JP4178610B2 - Oxide ion conductors and their applications - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention relates to a novel rare earth gallate oxide oxide conductor having a perovskite structure. The oxide ion conductor of the present invention exhibits a very high oxide ion conductivity or mixed oxide ion conductivity without being significantly affected by the partial pressure of oxygen, and is a gas for an electrolyte or air electrode of a fuel cell, an oxygen sensor, etc. It is useful as a sensor, an oxygen separation membrane such as an electrochemical oxygen pump, and a gas separation membrane.
[0002]
[Prior art]
Low electronic electrical conductivity, mainly oxide ions (O^2-) Oxide ion conductors that exhibit electrical conductivity by movement of^2-Consists of metal oxides doped with other metals to generate voids, solid oxide type (solid electrolyte type) fuel cell (SOFC) electrolyte, gas sensor such as oxygen sensor, oxygen separation membrane for electrochemical oxygen pump, etc. Application to has been attempted.
[0003]
A typical example of an oxide ion conductor is zirconium oxide (ZrO₂) Small amount of CaO, MgO, Y₂O_Three, Gd₂O_ThreeThis is a cubic fluorite-type solid solution called stabilized zirconia in which a divalent or trivalent metal oxide such as Stabilized zirconia has excellent heat resistance, and oxide ion conductivity is dominant under all oxygen partial pressures from oxygen atmosphere to hydrogen atmosphere, and even if the oxygen partial pressure decreases, the ion transport number ( The ratio of oxide ion conductivity to electric conductivity is difficult to decrease.
[0004]
Therefore, stabilized zirconia is widely used as a zirconia (oxygen) sensor for control of various industrial processes including steel making and combustion (air-fuel ratio) control of automobile engines. It is also used as an electrolyte in the solid oxide fuel cell (SOFC) operating at around 1000 ° C, which is under development. However, the oxide ion conductivity of stabilized zirconia is not sufficiently high, and the conductivity is insufficient particularly when the temperature is lowered. For example, Y₂O_ThreeThe ionic conductivity of stabilized zirconia is 10 at 1000 ° C.^-1 S / cm but 10 at 500 ° C^-Four Since it is reduced to S / cm, the operating temperature is limited to a high temperature of at least 800 ° C.
[0005]
As a fluorite-type oxide that exhibits extremely high oxide ion conductivity that surpasses stabilized zirconia, Bi₂O_ThreeY₂O_ThreeBi dissolved in₂O_ThreeThere are oxides. Although this oxide has a very high oxide ion conductivity, its heat resistance is insufficient because its melting temperature is as low as 850 ° C. In addition, it is weak in reducing atmosphere, and when the oxygen partial pressure decreases, Bi³⁺→ Bi²⁺As a result of this change, n-type electronic conductivity appears, and when the oxygen partial pressure is lowered and the atmosphere is close to a pure hydrogen atmosphere, it is reduced to a metal, so that it cannot be used for a solid oxide fuel cell.
[0006]
Among other fluorite-type oxide ion conductors, ThO₂The oxide oxide has much lower oxide ion conductivity than stabilized zirconia, and electronic conductivity becomes dominant at a low oxygen partial pressure, so that the ion transport number is remarkably lowered. CeO₂Oxide oxide exhibits oxide ion conductivity superior to that of stabilized zirconia, but oxygen partial pressure is 10^-12Ce when it falls below atmospheric pressure⁴⁺→ Ce³⁺As a result of the change, n-type electronic conductivity appears, and the ion transport number greatly decreases.
[0007]
PbWO as an oxide ion conductor with a crystal structure other than fluorite_Four, LaAlO_Three, CaTiO_ThreeHowever, all of these have low oxide ion conductivity, and semiconductivity appears under a low oxygen partial pressure, mainly resulting in electronic conductivity, resulting in a decrease in ion transport number.
[0008]
[Problems to be solved by the invention]
As explained above, oxide ion conductors with higher oxide ion conductivity than stabilized zirconia are known, but heat resistance is insufficient, and electronic conductivity is dominant at low oxygen partial pressure. Therefore, the ion transport number is greatly reduced, so that it is not suitable for applications such as an electrolyte of a solid oxide fuel cell or an oxygen sensor.
[0009]
The present invention shows higher oxide ion conductivity than stabilized zirconia, high heat resistance, high oxide ion conductivity even when the temperature is lowered as well as high temperature, more preferably from oxygen atmosphere to hydrogen atmosphere. Provide an oxide ion conductor in which the decrease in ion transport number is small at any oxygen partial pressure (that is, even when the oxygen partial pressure is low), oxide ion conduction is dominant, or mixed ion conductivity is exhibited. This is the issue.
[0010]
[Means for Solving the Problems]
The present inventors have studied ABO with a perovskite structure in order to solve the above problems._Three (Wherein A is one or more lanthanoid-based rare earth metals, B is Ga), a part of the rare earth metal at the A site is an alkaline earth metal, and / or It has been found that a material exhibiting high oxide ion conductivity can be obtained by substituting a part of Ga atoms at the B site with a non-transition metal such as Mg, In or Al. Above all, La_0.8Sr_0.2Ga_0.8Mg_0.2O_ThreeThe material indicated by shows high oxide ion conductivity.
[0011]
FIG. 1 shows a graph comparing the electrical conductivity of this compound with a conventional oxide ion conductor. As you can see from this graph, La_0.8Sr_0.2Ga_0.8Mg_0.2O_ThreeY is a typical representative stabilized zirconia₂O_ThreeCompared to stabilized zirconia and CaO stabilized zirconia, it exhibits very high conductivity (electrical conductivity, the same applies hereinafter). Bi₂O_ThreeAlthough the system oxide exhibits higher conductivity than this, it is difficult to put it into practical use as an oxide ion conductor because it has insufficient heat resistance as described above and is weak in a reducing atmosphere.
[0012]
As a result of searching for materials having higher oxide ion conductivity, the present inventors have further improved oxide ion conductivity when a small amount of a specific transition metal is contained in the B site of the rare earth gallate oxide. The present inventors have found that high oxide ion conductivity is exhibited even at a low temperature, and reached the present invention.
[0013]
Here, the present invention is an oxide ion conductor represented by the following general formula.
