JP7755535B2 - Cermet layer and hydrogen electrode for steam electrolysis - Google Patents

Description

The present invention relates to a cermet layer and a hydrogen electrode for steam electrolysis, and more specifically to a cermet layer containing two types of composite oxides with different compositions as oxide ion conductors, electron conductors, or Ni oxidation inhibitors, and a hydrogen electrode for steam electrolysis.

A solid oxide fuel cell (SOFC) is a fuel cell that uses an oxide ion conductor as an electrolyte. When fuel gases such as _H2 , CO, and _CH4 are supplied to the anode (fuel electrode) of the SOFC and _O2 is supplied to the cathode (oxygen electrode), an electrode reaction occurs and electricity can be generated. _CO2 and _H2O produced by the electrode reaction are discharged outside the SOFC.
On the other hand, solid oxide electrolysis cells (SOECs) have the same structure as SOFCs, but they operate in the opposite direction to the SOFCs: when _CO2 or _H2O is supplied to the cathode (hydrogen electrode) of an SOEC and a current is passed between the electrodes, CO and _H2 are produced.

An SOEC includes a single cell in which an anode (oxygen electrode) is bonded to one side of an electrolyte and a cathode (hydrogen electrode) is bonded to the other side. The following materials are generally used as materials for the components that make up such an SOEC (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), etc.
(b) Oxygen electrode: lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium cobaltite (LSC), etc.
(c) Hydrogen electrode: Ni/YSZ, Ni/SDC, Ni—Fe/SDC, etc.

Ni/YSZ cermet is generally used for the hydrogen electrode of an SOEC. The hydrogen electrode may also have a two-layer structure with an active layer on the electrolyte side and a diffusion layer on the separator side. However, water vapor, the raw material for hydrogen production, is supplied to the hydrogen electrode at high temperatures (above 700°C). This causes the Ni contained in the hydrogen electrode to easily oxidize, forming NiO. Because NiO is an insulator, when NiO forms in the hydrogen electrode, the electron path is interrupted in that area, preventing the electrolytic reaction from proceeding. As a result, the electrolytic characteristics deteriorate.

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 of the present invention is to provide a cermet layer that exhibits little decrease in electronic conductivity even when exposed to high-temperature water vapor.
Another object of the present invention is to provide a hydrogen electrode for steam electrolysis that exhibits little decrease in electronic conductivity even when exposed to high-temperature steam.

In order to solve the above problems, the cermet layer according to the present invention has the following configuration.
(1) The cermet layer is
Ni-containing particles;
first YCZ particles made of a composite oxide of _Y2O3 , _CeO2 , and _{ZrO2 ;}
second YCZ particles made of a composite oxide of _Y2O3 , _CeO2 , and _{ZrO2 ;}
The cermet comprises:
(2) The Ce content in the second YCZ particles is less than the Ce content in the first YCZ particles.

The hydrogen electrode for steam electrolysis according to the present invention is equipped with the cermet layer according to the present invention.

When the raw material mixture containing the Ni-containing particles, the first YCZ particles (e.g., third YCZ powder having a predetermined composition), and the second YCZ particles (e.g., YSZ powder having a predetermined composition) is fired as raw materials for producing the cermet layer, some of the Ce contained in the first YCZ particle raw material (e.g., third YCZ powder) diffuses into the second YCZ particle raw material (e.g., YSZ powder). As a result, first YCZ particles (YCZ particles derived from the third YCZ powder) containing a relatively large amount of Ce and second YCZ particles (YCZ particles derived from the YSZ powder) containing a relatively small amount of Ce are produced.

When steam electrolysis is performed using an SOEC equipped with a hydrogen electrode for steam electrolysis that includes such a cermet layer (particularly, a hydrogen electrode for steam electrolysis whose active layer is made of such a cermet layer), the deterioration of the electrode performance of the hydrogen electrode is suppressed.
(a) In a water vapor atmosphere, the Ni-containing particles are oxidized, but the first YCZ particles and/or the second YCZ particles occlude (trap) and remove oxygen adsorbed on Ni, thereby suppressing the oxidation of the Ni-containing particles; and
(b) This is thought to be because the first YCZ particles, which contain a relatively large amount of Ce, also function as mixed electron/ion conductors, and therefore electrons and oxide ions are conducted via the first YCZ particles even when the Ni-containing particles are oxidized.
Furthermore, the second YCZ particles also have the function of suppressing a decrease in the strength of the cermet layer, so that when such a cermet layer is used in the hydrogen electrode of an SOEC, the decrease in electronic conductivity and oxide ion conductivity is suppressed and the durability of the SOEC is improved.

FIG. 1 is a schematic diagram of a solid oxide electrolysis cell using Ni/{YCZ(1)+YCZ(2)} as the active layer. FIG. 1 is a schematic diagram of a conventional solid oxide electrolysis cell using Ni/YSZ as an active layer. FIG. 1 is a schematic diagram of the mechanism by which Ni oxidation is suppressed in a hydrogen electrode having a cermet layer containing YCZ(1) and YCZ(2). 1 shows elemental mapping of Ce obtained by SEM/EDX analysis of the active layer obtained in Example 1. 1 shows the electrolysis current deterioration rates of the electrolysis cells obtained in Example 1 and Comparative Example 1.

