Oxide ion conductive solid electrolyte

Abstract

To provide a solid electrolyte having a mayenite-type compound structure and significantly high oxide ion conductivity.SOLUTION: An oxide ion conducting solid electrolyte has a mayenite-type compound with a representative composition represented by Ca12Al14O33, and has at least one metal element M selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu) and gadolinium (Gd), the metal element M being contained in the range of 0.65 mol% to 6.0 mol% in terms of oxide relative to the whole of the oxide ion conducting solid electrolyte, the content of the mayenite-type compound in the oxide ion conducting solid electrolyte being higher than 60 mass%.SELECTED DRAWING: None

Description

Technical Field

[0001] The present invention relates to an oxide ion conductive solid electrolyte.

Background Art

[0002] Solid electrolytes having oxide ion conductivity have various applications such as solid oxide fuel cells (SOFCs), solid oxide electrolysis cells (SOECs), oxygen sensors, and oxygen pumps. In recent years, as one of the implementation forms of the technical concept of Power to Gas / Chemical, which has attracted attention due to the spread of renewable energy, a high-efficiency energy system combining SOFC and SOEC has attracted attention.

[0003] Both SOFC and SOEC are electrochemical cells that operate at high temperatures. The former can handle various fuels such as hydrogen, carbon monoxide, and methane, and the latter can electrolyze water and carbon dioxide generated by the operation of SOFC and return them to hydrogen and carbon monoxide.

[0004] SOFC and SOEC have a solid electrolyte provided between two electrodes, and operate by the conduction of oxide ions in this solid electrolyte.

Prior Art Documents

Non-Patent Documents

[0005]

Non-Patent Document 1

Non-Patent Document 2

Non-Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0006] As solid electrolytes for SOFCs and SOECs, materials such as yttria - stabilized zirconia (YSZ) and scandia - stabilized zirconia (ScSZ) have been proposed so far.

[0007] Recently, it has been reported that melilite - type compounds exhibit oxide - ion conductivity (Non - Patent Document 1). Melilite - type compounds have a crystal structure containing free oxide ions in cages. Therefore, there is a possibility that these free oxide ions can contribute to ion conduction.

[0008] However, according to the inventors of the present application, it has been recognized that the ion conductivity of conventional melilite - type compounds is not very high (for example, about 1 / 10 of YSZ).

[0009] Therefore, in order to apply melilite - type compounds to oxide - ion - conductive solid electrolytes, further improvement in oxide - ion conductivity is required.

[0010] The present invention has been made in view of such a background, and an object of the present invention is to provide a solid electrolyte having a melilite - type compound structure and having significantly high oxide - ion conductivity.

Means for Solving the Problems

[0011] In the present invention, an oxide - ion - conductive solid electrolyte, Ca 12 Al 14 O 33 having a melilite - type compound having a representative composition represented by is provided. It has at least one metal element M selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), and gadolinium (Gd), The metal element M is contained in the range of 0.65 mol% to 7.0 mol% in terms of oxide with respect to the whole of the oxide ion conductive solid electrolyte, An oxide ion conductive solid electrolyte is provided in which the content of the mayenite-type compound in the oxide ion conductive solid electrolyte is higher than 60% by mass.

[0012] In addition, in the present invention, an oxide ion conductive solid electrolyte, Ca 12 Al 14 O 33 It has a mayenite-type compound having a representative composition represented by, It has at least one metal element M selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), and gadolinium (Gd), The metal element M is contained in the range of 0.65 mol% to 7.0 mol% in terms of oxide with respect to the whole of the oxide ion conductive solid electrolyte, An oxide ion conductive solid electrolyte is provided in which the molar ratio (M / Ca) of the metal element M to Ca atoms satisfies 0.02 ≦ M / Ca ≦ 0.25.

Effects of the Invention

[0013] In the present invention, a solid electrolyte having a mayenite-type compound structure and significantly high oxide ion conductivity can be provided.

Brief Description of the Drawings

[0014]

Figure 1

Figure 2

Figure 3

Figure 4

Figure 5

Figure 6

Figure 7

Figure 8

Mode for Carrying Out the Invention

[0015] Hereinafter, an embodiment of the present invention will be described.

[0016] (Oxide Ion-Conductive Solid Electrolyte According to an Embodiment of the Present Invention) In one embodiment of the present invention, an oxide ion-conductive solid electrolyte, Ca 12 Al 14 O 33 has a melilite-type compound having a representative composition represented by, has at least one metal element M selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), and gadolinium (Gd), the metal element M is contained in the range of 0.65 mol% to 7.0 mol% in terms of oxide with respect to the whole of the oxide ion-conductive solid electrolyte, An oxide ion-conductive solid electrolyte is provided in which the content of the mayenite-type compound in the oxide ion-conductive solid electrolyte is higher than 60% by mass.

