U.S. patent application number 10/638917 was filed with the patent office on 2005-02-17 for composition and method for surface treatment of oxidized metal.
Invention is credited to Abd Elhamid, Mahmoud H., Mikhail, Youssef.
Application Number | 20050037935 10/638917 |
Document ID | / |
Family ID | 34135768 |
Filed Date | 2005-02-17 |
United States Patent
Application |
20050037935 |
Kind Code |
A1 |
Abd Elhamid, Mahmoud H. ; et
al. |
February 17, 2005 |
Composition and method for surface treatment of oxidized metal
Abstract
The present invention provides an electrically conductive
element for a proton exchange membrane fuel cell having low
electrical contact resistance and high corrosion resistance. The
conductive element comprises a corrosion susceptible metal
substrate with a surface, which is preferably treated to activate
the surface (i.e. to remove a passivation layer of oxides from the
surface) with an acidic treatment solution. The treated surface is
then overlaid with an electrically conductive, corrosion-resistant,
protective coating to protect the substrate re-forming a
passivation layer while exposed to the corrosive environment of the
fuel cell. The present invention also provides methods of preparing
an electrically conductive element to have low electrical contact
resistance and high corrosion resistance.
Inventors: |
Abd Elhamid, Mahmoud H.;
(Grosse Pointe Woods, MI) ; Mikhail, Youssef;
(Sterling Heights, MI) |
Correspondence
Address: |
HARNESS, DICKEY & PIERCE, P.L.C.
P.O. BOX 828
BLOOMFIELD HILLS
MI
48303
US
|
Family ID: |
34135768 |
Appl. No.: |
10/638917 |
Filed: |
August 11, 2003 |
Current U.S.
Class: |
510/175 |
Current CPC
Class: |
H01M 2008/1095 20130101;
Y02E 60/50 20130101; C11D 11/0029 20130101; H01M 8/021 20130101;
C23G 1/081 20130101; H01M 8/0254 20130101; H01M 8/0258 20130101;
C23G 1/086 20130101; C11D 7/10 20130101 |
Class at
Publication: |
510/175 |
International
Class: |
C11D 001/00 |
Claims
What is claimed is:
1. A composition for treating a surface of a metallic substrate
having metal oxide at the surface, said composition comprising: a
solution comprising a solvent and solute, said solution having a pH
equal to or less than about 4, wherein said solute comprises an
anionic species of one or more halogen ions present in said
solution at a concentration of from about 1.times.10.sup.-1 to
about 1.times.10.sup.-6 molarity.
2. The composition according to claim 1, wherein said concentration
of halogen ions in solution is from about 1.times.10.sup.-3 to
about 1.times.10.sup.-5 molarity.
3. The composition according to claim 1, wherein said solvent
comprises water.
4. The composition according to claim 1, wherein said solution
comprises sulfuric acid.
5. The composition according to claim 1, wherein said solution
comprises nitric acid.
6. The composition according to claim 1, wherein said halogen ion
is selected from the group consisting of: F.sup.-, Cl.sup.-,
Br.sup.-, I.sup.-, and mixtures thereof.
7. The composition according to claim 1, wherein said halogen ion
comprises fluorine.
8. The composition according to claim 7, wherein said solution
comprises hydrofluoric acid.
9. The composition according to claim 1, wherein said solution has
a pH of from about 2 to about 4.
10. The composition according to claim 1, wherein said solution
comprises water, sulfuric acid, and fluoride ions at a
concentration of between about 1.times.10.sup.-3 and about
1.times.10.sup.-5.
11. A method of treating a surface of a metallic substrate having
metal oxide at the surface, the method comprising: contacting the
surface with an acidic solution, having a pH of from 0 to about 4,
for reaction with the metal oxide, without an impressed electrical
current, to form a metal halide species soluble in said solution
thereby reducing the amount of the metal oxide; and, separating
said metal halide and said acidic solution from the surface.
12. The method of claim 11, wherein said contacting is conducted at
a temperature of from about 25.degree. C. to about 100.degree.
C.
13. The method of claim 11, wherein said contacting is conducted
for a duration less than 600 seconds.
14. The method of claim 11, wherein said separating comprises
rinsing the surface with a rinse solution, thereby removing said
acidic solution and said metal halide from the surface.
15. A method of treating a surface of a metallic substrate having
metal oxide at the surface, the method comprising: contacting the
surface with an acidic solution having an anionic species of one or
more halogen ions present at a concentration of from about
1.times.10.sup.-1 to about 1.times.10.sup.-6 molarity for reaction
with the metal oxide, without an impressed electrical current, to
form a metal halide species soluble in said solution; and
separating said metal halide and said acidic solution from the
surface.
16. The method of claim 15, wherein said contacting is conducted at
a temperature of from about 25.degree. C. to about 100.degree.
C.
17. The method of claim 15, wherein said contacting is conducted
for a duration less than 600 seconds.
18. The method of claim 15 wherein said separating comprises
rinsing the surface with a rinse solution, thereby removing said
acid solution and said metal halide from the surface.
19. The method of claim 15 wherein said metal oxide provides the
source of said metal in said metal halide and said halogen ions
provide the source of said halide in said metal halide, whereby the
amount of said metal oxide at the surface is lessened.
Description
FIELD OF THE INVENTION
[0001] The present invention relates to composition and method for
surface treatment of oxidized metal and more particularly to such
as applied to preparation of corrosion-resistant electrically
conductive elements.
BACKGROUND OF THE INVENTION
[0002] 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 MEA ("membrane-electrode-assembly") comprising a thin,
solid polymer membrane-electrolyte having an anode on one face and
a cathode on the opposite face. 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. The MEA is
sandwiched between a pair of electrically conductive contact
elements which serve as current collectors for the anode and
cathode, which may contain appropriate channels and openings
therein for distributing the fuel cell's gaseous reactants (i.e.,
H.sub.2 and O.sub.2/air) over the surfaces of the respective anode
and cathode.
[0003] Bipolar PEM fuel cells comprise a plurality of the MEAs
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 current collector. The current
collector or bipolar plate has two working surfaces, 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.
