JP2018142500A - Air electrode material, air electrode and solid oxide fuel cell - Google Patents

Abstract

PROBLEM TO BE SOLVED: To provide an air electrode material which makes Cr poisoning unlikely to occur even during use in a state in contact with a metal material containing Cr, an air electrode, and a solid oxide fuel cell including such an air electrode.SOLUTION: When a trivalent metal selected from rare earth and Bi is defined as R, a divalent metal with lower reactivity with Cr on a contact interface with a metal material containing Cr than Sr is defined as A, one or two or more metals selected from among Fe, Co, Ni and Mn are defined as B and an oxygen vacancy quantity is defined as δ, an air electrode material consists of a perovskite conductive oxide having a composition of RABOunder 0<x<1. And an air electrode and a solid oxide fuel cell are configured by using such an air electrode material.SELECTED DRAWING: Figure 1

Description

  The present invention relates to an air electrode material, an air electrode, and a solid oxide fuel cell. More specifically, the present invention relates to an air electrode material composed of a conductive oxide not containing Sr, and an air electrode and a solid formed using the same. The present invention relates to an oxide fuel cell.

A conductive oxide having mixed conductivity of electrons and oxide ions is used as a constituent material for an air electrode (cathode) of a solid oxide fuel cell (SOFC). As a conductive oxide having mixed conductivity of electrons and oxide ions, a perovskite-type composite oxide is generally used. Among these, La _1-x Sr _x MnO _3-δ , La _1-x Sr _x CoO ₃₋ Perovskite complex oxides containing Sr, such as _δ , La _1-x Sr _x Co _1-y Fe _y O _{3 -δ} , are often used as SOFC air electrode materials.

The SOFC is required to maintain high performance for a long period of time, and it is desirable that the air electrode material does not deteriorate due to use over time. For example, Patent Document 1 below has a perovskite crystal structure represented by the general formula ABO ₃ as an electrode material for SOFC that can maintain the performance of a low-temperature operation type SOFC air electrode and the like well over a long period of time. An electrode material containing an oxide and having a lattice distortion of 2.5% or less calculated based on Rietveld analysis is disclosed. In the electrode material disclosed as an example, La or Sm, or a combination of them and Sr is adopted as the metal occupying the A site of ABO ₃ .

JP, 2015-111534, A

  A major cause of the deterioration of the electrode due to long-term use is that the constituent material of the electrode causes an interaction or a chemical reaction with the constituent material of another member in contact with the electrode. The SOFC is usually used in the form of a cell stack in which single cells each having a fuel electrode and an air electrode bonded to both surfaces of an electrolyte layer are stacked via a separator (interconnector) made of stainless steel or the like. In this case, the fuel electrode and the air electrode are held at the operating temperature of the SOFC while being in contact with the separator. At this time, a phenomenon of chromium (Cr) poisoning is known in which the conductive oxide constituting the air electrode reacts with Cr contained in the stainless steel or the like constituting the separator, and the performance of the air electrode deteriorates.

  In SOFC, as a method for preventing Cr poisoning of the air electrode, a method is used in which the separator surface is coated to prevent mutual diffusion of constituent elements between the separator and the air electrode. However, the coating process of the separator requires a large cost. Therefore, a method that can suppress Cr poisoning of the air electrode without coating the separator is desired.

According to the study by the present inventors, the degree of Cr poisoning in the air electrode depends significantly on the material constituting the air electrode (see FIG. 1). As described above, as the conductive oxide constituting the air electrode, a perovskite complex oxide containing Sr at the A site, represented by La _1-x Sr _x Co _1-y Fe _y O ₃ , is generally used. Although Sr has high reactivity with Cr, Cr poisoning tends to be serious in this type of material. The electrode material of Patent Document 1 is intended to maintain good electrode performance over a long period of time, but durability in terms of suppressing Cr poisoning is not considered. Actually, in Patent Document 1, in the examples, Sr is adopted as an element contained in the crystal in addition to the rare earth metal, so that Cr poisoning is considered to be serious.

  The problem to be solved by the present invention is an air electrode material and an air electrode that hardly cause poisoning of Cr even when used in contact with a metal material containing Cr, and a solid oxide fuel having such an air electrode To provide a battery.