[0017] In the present application, the content of the mayenite-type compound contained in the oxide ion-conductive solid electrolyte can be determined by Rietveld analysis.

[0018] An oxide ion-conductive solid electrolyte (hereinafter referred to as "the first solid electrolyte") according to an embodiment of the present invention contains a mayenite-type compound having a C12A7 structure.

[0019] The mayenite-type compound has a representative composition represented by 12CaO·7Al2O3 and has a characteristic crystal structure having three-dimensionally connected voids (cages) with a diameter of about 0.4 nm.

[0020] The framework constituting this cage is positively charged and forms 12 cages per unit cell. One-sixth of this cage is occupied by oxide ions inside to satisfy the electrical neutrality condition of the crystal. However, the oxide ions in this cage have characteristics chemically different from those of other oxygen ions constituting the framework. Therefore, the oxide ions in the cage are particularly called free oxide ions.

[0021] The mayenite-type compound has a composition formula [Ca 24 Al 28 O 64 4+ (O 2- )2 (Non-Patent Document 2).

[0022] As described above, since the mayenite-type compound contains free oxide ions in the cage, it may function as an oxide ion conductor (Non-Patent Document 1).

[0023] ​However, according to the analysis by the inventors of the present application, it has been recognized that the ionic conductivity of a general melilite-type compound having the above composition formula is not very high (for example, about 1 / 10 of YSZ).

[0024] Therefore, in order to apply the melilite-type compound to an oxide ion conductive solid electrolyte, further improvement of the oxide ion conductivity is required.

[0025] So far, the inventors of the present application have been intensively researching and developing measures for enhancing the oxide ion conductivity of the melilite-type compound. And the inventors of the present application have found that when an oxide of a specific metal M is added to the melilite-type compound, the ionic conductivity of the melilite-type compound is significantly enhanced, leading to the present invention. Here, the metal M is selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), and gadolinium (Gd).

[0026] Therefore, the first solid electrolyte has at least one of Gd, Pr, Nd, europium Eu, and Sm.

[0027] The metal M is contained in the oxide ion conductive solid electrolyte in the range of 0.65 mol% to 7.0 mol% in terms of oxide.

[0028] When the content of the metal M is less than 0.65 mol%, no significant effect appears on the ionic conductivity of the melilite-type compound. Also, when the metal M is added in an amount exceeding 7.0 mol%, the number of heterogeneous phases increases, making it difficult to obtain an oxide ion conductive solid electrolyte mainly composed of the melilite-type compound. However, if a significant effect appears on the ionic conductivity, the mass fraction of the melilite-type compound in the first solid electrolyte may be less than 100%.

[0029] In particular, in the first solid electrolyte, the melilite-type compound itself may contain the metal M. In this case, the metal M may be arranged at the site of the Ca atom in the melilite-type compound. In particular, the molar ratio (M / Ca) of the metal M to the Ca atom may be in the range of 0.02 ≦ M / Ca ≦ 0.25, Ca 24-x M x Al 28 O 66+x / 2 may be satisfied.

[0030] Also, in the first solid electrolyte, the metal M may satisfy 0.84 ≦ (M + Ca) / Al ≦ 0.88 in terms of molar ratio.

[0031] In particular, in the first solid electrolyte, the metal M is preferably contained in the oxide ion-conductive solid electrolyte in the range of 0.5 mol% to 5.3 mol% in terms of oxide. By setting the content of the metal M to 5.3 mol% or less, an oxide ion-conductive solid electrolyte mainly composed of a melilite-type compound in which there are few heterogeneous phases and the metal M is included in the crystal structure can be obtained.

[0032] Such a first solid electrolyte has significantly higher ionic conductivity than conventional melilite-type compounds. Therefore, the first solid electrolyte can be expected to be used as a solid electrolyte in SOFC, SOEC, etc.

[0033] Also, in another embodiment of the present invention, an oxide ion-conductive solid electrolyte, Ca 12 Al 14 O 33 having a melilite-type compound having a representative composition represented by, having at least one metal element M selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), and gadolinium (Gd), the metal element M is contained in the oxide ion-conductive solid electrolyte as a whole in the range of 0.65 mol% to 7.0 mol% in terms of oxide, An oxide ion-conductive solid electrolyte is provided in which the molar ratio (M / Ca) of the metal element M to Ca atoms satisfies 0.02 ≤ M / Ca ≤ 0.25.

[0034] Also in the oxide ion-conductive solid electrolyte (hereinafter referred to as "the second solid electrolyte") according to another embodiment of the present invention, similar to the aforementioned first solid electrolyte, significantly higher ionic conductivity can be obtained compared to conventional mayenite-type compounds. Therefore, the second solid electrolyte can be expected to be used as a solid electrolyte in SOFCs, SOECs, etc.

