Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-11-24
2003-07-29
Ryan, Patrick (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000
Reexamination Certificate
active
06599651
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a separator for a proton exchange fuel cell using solid polymer membrane as electrolyte and to a method of manufacturing the separator. More particularly, the present invention relates to a method of manufacturing a compact, light weight, separator for a proton exchange fuel cell with improved reliability and at low cost.
2. Description of the Related Art
A fuel cell is a device that converts chemical energy into electric energy by bringing a fuel such as hydrogen and an oxidizing agent such as air to electrochemically react with each other.
Various types of fuel cells which differ according to type of electrolyte used as known; for instance, phosphoric acid type, fused carbonate type, solid oxide type and proton exchange type. Of these fuel cells, a proton exchange fuel cell is a fuel cell utilizing the fact that when a polymer resin membrane containing a proton exchange radical is saturated with water it acts as proton conductive electrolyte. The proton exchange fuel cell acts in a relatively low temperature range with excellent power generating efficiency and has attracted attention in recent years.
FIG. 6
is a diagram showing the structure of a unit cell that is a base unit of a conventional proton exchange fuel cell.
As shown in
FIG. 6
, a unit cell
1
is composed of an ion conductive solid polymer membrane
2
, an anode electrode
3
and a cathode electrode
4
arranged with solid polymer membrane
2
interposed between them. Further, at the outsides of these electrodes
3
,
4
, there are arranged an anode electrode side separator
5
and a cathode electrode side separator
6
, each of which is gas impermeable and has a gas supply groove for supplying reaction gas to one of electrodes
3
,
4
.
As ion conductive solid polymer membrane
2
, for instance, perfluorocarbon-sulfonic acid (Nafion-R: Du Pont, U.S.A.), which is a proton exchange membrane, is known. Solid polymer membrane
2
contains a hydrogen ion exchange radical, and functions as an ion conductive electrolyte when saturated in water. Solid polymer membrane
2
also functions to separate a fuel
7
supplied on the anode electrode
3
side of the solid polymer membrane
2
from an oxidizing agent
8
supplied on the cathode electrode
4
side of the solid polymer membrane
2
.
Anode electrode
3
arranged at one side of solid polymer membrane
2
is formed of a catalytic layer
3
a
and a porous carbon flat plate
3
b
. Further, cathode electrode
4
arranged opposing anode electrode
3
is formed of a catalytic layer
4
a
and a porous carbon flat plate
4
b.
Separator
5
at the anode electrode side is composed of a separator substrate
9
and fuel supply grooves
10
a
,
10
b
arranged at both sides of separator substrate
9
for supplying fuel.
On the other hand, separator
6
arranged at the cathode electrode side is composed of a separator substrate
11
, an oxidizing agent supply groove
12
for supplying an oxidizing agent arranged on one surface of separator substrate
11
at the surface side contacting cathode electrode
4
, and a fuel supply groove
10
for supplying fuel arranged on another surface of separator substrate
11
.
The principle of unit cell
1
will be described below.
When fuel
7
is supplied to anode electrode
3
and oxidizing agent
8
is supplied to cathode electrode
4
, the electromotive force is generated by the electrochemical reaction between a pair of electrodes
3
,
4
of unit cell
1
. Normally, hydrogen is used as fuel
7
and air is used as oxidizing agent
8
.
When hydrogen is supplied as fuel to anode electrode
3
, hydrogen is ionized into hydrogen ion and electron in anode catalytic layer
3
a
(Anode reaction). The hydrogen ion moves to cathode electrode
4
through solid polymer membrane
2
, and the electron moves to cathode electrode
4
through an external circuit. On the other hand, the oxygen contained in the air is supplied to cathode electrode
4
as oxidizing agent
8
causes the cathode reaction by the hydrogen ion and the electron in catalytic layer
4
a
to generate water. At this time, the electrons pass through the external circuit and become a current and is able to feed electric power. In other words, in anode electrode
3
and cathode electrode
4
, reactions shown below will progress. Further, the generated water is discharged together with not-reacted gas to the outside of unit cell
1
.
Anode Reaction: H
2
→2H
+
+2e
−
Cathode Reaction: 2H
+
+1/20
2
+2e
−
→H
2
O
In such unit cell
1
, if water content in solid polymer membrane
2
becomes less, ion resistance becomes high, and mixing of fuel
7
and oxidizing agent
8
(crossover) takes place, and unit cell
1
is not able to generate the electric power. So, it is desirable to keep solid polymer membrane
2
in the state saturated with water.
Further, when the hydrogen ion ionized in anode electrode
3
upon power generation moves to cathode electrode
4
through solid polymer membrane
2
, water also moves jointly. So, at the anode electrode
3
side, solid polymer membrane
2
tends to become dry. Further, if moisture contained in supplied fuel
7
or supplied air is less, solid polymer membrane
2
tends to become dry at around respective inlet ports of reaction gases. For this reason, pre-humidified fuel
7
and pre-humidified oxidizing agent
8
are generally supplied to unit cell
1
.
By the way, electromotive force of unit cell
1
is low as below 1 volt, and a cell stack is generally formed by laminating several tens to several hundreds of unit cells
1
via separators
5
,
6
arranged at the upper and lower sides of unit cells
1
. Cooling plates are inserted into respective unit cells
1
in order to control the temperature rise of the cell stack resulting from the power generation.
Separators
5
,
6
used in a proton exchange fuel cell are required to be impermeable to reaction gas and cooling water so as to separate each of unit cells
1
. On the other hand, separators
5
,
6
are also required to be electrically conductive in order to laminate unit cells
1
to provide a cell stack and to function as the fuel cell. Normally, a proton exchange fuel cell is operated at relatively low temperature of 70~90° C. Separators
5
,
6
inside the proton exchange fuel cell are under the severe environment where they are exposed to the air containing water vapor whose vapor pressure is close to a saturate vapor pressure at the temperature of 70~90° C., and at the same time, potential difference is generated between separators
5
,
6
pursuant to the electrochemical reaction. So, it is necessary to select a corrosion proof material for the separators
5
,
6
. As corrosion proof material, stainless steel, etc. are generally used. When stainless steel, etc. are applied to separators
5
,
6
, the surface thereof is oxidized and a passive state membrane is formed on the surface thereof. As a result, the resistance loss of the fuel cell becomes large and power generating efficiency drops to a large extent.
In the U.S.A., during 1970's, for the separators of the proton exchange fuel cell developed for the space shuttle, niobium which is excellent corrosion proof noble metal, was used. However, noble metal materials have such defects that they are extremely expensive and heavy. So, as disclosed in U.S. Pat. No. 5,521,018, Ballard Power Systems Inc. of Canada uses carbon plates for separators so as to reduce the weight and cost of a cell stack.
FIG. 7
shows the construction of a cell stack of a conventional proton exchange fuel cell using carbon plates for separators.
As sown in
FIG. 7
, a cell stack
13
is composed of, in an outer frame
14
, a cell portion
15
which generates electric power by reacting gas, and a humidifying portion
16
for humidifying reaction gas. In cell portion
15
, a plurality of unit cells
1
are arranged in outer frame
14
.
FIG. 8
is a schematic diagram showing the structure of conventional unit cell
1
in cell portion
1
Saito Kazuo
Saitou Masahiro
Takahashi Masashi
Takaishi Kazutoshi
Dove Tracy
Kabushiki Kaisha Toshiba
Ryan Patrick
LandOfFree
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