Polymer electrolyte fuel cells system

Details Polymer electrolyte fuel cells system Polymer electrolyte fuel cells system

2000-05-03

2003-09-02

Kalafut, Stephen (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S006000, C429S047000, C429S047000

Reexamination Certificate

active

06613467

ABSTRACT:

This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 10-251124, filed Sep. 4, 1998, the entire contents of which are incorporated herein by reference.
BACKGROUND OF THE INVENTION
This invention relates to a polymer electrolyte fuel cells system which makes use of a solid polymer as an electrolyte, and in particular to a polymer electrolyte fuel cells system having a mechanism for preventing the drying of a solid polymer electrolyte membrane.
A fuel cells system is designed such that a fuel gas such as hydrogen or a reactive gas is rendered to electrochemically react with an oxidizing gas such as air so as to directly convert the chemical energy of the fuel to an electric energy.
This fuel cells system can be classified into various types depending on the kind of electrolyte. As one type of fuel cells system, there is known a polymer electrolyte fuel cells system which makes use of a solid polymer as an electrolyte.
FIG. 1
shows a cross-sectional view of one example of the structure of such a polymer electrolyte fuel cells system.
Referring to
FIG. 1
, each cell
4
comprises a pair of gas-diffusing electrodes consisting of a fuel electrode (hereinafter, referred to as an anode)
1
a
and an oxidizing electrode (hereinafter, referred to as a cathode)
1
b
, and a polymer electrolyte membrane
3
having ion conductivity and gas-separating function which is interposed between the pair of gas-diffusing electrodes with a catalyst layer
2
a
or
2
b
(made of Pt for example) being interposed between the solid polymer electrolyte membrane
3
and each of the gas-diffusing electrodes. This cell
4
is capable of generating an electric output through a power generation by an electrochemical reaction between a fuel gas or a reactive gas and an oxidizing gas.
This fuel cells system is constituted by a plurality of cells
4
and a gas-impermeable separator
5
provided with grooves for feeding a reactive gas to each of the electrodes of the cells
4
.
According to this fuel cells system, a fuel gas such as hydrogen is fed to the anode
1
a
, while an oxidizing gas such as air is fed to the cathode
1
b
so as to allow an electrochemical reaction to take place, thereby generating an electromotive force at each cell
4
. This electromotive force of each cell
4
is as low as about 1V at most. Therefore, in order to obtain a high output, a cell stack comprising a laminate body of a plurality of cells
4
is put to practical use as a fuel cells system.
Since the electrochemical reaction in this fuel cells system is exothermic reaction, heat is caused to generate. For the purpose of removing this superfluous heat, a cooling plate
7
allowing a cooling medium to pass therethrough is disposed beside every cell laminate body
6
comprising a plurality of cells
4
which are laminated with a separator
5
being interposed between neighboring cells.
Further, if the fuel gas leaks outside the system, not only the utilization of fuel gas is deteriorated, but also there is a danger of explosion by the fuel gas. Therefore, a gas seal is applied, by making use of a sealing material
8
, to a space between the solid polymer electrolyte membrane
3
and the gas-impermeable separator
5
.
Additionally, at the location of the cathode
1
b
, water is generated due to an electrode reaction. When this water is condensed at the electrode reaction site, the diffusion of gas is badly affected. Therefore, the water thus produced is required to be discharged together with unreacted gas outside the cell.
On the other hand, as for the material for the solid polymer electrolyte membrane
3
, a perfluorosulfonate film which is a fluorinated ion-exchange membrane is known. This solid polymer electrolyte membrane
3
contains an exchange group or hydrogen ion in its molecule and hence, functions as an ion conductive material when it is saturated with water.
However, once this solid polymer electrolyte membrane
3
is dried on the contrary, the ion conductivity thereof is lowered, thus prominently deteriorating the performance of cell. Therefore, there have been taken various measures to prevent the drying of the solid polymer electrolyte membrane
3
.
For example, there is known a method wherein a humidifier constructed to allow water and a reactive gas to pass therethrough is disposed on both sides of a steam-permeating film such as the solid polymer electrolyte membrane
3
so as to allow the reactive gas to be wetted before it is fed to the solid polymer electrolyte membrane
3
.
In this case, the humidifier is generally formed integral with the cell stack. Further, it is also known that if a reactive gas to be fed to the anode
1
a
and cathode
1
b
is allowed to flow to face each other and at the same time, the operation temperature is controlled to not more than 60° C. so as to increase the relative humidity of the reactive gas, the generation of power can be achieved without necessitating to humidify the reactive gas.
On the other hand, as shown in Japanese Patent Unexamined Publication H6-132038, there has been also proposed a method of humidifying unreactive gas by introducing both reacted gas and unreacted gas into a gas chamber partitioned by means of a steam permeating film.
In this case, since water vapor is caused to generate on the cathode side
1
b
due to an electrode reaction, the reacted gas is rendered to contain saturated or nearly saturated water vapor.
On the other hand, since the quantity of water vapor contained in the unreacted gas is relatively small, a difference in partial pressure of water vapor is caused to generate between the reacted and unreacted gases, so that this difference in partial pressure of water vapor can be utilized as a driving force for effecting the concentration diffusion of water vapor.
Further, as shown in Japanese Patent Unexamined Publication H8-273687, there is also proposed to use a hollow fiber as a water vapor-permeating film, wherein unreacted gas is fed through the interior of the hollow fiber and the reacted gas is fed through the exterior of the hollow fiber, thereby humidifying the reactive gas.
Since the contact area between the reacted gas and the unreacted gas can be increased due to the employment of this hollow fiber, it becomes possible to provide a compact humidifier having a high humidification efficiency. Moreover, since a hollow fiber is employed, it becomes possible to incorporate the humidifier inside the gas manifold of the cell stack.
However, these conventional polymer electrolyte fuel cells systems as mentioned above are accompanied with various problems that when the reactive gas is to be humidified by means of a humidifier or through a humidity exchange, the resultant system becomes inevitably sophisticated and difficult to make it compact, and also may raise various problems when it is used in a low temperature environment such as an air atmosphere of 0° C. or less.
In the case of the humidifier having a structure wherein water and reactive gas are allowed to flow along both sides of a water vapor-permeating film, the freezing of water passageway may be caused to generate when an external temperature is lowered, thus possibly inviting the closing of the passageway, the fracturing of the water vapor-permeating film due to the expansion in volume of ice, and the deformation of the separator
5
.
On the other hand, if a non-humidifying operation is to be performed without employing a humidifier, it may become difficult to ensure the long term stability of the solid polymer electrolyte membrane
3
and of the cell performance. In addition to this problem, when the fuel cell is operated at a temperature of not more than 60° C. which is lower than the ordinary operating temperature of 70 to 90° C. by making use of a fuel gas containing CO as in the case of a reformed gas, the catalyst in the anode
1
a
is badly affected by this CO, thus resulting in the promotion of anodic polarization and hence, badly deteriorating the cell performance.
In the case of humidifying the reactive gas

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