Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
1999-07-28
2002-11-05
Brouillette, Gabrielle (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S006000, C429S006000, C429S006000, C429S047000, C427S115000
Reexamination Certificate
active
06475656
ABSTRACT:
This application is 371 of PCT/EP98/00479, filed Jan. 29, 1998.
BACKGROUND OF THE INVENTION
The present invention relates to a membrane-electrode unit for a polymer electrolyte membrane fuel cell having a polymer electrolyte membrane, an anode arranged on one surface of the membrane and a cathode arranged on the other surface of the membrane, as well as to a method of making the membrane-electrode unit.
Polymer electrolyte membrane fuel cells, as they are commonly employed for producing electric current, contain an anode, a cathode and an ion exchange membrane disposed therebetween. A plurality of fuel cells constitutes a fuel cell stack, with the individual fuel cells being separated from each other by bipolar plates acting as current collectors. The bipolar plate on the anode side of a cell constitutes at the same time the cathode side bipolar plate of the neighboring cell. For generating electricity, a burnable gas, e.g. hydrogen, is introduced into the anode region, and an oxidizing agent, e.g. air or oxygen, is introduced into the cathode region. Both the anode and cathode in the regions in contact with the polymer electrolyte membrane contain a catalyst layer. In the anode catalyst layer, the fuel is oxidized thereby forming cations and free electrons, and in the cathode catalyst layer, the oxidizing agent is reduced by taking up electrons. As an alternative, the two catalyst layers may also be applied on opposite sides of the membrane. The structure of an anode, a membrane, a cathode and the corresponding catalyst layers is referred to as membrane-electrode unit. The cations formed on the anode side migrate through the ion exchange membrane to the cathode and react with the reduced oxidizing agent, thereby forming water when hydrogen is used as burnable gas and oxygen is used as oxidizing agent. The heat created in the reaction of burnable gas and oxidizing agent is dissipated by exactly fitting manner for each individual membrane-electrode unit. Inexpensive manufacture of membrane-electrode units in the form of square-meter material is not possible. Moverover, the seals must be cut separately and then attached in an exactly fitting manner. cooling. For better distribution of the reaction gases and, possibly, for supporting the membrane-electrode unit, gas-conducting structures, e.g. grid-like nets, may be provided between electrodes and bipolar plates.
Upon installation in a fuel cell, the membrane-electrode unit is in contact on the anode side with the burnable gas and on the cathode side with the oxidizing agent. The polymer electrolyte membrane separates the regions containing the burnable gas and the oxidizing agent, respectively, from each other. For preventing contact of the burnable gas and oxidizing agent, which could cause explosion-like reactions, reliable sealing of the gas spaces from each other must be ensured. In this respect, a problem is present in particular for providing a sealing against burnable gas hydrogen that has excellent diffusion properties.
In order to prevent that a gas exchange can take place in the fuel cell along the edges of the membrane, the following measures are taken conventionally: in producing conventional membrane-electrode units, the dimensions for the membrane and electrodes are selected such that, with a sandwich-like arrangement of the membrane between the electrodes, the membrane projects on each side a good distance beyond the area of the electrodes. The conventional membrane-electrode unit thus comprises a membrane with the edge portions that are not covered by electrode material. Flat seals, e.g. of stretched PTFE, are attached around the periphery of the membrane-electrode unit on both sides of the membrane so as to cover the projecting portions of the membrane. In case of a square membrane-electrode unit, for example, square frames are pressed on and/or attached adhesively on both sides of the membrane, such that they at least partly cover the projecting portions of the membrane. These conventional membrane-electrode units on the one hand involve the disadvantage that they are quite complex in manufacture since the anode, cathode and membrane must each be cut separately and then must be assembled in an exactly fitting manner for each individual membrane-electrode unit. Inexpensive manufacture of membrane-electrode units in the form of square-meter material is not possible. Moreover, the seals must be cut separately and then attached in an exactly fitting manner.
A further disadvantage of the conventional membrane-electrode units becomes evident in mounting the same in a fuel cell. In the fuel cell, a gastight space must be provided at least on the anode side between membrane-electrode unit and the bipolar plate confining the cell. Conventionally, sealing rings or strips are employed here between membrane-electrode unit and bipolar plate, with several cells each being clamped together in series and being provided with a joint supply of burnable gas. The gastight spaces are formed upon such clamping together only. In case of a leak, it is difficult to locate the same, and it is not possible either to remove just one cell, but only the clamped together unit containing the leak. This involves considerable expenditure in work and loss of useful time of the fuel cell.
Occasionally, it is dispensed with providing the membrane-electrode unit with a pressed-on sealing frame. Sealing then is effected upon installation in a fuel cell by clamping a sealing ring between the membrane part not covered by electrode material and the adjacent bipolar plate. In both cases, a gap results between the electrode material and the seal, making the arrangement sensitive to mechanical damage, in particular in case of thin or brittle membranes. Furthermore, there is the risk that the membrane-electrode unit is not clamped in a completely planar manner so that the membrane contacts the metallic current lead-out conductor. The metal then may be partly removed by an acid membrane. The metal ions enter the membrane, thereby impairing the conductivity thereof.
The present invention allows to overcome the above-indicated disadvantages.
SUMMARY OF THE INVENTION
It is the object of the invention to make available a membrane-electrode unit for a polymer electrolyte membrane fuel cell, which on at least one side can be connected to a bipolar plate in such a manner that a gastight space is formed between membrane and bipolar plate.
Another object of the invention is to make available a membrane-electrode unit in which the assembly membrane-electrode unit/bipolar plate can be tested for gas tightness separately.
A further object of the invention consists in making available a simple, inexpensive method of making such membrane-electrode units.
In making the membrane-electrode unit, according to the invention, the anode, cathode and membrane are not cut separately and the individual parts then connected to each other, but rather a layer material is produced consisting of an anode material, a cathode material and a membrane material disposed therebetween, for example by means of a rolling method as employed in paper production. This provides square-meter material from which the individual membrane-electrode units can be cut, punched or severed in another manner in one operation in the desired size. A membrane-electrode unit obtained in this manner contains, apart from the end face, no free membrane area, but rather the membrane on both surfaces thereof is fully covered by the anode material and the cathode material, respectively. If desired, passages can be formed in the membrane-electrode unit, which is possible in one operation as well.
The membranes, electrodes and catalysts used for manufacturing the membrane-electrode unit according to the invention as such may be conventional materials, as they are commonly used for corresponding purposes. As electrodes, i.e. anodes and cathodes, there may be used, for example, diffusion electrodes of carbon paper or graphitized fabrics, containing a catalyst having an arbitrary distribution parallel and also vertical to
Koschany Arthur
Schwesinger Thomas
Brouillette Gabrielle
Crepeau Jonathan
Kinberg Robert
Proton Motor Fuel Cell GmbH
Venable
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