WO2018167889A1 - Oxygen electrode for electrochemical cell, and electrochemical cell - Google Patents

Abstract

The electrochemical cell relating to an embodiment of the present invention has: a first oxide having a perovskite structure, said first oxide being disposed in a predetermined region; and a second oxide, which is disposed by being dispersed in the predetermined region, and which has one constituent element at a density that is higher than that of the one constituent element in the first oxide.

Description

Oxygen electrode for electrochemical cell and electrochemical cell

Embodiments of the present invention relate to an oxygen electrode for an electrochemical cell and an electrochemical cell.

Solid oxide electrochemical cells are being developed as fuel cells for power generation, electrolysis cells for hydrogen production, and power storage systems that combine these. Since the solid oxide electrochemical cell uses a solid oxide as an electrolyte, the operating temperature is high (for example, 600 to 1000 ° C.), and a high reaction rate can be obtained without using an expensive noble metal catalyst. Is possible. For this reason, when this is operated as a fuel cell (solid oxide fuel cell: SOFC), high power generation efficiency is obtained, and when it is operated as an electrolysis cell (solid oxide type electrolysis cell: SOEC), it is high at a low electrolysis voltage. Hydrogen can be produced efficiently.

In many cases, solid oxide electrochemical cells have a hydrogen electrode side as a support and an oxygen electrode formed on the support (hydrogen electrode support type). In this case, the area of the oxygen electrode is smaller than that of the hydrogen electrode (the area of the electrode and effective reaction part is small), and the performance of the oxygen electrode effectively determines the performance of the entire cell. That is, it is important to improve the performance of the oxygen electrode in order to improve the performance or extend the life of the solid oxide electrochemical cell.

The problem to be solved by the present invention is to provide an oxygen electrode for an electrochemical cell and an electrochemical cell in which the performance of the oxygen electrode is improved.

The oxygen electrode for an electrochemical cell according to the embodiment is disposed in a predetermined region, is disposed in a predetermined region, and is dispersed in the predetermined region, and one constituent element is the first element. And a second oxide having a higher density than that of the oxide.

It is sectional drawing of the electrochemical cell which concerns on embodiment. 4 is a SEM photograph of a cross section of an electrochemical cell in Example 3. 4 is a Co distribution of an electrochemical cell cross section of Example 3. FIG. 4 is an Fe distribution in a cross section of an electrochemical cell of Example 3. 4 is a La distribution of a cross section of an electrochemical cell of Example 3. It is Sr distribution of the electrochemical cell cross section of Example 3. FIG. It is Gd distribution of the electrochemical cell cross section of Example 3. It is Ce distribution of the electrochemical cell cross section of Example 3. FIG. It is a SEM photograph of the electrochemical cell cross section of a comparative example. It is Co distribution of the electrochemical cell cross section of a comparative example. It is Fe distribution of the electrochemical cell cross section of a comparative example. It is La distribution of the electrochemical cell cross section of a comparative example. It is Sr distribution of the electrochemical cell cross section of a comparative example. It is Gd distribution of the electrochemical cell cross section of a comparative example. It is Ce distribution of the electrochemical cell cross section of a comparative example.

Hereinafter, although the solid oxide electrochemical cell according to the embodiment will be described, the present invention is not limited to the following embodiment and examples. Moreover, the schematic diagram referred in the following description is a figure which shows the positional relationship of each structure, The ratio of the magnitude | size of a particle | grain, the thickness of each layer, etc. do not necessarily correspond with an actual thing.

FIG. 1 is a cross-sectional view schematically showing a part of a cross-sectional structure of a flat plate type solid oxide electrochemical cell 10. The flat plate type solid oxide electrochemical cell 10 is a hydrogen electrode supported solid oxide electrochemical cell.

On the support substrate 11, a hydrogen electrode 12, an electrolyte layer 13, a reaction preventing layer 14, and an oxygen electrode 15 are sequentially laminated.
Among these, the support substrate 11, the hydrogen electrode 12, and the oxygen electrode 15 are porous and allow gas (gas) to pass therethrough. On the other hand, the electrolyte layer 13 and the reaction preventing layer 14 do not need to pass gas (pass ions) and are dense (non-porous).

