U.S. patent application number 11/324370 was filed with the patent office on 2007-07-05 for highly conductive composites for fuel cell flow field plates and bipolar plates.
Invention is credited to Bor Z. Jang, Lulu Song, Aruna Zhamu.
Application Number | 20070154771 11/324370 |
Document ID | / |
Family ID | 38224827 |
Filed Date | 2007-07-05 |
United States Patent
Application |
20070154771 |
Kind Code |
A1 |
Jang; Bor Z. ; et
al. |
July 5, 2007 |
Highly conductive composites for fuel cell flow field plates and
bipolar plates
Abstract
This invention provides a fuel cell flow field plate or bipolar
plate having flow channels on faces of the plate, comprising an
electrically conductive polymer composite. The composite is
composed of (A) at least 50% by weight of a conductive filler,
comprising at least 5% by weight reinforcement fibers, expanded
graphite platelets, graphitic nano-fibers, and/or carbon
nano-tubes; (B) polymer matrix material at 1 to 49.9% by weight;
and (C) a polymer binder at 0.1 to 10% by weight; wherein the sum
of the conductive filler weight %, polymer matrix weight % and
polymer binder weight % equals 100% and the bulk electrical
conductivity of the flow field or bipolar plate is at least 100
S/cm. The invention also provides a continuous process for
cost-effective mass production of the conductive composite-based
flow field or bipolar plate.
Inventors: |
Jang; Bor Z.; (Centerville,
OH) ; Zhamu; Aruna; (Centerville, OH) ; Song;
Lulu; (Centerville, OH) |
Correspondence
Address: |
Bor Z. Jang
9436 Parkside Drive
Centerville
OH
45458
US
|
Family ID: |
38224827 |
Appl. No.: |
11/324370 |
Filed: |
January 4, 2006 |
Current U.S.
Class: |
429/514 ;
252/511; 264/331.11; 429/518; 429/535 |
Current CPC
Class: |
H01M 8/0213 20130101;
C08J 5/04 20130101; H01M 8/0228 20130101; Y02E 60/50 20130101; H01B
1/24 20130101; Y02P 70/50 20151101; H01M 8/0221 20130101; H01M
8/0226 20130101 |
Class at
Publication: |
429/038 ;
252/511; 264/331.11 |
International
Class: |
H01M 8/02 20060101
H01M008/02; H01B 1/24 20060101 H01B001/24; C08J 5/00 20060101
C08J005/00 |
Goverment Interests
[0001] The present invention is based on the research results of a
project supported by the US Department of Energy SBIR-STTR Program.
The US government has certain rights on this invention.
Claims
1. A fuel cell flow field plate or bipolar plate having flow
channels on at least a face of the plate, comprising an
electrically conductive polymer composite having: (A) at least 50%
by weight of a conductive filler, comprising at least 5% by weight
reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes; (B) thermoplastic matrix
material at 1 to 49.9% by weight; and (C) thermoset resin binder at
0.1 to 10% by weight; wherein the sum of said conductive filler
weight %, thermoplastic matrix weight % and thermoset resin binder
weight % equals 100% and the bulk electrical conductivity of said
flow field or bipolar plate is at least 100 S/cm.
2. The flow field or bipolar plate as defined in claim 1, wherein
said plate has a major surface having a skin layer less than 100
.mu.m in thickness and said skin layer has a polymer volume
fraction less than 20% and a conductive filler greater than
80%.
3. The flow field or bipolar plate as defined in claim 1, wherein
the bulk conductivity is at least 200 S/cm.
4. The flow field or bipolar plate as defined in claim 1 wherein
said thermoset resin binder is selected from the group consisting
of unsaturated polyester resins, vinyl esters, epoxies, phenolic
resins, polyimide resins, bismaleimide resins, polyurethane resins,
and combinations thereof.
5. The flow field or bipolar plate as defined in claim 1 wherein
said conductive filler comprises a conductive material selected
from the group consisting of graphite powder, carbon/graphite
fibers, metal fibers, carbon nano-tubes, graphitic nano-fibers,
nano-scaled graphene plates, carbon blacks, metal particles, and
combinations thereof.
6. The flow field or bipolar plate as defined in claim 1, wherein
said reinforcement fibers, carbon nano-tubes, graphitic
nano-fibers, and/or expanded graphite platelets form an
overlapping, contiguous-strand backbone structure.
7. The flow field or bipolar plate as defined in claim 1 wherein
said reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes are bonded together by said
thermoset resin binder.
8. The flow field or bipolar plate as defined in claim 1 wherein
said reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes are bonded together by said
thermoset resin binder and said thermoplastic matrix material.
9. The flow field or bipolar plate as defined in claim 1 wherein
said reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes form a mat.
10. A fuel cell flow field plate or bipolar plate having flow
channels on at least a face of the plate, comprising an
electrically conductive polymer composite having: (A) at least 50%
by weight of a conductive filler, comprising at least 5% by weight
reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes; (B) a polymer matrix
material at 1 to 49.9% by weight; and (C) a polymer binder at 0.1
to 10% by weight; wherein the sum of said conductive filler weight
%, polymer matrix material weight % and polymer binder weight %
equals 100% and the bulk electrical conductivity of said flow field
plate or bipolar plate is at least 100 S/cm.
11. The flow field or bipolar plate as defined in claim 10 wherein
said polymer matrix material comprises a material selected from a
thermoset resin, an interpenetrating network, a
semi-interpenetrating network, an elastomer, or a combination
thereof.
12. The flow field or bipolar plate as defined in claim 10 wherein
said polymer binder comprises a water soluble polymer.
13. The flow field or bipolar plate as defined in claim 10, wherein
the bulk conductivity is at least 200 S/cm.
14. The flow field or bipolar plate as defined in claim 10 wherein
said polymer matrix material is selected from the group consisting
of unsaturated polyester resins, vinyl esters, epoxies, phenolic
resins, polyimide resins, bismaleimide resins, polyurethane resins,
and combinations thereof.
15. The flow field or bipolar plate as defined in claim 10 wherein
said conductive filler comprises a conductive material selected
from the group consisting of graphite powder, carbon/graphite
fibers, metal fibers, carbon nano-tubes, graphitic nano-fibers,
nano-scaled graphene plates, carbon blacks, metal particles, and
combinations thereof.
16. The flow field or bipolar plate as defined in claim 10, wherein
said conductive filler comprises nano-scaled graphene plates.
17. The flow field or bipolar plate as defined in claim 10, wherein
said reinforcement fibers, carbon nano-tubes, graphitic
nano-fibers, and/or expanded graphite platelets form an
overlapping, contiguous-strand backbone structure.
18. The flow field or bipolar plate as defined in claim 10 wherein
said reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes form a mat.
19. The flow field or bipolar plate as defined in claim 10, wherein
said plate has a major surface having a skin layer less than 100
.mu.m in thickness and said skin layer has a polymer volume
fraction less than 20% and a conductive filler volume fraction
greater than 80%.
