JPH0433266A - Oxide solid solution and mixed body suitable for electrode material for solid electrolyte fuel cell - Google Patents

Abstract

PURPOSE:To improve stability over a long period of time by forming an electrode mainly with a specific quantity of magnesium oxide against the total mole quantity and the remainder of nickel oxide. CONSTITUTION:An electrode is mainly made of magnesium oxide and nickel oxide, the content of the magnesium oxide is 5-40mol.% against the total mole quantity, and the remainder is an oxide solid solution of the nickel oxide. A mixed body is mainly made of this oxide solid solution and cubic zirconium oxide, and the mixing quantity of the cubic zirconium oxide is set to 60vol.% or below against the total volume of the mixed body. When the fuel electrode is formed with the oxide solid solution or the mixed body, the electrode with excellent long-term stability and a good catalyst function can be obtained.

Description

DETAILED DESCRIPTION OF THE INVENTION [Field of Industrial Application] The present invention relates to oxide solid solutions and mixtures suitable for electrode materials for solid oxide fuel cells.

[Conventional technology] Conventionally, solid oxide fuel cells (hereinafter referred to as 5OFC)
The material of the fuel side electrode (hereinafter referred to as fuel electrode) is nickel (Ni) or nickel and zirconium oxide (ZrO2).
) cermet (hereinafter referred to as Ni-ZrO2 cermet) was used.

[Problems to be Solved by the Invention] However, fuel electrodes manufactured using these materials do not have sufficient long-term stability. Furthermore, the particle size of Ni produced by reduction of nickel oxide (Nip) at the beginning of battery operation depends on the particle size of NiO before reduction, and there is a limit to the miniaturization of the particle size of NiO before reduction. It is difficult to make the Ni particles produced by reduction more fine and, as a result, to enhance the catalytic action.

Therefore, an object of the present invention is to provide an oxide solid solution and a mixture suitable as an electrode material for 5OFC. It also has excellent long-term stability, and it is possible to make the particle size of Ni produced by reduction of NiO during the initial stage of battery operation smaller than that of conventional products, and as a result, it is expected to have an even higher catalytic effect. It is possible.

[Means for Solving the Problems] In order to solve the above problems, the invention of claim 1 mainly consists of magnesium oxide and nickel oxide, and the magnesium oxide content is 5 mol% or more with respect to the total molar amount thereof.
It is an oxide solid solution suitable for an electrode material for 5OFC, characterized in that the content is 0 mol % or less, and the remainder is nickel oxide.

Here, the oxide that must contain 5 mol% or more of MgO with respect to the total molar amount of magnesium oxide (MgO) and nickel oxide (Nip) is an oxide in which MgO is less than 5 mol% and the balance is NiO. 5OF using solid solution as material
This is because when an electrode for C is manufactured, the shrinkage is large at the initial stage of battery operation, resulting in separation between the electrode and the electrolyte. In addition, the MgO content must be 40 mol% or less, because if the 5OFC electrode is manufactured using an oxide solid solution containing 40 mol% or more and the balance being NiO, the amount of Ni precipitated will decrease. However, as a result, sufficient catalytic action cannot be expected and conductivity due to Ni cannot be expected, resulting in a decrease in electrode performance.

The solid solution refers to one in which nickel atoms (Ni), magnesium atoms (Mg), and oxygen atoms (0) are uniformly mixed on the atomic order. By uniformly mixing Ni, Mg, and O atoms on the atomic order in this way, the Ni particles precipitated by the reduction of NiO are made finer.

The method for producing the oxide solid solution according to claim 1 includes Ni
Thermal decomposition of an acetate aqueous solution of Ni and Mg
Wet processes such as thermal decomposition of nitrate aqueous solutions (
dry process (wet process), method of mixing NiO powder and MgO powder and calcining method, method of mixing Ni salt powder and Mg salt powder and calcining method, etc.
Conventional methods such as cess) can be used.

The invention of claim 2 provides a mixture suitable for an electrode material for 5OFC, and the mixture mainly consists of the oxide solid solution of claim 1 and cubic zirconium oxide, and the cubic zirconium oxide is It is characterized in that the amount of the mixture is 60% by volume or less based on the total volume of the mixture.

