electrode

Abstract

To provide an electrode that undergoes minimal change in its structure even when exposed to high-temperature steam. [Solution] The electrode comprises electrolyte particles and Ni-based particles. The electrolyte particles contain CeO2 (LSDC) doped with La and Sm. The Ni-based particles consist of core-shell particles in which part or all of the surface of a core made of Ni or a Ni-based alloy is covered with a shell made of a composite oxide containing NiO or Ni. Preferably, the LSDC has a total content of La and Sm of more than 0 mol% and 20.0 mol% or less.

Description

[Technical Field]

[0001] The present invention relates to electrodes, and more particularly to electrodes suitable as fuel electrodes for solid oxide electrolytic cells (SOECs) or solid oxide fuel cells (SOFCs). [Background technology]

[0002] Solid oxide fuel cells (SOFCs) are fuel cells that use an oxide ion conductor as an electrolyte. 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), an electrode reaction proceeds, and electricity can be extracted. The CO2 and H2O generated by the electrode reaction are discharged outside the SOFC. On the other hand, solid oxide electrolytic cells (SOECs) have the same structure as SOFCs, but they cause the opposite reaction. That is, by supplying CO2 or H2O to the cathode (fuel electrode) of an SOEC and passing an electric current between the electrodes, CO or H2 can be produced.

[0003] SOECs consist of a single cell in which an anode (air electrode) is bonded to one side of the electrolyte and a cathode (fuel electrode) is bonded to the other side. Generally, the following materials are used as materials for the components that make up such SOECs (see Non-Patent Documents 1-5). (a) Electrolytes: Yttria-stabilized zirconia (YSZ), Scandia-stabilized zirconia (ScSZ), Scandia-yttria-stabilized zirconia (ScYSZ), Samaria-dope ceria (SDC), Lanthanum-strontium-gallium-magnesium oxide (LSGM), etc. (b) Air electrode: Lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium cobaltite (LSC), etc. (c) Fuel electrode: Ni / YSZ, Ni / ScYSZ, Ni-Cu / YSZ, etc.

[0004] Patent Document 1 describes a fuel electrode, but with respect to ceria, GdO 1.5 A reaction prevention layer for solid oxide fuel cells doped with more than 10 mol% but less than 30 mol% is disclosed. Patent Document 2 discloses an active layer equipped with a gas channel for diffusing gas from a diffusion layer toward an electrolyte layer, although this is not intended for the optimization of fuel electrode materials.

[0005] Ni / YSZ cermet is generally used for the fuel electrode of SOECs. However, the fuel electrode is supplied with steam, the raw material for hydrogen production, at high temperatures (above 700°C). Therefore, it is known that the electrolytic properties of SOECs using Ni / YSZ as the fuel electrode gradually deteriorate when used for a long period of time. This is thought to be because the Ni particles are oxidized when the fuel electrode is exposed to high-temperature steam, and evaporate as nickel hydroxide, which has a low vapor pressure. As a result, the structure of the fuel electrode is thought to change.

[0006] Various proposals have been made to solve this problem. For example, Patent Documents 3 to 6 propose a fuel electrode comprising Ni-containing particles and ACZ particles made of a composite oxide (ACZ) of A2O3 (where A=Y, La, and / or Sc), CeO2, and ZrO2. Furthermore, Patent Document 7 proposes an active layer comprising a cermet containing Ni-containing particles and YScCZ particles made of ZrO2 doped with Y, Sc, and Ce. However, there are limits to how much the durability of SOEC can be improved solely by optimizing the electrolyte used in the fuel electrode. [Prior art documents] [Patent Documents]

[0007] [Patent Document 1] Japanese Patent Publication No. 2018-142419 [Patent Document 2] Japanese Patent Publication No. 2018-085200 [Patent Document 3] Japanese Patent Application Laid-Open No. 2020-155349 [Patent Document 4] Japanese Patent Application Laid-Open No. 2021-085061 [Patent Document 5] Japanese Patent Application Laid-Open No. 2020-167052 [Patent Document 6] Japanese Patent Application Laid-Open No. 2021-161467 [Patent Document 7] Japanese Patent Application Laid-Open No. 2022-074189 [Non-Patent Document]

[0008] [Non-Patent Document 1] Ebbesen, S. D.; Hansen, J. B.; Morgensen, M. B. ECS Trans. 2013, 57, 3217. [Non-Patent Document 2] Jensen, S. H.; Larsen, P. H.; Mogensen, M. Int. J. Hydrogen Energy 2007, 32, 3253. [Non-Patent Document 3] Ullmann H.: Trofimenko N.: Tietz F.: Stoever D.: Ahmad-Khanlou A.: Solid State Ionics 2000, 138, 79. [Non-Patent Document 4] Laguna-Bercero, M. A.; Skinner, S. J.; Kilner, J. A. J. Power Sources 2009, 192, 126. [Non-Patent Document 5] Sune Dalgaard Ebbesen, Soeren Hoejgaard Jensen, Anne Hauch, and Morgens Bjerg Morgensen, Chem. Rev. 2014, 114, 10697 [Summary of the Invention] [Problems to be Solved by the Invention]

[0009] The problem to be solved by the present invention is to provide an electrode with little change in the electrode structure even when exposed to high-temperature water vapor.

