Electrode

Abstract

Provided is an electrode that suppresses reoxidation and reduction of Ni and has improved initial characteristics and durability. The electrode has a cermet layer containing Ni-containing particles and an Nb compound. The Nb compound may cover at least a portion of the surface of the Ni-containing particles. The mass ratio of Nb contained in the Nb compound relative to the mass of Ni contained in the Ni-containing particles may be 0.2-3.0 mass%. La may be added to the Nb compound. The cermet layer may include electrolyte particles having oxide ion conductivity or oxide ion-electron dual conductivity.

Description

[0001] This disclosure relates to electrodes, and more particularly to electrodes that can be used as hydrogen electrodes in solid oxide electrolyzers (SOECs) or fuel electrodes in solid oxide fuel cells (SOFCs).

[0002] Known electrochemical cells include solid oxide fuel cells (SOFC) and solid oxide electrolyzers (SOEC), which have a solid electrolyte layer with oxide ion conductivity.

[0003] SOFC is a fuel cell that uses an oxide ion conductor as the electrolyte. If fuel gases such as H2, CO, or CH4 are supplied to the anode (fuel electrode) of the SOFC, and O2 is supplied to the cathode (oxygen electrode), an electrode reaction occurs, allowing electricity to be extracted. The CO2 and H2O generated through the electrode reaction are discharged outside the SOFC.

[0004] On the other hand, SOEC has the same structure as SOFC and undergoes the reverse reaction with SOFC. That is, if CO2 and H2O are supplied to the cathode (hydrogen electrode) of SOEC and current flows between the electrodes, CO and H2 can be generated.

[0005] In SOEC hydrogen electrodes, cermets such as Ni / YSZ, Ni / SDC, and Ni-Fe / SDC are sometimes used. However, in the hydrogen electrode, water vapor, which is the raw material for hydrogen production, is supplied at high temperatures (above 700°C). Therefore, the Ni contained in the hydrogen electrode is easily oxidized to form NiO. Since NiO is an insulator, if NiO forms in the electrode, the electron pathway in that part is interrupted, and the electrolysis reaction stops. As a result, the electrolysis characteristics are reduced.

[0006] The same applies to SOFCs. That is, water is generated at the fuel electrode of an SOFC through an electrode reaction. Therefore, especially under high-load operating conditions, the generated water vapor sometimes reduces power generation characteristics because it oxidizes the Ni in the fuel electrode.

[0007] Therefore, in electrodes used in SOFCs or SOECs, there is a need for techniques to suppress Ni oxidation. As an example of such a technique, 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 is composed of a cermet (B) containing Ni particles (B) and YScCZ particles composed of ZrO2 doped with Y, Sc, and Ce.

[0008] Patent Document 1: Japanese Patent Application Publication No. 2022-74189 Summary of the Invention

[0009] However, SOEC operates at high temperatures of around 800°C. Therefore, the technology described in Patent Document 1 suffers from the problem that insufficient initial properties and sufficient durability cannot be obtained due to the repeated re-oxidation and reduction of Ni.

[0010] This disclosure was made to solve such problems and aims to provide an electrode that inhibits the re-oxidation and reduction of Ni and improves initial properties and durability.

[0011] The electrode disclosed herein has a cermet layer comprising Ni particles and Nb compounds.

[0012] According to this disclosure, the aim is to provide an electrode that inhibits the re-oxidation and reduction of Ni, and improves initial properties and durability.

[0013] The above and other objects, features and advantages of this disclosure will be more fully understood from the detailed description and accompanying drawings given below. Attached Figure Description

[0014] Figure 1 A coordinate graph showing the IV characteristics of each SOEC obtained in Examples 1 and 2 and Comparative Examples 1 and 2 during co-electrolysis.

[0015] Figure 2 The figures are Cole-Cole diagrams of each SOEC obtained in Example 1 and Comparative Example 1.

[0016] Figure 3 A coordinate graph showing the durability test results of each SOEC obtained in Example 1 and Comparative Example 1.

[0017] Figure 4 This is a SEM image of the hydrogen electrode after the durability test.

[0018] Figure 5 The coordinate graphs of the K-end XANES spectra and the coordinate graphs of the K-end radial distribution function are shown to represent the XANES measurement results of each SOEC obtained in Example 1 and Comparative Example 1.

[0019] Figure 6 The coordinate plot shows the K-end XANES spectra of Nb oxide-coated NiO particles before and after reduction treatment.

[0020] Figure 7 This is a coordinate graph representing the analytical results of the gas generated by SOEC obtained in Example 2. Detailed Implementation

[0021] Implementation Method 1

[0022] The following detailed description of specific embodiments applying this disclosure is provided with reference to the accompanying drawings. However, this disclosure is not limited to the following embodiments. Furthermore, for clarity, the following description and drawings will be simplified as appropriate.

[0023] The electrodes disclosed herein are used as hydrogen electrodes in solid oxide electrolyzers (SOECs) or fuel electrodes in solid oxide fuel cells (SOFCs). These electrodes, which can be reversibly used in solid oxide fuel cells / electrolyzers, can be used as hydrogen electrodes in SOECs or fuel electrodes in SOFCs. In this embodiment, the electrodes used as hydrogen electrodes in SOECs will be specifically described.

[0024] [electrode]

[0025] The electrode disclosed herein has a cermet layer comprising Ni particles and Nb compounds.

[0026] [Contains Ni particles]

[0027] The Ni-containing particles are metallic particles in which the mass ratio of Ni relative to the total mass of metallic elements contained in the particles is 90% or more by mass. Preferably, the mass ratio of Ni contained in the Ni-containing particles is 95% or more by mass.

