Method for producing coating composition, gadolinium-doped ceria layer, electrochemical element, electrochemical module, electrochemical device, energy system, solid oxide fuel cell, and solid oxide electrolysis cell

Abstract

Provided is a coating composition (b) with which a film can be formed at a low cost by a simple method using a cerium compound (o) and a gadolinium compound (p) as starting materials, and with which a dense gadolinium-doped ceria (GDC) layer can be obtained. A coating composition (b) is obtained that contains a cerium compound (o), a gadolinium compound (p), a hydroxycarboxylic acid (q), a polyalkylene glycol (r), and an organic solvent (s).

Description

Method for producing coating composition, gadolinium-doped ceria layer, electrochemical element, electrochemical module, electrochemical device, energy system, solid oxide fuel cell, and solid oxide electrolysis cell

[0001] The present invention relates to a method for producing a coating composition that can form a gadolinium-doped ceria (GDC) layer on the surface of various components by coating, and also to various functional components that are provided with a gadolinium-doped ceria (GDC) layer.

[0002] Conventionally, for example, in solid oxide fuel cells (SOFCs), it has been known to laminate a gadolinium-doped ceria (GDC) layer as a functional component such as an intermediate layer (reaction prevention layer) between a solid electrolyte (e.g., yttria-stabilized zirconia (YSZ)) and an air electrode (e.g., strontium-doped lanthanum cobalt oxide). The following techniques are available as methods for forming a ceria layer such as a gadolinium-doped ceria (GDC) layer.

[0003] Non-Patent Document 1 discloses a method for preparing a GDC coating solution by dissolving gadolinium nitrate hexahydrate, cerium nitrate hexahydrate, and polyvinylpyrrolidone (molecular weight 360,000) in a mixed solvent of ethanol and ethylene glycol, followed by the addition of acetylacetone as a chelating agent and acetic acid to promote dissolution of the precursor. Furthermore, the document discloses a gadolinium-doped ceria (GDC) film formed by spin-coating this GDC coating solution onto an yttria-stabilized zirconia (YSZ) film.

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

[0005] Non-Patent Document 3 discloses a method of dissolving cerium nitrate and gadolinium nitrate in ion-exchanged water, adding citric acid and ethylene glycol, and heating at 70° C. to obtain a viscous gel. This document also discloses that the obtained gel is heated to 250° C. and then to 700° C. to obtain GDC powder.

[0006] Kunho Lee, Juhyun Kang, Sangbeom Jin, Sanghum Lee, International Journal of Hydrogen Energy, 42, 6220-6230 (2017) F. Pinar Gokdemir, A. Evrim Saatci, Orhan Ozdemir, Bahadir Keskin, Kubilay Kutlu, Materials Science in Semiconductor Processing, 38, 300-305 (2015) 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 Technology, 18, 511-524 (2021)

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

[0008] (2) When ethylene glycol is used as a solvent in the coating solution, the boiling point of ethylene glycol is as high as 197°C, so the ethylene glycol may not dry and remain in the drying process after spray coating (usually at around 80 to 100°C).

[0009] (3) As will be explained later in a comparative example in comparison with the working examples, when polyvinylpyrrolidone is added to a coating solution, the amidocarbonyl groups of polyvinylpyrrolidone bond strongly with metal (cerium, gadolinium) ions, making the coating solution prone to gelation. If the coating is applied in this gelled state, the entire surface of the coating film will have a rough, granular texture. If the coating is baked in this state, the baked coating film may easily peel off even when lightly touched with a finger.

[0010] Furthermore, it has been found that the technology disclosed in Patent Document 2 has the following problems: (1) It is believed that a cerium salt of citric acid is produced in an ethanol solution in which cerium chloride and citric acid are dissolved. For this solution to polymerize, i.e., to form a sol, the only way to achieve this is through a condensation reaction between the free carboxyl groups of the cerium citrate and the hydroxyl groups of another cerium citrate molecule, and it is believed that the rate of sol formation is slow. While this document does not disclose coating film thickness, a single dip coating typically results in a coating film with a thickness on the order of 10 nm. Therefore, repeated dip coating is required to achieve a coating film with a thickness of 1 μm or more, which poses a problem in terms of productivity.

[0011] (2) As will be explained later in a comparative example in comparison with the examples, when this method is used, the coating solution is not sufficiently solated during spray coating, so that the coating film is repelled from the substrate and does not form a uniform coating film. After drying, the coating film may easily peel off even with a light touch of a finger, and the baking required after the formation of the GDC film may not be achieved.

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

[0013] (2) As will be explained later in a comparative example in comparison with the examples, when this method is used, because ion-exchanged water is used as a solvent, the coating film is repelled from the substrate when sprayed, 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 baked at 700°C in this state, the coating film had many defects such as cracks, and the coating film easily peeled off 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 commonly used silicon alkoxides, attempts to achieve a coating film thickness of 1 μm or more are prone to defects such as cracks and fractures on the surface. Furthermore, because the hydrolysis rate of cerium alkoxide is extremely fast, even if a gadolinium compound is added, the reaction is completed by the cerium alkoxide alone, raising concerns that the gadolinium will be unevenly distributed and a uniform gadolinium-doped ceria film will not be obtained. Furthermore, for example, cerium tetra(isopropoxide), which is sold as a reagent, is extremely expensive, costing approximately 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 an object of the present invention is to provide a coating composition that can be used to form a film at low cost by a simple method using a cerium compound such as cerium nitrate or a gadolinium compound such as gadolinium nitrate as a starting material, and that can give a dense gadolinium-doped ceria (GDC) film with a uniform composition.

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

[0017] The coating composition produced by this method contains a metal hydroxycarboxylic acid (cerium and gadolinium) salt formed by coordination with a cerium compound and a gadolinium compound, and a polyalkylene glycol capable of undergoing a dehydration condensation reaction with the free carboxyl group of the metal hydroxycarboxylic acid salt, thereby allowing for moderate polymerization, i.e., sol formation, within the coating composition. Therefore, a coating film of the desired thickness can be formed by a simple method of applying the coating composition, for example, using an air spray. The inventors' investigations have shown that the use of the coating composition of the present invention makes it possible to easily obtain a gadolinium-doped ceria film with few defects.

[0018] In a typical complex polymerization method, a monomer such as ethylene glycol or propylene glycol is used as a reagent for dehydration condensation with a hydroxycarboxylic acid. However, when a monomer is used, as in Comparative Example 4 described below, the rate of polymerization (sol formation) is slow when the monomer undergoes a dehydration condensation reaction with the free carboxyl groups of the hydroxycarboxylic acid coordinated to a cerium compound and a gadolinium compound, making it difficult to achieve sufficient polymerization (sol formation).

[0019] A second characteristic feature of the present invention is that in the method for producing a coating composition, the coating 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 is the organic solvent.

[0020] If the content of the cerium compound is less than 5% by mass, the raw material tends to be insufficient, delaying the production of the target product, whereas if it is more than 20% by mass, the quality of the GDC coating film may be deteriorated.

[0021] If the gadolinium compound content is lower than 0.5 mass%, the raw material tends to be insufficient, making it difficult to obtain the target product, whereas if it is higher than 2 mass%, the amount of gadolinium in the resulting GDC coating film may be excessive.

[0022] The hydroxycarboxylic acid coordinates with the cerium compound and the gadolinium compound to form a metal salt. If the hydroxycarboxylic acid content is less than 1 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, if the hydroxycarboxylic acid content is more than 10 mass %, the hydroxycarboxylic acid does not form a metal salt with the cerium compound and the gadolinium compound, and only the free hydroxycarboxylic acid and the polyalkylene glycol undergo a polycondensation reaction, which is not preferable.

[0023] Polyalkylene glycol is used as a reagent that undergoes dehydration condensation with the free carboxyl groups of the hydroxycarboxylic acid coordinated with the cerium compound and gadolinium compound to polymerize (solize). If the content is less than 1% by mass, the dehydration condensation reaction does not occur sufficiently, and the coating solution may not be sol-formed. If the content is more than 10% by mass, defects such as cracks may easily occur in the GDC coating film during drying and baking after coating, and the quality of the coating film may be reduced.

[0024] The organic solvent is used to dissolve the cerium compound, gadolinium compound, hydroxycarboxylic acid, and polyalkylene glycol to form a uniform composition.

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

[0026] That is, cerium compounds are the starting materials for the gadolinium-doped ceria. Among these, cerium nitrate, cerium chloride, and cerium acetate are preferred in terms of availability and reactivity. By combining these compounds with oxycarboxylic acids and polyalkylene glycols (described later), a gadolinium-doped ceria film free from cracking and peeling can be obtained.

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

[0028] That is, gadolinium compounds are the starting materials for the gadolinium-doped ceria. Among these, gadolinium nitrate, gadolinium chloride, and gadolinium acetate are preferred in terms of availability and reactivity. By combining these compounds with oxycarboxylic acids and polyalkylene glycols, which will be described later, a gadolinium-doped ceria film that is free from cracking and peeling can be obtained.

