively charged Fe(CN) solution dissolved in water. The negolite solution 1102 and the posolite solution 1104 can use potassium hydroxide, sodium hydroxide, or a mixture of both as a supporting electrolyte. The rebalancing cell 1110 comprises a first chamber 1111 bounded on one side by an ion selective membrane 1114 and a second chamber 1112, and on the other side by a bipolar membrane 1115 and a third chamber 1113. The bipolar membrane 1115 is arranged such that application of an appropriate electrical potential across the bipolar membrane 1115 produces hydroxide ions which enter the third chamber 1113 and protons (or hydronium ions) which enter the first chamber 1111. The ion selective membrane 1114 can be a cation exchange membrane that allows potassium or sodium ions to pass freely.

[0081] The second chamber 1112 contains a first electrode 1116 and the third chamber 1113 contains a second electrode 1117. The first electrode 1116 may contain a conductive carbon material such as carbon cloth without other electrocatalysts and the second electrode 1117 may contain nickel / iron oxyhydroxide. The two electrodes 1116, 1117 are connected to an external power source 1118.

[0082] The negolite solution 1102 is circulated from the negolite tank 1101 to both the first chamber 1111 and the second chamber 1112 and then back to the negolite tank 1101 from both chambers 1111, 1112. The negolite solution that has passed through the second chamber 1112 flows past the first electrode 1116.

[0083] The posolite solution 1104 is circulated from the posolite tank 1103 to the third chamber 1113 , passes through the second electrode 1117 , and then returns from the third chamber 1113 to the posolite tank 1103 .

[0084] When an appropriate potential is applied across the electrodes 1116, 1117 of the rebalancing cell 1110 using an external power source 1118, the negolite solution 1102 is depleted (SOC increases) at the first electrode 1116. At the same time, water (or hydroxide ions) in the posolite solution 1104 is oxidized at the second electrode 1117 to form oxygen gas, which escapes the rebalancing cell 1110 to the outside of the system 1120. At the same time, water splitting occurs within the bipolar membrane 1115, releasing hydroxide ions which enter the third chamber 1113 and protons (or hydronium ions) which enter the first chamber 1111.

[0085] This operation is preferably performed (a) when the SOC of the negolite solution 1102 is less than 100% (e.g., <99%, <90%, <80%, <60%, <40%, <20%, <10%, or 0%) so that the first electrode 1116 can continue to accept electrons, and (b) when the SOC of the posolite solution 1104 is close to or at 100% (e.g., >60%, >80%, >90%, >95%, >98%, >99%, >99.5%, >99.8%, >99.9%, or 100%) so that the proportion of the posolite solution 1104 that is reoxidized at the second electrode 1117 is small compared to the amount of oxygen produced. This also has the advantage of minimizing the opportunity for oxygen evolved from the second electrode 1117 to redissolve in the posolite solution 1104 rather than escaping 1120 the system and eventually diffusing back into the negolite solution 1102. When the SOC imbalance between the negolite solution 1102 and the posolite solution 1104 falls below a threshold, the potential supplied by the external power source 1118 can be altered, reduced, or turned off.

[0086] In other embodiments, the properties of the first electrode 1116 and the second electrode 1117 may be swapped. In these embodiments, an external voltage provided by an external power source 1118 causes oxygen evolution at the first electrode 1116 in contact with the negolite solution and reduction of the posolite solution in the third chamber 1113, thereby lowering the SOC.

[0087] Thus, operation is preferably performed (a) when the SOC of the posolite solution 1104 is greater than 0% (e.g., >1%, >10%, >20%, >40%, >60%, >80%, >90%, or 100%) so that the second electrode 1117 can continue to accept electrons, and (b) when the SOC of the negolite solution 1102 is close to or at 0% (e.g., <40%, <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%) so that the proportion of the negolite solution 1102 that is reoxidized at the first electrode 1116 is small compared to the amount of oxygen that is generated. This also has the advantage of minimizing the opportunity for the oxygen generated from the first electrode 1116 to react again with the negolite solution 1102 before escaping the system to 1120. When the SOC imbalance between the negolite solution 1102 and the posolite solution 1104 falls below a threshold, the potential supplied by the external power source 1118 may again be changed, reduced, or turned off.

