Member for hydrogen production and hydrogen production apparatus

A member for hydrogen production includes a ceramic composite in which a plurality of ceramic particles having an average particle diameter ranging from 5 nm to 200 nm are dispersed in a porous insulator having a different component from the ceramic particles. The ceramic particles comprise at least one substance selected from the group consisting of AXO3±δ (where 0≤δ≤1, A: at least one of rare earth elements, alkaline earth elements, and alkali metal elements, X: at least one of transition metal elements and metalloid elements, and O: oxygen), cerium oxide, and zirconium oxide as a main component.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application claiming priority to International Application No. PCT/JP2016/083771, filed on Nov. 15, 2016, which claims priority to Japanese Patent Application No. 2015-243303, filed on Dec. 14, 2015; and claims priority to Japanese Patent Application No. 2016-101596, filed on May 20, 2016; all of which are herein incorporated by reference for all purposes.

FIELD

This disclosure relates to a member for hydrogen production and a hydrogen production apparatus.

BACKGROUND

In recent years, as a solution to problems such as global warming due to an increase in carbon dioxide associated with the consumption of fossil fuels, clean renewable energy that does not emit carbon dioxide has attracted attention instead of the fossil fuels.

Solar energy, which is one of the renewable energy, has no worry of exhaustion. The use of the solar energy can contribute to the reduction in greenhouse gases. Under such circumstances, an energy system in which primary energy is sought from solar light and secondary energy is supported by hydrogen is one of the ideal clean energy systems. Establishment of such an energy system is an urgent matter.

As one method of converting solar energy into chemical energy, for example, the use of ceramic members such as ceria (CeO2) as a reaction system carrier has been developed. This method utilizes a two-step hydrolysis reactions occurring in the reaction system carrier (for example, refer to Patent Literature 1).

Specifically, first of all, in a first step, the ceramic member as the reaction system carrier is heated to 1400° C. to 1800° C. using solar energy. In this first step, oxygen is generated by the reduction of the ceramic member.

Subsequently, in a second step, the reduced ceramic member is cooled to 300° C. to 1200° C. In this second step, the ceramic member is reacted with water. In this reaction, the reduced ceramic member is oxidized to generate hydrogen.

CITATION LIST

Patent Literature

SUMMARY

A member for hydrogen production of the present disclosure includes a ceramic composite in which a plurality of ceramic particles having an average particle diameter of 5 nm to 200 nm are dispersed in a porous insulator having a different component from the ceramic particles, in which the ceramic particles comprise at least one substance selected from the group consisting of AXO3±δ(where 0≤δ≤1, A: at least one of rare earth elements, alkaline earth elements, and alkali metal elements, X: at least one of transition metal elements and metalloid elements, and O: oxygen), cerium oxide, and zirconium oxide as a main component.

A hydrogen production apparatus of the present disclosure includes a reaction unit configured to cause oxidation-reduction reactions by receiving solar energy, a water supply unit configured to supply water to the reaction unit, and a recovery unit configured to recover hydrogen gas generated from the reaction unit, in which the above member for the hydrogen production is provided in the reaction unit.

DESCRIPTION OF EMBODIMENTS

FIG. 1is a sectional view schematically illustrating one embodiment of the member for hydrogen production of this disclosure. The member for hydrogen production according to the present embodiment is made of a ceramic composite5in which fine ceramic particles1are dispersed in a porous insulator3.

The insulator3is formed of a material the main component of which is different from the material of the ceramic particles1. As the material of the insulator3, silicon oxide, aluminum oxide, zinc oxide, oxides of alkaline earth elements, oxides of rare earth elements, and composite oxides thereof are suitable materials. In this case, the insulator3has many open pores6and the open pores6extend in such a manner that the pores reach from an outer surface5aof the ceramic composite5to the inner ceramic particles1. In this case, the open porosity is 10% or higher. As the open porosity of the insulator3, a value measured for the ceramic composite5including the ceramic particles1is used. This is because the ceramic particles1are dense bodies and the porosity of the insulator3directly corresponds to the porosity of the ceramic composite5.