An embodiment of the present invention will be described in detail below.
[1. Cermet layer]
The cermet layer according to the present invention comprises:
Ni-containing particles;
first YCZ particles made of a composite oxide of _Y2O3 , _CeO2 , and _{ZrO2 ;}
second YCZ particles made of a composite oxide of _Y2O3 , _CeO2 , and _{ZrO2 ;}
The cermet comprises:

The cermet layer is particularly
(a) producing a compact using a raw material mixture including a raw material of Ni-containing particles, a third YCZ powder having a predetermined composition, and a YSZ powder having a predetermined composition;
(b) sintering the compact in an air atmosphere at a temperature of 1000°C or higher and 1400°C or lower;
(c) The sintered body is preferably further heated at a temperature of 600° C. or more and 800° C. or less in a hydrogen reducing atmosphere.

[1.1. Ni-containing particles]
The "Ni-containing particles" refer to particles containing Ni contained in a cermet layer, in which the mass ratio 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 is preferably 95 mass% or more.
The Ni-containing particles function as an electrode catalyst and an electron conductor in the cermet layer. The composition of the Ni-containing particles is not particularly limited as long as they exhibit these functions.

Examples of Ni-containing particles include Ni, Ni-Fe alloys, and Ni-Co alloys. Of these, Ni or Ni-Fe alloys are preferred as Ni-containing particles.

[1.2. 1st YCZ particle, 2nd YCZ particle]
[1.2.1. Definition]
The "first YCZ particles" refer to particles made of _a composite oxide of _Y2O3 , _CeO2 , and _ZrO2 , and having a higher Ce content than the second YCZ particles.
The "second YCZ particles" refer to particles made of _a composite oxide of _Y2O3 , _CeO2 , and _ZrO2 , and having a lower Ce content than the first YCZ particles.

When producing a cermet layer, the raw material of the first YCZ particles is
(a) a powder having the same composition as the first YCZ particles having the target composition (=first YCZ powder), or
(b) a powder capable of producing first YCZ particles having a desired composition by solid-state reaction (e.g., a third YCZ powder);
This also applies to the second YCZ particles.

When the latter method is used to produce a cermet layer, the first YCZ particles are particles derived from the raw material for the first YCZ particles (e.g., third YCZ powder) added as a starting material for producing the cermet layer. On the other hand, the second YCZ particles are particles derived from the raw material for the second YCZ particles (e.g., YSZ powder) added as a starting material for producing the cermet layer.

[1.2.2. Oxygen storage and release capacity]
The YCZ particles are made of _ZrO2 doped with Y and Ce. Generally, as the amount of Ce contained in the YCZ particles increases, the oxygen storage capacity increases, the oxide ion conductivity decreases, the electronic conductivity improves, and the strength decreases.
Therefore, the first YCZ particles containing a relatively large amount of Ce have the main function of suppressing oxidation of Ni contained in the Ni-containing particles and functioning as an electron conductor in the cermet layer.
On the other hand, the second YCZ particles containing a relatively small amount of Ce have, in the cermet layer, mainly the functions of suppressing the oxidation of Ni contained in the Ni-containing particles, the function of acting as an oxide ion conductor, and the function of maintaining the strength of the cermet layer.

The Y doped in _ZrO2 mainly has the function of imparting high ionic conductivity to _ZrO2 . The Ce doped in _ZrO2 mainly has the function of imparting oxygen storage/release capacity to _ZrO2 and the function of imparting electronic conductivity to _ZrO2 .
Therefore, when the first YCZ particles and the second YCZ particles are added to a hydrogen electrode containing Ni-containing particles, oxidation of Ni contained in the Ni-containing particles is significantly suppressed while maintaining high electrode activity. Furthermore, even if the Ni-containing particles are oxidized, electrons are conducted via the first YCZ particles.
Even when the Ni-containing particles contain a metal element other than Ni, if the oxidation of Ni can be suppressed at least, a three-phase boundary (TPB) can be ensured in the electrode.

The following formula (a) shows the oxygen storage and release reaction of _CeO2 -doped _ZrO2 (CZ): In formula (a), 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 …(a)

The Ce ions dissolved in _ZrO2 can reversibly take on a trivalent state (reduced state) or a tetravalent state (oxidized state) depending on the oxygen partial pressure in the surrounding atmosphere. Therefore, when CZ is exposed to an oxidizing atmosphere, it incorporates oxygen ions from the atmosphere into its crystal lattice. On the other hand, when CZ is exposed to a reducing atmosphere, it releases oxygen ions from the crystal lattice into the atmosphere. This is also true for YCZ.

[1.2.3. Composition of first YCZ particles]
The first YCZ particles preferably have a composition represented by the following formula (1):
Y _x Ce _y Zr _1-xy O _2-δ …(1)
however,
0<x≦0.2, 0<y≦0.2,
δ is the value at which electrical neutrality is maintained.

In formula (1), x represents the ratio of the number of moles of Y to the total number of moles of Y, Ce, and Zr contained in the first YCZ particles. If x is too small, the oxide ion conductivity of the first YCZ particles may decrease. Therefore, x must be greater than 0. x is preferably 0.04 or greater, and more preferably 0.08 or greater.
On the other hand, if x is too large, the oxide ion conductivity may decrease. Therefore, x is preferably 0.2 or less, more preferably 0.15 or less, and even more preferably 0.1 or less.

