Electrolytic cell and method for manufacturing an electrolytic cell

Abstract

This invention provides an electrolytic cell and a method for manufacturing the same, which enable both the electrolytic reaction of water vapor and mechanical strength by providing a support layer with increased strength while maintaining the gas permeability required for electrolytic cells. [Solution] The electrolytic cell comprises a hydrogen electrode that is gas permeable and capable of electrolyzing water vapor flowing into it into oxygen ions and hydrogen, a solid oxide electrolyte layer capable of conducting oxygen ions generated at the hydrogen electrode, an oxygen electrode that is gas permeable and capable of generating oxygen molecules from oxygen ions that have reached from the solid oxide electrolyte layer, and a support layer that supports the hydrogen electrode or the oxygen electrode. The support layer has a porous sintered layer having a plurality of nickel oxide particles and a plurality of partially stabilized zirconia particles. The frequency distribution curve showing the particle size distribution of the plurality of partially stabilized zirconia particles has a first peak in the particle size range of 30 μm to 70 μm and a second peak in the particle size range of 0.5 μm to 4.0 μm.

Description

[Technical Field]

[0001] Embodiments of the present invention relate to an electrolytic cell and a method for manufacturing an electrolytic cell. [Background technology]

[0002] In recent years, the introduction of renewable energy sources such as solar, wind, and geothermal power has been promoted from the perspectives of environmental issues such as the depletion of fossil fuels and global warming caused by the release of carbon dioxide into the atmosphere, as well as energy security. Furthermore, hydrogen energy is attracting attention as a secondary energy source from the perspective of storage and transportation. Hydrogen energy is expected to be applied to fuel cell vehicles, for example, and there is a need for the production and storage of low-cost, high-quality hydrogen.

[0003] Currently, the dominant method for hydrogen production, from a cost and technological standpoint, is the reforming of fossil fuels. However, hydrogen production through fossil fuel reforming inevitably generates carbon dioxide during the process. In contrast, methods of producing hydrogen using water as a raw material and renewable energy are known to produce no carbon dioxide and have a low environmental impact. Methods for generating hydrogen by electrolyzing water or steam include the PEM type, which uses a polymer electrolyte membrane (PEM), and the SOEC type, which uses a solid oxide electrolysis cell (SOEC). Among these, the SOEC type requires less electricity in principle to produce hydrogen and is expected to be a promising future hydrogen production method.

[0004] SOECs, used for the electrolysis of water or steam for hydrogen production, consist of a support layer for the hydrogen electrode that conducts electrons, a hydrogen electrode (active layer) for electrolyzing water, a solid oxide electrolyte layer that conducts oxygen ions, and an oxygen electrode that combines oxygen ions to form oxygen molecules. The electrolyte is important because it has oxygen ion conductivity and the role of separating hydrogen gas and oxygen gas. In addition to the function of conducting electrons, the support layer is required to have gas permeability to supply steam to the active layer and strength to maintain the shape of the electrolytic cell. Therefore, the support layer is required to have the contradictory properties of being a porous layer with pores that are considered defects, while achieving high strength.

[0005] In other words, while the support layer of SOEC requires mechanical reliability such as strength, it also requires gas permeability. As a result, it tends to have low strength because it contains pores that can be the starting point of fracture. Thus, the support layer is required to achieve both mechanical reliability and gas permeability, but in order to ensure the gas permeability performance of the electrolytic cell, there is a challenge in that the mechanical reliability based on the porous layer must be kept low. [Prior art documents] [Patent Documents]

[0006] [Patent Document 1] Patent No. 5498191 [Patent Document 2] Japanese Patent Application Publication No. 10-64565 [Patent Document 3] Japanese Patent Application Publication No. 8-306361 [Patent Document 4] Japanese Patent Application Publication No. 7-307159 [Patent Document 5] Japanese Patent Application Publication No. 4-118861 [Overview of the project] [Problems that the invention aims to solve]

[0007] The problem that this invention aims to solve is to provide an electrolytic cell and a method for manufacturing the same that enable both water vapor electrolytic reaction and mechanical strength by providing a support layer with increased strength while maintaining the gas permeability required for an electrolytic cell. [Means for solving the problem]

