Proton conductive solid electrolyte, proton conductive fuel cell, steam electrolysis cell, and method for producing proton conductive solid electrolyte

Abstract

Provided are: a proton conductive solid electrolyte in which deterioration of mechanical strength due to thermal expansion and hydration expansion is suppressed; a proton conductive fuel cell and a steam electrolysis cell, each of which comprises said proton conductive solid electrolyte; and a method for producing said proton conductive solid electrolyte. The proton conductive solid electrolyte comprises: a proton conductive ceramic that is composed of a perovskite type metal multiple oxide; and a dispersion material that is composed of a metal compound which is different from the proton conductive ceramic. The dispersion material is dispersed in the proton conductive ceramic.

Description

Proton-conductive solid electrolyte, proton-conductive fuel cell, steam electrolysis cell, and method for producing proton-conductive solid electrolyte

[0001] The present invention relates to a proton-conductive solid electrolyte, a proton-conductive fuel cell, a steam electrolysis cell, and a method for producing a proton-conductive solid electrolyte.

[0002] As solid oxide fuel cells (SOFCs), oxide ion-conductive types are common, but proton-conductive fuel cells (PCFCs) have attracted attention from the viewpoints of having high power generation characteristics even at low operating temperatures and being expected to achieve higher power generation efficiency. In conventional general oxide ion-conductive SOFCs, water is generated on the anode (fuel electrode) side, whereas in PCFCs, water is generated on the cathode (air electrode) side. Therefore, the fuel gas is not diluted by the generation of water, and no voltage drop occurs due to dilution of the fuel gas. Further, PCFCs can be operated at lower temperatures, and suffer less performance degradation due to temperature decrease. Furthermore, in addition to PCFCs, proton-conductive ceramic steam electrolysis cells (PCECs), which are devices using a proton-conductive solid electrolyte, have also attracted attention.

[0003] From the above viewpoints, development of proton-conductive solid electrolytes has been promoted. As a material having good proton conductivity, proton-conductive ceramics made of a perovskite-type complex metal oxide are known. Examples of proton-conductive ceramics that exhibit high proton conductivity even at relatively low temperatures include BaZrO ３ wherein a part of Zr is substituted with a rare earth element ３ -based proton-conductive ceramics. Specifically, Patent Document 1 discloses a proton conductor containing a compound represented by BaZr １－ｘ－ｙ Yb ｘ Sc ｙ O ３－δ (0 < x < 0.5, 0 < y < 0.5, x + y < 0.5, 0 < δ < 0.5), which was developed by the present inventors.

[0004] International Publication No. 2021 / 192548

[0005] As described above, BaZrO ３Proton-conductive ceramics including this series of materials are materials with excellent proton conductivity, but they are prone to reduction in mechanical strength due to thermal expansion and hydration expansion. In proton-conductive ceramics composed of perovskite-type metal composite oxides, metal cations (BaZrO ３ in the case of Zr ４＋ ), part of which are substituted with low-valence cations, thereby generating oxygen vacancies and imparting proton conductivity. When the proton-conductive ceramic comes into contact with water molecules in the atmosphere at the operating temperature of a PCFC, in addition to thermal expansion, it may sometimes cause expansion due to hydration. In the hydration process, protons derived from water molecules dissolve into the crystal (proton dissolution), and oxygen atoms are incorporated into oxygen vacancies. When hydration occurs, the proton-conductive ceramic expands, which may lead to reduced mechanical strength and even damage such as cracking. From the perspective of improving the durability of PCFCs, it is desirable to suppress the reduction in mechanical strength caused by thermal expansion and hydration expansion in proton-conductive ceramics.

[0006] In view of the above, the problem to be solved by the present invention is to provide a proton-conductive solid electrolyte that suppresses the reduction in mechanical strength caused by thermal expansion and hydration expansion, a proton-conductive fuel cell and a steam electrolysis cell including such a proton-conductive solid electrolyte, and a method for producing such a proton-conductive solid electrolyte.

[0007] In order to solve the above problems, the proton-conductive solid electrolyte according to the present invention has the following configuration.

[0008] [1] The proton-conductive solid electrolyte according to the present invention includes a proton-conductive ceramic composed of a perovskite-type metal composite oxide, and a dispersant composed of a metal compound different from the proton-conductive ceramic, wherein the dispersant is dispersed in the proton-conductive ceramic.

[0009] [2] In the aspect of [1] above, it is preferable that the dispersant does not form a solid solution or react with the proton-conductive ceramic.

[0010] [3] In the embodiment of [1] or [2] above, the dispersant may be composed of a metal oxide.

[0011] [4] In the embodiment of [3] above, the dispersant may be composed of MgO.

[0012] [5] In any of the embodiments of [1] to [4] above, the proton-conducting ceramic is BaZr １－ｘ－ｙ Yb ｘ Sc ｙ O ３－δ It is preferable to have a composition of the following values: (0 < x < 0.5, 0 < y < 0.5, x + y < 0.5, 0 < δ < 0.5).