Ln_1-xA_xGa_1-yzB1_yB2_zO_Three  ・ ・ ・ ▲ ▼
▲ 1 ▼
Ln = One or more of La, Ce, Pr, Nd, Sm;
A = one or more of Sr, Ca, Ba;
B1 = one or more of Mg, Al, In;
B2 = one or more of Co, Fe, Ni, Cu;
x = 0.05-0.3;
y = 0-0.29;
z = 0.01-0.3;
y + z = 0.025-0.3.
[0014]
In the present invention, the “oxide ion conductor” means an electrically conductive material exhibiting substantial oxide ion conductivity. That is, not only the oxide ion conductor in the narrow sense where the oxide ion conductivity occupies most of the electric conductivity, but also the electron conductivity, sometimes called an electron-ion mixed conductor (or oxide ion mixed conductor). In the present invention, a material in which both the oxide ion conductivity and the oxide ion conductivity account for a large proportion is also included in the oxide ion conductor as a material exhibiting oxide ion conductivity.
[0015]
In the case of oxide ion conductors in the narrow sense where the majority of electrical conductivity is occupied by oxide ion conductivity, the ion transport number (ratio of oxide ion conductivity in electrical conductivity) is preferably 0.7 or more, more Preferably it is 0.9 or more. On the other hand, in the case of an electron-ion mixed conductor, the ion transport number is preferably 0.1 to 0.7, more preferably 0.2 to 0.6.
[0016]
According to the present invention, a solid oxide fuel cell using the oxide ion conductor as an electrolyte or an air electrode, a gas sensor comprising the oxide ion conductor, an oxygen separation membrane for an electrochemical oxygen pump, Also provided is a gas separation membrane that utilizes gas concentration differences.
[0017]
DETAILED DESCRIPTION OF THE INVENTION
The oxide ion conductor of the present invention represented by the above formula (1) has a perovskite crystal structure, and is ABO._ThreeThe Ln atom and A atom of the above general formula occupy the A site of the perovskite crystal, and the remaining Ga atom, B1 atom, and B2 atom occupy the B site. The B1 atom may not be present.
[0018]
Originally a part of both A and B sites occupied by trivalent metals are divalent metals (for example, the above A atom occupying A site, B1 Mg occupying B site) or transition metal (B2 atom occupying B site). Occupancy causes oxygen vacancies, and oxide ion conductivity appears due to the oxygen vacancies. Accordingly, the number of oxygen atoms is reduced by this oxygen vacancy.
[0019]
That is, in the formula (1), the number of oxygen atoms is displayed as three, but the number of oxygen atoms is actually three or less. However, the number of oxygen vacancies varies not only with the type of added atoms (A, B1, B2) but also with temperature, oxygen partial pressure, and type and amount of B2 atoms, so it is difficult to display accurately. . Therefore, in the chemical formula showing the perovskite material of this specification, the numerical value of the oxygen atomic ratio is displayed as 3 for convenience.
[0020]
In the above formula (1), Ln is a lanthanoid rare earth metal, A is an alkaline earth metal, B1 is a non-transition metal, and B2 is a transition metal. That is, the oxide ion conductor of the present invention has a lanthanoid gallate (LnGaO_Three) As a basic structure, and three types of alkaline earth metal (A), non-transition metal (B1), and transition metal (B2), or two types of alkaline earth metal (A) and transition metal (B2) This is a ternary (Ln + A + Ga + B1 + B2) or quaternary (Ln + A + Ga + B2) complex oxide doped with the above atoms. Hereinafter, this composite oxide may be referred to as a 5/4 ternary composite oxide.
[0021]
Ln + A + Ga + B1 quaternary complex oxide (typical example is La_0.8Sr_0.2Ga_0.8Mg_0.2O_Three) Is an excellent oxide ion conductor that exhibits higher oxide ion conductivity than stabilized zirconia, as shown in FIG. In the present invention, this is referred to as a control quaternary complex oxide. According to the present invention, by replacing some or all of the B1 atoms of this control quaternary composite oxide with a transition metal (B2 atom), the oxide ionic conductivity is generally higher than that of the control quaternary composite oxide. An oxide ion conductor exhibiting properties can be obtained.
[0022]
In Figure 2, La_0.8Sr_0.2Ga_0.8Mg_0.2O_ThreeThe oxide ionic conductor of the present invention in which a part of Mg of the control quaternary composite oxide is replaced with a transition metal (indicated by M in the general formula of FIG. 2) to form a ternary system (Ln is La, A is Sr, B1 is Mg, and B2 is M atom).
[0023]
As can be seen from this figure, when the B2 atom (M in the general formula of FIG. 2) is Co or Fe, it exhibits much higher electrical conductivity than the control quaternary composite oxide at any temperature. In particular, in the control quaternary complex oxide, the decrease in conductivity is large on the low temperature side (value on the horizontal axis is 1.1 or more and about 630 ° C or less), so the conductivity is improved by the inclusion of Co or Fe on the low temperature side. . When the B2 atomic (M) atom is Ni, the conductivity exceeds the conductivity of the control quaternary composite oxide when the horizontal axis is about 0.9 or more (temperature is about 840 ° C. or less). When the B2 atom is Cu, the conductivity exceeds the conductivity of the control quaternary complex oxide when the horizontal axis is about 1.1 or higher (temperature is about 630 ° C or lower). As the temperature drops, the conductivity does not decrease and is almost constant. Therefore, when the horizontal axis is 1.3 or higher (temperature is about 500 ° C or lower), the highest conductivity is shown in the figure.
[0024]
Therefore, when the B2 atom is Ni or Cu, it is preferably used as an oxide ion conductor on the relatively low temperature side. However, the control quaternary complex oxide (La_0.8Sr_0.2Ga_0.8Mg_0.2O_Three1) As shown in Fig. 1, it shows much higher conductivity than stabilized zirconia even on the high temperature side where the abscissa exceeds 1.0. For example, it can be said that the conductivity is sufficiently high not only on the low temperature side but also on the high temperature side.
[0025]
In contrast, when the transition metal of the B2 atom is Mn, the conductivity is lower than the control quaternary complex oxide on the high temperature side where the horizontal axis is 1.1 or less, and the control quaternary system is used even on the low temperature side where the horizontal axis is 1.1 or more. The conductivity is almost the same as that of the composite oxide, and substantially no improvement in conductivity can be obtained at any temperature by substituting a part of Mg with a transition metal. Therefore, as transition metal B2 atoms, one or more selected from Co, Fe, Ni, and Cu can be obtained at least at some temperatures, as compared to the control quaternary composite oxide. And
[0026]
When the atomic ratio (x) of doped atoms at each site, that is, the A atom at the A site, or the total atomic ratio (y + z) of B1 atoms + B2 atoms at the B site falls outside the above range, 5/4 element of the present invention. The electrical conductivity or ion transport number of the composite oxide decreases.
