Method for manufacturing a paint composition, method for manufacturing a gadolinium-doped ceria layer, method for manufacturing an electrochemical element, method for manufacturing a solid oxide fuel cell, and method for manufacturing a solid oxide electrolytic cell

Abstract

To provide a coating composition (b) that enables film deposition in a simple manner and at a low cost using a cerium compound (o) and a gadolinium compound (p) as the starting materials, and can form a compact gadolinium-doped ceria (GDC) layer.SOLUTION: A coating composition (b) contains a cerium compound (o), a gadolinium compound (p), hydroxycarboxylic acid (q), polyalkylene glycol (r), and organic solvent (s).SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] The present invention relates to a method for producing a paint composition capable of forming a gadolinium-doped ceria (GDC) layer on the surface of various members by painting, and also relates to various functional members provided with a gadolinium-doped ceria (GDC) layer.

Background Art

[0002] Conventionally, for example, in a solid oxide fuel cell (SOFC), as a functional member such as an intermediate layer (reaction prevention layer) between its solid electrolyte (such as yttria-stabilized zirconia (YSZ)) and air electrode (such as strontium-doped lanthanum cobalt oxide), it is known to laminate a gadolinium-doped ceria (GDC) layer. As methods for forming a ceria layer such as a gadolinium-doped ceria (GDC) layer, there are the following techniques.

[0003] Non-Patent Document 1 discloses that gadolinium nitrate hexahydrate, cerium nitrate hexahydrate, and polyvinylpyrrolidone (molecular weight 360,000) are dissolved in a mixed solvent of ethanol and ethylene glycol, and then acetylacetone as a chelating agent and acetic acid for promoting the dissolution of the precursor are added to prepare a GDC coating solution. Further, a gadolinium-doped ceria (GDC) film formed by spin coating this GDC coating solution on an yttria-stabilized zirconia (YSZ) film is disclosed.

[0004] Non-Patent Document 2 discloses that a coating solution is prepared by dissolving cerium chloride heptahydrate and citric acid in ethanol. Further, a ceria film formed by dip coating this coating solution on a glass substrate with an FTO (fluorine-doped tin oxide) film is disclosed.

[0005] Non-patent document 3 discloses a method for obtaining a viscous gel by dissolving cerium nitrate and gadolinium nitrate in deionized water, adding citric acid and ethylene glycol, and heating at 70°C. This document discloses obtaining GDC powder by heating this obtained gel to 250°C and then to 700°C. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] Kunho Lee, Juhyun Kang, Sangbeom Jin, Sanghum Lee, International Journal of Hydrogen Energy, 42, 6220-6230(2017) [Non-Patent Document 2] F. Pinar Gokdemir, A. Evrim Saatci, Orhan Ozdemir, Bahadir Keskin, Kubilay Kutlu, Materials Science in Semiconductor Processing, 38, 300-305 (2015) [Non-Patent Document 3] Saeed Ur Rehman,Ahmad Shaur,Hye-Sung Kim,Dong Woo Joh,Rak-Hyun Song,Tak-Hyoung Lim,Jong-Eun Hong,Seok-Joo Park,Seung-Bok Lee,International Journal of Applied Ceramic Technolohy,18,511-524(2021) [Overview of the project] [Problems that the invention aims to solve]

[0007] However, it was found that the technology disclosed in Non-Patent Document 1 has the following problems. (1) When attempting to form the GDC film as a reaction prevention layer on the surface of the YSZ film as a solid electrolyte layer, it is necessary to repeatedly drop the GDC coating solution and spin coat it. Although this Non-Patent Document 1 does not describe the thickness of the GDC coating film, it does describe that a YSZ coating film formed by spin coating required five spin coatings to achieve a thickness of 1 μm, which presents a problem in terms of productivity.

[0008] (2) When ethylene glycol is used as a solvent in the coating solution, since ethylene glycol has a high boiling point of 197°C, it may remain undried during the drying process after spray painting (which usually takes place at around 80-100°C).

[0009] (3) Later, comparative examples will be explained in comparison with the examples, but when polyvinylpyrrolidone is added to the coating solution, the amide carbonyl group of polyvinylpyrrolidone and the metal (cerium, gadolinium) ions bond strongly, causing the coating solution to gel easily. When coating is performed in this gelled state, the entire surface of the coating film becomes rough and uneven. If firing is performed in this state, the coated film after firing may peel off easily with just a light touch of a finger.

[0010] Furthermore, it was found that the technology disclosed in Patent Document 2 has the following problems. (1) In an ethanol solution of cerium chloride and citric acid, it is thought that the cerium salt of citric acid is formed. For this solution to polymerize, i.e., become a sol, the only way is for the free carboxyl group of cerium citrate to condense with the hydroxyl group of another molecule of cerium citrate, and the rate of sol formation is thought to be slow. Although the coating film thickness is not described in this document, generally a single dip coat results in a coating film thickness of the order of 10 nm, so in order to obtain a coating film of 1 μm or more, it is necessary to repeat the dip coat, which presents a problem in terms of productivity.

[0011] (2) As will be explained later in comparison with the examples, when this method is used, the coating solution may not be sufficiently sol-formed when spray-painted, causing the coating film to be repelled from the substrate and resulting in an uneven coating. After drying, the coating film may easily peel off with just a light touch of a finger, and it may not be possible to proceed to the baking stage required after the formation of the GDC film.

[0012] Furthermore, it was found that the technology disclosed in Patent Document 3 has the following problems. (1) In this method, cerium and gadolinium are coordinated with citric acid to form citrate, and then twice the molar amount of ethylene glycol relative to the citric acid is added. The process of heating the resulting sol to 250°C and then calcining it at 700°C to produce GDC powder is difficult to apply to coating.

[0013] (2) As will be explained later in comparison with the examples, when this method is used, since deionized water is used as the solvent, the coating film is repelled from the substrate when spray painted, making it difficult to obtain a uniform coating film, and unreacted ethylene glycol (boiling point 197°C) remains even after drying at 80°C. Furthermore, when fired at 700°C in this state, the coating film has many defects such as cracks, and the coating film peels off easily with just a light touch of a finger.

[0014] On the other hand, a method of forming a coating film by a sol-gel reaction using cerium alkoxide as a starting material is also conceivable. However, because the hydrolysis rate of cerium alkoxide is extremely fast compared to that of ordinary silicon alkoxide, when attempting to achieve a coating film thickness of 1 μm or more, defects such as cracks and fissures are likely to occur on the surface. Furthermore, because the hydrolysis rate of cerium alkoxide is extremely fast, even if a gadolinium compound is added, the reaction may be completed with cerium alkoxide alone, raising concerns that gadolinium will be unevenly distributed and a uniform gadolinium-doped ceria film will not be formed. Furthermore, for example, cerium tetra (isopropoxide), which is sold as a reagent, is extremely expensive at around 36,000 yen per gram, and is therefore considered unsuitable for industrial use.

[0015] The present invention has been made in view of the above-mentioned problems, and the object of the present invention is to provide a coating composition that can be formed in a simple and low-cost manner using cerium compounds such as cerium nitrate and gadolinium compounds such as gadolinium nitrate as starting materials, and that can produce a dense gadolinium-doped ceria (GDC) film with a uniform composition. [Means for solving the problem]

[0016] The first characteristic feature of the present invention is that it is a method for producing a paint composition by mixing a composition containing a cerium compound, a gadolinium compound, a hydroxycarboxylic acid, a polyalkylene glycol, and an organic solvent.

[0017] The paint composition produced by this method contains a hydroxycarboxylate metal (cerium and gadolinium) salt formed by coordination with a cerium compound and a gadolinium compound, and a polyalkylene glycol that can undergo a dehydration condensation reaction with the free carboxyl group of the hydroxycarboxylate metal salt. This allows for moderate polymerization, i.e., solification, within the paint composition. Therefore, a coating film thickness can be formed to a desired level by a simple method, such as applying the paint composition using an air spray. The inventors' studies have shown that using the paint composition of the present invention makes it easy to obtain a gadolinium-doped ceria film with few defects.

[0018] In general complex polymerization methods, monomers such as ethylene glycol or propylene glycol are used as reagents for dehydration condensation with hydroxycarboxylic acids. However, when monomers are used, as shown in Comparative Example 4 later, the rate of polymerization (sol formation) during the dehydration condensation reaction with the free carboxyl group of the hydroxycarboxylic acid coordinated to the cerium compound and the gadolinium compound is slow, making it difficult to achieve sufficient polymerization (sol formation).

[0019] The second characteristic configuration of the present invention is that in the method for producing the coating composition, the content of the cerium compound in the coating composition is 5 to 20% by mass, the content of the gadolinium compound is 0.5 to 2% by mass, the content of the hydroxycarboxylic acid is 1 to 10% by mass, the content of the polyalkylene glycol is 1 to 10% by mass, and the content of the organic solvent is the balance.

[0020] When the content of the cerium compound is less than 5% by mass, the raw materials tend to be insufficient and the production of the target product is delayed. On the other hand, when it is higher than 20% by mass, the quality of the GDC coating film may deteriorate.

[0021] When the content of the gadolinium compound is less than 0.5% by mass, the raw materials tend to be insufficient and it becomes difficult to obtain the target product. On the other hand, when it is higher than 2% by mass, the amount of gadolinium in the produced GDC coating film may be excessive.

[0022] The hydroxycarboxylic acid coordinates with the cerium compound and the gadolinium compound to form a metal salt. When the content of the hydroxycarboxylic acid is less than 1% by mass, it is difficult to obtain the effect of coordinating with the cerium compound and the gadolinium compound to form a metal salt. On the other hand, when it is higher than 10% by mass, it is not preferable because only the free hydroxycarboxylic acid and the polyalkylene glycol undergo a polycondensation reaction without forming a metal salt with the cerium compound and the gadolinium compound.

[0023] The polyalkylene glycol is used as a reagent for dehydration condensation with the free carboxyl group of the hydroxycarboxylic acid coordinated with the cerium compound and the gadolinium compound to polymerize (sol-gel). When the content is less than 1% by mass, the dehydration condensation reaction does not occur sufficiently and the coating solution may not sol-gel. When more than 10% by mass is added, defects such as cracks are likely to occur in the GDC coating film during drying and firing after coating, and the quality of the coating film may deteriorate.

[0024] Organic solvents are used to dissolve cerium compounds, gadolinium compounds, hydroxycarboxylic acids, and polyalkylene glycols to achieve a homogeneous composition.

[0025] A third characteristic feature of the present invention is that, in the method for producing the above-mentioned paint composition, the cerium compound is one or more of cerium nitrate, cerium chloride, cerium sulfate, cerium phosphate, cerium acetate, and cerium carbonate.

[0026] In other words, cerium compounds serve as the starting materials for the gadolinium-doped ceria described above. Among these, cerium nitrate, cerium chloride, and cerium acetate are preferred due to their availability and reactivity. By combining these compounds with oxycarboxylic acids and polyalkylene glycols, which will be described later, a gadolinium-doped ceria film without cracking or peeling can be obtained.

[0027] A fourth characteristic feature of the present invention is that, in the method for producing the above-mentioned paint composition, the gadolinium compound is one or more of gadolinium nitrate, gadolinium chloride, gadolinium sulfate, gadolinium phosphate, gadolinium acetate, and gadolinium carbonate.