[0028] Ni-containing particles in a cermet layer function as both an electrode catalyst and an electron conductor. The composition of the Ni-containing particles is not particularly limited as long as they fulfill this function.

[0029] Examples of Ni-containing particles include Ni, Ni-Fe alloys, and Ni-Co alloys. Among these, Ni or Ni-Fe alloys are preferred.

[0030] Ni-containing particles have high electronic conductivity and are highly active as electrochemical catalysts, making them a preferred material for the electrode used as the hydrogen electrode. However, in SOEC, due to the passage of time, Ni-containing particles aggregate and become unevenly distributed, migrating away from the solid electrolyte layer, sometimes leading to electrode deterioration.

[0031] Therefore, it was considered to make the average particle size of Ni-containing particles within a range that could suppress the aggregation of Ni-containing particles. However, through repeated research, the inventors discovered that, regardless of the average particle size of the Ni-containing particles, the migration of Ni-containing particles occurs through repeated re-oxidation and reduction of Ni, resulting in the re-oxidation of Ni within the Ni-containing particles.

[0032] It is believed that the migration of Ni-containing particles occurs when an overvoltage is applied to the electrode to induce an electrode reaction. Since the electrode is a composite electrode, even in a reducing atmosphere, there are sometimes localized areas that are relatively oxidizing. Therefore, in these relatively oxidizing areas, Ni may be re-oxidized. If Ni is oxidized, its wettability to the electrolyte increases; if it is reduced, this wettability decreases. Therefore, if Ni is oxidized, the electrode becomes easier to peel off from the solid electrolyte layer, potentially reducing electrode durability. Furthermore, by oxidizing Ni, NiO, an insulator, is formed in the electrode, thereby interrupting the electron pathway in that area, halting the electrolysis reaction, and thus reducing the electrode's electrolytic properties.

[0033] Therefore, in order to suppress electrode degradation caused by the migration of Ni particles, it is desirable to suppress the re-oxidation and reduction of Ni.

[0034] Therefore, the electrode disclosed herein, by having a cermet layer containing Ni particles and Nb compounds, suppresses the re-oxidation and reduction of Ni contained in the Ni particles, thereby improving both initial characteristics (electrolytic characteristics) and durability.

[0035] [Nb compounds]

[0036] The Nb compound is a compound containing niobium (Nb). The mass percentage of Nb contained in the Ni compound is preferably 0.2% by mass or more and 3.0% by mass or less.

[0037] Nb compounds can acquire an indeterminate composition that corresponds to the oxygen partial pressure in the surrounding atmosphere. Therefore, Nb compounds can alter the oxidation state of Ni contained in Ni-containing particles present near the Nb compound. Thus, Nb compounds in a cermet layer can suppress the oxidation of Ni contained in Ni-containing particles.

[0038] The Nb compound preferably coats at least a portion of the surface containing Ni particles. By coating at least a portion of the surface containing Ni particles in the cermet layer, the Nb compound further inhibits the oxidation of Ni contained in the Ni particles.

[0039] The mass ratio of Nb in the Nb compound relative to the mass of Ni contained in the Ni-containing particles is not particularly limited and can be adjusted according to the mass of Ni contained in the coated Ni-containing particles. The preferred mass ratio of Nb in the Nb compound relative to the mass of Ni contained in the Ni-containing particles is 0.2 to 3.0% by mass. This ensures sufficient suppression of the oxidation of Ni contained in the Ni-containing particles.

[0040] Examples of Nb compounds include various Nb-containing compounds such as niobium alkoxides, niobium oxide, niobic acid, niobium hydroxide, niobium chloride, niobium nitrate, niobium sulfate, niobium oxalate, and niobium formate.

[0041] Among these, niobium oxide is preferred as the Nb compound. The niobium oxide Nb compound has high basicity, which helps to suppress the decrease in electrode activity caused by coking. Coking, in this context, refers to the phenomenon of carbon (C) deposition at the hydrogen electrode caused by an increased amount of CO generated from the co-electrolysis of CO2 / H2O in SOEC.

[0042] The niobium in niobium oxide can be in any of the following oxidation states: pentavalent, tetravalent, trivalent, divalent, or monovalent. Examples of niobium oxides include niobium monoxide (NbO), niobium dioxide (NbO2), and niobium pentoxide (Nb2O5). Niobium oxide is obtained, for example, by sintering.

[0043] For Nb compounds, elements other than Nb can be added. Lanthanum (La) is preferably added to Nb compounds. Compared with Nb compounds without added La, Nb compounds with added La can improve the catalytic activity and stability as Ni-particle-containing electrochemical catalysts.

[0044] More preferably, the Nb compound contains 0.8 to 1.2% La relative to the atomic weight of Nb1 in the Nb compound. This significantly improves the catalytic activity and stability of the Ni-containing particles.

[0045] The average thickness of the Nb compound coating on at least a portion of the Ni-containing surface is, for example, 1 to 20 nm, preferably uniform. Furthermore, the average thickness of the Nb compound can be calculated by observing the uniformly formed layer on the Ni-containing surface using electron microscopes such as scanning electron microscopes (SEM) and transmission electron microscopes (TEM), or by analyzing it using spectrometers such as those with accompanying energy dispersive X-ray spectrometers (EDS). It should be noted that the average thickness refers to the average value when measuring multiple locations.

[0046] In addition to containing Ni particles and Nb compounds, the cermet layer preferably contains electrolyte particles with oxide ionic conductivity or oxide ionic electronic conductivity. Materials suitable for the hydrogen electrode can be used as electrolyte particles without particular restriction.

