Chemistry: electrical current producing apparatus – product – and – Having earth feature
Reexamination Certificate
1999-05-26
2001-01-30
Bell, Bruce F. (Department: 1741)
Chemistry: electrical current producing apparatus, product, and
Having earth feature
C429S047000, C429S218100, C429S223000, C429S241000, C204S291000, C204S292000
Reexamination Certificate
active
06180277
ABSTRACT:
BACKGROUND OF THE INVENTION
The invention relates to an electrode with a net-like open pore system and with a ceramic and a metallic meshwork. The mesh works penetrate each other.
The invention also resides in a method of manufacturing such an electrode.
The open pore system facilitates a material transport through the electrode. The ceramic meshwork provides for the mechanical stability of the electrode. The metallic meshwork serves as electron conductor and as catalyst.
Such an electrode, that is, an anode of a high temperature fuel cell is disclosed in published German patent application No. 196 30 843.7. It has an open pore system, that is, it is porous throughout. Materials such as gaseous fuel, oxidation media or water may pass through the anode. The ceramic mesh work consists of zirconium oxide. It makes the electrode mechanically stable. The metallic meshwork consists of nickel. It conducts electrons and serves as a catalyst for the chemical reactions, which take place in the high temperature fuel cell.
It is however disadvantageous that there is a self-diffusion of the metal. Under high temperature conditions, a diffusion of metal in metal takes place in the anode, that is, for example nickel diffuses in nickel or platinum in platinum. The self-diffusion results in a coagulation that is in an increase of the metallic mesh work. The increased reduces the electrochemically active surface area. The electrochemically active surface area is the surface on which the electrochemical reactions take place which are necessary for the operation of the fuel cells. When the electrochemically active surface area of the fuel cell becomes smaller, the capacity of the fuel cell becomes smaller.
It is the object of the present invention to provide an electrode, which is efficient and effective over a long period of time.
SUMMARY OF THE INVENTION
In a dispersoid-reinforced electrode with a net-like open pore structure and a ceramic and a metallic meshwork, ceramic particles with an average particle diameter of less than 100 nm are homogeneously distributed in the metallic network thereby reinforcing the electrode.
It has been found that the dispersoid reinforcement of the electrode inhibits the occurrence of the disadvantageous self-diffusion. The increase of the electron-conductive network, which normally occurs under the given operating conditions, is counteracted. The electrochemically active surfaces in the electrode become more stable. The efficiency of the electrode is therefore maintained over an extended period.
A dispersoid reinforcement is to be understood as a homogeneous distribution of ceramic particles in the metal wherein the ceramic particles have an average diameter of less than 100 nm. Preferably, the diameter of the particles to a large extent is less than 30 nm.
The larger the content of the ceramic particles (below also called dispersoids) in the metal (below also called metal matrix), the more stable is the behavior of the electrode. The content is limited on the upside since a ceramic meshwork is formed by these particles in the metal when an upper limit of the particle content is exceeded. Then the desired homogeneous distribution of particles with an average diameter of less than 100 nm is no longer present.
The content of the ceramic particles in the metal is therefore between 0.5 and 15 vol. %, preferably at least 10 vol. %.
For the manufacturing and operating conditions of the electrodes, the following should be taken into consideration concerning the selection of the materials of which the dispersoids consist or are made:
The dispersoids should be inert in the metal matrix. That is, they should not dissolve therein. No chemical reactions should take place between the metal matrix and the dispersoid material. Otherwise, the advantageous dispersoid structure is changed.
The atoms or ions of the dispersoid material should have sufficiently low solubility in the metal matrix. This sufficiently low solubility exists particularly if the inter-diffusion of the atoms or ions in the metal matrix is so low that the advantageous dispersoid structure is not increased.
The dispersoids should be thermodynamically stable. They should be thermically stable so that they are not destroyed at operating temperatures. In high temperature fuel cells, where the electrodes according to the invention are preferably utilized, the operating temperature is presently between 600 and 1050° C.
The dispersoids should also be stable under reducing conditions, that is, at low oxygen partial pressures. In high temperature fuel cells, the typical oxygen partial pressure present in the atmosphere at the anode is between 10
−14
and 10
−20
bar.
The dispersoids should be as hard as possible in order to effectively inhibit permutation controlled high temperature deformation processes, which may result in a structure change of the metal matrix.
In a preferred embodiment of the invention, the material for the dispersoids is so selected that the mechanical compatibility between the metallic, catalytically effective meshwork and the ceramic meshwork is improved. Such an improvement is then present when, as a result of the dispersoid reinforcement, the linear expansion coefficient of the metallic meshwork is adapted to that of the ceramic network, that is, both expansion coefficients are closely matched. Consequently, the dispersoids should have an expansion coefficient, which is about the same as that of the ceramic meshwork.
If the expansion coefficient of the metallic meshwork is greater than that of the ceramic meshwork, the expansion coefficient of the dispersoids is preferably smaller than, or equal to, the expansion coefficient of the ceramic meshwork in order to achieve the adaptation referred to before. If the expanision coefficient of the metallic meshwork is smaller than that of the ceramic mesh work, the expansion coefficient of the dispersoids is preferably greater than, or equal to, the expansion coefficient of the ceramic meshwork.
For example: the ceramic meshwork of an anode of a solid electrolyte fuel cell consists of ZrO
2
, which is doped with 8 mol % Y
2
O
3
, that is, of so-called 8YSZ. The metallic meshwork of this anode consists of nickel. For the adaptation of the thermal expansion coefficients of the two mesh works, a dispersal reinforcement of the nickel with 8YSZ is suitable.
There are different possibilities for the manufacture of dispersoid-reinforced metallic powder:
Processes wherein the dispersoid material is so combined with the metallic material that, at the end, it is present in the metal as a ceramic:
Mechanical alloying of metal powder-like ceramic dispersoid material,
Mixing, by stirring, of powder-like ceramic dispersoid material into a metal melt. The melt alloyed in this manner is subsequently processed to a powder for example, by a nozzle spray process.
Processes, wherein the dispersoid material is introduced first in metallic form into the metallic material:
metallic alloying of metal powder with powder-like metallic dispersoid material,
melt metallurgical alloying of the metal with the dispersoid material. The melt alloyed in this way is subsequently processed to a powder by nozzle spraying,
implantation of ions of the dispersoid material, which is first present as a metal, into the metal (ion implantation).
By a subsequent heat treatment of the alloyed powder obtained only the content of dispersoid material in the powder particles is selectively oxidized (internal oxidation). Because of the selective oxidation, the dispersoid material is converted within the metal to a ceramic. The metallic material, which is to form the metallic meshwork remains chemically unchanged. Altogether, the metal is, in accordance with the method of the invention, dispersoid reinforced.
Methods, wherein the dispersoid material is introduced in ceramic form in oxidized metal called below matrix material:
Mechanical alloying of ceramic matrix and dispersoid materials,
spray drying of a salt solution which contains the metal ions of the matrix and the dispersoid materia
Buchkremer Hans Peter
Mallener Werner
Stover Detlev
Vassen Robert
Wilkenhöner Rolf
Bach Klaus J.
Bell Bruce F.
Forschungszentrum J{umlaut over (u)}lich GmbH
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