[0028] In other words, gadolinium compounds serve as the starting materials for the gadolinium-doped ceria described above. Among these, gadolinium nitrate, gadolinium chloride, and gadolinium acetate are preferred due to their availability and reactivity. By combining these compounds with oxycarboxylic acids and polyalkylene glycols, which will be described later, a gadolinium-doped ceria film without cracking or peeling can be obtained.

[0029] The fifth characteristic feature of the present invention is that, in the method for producing the above-mentioned coating composition, the hydroxycarboxylic acid is one or more of the following: 2-hydroxypropanediic acid (tartonic acid), 2-hydroxybutanediic acid (malic acid), 2,3-dihydroxybutanediic acid (tartaric acid), 3-hydroxypentanediic acid (3-hydroxyglutaric acid), 2,4-dihydroxypentanediic acid, 2,3,4-trihydroxypentanediic acid, 3-hydroxyhexanoic acid, 2,3,4,5-tetrahydroxyhexanoic acid (galactaric acid), 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), and 2-hydroxypropane-1,2,3-tricarboxylic acid (isocitric acid).

[0030] In other words, hydroxycarboxylic acids have both a hydroxyl group and a carboxyl group in their molecule. These hydroxycarboxylic acids coordinate with cerium compounds and gadolinium compounds to form metal salts of hydroxycarboxylic acids. Furthermore, the free carboxyl (-COOH) groups of the hydroxycarboxylic acid metal salt that are not coordinated to the metal undergo a dehydration condensation reaction with the terminal hydroxy (-OH) groups of the polyalkylene glycol, resulting in moderate polymerization and the formation of a stable sol. Therefore, it is preferable that the hydroxycarboxylic acid molecule contains two or more carboxyl groups and one or more hydroxyl groups.

[0031] Among these, 2-hydroxybutaneic acid (malic acid), 2,3-dihydroxybutaneic acid (tartaric acid), 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), and 2-hydroxypropane-1,2,3-tricarboxylic acid (isocitric acid) are preferred due to their availability and coordination ability with cerium and gadolinium compounds. These compounds form stable metal salts (chelate compounds) with cerium and gadolinium compounds, allowing for the creation of gadolinium-doped ceria films with a uniform composition that are free from cracking and peeling.

[0032] The sixth characteristic feature of the present invention is that, in the method for producing the above-mentioned paint composition, the polyalkylene glycol is one or more of the following: tetraethylene glycol, pentaethylene glycol, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, polyethylene glycol 1000, tripropylene glycol, tetrapropylene glycol, polypropylene glycol 400, polypropylene glycol 700, and polypropylene glycol 1000.

[0033] Specifically, examples of polyalkylene glycols include polyethylene glycol and polypropylene glycol, and the molecular weight of the polyalkylene glycol is preferably around 100 to 2000, and more preferably around 200 to 1000.

[0034] In the case of polyethylene glycol, it can be represented by the following general formula 1. H(OCH2CH2) n OH (general formula 1) [In the formula, n is an integer greater than or equal to 1.] Since the molecular weight is 194 for n=4 and 1030 for n=23, it is thought that polyethylene glycols with molecular weights in the range of approximately 200 to 1000 can undergo dehydration condensation reactions with the free carboxyl groups of hydroxycarboxylic acids coordinated to cerium and gadolinium compounds to moderately polymerize and form a stable sol.

[0035] On the other hand, polypropylene glycol can be represented by the following general formula 2. H(OCHCH3CH2) n OH (general formula 2) Since the molecular weight is 192 for n=3 and 1004 for n=17, it is thought that polypropylene glycols with molecular weights in the range of approximately 200 to 1000 can undergo dehydration condensation reactions with the free carboxyl groups of hydroxycarboxylic acids coordinated to cerium and gadolinium compounds to moderately polymerize and form a stable sol.

[0036] Among these, tetraethylene glycol, pentaethylene glycol, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, polyethylene glycol 1000, tripropylene glycol, tetrapropylene glycol, polypropylene glycol 400, polypropylene glycol 700, and polypropylene glycol 1000 can be mentioned in terms of ease of availability and film-forming properties.

[0037] The seventh characteristic feature of the present invention is that, in the method for producing the above-mentioned paint composition, the organic solvent is one or more of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, and 2-methyl-2-propanol.

[0038] The organic solvent may be used alone or as a mixture of two or more organic solvents. It will make up the remainder of the other components (cerium compounds, gadolinium compounds, hydroxycarboxylic acids, polyalkylene glycols), but the content in the paint composition is preferably 50 to 90% by mass while satisfying the conditions for the other components. Here, if the organic solvent content is lower than 50% by mass, sufficient mixing performance may not be obtained. If it is higher than 90% by mass, there is a tendency for the raw material components to be insufficient. The alcohol does not need to be particularly limited, as long as it can dissolve the cerium compound and the gadolinium compound.

[0039] [Usage patterns of the gadolinium-doped layer] The following describes the case in which the gadolinium-doped ceria layer according to the present invention is used in an "electrochemical element".

[0040] In the present invention, the "electrochemical element" is configured by having a counter electrode layer on the opposite side of the electrode layer, with an electrolyte layer in between.

[0041] In this case, when this electrochemical element is used as a fuel cell, for example, the electrode layer can be used as the fuel electrode layer and the counter electrode layer as the air electrode layer, thereby generating electricity between the two electrodes. That is, by supplying a reducing gas (typically a fuel gas containing hydrogen) to the fuel electrode layer and an oxidizing gas (typically air containing oxygen) to the air electrode layer, the electrochemical element can be used as a single fuel cell. Conversely, when a predetermined power is supplied between the two electrodes, this electrochemical element can function as an electrolytic element (electrolysis element) that receives water and decomposes it. Hereafter, electrolysis may be simply referred to as electrolysis.

[0042] Therefore, in the present invention, when the electrochemical element is used as a fuel cell, the electrochemical device becomes a fuel cell device, and when the electrochemical element is used as an electrolysis cell, it becomes an electrolysis device.

[0043] The paint composition produced by the method for producing the paint composition according to the present invention can be used to produce a gadolinium-doped ceria layer in an easy manner, and therefore can be used at least in the production of the reaction-preventing layer in the present invention. Furthermore, gadolinium-doped ceria can also be used as a material for components used in electrochemical elements, such as a material for an intermediate layer that may be provided between an electrode layer and an electrolyte layer.

[0044] In other words, the eighth characteristic feature of the present invention is that the electrochemical element according to the present invention comprises a gadolinium-doped ceria layer obtained by curing the paint composition produced by the method for producing the paint composition according to the present invention.

[0045] The ninth characteristic feature of the present invention is that it is an electrochemical element equipped with a gadolinium-doped ceria layer according to the present invention.

[0046] Furthermore, the tenth characteristic feature of the present invention is that the electrochemical element has a metal support.

[0047] According to the above-described configuration, the electrochemical elements can be supported by a robust metal support, while also ensuring the required electrical conductivity between the electrochemical elements.

[0048] The eleventh characteristic feature of the present invention is that it is an electrochemical module in which multiple electrochemical elements are arranged in an assembled state.

[0049] According to the above-described configuration, since multiple electrochemical elements are arranged in a cluster, it is possible to obtain a compact, high-performance electrochemical module with excellent strength and reliability while suppressing material and processing costs.

[0050] The twelfth characteristic configuration of the present invention is that it is an electrochemical device having the above-mentioned electrochemical element or electrochemical module, and a fuel converter that supplies a gas containing a reducing gas to the electrochemical element or electrochemical module, or a fuel converter that converts the gas containing a reducing gas generated by the electrochemical element or electrochemical module.

[0051] According to the above characteristic configuration, when the electrochemical element or electrochemical module is operated as a fuel cell, hydrogen can be generated from natural gas or other fuels supplied using existing raw material supply infrastructure such as city gas by a fuel converter such as a reformer, and then supplied to the fuel cell. Furthermore, when the electrochemical element or electrochemical module is operated as an electrolytic cell, it can be used as an electrochemical device that, for example, generates hydrogen through the electrolysis of water and then reacts it with carbon monoxide or carbon dioxide in a fuel converter to convert it into methane or other substances.

[0052] The thirteenth characteristic feature of the present invention is that it is an electrochemical device having at least the above-mentioned electrochemical element or electrochemical module, and a power converter that extracts power from the electrochemical element or electrochemical module, or a power converter that supplies power to the electrochemical element or electrochemical module.

[0053] According to the above characteristic configuration, the power converter can convert the power extracted from the electrochemical element or electrochemical module for external use, or supply power to both from an external source to use the electrochemical element or electrochemical module for electrolysis. Furthermore, it is preferable to use an inverter as the power converter, as the inverter can boost the voltage or convert DC to AC, making it easier to utilize the electrical output obtained from the electrochemical module. On the other hand, when used for electrolysis, DC can be obtained from the AC power source to construct an electrochemical apparatus that performs electrolysis.

[0054] The fourteenth characteristic feature of the present invention is that it is an energy system having the above-mentioned electrochemical apparatus and a waste heat utilization unit that reuses the heat discharged from the electrochemical apparatus.

[0055] According to the above-described configuration, since it has an electrochemical device and a waste heat utilization unit that reuses the heat discharged from the electrochemical device, it is possible to realize an energy system that is excellent in durability, reliability, and performance, as well as energy efficiency. Furthermore, it is also possible to realize an energy-efficient hybrid system by combining it with a power generation system that generates electricity using the combustion heat of unused fuel gas discharged from the electrochemical device.

[0056] Furthermore, the 15th characteristic feature of the present invention is that it is a solid oxide fuel cell equipped with the above-mentioned electrochemical element, which generates an electricity reaction.

[0057] On the other hand, the sixteenth characteristic feature of the present invention is that it is a solid oxide type electrolytic cell that includes the above-mentioned electrochemical element and generates an electrolytic reaction in the electrochemical element. [Brief explanation of the drawing]

[0058] [Figure 1] Diagram showing the manufacturing state of the paint composition. [Figure 2] Diagram showing the coating state of the paint composition. [Figure 3] Graph showing the XRD measurement results of the coating film in Example 1. [Figure 4] Graph showing the XRD measurement results of the coating film in Example 2. [Figure 5] Graph showing the XRD measurement results of the coating film in Example 3. [Figure 6] Graph showing the XRD measurement results of the coating film in Example 4. [Figure 7] Graph showing the XRD measurement results of the coating film in Example 5. [Figure 8] Graph showing the XRD measurement results of the coating film in Example 6. [Figure 9] Graph showing the XRD measurement results of the coating film in Example 7. [Figure 10] Graph showing the XRD measurement results of the coating film in Example 8. [Figure 11] Graph showing the XRD measurement results of the coating film in Example 9. [Figure 12] Graph showing the XRD measurement results of the coating film in Example 10. [Figure 13] Graph showing the XRD measurement results of the coating film in Example 11. [Figure 14] Graph showing the XRD measurement results of the coating film in Example 12. [Figure 15] Cross-sectional view of a key component showing an example of an electrochemical element configuration. [Figure 16] A diagram showing an example configuration of an electrochemical module. [Figure 17] This diagram shows an example configuration of an electrochemical device that functions as a fuel cell. [Figure 18] Cross-sectional view of key components showing an alternative configuration example of an electrochemical element. [Figure 19] This diagram shows an alternative usage configuration in which an electrochemical element is used in the electrolytic reaction section. [Figure 20] A diagram showing another embodiment comprising an electrolytic reaction section, a reverse water-gas shift reaction section, and a hydrocarbon synthesis reaction section. [Modes for carrying out the invention]

[0059] The present invention will be described in detail below based on examples, but the present invention is not limited to these examples. Figure 1 shows the manufacturing state of the coating composition b according to the present invention, and Figure 2 shows an example of its application state.

