JP2023067440A - High temperature steam electrolytic cell and production method thereof - Google Patents

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

Embodiments of the present invention relate to high temperature steam electrolysis cells and methods of making the same.

In recent years, the introduction of renewable energy such as solar, wind, and geothermal energy has been promoted from the viewpoint of environmental issues such as the depletion of fossil fuels, global warming due to the release of carbon dioxide into the atmosphere, and energy security. It is Hydrogen energy is attracting attention as a secondary energy from the viewpoint of storage and transportation. Hydrogen energy is expected to be applied to, for example, fuel cell vehicles, and low-cost, high-quality hydrogen production and storage are required.

At present, hydrogen is mainly produced by reforming fossil fuels from the viewpoint of cost and technology. However, hydrogen production by reforming fossil fuels inevitably generates carbon dioxide during the production process. On the other hand, it is known that the method of producing hydrogen using renewable energy from water as a raw material does not generate carbon dioxide and has less environmental impact. As a method of electrolyzing water or steam to generate hydrogen, there are a PEM type using a solid polymer electrolyte membrane (PEM) and a SOEC using a solid oxide electrolysis cell (Solid Oxide Electrolysis Cell: SOEC). type is known. Among them, the SOEC type is expected as a future hydrogen production method because it theoretically requires less electric power for producing hydrogen.

The SOEC used for the electrolysis of water and steam for hydrogen production consists of a hydrogen electrode support layer 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 is important because it has oxygen ion conductivity and a role of separating hydrogen gas and oxygen gas. In addition to the function of passing electrons, the support layer is required to have gas permeability for supplying water vapor to the active layer and strength for maintaining the form of the electrolytic cell. For this reason, a porous layer is used for the support layer in order to achieve gas permeability. Therefore, the support layer is required to have a contradictory characteristic of realizing high strength in a porous layer having pores which are regarded as defects.

In other words, the porous support layer of the SOEC is required to have mechanical reliability such as strength, but it is also required to have gas permeability. Cheap. In this way, the support layer is required to have both mechanical reliability and gas permeability. There are some issues that must be addressed.

JP 2018-154864 A JP 2016-071983 A Patent No. 5498191

The problem to be solved by the present invention is that by providing a support layer with increased strength while maintaining the gas permeability required for an electrolytic cell, it is possible to achieve both the electrolytic reaction characteristics of water vapor and the mechanical strength. To provide a high-temperature steam electrolysis cell and a method for manufacturing the same.

A high-temperature steam electrolysis cell of an embodiment comprises a gas-permeable support layer, a gas-permeable support layer provided on the support layer, and hydrogen capable of electrolyzing water vapor flowing into the interior into oxygen ions and hydrogen. an electrode, a solid oxide electrolyte layer capable of conducting the oxygen ions generated by the hydrogen electrode, and a gas permeable solid oxide electrolyte layer capable of generating oxygen molecules from the oxygen ions arriving from the solid oxide electrolyte layer. the support layer is a composite of nickel oxide and gadolinium solid solution ceria, wherein a part of the gadolinium solid solution ceria in the composite is replaced with ceria-stabilized zirconia and a porous sintered layer.

BRIEF DESCRIPTION OF THE DRAWINGS It is sectional drawing which shows the high temperature steam electrolysis cell of embodiment. 4 is a cross-sectional SEM image of a support layer of a sintered body having a grain size of 2 μm in Example 2. FIG. 4 is a cross-sectional SEM image of a support layer of a sintered body having a grain size of 25 μm in Example 2. FIG.

A high-temperature steam electrolysis cell of an embodiment will be described below with reference to the drawings. In each of the embodiments shown below, substantially the same components are denoted by the same reference numerals, and the description thereof may be partially omitted. The drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each part, and the like may differ from the actual ones.

