JP7592986B2 - Electrochemical Cell - Google Patents

Description

The present invention relates to an electrochemical cell.

Conventionally, electrochemical cells such as water electrolysis cells and fuel cells using a solid electrolyte layer with oxygen ion conductivity are known. For example, Patent Document 1 discloses a water electrolysis cell in which a part of an oxygen electrode power supply that supplies current is embedded inside the oxygen electrode. According to the document, this configuration makes it possible to maintain adhesion between the oxygen electrode and the oxygen electrode power supply for a long period of time, to supply electrolysis current uniformly to the oxygen electrode, and to suppress an increase in contact resistance.

JP 2008-045152 A

In the above-mentioned conventional technology, as long as the oxygen electrode and the oxygen electrode power supply are made of different materials, poor contact due to differences in thermal expansion and the like is unavoidable. In other words, in the conventional technology, there are localized areas of contact and areas of non-contact between the oxygen electrode and the oxygen electrode power supply in the oxygen electrode. Therefore, in the conventional technology, it is not possible to obtain a uniform reaction within the cell surface, and reaction unevenness is likely to occur within the cell surface.

The present invention was made in consideration of these problems, and aims to provide an electrochemical cell that can suppress uneven reactions within the cell surface.

One aspect of the present invention is a fuel cell comprising: a solid electrolyte layer (2) having oxygen ion conductivity; a first electrode (3) disposed on one side of the solid electrolyte layer and supplied with fuels (F, F1, F2); and a second electrode (4) disposed on the other side of the solid electrolyte layer and paired with the first electrode;
The second electrode includes a metal oxide having oxygen non-stoichiometry (41) and a decomposition product of the metal oxide having oxygen non-stoichiometry (42),
The metal oxide having oxygen non-stoichiometry is a material that does not decompose under an oxygen partial pressure of 10 ⁻¹ bar to less than 10 ^−0.5 bar during steady-state cell operation , but decomposes at a temperature during steady-state cell operation and an oxygen partial pressure of 10 ⁻⁶ bar or less, or 10 ^−0.5 bar or more.
Located in an electrochemical cell (1).

The electrochemical cell has the above configuration. Therefore, the electrochemical cell can suppress uneven reaction within the cell surface.

The reference characters in parentheses in the claims and the means for solving the problems indicate the corresponding relationship with the specific means described in the embodiments described below, and do not limit the technical scope of the present invention.

FIG. 1 is an explanatory diagram illustrating a schematic microstructure of an electrochemical cell according to the first embodiment. FIG. 2 is a schematic diagram showing how a part of a metal oxide having oxygen non-stoichiometry is converted into a decomposition product of a metal oxide having oxygen non-stoichiometry in a portion of the second electrode where current flows easily locally, and the reaction shifts to another reaction field, in which (a) shows the state before the decomposition of a metal oxide having oxygen non-stoichiometry, and (b) shows the state after the decomposition of a metal oxide having oxygen non-stoichiometry. FIG. 3 is an explanatory diagram for explaining a case where the electrochemical cell according to the first embodiment is operated as a solid oxide electrolysis cell (SOEC mode). FIG. 4 is an explanatory diagram for explaining a case where the electrochemical cell according to the first embodiment is operated as a solid oxide fuel cell (SOFC mode). FIG. 5 is an explanatory diagram illustrating a schematic microstructure of the electrochemical cell according to the second embodiment. FIG. 13 is a diagram showing the results of an XRD analysis of a second electrode of an electrochemical cell fabricated in an experimental example.

The electrochemical cell of the present embodiment includes a solid electrolyte layer having oxygen ion conductivity, a first electrode disposed on one side of the solid electrolyte layer and to which a fuel is supplied, and a second electrode disposed on the other side of the solid electrolyte layer and paired with the first electrode,
The second electrode includes a metal oxide having oxygen non-stoichiometry and a decomposition product of the metal oxide having oxygen non-stoichiometry.

In electrochemical cells using a solid electrolyte layer with oxygen ion conductivity, electrochemical reactions occurring at the three-phase interface in the first electrode to which fuel is supplied are often used, and if no measures are taken, reaction unevenness is likely to occur within the cell surface. In the above electrochemical cell, when a portion where electrolysis current or generation current is likely to flow locally occurs due to unevenness in the microstructure of the first electrode, a high oxygen concentration atmosphere or a low oxygen concentration atmosphere is locally generated in the second electrode paired with the first electrode due to a local overload electrolysis reaction or overload generation reaction (hereinafter, collectively referred to as an overload reaction). In the portion of the second electrode where such a high oxygen concentration atmosphere or low oxygen concentration atmosphere occurs, a metal compound having oxygen non-stoichiometry decomposes, generating decomposition products with reduced activity or inactive decomposition products. Therefore, the reaction moves to another reaction field, and the local overload reaction is suppressed. Therefore, with the above electrochemical cell, reaction unevenness within the cell surface can be suppressed. In addition, by suppressing reaction unevenness within the cell surface, deterioration due to microstructural destruction in the first electrode is also suppressed, and a highly reliable electrochemical cell can be obtained.

The electrochemical cell of this embodiment will be described in detail below with reference to the drawings. Note that the electrochemical cell of this embodiment is not limited to the following examples.

(Embodiment 1)
The electrochemical cell of the first embodiment will be described with reference to Figures 1 to 4. As illustrated in Figure 1, the electrochemical cell 1 of the present embodiment includes a solid electrolyte layer 2, a first electrode 3, and a second electrode 4. The first electrode 3 is disposed on one side of the solid electrolyte layer 2. The second electrode 4 is disposed on the other side of the solid electrolyte layer 2. Specifically, Figure 1 shows an example in which the first electrode 3, the solid electrolyte layer 2, and the second electrode 4 are stacked in this order and joined to each other.

