JPH05335021A - Air electrode for fuel cell with molten carbonate and manufacture of air electrode - Google Patents

Abstract

PURPOSE:To provide an air electrode and its method of manufacturing, with which elusion to a molten carbonate as electrolyte is lesser and the lifetime of resultant battery is prolonged. CONSTITUTION:A green sheet of two-layer structure is formed by laying a metal sheet of cobalt, iron, or alloy thereof on a nickel metal sheet. The obtained green sheet is baked in an inert atmosphere to form a porous plate, which is subjected to oxidation process to oxidize the surface of the porous plate.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an air electrode for a molten carbonate fuel cell which operates at about 500 to 750 ° C. and a method for producing the same, and particularly to molten carbonate having excellent molten salt resistance and high electric conductivity. TECHNICAL FIELD The present invention relates to an air electrode for a salt fuel cell and a manufacturing method thereof.

[0002]

2. Description of the Related Art Conventionally, in a molten carbonate fuel cell, a molten material of an alkali carbonate such as lithium carbonate, potassium carbonate and sodium carbonate is used as an electrolyte, and this electrolyte is used as a molten salt resistant material such as lithium aluminate. It is processed into a plate shape together with a holding material, which is arranged between the fuel electrode and the air electrode to form a cell.

The electrolyte used in this case generally has a composition of 62 mol% lithium carbonate and 38 mol% potassium carbonate.

The air electrode is required to have good stability in an oxidizing atmosphere and good molten salt resistance and electric conductivity. A nickel oxide porous plate obtained by oxidizing a porous plate made of nickel as a material inside the battery has been used as an air electrode because it has the above characteristics.

The fuel electrode is required to have stability in a reducing atmosphere, good molten salt resistance and good electrical conductivity, and also to exhibit a catalytic action. A porous plate made of an alloy containing nickel as a main component is used as a fuel electrode because it has the above characteristics.

That is, the structure of a general battery is as follows. (Air electrode: + side) Nickel oxide | Molten carbonate | Nickel alloy (Fuel electrode: -side)

However, lithium aluminate is used as the holding material for the molten carbonate.

[0008]

The molten carbonate fuel cell thus constructed has many problems in terms of long-term stability of cell characteristics, that is, long life.

For example, there are problems such as evaporation of the electrolyte, elution of the electrode into the electrolyte, and deterioration of the adhesion between the electrode and the electrolyte plate.

Particularly, in order to prolong the service life, an electric short circuit between the air electrode and the fuel electrode due to elution of the air electrode is a serious problem.

It is an object of the present invention to provide an air electrode and a method for producing the same that can improve the life of the battery by improving the elution of the air electrode into the molten carbonate as an electrolyte.

[0012]

The air electrode for a molten carbonate fuel cell according to the present invention and the method for producing the same include a cobalt metal, a nickel-cobalt alloy, iron, a nickel-iron alloy, an iron-cobalt alloy on a nickel metal sheet. A nickel-cobalt-iron alloy and a green sheet having a two-layer structure in which metal sheets made of these and additives are laminated are formed, and the green sheet is fired in an inert atmosphere to form a porous plate,
It is characterized by using this porous plate.

Further, in the manufacturing method according to the present invention, it is preferable that the surface of the porous plate is oxidized by pre-oxidizing the porous plate.

Further, in the air electrode for molten carbonate fuel cell according to the present invention, it is preferable that the surface of the calcined porous plate is lithiated.

[0015]

According to the present invention, Co,
A metal sheet of one member selected from the group consisting of Co-Ni alloy, Fe, Fe-Ni alloy, Fe-Co alloy, Fe-Co-Ni alloy and these and additives is laminated to form a two-layer porous structure. By forming a composite plate, and then firing the composite-formed porous plate in an inert atmosphere, the C
Based on the finding that a long-life battery can be obtained by using a porous electrode material having a two-layer structure composed of a layer of o, Fe or an alloy thereof and a Ni metal layer, and using this porous electrode material. There is.

