JP3325378B2 - Conductive ceramics and fuel cell using the same - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

BACKGROUND OF THE INVENTION This invention relates to ceramic and manufacturing method thereof conductivity comprising LaCrO ₃ system composition, in particular, relates to the improvement of sinterability, in particular a fuel cell separator, the gas diffuser And a conductive ceramic suitable for an interconnector, an electrode for MHD power generation, and the like.

[0002]

2. Description of the Related Art Lanthanum chromite oxide (LaCr)
O ₃ ) is used as a separator, a gas diffuser, and an interconnector of a solid oxide fuel cell because of its excellent chemical stability at high temperature and high electron conductivity.

FIG. 1 shows a solid oxide fuel cell having a flat plate shape. In a flat fuel cell, for example, LaMnO ₂ is used as one of the solid electrolytes 1 made of Y ₂ O ₃ stabilized ZrO _2.
A system air electrode 2 and a fuel electrode 3 such as Ni-zirconia are provided on the other side, and the connection between the cells is made by a LaCrO ₃ -based separator (current collecting member) 4. In the fuel cell, power is generated at a temperature of 1000 to 1050 ° C. while flowing a gas containing oxygen, for example, air on the air electrode side and flowing a fuel, for example, hydrogen gas on the fuel electrode side. As the above separator material, CaO, SrO or Mg
A LaCrO ₃ material in which O is dissolved is used.

[0004]

Problems to be Solved by the Invention In addition to the slow diffusion rate of cations, the LaCrO ₃ -based material volatilizes the Cr component from the material during the sintering process, and causes the Cr contact at the contact portion (neck portion) of the particles. _It condenses and deposits as ₂ O ₃ and hinders sintering. For this reason, it is necessary to perform sintering at a high temperature of 2000 ° C. or more in the atmosphere or sintering in a reducing atmosphere while suppressing the evaporation and condensation of Cr ₂ O _3. High temperature is required. The production of a material by such high-temperature sintering makes mass production of fuel cells extremely difficult from an economic viewpoint, and is a factor that increases the cost.

On the other hand, as a method for obtaining a LaCrO ₃ -based material at a low temperature, an electrochemical vapor deposition (EVD) method has been applied. However, although this method produces a LaCrO ₃ -based material at a relatively low temperature of 1400 ° C.,
Since the growth rate of LaCrO ₃ is low, mass productivity is lacking. In addition, this method has a problem in terms of economy because it is necessary to use an extremely expensive metal chloride as a main raw material.

[0006]

Means for Solving the Problems The inventors of the present invention have studied the method for solving the above-mentioned problems and improving the sinterability at a low temperature, and as a result, have found that La, Cr and Mg, C
a, a group of Ga, In, Sn, and Pb with respect to a system containing at least one alkaline earth element selected from the group of a and Sr
It has been found that by adding at least one metal compound selected from the group consisting of sintering, sinterability is improved and densification by firing at a low temperature becomes possible, and the present invention has been achieved.

That is, according to the present invention, the conductive ceramic is made of gold.
In atomic ratio of group element formula La _x A _y Cr _z B _u formula, A is less selected from Mg, Ca, the group of Sr
At least one type B is at least one selected from the group consisting of Ga, In, Sn, and Pb.
When both are represented by one type , x, y, z and u satisfy the condition of 0.0001 ≦ u / x + y + z ≦ 0.2 0.01 ≦ y / x + y + z ≦ 0.2.

Further, according to the present invention, the conductive ceramic is used as a current collecting member for electrically connecting single cells of a fuel cell.

In the conductive ceramics according to the present invention, La, Cr, Mg, C
Ga, In, S for a composite oxide containing at least one alkaline earth element selected from the group consisting of
A major feature is that at least one metal selected from the group consisting of n and Pb is contained. These metal elements are represented by 0.0001 ≦ u / x + y + z ≦ 0.
It is blended in an amount satisfying 2. This is because this value is 0.0
If it is less than 001, the sintered body cannot be sufficiently densified by low-temperature sintering, and an open porosity of 1% or more exists. This is because it cannot be applied. Further, the conductive ceramics of the present invention has a property that it is stable in an atmosphere of hydrogen and oxygen. However, if the above range is exceeded, there is a problem that stability in a hydrogen atmosphere is deteriorated and the material is decomposed during power generation. Even within this range, 0.005 ≦ u / x + y
The range of + z ≦ 0.1 is good.

