1. Field of the Invention
The present invention relates to a fuel cell that uses directly hydrogen, methanol, ethanol, dimethyl ether, isopropyl alcohol, ethylene glycol, glycerin, methane, dimethoxymethane and the like as a fuel and the air, oxygen or ozone as an oxidant.
2. Description of Related Art
A fuel cell generates electric power by an electrochemical reaction between a fuel capable of generating a hydrogen ion such as hydrogen and an oxygen containing oxidant such as the air. Its structure will be described herein. First, catalyst layers are formed respectively on both surfaces of a polymer electrolyte for transporting hydrogen ions selectively. Next, on outer surfaces of these catalyst layers, gas diffusion layers are formed using, for example, a water-repellent electrically conductive carbon particle paper that has both fuel gas permeability and electron conductivity. The catalyst layer and the gas diffusion layer form an electrode.
Then, a gas sealant or a gasket is disposed so as to surround the electrode and sandwich the polymer electrolyte so that a fuel to be supplied may not leak out and be mixed with the oxidant. This sealant or gasket is integrated with the electrode and the polymer electrolyte, thus forming a membrane electrode assembly (MEA).
In general, the catalyst layer of the fuel cell is produced by preparing a paste of a platinum-based precious metal catalyst as a catalyst with electrically conductive carbon particles such as carbon black or graphite (a catalyst carrier) and a polymer electrolyte, and forming a thin film of this paste.
Currently, “Nafion” (trade name; manufactured by DuPont.), which is a perfluorocarbon sulfonic acid polymer, is in general use as the polymer electrolyte. In order to provide the “Nafion” with hydrogen ion conductivity, it is necessary to humidify it.
The incoming fuel from an anode side is separated into hydrogen ions and electrons on the catalyst of the electrode, while hydrogen ions and electrons that have passed through the electrolyte react with the oxidant on the catalyst on a cathode side. At this time, electric energy can be obtained.
In the case where hydrogen is used as the fuel, the reactions below occur in the respective electrodes.    Anode: 2H2→4H++4e−    Cathode: O2+4H++4e−→2H2O
Alternatively, in the case where methanol is used as the fuel, the reactions below occur.    Anode: CH3OH+H2O→CO2+6H++6e−    Cathode: 3/2O2+6H++6e−→3H2O
On the catalyst layer inside the electrode, reactants and products are diffused, and the electrons and the hydrogen ions move. Thus, the size of a three-phase zone, which is a reaction point and serves as a passage of each of the fuel, the electrons and the hydrogen ions, becomes important.
The area of the three-phase zone is an effective area of the catalyst. As this area becomes larger, a utilization factor of the catalyst increases, leading to a higher cell performance. By including the polymer electrolyte in the catalyst layer as described above, the reaction area increases.
Conventionally, attempts have been made to provide a layer in which the electrode and the polymer electrolyte are mixed and dispersed at an interface between the electrode and the electrolyte. A conventional technology has suggested a method of applying a dispersed solution of the polymer electrolyte and a mixture of catalyst onto a polymer electrolyte membrane and hot-pressing with an electrode, followed by reduction of the catalyst compound, and a method including the reduction, the application and then the hot-pressing (for example, see JP 62(1987)-61118 B and JP 62(1987)-61119 B).
Further, there has been a method of forming a porous electrode, spraying the polymer electrolyte solution on the electrode, and then hot-pressing this electrode and the polymer electrolyte membrane (for example, see JP 2(1990)-48632 B and JP 3(1991)-184266 A). There also is a method of mixing powder prepared by coating a surface of polymeric resin with a polymer electrolyte into an electrode (for example, see JP 3(1991)-295172 A). Moreover, there is a method of mixing a polymer electrolyte, a catalyst, carbon powder and a fluorocarbon resin and forming a film to be an electrode (for example, see JP 5(1993)-36418 A).
However, the above-mentioned conventional catalyst layer uses the polymer electrolyte that is soluble in water and an alcohol solution such as ethanol.
When an alcohol such as methanol is used as the fuel, a reaction occurs such that alcohol:water=1:1. Accordingly, during power generation, the electrolyte elutes into the alcohol solution, so that the three-phase zone decreases, lowering the reaction efficiency, and causing a problem of voltage drop.
Furthermore, the electrolyte elutes into water generated in the cathode during power generation and humidifying water necessary for hydrogen ion conduction, so that the three-phase zone decreases, lowering the voltage.