Here, cubic zirconium oxide means zirconium oxide (ZrOz) whose crystal system is stabilized to a cubic system. Various methods can be used to stabilize the crystal system, such as stabilization using ytoz or calcium oxide (Cab). The reason why it is necessary to stabilize ZrO2 into a cubic system is that when the crystal system of ZrO changes due to temperature changes, its volume changes significantly, and as a result, separation between the electrode and the electrolyte occurs during electrode creation. This is because it will occur.

The reason why the amount of cubic zirconium oxide mixed is set to 60% by volume or less based on the total volume of the mixture is that when a mixture containing more than this amount is used as an electrode material for 5OFC, This is because the amount of Ni precipitated decreases because the amount of NiO contained in the metal decreases, and therefore, sufficient catalytic action cannot be expected and conductivity due to Ni cannot be expected.

The mixture in claim 2 is a simple mixture of the oxide solid solution powder of claim 1 and the cubic zirconium oxide powder, and 5OFC made from the oxide solid solution of claim 1 or the mixture of claim 2 as a material. Various conventional methods are used to manufacture the electrodes, including printing on an electrolytic surface and firing, dipping on an electrolytic surface and firing, chemical vapor deposition (CVD), or thermal spraying on an electrolyte. Vine.

Furthermore, although the oxide solid solution of claim 1 and the mixture of claim 2 are suitable for use as electrode materials for 5OFC, they also have other uses, for example, as catalysts.

[Example] Next, an example of the present invention will be described. In the following description, the Nio-Mgo solid solution means a solid solution in which Ni atoms, Mg atoms, and 0 atoms are uniformly mixed.

[Example 1] Production of Ni0-MgO solid solution and its evaluation (1-A)
Production of Ni0-MgO solid solution Nickel acetate tetrahydrate (N
i(C1bCOO)z 4 H2O, special grade, Wako Pure Chemical Industries, Ltd.) and magnesium acetate tetrahydrate (Mg(CHaC
OO) t・4H20, special grade, manufactured by Kishida Chemical), number of moles is 1
00:0.80:20.60:40.40:60.
The mixture was weighed to a ratio of 20:80 and O:100, and pure water was added thereto, followed by stirring with a magnetic stirrer to prepare a 0.5N aqueous solution. These aqueous solutions were dropped into a quartz tube maintained at 750 to 850°C at a rate of about 2 cc/min to perform thermal decomposition. Thereafter, heat treatment was performed in air at 1000° C. for 24 hours to obtain a desired powder. When examining the X-ray charts of the obtained powders using the alpha ray of copper as the X-ray source, only the diffraction peak of rock salt-type crystals was present in all powders, and this diffraction peak was due to the increase in the amount of MgO added. The powder obtained by the above manufacturing method was found to have a continuous shift to lower angles as the Ni
It was found that the O component and the MgO component were sufficiently dissolved in solid solution.

In the following description, the molar ratio of nickel acetate tetrahydrate and magnesium acetate tetrahydrate used during the production of the solid solution in (1-A) is 100:0.80:20.60:
40.40:60.20:80 and 0:100, the products were respectively NiO oxide and Mg020%.
Solid solution and Mg040% solid solution, Mg060% solid solution,
It is expressed as an 80% Mg0 solid solution and an oxide of MgO alone. Among these, MgO20, 40, 60 and 80% solid solution is Ni
This corresponds to a 0-MgO solid solution. For example, in a Mg020% solid solution, the amount of MgO solid solution is 20%.
It is expressed as mol%, and the solid solution amount of NiO is 80 mol%. Other NjO-MgO solid solutions are expressed in the same manner.

(1-B) Reactivity of Ni0-MgO solid solution and yttria-stabilized zirconia Ittria-stabilized zirconia (hereinafter referred to as YSZ) is a substance often used as an electrolyte. N
When forming a fuel electrode on a YSZ electrolyte using an i0-MgO solid solution, high resistance will occur if the solid solution and YSZ precipitate another phase, so the solid solution cannot be used as an electrode material for 5OFC. Therefore, the following experiment was conducted to investigate whether Ni0Mg0 solid solution and YSZ precipitate other phases under the electrode fabrication conditions.