[0035] At present, in one embodiment of the present invention, the following reasons are considered for the increase in ionic conductivity by incorporating a predetermined amount of metal M into the mayenite-type compound.

[0036] When the aforementioned metal M is added to the mayenite-type compound, it is considered that the atoms of metal M are preferentially substituted and arranged at the sites of Ca atoms.

[0037] Here, Ca atoms are divalent, while Pr atoms, Nd atoms, Sm atoms, Eu atoms, and Gd atoms are trivalent. Therefore, when Ca atoms are substituted by Pr atoms, Nd atoms, Sm atoms, Eu atoms, and Gd atoms, the concentration of oxide ions increases to maintain electrical neutrality. Along with this, as a result, the concentration of free oxide ions in the cage also increases, and it is considered that the ionic conductivity is improved.

[0038] However, the mechanism of the improvement in the above ionic conductivity is based on current experimental considerations, and the first solid electrolyte and the second solid electrolyte may have improved ionic conductivity by another mechanism.

[0039] Further, the first solid electrolyte and the second solid electrolyte may further contain titanium (Ti).

[0040] Ti may be contained in the first solid electrolyte and the second solid electrolyte in a range of 0.1 mol% to 30 mol% in terms of oxide.

[0041] In the first solid electrolyte and the second solid electrolyte, Ti is considered to be substituted and arranged at the site of aluminum (Al) atoms in the mayenite-type compound. The molar ratio Ti / Al of Ti atoms to Al atoms may be 0.015 ≦ Ti / Al ≦ 0.50. Further, when the first solid electrolyte contains Ti, the metal M may satisfy 0.84 ≦ (M + Ca) / (Al + Ti) ≦ 0.88 in terms of molar ratio, and Ca 24-x M x Al 28-y Ti y O 66+x / 2+y / 2 may also be satisfied.

[0042] When the first solid electrolyte and the second solid electrolyte contain Ti, the ionic conductivity is further improved. This is expected for the following reasons.

[0043] When Ti is added to the mayenite-type compound, Ti atoms are considered to be preferentially substituted and arranged at the site of Al atoms. However, although Al atoms are trivalent, Ti atoms are tetravalent. Therefore, when Al atoms are substituted by Ti atoms, for electrical neutrality, the concentration of oxide ions increases. Further, accordingly, the concentration of free oxide ions in the cage also increases, and as a result, the ionic conductivity is considered to be improved.

[0044] (Examination of ionic conductive species) In the mayenite-type compound having a C12A7 structure containing the metal M, the element that is the main body of ionic conduction was evaluated by simulation.

[0045] In the simulation, the relationship between the mean square displacement MSD of oxide ions in the material at 1 atm and 1200 K and the elapsed time was calculated by classical molecular dynamics calculation.

[0046] The code used for the calculation is LAMMPS. For the pair potential between the constituent elements of each material, which is an input parameter, the values reported by Pedone et al. (see Non-Patent Document 3) were used.

[0047] In a supercell with the unit cell expanded to 6×6×6, molecular dynamics simulations for 1 nanosecond were performed in the NPT ensemble for the system in which 2.7 mol% of Ca was replaced with Nd in terms of molar ratio. Also, after the temperature, internal energy, and lattice constant were stabilized, molecular dynamics simulations for 1 nanosecond were performed in the NVE ensemble.

[0048] Figure 1 shows the results of the simulation. In Figure 1, the horizontal axis is the elapsed time, and the vertical axis is the MSD of each element.

[0049] The slope of the MSD is proportional to the diffusion coefficient. Therefore, in Figure 1, it shows that the larger the slope of the MSD of an element, the easier it is for diffusion to occur inside the melilite-type compound.

[0050] As shown in Figure 1, in the compound in which a part of the Ca atoms in the melilite-type compound was replaced with Nd, it was found that the main body of ion conduction is oxide ions.

[0051] Figure 2 shows the results obtained when the same simulation was performed for the system in which 2.7 mol% of Ca was replaced with Gd.

[0052] Similar results were obtained in this system as well.

[0053] (Other features) Since the first solid electrolyte and the second solid electrolyte are ceramics, they have high high-temperature stability. For example, the first solid electrolyte and the second solid electrolyte can be stably used even in a temperature range of 800°C or higher.

[0054] (Applications) The first solid electrolyte and the second solid electrolyte are stable even at high temperatures and have significantly high oxide ion conductivity. Therefore, the first solid electrolyte and the second solid electrolyte can be applied, for example, as the solid electrolyte of a solid oxide fuel cell (SOFC) cell and as the solid electrolyte for an SOEC.