[0027]
FIG. 3 shows the conductivity when the ratio of A atom (Sr) is changed, and the atomic ratio (x) of A atom is out of the range of 0.05 to 0.3 (= the atomic ratio of Ln atom is 0.7 to 0.95). It can be seen that the conductivity decreases.
[0028]
FIG. 4 (a) shows the conductivity when the total atomic ratio of B1 atom + B2 atom (y + z, where y: z = 11.5: 8.5) is changed. As this sum increases, the conductivity increases. However, as shown in FIG. 4 (b), when the value of y + z increases, the ion transport number decreases, and when 0.3 (= Ga atomic ratio exceeds 0.7), the ion transport number falls below 0.7.
[0029]
Of the two types of doped atoms at the B site, as shown in FIG. 5, as the z value, which is the atomic ratio of B2 atoms (Co), increases, the electrical conductivity increases. This is because the B2 atom is a transition metal, and it easily develops n-type or p-type electronic electrical conduction due to valence fluctuations. This is because it becomes higher. Along with this, the ratio of oxide ion conductivity (ion transport number) decreases.
[0030]
As can be seen from FIG. 5, when the z value is 0.15 or less, the ion transport number is 0.7 or more, and particularly when the z value is 0.10 or less, the ion transport number is as high as 0.9 or more. It functions as a narrowly defined oxide ion conductor. However, in this case, if the B site does not contain B1 atoms, which are non-transition metals, to some extent, the contribution ratio of electronic conductivity cannot be maintained at 0.3 or less. As will be described later, such a material is useful as an electrolyte for a solid oxide fuel cell, a gas sensor, an oxygen separation membrane for an electrochemical oxygen pump, and the like.
[0031]
On the other hand, when the z value exceeds 0.15, the ion transport number decreases to 0.7 or less, and functions as an electron-ion mixed conductor. As described above, such a material is also included in the oxide ion conductor in the present invention. It should be noted that even if the z value is 0.2 (ie, y value = 0), that is, the quaternary complex oxide in which Mg (B1 atom) is completely substituted with Co (B2 atom), the ion transport number is It remains at about 0.3 and still functions sufficiently as an electron-ion mixed conductor (ie, oxide ion mixed conductor), and has the highest conductivity as described above. Such a mixed conductor is useful for an air electrode or a gas separation membrane of a solid oxide fuel cell, as will be described later.
[0032]
In the above formula (1), preferred compositions are as follows.
Ln = La, Nd or mixtures thereof, in particular La,
A = Sr,
B1 = Mg,
B2 = Co,
x = 0.10-0.25, especially 0.17-0.22,
y = 0 to 0.17, especially 0.09 to 0.13,
y + z = 0.10-0.25, especially 0.15-0.20.
[0033]
The z value is z = 0.02 to 0.15, particularly 0.07 to 0.10 when functioning as an oxide ion conductor in a narrow sense having high oxide ion conductivity (ion transport number of 0.7 or more, preferably 0.9 or more). It is preferable that On the other hand, when it is desired to function as an electron-ion mixed conductor, the z value is 0.15 <z ≦ 0.3, and preferably 0.15 <z ≦ 0.25.
[0034]
In one preferred embodiment of the present invention, Ln = La, A = Sr, B1 = Mg, B2 = Fe, x = 0.1 to 0.3, y = 0.025 to 0.29, z = 0.01 to 0.15, y + z = 0.035 to 0.3. It is. That is, this oxide ion conductor is represented by the following equation (2).
[0035]
La_1-xSr_x Ga_1-yz Mg_yFe_zO_Three  ... (2)
Where
x = 0.1-0.3;
y = 0.025-0.29;
z = 0.01-0.15;
y + z = 0.035-0.3.
[0036]
The oxide ion conductor represented by the formula (2) exhibits high electrical conductivity almost independent of the oxygen partial pressure. This electrical conductivity is slightly contributed by P-type semiconductivity under high oxygen partial pressure, 1 to 10^{-twenty one}Oxygen ion conductivity is dominant at a wide oxygen partial pressure of atm (that is, oxygen partial pressure extending from the reducing atmosphere to the oxidizing atmosphere), and the electric conductivity ion transport number is as high as 0.9 or more. Thus, the high ion transport number is exhibited regardless of the oxygen partial pressure, and at the same time, the electrical conductivity is high. Therefore, the improvement in the electrical conductivity of the oxide ion conductor of the present invention is mainly the improvement in the oxide ion conductivity. It is thought to be due to.
[0037]
Figure 6 shows La_0.8Sr_0.2Ga_0.8Mg_0.2O_ThreeA ternary composite oxide obtained by substituting a part of Mg of the control quaternary composite oxide shown in FIG._0.8Sr_0.2Ga_0.8Mg_0.2-zFe_zO_Three) Electrical conductivity (950 ° C, oxygen partial pressure = 10^-Five atm). As can be seen from this figure, compared with the control quaternary composite oxide with z = 0, when Mg is partially replaced with Fe, the electrical conductivity generally increases, and the electrical conductivity is in the range of z = 0.01 to 0.05. It is especially high, and it is understood that the maximum value is reached around z = 0.03.
[0038]
Figure 7 (a) shows La_0.8Sr_0.2Ga_0.8Mg_0.2-zFe_zO_Three (Arrhenius plot) Temperature change of electrical conductivity of quaternary composite oxide represented by formula (2) shown by (z = 0.03, 0.05, 0.1, 0.15) and quaternary composite oxide of z = 0 FIG. 7 (b) shows the oxygen partial pressure dependence of the electrical conductivity of the ternary composite oxide of z = 0.03 and related compounds in the formula (2). As can be seen from this figure, this ternary composite oxide exhibits high electrical conductivity over a wide temperature and oxygen partial pressure range, and the electrical conductivity exhibits almost no oxygen partial pressure dependence. It turns out that a high ion transport number is shown.
[0039]
Therefore, this ternary composite oxide is useful as an electrolyte, a gas sensor, an oxygen separation membrane for an electrochemical oxygen pump, etc. for a solid oxide fuel cell. Since oxide ion conductivity is higher than that of stabilized zirconia and changes due to temperature and oxygen partial pressure are small, a product superior in performance to stabilized zirconia can be provided.