In order to solve the above problems, an air electrode material according to the present invention is a material constituting an air electrode of a solid oxide fuel cell, wherein R is a trivalent metal selected from rare earth and Bi, and A is , A bivalent metal having a reactivity with Cr lower than that of Sr at the contact interface with a metal material containing Cr, and B is one or more selected from Fe, Co, Ni, Mn, It is made of a perovskite-type conductive oxide having a composition of R _x A _1-x BO _{3 -δ} , where _{δ is} the amount of oxygen vacancies and 0 <x <1.

  Here, it is preferable that R = La, A = Ca, and B = Fe. Furthermore, it is preferable that 0.5 <x <0.8. In particular, 0.55 ≦ x ≦ 0.70 is preferable.

  The air electrode according to the present invention comprises the above air electrode material and constitutes a solid oxide fuel cell.

  The solid oxide fuel cell according to the present invention includes an electrolyte layer made of a solid oxide electrolyte, a fuel electrode bonded to the electrolyte layer, and the above-described electrode bonded to the electrolyte layer facing the fuel electrode. A positive air electrode.

  Here, the solid oxide fuel cell may further include a separator made of a metal material containing Cr in contact with the air electrode.

  The conductive oxide constituting the air electrode material according to the present invention does not contain Sr having high reactivity with Cr, and the metal element A has lower reactivity with Cr than Sr. Contains. Therefore, even if it uses in the state which contacted the metal material containing Cr, it is hard to raise | generate degradation by reaction with Cr. Therefore, even if such an air electrode material is used to form an air electrode of a solid oxide fuel cell and is used in contact with a separator made of a metal material containing Cr, Cr poisoning of the air electrode occurs. It is difficult to form an air electrode with excellent long-term durability.

Here, when R = La, A = Ca, and B = Fe, Cr poisoning of the air electrode material can be effectively suppressed due to the low reactivity between Ca and Cr. Moreover, it is easy to replace Ca ²⁺ with La ³⁺ in a state where the perovskite structure is stably adopted, and it is easy to achieve both high oxide ion conductivity and electronic conductivity.

Furthermore, when 0.5 <x <0.8, the activation energy for oxygen permeation in the air electrode material is significantly lower than in regions where x is smaller or larger than that. Therefore, it can be suitably used to construct an SOFC air electrode operated at a relatively low temperature. In particular, when 0.55 ≦ x ≦ 0.70, not only the activation energy is small, but also the value of the oxygen permeation rate itself is large, so that it has particularly high power generation characteristics when used as an air electrode. A solid oxide fuel cell can be obtained.

  In the air electrode and the solid oxide fuel cell according to the above invention, the separator made of a metal material containing Cr, such as stainless steel, is used by making the air electrode from the above air electrode material. Even if it exists, Cr poisoning in an air electrode can be suppressed. Therefore, even after a long period of use, an air electrode and a solid oxide fuel cell with little deterioration in characteristics can be obtained.

It is a SEM-EPMA image which shows the verification result of the reactivity in the interface of a conductive oxide and Cr steel. (A) is the distribution of Ca at the La _x Ca _1-x FeO _3-δ (x = 0.5) / Cr steel interface, and (b) is La _0.6 Sr _0.4 Co _0.2 Fe _0.8. It shows the distribution of Sr at the O _3−δ / Cr steel interface. It is an X-ray diffraction pattern of La _x Ca _1-x FeO _3-δ (x = 0.4, 0.5, 0.6), and (b) is an enlarged view of a portion surrounded by a square in (a). It is. _{La x Ca 1-x FeO 3} -δ the X-ray diffraction pattern of (x = 0.5,0.6), a diagram provided for comparison between the simulation result of the cubic and Akira Nogata perovskite structure. (B) is an enlarged view of a portion surrounded by a square in (a). For _{La x Ca 1-x FeO 3} -δ (x = 0.6), the results of experiments to measure the oxygen transmission rate indicates a transmission rate of oxygen and nitrogen were measured while changing the temperature. For _{La x Ca 1-x FeO 3} -δ, it is an experimental result showing the temperature dependence of the oxygen transmission rate in the case of variously changing the x, is (a) a linear display, (b) the Arrhenius plot . For _{La x Ca 1-x FeO 3} -δ, it is a diagram showing a dependency on La doping amount x of activation energy for oxygen transmission.

  Hereinafter, an air electrode material, an air electrode, and a solid oxide fuel cell according to embodiments of the present invention will be described.

[Air electrode material]
First, an air electrode material according to an embodiment of the present invention will be described. The air electrode material according to one embodiment of the present invention is made of the following conductive oxide.

