U.S. patent application number 17/664562 was filed with the patent office on 2022-09-08 for cost-efficient high energy density redox flow battery.
The applicant listed for this patent is ESS Tech, Inc.. Invention is credited to Craig E. Evans, Yang Song.
Application Number | 20220285717 17/664562 |
Document ID | / |
Family ID | 1000006348518 |
Filed Date | 2022-09-08 |
United States Patent
Application |
20220285717 |
Kind Code |
A1 |
Song; Yang ; et al. |
September 8, 2022 |
COST-EFFICIENT HIGH ENERGY DENSITY REDOX FLOW BATTERY
Abstract
Methods and systems are provided for a redox flow battery
system. In one example, the redox flow battery is adapted with an
additive included in a battery electrolyte and an anion exchange
membrane separator dividing positive electrolyte from negative
electrolyte. An overall system cost of the battery system may be
reduced while a storage capacity, energy density and performance
may be increased.
Inventors: |
Song; Yang; (West Linn,
OR) ; Evans; Craig E.; (West Linn, OR) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
ESS Tech, Inc. |
Wilsonville |
OR |
US |
|
|
Family ID: |
1000006348518 |
Appl. No.: |
17/664562 |
Filed: |
May 23, 2022 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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16536237 |
Aug 8, 2019 |
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17664562 |
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62717625 |
Aug 10, 2018 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H01M 8/188 20130101;
H01M 4/582 20130101; H01M 4/368 20130101; H01M 8/02 20130101 |
International
Class: |
H01M 8/18 20060101
H01M008/18; H01M 4/58 20060101 H01M004/58; H01M 4/36 20060101
H01M004/36; H01M 8/02 20060101 H01M008/02 |
Claims
1-13. (canceled)
14. A method for a redox flow battery system comprising; plating a
metal from an electrolyte solution onto a negative electrode during
charging of the redox flow battery system; deplating the metal from
the negative electrode into the electrolyte solution during
discharging of the redox flow battery system; and transporting
anions across an anion exchange membrane separator positioned in
the electrolyte solution, the anion exchange membrane separator
configured to separate a negative electrode compartment from a
positive electrode compartment of the redox flow battery system;
wherein the electrolyte solution in the negative electrode
compartment comprises stearic acid as a plating additive to form
uniform and crack-free layers of metal at the negative
electrode.
15. The method of claim 14, wherein transporting the anions across
the anion exchange membrane separator includes transporting the
anions without transporting cations or complexes across the
separator.
16. The method of claim 14, wherein transporting the anions across
the anion exchange membrane separator includes transporting the
anions from a region of the redox flow battery system of lower
overall positive bias to a region of higher overall positive
bias.
17. The method of claim 14, wherein the electrolyte solution
consists of redox active species.
18. (canceled)
19. (canceled)
20. (canceled)
21. The method of claim 14, wherein the anion exchange membrane
separator is formed from one or more of a polymer network with ion
transport selectivity, a covalent organic framework, and a
pre-fabricated, commercially available material.
22. The method of claim 21, wherein the polymer network comprises
one or more of heteroaromatic compounds, aniline, olefins, and
sulfones.
23. The method of claim 14, wherein the anion exchange membrane
separator is fabricated by one of grafting, surface coating,
solvent, casting, and conformal coating.
24. The method of claim 14, wherein transporting the anions across
the anion exchange membrane separator comprises transporting
Cl.sup.- while inhibiting transport of iron cations.
25. The method of claim 14, wherein transporting the anions across
the anion exchange membrane separator comprises transporting
Cl.sup.- while inhibiting transport of the stearic acid.
26. The method of claim 14, wherein discharging the redox flow
battery system comprises providing up to 100 hours of energy to
power an external system.
27. The method of claim 14, wherein iron cations of the metal,
bound by the stearic acid, plate onto the negative electrode as a
stack of evenly spaced apart monolayers of iron that are separated
by layers formed of the trailing, chemically inert tail of the
stearic acid.
28. The method of claim 27, wherein the iron cations comprise
divalent and trivalent iron.
29. The method of claim 14, wherein the crack-free and uniform
layers of metal at the negative electrode are self-assembled
monolayers of iron, the iron bound by the stearic acid.
30. The method of claim 14, wherein a free energy of the metal is
lowered when bound by the stearic acid and the lowering of the free
energy causes the metal to self-assembly into layers separated by
the stearic acid.
31. The method of claim 14, wherein the crack-free and uniform
layers of metal have a thickness of over 1 cm.
32. The method of claim 14, wherein more than one molecule of the
stearic acid bonds with each metal center of the crack-free and
uniform layers of metal.
33. The method of claim 14, wherein the anion exchange membrane
separator comprises pH resistant functional groups.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] The present application claims priority to U.S. Provisional
Application No. 62/717,625, entitled "COST-EFFICIENT HIGH ENERGY
DENSITY REDOX FLOW BATTERY", and filed on Aug. 10, 2018. The entire
contents of the above-listed application are hereby incorporated by
reference for all purposes.
FIELD
[0002] The present description relates generally to methods and
systems for a redox flow battery.
BACKGROUND AND SUMMARY
[0003] Redox flow batteries are suitable for grid scale storage
applications due to their capability for scaling power and capacity
independently, as well as for charging and discharging over
thousands of cycles with reduced performance losses in comparison
to conventional battery technologies. An all-iron hybrid redox flow
battery is particularly attractive due to incorporation of low
cost, earth-abundant materials. The iron redox flow battery (IFB)
relies on iron, salt, and water for electrolyte, thus comprising
simple, earth abundant, and inexpensive materials and eliminates
incorporation of harsh chemicals thereby allowing the IFB to impose
minimal negative impact on the environment.
[0004] However, the inventors herein have recognized that further
reduction of overall system storage costs may be desirable in order
to expand a viable commercial application of the IFB. An increased
energy storage to unit cost ratio of the IFB may be achieved by
enhancing electrolyte energy density improving an ion transport
selectivity across a membrane separator separating positive
electrolyte from negative electrolyte in a battery cell of the IFB.
An accessibility and performance of the IFB may thus be
improved.
[0005] In one example, the issues described above may be addressed
by a battery cell with a positive electrolyte and a negative
electrolyte, the positive electrolyte in contact with a positive
electrode and the negative electrolyte in contact with a negative
electrode, and a membrane separator arranged between the negative
electrolyte and positive electrolyte, the membrane separator formed
from an anion exchange membrane (AEM) configured to maintain a
charge balance of the battery cell and increase an energy density
of the redox flow battery system In this way, an iron redox flow
battery (IFB) system may be manufactured with a reduced cost of
storage. Decreasing the cost of storage may include configuring the
IFB with an anionic exchange membrane separator to enable charge
balancing of the IFB during operation and increase a power density
of the IFB.
[0006] It should be understood that the summary above is provided
to introduce in simplified form a selection of concepts that are
further described in the detailed description. It is not meant to
identify key or essential features of the claimed subject matter,
the scope of which is defined uniquely by the claims that follow
the detailed description. Furthermore, the claimed subject matter
is not limited to implementations that solve any disadvantages
noted above or in any part of this disclosure.
BRIEF DESCRIPTION OF THE DRAWINGS
[0007] FIG. 1 shows a schematic of an example redox flow battery
system including a battery cell with electrodes and a membrane
separator.
[0008] FIG. 2 shows an example model of a self-assembled monolayer
of plated material for a plating electrode.
[0009] FIG. 3 shows a graph depicting an example comparison of a
maximum plating current density onto a negative electrode versus
electrolyte temperature between a conventional redox flow battery
and a redox flow battery configured to plate material as
self-assembled monolayers.
[0010] FIG. 4 shows an example of a power module of the redox flow
battery.
[0011] FIG. 5 shows a chart comparing storage costs of a
conventional redox flow battery system with storage coats of the
redox flow battery system adapted with the anion exchange membrane
separator and increased plating thickness.
[0012] FIG. 6 shows an example of a method for charging and
discharging the redox flow battery system.
[0013] FIG. 7 shows a schematic diagram of an example of the
battery cell of the redox flow battery including a negative
electrode compartment, a positive electrode compartment, and the
membrane separator.
[0014] FIG. 8 shows a first schematic diagram of an example of the
negative electrode compartment without a plating additive where the
negative electrode is coated with a cracked coating of plated
metal.
[0015] FIG. 9 shows a second schematic diagram of an example of the
negative electrode compartment with the plating additive where the
negative electrode is coated with crack-free coating of plated
metal.
[0016] FIG. 10 shows a graph comparing a relationship between
reactant concentration and temperature for a first solution
including a redox active species and supporting salts and for a
second solution including the redox active species without
supporting salts.
[0017] FIG. 4 is shown approximately to scale, however, other
dimensions may be used as desired.
DETAILED DESCRIPTION
[0018] The following description relates to systems and methods for
manufacturing a redox flow battery with reduced cost of storage.