[0029] A fifth characteristic feature of the present invention is that, in the method for producing a coating composition, the hydroxycarboxylic acid is one or more of 2-hydroxypropanedioic acid (tartronic acid), 2-hydroxybutanedioic acid (malic acid), 2,3-dihydroxybutanedioic acid (tartaric acid), 3-hydroxypentanedioic acid (3-hydroxyglutaric acid), 2,4-dihydroxypentanedioic acid, 2,3,4-trihydroxypentanedioic acid, 3-hydroxyhexanedioic acid, 2,3,4,5-tetrahydroxyhexanedioic acid (galactaric acid), 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), and 2-hydroxypropane-1,2,3-tricarboxylic acid (isocitric acid).

[0030] That is, hydroxycarboxylic acid has a hydroxy group and a carboxyl group in the molecule. This hydroxycarboxylic acid coordinates with a cerium compound and a gadolinium compound to form a metal salt of hydroxycarboxylic acid. Furthermore, the free carboxyl (-COOH) group that is not coordinated with the metal of the hydroxycarboxylic acid metal salt undergoes a dehydration condensation reaction with the terminal hydroxy (-OH) group of the polyalkylene glycol to moderately polymerize and form a stable sol. Therefore, it is preferable that the hydroxycarboxylic acid molecule has two or more carboxyl groups and one or more hydroxy groups.

[0031] Among these, 2-hydroxybutanedioic acid (malic acid), 2,3-dihydroxybutanedioic acid (tartaric acid), 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid), and 2-hydroxypropane-1,2,3-tricarboxylic acid (isocitric acid) are preferred in terms of availability and coordination ability with cerium compounds and gadolinium compounds. These compounds form stable metal salts (chelate compounds) with cerium compounds and gadolinium compounds, and therefore, a gadolinium-doped ceria film of uniform composition can be obtained without cracking or peeling.

[0032] A sixth characteristic feature of the present invention is that in the method for producing a coating composition, the polyalkylene glycol is one or more of 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] That is, examples of polyalkylene glycols include polyethylene glycol and polypropylene glycol, and the molecular weight of the polyalkylene glycol is preferably about 100 to 2,000, more preferably about 200 to 1,000.

[0034] In the case of polyethylene glycol, it can be represented by the following general formula 1: H(OCH ２ CH ２ ) ｎ OH (general formula 1) [wherein n is an integer of 1 or more] When n = 4, the molecular weight is 194, and when n = 23, the molecular weight is 1030. Therefore, it is considered that polyethylene glycol having a molecular weight in the range of about 200 to about 1000 can undergo a dehydration condensation reaction with the free carboxyl groups of the hydroxycarboxylic acid coordinated to the cerium compound and gadolinium compound to be appropriately polymerized, thereby forming a stable sol.

[0035] On the other hand, polypropylene glycol can be represented by the following general formula 2: H(OCHCH ３ CH２ ) ｎ OH (General Formula 2) When n = 3, the molecular weight is 192, and when n = 17, the molecular weight is 1004. Therefore, it is believed that polypropylene glycol having a molecular weight in the range of about 200 to about 1000 undergoes a dehydration condensation reaction with the free carboxyl groups of the hydroxycarboxylic acid coordinated to the cerium compound and the gadolinium compound to be appropriately polymerized and form a stable sol.

[0036] Among these, in terms of ease of availability and film-forming properties, 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.

[0037] A seventh characteristic feature of the present invention is that, in the method for producing a coating 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 in combination of two or more organic solvents. The organic solvent is the remainder of the other components (cerium compound, gadolinium compound, hydroxycarboxylic acid, polyalkylene glycol), and its content in the coating composition is preferably 50 to 90% by mass, while satisfying the requirements of the other components. If the organic solvent content is less than 50% by mass, sufficient mixing performance may not be obtained. If the amount exceeds 90% by mass, the raw material components tend to be insufficient. The alcohol is not particularly limited, as long as it can dissolve the cerium compound and gadolinium compound.

[0039] [Use of Gadolinium-Doped Ceria Layer] Hereinafter, the case where the gadolinium-doped ceria layer according to the present invention is used in an "electrochemical element" will be described.

[0040] The "electrochemical element" in the present invention is configured by providing a counter electrode layer on the opposite side of an electrode layer with an electrolyte layer sandwiched therebetween.

[0041] When this electrochemical element is operated as a fuel cell, for example, the electrode layer serves as the fuel electrode layer and the counter electrode layer serves as the air electrode layer, thereby generating electric power 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 operated as a single fuel cell. Conversely, when a predetermined amount of electric power is supplied between the two electrodes, the electrochemical element can be operated as an electrolytic element (electrolysis element) that receives a supply of water and decomposes it. Hereinafter, 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, the electrolysis device becomes an electrolysis device.

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

[0044] That is, the eighth characteristic configuration of the present invention is that the electrochemical element of the present invention is provided with a gadolinium-doped ceria layer obtained by curing the coating composition produced by the method for producing a coating composition of the present invention.

[0045] A ninth characteristic feature of the present invention is an electrochemical element including the gadolinium-doped ceria layer according to the present invention.

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

[0047] According to the above characteristic configuration, the electrochemical elements can be supported by a robust metal support, and the required electrical conductivity between the electrochemical elements can be ensured.

[0048] An eleventh characteristic feature of the present invention is that the present invention is an electrochemical module in which a plurality of the electrochemical elements are arranged in a group.

[0049] According to the above characteristic configuration, a plurality of the above-mentioned electrochemical elements are arranged in a cluster, so that a compact, high-performance electrochemical module with excellent strength and reliability can be obtained while reducing material costs and processing costs.

[0050] A twelfth characteristic configuration of the present invention is an electrochemical device having the above-mentioned electrochemical element or the electrochemical module, and a fuel converter that supplies a gas containing a reducing gas to the electrochemical element or the electrochemical module, or a fuel converter that converts a gas containing a reducing gas generated in the electrochemical element or the 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 the like supplied using an existing raw fuel supply infrastructure such as city gas by a fuel converter such as a reformer, and the hydrogen can be distributed to the fuel cell. Also, when the electrochemical element or electrochemical module is operated as an electrolysis cell, for example, it can be an electrochemical device in which hydrogen generated by the electrolysis reaction of water is reacted with carbon monoxide or carbon dioxide in a fuel converter to convert it into methane or the like.

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

[0053] According to the above characteristic configuration, the power converter can convert the power extracted from the electrochemical element or electrochemical module and provide it for external use, or can supply power from the outside to both, allowing the electrochemical element or electrochemical module to be used for electrolysis. For example, if an inverter is used as the power converter, this is preferable because it can boost the voltage and convert direct current to alternating current, making it easier to utilize the electrical output obtained from the electrochemical module. On the other hand, when used for electrolysis, an electrochemical device can be constructed that obtains direct current from an alternating current power source and performs electrolysis.

[0054] A fourteenth characteristic feature of the present invention is an energy system including the electrochemical device and a waste heat utilization section that reuses heat discharged from the electrochemical device.

[0055] According to the above-mentioned characteristic configuration, since the electrochemical device and the exhaust heat utilization unit that reuses the heat discharged from the electrochemical device are included, it is possible to realize an energy system that is excellent in durability, reliability, performance, and energy efficiency. Furthermore, it is also possible to realize a hybrid system that is excellent in energy efficiency by combining the electrochemical device with a power generation system that generates power by utilizing the combustion heat of unused fuel gas discharged from the electrochemical device.

[0056] A fifteenth characteristic feature of the present invention is a solid oxide fuel cell comprising the electrochemical element described above, in which a power generation reaction occurs in the electrochemical element.

[0057] On the other hand, a sixteenth characteristic configuration of the present invention is a solid oxide electrolysis cell comprising the above electrochemical element, in which an electrolytic reaction occurs.

[0058] Graph showing the XRD measurement results of the coating film of Example 1. Graph showing the XRD measurement results of the coating film of Example 2. Graph showing the XRD measurement results of the coating film of Example 3. Graph showing the XRD measurement results of the coating film of Example 4. Graph showing the XRD measurement results of the coating film of Example 5. Graph showing the XRD measurement results of the coating film of Example 6. Graph showing the XRD measurement results of the coating film of Example 7. Graph showing the XRD measurement results of the coating film of Example 8. Graph showing the XRD measurement results of the coating film of Example 10 Graph showing the XRD measurement results of the coating film of Example 11 Graph showing the XRD measurement results of the coating film of Example 12 Cross-sectional view of a main part showing an example of the configuration of an electrochemical element Diagram showing an example of the configuration of an electrochemical module Diagram showing an example of the configuration of an electrochemical device that functions as a fuel cell device Cross-sectional view of a main part showing another example of the configuration of an electrochemical element Diagram showing another use form in which an electrochemical element is used in an electrolysis reaction section Diagram showing another embodiment equipped with an electrolysis reaction section, a reverse water gas shift reaction section, and a hydrocarbon synthesis reaction section

[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 process of coating composition b according to the present invention, and Figure 2 shows an example of the application state of the coating composition b.