Further, according to the present invention, Mg, Ca, Sr
The amount y of at least one kind of alkaline earth element selected from the group represented by the formula is 0.01 ≦ y / x + y + z ≦
It must also be 0.2. This is because when these elements are less than the above ranges, the electric conductivity becomes small, and when these elements are more than the above ranges, they are decomposed in a reducing atmosphere such as hydrogen. The amount of the alkaline earth element y is 0.05
≦ y / x + y + z ≦ 0.15 is optimal.

The conductive ceramics of the present invention is La
_For metal oxides such as ₂ O ₃ , Cr ₂ O ₃ , MgCO ₃ , CaCO ₃ , or compound powders such as hydroxides, carbonates, nitrates, and chlorides that form oxides by heat treatment , Ga, At least selected from the group consisting of In, Sn, and Pb
Also, one kind of metal oxide powder, or a hydroxide or a carbonate capable of forming an oxide by heat treatment, and weighed and mixed so as to have the above ratio, and after shaping this into a predetermined shape, 1300 to 170 in an oxidizing atmosphere such as air
Densification can be achieved by firing at 0 ° C. for about 2 to 5 hours.

In addition, La ₂ O ₃ , Cr ₂ O ₃ , MgO,
Alkaline earth element compounds such as CaO
To produce a calcined after ABO ₃ type perovskite composite oxide at 1500 ° C., with respect to those obtained by pulverizing this, G
at least one selected from the group consisting of a, In, Sn, and Pb
Metal oxide powders, or hydroxides and carbonates capable of forming an oxide by heat treatment, and weighed and mixed so as to have the above ratio, and after shaping this into a predetermined shape, such as air 1300-1700 ° C in oxidizing atmosphere
For about 2 to 5 hours for densification. According to such a production method, the density can be reduced to an open porosity of 1% or less, particularly 0.5% or less.

[0013] The conductive ceramic of the present invention has a crystal structure in which a perovskite-type crystal containing at least La, Cr and the above-mentioned alkali earth element as a constituent element is 50% by volume or more. And Ga, In, Sn, and other phases other than the main crystal phase .
It is composed of a compound or a glass phase of at least one metal selected from the group of Pb and a constituent element of the perovskite crystal, particularly La ₂ O ₃ . This other phase
Excess components are precipitated from the stoichiometric composition of the perovskite crystal.

The conductive ceramic according to the present invention is suitably used, for example, as an electrode material in a fuel cell. FIG. 1 shows a typical structure of a flat fuel cell. According to FIG. 1, one side of a plate-like solid electrolyte 1 made of Y ₂ O ₃ stabilized ZrO ₂ or the like is provided with LaSrM
An air electrode 2 made of nO ₃ or LaCaMnO ₃ is formed, and a fuel electrode 3 made of Ni—ZrO ₂ (Y ₂ O ₃ stabilized) cermet is formed on the other surface. The current collecting member 4 is disposed at a position where the current collecting member 4 is connected to the fuel electrode of the cell adjacent to the air electrode of the cell. In such a cell, the air electrode 2 is in contact with an oxygen-containing gas such as air, the fuel electrode 3 is in contact with a fuel electrode such as hydrogen gas, and both the air electrode 2 and the fuel electrode 3 are made of a porous material. However, the current collecting member 4 is required to have high density and high electrical conductivity because one side thereof is in contact with the oxygen-containing gas and one side is in contact with the hydrogen gas to completely separate them. Is done.

As described above, the conductive ceramic is a dense body having an open porosity of 1% or less.
Since the electric conductivity, particularly when the fuel cell is operating (about 1000 ° C.), is as high as 15 s / cm or more,
The current collecting member sufficiently satisfies the required characteristics. In addition, this conductive ceramic is also excellent in durability against hydrogen, and is therefore excellent in long-term stability, which is one reason that it is suitable as a current collecting member.

[0016]

In the LaCrO ₃ -based material, in addition to the slow cation diffusion rate in the crystal, the Cr component is liable to evaporate preferentially. The so-called LaCrO ₃ -based material, which is deposited as Cr ₂ O ₃ and inhibits the diffusion of cations to deteriorate sinterability, is dominated by an evaporative condensation mechanism.

On the other hand, in the material of the present invention, Ga,
At least one type of gold selected from the group consisting of In, Sn, and Pb
By adding genus, liquid or reactant is generated in sintering, and simultaneously suppress evaporation of Cr, by promoting the diffusion of cations in component elements of the perovskite type crystal by firing at a low temperature Dense LaCrO
_{A 3} series sintered body can be obtained.