[0060] As previously explained, the paint composition b produced by the method for producing the paint composition according to the present invention is composed of a mixture of cerium compound o, gadolinium compound p, hydroxycarboxylic acid q, polyalkylene glycol r, and organic solvent s, and contains these.

[0061] Here, the cerium compound o is a starting material for the coating film that will become the gadolinium-doped ceria (GDC) layer, and examples include cerium nitrate, cerium chloride, cerium sulfate, cerium phosphate, cerium acetate, cerium carbonate, etc. The content of this cerium compound o in the paint composition b is preferably 5 to 20% by mass. The following examples and comparative examples show cases in which cerium nitrate is used.

[0062] Examples of gadolinium compounds p include gadolinium nitrate, gadolinium chloride, gadolinium sulfate, gadolinium phosphate, gadolinium acetate, and gadolinium carbonate. The content of gadolinium compound p in the paint composition b is preferably 0.5 to 2% by mass. The following examples and comparative examples show cases in which gadolinium nitrate is used.

[0063] Examples of hydroxycarboxylic acid q include 2-hydroxypropanediic acid (tartonic acid), 2-hydroxybutanediic acid (malic acid), 2,3-dihydroxybutanediic acid (tartaric acid), 3-hydroxypentanediic acid (3-hydroxyglutaric acid), 2,4-dihydroxypentanediic acid, 2,3,4-trihydroxypentanediic acid, 3-hydroxyhexandic acid, 2,3,4,5-tetrahydroxyhexandic acid (galactaric acid), 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), and 2-hydroxypropane-1,2,3-tricarboxylic acid (isocitric acid). The amount of hydroxycarboxylic acid q in paint composition b is preferably about 0.5 to 2 moles per mole of the total amount of cerium compound and gadolinium compound. Furthermore, the content of hydroxycarboxylic acid q in paint composition b is preferably 1 to 10% by mass. The following examples and comparative examples show the use of 2-hydroxybutaneoic acid (L-(-)-malic acid), 2,3-dihydroxybutaneoic acid (L-(+)-tartaric acid), and 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid).

[0064] Examples of polyalkylene glycols (r) include tetraethylene glycol, pentaethylene glycol, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, polyethylene glycol 1000, tripropylene glycol, tetrapropylene glycol, polypropylene glycol 400, polypropylene glycol 700, and polypropylene glycol 1000. Furthermore, the content of polyalkylene glycol r in the paint composition b is preferably 1 to 10% by mass. The following examples and comparative examples show the use of polyethylene glycol 200, polyethylene glycol 400, and polyethylene glycol 600.

[0065] Examples of organic solvents s include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, and 2-methyl-2-propanol. The amount of organic solvent s represents the remainder of the other components. The following examples and comparative examples show the use of ethanol, 2-propanol, 1-butanol, and a mixed solvent of ethanol and 1-butanol.

[0066] [Examples, Comparative Examples] Examples 1 to 11 and Comparative Examples 1 to 4 shown below are examples in which, as shown in Figure 2, the coating composition b according to the present invention was applied to the upper surface of a test piece B (50 × 50 × 1 mm, yttria-stabilized zirconia pellet substrate (simulating electrolyte layer 4)), and heat-treated at a predetermined temperature and time to evaluate whether the target layer, a gadolinium-doped ceria (GDC) layer, could be obtained. As will be described later, the gadolinium-doped ceria (GDC) layer can be used as a reaction prevention layer 5. It can also be used as an intermediate layer 3 (see Figure 15).

[0067] Figure 1 shows an example of mixing, where raw materials (cerium compound o, gadolinium compound p, hydroxycarboxylic acid q, polyalkylene glycol r, organic solvent s) are placed in a glass container V1 and mixed with a magnetic stirrer W1. Figure 2 shows the application of paint composition b to a predetermined surface using an air spray X. In the following descriptions of the examples and comparative examples, the reference numerals shown in the drawings have been omitted to clarify the names and amounts of each raw material used.

[0068] (Example 1) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 1.00 g of polyethylene glycol (average molecular weight 200), 19.73 g (25 ml) of ethanol, and 20.25 g (25 ml) of 1-butanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 46.39 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 700°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 3. Figures 3 to 13 below show the X-ray diffraction results of the examples. In Figures 3 to 13, "●" indicates the peak of gadolinium-doped ceria.

[0069] (Example 2) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 1.00 g of polyethylene glycol (average molecular weight 200), 19.73 g (25 ml) of ethanol, and 20.25 g (25 ml) of 1-butanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 46.39 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 800°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 4.

[0070] (Example 3) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 1.00 g of polyethylene glycol (average molecular weight 200), 19.73 g (25 ml) of ethanol, and 20.25 g (25 ml) of 1-butanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 46.39 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 900°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 5.

[0071] (Example 4) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 1.00 g of polyethylene glycol (average molecular weight 200), 19.73 g (25 ml) of ethanol, and 20.25 g (25 ml) of 1-butanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 46.39 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 6.

[0072] (Example 5) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 1.00 g of polyethylene glycol (average molecular weight 200), and 39.45 g (50 ml) of ethanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 45.86 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 7.

[0073] (Example 6) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 0.95 g of citric acid monohydrate, 0.90 g of polyethylene glycol (average molecular weight 200), and 39.0 g (50 ml) of 2-propanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 45.25 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 8.

[0074] (Example 7) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 1.00 g of polyethylene glycol (average molecular weight 200), and 40.5 g (50 ml) of 1-butanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 46.91 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 9.

[0075] (Example 8) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 2.00 g of polyethylene glycol (average molecular weight 400), 19.73 g (25 ml) of ethanol, and 20.25 g (25 ml) of 1-butanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 47.39 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 400) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 10.

[0076] (Example 9) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 3.00 g of polyethylene glycol (average molecular weight 600), 19.73 g (25 ml) of ethanol, and 20.25 g (25 ml) of 1-butanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 48.39 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 600) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 11.

[0077] (Example 10) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 0.75 g of L-(+)-tartaric acid, 1.00 g of polyethylene glycol (average molecular weight 200), and 39.45 g (50 ml) of ethanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 45.65 g of paint composition. The paint composition was prepared by mixing 0.5 moles of L-(+)-tartaric acid and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 12.

[0078] (Example 11) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 0.67 g of L-(-)-malic acid, 1.00 g of polyethylene glycol (average molecular weight 200), and 39.45 g (50 ml) of ethanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 45.48 g of paint composition. The paint composition was prepared by mixing 0.5 moles of L-(-)-malic acid and 0.5 moles of polyethylene glycol (average molecular weight 200) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 13.

[0079] (Example 12) In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 0.95 g of citric acid monohydrate, 0.87 g of tripropylene glycol (molecular weight 192), 19.73 g (25 ml) of ethanol, and 20.25 g (25 ml) of 1-butanol were mixed and stirred with a magnetic stirrer for 3 hours to prepare 46.15 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of tripropylene glycol (molecular weight 192) for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate. Separately, the paint composition was applied to a test piece using an air spray, and a coating film was prepared by heat treatment at 80°C for 30 minutes. Furthermore, a coating film was prepared by holding it at 1000°C for 60 minutes. The coating film showed no damage such as cracks or peeling, and the film thickness was approximately 2 μm. X-ray diffraction analysis of the coating film revealed clear peaks of gadolinium-doped ceria at 2θ values ​​of 28.5, 33.0, 47.4, 56.3, 59.0, 69.3, 76.6, and 79.0, as shown in Figure 14.

[0080] (Comparative Example 1) Comparative Example 1 is an example that reproduces the method disclosed in Non-Patent Document 1. In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.00 g of acetylacetone, 3.60 g of acetic acid, 0.22 g of polyvinylpyrrolidone (molecular weight 360,000), 19.73 g (25 ml) of ethanol, and 27.75 g (25 ml) of ethylene glycol were mixed and stirred at 100°C for 20 hours using a hot magnetic stirrer as described in Non-Patent Literature 1 to prepare 56.66 g of paint composition. A portion of the paint composition gelled and did not become a uniform solution. Based on Non-Patent Document 1, the paint composition was prepared with a ratio of 1 mole of acetylacetone and 1 mole of acetic acid for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Polyvinylpyrrolidone (molecular weight 360,000) was added at a concentration of 5% by mass of the total mass of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate. Separately, the paint composition was applied to a test piece using an air spray and heat-treated at 80°C for 30 minutes. However, because ethylene glycol (boiling point 197°C) was used as the solvent, it could not be dried. Therefore, when heat-treated at 200°C for 30 minutes, the entire surface of the coating film became rough and uneven. Subsequently, when baked at 500°C for 30 minutes as described in Non-Patent Document 1, cracks appeared and peeling was observed across the entire surface of the coating film. In this comparative example 1, polyethylene glycol (boiling point 197°C) was used as the solvent, It is believed that the need to dry at 200°C led to a deterioration of the coating film quality. Furthermore, the addition of polyvinylpyrrolidone as a solubilizing agent caused strong bonding between the amide carbonyl group of polyvinylpyrrolidone and the cerium and gadolinium ions, resulting in a rough, uneven surface across the entire coating. This likely led to cracking and peeling of the coating film during 500°C firing.

[0081] (Comparative Example 2) This Comparative Example 2 is an example that reproduces the method disclosed in Non-Patent Document 2. 3.35 g of cerium(III) chloride heptahydrate, 0.37 g of gadolinium(III) chloride hexahydrate, 0.95 g of citric acid monohydrate, and 39.45 g (50 ml) of ethanol were mixed in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to prepare 44.75 g of paint composition. Based on Non-Patent Document 2, the paint composition was prepared with a ratio of 0.5 moles of citric acid monohydrate to 1 mole of the total amount of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate. Separately, the paint composition was applied to a test piece using an air spray, but the coating was repelled from the substrate and did not form a uniform coating. When heat treatment was performed at 100°C for 15 minutes as described in Non-Patent Literature 2, the coating peeled off easily with just a light touch of a finger after drying. Since peeling of the coating was observed at the 100°C drying stage, firing was not performed. In this comparative example 2, since polyalkylene glycol was not used, the polymerization (solification) did not proceed sufficiently, which is thought to have caused the coating film to be repelled from the substrate and peel off after drying.

[0082] (Comparative Example 3) Comparative Example 3 is an example that reproduces the method disclosed in Non-Patent Document 3. In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 4.20 g of citric acid monohydrate, 2.48 g of ethylene glycol, and 50.0 g (50 ml) of deionized water were mixed and stirred at 70°C for 3 hours using a hot magnetic stirrer to prepare 61.05 g of paint composition. Based on Non-Patent Document 3, the paint composition was prepared with a ratio of 2 moles of citric acid monohydrate and 4 moles of ethylene glycol for every 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate combined. Separately, the paint composition was applied to a test piece using an air spray, but the coating repelled and did not form a uniform film. When heat-treated at 80°C for 30 minutes, a liquid, presumably ethylene glycol, remained undried. Further baking at 700°C for 1 hour caused the coating film to turn into a powder, and it peeled off easily with just a light touch of a finger. In this comparative example 3, the use of deionized water as the solvent resulted in an uneven coating film during spray painting. Furthermore, it is believed that the presence of unreacted ethylene glycol with the metal citrate caused the coating film to become brittle and peel off during 700°C firing.