In formula (1), y represents the ratio of the number of moles of Ce to the total number of moles of Y, Ce, and Zr contained in the first YCZ particles. If y is too small, the oxygen storage capacity decreases, and the oxidation inhibitory function of Ni contained in the Ni-containing particles may decrease. Furthermore, if y is too small, the electronic conductivity of the first YCZ particles may decrease. Therefore, y must be greater than 0. y is preferably 0.04 or more, more preferably 0.08 or more.
On the other hand, if y is too large, not only the oxide ion conductivity of the cermet layer decreases, but also the mechanical strength of the cermet layer may decrease. Therefore, y is preferably 0.2 or less. y is more preferably 0.15 or less, or 0.1 or less.

It is particularly preferable that the first YCZ particles satisfy the conditions 0.08≦x≦0.1 and 0.08≦y≦0.1.

[1.2.4. Composition of second YCZ particles]
The second YCZ particles preferably have a composition represented by the following formula (2).
Y _x' Ce _y' Zr _1-x'-y' O _2-δ' ...(2)
however,
0<x'≦0.2, 0<y'≦0.02, y'<y
δ' is the value at which electrical neutrality is maintained.

In formula (2), x' represents the ratio of the number of moles of Y to the total number of moles of Y, Ce, and Zr contained in the second YCZ particles. If x' is too small, the oxide ion conductivity of the second YCZ particles may decrease. Therefore, x' must be greater than 0. x' is preferably 0.05 or greater, and more preferably 0.09 or greater.
On the other hand, if x' is too large, the oxide ion conductivity may decrease. Therefore, x' is preferably 0.2 or less, more preferably 0.15 or less, and even more preferably 0.12 or less.

In formula (2), y' represents the ratio of the number of moles of Ce to the total number of moles of Y, Ce, and Zr contained in the second YCZ particles. If y' is too small, the oxygen storage capacity decreases, and the oxidation inhibitory function of Ni contained in the Ni-containing particles may decrease. Therefore, y' must be greater than 0. y is preferably 0.005 or more, more preferably 0.009 or more.
On the other hand, if y' is too large, not only the oxide ion conductivity of the cermet layer decreases, but also the mechanical strength of the cermet layer may decrease. Therefore, y' is preferably 0.02 or less. y' is more preferably 0.017 or less, and even more preferably 0.015 or less.

The second YCZ particles preferably satisfy the conditions 0.09≦x'≦0.12 and 0.009≦y'≦0.015.

[1.3. Composition of cermet layer]
The cermet layer preferably satisfies the following formulas (3) to (5).
30mass%≦X≦70mass%…(3)
1mass%≦Y≦20mass%…(4)
X+Y+Z=100mass%...(5)
however,
X (mass%) is the ratio of the mass of the Ni-containing particles to the total mass of the Ni-containing particles, the first YCZ particles, and the second YCZ particles;
Y (mass %) is the ratio of the mass of the first YCZ particles to the total mass of the Ni-containing particles, the first YCZ particles, and the second YCZ particles;
Z (mass %) is the ratio of the mass of the second YCZ particles to the total mass of the Ni-containing particles, the first YCZ particles, and the second YCZ particles.

If the content X of the Ni-containing particles is too small, the total cell resistance may increase and the efficiency of the electrode reaction may decrease. Therefore, X is preferably 30 mass% or more. X is more preferably 40 mass% or more.
On the other hand, if X is excessive, the content of the first YCZ particles and the second YCZ particles will decrease. As a result, the oxidation suppression function of Ni contained in the Ni-containing particles may decrease, or the strength of the cermet layer may decrease. Therefore, X is preferably 70 mass% or less.

If the content Y of the first YCZ particles is too small, the oxidation suppression function of Ni contained in the Ni-containing particles may be reduced, or the oxide ion conductivity of the cermet layer may be reduced. Therefore, Y is preferably 1 mass% or more. Y is more preferably 3 mass% or more, and even more preferably 5 mass% or more.
On the other hand, if Y is excessive, the content of the second YCZ particles becomes relatively small, which may result in a decrease in the strength of the cermet layer. Therefore, Y is preferably 20 mass% or less, more preferably 18 mass% or less, and even more preferably 15 mass% or less.

[1.4. Use]
The cermet layer according to the present invention can be used for various purposes. Examples of the uses of the cermet layer include:
(a) an active layer of a hydrogen electrode for steam electrolysis;
(b) a diffusion layer of a hydrogen electrode for steam electrolysis;
(c) an active layer of an anode of a solid oxide fuel cell;
(d) a diffusion layer of an anode of a solid oxide fuel cell;
etc.

1.5. Porosity
The porosity of the cermet layer is not particularly limited, and an optimum porosity can be selected depending on the purpose.

For example, when the cermet layer according to the present invention is used as an active layer of a hydrogen electrode for steam electrolysis, the porosity of the active layer affects the electrolytic characteristics. If the porosity of the active layer is too small, the gas diffusibility decreases, and the efficiency of the electrode reaction decreases. Therefore, the porosity of the active layer is preferably 15% or more. The porosity is preferably 20% or more, and 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 becomes relatively small, which in turn reduces the efficiency of the electrode reaction. Therefore, the porosity of the active layer is preferably 40% or less. The porosity is preferably 35% or less, and more preferably 30% or less.

