Patent Publication Number: US-2023144710-A1

Title: Condensation-based redox flow battery rebalancing

Description:
BACKGROUND 
     Flow batteries, also known as redox flow batteries or redox flow cells, are designed to convert electrical energy into chemical energy that can be stored and later released back as electrical energy when there is demand. As an example, a flow battery may be used with a renewable energy system, such as a wind-powered system, to store energy that exceeds consumer demand and later release that energy when there is greater demand. 
     A typical flow battery includes a redox flow cell that has 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 referred to as the anolyte or negolyte) is delivered to the negative electrode and a positive fluid electrolyte (sometimes referred to as the catholyte or posolyte) is delivered to the positive electrode to drive reversible redox reactions between redox pairs. Upon charging, the electrical energy supplied causes a reduction reaction in one electrolyte and an oxidation reaction in the other electrolyte. The separator prevents the electrolytes from freely and rapidly mixing but selectively permits ions to pass through to complete the redox reactions. Upon discharge, the chemical energy contained in the liquid electrolytes is released in the reverse reactions and electrical energy is drawn from the electrodes. 
     SUMMARY 
     A redox flow battery according to an example of the present disclosure includes a redox flow cell, a supply and storage system external of the redox flow cell, and a separator. The redox flow cell has a barrier layer arranged between first and second electrodes. The supply and storage system includes first and second vessels, first and second liquid electrolyte solutions in, respectively, the first and second vessels, fluid lines connecting the first and second vessels to, respectively, the first and second electrodes, and a plurality of pumps operable to circulate the first and second liquid electrolyte solutions via the fluid lines through the redox flow cell and the first and second vessels. The first and second electrolyte solutions include a base solvent that has a tendency to migrate across the barrier layer from the second electrode toward the first electrode and thereby cause an imbalance such that the second electrolyte solution increases in concentration and the first electrolyte solution decreases in concentration. The first electrolyte solution emanates a vapor phase containing the base solvent. The separator is in fluid connection with the first vessel to receive the vapor phase. The separator is operable to condense the base solvent from the vapor phase to produce recovered base solvent and return the recovered base solvent to the second electrolyte solution to thereby reverse the imbalance such that the concentration of the second electrolyte solution decreases and the concentration of the first electrolyte solution increases. 
     In a further example of the foregoing embodiment, the separator includes a heat exchanger condenser. 
     A further example of any of the foregoing embodiments further includes an evaporator that is operable to evaporate the base solvent from the first electrolyte solution to produce the vapor phase. 
     A further example of any of the foregoing embodiments further includes a feed line into the evaporator, and the feed line has an inlet located in a headspace of the first vessel above the first electrolyte solution. 
     A further example of any of the foregoing embodiments further includes a feed line into the evaporator, and the feed line has an inlet located below a headspace of the first vessel and submersed in the first electrolyte solution. 
     In a further example of any of the foregoing embodiments, the evaporator is outside of the first vessel. 
     In a further example of any of the foregoing embodiments, the separator includes a diffuser. 
     In a further example of any of the foregoing embodiments, the separator includes a gas phase return line connected to the first vessel. 
     In a further example of any of the foregoing embodiments, the gas phase return line includes an outlet that is submersed in the first electrolyte solution. 
     In a further example of any of the foregoing embodiments, the first and second electrolyte solutions are incompatible with each other in that, if mixed, they react to produce precipitate or react to become electrochemically inert. 
     In a further example of any of the foregoing embodiments, the separator is activated responsive to at least one of i) the concentration of the second electrolyte solution exceeding a preset concentration upper threshold, ii) the concentration of the first electrolyte solution falling below a preset concentration lower threshold, iii) the volume of the second electrolyte solution falling below a preset volume lower threshold, or iv) the volume of the first electrolyte solution exceeding a preset volume upper threshold. 
     A method for rebalancing a redox flow battery according to an example of the present disclosure includes providing a flow battery as in any of the foregoing examples, condensing a base solvent from the vapor phase of the first electrolyte solution to produce recovered base solvent, and returning the recovered base solvent to the second electrolyte solution to thereby cause a concentration rebalance in the first and second electrolyte solutions. 
     In a further example of any of the foregoing embodiments, the condensing is conducted via heat exchange with the vapor phase. 
     A further example of any of the foregoing embodiments further includes evaporating the base solvent from the first electrolyte solution into the vapor phase using an evaporator. 
     In a further example of any of the foregoing embodiments, the condensing is conducted via pressure increase of the vapor phase. 
     In a further example of any of the foregoing embodiments, the condensing produces a recovered gas phase, and returning the recovered gas phase to the first vessel. 
     In a further example of any of the foregoing embodiments, the returning of the recovered gas phase to the first electrolyte solution includes bubbling the recovered gas phase through the first electrolyte solution. 
