Gas diffusion electrode for polymer electrolyte membrane...

Details Gas diffusion electrode for polymer electrolyte membrane... Gas diffusion electrode for polymer electrolyte membrane...

1999-09-02

2001-02-06

Bell, Bruce F. (Department: 1741)

Chemistry: electrical current producing apparatus, product, and

Having earth feature

C429S047000, C502S101000, C204S283000, C029S623100

Reexamination Certificate

active

06183898

ABSTRACT:

The invention relates to a gas diffusion electrode and also a process for its production, a process for coating the gas diffusion electrode with a catalytically active layer and its use for fuel cells and electrolysis cells.
In polymer electrolyte membrane fuel cells, a gas diffusion electrode is used as electrode between polymer electrolyte membrane and current collectors, e.g. bipolar plates. It has the function of conducting away the current generated in the membrane and has to allow the reaction gases to diffuse through to the catalytic layer. In addition, the gas diffusion electrode should be water-repellent at least in the layer facing the membrane in order to prevent water formed in the reaction from flooding the pores of the gas diffusion electrode and thus blocking gas transport to the catalytically active layer. For many applications, for example in space travel and for use in automobiles, it is also important that the materials used for constructing the cell stack are light and take up little space but nevertheless have a high mechanical stability. Very inexpensive production of the materials is always of interest.
For such gas diffusion electrodes, use has hitherto typically been made of materials comprising graphitized fabric or graphitized papers which are produced via an expensive thermal treatment (up to over 200° C.) (E-Tek, Inc. 1995 Catalogue, E-Tek, Inc. Natick. Mass. 01760, USA). The gas diffusion electrodes comprising graphitized fabric often do not allow oxygen, particularly atmospheric oxygen under low pressure, to diffuse sufficiently well and are also relatively heavy. The dense structure is necessary to obtain sufficient mechanical strength and a sufficiently high conductivity of the fabric perpendicular to the fiber direction. Their production requires high temperatures and an exact reaction procedure which leads to a correspondingly high energy consumption and high prices. The graphitized papers have the disadvantage that they are brittle and not flexible and the pore structure of these papers is fixed and cannot be changed without influencing the conductivity.
Also known are gas diffusion electrodes which comprise a hydrophobic, porous support material which is sufficiently electrically conductive for fuel cells, an intermediate layer which is not catalytically active and comprises an electron conductor material, and a catalytically active layer (EP-A-0 687 023). The intermediate layer which is not catalytically active here comprises a mixture of an electron-conducting ionomer and a proton-conducting ionomer. At a platinum loading of 0.21 mg/cm
2
, an H
2
pressure of 1.25 bar (absolute) and an air pressure of 1.8 bar (absolute), a fuel cell using the gas diffusion electrodes described can only achieve a maximum output of 200 mW/cm
2
or an output of 163 mW/cm
2
at a cell voltage of 0.6 V (Example 2, Table).
SUMMARY OF THE INVENTION
It is an object of the present invention to provide a gas diffusion electrode which is inexpensive to produce but mechanically stable, allows oxygen, in particular oxygen from the air under a low superatmospheric pressure, to diffuse readily, also has the necessary high electrical conductivity and is mechanically stable and water-repellent.
It is an additional object of the invention to provide a process for producing such a gas diffusion electrode.
It is also an object of the invention to provide a process for coating a gas diffusion electrode with a catalytically active layer and to indicate the use of the gas diffusion electrodes of the invention in fuel cells and electrolysis cells.
These objects are achieved by the gas diffusion electrode as set forth below.


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Derwent Publication, Week 9437. Patent Abstracts of Japan JP6223835 dated Aug. 12, 1994.

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