JP7324168B2 - electrode - Google Patents

Description

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

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

A SOEC has a single cell with an anode (oxygen electrode) bonded to one side of an electrolyte and a cathode (hydrogen electrode) bonded to the other side. The following materials are generally used as materials for members constituting such SOECs (see Non-Patent Documents 1 to 6).
(a) Electrolyte: yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (SSZ), samaria-doped ceria (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 and the like.

A Ni/YSZ cermet is generally used for the SOEC hydrogen electrode. However, the hydrogen electrode is supplied with steam, which is a raw material for hydrogen production, 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 progress. As a result, the electrolytic properties are degraded.
This point is the same for the SOFC. That is, at the SOFC anode (fuel electrode), water is produced by an electrode reaction. Therefore, in particular, under high-load operating conditions, the generated steam may oxidize Ni in the fuel electrode, degrading 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.; Sune Dalgaard Ebbesen, Soren Hojgaard Jensen, Anne Hauch, and Morgens Bjerg Morgensen, Chem. Rev. 2014, 114, 1069

The problem to be solved by the present invention is to provide an electrode for SOFC or SOEC that exhibits little reduction in electronic conductivity even when exposed to high-temperature steam.

In order to solve the above problems, an electrode according to the present invention has the following configuration.
(1) the electrode,
a diffusion layer;
and an active layer formed on the electrolyte layer side surface of the diffusion layer.
(2) the active layer,
Ni-containing particles (B);
ScCZ particles composed of an Sc-doped CeO ₂ —ZrO ₂ solid solution;
A cermet (B) containing electrolyte particles (B) composed of a solid oxide other than the Sc-doped CeO ₂ --ZrO ₂ solid solution.

When ScCZ is added to an electrode containing Ni-containing particles (B), oxidation of Ni contained in the Ni-containing particles (B) is remarkably suppressed. this is,
(A) Oxygen ions spilled over from the Ni-containing metal oxide film (for example, NiO) formed on the surface of the Ni-containing particles (B) are fixed to ScCZ,
(B) the water splitting function of ScCZ decomposes water vapor into oxygen and hydrogen, exposing the surroundings of ScCZ to a reducing atmosphere; and/or
(C) The coexistence of the Ni-containing particles (B) and ScCZ is considered to change the electronic state of Ni contained in the Ni-containing particles (B).

FIG. 2 is a schematic diagram of the process of oxygen absorption and desorption of CZ. 1 is a schematic diagram of a solid oxide electrolytic cell using Ni/(ScCZ+YSZ) for an active layer. FIG. 1 is a schematic diagram of a conventional solid oxide electrolytic cell using Ni/YSZ for an active layer; FIG. FIG. 4 is a schematic diagram of a Ni oxidation suppression mechanism by ScCZ.

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

An embodiment of the present invention will be described in detail below.
[1. electrode]
The electrode according to the present invention is
a diffusion layer;
and an active layer formed on the electrolyte layer side surface of the diffusion layer.

[1.1. diffusion layer]
In the present invention, the diffusion layer is made 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 laminate of a diffusion layer and an active layer, electrode reactions occur mainly within the active layer. Therefore, the diffusion layer does not necessarily have to have high ionic conductivity.

That is, the diffusion layer has at least
(a) a function for supporting the active layer formed on the surface on the electrolyte layer side;
(b) the function of diffusing the fuel (raw material for electrolysis or power generation) to the active layer;
(c) a function of transporting electrons necessary for a reduction reaction from the current collector to the active layer, or transporting electrons generated in the oxidation reaction from the active layer to the current collector;
(d) It must 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 exhibits such functions.

