Coating processes – Electrical product produced – Fuel cell part
Reexamination Certificate
1999-06-09
2004-09-14
Talbot, Brian K. (Department: 1762)
Coating processes
Electrical product produced
Fuel cell part
C427S126300
Reexamination Certificate
active
06790474
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a fuel electrode of a solid oxide fuel cell and a process for producing the same.
2. Description of the Prior Art
Fuel cells have recently been noted as energy sources from the viewpoint of not only saving of resources but also influence on the environment. In a solid oxide fuel cell (SOFC), a fuel electrode is disposed on one surface of a solid electrolyte layer, while an air electrode is disposed on an opposite surface. The cell is constituted of a plurality of cells electrically connected to one another, with separators each laminated between the adjacent cells for distributing fuel and oxidizing gas to the cell, and has a higher operation temperature of 700 to 1000° C. than other fuel cells to provide a higher power generating efficiency. Since all constituting materials are solid, they can easily be handled. These advantages advance practical use.
A circuit structure of a conventional typical solid oxide fuel cell is diagrammatically shown in
FIG. 1
together with a sectional view of a cell. In
FIG. 1
, a fuel electrode
2
is formed on one surface of a central solid electrolyte layer
1
(upper surface of the solid electrolyte layer
1
in FIG.
1
), an air electrode
3
is formed on an opposite surface, and the electrodes have interfaces
4
and
5
with the solid electrolyte layer
1
, respectively. In the solid electrolyte layer
1
, mainly 8YSZ (YSZ denotes a yttria-stabilized zirconia) or 3YSZ is used. The fuel electrode
2
and air electrode
3
are connected by an external circuit via a load
6
. When fuel gases such as hydrogen (H
2
), methane (CH
4
) and the like are supplied to the fuel electrode
2
, and oxidizing agents such as air, oxygen (O
2
) and the like are supplied to the air electrode
3
, an electromotive force is generated between the electrodes, so that current flows to the load
6
connected to the external circuit. It is known that in this type of solid oxide fuel cell, electrode material composition and structure have a large influence on cell performance, and especially the influence of the fuel electrode
2
is large. In the fuel electrode
2
, a cermet of metal and oxide is generally used.
As the fuel electrode
2
, a mixture of Ni and YSZ particles, i.e., Ni—YSZ cermet, is known.
Since the electrode reaction of the solid oxide fuel cell is accompanied with a gas phase reaction, as shown in
FIG. 2
, the conventional electrode reaction field of the fuel electrode
2
is limited to the vicinity of three-phase lines in which the electrolyte, Ni particles in the cermet and gas phase coexist, the current is drawn to the vicinity of the three-phase lines, and current density locally increases. Moreover, in the conventional fuel electrode
2
, Ni particles are not uniformly dispersed in the cermet, and electrolyte films or other fine particles are not present in a small amount on surfaces of the Ni particles. Therefore, it is difficult to inhibit the Ni particles from being flocculated.
As a result, a solid oxide fuel cell having the conventional fuel electrode has the disadvantages that (1) since the electrode reaction of the fuel electrode is limited to the vicinity of the three-phase lines with the Ni particles in the cermet and the gas phase coexisting therein, the overvoltage is large, and (2) the flocculation of the Ni particles directly causes the deterioration of the fuel cell.
SUMMARY OF THE INVENTION
The present invention has been developed to solve the above-mentioned problems in the conventional fuel electrode, and an object thereof is to provide a fuel electrode of a solid oxide fuel cell and a producing process thereof, in which, by providing metal oxide particles in a cermet with electronic conductivity, the field of electrode reaction is largely enlarged, flocculation of Ni particles is minimized, and power generating performance of the cell can be enhanced.
To attain the above-mentioned object, according to a first basic aspect of the present invention, there is provided a process for producing a fuel electrode of a solid oxide fuel cell, each cell comprising a solid electrolyte layer, a fuel electrode disposed on one surface of the solid electrolyte layer, and an air electrode disposed on an opposite surface, by alternately laminating a plurality of cells, adjacent cells being electrically connected to each other, and a plurality of separators for distributing fuel gas to the fuel electrode of each cell and oxidizing gas to the air electrode, comprising the steps of: adding a solution of a metallo-organic compound of yttrium (Y) and a solution of a metallo-organic transition-metal compound to a solution of a metallo-organic compound of zirconium (Zr) to prepare a mixed solution of metallo-organic compounds of Zr—Y-transition metal; mixing NiO powder into the mixed solution of the metallo-organic compounds to prepare a slurry; and successively subjecting the slurry to hydrolysis, polycondensation, pyrolysis, annealing and reduction to obtain a cermet formed of yttria-stabilized zirconia (YSZ, i.e., CeO
2
—Y
2
O
3
—ZrO
2
) containing the transition metal dissolved therein, and having electronic conductivity in a fuel electrode operating atmosphere, and Ni.
According to a second basic aspect of the present invention, there is provided a process for producing a fuel electrode of a solid oxide fuel cell, each cell comprising a solid electrolyte layer, a fuel electrode disposed on one surface of the solid electrolyte layer, and an air electrode disposed on an opposite surface, by alternately laminating a plurality of cells, adjacent cells being electrically connected to each other, and a plurality of separators for distributing fuel gas to the fuel electrode of each cell and oxidizing gas to the air electrode, comprising the steps of: adding a solution of a metallo-organic compound of yttrium (Y) and a solution of a metallo-organic transition-metal compound to a solution of a metallo-organic compound of zirconium (Zr) to prepare a mixed solution of metallo-organic compounds of Zr—Y-transition metal; mixing NiO powder and cerium oxide powder containing a divalent or trivalent metal oxide dissolved therein to the mixed solution of the metallo-organic compounds to prepare a slurry; and successively subjecting the slurry to hydrolysis, polycondensation, pyrolysis, annealing and reduction to obtain a cermet formed of yttria-stabilized zirconia (YSZ) containing the transition metal dissolved therein, nickel (Ni) and cerium oxide containing the divalent or trivalent-metal dissolved therein.
The transition metal described in the first and second basic aspects is one selected from the group consisting of cerium (Ce), titanium (Ti) and praseodymium (Pr).
The metallo-organic compound described in the first and second basic aspects is one selected from the group consisting of metallic octylate, metallic naphthenate, metallic stearate and another metallic aliphatic acid salt, and metallic acetyl acetonate complex.
The fuel electrode described in the first and second basic aspects is formed on a solid electrolyte by a screen printing process.
The concentration of the transition metal in the yttria-stabilized zirconia (YSZ) containing the transition metal dissolved therein according to the first and second basic aspects is in the range of 1 mol % to 30 mol %.
The volume fraction of the cerium oxide containing the divalent or trivalent metal dissolved therein according to the second basic aspect is in the range of 1% to 70%.
The concentration of Ni in the fuel electrode according to the first and second basic aspects is in the range of 20% to 95% as a volume fraction.
The concentration of the yttria-stabilized zirconia containing the transition metal dissolved therein in the cermet according to the first and second basic aspects is in the range of 1% to 50% as the volume fraction.
The divalent or trivalent metal oxide according to the second basic aspect is one, or a combination of plural ones, of BeO, MgO, CaO, SrO, BaO, Sm
2
O
3
, Y
2
O
3
, La
2
O
3
, Gd
2
O
3
,
Hishinuma Masakazu
Matsuzaki Yoshio
Talbot Brian K.
Tokyo Gas Co. Ltd.
Towsend and Towsend and Crew LLP
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