U.S. patent application number 12/199949 was filed with the patent office on 2009-04-23 for mold for compression molding in the preparation of a unitized membrane electrode assembly.
Invention is credited to DAVID P RULE.
Application Number | 20090101276 12/199949 |
Document ID | / |
Family ID | 27613428 |
Filed Date | 2009-04-23 |
United States Patent
Application |
20090101276 |
Kind Code |
A1 |
RULE; DAVID P |
April 23, 2009 |
MOLD FOR COMPRESSION MOLDING IN THE PREPARATION OF A UNITIZED
MEMBRANE ELECTRODE ASSEMBLY
Abstract
The invention provides a mold for use in a compression molding
apparatus that has a frame part with a hole through its center; a
bottom plunger; and a top plunger; wherein the plungers are
fabricated to fit substantially snugly in the hole in the frame
part, and wherein at least one plunger comprises at least one
low-thermal conductivity insert. The mold is useful in compression
molding processes used in the preparation of unitized membrane
electrodes.
Inventors: |
RULE; DAVID P; (Wilmington,
DE) |
Correspondence
Address: |
E I DU PONT DE NEMOURS AND COMPANY;LEGAL PATENT RECORDS CENTER
BARLEY MILL PLAZA 25/1122B, 4417 LANCASTER PIKE
WILMINGTON
DE
19805
US
|
Family ID: |
27613428 |
Appl. No.: |
12/199949 |
Filed: |
August 28, 2008 |
Related U.S. Patent Documents
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
|
|
10498028 |
Jun 7, 2004 |
7431875 |
|
|
PCT/US03/01795 |
Jan 22, 2003 |
|
|
|
12199949 |
|
|
|
|
60350800 |
Jan 22, 2002 |
|
|
|
Current U.S.
Class: |
156/245 ;
425/116 |
Current CPC
Class: |
B29C 2043/3618 20130101;
B29L 2031/3468 20130101; B29C 33/306 20130101; B29C 43/003
20130101; B29K 2503/04 20130101; H01M 8/0286 20130101; H01M 8/0284
20130101; Y02E 60/50 20130101; H01M 4/881 20130101; H01M 8/0273
20130101; B29C 43/361 20130101; B29C 33/3828 20130101; H01M 8/0271
20130101; H01M 8/1004 20130101; B29C 2033/023 20130101; B29C 43/36
20130101; B29C 33/76 20130101; B29C 2043/3615 20130101; B29C 33/02
20130101 |
Class at
Publication: |
156/245 ;
425/116 |
International
Class: |
B28B 7/16 20060101
B28B007/16; B29C 43/18 20060101 B29C043/18 |
Claims
1. A mold for use in a compression molding apparatus comprising:
(a) a frame part with a hole through its center; (b) a bottom
plunger; and (c) a top plunger; wherein the plungers are fabricated
to fit substantially snugly in the hole in the frame part, and
wherein at least one plunger comprises at least one low-thermal
conductivity insert.
2. The mold of claim 1 wherein the at least one low-thermal
conductivity insert is present in the top plunger and the bottom
plunger.
3. The mold of claim 1 wherein the plunger comprises a plurality of
low-thermal conductivity inserts.
4. The mold of claim 3 wherein the at least low-thermal
conductivity insert is a ceramic.
5. The mold of claim 4 wherein the ceramic is selected from the
group consisting of alumina, alumina silicate, glass, zirconia, and
boron nitride.
6. The mold of claim 1 wherein the at least one low-thermal
conductivity insert is bonded to the at least one plunger with an
adhesive.
7. The mold of claim 1 wherein the adhesive is a fast cure
adhesive.
8-21. (canceled)
Description
FIELD OF THE INVENTION
[0001] This invention relates to a mold for use in compression
molding, and more particularly to the preparation of a unitized
membrane electrode assembly having fluid impermeable polymer seal
that is prepared using compression molding.
BACKGROUND OF THE INVENTION
[0002] A variety of electrochemical cells falls within a category
of cells often referred to as solid polymer electrolyte ("SPE")
cells. An SPE cell typically employs a membrane of a cation
exchange polymer that serves as a physical separator between the
anode and cathode while also serving as an electrolyte. SPE cells
can be operated as electrolytic cells for the production of
electrochemical products or they may be operated as fuel cells.
[0003] Fuel cells are electrochemical cells that convert reactants,
namely fuel and oxidant fluid streams, to generate electric power
and reaction products. A broad range of reactants can be used in
fuel cells and such reactants may be delivered in gaseous or liquid
streams. For example, the fuel stream may be substantially pure
hydrogen gas, a gaseous hydrogen containing reformate stream, or an
aqueous alcohol, for example methanol in a direct methanol fuel
cell (DMFC). The oxidant may, for example, be substantially pure
oxygen or a dilute oxygen stream such as air.
