JP7700025B2 - High temperature steam electrolysis cell and method of manufacturing same - Google Patents

Description

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

In recent years, the introduction of renewable energy sources such as solar, wind and geothermal power has been promoted from the perspective 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. In addition, hydrogen energy has attracted 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 demand for low-cost, high-quality hydrogen production and storage.

Currently, the mainstream method of producing hydrogen is to reform fossil fuels from the perspective of cost and technology. However, producing hydrogen by reforming fossil fuels inevitably generates carbon dioxide during the production process. In contrast, it is known that a method of producing hydrogen using renewable energy and water as a raw material does not generate carbon dioxide and has a small environmental impact. Known methods of generating hydrogen by electrolyzing water or water vapor include the PEM type, which uses a solid polymer electrolyte membrane (PEM), and the SOEC type, which uses a solid oxide electrolysis cell (SOEC). Among these, the SOEC type, in principle, requires less electricity to produce hydrogen, and is therefore expected to be a hydrogen production method of the future.

SOECs used in the electrolysis of water and water vapor for hydrogen production are composed of a hydrogen electrode support layer that passes electrons, a hydrogen electrode (active layer) that electrolyzes water, a solid oxide electrolyte layer that conducts oxygen ions, and an oxygen electrode that bonds the 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 passing electrons, the support layer is required to have gas permeability to supply water vapor to the active layer and strength to maintain the shape of the electrolysis cell. For this reason, a porous layer is used in the support layer to achieve gas permeability. Therefore, the support layer is required to have the contradictory characteristic of being a porous layer with pores that are considered defects and achieving high strength.

In other words, the porous support layer of an SOEC is required to have mechanical reliability such as strength, but also gas permeability, and therefore has pores that can be the starting point of breakdown, making it prone to low strength. In this way, the support layer is required to have both mechanical reliability and gas permeability, but there is a problem in that in order to ensure the electrolysis cell performance of gas permeability, the mechanical reliability based on the porous layer must be kept low.

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

The problem that the present invention aims to solve is to provide a high-temperature steam electrolysis cell and a method for manufacturing the same that achieve both steam electrolysis reaction characteristics and mechanical strength by providing a support layer with increased strength while maintaining the gas permeability required of an electrolysis cell.

The high-temperature steam electrolysis cell of the embodiment includes a gas-permeable support layer, a hydrogen electrode provided on the support layer, the hydrogen electrode being gas-permeable and capable of electrolyzing steam flowing into the cell 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 being gas-permeable and capable of generating oxygen molecules from the oxygen ions arriving from the solid oxide electrolyte layer. The support layer includes a porous sintered layer in which a part of the gadolinium-doped ceria in the composite of nickel oxide and gadolinium-doped ceria is replaced with ceria-stabilized zirconia.

FIG. 1 is a cross-sectional view showing a high temperature steam electrolysis cell according to an embodiment. 1 is a cross-sectional SEM image of a support layer of a sintered body having a grain size of 2 μm in Example 2. 1 is a cross-sectional SEM image of a support layer of a sintered body having a grain size of 25 μm in Example 2.

The high-temperature steam electrolysis cell of the embodiment will be described below with reference to the drawings. In each embodiment shown below, substantially identical components are given the same reference numerals, and some of their descriptions may be omitted. The drawings are schematic, and the relationship between thickness and planar dimensions, the thickness ratio of each part, etc. may differ from the actual ones.

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 an 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 provides the strength of the electrolysis cell 1, and is made of a gas-permeable porous sintered body and has a water vapor passage through which water vapor 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 (GDC) is replaced with ceria stabilized zirconia (Ceria (CeO ₂ ) Stabilized Zirconia (ZrO ₂ ): CSZ). The porous sintered body serving as the support layer 2 has a porosity of, for example, about 30% to 50%. The specific configuration of the porous sintered body serving as the support layer 2, including the component ratio, will be described in detail later.

