JPH07107854B2 - Solid oxide solutions and mixtures for solid oxide fuel cell electrode materials - Google Patents

Description

TECHNICAL FIELD The present invention relates to a solid solution and a mixture for an electrode material of a solid oxide fuel cell.

[Prior Art] Conventionally, as a material of a fuel electrode (hereinafter referred to as a fuel electrode) of a solid oxide fuel cell (hereinafter referred to as SOFC), nickel (Ni) or a cermet of nickel and zirconium oxide (ZrO ₂ ) ( Hereinafter, Ni-ZrO ₂ cermet) was used.

[Problems to be Solved by the Invention] However, long-term stability is not sufficient in the fuel electrode manufactured from these materials. Furthermore, since the particle size of Ni produced by the reduction of nickel oxide (NiO) at the beginning of battery operation depends on the NiO particle system before reduction, and there is a limit to the refinement of the NiO particle system before reduction, It is difficult to make the Ni particles generated by the reduction finer and consequently enhance the catalytic action.

Therefore, an object of the present invention is to provide an oxide solid solution and a mixture suitable for an SOFC electrode material, and an SOFC electrode manufactured using the oxide solid solution or the mixture as a material is more than a conventional SOFC electrode. Also has excellent long-term stability, and it is possible to make the particle size of Ni generated by reduction of NiO at the beginning of battery operation smaller than conventional ones, and as a result, expect a higher catalytic action. It is possible.

[Means for Solving the Problems] In order to solve the above problems, the invention of claim 1 is mainly composed of magnesium oxide and nickel oxide, and magnesium oxide is 5 mol% or more with respect to the total molar amount thereof.
The solid solution is an oxide solid solution for an SOFC electrode material, characterized in that the content is not more than mol% and the balance is nickel oxide.

Here, magnesium oxide (MgO) and nickel oxide (N
iO) must be contained in an amount of 5 mol% or more from MgO with respect to the total molar amount of iO) because MgO is less than 5 mol% and the balance is N
This is because when an SOFC electrode is manufactured using an oxide solid solution of iO as a material, the contraction at the beginning of battery operation is large and the electrode and the electrolyte are separated. Also, MgO is 40
The amount of MgO should be 40 mol% or more, and the balance should be 40% or more, and the balance should be NiO.
When an OFC electrode is manufactured, the amount of Ni deposited decreases,
As a result, a sufficient catalytic action cannot be expected, and the electrical conductivity due to Ni cannot be expected, which deteriorates the electrode performance.

The solid solution means one in which nickel atoms (Ni), magnesium atoms (Mg) and oxygen atoms (O) are uniformly mixed in atomic order. In this way, Ni, Mg and O atoms are uniformly mixed in the atomic order, so that
Ni particles precipitated by reduction of O can be made fine.

A method for producing the oxide solid solution according to claim 1 is a wet process such as a thermal decomposition method of an aqueous solution of Ni and Mg acetate or a thermal decomposition method of an aqueous solution of Ni and Mg nitrate.
ess) or NiO powder and MgO powder are mixed and calcined or N
Drying such as the method of mixing and calcining i salt powder and Mg salt powder
Conventional methods such as a dry process can be used.

The invention according to claim 2 provides a mixture for an electrode material of SOFC, which mixture mainly comprises the oxide solid solution according to claim 1 and cubic zirconium oxide, and the cubic zirconium oxide is The mixing amount is 60% by volume or less based on the total volume of the mixture.

Here, cubic zirconium oxide means zirconium oxide (ZrO ₂ ) whose crystal system is stabilized in a cubic system. As a method for stabilizing the crystal system, for example, yttria (Y ₂ O ₃ ) is used for stabilization or calcium oxide (Ca
Various methods can be used to stabilize with O). In this way, it is necessary to stabilize ZrO ₂ to the cubic system because the volume of the ZrO ₂ changes significantly when the crystal system of ZrO ₂ changes due to temperature change, and therefore the electrode and electrode quality are This is because peeling occurs.