[0055] FIG. 2 schematically shows a configuration example of an SOFC cell.

[0056] As shown in FIG. 2, the SOFC cell 100 has an oxygen electrode 110, a fuel electrode 120, and a solid electrolyte 130 between both electrodes.

[0057] At the oxygen electrode 110, for example, the following reaction occurs: O2 + 4e - → 2O 2- (Equation (1)) The oxide ions generated at the oxygen electrode 110 pass through the solid electrolyte 130 and reach the opposite fuel electrode 120. At the fuel electrode 120, for example, the following reaction occurs: 2H2 + 2O 2- → 2H2O + 4e - (Equation (2)) Therefore, when the SOFC cell 100 is connected to an external load 140, the reactions of Equation (1) and Equation (2) continue, and power can be supplied to the external load 140.

[0058] In such an SOFC cell 100, for example, the first solid electrolyte and the second solid electrolyte can be applied as the solid electrolyte 130. Also, the first solid electrolyte and the second solid electrolyte can be applied as the solid electrolyte constituting the oxygen electrode 110 and the fuel electrode 120.

[0059] In such an SOFC cell 100, since the solid electrolyte 130 has significantly high oxide ion conductivity, it is possible to obtain significantly high power generation efficiency.

[0060] FIG. 3 schematically shows an example configuration of the SOEC cell.

[0061] As shown in FIG. 3, the SOEC cell 200 has an oxygen electrode 210, a hydrogen electrode 220, and a solid electrolyte 230 between both electrodes.

[0062] At the oxygen electrode 210, for example, the following reaction occurs: 2O 2- →O2 + 4e - (Equation (3)) Also, at the hydrogen electrode 220, for example, the following reaction occurs: 2H2O + 4e - →2H2 + 2O 2- (Equation (4)) The oxide ions generated at the hydrogen electrode 220 pass through the solid electrolyte 230 and reach the opposite oxygen electrode 210. Therefore, when the SOEC cell 200 is connected to an external power source 240, the reactions of Equation (3) and Equation (4) continue.

[0063] In such an SOEC cell 200, for example, as the solid electrolyte 230, a first solid electrolyte and a second solid electrolyte can be applied. Also, as the solid electrolyte constituting the oxygen electrode 210 and the hydrogen electrode 220, the first solid electrolyte and the second solid electrolyte can be applied.

[0064] In such an SOEC cell 200, since the solid electrolyte 230 has significantly high oxide ion conductivity, it is possible to obtain significantly high electrolysis efficiency.

[0065] (Method for manufacturing an oxide ion conductive solid electrolyte according to an embodiment of the present invention) Next, with reference to FIG. 5, an example of a method for manufacturing an oxide ion conductive solid electrolyte according to an embodiment of the present invention will be described.

[0066] FIG. 5 schematically shows an example of the flow of a method for manufacturing an oxide ion conductive solid electrolyte according to an embodiment of the present invention (hereinafter referred to as the "first manufacturing method").

[0067] As shown in FIG. 5, the first manufacturing method includes (1) a step of mixing a Ca source, an Al source, and a metal M source at a predetermined ratio to obtain a mixed powder (step S110); (2) a step of calcining the mixed powder to obtain a calcined powder (step S120); (3) a step of sintering the calcined powder to obtain a sintered body (step S130). It has.

[0068] Hereinafter, each step will be described. Here, as an example, the manufacturing method of the first solid electrolyte described above will be described. However, it is obvious to those skilled in the art that the first manufacturing method can also be applied to the manufacturing method of the second solid electrolyte.

[0069] (Step S110) First, a mixed powder is prepared. For this purpose, a Ca source, an Al source, and a metal M source are mixed at a predetermined ratio.

[0070] The Ca source may be selected from, for example, metallic calcium, calcium carbonate, calcium oxide, calcium hydroxide, calcium nitrate, and calcium acetate.

[0071] The Al source may be selected from, for example, metallic aluminum, α-alumina, γ-alumina, aluminum hydroxide, aluminum nitrate, and aluminum sulfate.

[0072] The metal M source may be selected from, for example, pure metal M and / or an oxide of metal M. For example, when Nd is adopted as the metal M, the metal M source may be metallic Nd and / or neodymium oxide.

[0073] Each raw material is weighed and mixed so that a mellite-type compound having a target composition can be obtained.

[0074] The mixing method is not particularly limited as long as a uniform mixed powder is obtained.

[0075] (Step S120) Next, the mixed powder is calcined.

[0076] The calcination step is carried out to remove compounds such as carbonic acid and nitric acid contained in the mixed powder, and to facilitate the formation of the target meitnerite-type compound in the subsequent sintering step.