Furthermore, for example, when the cermet layer according to the present invention is used as a diffusion layer of a hydrogen electrode for steam electrolysis, the porosity of the diffusion layer affects the gas diffusivity, strength, electronic conductivity, and other properties of the hydrogen electrode. Generally, if the porosity of the diffusion layer is too low, the gas diffusivity decreases. Therefore, the porosity of the diffusion layer is preferably 40% or more. The porosity is more preferably 45% or more, and even more preferably 50% or more.
On the other hand, if the porosity of the diffusion layer is too high, the strength and electronic conductivity decrease. Therefore, the porosity of the diffusion layer is preferably 60% or less. The porosity is preferably 58% or less, and more preferably 55% or less.

[2. Hydrogen electrode for steam electrolysis]
The hydrogen electrode for steam electrolysis according to the present invention (hereinafter also referred to as "hydrogen electrode") includes the cermet layer according to the present invention.

[2.1. Active Layer and Diffusion Layer]
The hydrogen electrode for steam electrolysis may be composed of only an active layer, or may be provided with an active layer and a diffusion layer formed on the separator-side surface of the active layer.
The active layer is the reaction site for the electrolytic reaction, and is required to have high ionic conductivity because it must transport oxide ions generated by the electrolytic reaction to the electrolyte.
On the other hand, the diffusion layer is used to support the active layer. In a hydrogen electrode consisting of a laminate of a diffusion layer and an active layer, the electrode reaction occurs mainly in the active layer. Therefore, the diffusion layer does not necessarily need to have high ionic conductivity.

That is, the diffusion layer includes at least
(a) a function of supporting an active layer formed on the surface of the electrolyte layer side;
(b) the function of diffusing the raw materials for electrolysis to the active layer;
(c) transporting electrons required for the reduction reaction from the current collector to the active layer; and
(d) It is necessary to have a function of discharging hydrogen generated in the active layer by the electrode reaction to the outside of the hydrogen electrode.
The composition of the diffusion layer is not particularly limited as long as it performs the above functions. The diffusion layer is generally made of a cermet containing Ni-containing particles and electrolyte particles made of a solid oxide.

The hydrogen electrode for steam electrolysis according to the present invention comprises:
(a) comprising only an active layer, the active layer being made of the cermet layer according to the present invention;
(b) A two-layer structure of an active layer and a diffusion layer, in which the active layer is made of the cermet layer according to the present invention and the diffusion layer is made of a layer other than the cermet layer according to the present invention;
(c) A two-layer structure of an active layer and a diffusion layer, both of which are made of the cermet layer according to the present invention.
(d) It may have a two-layer structure of an active layer and a diffusion layer, in which the diffusion layer is made of the cermet layer according to the present invention, and the active layer is made of a layer other than the cermet layer according to the present invention.

When the diffusion layer or active layer is composed of a layer other than the cermet layer of the present invention (hereinafter also referred to as the "second layer"), the composition of the second layer is not particularly limited. The second layer typically consists of a cermet containing second Ni-containing particles and electrolyte particles made of a solid oxide.

The second Ni-containing particles contained in the second layer may have the same composition as the Ni-containing particles contained in the cermet layer according to the present invention, or may have a different composition.
The electrolyte particles contained in the second layer include, for example, yttria-stabilized zirconia (YSZ) containing 3 to 15 mol % of Y ₂ O ₃ (for example, 8YSZ containing 8 mol % of Y ₂ O ₃ ).

To prevent a decrease in electronic conductivity and a decrease in the efficiency of the electrolysis reaction due to oxidation of the Ni-containing particles, it is preferable that at least the active layer of the hydrogen electrode for steam electrolysis be made of the cermet layer according to the present invention. Other aspects of the cermet layer are as described above, so further explanation is omitted.

[2.2. Electrolytic current deterioration rate]
The "electrolysis current deterioration rate ΔI" is the deterioration rate of the electrolysis current per 100 hours of electrolysis time before and after the durability test, and is a value expressed by the following formula (8).
ΔI (%/100h) = (I ₀ - _{I 1} ) x 100/(I ₀ x 3)...(8)
however,
I ₀ (A) is the electrolysis current when steam electrolysis was performed under the conditions of a cell temperature of 700°C and a voltage of 1.3V before the durability test.
I ₁ (A) is the electrolysis current when steam electrolysis was carried out under the conditions of a cell temperature of 700° C. and a voltage of 1.3 V after the durability test.
The "durability test" refers to a test in which steam electrolysis is performed using an SOEC under conditions of a cell temperature of 700°C and a voltage of 1.3V for 300 hours.

The cermet layer of the present invention contains two types of YCZ particles with different compositions, and therefore exhibits little decrease in electronic conductivity even when exposed to high-temperature water vapor. Therefore, when the cermet layer of the present invention is used at least as the active layer of a hydrogen electrode for water vapor electrolysis, the electrolysis current degradation rate is lower than that of conventional hydrogen electrodes. When the composition of the active layer is optimized, the electrolysis current degradation rate is 10%/100 h or less. When the composition of the active layer is further optimized, the electrolysis current degradation rate is 7%/100 h or less, or 5%/100 h or less.

[3. Manufacturing method of cermet layer]
The cermet layer according to the present invention comprises:
(a) forming a compact using a raw material mixture containing a raw material for Ni-containing particles, a raw material for first YCZ particles, and a raw material for second YCZ particles;
(c) sintering the obtained compact;
(d) The resulting sintered body can be subjected to a reduction treatment to produce the desired product.

[3.1. 1st process]
First, a compact is produced using a raw material mixture containing the raw material of the Ni-containing particles, the raw material of the first YCZ particles, and the raw material of the second YCZ particles (first step).