[1.1.1. Ni-containing particles (A)]
“Ni-containing particles (A)” refer to metal particles in which the mass ratio of Ni to the total mass of metal elements contained in the particles is 10% by 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) function as electron conductors in the diffusion layer. The composition of the Ni-containing particles (A) is not particularly limited as long as it exhibits such functions. Ni-containing particles (A) include, for example, Ni, Ni--Fe alloys, Ni--Co alloys, and the like.
The content of the Ni-containing particles (A) is not particularly limited as long as it functions as a diffusion layer. Also, 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 30-70 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.
As the electrolyte particles (A), for example,
(a) yttria-stabilized zirconia ( _YSZ ) containing 3-15 mol% _Y2O3 ;
(b) a material with the same or similar composition as the electrolyte contained in the active layer;
and so on.
Among these, YSZ is suitable for 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, electronic conductivity, etc. of the electrode. In general, if the porosity of the diffusion layer is too small, the gas diffusibility will decrease. Therefore, the diffusion layer preferably has a porosity of 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 becomes a reaction field for electrode reaction. In the present invention, the active layer is
Ni-containing particles (B);
ScCZ particles consisting of an Sc-doped CeO ₂ —ZrO ₂ solid solution;
A cermet (B) containing electrolyte particles (B) composed of a solid oxide other than the Sc-doped CeO ₂ --ZrO ₂ solid solution.

[1.2.1. Ni-containing particles (B)]
“Ni-containing particles (B)” refer to metal particles in which the mass ratio of Ni to the total mass of metal elements contained in the particles is 90% by 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) function 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 functions. Ni-containing particles (B) include, for example, Ni, Ni--Fe alloys, Ni--Co alloys, and the like. In order to obtain high catalytic activity and high electronic conductivity, the Ni-containing particles (B) preferably consist essentially of Ni only, with the remainder consisting of unavoidable impurities.

[1.2.2. ScCZ particles]
[A. Oxygen storage/release capacity]
The ScCZ particles are composed of a CeO ₂ —ZrO ₂ solid solution doped with Sc, and function as oxide ion conductors in the active layer and suppress the oxidation of Ni contained in the Ni-containing particles (B). have. CeO ₂ has the effect of imparting oxygen storage/release capacity to ZrO ₂ . On the other hand, Sc replaces the Ce site of the CeO ₂ --ZrO ₂ solid solution (CZ). When CZ is doped with an appropriate amount of Sc, decomposition of CZ during firing can be suppressed. ScCZ also has the effect of changing the electronic state of Ni contained in the Ni-containing particles (B). Therefore, when the Ni-containing particles (B) and ScCZ coexist, oxidation of Ni contained in the Ni-containing particles (B) can be suppressed. Even when the Ni-containing particles (B) contain a metal element other than Ni, a three-phase boundary (TPB) can be secured in the electrode if at least the oxidation of Ni can be suppressed.

FIG. 1 shows a schematic diagram of the oxygen absorption/desorption process of CZ. The following formula (1) shows the reaction formula of the oxygen absorption/desorption reaction of CZ. In formula (1), the reaction proceeding to the right represents an oxidation reaction, and the reaction proceeding to the left 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) depending on the oxygen partial pressure in the surrounding atmosphere. can be done. Therefore, when CZ is exposed to an oxidizing atmosphere, CZ incorporates oxygen ions in the atmosphere into the crystal lattice. On the other hand, when CZ is exposed to a reducing atmosphere, CZ releases oxygen ions in the crystal lattice into the atmosphere. This point is the same for ScCZ.

[B. composition]
ScCZ particles preferably have a composition represented by the following formula (2).
_{ScxCeyZr1- (x+y) O2} ( ₂ )
However, 0.06≦x≦0.19 and 0.05≦y≦0.15.

In 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 is too small, the heat resistance of the ScCZ particles and the function of suppressing the oxidation of Ni contained in the Ni-containing particles (B) are lowered. Therefore, x is preferably 0.06 or more. x is preferably 0.1 or more.
On the other hand, when x is excessive, phase separation occurs, the CZ structure cannot be maintained, and structural stability decreases. Therefore, x is preferably 0.19 or less.

In 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 is too small, the oxygen storage capacity may decrease, and the function of suppressing oxidation of Ni contained in the Ni-containing particles (B) may decrease. Therefore, y is preferably 0.05 or more.
On the other hand, if y is excessive, not only the oxygen ion conductivity in the electrode may decrease, but also the mechanical strength of the electrode may decrease. Therefore, y is preferably 0.15 or less.

