JPH06349499A - Manufacture of electrode for molten carbonate fuel cell - Google Patents

Abstract

PURPOSE:To manufacture a cathode electrode having high dissolution resistance and capable of preventing the elution of NiO into a carbonate. CONSTITUTION:The powder of an iron oxide or a cobalt oxide 2 is added to Ni powder 1 in the mixing process I. Water 3, a binder 4, a plasticizer 5, and a dispersant 6 are added and mixed. The slurry 7 is molded into a tape shape in the tape molding process II. It is baked in the weak reducing atmosphere, and the iron oxide or cobalt oxide 2 is solution-diffused on the Ni surface by the baking process to surround the Ni surface with the oxide. The elution of NiO is suppressed by the oxide film on the surface even when a manufactured electrode 8 is oxidized into NiO in a battery.

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 for producing a cathode electrode of a molten carbonate fuel cell used in the energy sector for directly converting chemical energy of fuel into electric energy. The present invention relates to a method for manufacturing a manufacturing electrode.

[0002]

2. Description of the Related Art Among the fuel cells proposed so far, a molten carbonate fuel cell is a cathode (oxygen electrode) which is an electrolyte plate (tile) formed by impregnating a molten carbonate as an electrolyte into a porous material. It is sandwiched by both electrodes of the anode and the fuel (fuel electrode) from both sides, and by supplying the oxidizing gas to the cathode side and the fuel gas to the anode side, the reaction takes place between the cathode side and the anode side and occurs between the cathode and the anode. One that is configured to generate power by the potential difference
The cells are formed into a stack by stacking the cells in multiple layers via a separator.

As a method for molding the electrodes of the molten carbonate fuel cell, molding accuracy, surface smoothness, and mass production are excellent.
Since it is possible to increase the size, in recent years, it has come to be manufactured by a tape molding method by a doctor blade method which has been used as a method for manufacturing an electrolyte plate. A conventional cathode electrode manufacturing method using the tape molding method by the doctor blade method is as shown in FIG.
First, a simple substance of carbonyl Ni powder a as a raw material powder, a dispersant b, and a solvent c are crushed by a ball mill or the like (grinding step d) to disperse the Ni powder a to primary particles, and then an organic binder f And plasticizer g are added and mixed (mixing step e) to form a slurry h, which is molded into a tape shape (sheet shape) by a doctor blade device (tape molding step i),
Finally, in an electric furnace, reducing atmosphere furnace, vacuum furnace, etc.
By firing at a temperature of 900 ° C. (firing step j), a porous (porosity: 70 to 80%) cathode electrode is obtained.

N produced by the above conventional production method
The cathode electrode, which is an i-porous body, is manufactured from a single substance of Ni powder a and is oxidized into NiO when incorporated into a battery and operated. Although there is little creep deformation due to the clamping pressure the cathode electrode is once oxidized is confirmed, NiO ^{_{is, NiO + CO 2 → Ni ++}} + CO 3 - the reaction by eluting the slightly during carbonates go.

Dissolved Ni ⁺⁺ ions are reduced by hydrogen diffused from the anode side, and Ni in the electrolyte plate is reduced.
Precipitates in the form of metal particles.

The Ni ⁺⁺ ions in the carbonate do not saturate in the carbonate, and the N from the cathode to the carbonate continues.
The dissolution of i continued, which caused the collapse of the optimal microstructure (pore distribution, porosity) as the cathode electrode during the battery operation, and the short circuit between the cathode and the anode due to Ni deposited in the electrolyte plate. Is a factor that significantly affects the battery performance and affects the battery life.

Therefore, conventionally, as a measure for preventing the above NiO from being dissolved in carbonate and adversely affecting the battery, changing the temperature and gas composition to change the operating conditions of the battery, alkaline earth Add to reduce the basicity of carbonate, etc.
By changing the composition of carbonate, as alternative materials, LiFeO ₂ , LiCoO ₂ , Li
It is considered to develop Mn ₂ O ₄ .

