U.S. patent application number 10/716396 was filed with the patent office on 2004-09-02 for fuel cell separator and production method therefor.
Invention is credited to Nakata, Hiromichi.
Application Number | 20040170881 10/716396 |
Document ID | / |
Family ID | 32376173 |
Filed Date | 2004-09-02 |
United States Patent
Application |
20040170881 |
Kind Code |
A1 |
Nakata, Hiromichi |
September 2, 2004 |
Fuel cell separator and production method therefor
Abstract
A low-cost fuel cell separator having a metallic substrate which
is able to stably maintain low electric resistance (high electrical
conductivity) and high corrosion resistance for a long period is
provided. The separator has a metallic substrate having an oxide
film forming a surface thereof and made from an oxidization of a
metal of the substrate, and an electrically conductive thin film
formed on a surface of the oxide film of the substrate. Due to this
construction, low electric resistance (high electrical
conductivity) is achieved by the electrically conductive thin film.
Furthermore, even if the electrically conductive thin film has
pinholes, the oxide film substantially prevents or reduces elution
from the separator substrate, thereby achieving high corrosion
resistance. Still further, since the oxide film is formed by
oxidation of the substrate, the oxide film can be formed at a lower
cost than an oxide film formed from a different metal.
Inventors: |
Nakata, Hiromichi;
(Toyota-shi, JP) |
Correspondence
Address: |
KENYON & KENYON
1500 K STREET, N.W., SUITE 700
WASHINGTON
DC
20005
US
|
Family ID: |
32376173 |
Appl. No.: |
10/716396 |
Filed: |
November 20, 2003 |
Current U.S.
Class: |
429/520 ;
427/115; 427/122; 428/469; 429/517; 429/521; 429/522; 429/535 |
Current CPC
Class: |
H01M 8/0215 20130101;
H01M 8/0213 20130101; H01M 8/0206 20130101; Y02E 60/50 20130101;
H01M 8/0228 20130101; Y02P 70/50 20151101; H01M 8/0204
20130101 |
Class at
Publication: |
429/034 ;
427/115; 427/122; 428/469 |
International
Class: |
H01M 008/02; B05D
005/12; B32B 015/04 |
Foreign Application Data
Date |
Code |
Application Number |
Apr 12, 2002 |
JP |
2002-351933 |
Claims
What is claimed is:
1. A fuel cell separator comprising: a metallic substrate having an
oxide film forming a surface thereof and made by an oxidization of
a material of the substrate; and an electrically conductive thin
film formed on a surface of the oxide film of the substrate.
2. The separator according to claim 1, wherein the oxide film is
formed by placing the substrate in air or in an oxidizing
atmosphere.
3. The separator according to claim 1, wherein the electrically
conductive thin film is a metal thin film.
4. The separator according to claim 1, wherein the electrically
conductive thin film is a noble metal thin film.
5. The separator according to claim 1, wherein the electrically
conductive thin film is a carbon thin film formed of carbon (C) at
an atomic level.
6. The separator according to claim 1, further comprising an
intermediate layer for enhancing adhesion which is provided between
the oxide film of the substrate and the electrically conductive
thin film.
7. The separator according to claim 6, wherein the electrically
conductive thin film is a metal thin film, and wherein the
intermediate layer is an Me layer formed of at least one element
selected from the group consisting of the metal elements of Ti, Zr,
Hf, V, Nb, Ta, Cr, Mo and W and the metalloid elements of Si and
B.
8. The separator according to claim 6, wherein the electrically
conductive thin film is a carbon thin film formed of carbon (C) at
an atomic level, and wherein the intermediate layer is formed by at
least one layer of an Me layer formed of at least one element
selected from the group consisting of the metal elements of Ti, Zr,
Hf, V, Nb, Ta, Cr, Mo and W and the metalloid elements of Si and B,
and a carbon-Me gradient layer which is formed on the Me layer and
which contains carbon (C) and an a metal or metalloid element (Me)
and in which a proportion of carbon (C) increases with increasing
distance from the substrate.
9. The separator according to claim 1, further comprising a carbon
coating film on a surface of the electrically conductive thin
film.
10. A method of producing a fuel cell separator comprising:
providing a substrate; providing an oxide film forming a surface of
the substrate by oxidizing a material of the substrate; and forming
an electrically conductive thin film on a surface of the oxide film
of the substrate.
11. The method according to claim 10, wherein providing the oxide
film comprises placing the substrate in air or in an oxidizing
atmosphere.
12. The method according to claim 10, wherein the electrically
conductive thin film is a metal thin film.
13. The method according to claim 10, wherein the electrically
conductive thin film is a noble metal thin film.
14. The method according to claim 10, wherein the electrically
conductive thin film is a carbon thin film formed of carbon (C) at
an atomic level.
15. The method according to claim 10, further comprising forming an
adhesion-enhancing intermediate layer between the oxide film of the
substrate and the electrically conductive thin film.
16. The method according to claim 15, wherein the electrically
conductive thin film is a metal thin film, and wherein the
intermediate layer is an Me layer formed of at least one element
selected from the group consisting of the metal elements of Ti, Zr,
Hf, V, Nb, Ta, Cr, Mo and W and the metalloid elements of Si and
B.