Means for Solving the Problem

[0010] To solve the above problem, the electrode according to the present invention comprises electrolyte particles, Ni-based particles and the electrolyte particles contain CeO2 (LSDC) doped with La and Sm, the Ni-based particles are composed of core-shell particles in which part or all of the surface of a core made of Ni or a Ni-based alloy is coated with a shell made of a composite oxide containing NiO or Ni.

Effects of the Invention

[0011] When a Ni / YSZ electrode is used as the fuel electrode of an electrolytic cell, the Ni / YSZ electrode is liable to deteriorate over time. This is considered to be because the form of the electrode reaction points changes due to the occurrence of gas-phase diffusion of Ni during use.

[0012] On the other hand, in an electrode containing electrolyte particles made of LSDC and Ni-based particles, when a shell made of a composite oxide containing NiO or Ni (hereinafter also referred to as "Ni-based oxide") is formed on the surface of the Ni-based particles in advance, deterioration of the electrode over time can be suppressed without sacrificing the electrode performance. This is (a) In a region (triple-phase boundary) where the Ni-based oxide, LSDC, and voids overlap, since LSDC extracts oxygen from the Ni-based oxide, a Ni layer or a Ni-based alloy layer is always formed near the triple-phase boundary and continues to function as an electrode reaction point. Therefore, and (b) because the shell made of Ni-based oxide in a region other than near the triple-phase boundary suppresses the gas-phase diffusion of Ni and suppresses the change in the form of the electrode reaction points it is considered.

Brief Description of the Drawings

[0013] [Figure 1] This is a schematic diagram of electrode degradation in a conventional fuel electrode. [Figure 2] This is a schematic diagram illustrating the process by which electrode activity is maintained by Ni-based particles with a core-shell structure. [Figure 3] This is a schematic diagram of the electrolytic cell used for impedance measurement. [Figure 4] This figure shows an example of impedance analysis results.

[0014] [Figure 5] This figure shows an example of the change in resistance rate over time at 700°C for sample No. 1 (LSDC) and sample No. 2 (GDC). [Figure 6] This shows the degradation rates of sample No. 1 (LSDC) and sample No. 2 (GDC) at 700°C. [Figure 7] This is an SEM image of a portion of the electrode cross-section of sample No. 1 after the durability test (700°C). [Figure 8] This is a STEM / EDS image of sample No. 1 after the durability test (700°C).

[0015] [Figure 9] This is an SEM image of sample No. 1 after the durability test (700°C). [Figure 10] This is an SEM / EDS mapping of sample No. 1 after the durability test (700°C). [Figure 11] This is the area ratio of the electrode components of sample No. 1 after the durability test (700°C). [Modes for carrying out the invention]

[0016] [Configuration 1] Electrolyte particles and, N-type particles and Equipped with, The electrolyte particles include CeO2(LSDC) doped with La and Sm. The Ni-based particles consist of core-shell particles in which part or all of the surface of a core made of Ni or a Ni-based alloy is covered with a shell made of a composite oxide containing NiO or Ni. electrode.

[0017] [Configuration 2] The electrode according to configuration 1, wherein the shell area ratio is greater than 0 and less than or equal to 0.98. However, the "area ratio of the shell" refers to the area of ​​the core in the cross-section of the electrode (S core ) with respect to the area of ​​the shell (S shell ) ratio (S shell / S core ) refers to.

[0018] [Configuration 3] The electrode according to configuration 1 or 2, wherein the shell includes a region having a thickness of 200 nm or less.

[0019] [Structure 4] An electrode according to any one of configurations 1 to 3, wherein the degradation rate at 700°C is 5.0% / h or less. However, the aforementioned "deterioration rate" refers to the slope A of the straight line ΔR = A × t obtained by plotting the resistance change rate of the electrode before and after the durability test: ΔR (%) on the vertical axis and the durability test time: t (h) on the horizontal axis, and connecting the values ​​at t=0h and t=40h.

[0020] [Composition 5] The LSDC is an electrode according to any one of configurations 1 to 4, wherein the total content of La and Sm is greater than 0 mol% and less than or equal to 20.0 mol%. However, the "total content of La and Sm" refers to the ratio of the total number of moles of La and Sm to the total number of moles of Ce, La, and Sm contained in the LSDC.

[0021] [Composition 6] The aforementioned LSDC is The La content is 1.0 mol% or more and 10.0 mol% or less. The content of Sm is 1.0 mol% or more and 10.0 mol% or less. An electrode as described in any one of configurations 1 through 5. however, The term "La content" refers to the ratio of the number of moles of La to the total number of moles of Ce, La, and Sm contained in the LSDC. The term "Sm content" refers to the ratio of the number of moles of Sm to the total number of moles of Ce, La, and Sm contained in the LSDC.

[0022] [Composition 7] The electrode according to any one of configurations 1 to 6, wherein the content of the Ni-based particles is 30 mass% or more and 70 mass% or less. However, the "content of Ni-based particles" refers to the ratio of the mass of Ni-based particles to the total mass of the electrolyte particles and Ni-based particles.

[0023] [Structure 8] An electrode according to any one of configurations 1 to 7, wherein the porosity is 20% or more and 40% or less. However, the aforementioned "porosity" refers to the value measured by a mercury porosimeter.