[0060] As explained above, the coating composition b produced by the coating composition production method of the present invention contains a mixture of a cerium compound o, a gadolinium compound p, a hydroxycarboxylic acid q, a polyalkylene glycol r, and an organic solvent s.

[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 thereof include cerium nitrate, cerium chloride, cerium sulfate, cerium phosphate, cerium acetate, and cerium carbonate. The content of this cerium compound o in the coating composition b is preferably 5 to 20 mass %. In the examples and comparative examples described below, examples using cerium nitrate are shown.

[0062] Examples of gadolinium compound p include gadolinium nitrate, gadolinium chloride, gadolinium sulfate, gadolinium phosphate, gadolinium acetate, and gadolinium carbonate. The content of gadolinium compound p in coating composition b is preferably 0.5 to 2 mass %. In the examples and comparative examples described below, examples using gadolinium nitrate are shown.

[0063] Examples of hydroxycarboxylic acid q include 2-hydroxypropanedioic acid (tartronic acid), 2-hydroxybutanedioic acid (malic acid), 2,3-dihydroxybutanedioic acid (tartaric acid), 3-hydroxypentanedioic acid (3-hydroxyglutaric acid), 2,4-dihydroxypentanedioic acid, 2,3,4-trihydroxypentanedioic acid, 3-hydroxyhexanedioic acid, 2,3,4,5-tetrahydroxyhexanedioic 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 coating composition b is preferably about 0.5 to 2 moles per mole of the total amount of the cerium compound and the gadolinium compound. The content of hydroxycarboxylic acid q in coating composition b is preferably 1 to 10 mass%. In the examples and comparative examples described below, examples using 2-hydroxybutanedioic acid (L-(-)-malic acid), 2,3-dihydroxybutanedioic acid (L-(+)-tartaric acid), and 1-hydroxypropane-1,2,3-tricarboxylic acid (citric acid) will be shown.

[0064] Examples of polyalkylene glycol 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. The content of polyalkylene glycol r in coating composition b is preferably 1 to 10 mass%. In the examples and comparative examples described below, examples using polyethylene glycol 200, polyethylene glycol 400, and polyethylene glycol 600 are shown.

[0065] Examples of organic solvents include methanol, ethanol, 1-propanol, 2-propanol, 1-butanol, 2-methyl-1-propanol, 2-butanol, and 2-methyl-2-propanol. Organic solvents represent the remaining amount of other components. In the examples and comparative examples described below, examples are shown in which ethanol, 2-propanol, 1-butanol, and a mixed solvent of ethanol and 1-butanol are used.

[0066] Examples and Comparative Examples Examples 1 to 11 and Comparative Examples 1 to 4 shown below are examples in which, as shown in Figure 2, coating composition b according to the present invention was applied to the upper surface of test piece B (50 x 50 x 1 mm), an yttria-stabilized zirconia pellet substrate (simulating electrolyte layer 4), and heat treatment was performed at a predetermined temperature for a predetermined 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 reaction prevention layer 5. It can also be used as intermediate layer 3 (see Figure 15).

[0067] As an example of mixing, Figure 1 shows raw materials (cerium compound o, gadolinium compound p, hydroxycarboxylic acid q, polyalkylene glycol r, organic solvent s) being placed in a glass container V1 and mixed with a magnetic stirrer W1. Figure 2 shows the application of coating composition b to a predetermined surface using an air spray X. In the following explanations 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.

[0068] Example 1: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 46.39 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and then heat-treated at 80°C for 30 minutes to produce a coating film. The coating was then heated to 700°C for 60 minutes to produce a coating film. The coating film showed no cracks, peeling, or other damage, and had a thickness of approximately 2 μm. When the coating film was analyzed by X-ray diffraction, clear peaks of gadolinium-doped ceria were observed 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. The X-ray diffraction results of the examples are shown in Figures 3 to 13. In Figures 3 to 13, "●" indicates the peaks of gadolinium-doped ceria.

[0069] Example 2: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 46.39 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and then heat-treated at 80°C for 30 minutes to produce a coating film. The coating was then heated to 800°C for 60 minutes to produce a coating film. The coating film showed no cracks, peeling, or other damage, and had a thickness of 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 46.39 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and then heat-treated at 80°C for 30 minutes to produce a coating film. The coating was then heated to 900°C for 60 minutes to produce a coating film. The coating film showed no cracks, peeling, or other damage, and had a thickness of 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 46.39 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and then heat-treated at 80°C for 30 minutes to produce a coating film. The coating was then heated to 1000°C for 60 minutes to produce a coating film. The coating film showed no cracks, peeling, or other damage, and had a thickness of 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 45.86 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control by air spraying and heat-treated at 80°C for 30 minutes to produce a coating film. The mixture was then heated at 1000°C for 60 minutes to produce a coating film. The coating film was free of damage such as cracks or peeling, and had a thickness of approximately 2 μm. When the coating film was analyzed by X-ray diffraction, clear peaks of gadolinium-doped ceria were observed 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 45.25 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and heat-treated at 80°C for 30 minutes to produce a coating film. The mixture was then held at 1000°C for 60 minutes to produce a coating film. The coating film was free of damage such as cracks or peeling, and had a thickness of approximately 2 μm. When the coating film was analyzed by X-ray diffraction, clear peaks of gadolinium-doped ceria were observed 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 46.91 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and heat-treated at 80°C for 30 minutes to produce a coating film. The mixture was then held at 1000°C for 60 minutes to produce a coating film. The coating film had no damage such as cracks or peeling, and the thickness was approximately 2 μm. When the coating film was analyzed by X-ray diffraction, clear peaks of gadolinium-doped ceria were observed 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 47.39 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 400) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and then heat-treated at 80°C for 30 minutes to produce a coating film. The coating was then heated to 1000°C for 60 minutes to produce a coating film. The coating film showed no cracks, peeling, or other damage, and had a thickness of 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 48.39 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of polyethylene glycol (average molecular weight 600) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and then heat-treated at 80°C for 30 minutes to produce a coating film. The coating was then heated to 1000°C for 60 minutes to produce a coating film. The coating film showed no cracks, peeling, or other damage, and had a thickness of 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 45.65 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of L-(+)-tartaric acid and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control by air spraying and heat-treated at 80°C for 30 minutes to produce a coating film. The mixture was then held at 1000°C for 60 minutes to produce a coating film. The coating film was free of damage such as cracks or peeling, and had a thickness of approximately 2 μm. When the coating film was analyzed by X-ray diffraction, clear peaks of gadolinium-doped ceria were observed 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 45.48 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of L-(-)-malic acid and 0.5 moles of polyethylene glycol (average molecular weight 200) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control by air spraying and heat-treated at 80°C for 30 minutes to produce a coating film. The mixture was then held at 1000°C for 60 minutes to produce a coating film. The coating film was free of damage such as cracks or peeling, and had a thickness of approximately 2 μm. When the coating film was analyzed by X-ray diffraction, clear peaks of gadolinium-doped ceria were observed 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: 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 46.15 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of tripropylene glycol (molecular weight 192) per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray and heat-treated at 80°C for 30 minutes to produce a coating film. The coating was then heated to 1000°C for 60 minutes to produce a coating film. The coating film showed no cracks, peeling, or other damage, and had a thickness of 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 This Comparative Example 1 is an example of replicating the method disclosed in Non-Patent Document 1. 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 in a 150 ml glass container and stirred at 100°C for 20 hours using a hot magnetic stirrer as described in Non-Patent Document 1 to produce 56.66 g of a coating composition. A portion of the coating composition gelled, and a homogeneous solution was not obtained. Based on Non-Patent Document 1, the coating composition was formulated in a ratio of 1 mole of acetylacetone and 1 mole of acetic acid per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. The amount of polyvinylpyrrolidone (molecular weight 360,000) was 5% by mass of the total mass of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control by air spray and heat-treated at 80°C for 30 minutes. However, drying was not possible due to the use of ethylene glycol (boiling point 197°C) as a solvent. Therefore, when the coating was heat-treated at 200°C for 30 minutes, the entire surface of the coating film developed a rough, granular appearance. After that, when the coating was baked at 500°C for 30 minutes as described in Non-Patent Document 1, cracks occurred throughout the entire coating film, and peeling was observed. In Comparative Example 1, polyethylene glycol (boiling point 197°C) was used as the solvent, which required drying at 200°C, which is thought to have deteriorated the quality of the coating film. Furthermore, polyvinylpyrrolidone was added as a sol-forming agent, and the amide carbonyl groups of polyvinylpyrrolidone were strongly bonded to the cerium ions and gadolinium ions, giving the entire coating film a rough, granular appearance, which is thought to have led to cracks occurring all over the coating film when baked at 500°C, causing it to peel off.