The electrochemical cell 1 may further include an intermediate layer (not shown) between the solid electrolyte layer 2 and the second electrode 4. The intermediate layer is a layer for mainly suppressing a reaction between the material of the solid electrolyte layer 2 and the material of the second electrode 4. In this case, the electrochemical cell 1 may specifically have a configuration in which the first electrode 3, the solid electrolyte layer 2, the intermediate layer, and the second electrode 4 are stacked in this order and joined together. The electrochemical cell 1 may have a flat cell structure. The electrochemical cell 1 may be configured so that the first electrode 3 also functions as a support, or the solid electrolyte layer 2 also functions as a support, or may be configured to be supported by another support (not shown) such as a metal member.

The solid electrolyte layer 2 has oxygen ion conductivity. Specifically, the solid electrolyte layer 2 can be formed in layers from solid electrolytes having oxygen ion conductivity. The solid electrolyte layer 2 is usually formed to be dense in order to ensure gas tightness. As the solid electrolyte constituting the solid electrolyte layer 2, for example, zirconium oxide oxides such as yttria-stabilized zirconia (YSZ) and scandia-stabilized zirconia (ScSZ) can be preferably used from the viewpoints of excellent strength and thermal stability. As the solid electrolyte constituting the solid electrolyte layer 2, yttria-stabilized zirconia and the like are preferable from the viewpoints of oxygen ion conductivity, mechanical stability, compatibility with other materials, and chemical stability from oxidizing atmospheres to reducing atmospheres.

When the solid electrolyte layer 2 does not function as a support, the thickness of the solid electrolyte layer 2 can be preferably 3 to 20 μm, more preferably 3.5 to 15 μm, and even more preferably 4 to 10 μm, from the viewpoint of electrical resistance, etc. When the solid electrolyte layer 2 functions as a support, the thickness of the solid electrolyte layer 2 can be preferably 30 to 300 μm, more preferably 50 to 200 μm, and even more preferably 100 to 150 μm, from the viewpoint of strength, electrical resistance, etc.

The first electrode 3 is a portion to which fuel is supplied. The first electrode 3 can be formed to be porous. The first electrode 3 can be formed in a layered form, and may be composed of a single layer or multiple layers. FIG. 1 shows an example in which the first electrode 3 is composed of a single layer. When the first electrode 3 is composed of multiple layers, the first electrode 3 can be specifically configured to include, for example, an active layer (not shown) arranged on the solid electrolyte layer 2 side and a diffusion layer (not shown) arranged on the opposite side to the solid electrolyte layer 2 side. The active layer is a layer for enhancing the electrochemical reaction in the first electrode 3. The diffusion layer is a layer that can diffuse the supplied fuel F in the in-plane direction of the first electrode 3.

1, the first electrode 3 can be configured to include an electron conductive material 31 having electron conductivity and an oxygen ion conductive material 32 having oxygen ion conductivity. Examples of the electron conductive material 31 include metal materials having catalytic activity such as Ni and Ni alloys. These can be used alone or in combination of two or more. Examples of the oxygen ion conductive material used in the first electrode 3 include zirconium oxides such as yttria-stabilized zirconia and scandia-stabilized zirconia. These can be used alone or in combination of two or more. In this embodiment, the first electrode 3 can be configured to include a mixture of Ni and yttria-stabilized zirconia. In FIG. 1, the reference numeral 30 denotes a pore. In addition, both the electron conductive material 31 and the oxygen ion conductive material 32 in the first electrode 3 can be in the form of particles.

When the first electrode 3 is made to function as a support, the thickness of the first electrode 3 can be, for example, preferably 100 to 800 μm, more preferably 150 to 700 μm, and even more preferably 200 to 600 μm, from the viewpoints of strength, electrical resistance, gas diffusibility, etc. When the first electrode 3 is not made to function as a support, the thickness of the first electrode 3 can be, for example, preferably 10 to 40 μm, more preferably 15 to 35 μm, and even more preferably 20 to 30 μm, from the viewpoints of electrical resistance, gas diffusibility, etc.

The second electrode 4 is an electrode that forms a pair with the first electrode 3. Specifically, as shown in FIG. 1, the second electrode 4 can be disposed so as to face the first electrode 3 with the solid electrolyte layer 2 interposed therebetween. The outer shape of the second electrode 4 can be formed, for example, to be the same size as the outer shape of the first electrode 3, or can be formed to be smaller than the outer shape of the first electrode 3. The second electrode 4 can be formed to be porous. The second electrode 4 can be formed in a layered shape, and can be composed of a single layer or multiple layers. FIG. 1 shows an example in which the second electrode 4 is composed of a single layer.

Here, the second electrode 4 contains a metal oxide 41 having oxygen non-stoichiometry and a decomposition product 42 of the metal oxide having oxygen non-stoichiometry. The metal oxide 41 having oxygen non-stoichiometry is a composite metal oxide that contains metal elements with different valences and can have point defects at oxygen positions. The decomposition product 42 of the metal oxide having oxygen non-stoichiometry is a substance produced by decomposition of the metal oxide 41 having oxygen non-stoichiometry. The metal oxide 41 having oxygen non-stoichiometry may be one or more types contained in the second electrode 4. Also, the decomposition product 42 of the metal oxide having oxygen non-stoichiometry may be one or more types contained in the second electrode 4.

