FLOW BATTERY WITH CARBON PAPER
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 having a negative electrode
and a positive electrode separated by an ion-exchange membrane or a non-conductive separator
filled with electrolyte. A negative electrolyte is delivered to the negative electrode and a positive
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 ion-exchange membrane
prevents the electrolytes from mixing rapidly but permits selected ions to pass through to
complete the redox reactions while electrically isolating the two electrodes. Upon discharge, the
chemical energy contained in the electrolyte 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 on at
least on side that participate in a reversible electrochemical reaction.
SUMMARY
[0004] Disclosed is a flow battery that includes a liquid electrolyte having an
electrochemically active specie. A flow field plate includes a first flow field channel and a
second flow field channel that is separated from the first flow field channel by a rib. There is a
flow path for the liquid electrolyte to flow over the rib between the channels. An electrode is
arranged adjacent the flow field plate such that the liquid electrolyte that flows over the rib must
flow through the electrode. The electrode includes a carbon paper that is catalytically active with
regard to liquid electrolyte. The carbon paper defines a compressive strain of less than 20% at a
compressive stress of 0.8 MPa and an uncompressed porosity in the range 60-85%.
[0005] In another aspect, the carbon paper has an average uncompressed thickness of
150-400 micrometers and a porosity of 65-85 vol%.
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 illustrates an example flow battery.
[0008] Figure 2 illustrates an example flow battery cell of the flow battery of Figure
1.
[0009] Figure 3 illustrates a carbon paper of a flow battery cell.
[0010] Figure 4 illustrates another example carbon paper of a flow battery cell.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0011] Figure 1 illustrates selected portions of an example flow battery 20 for
selectively storing and discharging electrical energy. As an example, the flow battery 20 may be
used to convert electrical energy generated in a renewable energy system to chemical energy that
can be stored until a later time at which there is demand for the electrical energy. The flow
battery 20 may then convert the chemical energy into electrical energy for supply to an electric
grid, for example. As will be described, the flow battery 20 includes features for enhanced
performance and durability.
[0012] The flow battery 20 includes a liquid electrolyte 22 that has an
electrochemically active specie 24 that functions in a redox pair with regard to an additional
liquid electrolyte 26 and electrochemically active specie 30. 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 -may be aqueous or non
aqueous solutions that include one or more of electrochemically active species.
[0013] The liquid electrolytes 22 and 26 are contained in respective storage tanks 32
and 34. As shown, the storage tanks 32 and 34 are substantially equivalent cylindrical storage
tanks; however, the storage tanks 32 and 34 can alternatively have other shapes and sizes.
[0014] The liquid electrolytes 22 and 26 are delivered (e.g., pumped) to one or more
cells 36 through respective feed lines 38 and are returned from the cell or cells 36 to the storage
tanks 32 and 34 via return lines 40.
[0015] In operation, the liquid electrolytes 22 and 26 are delivered to the 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 cell 36
through an electrical pathway 42 that completes the circuit and allows the completion of the
electrochemical redox reactions.
[0016] Figure 2 shows an example of the cell 36. It is to be understood that the flow
battery 20 can include a plurality of the cells 36 in a stack, depending on the designed capacity of
the flow battery 20. As shown, the cell 36 includes a first flow field plate 50 and a second flow
field plate 52 spaced apart from the first flow field plate 50. The second flow field plate 52 may
be substantially similar to the first flow field plate 50, as will be described below.
[0017] The first flow field plate 50 includes a first flow field channel 54 and a second
flow field channel 56 that is separated from the first flow field channel 54 by a rib 58. The
arrangement of the flow field channels 54 and 56 is such that there is a flow path 60 for the
liquid electrolyte 22 or 26 to flow over the rib 58 between the channels 54 and 56.
First and second electrodes 62 and 64 are arranged adjacent the respective first and
second flow field plates 50 and 52 such that the liquid electrolyte 22 or 26 that flows over the
ribs 58 must flow through the corresponding electrode 62 or 64. In this example, an ionexchange
membrane 66 is arranged between the electrodes 62 and 64.
[0018] In the illustrated embodiment, one or both of the electrodes 62 and 64 include
a carbon paper 68, such as carbon fiber paper, that is catalytically active with regard to the liquid
electrolyte 22 and/or 26. That is, the surfaces of the carbon material of the carbon paper serve as
catalytically active surfaces 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 more expensive catalytic materials,
such as noble metals or alloys, are not required as with cells that utilize gaseous reactants. In one
embodiment, the carbon paper 68 is activated using a thermal treatment process to clean the
carbon material, increase the surface area, and produce oxides that serve as active catalytic sites.
