Polymer electrolyte fuel cell and method for prudoucing the same

Abstract

A polymer electrolyte fuel cell is provided comprising: a hydrogen ion conductive polymer electrolyte membrane; an anode and a cathode sandwiching the hydrogen ion conductive polymer electrolyte membrane; an anode side electroconductive separator having a gas channel for supplying a fuel gas to the anode; a cathode side electroconductive separator having a gas channel for supplying an oxidant gas to the cathode; characterized in that the anode and the cathode comprise a gas diffusion layer and a catalyst layer formed on the gas diffusion layer at the side in contact with the hydrogen ion conductive polymer electrolyte membrane, the catalyst layer has catalyst particles and a hydrogen ion conductive polymer electrolyte, and at least either of hydrogen ion conductivity and gas permeability of at least either of the anode and the cathode varies in a thickness direction of the anode or the cathode.

Description

TECHNICAL FIELD [0001] The present invention relates to polymer electrolyte fuel cells useful as consumer co-generation systems and mobile power-generating appliances. BACKGROUND ART [0002] 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. [0003]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...

AI Technical Summary

Benefits of technology

[0021] In view of the foregoing problems, it is an object of the present invention to provide a polymer electrolyte fuel cell having a large reaction area and proper gas permeability, which demonstrates high discharging performance even when the cell is operated at a high current density.

Problems solved by technology

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.

If the gas diffusion layer does not have proper gas permeability, the polymer electrolyte membrane cannot be kept wet in a proper degree.

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.

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.

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.

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.

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.

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