Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2000-12-15
2004-06-08
Cantelmo, Gregg (Department: 1745)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S006000, C429S047000
Reexamination Certificate
active
06746793
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
The present invention relates to a fuel cell comprising a polymer electrolyte for use in portable power sources, power sources for electric vehicles, cogeneration systems for home and the like and a method for producing the same. More specifically, the present invention relates to an electrode for a fuel cell.
2. Background Art
A fuel cell using a polymer electrolyte is an electrochemical device for generating electric power and heat at the same time by electrochemically reacting a fuel gas containing hydrogen with an oxidant gas containing oxygen such as air.
In constructing such a fuel cell, first, catalytic reaction layers mainly composed of a carbon powder with a platinum group metallic catalyst carried thereon are formed on both sides of a polymer electrolyte membrane which transfers selectively a hydrogen ion. Next, on the outer surfaces of the catalyst layers, a pair of gas diffusion layers having both fuel gas permeability and electronic conductivity are formed, and each electrode is formed in combining one of the gas diffusion layers with one of the catalytic reaction layers. Thus, the electrodes and the polymer electrolyte membrane are primarily configured in one unit. This is called membrane electrode assembly (hereinafter, this is also referred to as “MEA”). In order to avoid leaking outside of supplied fuel gas or oxidant gas, or mixing of the two kinds of gases together, a gas seal material or a gasket is placed in the circumference of the electrodes so that they sandwich the polymer electrolyte membrane. Then, a large number of MEA's are laminated with electroconductive separator plates interposed between each thereof, and thereby a fuel cell as so-called laminated cell is condigured.
Next, the catalytic reaction layers in the electrodes of the fuel cell will be described. The carbon powder with metallic catalyst carried thereon is in the form of particle falling in the range of several hundred angstroms to several microns. By using a mixture of this carbon powder and a dispersion of a polymer electrolyte, a catalytic reaction layer having a thickness of 30 to 100 microns is formed between the electrode and the solid electrolyte membrane by a coating process such as printing or the like. In this catalytic reaction layers, electrochemical reaction of the fuel gas and the oxidant gas proceeds.
For example, in the anode where hydrogen reacts, hydrogen gas is supplied to the electrode surface through a fuel gas supply path notched in the separator plate. The electrode is usually made of a gas-permeable electroconductive material such as carbon paper or carbon cloth, and hydrogen gas can reach the catalytic reaction layer by permeating the electrode. Onto the surface of the catalyst carrying carbon powder, a polymer electrolyte formed with a dried and solidified solution of the polymer electrolyte adheres. In a so-called three phase zone constituted by a vapor phase containing hydrogen gas, a solid phase of the catalyst carrying carbon powder, and a phase of the polymer electrolyte, all of which being close to each other, the hydrogen gas is oxidized to become hydrogen ions and is discharged into the polymer electrolyte. The electron generated by oxidation of the hydrogen gas moves to an outside electric circuit passing through the electroconductive carbon powder. This electrochemical reaction progresses in a broader area because of the hydrogen gas dissolved in the polymer electrolyte. The thickness of the catalytic reaction layer is varied according the production process thereof; however, in order to obtain a good cell performance, the catalytic reaction layer is usually designed to have a thickness of 30 to 100 microns.
(1) Utilization Rate of the Catalyst in the Catalytic Reaction Layer
Within the catalytic reaction layer, however, the area which contributes to the actual electrode reaction is considered only a part of 20 microns thick in contact with the polymer electrolyte membrane. This is because the generated hydrogen ion has a difficulty in reaching the polymer electrolyte membrane. Also, in the condition where the catalyst carrying carbon powder is not in electrical contact with other carbon powder or with the electroconductive electrode, although the hydrogen ion can easily move, the electron is prevented from moving to the outside circuit. As a result, there has been a problem that the catalytic reaction layer formed by coating comes to the state where a large part thereof does not contribute to the electrode reaction, which impairs its performance, and therefore a large quantity of platinum is needed for recovering its performance.
As a consequence, it is desired to improve the catalytic reaction layer and make the platinum catalyst effectively contribute to the electrode reaction, thereby to improve the utilization rate of the platinum catalyst.
(2) Contact Resistance Between the Catalytic Reaction Layer and the Gas Diffusion Layer
The electrode for use in the polymer electrolyte fuel cell is produced by forming catalytic reaction layer comprising a noble metal carrying carbon powder on electroconductive porous electrode supporting material serving as gas diffusion layer. As the porous electroconductive supporting material, a carbon paper, a carbon cloth or the like made of carbon fiber or the like is used. These electrodes are generally formed on the supporting materials by means of screen printing process or transcription process with a noble metal carrying carbon fine powder prepared into an ink using an organic solvent such as isopropyl alcohol.
In recent years, in the viewpoint of workability, inks for electrodes using aqueous solvents in place of organic solvents have been proposed. However, when these methods are used, a part of the noble metal carrying carbon powder serving as the catalyst in the electrode penetrates into the electrode supporting material, which constitutes the gas diffusion layer. For this reason, measures are required such as using a relatively large amount of electrode catalyst, or maintaining electroconductivity in the contact surface between the gas diffusion layer and the catalytic reaction layer by increasing cramping pressure of the cell. Alternatively, a method in which the electrode catalyst layers are primarily applied and formed on the polymer electrolyte membrane has been proposed. These electrodes and the polymer electrolyte membrane are bound to each other by means of hot-pressing or the like.
As described above, in the polymer electrolyte fuel cell, it is strongly required not only to increase the utilization rate of the catalyst in the catalytic reaction layer but also to decrease the contact resistance between the carbon paper or the carbon cloth, which constitutes the gas diffusion layers, and the catalytic reaction layers.
(3) Reaction Efficiency Between the Polymer Electrolyte and the Catalyst
In the electrode as constituent of a polymer electrolyte fuel cell, the surface area of a so-called three phase zone, which is formed by a micropore serving as a supply path for the reaction gases (fuel gas and oxidant gas), the hydrogen ion conductive polymer electrolyte, and the electronic conductive electrode supporting material, is an important factor, which influences the discharge characteristic of the cell.
Conventionally, with an aim to enlarge this three phase zone, attempts have been made to provide, on the interface between the polymer electrolyte membrane and the porous electrode supporting material, a layer comprising a material which constitute the electrode supporting material and the polymer electrolyte, mixed with each other and dispersed. For example, in the technique described in Japanese Patent Publications No. Sho 62-61118 and Japanese Patent Publication No. Sho 62-61119, a method is disclosed in which a mixture of a polymer electrolyte dispersion and a catalyst compound is applied on the polymer electrolyte membrane, and this is hot-pressed with the electrode supporting material and subsequently the catalyst compound is reduced. Also suggested
Gyoten Hisaaki
Hatoh Kazuhito
Kanbara Teruhisa
Morita Junji
Nishida Kazufumi
Akin Gump Strauss Hauer & Feld & LLP
Cantelmo Gregg
Matsushita Electric - Industrial Co., Ltd.
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