JPH0317958A - Air electrode material for solid electrolyte fuel cell and solid electrolyte fuel cell - Google Patents

Abstract

PURPOSE:To obtain a chemically stable air electrode material of a solid electrolyte fuel cell by using a specific ceramics lanthanum manganate as its main component. CONSTITUTION:A ceramics having (La(1-x)Srx)(Mn(1-y)Cry)O3 type solid solution as its main component, with the amount x, y of substitution that satisfies the expression I. A lanthanum manganate type perovskite oxide comprising strontium and chrome dissolved for substitution in the A site and B site of perovskite structure, respectively. Thus an electrode material provided with the electrode activity of lanthanum manganate and the chemical stability and sintering resistance of lanthanum chromite is obtained.

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention provides a solid electrolyte that does not easily lose its porosity due to sintering and does not cause a decrease in electrode activity even during long-term operation at high temperatures. This relates to the air electrode of type fuel cells.

(Prior Art and Problems to be Solved) A solid oxide fuel cell is constructed, for example, as shown in the partially exploded oblique view of FIG. 1(a). That is, a spacer (2) is provided with a space (2a) for supplying a fuel (anode) electrode and an air (cathode) electrode and a section (3a) for supplying air (oxidizing agent) on both main surfaces of a flat solid electrolyte plate. 3) are superimposed. Then, the unit batteries consisting of the single cell (1) and the sublayers (2) and (3) are stacked in electrical series with other unit batteries via separators (4) and (5), and the first Each spacer (
A fuel gas supply pipe (6) that opens only into the fuel gas supply space (2a) of 2) and an air supply pipe (7) that opens only into the air supply space (3a) of the spacer (3) are provided. Then, the fuel gas is brought into contact with the fuel electrode of each vehicle cell (1), and the air is brought into contact with the air electrode, thereby generating electricity.

By the way, in a solid electrolyte fuel cell using zirconia as the solid electrolyte (1), its operating temperature is 100°C.
The temperature is higher than 0°C. Therefore, as a forming material for the air electrode formed on one side of the solid electrolyte (1), that is, used in a strong oxidizing atmosphere, lanthanum manganate-based oxide, which has excellent chemical stability in a high-temperature oxidizing atmosphere, is used. Lanthanum manganate-based oxide is considered to be a suitable material because it not only has high electron conductivity but also has a good thermal expansion coefficient compatible with zirconia, and its powder or organic acid salt is applied to the electrolyte zirconia membrane surface. After burning out the material, it is doped with an alkali metal to further improve electronic conductivity, and used as a perovskite structure.

However, because this alkaline earth-doped lanthanum manganate peskite is easily sintered, it cannot be used for long periods of at least 40,000 hours at operating temperatures of 1000°C or higher, such as in solid oxide fuel cells. When it is required to be used, sintering progresses and the porosity, which is an essential requirement for an electrode, is quickly lost, leading to a decrease in activity.

In addition, the stoichiometric ratio of the so-called A site and B site of perovskite is complex, and if the manganese at the B site is insufficient compared to the lanthanum and alkaline earth at the A site, lanthanum oxide will precipitate, leading to a decrease in conductivity. disintegration due to hydrolysis,
Furthermore, it causes inconveniences such as the use of insulating materials such as alkaline earth zirconate or lanthanum zirconium virochlore due to reaction with electrolyte zirconia. Furthermore, if an attempt is made to increase the chemical stability by using an excessive amount of manganese, the sinterability will further increase and the porosity will be easily lost.

In addition, the easy sinterability of this alkaline earth metal-doped lanthanum manganate poses great difficulties in the manufacturing process of solid oxide fuel cells.

In other words, regarding the shape of the air electrode during single cell production,
After applying alkaline earth metal doped lanthanum manganate to one surface of the electrolyte zirconia, sintering is performed, but in this case, in order to avoid loss of porosity due to sintering,
The baking temperature should be 1200″C or less, preferably 1000″C or less.
It is necessary to carry out the test at a temperature below °C.

However, when forming an alkaline earth metal-doped lanthanum manganate film, there are two methods: stacking sintered films and, for example, pressure sintering, or stacking constituent films and unsintered green films by adhering them to each other. Regardless of the sintering method used, the temperature during sintering must be 1400°C or higher, preferably 1450°C or higher. For this reason, alkaline earth metal-doped lanthanum manganate is sintered and loses its porosity, making it unusable as an air electrode.

