Abstract:
A flow battery includes a membrane having a thickness of less than approximately one hundred twenty five micrometers; and a solution having a reversible redox couple reactant, wherein the solution wets the membrane.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is related to PCT Application No. PCT/US09/68681 filed on Dec. 18, 2009 and U.S. patent application Ser. No. 13/022,285 filed on Feb. 7, 2011, each of which is incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    1. Technical Field 
         [0003]    This disclosure relates generally to a flow battery system and, more particularly, to a flow battery having a low resistance membrane. 
         [0004]    2. Background Information 
         [0005]    A typical flow battery system includes a stack of flow battery cells, each having an ion-exchange membrane disposed between negative and positive electrodes. During operation, a catholyte solution flows through the positive electrode, and an anolyte solution flows through the negative electrode. The catholyte and anolyte solutions each electrochemically react in a reversible reduction-oxidation (“redox”) reaction. Ionic species are transported across the ion-exchange membrane during the reactions, and electrons are transported through an external circuit to complete the electrochemical reactions. 
         [0006]    The ion-exchange membrane is configured to be permeable to certain non-redox couple reactants (also referred to as “charge transportions” or “charge carrier ions”) in the catholyte and anolyte solutions to facilitate the electrochemical reactions. Redox couple reactants (also referred to as “non-charge transportions” or “non-charge carrier ions”) in the catholyte and anolyte solutions, however, can also permeate through the ion-exchange membrane and mix together. The mixing of the redox couple reactants can induce in a self-discharge reaction that can disadvantageously decrease the overall energy efficiency of the flow battery system, especially when the flow battery cells are operated at current densities less than 100 milliamps per square centimeter (mA/cm 2 ), which is the typical current density operating range of conventional flow battery cells. 
         [0007]    The permeability of the ion-exchange membrane to the redox couple reactants is typically inversely related to a thickness of the ion-exchange membrane. A typical flow battery cell, therefore, includes a relatively thick ion-exchange membrane (e.g., ≧approximately 175 micrometers (μm); ˜6889 micro inches (μin)) to reduce or eliminate redox couple reactant crossover and mixing in an effort to decrease the overall energy inefficiency of the flow battery system, especially when the flow battery cells are operated at current densities less than 100 mA/cm 2 . 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIG. 1  is a schematic diagram of one embodiment of a flow battery system, which includes a plurality of flow battery cells arranged in a stack. 
           [0009]      FIG. 2  is a sectional diagrammatic illustration of one embodiment of one of the flow battery cells in  FIG. 1 , which includes an ion-exchange membrane. 
           [0010]      FIG. 3  is a cross-sectional diagrammatic illustration of one embodiment of the ion-exchange membrane in  FIG. 2 . 
           [0011]      FIGS. 4A to 4C  are enlarged partial sectional diagrammatic illustrations of different embodiments of the ion-exchange membrane in  FIG. 2 . 
           [0012]      FIG. 5  is a graphical comparison of overall energy inefficiencies versus current densities for two different flow battery cells. 
       
    
    
     DETAILED DESCRIPTION 
       [0013]    Referring to  FIG. 1 , a schematic diagram of a flow battery system  10  is shown. The flow battery system  10  is configured to selectively store and discharge electrical energy. By “store” it is meant that electrical energy is converted into a storable form that can later be converted back into electrical energy and discharged. During operation, for example, the flow battery system  10  can convert electrical energy generated by a renewable or non-renewable power system (not shown) into chemical energy, which is stored within a pair of first and second electrolyte solutions (e.g., anolyte and catholyte solutions). The flow battery system  10  can later convert the stored chemical energy back into electrical energy. Examples of suitable first and second electrolyte solutions include vanadium/vanadium electrolyte solutions, or any other pair of anolyte and catholyte solutions of substantially similar redox species. The pair of first and second electrolyte solutions, however, is not limited to the aforesaid examples. 
         [0014]    The flow battery system  10  includes a first electrolyte storage tank  12 , a second electrolyte storage tank  14 , a first electrolyte circuit loop  16 , a second electrolyte circuit loop  18 , at least one flow battery cell  20 , a power converter  23  and a controller  25 . In some embodiments, the flow battery system  10  can include a plurality of the flow battery cells  20  arranged and compressed into at least one stack  21  between a pair of end plates  39 , which cells  20  can be operated to collectively store and produce electrical energy. 