[0013] [6] In any of the embodiments described in [1] to [5] above, the amount of the dispersant added is preferably 15% or more by volume of the total of the proton-conducting ceramics and the dispersant.

[0014] [7] The proton-conducting fuel cell according to the present invention has a single cell structure comprising an electrolyte layer composed of any of the proton-conducting solid electrolytes described in [1] to [6] above, an anode provided on one side of the electrolyte layer, and a cathode provided on the other side of the electrolyte layer.

[0015] [8] The water vapor electrolytic cell according to the present invention has a single cell structure comprising an electrolyte layer composed of any of the proton-conducting solid electrolytes described in [1] to [6] above, an anode provided on one side of the electrolyte layer, and a cathode provided on the other side of the electrolyte layer.

[0016] [9] The method for producing a proton-conducting solid electrolyte according to the present invention includes a step of firing a precursor comprising a citrate complex containing a metal element constituting the proton-conducting ceramic and a citrate complex containing a metal element constituting the dispersant, thereby producing the proton-conducting solid electrolyte described in [3] or [4] above.

[0017] The proton-conducting solid electrolyte according to the present invention, having the configuration described in [1] above, contains a dispersant composed of a metal compound in addition to proton-conducting ceramics, with the dispersant dispersed in the proton-conducting ceramics. Because the dispersant, which is made of a different material from the proton-conducting ceramics, is dispersed in the proton-conducting ceramics, even if the proton-conducting solid electrolyte comes into contact with water molecules at high temperatures, the stress caused by the thermal expansion and hydration expansion of the crystals of the proton-conducting ceramics is relieved by the dispersant. Therefore, the decrease in the mechanical strength of the proton-conducting solid electrolyte due to expansion, and the occurrence of damage such as cracking caused by it, are suppressed.

[0018] In the embodiment described in [2] above, the dispersant does not undergo solid solution or reaction with the proton-conducting ceramics. By dispersing the dispersant in the proton-conducting ceramics while maintaining an independent state from the proton-conducting ceramics without undergoing solid solution or reaction, the effect of suppressing the decrease in mechanical strength due to stress relaxation can be maintained at a high level. Furthermore, by maintaining the composition of the proton-conducting ceramics, the proton conductivity of the proton-conducting ceramics can be maintained at a high level.

[0019] In the embodiment described in [3] above, the dispersant is composed of a metal oxide. In this case, the addition of the dispersant can effectively suppress the decrease in the mechanical strength of the proton-conducting solid electrolyte due to thermal and hydration expansion.

[0020] In the embodiment described in [4] above, the dispersant is composed of MgO. MgO is less likely to undergo solid solution or reaction with proton-conducting ceramics, and it is highly effective in suppressing the decrease in mechanical strength of the proton-conducting solid electrolyte due to thermal and hydration expansion by being finely dispersed with high uniformity within the proton-conducting ceramics.

[0021] In the embodiment described in [5] above, the proton-conducting ceramic is BaZr １－ｘ－ｙ Yb ｘ Sc ｙ O ３－δIt has the following composition. This proton-conducting ceramic exhibits high proton conductivity even at relatively low operating temperatures, making it suitable for use as an electrolyte in PCFCs and steam electrolytic cells. Furthermore, the addition of a dispersant such as MgO effectively suppresses the decrease in mechanical strength due to thermal and hydration expansion.

[0022] In the embodiment described in [6] above, the amount of dispersant added is 15% or more of the total volume of the proton-conducting ceramics and the dispersant. This provides a high level of effectiveness in suppressing the decrease in the mechanical strength of the proton-conducting ceramics due to the addition of the dispersant.

[0023] The proton-conducting fuel cell according to the present invention having the configuration described in [7] above has a proton-conducting solid electrolyte according to the present invention, in which a dispersant is dispersed in proton-conducting ceramics, as an electrolyte layer. Therefore, during operation, the mechanical strength of the electrolyte layer is less likely to decrease due to thermal expansion or hydration expansion, and damage such as cracking of the electrolyte layer that occurs as a result is less likely to occur. Thus, it becomes a highly durable fuel cell.

[0024] The steam electrolytic cell according to the present invention having the configuration described in [8] above has a proton-conducting solid electrolyte according to the present invention, in which a dispersant is dispersed in proton-conducting ceramics, as the electrolyte layer. Therefore, during operation, the mechanical strength of the electrolyte layer is less likely to decrease due to thermal expansion or hydration expansion, and damage such as cracking of the electrolyte layer that occurs as a result is less likely to occur. Thus, it becomes a highly durable steam electrolytic cell.

[0025] In the method for producing a proton-conducting solid electrolyte according to the present invention having the configuration described in [9] above, a step is carried out to calcine a precursor containing both a citric acid complex containing a metal element constituting the proton-conducting ceramic and a citric acid complex containing a metal element constituting the dispersant. By this method, a proton-conducting solid electrolyte having a structure in which a dispersant made of a metal oxide is dispersed spherically within the proton-conducting ceramic can be easily produced.

