Title: Unitized fuel cell assembly
Abstract: A unitized fuel cell assembly includes a first flow field plate, a second flow field plate, and a membrane electrode assembly (MEA) provided between the first and second flow field plates. A hard stop arrangement, in one configuration, is provided between the first and second flow field plates. The hard stop arrangement is dimensioned to limit compressive forces imparted to the MEA upon establishment of contact between the first and second flow field plates under pressure. A sealing arrangement is provided between the first and second flow field plates and peripheral to the MEA. The unitized fuel cell assembly is configured as a stand-alone fuel cell unit that can be used alone or in a stack of fuel cell units.
Patent Number: 6,989,214 Issued on 01/24/2006 to Mao, et al.
| Inventors:
|
Mao; Shane S. (Woodbury, MN);
Pierpont; Daniel M. (North Saint Paul, MN);
Le; Jimmy M. (Saint Paul, MN);
Saulsbury; Kim B. (Lake Elmo, MN);
Mlinar; John R. (Coon Rapids, MN);
Moreau; Claude D. (Woodbury, MN)
|
| Assignee:
|
3M Innovative Properties Company (St. Paul, MN)
|
| Appl. No.:
|
295292 |
| Filed:
|
November 15, 2002 |
| Current U.S. Class: |
429/35; 429/32; 429/36; 429/38; 429/40 |
| Current Intern'l Class: |
H01M 2/08 (20060101); H01M 8/10 (20060101); H01M 2/14 (20060101); H01M 4/86 (20060101) |
| Field of Search: |
429/30,32,35,36,38,40
|
References Cited [Referenced By]
U.S. Patent Documents
| 6020083 | Feb., 2000 | Breault et al.
| |
| 6159628 | Dec., 2000 | Grasso et al.
| |
| 6399234 | Jun., 2002 | Bonk et al.
| |
| 2002/0081474 | Jun., 2002 | Foster.
| |
| 2002/0110720 | Aug., 2002 | Yang.
| |
| Foreign Patent Documents |
| 0 933 826 | Aug., 1999 | EP.
| |
| 1 156 546 | Nov., 2001 | EP.
| |
| 1 220 345 | Jul., 2002 | EP.
| |
| 1 223 629 | Jul., 2002 | EP.
| |
| 1 372 203 | Dec., 2003 | EP.
| |
| 1 391 956 | Feb., 2004 | EP.
| |
| WO 00/1021/6 | Feb., 2000 | WO.
| |
| WO 02/2784/7 | Apr., 2002 | WO.
| |
| WO 02/056407 | Jul., 2002 | WO.
| |
| WO 02/065572 | Aug., 2002 | WO.
| |
| WO 02/089240 | Nov., 2002 | WO.
| |
| WO 03/026049 | Mar., 2003 | WO.
| |
| WO 03/041205 | May., 2003 | WO.
| |
Primary Examiner: Ryan; Patrick Joseph
Assistant Examiner: Parsons; Thomas H.
Attorney, Agent or Firm: Dahl; Philip Y.
Claims
What is claimed is:
1. A unitary fuel cell assembly, comprising:
a first field plate;
a second flow field plate;
a membrane electrode assembly (MEA) provided between the first and second flow
field plates, the MEA comprising first and second fluid transport layers (FTLs)
and a membrane provided between anode and cathode catalytic layers;
a hard stop arrangement provided between the first and second flow field plates;
and
a thermoplastic material provided between at least a portion of the first and
second FTLs and within a gap defined between the first and second FTLs and the
hard stop arrangement.
2. The assembly of claim 1, wherein the hard stop defines a unitary hard stop structure.
3. The assembly of claim 1, wherein the thermoplastic material extends at least
from the first and second FTLs to the hard stop arrangement.
4. The assembly of claim 1, wherein the thermoplastic material is further provided
between the hard stop arrangement and the first and second flow field plates, respectively.
5. The assembly of claim 1, wherein an adhesive is provided between the hard
stop arrangement and the first and second flow field plates, respectively.
6. The assembly of claim 1, wherein the thermoplastic material provided between
the first and second FTLs is diffused into the first and second FTLs.
7. The assembly of claim 1, wherein the thermoplastic material comprises a plurality
of thermoplastic films.
8. The assembly of claim 1, wherein the thermoplastic material comprises:
a first thermoplastic film provided between at least a portion of the first and
second FTLs and extending within the gap; and
a second thermoplastic film provided between at least the portion of the first
and second FTLs and extending within the gap.
9. The assembly of claim 1, wherein the thermoplastic material comprises:
a first thermoplastic film provided between at least a portion of the first and
second FTLs, extending within the gap, and extending between the hard stop arrangement
and the first flow field plate; and
a second thermoplastic film provided between at least the portion of the first
and second FTLs, extending within the gap, and extending between the hard stop
arrangement and the second flow field plate.
10. The assembly of claim 1, wherein the hard stop arrangement comprises one
or more incompressible stop members.
11. The assembly of claim 1, wherein:
a portion of the membrane extends beyond the first and second FTLs; and
the hard stop arrangement comprises:
a first hard stop provided between a first surface of the membrane portion and
the first flow field plate; and
a second hard stop provided between a second surface of the membrane portion
and the second flow field plate.
12. The assembly of claim 11, wherein the thermoplastic material is further provided
between the first flow field plate and the first hard stop and between the second
flow field plate and the second hard stop.
13. The assembly of claim 11, wherein the thermoplastic material comprises:
a first thermoplastic film provided between at least the portion of the first
and second FTLs, extending within the gap, and extending between the first hard
stop arrangement and the first flow field plate; and
a second thermoplastic film provided between at least the portion of the first
and second FTLs, extending within the gap, and extending between the second hard
stop arrangement and the second flow field plate.
14. The assembly of claim 11, further comprising:
a first adhesive layer provided between the first flow field and the first hard
stop; and
a second adhesive layer provided between the second flow field and the second
hard stop.
15. The assembly of claim 1, wherein the hard stop arrangement comprises one
or more of polyethylene, polypropylene, polyester, fiberglass, nylon, PET, Delrin,
Mylar, Kapton, Lexan or Teflon.
16. The assembly of claim 1, wherein the hard stop arrangement comprises PEN.
17. The assembly of claim 1, wherein the thermoplastic material comprises a polyester material.
18. The assembly of claim 1, wherein the thermoplastic material comprises a polyethylene
or a fluoroplastic material.
