Chemistry: electrical and wave energy – Apparatus – Electrolytic
Reexamination Certificate
2000-05-19
2002-03-26
Bell, Bruce F. (Department: 1741)
Chemistry: electrical and wave energy
Apparatus
Electrolytic
C429S047000, C429S047000, C502S101000, C502S416000, C502S418000, C502S426000, C428S304400, C428S310500, C428S311110, C428S311710, C428S320200, C428S477400
Reexamination Certificate
active
06361666
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a gas diffusion electrode made of carbon, a process for producing an electrode and carbonizable composite.
Gas diffusion electrodes are used, in particular, in batteries and especially in fuel cells such as PEM fuel cells (PEM=polymer electrolyte membrane) . In fuel cells, for example, the energy stored in chemical form in the hydrogen and oxygen, which would be released in an explosive hydrogen-oxygen recombination reaction, can be converted into electric energy by means of an electrochemical process which represents a reversal of water electrolysis. PEM fuel cells have a central membrane/electrode unit which includes a sheet-like, proton-conducting solid state electrolyte on each side of which there is arranged a very smooth hydrophobic, porous gas diffusion electrode provided with a catalyst coating. Oxygen is supplied to the electrode on the cathode side and hydrogen is supplied to the electrode on the anode side. Electron exchange takes place on the catalyst-coated surfaces of the electrodes, resulting in an electric potential being built up. On the cathode side, water is formed as reaction product of the electrochemical process.
The electrodes have to meet the following requirements:
Good electrical conductivity, good gas permeability and mechanical stability; in addition, they should have a smooth outer surface. A smooth surface is very important because this achieves the best possible contact, and thus a low electrical contact resistance, between electrode, catalyst and electrolyte. The electrodes should therefore have at most a surface roughness in the micron range. In order to make sufficient gas flow possible, the nitrogen permeability of the electrodes should be >10
−6
m
2
/s at atmospheric pressure, preferably >10
−5
m
2
/s. For this reason, the largest pores should have a diameter of ≧100 nm, preferably from 0.5 to 10 &mgr;m. It is also important for the electrodes to have a hydrophobic character. This is because it prevents the water formed in the electrochemical reaction between hydrogen and oxygen from accumulating in the pores and blocking them.
To meet the above-mentioned requirements, gas diffusion electrodes are produced using modified carbon papers, i.e. carbon papers which are sealed on the surface by means of carbon black or graphite. However, these materials are not satisfactory in respect of surface smoothness and pore size.
U.S. Pat. No. 5,260,855 discloses the use of electrodes made of foam-type carbon in high-capacity capacitors (“supercapacitors”); the foam-type carbon can be an aerogel or xerogel. Such electrodes, too, do not meet the abovementioned requirements. This is because a carbon matrix is integrated into the aerogel to increase the electrical conductivity. For this purpose, the carbon matrix, for example in the form of carbon fibers, is incorporated into the gel before gelation and pyrolysis. Since aerogel and carbon matrix display different shrinkage behavior during pyrolysis, microcracks are formed. The pyrolyzed aerogel thus loses some of its adhesion to the surface of the carbon matrix. The pore size of the electrodes, which is determined by the matrix material, can therefore not be set and reproduced exactly. In addition, the surface of the electrodes is severely roughened during pyrolysis.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a gas diffusion electrode made of carbon, a process for producing an electrode and a carbonizable composite, which overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, in which the electrode is made of carbon having a smooth surface, in which the porosity can be regulated at will and in which no problems occur as a result of cracks between the electrode material and a supporting skeleton.
With the foregoing and other objects in view, there is provided, according to the invention, a thin, flat, and porous carbon gas diffusion electrode, comprising a side in contact with a supply of gas and a side in contact with an electrolyte, and a pyrolysis product of a composite of an organic polymer having a spatial globular structure (SGS polymer) and a reinforcing skeleton formed at least in part of organic material. The porosity of the carbon gas diffusion electrode according to the invention can be regulated at will while the surface of the electrode is smooth.
With the foregoing and other objects in view, there is also provided, according to the invention, a method of producing a thin, flat, and porous carbon gas diffusion electrode, which comprises the steps of providing a composite of an organic polymer having a spatial globular structure (SGS polymer) and a reinforcing skeleton formed at least in part of organic material, and pyrolyzing the composite under protective gas.
With the foregoing and other objects in view, there is further provided, according to the invention, a novel carbonizable composite affording a porous carbon gas diffusion electrode upon pyrolysis, the composite comprising an organic polymer having a spatial globular structure (SGS polymer) and a reinforcing skeleton formed at least in part of organic material, the SGS polymer and the reinforcing skeleton having comparable volumetric shrinkage upon pyrolysis.
The gas diffusion electrode of the invention, which is porous and has an extremely smooth surface, can be flat and thin, since it has been found that the porous, thin, flat characteristics of the reinforcing skeletons are largely preserved during pyrolysis according to this invention. For applications in the field of fuel cells, the gas diffusion electrode also has to be hydrophobic. In order to achieve this, it is additionally hydrophobicized during or after pyrolysis.
Polymers of spatial globular structure (SGS polymers) are known per se (see: “Isotopenpraxis, Environ. Health Stud.”), Vol. 29 (1993), pages 275 to 282, and also http://rzaix340. rz.uni-leipzig.de/kind/rgseng.htm, last update: Aug. 15, 1996). SGS polymers have a high porosity because of the globular structure and are used, for example, as filter materials. The size of the globules, and thus the porosity, of SGS polymers can be regulated easily, such that typical SGS polymers permit the passage of 800-3000 specific volumes per hour. The particle size of typical SGS polymers can range from 0.3 to 15 &mgr;m, with pores ranging from 0.01 to 50&mgr;m whose size can be controlled to within plus or minus 10%, thus affording a porosity in the range from 35 to 90%.
Since it has been found that the high porosity is retained even after pyrolysis, SGS polymers in pyrolyzed form are particularly suitable for porous gas diffusion electrodes according to the invention.
The reinforcing skeleton, which serves to stabilize the SGS polymer, is formed at least partly of organic material. This generally means that at least 80% of the reinforcing skeleton is organic material. Inorganic constituents can be, for example, glass fibers or flame retardants such as boron- containing salts. The organic material of the reinforcing skeleton has a volume shrinkage during pyrolysis which is comparable to that of the SGS polymer and the skeleton after pyrolysis is still sufficiently strong for it to be able to support the pyrolyzed SGS polymer.
It is advantageous for the reinforcing skeleton to be readily wettable by the SGS polymer. For this purpose, the organic material preferably has substructures which can form hydrogen bonds. These substructures are, in particular, functional groups such as OH, OR, CO, COOH, COOR, CN, NH
2
, NHR, NR
2
, CONH
2
, CONHR, CONR
2
, CO—NH—CO and CO—NR—CO. Hydroxyl and carboxamide groups have been found to be particularly advantageous.
The gas diffusion electrode of the invention displays significant improvements compared to known electrodes. This can be attributed to the use of a reinforcing skeleton of organic material which together with the SGS polymer forms, depending on the type and porosity (of the reinforcing skeleton),
Leuschner Rainer
Lipinski Matthias
Bell Bruce F.
Greenberg Laurence A.
Lerner Herbert L.
Siemens Aktiengesellschaft
Stemer Werner H.
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