20. A process for producing a fuel cell flow field plate or bipolar
plate as defined in claim 1, said process comprising: (A)
continuously or intermittently feeding and moving a sheet of porous
substrate toward a desired direction, said substrate having
through-thickness pores; (B) mixing and feeding said conductive
filler, said thermoset binder, said thermoplastic matrix material
and a carrier fluid onto said porous substrate and directing said
carrier fluid to substantially flow through said pores, leaving
behind a layer of a solid mixture of said filler, binder and matrix
material on said substrate; (C) moving said substrate so as to
allow said solid mixture layer to go through a compaction stage;
and (D) heating and consolidating said solid mixture layer and
generating flow channels on a surface of said solid mixture layer
to form said flow field or bipolar plate.
21. The process as defined in claim 20, wherein said carrier fluid
comprises water and said step of mixing and feeding comprises
slurry molding.
22. The process as defined in claim 20, wherein said carrier fluid
comprises compressed air.
23. The process as defined in claim 20, wherein said step of
heating and consolidating comprises a step of embossing or
matched-die molding said mixture layer.
24. The process as defined in claim 23, further comprising a step
of coating an embossing tool surface or mold surface with a layer
of fine graphite, expanded graphite and/or nano-scaled graphene
plate powder prior to embossing or molding.
25. The process as defined in claim 20, further comprising a step
of curing said thermoset resin binder before, during, and/or after
said compaction stage.
26. A process for producing a fuel cell flow field plate or bipolar
plate as defined in claim 10, said process comprising: (A)
continuously or intermittently feeding and moving a sheet of porous
substrate toward a desired direction, said substrate having
through-thickness pores; (B) mixing and feeding said conductive
filler, said polymer binder, said polymer matrix material and a
carrier fluid onto said porous substrate and directing said carrier
fluid to substantially flow through said pores, leaving behind a
layer of a solid mixture of said filler, binder and matrix material
on said substrate; (C) moving said substrate so as to allow said
solid mixture layer to go through a compaction stage; and (D)
heating and consolidating said solid mixture and generating flow
channels on at least a surface of said solid mixture layer to form
said flow field or bipolar plate.
27. The process as defined in claim 26, wherein said carrier fluid
comprises water and said step of mixing and feeding comprises
slurry molding.
28. The process as defined in claim 26, wherein said carrier fluid
comprises compressed air.
29. The process as defined in claim 26, wherein said step of
heating and consolidating comprises a step of embossing or
matched-die molding said mixture layer.
30. The process as defined in claim 29, further comprising a step
of coating an embossing tool surface or a mold surface with a layer
of fine graphite, expanded graphite and/or nano-scaled graphene
plate powder prior to embossing or molding.
Description
FIELD OF THE INVENTION
[0002] The present invention provides a highly electrically
conductive composite material for use in a fuel cell bipolar plate
or flow field plate.
BACKGROUND OF THE INVENTION
[0003] A proton exchange membrane (PEM) fuel cell is typically
composed of a seven-layered structure, including (a) a central PEM
electrolyte layer for proton transport; (b) two electro-catalyst
layers on the two opposite primary surfaces of the electrolyte
membrane; (c) two fuel or gas diffusion electrodes (GDEs,
hereinafter also referred to as diffusers) or backing layers
stacked on the corresponding electro-catalyst layers (each GDE
comprising porous carbon paper or cloth through which reactants and
reaction products diffuse in and out of the cell); and (d) two flow
field plates (or a bi-polar plate) stacked on the GDEs. The flow
field plates are typically made of graphite, metal, or conducting
composite materials, which also serve as current collectors.
Gas-guiding channels are defined on a GDE facing a flow field plate
or, more typically, on a flow field plate surface facing a GDE.
Reactants (e.g., H.sub.2 or methanol solution) and reaction
products (e.g., CO.sub.2 at the anode of a direct methanol fuel
cell, and water at the cathode side) are guided to flow into or out
of the cell through the flow field plates. The configuration
mentioned above forms a basic fuel cell unit. Conventionally, a
fuel cell stack comprises a number of basic fuel cell units that
are electrically connected in series to provide a desired output
voltage. If desired, cooling channels and humidifying plates may be
added to assist in the operation of a fuel cell stack.
[0004] In one common practice, a fuel flow field plate and an
oxidant gas flow field plate are separately made and then assembled
together to form a bipolar plate (one side of a bipolar plate
serving as a negative terminal and the other side as a positive
terminal, hence the name). In some cases, an additional separator
is sandwiched between the two flow field plates to form a
three-layer bipolar plate. It would be highly advantageous if the
flow filed plates and the separator can be mass-produced into an
integrated bipolar plate assembly. This could significantly reduce
the overall fuel cell production costs and reduce contact ohmic
losses across constituent plate interfaces. The bipolar plate is
known to significantly impact the performance, durability, and cost
of a fuel cell system. The bipolar plate, which is typically
machined from graphite, is one of the most costly components in a
PEM fuel cell.
[0005] Fluid flow field plates or bipolar plates have open-faced
channels formed in one or both opposing major surfaces for
distributing reactants to the gas diffuser plates which are the
anode and cathode backing layers, typically made of carbon paper or
fabric. The open-faced channels also provide passages for the
removal of reaction products and depleted reactant streams.
Optionally, a bipolar plate may have coolant channels to manage the
fuel cell temperature. A flow field plate or bipolar plate should
have the following desirable characteristics: high electrical
conductivity (e.g., preferably having a conductivity no less than
100 S/cm), low permeability to fuel or oxidant fluids, good
corrosion resistance, and good structural integrity.
[0006] Conventional methods of fabricating fluid flow field plates
or bipolar plates require the engraving or milling of flow channels
into the surface of rigid plates formed of a metal, graphite, or
carbon-resin composite. These methods of fabrication place
significant restrictions on the minimum achievable fuel cell
thickness due to the machining process, plate permeability, and
required mechanical properties. Further, such plates are expensive
due to high machining costs. The machining of channels into the
graphite plate surfaces causes significant tool wear and requires
significant processing times.
[0007] Alternatively, fluid flow field plates can be made by a
lamination process (e.g., U.S. Pat. No. 5,300,370, issued Apr. 5,
1994 to Washington, et al.), wherein an electrically conductive,
fluid impermeable separator layer and an electrically conductive
stencil layer are consolidated to form one open-faced channel.
Presumably, two conductive stencil layers and one separator layer
may be laminated to form a three-layer bipolar plate. Such
laminated fluid flow field assemblies tend to have higher
manufacturing costs than integrated plates, due to the number of
manufacturing steps associated with forming and consolidating the
separate layers. They are also prone to delamination due to poor
interfacial adhesion and vastly different coefficients of thermal
expansion between a stencil layer (typically a metal) and a
separator layer.
[0008] A variety of composite bipolar plates have been developed,
which are mostly made by compression molding of polymer matrices
(thermoplastic or thermoset resins) filled with conductive
particles such as graphite powders or fibers. Because most polymers
have extremely low electronic conductivity, excessive conductive
fillers have to be incorporated, resulting in an extremely high
viscosity of the filled polymer melt or liquid resin and, hence,
making it very difficult to process. Bi-polar plates for use in PEM
fuel cells constructed of graphite powder/fiber filled resin
composite materials and having gas flow channels are reviewed by
Wilson, et al (U.S. Pat. No. 6,248,467, Jun. 19, 2001).