19. The assembly of claim 1, wherein the thermoplastic material has a melting
temperature range between about 50° C. and about 180° C.
20. The assembly of claim 1, wherein the thermoplastic material has a melting
temperature about equal to a bonding temperature of the MEA.
21. The assembly of claim 1, wherein a plurality of the unitary fuel cell assemblies
are arranged in an interconnected stack of the unitary fuel cell assemblies to
define a fuel cell stack assembly.
22. The assembly of claim 21, wherein one or more of the fuel cell stack assemblies
are incorporated into a fuel cell system.
23. A unitary fuel cell assembly, comprising:
a first flow field plate;
a second flow field plate;
a membrane electrode assembly (MEA) provided between the first and second flow
field plates, the MEA comprising first and second fluid transport layers (FTLs)
and a membrane provided between anode and cathode catalytic layers;
a hard stop arrangement provided between the first and second flow field plates,
the hard stop arrangement dimensioned to control compressive forces imparted to
the MEA upon establishment of contact between the first and second flow field plates
under pressure; and
a sealing arrangement provided between the first and second flow field plates
and peripheral to the MEA.
24. The assembly of claim 23, wherein the sealing arrangement is situated peripheral
to the MEA and the hard stop arrangement.
25. The assembly of claim 23, wherein the sealing arrangement is situated between
the MEA and the hard stop arrangement.
26. The assembly of claim 23, wherein the sealing arrangement at least partially
encompasses the hard stop arrangement.
27. The assembly of claim 23, wherein the sealing arrangement comprises an elastomeric gasket.
28. The assembly of claim 23, wherein the sealing arrangement comprises an in-situ
formed seal.
29. The assembly of claim 23, wherein the sealing arrangement is formed from
a thermosetting material.
30. The assembly of claim 23, wherein the sealing arrangement comprises a silicone material.
31. The assembly of claim 23, wherein the sealing arrangement is formed from
a curable, liquefied sealing material.
32. The assembly of claim 23, wherein the first and second flow field plates
comprise trap channels for trapping excess sealing material.
33. The assembly of claim 23, wherein a portion of the membrane extends beyond
the first and second FTLs, and the sealing arrangement contacts the membrane portion.
34. The assembly of claim 23, wherein the sealing arrangement contacts at least
an peripheral edge of the MEA.
35. The assembly of claim 23, wherein the hard stop arrangement is separable
with respect to the first and second flow field plates.
36. The assembly of claim 23, wherein the hard stop arrangement is integrally
connected to one or both of the first and second flow field plates.
37. The assembly of claim 23, wherein the hard stop arrangement is provided between
the MEA and a peripheral edge of the first and second flow field plates.
38. The assembly of claim 23, wherein the hard stop arrangement is provided along
a peripheral edge of the first and second flow field plates, the stop arrangement
separable with respect to the first and second flow field plates.
39. The assembly of claim 23, wherein the hard stop arrangement comprises:
a protruding surface of a peripheral edge of one of the first and second flow
field plates;
a recessed surface of a peripheral edge of the other one of the first and second
flow field plates; and
an electrically insulating material provided between the respective protruding
and recessed surfaces.
40. The assembly of claim 23, wherein the hard stop arrangement comprises:
a channel provided on one of the first and second flow field plates;
an annular hard stop core; and
a curved recessed surface provided on the other one of the first and second flow
field plates.
41. The assembly of claim 40, wherein the sealing arrangement comprises a curable
liquefied sealant material, at least some of the liquefied sealant material provided
in the channel and between the curved recess and an adjacent surface of the annular
hard stop core.
42. The assembly of claim 23, wherein a plurality of the unitary fuel cell assemblies
are arranged in an interconnected stack of the unitary fuel cell assemblies to
define a fuel cell stack assembly.
43. The assembly of claim 42, wherein one or more of the fuel cell stack assemblies
are incorporated into a fuel cell system.
44. A recyclable unitary fuel cell assembly, comprising:
a first flow field plate;
a second flow field plate;
a membrane electrode assembly (MEA) provided between the first and second flow
field plates, the MEA comprising first and second fluid transport layers (FTLs)
and a membrane provided between anode and cathode catalytic layers;
a sealing arrangement provided between the first and second flow field plates;
and
an engagement arrangement releasably coupling together the first and second flow
field plates, the engagement arrangement configured to permit repeated coupling
and decoupling of the first and second flow field plates, whereby at least the
first and second flow field plates are recoverable for reuse with a replacement
component of the unitary fuel cell assembly.
45. The assembly of claim 44, wherein the engagement arrangement comprises
one or more protruding surfaces provided on one of the first and second flow
field plates; and
one or more recessed surfaces provided on the other one of the first and second
flow field plates.
46. The assembly of claim 44, wherein the engagement arrangement comprises
a protruding surface of a peripheral edge of one of the first and second flow
field plates; and
a recessed surface of a peripheral edge of the other one of the first and second
flow field plates.
47. The assembly of claim 44, wherein the engagement arrangement comprises a
microstructured pattern provided on one or more surfaces of the first and second
flow field plates.
48. The assembly of claim 44, wherein the engagement arrangement comprises a
plurality of releasably engaging fasteners provided on one or both of the first
and second flow field plates.
49. The assembly of claim 44, wherein the sealing arrangement is situated peripheral
to the MEA.
50. The assembly of claim 44, wherein the sealing arrangement is separable with
respect to the first and second flow field plates.
51. The assembly of claim 44, wherein the sealing arrangement comprises a pre-formed seal.
52. The assembly of claim 44, wherein the sealing arrangement comprises a curable,
liquefied sealant.
53. The assembly of claim 44, wherein the sealing arrangement comprises a curable,
liquefied silicone material.
54. assembly of claim 44, wherein the sealing arrangement comprises a thermoplastic material.
55. The assembly of claim 54, wherein the thermoplastic material comprises one
or more thermoplastic films.
56. The assembly of claim 55, wherein the thermoplastic material comprises a
polyester material.
57. The assembly of claim 55, wherein the thermoplastic material comprises a
polyethylene or fluoroplastic material.
58. The assembly of claim 44, wherein the MEA is separable with respect to the
first and second flow field plates.