[0047] Electrolyte particles exhibiting oxide ionic conductivity at least function as oxide ion conductors. Examples of such electrolyte particles include zirconium oxides and perovskite oxides, which are ceramic particles. Zirconium oxides may include, for example, zirconium oxide (ZrO2) and stabilized zirconium oxide. Stabilized zirconium oxide may be ZrO2 in which at least one stabilizer selected from Y2O3, Sc2O3, Yb2O3, Gd2O3, CaO, MgO, and CeO2 is dissolved. Perovskite oxides may include, for example, lanthanum strontium cobalt iron oxide (LSFM) and other perovskite oxides represented by ABO3. Among these, yttrium-stabilized zirconium oxide (YSZ) in which Y2O3 is dissolved is preferred.

[0048] Electrolyte particles exhibiting dual oxide-ionic-electronic conductivity possess at least the functions of both oxide-ionic and electron conductors. Examples of such electrolyte particles include ceramic particles such as cerium oxide. Cerium oxide may include, for example, cerium oxide (CeO2) and doped cerium oxide. Doped cerium oxide may include cerium oxide in which at least one rare earth element oxide selected from Sm2O3, Gd2O3, and Y2O3 is dissolved. Among these, gadolinium-doped cerium oxide (GDC) in which Gd2O3 is dissolved is preferred.

[0049] There are no particular limitations on the average particle size of the electrolyte particles, as long as the effects of the disclosed technology are achieved. The average particle size of the electrolyte particles is, for example, 0.1 to 5 μm, preferably 0.2 to 2 μm.

[0050] [Composition of the metal-ceramic layer]

[0051] The content of Ni particles is the ratio of the mass of Ni particles to the total mass of Ni particles, Nb compounds, and electrolyte particles.

[0052] If the content of Ni particles is too low, the total cell resistance increases, and sometimes the efficiency of the electrode reaction also decreases. Therefore, the content of Ni particles is preferably 30% by mass or more. More preferably, the content of Ni particles is 40% by mass or more.

[0053] On the other hand, if the content of Ni particles becomes excessive, the content of electrolyte particles decreases. As a result, the oxidation inhibition function of Ni contained in the Ni particles may decrease, or the strength of the cermet layer may decrease. Therefore, the content of Ni particles is preferably 70% by mass or less.

[0054] [Porosity of the metal-ceramic layer]

[0055] There are no particular limitations on the porosity of the metal-ceramic layer; the optimal porosity can be selected based on the purpose.

[0056] For example, when a cermet layer is used as the active layer of an electrode, the porosity of the active layer affects the electrolysis characteristics. If the porosity of the active layer is too small, the gas diffusivity decreases, and the efficiency of the electrode reaction decreases. Therefore, the porosity of the active layer is preferably 15% or more. More preferably, it is 20% or more, and even more preferably 25% or more.

[0057] On the other hand, if the porosity of the active layer becomes excessively large, the three-phase interface becomes relatively smaller, and the efficiency of the electrode reaction decreases. Therefore, the porosity of the active layer is preferably 40% or less. More preferably, it is 35% or less, and even more preferably 30% or less.

[0058] Furthermore, for example, when a cermet layer is used as the diffusion layer of the 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 small, the gas diffusivity decreases. Therefore, the porosity of the diffusion layer is preferably 40% or more. More preferably, it is 45% or more, and even more preferably 50% or more.

[0059] On the other hand, if the porosity of the diffusion layer becomes excessively large, the strength and electronic conductivity decrease. Therefore, the porosity of the diffusion layer is preferably 60% or less. More preferably, it is 58% or less, and even more preferably 55% or less.

[0060] [Active layer and diffusion layer]

[0061] The electrode disclosed herein has at least an active layer. In addition to the active layer, the electrode may have a diffusion layer formed on the electrolyte layer side surface of the active layer. That is, the electrode may sometimes have only an active layer, and sometimes it may have an active layer and a diffusion layer formed on the electrolyte layer side surface of the active layer.

[0062] The active layer serves as the reaction field for the electrolysis reaction. The active layer needs to transport oxide ions generated during the electrolysis reaction to the electrolyte, therefore requiring high ionic conductivity.

[0063] On the other hand, the diffusion layer serves to support the active layer. In a hydrogen electrode composed of a laminate of diffusion and active layers, the electrode reaction mainly occurs within the active layer. Therefore, the diffusion layer does not necessarily need to have high ionic conductivity.

[0064] That is, the diffusion layer needs to have at least

[0065] (a) Function for supporting the active layer formed on its electrolyte layer side surface,

[0066] (b) The function of diffusing the electrolytic raw materials to the active layer.

[0067] (c) The function of transporting electrons required for the reduction reaction from the current collector to the active layer, and

[0068] (d) Function to remove hydrogen generated in the active layer through electrode reaction to the outside of the hydrogen electrode.

[0069] The composition of the diffusion layer is not particularly limited as long as it serves this function.

[0070] The aforementioned metal-ceramic layer can be used as at least one of the active layer and the diffusion layer.

[0071] That is, the electrodes involved in this disclosure can be

[0072] (a) An electrode consisting only of an active layer, wherein the active layer is composed of the aforementioned metal-ceramic layer.

[0073] (b) An electrode comprising a two-layer structure of an active layer and a diffusion layer, wherein the active layer is composed of the aforementioned metal-ceramic layer, and the diffusion layer is composed of layers other than the aforementioned metal-ceramic layer.

