electrode

Abstract

To provide an electrode with improved initial properties and durability by suppressing the re-oxidation and reduction of Ni. [Solution] The electrode has a cermet layer containing Ni-containing particles and an Nb compound. The Nb compound preferably covers at least a portion of the surface of the Ni-containing particles. The ratio of the mass of Nb in the Nb compound to the mass of Ni in the Ni-containing particles is preferably 0.2 to 3.0 mass%. The Nb compound may contain La. The cermet layer may contain electrolyte particles that are conductive to oxide ions or conductive to both oxide ions and electrons.

Description

[Technical Field]

[0001] This disclosure relates to electrodes, and more particularly to electrodes that can be used as a hydrogen electrode in a solid oxide electrolytic cell (SOEC) or a fuel electrode in a solid oxide fuel cell (SOFC). [Background technology]

[0002] Electrochemical cells such as solid oxide fuel cell cells (SOFCs) and solid oxide electrolytic cells (SOECs), which have a solid electrolyte layer with oxide ion conductivity, are known.

[0003] SOFCs are fuel cells that use oxide ion conductors as electrolytes. When fuel gases such as H2, CO, and CH4 are supplied to the anode (fuel electrode) of an SOFC and O2 is supplied to the cathode (oxygen electrode), electrode reactions proceed and electricity can be extracted. The CO2 and H2O produced by the electrode reactions are discharged outside the SOFC.

[0004] On the other hand, SOEC has the same structure as SOFC, but it causes the opposite reaction. That is, by supplying CO2 or H2O to the cathode (hydrogen electrode) of SOEC and passing an electric current between the electrodes, CO or H2 can be produced.

[0005] In SOEC (Semiconductor Electrical Emission Control) systems, cermets such as Ni / YSZ, Ni / SDC, and Ni-Fe / SDC are sometimes used for the hydrogen electrodes. However, the hydrogen electrodes are supplied with steam, the raw material for hydrogen production, at high temperatures (above 700°C). As a result, the Ni contained in the hydrogen electrodes is easily oxidized, forming NiO. Since NiO is an insulator, when NiO is formed in the electrode, the electron path is interrupted in that area, and the electrolytic reaction stops. Consequently, the electrolytic properties deteriorate.

[0006] This point is also true for SOFCs. In other words, water is generated at the fuel electrode of an SOFC due to an electrode reaction. Therefore, especially under high-load operating conditions, the generated water vapor can oxidize the nickel in the fuel electrode, which can lead to a decrease in power generation characteristics.

[0007] Therefore, there is a need for a technology to suppress the oxidation of Ni in electrodes for SOFCs or SOECs. As an example of such a technology, Patent Document 1 discloses an electrode comprising a diffusion layer and an active layer formed on the electrolyte layer side surface of the diffusion layer. The active layer of this electrode consists of a cermet (B) containing Ni-containing particles (B) and YScCZ particles made of ZrO2 doped with Y, Sc, and Ce. [Prior art documents] [Patent Documents]

[0008] [Patent Document 1] Japanese Patent Publication No. 2022-74189 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, since SOEC operates at high temperatures of around 800°C, the technology described in Patent Document 1 had the problem that sufficient initial characteristics and sufficient durability could not be obtained due to repeated re-oxidation and reduction of Ni.

[0010] This disclosure was made to solve these problems and aims to provide an electrode with improved initial properties and durability by suppressing the re-oxidation and reduction of Ni. [Means for solving the problem]

[0011] An electrode according to one embodiment has a cermet layer containing Ni-containing particles and an Nb compound. [Effects of the Invention]

[0012] The purpose of this disclosure is to provide an electrode in which both initial characteristics and durability are improved by suppressing the re-oxidation and reduction of Ni. [Brief explanation of the drawing]

[0013] [Figure 1] It is a graph showing the IV characteristics during co - electrolysis of each SOEC obtained in Examples 1 and 2 and Comparative Examples 1 and 2. [Figure 2] It is the Cole - Cole plot of each SOEC obtained in Example 1 and Comparative Example 1. [Figure 3] It is a graph showing the durability test results of each SOEC obtained in Example 1 and Comparative Example 1. [Figure 4] It is a SEM image of the hydrogen electrode after the durability test. [Figure 5] It is a graph of the K - edge XANES spectrum and a graph of the K - edge radial distribution function showing the XANES measurement results for each SOEC obtained in Example 1 and Comparative Example 1. [Figure 6] It is a graph showing the K - edge XANES spectrum showing the XANES measurement results for Nb - oxide - coated NiO particles before and after the reduction treatment. [Figure 7] It is a graph showing the results of analyzing the gas generated by the SOEC obtained in Example 2.

Mode for Carrying Out the Invention

[0014] Embodiment 1 Hereinafter, specific embodiments to which the present disclosure is applied will be described in detail with reference to the drawings. However, the present disclosure is not limited to the following embodiments. Also, for clarity of explanation, the following description and drawings are simplified as appropriate.

[0015] The electrode according to the present disclosure is used for the hydrogen electrode of a solid oxide electrolysis cell (SOEC) or the fuel electrode of a solid oxide fuel cell (SOFC). The electrode may be used for the hydrogen electrode of an SOEC or the fuel electrode of an SOFC in a solid oxide fuel cell / electrolysis cell that uses them reversibly. In this embodiment, it will be described by being embodied as an electrode used for the hydrogen electrode of an SOEC.

[0016] [Electrode] The electrode according to this disclosure has a cermet layer containing Ni-containing particles and an Nb compound.

