JP3565696B2 - Method for manufacturing electrode of solid oxide fuel cell - Google Patents

Description

[0001]
[Industrial applications]
The present invention is a manufacturing method of an electrode of a solid oxide fuel cell, especially in further detail, the solid electrolyte type fuel cell (Solid Oxide Fuel Cell, hereinafter abbreviated as SOFC) relates to a method for manufacturing a fuel electrode.
[0002]
[Prior art]
SOFC is a general term for a fuel cell that generates electricity by supplying two kinds of gases, an oxidant and a fuel, to an oxidant electrode and a fuel electrode, and that uses a solid substance for all constituent materials. In the SOFC, the following ceramics are frequently used, and the SOFC is usually operated at a temperature around 1000 ° C.
[0003]
Electrolyte: Yttria stabilized zirconia (YSZ)
Fuel electrode: Nickel zirconia cermet (Ni-YSZ)
Oxidant electrode: strontium dopplantan manganite (LSM)
[0004]
Here, Ni is frequently used as the metal of the fuel electrode because Ni is excellent in stability against YSZ and also excellent in sulfur resistance when coal gas is used as fuel. Note that Co can be used as a metal other than Ni. As a method of manufacturing a fuel electrode of an SOFC having such a material configuration at low cost, usually, a raw material such as YSZ powder or NiO powder is mixed by a ball mill or the like, and the mixed powder is applied to the electrolyte as a paste. The method of sintering is used.
[0005]
The fuel electrode has a role as a catalyst for reacting the fuel gas with the oxidizing agent. At this time, it is said that the three-phase interface serves as an electrode reaction field. In the above-described Ni-YSZ (fuel electrode) / YSZ (electrolyte) material system, a portion where Ni, YSZ, and fuel gas are all in contact corresponds to a three-phase interface. At the three-phase interface, electrons are generated by the next electrode reaction, which is used as energy.
[0006]
H ₂ + O ²⁻ → H ₂ O + 2e ⁻
[0007]
Therefore, in order to improve the output characteristics of the SOFC, it is necessary to increase the amount of generated electrons due to an increase in the three-phase interface of the fuel electrode and to efficiently supply the generated electrons to an external circuit. Therefore, conventionally, by adjusting the particle size and the particle size ratio of the raw material powders NiO powder and YSZ powder, Ni particles and YSZ particles are highly dispersed to increase the three-phase interface. Studies have been made to improve the electron conductivity of the electrode by adjusting the mixing ratio.
[0008]
[Problems to be solved by the invention]
However, by optimizing the particle size and the particle size ratio of the NiO powder and the YSZ powder, a cell having excellent initial power generation characteristics can be obtained. The agglomeration of the Ni particles progresses, which leads to a decrease in the three-phase interface and a decrease in electron conductivity, and there is a problem that the output characteristics gradually decrease. This is the same when Co is used as the electrode material.
[0009]
Therefore, as a method of solving such a problem of sintering of metal particles, there is an example of using another kind of metal which has the same catalytic activity as Ni or Co and does not easily undergo sintering at a temperature around 1000 ° C. For example, in ruthenium zirconia cermet (Ru-YSZ) using Ru as an electrode metal, an electrode with little deterioration over time due to sintering is obtained. As described above, Ru has excellent catalytic ability and does not easily undergo sintering even at a temperature near 1000 ° C., but is expensive. Therefore, if a powder material is prepared by a conventional method of mixing powder, there is a problem that a large amount of Ru is used, the cost of the electrode material is increased, and practicality is poor.
[0010]
The present invention provides a method for producing an electrode using a fuel electrode powder that does not use such expensive noble metals, has many three-phase interfaces, has excellent electron conductivity, and does not easily cause sintering of the electrode active metal. It is intended to solve the problems of the conventional fuel electrode.
[0011]
[Means for Solving the Problems]
In order to solve the above problems, a method for manufacturing an electrode of a solid oxide fuel cell according to the present invention includes a step of immersing an oxide powder having oxygen ion conductivity in a solution containing nickel ions or cobalt ions,
Drying the oxide powder;
A step of holding nickel or cobalt in an oxide state on the surface of the dried oxide powder by heat treatment,
An electrode of a solid oxide fuel cell is prepared by mixing nickel or cobalt oxide powder with oxygen ion conductive oxide powder having nickel oxide or cobalt oxide particles retained on the surface obtained by the above steps. Producing a raw material powder of
Forming the raw material powder,
Sintering the formed raw material powder,
And the oxide powder having oxygen ion conductivity is yttria-stabilized zirconia or samarium-doped ceria.
[ 0012 ]
[Action]
The electrode structure of the solid oxide fuel cell using the fuel electrode raw material powder according to the production method of the present invention has the following effects .