Injection-molded composite-based bipolar plates are disclosed by
Saito, et al. (U.S. Pat. No. 6,881,512, Apr. 19, 2005 and U.S. Pat.
No. 6,939,638, Sep. 6, 2005). These thermoplastic or thermoset
composites exhibit a bulk conductivity significantly lower than 100
S/cm (the US Department of Energy target value), typically not much
higher than 10 S/cm.
[0009] Besmann, et al. disclosed a carbon/carbon composite-based
bipolar plate (U.S. Pat. No. 6,171,720 (Jan. 9, 2001) and U.S. Pat.
No. 6,037,073 (Mar. 14, 2000)). The manufacture process consists of
multiple steps, including production of a carbon fiber/phenolic
resin preform via slurry molding, followed by a compression-molding
step. The molded part is then pyrolyzed at a high temperature
(1,500.degree. C.-2,500.degree. C.) to obtain a highly porous
carbon/carbon composite. This is followed by chemical vapor
infiltration (CVI) of a carbon matrix into this porous structure.
It is well-known that CVI is a very time-consuming and
energy-intensive process and the resulting carbon/carbon composite,
although exhibiting a high electrical conductivity, is very
expensive.
[0010] Instead of using pyrolyzation and CVI to produce
carbon/carbon composites, Huang, et al. (US Patent Application Pub.
No. 2004/0229993, Nov. 18, 2004) discloses a process to produce a
thermoplastic composite with a high graphite loading. First,
polymer fibers, such as thermotropic liquid crystalline polymers or
polyester, reinforcing fibers such as glass fibers, and graphite
particles are combined with water to form a slurry. The slurry is
pumped and deposited onto a sieve screen. The sieve screen serves
the function of separating the water from the mixture of polymer
fibers, glass fibers and graphite. The mixture forms a wet-lay
sheet which is placed in an oven. Upon heating to a temperature
sufficient to melt the polymer fibers, the wet-lay sheet is allowed
to cool and have the polymer material solidify. Upon
solidification, the wet-lay sheet takes the form of a sheet
material with reinforcement glass fibers held together by globules
of thermoplastic material, and graphite particles adhered to the
sheet material by the thermoplastic material. Several of these
sheets are then stacked, preferably with additional graphite powder
interspersed between sheets, and compression-molded in a hot press.
After application of heat and pressure in the press, one or more
formed bipolar plates are obtained, where the bipolar plates are a
composite of glass fibers, thermoplastic matrix and graphite
particles. There are several drawbacks associated with this
composite composition and method: [0011] (1) The fabrication
process is tedious, consisting of many manual operations, and is
not readily amenable to mass production. [0012] (2) The composition
requires heating the mixture above the melting point of the
thermoplastic material twice--(a) the first time being to melt out
the thermoplastic solid, allowing the melt to flow to the contact
points between reinforcement fibers so as to bond the fibers
together when the thermoplastic is cooled and (b) the second time
to melt the thermoplastic so as to wet the remaining reinforcement
fibers and graphite powders and form the matrix of a structural
composite plate when the thermoplastic solidifies. Since
engineering thermoplastics typically have a high melting point
(e.g., >220.degree. C. for polyester), it would take some time
to heat up to that temperature and take some time to cool it down.
The cycle times are long and the process is energy-intensive.
[0013] (3) With this process, it appears difficult to achieve a
graphite proportion above 50% (and, hence, conductivity above 100
S/cm) without interspersing additional graphite powder between
layers of stacked preform sheets (an operation called "dry-lay")
prior to compression-molding. This is evidenced by FIG. 2 of
Huang's application, which indicates that all samples with the
resulting conductivity greater than 100 S/cm were prepared by a
combined wet-lay (slurry molding) and dry-lay procedure. Such
labor-dependent operations make the whole process time-consuming
and labor-intensive. Dry-laid graphite powder between layers,
although imparting high electrical conductivity to the composite,
tend to form graphite-rich interfacial layers which are brittle and
weak and tend to compromise the mechanical integrity of the
resulting composite laminate.
[0014] The flow field plate or bipolar plate should be constructed
from inexpensive starting materials, materials that are easily
formed into any plate configuration, preferably using a continuous
molding process, and materials that are corrosion resistant in low
temperature fuel cells and that do not require further processing
such as high temperature pyrolyzation treatments. Any laminated or
multi-layer plate should have adequate bonding between layers to
ensure structural integrity and reduced contact resistance (reduced
power loss due to joule heating).
[0015] Accordingly, a primary object of the present invention is to
provide a highly conductive composite composition and a fuel cell
flow field plate or bipolar plate from this composition that can be
made with a continuous process, which is suitable for mass
production. The resulting fuel cell component is highly conductive
and, hence, can be used as a current collector in a fuel cell with
reduced contact resistance.
[0016] Another object of the present invention is to provide a
highly conductive composite material for fuel cell bipolar plates
which can be made without involving high temperature
treatments.
[0017] Still another object of the present invention is to provide
a highly conductive composite material for fuel cell bipolar plates
which can be made without involving melting and cooling a
thermoplastic twice.
[0018] Another object of the present invention is to provide a
highly conductive composite material for fuel cell bipolar plates
which is based on a thermoset resin that can be molded with a fast
cycle.
[0019] Another object of the present invention is to provide a
process for continuously producing a highly conductive
composite-based flow field plate or bipolar plate.
SUMMARY OF THE INVENTION
[0020] This invention provides a fuel cell flow field plate or
bipolar plate having flow channels on faces of the plate,
comprising an electrically conductive polymer composite. In one
preferred embodiment, the composite is composed of (A) at least 50%
by weight of a conductive filler, comprising at least 5% by weight
reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes (this at least 5% is based on
the total weight f the composite); (B) thermoplastic at 1 to 49.9%
by weight; and (C) thermoset binder at 0.1 to 10% by weight;
wherein the sum of the conductive filler weight %, thermoplastic
weight % and thermoset binder weight % equals 100% and the bulk
electrical conductivity of the flow field or bipolar plate is at
least 100 S/cm and, preferably, at least 200 S/cm. The thermoset
binder resin has the advantage that it can be quickly cured so as
to hold the reinforcement elements together, typically without a
need to be heated to a high temperature and then cooled down
slowly. The resulting preform is very easy to handle during
subsequent molding operations. The thermoset resin is selected from
the group consisting of unsaturated polyester resins, vinyl esters,
epoxies, phenolic resins, polyimide resins, bismaleimide resins,
polyurethane resins, and combinations thereof. A fast-curing or
ultraviolet-curable resin is preferred.
[0021] The conductive filler comprises a conductive material
selected from the group consisting of graphite powder,
carbon/graphite fibers, metal fibers, carbon nano-tubes, graphitic
nano-fibers, expanded graphite platelets, carbon blacks, metal
particles, and combinations thereof. This filler may comprise some
non-conductive fibers, such as glass fibers and polymer fibers, for
the purpose of reinforcing or strengthening the composite without
significantly reducing the electrical conductivity. Preferably, the
thermoset binder is at 0.1 to 5% by weight and the thermoplastic is
at 10 to 40% by weight. This composition is such that reinforcement
fibers, carbon nano-tubes, graphitic nano-fibers, and/or expanded
graphite platelets (those reinforcement elements having a high
aspect ratio, such as a high length/thickness ratio or
length/diameter ratio) form an overlapping, contiguous-strand
backbone structure. Preferably, these high aspect-ratio elements
are bonded together by the thermoset resin binder, or a combination
of the thermoset binder and thermoplastic, to form a
shape-retaining backbone. This shape-retaining backbone or
"preform" makes it easily handleable for subsequent molding,
embossing and/or stamping operations to form a flow field or
bipolar plate.