The ceramic particles1includes at least one substance selected from the group consisting of AXO3±δ(where 0≤δ≤1, A: at least one of rare earth elements, alkaline earth elements, and alkali metal elements, X: at least one of transition metal elements and metalloid elements, and O: oxygen), cerium oxide, and zirconium oxide as a main component. In this case, the average particle diameter of the ceramic particles1(represented by a symbol D inFIG. 1) is 5 nm to 200 nm. The rare earth element may be or include a lanthanide element. The transition metal element may be or include, but is not limited to, Ti, V, Cr, Mn, Zr, Nb, Ta, or combinations thereof. The metalloid element may be or include, but is not limited to, B, Si, Ge, As, Se, Sb, Te, Po, At, or combinations thereof. As examples of a combination of plural species from the group consisting of AXO3±δ, cerium oxide, and zirconium oxide, a composite oxide in which a part of zirconium oxide is replaced by cerium oxide can be exemplified.

The main component refers to, for example, a ceramic composite5having a proportion of 60% by mass or larger determined from Rietveld analysis using X-ray diffraction.

When the ceramic particles1having the above main component are placed in a high temperature environment, a defective reaction expressed by Formula (1) occurs.

In this case, the ceramic particles1constituting the ceramic composite5are fine and thus the electrons generated in the ceramic particles1by the above defect reaction tend to stay on the surfaces of the ceramic particles1. This enhances the surface plasmon effect for the ceramic particles1. Therefore, the ceramic composite5itself can be changed into a high temperature state. As a result, the ceramic particles1themselves can have a function of absorbing light.

When the ceramic particles1causing such a reaction exist in the porous insulator3, the ceramic particles1cause a reaction in which oxygen is released (hereinafter referred to as an “oxygen release reaction” in some cases) in a high temperature state as represented by Formula (2). On the other hand, at a temperature lower than the temperature at which the oxygen release reaction occurs, the ceramic particles1cause a reaction that generates hydrogen (hereinafter referred to as a hydrogen production reaction in some cases) as represented by Formula (3).
MOox=MOred+½O2(2)

OX: Oxidation state

This is because the surface plasmon effect appears on the ceramic particles1due to the above defect reaction and, in addition, the above oxidation/reduction reactions occur in the insulator3constituting the ceramic composite5.

In this case, as the ceramic particles1, the smaller the average particle diameter becomes, the more the surface plasmon effect can be expected. However, the ceramic particles1having an average particle diameter of less than 5 nm are difficult to prepare currently. On the other hand, when the ceramic particles1have an average particle diameter of larger than 200 nm, the surface plasmon effect is difficult to be developed. Therefore, the ceramic composite5itself cannot be brought into the high temperature state. As a result, hydrogen is difficult to generate.

From the viewpoint of enhancing the surface plasmon effect of the ceramic particles1, the proportion of the ceramic particles1included in the ceramic composite5may range from 20% to 80% in a volume ratio. The ceramic particles1may exist dispersedly in an isolated state as a single particle in the insulator3in an amount of 90% or more. Namely, in the member for hydrogen production according to the present embodiment, the ceramic particles1individually exist at a ratio in the number of particles of 90% or more in the material constituting the insulator3.

The proportion of the ceramic particles1existing inside the ceramic composite5is obtained by using an electron microscope and an analyzer attached thereto (EPMA) by observing the section of the ceramic composite5. For example, the ceramic composite5is polished to expose the ceramic particles1and a predetermined region where 30 to 100 ceramic particles1exist in the section is specified. Subsequently, the area of this region and the total area of the ceramic particles1existing in this region are obtained and the total area of the ceramic particles1relative to the area is obtained. The area proportion thus obtained is defined as the volume proportion. Whether the ceramic particles1exist in the isolated state as a single particle in the insulator3is also determined by counting the number of the particles from the above observation.

FIGS. 2A and 2Billustrate other aspects of the present embodiment and are schematic views illustrating the member for hydrogen production in which a light absorbing member is provided on a ceramic composite.FIG. 2Aillustrates the case where the light absorbing member is a metal particle-containing composite andFIG. 2Billustrates the case where the light absorbing member is a metal film laminated product.

For the member for hydrogen production according to the present embodiment, as a method of further increasing the temperature of the ceramic composite5, a light absorbing member7being a heat medium from the outside may be provided on the ceramic composite5as illustrated inFIGS. 2A and 2B.