59. The assembly of claim 44, wherein the MEA is a pre-bonded MEA.
60. The assembly of claim 44, further comprising a hard stop arrangement provided
between the first and second flow field plates.
61. The assembly of claim 60, wherein the hard stop arrangement comprises one
or more incompressible stop members.
62. The assembly of claim 60, wherein the hard stop arrangement comprises one
or more of polyethylene, polypropylene, polyester, fiberglass, nylon, PET, Delrin,
Lexan, Mylar, Kapton or Teflon.
63. The assembly of claim 60, wherein the hard stop arrangement comprises PEN.
64. The assembly of claim 44, further comprising a hard stop arrangement provided
between the first and second flow field plates and separable with respect to the
first and second flow field plates.
65. The assembly of claim 44, wherein a plurality of the unitary fuel cell assemblies
are arranged in an interconnected stack of the unitary fuel cell assemblies to
define a fuel cell stack assembly.
66. The assembly of claim 65, wherein one or more of the fuel cell stack assemblies
are incorporated into a fuel cell system.
Description
FIELD OF THE INVENTION
The present invention relates generally to fuel cells and, more particularly,
to a unitized fuel cell assembly and packaging methodology.
BACKGROUND OF THE INVENTION
A typical fuel cell power system includes a power section in which one or more
stacks of fuel cells are provided. The efficacy of the fuel cell power system depends
in large part on the integrity of the various contacting and sealing interfaces
within individual fuel cells and between adjacent fuel cells of the stack.
Presently, the process of building a stack of fuel cells using conventional
approaches is tedious, time-consuming, and not readily adaptable for mass production.
By way of example, a typical 5 k kW fuel cell stack can include some 80 membrane
electrode assemblies (MEAs), some 160 flow field plates, and some 160 sealing gaskets.
These and other components of the stack must be carefully aligned and assembled.
Misalignment of even a few components can lead to gas leakage, hydrogen crossover,
and performance/durability deterioration.
Moreover, fuel cell MEAs are very fragile and need to be handled very carefully
to prevent electrical shorting, pinholes, and wrinkles formed on the membrane,
for example. MEA contamination is another significant concern during fuel cell
stack assembly. Presently known stack assembling processes are so labor intensive
that cost effective manufacturing of fuel cell systems may not be achievable using
conventional approaches.
There is a need for an improved fuel cell assembly and packaging methodology.
There is a further need for a fuel cell assembly that facilitates efficient assembling
and disassembling of fuel cell stacks. There is a further need for recycling useful
components in fuel cell stacks and systems. The present invention fulfills these
and other needs.
SUMMARY OF THE INVENTION
The present invention is directed to a unitized fuel cell assembly (UCA) and
packaging methodology. A unitized fuel cell system is unitary module or unit that
comprises one or more cells that can work as a functioning fuel cell alone or in
conjunction with other UCA's in a stack. According to an embodiment of the present
invention, a UCA includes a first flow field plate and a second flow field plate.
A membrane electrode assembly (MEA) is provided between the first and second flow
field plates. The MEA includes first and second fluid transport layers (FTLs) and
a membrane provided between anode and cathode catalytic layers. A hard stop arrangement
is provided between the first and second flow field plates. A thermoplastic material
is provided between at least a portion of the first and second FTLs and within
a gap defined between the first and second FTLs and the hard stop arrangement.
According to another embodiment, a UCA includes a first flow field plate,
a second flow field plate, and an MEA provided between the first and second flow
field plates. A hard stop arrangement is provided between the first and second
flow field plates. The hard stop arrangement is dimensioned to control compressive
forces imparted to the MEA upon establishment of contact between the first and
second flow field plates under pressure. A sealing arrangement is provided between
the first and second flow field plates and peripheral to the MEA.
In accordance with a further embodiment, a unitized fuel cell assembly is configured
for recyclable use. A recyclable unitary fuel cell assembly according to this embodiment
includes a first flow field plate, a second flow field plate, and an MEA provided
between the first and second flow field plates. A sealing arrangement is provided
between the first and second flow field plates. The assembly further includes an
engagement arrangement that releasably couples together the first and second flow
field plates. The engagement arrangement is configured to permit repeated coupling
and decoupling of the first and second flow field plates, whereby at least the
first and second flow field plates are recoverable for reuse with a replacement
component of the unitary fuel cell assembly.
In yet another embodiment of the present invention, a unitary fuel cell assembly
includes a first flow field plate, a second flow field plate, and an MEA provided
between the first and second flow field plates. The MEA includes first and second
fluid transport layers (FTLs) and a membrane provided between anode and cathode
catalytic layers. A thermoplastic material is provided between at least a portion
of the first and second FTLs, and further between the first and second flow field
plates for bonding together the first and second flow field plates to define a
unitary structure.