[0074] (c) A two-layer structure comprising an active layer and a diffusion layer, wherein both the active layer and the diffusion layer are composed of the aforementioned metal-ceramic layers;

[0075] (d) Any of the following electrodes: a two-layer structure comprising an active layer and a diffusion layer, wherein the diffusion layer is composed of the aforementioned metal-ceramic layer and the active layer is composed of a layer other than the aforementioned metal-ceramic layer.

[0076] When the diffusion layer or active layer is composed of a layer other than the aforementioned cermet layer (hereinafter also referred to as the "second layer"), there are no particular limitations on the composition of the second layer. The second layer is typically composed of a cermet containing second Ni-containing particles and electrolyte particles composed of solid oxides.

[0077] The second Ni-containing particles contained in the second layer may have the same composition as the Ni-containing particles contained in the aforementioned cermet layer, or they may have a different composition.

[0078] In addition, the electrolyte particles contained in the second layer may have the same composition as the electrolyte particles contained in the aforementioned metal-ceramic layer, or they may have a different composition.

[0079] To suppress electrode degradation caused by the oxidation of Ni particles, the electrode preferably has at least an active layer composed of a cermet layer. Other aspects related to the cermet layer are as described above and therefore omitted.

[0080] [use]

[0081] As described above, the electrode disclosed herein can be used not only as the hydrogen electrode of SOEC but also as the fuel electrode of SOFC. SOFC, aside from its different application, comprises the same structure as SOEC, therefore detailed description is omitted.

[0082] [Electrode manufacturing method]

[0083] The electrodes disclosed herein can be manufactured using various methods. For example, when the electrode has a single-layer structure, the electrode can be manufactured by...

[0084] (a) Firing a first mixture of raw materials containing Ni particles and Nb compounds.

[0085] (b) Using the obtained Nb compound to coat a second mixture of raw materials containing Ni particles and electrolyte particles, a molded body is formed.

[0086] (c) Sinter the resulting molded body.

[0087] (d) The obtained sintered body is subjected to reduction treatment to manufacture.

[0088] [First Process]

[0089] The first step is the process of calcining a first mixture of raw materials containing Ni particles and Nb compounds.

[0090] The raw material containing Ni particles is the raw material that becomes Ni-containing after sintering and reduction. There are no particular limitations on the type of raw material containing Ni particles, and the optimal raw material can be selected according to the purpose. Examples of raw materials containing Ni particles include NiO powder, Fe2O3 powder, Fe3O4 powder, mixtures of metallic Fe and NiO or metallic Ni, CoO powder, and Co2O3 powder.

[0091] The raw materials for Nb compounds are those that, upon firing, become Nb compounds. There are no particular limitations on the type of raw material for Nb compounds, as long as it can generate Nb compounds after firing. Examples of raw materials for Nb compounds include niobium alkoxides such as niobium isopropoxide. Such raw materials for Nb compounds can be used as solutions dissolved in solvents (hereinafter sometimes referred to as Nb raw material solutions). Examples of solvents include alcohols such as ethanol, water, and mixed solvents of alcohol and water. The firing temperature can be, for example, 500°C or higher and 1000°C or lower.

[0092] There are no particular limitations on the methods for obtaining Nb compounds from Nb compound starting materials; the optimal method can be selected based on the objective. Examples of methods for obtaining Nb compounds include synthesis using hydrolysis, impregnation, or sol-gel methods.

[0093] The raw materials containing Ni particles and Nb compounds are preferably combined in a manner that yields the target coating rate of Nb compounds after calcination.

[0094] Additionally, for example, in the case of a Nb compound with added La, the first mixture may contain a La source. There is no particular limitation on the type of La source, and the optimal source can be selected according to the purpose. Examples of La sources include lanthanum alkoxides such as lanthanum isopropoxide.

[0095] [Second Process]

[0096] The second step is to form a molded article by coating a second mixture containing Ni particles with the obtained Nb compound. The second mixture may contain raw materials containing electrolyte particles.

[0097] The raw materials for electrolyte particles are those that become electrolyte particles after sintering. There are no particular restrictions on the types of raw materials for electrolyte particles; the best raw materials can be selected according to the purpose.

[0098] For example, when the electrolyte particles are YSZ, as its raw material, there are...

[0099] (a) YSZ powder with the target composition,

[0100] (b) A mixture of ZrO2 powder and Y2O3 powder in a manner that is formulated as a target composition.

[0101] Additionally, the second mixture may contain pore-forming materials such as carbon powder. The NiO powder and other metal oxides contained in the Ni-particle-containing raw material added to the second mixture are reduced after the sintered body is formed. At this time, volume shrinkage occurs, introducing pores into the sintered body. Therefore, pore-forming materials are not necessarily required. However, if pore-forming materials are added to the second mixture, the degree of freedom in controlling the porosity increases.

[0102] The Nb compound coated with Ni particles and electrolyte particles is preferably formulated in such a way that a metal-ceramic layer with the desired composition is obtained after sintering and reduction.

[0103] There are no particular limitations on the method for manufacturing molded objects; the best method can be chosen based on the purpose. Examples of methods for manufacturing molded objects include...

[0104] (a) A method of forming a slurry containing a second mixture into strips, laminating the resulting green sheet onto a substrate (e.g., a molded body for making a second layer), isostatically pressing (CIP) the laminate, and then pressing the laminate together.

[0105] (b) A method for preparing a paste containing a second mixture and screen printing the paste onto the surface of a substrate, etc.

[0106] [Third Process]

[0107] The third step is to sinter the obtained molded body. The sintering conditions are preferably selected based on the composition of the raw materials. Sintering is generally preferably carried out in an atmospheric atmosphere at a temperature above 1000°C and below 1400°C for 1 to 5 hours.