[0026] This is a schematic diagram showing the structure of a proton-conducting solid electrolyte according to one embodiment of the present invention. For samples with different amounts of MgO added, (a) the X-ray diffraction measurement results and (b) the lattice constant are shown. For samples with different amounts of MgO added, scanning electron microscope images and elemental distribution images are shown along with sample photographs. For samples with different amounts of MgO added, photographs taken before and after heat treatment are shown. For samples with different amounts of MgO added, the temperature dependence of electrical conductivity is shown.

[0027] The following describes a proton-conducting solid electrolyte according to embodiments of the present invention, a method for producing the same, a proton-conducting fuel cell, and a steam electrolytic cell. In this specification, unless otherwise specified, all physical properties refer to values ​​at room temperature and in air.

[0028] [Proton-Conducting Solid Electrolytes] First, a proton-conducting solid electrolyte according to one embodiment of the present invention will be described.

[0029] Figure 1 schematically shows the microstructure of a proton-conducting solid electrolyte 1 according to one embodiment of the present invention. The proton-conducting solid electrolyte 1 contains a proton-conducting ceramic 2 and a dispersant 3. As shown in Figure 1, in the structure of the proton-conducting solid electrolyte 1, the dispersant 3 is dispersed within the proton-conducting ceramic 2. In other words, microcrystals of the dispersant 3 are scattered within the proton-conducting ceramic 2, which serves as the matrix material.

[0030] (1) Proton-conducting ceramics The proton-conducting ceramics that serve as the base material for the proton-conducting solid electrolyte according to this embodiment are composed of a perovskite-type metal complex oxide and possess proton conductivity. In the perovskite-type metal complex oxide, ABO ３ In a perovskite-type metal complex oxide represented by AB, a portion of metal B is substituted with metal M having a lower valence, thereby imparting proton conductivity. １－ｂ M ｂ O ３－δIt is written as (0 < b < 1, δ is the amount of oxygen vacancies). The specific types of perovskite-type metal complex oxides that constitute proton-conducting ceramics are not particularly limited, but BaZrO ３ The system material can be used suitably.

[0031] BaZrO ３ The proton-conducting ceramic system is BaZrO ３ In this form, a portion of the Ba is replaced with a low-valence metal such as a rare earth element. As a specific example, similar to that disclosed in Patent Document 1, BaZr １－ｘ－ｙ Yb ｘ Sc ｙ O ３－δ Compounds having the composition (0 < x < 0.5, 0 < y < 0.5, x + y < 0.5, 0 < δ < 0.5) can be suitably used. These compounds exhibit high proton conductivity even at relatively low temperatures of about 300 to 400°C. From the viewpoint of increasing proton conductivity, it is particularly preferable that x + y is 0.15 or higher, and more preferably 0.25 or higher. It is also preferable that x + y is 0.35 or lower. It is preferable that x is 0.15 or higher. Among these, it is preferable that x is 0.2 and y is 0.1, i.e., BaZr ０．７ Yb ０．２ Sc ０．１ O ３－δ When the composition is as described above, particularly high proton conductivity can be obtained. On the other hand, when the amount of Yb and Sc added (x + y) is 0.3 or more, the proton-conducting ceramic alone is prone to a decrease in mechanical strength due to hydration expansion, and the effect of adding a dispersant, as described later, becomes more pronounced.

[0032] BaZr １－ｘ－ｙ Yb ｘ Sc ｙ O ３－δ In addition, BaZr is a material that can be suitably applied as a proton-conducting ceramic constituting the proton-conducting solid electrolyte according to this embodiment. １－ｘ Sc ｘ O ３－δ (For example, x = 0.6), BaSn １－ｘ Sc ｘ O ３－δ (For example, x = 0.7), BaTi １－ｘ Scｘ O ３－δ (For example, x = 0.8), BaMo １－ｘ Sc ｘ O ３－δ (For example, x = 0.8) are some examples. These proton-conducting ceramics are prone to a decrease in mechanical strength due to hydration expansion, and the effects of adding dispersants, as described later, can be greatly enhanced. In particular, when the amount of low-valence metal added (x) is 0.3 or more, a decrease in mechanical strength due to hydration expansion is more likely to occur.

[0033] (2) Dispersant The dispersant that constitutes the proton-conducting solid electrolyte together with the proton-conducting ceramics is composed of a metal compound. The metal compound as a dispersant is a different metal compound from the proton-conducting ceramic as the base material and has a different composition from the proton-conducting ceramic. Preferably, the dispersant consists of a metal compound that does not conduct electricity or a metal compound that has proton conductivity. Since the proton-conducting ceramic as the base material exhibits sufficient proton conductivity, it is particularly preferable that the metal compound as a dispersant is composed of a compound that does not conduct electricity, including proton conductivity. Here, "not conducting electricity" means that the electrical resistivity is approximately 10 １０ This refers to a value of Ω·m or greater. Metallic compounds are typically composed of inorganic compounds containing metallic elements (including metalloids such as B and Si) and nonmetallic elements.