The above summary of the present invention is not intended to describe each embodiment
or every implementation of the present invention. Advantages and attainments, together
with a more complete understanding of the invention, will become apparent and appreciated
by referring to the following detailed description and claims taken in conjunction
with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1
a is an illustration of a fuel cell and its constituent layers;
FIG. 1
b illustrates a unitized cell assembly having a monopolar configuration
in accordance with an embodiment of the present invention;
FIG. 1
c illustrates a unitized cell assembly having a monopolar/bipolar
configuration in accordance with an embodiment of the present invention;
FIG. 2
a is a sectional view of a unitized cell assembly employing an
external hard stop arrangement and an in-situ formed sealing gasket in accordance
with an embodiment of the present invention;
FIG. 2
b is a sectional view of a unitized cell assembly employing an
internal hard stop arrangement and an in-situ formed sealing gasket in accordance
with an embodiment of the present invention;
FIGS. 3
a and 3
b are sectional views of a unitized cell
assembly employing a built-in hard stop arrangement and an in-situ formed sealing
gasket in accordance with an embodiment of the present invention;
FIGS. 4
a and 4
b are schematic sectional views of a unitized
cell assembly employing an internal hard stop arrangement and an in-situ formed
sealing gasket in accordance with another embodiment of the present invention;
FIGS. 5
a and 5
b are schematic sectional views of a unitized
cell assembly before and after a bonding process, respectively, the unitized cell
assembly employing an internal hard stop arrangement and an in-situ formed thermoplastic
sealing gasket in accordance with an embodiment of the present invention;
FIGS. 5
c and 5
d are schematic sectional views of a unitized
cell assembly before and after a bonding process, respectively, the unitized cell
assembly employing an internal hard stop arrangement and an in-situ formed thermoplastic
sealing gasket in accordance with another embodiment of the present invention;
FIGS. 5
e and 5
f are schematic sectional views of a unitized
cell assembly before and after a bonding process, respectively, the unitized cell
assembly employing an in-situ formed thermoplastic sealing gasket and excluding
a hard stop arrangement in accordance with a further embodiment of the present invention;
FIGS. 6
a-6
c show a unitized cell assembly system which
includes a monopolar unitized cell assembly and a separable cooling structure in
accordance with an embodiment of the present invention;
FIG. 6
d shows a unitized cell assembly system which includes a monopolar/bipolar
unitized cell assembly and a separable cooling structure in accordance with another
embodiment of the present invention;
FIGS. 7
a and 7
b illustrate a stack of unitized cell assemblies
disposed within a compression system in accordance with an embodiment of the present invention;
FIGS. 8
a-8
c illustrate various sectional views of a unitized
cell assembly which employs a locking or engagement capability in accordance an
embodiment of the present invention;
FIGS. 9
a-9
e illustrate various views of a unitized cell
assembly which incorporates an integral cooling arrangement in accordance with
an embodiment of the present invention;
FIG. 10 is an illustrative depiction of a simplified fuel cell stack that facilitates
an understanding of the manner in which fuels pass into and out of a stack of fuel
cells, wherein the fuel cells are preferably configured as unitized cell assemblies
in accordance with the principles of the present invention; and
FIG. 11 illustrates a fuel cell system within which one or more fuel cell stacks
employing unitized cell assemblies of the present invention can be employed.
While the invention is amenable to various modifications and alternative forms,
specifics thereof have been shown by way of example in the drawings and will be
described in detail. It is to be understood, however, that the intention is not
to limit the invention to the particular embodiments described. On the contrary,
the intention is to cover all modifications, equivalents, and alternatives falling
within the scope of the invention as defined by the appended claims.
DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS
In the following description of the illustrated embodiments, reference is made
to the accompanying drawings which form a part hereof, and in which is shown by
way of illustration, various embodiments in which the invention may be practiced.
It is to be understood that the embodiments may be utilized and structural changes
may be made without departing from the scope of the present invention.
The present invention is directed to an improved fuel cell assembly. Various
embodiments of the present invention are directed to a unitized fuel cell assembly
which provides for ease of fuel cell assembling and disassembling. A unitized fuel
cell package implemented in accordance with the present invention can further provide
for recycling of fuel cells configured for arrangement in a stack during fabrication,
repair, and maintenance of individual fuel cells and the fuel cell stack.
Certain embodiments are directed to a unitized fuel cell assembly implemented
in a monopolar or bipolar configuration. In other embodiments, a unitized fuel
cell is provided with a thermal management arrangement. In such embodiments, the
thermal management arrangement can be implemented integral to a unitized fuel cell
assembly or as a structure separate from the unitized fuel cell assembly. Further
embodiments of the present invention are directed to fuel cell stacks and systems
implemented using unitized fuel cell assemblies.
A fuel cell is an electrochemical device that combines hydrogen fuel and oxygen
from the air to produce electricity, heat, and water. Fuel cells do not utilize
combustion, and as such, fuel cells produce little if any hazardous effluents.
Fuel cells convert hydrogen fuel and oxygen directly into electricity, and can
be operated at much higher efficiencies than internal combustion electric generators,
for example.
A typical fuel cell is depicted in FIG. 1
a. The fuel cell 10 shown
in FIG. 1 includes a first fluid transport layer (FTL) 12 adjacent an anode
14. Adjacent the anode 14 is an electrolyte membrane 16. A
cathode 18 is situated adjacent the electrolyte membrane 16, and
a second fluid transport layer 19 is situated adjacent the cathode 18.
In operation, hydrogen fuel is introduced into the anode portion of the fuel cell
10, passing through the first fluid transport layer 12 and over the
anode 14. At the anode 14, the hydrogen fuel is separated into hydrogen
ions (H) and electrons (e).
The electrolyte membrane 16 permits only the hydrogen ions or protons
to pass through the electrolyte membrane 16 to the cathode portion of the
fuel cell 10. The electrons cannot pass through the electrolyte membrane
16 and, instead, flow through an external electrical circuit in the form
of electric current. This current can power an electric load 17, such as
an electric motor, or be directed to an energy storage device, such as a rechargeable battery.
Oxygen flows into the cathode side of the fuel cell 10 via the second
fluid transport layer 19. As the oxygen passes over the cathode 18,
oxygen, protons, and electrons combine to produce water and heat.
Individual fuel cells, such as that shown in FIG. 1
a, can be packaged
as unitized fuel cell assemblies as will be described in detail hereinbelow. The
unitized fuel cell assemblies, referred to herein as unitized cell assemblies or
UCAs for convenience, can be combined with a number of other UCAs to form a fuel
cell stack. The number of UCAs within the stack determines the total voltage of
the stack, and the active surface area of each of the cells determines the total
current. The total electrical power generated by a given fuel cell stack can be
determined by multiplying the total stack voltage by total current.
A number of different fuel cell technologies can be employed to construct UCAs
in accordance with the principles of the present invention. For example, a UCA
packaging methodology of the present invention can be employed to construct proton
exchange membrane (PEM) fuel cell assemblies. PEM fuel cells operate at relatively
low temperatures (about 175° F./80° C.), have high power density, can
vary their output quickly to meet shifts in power demand, and are well suited for
applications where quick startup is required, such as in automobiles for example.
The proton exchange membrane used in a PEM fuel cell is typically a thin plastic
sheet that allows hydrogen ions to pass through it. The membrane is typically coated
on both sides with highly dispersed metal or metal alloy particles (e.g., platinum
or platinum/ruthenium) that are active catalysts. The electrolyte used is typically
a solid organic polymer such as poly-perfluorosulfonic acid. Use of a solid electrolyte
is advantageous because it reduces corrosion and management problems.