     In a further example of any of the foregoing embodiments, the condensing is activated responsive to at least one of i) the concentration of the second electrolyte solution exceeding a preset concentration upper threshold, ii) the concentration of the first electrolyte solution falling below a preset concentration lower threshold, iii) the volume of the second electrolyte solution falling below a preset volume lower threshold, or iv) the volume of the first electrolyte solution exceeding a preset volume upper threshold. 
     The present disclosure may include any one or more of the individual features disclosed above and/or below alone or in any combination thereof. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       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 that accompany the detailed description can be briefly described as follows. 
         FIG.  1    illustrates a redox flow battery. 
         FIG.  2    illustrates a rebalancing system of the redox flow battery. 
         FIG.  3    illustrates a further example of the rebalancing system of  FIG.  2   . 
         FIG.  4    illustrates a rebalancing system that additionally has an evaporator. 
         FIG.  5    illustrates another rebalancing system that has an evaporator. 
         FIG.  6    illustrates a rebalancing system that includes a diffuser. 
     
    
    
     DETAILED DESCRIPTION 
       FIG.  1    schematically shows portions of an example system  10  that includes a redox flow battery  20  (“RFB  20 ”) for selectively storing and discharging electrical energy. As an example, the RFB  20  can be used to convert electrical energy to chemical energy. At a later time, the RFB  20  can be used to convert the chemical energy back into electrical energy that may be provided to an electric grid, for example. The RFB  20  thus provides for electrical energy storage. 
     The RFB  20  includes a first electrolyte solution  22  that has at least one electrochemically active species  24  that functions in a redox pair with regard to a second electrolyte solution  26  that has at least one electrochemically active species  28 . As will be appreciated, the terminology “first” and “second” is to differentiate that there are two distinct electrolytes. It is to be further understood that terms “first” and “second” as used herein are interchangeable in that a “first” could alternatively be termed a “second,” and vice versa. 
     The electrochemically active species  24 / 28  include ions that have 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 can be modified by bound chelating agents, including but not limited to ethylendiaminetetraacetic acid (EDTA) or other aminopolycarboxylic acids, acetylacetonates, bipyridyls, and phenanthrenes. In some examples, the multiple oxidation states can include the zero oxidation state if the element is readily soluble in the selected liquid solution in the zero oxidation state. Such elements can include the halogens, such as bromine, chlorine, and combinations thereof. The electrochemically active species  24 / 28  could also be organic molecules or macromolecules that contain groups that undergo electrochemically reversible reactions, such as quinones or nitrogen-containing organics, such as quinoxalines or pyrazines. The electrolytes  22 / 26  are solutions that include 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 first and second vessels  32 / 34 . 
     The electrolyte solutions  22 / 26  are circulated by pumps  35  to at least one redox flow cell  36  of the RFB  20  through respective feed lines  38 , and are returned from the cell  36  to the vessels  32 / 34  via respective return lines  40 . As can be appreciated, additional pumps  35  can be used if needed, as well as valves (not shown) at the inlets/outlets of the components of the RFB  20  to control flow. In this example, the feed lines  38  and the return lines  40  connect the vessels  32 / 34  in respective loops L 1 /L 2  with first and second electrodes  42 / 44 . Multiple cells  36  can be provided as a stack within the loops L 1 /L 2 . 
     The cell or cells  36  each include the first electrode  42 , the second electrode  44  spaced apart from the first electrode  42 , and a barrier layer  46  arranged between the first electrode  42  and the second electrode  44 . For example, the electrodes  42 / 44  may be porous electrically-conductive structures, such as carbon paper or felt. The electrodes  42 / 44  may also contain additional materials which are catalytically-active, for example a metal or metal oxide. In general, the cell or cells  36  can include bipolar plates, manifolds and the like for delivering the electrolytes  22 / 26  through flow field channels to the electrodes  42 / 44 . It is to be understood, however, that other configurations can be used. For example, the cell or cells  36  can alternatively be configured for flow-through operation where the electrolyte solutions  22 / 26  are pumped directly into the electrodes  42 / 44  without the use of flow field channels. 
     The barrier layer  46  can be, but is not limited to, an ionic-exchange membrane, a micro-porous polymer membrane, or an electrically insulating micro-porous matrix of a material, such as silicon carbide (SiC), that prevents the electrolyte solutions  22 / 26  from freely and rapidly mixing but permits selected ions to pass through to complete the redox reactions while electrically isolating the electrodes  42 / 44 . In this regard, the loops L 1 /L 2  are isolated from each other during normal operation, such as charge, discharge and shutdown states. 