[0017] [Ni-containing particles] Ni-containing particles are metal particles in which the ratio of Ni mass to the total mass of metal elements contained in the particles is 90 mass% or more. Preferably, the ratio of Ni mass in the Ni-containing particles is 95 mass% or more.

[0018] Ni-containing particles function as both an electrode catalyst and an electron conductor within the cermet layer. The composition of the Ni-containing particles is not particularly limited, as long as it performs these functions.

[0019] Examples of Ni-containing particles include Ni, Ni-Fe alloys, and Ni-Co alloys. Among these, Ni or Ni-Fe alloys are preferred for the Ni-containing particles.

[0020] Ni-containing particles have high electronic conductivity and high activity as electrochemical catalysts, making them suitable as constituent materials for electrodes, which are hydrogen electrodes. However, in SOEC, over time, the Ni-containing particles may aggregate and become unevenly distributed, or migrate away from the vicinity of the solid electrolyte layer, causing the electrode to deteriorate.

[0021] Therefore, it is conceivable to set the average particle size of the Ni-containing particles to a range that can suppress aggregation of the Ni-containing particles. However, as a result of repeated investigations by the inventors, it was found that the Ni contained in the Ni-containing particles is re-oxidized regardless of the average particle size of the Ni-containing particles, and that migration of the Ni-containing particles progresses as the Ni undergoes repeated re-oxidation and reduction.

[0022] Migration of Ni-containing particles is thought to occur when an overpotential for the electrode reaction is applied to the electrode. Since the electrode is a composite electrode, even if the electrode as a whole is in a reducing atmosphere, there may be areas in a relatively oxidizing atmosphere locally. Therefore, there is a risk that Ni will be re-oxidized in the areas of the electrode that are in a relatively oxidizing atmosphere. When Ni is oxidized, its wettability with respect to the electrolyte increases, and when it is reduced, this wettability decreases. Consequently, when Ni is oxidized, the electrode becomes more prone to peeling from the solid electrolyte layer, which may reduce the durability of the electrode. In addition, if NiO, an insulator, is formed in the electrode due to the oxidation of Ni, the electron path will be interrupted in that area, and the electrolytic reaction will not proceed, thus reducing the electrolytic properties of the electrode.

[0023] Therefore, in order to suppress electrode degradation due to migration of Ni-containing particles, it is desirable to suppress the re-oxidation and reduction of Ni.

[0024] Therefore, the electrode according to this disclosure has a cermet layer containing Ni-containing particles and an Nb compound, thereby suppressing the re-oxidation and reduction of Ni contained in the Ni-containing particles, improving both the initial properties (electrolytic properties) and durability.

[0025] [Nb compound] Nb compounds are compounds containing niobium (Nb). The mass percentage of Nb in the Ni compound is preferably 0.2 mass% to 3.0 mass%.

[0026] Nb compounds can adopt an unstoichiometric composition depending on the partial pressure of oxygen in the surrounding atmosphere. Therefore, Nb compounds have the effect of changing the oxidation state of Ni contained in Ni-containing particles present in the vicinity of the Nb compounds. Consequently, Nb compounds in the cermet layer have the function of suppressing the oxidation of Ni contained in Ni-containing particles.

[0027] It is preferable that the Nb compound coats at least a portion of the surface of the Ni-containing particles. By coating at least a portion of the surface of the Ni-containing particles contained in the cermet layer, the Nb compound further suppresses the oxidation of Ni contained in the Ni-containing particles.

[0028] The ratio of the mass of Nb in the Nb compound to the mass of Ni in the Ni-containing particles is not particularly limited, but can be adjusted according to the mass of Ni in the coated Ni-containing particles. Preferably, the ratio of the mass of Nb in the Nb compound to the mass of Ni in the Ni-containing particles is 0.2 to 3.0 mass%. This allows for sufficient suppression of the oxidation of Ni in the Ni-containing particles.

[0029] Examples of Nb compounds include various Nb-containing compounds such as niobium alkoxide, niobium oxide, niobium acid, niobium hydroxide, niobium chloride, niobium nitrate, niobium sulfate, niobium oxalate, and niobium formate.

[0030] Among these, the Nb compound is preferably niobium oxide. The Nb compound, which is niobium oxide, has high basicity and the function of suppressing the decrease in electrode activity due to coking. Here, coking is a phenomenon in which carbon (C) is deposited in the hydrogen electrode due to the large amount of CO produced by the co-electrolysis of CO2 / H2O in SOEC.

[0031] The oxidation state of niobium in niobium oxide can be pentavalent, tetravalent, trivalent, divalent, or monovalent. Examples of niobium oxide include niobium monoxide (NbO), niobium dioxide (NbO2), and niobium pentoxide (Nb2O5). Niobium oxide can be obtained, for example, by calcination.

[0032] Nb compounds may contain elements other than Nb. Preferably, lanthanum (La) is added to the Nb compound. Nb compounds with added La can enhance the catalytic activity and stability of Ni-containing particles as an electrochemical catalyst compared to Nb compounds without added La.

[0033] It is more preferable that the Nb compound contains 0.8 to 1.2 units of La per atomic weight of Nb. This allows for a sufficient increase in the catalytic activity and stability of the Ni-containing particles.

[0034] The average thickness of the Nb compound covering at least a portion of the surface of the Ni-containing particles is preferably uniform, for example, 1 to 20 nm. The average thickness of the Nb compound can be calculated by observing it with an electron microscope such as a scanning electron microscope (SEM) or a transmission electron microscope (TEM), or by analyzing it with a spectrometer such as an energy dispersive X-ray spectrometer (EDS) attached to these microscopes, and measuring the layer uniformly formed on the surface of the Ni-containing particles. The average thickness refers to the average value obtained when measuring multiple locations.