[0108] When the second mixture contains two or more oxides, a solid-phase reaction occurs during sintering, sometimes forming a solid solution with a specified composition. Furthermore, when the second mixture contains a pore-forming material, the pore-forming material disappears during sintering, forming pores within the sintered body.

[0109] [Fourth Process]

[0110] The fourth step is to perform a reduction treatment on the obtained sintered body. This forms a cermet layer, resulting in an electrode comprising the cermet layer. The reduction treatment is performed to reduce metal oxides such as NiO contained in the sintered body, generating Ni-containing particles. There are no particular limitations on the reduction conditions, but optimal conditions are preferably selected based on the composition of the cermet layer. Reduction is preferably performed in a hydrogen-reducing atmosphere at a temperature above 600°C and below 800°C.

[0111] Furthermore, SOECs are, for example, composed of a junction of a hydrogen electrode (cathode), an electrolyte layer, and an oxygen electrode (anode). The reduction of the cermet layer typically occurs after the layers are bonded together. This is also the case with SOFCs.

[0112] Example

[0113] The present disclosure is further illustrated by the following embodiments, but is not limited to these embodiments. In this embodiment, the results of co-electrolysis of CO2 / H2O in SOEC are shown. However, the electrode degradation mechanism is the same in the electrolysis of CO2 and H2O, therefore it is assumed that the same evaluation results will be obtained in the electrolysis of CO2 and H2O.

[0114] [1. Sample Preparation]

[0115] [Example 1]

[0116] [Coating of Nb compounds containing Ni particles]

[0117] [Synthesis example 1]

[0118] Using a hydrolysis method, niobium oxide (Nb oxide) was synthesized by coating the surface of NiO particles, which are Ni-containing particles, with Nb compounds.

[0119] First, niobium isopropoxide (0.33 mL), which is the raw material for Nb compounds, was dissolved in anhydrous ethanol (50 mL) as a solvent in an Nb raw material solution. NiO powder (1.98 g), which is the raw material containing Ni particles, was then added and stirred at room temperature for 1 hour to obtain a slurry of the first mixture.

[0120] Next, 1 mL of distilled water was added to the slurry of the first mixture to hydrolyze the niobium isopropoxide in the first mixture. The mixture was then concentrated at 60°C using an evaporator to obtain the powder of the first mixture.

[0121] Next, the powder of the first mixture was calcined at 400°C for 4 hours. Thus, NiO particles coated with Nb oxide (1% by mass) were obtained.

[0122] [Synthesis example 2]

[0123] Niobium oxide (Nb oxide) was synthesized by coating the surface of NiO particles, which are Ni-containing particles, with a Nb compound using an impregnation method.

[0124] First, niobium isopropoxide (0.33 mL), which is the raw material for Nb compounds, was dissolved in anhydrous ethanol (1 mL), which is the solvent, to form an Nb raw material solution. NiO powder (1.98 g), which is the raw material containing Ni particles, was then added and evaporated to solidify while stirring, resulting in a powder of the first mixture.

[0125] Next, the 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.

[0126] [Synthesis example 3]

[0127] Using the sol-gel method, niobium oxide (Nb oxide) was synthesized by coating the surface of NiO particles, which are Ni-containing particles, with Nb compounds.

[0128] First, in an Nb raw material solution in which 0.33 mL of niobium isopropoxide (as a raw material for Nb compounds) was dissolved in 50 mL of anhydrous butanol (as a solvent), NiO powder (1.98 g) and glycerol (10 mL) (as a raw material containing Ni particles) were added sequentially, and the mixture was stirred at 160 °C for 12 hours to obtain a gel of the first mixture.

[0129] Next, the gel of the first mixture was pre-calcined at 300°C to decompose the glycerol in the first mixture, and then calcined at 400°C for 4 hours. Thus, NiO particles coated with Nb oxide (1% by mass) were obtained.

[0130] For the NiO particles coated with Nb oxide obtained in Synthesis Examples 1 to 3, XPS (X-ray Photoelectron Spectroscopy), SEM-EDS, and XRD (X-Ray Diffraction) measurements were performed respectively.

[0131] XPS patterns were obtained by performing XPS measurements on the Nb oxide-coated NiO particles obtained in Synthesis Examples 1-3. Based on the XPS patterns, the Nb compound in each Nb oxide-coated NiO particle was confirmed to be Nb₂O₅.

[0132] Furthermore, based on the XPS pattern, the Nb oxide coating ratio of the NiO particles obtained in Synthesis Examples 1-3 was calculated relative to the NiO particles using the following formula (1). The resulting coating ratio was 1% by mass.

[0133] (Detection amount of Nb) / (Detection amount of Ni and Nb)×100···Equation (1)

[0134] SEM-EDS measurements were performed on the Nb oxide-coated NiO particles obtained in Synthesis Examples 1-3 to obtain SEM images and elemental distribution images. Then, the SEM images of the Nb oxide-coated NiO particles, the elemental distribution images of Ni, and the elemental distribution images of Nb were overlaid. Since there are elemental portions from Ni and elemental portions from Nb on the surface of the Nb oxide-coated NiO particles, it was confirmed that Nb₂O₅ coated NiO.

[0135] XRD patterns were obtained by XRD analysis of the NiO particles coated with Nb oxide obtained in Synthesis Examples 1-3. Based on the XRD patterns, no peaks from Nb were detected, thus confirming that Nb₂O₅ coated NiO with a thickness of several nm.