[1.2.3. Electrolyte particles (B)]
The electrolyte particles (B) consist of solid oxides other than the Sc-doped CeO ₂ --ZrO ₂ solid solution. In the present invention, 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, ScCZ changes its state depending on the oxygen partial pressure, which also changes its ionic conductivity. 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.
As the electrolyte particles (B), for example,
(a) YSZ particles consisting of yttria-stabilized zirconia;
(b) LaCZ particles made of La-doped CeO ₂ --ZrO ₂ solid solution; 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 consist of yttria-stabilized zirconia and function as oxide ion conductors in the active layer. The content of Y contained in the YSZ particles is not particularly limited, and an optimum amount can be selected depending on the purpose.
In order to obtain relatively high ionic conductivity and relatively high mechanical strength, the YSZ particles preferably contain 3 mol % or more and 15 mol % or less of Y ₂ O ₃ . 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 CeO ₂ —ZrO ₂ solid solution doped with La, and function as oxide ion conductors in the active layer and suppress the oxidation of Ni contained in the Ni-containing particles (B). have CeO ₂ has the effect of imparting oxygen storage/release capacity 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 CZ. Therefore, when CZ is doped with an appropriate amount of La, decomposition of CZ during firing can be suppressed.
LaCZ particles are inferior to ScCZ particles in terms of their ability to suppress oxidation. However, the additional inclusion of LaCZ particles in the electrode may improve thermal stability.

LaCZ particles are particularly preferably those having a composition represented by the following formula (3).
_{LaxCe2 -xZr2O7 ( 3} )
However, 0<x<0.2.

In 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 will decrease. Therefore x must be greater than zero. x is preferably 0.05 or more, more preferably 0.1 or more.
On the other hand, when x is excessive, the oxygen storage/release capacity of the LaCZ particles is lowered. Therefore, x is preferably less than 0.2. x is preferably 0.15 or less.

[1.2.4. Composition of active layer]
In the present invention, the active layer preferably contains predetermined amounts of ScCZ particles and electrolyte particles (B), with the remainder consisting of Ni-containing particles (B) and unavoidable impurities. Also, the active layer preferably satisfies the following formulas (4) to (7).
30mass%≤u≤70mass% (4)
10mass%≤v≤30mass% (5)
30mass%≤w≤70mass% (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 low, 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, if the content of ScCZ particles is excessive, the cell total resistance rather increases and the electrode reaction efficiency 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, resulting in a decrease in reaction active sites and an increase in ohmic resistance due to an increase in 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) is excessive, the ratio of the ScCZ particles in the active layer is reduced, 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 low, the catalytic activity and electronic conductivity of the active layer are lowered. 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 ratio of the ScCZ particles in the active layer decreases, so that the total cell resistance increases and the efficiency 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 properties or power generation properties. If the porosity of the active layer is too small, gas diffusibility is lowered and the electrode reaction efficiency is lowered. 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 is too high, the number of three-phase interfaces will be relatively small, and the electrode reaction efficiency will be lowered. 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. Application]
The electrode according to this embodiment can be used as the hydrogen electrode of a solid oxide electrolytic cell or the fuel electrode of a solid oxide fuel cell.

[2. Electrode manufacturing method]
The electrode according to the present invention is
(a) preparing a diffusion layer compact using a raw material mixture (A) containing a raw material for the Ni-containing particles (A) and a raw material for the electrolyte particles (A);
(b) forming an active layer compact on the surface of the diffusion layer compact using the raw material mixture (B) containing the raw material for the Ni-containing particles (B), the ScCZ powder, and the raw material for the electrolyte particles (B);
(c) sintering the resulting laminate;
(d) It can be produced by reducing the obtained sintered body.

[2.1. Diffusion Layer Forming Process]
First, a raw material mixture (A) containing a raw material for the Ni-containing particles (A) and a raw material for the electrolyte particles (A) is used to produce a diffusion layer molded body (diffusion layer molded body manufacturing step).

The type of raw material for the Ni-containing particles (A) is not particularly limited, and an optimum raw material can be selected according to the purpose. Raw materials for the Ni-containing particles (A) include, for example, NiO powder, Fe ₂ O ₃ powder, Fe ₃ O ₄ powder, a mixture of metallic Fe and NiO or metallic Ni, CoO powder, Co ₂ O ₃ powder, and the like. .
The type of raw material for the electrolyte particles (A) is not particularly limited, and an optimum raw material can be selected according to the purpose.
For example, when the electrolyte particles (A) are YSZ, the raw material is
(a) a YSZ powder having a desired composition;
(b) A mixture of ZrO ₂ powder and Y ₂ O ₃ powder blended so as to have the desired composition.