[0022] In another preferred embodiment, the composite comprises an
electrically conductive polymer composite having: (A) at least 50%
by weight of a conductive filler, comprising at least 5% by weight
reinforcement fibers, expanded graphite platelets, graphitic
nano-fibers, and/or carbon nano-tubes; (B) a polymer matrix
material at 1 to 49.9% by weight; and (C) a polymer binder at 0.1
to 10% by weight; wherein the sum of the conductive filler weight
%, polymer matrix material weight % and polymer binder weight %
equals 100% and the bulk electrical conductivity of the flow field
plate or bipolar plate is at least 100 S/cm. In this case, the
polymer matrix material is not a pure thermoplastic; instead, it
may comprise a material selected from a thermoset resin, an
interpenetrating network, a semi-interpenetrating network, an
elastomer, or a combination thereof. The polymer binder can be
advantageously selected from thermoset resins, but it does not have
to be a thermoset resin. For instance, it can be a thermoplastic
provided that heating and melting the thermoplastic to a high
temperature (e.g., >200.degree. C.) is not required. It is
convenient to have a binder comprising a water soluble polymer.
Vaporization of water allows the polymer to precipitate and bond to
the reinforcement elements quickly. In one further preferred
embodiment, the plate has a major surface having a skin layer less
than 100 .mu.m in thickness and having a polymer volume fraction
less than 20%, preferably less than 10%. In other words, the skin
layer is preferably composed of at least 80% conductive filler and
more preferably at least 90% conductive filler. Such a skin layer
prevents the formation of a resin-rich skin layer that otherwise
has a high, dominating electrical resistance.
[0023] Still another preferred embodiment of the present invention
is a process for producing a fuel cell flow field plate or bipolar
plate that has the aforementioned characteristics. The process
comprises (A) continuously or intermittently feeding and moving a
sheet of porous substrate (e.g., a web), preferably from a drum or
roller, toward a desired direction with the substrate having
through-thickness pores; (B) mixing and feeding a conductive
filler, a polymer binder (preferably a fast-curing thermosetting
resin or fast-solidifying thermoplastic), a polymer matrix material
(thermosetting, thermoplastic, elastomer, interpenetrating network,
semi-interpenetrating network, etc.) and a carrier fluid (water or
compressed air) onto the porous substrate and directing the carrier
fluid to substantially flow through the pores, leaving behind a
layer of a solid mixture of the filler, binder and matrix material
on the substrate; (C) moving the substrate forward to allow the
solid mixture layer to go through a compaction stage (e.g., between
a pair of compaction rollers); and (D) heating and consolidating
the solid mixture and generating flow channels on a surface (or two
surfaces) of the solid mixture layer to form the desired flow field
or bipolar plate. The step of heating and consolidating preferably
comprises a step of embossing or matched-die molding the mixture
layer.
BRIEF DESCRIPTION OF THE DRAWING
[0024] FIG. 1: A sectional view of a prior art PEM fuel cell
consisting of a membrane electrode assembly (MEA) sandwiched
between two flow field plates 21, 23.
[0025] FIG. 2: A sectional view of a fuel cell stack consisting of
two fuel cell units connected in series through a bipolar plate
19.
[0026] FIG. 3: A sectional view of (a) a bipolar plate 71 having
fluid flow channels 79, 83 formed on its two opposite surfaces; (b)
a flow field plate 71a having flow channels 79a formed on one of
its major surfaces.
[0027] FIG. 4: (a) Schematic of a slurry molding- or directed
fiber/binder spray-based process for producing a preform to a flow
field plate; (b) the resulting flow field plate 41 having coolant
channels 38, 38x; (c) schematic of a continuous process for
producing highly conducting flow field plates or bipolar plates;
(d) another version of a continuous process for producing highly
conducting flow field plates or bipolar plates; and (e) the
preform, with all ingredients held in place by a binder resin, may
be collected on a roller and molded later.
[0028] FIG. 5: (a) Schematic of a preform comprising reinforcement
elements (e.g., fibers) preferably forming a backbone of contiguous
strands; (b) reinforcements elements 33a, 33b, 33c are bonded by
resin binder 35a, 35b.
[0029] FIG. 6: (a) Schematic of two matting flow field plates each
with half of the coolant channels; (b) the two plates, after being
molded with the thermoset resin cured, are combined to form a
bi-polar plate with coolant channels.
[0030] FIG. 7: (a) Schematic of two matting preform sheets being
stacked and molded in a matched-die pressing process with molding
pins being inserted to produce coolant channels; (b) the resulting
integral bipolar plate with built-in coolant channels.
[0031] FIG. 8: A sectional view of stacked fuel cells using a
series of bipolar plates in accordance with the present
invention.
[0032] FIG. 9: Results of theoretical calculations on the effect of
the presence of a skin layer on the conductivity of a
filler-polymer composite, (a) the effect of skin layer resistivity
and thickness; (b) the effect of skin layer resistivity, thickness
and core layer conductivity.
[0033] FIG. 10: The effect of conductive filler type and proportion
on the flexural strength of the phenolic matrix composite
material.
DETAILED DESCRIPTION OF THE INVENTION
[0034] As shown in FIG. 1 and FIG. 2, a fuel cell typically
comprises a pair of fluid distribution plates (also referred to as
fluid flow field plates) 21 and 23, which are positioned on
opposite sides of a membrane electrode assembly 8. Plate 21, which
serves as a fuel distribution plate, is shaped to define fuel flow
channels 22 facing towards anode diffuser 10. Channels 22 are
designed to uniformly deliver the fuel to the diffuser, which
transports the fuel to the anode catalyst layer 16. An input port
and an output port (not shown), being in fluid communication with
channels 22, may also be provided in flow field plate 21 so that
carbon dioxide (in a DMFC) can be withdrawn from channels 22.
[0035] Flow field plate 23 is shaped to include fluid channels 24
for passage of a quantity of gaseous oxygen (or air). An input port
and an output port (not shown) are provided in plate 23, which are
in fluid communication with channels 24 so that oxygen (or air) can
be transported through the input port to the cathode diffuser 12
and cathode catalyst layer 18, and water and excess oxygen (or air)
can be withdrawn from channels 24 through the output port. Plate 23
is electrically conductive and in electrical contact with cathode
diffuser 12. It can be used as a uni-polar plate (the positive
terminal of the electrical current generated by the fuel cell unit)
or as a part of a bi-polar plate (if integrated with fuel flow
field plate 21). Shown in FIG. 2 is a fuel cell stack that consists
of two fuel cell units. On the two opposite sides of the stack are
two separate flow field plates 21a, 23a. Between the two MEAs (8a
and 8b) is a bipolar plate 19, which can be viewed as two flow
field plates integrated into one single component.