During power generation, a reducing agent such as hydrogen supplied to the hydrogen electrode 12 and an oxidizing agent such as oxygen supplied to the oxygen electrode 15 react electrochemically to generate electric energy and water vapor. During electrolysis, water vapor or the like is reduced by electrolysis at the hydrogen electrode 12, and oxygen is released from the oxygen electrode 15.

The support substrate 11 is a layer serving as a support for the electrochemical cell 10, and the mechanical strength of the electrochemical cell 10 can be maintained or improved. The constituent material of the support substrate 11 can be the same as the constituent material of the next hydrogen electrode 12.

The hydrogen electrode 12 can be composed of a sintered body containing metal (catalyst) particles and a metal oxide (oxygen ion conductive oxide).
Examples of the metal include one or more selected from the group consisting of nickel, cobalt, iron and copper, or alloys containing them.
Examples of the metal oxide include stabilized zirconia (SZ) and doped ceria (DC). In the stabilized zirconia, one or more kinds of stabilizers selected from the group consisting of Y ₂ O ₃ , Sc ₂ O ₃ , Yb ₂ O ₃ , Gd ₂ O ₃ , CaO, MgO, CeO _{2 and the} like are dissolved. Zirconia (zirconium dioxide: ZrO ₂ ). The doped ceria is ceria (cerium oxide: CeO ₂ ) in which one or more oxides selected from the group consisting of Sm ₂ O ₃ , Gd ₂ O ₃ , Y ₂ O _{3, and the} like are dissolved.

The electrolyte layer 13 is a solid oxide layer having electronic insulation and oxygen ion conductivity, and can be composed of, for example, stabilized zirconia (SZ) or doped ceria (DC). In the stabilized zirconia, one or more stabilizers selected from the group consisting of Y ₂ O ₃ , Sc ₂ O ₃ , Yb ₂ O ₃ , Gd ₂ O ₃ , CaO, MgO, CeO _{2 and the} like are dissolved. The In the doped ceria, one or more oxides selected from the group consisting of Sm ₂ O ₃ , Gd ₂ O ₃ , Y ₂ O _{3 and the} like are dissolved.

The reaction preventing layer 14 can be made of doped ceria (DC). In the doped ceria, one or more oxides selected from the group consisting of Sm ₂ O ₃ , Gd ₂ O ₃ and Y ₂ O ₃ are solid-dissolved.

The reaction preventing layer 14 prevents the reaction between the electrolyte layer 13 and the oxygen electrode 15. When the solid oxide electrochemical cell 10 is formed, the oxygen electrode 15 and the electrolyte layer 13 may react to reduce the performance of the solid oxide electrochemical cell 10. For example, when the oxygen electrode 15 and the electrolyte layer 13 are LaCoO ₃ -based perovskite oxide and stabilized zirconia, respectively, a solid phase reaction is caused by firing to form a high resistance phase such as La ₂ Zr ₂ O ₇ .
Thus, by disposing the reaction preventing layer 14 between the electrolyte layer 13 and the oxygen electrode 15, the solid phase reaction between the electrolyte layer 13 and the oxygen electrode 15 is prevented, and the performance of the solid oxide electrochemical cell 10 is ensured. it can.

The oxygen electrode 15 is composed of a sintered body containing a perovskite oxide. Perovskite oxide is mainly represented by the following composition formula (1).
_{Ln 1-x A x B 1} -y C y O 3-δ ... formula (1)
Examples of Ln include rare earth elements such as La. Examples of A include Sr, Ca, and Ba. Examples of B and C include Cr, Mn, Co, Fe, and Ni.
x, y, and δ satisfy the following relationship.
0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ δ ≦ 1

When Ln is lanthanum La and B or C is manganese Mn, lanthanum-manganese oxide (LaMnO ₃ system) is obtained. When Ln is lanthanum La and B or C is cobalt Co, lanthanum-cobalt oxide ( LaCoO ₃ system).

The perovskite oxide can also be represented by the following composition formula (2).
ABO ₃ Composition formula (2)
“A” in the composition formula (2) corresponds to “Ln” and “A” in the composition formula (1). Further, “B” in the composition formula (2) corresponds to “B” and “C” in the composition formula (1).