[3.1.1. Raw materials for Ni-containing particles]
The "raw material for Ni-containing particles" refers to a raw material that becomes Ni-containing particles after sintering and reduction. In the present invention, the type of raw material for Ni-containing particles is not particularly limited, and an optimal raw material can be selected depending on the purpose. Examples of raw materials for Ni-containing particles include _NiO powder, _Fe2O3 powder, _Fe3O4 powder, a mixture of metallic Fe _and NiO or metallic Ni _, CoO powder, and _Co2O3 powder.

[3.1.2. Raw material of first YCZ particles]
The "raw material for the first YCZ particles" refers to a raw material that becomes the first YCZ particles after sintering. The raw material for the first YCZ particles is not particularly limited as long as it can produce the first YCZ particles after sintering. The raw material for the first YCZ particles is preferably a third YCZ powder having a composition represented by the following formula (6):

Y _x" Ce _y" Zr _1-x"-y" O _2-δ" ...(6)
however,
0<x"≦0.2, 0<y"<1.0, y">y,
δ" is the value at which electrical neutrality is maintained.

In formula (6), x" represents the ratio of the number of moles of Y to the total number of moles of Y, Ce, and Zr contained in the third YCZ powder. If x" is too small, first YCZ particles having the desired composition may not be obtained. Therefore, x" is preferably greater than 0. x" is more preferably 0.04 or greater, and even more preferably 0.08 or greater.
Similarly, if x" is too large, first YCZ particles having the desired composition may not be obtained. Therefore, x" is preferably 0.2 or less. x is more preferably 0.15 or less, and even more preferably 0.1 or less.

In formula (6), y" represents the ratio of the number of moles of Ce to the total number of moles of Y, Ce, and Zr contained in the third YCZ powder. If y" is too small, first YCZ particles having the desired composition may not be obtained. Therefore, y" is preferably greater than 0. y" is more preferably 0.08 or greater.
Similarly, if y" is too large, first YCZ particles having the desired composition may not be obtained. Therefore, y" is preferably less than 1.0. y" is more preferably 0.9 or less, 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, 0.4 or less, 0.3 or less, 0.2 or less, or 0.1 or less.
The first YCZ particles are formed by diffusing a portion of Ce contained in the third YCZ powder into the raw material of the second YCZ particles. Therefore, the relationship y″>y holds.

[3.1.3. Raw material for second YCZ particles]
The "raw material for the second YCZ particles" refers to a raw material that becomes the second YCZ particles after sintering. The raw material for the second YCZ particles is not particularly limited as long as it can produce the second YCZ particles after sintering. The raw material for the second YCZ particles is preferably a YSZ powder having a composition represented by the following formula (7):

_{YzZr1 -zO2 -γ} ...(7)
however,
0<z≦0.2,
γ is the value at which electrical neutrality is maintained.

In formula (7), z represents the ratio of the number of moles of Y to the total number of moles of Y and Zr contained in the YSZ powder. If z is too small, second YCZ particles having the desired composition may not be obtained. Therefore, z is preferably greater than 0. z is more preferably 0.05 or greater, and even more preferably 0.09 or greater.
Similarly, if z is too large, second YCZ particles having the desired composition may not be obtained. Therefore, z is preferably 0.2 or less. z is more preferably 0.15 or less, and even more preferably 0.12 or less.

3.1.4. Pore-forming agent
The raw material mixture may contain a pore-forming material (e.g., carbon powder). Metal oxides (e.g., NiO powder) contained in the raw material of the Ni-containing particles added to the raw material mixture are reduced after the sintered body is produced. During this process, volumetric shrinkage occurs, introducing pores into the sintered body. Therefore, a pore-forming material is not necessarily required. However, adding a pore-forming material to the raw material mixture increases the degree of freedom in controlling the porosity.

[3.1.5. Raw material composition]
The raw materials are preferably blended so as to obtain a cermet layer having the desired composition after sintering and reduction.

[3.1.6. Method for producing molded body]
The method for producing the molded body is not particularly limited, and an optimum method can be selected depending on the purpose. Examples of the method for producing the molded body include:
(a) A method in which a slurry containing a raw material mixture is tape-cast, the resulting green sheet is laminated on a substrate (e.g., a molded body for producing a diffusion layer), and the laminate is isostatically pressed to bond the laminate;
(b) A method of preparing a slurry containing a raw material mixture and screen-printing the slurry on a surface of a substrate;
etc.

[3.2. 2nd process]
Next, the obtained compact is sintered (second step). The sintering conditions are preferably selected optimally depending on the raw material composition. Sintering is usually preferably carried out in an air atmosphere at a temperature of 1000°C or higher and 1400°C or lower for 1 hour to 5 hours.
When the raw material mixture contains two or more oxides, a solid-phase reaction may occur during sintering, resulting in the formation of a solid solution having a predetermined composition. Also, when the raw material mixture contains a pore-forming material, the pore-forming material disappears during sintering, resulting in the formation of pores in the sintered body.

[3.3. Reduction step]
Next, the obtained sintered body is subjected to a reduction treatment (reduction step). This results in the cermet layer according to the present invention. The reduction treatment is carried out to reduce metal oxides such as NiO contained in the sintered body and generate Ni-containing particles. The reduction conditions are not particularly limited, and it is preferable to select optimal conditions depending on the composition of the cermet layer. The reduction is preferably carried out at a temperature of 600°C or higher and 800°C or lower in a hydrogen reduction atmosphere.
As will be described later, a solid oxide electrolysis cell is composed of an assembly of a hydrogen electrode (cathode), an electrolyte layer, a reaction prevention layer, and an oxygen electrode (anode). The reduction of the cermet layer is usually carried out after the layers are joined together. This is also true for solid oxide fuel cells.