[0004] In SPE fuel cells, the solid polymer electrolyte membrane is
typically perfluorinated sulfonic acid polymer membrane in acid
form. Such fuel cells are often referred to as proton exchange
membrane ("PEM") fuel cells. The membrane is disposed between and
in contact with the anode and the cathode. Electrocatalysts in the
anode and the cathode typically induce the desired electrochemical
reactions and may be, for example, a metal black, an alloy or a
metal catalyst supported on a substrate, e.g., platinum on carbon.
SPE fuel cells typically also comprise a porous, electrically
conductive sheet material that is in electrical contact with each
of the electrodes, and permit diffusion of the reactants to the
electrodes. In fuel cells that employ gaseous reactants, this
porous, conductive sheet material is sometimes referred to as a gas
diffusion backing and is suitably provided by a carbon fiber paper
or carbon cloth. An assembly including the membrane, anode and
cathode, and gas diffusion backings for each electrode, is
sometimes referred to as a membrane electrode assembly ("MEA").
Bipolar plates, made of a conductive material and providing flow
fields for the reactants, are placed between a number of adjacent
MEAs. A number of MEAs and bipolar plates are assembled in this
manner to provide a fuel cell stack.
[0005] In fabricating unitized MEAs, multilayer MEAs may be sealed
using a fluid impermeable polymer seal. Several techniques may be
used to form these seals, including compression molding and
injection molding. With injection molding, the sealing polymer that
is used as the sealant material is applied in liquid or slurry form
and this is associated with its own disadvantages. In injection
molding, the sealing polymer sometimes does not flow onto both
sides of the membrane, and the relatively high pressures and flow
velocities may damage the gas diffusion backings. Balancing the
pressures on all edges of the gas diffusion backings may be
difficult. Another disadvantage of injection molding is the
difficulty of maintaining the position of the components of the MEA
in the mold. Clamping force on the components must be great enough
to impede motion due to the injection pressure and may damage the
fibers in the gas diffusion backing, creating debris and possible
shorting of the MEA if the debris punctures the membrane. Since
compression molding does not involve high-pressure gradients and
flow velocities, it does not generally have these problems.
[0006] A need exists for a mold useful in compression molding,
wherein membranes that are substantially dimensionally unstable are
used, that does not result in a damaged unitized MEA because of the
application of heat in the compression molding process.
SUMMARY OF THE INVENTION
[0007] In a first aspect, the invention provides a mold for use in
compression molding comprising: [0008] (a) a frame part with a hole
through its center; [0009] (b) a bottom plunger; and [0010] (c) a
top plunger; wherein the plungers are fabricated to fit
substantially snugly in the hole in the frame part, and wherein at
least one plunger comprises at least one low-thermal conductivity
insert.
[0011] In the first aspect, both plungers may be provided with at
least one low-thermal conductivity insert. Further, a plurality of
plungers may be used instead of a single plunger.
[0012] In a second aspect, the invention provides a process of
preparing a unitized membrane electrode assembly using
compression-molding comprising: [0013] (a) forming a multilayer
sandwich comprising a first gas diffusion backing having sealing
edges; a first electrocatalyst coating composition; a polymer
membrane; a second electrocatalyst coating composition; and a
second gas diffusion backing having sealing edges; and [0014] (b)
compression molding a sealing polymer to the multilayer sandwich,
wherein the mold used in the compression molding process comprises:
[0015] (c) a frame part with a hole through its center; [0016] (d)
a bottom plunger; and [0017] (e) a top plunger; wherein the
plungers are fabricated to fit substantially snugly in the hole in
the frame part, and wherein at least one plunger comprises at least
one low-thermal conductivity insert; whereby the sealing polymer is
impregnated into the sealing edges of the first and second gas
diffusion backings, and the sealing polymer envelops a peripheral
region of both the first and second gas diffusion backings and the
polymer membrane to form a polymer, fluid impermeable seal. The
sealing polymer may be a thermosetting or curable resin polymer or
a thermoplastic polymer.
BRIEF DESCRIPTION OF THE DRAWINGS
[0018] FIG. 1 is a schematic illustration of a mold (40) used in
compression molding showing the positioning of low-thermal
conductivity inserts 44 and 44'.
[0019] FIG. 2 is a schematic illustration of a expanded view of the
mold (40).
[0020] FIG. 3 is a schematic illustration of plungers (42) or (43)
showing more than one insert in the same plunger.
[0021] FIG. 4 is a schematic illustration of a multilayer sandwich
(10) comprising a first gas diffusion backing having sealing edges
(13); a first electrocatalyst coating composition (12); a polymer
membrane (11); a second electrocatalyst coating composition (12');
and a second gas diffusion backing having sealing edges (13') used
to form the membrane electrode assembly (MEA)
[0022] FIG. 5 is a schematic illustration of a unitized MEA (30)
after its removal from the mold in the compression molding
process.
[0023] FIG. 6 is a schematic illustration of an oblique view of a
unitized MEA (30) after its removal from the mold in the
compression molding process.
DETAILED DESCRIPTION OF THE INVENTION
Compression Molding:
[0024] The compression-molding apparatus consists of a mold (40)
and a heated press (not shown). The picture-frame mold (40) is
fabricated of a material having high thermal conductivity and
chosen to withstand the elevated temperatures of the process.