FIG. 1 shows a cross section of a high-temperature steam electrolysis cell according to an embodiment. A high-temperature steam electrolysis cell 1 shown in FIG. 1 is an SOEC that generates hydrogen and oxygen by electrolyzing high-temperature steam. and an oxygen electrode 6 . The support layer 2 is a strength member that mainly bears the strength of the electrolytic cell 1, is made of a porous sintered body having gas permeability, and has water vapor passages through which water vapor can flow. The support layer 2 _is composed of Ceria (CeO ₂ ) stabilized zirconia (ZrO ₂ ): Composed of a porous sintered body obtained by sintering a material substituted with CSZ). The porous sintered body as the support layer 2 has a porosity of, for example, about 30% or more and 50% or less. A specific configuration including the component ratio of the porous sintered body as the support layer 2 will be described in detail later.

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 more specifically, has gas permeability due to a skeleton of a network structure. The hydrogen electrode 3 has open pores at least partially surrounded by a skeleton of a network structure, and can electrolyze water vapor flowing into the open pores into oxygen ions and hydrogen. be. The hydrogen electrode 3 is made of, for example, a composite of NiO and GDC. Alternatively, a composite of an oxide such as Co, Fe, Cu, Ru, etc., and a rare earth element oxide or zirconia stabilized with a rare earth element may be applied to the hydrogen electrode 3 . The water vapor that has flowed into the open pores in the hydrogen electrode 3 from the water vapor passage of the support layer 2 is electrolyzed into oxygen ions and hydrogen mainly in the hydrogen electrode (active layer) 3 . Hydrogen gas (H ₂ ) generated by electrolysis is led out through a gas flow path (not shown) and stored, for example. The generated oxygen ions conduct into the solid oxide electrolyte layer 4 .

The solid oxide electrolyte layer 4 is laminated with the hydrogen electrode 3 on one side and with the oxygen electrode 6 on the other side with the intermediate layer 5 interposed therebetween. The solid oxide electrolyte layer 4 is made of a dense solid oxide electrolyte, and is an ionic conductor that allows ions such as oxygen ions to pass but does not conduct electricity. The solid oxide electrolyte layer 4 is made of stabilized zirconia, typically Y ₂ O ₃ stabilized, in which a stabilizer made of oxides of rare earth elements such as Y, Sc, Ce, Gd, and Sm is solid-dissolved. Zirconia (Yttria (Y ₂ O ₃ ) Stabilized Zirconia (ZrO ₂ ): YSZ), CSZ, or a composite thereof can be used.

The oxygen electrode 6 is made of ^an oxygen active material and is composed of a porous body having gas diffusibility and electronic conductivity. And electrons (e ⁻ ) supplied from an external power source can generate oxygen molecules (O ₂ ). The generated oxygen gas (O ₂ ) is led outside through a gas flow path (not shown) and stored, for example, as required. The oxygen electrode 6 is made of a porous sintered body containing an oxide having a perovskite structure represented by ABO ₃ (hereinafter referred to as perovskite oxide). For the oxygen electrode 6, for example, R _1-x A _x B _1-y C _y O _3-δ (here, R is a rare earth element such as La, A is an alkaline earth element such as Sr, Ca, Ba, B and C are metal elements such as Cr, Mn, Co, Fe and Ni, and x, y and δ are atomic ratios satisfying 0≦x≦1, 0≦y≦1 and 0≦δ≦1. A perovskite oxide represented by ) can be used. A representative example of the oxygen electrode 6 is (La _1-x Sr _x )(Co _1-y Fe _y )O _3-δ (LSCF).

The intermediate layer 5 is disposed between the solid oxide electrolyte layer 4 and the oxygen electrode 6 as necessary, and is a dense material that prevents diffusion and reaction of elements between the solid oxide electrolyte layer 4 and the oxygen electrode 6. It is a reaction-preventing layer. The SOEC 1 of the embodiment is constructed by laminating a supporting 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 SOEC 1 is constructed by forming a thin film 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 is 500 μm or more and 800 μm or less, the hydrogen electrode 3 is 30 μm or more and 50 μm or less, the solid oxide electrolyte layer 4 is 10 μm or more and 15 μm or less, the intermediate layer 5 is 5 μm or more and 10 μm or less, oxygen The pole 6 has a thickness of the order of 30 μm to 50 μm.

