Rebalancing of condensation-based redox flow batteries

Abstract

In redox flow batteries (RFBs), the base solvent of the electrolyte has a tendency to migrate from one electrode to the other across the barrier layer. This can result in a volume and concentration imbalance between the electrolytes that adversely affects the efficiency and capacity of the battery. Compatible electrolytes can be mixed to rebalance the system, but for incompatible electrolytes, mixing is not a viable option. To this end, the RFB herein includes a separator that recovers the base solvent from the vapor phase of one electrolyte and returns the recovered base solvent to the other electrolyte, thereby reversing the imbalance.

Description

[Technical Field]

[0001] (CROSS-REFERENCE TO RELATED APPLICATIONS) This disclosure claims priority to U.S. Patent Application No. 17 / 521,238, filed November 8, 2021. [Background technology]

[0002] Flow batteries, also known as redox flow batteries or redox flow cells, are designed to convert electrical energy into storable chemical energy and then re-release it as electrical energy when there is a later demand. For example, flow batteries may be used in conjunction with renewable energy systems, such as wind power systems, to store energy in excess of consumer demand and later release that energy when there is more demand.

[0003] A typical flow battery contains a redox flow cell with a negative electrode and a positive electrode separated by an electrolyte layer, which may include a separator such as an ion-exchange membrane. A negative fluid electrolyte (sometimes called an anolyte or negolyte) is delivered to the negative electrode, and a positive fluid electrolyte (sometimes called a catholyte or pozolyte) is delivered to the positive electrode, promoting a reversible redox reaction between the redox couple. Upon charging, the supplied electrical energy causes a reduction reaction in one electrolyte and an oxidation reaction in the other. The separator prevents the electrolytes from freely and rapidly mixing, allowing ions to selectively pass through to complete the redox reaction. Upon discharge, the chemical energy contained in the liquid electrolyte is released in a reverse reaction, drawing electrical energy from the electrodes. Summary of the Invention [Means for solving the problem]

[0004] A redox flow battery according to one embodiment of the present disclosure includes a redox flow cell, a supply and storage system external to the redox flow cell, and a separator. The redox flow cell has a barrier layer disposed between a first electrode and a second electrode. The supply and storage systems include first and second containers, a first liquid electrolyte solution in the first container and a second liquid electrolyte solution in the second container, fluid lines connecting the first and second containers to the first and second electrodes, respectively, and a plurality of pumps operable to circulate the first and second liquid electrolyte solutions through the redox flow cell and the first and second containers via the fluid lines. The first and second electrolyte solutions contain a base solvent, which tends to migrate across the barrier layer from the second electrode toward the first electrode, causing an imbalance such that the concentration of the second electrolyte solution increases and the concentration of the first electrolyte solution decreases. The first electrolyte solution emits a vapor phase containing the base solvent. The separator is fluidly connected to the first vessel for receiving the vapor phase, and is operable to condense the base solvent from the vapor phase to produce a recovered base solvent and return the recovered base solvent to the second electrolyte solution to reverse the imbalance such that the second electrolyte solution is reduced in concentration and the first electrolyte solution is increased in concentration.

[0005] In a further example of the aforementioned embodiment, the separator comprises a heat exchanger condenser.

[0006] A further example of any of the foregoing embodiments further includes an evaporator operable to evaporate the base solvent from the first electrolyte solution to produce a vapor phase.

[0007] A further example of any of the foregoing embodiments further includes a supply line to the evaporator, the supply line having an inlet disposed in a headspace of the first container above the first electrolyte solution.

[0008] A further example of any of the foregoing embodiments further includes a supply line to the evaporator, the supply line being positioned below the headspace of the first container and having an inlet immersed in the first electrolyte solution.

[0009] In a further example of any of the foregoing embodiments, the evaporator is external to the first vessel.

[0010] In a further example of any of the aforementioned embodiments, the separator includes a diffuser.

[0011] In a further example of any of the foregoing embodiments, the separator includes a gas phase return line connected to the first vessel.

[0012] In a further example of any of the foregoing embodiments, the gas phase return line includes an outlet immersed in the first electrolyte solution.

[0013] In a further example of any of the foregoing embodiments, the first and second electrolyte solutions are incompatible with each other and react upon mixing to form a precipitate or become electrochemically inactive.