That is, Mg020% produced in Example (1-A)
, MgO40%, Mg060%, and Mg080% solid solution powder, the X-ray charts before and after heat-treating equimolar mixtures with YSZ powder at 1400°C for 4 hours in the same manner as in Example (1-A). Examined. As a result, in all the Ni0Mg0 solid solutions investigated, the diffraction peaks after heat treatment were the same as before heat treatment, and Ni0-Mg0
There were only O solid solution and YSZ peaks. That is, Ni
Since the 0-MgO solid solution does not react with YSZ to precipitate other phases, it has been found that there is no problem when it is applied to the fuel electrode of the solid solution 5OFC.

(1-C) Coefficient of thermal expansion measurement The coefficient of thermal expansion of the oxide of NiO alone, the oxide of MgO alone, and various Ni0-MgO solid solutions produced in (1-A) was measured in air at room temperature using a conventional method. The measurement was carried out at a heating rate of 10°C/min from 1200°C to 1200°C.

The coefficient of thermal expansion was smaller for the NiO-MgO solid solution compared to the oxides of NiO alone or MgO alone. Ni0
-The coefficient of thermal expansion of MgO solid solution is 13.7 to 14.3X
10-1 of Ni-ZrO2 cermet at 10-'/℃
The value is equivalent to 5 × 10−′/°C, and therefore the present solid solution is compared to ZrO.

It was thought that there would be no problem even if it was applied alone to a 5OFC fuel electrode without being mixed with.

[Example 2] Preparation of Ni0-MgO solid solution fired body and change in properties due to hydrogen (H2) atmosphere treatment of the fired body (
2-A) Method of firing and hydrogen atmosphere treatment of Ni0-MgO solid solution powder.

In order to examine the properties of the 5OFC electrodes manufactured using the various Ni0-MgO solid solutions manufactured in (1-A), the various Ni0-MgO solid solutions were fired and treated in an H2 atmosphere as described below. Also for reference, Ni
Similar treatments were performed on oxides containing O alone and oxides containing MgO alone.

That is, a square bar of 5 x 5 x 15 mm was preformed by mold pressing the solid solution powder produced in (1-A), and then a sintered body was produced by isostatic pressing (CIF) L at 3 tons/crl. Firing was performed at 1400°C for 2 hours in air. Regarding the obtained sintered body, H! Atmosphere treatment was performed. The temperature schedule for this H3 atmosphere treatment is as follows: After replacing the inside of the furnace with H1 atmosphere, the temperature will rise from room temperature to 100% in 60 minutes.
After raising the temperature to 0°C and maintaining it at 1000°C for a predetermined time,
Furnace cooled. Furnace cooling reached 300°C or less in 5 minutes. 10
Holding time at 00℃ is 0.15.60 or 4/300
It was set as a standard.

(2-B) Microstructural observation Various Ni0-MgO solid solutions, NiO oxides and Mg fired as described in (2-A) and treated in H2 atmosphere
Scanning electron microscopy (SEM) of the microstructure of an oxide of O alone
and observed using an energy dispersive X-ray spectrometer (EDX).

In the case of a body, a porous body is formed in which fine spherical Ni particles of approximately 0.5 μm (particles that appear white in the photo) are surrounded by an MgO-rich solid solution (parts that appear blue in the photo).
It was found that coarsening due to sintering of i-particles was suppressed. A similar tendency was observed from the SEM-EDX photograph in the Mg040% solid solution. On the other hand, NiO
In the case of a single oxide, unlike the Ni0-MgO solid solution, it was observed from the SEM photograph that the Ni particles tended to rapidly sinter, coarsen, and become denser as a whole as the holding time was extended.

That is, in the Ni0-MgO solid solution, when Ni is produced by reduction of NiO, coarsening of Ni particles due to sintering is suppressed by the MgO-rich solid solution, and very fine Ni particles are produced. Therefore, high catalytic activity is expected in electrodes manufactured using the Nip-MgO solid solution as a material. It is also believed that peeling off from electrolytic purchase due to sintering of Ni particles together can be prevented. .