The conductive oxide constituting the air electrode material is a perovskite type conductive oxide having a composition of R _x A _1-x BO _3-δ . Here, R is a trivalent metal selected from rare earths and Bi. A is a divalent metal. B is one or more selected from Fe, Co, Ni, and Mn. x is the doping amount of R, and 0 <x <1. δ is the amount of oxygen vacancies. Each of R, A, and B may be made of a single metal species, or two or more selected from a predetermined group may be mixed.

In R _x A _1-x BO _3-δ , A is a divalent metal excluding Sr, and has a lower reactivity with Cr at the contact interface with a metal material containing Cr than Sr. For example, the reactivity between A and Cr at the contact interface when R _x A _1-x BO _3-δ is brought into contact with a metal material containing Cr such as stainless steel (Cr steel) is La _1-x. Sr and Co at the contact interface where Sr _x Co _1-y Fe _y O _3−δ (0 <x <1; 0 <y <1; δ is the amount of oxygen vacancies) is contacted with the same metal material containing Cr It is lower than the reactivity during.

If each of R, A, and B satisfies the above conditions such as valence and reactivity with Cr, R _x A _1-x BO _3-δ constitutes a perovskite structure, If it has electroconductivity, the specific metal seed | species of R, A, and B will not be specifically limited. However, preferred examples of R include rare earth elements such as La, Pr, Nd, Gd, Sm, and Y, and Bi. Among these, it is particularly preferable to use La. La can easily form a perovskite crystal structure when doped, and has a large ionic radius among rare earth elements, so that it becomes a perovskite crystal having a large lattice volume, resulting in high ionic conductivity, etc. Depending on the reason.

On the other hand, Mg, Ca, Ba, Pb etc. can be mentioned as a suitable example of the metal A. Among these, it is particularly preferable to use Ca in that the reactivity with Cr is particularly low at the contact interface with the metal material containing Cr. Also, since the ionic radius of La ³⁺ is close to 1.36 Å and the ionic radius of Ca ²⁺ is close to 1.34 Å, by setting R = La and A = Ca, in the perovskite structure, Ca ²⁺ in La ³⁺ Easy to replace.

Even if the metal B is any of Fe, Co, Ni, and Mn, a conductive perovskite oxide crystal can be formed. However, by setting B = Fe, high electronic conductivity can be easily achieved by utilizing a stable mixed valence state of Fe ³⁺ and Fe ⁴⁺ as described later. A part of Fe may be substituted with any one of Co, Ni, and Mn. For example, a form in which about 20% of Fe is substituted with these metals is preferable.

_{La x Ca 1-x FeO 3} -δ and other _{R x A 1-x BO 3} -δ , the electron-oxide ion mixed conducting oxides with electronic conductivity with both the oxide ion-conducting Can function as a material. In this material, electronic conductivity is brought about by the formation of a mixed valence state of B ³⁺ (eg Fe ³⁺ ) and B ⁴⁺ (eg Fe ⁴⁺ ) in the presence of oxygen vacancies. Furthermore, by replacing A ²⁺ with R ³⁺ in the crystal, the ratio of B ⁴⁺ increases in the mixed valence state depending on the charge neutrality condition. Thereby, high electronic conductivity is obtained. In the case of La _x Ca _1-x FeO _{3 -δ} , the electronic conductivity tends to be high when the La doping amount x is generally in the range of 0.4 to 0.95.

The oxide ion conductivity depends on both the kind of crystal serving as a base material of the conductive oxide and the doping amount x of R. For example, in the case of La _x Ca _1-x FeO _3-δ , the Ca ₂ Fe ₂ O ₅ itself that is a base material exhibits high oxide ion conductivity such as 2 × 10 ⁻³ S / cm. A relatively high oxide ion conductivity can be obtained in a wide range of the amount x. Furthermore, the oxide ion conductivity also depends on the La doping amount x, and as shown in a later example, in the region of 0.5 <x <0.8, particularly 0.55 ≦ x ≦ 0.70. A low activation energy is obtained in oxygen permeation in the form of oxide ions. For this reason, from the viewpoint of achieving high oxide ion conductivity, the La doping amount x is preferably in the range of 0.5 <x <0.8. In particular, in addition to having a low activation energy, the oxide ion conductivity value itself is preferably in a range of 0.55 ≦ x ≦ 0.70.