[0035] The cermet layer preferably contains electrolyte particles having oxide ion conductivity or both oxide ion electron conductivity, in addition to Ni-containing particles and Nb compounds. Any material that can be used in a hydrogen electrode can be used as the electrolyte particles without particular limitations.

[0036] Electrolyte particles having oxide ion conductivity have at least the function of an oxide ion conductor. Examples of electrolyte particles having oxide ion conductivity include ceramic particles such as zirconium oxide and perovskite-type oxides. Zirconium oxide may include, for example, zirconia (ZrO2) and stabilized zirconia. Examples of stabilized zirconia include ZrO2 in which at least one stabilizer selected from the group consisting of Y2O3, Sc2O3, Yb2O3, Gd2O3, CaO, MgO, and CeO2 is solid-dissolved. Examples of perovskite-type oxides include perovskite-type oxides represented by ABO3, such as lanthanum strontium cobalt iron oxide (LSFM). Among these, yttria-stabilized zirconia (YSZ) in which Y2O3 is solid-dissolved is preferably used.

[0037] Electrolyte particles having bielectronic conductivity of oxide ions have at least the function of an oxide ion conductor and an electronic conductor. Examples of electrolyte particles having bielectronic conductivity of oxide ions include ceramic particles such as cerium oxide. Cerium oxide may include, for example, ceria (CeO2) and doped ceria. Examples of doped ceria include ceria in which at least one rare earth element oxide selected from the group consisting of Sm2O3, Gd2O3, and Y2O3 is solid-dissolved. Among these, gadlinia-doped ceria (GDC) in which Gd2O3 is solid-dissolved is preferably used.

[0038] The average particle size of the electrolyte particles is not particularly limited insofar as the effects of the technology disclosed herein are achieved. The average particle size of the electrolyte particles is, for example, 0.1 to 5 μm, and preferably 0.2 to 2 μm.

[0039] [Composition of the cermet layer] The Ni-containing particle content is the ratio of the mass of Ni-containing particles to the total mass of Ni-containing particles, Nb compounds, and electrolyte particles. If the amount of Ni-containing particles is too low, the total cell resistance will increase, and the efficiency of the electrode reaction may decrease. Therefore, the Ni-containing particle content is preferably 30 mass% or more. More preferably, the Ni-containing particle content is 40 mass% or more. On the other hand, if the content of Ni-containing particles is excessive, the content of electrolyte particles decreases. As a result, the oxidation-inhibiting function of the Ni contained in the Ni-containing particles may decrease, or the strength of the cermet layer may decrease. Therefore, the content of Ni-containing particles is preferably 70 mass% or less.

[0040] [Porosity of the cermet layer] The porosity of the cermet layer is not particularly limited, and the optimal porosity can be selected depending on the purpose.

[0041] For example, when a cermet layer is used as the active layer of an electrode, the porosity of the active layer affects the electrolytic properties. If the porosity of the active layer is too low, the gas diffusivity decreases, and the efficiency of the electrode reaction decreases. Therefore, a porosity of 15% or more is preferable for the active layer. Preferably, the porosity is 20% or more, and more preferably, 25% or more. On the other hand, if the porosity of the active layer becomes too high, the number of three-phase interfaces decreases relatively, which actually reduces the efficiency of the electrode reaction. Therefore, the porosity of the active layer is preferably 40% or less. Preferably, the porosity is 35% or less, and more preferably 30% or less.

[0042] Furthermore, for example, when a cermet layer is used as the diffusion layer of an electrode, the porosity of the diffusion layer affects the gas diffusivity, strength, and electronic conductivity of the hydrogen electrode. Generally, if the porosity of the diffusion layer is too low, the gas diffusivity decreases. Therefore, a porosity of 40% or higher is preferable for the diffusion layer. More preferably, the porosity is 45% or higher, and even more preferably, 50% or higher. On the other hand, if the porosity of the diffusion layer becomes too high, the strength and electronic conductivity will decrease. Therefore, the porosity of the diffusion layer is preferably 60% or less. Preferably, the porosity is 58% or less, and more preferably 55% or less.

[0043] [Active layer and diffusion layer] The electrode according to this disclosure comprises at least an active layer. In addition to the active layer, the electrode may also comprise a diffusion layer formed on the electrolyte layer side surface of the active layer. That is, the electrode may comprise only an active layer, or it may comprise an active layer and a diffusion layer formed on the electrolyte layer side surface of the active layer.

[0044] The active layer is the layer that serves as the reaction site for the electrolytic reaction. The active layer is composed of oxides produced by the electrolytic reaction. Because ions need to be transported to the electrolyte, high ionic conductivity is required. On the other hand, the diffusion layer is for supporting the active layer. In a hydrogen electrode, the electrode reaction occurs mainly within the active layer. Therefore, the diffusion layer is necessary. It is not necessary for it to have particularly high ionic conductivity.

[0045] In other words, the diffusion layer is at least, (a) Functions for supporting the active layer formed on the electrolyte layer side surface, (b) Function to diffuse the electrolytic raw materials to the active layer, (c) The function of transporting electrons necessary for the reduction reaction from the current collector to the active layer, (d) Function to discharge hydrogen generated in the active layer by the electrode reaction to the outside of the hydrogen electrode. It must be equipped with [the necessary features / qualities]. The composition of the diffusion layer is not particularly limited, as long as it performs the function described above.