[0088] Since hydroxide ions are formed in the negolite solution 1102 by reaction with oxygen, but are consumed by the formation of protons (or hydronium ions) at the bipolar membrane 1115, there is no net change in the ionic strength (or osmotic potential) of the negolite solution 1102 as a result of oxygen absorption and subsequent rebalancing. The hydroxide ions generated at the bipolar membrane 1115 are conducted to the posolite solution 1104 in the third chamber 1113, and an equal amount is consumed at the second electrode 1117. Note that water molecules are generated at the negolite solution 1102 and at the second electrode 1117, but an equal amount is consumed within the bipolar membrane 1115. Thus, in this embodiment, no long-term net transport of water occurs from the negolite solution 1102 to the posolite solution 1104 or vice versa. With the bipolar membrane, osmotic neutrality can be maintained even when oxygen evolution from the second electrode 1117 is occurring in the posolite solution 1104. This has the added advantage that the oxygen evolution electrocatalyst on the second electrode 1117 is not exposed to a reducing chemical environment (eg, high SOC negolite solution 1102) that may render it unstable.

[0089] 12-14 show examples of methods of operating a redox flow battery with a rebalancing cell as described above in an energy storage system. In these examples, the flow battery is cycled (charged and discharged) using typical methods of operation for flow batteries. It should be understood that the cycling process also includes rest (idle) periods without the rebalancing cell and electrochemical capacity recovery techniques (e.g., venting the negolite, electrochemical reoxidation of the negolite, periodic deep discharge, etc.). The entire system with the redox flow battery and the rebalancing cell can be operated using one or a combination of these methods.

[0090] In the method of Figure 12, the rebalancing cell is operated (i.e., current is passed through it) at a constant voltage, constant current, or similar substantially always-on condition 1201, independent of the cycling of the redox flow battery, so that the SOC imbalance between negolite and posolite is maintained below a threshold value. In other words, the average rate of oxygen production from the rebalancing cell is approximately the same as the average rate of oxygen absorption into the posolite and negolite solutions.

[0091] In the method of FIG. 13, the redox flow battery is first discharged (1301) such that the cell voltage or discharge current density of the redox flow battery is below a threshold value. The cell voltage or current density can be used as a proxy for the approximate SOC of the flow battery, so that the SOC of the negolite solution can be confirmed to be low, close to 0% or 0% (e.g., <40%, <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%). Next, while the flow battery is maintained in this discharge state 1302, the rebalancing cell is operated (1303) (i.e., current is passed through it) until the cell voltage of the rebalancing cell exceeds a threshold value or the current density of the rebalancing cell falls below a threshold value. When this occurs, the SOC imbalance between the negolite and posolite solutions is reduced to a level close to or low at 0% (e.g., <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%). This method may be used at the end of a charge-discharge cycle of the redox flow battery, at the end of a certain number of cycles, or after a predetermined time has elapsed.

[0092] In the method of FIG. 14, the redox flow battery is first charged (1401) such that the cell voltage exceeds a threshold or the charging current density of the redox flow battery falls below a threshold. The cell voltage or current density can be used as a proxy for the approximate SOC of the flow battery, thereby ensuring that the SOC of the posolite solution is high and close to or at 100% (e.g., >60%, >80%, >90%, >95%, >98%, >99%, >99.5%, >99.8%, >99.9%, or 100%). The rebalancing cell is then operated (1403) (i.e., current is passed through it) while the flow battery is maintained in this state of charge 1402, until the cell voltage of the rebalancing cell exceeds a threshold or the current density of the rebalancing cell falls below a threshold. When this occurs, the SOC imbalance between the negolite and posolite solutions is reduced to a level close to or low at 0% (e.g., <20%, <10%, <5%, <2%, <1%, <0.5%, <0.2%, <0.1%, or 0%). This method may be used at the end of a discharge-charge cycle of a redox flow battery, at the end of a certain number of cycles, or after a predetermined time has elapsed.