[0081] Comparative Example 2 This Comparative Example 2 is an example of replicating 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 produce 44.75 g of a coating composition. Based on Non-Patent Document 2, the coating composition was formulated at a ratio of 0.5 moles of citric acid monohydrate per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray, but the coating was repelled from the substrate and did not form a uniform coating. After heat treatment at 100°C for 15 minutes as described in Non-Patent Document 2, the coating was easily peeled off with a light touch of a finger after drying. No baking was carried out because peeling of the coating film was observed when drying at 100° C. In Comparative Example 2, since polyalkylene glycol was not used, polymerization (sol formation) did not proceed sufficiently, and it is believed that this was the reason why the coating film was repelled from the substrate and peeled off after drying.

[0082] Comparative Example 3 This Comparative Example 3 is an example that reproduces the method disclosed in Non-Patent Document 3. 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 ion-exchanged water were mixed in a 150 ml glass container and stirred at 70°C for 3 hours using a hot magnetic stirrer to produce 61.05 g of a coating composition. Based on Non-Patent Document 3, the coating composition was formulated in a ratio of 2 moles of citric acid monohydrate and 4 moles of ethylene glycol per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control using an air spray, but repellency occurred in the coating film, resulting in an inconsistent coating film. When heat-treated at 80°C for 30 minutes, a liquid believed to be ethylene glycol remained without drying. Further baking at 700°C for 1 hour caused the coating film to turn powdery, and the coating film easily peeled off with a light touch of a finger. Because ion-exchanged water was used as the solvent in Comparative Example 3, a uniform coating film was not obtained when spray-applied. Furthermore, it is believed that the metal citrate and unreacted ethylene glycol remained, causing the coating film to become brittle upon baking at 700°C, resulting in peeling.

[0083] Comparative Example 4: In Comparative Example 4, ethylene glycol was used instead of polyethylene glycol 200. 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 in a 150 ml glass container and stirred for 3 hours using a magnetic stirrer to produce 45.17 g of a coating composition. The coating composition was mixed in a ratio of 0.5 moles of citric acid monohydrate and 0.5 moles of ethylene glycol per mole of the total amount of cerium (III) nitrate hexahydrate and gadolinium (III) nitrate hexahydrate. Separately, the coating composition was applied to a test piece as a control 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 turn powdery and peel off easily with just a light touch of a finger. In Comparative Example 4, monomeric ethylene glycol was used instead of polyethylene glycol 200 in Example 5, so unreacted ethylene glycol with the metal citrate remained in the coating film, which was not dried by heat treatment at 80°C, and the coating film became brittle upon baking at 700°C, causing peeling.

[0084] Examples of combinations that can be used to obtain a coating film that becomes a suitable coating composition b and a gadolinium-doped ceria layer using a cerium compound o, a gadolinium compound p, a hydroxycarboxylic acid q, a polyalkylene glycol r, and an organic solvent s include the combinations described in Examples 1 to 11, as well as the following combinations, although 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] [Use of Gadolinium-Doped Ceria Layer] Hereinafter, with reference to FIGS. 15 to 18 , a solid oxide fuel cell will be described in which the gadolinium-doped ceria layer described above is used to form an electrochemical element E, and further, the solid oxide fuel cell is configured to include this electrochemical element E.

[0086] The electrochemical element E is used, for example, as a component of a solid oxide fuel cell that generates electricity by inducing a power generation reaction when supplied with a fuel gas containing hydrogen and air. Hereinafter, when describing the positional relationship of layers in the explanation related to the electrochemical element E, the electrolyte layer 4 is used as the reference layer for describing the positions, and the side of the counter electrode layer 6 as viewed from the electrolyte layer 4 may be referred to as "top" or "upper side" (the upper side in FIG. 15 ), and the side of the electrode layer 2 may be referred to as "bottom" or "lower side." Furthermore, the side of the metal substrate 1 on which the electrode layer 2 is formed (the upper side in FIG. 15 ) may be referred to as the "front side," and the opposite side (the lower side in FIG. 15 ) may be referred to as the "back side."

[0087] 15 , the electrochemical element E has 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. The electrochemical element E further has 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, and the electrolyte layer 4 is dense.

[0088] As explained above, the main elements of the layers constituting the electrochemical element E are the electrolyte layer 4, the electrode layer 2 sandwiched between it, and the counter electrode layer 6, and the electrochemical element E can be operated by providing these three layers.

[0089] (Metal Substrate) The metal substrate 1 serves as a support that supports the electrode layer 2, intermediate layer 3, electrolyte layer 4, etc., and maintains the strength of the electrochemical element E. A plate-shaped metal substrate 1 is used as this metal substrate, but the metal support may have other shapes, such as a box shape, a cylinder shape, or a disk shape. The metal substrate 1 may be strong enough to serve as a support for forming the electrochemical element E, and may have a thickness of, for example, 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.

[0090] The metal substrate 1 has a plurality of through holes 1a extending from the front surface to the back surface. For example, the through holes 1a can be formed in the metal substrate 1 by mechanical, chemical, or optical drilling. The through holes 1a function to allow gas to pass from the back surface to the front surface of the metal substrate 1. Porous metal can also be used to impart gas permeability to the metal substrate 1. For example, sintered metal or foam metal can also be used for the metal substrate 1. Ferritic stainless steel (an example of an Fe—Cr alloy) can be used as the metal substrate material. Furthermore, as shown in FIG. 15 , a coating layer 1b may be formed on the outer surface of the metal substrate 1 (including the surfaces of the through holes 1a). This coating layer 1b can be a metal oxide layer. For example, a Fe—Cr alloy can be subjected to a Co coating process followed by an oxidation process to form a metal oxide layer.

[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) or GDC (gadolinium-doped ceria, also known as 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 susceptible to damage even when subjected to repeated temperature cycles of low and high temperatures. This is preferable because it allows for the realization of an electrochemical element E with excellent long-term durability.

[0092] (Electrode Layer) As shown in FIG. 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 1 a are formed. 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 reducing costs, while ensuring sufficient electrode performance. The entire area where the through holes 1 a are formed is covered by the electrode layer 2. In other words, the through holes 1 a are formed inside the area of ​​the metal substrate 1 where the electrode layer 2 is formed. In other words, all of the through holes 1 a are provided facing the electrode layer 2.

[0093] Examples of materials for the electrode layer 2 include NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, and CuO-CeO. ２ , Cu—CeO ２ Examples of such composite materials include YSZ, GDC, CeO ２ can be called the aggregate of the composite material. The electrode layer 2 is preferably formed by a low-temperature firing method (e.g., a wet method using firing in a low-temperature range without firing in a high-temperature range above 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. By using such a process that can be used in a low-temperature range, a good electrode layer 2 can be obtained without firing in a high-temperature range above 1100°C. This is preferable because it prevents damage to the metal substrate 1 and suppresses interdiffusion of elements between the metal substrate 1 and the electrode layer 2, thereby realizing an electrochemical element E with excellent durability. Furthermore, using a low-temperature firing method is even more preferable because it facilitates handling of the raw materials.

[0094] The electrode layer 2 has a plurality of pores inside and on its surface to provide gas permeability. That is, the electrode layer 2 is formed as a porous layer. For example, the electrode layer 2 is formed so that its density is 30% or more and less than 80%. The size of the pores can be appropriately selected so that the electrochemical reaction proceeds smoothly. Note that density is the proportion of the space occupied by the material constituting the layer, and can be expressed as (1 - porosity), and is equivalent to the relative density.

[0095] (Intermediate Layer) As shown in FIG. 15 , the intermediate layer 3 can be formed as a thin layer on 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, approximately 1 μm to 100 μm, preferably approximately 2 μm to 50 μm, and more preferably approximately 4 μm to 25 μm. This thickness reduces the amount of expensive intermediate layer material used, thereby reducing costs, while ensuring sufficient performance. Examples of materials that can be used for the intermediate layer 3 include YSZ (yttria-stabilized zirconia), SSZ (scandia-stabilized zirconia), GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), and SDC (samarium-doped ceria). Ceria-based ceramics are particularly preferred.

[0096] The intermediate layer 3 is preferably formed by a low-temperature firing method (e.g., a wet method using firing in a low-temperature range without firing in a high-temperature range above 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. These film-forming processes that can be used in low-temperature ranges allow the intermediate layer 3 to be obtained without firing in a high-temperature range above 1100°C, for example. Therefore, interdiffusion of elements between the metal substrate 1 and the electrode layer 2 can be suppressed without damaging the metal substrate 1, resulting in an electrochemical element E with excellent durability. Furthermore, using a low-temperature firing method is even more preferable because it facilitates handling of raw materials.