[0046] The cermet layer described above can be used as at least one of the active layer and the diffusion layer. In other words, the electrode relating to this disclosure is (a) A device comprising only an active layer, wherein the active layer consists of the above-mentioned cermet layer, (b) A two-layer structure comprising an active layer and a diffusion layer, wherein the active layer consists of the cermet layer and the diffusion layer consists of a layer other than the cermet layer. (c) A two-layer structure comprising an active layer and a diffusion layer, wherein both the active layer and the diffusion layer are made of the above-mentioned cermet layer. (d) A two-layer structure comprising an active layer and a diffusion layer, wherein the diffusion layer is made of the cermet layer and the active layer is made of a layer other than the cermet layer. Either of these is acceptable.

[0047] When the diffusion layer or active layer consists of a layer other than the cermet layer (hereinafter also referred to as the "second layer"), the composition of the second layer is not particularly limited. The second layer usually consists of a cermet containing secondary Ni-containing particles and electrolyte particles made of solid oxide.

[0048] The second Ni-containing particles in the second layer may have the same composition as the Ni-containing particles in the cermet layer, or they may have a different composition. Furthermore, the electrolyte particles contained in the second layer may have the same composition as the electrolyte particles contained in the cermet layer, or they may have a different composition.

[0049] To suppress electrode degradation caused by oxidation of Ni-containing particles, it is preferable that the electrode's active layer consists of a cermet layer. Further explanation regarding the cermet layer is omitted as it is as described above.

[0050] [Application] As described above, the electrode relating to this disclosure can be used not only as the hydrogen electrode of an SOEC but also as the fuel electrode of an SOFC. Since the SOFC has the same structure as the SOEC except for its application, a detailed explanation will be omitted.

[0051] [Method for manufacturing electrodes] The electrode relating to this disclosure can be manufactured by various methods. For example, if the electrode has a single-layer structure, the electrode is: (a) A first mixture containing Ni-containing particles and Nb compound raw materials is calcined. (b) A molded body is formed using the second mixture containing the raw materials for the obtained Nb compound coated Ni particles and electrolyte particles. (c) The obtained molded body is sintered, (d) The obtained sintered body can be manufactured by reduction treatment.

[0052] [1st step] The first step is to calcine a first mixture containing Ni-containing particles and Nb compounds.

[0053] The raw materials for Ni-containing particles are those that become Ni-containing particles after sintering and reduction. The types of raw materials for Ni-containing particles are not particularly limited, and the most suitable raw material can be selected according to the purpose. Examples of raw materials for Ni-containing particles include NiO powder, Fe2O3 powder, Fe3O4 powder, mixtures of metallic Fe and NiO or metallic Ni, CoO powder, Co2O3 powder, etc.

[0054] The raw material for the Nb compound is a raw material that becomes an Nb compound after calcination. The type of raw material for the Nb compound is not particularly limited as long as it can produce an Nb compound after calcination. Examples of raw materials for the Nb compound include niobalkoxides such as niobium isopropoxide. Such raw materials for the Nb compound can be used as a solution dissolved in a solvent (hereinafter sometimes referred to as the Nb raw material solution). Examples of solvents include alcohols such as ethanol, water, and mixed solvents of alcohol and water. The calcination temperature may be, for example, 500°C to 1000°C.

[0055] The method for obtaining Nb compounds from Nb compound raw materials is not particularly limited, and the most suitable method can be selected depending on the purpose. Examples of methods for obtaining Nb compounds include synthesis methods such as hydrolysis, impregnation, or sol-gel synthesis.

[0056] It is preferable to blend the raw materials for the Ni-containing particles and the Nb compound so that the desired coverage of the Nb compound is obtained after calcination.

[0057] Furthermore, for example, in the case of an Nb compound to which La has been added, the first mixture may contain a raw material for La. The type of raw material for La is not particularly limited, and the most suitable raw material can be selected according to the purpose. Examples of raw materials for La include lanthanum alkoxides such as lanthanum isopropoxide.

[0058] [Second process] The second step is to form a molded body using the second mixture containing the obtained Nb compound-coated Ni-containing particles. The second mixture may also contain raw materials for electrolyte particles.

[0059] The raw materials for electrolyte particles are those that become electrolyte particles after sintering. The types of raw materials for electrolyte particles are not particularly limited; the most suitable raw materials can be selected according to the purpose. For example, if the electrolyte particles are YSZ, the raw materials would be: (a) YSZ powder having the desired composition, (b) A mixture of ZrO2 powder and Y2O3 powder, formulated to achieve the desired composition. These include:

[0060] Furthermore, the second mixture may contain a pore-forming agent such as carbon powder. Metal oxides such as NiO powder contained in the raw materials of the Ni-containing particles added to the second mixture are subjected to reduction treatment after the sintered body is produced. During this process, volume shrinkage occurs, and pores are introduced into the sintered body. Therefore, a pore-forming agent is not necessarily required. However, adding a pore-forming agent to the second mixture increases the degree of freedom in controlling the porosity.

[0061] It is preferable to blend the Nb compound-coated Ni-containing particles and electrolyte particles in such a way that a cermet layer having the desired composition can be obtained after sintering and reduction.

[0062] The method for manufacturing the molded body is not particularly limited, and the most suitable method can be selected depending on the purpose. For example, the method for manufacturing the molded body may be: (a) A method of forming a slurry containing the second mixture into a tape, laminating the resulting green sheet onto a substrate (for example, a molded body for producing the second layer), and pressing the laminate together by hydrostatic pressing (CIP). (b) A method of preparing a slurry containing the second mixture and screen printing the slurry onto the surface of a substrate. These include:

[0063] [3rd step] The third step is to sinter the obtained molded body. It is preferable to select the optimal sintering conditions according to the raw material composition. Sintering is usually preferably carried out in an atmospheric environment at a temperature of 1000°C to 1400°C for 1 to 5 hours. If the second mixture contains two or more oxides, a solid-phase reaction will proceed during sintering. A solid solution with a specific composition may be formed. Also, the second mixture contains a pore-forming agent. If present, the pore-forming material disappears during sintering, and pores are formed within the sintered body.