         [0015]    Each of the first and second electrolyte storage tanks  12  and  14  is adapted to hold and store a respective one of the electrolyte solutions. 
         [0016]    The first and second electrolyte circuit loops  16  and  18  each have a source conduit  22 ,  24 , a return conduit  26 ,  28  and a flow regulator  27 ,  29 , respectively. The first and second flow regulators  27  and  29  are each adapted to regulate flow of one of the electrolyte solutions through a respective one of the electrolyte circuit loops  16 ,  18  in response to a respective regulator control signal. Each flow regulator  27 ,  29  can include a single device, such as a variable speed pump or an electronically actuated valve, or a plurality of such devices, depending upon the particular design requirements of the flow battery system. Each flow regulator  27 ,  29  can be connected inline within its associated source conduit  22 ,  24 . 
         [0017]    Referring to  FIG. 2 , a diagrammatic illustration of one embodiment of the flow battery cell  20  is shown. The flow battery cell  20  includes a first current collector  30 , a second current collector  32 , a first liquid-porous electrode layer  34  (hereinafter “first electrode layer”), a second liquid-porous electrode layer  36  (hereinafter “second electrode layer”), and an ion-exchange membrane  38 . 
         [0018]    The first and second current collectors  30  and  32  are each adapted to transfer electrons to and/or away from a respective one of the first or second electrode layers  34 ,  36 . In some embodiments, each current collector  30 ,  32  includes one or more flow channels  40  and  42 . In other embodiments, one or more of the current collectors can be configured as a bipolar plate (not shown) with flow channels. Examples of such bipolar plates are disclosed in PCT Application No. PCT/US09/68681 and which is hereby incorporated by reference in its entirety. 
         [0019]    The first and second electrode layers  34  and  36  are each configured to support operation of the flow battery cell  20  at relatively high current densities (e.g., ≧approximately 100 mA/cm 2 ; ˜645 mA/in 2 ). Examples of such electrode layers are disclosed in U.S. patent application No. 13/022,285 filed on Feb. 7, 2011, which is hereby incorporated by reference in its entirety. 
         [0020]    The ion-exchange membrane  38  is configured as permeable to certain non-redox couple reactants such as, for example, H +  ions in vanadium/vanadium electrolyte solutions in order to transfer electric charges between the electrolyte solutions. The ion exchange membrane  38  is also configured to substantially reduce or prevent permeation therethrough (also referred to as “crossover”) of certain redox couple reactants such as, for example, V 4+/5+  ions in a vanadium catholyte solution or V 2+/3+  ions in a vanadium anolyte solution. 
         [0021]    The ion-exchange membrane  38  has a first ion exchange surface  56 , a second ion exchange surface  58 , a thickness  60  and a cross-sectional area  59  (see  FIG. 3 ). The ion-exchange membrane also has certain material properties that include an ionic resistance, an area specific resistance, a conductivity and a resistivity. The membrane thickness  60  extends between the first ion exchange surface  56  and the second ion exchange surface  58 . The ionic resistance is measured, in ohms (Ω), along a path between the first ion exchange surface  56  and the second ion exchange surface  58 . The ionic resistance is a function of the membrane thickness  60 , the membrane cross-sectional area  59  (see  FIG. 3 ) and the bulk membrane resistivity. The ionic resistance can be determined, for example, using, the following equation. 
         [0000]        R =(ρ* L )/ A  
 
         [0000]    where “R” represents the ionic resistance, “ρ” represents the membrane bulk resistivity, “L” represents the membrane thickness  60 , “A” represents the membrane cross-sectional area  59  (see  FIG. 3 ). The area specific resistance is a function of the ionic resistance and the membrane cross-sectional area  59  (see  FIG. 3 ). The area specific resistance can be determined, for example, using the following equation: 
         [0000]    
       
      
       R 
       AS 
       =R*A  
      
     
         [0000]    where “R AS ” represents the area specific resistance of the ion-exchange membrane  28 . 