[0136] Fabrication of Solid Oxide Electrolyte (SOEC) Cells

[0137] La will be used as a solid electrolyte 1-x Sr x Ga 1-y Mg y Lanthanum strontium gallium magnesium oxide (LSGM) powder, represented by the formula O3 (0 < x < 0.3, 0.0 < y < 0.3), was cold isostatically pressed to fabricate an electrolyte layer molded body with a diameter of 1.5 mm and a thickness of 0.5 mm. One side of the fabricated electrolyte layer molded body will contain a layer composed of Ba... 0.6 La 0.4A 0.5 mm diameter, 30 μm thick oxygen electrode was fabricated by screen printing a paste containing barium lanthanum cobalt oxide (BLC) represented by the formula CoO3. Then, a hydrogen electrode was fabricated by screen printing a paste containing a second mixture of Nb oxide-coated NiO particles obtained in Synthesis Examples 1-3 on the other side of the fabricated electrolyte layer. Thus, a disc-shaped laminate consisting of a hydrogen electrode, an electrolyte layer, and an oxygen electrode was fabricated.

[0138] A platinum screen was placed on both sides of the laminated body, platinum leads were installed, and then the laminated body was sealed with glass and placed in an alumina tube. The laminated body was then dried and sintered at 1100°C for 12 hours in an atmospheric atmosphere. Next, the resulting sintered body was reduced at 800°C for 2 hours in a hydrogen reducing atmosphere. Through this reduction treatment, the NiO contained in the hydrogen-formed body was reduced to produce Ni.

[0139] As described above, an SOEC consisting of a hydrogen electrode (cathode), an electrolyte layer, and an oxygen electrode (anode) was fabricated.

[0140] [Example 2]

[0141] [Coating of Ni-containing Nb compounds]

[0142] [Synthesis Example 4]

[0143] La-Nb oxides, which are Ni-containing NiO particles coated with La-added Nb compounds, were synthesized using a hydrolysis method.

[0144] First, lanthanum isopropoxide (0.049 g), which is the raw material for La, and niobium isopropoxide (0.73 mL), which is the raw material for Nb compounds, were dissolved in anhydrous ethanol (50 mL) as a solvent. NiO powder (4.95 g), which is the raw material for Ni particles, was then added to the Nb raw material solution and stirred at room temperature for 1 hour to obtain a slurry of the first mixture.

[0145] In addition to obtaining the first mixture, Nb oxide-coated NiO particles were prepared by the same method as in Synthesis Example 1, resulting in Nb oxide-coated NiO particles in which NiO particles were coated with La-Nb oxide (coating rate 1 wt%). Hereinafter, Nb oxide-coated NiO particles in which NiO particles are coated with La-Nb oxide are sometimes referred to as La-Nb oxide-coated NiO particles.

[0146] The La-Nb oxide-coated NiO particles obtained in Synthesis Example 4 were subjected to XPS, SEM-EDS, and XRD measurements, respectively.

[0147] 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, the La-added Nb compound in the La-Nb oxide-coated NiO particles was confirmed to be LaNbO5.

[0148] Furthermore, based on the XPS pattern, the La-Nb oxide coating ratio of the NiO particles obtained in Synthesis Example 4 was calculated relative to the NiO particles using the following equation (2). The result showed that the coating ratio was 1% by mass.

[0149] (Detection amount of La and Nb) / (Detection amount of Ni, La and Nb)×100···Equation (2)

[0150] 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 distribution images. Then, the SEM images of the La-Nb oxide-coated NiO particles, the elemental distribution images of Ni, Nb, and La were overlaid. Since there are elemental portions from Ni on the surface of the La-Nb oxide-coated NiO particles, and elemental portions from Nb and La are present in the same location, it was confirmed that LaNbO5 coated NiO.

[0151] XRD patterns were obtained by XRD analysis of the La-Nb oxide-coated NiO particles obtained in Synthesis Example 4. Furthermore, based on the XRD pattern, no peaks originating from La were detected, thus confirming that LaNbO5 coated NiO with a thickness of several nm.

[0152] Fabrication of Solid Oxide Electrolyte (SOEC) Cells

[0153] The electrolyte layer molded body and the oxygen electrode molded body were fabricated using the same method as in Example 1. Then, the La-Nb oxide-coated NiO particles obtained in Synthesis Example 4 and the La-coated NiO particles were... 0.6 Sr 0.4 Fe 0.9 Mn 0.1 O 3-δ The LSFM powder represented by the formula was mixed in a mass ratio of 90:10, along with a plasticizer and a binder, and dispersed in an organic solvent in which the dispersant was dissolved to obtain a slurry of the second mixture. In addition to obtaining the second mixture, the SOEC was prepared using the same method as in Example 1, producing a disc-shaped laminate composed of a hydrogen electrode molded body / electrolyte layer molded body / oxygen electrode molded body.

[0154] Secondly, the same method as in Example 1 was used to fire the fabricated laminate and perform a reduction treatment on the fired body.

[0155] As described above, an SOEC consisting of a hydrogen electrode (cathode), an electrolyte layer, and an oxygen electrode (anode) was fabricated.

[0156] [Comparative Example 1]

[0157] SOEC was fabricated using the same method as in Example 1, except that NaO particles without Nb oxide coating were used instead of Nb oxide-coated NiO particles.

[0158] [Comparative Example 2]

[0159] SOEC was fabricated using the same method as in Example 2, except that NaO particles without La-Nb oxide coating were used instead of La-Nb oxide-coated NiO particles.

[0160] [2. Experimental Methods and Evaluation Results]

[0161] [Co-electrolysis test]

[0162] The obtained SOEC was used to conduct a co-electrolysis experiment of CO2 / H2O. The co-electrolysis conditions are described below.