[ 0013 ]
The electrode of the solid oxide fuel cell manufactured according to the present invention is basically composed of an oxide powder 2 having oxygen ion conductivity and a metal having electrode activity or an oxide of the metal as shown in FIG. Have been. The metal or the oxide of the metal is composed of metal fine particles (or oxide fine particles of the metal) 1 held on the surface of the oxide powder 2 and metal fine particles not held by the oxide powder 2 ( Or an oxide powder of the metal) 3. The metal powder (or the metal oxide powder) 3 has a particle size that is substantially the same as the oxide powder 2, and has a larger particle size than the metal fine particles (or the metal oxide fine particles) 1. is there.
[ 0014 ]
Referring to FIG. 1, a specific example of an electrode structure of a solid oxide fuel cell according to the present invention is schematically shown for a Ni-YSZ (fuel electrode) / YSZ (electrolyte) material system. In the embodiment described herein, this electrode has a structure in which YSZ powder (oxide powder) 2 holding Ni fine particles (metal fine particles) 1 and Ni powder (metal powder) 3 are mixed.
[ 0015 ]
As is well known, the fuel electrode is reduced with hydrogen during fuel cell operation. Therefore, before the operation, the YSZ powder and the NiO powder holding the NiO fine particles are mixed, but the operation changes the NiO fine particles and the NiO powder into the Ni fine particles and the Ni powder, respectively.
[ 0016 ]
Since the Ni fine particles 1 held on the YSZ powder 2 are highly dispersed on the surface of the YSZ powder not having a particle size on the order of several nm to several tens nm, a very large three-phase interface can be obtained. In addition, since the Ni fine particles 1 are bound by the YSZ powder 2, sintering of the Ni fine particles 1 hardly occurs, and the electrode has excellent long-term stability. Furthermore, since the same type of Ni powder 3 as the Ni fine particles 1 held in the YSZ powder 2 exists in the electrode, electrons generated at the three-phase interface can be collected without developing a contact potential difference, It becomes an electrode having excellent electron conductivity.
[ 0017 ]
The particle diameter of the oxide powder having oxygen ion conductivity is preferably 0.2 to 1 μm. As will be apparent from the examples described later, if the ratio deviates from this range, there is a possibility that an electrode material having many three-phase interfaces, having excellent electron conductivity, and hardly causing sintering of an electrode active metal may not be obtained. As such an oxide powder having ionic conductivity, thermalia- doped ceria can be effectively used in addition to the above-mentioned YSZ.
[ 0018 ]
As described above, cobalt as well as nickel can be used as the metal fine particles or metal oxide fine particles held by the oxide powder having oxygen ion conductivity. The particle diameter of such metal fine particles or metal oxide fine particles is on the order of several nm to several tens nm as described above.
[ 0019 ]
The particle diameter of the metal powder or the metal oxide powder mixed with the oxide powder is preferably 0.1 to 3 μm. As will be apparent from the examples described later, if the ratio deviates from this range, there is a possibility that an electrode material having many three-phase interfaces, having excellent electron conductivity, and hardly causing sintering of an electrode active metal may not be obtained.
[ 0020 ]
According to the method for producing an electrode of a solid oxide fuel cell according to the present invention, first, an oxide powder having oxygen ion conductivity is immersed in a solution containing nickel ions or cobalt ions, and then the oxide powder is dried. By heating the dried oxide powder, nickel or cobalt is held in an oxide state on the surface of the oxide powder. By heating and thermally decomposing the nickel or cobalt compound attached to the surface of the oxide powder, nickel or cobalt oxide is formed on the surface of the oxide powder. The heating temperature at this time is equal to or higher than the temperature at which the nickel or cobalt compound is thermally decomposed into nickel or cobalt oxide.
[ 0021 ]
Next, an oxide powder having oxygen ion conductivity holding nickel oxide or cobalt oxide fine particles on the surface obtained by the above steps is mixed with nickel or cobalt oxide powder to obtain a solid oxide fuel cell. A raw material powder for an electrode is produced, the raw material powder is formed, and the formed raw material powder is sintered. It is preferable that the weight of the oxide powder having oxygen ion conductivity relative to the total weight of the raw material powder is 40% to 60%. This is because, as is clear from the examples described later, if the ratio deviates from this range, there is a possibility that an electrode material having many three-phase interfaces, excellent electron conductivity, and hardly causing sintering of the electrode active metal will not be obtained. The firing can be performed at a temperature in the range of 1200 ° C. to 1600 ° C.
[ 0022 ]
【Example】
In the present embodiment, a description will be given of an example of a cell power generation test result using a fuel electrode using a Ni-YSZ material system. First, 8 mol% stabilized zirconia powder having an average particle size of 0.4 μm (hereinafter referred to as “8YSZ powder”) was immersed in a saturated nickel nitrate aqueous solution at room temperature for 1 hour. Next, this was filtered, and the 8YSZ powder remaining on the filter paper was dried. The dried 8YSZ powder was green, and it was confirmed that nickel nitrate covered the surface of the 8YSZ powder.