In addition, 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 necessarily required. However, adding a pore-forming material to the raw material mixture (A) increases the degree of freedom in controlling the porosity.

The method for producing the diffusion layer compact is not particularly limited, and an optimum method can be selected according to the purpose. Examples of methods for producing a diffusion layer compact include:
(a) A method of forming a slurry containing the raw material mixture (A) into a tape, laminating a plurality of the obtained green sheets, and isostatically pressing the laminated body to bond it together;
(b) a method of press-molding the raw material mixture (A) with a mold;
and so on.

[2.2. Active Layer Forming Forming Process]
Next, an active layer compact is formed on the surface of the diffusion layer compact using the raw material mixture (B) containing the raw material for the Ni-containing particles (B), the ScCZ powder, and the raw material for the electrolyte particles (B) (active layer compact). step of producing a layer compact).

If a mixture of CZ powder and Sc source is used as a raw material for forming the active layer, CZ may decompose during firing. Therefore, it is preferable to use ScCZ powder doped with a predetermined amount of Sc as the raw material for forming the active layer.
Furthermore, 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, so the description is omitted. do.

The method for producing the active layer compact is not particularly limited, and an optimum method can be selected according to the purpose. Examples of methods for producing the active layer compact include:
(a) A method of tape-molding a slurry containing the raw material mixture (B), laminating the obtained green sheet on the diffusion layer compact, and isostatically pressing the laminate to bond it;
(b) a method of preparing a slurry containing the raw material mixture (B) and screen-printing the slurry onto the surface of the diffusion layer compact;
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 air 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 raw material mixture contains a pore-forming material, the pore-forming material disappears during sintering, and pores are formed in the sintered body.

[2.4. Reduction process]
Next, the obtained sintered body is subjected to reduction treatment (reduction step). Thereby, an electrode according to the present invention is obtained. The reduction treatment is performed to reduce metal oxides such as NiO contained in the sintered body to produce Ni-containing particles (A) and Ni-containing particles (B). Reduction conditions are not particularly limited, and it is preferable to select optimum conditions according to the composition of the electrode.
As will be described later, the solid oxide electrolytic cell is composed of a hydrogen electrode (cathode)/electrolyte layer/reaction prevention layer/oxygen electrode (anode) assembly. Reduction of the hydrogen electrode is usually performed after bonding the layers. This point also applies to solid oxide fuel cells.

[3. Solid oxide electrolytic cell and polymer electrolyte fuel cell]
FIG. 2 shows a schematic diagram of a solid oxide electrolytic cell using Ni/(ScCZ+YSZ) for the active layer. In FIG. 2, a solid oxide electrolysis cell (SOEC) 10 is
an electrolyte 12;
a hydrogen electrode 14 bonded to one side of the electrolyte 12;
an oxygen electrode 16 joined to the other surface of the electrolyte 12;
It has an intermediate layer 18 interposed between the electrolyte 12 and the oxygen electrode 16 .

An 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 an active layer 14a and a diffusion layer 14b supporting the active layer 14a. In FIG. 2, the active layer 14a is made of cermet (B) containing Ni particles, ScCZ particles, and YSZ particles.
Materials for the electrolyte 12, the oxygen electrode 16, and the intermediate layer 18 are not particularly limited, and optimum materials can be selected according to the purpose.

For example, YSZ or the like can be used for the electrolyte 12 .
(La,Sr)CoO ₃ (LSC), (La,Sr)(Co,Fe)O ₃ (LSCF), (La,Sr)MnO ₃ (LSM), or the like can be used for the oxygen electrode 16 .
The intermediate layer 18 is a layer for preventing reactions 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 .

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

[3. action]
FIG. 3 shows a schematic diagram of a conventional solid oxide electrolytic cell using Ni/YSZ for the active layer. A conventional SOEC 10' includes an electrolyte 12, a hydrogen electrode 14' bonded to one side of the electrolyte 12, an oxygen electrode 16 bonded to the other side of the electrolyte 12, and a and an intermediate layer 18 inserted into the . The conventional SOEC 10' uses Ni--YSZ cermet as the hydrogen electrode 14'.