[0040]
In the above formula (2), preferred compositions are as follows.
x = 0.15 to 0.25, especially 0.17 to 0.22.
y = 0.09-0.24, especially 0.10-0.20,
z = 0.01-0.05, especially about 0.03,
y + z = 0.10-0.25, especially 0.15-0.22.
[0041]
The oxide ion conductor of the present invention can be produced by forming a mixture in which the powders of the oxides of the component elements are mixed well at a predetermined blending ratio by appropriate means, firing and sintering. As the raw material powder, in addition to oxides, precursors (eg, carbonates, carboxylic acids, etc.) that are thermally decomposed during firing to become oxides can be used. The firing temperature for sintering is 1200 ° C. or more, preferably 1300 ° C. or more, and the firing time is several hours to several tens of hours. In order to shorten the firing time, the raw material mixture may be pre-fired at a temperature lower than the sintering temperature. This pre-baking can be carried out, for example, by heating at 500 to 1300 ° C. for about 1 to 10 hours. The pre-fired mixture is crushed if necessary, then shaped and finally sintered. For the molding, an appropriate powder molding means such as uniaxial compression molding, isostatic pressing, extrusion molding, tape cast molding or the like can be adopted. The firing atmosphere including pre-firing is preferably an oxidizing atmosphere such as air or an inert gas atmosphere.
[0042]
Among the oxide ion conductors of the present invention, those having a y value of 0.025 or more and a z value of 0.15 or less have a dominant oxide ion conductivity in electrical conductivity (that is, ion transport number). Is 0.7 or more), and the oxide ion conductor in the narrow sense is obtained. This material can be used for various oxide ion conductor applications (eg, SOFC electrolytes, gas sensors) that have traditionally used stabilized zirconia. This type of oxide ion conductor of the present invention is expected to give a product with higher performance than stabilized zirconia because it has higher oxide ion conductivity than stabilized zirconia and can operate at low temperatures. .
[0043]
Although the application field of oxide ion conductors such as YSZ is widespread, one important application is the electrolyte of solid oxide (solid electrolyte) fuel cells (SOFC). The most developed SOFC at present is Y₂O_ThreeA perovskite-type material (e.g., Sr-containing LaMnO) that exhibits electronic electrical conductivity at the cathode (cathode) using a thin film of stabilized zirconia (YSZ) as an electrolyte._ThreeThe fuel electrode (anode) has a battery configuration using a metal such as Ni or a cermet such as Ni-YSZ. Since the conductivity of YSZ is low at low temperatures and power generation efficiency can be increased by cogeneration that operates the turbine generator using the heat of exhaust gas at around 1000 ° C, SOFCs using YSZ as an electrolyte are 1000 Designed to operate at high temperatures around ℃.
[0044]
SOFC has a large voltage drop due to the resistance loss of the electrolyte, and the thinner the film, the higher the output. Therefore, the electrolyte YSZ is used in a thin film of about 30-50 μm. However, since the oxide ion conductivity of YSZ is still small, it is necessary to heat to about 1000 ° C. in order to obtain practically sufficient performance. A practical power density of 0.35 W / cm at a working temperature of 1000 ° C with a thin film YSZ of 30 μm thickness²It is reported as a degree. In order to increase the battery output or lower the operating temperature, experimental examples using YSZ thin films with a thickness of several μm to 10 μm have been reported, but such thin films are required for electrolytes. The gas impermeability becomes uncertain, which is undesirable in terms of reliability.
[0045]
The narrowly defined oxide ion conductor composed of the ternary perovskite oxide of the present invention can be obtained with oxide oxide conductivity much higher than that of YSZ. For example, the thickness is 0.5 mm (= 500 μm). Even when the SOFC is configured using a thick film electrolyte that can be manufactured by the sintering method, a higher output than the YSZ thin film can be obtained. The maximum power density in this case varies depending on the type and atomic ratio of B2 atoms, but surpasses this even at an operating temperature of 1000 ° C compared to SOFC using a 30 μm-thick YSZ thin film, and several times at an operating temperature of 800 ° C ( Eg 3 times or more). Alternatively, when used in a film having a thickness of about 200 μm, a power density equivalent to that produced by a 30 μm-thick YSZ film at 1000 ° C. can be obtained at a low temperature of 600 ° C. to 700 ° C.
[0046]
When the oxide ion conductor of the present invention is used for an SOFC electrolyte, the specific material to be used may be selected according to the operating temperature. For example, when turbine power generation using exhaust gas is performed simultaneously as cogeneration, a high operating temperature of about 1000 ° C. is required. Therefore, B2 atoms exhibit high oxide ion conductivity at such a high temperature, Co2 or Fe, In particular, it is preferable to use a ternary composite oxide of Co as an electrolyte. On the other hand, if the operating temperature is about 800 ° C., those having B2 atoms of Ni can be used in addition to the above, and if the operating temperature is 600 ° C. or lower, those having B2 atoms of Cu can be used.
[0047]
Even if the operating temperature is as low as 600 to 700 ° C., for example, the power generation efficiency of SOFC does not decrease so much by simultaneously performing power generation with water vapor or other exhaust gas or simultaneously achieving effective use of energy as a heat source. When the operating temperature is lowered in this way, steel materials such as stainless steel can be used for the SOFC structural material, and material costs are significantly reduced compared to materials such as Ni-Cr alloys and ceramics when the operating temperature is around 1000 ° C. There is also an advantage. In the conventional YSZ, it was not possible to build a SOFC that operates at such a low temperature. However, according to the present invention, a variety of SOFCs can be built from the low-temperature operation type to the high-temperature operation type according to the usage environment. It becomes possible to do.
[0048]
In particular, the oxide ion conductor composed of the ternary complex oxide represented by the above formula (2) has a wide temperature range exhibiting high oxide ion conductivity. It functions well as an SOFC electrolyte at any high operating temperature around ℃. Therefore, when this oxide ion conductor is selected as the electrolyte, various SOFCs from a low temperature operation type to a high temperature operation type can be constructed with this material alone.
[0049]
As described above, the ternary composite oxide of the present invention has a very high oxide ion conductivity compared to YSZ, and therefore can be manufactured from a sintered body having a thick electrolyte and, for example, about 0.5 mm. As a result, it is possible to produce a SOFC with significantly improved mechanical strength and longevity and a higher maximum power density than when YSZ is used as the electrolyte.
