electrode

Abstract

To provide an electrode in which a structure thereof is less changed even when it is exposed in a high-temperature steam.SOLUTION: An electrode comprises electrolyte particles and Ni-based particles. The electrolyte particles include Gd-doped CeO2(GDC) and / or Gd- and La-doped CeO2(La-GDC). The Ni-based particles comprise core-shell particles, in which a part or all of the core surface made of Ni or a Ni-based alloy is coated with a shell made of a complex oxide including a NiO or Ni. The GDC is preferred to have the content of Gd of more than 0 mol% and 20 mol% or less. The La-GDC is preferred to have the content of Gd of more than 0 mol% and 20 mol% or less, and the content of La of more than 0 mol% and 20 mol% or less. The content of the Ni-based particles is preferred to be 30 mass% or more and 70 mass% or less.SELECTED DRAWING: Figure 6

Description

[Technical Field]

[0001] The present invention relates to an electrode, and more particularly to an electrode suitable as an anode for a solid oxide electrolysis cell (SOEC) or anode for a solid oxide fuel cell (SOFC). [Background technology]

[0002] A solid oxide fuel cell (SOFC) is a fuel cell that uses an oxide ion conductor as an electrolyte. When fuel gases such as H2, CO, and CH4 are supplied to the anode (fuel electrode) of the SOFC and O2 is supplied to the cathode (oxygen electrode), an electrode reaction occurs and electricity can be generated. The CO2 and H2O produced by the electrode reaction are discharged outside the SOFC. On the other hand, solid oxide electrolysis cells (SOECs) have the same structure as SOFCs, but they operate in the opposite direction to the SOFCs: when CO2 or H2O is supplied to the cathode (fuel electrode) of an SOEC and an electric current is passed between the electrodes, CO and H2 are produced.

[0003] An SOEC includes a single cell in which an anode (air electrode) is bonded to one side of an electrolyte and a cathode (fuel electrode) is bonded to the other side. The following materials are generally used as materials for the components that make up such an SOEC (see Non-Patent Documents 1 to 5). (a) Electrolytes: Yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), scandia-yttria-stabilized zirconia (ScYSZ), samaria-doped 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] In Patent Document 1, although it is not a fuel electrode, GdO 1.5 A reaction preventive layer for a solid oxide fuel cell is disclosed, which is doped with more than 10 mol % and less than 30 mol % of a metal. Patent Document 2 does not aim to optimize the fuel electrode material, but discloses an active layer provided with gas flow paths for diffusing gas from the diffusion layer toward the electrolyte layer.

[0005] Ni / YSZ cermet is generally used for the anode of an SOEC. However, water vapor, the raw material for hydrogen production, is supplied to the anode at high temperatures (over 700°C). For this reason, it is known that when an SOEC using Ni / YSZ as the anode is used for a long period of time, its electrolytic properties gradually deteriorate. This is thought to be because when the anode is exposed to high-temperature water vapor, the Ni particles are oxidized, becoming nickel hydroxide, which has a low vapor pressure, and then evaporating. This is thought to result in a change in the structure of the anode.

[0006] In order to solve this problem, various proposals have been made. For example, Patent Documents 3 to 6 propose a fuel electrode containing 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 made of a cermet containing Ni-containing particles and YScCZ particles made of ZrO2 doped with Y, Sc, and Ce. However, there is a limit to how much the durability of SOEC can be improved by simply optimizing the electrolyte used in the anode. [Prior art documents] [Patent documents]

[0007] [Patent Document 1] Japanese Patent Application Laid-Open No. 2018-142419 [Patent Document 2] JP 2018-085200 A [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]<​​​​​​​​​​​​​​​​​​​​

[0009] The problem to be solved by the present invention is to provide an electrode that undergoes minimal change in electrode structure even when exposed to high-temperature water vapor. [Means for solving the problem]

[0010] In order to solve the above problems, the electrode according to the present invention comprises: Electrolyte particles; Ni-based particles and Equipped with the electrolyte particles include Gd-doped CeO2 (GDC) and / or Gd- and La-doped CeO2 (La-GDC); The Ni-based particles are core-shell particles in which a part or all of the surface of a core made of Ni or a Ni-based alloy is covered with a shell made of NiO or a composite oxide containing Ni. [Effects of the Invention]

[0011] When used as the fuel electrode in an electrolysis cell, the Ni / YSZ electrode is prone to deterioration over time, which is thought to be due to changes in the morphology of the electrode reaction sites caused by the vapor phase diffusion of Ni during use.

[0012] In contrast, in an electrode containing electrolyte particles made of GDC and / or La-GDC and Ni-based particles, if a shell made of NiO or a composite oxide containing Ni (hereinafter also referred to as "Ni-based oxide") is formed on the surface of the Ni-based particles in advance, the deterioration of the electrode over time can be suppressed without sacrificing the electrode performance. (a) In the region where the Ni-based oxide, GDC or La-GDC, and voids overlap (three-phase interface), GDC or La-GDC extracts oxygen from the Ni-based oxide, so that a Ni layer or Ni-based alloy layer is always formed near the three-phase interface and continues to function as an electrode reaction site; and (b) The shell consisting of Ni-based oxides in the area other than the three-phase interface suppresses the vapor phase diffusion of Ni and suppresses the morphological changes of the electrode reaction points. It is thought that... [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1 is a schematic diagram of electrode deterioration in a conventional fuel electrode. [Figure 2] This is a schematic diagram of the process by which electrode activity is maintained by Ni-based particles with a core-shell structure. [Figure 3] FIG. 1 is a schematic diagram of an electrolytic cell used for impedance measurement. [Figure 4] FIG. 10 is a diagram showing an example of an impedance analysis result.