17. The method according to claim 15, wherein the electrically
conductive thin film is a carbon thin film formed of carbon (C) at
an atomic level, and wherein the intermediate layer is formed by at
least one layer of an Me layer formed of at least one element
selected from the group consisting of the metal elements of Ti, Zr,
Hf, V, Nb, Ta, Cr, Mo and W and the metalloid elements of Si and B,
and a carbon-Me gradient layer which is formed on the Me layer and
which contains carbon (C) and a metal or metalloid element (Me) and
in which a proportion of carbon (C) increases with increasing
distance from the substrate.
18. The method according to claim 10, further comprising forming a
carbon coating film on a surface of the electrically conductive
thin film.
Description
INCORPORATION BY REFERENCE
[0001] The disclosure of Japanese Patent Application No.
2002-351933 filed on Dec. 4, 2002, including the specification,
drawings and abstract thereof, are incorporated herein by reference
in their entirety.
BACKGROUND OF THE INVENTION
[0002] 1. Field of the Invention
[0003] The invention relates to a fuel cell separator and
production method therefor and, more particularly, to a structure
of a surface-treated layer of a metal separator for a solid polymer
electrolyte type fuel cell.
[0004] 2. Description of the Related Art
[0005] A solid polymer electrolyte type fuel cell battery is formed
by stacking modules each of which is formed by stacking at least
one cell made up of a membrane-electrode assembly (hereinafter,
referred to as "MEA") and a separator.
[0006] Each MEA is made up of an electrolyte membrane formed by an
ion-exchange membrane, an electrode (anode) formed by a catalyst
layer that is disposed on a surface of the electrolyte membrane,
and an electrode (cathode) formed by a catalyst layer that is
disposed on another surface of the electrolyte membrane. Normally,
a diffusion layer is provided between the MEA and the separator.
The diffusion layer facilitates the diffusion of a reaction gas
into the catalyst layer. The separator has a fuel gas channel for
supplying a fuel gas (hydrogen) to the anode, and an oxidizing gas
channel for supplying an oxidizing gas (oxygen, or air in ordinary
cases) to the cathode. The separator forms a passageway of
electrons between adjacent cells.
[0007] Terminals (electrode plates), insulators, and end plates are
disposed on two opposite ends of a cell stack in the cell stacking
direction. The cell stack is clamped in the cell stacking
direction, and is fixed through the use of fastener members (e.g.,
tension plates) that extend outside the cell stack in the cell
stacking direction, and also through the use of bolts and nuts. In
this manner, a stack is formed. On the anode side of a solid
polymer electrolyte type fuel cell, a reaction occurs in which
hydrogen is separated into hydrogen ions (protons) and electrons.
The hydrogen ions migrate through the electrolyte membrane to the
cathode side. On the cathode side, the hydrogen ions participate in
a reaction with oxygen and electrons (i.e., electrons produced on
the anode side of the adjacent MEA come to the cathode through the
separator, or electrons produced on the anode side of the cell
disposed at an end of the cell stack come to the cathode of the
cell at the opposite end via an external circuit), thereby
producing water.
[0008] Anode side: H.sub.2.fwdarw.2H.sup.++2e.sup.-
[0009] Cathode side:
2H.sup.++2e.sup.-+(1/2)O.sub.2.fwdarw.H.sub.2O
[0010] Since the separators need to have electrical conductivity,
separators are normally formed of a metal, carbon, or an
electrically conductive resin, or are formed by a combination of a
metal separator and a resin frame. Carbon separators and
electrically conductive resin separators are chemically stable and
therefore maintain electrical conductivity even during contact with
acid water. However, due to a strength requirement of bottom
surfaces of channels formed in separators, the carbon separators
and electrically conductive resin separators need to have
relatively great thickness, thus resulting in an increased stack
length. In contrast, the metal separators, having relatively high
strength, can be made relatively thin despite grooves and ridges
being formed to provide channels. Thus, the stack length can be
reduced. However, corrosion by acid water becomes a problem leading
to a reduced electrical conductivity and a reduced output. That is,
to adopt metal separators, it is necessary that the metal
separators be able to maintain good electrical conductivity and
good corrosion resistance for a long period.
[0011] As a related-art technology, Japanese Patent Application
Laid-Open Publication No. 2001-93538 discloses a technology in
which a surface of a substrate (stainless steel) of a metal
separator of a fuel cell is provided with an electrically
conductive film and an acid-resistant film of a metal material
different from that of the substrate.
[0012] However, the conventional metal separator has a problem of
increased cost since the acid-resistant film and the substrate are
formed from different metal materials.
SUMMARY OF THE INVENTION
[0013] An aspect of the invention provides a separator for a fuel
cell. This fuel cell separator includes a metal substrate having an
oxide film forming a surface of the substrate and made by an
oxidization of a material of the substrate and an electrically
conductive thin film formed on a surface of the oxide film. Another
aspect of the invention provides a production method for a fuel
cell separator. The production method includes the step of forming,
on a surface of a substrate, an oxide film from a material of the
substrate, and the step of forming an electrically conductive thin
film on a surface of the oxide film. According to the fuel cell
separator and the production method described above, the
electrically conductive thin film achieves low electrical
resistance (high electrical conductivity), and the oxide film
substantially prevents or reduces elution from the separator
substrate and therefore achieves high corrosion resistance even if
the electrically conductive thin film has pinholes. Furthermore,
since the oxide film is an oxide film made by an oxidization of a
material of the substrate, the formation of the oxide film costs
less than the formation of an oxide film by an oxidization of a
different material.