[0050]
The SOFC electrode using the ternary oxide ion conductor of the present invention as an electrolyte is not particularly limited, and an electrode material used for a conventional SOFC can be used. For example, if the air electrode is Sm_0.5-0.7Sr_0.3-0.5CoO_ThreeTherefore, the fuel electrode can be made of Ni metal. This battery configuration increases output, especially at low temperatures, 1.5 W / cm even at 800 ° C.²Therefore, a solid oxide fuel cell capable of operating at a low temperature of 600 ° C or lower, which was impossible in the past, has been achieved. It is expected to be possible to produce. The fuel electrode uses Ni-CeO to reduce electrode overvoltage.₂A cermet such as Particularly preferred air electrodes and fuel electrodes will be described later.
[0051]
The largest application of YSZ at present is oxygen sensors, which are used in large quantities for air-fuel ratio control of automobiles and also for industrial processes such as steelmaking. This oxygen sensor is called a solid electrolyte oxygen sensor and measures the acidity concentration based on the principle of an oxygen concentration cell. That is, if there is a difference in oxygen gas partial pressure at both ends of the material made of oxide ion conductor, oxide ions diffuse into the material to form an oxygen concentration cell. By measuring, the oxygen partial pressure can be measured. In addition to oxygen gas, the solid electrolyte oxygen sensor_x, NO_xIt can also be used as an oxygen-containing gas sensor.
[0052]
Although the oxygen sensor made of YSZ is relatively inexpensive, the oxide ion conductivity decreases at low temperatures, so the sensor can only be used at high temperatures of 600 ° C or higher, and its application has been limited. On the other hand, the ternary oxide ion conductor (y ≧ 0.025, z ≦ 0.15, including those represented by the above formula (2)) of the present invention in which the oxide ion conductivity is dominant is obtained from YSZ. Since it exhibits high oxide ion conductivity, it is useful as a gas sensor, particularly an oxygen sensor, and since it has high oxide ion conductivity even at low temperatures, it is a gas sensor that can be sufficiently used at 600 ° C. or lower.
[0053]
The ternary oxide ion conductor of the present invention (y ≧ 0.025, z ≦ 0.15, including those represented by the above formula (2)) in which the oxide ion conductivity is dominant is an electrochemical oxygen pump. It can also be used as an oxygen separation membrane. When a potential difference is applied to both sides of the separation membrane made of an oxide ion conductor, the oxide ions move inside to flow current, and oxygen flows in one direction from one surface to the opposite surface. This is an oxygen pump. For example, when air is flown, oxygen-enriched air is obtained from the opposite surface, and therefore, it is used as an oxygen separation membrane.
[0054]
Such oxygen separation membranes are used, for example, in military aircraft and helicopters to produce oxygen-enriched air from the surrounding rare air. It can be applied as an alternative to medical oxygen cylinders.
[0055]
In addition, the 5/4 quaternary perovskite oxide ion conductor (z> 0.15) of the present invention that exhibits electron-ion mixed conductivity (ie, an ion transport number of 0.7 or less) is an ionization catalyst that imparts electrons to oxygen. It exhibits both the electronic conductivity necessary to function and function as a current collector, and sufficient oxide ion conductivity to function as an oxide ion conductor for delivering oxide ions to the electrolyte, It is suitable for the material of the SOFC air electrode described above, and at least a part of the air electrode is preferably composed of this material.
[0056]
In particular, the SOFC electrolyte is a ternary material of the present invention which is an oxide ion conductor in a narrow sense in which the oxide ion conductivity is dominant as described above (where y ≧ 0.025, z ≦ 0.15, especially z ≦ 0.10, including those represented by the above formula (2)) 5/4 quaternary material of the present invention that exhibits electron-ion mixed conductivity in the air electrode (in formula (1)) z> 0.15, including the case of y = 0), the SOFC electrolyte and the air electrode are made of the same material, and the performance of the SOFC is improved.
[0057]
To explain this point in detail, in the conventional SOFC, the electrolyte and the air electrode are made of different materials. [For example, the electrolyte is YSZ and the air electrode is La (Sr) CoO._Three] In this case, when viewed microscopically at the atomic level, a very thin interface layer is formed in which the materials of both layers are mixed at the interface between the electrolyte and the air electrode, and the output loss due to the voltage loss due to the interface resistance. Decreases. When the electrolyte and the air electrode are made of the same material, the generation of the interface layer is suppressed and the interface resistance is reduced.
[0058]
In addition to the problem of interfacial resistance, when the electrolyte and the air electrode are made of different materials, the thermal expansion coefficients of the two are different, so that the thermal stress applied when the temperature is raised or lowered is increased. This problem is also significantly reduced by constituting the electrolyte and the air electrode from the same kind of material.
[0059]
The interface resistance and thermal stress are gradually changed from the electrolyte to the air electrode by providing one or more intermediate layers having an intermediate composition between these two materials between the electrolyte and the air electrode. By doing so, it can be further suppressed.
[0060]
As described above, the fuel electrode can be composed of various materials that have been conventionally used. Particularly preferable materials for the fuel electrode are (1) Ni and (2) General formula: Ce._1-mC_mO₂ (Wherein C represents one or more of Sm, Gd, Y and Ca, and m = 0.05 to 0.4). The ratio of the two is preferably such that the volume ratio of (1) :( 2) is in the range of 95: 5 to 20:80. More preferably, the m value is 0.1 to 0.3, and the volume ratio of (1) :( 2) is 90:10 to 40:60.
[0061]
The structure of the SOFC is not particularly limited, and may be a cylindrical type or a flat plate type. In the case of a flat plate type, either a stack type or an integral sintered type (monolith type) may be used. In any case, a three-layered structure in which an electrolyte layer is sandwiched between an air electrode and a fuel electrode (one side of the electrolyte layer is in contact with the air electrode layer and the other surface is in contact with the fuel electrode layer) has a basic cell structure. The electrolyte layer is gas impermeable, and the air electrode and fuel electrode layers are porous so that gas can pass through. In the case of a cylindrical type, an interconnector in which fuel gas (for example, hydrogen) and air (or oxygen) are separately supplied to the inside and outside of the cylinder, and a large number of cylindrical cells are provided on a part of the outer surface. Connected through. In the case of the flat plate type, the gas is supplied using a generally flat plate type interconnector provided with a flow path capable of supplying fuel gas and air separately. This interconnector is stacked in layers with the above-described flat plate cells having a three-layer structure so as to be multilayered.