The 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 network structure skeleton. The hydrogen electrode 3 has open pores at least partially surrounded by the network structure skeleton inside, and can electrolyze water vapor that flows into the open pores into oxygen ions and hydrogen. The hydrogen electrode 3 is made of, for example, a complex of NiO and GDC. In addition, a complex of an oxide of Co, Fe, Cu, Ru, etc., and an oxide of a rare earth element or zirconia stabilized with a rare earth element may be applied to the hydrogen electrode 3. The water vapor that flows into the open pores in the hydrogen electrode 3 from the water vapor passage of the support layer 2 is mainly electrolyzed into oxygen ions and hydrogen in the hydrogen electrode (active layer) 3. Hydrogen gas (H ₂ ) generated by electrolysis is led to the outside from a gas flow path not shown, and is stored, for example. The generated oxygen ions are conducted into the solid oxide electrolyte layer 4 .

One surface of the solid oxide electrolyte layer 4 is laminated with the hydrogen electrode 3, and the other surface is 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 passes ions such as oxygen ions but does not pass electricity. For the solid oxide electrolyte layer 4, for example, stabilized zirconia in _which a stabilizer made of an oxide of a rare earth element such as Y, Sc, Ce, Gd, or Sm is dissolved, typically _Y2O3 stabilized zirconia (Yttria( _{Y2O3 )} Stabilized Zirconia ( _ZrO2 ):YSZ) or CSZ, or a composite of these can be used.

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, and is capable of generating oxygen molecules (O ₂ ) from oxygen ions (O ^2- ) arriving in the porous body from the solid oxide electrolyte layer 4 and electrons (e ^- ) supplied from an external power source. The generated oxygen gas (O ₂ ) is led to the outside through a gas flow passage (not shown) and is 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 example, a perovskite oxide represented by R1 _{-xAxB1 - yCyO3 - δ} (where R is a rare earth element such as La, A is an alkaline earth element such as Sr, Ca or Ba, B and C are metal elements such as Cr, Mn, Co, Fe or Ni, and x, y and δ are atomic ratios satisfying 0≦x≦1, 0≦y≦1 and 0≦δ≦1) can be used for the oxygen electrode 6. A representative example of the oxygen electrode 6 is ( _{La1- xSrx} )(Co1 _{- yFey} )O3 _-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 reaction prevention layer that prevents diffusion and reaction of elements between the solid oxide electrolyte layer 4 and the oxygen electrode 6. The SOEC 1 of the embodiment is constructed by sequentially stacking the support layer 2, the hydrogen electrode 3, the solid oxide electrolyte layer 4, the intermediate layer 5, and the oxygen electrode 6. 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. The SOEC 1 is further constructed by forming a thin film of the intermediate layer 5 and the oxygen electrode 6 on the solid oxide electrolyte layer 4. With regard to 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.

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, pore-forming agent, and a solvent to raw material powder to form a slurry, which is then formed into a sheet, laminated, and pressed into a compact, which is then subjected to a degreasing process (a binder removal 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, making the support layer 2 porous. Generally, the porosity is around 40%, and this high porosity allows for gas permeability.

However, when a load is applied to the porous sintered body as described above, stress is generated in the surrounding matrix around the pores according to their shape. If the generated stress becomes higher than the rupture strength of the matrix, cracks are generated and the cracks propagate, causing the destruction of the support layer 2 and eventually the large-scale destruction of the entire electrolytic cell 1. Therefore, the pore shape, such as the diameter and shape of the pores, is important. Spherical particles, which are the least likely to cause stress concentration, are often used as pore-forming agents. Ceramics (sintered bodies), which are brittle materials, require high strength for ease of handling and setting. If the Young's modulus is high, even a slight deformation will generate high stress and cause easy destruction, so a low Young's modulus is preferable. Since cracks often propagate along grain boundaries, it is effective to improve grain boundary strength or add additives to increase strength.

Therefore, in the embodiment, the support layer 2 uses 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, but also replaces part of the GDC of the composite with CSZ. Among the composites of NiO and GDC, the ceria of GDC has excellent affinity with the ceria of CSZ, and therefore shows mutual reaction in the porous sintered body. Since CSZ has superior strength characteristics compared to GDC, the strength of the porous sintered body can be improved by replacing part of GDC in the composite of NiO and GDC to contain CSZ. 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 therefore the strength of the entire electrolysis cell 1 can be increased and mechanical reliability can be improved. 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, and therefore the characteristics of the electrolysis cell 1 can be maintained. In other words, it is possible to provide a high-temperature steam electrolysis cell 1 with excellent cell characteristics and mechanical strength.