R _x A _1-x BO _3-δ is La _1-x Sr _x Co _1-, which is commonly used as an air electrode material of a solid oxide fuel cell (SOFC) in oxygen permeation in the form of oxide ions. It is preferable to have an activation energy lower than _y Fe _y O _{3 -δ} . The activation energy of La _1-x Sr _x Co _1-y Fe _y O _3-δ depends on the values of x and y, but is approximately 120 kJ / when x = 0.4 and y = 0.2. R _x A _1-x BO _3-δ is preferably less than 120 kJ / mol, more preferably 100 kJ / mol or less, and 70 kJ / mol or less.

As described above, R _x A _1-x BO _3-δ has a perovskite crystal structure. In this case, a part of A ²⁺ occupying the A site of the perovskite structure is replaced with R ³⁺ . In the case of La _x Ca _1-x FeO _3-δ , the base material Ca ₂ Fe ₂ O ₅ has a brown miralite structure, but as the La doping amount x increases, a structural change to a perovskite structure occurs. . As will be shown later in the examples, at least x = 0.4 or more, the perovskite structure becomes dominant. More specifically, it is considered that there is a high possibility that a tetragonal perovskite structure close to cubic is taken.

R _x A _1-x BO _3-δ can be appropriately produced by a solid phase method, a liquid phase method, or the like. La _x Ca _1-x FeO _3-δ is suitable, for example, by synthesizing and firing a precursor by an organic complex polymerization method (Pecini method; US Pat. No. 3,330,697) which is a kind of liquid phase method. Can be manufactured. When this oxide is manufactured by the solid phase method, La ₂ O ₃ is used as the La source. However, since the crystal grain size of La ₂ O ₃ is large, it is difficult to dope La uniformly. is there. On the other hand, uniform dope of La can be easily achieved by using the liquid phase method.

The air electrode material made of the conductive oxide R _x A _1-x BO _{3-δ according} to the present embodiment may be used alone to form the SOFC air electrode, but other types of conductive oxides, etc. The air electrode may be mixed with other materials. However, even when other materials are mixed, it is better not to use a material containing Sr from the viewpoint of reducing Cr poisoning as the entire air electrode. Further, as an air electrode material is represented by the general formula _{R x A 1-x BO 3} -δ, even with only one material may be used as a mixture of two or more.

[Air electrode and solid oxide fuel cell]
Next, an air electrode and an SOFC according to an embodiment of the present invention will be described.

  The air electrode according to one embodiment of the present invention comprises the air electrode material according to one embodiment of the present invention described above. And SOFC concerning one Embodiment of this invention is comprised including such an air electrode.

  The SOFC according to one embodiment of the present invention is based on a single cell structure comprising an electrolyte layer made of a solid oxide electrolyte material and two electrodes, a fuel electrode (anode) and an air electrode (cathode). . In a single cell, a fuel electrode and an air electrode are joined to an electrolyte layer so as to face each other. The single cell may have various shapes such as a flat plate type and a cylindrical type. Further, as a single cell having each shape, there can be one having an electrolyte layer as a support, and a thick fuel electrode as a support. The air electrode according to the present embodiment can be applied both when the electrolyte layer is used as a support and when the fuel electrode is used as a support. In the following description, a flat plate type single cell is mainly assumed from the viewpoint of forming a cell stack by bringing a separator into close contact with an air electrode.

  As an air electrode which comprises a single cell, what consists of air electrode material concerning one Embodiment of this invention demonstrated above is used. Although the material which comprises an electrolyte layer and a fuel electrode is not specifically limited, The following can be illustrated.

The electrolyte layer is made of a solid oxide electrolyte material exhibiting oxide ion conductivity. Specific examples include scandia (Sc ₂ O ₃ ), yttria (Y ₂ O ₃ ), ceria (CeO ₂ ), and calcia (CaO). , Stabilized zirconia (ZrO ₂ ), samaria (Sm ₂ O ₃ ), gadolinia (Gd ₂ O ₃ ), yttria stabilized by one or more oxides selected from magnesia (MgO) and the like Examples thereof include ceria (CeO ₂ ) -based solid solutions containing one or more oxides selected from the above, stabilized zirconia and / or composites of ceria-based solid solutions and alumina, and the like. Of these, stabilized zirconia and a composite of stabilized zirconia and alumina are preferable.

  Moreover, as a material of a fuel electrode, the cermet formed from a metal catalyst or these metal catalysts and a solid electrolyte can be illustrated. Specific examples of the metal catalyst include Ni, Ni alloy, nickel oxide (NiO), Co, Ru, Pt, and Pd. Two or more of these may be mixed. On the other hand, as the solid electrolyte constituting the cermet, stabilized zirconia, ceria-based solid solution, and the like as listed as solid oxide electrolyte materials that can constitute the electrolyte layer can be employed. In the cermet, the ratio (mass ratio) between the metal catalyst and the solid electrolyte is preferably in the range of catalyst: solid electrolyte = 30: 70 to 70:30.