BRIEF DESCRIPTION OF THE DRAWINGS
[0014] The above mentioned embodiment and other embodiments,
objects, features, advantages, technical and industrial
significance of this invention will be better understood by reading
the following detailed description of the exemplary embodiments of
the invention, when considered in connection with the accompanying
drawings, in which:
[0015] FIG. 1 is an enlarged sectional view of a portion of a fuel
cell separator in accordance with Embodiment 1 of the
invention;
[0016] FIG. 2 is an enlarged sectional view of a portion of a fuel
cell separator in accordance with Embodiment 2 of the
invention;
[0017] FIG. 3 is an enlarged sectional view of a portion of a fuel
cell separator in accordance with Embodiment 3 of the
invention;
[0018] FIG. 4 is a sectional view illustrating the performance of a
corrosion resistance test (couple electric current test method) of
a fuel cell separator of the invention;
[0019] FIG. 5 is a bar graph indicating results of the corrosion
resistance test of fuel cell separators of the invention (the
amounts of ion elution from test pieces 1 of Condition 1 to
Condition 5);
[0020] FIG. 6 is a bar graph indicating results of the corrosion
resistance test of fuel cell separators of the invention (the
amounts of ion elution from test pieces 2 of Condition 1 to
Condition 4);
[0021] FIG. 7 is a sectional view illustrating the performance of
an electrical conductivity test (contact electrical resistance test
method) of a fuel cell separator of the invention;
[0022] FIG. 8 is a bar graph indicating results of the electrical
conductivity test of fuel cell separators of the invention (the
contact electrical resistances of test pieces 1 of Condition 1 to
Condition 5 before and after the corrosion);
[0023] FIG. 9 is a bar graph indicating results of the electrical
conductivity test of fuel cell separators of the invention (the
contact electrical resistances of test pieces 2 of Condition 1 to
Condition 4 before and after the corrosion);
[0024] FIG. 10 is a conceptual bar graph indicating the proportions
of the contact electrical resistance between the layers or the
specific resistance of the layers of each of fuel cell separator
test pieces of Condition 1, Condition 4 and Condition 5 determined
from the electrical conductivity test;
[0025] FIG. 11 is a sectional view illustrating the performance of
an adhesion test (a test method using water jet) of a fuel cell
separator of the invention;
[0026] FIG. 12 is a bar graph indicating results of the adhesion
test of fuel cell separators of the invention (the rates of
remaining Au thin film in test pieces 1 of Condition 2 to Condition
5 before and after the adhesion test);
[0027] FIG. 13 is a bar graph indicating results of the adhesion
test of fuel cell separators of the invention (the rates of
remaining C thin film in test pieces 2 of Condition 2 to Condition
4 before and after the adhesion test); and
[0028] FIG. 14A shows sectional views of variation I in fuel cell
separators of the invention.
[0029] FIG. 14B shows sectional views of variation II in fuel cell
separators of the invention.
[0030] FIG. 14C shows sectional views of variation III in fuel cell
separators of the invention.
[0031] FIG. 14D shows sectional views of variation IV in fuel cell
separators of the invention.
[0032] FIG. 14E shows sectional views of variation V in fuel cell
separators of the invention.
[0033] FIG. 14F shows sectional views of variation VI in fuel cell
separators of the invention.
[0034] FIG. 14G shows sectional views of variation VII in fuel cell
separators of the invention.
[0035] FIG. 14H shows sectional views of variation VIII in fuel
cell separators of the invention.
[0036] FIG. 14I shows sectional views of variation IX in fuel cell
separators of the invention.
[0037] FIG. 14J shows sectional views of variation X in fuel cell
separators of the invention.
[0038] FIG. 14K shows sectional views of variation XI in fuel cell
separators of the invention.
[0039] FIG. 14L shows sectional views of variation XII in fuel cell
separators of the invention.
DESCRIPTION OF THE EXEMPLARY EMBODIMENTS
[0040] In the following description and the accompanying drawings,
the present invention will be described in more detail in terms of
exemplary embodiments.
[0041] FIG. 1 illustrates Embodiment 1 (in which the electrically
conductive thin film is a metal thin film) of the invention. FIG. 2
illustrates Embodiment 2 (in which the electrically conductive thin
film is a carbon thin film formed of carbon (C) at an atomic level)
of the invention. FIG. 3 illustrates Embodiment 3 (in which a
carbon coating film is formed on the electrically conductive thin
film) of the invention. FIGS. 4 to 13 indicate specifications of
tests and results of the tests. In Embodiments 1 to 3 of the
invention, like portions are represented by like reference
characters in the drawings.
[0042] The same or similar portions of Embodiments 1 to 3 of the
invention will first be described. Fuel cell batteries
incorporating separators to which the invention is applied are
installed in fuel cell motor vehicles and the like. However, such
fuel cell batteries may also be installed in other vehicles or
apparatuses. The fuel cells incorporating separators to which the
invention is applied are solid polymer electrolyte fuel cells. The
structure of a stack formed by stacking MEAs and separators
conforms to the structure of ordinary solid polymer electrolyte
fuel cell batteries described above in conjunction with the
related-art technology.