[0077] The conditions for calcination are not particularly limited, but in order to obtain the target mixed oxide, the calcination temperature is preferably 1000 °C or higher. However, if the calcination temperature is too high, excessive crystallization will proceed in the mixed powder. Therefore, the calcination temperature is preferably 1300 °C or lower.

[0078] The calcination time is, for example, about 5 hours to 24 hours. However, the calcination time also varies depending on the calcination temperature, and the higher the calcination temperature, the shorter the calcination time can be.

[0079] Thereby, calcined powder is obtained.

[0080] Note that the calcined powder may be pulverized as necessary. The average particle size after pulverization may be, for example, in the range of 0.1 μm to 100 μm.

[0081] (Step S130) Next, the calcined powder is sintered.

[0082] The sintering step is carried out to obtain a dense sintered body having the target crystal phase.

[0083] Note that before the sintering step, the calcined powder may be formed, and the sintering step may be carried out using the obtained formed body.

[0084] The conditions for forming and the like are not particularly limited, and a general forming method such as a uniaxial forming method or a hydrostatic pressure forming method may be adopted.

[0085] The sintering method is not particularly limited. For example, the green powder or the formed body may be sintered by a non-pressure sintering method under normal pressure.

[0086] Alternatively, the green powder may be sintered using a pressure sintering method such as hot press sintering or spark plasma sintering. In this case, forming and sintering may be carried out at once.

[0087] The sintering temperature is not particularly limited as long as a proper sintered body can be obtained, but the range of 1200°C to 1400°C is preferable. If the sintering temperature is too low, a dense sintered body may not be obtained. Also, if the sintering temperature is too high, melting of the object to be processed may occur.

[0088] It is desirable to select an optimal time according to the sintering temperature for the sintering time. Generally, the higher the sintering temperature, the shorter the time required for sintering to be completed.

[0089] The optimal sintering time varies depending on the sintering temperature. In the case of non-pressure sintering treatment, for example, it is about 5 hours to 48 hours, and in the case of pressure sintering treatment by spark plasma, for example, it is about 5 minutes to 60 minutes.

[0090] Note that when performing the pressure sintering treatment using a carbon container, carbon may adhere to the surface of the sintered body. In such a case, the adhered carbon can be removed by heat treatment at 800°C to 1000°C for about 5 hours in the atmosphere.

[0091] By the above steps, an oxide ion conductive solid electrolyte according to an embodiment of the present invention can be manufactured.

[0092] (Another manufacturing method of the oxide ion conductive solid electrolyte according to an embodiment of the present invention) Next, with reference to FIG. 6, an example of another manufacturing method of the oxide ion conductive solid electrolyte according to an embodiment of the present invention will be described.

[0093] FIG. 6 schematically shows an example of the flow of another manufacturing method of an oxide ion conductive solid electrolyte according to an embodiment of the present invention (hereinafter referred to as "the second manufacturing method").

[0094] As shown in FIG. 6, the second manufacturing method includes (1) a step of mixing a Ca source and an Al source at a predetermined ratio to obtain a first mixed powder (step S210); (2) a step of calcining the first mixed powder to obtain a first calcined powder (step S220); (3) a step of mixing the first calcined powder and a metal M source at a predetermined ratio to obtain a second mixed powder (step S230); (4) a step of calcining the second mixed powder to obtain a second calcined powder (step S240); (5) a step of sintering the second calcined powder to obtain a sintered body (step S250). The second manufacturing method has the above steps.

[0095] Each step included in the second manufacturing method can be easily understood by those skilled in the art from the descriptions of steps S110 to S130 in the above-mentioned first manufacturing method. Therefore, detailed descriptions of each step are omitted here.

[0096] However, in the second manufacturing method, different from the first manufacturing method, an oxide ion conductive solid electrolyte is manufactured through two calcination steps (step S220 and step S240).

[0097] In this case, compared with the case of calcining a mixed powder containing a Ca source, an Al source, and a metal M source at once as in the first manufacturing method, an oxide ion conductive solid electrolyte with a more homogeneous composition can be manufactured.

[0098] That is, in the first manufacturing method, highly reactive Ca may react with metal M to form a heterogeneous phase. In contrast, in the second manufacturing method, in the first calcination step (step S220), Ca and Al can react to prepare a calcined powder that is bonded in advance. Therefore, in the second calcination step (step S240), metal M can be more reliably introduced into a desired site in the melilite-type compound.

[0099] As described above, the manufacturing method of the oxide ion conductive solid electrolyte according to an embodiment of the present invention has been described by taking the first manufacturing method and the second manufacturing method as examples. However, the above description is merely an example, and the oxide ion conductive solid electrolyte according to an embodiment of the present invention may be manufactured by another method such as a hydrothermal method, a sol-gel method, and a liquid-phase combustion method.

Examples

[0100] Hereinafter, examples of the present invention will be described.