[0097] The intermediate layer 3 preferably has oxygen ion (oxide ion) conductivity. It is more preferable that the intermediate layer 3 has mixed conductivity of oxygen ions (oxide ions) and electrons. The intermediate layer 3 having these properties is suitable for application to the electrochemical element E.

[0098] (Electrolyte Layer) As shown in FIG. 15 , 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. It can also be formed as a thin film with a thickness of 10 μm or less. Specifically, the electrolyte layer 4 is provided over (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 electrochemical element E as a whole can be made to have excellent robustness. Furthermore, the electrolyte layer 4 is provided on the front surface of the metal substrate 1 in an area larger than the area in which the through-hole 1 a is provided. In other words, the through-hole 1 a is formed inside the area in the metal substrate 1 in which 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 an SOFC, gas is supplied to the electrode layer 2 from the back side of the metal substrate 1 through the through holes 1 a during SOFC operation. Gas leakage can be suppressed in the area where the electrolyte layer 4 contacts the metal substrate 1 without providing a separate member such as a gasket. Note that, although the electrolyte layer 4 completely covers the periphery of the electrode layer 2 in this embodiment, a configuration in which the electrolyte layer 4 is provided on top of the electrode layer 2 and intermediate layer 3 and a gasket or the like is provided around the periphery may also be adopted.

[0100] Examples of materials that can be used for the electrolyte layer 4 include YSZ (yttria-stabilized zirconia), SSZ (scandia-stabilized zirconia), GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), SDC (samarium-doped ceria), and LSGM (strontium-magnesium-doped lanthanum gallate). Zirconia-based ceramics are particularly suitable. Using zirconia-based ceramics for the electrolyte layer 4 allows the operating temperature of an SOFC using the electrochemical element E to be higher than that of ceria-based ceramics. For example, when the electrochemical element E is used in an SOFC, a material such as YSZ that can exhibit high electrolyte performance even at high temperatures of approximately 650°C or higher is used for the electrolyte layer 4. A hydrocarbon-based raw fuel, such as city gas or LPG, is used as the raw fuel for the system, and the raw fuel is converted into the SOFC anode gas by steam reforming or the like. This allows for the construction of a highly efficient SOFC system in which 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 a firing treatment in a low-temperature range that does not involve firing at a high temperature above 1100°C). For example, the electrolyte layer 4 can be formed by applying a liquid composition by air spraying, bar coating, dispenser coating, brushing, or spatula coating, followed by firing at a temperature below 1100°C. Alternatively, the electrolyte layer 4 is preferably 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. By using these film-forming processes that can be used in a low-temperature range, an electrolyte layer 4 that is dense and has high airtightness and gas barrier properties can be obtained without firing at a high temperature above 1100°C. Therefore, damage to the metal substrate 1 can be suppressed, and interdiffusion of elements between the metal substrate 1 and the electrode layer 2 can be suppressed, resulting in an electrochemical element E with excellent performance and durability. In particular, using a low-temperature firing method or a spray coating method is preferable because it allows for the realization of a low-cost element. Furthermore, using a low-temperature firing method or a spray coating method is even more preferable because it makes it easy to obtain a dense electrolyte layer 4 that is airtight and has high gas barrier properties at a low temperature.

[0102] The electrolyte layer 4 is densely configured to prevent gas leakage of the anode gas and the cathode gas and to exhibit high ionic conductivity. The density of the electrolyte layer 4 is preferably 90% or more, more preferably 95% or more, and even more preferably 98% or more. When the electrolyte layer 4 is a uniform layer, the density thereof is preferably 95% or more, and more preferably 98% or more. Furthermore, when the electrolyte layer 4 is configured in a multi-layer structure, it is preferable that at least a portion of the layers include a layer having a density of 98% or more (a dense electrolyte layer), and more preferably a layer having a density of 99% or more (a dense electrolyte layer). When such a dense electrolyte layer is included as part of the electrolyte layer 4, it is easy to form an electrolyte layer 4 that is dense and has high airtightness and gas barrier properties, even when the electrolyte layer 4 is configured in a multi-layer structure.

[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, approximately 1 μm to 100 μm, preferably approximately 2 μm to 50 μm, and more preferably approximately 4 μm to 25 μm. This thickness reduces the amount of expensive reaction prevention layer material used, thereby reducing costs, while ensuring sufficient performance. The material for the reaction prevention layer 5 may be any material that can prevent reactions between the components of the electrolyte layer 4 and the components of the counter electrode layer 6, such as ceria-based materials such as GDC (gadolinium-doped ceria). By introducing the reaction prevention layer 5 between the electrolyte layer 4 and the counter electrode layer 6, reactions between the constituent materials of the counter electrode layer 6 and the electrolyte layer 4 are effectively suppressed, thereby improving the long-term stability of the performance of the electrochemical element E. Forming the reaction prevention layer 5 using an appropriate method capable of forming the layer at a processing temperature of 1100°C or less is preferable because it can suppress damage to the metal substrate 1 and also suppress interdiffusion of elements between the metal substrate 1 and the electrode layer 2, thereby realizing an electrochemical element E with excellent performance and durability. For example, low-temperature firing (e.g., a wet method using firing at a low temperature range without firing at a high temperature range exceeding 1100°C), spray coating (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, etc.), PVD (sputtering, pulsed laser deposition, etc.), CVD, etc. can be appropriately used. In particular, low-temperature firing and spray coating methods are preferable because they allow for low-cost elements to be realized. Furthermore, low-temperature firing is even more preferable because it facilitates 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, approximately 1 μm to 100 μm, preferably 5 μm to 50 μm. This thickness reduces the amount of expensive counter electrode layer material used, thereby reducing costs, while ensuring sufficient electrode performance. Examples of materials that can be used for the counter electrode layer 6 include composite oxides such as lanthanum strontium cobalt ferrite (LSCF) and lanthanum strontium manganate (LSM), ceria-based oxides, and mixtures thereof. It is particularly 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 formed using these materials functions as a cathode.

[0105] The counter electrode layer 6 is preferably formed using a method capable of forming the counter electrode layer 6 at a processing temperature of 1100°C or less, since this suppresses damage to the metal substrate 1 and also suppresses interdiffusion of elements between the metal substrate 1 and the electrode layer 2, thereby realizing an electrochemical element E with excellent performance and durability. For example, low-temperature firing (e.g., a wet method using firing in a low-temperature range without firing in a high-temperature range exceeding 1100°C), spray coating (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, etc.), PVD (sputtering, pulsed laser deposition, etc.), CVD, etc., can be appropriately used. In particular, low-temperature firing and spray coating methods are preferred because they allow for the realization of low-cost elements. Furthermore, low-temperature firing is even more preferred because it facilitates 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, the reaction prevention layer 5, etc., but 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 application can also be used for the electrolyte layer 4. Furthermore, a gadolinium-doped ceria layer obtained by curing the coating composition b of the present application can also be used for 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 of a solid oxide fuel cell. For example, a reducing gas (typically a fuel gas containing hydrogen) is supplied to the electrode layer 2 from the rear surface of the metal substrate 1 through the through-holes 1a, and an oxidizing gas (typically air containing oxygen) is supplied to the counter electrode layer 6, which is the counter electrode of the electrode layer 2, and the electrochemical element E is operated at a temperature of, for example, 600°C or higher and 850°C or lower. In this case, the oxygen O contained in the air is oxidized in the counter electrode layer 6. ２ is electron e － reacts with oxygen ions O ２－ The oxygen ions O ２－ The hydrogen H contained in the supplied fuel gas moves through the electrolyte layer 4 to the electrode layer 2 (see FIG. 15). ２ is oxygen ion O ２－ reacts with water H ２ O and electron e － The above reaction generates an electromotive force between the electrode layer 2 and the counter electrode layer 6. In this case, the electrode layer 2 functions as the fuel electrode (anode) of the SOFC, and the counter electrode layer 6 functions as the air electrode (cathode).

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

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

[0110] (Electrode Layer Formation Step) In the electrode layer formation step, a thin layer of electrode layer 2 is formed on the front surface of metal substrate 1 obtained in the metal substrate preparation step in an area larger than the area where through holes 1 a are formed. The through holes 1 a in metal substrate 1 can be formed by laser processing or the like. As described above, the electrode layer 2 can be formed by low-temperature firing (a wet method in which firing is performed at a low temperature of 1100°C or less), spray coating (thermal spraying, aerosol deposition, aerosol gas deposition, powder jet deposition, particle jet deposition, cold spray, or the like), or PVD (sputtering, pulsed laser deposition, or the like). Whichever method is used, it is preferable to perform the process at a temperature of 1100°C or less to prevent deterioration of metal substrate 1.