[0064] [4th step] The fourth step is to reduce the obtained sintered body. This forms a cermet layer, and an electrode having a cermet layer is obtained. The reduction treatment is performed to reduce metal oxides such as NiO contained in the sintered body and generate Ni-containing particles. The reduction conditions are not particularly limited, and it is preferable to select the optimal conditions according to the composition of the cermet layer. The reduction is preferably carried out at a temperature of 600°C to 800°C under a hydrogen reduction atmosphere.

[0065] SOECs, for example, consist of a combination of a hydrogen electrode (cathode), an electrolyte layer, and an oxygen electrode (anode). The reduction of the cermet layer is usually performed after the layers are joined together. This is also the case for SOFCs. [Examples]

[0066] The present disclosure will be explained in more detail below with reference to examples, but the present disclosure is not limited to the following examples. In addition, these examples show the results of evaluating co-electrolysis of CO2 / H2O in SOEC. However, since the electrode degradation mechanism is similar in the electrolysis of CO2 and H2O, it is expected that similar evaluation results can be obtained in the electrolysis of CO2 and H2O as well.

[0067] [1. Sample Preparation] [Example 1]

[0068] [Coating of Ni-containing particles with Nb compounds] [Synthesis Example 1] Using a hydrolysis method, niobium oxide (Nb oxide) was synthesized as an Nb compound to coat the surface of NiO particles, which are Ni-containing particles.

[0069] First, niobium isopropoxide (0.33 mL), used as a raw material for the Nb compound, was dissolved in anhydrous ethanol (50 mL) as a solvent to obtain an Nb raw material solution. NiO powder (1.98 g), used as a raw material for Ni-containing particles, was then added to this solution and stirred at room temperature for 1 hour to obtain a slurry of the first mixture. Next, 1 mL of distilled water was added to the slurry of the obtained first mixture to hydrolyze the niobium isopropoxide in the first mixture, and then the mixture was concentrated at 60°C using an evaporator to obtain the powder of the first mixture. Next, the resulting powder of the first mixture was calcined at 400°C for 4 hours. This yielded Nb oxide-coated NiO particles, in which NiO particles were coated with Nb oxide (coverage 1 mass%).

[0070] [Synthesis Example 2] Using an impregnation method, niobium oxide (Nb oxide) was synthesized as an Nb compound to coat the surface of NiO particles, which are Ni-containing particles.

[0071] First, niobium isopropoxide (0.33 mL), used as a raw material for the Nb compound, was dissolved in anhydrous ethanol (1 mL) as a solvent to create an Nb raw material solution. NiO powder (1.98 g), used as a raw material for Ni-containing particles, was then added to this solution. The mixture was then evaporated to dryness while stirring to obtain the first mixture powder. Next, the resulting powder of the first mixture was calcined at 400°C for 4 hours. This yielded Nb oxide-coated NiO particles, in which NiO particles were coated with Nb oxide.

[0072] [Synthesis Example 3] Using the sol-gel method, niobium oxide (Nb oxide) was synthesized as an Nb compound to coat the surface of NiO particles, which are Ni-containing particles.

[0073] First, niobium isopropoxide (0.33 mL), a raw material for the Nb compound, was dissolved in anhydrous butanol (50 mL) as a solvent to form the Nb raw material solution. NiO powder (1.98 g) and glycerin (10 mL), raw materials for the Ni-containing particles, were added in that order, and the mixture was stirred at 160°C for 12 hours to obtain the first mixture gel.

[0074] Next, the resulting gel of the first mixture was calcined at 300°C to decompose the glycerin in the first mixture, and then calcined at 400°C for 4 hours. This yielded Nb oxide-coated NiO particles, in which NiO particles were coated with Nb oxide (coverage 1 mass%).

[0075] XPS (X-ray Photoelectron Spectroscopy), SEM-EDS, and XRD (X-Ray Diffraction) measurements were performed on each Nb oxide-coated NiO particle obtained in Synthesis Examples 1-3. XPS patterns were obtained by performing XPS measurements on each Nb oxide-coated NiO particle obtained in Synthesis Examples 1-3. Based on the XPS patterns, it was confirmed that the Nb compound in each Nb oxide-coated NiO particle was Nb2O5.

[0076] Further, based on the XPS pattern, for each of the Nb-oxide-coated NiO particles obtained in Synthesis Examples 1 to 3, the coating rate of the Nb oxide on the NiO particles was calculated according to the following formula (1). As a result, the coating rate was 1 mass%. (Amount of detected Nb) / (Amounts of detected Ni and Nb) × 100 ··· Formula (1)

[0077] SEM images and their elemental mapping images were obtained by SEM-EDS measurement performed on each of the Nb-oxide-coated NiO particles obtained in Synthesis Examples 1 to 3. Then, the SEM image of the Nb-oxide-coated NiO particles, the elemental mapping image of Ni, and the elemental mapping image of Nb were overlaid, and since the elemental part derived from Nb was present at the same location as the elemental part derived from Ni on the surface of the Nb-oxide-coated NiO particles, it was confirmed that Nb2O5 coated NiO. XRD patterns were obtained by XRD measurement performed on each of the Nb-oxide-coated NiO particles obtained in Synthesis Examples 1 to 3. Then, based on the XRD patterns, since no peak derived from Nb was detected, it was confirmed that Nb2O5 coated NiO with a thickness of several nanometers.

