JP7114555B2 - Electrodes for water vapor electrolysis - Google Patents

Description

TECHNICAL FIELD The present invention relates to a steam electrolysis electrode, and more particularly to a steam electrolysis electrode suitable as a hydrogen electrode for a solid oxide electrolysis cell (SOEC).

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 of the SOFC, and O ₂ is supplied to the cathode, 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 oxygen electrode bonded to one side of the electrolyte and a 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, steam as a fuel is supplied to the hydrogen electrode at a high temperature (700° C. or higher). Therefore, Ni contained in the 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 electrode reaction does not proceed. As a result, the electrolytic properties are degraded.

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 steam electrolysis in which the decrease in electronic conductivity is small even when exposed to high-temperature steam.

In order to solve the above problems, the steam electrolysis electrode according to the present invention has the following configuration.
(1) The water vapor electric field electrode is
a diffusion layer;
and an active layer formed on the electrolyte layer side surface of the diffusion layer.
(2) the diffusion layer,
Ni particles (A);
A cermet (A) containing electrolyte particles (A) made of a solid oxide.
(3) the active layer,
Ni particles (B);
YSZ particles made of yttria-stabilized zirconia;
cermet (B) containing LCZ particles made of La-doped CeO ₂ --ZrO ₂ solid solution.

A CeO ₂ —ZrO ₂ solid solution (CZ) has an oxygen storage/release capability 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.

In contrast, La-doped CeO ₂ --ZrO ₂ solid solution (LCZ) has a higher thermal stability of the crystal structure than CZ. Therefore, decomposition of LCZ is suppressed even when a mixture of Ni, YSZ, and LCZ is fired at a high temperature. An electrode in which decomposition of LCZ is suppressed maintains a high oxygen storage/release capacity. Therefore, when steam electrolysis is performed using this, the oxidation of Ni is suppressed and the steam electrolysis performance is improved.

FIG. 3 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/(YSZ+LCZ) 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. It is a schematic diagram of the Ni oxidation suppression mechanism by LCZ. 4 shows IV characteristics of SOECs obtained in Example 1 and Comparative Example 1. FIG. 1 is a comparison of the electrolysis efficiency of SOECs obtained in Example 1 and Comparative Example 1. FIG.

FIG. 7A is an X-ray diffraction pattern of La _x Ce _2-x Zr ₂ O ₇ (x=0.125) before heat treatment. FIG. 7(B) is an X-ray diffraction pattern after heat treatment at 1450° C. of La _x Ce _2-x Zr ₂ O ₇ (x=0.125). FIG. 8A is an X-ray diffraction pattern of La _x Ce _2-x Zr ₂ O ₇ (x=0.25) before heat treatment. FIG. 8(B) is an X-ray diffraction pattern after heat treatment at 1450° C. of La _x Ce _2-x Zr ₂ O ₇ (x=0.25).

An embodiment of the present invention will be described in detail below.
[1. Electrodes for water vapor electrolysis]
The water vapor electric field 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 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, the reduction reaction occurs 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 fuel (raw material for electrolysis) to the active layer;
(c) the function of transporting electrons necessary for the reduction reaction from the current collector to the active layer, and
(d) It must have a function of discharging hydrogen generated in the active layer by a reduction 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 particles (A)]
The content of the Ni particles (A) is not particularly limited as long as it functions as a diffusion layer. Also, the content of the Ni particles (A) contained in the diffusion layer may be the same as or different from the content of the Ni particles (B) contained in the active layer. The content of Ni particles (A) is usually 30 to 70 wt%.

[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 made of a cermet (B) containing Ni particles (B), YSZ particles made of yttria-stabilized zirconia, and LCZ particles made of La-doped CeO ₂ --ZrO ₂ solid solution.

[1.2.1. Ni particles (B)]
The Ni particles (B) function as an electrode catalyst and an electron conductor in the active layer. In order to obtain high catalytic activity and high electronic conductivity, it is preferable that the Ni particles (B) consist essentially of Ni only, with the remainder consisting of unavoidable impurities.

