FLOW BATTERY WITH ENHANCED DURABILITY
BACKGROUND
[0001] This disclosure relates to flow batteries for selectively storing and discharging
electric energy.
[0002] Flow batteries, also known as redox flow batteries or redox flow cells, are
designed to convert electrical energy into chemical energy that can be stored and later released
when there is demand. As an example, a flow battery may be used with a renewable energy
system, such as a wind-powered system, to store energy that exceeds consumer demand and later
release that energy when there is greater demand.
[0003] A basic flow battery includes a redox flow cell that has a negative electrode
and a positive electrode separated by an electrolyte layer, which may include separator such as
an ion-exchange membrane. A negative liquid electrolyte is delivered to the negative electrode
and a positive liquid electrolyte is delivered to the positive electrode to drive an
electrochemically reversible redox reaction. Upon charging, the electrical energy supplied causes
a chemical reduction reaction in one electrolyte and an oxidation reaction in the other electrolyte.
The separator prevents the electrolytes from mixing but permits selected ions to pass through to
complete the redox reactions. Upon discharge, the chemical energy contained in the liquid
electrolytes is released in the reverse reactions and electrical energy can be drawn from the
electrodes. Flow batteries are distinguished from other electrochemical devices by, inter alia, the
use of externally-supplied, liquid electrolytes that participate in a reversible electrochemical
reaction.
SUMMARY
[0004] Disclosed is a flow battery that includes at least one electrochemical cell that
has a first electrode, a second electrode spaced apart from the first electrode and separator
arranged between the first electrode and the second electrode. A first storage portion and a
second storage portion are respectively fluidly connected with the at least one cell. A first liquid
electrolyte and a second liquid electrolyte are located in the respective first storage portion and
the second storage portion. The first electrode has an area over which it is catalytically active
with regard to the first liquid electrolyte and the second electrode has an area over which it is
catalytically active with regard to the second liquid electrolyte such that the area of the first
electrode is greater than the area of the second electrode.
[0005] Also disclosed is a method for controlling degradation in a flow battery. The
method includes controlling a potential for corrosion by establishing a first electrode to have an
area over which it is catalytically active with regard to a first liquid electrolyte and a second
electrode to have an area over which it is catalytically active with regard to a second liquid
electrolyte such that the area of the first electrode is greater than the area of the second electrode.
BRIEF DESCRIPTION OF THE DRAWINGS
[0006] The various features and advantages of the disclosed examples will become
apparent to those skilled in the art from the following detailed description. The drawings that
accompany the detailed description can be briefly described as follows.
[0007] Figure 1 shows an example flow battery.
[0008] Figure 2 shows an example electrochemical cell.
[0009] Figure 3 shows a first electrode and a second electrode of different sizes.
[0010] Figure 4 shows another arrangement of the electrodes shown in Figure 3.
[0011] Figure 5 shows frame seals around the periphery of a first electrode and a
second electrode.
[0012] Figure 6 shows an indentation of a perimeter of an area of a second electrode
from the perimeter of an area of a first electrode.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0013] Figure 1 schematically illustrates selected portions of an example flow battery
20 for selectively storing and discharging electrical energy. As an example, the flow battery 20
can be used to convert electrical energy generated in a renewable energy system to chemical
energy that is stored until a later time when there is greater demand at which the flow battery 20
then converts the chemical energy back into electrical energy. The flow battery 20 can supply the
electric energy to an electric grid, for example. As will be described, the disclosed flow battery
20 includes features for enhanced durability.
[0014] The flow battery 20 includes at least one liquid electrolyte 22 that has an
electrochemically active specie 24 that functions in a redox pair with regard to a second reactant
26, which can be another liquid electrolyte with electrochemically active specie, or any other
electrochemically active specie such as hydrogen or air, for example. For example, the
electrochemically active species are based on vanadium, bromine, iron, chromium, zinc, cerium,
lead or combinations thereof. In embodiments, the liquid electrolytes 22 and 26 are aqueous
solutions that include one or more of the electrochemically active species 24 and 30.
[0015] The liquid electrolyte 22 and reactant 26 are contained in respective storage
portions 32 and 34, such as tanks. As shown, the storage portions 32 and 34 are substantially
equivalent cylindrical storage tanks; however, the storage portions 32 and 34 can alternatively
have other shapes and sizes.
