U.S. patent application number 13/724505 was filed with the patent office on 2014-06-26 for unique pre-form design for two-step forming of stainless steel fuel cell bipolar plates.
This patent application is currently assigned to GM Global Technology Operations LLC. The applicant listed for this patent is GM GLOBAL TECHNOLOGY OPERATIONS LLC. Invention is credited to John R. Bradley, Gerald W. Fly, Arianna T. Morales, Steven J. Spencer, Siguang Xu.
Application Number | 20140178802 13/724505 |
Document ID | / |
Family ID | 50956334 |
Filed Date | 2014-06-26 |
United States Patent
Application |
20140178802 |
Kind Code |
A1 |
Xu; Siguang ; et
al. |
June 26, 2014 |
Unique Pre-Form Design For Two-Step Forming Of Stainless Steel Fuel
Cell Bipolar Plates
Abstract
A bipolar plate used in a fuel cell and a method of making a
bipolar plate. The sheet is made from a ferritic or austenitic
stainless steel, and defines an undulated surface pattern such that
the patterned sheet may be formed into the bipolar plate. In one
configuration, a stamping or related metal forming tool operation
will further deform the patterned sheet into the bipolar plate
shape such that the wall thickness is substantially uniform
throughout the surface. In this way, there is a substantial
reduction in stretching/thinning/necking at an intersection between
bends and side walls that define the undulations of the pattern. In
one form, the pattern defines a repeating serpentine shape. In a
particular embodiment, the bends may include surface modifications
to reduce the effects of sheet-to-tool misalignment.
Inventors: |
Xu; Siguang; (Rochester
Hills, MI) ; Spencer; Steven J.; (Rochester, NY)
; Bradley; John R.; (Clarkston, MI) ; Fly; Gerald
W.; (Geneseo, NY) ; Morales; Arianna T.;
(Royal Oak, MI) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
GM GLOBAL TECHNOLOGY OPERATIONS LLC |
Detroit |
MI |
US |
|
|
Assignee: |
GM Global Technology Operations
LLC
Detroit
MI
|
Family ID: |
50956334 |
Appl. No.: |
13/724505 |
Filed: |
December 21, 2012 |
Current U.S.
Class: |
429/535 |
Current CPC
Class: |
H01M 2008/1095 20130101;
Y02E 60/50 20130101; H01M 8/0247 20130101; H01M 8/1006 20130101;
H01M 8/021 20130101; Y02P 70/50 20151101; H01M 8/0263 20130101 |
Class at
Publication: |
429/535 |
International
Class: |
H01M 8/02 20060101
H01M008/02 |
Claims
1. A method of preparing a stainless steel bipolar plate for a fuel
cell, said method comprising: configuring a bipolar plate forming
apparatus to progressively form said bipolar plate, said
progressively forming comprising: using a first shaping step to
pre-form a sheet of stainless steel such that a portion of the
material making up said sheet is selectively moved, said pre-formed
sheet defining a nominal thickness and a plurality of undulations
representative of bipolar plate reactant channel flowpaths therein;
and using a second shaping step to change the shape of said
pre-formed sheet such that a maximum deviation from said nominal
thickness as a result of said first and second shaping steps is
substantially reduced relative to formation of a bipolar plate from
a single-step forming process; and providing said sheet to said
bipolar plate forming apparatus such that upon operation of said
bipolar plate forming apparatus on said sheet, the shape of said
sheet is substantially converted into that of said bipolar
plate.
2. The method of claim 1, wherein said stainless steel used in said
bipolar plate comprises an austenitic stainless steel.
3. The method of claim 1, wherein said stainless steel used in said
bipolar plate comprises a ferritic stainless steel.
4. The method of claim 1, wherein said selectively moved material
corresponds to discrete portions of said sheet that substantially
coincide with locations therein where at least one of a land and
said channel are subsequently formed.
5. The method of claim 4, wherein said discrete portions will, upon
formation by said first shaping step, define a substantially
V-shape.
6. The method of claim 5, wherein said substantially V-shape
defines a flattened region at the apex thereof.
7. The method of claim 4, wherein an angle defined at the apex of
said substantially V-shape is between about 114 degrees and 120
degrees.
8. The method of claim 3, wherein a depth of draw of said changing
the shape is at least about 350 microns.
9. The method of claim 8, wherein said pre-formed sheet defines a
pre-form draw depth sufficient to substantially match a final form
depth.
10. The method of claim 1, wherein said maximum deviation from said
nominal thickness is less than about twenty percent.
11. The method of claim 1, wherein said reduced maximum deviation
corresponds to an attendant decrease in thinning and necking in
regions of said sheet that correspond to said channel.
12. A method of preparing a bipolar plate for a fuel cell, said
method comprising: using a first shaping step to form a
substantially planar sheet of ferritic stainless steel into a
substantially non-planar serpentine pattern shape, wherein said
substantially planar sheet defines a nominal thickness
substantially throughout the portion of its surface that defines
said bipolar plate; and using a second shaping step to form said
serpentine pattern into a bipolar plate such that upon creation
thereof, it defines a substantially constant wall thickness with a
substantial reduction in deviation from said nominal thickness
relative to formation of a bipolar plate from a single-step forming
process.