  In a single cell, for the purpose of suppressing the reaction between the electrolyte material and the electrode material between the electrode (fuel electrode, air electrode) and the electrolyte layer, or increasing the catalytic activity of the electrode An intermediate layer or the like may optionally be interposed. Specific examples of the material constituting the intermediate layer include at least one oxide selected from gadria, yttria, and ceria.

  The single cell as described above can be manufactured, for example, as follows. That is, first, a solid oxide electrolyte material is formed into a desired shape such as a flat plate shape by a press molding method or a tape molding method, and sintered at an optimum temperature according to the composition to form an electrolyte layer. Next, the slurry containing the fuel electrode material is applied to one surface of the electrolyte layer by a screen printing method, a doctor blade method, a brush coating method, a spray method, a dipping method, etc., and at an optimum temperature according to its composition. Sinter to make the fuel electrode. Similarly, the slurry containing the air electrode material is applied to the other surface of the electrolyte layer and sintered to form an air electrode.

  It is preferable to use a single cell by electrically connecting a plurality of single cells via a separator (interconnector). The separator is made of a conductive material in which a gas flow path is formed. Examples of the material constituting the separator include stainless steel typified by Cr steel, or heat-resistant metal materials such as Cr-based alloys and Ni-based alloys. A gas flow path can be formed in a plate-like material made of these materials by pressing, etching, or the like to form a separator. A cell stack can be constructed by stacking single cells and separators alternately with a current collector appropriately interposed.

  The present SOFC can be operated at a temperature of 600 to 1000 ° C. With the SOFC raised to the operating temperature, fuel gas is supplied to the fuel electrode and air is supplied to the air electrode through the gas flow path of the separator. Examples of the fuel gas include hydrogen, methane, those diluted with an inert gas such as nitrogen, city gas, and the like.

In the present SOFC, the air electrode includes an air electrode material having a composition of R _x A _1-x BO _3-δ typified by La _x Ca _1-x FeO _3-δ described above. Yes. Due to the low reactivity of the conductive oxide to Cr, a cell stack is formed by bringing a separator made of a metal material containing Cr into close contact with the air electrode, and the cell stack is exposed to a temperature environment of 600 to 1000 ° C. Even if it does, the fall of the power generation performance by Cr poisoning of an air electrode does not occur easily.

When a SOFC having an air electrode made of conventional La _1-x Sr _x Co _1-y Fe _y O _3-δ is brought into close contact with a separator made of a metal material containing Cr to form a cell stack, and the SOFC is operated It is known that Cr poisoning occurs in the air electrode, and the power generation performance of SOFC decreases. Specifically, a reaction between Sr and Cr occurs at the contact interface between the air electrode and the separator (see FIG. 1B), and SrCrO ₄ is formed at the interface. This SrCrO ₄ becomes a resistance component, and it is considered that the power generation performance of the entire cell stack is lowered. Note that Cr poisoning of the air electrode by the separator can also occur when a current collector is interposed between the separator and the air electrode.

On the other hand, the R _x A _1-x BO _3-δ does not contain Sr, and the reactivity of the metal A to Cr is lower than that of Sr, so that the separator made of a metal material containing the air electrode and Cr It is possible to suppress the reaction between the metal A and Cr. As a _{result, La 1-x Sr x Co} 1-y Fe y O 3-δ as a problem in the case of a reduction in the power generation performance of the SOFC according to Cr poisoning can be reduced. As a result, the long-term durability of the SOFC can be improved.

In a conventional general cell stack, a coating made of a conductive oxide such as spinel oxide such as MnCo ₂ O ₄ may be applied on the surface of the separator in order to prevent Cr poisoning in the air electrode. Many. However, in the present SOFC, such coating can be omitted because Cr poisoning in the air electrode is suppressed. From the standpoint of reducing the cost required for coating, it is preferable not to perform coating.