[0016] The liquid electrolytes 22 and 26 are delivered (e.g., pumped) to one or more
electrochemical cells 36 of the flow battery 20 through respective feed lines 38 and are returned
from the electrochemical cell 36 to the storage portions 32 and 34 via return lines 40. Thus, the
storage portions 32 and 34 are external of the electrochemical cell 36 and are fluidly connected
with the electrochemical cell 36 to circulate the liquid electrolytes 22 and 26 there through.
[0017] In operation, the liquid electrolytes 22 and 26 are delivered to the
electrochemical cell 36 to either convert electrical energy into chemical energy or convert
chemical energy into electrical energy that can be discharged. The electrical energy is
transmitted to and from the electrochemical cell 36 through an electrical pathway 42 that
completes the circuit and allows the completion of the electrochemical redox reactions.
[0018] Figure 2 shows a cross-section of a portion of one of the electrochemical cells
36. It is to be understood that the flow battery 20 can include a plurality of such electrochemical
cells 36 in a stack, depending on the designed capacity of the flow battery 20.
[0019] In this example, the electrochemical cell 36 includes a first bipolar plate 50
and a second bipolar plate 52 spaced apart from the first bipolar plate 50. The bipolar plates 50
and 52 are electrically conductive and can be graphite plates or metallic plates, for example. The
bipolar plates 50 and 52 may include a plurality of channels 54 that serve as flow fields for
delivering the liquid electrolytes 22 and 26 within the electrochemical cell 36.
[0020] A first electrode 62 is arranged adjacent the first bipolar plate 50 and a second
electrode 64 is arranged adjacent the second bipolar plate 52. In the illustrated example, the first
electrode 62 is in contact with the face of the first bipolar plate 50 and the second electrode 64 is
in contact with the face of the second bipolar plate 52. A separator, such as an ion-exchange
membrane, 66 is arranged between, and in contact with, the electrodes 62 and 64. In this
example, the first electrode 62 is an anode electrode and the second electrode 64 is a cathode
electrode.
[0021] The first electrode 62 and the second electrode 64 are porous materials that
are electrically conductive and catalytically active for the desired redox reactions, such as porous
carbon electrodes. As an example, one or both of the electrodes 62 and 64 include a carbon paper
or felt materials that are catalytically active with regard to the liquid electrolytes 22 and 26. That
is, the surfaces of the carbon material are catalytically active in the flow battery 20. In the redox
reactions of the flow battery 20, the energy barrier to the reaction is relatively low, and thus
stronger catalytic materials, such as noble metals or alloys, are typically not required as with
electrochemical devices that utilize gaseous reactants such as oxygen or hydrogen. The carbon
material can be activated using a thermal and/or chemical treatment processes to clean the carbon
material and produce oxides that serve as active catalytic sites.
[0022] In operation, the liquid electrolytes 22 and 26 are pumped from the storage
portions 32 and 34 into the respective bipolar plates 50 and 52. In this regard, the bipolar plates
can include manifolds and the like for delivering the liquid electrolytes 22 and 26 into the
channels 54. The liquid electrolytes 22 and 26 flow through the channels 54 and are delivered to
the electrodes 62 and 64. It is to be understood that the bipolar plates 50 and 52 and the channels
54 are optional in the flow battery 20. That is, the flow battery 20 can alternatively be configured
for "flow-through" operation where the liquid electrolytes 22 and 30 are pumped directly into the
electrodes 62 and 64 without the use of flow field channels.
[0023] In operation, there can be a high electrical over-potential at the second
electrode 64 (the cathode), which results in corrosion degradation of materials within the flow
battery 20. Thus, the term "high electrical over-potential" refers to a potential that is sufficient to
drive corrosive degradation of one or more materials within the electrochemical cell 36. Under
designed operation conditions in the electrochemical cell 36, the potential is maintained below a
potential that drives corrosion degradation of the selected materials. However, localized
starvation of anolyte is one condition that can cause the electrical potential to change in localized
areas of an anode electrode in a flow battery. Carbon material, such as carbon in the second
electrode 64 and/or the bipolar plate 52, any metal or metal alloys or other materials subject to
corrosion degradation in the electrochemical cell 36 can degrade in this over-potential condition
and ultimately debit the durability of the flow battery 20. The electrochemical mechanisms and
operational circumstances responsible for the degradation are understood by those skilled in the
art and are therefore not further discussed herein.