13. The method of claim 12, wherein each of said first and second
shaping steps comprise stamping.
14. The method of claim 13, further comprising reducing the
likelihood of misalignment between said serpentine pattern and a
tool used to perform said stamping by providing a flattened region
around the apex of each of a plurality of substantially V-shaped
peaks that are formed in said serpentine pattern.
15. The method of claim 14, wherein said reducing the likelihood of
misalignment comprises self-adjusting said serpentine pattern prior
to said second shaping step by placement of a flattened bend region
therein into a bipolar plate forming apparatus with a lateral
misalignment of up to about 400 microns.
16. The method of claim 12, wherein said reduction in deviation
from said nominal thickness relative to formation of a bipolar
plate from a single-step forming process is at least about 50%.
17. The method of claim 12, wherein a depth of draw associated with
at least one of said serpentine pattern and said bipolar plate is
between about 300 microns and about 400 microns.
18. A method of preparing a fuel cell, said method comprising:
arranging a membrane electrode assembly to comprise an anode, a
cathode and a membrane disposed between said anode and cathode such
that a respective anode reactant and cathode reactant may be placed
in fluid communication therewith; and placing a stainless steel
bipolar plate that defines a plurality of flow channels therein in
adjacent each of said anode and said cathode such that upon
operation of said fuel cell, reactants introduced from a fuel
source and an oxygen source respectively can be delivered to said
anode and said cathode through said flow channels, said bipolar
plates formed by: shaping a pre-formed sheet of stainless steel
such that it defines a nominal thickness and a plurality of
reactant channel flowpaths therein; and changing the shape of said
pre-formed sheet such that a maximum deviation from said nominal
thickness is substantially reduced relative to formation of a
bipolar plate from a single-step forming process.
19. The method of claim 18, wherein said shaping said pre-formed
sheet and said changing the shape of said pre-formed sheet is
performed by a bipolar plate forming apparatus.
20. The method of claim 19, wherein said substantial reduction in
said maximum deviation is at least about 50%.
21. The method of claim 19, further comprising reducing the
likelihood of misalignment of said pre-formed sheet and said
bipolar plate forming apparatus by providing a flattened region
about an apex formed in each of a plurality of substantially
V-shaped peaks that define a serpentine pattern in said pre-formed
sheet.
22. The method of claim 18, wherein said stainless steel used in
said bipolar plate comprises a ferritic stainless steel.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates generally to a stainless steel
alloy bipolar plate for use in a fuel cell environment that
exhibits ease of manufacturability, and more particularly to such a
bipolar plate that is easy and inexpensive to manufacture while
preserving the best mechanical/structural properties possible.
[0002] In many fuel cell systems, hydrogen or a hydrogen-rich gas
is supplied through a flowpath to the anode side of a fuel cell
while oxygen (such as in the form of atmospheric oxygen) is
supplied through a separate flowpath to the cathode side of the
fuel cell. An appropriate catalyst (for example, platinum) is
typically disposed to form on these respective sides an anode to
facilitate hydrogen oxidation and as a cathode to facilitate oxygen
reduction. From this, electric current is produced with high
temperature water vapor as a reaction byproduct. In one form of
fuel cell, called the proton exchange membrane or polymer
electrolyte membrane (in either event, PEM) fuel cell, an
electrolyte in the form of an ionomer membrane is situated between
the anode and cathode to form a membrane electrode assembly (MEA)
which is further layered between diffusion layers that allow both
gaseous reactant flow to and electric current flow from the MEA.
The aforementioned catalyst layer may be disposed on or as part of
the diffusion layer or the membrane.
[0003] To increase electrical output, individual fuel cell units
are stacked with bipolar plates disposed between the diffusion
layer and anode electrode of one MEA and the diffusion layer and
cathode electrode of an adjacent MEA. Typically, the bipolar plates
are made from an electrically-conductive material in order to form
an electrical pathway between the MEA and an external electric
circuit. In such a stacked configuration, the bipolar plates
separating adjacently-stacked MEAs have opposing surfaces each of
which include flow channels separated from one another by raised
lands. The channels act as conduit to convey hydrogen and oxygen
reactant streams to the respective anode and cathode of the MEA,
while the lands, by virtue of their contact with the electrically
conductive diffusion layer that is in turn in electrical
communication with current produced at the catalyst sites, act as a
transmission path for the electricity generated in the MEA. In this
way, current is passed through the bipolar plate and the
electrically-conductive diffusion layer.
[0004] Because the bipolar plate operates in a high temperature and
corrosive environment, conventional metals, such as plain carbon
steel, may not be suitable for certain applications (such as in
automotive environments) where long life (for example, about 10
years with 6000 hours of life) is required. During typical PEM fuel
cell stack operation, the proton exchange membranes are at a
temperature in the range of between about 75.degree. C. and about
175.degree. C., and at a pressure in the range of between about 100
kPa and 200 kPa absolute. Under such conditions, plates made from
alloyed metals such as stainless steel may be advantageous, as they
have desirable corrosion-resistant properties. In situations where
cost of fuel cell manufacture is an important consideration,
metal-based bipolar plates may be preferable to other
high-temperature, electrically conductive materials, such as
graphite. In addition to being relatively inexpensive, stainless
steel plates can be formed into relatively thin members (for
example, between 0.1 and 1.0 millimeters in thickness).