[0004] Contact elements are often constructed from electrically
conductive metallic materials. In an H.sub.2 and O.sub.2/air PEM
fuel cell environment, the bipolar plates and other contact
elements (e.g., end plates) are in constant contact with moderately
acidic solutions (pH 3-5) and operate in a highly oxidizing
environment, being polarized to a maximum of about +1 V (vs. the
normal hydrogen electrode). On the cathode side the contact
elements are exposed to pressurized air, and on the anode side
exposed to atmospheric hydrogen. Unfortunately, many 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 electrically 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. Thus, there is a need
to provide electrically conductive elements that maintain
electrical conductivity, resist the fuel cell aggressive
environment, and improve overall operational efficiency of a fuel
cell.
SUMMARY OF THE INVENTION
[0005] The present invention relates to metallic substrate such as
an electrically conductive element which in one preferred
embodiment comprises an electrically conductive corrosion
susceptible metal substrate having a contact resistance of less
than or equal to about 10 m.OMEGA.-cm.sup.2 under a compressive
force of about 2700 kPa, and an electrically conductive corrosion
resistant coating overlying one or more regions of a surface of the
metal substrate.
[0006] Other preferred embodiments according to the present
invention include methods of treating an oxidized metal substrate
such as when making an electrically conductive element. One method
comprises providing a conductive metal substrate having a surface
susceptible to passivation in the presence of oxygen by forming
oxides. An acidic treatment solution is applied to the surface to
remove the oxides and reduce contact resistance of the surface as
compared to its pre-treated state. The surface is rinsed to remove
substantially all of the treatment solution, and then coated with a
corrosion resistant electrically conductive coating.
[0007] Alternate preferred embodiments of the present invention
contemplate a composition for treating a surface of a metallic
substrate having a metal oxide at the surface. The composition
comprises a solution comprising a solvent and a solute, where the
solution has a pH equal to or less than about 4, and where the
solute comprises an anionic species of one or more halogen ions
present in the solution at a concentration of from about
1.times.10.sup.-1 to about 1.times.10.sup.-6 molar.
[0008] Other alternate embodiments of the present invention include
methods of treating a surface of a metallic substrate having metal
oxide at the surface, the method comprising contacting the surface
with an acidic solution to react with the metal oxide without an
impressed electrical current to form a metal halide species soluble
in the solution where the solution has a pH of from 0 to about 4.
The metal halide is separated from the surface. The surface is
rinsed with a rinse solution, which thus, removes the solution and
the metal halide from the surface.
[0009] Further the present invention contemplates in other
preferred alternate embodiments a method of treating a surface of a
metallic substrate having a metal oxide at the surface by
contacting the surface with an acidic solution to react with the
metal oxide without an impressed electrical current to form a metal
halide species soluble in the solution wherein the solution has an
anionic species of one or more halogen ions present in the solution
at a concentration of from about 1.times.10.sup.-1 to about
1.times.10.sup.-6 molar. The metal halide is separated from the
surface and the surface is rinsed with a rinse solution, thus
removing the solution and the metal halide from the surface.
[0010] Further areas of applicability of the present invention will
become apparent from the detailed description provided hereinafter.
It should be understood that the detailed description and specific
examples, while indicating the preferred embodiment of the
invention, are intended for purposes of illustration only and are
not intended to limit the scope of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The present invention will become more fully understood from
the detailed description and the accompanying drawings,
wherein:
[0012] FIG. 1 is a schematic, exploded, isometric, illustration of
a liquid-cooled PEM fuel cell stack (only two cells shown);
[0013] FIG. 2 is an exploded, isometric view of a bipolar plate
useful with PEM fuel cell stacks like that illustrated in FIG.
1;
[0014] FIG. 3 is a partial cross-sectional view in the direction
3-3 of FIG. 2;
[0015] FIG. 4 is a terminal collector end plate;
[0016] FIG. 5 is a magnified portion of the bipolar plate of FIG.
3;
[0017] FIG. 6 is an exemplary testing apparatus used to measure the
contact resistance of a sample; and
[0018] FIG. 7 is a graph comparing the electrical contact
resistance of 316 L stainless steel samples at various applied
pressure values prepared according to the present invention and by
prior art methods of preparation.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0019] The following description of the preferred embodiment(s) is
merely exemplary in nature and is in no way intended to limit the
invention, its application, or uses.
[0020] The present invention relates to composition and method for
surface treatment of oxidized metal and more particularly to such
as applied to preparation of corrosion-resistant electrically
conductive elements.
[0021] The present invention relates to 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 element confronting at
least one of the electrodes. The conductive element has a working
surface that serves to conduct electrical current from that
electrode. The conductive element comprises a metal substrate with
a surface, which is preferably treated according to the present
invention, to activate the surface, that is, to remove a
passivation or oxide layer from the surface. The treated surface is
then overlaid with an electrically conductive, corrosion-resistant,
protective coating to protect the substrate from the corrosive
environment of the fuel cell. By "corrosion susceptible metal" is
meant that the metal is either oxidized/passivated or dissolved by
the cell's environment.
[0022] To gain a better understanding of the present invention, an
exemplary fuel cell where the present invention may be employed is
shown in FIG. 1, which depicts two individual proton exchange
membrane (PEM) fuel cells connected to form a stack having a pair
of membrane-electrode-assemblies (MEAs) 4, 6 separated from each
other by an electrically conductive, liquid-cooled, bipolar
separator plate conductive element 8. An individual fuel cell,
which is not connected in series within a stack, has a separator
plate 8 with a single electrically active side. In a stack, a
preferred bipolar separator plate 8 typically has two electrically
active sides 20, 21 within the stack, each active side 20, 21
respectively facing a separate MEA 4, 6 with opposite charges that
are separated, hence the so-called "bipolar" plate. As described
herein, the fuel cell stack is described as having conductive
bipolar plates, however the present invention is equally applicable
to stacks having only a single fuel cell.
[0023] The MEAs 4,6 and bipolar plate 8 are stacked together
between stainless steel clamping terminal plates 10,12 and end
contact fluid distribution elements 14,16. The end fluid
distribution elements 14, 16, as well as both working faces or
sides 20,21 of the bipolar plate 8, contain a plurality of lands
adjacent to grooves or channels on the active faces 18, 19, 20, 21,
22, and 23 for distributing fuel and oxidant gases (i.e., H.sub.2
and O.sub.2) to the MEAs 4,6. Nonconductive gaskets or seals 26,
28, 30, 32, 33, and 35 provide seals and electrical insulation
between the several components of the fuel cell stack.