[0034] Typical examples of metal compounds that constitute dispersants include metal oxides, metal nitrides, and metal carbides. When dispersed in proton-conducting ceramics, these metal compounds are highly effective in suppressing the decrease in the mechanical strength of the proton-conducting solid electrolyte described later. The influence on the physical properties of the proton-conducting ceramics due to solid solution or reaction is also easily kept to a minimum. Furthermore, the metal compounds themselves exhibit high fracture toughness, which also contributes to improving the mechanical strength of the proton-conducting solid electrolyte. Among these, metal oxides have excellent properties and can be suitably used.

[0035] MgO is a metal oxide that can be particularly suitable as a dispersant. As shown in later examples, MgO is highly effective in suppressing the decrease in the mechanical strength of proton-conducting solid electrolytes. BaZrO ３ It does not undergo solid solution or reaction with proton-conducting ceramics such as system materials. In addition to MgO, other metal oxides that can be used as dispersants include ZnO, CuO, and Al. ２ O ３ , ZrO ２ Also, MgAl ２ O ３ Examples of composite oxides containing these oxides in their composition include SiC and B. ４ Examples include C, WC, etc. Examples of metal nitrides include Si ３ N ４ Examples include the following.

[0036] (3) Structure and Characteristics of the Proton-Conducting Solid Electrolyte As shown in Figure 1, in the proton-conducting solid electrolyte 1 according to this embodiment, the dispersant 3 is dispersed in the proton-conducting ceramic 2. That is, the regions occupied by the proton-conducting ceramic 2 and the regions occupied by the dispersant 3 are mixed, and the microcrystals of the dispersant 3 are scattered in the proton-conducting ceramic 2 as the matrix material. Preferably, the dispersant 3 is dispersed in spherical or nearly spherical regions.

[0037] In the proton-conducting solid electrolyte according to this embodiment, the dispersion of a dispersant within the proton-conducting ceramics suppresses the reduction in mechanical strength due to thermal expansion and hydration expansion, even when the electrolyte is heated or comes into contact with steam during operation, for example, in a proton-conducting fuel cell or steam electrolytic cell. Furthermore, damage such as cracking due to the reduction in mechanical strength becomes less likely. The suppression of the reduction in mechanical strength due to thermal and hydration expansion is thought to be because the dispersion, which is made of a different material from the proton-conducting ceramics and has a different coefficient of thermal expansion and / or presence and degree of hydration expansion than the proton-conducting ceramics, is dispersed within the proton-conducting ceramics, thereby mitigating the expansion stress associated with heating and hydration. From the viewpoint of enhancing the stress relaxation effect, it is preferable that the dispersion does not undergo hydration expansion and has a coefficient of thermal expansion close to that of the proton-conducting ceramics. In particular, it is preferable that the linear expansion coefficient of the dispersion is within a range of ±50%, or even ±40%, of the linear expansion coefficient of the proton-conducting ceramics. Furthermore, MgO does not undergo hydration expansion, and its coefficient of linear expansion is 13.5 × 10⁻⁶. －６ K －１ That is. BaZr ０．８ Yb ０．２ O ３ The coefficient of linear expansion is 10.35 × 10⁻⁶ －６ K －１ Furthermore, in addition to the contribution of stress relaxation, when the dispersant is composed of compounds with excellent mechanical strength, including fracture toughness, such as metal oxides, metal carbides, and metal nitrides, the high mechanical strength of these compounds also contributes to improving the overall mechanical strength of the proton-conducting solid electrolyte.

[0038] Furthermore, the proton-conducting solid electrolyte according to this embodiment exhibits high electrical conductivity, similar to proton-conducting ceramics without added dispersants. This is because the dispersion of dispersants in proton-conducting ceramics does not significantly affect the physical properties of the proton-conducting ceramics, including their proton conductivity.

[0039] In a proton-conducting solid electrolyte, a portion of the dispersant may undergo solid solution or reaction with the proton-conducting ceramics. However, it is preferable that the dispersant is dispersed without undergoing solid solution or reaction with the proton-conducting ceramics. By preventing solid solution or reaction of the dispersant, a significant effect in suppressing the decrease in mechanical strength due to the addition of the dispersant can be obtained. Furthermore, changes in physical properties such as electrical conductivity due to alteration of the proton-conducting ceramics can be suppressed. Here, it can be determined, for example, that the dispersant has not undergone solid solution or reaction with the proton-conducting ceramics by ensuring that the peak position of the X-ray diffraction (XRD) and the lattice constant of the proton-conducting ceramics do not change with the addition of MgO, or that the change is negligible (for example, the rate of change of the lattice constant is 0.5% or less).