The redox flow battery is shown in FIG. 1 with an integrated
multi-chamber tank having separate positive and negative
electrolyte chambers. The electrolyte chambers may be coupled to
one or more battery cells, each cell comprising a negative
electrode and a positive electrode. The positive and negative
electrolytes may be separated within each of the one or more
battery cells by a membrane separator that selectively allows
transport of ions across the separator to maintain charge balance
across the battery cells. Battery performance may be increased by
incorporating an additive into a material of the negative, or
plating, electrode. The additive may result in a self-imposed
monolayer arrangement of plated material, as illustrated in FIG. 2
by an example of a model of self-assembled plated monolayers. When
adapted with the additive-promoted plating as self-assembled
monolayers, a redox flow battery may sustain higher plating current
density at lower temperatures compared to a conventional redox flow
battery, as depicted in FIG. 3. Furthermore, an energy density of a
battery electrolyte may be enhanced and battery efficiency improved
by implementing an anion exchange membrane separator in the redox
flow battery in addition to incorporation of the additive. The
plated negative electrode and the anion exchange membrane separator
may be installed in a power module of the redox flow battery in a
configuration shown in FIG. 4. Overall system costs for a
conventional redox flow battery are compared with estimated system
costs for the redox flow battery of FIG. 4 in a prophetic chart
shown in FIG. 5. An example of a method for operating the redox
flow battery is provided in FIG. 6, showing events occurring during
charging and discharging of the battery when equipped with an AEM
and the plating additive. A schematic diagram of a battery cell of
the IFB, including positive and negative electrode compartments
separated by a membrane separator, is illustrated in FIG. 7 to show
a formation of uniform plated layers around a negative electrode of
the battery cell resulting from presence of the additive. In the
absence of the additive the coating of metal on to the negative
electrode may include cracks, as shown in a first schematic diagram
of the negative electrode compartment in FIG. 8, which may lead to
degradation of the negative electrode. By including the additive in
the negative electrode compartment, formation of the crack-free
metal coating may be enabled, as shown in FIG. 9 in a second
schematic diagram of the negative electrode compartment. By
including the AEM in the IFB, use of costly supporting salts in the
IFB electrolytes may be precluded which may increase a solubility
of reactants (e.g., redox active species) in the electrolytes. A
graph depicting an effect of the AEM, and concomitant absence of
supporting salts, on reactant solubility is shown in FIG. 10.
[0019] FIGS. 4 and 7-9 show example configurations with relative
positioning of the various components. If shown directly contacting
each other, or directly coupled, then such elements may be referred
to as directly contacting or directly coupled, respectively, at
least in one example. Similarly, elements shown contiguous or
adjacent to one another may be contiguous or adjacent to each
other, respectively, at least in one example. As an example,
components laying in face-sharing contact with each other may be
referred to as in face-sharing contact. As another example,
elements positioned apart from each other with only a space
there-between and no other components may be referred to as such,
in at least one example. As yet another example, elements shown
above/below one another, at opposite sides to one another, or to
the left/right of one another may be referred to as such, relative
to one another. Further, as shown in the figures, a topmost element
or point of element may be referred to as a "top" of the component
and a bottommost element or point of the element may be referred to
as a "bottom" of the component, in at least one example. As used
herein, top/bottom, upper/lower, above/below, may be relative to a
vertical axis of the figures and used to describe positioning of
elements of the figures relative to one another. As such, elements
shown above other elements are positioned vertically above the
other elements, in one example. As yet another example, shapes of
the elements depicted within the figures may be referred to as
having those shapes (e.g., such as being circular, straight,
planar, curved, rounded, chamfered, angled, or the like). Further,
elements shown intersecting one another may be referred to as
intersecting elements or intersecting one another, in at least one
example. Further still, an element shown within another element or
shown outside of another element may be referred as such, in one
example.
[0020] Hybrid redox flow batteries are redox flow batteries that
are characterized by the deposition of one or more of the
electro-active materials as a solid layer on an electrode. Hybrid
redox flow batteries may, for instance, include a chemical that
plates via an electrochemical reaction as a solid on a substrate
throughout the battery charge process. During battery discharge,
the plated species may ionize via an electrochemical reaction,
becoming soluble in the electrolyte. In hybrid battery systems, the
charge capacity (e.g., a maximum amount of energy stored) of the
redox battery may be limited by the amount of metal plated during
battery charge and may depend accordingly on the efficiency of the
plating system as well as the available volume and surface area
available for plating.
[0021] As shown in FIG. 1, in a redox flow battery system 10, a
negative electrode 26 may be referred to as a plating electrode and
a positive electrode 28 may be referred to as a redox electrode. A
negative electrolyte within a plating side (e.g., a negative
electrode compartment 20) of a battery cell 18 may be referred to
as a plating electrolyte, and a positive electrolyte on a redox
side (e.g. a positive electrode compartment 22) of the battery cell
18 may be referred to as a redox electrolyte.
[0022] Anode refers to the electrode where electro-active material
loses electrons and cathode refers to the electrode where
electro-active material gains electrons. During battery charge, the
positive electrolyte gains electrons at the negative electrode 26;
therefore the negative electrode 26 is the cathode of the
electrochemical reaction. During discharge, the positive
electrolyte loses electrons; therefore the negative electrode 26 is
the anode of the reaction. Alternatively, during discharge, the
negative electrolyte and negative electrode may be respectively
referred to as an anolyte and anode of the electrochemical
reaction, while the positive electrolyte and the positive electrode
may be respectively referred to as a catholyte and cathode of the
electrochemical reaction. In contrast, during charge, the negative
electrolyte and negative electrode may be respectively referred to
as the catholyte and cathode of the electrochemical reaction, while
the positive electrolyte and the positive electrode may be
respectively referred to as the anolyte and anode of the
electrochemical reaction. For simplicity, the terms positive and
negative are used herein to refer to the electrodes, electrolytes,
and electrode compartments in redox battery flow systems.
[0023] One example of a hybrid redox flow battery is an all iron
redox flow battery (IFB), in which the electrolyte comprises iron
ions in the form of iron salts (e.g., FeCl.sub.2, FeCl.sub.3, and
the like), wherein the negative electrode comprises metal iron. For
example, at the negative electrode 26, ferrous ion, Fe.sup.2+,
receives two electrons and plates as iron metal on to the negative
electrode 26 during battery charge, and iron metal, Fe.sup.0, loses
two electrons and re-dissolves as Fe.sup.+ during battery
discharge. At the positive electrode, Fe.sup.2+ loses an electron
to form ferric ion, Fe.sup.3+, during charge, and during discharge
Fe.sup.3+ gains an electron to form Fe.sup.2+. The electrochemical
reaction is summarized in equations (1) and (2), wherein the
forward reactions (left to right) indicate electrochemical
reactions during battery charge, while the reverse reactions (right
to left) indicate electrochemical reactions during battery
discharge:
Fe.sup.2++2e-.revreaction.Fe.sup.0-0.44 V (Negative Electrode)
(1)
Fe.sup.2++2 Fe.sup.3++2e-+0.77 V (Positive Electrode) (2)
[0024] As discussed above, the negative electrolyte used in the IFB
may provide a sufficient amount of Fe.sup.2+ so that, during
charge, Fe.sup.2+ can accept two electrons from the negative
electrode to form Fe.sup.0 and plate onto a substrate. During
discharge, the plated Fe.sup.0 may then lose two electrons,
ionizing into Fe.sup.2+ and may be dissolved back into the
electrolyte. The equilibrium potential of the above reaction is
-0.44 V and thus, this reaction provides a negative terminal for
the desired system. On the positive side of the IFB, the
electrolyte may provide Fe.sup.2+ during charge which loses
electron and oxidizes to Fe.sup.3+. During discharge, Fe.sup.3+
provided by the electrolyte becomes Fe.sup.2+ by absorbing an
electron provided by the electrode. The equilibrium potential of
this reaction is +0.77 V, creating a positive terminal for the
desired system.
[0025] The IFB provides the ability to charge and recharge its
electrolytes in contrast to other battery types utilizing
non-regenerating electrolytes. Charge is achieved by applying a
current across the electrodes via terminals 40 and 42. The negative
electrode 26 may be coupled via terminal 40 to the negative side of
a voltage source so that electrons may be delivered to the negative
electrolyte via the positive electrode (e.g., as Fe.sup.2+ is
oxidized to Fe.sup.3+ in the positive electrolyte in the positive
electrode compartment 22). The electrons provided to the negative
electrode 26 (e.g., plating electrode) can reduce the Fe.sup.2+ in
the negative electrolyte to form Fe.sup.0 at the plating substrate,
causing it to plate onto the negative electrode 26.