[0014] [Figure 5] FIG. 10 is a graph showing an example of the change in the resistance change rate over time of Sample No. 1. [Figure 6] This shows the deterioration rate of the electrodes of Sample No. 1 and Sample No. 2 at each temperature. [Figure 7] 1 is an SEM image of a cross section (part) of the electrode of Sample No. 1 after a durability test (800°C). [Figure 8] This is an example of SEM / EDS mapping of the electrode cross section (entire: from the electrolyte interface to the electrode surface) of sample No. 1 after a durability test (800°C). [Figure 9] This is the area ratio of the electrode constituent material of sample No. 1 after the durability test (800°C). [Figure 10] This is the area ratio of the electrode constituent material of sample No. 1 after the durability test (850°C).

[0015] [Figure 11] TEM image (left) and EDS mapping (right) of the electrode of sample No. 1 after the durability test (800°C). [Figure 12] Fig. 12(A) shows the results of line analysis when the line analysis was performed along the direction of arrow A in Fig. 11. Fig. 12(B) shows the results of line analysis when the line analysis was performed along the direction of arrow B in Fig. 11. [Figure 13] 1 shows the oxygen storage capacity of the electrodes of Sample No. 1 and Sample No. 2 at various temperatures. [Figure 14]This shows the rate of change of Ni particles at each temperature for the electrode of sample No. 1.

[0016] [Figure 15] This shows the degradation rate of the electrodes of Sample No. 3 (La-GDC) and Sample No. 1 (GDC) at each temperature. [Figure 16] Oxygen storage capacity of La-GDC and GDC. [Figure 17] 1 is an SEM image of a cross section (part) of the electrode of sample No. 3 after a durability test (700°C). [Figure 18] This is an example of SEM / EDS mapping of the electrode cross section (region from the electrolyte interface to approximately 20 μm toward the electrode surface) of sample No. 3 after a durability test (700°C). [Figure 19] This shows the area ratio of the electrode constituent material of sample No. 3 after the durability test (700°C). [Figure 20] This shows the area ratio of the electrode constituent material of sample No. 3 after the durability test (800°C). DETAILED DESCRIPTION OF THE INVENTION

[0017] [Configuration 1] Electrolyte particles; Ni-based particles and Equipped with the electrolyte particles include Gd-doped CeO2 (GDC) and / or Gd- and La-doped CeO2 (La-GDC); The Ni-based particles are core-shell particles in which a part or all of the surface of a core made of Ni or a Ni-based alloy is covered with a shell made of NiO or a composite oxide containing Ni. electrode.

[0018] [Configuration 2] The electrode according to aspect 1, wherein the area ratio of the shell is greater than 0 and not greater than 0.98. However, the "shell area ratio" refers to the area of ​​the core in the cross section of the electrode (S core ) to the area of ​​the shell (S shell ) ratio (S shell / Score )

[0019] [Configuration 3] 3. The electrode of claim 1 or 2, wherein the shell includes a region having a thickness of 200 nm or less.

[0020] [Configuration 4] 4. The electrode according to any one of aspects 1 to 3, wherein the degradation rate at 700°C is 10% / h or less. Here, the "deterioration rate" refers to the slope A of the 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, connecting the values ​​at t=0 h and t=40 h.

[0021] [Configuration 5] 5. The electrode according to any one of aspects 1 to 4, wherein the GDC has a Gd content of more than 0 mol % and not more than 20 mol %. Here, the "content of Gd" refers to the ratio of the number of moles of Gd to the total number of moles of Ce and Gd contained in the GDC.

[0022] [Configuration 6] The La-GDC is The Gd content is more than 0 mol% and less than 10 mol%; La content is more than 0 mol% and less than 10 mol% 6. The electrode of any one of configurations 1 to 5. however, The "Gd content" refers to the ratio of the number of moles of Gd to the total number of moles of Ce, Gd, and La contained in the La-GDC, The "La content" refers to the ratio of the number of moles of La to the total number of moles of Ce, Gd, and La contained in the La-GDC.

[0023] [Configuration 7] 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. Here, the "content of Ni-based particles" refers to the ratio of the mass of the Ni-based particles to the total mass of the electrolyte particles and the Ni-based particles.

[0024] [Configuration 8] 8. The electrode of any one of aspects 1 to 7, wherein the porosity is 20% or more and 40% or less. The "porosity" mentioned above refers to a value measured by a mercury porosimeter.

[0025] [Configuration 9] 9. The electrode according to any one of configurations 1 to 8, used as an anode for a solid oxide electrolysis cell (SOEC) or an anode for a solid oxide fuel cell (SOFC).

[0026] An embodiment of the present invention will be described in detail below. [1. Electrode] The electrode according to the present invention comprises: Electrolyte particles; Ni-based particles and It is equipped with:

[0027] [1.1. Electrolyte particles] The electrolyte particles include Gd-doped CeO2 (GDC) and / or Gd- and La-doped CeO2 (La-GDC).

[0028] [1.1.1. GDC] GDC not only functions as an oxide ion conductor but also as an oxygen storage material. Therefore, when GDC is used as an electrolyte particle, it not only allows oxide ions to be exchanged between GDC and Ni-based particles during electrode use, but also suppresses morphological changes at the electrode reaction sites.

[0029] In the present invention, the content of Gd in GDC is not particularly limited, and an optimum value can be selected depending on the purpose. Here, the "Gd content" refers to the ratio of the number of moles of Ce to the total number of moles of Ce and Gd contained in GDC.