[0163] Temperature: 800℃

[0164] Oxygen electrode atmosphere: dry air (flow rate 100cc / min)

[0165] Hydrogen electrode atmosphere: CO2 / H2O ratio = 1 (30% CO2 - 30% H2O - 1% H2 - 39% Ar) gas (flow rate 100 cc / min)

[0166] [IV Characteristics]

[0167] Figure 1 This is a coordinate graph showing the IV characteristics of each SOEC obtained in Examples 1 and 2 and Comparative Examples 1 and 2 during co-electrolysis. Figure 1 In coordinate graph G1, the OCV to IV curves near 1.5V during co-electrolysis of each SOEC obtained in Example 1 and Comparative Example 1 are superimposed. Figure 1 In the coordinate graph G2, the OCV to IV curves near 1.5V during co-electrolysis of each SOEC obtained in Example 2 and Comparative Example 2 are superimposed.

[0168] As by Figure 1 As shown in coordinate graph G1, when comparing the current density at a voltage of 1.5V, the current density in Comparative Example 1 is approximately 0.17 A / cm². 2On the other hand, in Example 1, the current density is approximately 0.2 A / cm². 2 Therefore, it can be seen that Example 1 has higher IV characteristics compared to Comparative Example 1. This is believed to be because the hydrogen electrode in Example 1 contains Nb oxide, thereby suppressing the oxidation of Ni contained in the NiO particles.

[0169] Additionally, as by Figure 1 As shown in coordinate graph G2, 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, the current density in Example 2 is approximately 0.3 A / cm². 2 Therefore, it can be seen that Example 2, which uses La-Nb oxide to coat NiO particles, has higher IV characteristics compared to Comparative Example 2, which does not use La-Nb oxide to coat NiO particles. This is believed to be because, in Example 2, the oxidation of Ni contained in the Ni particles is suppressed by including NiO particles and La-Nb oxide through the hydrogen electrode.

[0170] [AC impedance]

[0171] Impedance measurements were performed on each SOEC obtained in Example 1 and Comparative Example 1. Figure 2 The Cole-Cole diagram shown. Figure 2 The figures are Cole-Cole diagrams of each SOEC obtained in Example 1 and Comparative Example 1.

[0172] Depend on Figure 2 It can be seen that, comparing the change in the real part of the complex impedance, the change in Example 1, where NiO particles are coated with Nb oxide, is smaller than that in Comparative Example 1, where NiO particles are not coated with Nb oxide. This indicates that the resistance of the activation polarization of the hydrogen electrode is significantly reduced in Example 1. This is believed to be because, in Example 1, the hydrogen electrode contains both NiO particles and Nb oxide, thereby increasing the catalytic activity of the NiO particles.

[0173] [Durability Testing]

[0174] Durability tests were conducted using the SOECs obtained in Example 1 and Comparative Example 1. The durability test conditions are as follows.

[0175] Pool temperature: 800℃

[0176] Air atmosphere: 79% N2, 21% O2

[0177] Hydrogen electrode atmosphere: 30% CO2 - 30% H2O - 40% Ar

[0178] Figure 3 This is a coordinate graph representing the durability test results of each SOEC obtained in Example 1 and Comparative Example 1. Figure 3 In coordinate graph G3, for the SOEC obtained in Example 1, it is shown that 600 mA / cm 2 The shift in cell voltage during co-electrolysis with constant current. Figure 3 In coordinate graph G4, for the SOEC obtained in Comparative Example 1, the figure shows the results using 600 mA / cm². 2 The shift in cell voltage during co-electrolysis with constant current.

[0179] As by Figure 3 As shown in coordinate graph G4, in Comparative Example 1, the cell voltage became unstable approximately 7 hours after the evaluation began, indicating that degradation had occurred. On the other hand, as shown in... Figure 3 As shown in coordinate graph G3, in Example 1, the cell voltage stabilized approximately 15 hours after the evaluation began. Therefore, it can be concluded that the SOEC of Example 1 has higher durability compared to the SOEC of Comparative Example 1. This is believed to be because, in Example 1, the oxidation of Ni contained in the NiO particles was suppressed by including NiO particles and Nb oxide at the hydrogen electrode.

[0180] In addition, the same durability test was performed using each SOEC obtained in Example 2 and Comparative Example 2. Furthermore, the degradation rate (%) was calculated using the following formula (3) from the cell voltage values ​​before and after the durability test.

[0181] (V1-V0) / V0×100···Equation (3)

[0182] Where V0 is the cell voltage at the start of the durability test, and V1 is the cell voltage after 30 hours of durability test.

[0183] As a result, the degradation rate was 23% in Comparative Example 2, while it was 0.4% in Example 2. Therefore, it can be seen that the SOEC of Example 2 has higher durability compared to the SOEC of Comparative Example 2. This is believed to be because the oxidation of Ni contained in the NiO particles is suppressed by including NiO particles and La-Nb oxide at the hydrogen electrode.

[0184] [SEM image of the hydrogen electrode after durability testing]

[0185] Figure 4 This is a SEM image of the hydrogen electrode after the durability test. Figure 4 SEM image P1 is an SEM image of the surface of the hydrogen electrode after the durability test of the SOEC obtained in Example 1. Figure 4 SEM image P2 is an SEM image of the surface of the hydrogen electrode after the durability test of the SOEC obtained in Comparative Example 1.

