JP2024137341A - Fuel electrode for solid oxide water electrolysis cell and solid oxide water electrolysis cell - Google Patents

Abstract

The present invention provides a fuel electrode for a solid oxide water electrolysis cell, which undergoes little change in electrode structure even when exposed to high-temperature water vapor, and a solid oxide water electrolysis cell including the same.
[Solution] The fuel electrode for a solid oxide water electrolysis cell includes Ni-based particles and electrolyte particles made of solid oxide electrolyte A. The electrolyte particles are made of a solid solution or mixture of Y-doped _ZrO2 (YSZ) and Gd-doped _CeO2 (GDC). The solid oxide water electrolysis cell uses such a fuel electrode for a solid oxide water electrolysis cell as the fuel electrode.
[Selected figure] Figure 4

Description

The present invention relates to a fuel electrode for a solid oxide water electrolysis cell and a solid oxide water electrolysis cell, and more specifically, to a fuel electrode for a solid oxide water electrolysis cell that contains a solid solution or mixture of Y-doped ZrO ₂ (YSZ) and Gd-doped CeO ₂ (GDC) as a solid oxide electrolyte, and a solid oxide water electrolysis cell equipped with the same.

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

The SOEC is equipped with a single cell in which an anode (air electrode) is bonded to one side of an electrolyte, and a cathode (fuel electrode) is bonded to the other side. The following materials are generally used as materials for the components that constitute 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) Air electrode: lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium cobaltite (LSC), etc.
(c) Fuel electrode: Ni/YSZ, Ni/SDC, Ni-Fe/SDC, etc.

In addition, Patent Document 1 states:
an electrode skeleton obtained by sintering electronically conductive oxide particles ₍ e.g., _{La0.1Sr0.9TiO3} (LST)) and ionically conductive oxide particles (e.g., _{Gd0.1Ce0.9O2 ( GDC} )) _;
A fuel electrode is disclosed that includes a composite electrode catalyst (eg, Ni-GDC cermet) supported on the surface of an electrode framework.

The same document states:
(A) In a conventional fuel electrode made of a sintered body of a metal such as Ni and an ion-conductive oxide, when volume expansion due to oxidation of the metal and volume contraction due to reduction of the metal oxide are repeated, the framework of the fuel electrode is easily destroyed; and
(B) It is described that when the electrode skeleton of the fuel electrode is made only of oxide, almost no volume change occurs, so that destruction of the skeleton of the fuel electrode can be avoided.

Patent Document 2 discloses a substrate for an electrochemical cell in which an active layer made of a cermet of Ni and YSZ is provided between a fuel electrode body and a solid electrolyte membrane.
The same document states:
(a) the thickness of the active layer is set to 3 μm or more and 15 μm or less;
(b) the average particle size (d) of the Ni particles and the average particle size (d) of the YSZ particles contained in the active layer are each set to 0.5 μm or more and 1.0 μm or less;
(c) It is described that when dm/ds is set to 0.8 to 1.2, the adhesion between the fuel electrode and the solid electrolyte membrane is improved, and peeling and cracking of the solid electrolyte membrane are suppressed.

Ni/YSZ cermet is generally used for the fuel electrode of SOEC. However, water vapor, which is the raw material for hydrogen production, is supplied to the fuel electrode at high temperatures (700°C or higher). As a result, the Ni contained in the fuel electrode is easily oxidized, causing a change in the electrode structure. As a result, the electrolytic characteristics are degraded.

JP 2022-137626 A JP 2004-362849 A

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 fuel electrode for a solid oxide water electrolysis cell which exhibits little change in electrode structure even when exposed to high-temperature water vapor.
Another object of the present invention is to provide a solid oxide water electrolysis cell including such a solid oxide water electrolysis cell fuel electrode.

In order to solve the above problems, the present invention provides a fuel electrode for a solid oxide water electrolysis cell, comprising:
Ni-based particles;
and electrolyte particles made of a solid oxide electrolyte A.
The electrolyte particles consist of a solid solution or mixture of Y-doped ZrO ₂ (YSZ) and Gd-doped CeO ₂ (GDC).

The solid oxide water electrolysis cell according to the present invention comprises:
an electrolyte layer made of a solid oxide electrolyte C;
a fuel electrode formed on one surface of the electrolyte layer;
an air electrode formed on the other surface of the electrolyte layer;
a reaction prevention layer inserted at the interface between the electrolyte layer and the air electrode;
The fuel electrode comprises the solid oxide water electrolysis cell fuel electrode according to the present invention.