[0024] [Composition 9] An electrode according to any one of configurations 1 to 8, used as a fuel electrode for a solid oxide electrolytic cell (SOEC) or a solid oxide fuel cell (SOFC).

[0025] One embodiment of the present invention will be described in detail below. [1. Electrode] The electrode according to the present invention is Electrolyte particles and, N-type particles and It is equipped with.

[0026] [1.1. Electrolyte particles] The electrolyte particles contain CeO2 (LSDC) doped with La and Sm. LSDC functions not only as an oxide ion conductor but also as an oxygen storage material. Therefore, when using LSDC as electrolyte particles, during the use of the electrode, not only can the transfer of oxide ions with Ni-based particles be carried out, but also the morphological changes of the electrode reaction points can be suppressed. Furthermore, CeO₂ doped with Gd (GDC) also functions as an oxygen storage material, but LSDC has a greater effect of suppressing the morphological changes of the electrode reaction points compared to GDC. This is presumably because LSDC has a higher oxygen storage capacity than GDC.

[0027] In LSDC, La and Sm take a trivalent valence. Therefore, when an appropriate amount of La and Sm is doped into CeO₂, oxygen vacancies are introduced into the crystal lattice of CeO₂. As a result, the oxide ion conductivity of LSDC is improved. Furthermore, in LSDC, La and Sm have the effect of improving the oxygen storage capacity of LSDC. This is presumably because when La 3+ (ionic radius: 0.116 nm) and Sm 3+ (ionic radius: 0.108 nm) are doped, the strain of the crystal lattice that occurs when Ce 4+ (ionic radius: 0.097 nm) is reduced to Ce 3+ (ionic radius: 0.114 nm) is relaxed.

[0028] In the present invention, the total content of La and Sm, the content of La, and the content of Sm in LSDC are not particularly limited, and optimal values can be selected according to the purpose. Here, the "total content of La and Sm (mol%)" refers to the ratio of the total number of moles of La and Sm to the total number of moles of Ce, La, and Sm contained in LSDC. The "content of La (mol%)" refers to the ratio of the number of moles of La to the total number of moles of Ce, La, and Sm contained in LSDC. The "content of Sm (mol%)" refers to the ratio of the number of moles of Sm to the total number of moles of Ce, La, and Sm contained in LSDC.

[0029] In LSDC, the oxygen storage capacity of the LSDC increases as the total content of La and Sm increases. To obtain this effect, the total content of La and Sm is preferably greater than 0 mol%. More preferably, the total content is 2.0 mol% or more, 4.0 mol% or more, or 6.0 mol% or more. On the other hand, if the total content of La and Sm is excessive, the oxygen storage capacity may decrease, or the oxide ion conductivity may plateau. Therefore, the total content of La and Sm is preferably 20.0 mol% or less. More preferably, the total content is 18.0 mol% or less, 16.0 mol% or less, or 14.0 mol% or less.

[0030] In LSDC, the oxygen storage capacity of the LSDC increases as the La content increases. To obtain this effect, the La content is preferably 1.0 mol% or more. More preferably, the content is 2.0 mol% or more, or 3.0 mol% or more. On the other hand, if the La content is excessive, the oxygen storage capacity may decrease, or the oxide ion conductivity may plateau. Therefore, the La content is preferably 10.0 mol% or less. More preferably, the content is 9.0 mol% or less, 8.0 mol% or less, or 7.0 mol% or less.

[0031] In LSDC, the oxygen storage capacity of the LSDC increases as the Sm content increases. To obtain this effect, the Sm content is preferably 1.0 mol% or more. More preferably, the content is 2.0 mol% or more, or 3.0 mol% or more. On the other hand, if the Sm content is excessive, the oxygen storage capacity will actually decrease. Therefore, the Sm content is preferably 10.0 mol% or less. More preferably, the content is 9.0 mol% or less, 8.0 mol% or less, or 7.0 mol% or less.

[0032] In particular, LSDC is preferred if it has a La content of 2.5 mol% or more and a Sm content of 2.5 mol% or more and 7.5 mol% or less.

[0033] [1.2. Ni-based particles] In the present invention, Ni-based particles consist of core-shell particles in which part or all of the surface of a core made of Ni or a Ni-based alloy is coated with a shell made of a composite oxide containing NiO or Ni.

[0034] [1.2.1. Core] The core functions as a catalyst and an electron conductor within the electrode. In this invention, the core is made of Ni or a Ni-based alloy. When the core is made of a Ni-based alloy, the type of alloying element is not particularly limited. Examples of alloying elements include Fe and Co. Furthermore, when the core is made of a Ni-based alloy, the Ni content in the core is preferably 90 mass% or more. More preferably, the Ni content is 95 mass% or more. The core is preferably made of Ni or a Ni-Fe alloy.

[0035] [1.2.2. Shell] [A. Shell composition] Part or all of the core surface is covered with a shell. The shell has the function of suppressing gas-phase diffusion of Ni contained in Ni-based particles. Here, "gas-phase diffusion of Ni" refers to the phenomenon in which, when an electrode containing Ni is exposed to high-temperature water vapor, the Ni reacts with the water vapor to form nickel hydroxide, and the nickel hydroxide or its decomposition products diffuse within the electrode via the gas phase.