[0078] [Fabrication of Solid Oxide Electrolysis Cell (SOEC)] La as a solid electrolyte 1-x Sr x Ga 1-y Mg y Powder of lanthanum strontium gallium magnesium oxide (LSGM) represented by the formula La 0.6 Sr 0.4A slurry containing barium lanthanum cobalt oxide (BLC), represented by the formula CoO3, was screen printed to produce an oxygen electrode molded body with a diameter of 0.5 mm and a thickness of 30 μm. Then, a slurry of a second mixture containing Nb oxide-coated NiO particles obtained in Synthesis Examples 1-3 was screen printed onto the other side of the fabricated electrolyte layer molded body to produce a hydrogen electrode molded body. This resulted in the creation of a disc-shaped laminate consisting of a hydrogen electrode molded body, an electrolyte layer molded body, and an oxygen electrode molded body.

[0079] After attaching platinum lead wires to both sides of the laminate fabricated in this manner via a platinum mesh, the laminate was set in an alumina tube using a glass seal. The laminate was then dried and fired in an air atmosphere at 1100°C for 12 hours. Next, the resulting sintered body was subjected to a reduction treatment in a hydrogen reduction atmosphere at 800°C for 2 hours. The reduction treatment reduced the NiO contained in the hydrogen electrode molded body to Ni.

[0080] As described above, an SOEC (Single-Oxygen Electrolyte Layer) consisting of a hydrogen electrode (cathode electrode), an electrolyte layer, and an oxygen electrode (anode electrode) was fabricated.

[0081] [Example 2] [Coating of Ni-containing particles with Nb compounds] [Synthesis Example 4] Using a hydrolysis method, La-Nb oxides were synthesized as Nb compounds to which La was added to coat the surface of NiO particles, which are Ni-containing particles. First, lanthanum isopropoxide (0.049 g), a raw material for La, and niobium isopropoxide (0.73 mL), a raw material for Nb compounds, were dissolved in anhydrous ethanol (50 mL) as a solvent to form an Nb raw material solution. NiO powder (4.95 g), a raw material for Ni-containing particles, was then added to this solution, and the mixture was stirred at room temperature for 1 hour to obtain the slurry of the first mixture. The preparation of Nb oxide-coated NiO particles was carried out in the same manner as in Synthesis Example 1, except that the first mixture was obtained in this manner, by coating NiO particles with La-Nb oxide (coverage rate 1 mass%). Hereinafter, Nb oxide-coated NiO particles coated with La-Nb oxide may be referred to as La-Nb oxide-coated NiO particles.

[0082] The La-Nb oxide-coated NiO particles obtained in Synthesis Example 4 were subjected to XPS, SEM-EDS, and XRD measurements, respectively.

[0083] XPS patterns were obtained by performing XPS measurements on the La-Nb oxide coated NiO particles obtained in Synthesis Example 4. Based on the XPS patterns, it was confirmed that the Nb compound with added La in the La-Nb oxide coated NiO particles was LaNbO5.

[0084] Furthermore, based on the XPS pattern, the coverage of the La-Nb oxide on the NiO particles obtained in Synthesis Example 4 was calculated using the following formula (2). As a result, the coverage was found to be 1 mass%. (Amount of La and Nb detected) / (Amount of Ni, La, and Nb detected) × 100 ... Equation (2)

[0085] SEM-EDS measurements were performed on the La-Nb oxide coated NiO particles obtained in Synthesis Example 4 to obtain SEM images and their elemental mappings. By superimposing the SEM images of the La-Nb oxide coated NiO particles, the elemental mappings of Ni, Nb, and La, it was confirmed that LaNbO5 is coated with NiO, as Nb-derived and La-derived elemental regions were present in the same locations as Ni-derived elemental regions on the surface of the La-Nb oxide coated NiO particles.

[0086] An XRD pattern was obtained by XRD measurement performed on the La-Nb oxide-coated NiO particles obtained in Synthesis Example 4. Then, based on the XRD pattern, since no peak derived from La was detected, it was confirmed that LaNbO5 coated NiO with a thickness of several nanometers.

[0087] [Fabrication of Solid Oxide Electrolysis Cell (SOEC)] By the same method as in Example 1, an electrolyte layer green body and an oxygen electrode green body were fabricated. Then, the La-Nb oxide-coated NiO particles obtained in Synthesis Example 4 and La 0.6 Sr 0.4 Fe 0.9 Mn 0.1 O 3-δ A mixed powder obtained by mixing powders of LSFM represented by the formula in a mass ratio of 90:10, a plasticizer, and a binder were dispersed in an organic solvent in which a dispersant was dissolved to obtain a slurry of a second mixture. The SOEC was fabricated by the same method as in Example 1 except that the second mixture was obtained in this way, and a disk-shaped laminate composed of a hydrogen electrode green body / electrolyte layer green body / oxygen electrode green body was fabricated.

[0088] Next, by the same method as in Example 1, the fabricated laminate was fired and the fired body obtained by firing was subjected to reduction treatment.

[0089] As described above, a SOEC of hydrogen electrode (cathode electrode) / electrolyte layer / oxygen electrode (anode electrode) was fabricated.

[0090] [Comparative Example 1] A SOEC was fabricated by the same fabrication method as in Example 1 except that NaO particles not coated with Nb oxide were used instead of Nb oxide-coated NiO particles.

[0091] [Comparative Example 2] A SOEC was fabricated by the same fabrication method as in Example 2 except that NaO particles not coated with La-Nb oxide were used instead of La-Nb oxide-coated NiO particles.