[0026] Discharge can be sustained while Fe.sup.0 remains available
to the negative electrolyte for oxidation and while Fe.sup.3+
remains available in the positive electrolyte for reduction. As an
example, Fe.sup.3+ availability can be maintained by increasing the
concentration or the volume of the positive electrolyte to the
positive electrode compartment 22 side of cell 18 to provide
additional Fe.sup.3+ ions via an external source, such as an
external positive electrolyte tank 52. More commonly, availability
of Fe.sup.0 during discharge may be an issue in IFB systems,
wherein the Fe.sup.0 available for discharge may be proportional to
the surface area and volume of the negative electrode substrate as
well as the plating efficiency. Charge capacity may be dependent on
the availability of Fe.sup.2+ in the negative electrode compartment
20. As an example, Fe.sup.2+ availability can be maintained by
providing additional Fe.sup.2+ ions via an external source, such as
an external negative electrolyte chamber 50 to increase the
concentration or the volume of the negative electrolyte to the
negative electrode compartment 20 side of cell 18.
[0027] In an IFB, the positive electrolyte comprises ferrous ion,
ferric ion, ferric complexes, or any combination thereof, while the
negative electrolyte comprises ferrous ion or ferrous complexes,
depending on the state of charge of the IFB system. As previously
mentioned, utilization of iron ions in both the negative
electrolyte and the positive electrolyte allows for utilization of
the same electrolytic species on both sides of the battery cell,
which can reduce electrolyte cross-contamination and can increase
the efficiency of the IFB system, resulting in less electrolyte
replacement as compared to other redox flow battery systems.
[0028] Plating quality at the negative electrode 26 may impact a
performance of the battery system 10. For example, iron may plate
from electrolyte onto a surface of the negative electrode 26
according to electrolyte current flow. As a result, deposition of
iron may be pronounced on features such as corners, bends or
protrusions. Uneven plating may lead to loss of battery efficiency
and/or capacity by forming gaps along the surface of the negative
electrode 26 where iron is less likely to plate, an effect that
becomes further exacerbated by continued accumulation of iron metal
onto the protruding features or corners or bends. A uniform
deposition of iron onto the negative electrode surface throughout
the charging cycle of the battery system 10 may allow for rapid
charging and discharging. Increasing a plating thickness of the
iron deposited onto the negative electrode 26 may enable prolonged
energy storage for the discharge cycle occurring during coupling of
the battery system 10 to an electrically powered external device or
system. Furthermore, a consistent distribution of iron across the
electrode surface may provide even heat distribution, thereby
simplifying thermal management of the battery system 10 and leading
to faster charging and discharging of the battery system 10 at
ambient temperature. Methods to promote uniform iron plating via
additive-assisted self-assembly of monolayers will be discussed
further below with reference to FIGS. 2, 4-6.
[0029] Efficiency losses in an IFB may result from electrolyte
crossover through a separator 24 (e.g., ion-exchange membrane
barrier, micro-porous membrane, and the like). For example, ferric
ions in the positive electrolyte may be driven toward the negative
electrolyte by a ferric ion concentration gradient and an
electrophoretic force across the separator. Subsequently, ferric
ions penetrating the membrane barrier and crossing over to the
negative electrode compartment 20 may result in coulombic
efficiency losses. Ferric ions crossing over from the low pH redox
side (e.g., more acidic positive electrode compartment 22) to high
pH plating side (e.g., less acidic negative electrode compartment
20) may result in precipitation of Fe(OH).sub.3. Precipitation of
Fe(OH).sub.3 may degrade the separator 24 and cause permanent
battery performance and efficiency losses. For example,
Fe(OH).sub.3 precipitate may chemically foul the organic functional
group of an ion-exchange membrane or physically clog the small
micro-pores of an ion-exchange membrane. In either case, due to the
Fe(OH).sub.3 precipitate, membrane ohmic resistance may rise over
time and battery performance may degrade. Precipitate may be
removed by washing the battery with acid, but the constant
maintenance and downtime may be disadvantageous for commercial
battery applications. Furthermore, washing may be dependent on
regular preparation of electrolyte, contributing to additional
processing costs and complexity. Alternatively, adding specific
organic acids to the positive electrolyte and the negative
electrolyte in response to electrolyte pH changes may mitigate
precipitate formation during battery charge and discharge cycling
without driving up overall costs. Additionally, implementing a
membrane barrier that inhibits ferric ion cross-over may also
mitigate fouling.
[0030] Additional coulombic efficiency losses may be caused by
reduction of H.sup.+ (e.g., protons) and subsequent formation of
H.sub.2 (e.g., hydrogen gas), and the reaction of protons in the
negative electrode compartment 20 with electrons supplied at the
plated iron metal electrode to form hydrogen gas.
[0031] The IFB electrolyte (e.g., FeCl.sub.2, FeCl.sub.3,
FeSO.sub.4, Fe.sub.2(SO.sub.4).sub.3, and the like) is readily
available and can be produced at low costs. The IFB electrolyte
offers higher reclamation value because the same electrolyte can be
used for the negative electrolyte and the positive electrolyte,
consequently reducing cross contamination issues as compared to
other systems. Furthermore, owing to its electron configuration,
iron may solidify into a generally uniform solid structure during
plating thereof on the negative electrode substrate. For zinc and
other metals commonly used in hybrid redox batteries, solid
dendritic structures may form during plating. The stable electrode
morphology of the IFB system may increase the efficiency of the
battery in comparison to other redox flow batteries. Further still,
iron redox flow batteries reduce the use of toxic raw materials and
can operate at a relatively neutral pH as compared to other redox
flow battery electrolytes. Accordingly, IFB systems reduce
environmental hazards as compared with all other current advanced
redox flow battery systems in production.
[0032] Continuing with FIG. 1, a schematic illustration of the
redox flow battery system 10 is shown. The redox flow battery
system 10 may comprise the redox flow battery cell 18 fluidly
connected to a multi-chambered electrolyte storage tank 110. The
redox flow battery cell 18 may generally include the negative
electrode compartment 20, separator 24, and positive electrode
compartment 22. The separator 24 may comprise an electrically
insulating ionic conducting barrier which prevents bulk mixing of
the positive electrolyte and the negative electrolyte while
allowing conductance of specific ions therethrough. For example,
the separator 24 may comprise an ion-exchange membrane and/or a
microporous membrane.
[0033] In some examples, the separator 24 may be formed from an
anion exchange membrane that conducts a target anion, such as
Cl.sup.-, across the separator 24 while inhibiting flow of iron
cations (Fe.sup.2+, Fe.sup.3+) and iron cation complexes, e.g.,
FeCl.sub.4.sup.-. By configuring the battery system 10 with the
anion exchange membrane, an amount of electrolyte solution and
number of chemical species may be reduced. In other words, by
adapting the separator 24 as an anion exchange membrane, the
electrolytic species include active materials such as FeCl.sub.2
and FeCl.sub.3 without additional supporting redox inactive
electrolytes. In another example, by adapting the separator 24 as
an anion exchange membrane, the electrolytic species may include
only active materials such as FeCl.sub.2 and FeCl.sub.3, which,
when dissolved in aqueous solution, provides iron cations that
undergo redox reactions as well as chloride anions for charge
balance, without additional supporting redox inactive electrolytes.
As a result, a volume of electrolyte may be reduced while a
concentration of active materials is increased, allowing for a
smaller and less expensive storage tank to be used. Further details
of the anion exchange membrane separator will be provided with
reference to FIGS. 3-6.
[0034] The negative electrode compartment 20 may comprise the
negative electrode 26, and the negative electrolyte may comprise
electroactive materials. The positive electrode compartment 22 may
comprise the positive electrode 28, and the positive electrolyte
may comprise electroactive materials. In some examples, multiple
redox flow battery cells 18 may be combined in series or parallel
to generate a higher voltage or current in a redox flow battery
system. Further illustrated in FIG. 1 are negative and positive
electrolyte pumps 30 and 32, both used to pump electrolyte solution
through the flow battery system 10. Electrolytes are stored in one
or more tanks external to the cell, and are pumped via negative and
positive electrolyte pumps 30 and 32 through the negative electrode
compartment 20 side and the positive electrode compartment 22 side
of the battery, respectively.
[0035] The redox flow battery system 10 may also include a first
bipolar plate 36 and a second bipolar plate 38, each positioned
along a rear-facing side, e.g., opposite of a side facing the
separator 24, of the negative electrode 26 and the positive
electrode 28, respectively. The first bipolar plate 36 may be in
contact with the negative electrode 26 and the second bipolar plate
38 may be in contact with the positive electrode 28. In other
examples, however, the bipolar plates may be arranged proximate but
spaced away from the electrodes within the respective electrode
compartments. The IFB electrolytes may be transported to reaction
sites at the negative and positive electrodes 26 and 28 by the
first and second bipolar plates 36 and 38, resulting from
conductive properties of a material of the bipolar plates 36, 38.
Electrolyte flow may also be assisted by the negative and positive
electrolyte pumps 30 and 32, facilitating forced convection through
the redox flow battery cell 18. Reacted electrochemical species may
also be directed away from the reaction sites by the combination of
forced convection and the presence of the first and second bipolar
plates 36 and 38.