[0062]
One of the rate-limiting reactions in the SOFC electrode reaction is the ionization of oxygen at the air electrode represented by the following formula.
1 / 2O₂+ 2e^-→ O^2-
Since this reaction occurs at the interface between the air electrode, the electrolyte, and the air, the reaction amount increases as the area of this interface increases. For this reason, for example, the above three-layer structure has not been made flat but corrugated.
[0063]
In the preferred embodiment of the present invention, as shown in FIG. 8, a cell structure is used in which irregularities are formed on both surfaces of the electrolyte layer, and the material of the air electrode or fuel electrode is adhered to the surface irregularities. In this case, the main body portion of the electrolyte layer needs to be gas-impermeable, but the uneven portions formed on both surfaces may be porous. The material of the concavo-convex portion may be the same material as the electrolyte (that is, an oxide ion conductor in a narrow sense), but is preferably a material that exhibits electron-ion mixed conductivity. For example, the concavo-convex portion on the air electrode side can be made of the material (z> 0.15) showing the electron-ion mixed conductivity according to the present invention. In that case, it is preferable that the individual particles adhered to the concavo-convex portions are made of a material having a dominant electronic electrical conductivity, such as a conventional air electrode material.
[0064]
Such a structure can be formed by first baking the ion-electron mixed conductor particles on the surface of the electrolyte layer, and further attaching and baking finer electron conductor particles on the surface. Alternatively, the same structure can be realized at a certain ratio by simply adhering a mixture of ion-electron mixed conductor particles and electron conductor particles to the surface of the electrolyte layer and baking it.
[0065]
Conventional air electrode material is La (Sr) CoO_Three, La (Sr) MnO_ThreeIs an electronic conductor with a dominant electronic conductivity (low ion transport number), so even if oxygen in the air is ionized to oxide ions, it passes through the air electrode material and becomes oxide into the electrolyte. I cannot send in ions. Therefore, when this air electrode material is used, the surface uneven portion on the air electrode side in FIG. 8 is made of an electrolyte material, and the air electrode material is adhered to the surface uneven portion in the form of particles. In this case, as shown in FIG. 9A, oxygen ionization occurs at the interface between the electrolyte layer, the air electrode particles, and the air, that is, the outer edge (circumference) of the interface between the electrolyte layer and the air electrode particles. It only happens in a one-dimensional area along. As a result, the polarization of the air electrode increases and the output of SOFC decreases. In addition, since the electrolyte layer needs to be in contact with air in order to take in oxide ions, the air electrode cannot completely cover the electrolyte layer, and the amount of adhesion is limited. Therefore, the electrical connection to the external terminal depending on the electronic electrical conduction of the air electrode is likely to be incomplete. Alternatively, in order to obtain a sufficient electrical connection, a bridging structure rich in voids of a conductive material that roughly covers the three-phase interface and connects the air electrode particles is required. Resistance to passage.
[0066]
On the other hand, since the material for the air electrode of the present invention exhibits ion-electron mixed conductivity, the material itself can ionize oxygen in the air to oxide ions. Therefore, as described above, the surface uneven portion on the air electrode side in FIG. 8 is composed of this mixed conductive air electrode material, and individual particles adhered to the uneven portion are air electrode materials of conventional electron conductors. It can consist of In this case, as shown in FIG. 9 (b), oxygen ionization occurs at a two-dimensional region of the two-phase interface between the surface irregularities of the mixed conductive material and the air, that is, the entire outer surface of the material. The ionization efficiency is dramatically increased and the polarization of the air electrode can be prevented, thereby improving the SOFC output. Oxide ions generated by ionization flow to the electrolyte through the air electrode material due to the oxide ion conductivity of the mixed conductive air electrode material. In addition, the mixed conductive air electrode material that forms the surface irregularities can also be electronically conductive, and electricity can flow to the external terminals. It is made to adhere to the surface of the uneven part.
[0067]
The anode is made of Ni and ceria-based material (Ce_1-mC_mO₂). Also in this case, the ceria-based material that is the oxide ion mixed conductor constitutes the surface irregularity portion on the fuel electrode side, and individual particles on the surface are composed of Ni that is the electron conductor. With this configuration, as in the case of the air electrode described above, H in a two-dimensional region is obtained.₂The oxide ions are delivered to₂The efficiency of the O production reaction is significantly improved.
[0068]
The oxide ion conductor (z> 0.15) of the present invention exhibiting mixed electron-ion conductivity can also be used as a gas separation membrane utilizing a gas concentration difference. In the case of a gas separation membrane, it is not necessary to apply a potential difference from the outside to both sides of the membrane, and the oxygen concentration difference in the gas on both sides of the separation membrane becomes the driving force for separation. Due to this oxygen concentration difference, oxide ions flow from the high concentration side to the low concentration side, and electrons flow in the opposite direction to electrically compensate for this flow. Therefore, if there is no electronic conductivity along with oxide ion conductivity (that is, if it is not an electron-ion mixed conductor), it will not function because electrons do not flow.
[0069]
This gas separation membrane is not only oxygen but also, for example, water or NO_XCan also be used for disassembling. In the case of water, when it is decomposed into oxide ions and hydrogen on the surface of the separation membrane, there is a difference in the oxide ion concentration on both sides of the membrane, which can be used as a driving force to allow the flow of oxide ions, without hydrogen flowing. Since it remains, hydrogen can be produced from water. NO_XIn the case of_XIs detoxified and separated into nitrogen and oxygen.
[0070]
In addition, the oxide ion conductor of the present invention can be used for electrochemical reactors, oxygen isotope separation membranes, and the like.
[0071]
【Example】
(Example 1)
La₂O_Three, SrCO_Three, Ga₂O_Three, MgO and CoO, Fe₂O_Three, Ni₂O_Three, CuO, MnO₂Each transition metal oxide powder selected from_0.8Sr_0.2Ga_0.8Mg_0.1M_0.1O_Three (M is a transition metal) was added at a ratio to generate and mixed well, followed by pre-baking at 1000 ° C. for 6 hours. This pre-fired mixture was pulverized and compression-molded into a disk shape having a thickness of 0.5 mm and a diameter of 15 mm by an isostatic press, and the compact was fired at 1500 ° C. for 6 hours to be sintered. When the crystal structure of the obtained sintered body was examined by X-ray diffraction, all had a perovskite crystal structure.