[0005] Of the various types of stainless steels, those with
ferritic microstructures, which typically have a high chromium
content and are typically devoid of nickel, exhibit body-centered
cubic (BCC) crystal structure and tend to have the desirable
attributes of being relatively low cost and high in corrosion
resistance (the latter due to chromium oxide barrier formation).
Nevertheless, their hardening curves are such that they are more
susceptible to necking, stretching, thinning and consequent
cracking when exposed to conventional stamping or related one-step
metal-forming operations than their more conventional (for example,
304 stainless steel) counterparts. These difficulties are
especially prevalent in single-step deep-draw operations (for
example, those involving relative large--such as between about 300
microns and 400 microns in depth--out-of-plane deformations) where
significant side wall deformation may take place. This early
necking and fracture is especially prevalent in tight radii used to
form the adjacent walls of the reactant flow channels. While the
hardening curves of other more formable stainless steels (such as
the aforementioned austenitics) generally allows for the more harsh
bending conditions imposed by the conventional one-step approach,
early necking and fracture from such single-step forming is also
prevalent in situations where the draw depth is comparatively large
(such as between about 400 and 500 microns, or more).
[0006] Moreover, current bipolar plate manufacturing accounts for
high portion of overall fuel cell stack cost. While using stamped
stainless steel bipolar plates would be beneficial in addressing a
significant portion of this cost, the low formability of stainless
steel in general (and ferritic stainless steel in particular) is a
significant challenge, especially for stamping very thin (for
example, 0.100 millimeters or thinner) sheets that are possessive
of the required channel strength and depth to satisfy functional
requirements.
[0007] To improve the formability of thin stainless steel sheet, a
hydro-forming process could be used. Nevertheless, such a process
is slow, and requires expensive special equipment that would make
it hard to meet either the required production rate or production
cost Likewise, electro-magnetic forming could be used, but is a
process that is still under development and not suitable for
low-cost mass production.
BRIEF SUMMARY OF THE INVENTION
[0008] According to an aspect of the present invention, a method of
forming a stainless steel bipolar plate for a fuel cell is
disclosed. The method includes a two-step (i.e., progressive)
process of providing a pre-formed sheet of stainless steel to a
plate forming tool, where the pre-formed sheet is possessive of a
nominal thickness. The tool (which in one form may be a pair of
cooperatively-shaped dies made to come together under pressure) can
form the final shape of the bipolar plate and (if appropriately
configured with the necessary dies) also be used to prepare the
pre-formed sheet. The first step changes the shape of a generally
flat sheet into an undulated pre-formed sheet to better define the
shapes that will--upon formation in the second step--form the flow
channels and lands that make up the bipolar plate. By the present
pre-forming step, selective movement of the material making up the
thickness of the sheet will help ensure that a maximum deviation
from the nominal thickness as a result of the shape-changing
operation is kept to a minimum. In this way, undue necking,
stretching or thinning that accompanies conventional forming
processes (such as the aforementioned one-step forming process) is
avoided. As will be discussed in more detail below, in one form,
such thickness deviation may be kept to an amount no more than
about 20% for ferritic stainless steels using draw depths of about
365 microns. By the present invention, the thickness deviation
exhibits significant reductions compared to conventional one-step
approaches that use the same material, the same nominal sheet
thickness and the same depth of draw. Reasons for this include (1)
the preform die (which in one particular form may be generally
V-shaped) bends/deforms the sheet at the land that gives definition
to the channel, where the material otherwise cannot get enough
plastic deformation in the one-step forming, (2) the V-shaped
preform design increases the sheet length of line before the sheet
goes into the final form die while keeping the thickness change of
the V-shaped straight wall uniform and minimal, and (3) the apexes
in the bent areas of the preform tend to feed additional metal to
the bending radii in the final form, thus reducing the
stretching/pulling of the metal from the wall area of the final
form. These features all help move the metal in a favorable way and
form a deeper channel with less thinning and necking.
[0009] According to another aspect of the invention, a method of
forming a ferritic stainless steel bipolar plate is disclosed. The
method includes forming a substantially planar sheet or sheet of
ferritic stainless steel into an undulated pattern shape, and then
forming that shape into a bipolar plate such that upon creation of
the plate, the wall thickness is substantially constant; in this
way, there is a substantial reduction in deviation from the nominal
thickness compared to that used by the one-step approach of the
prior art. Both the substantially planar sheet and the pre-formed
serpentine pattern define a nominal thickness; this thickness is
substantially constant throughout at least the portion of the
sheet's surface that will end up corresponding to the bipolar
plate.
