U.S. patent number RE42,434 [Application Number 10/720,005] was granted by the patent office on 2011-06-07 for corrosion resistant pem fuel cell.
This patent grant is currently assigned to GM Global Technology Operations LLC. Invention is credited to Rodney Lynn Borup, Brian K. Brady, Kevin M. Cunningham, Matthew Howard Fronk, Jay S. Hulett.
United States Patent |
RE42,434 |
Fronk , et al. |
June 7, 2011 |
Corrosion resistant PEM fuel cell
Abstract
A PEM fuel cell having electrical contact elements comprising a
corrosion-susceptible substrate metal coated with an electrically
conductive, corrosion-resistant polymer containing a plurality of
electrically conductive, corrosion-resistant filler particles. The
substrate may have an oxidizable metal first layer (e.g., stainless
steel) underlying the polymer coating.
Inventors: |
Fronk; Matthew Howard (Honeoye
Falls, NY), Borup; Rodney Lynn (East Rochester, NY),
Hulett; Jay S. (Rochester, NY), Brady; Brian K. (North
Chili, NY), Cunningham; Kevin M. (Romeo, MI) |
Assignee: |
GM Global Technology Operations
LLC (Detroit, MI)
|
Family
ID: |
23812918 |
Appl.
No.: |
10/720,005 |
Filed: |
November 21, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
Reissue of: |
09456478 |
Dec 7, 1999 |
6372376 |
Apr 16, 2002 |
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Current U.S.
Class: |
429/529; 429/532;
429/530; 429/479; 429/522 |
Current CPC
Class: |
H01M
8/0228 (20130101); H01M 8/0206 (20130101); H01M
2008/1095 (20130101); H01M 8/0213 (20130101); H01M
8/0226 (20130101); Y02E 60/50 (20130101) |
Current International
Class: |
H01M
4/86 (20060101); H01M 2/14 (20060101) |
Field of
Search: |
;429/522,529,530,532,479 |
References Cited
[Referenced By]
U.S. Patent Documents
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5272017 |
December 1993 |
Swathirajan et al. |
5578388 |
November 1996 |
Faita et al. |
5728283 |
March 1998 |
Reuter et al. |
5798188 |
August 1998 |
Mukohyama et al. |
5952118 |
September 1999 |
Ledjeff et al. |
|
Foreign Patent Documents
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0360219 |
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Sep 1989 |
|
EP |
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0780916 |
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Nov 1996 |
|
EP |
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0949704 |
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Apr 1999 |
|
EP |
|
0955686 |
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May 1999 |
|
EP |
|
2336712 |
|
Oct 1999 |
|
GB |
|
WO96/37005 |
|
Nov 1996 |
|
WO |
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WO 96/37005 |
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Nov 1996 |
|
WO |
|
Other References
"Electrically Conducting Polymers: Science and Technolgy", Arthur
J. Epstein, MRS Bulletin/Jun. 1997 pp. 16-23. cited by examiner
.
"Cathode Electrodeposition", A Journal of Coatings Technology
Reprint, M. Wimser et al. pp. 35-44, May 1982. cited by examiner
.
European Search Report mailed Aug. 4, 2005, 3 pages. cited by
other.
|
Primary Examiner: Dove; Tracy
Attorney, Agent or Firm: Reising Ethington P.C.
Government Interests
The Government of the United States of America has rights in this
invention pursuant to contract No. DE-AC02-90CH10435 awarded by the
United States Department of Energy.
Claims
What is claimed is:
1. In a PEM fuel cell having at least one cell comprising a pair of
opposite polarity electrodes, a membrane electrolyte
.[.intedacent.]. .Iadd.interjacent .Iaddend.said electrodes for
conducting ions therebetween, and an electrically conductive
contact element having a working face confronting at least one of
said .[.electrodessfor.]. .Iadd.electrodes for .Iaddend.conducting
electrical current from said one electrode, the improvement
comprising: said contact element comprising a corrosion-susceptible
metal substrate and an electrically conductive, corrosion-resistant
protective coating on said face to protect said substrate from the
corrosive environment of said fuel cell, said protective coating
comprising a mixture of electrically conductive particles dispersed
throughout an oxidation-resistant and acid-resistant,
water-insoluble polymeric matrix and having a resistivity .Iadd.no
.Iaddend.greater than about 50 ohm-cm, said mixture comprising
graphite particles having a first particle size and other
electrically conductive particles selected from the group
consisting of gold, platinum, nickel, palladium, rhodium, niobium,
titanium carbide, titanium nitride, titanium diboride,
chromium-alloyed titanium, nickel-alloyed titanium, rare earth
metals and carbon, said other particles having a second particle
size less than said first particle size to enhance the packing
density of said particles.
