High-temperature steam electrolysis cell and method for manufacturing the same

Abstract

To provide a high temperature steam electrolytic cell that can have both an electrolytic reaction of steam and a mechanical strength by arranging a support layer with high strength, while keeping gas permeability of its electrolytic cell.SOLUTION: A high temperature steam electrolytic cell 1 of an embodiment of the present invention comprises: a support layer 2 having gas permeability; a hydrogen electrode 3 which is mounted on the support layer 2, has gas permeability, and can electrolyze steam introduced inside into oxygen ions and hydrogen; a solid oxide electrolyte layer 4 electrically conductive to the oxygen ions generated by the hydrogen electrode; and an oxygen electrode 6 which has gas permeability and can generate oxygen molecules from the oxygen ions through the solid oxide electrolyte layer. The support layer 2 comprises a porous sinter layer in which, in a composite of a nickel oxide and a gadolinium solid solution ceria, part of the gadolinium solid solution ceria is replaced by a ceria stabilized zirconia.SELECTED DRAWING: Figure 1

Description

Technical Field

[0001] Embodiments of the present invention relate to a high-temperature steam electrolysis cell and a method for manufacturing the same.

Background Art

[0002] In recent years, from the viewpoints of depletion of fossil fuels, environmental problems such as global warming due to carbon dioxide emissions into the atmosphere, and energy security, the introduction of renewable energy represented by solar power, wind power, geothermal energy, etc. has been promoted. Further, as secondary energy, hydrogen energy has attracted attention from the viewpoints of storage and transportation. Hydrogen energy is expected to be applied to, for example, fuel cell vehicles, and the production and storage of hydrogen at low cost and high quality are required.

[0003] Currently, from the viewpoints of cost and technology, a method of reforming fossil fuels for hydrogen production is the mainstream. However, hydrogen production by reforming fossil fuels inevitably generates carbon dioxide in the production process. On the other hand, a method of producing hydrogen using renewable energy with water as a raw material is known to generate no carbon dioxide and have a low environmental impact. As a method of electrolyzing water or steam to generate hydrogen, a PEM type using a polymer electrolyte membrane (PEM) and an SOEC type using a solid oxide electrolysis cell (SOEC) are known. Among them, the SOEC type is expected as a future hydrogen production method because it requires less power for hydrogen production in principle.

[0004] The SOEC used for the electrolysis of water or steam for hydrogen production is composed of a support layer of a hydrogen electrode that conducts electrons, a hydrogen electrode (active layer) that electrolyzes water, a solid oxide electrolyte layer that conducts oxygen ions, and an oxygen electrode that combines oxygen ions into oxygen molecules. The electrolyte has the role of conducting oxygen ions and separating hydrogen gas and oxygen gas, which is important. In addition to the function of conducting electrons, the support layer is required to have gas permeability for supplying steam to the active layer and strength for maintaining the form of the electrolysis cell. For this reason, a porous layer is used for the support layer to achieve gas permeability. Therefore, the support layer is required to have the antinomic characteristics of a porous layer having pores regarded as defects and realizing high strength.

[0005] That is, while mechanical reliability such as strength is required for the porous support layer of the SOEC, gas permeability is also required. Therefore, due to the presence of pores that are the starting points of fracture, the strength is likely to be low. In this way, both mechanical reliability and gas permeability are required for the support layer. However, in order to ensure the electrolysis cell performance of gas permeability, there is a problem that the mechanical reliability based on the porous layer has to be kept low.

Prior Art Documents

Patent Documents

[0006]

Patent Document 1

Patent Document 2

Patent Document 3

Summary of the Invention

Problems to be Solved by the Invention

[0007] The problem to be solved by the present invention is to provide a high-temperature steam electrolysis cell and a method for manufacturing the same, which enable both the electrolysis reaction characteristics of steam and the mechanical strength by providing a support layer with enhanced strength while maintaining the gas permeability required for the electrolysis cell.