[0101] In the following description, Examples 1 to 8 and Examples 11 to 12 are examples, and Examples 21 to 23 are comparative examples.

[0102] (Example 1) A sintered body was produced by the following method.

[0103] [Formulation step] Calcium carbonate powder (4.16 g), α-alumina powder (2.52 g), and neodymium oxide powder (0.149 g) were each weighed. These were put into a pot containing φ5 mm zirconia balls and 10 cc of isopropanol, and pulverized and mixed for 3 hours by the planetary ball mill method. Next, the mixed powder was dried at 100 ° C. to remove isopropanol. Further, the mixed powder was separated from the zirconia balls by a sieve.

[0104] [Calcination step] The obtained mixed powder was put into an alumina crucible and calcined at 1200 ° C. for 5 hours in the air. The obtained sample was pulverized in an agate mortar to prepare a calcined powder.

[0105] [Sintering process] 1 g of the calcined powder was placed in a cemented carbide die with a diameter of 1.5 cm, and a pressure of 20 kN was applied with a hydraulic press to perform uniaxial forming. Furthermore, isostatic pressing treatment was carried out at 196 MPa to produce a pellet with a diameter of 1.5 cm. The pellet was heat-treated in the atmosphere at 1200 °C for 12 hours to obtain a sintered body with a diameter of 1.3 cm and a thickness of 2 mm.

[0106] The obtained sintered body is designated as "Sample 1". In Sample 1, the content of Nd is 0.67 mol% in terms of Nd2O3 conversion, and the molar ratio Nd / Ca is 0.021.

[0107] (Example 2) A sintered body was produced in the same manner as in Example 1. However, in this Example 2, the mixing ratio of each raw material in the above-mentioned [Formulation process] was changed from that in Example 1 to prepare a mixed powder. Other processes are the same as in Example 1.

[0108] The obtained sintered body is designated as "Sample 2".

[0109] (Examples 3 - 4) A sintered body was produced in the same manner as in Example 1. However, in Examples 3 - 4, the mixing ratio of each raw material in the above-mentioned [Formulation process] was changed from that in Example 1 to prepare a mixed powder. Also, in the [Sintering process], the heat treatment temperature was set at 1300 °C. Other processes are the same as in Example 1.

[0110] The obtained sintered bodies are respectively designated as "Sample 3" - "Sample 4".

[0111] (Example 5) Calcium carbonate powder (3.64 g), α-alumina powder (2.36 g), and gadolinium oxide powder (0.599 g) were weighed respectively. These were put into a pot containing φ5 mm zirconia balls and 10 cc of isopropanol, and pulverized and mixed for 3 hours by the planetary ball mill method. Next, the mixed powder was dried at 100 °C to remove isopropanol. Further, the mixed powder was separated from the zirconia balls by a sieve.

[0112] [Calcination process] The obtained mixed powder was put into an alumina crucible and calcined in the air at 1300 °C for 5 hours. The obtained sample was pulverized in an agate mortar to prepare a calcined powder.

[0113] [Sintering process] 1 g of the calcined powder was put into a cemented carbide die with a diameter of φ1.5 cm, and a pressure of 20 kN was applied with a hydraulic press to perform uniaxial molding. Further, isostatic pressing treatment was carried out at 196 MPa to prepare a pellet with a diameter of φ1.5 cm. The pellet was heat-treated in the air at 1300 °C for 12 hours to obtain a sintered body with a diameter of φ1.3 cm and a thickness of 2 mm.

[0114] The obtained sintered body was designated as "Sample 5". In Sample 5, the content of Gd is 2.7 mol% in terms of Gd2O3, and the molar ratio Gd / Ca is 0.091.

[0115] (Examples 6 to 8) Sintered bodies were prepared in the same manner as in Example 5. However, in Examples 6 to 8, praseodymium oxide, europium oxide, and samarium oxide were added as the aforementioned metal M respectively to prepare mixed powders. Other steps were the same as in the case of Example 5.

[0116] The obtained sintered bodies were designated as "Sample 6" to "Sample 8" respectively.

[0117] (Example 21) A sintered body was produced in the same manner as in Example 1. However, in this Example 21, in the above-mentioned [mixing step], only calcium carbonate (4.33 g) and α-alumina (2.57 g) were mixed to prepare a mixed powder. That is, the mixed powder was prepared without adding the metal M source. The other steps were the same as in the case of Example 1.

[0118] The obtained sintered body is referred to as "Sample 21".

[0119] (Example 22) A sintered body was produced in the same manner as in Example 1. However, in this Example 22, the mixing ratio of each raw material in the above-mentioned [mixing step] was changed from that in the case of Example 1 to prepare a mixed powder. Specifically, calcium carbonate powder (2.61 g), α-alumina powder (2.07 g), and neodymium oxide powder (1.46 g) were weighed respectively. Also, the content of Nd was 8.6 mol% in terms of Nd2O3, and the molar ratio Nd / Ca was 0.33.