[0111] When the electrode layer formation step is performed using a low-temperature firing method, specifically, a material powder and a solvent (dispersion medium) are mixed to form a material paste, which is then 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. The electrode layer 2 is then compression-molded (electrode layer smoothing step) and fired at 1100°C or less (electrode layer firing step). Compression molding of the electrode layer 2 can be performed, for example, by CIP (cold isostatic pressing), roll pressing, RIP (rubber isostatic pressing), or the like. Furthermore, firing of the electrode layer 2 is preferably performed at a temperature of 800°C or higher and 1100°C or lower. The order of the electrode layer smoothing step and the electrode layer firing step can also be reversed.

[0112] In the case of forming an electrochemical element E having an intermediate layer 3, the electrode layer smoothing step and the electrode layer firing step may be omitted, or the electrode layer smoothing step and the electrode layer firing step may be included in the intermediate layer smoothing step and the intermediate layer firing step described later. The electrode layer smoothing step may also be performed by performing lapping, leveling, surface cutting, polishing, or the like.

[0113] (Diffusion-Inhibiting Layer Formation Step) The diffusion-inhibiting layer becomes 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. Incidentally, the firing process preferably includes a firing step in which the firing atmosphere is set to a low oxygen partial pressure, since this effectively inhibits interdiffusion 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-inhibiting layer formation step, including cases where a coating method without firing is used. For example, in the separate diffusion-inhibiting layer formation step, the metal oxide layer 1b is formed by coating Co on the metal substrate 1 and then oxidizing it. Alternatively, in the separate diffusion-inhibiting layer formation step, the metal oxide layer 1b can be formed by coating Co on an intermediate layer formed on the metal substrate 1 and then oxidizing it. In either case, it is desirable to perform the process at a temperature of 1100°C or less, which can prevent damage to the metal substrate 1. Furthermore, the metal oxide layer 1b (diffusion-inhibiting layer) may be formed on the surface of the metal substrate 1 during the firing process in the intermediate layer formation step described below.

[0114] (Intermediate Layer Forming Step) In the intermediate layer forming step, the intermediate layer 3 is formed in a thin layer state on the electrode layer 2 so as to cover the electrode layer 2. As described above, the intermediate layer 3 can be formed by a low-temperature firing method (a wet method in which firing is performed in a low temperature range of 1100°C or less), a spray coating method (a thermal spray method, an aerosol deposition method, an aerosol gas deposition method, a powder jet deposition method, a particle jet deposition method, a cold spray method, or the like), a PVD method (a sputtering method, a pulsed laser deposition method, or the like), a CVD method, or the like. Whichever method is used, it is preferable to perform the method at a temperature of 1100°C or less in order to suppress deterioration of the metal substrate 1.

[0115] When the intermediate layer formation step is performed using a low-temperature firing method, the process is specifically as follows. First, a material powder for the intermediate layer 3 is mixed with a solvent (dispersion medium) to form a material paste, which is then applied to the upper surface of the electrode layer 2. Ceria-based ceramics such as GDC (gadolinium-doped ceria), YDC (yttrium-doped ceria), and SDC (samarium-doped ceria) are preferably used as the material for the intermediate layer 3. The coating 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 less (intermediate layer firing step). The intermediate layer 3 can be formed, for example, by CIP (cold isostatic pressing), roll pressing, or RIP (rubber isostatic pressing). Furthermore, it is preferable to perform the firing of the intermediate layer 3 at a temperature of 800°C or higher and 1100°C or lower. 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. Furthermore, it is more preferable to perform the firing of the intermediate layer 3 at 1050°C or lower, and even more preferable to perform it at 1000°C or lower. This is because the lower the firing temperature of the intermediate layer 3, the more effectively the electrochemical element E can be formed while suppressing damage and deterioration of the metal substrate 1. The order of the intermediate layer smoothing step and the intermediate layer firing step can also be reversed. The intermediate layer smoothing step can also be performed by performing lapping, leveling, surface cutting, polishing, etc.

[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 while covering the electrode layer 2 and the intermediate layer 3. Alternatively, the electrolyte layer 4 may be formed as a thin film having 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 in which firing is performed at a low temperature of 1100°C or less). However, the electrolyte layer 4 can also be formed by, for example, applying the coating composition b according to the present invention by air spraying, bar coating, dispenser coating, brush coating, or spatula coating, followed by firing at a temperature of 1100°C or less. Other methods that can be used include 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. Regardless of the method used, it is preferable to perform the process at a temperature of 1100°C or less to prevent 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 by a low-temperature firing method, 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. Whichever method is used, it is preferable to perform the process at a temperature of 1100°C or less to suppress deterioration of the metal substrate 1. Note that, in order to flatten the upper surface of the reaction prevention layer 5, for example, a leveling process, a cutting process, or a polishing process may be performed on the surface after the formation of the reaction prevention layer 5, or a press process may be performed after wet formation and before firing.

[0118] When the reaction prevention layer formation step is performed using a low-temperature firing method, the following specific example is performed. First, a material powder for the reaction prevention layer 5 is mixed with a solvent (dispersion medium) to form a material paste, which is then applied to the upper surface of the electrolyte layer 4. Ceria-based ceramics such as GDC (gadolinium-doped ceria) are preferably used as the material for the reaction prevention layer 5. Here, the coating composition b according to the present invention can be used in this material paste. The reaction prevention layer 5 is then compression-molded (reaction prevention layer smoothing step) and fired at 1100°C or less (reaction prevention layer firing step). The reaction prevention layer 5 can be formed, for example, by CIP (cold isostatic pressing), roll pressing, or RIP (rubber isostatic pressing). Furthermore, the reaction prevention layer 5 is preferably fired at a temperature of 800°C or higher and 1100°C or lower. This is because such a temperature makes it possible to form a strong reaction prevention layer 5 while suppressing damage and deterioration of the metal substrate 1. Furthermore, it is more preferable to perform the firing of the reaction prevention layer 5 at 1050°C or less, and even more preferable to perform it at 1000°C or less. This is because the electrochemical element E can be formed while further suppressing damage and deterioration of the metal substrate 1 as the firing temperature of the intermediate layer 3 is lowered. Furthermore, the order of the reaction prevention layer smoothing step and the reaction prevention layer firing step can be reversed. The reaction prevention layer smoothing step can also be performed by lapping, leveling, surface cutting, polishing, or the like.

[0119] (Counter Electrode Layer Forming Step) In the counter electrode layer forming step, the counter electrode layer 6 is formed in a thin layer state on the reaction prevention layer 5. As described above, the counter electrode layer 6 can be formed by a low-temperature firing method, a spray coating method (such as a thermal spraying method, an aerosol deposition method, an aerosol gas deposition method, a powder jet deposition method, a particle jet deposition method, or a cold spray method), a PVD method (such as a sputtering method or a pulsed laser deposition method), or a CVD method. Whichever method is used, it is preferable to perform the method at a temperature of 1100° C. or less in order 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 powder and a solvent (dispersion medium) are mixed to prepare a material paste, which is then 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 a part of this material paste.

[0121] In this manner, the electrochemical element E can be manufactured. The electrode layer-forming step and intermediate layer-forming step described above can be performed to manufacture a substrate with an electrode layer for a metal-supported electrochemical element. The electrochemical element E can also be configured to lack either or both of the intermediate layer 3 and the reaction prevention layer 5. That is, the electrode layer 2 and the electrolyte layer 4 can be formed in contact with each other, or the electrolyte layer 4 and the counter electrode layer 6 can be formed in contact with each other. In this case, the intermediate layer-forming step and the reaction prevention layer-forming step are omitted from the manufacturing method described above.

[0122] [Electrochemical Element, Electrochemical Module, Electrochemical Device, Energy System] Using Figures 16 and 17, an example of a solid oxide fuel cell will be described for an electrochemical element E, an electrochemical module M, an electrochemical device Y, and an 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 having an Fe-Cr alloy base and a Co composite oxide coating 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 serves as a supply path for a reducing gas (typically, a fuel gas containing hydrogen). As shown in Figure 16, a plurality of electrochemical elements E are stacked with a current collecting member 26 sandwiched therebetween to form an electrochemical module M. Here, a stack is an example of an assembly. The current collecting member 26 is joined to the counter electrode layer 6 of the electrochemical element E and the U-shaped member 7, electrically connecting them together. Therefore, the electrochemical module M is electrically connected as a whole in the stacking direction. The current collecting member 26 can also be made of the same material as the metal substrate 1 (an alloy material having an Fe-Cr alloy base material and a Co-containing complex oxide coating on its outer surface). The above-mentioned alloy member is used.

[0123] 17 , the electrochemical module M includes a gas manifold 17, a terminal member m1, and a current extraction portion m2. A plurality of electrochemical elements E, each stacked with a current collecting member 26 sandwiched therebetween as shown in FIG. 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 interior of the cylindrical support TS and passes through the through-holes 1 a of the metal substrate 1 to be supplied to the electrode layer 2.