Typically, a metal such as tool steel or aluminum may be used. Some
examples of metals that are useful include metals having American
Iron and Steel Institute (AISI) specifications of H-13, H-19, P-4,
P-5, and P-6. Some additional materials may include 400 series
steels such as AISI 410, 416, 420, 431 and 400. Some useful types
of aluminum include Aluminum Association (AA) designations AA 5086,
AA 5454, AA 2024, and AA 7075.
[0025] As shown in FIGS. 1 and 2, the mold (40) generally consists
of three parts--a frame part with a hole through its center (41), a
bottom plunger (42), and a top plunger (43). The plungers are
fabricated to fit snuggly into the frame, and one of the plungers,
typically the bottom plunger (42), may actually be integral with
it. Typically, a hole (45) or (45') is provided in the frame or
plungers where a thermocouple may be inserted for the purpose of
monitoring the sealing polymer temperature.
[0026] The plungers are typically to be heated and cooled at the
same rate in order to minimize warping of the product part. In one
embodiment, this can be most readily achieved by making their
masses essentially equal. Since the polymer membrane (11), such as
an ion exchange membrane, that is used to make the membrane
electrode assembly is substantially dimensionally unstable, it is
important that the plungers be kept cooler in their centers than at
their periphery. to prevent it from degrading or wrinkling.
Plungers made with low-thermal conductivity inserts (44) and (44')
in their central areas can achieve this, as the center of the mold
may be insulated from the heat of the press, and therefore remain
at a lower temperature than the metal parts throughout the process.
Alternately, as shown in FIG. 3, the insert may be made up of a
plurality of smaller inserts (44a') with the proviso that the
amount of metal kept in contact with the membrane electrode
assembly is sufficiently low so warping resulting from the metal
contact is minimized. Any material may be used as the inserts (44)
and (44') with the proviso that they keep the MEA components at a
lower temperature. Some useful materials include ceramics selected
from the group consisting of alumina, alumina silicate, glass,
zirconia, and boron nitride. Some useful ceramic materials may be
purchased from Cotronics Corporation, Brooklyn, N.Y., e.g. glass
ceramics; Corning, Inc., Corning, N.Y., e.g. ceramics sold under
the tradename Macor.RTM.; Maryland Lava Company, Street, Md., and
Hottec, Inc., Norwich, Conn., e.g. cementous aluminate materials
sold under the tradename Fabcram.RTM.. Adhesives may be used to
bond the inserts in place. Some useful adhesives in fast cure
adhesives such as Zircon.RTM. adhesives, and the Resbond.TM. family
of adhesives from Cotronics Corporation, Brooklyn, N.Y. After
bonding with the plunger the face of the plunger (46) is then
polished so the insert is flush with the surface (46) of the
plunger (42) or (43).
Membrane Electrode Assembly:
[0027] The unitized MEA is prepared using a multilayer sandwich
(10), shown in FIG. 4, comprising a first gas diffusion backing
having sealing edges (13); a first electrocatalyst coating
composition (12); a polymer membrane (11); a second electrocatalyst
coating composition (12'); and a second gas diffusion backing
having sealing edges (13'). The unitized MEA also comprises a
polymer fluid impermeable seal (14), shown in FIGS. 5 and 6,
wherein the sealing polymer is either a thermoplastic polymer or a
thermosetting or curable resin, and wherein the sealing polymer is
impregnated into the at least a portion of the sealing edges of the
first and second gas diffusion backings (13) and (13'), and the
seal envelops a peripheral region of both the first and second gas
diffusion backings (13) and (13'), and the polymer membrane
(11).
Gas Diffusion Backing:
[0028] The gas diffusion backings having sealing edges (13) and
(13') include a porous electrically conductive material, typically
having an interconnected pore or void structure. Typically, the
sealing edge of the gas diffusion backing is the cut edge. The
electrically conductive material typically comprises a
corrosion-resistant material such as carbon, which may be formed
into fibers. Such fibrous carbon structures may be in the form of a
paper, woven fabric, or nonwoven web. Alternatively, the
electrically conductive material may be in particle form. Mixtures
of the fibrous carbon structures and the electrically conductive
material in particulate form may be used. The electrically
conductive material may further be optionally surface-treated to
either increase or decrease its surface energy, allowing it to have
either increased or decreased hydrophobicity.
[0029] A binder is optionally used to provide the structure with
desired mechanical properties such as strength or stiffness. The
binder itself may be chosen to serve the additional purpose of a
surface treatment as mentioned above.
[0030] A microporous composition may also be optionally included
with one or both of the gas diffusion backings. This composition
may be located on one or both surfaces of the gas diffusion backing
or impregnated into it or both. It serves, among other purposes, to
afford electrical and/or fluid contact on a fine scale with the
electrocatalyst coating. It may further enhance the ability of the
gas diffusion backing to permit two-phase fluid flow during fuel
cell operation, such as shedding liquid water in the cathode
oxidant stream or shedding carbon dioxide bubbles in the anode
stream of a direct-methanol fuel cell. It typically comprises
electrically conductive particles and a binder. The particles may
be, for example, high-structure carbon black such as Vulcan.RTM.