When a cermet containing Ni-based particles and electrolyte particles is used in the fuel electrode of a solid oxide water electrolysis cell, if a solid solution or mixture of YSZ and GDC is used as the electrolyte particles, changes in the electrode structure caused by the oxidation of Ni are suppressed. This is thought to be because the GDC component, which has oxygen storage capacity, suppresses the oxidation of the Ni-based particles.

Furthermore, the GDC component contained in the electrolyte particles has electronic conductivity in addition to oxide ion conductivity. Therefore, a fuel electrode containing a GDC component can expand the number and/or area of electrode reaction points compared to a conventional fuel electrode that does not contain a GDC component. As a result, the reaction resistance (Rct2) increase rate of the fuel electrode of the present invention is lower than that of a conventional fuel electrode, and the fuel electrode exhibits high durability.

FIG. 1 is a schematic diagram of a solid oxide water electrolysis cell using Ni/GDC-YSZ as the fuel electrode (active layer). FIG. 1 is a schematic diagram of a conventional solid oxide water electrolysis cell using Ni/YSZ as the fuel electrode (active layer). FIG. 1 is a schematic diagram of the water electrolysis cells of Samples No. 1 and 2. 1 shows the rate of increase in reaction resistance (Rct2) of the fuel electrodes of samples 1 to 3.

[Configuration 1]
Ni-based particles;
and electrolyte particles made of a solid oxide electrolyte A.
The electrolyte particles are a solid solution or a mixture of Y-doped ZrO ₂ (YSZ) and Gd-doped CeO ₂ (GDC), and the electrolyte particles are a fuel electrode for a solid oxide water electrolysis cell.

[Configuration 2]
2. The fuel electrode for a solid oxide water electrolysis cell according to claim 1, wherein the electrolyte particles have an average composition represented by the following formula (1):
{Y _y Zr _1-y O _2-δ' } _1-z {Gd _x Ce _1-x O _2-δ } _z (1)
however,
0<x≦0.30, 0<y≦0.30, 0<z<1.0,
δ and δ' are the values at which electrical neutrality is maintained.

[Configuration 3]
3. The fuel electrode for a solid oxide water electrolysis cell according to claim 1 or 2, having a thickness of 15 μm or more and 200 μm or less.

[Configuration 4]
4. A fuel electrode for a solid oxide water electrolysis cell according to any one of claims 1 to 3, wherein the porosity is 15% or more and 40% or less.

[Configuration 5]
5. The fuel electrode for a solid oxide water electrolysis cell according to any one of configurations 1 to 4, wherein the content of the Ni-based particles is 30.0 mass % or more and 70.0 mass % or less.

[Configuration 6]
an electrolyte layer made of a solid oxide electrolyte C;
a fuel electrode formed on one surface of the electrolyte layer;
an air electrode formed on the other surface of the electrolyte layer;
a reaction prevention layer inserted at the interface between the electrolyte layer and the air electrode;
6. A solid oxide water electrolysis cell, wherein the fuel electrode is the solid oxide water electrolysis cell fuel electrode according to any one of configurations 1 to 5.

[Configuration 7]
An air electrode-side current collecting layer formed on the surface of the air electrode, and/or
7. The solid oxide water electrolysis cell according to configuration 6, further comprising an anode-side current collecting layer formed on a surface of the anode.

An embodiment of the present invention will be described in detail below.
[1. Fuel electrode for solid oxide water electrolysis cell]
The fuel electrode for a solid oxide water electrolysis cell according to the present invention (hereinafter, simply referred to as "fuel electrode") comprises:
Ni-based particles;
and electrolyte particles made of a solid oxide electrolyte A.

[1.1. Ni-based particles]
The term "Ni-based particles" refers to metal particles in which the ratio of the mass of Ni to the total mass of metal elements contained in the particles is 90 mass% or more. The mass ratio of Ni contained in the Ni-based particles is preferably 95 mass% or more. % or more.
The Ni-based particles function as an electrode catalyst and an electron conductor in the fuel electrode. The composition of the Ni-based particles is not particularly limited as long as they exhibit such functions.
Examples of Ni-based particles include Ni, Ni-Fe alloys, Ni-Co alloys, etc. Among these, the Ni-based particles are preferably Ni or Ni-Fe alloys.

[1.2. Electrolyte particles]
[1.2.1. Definition]
In the present invention, the term "electrolyte particles" refers to particles made of a solid solution or a mixture of Y-doped ZrO ₂ (YSZ) and Gd-doped CeO ₂ (GDC).