In the constituent materials 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 this mass ratio of NiO to GDC, the function and characteristics of the support layer 2 can be satisfactorily satisfied. In such a composite, it is preferable to replace 1% to 40% by mass of GDC with CSZ. If the amount of GDC substituted by CSZ is less than 1% by mass, the strength improvement effect of the porous sintered body due to the composite of NiO and GDC may not be sufficiently obtained. If the amount of GDC substituted by CSZ exceeds 40% by mass, the characteristics (cell characteristics, etc.) of the support layer 2 due to the composite of NiO and GDC may be deteriorated.

In the porous sintered body made of a composite of NiO, GDC, and CSZ that constitutes the support layer 2, the particle size of the aggregates containing at least one of GDC and CSZ is preferably 20 μm or less. Aggregates with a particle size exceeding 20 μm become the starting point of cracks, and the cracks that occur tend to extend further, which causes a decrease in strength. Therefore, it is preferable that the particle size of the aggregates is 20 μm or less. As described above, the porosity of the porous sintered body that constitutes 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 electrolysis cell 1 tend to deteriorate. If the porosity of the porous sintered body exceeds 50%, the gas permeability improves, but the mechanical properties such as the strength of the support layer 2 tend to deteriorate.

The manufacturing method of the high-temperature steam electrolysis cell 1 of the embodiment is not particularly limited, but can be prepared, for example, as follows. First, the powders of nickel oxide (NiO), gadolinium-doped ceria (GDC), and ceria-stabilized zirconia (CSZ) are mixed in the above-mentioned ratio to prepare a raw material powder. At this time, it is preferable that the CSZ powder has an average particle size of 0.1 μm or more and 1 μm or less. If the average particle size of the CSZ powder exceeds 1 μm, agglomerated particles with a large particle size are likely to occur in the porous sintered body to be produced. If 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 size of the NiO powder and the GDC powder is not necessarily limited, but it is preferable that they 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, and a solvent is further added as necessary to mix the raw material slurry. The raw material slurry is formed into a sheet to produce a sheet. Next, the slurry for forming the hydrogen electrode 3 and the slurry for forming the solid oxide electrolyte layer 4 are formed into sheets on the obtained sheet in order to produce a laminated molded body. The laminated molded body is subjected to the steps of thermocompression bonding, degreasing, and sintering to produce a porous laminated sintered body. The high-temperature steam electrolysis cell 1 is obtained by carrying out the steps of forming and baking the materials for forming the intermediate layer 5 and the oxygen electrode 6 on the laminated sintered body. Note that each of these constituent layers may be degreased and sintered separately.

Next, we will describe specific examples of electrolysis cells according to the embodiment and their evaluation results.

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

100 parts by mass of each of the raw material powders with the above-mentioned raw material composition ratios were added with 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-forming agent, and then ethanol was added as a solvent, and the mixture was pot-mixed for 24 hours to produce a slurry. Each of the slurries produced in this way was formed into a sheet with a thickness of 1 mm to produce a sheet for the support layer.

Next, NiO-GDC-based slurry was formed on each of the above-mentioned sheets as a material for forming the hydrogen electrode active layer. For the electrolyte layer, yttria-stabilized zirconia (YSZ) slurry was screen-printed in a sheet shape with a thickness of 20 μm using a polyethylene terephthalate (PET) film. The prepared support layer, sheets for the hydrogen electrode active layer, and sheets for the electrolyte layer were then thermocompressed at a temperature of 70°C to form a laminated molded body, which was then degreased at 400°C for 2 hours. The degreased body was then sintered at a temperature of 1300°C or higher at which sintering proceeded sufficiently to obtain a sintered body. The atmosphere was air. LSCF was screen-printed on the electrolyte side of the sintered body as an oxygen electrode, and then baked.

In this way, a 50 x 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, and the breaking strength of each was measured. The amount of GDC substituted by CSZ and the breaking strength are shown in Table 1. For comparison, a sintered body for an electrolytic cell was also produced by preparing a slurry for a support layer for a material in which GDC was not substituted by CSZ (CSZ: 0 mass%). 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 of a NiO-GDC-based material was substituted with CSZ has a higher strength than a material in which GDC was not substituted with CSZ. However, when the amount of GDC substituted by CSZ is 50 mass%, the strength is slightly reduced, and therefore it can be seen that the amount of GDC substituted by CSZ is preferably 40 mass% or less.