         [0022]    The membrane thickness  60  can be sized and/or the area specific resistance can be selected to reduce overall energy inefficiency of the flow battery cell  20  as a function of an average current density at which the flow battery cell  20  is to be operated, which will be described below in further detail. In one embodiment, the membrane thickness  60  is sized less than approximately 125 μm (˜4921 μin) ( e.g., &lt; 100 μm; ˜3937 μin) where the flow battery cell  20  is to be operated at an average current density above approximately 100 mA/cm 2  (˜645 mA/in 2 ) ( e.g., &gt;approximately  200 mA/cm 2 ; ˜1290 mA/in 2 ). In another embodiment, the area specific resistance is selected to be less than approximately 425 mΩ*cm 2  (˜2742 mΩin 2 ) where the flow battery cell  20  is to be operated at an average current density above approximately 100 mA/cm 2  ( e.g., &gt;approximately  200 mA/cm 2 ). 
         [0023]    Referring to  FIGS. 4A to 4C , the ion-exchange membrane  38  includes one or more membrane layers  61 . In the embodiment shown in  FIG. 4A , for example, the ion-exchange membrane  38  is constructed from a single layer  62  of a polymeric ion-exchange material (also referred to as an “ionomer”) such as perfluorosulfonic acid (also referred to as “PSFA”) (e.g., Nafion® polymer manufactured by DuPont of Wilmington, Del., United States) or perfluoroalkyl sulfonimide ionomer (also referred to as “PFSI”). Other suitable ionomer materials include any polymer with ionic groups attached, which polymer can be fully or partially fluorinated for increased stability, as compared to hydrocarbon-based polymers. Examples of suitable polymers include polytetrafluoroethylenes (also referred to as “PTFE”) such as Teflon® (manufactured by DuPont of Wilmington, Del., United States), polyvinylidene fluorides (also referred to as “PVDF”) and polybenzimidazoles (also referred to as “PBI”). Examples of suitable ionic groups include sulfonates, sulfonimides, phosphates, phosphonic acid groups, sulfonic groups, as well as various anionic groups. 
         [0024]    In the embodiment shown in  FIG. 4B , the ion-exchange membrane  38  is constructed from a composite layer  64 . The composite layer  64  can include a matrix of nonconductive fibrous material (e.g., fiberglass), or a porous sheet of PTFE (such as Gore-Text material manufactured by W. L. Gore and Associates of Newark, Del., United States), impregnated with an ion-exchange binder or ionomer (e.g., PFSA, PFSI, etc.). Alternatively, the composite layer  64  can be constructed from a mixture of nonconductive fibrous material or PTFE and an ion-exchange ionomer (e.g., PFSA). 
         [0025]    In the embodiment shown in  FIG. 4C , the ion-exchange membrane  38  is constructed from a composite layer  66  disposed between two polymeric layers  68  and  69 . The composite layer  66  can be constructed from, as indicated above, a matrix of nonconductive fibrous material impregnated with an ion-exchange binder. The polymeric layers  68  and  69  can each be constructed from a polymeric ion-exchange material such as PFSA, PFSI or some other fluoropolymer-based ionomer, or a copolymer-based ionomer. Alternatively, each polymeric layer  68 ,  69  can each be constructed from a different type of ionomer. The polymeric layer that is proximate the anolyte solution, for example, can be constructed from an ionomer that is less stable to oxidation such as a hydrocarbon-based ionomer. The polymeric layer that is proximate the catholyte solution, on the other hand, can be constructed from an ionomer that is more stable to oxidation such as a fully fluorinated ionomer. In an alternative embodiment, a polymeric ion-exchange material layer (e.g., a layer of PFSA) can be disposed between two porous layers of polymers that are not ionomer materials (e.g., porous polyethylene or porous PTFE, such as Gore-Tex® material manufactured by W. L. Gore and Associates of Newark, Del., United States). In some embodiments, hydrophobic materials such as PTFE can be pretreated to make them hydrophilic. An example of such a treated porous PTFE layer is a GORE™ polytetrafluoroethylene (PTFE) separator (formerly known as EXCELLERATOR®) manufactured by W. L. Gore and Associates of Newark, Del., United States. The ion-exchange membrane  38 , however, is not limited to the aforesaid configurations and materials. 
         [0026]    Referring again to  FIG. 2 , the ion-exchange membrane  38  is disposed between the first and second electrode layers  34  and  36 . In one embodiment, for example, the first and second electrode layers  34  and  36  are hot pressed or otherwise bonded onto opposite sides of the ion-exchange membrane  38  to attach and increase interfacial surface area between the aforesaid layers  34 ,  36  and  38 . The first and second electrode layers  34  and  36  are disposed between, and are connected to the first and second current collectors  30  and  32 . 