[1.2.2. YSZ particles]
The YSZ grains consist of yttria-stabilized zirconia and function as oxygen 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 .

[1.2.3. LCZ particles]
[A. Oxygen storage/release capacity]
The LCZ particles consist of a La-doped CeO ₂ --ZrO ₂ solid solution and have the function of suppressing the oxidation of the Ni particles (B) in the active layer. 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.

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 LCZ.

[B. composition]
The LCZ particles particularly preferably have a composition represented by the following formula (2).
_{LaxCe2 - xZr2O7} ( ₂ )
However, 0<x<0.2.

In formula (2), x represents the number of moles of La with respect to the total number of moles of La and Ce contained in the LCZ particles. If x becomes too small, the heat resistance of the LCZ 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 LCZ 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]
The active layer preferably satisfies the following formulas (3) to (6).
40mass%≤u≤70mass% (3)
30mass%≤v≤70mass% (4)
0<w≤20mass% (5)
u+v+w=100 mass% (6)
however,
u is the mass ratio (mass%) of the Ni particles (B) contained in the active layer of the electrode for steam electrolysis;
v is the mass ratio (mass%) of the YSZ particles contained in the active layer of the electrode for steam electrolysis;
w is the mass ratio (mass%) of the LCZ particles contained in the active layer of the electrode for steam electrolysis.

[A. Content of Ni particles (B)]
"u" represents the ratio of the mass of Ni particles (B) to the total mass of Ni particles (B), YSZ particles, and LCZ particles. If u becomes too small, the total cell resistance will increase and the electrolysis efficiency will also decrease. Therefore, u is preferably 40 mass% or more.

On the other hand, if u is too large, the proportion of YSZ particles in the active layer will decrease, so that the total cell resistance will rather increase and the electrolysis efficiency will decrease. Therefore, u is preferably 70 mass% or less. u is preferably 60 mass% or less.

[B. Content of YSZ particles]
"v" represents the ratio of the mass of YSZ particles (B) to the total mass of Ni particles (B), YSZ particles, and LCZ particles. If v becomes too small, the total cell resistance will increase and the electrolysis efficiency will also decrease. Therefore, v is preferably 30 mass% or more. v is preferably 40 mass% or more.
On the other hand, if v becomes too large, the ratio of Ni particles decreases, resulting in a decrease in electrolysis efficiency. Therefore, v is preferably 70 mass% or less. v is preferably 60 mass% or less.

[C. Content of LCZ particles]
"w" represents the ratio of the mass of LCZ particles (B) to the total mass of Ni particles (B), YSZ particles and LCZ particles. If w is too small, the oxidation of the Ni particles (B) cannot be sufficiently suppressed. Therefore, w is preferably more than 0 mass%. w is preferably 5 mass % or more.
On the other hand, if w is too large, the cell total resistance rather increases and the electrolysis efficiency decreases. Therefore, w is preferably 20 mass% or less.

[1.2.5. Porosity]
The porosity of the active layer affects the electrolytic properties. If the porosity of the active layer is too small, gas diffusibility is lowered and the electrolysis 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 large, the number of three-phase interfaces will be relatively small, resulting in deterioration of the electrolytic properties. Therefore, the porosity of the active layer is preferably 40% or less. The porosity is preferably 35% or less, more preferably 30% or less.

[2. Method for producing electrode for water vapor electrolysis]
The steam electrolysis electrode according to the present invention is
(a) preparing a diffusion layer compact using a raw material mixture (A) containing NiO powder and raw materials for electrolyte particles (A);
(b) forming an active layer compact on the surface of the diffusion layer compact using a raw material mixture (B) containing NiO powder, YSZ powder, and LCZ powder;
(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 NiO powder and raw materials for electrolyte particles (A) is used to produce a diffusion layer compact (diffusion layer compact producing step).