[0008]

However, regarding the above measures, there are restrictions on the operating conditions, and regarding the above, there is a change in the composition due to the diffusion and precipitation of alkaline earth in the carbonate bath due to the potential difference in the electrolyte plate. In addition, the alternative material of (1) is more stable than NiO, but has a problem that it has low electronic conductivity and cannot be used as an electrode. Further, as an alternative material, Ni × Fe ₃ -xO ₄
Although it has been proposed to use, it is not stable as an electrode although it has excellent stability in carbonate, but it cannot be used as an electrode.

Therefore, the present invention is intended to provide a method for producing a molten carbonate fuel cell electrode for producing a cathode electrode having high dissolution resistance so as to reduce the equilibrium solubility of NiO in a carbonate. is there.

[0010]

In order to solve the above-mentioned problems, the present invention adds a small amount of fine powder of iron oxide or cobalt oxide to Ni powder, and mixes them together with water, a binder and a plasticizer. After mixing an appropriate amount of a dispersant to form a slurry and molding the slurry into a tape, 900-1 in a weak reducing atmosphere
This is a method of producing a porous electrode by firing at 050 ° C. for 10 to 30 minutes.

Further, the powder of the above-mentioned iron oxide or cobalt oxide, which is obtained by adding lithium oxide or lithium carbonate and then firing in the air to cause a reaction to form a composite oxide, is mixed with Ni powder. You may do it.

Alternatively, a powder of a composite oxide obtained by adding an alkaline earth oxide to the above iron oxide or cobalt oxide and mixing them, and then firing and reacting in the atmosphere to form a composite oxide is used.
It may be mixed with powder.

Further, the iron oxide or cobalt oxide, the lithium oxide, and the alkaline earth oxide are mixed, and then the powder is obtained by reacting by firing in the air to form a composite oxide, and mixed with Ni powder. You can also do so.

[0014]

[Function] When fine powder of iron oxide or cobalt oxide is added to Ni powder to form a slurry, and the mixture is formed into a tape and then baked, the iron oxide or cobalt oxide forms a solid solution on the Ni surface to cover the Ni surface. As a result, an oxide film having excellent dissolution resistance is formed, and this oxide film can suppress the elution of NiO.

When fine powder of a composite oxide obtained by firing and pulverizing a mixture of the above-mentioned iron oxide or cobalt oxide with lithium oxide or lithium carbonate is added to Ni powder, the iron oxide or cobalt oxide is converted into Ni. It is possible to promote the solid solution diffusion of Al and improve the electronic conductivity of the oxide formed on the Ni surface.

Further, when fine powder of a composite oxide obtained by firing a mixture of the above-mentioned iron oxide or cobalt oxide mixed with an alkaline earth oxide and then pulverizing it is added to N powder, N
It is possible to suppress the elution of iO and reduce the basicity of the carbonate impregnated in the electrode.

Furthermore, when fine powder of iron oxide or cobalt oxide mixed with lithium oxide and alkaline earth oxide to form a composite oxide is added to Ni powder, the above iron oxide or cobalt oxide is converted into lithium oxide. The effect of adding the oxide and the effect of adding the alkaline earth oxide can be obtained.

[0018]

Embodiments of the present invention will be described below with reference to the drawings.

FIG. 1 shows a process flow of one embodiment of the manufacturing method of the present invention.
Iron oxide (Fe ₂ O ₃ , iron hydroxide containing Fe ₃ O ₄ ) or cobalt oxide (CoO) 2 is added and mixed in the mixing step I, and further water 3, binder 4, plasticizer 5, dispersant 6 was added and mixed to form a slurry 7. This slurry 7 was tape-formed in a tape forming step II, and the formed tape was then baked in a baking step III to form a porous electrode (cathode electrode) 8. To get it.

In the above method, the optimum amount of iron oxide or cobalt oxide 2 added to Ni powder 1 is 1 to 10%.
/ Ni. The amount of addition to Ni powder is set to 1 to 10% because the desired effect cannot be expected if it is 1% or less, and 10%.
This is because Ni or NiO will interfere with the sintering and the strength of the electrode will be insufficient.