[0093] In all of the methods shown in Figures 12-14, specifically steps 1203, 1303, or 1403, a portion of the current passing through the rebalancing cell can be directed to oxidizing water (i.e., generating oxygen) as originally intended, while a portion of the current can be further directed to oxidizing depleted NEGOlite reactants back to the original NEGOlite reactants, thereby restoring the charge capacity of the NEGOlite.

[0094] [Example 1] Two separate cells were constructed. The first cell was called the redox flow battery cell and was used to charge / discharge the flow battery, which consisted of DCDHAQ (1,8-dihydroxy-2,7-bis(carboxymethyl)-9,10-anthraquinone) as the negolite active material and a 1:1 molar combination of sodium ferrocyanide and potassium ferrocyanide as the posolite active materials. The second cell was called the rebalancing cell and was used intermittently to offset the imbalance caused by exposure to oxygen. Both cells were operated exposed to air, not in an inert atmosphere glove box or under a protective blanket of inert gas.

[0095] Both the redox flow battery cell and the rebalancing cell were constructed with hardware purchased from Fuel Cell Technologies (Albuquerque, NM). Both cells were constructed with a serpentine flow design and a 50 cm 2 Resin-impregnated graphite flow plates with a geometric surface area of ​​100 nm were used. Two sheets of heat-activated AvCarb carbon paper (EP-40) were used on both sides of the redox flow cell and on the cathode (negative) side of the rebalancing cell. Nickel wire mesh with a wire diameter of 0.016 inches and a mesh size of 20x20 (number of openings per inch) was used on the anode (positive) side of the rebalancing cell, where the oxygen evolution reaction occurs. FKE-50 membranes were used as ion-selective membranes in the redox flow cells. Fumatech FBM bipolar membranes were used in the rebalancing cells. The FBM membranes were positioned so that protons were produced on the cathode side of the rebalancing cell and hydroxide ions were produced on the anode side of the rebalancing cell. Viton sheets were used to cover the outer space between the electrodes. The torque used to assemble the cells was 60 lb-in (6.78 Nm) for each of eight 1 / 4-28 volts.

[0096] The redox flow battery cell and rebalance cell were set up using reservoirs with the same configuration as in Figure 2. The flow rates of all electrolytes were set at 225 mL / min and driven by a KNF NF60 pump. In some cases, a blanket of nitrogen was maintained over the negolite of the redox flow battery cell. Charge / discharge of the cycling cell was performed using an Arbin battery tester, and the rebalance cell was electrically connected to and operated using a Bio-Logic VSP-300 potentiostat.

[0097] The negolite for the redox flow battery cell consisted of 100 mL of 0.08 M DCDHAQ with a 1:1 molar ratio of sodium hydroxide to potassium hydroxide adjusted to a pH of 14. The negolite for the redox flow battery cell consisted of 55 mL of 0.15 M sodium ferrocyanide, 0.15 M potassium ferrocyanide, and 0.10 M potassium ferricyanide with a 1:1 molar ratio of sodium hydroxide to potassium hydroxide adjusted to a pH of 14. For simplicity, the anode chamber of the rebalancing cell contained 100 mL of a 1:1 molar ratio of sodium hydroxide to potassium hydroxide with a total hydroxide concentration of 3 M.