Means for Solving the Problem

[0008] In the high-temperature steam electrolysis cell of the embodiment, a support layer having gas permeability, a hydrogen electrode provided on the support layer, having gas permeability, and capable of electrolyzing steam flowing into the interior into oxygen ions and hydrogen, a solid oxide electrolyte layer capable of conducting the oxygen ions generated at the hydrogen electrode, and a gas permeability, and an oxygen electrode capable of generating oxygen molecules from the oxygen ions reaching from the solid oxide electrolyte layer are provided. In the high-temperature steam electrolysis cell, the support layer includes a porous sintered layer in a composite of nickel oxide and gadolinium-doped ceria, in which a part of the gadolinium-doped ceria in the composite is replaced with ceria-stabilized zirconia.

Brief Description of the Drawings

[0009]

Figure 1

Figure 2

Figure 3

Modes for Carrying Out the Invention

[0010] Hereinafter, the high-temperature steam electrolysis cell of the embodiment will be described with reference to the drawings. In each of the embodiments shown below, substantially the same constituent parts are denoted by the same reference numerals, and the description thereof may be partially omitted. The drawings are schematic, and the relationship between the thickness and the planar dimensions, the ratio of the thicknesses of the respective parts, etc. may be different from the actual ones.

[0011] FIG. 1 shows a cross-section of a high-temperature steam electrolysis cell according to an embodiment. The high-temperature steam electrolysis cell 1 shown in FIG. 1 is a SOEC that generates hydrogen and oxygen by electrolyzing high-temperature steam, and includes a support layer 2, a hydrogen electrode 3, a solid oxide electrolyte layer 4, an intermediate layer 5, and an oxygen electrode 6. The support layer 2 is a strength member that mainly bears the strength of the electrolysis cell 1, and is made of a porous sintered body having gas permeability and has a steam passage through which steam can flow. The support layer 2 is made of a porous sintered body obtained by sintering a material in which a part of GDC of a composite material of nickel oxide (NiO) and gadolinium-doped ceria (Gadolinia Doped Ceria (CeO2): GDC) is replaced with ceria-stabilized zirconia (Ceria (CeO2) Stabillized Zirconia (ZrO2): CSZ). The porous sintered body as the support layer 2 has a porosity of, for example, about 30% or more and 50% or less. Specific configurations including the component ratio of the porous sintered body as the support layer 2 will be described in detail later.

[0012] A hydrogen electrode 3 is provided on the support layer 2. The hydrogen electrode 3 is composed of a porous layer made of a hydrogen electrode active material, and specifically has gas permeability due to a skeleton of a network structure. The hydrogen electrode 3 has open pores internally at least partially surrounded by the skeleton of the network structure, and can electrolyze the steam flowing into the open pores into oxygen ions and hydrogen. The hydrogen electrode 3 is made of, for example, a composite of NiO and GDC. In addition, a composite of oxides such as Co, Fe, Cu, Ru, etc. and oxides of rare earth elements or zirconia stabilized with rare earth elements may be applied to the hydrogen electrode 3. The steam flowing from the steam passage of the support layer 2 into the open pores in the hydrogen electrode 3 is mainly electrolyzed into oxygen ions and hydrogen in the hydrogen electrode (active layer) 3. The hydrogen gas (H2) generated by electrolysis is led out to the outside through a gas flow path (not shown) and is stored, for example. The generated oxygen ions conduct in the solid oxide electrolyte layer 4.

[0013] The solid oxide electrolyte layer 4 has one surface laminated with the hydrogen electrode 3 and the other surface laminated with the oxygen electrode 6 via the intermediate layer 5. The solid oxide electrolyte layer 4 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. In the solid oxide electrolyte layer 4, for example, stabilized zirconia in which a stabilizer composed of an oxide of a rare earth element such as Y, Sc, Ce, Gd, Sm, etc. is solid-solved, typically Y2O3 stabilized zirconia (Yttria (Y2O3) Stabillized Zirconia (ZrO2): YSZ), CSZ, or a composite of these can be used.