[ 0023 ]
Next, the 8YSZ powder on which the nickel nitrate was adsorbed was kept at 450 ° C. for 2 hours to thermally decompose the nickel nitrate. X-ray diffraction analysis confirmed that all of the nickel nitrate was converted to NiO by thermal decomposition. Further, as a result of elementary analysis of the surface of the 8YSZ powder that was not yet subjected to EPMA, it was confirmed that the Ni element was dispersed at a high density on the surface of the 8YSZ powder. Here, the amount of NiO held by the 8YSZ powder was determined by ICP analysis separately from the production process of the electrode to be 15 wt%. During operation of the fuel cell, the fuel electrode is reduced by hydrogen, so that the NiO particles are reduced to Ni particles. Then, when the state after performing the reduction treatment on the NiO held by the 8YSZ powder was observed by TEM, the particle diameter of the Ni particles held by the 8YSZ powder was several nm to several tens nm. (FIG. 2).
[ 0024 ]
Next, the 8YSZ powder holding NiO was further mixed with NiO powder having an average particle size of 2 μm. The mixing ratio was such that the weight ratio of the total NiO (the total weight of NiO and NiO powder held in the 8YSZ powder) to the YSZ powder was 50:50 wt%. In the method of powder mixing, 8YSZ powder holding NiO, NiO powder, ethanol, and zirconia balls were placed in a poly container, and rotated by a ball mill for 24 hours. After completion of the ball mill, the mixed powder was dried, and polyvinyl butyral as a binder and tvneol as a solvent were added thereto to form a slurry.
[ 0025 ]
This slurry is formed into a square (1.5 cm × 1.5 cm) having a thickness of 0.1 mm on one surface of a disk-shaped 8YSZ substrate (3.5 cm in diameter, 0.5 mm in thickness) serving as a solid electrolyte. Was applied. On the other side of the 8YSZ substrate, a slurry obtained by adding polyvinyl butyral and terpineol to a strontium do plantan manganite powder having an average particle diameter of 0.2 μm, and having a thickness of 0.1 mm and a square (2 cm) × 2 cm) to form an oxidant electrode. In this way, the fuel electrode and the oxidizing electrode were sintered by subjecting the 8YSZ substrate coated with the slurry on both surfaces to a baking treatment at 1250 ° C. for 2 hours to obtain a cell for a power generation test. The sintering temperature for this sintering can be performed at a temperature in the range of 1200 ° C to 1600 ° C.
[ 0026 ]
Further, as a comparative sample, a raw material powder prepared by a conventional method in which 8YSZ powder (average particle diameter 0.4 μm) and NiO powder (average particle diameter 2 μm) were mixed in a ball mill at a ratio of 50:50 wt% was prepared. Electrodes were also made. The conditions for forming the electrodes on the 8YSZ substrate in the conventional method are the same as those in the case of using the sample according to the present invention in which NiO is held in the 8YSZ powder.
[ 0027 ]
Next, a power generation test was performed on cells using the fuel electrode according to the present invention (Example) and the fuel electrode according to the conventional method (Comparative Example). The test cell was sandwiched between alumina tubes, air was supplied to the oxidant electrode as an oxidant, and hydrogen was supplied to the fuel electrode as a fuel gas. A Pt mesh was used for the current collector of the oxidant electrode, and a Ni mesh was used for the current collector of the fuel electrode. The test temperature was 1000 ° C. FIG. 3 shows the change over time of the battery voltage in the continuous power generation test for 1000 hours for each cell. Here, the current value was kept constant at 0.3 A / cm ² . In the figure, A shows the fuel electrode according to the embodiment of the present invention, and B shows the change over time of the fuel electrode according to the comparative example.
[ 0028 ]
When the fuel electrode according to the conventional method was used, the initial voltage of the power generation was 0.74 V, and the voltage was observed to decrease with time, and the voltage after 1000 hours had decreased to 0.37 V. On the other hand, when the fuel electrode according to the method of the present invention was used, the initial voltage of the power generation was 0.87 V, and the initial performance was almost maintained at 0.82 V even after 1000 hours.
[ 0029 ]
With respect to the cells using the fuel electrodes according to the present invention method and the conventional method, those in which the power generation test was completed in 1 hour were also prepared. Cross-sectional SEM images of the fuel electrodes of the cells were compared. As a result, in the case of the conventional method, the fuel electrode after 1000 hours of the power generation test showed coarser Ni particles due to sintering between the Ni particles compared to the fuel electrode after 1 hour of the power generation test, In the fuel electrode using the manufacturing method of the present invention, even after 1000 hours of the power generation test, the state of the fuel electrode 1 hour after the power generation test was almost maintained. As described above, the fuel electrode using the manufacturing method of the present invention has an effect of suppressing sintering of Ni particles, and can improve the life characteristics of the fuel electrode as compared with the conventional method.