1 to 10% of iron oxide or cobalt oxide 2 is added to Ni powder 1 and mixed in mixing step I, and appropriate amounts of water 3, binder 4, plasticizer 5 and dispersant 6 are added. Are mixed to form a slurry 7, and the slurry 7 is tape-formed in a tape forming step II, and then is baked in a weak reducing atmosphere at a temperature of 900 to 1050 ° C. for 10 to 30 minutes in a baking step III. As a result, Fe or Co is reduced and activated, and iron oxide or cobalt oxide 2 is solid-solution diffused on the surface of the Ni powder 1 to form Ni-, which has excellent dissolution resistance, on the surface of the Ni powder 1.
A film of Fe (or Co) oxide is produced, and only the Ni surface can be wrapped with Fe ₂ O ₃ or CoO.

Even if the electrode manufactured in this manner is oxidized into NiO in the battery, the oxide film 2a is formed on the surface as shown in FIG. 2, so that the elution of NiO into carbonate is suppressed. In addition, it is possible to improve the compressive strength of the cathode electrode and prevent the microstructure of the electrode from collapsing.

Although Ni and oxide have low electron conductivity, they do not impair the performance as electrodes because they are thin films.

In the above, the firing temperature in the firing step III is set to 900 to 1050 ° C. because N is below 900 ° C.
This is because the i powders are not sufficiently sintered and the sintering proceeds excessively at 1050 ° C. or higher, resulting in a dense structure and a small porosity, which makes the powder useless as an electrode.

Further, water 3, a binder (for example, polyvinyl butyral as an organic solvent type) 4, a plasticizer (phthalic acid) 5, and a dispersant (for example, a nonionic surfactant) 6 are as follows. Add such an amount.

200 g of water, 10 g of binder, 2 g of plasticizer and 0.5 g of dispersant are added to 100 g of Ni powder.

In the embodiment shown in FIG. 1, Fe,
Although it is possible to use a metal component of Co, the amount of solid solution in Ni increases during the sintering process, the Ni-oxide layer having low electron conductivity becomes thick, and the electrical resistance of the electrode increases.

FIG. 3 shows another embodiment of the present invention.
In the same configuration as the process flow shown in FIG. 1, instead of adding the iron oxide or the cobalt oxide 2 alone to the Ni powder 1 in the mixing step I, the iron oxide or the cobalt oxide 2 and the lithium oxide 9 or the carbonic acid are added. A lithium complex oxide is prepared and its fine powder is added to the Ni powder 1. Other configurations are the same as in the case of FIG.

More specifically, iron oxide (eg, Fe ₂ O
₃ ) or cobalt oxide (CoO) 2 with lithium oxide (LiO) 9 added at an optimum addition amount of 0.5 to 10% and mixed in mixing step IV, and then in the firing step V, Fe or Co is added. Oxide of Li and Li oxide in the atmosphere at 900-1
The reaction is carried out at a temperature of 100 ° C. for 30 to 60 minutes to generate a composite oxide (for example, Li ₂ Fe ₃ O ₅ ), which is crushed in a smashing step VI into fine powder. The Ni powder 1 is added and mixed, and the electrode (cathode electrode) 8 is manufactured through the same steps as those shown in FIG.

In the above method, the temperature of 900 to 1100 ° C. is set as the firing condition in the firing step V is 900 ° C.
In the following, the conversion of Fe ₂ O ₃ into Li is insufficient, and 1100
This is because the grain growth of the oxide progresses too much at temperatures above ℃.

The optimum amount of the lithium oxide 9 added to the above-mentioned iron oxide or cobalt oxide 2 is 0.5 to 10% because iron oxide Fe ₂ O ₃ and cobalt oxide are 0.5% or less. The solid solution diffusion of CoO into Ni becomes insufficient and becomes 10%.
This is because, when the composite oxide is produced by preheating, the sintering progresses and the grain growth of the oxide proceeds too much, which makes the pulverization process in the subsequent pulverization step VI difficult.

The crushing in the crushing step VI is carried out by a crusher, a ball mill, etc., and the particle size after crushing is 1 to 15 μm.
Let m. The smaller the particle size is, the better, but if it is 15 μm or more, it becomes difficult to uniformly mix with Ni powder 1.
Less than μm is desirable.