However, the present material is required to have essentially electrical conductivity, and after sintering, consists of a main crystal phase of perovskite crystals, but the above-mentioned liquid phase partially precipitates an insulating phase at grain boundaries. It seems to be. For this reason, if there are many grain boundaries, electric conductivity will be reduced, and this will cause the power generation characteristics of the fuel cell to deteriorate. For these reasons, the upper limit of the amount of the specific metal added is set. Since such crystals have a small amount and do not affect the electrical conductivity, they do not pose any problem even if they are used as a current collecting member as long as they satisfy the above-mentioned addition amount range.

[0019]

EXAMPLES La ₂ O ₃ , MgCO _{3 having} a purity of 99.9% commercially available
₃ , Cr ₂ O ₃ , SrCO ₃ , and CaCO ₃ are mixed at a predetermined ratio, and then heat-treated in an oxidizing atmosphere at 1300 ° C. for 24 hours, and then pulverized to obtain a powder having an average particle size of 3 μm.
_{a (Mg 0.1 Cr 0.9) O} 3, (La 0.9 Sr 0.1) Cr
O ₃ , (La _0.9 Ca _0.1 ) CrO ₃ solid solution powder was obtained. Next, this solid solution powder was added from the group of Ga, In, Sn, and Pb.
After adding a predetermined amount of at least one selected metal oxide powder and mixing for 24 hours in a ball mill, 5 mm × 5 m
m, formed into a square prism with a length of 45 mm.
It was fired at a temperature of 1600 ° C.

With respect to the obtained sintered body, the open porosity of the sample was measured by the Archimedes method to determine the sinterability. Further, a sample piece having a size of 3 mm × 3 mm × 20 mm was prepared as described above, and 100
The electrical conductivity was measured at 0 ° C.

For comparison, a commercially available La _0.9 Sr _0.1 CrO
_The open porosity and the electric conductivity were measured using the raw materials of the _three compositions fired in the air at 2000 ° C. for 2 hours. Also,
After the sintered body was kept in a hydrogen atmosphere at 1000 ° C. for 24 hours, the stability of the sample was examined. If no change was observed, ○ was given, and if the surface was decomposed, × was given. The results are shown in Table 1.

[0022]

[Table 1]

As is clear from the results shown in Table 1 , in the sample No. 1 of the conventional product, the open porosity was as large as 28% at the firing temperature of 1550 ° C., and the densification was not possible. And contrast, the products of the present invention either can be densified to less than 1% open porosity at a firing temperature of 1,450 to 1,600 ° C., yet 10
The electric conductivity at 00 ° C. was 15 s / cm or more, and the stability was excellent even in hydrogen at 1000 ° C.

[0024]

As described in detail above, the sinterability of the LaCrO ₃ -based composition can be improved, and the LaCrO ₃ composition has high conductivity and can be densified at a firing temperature of 1600 ° C. or less. In addition, it is excellent in stability in a high-temperature hydrogen atmosphere, and is preferably used as a current collecting member such as an interconnector, a separator, and a gas diffuser that comes into contact with hydrogen such as a fuel cell, so that it is inexpensive and has low fuel consumption. An electrode material that can cope with long-term stability as a battery can be provided.

[Brief description of the drawings]

FIG. 1 is a schematic view of the structure of a flat fuel cell.

[Explanation of symbols]

 DESCRIPTION OF SYMBOLS 1 Solid electrolyte 2 Air electrode 3 Fuel electrode 4 Current collecting member

────────────────────────────────────────────────── ─── Continuation of the front page (56) References JP-A-4-219366 (JP, A) JP-A-4-219364 (JP, A) JP-A-4-108667 (JP, A) JP-A-3-219 146465 (JP, A) (58) Field surveyed (Int. Cl. ⁷ , DB name) C04B 35/42-35/50

Claims (2)

(57) [Claims] 1. A in atomic ratio the formula La _x A _y Cr _z B _u type metal elements, A is, Mg, Ca, at least one B is selected from the group consisting of Sr is, Ga, In, Sn, Pb And x, y, z and u are represented by at least one member selected from the group consisting of: 0.0001 ≦ u / x + y + z ≦ 0.2 0.01 ≦ y / x + y + z ≦ 0.2 Conductive ceramics characterized by becoming. 2. A fuel cell according to claim 1, wherein a current collecting member for electrically connecting the cells is made of the conductive ceramic according to claim 1.