[0036] The preparation of a bipolar plate (71 in FIG. 3(a)
containing surface flow field channels 79, 83 on two opposite major
surfaces) or a flow field plate (71a in FIG. 3(b) having flow field
channels 79a on only one major surface) begins with the fabrication
of a porous preform from a conductive filler. This filler
preferably comprises at least 5% by weight reinforcement fibers
(glass fibers, polymer fibers or, preferably carbon/graphite
fibers), expanded graphite platelets, graphitic nano-fibers (GNFs),
and/or carbon nano-tubes (CNTs). These reinforcement elements have
very high aspect ratios (length-to-thickness or length-to-diameter
ratios) and, hence, are more amenable to the formation of a
backbone structure. This backbone structure is typically
characterized by having the elements forming a network of
contiguous strands bonded by a binder polymer. This backbone
structure, possibly with other conductive ingredients, makes a
"preform" which is sufficiently rigid to enable subsequent molding
operations. The conductive filler may comprise a conductive
material selected from the group consisting of graphite powder,
carbon/graphite fibers, metal fibers, carbon nano-tubes, graphitic
nano-fibers, expanded graphite platelets, carbon blacks, metal
particles, and combinations thereof. Some of these elements (e.g.,
powder or metal particles) have a lower aspect ratio, but they
could impart good electrical conductivity to the resulting
composite.
[0037] The elements of a preform (such as reinforcement fibers,
expanded graphite platelets, graphitic nano-fibers, and/or carbon
nano-tubes) will be bonded by a thermoset binder at 0.1 to 10% by
weight. The thermoset resin binder is selected from the group
consisting of unsaturated polyester resins, vinyl esters, epoxies,
phenolic resins, polyimide resins, bismaleimide resins,
polyurethane resins, and combinations thereof. A selected amount of
thermoplastic, at 1 to 49.9% by weight; preferably in the form of
short fibers or filaments (e.g., nylon fiber, polyester fiber, or
polypropylene fiber) may also be incorporated in the preform during
the preform fabrication process. Any thermoplastic, preferably
those that can be easily made into fibrous form, can be used as
part of the present composition. This thermoplastic material will
be melted during the subsequent molding operation and fill the
interstices between reinforcement elements. Upon solidification,
the thermoplastic becomes the matrix material of the resulting
conductive composite.
[0038] Several fabrication techniques can be employed to fabricate
a conductive preform--a monolithic body having a desired porosity.
In one preferred embodiment of the present invention, the porous
preform material is made to an appropriate shape by a conventional
slurry molding technique using chopped or milled carbon fibers of
various lengths. In another preferred embodiment, the porous
preform can be made by using a fiber/binder spraying technique. In
yet another preferred embodiment, the preform may be made by adding
fine-scale conductive fillers (such as nano-scaled graphene plates
(NGPs), sub-micron graphite powder particles, graphitic nano fibers
(GNFs), carbon blacks, metal nano particles, and carbon nano-tubes
(CNTs)) to a pre-made fiber mat. These methods can be carried out
as follows:
A. Slurry Molding Route:
[0039] An aqueous slurry is prepared which comprises a mixture of
carbon fibers having lengths typically in the range of about 0.1 mm
to about 10 mm and about 0.1 wt % to about 10 wt % thermoset resin
powder binder (e.g., phenolic resin). In addition to carbon fibers,
other conductive ingredients such as metal fibers, CNTs, GNFs,
NGPs, expanded graphite plates, carbon blacks, metal particles, or
a combination thereof can be a part of the slurry. A desired
proportion of a thermoplastic (in powder, granule, or, preferably,
fibrous or filamentous form) is also added to the slurry. The
slurry is forced through a sieve or mold screen of a desired mesh
size to trap the solids, thus producing a wet monolithic, which is
subsequently dried at a temperature of less than 80.degree. C. This
mold screen is a part of a mold 37 (FIG. 4(a)) which, along with
optional molding pins (e.g., 39z in the Z-direction and 39x in the
X-direction as defined in FIG. 4(a)), helps define the fuel or
oxidant transport and distribution channels 35 and optional coolant
channels (e.g., 38, 38x in the resulting preform 41, FIG. 4(b)).
Alternatively, these channels can be produced at a later stage
during the subsequent composite molding.
[0040] The initial porosity of the preform, in the slurry molded
and dried condition, is typically in the range 50-90%. If
necessary, the dried monolith preform is further densified. The
phenolic resin binder is cured in a shaped steel mold at a
temperature in the range of about 120.degree. C. to about
160.degree. C., preferably about 130.degree. C. (sufficient to cure
the thermoset binder, but not high enough to melt the
thermoplastic). Other alternative types of binder material (such as
fast curing epoxy resins and ultraviolet curable resins) may be
used, which serve to provide rigidity or some integrity to the
resulting preform (FIG. 5(a)) prior to thermoplastic matrix
material consolidation.
[0041] In the above example, only about 0.1 wt % to about 10 wt %
binder resin (more typically about 0.5 wt % to 3 wt. %) was
typically used for the primary purpose of providing a desired level
of rigidity to the fiber preform, prior to the next step of
thermoplastic matrix consolidation. The reinforcement elements 33a,
33b, 33c (fibers, plates, etc.) are bonded together by the
thermoset binder 35a, 35b, as illustrated in FIG. 5(b). These
reinforcement elements preferably form an overlapping,
contiguous-strand backbone structure, as illustrated in FIG. 5(a),
which is a preferred form of the "preform". The backbone structure
may comprise glass fibers, high-strength polymer fibers (e.g.,
aromatic polyamide and ultra high molecular weight polyethylene),
ceramic fibers and the like for the sole purpose of providing
structural integrity to the preform. However, they are not
electrically conductive materials.
B. Fiber/Binder Spraying Route:
[0042] The directed fiber spray-up process utilizes an air-assisted
chopper/binder guns (or fiber/binder spraying guns) which convey
carbon fibers (and/or other reinforcement elements) and a binder to
a molding tool (e.g., a perforated metal screen shaped identical or
similar to the part to be molded). In addition to carbon fibers,
other conductive ingredient such as metal fibers, carbon
nano-tubes, graphitic nano-fibers, nano-scaled graphene plates,
expanded graphite plates, carbon blacks, or a combination thereof
(plus thermoplastic fibers or granules) can be a part of the
air-driven stream of preform ingredients that impinges upon the
metal screen. This shaped screen is a part of a mold 37 (FIG.
4(a)), which also contains molding pins (e.g., 39z in the
Z-direction and 39x in the X-direction as defined in FIG. 4(a)).
These pins will help define the fuel or oxidant
transport/distribution channels 35 and optional coolant channels
(e.g., 38, 38x in the resulting preform 41, FIG. 4(b)). The chopped
fibers may be held in place on the screen by a large blower drawing
air through the screen. Once the desired thickness of reinforcement
has been achieved, the chopping system is turned off and the
preform is formed by polymerizing or curing the binder. The binder
resin does not have to be added to the mixture during the
fiber/filler blowing step; instead, it can be blown into the
preform once all the reinforcement elements are in place. The
binder can be an ultraviolet-curable resin or other fast-curing
resins. Once stabilized, the preform is cooled and removed from the
screen. It may be noted that the coolant channels or fluid flow
field channels can be built in the bipolar plate or flow field
plate at a later stage using matched-die molding, for instance. The
surface flow channels may also be created by embossing.