In the method shown in FIG. 3, the slurry 7 is subjected to the mixing step I without reacting the lithium oxide 9 with the iron oxide or cobalt oxide 2 to form a composite oxide.
However, if lithium oxide 9 is added alone at this time, it will dissolve in water due to the aqueous slurry and the pH will increase, and the dispersion of the Ni powder will deteriorate. Since the fine powder is used after the composite oxide including the lithium oxide 9 is generated, the problem of deteriorating the dispersion of the Ni powder is eliminated.

In the method shown in FIG. 3, since lithium oxide 9 is added to iron oxide or cobalt oxide 2 to form a composite oxide and then added to Ni powder 1, Fe ₂ O ₃ is added by firing in firing step III. , CoO can be promoted to form a solid solution with Ni, and the electronic conductivity of the oxide formed on the surface of the Ni powder can be improved.

We have found that LiO / Fe ₂ O ₃ = 5 /
When a powder of a composite oxide consisting of 95 (weight ratio) was added to 5% of Ni powder 1 and mixed, tape-formed and fired to manufacture an electrode, the microstructure of the manufactured electrode was changed. There wasn't.

In the manufactured electrode 8, the oxidation of Ni is promoted by the presence of Li.

Even if Li ₂ CO ₃ or Li (OH) is used as the lithium oxide 9 instead of LiO, the same effect as described above can be obtained.

Since there are Li ions in the battery,
It is considered that the same effect can be expected by the Li ions in the battery without adding LiO, but if LiO is not added, the solid solution diffusion of the oxide into Ni will be delayed, and an oxide film will be formed on the Ni surface. As a result of the delay, the compression stress easily occurs due to the tightening stress of the battery in the battery, so that the tightening stress needs to be adjusted.

Next, FIG. 4 shows another embodiment of the present invention. Instead of the lithium oxide 9 in the embodiment of FIG. 3, an alkaline earth oxide 10 is replaced with iron oxide or cobalt oxide 2. Ni powder 1 after being added to to generate a complex oxide
It is designed to be mixed with.

As the alkaline earth oxide 10, Mg
O, CaO, SrO or the like is used, and the addition amount to iron oxide or cobalt oxide 2 is added as an optimum value of 10 to 80%, and after mixing in mixing step IV, firing step V
Then, by reacting the iron oxide or cobalt oxide 2 and the lithium oxide 9 shown in FIG. 3 under the same reaction condition, a composite of the iron oxide or cobalt oxide and the alkaline earth oxide is formed. An oxide (for example, MgFe ₂ O ₃ ) is produced, and this composite oxide is pulverized in a pulverization step VI using a ball mill or the like and added to the Ni powder 1 to form a slurry 7.
The tape 8 is formed into a tape in the tape forming step II and is baked in the baking step III to manufacture the electrode 8.

The amount of the alkaline earth oxide 10 added to the iron oxide or cobalt oxide 2 is set to 10 to 80% because the effect of the alkaline earth cannot be expected at 10% or less and 80%. This is because the effect is the same even if added.

According to this method, MgO is present around the oxide film 2a on the surface of Ni produced as shown in FIG. 2 by the addition of the alkaline earth oxide 10, and the carbonate adheres to the NiO. It becomes difficult to prevent the elution of NiO.

As shown in FIG. 4, oxide 1 of alkaline earth is used.
When 0 is added to iron oxide or cobalt oxide 2 to form a composite oxide, and alkaline earth oxide 10 is added alone during slurry production, the same as in the case of lithium oxide 9 described above. There is an effect on the pH during slurry production.

FIG. 5 shows still another embodiment of the present invention. To the iron oxide or cobalt oxide 2, the lithium oxide 9 shown in FIG. 3 and the alkaline earth oxide 10 shown in FIG. 4 are added. Then, the electrode 8 is manufactured by using a composite oxide obtained by mixing, mixing and firing.