[0010] According to yet another aspect of the invention, a method
of preparing a fuel cell is disclosed. The method includes
arranging an MEA to comprise an anode, a cathode and a membrane
disposed between the anode and cathode such that a respective anode
reactant and cathode reactant may be placed in fluid communication
therewith. In addition, the method includes placing one or more
stainless steel bipolar plates formed by (a) providing a pre-formed
sheet of stainless steel defined by a nominal thickness and a
plurality of reactant channel flowpaths therein, and (b) changing
the shape of the pre-formed sheet such that a maximum deviation
from the nominal thickness as a result of one or both operations is
substantially reduced relative to formation of a bipolar plate from
a forming process of the prior art. In one form, the shape changing
takes place by operation of a bipolar plate forming apparatus (such
as the aforementioned tool) where pre-shaped dies may stamp or
otherwise form the intermediate and final shapes.
[0011] It will be appreciated by those skilled in the art that
other components may make up the fuel cell, such as one or more gas
diffusion layers (GDLs) that may be placed between the respective
electrodes (i.e., anodes and cathodes) and the bipolar plates to
provide one or both of a reactant flowpath and an electrical
current path to an external load-consuming circuit Likewise, it
will be appreciated that functions shared by the GDLs and
electrodes may be combined into hybrid structure. Thus, for
example, the catalytic material may be formed on either or both of
the GDL and a substrate used to define the anode and cathode.
Furthermore, placement of the bipolar plate adjacent the anode or
cathode may include having the respective GDL placed therebetween
such that the outwardly-projecting lands of the bipolar plate are
in intermediate contact with the electrodes via direct contact
between the plate and the GDL on one side and the anode or cathode
and the GDL on the other side;. Within the present context, so long
as such indirect contact maintains all of the reactant flow
attributes between the plate and the respective electrodes, it is
deemed to be adjacent.
BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS
[0012] The following detailed description of the preferred
embodiments of the present invention can be best understood when
read in conjunction with the following drawings, where like
structure is indicated with like reference numerals and in
which:
[0013] FIG. 1 is an illustration of a partially exploded, sectional
view of a portion of a fuel cell with surrounding bipolar
plates;
[0014] FIG. 2 is side elevation view of one side of a bipolar plate
made in accordance with the prior art with one-step forming of 75
micron ferritic sheets shows necking failure;
[0015] FIG. 3 is side elevation view of a pre-formed sheet prior to
being formed into a bipolar plate in accordance with the present
invention;
[0016] FIG. 4 is side elevation view of the sheet of FIG. 3 once it
has been subjected to a forming step in a die in accordance with
the present invention such that formation-induced thinning along
the surface thereof is reduced;
[0017] FIG. 5 shows a side-by-side comparison of two different
embodiments of a sheet made in accordance with the disclosed
invention, where the right side shows the sheet of FIG. 3 and the
left side shows a modified version where the generally V-shaped
bends formed in the serpentine pattern of the sheet have been
flattened at their apex in the pre-formed shape prior to placement
in the bipolar plate forming apparatus;
[0018] FIG. 6 shows how the pre-formed sheet self-adjusts in the
bipolar plate form apparatus, even when exposed to a 400 micron
lateral misalignment over the course of several snapshots of a
forming time sequence;
[0019] FIG. 7 shows a nominal 75 micron thick pre-form sheet with a
305 micron pre-form draw depth showing a maximum thickness
deviation of 15.3% near the apex;
[0020] FIGS. 7A and 7B show the pre-form sheet of FIG. 7 being
subjected to deformation under dies of the tool of the present
invention such that they are converted into pre-form sheet with a
shallower draw depth;
[0021] FIG. 8A shows the results of a one-step forming process of a
nominal 75 micron thick pre-form sheet with a 365 micron designed
draw depth of the prior art;
[0022] FIG. 8B shows the results of a two-step forming process
according to an aspect of the present invention involving a nominal
75 micron thick pre-form sheet with a 365 micron draw depth
according to an aspect of the present invention; and
[0023] FIG. 9 shows the various steps used in the progressive
formation of a bipolar plate according to an aspect of the present
invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Referring initially to FIG. 1, a partial, sectional view of
a conventional PEM fuel cell 1 in exploded form is shown. The fuel
cell 1 includes a substantially planar proton exchange membrane 10,
anode catalyst layer 20 in facing contact with one face of the
proton exchange membrane 10, and cathode catalyst layer 30 in
facing contact with the other face. Collectively, the proton
exchange membrane 10 and catalyst layers 20 and 30 are referred to
as the MEA 40. An anode diffusion layer 50 is arranged in facing
contact with the anode catalyst layer 20, while a cathode diffusion
layer 60 is arranged in facing contact with the cathode catalyst
layer 30. Each of diffusion layers 50 and 60 are made with a
generally porous construction to facilitate the passage of gaseous
reactants to the catalyst layers 20 and 30. Collectively, anode
catalyst layer 20 and cathode catalyst layer 30 are referred to as
electrodes, and can be formed as separate distinct layers as shown,
or in the alternate (as mentioned above), as embedded at least
partially in diffusion layers 50 or 60 respectively, as well as
embedded partially in opposite faces of the proton exchange
membrane 10.