Next, the support layer 2 will be described in detail. For the high-temperature steam electrolysis cell 1, for example, a raw material powder is added with a binder, a pore-forming agent, and a solvent to form a slurry, and the slurry is sheet-formed, laminated, and pressed to form a molded body, and the molded body is subjected to degreasing, which is a binder removal treatment. It is manufactured by applying a process and a sintering process. The pore-forming agent added to make the support layer 2 porous is thermally decomposed and removed in the degreasing process, and the remaining holes remain even after sintering to realize the support layer 2 to be porous. there is Generally, the porosity is around 40%, and gas permeability is exhibited at this high porosity.

However, when a load is applied to the porous sintered body as described above, a stress corresponding to the shape of the pores is generated in the surrounding matrix. When this generated stress exceeds the breaking strength of the matrix, cracking occurs and the cracking extends, causing destruction of the support layer 2 and, in turn, large-scale destruction of the entire electrolytic cell 1 . Therefore, pore morphology such as pore diameter and shape is important. Spherical particles are often used as pore-forming agents to minimize stress concentration. Ceramics (sintered bodies), which are brittle materials, are required to have high strength in order to facilitate handling and setting. If the Young's modulus is high, even a slight deformation will generate a high stress and break easily, so 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.

Therefore, for the support layer 2 in the embodiment, in addition to using 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, a part of the GDC of the composite is replaced with CSZ. Among the composites of NiO and GDC, since ceria of GDC has excellent affinity with ceria of CSZ, mutual reaction occurs in the porous sintered body. Since CSZ is superior in strength characteristics to GDC, the strength of the porous sintered body can be improved by including CSZ in the composite of NiO and GDC by substituting part of GDC. . In particular, by including CSZ in the composite of NiO and GDC, the grain boundary strength of the porous sintered body can be improved. Mechanical reliability can be improved. Furthermore, since the support layer 2 is mainly composed of a composite of NiO and GDC, the properties and functions of the support layer 2 are maintained, so the properties of the electrolytic cell 1 can be maintained. That is, it becomes possible to provide a high-temperature steam electrolysis cell 1 that is excellent in cell characteristics and mechanical strength.

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 and GDC, the functions and characteristics of the support layer 2 can be satisfactorily satisfied. In such a complex, it is preferable to replace 1% by mass or more and 40% by mass or less of GDC with CSZ. If the amount of GDC replaced by CSZ is less than 1% by mass, the composite of NiO and GDC may not sufficiently improve the strength of the porous sintered body. If the amount of GDC replaced by CSZ exceeds 40% by mass, the properties (cell properties, etc.) of the support layer 2 made of the composite of NiO and GDC may be degraded.

In the porous sintered body composed of the composite of NiO, GDC, and CSZ that constitutes the support layer 2, the particle size of aggregates containing at least one of GDC and CSZ is preferably 20 μm or less. Agglomerates with a particle size exceeding 20 μm serve as starting points for cracks, and the cracks that have occurred tend to extend, resulting in a reduction in strength. Therefore, it is preferable that the particle size of the aggregates is 20 μm or less. The porosity of the porous sintered body forming the support layer 2 is preferably about 30% or more and 50% or less as described above. If the porosity of the porous sintered body is less than 30%, the gas permeability is lowered and the characteristics of the electrolytic cell 1 are likely to be lowered. If the porosity of the porous sintered body exceeds 50%, the gas permeability is improved, but the mechanical properties such as the strength of the support layer 2 tend to deteriorate.

The method for manufacturing the high-temperature steam electrolysis cell 1 of the embodiment is not particularly limited, but it can be manufactured, for example, as follows. First, powders of nickel oxide (NiO), gadolinium solid-solution ceria (GDC), and ceria-stabilized zirconia (CSZ) are mixed in the above ratio to prepare a raw material powder. At this time, the CSZ powder preferably has an average particle size of 0.1 μm or more and 1 μm or less. If the CSZ powder has an average particle size of more than 1 μm, agglomerated particles having a large particle size tend to occur in the produced porous sintered body. When the CSZ powder has an average particle size of less than 0.1 μm, the crystal grains do not grow sufficiently, and the strength of the porous sintered body tends to decrease. Although the average particle size of the NiO powder and the GDC powder is not necessarily limited, it is preferable to have an average particle size of 0.1 μm or more and 1 μm or less, like the CSZ powder.