The metal oxide 41 having oxygen non-stoichiometry can be configured to include an oxygen-deficient metal oxide 411. The oxygen-deficient metal oxide 411 is a metal oxide with oxygen vacancies, and the above-mentioned point defect positions are vacant. The oxygen-deficient metal oxide 411 becomes unstable when excess oxygen enters, and tends to change into a more stable oxide or the like. Therefore, the configuration in which the metal oxide 41 having oxygen non-stoichiometry includes the oxygen-deficient metal oxide 411 has the following advantages. That is, the oxygen-deficient metal oxide 411 increases sensitivity to a high oxygen concentration atmosphere, and as shown in FIG. 2(a), in the part where a high oxygen concentration atmosphere occurs, the oxygen-deficient metal oxide 411 is decomposed by the excess oxygen that has entered, and a decomposition product with reduced activity or an inactive decomposition product is generated. That is, as shown in FIG. 2(b), a decomposition product 421 of the oxygen-deficient metal oxide is generated. Since the decomposition product 421 of the oxygen-deficient metal oxide has poor electrode activity, as shown in Figures 2(a) and (b), the reaction moves to another reaction field 44, suppressing local overload reactions (local overload water electrolysis reactions in the SOEC mode described below, and local overload power generation reactions in the SOFC mode described below), and suppressing reaction unevenness within the cell surface.

The metal oxide 41 having oxygen non-stoichiometry can be configured to have a perovskite structure or a double perovskite structure. According to this configuration, metal elements with different valences are fixed, so that in a steady-state operating environment of a solid oxide electrochemical cell (SOEC) such as a water electrolysis cell (including a steam electrolysis cell, omitted below) or a solid oxide fuel cell (SOFC), the structure can be changed in response to oxygen concentration fluctuations while maintaining the structure having oxygen non-stoichiometry, and a stable compound can be formed. Therefore, according to this configuration, even if the second electrode 4 is placed in an environment with a different oxygen concentration during steady-state operation of the electrochemical cell 1, it is easy to ensure electrode activity. The metal oxide 41 having oxygen non-stoichiometry can preferably be configured to have a perovskite structure. According to this configuration, the metal in the A site or B site can form a stable compound due to oxygen concentration fluctuations. Therefore, this configuration is advantageous for achieving a uniform electrolysis reaction while maintaining power generation performance.

Examples of oxygen-deficient metal oxide 411 include (La,Sr)CoO _3-δ , (La,Sr)(Co,Fe)O _3-δ , (La,Sr)(Mn,Fe)O _3-δ , (La,Sr)FeO _3-δ , (Sm,Sr)CoO _3-δ , (La,Ca)CrO _3-δ , (Ba,La)CoO _{3-δ ,} and the like. These can be used alone or in combination of two or more. Here, δ indicates the oxygen non-stoichiometric amount. In addition, the above (A,B) means that at least one of element A and element B is included, and element A and element B can be partially substituted for each other so that the total chemical composition of element A and element B is 1 (the same applies below). Of the oxygen-deficient metal oxides 411 described above, from the viewpoints of electrical resistivity, low toxicity to other functional layers, raw material costs, etc., preferred are (La,Sr)CoO _3-δ , (La,Sr)(Co,Fe)O _3-δ , (Sm,Sr)CoO _3-δ , etc., and more preferably (La,Sr)CoO _3-δ .

Specifically, (La,Sr)CoO _3-δ is La _1-x Sr _x CoO _3-δ (0≦x≦1, preferably 0.1≦x≦0.5), specifically, (La,Sr)(Co,Fe)O _3-δ is La _1-x Sr _x Co _1-y Fe _y O _3-δ (0≦x≦1, 0≦y≦1, preferably 0.1≦x≦0.5, 0.5≦y≦0.9), specifically, (La,Sr)(Mn,Fe)O _3-δ is La _1-x Sr _x Mn _1-y Fe _y O _3-δ. (0≦x≦1, 0≦y≦1, preferably 0.1≦x≦0.5, 0.5≦y≦0.9), (La,Sr)FeO _3-δ is specifically La _1-x Sr _x FeO _3-δ (0≦x≦1, preferably 0.1≦x≦0.5), (Sm,Sr)CoO _3-δ is specifically Sm _1-x Sr _x CoO _3-δ (0≦x≦1, preferably 0.4≦x≦0.6), (La,Ca)CrO _3-δ is specifically La _1-x Ca _x CoO _3-δ (0≦x≦1, preferably 0.1≦x≦0.5), (Ba,La)CoO _3-δ is specifically Ba _1-x La _x CoO _3-δ (0≦x≦1, preferably 0.1≦x≦0.5), etc. These may be used alone or in combination of two or more. These metal oxides have a perovskite structure.

Specific examples of the decomposition product 421 of the oxygen-deficient metal oxide include inactive oxides containing one or more of the metal elements constituting the oxygen-deficient metal oxide 411. Examples of the decomposition product of (La,Sr)CoO _3-δ include oxides containing Sr and Co, such as Sr ₃ Co ₂ O _6.6 and Sr ₂ Co ₂ O _5. Examples of the decomposition product of (La,Sr)(Co,Fe)O _3-δ include oxides containing Sr and Co and/or Fe, such as Sr ₃ Co ₂ O _6.6 , Sr ₂ Co ₂ O ₅ and SrFeO ₂ . Examples of decomposition products of (Sm, Sr)CoO _3-δ include oxides containing Sr and Co, such as Sr ₃ Co ₂ O _6.6 and Sr ₂ Co ₂ O ₅ .

In FIG. 1, an example is shown in which the metal oxide 41 having oxygen non-stoichiometry is composed of an oxygen-deficient metal oxide 411. Therefore, in this embodiment, as illustrated in FIG. 1, the second electrode 4 contains the oxygen-deficient metal oxide 411 and a decomposition product 421 of the oxygen-deficient metal oxide. The decomposition product 421 of the oxygen-deficient metal oxide is a substance produced by decomposition of a part of the oxygen-deficient metal oxide 411 contained in the second electrode 4. Note that the above-mentioned metal oxide 41 having oxygen non-stoichiometry, the decomposition product 42 of the metal oxide having oxygen non-stoichiometry, the oxygen-deficient metal oxide 411, and the decomposition product 421 of the oxygen-deficient metal oxide can all be in the form of particles.