2. A fuel cell according to claim 1 wherein said carbon comprises
carbon black.
3. A fuel cell according to claim 1 wherein said coating is
electrophoretically deposited onto said substrate from a suspension
of said particles in an aqueous solution of acid-solubilized
polymer.
4. A fuel cell according to claim 1 wherein a discrete film of said
coating is laminated onto said substrate to form said electrically
conductive contact element.
5. A fuel cell according to claim 1 wherein a precursor of said
coating is deposited onto said substrate from a solution thereof,
dried and cured to form said coating.
6. A fuel cell according to claim 1 wherein said substrate
comprises a first acid-soluble metal underlying a second
acid-insoluble, passivating metal layer susceptible to oxidation in
said environment.
7. A fuel cell according to claim 1 wherein said polymer matrix is
selected from the group consisting of epoxies, silicones,
polyamide-imides, polyether-imides, polyphenols, fluro-elastomers,
polyesters, phenoxy-phenolics, epoxide-phenolics, acrylics and
urethanes.
8. In a PEM fuel cell having at least one cell comprising a pair of
opposite polarity electrodes, a membrane electrolyte
.[.intedjacent.]. .Iadd.interjacent .Iaddend.said electrodes for
conducting ions therebetween, and an electrically conductive
contact element having a working face confronting at least one of
said electrodes for conducting electrical current from said one
electrode, the improvement comprising: said contact element
comprising a corrosion-susceptible metal substrate and an
electrically conductive, corrosion-resistant protective coating on
said face to protect said substrate from the corrosive environment
of said fuel cell, said protective coating comprising a plurality
of electrically conductive particles dispersed throughout an
oxidation-resistant and acid-resistant, water-insoluble polymeric
matrix and having a resistivity no greater than about 50 ohm-cm,
said substrate comprising a first acid-soluble metal underlying a
second acid-insoluble, passivating layer susceptible to oxidation
in said environment.
Description
TECHNICAL FIELD
This invention relates to PEM fuel cells, and more particularly to
corrosion-resistant electrical contact elements therefor.
BACKGROUND OF THE INVENTION
Fuel cells have been proposed as a power source for electric
vehicles and other applications. One known fuel cell is the PEM
(i.e., Proton Exchange Membrane) fuel cell that includes a
so-called "membrane-electrode-assembly" comprising a thin, solid
polymer membrane-electrolyte having an anode on one face of the
membrane-electrolyte and a cathode on the opposite face of the
membrane-electrolyte. The anode and cathode typically comprise
finely divided carbon particles, very finely divided catalytic
particles supported on the internal and external surfaces of the
carbon particles, and proton conductive material intermingled with
the catalytic and carbon particles. One such
membrane-electrode-assembly and fuel cell is described in U.S. Pat.
No. 5,272,017 issued Dec. 21, 1993 and assigned to the assignee of
the present invention. The membrane-electrode-assembly is
sandwiched between a pair of electrically conductive contact
elements which serve as current collectors for the anode and
cathode, and may contain appropriate channels and openings therein
for distributing the fuel cell's gaseous reactants (i.e., H.sub.2
& O.sub.2/air) over the surfaces of the respective anode and
cathode.
Bipolar PEM fuel cells comprise a plurality of the
membrane-electrode-assemblies stacked together in electrical series
while being separated one from the next by an impermeable,
electrically conductive contact element known as a bipolar plate or
septum. The septum or bipolar plate has two working faces, one
confronting the anode of one cell and the other confronting the
cathode on the next adjacent cell in the stack, and electrically
conducts current between the adjacent cells. Contact elements at
the ends of the stack contact only the end cells and are referred
to as end plates.
