JP2021161467A - electrode - Google Patents

Abstract

To provide an electrode for SOFC or SOEC which has electron conductivity less reduced even when being exposed to high-temperature steam.SOLUTION: The electrode comprises a diffusion later and an active layer formed on an electrolyte layer-side surface of the diffusion layer. The active layer comprises a cermet (B) including Ni-containing particles (B), ScCZ particles formed of an Sc-doped CeO2-ZrO2 solid solution, and electrolyte particles (B) formed of a solid oxide other than the Sc-doped CeO2-ZrO2 solid solution. Preferably, the ScCZ particles have a composition represented by ScxCeyZr1-(x+y)O2 (wherein 0.06≤x≤0.19 and 0.05≤y≤0.15).SELECTED DRAWING: Figure 5

Description

The present invention relates to an electrode, and more specifically, it can be used as a hydrogen electrode of a solid oxide fuel cell (SOEC) or a fuel electrode of a solid oxide fuel cell (SOFC), and has excellent oxidation resistance. Regarding the electrodes.

A solid oxide fuel cell (SOFC) is a fuel cell that uses an oxide ion conductor as an electrolyte. _{When fuel gas such as H 2} , CO, and CH ₄ is supplied to the anode (fuel electrode) of the SOFC _{and O 2} is supplied to the cathode (oxygen electrode), the electrode reaction proceeds and electric power can be taken out. _{CO 2} and H ₂ O generated by the electrode reaction are discharged to the outside of the SOFC.
On the other hand, the solid oxide fuel cell (SOEC) has the same structure as SOFC, but causes a reaction opposite to that of SOFC. _{That is, when CO 2} or H ₂ O is supplied to the cathode (hydrogen electrode) of the SOEC and a current is passed between the electrodes, CO or H ₂ can be generated.

SOEC includes a single cell in which an anode (oxygen electrode) is bonded to one surface of an electrolyte and a cathode (hydrogen electrode) is bonded to the other surface. The following materials are generally used as materials for the members constituting such SOEC (see Non-Patent Documents 1 to 6).
(A) Electrolytes: yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (SSZ), sammalia-doped toseria (SDC), lanthanum strontium gallium magnesium oxide (LSGM) and the like.
(B) Oxygen electrode: Lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium cobaltite (LSC) and the like.
(C) Hydrogen electrode: Ni / YSZ, Ni / SDC, Ni-Fe / SDC, etc.

Ni / YSZ cermet is generally used for the hydrogen electrode of SOEC. However, steam, which is a raw material for hydrogen production, is supplied to the hydrogen electrode at a high temperature (700 ° C. or higher). Therefore, Ni contained in the hydrogen electrode is easily oxidized to form NiO. Since NiO is an insulator, when NiO is formed in the electrode, the electron path is interrupted at that portion and the electrolytic reaction does not proceed. As a result, the electrolytic characteristics deteriorate.
This point is the same for SOFC. That is, at the anode (fuel electrode) of the SOFC, water is generated by the electrode reaction. Therefore, in particular, under high load operating conditions, the generated steam may oxidize Ni in the fuel electrode, resulting in deterioration of power generation characteristics.

Ebbesen, S. D .; Hansen, J. B .; Morgensen, M. B. ECS Trans. 2013, 57, 3217. Jensen, S. H .; Larsen, P. H .; Mogensen, M. Int. J. Hydrogen Energy 2007, 32, 3253. Katahira, K .; Kohchi, Y .; Shimura, T .; Iwahara, H. Solid State Ionics 2000, 138, 91. Languna-Bercero, M. A .; Skinner, S. J .; Kilner, J. A. J. Power Sources 2009, 192, 126. O'Brien, J. E .; Stoots, C. M .; Herring, J. S .; Lessing, P. A .; Hartvigsen, J. J .; Elangovan, S. J. Fuel Cell Sci. Technol. 2005, 2, 156. Sune Dalgaard Ebbesen, Soren Hojgaard Jensen, Anne Hauch, and Morgens Bjerg Morgensen, Chem. Rev. 2014, 114, 1069

An object to be solved by the present invention is to provide an electrode for SOFC or SOEC in which the decrease in electron conductivity is small even when exposed to high temperature water vapor.

In order to solve the above problems, the electrode according to the present invention has the following configuration.
(1) The electrode is
Diffusion layer and
It includes an active layer formed on the surface of the diffusion layer on the electrolyte layer side.
(2) The active layer is
Ni-containing particles (B) and
ScCZ particles composed of Sc-doped CeO _{2- ZrO 2 solid solution,}
It is composed of a cermet (B) containing electrolyte particles (B) made of a solid oxide other than the Sc-doped CeO _2- ZrO _{2 solid solution.}

When ScCZ is added to the electrode containing the Ni-containing particles (B), the oxidation of Ni contained in the Ni-containing particles (B) is remarkably suppressed. this is,
(A) Oxygen ions spilled over from a Ni-containing metal oxide film (for example, NiO) formed on the surface of the Ni-containing particles (B) are fixed to ScCZ.
(B) Water vapor is decomposed into oxygen and hydrogen by the water decomposition function of ScCZ, and the surroundings of ScCZ are exposed to a reducing atmosphere, and / or.
(C) It is considered that the coexistence of the Ni-containing particles (B) and ScCZ changes the electronic state of Ni contained in the Ni-containing particles (B).