Hydrogen is fed to the anode side of the fuel cell where the catalyst promotes
the hydrogen atoms to release electrons and become hydrogen ions (protons). The
electrons travel in the form of an electric current that can be utilized before
it returns to the cathode side of the fuel cell where oxygen has been introduced.
At the same time, the protons diffuse through the membrane to the cathode, where
the hydrogen ions are recombined and reacted with oxygen to produce water.
A membrane electrode assembly (MEA) is the central element of PEM fuel cells,
such
as hydrogen fuel cells. As discussed above, typical MEAs comprise a polymer electrolyte
membrane (PEM) (also known as an ion conductive membrane (ICM)), which functions
as a solid electrolyte.
One face of the PEM is in contact with an anode electrode layer and the opposite
face is in contact with a cathode electrode layer. Each electrode layer includes
electrochemical catalysts, typically including platinum metal. Fluid transport
layers (FTLs) facilitate gas transport to and from the anode and cathode electrode
materials and conduct electrical current.
In a typical PEM fuel cell, protons are formed at the anode via hydrogen oxidation
and transported to the cathode to react with oxygen, allowing electrical current
to flow in an external circuit connecting the electrodes. The FTL may also be called
a gas diffusion layer (GDL) or a diffuser/current collector (DCC). The anode and
cathode electrode layers may be applied to the PEM or to the FTL during manufacture,
so long as they are disposed between PEM and FTL in the completed MEA.
Any suitable PEM may be used in the practice of the present invention. The PEM
typically has a thickness of less than 50 μm, more typically less than 40
μm, more typically less than 30 μm, and most typically about 25 μm.
The PEM is typically comprised of a polymer electrolyte that is an acid-functional
fluoropolymer, such as Nafion® (DuPont Chemicals, Wilmington DE) and Flemion®
(Asahi Glass Co. Ltd., Tokyo, Japan). The polymer electrolytes useful in the present
invention are typically preferably copolymers of tetrafluoroethylene and one or
more fluorinated, acid-functional comonomers.
Typically, the polymer electrolyte bears sulfonate functional groups.
Most typically, the polymer electrolyte is Nafion®. The polymer electrolyte
typically has an acid equivalent weight of 1200 or less, more typically 1100 or
less, more typically 1050 or less, and most typically about 1000.
Any suitable FTL may be used in the practice of the present invention. Typically,
the FTL is comprised of sheet material comprising carbon fibers. The FTL is typically
a carbon fiber construction selected from woven and non-woven carbon fiber constructions.
Carbon fiber constructions which may be useful in the practice of the present invention
may include: Toray Carbon Paper, SpectraCarb Carbon Paper, AFN non-woven carbon
cloth, Zoltek Carbon Cloth, and the like. The FTL may be coated or impregnated
with various materials, including carbon particle coatings, hydrophilizing treatments,
and hydrophobizing treatments such as coating with polytetrafluoroethylene (PTFE).
Any suitable catalyst may be used in the practice of the present invention. Typically,
carbon-supported catalyst particles are used. Typical carbon-supported catalyst
particles are 50-90% carbon and 10-50% catalyst metal by weight, the catalyst metal
typically comprising Pt for the cathode and Pt and Ru in a weight ratio of 2:1
for the anode. The catalyst is typically applied to the PEM or to the FTL in the
form of a catalyst ink. The catalyst ink typically comprises polymer electrolyte
material, which may or may not be the same polymer electrolyte material which comprises
the PEM.
The catalyst ink typically comprises a dispersion of catalyst particles in a
dispersion of the polymer electrolyte. The ink typically contains 5-30% solids
(i.e. polymer and catalyst) and more typically 10-20% solids. The electrolyte dispersion
is typically an aqueous dispersion, which may additionally contain alcohols, polyalcohols,
such a glycerin and ethylene glycol, or other solvents such as N-methylpyrilidon
(NMP) and dimethylformaldehyde (DMF). The water, alcohol, and polyalcohol content
may be adjusted to alter rheological properties of the ink. The ink typically contains
0-50% alcohol and 0-20% polyalcohol. In addition, the ink may contain 0-2% of a
suitable dispersant. The ink is typically made by stirring with heat followed by
dilution to a coatable consistency.
The catalyst may be applied to the PEM or the FTL by any suitable means, including
both hand and machine methods, including hand brushing, notch bar coating, fluid
bearing die coating, wire-wound rod coating, fluid bearing coating, slot-fed knife
coating, three-roll coating, or decal transfer. Coating may be achieved in one
application or in multiple applications.
Direct methanol fuel cells (DMFC) are similar to PEM cells in that they both
use a polymer membrane as the electrolyte. In a DMFC, however, the anode catalyst
itself draws the hydrogen from liquid methanol fuel, eliminating the need for a
fuel reformer. DMFCs typically operate at a temperature between 120-190° F./49-88°
C. A direct methanol fuel cell can be subject to UCA packaging in accordance with
the principles of the present invention.
Referring now to FIG. 1
b, there is illustrated an embodiment of
a UCA implemented in accordance with a PEM fuel cell technology. As is shown in
FIG. 1
b, a membrane electrode assembly (MEA) 25 of the UCA 20
includes five component layers. A PEM layer 22 is sandwiched between a pair
of fluid transport layers 24 and 26, such as diffuse current collectors
(DCCs) or gas diffusion layers (GDLs) for example. An anode 30 is situated
between a first FTL 24 and the membrane 22, and a cathode 32
is situated between the membrane 22 and a second FTL 26.
In one configuration, a PEM layer 22 is fabricated to include an anode
catalyst coating 30 on one surface and a cathode catalyst coating 32
on the other surface. This structure is often referred to as a catalyst-coated
membrane or CCM. According to another configuration, the first and second FTLs
24, 26 are fabricated to include an anode and cathode catalyst coating
30, 32, respectively. In yet another configuration, an anode catalyst
coating 30 can be disposed partially on the first FTL 24 and partially
on one surface of the PEM 22, and a cathode catalyst coating 32 can
be disposed partially on the second FTL 26 and partially on the other surface
of the PEM 22.
The FTLs 24, 26 are typically fabricated from a carbon fiber paper
or non-woven material or woven cloth. Depending on the product construction, the
FTLs 24, 26 can have carbon particle coatings on one side. The FTLs
24, 26, as discussed above, can be fabricated to include or exclude
a catalyst coating.