[0040] The amount of dispersant added to a proton-conducting solid electrolyte is not particularly limited. However, it is preferable that the amount of dispersant added be 10% or more, and even 15% or more, of the total volume ratio of the proton-conducting ceramics and the dispersant. This allows for a high degree of suppression of the decrease in mechanical strength due to thermal and hydration expansion caused by the addition of the dispersant. On the other hand, it is preferable to keep the amount of dispersant added to 30% or less, and even 20% or less. This makes it easier to maintain high levels of properties provided by the proton-conducting ceramics, such as proton conductivity, in the proton-conducting solid electrolyte.

[0041] In proton-conducting solid electrolytes, the particle size of the dispersed microcrystals is not particularly specified. However, from the viewpoint of enhancing the effect of suppressing the decrease in mechanical strength due to expansion, it is preferable to set the average particle size to 0.1 μm or more, and more preferably 0.3 μm or more. It is also preferable to set it to 10 μm or less, and more preferably 1 μm or less. The particle size of the dispersed microcrystals can be measured, for example, as the area circle equivalent diameter in a scanning electron microscope (SEM) image.

[0042] A proton-conducting solid electrolyte may consist only of proton-conducting ceramics and a dispersant, excluding unavoidable impurities derived from the raw materials, but it may also contain other components as long as they do not impair the properties of the material containing the proton-conducting ceramics and the dispersant. Examples of other components include conductive materials other than proton-conducting ceramics. Furthermore, the proton-conducting ceramics and the dispersant may consist of only one type, or two or more types may be used.

[0043] (4) Method for producing a proton-conducting solid electrolyte The proton-conducting solid electrolyte according to this embodiment can be produced by a solid-phase method or a liquid-phase method. When using the solid-phase method, a powder of a starting material containing each component element constituting the proton-conducting ceramic and a powder of a starting material containing the component elements constituting the dispersant are mixed in a predetermined ratio, the resulting mixed powder is appropriately molded, and then fired to obtain a proton-conducting solid electrolyte. When using the liquid-phase method, a precursor such as a complex containing the component elements of the proton-conducting ceramic and the component elements of the dispersant in a predetermined component ratio is formed by a liquid-phase reaction, the precursor is appropriately molded, and then fired to obtain a proton-conducting solid electrolyte.

[0044] In particular, when the dispersant consists of a metal oxide, the citric acid complex method, a type of liquid-phase method, can be suitably applied. In the citric acid complex method, a precursor is prepared that contains both a citric acid complex containing the metal elements constituting the proton-conducting ceramics and a citric acid complex containing the metal elements constituting the dispersant. A proton-conducting solid electrolyte is obtained by calcining this precursor. A specific method for producing a proton-conducting solid electrolyte using the citric acid complex method involves mixing starting materials such as nitrates containing the respective metal elements constituting the proton-conducting ceramics and the dispersant in predetermined ratios, dissolving them in distilled water, and forming a complex with citric acid and ethylenediaminetetraacetic acid (EDTA) to produce a precursor. The precursor is then calcined to obtain a powder material, which can then be appropriately processed through steps such as pulverization, drying, and sizing, before being molded and calcined.

[0045] In the citric acid complex method, a proton-conducting solid electrolyte having a structure in which the dispersant is uniformly dispersed within the proton-conducting ceramic can be easily produced by preparing a precursor containing both the complex that serves as the raw material for the proton-conducting ceramic and the complex that serves as the raw material for the dispersant, and then sintering it. Even if the precursor contains a mixture of citric acid complexes containing metal elements that constitute the proton-conducting ceramic and citric acid complexes containing metal elements that constitute the dispersant, microcrystals of the dispersant can be precipitated within the proton-conducting ceramic matrix during the firing process. The dispersant tends to form a dispersed state in a spherical or near-spherical shape. In particular, the dispersant is more likely to precipitate as microcrystals when the substances constituting the dispersant do not undergo solid solution or reaction with the proton-conducting ceramic.

[0046] As described above, proton-conducting solid electrolytes can be manufactured by solid-phase or liquid-phase methods. Alternatively, one of the raw materials for the proton-conducting ceramics and the raw materials for the dispersant may be prepared by solid-phase method, and the other by liquid-phase method, and then these raw materials may be combined and sintered. For example, a precursor to the proton-conducting ceramics may be prepared by liquid-phase method, and then the dispersant itself, or a raw material that becomes a dispersant after firing, may be added to the precursor in solid phase.

[0047] When producing a proton-conducting solid electrolyte containing a substance that readily undergoes solid solution in proton-conducting ceramics as a dispersant using a liquid-phase method, solid solution can be suppressed by performing firing at a relatively low temperature. Furthermore, solid solution can also be suppressed by precipitating the dispersant through reduction or electrolysis.

[0048] [Proton-Conducting Fuel Cell] Next, a proton-conducting fuel cell according to one embodiment of the present invention will be described.

[0049] A proton-conducting fuel cell according to one embodiment of the present invention (hereinafter sometimes simply referred to as a fuel cell or PCFC) has an electrolyte layer composed of a proton-conducting solid electrolyte according to an embodiment of the present invention, in which a dispersant is dispersed in proton-conducting ceramics as described above. The single cell structure has an anode (fuel electrode) on one side of the electrolyte layer and a cathode (air electrode) on the other side. Because the electrolyte layer is proton-conducting, the single cell functions as a proton-conducting fuel cell.