[0072]
The electrical conductivity of the obtained sintered body was determined by applying a platinum paste serving as an electrode to a rectangular parallelepiped sample cut from a disk-shaped sintered body, and then connecting a platinum wire at 950 to 1200 ° C. for 10 to 60 minutes. The resistance value was obtained by measuring the resistance value by the DC four-terminal method or the AC two-terminal method in an apparatus that can be baked and adjusted to an arbitrary oxygen partial pressure and temperature. O partial pressure adjustment is O₂−N₂, CO-CO₂, H₂−H₂This was carried out using O mixed gas.
[0073]
The measurement results are shown in FIGS. Figure 2 shows a constant oxygen partial pressure (10^-Five Atm) shows the electrical conductivity (Arrhenius plot of conductivity) when the temperature is changed. FIG. 10 shows the electric conductivity (dependence of conductivity on oxygen partial pressure) when the temperature is constant (950 ° C.) and the oxygen partial pressure is changed. Although FIG. 2 has already been described, it can be seen that when the transition metal is Co, Fe, Ni, or Cu, the conductivity is greatly improved at least on the low temperature side by substituting a part of Mg with the transition metal.
[0074]
From FIG. 10, when the transition metal is Ni, Cu or Mn, the conductivity varies depending on the oxygen partial pressure, but when the transition metal is Co or Fe, even if the oxygen partial pressure varies, it is almost constant and high. It can be seen that the conductivity is maintained.
[0075]
The result of measuring the ion transport number of the compound whose transition metal is Co is shown together with the electric conductivity in FIG. This ion transport number was determined by preparing an oxygen concentration cell with partitioning the oxygen partial pressures of the atmosphere at both ends of the sample different from each other, measuring the electromotive force of this battery, and applying the theoretical electromotive force under the same conditions to the Nernst It calculated | required from the type | formula and calculated | required as ratio with respect to the theoretical electromotive force of the measured value of an electromotive force. Even when the transition metal was other than Co, the same tendency as in FIG. 5 was observed. When the ratio of the transition metal to Mg increased, the electrical conductivity increased and the ion transport number decreased. However, since the increase in electrical conductivity is a logarithmic increase, it is much greater than the decrease in ion transport number. Therefore, even if the ion transport number decreases, the absolute value of the oxide ion conductivity increases.
[0076]
(Example 2)
In the same manner as in Example 1, La_0.8Sr_0.2Ga_0.8Mg_0.115Co_0.085 O_ThreeAn oxide ion conductor made of a sintered body of^-Five The electrical conductivity was measured by changing the temperature with atm. The measurement results (Arrhenius plot of conductivity) are shown in FIG.
[0077]
(Example 3)
In the same manner as in Example 1, La_1-xSr_xGa_0.8Mg_0.115Co_0.085 O_Three An oxide ion conductor made of a sintered body (wherein x = 0.05, 0.1, 0.15, 0.2, 0.25, or 0.3) was produced, and the electric conductivity was measured by changing the temperature or the oxygen partial pressure. FIG. 3 shows the relationship between the value of x at 950 ° C. and electrical conductivity. Arrhenius plot of conductivity (oxygen partial pressure 10^-Five Atm) and oxygen partial pressure dependence (temperature 950 ° C) are shown in Fig. 12 (a) and (b), respectively. It is noted that the oxygen partial pressure dependency changes depending on the x value.
[0078]
Example 4
In the same manner as in Example 1, La_0.8Sr_0.2Ga_(1-yz)Mg_yCo_zO_Three An oxide ion conductor made of a sintered body of (where y + z = 0.05, 0.1, 0.15, 0.2, 0.25 or 0.3, y: z = 11.5: 8.5) is produced, and the electric conductivity and ions are changed at different temperatures. The transportation number was measured. Arrhenius plot of conductivity at each (y + z) value (oxygen partial pressure 10^-Fiveatm) is shown in FIG. The relationship between the (y + z) value at 950 ° C. and the electrical conductivity is shown in FIG. 4 (a), and the relationship with the ion transport number is shown in FIG. 4 (b).
[0079]
(Example 5)
In the same manner as in Example 1, Ln_0.9A_0.1Ga_0.8B1_0.1Co_0.1O_ThreeThe oxide ion conductor which consists of the sintered compact which changed each metal atom of Ln, A, and B1 by the composition which becomes this was produced, and the electrical conductivity was measured. Oxygen partial pressure 10^-Five atm, conductivity at 950 ° C (σ / Scm^-1) Was as follows.
[0080]
▲ 1 ▼ Ln_0.9Sr_0.1Ga_0.8Mg_0.1Co_0.1O_Three
Ln = La: 0.53
= Pr: 0.49
= Nd: 0.36
= Ce: 0.08
= Sm: 0.05
▲ 2 ▼ La_0.9A_0.1Ga_0.8Mg_0.1Co_0.1O_Three
A = Sr: 0.53
= Ca: 0.24
= Ba: 0.22
▲ 3 ▼ La_0.9Sr_0.1Ga_0.8B1_0.1Co_0.1O_Three
B1 = Al: 0.12
= Mg: 0.53
= In: 0.23
Example 6
In the same manner as in Example 1, La_0.8Sr_0.2Ga_0.8Mg_0.2-zFe_zO_Three An oxide ion conductor made of a sintered body (z = 0 to 0.2) was produced. When the crystal structure of the obtained sintered body was examined by X-ray diffraction, all had a perovskite crystal structure.
[0081]
This oxide ion conductor has a temperature of 950 ° C and an oxygen partial pressure of 10^-Five The measurement results of electrical conductivity at atm are as shown in FIG. From this figure, it can be seen that high electrical conductivity is obtained when z = 0.01 to 0.15 as described above. In addition, the temperature dependence and oxygen partial pressure dependence of electrical conductivity when z = 0.03 in the above composition are as shown in FIGS. 7 (a) and 7 (b), respectively. It can be seen that this oxide ion conductor exhibits high electrical conductivity and ion transport number in a wide temperature range and oxygen partial pressure range.
[0082]
【The invention's effect】
According to the present invention, the oxide ion conductivity is higher than that of the stabilized zirconia which is a typical representative oxide ion conductor, and of course, there is no transition between the A site and the B site which have higher oxide ion conductivity. Compared to quaternary complex oxides doped only with metal, oxide ion conductivity is higher, and the ratio of oxide ion conductivity and electron conductivity, that is, ion transport number, can be controlled easily and freely. An oxide ion conductor that can be used can be realized. Therefore, not only a material useful as a narrowly defined oxide ion conductor having high electrical conductivity and an ion transport number as high as 0.9 or more, but also a material useful as an electron-ion mixed conductor can be obtained.