[0186] As by Figure 4As can be seen from SEM image P2, in Comparative Example 1, the presence of scattered needle-like carbon deposits within the hydrogen electrode indicates that coking has occurred. On the other hand, as shown by... Figure 4 As shown in SEM image P1, no carbon precipitation was confirmed within the hydrogen electrode in Example 1. Therefore, it can be concluded that the hydrogen electrode of Example 1 suppressed coking compared to the hydrogen electrode of Comparative Example 1. This is believed to be because, in the Comparative Example, the atmosphere within the electrode was non-uniform, with locally enhanced reduction, further reducing CO and causing carbon precipitation. On the other hand, in the Example, the niobium compound released oxygen under a strong reducing atmosphere and absorbed oxygen under a strong oxidizing atmosphere, thus maintaining a uniform redox atmosphere within the electrode.

[0187] Similarly, although not illustrated, SEM images of the hydrogen electrodes after durability tests of each SOEC obtained in Example 2 and Comparative Example 2 show scattered needle-like carbon deposits within the hydrogen electrode in Comparative Example 2, while no carbon deposits were confirmed within the hydrogen electrode in Example 2. Therefore, it can be concluded that the hydrogen electrode of Example 2 suppressed coking compared to the hydrogen electrode of Comparative Example 2. This is believed to be because it achieved the same effect as Example 1.

[0188] [XANES score]

[0189] 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-end XANES spectrum and K-end radial distribution function of Nb were obtained from the XANES measurements of each SOEC obtained in Examples 1 and 2 and Comparative Examples 1 and 2.

[0190] first, Figure 5 To illustrate the coordinate plots of the K-end XANES spectra and the K-end radial distribution function for the XANES measurements of each SOEC obtained in Example 1 and Comparative Example 1. Figure 5 In coordinate graph G5, the K-end XANES spectra of Nb are overlaid for each SOEC obtained in Example 1 and Comparative Example 1. Figure 5 In the coordinate graph G6, for each SOEC obtained in Example 1 and Comparative Example 1, the radial distribution function of the K end of Nb is represented by overlap.

[0191] Depend on Figure 5 The K-terminal XANES spectrum and K-terminal radial distribution function shown in Example 1 and Comparative Example 1 exhibit almost identical spectra and distributions. Therefore, it can be concluded that the electronic state of Ni contained in the NiO particles is not changed by the coating of Nb oxide.

[0192] Similarly, although not illustrated, Example 2 and Comparative Example 2 show almost identical spectra and distributions based on the K-terminal radial distribution function of Nb's K-terminal XANESN, representing the XANES measurements of each SOEC obtained in Example 2 and Comparative Example 2. Therefore, it can be concluded that the electronic state of Ni contained in NiO particles is not changed by the La-Nb oxide coating.

[0193] Secondly Figure 6 A coordinate graph showing the K-end XANES spectra of Nb oxide-coated NiO particles before and after reduction treatment is provided. The K-end XANES spectra of Nb were obtained by XANES measurements of each Nb oxide-coated NiO particle before and after the reduction treatment performed during the fabrication of the SOEC in Example 1. Figure 6 In the coordinate diagram, the K-end XANES spectra of Nb are superimposed for each Nb oxide-coated NiO particle before and after reduction treatment. Furthermore, in... Figure 6 The coordinate graph also shows the overlapping spectra of the reference NbO and Nb2O5.

[0194] Depend on Figure 6 The K-terminal XANES spectrum shown indicates absorption between NbO and Nb₂O₅, suggesting that the Nb valence in the Nb oxide coated with NiO particles is 3–4. It is assumed that the Nb valence changes reversibly due to oxygen adsorption and desorption.

[0195] Similarly, although not illustrated, K-terminal XANES spectra of Nb and La were obtained, representing the XANES measurement results of the La-Nb oxide-coated NiO particles before and after the reduction treatment performed during the preparation of the SOEC in Example 2. The obtained K-terminal XANES spectra show absorption between NbO and Nb₂O₅, indicating that the Nb valence in the La-Nb oxide coated with NiO particles is 3 to 4. Furthermore, the obtained K-terminal XANES spectra show that the La valence in the La-Nb oxide coated with NiO particles is 3.

[0196] [Analysis of gases generated by SOEC]

[0197] Figure 7 This is a graph showing the analytical results of the gases generated by the SOEC obtained in Example 2. By supplying CO2 and H2O to the hydrogen electrode of the SOEC obtained in Example 2, and flowing current between the electrodes, CO and H2 were generated. Figure 7 As shown in the coordinate graph, the CO and H2 generation rates of the SOEC obtained in Example 2 increase with increasing current density. Furthermore, it is evident that the SOEC obtained in Example 2 can generate CO and H2 with 100% Faraday efficiency.

[0198] The above examples confirm that by using Nb oxide-coated NiO particles or La-Nb oxide-coated NiO particles as the hydrogen electrode of SOEC, it is possible to suppress the re-oxidation and reduction of Ni and inhibit coking, thereby improving the durability of SOEC.

[0199] It should be noted that this disclosure is not limited to the above-described embodiments and can be appropriately modified without departing from the spirit of the invention.

[0200] As will be apparent from the disclosure described herein, embodiments of this disclosure can be varied in many ways. These variations should not be considered a departure from the spirit and scope of this disclosure, and it will be apparent to those skilled in the art that all such modifications are intended to be included within the scope of the appended claims.

Claims

1. An electrode having a cermet layer containing Ni particles and Nb compounds.

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

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

4. The electrode according to claim 1, wherein, The Nb compound was enriched with La.

5. The electrode according to claim 1, wherein, The metal-ceramic layer contains electrolyte particles that have oxide ionic conductivity or oxide ionic electronic conductivity.

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