(2-C) Specific surface area measurement When the sintered body prepared in (2-A) was treated with H2 atmosphere as described in (2-A), the change in specific surface area was measured with N2
It was investigated using the adsorption method (Method B, E, T). The results are shown in FIG. Note that the specific surface area is usually expressed as the surface area per unit mass (rrr/g), but in this case, the density differs when the amount of solid solution of MgO differs, making it difficult to compare data, so it is expressed as the surface area per unit volume (rrr/g). rd/cc).

In the symbols in Figure 1, ○ is NiO alone oxide,
is Mg020% solid solution, △ is Mg040% solid solution, m is Mg060% solid solution, mouth is Mg080% solid solution, and ■
shows the results for an oxide of MgO alone. The vertical axis indicates the surface area per unit volume, and its numerical unit is rd/cc. The horizontal axis represents the time after H2 atmosphere treatment, and the numerical unit is minutes. Below the horizontal axis, 60 minutes on the horizontal axis is set to 0 minutes, and the other horizontal axis is 1000 minutes.
It represents the holding time in °C and its numerical unit is minutes.

As shown in Figure 1, the specific surface area of the oxides of NiO alone and MgO alone (symbols ○ and ■) is very small both before and after treatment in two atmospheres.In comparison, for the Ni0-MgO solid solution, , the specific surface area increased by H2 atmosphere treatment.The degree of increase was greater for solid solutions with a larger amount of NiO solid solution.That is, Mg080%, Mg060%, Mg040%,
The degree of increase in specific surface area due to Ht atmosphere treatment increases in the order of Mg020% solid solution, and in particular, in Mg020% solid solution, the specific surface area is 17% at a holding time of 60 minutes.
．． It was a very large value of 5rrr/cc.

After a holding time of 300 minutes, it slightly decreased to 15.4rd/cc, but still maintained a high value. Therefore, it can be expected that electrodes manufactured using the Ni0-MgO solid solution have high catalytic activity.

(2-D) Measurement of shape change rate In (2-A), the thickness, width and length of the square bar, which is the sintered body, was treated in H2 atmosphere as described in (2-A). Measured with a micrometer before and after, H2
The average value of the percent change in thickness, width, and length due to the atmosphere treatment was calculated and used as the shape change rate.

The results are shown in FIG. The meanings of the symbols in FIG. 2 are the same as in FIG. The horizontal axis is the same as in FIG. 1, and the vertical axis represents the rate of shape change, with a positive value representing expansion due to the H2 atmosphere treatment and a negative value representing contraction.

As shown in Figure 2, the temperature of NiO alone is 1000℃.
It had already shrunk by 9.2% due to the increased temperature, and by holding the temperature at 1000°C for 300 minutes, it shrunk by 15.7%.

On the other hand, various Ni0-MgO solid solutions showed only slight changes in shape due to H2 atmosphere treatment.

For example, a 20% MgO solid solution expanded by only 0.5% even after being maintained at 100°C for 300 minutes.

As the MgO solid solution amount increased, this expansion amount became smaller. That is, no change in shape was observed in the 80% Mg0 solid solution and the oxide of MgO alone.

From the above results, the solid solution of MgO in NiO is
It was found to be effective in preventing shrinkage due to sintering in an atmosphere.

In order to investigate in more detail the relationship between the amount of MgO dissolved in the Ni0-MgO solid solution and the rate of shape change, the holding time was 6.
The relationship between the amount of magnesium oxide solid solution (mol %) and the rate of shape change after 0 minutes was investigated in the same manner. The results are shown in FIG.

In Figure 3, the vertical axis indicates the shape change rate, and the numerical unit is %.
It is. The horizontal axis is Mg contained in the Ni0-MgO solid solution.
It shows the amount of O solid solution, and its numerical unit is mol%.

As shown in FIG. 3, when the MgO solid solution amount is less than 5 mol%, H! The Ni0-MgO solid solution thickens (shrinks) due to sintering in the atmosphere. Therefore, the amount of MgO solid solution becomes 5 mol%.
When using a Ni0-MgO solid solution as an electrode material, it is found that it is unsuitable as an electrode material because it shrinks greatly in the early stages of battery operation and peels off from the electrolyte.