[0014] In a further example of any of the foregoing embodiments, the separator is activated in response to at least one of: i) the concentration of the second electrolyte solution exceeding a predetermined upper concentration threshold; ii) the concentration of the first electrolyte solution falling below a predetermined lower concentration threshold; iii) the volume of the second electrolyte solution falling below a predetermined lower volume threshold; or iv) the volume of the first electrolyte solution exceeding a predetermined upper volume threshold.

[0015] A method of rebalancing a redox flow battery according to one example of the present disclosure includes providing a flow battery such as any of the previous examples, condensing a base solvent from the vapor phase of a first electrolyte solution to produce a recovered base solvent, and returning the recovered base solvent to a second electrolyte solution, thereby causing a rebalance in the concentrations of the first and second electrolyte solutions.

[0016] In a further example of any of the foregoing embodiments, the condensation is performed by heat exchange with a vapor phase.

[0017] A further example of any of the foregoing embodiments further includes evaporating the base solvent from the first electrolyte solution into a gas phase using an evaporator.

[0018] In a further example of any of the foregoing embodiments, the condensation is effected by increasing the pressure of the vapor phase.

[0019] In a further example of any of the foregoing embodiments, condensation produces a recovered gas phase, and the recovered gas phase is returned to the first vessel.

[0020] In a further example of any of the foregoing embodiments, returning the recovered gas phase to the first electrolyte solution comprises bubbling the recovered gas phase through the first electrolyte solution.

[0021] In a further example of any of the foregoing embodiments, condensation is prompted in response to at least one of: i) the concentration of the second electrolyte solution exceeding a predetermined upper concentration threshold; ii) the concentration of the first electrolyte solution falling below a predetermined lower concentration threshold; iii) the volume of the second electrolyte solution falling below a predetermined lower volume threshold; or iv) the volume of the first electrolyte solution exceeding a predetermined upper volume threshold.

[0022] The present disclosure may include any one or more of the individual features disclosed above and / or below, taken alone or in any combination thereof.

[0023] The various features and advantages of the present disclosure will become apparent to those skilled in the art from the following detailed description. The drawings accompanying the detailed description can be briefly described as follows. [Brief explanation of the drawings]

[0024] [Figure 1] 1 shows a redox flow battery. [Figure 2] 1 shows a rebalancing system for a redox flow battery. [Figure 3] 3 illustrates a further example of the rebalancing system of FIG. 2. [Figure 4] 1 shows a rebalancing system additionally having an evaporator. [Figure 5] 1 shows another rebalancing system with an evaporator. [Figure 6] 1 shows a rebalancing system including a diffuser. DETAILED DESCRIPTION OF THE INVENTION

[0025] FIG. 1 schematically illustrates a portion of an exemplary system 10 including a redox flow battery 20 ("RFB 20") for selectively storing and discharging electrical energy. As an example, the RFB 20 may be used to convert electrical energy into chemical energy. The RFB 20 may then be used to reconvert the chemical energy into electrical energy, for example, to supply to a power grid. Thus, the RFB 20 provides storage of electrical energy.

[0026] RFB 20 includes a first electrolyte solution 22 having at least one electrochemically active species 24 that functions as a redox couple with a second electrolyte solution 26 having at least one electrochemically active species 28. As will be understood, the terms "first" and "second" are used to distinguish between two different electrolytes. Furthermore, it is understood that the terms "first" and "second" as used herein are interchangeable in that a "first" can also be referred to as a "second," and vice versa.

[0027] Electrochemically active species 24 / 28 include ions with multiple reversible oxidation states in a selected base solvent (such as, but not limited to, water, acetonitrile, dimethoxyethane, and propylene carbonate). In some examples, the multiple oxidation states are non-zero oxidation states, such as transition metals, including, but not limited to, vanadium, iron, manganese, chromium, zinc, molybdenum, sulfur, cerium, lead, tin, titanium, germanium, and functional combinations thereof. In some cases, the transition metals may be modified by binding chelators, including, but not limited to, ethylenediaminetetraacetic acid (EDTA) or other aminopolycarboxylic acids, acetylacetonates, bipyridyl, and phenanthrene. In some examples, the multiple oxidation states may include the zero oxidation state if the element is readily soluble in the selected liquid solution in the zero oxidation state. Such elements may include halogens, such as bromine and chlorine, and combinations thereof. The electrochemically active species 24 / 28 may be organic molecules or polymers containing groups that undergo electrochemically reversible reactions, such as quinones or nitrogen-containing organics such as quinoxalines or pyrazines. The electrolyte 22 / 26 is a solution containing one or more of the electrochemically active species 24 / 28. The first electrolyte solution 22 and the second electrolyte solution 26 are contained in a supply / storage system 30 that includes a first container 32 and a second container 34.