     The electrolyte solutions  22 / 26  may be delivered to, and circulate through, the cell or cells  36  during an active charge mode and discharge mode to either convert electrical energy into chemical energy or, in the reverse reaction, convert chemical energy into electrical energy that is discharged. The electrical energy is transmitted to and from the cell or cells  36  through an electric circuit  48  that is electrically coupled with the electrodes  42 / 44 . 
     The base solvent of the electrolytes  22 / 26  has a tendency to migrate across the barrier layer  46 , such as by diffusion. For example, differences in concentrations of the species  24 / 28  in the electrolyte solutions  22 / 26 , differences in ionic strengths of the species  24 / 28 , electro-osmotic drag of the base solvent across barrier layer  46  with charge carriers during charge and discharge, or combinations of these phenomena can drive diffusion. The result is that over time there can be a net gain of the base solvent in one of the electrolyte solutions  22 / 26  and a net loss of the base solvent in the other of the electrolyte solutions  22 / 26 , which ultimately reduces efficiency and limits the electrical storage capacity of the RFB  20 . In systems where the electrolyte solutions are compatible, such as an all vanadium system, some of the electrolyte solution that gained solvent can be transferred back into, and mixed with, the electrolyte solution that lost solvent in order to rebalance the electrolyte volumes and concentrations. However, in systems where the electrolyte solutions are incompatible (but use 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 will be discussed in further detail below, the RFB  20  includes a rebalancing system  50 , having a separator  52  and a feed line  54 . Rebalancing system  50  is operable to rebalance the electrolyte solutions  22 / 26  without mixing them together. While such a rebalancing system  50  is expected to be most beneficial for systems that utilize incompatible electrolyte solutions, the examples herein could also be applied to systems that utilize compatible electrolytes. 
     An example of the rebalancing system  50  is represented in  FIG.  2   . The examples herein are based on diffusion that tends to causes a net gain of the base solvent in the first electrolyte solution  22  and a net loss of the base solvent in the second electrolyte solution  26 . It is to be understood, however, that the examples are equally applicable for the inverse scenario, in which the diffusion tends to causes a net gain of the base solvent in the second electrolyte solution  26  and a net loss of the base solvent in the first electrolyte solution  22 . As those of ordinary skill in the art will be aware of, the “direction” in which there is a net gain or loss of the base solvent will depend on the configuration and operating parameters of the particular RFB and can be readily determined via experimentation and/or operation of the RFB. 
     The second electrolyte solution  26  emanates a vapor phase (V) that contains the base solvent (e.g., a “wet” gas). For example, the RFB  20  generates heat during operation. This heat tends to cause evaporation of some of the base solvent from the electrolyte solutions  22 / 26 , thereby naturally producing the vapor phase (V) with RFB operation. The rebalancing system  50  includes a separator  52  that is in fluid connection with the first vessel  32 , to collect the vapor phase (V). For instance, the vapor phase (V) tends to collect in the headspace in the vessel  32  above the liquid level of the first electrolyte solution  22 , and there is a feed line  54  that has an inlet at the headspace and that leads into the separator  52 . A fan, a pump, or other mover  56  may be provided in the feed line  54  in order to transfer the vapor phase (V) from the vessel  32  into the separator  52  that is outside of the vessel  32 . Alternatively, for a more compact system, the separator  52  may be located inside of the vessel  32 . 
     In the illustrated example, the separator  52  is a heat exchanger condenser. The condenser is operable to receive a working fluid (coolant) to reduce the temperature of the vapor phase (V) that is circulated through it. The base solvent condenses to produce recovered base solvent (R) that is relatively pure. The type of condenser is not particularly limited, as long as the vapor phase (V) can be circulated through it and the condensed base solvent can be collected. The recovered base solvent (R) is then returned to the second electrolyte solution  26 , such as via a return line  58  to the vessel  34 . The remaining “dry” gas phase after the base solvent is condensed is returned via gas phase return line  60  to the first vessel  32 . 
     In the illustrated example, the outlet of the gas phase return line  60  is 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 submersed in the first electrolyte solution  22  such that the dry gas phase is bubbled through the first electrolyte solution  22 . To ensure submersion, the outlet is placed at a level that is substantially below the expected minimum level of the first electrolyte solution  22  in the vessel  32 . The bubbling enhances capture of base solvent vapor from the first electrolyte solution  22 . For increased bubbling, the outlet may include a porous element, such as a porous frit. The pores of the porous element divide the gas bubbles into smaller bubbles, which increases surface area for evaporation. 