And the mixing amount of cubic zirconium oxide is set to 60% by volume or less with respect to the total volume of the mixture, when a mixture containing more than this is used as an SOFC electrode material,
This is because the amount of NiO contained in the mixture is reduced and the amount of Ni deposited is reduced, so that a sufficient catalytic action cannot be expected and the electrical conductivity due to Ni cannot be expected.

The mixture in claim 2 means a mixture of the oxide solid solution powder of claim 1 and cubic zirconium oxide powder.

A method for producing an SOFC electrode using the oxide solid solution according to claim 1 or the mixture according to claim 2 as a material is a method of printing on an electrolyte and firing, a method of dipping and firing on the electrolyte, and a chemical vapor deposition method. Various ordinary methods such as (CVD) or a method of spraying on an electrolyte can be used.

The oxide solid solution of claim 1 and the mixture of claim 2 are
Although it is suitable for use as an electrode material for SOFCs, it has other uses as well, and can be used, for example, as a catalyst for steam reforming or carbon dioxide reforming of methane.

[Examples] Next, examples 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 O atoms are uniformly mixed.

[Example 1] NiO-MgO solid solution preparation and evaluation (1-A) NiO-MgO solid solution produced nickel acetate tetrahydrate _{(Ni (CH 3 COO) 2} 4H 2 O, special grade, manufactured by Wako Pure Chemical Industries, ) And magnesium acetate tetrahydrate (Mg (CH
_{3 COO) 2 · 4H 2 O} , special grade, manufactured by Kishida Chemical Co., Ltd.) and the molar number 100:
Weighed at 0, 80:20, 60:40, 40:60, 20:80, 0: 100, added pure water, stirred with a magnetic stirrer, then 0.5
A normal aqueous solution was prepared. These aqueous solutions were dropped into a quartz tube kept at 750-850 ℃ at a rate of about 2 cc / min for thermal decomposition. Then, heat treatment was performed in air at 1000 ° C. for 24 hours to obtain a predetermined powder. When the X-ray chart of the obtained powder was examined using the Kα line of copper as the X-ray source, only the diffraction peaks of rock salt type crystals were present in all the powders, and this diffraction peak was the amount of MgO added. The powder obtained by the above-mentioned production method, because it continuously shifts to the lower angle side with increase, each diffraction peak is one without separation, and the diffraction peak is sharp. It was found that NiO component and MgO component were sufficiently dissolved.

In the following description, the molar ratio of nickel acetate tetrahydrate and magnesium acetate tetrahydrate used in the preparation of the solid solution in (1-A) is 100: 0, 80:20, 60:40, 40:60. , 2
The products with 0:80 and 0: 100 are represented as oxides of NiO alone, MgO 20% solid solution and MgO 40% solid solution, MgO 60% solid solution, MgO 80% solid solution, and MgO alone oxide, respectively. Among them, MgO 20, 40, 60 and 80% solid solutions correspond to NiO-MgO solid solutions. For example, in a MgO 20% solid solution, the solid solution amount of MgO is 20 mol% and the solid solution amount of NiO is 80%.
It is expressed as being in mol%. The same applies to other NiO-MgO solid solutions.

(1-B) Reactivity of NiO-MgO Solid Solution with Yttria-Stabilized Zirconia Yttria-stabilized zirconia (hereinafter referred to as YSZ) is a substance often used as an electrolyte. NiO
-When forming a fuel electrode on an electrolyte of YSZ using a MgO solid solution, the solid solution and YSZ cannot be used as an SOFC electrode material because a high resistance occurs when another phase is precipitated. Therefore, the NiO-MgO
It was investigated whether the solid solution and YSZ precipitate other phases under the conditions of electrode preparation.

That is, MgO 20% and MgO produced in Example (1-A)
X-ray charts before and after heat-treating an equimolar mixture of 40%, MgO60% and MgO80% solid solution powders with YSZ powder for 4 hours at 1400 ° C were examined by the same method as in Example (1-A). It was As a result, in all the investigated NiO-MgO solid solutions, the diffraction peaks after the heat treatment were the same as those before the heat treatment, and were only the peaks of the NiO-MgO solid solution and YSZ.
That is, since the NiO-MgO solid solution does not react with YSZ to precipitate other phases, it was found that there is no problem even if it is applied to the fuel electrode of the solid solution SOFC.