[0120] In addition, the firing temperature in the above-mentioned [sintering step] was 1300 °C.

[0121] The other steps were the same as in the case of Example 1.

[0122] The obtained sintered body is referred to as "Sample 22".

[0123] (Example 23) A sintered body was produced in the same manner as in Example 1. However, in this Example 23, the mixing ratio of each raw material in the above-mentioned [mixing step] was changed from that in the case of Example 1 to prepare a mixed powder. Specifically, calcium carbonate powder (3.09 g), α-alumina powder (1.92 g), and neodymium oxide powder (1.35 g) were weighed respectively. Also, the content of Nd was 7.5 mol% in terms of Nd2O3, and the molar ratio Nd / Ca was 0.26.

[0124] In addition, the firing temperature in the above-mentioned [sintering step] was 1200 °C.

[0125] The other processes are the same as in Example 1.

[0126] The obtained sintered body is designated as "Sample 23".

[0127] The raw materials and firing conditions of each sample are summarized in Table 1 below.

[0128]

Table 1

[0129] (X-ray diffraction analysis) Using each sample, X-ray diffraction analysis was performed. Also, from the obtained results, the content of the melilite-type compound contained in the sample was calculated by Rietveld analysis.

[0130] Figure 7 shows the X-ray diffraction pattern obtained in Sample 2. In Figure 7, each peak without a mark corresponds to the crystal phase of the melilite-type compound. Also, each peak indicated by a wedge symbol corresponds to the crystal phase of the heterogeneous NdCaAl3O7.

[0131] Thus, it was confirmed that Sample 2 contains a melilite-type compound. As a result of Rietveld analysis, the content of the melilite-type compound in Sample 2 was 88.0 mass%.

[0132] In the column of "Content of melilite-type compound" in Table 2 below, the evaluation results of the content of the melilite-type compound obtained in each sample are shown. In Table 2, "○" indicates that the content of the melilite-type compound is 60 mass% or more, and "×" indicates that the content of the melilite-type compound is less than 60 mass%.

[0133]

Table 2

[0134] (Resistivity measurement) Using each sample, the resistivity was measured. The impedance method was used for the resistivity measurement.

[0135] First, the surface of each sample was polished with #80 to #1000 sandpaper to remove the surface layer and smooth it.

[0136] Next, a platinum electrode with a diameter of φ6 mm and a thickness of 10 μm was installed on the polished surface via a platinum paste. This sample was heat-treated at 1000 °C for 15 minutes in an air atmosphere to solidify the platinum paste.

[0137] Next, the sample was placed in an electric furnace in an air atmosphere. Also, the sample was connected to a potentiostat (BioLogic SP-150) via a platinum wire connected to the platinum electrode.

[0138] Next, after heating the sample to 850 °C, it was held for 120 minutes to stabilize the temperature of the sample.

[0139] After the temperature of the sample was stabilized, impedance measurement was carried out to create a Cole-Cole plot. The measurement frequency was set to 1 MHz to 100 mHz.

[0140] In the obtained Cole-Cole plot, the resistivity was determined from the intersection with the horizontal axis (real axis).

[0141] Figure 8 shows, as an example, the Cole-Cole plot obtained for Sample 4. In this example, the resistivity of Sample 4 was determined from the position of the arrow.

[0142] In the column of "resistivity" in Table 2 above, the results obtained for each sample are summarized. In Table 2, the resistivity of each sample is shown as the ratio of the resistivity obtained in Sample 21.

[0143] From these results, it was found that the resistivity (relative ratio) of Sample 22 was 0.87, and the resistivity did not decrease significantly compared to Sample 21, which is a melilite-type compound containing no metal M. Also, the resistivity (relative ratio) of Sample 23 was 1.43, and the resistivity increased compared to Sample 21. On the other hand, in Samples 1 to 8, it was found that the resistivity decreased significantly compared to Sample 21.

[0144] Thus, it was found that in the melilite-type compound, in terms of oxide conversion, in the samples where metal M was added in the range of 0.65 mol% to 7.0 mol%, the resistivity decreased significantly.

[0145] From these results, it is considered that in the solid electrolyte composed of adding metal M to the melilite-type compound, better ionic conductivity can be exhibited compared to the conventional melilite-type compound.

[0146] (Example 11) A sintered body was produced in the same manner as in Example 1. However, in this Example 11, in the above-mentioned [Formulation Step], in addition to calcium carbonate powder, α-alumina powder, and neodymium oxide powder, titanium dioxide powder was used as a raw material. The titanium dioxide powder was added so as to be 10 mol% with respect to the whole. The Ti / Al ratio in the mixed powder is 0.17.