That is, according to the present invention, Co, Co-Ni alloy, Fe, Fe-Ni alloy, Fe-Co alloy, Fe-C are used.
A slurry composed of an o-Ni alloy or a metal composed of one member selected from these and other additives and an organic component (a binder, a dispersant, etc.) is formed, and the slurry is stacked on a Ni sheet. Molded into a sheet,
Molten carbonate fuel comprising a sheet having a two-layer structure of Ni with Ni, Fe, or an alloy thereof, and firing the sheet having the two-layer structure in an inert atmosphere to form a porous plate An air electrode for a battery and a method for manufacturing the same.

In any of the embodiments of the present invention, Co, Co--Ni alloy, Fe, Fe--Ni alloy, F
The Ni sheet used here is a green sheet containing an organic component (a binder, a solvent, a dispersant, etc.) as long as a metal sheet such as an e-Co alloy or an Fe-Co-Ni alloy can be formed. However, there is no particular problem even if it is a porous plate after firing.

In another aspect of the present invention, the surface of each of the porous plates obtained in the first and second aspects is previously oxidized before being incorporated into a battery as an air electrode. Thus, a method for producing a molten carbonate fuel cell air electrode using a material obtained by oxidizing the surface of the porous plate.

Specifically, the surface of the porous air electrode is oxidized by heating the air electrode material of the present invention in air at 400 to 650 ° C. for 20 to 50 hours to perform oxidation treatment. In such a case, when used as an air electrode, LiCoO ₂ or LiF with respect to the surface of the air electrode due to the reaction with Li forming a part of the electrolyte composition.
It is possible to effectively generate eO ₂ , which improves the corrosion resistance of the air electrode.

In a composite material used as an air electrode in the present invention, a Co layer, a Co-Ni alloy layer, a Fe layer, a Fe-Ni alloy layer, a Fe-Co alloy layer or a Fe-layer formed on a Ni sheet. The Co-Ni alloy layer is preferably thin in consideration of its electrical characteristics, but it is necessary to have a certain thickness in view of resistance to molten salt. The thickness of the layer is preferably about ½ of the thickness of the porous electrode material, although a difference may occur depending on how the electrolyte permeates into the air electrode, that is, the electrolytic mass during battery operation.

In the present invention, looking at the influence of the other component additive which is also a part of the material constituting the air electrode, in view of the molten salt resistance property, in any case, it constitutes a part of the electrolyte composition. Due to the reaction with existing Li, LiC is formed on the surface layer of the air electrode.
oO _2, LiFeO ₂ or better unless preclude the mixture thereof is produced, as an additive to be used in this case, 5 wt% or less, for example Sn, M
n, Cu, etc. are mentioned.

Particularly in the case of Mn, it can be said that it is a suitable additive because it forms a compound having a molten salt resistance.

Needless to say, when incorporating this air electrode into a battery, a layer of Co, Fe or an alloy thereof must be arranged on the electrolyte side.

The air electrode of the present invention is in a cathode atmosphere and the layers of Co, Fe and alloys thereof are oxidized. In addition, LiCoO ₂ , L
iFeO ₂ and a mixture thereof are generated.
The LiCoO ₂ , LiFeO ₂ and a mixture thereof have electrical conductivity and are excellent in molten salt resistance. As described above, when the oxidation treatment is performed in advance, this compound is surely produced.

However, when this compound is used as an air electrode, it has lower conductivity than nickel oxide and is inferior in electrode characteristics.

Therefore, in the present invention, a composite material having a two-layer structure of the layers of Co, Fe and alloys thereof is used. As described above, the composite material having the two-layer structure is oxidized inside the battery and further converted into Li, so that LiCoO ₂ / Ni
_{O, LiFeO 2 / NiO, LiCoO} 2 -NiO / N
2 such as iO or LiFeO ₂ —NiO / NiO
It becomes a layered air electrode. In the case of using an electrode in which an oxide having cobalt as a main component is formed on the electrode surface by further oxidizing the above-mentioned two-layer structure electrode,
When the battery is used, LiCoO ₂ , L is added to the surface layer of the electrode.
iFeO ₂ or a mixture thereof can be produced more effectively.

As a result, it becomes possible to form a stable air electrode which is excellent in molten salt resistance and has little change over time.

[0028]

EXAMPLES Examples of the present invention will be described in detail below.