[0008] The electrolytic cell of the embodiment comprises a hydrogen electrode that is gas permeable and capable of electrolyzing water vapor flowing into it into oxygen ions and hydrogen, a solid oxide electrolyte layer capable of conducting oxygen ions generated at the hydrogen electrode, an oxygen electrode that is gas permeable and capable of generating oxygen molecules from oxygen ions that have reached from the solid oxide electrolyte layer, and a support layer that supports the hydrogen electrode or the oxygen electrode. The support layer has a porous sintered layer having a plurality of nickel oxide particles and a plurality of partially stabilized zirconia particles. The frequency distribution curve showing the particle size distribution of the plurality of partially stabilized zirconia particles has a first peak in the particle size range of 30 μm to 70 μm and a second peak in the particle size range of 0.5 μm to 4.0 μm. [Brief explanation of the drawing]

[0009] [Figure 1] This is a schematic diagram showing an example of the structure of an electrolytic cell. [Figure 2] This is a schematic diagram illustrating an example of a frequency distribution curve. [Modes for carrying out the invention]

[0010] The embodiments will be described below with reference to the drawings. In each of the embodiments shown below, substantially identical components are denoted by the same reference numerals, and their descriptions may be partially omitted. The drawings are schematic, and the relationship between thickness and planar dimensions, the ratio of the thickness of each part, etc., may differ from those in reality.

[0011] Figure 1 is a schematic diagram showing an example of the structure of an electrolytic cell. Figure 1 shows an electrolytic cell 10. The electrolytic cell 10 is a SOEC that produces hydrogen and oxygen by electrolysis of high-temperature steam. The electrolytic cell 10 comprises a hydrogen electrode 11, an oxygen electrode 12, a solid oxide electrolyte layer 13, and a support layer 14. Although a flat-plate type SOEC is shown here, it is not limited to this, and an electrolytic cell 10 of a non-flat-plate type, such as a cylindrical type, may also be used.

[0012] The hydrogen electrode 11 is provided on the support layer 14. The hydrogen electrode 11 is composed of a porous layer made of a hydrogen electrode active material, and specifically has gas permeability due to its network structure framework. The hydrogen electrode 11 has open pores inside that are at least partially surrounded by the network structure framework, and is capable of electrolyzing water vapor flowing into these open pores into oxygen ions and hydrogen. The hydrogen electrode 11 is made of, for example, a composite of nickel oxide (NiO) and gadolinia-doped ceria (CeO2: GDC). In addition, composites of oxides such as cobalt (Co), iron (Fe), copper (Cu), and ruthenium (Ru) with oxides of rare earth elements or zirconia stabilized with rare earth elements may also be applied to the hydrogen electrode 11. Water vapor flowing from the water vapor passage of the support layer 14 into the open pores in the hydrogen electrode 11 is mainly electrolyzed into oxygen ions and hydrogen within the hydrogen electrode (active layer) 11. The hydrogen gas (H2) produced by electrolysis is discharged to the outside through a gas channel (not shown) and stored, for example. The generated oxygen ions are conductible into the solid oxide electrolyte layer 13.

[0013] The oxygen electrode 12 is made of an oxygen electrode active material and is composed of a porous body having gas diffusivity and electron conductivity, and oxygen ions (O) that reach the porous body from the solid oxide electrolyte layer 13 2- ) and electrons (e) supplied from an external power source -) can generate oxygen molecules (O2). The generated oxygen gas (O2) is led out to the outside through a gas flow path not shown in the figure and is stored, for example, as necessary. The oxygen electrode 12 is made of a porous sintered body containing an oxide having a perovskite structure represented by ABO3 (hereinafter referred to as perovskite oxide). For the oxygen electrode 12, for example, R 1-x A x B 1-y C y O 3-δ , (where R is a rare earth element such as La, A is an alkaline earth element such as scandium (Sr), calcium (Ca), barium (Ba), etc., boron (B) and carbon (C) are metal elements such as chromium (Cr), manganese (Mn), cobalt (Co), iron (Fe), nickel (Ni), etc., and x, y, and δ are atomic ratios satisfying 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, and 0 ≦ δ ≦ 1) can be used. As a representative example of the oxygen electrode 12, (La 1-x Sr x )(Co 1-y Fe y )O 3-δ (LSCF) can be mentioned.