[0036] As described later, the shell is formed by oxidizing an electrode containing Ni-based particles reduced to a metallic state under a controlled oxidizing atmosphere. Therefore, the shell consists of an oxide containing the metal elements that make up the core. More specifically, the shell consists of a composite oxide (Ni-based oxide) containing NiO or Ni. Furthermore, in the initial state, the entire surface of the core may be covered with a shell. This is thought to be because, when current is applied to the electrodes, in the region where Ni-based particles, LSDC, and voids (gas phase) overlap (three-phase interface), the LSDC extracts oxygen from the shell, and a Ni layer or Ni-based alloy layer is always formed near the three-phase interface.

[0037] [B. Shell thickness] The thickness of the shell may vary depending on the location. The shell does not necessarily have to completely cover the surface of the core; it may cover only a portion of the core. Even if only a portion of the core is covered by the shell, the vapor phase diffusion of Ni can be effectively suppressed depending on the coverage. On the other hand, if the entire core is covered with a thick shell, the electrode activity may be excessively reduced. Therefore, it is preferable that the shell includes a region that is thinner than the surrounding area (hereinafter referred to as the "thin film region"). To obtain high electrode activity, the thickness of the thin film region is preferably 200 nm or less. More preferably, the thickness of the thin film region is 150 nm or less, 100 nm or less, 50 nm or less, 40 nm or less, or 30 nm or less.

[0038] The thin film region is preferably formed at the three-phase interface. When the thin film region is at the three-phase interface, the area near the three-phase interface can continue to function as an electrode active site due to the oxygen storage capacity of the electrolyte particles. Here, "three-phase interface" refers to the electrolyte particle / Ni-based particle interface near the three-phase interface, or the Ni-based particle / gas-phase interface near the three-phase interface. "Near the three-phase interface" refers to the region within 300 nm of the electrode reaction point where the three phases—electrolyte particles, Ni-based particles, and the gas phase—intersect.

[0039] [C. Shell area ratio] "Shell area ratio" refers to the area of ​​the core in the cross-section of the electrode (S core ) relative to the shell area (S shell ) ratio (S shell / S core ) refers to S coreand S shell These can each be calculated from the SEM / EDS mapping of the electrode cross-section.

[0040] The shell area ratio correlates with the shell coverage on the core surface. If the shell area ratio becomes too small, the proportion of the core that is exposed increases. This can make it difficult to suppress gas-phase diffusion of Ni. Therefore, a shell area ratio greater than 0 is preferable. More preferably, the area ratio is 0.30 or higher, 0.40 or higher, 0.50 or higher, or 0.56 or higher. On the other hand, if the shell area ratio becomes too large, the proportion of Ni-based oxide in the electrode increases, which may degrade the electrode characteristics. Therefore, the shell area ratio is preferably 0.98 or less. More preferably, the area ratio is 0.95 or less, or 0.90 or less.

[0041] [1.3. Electrode Composition] [1.3.1. Ni-based particle content] "Ni-based particle content" refers to the ratio of the mass of Ni-based particles to the total mass of electrolyte particles and Ni-based particles.

[0042] If the Ni-based particle content is too low, the total cell resistance increases, and the efficiency of the electrode reaction decreases. Therefore, the Ni-based particle content is preferably 30 mass% or more. More preferably, the content is 40 mass% or more. On the other hand, if the content of Ni-based particles is excessive, the content of electrolyte particles decreases, which may actually reduce the efficiency of the electrode reaction. Therefore, the content of Ni-based particles is preferably 70 mass% or less.

[0043] [1.3.2. LSDC content] "LSDC content (mass%)" refers to the ratio of the mass of LSDC to the total mass of electrolyte particles contained in the electrode. The electrolyte particles may consist solely of LSDC, or they may contain other components. Generally, the higher the total LSDC content, the better the electrode's durability. To obtain high durability, a total LSDC content of 80 mass% or more is preferable. More preferably, the total content is 90 mass% or more, 95 mass% or more, or 99 mass% or more.

[0044] [1.4. Porosity] "Porosity" refers to the value measured by a mercury porosimeter.

[0045] The porosity of the electrode affects its characteristics. If the porosity of the electrode is too low, the diffusivity of the gas decreases, which may reduce the efficiency of the electrode reaction. Therefore, the porosity of the electrode is preferably 20% or higher. More preferably, the porosity is 25% or higher. On the other hand, if the porosity of the electrode becomes too large, the three-phase interface becomes relatively smaller, which may actually decrease the efficiency of the electrode reaction. Therefore, the porosity of the electrode is preferably 40% or less. More preferably, the porosity is 35% or less, or 30% or less.

[0046] [1.5. Characteristics: Deterioration rate” "Degradation rate" refers to the slope A of the straight line ΔR = A × t obtained by plotting the resistance change rate of the electrode before and after the durability test: ΔR (%) on the vertical axis and the durability test time: t (h) on the horizontal axis, and connecting the values ​​at t=0h and 40h.