[0036] As illustrated in FIG. 1, the redox flow battery cell 18 may
further include negative battery terminal 40, and positive battery
terminal 42. When a charge current is applied to the battery
terminals 40 and 42, the positive electrolyte is oxidized (lose one
or more electrons) at the positive electrode 28, and the negative
electrolyte is reduced (gain one or more electrons) at the negative
electrode 26. During battery discharge, reverse redox reactions
occur on the electrodes. In other words, the positive electrolyte
is reduced (gain one or more electrons) at the positive electrode
28, and the negative electrolyte is oxidized (lose one or more
electrons) at the negative electrode 26. The electrical potential
difference across the battery is maintained by the electrochemical
redox reactions in the positive electrode compartment 22 and the
negative electrode compartment 20, and may induce a current through
a conductor while the reactions are sustained. The amount of energy
stored by a redox battery is limited by the amount of
electro-active material available in electrolytes for discharge,
depending on the total volume of electrolytes and the solubility of
the electro-active materials.
[0037] The flow battery system 10 may further comprise the
integrated multi-chambered electrolyte storage tank 110. The
multi-chambered storage tank 110 may be divided by a bulkhead 98.
The bulkhead 98 may create multiple chambers within the storage
tank so that both the positive and negative electrolyte may be
included within a single tank. The negative electrolyte chamber 50
holds negative electrolyte comprising electroactive materials, and
the positive electrolyte chamber 52 holds positive electrolyte
comprising electroactive materials. The bulkhead 98 may be
positioned within the multi-chambered storage tank 110 to yield a
desired volume ratio between the negative electrolyte chamber 50
and the positive electrolyte chamber 52. In one example, the
bulkhead 98 may be positioned to set the volume ratio of the
negative and positive electrolyte chambers according to the
stoichiometric ratio between the negative and positive redox
reactions. The figure further illustrates the fill height 112 of
storage tank 110, which may indicate the liquid level in each tank
compartment. The figure also shows gas head space 90 located above
the fill height 112 of negative electrolyte chamber 50, and gas
head space 92 located above the fill height 112 of positive
electrolyte chamber 52. The gas head space 92 may be utilized to
store hydrogen gas generated through operation of the redox flow
battery (e.g., due to proton reduction and corrosion side
reactions) and conveyed to the multi-chambered storage tank 110
with returning electrolyte from the redox flow battery cell 18. The
hydrogen gas may be separated spontaneously at the gas-liquid
interface (e.g., fill height 112) within the multi-chambered
storage tank 110, thereby precluding having additional gas-liquid
separators as part of the redox flow battery system. Once separated
from the electrolyte, the hydrogen gas may fill the gas head spaces
90 and 92. A such, the stored hydrogen gas can aid in purging other
gases from the multi-chamber storage tank 100, thereby acting as an
inert gas blanket for reducing oxidation of electrolyte species,
which can help to reduce redox flow battery capacity losses. In
this way, utilizing the integrated multi-chambered storage tank 110
may forego having separate negative and positive electrolyte
storage tanks, hydrogen storage tanks, and gas-liquid separators
common to conventional redox flow battery systems, thereby
simplifying the system design, reducing the physical footprint of
the system, and reducing system costs.
[0038] FIG. 1 also shows the spill over-hole 96, which creates an
opening in the bulkhead 98 between gas head spaces 90 and 92, and
provides a means of equalizing gas pressure between the two
chambers. The spill over hole 96 may be positioned a threshold
height above the fill height 112. The spill over hole further
enables a capability to self-balance the electrolytes in each of
the positive and negative electrolyte chambers in the event of a
battery crossover. In the case of an all iron redox flow battery
system, the same electrolyte (Fe.sup.2+) is used in both negative
and positive electrode compartments 20 and 22, so spilling over of
electrolyte between the negative and positive electrolyte chambers
50 and 52 may reduce overall system efficiency, but the overall
electrolyte composition, battery module performance, and battery
module capacity are maintained. Flange fittings may be utilized for
all piping connections for inlets and outlets to and from the
multi-chambered storage tank 110 to maintain a continuously
pressurized state without leaks. The multi-chambered storage tank
110 can include at least one outlet from each of the negative and
positive electrolyte chambers, and at least one inlet to each of
the negative and positive electrolyte chambers. Furthermore, one or
more outlet connections may be provided from the gas head spaces 90
and 92 for directing hydrogen gas to rebalancing reactors 80 and
82.
[0039] Although not shown in FIG. 1, integrated multi-chambered
electrolyte storage tank 110 may further include one or more
heaters thermally coupled to each of the negative electrolyte
chamber 50 and the positive electrolyte chamber 52. In alternate
examples, only one of the negative and positive electrolyte
chambers may include one or more heaters. In the case where only
the positive electrolyte chamber 52 includes one or more heaters,
the negative electrolyte may be heated by transferring heat
generated at the battery cells of the power module to the negative
electrolyte. In this way, the battery cells of the power module may
heat and facilitate temperature regulation of the negative
electrolyte. The one or more heaters may be actuated by the
controller 88 to regulate a temperature of the negative electrolyte
chamber 50 and the positive electrolyte chamber 52 independently or
together. For example, in response to an electrolyte temperature
decreasing below a threshold temperature, the controller 88 may
increase a power supplied to one or more heaters so that a heat
flux to the electrolyte is increased. The electrolyte temperature
may be indicated by one or more temperature sensors mounted at the
multi-chambered electrolyte storage tank 110, including sensors 60
and 62. As examples, the one or more heaters may include coil type
heaters or other immersion heaters immersed in the electrolyte
fluid, or surface mantle type heaters that transfer heat
conductively through the walls of the negative and positive
electrolyte chambers to heat the fluid therein. Other known types
of tank heaters may be employed without departing from the scope of
the present disclosure. Furthermore, controller 88 may deactivate
one or more heaters in the negative and positive electrolyte
chambers 50, 52 in response to a liquid level decreasing below a
solids fill threshold level. Said in another way, controller 88 may
activate the one or more heaters in the negative and positive
electrolyte chambers 50, 52 only in response to a liquid level
increasing above the solids fill threshold level. In this way,
activating the one or more heaters without sufficient liquid in the
positive and/or negative electrolyte chambers can be averted,
thereby reducing a risk of overheating or burning out the
heaters.
[0040] Further still, one or more inlet connections may be provided
to each of the negative and positive electrolyte chambers 50, 52
from a field hydration system (not shown). In this way, the field
hydration system can facilitate commissioning of the redox flow
battery system, including installing, filling, and hydrating the
system, at an end-use location. Furthermore, prior to its
commissioning at the end-use location, the redox flow battery
system may be dry-assembled at a battery manufacturing facility
different from end-use location without filling and hydrating the
system, before delivering the system to the end-use location. In
one example, the end-use location may correspond to the location
where the redox flow battery system 10 is to be installed and
utilized for on-site energy storage. Said in another way, it is
anticipated that, once installed and hydrated at the end-use
location, a position of the redox flow battery system 10 becomes
fixed, and the redox flow battery system 10 is no longer deemed a
portable, dry system. Thus, from the perspective of a redox flow
battery system end-user, the dry portable redox flow battery system
10 may be delivered on-site, after which the redox flow battery
system 10 is installed, hydrated and commissioned. Prior to
hydration the redox flow battery system 10 may be referred to as a
dry, portable system, the redox flow battery system 10 being free
of or without water and wet electrolyte. Once hydrated, the redox
flow battery system 10 may be referred to as a wet non-portable
system, the redox flow battery system 10 including wet
electrolyte.
[0041] Further illustrated in FIG. 1, electrolyte solutions
typically stored in the multi-chambered storage tank 110 are pumped
via negative and positive electrolyte pumps 30 and 32 throughout
the flow battery system 10. Electrolyte stored in negative
electrolyte chamber 50 is pumped via negative electrolyte pump 30
through the negative electrode compartment 20 side, and electrolyte
stored in positive electrolyte chamber 52 is pumped via positive
electrolyte pump 32 through the positive electrode compartment 22
side of the battery.
[0042] Two electrolyte rebalancing reactors 80 and 82, may be
connected in-line or in parallel with the recirculating flow paths
of the electrolyte at the negative and positive sides of the
battery cell 18, respectively, in the redox flow battery system 10.