[0014] The solid oxide electrolyte layer 13 has one side facing the hydrogen electrode 11 and the other side facing the oxygen electrode 12. The solid oxide electrolyte layer 13 is made of a dense solid oxide electrolyte and is an ion conductor that allows ions such as oxygen ions to pass through but does not conduct electricity. For the solid oxide electrolyte layer 13, for example, stabilized zirconia in which a stabilizer composed of an oxide of a rare earth element such as yttrium (Y), scandium (Sc), cerium (Ce), gadolinium (Gd), samarium (Sm), etc. is solid-dissolved, typically yttria-stabilized zirconia (YSZ) or ceria-stabilized zirconia (CSZ), or a composite of these can be used.

[0015] The electrolytic cell 10 may have an intermediate layer between the oxygen electrode 12 and the solid oxide electrolyte layer 13. The intermediate layer is a dense reaction prevention layer that prevents the diffusion and reaction of elements between the oxygen electrode 12 and the solid oxide electrolyte layer 13.

[0016] The support layer 14 is a strength member mainly responsible for the strength of the electrolytic cell 10, and supports the hydrogen electrode 11 or the oxygen electrode 12. The support layer 14 is made of, for example, a porous sintered body (porous ceramics) having gas permeability, and has a water vapor passage through which water vapor can flow. The support layer 14 has a porous sintered layer containing nickel oxide (NiO) particles and partially stabilized zirconia (Partially Stabilized Zirconia (ZrO2): PSZ) particles. The support layer 14 contains, for example, NiO particles and PSZ particles as main constituent components.

[0017] Unlike stabilized zirconia, PSZ is zirconia containing a stable part where the crystal is stabilized and an unstable part where the crystal is not stabilized. By using PSZ, high strength can be achieved. Examples of PSZ particles include yttria partially stabilized zirconia (Yttria (Y2O3) Partially Stabilized Zirconia (ZrO2): Y-PSZ) particles.

[0018] The porous sintered body as the support layer 14 preferably has a porosity of about 30% or more and 50% or less. When the porosity of the porous sintered body is less than 30%, the gas permeability decreases and the characteristics of the electrolytic cell 10 tend to deteriorate. When the porosity of the porous sintered body exceeds 50%, although the gas permeability improves, the mechanical properties such as the strength of the support layer 14 tend to deteriorate.

[0019] The electrolytic cell 10 can be manufactured, for example, by adding a binder, a pore former, etc. and a solvent to raw material powder to form a slurry, subjecting the slurry to sheet forming, lamination, and pressure bonding to form a molded body, and then subjecting the molded body to a debinding process (a degreasing process) and a sintering process. The pore former added for the porosity of the support layer 14 is thermally decomposed and removed in the debinding process, and the remaining holes remain even after sintering, realizing the porosity of the support layer 14. Generally, the porosity is around 40%, and gas permeability is exhibited at this high porosity.

[0020] However, when a load is applied to a porous sintered body as described above, stress is generated in the matrix surrounding the pores, according to their shape. If this generated stress exceeds the fracture strength of the matrix, cracks will occur, and further crack propagation will cause the failure of the support layer 14, and ultimately the large-scale failure of the entire electrolytic cell 10. Therefore, controlling the pore morphology, such as the diameter and shape of the pores, is important. Spherical particles, which are least likely to cause stress concentration, are often used as pore-forming agents. Ceramics (sintered bodies), being brittle materials, require high strength for ease of handling and setting. A low Young's modulus is preferable because a high Young's modulus generates high stress even with slight deformation, leading to easy fracture. Since cracks often propagate along grain boundaries, it is effective to increase strength by adding additives to improve grain boundary strength or the strength of the matrix material itself, or by applying compressive stress.

[0021] Therefore, the support layer 14 in this embodiment includes a porous sintered body (porous sintered layer) having a frequency distribution curve showing the particle size distribution of multiple PSZ particles, which has multiple peaks with different particle size ranges. By adding PSZ particles exhibiting a frequency distribution curve with multiple peaks together with NiO particles and generating compressive stress at the grain boundaries, the strength of the porous sintered layer constituting the support layer 14 can be increased. Consequently, it becomes possible to improve the strength of the support layer 14, its mechanical reliability, and ultimately the mechanical reliability of the entire electrolytic cell 10.

