Metal treatment – Stock – Ferrous
Reexamination Certificate
2000-04-13
2002-04-30
Yee, Deborah (Department: 1742)
Metal treatment
Stock
Ferrous
C148S326000, C148S327000, C134S003000, C134S041000, C134S027000, C429S006000
Reexamination Certificate
active
06379476
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a stainless steel product having low contact electrical resistance, and to a method for producing the stainless steel product. The invention also relates to a bipolar plate produced from the stainless steel product and to a polymer electrode fuel cell (hereinafter may be abbreviated as PEFC) containing the bipolar plate.
2. Description of the Related Art
Stainless steel has excellent corrosion resistance due to passive film formed on the surface thereof. However, stainless steel is not suitable for producing electrically conductive elements requiring low contact electrical resistance, since the passive film formed on the surface has high electrical resistance. In general, the more excellent the corrosion resistance of passive film, the higher the electrical resistance thereof.
Therefore, reduction in contact electrical resistance of stainless steel enables stainless steel to serve as an electrically conductive element such as a terminal, the element being employed in a circumstance requiring corrosion resistance. One example of an electrically conductive element exhibiting excellent corrosion resistance and low contact electrical resistance is a bipolar plate (also called a “separator”) of a PEFC.
A fuel cell generates DC power, and examples of fuel cells include a solid oxide fuel cell (abbreviated as SOFC), a molten carbonate fuel cell (abbreviated as MCFC), and a phosphoric acid fuel cell (abbreviated as PAFC). These fuel cells are named after a component material of an electrolyte, which is the most important portion of a fuel cell.
At present, fuel cells which have attained a commercially satisfactory level include a PAFC and an MCFC.
Approximate operation temperatures of an SOFC, an MCFC, a PAFC, and a PEFC are 1000° C., 650° C., 200° C., and 80° C., respectively.
A PEFC operates at approximately 80° C. and is easy to start and stop. The expected energy efficiency thereof is approximately 40%. Therefore, there is worldwide demand for PEFCs, which can be employed practically in an on-site power source used in a small-scale power plant, a telephone office, or a similar site; a domestic small on-site power source making use of city gas as a fuel; and a power source incorporated in a low-pollution electric automobile making use of hydrogen, methanol, or gasoline as a fuel.
Although the aforementioned fuel cells are categorized as fuel cells, i.e., their names include the term “fuel cell,” they must be considered individually when a component material of a fuel cell is designed, since performance required for a component material, particularly anti-corrosion performance, varies with the type of fuel cell.
Specifically, the performance depends on corrosion of a component material caused by an employed electrolyte;
oxidation at high temperature predominantly occurring above approximately 380° C.; and sublimation and re-deposition of an electrolyte, and condensation.
In practice, a variety of materials are employed as component materials of a fuel cell; e.g., graphite materials, Ni cladding, alloys having a high alloying element content, and stainless steel.
Thus, materials per se employed in commercialized PAFCs and MCFCs cannot be applied to a component material of PEFCs.
FIGS. 1A and 1B
shows the structure of a PEFC; i.e.,
FIG. 1A
is an exploded view of a fuel cell (membrane electrode assemblies) and
FIG. 1B
is a perspective view of an entire fuel cell. As shown in
FIGS. 1A and 1B
, a fuel cell
1
is an assembly of membrane electrode assemblies. The membrane electrode assembly comprises a solid polymer electrolyte membrane
2
, a fuel electrode (anode) membrane
3
being laminated on one surface of the solid polymer electrolyte membrane
2
and an oxidizing agent electrode (cathode) membrane
4
being laminated on the other surface. The membrane
3
is further layered with a bipolar plate
5
a
, while the membrane
4
is further layered with a bipolar plate
5
b.
The solid polymer electrolyte membrane
2
comprises a proton-conductive fluoride membrane having a hydrogen-ion (proton)-exchange group.
Each of the anode membrane
3
and the cathode membrane
4
is provided with a catalyst layer comprising a granular platinum catalyst, graphite powder, and an optional fluororesin having a hydrogen-ion (proton)-exchange group, which is to come into contact with a fuel gas or an oxidizing gas.
A fuel gas A (hydrogen or a hydrogen-containing gas) is fed through channels
6
a
provided in the bipolar plate
5
a
, to thereby supply hydrogen to the anode membrane, while an oxidizing gas B such as air is fed through channels
6
b
provided in the bipolar plate
5
b
, to thereby supply oxygen. The thus-supplied gasses induce electrochemical reaction, to thereby generate DC power.
Functions required of a bipolar plate of a PEFC are as follows:
(1) a function of a channel which supplies a fuel gas and an oxidizing gas uniformly in inner planes of a cell;
(2) a function of a channel which effectively discharges water formed in cathode portions to outside a fuel cell along with a carrier gas such as air or oxygen after reaction;
(3) a function of an electrical connector between membrane electrode assemblies so as to maintain low resistance and high conductivity suitable for an electrode for a long period of time;
(4) a function of a separator which separates a cathode chamber and an anode chamber in adjacent assemblies; and
(5) a function of a separator which isolates cooling water channels and separates adjacent assemblies.
Hitherto, there has been earnestly investigated application of a carbon sheet as a material of a bipolar plate of a PEFC. However, a carbon sheet is disadvantageous in that the sheet is easily fractured and severely elevates cost of mechanical processing for producing a flat surface and forming gas channels. These fatal problems might make commercialization of a fuel cell difficult.
Among carbonaceous materials, thermally expandable graphite has been most attractive material for producing a bipolar plate of a PEFC, in that the graphite is considerably inexpensive. However, in order to provide functions of the aforementioned separators by means of reducing gas-permeability, thermally expandable graphite must be subjected to a plurality of steps of resin impregnation and firing. In addition, there still remain problems in cost of mechanical processing for ensuring surface flatness and forming channels. Thus, commercialization of thermally expandable graphite has not yet been attained.
In contrast to investigation of application of graphite materials, stainless steel has been applied to a bipolar plate, in view of cost reduction.
Japanese Patent Application Laid-Open (kokai) No. 10-228914 discloses a fuel cell bipolar plate which is formed of a metallic material, in which a surface of the bipolar plate which contacts with a membrane electrode assembly is plated directly with gold. Examples of metallic materials include stainless steel, aluminum, and Ni-Fe alloy, with Type 304 being employed as stainless steel. According to the disclosure, the bipolar plate is plated with gold, to thereby lower contact resistance between the bipolar plate and an electrode and enhance electric conduction from the bipolar plate to the electrode. Thus, a fuel cell containing such bipolar plates is considered to generate high output power.
Japanese Patent Application Laid-Open (kokai) No. 8-180883 discloses a PEFC employing bipolar plates formed of a metallic material which is easily coated with passive film in air. According to the disclosure, the metallic surface of the bipolar plates is completely coated with passive film, to thereby make the surface resistant to chemical substances. Thus, ionization of water formed in the fuel cell is suppressed, to thereby suppress lowering efficiency of electrochemical reaction. It is also disclosed that passive film on a portion contacting with an electrode membrane of a bipolar plate is removed and a layer of a noble metal is formed, to thereby lower contact el
Doi Takashi
Fukuta Shinji
Seki Akira
Tarutani Yoshio
Burns Doane , Swecker, Mathis LLP
Sumitomo Metal Industries Ltd.
Yee Deborah
LandOfFree
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