[0024] The electrochemical cell 36 includes features for controlling (e.g., limiting)
the potential for corrosion degradation. Figure 3 shows a view of the first electrode 62 and the
second electrode 64 according to the section shown in Figure 2. The first electrode 62 has an area
Ai over which it is catalytically active with regard to the first liquid electrolyte 22 and the second
electrode 64 has an area A2 over which it is catalytically active with regard to the second liquid
electrolyte 26. The area Ai is greater than the area A2. The separator 66 also has an area, A3,
which in this example is equal to the area Ai.
[0025] Selecting the area Ai to be greater than the area A2 ensures that wherever
there is catholyte (liquid electrolyte 26) present within the second electrode 64 there will also be
anolyte (first electrolyte 22) present in the first electrode 62 such that localized starvation of the
anolyte is limited or prevented. Thus, establishing the area Ai to be greater than the area A2
controls the potential for corrosion degradation and thereby enhances the durability of the flow
battery 20.
[0026] The areas Ai and A2 over which the respective electrodes 62 and 64 are
catalytically active, or "effective areas," are areas that have open porosity for receiving the
respective liquid electrolytes 22 and 26. As will be described, the areas Ai and A2 can be defined
by the physical sizes of the electrodes 62 and 64, by the sizes of the perimeters of the effective
areas, or both.
[0027] In a further embodiment, in accordance with the differential areas Ai and A2,
the first bipolar plate 50 optionally includes a larger flow field than the second bipolar plate 52.
The areas of the flow fields are defined by a perimeter that circumscribes all of the channels 54
in the respective bipolar plates 50 and 52. In Figure 2, the area of the flow field of the first
bipolar plate 50 is denoted at 50a and the area of the flow field of the second bipolar plate 52 is
denoted at 52a.
[0028] In the example shown in Figure 3, the electrodes 62 and 64 extend between
respective edges 62a and 64a. As shown, the edges 62a and 64a extend on four sides and thus the
electrodes 62 and 64 are each provided in a square shape, although other shapes can alternatively
be selected. The edges 64a of the second electrode 64 are indented inwardly with regard to the
edges 62a of the first electrode 62. Thus, the areas Ai and A2 in this embodiment are defined by
the edges 62a and 64a and thus the physical sizes of the electrodes 62 and 64. In a further
embodiment, the edges 64a are indented a uniform distance inwardly of the edges 62a such that
there is a substantially uniform band 65 around the periphery of the second electrode 64.
[0029] In another example shown in Figure 4, the indentation distance is non-uniform
around the second electrode 64 such that the indentation distance on at least one side is greater
than an indentation distance on another side. For example, the indentation distance between the
edges 62a and 64a is non-uniform around the electrodes 62 and 64. In comparison, in the
example shown in Figure 3, the edges 64a are indented a uniform distance inwardly of the edges
62aln a further example, the designed indentation distance should be slightly larger than the
maximum indentation that can result from the anticipated manufacturing tolerances.
[0030] In another example shown in Figure 5, frame seals 66 establish the areas Ai
and A2. The frame seals 66 extend around the outside periphery of each of the electrodes 62 and
64. As an example, each frame seal 66 includes a seal material, such as a polymeric material, that
is impregnated into the edges 62a and 64a of the electrodes 62 and 64 to prevent escape of the
liquid electrolytes 22 and 26. The seal material infiltrates into the pores of the electrodes 62 and
64 and, upon solidification, prevents or substantially prevents liquid electrolyte from flowing in
that portion of the electrode 62 or 64. Thus, the areas of the frame seals 66 of the electrodes 62
and 64 are not catalytically active and therefore are not part of the effective area of the electrodes
62 and 64 for participating in the electrochemical reactions.