[0047] The resistance change rate, ΔR(%), is the value expressed by the following equation (1). ΔR(%)={Rct2(t)-Rct2(0)}×100 / Rct2(0) …(1) however, Rct2(0) is the electrode reaction resistance of the electrode before the durability test. Rct2(t) is the electrode reaction resistance of the electrode after a durability test of time t.

[0048] "Electrode reaction resistance (Rct2)" refers to the reaction resistance at the three-phase interface within the fuel electrode, obtained by performing impedance measurements on an electrolytic cell using the aforementioned electrode as the fuel electrode, and is defined as the diameter of the arc (second arc) that contributes to the electrode reaction. A "durability test" refers to a test in which steam electrolysis is performed for a predetermined time under the conditions shown in Table 1 using an electrolytic cell with the aforementioned electrode as the fuel electrode.

[0049] [Table 1]

[0050] Conventional fuel electrodes (Ni / YSZ) have a high degradation rate because the morphology of the electrode reaction site is prone to change. In contrast, the electrode according to the present invention has a lower degradation rate compared to conventional electrodes, regardless of the temperature during durability testing, because the Ni-based particles are core-shell particles. For example, the degradation rate of a conventional electrode at 700°C is 17.18% / h. In contrast, the degradation rate of the electrode according to the present invention at 700°C is 5.0% / h or less. Further optimization of the manufacturing conditions reduces the degradation rate at 700°C to 4.0% / h or less, or even 3.0% / h or less.

[0051] [1.6. Usage] Solid oxide electrolytic cells (SOECs) and solid oxide fuel cells (SOFCs) are generally, An electrolyte layer containing a solid oxide electrolyte, A fuel electrode bonded to one side of the electrolyte layer, An air electrode bonded to the other side of the electrolyte layer, An intermediate layer (reaction prevention layer) inserted between the electrolyte layer and the air electrode and It is equipped with. In addition, the fuel electrode side current collector layer may be located outside the fuel electrode, or the air electrode side current collector layer may be located outside the air electrode.

[0052] The electrode according to the present invention is particularly suitable as a fuel electrode for a solid oxide electrolytic cell (SOEC) or a fuel electrode for a solid oxide fuel cell (SOFC). When the electrode according to the present invention is used as a fuel electrode for SOEC or SOFC, the materials of the other components are not particularly limited, and the most suitable materials can be selected according to the purpose.

[0053] For example, the solid oxide electrolyte constituting the electrolyte layer can be yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), scandia-yttria-stabilized zirconia (ScYSZ), samaria-dope ceria (SDC), lanthanum-strontium-gallium-magnesium oxide (LSGM), or the like.

[0054] For the air electrode, materials such as lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), and lanthanum strontium cobaltite (LSC) can be used.

[0055] The intermediate layer is a layer that prevents reactions that occur when the electrolyte layer and the air electrode come into direct contact, and is inserted as needed. For example, if the electrolyte layer is YSZ and the air electrode is LSC, it is preferable to use Gd-doped CeO2 (GDC) for the intermediate layer.

[0056] The materials for the fuel electrode current collector and the air electrode current collector are not particularly limited, as long as they are capable of transferring electrons, supplying reactants, and discharging reaction products. Typically, the current collectors are made of materials having the same or similar composition as the electrodes, but with a higher porosity than the electrodes.

[0057] [2. Method for manufacturing electrodes] The electrode according to the present invention is (a) A molded body is prepared using a raw material mixture containing Ni-based particle raw materials and electrolyte particle raw materials. (b) The obtained molded body is sintered, (c) The obtained sintered body is subjected to reduction treatment, (d) The obtained reduced product is oxidized under a controlled oxidizing atmosphere. It can be manufactured by doing so.

[0058] [2.1. Molded Body Manufacturing Process] First, a molded body is produced using a raw material mixture containing Ni-based particles and electrolyte particles.

[0059] "Raw material for Ni-based particles" refers to a raw material that becomes Ni-based particles after sintering, reduction, and controlled oxidation treatment. In this invention, the type of raw material for Ni-based particles is not particularly limited, and the most suitable raw material can be selected according to the purpose. Examples of raw materials for Ni-based particles include NiO powder, Fe2O3 powder, Fe3O4 powder, a mixture of metallic Fe and NiO or metallic Ni, CoO powder, Co2O3 powder, etc.

[0060] "The raw material for electrolyte particles" refers to the raw material that becomes electrolyte particles after sintering, reduction, and controlled oxidation treatment. In this invention, LSDC powder is used as the raw material for electrolyte particles. The composition of LSDC is as described above, so no further explanation is given.

[0061] The raw material mixture may contain a pore-forming agent (e.g., carbon powder). The metal oxide (e.g., NiO powder) contained in the raw materials of the Ni-based particles added to the raw material mixture is 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 always necessary. However, adding a pore-forming agent to the raw material mixture increases the degree of freedom in controlling the porosity. Furthermore, it is preferable that each raw material be blended such that a fuel electrode having the desired composition is obtained after sintering, reduction, and controlled oxidation treatment.

[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 a raw material mixture into a tape, laminating the resulting green sheet onto a substrate (for example, a molded body that will become the fuel electrode side current collector layer after sintering, or a molded body that will become the electrolyte layer after sintering), and pressing the laminated body together by hydrostatic pressure. (b) A method of preparing a slurry containing a raw material mixture and screen printing the slurry onto the surface of a substrate. These are some examples.