[0028] The electrolyte solution 22 / 26 is circulated by a pump 35 to at least one redox flow cell 36 of the RFB 20 via a respective supply line 38 and returned from the cell 36 to the reservoir 32 / 34 via a respective return line 40. As can be appreciated, additional pumps 35 can be used as needed, as can valves (not shown) at the inlets / outlets of the components of the RFB 20 to control flow rates. In this example, the supply line 38 and return line 40 connect the reservoir 32 / 34 in each loop L1 / L2 to a first electrode 42 and a second electrode 44. Multiple cells 36 can be provided as a stack in the loop L1 / L2.

[0029] Each of the one or more cells 36 includes a first electrode 42, a second electrode 44 spaced apart from the first electrode 42, and a barrier layer 46 disposed between the first electrode 42 and the second electrode 44. For example, the electrodes 42 / 44 may be porous, conductive structures such as carbon paper or felt. The electrodes 42 / 44 may also contain additional catalytically active materials, such as metals or metal oxides. Generally, the one or more cells 36 may include bipolar plates, manifolds, or the like for delivering the electrolyte 22 / 26 to the electrodes 42 / 44 through flow field channels. However, it should be understood that other configurations may also be used. For example, the one or more cells 36 may alternatively be configured for flow-through operation, in which the electrolyte solution 22 / 26 is pumped directly to the electrodes 42 / 44 without the use of flow field channels.

[0030] Barrier layer 46 may be, but is not limited to, an ion exchange membrane, a microporous polymer membrane, or an electrically insulating microporous matrix of a material such as silicon carbide (SiC), which prevents the electrolyte solutions 22 / 26 from freely and rapidly mixing, but allows selected ions to pass through while electrically insulating electrodes 42 / 44 to complete the redox reaction. In this regard, loops L1 / L2 are isolated from one another during normal operation, such as charging, discharging, and shutdown states.

[0031] The electrolyte solution 22 / 26 is supplied to and circulated through one or more cells 36 during active charge and discharge modes, converting electrical energy to chemical energy or, in the reverse reaction, converting chemical energy to electrical energy that is discharged. Electrical energy is transferred to and from one or more cells 36 via an electrical circuit 48 electrically coupled to the electrodes 42 / 44.

[0032] The base solvent of the electrolyte 22 / 26 tends to migrate across the barrier layer 46, such as by diffusion. For example, diffusion can be driven by differences in the concentrations of the species 24 / 28 in the electrolyte solution 22 / 26, differences in the ionic strength of the species 24 / 28, electroosmotic resistance of the base solvent across the barrier layer 46 containing charge carriers during charging and discharging, or a combination of these phenomena. As a result, over time, a net gain of base solvent can occur in one of the electrolyte solutions 22 / 26 and a net loss of base solvent in the other electrolyte solution 22 / 26, ultimately reducing the efficiency and limiting the electrical storage capacity of the RFB 20. In systems with compatible electrolyte solutions, such as an all-vanadium system, a portion of the electrolyte solution that has gained solvent can be mixed back into the electrolyte solution that has lost solvent to rebalance the electrolyte volume and concentration. However, in systems with incompatible electrolyte solutions (but using the same base solvent), such as systems with species that react to form precipitates or inert reaction products (e.g., sulfur / manganese), mixing the electrolytes is not an option. In this regard, as described in more detail below, RFB 20 includes a rebalancing system 50 having a separator 52 and a supply line 54. Rebalancing system 50 operates to rebalance electrolyte solutions 22 / 26 without mixing them. While such a rebalancing system 50 is expected to be most beneficial for systems utilizing incompatible electrolyte solutions, the examples herein are also applicable to systems utilizing compatible electrolytes.