[0147] Also, in this Example 11, in the [Sintering Step], the heat treatment temperature was set to 1300 °C. The other steps are the same as in the case of Example 1.

[0148] The obtained sintered body is referred to as "Sample 11". In Sample 11, in terms of Nd2O3 conversion, the content of Nd is 1.3 mol%, and the molar ratio Nd / Ca is 0.043.

[0149] (Example 12) A sintered body was produced in the same manner as in Example 11. However, in this Example 12, the mixing ratio of each raw material in the [mixing process] was changed from that in Example 11 to prepare a mixed powder. The titanium dioxide powder was added so as to be 2.7 mol% with respect to the whole. The Ti / Al ratio in the mixed powder is 0.037.

[0150] The obtained sintered body is referred to as "Sample 12". In Sample 12, in terms of Nd2O3 conversion, the content of Nd is 2.7 mol%, and the molar ratio Nd / Ca is 0.091.

[0151] The raw materials and firing conditions of Sample 11 and Sample 12 are collectively shown in Table 3 below.

[0152]

Table 3

[0153] As a result of the measurement, the resistivity of Sample 11 was 0.10 times that of Sample 21. Also, the resistivity of Sample 12 was 0.07 times that of Sample 21.

[0154] From these results, it was found that the resistivity was greatly reduced in Sample 11 and Sample 12.

Explanation of Signs

[0155] 100 SOFC cell 110 Oxygen electrode 120 Fuel electrode 130 Solid electrolyte 140 External load 200 SOEC cell 210 Oxygen electrode 220 Hydrogen electrode 230 Solid electrolyte 240 External power supply

Claims

1. An oxide ion-conductive solid electrolyte, Ca １２ Al １４ O ３３ and having a melilite-type compound having a representative composition represented by having at least one metal element M selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), and gadolinium (Gd), wherein the metal element M is contained in a range of 0.65 mol% to 7.0 mol% in terms of oxide with respect to the total value of the number of moles obtained by converting each metal contained in the oxide ion-conductive solid electrolyte into oxide, a part of the metal element M is contained in the melilite-type compound, a part of the metal element M is arranged at the site of Ca atoms in the melilite-type compound, and the content of the melilite-type compound in the oxide ion-conductive solid electrolyte is higher than 60% by mass. An oxide ion-conductive solid electrolyte.

2. The oxide ion-conductive solid electrolyte according to claim 1, wherein the metal element M is contained in the melilite-type compound.

3. The oxide ion-conductive solid electrolyte according to claim 2, wherein the metal element M is arranged at the site of Ca atoms in the melilite-type compound.

4. The oxide ion-conductive solid electrolyte according to claim 3, wherein the molar ratio (M / Ca) of the metal element M to Ca atoms satisfies 0.02 ≦ M / Ca ≦ 0.

25.

5. An oxide ion-conductive solid electrolyte, Ca １２ Al １４ O ３３ and has a melilite-type compound having a representative composition represented by having at least one metal element M selected from praseodymium (Pr), neodymium (Nd), samarium (Sm), europium (Eu), and gadolinium (Gd), wherein the metal element M is contained in a range of 0.65 mol% to 7.0 mol% in terms of oxide with respect to the total value of the number of moles obtained by converting each metal contained in the oxide ion-conductive solid electrolyte into oxide, a part of the metal element M is contained in the melilite-type compound, a part of the metal element M is arranged at the site of Ca atoms in the melilite-type compound, and the molar ratio (M / Ca) of the metal element M to Ca atoms satisfies 0.02 ≦ M / Ca ≦ 0.

25. An oxide ion-conductive solid electrolyte.

6. The oxide ion-conductive solid electrolyte according to any one of claims 1 to 5, wherein the metal element M satisfies 0.84 ≦ (M + Ca) / Al ≦ 0.88 in terms of molar ratio.

7. The metal element M is contained in the oxide ion-conductive solid electrolyte in an amount of 5.3 mol% or less in terms of oxide, and the oxide ion-conductive solid electrolyte according to any one of claims 1 to 6. **Claim 8** Furthermore, the oxide ion-conductive solid electrolyte according to any one of claims 1 to 7, further containing titanium (Ti) in an amount of 0.1 mol% to 30 mol% in terms of oxide. **Claim 9** The oxide ion-conductive solid electrolyte according to claim 8, wherein the Ti is arranged at the site of the Al atom in the mayenite-type compound. **Claim 10** A fuel cell comprising the oxide ion-conductive solid electrolyte according to any one of claims 1 to 9. **Claim 11** An electrolytic cell comprising the oxide ion-conductive solid electrolyte according to any one of claims 1 to 9.

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