[0050] In a single cell, the anode and cathode electrodes may be directly bonded to the electrolyte layer, or an intermediate layer may be optionally interposed between the electrodes and the electrolyte layer. Furthermore, single cells are typically used as fuel cells when multiple cells are integrated together with appropriate separators. Current collectors may be placed between the separators and each electrode as appropriate.

[0051] As described above, a proton-conducting solid electrolyte in which a dispersant is dispersed in proton-conducting ceramics exhibits high proton conductivity, and due to the contribution of the dispersant, it is less susceptible to a decrease in mechanical strength due to thermal expansion and hydration expansion, and to damage such as cracking caused by these factors. By using a material with such properties as the electrolyte layer of a PCFC, it becomes possible to use the PCFC while maintaining good properties over a long period of time.

[0052] In this PCFC, the materials constituting the anode and cathode are not specifically designated, and materials that can be used as anodes and cathodes in general solid oxide fuel cells may be appropriately selected. For example, as the constituting material for the anode, examples include metals such as Ni, Cu, Pt, Pd, Ru, Ag, or alloys containing at least one of these metals. A cermet made of these metal materials and ceramic materials may also be used. Similarly, as the constituting material for the cathode, examples include metals such as Ni, Co, Pt, Pd, Ru, or alloys containing at least one of these metals. A cermet made of these metal materials and ceramic materials may also be used. Alternatively, the cathode may be constructed from a conductive ceramic material. For the constituting materials of the anode and cathode, only one type may be used, or two or more types may be used in combination.

[0053] [Steam Electrolysis Cell] Finally, a steam electrolysis cell according to one embodiment of the present invention will be described.

[0054] A water vapor electrolytic cell according to one embodiment of the present invention has an electrolyte layer composed of a proton-conducting solid electrolyte according to an embodiment of the present invention, in which a dispersant is dispersed in proton-conducting ceramics as described above. The single cell structure has an anode (fuel electrode) on one side of the electrolyte layer and a cathode (air electrode) on the other side. Because the electrolyte layer is proton-conducting, the single cell functions as a proton-conducting ceramic water vapor electrolytic cell (PCEC).

[0055] In a water vapor electrolytic cell, similar to the fuel cell described above, the anode and cathode electrodes in a single cell may be directly bonded to the electrolyte layer, or an intermediate layer may be optionally interposed between the electrodes and the electrolyte layer. Furthermore, single cells are usually used as electrolytic cells when multiple cells are assembled together via appropriate separators. The materials listed above for the cathode of a fuel cell are suitably used as the constituent material for the anode of the water vapor electrolytic cell. Similarly, the materials listed above for the anode of a fuel cell are suitably used as the constituent material for the cathode of a water vapor electrolytic cell.

[0056] As described above, a proton-conducting solid electrolyte in which a dispersant is dispersed in proton-conducting ceramics exhibits high proton conductivity, and due to the contribution of the dispersant, it is less susceptible to a decrease in mechanical strength due to thermal expansion and hydration expansion, and to damage such as cracking caused by these factors. By using a material with such properties as the electrolyte layer of a PCEC, it is possible to use the PCEC while maintaining good properties over a long period of time. PCECs perform electrolysis using water vapor, and hydration expansion of the electrolyte layer due to water vapor is likely to occur, but by using a proton-conducting solid electrolyte in which the decrease in mechanical strength due to hydration expansion is suppressed by the dispersion of a dispersant, damage to the electrolyte layer due to hydration can be effectively suppressed.

[0057] The present invention will be described in detail below with reference to examples.

[0058] [Sample preparation] BaZr as a proton-conducting ceramic ０．７ Yb ０．２ Sc ０．１ O ３－δ A sample was prepared by adding MgO as a dispersant to BZYbSc721 (hereinafter sometimes referred to as BZYbSc721) using the citric acid complex method. Specifically, as starting materials, the nitrates of Ba, ZrO, Yb, Sc, and Mg were mixed in ratios corresponding to the composition of the sample to be prepared, and a precursor was prepared by dissolving them in pure water with citric acid and EDTA and stirring. The obtained precursor was degassed and calcined at 900°C for 10 hours. The obtained calcined body was pulverized with a roller mill, lamp dried, and sieved with a 150 μm mesh sieve. The obtained powder material was formed into a φ20 mm disc shape. The forming was done using a powder press (load 20 kN) and a hydrostatic press (pressure 250 MPa). Next, the molded body was subjected to final calcination. Final calcination was performed by immersion calcination, O ２ The process was carried out at 1650°C for 10 hours.

[0059] Several samples with different amounts of added MgO were prepared. The amount of MgO added was adjusted by changing the mixing ratio of the starting materials to obtain the desired BZYbSc721:MgO volume ratio. Specifically, five samples were prepared with BZYbSc721:MgO volume ratios of 10:0, 9.5:0.5, 9:1, 8.5:1.5, and 8:2. Of these, the 10:0 sample did not have any added MgO, and did not contain Mg nitrate as a starting material.