The mixing ratio (weight ratio) of the iron oxide or cobalt oxide 2, the lithium oxide 9 and the alkaline earth oxide 10 is, for example, LiO / MgO / Fe ₂ O ₃ = 2 /
After mixing in the mixing step IV as 48/50 (weight ratio),
It was obtained by sintering the oxide of LiO / MgO / Fe ₂ O ₃ = 2/48/50 (weight ratio) in a firing step V to produce a composite oxide, and then pulverizing it in a pulverizing step VI. An electrode (cathode electrode) is obtained by adding fine powder to Ni powder 1 in a mixing step I to mix it into a slurry 7 and then forming a tape in a tape forming step II and firing it in a firing step III.
8 can be manufactured.

The inventors of the present invention used, as a cathode, an electrode 8 manufactured by adding 5% of fine powder of the composite oxide produced in the above weight ratio to Ni, while using 8% Cr to Ni for the anode. As a result of examining the power generation result of the battery used, FIG.
The results shown by the circles were obtained. In FIG. 6, the mark ● indicates the power generation result when the conventional Ni-only electrode was used as the cathode.

As is apparent from FIG. 6, when the electrode 8 manufactured by the method of the present invention is used as the cathode, the initial performance of the battery is equivalent to that of the conventional cathode, and 20
Almost no deterioration in performance was observed even after 00 hours. The amount of Ni in the carbonate after 2000 hours was about half that in the case of using the conventional cathode. This gives N
It was found that it was possible to manufacture a cathode electrode having high dissolution resistance in which elution of iO into carbonate was suppressed.

Further, the above LiO / MgO / Fe ₂ O ₃ = 2
/ 48/50 (weight ratio) mixed oxide powder
The microstructure of the electrode manufactured by mixing the powder with the addition amount of 5% was equivalent to the electrode manufactured by the method of FIG.

LiO / MgO / Fe ₂ O ₃ = 2/4
The electrode 8 manufactured by using the composite oxide in a weight ratio of 8/50 was used as a cathode in a battery, and as a result of X-ray diffraction of the cathode electrode after power generation, the electrodes were NiO, Li ₂ Fe.
₃ O ₅ , NiFe ₂ O ₄ , Li ₂ NiFe ₂ O ₄ , Li
It consisted of Fe ₅ O ₈ .

In the embodiment shown in FIG. 5, Li ₂ CO ₃ is used as the lithium oxide 9, and Li ₂ CO ₃ / MgO is used.
/ Fe ₂ O ₃ = 1/12/12 (weight ratio) was mixed for 60 minutes in the air at a temperature of 1000 ° C., and the powder was calcined for 60 minutes. As a result of X-ray diffraction, Li ₂ Fe ₃ O ₅ and LiFe were obtained. ₅ O
₈ , MgFe ₂ O ₄ , and Fe ₃ O ₄ were detected.

The inventors of the present invention also applied LiO / M on a Ni plate.
A test piece coated with complex oxide powder of gO / Fe ₂ O ₃ = 2/48/50 (weight ratio) and LiO / Fe ₂ O ₃ = 5/95 (weight ratio) according to the example shown in FIG. A test piece obtained by coating the Ni plate with the complex oxide powder of 1.
After heating under the same conditions as in III to cause solid solution diffusion of the composite oxide on the Ni surface, a corrosion test in carbonate was carried out.

6 in an atmosphere of air / CO ₂ = 80/20
In cross-section observation after heating at 50 ° C. for 20 hours, the thinning due to corrosion in both of these test pieces was as small as 2 to 4 μm. On the other hand, the Ni plate used as the comparative test piece was found to be thinned by about 10 μm.

[0053]