Next, a binder and a pore-forming agent are added to the raw material powder described above, and if necessary, a solvent is added and mixed to prepare a raw material slurry. A sheet is produced by forming such a raw material slurry into a sheet. Then, the slurry for forming the hydrogen electrode 3 and the slurry for forming the solid oxide electrolyte layer 4 are formed in order into a sheet on the obtained sheet, thereby producing a laminate molded body. A porous laminated sintered body is produced by subjecting such a laminated molded body to the steps of thermocompression bonding, degreasing, and sintering. The high-temperature steam electrolysis cell 1 is obtained by carrying out each step of molding and baking the materials for forming the intermediate layer 5 and the oxygen electrode 6 on the laminated sintered body. These constituent layers may be individually degreased and sintered.

Next, specific examples of the electrolytic cell of the embodiment and evaluation results thereof will be described.

(Example 1)
In manufacturing the high-temperature steam electrolysis cell, nickel oxide (NiO) with an average particle size of 0.5 μm and gadolinium-dissolved ceria (GDC) with an average particle size of 0.2 μm were prepared as raw materials for the support layer. A raw material powder was prepared by replacing part of the GDC with ceria-stabilized zirconia (CSZ) having an average particle size of 0.2 μm when blending these in a mass ratio of 6:4. The amount of GDC substituted with CSZ was adjusted in the range of 10% by mass to 50% by mass. The amount of substitution with CSZ is shown in Table 1 below.

10 parts by weight of polyvinyl acetal resin as a binder and 5 parts by weight of graphite having an average particle size of 10 μm as a pore-forming agent are added to 100 parts by weight of the plurality of raw material powders having the raw material composition ratio adjusted as described above, and a solvent is added. Ethanol was added as a solution and pot-mixed for 24 hours to prepare slurries, respectively. Each slurry thus prepared was formed into a sheet having a thickness of 1 mm to prepare a support layer sheet.

Next, a NiO-GDC-based slurry as a material for forming a hydrogen electrode active layer was sheet-formed on each of the above-described sheets. For the electrolyte layer, a polyethylene terephthalate (PET) film was used, and yttria-stabilized zirconia (YSZ) slurry was screen-printed into a sheet with a thickness of 20 μm. Further, the sheets for the support layer and the hydrogen electrode active layer and the sheets for the electrolyte layer were thermocompressed at a temperature of 70° C. to form a laminate molded body, and then degreased at 400° C. for 2 hours. . Further, the degreased body was sintered at a temperature of 1300° C. or higher, at which sintering proceeds sufficiently, 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.

Thus, a 50×50 mm sintered body for an electrolytic cell was obtained. Using such a sintered body, a three-point bending test was performed at a load rate of 0.5 mm/min to measure the breaking strength. Table 1 shows the amount of GDC substituted by CSZ and the breaking strength. For comparison, a support layer slurry was similarly prepared for a material in which GDC was not replaced with CSZ (CSZ: 0% by mass) to produce a sintered body for an electrolytic cell. As shown in Table 1, the sintered body for an electrolytic cell having a support layer formed using raw material powder in which part of GDC of the NiO-GDC-based material was replaced with CSZ did not replace GDC with CSZ. It can be seen that the strength is greater than when the material is used. However, when the amount of GDC substituted with CSZ is 50% by mass, the strength is slightly reduced, so it can be seen that the amount of GDC substituted with CSZ is preferably 40% by mass or less.

(Example 2)
In producing a compact obtained by laminating the sheet for the support layer and the hydrogen electrode active layer and the sheet for the electrolyte layer in Example 1, the holding time during sintering of the compact was changed (longer or shorter). By aging), sintered bodies were produced in which the particle size of the particles containing at least one of GDC and CSZ was changed. A sintered body for an electrolytic cell was produced by applying the same steps as in Example 1 except for the above. The breaking strength of these sintered bodies for electrolytic cells was measured in the same manner as in Example 1. Table 2 shows the particle size and breaking strength of aggregates containing at least one of GDC and CSZ.