XC72 manufactured by Cabot Corporation, or acetylene carbon black.
The binder may be, for example, a polymer such as Teflon.RTM.
polytetrafluoroethylene manufactured by DuPont.
First and Second Electrocatalyst Coating Compositions:
[0031] The electrocatalyst coating compositions (12) and (12')
include an electrocatalyst and an ion exchange polymer; the two
coating compositions may be the same or different. The ion exchange
polymer may perform several functions in the resulting electrode
including serving as a binder for the electrocatalyst and improving
ionic conductivity to catalyst sites. Optionally, other components
are included in the composition, e.g., PTFE in particle form.
[0032] Electrocatalysts in the composition are selected based on
the particular intended application for the catalyst layer.
Electrocatalysts suitable for use in the present invention include
one or more platinum group metal such as platinum, ruthenium,
rhodium, and iridium and electroconductive oxides thereof, and
electroconductive reduced oxides thereof. The catalyst may be
supported or unsupported. For direct methanol fuel cells, a
(Pt--Ru)O.sub.x electocatalyst has been found to be useful. One
particularly preferred catalyst composition for hydrogen fuel cells
is platinum on carbon, for example, 60-wt % carbon, 40-wt %
platinum, obtainable from E-Tek Corporation of Natick, Mass. These
compositions when employed accordance with the procedures described
herein, provided particles in the electrode which are less than 1
.mu.m in size.
[0033] Since the ion exchange polymer employed in the
electrocatalyst coating composition serves not only as binder for
the electrocatalyst particles but also may assist in securing the
electrode to the membrane, it is preferable for the ion exchange
polymers in the composition to be compatible with the ion exchange
polymer in the membrane. Most preferably, exchange polymers in the
composition are the same type as the ion exchange polymer in the
membrane.
[0034] Ion exchange polymers for use in accordance with the present
invention are preferably highly fluorinated ion-exchange polymers.
"Highly fluorinated" means that at least 90% of the total number of
univalent atoms in the polymer are fluorine atoms. Most preferably,
the polymer is perfluorinated. It is also preferred for use in fuel
cells for the polymers to have sulfonate ion exchange groups. The
term "sulfonate ion exchange groups" is intended to refer to either
sulfonic acid groups or salts of sulfonic acid groups, preferably
alkali metal or ammonium salts. For applications where the polymer
is to be used for proton exchange as in fuel cells, the sulfonic
acid form of the polymer is preferred. If the polymer in the
electrocatalyst coating composition is not in sulfonic acid form
when used, a post treatment acid exchange step will be required to
convert the polymer to acid form prior to use.
[0035] Preferably, the ion exchange polymer employed comprises a
polymer backbone with recurring side chains attached to the
backbone with the side chains carrying the ion exchange groups.
Possible polymers include homopolymers or copolymers of two or more
monomers. Copolymers are typically formed from one monomer which is
a nonfunctional monomer and which provides carbon atoms for the
polymer backbone. A second monomer provides both carbon atoms for
the polymer backbone and also contributes the side chain carrying
the cation exchange group or its precursor, e.g., a sulfonyl halide
group such a sulfonyl fluoride (--SO.sub.2F), which can be
subsequently hydrolyzed to a sulfonate ion exchange group. For
example, copolymers of a first fluorinated vinyl monomer together
with a second fluorinated vinyl monomer having a sulfonyl fluoride
group (--SO.sub.2F) can be used. Possible first monomers include
tetrafluoroethylene (TFE), hexafluoropropylene, vinyl fluoride,
vinylidine fluoride, trifluoroethylene, chlorotrifluoroethylene,
perfluoro (alkyl vinyl ether), and mixtures thereof. Possible
second monomers include a variety of fluorinated vinyl ethers with
sulfonate ion exchange groups or precursor groups which can provide
the desired side chain in the polymer. The first monomer may also
have a side chain that does not interfere with the ion exchange
function of the sulfonate ion exchange group. Additional monomers
can also be incorporated into these polymers if desired.
[0036] Especially preferred polymers for use in the present
invention include a highly fluorinated, most preferably
perfluorinated, carbon backbone with a side chain represented by
the formula
--(O--CF.sub.2CFR.sub.f).sub.a--O--CF.sub.2CFR'.sub.fSO.sub.3H,
wherein R.sub.f and R'.sub.f are independently selected from F, Cl
or a perfluorinated alkyl group having 1 to 10 carbon atoms, a=0, 1
or 2. The preferred polymers include, for example, polymers
disclosed in U.S. Pat. No. 3,282,875 and in U.S. Pat. Nos.
4,358,545 and 4,940,525.