Gas-permeable conductive diffusion media 34, 36, 38, and 40 press
up against the electrode faces of the MEAs 4,6. Additional layers
of conductive media 43, 45 are placed between the end contact fluid
distribution elements 14,16 and the terminal collector plates 10,12
to provide a conductive pathway therebetween when the stack is
compressed during normal operating conditions. The end contact
fluid distribution elements 14,16 press up against the diffusion
media 34,43 and 40,45 respectively.
[0024] 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 41 for both the H.sub.2 and O.sub.2/air
sides of the MEAs is also provided. Additional plumbing 50 is
provided for circulating coolant from a storage area 52 through the
bipolar plate 8 and end plates 14, 16 and out the exit plumbing
54.
[0025] 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 interjacent the
first metal sheet 58 and the second metal sheet 60. The exterior
metal sheets 58,60 are made as thin as possible (e.g., about
0.002-0.02 inches thick), which may be formed by stamping, by photo
etching (i.e., through a photolithographic mask), electroforming,
or any other conventional process for shaping sheet metal. The
external sheet 58 has a first working surface 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
(such as 36 or 38 in FIG. 1) which, in turn, press against the MEAs
(such as 4 or 6 in FIG. 1, respectively). For drafting simplicity,
FIG. 2 depicts only two arrays of lands 64 and grooves 66. In
reality, the lands and grooves 64,66 will cover the entire external
surfaces of the metal sheets 58, 60 that engage the carbon/graphite
papers. 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.
[0026] 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 surface 61 (i.e., coolant
side) of sheet 60 is shown in FIG. 2.
[0027] 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 surface 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 58,60 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 inside surfaces 90, 92 of the
exterior sheets 58, 60 respectively.
[0028] The spacer sheet 62 is positioned between the first sheet 58
and second sheet 60, where the ridges 76 on the first sheet 58 and
the ridges 80 on the second sheet 60 are bonded (e.g. by a bonding
layer 85, such as brazement or adhesives) to the spacer sheet 62.
As recognized by one of skill in the art, the current collectors of
the present invention may vary in design from those described
above, such as for example, in the configuration of flow fields,
placement and number of fluid delivery manifolds, and the coolant
circulation system, however, the function of conductance of
electrical current through the surface and body of the current
collector functions similarly between all designs.
[0029] The present invention is also applicable to other conductive
elements in a fuel cell, such as terminal collector end plates like
the exemplary one shown in FIG. 4. A terminal collector end plate
99 (such as 10 or 12 of FIG. 1) has an electrically non-conductive
region 100, as well as an electrically conductive region 102. The
conductive regions 102 of the terminal plate 99 are typically
separated from the non-conductive region 100 by sealing gaskets
33,35 (FIG. 1). Apertures 104 within the non-conductive region 100
extend through the body, or substrate, 128 of the terminal plate 99
and permit fluid transport (e.g. H.sub.2, O.sub.2, coolant, anode
and cathode effluent) both into and out of the stack during
operating conditions. The particular quantity or sequence of the
apertures 104 is not limiting, and is merely exemplary as described
herein, as numerous configurations are possible as recognized by
one of skill in the art. A bipolar plate flow field design may
dictate the inlet and outlet aperture 104 configurations and fluid
delivery placement. An electrically conductive collector tab 120
can be attached to external leads, facilitating the external
collection of current from the stack.
[0030] Selection of the material of construction for an
electrically conductive element within a fuel cell, such as bipolar
plates or terminal plates, includes weighing such parameters as
overall density (mass and volume), electrical contact resistance of
the substrate measured at the surface, bulk conductivity, and
corrosion and oxidation resistance. Thus, the important
considerations for an electrical element include surface and bulk
intrinsic conductivity of the material to perform as an electrical
current collector, while withstanding the potentially corrosive
conditions experienced within the fuel cell. It is preferred that
current collectors according to the present invention comprise a
conductive metal.
[0031] Metal materials undergo a variety of chemical reactions that
impact functionality within the fuel cell. Passivation, as used
herein, generally refers to a conversion process of treating a
metal to render the surface less chemically reactive and falls
within the process of corrosion, where the metal is attacked by a
corrosive agent. After passivation has occurred in certain metals,
the surface is coated with a protective passivation film that
renders the surface of the plate more electrochemically inert than
its pre-treated state. Such a passivation layer may protect the
underlying metal by making it less prone to corrosion when compared
with the pre-treated metal surface, such as is the case with
stainless steel alloys.
[0032] Although metal surfaces are often intentionally processed to
form the passivation layer, mere exposure to oxygen (or air) also
passivates certain metallic surfaces. While having corrosion
resistance benefits, passivated surfaces in conductive metals also
exhibit high electrical contact resistance values, generally making
such metals having passivated surfaces poor electrical conductors.
Although not limiting to the manner in which the present invention
operates, it is believed that one of the primary reasons for poor
electrical surface conductivity after passivation of a metal
surface is due to the formation of metal oxides in a passivation
layer. Activation, in contrast to passivation, is the conversion
process of making a surface relatively more active. Thus,
activation often entails removing or thinning the passivation layer
from a surface, by subjecting the surface to reducing conditions,
which renders it more electrochemically active and hence reduces
electrical contact resistance.
[0033] Stainless steel is generally defined as an iron-chromium
alloy with a minimum of 9% chromium. Other ferritic, martensitic,
or austenitic alloys are contemplated for use in PEM fuel cells.
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 particularly desirable metals for use
within a fuel cell, due to their relatively high bulk electrical
conductivity and corrosion resistance provided by a dense
passivation (i.e., metal oxide) layer at the surface. Thin
stainless steel plates can be used to decrease the volumetric and
weight power density of the fuel cell stack. Further, stainless
steel materials have relatively high strength, physical durability,
adherence to protective coatings, and are less expensive than many
other conductive metal alternatives. However, the oxide layer at
the surface impermissibly increases electrical contact resistance
of the substrate, which has previously prevented its independent
use as an electrical contact element or current collector. Further
many other relatively lightweight metals are susceptible to
corrosive attack (e.g. aluminum and titanium), and in light of such
corrosion sensitivity and similar propensity for oxidation, various
protective coatings are used for the metal substrate. Often such
protective coatings increase the electrical resistance of the metal
plate to unacceptable levels or are very costly, such as with gold
or platinum coatings. Thus, there is a trade-off between
conductivity and corrosion protection. It is an object of the
present invention to reduce the overall electrical contact
resistance of an electrically conductive element, while providing
corrosion and oxidation resistance for the underlying metal
substrate of the conductive element.