In the particular embodiment shown in FIG. 1
b, MEA 25 is shown sandwiched
between a first edge seal system 34 and a second edge seal system 36.
Adjacent the first and second edge seal systems 34 and 36 are flow
field plates 40 and 42, respectively. Each of the flow field plates
40, 42 includes a field of gas flow channels 43 and ports
through which hydrogen and oxygen feed fuels pass. In the configuration depicted
in FIG. 1
b, flow field plates 40, 42 are configured as monopolar
flow field plates, in which a single MEA 25 is sandwiched there between.
The flow field in this and other embodiments may be a low lateral flux flow field
as disclosed in co-pending application Ser. No. 09/954,601, filed Sep. 17, 2001,
and incorporated herein by reference.
The edge seal systems 34, 36 provide the necessary sealing within
the UCA package to isolate the various fluid (gas/liquid) transport and reaction
regions from contaminating one another and from inappropriately exiting the UCA
20, and may further provide for electrical isolation and hard stop compression
control between the flow field plates 40, 42. The term "hard stop"
as used herein generally refers to a nearly or substantially incompressible material
that does not significantly change in thickness under operating pressures and temperatures.
More particularly, the term "hard stop" refers to a substantially incompressible
member or layer in an membrane electrode assembly (MEA) which halts compression
of the MEA at a fixed thickness or strain. A "hard stop" as referred to herein
is not intended to mean an ion conducting membrane layer, a catalyst layer, or
a gas diffusion layer.
In one configuration, the edge seal systems 34, 36 include a gasket
system formed from an elastomeric material. In other configurations, as will be
described below, one, two or more layers of various selected materials can be employed
to provide the requisite sealing within UCA 20. Other configurations employ
an in-situ formed seal system.
In certain embodiments, the gasket may be a closed-cell foam rubber gasket as
disclosed in co-pending application Ser. No. 10/294,098, filed Nov.14, 2002 incorporated
herein by reference. In other embodiments, the gasket may be formed with a contact
face having a raised-ridge microstructured sealing pattern as disclosed in co-pending
application Ser. No. 10/143,273, filed May 10, 2002, and incorporated herein by reference.
FIG. 1
c illustrates a UCA 50 which incorporates multiple MEAs
25 through employment of one or more bipolar flow field plates 56.
In the configuration shown in FIG. 1
c, UCA 50 incorporates two MEAs
25
a and 25
b and a single bipolar flow field plate 56.
MEA 25
a includes a cathode 62
a/membrane 61
a/anode
60
a layered structure sandwiched between FTLs 66
a and
64
a. FTL 66
a is situated adjacent a flow field end
plate 52, which is configured as a monopolar flow field plate. FTL 64
a
is situated adjacent a first flow field surface 56
a of bipolar
flow field plate 56.
Similarly, MEA 25
b includes a cathode 62
b/membrane
61
b/anode 60
b layered structure sandwiched between
FTLs 66
b and 64
b. FTL 64
b is situated
adjacent a flow field end plate 54, which is configured as a monopolar flow
field plate. FTL 66
b is situated adjacent a second flow field surface
56
b of bipolar flow field plate 56. It will be appreciated
that N number of MEAs 25 and N-1 bipolar flow field plates 56 can
be incorporated into a single UCA 50. It is believed, however, that, in
general, a UCA 50 incorporating one or two MEAs 56 (N=1, bipolar
plates=0 or N=2, bipolar plates=1) is preferred for more efficient thermal management.
The UCA configurations shown in FIGS. 1
b and 1
c are representative
of two particular arrangements that can be implemented for use in the context of
the present invention. These two arrangements are provided for illustrative purposes
only, and are not intended to represent all possible configurations coming within
the scope of the present invention. For example, the seal system 34 shown
in FIG. 1
b can be replaced or supplemented with other sealing systems, such
as those disclosed herein. Rather, FIGS. 1
b and 1
c are intended
to illustrate various components that can be selectively incorporated into a unitized
fuel cell assembly packaged in accordance with the principles of the present invention.
By way of further example, a variety of sealing methodologies implemented in
accordance
with the present invention can be employed to provide the requisite sealing of
a UCA comprising a single MEA disposed between a pair of monopolar flow field plates,
and can also be employed to seal a UCA comprising multiple MEAs, a pair of monopolar
flow field plates and one or more bipolar flow field plates. For example, a UCA
having a monopolar or bipolar structure can be constructed to incorporate an in-situ
formed solid gasket, such as a flat solid silicone gasket.
In particular embodiments, a UCA, in addition to including a sealing gasket,
can
incorporate a hard stop arrangement. The hard stop(s) can be built-in, disposed
internal to the UCA, or integrated into the monopolar and/or bipolar flow field
plates. Other features can be incorporated into a UCA, such as an excess gasket
material trap channel and a micro replicated pattern provided on the flow field
plates. Incorporating a hard stop into the UCA packaging advantageously limits
the amount of compressive force applied to the MEA during fabrication (e.g., press
forces) and during use (e.g., external stack pressure system). For example, the
height of a UCA hard stop can be calculated to provide a specified amount of MEA
compression, such as 30%, during UCA construction, such compression being limited
to the specified amount by the hard stop. Incorporating a hard stop into the flow
field plates can also act as a registration aid for the two flow field plates.
Moreover, a variety of UCA configurations can be implemented with a thermal
management capability in accordance with other embodiments of the present invention.
By way of example, a given UCA configuration can incorporate an integrated thermal
management system. Alternatively, or additionally, a given UCA can be configured
to mechanically couple with a separable thermal management structure, embodiments
of which will be describe below. Accordingly, a fuel cell assembly of the present
invention is not to be limited to a specific UCA configuration or to a particular
thermal management system as described herein.
It is appreciated by one skilled in the art that advancements in fuel cell technology
are needed in order to mass produce fuel cells and systems at marketable prices.
Conventional fuel cell packaging approaches presently limit the ability to achieve
high levels of fuel cell stack assembling efficiency. Moreover, current packaging
and stacking approaches are presently not amenable to fuel cell component recycling,
which results in wasteful scrapping of an entire fuel cell assembly once the fuel
cell has been identified as a poor performer. Fuel cell recycling permits reuse
of certain fuel cell assembly components once a defective fuel cell has been removed
and subject to disassembly. A UCA packaging approach consistent with the principles
of the present invention provides for efficient assembling and disassembling of
fuel cell stacks and, further, provides for recycling of various UCA components.