When making an anode, the raw material for the electrolyte particles is:
(a) a powder having the same composition as the electrolyte particles having a desired average composition;
(b) A mixture of YSZ powder and GDC powder that can produce electrolyte particles having the desired average composition is used.
In the latter case,
(b1) Electrolyte particles having a uniform composition are obtained,
(b2) Electrolyte particles consisting of a mixture of YSZ and GDC may be obtained.

When the electrolyte particles are made of a mixture of YSZ and GDC, the YSZ and GDC constituting the electrolyte particles may have compositions different from the YSZ powder and GDC powder used as the raw materials due to solid-phase diffusion.
The history of the electrolyte particles contained in the fuel electrode is not particularly limited, and they may be produced by any method.

[1.2.2. Oxygen storage and release capacity]
The GDC component contained in the electrolyte particles has a function as an oxide ion conductor, a function as an electron conductor, and a function of suppressing the oxidation of Ni contained in the Ni-based particles in the fuel electrode.

Gd doped in _CeO2 mainly has the function of imparting high ionic conductivity to _CeO2 and further improving the inherent oxygen storage and release capacity of _CeO2 . Furthermore, _CeO2 itself also functions as an electronic conductor.
Therefore, when electrolyte particles containing GDC components are added to a fuel electrode containing Ni-based particles, the oxidation of Ni contained in the Ni-based particles is significantly suppressed while maintaining high electrode activity. Even if the Ni-based particles contain metal elements other than Ni, if the oxidation of Ni can be suppressed at least, a three-phase boundary (TPB) can be secured in the electrode. Furthermore, even if a part of the Ni-based particles is oxidized, electrons are conducted through the electrolyte particles.

The following formula (a) shows the reaction of oxygen storage and release reaction of _CeO2 . In formula (a), the reaction proceeding to the right side represents an oxidation reaction, and the reaction proceeding to the left side represents a reduction reaction.
CeO _2-x +(x/2)O ₂ ⇔ CeO ₂ …(a)

Ce ions can reversibly take a trivalent state (reduced state) or a tetravalent state (oxidized state) depending on the oxygen partial pressure in the surrounding atmosphere. Therefore, when _CeO2 is exposed to an oxidizing atmosphere, _CeO2 takes in oxygen ions in the atmosphere into its crystal lattice. On the other hand, when _CeO2 is exposed to a reducing atmosphere, _CeO2 releases oxygen ions in the crystal lattice into the atmosphere. This is also true for GDC and electrolyte particles containing a GDC component.

[1.2.3. composition]
The electrolyte particles preferably have an average composition represented by the following formula (1).
{Y _y Zr _1-y O _2-δ' } _1-z {Gd _x Ce _1-x O _2-δ } _z (1)
however,
0<x≦0.30, 0<y≦0.30, 0<z<1.0,
δ and δ' are the values at which electrical neutrality is maintained.

[A.x]
In formula (1), x represents the ratio of the number of moles of Gd to the total number of moles of Gd and Ce contained in the electrolyte particle. If x is too small, the oxygen storage capacity of the electrolyte particle may decrease. Therefore, x must be greater than 0. x is preferably 0.05 or more.
On the other hand, if x is excessive, the oxygen storage capacity of the electrolyte particles may be decreased. Therefore, x is preferably 0.30 or less, and more preferably 0.25 or less.

[B.y]
In formula (1), y represents the ratio of the number of moles of Y to the total number of moles of Y and Zr contained in the electrolyte particles. Y has the effect of improving the ionic conductivity of _ZrO2 and stabilizing the high-temperature phase (tetragonal, cubic) of _ZrO2 at room temperature. In order to obtain such an effect, y must be greater than 0. y is preferably 0.05 or more.

On the other hand, in the present invention, the ionic conductivity of the electrolyte particles is controlled mainly by the doping amount of Y (y). In this case, the ionic conductivity is highest when the composition of the YSZ component contained in the electrolyte particles is in the vicinity of the 8 mol % Y ₂ O ₃ -ZrO ₂ (8YSZ) composition. Therefore, if y becomes excessive, the ionic conductivity decreases. Therefore, y is preferably 0.3 or less. y is more preferably 0.25 or less.