In an H.sub.2-O.sub.2/air PEM fuel cell environment, the bipolar
plates and other contact elements (e.g., end plates) are in
constant contact with highly acidic solutions (pH 3-5) containing
F.sup.-, .[.SO.sub.4.sup.--,.]. .Iadd.SO.sub.4.sup.- -,
.Iaddend.SO.sub.3.sup.-, HSO.sub.4.sup.-, .[.CO.sub.3.sup.--,.].
.Iadd.CO.sub.3.sup.- -, .Iaddend.and HCO.sub.3.sup.-, etc.
Moreover, the cathode operates in a highly oxidizing environment,
being polarized to a maximum of about +1 V (vs. the normal hydrogen
electrode) while being exposed to pressurized air. Finally, the
anode is constantly exposed to super atmospheric hydrogen. Hence,
contact elements made from metal must be resistant to acids,
oxidation, and hydrogen embrittlement in the fuel cell environment.
As few metals exist that meet this criteria, contact elements have
often been fabricated from large pieces of graphite which is
corrosion-resistant, and electrically conductive in the PEM fuel
cell environment. However, graphite is quite fragile, and quite
porous making it extremely difficult to make very thin gas
impervious plates therefrom.
Lightweight metals such as aluminum and titanium and their alloys
have also been proposed for use in making fuel cell contact
elements. Such metals are more conductive than graphite, and can be
formed into very thin plates. Unfortunately, such light weight
metals are susceptible to corrosion in the hostile PEM fuel cell
environment, and contact elements made therefrom either dissolve
(e.g., in the case of aluminum), or form highly electronically
resistive, passivating oxide films on their surface (e.g., in the
case of titanium or stainless steel) that increases the internal
resistance of the fuel cell and reduces its performance. To address
this problem it has been proposed to coat the lightweight metal
contact elements with a layer of metal or metal compound which is
both electrically conductive and corrosion resistant to thereby
protect the underlying metal. See for example, Li et al U.S. Pat.
No. 5,624,769, which is assigned to the assignee of the present
invention, and discloses a light metal core, a stainless steel
passivating layer atop the core, and a layer of titanium nitride
(TiN) atop the stainless steel layer.
SUMMARY OF THE INVENTION
The present invention comprehends a PEM fuel cell having at least
one cell comprising a pair of opposite polarity electrodes, a
membrane electrolyte interjacent the electrodes for conducting ions
therebetween, and an electrically conductive contact element
confronting at least one of the electrodes. The contact element has
a working face that serves to conduct electrical current from that
electrode. The contact element comprises a corrosion-susceptible
metal substrate, having an electrically conductive,
corrosion-resistant, protective polymer coating on the working face
to protect the substrate from the corrosive environment of the fuel
cell. By "corrosion susceptible metal" is meant a metal that is
either dissolved by, or oxidized/passivated by, the cell's
environment. An oxidizable metal layer may cover a dissolvable
metal substrate, and underlie the conductive polymer layer.
More specifically, the protective coatings of the present invention
comprises a plurality of electrically conductive, corrosion-proof
(i.e., oxidation-resistant and acid-resistant) filler particles
dispersed throughout a matrix of an acid-resistant,
water-insoluble, oxidation resistant polymer that binds the
particles together and holds them on the surface of the metal
substrate. The coating contains sufficient filler particles to
produce a resistivity no greater than about 50 ohm-cm, and has a
thickness between about 5 microns and about 75 microns depending on
the composition, resistivity and integrity of the coating. Thinner
coatings (i.e., about 15-25 microns) are preferred for minimizing
the IR drop through the stack. Impervious protective coatings are
used directly on metals that are dissolvable by the system acids.
Pervious coatings may be used on metals that are only
oxidized/passivated, or on dissolvable metals covered with a layer
of oxidizable/passivatable metal.