[0034] Previous methods to overcome such high electrical contact
resistance include treating a conductive metal substrate to clean
the surface of a passivation layer (e.g. metal oxides), and then
coating with a prophylactic coating. Current cleaning methods often
employ cathodic cleaning where electrical current (e.g. a current
density of 4 A/cm.sup.2) is impressed onto the conductive substrate
which is in contact with an electrolyte to facilitate the
generation of gas bubbles at the surface, such as cathodic cleaning
described in ASTM B254 7.4.1, for example. Such cathodic cleaning
generally takes a minimum processing time of 10 minutes (typically
much longer) to effectively clean the metal substrate, and is not
generally suitable for high production coil coating processes.
Reducing processing time is an important consideration when using
continuous manufacturing methods for creating electrically
conductive elements or other components within an electrochemical
cell.
[0035] Other cleaning methods include mechanical abrasion of the
surface, or cleaning the substrate with commercially available
alkaline cleaners, acidic solvents, or pickle liquors. Other
methods of treating a conductive metal substrate include
pre-sputtering the surface of the metal in a physical vapor
deposition (PVD) chamber, and radio-frequency glow-discharge
treatment. The above described methods of removing the passivation
layer from the metal are not thought to sufficiently reduce contact
resistance to an acceptable value and simultaneously reduce
processing time, to allow for the use of metals susceptible to
passivation as current collector elements.
[0036] According to one aspect of the present invention, an
electrically conductive element or current collector made of a
corrosion-susceptible metal is treated to impart a low contact
resistance and the ability to withstand corrosion and oxidative
attack, while minimizing the amount of substrate metal lost during
the activation process. Such a treatment permits the use of metals,
such as stainless steel, which previously had too high of an
electrical contact resistance for practical use in a fuel cell.
[0037] Corrosion-susceptible metal substrates treated according to
the present invention have significantly reduced contact resistance
values as compared with their pre-treated state. In one preferred
embodiment of the present invention, a surface of the electrically
conductive element is activated by treatment with an acidic
solution having a composition such that metal oxides in a
passivation layer at the surface are soluble therein, and thus are
removed from the surface of the metal substrate. The acid solution
is designed to react with and convert metal oxides to soluble
constituents that may be readily removed from the surface. In
selecting the preferred composition of the acid solution, the
thermodynamics for various constituents in the metal substrate are
evaluated over a range of pH values. Thus, for a preferred
embodiment, the metal substrate is stainless steel, such as for
example 316L (UNS S31603), an alloy which contains iron, chromium,
nickel, and molybdenum. By evaluating the thermodynamics of each
constituent metal oxide, it becomes possible to select a desirable
pH range where the metal oxide phases might form unstable species.
While evaluating such thermodynamic data, certain preferable anions
have been discovered which alter the constituent metal oxide phases
at preferred conditions, thus creating soluble metal species that
react with the anions, thus dissolving the metal oxide phases by
ionic interaction at the metal surface.
[0038] In preferred embodiments of the present invention, the pH of
the acidic solution is less than 7, preferably between 0 and about
4, and most preferably between about 2 to about 4. The acidic
solution comprises a solvent and a solute. The solute preferably
comprises at least one acidic anion and a corresponding conjugate
cation. As recognized by one of skill in the art, the selection of
anion(s)/cation(s) for the acidic solution effects the pH (due to
the PKa value of the anion and cation). Thus, anions that provide
the requisite pH, while also enabling formation of soluble metal
species from the metal oxides are preferred for use with the
present invention.
[0039] Preferred anions according to the present invention are
halogen ions, including fluoride (F.sup.-), chloride (Cl.sup.-),
bromide (Br.sup.-), iodide (I.sup.-), as well as sulfate
(SO.sub.4.sup.2-), nitrate (NO.sub.3.sup.-), and mixtures thereof.
The most preferred anions according to the present invention are
fluoride (F.sup.-) and sulfate (SO.sub.4.sup.2-). The preferred
cations comprise hydrogen protons. Preferred solvents in the acidic
solution according to the present invention comprise water. Thus,
preferred solutes (with both anions and cations) according to the
present invention include halides, such as hydrofluoric acid,
hydrochloric acid, hydrobromic acid, hydriodic acid, as well as,
sulfuric acid, nitric acid, and mixtures thereof.
[0040] As an example, when the electrically conductive element is
selected to be 316L stainless steel (a corrosion-susceptible metal
substrate), the nickel and ferric oxides have limited domains of
stability across the range of all pH values, and will react to form
other species at low pH values and high potentials. However, the
chromium oxide species are stable and exist even at very low pHs.
It has been found that when a fluoride anion is incorporated at
concentrations of greater than 1.times.10.sup.-6 into an acidic
treatment solution (preferably in a solution also comprising
sulfuric acid that has a pH equal to or below 4) the chromic oxides
will form chromium fluoride. Chromium fluoride is soluble in the
acidic treatment solution. The metal oxides are thus transformed by
reaction with the anions to metal compounds that are soluble in the
solvent at the appropriate pH conditions, such as metal halides
when the anion is a halogen.
[0041] In one preferred embodiment of the present invention, where
the halogen ion in the solute is fluoride, the preferred
concentration of fluoride anions in the acidic solution of the
present invention is from about 1.times.10.sup.-1 to about
1.times.10.sup.-6 molarity. This concentration is selected to
optimize the pH of the solution and the amount of anions needed to
achieve the necessary reaction with metal oxides at the surface.
From empirical observation, the greater the concentration of
preferred anions (i.e., closer to 1.times.10.sup.-1), the higher
the pH value may be while still achieving the necessary reduction
in metal oxides at the surface. Likewise, it has been observed that
the less the concentration of preferred anions (i.e., closer to
1.times.10.sup.-6) in the acid treatment solution, the lower the pH
value must be. It should be noted that other solutes may be present
to achieve the necessary low pH value, but may not contain anions
according to the present invention that react with metal oxides to
create the desirable soluble metal constituent at the surface of
the metal substrate. In certain preferred embodiments of the
present invention, the concentration of the halide is from between
about 1.times.10.sup.-3 and about 1.times.10.sup.-5 molarity in the
acidic treatment solution, which has a pH of between about 2 and
about 4.