[C.z]
z represents the ratio of the number of moles of the GDC component to the total number of moles of the YSZ component and the GDC component contained in the electrolyte particle. In general, the larger z is, the more durable the fuel electrode is. To obtain such an effect, z must be greater than 0. z is preferably 0.5 or more, 0.6 or more, or 0.7 or more.
On the other hand, if z is excessive, there is a risk that the mechanical strength cannot be ensured due to reduction expansion. Therefore, z must be less than 1.0. z is preferably 0.95 or less.

[1.3. Composition of the fuel electrode]
The "content of Ni-based particles" refers to the ratio of the mass of Ni-based particles to the total mass of the electrolyte particles and Ni-based particles contained in the fuel electrode.

If the content of the Ni-based particles is too small, the total cell resistance increases and the efficiency of the electrode reaction decreases. Therefore, the content of the Ni-based particles is preferably 30 mass% or more. The content is more preferably 40 mass% or more.
On the other hand, if the content of the Ni-based particles is excessive, the content of the electrolyte particles becomes small, and the oxidation of Ni contained in the Ni-based particles cannot be sufficiently suppressed. Therefore, the content of the Ni-based particles is preferably 70 mass% or less.

[1.4. Porosity]
The porosity of the fuel electrode affects the electrolysis characteristics. If the porosity of the fuel electrode is too small, the gas diffusion decreases, and the efficiency of the electrode reaction decreases. Therefore, the porosity of the fuel electrode is preferably 15% or more. The porosity is more preferably 20% or more, or 25% or more.
On the other hand, if the porosity of the fuel electrode is too large, the number of three-phase interfaces becomes relatively small, and the efficiency of the electrode reaction decreases. Therefore, the porosity of the fuel electrode is preferably 40% or less. The porosity is more preferably 35% or less, or 30% or less.

1.5. Thickness
The thickness of the fuel electrode is not particularly limited, and an optimal thickness can be selected according to the purpose. If the thickness of the fuel electrode is too thin, the total amount of Ni-based particles that function as an electrode catalyst decreases, and the efficiency of the electrode reaction may decrease. Therefore, the thickness of the fuel electrode is preferably 15 μm or more. The thickness is more preferably 25 μm or more, 30 μm or more, 40 μm or more, or 50 μm or more.
On the other hand, even if the thickness of the fuel electrode is made thicker than necessary, there is no difference in the effect and no practical benefit. Therefore, the thickness of the fuel electrode is preferably 200 μm or less. The thickness is more preferably 180 μm or less, 160 μm or less, or 140 μm or less.

[1.6. Characteristics: Reaction Resistance (Rct2) Increase Rate]
"Reaction resistance (Rct2) increase rate (%/h)" refers to the value expressed by the following formula (2).
Reaction resistance (Rct2) increase rate = { _R2 x 100/R1 _- 100} ÷ 100 (2)
however,
_R1 is the electrochemical reaction resistance of the solid oxide water electrolysis cell fuel electrode immediately after production,
_R2 is the electrochemical reaction resistance of the solid oxide water electrolysis cell fuel electrode after the durability test.
"Electrochemical reaction resistance (Rct2)" refers to the reaction resistance at the three-phase interface in the fuel electrode, which is obtained by measuring the impedance of the fuel electrode using a reference electrode for a solid oxide water electrolysis cell, and is the diameter of the arc (second arc) that contributes to the electrode reaction,
The "durability test" refers to a test in which steam electrolysis is performed for 100 hours under the conditions shown in Table 1.

Conventional fuel electrodes have low oxidation resistance, so the reaction resistance (Rct2) increases at a rate exceeding 20%/h. In contrast, the fuel electrode of the present invention has high oxidation resistance, so the reaction resistance (Rct) increases at a rate lower than that of conventional fuel electrodes. By further optimizing the manufacturing conditions, the reaction resistance (Rct2) increases at a rate of 15%/h or less, 10%/h or less, or 5%/h or less.

1.7. Uses
The solid oxide water electrolysis cell fuel electrode according to the present invention is suitable as a fuel electrode for a water electrolysis cell, but can also be used as a fuel electrode for a solid oxide fuel cell.

[2. Method for manufacturing fuel electrode for solid oxide water electrolysis cell]
The fuel electrode for a solid oxide water electrolysis cell according to the present invention comprises:
(a) preparing a compact using a raw material mixture containing raw materials for Ni-based particles and raw materials for electrolyte particles;
(b) sintering the obtained compact;
(c) The resulting sintered body is subjected to a reduction treatment to produce the desired product.

[2.1. Molded object forming process]
First, a compact is produced using a raw material mixture containing raw materials for the Ni-based particles and raw materials for the electrolyte particles.