In the SOEC 10' where the hydrogen electrode 14' is made of Ni--YSZ cermet, when H ₂ O is supplied to the hydrogen electrode 14' and a current is passed between the hydrogen electrode 14' and the oxygen electrode 16, the following occurs at the hydrogen electrode 14': A reductive reaction represented by the formula (8) proceeds.
_H2O +2e ^-- > _H2 + ^O2- (8)

However, in water electrolysis using the SOEC 10', high-temperature (700° C. or higher) water vapor 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 degraded.
This point is the same for the SOFC. That is, at the SOFC anode (fuel electrode), water is produced by an electrode reaction. Therefore, especially under high-load operating conditions, the generated water may oxidize Ni in the fuel electrode, degrading 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. CZ has an oxygen storage/release capacity in addition to its function as an oxide ion conductor. Therefore, when CZ is added to an electrode made of Ni/YSZ cermet, oxidation of Ni can be suppressed to some extent.
However, when sintering a mixture of Ni, YSZ, and CZ to produce an electrode, the CZ is exposed to high temperatures, and some of the CZ may decompose to form another phase. 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, La-doped CeO ₂ --ZrO ₂ solid solution (LaCZ) has a higher thermal stability of the crystal structure than CZ. Therefore, decomposition of LaCZ is suppressed even when a mixture of Ni, YSZ, and LaCZ is fired at a high temperature. The electrode in which decomposition of LaCZ is suppressed maintains high oxygen storage/release capacity. Therefore, when steam electrolysis or power generation is performed using this, the oxidation of Ni is suppressed, and the electrolysis performance or power generation performance is improved. However, the oxidation resistance of electrodes containing LaCZ is poor.

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

FIG. 4 shows a schematic diagram of the Ni oxidation suppression mechanism by ScCZ. In the SOEC cell, the addition of ScCZ to the hydrogen electrode significantly suppresses the oxidation of Ni for the following reasons.

[A. Fixation of oxygen ions by ScCZ]
When the hydrogen electrode containing Ni and ScCZ is exposed to high-temperature steam, Ni is oxidized and the surfaces of Ni particles are coated with NiO (step 1). At this time, if there are ScCZ particles in the vicinity of 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 under the reducing atmosphere, ScCZ easily releases the trapped oxygen atoms or oxygen ions as oxygen molecules. Oxygen molecules released from ScCZ are discharged out of the system through the diffusion layer before re-oxidizing Ni (step 4). It is considered that Ni oxidation is suppressed by repeating steps 1 to 4 as described above.

[C. Change in electronic state of Ni]
As will be described later, the electronic state of Ni present in the Ni/YSZ cermet is almost the same as the electronic state of Ni present alone. On the other hand, the electronic state of Ni present in the Ni/ScCZ cermet is different from the electronic state when Ni exists alone. The reason why the Ni/ScCZ cermet exhibits much higher oxidation resistance than the Ni/YSZ cermet and the Ni/LaCZ cermet is considered to be that the electronic state of Ni changes due to the coexistence of Ni and ScCZ. .
This point is the same when using Ni-containing particles (B) containing an element other than Ni as an electrode catalyst. It is thought that the electronic state of Ni contained in the Ni-containing particles (B) changes, thereby suppressing 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]
Electrolyte sources include:
(a) ScCZ powder _consisting of _{Sc0.132Ce0.104Zr0.764O2 solid} solution (Experiment No. ₁ ),
_{(b) YSZ powder consisting of Y0.158Zr0.842O2 solid solution} (Experiment No. 2),
( _c ) LaCZ powder consisting of _{La0.25Ce0.25Zr0.5O2} solid solution ( _Experiment No. 3), _or
( _d ) CZ powder _composed of _{Ce0.118Zr0.882O2} solid solution (Experiment No. 4)
was used.
NiO powder was used as the Ni source.

An electrolyte source and NiO powder were blended at 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 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 storage capacity]
The electrolyte powder was directly used for measurement of oxygen storage capacity.

[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 X-ray absorption microstructure (XAFS) measurement. A 4% H ₂ /He balance gas (100 cc/min total) was introduced into the cell and the temperature was raised to 780° C. (60° C./min). After that, the temperature was lowered to 700° C. and 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).