[0030] Generally, the higher the Gd content, the higher the oxide ion conductivity and oxygen storage capacity of GDC. To achieve these effects, the Gd content is preferably greater than 0 mol%, more preferably 4 mol% or more, or even 8 mol% or more. On the other hand, if the Gd content is excessive, the oxide ion conductivity may decrease or the oxygen storage capacity may decrease. Therefore, the Gd content is preferably 20 mol% or less. The content is more preferably 15 mol% or less, or 10 mol% or less. It is particularly preferable that the GDC contains Gd in an amount of 4 mol % or more and 15 mol % or less.

[0031] [1.1.2. La-GDC] Similarly, La-GDC not only functions as an oxide ion conductor but also as an oxygen storage material. Therefore, when La-GDC is used as an electrolyte particle, it not only allows oxide ions to be exchanged between the Ni-based particles and the electrode during use, but also suppresses morphological changes at the electrode reaction sites. Furthermore, La-GDC is more effective at suppressing morphological changes at the electrode reaction sites than GDC.

[0032] In the present invention, the Gd content and La content in La-GDC are not particularly limited, and optimal values ​​can be selected depending on the purpose. Here, the "Gd content" refers to the ratio of the number of moles of Gd to the total number of moles of Ce, Gd, and La contained in La-GDC. The "La content" refers to the ratio of the number of moles of La to the total number of moles of Ce, Gd, and La contained in La-GDC.

[0033] The higher the Gd content in La-GDC, the higher the oxide ion conductivity of La-GDC. To achieve this effect, the Gd content is preferably greater than 0 mol%. The content is more preferably 1 mol% or more, or 2.5 mol% or more. On the other hand, if the Gd content is excessive, the oxygen storage capacity may decrease or the oxide ion conductivity may plateau. Therefore, the Gd content is preferably less than 10 mol%. The content is more preferably 8.5 mol% or less, or 7.5 mol% or less.

[0034] The higher the La content in La-GDC, the higher the oxygen storage capacity of La-GDC. This is because La is present in the crystal lattice of CeO2. 3+ (ionic radius: 0.116 nm) is doped, 4+ (ionic radius: 0.097 nm) is Ce 3+ This is thought to be because the distortion of the crystal lattice that occurs when La is reduced to a crystalline form (ionic radius: 0.114 nm) is alleviated. To achieve this effect, the La content is preferably greater than 0 mol %. The content is more preferably 1 mol % or more, or 2.5 mol % or more. On the other hand, if the La content is excessive, the oxygen storage capacity may decrease. Therefore, the La content is preferably less than 10 mol %. The content is more preferably 8.5 mol % or less, or 7.5 mol % or less. It is particularly preferable that the La-GDC has a Gd content of 2.5 mol % or more and 7.5 mol % or less and a La content of 2.5 mol % or more and 7.5 mol % or less.

[0035] [1.2. Ni-based particles] In the present invention, the Ni-based particles are comprised 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 NiO or a composite oxide containing Ni.

[0036] Core The core functions as a catalyst and an electron conductor in the electrode. In the present 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, and examples of the alloying element include Fe and Co. When the core is made of a Ni-based alloy, the Ni content in the core is preferably 90 mass % or more, and more preferably 95 mass % or more. The core is particularly preferably made of Ni or a Ni-Fe alloy.

[0037] Shell A. Shell Composition The surface of the core is partially or entirely covered with a shell, which has the function of suppressing vapor phase diffusion of Ni contained in the Ni-based particles. Here, "vapor phase diffusion of Ni" refers to a phenomenon in which, when an electrode containing Ni is exposed to high-temperature water vapor, 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.

[0038] As described below, 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 is made of an oxide containing the metal element that constitutes the core. More specifically, the shell is made of a composite oxide containing NiO or Ni (Ni-based oxide). In the initial state, the entire surface of the core may be covered with a shell. This is thought to be because, when a current is applied to the electrode, GDC or La-GDC extracts oxygen from the shell in the region (three-phase interface) where the Ni-based particles, GDC or La-GDC, and voids (gas phase) overlap, resulting in the formation of a Ni layer or Ni-based alloy layer near the three-phase interface.

[0039] B. Shell Thickness The thickness of the shell may vary depending on the location. The shell does not necessarily need to completely cover the surface of the core, but may cover only a portion of the core. Even when only a portion of the core is covered with the shell, the vapor phase diffusion of Ni can be effectively suppressed depending on the coverage rate. On the other hand, if the entire core is covered with a thick shell, the activity of the electrode 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 a "thin film region"). To obtain high electrode activity, the thickness of the thin film region is preferably 200 nm or less. The thickness of the thin film region is more preferably 150 nm or less, 100 nm or less, 50 nm or less, 40 nm or less, or 30 nm or less.

[0040] The thin film region is preferably formed in a three-phase interface, because the presence of the thin film region in the three-phase interface allows the vicinity of the three-phase interface to continue to function as an electrode active site due to the oxygen storage capacity of the electrolyte particles. Here, the "three-phase interface" refers to the electrolyte particle / Ni-based particle interface in the vicinity of the three-phase interface, or the Ni-based particle / gas phase interface in the vicinity of the three-phase interface. The term "vicinity of the three-phase interface" refers to a region 300 nm or less from the electrode reaction point where the three phases of the electrolyte particles, Ni-based particles, and gas phase intersect.

[0041] [C. Shell Area Ratio] The "shell area ratio" is the ratio of the core area (S core ) to the shell area (S shell ) ratio (S shell / S core ) S core and S shell can be calculated from SEM / EDS mapping of the cross section of the electrode, respectively.