Example 2
In producing a compact in which the sheets for the support layer and the hydrogen electrode active layer in Example 1 and the sheet for the electrolyte layer were laminated, the retention time during sintering of the compact was changed (increasing or decreasing the length of time) to produce a sintered body in which the particle size of the particles containing at least one of GDC and CSZ was changed. Otherwise, the same steps as in Example 1 were applied to produce a sintered body for an electrolytic cell. The fracture strength of these sintered bodies for an electrolytic cell was measured in the same manner as in Example 1. The particle size and fracture strength of the aggregates containing at least one of GDC and CSZ are shown in Table 2.

As shown in Table 2, in a sintered body for an electrolytic cell using a composite in which part of the GDC in a NiO-GDC-based material is replaced with CSZ, when the particle size of the aggregates containing at least one of GDC and CSZ is 25 μm, the strength is slightly reduced, and therefore it is preferable that the particle size of the aggregates is 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.

Example 3
In preparing the sheet for the support layer in Example 1, except for changing the average particle size of the raw material powder of CSZ, the sheets for the support layer and the hydrogen electrode active layer were prepared, and a molded body was prepared by laminating such a sheet and a sheet for the electrolyte layer, and the molded body was degreased and sintered. Furthermore, LSCF was printed and baked as an oxygen electrode to prepare a sintered body for an electrolytic cell. The fracture strength of these sintered bodies for an electrolytic cell was measured in the same manner as in Example 1. The average particle size and fracture strength of the raw material powder of CSZ are shown in Table 3. As shown in Table 3, when the average particle size of the raw material powder of CSZ is less than 0.1 μm or exceeds 1 μm, the strength is slightly reduced. Therefore, it can be seen that the average particle size of CSZ is preferably 0.1 μm or more and 1 μm or less when preparing a porous sintered body of a NiO-GDC-CSZ composite.

The configurations of the above-mentioned embodiments can be applied in combination with each other, and some of them can be replaced. Several embodiments of the present invention have been described here, 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, substitutions, modifications, 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 are also included in the scope of the invention and its equivalents described in the claims.

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 high-temperature steam electrolysis cell comprising: a gas-permeable support layer; a hydrogen electrode provided on the support layer, the hydrogen electrode having gas permeability and capable of electrolyzing water vapor flowing therein 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 arriving from the solid oxide electrolyte layer,
1. A high-temperature steam electrolysis cell, wherein the support layer comprises a porous sintered layer of a composite of nickel oxide and gadolinium-doped ceria, in which a portion of the gadolinium-doped ceria in the composite is substituted with ceria-stabilized zirconia. The high-temperature steam electrolysis cell according to claim 1, wherein the composite contains the nickel oxide and the gadolinium-doped ceria in a mass ratio of 5:5 to 7:3, and 1 mass % to 40 mass % of the gadolinium-doped ceria is replaced with ceria-stabilized zirconia. The high-temperature steam electrolysis cell according to claim 1 or 2, wherein the particle size of the aggregates containing at least one of the gadolinium-doped ceria and the ceria-stabilized zirconia present in the composite is 20 μm or less. 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. 2. A method for producing a high temperature steam electrolysis cell according to claim 1, comprising the steps of:
A step of preparing a raw material powder including nickel oxide powder, gadolinium -doped ceria powder, and ceria- stabilized zirconia powder;
preparing a raw material slurry by adding a binder, a pore forming agent and a solvent to the raw material powder;
forming the raw material slurry into a sheet shape 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 method for producing a high-temperature steam electrolysis cell according to claim 5, wherein the raw material powder contains the nickel oxide and the gadolinium-doped ceria in a mass ratio of 5:5 to 7:3, and 1 mass % to 40 mass % of the gadolinium-doped ceria is replaced with ceria-stabilized zirconia. The method for producing a high-temperature steam electrolysis cell according to claim 5 or 6, wherein the ceria-stabilized zirconia powder has an average particle size of 0.1 μm or more and 1 μm or less. The method for producing a high-temperature steam electrolysis cell according to any one of claims 5 to 7, wherein the forming process of the green body is a process for forming the slurry for forming the hydrogen electrode and the slurry for forming the solid oxide electrolyte layer in sheet form in order on the sheet layer of the raw material slurry of the green body to produce a laminated green body, and the degreasing process and the sintering process are performed on the laminated green body.

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