Water decomposition device

Abstract

To provide a water decomposition apparatus in which generated hydrogen is easily collected and which can be simply and compactly constituted as a whole.SOLUTION: A water decomposition device (1) includes an electrolytic bath (5) in which an electrolytic aqueous solution (3) is accommodated, an oxygen generation electrode (9) that is a photoelectrode including an n-type semiconductor layer (7) immersed in the electrolytic aqueous solution (3) in the electrolytic bath (5), and a hydrogen generation electrode (13) including a hydrogen storage alloy layer (11) immersed in the electrolytic aqueous solution (3) in the electrolytic bath (5).SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a water splitting device that electrolyzes water using light such as sunlight. [Background technology]

[0002] In recent years, efforts to utilize hydrogen, a clean energy source, have been promoted as one solution to environmental problems. The most common method of generating hydrogen is by electrolyzing water. However, it goes without saying that electrolyzing water using electricity derived from fossil fuels such as thermal power plants does not reduce the burden on the environment, and using electricity generated by solar power, a renewable energy source, to electrolyze water is not necessarily beneficial overall in terms of energy efficiency.

[0003] Therefore, it has been proposed to produce hydrogen using a photoelectrode made of a semiconductor that can directly electrolyze water when irradiated with sunlight. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-070850 Summary of the Invention [Problem to be solved by the invention]

[0005] However, in conventional water splitting devices, oxygen gas generated from the anode (photoelectrode) and hydrogen gas generated from the cathode are each released directly into the water, so in order to separate and collect the hydrogen from the oxygen, the anode and cathode had to be positioned horizontally at a certain distance apart, making it difficult to reduce the overall size of the water splitting device or simplify the structure by stacking the electrodes.

[0006] Therefore, in order to solve the above problems, an object of the present invention is to provide a water decomposition apparatus that can easily collect generated hydrogen and can be configured in a simple and compact manner as a whole. [Means for solving the problem]

[0007] In order to achieve the above object, the water decomposition apparatus according to the present invention comprises: an electrolytic cell containing an aqueous electrolytic solution; an oxygen generating electrode which is a photoelectrode including an n-type semiconductor layer immersed in the electrolytic aqueous solution in the electrolytic cell; a hydrogen generation electrode including a hydrogen storage alloy layer immersed in the electrolytic aqueous solution in the electrolytic cell; Equipped with.

[0008] According to this configuration, hydrogen generated at the hydrogen generating electrode can be temporarily stored in the hydrogen storage alloy, which makes it easier to separate and collect hydrogen from oxygen, for example, allowing hydrogen to be collected during times when there is no sunlight. Furthermore, since the hydrogen generating electrode is provided with a hydrogen storage alloy layer, it is easier to separate hydrogen from oxygen, making it easier to shorten the distance between electrodes than before and simplify the configuration by stacking electrodes. Therefore, the entire water decomposition device can be configured simply and compactly.

[0009] In one embodiment of the present invention, the oxygen generating electrode is in the form of a flat plate or a sheet, and the oxygen generating electrode is disposed in the electrolytic cell in a substantially horizontal direction and in a position closest to other components. According to this configuration, the oxygen generating electrode, which is a photoelectrode, can be disposed at the top in the electrolytic cell and in a horizontal direction, so that the light-receiving area of ​​the oxygen generating electrode can be easily increased.

[0010] In one embodiment of the present invention, the hydrogen generating electrode may be in the form of a flat plate or a sheet, and the hydrogen generating electrode may be disposed below the oxygen generating electrode and substantially parallel to the oxygen generating electrode in the electrolytic cell. Furthermore, a flat plate or sheet-like insulating member may be interposed between the oxygen generating electrode and the hydrogen generating electrode, and the oxygen generating electrode, the insulating member, and the hydrogen generating electrode may be stacked. With this configuration, the entire electrode group can be configured compactly, resulting in a simple and compact configuration.

[0011] In one embodiment of the present invention, the hydrogen generation electrode may be replaceable. With this configuration, the hydrogen generation electrode in a state where hydrogen is stored can be removed from the water decomposition apparatus and transported to a hydrogen storage facility.