[0037] The electrocatalyst coating or catalyst layer may be formed
from a slurry or ink. The liquid medium for the ink is one selected
to be compatible with the process of application. The inks may be
applied to the membrane by any known technique to form a
catalyst-coated membrane. Alternately, the inks may be applied to
the gas diffusion backing. Some known application techniques
include screen, offset, gravure, flexographic or pad printing, or
slot-die, doctor-blade, dip, or spray coating. It is advantageous
for the medium to have a sufficiently low boiling point that rapid
drying of electrode layers is possible under the process conditions
employed. When using flexographic or pad printing techniques, it is
important that the composition not dry so fast that it dries on the
flexographic plate or the cliche plate or the pad before transfer
to the membrane film.
[0038] A wide variety of polar organic liquids or mixtures thereof
can serve as suitable liquid media for the ink. Water in minor
quantity may be present in the medium if it does not interfere with
the printing process. Some preferred polar organic liquids have the
capability to swell the membrane in large quantity although the
amount of liquids the electrocatalyst coating composition applied
in accordance with the invention is sufficiently limited that the
adverse effects from swelling during the process are minor or
undetectable. It is believed that solvents with the capability to
swell the polymer membrane can provide better contact and more
secure application of the electrode to the membrane. A variety of
alcohols are well suited for use as the liquid medium.
[0039] Preferred liquid media include suitable C4 to C8 alkyl
alcohols including, n-, iso-, sec- and tert-butyl alcohols; the
isomeric 5-carbon alcohols, 1,2- and 3-pentanol,
2-methyl-1-butanol, 3-methyl, 1-butanol, etc., the isomeric
6-carbon alcohols, e.g. 1-, 2-, and 3-hexanol, 2-methyl-1-pentanol,
3-methyl-1-pentanol, 2-methyl-1-pentanol, 3-methyl, 1-pentanol,
4-methyl-1-pentanol, etc., the isomeric C7 alcohols and the
isomeric C8 alcohols. Cyclic alcohols are also suitable. Preferred
alcohols are n-butanol and n-hexanol. Most preferred is
n-hexanol.
[0040] If the polymer in the electrocatalyst coating composition is
not in sulfonic acid form when used, a different liquid medium may
be preferred in the ink. For example, if the one of the preferred
polymers described above has its sulfonated groups in the form of
sulfonyl fluoride, a preferred liquid medium is a high-boiling
fluorocarbon such as "Fluorinert" FC-40 manufactured by 3M.
[0041] Handling properties of the ink, e.g. drying performance, can
be modified by the inclusion of compatible additives such as
ethylene glycol or glycerin up to 25% by weight based on the total
weight of liquid medium.
[0042] It has been found that the commercially available dispersion
of the acid form of the perfluorinated sulfonic acid polymer, sold
by E.I. du Pont de Nemours and Company under the trademark
Nafion.RTM., in a water/alcohol dispersion, can be used, as
starting material, for the preparation of an electrocatalyst
coating composition suitable for use in flexographic or pad
printing.
[0043] In the electrocatalyst coating composition, it is preferable
to adjust the amounts of electrocatalyst, ion exchange polymer and
other components, if present, so that the electrocatalyst is the
major component by weight of the resulting electrode. Most
preferably, the weight ratio of electrocatalyst to ion exchange
polymer in the electrode is about 2:1 to about 10:1.
[0044] Utilization of the electrocatalyst coating technique in
accordance with the process of the present invention can produce a
wide variety of printed layers which can be of essentially any
thickness ranging from very thick, e.g., 20 .mu.m or more very
thin, e.g., 1 .mu.m or less. This full range of thickness can be
produced without evidence of cracking, loss of adhesion, or other
inhomogenieties. Thick layers, or complicated multi-layer
structures, can be easily achieved by utilizing the pattern
registration available using flexographic or pad printing
technology to provide multiple layers deposited onto the same area
so that the desired ultimate thickness can be obtained. On the
other hand, only a few layers or perhaps a single layer can be used
to produce very thin electrodes. Typically, a thin layer ranging
from 1 to 2 .mu.m may be produced with each printing with lower %
solids formulations. Some typical electrostatic coating
compositions or inks are disclosed in U.S. Pat. No. 5,330,860.
[0045] The multilayer structures mentioned above permit the
electrocatalyst coating to vary in composition, for example the
concentration of precious metal catalyst can vary with the distance
from the substrate, e.g. membrane, surface. In addition,
hydrophilicity can be made to change as a function of coating
thickness, e.g., layers with varying ion exchange polymer EW can be
employed. Also, protective or abrasion-resistant top layers may be
applied in the final layer applications of the electrocatalyst
coating.
[0046] Composition may also be varied over the length and width of
the electrocatalyst coated area by controlling the amount applied
as a function of the distance from the center of the application
area as well as by changes in coating applied per pass. This
control is useful for dealing with the discontinuities that occur
at the edges and corners of the fuel cell, where activity goes
abruptly to zero. By varying coating composition or plate image
characteristics, the transition to zero activity can be made
gradual. In addition, in liquid feed fuel cells, concentration
variations from the inlet to the outlet ports can be compensated
for by varying the electrocatalyst coating across the length and
width of the membrane.
