Coating processes – Electrical product produced – Fuel cell part
Reexamination Certificate
2002-09-27
2004-09-07
Barr, Michael (Department: 1762)
Coating processes
Electrical product produced
Fuel cell part
C427S458000, C427S372200, C427S402000
Reexamination Certificate
active
06787183
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of and an apparatus for producing an electrode of a fuel cell. The output of the fuel cell with the electrode is high even when current density is high.
2. Description of the Related Art
In general, a fuel cell comprises a stack including a predetermined number of cell units which are electrically connected in series to one another. A collector is electrically connected to each of the cell units which are disposed at both ends of the stack. Further, an end plate is arranged outside of each of the collectors with an insulating plate interposed between the end plate and the collector to avoid electric leakage. Additionally, a backup plate may be arranged outside of each of the end plates in some cases. The stack, the insulating plates, and the collectors are interposed between the end plates or between the backup plates connected to each other by tie rods or the like.
The cell unit has an electrolyte electrode assembly
1
as shown in FIG.
8
. The electrolyte electrode assembly
1
comprises an anode
2
and a cathode
3
which are connected to end surfaces of an electrolyte layer
4
respectively.
In this arrangement, the anode
2
comprises a water-repellent layer
6
a
and an electrode catalyst layer
7
a
which are stacked in order of mention on an electrode base material
5
a
. In general, the electrode base material
5
a
is made of carbon paper, carbon cloth, or the like. The water-repellent layer
6
a
is made of carbon black and polytetrafluoroethylene (PTFE). The electrode catalyst layer
7
a
is made of carbon black with Pt supported thereon. On the other hand, the cathode
3
is constructed in the same manner as the anode
2
. Therefore, the constitutive elements of cathode
3
are denoted by reference symbols of the elements of the anode
2
but replacing “a” with “b”, and detailed explanation thereof is omitted.
The electrolyte layer
4
comprises a membrane
8
impregnated with a liquid electrolyte. The membrane
8
may be a polymer membrane (see U.S. Pat. No. 5,525,436) made of basic polymer such as polybenzimidazole or porous SiC. On the other hand, the liquid electrolyte may be liquids capable of electrically conducting hydrogen ion such as phosphoric acid, sulfuric acid, and methanesulfonic acid.
The cell unit comprises the electrolyte electrode assembly
1
which is interposed between a pair of separators (not shown).
When the fuel cell is operated, a fuel gas such as a hydrogen-containing gas is supplied to the anode
2
via a gas flow passage in the separator, and an oxygen-containing gas such as air is supplied to the cathode
3
. During this process, hydrogen in the fuel gas is ionized in the electrode catalyst layer
7
a
of the anode
2
, and thus hydrogen ions and electrons are produced.
The produced hydrogen ions are moved in the cell unit, and arrive at the electrode catalyst layer
7
b
of the cathode
3
. During this process, the electrons are extracted by an external circuit which is electrically connected to the collectors. The electrons are utilized as DC electric energy to energize the external circuit. After that, the electrons arrive at the cathode
3
. The hydrogen ions and the electrons reaching the cathode
3
react in the electrode catalyst layer
7
b
with the oxygen in the oxygen-containing gas supplied to the cathode
3
. Thus, H
2
O is produced. H
2
O is repelled by the water-repellent layers
6
a
,
6
b
of both electrodes
2
,
3
, and is promptly discharged. Accordingly, it is possible to prevent the liquid electrolyte in the electrolyte layer
4
from flowing outside together with H
2
O.
The anode
2
is produced, for example, by using a screen printing apparatus
10
shown in FIG.
9
. At first, the electrode base material
5
a
(carbon paper or carbon cloth) is placed on a vacuum suction plate
12
through which a plurality of suction holes
11
each having a diameter of about 1 to 2 mm are provided at intervals of about 10 mm.