(1-C) Measurement of coefficient of thermal expansion The coefficient of thermal expansion of the oxide of NiO alone, the oxide of MgO alone, and various NiO-MgO solid solutions produced in (1-A) was measured in air at room temperature using a usual method. To 1200 ° C under the conditions of a heating rate of 10 ° C / min.

The coefficient of thermal expansion of NiO is higher than that of NiO or MgO alone.
-It became smaller in MgO solid solution. The coefficient of thermal expansion of the NiO-MgO solid solution is 13.7 to 14.3 × 10 ^-6 / ° C, which is equivalent to the value of 10 to 15 × 10 ^-6 ° C of Ni-ZrO ₂ cermet, and therefore this solid solution is mixed with ZrO _2. It was considered that there would be no problem if it was applied alone to the SOFC fuel electrode.

EXAMPLE 2] NiO-MgO solid solution fired bodies manufactured and said heat-hydrogen (H ₂₎ characteristic change due to ambient processing (2-A) The method of firing and hydrogen atmosphere processing of NiO-MgO solid solution powder.

In order to investigate each property of the SOFC electrode manufactured by using various NiO-MgO solid solutions manufactured in (1-A),
The NiO-MgO solid solution was fired and treated with H ₂ atmosphere as described below. For reference, the same treatment was performed on the oxide of NiO alone and the oxide of MgO alone.

That is, after the solid solution powder produced in (1-A) is die-pressed to preform a 5 × 5 × 15 mm square bar,
A hydrostatic press (CIP) was performed at 3 ton / cm ² to prepare a sintered body. Baking was performed at 1400 ° C. for 2 hours in air. H ₂ atmosphere treatment was performed on the obtained sintered body. The temperature schedule for this H ₂ atmosphere treatment is as follows: after the furnace is replaced with the H ₂ atmosphere, the temperature is raised from room temperature to 1000 ° C. in 60 minutes, and the predetermined time is set to 1000.
After maintaining the temperature at 0 ° C, the furnace was cooled. Furnace cooling reached below 300 ° C in 5 minutes. The holding time at 1000 ° C. was set to 0, 15, 60 or 4/300 levels.

(2-B) Microstructural observation Various types of material that were fired as described in (2-A) and treated with H ₂ atmosphere
The microstructures of the NiO-MgO solid solution, the oxide of NiO alone and the oxide of MgO alone were observed using a scanning electron microscope (SEM) and an energy dispersive X-ray analyzer (EDX).

FIG. 6 shows a SEM-EDX photograph of the MgO 20% solid solution after a retention time of 300 minutes. In Fig. 6, ED is the amount of Ni and Mg on the straight line.
Line analysis was performed by X.

As shown in Fig. 6, in the case of MgO 20% solid solution, about 0.5μ
Fine spherical Ni particles of m (white particles in the photo)
It was found that the O-rich solid solution (the part that appears gray in the photograph) surrounds the porous body, which suppresses the coarsening of the Ni particles by sintering. The same tendency was observed from the SEM-EDX photograph for the MgO 40% solid solution. On the other hand, in the oxide of NiO alone, unlike the NiO-MgO solid solution, the Ni particles rapidly sinter due to the extension of the retention time, and the tendency of coarsening and densification as a whole SEM
Observed from the photograph.

That is, in the NiO-MgO solid solution, NiO is reduced by reduction of NiO.
When Mg is generated, the coarsening of the Ni particles due to sintering is suppressed by the MgO-rich solid solution, and very fine Ni particles are generated. Therefore, high catalytic activity is expected in an electrode manufactured using a NiO-MgO solid solution as a material. Also, Ni
It is considered that separation of particles from each other due to sintering can be prevented.

(2-C) Specific surface area measurement The specific surface area change when the sintered body prepared in (2-A) is treated with H ₂ atmosphere as described in (2-A) is the N ₂ adsorption method (BET method). It investigated using. The results are shown in FIG. The specific surface area is usually the surface area per unit mass (m ²
/ g), but this time, different amounts of MgO solid solution cause different densities, making it difficult to compare data. Therefore, the surface area per unit volume (m ² / cc) was used.