[0083] (Comparative Example 4) Comparative Example 4 is an example in which ethylene glycol was used instead of polyethylene glycol 200. In a 150 ml glass container, 3.91 g of cerium(III) nitrate hexahydrate, 0.45 g of gadolinium(III) nitrate hexahydrate, 1.05 g of citric acid monohydrate, 0.31 g of ethylene glycol, and 39.45 g (50 ml) of ethanol were mixed and stirred for 3 hours using a magnetic stirrer to prepare 45.17 g of paint composition. The paint composition was prepared by mixing 0.5 moles of citric acid monohydrate and 0.5 moles of ethylene glycol with a total of 1 mole of cerium(III) nitrate hexahydrate and gadolinium(III) nitrate hexahydrate. Separately, when the paint composition was applied to a test piece using an air spray and heat-treated at 80°C for 30 minutes, a liquid, believed to be ethylene glycol, remained undried. Further baking at 700°C for 1 hour caused the coating film to become powdery, and the film peeled off easily with just a light touch of a finger. In this Comparative Example 4, since ethylene glycol monomer was used instead of polyethylene glycol 200 as in Example 5, it is believed that unreacted ethylene glycol with the metal citrate remained in the coating film, was not dried by the 80°C heat treatment, and the coating film became brittle and peeled off after firing at 700°C.

[0084] Using cerium compound o, gadolinium compound p, hydroxycarboxylic acid q, polyalkylene glycol r, and organic solvent s, combinations that yield a suitable coating film that forms a gadolinium-doped ceria layer include, in addition to the combinations described in Examples 1 to 11, the following combinations, for example. However, the present invention is not limited to these combinations. (a) Cerium(III) nitrate hexahydrate, gadolinium(III) nitrate hexahydrate, citric acid, polyethylene glycol 1000, ethanol, 1-butanol (b) Cerium(III) nitrate hexahydrate, gadolinium(III) nitrate hexahydrate, citric acid, polypropylene glycol 400, ethanol, 1-butanol (c) Cerium(III) nitrate hexahydrate, gadolinium(III) nitrate hexahydrate, citric acid, polypropylene glycol 700, ethanol, 1-butanol (d) Cerium(III) nitrate hexahydrate, gadolinium(III) nitrate hexahydrate, citric acid, polypropylene glycol 1000, ethanol, 1-butanol (e) Cerium(III) chloride heptahydrate, gadolinium(III) chloride heptahydrate, citric acid, polyethylene glycol 200, ethanol, 1-butanol (f) Cerium(III) acetate monohydrate, gadolinium(III) acetate tetrahydrate, citric acid, polyethylene glycol 200, ethanol, 1-butanol (g) Cerium(IV) sulfate monohydrate, gadolinium(III) sulfate octahydrate, citric acid, polyethylene glycol 200, ethanol, 1-butanol (h) Cerium(III) phosphate, gadolinium(III) phosphate, citric acid, polyethylene glycol 200, ethanol, 1-butanol (i) Cerium(III) chloride heptahydrate, gadolinium(III) chloride heptahydrate, tartaric acid, polyethylene glycol 400, ethanol, 1-butanol (j) Cerium(III) acetate monohydrate, gadolinium(III) acetate tetrahydrate, tartaric acid, polyethylene glycol 400, ethanol, 1-butanol (k) Cerium(IV) sulfate monohydrate, gadolinium(III) sulfate octahydrate, tartaric acid, polyethylene glycol 400, ethanol, 1-butanol (l) Cerium(III) phosphate, gadolinium(III) phosphate, tartaric acid, polyethylene glycol 400, ethanol

[0085] [Usage patterns of the gadolinium-doped layer] The following describes a solid oxide fuel cell that uses the gadolinium-doped ceria layer described above to form the electrochemical element E, with reference to Figures 15 to 18.

[0086] The electrochemical element E is used, for example, as a component of a solid oxide fuel cell that generates electricity by receiving a hydrogen-containing fuel gas and air to trigger a power generation reaction. Furthermore, in the following explanations related to the electrochemical element E, when describing the positional relationships of the layers, the reference layer for positional notation will be the electrolyte layer 4. The side of the counter electrode layer 6 relative to the electrolyte layer 4 will be referred to as "upper" or "upper side" (upper side in Figure 15), and the side of the electrode layer 2 will be referred to as "lower" or "lower side." In addition, the side of the metal substrate 1 on which the electrode layer 2 is formed (upper side in Figure 15) will be referred to as the "front side," and the opposite side (lower side in Figure 15) will be referred to as the "back side."

[0087] (Electrochemical element) As shown in Figure 15, the electrochemical element E comprises a metal substrate 1 (an example of a metal support), an electrode layer 2 formed on the metal substrate 1, an intermediate layer 3 formed on the electrode layer 2, and an electrolyte layer 4 formed on the intermediate layer 3. Furthermore, the electrochemical element E comprises a reaction prevention layer 5 formed on the electrolyte layer 4 and a counter electrode layer 6 formed on the reaction prevention layer 5. In other words, the counter electrode layer 6 is formed on the electrolyte layer 4, and the reaction prevention layer 5 is formed between the electrolyte layer 4 and the counter electrode layer 6. The electrode layer 2 and the counter electrode layer 6 are porous, while the electrolyte layer 4 is dense.

[0088] As explained previously, the main elemental components of each layer constituting the electrochemical element E are the electrolyte layer 4, the electrode layer 2 sandwiched between it, and the counter electrode layer 6. By providing these three layers, the electrochemical element E can be made to function.

[0089] (Metal substrate) The metal substrate 1 serves as a support to maintain the strength of the electrochemical element E by supporting the electrode layer 2, the intermediate layer 3, and the electrolyte layer 4, etc. A plate-shaped metal substrate 1 is used as this metal substrate, but other shapes such as box-shaped, cylindrical, or disc-shaped metal supports are also possible. Furthermore, the metal substrate 1 only needs to have sufficient strength to form the electrochemical element E as a support. For example, a substrate with a thickness of about 0.1 mm to 2 mm, preferably about 0.1 mm to 1 mm, and more preferably about 0.1 mm to 0.5 mm can be used.

[0090] The metal substrate 1 has a plurality of through holes 1a that penetrate through the front surface and the back surface. For example, the through holes 1a can be made in the metal substrate 1 by mechanical, chemical, or optical drilling. The through holes 1a have the function of allowing gas to pass from the back surface to the front surface of the metal substrate 1. It is also possible to use a porous metal to give the metal substrate 1 gas permeability. For example, the metal substrate 1 can be made of sintered metal or foamed metal. As the metal substrate material, a ferritic stainless steel material (an example of an Fe-Cr alloy) is used. Furthermore, as shown in Figure 15, a coating layer 1b may be formed on the outer surface of the metal substrate 1 (including the surface of the through holes 1a). This coating layer 1b can be a layer of metal oxide. For example, a layer of metal oxide can be formed by applying a Co coating treatment to an Fe-Cr alloy and then performing an oxidation treatment.

[0091] When an Fe-Cr alloy is used as the material for the metal substrate 1, the thermal expansion coefficient of this material becomes close to that of YSZ (yttria-stabilized zirconia) and GDC (gadolinium-doped ceria, also called CGO), which are used as materials for the electrode layer 2, intermediate layer 3, electrolyte layer 4, and reaction prevention layer 5. As a result, the electrochemical element E is less likely to be damaged even when subjected to repeated low-temperature and high-temperature cycles. Therefore, it is preferable because it is possible to realize an electrochemical element E with excellent long-term durability.

[0092] (electrode layer) As shown in Figure 15, the electrode layer 2 can be formed as a thin layer on the front surface of the metal substrate 1 in an area larger than the area where the through-holes 1a are provided. When the electrode layer 2 is formed as a thin layer, its thickness can be, for example, about 1 μm to 100 μm, preferably 5 μm to 50 μm. Such a thickness makes it possible to reduce the amount of expensive electrode layer material used, thereby lowering costs while ensuring sufficient electrode performance. The entire area where the through-holes 1a are provided is covered by the electrode layer 2. In other words, the through-holes 1a are formed inside the area on the metal substrate 1 where the electrode layer 2 is formed. To put it another way, all the through-holes 1a are provided facing the electrode layer 2.

[0093] For the electrode layer 2, composite materials such as NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, CuO-CeO2, and Cu-CeO2 can be used. In these examples, YSZ, GDC, and CeO2 can be called aggregates of the composite material. The electrode layer 2 is preferably formed by low-temperature firing (for example, a wet method using firing in a low temperature range without firing in a high temperature range higher than 1100°C), spray coating (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, or cold spray), PVD (such as sputtering or pulsed laser deposition), or CVD. These low-temperature processes allow for the creation of a good electrode layer 2 without using firing in a high temperature range higher than 1100°C. Therefore, it is preferable to use a low-temperature firing method because it avoids damaging the metal substrate 1 and suppresses elemental interdiffusion between the metal substrate 1 and the electrode layer 2, thereby realizing a highly durable electrochemical element E. Furthermore, using a low-temperature firing method is even preferable because it facilitates the handling of raw materials.

[0094] The electrode layer 2 has multiple pores on its interior and surface to allow gas permeability. In other words, the electrode layer 2 is formed as a porous layer. For example, the electrode layer 2 is formed so that its density is between 30% and less than 80%. The size of the pores can be appropriately selected to ensure that the electrochemical reaction proceeds smoothly. Density is the ratio of the material constituting the layer to the surrounding space, and can be expressed as (1 - porosity), and is equivalent to relative density.

[0095] (Middle class) As shown in Figure 15, the intermediate layer 3 can be formed as a thin layer on top of the electrode layer 2, covering the electrode layer 2. When the intermediate layer 3 is formed as a thin layer, its thickness can be, for example, about 1 μm to 100 μm, preferably about 2 μm to 50 μm, and more preferably about 4 μm to 25 μm. Such a thickness makes it possible to reduce the amount of expensive intermediate layer material used, thereby lowering costs while ensuring sufficient performance. As the material for the intermediate layer 3, for example, YSZ (yttria-stabilized zirconia), SSZ (scandia-stabilized zirconia), GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), SDC (samarium-doped ceria), etc. Ceria-based ceramics are particularly suitable.

[0096] The intermediate layer 3 is preferably formed by low-temperature firing (for example, a wet method using firing in a low temperature range without firing in a high temperature range higher than 1100°C), spray coating (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, and cold spray), PVD (such as sputtering and pulsed laser deposition), or CVD. These film formation processes, which can be used in a low temperature range, allow the intermediate layer 3 to be obtained without using firing in a high temperature range higher than 1100°C, for example. Therefore, elemental interdiffusion between the metal substrate 1 and the electrode layer 2 can be suppressed without damaging the metal substrate 1, and a highly durable electrochemical element E can be realized. Furthermore, using a low-temperature firing method is even more preferable because it simplifies the handling of raw materials.

[0097] The intermediate layer 3 preferably has oxygen ion (oxide ion) conductivity. It is even more preferable if it has mixed conductivity between oxygen ions (oxide ions) and electrons. An intermediate layer 3 having these properties is suitable for application to an electrochemical element E.

