FLOW BATTERY HAVING A LOW RESISTANCE MEMBRANE
CROSS-REFERENCE TO RELATED APPLICATIONS
[0001] Applicant hereby claims priority to U.S. Patent Application No. 13/023 ,101 filed
February 8, 201 1, the disclosure of which is herein incorporated by reference. This application is
also related to PCT Application No. PCT/US09/68681 filed on December 18, 2009 and U.S.
Patent Application No. 13/022,285 filed on February 7, 201 1, each of which is incorporated by
reference in its entirety.
BACKGROUND
1. Technical Field
[0002] This disclosure relates generally to a flow battery system and, more particularly,
to a flow battery having a low resistance membrane.
2. Background Information
[0003] 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.
[0004] The ion-exchange membrane is configured to be permeable to certain non-redox
couple reactants (also referred to as "charge transport ions" or "charge carrier ions") in the
catholyte and anolyte solutions to facilitate the electrochemical reactions. Redox couple
reactants (also referred to as "non-charge transport ions" 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/cm2), which is the typical current density operating range
of conventional flow battery cells.
[0005] 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 (mhi); ~ 6889 micro inches (m h)) 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 .
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] 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.
[0007] 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.
[0008] FIG. 3 is a cross-sectional diagrammatic illustration of one embodiment of the
ion-exchange membrane in FIG. 2.
[0009] FIGS. A to 4C are enlarged partial sectional diagrammatic illustrations of
different embodiments of the ion-exchange membrane in FIG. 2.
[0010] FIG. 5 is a graphical comparison of overall energy inefficiencies versus current
densities for two different flow battery cells.
DETAILED DESCRIPTION
[001 1] 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.
[0012] 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 2 1 between a pair of end plates 39, which cells
20 can be operated to collectively store and produce electrical energy.
[0013] 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.
[0014] 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.
[0015] 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 ionexchange
membrane 38.
[0016] 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.
[0017] 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 niA/cm2; ~ 645 mA/in2) . Examples of such electrode layers are disclosed in U.S. Patent
Application No. 13/022,285 filed on February 7, 201 1, which is hereby incorporated by
reference in its entirety.
[0018] 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, v4+ 5+ ions in a vanadium
catholyte solution or V ions in a vanadium anolyte solution.
where "R " represents the area specific resistance of the ion-exchange membrane 28.
[0020] 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 mh (~ 4921 m h) (e.g., < 100 mh ; ~ 3937 m h) where the flow battery cell
20 is to be operated at an average current density above approximately 100 mA/cm (-645
mA/in2) (e.g., > approximately 200 mA/cm2; ~ 1290 mA/in2). In another embodiment, the area
specific resistance is selected to be less than approximately 425 *c 2 (~ 2742 hW* h2)
where the flow battery cell 20 is to be operated at an average current density above
approximately 100 mA/cm2 (e.g., > approximately 200 mA/cm2) .
[0021] Referring to FIGS. 4A to 4C, the ion-exchange membrane 38 includes one or
more membrane layers 6 1. 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, Delaware, 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 polytetrafiuoroethylenes (also referred to as "PTFE")
such as Teflon® (manufactured by DuPont of Wilmington, Delaware, 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.
[0022] In the embodiment shown in FIG. 4B, the ion-exchange membrane 3 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-Tex®
material manufactured by W. L. Gore and Associates of Newark, Delaware, 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).
[0023] 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 ionexchange
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, Delaware, 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, Delaware, United States. The ionexchange
membrane 38, however, is not limited to the aforesaid configurations and materials.
[0024] Referring again to FIG. 2, the ion-exchange membrane 3 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.
[0025] 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.
[0026] 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.
[0027] Referring to FIGS. 1 and 2, the source conduit 22 of the first electrolyte circuit
loop 16 fiuidly 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 fiuidly 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 fiuidly 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 fiuidly 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 2 1 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.
[0028] 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.
[0029] 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 1 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.
[0030] 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.
[003 1] 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.
[0032] 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 overpotential
inefficiency can be determined using, for example, the following equations:
nv = (V- Vocv)/Vocv,
V =f(iRAS)
where " " represents the over potential inefficiency, " ' represents the voltage potential of the
flow battery cell 20, "Vocv" represents open circuit voltage, "f0" represents a functional
relationship, and " " represents ionic current across the ion-exchange membrane 38.