Polymer Membrane:
[0047] A polymer membrane (11), for use in accordance with the
invention, can be made of the same ion exchange polymers discussed
above for use in the electrocatalyst coating compositions. The
membranes can be made by known extrusion or casting techniques and
have thickness which can vary depending upon the application and
typically have a thickness of about 350 .mu.m or less. The trend is
to employ membranes that are quite thin, i.e., about 50 .mu.m or
less. The process in accordance with the present in invention is
well-suited for use in forming electrodes on such thin membranes
where the problem associated with large quantities of solvent
during coating are especially pronounced. While the polymer may be
in alkali metal or ammonium salt form during the flexographic or
pad printing process, it is preferred for the polymer in the
membrane to be in acid form to avoid post treatment acid exchange
steps. Suitable perfluorinated sulfonic acid polymer membranes in
acid form are available under the trademark Nafion.RTM. by E.I. du
Pont de Nemours and Company. Alternatively, membranes made from a
variety of other ion-conducting polymers could be used, for example
sulfonated polyaromatics as described in World Patent WO
00/15691.
[0048] Reinforced perfluorinated ion exchange polymer membranes can
also be utilized in catalyst coated membrane (CCM) manufacture by
the inventive printing process. Reinforced membranes can be made by
impregnating porous, expanded PTFE (ePTFE) with ion exchange
polymer. Expanded PTFE is available under the tradename "Goretex"
from W. L. Gore and Associates, Inc., Elkton Md., and under the
tradename "Tetratex" from Tetratec, Feasterville Pa. Impregnation
of ePTFE with perfluorinated sulfonic acid polymer is disclosed in
U.S. Pat. Nos. 5,547,551 and 6,110,333.
[0049] Catalyst coated membranes or gas diffusion backings coated
with electrocatalyst coating compositions may be provided with post
treatments such as calendering, vapor treatment to affect water
transport, or liquid extraction to remove trace residuals from any
of the above earlier steps. If the membrane dispersion or solution
used was the precursor of the highly fluorinated ionomer, after
application of the solution or dispersion the sandwich formed may
be subjected to a chemical treatment to convert the precursor to
the ionomer.
Sealing Polymer:
[0050] Thermosetting or thermoplastic polymers may be used as the
sealing polymer. FIG. 5 is a schematic illustration of a unitized
MEA (30) after its removal from the mold in the compression molding
process.
[0051] Thermoplastic polymers are "materials that soften and flow
upon application of pressure and heat. Thus, most thermoplastic
materials can be remolded many times. The obvious advantage is that
a piece that is rejected or broken after molding can be ground up
and remolded. In case of a mis-molded part, thermoplastic materials
also offer the option of repair through application of heat. Some
techniques for this include, for example, contact heating, infrared
energy, and ultrasonic welding. The presence of electrical
conductors in a fuel cell also offers the possibility of electrical
resistance or induction welding to re-melt and re-form a
thermoplastic component.
[0052] Chemically, thermoplastic processing is essentially inert,
with very low emissions and little or no appreciable chemical
reaction-taking place. Thus, problems such as environmental impact,
worker exposure, and bubble formation in the parts are minimal.
Thermoplastics as a class include some of the most chemically inert
materials in common usage, such as fluoropolymers and aromatic
poly(ether ketone)s. Such sealing polymers are available with
extremely low levels of any potential fuel cell contaminants, such
as metals, catalysts, and reactive functional groups.
[0053] Thermoplastic polymers offer a wide range of physical
properties of interest to the fuel cell designer. Semicrystalline
forms such as high-density polyethylene and polyvinylidene fluoride
have particularly low permeability to gases and liquids, and high
mechanical toughness. Many have high compressive moduli, either in
the neat or reinforced forms, and so can be used to rigidly support
fuel cell stack pressure without significantly changing the MEA
thickness. Finally, thermoplastics such as melt-processible
fluoropolymers offer very durable electrical properties, including
dielectric strength and electrical resistance.
[0054] One of the most significant advantages for thermoplastics in
this application is their flow properties. In the process of
injection molding, the mold and MEA are held below the melt
temperature of the injected sealing polymer as it is introduced.
The sealing polymer solidifies almost instantly upon contact with
these relatively cool surfaces, and additional sealing polymer
continues to flow past these areas in the interior of the cavity
only. As this material reaches the flow front, it spreads apart,
contacts cooler surface, and solidifies there. This phenomenon,
referred to as "fountain flow" in polymer-processing literature,
offers a unique advantage for thermoplastics in this invention. The
spreading-apart effect tends to separate electrodes that were
initially in or near short-circuit contact. Further, the rapid
solidification on contact with the MEA layers tends to prevent
sealing off of the catalyst layers. The electrode-separating action
of thermoplastic flow has been clearly seen in the products of this
invention through microscopic examination.