It is a schematic diagram of the oxygen occlusion / release process of CZ. It is a schematic diagram of the solid oxide type electrolytic cell which used Ni / (ScCZ + YSZ) as an active layer. It is a schematic diagram of the conventional solid oxide fuel cell electrolytic cell using Ni / YSZ as an active layer. It is a schematic diagram of the Ni oxidation suppression mechanism by ScCZ.

The Ni oxidation rate of various electrodes under an oxidizing atmosphere (with or without humidification) at a temperature of 700 ° C. It is an XAFS spectrum of Ni / YSZ (Y _0.158 Zr _0.842 O _2). It is an XAFS spectrum of Ni / ScCZ (Sc _0.132 Ce _0.104 Zr _0.764 O _2). It is the oxygen occlusion amount of ScCZ (Sc _0.132 Ce _0.104 Zr _0.764 O ₂ ) and CZ (Ce _0.118 Zr _0.882 O _2).

Hereinafter, an embodiment of the present invention will be described in detail.
[1. electrode]
The electrode according to the present invention is
Diffusion layer and
It includes an active layer formed on the surface of the diffusion layer on the electrolyte layer side.

[1.1. Diffusion layer]
In the present invention, the diffusion layer is composed of a cermet (A) containing Ni-containing particles (A) and electrolyte particles (A) made of a solid oxide. The diffusion layer is for supporting the active layer. In an electrode composed of a laminated body of a diffusion layer and an active layer, an electrode reaction mainly occurs in the active layer. Therefore, the diffusion layer does not necessarily have to have high ionic conductivity.

That is, the diffusion layer is at least
(A) Function for supporting the active layer formed on the surface on the electrolyte layer side,
(B) A function of diffusing fuel (raw material for electrolysis or power generation) to the active layer,
(C) The function of transporting the electrons required for the reduction reaction from the current collector to the active layer, or the function of transporting the electrons generated by the oxidation reaction from the active layer to the current collector, and
(D) It is necessary to have a function of discharging hydrogen or water vapor generated in the active layer by the electrode reaction to the outside of the electrode.
The composition of the diffusion layer is not particularly limited as long as it performs such a function.

[1.1.1. Ni-containing particles (A)]
The “Ni-containing particles (A)” refers to metal particles in which the ratio of the mass of Ni to the total mass of metal elements contained in the particles is 10 mass% or more. The mass ratio of Ni contained in the Ni-containing particles (A) is preferably 50 mass% or more, more preferably 90 mass% or more.
The Ni-containing particles (A) have a function as an electron conductor in the diffusion layer. The composition of the Ni-containing particles (A) is not particularly limited as long as it exhibits such a function. Examples of the Ni-containing particles (A) include Ni, Ni—Fe alloy, Ni—Co alloy and the like.
The content of the Ni-containing particles (A) is not particularly limited as long as it functions as a diffusion layer. Further, the content of the Ni-containing particles (A) contained in the diffusion layer may be the same as or different from the content of the Ni-containing particles (B) contained in the active layer. The content of the Ni-containing particles (A) is usually 30 to 70 mass%.

[1.1.2. Electrolyte particles (A)]
The composition of the electrolyte particles (A) is not particularly limited as long as it functions as a diffusion layer. The electrolyte particles (A) may have the same composition as the electrolyte contained in the active layer, or may have a different composition.
Examples of the electrolyte particles (A) include, for example.
(A) Yttria-stabilized zirconia (YSZ) containing ₃ to 15 mol% Y ₂ O 3,
(B) A material having the same or similar composition as the electrolyte contained in the active layer,
and so on.
Among these, YSZ is preferable as the electrolyte particles (A). This is because the mechanical strength is stable.

[1.1.3. Porosity]
The porosity of the diffusion layer affects the gas diffusivity, strength, electron conductivity, etc. of the electrode. In general, if the porosity of the diffusion layer is too small, the gas diffusivity decreases. Therefore, the porosity of the diffusion layer is preferably 40% or more. The porosity is preferably 45% or more, more preferably 50% or more.
On the other hand, if the porosity of the diffusion layer becomes too large, the strength and electron conductivity will decrease. Therefore, the porosity of the diffusion layer is preferably 60% or less. The porosity is preferably 58% or less, more preferably 55% or less.

[1.2. Active layer]
The active layer is a portion that serves as a reaction field for the electrode reaction. In the present invention, the active layer is
Ni-containing particles (B) and
ScCZ particles composed of Sc-doped CeO _{2- ZrO 2 solid solution,}
It is composed of a cermet (B) containing electrolyte particles (B) made of a solid oxide other than the Sc-doped CeO _2- ZrO _{2 solid solution.}

[1.2.1. Ni-containing particles (B)]
The “Ni-containing particles (B)” refers to metal particles in which the ratio of the mass of Ni to the total mass of metal elements contained in the particles is 90 mass% or more. The mass ratio of Ni contained in the Ni-containing particles (B) is preferably 95 mass% or more.
The Ni-containing particles (B) have functions as an electrode catalyst and an electron conductor in the active layer. The composition of the Ni-containing particles (B) is not particularly limited as long as it exhibits such a function. Examples of the Ni-containing particles (B) include Ni, Ni—Fe alloy, Ni—Co alloy and the like. In order to obtain high catalytic activity and high electron conductivity, it is preferable that the Ni-containing particles (B) are substantially composed of only Ni and the balance is composed of unavoidable impurities.