[0063] [2.2. Sintering Process] Next, the resulting molded body is sintered (sintering step). It is preferable to select the optimal sintering conditions according to the raw material composition. Sintering is usually carried out in an atmospheric environment at 1000°C to 1500°C (preferably 1000°C to 1300°C) for 1 to 5 hours. If a pore-forming agent is included in the raw material mixture, the pore-forming agent disappears during sintering, and pores are formed in the sintered body.

[0064] [2.3. Reduction Process] Next, the obtained sintered body is subjected to a reduction treatment (reduction step). The reduction treatment is performed to reduce metal oxides such as NiO contained in the sintered body and generate metallic Ni-based particles (i.e., core particles). The reduction conditions are not particularly limited, as long as they are capable of generating core particles.

[0065] [2.4. Oxidation process] Next, the obtained reduced product is oxidized under a controlled oxidizing atmosphere. This yields the electrode according to the present invention. The controlled oxidation treatment is performed to form a shell on the surface of the core particles. The conditions for the controlled oxidation treatment are not particularly limited, as long as they allow for the selective oxidation of only the surface portion of the core particles.

[0066] The following methods can be used to form core-shell particles. The first method is, (a) A cell is fabricated using the electrode immediately after reduction (the electrode containing core particles) as the fuel electrode, (b) With the cell heated to a temperature of 650°C or higher, adjust the gas atmosphere on the fuel electrode side to H2O / H2=4 (volume ratio), (c) In this state, apply a current of 30mA to the cell for at least 50 hours. (d) After stopping the application of current, fill the fuel electrode with an inert gas (e.g., N2 gas). It is a method.

[0067] The second method is, (a) A cell is fabricated using the electrode immediately after reduction (the electrode containing core particles) as the fuel electrode, (b) With the cell heated to a temperature of 400°C or higher, adjust the gas atmosphere on the fuel electrode side to H2O / H2 = 1.0 or higher (by volume), (c) Hold the cell for at least 1 hour without applying current, (d) After holding, fill the fuel electrode with an inert gas (e.g., N2 gas) and hold it in that state. It is a method.

[0068] In this invention, any method may be used. Furthermore, regardless of the method used, the shell area ratio can be controlled by optimizing the temperature, atmosphere, current, processing time, etc.

[0069] As described above, an electrolytic cell consists of a composite of a fuel electrode (cathode), an electrolyte layer, a reaction prevention layer, and an air electrode (anode). Furthermore, a fuel electrode-side current collector layer may be further bonded to the outside of the fuel electrode, and / or an air electrode-side current collector layer may be further bonded to the outside of the air electrode. Sintering and bonding of each layer are performed by stacking molded bodies and then heating the laminate to a predetermined temperature. If the optimal sintering temperature differs for each layer, sintering is usually performed in multiple stages. Furthermore, the reduction of the fuel electrode is usually performed after all layers have been bonded. When the electrode according to the present invention is used as the fuel electrode of an electrolytic cell, the controlled oxidation treatment of the fuel electrode is performed after all layers are joined and the fuel electrode has been subjected to reduction treatment.

[0070] [3. Effect] [3.1. Electrode Degradation] Figure 1 shows a schematic diagram of electrode degradation in a conventional fuel electrode. In a conventional electrolytic cell using Ni / YSZ as the fuel electrode, the degradation of the fuel electrode over time is thought to be caused by vapor-phase diffusion of Ni.

[0071] The degradation of the fuel electrode over time due to vapor-phase diffusion of Ni is thought to proceed as follows: As shown in Figure 1, first, when the fuel electrode is exposed to high-temperature steam, Ni reacts with the steam to produce nickel hydroxide. The produced nickel hydroxide or its decomposition products diffuse through the gas phase within the electrode and adhere to the surface of larger Ni particles. As a result, the morphology of the electrode reaction sites changes, and the electrode is thought to degrade.

[0072] [3.2. Maintaining electrode activity with core-shell particles] In contrast, in an electrode containing electrolyte particles made of LSDC and Ni-based particles, if a shell made of a composite oxide (Ni-based oxide) containing NiO or Ni is formed on the surface of the Ni-based particles beforehand, the degradation of the electrode over time can be suppressed without sacrificing electrode performance. This is thought to be due to the following reasons.

[0073] [3.2.1. Maintaining the electrode reaction point using LSDC] Figure 2 shows a schematic diagram of the process by which electrode activity is maintained by Ni-based particles with a core-shell structure. First, when an electrode containing pure Ni particles and LSDC is oxidized under a controlled oxidizing atmosphere, only the surface portion of the pure Ni particles is selectively oxidized. As a result, core-shell particles are obtained in which the surface of the core made of pure Ni is coated with NiO. See the upper left diagram in Figure 2.

[0074] Because LSDC has oxygen storage capacity, in the region where NiO / LSDC / void overlap (three-phase interface), LSDC extracts oxygen from NiO. As a result, near the three-phase interface, NiO may be reduced to metallic Ni, but in other regions, the core surface often remains covered with NiO. See the upper right and lower right diagrams of Figure 2.