[0042] Preferred temperature conditions for applying the acid
treatment solution to the metal surface range from about 25.degree.
C. (room temperature) to about 100.degree. C. at ambient
atmospheric pressure. The acid treatment solutions of the present
invention are not only highly effective in surface activation, but
also significantly reduce the treatment time to achieve the
activation. Shorter processing times prevent excessive and
unnecessary removal of the underlying substrate metal, and further
makes the present invention well suited for continuous
manufacturing processes. The preferred duration for exposing the
metal surface to the acid solution ranges from approximately 3
seconds up to about 600 seconds (10 minutes). The most preferred
duration of treatment is between about 3 to about 120 seconds (2
minutes). As appreciated by one of skill in the art, the processing
time is dependent on a variety of factors, including the
aggressiveness of the acidic treatment solution, the character and
extent of the passivation layer on the metal, and the relative
amount of surface area to be activated. The present invention is
readily adaptable to continuous manufacturing processes, which
provides increased productivity by requiring less processing time,
while enhancing the overall quality of surface activation.
[0043] Methods of treating the metal substrate according to
preferred embodiments of the present invention comprise providing a
conductive metal substrate with a surface susceptible to
passivation in the presence of oxygen (i.e.,
corrosion-susceptible). The acidic treatment solution is applied to
the conductive metal substrate. Such application of treatment
solution may include submerging the metal substrate in a container
filled with acidic treatment solution for the requisite length of
time necessary to remove the metal oxides. In alternate preferred
embodiments, the application of treatment solution may include
spraying the conductive metal substrate with acidic treatment
solution. The surface of the metal may also be sprayed and
submerged, either simultaneously or in successive processing steps.
The application of the acidic treatment solution ensures that the
passivation layer of metal oxides is removed from the surface of
the substrate. The removal of the passivation layer provides
reduced electrical contact resistance of the substrate when
compared to the pre-treated state of the substrate.
[0044] After the acidic treatment solution has been applied, the
metal oxides are transformed by ionic interaction with the
preferred anions of the acidic treatment solution to form metal
species soluble in the solvent of the acidic treatment solution.
One preferred solvent in the acidic treatment solution according to
the present invention is water. Thus, the preferred anions
preferably create a metal anion species that is hydrophilic and
ionic, which is soluble in water. Hence, during the application
process, the soluble metal species is likewise dissolved in solvent
and removed from the metal surface and dispersed into the acidic
treatment solution.
[0045] The surface of the metal substrate is preferably rinsed
after applying the treatment solution. The rinsing of the metal
substrate surface removes substantially all of any residual acidic
treatment solution remaining on the metal substrate. By
"substantially all" it is meant that a large portion of the acidic
treatment solution is removed, so that the metal surface is not
detrimentally impacted by the presence of residual acidic treatment
solution. Long term exposure to residual acidic treatment solution
may cause physical deformities in the metal surface, such as
pitting or embrittlement, for example, or may impede the efficacy
of subsequent processing. Thus, it is preferred that the metal
surface is rinsed to remove most, or substantially all, of the
residual acidic treatment solution.
[0046] The rinsing is preferably conducted with a similar solvent
as that used in the acidic treatment solution, which is preferably
deionized water. After exposure to a low pH acid (i.e., the acidic
treatment solution) during application, it is preferred that a
first rinse solution is mildly acidic rather than neutral (i.e.,
has a pH of less than 7 and greater than about 4) to prevent
shocking of the metal surface, which could cause precipitates to
form. In certain preferred embodiments, use of the first rinse
solution is sufficient to remove substantially all of any residual
acidic treatment solution on the surface. The first rinse solution
may be used for multiple rinse sequences, if necessary. The first
rinse solution preferably comprises deionized water and a mild and
inexpensive acid solute, such as acetic acid, carbonic acid, and
the like, or very low concentrations of more aggressive acids, such
as sulfuric acid.
[0047] In alternate preferred embodiments, an additional sequential
rinsing step may be employed with a second rinse solution. The
second rinse solution is preferably neutral with a pH of
approximately 7, which can be used after exposure to the first
mildly acidic rinse, so that there is no danger of shocking the
metal surface. A second neutral rinse solution preferably comprises
deionized water. Each of the respective first and second rinse
solutions may be used multiple times to rinse the surface of the
metal substrate, if necessary, to remove substantially all of the
acidic treatment solution.
[0048] Thus, the treatment of the surface of the metal substrate
with an acidic solution, activates the surface by transforming
metal oxides to soluble metal anion species. The rinsing of the
acidic solution and metal anion species from the surface according
to the present invention, further ensures removal of the
passivation layer. The present invention provides electrically
conductive elements where the contact resistance of the metal is
drastically reduced from its pre-treated state to a level such that
metal prone to forming electrically insulating passivation layers
of metal oxide, such as stainless steel, may be activated to the
extent that they may be employed as a metal substrate for an
electrically conductive element in a fuel cell.
[0049] In certain preferred alternate embodiments of the present
invention, the metal substrate is pre-cleaned prior to applying the
acidic treatment solution. Such cleaning typically serves to remove
any loosely adhered contaminants, such as oils, grease, waxy
solids, particles (including metallic particles, carbon particles,
dust, and dirt), silica, scale, and mixtures thereof. Many
contaminants are added during the manufacturing of the metal
material, and may also accumulate on the surface during transport
or storage. Thus, pre-cleaning is preferred in circumstances where
the metal substrate provided for processing is soiled with
contaminants. Pre-cleaning may entail mechanical abrasion; cleaning
with traditional alkaline cleaners, surfactants, mild acid washes;
or ultrasonic cleaning. The choice of the appropriate cleaning
process or sequence of cleaning processes is selected based upon
both the nature of the contaminant and the metal.
[0050] Mechanical cleaning or polishing may include abrading the
surface with a pad or roller comprising abrasive particles.
Examples of abrasive pads may include those having silicon carbide
or aluminum oxide dispersed on a nylon matrix, or paper with sand
or diamond particles, for example. A commercially available
abrasive pad having SiC on a nylon matrix is Scotch-Brite.RTM.
manufactured by 3M corporation of St. Paul, Minn. Other methods of
mechanically cleaning may include abrasive blasting of the surface,
which can be done with plastic (polymer) blast beads, walnut
shells, sand, or glass particles. Other mechanical cleaning or
polishing methods known to those of skill in the art may also be
used.