[1.2.2. ScCZ particles]
[A. Oxygen storage / release capacity]
The ScCZ particles are composed of a Sc-doped CeO _{2- ZrO 2} solid solution, and have a function as an oxide ion conductor and a function of suppressing the oxidation of Ni contained in the Ni-containing particles (B) in the active layer. Have. CeO ₂ has the effect of imparting oxygen storage / release ability to _{ZrO 2.} On the other hand, Sc _{replaces the Ce site of the CeO 2-} ZrO ₂ solid solution (CZ). By doping CZ with an appropriate amount of Sc, decomposition of CZ during firing can be suppressed. Further, ScCZ has an action of changing the electronic state of Ni contained in the Ni-containing particles (B). Therefore, when the Ni-containing particles (B) and ScCZ coexist, the oxidation of Ni contained in the Ni-containing particles (B) can be suppressed. Further, even when the Ni-containing particles (B) contain a metal element other than Ni, a three-phase interface (TPB) can be secured in the electrode as long as the oxidation of Ni can be suppressed at least.

FIG. 1 shows a schematic diagram of the oxygen occlusion / release process of CZ. Further, the following formula (1) shows the reaction formula of the oxygen storage / release reaction of CZ. In formula (1), the reaction that proceeds to the right side represents an oxidation reaction, and the reaction that proceeds to the left side represents a reduction reaction.
CeO _2-x −ZrO ₂ + (x / 2) O ₂ ⇔ CeO ₂ −ZrO ₂ … (1)

Ce ions dissolved in ZrO ₂ reversibly take a trivalent state (reduced state) and a tetravalent state (oxidized state) according to the partial pressure of oxygen in the surrounding atmosphere. Can be done. Therefore, when the CZ is exposed to an oxidizing atmosphere, the CZ takes in oxygen ions in the atmosphere into the crystal lattice. On the other hand, when the CZ is exposed to a reducing atmosphere, the CZ releases oxygen ions in the crystal lattice into the atmosphere. This point is the same for ScCZ.

[B. composition]
The ScCZ particles preferably have a composition represented by the following formula (2).
_{Sc x Ce y Zr 1- (x} + y) O 2 ... (2)
However, 0.06 ≦ x ≦ 0.19 and 0.05 ≦ y ≦ 0.15.

In the formula (2), x represents the number of moles of Sc with respect to the total number of moles of Sc, Ce, and Zr contained in the ScCZ particles. If x becomes too small, the heat resistance of the ScCZ particles and the oxidation suppressing function of Ni contained in the Ni-containing particles (B) deteriorate. Therefore, x is preferably 0.06 or more. x is preferably 0.1 or more.
On the other hand, when x becomes excessive, phase separation occurs, the CZ structure cannot be maintained, and the structural stability deteriorates. Therefore, x is preferably 0.19 or less.

In the formula (2), y represents the number of moles of Ce with respect to the total number of moles of Sc, Ce, and Zr contained in the ScCZ particles. If y becomes too small, the oxygen occlusion amount may decrease, and the oxidation suppressing function of Ni contained in the Ni-containing particles (B) may decrease. Therefore, y is preferably 0.05 or more.
On the other hand, when y is excessive, not only the oxygen ion conductivity in the electrode is lowered, but also the mechanical strength of the electrode may be lowered. Therefore, y is preferably 0.15 or less.

[1.2.3. Electrolyte particles (B)]
The electrolyte particles (B) are composed of solid oxides other than the _{Sc-doped CeO 2-} ZrO _{2 solid solution.} In the present invention, the electrolyte particles (B) are added to improve and / or stabilize the ionic conductivity of the electrode. ScCZ also functions as an oxygen ion conductor, but its oxygen ion conductivity is relatively low. In addition, the state of ScCZ changes due to the partial pressure of oxygen, and the ionic conductivity also changes accordingly. On the other hand, when the electrode further contains the electrolyte particles (B), the ionic conductivity of the entire electrode is improved and / or the destabilization of the ionic conductivity due to the state change of the ScCZ particles is alleviated. Can be done.
Examples of the electrolyte particles (B) include, for example.
(A) YTSZ particles composed of yttria-stabilized zirconia,
(B) LaCZ particles composed of La-doped CeO _{2- ZrO 2} solid solution and the like. The active layer may contain any one of these electrolyte particles (B), or may contain two or more of them.