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

(Example 3)
In producing the sheet for the support layer in Example 1, the sheets for the support layer and the hydrogen electrode active layer were produced in the same manner as in Example 1, except that the average particle size of the raw material powder of CSZ was changed. A compact obtained by laminating such a sheet and a sheet for the electrolyte layer was produced, the compact was degreased and sintered, and LSCF as an oxygen electrode was printed and baked to produce an electrolytic cell. A body was produced. The breaking strength of these sintered bodies for electrolytic cells was measured in the same manner as in Example 1. Table 3 shows the average particle size and breaking strength of the raw material powder of CSZ. As shown in Table 3, when the average particle size of the raw CSZ powder is less than 0.1 μm or more than 1 μm, the strength is slightly lowered. Therefore, it can be seen that the average grain size of CSZ is preferably 0.1 μm or more and 1 μm or less when producing a porous sintered body of a NiO-GDC-CSZ composite.

It should be noted that the configurations of the respective embodiments described above can be combined and applied, and can also be partially replaced. Although several embodiments of the invention have been described herein, these embodiments are provided by way of example 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 scope of the invention. These embodiments and their modifications are included in the scope and gist of the invention, and are also included in the scope of the invention described in the claims and equivalents thereof.

DESCRIPTION OF SYMBOLS 1... High temperature steam electrolysis cell, 2... Support layer, 3... Hydrogen electrode, 4... Solid oxide electrolyte layer, 5... Intermediate layer, 6... Oxygen electrode.

Claims (8)

a support layer having gas permeability; a hydrogen electrode provided on the support layer and having gas permeability and capable of electrolyzing water vapor that has flowed into the interior into oxygen ions and hydrogen; a solid oxide electrolyte layer capable of conducting oxygen ions; and an oxygen electrode having gas permeability and capable of generating oxygen molecules from the oxygen ions arriving from the solid oxide electrolyte layer. in the electrolytic cell,
The support layer comprises a porous sintered layer in a composite of nickel oxide and gadolinium solid solution ceria in which a part of the gadolinium solid solution ceria in the composite is replaced with ceria-stabilized zirconia. electrolytic cell. The composite contains the nickel oxide and the gadolinium solid-solution 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 solid-solution ceria is ceria-stabilized. A high temperature steam electrolysis cell according to claim 1 substituted with zirconia. 3. The high-temperature steam electrolysis cell according to claim 1, wherein aggregates containing at least one of said gadolinium solid-solution ceria and said ceria-stabilized zirconia present in said composite have a particle size of 20 μm or less. The high-temperature steam electrolysis cell according to any one of claims 1 to 3, wherein said porous sintered layer has a porosity of 30% or more and 50% or less. A method for manufacturing a high-temperature steam electrolysis cell according to claim 1,
preparing a raw material powder containing the nickel oxide powder, the gadolinium solid-solution ceria powder, and the ceria-stabilized zirconia powder;
adding a binder, a pore-forming agent, and a solvent to the raw material powder to prepare a raw material slurry;
a step of forming the raw material slurry into a sheet to obtain a formed body;
a step of degreasing the molded body to obtain a degreased body;
and a step of sintering the degreased body to obtain a porous sintered layer as the support layer. The raw material powder contains the nickel oxide and the gadolinium solid-solution 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 solid-solution ceria is ceria-stabilized. 6. The method for manufacturing a high-temperature steam electrolysis cell according to claim 5, wherein the zirconia is substituted. 7. The method for manufacturing a high-temperature steam electrolysis cell according to claim 5, wherein said ceria-stabilized zirconia powder has an average particle size of 0.1 [mu]m or more and 1 [mu]m or less. In the step of forming the molded body, the slurry for forming the hydrogen electrode and the slurry for forming the solid oxide electrolyte layer are sequentially formed into sheets on the sheet layer of the raw material slurry of the molded body to produce a laminated molded body. The method for manufacturing a high-temperature steam electrolysis cell according to any one of claims 5 to 7, wherein said degreasing step and said sintering step are carried out on said laminated compact.

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