(1-E) Measurement of the amount of nickel precipitation The weight of the sample after the H22 atmosphere treatment in (2-A) was measured, and the weight change rate was investigated.

As a result, the longer the retention time, the lower the weight, but the MgO
As the amount of solid solution increased, the weight reduction rate due to H2 atmosphere treatment became smaller. Regarding the oxide of MgO alone, no weight change was observed even after holding for 300 minutes.

On the other hand, from the results of X-ray diffraction of each sample after H22 atmosphere treatment, it was observed that Ni was precipitated due to the reduction of NiO after H22 atmosphere treatment, and the amount of Ni precipitated increased as the holding time was longer. Moreover, it was found that the amount of MgO solid solution decreased as the amount of MgO solid solution increased. That is, from the above X-ray diffraction results, it was found that the weight reduction due to the H2 atmosphere treatment corresponded to the reduction of NiO, so the amount of Ni precipitation was calculated from this weight reduction rate.

For example, the result for Mg020% solid solution after a holding time of 60 minutes is 1.2486 g before H2 atmosphere treatment and 1.0824 g after treatment, a decrease of 13.3%. Therefore, the weight of 1 mole of Mg020% solid solution is approximately 67.83
g, so 9.02 g, which is equivalent to 13.3% of that.
divided by 16g, which is the weight of 1 mole of oxygen atoms, is 0.5
64, so in this case, the amount of Ni precipitated is 56.4 mol%.

FIG. 4 shows a graph showing the relationship between the amount of solid solution of magnesium oxide and the amount of nickel precipitation for a holding time of 60 minutes.

In FIG. 4, the vertical axis indicates the amount of nickel precipitation determined as described above, and the numerical unit is mol%.

The horizontal axis indicates the amount of solid solution of MgO, and its numerical unit is mol%.

As shown in FIG. 4, when the amount of MgO solid solution in the Ni0-MgO solid solution exceeds 40 mol %, the amount of Ni precipitated decreases significantly. Therefore, Ni with MgO solid solution amount of 40 mol% or more
When a 0-MgO solid solution is used as an electrode material, sufficient electrode activity and conductivity due to Ni cannot be expected due to the small amount of Ni precipitated, and the electrode performance deteriorates, so it is found to be unsuitable as an electrode material.

(Example 3) Manufacture of fuel electrode cell and its evaluation (3-A) Manufacture of fuel electrode cell (1-A) PJi(CHsCOO)2 4 H
A Mg020% solid solution powder produced by weighing 2O and Mg(C)I3Coo)z.4H70 in a molar ratio of 80:20 was calcined at 1400° C. for 4 hours to obtain a calcined powder. A binder and a dispersant were added to this calcined powder, and after stirring in an automatic mortar for 15 minutes, a screen (# (mesh) 200) was placed on YSZ pellets (diameter 13, thickness 1) made by Nippon Kagaku Togyo Co., Ltd. Printed. This is heated to 1400°C12
Baked in time. The thickness of the fuel electrode in this case was 15 μm. Next, an air electrode ((La
o, 5sro2L, eMnos) were screen printed and baked at 1200°C for 4 hours. Finally, set the reference pole to 1
A cell for performance evaluation was obtained by baking at 000°C for 2 hours. The temperature increase/decrease rate during baking was all set to 200°C/hour. Polyethylene glycol and ethanol were used as the binder and dispersant.

(3-B) Evaluation of initial performance The relationship between current density and fuel electrode polarization value was measured for the fuel electrode cell manufactured in (3-A), and its initial performance was evaluated. The measurement was carried out using the current interruption method, and the polarization value was determined based on the IR acid component η.
The components were separated and measured.

The results are shown in FIG.

In Figure 5, the vertical axis shows the polarization value, whose numerical unit is m
It is V. The horizontal axis shows the current density, whose numerical unit is mA
/cf. The symbols in FIG. 5 indicate the overall polarization value, which corresponds to the total value of the IR acid component and the η component. On the other hand, △ indicates the value of the resistance component of the overall polarization value, and IR
Corresponds to the acid component value.