[0012] In one embodiment of the present invention, the top of the electrolytic cell may be open. With this configuration, the amount and intensity of light reaching the oxygen generating electrode can be ensured sufficiently, and oxygen gas generated at the oxygen generating electrode is released into the atmosphere, so that a decrease in the amount of received light due to bubbles adhering to the oxygen generating electrode can be suppressed.

[0013] In one embodiment of the present invention, the device may include an electricity storage element 21 having a positive electrode connected to the oxygen generating electrode and a negative electrode connected to the hydrogen generating electrode. According to this configuration, the electricity generated during water splitting can be stored in the electricity storage element 21, and then this electricity can be used to release hydrogen. [Effects of the Invention]

[0014] As described above, the water decomposition apparatus according to the present invention makes it easy to collect the generated hydrogen and allows the entire apparatus to be configured simply and compactly. [Brief explanation of the drawings]

[0015] [Figure 1] 1 is a schematic diagram showing the general configuration of a water splitting apparatus according to one embodiment of the present invention. [Figure 2] FIG. 2 is a perspective view showing an example of electrode arrangement configuration of the water splitting apparatus of FIG. [Figure 3] 1 is a schematic diagram showing the configuration of a test water splitting apparatus according to one embodiment of the present invention. [Figure 4] 4 is a graph showing test results of the water splitting apparatus of FIG. 3. DETAILED DESCRIPTION OF THE INVENTION

[0016] Hereinafter, an embodiment of the present invention will be described with reference to the drawings, but the present invention is not limited to this embodiment.

[0017] FIG. 1 shows a water splitting apparatus 1 according to one embodiment of the present invention. The water splitting apparatus 1 according to this embodiment is an apparatus that electrolyzes water using light L such as sunlight. The water splitting apparatus 1 includes an electrolytic cell 5 containing an aqueous electrolytic solution 3, an oxygen generating electrode (anode) 9 which is a photoelectrode including an n-type semiconductor layer 7, and a hydrogen generating electrode (cathode) 13 which includes a hydrogen storage alloy layer 11. The oxygen generating electrode 9 and the hydrogen generating electrode 13 are both immersed in the aqueous electrolytic solution 3 in the electrolytic cell 5.

[0018] As shown in Fig. 2, in this embodiment, both the oxygen generating electrode 9 and the hydrogen generating electrode 13 are formed in a sheet shape. The oxygen generating electrode 9 and the hydrogen generating electrode 13 are stacked with a sheet-shaped insulating member 15 interposed therebetween, and are arranged so that the entire structure is substantially parallel to the horizontal direction. The oxygen generating electrode 9, the hydrogen generating electrode 13, and the insulating member 15 are flat. The oxygen generating electrode 9, the hydrogen generating electrode 13, and the insulating member 15 may be in a plate shape. By making the oxygen generating electrode 9, the hydrogen generating electrode 13, and the insulating member 15 in a sheet or plate shape in this way, the structure can be simplified as a laminated structure as shown in the figure. However, the shapes of the oxygen generating electrode 9, the hydrogen generating electrode 13, and the insulating member 15 are not limited to this example.

[0019] Specifically, in this example, the oxygen generating electrode 9 comprises a sheet-shaped anode current collector 17 and an n-type semiconductor layer 7 attached to the anode current collector 17. The hydrogen generating electrode 13 comprises a sheet-shaped cathode current collector 19 and a hydrogen storage alloy layer 11 attached to the cathode current collector 19. In the electrolytic cell 5, the oxygen generating electrode 9 is disposed on top of these members, with the n-type semiconductor layer 7 facing upward. The hydrogen generating electrode 13 is disposed below the oxygen generating electrode 9 via an insulating member 15, with the hydrogen storage alloy layer 11 facing upward (the insulating member 15 side). In this example, the insulating member 15 is a sheet-shaped member, and the oxygen generating electrode 9 (the anode current collector 17 of the oxygen generating electrode 9) and the hydrogen generating electrode 13 (the hydrogen storage alloy layer 11 of the hydrogen generating electrode 13) are stacked in such a manner that these members are in contact with the upper and lower surfaces of the insulating member 15, respectively.