The metal oxide 41 having oxygen non-stoichiometry is a material that does not decompose under an oxygen partial pressure of 10 ⁻¹ bar to less than 10 ^−0.5 bar during steady cell operation , but decomposes at the temperature during steady cell operation and an oxygen partial pressure of 10 ⁻⁶ bar or less, or 10 ^−0.5 bar or more. Usually, during steady cell operation of SOEC or SOFC, the oxygen concentration to which the oxygen electrode of the SOEC or the air electrode (oxidizer electrode) of the SOFC is exposed is 10 ⁻¹ bar to less than 10 ^−0.5 bar. Therefore, according to the above configuration, when the electrochemical cell 1 is operated in steady state as at least one of the SOEC and the SOFC, the metal oxide 41 having oxygen non-stoichiometry is unlikely to decompose in the oxygen concentration atmosphere to which the second electrode 4 is exposed. Therefore, according to the above configuration, an electrochemical cell 1 that is easy to ensure stable output during steady operation can be obtained.

For example, the above-mentioned (La,Sr)CoO _3-δ and (La,Sr)(Co,Fe)O _3-δ decompose at an oxygen concentration of 10 ⁻⁸ bar, as understood from the literature "K.Kendall, M.Kendall, "Chapter 6-Cathodes" High-temperature Solid Oxide Fuel Cells for the 21st Century, p166, Figure 6.4 (2015)". Also, (La,Sr)MnO _3+δ , which will be described later in embodiment 2, decomposes at an oxygen concentration of 10 ⁻⁷ bar, as understood from the literature "T.Ishihara "Perovskite Oxide for Cathode of SOFCs" Perovskite Oxide for Solid Oxide Fuel Cells, p160, Figure 7.9 (2009)".

As illustrated in Fig. 1, the second electrode 4 may further include an oxygen ion conductive material 43 having oxygen ion conductivity. Examples of the oxygen ion conductive material 43 used in the second electrode 4 include ceria ( _CeO2 ), ceria doped with one or more elements selected from Gd, Y, Sm, La, Nd, Yb, Ca, and Ho, zirconia doped with one or more elements selected from Y, Ca, and Sc, (La,Sr)(Ga,Mg) _O3 , (La,Sr)(Sc,Mg) _O3 , and (La,Sr)(Al,Mg) _O3 . These may be used alone or in combination of two or more. Specifically, (La,Sr)(Ga,Mg) _O3 is La1 _{-xSrxGa1 - yMgyO3 (} 0≦x≦1, 0≦y≦1, preferably 0.1≦x≦0.3, 0.1≦y _≦ 0.3), specifically, (La,Sr)(Sc, _Mg ) _O3 is La1 _{-xSrxSc1 - yMgyO3 (} 0≦x≦1, 0≦y≦1, preferably 0.1≦x≦0.5, 0.1≦y≦0.3), specifically, (La,Sr)(Al,Mg) _{O3 is La1- xSrxAl1 - yMgyO3} (0≦x≦1, 0≦y≦1, preferably 0.1≦x≦0.5, 0.1≦y≦0.5) and the like can be exemplified.

Of the above-mentioned oxygen ion conductive materials 43 used in the second electrode 4, Gd-doped ceria is preferable from the viewpoints of small temperature dependence of oxygen ion conductivity, electronic conductivity, and suppression of reaction with other functional layers. In FIG. 1, the reference numeral 40 denotes pores. The oxygen ion conductive material 43 used in the second electrode 4 can be in the form of particles.

In the second electrode 4, the amount of the metal oxide 41 having oxygen uncertainty in the total of the metal oxide 41 having oxygen uncertainty and the decomposition product 42 of the metal oxide having oxygen uncertainty is preferably 95% by volume or more, from the viewpoint of ensuring a sufficient number of reaction points for the electrolytic reaction and the power generation reaction. The above volume percentage can be calculated using the results of 3D elemental mapping using FIB-SEM-EDS (energy dispersive X-ray spectroscopy using a focused ion beam scanning electron microscope) or the like.

The thickness of the second electrode 4 can be 5 μm or more. In the electrochemical cell 1, in addition to the metal oxide 41 having oxygen non-stoichiometry, the second electrode 4 contains decomposition products 42 of the metal oxide having oxygen non-stoichiometry. In other words, the second electrode 4 loses some of its electrode activity due to the decomposition of a part of the metal oxide 41 having oxygen non-stoichiometry, compared to an electrode that contains the metal oxide 41 having oxygen non-stoichiometry and does not contain the decomposition products 42 of the metal oxide having oxygen non-stoichiometry. Therefore, if the thickness of the second electrode 4 is within the above range, it becomes easier to ensure reaction points where sufficient reaction can be obtained by ensuring the electrode thickness, and it becomes easier to suppress the performance deterioration of the second electrode 4.

The thickness of the second electrode 4 can be preferably 10 μm or more, more preferably 15 μm or more, even more preferably 20 μm or more, and even more preferably 25 μm or more, from the viewpoint of ensuring sufficient reaction sites. The thickness of the second electrode 4 can be preferably 100 μm or less, more preferably 60 μm or less, and even more preferably 50 μm or less, from the viewpoint of gas diffusion, electrical resistance, and the like.

When the electrochemical cell 1 has an intermediate layer, the intermediate layer can be specifically formed in a layer shape from a solid electrolyte having oxygen ion conductivity. Examples of the solid electrolyte used in the intermediate layer include ceria (CeO ₂ ) and ceria doped with one or more elements selected from Gd, Sm, Y, La, Nd, Yb, Ca, and Ho. These can be used alone or in combination. Gd-doped ceria is preferable as the solid electrolyte used in the intermediate layer.