C. Fiber Mat Route:
[0043] This route may begin with provision of a highly porous fiber
mat, which is basically composed of carbon and/or glass fibers
bonded at their points of contact by a binder. The mat has
interconnected interstices or voids between fibers. Slurry molding-
or directed fiber blowing-type procedure is then used to add
thermoplastic fibers/powders and fine-scaled (preferably
nano-scaled) conductive elements such as CNTs, NGPs, GNFs, graphite
powders, metal nano particles and carbon black) into the voids of
the fiber mat to produce a preform. Glass or carbon fiber mats are
commercially available. They typically contain some pre-applied
binder resin to impart rigidity and strength to the mat.
[0044] Each one of the above three routes can be implemented as a
continuous process. For instance, as shown in FIG. 4(c), the
process begins with pulling a web 86 (porous sheet) from a roller
84. The moving web receives a stream of slurry 88 (as described in
the above-described Slurry Molding Route) from above the web. Water
sieves through the web with all other ingredients (a mixture of
fillers, binder, thermoplastic fibers, etc.) remaining on the
surface of the web being moved forward to go through a compaction
stage by a pair of compaction rollers 90a, 90b. Heat may be
supplied to the mixture before, during, and after compaction to
help cure the thermoset binder for retaining the shape of the
resulting preform. The preform then goes through embossing or
matched-die molding by a pair of embossing rollers or matting mold
platens 92a, 92b to create flow channels on one or both major
surfaces of the preform, which is also consolidated to become a
flow field plate or bipolar plate. Preferably, the moving web 86a
is separated from the preform 91 and collected by a winding roller
84a. It is also preferred that the mold platens 92a, 92b are
replaced by a pair of embossing rollers (not shown). Alternatively,
the compaction rollers 90a, 90b may also serve as embossing rollers
for creating flow channels. In this latter case, the mold platens
92a, 92b will not be needed. A cutting device may be installed to
separate individual plates in-line.
[0045] Similar procedures may be followed for the case where the
mixture 88 of fillers, binder and thermoplastic is delivered to the
surface of a moving web 86 by compressed air, like in a directed
fiber/binder spraying route described above (FIG. 4(c)). Air will
permeate through the web with other solid ingredients trapped on
the surface of the web, which are conveyed forward. The subsequent
operations are similar than those involved in the slurry molding
route.
[0046] Alternatively, as indicated in FIG. 4(d), a continuous fiber
mat 87 may be pulled from a roller 84. A mixture 89 of ultra-fine
fillers and thermoplastic powders is then delivered, either through
slurry pouring or directed spraying, to enter the macro pores of
the fiber mat structure. Compaction rollers 90a, 90b also help to
work the mixture into the pores of the mat. The resulting preform
is then embossed/molded into flow field plates or bipolar plates.
Embossing or molding may involve heating the preform to melt out
the thermoplastic resin and cure the thermoset resin binder, and
then cooling the structure down to room temperature.
[0047] Alternatively, as schematically shown in FIG. 4(e), the
preform 91, with all ingredients held in place by the thermoset
binder, may be stored first (e.g., wrapped around a roller 93). At
a later time, the preform may then be cut and fit into a mold (if
so desired, a plurality of layers of cut preform may be stacked
together) for consolidation of the plate and formation of surface
flow channels and, possibly, coolant channels.
[0048] If coolant channels are needed, they can be created during
the molding process in several ways. For instance, during the flow
field plate molding process, the mold surface may be shaped to
produce a part of a channel groove (e.g., 52a in FIG. 6(a)). Two
matting flow field plates may then be positioned together to form a
bipolar plate 54 (FIG. 6(b)) having complete coolant channels
(e.g., 52).
[0049] Preferably, coolant channels are built into a bipolar plate
when it is molded. For instance, as schematically shown in FIG.
7(a), two composite preform sheets 63a or 63b may be molded between
a pair of matched molds (61a, 61b) and a number of molding pins 67.
Upon completion of the molding procedure, these pins, pre-coated
with a mold release agent, may be pulled out of the composite
structure to obtain an integral bipolar plate 54 (FIG. 7(b)) with
built-in coolant channels 67a. Optionally, coolant channels may be
fitted with connectors, preferably before the resin matrix material
is solidified. FIG. 8 shows back-to-back flow field plates that are
fabricated as one monolithic component 54, with coolant channels 52
formed as complete channels within the component, as well as
reactant channels 60 & 62. The two outer surfaces of bipolar
plate 54 are stacked against respective diffuser layers 56, 58
(preferably made of carbon paper), which are in turn connected to
catalyst-coated membrane (e.g., 70) to complete a fuel cell
stack.
[0050] The type and proportion of the conductive filler are
preferably chosen in such a way that the bulk conductivity of the
resulting resin mixture is greater than 100 S/cm and further
preferably greater than 200 S/cm. The US Department of Energy
conductivity target for composite bipolar plates is 100 S/cm.
[0051] As indicated earlier, the conducting filler material may be
selected from carbon fibers, metal fibers, metal particles
(preferably nano-scaled), carbon nano-tubes (CNTs), graphitic
nano-fibers (GNFs), nano-scaled graphene plates (NGPs), carbon
blacks, or a combination thereof. Individual nano-scaled graphite
planes (individual graphene sheets) and stacks of multiple
nano-scaled graphene sheets are collectively called nano-sized
graphene plates (NGPs). The structures of these materials may be
best visualized by making a longitudinal scission on the
single-wall or multi-wall of a nano-tube along its tube axis
direction and then flattening up the resulting sheet or plate.
These nano materials have strength, stiffness, and electrical
conductivity that are comparable to those of carbon nano-tubes, but
NGPs can be mass-produced at lower costs. They can be produced by
reducing the expanded graphite particles to much smaller sizes (100
nanometers or smaller). The preparation of other nano-scaled
carbon-based materials, including CNTs, GNFs, and carbon black, is
well-known in the art. They are all commercially available, along
with nano-scaled metal particles.
[0052] It may be noted that the matrix material does not have to be
a thermoplastic and the binder resin does not have to be a
thermoset. The matrix material can be a thermoset (including an
interpenetrating network), a thermoplastic, a thermoplastic
elastomer, a combined thermoset/thermoplastic (e.g., a
semi-interpenetrating network), a rigid rubber or elastomer. A
thermoset resin matrix can be advantageous since a molded part can
be separated from a mold as soon as curing is achieved to a desired
extent; no cooling is required. By contrast, cooling is required of
a thermoplastic matrix composite after hot molding. Thermoplastic
melts are also of higher viscosity and more difficult to
process.
[0053] Hence, another preferred embodiment of the present invention
is a fuel cell flow field plate or bipolar plate having flow
channels on faces of the plate, comprising an electrically
conductive polymer composite having: (A) at least 50% by weight of
a conductive filler, comprising at least 5% by weight reinforcement
fibers, expanded graphite platelets, graphitic nano-fibers, and/or
carbon nano-tubes; (B) a polymer matrix material (not a pure
thermoplastic) at 1 to 49.9% by weight; and (C) a polymer binder at
0.1 to 10% by weight; wherein the sum of the conductive filler
weight %, polymer matrix material weight % and polymer binder
weight % equals 100% and the bulk electrical conductivity of the
flow field plate or bipolar plate is at least 100 S/cm, typically
or preferably greater than 200 S/cm.
