JPH09213345A - Material for air electrode in fused carbonate fuel cell and air electrode - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air electrode material for a fused carbonate fuel cell excellent in fused salt resistance also in conductivity, by making a third metal element in solid solution in LiCoO2 . SOLUTION: A solid solution of oxide, containing lithium Li, cobalt Co and magnesium Mg as a metal element so as to have 1 or more 1.2 or less mole ratio of lithium relating to cobalt and prevent exceeding 0.2 of mole ratio of magnesium relating to cobalt, is used as a material for an air electrode of a fused carbonate fuel cell.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a molten carbonate fuel cell (MCFC) air electrode, and more particularly to a molten salt resistant air electrode material.

[0002]

2. Description of the Related Art A molten carbonate fuel cell normally uses nickel (Ni) as a fuel electrode (negative electrode), nickel oxide (NiO) as an air electrode (anode), and molten carbonate as an electrolyte. . In order for such a molten carbonate fuel cell to be put into practical use, a life of 40,000 hours or more is required, but since it has problems such as loss of electrolyte, elution of air electrode and corrosion of separator, That is difficult to achieve.
Currently, Li-doped NiO, which is relatively stable in a molten salt carbonate, is used as an air electrode material.

[0003]

However, even when NiO is used as the air electrode material, the formula (1) is not satisfied during battery operation.
Due to the elution of Ni due to the reaction of 1 and the precipitation of Ni due to the reduction of the eluted Ni in the electrolyte, an internal short circuit of the cell occurs during long-term operation, and the battery cannot operate. NiO + CO ₂ → Ni ²⁺ + CO ₃ ^2-- Equation (1) Therefore, it is considered necessary to develop a stable cathode material that is difficult to elute in molten salt carbonate.

[0004] Here, LiCoO ₂ is known as a material having a higher resistance to molten salts than NiO. However, it is known that the conductivity of LiCoO ₂ changes depending on the molar ratio of lithium to cobalt (the molar ratio of Li / Co), and in general, the conductivity is lower than that of NiO. For example, when the Li / Co molar ratio is 1.2, the conductivity at the battery operating temperature of 650 ° C. is 3 to 4 S / cm.
However, it is still lower than the conductivity (about 9 S / cm) at the same temperature of Li-doped NiO which is a conventional material.

Therefore, LiCo excellent in molten salt resistance
In order to use O ₂ as a practical air electrode material, it is necessary to improve the electric conductivity.

Therefore, in the present invention, by using an oxide solid solution in which a third metal element is solid-solubilized in addition to Li and Co, the molten carbonate fuel excellent in the resistance to molten salt and the conductivity is excellent. It is an object to provide a material for an air electrode of a battery. Further, in the present invention, by using an oxide solid solution obtained by solidifying a third metal element in addition to Li and Co,
It is an object of the present invention to provide an air electrode of a molten carbonate fuel cell which has excellent molten salt resistance and can be operated for a long time.

[0007]

[Means for Solving the Problems] As means for solving the above-mentioned problems, the first invention relates to lithium (Li), cobalt (Co) and magnesium (M) as metal elements.
It is a material for an air electrode of a molten carbonate fuel cell, which is composed of an oxide solid solution containing g). According to this invention, Li and Co
By forming a solid solution with Mg and Mg, it has resistance to molten salt, and further, the conductivity is improved as compared with the conventional LiCoO ₂ .

In particular, by solid solution of Mg as the third element, a phase change such that the Li / Co molar ratio in the molten carbonate becomes 1 when the Li / Co molar ratio is less than 1, that is, Li conversion is suppressed. Therefore, conventionally, in LiCoO ₂ , when the Li / Co molar ratio is smaller than 1, the problem that the conductivity is lowered during operation due to Li formation is solved by solid solution of Li, Co and Mg. Become. In addition, in LiCoO ₂ , when the molar ratio of Li / Co is smaller than 1, Li / Co
As the molar ratio of Co is decreased, the conductivity is improved and L
As the i / Co molar ratio approaches 1, the conductivity decreases. Further, by solid solution of Li, Co and Mg,
When the Li / Co molar ratio is less than 1, the change in the fine structure (pore distribution) due to the above phase change is suppressed. Therefore, conventionally, in LiCoO ₂ , the problem that LiCoO ₂ particles are likely to be lost and become brittle due to such a phase change and the air electrode is structurally unstable is solved.