4. Solid Oxide Electrolysis Cell
FIG. 1 shows a schematic diagram of a solid oxide electrolysis cell using Ni/{YCZ(1)+YCZ(2)} as the active layer. In FIG. 1, a solid oxide electrolysis cell (SOEC) 10 includes:
an electrolyte 12;
a hydrogen electrode 14 bonded to one surface of the electrolyte 12;
an oxygen electrode 16 bonded to the other surface of the electrolyte 12;
The electrolyte 12 and the oxygen electrode 16 are sandwiched between an intermediate layer 18 .

The hydrogen electrode 14 used is a hydrogen electrode for steam electrolysis according to the present invention. In Fig. 1, the hydrogen electrode 14 is made of a laminate of an active layer 14a and a diffusion layer 14b that supports the active layer 14a. The active layer 14a is made of a cermet containing Ni particles, first YCZ particles (hereinafter also referred to as "YCZ(1)"), and second YCZ particles (hereinafter also referred to as "YCZ(2)"). Furthermore, the diffusion layer 14b is made of a cermet containing Ni particles and electrolyte particles.
The materials for the electrolyte 12, the oxygen electrode 16, and the intermediate layer 18 are not particularly limited, and the most suitable materials can be selected depending on the purpose.

For example, the electrolyte 12 may be made of YSZ or the like.
The oxygen electrode 16 may be made of (La,Sr)CoO ₃ (LSC), (La,Sr)(Co,Fe)O ₃ (LSCF), (La,Sr)MnO ₃ (LSM), or the like.
The intermediate layer 18 is inserted as needed to prevent a reaction that would occur due to direct contact between the electrolyte 12 and the oxygen electrode 16. For example, when the electrolyte 12 is YSZ and the oxygen electrode 16 is LSC, it is preferable to use Gd-doped _CeO2 (GDC) for the intermediate layer 18.

As mentioned above, the hydrogen electrode according to the present invention can be used as the fuel electrode of a solid oxide fuel cell (SOFC). Since SOFCs have the same structure as the SOEC 10 except for their intended use, detailed explanations will be omitted.

[5. Effect]
2 shows a schematic diagram of a conventional solid oxide electrolysis cell using Ni/YSZ as the active layer. The 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 an intermediate layer 18 inserted between the electrolyte 12 and the oxygen electrode 16. The conventional SOEC 10' uses Ni/YSZ cermet as the hydrogen electrode 14'.

In an SOEC 10' in which 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, a reduction reaction shown in the following formula (9) proceeds in the hydrogen electrode 14'.
_H2O + ^2e- → _H2 +O2 ^-... (9)

However, in water electrolysis using the SOEC 10', high-temperature (700°C or higher) steam, which serves as a raw material, is supplied to the hydrogen electrode 14'. As a result, Ni contained in the hydrogen electrode 14' is easily oxidized, forming NiO. Because NiO is an insulator, when NiO is formed, the electron path is interrupted in that area, and the electrode reaction does not proceed. As a result, the electrolysis characteristics deteriorate.
This is also true for SOFCs. That is, water is generated by an electrode reaction at the anode (fuel electrode) of an SOFC. Therefore, particularly under high-load operating conditions, the generated water may oxidize Ni in the fuel electrode, resulting in a deterioration in power generation performance.

In contrast, when a raw material mixture containing a raw material for Ni-containing particles, a raw material for first YCZ particles (e.g., third YCZ powder having a predetermined composition), and a raw material for second YCZ particles (e.g., YSZ powder having a predetermined composition) is used as raw materials for producing a cermet layer and then fired, some of the Ce contained in the raw material for the first YCZ particles (e.g., third YCZ powder) diffuses into the raw material for the second YCZ particles (e.g., YSZ powder). As a result, first YCZ particles (YCZ particles derived from the third YCZ powder) containing a relatively large amount of Ce and second YCZ particles (YCZ particles derived from the YSZ powder) containing a relatively small amount of Ce are produced.

When steam electrolysis is performed using an SOEC equipped with a hydrogen electrode for steam electrolysis that includes such a cermet layer (particularly, a hydrogen electrode for steam electrolysis whose active layer is made of such a cermet layer), the deterioration of the electrode performance of the hydrogen electrode is suppressed.
(a) In a water vapor atmosphere, the Ni-containing particles are oxidized, but the first YCZ particles and/or the second YCZ particles occlude (trap) and remove oxygen adsorbed on Ni, thereby suppressing the oxidation of the Ni-containing particles; and
(b) This is thought to be because the first YCZ particles, which contain a relatively large amount of Ce, also function as mixed electron/ion conductors, and therefore electrons and oxide ions are conducted via the first YCZ particles even when the Ni-containing particles are oxidized.
Furthermore, the second YCZ particles also have the function of suppressing a decrease in the strength of the cermet layer, so that when such a cermet layer is used in the hydrogen electrode of an SOEC, the decrease in electronic conductivity and oxide ion conductivity is suppressed and the durability of the SOEC is improved.