[0098] The redox flow battery cell is charged / discharged at ±100mA / cm until the charge potential reaches 1.55V or the discharge potential reaches 0.65V. 2 At that point, the redox flow cell was charged at a constant current of 5mA / cm 2 The rebalance cells were held at these potentials until the potential dropped to 0.05 V, at which point the next half-cycle (discharge / charge) was started. The rebalance cells were kept off most of the time, but during operation, they were charged at 10 mA / cm until the potential reached 2.3 V. 2 After the current was applied at a constant current of 0.01V, the rebalance cell current was then applied at a constant potential with a current density of 3mA / cm. 2The current density of the rebalancing cell was below 1000 mA. Typically, at this point, hydrogen gas bubbles start to form at the cathode (negative electrode) side of the rebalancing cell. Increasing the cutoff threshold current density of the rebalancing cell would prevent hydrogen gas from forming at the cathode of the rebalancing cell. The cycle sequence during the rebalancing operation was: (1) charge the redox flow battery cell until the threshold is reached, (2) apply current to the rebalancing cell until the threshold is reached, and (3) discharge the redox flow battery cell until the threshold is reached. Note that many other cycle sequences involving simultaneous operation of the redox flow battery cell and the rebalancing cell are possible but were not tested here.

[0099] In this cycle sequence, during the charging step of the redox flow battery cell, a portion of negolite is constantly oxidized by the oxygen present in the air. Thus, more ferrocyanide is oxidized to fully charge negolite. This process continues until, due to contact with air, posolite no longer has a surplus to accommodate the oxidation of negolite. From this point onwards, the capacity of the cell, originally limited by negolite, becomes limited by posolite.

[0100] To counteract the effects of oxygen, the rebalancing cell was operated intermittently. After fully charging the catholyte in step (1) of the cycle sequence described above, with the capacity limited by the anolyte as described above, the valve was opened to circulate the anolyte to the cathode chamber of the rebalancing cell, and step (2) was initiated. After step (2) was completed, the valve was closed and step (3) followed.

[0101] Figure 15 shows the capacity and coulombic efficiency of the cycled cell over time. For the first few cycles (~0.1 days), the capacity appears to be stable since there is a slight excess of posolite. Beyond that, the capacity appears to drop rapidly as oxygen continues to reduce the state of charge of the negolite (i.e., oxidize it) and there is no longer enough posolite to fully charge the negolite.

[0102] As shown in Figure 15, after each rebalancing cycle, the capacity of the redox flow battery cell increases sharply. The capacity also drops sharply after rebalancing, likely because aerobic oxidation occurs at a faster rate with higher concentrations of reduced DCDHAQ. To prove that the capacity loss is primarily due to the effect of atmospheric oxygen, we placed the negolite reservoir (but not the posolite or the rest of the system) under a blanket of ultra-high purity nitrogen about 2.5 days before the last rebalancing procedure, and immediately observed a slowdown in the rate of capacity loss. No changes were observed in the liquid levels of the posolite, negolite, or anode reservoirs of the rebalanced cell throughout the entire period. The results show that the rebalanced cell with a bipolar membrane can recover lost capacity in a redox flow battery system caused by imbalances in the state of charge of the posolite and negolite, caused by oxygen or other processes.

[0103] Unless otherwise indicated, all numerical values ​​expressing size, amount, volume, charge capacity, state of charge, and other chemical and physical properties of features used in the specification and claims are to be understood in all instances as being modified by the term "about." Thus, unless otherwise indicated, the numerical parameters set forth in the foregoing specification, as well as in the appended claims, are approximations that may vary depending on the desired properties one of ordinary skill in the art seeks to obtain using the techniques disclosed herein. The use of numerical ranges with endpoints includes all numerical values ​​within that range (e.g., 1-5 includes 1, 1.5, 2, 2.75, 3, 3.80, 4, 5; <10% includes 10%, 9.8%, 5.5%, 2%, 0.01%, 0%; >90% includes 90%, 90.2%, 94.5%, 98%, 99.99%, 100%) and any ranges therein.