[0022] The particle size distribution of multiple PSZ particles is measured using, for example, a scanning electron microscope (SEM) or X-ray diffraction (XRD). The frequency distribution curve, based on the number of particles, which shows the particle size distribution of multiple PSZ particles, preferably has a peak P1 in the range of particle size 30 μm to 70 μm and a peak P2 in the range of particle size 0.5 μm to 4.0 μm. The frequency distribution curve can be obtained, for example, by measuring the particle size of multiple PSZ particles in accordance with JIS R 1670. Figure 2 shows an example of a frequency distribution curve. The horizontal axis in Figure 2 represents the particle size of the PSZ particles, with increasing particle size indicated from the left end to the right end of the horizontal axis. The vertical axis in Figure 2 represents the frequency of PSZ particles based on the number of particles, with increasing frequency indicated from the bottom end to the top end of the vertical axis.

[0023] The mode (mode diameter) of peak P1 is 30 μm or more and 70 μm or less, preferably 40 μm or more and 50 μm or less, for example, 50 μm.

[0024] The mode (mode diameter) of peak P2 is between 0.5 μm and 4.0 μm, preferably between 1.0 μm and 3.0 μm, for example, 1.0 μm.

[0025] It is preferable that the mode (highest frequency) of peak P2 is higher than the mode (highest frequency) of peak P1. This allows the function and characteristics of the support layer 14 to be well satisfied.

[0026] Preferably, the frequency distribution curve does not have any peaks in the particle size range of over 4.0 μm and less than 30 μm. Furthermore, it is preferable that the frequency distribution curve does not have any peaks in the particle size range of less than 4.0 μm. Moreover, it is preferable that the frequency distribution curve does not have any peaks in the particle size range of over 70 μm.

[0027] The particle size distribution described above can be achieved by constructing a plurality of PSZ particles, which include a first plurality of PSZ particles (PSZ1) with a particle size of 30 μm to 70 μm and a second plurality of PSZ particles (PSZ2) with a particle size of 0.5 μm to 4.0 μm. Peak P1 is attributed to PSZ1. Peak P2 is attributed to PSZ2. Preferably, the plurality of PSZ particles consist only of PSZ1 and PSZ2, and preferably do not include PSZ particles with a particle size of less than 4.0 μm, particles with a particle size of more than 4.0 μm but less than 30 μm, or particles with a particle size of more than 70 μm. PSZ1 and PSZ2 may be dispersed among each other or in contact with each other in the porous sintered layer. For example, one PSZ2 may be embedded between adjacent PSZ1s, or PSZ2 may be provided in contact with the surface of a PSZ1.

[0028] In the porous sintered layer, the volume ratio of PSZ1 to PSZ (PSZ1 / PSZ2) is preferably 25% to 75%, more preferably 30% to 70%, and more preferably 40% to 60%. By adjusting PSZ1 / PSZ2 to 25% to 75%, the functions and characteristics of the support layer 14 can be well satisfied. PSZ1 / PSZ2 can be appropriately adjusted, for example, by adjusting the amount of PSZ1 and PSZ2 added when manufacturing the electrolytic cell 10.

[0029] The average particle size of the NiO particles is, for example, 0.2 μm to 60 μm, preferably 1 μm to 30 μm. By controlling the average particle size of the NiO particles to 0.2 μm to 60 μm, the function and characteristics of the support layer 14 can be well satisfied. The average particle size of the NiO particles can be measured, for example, using an SEM in accordance with JIS R 1670.

[0030] In the porous sintered layer, the mass ratio of PSZ particles (total particles of PSZ1 and PSZ2) to NiO particles (PSZ / NiO) is preferably less than 80%, and more preferably 70% or less. By adjusting PSZ / NiO to 70% or less, the function and characteristics of the support layer 14 can be well satisfied. The lower limit of PSZ / NiO is not particularly limited, but for example, it is 50% or more. PSZ / NiO can be appropriately adjusted, for example, by adjusting the amount of PSZ1, PSZ2, and NiO particles added when manufacturing the electrolytic cell 10.