Turning now to FIG. 2
a, there is illustrated a cross-sectional view
of a UCA in accordance with one embodiment of the present invention. According
to this embodiment, UCA 80 incorporates in-situ formed flat, solid silicone
gaskets and a hard stop arrangement. In the embodiment shown in FIG. 2
a,
and in other embodiments described herein, a liquefied silicone sealant can be
employed. It is understood that silicone sealant material represents one of several
types of materials suitable for use in the construction of a UCA in accordance
with the present invention. Other sealing materials can alternatively be employed,
assuming such materials exhibit appropriate elastic properties for sealing and
are sufficiently durable for fuel cell environments.
The UCA 80 shown in FIG. 2
a can be constructed according to the
following illustrative process. Flow field plate 84 is placed on a flat
surface with the flow channels 85 facing upwardly. The flow field plate
84, for purposes of example, is a 13 cm×13 cm plate having a 10 cm×10
cm flow channel area. It is noted that the flow field plates 84, 82
can be fabricated from a carbon/polymer composite material, graphite, metal or
metal coated with conductive material.
A liquefied silicone sealant material is dispensed at a predetermined rate, such
as a rate of about 0.35 mg/min, onto the surfaces of the flow field plate 84
where the gasket of the MEA will be formed. A suitable silicone material is D98-55,
parts A and B, available from Dow Corning. The flow channel area 85 is covered
by an 11 cm×11 cm FTL 88. A catalyst-coated membrane (CCM) 90,
which represents a PEM coated with an anode catalyst material on one surface and
a cathode catalyst material on the other surface, is placed on the lower FTL 88
with the CCM 90 center aligned to the FTL center.
An upper 11 cm×11 cm FTL 86 is placed on the CCM 90 with alignment
of the respective centers. The FTLs 86, 88 are slightly larger than
the CCM 90 to provide a space into which the silicone can flow and infiltrate
into the porous carbon fiber of the FTLs 86, 88 to create an edge
seal. This oversizing of the FTLs 86, 88 relative to the CCM 90
also prevents silicone from flowing into the flow channels 85, which would
otherwise plug up the outer flow channels.
As shown, a membrane 91 of CCM 90 or the entire CCM 90 extends
outwardly from the MEA to a position proximate a hard stop 92. This extended
membrane or CCM portion provides for enhanced electrical isolation between the
flow field plates 84 and 82.
It is understood, however, that membrane 91 or CCM 90 need not extend
from the MEA as is illustrated in FIG. 2
a and other figures. Further, is
it understood that membrane 91 or CCM 90 can extend from the MEA
to a position at some desired distance between the MEA and hard stop 92.
An external hard stop 92 is used within the UCA 80 as a shim to
control MEA compression. The hard stop 92 can be fabricated from a variety
of materials, including, for example, polyethylene napthalate (PEN), polyethylene
terephthalate (PET), Teflon, or other incompressible material or a combination
of such materials. In the embodiment shown in FIG. 2
a, the external hard
stop 92 is fabricated from PEN and coated with Teflon to ensure non-stickiness
and removability after the UCA has formed. The thickness of hard stop 92
can be selected to achieve a desired amount of MEA compression. In FIG. 2
a,
the thickness of hard stop 92 is selected to ensure 30% compression of the MEA.
Liquefied silicone with two parts (A and B) premixed is dispensed at a
rate of about 0.35 mg/min onto the surfaces of the upper flow field plate 82
and the lower flow field plate 84 where the gaskets of the MEA will be formed.
The MEA components and external hard stops 92 are sandwiched between the
two flow field plates 82 and 84 with the dispensed silicone. The
entire sandwich structure 80 is then placed into a press. The sandwich structure
80 is preferably subject to press conditions of 270° F. at 3 tons for
10 minutes, which results in formation of UCA 80 with in-situ formed flat,
solid gaskets. During the UCA forming process, FTLs 88, 86 and CCM
90 are bonded to form an MEA with good interfaces. It is noted that a full
10 minutes of bonding time is typically needed if the MEA has not previously been
bonded. It is further noted that the silicone material may cure after a time much
shorter than 10 minutes, and that the typical press/bonding time of 10 minutes
cart be reduced in cases where the subject MEA is a previously bonded MEA.
FIG. 2
b illustrates another embodiment of a UCA in accordance with the
principles of the present invention. In this embodiment, an internal hard stop
arrangement is employed, in addition to use of an in-situ formed silicone gasket.
A 13 cm×13 cm flow field plate 84 with a 10 cm×10 cm flow channel
area 85 is placed on a flat surface with the flow channels facing upwardly.
This UCA configuration includes a trap channel 95 provided on each of the
flow field plates 82, 84 within the silicone gasket formation region.
As shown, the trap channel 95 is located between the hard stop arrangement
93
a/93
b and the outer periphery of the respective flow
field plates 82, 84. The trap channels 95 provide a space
for the excess liquefied silicone to flow into so as not to plug the flow channels.
This can also provide an internal locking mechanism that enhances UCA packaging
integrity, in addition to the requisite MEA sealing.
A liquefied silicone is dispensed at a rate of about 0.35 mg/min onto the surfaces
of the flow field plate 84 where the gasket of the MEA will be formed. The
amount of silicone dispensed on the plate surfaces can be reduced by about 50%
of the amount calculated for FIG. 2
a due to the presence of the integral
hard stop arrangement.
The hard stop arrangement of the instant embodiment includes frames 93
a
and 93
b formed from a suitable material such as PEN, PET, polyethylene,
polypropylene, polyester, fiberglass, nylon, Delrin, Lexan, Mylar, Kapton, Teflon,
or the like. Blends of these materials or composite materials of these with fillers
such as carbon, glass, ceramic, etc. may also be used as hard stops. It is understood
that the hard stop arrangement need not be a single continuous member, but may
instead be defined by a number of unconnected or loosely connected discrete hard
stop elements.
The frames 93
a and 93
b shown in FIG. 2
b are
fashioned from PEN. The PEN frame 93
b in this embodiment has an outer
dimension of 12.5 cm×12.5 cm and an 11 cm×11cm window. The frame 93
b
is placed on the flow field plate 84, such that the frame 93
b
covers much of the liquefied silicone 94. The thickness of the PEN frame
94 is selected to ensure 30% compression of the MEA.