[0098] (electrolyte layer) As shown in Figure 15, the electrolyte layer 4 is formed as a thin layer on top of the intermediate layer 3, covering the electrode layer 2 and the intermediate layer 3. It can also be formed as a thin film with a thickness of 10 μm or less. Specifically, the electrolyte layer 4 is provided across (straddles) the intermediate layer 3 and the metal substrate 1. By configuring it in this way and bonding the electrolyte layer 4 to the metal substrate 1, the entire electrochemical element E can be made highly robust. Furthermore, the electrolyte layer 4 is provided on the front surface of the metal substrate 1 in a region larger than the region where the through-hole 1a is provided. In other words, the through-hole 1a is formed inside the region of the metal substrate 1 where the electrolyte layer 4 is formed.

[0099] Furthermore, gas leakage from the electrode layer 2 and intermediate layer 3 can be suppressed around the electrolyte layer 4. That is, when the electrochemical element E is used as a component of the SOFC, gas is supplied to the electrode layer 2 from the back side of the metal substrate 1 through the through hole 1a when the SOFC is operating. Gas leakage can be suppressed in the area where the electrolyte layer 4 is in contact with the metal substrate 1 without providing a separate component such as a gasket. In this embodiment, the electrolyte layer 4 completely covers the periphery of the electrode layer 2, but it is also possible to provide the electrolyte layer 4 on top of the electrode layer 2 and intermediate layer 3, and provide a gasket or the like around it.

[0100] As the material for the electrolyte layer 4, YSZ (yttria-stabilized zirconia), SSZ (scandia-stabilized zirconia), GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), SDC (samarium-doped ceria), LSGM (strontium-magnesium-doped lanthanum gallate), etc., can be used. Zirconia-based ceramics are particularly preferred. If the electrolyte layer 4 is made of zirconia-based ceramics, the operating temperature of the SOFC using the electrochemical element E can be made higher compared to ceria-based ceramics. For example, when using the electrochemical element E in an SOFC, if the electrolyte layer 4 is made of a material such as YSZ that can exhibit high electrolyte performance even in high temperature ranges of around 650°C or higher, and the system uses hydrocarbon-based raw fuels such as city gas or LPG as the raw fuel, and the raw fuel is converted into the anode gas of the SOFC by steam reforming, etc., a highly efficient SOFC system can be constructed in which the heat generated in the SOFC cell stack is used to reform the raw fuel gas.

[0101] The electrolyte layer 4 is preferably formed by a low-temperature firing method (a wet method using firing in a low temperature range without firing in a high temperature range exceeding 1100°C). For example, it can be formed by coating a liquid composition using an air spray method, bar coating method, dispenser method, brush application, or spatula application, and then firing it in a temperature range of 1100°C or lower. Alternatively, it can be formed by a spray coating method (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, or cold spray), a PVD method (such as sputtering or pulsed laser deposition), or a CVD method. These low-temperature film formation processes allow for the creation of a dense, airtight, and highly gas-barrier electrolyte layer 4 without using firing in a high temperature range exceeding 1100°C. This suppresses damage to the metal substrate 1 and inhibits elemental interdiffusion between the metal substrate 1 and the electrode layer 2, enabling the realization of an electrochemical element E with excellent performance and durability. In particular, low-temperature firing methods and spray coating methods are preferable because they enable the realization of low-cost devices. Furthermore, using low-temperature firing methods and spray coating methods is even preferable because a dense electrolyte layer 4 with high airtightness and gas barrier properties can be easily obtained in the low-temperature range.

[0102] The electrolyte layer 4 is densely constructed to shield against gas leaks of anode and cathode gases and to exhibit high ionic conductivity. The density of the electrolyte layer 4 is preferably 90% or higher, more preferably 95% or higher, and even more preferably 98% or higher. If the electrolyte layer 4 is a uniform layer, its density is preferably 95% or higher, and more preferably 98% or higher. Furthermore, if the electrolyte layer 4 is composed of multiple layers, it is preferable that at least a portion of it includes a layer with a density of 98% or higher (a dense electrolyte layer), and more preferably a layer with a density of 99% or higher (a dense electrolyte layer). Including such a dense electrolyte layer in a portion of the electrolyte layer 4 makes it easier to form a dense electrolyte layer 4 with high airtightness and gas barrier properties, even if the electrolyte layer 4 is composed of multiple layers.

[0103] (Reaction prevention layer) The reaction prevention layer 5 can be formed as a thin layer on the electrolyte layer 4. When the reaction prevention layer 5 is formed as a thin layer, its thickness can be, for example, about 1 μm to 100 μm, preferably about 2 μm to 50 μm, and more preferably about 4 μm to 25 μm. Such a thickness makes it possible to reduce the amount of expensive reaction prevention layer material used, thereby lowering costs while ensuring sufficient performance. The material of the reaction prevention layer 5 can be any material that can prevent the reaction between the components of the electrolyte layer 4 and the components of the counter electrode layer 6, but for example, ceria-based materials such as GDC (gadolinium-doped ceria) can be used. By introducing the reaction prevention layer 5 between the electrolyte layer 4 and the counter electrode layer 6, the reaction between the constituent materials of the counter electrode layer 6 and the constituent materials of the electrolyte layer 4 is effectively suppressed, and the long-term stability of the performance of the electrochemical element E can be improved. The formation of the reaction prevention layer 5 is preferably carried out using a method that can be performed at a processing temperature of 1100°C or lower, as this suppresses damage to the metal substrate 1 and inhibits elemental interdiffusion between the metal substrate 1 and the electrode layer 2, thereby realizing an electrochemical element E with excellent performance and durability. For example, this can be done using a low-temperature firing method (e.g., a wet method using firing in a low temperature range without firing in a high temperature range exceeding 1100°C), a spray coating method (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, or cold spray), a PVD method (such as sputtering or pulsed laser deposition), or a CVD method. In particular, using a low-temperature firing method or a spray coating method is preferable because it enables the realization of a low-cost element. Furthermore, using a low-temperature firing method is even preferable because it simplifies the handling of raw materials.

[0104] (Counter electrode layer) The counter electrode layer 6 can be formed as a thin layer on the electrolyte layer 4 or the reaction prevention layer 5. When the counter electrode layer 6 is formed as a thin layer, its thickness can be, for example, about 1 μm to 100 μm, preferably 5 μm to 50 μm. Such a thickness makes it possible to reduce the amount of expensive counter electrode layer material used, thereby lowering costs while ensuring sufficient electrode performance. As the material for the counter electrode layer 6, for example, composite oxides such as LSCF (lanthanum strontium cobalt ferrite) and LSM (lanthanum strontium manganate), ceria oxides, and mixtures thereof can be used. In particular, it is preferable that the counter electrode layer 6 contains a perovskite-type oxide containing two or more elements selected from the group consisting of La, Sr, Sm, Mn, Co, and Fe. The counter electrode layer 6 constructed using the above materials functions as a cathode.

[0105] Furthermore, it is preferable to form the counter electrode layer 6 using a method that can be performed at a processing temperature of 1100°C or lower, as this suppresses damage to the metal substrate 1 and inhibits elemental interdiffusion between the metal substrate 1 and the electrode layer 2, thereby realizing an electrochemical element E with excellent performance and durability. For example, this can be done using a low-temperature firing method (e.g., a wet method using firing in a low temperature range without firing in a high temperature range exceeding 1100°C), a spray coating method (such as thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, or cold spray), a PVD method (such as sputtering or pulsed laser deposition), or a CVD method as appropriate. In particular, using a low-temperature firing method or a spray coating method is preferable because it enables the realization of a low-cost element. Moreover, using a low-temperature firing method is even preferable because it simplifies the handling of raw materials.

[0106] As explained above, in the electrochemical element E according to the present invention, a GDC (gadolinium-doped ceria) layer can be used for the intermediate layer 3 and the reaction prevention layer 5, and by using the coating composition b according to the present invention, the intermediate layer 3 and the reaction prevention layer 5 can be formed inexpensively and efficiently. In another embodiment, a gadolinium-doped ceria layer obtained by curing the coating composition b of the present invention can be used in the electrolyte layer 4. Furthermore, a gadolinium-doped ceria layer obtained by curing the coating composition b of the present invention can also be used in part of the electrode layer 2 or the counter electrode layer 6.

[0107] (Operation as a solid oxide fuel cell) By configuring the electrochemical element E as described above, the electrochemical element E can be used as a fuel cell in a solid oxide fuel cell. For example, a reducing gas (typically a fuel gas containing hydrogen) is supplied to the electrode layer 2 through a through hole 1a from the back side of the metal substrate 1, and an oxidizing gas (typically air containing oxygen) is supplied to the counter electrode layer 6, which is the opposite electrode of the electrode layer 2, and the system is operated at a temperature of, for example, 600°C to 850°C. In this case, the oxygen O2 contained in the air in the counter electrode layer 6 will produce electrons e - It reacts with oxygen ions O 2- This is produced. The oxygen ion O 2- The hydrogen (H2) contained in the supplied fuel gas moves through the electrolyte layer 4 to the electrode layer 2 (see Figure 15). In the electrode layer 2, the hydrogen (H2) is converted into oxygen ions (O2). 2- It reacts with water (H2O) and electrons (e). - This is generated. The above reaction generates an electromotive force between electrode layer 2 and counter electrode layer 6. In this case, electrode layer 2 functions as the fuel electrode (anode) of the SOFC, and counter electrode layer 6 functions as the air electrode (cathode).

[0108] (Method of manufacturing electrochemical elements) Next, a method for manufacturing the electrochemical element E according to this embodiment will be described. The following description mainly explains an example in which a gadolinium-doped ceria layer is formed on the intermediate layer 3 and the reaction-preventing layer 5 using the coating composition b according to the present invention.

[0109] (Metal substrate preparation step) In the metal substrate preparation step, a plate material made of an Fe-Cr alloy of a predetermined shape is prepared, and a number of through holes 1a can be formed at predetermined positions on this plate material by laser processing or the like. Alternatively, this plate material may be subjected to a Co plating treatment, and then an oxidation treatment may be performed after the plating treatment to form a metal oxide layer containing Co. This metal oxide layer will be formed as a metal oxide layer 1b (coating layer) in the electrode layer formation step described later.

[0110] (Electrode layer formation step) In the electrode layer formation step, the electrode layer 2 is formed in a thin layer over an area wider than the area where the through-holes 1a are provided on the front surface of the metal substrate 1 obtained in the metal substrate preparation step. The through-holes 1a in the metal substrate 1 can be provided by laser processing or the like. As described above, the electrode layer 2 can be formed using methods such as low-temperature firing (wet firing method at a low temperature of 1100°C or lower), spray coating (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, etc.), or PVD (sputtering, pulsed laser deposition, etc.). In any case, it is desirable to perform the process at a temperature of 1100°C or lower to suppress deterioration of the metal substrate 1.

[0111] When the electrode layer formation step is performed by a low-temperature firing method, specifically, a material paste is prepared by mixing the material powder with a solvent (dispersion medium) and applied to the front surface of the metal substrate 1. Here, the coating composition b according to the present invention can also be used as part of this material paste. Then, the electrode layer 2 is compression molded (electrode layer smoothing step) and fired at a temperature of 1100°C or lower (electrode layer firing step). The compression molding of the electrode layer 2 can be carried out by methods such as CIP (Cold Isostatic Pressing), roll press molding, or RIP (Rubber Isostatic Pressing). Furthermore, the firing of the electrode layer 2 is preferably carried out at a temperature between 800°C and 1100°C. The order of the electrode layer smoothing step and the electrode layer firing step can also be reversed.