[0054] The binder resin serves a primary function of rapidly
bonding together the reinforcement fibers and other conductive
elements to produce a preform that is rigid enough for easy
handling. With that purpose in mind, other types of polymer can be
used as a binder. For instance, a water-soluble polymer like poly
(vinyl alcohol) and polyethylene oxide can be dissolved in water or
a mixture of water and ethanol. The resulting solution can be used
as a dispersing medium for the slurry in the process of slurry
molding of the preform. In the case of compressed air-assisted
directed fiber spraying process, a dilute polymer-water solution
may be blown through the stacked fibers/fillers (preform) with
water and alcohol being quickly removed with heat. The remaining
polymer residues will serve to bond together the reinforcement
elements.
[0055] In one preferred embodiment, the polymer matrix material
comprises a material selected from a thermoset resin, an
interpenetrating network, a semi-interpenetrating network, an
elastomer, or a combination thereof. This matrix material is
preferably added to the preform in a solid powder form. Even a
thermosetting resin with a proper degree of curing can be made into
a powder form (e.g., epoxy, phenolic, and polyimide resins). These
powders may be incorporated to a preform using any of the above
three routes: slurry molding (wet-lay), directed fiber/binder
spraying, and fiber mat. Again, conductive filler may comprise a
conductive material selected from the group consisting of graphite
powder, carbon/graphite fibers, metal fibers, carbon nano-tubes,
graphitic nano-fibers, nano-scaled graphene plates, carbon blacks,
metal particles, and combinations thereof. Preferably, the
conductive filler is at 50 to 75% by weight, which gives a good
balance of electrical conductivity and mechanical properties
(strength, stiffness, and flexibility). The preform may be composed
of reinforcement fibers, carbon nano-tubes, graphitic nano-fibers,
and/or expanded graphite platelets that form an overlapping,
contiguous-strand backbone structure or a mat. These reinforcement
elements are bonded together by the polymer binder material at
their points of contact. The final consolidation process involves
heating to cure the resin and impressing the flow channels and
other surface features to the composite for forming the final
plates.
[0056] The present invention also provides a fuel cell or a stack
of fuel cells that comprises a highly conductive flow field plate
or bipolar plate component as defined in any of the aforementioned
preferred embodiments. The resulting fuel cell system is of lower
costs (due to their amenability to mass production) and better
performance (due to lower contact resistance and internal
resistance and, hence, higher voltage).
[0057] Conductivity measurements can be made by using the
four-point probe method on small-sized plate samples. Table 1
summarizes the parameters and properties of the samples prepared in
the present study. These examples have clearly demonstrated the
superior electrical conductivity of the presently invented
composite compositions and the composite-based flow field plate or
bipolar plate products. These conductivity values are far superior
to those of most of prior art bipolar plates. TABLE-US-00001 TABLE
1 Composition and properties of highly conductive polymer
composites (Gr = graphite powder, GNF = graphitic nano-fibers, NGP
= nano graphene plate, EG = expanded graphite platelets). Flexural
Preform Conductive .sigma. strength, Example Matrix Binder Backbone
Fillers (S/cm) MPa Thermoset 1 48% Phenolic 2% UV 10% carbon 40% Gr
75 32.5 curable epoxy fiber powder 2 38% Phenolic 2% UV 10% carbon
45% Gr + 5% 184 35.4 curable epoxy fiber NGP 3 33% Phenolic 2% UV
10% carbon 50% Gr + 5% 221 35.2 curable epoxy fiber NGP 4 28%
Phenolic 2% UV 10% carbon 50% Gr + 10% 250 38.7 curable epoxy fiber
NGP 5 23% Phenolic 2% UV 10% carbon 55% Gr + 10% 277 37.5 curable
epoxy fiber NGP Thermoplastic 6 48% Nylon 2% UV 8% carbon 40% Gr 76
62 6/6 curable epoxy fiber + 2% powder GNF 7 38% Nylon 2% UV 8%
carbon 50% Gr 186 54 6/6 curable epoxy fiber + 2% powder GNF 8 33%
Nylon 2% phenolic 10% carbon 55% Gr 228 53 6/6 fiber powder 9 28%
Nylon 2% phenolic 10% carbon 60% Gr 256 6/6 fiber powder 10 23%
Nylon 2% phenolic 10% carbon 55% Gr + 10% 280 6/6 fiber EP 11 23%
Nylon 2% phenolic 10% glass 65% Gr 203 6/6 fiber powder 8a 35%
Nylon 2% phenolic 10% carbon 53% Gr 107 No Gr or 6/6 fiber powder
EG skin 9a 30% Nylon 2% phenolic 10% carbon 58% Gr 121 No Gr or 6/6
fiber EG skin 10a 25% Nylon 2% phenolic 10% carbon 55% Gr + 8% 132
No Gr or 6/6 fiber EP EG skin
[0058] It may be noted that Samples 1-11 were prepared in such a
way that a thin layer of fine powder of graphite, expanded graphite
platelets, or nano-scale graphene plates were sprayed between the
preform surface and the surface of a mold, for both major surfaces
of a preform, prior to heating and consolidating. This is easily
achieved by spraying a thin layer of fine graphite powder, NGPs, or
expanded graphite platelets on the surface of the molds, instead of
spraying a mold releasing agent. Since a mold releasing agent is
normally needed in all polymer and composite molding operations
anyway, this does not add an extra step to the process.
Graphite-type materials are surprisingly great mold-releasing
agents. This step effectively created a thin, graphite-rich,
substantially polymer-free skin layer that is highly conductive.
(This layer also makes it easy to remove the molded plate from the
mold surface.) By contrast, those samples (e.g., Examples 8a, 9a,
and 10a) prepared without such a step, tend to form a polymer-rich
skin layer that could significantly increase the total resistance
of a plate. As compared to Samples 8-10, the measured conductivity
values of corresponding Samples 8a-10a are significantly lower.
This is a highly surprising, yet very important observation because
the presence of the two polymer-rich skin layers of a bipolar plate
could significantly increase the contact resistance and joule loss
of the whole fuel cell stack.
[0059] This dramatic reduction in electrical conductivity of a
molded composite plate without a graphite coating skin may be
understood as follows: A molded composite plate may be viewed as a
three-layer structure with the skin, core and skin layers
electrically connected in series. The total resistance is the sum
of the resistance values of the three layers:
R=R.sub.1+R.sub.2+R.sub.3=.rho..sub.1(t.sub.1/A.sub.1)+.rho..sub.2(t.sub.-
2/A.sub.2)+.rho..sub.3(t.sub.3/A.sub.3)=(1/.sigma..sub.1)(t.sub.1/A.sub.1)
+(1/.sigma..sub.2)(t.sub.2/A.sub.2)+(1/.sigma..sub.3)(t.sub.3/A.sub.3),
where .rho.=resistivity, .sigma.=conductivity, t=thickness, and
A=area of a layer, and, approximately, A.sub.1=A.sub.2=A.sub.3.