         [0027]    Referring again to  FIG. 1 , the power converter  23  is adapted to regulate current density at which the flow battery cells operate, in response to a converter control signal, by regulating exchange of electrical current between the flow battery cells  20  and, for example, an electrical grid (not shown). The power converter  23  can include a single two-way power converter or a pair of one-way power converters, depending upon the particular design requirements of the flow battery system. Examples of suitable power converters include a power inverter, a DC/DC converter connected to a DC bus, etc. The present system  10 , however, is not limited to any particular type of power conversion or regulation device. 
         [0028]    The controller  25  can be implemented by one skilled in the art using hardware, software, or a combination thereof. The hardware can include, for example, one or more processors, analog and/or digital circuitry, etc. The controller  25  is adapted to control storage and discharge of electrical energy from flow battery system  10  by generating the converter and regulator control signals. The converter control signal is generated to control the current density at which the flow battery cells are operated. The regulator control signals are generated to control the flow rate at which the electrolyte solutions circulate through the flow battery system  10 . 
         [0029]    Referring to  FIGS. 1 and 2 , the source conduit  22  of the first electrolyte circuit loop  16  fluidly connects the first electrolyte storage tank  12  to one or both of the first current collector  30  and the first electrode layer  34  of each flow battery cell. The return conduit  26  of the first electrolyte circuit loop  16  reciprocally fluidly connects the first current collector  30  and/or the first electrode layer  34  of each flow battery cell to the first electrolyte storage tank  12 . The source conduit  24  of the second electrolyte circuit loop  18  fluidly connects the second electrolyte storage tank  14  to one or both of the second current collector  32  and the second electrode layer  36  of each flow battery cell. The return conduit  28  of the second electrolyte circuit loop  18  reciprocally fluidly connects the second current collector  32  and/or the second electrode layer  36  of each flow battery cell to the second electrolyte storage tank  14 . The power converter  23  is connected to the flow battery stack through a pair of first and second current collectors  30  and  32 , each of which can be disposed in a different flow battery cell  20  on an opposite end of the stack  21  where the cells are serially interconnected. The controller  25  is in signal communication (e.g., hardwired or wirelessly connected) with the power converter  23 , and the first and second flow regulators  27  and  29 . 
         [0030]    Referring still to  FIGS. 1 and 2 , during operation of the flow battery system  10 , the first electrolyte solution is circulated (e.g., pumped via the flow regulator  27 ) between the first electrolyte storage tank  12  and the flow battery cells  20  through the first electrolyte circuit loop  16 . More particularly, the first electrolyte solution is directed through the source conduit  22  of the first electrolyte circuit loop  16  to the first current collector  30  of each flow battery cell  20 . The first electrolyte solution flows through the channels  40  in the first current collector  30 , and permeates or flows into and out of the first electrode layer  34 ; i.e., wetting the first electrode layer  34 . The permeation of the first electrolyte solution through the first electrode layer  34  can result from diffusion or forced convection, such as disclosed in PCT Application No. PCT/US09/68681, which can facilitate relatively high reaction rates for operation at relatively high current densities. The return conduit  26  of the first electrolyte circuit loop  16  directs the first electrolyte solution from the first current collector  30  of each flow battery cell  20  back to the first electrolyte storage tank  12 . 
         [0031]    The second electrolyte solution is circulated (e.g., pumped via the flow regulator  29 ) between the second electrolyte storage tank  14  and the flow battery cells  20  through the second electrolyte circuit loop  18 . More particularly, the second electrolyte solution is directed through the source conduit  24  of the second electrolyte circuit loop  18  to the second current collector  32  of each flow battery cell  20 . The second electrolyte solution flows through the channels  42  in the second current collector  32 , and permeates or flows into and out of the second electrode layer  36 ; i.e., wetting the second electrode layer  36 . As indicated above, the permeation of the second electrolyte solution through the second electrode layer  36  can result from diffusion or forced convection, such as disclosed in PCT Application No. PCT/US09/68681, which can facilitate relatively high reaction rates for operation at relatively high current densities. The return conduit  28  of the second electrolyte circuit loop  18  directs the second electrolyte solution from the second current collector  32  of each flow battery cell  20  back to the second electrolyte storage tank  14 . 