Preferably, the conductive particles comprise carbon or graphite
having a particle size less than about 50 microns. Most preferably,
the particles comprise a mixture of graphite with smaller carbon
black particles (i.e., about 0.5-1.5 microns) that fill the
interstices between larger graphite particles (i.e., about 5-20
microns) to optimize the packing density of said particles for
improved conductivity. Other oxidation-resistant and acid-resistant
conductive particles may be substituted for the small carbon black
particles. The polymer matrix comprises any water-insoluble polymer
that (1) is resistant to acids and oxidation, (2) can be readily
coated or formed into thin films, and (3) can withstand the
operating temperatures of the fuel cell (i.e. up to about
120.degree. C.
The coating may be applied in a variety of ways including: (1)
laminating a preformed discrete film of the coating material onto
the working face(s) of the conductive element; or (2) applying
(e.g. spraying, brushing, doctor blading etc.) a precursor layer of
the coating material (i.e. a slurry of conductive particles in
solvated polymer) to the working face followed by drying and curing
the film, or (3) electrophoretically depositing the coating onto
the working face(s).
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will better be understood when considered in the
light of the following detailed description of certain specific
embodiments thereof which is given hereafter in conjunction with
the several figures in which:
FIG. 1 is a schematic, exploded, isometric, illustration of a
liquid-cooled PEM fuel cell stack (only two cells shown);
FIG. 2 is an exploded, isometric view of a bipolar plate useful
with PEM fuel cell stacks like that illustrated in FIG. 1;
FIG. 3 is a sectioned view in the direction 3-3 of FIG. 2; and
FIGS. 4 and 5 are magnified portions of the bipolar plate of FIG.
3;
DESCRIPTION OF THE PREFERRED EMBODIMENT
FIG. 1 depicts a two cell, bipolar PEM fuel cell stack having a
pair of membrane-electrode-assemblies (MEAs) 4 and 6 separated from
each other by an electrically conductive, liquid-cooled, bipolar
plate 8. The MEAs 4 and 6, and bipolar plate 8, are stacked
together between stainless steel clamping plates 10 and 12, and end
contact elements 14 and 16. The end contact elements 14 and 16, as
well as both working faces of the bipolar plate 8, contain a
plurality of grooves or channels 18, 20, 22, and 24 for
distributing fuel and oxidant gases (i.e., H.sub.2 & O.sub.2)
to the MEAs 4 and 6. Nonconductive gaskets 26, 28, 30, and 32
provide seals and electrical insulation between the several
components of the fuel cell stack. Gas permeable carbon/graphite
diffusion papers 34, 36, 38 and 40 press up against the electrode
faces of the MEAs 4 and 6. The end contact elements 14 and 16 press
up against the carbon/graphite papers 34 and 40 respectively, while
the bipolar plate 8 presses up against the carbon/graphite paper 36
on the anode face of MEA 4, and against carbon/graphite paper 38 on
the cathode face of MEA 6. Oxygen is supplied to the cathode side
of the fuel cell stack from storage tank 46 via appropriate supply
plumbing 42, while hydrogen is supplied to the anode side of the
fuel cell from storage tank 48, via appropriate supply plumbing 44.
Alternatively, air may be supplied to the cathode side from the
ambient, and hydrogen to the anode from a methanol or gasoline
reformer, or the like. Exhaust plumbing (not shown) for both the
H.sub.2 and O.sub.2/air sides of the MEAs will also be provided.
Additional plumbing 50, 52 and 54 is provided for supplying liquid
coolant to the bipolar plate 8 and end plates 14 and 16.
Appropriate plumbing for exhausting coolant from the plate 8 and
end plates 14 and 16 is also provided, but not shown.