[0075] When an electric current is applied to the electrode (for example, during electrolysis), the oxygen absorbed by the LSDC diffuses within the LSDC, forming lattice vacancies that give rise to oxygen storage capacity. As a result, even when the electrode is exposed to high-temperature steam, a Ni layer is always formed in the region where NiO / LSDC / vacancies overlap, and it continues to function as an electrode reaction site. See the lower left diagram in Figure 2. This point is also true when the core of the Ni-based particle is made of a Ni-based alloy and the shell is a composite oxide containing Ni.

[0076] [3.2.2. Suppression of morphological changes of electrode reaction sites by shells] The vapor-phase diffusion of Ni is thought to be caused by direct contact between Ni and high-temperature water vapor. In contrast, when the surface of Ni outside the vicinity of the three-phase interface is coated with NiO, the formation of nickel hydroxide, a factor in electrode degradation, is suppressed in that region (i.e., the region where NiO and voids overlap). As a result, vapor-phase diffusion of Ni is thought to be suppressed. See the lower left diagram in Figure 2. This point is also true when the core of the Ni-based particle is made of a Ni-based alloy and the shell is a composite oxide containing Ni.

[0077] In electrodes composed of Ni-based particles and LSDC, the electrodes function even if the Ni-based particles do not contain a shell. However, when the Ni-based particles do not contain a shell, excessive changes in the morphology of the electrode reaction site may occur if the electrodes are used under harsh conditions immediately after initial use. In contrast, when the Ni-based particles contain a shell, the likelihood of such problems occurring is low.

[0078] [3.3. LSDC Effects] GDC also exhibits oxygen storage capacity. However, electrodes containing LSDC show higher durability compared to electrodes containing GDC alone. This is thought to be because LSDC has a higher oxygen storage capacity than GDC. [Examples]

[0079] [A. Experiment 1] [1. Fabrication of electrolytic cells] [1.1. Sample No. 1] An 8YSZ electrolyte pellet (diameter: 22 mm, thickness: 500 μm) with a reference electrode attached to its side was topped with a GDC sheet (reaction prevention layer, diameter 22 mm) on one side, and then fired at 1380°C. Next, a fuel electrode was formed on the other side of the 8YSZ electrolyte pellet (the side opposite to the side where the reaction prevention layer was baked on). Specifically, a NiO / LSDC paste (NiO:LSDC = 36:64, mass ratio) was applied by screen printing and fired at 1340°C. The LSDC powder used had a La content and Sm content of 5.0 mol%, respectively. Furthermore, LSC / GDC paste was applied to the reaction prevention layer using a screen printing method and fired at 1125°C to form an air electrode.

[0080] The resulting electrolytic cell was subjected to a reduction treatment of the fuel electrode. The reduction treatment was carried out by holding the cell at 700°C for 20 minutes in a 100% hydrogen atmosphere.

[0081] [1.2. Sample No. 2] NiO / GDC paste (NiO:GDC = 1:1, mass ratio) was used as the paste for manufacturing the fuel electrode. The GDC powder used had a Gd content of 10 mol%. The electrolytic cell was then prepared in the same manner as for sample No. 1.

[0082] [2. Test Method] [2.1. Deterioration rate] Figure 3 shows a schematic diagram of the electrolytic cell used for impedance measurement. A reference electrode is attached to the side of the electrolyte layer. The reference electrode is used to measure the voltage V1 between the electrolyte layer and the air electrode, and the voltage V2 between the electrolyte layer and the fuel electrode. By attaching the reference electrode to the electrolyte layer, the fuel electrode and the air electrode can be evaluated separately.

[0083] Using the electrolytic cell shown in Figure 3, a 100-hour steam electrolysis test (durability test) was conducted under the conditions shown in Table 1. Impedance measurements were performed during the durability test to determine the electrode reaction resistance (Rct2). Furthermore, the resistance change rate expressed by equation (1) was calculated using the electrode reaction resistance (Rct2) before the durability test and after the predetermined duration of the durability test. In addition, the degradation rate was calculated from the resistance change rate.

[0084] [2.2. SEM observation, EDS analysis] After the durability test (700°C), the fuel electrode was observed using SEM and underwent EDS analysis.

[0085] [2.3. Shell Area Ratio] SEM observation and EDS mapping were performed on the cross-section of the fuel electrode before the durability test and after the 100-hour durability test. From the EDS mapping results, the area of ​​each component material contained in the fuel electrode was calculated, and the area ratio was determined. The procedure for calculating the area ratio is as follows.

[0086] Specifically, the cross-section of the electrode was observed at a magnification of 4000x, and the electrode cross-section was divided into four areas (thickness: approximately 5 μm) along the thickness direction. For each area, the area percentage of Ni (core), NiO (shell), void area percentage, and electrolyte area percentage were calculated. Next, the average area ratio of each component was calculated for each area. Furthermore, using the "average area ratio of each component" calculated for each area, the "average area ratio of each component" for all four areas was calculated.

[0087] [3. Results] [3.1. Deterioration rate] Figure 4 shows an example of impedance analysis results. In Figure 4, the second arc (Rct2) represents the magnitude of the electrochemical reaction resistance of the fuel electrode. A larger diameter of the second arc indicates greater electrochemical reaction resistance. The arc to the left of the second arc is the first arc (Rct1), which represents the magnitude of the resistance of the electrolyte ion path within the hydrogen electrode.

