Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
2000-05-19
2003-01-07
Bell, Bruce F. (Department: 1741)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C429S047000, C204S283000, C204S284000, C423S44500R, C423S447100, C423S447200, C423S447400, C423S447900, C502S101000, C502S416000, C502S418000, C502S420000, C428S304400, C428S311710, C428S474700, C428S480000, C521S064000, C521S181000
Reexamination Certificate
active
06503655
ABSTRACT:
BACKGROUND OF THE INVENTION
Field of the Invention
The invention relates to a gas diffusion electrode comprising carbon and to a process for producing it.
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 chemically in hydrogen and oxygen which would be released in a hydrogen-oxygen reaction, can be converted into electrical energy by means of an electrochemical process which represents a reversal of the electrolysis of water.
PEM fuel cells have a central membrane/electrode unit comprising a thin, proton-conducting solid state electrolyte on both sides of which very smooth, hydrophobic, porous gas diffusion electrodes having a catalyst coating are located. Oxygen is fed to the electrode on the cathode side while hydrogen is fed to the electrode on the anode side. An electron exchange takes place on the catalyst-coated surfaces of the electrodes, as a result of which an electric potential is 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 gives the best possible contact, and thus a low electrical contact resistance, between electrode, catalyst and electrolyte. The electrodes should therefore have a surface roughness which is at most in the micron range. To make sufficient gas flow possible, the permeability of the electrodes for nitrogen should be >10
−6
m
2
/s at atmospheric pressure, preferably >10
−5
m
2
/s. For this purpose, 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, since this prevents the water formed in the electrochemical reaction between hydrogen and oxygen from accumulating in the pores and blocking them.
In order to meet the abovementioned requirements, modified carbon papers, i.e. carbon papers which are densified on the surface by means of carbon black or graphite, are used in gas diffusion electrodes. 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 carbon foam in supercapacitors; the carbon foam can be an aerogel or a xerogel. Such electrodes also do not satisfy the abovementioned requirements. To increase the electrical conductivity, a carbon matrix is integrated into the aerogel For this purpose, the carbon matrix, for example in the form of carbon fibers, is introduced 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 looses 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 precisely set and reproduced; in addition, the surface of the electrodes is greatly roughened during the pyrolysis.
Carbon aerogels produced from aerogels based on organic compounds by pyrolysis (see: R. W. Pekala, C. T. Alviso, Materials Research Society 1992 Spring Meeting San Francisco, April 1992, Proceedings 270 (1992), page 3) do have, owing to their high porosity, properties which allow their use in gas diffusion electrodes, but since they are naturally brittle they have to be mechanically stabilized for this purpose. However, the support skeletons in the form of carbon fibers (WO 95/06002) or inorganic fiber material comprising aluminum oxide, silicon dioxide or zirconium dioxide (DE 195 23 382 A1) which have hitherto been used for this purpose meet the condition of a matched coefficient of expansion only unsatisfactorily, since these materials are not able to follow the strong shrinkage of the organic aerogel precursor. This results in defects and cracks between the fibers and the aerogel; microscopically, the thin, flat carbon aerogels therefore appear wavy.
SUMMARY OF THE INVENTION
It is accordingly an object of the invention to provide a gas diffusion electrode, and a process for its production, that overcome the above-mentioned disadvantages of the prior art devices and methods of this general type, which is made of carbon having a smooth surface and 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 having a side in contact with a supply of gas and a side in contact with an electrolyte, comprising a pyrolysis product of a composite of an organic aerogel or xerogel and a reinforcing skeleton consisting 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 comprising the steps of providing carbonizable composite of an organic aerogel or xerogel and a reinforcing skeleton consisting at least in part of organic material, and pyrolyzing the composite under a protective gas, preferably at a temperature in the range from 600° C. to 2000° C.
With the foregoing and other objects in view, there is further provided, according to the invention, a novel carbonizable composite affording upon pyrolysis a porous carbon electrode, comprising an organic aerogel or xerogel and a reinforcing skeleton consisting at least in part of organic material, such that the volumetric shrinkage upon pyrolysis of the organic aerogel or xerogel and the reinforcing skeleton are within plus or minus 10% of one another.
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 after pyrolysis.
The reinforcing skeleton which serves to stabilize the aerogel or xerogel consists at least partially of organic material. This generally means that at least 80% of the reinforcing skeleton is organic material. Inorganic components can be, for example, glass fibers or flame retardants such as boron-containing salts. The organic material of the reinforcing skeleton has comparable shrinkage (volume shrinkage) to the aerogel or xerogel during pyrolysis, such that the shrinkage of the aerogel or xerogel and that of the reinforcing skeleton are within plus or minus 10% of one another, and the skeleton is, after pyrolysis, still so strong that it can assume a support function for the pyrolyzed aerogel or xerogel.
It is advantageous for the reinforcing skeleton to be readily wettable by the aerogel or xerogel. For this purpose, the organic material preferably contains substructures which can form hydrogen bonds. These 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 over known electrodes. This is attributable to the use of a reinforcing skeleton comprising organic material, which, during production of the electrode, forms, together with the aerogel or xerogel, a completely new structure, depending on the type and porosity of the reinfo
Fricke Jochen
Leuschner Rainer
Lipinski Matthias
Petricevic Raino
Bell Bruce F.
Greenberg Laurence A.
Mayback Gregory L.
Siemens Aktiengesellschaft
Stemer Werner H.
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