FIG. 2 is an isometric, exploded view of a bipolar plate 56
comprising a first exterior metal sheet 58, a second exterior metal
sheet 60, and an interior spacer metal sheet 62 inteijacent the
first metal sheet 58 and the second metal sheet 60. The exterior
metal sheets 58 and 60 are made as thin as possible (e.g., about
0.002-0.02 inches thick), may be formed by stamping, by photo
etching (i.e., through a photolithographic mask) or any other
conventional process for shaping sheet metal. The external sheet 58
has a first working face 59 on the outside thereof which confronts
a membrane-electrode-assembly (not shown) and is formed so as to
provide a plurality of lands 64 which define therebetween a
plurality of grooves 66 known as a "flow field" through which the
fuel cell's reactant gases (i.e., H.sub.2 or O.sub.2) flow in a
tortuous path from one side 68 of the bipolar plate to the other
side 70 thereof. When the fuel cell is fully assembled, the lands
64 press against the carbon/graphite papers 36 or 38 (see FIG. 1)
which, in turn, press against the MEAs 4 and 6 respectively. For
drafting simplicity, FIG. 2 depicts only two arrays of lands and
grooves. In reality, the lands and grooves will cover the entire
external faces of the metal sheets 58 and 60 that engage the
carbon/graphite papers 36 and 38. The reactant gas is supplied to
grooves 66 from a header or manifold groove 72 that lies along one
side 68 of the fuel cell, and exits the grooves 66 via another
header/manifold groove 74 that lies adjacent the opposite side 70
of the fuel cell. As best shown in FIG. 3, the underside of the
sheet 58 includes a plurality of ridges 76 which define
therebetween a plurality of channels 78 through which coolant
passes during the operation of the fuel cell. As shown in FIG. 3, a
coolant channel 78 underlies each land 64 while a reactant gas
groove 66 underlies each ridge 76. Alternatively, the sheet 58
could be flat and the flow field formed in a separate sheet of
material.
Metal sheet 60 is similar to sheet 58. The internal face 61 (i.e.,
coolant side) of sheet 60 is shown in FIG. 2. In this regard, there
is depicted a plurality of ridges 80 defining therebetween a
plurality of channels 82 through which coolant flows from one side
69 of the bipolar plate to the other 71. Like sheet 58 and as best
shown in FIG. 3, the external side of the sheet 60 has a working
face 63 having a plurality of lands 84 thereon defining a plurality
of grooves 86 through which the reactant gases pass. An interior
metal spacer sheet 62 is positioned interjacent the exterior sheets
58 and 60 and includes a plurality of apertures 88 therein to
permit coolant to flow between the channels 82 in sheet 60 and the
channels 78 in the sheet 58 thereby breaking laminar boundary
layers and affording turbulence which enhances heat exchange with
the inside faces 90 and 92 of the exterior sheets 58 and 60
respectively.
FIG. 4 is a magnified view of a portion of FIG. 3 and shows the
ridges 76 on the first sheet 58, and the ridges 80 on the second
sheet 60 bonded (e.g. by brazement 85) to the spacer sheet 62.
In accordance with the present invention, and as best shown in FIG.
4, the working faces 59 and 63 of the bipolar plate are covered
with an electrically conductive, .[.oxidation resistant.].
.Iadd.oxidation-resistant.Iaddend., and acid-resistant protective
coating 94 having a resistivity less than about 50 ohm-cm, and
comprising a plurality of oxidation-resistant, acid-insoluble,
conductive particles (i.e. less than about 50 microns) dispersed
throughout an acid-resistant, oxidation-resistant polymer matrix.
Preferably, the conductive filler particles are selected from the
group consisting of gold, platinum, graphite, carbon, nickel,
conductive metal borides, nitrides and carbides (e.g. titanium
nitride, titanium carbide, titanium diboride), titanium alloyed
with chromium and/or nickel, palladium, niobium, rhodium, rare
earth metals, and other nobel metals. Most preferably, the
particles will comprise carbon or graphite (i.e. hexagonally
crystallized carbon). The particles comprise varying weight
percentages of the coating depending on the density and
conductivity of the particles (i.e., particles having a high
conductivity and low density can be used in lower weight
percentages). Carbon/graphite containing coatings will typically
contain 25 percent by weight carbon/graphite particles. The polymer
matrix comprises any water-insoluble polymer that can be formed
into a thin adherent film and that can withstand the hostile
oxidative and acidic environment of the fuel cell. Hence, such
polymers, as epoxies, silicones, polyamide-imides,
polyether-imides, polyphenols, fluro-elastomers (e.g.,
polyvinylidene flouride), polyesters, phenoxy-phenolics,
epoxide-phenolics, acrylics, and urethanes, .[.inter alia.].
.Iadd.inter alia .Iaddend.are seen to be useful with the present
invention. Cross-linked polymers are preferred for producing
impermeable coatings.