The elements of Ln, A in the composition formula (1) (A in the composition formula (2)) belong to the A site, and elements B and C in the composition formula (1) (B in the composition formula (2)) belong to the B site. Belongs. As shown in the composition formula (1), when the amount of the element E1 at either the A site or the B site is large, the amount of the other element E2 belonging to the same site is reduced. This is to stabilize the perovskite structure.

The oxygen electrode 15 may contain doped ceria (DC) in addition to the perovskite oxide. In the doped ceria, one or more oxides selected from the group consisting of Sm ₂ O ₃ , Gd ₂ O ₃ and Y ₂ O ₃ are solid-dissolved.

Here, the oxide constituting the oxygen electrode 15 is divided into a first phase 151 (first oxide) and a second phase 152 (second oxide).

The first phase 151 is disposed in a continuous region of the entire layer of the oxygen electrode 15.
The second phase 152 is discretely (distributed) disposed in the first phase 151.
As will be described later, by mixing the first and second phases 151 and 152, the performance of the oxygen electrode 15 can be improved and the life can be extended.

The first and second phases 151 and 152 have different element ratios in at least one of the A site and B site of the perovskite oxide as shown below.

(1) Difference in element ratio in the same site Consider the case where the first and second phases 151 and 152 both have the Ln _1-x A _x B _1-y C _y O _3-δ structure. In this case, the structures of the first and second phases 151 and 152 are expressed as follows.
The first phase _{151: Ln 1-x1 A x1} B 1-y1 C y O 3-δ1
The first phase _{152: Ln 1-x2 A x2} B 1-y2 C y O 3-δ2
That is, if any of x1 ≠ x2 and y1 ≠ y2 is established, the ratio of elements is different in either the A site (Ln, A) or the B site (B, C).
For example, when Ln is lanthanum La and A, B, and C are Sr, Co, and Fe, respectively, the second phase 152 is more Co and less Fe than the first phase 151 (x1 ≠ x2 ), Or when La is large and Sr is small (y1 ≠ y2).

(2) Influence on another site The above explanation is that the Ln _1-x A _x B _1-y C _y O _3-δ structure is maintained in both the first and second phases 151, 152. Is assumed. That is, the ratio of elements only within the same site of A and B is a problem.

However, when the proportion of a certain element increases, the effect does not stop at the same site, but may affect another site. For example, when Ln is lanthanum La and A, B, and C are Sr, Co, and Fe, respectively, the second phase 152 is more Co and less Fe, La, and Sr than the first phase 151. It is possible.

In this case, the coexistence of the first and second phases 151 and 152 may be difficult (the second phase 152 may become unstable).
However, even in such a case, it has been experimentally shown that the first and second phases 151 and 152 can coexist. Basically, the first phase 151 has an ABO ₃ structure (Ln _1-x A _x B _1-y C _y O _3-δ structure). In contrast, the second phase 152 may have an A ₂ BO ₄ structure or a B ₃ O ₄ structure in addition to the ABO ₃ structure. Thus, first, second phases 151 and 152, in addition to a combination of ABO ₃ structure -ABO ₃ structure, ABO ₃ structure _-A 2 BO ₄ structure, a combination of ABO ₃ structure _-B 3 _{O 4} structure Even if it exists, the 1st, 2nd phase 151,152 may exist stably. This is considered to be because the first and second phases 151 and 152 settle in a stable state during the firing process when the solid oxide electrochemical cell 10 is formed.

Examples of the A ₂ BO ₄ structure include La _2-x Sr _x Co _1-y Fe _y O _{4 and} are represented by the following composition formula (3).
_{Ln 2-x A x B 1} -y C y O 4-δ ... formula (3)
An example of the B ₃ O ₄ structure is Co _3-x Fe _x O _{4 and} is represented by the following composition formula (4).
B _3-x C _x O _4-δ Composition formula (4)
Here, Ln is, for example, a rare earth element such as La. Examples of A include Sr, Ca, and Ba. Examples of B and C include Cr, Mn, Co, Fe, and Ni.
x, y, and δ satisfy the following relationship.
0 ≦ x ≦ 1, 0 ≦ y ≦ 1, 0 ≦ δ ≦ 1

The second phase 152 may have a single structure of any one of ABO ₃ , A ₂ BO ₄ , and B ₃ O ₄ structures. Further, two or more of ABO ₃ , A ₂ BO ₄ , and B ₃ O ₄ structures may be included.
The second phase 152 may exist in the ABO ₃ , A ₂ BO ₄ , and B ₃ O ₄ structure with oxides having a plurality of composition ratios intermingled. That is, the variables x, y, and δ in the composition formulas (1), (3), and (4) may not be constant within the same structure of the second phase 152.