[A. YSZ particles]
The YSZ particles are composed of yttria-stabilized zirconia and have a function as an oxide ion conductor in the active layer. The content of Y contained in the YSZ particles is not particularly limited, and the optimum amount can be selected according to the purpose.
In order to obtain relatively high ionic conductivity and relatively high mechanical strength, the YSZ particles preferably contain _{Y 2} O _{3 of 3 mol% or more and 15 mol% or less.} Further inclusion of YSZ particles in the electrode may improve the mechanical strength of the electrode.

[B. LaCZ particles]
The LaCZ particles are composed of a La-doped CeO _{2- ZrO 2} solid solution, and have a function as an oxide ion conductor in the active layer and a function of suppressing the oxidation of Ni contained in the Ni-containing particles (B). Has. CeO ₂ has the effect of imparting oxygen storage / release ability to _{ZrO 2.} On the other hand, La has the _{effect of substituting the Ce site of the CeO 2-} ZrO ₂ solid solution (CZ) and improving the heat resistance of the CZ. Therefore, if an appropriate amount of La is doped in CZ, decomposition of CZ at the time of firing can be suppressed.
The oxidation inhibitory function of LaCZ particles is inferior to that of ScCZ particles. However, if the electrode further contains LaCZ particles, the thermal stability may be improved.

The LaCZ particles are particularly preferably those having a composition represented by the following formula (3).
La _x Ce _2-x Zr ₂ O ₇ … (3)
However, 0 <x <0.2.

In the formula (3), x represents the number of moles of La with respect to the total number of moles of La and Ce contained in the LaCZ particles. If x becomes too small, the heat resistance of the LaCZ particles decreases. Therefore, x needs to be greater than 0. x is preferably 0.05 or more, more preferably 0.1 or more.
On the other hand, when x becomes excessive, the oxygen storage / release capacity of LaCZ particles decreases. Therefore, x is preferably less than 0.2. x is preferably 0.15 or less.

[12.4. Composition of active layer]
In the present invention, the active layer preferably contains a predetermined amount of ScCZ particles and electrolyte particles (B), and the balance is composed of Ni-containing particles (B) and unavoidable impurities. Further, the active layer preferably satisfies the following formulas (4) to (7).
30 mass% ≤ u ≤ 70 mass% ... (4)
10 mass% ≤ v ≤ 30 mass% ... (5)
30 mass% ≤ w ≤ 70 mass% ... (6)
u + v + w = 100 mass% ... (7)
However,
u is the mass ratio (mass%) of the ScCZ particles contained in the active layer.
v is the mass ratio (mass%) of the electrolyte particles (B) contained in the active layer.
w is the mass ratio (mass%) of the Ni-containing particles (B) contained in the active layer.

[A. Content of ScCZ particles]
If the content of the ScCZ particles is too small, the oxidation of Ni contained in the Ni-containing particles (B) cannot be sufficiently suppressed. Therefore, the content of ScCZ particles is preferably 30 mass% or more.

On the other hand, when the content of ScCZ particles becomes excessive, the total cell resistance increases and the efficiency of the electrode reaction also decreases. Therefore, the content of ScCZ particles is preferably 70 mass% or less. The content of ScCZ particles is preferably 60 mass% or less.

[B. Content of electrolyte particles (B)]
If the content of the electrolyte particles (B) is too low, the ionic conductivity is lowered, which leads to a decrease in the reaction active sites and an increase in the ohmic resistance due to an increase in the ionic conduction distance. Therefore, the content of the electrolyte particles (B) is preferably 10 mass% or more.

On the other hand, when the content of the electrolyte particles (B) becomes excessive, the proportion of ScCZ particles in the active layer decreases, so that the oxidation of Ni contained in the Ni-containing particles (B) cannot be sufficiently suppressed. Therefore, the content of the electrolyte particles (B) is preferably 30 mass% or less.

[C. Content of Ni-containing particles (B)]
If the content of the Ni-containing particles (B) is too small, the catalytic activity and electron conductivity of the active layer will decrease. Therefore, the content of the Ni-containing particles (B) is preferably 30 mass% or more.
On the other hand, when the content of the Ni-containing particles (B) becomes excessive, the proportion of ScCZ particles in the active layer decreases, so that the total cell resistance increases and the efficiency also decreases. Therefore, the content of the Ni-containing particles (B) is preferably 70 mass% or less.

[1.2.5. Porosity]
The porosity of the active layer affects the electrolytic or power generation characteristics. If the porosity of the active layer is too small, the diffusivity of the gas will decrease and the efficiency of the electrode reaction will decrease. Therefore, the porosity of the active layer is preferably 15% or more. The porosity is preferably 20% or more, more preferably 25% or more.
On the other hand, if the porosity of the active layer becomes too large, the three-phase interface becomes relatively small, and the efficiency of the electrode reaction decreases. Therefore, the porosity of the active layer is preferably 40% or less. The porosity is preferably 35% or less, more preferably 30% or less.

[1.3. Use]
The electrode according to this embodiment can be used as a hydrogen electrode of a solid oxide fuel cell or a fuel electrode of a solid oxide fuel cell.