XAFS spectra were taken from the time the He purge was initiated. Also, Ni and NiO were used as reference spectra. Table 1 shows the details of the XAFS measurement conditions.
Furthermore, 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 storage capacity]
The oxygen storage capacity of the ScCZ powder and the CZ powder was measured using a thermogravimetric analyzer. Under a fluctuating atmosphere in which the H ₂ (5%)/N ₂ atmosphere and the O ₂ (5%)/N ₂ atmosphere are switched three times every 5 minutes (that is, hold for 5 minutes in the H ₂ atmosphere → 5 minutes in the O ₂ atmosphere). Hold→Hold for 5 minutes under H ₂ atmosphere) to measure the change in the weight of the sample. The weight loss during the second H ₂ (5%)/N ₂ reduction was taken as the representative value of the oxygen storage capacity. 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 rate of various electrodes at a temperature of 700° C. under an oxidizing atmosphere (with or without humidification). From FIG. 5, the following can be understood.

(1) Without humidification, Ni/YSZ (Experiment No. 2) increased in Ni oxidation rate over time. Ni/LaCZ (Experiment No. 3) suppressed Ni oxidation as compared to Ni/YSZ, but could not completely suppress Ni oxidation.
(2) When water vapor (3% H ₂ O) was added to the atmosphere, Ni oxidation progressed further in Ni/LaCZ compared to no humidification. Although not shown, this point was the same for Ni/YSZ. For both Ni/YSZ and Ni/LaCZ, the Ni oxidation rate with humidification increased by about 20% compared to that without humidification.
(3) Ni/ScCZ (Experiment No. 1) significantly inhibited Ni oxidation compared to 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 ). 6 and 7 show the following.
(1) The XAFS spectrum of Ni (0 min) in Ni/YSZ showed the same spectrum as the reference Ni metal.
(2) XAFS spectrum of Ni (0 min) in Ni/ScCZ showed a spectrum distinct from Ni metal. The reason why the oxidation of Ni is suppressed by ScCZ is thought to be that the coexistence of Ni and ScCZ changes the electronic state of Ni, making it less likely to become NiO even in an oxidizing atmosphere (that is, the oxidation-reduction potential of Ni changes). be done.

[3.3. Oxygen storage capacity]
FIG. 8 shows the oxygen storage capacity 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 understood.
(1) The oxygen storage capacity of CZ (Experiment No. 4) at 700°C was 0.033 mg. On the other hand, the oxygen storage capacity 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 storage effect due to Ce ³⁺ ⇔ Ce ⁴⁺ , The oxygen storage capacity was zero.
(3) Replacing a part of Ce contained in CZ with Sc increases the oxygen storage capacity and also changes the electronic state of Ni. As a result, it is presumed that Ni became difficult to be oxidized.

Although the embodiments of the present invention have been described in detail above, the present invention is by no means limited to the above-described embodiments, and various modifications are possible without departing from the gist of the present invention.

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

Claims (6)

An electrode with the following configuration.
(1) the electrode,
a diffusion layer;
and an active layer formed on the electrolyte layer side surface of the diffusion layer.
(2) the active layer,
Ni-containing particles (B);
ScCZ particles consisting of an Sc-doped CeO ₂ —ZrO ₂ solid solution;
A cermet (B) containing electrolyte particles (B) made of a solid oxide other than the Sc-doped CeO ₂ --ZrO ₂ solid solution.
(3) The ScCZ particles have a composition represented by the following formula (2).
ScxCeyZr1- _{( x} + _{y) O2} ( 2 )
However, 0.06≦x≦0.19 and 0.05≦y≦0.15. 2. The electrode according to claim 1 , wherein said Ni-containing particles (B) are made of Ni or a Ni--Fe alloy. The electrolyte particles (B) are
YSZ particles made of yttria-stabilized zirconia, and/or
3. The electrode according to claim 1, comprising LaCZ particles of La-doped CeO ₂ --ZrO ₂ solid solution. _4. The electrode according to claim 3 , wherein the YSZ particles contain 3 mol % or more and 15 mol % or less of _Y2O3 . 5. The electrode according to any one of claims 1 to 4, wherein the active layer satisfies the following formulas (4) to (7).
30mass% ≤u≤60mass% (4)
10mass%≤v≤30mass% (5)
30mass% ≤w≤60mass% (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. 6. The electrode according to any one of claims 1 to 5, which is used for a hydrogen electrode of a solid oxide electrolytic cell or a fuel electrode of a solid oxide fuel cell.

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