[0051] Alkaline cleaners are preferred for pre-cleaning, especially
to remove organic oils and materials from the surface of the metal.
Preferred alkaline cleaners have a pH in the range of about 9 to
14. The alkaline cleaners preferred for the present invention
generally comprise a builder composition and a surfactant to
facilitate emulsification of organic compounds. Other additives may
also be included in an alkaline cleaner, including organic or
inorganic additives, chelating agents, and sequestrants which
enhance cleaning of the surface. Alkaline cleaners are typically
applied by immersion or spray. The cleaning step is usually
followed by a rinse with deionized water, prior to treating with
the acidic treatment solution.
[0052] Alkaline cleaners may operate by three mechanisms, including
saponification, emulsificiation, or dispersion. Often all three
mechanisms are employed in one particular alkaline cleaner. The
builder composition is generally the alkaline salt, and may
comprise blends of alkaline salts, usually selected from the group
consisting of: alkali metal orthophosphates, alkali metal condensed
phosphates, alkali metal hydroxides, alkali metal silicates, alkali
metal carbonates, alkali metal bicarbonates, and alkali metal
borates. Preferred alkali metals for the builder composition are
sodium and potassium. Surfactants are preferably organic compounds
that provide detergency, emulsification and wetting in an alkaline
cleaner, and are well known in the art. In one preferred alkaline
cleaner, the builder constituents are potassium hydroxide and
tetrapotassium pyrophosphate, present in a water solvent from about
10-30 wt. % and 1-10 wt. %, respectively. Such an alkaline cleaner
is commercially available as PARCO.RTM. Clean 1200 from Henkel
Surface Technologies of Madison Heights, Mich. Other surface
cleaning methods may include ultrasonic cleaning (generally coupled
with either an alkaline or acidic cleaner) where electrical energy
is converted by transducers to ultrasonic waves; corona discharge
or radio-frequency glow-discharge, both of which treat the target
surface to an electrical discharge (i.e., corona) which disperses
reactive oxygen or other gas molecules to activate the surface. All
of the above cleaning methods are suitable for the pre-cleaning
process.
[0053] As previously discussed, the acidic treatment solution
application activates the surface of an electrically conductive
element substrate by removing the passivation layer (metal oxides).
However, although the underlying metal substrate is restored to its
pre-passivated state with an activated surface, the metal substrate
still remains susceptible to new corrosion where a new passivation
layer comprising metal oxides may re-form at the surface. The
passivation layer will form upon exposure to a corrosion agent,
such as oxygen, present in the ambient atmosphere and within the
fuel cell. Thus, removing the passivation layer promotes electrical
conductivity by reducing overall contact resistance. However, if
the metal substrate is exposed to oxygen, the passivation layer
will re-form at the surface, counteracting the activation
process.
[0054] Hence, after the passivation layer is removed, the
electrically conductive element is preferably protected from
further corrosion/passivation by physically protecting it from
corrosive agents to prevent the re-formation of a passivation layer
at the surface of the metal substrate. As shown in FIG. 5, the
electrically conductive element (e.g. bipolar plate 56) has a
protective coating 130 overlaying the corrosion susceptible metal
substrate 132 on both the first surface 59 of the first sheet 58
and the second surface 63 of the second sheet 60. Further, the
protective coating 130 is applied to the interior first surface 90
of the first sheet 58 and second surface 92 of the second sheet 60,
to protect the substrate 132 from corrosive oxidative attack by the
coolant. From a practical point of view, it is not necessary to
coat the interior or coolant passages of the bipolar plate in the
case of stainless steel or titanium applications. The protective
coating 130 is a practical way to isolate the electrically
conductive element from corrosive agents that are prevalent both in
the processing environment and in the fuel cell itself. Thus, it is
preferred that the protective coating 130 is applied to the
conductive and corrosion-susceptible regions (e.g. surfaces 59, 60,
90, 92) of the electrically conductive element 56, such that the
underlying metal substrate 132 is protected from corrosive agents
that would react with the metal to electrically
inactivate/passivate each surface. Thus, select regions may include
only the electrically conductive areas that form conductive
pathways across the electrically conductive element or such regions
may coincide to the entire surface of the substrate.
[0055] According to preferred embodiments of the present invention,
the protective coating 130 is applied within a short duration of
time after the metal substrate 132 is removed from the rinsing
process, to minimize the re-formation of metal oxides in a
passivation layer at the surface. Thus, it is preferred that the
protective coating 130 is applied within a period of two hours, and
most preferable that the protective coating 130 is applied to the
metal substrate 132 surface in 30 minutes or less. A short time
between the treatment with the acidic solution and the application
of the protective coating 130 minimizes the exposure to oxygen to
reduce the amount of metal oxides or thickness of a passivation
layer forming at the surface. As appreciated by one of skill in the
art, various processing conditions and material characteristics may
impact the rate of passivation at the surface, and hence may impact
how quickly the protective coating 130 should be applied to the
metal surface. Ideal processing times may be discerned based on
material characteristics, physical testing, and other empirical
results.
[0056] The protective coatings 130 of the present invention are
preferably corrosion resistant electrically conductive coatings,
that protect the underlying metal substrate 132 from exposure to
corrosion agents. More specifically, the protective coatings 130
preferably have a resistance less than about 50 ohm-cm.sup.2
(.OMEGA.-cm.sup.2) and comprise a plurality of oxidation-resistant,
acid-insoluble, conductive particles (i.e., on the order of about
50 microns or less than about 50 microns) dispersed throughout an
acid-resistant, oxidation-resistant polymer matrix, where the
polymer binds the particles together and holds them on the surface
of the metal substrate 132. The coating contains sufficient
conductive filler particles to produce a total resistance no
greater than about 50 ohm-cm.sup.2 and has a thickness between
about 2 microns and about 75 microns, preferably between 2 and 30
microns, depending on the composition, resistivity and integrity of
the coating. Thinner coatings (i.e., about 15-25 microns) are most
preferred for minimizing the IR drop through the stack. Impervious
protective coatings 130 are preferred for the present invention to
protect the underlying metal substrate 132 surface from permeation
of corrosive agents.