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 NiO powder added to the raw material mixture (A) is subjected to a reduction treatment 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.
(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, a raw material mixture (B) containing NiO powder, YSZ powder, and LCZ powder is used to form an active layer compact on the surface of the diffusion layer compact (active layer compact manufacturing step).

If CZ powder is used as a raw material for forming the active layer, CZ may decompose during firing. Therefore, it is preferable to use LCZ powder doped with a predetermined amount of La 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 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, the electrode for steam electrolysis according to the present invention is obtained. The reduction treatment is performed to reduce NiO particles contained in the sintered body to Ni particles. 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.

[3. Solid oxide electrolytic cell]
FIG. 2 shows a schematic diagram of a solid oxide electrolytic cell using Ni/(YSZ+LCZ) 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 .

The electrode for steam electrolysis 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. The active layer 14a is made of cermet (B) containing Ni particles, YSZ particles and LCZ 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 .

[4. 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 (7) proceeds.
_H2O +2e ^-- >H2 ₊ ^O2- (7)

However, in the water electrolysis using the SOEC 10', the hydrogen electrode 14' is supplied with high-temperature (700°C or higher) water vapor as a raw material. 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 progress. As a result, the electrolytic properties are degraded.

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.

In contrast, La-doped CeO ₂ --ZrO ₂ solid solution (LCZ) has a higher thermal stability of the crystal structure than CZ. Therefore, decomposition of LCZ is suppressed even when a mixture of Ni, YSZ, and LCZ is fired at a high temperature. An electrode in which decomposition of LCZ is suppressed maintains a high oxygen storage/release capacity. Therefore, when steam electrolysis is performed using this, the oxidation of Ni is suppressed and the steam electrolysis performance is improved.

FIG. 4 shows a schematic diagram of the Ni oxidation suppression mechanism by LCZ. Suppression of Ni oxidation by LCZ is thought to proceed as follows.
That is, when the hydrogen electrode composed of Ni/(YSZ+LCZ) 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 LCZ 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 LCZ.

Oxygen spilled over from NiO is temporarily trapped in the LCZ (step 3). The hydrogen electrode is in a reducing atmosphere due to the generated H ₂ , but under the reducing atmosphere, the LCZ easily releases the trapped oxygen atoms or oxygen ions as oxygen molecules. Oxygen molecules released from the LCZ 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.

(Example 1, Comparative Example 1)
[1. Production of SOEC]
[1.1. Example 1]
[1.1.1. Production of Diffusion Layer Molded Body]
YSZ powder containing 8 mol % Y ₂ O ₃ (hereinafter also referred to as “8YSZ powder”) was used as the raw material for the electrolyte particles (A). NiO powder was used as the Ni source. 8YSZ powder and NiO powder were blended so that 8YSZ:Ni=50:50 (weight ratio), and a solvent and a binder were added to the mixed powder to prepare a slurry. Tape molding was performed using this slurry to obtain a green sheet (diffusion layer molding).

[1.1.2. Preparation of Active Layer Molded Body]
8YSZ powder was used as the YSZ source. An LCZ powder composed of a CeO ₂ —ZrO ₂ solid solution containing La (x=0.125) was used as the LCZ source. Furthermore, NiO powder was used as the Ni source. 8YSZ powder, LCZ powder, and NiO powder were blended at a mass ratio of 48:4:48, and a solvent and a binder were added to the mixed powder to obtain slurry. Tape molding was performed using this slurry to obtain a green sheet (active layer molding).

[1.1.3. Production of Electrolyte Layer Molded Body]
A solvent and a binder were added to the 8YSZ powder to obtain a slurry. Tape molding was performed using this slurry to obtain a green sheet (electrolyte layer molding).

[1.1.4. Production of Intermediate Layer Molded Body]
A solvent and a binder were added to the GDC powder to obtain a slurry. Tape molding was performed using this slurry to obtain a green sheet (intermediate layer molding).