"Raw material for Ni-based particles" refers to a raw material that becomes Ni-based particles after sintering and reduction. In the present invention, the type of raw material for Ni-based particles is not particularly limited, and the most suitable raw material can be selected according to the purpose. Examples of raw materials for Ni-based particles include _NiO powder, _Fe2O3 powder, _{Fe3O4 powder} , a mixture of metal Fe and NiO or metal Ni, CoO powder, and _{Co2O3 powder} .

The term "raw material for electrolyte particles" refers to a raw material that will become electrolyte particles after sintering. The type of raw material for electrolyte particles is not particularly limited, and an optimal raw material can be selected depending on the purpose. Examples of raw materials for electrolyte particles include:
(a) an electrolyte powder having a desired composition;
(b) a mixture of YSZ and GDC powders blended to provide a desired composition;
(c) a mixture of a Y-containing salt, a Zr-containing salt, a Gd-containing salt, and a Ce-containing salt (e.g., nitrates) formulated to provide a desired composition;
etc.

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-based particles added to the raw material mixture are 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 increases the degree of freedom in controlling the porosity.
Furthermore, the raw materials are preferably mixed so as to obtain a fuel electrode having a desired composition after sintering and reduction.

The method for producing the molded body is not particularly limited, and an optimal method can be selected depending on the purpose. Examples of the method for producing the molded body include
(a) A method of tape-casting a slurry containing a raw material mixture, laminating the obtained green sheet on a substrate (e.g., a green sheet to be a current collecting layer on the anode side after sintering, or a green sheet to be an electrolyte layer after sintering), and isostatically pressing the laminate;
(b) preparing a slurry containing the raw material mixture, and screen printing the slurry on a surface of a substrate;
etc.

[2.2. Sintering process]
Next, the obtained molded body is sintered (sintering step). It is preferable to select the optimum sintering conditions according to the raw material composition. Sintering is usually preferably performed in an air atmosphere at 1000°C to 1500°C for 1 hour to 5 hours.
When the raw material mixture contains two or more oxides, a solid-phase reaction may proceed during sintering to produce a solid solution having a predetermined composition. 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.

[2.3. Reduction step]
Next, the obtained sintered body is subjected to a reduction treatment (reduction step). As a result, the fuel 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 and generate Ni-based particles. The reduction conditions are not particularly limited, and it is preferable to select optimal conditions depending on the composition of the electrode.

As described below, a solid oxide water electrolysis cell is composed of an assembly of a fuel electrode (cathode), an electrolyte layer, a reaction prevention layer, and an air electrode (anode). A fuel electrode-side current collecting layer may be further joined to the outside of the fuel electrode, and/or an air electrode-side current collecting layer may be further joined to the outside of the air electrode. Sintering and joining of each layer is performed by stacking the compacts and then heating the stack to a specified temperature. When the optimal sintering temperature for each layer is different, sintering is usually performed in multiple stages. Furthermore, reduction of the fuel electrode is usually performed after all layers have been joined.

[3. Solid oxide water electrolysis cell]
3.1. Configuration
A schematic diagram of a solid oxide water electrolysis cell using Ni/GDC-YSZ as the fuel electrode (active layer) is shown in Fig. 1. In Fig. 1, a solid oxide water electrolysis cell (hereinafter, simply referred to as "SOEC") 10 is
an electrolyte layer 12 made of a solid oxide electrolyte C;
an anode 14 bonded to one surface of the electrolyte layer 12;
a cathode 16 bonded to the other surface of the electrolyte layer 12;
The battery includes an intermediate layer (reaction prevention layer) 18 inserted between the electrolyte layer 12 and the air electrode 16 .

The solid oxide water electrolysis cell fuel electrode (Ni/GDC-YSZ) according to the present invention is used for the fuel electrode 14. Details of the fuel electrode 14 are as described above, and therefore will not be described here.
1, an anode-side current collecting layer 20 is further disposed on the outer side of the anode 14. The anode-side current collecting layer 20 is for supporting the anode 14 when the thickness of the anode 14 is thin. The anode-side current collecting layer 20 may be omitted.
Similarly, although not shown, an air electrode side current collecting layer may be further disposed on the outside of the air electrode 16. The air electrode side current collecting layer (not shown) is for supporting the air electrode 16 when the thickness of the air electrode 16 is thin. The air electrode side current collecting layer may be omitted.