[0060] [Evaluation Method and Results] (1) Confirmation of Crystal State For each sample prepared as described above, X-ray diffraction (XRD) measurements were performed to evaluate the crystal state. The measurement was performed using the θ-2θ method with Cu-Kα rays as the radiation source, after polishing the surface of the disk-shaped sample with abrasive paper, using a Rigaku MiniFlex 600. From the obtained XRD measurement results, the lattice constant of BZYbSc721 in each sample was estimated. In addition, the density of each sample was calculated from the volume, weight, and lattice constant of the disk-shaped sample.

[0061] Furthermore, for samples with a volume ratio of BZYbSc721:MgO of 10:0, hydration was performed, and the lattice constant was estimated using XRD for the hydrated samples as well. Hydration was carried out by heat treatment under the same conditions as the crack verification test by heat treatment described later.

[0062] Figure 2(a) shows the results of XRD measurements for some of the samples. The figure also shows Ba(ZrO) obtained from the Inorganic Crystal Structure Database (ICSD). ３ Figure 2(b) and Table 1 below show the diffraction patterns of ) and MgO. Also, Figure 2(b) and Table 1 below show the lattice constants of BZYbSc721 obtained from the XRD measurement results. Table 1 also shows the density evaluation results.

[0063]

[0064] As shown in Figure 2(a), in all samples in which MgO was added to BZYbSc721 in three different ratios, diffraction peaks appeared at the same position as in the case of BZYbSc721 alone (10:0). According to the diffraction pattern in the database, Ba(ZrO３ Since ) and MgO have peaks at almost the same position, it is difficult to evaluate the effect of MgO addition in detail from the XRD pattern alone. However, it can be said that the addition of MgO has not caused phenomena that would significantly change the crystal structure of BZYbSc721, such as solid solution or reaction of a large amount of MgO.

[0065] Furthermore, comparing the lattice constants shown in Figure 2(b) and Table 1, the lattice constant of BZYbSc721 changes only slightly with the addition of MgO and with changes in the amount of MgO added. From this, it can be concluded that MgO does not undergo solid solution with BZYbSc721. Figure 2(b) and Table 1 also show the lattice constant of BZYbSc721 after hydration without the addition of MgO. It can be seen that the lattice constant increases by about 0.7% due to hydration, indicating that hydration expansion is occurring. Compared to this change in the lattice constant due to hydration expansion, the change in the lattice constant due to the addition of MgO is very small. Furthermore, according to Table 1, the density of the sample does not change significantly even with the addition of MgO, and it can be confirmed that the sinterability does not change much with the addition of MgO.

[0066] (2) Confirmation of the distribution of the dispersant For each sample, the distribution of MgO was evaluated by observation using a scanning electron microscope (SEM) and energy-dispersive X-ray spectroscopy (EDX). SEM observation and EDX analysis were performed using JEOL's "JCM-6000Plus" after polishing the surface of the disc-shaped sample with diamond paste (#8000).

[0067] Figure 3 shows, for each sample, a photograph of the sample, an SEM observation image (magnification 10,000x), and elemental distribution images of Mg, Ba, Zr, Yb, and Sc obtained by EDX analysis. In the SEM images of each sample with added MgO, numerous black, spot-like regions appear within the gray areas. As the MgO content increases, the area occupied by these spots becomes larger. Looking at the elemental distribution image of Mg, regions with high Mg concentration are distributed in a point-like manner, and these regions roughly correspond to the positions of the black spots in the SEM image. From this, it can be seen that MgO is distributed in the black areas scattered in the SEM image. On the other hand, in the elemental distribution image, the concentrations of Ba, Zr, Yb, and Sc are high outside the point-like regions with high Mg concentration. From this, it can be seen that the base material observed in gray in the SEM image is occupied by BZYbSc721. Based on the above results, it can be confirmed that in the sample to which MgO was added to BZYbSc721, MgO did not solid dissolve with BZYbSc721 but formed independent regions and was dispersed with high uniformity within BZYbSc721.

[0068] (3) Verification of cracking due to heat treatment The mechanical strength of the samples was evaluated by the presence or absence of cracking after heat treatment. For the evaluation, the discs of each sample were polished to a thickness of 0.700 mm with abrasive paper (#240, #500, #1000). The samples were placed in an alumina crucible and heat-treated in a small electric furnace at 900°C in air for 1 hour. The condition of the samples before and after heat treatment was recorded by photographs and the presence or absence of cracking due to heat treatment was evaluated.

[0069] Figure 4 shows photographs of sample disks before and after heat treatment for some samples. As can be seen, in the sample without MgO addition (BZYbSc721:MgO = 10:0), the disk shattered into pieces after heat treatment. This is thought to be due to a decrease in the mechanical strength of the sample caused by thermal expansion and hydration expansion. As shown in Figure 2(b) and Table 1, the lattice constant of BZYbSc721 increased significantly due to hydration expansion, and this increase in the lattice constant is thought to be the cause of the decrease in mechanical strength and cracking.