[0043] A fuel cell separator 10 in accordance with the invention is
a metal separator as shown in FIG. 1 which has a metal substrate 11
that has, on a surface thereof, an oxide film (passive film) 11a
made by an oxidization of a material of the substrate, and an
electrically conductive thin film 12 formed on a surface of the
oxide film 11a of the substrate 11. Examples of the metal material
of the separator substrate 11 include stainless steel (SUS), steel,
aluminum (Al), aluminum alloys, titanium (Ti), titanium alloys,
etc. Examples of the material of the oxide film 11a in the case
where the substrate 11 is made of stainless steel include
Cr.sub.2O.sub.3, NiO, Fe.sub.2O.sub.3, etc. If the substrate is
made of Al, the oxide film 11a may be made of Al.sub.2O.sub.3. If
the substrate is made of Ti, the oxide film 11a may be made of
TiO.sub.2. If the substrate is made of steel, the oxide film 11a
may be made of Fe.sub.2O.sub.3. The oxide film 11a may be formed
naturally by letting the substrate 11 stand in air, or may also be
formed by placing the substrate 11 in an oxidizing atmosphere
(oxidizing solution). The oxide film 11a of the substrate improves
the corrosion resistant performance of the substrate 11.
[0044] The electrically conductive thin film 12 is a metal thin
film 12A, or a carbon thin film 12B formed of carbon (C) at an
atomic level. Examples of the metal forming the metal thin film 12A
include noble metals, an example of which is Au. The carbon thin
film 12B is a film built at an atomic level that is, the film 12B
is bonded to the substrate oxide film 11a and the like at an atomic
level, and does not include a coating film 14 that is formed of a
carbon powder and a resin binder. The noble metals and carbon are
highly resistant to corrosion. The thickness of the electrically
conductive thin film 12 is of the order of nanometers (nm) (it is
to be noted that the range of 0.01 to 10 .mu.m is appropriate), and
is less than the thickness of the coating film 14, which is several
ten micrometers (.mu.m). The electrically conductive thin film 12
may be formed by CVD (chemical vapor deposition) or PVD (physical
vapor deposition), including vapor deposition, sputtering, ion
plating, etc. To form the metal-based thin film 12A, plating may
also be employed.
[0045] It is desirable that an intermediate layer 13 for enhancing
the adhesion and the corrosion resistance be formed between the
oxide film 11a of the substrate and the electrically conductive
thin film 12. The formation of the intermediate layer 13 on the
oxide film 11a of the substrate is desirable for increasing
adhesion and corrosion resistance because the provision of the
electrically conductive thin film 12Alone will result in
insufficient corrosion resistance and insufficient adhesion. The
provision of the intermediate layer 13 alone will also result in
insufficient corrosion resistance and insufficient adhesion.
Therefore, it is also desirable to form the electrically conductive
thin film 12 on the intermediate layer.
[0046] As for the intermediate layer 13, a material and a structure
are selected so as to achieve good adhesion to the substrate oxide
film 11a and good bonding characteristic thereto, that is, good
bonding characteristic to oxygen atom (O), and so as to achieve
good adhesion and good bonding characteristic to the electrically
conductive thin film 12. If the electrically conductive thin film
12 is a metal thin film 12A as shown in FIG. 1, an Me layer 13a
formed of at least one element selected from the group consisting
of the metal elements of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W and
the metalloid elements of Si and B is selected as an intermediate
layer 13. These elements have good bonding characteristics to
oxygen, and have good adhesion and good bonding characteristic to
the oxide film 11a of the substrate. The intermediate layer 13 has
good bonding characteristic to the metal thin film 12A since the
intermediate layer 13 is formed of metal or metalloid. If the
electrically conductive thin film 12 is a carbon thin film 12B
formed of carbon (C) at the atomic level, a layer formed by at
least one of the following two types of layers is selected as the
intermediate layer 13. The two types of layers are: (a) a Me layer
13a formed of at least one element selected from the metal elements
of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W and the metalloid elements
of Si and B; (b) a (carbon-Me) gradient layer 13b which is formed
on top of the Me layer 13a and which contains carbon (C) and an
element (Me) of metal or metalloid and in which the proportion of
carbon (C) increases with increasing distance from the substrate
11. The layer 13a of type (a) has good bonding characteristics to
oxygen atom, and has good adhesion and good bonding characteristics
to the substrate oxide film 11a. The layer 13b of type (b) has good
bonding characteristics to the layer 13a of type (a), and has good
adhesion and good bonding characteristics to the carbon thin film
12B.
[0047] As shown in FIG. 3, a carbon (c) coating film 14 may be
formed on top of the electrically conductive thin film 12. In that
case, the structure of the surface-treated layer is formed by the
substrate 11, the substrate oxide film 11a on a substrate surface,
the intermediate layer 13 provided if necessary, the electrically
conductive thin film 12 formed by the metal thin film 12A or the
carbon thin film 12B formed of carbon (C) at the atomic level, and
the C coating film 14 in that order. However, the provision of the
C coating film 14 is not essential.
[0048] If the electrically conductive thin film 12 is the carbon
thin film 12B built of carbon (C) at the atomic level as shown in
FIG. 2, the intermediate layer 13 and the carbon thin film 12B may
be formed as in Variations (modifications) I to XII shown in FIG.