[0033] An example of a rebalancing system 50 is shown in FIG. 2. The examples herein are based on diffusion tending to cause a net gain of base solvent in the first electrolyte solution 22 and a net loss of base solvent in the second electrolyte solution 26. However, it should be understood that these examples equally apply to the reverse scenario, where diffusion tends to cause a net gain of base solvent in the second electrolyte solution 26 and a net loss of base solvent in the first electrolyte solution 22. As will be apparent to those skilled in the art, the "direction" of the net gain or loss of base solvent will depend on the configuration and operating parameters of the particular RFB and can be readily determined through experimentation and / or operation of the RFB.

[0034] The second electrolyte solution 26 releases a vapor phase (V) containing a base solvent (e.g., a "wet" gas). For example, the RFB 20 generates heat during operation. This heat tends to evaporate some of the base solvent from the electrolyte solution 22 / 26, thereby naturally generating the vapor phase (V) during RFB operation. The rebalancing system 50 includes a separator 52 fluidly connected to the first vessel 32 to collect the vapor phase (V). For example, the vapor phase (V) tends to collect in the headspace within the vessel 32 above the liquid level of the first electrolyte solution 22, and a feed line 54 having an inlet in the headspace and leading to the separator 52 is provided. A fan, pump, or other moving device 56 may be provided in the feed line 54 to transfer the vapor phase (V) from the vessel 32 to the separator 52, which is located outside the vessel 32. Alternatively, for a more compact system, the separator 52 may be located inside the vessel 32.

[0035] In the illustrated example, separator 52 is a heat exchanger condenser. The condenser receives a working fluid (refrigerant) and is operable to reduce the temperature of the vapor phase (V) circulating therethrough. The base solvent is condensed to produce a relatively pure recovered base solvent (R). The type of condenser is not particularly limited, so long as the vapor phase (V) can be circulated through the condenser and the condensed base solvent can be recovered. The recovered base solvent (R) is then returned to second electrolyte solution 26, such as via return line 58 to vessel 34. The "dry" gas phase remaining after the base solvent is condensed is returned to first vessel 32 via gas phase return line 60.

[0036] In the illustrated example, the outlet of the gas phase return line 60 is located in the headspace above the first electrolyte solution 22. Alternatively, as shown in FIG. 3, the outlet of the gas phase return line 60 is immersed in the first electrolyte solution 22, allowing the dry gas phase to bubble through the first electrolyte solution 22. To ensure immersion, the outlet is positioned at a level significantly below the minimum expected level of the first electrolyte solution 22 in the container 32. The bubbling promotes the capture of base solvent vapor from the first electrolyte solution 22. To enhance bubbling, the outlet may include a porous element, such as a porous frit. The pores in the porous element break up the gas bubbles into smaller bubbles, increasing the surface area for evaporation.

[0037] Returning the recovered base solvent (R) to the second electrolyte solution 26 reverses the imbalance, decreasing the concentration of the second electrolyte solution 26 and increasing the concentration of the first electrolyte solution 22. Condensation and return can be performed continuously in accordance with a known or estimated diffusion rate of the base solvent across the barrier layer 46 to maintain the balance within a desired range, or selectively as needed when the balance falls outside the desired range. For example, the separator 52 is activated (re-condensed) in response to one of the following: i) the concentration of the second electrolyte solution 26 exceeds a predetermined upper concentration threshold; ii) the concentration of the first electrolyte solution 22 falls below a predetermined lower concentration threshold; iii) the volume of the second electrolyte solution 26 falls below a predetermined lower concentration threshold; or iv) the volume of the first electrolyte solution 22 exceeds a predetermined upper volume threshold. Concentrations can be determined using known devices and techniques, such as, but not limited to, spectroscopy and wet electrochemistry. The volume may be determined from the fill level or fill gauge of the vessel 32 / 34. In this regard, the RFB 20 may incorporate an electronic controller configured to operate (via software, hardware, or both) at least the separator 52 in accordance with the control strategies described above. Such an electronic controller may be connected to, control the operation of, and provide feedback to, the concentration measuring device, the fill level measuring device, the pump 35, the valves, or other components within the RFB 20.