[0104] The foregoing description has been presented for purposes of illustration and description, and is not intended to be exhaustive or to limit the embodiments to the precise form disclosed. Many modifications and variations are possible in light of the above teachings. Any or all features of the disclosed embodiments may be applied individually or in any combination and are not intended to be limiting, but are merely exemplary. It is intended that the scope of the invention be determined not by this detailed description, but rather by the claims appended hereto. [Explanation of symbols]

[0105] 200 Redox flow battery 201 Negolite Tank 202 Negolite solution 203 Posolite Tank 204 Posolite solution 205 Redox flow battery stack 206 Power supply 210 Rebalancing Cell 211 First Chamber 212 Second Chamber 213 Ion-selective membranes 214 First electrode 215 Second Electrode 216 External power supply 220 Supporting electrolyte tank 221 Supporting electrolyte 300 Redox flow battery 301 Negolite Tank 302 Negolite solution 303 Posolite Tank 304 Posolite solution 305 Redox flow battery stack 306 Power supply 310 Rebalancing Cell 311 First Chamber 312 Second Chamber 313 Ion-selective membranes 314 First electrode 315 Second Electrode 316 External power supply 320 Supporting electrolyte tank 321 Supporting Electrolyte 400 Redox flow battery 401 Negolite Tank 402 Negolite Solution 403 Posolite Tank 404 Posolite solution 405 Redox flow battery stack 406 Power supply 410 Rebalancing Cell 411 First Chamber 412 Second Chamber 413 Ion-selective membrane 414 First Electrode 415 Second Electrode 416 External power supply 500 Redox flow battery 501 Negolite Tank 502 Negolite solution 503 Posolite Tank 504 Posolite solution 505 Redox flow battery stack 506 Power supply 510 Rebalancing Cell 511 First Chamber 512 Second Chamber 513 Ion-selective membrane 514 First electrode 515 Second Electrode 516 External power supply 600 Integrated Rebalancing Cell 601 Negolite Tank 602 Negolite Solution 603 Posolite Tank 604 Posolite solution 605 Redox flow battery stack 606 Power supply 614 First electrode 615 Second electrode 616 External power supply 620 Conduit 700 Integrated Rebalancing Cell 701 Nego Light Tank 702 Negolite solution 703 Posolite Tank 704 Posolite solution 705 Redox flow battery stack 706 Power supply 714 First Electrode 715 Second Electrode 716 External power supply 720 Conduit 800 Redox flow battery 801 Negolite Tank 802 Negolite solution 802 Negolite 803 Posolite Tank 804 Posolite solution 805 Redox Flow Battery Stack 806 Power supply 810 Rebalancing Cell 811 First Chamber 812 Second Chamber 813 Ion-selective membrane 814 First electrode 815 Second electrode 816 External power supply 900 Redox Flow Battery 900 Flow Battery 901 Nego Light Tank 902 Negolite Solution 903 Posolite Tank 904 Posolite solution 905 Redox flow battery stack 906 Power supply 910 Rebalancing Cell 911 First Chamber 912 Second Chamber 913 Bipolar membrane 914 First electrode 915 Second electrode 916 External power supply 1000 Redox Flow Battery 1000 Flow Battery 1001 Nego Light Tank 1002 Negolite solution 1003 Posolite Tank 1004 Posolite solution 1005 Redox flow battery stack 1006 Power supply 1010 Rebalancing Cell 1011 First Chamber 1012 Second Chamber 1013 Third Chamber 1014 Bipolar membrane 1015 Ion selective membrane 1016 First electrode 1017 Second electrode 1018 External power supply 1100 Redox flow battery 1100 Flow Battery 1101 Negolite Tank 1102 Negolite solution 1103 Posolite Tank 1104 Posolite solution 1105 Redox flow battery stack 1106 Power supply 1110 Rebalancing Cell 1111 First Chamber 1112 Second Chamber 1113 Third Chamber 1114 Ion selective membrane 1115 Bipolar membrane 1116 First electrode 1117 Second electrode 1118 External power supply