As described above, in the first and second phases 151 and 152, one of the constituent elements (element E1) is more in the second phase 152 than in the first phase 151.

At this time, the element E2 at the same site as the element E1 is smaller in the second phase 152 than in the first phase 151 as described above.
For this reason, the density ratio α of the elements E1 and E2 (the ratio α (= d1 / d2) of the density d2 of the element E1 to the density d1 of the element E2) is different between the first and second phases 151 and 152. When the density ratios of the first and second phases 151 and 152 are α1 and α2, respectively, the density ratio α2 is larger than the density ratio α1 (α2> α1).

At this time, the density ratio ratio R (= α2 / α1) can be defined. This ratio R is greater than 1. It is preferable that the ratio R is a somewhat large value, for example, 1.5 or more (more preferably 2.0 or more). When the ratio R is close to 1, the difference between the first and second phases 151 and 152 is small, and even if the phases are mixed, it is difficult to improve performance.

Here, for the sake of convenience, the outer shape of the second phase 152 is circular (spherical), but actually, the outer shape of the second phase 152 is an intricate shape with irregularities due to stabilization such as a perovskite structure. Rather, it is considered that an intricate shape is preferable from the viewpoint of improving the performance of the oxygen electrode 15 by increasing the interface between the first and second phases 151 and 152.

At this time, the size of the second phase 151 can be evaluated by a virtual diameter (particle diameter, diameter) D. Here, the virtual diameter D is defined by the following equation (1).
D = 2 · (S / π ) 1/2 ... formula (1)
The area S is the area of each second phase 152 on the sample cross section.
A set of bright spots in FIG. 2B described later corresponds to the second phase 152. In addition, the circle | round | yen (dotted line) surrounding the 2nd phase 152 in FIG. 2B shows each 2nd phase 152 (2nd oxide), and is unrelated to the virtual diameter D. FIG.

The size (virtual diameter D) of the second phase 152 is preferably 100 nm or more and 5000 nm or less (more preferably 0.3 μm or more and 3 μm or less). If the size is too small, the stability of the phase may be lacking. If the size is too large, the performance of the oxygen electrode 15 may be improved due to an increase in the interface between the first and second phases 151 and 152.

The density of the second phase 152 is preferably 10 pieces / mm ² or more and about 10,000 pieces / mm ² or less (more preferably, 500 pieces / mm ² or more and about 5000 pieces / mm ² or less). Even if the density is too small or too large, the performance of the oxygen electrode 15 is reversed.

In addition to the first and second phases 151 and 152, third phases (third oxides) having different compositions may be dispersedly arranged. Similarly to the second phase 152, the _third phase can be an oxide having any one of the ABO ₃ structure, the A ₂ BO ₄ structure, and the B ₃ O ₄ structure.

The presence of the second phase 152 can be confirmed by measuring the composition distribution. For example, a scanning electron microscope (SEM) capable of energy dispersive X-ray spectroscopy (EDX) can be used.
For example, a solid oxide electrochemical cell is cleaved and the cross section is smoothed by ion milling or the like. This sample cross section is observed by SEM at a magnification of about 1000 to 100,000 times and measured by EDX. At this time, it is preferable that the minimum composition measurement region (for example, a radius of about 500 nm or less) and the energy resolution (for example, about 100 eV or less) are small.

Considering the measurement limit of the apparatus, the second phase 152 can be detected as follows, for example.
(1) Element Aggregation Detection As described above, the composition of the sample cross-section is analyzed, and the region of the element E1 aggregate (the first virtual diameter D) is a certain size (eg, 0.3 μm or more) 2 phase 152) (for example, in a circle (dotted line) in FIG. 2B described later).

(2) Confirmation of element ratio In the region (second phase 152) where the size is more than a certain level, outside (first phase 151), element E1, this element E1 and element E2 (the same site as element E1) Density ratios α2 and α1 are obtained. The density ratio α2 is the ratio of the element E1 to the element E2 within the region, and the density ratio α1 is the ratio of the element E1 to the element E2 outside the region.
If the density ratio ratio R (= α2 / α1) is larger than 1 (for example, 2.0 or more), it can be said that the second phase 152 is detected (the element E1 is locally distributed).