One or more rebalancing reactors may be connected in-line with the
recirculating flow paths of the electrolyte at the negative and
positive sides of the battery, and other rebalancing reactors may
be connected in parallel, for redundancy (e.g., a rebalancing
reactor may be serviced without disrupting battery and rebalancing
operations) and for increased rebalancing capacity. In one example,
the electrolyte rebalancing reactors 80 and 82 may be placed in the
return flow path from the positive and negative electrode
compartments 20 and 22 to the positive and negative electrolyte
chambers 50 and 52, respectively. Electrolyte rebalancing reactors
80 and 82 may serve to rebalance electrolyte charge imbalances in
the redox flow battery system occurring due to side reactions, ion
crossover, and the like, as described herein. In one example,
electrolyte rebalancing reactors 80 and 82 may include trickle bed
reactors, where the hydrogen gas and electrolyte are contacted at
catalyst surfaces in a packed bed for carrying out the electrolyte
rebalancing reaction. In other examples the rebalancing reactors 80
and 82 may include flow-through type reactors that are capable of
contacting the hydrogen gas and the electrolyte liquid and carrying
out the rebalancing reactions in the absence a packed catalyst
bed.
[0043] During operation of the redox flow battery system 10,
sensors and probes may monitor and control chemical properties of
the electrolyte such as electrolyte pH, concentration, state of
charge, and the like. For example, as illustrated in FIG. 1,
sensors 62 and 60 maybe be positioned to monitor positive
electrolyte and negative electrolyte conditions at the positive
electrolyte chamber 52 and the negative electrolyte chamber 50,
respectively. In another example, sensors 62 and 60 may each
include one or more electrolyte level sensors to indicate a level
of electrolyte in the positive electrolyte chamber 52 and the
negative electrolyte chamber 50, respectively. As another example,
sensors 72 and 70, also illustrated in FIG. 1, may monitor positive
electrolyte and negative electrolyte conditions at the positive
electrode compartment 22 and the negative electrode compartment 20,
respectively. Sensors may be positioned at other locations
throughout the redox flow battery system 10 to monitor electrolyte
chemical properties and other properties. For example a sensor may
be positioned in an external acid tank (not shown) to monitor acid
volume or pH of the external acid tank, wherein acid from the
external acid tank is supplied via an external pump (not shown) to
the redox flow battery system 10 in order to reduce precipitate
formation in the electrolytes. Additional external tanks and
sensors may be installed for supplying other additives to the redox
flow battery system 10. For example, various sensors including,
temperature, conductivity, and level sensors of a field hydration
system may transmit signals to the controller 88. Furthermore,
controller 88 may send signals to actuators such as valves and
pumps of the field hydration system during hydration of the redox
flow battery system 10. Sensor information may be transmitted to a
controller 88 which may in turn actuate pumps 30 and 32 to control
electrolyte flow through the cell 18, or to perform other control
functions, as an example. In this manner, the controller 88 may be
responsive to, one or a combination of sensors and probes.
[0044] Redox flow battery system 10 may further comprise a source
of hydrogen gas. In one example the source of hydrogen gas may
comprise a separate dedicated hydrogen gas storage tank. In the
example of FIG. 1, hydrogen gas may be stored in and supplied from
the integrated multi-chambered electrolyte storage tank 110.
Integrated multi-chambered electrolyte storage tank 110 may supply
additional hydrogen gas to the positive electrolyte chamber 52 and
the negative electrolyte chamber 50. Integrated multi-chambered
electrolyte storage tank 110 may alternately supply additional
hydrogen gas to the inlet of electrolyte rebalancing reactors 80
and 82. As an example, a mass flow meter or other flow controlling
device (which may be controlled by controller 88) may regulate the
flow of the hydrogen gas from integrated multi-chambered
electrolyte storage tank 110. The integrated multi-chambered
electrolyte storage tank 110 may supplement the hydrogen gas
generated in redox flow battery system 10. For example, when gas
leaks are detected in redox flow battery system 10 or when the
reduction reaction rate is too low at low hydrogen partial
pressure, hydrogen gas may be supplied from the integrated
multi-chambered electrolyte storage tank 110 in order to rebalance
the state of charge of the electro-active species in the positive
electrolyte and negative electrolyte. As an example, controller 88
may supply hydrogen gas from integrated multi-chambered electrolyte
storage tank 110 in response to a measured change in pH or in
response to a measured change in state of charge of an electrolyte
or an electro-active species. For example an increase in pH of the
negative electrolyte chamber 50, or the negative electrode
compartment 20, may indicate that hydrogen is leaking from the
redox flow battery system 10 and/or that the reaction rate is too
slow with the available hydrogen partial pressure, and controller
88, in response to the pH increase, may increase a supply of
hydrogen gas from integrated multi-chambered electrolyte storage
tank 110 to the redox flow battery system 10. As a further example,
controller 88 may supply hydrogen gas from integrated
multi-chambered electrolyte storage tank 110 in response to a pH
change, wherein the pH increases beyond a first threshold pH or
decreases beyond second threshold pH. In the case of an IFB,
controller 88 may supply additional hydrogen to increase the rate
of reduction of ferric ions and the rate of production of protons,
thereby reducing the pH of the positive electrolyte. Furthermore,
the negative electrolyte pH may be lowered by hydrogen reduction of
ferric ions crossing over from the positive electrolyte to the
negative electrolyte or by protons, generated at the positive side,
crossing over to the negative electrolyte due to a proton
concentration gradient and electrophoretic forces. In this manner,
the pH of the negative electrolyte may be maintained within a
stable region, while reducing the risk of precipitation of ferric
ions (crossing over from the positive electrode compartment) as
Fe(OH).sub.3. Other control schemes for controlling the supply rate
of hydrogen gas from integrated multi-chambered electrolyte storage
tank 110 responsive to a change in an electrolyte pH or to a change
in an electrolyte state of charge, detected by other sensors such
as an oxygen-reduction potential (ORP) meter or an optical sensor,
may be implemented. Further still, the change in pH or state of
charge triggering the action of controller 88 may be based on a
rate of change or a change measured over a time period. The time
period for the rate of change may be predetermined or adjusted
based on the time constants for the redox flow battery system 10.
For example the time period may be reduced if the recirculation
rate is high, and local changes in concentration (e.g., due to side
reactions or gas leaks) may quickly be measured since the time
constants may be small.
[0045] As described above, an overall performance of a redox flow
battery may be improved by a combination of uniform, thick,
reversible plating of metal onto a negative (plating) electrode and
presence of an anion exchange membrane separator controlling flow
of ions between a positive electrode compartment and a negative
electrode compartment of a battery cell. In an iron redox flow
battery (IFB) system, ferrous iron, Fe.sup.2+ is reduced to iron
metal, Fe.sup.0, at the negative electrode, resulting in deposition
of the metal during the IFB charging cycle. Formation of
continuous, uniform, and crack-free plated layers that coat a
surface of the negative electrode may reduce a likelihood of uneven
current density across a surface of the negative electrode which
may lead to development of localized heating that degrades the
electrode and diminishes the electrode lifetime. In addition, as
the IFB may undergo tens of thousands of charge/discharge cycles,
maintaining uniform plating over the cycles is highly
desirable.
[0046] Furthermore, in order to achieve 100 hours of energy storage
in the IFB at a target power level, a desirable plating thickness
may be greater than 1 cm. The increased plating thickness, as
compared to plating in conventional redox flow battery systems, may
provide a sufficient amount of plated iron on the negative
electrode to continuously oxidize elemental iron to ferrous ion
over a period of 100 hours during battery discharge. To obtain
consistently uniform and reversible plating, an additive may be
used to induce deposition of plated iron in self-assembled
monolayers, as shown in FIGS. 2 and 7.
[0047] A small quantity of an additive may be added to a negative
electrolyte solution, e.g., the negative electrolyte stored in the
negative electrode compartment 20 of FIG. 1, at millimolar (mM)
concentrations so that the presence of the additive does not affect
system costs or overall energy and power density. A model 200 of a
self-assembly of monolayers with an additive 202 is shown in FIG.
2. The additive 202 may be a fatty acid, such as stearic acid 202,
which may interact with an iron center 204 at an electron-rich
carboxylate functional end group 206 of the stearic acid molecule.
For the redox flow battery system 10 of FIG. 1, in the case of an
IFB system, the negative (plating) electrode 26 may include an iron
substrate (including one or more iron center 204). The functional
end group 206 may form a chemical bond with the iron center 204. A
long-chain carbon tail 208 of stearic acid 202 may trail from the
adsorbed functional end group 206, away from the iron center 204.
The trailing carbon tail 208 may be much longer in length than the
functional end group 206 of stearic acid 202. For example, the
carbon tail 208 may be a first length 203, such as 2.15 nm, while
the functional end group 206 may be a second, shorter length 205,
such as 0.15 nm, as shown in FIG. 2. Van der Waals interactions
between each stearic acid carbon tail 208 and adjacent carbon tails
may result in a tight packing arrangement of each layer of plated
iron to reduce a free energy of each layer. Each consecutive plated
layer may be similarly packed as a new monolayer adjacent to a
previously plated monolayer, forming a stack 210 of evenly spaced
apart monolayers of iron that are separated by layers of stearic
acid carbon tails surrounding each iron center 204. A stacking of
monolayers of iron are depicted in a schematic diagram 700 in FIG.
7.