[1.1.5. Primary firing]
A predetermined number of diffusion layer moldings were superimposed. Next, an active layer molded body, an electrolyte layer molded body, and an intermediate layer molded body were further superposed thereon in this order. The resulting laminate was isostatically pressed. Further, the obtained compact was fired at 1400°C.

[1.1.6. Preparation of oxygen electrode]
A solvent and a binder were added to the LSC powder to obtain a slurry. This slurry was applied to the surface of the intermediate layer. Furthermore, the coating film was baked at 1000° C. to form an oxygen electrode. The area of the oxygen electrode was 0.5 cm ² .

[1.2. Comparative Example 1]
An SOEC was fabricated in the same manner as in Example 1, except that no LCZ powder was used in fabricating the active layer.

[2. Test method]
Steam electrolysis was performed using the obtained SOEC. The cell temperature was 700°C. Moreover, the electrolysis was performed under the condition that the ratio of H ₂ O/H ₂ =1.

[3. result]
FIG. 5 shows IV characteristics of SOECs obtained in Example 1 and Comparative Example 1. FIG. 5 that Example 1 has higher IV performance than Comparative Example 1. FIG. This is probably because the addition of LCZ to the active layer suppressed the oxidation of Ni.

FIG. 6 shows a comparison of the electrolysis efficiencies of the SOECs obtained in Example 1 and Comparative Example 1. In FIG. The electrolysis efficiency of Comparative Example 1 was 68.7%. On the other hand, the electrolysis efficiency of Example 1 was 94.9%, which was about 1.4 times that of Comparative Example 1.

(Example 2)
[1. Preparation of sample]
Two types of La _x Ce _2-x Zr ₂ O ₇ (LCZ) powders with x=0.125 or 0.25 were tested.

[2. Test method]
The LCZ powder was heat treated at 1450° C. for 1 hour in air. Before and after the heat treatment, the XRD pattern of the LCZ powder was measured to identify the generated phase.

[3. result]
7(A) and 7(b) show the X-ray diffraction patterns of La _x Ce _2-x Zr ₂ O ₇ (x=0.125) before heat treatment and after heat treatment at 1450, respectively. 8A and 8B show the X-ray diffraction patterns of La _x Ce _2-x Zr ₂ O ₇ (x=0.25) before heat treatment and after heat treatment at 1450° C., respectively. 7 and 8, when x = 0.125, the single phase is maintained even after the heat treatment, whereas when x = 0.25, the LCZ is separated into two phases after the heat treatment. I understand. These results show that x is preferably less than 0.2.

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

Claims (4)

An electrode for water vapor electrolysis having the following configuration.
(1) The water vapor electric field electrode is
a diffusion layer;
and an active layer formed on the electrolyte layer side surface of the diffusion layer.
(2) the diffusion layer,
Ni particles (A);
A cermet (A) containing electrolyte particles (A) made of a solid oxide.
(3) the active layer,
Ni particles (B);
YSZ particles made of yttria-stabilized zirconia;
cermet (B) containing LCZ particles made of La-doped CeO ₂ --ZrO ₂ solid solution. The electrode for steam electrolysis according to claim ₁ , wherein the YSZ particles contain ₃ mol% or more and 15 mol% or less of Y2O3. 3. The electrode for steam electrolysis according to claim 1, wherein the LCZ particles have a composition represented by the following formula (2).
_{LaxCe2 - xZr2O7} ( ₂ )
However, 0<x<0.2. The electrode for steam electrolysis according to any one of claims 1 to 3, which satisfies the following formulas (3) to (6).
40mass%≤u≤70mass% (3)
30mass%≤v≤70mass% (4)
0<w≤20mass% (5)
u+v+w=100 mass% (6)
however,
u is the mass ratio (mass%) of the Ni particles (B) contained in the active layer of the electrode for steam electrolysis;
v is the mass ratio (mass%) of the YSZ particles contained in the active layer of the electrode for steam electrolysis;
w is the mass ratio (mass%) of the LCZ particles contained in the active layer of the electrode for steam electrolysis.

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