[0112] Furthermore, when forming an electrochemical element E having an intermediate layer 3, the electrode layer smoothing step and the electrode layer firing step can be omitted, or the electrode layer smoothing step and the electrode layer firing step can be included in the intermediate layer smoothing step and the intermediate layer firing step described later. The electrode layer smoothing step can also be performed by lapping, leveling, surface cutting, polishing, etc.

[0113] (Diffusion suppression layer formation step) The diffusion-suppressing layer is a metal oxide layer 1b (coating layer) formed on the surface of the metal substrate 1 during the firing process in the electrode layer formation step described above. It is preferable that the firing process includes a firing process in which the firing atmosphere is under conditions of low oxygen partial pressure, as this enhances the effect of suppressing the mutual diffusion of elements and forms a high-quality metal oxide layer 1b with low resistance. The electrode layer formation step may also include a separate diffusion-suppressing layer formation step, including a case where the electrode layer formation step is a coating method that does not involve firing. For example, in the separate diffusion-suppressing layer formation step, the metal oxide layer 1b is formed by coating the metal substrate 1 with Co and then performing an oxidation treatment. Alternatively, for example, in the separate diffusion-suppressing layer formation step, the metal oxide layer 1b can be formed by coating an intervening layer formed on the metal substrate 1 with Co and then performing an oxidation treatment. In all cases, it is desirable to carry out the process at a processing temperature of 1100°C or lower, which can suppress damage to the metal substrate 1. Furthermore, a metal oxide layer 1b (diffusion suppression layer) may be formed on the surface of the metal substrate 1 during the firing process in the intermediate layer formation step described later.

[0114] (Intermediate layer formation step) In the intermediate layer formation step, an intermediate layer 3 is formed on top of the electrode layer 2 in a thin layer, covering the electrode layer 2. As described above, the intermediate layer 3 can be formed using methods such as low-temperature firing (wet firing method at a low temperature of 1100°C or below), spray coating (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, etc.), PVD (sputtering, pulsed laser deposition, etc.), and CVD. In any case, it is desirable to perform the process at a temperature of 1100°C or below in order to suppress the deterioration of the metal substrate 1.

[0115] When the intermediate layer formation step is performed by low-temperature firing, the procedure is as follows: First, the material powder for the intermediate layer 3 and the solvent (dispersion medium) are mixed to create a material paste, which is then applied to the upper surface of the electrode layer 2. As the material for the intermediate layer 3, ceria-based ceramics such as GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), and SDC (samarium-doped ceria) are preferably used. Here, the paint composition b according to the present invention can be used in this material paste. The intermediate layer 3 is then compression-molded (intermediate layer smoothing step) and fired at 1100°C or below (intermediate layer firing step). The intermediate layer 3 can be molded by, for example, CIP (Cold Isostatic Pressing), roll press molding, or RIP (Rubber Isostatic Pressing). Furthermore, it is preferable to fire the intermediate layer 3 at a temperature of 800°C to 1100°C. This is because at such a temperature, a high-strength intermediate layer 3 can be formed while suppressing damage and deterioration of the metal substrate 1. It is even more preferable to fire the intermediate layer 3 at 1050°C or below, and even more preferable to fire it at 1000°C or below. This is because lowering the firing temperature of the intermediate layer 3 allows for the formation of the electrochemical element E while further suppressing damage and deterioration of the metal substrate 1. Furthermore, the order of the intermediate layer smoothing process and the intermediate layer firing process can be reversed. The intermediate layer smoothing process can also be performed by methods such as lapping, leveling, surface cutting, or polishing.

[0116] (Electrolyte layer formation step) In the electrolyte layer formation step, the electrolyte layer 4 is formed as a thin layer on the intermediate layer 3, covering the electrode layer 2 and the intermediate layer 3. Alternatively, it may be formed as a thin film with a thickness of 10 μm or less. As described above, the electrolyte layer 4 is preferably formed by a low-temperature firing method (a wet method that performs firing in a low-temperature range of 1100°C or less). However, it can also be formed by applying the paint composition b according to the present invention by an air spray method, bar coating method, dispenser method, brush application, or spatula application, and then firing it in a temperature range of 1100°C or less. Alternatively, methods such as spray coating (thermal spraying, aerosol deposition method, aerosol gas deposition method, powder jet deposition method, particle jet deposition method, cold spray method, etc.), PVD method (sputtering method, pulsed laser deposition method, etc.), and CVD method can be used. In any case, it is desirable to perform the process at a temperature of 1100°C or less in order to suppress deterioration of the metal substrate 1.

[0117] (Reaction prevention layer formation step) In the reaction prevention layer formation step, the reaction prevention layer 5 is formed as a thin layer on the electrolyte layer 4. As described above, the reaction prevention layer 5 can be formed using methods such as low-temperature firing, spray coating (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, etc.), PVD (sputtering, pulsed laser deposition, etc.), or CVD. In any case, it is desirable to perform the process at a temperature of 1100°C or lower to suppress deterioration of the metal substrate 1. In order to make the upper surface of the reaction prevention layer 5 flat, for example, leveling treatment, surface cutting or polishing may be performed after the formation of the reaction prevention layer 5, or press processing may be performed after wet formation and before firing.

[0118] When the reaction prevention layer formation step is performed by low-temperature firing, it is carried out specifically as shown in the following example. First, the material powder of the reaction prevention layer 5 and the solvent (dispersion medium) are mixed to create a material paste, which is then applied to the upper surface of the electrolyte layer 4. As the material for the reaction prevention layer 5, ceria-based ceramics such as GDC (gadolinium-doped ceria) are preferably used. Here, the paint composition b according to the present invention can be used as the material paste. The reaction prevention layer 5 is then compression molded (reaction prevention layer smoothing step) and fired at 1100°C or below (reaction prevention layer firing step). The reaction prevention layer 5 can be molded by, for example, CIP (Cold Isostatic Pressing), roll press molding, RIP (Rubber Isostatic Pressing), etc. Furthermore, the firing of the reaction prevention layer 5 is preferably carried out at a temperature of 800°C to 1100°C. This is because at such a temperature, a highly strong reaction prevention layer 5 can be formed while suppressing damage and deterioration of the metal substrate 1. Furthermore, it is more preferable to fire the reaction prevention layer 5 at 1050°C or below, and even more preferable to fire it at 1000°C or below. This is because lowering the firing temperature of the intermediate layer 3 allows for the formation of the electrochemical element E while further suppressing damage and deterioration of the metal substrate 1. Furthermore, the order of the reaction prevention layer smoothing process and the reaction prevention layer firing process can be reversed. The reaction prevention layer smoothing process can also be performed by methods such as lapping, leveling, surface cutting, or polishing.

[0119] (Counter electrode layer formation step) In the counter electrode layer formation step, the counter electrode layer 6 is formed as a thin layer on the reaction prevention layer 5. As described above, the counter electrode layer 6 can be formed using methods such as low-temperature firing, spray coating (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, etc.), PVD (sputtering, pulsed laser deposition, etc.), or CVD. In any case, it is desirable to perform the process at a temperature of 1100°C or lower to suppress deterioration of the metal substrate 1.

[0120] When the counter electrode layer formation step is performed by a low-temperature firing method, specifically, a material paste is prepared by mixing the material powder with a solvent (dispersion medium) and applied to the upper surface of the reaction prevention layer 5. Here, the coating composition b according to the present invention can also be used as part of this material paste.

[0121] In this manner, the electrochemical element E can be manufactured. Furthermore, by performing the electrode layer formation step and the intermediate layer formation step described above, an electrode layer substrate for a metal-supported electrochemical element can be manufactured. Furthermore, in the electrochemical element E, it is possible to omit either the intermediate layer 3 or the reaction prevention layer 5, or both. That is, it is also possible to have a configuration in which the electrode layer 2 and the electrolyte layer 4 are in contact, or a configuration in which the electrolyte layer 4 and the counter electrode layer 6 are in contact. In this case, the intermediate layer formation step and the reaction prevention layer formation step are omitted in the above-described manufacturing method.

[0122] [Electrochemical elements, electrochemical modules, electrochemical devices, energy systems] Using Figures 16 and 17, an example of a solid oxide fuel cell will be explained for the electrochemical element E, electrochemical module M, electrochemical apparatus Y, and energy system Z. In this form of electrochemical element E, as shown in Figure 16, a U-shaped member 7 is attached to the back surface of a metal substrate 1, and the metal substrate 1 and the U-shaped member 7 form a cylindrical support TS. The U-shaped member 7 (separator) can be made of the same material as the metal substrate 1 (an alloy material in which the base material is an Fe-Cr alloy and a composite oxide film of Co is formed on its outer surface). When the electrochemical element E is used as a fuel cell, the flow path formed between the metal substrate 1 and the U-shaped member 7 becomes the supply path for reducing gas (typically a fuel gas containing hydrogen). As shown in Figure 16, multiple electrochemical elements E are stacked with a current collector 26 in between to form an electrochemical module M. Here, stacking is just one example of assembly. The current collector 26 is joined to the counter electrode layer 6 of the electrochemical element E and to the U-shaped member 7, electrically connecting the two. Therefore, the electrochemical module M is electrically connected as a whole in its stacking direction. The current collector 26 can also be made of the same material as the metal substrate 1 (an alloy material in which the base material is an Fe-Cr alloy and a composite oxide coating containing Co is formed on its outer surface). The above-mentioned alloy material is used.

[0123] As shown in Figure 17, the electrochemical module M includes a gas manifold 17, a terminal member m1, and a current extraction section m2. Multiple electrochemical elements E, stacked with a current collector 26 in between as shown in Figure 16, have one open end of a cylindrical support TS connected to the gas manifold 17, and receive a supply of gas (in this example, reformed gas reformed by a reformer 34) from the gas manifold 17. The supplied gas flows through the inside of the cylindrical support TS and is supplied to the electrode layer 2 through the through-hole 1a of the metal substrate 1.

[0124] Figure 17 shows an overview of the energy system Z and the electrochemical apparatus Y. The energy system Z includes an electrochemical device Y and a heat exchanger 53 which serves as a waste heat utilization unit for reusing the heat discharged from the electrochemical device Y. The electrochemical apparatus Y comprises an electrochemical module M, a fuel supply unit having a desulfurizer 31 and a reformer 34 that supplies fuel gas containing reducing gas to the electrochemical module M, and an inverter 38 which serves as a power conversion unit that extracts power from the electrochemical module M. Therefore, this electrochemical apparatus Y is a power generation device that generates electricity when fuel is supplied. More specifically, the electrochemical apparatus Y includes a desulfurizer 31, a reformed water tank 32, a vaporizer 33, a reformer 34, a blower 35, a combustion unit 36, an inverter 38, a control unit 39, a storage container 40, and an electrochemical module M.

[0125] The desulfurizer 31 removes (desulfurizes) sulfur compounds contained in hydrocarbon raw fuels such as city gas. When sulfur compounds are present in the raw fuel, the desulfurizer 31 can suppress the effect of sulfur compounds on the reformer 34 or the electrochemical element E. The vaporizer 33 generates steam from reformed water supplied from the reformed water tank 32. The reformer 34 uses the steam generated in the vaporizer 33 to steam reform the raw fuel that has been desulfurized in the desulfurizer 31, generating a reformed gas containing hydrogen.