Claims

1. a first tank containing a negolite solution containing a quinone; a second tank containing a posolite solution; A flow battery stack capable of extracting electrical energy from the chemical reaction of the negolite solution and the posolite solution and supplying electrical energy to cause a reverse reaction of the negolite solution and the posolite solution. A redox flow battery device comprising: a rebalancing device comprising a first electrode and a second electrode having an oxygen evolution reaction catalyst, the rebalancing device being configured to receive at least one negolite or posolite solution from a source tank, the source tank being one of the first tank and the second tank, reforming the negolite or posolite solution in the source tank, and returning the reformed solution to the source tank; A redox flow battery system comprising: the flow battery stack is configured to receive a negolite solution from the first tank and return the negolite solution to the first tank, receive a posolite solution from the second tank and return the posolite solution to the second tank; A redox flow battery system, wherein the first chamber and the second chamber are configured to receive a negolite solution from a first tank of the redox flow battery device and to return the reformed negolite solution from the first chamber and the second chamber to the first tank of the redox flow battery device.

2. The system of claim 1 , wherein the pH of each of the negolite solution and the posolite solution is greater than 7.

3. The system of claim 2 , wherein the second electrode comprises platinum, nickel, nickel-iron, nickel oxyhydroxide, or nickel-iron oxyhydroxide.

4. The system of claim 1 , wherein the pH of the negolite solution and the posolite solution is each less than 7.

5. The system of claim 4 , wherein the second electrode comprises platinum, ruthenium oxide, or iridium oxide.

6. The system of any one of claims 1 to 5, wherein the rebalancing device is further configured to perform an oxidation reaction on the negolite solution via the second electrode.

7. The system of any one of claims 1 to 5, further comprising an outlet for venting or releasing gaseous products produced at the second electrode.

8. 6. The system of claim 1, wherein the first electrode and the second electrode of the rebalancing device are integrated into either a first tank or a second tank of the redox flow battery device.

9. 10. The system of claim 8, further comprising an outlet for venting or releasing gas product produced at the second electrode, and a conduit for collecting and directing bubbles of gas product produced at the second electrode to the outlet.

10. 10. The system of claim 1, further comprising a first separator dividing the rebalancing device to define a first chamber and a second chamber, the first chamber comprising the first electrode and the second chamber comprising the second electrode.

11. The system of claim 10 , wherein the first separator is an anion exchange membrane or a cation exchange membrane.

12. The system of claim 11, wherein the first separator is a bipolar membrane.

13. 11. The system of claim 10, wherein the first chamber and the second chamber are configured to receive a posolite solution from a second tank of the redox flow battery device and return a reformed posolite solution from the first chamber and the second chamber to the second tank of the redox flow battery device.

14. a supporting electrolyte tank and a supporting electrolyte solution; a second chamber configured to receive the supporting electrolyte solution from the supporting electrolyte tank and return the supporting electrolyte solution to the supporting electrolyte tank; a first chamber configured to receive a negolite solution from a first tank of the redox flow battery device and return a reformed negolite solution to the first tank of the redox flow battery device; The system according to any one of claims 10 to 13, further comprising: an osmotic pressure of the supporting electrolyte solution higher than the osmotic pressure of the negolite solution.

15. a supporting electrolyte tank and a supporting electrolyte solution; a second chamber configured to receive the supporting electrolyte solution from the supporting electrolyte tank and return the supporting electrolyte solution to the supporting electrolyte tank; a first chamber configured to receive a modified potholite solution from the second tank of the redox flow battery device and return the modified potholite solution to the second tank of the redox flow battery device; The system according to any one of claims 10 to 13, further comprising: an osmotic pressure of the supporting electrolyte solution higher than an osmotic pressure of the posolite solution.