     The return of the recovered base solvent (R) to the second electrolyte solution  26  reverses the imbalance such that the concentration of the second electrolyte solution  26  decreases and the concentration of the first electrolyte solution  22  increases. The condensing and returning can be conducted continuously in coordination with a known or estimated diffusion rate of the base solvent across the barrier layer  46  in order to maintain a balance within a desired margin, or conducted selectively as needed when it falls outside of a desired margin. For instance, the separator  52  is activated (to condense and return) responsive to at least one of i) the concentration of the second electrolyte solution  26  exceeding a preset concentration upper threshold, ii) the concentration of the first electrolyte solution  22  falling below a preset concentration lower threshold, iii) the volume of the second electrolyte solution  26  falling below a preset volume lower threshold, or iv) the volume of the first electrolyte solution  22  exceeding a preset volume upper threshold. Concentrations can be determined using known equipment and techniques, such as but not limited to, spectroscopy or wet electrochemistry. Volumes can be determined from fill levels or fill gauges of the vessels  32 / 34 . In these regards, the RFB  20  may incorporate an electronic controller that is configured (via software, hardware, or both) to operate at least the separator  52  in accordance with the above control strategy. Such an electronic controller may be connected to concentration measurement equipment, fill level measurement equipment, the pumps  35 , valves, or other components in the RFB  20  to control operation and provide feedback. 
       FIG.  4    illustrates a further example in which the rebalancing system  50  is the same as in  FIG.  3    except that there is an evaporator  62  in the feed line  54  outside of the vessel  32 . The evaporator  62  is operable to enhance evaporation (versus natural evaporation as in the example of  FIG.  3   ) of the first electrolyte solution  22  to produce the vapor phase (V). For instance, the gas from the headspace of the vessel  32  may already carry some vapor phase (V) of the base solvent due to natural evaporation from the heat generated by operation of the RFB  20 . The first electrolyte solution  22  is circulated via circulation lines  64  through the evaporator  62 . The evaporator  62  may include a porous element that increases liquid/gas interface surface area for the gas to capture additional vapor phase. In a further example, the evaporator  62  may also include a heater that may be used to counteract evaporative cooling. The resulting vapor phase (V) is then fed from the evaporator  62  into the separator  52  (condenser) to recover the base solvent, as discussed above. By capturing additional base solvent, a relatively higher amount of base solvent can be recovered for return to the second electrolyte  26 . As for the separator  52 , the evaporator  62  may be located inside of the vessel  32  for a more compact system. 
     In another configuration illustrated in  FIG.  5   , the rebalancing system includes supplemental heating of the first electrolyte solution  22  (in addition to natural heating) to generate the vapor phase (V). In this example, instead of the inlet of the feed line  54  being located in the headspace of the vessel  32 , the inlet of the feed line  54  is below the headspace and submersed in the first electrolyte solution  22  in order to feed the first electrolyte solution  22  to the evaporator  62 . The first electrolyte solution  22  is circulated through the evaporator  62  via lines  64 . The evaporator  62  provides supplemental heat to the first electrolyte solution  22 , causing evaporation of some of the base solvent to produce the vapor phase (V). The vapor phase (V) is then fed to the condenser  52  via feed line  54  to recover the base solvent, as discussed above, and return it via return line  58  to the second electrolyte solution  26 . The dry gas phase after the base solvent is condensed is returned via gas phase return line  60  to the evaporator  62  and bubbled through the first electrolyte solution  22  therein to enhances capture of base solvent vapor. Alternatively, for a more compact system, the evaporator  62  may be located inside of the vessel  32 . 
     In the examples above, the rebalancing system  50  is temperature-driven, i.e., reduce temperature to drive condensation. In the rebalancing system  150  in following example of  FIG.  6   , however, the recovery of the base solvent is pressure-driven, i.e., increase pressure to drive condensation. In this regard, in the rebalancing system  150  the separator  152  is a diffuser instead of a condenser as in the prior examples. The vapor phase (V) gas from the headspace of the vessel  32  is provided into the diffuser. The diffuser provides an increase in volume for the gas flow, which has the effect of reducing the flow velocity. The velocity-decrease causes an increase in pressure that results in condensation of the base solvent. As in the prior examples, the recovered base solvent (R) is then fed back into the second electrolyte solution  26  via return line  58  for rebalancing. In additional examples, the condenser in each of the examples of  FIGS.  2 - 5    is replaced with the diffuser. Use of the diffuser eliminates the need for a coolant that would be used in the condenser. 
     Although a combination of features is shown in the illustrated examples, not all of them need to be combined to realize the benefits of various embodiments of this disclosure. In other words, a system designed according to an embodiment of this disclosure will not necessarily include all of the features shown in any one of the Figures or all of the portions schematically shown in the Figures. Moreover, selected features of one example embodiment may be combined with selected features of other example embodiments. 
     The preceding description is exemplary rather than limiting in nature. Variations and modifications to the disclosed examples may become apparent to those skilled in the art that do not necessarily depart from this disclosure. The scope of legal protection given to this disclosure can only be determined by studying the following claims.