Further, when the Li / Co molar ratio exceeds 1, if Mg is solid-solubilized, L in solid solution synthesis is increased.
Remaining i component (Li ₂ CO _3, etc.) is suppressed. As a result, the problem of LiCoO ₂ that the conductivity of the air electrode varies due to the degree of remaining Li component during solid solution synthesis is solved.

In the present invention, the molar ratio of lithium to cobalt in the oxide solid solution is 1 or more.
In a preferred embodiment, it is 2 or less and the molar ratio of magnesium to cobalt does not exceed 0.2. Li /
This is because when the Co molar ratio is less than 1, the increase in conductivity is small even if Mg is solid-solved.
This is because the i component is likely to remain. Also, Mg / C
The reason that the o molar ratio does not exceed 0.2 is that the conductivity decreases when the Mg / Co molar ratio exceeds 0.2. More preferably, the Mg / Co molar ratio does not exceed 0.1.

The second invention is lithium (Li),
In addition to cobalt (Co), silver (Ag), nickel (N
A molten carbonate fuel cell air electrode material comprising an oxide solid solution containing one or more metal elements selected from i), copper (Cu), and strontium (Sr). According to the present invention, in addition to Li and Co, Ag, Ni, Cu,
Alternatively, the oxide solid solution containing one or more metal elements selected from Sr improves the conductivity as compared with the conventional LiCoO ₂ . Further, the structure of the air electrode can be stabilized and the variation in conductivity can be reduced. For example, LiCoO ₂ 65
Li _1.05 CoA for conductivity of 0.1 S / cm at 0 ° C
It is 1.2 S / cm at g _0.1 O ₂ , and Li _1.05 Co
Ni _0.1 O ₂ has 0.7 S / cm, and Li _1.05 C
With oCu _0.1 O ₂ , it is 0.5 S / cm, and Li _1.05
For CoSr _0.1 O ₂ , it is 0.4 S / cm. In addition,
The conductivity is measured by the DC 4-terminal method, and the conditions are shown in Table 1.
The same as above.

The third aspect of the invention is to use lithium (Li), cobalt (Co) and magnesium (M) as metal elements.
g), or a molar ratio of lithium to cobalt in the oxide solid solution of 1 or more.
An air electrode for a molten carbonate fuel cell, which is formed by using an oxide solid solution having a molar ratio of magnesium to cobalt of 2 or less and not exceeding 0.2. According to the present invention, in addition to Li and Co, the air electrode of a carbonated molten salt fuel cell using an oxide solid solution containing Mg as a material has molten salt resistance and sufficient electrical conductivity. It is an air electrode. Therefore, the air electrode can withstand a long operating time. Further, M
Since it contains g, it is an air electrode in which the change of the fine structure in the molten carbonate is suppressed, and at the same time, it is an air electrode with no variation in conductivity. In addition to lithium (Li) and cobalt (Co), silver (Ag), nickel (Ni), copper (Cu), or strontium (S).
The air electrode for a molten carbonate fuel cell, which is formed by using an oxide solid solution containing one or more metal elements selected from among r), also has excellent molten salt resistance and improved conductivity. An air electrode can be provided.

[0013]

BEST MODE FOR CARRYING OUT THE INVENTION The present invention will be described in detail below.
In the present invention, in order to obtain an oxide solid solution of a metal element,
Usually, a solid solution is obtained, such as a method of pyrolyzing an aqueous solution of acetate or nitrate of a metal element to form a mixture of oxides and then calcining, a method of mixing oxide powder or chloride powder of a metal element and calcining. The method used for can be used. Therefore, Mg acetate or the like may be added to LiCoO ₂ powder to synthesize an oxide solid solution containing three metal elements of Li, Co, and Mg. May be synthesized. In addition,
The calcination conditions need to be set so that a solid solution can be formed.