[0014] The oxygen electrode 6 is made of an oxygen electrode active material and is composed of a porous body having gas diffusibility and electronic conductivity. In the porous body, oxygen ions (O 2- ) reaching from the solid oxide electrolyte layer 4 and electrons (e - ) supplied from an external power source can generate oxygen molecules (O2). The generated oxygen gas (O2) is led to the outside from a gas flow path (not shown) and is stored, for example, as necessary. The oxygen electrode 6 is composed 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 6, for example, a perovskite oxide represented by 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 Sr, Ca, Ba, etc., B and C are metal elements such as Cr, Mn, Co, Fe, Ni, etc., and x, y, and δ are atomic ratios satisfying 0 ≦ x ≦ 1, 0 ≦ y ≦ 1, and 0 ≦ δ ≦ 1) can be used. A typical example of the oxygen electrode 6 is (La 1-x Sr x )(Co 1-y Fe y )O 3-δ (LSCF).

[0015] The intermediate layer 5 is disposed between the solid oxide electrolyte layer 4 and the oxygen electrode 6 as needed, and is a dense reaction prevention layer that prevents the diffusion and reaction of elements between the solid oxide electrolyte layer 4 and the oxygen electrode 6. The SOEC1 of the embodiment is configured by laminating a support layer 2, a hydrogen electrode 3, a solid oxide electrolyte layer 4, an intermediate layer 5, and an oxygen electrode 6 in this order. More specifically, a thin film of the hydrogen electrode 3 is formed on the support layer 2, and a thin film of the solid oxide electrolyte layer 4 is further formed on the hydrogen electrode 3. Further, the SOEC1 is configured by forming thin films of the intermediate layer 5 and the oxygen electrode 6 on the solid oxide electrolyte layer 4. Regarding the thickness of each component, for example, the support layer 2 has a thickness of 500 μm or more and 800 μm or less, the hydrogen electrode 3 has a thickness of 30 μm or more and 50 μm or less, the solid oxide electrolyte layer 4 has a thickness of 10 μm or more and 15 μm or less, the intermediate layer 5 has a thickness of 5 μm or more and 10 μm or less, and the oxygen electrode 6 has a thickness of about 30 μm or more and 50 μm or less.

[0016] Next, the support layer 2 will be described in detail. The high-temperature steam electrolysis cell 1 is manufactured, for example, by adding a binder, a pore former, etc. and a solvent to raw material powder to form a slurry, sheet-forming, laminating, and pressing this slurry to form a molded body, and then subjecting the molded body to a debinding process and a sintering process which are debinding treatments. The pore former added for the purpose of making the support layer 2 porous is thermally decomposed and removed in the debinding process, and the remaining holes remain even after sintering to realize the porosity of the support layer 2. Generally, the porosity is around 40%, and this high porosity exhibits gas permeability.

[0017] However, when a load is applied to the porous sintered body as described above, stress corresponding to its shape is generated in the matrix around the pores. When this generated stress becomes higher than the fracture strength of the matrix, cracks occur, and the propagation of the cracks causes the destruction of the support layer 2 and, ultimately, the large-scale destruction of the entire electrolytic cell 1. Therefore, pore morphology such as the pore diameter and shape is important. Spherical particles, which are least likely to cause stress concentration, are often used as the pore-forming agent. For the brittle material ceramics (sintered body), high strength is required due to ease of handling and setting. Since a high Young's modulus causes high stress even with slight deformation and easily leads to fracture, a lower Young's modulus is preferable. Since cracks often propagate along grain boundaries, it is effective to improve the grain boundary strength or add additives to increase the strength.

[0018] Therefore, in the support layer 2 of the embodiment, not only a sintered body of a composite of NiO and GDC, which is effective as a constituent material of the support layer 2 in contact with the hydrogen electrode 3, is used, but also a part of the GDC in the composite is replaced with CSZ. Among the composite of NiO and GDC, since the ceria of GDC has excellent affinity with the ceria of CSZ, they show mutual reaction in the porous sintered body. Since CSZ has superior strength characteristics compared to GDC, by replacing a part of GDC in the composite of NiO and GDC with CSZ and containing CSZ, the strength of the porous sintered body can be improved. In particular, by containing CSZ in the composite of NiO and GDC, the grain boundary strength of the porous sintered body can be improved, so that the strength of the support layer 2 and, ultimately, the strength of the entire electrolytic cell 1 can be increased, improving the mechanical reliability. Furthermore, since the support layer 2 is mainly composed of a composite of NiO and GDC, the characteristics and functions of the support layer 2 are maintained, making it possible to maintain the characteristics of the electrolytic cell 1. That is, it becomes possible to provide a high-temperature steam electrolytic cell 1 with excellent cell characteristics and mechanical strength.