The thickness of the intermediate layer can be preferably 1 to 20 μm, and more preferably 2 to 10 μm, from the viewpoints of reducing ohmic resistance and suppressing element diffusion from the second electrode.

The thickness of each portion of the electrochemical cell 1 described above is the arithmetic average of the thickness measurements of 10 points at each portion measured by observing a cross section perpendicular to the cell surface of the electrochemical cell 1 with a scanning electron microscope (SEM).

The electrochemical cell 1 can be manufactured, for example, as follows.

A cell (not shown) is prepared, which includes a solid electrolyte layer 2 having oxygen ion conductivity, a first electrode 3 disposed on one side of the solid electrolyte layer 2, and a second electrode precursor (not shown) disposed on the other side of the solid electrolyte layer 2, which includes a metal oxide 41 having oxygen non-stoichiometry but does not include a decomposition product 42 of the metal oxide having oxygen non-stoichiometry. In this embodiment, specifically, for example, a second electrode precursor containing an oxygen-deficient metal oxide 411 but not a decomposition product 421 of the oxygen-deficient metal oxide can be used. Next, a leveling process is performed on this cell, which is operated at a current density higher than the current density during steady operation of the electrochemical cell 1, that is, in a current density region in which non-uniform reactions are more evident. As a result, in a portion in the second electrode precursor where current is likely to flow locally, the metal oxide 41 having oxygen non-stoichiometry decomposes, and a decomposition product 42 of the metal oxide having oxygen non-stoichiometry is generated. On the other hand, in a portion in the second electrode precursor where local current does not flow, decomposition of the metal oxide 41 having oxygen non-stoichiometry hardly progresses. The leveling process converts the second electrode precursor into the second electrode 4 containing the metal oxide 41 having oxygen non-stoichiometry and the decomposition product 421 of the metal oxide having oxygen non-stoichiometry, and the electrochemical cell 1 described above is obtained. That is, in the manufacturing method of the electrochemical cell 1, the leveling process is performed on the cell, and a part of the metal oxide 41 having oxygen non-stoichiometry is intentionally decomposed in the cell state, thereby mitigating the reaction distribution in the cell surface of the electrochemical cell 1 obtained. That is, the reaction in the cell surface of the electrochemical cell 1 is leveled. Note that, as the leveling process, for example, a process of supplying a mixed gas of H ₂ and H ₂ O to the first electrode 3, supplying air to the second electrode precursor, and operating at a leveling temperature of 650° C. to 850° C. in a current density region higher than the current density during steady operation can be exemplified. As the current density region higher than the current density during steady operation, for example, a current density range of 2 to 4 times the current density (described later) when the electrochemical cell 1 is operated for steady power generation can be exemplified. The treatment time for the power generation operation can be, for example, 2 to 10 hours. The above treatment can be carried out once, or can be carried out multiple times under the same or different conditions. The electrochemical cell 1 thus obtained is preferably operated in a steady state at a current density equal to or lower than the current density during the above-mentioned leveling treatment.

The electrochemical cell 1 can be used as at least one of an SOEC and an SOFC. That is, the electrochemical cell 1 may be operated as an SOEC, or may be operated as an SOFC, or may be configured to be switchable between an SOEC mode in which the electrochemical cell 1 is operated as an SOEC and an SOFC mode in which the electrochemical cell 1 is operated as an SOEC and an SOFC. An example of the operation of the electrochemical cell 1 will be described below.

First, an example of operating the electrochemical cell 1 as an SOEC will be described with reference to FIG. 3. In this case, the first electrode 3 is used as a hydrogen electrode, and the second electrode 4 is used as an oxygen electrode. The first electrode 3 can be supplied with a water (H ₂ O)-containing gas F1, such as water vapor gas, as the fuel F. The second electrode 4 can be supplied with a gas A1, such as air, or no gas A1. FIG. 3 shows an example of supplying the gas A1 to the second electrode 4. A first electrode side gas flow passage 5 can be arranged in contact with the first electrode 3 on the first electrode 3 side. A second electrode side gas flow passage 6 can be arranged in contact with the second electrode 4 on the second electrode 4 side. A power source 7 is connected between the first electrode 3 and the second electrode 4, as illustrated in FIG. 3. In the SOEC mode, the fuel F supplied from the supply port 51 of the first electrode side gas flow passage 5 passes through the first electrode side gas flow passage 5 and reaches the first electrode 3. At the first electrode 3, an electrode reaction of H ₂ O+2e ^- →H ₂ +O ^2- occurs. The remaining fuel F is discharged from the outlet 52 together with the generated hydrogen gas (H ₂ ) through the first electrode side gas flow passage 5. On the other hand, at the second electrode 4, an electrode reaction of O ^2- →1/2O ₂ +2e ^- → occurs. The generated oxygen gas (O ₂ ) is discharged from the outlet 62. When the gas A1 is not supplied to the second electrode 4, the generated oxygen gas is discharged from the outlet 62 due to the difference in oxygen concentration with the outside. When the gas A1 is supplied to the second electrode 4, the generated oxygen gas has an advantage that its concentration is diluted by the gas A1 and discharged to the outside. In the solid electrolyte layer 2, oxygen ions (O ^2- ) move from the first electrode 3 side toward the second electrode 4 side. In this way, the electrochemical cell 1 can be operated as an SOEC. In addition, in the first electrode 3, a conditioning gas (reducing gas) (not shown) such as hydrogen gas can be mixed with the water-containing gas F1 as the fuel F. In this case, oxidation of the metal material that may be contained in the first electrode 3 can be suppressed during the SOEC operation.