Example 1 Ni metal powder (INCO-2
55) and Co metal powder, a Ni sheet and a Ni-Co two-layered sheet were prepared as follows.

200 g of Ni raw material powder, 50 g of gum arabic, 10% by weight of polyvinyl butyral (PVB) as a binder, 50 g of a solution containing alcohol and toluene in a ratio of 1: 1 and a dispersant. As a result, 3.5 g of Nissan nonion (OP-85R) and 7.0 g of dibutyl phthalate (DBP) as a plasticizer were weighed. The weighed raw materials are mixed into a slurry using a ball mill, and the resulting slurry is used to
The thickness of the Ni sheet is about 1.
It was made 3 mm. Further, on this Ni sheet, a Co sheet was formed using a Co slurry prepared by replacing the Ni metal powder with a Co raw material powder, and the thickness of the entire sheet having a two-layer structure including a Ni layer and a Co layer was formed. Was about 2 mm.

At this time, a film is used as a base for preparing a sheet having a two-layer structure. In this film, alcohol containing 2% by weight of binder and toluene are present at a ratio of 1: 1. Solution of nickel mesh (0.1 NiO.12-M20 manufactured by Katsura Grating Co., Ltd.
F) was previously attached. Then, the slurry was applied to the nickel mesh side, and the film was peeled off before firing.

After degreasing the green sheet having a two-layer structure manufactured as described above, H ₂ and A
The gas flow rate of the mixed gas when the ratio of r is 1: 5 is about 600
It is fired in an atmosphere of ml / min and Co / Ni 2
It was a layered electrode.

The porosity of the sheet obtained in this case was about 68%.

The Ni / Co two-layer structure sheet obtained as described above was oxidized in air at about 420 ° C. for about 20 hours. Then, at a temperature of 650 ° C, P
_co2 is at 0.3 atm, P _o2 is under an air electrode atmosphere 0.16atm, Li is 62 mol%, K were immersed for 300 hours in a bath of 38 mol% of the molten carbonate. According to the result of the X-ray diffraction inspection on the surface of the sample taken after the immersion treatment in the bath of the molten carbonate, it was revealed that the stable layer of LiCoO ₂ was formed on the surface of the sample. became.

A cell having a size of 10 cm ² was cut out from the porous plate alloy sheet having a two-layer structure composed of Ni and Co, which was obtained as described above, to obtain an air electrode, and the Ni electrode was used as a fuel electrode. When the test was conducted, the results as shown in FIG. 1 were revealed, and it was revealed that the current-voltage characteristics and the positive and negative polarization characteristics after 100 hours from the start of operation were excellent.

In this case, the test conditions when the battery is operated are as follows: air is 70, carbon dioxide is a mixed gas of 30 at a flow rate of 100 ml / min as the cathode gas, hydrogen is 80, and carbon dioxide is carbon dioxide. A mixed gas with a gas ratio of 20 was supplied as a fuel electrode gas at a flow rate of 100 ml / min, and the operating temperature was 650 ° C.

As the electrolyte, Li is 62 mol%
Then, a carbonate having K of 38 mol% was used, and a lithium aluminate (LiAlO ₂ ) plate was used as a holding material for this electrolyte.

The electromotive force (V) when 2 hours have passed after the battery was operated as described above was measured at various current densities (mA / c).
The result of measurement for m ² ) is shown in FIG.

Further, as shown in FIG. 1, when the above fuel cell was operated under a load condition of 150 mA / cm ² , the electromotive force of the cell measured 2 hours after starting was 0.8.
It showed 41V, but when the operation was continued, the electromotive force after 10 hours was 0.89V.
The electromotive force after the lapse of 00 hours was 0.847 V, indicating a stable state.

As described above, Ni and Co of 2 are added to the air electrode.
In a fuel cell using a layered porous plate, the deterioration of cell performance that is affected by changes in the state of the surface layer of the material that makes up the air electrode is small, and the characteristics of the fuel cell are stable over a long period of time. Was confirmed.