[0057] 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 palladium, niobium, rhodium, rare
earth metals, and other noble 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, polyesters,
phenoxy-phenolics, epoxide-phenolics, acrylics, and urethanes,
inter alia are seen to be useful with the present invention. Both
thermoset and thermoplastic polymers are suitable for producing
impermeable coatings.
[0058] In accordance with one embodiment of the invention, the
conductive polymer coating 130 is applied directly to the substrate
metal 132 and allowed to dry/cure thereon. The coating 130 may be
applied in a variety of ways, and examples of such methods are
described in detail in U.S. Pat. No. 6,372,376 to Fronk et al. and
may include (1) electrophoretic deposition, (2) brushing, spraying
or spreading, or (3) laminating. The present invention is adaptable
for use in continuous manufacturing process such as coil coating.
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 132 having complex and recessed surfaces
like those used to form the reactant flow fields on the working
surface(s) of the conductive elements. Electrophoretic deposition
is a well-known process used to apply polymers to conductive
substrates. When cross-linkable polymers are used, the suspension
also includes a catalyst for promoting the cross-linking.
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.
Cathodic epoxies, acrylics, urethanes and polyesters are useful
with this method of depositing the coating. Other examples of
suitable polymers include thermoset and thermoplastic resins, such
as those disclosed in the U.S. Pat. No. 6,372,376 to Fronk, et al.
and the references cited therein. Subsequent baking of the coated
conductive element cures and densifies the coating.
[0059] According to another embodiment of the invention, the
coating 130 is first formed as a discrete film (e.g. by solvent
casting, extrusion etc.), and then laminated onto the working
surface of the conductive element, e.g., by hot rolling. This
technique will preferably be used to make laminated sheet stock
from which the conductive 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 132 that is supple
enough so that it can be readily shaped, (e.g. stamped) without
tearing or disrupting the film when the conductive element is
formed as by stamping. To ensure adherence of the coating 130 to
the substrate 132, the surface of the substrate 132 to which the
film is applied should be pre-cleaned as described above including
removing all undesirable surface films (e.g., oil). It is also
preferred that the surface is treated with a conversion coating,
roughened, or abraded to roughen the surface for anchoring the
polymer film thereto. Fluoropolymers such as polyvinylidene
difluoride and a fluoroelastomer, an FPM fluoroelastomer, such as
Viton vinylidene fluoride hexafluoropropylene copolymer from DuPont
Dow are exemplary, or the like are useful with this embodiment, and
may be used with conventional plasticizers such as dibutyl
phthalate.
[0060] According to another embodiment of the invention, the
electrically conductive polymer film 130 is applied to the surface
of the metal substrate 132 by spraying, brushing or spreading (e.g.
with a doctor blade). In this embodiment, a precursor of the
coating 130 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 132. 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 surface by means of the solvent-free polymer.
[0061] 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.
[0062] The mix is applied to the substrate 132, 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., <200.degree. C.), or
more quickly in a two step process wherein the solvent is first
removed by heating for ten minutes at about 150.degree. C. to
175.degree. C. (i.e., dried) followed by higher temperature heating
(250.degree. C. to 400.degree. C.) for various times ranging from
about 30 seconds to about 900 seconds (15 min), the time being
dependent on the temperature used to cure the polymer.
EXAMPLE 1
[0063] An electrically conductive element according to a preferred
embodiment of the present invention is prepared by providing
stainless steel strips samples measuring approximately 31
cm.times.18 cm.times.0.02 cm. The stainless steel is Type 316L,
cold rolled, bright annealed, strip, UNS S31603 in accordance with
ASTM A480 from Arcelor, S.A. located in Grand-Duch, Luxembourg. The
steel substrate is pre-cleaned by spraying an alkaline cleaning
solution of PARCO.RTM. Clean 1200 (an alkaline cleaner with a
builder constituent comprising potassium hydroxide and
tetrapotassium pyrophosphate) at 5 wt % and 95 wt % deionized water
for 10 seconds at 71.degree. C. A deionized water rinse is sprayed
onto the surface to remove any residual alkaline cleaning
solution.
[0064] An acidic treatment solution is prepared by admixing 3 wt %
sulfuric acid solution; 97 wt % deionized water solvent; and 284
ppm of HF. The sulfuric acid can be purchased as Ridolene.RTM. 123,
available from Henkel Surface Technologies of Madison Heights,
Mich., which has a sulfuric acid concentration range of between 30
to 60 wt %, as well as a surfactant. Ridolene.RTM. 123 also
contains up to about 10% surfactant. The hydrofluoric acid can be
purchased from VWR Scientific Products Inc. The acidic treatment
solution is sprayed on the steel substrate surface for 10 seconds
at 66.degree. C. to activate the surface and remove metal oxides.
The steel substrate is then sprayed for about 10 seconds at about
66.degree. C. with a first rinse solution comprising 1 wt %
H.sub.2SO.sub.4 and 99 wt % deionized water. The steel substrate is
further sprayed for about 10 seconds at about 66.degree. C. with a
second rinse solution comprising 100 wt % deionized water. The
surface of the steel substrate is then dried by a pair of squeegee
rollers at 26.degree. C., such as 30-90 Shore A Durometer elastomer
such as ethylene propylene rubber or polychloroprene.
[0065] An electrically-conductive protective polymer coating matrix
supplied by Acheson Colloids Company, Port Huron, Mich. as
Electrodag.RTM. EB-008 is applied to the stainless steel substrate
by spray coating at room temperature (26.degree. C.). The stainless
steel substrate having a spray coating of polymer matrix is placed
in an oven at 150.degree. C. for 10 minutes to vaporize the
solvents and is then placed in an oven at 260.degree. C. for 30
minutes to cure the polymeric matrix.
EXAMPLE 2
[0066] An electrically conductive element prepared according to an
alternate preferred embodiment of the present invention is prepared
by providing the same stainless steel strips samples as those in
Example 1. The steel substrate is pre-cleaned with the same
alkaline cleaning solution and rinse solution as in Example 1.
[0067] An acidic treatment solution is prepared by admixing 3 wt %
sulfuric acid solution; 97 wt % deionized water solvent; and 568
ppm of HF. The sulfuric acid is provided as Ridolene.RTM. 123, as
used in Example 1. The hydrofluoric acid can be purchased from VWR
Scientific Products Inc. The acidic treatment solution is sprayed
on the steel substrate surface for 10 seconds at 66.degree. C. to
activate the surface and remove metal oxides. The steel substrate
is then rinsed, dried, and coated in the same manner as the
conductive element described in Example 1.