[0048] The schematic diagram 700 shows a battery cell 702, which
may be a non-limiting example of the battery cell 18 of FIG. 1. The
battery cell 702 includes a negative electrode compartment 704 with
a negative electrode 706 submerged in a negative electrolyte 708.
The negative electrode compartment 704 may be separated from a
positive electrode compartment 710, containing a positive electrode
712 submerged in a positive electrolyte 714, by a membrane
separator 716. In the example shown in FIG. 7, the membrane
separator 716 is an anion exchange membrane (AEM) separator 716,
allowing only anions 720 to pass therethrough. In other examples,
however, the membrane separator 716 may be configured to be a
cation exchange membrane separator or a microporous substrate.
[0049] Both the negative electrolyte 708 and the positive
electrolyte 714 comprise cations 718 (e.g., Fe.sup.2+, Fe.sup.3+),
which may each be the iron center 204 of FIG. 2, and anions 720
(e.g., Cl.sup.-). The negative electrolyte 708 may additionally
include additive molecules 722, such as the stearic acid 202 of
FIG. 2, which bind with the cations 718 at one end to form
complexes 724. A number of additive molecules 722 bound to each of
the cations 718 may vary. For example, complexes 724 with one, two,
or four bound additive molecules 722 are shown in FIG. 7 but
complexes with other quantities of bound additive molecules 722,
such as three, five, or six, have been contemplated.
[0050] The complexes 724 may plate onto the negative electrode 706
and form consecutive self-assembled monolayers 726, similar to the
stack 210 of iron monolayers shown in FIG. 2. Each layer of the
monolayers 726 of iron may have a uniform thickness, defined in a
direction perpendicular from a surface 728 of the negative
electrode 706, along a portion of the surface 728 of the negative
electrode 706 that is in contact with the negative electrolyte 708,
e.g., submerged in the negative electrolyte 708. A stacking of
uniform monolayers 726 of iron onto the negative electrode 706 may
allow for a plating thickness 730, also defined in a direction
perpendicular to the surface 728 of the negative electrode 706, of
greater than 1 cm while maintaining a consistent thickness of the
plating thickness 730 across the surface 728 of the negative
electrode. The plating thickness 730 may represent a sum of the
thicknesses of each layer of the stacked monolayer 726 of iron.
[0051] It will be appreciated that while use of stearic acid in the
IFB is described, with respect to FIG. 2, numerous other types of
fatty acids or surfactants may be used to promote self-assembly of
the plated iron into monolayers without departing from the scope of
the present disclosure. For example, various other additives which
include a functional end group interacting and binding to iron
cations, as well as a chemically inert tail formed from a
hydrocarbon chain, may be used in place of stearic acid.
[0052] The additive, which may be stearic acid in one example, may
promote self-assembled monolayers that form an overall crack-free
coating of iron on the negative electrode. For example, a first
schematic diagram 800 of a negative electrode compartment 802,
similar to the negative electrode compartment 20 of FIGS. 1 and 704
of FIG. 7, is shown in FIG. 8. A portion of a negative electrode
804 is submerged in a first negative electrolyte 806 which may
contain a mixture of metal cations and anions in aqueous solution.
Upon reduction, the metal may plate onto the negative electrode 804
forming a coating 808. The coating 808 includes cracks 810, which
may be gaps in the coating 808 forming as a result of topographic
features or an uneven texture of a surface of the negative
electrode 804.
[0053] A presence of cracks 810 in the coating 808 disrupts
electrical conductivity through the coating 808. Uneven
distribution of current may result, causing uneven heating and
formation of hotspots along the surface of the negative electrode
804. Degradation of the negative electrode 804 may occur due to the
uneven heating.
[0054] In contrast, a second schematic diagram 900 shown in FIG. 9
of the negative electrode compartment 802 may store a second
negative electrolyte 902. The second negative electrolyte 902 may
be a mixture of metal cations, anions, and an additive, such as
stearic acid. Upon reduction of ferrous iron at the negative
electrode 804, the metal may plate onto the negative electrode 804
in self-assembled monolayers, as described in FIGS. 2 and 7,
resulting in deposition of a smooth, continuous, and crack-free
coating 904 of metal. A continuity of current flow through the
coating 904 is maintained, allowing for even heating of the
negative electrode 804 and preserving and prolonging an integrity
of the negative electrode 804.
[0055] The additive may, in addition to promoting formation of
organized and crack-free layers of plated iron, inhibit corrosion
of the iron-plated negative electrode. For example, as shown in
FIG. 7, each iron center may adsorb more than one stearic acid
functional end group and be surrounded by more than one attached
stearic acid tail, the plurality of stearic acid tails around each
iron center providing a coating around the iron centers that
reduces oxidation of iron during storage or when the IFB is in an
idle state.
[0056] Compared to IFB systems operating without plating additives,
the self-assembled monolayer plating of iron, as described above,
may allow plating and deplating (e.g., electron exchange between
Fe.sup.0 to Fe.sup.2+) to occur at lower temperature, such as
ambient temperature, in the IFB when implemented with suitable
system control logics. Conventional iron flow batteries may operate
at elevated temperatures of 50-65.degree. C., for example, to
achieve high charging efficiency, adding complexity and consuming
energy in order to heat the IFB to provide a desirable power
output. Operation of the IFB at ambient temperature rather than
higher temperatures may simplify the system and reduce overall
costs.
[0057] A comparison of plating current density between a
conventional IFB and the IFB configured to plate iron in
self-assembled monolayers is shown in a graph 300 in FIG. 3. Graph
300 depicts a maximum plating current density in mA/cm.sup.2
increasing upwards along the y-axis and electrolyte temperature
increasing to the right along the x-axis. A first plot 302 of an
IFB with a conventional plating system, e.g., no additive, is given
in graph 300 along with a second plot 304 of an IFB with an
additive, such as stearic acid, to promote self-assembly of
monolayers during iron plating.
[0058] At ambient temperature (T.sub.amb), a difference in maximum
plating current density between the first plot 302 and the second
plot 304 is greatest. The second plot 304 indicates that the
additive-equipped IFB may support a maximum plating current density
that is higher than a maximum plating current density shown by the
first plot 302 when equal reversibility and plating quality is
demonstrated by both systems. The difference in plating current
density between the first plot 302 and second plot 304 decreases as
temperature increases, with the second plot 304 consistently higher
than the first plot 302 until the plots merge at a terminal high
end temperature. The results shown in graph 300 indicate that
uniform, crack-free plating of iron together with pulse charging
allows significantly thicker plating of iron onto a negative
electrode of the IFB without adversely affecting battery
performance at relatively low operating temperatures. A deviation
between plating current density of the second plot 304 versus the
first plot 302 is most pronounced at ambient temperature. Thus, the
system represented in plot 304 may support greater plating
thickness on the negative electrode without causing significant
electrolyte volume change, drop in pressure, or cell clogging over
100 hours of storage.
[0059] Furthermore, high efficiency charging of conventional redox
flow batteries may be associated with high plating overpotentials
which may lead to hydrogen evolution at the negative electrode. In
the IFB system with self-assembled monolayers of plated iron, pulse
charging may reduce plating overpotential and further assist in
uniform iron plating by decreasing iron grain size. In some
example, pulse charging, in combination with the additive, may
assist in reducing electrode overpotential, thus reducing a
likelihood of hydrogen generation at the negative electrode.
[0060] Energy density of the IFB may also be enhanced by
implementing an anion exchange membrane (AEM) as a separator
between a negative electrode compartment and a positive electrode
compartment. Incorporation of the AEM, in addition to an additive
to control iron plating, may also decrease storage costs to, for
example, $10 per kilowatt hour by eliminating supporting
electrolytes, such as KCl, and demonstrate greater than 60% round
trip efficiency of the IFB. The AEM may interact with anions, such
as Cl.sup.-, and not cations, such as Fe.sup.2+ and Fe.sup.3+,
allowing anion transport across electrode compartments via an anion
exchange mechanism. In addition, with application of AEM and
elimination of supporting electrolytes, reactants concentration in
water can be significantly increased, i.e. the overall system
energy efficiency can be increased.
[0061] For example, as shown in FIG. 7, anions 720 may flow across
the AEM separator 716 between the negative electrode compartment
704 and positive electrode compartment 710 but not the metal
cations 718, the additive molecules 722, or the complexes 724. In
one example, the anions 720 may be Cl.sup.- in an IFB system. As
described above, the anions 720 may be transported across the AEM
separator 716 in response to the charge imbalance in the battery
cell 702. For example, generation of H.sub.2 during IFB operation,
as described above with reference to FIG. 1, may result in uneven
distribution of charges between the positive electrode compartment
710 and the negative electrode compartment 704. The charge
imbalance may be alleviated by enabling the anions 720 to flow
across the AEM separator 716, either in a first direction from the
negative electrode compartment 704 to the positive electrode
compartment 710 as indicated by arrow 703 or in a second direction
from the positive electrode compartment 710 to the negative
electrode compartment 704 as indicated by arrow 705. Anion
transport may occur during regular charge and discharge reaction of
the IFB, allowing, for example, only or mostly Cl.sup.- movement
across the AEM separator 716.