Figure 3 shows a schematic diagram of the mechanism by which Ni oxidation is suppressed in a hydrogen electrode equipped with a cermet layer containing YCZ(1) and YCZ(2). Note that in Figure 3, for ease of understanding, YSZ and YCZ(2) derived from YSZ are depicted as plates, but this is merely an example. In an actual hydrogen electrode, they exist as particles.

3A, when the hydrogen electrode has a cermet layer made of a mixture of Ni particles, YCZ particles, and YSZ particles, some of the Ni particles are in contact with the YCZ particles, but other parts of the Ni particles are not in contact with the YCZ particles. Hereinafter, the contact points between the Ni particles and the YCZ particles are also referred to as "reaction points."
As shown in Figure 3(B), the suppression of Ni oxidation by YCZ particles is thought to occur when oxygen adsorbed on Ni particles is extracted by YCZ particles, which have oxygen storage and release capacity, via reaction sites. However, when the hydrogen electrode is composed of a mixture of Ni particles, YCZ particles, and YSZ particles, the number of reaction sites becomes relatively small. That is, the number of Ni particles not in contact with YCZ particles becomes relatively large. As a result, oxidation of Ni particles not in contact with YCZ particles is more likely to proceed in a steam electrolysis environment.

In contrast, as shown in FIG. 3(C), when a mixture of Ni particles, YCZ particles (third YCZ powder), and YSZ particles is heated to a predetermined temperature (e.g., 1400°C), some of the Ce contained in the YCZ particles diffuses into the YSZ particles.
As a result, as shown in Figure 3(D), YCZ particles containing a relatively large amount of Ce (first YCZ particles, YCZ(1)) and YCZ particles containing a relatively small amount of Ce (second YCZ particles, YCZ(2)) are generated in the hydrogen electrode. This also increases the number of reaction sites and reduces the proportion of Ni particles that are not in contact with YCZ(1) or YCZ(2).

Therefore, when steam electrolysis is performed using such a hydrogen electrode, oxygen is extracted not only from the Ni particles in contact with YCZ(1), but also from the Ni particles in contact with YCZ(2), as shown in Figure 3(E). As a result, oxidation of the Ni particles is suppressed compared to Figure 3(B).

(Example 1, Comparative Example 1)
1. Sample Preparation
[1.1. Preparation of third YCZ powder]
Cerium nitrate was used as the Ce source. Zirconium oxynitrate was used as the Zr source. Yttrium nitrate hexahydrate was used as the Y source. Yttrium nitrate hexahydrate and zirconium oxynitrate were added to an aqueous cerium nitrate solution and mixed. 30 mass% hydrogen peroxide water was further added and mixed to obtain mixed solution A. The mixing ratio of each raw material was set to obtain 3YCZ powder having a composition of Y _x" Ce _y" Zr _1-x"-y" O _2-δ" (x" = 0.135, y" = 0.1).
Separately, a mixture B was obtained by mixing 25% aqueous ammonia and ion-exchanged water.
Mixture A was added to mixture B with stirring. After a predetermined time had passed, the precipitate was collected.

Next, the precipitate was heat-treated in an air atmosphere at 150°C for 7 hours and 400°C for 5 hours and then dried. The dried powder was then calcined in an air atmosphere at 1400°C for 5 hours to obtain a third YCZ powder.

[1.2. Preparation of electrolytic cell]
1.2.1. Example 1
NiO powder, 8YSZ powder, and 3YCZ powder were weighed out to a mass ratio of 50:40:10 and mixed together to obtain a raw material mixture A.
Next, a diffusion layer sheet was fabricated using a tape casting method. The composition of the diffusion layer sheet was set to a composition such that the composition after sintering would be 30 to 60 mass% Ni-8YSZ. Next, an active layer sheet containing raw material mixture A, an electrolyte sheet containing 8YSZ, and an intermediate layer sheet containing GDC were formed in this order on the surface of the diffusion layer sheet using a screen printing method. After drying the laminate, it was sintered in an air atmosphere at 1400°C for 15 hours.
Next, an oxygen electrode sheet containing LSC was formed on the surface of the intermediate layer. After drying, the laminate was sintered in air at 1100°C for 15 hours. The resulting sintered body was then subjected to reduction treatment at 800°C for 2 hours.

[1.2.2. Comparative Example 1]
An electrolytic cell was produced in the same manner as in Example 1, except that a raw material mixture prepared by weighing out NiO powder and 8YSZ powder to give a mass ratio of 50:50 was used instead of raw material mixture A for forming the active layer.

[2. Test Method and Results]
[2.1. SEM/EDX Observation]
The elemental distribution in the cross section of the active layer was evaluated using SEM/EDX. Figure 4 shows elemental mapping of Ce obtained by SEM/EDX analysis of the active layer obtained in Example 1. YCZ(1) and YCZ(2) were constructed by reacting the third YCZ powder A with the 8YSZ powder at 1400°C. In Figure 4, YCZ(1) shows the state after the reaction of the third YCZ powder A added to the raw materials. YCZ(2) shows the state after the reaction of the 8YSZ powder added to the raw materials.
In the case of Figure 4, the composition of YCZ(1) was identified as _{Y0.08-0.1Ce0.08-0.1Zr0.85-0.87O2 +α ,} and the composition of YCZ(2 ₎ was identified _{as Y0.09-0.12Ce0.009-0.015Zr0.85-0.9O2 +α .}

[2.2. Durability test]
The electrolytic cell thus obtained was subjected to a durability test under the following conditions.
Cell temperature: 700°C
Air electrode atmosphere: _N2 (flow rate: 160 ccm) + _O2 (flow rate: 40 ccm) ( _N2 : 80%, _O2 : 20% (almost air composition), total flow rate: 200 ccm)
Anode atmosphere: A mixed gas of _N2 (flow rate: 125 ccm) + _H2 (flow rate: 15 ccm) was supplied through a humidifier at 70°C (approximately _H2 : 7.5%, _N2 : 62.5%, _H2O : 30% ( _H2O / _H2 volume ratio: 4), total flow rate: 200 ccm).