[ 0030 ]
For the purpose of evaluating the electron conductivity of the electrode, a molded sintered body was produced using each of the fuel electrode raw material powders produced by the method of the present invention and the conventional production method, and the conductivity of the sintered body was measured. It was measured. First, 4 g of each raw material powder was press-formed at a compression strength of ² t / cm ² so as to form a disk having a diameter of 25 mm, and then the pressed body was formed at 1250 ° C. It was fired. A sample for measuring conductivity having a width of 3 mm and a length of about 15 mm was cut out from the sintered sample, and the conductivity was measured by a DC four-terminal method in a hydrogen reducing atmosphere at 1000 ° C.
[ 0031 ]
FIG. 4 shows the change over time of the electrical conductivity of the molded sintered body using the raw material powder according to the method of the present invention and the conventional method. In the figure, A is the change over time in the conductivity of the example according to the present invention and B is the change over time in the conductivity of the comparative example. From such characteristics, it can be seen that in the fuel electrode using the manufacturing method according to the present invention, the decrease in conductivity with time is small, and sintering between Ni particles is suppressed. Also, the value of the conductivity according to the present invention is more than two orders of magnitude higher than that according to the conventional method, and exhibits high electron conductivity, so that electrons generated at the three-phase interface can be efficiently collected. .
[ 0032 ]
In this embodiment, the case where the average particle size of the powder of the oxide 8YSZ having oxygen ion conductivity is 0.4 μm is shown. However, similar good results are obtained when the particle size is in the range of 0.2 μm to 1 μm. Was done. The case where the average particle size of the NiO powder was 2 μm was shown, but similar good results were obtained when the particle size was in the range of 0.1 μm to 3 μm. Further, in the present embodiment, the case where the weight ratio of the total NiO (total weight of NiO and NiO powder held in the 8YSZ powder) and the 8YSZ powder is 50:50 wt% is shown, but the total weight of both is added. Similar good results were obtained when the weight ratio of 8YSZ to the mixture was in the range of 40 wt% to 60 wt%.
[ 0033 ]
【The invention's effect】
The electrode structure manufactured by the present invention has the following effects. First, since the metal fine particles are highly dispersed on the surface of the oxide powder, the three-phase interface becomes very large, and an electrode having a small voltage drop due to the electrode reaction and excellent output characteristics can be obtained. In addition, since the metal fine particles are held by the oxide powder, sintering between the metal particles hardly occurs, and an electrode having excellent long-term stability and little deterioration over time can be obtained. Furthermore, high electron conductivity is exhibited by the presence of the metal powder which is not retained by the oxide powder.
[Brief description of the drawings]
FIG. 1 is a schematic view of a fuel electrode using a raw material powder according to a production method of the present invention.
FIG. 2 is a TEM image photograph of a YSZ powder holding Ni fine particles produced by the production method of the present invention.
FIG. 3 is a diagram showing a change over time of a voltage in a power generation test of a cell using a fuel electrode manufactured from a raw material powder according to the manufacturing method of the present invention.
FIG. 4 is a diagram showing a change over time in the electrical conductivity of a fuel electrode produced from a raw material powder according to the production method of the present invention.
[Explanation of symbols]
1 Metal fine particles 2 Oxide powder 3 Metal oxide powder

Claims (4)

Immersing the oxide powder having oxygen ion conductivity in a solution containing nickel ions or cobalt ions,
Drying the oxide powder;
A step of holding nickel or cobalt in an oxide state on the surface of the dried oxide powder by heat treatment,
An electrode of a solid oxide fuel cell is prepared by mixing nickel or cobalt oxide powder with oxygen ion conductive oxide powder having nickel oxide or cobalt oxide particles retained on the surface obtained by the above steps. Producing a raw material powder of
Forming the raw material powder,
Sintering the formed raw material powder,
And wherein the oxide powder having oxygen ion conductivity is yttria-stabilized zirconia or samarium-doped ceria . The method for producing an electrode for a solid oxide fuel cell according to claim 7, wherein the weight of the oxide powder having oxygen ion conductivity in the total weight of the raw material powder is 40% to 60%. The temperature of the heat treatment is a temperature at which a nickel compound or a cobalt compound attached to the particle surface of the oxide powder by being immersed in a solution containing nickel ions or cobalt ions becomes nickel oxide or cobalt oxide by thermal decomposition. method for producing a solid electrolyte fuel cell electrode according to claim 1 or 2, characterized in that the above temperature. The method for producing an electrode of a solid oxide fuel cell according to any one of claims 1 to 3 , wherein the sintering temperature is in a range of 1200C to 1600C.

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