[0083]
The oxide ion conductor of the present invention having a high ion transport number can be used at a lower temperature than stabilized zirconia and exhibits high oxide ion conductivity under all oxygen partial pressures from an oxygen atmosphere to a hydrogen atmosphere. It is useful as an electrolyte for a physical fuel cell, a gas sensor such as an oxygen sensor, and an oxygen separation membrane for an electrochemical oxygen pump. In particular, the oxide ion conductor represented by the above formula (2) has high oxide ion conductivity in a wide temperature range and a very wide oxygen partial pressure ranging from a pure oxygen atmosphere to a substantial hydrogen atmosphere. Very advantageous.
[0084]
In addition, the oxide ion conductor of the present invention exhibiting electron-ion mixed conductivity can be used as an air electrode of a solid oxide fuel cell or a gas separation membrane utilizing a gas concentration difference. In particular, when the oxide ion conductor of the present invention showing this electron-ion mixed conductivity is used as an air electrode, and the SOFC is constructed using the above-defined oxide ion conductor of the present invention having a high ion transport number as an electrolyte, Since the interface resistance is reduced, SOFC output can be increased.
[Brief description of the drawings]
FIG. 1 shows a conventional representative oxide ion conductor and La_0.8Sr_0.2Ga_0.8Mg_0.2O_ThreeIt is a graph which shows the electrical conductivity of the perovskite type oxide ion conductor which consists of a quaternary type complex oxide with the composition which becomes.
FIG. 2 compares the electrical conductivity of a ternary composite oxide in which a part of Mg in the quaternary perovskite oxide ion conductor of FIG. It is a graph shown.
FIG. 3 is a graph showing the relationship between the electric conductivity and the ratio (x value) of A atoms (Sr) which are doped atoms at the A site of an oxide ion conductor made of a ternary composite oxide according to the present invention. It is.
FIG. 4 (a) shows the ratio of doped atoms at the B site of the oxide ion conductor made of the ternary composite oxide according to the present invention, that is, the total atomic ratio of B1 atoms + B2 atoms (y + z, However, y: z = 11.5: 8.5) is a graph showing the relationship between the electrical conductivity and FIG. 4 (b) is a graph showing the relationship between the (y + z) value and the ion transport number. (c) is a graph showing the relationship between the electrical conductivity and temperature of an oxide ion conductor made of a ternary composite oxide having various (y + z) values.
FIG. 5 shows the ratio (z value) of the B2 atoms that are transition metals out of the B-site doped atoms of the oxide ion conductor made of the quinary composite oxide according to the present invention, the electrical conductivity and the ion transport. It is a graph which shows the relationship with a rate.
[Figure 6] La_0.8Sr_0.2Ga_0.8Mg_0.2-zFe_zO_ThreeThe electrical conductivity of the composite oxide represented by (950 ° C, oxygen partial pressure = 10^-Five It is a graph which shows atm) with respect to z value.
[Figure 7] La_0.8Sr_0.2Ga_0.8Mg_0.2-zFe_zO_ThreeThe electrical conductivity oxygen partial pressure of the oxide ion conductor of the present invention represented by 10^-Five Temperature change at atm (a) and oxygen partial pressure dependence at 950 ° C (b) are shown.
FIG. 8 is a schematic cross-sectional view of a cell structure of a solid oxide fuel cell provided with surface irregularities.
FIG. 9 is an explanatory view showing an interface between the electrolyte layer having the cell structure and an air electrode.
FIG. 10 is a graph showing the oxygen partial pressure dependence of the electrical conductivity of an oxide ion conductor made of a ternary composite oxide according to the present invention.
FIG. 11 is a graph showing a temperature change of electrical conductivity of an oxide ion conductor made of another ternary composite oxide according to the present invention.
FIG. 12 is a graph showing temperature change (a) and oxygen partial pressure dependency (b) of electrical conductivity of an oxide ion conductor made of another ternary composite oxide according to the present invention.

Claims (13)

General _{formula: Ln 1-x A x Ga} 1-yz B1 y B2 oxide represented by _z O ₃ ion conductor.
Where
Ln = One or more of La, Ce, Pr, Nd, Sm;
A = one or more of Sr, Ca, Ba;
B1 = one or more of Mg, Al, In;
B2 = one or more of Co, Fe, Ni, Cu;
x = 0.05-0.3;
y = 0-0.29;
z = 0.01-0.3;
y + z = 0.025-0.3. The oxide ion conductor according to claim 1, wherein y ≧ 0.025 and z ≦ 0.15 are high in oxide ion conductivity. 2. The oxide ion conductor according to claim 1, which exhibits electron-ion mixed conductivity, wherein z> 0.15. 2. Ln = La and / or Nd, A = Sr, B1 = Mg, B2 = Co, x = 0.10 to 0.25, y = 0 to 0.17, z = 0.02 to 0.15, y + z = 0.10 to 0.25. Oxide ion conductor. The oxide ion according to claim 2, wherein Ln = La, A = Sr, B1 = Mg, B2 = Fe, x = 0.1 to 0.3, y = 0.025 to 0.29, z = 0.01 to 0.15, y + z = 0.035 to 0.3 Conductor. The oxide ion conductor according to claim 5, wherein x = 0.15 to 0.25, y = 0.09 to 0.24, z = 0.01 to 0.05, and y + z = 0.10 to 0.25. A solid oxide fuel cell comprising an electrolyte comprising the oxide ion conductor according to claim 2, 4, 5 or 6. A solid oxide fuel cell comprising an air electrode comprising the oxide ion conductor according to claim 3. A solid oxide fuel cell comprising an electrolyte comprising the oxide ion conductor according to claim 2, 4, 5 or 6 and an air electrode containing the oxide ion conductor according to claim 3. (1) Ni and (2) General formula: Ce _1-m C _m O ₂ (wherein C represents one or more of Sm, Gd, Y and Ca, and m = 0.05 to 0.4) The solid oxide fuel cell of any one of Claims 7-9 provided with the fuel electrode which consists of a compound shown by these. A gas sensor comprising the oxide ion conductor according to claim 2, 4, 5 or 6. An oxygen separation membrane for an electrochemical oxygen pump comprising the oxide ion conductor according to claim 2, 4, 5 or 6. A gas separation membrane comprising the oxide ion conductor according to claim 3.

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