[2. Electrode manufacturing method]
The electrode according to the present invention is
(A) A diffusion layer molded body was prepared using the raw material of the Ni-containing particles (A) and the raw material mixture (A) containing the raw materials of the electrolyte particles (A).
(B) An active layer molded body is formed on the surface of the diffusion layer molded body using a raw material mixture (B) containing a raw material of Ni-containing particles (B), ScCZ powder, and a raw material of electrolyte particles (B).
(C) The obtained laminate is sintered and
(D) It can be produced by reducing the obtained sintered body.

[2.1. Diffusion layer molded product manufacturing process]
First, a diffusion layer molded product is produced using the raw material of the Ni-containing particles (A) and the raw material mixture (A) containing the raw material of the electrolyte particles (A) (diffusion layer molded product manufacturing step).

The type of raw material for the Ni-containing particles (A) is not particularly limited, and the optimum raw material can be selected according to the purpose. Examples of the raw material of the Ni-containing particles (A) include NiO powder, Fe ₂ O ₃ powder, Fe ₃ O ₄ powder, a mixture of metal Fe and NiO or metal Ni, CoO powder, and Co ₂ O ₃ powder. ..
The type of raw material for the electrolyte particles (A) is not particularly limited, and the optimum raw material can be selected according to the purpose.
For example, when the electrolyte particle (A) is YSZ, the raw material thereof is
(A) YSZ powder having the desired composition,
(B) _{There is a mixture of ZrO 2} powder and Y ₂ O ₃ powder which are blended so as to have a desired composition.

Further, the raw material mixture (A) may contain a pore-forming material (for example, carbon powder). The metal oxide (for example, NiO powder) contained in the raw material of the Ni-containing particles (A) added to the raw material mixture (A) is reduced after the sintered body is produced. At that time, volume shrinkage occurs and pores are introduced into the sintered body. Therefore, the pore-forming material is not always necessary. However, when the pore-forming material is added to the raw material mixture (A), the degree of freedom in controlling the porosity is increased.

The method for producing the diffusion layer molded product is not particularly limited, and the optimum method can be selected according to the purpose. As a method for producing the diffusion layer molded product, for example,
(A) A method in which a slurry containing a raw material mixture (A) is tape-molded, a plurality of obtained green sheets are laminated, and the laminated body is pressure-bonded by hydrostatic pressure pressing.
(B) A method of press-molding the raw material mixture (A) with a mold,
and so on.

[2.2. Active layer molded product manufacturing process]
Next, an active layer molded body is formed on the surface of the diffusion layer molded body using a raw material mixture (B) containing a raw material of Ni-containing particles (B), ScCZ powder, and a raw material of electrolyte particles (B) (activity). Layer molded body manufacturing process).

If a mixture of CZ powder and Sc source is used as a raw material for forming the active layer, CZ may be decomposed during firing. Therefore, it is preferable to use ScCZ powder doped with a predetermined amount of Sc as a raw material for producing the active layer.
Further, for the same reason as the raw material mixture (A), the raw material mixture (B) may contain a pore-forming material (for example, carbon powder).
The details of the raw material of the Ni-containing particles (B) and the raw material of the electrolyte particles (B) are the same as those of the raw material of the Ni-containing particles (A) and the raw material of the electrolyte particles (A), respectively, and thus the description thereof is omitted. do.

The method for producing the active layer molded product is not particularly limited, and the optimum method can be selected according to the purpose. As a method for producing the active layer molded product, for example,
(A) A method in which a slurry containing a raw material mixture (B) is tape-molded, the obtained green sheet is laminated on a diffusion layer molded body, and the laminated body is pressure-bonded by hydrostatic pressure pressing.
(B) A method of preparing a slurry containing the raw material mixture (B) and screen-printing the slurry on the surface of the diffusion layer molded product.
and so on.

[2.3. Sintering process]
Next, the obtained laminate is sintered (sintering step). As for the sintering conditions, it is preferable to select the optimum conditions according to the raw material composition. Sintering is usually preferably carried out at 1000 ° C. to 1500 ° C. for 1 hour to 5 hours in an atmospheric atmosphere.
When two or more kinds of oxides are contained in the raw material mixture, a solid phase reaction may proceed during sintering to form a solid solution having a predetermined composition. Further, when the pore-forming material is contained in the raw material mixture, the pore-forming material disappears at the time of sintering, and pores are formed in the sintered body.

[2.4. Reduction process]
Next, the obtained sintered body is reduced (reduction step). As a result, the electrode according to the present invention can be obtained. The reduction treatment is performed to reduce metal oxides such as NiO contained in the sintered body to generate Ni-containing particles (A) and Ni-containing particles (B). The reduction conditions are not particularly limited, and it is preferable to select the optimum conditions according to the composition of the electrode.
As will be described later, the solid oxide type electrolytic cell is composed of a conjugate of a hydrogen electrode (cathode) / electrolyte layer / reaction prevention layer / oxygen electrode (anode). The reduction of the hydrogen electrode is usually carried out after joining the layers. This point is the same for the solid oxide fuel cell.