In order to create the oxygen electrode 15 having the first and second phases 151 and 152, the following methods (a) and (b) can be used.

(A) Raw material The structure of the first and second phases 151 and 152 can be created by mixing a plurality of materials M1 and M2. For example, the powder material M1 corresponding to the first phase 151 and the powder material M2 corresponding to the second phase 152 are mixed and spray coated.
At this time, the powder material M2 has a larger proportion of at least a specific element, a larger particle size, and a smaller mixing amount than the powder material M1. When this mixture is spray-coated, the powder material M1 has a structure in which particles of the powder material M2 are dispersed. By baking this, the oxygen electrode 15 having the first and second phases 151 and 152 can be produced.

(B) Heat treatment conditions As will be described in detail in Examples, even if the material used is uniform, the oxygen electrode 15 having the first and second phases 151 and 152 can be created by adjusting the heat treatment temperature. Specifically, heat treatment is performed in a state where the constituent materials of the electrolyte layer 13 and the reaction preventing layer 14 are laminated (primary heat treatment), and further, the heat treatment is performed by laminating the constituent materials of the oxygen electrode 15 (secondary heat treatment).

At this time, the temperature of the primary and secondary heat treatment is made higher than the normal temperature for forming the perovskite oxide. As a result, as shown in the embodiment, the oxygen electrode 15 having the first and second phases 151 and 152 can be formed. This is due to thermal diffusion in two stages (thermal diffusion occurs between the electrolyte layer 13 and the reaction prevention layer 14 in the primary heat treatment, and thermal diffusion occurs between the reaction prevention layer 14 and the oxygen electrode 15 in the secondary heat treatment). It is considered a thing.

Examples will be specifically described below.
Example 1
A. Preparation Sample 1 was prepared by the following procedures (1) to (5).

(1) Preparation of Precursor for Support Substrate 11 Nickel oxide (NiO) powder and gadolinium-doped ceria (GDC) powder are mixed at a weight ratio of 6: 4 to prepare a mixed powder. The GDC is ceria (cerium oxide (IV): CeO ₂₎ Gadoria to (gadolinium oxide: _Gd 2 _{O 3)} a _{(Gd 2} O ₃₎ 0.1 mixed so that the composition of _{(CeO 2) 0.9} It is created by firing.
The precursor of the support substrate 11 is created by mixing the mixed powder with a solvent to form a paste and making this into a sheet shape.

(2) Preparation of primary laminated body On this precursor, the hydrogen electrode 12, the electrolyte layer 13, and the reaction prevention layer 14 are produced in order by spray coating, and the precursor 110, the hydrogen electrode 12, the electrolyte layer 13, and the reaction prevention layer 14 are produced. Is formed.
The hydrogen electrode 12 is formed by mixing a mixed powder of nickel oxide (NiO) and gadolinium doped ceria (GDC) with a solvent and spray coating.
The electrolyte layer 13 is formed by mixing yttria-stabilized zirconia (YSZ) powder with a solvent and spray coating.
The reaction preventing layer 14 is formed by mixing GDC with a solvent and spray coating.

(3) Firing of the primary laminate (primary heat treatment)
The primary laminate is fired at 1200 ° C. to 1600 ° C. (here, 1400 ° C.) until each layer and each layer have sufficient strength. By making the temperature in the primary heat treatment higher than usual, thermal diffusion occurs between the electrolyte layer 13 and the reaction preventing layer 14.

(4) Addition of the oxygen electrode 15 The La (Sr) Co (Fe) O _3-δ is spray coated on the reaction preventing layer 14 to form the oxygen electrode 15, and the precursor 110, the hydrogen electrode 12, the electrolyte layer 13, A secondary laminate of the reaction preventing layer 14 and the oxygen electrode 15 is formed.

(5) Firing of secondary laminate (secondary heat treatment)
The secondary laminate is fired at 900 ° C. or higher and 1300 ° C. or lower (here, 1100 ° C.) so that the oxygen electrode 15 is firmly bonded to the reaction preventing layer 14. By making the temperature in the secondary heat treatment higher than usual, thermal diffusion occurs between the reaction preventing layer 14 and the oxygen electrode 15.
As a result, the solid oxide electrochemical cell 10 according to the example is produced.