[0042] The shell area ratio correlates with the coverage of the core surface with the shell. If the shell area ratio is too small, the proportion of exposed cores increases. This may make it difficult to suppress the vapor phase diffusion of Ni. Therefore, the shell area ratio is preferably greater than 0. The area ratio is more preferably 0.30 or more, 0.40 or more, 0.50 or more, or 0.56 or more. On the other hand, if the shell area ratio is too large, the proportion of Ni-based oxide in the electrode increases, which may result in a deterioration in electrode properties. Therefore, the shell area ratio is preferably 0.98 or less. The area ratio is more preferably 0.95 or less, or even 0.90 or less.

[0043] 1.3. Electrode Composition [1.3.1. Ni-based particle content] 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.

[0044] If the content of Ni-based particles is too low, the total cell resistance increases and the efficiency of the electrode reaction decreases. Therefore, the content of Ni-based particles is preferably 30 mass% or more. The content is more preferably 40 mass% or more. On the other hand, if the content of Ni-based particles is excessive, the content of electrolyte particles will be reduced, which may result in a decrease in the efficiency of the electrode reaction. Therefore, the content of Ni-based particles is preferably 70 mass% or less.

[0045] [1.3.2. La-GDC content] "La-GDC content" refers to the ratio of the mass of La-GDC to the total mass of GDC and La-GDC contained in the electrode. In the present invention, the content of La-GDC is not particularly limited, and an optimal content can be selected depending on the purpose. That is, the electrode according to the present invention may contain only one of GDC or La-GDC as electrolyte particles, or may contain both. To obtain high durability, the content of La-GDC is preferably 50 mass% or more. The content is more preferably 70 mass% or more, or 90 mass% or more.

[0046] [1.3.3. Total content of GDC and La-GDC] "Total content of GDC and La-GDC" refers to the ratio of the total mass of GDC and La-GDC to the total mass of electrolyte particles contained in the electrode. The electrolyte particles may consist solely of GDC and / or La-GDC, or may contain other components. Generally, the greater the total content of GDC and La-GDC, the better the durability of the electrode. To achieve high durability, the total content of GDC and La-GDC is preferably 80 mass% or more. The total content is more preferably 90 mass% or more, 95 mass% or more, or 99 mass% or more.

[0047] [1.4. Porosity] "Porosity" refers to a value measured by a mercury porosimeter.

[0048] The porosity of an electrode affects the electrode characteristics. If the porosity of an electrode is too small, the gas diffusion property may decrease, and the efficiency of the electrode reaction may decrease. Therefore, the porosity of an electrode is preferably 20% or more. The porosity is more preferably 25% or more. On the other hand, if the porosity of the electrode is too large, the number of three-phase interfaces becomes relatively small, which may result in a decrease in the efficiency of the electrode reaction. Therefore, the porosity of the electrode is preferably 40% or less. The porosity is more preferably 35% or less, or even 30% or less.

[0049] [1.5. Characteristics: Deterioration rate” The "deterioration rate" refers to the slope A of the line ΔR=A×t obtained by connecting the values ​​at t=0 h and t=40 h, with the vertical axis representing the rate of change in resistance of the electrode before and after the durability test: ΔR (%) and the horizontal axis representing the durability test time: t (h).

[0050] The resistance change rate: ΔR (%) is a value expressed by the following formula (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 the durability test for t hours.

[0051] "Electrode reaction resistance (Rct2)" is the reaction resistance of the three-phase interface in the fuel electrode, obtained by performing impedance measurements on an electrolytic cell using the electrode as the fuel electrode, and refers to the diameter of the arc (second arc) that contributes to the electrode reaction. The "durability test" refers to a test in which steam electrolysis is carried out for a predetermined time under the conditions shown in Table 1 using an electrolysis cell in which the electrode is used as the fuel electrode.

[0052] [Table 1]

[0053] Conventional fuel electrodes (Ni / YSZ) deteriorate quickly because the morphology of the electrode reaction sites changes easily. In contrast, the electrode of the present invention uses Ni-based particles in the form of core-shell particles, so its deterioration rate is slower than that of conventional electrodes, regardless of the temperature during durability tests. For example, the degradation rate of a conventional electrode at 700°C is 17.18% / h. In contrast, when the electrolyte particles consist of only GDC, the degradation rate of the electrode according to the present invention at 700°C is 10% / h or less. By further optimizing the manufacturing conditions, the degradation rate at 700°C can be reduced to 9% / h or less, or even 8% / h or less.

[0054] Similarly, the degradation rate of a conventional electrode at 750°C is 10.25% / h. In contrast, when the electrolyte particles consist of only GDC, the degradation rate of the electrode according to the present invention at 750°C is 6% / h or less. By further optimizing the manufacturing conditions, the degradation rate at 750°C is 5% / h or less. Furthermore, the degradation rate of a conventional electrode at 800°C is 2.67% / h. In contrast, when the electrolyte particles consist of only GDC, the degradation rate of the electrode according to the present invention at 800°C is 2% / h or less. By further optimizing the manufacturing conditions, the degradation rate at 800°C is 1.5% / h or less.

[0055] When the electrolyte particles consist only of La-GDC, the degradation rate of the electrode according to the present invention is 5% / h or less at 700°C. By further optimizing the manufacturing conditions, the degradation rate at 700°C can be reduced to 4% / h or less, or even 3% / h or less. Furthermore, when the electrolyte particles consist only of La-GDC, the degradation rate of the electrode according to the present invention at 800°C is 1% or less. By further optimizing the manufacturing conditions, the degradation rate at 800°C is 0.5% or less. The same is true when the electrolyte particles consist of a mixture of GDC and La-GDC; optimizing the composition results in an electrode with a slow degradation rate.