[3. Solid oxide electrolytic cell and polymer electrolyte fuel cell]
FIG. 2 shows a schematic diagram of a solid oxide type electrolytic cell using Ni / (ScCZ + YSZ) as an active layer. In FIG. 2, the solid oxide fuel cell (SOEC) 10 is
Electrolyte 12 and
A hydrogen electrode 14 bonded to one surface of the electrolyte 12 and
An oxygen electrode 16 bonded to the other surface of the electrolyte 12 and
It includes an intermediate layer 18 inserted between the electrolyte 12 and the oxygen electrode 16.

The electrode according to the present invention is used for the hydrogen electrode 14. That is, the hydrogen electrode 14 is composed of a laminate of the active layer 14a and the diffusion layer 14b supporting the active layer 14a. In FIG. 2, the active layer 14a is composed of a cermet (B) containing Ni particles, ScCZ particles, and YSZ particles.
The materials of the electrolyte 12, the oxygen electrode 16, and the intermediate layer 18 are not particularly limited, and the optimum material can be selected according to the purpose.

For example, YSZ or the like can be used as the electrolyte 12.
As the oxygen electrode 16, (La, Sr) CoO ₃ (LSC), (La, Sr) (Co, Fe) O ₃ (LSCF), (La, Sr) MnO ₃ (LSM) and the like can be used.
The intermediate layer 18 is a layer for preventing a reaction caused by direct contact between the electrolyte 12 and the oxygen electrode 16, and is inserted as necessary. For example, when the electrolyte 12 is YSZ and the oxygen electrode 16 is LSC, it is _{preferable to use Gd-doped CeO 2} (GDC) for the intermediate layer 18.

As described above, the electrode according to the present invention can be used as a fuel electrode of a solid oxide fuel cell (SOFC). Since the SOFC has the same structure as the SOEC 10 except for different uses, detailed description thereof will be omitted.

[3. Action]
FIG. 3 shows a schematic diagram of a conventional solid oxide fuel cell electrolytic cell using Ni / YSZ as an active layer. The conventional SOIC 10'is between the electrolyte 12, the hydrogen electrode 14'bonded to one surface of the electrolyte 12, the oxygen electrode 16 bonded to the other surface of the electrolyte 12, and the electrolyte 12 and the oxygen electrode 16. It has an intermediate layer 18 inserted into the. In the conventional SOCC 10', Ni-YSZ cermet is used as the hydrogen electrode 14'.

_{When H 2} O is supplied to the hydrogen pole 14'and a current is passed between the hydrogen pole 14' and the oxygen pole 16 in the SOC 10'where the hydrogen pole 14'is made of Ni-YSZ cermet, the next hydrogen pole 14' The reduction reaction represented by the formula (8) proceeds.
^{_{H 2 O + 2e - → H}} 2 + O 2- ... (8)

However, in water electrolysis using SOCE10', high-temperature (700 ° C. or higher) steam as a raw material is supplied to the hydrogen electrode 14'. Therefore, Ni contained in the hydrogen electrode 14'is easily oxidized to form NiO. Since NiO is an insulator, when NiO is formed, the electron path is interrupted at that portion and the electrode reaction does not proceed. As a result, the electrolytic properties are reduced.
This point is the same for SOFC. That is, at the anode (fuel electrode) of the SOFC, water is generated by the electrode reaction. Therefore, in particular, under high load operating conditions, the generated water may oxidize Ni in the fuel electrode, resulting in deterioration of power generation characteristics.

In order to solve this problem, it is effective to add a _{CeO 2-} ZrO _{2 solid solution (CZ) to the hydrogen electrode.} In addition to its function as an oxide ion conductor, CZ has the ability to store and release oxygen. Therefore, when CZ is added to the electrode made of Ni / YSZ cermet, the oxidation of Ni can be suppressed to some extent.
However, when a mixture of Ni, YSZ, and CZ is fired to prepare an electrode, a part of CZ may be decomposed to form another phase because the CZ is exposed to a high temperature. As the decomposition of CZ progresses, the oxygen storage / release capacity decreases, and the effect of suppressing the oxidation of Ni decreases.

On the other hand, the La-doped CeO _{2- ZrO 2} solid solution (LaCZ) has a higher thermal stability of the crystal structure than the CZ. Therefore, even when the mixture of Ni, YSZ, and LaCZ is fired at a high temperature, the decomposition of LaCZ is suppressed. The electrode in which the decomposition of LaCZ is suppressed maintains a high oxygen storage / release capacity. Therefore, when steam electrolysis or power generation is performed using this, oxidation of Ni is suppressed, and electrolysis performance or power generation performance is improved. However, the oxidation resistance of the electrode containing LaCZ is insufficient.

On the other hand, the Sc-doped CeO _{2- ZrO 2} solid solution (ScCZ) has a higher thermal stability of the crystal structure than the CZ. Furthermore, ScCZ is significantly more effective in suppressing the oxidation of Ni than LaCZ. Therefore, when steam electrolysis or power generation is performed using an electrode containing Ni and ScCZ, the oxidation of Ni is remarkably suppressed.

FIG. 4 shows a schematic diagram of the Ni oxidation suppression mechanism by ScCZ. It is considered that the reason why the oxidation of Ni is remarkably suppressed by the addition of ScCZ to the hydrogen electrode in the SOEC cell is as follows.