(Objective of the Invention) The object of the present invention is to provide a lanthanum manganate for air electrodes that does not lose its porosity even during long-term use at high temperatures and during the sintering process during manufacturing. Recently, interest as a clean local power source has been increasing. The aim is to advance the realization of solid oxide fuel cells.

(Means of the Invention for Solving the Problems) The present inventors have discovered that in order to prevent sintering of easily sinterable lanthanum manganate-based perovskite oxides and to make electrodes that can withstand long-term use, An effective method for preventing sintering may be to mix lanthanum manganate with oxidized zirconia or lanthanum chromite particles and coat the resulting hard-to-sinter particles with lanthanum manganate. I thought about it and did a lot of research. As a result, we were able to discover a completely new type of strontium chromium-doped lanthanum manganate that, when mixed with lanthanum chromite, has an excellent effect on preventing sintering that was previously unknown. The present invention has been made based on this knowledge. Like lanthanum manganate, lanthanum chromite has electronic conductivity and is stable not only in an oxidizing atmosphere but also in a reducing atmosphere. Therefore, it has been studied as an electrode material for fuel cell electrodes or MHD power generation, but it has not been given much importance as an electrode for fuel cells because of its low electrode activity.

However, according to research conducted by the present inventors, strontium, strontium, and
This is a lanthanum manganate-based perovskite oxide of the present invention in which chromium is substituted as a solid solution at the B site (La(1-x
) Srx) (Mn(IV).C ry) 0 3-based solid solution is suitable as an electrode material having the electrode activity of lanthanum manganate and the chemical stability and sinterability of lanthanum chromite. was detected, and it was revealed that the above purpose could be achieved by having the following set of precepts.

(1) (La(1-x)Srx)(Mn(1-y)C
ry) Ceramics mainly composed of O= system solid solution, and substituted Nx. This can be achieved using a lanthanum manganate air electrode material in which y is expressed by the following formula (1), and (2) it can also be achieved using a solid electrolyte fuel cell equipped with an air electrode made of this air electrode material.

Figure 2 shows shrinkage rate and chromium substitution amount y, which are indicators of sinterability.
FIG. 3 is a diagram showing an example of the relationship between the shrinkage rate due to sintering at 1500''C and the shrinkage rate of the assembly, and these are based on pellets manufactured in the following manner.

Lanthanum nitrate (La(No:+)+), strontium nitrate (SrN○3), manganese nitrate (M n (N
0 3) Z) and chromium nitrate (Cr(N Oi)i
) are collected at a predetermined ratio, mixed, and added dropwise to a 4-fold equivalent or more oxalic acid ethanol solution. At this time, chromium is not co-precipitated and is evaporated to dryness on a hot plate. The resulting solid is heated at 500°C for 5 hours to burn off the organic matter.Then, the residue is thoroughly ground and mixed, and then heated at 1250°C for 5 hours and burned to crush the organic matter. Then, the obtained powder is lltor/c
Pellets with a diameter of 40 mm and a thickness of 2 to 3 mm were made by pressure molding using J, and the pellets were fired at 1500°C for 5 hours.

As is clear from FIG. 2, even by substituting about 0.1 mole of Cr at the B site, a significant decrease in sinterability is observed. However, in the range beyond this range (0.1 < y < 0.4 mol), even if the amount of substitution is increased, there is hardly any decrease in sinterability.

Furthermore, since La(Mn(1 y)Cry)Oz is originally an insulator, its electrical conductivity must be improved when used as an air electrode. From this point of view, Figure 3 shows a more detailed investigation of the sinterability and conductivity in the range of x, y. The results are shown in Figure 4. As is clear from Figure 3, O < x < 0.3.0 < y < 0.2
The sinterability of the range of x and y is particularly low in the upper part (shaded area) of the range of x and y. In addition, the electrical conductivity in a certain range of the set shown in FIG. 3 is determined by the curve (2) (L
ao. II S rO. zMno. q C ro
．． Curve (1) showing the conventional one, like 10,)
(Lao., Sro.zMnoi).

The results also show from curves (1), (2), and (3) that as the amount of Cr substitution increases, the electrical conductivity decreases. Note that curve (3) is La6. ISro. zMno. eC
ro. In the case of z○3, and in the case of y≧0.2, a rapid decrease in electrical conductivity occurs.