[0020] From the viewpoint of maximizing the amount of light received, the depth of the upper surface of the n-type semiconductor layer 7 of the oxygen generating electrode 9 from the water surface is preferably shallow within a range that ensures complete immersion in the electrolytic aqueous solution 3. Specifically, the depth of the upper surface of the n-type semiconductor layer 7 of the oxygen generating electrode 9 from the water surface is preferably 1 mm or more and 100 mm or less, more preferably 50 mm or less, and even more preferably 10 mm or less. However, the depth of the upper surface of the n-type semiconductor layer 7 of the oxygen generating electrode 9 from the water surface is not limited to the above range.

[0021] The n-type semiconductor of the n-type semiconductor layer 7 is not particularly limited as long as it has properties as a photoelectrode, i.e., absorbs light energy and generates carriers (electrons and holes). In this embodiment, TiO2 is used as the n-type semiconductor. Other specific examples of n-type semiconductors include, but are not limited to, tungsten oxide (WO3) and zinc oxide (ZnO).

[0022] The hydrogen storage alloy of the hydrogen storage alloy layer 11 is not particularly limited as long as it can reversibly store and release hydrogen, and for example, a material generally known as a negative electrode active material for nickel-metal hydride secondary batteries can be used. In this embodiment, LaNi5 is used. Other specific examples of hydrogen storage alloys include AB5-type alloys such as MmNi5 (Mm is misch metal), AB3-type alloys such as rare earth-magnesium-nickel alloys, A2B7-type alloys such as rare earth-magnesium-nickel alloys with a superlattice structure, and AB2-type alloys such as (Zr,Ti)Ni2. These include, but are not limited to:

[0023] In this embodiment, the electrolytic cell 5 has an open top 5a. More specifically, the electrolytic cell 5 according to this embodiment is formed in a substantially rectangular parallelepiped shape, and has only a bottom wall 5b and a side wall 5c, with no top wall. As such, the open top of the electrolytic cell 5 can ensure a sufficient amount and intensity of light L reaching the oxygen generating electrode 9. Furthermore, since oxygen gas generated at the oxygen generating electrode 9 is released into the atmosphere, a decrease in the amount of light received due to bubbles adhering to the oxygen generating electrode 9 can be suppressed.

[0024] In this embodiment, a sheet-like member made of an insulating material, for example, a nonwoven fabric made of polypropylene, is used as the insulating member 15. However, the insulating member 15 is not limited to this example, and any material and structure that can ensure electrical insulation between the oxygen generating electrode 9 and the hydrogen generating electrode 13 can be used. For example, a flat member made of porous ceramic may be used as the insulating member 15. Furthermore, when a structure is adopted that holds the oxygen generating electrode 9 and the hydrogen generating electrode 13 so as to ensure electrical insulation between them, it is not necessary to interpose the insulating member 15 therebetween.

[0025] An aqueous solution in which an electrolyte such as NaCl is dissolved in fresh water is used as the aqueous electrolytic solution 3. Instead of such an artificially prepared solution, seawater may be used as the aqueous electrolytic solution 3. When seawater is used as the aqueous electrolytic solution 3 as described above, for example, the water splitting apparatus 1 may be used to concentrate the seawater as a pre-process for salt production.

[0026] The hydrogen generating electrode 13 may be provided so as to be replaceable, that is, so as to be easily detachable. With this configuration, the hydrogen generating electrode 13 in a state in which hydrogen is stored can be removed from the water decomposition apparatus 1 and transported to a hydrogen storage facility. When the hydrogen generating electrode 13 is provided so as to be replaceable, unlike the example shown in FIG. 1, it is preferable that the cathode current collector 19 is disposed on the upper side (the insulating member 15 side) and the hydrogen storage alloy layer 11 is disposed on the lower side of the hydrogen generating electrode 13. With this configuration, the hydrogen generating electrode 13 in a state in which hydrogen is stored can be removed from the water decomposition apparatus 1 and transported to a hydrogen storage facility.

[0027] However, the collection of hydrogen stored in the hydrogen storage alloy layer 11 of the hydrogen generating electrode 13 may be performed while the hydrogen generating electrode 13 is installed in the water decomposition apparatus 1. In this case, the hydrogen generating electrode 13 may be fixed to the electrolytic cell 5 in such a manner that it cannot be easily attached or detached.