[0031] The frame seals 66 serve as a perimeter boundary of the areas Ai and A2. The
impregnation of the seal material can be controlled such that the seal material penetrates a
greater in-plane distance into the second electrode 64 than into the first electrode 62. That is, in
the in-plane direction, the thickness of the frame seal 66 of the first electrode 62 is less than the
thickness of the frame seal 66 of the second electrode 64. In the drawing, 62b denotes the
perimeter of the area Ai and 64b denotes the perimeter of the area A2.
[0032] Figure 6 shows another view of the perimeters 62b and 64b. As shown,
similar to the indentation of the edges 64a and 62a, the perimeter 64b of the second electrode 64
is indented inwardly of the perimeter 62b of area Ai of the first electrodes 62. Thus, although the
edges 62a and 64a may or may not be flush, the perimeters 62b and 64b are non-flush to achieve
the difference in area between Ai and A2.
[0033] Although a combination of features is shown in the illustrated examples, not
all of them need to be combined to realize the benefits of various embodiments of this disclosure.
In other words, a system designed according to an embodiment of this disclosure will not
necessarily include all of the features shown in any one of the Figures or all of the portions
schematically shown in the Figures. Moreover, selected features of one example embodiment
may be combined with selected features of other example embodiments.
[0034] The preceding description is exemplary rather than limiting in nature.
Variations and modifications to the disclosed examples may become apparent to those skilled in
the art that do not necessarily depart from the essence of this disclosure. The scope of legal
protection given to this disclosure can only be determined by studying the following claims.

CLAIMS
What is claimed is:
1. A flow battery comprising:
at least one electrochemical cell including a first electrode, a second electrode spaced
apart from the first electrode, and a separator layer arranged between the first electrode and the
second electrode;
a first storage portion and a second storage portion respectively fluidly connected with
the at least one electrochemical cell;
at least one electrolyte in one of the first storage portion or the second storage portion,
wherein the first electrode has an area Ai over which it is catalytically active with regard
to the first liquid electrolyte and the second electrode has an area A2 over which it is catalytically
active with regard to the second liquid electrolyte, and the area Ai is greater than the area A2.
2. The flow battery as recited in claim 1, wherein the first electrode is an anode electrode
and the second electrode is a cathode electrode.
3. The flow battery as recited in claim 1, wherein the first electrode is non-flush with the
second electrode with regard to edge alignment of the electrodes.
4. The flow battery as recited in claim 1, wherein the perimeter of the area A2 is indented
inwardly of the perimeter of the area Ai.
5. The flow battery as recited in claim 4, wherein the perimeter of the area A2 is indented a
uniform distance around the perimeter of the area A .
6. The flow battery as recited in claim 4, wherein the perimeter of the area A2 is indented a
non-uniform distance around the perimeter of the area Ai.
7. The flow battery as recited in claim 1, wherein the edges of the second electrode are
indented inwardly of the edges of the first electrode.
8. The flow battery as recited in claim 1, wherein the first electrode and the second
electrode each include a frame seal that comprises a seal material that is impregnated into the
respective first electrode and second electrode to provide the respective area Ai and area A2, and
the frame seals have an in-plane thickness at the edges of the respective first electrode and
second electrode such that the in-plane thickness of the frame seal of the first electrode is less
than the in-plane thickness of the frame seal of the second electrode.
9. The flow battery as recited in claim 1, wherein the at least one electrochemical cell
includes a first bipolar plate arranged adjacent the first electrode and a second bipolar plate
arranged adjacent the second electrode, the first bipolar plate including a first flow field and the
second bipolar plate including a second flow field that extends over an area that is less than the
area over which the first flow field extends.
10. The flow battery as recited in claim 1, wherein the separator is an ion-exchange material.
11. The flow battery as recited in claim 1, wherein the separator has an area A3 that is equal
to the area Ai.
12. The flow battery as recited in claim 1, including a gas selected from hydrogen and air in
the other of the first storage portion or the second storage portion.
13. A method for controlling degradation in a flow battery, the method comprising:
controlling a potential for corrosion by establishing a first electrode to have an area Ai
over which it is catalytically active with regard to a first liquid electrolyte and a second electrode
to have an area A2 over which it is catalytically active with regard to a second liquid electrolyte
such that the area Ai is greater than the area A2.