[0062] For example, when an overall charge balance of the positive
electrode compartment 710 is biased positive while an overall
charge balance of the negative electrode compartment 710 is
neutral, the anions 720 may flow along the first direction as
indicated by arrow 703 to neutralize the overall positive charge.
Similarly, when the overall charge balance of the negative
electrode compartment 710 is biased positive while the overall
charge balance of the positive electrode compartment 703 is
neutral, the anions 720 may flow along the second direction, as
indicated by arrow 705. As such, charge balance in the battery cell
702 may be restored and stable operation of the IFB is
maintained.
[0063] Furthermore, implementation of the AEM separator 716 may
hinder cross-over of iron cations between the positive electrode
compartment 710 and the negative electrode compartment 704. By
inhibiting migration of iron cations across the AEM separator 716,
precipitation of Fe(OH).sub.3 is circumvented and a likelihood of
membrane fouling at the AEM separator 716 is reduced. As well, loss
of iron due to Fe(OH).sub.3 formation may be mitigated.
[0064] Equipping the IFB with the AEM separator may also decrease
an overall cost of the IFB. By inhibiting cation flow, use of
supporting electrolyte to increase a conductivity of the solution
may be eliminated. In conventional redox flow battery systems,
supporting electrolyte, comprising electrically conductive species
in solution that do not participate in the redox reactions of the
redox flow battery, may impose significant additional costs.
However, the IFB may be configured to instead rely on electrolyte
containing exclusively electroactive species involved in iron redox
chemistry when the AEM separator is installed, thereby increasing
energy density and decreasing system costs. A lesser volume of
electrolyte may be used with up to a twofold increase in
concentration of redox active iron species providing a suitable
amount of Fe.sup.2+ to enable an iron plating of greater than 1 cm
thickness.
[0065] Various types of AEMs may be considered to form the
separator. For example, a polymer network of the separator membrane
may include heteroaromatic compounds, aniline, olefins, or sulfones
as building blocks. Alternatively, the membrane may be formed from
a covalent organic framework or include pH resistant functional
groups. The AEM may be fabricated by numerous methods including
grafting, surface coating, solvent casting, conformal coating or,
as another example, may be commercially available.
[0066] Eliminating supporting salts from electrolyte of an IFB
system may result in increased energy density of the IFB. As
described above, implementation of an AEM allows exclusive
dissolution of redox active species in the electrolyte thereby
enhancing a solubility of the redox active species in a given
volume of electrolyte. As an example, solubility of the redox
active species without presence of supporting salts is compared
with solubility of the redox active species with supporting salts
included in an electrolyte in graph 1000, as depicted in FIG.
10.
[0067] Graph 1000 shows a temperature of the electrolyte increasing
to the right along the x-axis and a concentration of a reactant or
redox active species in solution, e.g., an amount of the redox
active species dissolved in water, increasing upwards along the
y-axis. The redox active species may be FeCl.sub.2 and/or
FeCl.sub.3. A first plot 1002 represents a concentration of the
reactant with supporting salts also dissolved in the solution and a
second plot 1004 represents a concentration of the reactant without
presence of the supporting salts. When the temperature is low, such
as at -10.degree. C., a solubility of the reactant may be similar
with or without the presence of the supporting salts.
[0068] As the temperature increases, the first plot 1002 and the
second plot 1004 diverge, with the concentration of the reactant
rising more rapidly in the second plot 1004 than the first plot
1002. Above 0.degree. C., solubility of the reactant increases in
both plots but reactant solubility is consistently higher in the
second plot. For example, in an operating temperature range of an
IFB of between 50-60 .degree. C., as indicated by shaded area 1006,
the concentration of the reactant is at least three times higher
than the reactant concentration when the supporting salts are
present. Thus, the presence of supporting salts may suppress
reactant solubility, decreasing an amount of redox active species
able to engage in charge/discharge cycles of the IFB.
[0069] The solubility of the redox active species may be directly
correlated to an energy density of the IFB. Increasing the
solubility of the redox active species may result in higher energy
density while reducing the solubility of the redox active species
may decrease the IFB energy density. With respect to graph 1000,
the greater solubility of the reactant at temperatures between
50-60.degree. C., as indicated by shaded area 1006, may result in a
greater than twofold increase in energy density of the IFB.
Elimination of the supporting salts may thereby enhance an
efficiency of the IFB system.
[0070] An IFB system may include a power module adapted with both
an additive to encourage uniform plating and an AEM as a separator
between a negative electrode and a positive electrode, the AEM
having inherent ion selectivity. An example of a power module 400
that may be used in a redox flow battery system, such as the redox
flow battery system of FIG. 1, is shown in FIG. 4. A set of
reference axis 401 is provided, indicating a y-axis, an x-axis, and
a z-axis. The power module 400 comprises a series of components
arranged as layers within the power module 400. The layers may be
positioned co-planar with a y-x plane and stacked along the
z-axis.
[0071] Pressure plates 402 may be arranged at a first end 403 and a
second end 405 of the power module 400 that provide rigid end walls
that define boundaries of the power module 400. The pressure plates
402 allow layers of the power module 400 to be pressed together
between the pressure plates 402 to seal components of the power
module within an interior 407 of the power module 400. Picture
frames 404 may be arranged inside of the pressure plates, e.g.,
against sides of the pressure plates facing inwards along the
z-axis, towards the interior 407 of the power module 400, the
picture frames 404 adapted to interface with one another to seal
fluids within the interior 407 of the power module 400.
[0072] Elements of the power module 400 are now described along a
direction from the first end 403 towards the second end 405. A
negative spacer 406 is arranged adjacent to one of the picture
frames 404 positioned at the first end 403, the negative spacer 406
defining flow channels along a surface of a negative electrode. A
bipolar plate 408, which may have an integrated negative electrode
along a surface of the bipolar plate 408 in face-sharing contact
with the negative spacer 406, is positioned between the negative
spacer 406 and surrounded by a bipolar plate frame plate 410 that
provides structural support.
[0073] A positive electrode 412, which may be a sheet of graphite
felt, is arranged along a face of the bipolar plate 408 opposite of
the negative spacer 406. A membrane 414, surrounded by a membrane
frame plate 416 for structural support, may be positioned adjacent
to the positive electrode 412, on a side of the positive electrode
facing the second end 405 of the power module 400. The membrane 414
may be adapted as an anion exchange membrane, transporting anions
across the membrane 414 but not cations or complexes. The
components described above, e.g., the negative spacer 406, the
bipolar plate 408, the positive electrode 412, and the membrane 414
may repeat within the power module, from the first end 403 to the
second end 405, a number of times, forming a battery stack.
Negative electrolyte, including an additive such as stearic acid,
may be contained between another membrane, arranged on a side of
the bipolar plate 408 towards the first end 403 of the power module
400, and the bipolar plate 408, the negative electrolyte in contact
with both the negative spacer 406 and integrated negative electrode
(e.g., integrated into the surface of the bipolar plate 408).
Positive electrolyte may be contained between the bipolar plate 408
and the membrane 414, in contact with the positive electrode
412.
[0074] Estimated reductions in system costs resulting from adapting
an IFB with an AEM and an additive for uniform iron plating is
compared to costs of a conventional IFB in a prophetic chart 500
illustrated in FIG. 5. System costs in dollars per kilowatt hour is
shown increasing upwards along a y-axis of chart 500 over a 100
hour storage period at a rated power. A first column 502 shows a
system cost breakdown for a conventional IFB. A majority of system
costs may be attributed to electrolyte, represented by a first
hatched area 504 in the first column 502. In a second column 506, a
majority portion of a system cost of an AEM and additive-adapted
IFB is similarly attributable to electrolyte and represented by a
second hatched area 508. The electrolyte may include both redox
active species and supporting salts. However, the amount due to
electrolyte in the second column 506 is reduced compared to the
first column 502, indicating a cost savings of, for example, 30%.
The lower electrolyte costs for the second column 506 may results
from a decreased volume of electrolyte in the IFB when the AEM is
implemented, as described above. The decreased volume of
electrolyte may increase an energy density of the electrolyte by
increasing a concentration of the redox active species such as
FeCl.sub.2 and FeCl.sub.3. In addition, use of expensive supporting
salts, e.g., electrically conductive species that do not
participate in redox reactions of the IFB, may be precluded,
thereby further reducing system costs.
[0075] Other variables contributing to system costs, such as
thermal management systems, represented by a first shaded area 510
in the first column 502 and by a second shaded area 512 in the
second column 506 may show a smaller relative area in the second
column 506 relative to the first column 502 as a result of additive
use to plate uniform layers onto a negative electrode of the IFB.