3.2 Materials
The materials for the electrolyte layer 12, the air electrode 16, the intermediate layer 18, the fuel electrode side current collecting layer 20, and the air electrode side current collecting layer (not shown) are not particularly limited, and an optimum material can be selected depending on the purpose.

[3.2.1. Electrolyte layer, air electrode, intermediate layer]
For example, the electrolyte layer 12 may be made of YSZ.
The cathode 16 can be made of (La,Sr) _CoO3 (LSC), (La,Sr)(Co,Fe) _O3 (LSCF), (La,Sr) _MnO3 (LSM), a composite of LSC and Gd-doped _CeO2 (GDC), or the like.
The intermediate layer 18 is a layer for preventing a reaction caused by direct contact between the electrolyte layer 12 and the air electrode 16, and is inserted as necessary. For example, when the electrolyte layer 12 is YSZ and the air electrode 16 is LSC, it is preferable to use Gd-doped _CeO2 (GDC) for the intermediate layer 18.

[3.2.2. Fuel electrode side current collecting layer]
The fuel electrode side current collecting layer 20 includes at least
(a) a function of supporting the fuel electrode 14 formed on the surface of the electrolyte layer 12;
(b) A function of diffusing the raw material for electrolysis to the fuel electrode 14;
(c) a function of transporting electrons required for the reduction reaction from the current collector to the fuel electrode 14; and
(d) It is necessary to have a function of discharging hydrogen generated at the fuel electrode 14 by the electrode reaction to the outside of the fuel electrode 14.
There are no particular limitations on the composition of the fuel electrode side current collecting layer 20 so long as it exhibits the above-mentioned functions. The current collecting layer is preferably a cermet containing Ni-based particles and electrolyte particles made of a solid oxide electrolyte.

[A. Ni-based particles contained in current collecting layer]
The "Ni-based particles (hereinafter also referred to as "Ni-based particles B")" contained in the fuel electrode-side current collecting layer 20 (hereinafter also simply referred to as the "current collecting layer") refer to metal particles in which the ratio of the mass of Ni to the total mass of metal elements contained in the particles is 10 mass% or more. The mass ratio of Ni contained in the Ni-based particles B is preferably 50 mass% or more, and more preferably 90 mass% or more.

The Ni-based particles B function as an electronic conductor in the current collecting layer. The composition of the Ni-based particles B is not particularly limited as long as it exhibits such a function. Examples of Ni-based particles B include Ni, Ni-Fe alloys, and Ni-Co alloys.

B. Electrolyte Particles
The composition of the electrolyte particles contained in the fuel electrode side current collecting layer 20 (hereinafter also referred to as "electrolyte particles B") is not particularly limited as long as it functions as a current collecting layer.
Examples of the electrolyte particles B include:
(a) yttria-stabilized zirconia ( _YSZ ) containing 3 to 15 mol % _Y2O3 ;
(b) a material having the same or similar composition as the electrolyte particles contained in the anode;
etc.
Among these, YSZ is preferable for the electrolyte particles B because it has stable mechanical strength.

C. Composition of the current collecting layer
The "content of Ni-based particles B" refers to the ratio of the mass of Ni-based particles B to the total mass of electrolyte particles B and Ni-based particles B contained in the current collecting layer.
The content of the Ni-based particles B is not particularly limited as long as the current collecting layer functions as such. The content of the Ni-based particles B contained in the current collecting layer may be the same as or different from the content of the Ni-based particles contained in the fuel electrode. The content of the Ni-based particles B contained in the current collecting layer is usually 30 to 70 mass%.

D. Porosity
The porosity of the current collecting layer affects the gas diffusibility, strength, electronic conductivity, etc. of the current collecting layer. In general, if the porosity of the current collecting layer is too small, the gas diffusibility decreases. Therefore, the porosity of the current collecting 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 current collecting layer is too high, the strength and electronic conductivity may decrease. Therefore, the porosity of the current collecting layer is preferably 60% or less. The porosity is preferably 58% or less, and more preferably 55% or less.

[3.2.3. Cathode-side current collecting layer]
The function of the air electrode side current collecting layer is almost the same as that of the anode side current collecting layer 20 , except that the reaction form at the air electrode 16 is different from that at the anode 14 .
The material of the electrolyte particles in the air electrode side current collecting layer is LSC, LSCF, etc. Other points regarding the air electrode side current collecting layer are similar to those of the fuel electrode side current collecting layer 20, and therefore a description thereof will be omitted.