Example 2 Instead of cobalt metal powder, N
When the fuel cell was treated in the same manner as in Example 1 except that the i-Co alloy powder was used and the fuel cell was operated under a load condition of 150 mA / cm ² , the electromotive force of the cell measured 2 hours after starting was measured. Shows 0.84V, but after 10 hours, the electromotive force is 0.90V, and after 1000 hours, the electromotive force is 0.84V, showing a stable state. Was there.

Further, in a fuel cell using a porous plate having a two-layer structure of Ni—Co alloy and Ni for the air electrode, the cell is affected by the change in the state of the surface layer of the material forming the air electrode. It was confirmed that the performance of the fuel cell was stable with little deterioration in performance.

[Example 3] Ni metal powder (INCO-2
55) and Fe metal powder, a sheet having a two-layer structure of Ni and Fe was produced as follows.

200 g of Ni raw material powder, 50 g of gum arabic, 10% by weight of polyvinyl butyral (PVB) as a binder, 51 g of a solution containing alcohol and toluene at a ratio of 1: 1 and a dispersant. As a result, 3.4 g of Nissan nonion (OP-85R) and 7.0 g of dibutyl phthalate (DBP) as a plasticizer were weighed. The weighed raw materials are mixed into a slurry using a ball mill, and the resulting slurry is used to
The thickness of the Ni sheet is about 1.
It was made 3 mm. Further, an Fe sheet was formed on the Ni sheet by using an Fe slurry prepared by replacing the Ni metal powder with the Fe raw material powder, and the total thickness of the sheet having a two-layer structure including a Ni layer and an Fe layer was formed. Was about 2 mm.

At this time, the film used as an underlayer for producing the two-layer structure sheet was a nickel mesh in a solution in which alcohol containing 2% by weight of binder and toluene were present at a ratio of 1: 1. (0.1 NiO.12-M20F manufactured by Katsura Grating) was attached in advance. Then, the slurry was applied to the nickel mesh side, and the film was peeled off before firing.

The green sheet having a two-layer structure produced as described above has an H ₂ and Ar ratio of 1: ₂ at an absolute temperature of about 1225 ° C.
In a mixed gas atmosphere of a ratio of 5, the gas flow rate is about 600
An electrode having a two-layer structure of Fe / Ni was prepared at a flow rate of ml / min. The porosity of the sheet obtained in this case is about 70.
%Met.

Ni and Fe 2 obtained as described above
The layered sheet was oxidized in air at about 420 ° C. for about 20 hours. Then, at a temperature of 650 ° C, P
_co2 is at 0.3 atm, P _o2 is under an air electrode atmosphere 0.16atm, Li is 62 mol%, K were immersed for 300 hours in a bath of 38 mol% of the molten carbonate. According to the result of the X-ray diffraction inspection on the surface of the sample taken after the immersion treatment in the bath of the molten carbonate, it was revealed that the stable layer of LiFeO ₂ was formed on the surface of the sample. became.

A cell having a size of 10 cm ² was cut out from the porous plate alloy sheet having a two-layer structure composed of Fe and Ni thus obtained, and used as an air electrode, and the Ni electrode was used as a fuel electrode. The test was conducted.

The test conditions when operating the battery are:
70 ml / min of air and 30 ml / m of carbon dioxide
The mixed gas obtained by introducing it into the gas mixing tube at a ratio of in was used as the air electrode gas. A mixed gas obtained by introducing hydrogen into the gas mixing tube at a rate of 80 ml / min and carbon dioxide at a rate of 20 ml / min was used as the fuel electrode gas, and the operating temperature was 650 ° C.

Further, as an electrolyte, Li is 62 mol%
And a carbonate having K of 38 mol% is used, and a lithium aluminate (LiAlO 2) is used as a holding material for this electrolyte.
₂ ) A plate was used.

Furthermore, the above fuel cell was used at 150 mA / c.
FIG. 2 shows the changes with time in the battery voltage when operated under a load of m ² . Specifically, the electromotive force of the battery measured 0.841V after 2 hours from the start was 0.841V, but the electromotive force after 10 hours was 0.898V, and 1000 hours later. Electromotive force after passing is 0.835V
And showed a stable state.

