In the electrodes of fuel cells, a fuel gas such as hydrogen and an oxidant gas such as air react electrochemically to generate electricity and heat simultaneously. Owing to the variety of electrolytes with which they are equipped, there are several types of fuel cells.
FIG. 1 is a sectional view illustrating a structure of conventional polymer electrolyte fuel cells. Polymer electrolyte fuel cells comprise electrolyte membrane-electrode assemblies 5 (MEAs), comprising a hydrogen ion conductive polymer electrolyte membrane 1 and a pair of electrodes 4 sandwiching the membrane. The pair of electrodes comprise an anode and a cathode, wherein a fuel gas is supplied to the anode and an oxidant gas is supplied to the cathode. The polymer-electrolyte membrane, for example, has a —CF2-skeleton and comprises a perfluorocarbon sulfonate having sulfonic acids on the terminal ends of its side chains.
The anode and the cathode comprise a catalyst layer 2 contiguous with the hydrogen ion conductive polymer electrolyte membrane and a gas diffusion layer 3 having gas-permeability and electroconductivity arranged on the outer face of the catalyst layer.
Electroconductive separators 7 for affixing an MEA, and at the same time electrically interconnecting in series neighboring MEAs, are arranged on the outer faces of the MEA. The electroconductive separator has a gas channel 6 for supplying the fuel gas or the oxidant gas to the anode or the cathode, and for conveying a surplus gas and water created by the reaction of hydrogen and oxygen. The gas channels can be provided independently of the electroconductive separator, but the gas channel is generally formed by providing ribs or grooves on the surface of the electroconductive separator.
A cooling water channel 8 can be formed on some of the electroconductive separators. For example, electroconductive separators, each having a gas channel on one side thereof and a prescribed groove on the other side thereof, are bonded together with a sealant 10 in such a manner that the side having the gas channel faces outside, thereby the prescribed grooves form a cooling water channel as shown in FIG. 1.
Gaskets 9 are arranged between both peripheries of the electroconductive separators and the MEA, in order to prevent gases from mixing with each other and from leaking outside.
To increase output voltage in procuring power-generating devices, a plurality of unit cells, comprising an MEA and a pair of electroconductive separators having gas channels, are laminated. A fuel gas or an oxidant gas is supplied from the exterior through a manifold to the inlet of the gas channel within each unit cell. Electric current generated through the electrode reactions is then collected to the gas diffusion layers and taken out to the exterior through the electroconductive separators.
During the operation of the cell, for example, oxygen moves from the gas channel to the catalyst layer through the gas diffusion layer in the cathode while hydrogen moves from the gas channel to the catalyst layer through the gas diffusion layer in the anode. Herein, if the contact between the catalyst particles and the hydrogen ion conductive polymer electrolyte in the catalyst layer is insufficient, the reaction area becomes small, leading to deterioration of discharging performance of the cell.
Conversely, water produced through the cell reaction moves from the catalyst layer to the gas channel through the gas diffusion layer to be removed outside of the cell with the surplus gas. If the gas diffusion layer does not have proper gas permeability, the polymer electrolyte membrane cannot be kept wet in a proper degree. If the water content in the polymer electrolyte membrane is decreased, its hydrogen ion conductivity will be lowered. On the other hand, if the water content in the polymer electrolyte membrane is extremely high, condensed water will clog micropores of the gas diffusion layer or gas channels of the electroconductive separators, resulting in considerable degradation of the cell performance. This condition is called “flooding”.
Therefore, the contact condition of the catalyst particles with the hydrogen ion conductive polymer electrolyte and the gas permeability in the anode and the cathode considerably affect discharging performance of the fuel cell.
In order to enlarge the reaction area of the anode and the cathode, it is effective that the hydrogen ion conductive polymer electrolyte is included in the catalyst layer (Japanese Examined Patent Publication No. Sho 62-61118, U.S. Pat. No. 5,211,984). Likewise, in order to increase gas permeability of the anode and the cathode, it is effective that a water-repellent is included in the catalyst layer (Japanese Laid-Open Patent Publication No. Hei 5-36418, J. Electroanal. Chem. 197, 195(1986)). Therefore, the catalyst layer generally contains catalyst particles, a hydrogen ion conductive polymer electrolyte and, if necessary, water repellent. Further, carbon powders carrying a platinum-group metal are utilized as the catalyst particles.
Generally, the anode and the cathode are obtained by forming a catalyst layer on one side of the gas diffusion layer. The catalyst layer is usually formed by applying an ink, which comprises catalyst particles, a dispersion of a hydrogen ion conductive polymer electrolyte and an organic solvent such as isopropyl alcohol, onto the gas diffusion layer by using a screen printing method or a transfer printing method. The above-mentioned ink normally contains a pore-producing agent, but the pore-producing agent is to be removed during calcination of the electrode after forming the catalyst layer; thereby, micropores for passing a gas through are formed inside of the catalyst layer. The catalyst layer thus obtained has a constant mixing ratio of the catalyst particles to the hydrogen ion conductive polymer electrolyte in its thickness direction.
Conventional polymer electrolyte fuel cells as mentioned above have following problems.
First, it is considered to be effective that the mixing ratio of catalyst particles to a hydrogen ion conductive polymer electrolyte in a thickness direction of the catalyst layer varies in the thickness direction of the catalyst layer in order for hydrogen ions and electrons in the catalyst layer to move smoothly. It would be theoretically possible to vary the structure of the catalyst layer step by step, by preparing a plurality of inks each having different compositions and applying them over and over using a screen printing method or a transfer printing method, but it is practically very difficult and such a catalyst layer is not yet obtained, let alone seamlessly varying the structure of the catalyst layer by a screen printing method or a transfer printing method.
Conventional production process of anode and cathode has a problem of becoming complicated because it has a calcination process or a washing process for removing a pore-forming agent.
If an ink containing a solvent such as alcohols is screen-printed on a porous conductive base material, the ink is permeated inside the base material or passes through the base material. Accordingly, there is also a problem that a catalyst layer cannot be formed directly on the surface of a porous conductive base material. On the other hand, if an ink is screen-printed on a polymer electrolyte membrane, there are problems such as the polymer electrolyte membrane is swelled with the solvent in the ink and the polymer electrolyte membrane is difficult to be fixed on a device.
If catalyst particles and a water repellent or a carbon powder which is made water repellent are mixed with a dispersion of a polymer electrolyte, a plenty of the polymer electrolyte is adsorbed on the surface of the water repellent or the carbon powder which is made water repellent. Therefore, the contact condition of the polymer electrolyte with the catalyst particles becomes uneven and sufficient reaction area cannot be retained. Further, if a water repellent is added to an ink, catalyst particles are excessively covered with the water repellent, thereby decreasing a reaction area.
Since porous conductive base materials such as carbon paper, carbon cloth and carbon felt are conventionally used as the gas diffusion layer, it is difficult to adjust the porosity of the gas diffusion layer to the appropriate range.
As constant pressure is applied to the unit cells in laminating direction in order to decrease the contact resistance of each part and ensuring the gas sealing property, there is also such a problem that, if porosity of the gas diffusion layer is too large, the gas diffusion layer is crushed at the portion where the electroconductive separator and the gas diffusion layer are in contact and the gas permeability of the gas diffusion layer turns out to be uneven in a plane direction.