[4. Action]
[4.1. Oxidation of Ni]
Fig. 2 shows a schematic diagram of a conventional solid oxide water electrolysis cell (SOEC) using Ni/YSZ as the anode (active layer). In Fig. 2, the conventional SOEC 10' includes an electrolyte layer 12, an anode 14' bonded to one surface of the electrolyte layer 12, a cathode 16 bonded to the other surface of the electrolyte layer 12, and an intermediate layer 18 inserted between the electrolyte layer 12 and the cathode 16. The conventional SOEC 10' uses Ni/YSZ cermet as the anode 14'.

In an SOEC 10' having an anode 14' made of Ni/YSZ cermet, when H ₂ O is supplied to the anode 14' and a current is passed between the anode 14' and the air electrode 16, a reduction reaction shown in the following formula (3) proceeds in the anode 14'.
_H2O + ^2e- → _H2 +O2 ^-... (3)

However, in water electrolysis using the SOEC 10', high-temperature (700°C or higher) water vapor, which is the raw material, is supplied to the fuel electrode 14'. As a result, Ni contained in the fuel electrode 14' is easily oxidized to form Ni(OH) ₂ . Since Ni(OH) ₂ is easily evaporated, the morphology of the three-phase interface, which is the electrode reaction site, changes due to the evaporation of Ni(OH) ₂ . As a result, the electrolysis characteristics are degraded.
The same is true for SOFCs. That is, water is generated by an electrode reaction at the anode (fuel electrode) of an SOFC. Therefore, especially under high-load operating conditions, Ni in the fuel electrode may be oxidized by the generated water, resulting in a decrease in power generation characteristics.

[4.2. Suppression of Ni oxidation by GDC components]
In contrast, GDC exhibits high oxygen storage capacity. Therefore, when steam electrolysis is performed using an SOEC equipped with a fuel electrode containing Ni-based particles and electrolyte particles containing a GDC component, the oxidation of Ni is significantly suppressed. In the SOEC, the reason why the oxidation of Ni is significantly suppressed by adding the GDC component to the fuel electrode is considered to be as follows.

[4.2.1. Fixation of oxygen by GDC components]
When a hydrogen electrode containing Ni and GDC components is exposed to high-temperature water vapor, the Ni is oxidized and the surface of the Ni particles becomes Ni(OH) ₂ (step 1). At this time, if there are GDC components in the vicinity of the Ni particles, oxygen is extracted from the Ni(OH) ₂ and the extracted oxygen is fixed to the GDC components (step 2). It is thought that the trigger for this is the oxygen storage capacity of the GDC components.

[4.2.2. Generation of reducing atmosphere by GDC components]
The oxygen extracted from Ni(OH) ₂ is temporarily trapped by the GDC component (step 3). The fuel electrode is in a reducing atmosphere due to _H2 generated from the Ni catalyst. The GDC component itself has water decomposition ability and generates hydrogen from it, so the GDC component also functions as a _H2 source. Furthermore, in a reducing atmosphere, the GDC component easily releases the trapped oxygen atoms or oxygen ions as oxygen molecules. The oxygen molecules released from the GDC component are discharged outside the system before Ni is reoxidized (step 4). Alternatively, the oxygen ions in the electrolyte particles containing the GDC component diffuse within the electrolyte particles due to an electrolytic reaction, and oxygen vacancies are formed (step 5). It is considered that Ni oxidation is suppressed by repeating steps 1 to 5.

(Samples No. 1 to 3)
1. Preparation of Samples
[1.1. Preparation of GDC powder (samples No. 1 and 2)]
Predetermined amounts of cerium nitrate Ce _{(NO3)3.6H2O and gadolinium nitrate Gd(NO3)3.5H2O were weighed and dissolved} in ion-exchanged water. Further, a predetermined amount _of an aqueous solution of cerium nitrate and a predetermined amount of an aqueous solution of gadolinium nitrate were added to a mixture of ion-exchanged water and 25% ammonia water, and the mixture was stirred for 15 minutes to obtain a precipitate.
Next, the precipitate was heat-treated in an air atmosphere at 150°C for 7 h and 400°C for 5 h, and then dried to obtain GDC powders. The Gd doping amount was 10 at% (sample No. 1) or 20 at% (sample No. 2).