[0019] In the constituent material of the support layer 2 of the hydrogen electrode 3, the mass ratio of NiO to GDC is preferably in the range of 5:5 to 7:3. By satisfying such a mass ratio of NiO to GDC, the functions and characteristics of the support layer 2 can be satisfactorily satisfied. In such a composite, it is preferable to replace 1% by mass or more and 40% by mass or less of GDC with CSZ. If the replacement amount of GDC by CSZ is less than 1% by mass, there is a possibility that the effect of improving the strength of the porous sintered body by the composite of NiO and GDC cannot be sufficiently obtained. If the replacement amount of GDC by CSZ exceeds 40% by mass, there is a possibility of degrading the characteristics (such as cell characteristics) of the support layer 2 by the composite of NiO and GDC.

[0020] In the porous sintered body composed of the composite of NiO, GDC, and CSZ constituting the support layer 2, the particle diameter of the aggregate containing at least one of GDC and CSZ is preferably 20 μm or less. Aggregates with a particle diameter exceeding 20 μm become the starting points of crack generation, and the generated cracks are more likely to spread, which is a factor in reducing the strength. Therefore, it is preferable that the particle diameter of the aggregate is 20 μm or less. As described above, the porosity of the porous sintered body constituting the support layer 2 is preferably about 30% or more and 50% or less. If the porosity of the porous sintered body is less than 30%, the gas permeability decreases and the characteristics of the electrolytic cell 1 are likely to deteriorate. If the porosity of the porous sintered body exceeds 50%, although the gas permeability improves, the mechanical characteristics such as the strength of the support layer 2 are likely to deteriorate.

[0021] The manufacturing method of the high-temperature steam electrolysis cell 1 of the embodiment is not particularly limited, but can be created, for example, as follows. First, powders of nickel oxide (NiO), gadolinium-doped ceria (GDC), and ceria-stabilized zirconia (CSZ) are mixed at the above-described ratios to prepare raw material powders. At this time, the CSZ powder preferably has an average particle size of 0.1 μm or more and 1 μm or less. When the average particle size of the CSZ powder exceeds 1 μm, large aggregated particles are likely to occur in the porous sintered body to be produced. When the average particle size of the CSZ powder is less than 0.1 μm, the crystal grains do not grow sufficiently, and the strength of the porous sintered body is likely to decrease. The average particle sizes of the NiO powder and the GDC powder are not necessarily limited, but preferably have an average particle size of 0.1 μm or more and 1 μm or less, similar to the CSZ powder.

[0022] Next, a binder and a pore former are added to the above-described raw material powders, and a solvent is further added and mixed as necessary to prepare a raw material slurry. Such a raw material slurry is formed into a sheet to produce a sheet. Next, a hydrogen electrode 3 forming slurry and a solid oxide electrolyte layer 4 forming slurry are sequentially formed into sheets on the obtained sheet to produce a laminated formed body. By subjecting such a laminated formed body to each process of thermocompression bonding, degreasing, and sintering, a porous laminated sintered body is produced. By performing each process of forming and baking the intermediate layer 5 and the oxygen electrode 6 forming material on the laminated sintered body, the high-temperature steam electrolysis cell 1 is obtained. Note that each of these constituent layers may be degreased and sintered individually.

Example

[0023] Next, a specific example of the electrolysis cell of the embodiment and its evaluation results will be described.

[0024] (Example 1) In manufacturing a high-temperature water vapor electrolysis cell, first, nickel oxide (NiO) with an average particle size of 0.5 μm and gadolinium-doped ceria (GDC) with an average particle size of 0.2 μm were prepared as raw materials for the support layer. When these were blended so that the mass ratio was 6:4, the raw material powder was adjusted by replacing a part of GDC with ceria-stabilized zirconia (CSZ) with an average particle size of 0.2 μm. The replacement amount of GDC with CSZ was adjusted in the range of 10% to 50% by mass. The replacement amount with CSZ is shown in Table 1 described later.