[0025] In addition to providing a substantially porous flowpath for
reactant gases to reach the appropriate side of the proton exchange
membrane 10, the diffusion layers 50 and 60 provide electrical
contact between the electrode catalyst layers 20, 30 and a bipolar
plate 70 (through lands 74) that in turn acts as a current
collector. Moreover, by its generally porous nature, the diffusion
layers 50 and 60 also form a conduit for removal of product gases
generated at the catalyst layers 20, 30. Furthermore, the cathode
diffusion layer 60 generates significant quantities of water vapor
in the cathode diffusion layer. Such feature is important for
helping to keep the proton exchange membrane 10 hydrated. Water
permeation in the diffusion layers can be adjusted through the
introduction of small quantities of polytetrafluoroethylene (PTFE)
or related material.
[0026] Simplified opposing surfaces 70A and 70B of a pair of
bipolar plates 70 are provided to separate each MEA 40 and
accompanying diffusion layers 50, 60 from adjacent MEAs and layers
(neither of which are shown) in a stack. One plate 70A engages the
anode diffusion layer 50 while a second plate 70B engages the
cathode diffusion layer 60. Each plate 70A and 70B (which upon
assembly as a unitary whole would make up the bipolar plate 70)
defines numerous reactant gas flow channels 72 along a respective
plate face. Lands 74 separate adjacent sections of the reactant gas
flow channels 72 by projecting toward and making direct contact
with the respective diffusion layers 50, 60. Although bipolar plate
70 is shown (for stylized purposes) defining purely rectangular
reactant gas flow channels 72, it will be appreciated by those
skilled in the art that a more accurate (and preferable) embodiment
will be shown below in conjunction with FIGS. 4 and 6, where
generally serpentine-shaped channels 172, 272 (along with their
respective generally planar apexes 173, 273) are formed.
[0027] In operation, a first gaseous reactant, such as hydrogen, is
delivered to the anode 20 side of the MEA 40 through the channels
72 from plate 70A, while a second gaseous reactant, such as oxygen
(typically in the form of air) is delivered to the cathode 30 side
of the MEA 40 through the channels 72 from plate 70B. Catalytic
reactions occur at the anode 20 and the cathode 30 respectively,
producing protons that migrate through the proton exchange membrane
10 and electrons that result in an electric current that may be
transmitted through the diffusion layers 50 and 60 and bipolar
plate 70 by virtue of contact between the lands 74 and the layers
50 and 60.
[0028] Referring next to FIG. 2, the results of forming three
commercially-available stainless steel sheets 70A, 70B and 70C into
bipolar plates 70 according to the conventional one-step prior art
approach are shown. In each of the three (all of which define a
generally continuous surface profile made up of generally straight
side walls 71A, 71B and 71C separated by bends 73A, 73B and 73C,
respectively), the stainless steel sheets are ferritic, and about
75 microns thick, while the die draw depth is set to 365 microns.
It will be appreciated by those skilled in the art that different
bipolar plate designs may dictate other thicknesses, and that
typical values are between about 350 and 400 microns. As can be
seen, significant necking occurs at the intersections 75A, 75B and
75C between the corresponding side walls 71A, 71B and 71C and bends
73A, 73B and 73C, where the first sheet 70A exhibits about 42%
necking (i.e., thinning), the second sheet 70B exhibits about 63%
necking and the third sheet 70C exhibits about 38% necking.
Likewise, in comparable tests where the draw depth was set to 350
(rather than 365) microns, the necking (which is a measure of the
thickness deviation) was 34.4%, 38% and 41.3%, respectively.
[0029] In either event, such unacceptably high levels of thinning
or stretching may lead to fracture, especially for ferritic
stainless steels, which are particularly prone due to their lower
hardening curve than that of austenite or other stainless steels.
The present inventors are likewise aware that a similar high
likelihood of fracturing or weakening (neither of which are shown)
will occur with the generally more robust austenitic stainless
steels in situations where the draw depth may be greater (such as
above about 400 microns). Thus, while much of the present
disclosure is especially useful for ferritic stainless steels, it
will be appreciated by those skilled in the art that the invention
discussed herein is also applicable to austenitic stainless steels,
particularly in those situations where larger (i.e., 400 microns or
greater) draw depths may be required.
[0030] Referring next to FIGS. 3, 4 and 9, the results of a
two-step forming process (FIGS. 3 and 4), as well as the steps used
in preparing a bipolar plate (FIG. 9) according to an aspect of the
present invention are shown. These include first preparing a
pre-form sheet 170A (FIG. 3), and second subjecting the sheet 170A
to stamping or related deformation in a die or related bipolar
plate forming apparatus (not shown)) such that by this second
deformation is converted into a shape suitable for use a bipolar
plate 170 (FIG. 4). The pre-form shape of FIG. 3 defines a
generally triangular cross-sectional profile, while the final form
of FIG. 4 defines a serpentine shape with trapezoidal features. In
a preferred form, the sheet 170A is made from a ferritic or
austenitic stainless steel alloy where corrosion-inhibiting
materials (such as chromium, nickel, molybdenum, copper or the
like) are added to the iron base. By way of example, the ferritic
stainless steel may contain chromium and (possibly up to about 30
weight percent) with substantially no nickel. Likewise, an
austenitic stainless steel may contain between about 16 and 26
weight percent chromium and about 8 to 22 weight percent nickel,
where well-known examples include the American Iron and Steel
Institute (AISI) Type 304 and Type 316 varieties. Other alloys,
such as iron-nickel alloys, will less likely be used for the base
material because of their lack of corrosion resistance.