[0031] The method for manufacturing the electrolytic cell 10 of the embodiment is not particularly limited, but for example, the electrolytic cell 10 can be manufactured as follows. First, PSZ1 powder and PSZ2 powder are mixed to prepare PSZ mixed powder, and the PSZ mixed powder and NiO powder are mixed in the above-mentioned ratio to prepare raw material powder. PSZ1 powder and PSZ2 powder may also be prepared by preparing and classifying PSZ powder.

[0032] Next, a binder and a pore-forming agent are added to the raw material powder described above, and a solvent is added as needed and mixed to prepare a raw material slurry. A sheet is produced by forming this raw material slurry into a sheet. Then, a laminated molded body is produced by sequentially forming the hydrogen electrode 11 formation slurry and the solid oxide electrolyte layer 13 formation slurry into sheets on the obtained sheet. A porous laminated sintered body is produced by applying the processes of thermocompression bonding, degreasing, and sintering to this laminated molded body. An electrolytic cell 10 is obtained by performing the processes of forming the oxygen electrode 12 material and baking on the laminated sintered body. Note that each of these constituent layers may be degreased and sintered individually. Note that the particle sizes of the NiO powder, PSZ1 powder, and PSZ2 powder do not change particularly even after the sintering process, and can therefore be considered equivalent to the particle sizes of the NiO particles, PSZ1, and PSZ2 in the porous sintered layer. [Examples]

[0033] Next, we will describe specific examples of the electrolytic cell of the embodiment and the evaluation results thereof.

[0034] (Example 1) To adjust the particle size distribution of yttrium-stabilized zirconia (Y-PSZ) powder, Y-PSZ powder with an average particle size of 0.1 μm was calcined at a temperature of 600°C to 1100°C for 2 hours and then classified to obtain Y-PSZ (PSZ1) powder with a particle size of 30 μm to 70 μm and Y-PSZ (PSZ2) powder with a particle size of 0.5 μm to 4.0 μm. Next, PSZ powder (PSZ mixed powder) was prepared by blending PSZ1 powder and PSZ2 powder in a volume ratio of 1:1, and then raw material powder was prepared by blending it with nickel oxide (NiO) powder with an average particle size of 0.5 μm in a mass ratio of 6:4.

[0035] To 100 parts by mass of the above raw material powder, 10 parts by mass of polyvinyl acetal resin as a binder and 5 parts by mass of graphite with an average particle size of 10 μm as a pore-forming agent were added, and ethanol was added as a solvent and the mixture was pot-mixed for 24 hours to prepare a slurry. Next, the slurry was formed into a sheet with a thickness of 1 mm to prepare a support layer sheet.

[0036] Next, a sheet for the hydrogen electrode active layer was formed by sheet molding of a NiO-GDC slurry as the material for forming the hydrogen electrode active layer. A polyethylene terephthalate (PET) film was used as the electrolyte layer, and an electrolyte sheet was fabricated by screen printing a yttria-stabilized zirconia (YSZ) slurry to a thickness of 0.020 mm. Furthermore, the fabricated support layer sheet, hydrogen active layer sheet, and electrolyte sheet were heat-pressed together at a temperature of 70°C to form a laminated molded body, which was then degreased at a temperature of 400°C for 2 hours. The degreased body was then sintered at a temperature of 1300°C or higher for a predetermined time to obtain a sintered body. Degreasing and sintering were performed in an atmospheric environment. An LSCF oxygen electrode was formed on the electrolyte side of this sintered body by screen printing, and then baked.

[0037] In this way, a 50 mm x 50 mm sintered body for electrolytic cells was obtained. Using this sintered body, a three-point bending test was performed at a load application rate of 0.5 mm / min, and the fracture strength (bending strength) was measured. In addition, the electrical resistance of the laminate was measured using the three-terminal method. The results are shown in Table 1. In Table 1, the electrical resistance ratio is the relative value when the electrical resistance of the above laminate, where the mass fraction of PSZ mixed powder to NiO powder (PSZ / NiO) is 50%, is set to 1.00.