In particular, by constituting SOFC using La _x Ca _1-x FeO _3-δ as an air electrode material, not only the suppression of Cr poisoning but also the electronic conductivity and oxide ion conductivity in the air electrode The SOFC can be provided with an excellent air electrode. In particular, when the activation energy for oxygen permeation decreases in the region of 0.5 <x <0.8, the oxide ion conductivity in the air electrode can be increased, and the power generation performance as a whole SOFC can be improved. Furthermore, the low activation energy for oxygen permeation means that the temperature dependence of the oxide ion conductivity is small, indicating that there is a possibility that sufficient power generation performance can be obtained even if the SOFC is operated at a low temperature. is there. For example, the present SOFC can be operated in a relatively low temperature region such as 600 to 850 ° C. Lowering the operating temperature contributes to a reduction in the cost required for operation and a longer life of each component. In particular, in the region of 0.55 ≦ x ≦ 0.70, not only the activation energy is low, but also the oxide ion conductivity value itself is large, so that these effects are particularly excellent.

  Hereinafter, the present invention will be described in detail using examples. The present invention is not limited to the following examples.

[Sample preparation]
La _x Ca _1-x FeO _{3 -δ} crystals were produced by changing the La doping amount x in various ways for use in the subsequent experiments.

For x> 0, the Pechini method using La ₂ O ₃ (99.99%), CaCO ₃ (99.9%), Fe (NO ₃ ) · 9H ₂ O (99.9%) as starting materials Then, the precursor was synthesized and calcined at 900 ° C. for 5 hours. Further, it was formed into a disk shape and fired at 1300 ° C. for 10 hours.

In the case of x = 0, CaCO ₃ (99.9%) and Fe ₂ O ₃ (99.9%) were mixed in a solid phase and then calcined at 900 ° C. for 5 hours. Further, it was formed into a disk shape and fired at 1150 ° C. for 10 hours.

For comparison, a sample prepared by calcining calcined powder of La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O _3-δ (hereinafter sometimes referred to as LSCF) at 1200 ° C. for 12 hours is also prepared. did.

[Experiment 1: Evaluation of Cr poisoning]
First, an experiment on comparison of Cr poisoning of La _x Ca _1-x FeO _3-δ and La _0.6 Sr _0.4 Co _0.2 Fe _0.8 O _3-δ was performed.

(Test method)
La _x Ca _1-x FeO _3-δ (x = 0.5) prepared above and each sample disk of LSCF were brought into close contact with a flat plate of Cr steel (Cr content: 22% by mass), and in the atmosphere And left at 800 ° C. for 1000 hours. Thereafter, the cross section of the contact interface was observed using a scanning electron microscope (SEM), and the distribution of each constituent element was mapped by elemental analysis using an electron beam microanalyzer (EPMA).

(result)
FIG. 1 shows the result of element mapping by SEM-EPMA. (A) shows the distribution of Ca when La _x Ca _1-x FeO _3-δ is used as a sample, and (b) shows the distribution of Sr when LSCF is used as a sample. La _x Ca _1-x FeO _{3 -δ} or LSCF is located in the upper part of the figure, and Cr steel is located in the lower part. Further, the broken lines shown in the figure (white in (a) and gray in (b)) indicate the distribution concentrations of Sr and Ca, respectively. The upper scale in the figure displays the values of the distribution concentrations in units of atomic%.

  In the case of LSCF in FIG. 1B, a sharp peak is observed in the Sr distribution at the interface between LSCF and Cr. That is, Sr is aggregated at the interface. Although illustration is omitted, the Cr concentration is high at the interface where the Sr concentration is high. On the other hand, the Fe concentration is low at positions where the Sr and Cr concentrations are high. These results are interpreted as Sr and Cr reacting locally to form a compound at the contact interface between LSCF and Cr steel, and replacing the perovskite oxide containing Fe.

On the other hand, in the case of La _x Ca _1-x FeO _3-δ in FIG. 1A, no behavior is observed in which Ca is distributed at a high density at the contact interface. This indicates that the reaction due to mutual diffusion between Ca and Cr does not substantially occur at the contact interface between La _x Ca _1-x FeO _3-δ and Cr steel. That is, La _x Ca _1-x FeO _3-δ is unlikely to cause Cr poisoning at the contact interface with Cr steel, unlike LSCF.

[Experiment 2: Confirmation of the crystal structure of La _x Ca _1-x FeO _3-δ ]
Next, the crystal structure of La _x Ca _1-x FeO _3-δ was confirmed.

(experimental method)
X-ray diffraction measurement was performed on three samples of x = 0.4, 0.5, and 0.6 using CuKα rays.