[0124] 17 shows an overview of an energy system Z and an electrochemical device Y. The energy system Z includes an electrochemical device Y and a heat exchanger 53 as a waste heat utilization unit that reuses heat discharged from the electrochemical device Y. The electrochemical device Y includes an electrochemical module M, a fuel supply unit that has a desulfurizer 31 and a reformer 34 and supplies a fuel gas containing a reducing gas to the electrochemical module M, and an inverter 38 as a power conversion unit that extracts electric power from the electrochemical module M. Therefore, the electrochemical device Y is a power generation device that generates electric power when fuel is supplied. In more detail, the electrochemical device Y includes a desulfurizer 31, a reforming water tank 32, a vaporizer 33, the reformer 34, a blower 35, a combustion unit 36, an inverter 38, a control unit 39, a storage container 40, and the electrochemical module M.

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

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

[0127] The electrochemical module M has a plurality of electrochemical elements E and a gas manifold 17. The plurality of electrochemical elements E are arranged in parallel while electrically connected to each other, and one end (lower end) of each electrochemical element E is fixed to the gas manifold 17. The electrochemical elements E generate electricity by electrochemically reacting the reformed gas supplied through the gas manifold 17 with 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 a commercial grid (not shown). Thus, the inverter 38 serves 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 device Y and the energy system Z.

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

[0130] The raw fuel is supplied to the desulfurizer 31 through a raw fuel supply path 42 by operation of a booster pump 41. The reforming water in the reforming water tank 32 is supplied to the vaporizer 33 through a reforming water supply path 44 by operation of a reforming water pump 43. The raw fuel supply path 42 merges with the reforming water supply path 44 at a location downstream of the desulfurizer 31, and the reforming water and raw fuel that have merged outside the storage container 40 are supplied to the vaporizer 33 provided inside the storage container 40.

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

[0132] The reformed gas supplied to the gas manifold 17 is distributed to the plurality of electrochemical elements E and supplied to the electrochemical elements E from the lower end, which is the connection between the electrochemical elements E and the gas manifold 17. Mainly hydrogen in the reformed gas is used for the electrochemical reaction in the electrochemical elements E. Therefore, the reformer 34 serves as a fuel converter that supplies gas containing a reducing gas to the electrochemical elements E or the electrochemical modules M. Reaction exhaust gas containing 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 to become combustion exhaust gas, which is discharged to the outside of the storage container 40 through a combustion exhaust gas outlet 50. A combustion catalyst section 51 (e.g., a platinum-based catalyst) is arranged in the combustion exhaust gas outlet 50, and combusts and removes 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 a heat exchanger 53 through a combustion exhaust gas discharge path 52.

[0134] The heat exchanger 53 exchanges heat between the combustion exhaust gas generated by combustion in the combustion section 36 and the supplied cold water to generate hot water. That is, the heat exchanger 53 operates as a waste heat utilization section that reuses the heat discharged from the electrochemical device Y.

[0135] Instead of the exhaust heat utilization section, a reaction exhaust gas utilization section (not shown) may be provided that utilizes the reaction exhaust gas discharged (without being burned) from the electrochemical module M. The reaction exhaust gas contains the remaining hydrogen gas that was not used in the reaction in the electrochemical element E. In the reaction exhaust gas utilization section, the remaining hydrogen gas is utilized for heat utilization by combustion or for power generation using a fuel cell or the like, thereby making effective use of energy.

[0136] 18 shows another embodiment of the electrochemical module M. The electrochemical module M according to this another embodiment is configured by stacking the above-described electrochemical elements E with inter-cell connection members 71 sandwiched therebetween.

[0137] The inter-cell connection member 71 is a plate-shaped member that is electrically conductive and gas impermeable, and has grooves 72 formed on the front and back surfaces that are perpendicular to each other. The inter-cell connection member 71 can be made of a metal such as stainless steel or a metal oxide.

[0138] 18 , when electrochemical elements E are stacked with this inter-cell connection member 71 sandwiched therebetween, gas can be supplied to the electrochemical elements E through the grooves 72. More specifically, one groove 72 serves as a first gas flow path 72a, which supplies gas to the front side of the electrochemical element E, i.e., the counter electrode layer 6. The other groove 72 serves as a second gas flow path 72b, which supplies gas from the back side of the electrochemical element E, i.e., the back surface of the metal substrate 1, to the electrode layer 2 through the through-holes 1a.

[0139] When this electrochemical module M is operated as a fuel cell, an oxidizing gas (typically, air containing oxygen) is supplied to the first gas flow path 72a, and a reducing gas (typically, fuel gas containing hydrogen) is supplied to the second gas flow path 72b. This causes a reaction as a fuel cell to proceed in the electrochemical elements E, generating electromotive force and 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 elements E.

[0140] In this embodiment, grooves 72 that are perpendicular to each other are formed on the front and back surfaces of the inter-cell connection member 71, but grooves 72 that are parallel to each other can also be formed on the front and back surfaces of the inter-cell connection member 71.

[0141] [Other Embodiments] (1) In the above embodiment, the electrochemical element E is used in a solid oxide fuel cell. However, the electrochemical element E can also be used in a solid oxide electrolysis cell that causes an electrolytic reaction, an oxygen sensor that uses a solid oxide, or the like.

[0142] An example of using the electrochemical element E as a solid oxide electrolysis cell will be described below with reference to the drawings. As previously mentioned, in this example, the electrochemical device Y according to the present invention is used as a hydrocarbon production system 100, and the electrochemical element E is made to function as an electrolysis reaction section 10 by supplying predetermined raw material gas and electric power. That is, water H ２ O and carbon dioxide CO ２ These are decomposed to produce carbon monoxide (CO) and hydrogen (H) as raw materials for synthesizing hydrocarbons. ２ Then, hydrocarbons can be obtained in the hydrocarbon synthesis reaction section 30.

[0143] Fig. 19 is a system diagram showing the overall configuration of a hydrocarbon production system 100, and Fig. 20 shows an example of the configuration of an electrolytic cell unit U that constitutes the hydrocarbon production system 100. As can be seen from Fig. 19, the hydrocarbon production system 100 includes an electrolytic reaction section 10, a first catalytic reaction section 20, a second catalytic reaction section 30, a heavy hydrocarbon separation section 70 (illustrated as a CnHm separation section), a water separation section 80 (illustrated as a H ２ O separation section) and carbon dioxide separation section 90 (CO ２ 19 illustrates the electrolysis reaction section 10, the first catalytic reaction section 20, and the second catalytic reaction section 30 as separate sections, but as shown in FIG. 20, these sections 10, 20, and 30 can be provided as a single electrolysis cell unit U.

[0144] The electrolysis reaction section 10 is a section where at least a part of the inflowing gas is electrolyzed, the first catalytic reaction section 20 is a reverse water-gas shift reaction section where at least a part of the inflowing gas undergoes a reverse water-gas shift reaction, and the second catalytic reaction section 30 is a hydrocarbon synthesis reaction section where at least a part of the inflowing gas is synthesized into hydrocarbons. ４ The primary hydrocarbons synthesized in the second catalytic reaction section 30 are hydrocarbons with one carbon atom (carbon number 1), but may also include lower saturated hydrocarbons with two to four carbon atoms. Furthermore, hydrocarbons with a carbon number greater than that of 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 is cooled.

[0145] The water separation unit 80 and the carbon dioxide separation unit 90 separate predetermined components (H ２ O and CO ２ The components removed and recovered by these sections are returned to a predetermined section of the system via a water return line 81 and a carbon dioxide return line 91 as shown in the figure, and are reused. ２ O and CO ２ As a result, the hydrocarbon production system 100 is substantially CO ２In the figure, the gas flowing into each part is shown before each part, and the gas released from that part is shown after that part.

[0146] In the electrolysis reaction section 10, H ２ O and CO ２ is introduced and electrolyzed inside, ２ O is H ２ and O2, and some CO ２ CO and O ２ The reaction is described as follows: ２ O → 2H ２ +O ２ (Formula 1) 2CO ２ →2CO+O ２ (Equation 2) These equations 1 and 2 are also shown in the box representing the electrolytic reaction section 10 in FIG.