[1.2. Preparation of water electrolysis cell]
[1.2.1. Samples No. 1 and 2]
3 shows a schematic diagram of the water electrolysis cells of Samples No. 1 and 2. A GDC sheet (intermediate layer) was placed on one side of an 8YSZ electrolyte pellet (thickness: 500 μm) with a reference electrode (not shown) attached to the side, and the resulting mixture was fired at 1,380° C.
Next, a fuel electrode was formed on the other surface of the 8YSZ electrolyte pellet (the surface opposite to the surface on which the intermediate layer was baked) by applying Ni/GDC paste (NiO:GDC=1:1, mass ratio) by screen printing and firing at 1340°C.

Next, the LSC/GDC paste was applied onto the intermediate layer by screen printing and fired at 1125° C. to form a cathode.
Furthermore, the LSC paste was applied onto the LSC/GDC electrode (air electrode) by screen printing and fired at 950° C. to form an air electrode-side current collecting layer.

[1.2.2. Sample No. 3]
A water electrolysis cell was fabricated in the same manner as in Sample No. 1, except that a Ni/8YSZ paste was used as the paste for forming the fuel electrode.

2. Test Method
Using the obtained water electrolysis cell, a steam electrolysis test (durability test) was carried out for 100 hours under the conditions shown in Table 1.
In addition, impedance measurements were performed during the durability test to measure the reaction resistance (Rct2). Furthermore, the reaction resistance (Rct2) increase rate was calculated using the reaction resistance (Rct2) before and after the durability test.

3. Results
The fuel electrode according to the present invention is characterized in that the change in particle size of Ni particles in the current collecting region (region away from the electrolyte layer/fuel electrode interface) is suppressed. This result means that the shape of the Ni particles, which are the electron paths, does not change and the electrolytic reaction can be stably guaranteed. The reason that the change in particle size of Ni particles is suppressed is that the GDC component has oxygen storage capacity and suppresses the oxidation of Ni in the current collecting region. If the oxygen storage capacity is high, the effect extends not only to the current collecting region but also to the electrode active region (region near the electrolyte layer/fuel electrode interface).

Figure 4 shows the reaction resistance (Rct2) increase rate of the anodes of samples No. 1 to 3. The reaction resistance increase rate of sample No. 3 was 23.5%. On the other hand, the reaction resistance increase rates of samples No. 1 and 2 were 3.2% and 2.6%, respectively. From Figure 4, it can be seen that when Ni/GDC is used as the anode, the reaction resistance increase rate can be reduced to about 1/8 compared to Ni/YSZ. This result shows that when a solid solution or mixture of a YSZ component and a GDC component is used as the anode, the reaction resistance increase rate decreases depending on the amount of the GDC component.
4, it can be seen that the reaction resistance increase rate further decreases as the doping amount of Gd increases, which is believed to be because the oxygen storage capacity of GDC increases as the doping amount of Gd increases.

Although the embodiment of the present invention has been described in detail above, the present invention is not limited to the above embodiment, and various modifications are possible without departing from the gist of the present invention.

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

Claims (7)

Ni-based particles;
and electrolyte particles made of a solid oxide electrolyte A.
The electrolyte particles are a solid solution or a mixture of Y-doped ZrO ₂ (YSZ) and Gd-doped CeO ₂ (GDC), and the electrolyte particles are a fuel electrode for a solid oxide water electrolysis cell. 2. The fuel electrode for a solid oxide water electrolysis cell according to claim 1, wherein the electrolyte particles have an average composition represented by the following formula (1):
{Y _y Zr _1-y O _2-δ' } _1-z {Gd _x Ce _1-x O _2-δ } _z (1)
however,
0<x≦0.30, 0<y≦0.30, 0<z<1.0,
δ and δ' are the values at which electrical neutrality is maintained. The fuel electrode for a solid oxide water electrolysis cell according to claim 1, having a thickness of 15 μm or more and 200 μm or less. The fuel electrode for a solid oxide water electrolysis cell according to claim 1, having a porosity of 15% or more and 40% or less. The fuel electrode for a solid oxide water electrolysis cell according to claim 1, wherein the content of the Ni-based particles is 30.0 mass% or more and 70.0 mass% or less. an electrolyte layer made of a solid oxide electrolyte C;
a fuel electrode formed on one surface of the electrolyte layer;
an air electrode formed on the other surface of the electrolyte layer;
a reaction prevention layer inserted at the interface between the electrolyte layer and the air electrode;
A solid oxide water electrolysis cell, wherein the fuel electrode is the solid oxide water electrolysis cell fuel electrode according to claim 1 . An air electrode-side current collecting layer formed on the surface of the air electrode, and/or
7. The solid oxide water electrolysis cell according to claim 6, further comprising an anode-side current collecting layer formed on a surface of the anode.

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