[0055] The thermoplastic polymers useable in this invention may be
from any of a number of classes. Melt-processible fluoropolymers
such as DuPont Teflon.RTM. FEP 100 and DuPont Teflon.RTM. PFA 340
may be used, as well as partially fluorinated polymers, an example
being polyvinylidene fluoride such as Kynar.RTM. 710 and Kynar
Flex.RTM. 2801 manufactured by Atofina Chemicals, King of Prussia,
Pa. Thermoplastic fluoroelastomers such as Kalrez.RTM. and
Viton.RTM., manufactured by E. I. Du Pont de Nemours & Company,
Inc., Wilmington, Del., also fall into this class. Aromatic
condensation polymers such as polyaryl(ether ketone)'s, an example
being polyaryl(ether ether ketone) manufactured by Victrex
Manufacturing Limited, Lancashire, Great Britain; modified
polyethylene such as Bynel.RTM. 40E529, modified polypropylene such
as Bynel.RTM. 50E561, both manufactured by DuPont; polyethylene
such as Sclair.RTM. 2318 manufactured by NOVA Chemicals
Corporation, Calgary, Alberta, Canada; thermoplastic elastomers
such as Hytrel.RTM. (DuPont); liquid-crystal polymers such as
Zenite.RTM. liquid-crystal polyester (DuPont), and aromatic
polyamides such as Zytel.RTM. HTN (DuPont) can also be used.
Thermosetting materials are materials that, once heated, react
irreversibly so that subsequent applications of heat and pressure
do not cause them to soften and flow. Thermoplastic polymers are
preferred over thermosetting materials because a rejected or
scrapped piece prepared with a thermosetting material cannot be
ground up and remolded. Some examples of thermosetting materials
include epoxies, urethane resins, and vulcanized natural
rubber.
[0056] The sealing polymer may also be optionally reinforced with
fibers, fabrics, or inorganic fillers, which may either be placed
in the mold during the compression molding process or compounded
into the sealing polymer beforehand. Such reinforcements can reduce
warpage in the final part.
Process:
[0057] The multilayer MEA sandwich (10) is placed in the center of
the bottom plunger (42) with the frame part (41) around it. The
plunger may have a release surface or be optionally coated or lined
with a release agent, such as PTFE film, to allow easy removal of
the part after molding. The CCM or membrane is typically cut to be
larger than the gas diffusion backings. Several layers of the
sealing polymer film are cut to the shape of a frame to surround
the gas diffusion backings (13) and (13') but partly overlap the
extended portion of the membrane (11) all around its perimeter.
Alternatively, the sealing polymer may be introduced to the process
in a number of other forms, including powders, strips, fibers,
fabric, liquid, or paste. It is preferable that it be introduced in
a precisely metered manner, such as a die-cut film of controlled
thickness or a metering pump with robotic control for a liquid. The
sealing polymer is placed in the mold, above and below the membrane
but surrounding the gas diffusion backings. As with the bottom
plunger (42), the top plunger (43) may have a release surface or be
optionally coated or lined with a release agent, such as PTFE
film.
[0058] The top plunger (43) of the tool is put in place, fitted
into the frame part (41). The tool (40) with the materials within
is put in a press, allowed to heat to above the melting point of
the sealing polymer, compressed by mechanical action of the press,
for example hydraulically, and cooled in place. The press may be
heated on only one side in which case on the plunger on the side
that is heated needs a low-thermal conductivity insert. Any press
suitable for heating and melting the thermoplastic seal material
may be used in this invention. Some known presses include presses
from Carver Inc., Wabash, Ind.; PHI, City Of Industry, Calif.; and
Johnson Machinery Company, Bloomfield, N.J. A shim (not shown) may
be placed on the frame between the top plunger and the frame to
determine the extent to which the MEA components are compressed. If
a shim is not used a compression pressure of about 0.1 to about 10
MPa, more typically a compression pressure of about 2 to about 3
MPa may be used. The sealing polymer is preferably heated to just
the point of complete melting throughout before cooling is
initiated. After the sealing polymer is cooled sufficiently for it
to have structural integrity, the unitized MEA, shown in FIGS. 5
and 6 was removed from the tool. As can be clearly seen the
unitized MEA (30) comprises the MEA sandwich (11) and an integral
seal (14). The unitized MEA may also be cooled to lower
temperatures if necessary, for example to reduce warpage.
[0059] Ridges, ribs and other features (not shown) may be provided
on the seal by having recesses in the plunger area adjacent the
seal.
[0060] An example of a well-known industrial process of compression
molding was the production of phonograph records, which were
typically made from compounded polyvinyl chloride. An example of
such a process is described in Principles of Polymer Systems, 2nd
Ed., Ferdinand Rodriguez, McGraw-Hill, New York, 1982.
Fuel Cell:
[0061] The unitized MEA (30) may be used to assemble a fuel cell.
Bipolar plates (not shown) are positioned on the outer surfaces of
the first and second (cathode and anode) gas diffusion backings
having sealing edges (13) and (13'). If the seals (14) and (14')
are provided with ridges, domes, ribs, or other structural features
(not shown), the bipolar plates may be provided with recesses that
mesh with these features on the seals (14) and (14').
[0062] Several fuel cells may be connected together, typically in
series, to increase the overall voltage of the assembly. This
assembly is typically known as a fuel cell stack.
[0063] Use of the mold having a low conductivity insert and
manufacture of the unitized MEA of the invention will be further
clarified with reference to the following examples. The examples
are merely illustrative and are not intended to limit the scope of
the invention.