As described above, according to the method for producing a molten carbonate fuel cell electrode of the present invention, a small amount of iron oxide or cobalt oxide powder is added to Ni powder and mixed to form a slurry. After forming into a tape shape and firing in a weak reducing atmosphere to manufacture the electrode, iron oxide or cobalt oxide is solid-solution diffused on the surface of the Ni powder during firing to form an oxide film to cover the Ni surface. Therefore, even if the electrode is oxidized in the battery, elution of NiO into carbonate can be suppressed, and the equilibrium solubility of NiO in carbonate can be reduced. It is possible to improve the compressive strength of the electrode and not change the microstructure of the electrode, and it is possible to obtain the excellent effect. Moreover, a small amount of lithium oxide is mixed with the iron oxide or cobalt oxide. Calcination and compound oxide By adding a small amount of this composite oxide powder to Ni powder, tape-forming the same as above, and firing it to produce an electrode, by adding lithium oxide, iron oxide or cobalt on the Ni surface can be obtained. The effect that the solid solution of the oxide can be promoted and the electron conductivity of the oxide film on the Ni surface can be improved is obtained, and the iron oxide or cobalt oxide may be mixed with an alkaline earth oxide. The powder of the mixed oxide obtained by mixing and firing is added to Ni powder, mixed, and tape-formed in the same manner as above, and then fired to produce an electrode, whereby iron oxide or cobalt oxide on the Ni surface is oxidized. Since the alkaline earth is present around the film of the substance, the carbonate is unlikely to adhere to NiO, and the effect of preventing the elution of NiO can be obtained. Further, iron oxide or cobalt oxide, lithium oxide and alkaline earth oxide are mixed at a required mixing ratio and fired to form a composite oxide, and the powder of this composite oxide is mixed with Ni powder. An electrode with high dissolution resistance that is excellent in promoting solid solution of iron oxide or cobalt oxide on the Ni surface and preventing elution of NiO by manufacturing an electrode by firing after tape molding, and is resistant to compression deformation There is an effect that can be obtained.

[Brief description of drawings]

FIG. 1 is a process flow showing an example of a method for producing an electrode for a molten carbonate fuel cell of the present invention.

FIG. 2 is a schematic view showing a part of a crystal structure of an electrode manufactured by the manufacturing method of FIG.

FIG. 3 is a process flow showing another embodiment of the manufacturing method of the present invention.

FIG. 4 is a process flow showing another embodiment of the manufacturing method of the present invention.

FIG. 5 is a process flow showing still another embodiment of the manufacturing method of the present invention.

FIG. 6 is a diagram of power generation results comparing the electrode manufactured by the manufacturing method of FIG. 5 and a conventional electrode.

FIG. 7 is a process flow showing an example of a conventional cathode electrode manufacturing method.

[Explanation of symbols]

 1 Ni powder 2 Iron oxide or cobalt oxide 3 Water 4 Binder 5 Plasticizer 6 Dispersant 7 Slurry 8 Electrode 9 Lithium oxide 10 Alkaline earth oxide I Mixing step II Tape forming step III Firing step IV Mixing step V Baking process VI Crushing process

 ─────────────────────────────────────────────────── ─── Continuation of the front page (72) Inventor Shinichiro Sakai 3-15-1, Toyosu, Koto-ku, Tokyo Ishikawajima Harima Heavy Industries Ltd. Technical Research Institute

Claims (4)

[Claims] 1. A small amount of iron oxide or cobalt oxide powder is added to and mixed with Ni powder, and further water, a binder, a plasticizer,
A suitable amount of a dispersant is added to form a slurry, and the slurry is formed into a tape, and then 900 to 105 in a weak reducing atmosphere.
A method for producing an electrode for a molten carbonate fuel cell, comprising producing a porous cathode electrode by firing at 0 ° C. for 10 to 30 minutes. 2. Iron oxide or cobalt oxide, to which a small amount of lithium oxide or lithium carbonate is added and mixed, followed by firing in the air to react the two into a composite oxide, and a powder of this composite oxide. 2. The method for producing an electrode for a molten carbonate fuel cell according to claim 1, characterized in that is added to Ni powder and mixed. 3. An iron oxide or cobalt oxide to which an alkaline earth oxide is added and mixed, followed by firing in the air to react the two into a composite oxide. The method for producing an electrode for a molten carbonate fuel cell according to claim 1, wherein the Ni powder is added and mixed. 4. An iron oxide or cobalt oxide, a lithium oxide and an alkaline earth oxide are mixed and fired in the atmosphere,
The method for producing an electrode for a molten carbonate fuel cell according to claim 1, wherein these are reacted to form a composite oxide, and the powder of the composite oxide is added to Ni powder and mixed.