[0056] [1.6. Usage] Solid oxide electrolysis cells (SOECs) and solid oxide fuel cells (SOFCs) are generally an electrolyte layer including a solid oxide electrolyte; an anode bonded to one surface of the electrolyte layer; an air electrode bonded to the other surface of the electrolyte layer; An intermediate layer (reaction prevention layer) inserted between the electrolyte layer and the air electrode It is equipped with: In some cases, a fuel electrode-side current collecting layer is disposed on the outside of the fuel electrode, or a cathode-side current collecting layer is disposed on the outside of the air electrode.

[0057] The electrode according to the present invention is particularly suitable as an anode for a solid oxide electrolysis cell (SOEC) or a cathode for a solid oxide fuel cell (SOFC). When the electrode according to the present invention is used as an SOEC anode or an SOFC anode, the materials of the other components are not particularly limited, and an optimum material can be selected depending on the purpose.

[0058] For example, the solid oxide electrolyte that constitutes the electrolyte layer can be made of yttria-stabilized zirconia (YSZ), scandia-stabilized zirconia (ScSZ), scandia-yttria-stabilized zirconia (ScYSZ), samaria-doped ceria (SDC), lanthanum strontium gallium magnesium oxide (LSGM), etc.

[0059] For the air electrode, lanthanum strontium manganite (LSM), lanthanum strontium cobalt ferrite (LSCF), lanthanum strontium cobaltite (LSC), etc. can be used.

[0060] The intermediate layer is inserted as needed to prevent reactions that may occur due to direct contact between the electrolyte layer and the air electrode. For example, when the electrolyte layer is YSZ and the air electrode is LSC, it is preferable to use Gd-doped CeO (GDC) for the intermediate layer.

[0061] The materials for the anode-side current collecting layer and the cathode-side current collecting layer are not particularly limited as long as they are capable of donating and receiving electrons, supplying reactants, and discharging reaction products. The current collecting layers are typically made of a material that has the same or similar composition as the electrodes and has a higher porosity than the electrodes.

[0062] 2. Electrode manufacturing method The electrode according to the present invention comprises: (a) preparing a compact using a raw material mixture containing raw materials for Ni-based particles and raw materials for electrolyte particles; (b) sintering the obtained green body; (c) subjecting the obtained sintered body to a reduction treatment; (d) The resulting reduced product is oxidized in a controlled oxidizing atmosphere. It can be produced by

[0063] [2.1. Molded object forming process] First, a compact is produced using a raw material mixture containing raw materials for Ni-based particles and raw materials for electrolyte particles.

[0064] The term "raw material for Ni-based particles" refers to a raw material that becomes Ni-based particles after sintering, reduction, and controlled oxidation treatment. In the present invention, the type of raw material for Ni-based particles is not particularly limited, and an optimal raw material can be selected depending on 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, and Co2O3 powder.

[0065] The term "electrolyte particle raw material" refers to a raw material that becomes electrolyte particles after sintering, reduction, and controlled oxidation. In the present invention, GDC powder and / or La-GDC powder are used as the electrolyte particle raw material. The compositions of GDC and La-GDC are as described above, so further explanation is omitted.

[0066] The raw material mixture may contain a pore-forming material (e.g., carbon powder). The metal oxide (e.g., NiO powder) contained in the raw material of the Ni-based particles added to the raw material mixture is reduced after the sintered body is produced. During this process, volumetric shrinkage occurs, introducing pores into the sintered body. Therefore, a pore-forming material is not necessarily required. However, adding a pore-forming material to the raw material mixture increases the degree of freedom in controlling the porosity. Furthermore, the raw materials are preferably blended so as to obtain an anode having the desired composition after sintering, reduction, and controlled oxidation treatment.

[0067] The method for producing the molded body is not particularly limited, and an optimum method can be selected depending on the purpose. Examples of the method for producing the molded body include: (a) A method in which a slurry containing a raw material mixture is tape-cast, the resulting green sheet is laminated on a substrate (for example, a green body that will become the anode-side current collecting layer after sintering, or a green body that will become the electrolyte layer after sintering), and the laminate is isostatically pressed to bond the green sheet; (b) A method of preparing a slurry containing a raw material mixture and screen-printing the slurry onto a surface of a substrate; etc.

[0068] 2.2. Sintering process Next, the obtained molded body is sintered (sintering step). It is preferable to select the optimum sintering conditions depending on the raw material composition. Sintering is usually carried out in an air atmosphere at 1000°C to 1500°C (preferably 1000°C to 1300°C) for 1 hour to 5 hours. If a pore-forming material is contained in the raw material mixture, the pore-forming material disappears during sintering, and pores are formed in the sintered body.

[0069] [2.3. Reduction process] Next, the obtained sintered body is subjected to a reduction treatment (reduction step). The reduction treatment is carried out 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 allow the generation of core particles.

[0070] [2.4. Oxidation process] Next, the resulting reduced product is subjected to an oxidation treatment in a controlled oxidizing atmosphere, thereby obtaining an electrode according to the present invention. The controlled oxidation treatment is carried out in order to form a shell on the surface of the core particle. The conditions for the controlled oxidation treatment are not particularly limited, as long as they are capable of selectively oxidizing only the surface layer portion of the core particle.