14. In FIG. 14, the term "metal separator" refers to a unit of the
substrate 11 and the substrate oxide film 11a provided on the
surface of the substrate 11. Furthermore, "Me" indicates a layer
formed of at least one element selected from the metal elements of
Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W and the metalloid elements of
Si and B. The term "MeC" indicates a carbide of the aforementioned
Me element. The "MeC gradient layer" is a layer in which the
composition changes from Me to C in a gradient fashion in the
direction of thickness. A combination of two kinds of elements,
such as "Me(A)" and "Me(B)" where Me(A)=W (tungsten), Me(B).dbd.Cr
(chrome), and a combination of more than two kinds of elements are
also possible. The "carbon+Me or MeC layer" is a layer in which Me
or MeC is compounded by an atomic level fabrication in an outermost
surface carbon layer. The symbol "+" indicates compounding.
[0049] Variations I to XII shown in FIG. 14 will be described.
[0050] Variation I is formed of a metal separator, an Me or MeC
layer, a carbon-Me or MeC gradient layer, and a carbon layer (C
thin film).
[0051] Variation II is formed of a metal separator, a carbon-Me or
MeC gradient layer, and a carbon layer (C thin film).
[0052] Variation III is formed of a metal separator, an Me or MeC
layer, and a carbon layer (C thin film).
[0053] Variation IV is formed of a metal separator, and a carbon-Me
or MeC gradient layer.
[0054] Variation V is formed of a metal separator, an Me(B) or
Me(B)C layer, an Me(A) or Me(A)C-Me(B) or Me(B)C gradient layer, a
carbon-Me(A) or Me(A)C gradient layer, and a carbon layer (C thin
film).
[0055] Variation VI is formed of a metal separator, an Me(A) or
Me(A)C-Me(B) or Me(B)C gradient layer, a carbon-Me(A) or Me(A)C
gradient layer, and a carbon layer (C thin film).
[0056] Variation VII is formed of a metal separator, an Me(B) or
Me(B)C layer, an Me(A) or Me(A)C layer, and a carbon layer (C thin
film).
[0057] Variation VIII is formed of a metal separator, an Me(A) or
Me(A)C-Me(B) or Me(B)C gradient layer, a carbon-Me(A) or Me(A)C
gradient layer, and a carbon layer (C thin film).
[0058] Variation IX is formed of a metal separator, an Me or MeC
layer, a carbon+Me or MeC-Me or MeC gradient layer, and a carbon+Me
or MeC layer.
[0059] Variation X is formed of a metal separator, a carbon+Me or
MeC-Me or MeC gradient layer, and a carbon+Me or MeC layer.
[0060] Variation XI is formed of a metal separator, an Me or MeC
layer, and a carbon+Me or MeC layer.
[0061] Variation XII is formed of a metal separator, and a
carbon+Me or MeC-Me or MeC gradient layer.
[0062] Constructions of various embodiments of the invention will
next be described.
Embodiment 1
[0063] A fuel cell separator 10 in accordance with Embodiment 1 of
the invention includes a metal substrate 11 having, on its surface,
an oxide film 11a made by an oxidization of a material of the
substrate itself, an intermediate layer 13 formed on the substrate
11, and an electrically conductive thin film 12 formed on the
intermediate layer 13. The electrically conductive thin film 12 is
a metal thin film 12A. The intermediate layer 13 is a Me layer 13a
formed of at least one element selected from the metal elements of
Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W and the metalloid elements of
Si and B.
Embodiment 2
[0064] A fuel cell separator 10 in accordance with Embodiment 2 of
the invention includes a metal substrate 11 having, on its surface,
an oxide film 11a made by an oxidization of a material of the
substrate itself, an intermediate layer 13 formed on the substrate
11, and an electrically conductive thin film 12 formed on the
intermediate layer 13. The electrically conductive thin film 12 is
a carbon thin film 12B built of carbon (C) at the atomic level. The
intermediate layer 13 is formed by at least one the following two
layers: a layer (Me) 13a formed of at least one element selected
from the metal elements of Ti, Zr, Hf, V, Nb, Ta, Cr, Mo and W and
the metalloid elements of Si and B; and a (carbon-Me) gradient
layer 13b which is formed on top of the layer (Me) 13a and which
contains carbon (C) and an element (Me) of metal or metalloid and
in which the compounding proportion of carbon (C) increases with
increasing distance from the substrate.
Embodiment 3
[0065] In a fuel cell separator 10 in accordance with Embodiment 3
of the invention, a C coating film 14 is formed on the electrically
conductive thin film 12 of a fuel cell separator similar to the
separator of Embodiment 1 or 2 of the invention.
[0066] Operation of the invention will next be described. In the
fuel cell separator 10 in accordance with the invention, the
substrate 11 of the metal separator has, on its surface, the oxide
film 11a made by an oxidization of a material of the substrate
itself, and the oxide film 11a has, on its surface, the
electrically conductive thin film 12. Therefore, low electrical
resistance is achieved by the electrically conductive thin film 12,
thereby providing a highly electrically conductive separator. Even
if the electrically conductive thin film 12 has pinholes, the oxide
film 11a substantially prevents or reduces elution from the
separator substrate 11, thereby achieving high corrosion resistance
of the separator. Furthermore, since the oxide film 1 la is an
oxide film made by an oxidization of a material of the substrate 11
itself, the formation of the oxide film 11a costs less than the
formation of an oxide film from a different metal as in the
aforementioned related-art technology (patent literature 1).
[0067] Furthermore, since the intermediate layer 13 for enhancing
the adhesion is formed between the oxide film 11a of the substrate
itself and the electrically conductive thin film 12, good adhesion
of the surface-treated layer 12, 13 to the substrate 11 is secured.