In the symbol of Fig. 1, ○ means oxide of NiO alone, ● means M
gO 20% solid solution, △: MgO 40% solid solution, ▲: MgO 60% solid solution, □: MgO 80% solid solution, and ■: results for oxides of MgO alone. The vertical axis represents the surface area per unit volume, and its numerical unit is m ² / cc. Horizontal axis is H ₂
It is the time after the atmosphere treatment, and the numerical unit is minutes. Below the abscissa, the other abscissa, which has 60 minutes on the abscissa as 0 minute, represents the holding time of 1000 ° C., and its numerical unit is minutes.

As shown in FIG. 1, the specific surface area of oxides of NiO alone and MgO alone (symbols ○ and ■) is very small both before and after H ₂ atmosphere treatment. NiO in comparison
For the MgO solid solution, the specific surface area was increased by the H ₂ atmosphere treatment. And the solid solution with a large amount of NiO solid solution increased the degree of increase. That is, MgO 80%, M
In the order of 60% gO, 40% MgO, and 20% MgO solid solution, the specific surface area increases with the H ₂ atmosphere treatment, especially MgO 20%
The specific surface area of the solid solution at a retention time of 60 minutes is
It was a very large value of 17.5m ² / cc. After a holding time of 300 minutes, it decreased to 15.4 m ² / cc, but remained high. Therefore, it can be expected that the electrode manufactured using the NiO-MgO solid solution as a material has high catalytic activity.

(2-D) Measurement of shape change rate In (2-A), the thickness, width, and length of the square rod that was a sintered body were treated with H ₂ atmosphere as described in (2-A). Before and after the measurement, a micrometer was used to measure, and the average value of the respective change rates% of the thickness, width, and length by the H ₂ atmosphere treatment was calculated to obtain 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, the vertical axis represents the rate of shape change, plus means expansion by the H ₂ atmosphere treatment, and minus means contraction.

As shown in FIG. 2, the oxide of NiO alone contracted 9.2% already at 1000 ° C. elevated temperature, and contracted 15.7% by keeping 1000 ° C. for 300 minutes. On the other hand, various NiO-MgO solid solutions showed only a slight change in shape by the H ₂ atmosphere treatment. For example, MgO 20% solid solution expanded only 0.5% even after holding at 100 ° C for 300 minutes. And as the MgO solid solution amount increased, this expansion amount became smaller.
That is, no shape change was observed in the MgO 80% solid solution and MgO alone oxides.

From the above results, it was found that the solid solution of MgO with NiO is effective for preventing shrinkage due to sintering in H ₂ atmosphere.

In order to further investigate the relationship between the amount of MgO dissolved in the NiO-MgO solid solution and the shape change rate, the relationship between the magnesium oxide solid solution amount (mol%) and the shape change rate after a retention time of 60 minutes was determined by the same method. I looked it up. The results are shown in FIG.

In FIG. 3, the vertical axis represents the shape change rate, and the numerical unit is%.
Is. The horizontal axis represents the amount of MgO solid solution contained in the NiO-MgO solid solution, and the numerical unit is mol%.

As shown in FIG. 3, when the MgO solid solution amount is less than 5 mol%, the NiO-MgO solid solution contracts greatly due to sintering in an H ₂ atmosphere. Therefore, the amount of MgO solid solution is less than 5 mol% NiO
It can be seen that when a solid solution of MgO is used as the electrode material, it contracts greatly at the beginning of battery operation and peels off from the electrolyte, making it unsuitable as an electrode material.

(2-E) Measurement of nickel deposition amount The weight of the sample before and after the H ₂ atmosphere treatment in (2-A) was measured, and the weight change rate was examined. As a result, the weight decreased as the holding time increased, but the weight reduction rate due to the H ₂ atmosphere treatment decreased as the MgO solid solution amount increased. Regarding the oxide of MgO alone, no weight change was observed even after 300 minutes of holding.