(result)
FIG. 2 shows each measurement result. FIG. 3 shows the simulation results for the case of x = 0.5, 0.6 when the cubic perovskite structure is adopted (Cubic Pm-3m) and when the tetragonal perovskite structure is adopted (Orthohombic Pbnm). Show.

  In FIG. 3, the actual measurement result and the simulation result are compared. Here, the peaks peculiar to the tetragonal perovskite structure are indicated by “▲”, but these peaks are clearly observed in the measurement results of x = 0.5 and x = 0.6. The intensity ratio between these peaks and other peaks is different from the simulation results of tetragonal crystals. From these results, it can be said that at least when x = 0.5 and x = 0.6, there is a high possibility that a cubic perovskite structure close to cubic is taken.

  Further, in FIG. 2, when x = 0.4, a peak is basically observed at the same position as when x = 0.5 and 0.6, and it is interpreted that a perovskite structure is formed. The However, as clearly shown in FIG. 2 (b), the peak is broader than in the case of x = 0.5, 0.6, suggesting that there is disorder in the crystal structure. The

[Experiment 3: Evaluation of oxygen permeation characteristics]
Finally, the oxygen permeation characteristics of La _x Ca _1-x FeO _3-δ were evaluated.

(experimental method)
Prior to the evaluation of oxygen permeation characteristics, the density of the sample was evaluated. As a representative, relative density was measured by Archimedes method for sample disks of x = 0.4 and x = 0.6. As a result, the relative densities were 97% (x = 0.4) and 98% (x = 0.6). It was confirmed that the oxygen permeation characteristics can be evaluated sufficiently accurately by having such a high density. In other words, molecular oxygen transmission can be substantially eliminated, and the oxygen transmission rate in the form of oxide ions can be accurately evaluated.

  Next, the oxygen transmission rate was measured using a sample disk having a thickness of 1.0 mm in which the La doping amount x was varied. In the measurement, the space closed by the alumina tube is divided into two spaces by the sample disk, air is supplied to one space (upstream side), and helium gas is circulated to the other space (downstream side). (Flow rate: 20 sccm). In this state, the speed at which oxygen permeates through the sample disk from the upstream space to the downstream space was measured by gas chromatography in the downstream space. In the measurement, the temperature inside the space closed by the alumina tube including the sample disk was changed in the range of 800 to 1000 ° C. At this time, as shown in FIG. 4, “1000 ° C.”, “975 ° C.”, etc., the oxygen permeation rate was measured while maintaining a constant temperature for a predetermined time, and then the temperature was The process of changing to continuous was repeated and the oxygen permeation rate was measured again while maintaining a constant temperature for a predetermined time. The measurement temperature was changed by a method of decreasing the temperature from 1000 ° C. in order. In addition, the nitrogen permeation rate was also measured in order to confirm the presence or absence of physical gas leaks due to cracks in the sample along with the oxygen permeation rate.

(result)
FIG. 4 shows the result of measuring the oxygen transmission rate J _O2 (mol / cm ² / sec) per unit area of the sample disk while changing the temperature in the case of x = 0.6. First, since the nitrogen permeation rate is almost zero, it is confirmed that there is no gas leak. In other words, it can be seen that the obtained oxygen permeation rate does not indicate molecular permeation of oxygen but oxygen contributes to oxygen permeation as oxide ions. As the measurement temperature is decreased discontinuously with respect to time, the oxygen permeation rate decreases stepwise. This shows the temperature dependence of the oxygen transmission rate. It can be confirmed that the oxygen transmission rate at each temperature is stable, and the oxygen transmission rate in the form of oxide ions is measured with high reliability.

  FIG. 5 (a) shows the results of measuring the oxygen transmission rate at various temperatures for samples with various La doping amounts x. Each sample shows temperature dependence in which the oxygen permeation rate increases as the temperature increases, but the absolute value of the oxygen permeation rate and the degree of temperature dependence vary greatly from sample to sample. In the case of x = 0.55, 0.60, 0.65, and 0.70, it can be seen that the oxygen transmission rate is higher than that in the case of LSCF in almost the entire temperature range in which the measurement was performed.

Further, FIG. 5B shows the same data as an Arrhenius plot. That is, the logarithm LogJ _O2 of oxygen transmission rate on the vertical axis, the horizontal axis represents the reciprocal 1 / T of the measuring temperature plots the temperature dependence of the oxygen transmission rate. The activation energy of oxygen permeation can be calculated from the slope obtained by linear approximation of the Arrhenius plot. In FIG. 5B, the linear approximation is well established for the data group of any sample. The value of the activation energy obtained is shown in FIG. 6 as a function of the La doping amount x. In LSCF, the activation energy is about 120 kJ / mol, but when x = 0, 0.4, 0.55, 0.60, 0.65, 0.70, the activation energy below that is Has been obtained. In particular, when x = 0.55, 0.60, 0.65, and 0.70, the activation energy is 48 to 72 kJ / mol, which is less than half that of LSCF.