The molten carbonate fuel cell of the present invention may be an external reforming type or an internal reforming type. Further, the air electrode of the molten carbonate fuel cell of the present invention,
It does not question its form.

[0015]

EXAMPLES Hereinafter, the present invention will be described specifically with reference to examples. It should be noted that the present invention is by no means limited to the following examples. (Example 1) (Mg / Co molar ratio and conductivity) LiCoO ₂ (Li / C
o molar ratio = 1.05) powder and a predetermined amount _{Mg (CH 3 COO) 2 ·} 4
H ₂ O (special grade reagent) is mixed using a ball mill,
After calcination for 12 hours at 0 ° C, crushing treatment for 24 hours,
Li _1.05 CoMg _x O ₂ (composition of four different Mg / Co molar ratios (Mg addition amount: x in FIG. 1))
x = 0, 0.05, 0.1, 0.2) was obtained. After powder compaction of each composition, it is baked at 900 ℃, 5mm × 5mm × 25
mm porous sintered body samples 1 to 4 were obtained.

Gold (Au) electrodes were baked on these samples 1 to 4 and the electrical conductivity was measured by the DC 4-terminal method according to the measurement conditions shown in Table 1. FIG. 1 shows the relationship between the amount of added Mg and the conductivity. As is clear from FIG. 1, the conductivity increases until the Mg / Co molar ratio reaches 0.2 (20 mol%), and the conductivity increases to 0.1 (10 mol%).
The maximum (about 9 S / cm) was shown in the vicinity of (mol%), and it decreased when it was higher than that. The maximum value in this example was almost comparable to that of Li-doped NiO, which is a conventional air electrode material. When these samples 1 to 4 were subjected to X-ray diffraction, as shown in FIG. 2, they were completely solid until the Mg / Co molar ratio was 0.1 (the amount of Mg added was 10 mol%) (sample 3). Although dissolved, 0.2 (20 mol% in the same) (Sample 4) did not completely solidify Mg, and a peak corresponding to MgO was observed. Therefore, the molar ratio of Mg to Co in the oxide solid solution of Li, Co, and Mg is 0.
A range not exceeding 2 is preferable, and a range of 0.
It was clear that the range did not exceed 1.

[0017]

[Table 1]

(Example 2) (LiCoMgO_TwoLi / Co molar ratio and conductivity)
And Li / Co molar ratios of 0.5, 0.9, 1.0, 1.1 and 1.
2 various LiCoO _TwoAnd the Mg / Co molar ratio is 0.1
(Mg addition: 10mol%)_ThreeCO
O)_Two・ 4H_TwoO (special grade reagent) is the same as in Example 1.
Operate various LiCoMgO_TwoOf powder was obtained. these
After pressure molding the powder of No. 1, the sintered body sample was processed in the same manner as in Example 1.
5-9 were obtained. In addition, as a comparative example, Li / Co molar ratio
Of LiCoO in the range of 0.4 to 1.1_TwoUsing powder, Mg (CH_Three
COO)_Two・ 4H_TwoExample except that O (special grade reagent) was not added
A sintered body sample of a comparative example was prepared in the same manner as in 1.

Gold electrodes were baked on these samples, and the results are shown in Table 1.
The electrical conductivity was measured by the direct current 4-terminal method according to the measurement conditions of. This result is shown in FIG. As is clear from FIG.
When the Li / Co molar ratio is less than 1, the conductivity increases as the Li / Co molar ratio decreases. On the other hand, L
When the i / Co molar ratio is 1 or more, the conductivity is remarkably increased as the Li / Co molar ratio increases, and
In the vicinity, the rate of increase in conductivity decreased.