[0147] In the first catalytic reaction section 20 (reverse water gas shift reaction section), at least H ２ and CO ２ is introduced, the reverse water gas shift reaction occurs inside, and CO ２ CO, H ２ is released as HO. 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 (CO ２ and H ２ reacts to form CO and H ２ The reaction proceeds in the direction of producing CO. ２ +H ２ ⇔CO + H ２O (Equation 3) This equation 3 is also shown in the box representing the first catalytic reaction section 20 (reverse water gas shift reaction section) in Figure 19. The box also schematically shows a reverse water gas shift catalyst cat1 used in the reaction. The inventors believe that this type of reverse water gas shift catalyst cat1 is preferably a catalyst in which either nickel or iron, or both of them, is supported as a catalytically active component ca1 (metal catalyst) on one or more supports cb1 (metal oxide supports) 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 H ２ and CO are introduced and hydrocarbons are synthesized through a catalytic reaction. ２ From CH ４ The reaction by which CO and H are synthesized is described as the following equilibrium reaction. ２ From CH ４ The reaction in which is synthesized is the reaction described in the following formula 4, which proceeds to the right (CO and H2 react to form CH ４ and H ２ The reaction proceeds in the direction of producing O. ２ ⇔CH ４ +H ２ O (Equation 4) This equation 4 is also shown in the box representing the second catalytic reaction section 30 (hydrocarbon synthesis reaction section) in FIG. 19. The box also shows a schematic representation of the hydrocarbon synthesis catalyst cat2 used in the reaction. The inventors believe that this type of hydrocarbon synthesis catalyst cat2 is preferably a catalyst in which at least ruthenium, as the catalytically active component ca2, is supported on a carrier cb2 (metal oxide carrier), such as alumina. Furthermore, the equilibrium reaction of (Equation 3) also occurs at this site. Depending on the type of catalyst used in the second catalytic reaction section 30, it is possible to proceed with the FT (Fischer-Tropsch) synthesis reaction, so that CO and H ２ It is also possible to synthesize hydrocarbons such as ethane and propane from this.

[0149] H generated in the water separation section 80 ２O is separated and returned to the upstream side of the electrolysis reaction unit 10 via a water return line 81 (water recycle line). ２ is separated and returned to the upstream side of the electrolysis reaction section 10 via a carbon dioxide return line 91 (carbon dioxide recycle line). As a result, in this hydrocarbon production system 100, hydrocarbons are ultimately synthesized and can be supplied to the outside.

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

[0151] This electrolysis cell unit U also includes an electrochemical element E in which an electrode layer 2 and a counter electrode layer 6 are formed with an electrolyte layer 4 sandwiched therebetween, a metal substrate 1 serving as a metal support and acting as a separator, a U-shaped member 7 serving as a supply path forming member, and a current collecting member 26 serving as a supply path forming member, and is configured to form an electrode layer side gas supply path 7a and a counter electrode layer side gas supply path 26a. ２ O and CO ２ On the other hand, air g2 (O ２ ) is supplied. The electrochemical module M can be constructed by stacking (assembling) these electrolytic cell units U in the unit thickness direction, which is the left-right direction in FIG. 20 . As described above, the electrolyte layer 4 and, as a part thereof, the electrode layer 2 can be constructed using the coating composition b according to the present invention.

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

[0153] (2) In the above embodiment, the present invention is used in a metal-supported solid oxide fuel cell using the metal substrate 1 as the support, but the present invention can also be used in an electrode-supported solid oxide fuel cell using the electrode layer 2 or the counter electrode layer 6 as the support, or an electrolyte-supported solid oxide fuel cell using the electrolyte layer 4 as the support. In these cases, the electrode layer 2 or the counter electrode layer 6, or the electrolyte layer 4 can function as a support by being adjusted to a required thickness.

[0154] (3) In the above embodiment, the material of the electrode layer 2 is, for example, NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, or CuO-CeO ２ , Cu—CeO ２A composite material such as LSCF or LSM is used, and a composite oxide such as LSCF or LSM is used as the material for the counter electrode layer 6. The electrochemical element E configured in this manner can be used as a solid oxide fuel cell by supplying hydrogen gas to the electrode layer 2 to make it a fuel electrode and by supplying air to the counter electrode layer 6 to make it an air electrode. This configuration can also be modified to configure the electrochemical element E so that the electrode layer 2 can be made to be an air electrode and the counter electrode layer 6 can be made to be a fuel electrode. That is, a composite oxide such as LSCF or LSM is used as the material for the electrode layer 2, and a composite oxide such as NiO-GDC, Ni-GDC, NiO-YSZ, Ni-YSZ, or CuO-CeO is used as the material for the counter electrode layer 6. ２ , Cu—CeO ２ In the electrochemical element E configured in this manner, air is supplied to the electrode layer 2 to serve as an air electrode, and hydrogen gas is supplied to the counter electrode layer 6 to serve as a fuel electrode, so that the electrochemical element E can be used as a solid oxide fuel cell.

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

[0156] The configurations disclosed in the above embodiments can be applied in combination with configurations disclosed in other embodiments as long as no contradiction occurs. Furthermore, the embodiments disclosed in this specification are merely examples, and the present invention is not limited to these, and can be modified as appropriate within the scope of the object of the present invention.

[0157] The present invention relates to a method for producing a coating composition that can form a film at low cost using a simple method from a cerium compound such as cerium nitrate or a gadolinium compound such as gadolinium nitrate as a starting material, and that can yield a dense gadolinium-doped ceria (GDC) film with a uniform composition. The present invention is suitable for use in a gadolinium-doped ceria layer, an electrochemical element, an electrochemical module, an electrochemical device, an energy system, a solid oxide fuel cell, and a solid oxide electrolysis cell.

[0158] REFERENCE SIGNS LIST 1 Metal substrate (metal support) 1a Through-hole 2 Electrode layer 3 Intermediate layer 4 Electrolyte layer 5 Reaction prevention layer 6 Counter electrode layer 7 U-shaped member (separator) 10 Electrolysis reaction section 17 Gas manifold (manifold) 20 First catalytic reaction section (reverse water gas shift reaction section) 26 Current collecting member 30 Second catalytic reaction section (hydrocarbon synthesis reaction section, fuel converter) 31 Desulfurizer 32 Reforming water tank 33 Vaporizer 34 Reformer (fuel converter) 35 Blower 36 Combustion section 38 Inverter (power converter) 39 Control section 40 Storage container 41 Booster pump 42 Raw fuel supply channel 43 Reforming water pump 44 Reforming water supply channel 45 Steam-containing raw fuel supply channel 46 Reformed gas supply channel 50 Combustion exhaust gas outlet 51 Combustion catalyst section 52 Combustion exhaust gas discharge path 53 Heat exchanger (exhaust heat utilization section) 70 Heavy hydrocarbon separation section 71 Inter-cell connection member (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 system E Electrochemical element M Electrochemical module TS Cylindrical support U Electrolytic cell unit Y Electrochemical device Z Energy system o Cerium compound p Gadolinium compound q Hydroxycarboxylic acid r Polyalkylene glycol s Organic solvent

Claims

1. A method for producing a coating composition, comprising mixing a composition containing a cerium compound, a gadolinium compound, a hydroxycarboxylic acid, a polyalkylene glycol, and an organic solvent to produce a coating composition.

2. A method for producing a coating composition according to claim 1, wherein the coating 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 is the organic solvent.

3. The method for producing a coating 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 coating 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 coating composition according to any one of claims 1 to 4, wherein the hydroxycarboxylic acid is one or more of 2-hydroxypropanedioic acid (tartronic acid), 2-hydroxybutanedioic acid (malic acid), 2,3-dihydroxybutanedioic acid (tartaric acid), 3-hydroxypentanedioic acid (3-hydroxyglutaric acid), 2,4-dihydroxypentanedioic acid, 2,3,4-trihydroxypentanedioic acid, 3-hydroxyhexanedioic acid, 2,3,4,5-tetrahydroxyhexanedioic 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 coating composition described in any one of claims 1 to 5, wherein the polyalkylene glycol is one or more of 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.

7. A method for producing a coating composition according to any one of claims 1 to 6, 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.

8. A gadolinium-doped ceria layer obtained by curing a coating composition produced by the method for producing a coating composition according to any one of claims 1 to 7.

9. An electrochemical device comprising the gadolinium-doped ceria layer according to claim 8.

10. The electrochemical device according to claim 9, which has a metal support.

11. An electrochemical module in which a plurality of electrochemical elements according to claim 9 or 10 are arranged in an assembled state.

12. An electrochemical device comprising: an electrochemical element as defined in claim 9 or 10; or an electrochemical module as defined in claim 11; and a fuel converter which supplies a gas containing a reducing gas to said electrochemical element or said electrochemical module, or a fuel converter which converts a gas containing a reducing gas generated in said electrochemical element or said electrochemical module.

13. An electrochemical device comprising at least an electrochemical element according to claim 9 or 10 or an electrochemical module according to claim 11, and a power converter for extracting power from the electrochemical element or the electrochemical module, or a power converter for supplying power to the electrochemical element or the electrochemical module.

14. An energy system comprising the electrochemical device according to claim 13 and a waste heat utilization section for reusing heat discharged from the electrochemical device.

15. A solid oxide fuel cell comprising the electrochemical element according to claim 9 or 10, in which an electricity-generating reaction occurs in the electrochemical element.

16. A solid oxide electrolysis cell comprising the electrochemical element according to claim 9 or 10, in which an electrolytic reaction occurs.

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