[0071] The following methods can be used to form core-shell particles. The first method is (a) A cell was prepared using the electrode immediately after reduction (the electrode containing the core particles) as the fuel electrode. (b) With the cell heated to a temperature of 650°C or higher, the gas atmosphere on the fuel electrode side was adjusted to H2O / H2 = 4 (volume ratio), (c) Apply a current of 30 mA to the cell in this state for at least 50 hours. (d) After the current application is stopped, fill the inside of the fuel electrode with an inert gas (e.g., N2 gas). It is a method.

[0072] The second method is (a) A cell was prepared using the electrode immediately after reduction (the electrode containing the core particles) as the fuel electrode. (b) With the cell heated to a temperature of 400°C or higher, the gas atmosphere on the fuel electrode side is adjusted to H2O / H2 = 1.0 or more (volume ratio), (c) holding the cell without applying current for at least 1 hour; (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.

[0073] In the present invention, either method may be used. In addition, regardless of which method is used, the area ratio of the shell can be controlled by optimizing the temperature, atmosphere, current, treatment time, etc.

[0074] As described above, the electrolysis cell is composed of an assembly of a fuel electrode (cathode), an electrolyte layer, a reaction prevention layer, and an air electrode (anode). An anode-side current collecting layer may be further joined to the outside of the fuel electrode, and / or an air electrode-side current collecting layer may be further joined to the outside of the air electrode. Sintering and joining of each layer is performed by stacking the compacts and then heating the stack to a predetermined temperature. When the optimal sintering temperatures for each layer are different, sintering is usually performed in multiple stages. Furthermore, reduction of the fuel electrode is usually performed after all layers have been joined. When the electrode according to the invention is used as an anode in an electrolysis cell, the controlled oxidation of the anode is carried out after all layers have been bonded and the anode has been reduced.

[0075] [3. Effect] 3.1. Electrode Deterioration A schematic diagram of electrode degradation in a conventional anode is shown in Figure 1. In conventional electrolysis cells using Ni / YSZ as the anode, the degradation of the anode over time is thought to be caused by the vapor phase diffusion of Ni.

[0076] The deterioration of the anode over time due to vapor-phase diffusion of Ni is thought to proceed as follows. Specifically, as shown in Figure 1, when the anode is exposed to high-temperature steam, Ni reacts with the steam to produce nickel hydroxide. The resulting nickel hydroxide or its decomposition products diffuse through the electrode via the gas phase and adhere to the surfaces of larger Ni particles. This is thought to result in a change in the morphology of the electrode reaction sites, leading to electrode deterioration.

[0077] 3.2. Maintaining electrode activity with core-shell particles In contrast, in an electrode containing electrolyte particles made of GDC and / or La-GDC and Ni-based particles, if a shell made of NiO or a composite oxide containing Ni (Ni-based oxide) is formed on the surface of the Ni-based particles in advance, the deterioration of the electrode over time can be suppressed without sacrificing the electrode performance. This is thought to be due to the following reasons.

[0078] [3.2.1. Maintenance of electrode reaction sites by GDC and La-GDC] 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, an electrode containing pure Ni particles and GDC is oxidized in a controlled oxidizing atmosphere, and only the surface layer of the pure Ni particles is selectively oxidized. As a result, core-shell particles are obtained in which the surface of a core made of pure Ni is coated with NiO. See the upper left diagram in Figure 2.

[0079] GDC has the ability to store oxygen, so in the "region where NiO / GDC / voids overlap (three-phase interface)," GDC extracts oxygen from NiO. As a result, NiO may be reduced to metallic Ni near the three-phase interface, but in other regions, the surface of the core often remains covered with NiO. See the upper right and lower right figures in Figure 2.

[0080] When a current is applied to the electrode (for example, during electrolysis), the oxygen stored in the GDC diffuses within the GDC, forming lattice vacancies that provide the oxygen storage capacity. As a result, even when the electrode is exposed to high-temperature steam, a Ni layer is always formed in the "NiO / GDC / vacancy overlapping region," which continues to function as an electrode reaction site. See the bottom left diagram in Figure 2. In this regard, (a) The electrolyte particles are La-GDC, or the electrolyte particles are a mixture of GDC and La-GDC, or (b) When the core of a Ni-based particle is made of a Ni-based alloy and the shell is made of a composite oxide containing Ni The same is true.

[0081] [3.2.2. Suppression of morphological changes at electrode reaction sites by the shell] It is thought that the vapor phase diffusion of Ni is caused by direct contact of Ni with high-temperature water vapor. In contrast, when the surface of Ni in areas other than the vicinity of the three-phase interface is covered with NiO, the generation of nickel hydroxide, which is a cause of electrode degradation, is suppressed in that area (i.e., the "area where NiO overlaps with voids"). As a result, it is thought that the vapor phase diffusion of Ni is suppressed. See the bottom left diagram in Figure 2. This also applies to the case where the core of the Ni-based particle is made of a Ni-based alloy and the shell is made of a composite oxide containing Ni.

[0082] An electrode composed of Ni-based particles and GDC and / or La-GDC functions even when the Ni-based particles do not contain a shell. However, when the Ni-based particles do not contain a shell, if the electrode is used under harsh conditions immediately after starting use, the morphology of the electrode reaction points may change excessively. In contrast, when the Ni-based particles contain a shell, such problems are less likely to occur.