EXAMPLES
Control 1
[0064] A picture-frame mold was fabricated of tool steel, having an
American Iron and Steel Institute (AISI) specification of H-13 heat
treated to RC 40-44, and manufactured by Carpenter Technology
Corporation, Reading, PA. The mold consisted of three parts--a
frame with 7.6-cm-square hole, a 0.95-cm-thick 7.6-cm-square bottom
plunger, and a 4.1-cm-thick, 7.6-cm-diameter square top plunger. A
hole was drilled into one side of the frame where a thermocouple
was inserted for the purpose of reading the mold temperature at
this interface. Steel shims having an equal thickness are placed on
opposite sides of the frame between the frame and the top plunger
to limit the amount of compression in the MEA.
[0065] A three-layer sandwich comprising 0.2-mm-thick Nafion.RTM.
117, DuPont, Wilmington, Del., between two layers of a
carbon-fiber-based diffusion backing, SGL "Sigracet" GDL 10AA,
manufactured by SGL Carbon Group, Manheim, Germany, was placed in
the center of the bottom plunger atop a 0.08-mm-thick PTFE release
film, with the frame part around it. This sandwich was in essence a
"dummy" MEA, in that it lacked the electrocatalysts necessary for
fuel cell function, but could serve mechanically and electrically
to work in the same way.
[0066] The membrane had been cut to be about 7 mm larger than the
diffusion backings. Several layers of thermoplastic polymer film,
Bynel.RTM. 40E529 polyethylene-containing seal material,
manufactured by DuPont, Wilmington, Del., were die-cut to square
dimensions of 7.6 cm outside diameter and 5.1 cm inside diameter;
the films thus formed frames that would surround the diffusion
backings but partly overlap the extended portion of the membrane
all around its perimeter. These layers of sealing polymer were also
placed in the mold, above and below the membrane. A second piece of
the release film was placed on top of the sandwich.
[0067] The top plunger of the tool was put in place, fitted into
the frame. The tool with the materials within was placed in a
press, allowed to heat to above the melting point of the
thermoplastic polymer, compressed hydraulically and cooled in
place. Just before cooling, the temperature in the frame was
measured to be approximately 185.degree. C., and the set point for
the press-platens temperature was 200.degree. C. After the frame
temperature was below 60.degree. C., the part was removed from the
tool.
[0068] The sandwich components were held together by the
consolidated thermoplastic polymer seal thus formed. Further, the
MEA sandwich was fully encapsulated at its edges; the edge of the
membrane was not visible around any of the specimen. The sealing
polymer was able to contact and slightly ingress into both of the
diffusion backing layers all around their perimeters. However, the
product specimen was significantly warped, and the central part of
the membrane appeared rippled and was found to be dark in color,
indicating it had been overheated.
Example 1
[0069] Control 1 was repeated with the following exception: the top
and bottom plungers of the tool were modified so only the areas
contacting the Bynel.RTM. 40E529 polyethylene-containing seal
material, manufactured by DuPont, Wilmington, Del., were steel. The
inner square area adjacent the gas diffusion backings, were made of
a low-thermal-conductivity ceramic Cotronics 914 machinable glass
ceramic, manufactured Cotronics Corporation, Brooklyn, N.Y., and
the inner surface of this ceramic was maintained at a much lower
temperature than the steel portion throughout the molding process.
Further, the molding temperature was reduced; the set point
temperature was maintained at 145.degree. C. and the maximum frame
temperature was maintained at 137.degree. C.
[0070] The specimen thus made was fully encapsulated, and the
central part of the membrane was smooth and flat, showing no
ripples. However, the specimen was still somewhat warped indicating
that depending on the components of the sandwich the mass of the
top and bottom plungers may have to adjusted to avoid warping of
the unitized MEA formed.
Example 2
[0071] Example 1 was repeated with the following exception: the
tooling was modified such that the top and bottom plungers of the
tool were the same thickness, 4.1 cm, and mass. The frame was
suspended high enough, using shims, such that its midpoint was near
the midpoint of the MEA materials.
[0072] The specimen thus made was fully encapsulated, and was not
warped, and the central part of the membrane was smooth and flat,
showing no ripples.
Example 3
[0073] Example 2 was repeated with the following exception: the
bottom plunger was replaced with an unmodified plunger containing
no ceramic material.
[0074] The specimen thus made was fully encapsulated, but exhibited
minor warping, and the central part of the membrane was smooth and
flat, showing no ripples.
Example 4
[0075] Example 2 was repeated with the following exception: instead
of a 0.2-mm-thick Nafion.RTM. membrane, a 0.05-mm-thick membrane
coated on both sides with a platinum-based catalyst layer was used.
This catalyst-coated membrane (CCM) was designed for use in a PEM
fuel cell.
[0076] The specimen thus made was placed in a hydrogen-fueled test
fuel cell and found to generate electric current. A polarization
curve was generated for this specimen and found to match that of a
similar CCM assembled into a similar cell with traditional gaskets
and separate diffusion backings.
* * * * *