Next, an example of operating the electrochemical cell 1 as an SOFC will be described with reference to FIG. 4. In this case, the first electrode 3 is used as a fuel electrode, and the second electrode 4 is used as an air electrode (oxidizer electrode). The first electrode 3 can be supplied with a hydrogen-containing gas F2 such as hydrogen gas as a fuel F. The second electrode 4 can be supplied with an oxygen-containing gas A2 such as air or oxygen gas as an oxidizer. A first electrode side gas flow passage 5 can be arranged in contact with the first electrode 3 on the first electrode 3 side. A second electrode side gas flow passage 6 can be arranged in contact with the second electrode 4 on the second electrode 4 side. As illustrated in FIG. 4, a load 8 is connected between the first electrode 3 and the second electrode 4. In the SOFC mode, the fuel F supplied from the supply port 51 of the first electrode side gas flow passage 5 passes through the first electrode side gas flow passage 5 and reaches the first electrode 3. At the first electrode 3, an electrode reaction of H ₂ +O ²⁻ →H ₂ O+2e ⁻ occurs. The remaining fuel F is discharged from the exhaust port 52 through the first electrode side gas flow passage 5 together with the generated water vapor (H ₂ O). On the other hand, the oxygen-containing gas A2 supplied from the supply port 61 of the second electrode side gas flow passage 6 passes through the second electrode side gas flow passage 6 and reaches the second electrode 4. At the second electrode 4, an electrode reaction of 1/2O ₂ +2e ^- →O ^2- occurs. The remaining oxygen-containing gas A2 is discharged from the exhaust port 62 through the second electrode side gas flow passage 6. At the solid electrolyte layer 2, oxygen ions (O ^2- ) move from the second electrode 4 side toward the first electrode 3 side. In this way, the electrochemical cell 1 can be operated as a SOFC. Note that the hydrogen-containing gas F2 as the fuel F can also be mixed with water vapor or the like (not shown) at the first electrode 3.

Next, an example of operating the electrochemical cell 1 as both an SOEC and an SOFC (operating the electrochemical cell 1 reversibly) will be described. In this case, the electrochemical cell 1 may be configured to operate as both an SOEC and an SOFC as described above. With this configuration, it is possible to obtain an electrolysis/power generation device that uses electricity from surplus power, renewable energy, etc., operates the electrochemical cell 1 as an SOEC, stores the generated hydrogen gas separately, and uses the stored hydrogen gas as fuel to operate the electrochemical cell 1 as an SOFC to generate electricity.

The steady-state operating temperature of the electrochemical cell 1 can be, for example, 400° C. or more and 800° C. or less when functioning as an SOEC, and 400° C. or more and 800° C. or less when functioning as an SOFC, similar to the SOEC. The current density during steady-state operation of the electrochemical cell 1 can be, specifically, 0.4 A/cm ² or more and 2.0 A/cm ² or less when functioning as an SOEC, and 0.2 A/cm ² or more and 1.0 A/cm ² or less when functioning as an SOFC.

(Embodiment 2)
The electrochemical cell of the second embodiment will be described with reference to Fig. 5. Of the symbols used in the second and subsequent embodiments, those that are the same as those used in the previous embodiments represent the same components, etc., as those in the previous embodiments, unless otherwise specified.

As illustrated in FIG. 5, the electrochemical cell 1 of this embodiment is different from the electrochemical cell 1 of embodiment 1 in the configuration of the second electrode 4. Specifically, in this embodiment, the metal oxide 41 having oxygen non-stoichiometry in the second electrode 4 can be configured to further include an oxygen-excess metal oxide 412.

The oxygen-excess metal oxide 412 is a metal oxide with interstitial oxygen, and the above-mentioned point defect positions are excessively filled. When the interstitial oxygen is removed, the oxygen-excess metal oxide 412 becomes unstable and tries to change into a more stable oxide. Therefore, according to a configuration in which the metal oxide 41 having oxygen non-stoichiometry includes not only the oxygen-deficient metal oxide 411 but also the oxygen-excess metal oxide 412, there are the following advantages. That is, when a high oxygen concentration atmosphere is locally generated in the second electrode 4, even if the oxygen-deficient metal oxide 411 contained in the second electrode 4 is excessively decomposed, the oxygen-excess metal oxide 412 does not decompose. Conversely, when a low oxygen concentration atmosphere is locally generated in the second electrode 4, even if the oxygen-excess metal oxide 412 contained in the second electrode 4 is excessively decomposed, the oxygen-deficient metal oxide 411 does not decompose. Therefore, according to the above configuration, it is possible to suppress the amount of catalytically active material in the second electrode 4 from being excessively reduced, and it is possible to suppress a decrease in the electrode activity of the second electrode 4. In other words, with the above configuration, even if a high oxygen concentration atmosphere or a low oxygen concentration atmosphere occurs locally within the second electrode 4, excessive decomposition of the metal oxide 41 having oxygen instability that exerts catalytic activity can be suppressed.

Examples of the oxygen-excess metal oxide 412 include (La, Sr) _MnO3+δ , _{PrBaCo2O5 +δ} , and _{PrBaFe2O5 +δ} . These may be used alone or in combination of two or more. Here, δ indicates the non-stoichiometric amount of oxygen. Among the above-mentioned oxygen-excess metal oxides 412, from the viewpoints of electrical resistivity, costs required for material purification, and the like, preferred are (La, Sr) _MnO3+δ , _{PrBaCo2O5 +δ} , and the like, and more preferred is (La, Sr)MnO3 _+δ .

Specific examples of (La,Sr)MnO _3+δ include La _1-x Sr _x MnO _3+δ (0≦x≦1, preferably 0.1≦x≦0.4). These can be used alone or in combination of two or more. These metal oxides have a perovskite structure or a double perovskite structure.