[0038] (Examples 2 and 3) In preparing the support layer sheet in Example 1, the only difference was that the mass ratio of PSZ mixed powder and NiO powder was changed. A sintered body for an electrolytic cell was then fabricated and its fracture strength and electrical resistance were evaluated in the same manner as in Example 1. The results are shown in Table 1.

[0039] (Comparative Example 1) In preparing the support layer sheet in Example 1, the sintered body for the electrolytic cell was fabricated in the same manner as in Example 1, except that the PSZ / NiO ratio was changed to 80%, and its fracture strength and electrical resistance were evaluated. The results are shown in Table 1.

[0040] [Table 1]

[0041] (Examples 4, 5, 6, Comparative Examples 2, 3) In preparing the support layer sheet in Example 1, the only difference was that the volume fraction of PSZ1 powder to PSZ powder (PSZ1 / PSZ2) was changed as shown in Table 2. A sintered body for an electrolytic cell was then prepared and its fracture strength evaluated in the same manner as in Example 1. The results are shown in Table 2. In Table 2, the bending strength ratio is a relative value, with the bending strength of the sintered body with a PSZ1 / PSZ2 ratio of 40% set to 1.0.

[0042] [Table 2]

[0043] Table 1 shows that by controlling the PSZ / NiO ratio to less than 80%, for example to 70% or less, it is possible to realize a support layer that is highly mechanically reliable and achieves both appropriate bending strength and appropriate electrical resistance.

[0044] Furthermore, Table 2 shows that by controlling PSZ1 / PSZ2 to more than 20% but less than 70%, for example, between 25% and 65%, or even between 30% and 60%, a support layer with appropriate bending strength can be achieved.

[0045] Although several embodiments of the present invention have been described above, these embodiments are presented as examples only and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, substitutions, and modifications can be made without departing from the spirit of the invention. These embodiments and their variations are included in the scope and spirit of the invention, as well as in the claims of the invention and its equivalents. [Explanation of symbols]

[0046] 10...Electrolytic cell, 11...Hydrogen electrode, 12...Oxygen electrode, 13...Solid oxide electrolyte layer, 14...Support layer.

Claims

1. A hydrogen electrode that is permeable to gas and capable of electrolyzing water vapor that flows into the interior into oxygen ions and hydrogen, A solid oxide electrolyte layer capable of conducting the oxygen ions generated at the hydrogen electrode, An oxygen electrode having gas permeability and capable of generating oxygen molecules from the oxygen ions that have reached from the solid oxide electrolyte layer, A support layer for supporting the hydrogen electrode or the oxygen electrode, It is equipped with, The aforementioned support layer is It has a porous sintered layer having multiple nickel oxide particles and multiple partially stabilized zirconia particles, The frequency distribution curve showing the particle size distribution of the plurality of partially stabilized zirconia particles has a first peak in the particle size range of 30 μm to 70 μm and a second peak in the particle size range of 0.5 μm to 4.0 μm. Electrolytic cell.

2. The mass ratio of the plurality of partially stabilized zirconia particles to the plurality of nickel oxide particles in the porous sintered layer is 70% or less. The electrolytic cell according to claim 1.

3. The aforementioned plurality of partially stabilized zirconia particles are A first plurality of partially stabilized zirconia particles having a particle size of 30 μm or more and 70 μm or less, A second set of partially stabilized zirconia particles with a particle size of 0.5 μm or more and 4.0 μm or less, It was divided into, The volume ratio of the first plurality of partially stabilized zirconia particles to the plurality of partially stabilized zirconia particles is 25% or more and 75% or less. The electrolytic cell according to claim 1.

4. The aforementioned plurality of partially stabilized zirconia particles are a plurality of yttrium partially stabilized zirconia particles. The electrolytic cell according to claim 1.

5. The average particle size of the plurality of nickel oxide particles is 0.2 μm or more and 60 μm or less. The electrolytic cell according to claim 1.

6. A method for manufacturing an electrolytic cell according to claim 1, A slurry is formed containing a first partially stabilized zirconia powder with a particle size of 30 μm to 70 μm and a second partially stabilized zirconia powder with a particle size of 0.5 μm to 4.0 μm. The slurry is molded to form a sheet-like molded body. The laminate including the molded body is sintered. A method for manufacturing electrolytic cells.

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