A metal particle-containing composite7A illustrated inFIG. 2Ais obtained by dispersing metal particles7bin a dense sintered ceramic product7a. In this case, the metal particle-containing composite7A absorbs sunlight and thus the free electrons existing in the metal particles7bdevelop a surface plasmon effect, whereby the metal particle-containing composite7A itself generates heat. As the metal particles7b, one of the metals selected from the group consisting of tungsten, molybdenum, niobium, nickel, copper, silver, gold, platinum, and palladium can be selected. As the metal particles7b, a compound in which carbon (C) or nitrogen (N) is bonded to a metal or both of carbon (C) and nitrogen (N) are bonded to a metal can be applied instead of the above metals. As the compound, at least one compound selected from the group consisting of tantalum carbide (TaC), vanadium carbide (VC), titanium nitride (TiN), titanium carbide (TiC), titanium carbonitride (TiCN), niobium carbide (NbC), and niobium nitride (NbN) can be exemplified.

Also in this case, the size (average particle diameter) of the metal particles7bis fine from the viewpoint of enhancing the surface plasmon effect similar to the case of the ceramic composite5described above. The average particle diameter of the metal particles7branges from 5 nm to 50 nm, for example. On the other hand, the open porosity of the sintered ceramic product7asurrounding the metal particles7bmay be 5% or lower. As the sintered ceramic product7a, a low thermal expansion glass containing silicon oxide as a main component is suitable because the glass has high light transparency and excellent heat resistance. As the material for the sintered ceramic product7a, the material composed of the same components as the components of the insulator3constituting the ceramic composite5can also be used.

Examples of a metal-based film laminated product7B illustrated inFIG. 2Binclude, but not limited to, a laminated product in which a tungsten film7c, an iron silicide (FeSi) film7d, and a silicon oxide film7eare formed in layers, for example. In this case, the tungsten film7cand the iron silicide film7dformed on the lower layer side of the silicon oxide film7ehave a role of absorbing light in a specific wavelength region. At the same time, the silicon oxide film7ebeing a dielectric substance achieves a function to reduce radiation from the metal-based film laminated product7B.

In the member for hydrogen production according to the present embodiment, a metal film9may be interposed between the ceramic composite5and the light absorbing member7. As illustrated inFIGS. 3A and 3B, when the metal film9is provided between the ceramic composite5and the light absorbing member7, the light incident to the light absorbing member7is reflected at the surface of the metal film9. Therefore, the light incident to the light absorbing member7is difficult to permeate to the ceramic composite5side. This allows the light to concentrate inside the light absorbing member7. Therefore, the amount of heat generated by the light absorbing member7can be increased. As the material of the metal film9, any metal having high light reflectivity may be used. For example, tungsten, molybdenum, nickel, copper, silver, gold, platinum, palladium, and the like are suitable.

FIGS. 4A and 4Bare sectional views schematically illustrating states when the hydrogen production apparatus according to the present embodiment is operated.FIG. 4Aillustrates the state where oxygen is generated from the member for hydrogen production andFIG. 4Billustrates the state where hydrogen is generated from the same member for hydrogen production.

A hydrogen production apparatus20according to the present embodiment includes a reaction unit21configured to cause oxidation-reduction reactions by receiving solar energy (a white arrow illustrated inFIG. 4A), a water supply unit23configured to supply water to the reaction unit21, and a recovery unit25configured to recover hydrogen gas generated from the reaction unit21. In this case, the reaction unit21has a light absorbing member7configured to convert sunlight into heat and a member for hydrogen production configured to generate hydrogen when the reaction unit21is heated. The member for hydrogen production has the ceramic composite5described above. A discharge port27configured to discharge oxygen is attached to the reaction unit21. In the hydrogen production apparatus20, a shielding plate29configured to allow the reaction unit21to receive or shield sunlight is provided. The shielding plate29may be an opaque plate and any material such as plastic, metal, and wood can be used.

As illustrated inFIG. 4A, when the shielding plate29is moved from the upper surface of the reaction unit21, the ceramic composite5being the member for hydrogen production receives light (sunlight). As a result, the ceramic composite5being the member for hydrogen production located in the reaction unit21is in a high temperature state and thus the ceramic composite5causes the reduction reaction represented by Formula (2) to generate oxygen.

