JP7309642B2 - High-temperature steam electrolysis hydrogen production cell and method for producing high-temperature steam electrolysis hydrogen production cell - Google Patents

Description

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

In recent years, from the perspective of environmental issues such as the depletion of fossil fuels, global warming caused by the release of carbon dioxide into the atmosphere, and energy security, the introduction of renewable energy represented by solar, wind, and geothermal power is increasing. being promoted. Hydrogen energy is attracting attention as a secondary energy from the viewpoint of storage and transportation. This 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.

In terms of cost and technology, currently the mainstream method of hydrogen production is to reform and produce hydrogen from fossil fuels. However, hydrogen production by fossil fuel reforming inevitably generates carbon dioxide during the production process.

On the other hand, it is known that the method of producing hydrogen using water or steam as a raw material and using renewable energy does not generate carbon dioxide and has a low environmental load. A PEM type using a solid polymer electrolyte membrane and an SOEC type using a solid oxide are known as methods for producing hydrogen by electrolyzing water or steam.

Among them, the SOEC type, which uses a solid oxide, is expected as a future hydrogen production method because it theoretically requires less electric power for producing hydrogen. The solid oxide cell, which is essential for the electrolysis of hydrogen production, consists of a support layer that conducts electrons, an active layer that electrolyzes water, an electrolyte layer that conducts oxygen ions, and an oxygen electrode that combines oxygen ions into oxygen molecules. ing.

Since solid oxide cells operate at high temperatures of about 600 to 1000° C., each constituent layer is required to have thermal stability. In particular, the support layer, which is a thermally unstable porous ceramic and has the largest volume ratio, is required to have high stability. Therefore, in addition to the first component that allows electrons to pass through, the support layer often contains a second component that suppresses the growth of crystal grains and pore diameters. There was a problem of declining performance.

Patent No. 5498191

Since the high-temperature steam electrolysis hydrogen production cell produces hydrogen by electrolysis, it is desired that the electron conductivity is excellent. On the other hand, it is necessary to add a second component for structural stability in order to stabilize the cell at high temperatures. Stabilization of conductivity was a big issue.

Accordingly, an object of the present invention is to provide a cell for producing hydrogen by high-temperature steam electrolysis, which has high stability at high temperatures and excellent electron conductivity, and a method for producing a cell for producing hydrogen by high-temperature steam electrolysis.

The cell for high-temperature steam electrolysis hydrogen production of the embodiment comprises: a support layer which is capable of conducting electrons and allows gas to pass through and plays a major role in maintaining the strength of the cell; an active layer which allows gas and oxygen ions to pass through and electrolyzes water; A cell for high-temperature steam electrolysis hydrogen production, comprising an electrolyte layer that conducts oxygen ions but impermeable to gas, and an oxygen electrode that binds oxygen ions to form oxygen molecules, wherein the support layer has electrical conductivity. a second component made of a material for maintaining high-temperature strength; and pores having gas permeability; The coverage with crystal grains is in the range of 10% to 55%.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a view showing a schematic cross-sectional structure of a high-temperature steam electrolysis hydrogen production cell according to an embodiment; The figure which expands and shows the structure of a support layer typically.

A high-temperature steam electrolysis hydrogen production cell according to an embodiment will be described below with reference to the drawings.

FIG. 1 shows a schematic cross-sectional configuration of a high-temperature steam electrolysis hydrogen production cell (SOEC) 100 according to an embodiment. As shown in FIG. 1, a high-temperature steam electrolysis hydrogen production cell 100 includes a support layer 101 that can conduct electrons and allows gas to pass through, an active layer 102 that allows gas and oxygen ions to pass through to electrolyze water, and oxygen ions. It has an electrolyte layer 103 that is conductive and impermeable to gas, and an oxygen electrode 104 that combines oxygen ions into oxygen molecules.

FIG. 2 is an enlarged diagram schematically showing the structure of the support layer 101 shown in FIG. The support layer 101 is made of porous ceramics, and as shown in FIG. 2, the first component (the hatched portion in FIG. 2) that allows electrons to pass through and the structural stability at high temperatures are added. and a second component (part not hatched in FIG. 2). The second component is for stabilizing the structure at high temperatures by suppressing the growth of crystal grains and pore diameters.

As the first component, for example, at least one oxide selected from the group consisting of Ni (nickel), Co (cobalt), Fe (iron), Cu (copper), and Ru (ruthenium) can be used, Alloys containing these elements can also be used.

As the second component, for example, at least one oxide selected from the group consisting of Ce (cerium), Gd (gadolinium), Sm (samarium), Y (yttrium), Zr (zirconium), and Sc (scandium). can be used, and alloys containing these elements can also be used.

As the second component, for example, stabilized zirconia in which a stabilizer such _{as Y2O3 , Sc2O3} , _{Yb2O3 , Gd2O3} , CaO, MgO, _{CeO2 is} dissolved is used. be able to. As the second component, for example, gadolinia-doped ceria (GDC) in which an oxide such as Gd ₂ O ₃ and CeO ₂ are solid-dissolved can be used.