[0092] [2. Test Methods and Evaluation Results] [Co-electrolysis test] Using the obtained SOEC, a CO2 / H2O co-electrolysis test was performed. The co-electrolysis conditions were as follows: Temperature: 800°C Oxygen electrode atmosphere: Dry air (flow rate 100cc / min) Hydrogen electrode atmosphere: CO2 / H2O ratio = 1 (30%CO2-30%H2O-1%H2-39%Ar) gas (flow rate 100cc / min)

[0093] [IV characteristics] Figure 1 is a graph showing the IV characteristics during co-electrolysis for each SOEC obtained in Examples 1 and 2 and Comparative Examples 1 and 2. Graph G1 in Figure 1 shows the IV curves from OCV to approximately 1.5V during co-electrolysis for each SOEC obtained in Example 1 and Comparative Example 1, superimposed on the graph. Graph G2 in Figure 1 shows the IV curves from OCV to approximately 1.5V during co-electrolysis for each SOEC obtained in Example 2 and Comparative Example 2, superimposed on the graph.

[0094] As can be seen from graph G1 in Figure 1, when comparing the current density at a voltage of 1.5V, the current density in Comparative Example 1 is approximately 0.17 A / cm². 2 On the other hand, in Example 1, the current density was approximately 0.2 A / cm². 2 Therefore, it was found that Example 1 had better IV characteristics than Comparative Example 1. This is thought to be because the inclusion of Nb oxide in the hydrogen electrode suppressed the oxidation of Ni contained in the NiO particles.

[0095] Furthermore, as can be seen from graph G2 in Figure 1, when comparing the current density at a voltage of 1.5V, the current density in Comparative Example 2 is approximately 0.12 A / cm². 2 On the other hand, in Example 2, the current density was approximately 0.3 A / cm². 2 Therefore, it was found that Example 2, in which NiO particles were coated with La-Nb oxide, had higher IV characteristics than Comparative Example 2, in which NiO particles were not coated with La-Nb oxide. This is thought to be because the inclusion of NiO particles and La-Nb oxide in the hydrogen electrode suppressed the oxidation of Ni contained in the Ni-containing particles.

[0096] [AC impedance] Impedance measurements were performed on each SOEC obtained in Example 1 and Comparative Example 1, and the Cole-Cole plots shown in Figure 2 were obtained. Figure 2 shows the Cole-Cole plots of each SOEC obtained in Example 1 and Comparative Example 1.

[0097] As can be seen from Figure 2, when comparing the change in the real part of the complex impedance, Example 1, in which NiO particles were coated with Nb oxide, showed a smaller change than Comparative Example 1, in which NiO particles were not coated with Nb oxide, indicating a significant reduction in the resistance of the activation polarization of the hydrogen electrode. This is thought to be because the catalytic activity of the NiO particles was increased by including NiO particles and Nb oxide in the hydrogen electrode.

[0098] [Durability Test] Durability tests were conducted using the SOECs obtained in Example 1 and Comparative Example 1. The durability test conditions were as follows: Cell temperature: 800℃ Air electrode atmosphere: 79% N2, 21% O2 Hydrogen electrode atmosphere: 30%CO2-30%H2O-40%Ar

[0099] Figure 3 is a graph showing the durability test results for each SOEC obtained in Example 1 and Comparative Example 1. Graph G3 in Figure 3 shows the SOEC obtained in Example 1 at 600 mA / cm². 2 This shows the cell voltage changes during co-electrolysis performed with a constant current. Graph G4 in Figure 3 shows the SOEC obtained in Comparative Example 1 at 600 mA / cm². 2 This shows the cell voltage changes during co-electrolysis performed with a constant current.

[0100] As can be seen from graph G4 in Figure 3, in Comparative Example 1, the cell voltage became unstable after about 7 hours from the start of evaluation, indicating degradation. On the other hand, as can be seen from graph G3 in Figure 3, the cell voltage in Example 1 remained stable for about 15 hours from the start of evaluation. Therefore, it was found that the SOEC of Example 1 had higher durability than the SOEC of Comparative Example 1. This is thought to be because the inclusion of NiO particles and Nb oxide in the hydrogen electrode suppressed the oxidation of Ni contained in the NiO particles.

[0101] Furthermore, similar durability tests were conducted using each SOEC obtained in Example 2 and Comparative Example 2. The degradation rate (%) was then calculated from the cell voltage values ​​before and after the durability test using the following formula (3). (V1-V0) / V0×100...Equation (3) Here, V0 is the cell voltage at the start of the endurance test, and V1 is the cell voltage after 30 hours of endurance testing.

[0102] As a result, the degradation rate in Comparative Example 2 was 23%, while the degradation rate in Example 2 was 0.4%. Therefore, it was found that the SOEC of Example 2 had higher durability than the SOEC of Comparative Example 2. This is thought to be because the inclusion of NiO particles and La-Nb oxide in the hydrogen electrode suppressed the oxidation of Ni contained in the NiO particles.

[0103] [SEM image of the hydrogen electrode after durability testing] Figure 4 shows SEM images of the hydrogen electrode after the durability test. SEM image P1 in Figure 4 is an SEM image of the surface of the hydrogen electrode obtained in Example 1 after the durability test. SEM image P2 in Figure 4 is an SEM image of the surface of the hydrogen electrode obtained in Comparative Example 1 after the durability test.