[0025] To 100 parts by mass of a plurality of raw material powders with the above-described adjusted raw material composition ratios, 10 parts by weight of polyvinyl acetal resin as a binder and 5 parts by weight of graphite with an average particle size of 10 μm as a pore former were added respectively, and further ethanol was added as a solvent and they were pot-mixed for 24 hours to prepare slurries respectively. Each slurry thus prepared was sheet-molded to a thickness of 1 mm to prepare a sheet for the support layer.

[0026] Next, an NiO-GDC-based slurry was sheet-molded on each of the above-described sheets as a forming material for the hydrogen electrode active layer. For the electrolyte layer, using a polyethylene terephthalate (PET) film, a yttria-stabilized zirconia (YSZ) slurry was screen-printed in a sheet form with a thickness of 20 μm. Further, the prepared sheet for the support layer and the hydrogen electrode active layer and the sheet for the electrolyte layer were thermocompression-bonded at a temperature of 70 °C to form a laminated molded body, and then debinding was performed at 400 °C for 2 hours. Further, the debound body was sintered at a temperature at which sufficient sintering proceeds at 1300 °C or higher to obtain a sintered body. The atmosphere was an air atmosphere. After screen-printing LSCF as an oxygen electrode on the electrolyte side of such a sintered body, baking was performed.

[0027] In this way, a 50×50 mm sintered body for an electrolytic cell was obtained. Using such a sintered body, a three-point bending test was conducted at a load application rate of 0.5 mm / min, and the fracture strength was measured for each. Table 1 shows the replacement amount of GDC by CSZ and the fracture strength. Also, for comparison, a material in which GDC was not replaced by CSZ (CSZ: 0 mass%) was used to prepare a slurry for the support layer in the same manner to produce a sintered body for an electrolytic cell. As shown in Table 1, it can be seen that the sintered body for an electrolytic cell having a support layer formed using a raw material powder in which a part of GDC in the NiO-GDC-based material was replaced with CSZ has a higher strength than the case where a material without replacement of GDC by CSZ was used. However, when the replacement amount of GDC by CSZ was 50 mass%, the strength was slightly decreased, so it can be seen that the replacement amount of GDC by CSZ is preferably 40 mass% or less.

[0028]

Table 1

[0029] (Example 2) In producing a molded body in which the sheet for the support layer and the hydrogen electrode active layer and the sheet for the electrolyte layer in Example 1 were laminated, by changing the holding time during sintering of the molded body (lengthening or shortening), a sintered body having a changed particle size of at least one of GDC and CSZ was produced. Otherwise, by applying the same steps as in Example 1, a sintered body for an electrolytic cell was produced. The fracture strength of these sintered bodies for electrolytic cells was measured in the same manner as in Example 1. Table 2 shows the particle size of the aggregate containing at least one of GDC and CSZ and the fracture strength.

[0030] As shown in Table 2, in the sintered body for an electrolytic cell using a composite in which a part of GDC in the NiO-GDC-based material was replaced with CSZ, when the particle size of the aggregate containing at least one of GDC and CSZ is 25 μm, the strength is slightly decreased, so it can be seen that the particle size of the aggregate is preferably 20 μm or less. Figure 2 shows a cross-sectional SEM image of a sintered body with a particle size of 2 μm, and Figure 3 shows a cross-sectional SEM image of a sintered body with a particle size of 25 μm.