Scanning electron microscopic examinations reveal that the
resin-rich skin layers in composite materials are typically 0.1-10
.mu.m thick. The resistivity of Nylon 6/6 (a thermoplastic) is
typically in the range of 10.sup.12-10.sup.15 ohm-cm and that of
epoxy is slightly higher, 10.sup.15 or greater. In contrast, the
resistivity of graphite powder or expanded graphite platelets is
typically in the range of 10.sup.-5-10.sup.-3 ohm-cm. A skin layer,
even as thin as 1 nm-0.1 .mu.m could completely dominate the
over-all resistivity of the composite bipolar plate. TABLE-US-00002
TABLE 2 Composite conductivity as a function of skin layer
resistivity (skin thickness = 1 nm). Top layer Top layer Core layer
Core layer Bipolar plate Bipolar plate Bipolar plate resistivity
thickness resistivity thickness Active area resistance resistivity
conductivity .rho..sub.1 (.OMEGA.-cm) t.sub.1(cm) .rho..sub.2
(.OMEGA.-cm) t.sub.2(cm) A (cm.sup.2) R (.OMEGA.) .rho.
(.OMEGA.-cm) .sigma. (S/cm) 1.00E+12 1.00E-07 0.01 1 100 2000.0001
199999.97 5E-06 1.00E+10 1.00E-07 0.01 1 100 20.0001 2000.0096
0.000499998 1.00E+08 1.00E-07 0.01 1 100 0.2001 20.009996
0.049975022 1.00E+06 1.00E-07 0.01 1 100 0.0021 0.20999996
4.761905714 1.00E+04 1.00E-07 0.01 1 100 0.00012 0.012 83.33335
1.00E+02 1.00E-07 0.01 1 100 0.0001002 0.01002 99.80041916 1.00E+00
1.00E-07 0.01 1 100 0.0001 0.0100002 99.99802004
[0060] This is further illustrated in Table 2, FIG. 9(a) and FIG.
9(b), which are results of some simple calculations. As
demonstrated in Table 2 and FIG. 9(a), with a core layer of 1 cm in
thickness and 100 S/cm in conductivity and with a skin layer
resistivity of 10.sup.12 ohm-cm (assuming a pure resin skin), the
over-all conductivity of the composite is only 5.times.10.sup.-6
S/cm even with a skin layer thickness as small as 1 nm. By
decreasing the skin layer resistivity to 10.sup.6 ohm-cm and with
the same skin layer thickness of 1 nm, the composite conductivity
would still be relatively low (4.76 S/cm). By contrast, if the skin
layer is relatively polymer-free and graphite-rich, this layer
would have a relatively high conductivity (10.sup.2-10.sup.5 S/cm)
and the over-all composite conductivity will be dominated by the
core layer conductivity, which is illustrated in FIG. 9(b). The
over-all composite conductivity would be comparable to the
conductivity of the core layer. Hence, it is reasonable to say that
the measured conductivity values for Samples 1-11, as listed in
Table 1, are good assessment of not only the over-all, but also the
core layer conductivity.
[0061] It may be further noted that the composite bipolar plates
prepared in the present study (without a sprayed layer of graphite
powder; e.g., Samples 8a-10a) were usually found to have a
resin-rich layer which is deficient in, but not totally free from,
conductive elements. Some conductive elements were found to
sporadically protrude out of the plate surface. Hence, the
effective conductivity of this skin layer is expected to be much
higher than that of a neat resin and estimated to be in the range
of 0.01-1 S/cm. The volume fraction of these protruding conductive
elements is typically less than 20% and more typically less than
10%. With a sprayed layer of fine graphite powder-, expanded
graphite powder-, or NGP-based mold releasing agent, a portion of
this agent (fine particles) are incorporated in the surface of the
composite during the composite molding or embossing procedure. The
resulting graphite-rich skin layer is estimated to have a
conductivity in the range of 100-1000 S/cm. In this case, the skin
layer typically has a thickness thinner than 200 .mu.m (more
typically thinner than 100 .mu.m) and a polymer volume fraction
less than 20% (more typically less than 10%). There is no need to
interleaf graphite powders between preform sheets (which could
complicate the production process and compromise the composite
strength). There is also no need to intentionally produce a
laminated sandwich structure with a core layer of one thermoplastic
composite (e.g., polyethylene terephthalate-based) to provide
mechanical strength and extra top and bottom layers of a different
thermoplastic composite (e.g., poly vinylidene fluoride-based) to
enhance electrical conductivity, as suggested by Huang, et al. (US
Patent Application Pub. No. 2004/0229993, Nov. 18, 2004). The
sandwich structure clearly would significantly increase the process
complexity and final product costs. In contrast, our inventive
technology was able to achieve the desired properties without
creating such a sandwich structure.
[0062] Another noteworthy feature is the surprising observation
that, within the conductive filler proportion range studied (e.g.,
Examples 1-5), an increase in the NGP percentage always leads to an
increase in the composite flexural strength and electrical
conductivity. However, this is not the case with graphite powder,
which tends to increase the electrical conductivity, but could
decrease the composite strength (FIG. 10). This indicates the
superiority of NGPs in terms of imparting both electrical
conductivity and mechanical strength to the composite material.
[0063] In all of the samples prepared in the present study, the
over-all conductivity values of the composite plates are very
impressive. The processes for preparing these composites are
continuous and can be automated. The processing costs are
relatively low.
[0064] Thus, in summary, another preferred embodiment of the
present invention is a process for producing a fuel cell flow field
plate or bipolar plate as described above. The process comprises
(A) continuously or intermittently feeding and moving a sheet of
porous substrate (e.g., a web) toward a desired direction with the
substrate having through-thickness pores; (B) mixing and feeding a
conductive filler, a polymer binder, a polymer matrix material
(thermosetting, thermoplastic, elastomer, interpenetrating network,
semi-interpenetrating network, etc.) and a carrier fluid (water or
compressed air) onto the porous substrate and directing the carrier
fluid to substantially flow through the pores, leaving behind a
layer of a solid mixture of the filler, binder and matrix material
on the substrate; (C) moving the substrate forward so as to allow
the solid mixture layer to go through a compaction stage (e.g.,
between a pair of compaction rollers); and (D) heating and
consolidating the solid mixture and generating flow channels or
other features on a surface of the mixture layer to form the
desired flow field or bipolar plate. The step of heating and
consolidating preferably comprises embossing or matched-die molding
the mixture layer.
[0065] Preferably, the process includes a step of coating an
embossing surface or mold surface with a layer of graphite or NGP
powder prior to embossing or molding. This layer of graphite-based
powder material, positioned between a mold or embossing roller
surface and a surface of the solid mixture layer, will be
incorporated as a graphite-rich skin layer on the plate surface
after melting and consolidation of the polymer matrix. If the
binder is a thermosetting material, the process should preferably
further comprise a step of curing the thermoset resin binder
before, during, and/or after the compaction stage. If the binder
material is a water-soluble polymer material, the process should
preferably further comprise a step of rapidly removing water from
the mixture, allowing the polymer to precipitate and bond the
reinforcement elements together.
* * * * *