Therefore, the durability of the separator improves, and high
electrical conductivity and high corrosion resistance of the
separator are maintained for a long period.
[0068] Furthermore, if the C coating film 14 is formed on the
electrically conductive thin film 12 of the surface-treated layer,
the reliability associated with durability further improves
corresponding to the formation of the C coating film 14, in
addition to improvements of the surface-treated layer in electrical
conductivity, corrosion resistance and adhesion.
[0069] Fuel cell separators 10 in accordance with the invention and
comparative examples were tested with respect to corrosion
resistance, contact electrical resistance and adhesion. Results of
the tests concerning the separators of the invention on the one
hand and the comparative examples on the other hand were compared.
The comparison confirmed that the fuel cell separators of the
invention had sufficiently high corrosion resistance, sufficiently
low contact resistance (high electrical conductivity), and high
adhesion of the surface-treated layer, compared with the
comparative examples. This will be further described in detail
below.
[0070] Test pieces used for the tests will first be described. The
separators having an electrically conductive thin film of Au, that
is, a noble metal, will be referred to as "test pieces 1", and the
separators having an electrically conductive thin film of carbon
(C) will be referred to as "test pieces 2". In the description
below, Condition 4 regarding test pieces 1 and Condition 3 and
Condition 4 regarding test pieces 2 represent the invention, and
the other conditions represent comparative examples.
[0071] Test Pieces 1: Au was used for the electrically conductive
thin film
[0072] <Condition 1> not treated/substrate is SUS316L (with
an oxide film)
[0073] <Condition 2> 10 nm Au sputtering (the electrically
conductive thin film of Au with a thickness of 10 nm by a
sputtering)/substrate is SUS316L (without ion etching (IE), and
with an oxide film)
[0074] <Condition 3> 10 nm Au sputtering/substrate is SUS316L
(with IE in an Ar atmosphere (Ar--IE), and without an oxide
film)
[0075] <Condition 4 (invention)> 10 nm Au sputtering/10 nm Cr
sputtering (the electrically conductive thin film of Cr with a
thickness of 10 nm by a sputtering)/substrate is SUS316L (without
IE, and with an oxide film)
[0076] <Condition 5> 10 nm Au sputtering/10 nm Cr
sputtering/substrate is SUS316L (with Ar--IE, and without an oxide
film)
[0077] Test Pieces 2: carbon was used for the electrically
conductive thin film
[0078] <Condition 1> not treated/substrate is SUS316L (with
an oxide film)
[0079] <Condition 2> 50 nm C sputtering (the electrically
conductive thin film of carbon with a thickness of 50 nm by a
sputtering)/50 nm C--Cr gradient sputtering (the carbon-chromium
gradient layer, in a proportion of carbon increasing with
increasing distance from substrate, with a thickness of 50 nm by a
sputtering)/substrate is SUS316L (with Ar--IE, and without an oxide
film)
[0080] <Condition 3 (invention)> 50 nm C sputtering/50 nm
C--Cr gradient sputtering/substrate is SUS316L (with Ar-light
etching (LE, meaning light IE), and with a thin oxide film)
[0081] <Condition 4 (invention)> 50 nm C sputtering/50 nm
C--Cr gradient sputtering/substrate is SUS316L (without IE, and
without a thick oxide film (the same as in Condition 1))
[0082] 1. Corrosion Resistance Test
[0083] (1) Corrosion Resistance Test Condition
[0084] The corrosion resistance test was performed by a couple
electric current test method illustrated in FIG. 4. A test piece
and a counter electrode (a burned carbon, i.e., graphite,
contacting the separator in a fuel cell) were placed in an acidic
aqueous solution (an acidic aqueous solution simulating an
environment to which the separator of a fuel cell is exposed, and a
sulfuric acid solution of pH 2 was used in this test). While the
temperature was set at 80.degree. C., that is, an operation
temperature of the fuel cell, the electric current density at the
time of elution of separator metal Me in the form of ions was
measured by an ammeter set on an external circuit of the counter
electrode. The corrosion time was set at 100 hours. The plus
current is a current occurring at the time of elution of Me in the
form of positive ions, and means induction of corrosion. Zero or
minus current means freedom from problems in corrosion or corrosion
resistance.
[0085] (2) Quantification of Elution of Ions
[0086] The corrosive solution was analyzed by ICP (induced coupled
plasma emission spectroscopy) to determine the quantity of the
aforementioned ion elution.
[0087] (3) Results of Corrosion Resistance Test
[0088] Results of the test regarding the amount of ion elution from
the SUS substrates of the Au thin film type test pieces 1 of
Condition 1 to Condition 5 are indicated in FIG. 5. In the bar
chart of FIG. 5, the test pieces 1 of Condition 1, Condition 2,
Condition 3, Condition 4 and Condition 5 are sequentially indicated
from the left. Of the three segments of each bar in FIG. 5, the top
segment indicates the amount of Ni elution from the SUS substrate,
and the intermediate segment indicates the amount of Cr elution
from the SUS substrate, and the bottom segment indicates the amount
of Fe elution from the SUS substrate. With regard to the test piece
1 of the invention of Condition 4, the amounts of elution of Ni, Cr
and Fe ions were less than the measuring range lower limit, and can
be considered substantially no elution.
