Separator for fuel cell and manufacture thereof

Details Separator for fuel cell and manufacture thereof Separator for fuel cell and manufacture thereof

2000-03-09

2002-10-29

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000

Reexamination Certificate

active

06472094

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a fuel cell separator and a method for producing the same, and more particularly to the fuel cell separator and a method for producing the same for use in a contact current-collection and for forming a gas passage.
BACKGROUND OF THE INVENTION
In a solid polymer fuel cell, a solid polymer electrolyte membrane (simply referred to as “an electrolyte membrane” hereinbelow) is used as an electrolyte. The fuel cell has been used for space development and military because of its characteristics such that it has a high output density, its structure is simple, an operating temperature is relatively low, it has quietness and the like. In the case that hydrogen is used as a fuel, the fuel-cell essentially exhausts no nitrogen oxide and no carbon dioxide, thus it has become center of public attention for use as a low-pollution power source for automobile.
FIGS.
24
(
a
) and (
b
) show an example of a basic structure of a solid polymer fuel cell. A solid polymer fuel cell
1
shown in FIGS.
24
(
a
) and (
b
) has such basic structure that separators
20
,
20
are disposed at both sides of a unit cell
10
, which is fixed by separator frames
18
,
18
made of an insulating resin material, and a water cooler
26
, which controls a cell temperature, is incorporated into one separator
20
.
The unit cell
10
is formed by electrodes
14
,
16
, which are bonded on both surfaces of an electrolyte membrane
12
having a thickness of 50 to 200 &mgr;m. In general, a fluoro polymer electrolyte membrane, typically represented by a perfluoro sulfonic acid membrane, is used as the electrolyte membrane
12
, being known under the trade name of Nafion (registered trademark for products manufactured by Du Pont Co.).
In electrodes
14
and
16
, one side of a carbon paper or a carbon cloth is coated with mixture of a carbon particle on which platinum or the like is loaded, and a perfluoro sulfonic acid polymer solution. Then, the surface with which the mixture is coated is crimped to the electrolyte membrane
12
to form a membrane-electrode assembly (MEA). The electrodes
14
,
16
are composed of two layers, namely, one is a porous and hydrophobic catalyst bed (not shown) composed of a carbon particle on which platinum is loaded, and of an electrolyte, and the other is a porous diffusion layer (not shown) composed of a carbon paper and the like.
In the separator
20
, so as to supply process gases to the electrodes
14
,
16
, which are bonded on both surfaces of the electrolyte membrane
12
, there are a great number of isolated or combined projections
24
,
24
(simply referred to as “a projection” hereinbelow) on a flat plate-shaped separator base
22
, namely, on the surface facing with the electrodes
14
,
16
. The projection
24
forms a gas passage
27
. The separators
20
,
20
collect the generated electric power and then take out it to the outside by means of a part in contact with the electrodes
14
,
16
; the part is formed on the upper surface of the projection
24
. Furthermore, the separator
20
prevents fuel gases and oxidants gases from mixing. For the separator
20
, therefore, such material as to have gas impermeability and conductivity is employed.
Then, a great number of basic structures shown in FIG.
24
(
a
) are laminated and compressed by the given pressure, and the electrodes
14
,
16
are made to be contacted with the projections
24
,
24
formed on the surface of the separator
20
,
20
, whereby the solid polymer fuel cell
1
is produced.
In the solid polymer fuel cell having above-mentioned structure, fuel gases such as reformate gases containing hydrogen flow toward the electrode
14
(anode) and oxidant gases such as air containing oxygen flow toward the electrode
16
(cathode) at a state such that both ends of the solid polymer fuel cell are connected to a load, whereby supplied gases pass through the diffusion layer and reach to the catalyst bed. Then, a hydrogen ion generated at the catalyst bed on the anode
14
moves to the cathode
16
through an electrolyte radical in the electrolyte membrane
12
, and reacts with oxygen in the catalyst bed on the cathode
16
, whereby water is generated. Electric power generated during reaction collects at the projections
24
,
24
in contact with the electrodes
14
,
16
, then is taken out to the outside through the separators
20
,
20
arranged at both ends of the solid polymer fuel cell
1
.
The electrolyte membrane
12
employed in a solid polymer fuel cell needs water in order to show conductivity, therefore, process gases supplied to the electrodes
14
,
16
are humidified generally. Furthermore, an operation temperature of the solid polymer fuel cell is about 80 to 90° C. Thus, the separators
20
,
20
employed in a solid polymer fuel cell are required to have not only excellent gas impermeability and conductivity but also have the stable and low contact resistance even under an oxidizing water vapor atmosphere.
For the fuel cell separator, therefore, a thin plate made of dense carbon graphite that a projection is formed thereon by means of a machine work is employed generally. Furthermore, Japanese Patent Laid-Open No. Hei 4-95354 discloses a separator of which the contact resistance with a carbon matrix electrode is made to be lowered by way of depositing Au, Ta, W, Mo or the like, on the surface of a dense carbon plate.
Additionally, a metallic separator has been also proposed. Taking account of the corrosion resistance and an electric conductivity, for example, stainless steel, Ti, Cu, Al or the like, are employed as a material for a separator. Furthermore, Japanese Patent Laid-Open No. Hei 8-222237 discloses a separator that has a thin plate having a great number of projections spaced at regular intervals of a few millimeters on both surfaces; the projections are formed by processing the thin plate such as to be composed of stainless steel, cold-rolled steel, Al or the like, that are coated with carbon graphite in a process of an embossing or a dimple forming.
Dense carbon graphite has the excellent current-collecting performance and is stable even under an oxidizing water vapor atmosphere, so it is suitable for a material of a separator. Dense carbon graphite, however, is expensive and brittle, and has the poor workability performance. A cutting work was, therefore, only known as a process for forming a projection on a surface of a thin plate composed of dense carbon graphite, thus, mass production was difficult, disadvantageously.
In contrast, a metallic material has more excellent workability than dense carbon graphite, thus a deformation processing such as a press forming may be employed as a simple method for forming a projection thereon.
However, so as to form a great number of projections on a surface of a thin plate by a press forming, a die with high accuracy is required, thereby the increase of expense is induced. In addition to this, if a projection is formed by a press forming, then a thickness of a side wall of a projection is decreased, thus causing micro cracks.
In addition, when inexpensive metal such as stainless steel, cold-rolled steel, Al, or the like, is employed as material for a separator, then an oxide film is generated on a surface of a metallic separator at a state exposed to an oxidizing water vapor atmosphere, whereby the contact resistance between an electrode and a separator is greatly increased. Therefore, the inner electrical resistance is greatly increased, thus causing the energy conversion efficiency to be decreased, disadvantageously.
To overcome this problem, as disclosed in Japanese Patent Laid-Open No. Hei 8-222237, there is such an idea as to coat a surface of a metallic separator with dense carbon graphite by means of processes such as impregnation, thermal spraying, electrocoating, sputtering or the like. According to these processes, however, tightness and adhesiveness of coating are insufficient, thus resulting in poor reliability, disadvantageously. Furthermore, due to a coating process, it results in the in

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