Seal for fuel cell and forming method therefor

Details Seal for fuel cell and forming method therefor Seal for fuel cell and forming method therefor

2001-07-17

2003-12-23

Ruthkosky, Mark (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S006000, C277S628000, C425S412000

Reexamination Certificate

active

06667124

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to seals for gas sealing in solid polymer electrolyte fuel cells, and relates to a forming method therefor.
2. Related Art
In solid polymer electrolyte fuel cells, a separator is layered on both sides of a plate-shaped membrane electrode assembly to form a unit of the layered structure, and the plural units are layered to form a fuel cell stack. The membrane electrode assembly is a layered structure, in which a polymerized electrolytic membrane is held by a positive catalytic electrode (cathode electrode plate) and a negative catalytic electrode (anode electrode plate), and a gas diffusion layer is layered on the outer surface of each catalytic electrode. The separator is made from a material having electron transmitting characteristics, and has plural grooved gas passages in which a fuel gas such as hydrogen gas, an oxidizing gas such as oxygen or air, and a coolant flow individually. The separator is layered on the membrane electrode assembly such that linear protrusions between the gas passages are contacted with the gas diffusion layer.
According to the fuel cell, a fuel gas is provided to the gas passage of the separator at the negative electrode side, and an oxidizing gas is provided to the gas passage of the separator at the positive electrode side, whereby electricity is generated by electrochemical reaction. During the operation of the fuel cell, the gas diffusion layers transmit the electrons generated by the electrochemical reaction between the catalytic electrode layers and the separators, and diffuse the fuel gas and the oxidizing gas. The catalytic electrode layer in the negative electrode side results in a chemical reaction for the fuel gas so as to generate protons and electrons. The catalytic electrode layer in the positive electrode side generates water from oxygen, the proton, and the electron, and the polymerized electrolytic membrane facilitates ionic migration for the proton, whereby the electric power is provided via the positive and negative catalytic electrode layer.
In the above-described fuel cell, the fuel gas, the oxidizing gas, and the coolant must be flowed in the individual gas passages, so that the gas passages are separated from each other by a seal. The sealing portion varies according to the structure of the fuel cell stack. For example, a seal is provided around a communicating opening of the gas passages penetrating the fuel cell stack, around the membrane electrode assembly, around a coolant passage provided on the outer surface of the separator, and around the circumference of the outer surface of the separator.
According to conventional sealing technology, in general, an elastic material made from an organic rubber of the fluorine type, silicone type, ethylene propylene type, or the like, is formed into a shape of a sheet or an O-ring, and is mounted to a sealing portion. The sealing member seals the sealing portion by a reaction force generated by being compressed in a stacked condition. As other sealing structures, a seal in which an inorganic material formed by carbon or ceramics is compressed, a mechanical seal using caulking, adhering, and the like have been provided.
Fuel cells are often carried or installed in automobiles for use. In these cases, the cells are stringently required to be small and thin. Since separators are usually made from brittle carbon, they are readily broken during assembling of a fuel cell stack. Therefore, seals made from organic rubbers are widely used, since they are flexible and have suitable reaction force, thereby preventing breakage of the separator in the assembly a fuel cell stack.
FIG.
6
A through
FIG. 6E
show a related method for providing a seal, which is made from an organic rubber and tightly contacts with a separator (not shown), over the circumference of the membrane electrode assembly
1
. The membrane electrode assembly
1
is formed such that a polymerized electrolytic membrane
4
is held by a cathode electrode plate
2
and an anode electrode plate
3
, and a gas diffusion layer
5
is layered on the outer surface of each electrode plate
2
or
3
. The center electrolytic membrane
4
has a larger area than that of each electrode plate
2
or
3
and each gas diffusion layer
5
, and the circumference
4
a
thereof projects from them. As shown in
FIG. 6E
, a seal
60
is integrally formed with the circumference
4
a
. Reference numerals
70
and
80
in
FIGS. 6A through 6E
are an upper die and a lower die of a forming die. In the forming die, recesses
71
and
81
into which the membrane electrode assembly
1
is fitted and grooves
72
and
82
forming a cavity
90
are formed in vertical symmetry. A gate
73
communicated to the cavity
90
from outside is formed in the upper die.
In order to provide the seal
60
to the membrane electrode assembly
1
, first, as shown in
FIG. 6A
, the membrane electrode assembly
1
is fitted into the recess
81
of the lower die
80
, and next, as shown in
FIG. 6B
, the upper die
70
is lowered and the membrane electrode assembly
1
is fitted into the recess
71
of the upper die
70
. Then, as shown in
FIG. 6C
, the upper and lower dies
70
and
80
are clamped to each other, and next, as shown in
FIG. 6D
, a sealing material
60
A is charged into the cavity
90
from the gate
73
. The sealing material
60
A is vulcanized when the material is a vulcanizing rubber, is heated when the material is of the heat-cure type, or is then removed from the opened upper and lower dies
70
and
80
without heating when the material is of the thermoplastic type, a membrane electrode assembly
1
as shown in
FIG. 6E
is then obtained. The electrolytic membrane
4
of the membrane electrode assembly
1
is integrally formed with the seal
60
which surrounds the electrode plates
2
and
3
, and the gas diffusion layer
5
.
The thickness of the membrane electrode assembly
1
is not strictly constant, and is sometimes thicker or thinner than the regular thickness. According to the forming method shown in
FIGS. 6A through 6E
, when the thickness of the membrane electrode assembly
1
is in the regular range, suitable tightening thickness of the seal, namely, a sealing pressure and a sealing height can be obtained. However, when the thickness of the membrane electrode assembly
1
exceeds the regular range, as shown in
FIG. 7A
, the circumferences of the upper and lower dies
70
and
80
do not contact sufficiently with each other and form a clearance S
1
, into which the material for sealing inserts. As a result, as shown in
FIG. 7B
, a burr
61
is formed at the outer circumference of the seal
60
, and the thickness of the seal
60
is greater than the regular range thereof, and the burr must be removed.
In contrast, when the thickness of the seal
60
is thinner than the regular range, as shown in
FIG. 8A
, a clearance S
2
is formed between the upper die
70
and the membrane electrode assembly
1
, the seal
60
is formed without compression of the membrane electrode assembly
1
. As a result, as shown in
FIG. 8B
, the height H
2
of the seal
60
from the membrane electrode assembly
1
is greater than the regular range. Therefore, the seal
60
is excessively compressed when a fuel cell stack is assembled, so that the separator is damaged or deformed. Furthermore, the material for sealing may flow into the clearance S
2
between the upper die
70
and the membrane electrode assembly
1
so as to damage the membrane electrode assembly, and the sealing material may adhere to the membrane electrode assembly
1
, so that the power generation performance may be deteriorated or energizing may be impossible in some cases.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a seal for fuel cell and a forming method therefor, in which constant sealing pressure and sealing height can be obtained even if the thickness of the membrane electrode assembly varies, whereby problems such as damages to members such as separators closely contact

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