[0031] By the present approach, the initial pre-forming step
followed by the forming step promotes the selective movement of the
sheet material such that the overall operation keeps a deviation
from the nominal thickness of the original sheet to a minimum. In
the present context, such selective movement corresponds to the
localized transfer of discrete portions of the material that makes
up the initially-flat original sheet to adjacent sheet regions. In
a more particular form (as will be discussed in more detail below),
such discrete portions may coincide with those areas in or around
where bends that define the channels are formed. One advantage
associated with the approach of the present invention is achieved
because the V-shaped preform 170A shown in FIG. 3 includes bends or
related deformations to the sheet at both the portion that will
(upon final forming) include the land 74 (as shown in FIG. 1), as
well as at the portion that will define the bottom of the channel
72 (also as shown in FIG. 1). In comparison, the one-step forming
of the known art is such that the material making up the original
sheet does not experience enough plastic deformation; this in turn
leads to undue necking, thinning and related large deviations from
the original nominal thickness. Another advantage associated with
the present invention is that the V-shape preform design 170A
increases the original sheet length of line prior to the sheet
going into the final forming tool die while keeping the thickness
change of the V-shape's straight (i.e., diagonal) walls 171A
uniform and minimal. A further advantage comes from apexes 173A in
the bent areas of the preform feed additional metal to the bending
radii 175 that are shown in the final form of FIG. 4; this movement
or feeding of additional material is useful in reducing the
stretching or pulling of the metal from the wall area 171 of the
final form 170. These features all help form a deeper channel with
less thinning and necking. In other words, the preform design
allows the stretch to occur across a longer length of material than
if the same deformations were imparted to the sheet in a
single-step forming process where any stretching is highly
localized. Because the material is not conducive to this amount of
localized stretch, the preform approach of the present invention
moves the metal only a part of the way while minimizing stretch;
this then allows the final form to complete the desired final
geometry without the exceeding local material properties.
[0032] In essence, the first step of the two-step approach
disclosed herein pre-stretches the sheet of material, and more
particularly performs this pre-stretching at fracture-vulnerable
regions of the formed bipolar plate. In the embodiment depicted,
the draw depth D of the tooling corresponding to the formation of
the pre-form sheet 170A is 350 microns (although it will be
appreciated by those skilled in the art that other depths may be
employed, depending on the desired dimensions of the resulting
reactant channels), while the lateral channel repeat length L is
1.6 millimeters. Likewise, the draw depth of the tool used to make
the final bipolar plate shape is 365 microns. Furthermore, in
situations where austentic (rather than ferritic) stainless steel
is used (for example, where one or both of the draw depths is over
about 400 microns), a same preform tool can be used and different
preform draw shapes can be obtained by adjusting the draw depth
(punch stroke); such an approach may help to reduce tool redundancy
and related costs.
[0033] The surface of the sheet 170A that will subsequently be
formed into bipolar plate 170 is undulated such that it defines a
generally wavy, repeating serpentine pattern made up of a
continuous length of stainless steel material that includes a
repeating pattern of side walls 171 and bends (now in the shape of
lands similar to that depicted in FIG. 1) 173 that meet at
intersection 175. The V-shaping may optionally include a generally
flattened-out region 173A; this helps promote proper alignment of
the preformed sheet 170A in the final form die. In this way, the
sheet 170A can tolerate potentially large misalignment relative to
the tool 200 (shown later) while preserving the desired final form.
The equivalent structure is discussed in more detail below and
shown with particularity in FIG. 6, where finished bipolar plate
270 mimics the other remaining features of finished bipolar plate
170 of FIG. 4. The enhanced alignment helps provide a better
transition from the feed region to the active area. Moreover, such
a configuration reduces (and therefore, better controls) elastic
deformation springback of the material making up sheet 170A after
the forming operation because it provides additional plastic
deformation in the preform stage. In another particular form, the
side walls 171 are generally straight. In yet another form, the
V-shape defines a wide-open angle to allow relatively large
misalignment of the sheet 170A and the tool 200 in the final
forming process. Preferably, the range of this angle is between 114
degrees and 120 degrees for normal bipolar plate 170 channel
designs. In still another form, the channels formed by the side
walls 171 and bends 173 of sheet 170A may include a large pre-form
draw depth to match final form depth as closely as possible (and
will be compatible with the depth dictated by the depth of the die
of the tool); such a configuration will result in large plastic
deformation Likewise, a small punch radius may be used to stretch
and deform the corners of the channel areas that generally coincide
with the intersections 175 of the finally-formed bipolar plate 170
that otherwise cannot sustain enough plastic deformation in the
final form due to the low strain hardening rate (and consequent
tendency to stretch out and become too thin) of ferrific stainless
steel material. The disclosed invention will allow the sheet being
used to form the bipolar plate 170 to provide a substantially
uniform thickness distribution at the end of final form channel
geometries without necking and fracture. Such an approach is
suitable for both ferritic and austenitic stainless steels.