On the other hand, from the results of X-ray diffraction of each sample before and after an H ₂ atmosphere treatment, by reduction of NiO is after an H ₂ atmosphere treatment Ni
It was found that the precipitation amount of Ni was larger, and that the precipitation amount of Ni was larger as the holding time was longer, and decreased as the solid solution amount of MgO increased. That is, from the above X-ray diffraction results, it was found that the weight reduction due to the H ₂ atmosphere treatment corresponds to the reduction of NiO. Therefore, the amount of Ni precipitation was calculated from this weight reduction rate.

For example, the result after a retention time of 60 minutes for MgO 20% solid solution is H
₂ Atmosphere before treatment 1.2486g, after treatment 1.0824g, 13.
It has decreased by 3%. Therefore, since the weight of 1 mol of MgO 20% solid solution is about 67.83 g, if 9.02 g corresponding to 13.3% thereof is divided by 16 g which is the weight of 1 mol of oxygen atom, it becomes 0.564, which is 56.4 mol% in this case. Is the precipitation amount of Ni.

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

In FIG. 4, the vertical axis represents the nickel deposition amount obtained as described above, and its numerical unit is mol%. The horizontal axis represents the amount of MgO solid solution, and the numerical unit is mol%.

As shown in Fig. 4, the amount of MgO solid solution of NiO-MgO solid solution is 40
If it exceeds mol%, the amount of Ni deposited is significantly reduced. Therefore
When the NiO-MgO solid solution with a MgO solid solution amount of 40 mol% or more is used as the electrode material, sufficient electrode activity and electrical conductivity due to Ni cannot be expected because the Ni precipitation amount is small, and the electrode performance deteriorates. It turns out that it is not suitable as an electrode material.

(Example 3) Production and Evaluation of the fuel electrode cell (3-A) Preparation of fuel electrode cell at (1-A) Ni (CH 3 COO) 2 · 4H 2 O and Mg (CH _{3 COO) 2}
The MgO 20% solid solution powder produced by weighing 4H ₂ O at 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 the calcined powder, the mixture was stirred for 15 minutes in an automatic mortar, and then screened (# (mesh) 200) on YSZ pellets (diameter 13 mm, thickness 1 mm) manufactured by Nippon Kagaku Sangyo Co., Ltd. . This was baked at 1400 ° C. for 2 hours. In this case, the fuel electrode wall thickness was 15 μm. next,
Air electrode ((La 0.8 Sr 0.2 ) 0.9 MnO ₃ ) on the back surface of YSZ pellets
Was screen printed and baked at 1200 ° C. for 4 hours. Finally, the back lighting electrode was baked at 1000 ° C. for 2 hours to obtain a cell for performance evaluation. The rate of temperature increase / decrease during baking was 200 ° C./hour. Polyethylene glycol and ethanol were used as the binder and the dispersant.

(3-B) Evaluation of Initial Performance The relationship between the current density and the fuel electrode component value of the fuel electrode cell manufactured in (3-A) was measured, and the initial performance was evaluated. The polarization value was separated into IR and η components by the current interruption method. The result is shown in FIG.

In Fig. 5, the vertical axis represents the polarization value, and its numerical unit is mV.
Is. The horizontal axis shows the current density, and the numerical unit is mA / cm ²
Is. The symbol in FIG. 5 shows the polarization value of the whole, and corresponds to the total value of the IR component and the η component. On the other hand, Δ indicates the value of the resistance component of the entire polarization value and corresponds to the value of the IR component. Therefore, the difference between the two values becomes the value of the η component.
Here, the IR component is a combination of 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. The η component is composed of activation polarization of the fuel electrode and concentration polarization of the fuel electrode, and as shown in FIG. 5, it was about 70 mV when the current density was 200 mA / cm ² . The value of the η component is equivalent to the value of the η component under the same conditions for the high-performance Ni-ZrO ₂ cermet fuel electrode obtained by the coating and sintering method.

Therefore, it was found that the fuel electrode manufactured by using the oxide solid solution according to the present invention has the same initial performance as the conventional high-performance Ni-ZrO ₂ cermet fuel electrode.