[0089] Results of the test regarding the amount of ion elution from
the SUS substrates of the C thin film type test pieces 2 under
Condition 1 to Condition 4 are indicated in FIG. 6. In the bar
chart of FIG. 6, the test pieces 2 of Condition 1, Condition 2,
Condition 3 and Condition 4 are indicated sequentially from the
left. Of the three segments of each bar in FIG. 6, the top segment
indicates the amount of Ni elution from the SUS substrate, and the
intermediate segment indicates the amount of Cr elution from the
SUS substrate, and the bottom segment indicates the amount of Fe
elution from the SUS substrate. With regard to the test pieces of
the invention of Condition 3 and Condition 4, the amounts of
elution of Ni, Cr and Fe ions were less than in the cases of the
test pieces of Condition 1 and Condition 2, thus indicating
improvements in corrosion resistance.
[0090] The results of the corrosion resistance test can be
summarized as follows.
[0091] The test pieces of the invention with the oxide film 11a had
good corrosion resistance (elution resistance), regardless of the
presence or absence of the intermediate Cr layer 13.
[0092] As for the C thin film type test pieces, the corrosion
resistance improved with increases in the amount of the oxide film
11a remaining.
[0093] 2. Electrical Conductivity Test
[0094] (1) Electrical Conductivity Test Condition
[0095] The electrical conductivity test was performed by a contact
resistance test method illustrated in FIG. 7. The electrical
contact resistance was measured before and after the corrosion
resistance test. The test pieces used for the resistance
measurement were the same as the test pieces 1 of Condition 1 to
Condition 5 and the test pieces 2 of Condition 1 to Condition 4 in
the aforementioned corrosion resistance test, in which the
substrate 11 (having, an oxide film 11a on a surface) was
surface-treated. Each test piece was sandwiched with an intervening
diffusion cloth (a carbon cloth identical to the one incorporated
as a diffusion layer in a fuel cell) between polar plates. After
the planar contact pressure was set at 20 kgf/cm.sup.2, which is
close to the planar contact pressure of a fuel cell, and the
current was set at 1 A, the voltage V between the two polar plates
was measured. Then, a contact resistance was determined as in the
following equation:
Resistance R=V/I(I=1 A)
[0096] (2) Results of Electrical Conductivity Test
[0097] Results of measurement of the contact electrical resistance
of the Au thin film type test pieces 1 of Condition 1 to Condition
5 (the same specifications as in Condition 1 to Condition 5 of the
Au thin film type test pieces 1 in the corrosion resistance test)
are indicated in FIG. 8. The five pairs of bars in FIG. 8 indicate
the test pieces 1 of Condition 1, Condition 2, Condition 3,
Condition 4 and Condition 5 in that order from the left. Of the two
bars for each condition in FIG. 8, the left side bar indicates the
contact resistance before the corrosion, and the right side bar
indicates the contact resistance after the corrosion. The test
piece of the invention of Condition 4 exhibited low contact
resistance.
[0098] Results of measurement of the contact electrical resistance
of the C thin film type test pieces 2 of Condition 1 to Condition 4
(the same specifications as in Condition 1 to Condition 4 of the C
thin film type test pieces 2 in the corrosion resistance test) are
indicated in FIG. 9. The four pairs of bars in FIG. 9 indicate the
test pieces 2 of Condition 1, Condition 2, Condition 3 and
Condition 4 in that order from the left. Of the two bars for each
condition in FIG. 9, the left side bar indicates the contact
resistance before the corrosion, and the right side bar indicates
the contact resistance after the corrosion. The test pieces of the
invention of Condition 3 and Condition 4 exhibited low contact
resistance.
[0099] The results of the electrical conductivity test can be
summarized as follows.
[0100] The results indicate that the surface treatment (formation
of the electrically conductive thin film 12) achieved lower contact
resistances, regardless of the presence or absence of the oxide
film 11a on the surface of the SUS substrate 11.
[0101] The reason for this can be considered as follows. As
indicated in the conceptual diagram of FIG. 10, the resistance in
the direction perpendicular to the plane of the separator includes
the contact resistance between the diffusion layer and the
separator, the specific resistance of the substrate oxide film,
etc. However, it is the contact resistance between the diffusion
layer and the separator that makes up a major portion of the
resistance. The specific resistance of the substrate oxide film or
the like contributes merely to a small extent since the thickness
of the substrate oxide film is as small as several nanometers.
Therefore, as the contact resistance between the diffusion layer
and the separator is reduced by formation of the electrically
conductive thin film 12, the resistance in the direction
perpendicular to the plane of the separator considerably reduces.
In contrast, the resistance increase attributed to the oxide film
11a does not considerably affect the increase in resistance in the
direction perpendicular to the plane of the separator. It is
considered that the electrical resistance in the direction
perpendicular to the plane of the separator reduces as a whole.
[0102] Of the three bars shown in FIG. 10, the left side bar
indicates the test piece 1 of Condition 1 according to a
comparative example, and the middle bar indicates the test piece 1
of Condition 4 according to the invention, and the right side bar
indicates the test piece 1 of Condition 5 according to another
comparative example. Of the three segments of the bar of the test
piece 1 of Condition 1, the top segment indicates the contact
resistance between the diffusion cloth (carbon cloth) and the
substrate oxide film, and the intermediate segment indicates the
resistance of the substrate oxide film, and the bottom segment
indicates the resistance of the SUS substrate. Of the five segments
of the bar of the test piece 1 of Condition 4 according to the
invention, the first segment from top indicates the contact
resistance between the diffusion cloth (carbon cloth) and the
electrically conductive thin film, and the second segment from top
indicates the resistance of the electrically conductive thin film,
and the third segment from top indicates the resistance of a base
film (intermediate Me layer), and the fourth segment from top
indicates the resistance of the substrate oxide film, and the fifth
segment from top indicates the resistance of the SUS substrate. Of
the four segments of the bar of the test piece 1 of Condition 5,
the first segment from top indicates the contact resistance between
the diffusion cloth (carbon cloth) and the electrically conductive
thin film, and the second segment from top indicates the resistance
of the electrically conductive thin film, and the third segment
from top indicates the resistance of a base film (intermediate Me
layer), and the fourth segment from top indicates the resistance of
the SUS substrate.