In a further embodiment, the carbon paper is a carbon/carbon composite including carbon fibers
and a carbon binder residue. Polyacrolynitrile is one example precursor for carbon fibers used in
the carbon paper 68. A phenolic resin is one example precursor for carbon binder.
[0019] There is a tradeoff in flow batteries between performance and pressure drop of
the flow of the liquid electrolytes through a cell. For example, in flow batteries that do not utilize
flow fields and force the flow of the liquid electrolytes through carbon felt electrodes ("flowthrough"),
there is relatively good performance but high pressure drop (which requires more
input energy to move electrolyte through the cell) and relatively low durability because of stack
compression on the carbon felt and ion-exchange membrane. In flow batteries that utilize a flow
field (i.e., "flow-by"), there is less of a pressure drop because the liquid electrolytes are not
forced through the electrodes, but the performance is relatively lower because the enhanced mass
transport afforded by the convective flow through the electrodes is not present to a significant
degree.
[0020] The flow battery 20 utilizes a "mixed flow" flow field and the carbon paper 68
to provide a beneficial balance between pressure drop and performance. The term "mixed flow"
refers to a combination of "flow-through" and "flow-by". In embodiments, the "mixed flow" is
achieved through the arrangement of the flow field channels 54 and 56 on the first flow field
plate 50 (and optionally also the second flow field plate 52). For example, the second flow field
channel 56 is downstream from the first flow field channel 54, and thus the liquid electrolyte
22/26 in the second flow field channel 56 is at a lower pressure than the liquid electrolyte 22/26
in the first flow field channel 54 due to pressure loss. The difference in pressure causes a
pressure gradient between the channels 54 and 56 that drives at least a portion of the liquid
electrolyte 22/26 to flow over the rib 58 in the flow path 60 from the first flow field channel 54
into the second flow field channel 56. In a few examples, the flow field channels 54 and 56 are
channels of a serpentine channel arrangement, interdigitated channel arrangement, partially
interdigitated channel arrangement or combination thereof to provide the pressure gradient.
[0021] The characteristics of the carbon paper 68 are selected in accordance with the
"mixed flow" design of the channels 54 and 56 to enhance performance and durability. For
example, the carbon paper 68 has a predetermined compressive strain of less than 20% at a
compressive stress of 0.8 MPa, an uncompressed porosity in the range 60-85%, and a thickness
(t) in the range 150 to 400 mih (micrometers). A Young's modulus of compression could be
specified instead of compressive strain provided the stress-strain response is linear. In
comparison, carbon felts commonly used in flow batteries are relatively pliable and can intrude
into channels to restrict flow and cause inconsistent performance. The relatively stiff carbon
paper 68 reduces intrusion and thereby reduces flow restriction and increases performance
consistency. Carbon felt is also relatively thick and increases stack size and the average distance
that ions must move to reach an ion-exchange membrane. The relatively thin carbon paper 68
reduces stack size and the average distance of ion movement. Moreover, the carbon paper 68 is
less compressible than felt and therefore does not require high stack compression, which
improves stack durability. Additionally, carbon felt compresses over the ribs and thereby has an
inconsistent porosity that debits flow distribution. The carbon paper 68 is relatively less
compressible and therefore provides more uniform compression and flow distribution.
[0022] The predetermined compressive strength, thickness (t) and uncompressed
porosity enable the forced flow component of the mixed flow of the liquid electrolyte 22 or 26
over the rib 58 between channels 54 and 56. For example, the compressive strength is greater
than 0.8 MPa at 20% compressive strain, the uncompressed porosity is from 65-85 vol% and the
thickness is from 150-400 micrometers. In a further example, the compressive strength is greater
than 0.8 MPa at 10% compressive strain and the thickness is from 150-250 micrometers.
[0023] Figure 3 shows a portion of another example of a carbon paper 168 for use in
the flow battery 20. In this disclosure, like reference numerals designate like elements where
appropriate, and reference numerals with the addition of one-hundred or multiples thereof
designate modified elements that are understood to incorporate the same features and benefits of
the corresponding elements. In this example, the carbon paper 168 includes catalytically active
carbon fibers 170. The carbon fibers 170 are arranged randomly or in a pattern, such as a woven
structure. Carbon particles 172 are disposed on the carbon fibers 170. The carbon particles 172
increase the surface area of the carbon paper 168 for enhanced catalytic activity. In one example,
the carbon particles 172 have an average diameter of 10-100 nanometers and the carbon paper
168 includes 1-10 wt of the carbon particles 172.
