Chemistry: electrical current producing apparatus – product – and – With pressure equalizing means for liquid immersion operation
Reexamination Certificate
2002-02-15
2004-11-16
Barr, Michael (Department: 1746)
Chemistry: electrical current producing apparatus, product, and
With pressure equalizing means for liquid immersion operation
C429S047000, C429S209000
Reexamination Certificate
active
06818339
ABSTRACT:
TECHNICAL FIELD
The present invention relates to a room temperature-activating polymer electrolyte fuel cell used in portable power sources, power sources for electric vehicles, and domestic co-generation systems.
BACKGROUND ART
Polymer electrolyte fuel cells simultaneously generate electric power and heat through electrochemical reactions of a hydrogen-containing fuel gas and an oxygen-containing oxidant gas, such as the air.
To manufacture this fuel cell, a catalyst layer mainly composed of carbon powder with a platinum metal catalyst carried thereon is formed in first on both faces of a polymer electrolyte membrane that selectively transports hydrogen ions. Subsequently, a gas diffusion layer having both gas permeability and electron conductivity relative to either the fuel gas or the oxidant gas is formed on outside each catalyst layer, giving an electrode composed of the catalyst layer and the gas diffusion layer. The joint body of the electrodes and the electrolyte membrane is referred to as an MEA.
In order to prevent leakage of the supplied gas and mixing of the fuel gas and the oxidant gas, gaskets are disposed across the polymer electrolyte membrane to surround the electrodes. In some cases, the gaskets, the electrodes, and the polymer electrolyte membrane are integrated with one another as the MEA.
Each MEA is interposed between a pair of conductive separator plates, which mechanically fix the MEA and electrically connect the adjoining MEAs with each other in series. A gas flow path is formed in a portion of the separator plate that is in contact with the MEA to feed a supply of the reaction gas to the electrode plane and flow out the produced gas and the remaining excess gas. The gas flow path may be separately attached to the separator plate, but generally a groove is formed on the surface of the separator plate to function as a gas flow path.
The conductive separator plate is required to have high electron conductivity, gas tightness, and high corrosion resistance and the prior art technique thus forms a groove in a dense carbon plate by cutting or another adequate working process to complete the conductive separator plate.
The gas flow path formed in the prior art conductive separator plate generally has multiple linear gas flow paths (straight flow paths) in parallel running from the gas inlet to the gas outlet. In the polymer electrolyte fuel cell, water is produced on the cathode during the operation and, therefore, there is a problem that efficient removal of the product water is required for exertion of the sufficient cell performance. The length of each gas flow path is accordingly extended by decreasing the sectional area of the gas flow path formed in the conductive separator plate and forming a meandering gas flow path (serpentine flow path). This practically raises the gas flow rate and forcibly removes the product water, thereby improving the cell performance.
A plurality of the above single cells are laid one upon another to form the structure of cell stack when fuel cell is used. Heat as well as electric power is generated during the operation of the fuel cell, and a cooling plate is accordingly provided for every one or two single cells in the cell stack, in order to keep the cell temperature at a substantially fixed level and allow utilization of the simultaneously generated thermal energy.
A thin metal plate having the structure that allows circulation of a thermal medium inside thereof, such as cooling water, is generally used for the cooling plate. A flow path for cooling water is formed on the rear face of the conductive separator plate included in the single cell, that is, on a specific face of the conductive separator plate where a cooling water may be flowed, and this conductive separator plate may be used as the cooling plate. At that time, an O-ring or a gasket is required for sealing the thermal medium like cooling water. In this case, the O-ring should be pressed completely to ensure the sufficient electrical conductivity across the cooling plate.
The cell stack requires an aperture called manifold for supplying and discharging the fuel gas and the oxidant gas to the respective single cells. The inner manifold structure, in which an aperture for supplying and discharging the cooling water is formed inside the cell stack, is typically adopted.
In the either of the inner manifold structure and the outer manifold structure, it is necessary that a plurality of single cells including the cooling sections are stackd in one direction to form a stack (cell stack) and a pair of end plates are arranged on both ends of the stack and are fixed with a clamping rod.
In case of clamping by using the clamping rod, it is desirable to evenly clamp the single cells in the plane direction. From the viewpoint of mechanical strength, a metal material such as stainless steel is typically used for the end plates and the clamping rod. These end plates and the clamping rod are electrically insulated from the stack via an insulator, so that the electric current does not leak outside via the end plates. The clamping rod may be inserted in a through hole formed in the separator plates. It is also proposed that the whole stack may be clamped via the end plates with a metal belt.
In the polymer electrolyte fuel cell thus obtained, the electrolyte membrane functions as the electrolyte in the wet state, and the supplies of the fuel gas and the oxidant gas should thus be humidified. In the temperature range up to at least 100° C., the higher water content of the polymer electrolyte membrane increases the ion conductivity and lowers the inner resistance of the cell, thus ensuring the high performance.
Supply of a high humid gas at temperatures higher than the cell driving temperature, however, causes sweating inside the cell and the water drops undesirably interfere with the smooth gas supply. Water is produced by power generation on the cathode, to which the supply of the oxidant gas is fed, and thus there is a problem that the efficiency of removal of the product water and the cell performance are lowered. The supply of gas is thus generally humidified to have the dew point a little lower than the cell driving temperature.
As typical method of humidifying the gas supply, there are a bubbler humidification process that bubbles the supply of gas in deionized water kept at a predetermined temperature for humidification and a membrane humidification process that makes a flow of deionized water kept at a predetermined temperature in one face of a membrane that allows easy transfer of the water content, such as an electrolyte membrane, while making a flow of the gas supply in the other face for humidification. When a gas obtained by steam reforming a fossil fuel like methanol or methane is used for the fuel gas, steam is included in the reformed gas and humidification is thus not required in such cases.
The humidified fuel gas and oxidant gas are supplied to the polymer electrolyte fuel cell for power generation. There is a current density distribution in a single plane of an arbitrary single cell in the cell stack. The fuel gas is humidified and then fed into the fuel cell via a gas inlet and power generation consumes hydrogen included the fuel gas. This leads to a phenomenon of a higher hydrogen partial pressure and a lower steam partial pressure in the upper stream of the gas flow path and a lower hydrogen partial pressure and a higher steam partial pressure in the lower stream of the gas flow path.
The oxidant gas is humidified and then fed into the fuel cell via a gas inlet and power generation consumes oxygen included in the oxidant gas, while producing water. This leads to a phenomenon of a higher oxygen partial pressure and a lower steam partial pressure in the upper stream of the gas flow path and a lower oxygen partial pressure and a higher steam partial pressure in the lower stream of the gas flow path.
The temperature of cooling water used for cooling down the cell is lower in the vicinity of the inlet and the higher in the vicinity of the outlet and there is ac
Gyoten Hisaaki
Hori Yoshihiro
Hosaka Masato
Kanbara Teruhisa
Kusakabe Hiroki
Barr Michael
Wills Monique
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