Other elements contributing to overall system costs, represented by
a third shaded area 514 in the first column 502 may represent a
larger area, and therefore cost, compared to a fourth shaded area
516 in the second column 506.
[0076] An example of a method 600 for operating an iron redox flow
battery (IFB) system is shown in FIG. 6. The IFB may include a
negative electrolyte with a small amount (e.g., millimolar
concentration) of an additive. The additive may be a fatty acid,
such as stearic acid, with a functional end group configured to
interact with iron as well as a chemically inert carbon tail that
does not interact with electroactive species in the electrolyte.
The IFB may also include a membrane separator positioned between a
negative electrode and a positive electrode and configured to
moderate exchange of ions between the negative electrolyte, in
contact with the negative electrode, and a positive electrolyte, in
contact with the positive electrode. In one example, the membrane
separator may be an anion exchange membrane (AEM) separator, such
as the AEM separator 716 of FIG. 7, enabling flow of anions across
the AEM.
[0077] It will be appreciated that method 600 may be similarly
applied to an IFB system using the plating additive and
incorporating a cation exchange membrane separator or a microporous
substrate in place of the AEM separator. Such alternatives to the
AEM separator may enable transport of chemical species other than
anions across the membrane separator.
[0078] At 602, the method includes charging the IFB to store energy
via a charging process. In some examples, charging the IFB may be
achieved by pulse charging. Application of pulse charging, where a
series of voltage or current pulses is applied to the IFB, may
result in a reduction of an overpotential of the negative
electrode. The use of pulse charging may mitigate overheating of
the battery during charging. The charging process may include
applying a current to the IFB to reduce ferrous iron in the
negative electrolyte to iron metal and plate the iron metal onto
the negative electrode at 604. Simultaneously, ferrous iron may be
oxidized to ferric iron in the positive electrolyte at the positive
electrode. The additive may interact with the iron so that the iron
plates onto the electrode in self-assembled monolayers. The plated
monolayers of iron form an even, uniform coating on the negative
electrode, allowing for even heat distribution across the negative
electrode. The plated iron may form a coating greater than 1 cm
thick.
[0079] Charging the IFB may also include transporting ions,
specifically, anions such as chloride, across the AEM separator at
606. The anions may flow across the AEM separator by an anion
exchange mechanism while exchange of cations across the AEM
separator is inhibited. By allowing anions to be exchanged between
the positive electrolyte and negative electrolyte, a charge balance
of the IFB system may be maintained.
[0080] In another example where the membrane separator is
configured as the cation exchange membrane, cations such as H.sup.+
may be allowed to flow across the membrane separator while exchange
of anions is inhibited. In yet another example where the membrane
separator is replaced by a microporous substrate, both cations,
such as K.sup.+, H.sup.+, and anions, such as Cl.sup.-,
FeCl.sub.4.sup.-, are transported. In each of the alternate
embodiments of the membrane separator, the charge balance of the
IFB system may be similarly maintained.
[0081] At 608, the method includes discharging the IFB to provide
power to an external system. Discharging the IFB may include, at
610, flowing a current to the external system from the IFB as iron
metal is oxidized to ferrous iron at the negative electrode and
ferric iron is reduced to ferrous iron at the positive electrode.
Anions may be transported across the AEM separator at 612 of the
method during discharge, providing charge balance between the
positive and negative electrolytes. By adapting the IFB with the
additive in the negative electrolyte and the AEM separator,
discharge of the IFB may provide, for example, up to 100 hours of
energy storage to power the external system.
[0082] In this way, a performance of a redox flow battery may be
improved while reducing system costs. For example, an energy
density of the electrolyte solution may be increased by configuring
the redox flow battery with an anion exchange membrane separator
that selectively transports anions across the separator while
inhibiting cation flow. Incorporation of the anion exchange
membrane separator may allow a concentration of electroactive
species in the electrolyte to be increased by eliminating use of
supporting salts, thereby enhancing the energy density of the redox
flow battery. Furthermore, a smaller volume of electrolyte may be
used and overall system costs are decreased. The technical effect
of adapting the redox flow battery with the additive and the anion
exchange membrane separator is that an energy density of the
battery is increased while maintaining charge balance within the
battery and cost per energy unit is decreased.
[0083] In one embodiment, a redox flow battery system includes a
battery cell with a positive electrolyte and a negative
electrolyte, the positive electrolyte in contact with a positive
electrode and the negative electrolyte in contact with a negative
electrode, and a membrane separator arranged between the negative
electrolyte and positive electrolyte, the membrane separator formed
from an anion exchange membrane (AEM) configured to maintain a
charge balance of the battery cell and increase an energy density
of the redox flow battery system. In a first example of the system,
the AEM transports anions between the positive electrolyte and
negative electrolyte. A second example of the system optionally
includes the first example, and further includes, wherein the AEM
blocks passage of cations between the positive electrolyte and
negative electrolyte. A third example of the system optionally
includes one or more of the first and second examples, and further
includes, wherein the positive electrolyte and the negative
electrolyte are formed exclusively from a redox active species and
free of supporting, redox inactive materials. A fourth example of
the system optionally includes one or more of the first through
third examples, and further includes, wherein a concentration of
the redox active species is increased by two times or more when the
membrane separator is formed from the AEM. A fifth example of the
system optionally includes one or more of the first through fourth
examples, and further includes, wherein a solubility of the redox
active species is higher when the electrolyte is formed exclusively
from redox active species than when the electrolyte includes both
the redox active species and the supporting, redox inactive
materials. A sixth example of the system optionally includes one or
more of the first through fifth examples, and further includes,
wherein the anions flow across the AEM in response to a charge
imbalance between the positive electrolyte and the negative
electrolyte. A seventh example of the system optionally includes
one or more of the first through sixth examples, and further
includes, wherein both a volume of positive electrolyte and a
volume of negative electrolyte in the redox flow battery system are
decreased when the membrane separator is formed from the AEM than
when the membrane separator is not formed from the AEM. An eighth
example of the system optionally includes one or more of the first
through seventh examples, and further includes, wherein the AEM is
formed from a covalent organic framework. A ninth example of the
system optionally includes one or more of the first through eighth
examples, and further includes, wherein the AEM includes pH
resistant functional groups. A tenth example of the system
optionally includes one or more of the first through ninth
examples, and further includes, wherein the AEM is fabricated by
one of grafting, surface coating, solvent, casting, and conformal
coating. An eleventh example of the system optionally includes one
or more of the first through tenth examples, and further includes,
wherein the AEM is formed of a pre-fabricated, commercially
available material.
[0084] In another embodiment, a method includes plating a metal
from an electrolyte solution onto a negative electrode during
charging of the redox flow battery system, deplating the metal from
the negative electrode into the electrolyte solution during
discharging of the redox flow battery system and transporting
anions across an anion exchange membrane separator positioned in
the electrolyte solution, the anion exchange membrane separator
configured to separate a negative electrode compartment from a
positive electrode compartment of the redox flow battery system. In
a first example of the method, transporting anions includes
transporting anions without transporting cations or complexes
across the separator. A second example of the method optionally
includes the first example, and further includes, wherein
transporting anions across the anion exchange membrane separator
includes transporting anions from a region of the redox flow
battery system of lower overall positive bias to a region of higher
overall positive bias. A third example of the method optionally
includes one or more of the first and second examples, and further
includes, forming the electrolyte solution from only redox active
species and no supporting salts and wherein forming the electrolyte
solution from only the redox active species includes increasing a
concentration of the redox active species relative to when the
supporting salts are present in the electrolyte solution.
[0085] In yet another embodiment, an iron redox flow battery
includes an electrolyte formed from iron chloride complexes in
aqueous solution in contact with a positive electrode and a
negative electrode, and an anion exchange membrane separator
positioned between the positive electrode and negative electrode
and in contact with the electrolyte, the anion exchange membrane
separator configured to selectively interact with chloride anions
from the iron chloride complexes. In a first example of the
battery, an energy density of the iron redox flow battery is at
least two times higher when the anion exchange membrane separator
is implemented in the iron redox flow battery. A second example of
the battery optionally includes the first example, and further
includes, wherein the anion exchange membrane separator blocks
cross-over of iron cations from the iron chloride complexes between
a positive electrode compartment housing the positive electrode and
a negative electrode compartment housing the negative
electrode.
[0086] The following claims particularly point out certain
combinations and sub-combinations regarded as novel and
non-obvious. These claims may refer to "an" element or "a first"
element or the equivalent thereof. Such claims should be understood
to include incorporation of one or more such elements, neither
requiring nor excluding two or more such elements. Other
combinations and sub-combinations of the disclosed features,
functions, elements, and/or properties may be claimed through
amendment of the present claims or through presentation of new
claims in this or a related application. Such claims, whether
broader, narrower, equal, or different in scope to the original
claims, also are regarded as included within the subject matter of
the present disclosure.
* * * * *