B. Evaluation Sample 1 was evaluated by the following procedures (1) and (2).

(1) Measurement of initial IV characteristics The initial IV characteristics of the solid oxide electrochemical cell 10 prepared using a hydrogen electrode output characteristic evaluation apparatus were evaluated.
-Reduction of support substrate 11 and hydrogen electrode 12 The solid oxide electrochemical cell 10 was installed in a hydrogen electrode output characteristic evaluation apparatus. Thereafter, the temperature is maintained at 700 ° C., dry hydrogen is supplied to the hydrogen electrode 12 side, and an N ₂ / O ₂ mixed gas (mixed at a volume ratio of 4: 1) is supplied to the oxygen electrode 15 side. Reduce.

Measurement The hydrogen electrode output characteristic evaluation apparatus can evaluate the IV characteristics of the solid oxide electrochemical cell 10. That is, the water vapor concentration on the hydrogen electrode side is controlled, the solid oxide electrochemical cell 10 is operated in the SOFC mode and the SOEC mode, and the initial IV characteristics are measured. Thereafter, the temperature is lowered in a hydrogen atmosphere and cooled to room temperature, and the solid oxide electrochemical cell 10 is taken out from the hydrogen electrode output characteristic evaluation apparatus.

(2) Cross-sectional enlarged observation / composition analysis The cross-sectional enlarged observation and composition analysis of the solid oxide electrochemistry cell were performed.
The solid oxide electrolysis cell was cleaved and the cross section was smoothed by ion milling to obtain a measurement sample. For ion milling, an ion milling device (Hitachi High-Technologies, IM-4000) was used.
Using a scanning electron microscope (Hitachi High-Technologies, SU8000, S-5200), the cross section was observed (SEM observation and EDX composition analysis magnified 10,000 times).

(Example 2)
A solid oxide electrochemical cell 10 was prepared in the same manner as in Example 1, and the IV characteristics in the initial state were evaluated. Thereafter, the solid oxide electrochemical cell 10 was continuously operated in the SOEC mode for about 250 hours. Further, the cross section was observed in the same manner as in Example 1.

(Example 3)
A solid oxide electrochemical cell 10 was prepared in the same manner as in Example 1, and the IV characteristics in the initial state were evaluated. Thereafter, the solid oxide electrochemical cell 10 was continuously operated in the SOEC mode for about 3000 hours. Further, the cross section was observed in the same manner as in Example 1.

(Comparative example)
A solid oxide electrochemical cell was fabricated in substantially the same manner as in Example 1. However, the temperatures of the first and second heat treatments were lowered to 1300 ° C. and 1050 ° C., respectively.
After evaluating the IV characteristics in the initial state, the solid oxide electrochemical cell 10 was continuously operated in the SOEC mode for about 3000 hours. Further, the cross section was observed in the same manner as in Example 1.

Hereinafter, the evaluation results of Examples 1 to 3 and the comparative example will be described together.
(1) Results of Surface Observation FIGS. 2A to 2G show the SEM photographs of the sample cross section of Example 3 and the results of surface analysis of Co, Fe, La, Sr, Gd, and Ce, respectively.
3A to 3G respectively show SEM photographs of the cross-section of the sample of the comparative example and the results of surface analysis (EDX) of Co, Fe, La, Sr, Gd, and Ce.

2A and 3A show states in which the cross sections of the electrolyte layer 13, the reaction preventing layer 14, and the oxygen electrode 15 are enlarged. For ease of understanding, a dotted line is drawn at the boundary between the electrolyte layer 13, the reaction preventing layer 14, and the oxygen electrode 15. It can also be seen that the layer structure of the electrolyte layer 13, the reaction preventing layer 14, and the oxygen electrode 15 is formed, and the oxygen electrode 15 is porous.

In FIG. 2B, the Co aggregate is clearly represented as a collection of bright spots. For ease of understanding, this Co aggregate is surrounded by a circle (dotted line).
As shown in FIGS. 2C to 2E, a portion (dark portion) where the density of Fe, La, and Sr is low exists at a position corresponding to the Co aggregate (circle (dotted line)).
Thus, the density of Co is locally high, and not only the Fe at the same site as Co but also the density of La and Sr at another site is low at that location.