[0028] As shown in FIG. 1 , the water decomposition apparatus 1 according to this embodiment includes an electric storage element 21 having a positive electrode connected to the oxygen generating electrode 9 and a negative electrode connected to the hydrogen generating electrode 13. The electric storage element 21 may be, for example, a secondary battery such as a lithium-ion secondary battery or a nickel-metal hydride secondary battery, or a capacitor. While the provision of the electric storage element 21 is not essential, the provision of the electric storage element 21 in the water decomposition apparatus 1 enables the electric power generated during water decomposition upon receiving the energy of light L to be stored in the electric storage element 21, and then, during a time period when water decomposition is stopped (for example, during nighttime when there is no sunlight), this electric power can be used to release hydrogen from the hydrogen storage alloy layer 11. The electric power stored in the electric storage element 21 may be used as a power source for a device external to the water decomposition apparatus 1.

[0029] The present invention will be explained in more detail with reference to the following examples, but the present invention is not limited to these examples. [Example]

[0030] For testing purposes, a water splitting apparatus 1 shown in Figure 3 was fabricated. The fabrication method is described below.

[0031] (1) Preparation of n-type semiconductor layer and oxygen evolution electrode Titanium dioxide powder (80% or more rutile crystal) was thoroughly mixed with 99.7 parts by weight of this titanium dioxide powder and 0.3 parts by weight of acetylene black to produce a light gray mixed powder. The powder was heated to 1000°C in a nitrogen atmosphere, and after the temperature was raised, heat treatment was carried out for 2 hours. 2 parts by weight of acetylene black was further added to 96.0 parts by weight of the mixed powder after the heat treatment, and mixed, and then kneaded. In the apparatus, 2 parts by weight of polyvinylidene fluoride was added as a binder, and then NMP solvent was added. The mixture was thoroughly kneaded to obtain a slurry.

[0032] The obtained slurry is mixed with water to a weight of 50 to 60 mg / cm 2 The coating was applied to a 50 μm thick stainless steel (SUS304) foil, which was the current collector 17, and dried at 120° C. At this time, uncoated areas were left. Thereafter, the electrode was pressed so that the thickness of the n-type semiconductor layer 7 became 100±10 μm. The electrode was punched out so that the coated portion was 20×20 mm and the uncoated portion was 20×10 mm or more, and the resulting electrode was used as the oxygen generating electrode 9 serving as the anode.

[0033] (2) Preparation of the hydrogen storage alloy layer 11 and the hydrogen generation electrode 13 97 parts by weight of the hydrogen storage alloy LaNi5, 0.5 parts by weight of CMC, 1.5 parts by weight of Al Cetylene black and 1.0 part by weight of SBR were added, and water was further added and kneaded to form a mixture slurry. The mixture slurry was applied to the surface of a current collector 19 made of stainless steel foil (SUS304: thickness 50 μm). After drying, the layer was compressed with a roller to make the thickness of the hydrogen storage alloy layer 11 0.4 mm. The coated portion was punched out to have a size of 20×20 mm and an uncoated portion of 20×10 mm or more, and the punched portion was used as the hydrogen generation electrode 13, which served as the cathode.

[0034] (3) Electrode group configuration The insulating material 15 is made of polypropylene nonwoven fabric with a thickness of 0.5 mm and a porosity of 60% or more. The n-type semiconductor layer 7 was placed facing outward via an insulating member 15, and the hydrogen storage alloy layer 11 was layered so as to be in contact with the surface of the insulating member. The current collectors 17 and 19 were placed so as to protrude in opposite directions. A stainless steel tab with a thickness of 0.1 mm, a width of 3 mm, and a length of 20 mm was welded to the current collector. A lead wire was welded to the opposite side of the tab from the current collector.

[0035] (4) Cell preparation An acrylic container having a length of 150 mm and a depth of 50 mm was prepared as the electrolytic cell 5. As the liquid 3, 200 ml of a 3 wt % NaCl aqueous solution was added, and the n-type semiconductor layer 7 was placed in the water so that it faced upward. The electrode was immersed flat in the solution, and the surface of the n-type semiconductor layer 7 was adjusted to a depth of 1 mm from the water surface. The lead wires were positioned so as not to be immersed in the aqueous electrolytic solution 3.