EXAMPLE 3
[0068] An electrically conductive element prepared according to
another alternate preferred embodiment of the present invention is
prepared by providing the same stainless steel strips samples as
those described in Example 1. The steel substrate is pre-cleaned at
the same conditions with the same alkaline cleaning solution and
rinse solution as in Example 1.
[0069] An acidic treatment solution is prepared by admixing 3 wt %
sulfuric acid solution and 97 wt % deionized water solvent. The
sulfuric acid is provided as Ridolene.RTM. 123, as used in Example
1. The acidic treatment solution is sprayed on the steel substrate
surface for 10 seconds at 66.degree. C. to activate the surface and
remove metal oxides. The steel substrate is then rinsed, dried, and
coated in the same manner as the conductive element described in
Example 1.
[0070] Immediately after the application of acidic treatment
solution and before the protective coating in Examples 1, 2, and 3,
the contact resistance measurements of the activated stainless
steel samples were measured via a testing apparatus as shown in
FIG. 6. The testing apparatus comprises a carver press 200 with
gold coated platens 202 and a first and second electrically
conductive activated carbon paper media 204,206 respectively,
pressed between a sample 208 and the gold coated platens 202. A
surface area of 49 cm.sup.2 was tested using 50 A/cm.sup.2 current
which is applied by a direct current supply. The resistance is
measured using a four-point method and calculated from measured
voltage drops and from known applied currents and sample 208
dimensions. The voltage drop is measured across either two
diffusion media 204,206 through the sample 208 (total contact
resistance) or across two points on the sample 208 surface 209
(bulk contact resistance). The sample 208 may comprise a single
stainless steel plate or a bipolar plate having two plates 210
joined (e.g. brazed) together, which is shown in FIG. 6. In the
circumstance where the bipolar plate is tested, the bulk contact
resistance is measured from a midpoint 212 between the two plates
210 to an exterior 214 of either plate 210, to establish a value
for a single plate 210. Contact resistance measurements were
measured as milli-Ohm square centimeter (m.OMEGA.-cm.sup.2) with
incremental force applied at the following pressures: 200 p.s.i.
(1400 kPa), 300 p.s.i. (2000 kPa), 400 p.s.i. (2700 kPa).
[0071] It should be noted that the contact resistance of the
conductive carbon paper 204,206 is generally a known value, which
can be subtracted from the measurement to establish the contact
resistance of the metal plate 210 only. During testing of the
samples, a 1 mm thick Toray carbon paper (commercially available
from Toray as TGP-H-0.1T) was used for the first and second carbon
paper media 204,206. However, in many circumstances the contact
resistance of the conductive paper 204,206 is negligible and adds
such a small incremental value to the contact resistance value,
that it need not be subtracted. The values referred to herein are
for the interfacial electrical contact resistance of the sample 208
only, and reflect contact resistance values for the metal plate 210
alone.
[0072] Results of such testing are shown in FIG. 7 and Table 1.
FIG. 7 shows the contact resistance for the sample in Example 1, as
well as Control 1, which is an untreated sample of the stainless
steel strip as described above in the Examples, as received from
the manufacturer. FIG. 7 also shows Control 2, which is a similar
sample of 316L stainless steel, however it was treated with only
the known method of cleaning with an alkaline cleaner, PARCO.RTM.
1200 for 10 seconds at 66.degree. C. The contact resistance
measurements were taken for both the upper first paper 204 through
the sample 208 and the second lower paper 206 through the sample
208, with both values averaged to provide the resulting contact
resistance values indicated below in Table 1 as average bulk
contact resistance values in m.OMEGA.-cm.sup.2 for Examples 1-3 and
Controls 1-2.
1TABLE 1 Applied Pressure Control 1 Control 2 Example 1 Example 2
Example 3 200 p.s.i 637.0 399.4 4.5 5.2 5.9 (1400 kPa) 300 p.s.i.
499.8 277.3 3.7 4.4 4.9 (2000 kPa) 400 p.s.i. 377.8 158.3 3.4 4.2
4.6 (2700 kPa)
[0073] As can be observed from FIG. 7 and Table 1, the samples that
were treated according to the present invention have a
significantly reduced (2 orders of magnitude less) contact
resistance. Although the prior art cleaning with an alkaline
cleaner shows a reduction in contact resistance, the contact
resistance still remains impermissibly high for use as an
electrically conductive element. Further, it should be noted that
the treatment times with acidic treatment solution are 10 seconds
in Examples 1, 2, and 3, and achieve contact resistance values
within a desirable range.
[0074] Although, the description of the present invention in the
Examples above is merely exemplary, it is preferred for conductive
elements according to the present invention that the contact
resistance for the electrically conductive substrate is less than
about 10 m.OMEGA.-cm.sup.2 under a compressive force. Based on the
trends exhibited here for contact resistance as a function of
compressive force, even a contact resistance corresponds to a
contact resistance measurement with low compressive force
corresponding to firm, forced engagement (i.e., up to 200 psi)
according to the present invention is better than that of the
controls under significant compressive force (i.e., 300 to 400
psi). Thus, in preferred embodiments of the present invention,
where the compressive force is applied at 400 psi applied pressure
(i.e., 2700 kPa) as measured across the conductive element sample
and one piece of conductive paper, is less than 10
m.OMEGA.-cm.sup.2, and most preferably less than 5
m.OMEGA.-cm.sup.2. When the protective coating is applied over the
treated surface, it is preferred that the overall contact
resistance, through the conductive protective coating, and treated
surface of the metal substrate is less than 30 m.OMEGA.-cm.sup.2
under a compressive force applied at a pressure of about 400 psi.
(2700 kPa) and most preferably below total contact resistance of
less than about 5 m.OMEGA.-cm.sup.2. Thus, the present invention
provides a method and composition for treating an electrically
conductive element for a fuel cell, that reduces the metal
substrate contact resistance to levels that permit the use of
previously unavailable metals in a shorter duration than was
previously feasible.
[0075] The description of the invention is merely exemplary in
nature and, thus, variations that do not depart from the gist of
the invention are intended to be within the scope of the invention.
Such variations are not to be regarded as a departure from the
spirit and scope of the invention.
* * * * *