[0126] The electrochemical module M generates electricity by using reformed gas supplied from the reformer 34 and air supplied from the blower 35 to perform an electrochemical reaction. The combustion unit 36 ​​mixes the reaction exhaust gas discharged from the electrochemical module M with air and burns the combustible components in the reaction exhaust gas.

[0127] The electrochemical module M comprises a plurality of electrochemical elements E and a gas manifold 17. The plurality of electrochemical elements E are arranged in parallel and electrically connected to each other, with one end (lower end) of each electrochemical element E fixed to the gas manifold 17. The electrochemical elements E generate electricity by causing an electrochemical reaction between the reformed gas supplied through the gas manifold 17 and the air supplied from the blower 35.

[0128] The inverter 38 adjusts the output power of the electrochemical module M to the same voltage and frequency as the power received from the commercial grid (not shown). Therefore, this inverter 38 acts as a power converter that extracts power from the electrochemical element E or the electrochemical module M. The control unit 39 controls the operation of the electrochemical apparatus Y and the energy system Z.

[0129] The vaporizer 33, reformer 34, electrochemical module M, and combustion section 36 are housed in a storage container 40. The reformer 34 uses the heat of combustion generated by the combustion of the reaction exhaust gas in the combustion section 36 to reform the raw fuel.

[0130] The raw fuel is supplied to the desulfurizer 31 through the raw fuel supply line 42 by the operation of the booster pump 41. The reformed water from the reformed water tank 32 is supplied to the vaporizer 33 through the reformed water supply line 44 by the operation of the reformed water pump 43. The raw fuel supply line 42 then merges with the reformed water supply line 44 downstream of the desulfurizer 31, and the reformed water and raw fuel, which have merged outside the storage container 40, are supplied to the vaporizer 33 located inside the storage container 40.

[0131] The reformed water is vaporized into steam in the vaporizer 33. The raw fuel containing the steam generated in the vaporizer 33 is supplied to the reformer 34 through the steam-containing raw fuel supply line 45. In the reformer 34, the raw fuel is steam reformed, and reformed gas (which is a fuel gas containing hydrogen, the reducing gas described above) is produced, with hydrogen gas as the main component. The reformed gas produced in the reformer 34 is supplied to the gas manifold 17 of the electrochemical module M through the reformed gas supply line 46.

[0132] The reformed gas supplied to the gas manifold 17 is distributed to multiple electrochemical elements E and supplied to the electrochemical elements E from the lower end, which is the connection point between the electrochemical elements E and the gas manifold 17. The hydrogen in the reformed gas is mainly used for the electrochemical reaction in the electrochemical elements E. Therefore, this reformer 34 acts as a fuel converter, supplying gas containing reducing gas to the electrochemical elements E or electrochemical module M. The reaction exhaust gas, including the remaining hydrogen gas not used in the reaction, is discharged from the upper end of the electrochemical elements E to the combustion section 36.

[0133] The reaction exhaust gas is combusted in the combustion section 36 and discharged as combustion exhaust gas to the outside of the storage container 40 from the combustion exhaust gas outlet 50. A combustion catalyst section 51 (for example, a platinum-based catalyst) is placed at the combustion exhaust gas outlet 50 to burn and remove reducing gases such as carbon monoxide and hydrogen contained in the combustion exhaust gas. The combustion exhaust gas discharged from the combustion exhaust gas outlet 50 is sent to the heat exchanger 53 via the combustion exhaust gas discharge passage 52.

[0134] The heat exchanger 53 exchanges heat between the combustion exhaust gas generated by combustion in the combustion section 36 and the supplied chilled water to produce hot water. In other words, the heat exchanger 53 operates as a waste heat utilization unit that reuses the heat discharged from the electrochemical apparatus Y.

[0135] Alternatively, instead of a waste heat utilization unit, a reaction exhaust gas utilization unit (not shown in the diagram) may be provided that utilizes the reaction exhaust gas discharged (without combustion) from the electrochemical module M. The reaction exhaust gas contains residual hydrogen gas that was not used in the reaction at the electrochemical element E. In the reaction exhaust gas utilization unit, the residual hydrogen gas is used for heat utilization through combustion or power generation using fuel cells, etc., thereby enabling the efficient use of energy.

[0136] [Another embodiment of the electrochemical module] Figure 18 shows another embodiment of the electrochemical module M. In this alternative embodiment, the electrochemical module M is constructed by stacking the above-mentioned electrochemical elements E with inter-cell connecting members 71 in between.

[0137] The inter-cell connecting member 71 is a plate-shaped member that is conductive and impermeable to gases, and has mutually perpendicular grooves 72 formed on its front and back surfaces. The inter-cell connecting member 71 can be made of a metal such as stainless steel or a metal oxide.

[0138] As shown in Figure 18, when the electrochemical elements E are stacked with this inter-cell connecting member 71 in between, gas can be supplied to the electrochemical elements E through the groove 72. Specifically, one groove 72 becomes the first gas flow path 72a, supplying gas to the front side of the electrochemical element E, i.e., the counter electrode layer 6. The other groove 72 becomes the second gas flow path 72b, supplying gas to the electrode layer 2 from the back side of the electrochemical element E, i.e., the back surface of the metal substrate 1, through the through hole 1a.

[0139] When operating this electrochemical module M as a fuel cell, an oxidizing gas (typically air containing oxygen) is supplied to the first gas channel 72a, and a reducing gas (typically fuel gas containing hydrogen) is supplied to the second gas channel 72b. This causes the reaction to proceed in the electrochemical element E, generating electromotive force and electric current. The generated power is extracted from the electrochemical module M through the inter-cell connecting members 71 at both ends of the stacked electrochemical element E.

[0140] In this embodiment, grooves 72 perpendicular to each other are formed on the front and back surfaces of the inter-cell connecting member 71, but it is also possible to form grooves 72 parallel to each other on the front and back surfaces of the inter-cell connecting member 71.

[0141] [Another embodiment] (1) In the above embodiment, the electrochemical element E was used in a solid oxide fuel cell, but the electrochemical element E can also be used in a solid oxide electrolytic cell that generates an electrolytic reaction, or in an oxygen sensor that uses solid oxides.

[0142] The following describes an example of using the electrochemical element E as a solid oxide type electrolytic cell, with reference to the drawings. As previously shown, this example is one in which the electrochemical apparatus Y according to the present invention is a hydrocarbon production system 100, and by supplying predetermined raw material gas and electricity, the electrochemical element E is made to function as an electrolytic reaction unit 10. That is, by supplying water H2O and carbon dioxide CO2 to this electrolytic reaction unit 10, carbon monoxide CO and hydrogen H2 are obtained as raw materials for the synthesis of hydrocarbons by decomposing them. Then, hydrocarbons can be obtained in the hydrocarbon synthesis reaction unit 30.

[0143] Figure 19 is a system diagram showing the overall configuration of the hydrocarbon production system 100, and Figure 20 shows an example configuration of an electrolytic cell unit U that constitutes the hydrocarbon production system 100. As can be seen from Figure 19, the hydrocarbon production system 100 is composed of an electrolytic reaction section 10, a first catalytic reaction section 20, a second catalytic reaction section 30, a heavy hydrocarbon separation section 70 (shown as a CnHm separation section), a water separation section 80 (shown as a H2O separation section), and a carbon dioxide separation section 90 (shown as a CO2 separation section), in that order. In Figure 19, the electrolytic reaction section 10, the first catalytic reaction section 20, and the second catalytic reaction section 30 are depicted separately, but as shown in Figure 20, these sections 10, 20, and 30 can be provided as a single electrolytic cell unit U.

[0144] The electrolytic reaction section 10 is a section that electrolyzes at least a portion of the incoming gas, the first catalytic reaction section 20 is a reverse water-gas shift reaction section that performs a reverse water-gas shift reaction on at least a portion of the incoming gas, and the second catalytic reaction section 30 is a hydrocarbon synthesis reaction section that synthesizes at least a portion of the incoming gas into hydrocarbons. The hydrocarbons synthesized here are mainly CH4 (carbon hydrocarbons with 1 carbon atom), but may also include lower saturated hydrocarbons with 2 to 4 carbon atoms. Furthermore, hydrocarbons with a larger number of carbon atoms than the lower saturated hydrocarbons and that are not saturated are also synthesized. These heavy hydrocarbons can be collected and separated in the heavy hydrocarbon separation section 70 as the gas released from the second catalytic reaction section 30 cools.

[0145] The water separation section 80 and the carbon dioxide separation section 90 are parts that remove at least a portion of predetermined components (H2O and CO2, in that order) from the gas flowing inside. The components removed and recovered by these parts are returned to predetermined parts of the system for reuse via the water return path 81 and the carbon dioxide return path 91, as shown in the figure. The H2O and CO2 returned via the respective return paths 81 and 91 are shown above them. As a result, this hydrocarbon production system 100 functions as a carbon closed system that does not release CO2 outside the system. In the figure, the gas flowing into each part is shown before each part, and the gas released from that part is shown after it.

[0146] In the electrolytic reaction section 10, H2O and CO2 are introduced as raw material gases and are electrolyzed inside, causing H2O to decompose into H2 and O2, while some of the CO2 is decomposed into CO and O2 and released. The reaction is described as follows: 2H2O→2H2+O2 (formula 1) 2CO2→2CO+O2 (formula 2) These equations 1 and 2 are also shown inside the box representing the electrolytic reaction section 10 in Figure 19.

[0147] In the first catalytic reaction section 20 (reverse water-gas shift reaction section), at least H2 and CO2 are introduced, and a reverse water-gas shift reaction occurs inside, releasing CO2 to CO and H2 to H2O. The reaction is described as the following equilibrium reaction, but the reverse water-gas shift reaction is a reaction in which the reaction described in Equation 3 below proceeds to the right (the reaction proceeds in the direction in which CO2 and H2 react to produce CO and H2O). CO2+H2⇔CO+H2O (Formula 3) Equation 3 is also shown inside the box representing the first catalytic reaction section 20 (reverse water-gas shift reaction section) in Figure 19. The reverse water-gas shift catalyst cat1 used in the reaction is also schematically shown inside the box. As this type of reverse water-gas shift catalyst cat1, the inventors prefer a catalyst in which nickel and iron, or both, are supported as the catalytically active component ca1 (metal catalyst) on one or more support cb1 (metal oxide support) selected from ceria-based metal oxides, zirconia-based metal oxides, and alumina-based metal oxides.

[0148] In the second catalytic reaction section 30 (hydrocarbon synthesis reaction section), at least H2 and CO flow in. Hydrocarbons are then synthesized through catalytic reactions. For example, the reaction in which CH4 is synthesized from CO and H2 is described as the following equilibrium reaction, but the reaction in which CH4 is synthesized from CO and H2 is a reaction in which the reaction described in equation 4 below proceeds to the right (the reaction proceeds in the direction in which CO and H2 react to produce CH4 and H2O). CO+3H2⇔CH4+H2O (Formula 4) Equation 4 is also shown inside the box representing the second catalytic reaction section 30 (hydrocarbon synthesis reaction section) in Figure 19. The hydrocarbon synthesis catalyst cat2 used in the reaction is also schematically shown inside the box. As a hydrocarbon synthesis catalyst cat2 of this type, the inventors believe that a catalyst is preferable in which at least ruthenium is supported as the catalytic active component ca2 on a support cb2 (metal oxide support), such as alumina. Furthermore, the equilibrium reaction shown in (Equation 3) also occurs in this region. Furthermore, depending on the type of catalyst used in the second catalytic reaction section 30, it is possible to carry out the FT (Fischer-Tropsch) synthesis reaction, thereby enabling the synthesis of hydrocarbons such as ethane and propane from CO and H2.