[0034] Referring with particularity to FIG. 4, significant
reductions in localized thinning are shown relative to the one-step
approach of the prior art, showing with particularity an even
distribution of the thickness (shown italicized in microns) across
the surface with a maximum reduction in thickness of about 20%
(more particularly, 20.5%) at location 173 that corresponded to
bend 173A of sheet 170A for a draw depth of the final forming stage
of 350 microns for a ferrific stainless steel sample with an
initial thickness of 75 microns. This optimum result took into
consideration various pre-form shapes and dimensions, including the
land width from 0 millimeters to 0.4 millimeters, apex radii of
curvature between 0.24 millimeters and 0.4 millimeters, and a depth
of draw between 320 microns and 350 microns. Likewise, this optimum
design was repeated for a final forming stage of 365 microns, and
produced a maximum reduction in thickness of about 23.2% at a
location (not shown) that corresponded to the intersection of the
bend and straight side walls. Compared to the single-step
approaches discussed above, the optimized thickness deviation
reduction is approximately 50% better than that of FIG. 2's
best-performing sheet 70C, and about 67% for the worst-performing
sheet 70B.
[0035] Referring with particularity to FIG. 9, the steps 300 used
to form a bipolar plate are shown. Initially, a generally flat
sheet S is fed (such as by continuous conveyor or other known
means) into a pre-form set of dies 210, 220. The operation of the
dies 210, 220 in the first shaping step 310 ensures that a
generally undulated pre-form shape 170A, 270A (the former of which
is similar to that depicted in FIG. 2) is formed. From there, a
second shaping step 320 puts them into their final shape. Although
shown notionally as having the same general shape, the dies 210,
220 used in the pre-form step 310 are different from those used in
the second step used in the final forming. As will be appreciated
by those skilled in the art, both the pre-form and final die sets
are engineered for the particular part function and material type.
Thus, alternate part designs or material type may require a
slightly different shape/design, and such sets may be modified
accordingly. From these two shaping steps, step 330 may be used to
form additional bipolar plate features, such as piercing of
internal features, port openings or the like. Such pierced features
may include header openings to distribute fluids to a finished the
stack assembly (not shown), while port (i.e., anode and cathode)
openings provide pathways for reactant fluids to enter or exit each
cell. Other features, such as openings to create or support the
datum structure for assembly and other knockouts to facilitate
assembly and integration (for example, to facilitate cell voltage
monitoring) may also be formed. After that, an additional step 340
may be used to provide cutting, trimming, perimeter piercing or
related separation of excess to be discarded. For example, the last
station that corresponds to step 340 may be used to punch away the
perimeter material to cut the finished plate 170, 270 out of the
strip for collection to a downstream assembly. As will be
appreciated by those skilled in the art, an apparatus used to form
the final shape bipolar plate 170, 270 may include these and other
various fabricating stages such that all are integrated into a
single forming tool or machine. Both the preform step & die
(that corresponds to the first shaping step 310) and the final form
step & die (that corresponds to the second shaping step 320)
have radii cut into the general shape of the tooling to prevent
tearing of the fully-formed bipolar plates 170, 270. Thus, as noted
above, while the preform shapes of FIG. 3 and the first stage of
FIG. 6 generally define a more of triangular cross-sectional shape,
the final form (such as that depicted in FIG. 4 and the last stage
of FIG. 6) defines more of a trapezoidal shape. In either event,
both sections are blended and smoothed at corners to minimize
stress concentration factors and related stress risers. As such,
sharp transitions, corners or the like are avoided as being less
than optimal for stamping operations such as those imparted in
shaping steps 310, 320.
[0036] Referring next to FIGS. 5 through 7B, a variation on the
pre-form sheet 170A may be used to reduce the chance of
misalignment with the dies used as part of the bipolar plate
forming apparatus. Because increased misalignment can be a
significant contributor to higher degrees of maximum thinning, it
would be advantageous to create a pre-form sheet that reveals a
tendency to self-correct. Referring with particularity to FIG. 5,
one way that the alignment may be improved is by promoting a more
conformal fit between the apex at the bend 173A of sheet 170A and
the top of the bottom die 210. A comparison between one form of the
sheet 170A and an optional form 270A is shown on the right and
left, respectively. Sheet 270A includes a flattened region at bend
273A whereas the bend 173A of sheet 170A has a more pronounced
(i.e., sharper) V-shape. This flattened region is generally
coincident with the shape of shoulder 210A of bottom die 210. This
shaping allows thickness strain to be slightly more evenly
distributed along the arc length of the channel (as dictated by the
repeating length defined along the surface by the side wall 271A,
the bend 273A and the intersection 275A. This also reduces the
chance of misalignment between the sheet 270A and the bottom die
210, thereby making it possible to form a bipolar plate 270 with a
deep channel design. In one form, the nominal thickness of a
ferritic stainless steel sheet 270A is 75 microns, although it will
be appreciated by those skilled in the art that other thicknesses
may be used. As with the V-shape design mentioned above in
conjunction with FIGS. 3 and 4, this modified pre-form design of
sheet 270A also has self-adjustment capability for relatively large
lateral misalignment. This modification of the pre-form design of
sheet 270A will slightly change the angle of the bend 273A;
however, this has very little effect on the formability of both the
pre-form of sheet 270A and the final bipolar plate 270. With the
optional design that corresponds to pre-form sheet 270A, large
amounts of lateral misalignment (for example, by about 200 to 400
microns) relative to the bottom die 210 of tool 200 may tend to
realign as the deformation of the sheet progresses.