[0088] Figure 5 shows an example of the change in resistance rate over time at 700°C for sample No. 1 (LSDC) and sample No. 2 (GDC). In the case of sample No. 2 (GDC), a tendency was observed for the rate of change in resistance rate to decrease when the durability test time exceeded 40 hours. In contrast, in the case of sample No. 1 (LSDC), when the durability test time exceeded 80 hours, a tendency was observed for the rate of change in resistance to decrease. Furthermore, the rate of change in resistance of sample No. 1 (LSDC) was significantly lower than that of sample No. 2 (GDC).

[0089] Figure 6 shows the degradation rates of sample No. 1 (LSDC) and sample No. 2 (GDC) at 700°C. The degradation rate of sample No. 2 (GDC) at 700°C was 6.22% / hr. In contrast, the degradation rate of sample No. 1 (LSDC) at 700°C was 1.08% / hr, which is approximately 1 / 6 of the degradation rate of sample No. 2. The results in Figures 5 and 6 suggest that when the fuel electrode is subjected to oxidation treatment under a controlled oxidation atmosphere immediately after manufacturing, and the Ni-based particles are pre-formed into a core-shell structure, the degradation of the Ni-based particles during electrolysis is suppressed.

[0090] [3.2. SEM observation, STEM / EDS mapping] Figure 7 shows an SEM image of a portion of the electrode cross-section of sample No. 1 after the durability test (700°C). Figure 8 shows a STEM / EDS image of sample No. 1 after the durability test (700°C). From Figures 7 and 8, it can be seen that a NiO layer with a thickness of 100 nm or less is formed on the surface of the Ni particles.

[0091] [3.3. Shell Area Ratio] Figure 9 shows the SEM image of sample No. 1 after the durability test (700°C). Figure 10 shows the SEM / EDS mapping of sample No. 1 after the durability test (700°C). Figure 11 shows the area ratio of the electrode components of sample No. 1 after the durability test (700°C).

[0092] For sample No. 1, the ratio of Ni particles (core) to NiO layer (shell) after durability testing (700°C) in each area was 16:1.7, 17.5:2.1, 16.9:2.1, and 16.7:2.1, respectively. The average ratio of Ni particles to NiO layer was 16.8:2.0 (=1:0.119).

[0093] Although embodiments of the present invention have been described in detail above, the present invention is not limited in any way to the above embodiments, and various modifications are possible without departing from the spirit of the present invention. [Industrial applicability]

[0094] The electrode according to the present invention can be used as a fuel electrode for a solid oxide electrolytic cell (SOEC) or a fuel electrode for a solid oxide fuel cell (SOFC).

Claims

1. Electrolyte particles and, Ni-based particles and Equipped with, The electrolyte particles are CeO2 doped with La and Sm. 2 (LSDC) The Ni-based particles consist of core-shell particles in which part or all of the surface of a core made of Ni or a Ni-based alloy is covered with a shell made of a composite oxide containing NiO or Ni. electrode.

2. The electrode according to claim 1, wherein the shell area ratio is greater than 0 and less than or equal to 0.

98. However, the "area ratio of the shell" refers to the area of ​​the core in the cross-section of the electrode (S core ) with respect to the area of ​​the shell (S shell ) ratio (S shell / S core ) refers to.

3. The electrode according to claim 1, wherein the shell includes a region having a thickness of 200 nm or less.

4. The electrode according to claim 1, wherein the degradation rate at 700°C is 5.0% / h or less. However, the aforementioned "deterioration rate" refers to the slope A of the straight line ΔR = A × t obtained by plotting the resistance change rate of the electrode before and after the durability test: ΔR (%) on the vertical axis and the durability test time: t (h) on the horizontal axis, and connecting the values ​​at t = 0h and 40h.

5. The electrode according to claim 1, wherein the LSDC has a total content of La and Sm that is greater than 0 mol% and less than or equal to 20.0 mol%. However, the "total content of La and Sm" refers to the ratio of the total number of moles of La and Sm to the total number of moles of Ce, La, and Sm contained in the LSDC.

6. The aforementioned LSDC is The La content is 1.0 mol% or more and 10.0 mol% or less. The Sm content is 1.0 mol% or more and 10.0 mol% or less. The electrode according to claim 1. however, The "La content" refers to the ratio of the number of moles of La to the total number of moles of Ce, La, and Sm contained in the LSDC. The term "Sm content" refers to the ratio of the number of moles of Sm to the total number of moles of Ce, La, and Sm contained in the LSDC.

7. The electrode according to claim 1, wherein the content of the Ni-based particles is 30 mass% or more and 70 mass% or less. However, the "content of Ni-based particles" refers to the ratio of the mass of Ni-based particles to the total mass of the electrolyte particles and the Ni-based particles.

8. The electrode according to claim 1, wherein the porosity is 20% or more and 40% or less. However, the aforementioned "porosity" refers to the value measured by a mercury porosimeter.

9. The electrode according to claim 1, used as a fuel electrode for a solid oxide electrolytic cell (SOEC) or a fuel electrode for a solid oxide fuel cell (SOFC).

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