[0149] The H2O generated in the water separation section 80 is separated and returned to the upstream side of the electrolytic reaction section 10 via the water return path 81 (water recycling line). The CO2 generated in the carbon dioxide separation section 90 is separated and returned to the upstream side of the electrolytic reaction section 10 via the carbon dioxide return path 91 (carbon dioxide recycling line). As a result, in this hydrocarbon production system 100, hydrocarbons are ultimately synthesized and can be supplied externally.

[0150] Figure 20 shows an electrolytic cell unit U having such an electrochemical element E in the electrolytic reaction section 10, and also including a reverse water-gas shift reaction section 20 and a hydrocarbon synthesis reaction section 30. Figure 20 is a diagram of the electrolytic cell unit U, including the direction of gas advection. Furthermore, in this figure, the intermediate layer 3 and reaction prevention layer 5 of the electrochemical element E in Figure 15 are omitted in order to clarify the electrochemical reaction system. In addition, in comparison with Figure 19, the heavy hydrocarbon separation section 70, the water separation section 80, and the carbon dioxide separation section 90 are omitted.

[0151] This electrolytic cell unit U is also composed of an electrochemical element E in which an electrode layer 2 and a counter electrode layer 6 are formed with an electrolyte layer 4 in between, a metal substrate 1 acting as a metal support that functions as a separator, a U-shaped member 7 acting as a supply path forming member, and a current collector member 26 acting as a supply path component, forming an electrode layer side gas supply path 7a and a counter electrode layer side gas supply path 26a. The electrode layer side gas supply path 7a is supplied with H2O and CO2, which are the targets of electrolysis. On the other hand, the counter electrode layer side gas supply path 26a is supplied with air g2(O2), which is an example of an oxygen-containing gas. By stacking (assembling) these electrolytic cell units U in the thickness direction of the unit, which is the left-right direction in Figure 20, an electrochemical module M can be constructed. As described above, the electrolyte layer 4 and the electrode layer 2 as a part thereof can be constructed using the coating composition b according to the present invention.

[0152] This alternative configuration is an example where the electrochemical element E acts as the electrolytic reaction section 10. As can be seen from Figure 20, DC power is supplied between the electrode layer 2 and the counter electrode layer 6. The illustrated example shows that power obtained from the AC power source 37 is converted to AC / DC by the inverter 38 and supplied to the electrochemical element E. Therefore, this inverter 38 becomes a power converter that supplies power to the electrochemical element E or, if they are integrated, to the electrochemical module M which is an assembly of them. However, the reverse water-gas shift catalyst cat1 is applied to the inner surface of the electrode layer-side gas supply passage 7a (the supply passage-side inner surface of the U-shaped member 7, the surface of the metal substrate 1 opposite to the surface on which the electrode layer 2 is formed, and the surfaces of the multiple through holes 1a). This coating layer 20b is shown by a thick solid line. Furthermore, the electrode layer-side gas supply passage 7a extends beyond the electrolytic reaction section 10, and the coating layer 20b is also provided on this extended side. Further on, the hydrocarbon synthesis catalyst cat2 is applied to provide a coating layer 30b, forming the hydrocarbon synthesis reaction section 30. As a result, in this configuration, the hydrocarbon synthesis reaction unit 30 becomes a power converter that supplies power to the electrochemical element E, or, if they are integrated, to the electrochemical module M which is an assembly thereof. Then, using the gas obtained in the electrolytic reaction section 10 and the reverse water-gas shift reaction section 20, hydrocarbons can be obtained in the hydrocarbon synthesis reaction section 30.

[0153] (2) In the above embodiment, the present invention was used in a metal-supported solid oxide fuel cell with a metal substrate 1 as the support. However, the present invention can also be used in an electrode-supported solid oxide fuel cell with an electrode layer 2 or a counter electrode layer 6 as the support, or in an electrolyte-supported solid oxide fuel cell with an electrolyte layer 4 as the support. In those cases, the electrode layer 2 or the counter electrode layer 6, or the electrolyte layer 4 can be made to the required thickness to obtain the function of a support.

[0154] (3) In the above embodiment, composite materials such as NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, CuO-CeO2, and Cu-CeO2 were used as the material for the electrode layer 2, and composite oxides such as LSCF and LSM were used as the material for the counter electrode layer 6. The electrochemical element E configured in this way can be used as a solid oxide fuel cell by supplying hydrogen gas to the electrode layer 2 to serve as the fuel electrode and supplying air to the counter electrode layer 6 to serve as the air electrode. It is also possible to modify this configuration so that the electrochemical element E can be used as an air electrode with the electrode layer 2 and as a fuel electrode with the counter electrode layer 6. That is, composite oxides such as LSCF and LSM are used as the material for the electrode layer 2, and composite materials such as NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, CuO-CeO2, and Cu-CeO2 are used as the material for the counter electrode layer 6. With the electrochemical element E configured in this way, air can be supplied to the electrode layer 2 to form an air electrode, and hydrogen gas can be supplied to the counter electrode layer 6 to form a fuel electrode, allowing the electrochemical element E to be used as a solid oxide fuel cell.

[0155] (4) In the above embodiment, an example was shown in which a composite oxide film containing Co is formed on the surface of the Fe-Cr alloy of the metal substrate 1 to suppress the volatilization of Cr and other elements from this material. However, one or more of the U-shaped member 7 as a separator, the gas manifold 17, the current collector 26, and the inter-cell connecting member 71 as an interconnector may be made of this type of alloy material. On the other hand, as this type of oxide film, a Co-only oxide film may be formed on the surface of the Fe-Cr alloy. This is effective against Cr volatilization.

[0156] Furthermore, the configurations disclosed in the above embodiments can be applied in combination with configurations disclosed in other embodiments, provided that no inconsistencies arise. Also, the embodiments disclosed herein are illustrative, and the embodiments of the present invention are not limited thereto; they can be modified as appropriate without departing from the object of the present invention. [Explanation of symbols]

[0157] 1. Metal substrate (metal support) 1a Through hole 2 electrode layer 3. Middle Class 4 Electrolyte layer 5. Reaction prevention layer 6 Counter electrode layer 7 U-shaped member (separator) 10 Electrolytic reaction section 17. Gas manifold (manifold) 20 First catalytic reaction section (reverse water-gas shift reaction section) 26 Current collector 30. Second catalytic reaction section (hydrocarbon synthesis reaction section, fuel converter) 31 Desulfurizer 32. Water treatment tank 33. Vaporizer 34. Reformer (Fuel Converter) 35 Blower 36 Combustion section 38. Inverter (Power Converter) 39 Control Unit 40 storage containers 41. Booster pump 42 Raw fuel supply route 43. Water reforming pump 44. Reconditioned water supply channel 45. Water vapor-containing raw fuel supply channels 46. ​​Reformed gas supply channels 50 Combustion exhaust gas outlet 51 Combustion catalyst section 52 Combustion exhaust gas discharge channel 53 Heat exchanger (waste heat utilization section) 70 Heavy hydrocarbon separation section 71 Inter-cell connecting component (interconnector) 72 Groove 72a First gas flow path 72b Second gas flow path 80 Water separation section 90 Carbon dioxide separation section 100 Hydrocarbon Production Systems E Electrochemical element M Electrochemical Module TS tubular support U Electrolytic Cell Unit Y Electrochemical apparatus Z Energy System o Cerium compounds p-gadolinium compounds q hydroxycarboxylic acid r Polyalkylene glycol s Organic solvent

Claims

1. A paint composition is manufactured by stirring and mixing all the components of a composition containing a cerium compound, a gadolinium compound, a hydroxycarboxylic acid, a polyalkylene glycol, and an organic solvent. A method for producing a paint composition in which a gadolinium-doped ceria layer is formed by curing the paint composition alone for use in an electrochemical element, The polyalkylene glycol is one or more of the following with a molecular weight in the range of 100 to 2000: tetraethylene glycol, pentaethylene glycol, polyethylene glycol 200, polyethylene glycol 400, polyethylene glycol 600, polyethylene glycol 1000, tripropylene glycol, tetrapropylene glycol, polypropylene glycol 400, polypropylene glycol 700, and polypropylene glycol 1000. A method for producing a paint composition, wherein the organic solvent is one or more of methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, and 2-methyl-2-propanol.

2. A method for producing the paint composition according to claim 1, wherein the paint composition contains 5 to 20% by mass of the cerium compound, 0.5 to 2% by mass of the gadolinium compound, 1 to 10% by mass of the hydroxycarboxylic acid, 1 to 10% by mass of the polyalkylene glycol, and the remainder of the organic solvent.

3. A method for producing a paint composition according to claim 1 or 2, wherein the cerium compound is one or more of cerium nitrate, cerium chloride, cerium sulfate, cerium phosphate, cerium acetate, and cerium carbonate.

4. A method for producing a paint composition according to any one of claims 1 to 3, wherein the gadolinium compound is one or more of gadolinium nitrate, gadolinium chloride, gadolinium sulfate, gadolinium phosphate, gadolinium acetate, and gadolinium carbonate.

5. A method for producing a paint composition according to any one of claims 1 to 4, wherein the hydroxycarboxylic acid is one or more of the following: 2-hydroxypropanediic acid (tartronic acid), 2-hydroxybutanediic acid (malic acid), 2,3-dihydroxybutanediic acid (tartaric acid), 3-hydroxypentanediic acid (3-hydroxyglutaric acid), 2,4-dihydroxypentanediic acid, 2,3,4-trihydroxypentanediic acid, 3-hydroxyhexanoic acid, 2,3,4,5-tetrahydroxyhexanoic acid (galactaric acid), 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), and 2-hydroxypropane-1,2,3-tricarboxylic acid (isocitric acid).

6. A method for producing a gadolinium-doped ceria layer, comprising curing the paint composition produced by the method for producing a paint composition according to any one of claims 1 to 5, thereby forming a gadolinium-doped ceria layer.

7. A method for manufacturing an electrochemical element having at least an electrode layer, an electrolyte layer, and a counter electrode layer, A method for manufacturing an electrochemical element, comprising curing a paint composition produced by the method for manufacturing a paint composition according to claim 1 to form a gadolinium-doped ceria layer as the electrolyte layer.

8. A method for manufacturing an electrochemical element having at least an electrode layer, an electrolyte layer, a counter electrode layer, and an intermediate layer, A method for producing an electrochemical element, comprising curing a paint composition produced by the method for producing a paint composition according to claim 1 to form a gadolinium-doped ceria layer as the intermediate layer.

9. A method for manufacturing an electrochemical element according to claim 7 or 8, wherein the paint composition is applied to a metal support and cured.

10. A method for manufacturing a solid oxide fuel cell, comprising providing the electrochemical element manufactured by the method for manufacturing an electrochemical element described in claim 7 or 8 to a solid oxide fuel cell that generates an electric power reaction with the electrochemical element.

11. A method for manufacturing a solid oxide electrolytic cell, comprising providing the electrochemical element manufactured by the method for manufacturing an electrochemical element described in claim 7 or 8 to a solid oxide electrolytic cell that generates an electrolytic reaction with the electrochemical element.

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