         [0032]    During an energy storage mode of operation, electrical energy is input into the flow battery cell  20  through the current collectors  30  and  32 . The electrical energy is converted to chemical energy through electrochemical reactions in the first and second electrolyte solutions, and the transfer of non-redox couple reactants from, for example, the first electrolyte solution to the second electrolyte solution across the ion-exchange membrane  38 . The chemical energy is then stored in the electrolyte solutions, which are respectively stored in the first and second electrolyte storage tanks  12  and  14 . During an energy discharge mode of operation, on the other hand, the chemical energy stored in the electrolyte solutions is converted back to electrical energy through reverse electrochemical reactions in the first and second electrolyte solutions, and the transfer of the non-redox couple reactants from, for example, the second electrolyte solution to the first electrolyte solution across the ion-exchange membrane  38 . The electrical energy regenerated by the flow battery cell  20  passes out of the cell through the current collectors  30  and  32 . 
         [0033]    Energy efficiency of the flow battery system  10  during the energy storage and energy discharge modes of operation is a function of the overall energy inefficiency of each flow battery cell  20  included in the flow battery system  10 . The overall energy inefficiency of each flow battery cell  20 , in turn, is a function of (i) over-potential inefficiency and (ii) coulombic cross-over inefficiency of the ion-exchange membrane  38  in the respective cell  20 . 
         [0034]    The over-potential inefficiency of the ion-exchange membrane  38  is a function of the area specific resistance and the thickness  60  of the ion-exchange membrane  38 . The over-potential inefficiency can be determined using, for example, the following equations: 
         [0000]        n   v =( V−V   OCV )/ V   OCV , 
         [0000]        V=f ( iR   AS ) 
         [0000]    where “n v ” represents the over potential inefficiency, “V” represents the voltage potential of the flow battery cell  20 , “V OCV ” represents open circuit voltage, “ƒ(•)” represents a functional relationship, and “i” represents ionic current across the ion-exchange membrane  38 . 
         [0035]    The coulombic cross-over inefficiency of the ion-exchange membrane  38  is a function of redox couple reactant cross-over and, therefore, the membrane thickness  60 . The coulombic cross-over inefficiency can be determined using, for example, the following equations: 
         [0000]        n   c =Flux cross-over /Consumption 
         [0000]        Flux   cross-over   =f ( L ) 
         [0000]    where “n c ” represents the coulombic cross-over inefficiency, “Flux cross-over ” represents the flux rate of redox couple species that diffuses through the ion-exchange membrane  38  and “Consumption” represents the rate of redox couple species converted by the ionic current across the ion-exchange membrane  38 . 
         [0036]    Referring to  FIG. 5 , a graphical comparison is shown of overall energy inefficiencies versus current densities for first and second embodiments of the flow battery cell  20 . The first embodiment of the flow battery cell  20  (shown via the dashed line  70 ) has an ion-exchange membrane with a thickness of approximately 160 μm (˜6299 μin). The second embodiment of the flow battery cell  20  (shown via the solid line  72 ) has an ion-exchange membrane with a thickness of approximately 50 μm (˜1968 μin). The second embodiment of the flow battery cell  20  with the thinner membrane thickness has a lower overall energy inefficiency, relative to the energy inefficiency of the first embodiment of the flow battery cell, when the cell  20  is operated at a current density above approximately 150 mA/cm 2  (˜967 mA/in 2 ). The lower overall energy inefficiency is achieved, at least in part, by operating the flow battery cell  20  above the aforesaid relatively high current density to mitigate additional redox couple reactant crossover due to the thinner membrane thickness and lower area specific resistance. A lower overall energy inefficiency of a flow battery cell, in other words, is achieved when the magnitude of an increase in coulombic cross-over inefficiency due to a thin membrane thickness is less than the magnitude of a decrease in over-potential inefficiency due to a corresponding low area specific resistance of the ion-exchange membrane. 
         [0037]    While various embodiments of the present flow battery have been disclosed, it will be apparent to those of ordinary skill in the art that many more embodiments and implementations are possible within the scope thereof. Accordingly, the present flow battery is not to be restricted except in light of the attached claims and their equivalents.