A suction jig
14
, which is provided with a recess
13
, is arranged under the vacuum suction plate
12
. A tube section
15
protrudes on one side of the suction jig
14
. An unillustrated suction mechanism is connected to the tube section
15
. The atmospheric air around the vacuum suction plate
12
is sucked through the recess
13
by the suction mechanism. Accordingly, the vacuum suction plate
12
is positioned and fixed on the suction jig
14
. Similarly, the atmospheric air around the electrode base material
5
a
is sucked through the suction holes
11
. Accordingly, the electrode base material
5
a
is positioned and fixed on the vacuum suction plate
12
.
A paste P
1
for the water-repellent layer (see FIG.
10
), which is prepared by dispersing carbon black particles and PTFE particles in a solvent such as ethylene glycol together with a surfactant, is applied onto the electrode base material
5
a
in this state.
Specifically, the paste P
1
is supplied onto a screen
17
of a screen section
16
of a screen printing apparatus
10
. Subsequently, as shown in
FIG. 10
, a squeegee
18
is displaced from the right end to the left end. Accordingly, the screen
17
is expanded from a frame member
19
toward the electrode base material
5
a
, and the paste P
1
is applied onto the electrode base material
5
a
through the screen
17
.
The paste P
1
is coated on the electrode base material
5
a
by a predetermined thickness. The paste P
1
is heated to remove the solvent by volatilization. Accordingly, the water-repellent layer
6
a
(see
FIG. 8
) is formed.
Subsequently, the electrode base material
5
a
, on which the water-repellent layer
6
a
has been formed, is positioned and fixed again on the vacuum suction plate
12
(see
FIG. 9
) in the same manner as described above, and a paste P
2
for the electrode catalyst layer is supplied onto the screen
17
. The paste P
2
is prepared by dispersing carbon black particles with Pt supported thereon in a solvent such as ethylene glycol. The squeegee
18
of the screen printing apparatus
10
is displaced from the right end to the left end in the same manner as described above. Accordingly, the paste P
2
is coated onto the water-repellent layer
6
a
through the screen
17
.
Finally, the electrode base material
5
a
, the water-repellent layer
6
a
, and the paste P
2
are pressed and heated. During this process, the solvent in the paste P
2
is removed by vaporization. Accordingly, the electrode catalyst layer
7
a
(see
FIG. 8
) is formed. As a result, the anode
2
is completed. The cathode
3
is produced in the same manner as the anode
2
.
If the water-repellent layers
6
a
,
6
b
are unnecessary, the steps of applying and drying the paste P
1
are omitted. That is, the electrode catalyst layers
7
a
,
7
b
are directly formed on the electrode base materials
5
a
,
5
b
by applying the paste P
2
onto the electrode base materials
5
a
,
5
b
, and then drying the paste P
2
.
The carbon paper or the carbon cloth used for the electrode base material
5
a
,
5
b
is a porous member having a porosity of 70 to 90%. For this reason, when the paste P
1
is applied after positioning and fixing the electrode base material
5
a
,
5
b
by suction as described above, the paste P
1
enters pores
20
of the electrode base material
5
a
,
5
b
as shown in FIG.
11
. Therefore, a large number of depressions
21
are formed on the water-repellent layer
6
a
,
6
b
. If the paste P
2
is applied and dried in this state, a large number of depressions
21
are also formed on the electrode catalyst layer
7
a
,
7
b.
The electric charge distribution is not uniform in the electrode catalyst layer
7
a
,
7
b
having the large number of depressions
21
. The conductivity of the electrodes
2
,
3
of the fuel cell is lowered, and the internal resistance of the fuel cell is increased. Therefore, the output of the fuel cell is lowered, especially when current density is high.
In order to solve the problem as described above, it is considered to be effective tha
Iwasaki Kazuhiko
Okada Noboru
Tanaka Ichiro
Barr Michael
Honda Giken Kogyo Kabushiki Kaisha
Lahive & Cockfield LLP
Laurentano Anthony L.
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