Chemistry: electrical current producing apparatus – product – and – Having magnetic field feature
Reexamination Certificate
2001-10-23
2004-12-14
Ruthkosky, Mark (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having magnetic field feature
C429S006000
Reexamination Certificate
active
06830840
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a polymer electrolyte fuel cell of the room temperature operation type, which is used for a portable power supply, a power supply of electric cars, a domestic cogeneration system, or the like.
BACKGROUND ART
A polymer electrolyte fuel cell produces both electric power and heat at the same time by making a hydrogen-containing fuel gas and an oxygen-containing oxidizing gas such as air react electrochemically. The fuel cell is fabricated as follows. First, a catalytic reaction layer mainly composed of carbon powder carrying a platinum type metallic catalyst is formed on both sides of a polymer electrolyte membrane which selectively transports hydrogen ions. Then, a diffusion layer provided with both permeability for the fuel gas or the oxidizing gas and electron conductivity is formed on the outer surface of each catalytic reaction layer. The catalytic reaction layer and this diffusion layer integrally function as an electrode. The assembly comprising the electrodes and the electrolyte membrane called as an MEA (Membrane Electrolyte Assembly).
Then, in order to avoid the supplied gases from leaking outside the fuel cell, or the fuel gas and the oxidizing gas from mixing each other, gaskets are arranged around the electrode in such a manner as to sandwich the polymer electrolyte membrane. The gaskets may be previously formed to be integral with the electrode and the polymer electrolyte membrane, and the integrated body is called an MEA in some cases.
Furthermore, a conductive separator plate is provided outside the MEA to fix the MEA mechanically and to connect adjacent MBAs electrically in serial. At the portions of the separator plate that are in contact with the MEAs, gas channels are formed to supply reaction gases to and to carry away a produced gas or excessive gases from the surfaces of the electrodes. The gas channel could be provided individually and separately from the separator plate; however, it is common that a groove is made on the surface of the separator plate and used as the gas channel.
In general, in the actual use of a fuel cell, a laminated structure where a plurality of the above-mentioned unit cells is stacked is adopted. During the operation of the fuel cell, not only electric power but also heat is produced; in the laminated structure, a cooling plate is provided for every one or two unit cells to make it possible to keep the cell temperature constant and to use the produced heat energy in the form of hot water or the like. The common structure of a cooling plate is composed of a thin metallic plate and a heat medium such as cooling water circulating through inside thereof. Moreover, there is another structure where channels are provided on the rear side of the separator which composes a unit cell, i.e., on the side where the cooling water is intended to circulate, thereby making the separator plate function as the cooling plate. In this case, an O-ring or a gasket are needed for sealing a heat medium such as cooling water. In this sealing method, it is necessary to secure sufficient electrical conductivity between the top and bottom of the cooling plate by completely compressing the O-ring or the like.
Furthermore, in such a laminated structure, apertures called manifolds to provide and discharge the fuel gas to and from each unit cell are necessary. As the manifolds, the common one is a so-called internal manifold type provided with apertures for supplying and discharging cooling water inside the cell stack.
Which of the internal manifold type or the external manifold type may be used, it is necessary that the plurality of unit cells including cooling plates is stacked in one direction; a pair of end plates are arranged at both ends of the stacked cell; and the stacked cell is pressed and fixed from outside the two end plates by using a fastening rod. In the fastening, it is preferable to fasten the unit cells as uniformly as possible within the plane thereof. From the viewpoint of mechanical strength; the end plates and the fastening rod are usually made from a metallic material such as stainless steel. These end plates and the fastening rod are electrically isolated from the stacked cell by an insulating plate so as to produce a structure where there is no current leakage outside through the end plates. As for the fastening rod, it has been suggested to penetrate it through a through-hole inside the separator and to fasten the whole stacked cell including the end plates by means of a metallic belt.
In the aforementioned polymer electrolyte fuel cell the electrolyte membrane functions as an electrolyte when it contains some water and, therefore, it is necessary to moisturize and supply the fuel gas and the oxidizing gas. In; addition, the polymer electrolyte membrane has the effect of increasing the ion conductivity thereof with increasing water content, thereby reducing the internal resistance of the cell, and exhibiting high performance at least within a temperature range up to 100° C. Therefore, in order to increase the water content in the electrolyte membrane, it is necessary to supply the supplied gas after being highly moisturized.
However, supplying an excessively moisturized gas at the cell operating temperature causes condensed water inside the cell, and the water drops disturb a smooth gas supply. Furthermore, on the electrode (air electrode) side to which the oxidizing gas is supplied, electric power generation produces water, thereby causing a problem that the efficiency of eliminating the produced water decreases, and the cell performance deteriorates. For this, it is general that a moisturized gas having a dew point slightly lower than the cell operating temperature is prepared and supplied into the cell.
Commonly used as methods for moisturizing a gas are a bubbler moisturizing system where a gas is supplied to deionized water maintained at a predetermined temperature and moisturized by ventilating (bubbling), and a membrane moisturizing system where deionized water maintained at a predetermined temperature is flown on one side of a membrane such as an electrolyte membrane, which allows water to move easily, whereas a gas is flown on the other side to moisturize a gas. When a gas prepared by steam reforming a fossil fuel such as methanol or methane is used as a fuel gas, there are cases where moisturizing is unnecessary because steam is contained in the reformed gas.
The moisturized fuel and oxidizing gases are supplied to the polymer electrolyte fuel cell and used for electric power generation. At this time, in a single plane of any unit cell in the stacked cell, a current density distribution occurs.
To be more specific, the fuel gas is moisturized in a predetermined manner at the gas supplying inlet and supplied; however, hydrogen in the fuel gas is consumed in electric power generation, Which causes a phenomenon that a hydrogen partial pressure is high and a steam partial pressure is low in the upstream parts of the fuel gas, whereas the hydrogen partial pressure is low and the steam partial pressure is high in the downstream parts of the fuel gas. Furthermore, the oxidizing gas is also moisturized in a predetermined manner at the gas supplying inlet and supplied; however, oxygen in the oxidant gas is consumed in electric power generation, and water is produced by electric power generation, which brings about a phenomenon that the oxygen partial pressure is high and the steam partial pressure is low in the upstream parts of the oxidizing gas, whereas the oxygen partial pressure is low and the steam partial pressure is high in the downstream parts of the oxidizing gas. Moreover, the temperature of the cooling water for cooling the cell becomes low on the inlet side and high on the outlet side of the fuel cell so as to cause a temperature distribution in a single plane of a unit cell. From these reasons, in a single plane of the cell, a current density distribution (performance distribution) occurs.
When an unevenness of the hydrogen and steam partial pressures in the fuel gas; an unev
Gyoten Hisaaki
Hatoh Kazuhito
Kanbara Teruhisa
Matsumoto Toshihiro
Niikura Junji
Akin Gump Strauss Hauer & Feld,L.L.P.
Ruthkosky Mark
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