Specific examples of the decomposition product 422 of the oxygen-excess metal oxide include inactive oxides containing one or more metal elements constituting the above-mentioned oxygen-excess metal oxide _412. Examples of the decomposition product of (La,Sr) _{MnO3 +δ} include oxides containing Sr and Mn, such as _{Sr2Mn2O5 and Sr2MnO4} .

In this embodiment, the second electrode 4 may include an oxygen-deficient metal oxide 411, a decomposition product 421 of the oxygen-deficient metal oxide, and an oxygen-excess metal oxide 412, or may include an oxygen-deficient metal oxide 411, a decomposition product 421 of the oxygen-deficient metal oxide, an oxygen-excess metal oxide 412, and a decomposition product 422 of the oxygen-excess metal oxide. The decomposition product 422 of the oxygen-excess metal oxide is a substance produced by decomposition of a part of the oxygen-excess metal oxide 412 contained in the second electrode 4. The metal oxide 41 having oxygen non-stoichiometry, the decomposition product 42 of the metal oxide having oxygen non-stoichiometry, the oxygen-deficient metal oxide 411, the decomposition product 421 of the oxygen-deficient metal oxide, the oxygen-excess metal oxide 412, and the decomposition product 422 of the oxygen-excess metal oxide can all be particulate. In addition, the electrochemical cell 1 having the second electrode 4 in this embodiment can be manufactured, for example, as follows. Specifically, the second electrode precursor described in the first embodiment includes an oxygen-deficient metal oxide 411 and an oxygen-excess metal oxide 412, but does not include a decomposition product 421 of the oxygen-deficient metal oxide and a decomposition product 422 of the oxygen-excess metal oxide. As described in the first embodiment, the electrochemical cell 1 is operated at a current density in the range of 2 to 4 times the current density during steady-state power generation, and then the electrochemical cell 1 is further operated at a current density in the range of 2 to 4 times the current density during steady-state electrolysis. The treatment time for the electrolysis operation can be, for example, 2 to 10 hours.

The second electrode 4 may also include an oxygen ion conductive material 43 having oxygen ion conductivity as described above in embodiment 1. FIG. 5 shows an example in which the second electrode 4 includes an oxygen-deficient metal oxide 411, a decomposition product of the oxygen-deficient metal oxide 421, an oxygen-excess metal oxide 412, a decomposition product of the oxygen-excess metal oxide 422, and an oxygen ion conductive material 43.

The other configurations and effects are the same as those of embodiment 1.

(Experimental Example)
<Material preparation>
NiO powder (average particle size: 1.0 μm), yttria-stabilized zirconia (hereinafter, YSZ) powder (average particle size: 0.5 μm) containing 8 mol% Y ₂ O ₃ , carbon (pore former), polyvinyl butyral, isoamyl acetate, and 1-butanol were mixed in a ball mill to prepare a slurry. The mass ratio of NiO powder to YSZ powder was 65:35. The slurry was applied in layers on a resin sheet using a doctor blade method, dried, and then the resin sheet was peeled off to prepare a sheet for forming a first electrode. The average particle size is the particle size (diameter) d50 when the cumulative frequency distribution based on the volume measured by the laser diffraction/scattering method shows 50% (hereinafter the same).

A slurry was prepared by mixing YSZ powder (average particle size: 0.5 μm), polyvinyl butyral, isoamyl acetate, and 1-butanol in a ball mill. Thereafter, a sheet for forming a solid electrolyte layer was prepared in the same manner as the sheet for forming the first electrode.

A slurry was prepared by mixing Gd-doped CeO ₂ (hereinafter, GDC) powder (average particle size: 0.3 μm), polyvinyl butyral, isoamyl acetate, 2-butanol, and ethanol in a ball mill. In this experimental example, CeO ₂ doped with 10 mol % Gd was used as the GDC. Thereafter, an intermediate layer-forming sheet was prepared in the same manner as in the production of the first electrode-forming sheet.

(La, Sr)CoO _3-δ powder (average particle size: 1.5 μm) as an oxygen-deficient metal oxide and GDC powder (average particle size: 2.0 μm) as an oxygen ion conductive material were mixed and dispersed in terpineol as a solvent together with a dispersant. Ethyl cellulose as a binder was added to the obtained dispersion and mixed to prepare a paste for forming a second electrode. Specifically, La _0.6 Sr _0.4 CoO _3-δ powder was used as the (La, Sr)CoO _3-δ powder. The mass ratio of the (La, Sr)CoO _3-δ powder to the GDC powder was 50:50.

<Preparation of electrochemical cell>
A plurality of sheets for forming the first electrode, a sheet for forming the solid electrolyte layer, and a sheet for forming the intermediate layer were laminated in this order and pressed together using a hydrostatic press (WIP) molding method. The WIP molding conditions were a temperature of 85° C., a pressure of 50 MPa, and a pressing time of 10 minutes. The obtained molded body was degreased.

The resulting molded body was then sintered in an air atmosphere at 1,350°C for 2 hours. This resulted in a sintered body in which a layered first electrode (thickness 400 μm), a solid electrolyte layer (3.5 μm), and an intermediate layer (3 μm) were laminated in this order.

Next, a paste for forming a second electrode was applied to the surface of the intermediate layer of the fired body by screen printing, and fired (baked) in an air atmosphere at 950°C for 2 hours to form a layered second electrode precursor (thickness 50 μm). The outer shape of the second electrode precursor was smaller than that of the first electrode. This resulted in a flat cell. The formed cell is usually assembled to a metal stack member and can be operated as a system. In this experimental example, in order to eliminate various environmental factors such as poisonous substances scattered from metal members and temperature distribution due to large size, a small cell was formed, and only this cell was used to perform the following processing and evaluation using an element evaluation bench.