An 11 cm×11 cm FTL 88 is placed into the inner window of the PET frame
93
b. A CCM 90 is placed on the FTL 88, with the CCM
center aligned to the FTL center. Another PET frame 93
a with the
same dimensions as frame 93
b is placed on the CCM 90 with
centers respectively aligned. The second 11 cm×11 cm FTL 86 is placed
into the window of PEN frame 93
a.
A liquefied silicone 94 is dispensed at a rate of about 0.35 mg/min onto
the surfaces of The second flow field plate 82 where the gasket of the MEA
will be formed. The second flow field plate 82 is placed on top of the flow
field plate 84/FTL 88/CCM 9O/FTL 86 structure, and
placed into a press, preferably under press conditions of 270° F., 3 tons
for 10 minutes.
FIG. 3
a illustrates another embodiment in which a built-in hard stop
is employed in addition to an in-situ formed silicone gasket. The basic construction
of UCA 80 shown in FIG. 3
a is similar to that shown in FIG. 2
b,
with the exception of the hard stop configuration. In the embodiment shown in FIG.
3
a, the hard stop feature is built into the flow field plates 82,
84. As shown, each of the flow field plates 82, 84 has a protruding
peripheral edge 82
a, 84
a, best seen in FIG. 3
b.
The edges 82
a, 84
a are formed to register with one
another and to provide a gap of a predetermined size between internal flow field
plate surfaces sufficient to accommodate the silicone seal 94. The heights
of the protruding peripheral edges 82
a, 84
a are selected
to provide an appropriate degree of MEA compression.
As shown in FIG. 3
b, the peripheral edge 82
a includes a protruding
interface and the peripheral edge 84
a includes a recessed interface.
The protruding interface of edge 82
a is received by the recessed
interface of edge 84
a when the two flow field plates 82, 84
are brought together under pressure within the press. An insulating layer 89,
such as an insulating film, is disposed between the peripheral edges 82
a,
84
a to provide the requisite electrical isolation between the two
flow field plates 82, 84.
In accordance with another sealing approach, surfaces of the flow field plates
can be machined to include a micro replicated pattern, often referred to as a microstructured
surface. Various microstructure patterns and methods of producing same are known
in the art. The microstructured patterns can be machined into particular regions
of the flow field plates to provide mechanical coupling between flow field plates
of the UCA upon engagement of the patterns provided on opposing flow field plate
surfaces. The patterns, for example, can have a ridge having a width which can
vary between 5 and 25 mils, and a height that can vary between about 1.5 and 2.5 mils.
For example, microstructured patterns can be machined into the flow field surfaces
within the gasket region to form many small semi-ridges on the surface of the gasket.
Microstructured patterns can also be machined into the flow field lands. As will
be discussed in greater detail below, UCA sealing can be accomplished by the combined
use of microstructured patterns and polymeric gaskets (e.g., in-situ formed silicone
gaskets or separate elastomeric gaskets) or by sole use of microstructured patterns
or other mechanical arrangements (e.g., locator pins, screws, bolts/nuts, interlocking
surface features).
FIGS. 4
a and 4
b illustrate a further embodiment of a UCA
which employs an internal hard stop and in-situ formed silicone seal or gasket.
In accordance with this embodiment, the UCA 100 includes an upper flow field
plate 102 that represents the cathode side of the fuel cell, and a lower
flow field plate 104 that represents the anode side of the fuel cell. The
hard stop arrangement 110, as best seen in FIG. 4
b, includes a one-piece
hard stop core or coil 112 positioned within a slot 114 provided
in the lower flow field plate 104.
The slot 114 can be pre-machined or molded in place during the plate making
process. The depth of the slot 114 can be varied according to the diameter
of the hard stop core 112. A curved recess 116 is provided in the
upper flow field plate 102 and has a radius matching that of the hard stop
core 112. The lower flow field plate 104 can include a trap channel
105 for accommodating excess sealant material that may flow during gasket formation.
The hard stop coil 112, as with other hard stop embodiments described
herein, can be formed from an incompressible material, such as PET, PEN, or Teflon.
The thickness of the hard stop coil 112 typically ranges between 0.5 mm
and 2.0 mm. In general, the thickness of the hard stop coil 112 should be
about 70% of the MEA's thickness, which is typically about 0.012 inch thick.
A silicone gasket is formed by dispensing liquid silicone on top of the hard
stop
coil 112 prior to positioning the coil 112 within the slot 114.
The hard stop coil 112 will then sink into the slot 114 and remain
orientated along the centerline of the slot 114. This helps to maintain
the same thickness of silicone layer proximate the hard stop coil 112. The
MEA 106 and upper flow field plate 102 are properly situated, and
the sandwich structure 100 is placed in a press under appropriate temperature
and pressure conditions for a predetermined duration of time, as discussed previously.
It is noted that the size of the membrane can be the same as the FTLs. Even if
the catalyst were unexpectedly exposed, this would not be a problem since the silicone
forms to protect against exposing the catalysts to the fuels. If it is intended
that the UCA be subject to recycling, an additional release coating can be applied
on the surface of the flow field plates 102, 104 which will come
into direct contact with the silicone gasket/sealing material. As such, the MEA
and seal/gasket of a failed UCA can be readily separated from the reusable flow
field plates 102, 104.
Turning now to FIGS. 5
a-5
f, there is illustrated a portion
of a UCA which employs a sealing arrangement in accordance with another embodiment
of the present invention. The embodiments depicted in FIGS. 5
a-5
f
incorporate a thermoplastic sealing material, which is typically dispensed
in the form of a film, tape, or other solid form. The thermoplastic can be a fluoroplastic
like THV (terpolymer of tetrafluoroethylene, hexafluoropropylene and vinylidene
difluoride); polyethylene; copolymers of polyethylene such as those of ethylene
and acrylic acid; Thermo-Bond 845 (manufactured by 3M, e.g., a polyethylene maleic
anhydride copolymer) and Thermo-Bond 668 (manufactured by 3M, e.g., a polyester).
Blends of these materials or composite materials of these with fillers such as
carbon, glass, ceramic, etc. may also be used as thermoplastics. Preferably, the
melt range i