[0083] [3.3. Effect of La] Electrodes containing La-GDC exhibit higher durability than electrodes containing only GDC, which is thought to be due to the improved oxygen storage capacity of the GDC electrode due to the substitution of La for some of the Gd in the GDC electrode. [Example]

[0084] [A. Experiment 1] 1. Preparation of electrolytic cell [1.1. Sample No. 1] A GDC sheet (reaction prevention layer, diameter 22 mm) was placed on one side of an 8YSZ electrolyte pellet (diameter: 22 mm, thickness: 500 μm) with a reference electrode attached to the 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 on which the anti-reaction layer was baked). Specifically, NiO / GDC paste (NiO:GDC = 1:1, mass ratio) was applied by screen printing and fired at 1340°C. The GDC powder used had a Gd content of 10 mol%. Furthermore, LSC / GDC paste was applied onto the reaction prevention layer by screen printing and fired at 1125°C to form an air electrode.

[0085] The resulting electrolytic cell was subjected to a reduction treatment of the fuel electrode by holding it at 700°C for 20 minutes in a 100% hydrogen atmosphere.

[0086] [1.2. Sample No. 2] An electrolytic cell was fabricated in the same manner as in Sample No. 1, except that the fuel electrode was fabricated using NiO / 8YSZ paste.

[0087] 2. Test Method [2.1. Deterioration rate] Figure 3 shows a schematic diagram of the electrolytic cell used for impedance measurements. 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.

[0088] 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. During the durability test, impedance measurements were performed to measure the electrode reaction resistance (Rct2). Furthermore, the electrode reaction resistance (Rct2) before the durability test and after the durability test for a predetermined period of time was used to calculate the resistance change rate expressed by equation (1). Furthermore, the degradation rate was calculated from the resistance change rate.

[0089] [2.2. Shell area ratio] The cross section of the anode was observed by SEM and EDS mapped before and after the durability test. From the results of the EDS mapping, the area of ​​each component in the anode was calculated and the area ratio was determined. The procedure for calculating the area ratio is as follows:

[0090] Specifically, the cross section of the electrode was observed at 2000 to 3500 magnifications, and the electrode cross section was divided into eight areas (thickness: approximately 5 μm) along the thickness direction. For each area, the area ratio of Ni (core), NiO (shell), void area, and electrolyte area ratio were calculated. Next, the average area ratio of each component was calculated for each area. Furthermore, the "average area ratio of each component" calculated for each area was used to calculate the "average area ratio of each component" for the entire electrode.

[0091] [2.3. TEM / EDS analysis] The fuel electrode after the durability test was subjected to TEM / EDS analysis.

[0092] [3. Results] [3.1. Deterioration rate] Figure 4 shows an example of the impedance analysis results. In Figure 4, the second arc (Rct2) represents the magnitude of the electrochemical reaction resistance of the fuel electrode. The larger the diameter of the second arc, the greater the 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 in the hydrogen electrode.

[0093] Figure 5 shows an example of the change over time in the rate of resistance change of Sample No. 1. Figure 5 shows that the rate of change in the rate of resistance change decreases when the durability test time exceeds 40 hours. Figure 6 shows the degradation rates of the electrodes of Samples 1 and 2 at various temperatures. For Sample 1, the degradation rates were 7.44% / h (650°C), 6.22% / h (700°C), 3.99% / h (750°C), 0.86% / h (800°C), and 0.79% / h (850°C), respectively. Figure 6 shows that the degradation rate of Sample 1 is approximately half that of Sample 2. These results also suggest that the degradation of Ni-based particles during electrolysis can be suppressed by subjecting the anode immediately after fabrication to an oxidation treatment in a controlled oxidizing atmosphere to form the Ni-based particles into a core-shell structure.

[0094] [3.2. Shell area ratio] Figure 7 shows an SEM image of a portion of the electrode cross section of Sample No. 1 after a durability test (800°C). Figure 8 shows an example of SEM / EDS mapping of the entire electrode cross section (from the electrolyte interface to the electrode surface) of Sample No. 1 after a durability test (800°C). Figure 9 shows the area ratio of the electrode constituent material of Sample No. 1 after a durability test (800°C). Furthermore, Figure 10 shows the area ratio of the electrode constituent material of Sample No. 1 after a durability test (850°C).

[0095] For sample No. 1, the ratio of Ni particles (core) to NiO layer (shell) across the entire electrode cross section after the durability test (800°C) was 17.7:9.9 (=1:0.56). See the whole section in the upper part of Figure 9. Also, the ratio of Ni particles (core) to NiO layer (shell) across the entire electrode cross section after the durability test (850°C) was 14.3:13.9 (=1:0.97). See the whole section in the upper part of Figure 10. The area that actually acts as an electrode reaction site is the region up to 20 μm from the fuel electrode / electrolyte interface (areas 5-8 in Figures 8-10), and the ratio of Ni particles (core) to NiO layer (shell) in this region after the durability test (800°C) was 17.9:8.8 (=1:0.49).The ratio of Ni particles (core) to NiO layer (shell) in this region after the durability test (850°C) was 14.4:13.0 (=1:0.90).

[0096] [3.3. TEM / EDS analysis] Figure 11 shows a TEM image (left) and EDS mapping (right) of the electrode of sample No. 1 after the durability test (800°C). Figure 12(A) shows the results of line analysis performed along the direction of arrow A in Figure 11. Figure 12(B) shows the results of line analysis performed along the direction of arrow B in Figure 11. Furthermore, Figure 13 shows the oxygen storage capacity of the electrodes of sample No. 1 and sample No. 2 at each temperature.

[0097] As shown in the left image of Figure 11, NiO layers were confirmed at the Ni particle / vapor phase interface and the Ni particle / GDC particle interface. However, the thickness of the NiO layer formed at the Ni particle / vapor phase interface and the Ni particle / GDC particle interface was not uniform. Figure 11 shows that there are regions near the three-phase interface where the NiO layer is extremely thin.