In this embodiment, the support layer 101 is configured so that the coverage shown below is in the range of 10% to 55%.
Coverage (%) = (length of contact between the first component and the second component/peripheral length of the first component crystal grain) x 100

As described above, by setting the coverage within the range of 10% to 55%, the electrical conductivity of the support layer 101 can be increased (the electrical resistance can be decreased).

As a material forming the electrolyte layer 103, for example, the above-described stabilized zirconia, gadolinia-doped ceria (GDC), or the like can be used.

As a material forming the oxygen electrode 104, for example, a sintered body containing an oxide having a perovskite structure, gadolinia-doped ceria (GDC), or the like can be used.

(Example)
Next, examples will be described.
In manufacturing the support layer 101 shown in FIG. 1, nickel oxide with an average particle size of 0.7 μm and stabilized zirconia powder with an average particle size of 0.05 μm to 4 μm were mixed at a weight ratio of 6:4. Further, an organic solvent (ethanol) was added and mixed in a ball mill for 24 hours to obtain a slurry.

Next, 5% by weight of a pore-forming agent having a particle size of 1 μm was added to the slurry prepared as described above, and 10% of a binder was added, and after sufficient stirring, the slurry was formed into a sheet having a thickness of 0.6 mm. A support layer sheet composed of a molded body was obtained.

Next, in forming the electrolyte layer 103 on the support layer sheet produced as described above, the stabilized zirconia slurry was screen-printed on the support layer sheet to a thickness of 0.01 mm to form the layer.

Next, after drying the support layer sheet, it was degreased at 600° C. for 2 hours to obtain a degreased body from which the pore-forming agent and the binder were removed. Further, sintering was performed at 1400° C. for 2 hours. During this sintering, the rate of temperature increase in the temperature range of 1000° C. or higher was changed to change the aggregation state of the second component, thereby controlling the coverage of the first component.

The control of the coverage of the first component during the sintering is performed by increasing the temperature increase rate per hour in the temperature range of 1000 ° C. or higher in seven ways in the range of 10 ° C./h to 60 ° C./h. It was carried out as a temperature rate. Note that the rate of temperature increase below 1000°C is 25°C/h in all cases. Therefore, when the temperature increase rate in the temperature range of 1000° C. or higher is 25° C./h, the temperature increase rate of less than 1000° C. and the temperature range of 1000° C. or higher are the same.

As described above, there is a tendency that when the heating rate is lowered in the temperature range of 1000° C. or higher, the coverage increases, and when the heating rate is raised, the coverage decreases. The coverage rate is 40% when the temperature increase rate in the temperature range of 1000 ° C. or higher is 25 ° C./h, and the coverage rate is higher than 25 ° C./h in the temperature range of 1000 ° C. or higher. was less than 40%, and when lowered the coverage was greater than 40%.

Next, the obtained sintered body was cut into pieces having a width of 20 mm and a thickness of 0.5 mm, and the electric conductivity was measured by the four-probe method. The electrical conductivity was measured five times, and the average value was calculated and normalized by the value at an average coverage of 40%.

Moreover, the coverage was evaluated as follows.
First, after the cross section of the cell is mirror-polished, the structure is observed with a SEM (scanning electron microscope), and 20 crystal grains of the first component having a typical grain size are selected. The perimeter of the particle was measured (length A).

Next, the length of the particles of the second component (because they are heavy elements, they look bright and can be easily identified) in contact with the particles of the first component were measured (length B).

The coverage is
Coverage = B/A x 100
, and the values were obtained and averaged for 20 particles.

The theoretical relative density at this time was 72%. As the theoretical relative density increases, the porosity for gas passage decreases, so the theoretical relative density is preferably 80% or less at the maximum. In addition, if the difference in thermal expansion coefficient between the first component and the second component is large, cracks may occur during cooling after sintering. Therefore, the difference in thermal expansion coefficient between the first component and the second component is preferably within 1 to 4×10 ⁻⁶ /K. In this case, the coefficient of thermal expansion of the first component is generally greater than the coefficient of thermal expansion of the second component.

Table 1 shows the relationship between the electrical conductivity ratio (normalized by the value at a coverage of 40%) obtained by measurement and the coverage.

In Table 1, the coverage of the samples is different from 60%, 55%, 50%, 40%, 30%, 20% and 10%. This is because the heating rate per hour in the temperature range of 1000° C. or higher was varied. It can also be seen that the electrical conductivity is good (the electrical resistance is low) when the coverage is in the range of 10 to 55%. Further, in the range of 10 to 30% coverage, the electrical conductivity is even better (the electrical resistance value is low).

That is, by increasing the temperature increase rate in the temperature range of 1000° C. or higher, the coverage can be reduced and the electrical conductivity can be increased. From the viewpoint of coverage, the coverage is preferably about 10% to 55%, more preferably about 10% to 30%.