[A. Fixation of oxygen ions by ScCZ]
When the hydrogen electrode containing Ni and ScCZ is exposed to high-temperature water vapor, Ni is oxidized and the surface of the Ni particles is covered with NiO (step 1). At this time, if there are ScCZ particles in the vicinity of the Ni particles, oxygen atoms or oxygen ions spill over from NiO (step 2). The trigger is considered to be the oxygen storage capacity of ScCZ.

[B. Generation of reducing atmosphere by ScCZ]
Oxygen spilled over from NiO is temporarily trapped in ScCZ (step 3). The hydrogen electrode is in a _{reducing atmosphere due to the generated H 2} , but in the reducing atmosphere, ScCZ easily releases trapped oxygen atoms or oxygen ions as oxygen molecules. The oxygen molecules released from ScCZ are discharged out of the system through the diffusion layer before reoxidizing Ni (step 4). Hereinafter, it is considered that Ni oxidation is suppressed by repeating such steps 1 to 4.

[C. Changes in the electronic state of Ni]
As will be described later, the electronic state of Ni existing in the Ni / YSZ cermet is almost the same as the electronic state when Ni is present alone. On the other hand, the electronic state of Ni existing in the Ni / ScCZ cermet is different from the electronic state when Ni is present alone. It is considered that the reason why Ni / ScCZ cermet shows superior oxidation resistance to Ni / YSZ cermet and Ni / LaCZ cermet is that the electronic state of Ni changes due to the coexistence of Ni and ScCZ. ..
This point is the same when Ni-containing particles (B) containing an element other than Ni are used as the electrode catalyst, and when Ni-containing particles (B) and ScCZ coexist, they are contained in Ni-containing particles (B). It is considered that the electronic state of Ni is changed, which suppresses the oxidation of Ni contained in the Ni-containing particles (B).

(Experiment Nos. 1 to 4)
[1. Preparation of sample]
[1.1. Preparation of sample for Ni oxidation rate and XAFS measurement]
For the electrolyte source,
(A) ScCZ powder (Experiment No. 1) composed of _{Sc 0.132} Ce _0.104 Zr _0.764 O _{2 solid solution,}
(B) YSZ powder (Experiment No. 2) composed of _{Y 0.158} Zr _0.842 O _{2 solid solution,}
(C) LaCZ powder (Experiment No. 3) composed of _{La 0.25} Ce _0.25 Zr _0.5 O _{2 solid solution, or}
(D) _{CZ powder composed of Ce 0.118} Zr _0.882 O ₂ solid solution (Experiment No. 4)
Was used.
Further, NiO powder was used as the Ni source.

The electrolyte source and NiO powder were blended in a mass ratio of 1: 1 to obtain a mixed powder (hereinafter, also referred to as “electrode material”).
0.8 mg of the electrode material and 40 mg of the θ-alumina powder were mixed and pressed to prepare a powder compact having a diameter of 10 mm and a thickness of 0.5 mm.

[1.2. Preparation of sample for measuring oxygen occlusion]
The electrolyte powder was used as it was for measuring the oxygen occlusion amount.

[2. Test method]
[2.1. Measurement of Ni oxidation rate and XAFS spectrum]
The powder compact containing the electrode material was set in a quartz cell for measuring X-ray absorption fine structure (XAFS). A 4% H ₂ / He balance gas (total amount 100 cc / min) was introduced into the cell and the temperature was raised to 780 ° C. (60 ° C./min). Then, the temperature was lowered to 700 ° C., and the mixture was purged with He gas for 1 minute. Next, the atmosphere in the cell _{was switched to an oxidizing atmosphere of 2% O 2} / He gas (without humidification) or 2% O ₂ + 3% H ₂ O / He gas (with humidification).

The XAFS spectrum was measured from the time the He purge was started. In addition, Ni and NiO were used as reference spectra. Table 1 shows the details of the XAFS measurement conditions.
Further, the Ni oxidation rate was calculated from the XAFS spectrum. Using the XAFS spectra of Ni and NiO, the NiO / Ni ratio was obtained by linear least squares fitting, and this was taken as the Ni oxidation rate.

[2.2. Oxygen occlusion]
The oxygen occlusion of ScCZ powder and CZ powder was measured using a thermogravimetric analyzer. H ₂ (5%) / N ₂ atmosphere and O ₂ (5%) / N ₂ atmosphere are switched 3 times every 5 minutes. Under a fluctuating atmosphere (that is, _{hold for 5 minutes in H 2} atmosphere → 5 minutes in O ₂ atmosphere. holding → were measured an increase or decrease in weight of the sample with H ₂ in an atmosphere 5 min hold). _{The weight loss at the time of the second} H ₂ (5%) / N 2 reduction was taken as the representative value of the oxygen storage amount. The sample amount was 15 mg, the gas flow rate was 100 mL / min, and the measurement temperature was 700 ° C.

[3. result]
[3.1. Ni oxidation rate]
FIG. 5 shows the Ni oxidation rates of various electrodes under an oxidizing atmosphere (with or without humidification) at a temperature of 700 ° C. From FIG. 5, the following can be seen.