Therefore, in order to obtain practical sinterability while satisfying electrical conductivity, it is necessary that the shrinkage rate be approximately 10% or less. In the above equation (1), x+2y is a linear equation that divides the region where the sinterability changes, and 0.2y where X<0.25.
25 is the intersection of the x+2y straight line and the substitution amount X axis (y =
0.12 of O ) + y < 0.12 is given by the intersection (X=O) of the x+2y straight line and the axis of the displacement amount y.

However, the conventional one is 150″C5, which is the same as in the above experiment.
Over time, it shrinks and loses most of its porosity.

The details of why the solid solution of the present invention is suitable as an air electrode material are currently unknown, but it is thought to be based on the following. In the above-mentioned solid solution according to the present invention, it is considered that strontium is substituted for lanthanum at the A site and chromium is substituted for manganese at the B site due to the ionic radius. According to xl analysis, while the amount of chromium substitution is small, it has the same perovskite structure as rhombohedral lanthanum and manganese. On the other hand, La where A and B sites are not replaced
In LnO3, both La and Mn are trivalent. However, when the A site is substituted with a divalent alkaline earth metal, some Mn ions are oxidized to tetravalent Mn ions, but trivalent Mn that is not oxidized
may be in a non-stoichiometric configuration due to the presence of divalent alkaline earth ions, which may cause lattice defects at the A site.

It is thought that this increases the electronic conductivity, and that cations move faster due to lattice defects, resulting in better sinterability.

That is, if Cr is partially substituted with Mn, the polymeric value of Cr is more stable than that of Mn, so lattice defects at the A site are less likely to occur, and this prevents cation movement and leads to sintering. It seems that it will be difficult to do.

Furthermore, since Mn and Cr coexist at the B site, the crystal structure changes from orthorhombic to rhombohedral in lanthanum manganate, which is thought to increase distortion of the crystal structure and cause cation migration. In addition to this, La and alkaline earth metals are present at the A site, and M at the B site.
It is thought that the electric conductivity is also good because n and Cr are mixed and have different valences.

(Effects of the Invention) As described above, according to the present invention, a chemically stable air electrode material can be obtained. Therefore, when producing a solid oxide fuel cell, the temperature at which the air electrode is baked into the zirconia electrolyte is no longer restricted by the temperature at which the fuel electrode is baked into the zirconia electrolyte, and it is possible to use the same temperature as the zirconia, lanthanum chromite separator, and Ni Zr02 cermet fuel electrode. Can be baked at temperatures that would cause sintering. Therefore, manufacturing temperature conditions are relaxed and manufacturing costs can be significantly reduced. Furthermore, since it has sinterability, by applying it as an air electrode for a solid electrolyte fuel cell as shown in FIG. 1, it becomes possible to realize a fuel cell with less reduction in activity due to loss of porosity.

6B Figure 1

[Brief explanation of the drawing]

Figures 1 (a) and (b) are a partially exploded perspective view and assembled sectional view of a flat solid oxide fuel cell, and Figures 2 and 3 are relationship diagrams of assembly and shrinkage rate for strontium-chromium-doped lantern chromite. , FIG. 4 is a diagram using the air electrode material of the present invention. (1)...Single cell, (2)(3)...Spacer,
(2a)...fuel gas supply space, (3a)...oxidizer supply space, (4)(5)...separator, (6
)... Fuel gas supply pipe, (7)... Oxidizer supply pipe.

Claims (2)

[Claims] (1) (La(1-x)Srx)(Mn(1-y)Cr
y) A solid electrolyte type ceramic lanthanum manganate mainly composed of O_3 system, characterized in that the values of x and y are 0<x+2y<0.25 x<0.25 y<0.12. Air electrode material for fuel cells. (2) In a fixed electrolyte fuel cell in which single cells in which a fuel electrode and an air electrode are arranged on both main surfaces of a solid electrolyte plate are stacked via a spacer and a separator, the air electrode is (La(1-x
)Srx)(Mn(1-y)Cry)O_3-based ceramic lanthanum manganate as a main component, wherein the values of x and y are 0<x+2y<0.25 x<0.25 y<0. 12. A solid oxide fuel cell characterized in that it is formed of an air electrode material.