Next, the holding time during sintering was changed in the range of 5 hours to 100 hours to change the size of the crystal grains (average grain size) of the first component in the sintered body, and to change the grain size of the pore-forming agent. We prepared samples with different pore sizes by changing the diameter, and investigated the effect of these differences on the electrical conductivity. The manufacturing conditions other than the holding time during sintering and the particle size of the pore-forming agent are the same as those for the sample with an electrical conductivity ratio of 1 shown in Table 1. Also, the electrical conductivity was measured by the method described above.

The average particle size and pore size were measured by extracting 20 typical particles by SEM, randomly measuring lengths and widths, and obtaining average values. In addition, when the aspect ratio is large, the actual situation is not reflected, but there was no such example in the actual measurement.

Table 2 below shows the above results. The electrical conductivity ratio was standardized based on the values for a particle size of 0.3 μm and a pore size of 0.5 μm.

As shown in Table 2, the crystal grain size (average grain size) of the first component in the sintered body was 0.3 μm, 1 μm, 5 μm, 10 μm, and 20 μm. These are the cases of 5 hours, 10 hours, 20 hours, 50 hours, and 100 hours, and the longer the holding time, the larger the grain size (average grain size).

The pore diameters (average diameter) were 0.5 μm, 1 μm, 6 μm, 12 μm, 20 μm and 30 μm. is the case.

When the crystal grain size (average grain size) of the first component in the sintered body was 20 μm, the electrical conductivity tended to decrease. Therefore, the size of crystal grains (average grain size) in the sintered body is preferably in the range of 0.3 μm to 10 μm.

Also, with respect to the pore diameter, there was a tendency for the electrical conductivity to decrease when the pore diameter was 0.5 μm and 30 μm. Therefore, the pore diameter is preferably in the range of 1 μm to 20 μm.

As described above, by setting the crystal grain size (average grain size) of the first component in the range of 0.3 μm to 10 μm and the pore diameter in the range of 1 μm to 20 μm, the electrical conductivity of the support layer is increased. be able to.

It is known that SOEC, which uses a flat-plate solid oxide electrochemical cell that generates hydrogen by electrolyzing hot steam, has higher water electrolysis efficiency than the PEM method and alkaline water electrolysis, and can keep hydrogen production costs low. ing. On the other hand, due to the high temperature operation, the material of the support layer of the cell is porous ceramic, and the reliability as a structural member is low due to its low strength, and the electronic conductivity and gas permeability are required. It has become.

The electronic conductivity of the cell is required to be low because it is the internal resistance that is loss in hydrogen production. This embodiment is a configuration for maintaining the electronic conductivity of the support layer, which is the main component member of the porous ceramics, and provides structural stability at high temperatures around the crystal grains of the first component responsible for electronic conduction. By defining the coverage of the second component, it is possible to maintain high electronic conductivity of the cell.

Although several embodiments of the invention have been described above, these embodiments are presented by way of example and are not intended to limit the scope of the invention. These novel embodiments can be implemented in various other forms, and various omissions, replacements, and modifications can be made without departing from the scope of the invention. These embodiments and modifications thereof are included in the scope and gist of the invention, and are included in the scope of the invention described in the claims and equivalents thereof.

100... High-temperature steam electrolysis hydrogen production cell, 101... Support layer, 102... Active layer, 103... Electrolyte layer, 104... Oxygen electrode.

Claims (6)

A support layer capable of conducting electrons and permeable to gas and having the primary role of maintaining the strength of the cell, an active layer permeable to gas and oxygen ions to electrolyze water, and an electrolyte layer that conducts oxygen ions and is impermeable to gas. and an oxygen electrode that combines oxygen ions into oxygen molecules, the cell for producing hydrogen by high-temperature steam electrolysis,
The support layer has a first component made of a material having electrical conductivity, a second component made of a material for maintaining high-temperature strength, and pores having gas permeability,
A cell for high-temperature steam electrolysis hydrogen production, wherein the coverage of the crystal grains of the first component with the crystal grains of the second component is in the range of 10% to 55%. 2. The high-temperature steam according to claim 1, wherein the average particle diameter of the crystal particles of the first component is in the range of 0.3 μm to 10 μm, and the average diameter of the pores is in the range of 1 μm to 20 μm. Cell for electrolytic hydrogen production. 3. The high-temperature steam electrolysis hydrogen production cell according to claim 1, wherein the support layer has a theoretical relative density of 80% or less. The high-temperature steam electrolysis hydrogen production according to any one of claims 1 to 3, characterized in that the difference in thermal expansion coefficient between the first component and the second component is within 1 to 4 × 10 ^-6 /K. cell for. The high temperature according to any one of claims 1 to 4, wherein the coverage of the crystal grains of the first component with the crystal grains of the second component is in the range of 10% to 30%. Cell for steam electrolysis hydrogen production. A method for manufacturing a cell for high-temperature steam electrolysis hydrogen production according to any one of claims 1 to 5,
The rate of temperature increase during sintering of the support layer is adjusted so that the coverage of the crystal grains of the first component with the crystal grains of the second component is in the range of 10% to 55%. A method for manufacturing a cell for high-temperature steam electrolysis hydrogen production.

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