[0031] [Table 2]

[0032] (Example 3) In producing the sheet for the support layer in Example 1, except for changing the average particle diameter of the raw material powder of CSZ, in the same manner as in Example 1, the production of the sheets for the support layer and the hydrogen electrode active layer, the production of a molded body obtained by laminating such a sheet and the sheet for the electrolyte layer, the debinding and sintering of the molded body were carried out, and further printing and baking of LSCF as the oxygen electrode were performed to produce a sintered body for an electrolysis cell, respectively. The fracture strength of these sintered bodies for electrolysis cells was measured in the same manner as in Example 1. The average particle diameter of the raw material powder of CSZ and the fracture strength are shown in Table 3. As shown in Table 3, when the average particle diameter of the raw material powder of CSZ is less than 0.1 μm or exceeds 1 μm, the strength is slightly decreased. Therefore, it can be understood that when producing the porous sintered body of the NiO-GDC-CSZ composite, the average particle diameter of CSZ is preferably 0.1 μm or more and 1 μm or less.

[0033] [Table 3]

[0034] In addition, the configurations of the above-described respective embodiments can be applied in combination with each other, and partial replacement is also possible. Here, several embodiments of the present invention have been described, but these embodiments are presented as examples and are not intended to limit the scope of the invention. These embodiments can be implemented in various other forms, and various omissions, replacements, changes, etc. can be made without departing from the gist of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and at the same time, are included in the invention described in the claims and the equivalent scope thereof. [Explanation of Reference Numerals]

[0035] 1…High-temperature steam electrolysis cell, 2…Support layer, 3…Hydrogen electrode, 4…Solid oxide electrolyte layer, 5…Intermediate layer, 6…Oxygen electrode.

Claims

1. In a high-temperature steam electrolysis cell comprising a support layer having gas permeability, a hydrogen electrode provided on the support layer, having gas permeability, and capable of electrolyzing water vapor flowing into the interior into oxygen ions and hydrogen, a solid oxide electrolyte layer capable of conducting the oxygen ions generated at the hydrogen electrode, and an oxygen electrode having gas permeability and capable of generating oxygen molecules from the oxygen ions reaching from the solid oxide electrolyte layer, the support layer is a porous sintered layer in a composite of nickel oxide and gadolinium-doped ceria, wherein a part of the gadolinium-doped ceria in the composite is replaced by ceria-stabilized zirconia. The high-temperature steam electrolysis cell.

2. The composite contains nickel oxide and gadolinium-doped ceria in a mass ratio of 5:5 to 7:3, and 1% by mass or more and 40% by mass or less of the gadolinium-doped ceria is replaced by ceria-stabilized zirconia. The high-temperature steam electrolysis cell according to Claim 1.

3. The high-temperature steam electrolysis cell according to Claim 1 or Claim 2, wherein the particle size of an aggregate containing at least one of the gadolinium-doped ceria and the ceria-stabilized zirconia present in the composite is 20 μm or less.

4. The high-temperature steam electrolysis cell according to any one of Claims 1 to 3, wherein the porous sintered layer has a porosity of 30% or more and 50% or less.

5. A method for manufacturing the high-temperature steam electrolysis cell according to Claim 1, comprising: preparing a raw material powder containing nickel oxide powder, gadolinium-doped ceria powder, and ceria-stabilized zirconia powder; adding a binder, a pore former, and a solvent to the raw material powder to prepare a raw material slurry; forming the raw material slurry into a sheet shape to obtain a formed body; debinding the formed body to obtain a debound body; and sintering the debound body to obtain a porous sintered layer as the support layer. A method for manufacturing a high-temperature steam electrolysis cell.

6. The raw material powder contains nickel oxide and gadolinium-doped ceria in a mass ratio of 5:5 to 7:3, and 1% by mass or more and 40% by mass or less of the gadolinium-doped ceria is replaced by ceria-stabilized zirconia. The method for manufacturing a high-temperature steam electrolysis cell according to Claim 5.

7. The method for manufacturing a high-temperature steam electrolysis cell according to Claim 5 or Claim 6, wherein the ceria-stabilized zirconia powder has an average particle size of 0.1 μm or more and 1 μm or less.

8. The step of forming the molded body is a step of producing a laminated molded body by sequentially forming the forming slurry of the hydrogen electrode and the forming slurry of the solid oxide electrolyte layer in a sheet shape on the sheet layer of the raw material slurry of the molded body, and the debinding step and the sintering step are performed on the laminated molded body. The method for manufacturing a high-temperature steam electrolysis cell according to any one of claims 5 to 7.

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