Thus, in the region of 0.5 <x <0.8, the activation energy for oxygen permeation is small. Further, in this region of 0.5 <x <0.8, more specifically, in the region of 0.55 ≦ x ≦ 0.70, as shown in FIG. 5A, the activation energy is low. Not only that, the absolute value of the oxygen permeation rate is generally higher than that of LSCF in the entire temperature range. Accordingly, the La _x Ca _1-x FeO _3-δ material in the region of 0.5 <x <0.8, particularly 0.55 ≦ x ≦ 0.70 is used as the air electrode, so that the temperature is relatively low. It is expected that high power generation performance can be obtained even when the SOFC is operated at the same time.

  In FIG. 6, as the La doping amount x increases, the activation energy rises near x = 0.5 except for a region where x is very small, and then decreases once and then rises again. ing. Although the details of the mechanism that takes such a unique behavior are unclear, there is a possibility that a change in the crystal structure as confirmed in Experiment 2 is related.

  In this experiment, as shown in FIG. 4, a series of processes for measuring the oxygen transmission rate by changing the measurement temperature in a predetermined range is continuously performed. Strictly speaking, helium is circulated. The oxygen concentration in the downstream space increases with time. The change in the oxygen concentration may affect the estimated activation energy value through the change in the oxygen partial pressure difference between the two spaces. Therefore, in order to confirm the degree of such influence, the activation energy was estimated by another method, and the values were compared. Specifically, the oxygen transmission rate was measured while changing the oxygen partial pressure on the downstream side to evaluate the oxygen partial pressure dependency of the oxygen transmission rate, and the oxygen transmission rate at each temperature under the same oxygen partial pressure differential condition The activation energy was estimated from As a result, for example, the activation energy when x = 0.6 is 60 kJ / mol, which is larger than the value in FIG. 6 estimated by the above measurement, but the behavior of the activation energy with respect to the value of x. In comparison with the activation energy of LSCF and LSCF, the relationship shown in FIG. 6 was maintained. From this, the results of FIGS. 5 and 6 obtained by the above measurement show, at least semi-quantitatively, an accurate analysis of the dependence of the oxygen permeation rate activation energy on the value of x and the comparison with LSCF. It is shown in

The present invention is not limited to the above-described embodiments and examples, and various modifications can be made without departing from the spirit of the present invention. In the present invention, although using a conductive oxide having a composition of _{R x A 1-x BO 3} -δ as an air electrode material of SOFC, the conductive oxide is limited to the air electrode of the SOFC In addition, Cr is contained in various applications that require mixed conductivity of electrons and oxide ions, such as air electrodes of solid oxide electrolytic cells (SOEC), electrodes of various devices represented by oxygen sensors, oxygen permeable membranes, etc. It can be used as a conductive oxide having low reactivity with Cr at the contact interface with the metal material to be used.

Claims (7)

A material constituting the air electrode of a solid oxide fuel cell,
R is a trivalent metal selected from rare earths and Bi,
A is a divalent metal whose reactivity with Cr at a contact interface with a metal material containing Cr is lower than Sr,
B is one or more selected from Fe, Co, Ni, Mn,
where δ is the amount of oxygen vacancies, 0 <x <1,
R x _{A 1-x BO} air electrode material, characterized by consisting of a perovskite-type conductive oxide having a composition of _3-[delta].   The air electrode material according to claim 1, wherein R = La, A = Ca, and B = Fe.   The air electrode material according to claim 2, wherein 0.5 <x <0.8.   The air electrode material according to claim 2, wherein 0.55 ≦ x ≦ 0.70.   An air electrode for a solid oxide fuel cell, comprising the air electrode material according to any one of claims 1 to 4. An electrolyte layer comprising a solid oxide electrolyte;
A fuel electrode joined to the electrolyte layer;
A solid oxide fuel cell comprising: the air electrode according to claim 5, wherein the air electrode is bonded to the electrolyte layer so as to face the fuel electrode.   The solid oxide fuel cell according to claim 6, further comprising a separator made of a metal material containing Cr in contact with the air electrode.