On the other hand, in the comparative example, when the Li / Co molar ratio is less than 1, the conductivity increases as the Li / Co molar ratio decreases, similar to the example, but compared with the example. It had a low conductivity. Further, when the Li / Co molar ratio is 1 or more, as the Li / Co molar ratio increases,
Although the conductivity was increased, the conductivity was lower than that in the examples. Therefore, Li / Co in LiCoMgO ₂
It was clear that the molar ratio was preferably 1 or more and 1.2 or less, and more preferably 1.05 or more and 1.2 or less.

(Example 3) (LiCoMgO ₂ elution property) Next, a Li _1.05 CoMg _0.1 O ₂ sintered sample was prepared in the same manner as the sample 3 of Example 1 above, and this sample was melted. An elution test was conducted to confirm the degree of elution in carbonate. Table 2 shows the test conditions. The test was carried out at 1 atm of molten salt carbonate (Li ₂
CO ₃ : K ₂ CO ₃ = 62: 38 mol%) The complete elution method was performed, and the amount of Co elution was measured by ICP emission spectroscopy. As a comparative example, an elution test was also performed on Li-doped NiO to measure the amount of Ni elution.

[0022]

[Table 2]

As a result, as shown in FIG. 4, the amount of Co elution and the amount of Ni elution reached the equilibrium level in about 300 hours in all cases. The elution amount of Li _1.05 CoMg _0.1 O ₂ was about the same as the elution amount of LiCoO ₂ without Mg shown by the dotted line in FIG. 4, which was about 1 / n of the elution amount of Ni of Li-doped NiO as a control example.
It was 3. Therefore, when Li _1.05 CoMg _0.1 O ₂ is used as the material of the air electrode, the elution of the air electrode in the molten carbonate is suppressed and the molten salt resistance is secured, so that the change in the pore structure is suppressed. In addition, inconvenience such as short circuit is prevented even during long-term use, and a long battery operation time is secured.

[0024]

EFFECT OF THE INVENTION When an air electrode is formed from the air electrode material of the molten carbonate fuel cell of the present invention, it becomes an air electrode which is excellent in molten salt resistance which is difficult to elute in the molten carbonate and is also excellent in conductivity. Further, according to the air electrode of the present invention, it is possible to form a molten carbonate fuel cell that can withstand long-term operation.

[Brief description of drawings]

FIG. 1 is a graph showing the relationship between the amount of Mg added to Li _1.05 CoMg _x O ₂ and the electrical conductivity.

FIG. 2 is a diagram showing a Mg addition amount of Li _1.05 CoMg _x O ₂ and an X-ray diffraction result.

FIG. 3 is a graph showing the relationship between the Li / Co ratio and the conductivity in LiCoMg _0.1 O ₂ .

4 is a graph showing the relationship between the immersion time of the Li _1.05 CoMg _0.1 O ₂ into the molten carbonate and Li _1.05 elution amount of CoMg _{0. 1} O _2.

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Tsutomu Kadooka 2-4-1 Rokuno, Atsuta-ku, Nagoya, Aichi Prefecture Fine Ceramics Center

Claims (4)

[Claims] 1. A material for an air electrode of a molten carbonate fuel cell, which comprises an oxide solid solution containing lithium (Li), cobalt (Co) and magnesium (Mg) as metal elements. 2. The material for an air electrode of the molten carbonate fuel cell according to claim 1, wherein a molar ratio of lithium to cobalt in the oxide solid solution is 1 or more and 1.2 or less, and magnesium to cobalt is used. A material for an air electrode of a molten carbonate fuel cell, which has a molar ratio of not more than 0.2. 3. In addition to lithium (Li) and cobalt (Co), at least one metal element selected from silver (Ag), nickel (Ni), copper (Cu), or strontium (Sr) is used. A material for an air electrode of a molten carbonate fuel cell, which comprises an oxide solid solution containing. 4. An air electrode for a molten carbonate fuel cell, which is formed by using the material for an air electrode for a molten carbonate fuel cell according to claim 1 or 2.