[0033] 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:
F s-ov r / Consumption
FluXcross-over =f(L)
where " " represents the coulombic cross-over inefficiency, "F l Xcro -o e " 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 .
[0034] 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 ionexchange
membrane with a thickness of approximately 160 m h (~ 6299 mίh) . 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 m (~ 1968 m h). 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/cm2 (~ 967
mA/in2) . 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.
[0035] 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.
What is claimed is:
1. A flow battery, comprising:
a membrane having an area specific resistance of less than approximately four hundred
twenty five milliohms - square centimeter across the membrane; and
a solution having a reversible redox couple reactant, wherein the solution wets the
membrane.
2. The flow battery of claim 1, further comprising a first electrode and a second electrode,
wherein the membrane is operable to transfer ionic current between the first electrode and the
second electrode at a current density greater than one hundred milliamps per square centimeter.
3. The flow battery of claim 1, wherein the membrane is configured as permeable to a nonredox
couple reactant within the solution.
4. The flow battery of claim 1, wherein the membrane has a thickness of less than
approximately one hundred twenty five micrometers.
5. The flow battery of claim 1, wherein the membrane comprises a composite of a first ion
exchange material and a material different than the first ion exchange material.
6. The flow battery of claim 1, wherein the membrane comprises a first layer and a second
layer, wherein the first layer has a first ion exchange material, and wherein the second layer has a
material different than the first ion exchange material.
7. A flow battery, comprising:
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.
8. The flow battery of claim 7, further comprising a first electrode and a second electrode,
wherein the membrane is operable to transfer ionic current between the first electrode and the
second electrode at a current density greater than one hundred milliamps per square centimeter.
9. The flow battery of claim 7, wherein the membrane is configured as permeable to a nonredox
couple reactant within the solution.
10. The flow battery of claim 7, wherein the membrane has an area specific resistance of less
than approximately four hundred twenty five milliohms - square centimeter across the
membrane.
11. The flow battery of claim 7, wherein the membrane comprises a composite of a first ion
exchange material and a material different than the first ion exchange material.
12. The flow battery of claim 10, wherein the membrane comprises a first layer and a second
layer, wherein the first layer has an ion exchange material, and wherein the second layer has a
material different than the ion exchange material.
13. A flow battery, comprising:
a membrane having an ion exchange material and a matrix; and
a solution having a reversible redox couple reactant, wherein the solution wets the
membrane.
14. The flow battery of claim 13, wherein the matrix comprises a nonconductive fibrous
material.
15. The flow battery of claim 14, wherein the nonconductive fibrous material comprises one
of fiber glass, polytetrafluoroethylene fibers, and a porous sheet of polytetrafluoroethylene.
16. The flow battery of claim 13, wherein the ion exchange material is a binder that is
impregnated into the matrix.
17. The flow battery of claim 13, wherein the ion exchange material comprises one of a
perfluorosulfonic acid and a perfluoroalkyl sulfonimide ionomer.
18. The flow battery of claim 13, wherein the membrane has at least one of:
a thickness of less than approximately one hundred twenty five micrometers; and
an area specific resistance of less than approximately four hundred twenty five milliohms
- square centimeter across the membrane.
19. A flow battery, comprising:
a membrane having a first layer and a second layer, wherein the first layer has an ion
exchange material, and wherein the second layer has a material different than the ion exchange
material; and
a solution having a reversible redox couple reactant, wherein the solution wets the
membrane.
20. The flow battery of claim 19, wherein the material in the second layer that is different
than the ion exchange material in the first layer comprises a matrix of nonconductive fibrous
material.
2 1. The flow battery of claim 20, wherein the matrix is impregnated with an ion exchange
binder.
22. The flow battery of claim 19, wherein the material in the second layer that is different
than the ion exchange material in the first layer comprises a hydrophobic porous material.
23. The flow battery of claim 19, wherein the ion exchange material in the first layer
comprises one type of ionomer and the second layer comprises a second type of ionomer.
24. The flow battery of claim 19, wherein the second layer is disposed between the first layer
and a third layer, and wherein the third layer has a second ion exchange material.
25. The flow battery of claim 19, wherein the membrane has at least one of:
a thickness of less than approximately one hundred twenty five micrometers; and
an area specific resistance of less than approximately four hundred twenty five milliohms
- square centimeter across the membrane.