Subsequently, as illustrated inFIG. 4B, when the reaction unit21is covered with the shielding plate29, the reaction unit21is shielded from sunlight. At this time, when water is supplied to the reaction unit21to bring into contact with the ceramic composite5, the ceramic composite5is cooled from the state illustrated inFIG. 4Awhere the reduction reaction has occurred. This allows the reduction reaction to be completed.

Subsequently, in the reaction unit21, the oxidation reaction represented by Formula (3) occurs to generate hydrogen gas inside the ceramic composite5. According to the hydrogen production apparatus according to the present embodiment, the heat from the sunlight can be efficiently absorbed to increase the hydrogen generation efficiency.

As illustrated inFIGS. 4A and 4B, the reaction unit21may be contained in a container22in a reduced pressure state. This can prevent the heat generated by the light absorbing member7from moving to the outside other than the reaction unit21. The heat can be efficiently supplied to the ceramic composite5in the reaction unit21.

When the reaction unit21is in a state of reduced pressure, an oxygen defect tends to be formed on the ceramic composite5side of the reaction unit21and thus the reduction reaction of the ceramic composite5proceeds and the amount of oxygen generated from the ceramic composite5can be increased.

Subsequently, when steam is supplied to the ceramic composite5after the reaction unit21is reduced, the probability that oxygen becomes free from the molecules of water increases and the amount of generated oxygen can be increased. This allows the proportion of the generated hydrogen gas relative to the amount of supplied water vapor to be increased.

In this case, an intake port for sucking the gas from inside the container22is obviously required to be provided in the container22provided with the reaction unit21. In the hydrogen production apparatus according to the present embodiment, the container22may have two intake ports22aand22b. In this case, when the schematic views illustrated inFIGS. 4A and 4Bare used as an example, one of the two intake ports22aand22b(in this case, the intake port22a) is used for reducing the pressure inside the whole container22. The purpose of this arrangement is to reduce the transfer of the heat from the surroundings of the light absorbing member7to the surroundings of the container22.

The other intake port (in this case, the intake port22b) is provided in such a manner that the intake port passes through the light absorbing member7to reach the ceramic composite5. The purpose of this arrangement is to control the internal pressure of the ceramic composite5being a member for hydrogen production in the reaction unit21located in the container22. The purpose of this pressure control is, for example, to change the pressure to be higher than that of the surroundings of the light absorbing member7. By this arrangement, when steam is supplied to the inside of the ceramic composite5, the oxidation reaction tends to proceed inside the ceramic composite5and the amount of hydrogen gas to be generated can be further increased.

In the hydrogen production apparatus20according to the present embodiment, an upper surface plate22cfacing the shielding plate29side of the container22may have a window22dfor taking in sunlight. A transparent plate may be located in the window22d. Glass having high heat resistance is suitable as the transparent plate. This allows sunlight to be efficiently taken in the container22. In addition, the light absorbing member7and the ceramic composite5can be heated to a higher temperature. As a result, the temperature change of the light absorbing member7and the porous composite5is increased between the state in which the container22is not covered with the shielding plate29(FIG. 4A) and the state in which the container22is covered with the shielding plate29(FIG. 4B), whereby the reactivity of reduction and oxidation of the ceramic composite5can be further enhanced. This allows the amount of generated hydrogen gas to be further increased.

FIGS. 5A, 5B and 5Care perspective views schematically illustrating the appearance structures of reaction units.FIGS. 5A, 5B and 5Cillustrate a flat plate type structure, a coaxial type of a hollow cylindrical structure, and a non-coaxial type of a hollow cylindrical structure, respectively.

As the reaction unit21constituting the hydrogen production apparatus20according to the present embodiment, a flat plate type laminated structure illustrated inFIG. 5A, a coaxial type of a hollow cylindrical tube type laminated structure illustrated inFIG. 5B, or a non-coaxial type of a hollow cylindrical tube type laminated structure illustrated inFIG. 5Cis suitable.