(1) In the case of no humidification, the Ni oxidation rate of Ni / YSZ (Experiment No. 2) increased with the passage of time. Ni / LaCZ (Experiment No. 3) suppressed Ni oxidation as compared with Ni / YSZ, but could not completely suppress Ni oxidation.
(2) When water vapor was added to the atmosphere (3% H ₂ O), Ni / LaCZ further proceeded with Ni oxidation as compared with no humidification. Although not shown, this point was the same for Ni / YSZ. In both Ni / YSZ and Ni / LaCZ, the Ni oxidation rate with humidification increased by about 20% as compared with that without humidification.
(3) In Ni / ScCZ (Experiment No. 1), Ni oxidation was remarkably suppressed as compared with Ni / YSZ and Ni / LaCZ. The Ni oxidation rate of Ni / ScCZ after holding for 50 minutes in an oxidizing atmosphere was less than 1% regardless of the presence or absence of humidification.

[3.2. XAFS spectrum]
FIG. 6 shows the XAFS spectrum of Ni / YSZ (Y _0.158 Zr _0.842 O _2). FIG. 7 shows the XAFS spectrum of Ni / ScCZ (Sc _0.132 Ce _0.104 Zr _0.764 O _2). The following can be seen from FIGS. 6 and 7.
(1) The XAFS spectrum of Ni (0 minutes) in Ni / YSZ showed the same spectrum as the reference Ni metal.
(2) The XAFS spectrum of Ni (0 minutes) in Ni / ScCZ clearly showed a spectrum different from that of Ni metal. It is considered that the reason why Ni oxidation is suppressed by ScCZ is that the coexistence of Ni and ScCZ changes the electronic state of Ni and makes it difficult to become NiO even in an oxidizing atmosphere (that is, the oxidation-reduction potential of Ni changes). Be done.

[3.3. Oxygen occlusion]
FIG. 8 shows the oxygen occlusion of _{ScCZ (Sc 0.132} Ce _0.104 Zr _0.764 O ₂ ) and CZ (Ce _0.118 Zr _0.882 O _2). From FIG. 8, the following can be seen.
(1) The oxygen occlusion amount of CZ (Experiment No. 4) at 700 ° C. was 0.033 mg. On the other hand, the oxygen occlusion amount of ScCZ at 700 ° C. was 0.047 mg. It is considered that the magnitude of the oxygen storage capacity of each electrode material corresponds to the magnitude of the effect of suppressing Ni oxidation.
(2) Although not shown, none of YSZ (Y-doped ZrO ₂ ), ScZ (Sc-doped ZrO ₂ ), and ScC (Sc-doped CeO ₂ ) has an oxygen occlusion effect due to ^{Ce 3+} ⇔ ^{Ce 4+.} The amount of oxygen stored was zero.
(3) When a part of Ce contained in CZ is replaced with Sc, the oxygen occlusion amount increases and the electronic state of Ni also changes. As a result, it is presumed that Ni is less likely to be oxidized.

Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications can be made without departing from the gist of the present invention.

The electrode according to the present invention can be used as a hydrogen electrode of a solid oxide fuel cell or a fuel electrode of a solid oxide fuel cell.

Claims (7)

An electrode having the following configuration.
(1) The electrode is
Diffusion layer and
It includes an active layer formed on the electrolyte layer side surface of the diffusion layer.
(2) The active layer is
Ni-containing particles (B) and
ScCZ particles composed of Sc-doped CeO _{2- ZrO 2 solid solution,}
It is composed of a cermet (B) containing electrolyte particles (B) made of a solid oxide other than the Sc-doped CeO _2- ZrO _{2 solid solution.} The electrode according to claim 1, wherein the ScCZ particles have a composition represented by the following formula (2).
_{Sc x Ce y Zr 1- (x} + y) O 2 ... (2)
However, 0.06 ≦ x ≦ 0.19 and 0.05 ≦ y ≦ 0.15. The electrode according to claim 1 or 2, wherein the Ni-containing particles (B) are made of a Ni or Ni—Fe alloy. The electrolyte particles (B) are
YTSZ particles consisting of yttria-stabilized zirconia and / or
The electrode according to any one of claims 1 to 3, which is made of LaCZ particles made of a La-doped CeO _{2- ZrO 2 solid solution.} The electrode according to claim 4, _{wherein the YSZ particles contain Y 2} O ₃ of 3 mol% or more and 15 mol% or less. The electrode according to any one of claims 1 to 5, wherein the active layer satisfies the following formulas (4) to (7).
30 mass% ≤ u ≤ 70 mass% ... (4)
10 mass% ≤ v ≤ 30 mass% ... (5)
30 mass% ≤ w ≤ 70 mass% ... (6)
u + v + w = 100 mass% ... (7)
However,
u is the mass ratio (mass%) of the ScCZ particles contained in the active layer.
v is the mass ratio (mass%) of the electrolyte particles (B) contained in the active layer.
w is the mass ratio (mass%) of the Ni-containing particles (B) contained in the active layer. The electrode according to any one of claims 1 to 6, which is used for a hydrogen electrode of a solid oxide fuel cell or a fuel electrode of a solid oxide fuel cell.

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