[0098] In the case of sample No. 1, a Ni-rich layer (approximately 5 nm thick) was confirmed in the NiO region near the three-phase interface (electrode reaction point). See Figure 12. This Ni-rich layer is thought to be a trace of GDC absorbing oxygen from NiO. This result suggests that during electrolysis, GDC absorbs oxygen from the NiO layer (see Figure 13), causing the NiO layer to become Ni-like. This also suggests that the surface of the Ni-based particles maintains its function as an electrode reaction site even when exposed to high-temperature steam.

[0099] Figure 14 shows the Ni particle change rate at each temperature for the electrode of Sample No. 1. The "Ni particle change rate" refers to the rate of change in the average particle size of Ni particles before and after the durability test. Figure 14 shows that the higher the temperature during the durability test, the smaller the Ni particle change rate. This result is thought to indicate that increasing the treatment temperature of the controlled oxidation treatment makes it easier for a shell to form on the surface of the Ni-based particles. Furthermore, these results suggest that increasing the treatment temperature of the controlled oxidation treatment suppresses the generation of Ni(OH)2, thereby suppressing morphological changes at the three-phase interface and making it easier to maintain electrode activity.

[0100] [B. Experiment 2] 1. Preparation of electrolytic cell [1.1. Sample No. 3] As in Sample No. 1, a reaction prevention layer was formed on one side of an 8-YSZ electrolyte pellet. La-GDC powder was used as the electrolyte powder for the anode, and NiO / La-GDC paste (NiO:La-GDC = 36:64, mass ratio) was applied by screen printing to the other side of the 8YSZ electrolyte pellet (the side opposite to the side on which the reaction prevention layer was baked). An electrolytic cell was then fabricated in the same manner as in Sample No. 1. The La-GDC powder used contained 5 mol% La and 5 mol% Gd.

[0101] 2. Test Method [2.1. Deterioration rate] In the same manner as in Sample No. 1, the electrode reaction resistance (Rct2) was measured, and the rate of resistance change and the rate of deterioration were calculated.

[0102] [2.2. Shell area ratio] The shell area ratio was determined in the same manner as in Sample No. 1.

[0103] [3. Results] [3.1. Deterioration rate] The degradation rate of the electrodes of sample No. 3 (La-GDC) and sample No. 1 (GDC) at each temperature is shown in Figure 15. It can be seen from Figure 15 that the degradation rate of sample No. 3 is less than half that of sample No. 1.

[0104] The oxygen storage capacities of La-GDC and GDC are shown in Figure 16. It can be seen from Figure 16 that the oxygen storage capacity of La-GDC is higher than that of GDC. The significantly improved durability of sample No. 3 compared to sample No. 1 is thought to be due to the improved oxygen storage capacity achieved by substituting a portion of the Gd contained in GDC with La.

[0105] [3.2. Shell area ratio] Figure 17 shows an SEM image of a portion of the electrode cross section of sample No. 3 after the durability test (700°C). Figure 18 shows an example of SEM / EDS mapping of the electrode cross section (region from the electrolyte interface to approximately 20 μm toward the electrode surface) of sample No. 3 after the durability test (700°C). Figure 19 shows the area ratio of the electrode constituent material of sample No. 3 after the durability test (700°C). Furthermore, Figure 20 shows the area ratio of the electrode constituent material of sample No. 3 after the durability test (800°C).

[0106] In the case of sample No. 3, the ratio (average) of Ni particles (core) to NiO layer (shell) after the durability test (700°C) was 14.5:5.0 (=1:0.34), and the ratio (average) of Ni particles (core) to NiO layer (shell) after the durability test (800°C) was 10.9:10.3 (=1:0.94).

[0107] Although the embodiments of the present invention have been described in detail above, the present invention is not limited to the above embodiments, and various modifications are possible within the scope of the gist of the present invention. [Industrial Applicability]

[0108] The electrode according to the present invention can be used as an anode for a solid oxide electrolysis cell (SOEC) or an anode for a solid oxide fuel cell (SOFC).

Claims

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

2. 2. The electrode according to claim 1, wherein the area ratio of the shell is greater than 0 and not greater than 0.

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

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

4. 2. The electrode according to claim 1, wherein the rate of deterioration at 700° C. is 10% / h or less. Here, the "deterioration rate" refers to a line obtained by connecting the values ​​at t = 0 h and t = 40 h, namely, ΔR = A × t, with 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 the slope A of the line is ΔR = A × t.

5. The electrode according to claim 1 , wherein the GDC has a Gd content of more than 0 mol % and not more than 20 mol %. Here, the "content of Gd" refers to the ratio of the number of moles of Gd to the total number of moles of Ce and Gd contained in the GDC.

6. The La-GDC is The Gd content is more than 0 mol% and less than 10 mol%, The La content is more than 0 mol% and less than 10 mol% 10. The electrode of claim 1. however, The "Gd content" refers to the ratio of the number of moles of Gd to the total number of moles of Ce, Gd, and La contained in the La-GDC, The "La content" refers to the ratio of the number of moles of La to the total number of moles of Ce, Gd, and La contained in the La-GDC.

7. 2. The electrode according to claim 1, wherein the content of the Ni-based particles is 30 mass % or more and 70 mass % or less. Here, the "content of Ni-based particles" refers to the ratio of the mass of the 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. The "porosity" mentioned above refers to a value measured by a mercury porosimeter.

9. 2. The electrode according to claim 1, which is used as an anode for a solid oxide electrolysis cell (SOEC) or an anode for a solid oxide fuel cell (SOFC).

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