With regard to the hollow cylindrical tube type laminated structure, the coaxial type laminated structure refers to a structure in which the central axis C1of the section of the cylindrical ceramic composite5and the central axis C2of the section of the cylindrical light absorbing member are on the same straight line. In other words, the structure is a structure in which the central axis C1of the ceramic composite5located inside the reaction unit21overlaps with the central axis C0when the circumference of the reaction unit21is a peripheral edge.

On the other hand, the non-coaxial type laminated structure refers to a structure in which the central axis C1of the section of the cylindrical ceramic composite5and the central axis C2of the section of the cylindrical light absorbing member do not overlap (non-coaxial structure). In other words, the center axis C1of the ceramic composite body5located inside the reaction unit21exists on a line shifted from the center axis C0when the circumference of the reaction unit21is the peripheral edge.

Namely, in the non-coaxial type structure, for example, the sectional thicknesses of the light absorbing member7have a relation of t1>t2and are different as illustrated inFIG. 5C. In this case, the section of the cylindrical ceramic composite5may have a similar shape to the sectional shape of the light absorbing member7.

In the case of a flat plate type laminated structure, the structure has a shape in which the flat plate-like member for hydrogen production (ceramic composite5) is sandwiched between the light absorbing members7from the upper side and the lower side and thus the thickness of the apparatus can be reduced. This allows the weight to be reduced and thus the apparatus is suitable for installation on the roof of a house or the like.

The hollow cylindrical tube type laminated structure has a structure in which the cylindrical light absorbing member7surrounds the outside of the cylindrical ceramic composite5and thus the surface area of the light absorbing member7can be increased when the structure is formed in such a manner that a plurality of hollow cylindrical tube type reaction units21are arranged in parallel. This allows the reaction unit21having a high light absorption ratio to be completed.

With regard to the hollow cylindrical tube type laminated structure, the non-coaxial type of the hollow cylindrical tube type laminated structure may improve the light absorption rate as compared to the coaxial type of the hollow cylindrical tube type laminated structure; however, both types of structures may be used. In the case of the non-coaxial type of the hollow cylindrical tube type laminated structure, as illustrated inFIG. 5C, when the thicker section of the light absorbing member7is placed at the upper side irradiated with sunlight, the volume proportion of the light absorbing member7can be increased. This allows the light absorption amount to be increased.

At this time, when the shape of the section of the cylindrical ceramic composite5and the shape of the section of the light absorbing member7are similar and the thicker side of the ceramic composite5and the thicker side of the section of the light absorbing member7are at the same side with respect to the central axis C0, both of the amount of light absorption and the amount of generated hydrogen can be simultaneously increased in the direction where the side is irradiated with sunlight.

FIGS. 6A and 6Billustrate other aspects of the hydrogen production apparatus according to the present embodiment and schematic sectional views illustrating the constitution in which a hydrogen absorbing member is provided adjacent to the reaction unit of the hydrogen production apparatus illustrated inFIGS. 4A and 4B.

In the hydrogen production apparatus according to the present embodiment, a hydrogen absorbing member31may be provided in the reaction unit21for generating hydrogen. In this case, a hydrogen absorption alloy exemplified below may be applied as the hydrogen absorbing member31. When the hydrogen absorbing member31is located adjacent to the reaction unit21, the hydrogen gas discharged from the reaction unit21can be temporarily stored in the hydrogen absorbing member31being a solid.

In this case, the amount of hydrogen in the container22containing the reaction unit21can be temporarily reduced and thus the hydrogen generation reaction represented by Formula 3 can be moved to the right side. This allows the generation rate of hydrogen to be increased. In addition, the volume of the generated hydrogen gas can be reduced and thus the size of the container22in which the reaction unit21is contained and recovery unit25can be reduced.

In the hydrogen production apparatus, the hydrogen absorbing member31is not located in contact with the reaction unit21but may be located at a location such that there is a space33between the hydrogen absorbing member31and the reaction unit21as illustrated inFIGS. 6Aand6B. When the hydrogen absorbing member31is located at a location such that the space33is between the hydrogen absorbing member31and the reaction unit21, hydrogen generated in the reaction unit21can be temporarily stored in the space33and thus the speed at the time of absorption of hydrogen into the hydrogen absorbing member31can be controlled and the hydrogen production reaction represented by Formula 3 can be stabilized.