In the fuel cell using a porous plate having a two-layer structure of Fe and Ni as the air electrode, the deterioration of the cell performance which is influenced by the change of the state of the surface layer of the material forming the air electrode is small. It was confirmed that the characteristics of the fuel cell were stable over the long term.

Example 4 Instead of iron metal powder, Fe--N
When the fuel cell was treated in the same manner as in Example 1 except that the i alloy powder was used and the fuel cell was operated under a load of 150 mA / cm ² , the electromotive force of the cell measured 2 hours after the start was 0. Although it showed 0.84V, the electromotive force after 10 hours passed was 0.90V and the electromotive force after 1000 hours passed was 0.84V, showing a stable state. ..

Further, in a fuel cell in which a porous plate having a two-layer structure of Fe-Ni alloy and Ni is used for the air electrode, the battery is affected by the change in the state of the surface layer portion of the material forming the air electrode. It was confirmed that the performance of the fuel cell was stable with little deterioration in performance.

[Comparative Example 1] For comparison, Ni was used as the fuel electrode.
A cell test was conducted in the same manner as in Example 1 except that a fuel cell in which a porous Ni plate was oxidized inside the cell as an air electrode and nickel oxide was used as the surface of the electrode was used.

In this case, under the load condition of the current density of 150 mA / cm ² , the electromotive force of the battery measured after 2 hours from the start of the battery was 0.871 V, but after 10 hours, The electromotive force is 0.901V, and the electromotive force after 1000 hours is 0.827V.
And the deterioration of the power generation characteristics was observed when the electromotive force was used as the reference.

In this case, the deterioration of the power generation characteristics recognized is
It is considered that the porosity characteristics of the fuel cell changed due to the nickel elution phenomenon generated at the air electrode. Furthermore, since elution contamination of NiO was also confirmed on the surface of the collected electrolyte plate, it was confirmed that nickel was eluted on the surface of the electrolyte plate.

[Comparative Example 2] A battery test was conducted in the same manner as in Example 1 except that LiCoO ₂ produced as described below was used as the air electrode.

That is, 200 g of LiCoO ₂ powder, 100 g of gum arabic, 2.8 g of surfactant and 10% by weight of polyvinyl butyral were added, and ethanol and toluene were 1: 1. Using a ball mill, 50 g of a mixed solution of ethanol and toluene having a volume ratio and 50 ml of a mixed solution of ethanol and toluene having a volume ratio of 1: 1 of ethanol and toluene were mixed. The slurry obtained by this mixing treatment was formed into a sheet by the doctor blade method.

The sheet thus obtained is treated at 420 ° C.
After degreasing for 2 hours at 1140 ° C., a porous electrode plate base material was prepared by further firing at 1140 ° C. for 1 hour. The electrode plate mother of the LiCoO ₂ porous plate thus obtained was prepared. The material was cut into 10 cm ² and used as an air electrode.

In the battery test (anode gas 80/20, cathode gas 70/30; ml), current-voltage characteristics, anodic polarization, and cathodic polarization after 100 hours have elapsed from the start of the battery are shown in FIG. ..

However, the current-voltage characteristic of the fuel cell measured in this case is only low, and the obtained current density is 120 mA / cm ² at most as shown in FIG.
Electric power could not be taken out in the state where the current density was 150 mA / cm ² .

Comparative Example 3 A battery test was conducted in the same manner as in Example 1 except that LiFeO ₂ produced as described below was used as the air electrode.

That is, 200 g of LiFeO ₂ powder, 100 g of gum arabic, 2.7 g of surfactant, and 10% by weight of polyvinyl butyral were added, and ethanol and toluene were 1: 1. Using a ball mill, 50 g of a mixed solution of ethanol and toluene having a volume ratio and 50 ml of a mixed solution of ethanol and toluene having a volume ratio of 1: 1 of ethanol and toluene were mixed. The slurry obtained by this mixing treatment was formed into a sheet by the doctor blade method.

The sheet thus obtained was degreased at 420 ° C. for 2 hours, and then calcined at 1100 ° C. for 1 hour to prepare a porous electrode plate base material. 10 cm ² was cut from the electrode plate base material of the obtained porous LiFeO ₂ plate and used as an air electrode.