[0070] On the other hand, in the two samples to which MgO was added, no cracks occurred in the disks even after heat treatment, and the appearance of the samples remained almost unchanged. This confirms that cracking of the samples is suppressed by the addition of MgO. It is thought that the decrease in mechanical strength due to thermal expansion and hydration expansion was suppressed by the addition of MgO.

[0071] (4) Verification of the effect of adding a dispersant on electrical conductivity The effect of adding MgO as a dispersant on electrical conductivity was evaluated by measuring the impedance of each sample and determining the electrical conductivity. For the impedance measurement sample, an Ag electrode was baked onto the disc-shaped sample prepared above. Specifically, Ag paste (φ6-8 mm) was placed on both sides of the sample disc and then sandwiched with an Ag mesh. In this state, it was baked at 900°C for 1 hour.

[0072] Impedance measurements were performed on the obtained sample using the AC four-terminal method. The measurement was performed at a pressure ratio of 1.9%H. ２ O-1%H ２ The measurements were performed in an Ar atmosphere, varying the temperature within the range of 800°C to 100°C. The measurement frequency was set to 4 to 8 Hz. The real and imaginary components of the measured impedance were plotted on the complex plane to create a Nyquist plot.

[0073] The electrolyte resistance (R) was estimated from the obtained Nyquist plot. Specifically, if the ionic conductivity was high and a real axial intercept was present in the Nyquist plot, the value of that real axial intercept was read and used as the electrolyte resistance (R). On the other hand, if the ionic conductivity was low and no real axial intercept appeared in the Nyquist plot, the electrolyte resistance (R) was calculated by applying a fitting analysis using an equivalent circuit. Furthermore, the electrical conductivity (σ) was calculated from the obtained electrolyte resistance (R) value. The calculation was performed using the relationship σ = l / RA, where A is the electrode area (φ6-8 mm) and l is the electrolyte thickness (0.05 cm).

[0074] Figure 5 shows the conductivity (σ) obtained at each measurement temperature for some samples, displayed logarithmically against the reciprocal of the temperature (T). According to this, compared to BZYbSc721 without MgO (■), adding MgO (▲ and ●) does not significantly change the conductivity value or its behavior with respect to temperature. Therefore, it can be said that the effect of MgO addition on the proton conductivity of BZYbSc721 is very small. Furthermore, even after adding MgO, the conductivity at 300°C (T) remains the same as that of BZYbSc721 without MgO. －１ = 1.75 × 10 －３ K －１ ) to 400℃ (T －１ = 1.49 × 10 －３ K －１ High electrical conductivity is obtained in the low-temperature range of approximately 5°C, and it is considered suitable for use as an electrolyte in fuel cells that operate at relatively low temperatures.

[0075] The present invention is not limited in any way to the embodiments and examples described above, and various modifications are possible without departing from the spirit of the invention.

[0076] 1. Proton-conducting solid electrolyte 2. Proton-conducting ceramics 3. Dispersant

Claims

1. A proton-conducting solid electrolyte comprising a proton-conducting ceramic composed of a perovskite-type metal complex oxide, and a dispersant composed of a metal compound different from that of the proton-conducting ceramic, wherein the dispersant is dispersed within the proton-conducting ceramic.

2. The proton-conducting solid electrolyte according to claim 1, wherein the dispersant does not undergo solid solution or reaction with the proton-conducting ceramics.

3. The proton-conducting solid electrolyte according to claim 1 or claim 2, wherein the dispersant is composed of a metal oxide.

4. The proton-conducting solid electrolyte according to claim 3, wherein the dispersant is composed of MgO.

5. The proton-conducting ceramic is BaZr １－ｘ－ｙ Yb ｘ Sc ｙ O ３－δ A proton-conducting solid electrolyte according to claim 1 or claim 2, having a composition of (0 < x < 0.5, 0 < y < 0.5, x + y < 0.5, 0 < δ < 0.5).

6. The amount of the dispersant added is 15% or more by volume of the total amount of the proton-conducting ceramics and the dispersant, as described in claim 1 or claim 2.

7. A proton-conducting fuel cell having a single cell structure comprising: an electrolyte layer composed of a proton-conducting solid electrolyte according to claim 1 or claim 2; an anode provided on one side of the electrolyte layer; and a cathode provided on the other side of the electrolyte layer.

8. A water vapor electrolytic cell having a single-cell structure comprising: an electrolyte layer composed of a proton-conducting solid electrolyte according to claim 1 or claim 2; an anode provided on one side of the electrolyte layer; and a cathode provided on the other side of the electrolyte layer.

9. A method for producing a proton-conducting solid electrolyte according to claim 3, comprising the step of firing a precursor comprising a citrate complex containing a metal element constituting the proton-conducting ceramic and a citrate complex containing a metal element constituting the dispersant.

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