[0024] The carbon particles 172 can be substantially uniformly distributed through
the carbon paper 168 such that the carbon paper 168 has a relatively uniform porosity.
Alternatively, as shown, there is a concentration gradient 174 of the carbon particles 172 through
the thickness (t) of the carbon paper 168 such that the carbon paper 168 has a graded porosity. In
this example, the concentration decreases as a function of distance from the membrane side of
the carbon paper 168. The concentration gradient 174 enables greater flow of the liquid
electrolyte 22/26 near the flow field plate side of the carbon paper 168 and increased catalytic
activity near the membrane side of the carbon paper 168 to reduce the average distance of ion
movement relative to the ion-exchange membrane 66.
[0025] In embodiments, the carbon particles 172 are deposited onto the carbon fibers
170 using a liquid suspension of the carbon particles 172 in a carrier fluid. The liquid suspension
is applied to the carbon paper 168, such as by spraying or dipping or painting, and then dried to
remove the carrier fluid such that the carbon particles 172 remain in the carbon paper 168. The
application and drying process can be repeated to achieve a desired loading level of the carbon
particles 172. Additionally, a vacuum can be applied to one side of the carbon paper 168 during
the application and/or drying process to achieve the concentration gradient 174.
[0026] Figure 4 shows a portion of another example of a carbon paper 268 for use in
the flow battery 20. In this example, the carbon paper 268 includes the catalytically active carbon
fibers 170 and carbon particles 272 disposed on the carbon fibers 170. The carbon particles 272
have a multi-modal size distribution to further enhance activity and control porosity and
conductivity. In this example, the carbon particles 272 include carbon particles 272a that have a
first average diameter and carbon particles 272b that have a second average diameter that is
greater than the first average diameter. In other examples, the carbon particles can differ in other
aspects, such as micro structure and/or composition. For example, one could have two particles
with same diameter but very different density and fraction and size of micropores.
[0027] 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.
[0028] 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:
a liquid electrolyte including an electrochemically active specie;
a flow field plate including a first flow field channel and a second flow field channel
separated from the first flow field channel by a rib;
a flow path of the liquid electrolyte over the rib between the channels; and
an electrode arranged adjacent the flow field plate such that the liquid electrolyte that
flows over the rib must flow through the electrode, the electrode including a carbon paper that is
catalytically active with regard to liquid electrolyte and that defines a compressive strain of less
than 20% at a compressive stress of 0.8 MPa and an uncompressed porosity in the range 60-85%.
2. The flow battery as recited in claim 1, wherein the carbon paper comprises carbon fibers
and carbon binder residue and has a uniform porosity through the thickness.
3. The flow battery as recited in claim 1, wherein the carbon paper comprises carbon fibers
and carbon binder residue and has a graded porosity through the thickness.
4. The flow battery as recited in claim 3, wherein the carbon paper has a maximum porosity
at the side near the flow field plate.
5. The flow battery as recited in claim 1, wherein the carbon paper includes carbon fibers
and carbon binder residue and carbon particles disposed on the carbon fibers.
6. The flow battery as recited in claim 5, wherein the carbon paper includes l-10wt% of the
carbon particles.
7. The flow battery as recited in claim 5, wherein the carbon particles have an average
diameter of 10-100 nanometers.
8. The flow battery as recited in claim 5, wherein the carbon particles have a multi-modal
size distribution.
9. The flow battery as recited in claim 1, wherein the carbon paper has a thickness of 150-
400 micrometers and the maximum porosity is 65-85 vol%.
10. A flow battery comprising:
a liquid electrolyte including an electrochemically active specie;
a flow field plate including a first flow field channel and a second flow field channel
separated from the first flow field channel by a rib;
a flow path of the liquid electrolyte over the rib between the channels; and
an electrode arranged adjacent the flow field plate such that the liquid electrolyte that
flows over the rib must go through the electrode, the electrode including a carbon paper that is
catalytically active with regard to liquid electrolyte and that has an average thickness of 150-400
micrometers and an uncompressed porosity of 65-85 vol%.
11. The flow battery as recited in claim 10, wherein the thickness is 150-250 micrometers.
12. The flow battery as recited in claim 10, wherein the carbon paper has a compressive
strain of less than 20% at a compressive stress of 0.8 MPa.
13. The flow battery as recited in claim 10, wherein the carbon paper comprises carbon fibers
and carbon binder residue and has a graded porosity through the thickness.
14. The flow battery as recited in claim 10, wherein the carbon paper includes carbon fibers
and carbon binder residue and carbon particles disposed on the carbon fibers.