In the battery test (anode gas 80/20, cathode gas 70/30; ml), current-voltage characteristics, anodic polarization, and cathodic polarization after 100 hours have elapsed from the start of the battery are shown in FIG. ..

[0067] However, current-voltage characteristics of the measured fuel cell in this case is only a low, resulting current density is at most 120 mA / cm ² as shown in FIG. 4, the current density is 150 mA / cm ² It was not possible to take out the power under the condition.

In the case of this example, the polarization of the cathode, which is the air electrode, is large, and the conductivity exhibited by LiFeO ₂ does not sufficiently satisfy the conductivity required for the electrode, so that the battery characteristics are deteriorated. I am letting you.

As described above, Co, Fe, and
Ni-Co alloy or Fe-Ni alloy layer and Ni
Since the air electrode using a material having a two-layer structure with the above layer has excellent corrosion resistance to electrolyte substances, it greatly contributes to stable supply of fuel cell output over a long period of time. At the same time, it can greatly contribute to the improvement of economic efficiency.

[0070]

EFFECT OF THE INVENTION The air electrode according to the present invention is excellent in molten salt resistance and has higher electric conductivity than the conventional alternative materials, and therefore, it is very effective in prolonging the life of the battery.
Since the air electrode according to the present invention can improve the economical efficiency of the fuel cell, it greatly contributes to the industry using such a cell.

[Brief description of drawings]

FIG. 1 is a graph showing characteristics of a battery using an air electrode according to an example of the present invention.

FIG. 2 is a graph showing a change in voltage of a battery using an air electrode according to an embodiment of the present invention over 1000 hours after the start of operation.

FIG. 3 is a graph showing current-voltage characteristics of a battery using a LiCoO ₂ air electrode as a comparative example.

FIG. 4 is a graph showing current-voltage characteristics of a battery using another LiFeO ₂ air electrode as a comparative example.

Claims (5)

[Claims] 1. A member selected from the group consisting of cobalt, a nickel-cobalt alloy, iron, a nickel-iron alloy, an iron-cobalt alloy, a nickel-cobalt-iron alloy, and these and additives on a nickel metal sheet. The air electrode for a molten carbonate fuel cell, comprising the steps of forming a green sheet having a two-layer structure by laminating the above metal sheets and firing the green sheet into an inactive atmosphere to form a porous plate. Manufacturing method. 2. A member selected from the group consisting of cobalt, nickel-cobalt alloys, iron, nickel-iron alloys, iron-cobalt alloys, nickel-cobalt-iron alloys, and additives on a nickel metal sheet. A green sheet having a two-layer structure is formed by laminating the metal sheets of 1., the green sheet is fired in an inert atmosphere to form a porous plate, and the porous plate is subjected to an oxidation treatment to remove the surface of the porous plate. A method for producing an air electrode for a molten carbonate fuel cell, which is characterized by oxidizing. 3. A nickel metal sheet, cobalt, nickel-cobalt alloy, iron, nickel-iron alloy, iron-cobalt alloy, nickel-cobalt-iron alloy, and these and additives laminated on the nickel metal sheet. An air electrode for a molten carbonate fuel cell, comprising a fired porous plate having a two-layer structure with a metal sheet which is a member selected from the group consisting of: 4. A nickel metal sheet and cobalt, nickel-cobalt alloy, iron, nickel-iron alloy, iron-cobalt alloy, nickel-cobalt-iron alloy, and these and additives laminated on the nickel metal sheet. An air electrode for a molten carbonate fuel cell, which is composed of a fired porous plate having a two-layer structure with a metal sheet which is one member selected from the group consisting of: and the surface of the fired porous plate is oxidized. .. 5. A nickel metal sheet, and cobalt, nickel-cobalt alloy, iron, nickel-iron alloy, iron-cobalt alloy, nickel-cobalt-iron alloy, and these and additives laminated on the nickel metal sheet. It is composed of a fired porous plate having a two-layer structure with a metal sheet of one member selected from the group consisting of: the surface of the fired porous plate is oxidized, and further, LiCoO ₂ , LiFeO _{2 is} formed on the oxidized surface.
An air electrode for a molten carbonate fuel cell, characterized in that ₂ or a mixture thereof is present.