[0038] FIG. 4 shows a further example in which the rebalancing system 50 is the same as that of FIG. 3 except for the inclusion of an evaporator 62 in the supply line 54 outside the vessel 32. The evaporator 62 is operable to promote evaporation of the first electrolyte solution 22 (as opposed to natural evaporation as in the example of FIG. 3) to produce a vapor phase (V). For example, the gas from the headspace of the vessel 32 may already contain some base solvent vapor phase (V) due to natural evaporation caused by heat generated by operation of the RFB 20. The first electrolyte solution 22 circulates through the evaporator 62 via a circulation line 64. The evaporator 62 may include a porous element to increase the liquid / gas interfacial surface area for the gas to capture additional vapor phase. In a further example, the evaporator 62 may also include a heater that can be used to counter evaporative cooling. The resulting vapor phase (V) is then fed from the evaporator 62 to the separator 52 (condenser), where the base solvent is recovered as described above. By capturing the additional base solvent, a relatively large amount of base solvent can be recovered for return to the second electrolyte 26. As with the separator 52, the evaporator 62 can be placed inside the vessel 32 for a more compact system.

[0039] In another configuration, shown in FIG. 5 , the rebalancing system includes supplemental heating (in addition to natural heating) of the first electrolyte solution 22 to generate the vapor phase (V). In this example, instead of the inlet of the supply line 54 being located within the headspace of the vessel 32, the inlet of the supply line 54 is below the headspace and immersed in the first electrolyte solution 22 to supply the first electrolyte solution 22 to the evaporator 62. The first electrolyte solution 22 circulates through the evaporator 62 via line 64. The evaporator 62 supplies supplemental heat to the first electrolyte solution 22, evaporating a portion of the base solvent to generate the vapor phase (V). The vapor phase (V) is then supplied via supply line 54 to the condenser 52, where the base solvent is recovered and returned to the second electrolyte solution 26 via return line 58, as described above. The dry gas phase, after the base solvent has been condensed, is returned to the evaporator 62 via gas phase return line 60 and bubbled through the first electrolyte solution 22 therein to enhance capture of the base solvent vapor. Alternatively, in a more compact system, the evaporator 62 may be located inside the vessel 32 .

[0040] In the above-described example, the rebalancing system 50 is temperature-driven, i.e., temperature is reduced to induce condensation. However, in the next example of the rebalancing system 150 shown in FIG. 6, the recovery of the base solvent is pressure-driven, i.e., pressure is increased to induce condensation. In this regard, in the rebalancing system 150, the separator 152 is a diffuser rather than a condenser as in the previous example. Vapor-phase (V) gas from the headspace of the vessel 32 is fed to the diffuser. The diffuser increases the volume of the gas flow, effectively slowing the flow rate. The reduced rate increases the pressure, causing the base solvent to condense. As in the previous example, the recovered base solvent (R) is then sent back to the second electrolyte solution 26 via the return line 58 for rebalancing. In an additional example, the condenser in each of the examples shown in FIGS. 2-5 is replaced with a diffuser. The use of a diffuser eliminates the need for a coolant used in the condenser.

[0041] Although combinations of features are shown in the illustrated examples, not all features need to be combined to realize the benefits of various embodiments of the present disclosure. In other words, a system designed in accordance with an embodiment of the present disclosure will not necessarily include all of the features shown in any of the figures or all of the portions schematically shown in the figures. Furthermore, selected features of one exemplary embodiment may be combined with selected features of other exemplary embodiments.

[0042] The foregoing description is exemplary in nature, rather than limiting. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from the disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.

Claims

1. a redox flow cell including a barrier layer disposed between a first electrode and a second electrode; a supply and storage system external to the redox flow cell, the supply and storage system including a first container and a second container, a first electrolyte solution and a second electrolyte solution in the first container and the second container, respectively, fluid lines connecting the first container and the second container to the first electrode and the second electrode, respectively, and a plurality of pumps operable to circulate the first electrolyte solution and the second electrolyte solution through the redox flow cell and the first container and the second container via the fluid lines; the supply and storage system, wherein the first electrolyte solution and the second electrolyte solution contain a base solvent, and the base solvent tends to migrate across the barrier layer from the second electrode toward the first electrode, thereby causing an imbalance such that the concentration of the second electrolyte solution increases and the concentration of the first electrolyte solution decreases, and the first electrolyte solution gives off a vapor phase containing the base solvent; a separator in fluid communication with the first vessel for receiving the vapor phase, the separator operable to condense the base solvent from the vapor phase to produce a recovered base solvent and return the recovered base solvent to the second electrolyte solution, thereby reversing the imbalance and decreasing the concentration of the second electrolyte solution and increasing the concentration of the first electrolyte solution; Redox flow batteries, including:

2. 10. The redox flow battery of claim 1, wherein the separator comprises a heat exchanger condenser.