[0036] (5) Measurement An overview of the measurement system is shown in Figure 3. A solar simulator was used as the light source 23, and the output was 100 mW / cm 2 After measuring the dark voltage and current for 5 minutes, light was irradiated for 30 minutes, and then the light was shut off. The voltage and current monitored during this period are shown in Figure 4. A current of 0.62 mAh was flowing in 30 minutes. This is the calculated result.

[0037] After the measurement, the hydrogen storage alloy layer 11 was taken out and placed in pure water at 50°C to capture the gas. The amount of gas captured in 10 minutes was 0.54 ml, and gas analysis confirmed that 88% of the gas was hydrogen gas.

[0038] The specific specifications of both electrodes are not limited to the examples described in the above embodiments. For example, the n-type semiconductor layer 7 of the oxygen generating electrode 9 does not need to store electric charges, so it is sufficient that it has a thickness that allows it to exhibit photoexcitation function, and it can also be produced by a thin film formation method other than coating, such as anodization of a metal plate or vapor deposition. Furthermore, the amount of hydrogen storage alloy filled in the hydrogen storage alloy layer 11 of the hydrogen generating electrode 13 is sufficient if it is an amount that can absorb the amount of hydrogen generated calculated from the expected maximum amount of sunlight in one day, but it may also be filled with an amount equivalent to several days' worth, taking into account the frequency of hydrogen gas extraction, etc.

[0039] As described above, in the water decomposition apparatus 1 according to this embodiment, the hydrogen generation electrode 13 is provided with the hydrogen storage alloy layer 11, which makes it easy to temporarily store hydrogen generated at the hydrogen generation electrode 13 in the hydrogen storage alloy layer 11 and separate it from oxygen for collection. For example, this makes it easy to collect hydrogen during times of day when there is no sunlight. This makes it easy to shorten the inter-electrode distance compared to conventional methods and simplify the configuration by stacking the electrodes. Therefore, the entire water decomposition apparatus 1 can be configured simply and compactly.

[0040] Although the preferred embodiments of the present invention have been described above with reference to the drawings, various additions, modifications, and omissions can be made without departing from the spirit of the present invention. Therefore, such additions, modifications, and omissions are also included within the scope of the present invention. [Explanation of symbols]

[0041] 1 Water splitting device 3 Electrolyte aqueous solution 5 Electrolytic cell 7 n-type semiconductor layer 9. Oxygen Evolving Electrode 11 Hydrogen storage alloy layer 13 Hydrogen evolution electrode 21 Energy storage element

Claims

1. An electrolytic cell containing an electrolytic aqueous solution, An oxygen-generating electrode, which is a photoelectrode containing an n-type semiconductor layer immersed in the electrolytic aqueous solution within the electrolytic cell, A hydrogen generation electrode comprising a hydrogen storage alloy layer immersed in the electrolytic aqueous solution within the electrolytic cell, Equipped with, The hydrogen generation electrode is provided in a replaceable manner. The oxygen generating electrode is positioned within the electrolytic cell in a substantially horizontal direction and at the uppermost position relative to the other components. The hydrogen generation electrode is positioned within the electrolytic cell below the oxygen generation electrode, substantially parallel to the oxygen generation electrode, and the cathode current collector supporting the hydrogen storage alloy layer is positioned on the upper side, with the hydrogen storage alloy layer on the lower side. Water splitting equipment.

2. In the water splitting apparatus according to claim 1, The oxygen-generating electrode and the hydrogen-generating electrode are flat plates or sheets. Water splitting equipment.

3. The water splitting apparatus according to claim 2, comprising a flat plate-shaped or sheet-shaped insulating member interposed between the oxygen generation electrode and the hydrogen generation electrode, The oxygen generating electrode, the insulating member, and the hydrogen generating electrode are stacked together. Water splitting equipment.

4. In the water splitting apparatus according to any one of claims 1 to 3, The top of the electrolytic cell is open. Water splitting equipment.

5. In the water splitting apparatus according to any one of claims 1 to 4, The energy storage element comprises a positive electrode connected to the oxygen generation electrode and a negative electrode connected to the hydrogen generation electrode. Water splitting equipment.

6. In the water splitting apparatus according to any one of claims 1 to 5, The depth from the water surface of the upper surface of the n-type semiconductor layer of the oxygen generation electrode is 1 mm or more and 100 mm or less. Water splitting equipment.

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