(3-C) Evaluation of long-term stability Regarding the fuel electrode cell manufactured in (3-A), a continuous energization test of the cell was performed in order to evaluate the long-term stability.
As the test conditions, H ₂ was used as the fuel gas, and air was introduced into the air electrode by a compressor. The flow rates of H ₂ and air were controlled at 50 cc / min with a mass flow controller. Then, by the current interruption method, the polarization value at the time of 200 mA / cm ² energization was measured after the battery was operated and after 100 hours of energization,
The polarization increase rate of the fuel electrode was obtained. The results are shown in Table 1. As a comparative example, the measurement results of Ni-ZrO ₂ cermet under the same conditions are also shown.

As shown in Table 1, compared with the electrode using the conventional Ni-ZrO ₂ cermet, the electrode using the oxide solid solution of the present invention is
It can be seen that the long-term stability is remarkably improved.

Further, the mixture of the oxide solid solution according to the present invention and cubic zirconium oxide has the same properties as the oxide solid solution of the present invention, and is therefore suitable as an electrode material for SOFC. In the mixture, the mixing amount of cubic zirconium oxide was 60.
The content is set to be not more than volume% because when the mixing amount of cubic zirconium oxide exceeds 60% by volume, the amount of nickel deposited is reduced, and sufficient electrode activity and conductivity cannot be expected. In the actual mixture of MgO 20% solid solution and cubic zirconium oxide, if the mixing amount of cubic zirconium oxide was set to 60% by volume, the DC four-terminal method would be applied.
The conductivity measured in H ₂ gas at ℃ was 1 × 10 ¹ Ωcm.

[Effects of the Invention] SOFC using the oxide solid solution or mixture according to the present invention as a material
When the fuel electrode is manufactured, an electrode having excellent long-term stability and expecting a good catalytic action can be obtained.

[Brief description of drawings]

Figure 1 is a graph showing the relationship between the surface area per unit volume of the sintered body created in the atmosphere of H ₂ processing time (2-A), the shape of Fig. 2 _a H ₂ atmosphere processing time and the sintered body FIG. 3 is a graph showing the relationship with the change rate, FIG. 3 is a graph showing the relationship between the shape change rate and the amount of MgO solid solution at 1000 ° C. holding time 60 minutes, and FIG. 4 is 1000 ° C. holding time 60 minutes. 5 is a graph showing the relationship between the amount of nickel deposited and the amount of MgO solid solution in FIG. 5, and FIG. 5 is a graph showing the relationship between the current density and the polarization value for the cell manufactured in (3-A).
The IR component and the η component are shown separately. Figure 6 shows Mg
3 is an electron micrograph of a crystal structure of an O20% solid solution after a retention time of 300 minutes.

Continuation of front page (74) The above two agents Attorney Hidehiko Okada (1 person outside) (72) Inventor Shinji Kawasaki 2-38-2 Okamachi, Mizuho-ku, Nagoya-shi, Aichi Prefecture (Gaishi City Hill Residence) (72) Inventor Keiichi Katayama 194, Hakoda, Kumagaya-shi, Saitama (Koukouitsu 1-2) (72) Inventor Takehito Bogauchi 3-6, Nekodo-dori, Chikusa-ku, Nagoya-shi, Aichi (Yuasa Battery Nekodo Dormitory) ( 72) Inventor Hiromitsu Ota 2-4-1 Nishitsujigaoka, Chofu City, Tokyo (72) Inventor Shinji Takeuchi 3-11-20 Wakaoji, Amagasaki City, Hyogo Prefecture (72) Inventor Yoshimi Esaki Midori Ward, Nagoya City, Aichi Prefecture Otakacho Kitakanzan 20-1 Examiner Kenichi Hara

Claims (2)

[Claims] 1. A solid electrolyte type which is mainly composed of magnesium oxide and nickel oxide, wherein magnesium oxide is 5 mol% or more and 40 mol% or less with respect to the total molar amount thereof and the balance is nickel oxide. Oxide solid solution for fuel cell electrode material. 2. A mixture mainly composed of an oxide solid solution and cubic zirconium oxide, wherein the oxide solid solution is mainly composed of magnesium oxide and nickel oxide, and 5 mol of magnesium oxide is contained with respect to the total molar amount thereof. % To 40 mol% and the balance is nickel oxide, while the cubic zirconium oxide has a mixed amount of 60 vol% or less based on the total volume of the mixture. Mixtures for fuel cell electrode materials.