Polymer electrolyte fuel cell stack

Details Polymer electrolyte fuel cell stack Polymer electrolyte fuel cell stack

2000-02-02

2003-03-11

Ryan, Patrick (Department: 1745)

Chemistry: electrical current producing apparatus, product, and

With pressure equalizing means for liquid immersion operation

C429S010000, C429S006000, C429S006000

Reexamination Certificate

active

06531236

ABSTRACT:

TECHNICAL FIELD
The present invention relates to a polymer electrolyte fuel cell stack that works at ordinary temperature and is used for portable power sources, electric vehicle power sources, and domestic cogeneration systems.
BACKGROUND ART
The polymer electrolyte fuel cell generates the electricity and the heat simultaneously by reacting a fuel such as hydrogen and an oxidant gas such as the air, electrochemically at gas diffusion electrodes with a catalyst like platinum carried thereon.
One example of the polymer electrolyte fuel cell stack is shown in the partially omitted perspective view of FIG.
4
.
On the opposite faces of a polymer electrolyte membrane
3
, which selectively transports hydrogen ions, catalytic reaction layers
2
. which are constituted by carbon powder with a platinum metal catalyst carried thereon, are closely formed. And, according to the requirement, a fluorocarbon water repellent may be added.
The polymer electrolyte used here may be a fluorocarbon polymer with sulfonate groups introduced into the ends of their side chains. This electrolyte has proton conductivity in the wet state. In order to activate the fuel cell, it is accordingly required to keep the polymer electrolyte in the wet state. The polymer electrolyte in the wet state has strong acidity due to H
30
dissociated from the sulfonate groups at the ends. Accordingly, the acid resistance is required for the material of the portions that are in direct contact with the electrolyte. The equivalent material to the electrolyte is also admixed to the reaction electrodes, so that the acid resistance is required for the material of the portions that are in direct contact with the reaction electrodes.
Further, on the respective outer faces of the catalytic reaction layers
2
A, a pair of diffusion layers
1
having both the gas permeability and the electrical conductivity are closely formed. This catalytic reaction layer
2
and the diffusion layer
1
constitute an electrode (an anode or a cathode).
In the case where pure hydrogen is used as the fuel, as a material constituting the anode and cathode, the same material can be used. In the case where the fuel is a gas mainly containing hydrogen, which is obtained by reforming a hydrocarbon fuel, carbon monoxide is naturally contained in the reformed gas. In order to prevent the noble metal catalyst from being poisoned with carbon monoxide, there is a proposal to add an anti-CO poisoning substance, such as ruthenium only to the anode side.
Outsides of the electrode, conductive separator (bi-polar) plates
4
are further arranged so as to mechanically fix the assembly of these electrolyte membrane and the electrodes and connect adjoining assemblies electrically with each other in series. In a portion of the separator plate
4
that is in contact with the electrode, a gas flow path
5
is formed to feed the supply of the reaction gas to the surface of the electrode and flow out the gas evolved by the reaction and the remaining excess gas. And, the gas manifolds
8
, which supply a gas to and exhaust a gas from the fuel cells, and water manifolds
14
, which supply water for cooling the fuel cell stack down and exhausts the water. A cooling means such as a cooling plate may be provided to the separator plate
4
may have.
In order to prevent the hydrogen gas and the air from being leaked from the cell laminate or from being undesirably mixed with each other, an internal sealing structure is general one, in which sealing portions or O-rings are disposed around the electrodes across the polymer electrolyte membrane.
Since the above-mentioned proton-conductive electrolyte has strong acidity, a fluorocarbon polymer material having high acid resistance is employed for sealing portions like gaskets that are in direct contact with the electrolyte.
With a view to maximizing the area of the electrodes, an external sealing structure may in adopted, which does not use the sealing portions or O-rings around the electrodes but extends the ends of the electrodes to the side face of the cell laminate and seal the side face of the cell laminate with an air-tight non-conductive material.
The polymer electrolyte fuel cell stacks of the external sealing structure are divided into an internal manifold type and an external manifold type. In the internal manifold type, the manifolds or gas flow paths for feeding a supply of gas to the respective unit cells are formed inside the cell laminate in the form of through apertures that pass through the constituents of the cell laminate such as separators. In the external manifold type, on the other hand, the manifolds are arranged outside the cell laminate.
In the prior art method wherein a solution obtained by dissolving a resin in a solvent is applied and dried or a reactive resin is applied and solidified in order to form the gas seal portion that covers the side face of the cell laminate, however, there is a problem that the sufficient gas sealing property cannot be given.
When the manifolds, which connects with gas inlets and outlets, are provided, the significant unevenness on the surface of the gas seal formed by the resin makes it difficult to ensure the favorable gas sealing property at a portion where the side face of the cell laminate is in contact with the manifold.
For example, there is a method which casts a thermosetting resin such as an epoxy resin into a cast mold which envelopes the cell laminate to integrally mold, but solidification of the resin takes time to bring about poor productivity.
Any of the above method has another problem that the gas inlets and outlets are closed by the air-tight non-conductive material.
Around the electrodes, sealing portions like gaskets are disposed and sandwiched between a pair of separator plates in order to prevent the reaction gases fed to the cathode and the anode from being leaked. The prior art technique arranges hard gaskets composed of, for example, a fluorocarbon resin, around the peripheral portion of the electrodes and subsequently places a pair of separator plates across the gaskets and, therefore, there needs the accurate adjustment of the thickness of the electrodes and the gaskets.
In the case where the gaskets have rubber-like elasticity, however, the strict size accuracy is not required, but the function of the gaskets can be attained by ascertain level of adjustment of the thickness. The properties required for the gaskets thus include acid resistance and the rubber-like elasticity. Although having the poorer acid resistance than the fluorocarbon resin, ethylene-propylene-diene rubber (EPDM) having elasticity is sometimes used for the material of the gaskets.
The separator plates are directly in contact with the electrodes and are thereby required to have high gas tightness and electrical conductivity, as well as the acid resistance. When the air is used as the oxidant gas, it is required to enhance the flow rate of the air supplied to the cathode and to efficiently remove liquid water or water vapor evolved at the cathode. A complicated structure generally called the serpentine-type as shown in
FIG. 5
is typically applied for the gas flow path structure in the separator plate. The separator plate is obtained by cutting a carbon material such as a dense carbon plate having gas tightness, a carbon plate impregnated with a resin, or glassy carbon to a desired shape and forming grooves for gas flow paths. In another example, the separator may be obtained by processing and plating a corrosion-resistant alloy plate with a noble metal on demand.
Also, on demand, the carbon material or the corrosion-resistant metal material may be used only for the portions that are in contact with the electrodes and require the sufficient electrical conductivity. Further, there has been an attempt that the separator plates of a resin-containing composite material may be used for the peripheral portions such as manifolds, which do not require the electrical conductivity. Also, there is suggested that a resin is mixed with carbon powder or metal power and press-molds or injection

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