Therefore, the difference between the two values becomes the value of the η component. Here, the IR acid component, the resistance polarization of the solid electrolyte, the resistance polarization of the fuel electrode, and the resistance polarization of the interface between the solid electrolyte and the fuel electrode are combined. The η component consists of the activation polarization of the fuel electrode and the concentration polarization of the fuel electrode, and as shown in FIG. 5, the current density is 200 mA/cy! In this case, it was about 70 mV.

The value of this η component is based on the high performance Ni obtained by the coating sintering method.
- This value is equivalent to the value of the η component under the same conditions for the ZrO2 cermet fuel electrode.

Therefore, it has been found that the fuel electrode manufactured using the oxide solid solution according to the present invention has an initial performance equivalent to that of the conventional high-performance Ni-ZrO2 cermet fuel electrode.

(3-C) Evaluation of long-term stability In order to evaluate the long-term stability of the fuel electrode cell manufactured in (3-A), a continuous current test was conducted on the fuel electrode cell.

As for the test conditions, Hl was used as the fuel gas, and air was introduced into the air electrode by a compressor. The flow rates of H3 and air were each controlled at 50 cc/min using a mass flow controller. Then, by current interruption method, 200 mA/
The polarization value during cr1 energization was measured during battery operation and after 100 hours of energization, and the rate of increase in polarization of the fuel electrode was determined. The results are shown in Table 1. As a comparative example, measurement results of Ni-Zr0* cermet under the same conditions are also shown.

Table 1 As shown in Table 1, it can be seen that the long-term stability of the electrode using the oxide solid solution of the present invention is significantly improved compared to the conventional electrode using Ni-ZrO2 cermet.

Furthermore, the mixture of the oxide solid solution and cubic zirconium oxide according to the present invention has properties similar to those of the oxide solid solution according to the present invention, and is therefore suitable as an electrode material for 5OFC.

In the above mixture, the mixing amount of cubic zirconium oxide was set to 60% by volume or less, but this is because if the mixing amount of cubic zirconium oxide exceeds 60% by volume, the amount of nickel precipitated decreases, and a sufficient electrode cannot be obtained. This is because activity and conductivity cannot be expected. Actually, in a mixture of Mg020% solid solution and cubic zirconium oxide, when the mixing amount of cubic zirconium oxide is 60% by volume, the electrical conductivity measured in H2 gas at 1000°C by the DC four-terminal method is It was lXl0'Ω country.

[Effect of the invention] Using the oxide solid solution or mixture according to the present invention as a material, 5O
When an FC fuel electrode is manufactured, an electrode that has excellent long-term stability and can be expected to have good catalytic action can be obtained.

[Brief explanation of drawings]

Figure 1 is a graph showing the relationship between the H2 atmosphere treatment time and the surface area per unit volume of the fired body prepared in (2-A), and Figure 2 is a graph showing the relationship between the H2 atmosphere treatment time and the shape change rate of the fired body. This is a graph showing the relationship between 1000 and Fig. 3.
FIG. 4 is a graph showing the relationship between the shape change rate and the amount of MgO solid solution at 60 minutes of holding time at 1000°C.
FIG. 5 is a graph showing the relationship between the amount of nickel precipitation and the amount of MgO solid solution at a holding time of 60 minutes, and FIG. 5 is a graph showing the relationship between current density and polarization value for the cell manufactured in (3-A). , the polarization values are shown separately for the IR acid component and the η component.雛6 fed 12 M, 0201/. 30 ke f ship 9
There is 55 Seki-EDX photo 17'.

Claims (2)

[Claims] (1) Mainly composed of magnesium oxide and nickel oxide, with 5% of the total molar amount of magnesium oxide being
An oxide solid solution suitable for an electrode material for a solid electrolyte fuel cell, characterized in that the content is mol % or more and 40 mol % or less, and the remainder is nickel oxide. (2) A mixture mainly consisting of an oxide solid solution and cubic zirconium oxide, the oxide solid solution mainly consisting of magnesium oxide and nickel oxide, and the magnesium oxide content is 5 mol% or more with respect to the total molar amount thereof. 40 mol% or less, and the balance is nickel oxide, while the amount of the cubic zirconium mixed is 60 vol% or less based on the total volume of the mixture. Mixture suitable for electrode materials.