Examples of hydrogen absorbing alloys applicable to the hydrogen absorbing member31include alloys based on alloys of transition elements such as titanium, manganese, zirconium, and nickel, which are AB2 types, alloys based on alloys (LaNi5, ReNi5, and the like) including transition elements (nickel, cobalt, aluminum, and the like) having a catalytic effect on rare earth elements, niobium, and zirconium, which are AB5 types, Ti—Fe-based ally, V-based alloy, Mg alloy, Pd-based alloy, Ca-based alloy, and/or the like.

EXAMPLES

Hereinafter, members for hydrogen production were prepared so as to have the constitution listed in Table 1 and whether the members for hydrogen production produced hydrogen was evaluated.

In this case, a perovskite material containing La0.8Sr0.2MnO3as the main component and substituting the Mn site with 0.5 mol of Fe was used for the ceramic composite. This perovskite material was synthesized by providing each metal alkoxide, preparing these metal alkoxides so as to be the above composition, and thereafter carrying out spray thermal decomposition. Subsequently, the synthesized powder was poured into water and classification operation was carried out by checking a sedimentation state every predetermined time to give the powder of the perovskite material (ceramic particles) having an average particle diameter listed in Table 1.

Subsequently, a glass powder (borosilicate glass) was mixed with the obtained perovskite material powder to prepare a composite powder. In this case, the composition of the mixed powder was prepared in such a manner that the perovskite material powder was 70% by mass and the glass powder was 30% by mass. This proportion is a proportion in which the proportion of the perovskite material in the ceramic composite is 45% by volume.

Subsequently, 10% by mass of PVA (polyvinyl alcohol) as an organic binder was added to the obtained composite powder to prepare a molded article. After the molded article was degreased, a ceramic composite was prepared by heating the molded article in the air using an infrared image furnace under conditions of a maximum temperature of 1400° C. and a retention time of about 1 second. As Comparative Example, a sample prepared from the molded article using the perovskite material powder alone was prepared and evaluated in the same manner (Sample No. 9).

The prepared ceramic composite was polished in such a manner that the ceramic composite had a size of 10 mm×10 mm×5 mm. The section of the prepared ceramic composite was analyzed using an electron microscope and an analyzer (EPMA) provided to the electron microscope. In this case, the ceramic particles constituting the ceramic composite had little particle growth and the average particle diameter was almost equal to the values listed in Table 1. Except for Sample No. 9, it was confirmed that the ceramic particles existed in an isolated state in a ratio in the number of particles of 90% or more in the glass phase.

For the light absorbing member, a metal particle-containing composite in which about 30% by mass of tungsten particles having an average particle diameter of 40 nm were dispersed in silica glass was applied. A sample (Sample No. 8) in which a metal film (Au) was formed between layers of the ceramic composite and the light absorbing member was also prepared and evaluated in the same manner. As the reaction unit, the structure illustrated in (a) and (b) ofFIG. 2was applied.

The amount of generated hydrogen gas was measured by installing a gas chromatograph apparatus in the recovery unit of the hydrogen production apparatus. In this case, in the hydrogen production apparatus, the pressure of the reaction unit formed with the light absorbing member and the ceramic composite was reduced and then the reaction part received sunlight in a state of 1 SUN. The generated amount obtained through 10 cycles is listed in Table 1.

As is clear from the results of Table 1, it was confirmed that the amount of generated hydrogen of Sample Nos. 1 to 5, 7 and 8 had an amount of 0.5 ml/g or larger, whereas the amount of generated hydrogen was 0.01 ml/g or smaller in the case of the sample (Sample No. 6) having an average particle diameter of 280 nm or larger and the sample (Sample No. 9) prepared by sintering the ceramic particles alone.

The samples (Sample Nos. 7 and 8) provided with the light absorbing member generated a larger amount of hydrogen than the sample (Sample No. 3) prepared using the same ceramic particles.

In the hydrogen production apparatus using the ceramic composite of Sample No. 8 as the member for hydrogen production, LaNi5was attached to the reaction unit as the hydrogen absorbing member in the constitution illustrated inFIG. 6and the hydrogen production apparatus was operated. In this case, the amount of generated hydrogen was 1.5 times higher than that of the Sample No. 8.