3. 3. The redox flow battery of claim 2, further comprising an evaporator operable to evaporate the base solvent from the first electrolyte solution to produce the vapor phase.

4. 4. The redox flow battery of claim 3, further comprising a supply line to the evaporator, the supply line having an inlet disposed in a headspace of the first container above the first electrolyte solution.

5. 4. The redox flow battery of claim 3, further comprising a supply line to the evaporator, the supply line being positioned below a headspace of the first container and having an inlet immersed in the first electrolyte solution.

6. 4. The redox flow battery of claim 3, wherein the evaporator is outside the first container.

7. 10. The redox flow battery of claim 1, wherein the separator comprises a diffuser.

8. 10. The redox flow battery of claim 1, wherein the separator includes a vapor return line connected to the first vessel.

9. 9. The redox flow battery of claim 8, wherein the vapor return line includes an outlet immersed in the first electrolyte solution.

10. 10. The redox flow battery of claim 1, wherein the first electrolyte solution and the second electrolyte solution are incompatible with each other in that, when mixed, they react to form a precipitate or react to become electrochemically inactive.

11. 10. The redox flow battery of claim 1, wherein the separator activates in response to at least one of: i) the concentration of the second electrolyte solution exceeding a predetermined upper concentration threshold; ii) the concentration of the first electrolyte solution falling below a predetermined lower concentration threshold; iii) the volume of the second electrolyte solution falling below a predetermined lower volume threshold; or iv) the volume of the first electrolyte solution exceeding a predetermined upper volume threshold.

12. 1. A method for rebalancing a redox flow battery, comprising: a flow battery having a redox flow cell including a barrier layer disposed between a first electrode and a second electrode; a supply and storage system external to the redox flow cell, the supply and storage system including a first container and a second container having a first electrolyte solution and a second electrolyte solution in the first container and the second container, respectively; fluid lines connecting the first container and the second container to the first electrode and the second electrode, respectively; and a plurality of pumps operable to circulate the first electrolyte solution and the second electrolyte solution through the redox flow cell and the first container and the second container via the fluid lines; providing the flow battery, wherein the first electrolyte solution and the second electrolyte solution include a base solvent, the base solvent having a tendency to migrate across the barrier layer from the second electrode to the first electrode, causing a concentration imbalance between the first electrolyte solution and the second electrolyte solution; condensing a base solvent from the vapor phase of the first electrolyte solution to produce a recovered base solvent, the condensing returning the recovered base solvent to the second electrolyte solution, thereby causing a rebalance of concentrations in the first electrolyte solution and the second electrolyte solution; The method comprising:

13. 13. The method of claim 12, wherein the condensing is performed via heat exchange with the vapor phase.

14. The method of claim 13 , further comprising evaporating the base solvent from the first electrolyte solution to the vapor phase using an evaporator.

15. The method of claim 12 , wherein the condensing is effected by increasing the pressure of the vapor phase.

16. 13. The method of claim 12, wherein the condensing produces a recovered vapor phase, and the recovered vapor phase is returned to the first vessel.

17. 17. The method of claim 16, wherein said returning said recovered gas phase to said first electrolyte solution comprises bubbling said recovered gas phase through said first electrolyte solution.

18. 13. The method of claim 12, wherein the condensing is activated in response to at least one of: i) the concentration of the second electrolyte solution exceeding a predetermined upper concentration threshold; ii) the concentration of the first electrolyte solution falling below a predetermined lower concentration threshold; iii) the volume of the second electrolyte solution falling below a predetermined lower volume threshold; or iv) the volume of the first electrolyte solution exceeding a predetermined upper volume threshold.

Images