16. the first chamber is configured to receive the posolite solution from the second tank of the redox flow battery device and return the posolite solution from the second chamber to the second tank of the redox flow battery device; The system described in any one of claims 10 to 13, wherein the second chamber is configured to receive the negolite solution from the first tank of the redox flow battery device and return the reformed negolite solution to the first tank of the redox flow battery device.

17. 14. The system of claim 10, further comprising a second separator disposed between the first separator and the second electrode, whereby a third chamber is defined between the first separator and the second separator, the third chamber being disposed between the first chamber and the second chamber.

18. 18. The system of claim 17, wherein the first separator is a bipolar membrane configured to supply protons to the first chamber and hydroxide ions to the third chamber, and the second separator is an anion exchange membrane or a cation exchange membrane.

19. the first chamber is configured to receive a negolite solution from a first tank of the redox flow battery device and return a reformed negolite solution to the first tank of the redox flow battery device; the second chamber is configured to receive the modified posolite solution from the second tank of the redox flow battery device and return the modified posolite solution to the second tank of the redox flow battery device; 20. The system of claim 18, wherein the third chamber is configured to receive either a posolite solution or a negolite solution from a first tank or a second tank of the redox flow battery device and return the reformed solution to a source tank of the redox flow battery device.

20. 18. The system of claim 17, wherein the first separator is an anion exchange membrane or a cation exchange membrane and the second separator is a bipolar membrane configured to supply protons to the third chamber and hydroxide ions to the second chamber.

21. the first chamber is configured to receive a negolite solution from a first tank of the redox flow battery device and return a reformed negolite solution to the first tank of the redox flow battery device; the second chamber is configured to receive the modified posolite solution from the second tank of the redox flow battery device and return the modified posolite solution to the second tank of the redox flow battery device; 21. The system of claim 20, wherein the third chamber is configured to receive either the posolite solution or the negolite solution from the first tank or the second tank of the redox flow battery device and return the reformed solution to the source tank of the redox flow battery device.

22. a first tank containing a negolite solution containing a quinone; a second tank containing a posolite solution; A flow battery stack capable of extracting electrical energy from a chemical reaction of the negolite solution and the posolite solution and supplying electrical energy for causing a reverse reaction of the negolite solution and the posolite solution, the flow battery stack being configured to receive the negolite solution from the first tank and return the negolite solution to the first tank, and to receive the negolite solution from the second tank and return the negolite solution to the second tank; A redox flow battery device comprising: a rebalancing device comprising a first electrode and a second electrode having an oxygen evolution reaction catalyst, the rebalancing device being configured to receive at least one negolite solution or posolite solution from a source tank and return the reformed solution(s) to the source tank(s) receiving the negolite solution or posolite solution, respectively, and further configured to perform an oxidation reaction on the negolite solution via the second electrode; providing a redox flow battery system comprising: applying a current to the rebalancing device to maintain a state of charge imbalance between Negolite and Posolite below a threshold; The method comprising:

23. providing the redox flow battery system of claim 22; discharging the redox flow battery device until a cell voltage or discharge current density falls below a threshold; maintaining the redox flow battery device in a discharged state; applying a current to the rebalancing device until a rebalancing cell voltage exceeds a threshold or a rebalancing current density falls below a threshold; The method comprising:

24. Providing a redox flow battery system according to claim 22; charging the redox flow battery device until the cell voltage exceeds a threshold or the charging current density falls below a threshold; maintaining a state of charge of the redox flow battery device; applying a current to the rebalancing device until the rebalancing cell voltage exceeds a threshold or the rebalancing current density falls below a threshold; The method comprising:

25. Providing a redox flow battery system according to claim 22; A step of passing a current through the rebalancing device, and directing a portion of the current through the rebalancing device to oxidize water at the second electrode, wherein a portion of the current through the rebalancing device is directed to oxidize depleted negolite to restore the charge capacity of the negolite solution; The method comprising:

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