Electrolysis was performed for 300 hours with the cell voltage fixed at 1.3 V, and the change in current density was evaluated. Furthermore, the electrolysis current degradation rate was calculated from the current values before and after the durability test. Figure 5 shows the electrolysis current degradation rates of the electrolysis cells obtained in Example 1 and Comparative Example 1. Figure 5 shows that the electrolysis current degradation rate in Example 1 was reduced to approximately one-third of that in Comparative Example 1.

Example 2
[1. Preparation of electrolytic cell]
A pore-forming agent was added to the raw material mixture A prepared in Example 1 to prepare raw material mixture B. A diffusion layer sheet was prepared using the obtained raw material mixture B. Thereafter, in the same manner as in Example 1, an electrolytic cell was prepared that included a hydrogen electrode in which both the diffusion layer and the active layer were made of the cermet layer according to the present invention.

[2. Test Method and Results]
A durability test was carried out under the same conditions as in Example 1, and the electrolysis current deterioration rate was calculated. The electrolysis current deterioration rate of Example 2 was equal to or lower than that of Example 1.

The above describes in detail an embodiment of the present invention, but the present invention is not limited to the above embodiment, and various modifications are possible without departing from the spirit of the present invention.

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

Claims (11)

A cermet layer having the following composition:
(1) The cermet layer is
Ni-containing particles;
first YCZ particles made of a composite oxide of _Y2O3 , _CeO2 , and _{ZrO2 ;}
second YCZ particles made of a composite oxide of _Y2O3 , _CeO2 , and _{ZrO2 ;}
The cermet comprises:
(2) The Ce content in the second YCZ particles is less than the Ce content in the first YCZ particles. The first YCZ particles have a composition represented by the following formula (1):
The cermet layer according to claim 1 , wherein the second YCZ particles have a composition represented by the following formula (2):
Y _x Ce _y Zr _1-xy O _2-δ …(1)
however,
0<x≦0.2, 0<y≦0.2,
δ is the value at which electrical neutrality is maintained.
Y _x' Ce _y' Zr _1-x'-y' O _2-δ' ...(2)
however,
0<x'≦0.2, 0<y'≦0.02, y'<y
δ' is the value at which electrical neutrality is maintained. the first YCZ particles satisfy 0.08≦x≦0.1 and 0.08≦y≦0.1;
The cermet layer according to claim 2 , wherein the second YCZ particles satisfy 0.09≦x′≦0.12 and 0.009≦y′≦0.015. The cermet layer according to any one of claims 1 to 3, wherein the following formulas (3) to (5) are satisfied:
30mass%≦X≦70mass%…(3)
1mass%≦Y≦20mass%…(4)
X+Y+Z=100mass%...(5)
however,
X (mass%) is the ratio of the mass of the Ni-containing particles to the total mass of the Ni-containing particles, the first YCZ particles, and the second YCZ particles;
Y (mass %) is the ratio of the mass of the first YCZ particles to the total mass of the Ni-containing particles, the first YCZ particles, and the second YCZ particles;
Z (mass %) is the ratio of the mass of the second YCZ particles to the total mass of the Ni-containing particles, the first YCZ particles, and the second YCZ particles. The cermet layer according to any one of claims 1 to 4 , which is used as an active layer of a hydrogen electrode for steam electrolysis. The cermet layer according to claim 5 , wherein the porosity is 20% or more and 40% or less. The cermet layer according to any one of claims 1 to 4 , which is used as a diffusion layer of a hydrogen electrode for steam electrolysis. The cermet layer according to claim 7 , wherein the porosity is 40% or more and 60% or less. A hydrogen electrode for steam electrolysis, comprising the cermet layer according to any one of claims 1 to 8 . an active layer made of the cermet layer,
10. The hydrogen electrode for steam electrolysis according to claim 9 , wherein the electrolysis current deterioration rate is 10%/100 h or less. a step of producing a compact using a raw material mixture including a raw material of Ni-containing particles, a third YCZ powder having a composition represented by the following formula (6), and a YSZ powder having a composition represented by the following formula (7);
sintering the compact at a temperature of 1000°C or higher and 1400°C or lower in an air atmosphere;
a step of further heating the obtained sintered body at a temperature of 600° C. or higher and 800° C. or lower in a hydrogen reducing atmosphere to obtain the cermet layer according to any one of claims 1 to 4;
A method for manufacturing a cermet layer comprising the steps of:
Y _x" Ce _y" Zr _1-x"-y" O _2-δ" …(6)
however,
0<x"≦0.2, 0<y"<1.0, y">y,
δ" is the value at which electrical neutrality is maintained.
Y _z Zr _1-z O _2-γ …(7)
however,
0<z≦0.2,
γ is the value at which electrical neutrality is maintained.