The substrate metal forming the contact element comprises a
corrosion-susceptible metal such as (1) aluminum which is
dissolvable by the acids formed in the cell, or (2) titanium or
stainless steel which are oxidized/passivated by the formation of
oxide layers on their surfaces. In accordance with one embodiment
of the invention, the conductive polymer coating is applied
directly to the substrate metal and allowed to dry/cure thereon.
According to another embodiment of the invention, the substrate
metal comprises an acid soluble metal (e.g., Al) that is covered
with an oxidizable metal (e.g., stainless steel) before the
electrically conductive polymer topcoat is applied.
The coating may be applied in a variety of ways, e.g., (1)
electrophoretic deposition, (2) brushing, spraying or spreading, or
(3) laminating. Electrophoretically deposited coatings are
particularly advantageous because they can be quickly deposited in
an automated process with little waste, and can be deposited
substantially uniformly onto substrates having complex and recessed
surfaces like those used to form the reactant flow fields on the
working face(s) of the contact elements. Electrophoretic deposition
is a well-known process useful to coat a variety of conductive
substrates such as automobile and truck bodies. Electrophoretic
deposition technology is discussed in a variety of publications
including "Cathodic Electrodeposition", Journal of Coatings
Technology, Volume 54, No. 688, pages 35-44 (May 1982). Briefly, in
electrophoretic deposition processes, a direct current is passed
through a suspension of the conductive particles in an aqueous
solution of a charged acid-soluble polymer. Under the influence of
the applied current, the polymer migrates to, and precipitates
upon, a conductive substrate of opposing charge, and carries with
it the conductive particles. When cross-linkable polymers are used,
the suspension also includes a catalyst for promoting the
cross-linking. Cathodic and anodic electrophoretic processes are
both known. Cathodically deposited coatings are preferred for fuel
cell applications, and are deposited by a process wherein
positively charged polymer is deposited onto a negatively charged
substrate. Anodically deposited coatings are less desirable since
they tend to dissolve some of the substrate metal and contaminate
the coating therewith. In cathodic electrophoretic coating, the
passage of electrical current causes the water to electrolyze
forming hydroxyl ions at the cathode and establishing an alkaline
diffusion layer contiguous therewith. The alkalinity of the
diffusion layer is proportional to the cathode current density.
Under the influence of the applied voltage, the positively charged
polymer migrates to the cathode and into the alkaline diffusion
layer where the hydroxyl ions react with the acid-solubilized
polymer and cause the polymer to precipitate onto the cathodic
substrate. The conductive filler particles become trapped in the
precipitate and co-deposit onto the cathodic substrate. Cathodic
epoxies, acrylics, urethanes and polyesters are useful with this
method of depositing the coating as well as other polymers such as
those disclosed in the "Cathodic Electrodeposition" publication
(supra), and in Reuter et al. U.S. Pat. No. 5,728,283 and the
references cited therein. Subsequent baking of the coated contact
element cures and densities the coating.
According to another embodiment of the invention, the coating is
first formed as a discrete film (e.g. by solvent casting, extrusion
etc.), and then laminated onto the working surface of the contact
element, e.g., by hot rolling. This technique will preferably be
used to make laminated sheet stock from which the contact elements
are subsequently formed, e.g. as by stamping. In this embodiment,
the discrete film will preferably contain a plasticizer to improve
handling of the film and to provide a coating layer atop the
substrate that is supple enough so that it can be readily shaped,
(e.g. stamped) without tearing or disrupting the film when the
contact element is formed as by stamping. To insure adherence of
the coating to the substrate, the surface of the substrate to which
the film is applied is (1) cleaned of all undesirable surface films
(e.g., oil), (2) oxides are removed by acid etching, and (3), most
preferably, roughened or abraded to roughen the surface for
anchoring the film thereto. Fluroelastomers such as polyvinyladiene
diflouride or the like are useful with this embodiment, and may be
used with conventional plasticizers such as dibutyl phthalate.