[0103] 3. Adhesion Test (Test of Adhesion Between the
Surface-Treated Layer 12, 13 and the Substrate)
[0104] (1) Adhesion Test Condition
[0105] The adhesion test was performed by a water jet test method
illustrated in FIG. 11. In the test, the water pressure at a nozzle
was about 200 MPa (2000 atm.) The rate of thin film remaining in
the water jet test was determined as in the following equation:
Rate of remaining thin film (%)=M/Mo.times.100
[0106] where
[0107] M is the amount of thin film after water jet (FIG.
14(b)),
[0108] M.sub.0 is the amount of thin film before water jet (FIG.
14(a)).
[0109] Elements were provided by fluorescent X-ray or the like.
[0110] (2) Results of Adhesion Test
[0111] Results of the adhesion test of the Au thin film type test
pieces 1 of Condition 2 to Condition 5 (the same specifications as
in Condition 2 to Condition 5 of the Au thin film type test pieces
1 in the corrosion resistance test) are indicated in FIG. 12. The
four pairs of bars of the chart of FIG. 12 indicate the test pieces
1 of Condition 2, Condition 3, Condition 4 and Condition 5 in that
order from the left. Of the two bars for each condition in FIG. 12,
the left side bar indicates the rate of remaining Au thin film
before the adhesion test, and the right side bar indicates the rate
of remaining Au thin film after the adhesion test. The test piece
of Condition 4 according to the invention exhibited high rate of
remaining Au thin film (good adhesion).
[0112] Results of the adhesion test of the C thin film type test
pieces 1 of Condition 2 to Condition 4 (the same specifications as
in Condition 2 to Condition 4 of the C thin film type test pieces 1
in the corrosion resistance test) are indicated in FIG. 13. The
three pairs of bars of the chart of FIG. 13 indicate the test
pieces 2 of Condition 2, Condition 3 and Condition 4 in that order
from the left. Of the two bars for each condition in FIG. 13, the
left side bar indicates the rate of remaining C thin film before
the adhesion test, and the right side bar indicates the rate of
remaining C thin film after the adhesion test. The test piece of
Condition 3 according to the invention exhibited high rate of
remaining C thin film (good adhesion).
[0113] The results of the adhesion test can be summarized as
follows.
[0114] Before Adhesion Test
[0115] If a base Me layer 13 is not provided, good adhesion can be
secured by removing the oxide film 11a via Ar--IE.
[0116] If a base Me layer 13a is provided, good adhesion can be
secured, regardless of the presence or absence of the substrate
oxide film 11a.
[0117] After Adhesion Test
[0118] If a base Me layer 13a is not provided, the adhesion reduces
due to the corrosion of the Au thin film/substrate interface.
[0119] If a base Me layer 13a is provided, good corrosion
resistance (due to the barrier effect of the base Me layer and
improvement in corrosion resistance caused by the substrate oxide
film 11a) can be maintained even after the corrosion.
[0120] In a fuel cell separator in accordance with the invention,
an oxide film is formed, on a surface of a metal separator, from a
substrate metal, and an electrically conductive thin film is formed
on a surface of the oxide film. Therefore, the fuel cell separator
achieves low electrical resistance (high electrical conductivity)
due to the electrically conductive thin film. Even if the
electrically conductive thin film has pinholes, the oxide film
substantially prevents or reduces elution from the separator
substrate, thereby achieving high corrosion resistance.
Furthermore, since the oxide film is an oxide film of the separator
substrate itself, the oxide film can be formed at a lower cost than
an oxide film formed from a different metal as in the
aforementioned related-art technology (patent literature 1).
Furthermore, in a fuel cell separator in accordance with the
invention, an intermediate layer for enhancing adhesion is formed
between the oxide film made of an oxidization of the substrate
material itself and the electrically conductive thin film, so that
good adhesion of the surface-treated layer is secured. Therefore,
good durability of the separator is achieved, and high electrical
conductivity and high corrosion resistance of the separator are
maintained for a long period. Still further, in a fuel cell
separator in accordance with the invention, a C coating film is
formed on top of the electrically conductive thin film. Therefore,
the reliability associated with durability further improves
corresponding to the formation of the C coating film, in addition
to the aforementioned good property of the surface-treated layer
(high electrical conductivity).
[0121] While the invention has been described with reference to
exemplary embodiments thereof, it is to be understood that the
invention is not limited to the exemplary embodiments or
constructions. To the contrary, the invention is intended to cover
various modifications and equivalent arrangements. In addition,
while the various elements of the exemplary embodiments are shown
in various combinations and configurations, which are exemplary,
other combinations and configurations, including more, less or only
a single element, are also within the spirit and scope of the
invention.
* * * * *