[0037] Referring with particularity to FIG. 6, the position of the
blank of sheet 270A (which could also be applied to the sheet 170A
of FIG. 3) is shown with a tendency to self-adjust as it moves
down; this is shown by arrow B as the bottom and top dies 210 and
220 close in upon one another corresponding to times t1 through t6.
Significantly, the forming takes place between the aligned sheet
270A (or 170A) and the bottom die 210; this in turn keeps the
thinning relatively uniform. While the misalignment shown is
particularly pronounced (for example, approximately 400 microns),
it will be appreciated that lesser degrees or misalignment are
equally resolvable. Significantly, this self-alignment ensures that
the work-hardening that was previously performed on the pre-formed
sheet 270A (or 170A) reduces the chance for necking, thinning or
the like. The final shape 270 of FIG. 6 analogizes the final shape
170 of FIG. 4.
[0038] Referring with particularity to FIGS. 7, 7A and 7B, a
pre-form sheet is shown, along with its conformal positioning
relative to a comparably-shaped bottom die 210. A sheet S (in FIG.
7) is subjected to deformation under dies 210 and 220 of tool 200
such that they are converted into pre-form sheet 170A (in FIG. 7A)
with a shallower draw depth D of 305 microns versus the 350 micron
depth of the previous version. In this example, the maximum
thinning took place around the apex of the V-shaped bend, and was
no more than 15.3% (as shown with particularity in FIG. 7B). It
will be appreciated that the same preform tool design used for a
350 micron depth (such as that discussed above in conjunction with
FIGS. 3 and 4) can be also used in tools employing shallower
preforms, where in another trial (not shown), the present inventors
used the same preform design to produce a maximum thinning of 19%
in a region generally similar to that of side wall 171 in FIG.
4.
[0039] Referring next to FIGS. 8A and 8B, a comparison between a
prior art one-step forming process 1000 (FIG. 8A) and a two-step
forming process 2000 according to the present invention (FIG. 8B)
that is made up of a pre-forming step 2000A and a forming step
2000B is shown. In both cases, a nominal 75 micron thick sheet is
subjected to forming to convert the sheet into a respective bipolar
plate with a 365 micron draw depth. In the prior art process 1000,
the conversion of a flat sheet into the bipolar plate exhibits a
necking-induced thickness reduction R at a location (corresponding
to about 1300 microns from the left end) of greater than 50% of its
75 micron original thickness; such a reduction is indicative of
significant compromise in plate structural capacity. By contrast,
the conversion of the sheet to the bipolar plate according to the
process 2000 of an aspect of the present invention shows in both
the conversion from a flat sheet to the pre-formed sheet, as well
as from the pre-formed sheet to the final bipolar plate thickness
reductions of no more than about 20%. The more gradual
pre-stretching movement of the material made possible by the
pre-forming step (along with the optional flattening of the bend
apex discussed above) helps push or relocate material to other
locations within the pre-form sheet 170A, thereby resulting in less
stretching in the final forming operation of the second step. It
will be appreciated by those skilled in the art that the preform
depth does not have to be same as final form as long as it can
reduce the thinning in the final draw to the design requirement. In
one preferred form, the depth of the preform is determined based on
computer simulation. Moreover, the final form quality is not very
sensitive to minor variations in preform depth variations inherent
in the manufacturing process.
[0040] A fuel cell made in accordance with an embodiment of the
present invention may be part of a larger fuel cell stack, which
may in turn form at least a portion of a propulsion system for a
vehicle, such as car. The fuel cell stack may be configured to
provide at least a portion of the motive or related propulsive
needs of the vehicle. It will also be appreciated by those skilled
in the art that other vehicular forms may be used in conjunction
with the fuel cell stack; such vehicles may include a truck,
motorcycle, aircraft, spacecraft or watercraft.
[0041] Unless otherwise indicated, all numbers expressing
quantities are approximations that may vary depending on the
desired properties sought to be obtained in embodiments of the
present invention. As such, they may all be understood to be
modified by the approximation "about". It is likewise noted that
terms such as "preferably," "commonly," and "typically" are not
utilized herein to limit the scope of the claimed invention or to
imply that certain features are critical, essential, or even
important to the structure or function of the claimed invention,
but rather to highlight alternative or additional features that may
or may not be utilized in a particular embodiment of the present
invention. Furthermore, the term "substantially" is utilized herein
to represent the inherent degree of uncertainty that may be
attributed to any quantitative comparison, value, measurement, or
other representation, and as such may represent the degree by which
a quantitative representation may vary from a stated reference
without resulting in a change in the basic function of the subject
matter at issue.
[0042] Having described the invention in detail and by reference to
specific embodiments thereof, it will be apparent that
modifications and variations are possible without departing from
the scope of the invention defined in the appended claims. More
specifically, although some aspects of the present invention are
identified herein as preferred or particularly advantageous, it is
contemplated that the present invention is not necessarily limited
to these preferred aspects of the invention.
* * * * *