According to another embodiment of the invention, the electrically
conductive polymer film is applied to the working face of the
substrate by spraying, brushing or spreading (e.g. with a doctor
blade). In this embodiment, a precursor of the coating is formed by
dissolving the polymer in a suitable solvent, mixing the conductive
filler particles with the dissolved polymer and applying it as a
wet slurry atop the substrate. The wet coating is then dried (i.e.
the solvent removed) and cured as needed (e.g., for thermosets).
The conductive particles adhere to the substrate by means of the
solvent-free polymer. A preferred polymer useful with this
embodiment comprises a polyamide-imide thermosetting polymer. The
polyamide-imide is dissolved in a solvent comprising a mixture of
N-methylpyrrolidone, propylene glycol and methyl ether acetate. To
this solution is added about 21% to about 23% by weight of a
mixture of graphite and carbon black particles wherein the graphite
particles range in size from about 5 microns to about 20 microns
and the carbon black particles range in size from about 0.5 micron
to about 1.5 microns with the smaller carbon black particles
serving to fill the voids between the larger graphite particles and
thereby increase the conductivity of the coating compared to
all-graphite coatings. The mix is applied to the substrate, dried
and cured to provide 15-30 micron thick coatings (preferably about
17 microns) having a carbon-graphite content of about 38% by
weight. It may be cured slowly at low temperatures (i.e.
<400.degree. F.), or more quickly in a two step process wherein
the solvent is first removed by heating for ten minutes at about
300.degree. F.-350.degree. F. (i.e., dried) followed by higher
temperature heating (500.degree. F.-750.degree. F.) for various
times ranging from about 1/2 min to about 15 min (depending on the
temperature used) to cure the polymer.
Some coatings may be pervious to the cell's hostile environment.
.[.Previous.]. .Iadd.Pervious .Iaddend.coatings are used directly
only on oxidizable metals (e.g., titanium or stainless steel) and
not directly on metals that are susceptible to dissolution in the
fuel cell environment (e.g., aluminum). Pervious coatings could
however be used on dissolvable metal substrates (e.g., Al) which
have first been coated or clad with an oxidizable/passivating metal
layer (e.g., titanium or stainless steel). When pervious coatings
are used on an oxidizable/passivating substrate or coating, oxides
will form at the sites (i.e., micropores) where the coating is
pervious, but not at sites where the polymer engages the substrate
metal. As a result, only a small portion of the surface is
oxidized/passivated (.[.i.e..]. .Iadd.i.e., .Iaddend.at the
micropores in the coating) resulting in very little increase in
electrical resistance attributable to the oxide formation.
According to one embodiment of the invention, the electrically
conductive polymer coating is applied to an acid-dissolvable
substrate metal (e.g., Al) which had previously been coated with a
layer of oxidizable/passivating metal such as stainless steel. In
this regard, a barrier/protective layer 96 of a metal that forms a
low resistance, passivating oxide film is deposited onto the
substrate 98, and is covered with a topcoat of conductive polymer
54 in accordance with the present invention. Stainless steels rich
in chromium (i.e., at least 16% by weight), nickel (i.e., at least
20% by weight), and molybdenum (i.e., at least 3% by weight) are
seen to be excellent such barrier/protective layers 96 as they form
a dense oxide layer at the sites of the micropores in the polymer
coating which inhibits further corrosion, but which does not
significantly increase the fuel cell's internal resistance. One
such stainless steel for this purpose is commercially available
from the Rolled Alloy Company as alloy Al-6XN, and contains
23.+-.2% by weight chromium, 21.+-.2% by weight nickel, and 6.+-.2%
by weight molybdenum. The barrier/protective stainless steel layer
is preferably deposited onto the metal substrate using conventional
physical vapor deposition (PVD) techniques (e.g., sputtering), or
chemical vapor deposition (CVD) techniques known to those skilled
in .[.these.]. .Iadd.the .Iaddend.art. Alternatively, electrolessly
deposited nickel-phosphorous alloys appear to have good potential
as a substitute for the stainless steel in that they readily form a
passivating film when exposed to the fuel cell environment which
provides a barrier to further oxidation/corrosion of the underlying
coating.
While the invention has been described in terms of specific
embodiments thereof it is not intended to be limited thereto but
rather only to the extent set forth hereafter in the claims which
follow.
* * * * *