[0104] As can be seen from the SEM image P2 in Figure 4, needle-shaped carbon deposits were observed within the hydrogen electrode in Comparative Example 1, indicating that coking had occurred. On the other hand, as can be seen from the SEM image P1 in Figure 4, no carbon deposits were observed within the hydrogen electrode in Example 1. Therefore, it was found that the hydrogen electrode of Example 1 exhibited suppressed coking compared to the hydrogen electrode of Comparative Example 1. This is thought to be because, in the comparative example, the atmosphere within the electrode was non-uniform, leading to localized strong reduction and further reduction of CO, resulting in carbon deposition, while in the example, the niobium compound released oxygen when the reducing atmosphere was strong and absorbed oxygen when the oxidizing atmosphere was strong, thus maintaining a uniform oxidation-reduction atmosphere within the electrode.

[0105] Similarly, although not shown in the figures, SEM images of the hydrogen electrodes after durability tests of each SOEC obtained in Example 2 and Comparative Example 2 showed that needle-shaped carbon deposits were scattered within the hydrogen electrode in Comparative Example 1, while no carbon deposits were observed in the hydrogen electrode of Example 2. Therefore, it was found that the hydrogen electrode of Example 2 exhibited suppressed coking compared to the hydrogen electrode of Comparative Example 2. This is thought to be due to the same effect as in Example 1.

[0106] [XANES spectrum] XANES (X-ray Absorption Near Edge Structure) measurements were performed on each SOEC obtained in Examples 1 and 2 and Comparative Examples 1 and 2. The K-edge XANES spectra and K-edge radial distribution functions of Nb were obtained from the XANES measurements performed on each SOEC obtained in Examples 1 and 2 and Comparative Examples 1 and 2.

[0107] First, Figure 5 shows graphs of the K-edge XANES spectra and K-edge radial distribution functions for each SOEC obtained in Example 1 and Comparative Example 1, illustrating the XANES measurement results. Graph G5 in Figure 5 shows the K-edge XANES spectra of Nb superimposed for each SOEC obtained in Example 1 and Comparative Example 1. Graph G6 in Figure 5 shows the K-edge radial distribution functions of Nb superimposed for each SOEC obtained in Example 1 and Comparative Example 1.

[0108] From the K-edge XANES spectra and K-edge radial distribution functions shown in Figure 5, Example 1 and Comparative Example 1 showed almost identical spectra and distributions. Therefore, it was found that the electronic state of Ni contained in NiO particles does not change due to coating with Nb oxide.

[0109] Similarly, although not shown in the figures, the XANES measurement results for each SOEC obtained in Example 2 and Comparative Example 2, specifically the K-edge XANES Nb radial distribution function, show that Example 2 and Comparative Example 2 exhibited almost identical spectra and distributions. Therefore, it was found that the electronic state of Ni contained in NiO particles does not change due to the coating with La-Nb oxide.

[0110] Next, Figure 6 is a graph showing the K-edge XANES spectra of Nb oxide-coated NiO particles before and after reduction treatment. The K-edge XANES spectra of Nb were obtained by XANES measurements performed on each Nb oxide-coated NiO particle before and after the reduction treatment carried out during the preparation of SOEC in Example 1. The graph in Figure 6 shows the K-edge XANES spectra of Nb for each Nb oxide-coated NiO particle before and after reduction treatment, superimposed on the graph. In addition, the spectra of reference NbO and Nb2O5 are also superimposed on the graph in Figure 6.

[0111] From the K-edge XANES spectrum shown in Figure 6, an absorption was observed between NbO and Nb2O5, indicating that the valency of Nb in the Nb oxide coating the NiO particles was 3-4. The valency of Nb is thought to change reversibly through oxygen adsorption and desorption.

[0112] Similarly, although not shown in the figures, K-edge XANES spectra of Nb and La were obtained for La-Nb oxide-coated NiO particles before and after the reduction treatment performed during the preparation of SOEC in Example 2. From the obtained K-edge XANES spectra, absorption was observed between NbO and Nb2O5, indicating that the valency of Nb in the La-Nb oxide coating the NiO particles was 3-4. Furthermore, from the obtained K-edge XANES spectra, it was found that the valency of La in the La-Nb oxide coating the NiO particles was 3.

[0113] [Analysis of gases produced by SOEC] Figure 7 is a graph showing the results of the analysis of the gas produced by the SOEC obtained in Example 2. CO2 and H2O were supplied to the hydrogen electrode of the SOEC obtained in Example 2, and CO and H2 were produced by passing an electric current between the electrodes. As shown in the graph in Figure 7, the production rate of CO and H2 in the SOEC obtained in Example 2 increased as the current density increased. It was found that the SOEC obtained in Example 2 was capable of producing CO and H2 with 100% Faraday efficiency.

[0114] The above examples confirm that using Nb oxide-coated NiO particles or La-Nb oxide-coated NiO particles in the hydrogen electrode of SOEC can suppress the re-oxidation and reduction of Ni, as well as the occurrence of coking, thereby improving the durability of SOEC.

[0115] This disclosure is not limited to the embodiments described above, and may be modified as appropriate without departing from its spirit. [Explanation of Symbols]

[0116] G1-G6 Graphs

Claims

1. An electrode having a cermet layer containing Ni-containing particles and an Nb compound.

2. The electrode according to claim 1, wherein the Nb compound coats at least a portion of the surface of the Ni-containing particles.

3. The electrode according to claim 1, wherein the ratio of the mass of Nb contained in the Nb compound to the mass of Ni contained in the Ni-containing particles is 0.2 to 3.0 mass%.

4. The electrode according to claim 1, wherein the Nb compound is to which La is added.

5. The electrode according to claim 1, wherein the cermet layer comprises electrolyte particles having conductivity for oxide ions or for both oxide ions and electrons.

Images