As shown in FIG. 3D, regarding Sr, there is another place where the density is locally high near the reaction preventing layer 14. At that location, the La density at the same site is low.
As shown in FIGS. 3F and 3G, Gd and Ce that are constituent elements of the reaction preventing layer 14 remain in the reaction preventing layer 14.

As described above, in the sample of Example 3, the following local regions (a) and (b) existed in the oxygen electrode 15.
(A) Region where Co density is large and Fe, La, Sr density is low (b) Region where Sr density is large and La density is low As a result of more detailed analysis, region (a) contains Co _{3 -x} Fe _x O _{4-δ (e.g., Co} 3 _{O 4)} is, _{La 2-x Sr} in the area _{(b) x Co 1-y} Fe y O 4-δ ( _e.g., La 1.2 _{Sr 0. 8} Co _0.5 Fe _0.5 O _4-δ ).

Such local regions (a) and (b) were also found in Examples 1 and 2. Further, no clear difference was found between the state after 3000 hours of operation in Example 3 and the state before operation of Example 1 and the state after 250 hours of operation of Example 2.

In contrast, as shown in FIGS. 3A to 3G, in the sample of the comparative example, La, Sr, Co, and Fe are relatively uniformly distributed in the oxygen electrode 15.

(2) Results of Initial IV Characteristics and Continuous Operation Test Table 1 shows the current densities of Examples 1 to 3 and Comparative Example and the deterioration rate in the continuous operation test of Example 3 and Comparative Example 1.

This current density is measured in the EC mode and the same cell voltage.
The current density in the initial state of Example 1-3 is an equivalent value and is in good agreement. On the other hand, the current density in the initial state of the comparative example is lower than the values of Examples 1 to 3. Further, the deterioration rate of the comparative example was larger than the deterioration rate of Example 3.

This result relates to the presence of a plurality of phases coexisting in the oxygen electrode 15, for example, the existence of “regions where the density of Co is high and the density of Fe, La, and Sr is low” (second phase). Conceivable.
The presence of the second phase 152 improves the initial characteristics and also improves the life (deterioration rate). Considering simply, the uniform oxygen electrode 15 as in the comparative example is considered to be in a more stable state and have a longer lifetime, but the embodiment in which the phases are not uniform has a longer lifetime.
Thus, the presence of a plurality of phases is considered to contribute to the performance and stabilization of the oxygen electrode.
As described above, according to at least one embodiment, the performance of the oxygen electrode can be improved.

Although several embodiments of the present invention have been described, these embodiments are presented as examples and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and changes can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the invention described in the claims and the equivalents thereof.

Claims (7)

1. A first oxide having a perovskite structure disposed in a predetermined region;
   A second oxide that is dispersed in the predetermined region and in which one constituent element has a higher density than the first oxide;
   An oxygen electrode for electrochemical cells.
2. The first oxide is an ABO ₃ type oxide;
   The second oxide is an ABO ₃ type, A ₂ BO ₄ type, or B ₃ O ₄ type oxide;
   The one constituent element is included in the A site or the B site,
   The oxygen electrode for an electrochemical cell according to claim 1, wherein the density of an element at the same site as the one constituent element is smaller in the second oxide than in the first oxide.
3. The oxygen electrode for an electrochemical cell according to claim 2, wherein the density of either the one constituent element or the element at another site is smaller in the second perovskite oxide than in the first perovskite oxide.
4. The A-site element is at least one selected from rare earth elements, Ca, Sr or Ba;
   The oxygen electrode for an electrochemical cell according to claim 2, wherein the B site element is at least one selected from Cr, Mn, Fe, Co, Ti, or Ni.
5. The oxygen electrode for an electrochemical cell according to claim 1, wherein the diameter of the second oxide is 100 nm or more and 5000 nm or less.
6. A support substrate;
   A hydrogen electrode disposed on the support substrate;
   An electrolyte layer disposed on the hydrogen electrode;
   The oxygen electrode for an electrochemical cell according to claim 1 disposed on the electrolyte layer;
   An electrochemical cell having:
7. A reaction preventing layer disposed between the electrolyte layer and the oxygen electrode;
   The electrochemical cell according to claim 6, further comprising:

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