Next, sealing glass was applied to the end faces of the obtained cell. The cell was heated to 850°C in an air atmosphere, and the first electrode and the fuel pipe were sealed with glass to form a gas seal structure. Next, the first electrode was subjected to a reduction treatment. The reduction treatment conditions were as follows: reduction temperature: 800°C, reduction treatment gas: hydrogen gas, reduction time: 3 hours.

Next, after the reduction treatment, the temperature was slightly lowered to the power generation temperature, and the cell was operated for 4 hours at a current density (specifically, 1.5 A/cm ² ) twice the current density during steady power generation. At this time, a mixed gas of H ₂ and H ₂ O (H ₂ :H ₂ O=6:4 in volume ratio) was supplied to the first electrode, and air was supplied to the second electrode precursor. The power generation temperature was set to 700° C. In this experimental example, the cell operated for power generation was further operated for 4 hours at the same current density as above. At this time, a mixed gas of H ₂ and H ₂ O (H ₂ :H ₂ O=6:4 in volume ratio) was supplied to the first electrode, and air was supplied to the second electrode precursor. The water electrolysis temperature was set to 700° C. Thereafter, the temperature was lowered while maintaining the atmosphere of the first electrode. In this manner, an electrochemical cell according to this experimental example was produced. In this experimental example, both the process by the power generation operation and the process by the electrolysis operation were performed as the leveling process. This is to ensure that a uniform reaction is obtained in both modes, because the reaction distribution caused by the unevenness of the voids in the cell plane, etc., creates areas that are difficult to react if there is a rate-limiting part for the hydrogen supply during power generation operation, whereas creates areas that are difficult to react if there is a rate-limiting part for the water vapor supply during electrolysis operation.

<XRD analysis of second electrode>
The electrochemical cell was taken out of the element evaluation bench and the structure of the second electrode was analyzed using an X-ray diffraction apparatus (RIGAKU Corporation, fully automated multipurpose X-ray diffraction apparatus "SmartLab"). In this experimental example, in order to exclude various environmental factors during the production of the electrochemical cell, the leveling process was performed using only a small cell. Therefore, the substances that can be contained in the second electrode can be basically broadly divided into (La, Sr)CoO _3-δ and GDC (including impurities in the raw materials) used as the raw materials for the second electrode, and other substances, that is, compounds formed during production.

FIG. 6 shows the XRD analysis result of the second electrode. According to FIG. 6, in the second electrode of the electrochemical cell, in addition to the peaks of (La, Sr) CoO _3-δ and GDC used as the raw material of the second electrode, the peaks of La ₂ O ₃ and Sr ₂ Co ₂ O ₅ were detected. The reason why the peaks of La ₂ O ₃ and Sr ₂ Co ₂ O ₅ were detected is because (La, Sr) CoO _3-δ , which is an oxygen non-stoichiometric metal oxide, was decomposed by the above-mentioned leveling treatment to produce La ₂ O ₃ and Sr ₂ Co ₂ O _5. For comparison, the second electrode precursor in the cell that was not subjected to the above-mentioned leveling treatment was subjected to the XRD analysis in the same manner, and the peaks of La ₂ O ₃ and Sr ₂ Co ₂ O ₅ were not detected in the second electrode precursor.

The electrochemical cell and a cell that had not been subjected to the leveling treatment were used as SOFCs and were operated steadily at 700°C and a current density of 0.5 A/ ^cm2 . As a result, the electrochemical cell had a reduced reaction temperature distribution within the cell surface compared to the cell that had not been subjected to the leveling treatment. The electrochemical cell was operated as an SOFC to confirm the reaction temperature distribution within the cell surface because the SOFC generates electricity and the reaction temperature distribution within the cell surface is easier to confirm than with an SOEC. From these results, it was confirmed that the electrochemical cell can suppress reaction unevenness within the cell surface.

The present invention is not limited to the above-described embodiments and experimental examples, and various modifications are possible without departing from the spirit of the invention. Furthermore, the configurations shown in the embodiments and experimental examples can be combined in any manner.

1 Electrochemical cell 2 Solid electrolyte layer 3 First electrode 4 Second electrode F Fuel F1 Water-containing gas F2 Hydrogen-containing gas

Claims (6)

a solid electrolyte layer (2) having oxygen ion conductivity, a first electrode (3) disposed on one side of the solid electrolyte layer and supplied with fuels (F, F1, F2), and a second electrode (4) disposed on the other side of the solid electrolyte layer and paired with the first electrode;
The second electrode includes a metal oxide having oxygen non-stoichiometry (41) and a decomposition product of the metal oxide having oxygen non-stoichiometry (42),
The metal oxide having oxygen non-stoichiometry is a material that does not decompose under an oxygen partial pressure of 10 ⁻¹ bar to less than 10 ^−0.5 bar during steady-state cell operation , but decomposes at a temperature during steady-state cell operation and an oxygen partial pressure of 10 ⁻⁶ bar or less, or 10 ^−0.5 bar or more.
Electrochemical cell (1). The metal oxide having oxygen non-stoichiometry includes an oxygen-deficient metal oxide (411),
10. The electrochemical cell of claim 1. The metal oxide having oxygen non-stoichiometry further includes an oxygen-excess metal oxide (412),
3. The electrochemical cell of claim 2. The metal oxide having oxygen non-stoichiometry has a perovskite structure.
4. An electrochemical cell according to any one of claims 1 to 3. The thickness of the second electrode is 5 μm or more.
5. An electrochemical cell according to any one of claims 1 to 4. Used as at least one of a solid oxide electrolysis cell and a solid oxide fuel cell,
6. An electrochemical cell according to any one of claims 1 to 5.

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