Photocatalytic hydrogen gas production device

Abstract

To provide a hydrogen gas production apparatus by a water decomposition reaction using a photocatalyst configured such that a light source device is arranged in water to heat water for improving hydrogen gas production efficiency and energy utilization efficiency, the apparatus being configured to avoid total reflection of light rays at an interface between a light transmitting surface and a water layer, thereby improving energy utilization efficiency contributing to the generation of hydrogen gas.SOLUTION: A hydrogen gas production apparatus 1 includes a container part 2 for receiving water W, and a light source device 3 arranged in water to emit light to induce the water decomposition reaction. The apparatus is configured such that a light transmitting surface 6, through which light of the light source device is transmitted, is covered with a photocatalyst layer 7 having a refractive index higher than the refractive index of the light transmitting surface and comprising a photocatalyst material for generating hydrogen gas by generating the water decomposition reaction that generates excited electrons and holes and decomposes water into hydrogen and oxygen when light is applied thereto, and that the photocatalyst layer is in contact with water.SELECTED DRAWING: Figure 1

Description

[Technical Field]

[0001] The present invention relates to a hydrogen gas production device, and more particularly to a device that produces hydrogen gas by a water decomposition reaction using a photocatalyst. [Background technology]

[0002] Hydrogen gas, which is expected to be used as a clean next-generation fuel that does not produce carbon dioxide when burned, can be produced by a water decomposition reaction using light energy with a photocatalyst. Therefore, various technologies for producing hydrogen gas using a photocatalyst have been proposed. For example, Patent Document 1 proposes a hydrogen generation device structure in which water containing dispersed photocatalyst particles is circulated within a housing having a light-receiving window, causing a water decomposition reaction due to light to generate hydrogen gas. Patent Document 2, filed by the same applicant as the present application, proposes a device in which a photocatalyst member carrying a photocatalyst is immersed in water within a container, and light is irradiated from a light source onto the photocatalyst member. The photocatalyst member generates excited electrons and holes when irradiated with light, causing a water decomposition reaction that decomposes water into hydrogen and oxygen, generating hydrogen gas. The device then uses the exhaust heat from the light source to heat the water in the container to be decomposed by the photocatalyst member, thereby increasing the water temperature and improving the efficiency of hydrogen gas production. [Prior art documents] [Patent documents]

[0003] [Patent Document 1] Patent Publication No. 2015-218103 [Patent Document 2] Patent Publication No. 2022-63186 [Non-patent literature]

[0004] [Non-Patent Document 1] T. Takata, et al. Nature, volume 581, pages 411-414, 2020 Summary of the Invention [Problem to be solved by the invention]

[0005] As described in Patent Document 2, using the exhaust heat of a light source to heat water to increase hydrogen gas production efficiency also improves the energy utilization efficiency related to hydrogen gas production. One such configuration is to place a light source device in water and have the water directly contact the outer surface of the light source device. In such a configuration, light is intended to be transmitted from the outer surface of the light source device into the water layer. However, if the refractive index of the outer surface of the light source device through which light passes (the light-transmitting surface) is higher than the refractive index of water, light rays whose incident angle exceeds the critical angle at the interface between the light-transmitting surface and the water layer will be totally reflected and will not penetrate into the water layer, thereby not contributing to hydrogen gas generation, thereby reducing energy utilization efficiency. In this regard, total reflection at the interface between the light-transmitting surface and the water layer can be avoided by simply making the refractive index of the light-transmitting surface lower than that of water. However, light-transmitting surfaces are generally made of transparent, hard materials such as silicone resin, epoxy resin, or glass, and therefore have a higher refractive index than water. Therefore, a new configuration is needed to avoid total reflection at the interface between the light-transmitting surface and the water layer.

[0006] Thus, the object of the present invention is to provide an apparatus for producing hydrogen gas through a water decomposition reaction using a photocatalyst, in which a light source device is placed in water to heat the water, thereby improving the efficiency of hydrogen gas production and energy efficiency, by avoiding total reflection of light rays at the interface between the light-transmitting surface and the water layer, and improving the efficiency of energy utilization that contributes to the generation of hydrogen gas. [Means for solving the problem]

[0007] According to one aspect of the present invention, the above problem is solved by a hydrogen gas production apparatus, a container portion for receiving water; a light source device that is placed in the water in the container and emits light into the water to induce a decomposition reaction of the water; Including, A device in which a light-transmitting surface of the light source device through which light passes is coated with a photocatalytic layer made of a photocatalytic substance that has a refractive index higher than that of the light-transmitting surface, and that, when irradiated with light, generates excited electrons and holes, initiates a water decomposition reaction that decomposes water into hydrogen and oxygen, and generates hydrogen gas, and the photocatalytic layer is in contact with the water. This is achieved by:

[0008] In the above configuration, the "photocatalytic substance" is a substance that, when irradiated with light (typically light with a wavelength of 365 nm or less), can induce a water decomposition reaction, reducing water to generate hydrogen gas. Specifically, it may be a photocatalytic substance available in this field, such as strontium titanate. The "light source device" may be any type that receives a supply of power and emits light that is absorbed by the photocatalytic substance to induce a water decomposition reaction. Examples of the light source include an LED, an organic electroluminescent (EL) light source, an incandescent lamp, and a xenon lamp. In the light source device, the light source is supported on a substrate, and its outer periphery is covered with a sealant or a jacket member to provide a waterproofing. The entire device is placed underwater. In particular, the portion of the sealant or jacket member through which light from the light source passes, i.e., the light-transmitting surface, is made of a transparent, hard material such as silicone resin, epoxy resin, or glass. The substrate supporting the light source is preferably made of a material with high thermal conductivity, typically a metal material such as aluminum or a glass epoxy resin. In particular, in the configuration of the present invention, the light-transmitting surface is covered with a photocatalytic layer made of the above-mentioned photocatalytic substance.

[0009] As described above, in a configuration in which the light-transmitting surface of a light source device is coated with a photocatalytic layer made of a photocatalytic material, the refractive index of the photocatalytic material (e.g., 2.4) is typically higher than the refractive index of the transparent, hard material forming the light-transmitting surface (approximately 1.4 to 1.6). Therefore, when light from the light source reaches the light-transmitting surface, it penetrates the photocatalytic layer without undergoing total reflection, excits the photocatalytic material, and generates excited electrons and holes. Furthermore, since the photocatalytic layer is in contact with water, the generated excited electrons and holes react with water, resulting in a water decomposition reaction. In this case, the exhaust heat from the light source is conducted to the water, warming it, thereby increasing the efficiency of hydrogen gas production. Furthermore, since light reaching the light-transmitting surface is irradiated onto the photocatalyst without total reflection, the light energy contributing to hydrogen gas production is increased, and energy utilization efficiency is also improved.

[0010] In the above configuration, the coating of the photocatalytic layer on the light-transmitting surface can be achieved in any manner. For example, a solution in which a powdered photocatalytic substance is dispersed in an organic solvent is applied to the light-transmitting surface and dried, resulting in the photocatalytic substance adsorbing onto the light-transmitting surface and coating the light-transmitting surface with a powder of the photocatalytic substance. Alternatively, the powdered photocatalytic substance may be dispersed in any transparent resin solution, which is applied to the light-transmitting surface and cured to form a photocatalytic layer. Photocatalytic substances are generally used by adding a co-catalyst to semiconductor particles that generate excited electrons and holes when irradiated with light, and that accept the excited electrons and holes and react with water (see, for example, Non-Patent Document 1). Therefore, the photocatalytic substance used in the photocatalyst layer may be semiconductor particles to which a co-catalyst has been added. Alternatively, a layer of semiconductor particles may be formed on the light-transmitting surface, and then a layer of the co-catalyst may be formed on the semiconductor particle layer. [Effects of the Invention]

[0011] Thus, in the present invention, in the production of hydrogen gas through a water decomposition reaction using a photocatalyst, a light source device is submerged in water, and the exhaust heat from the light source is released into the water to heat the water and improve photocatalytic efficiency. By coating the light-transmitting surface of the light source device with a photocatalyst layer having a higher refractive index than the light source, total reflection of the light from the light source at the light-transmitting surface is prevented, thereby increasing the amount of light irradiated onto the photocatalyst. This configuration reduces the amount of light trapped within the light source device and increases the light energy contributing to hydrogen gas generation in the photocatalyst layer, thereby improving energy utilization efficiency. Furthermore, when the light source of the device of the present invention is operated using electricity derived from solar energy, hydrogen energy can be efficiently obtained without emitting carbon dioxide.

[0012] Other objects and advantages of the present invention will become apparent from the following description of preferred embodiments of the invention. [Brief explanation of the drawings]

[0013] [Figure 1] FIG. 1(A) is a schematic diagram of one example of a hydrogen gas production device according to this embodiment, and FIGS. 1(B) and 1(C) are schematic cross-sectional side views of the light source device thereof. [Figure 2] Fig. 2(A) is a diagram showing a schematic diagram of the path of a light ray near the light-transmitting surface of a light source device according to this embodiment, and Fig. 2(B) is a diagram showing a schematic diagram of the path of a light ray near the light-transmitting surface of a light source device when this embodiment is not applied. [Figure 3] FIG. 3 is a diagram showing the range of light rays emitted from a light source, and explains the range of light rays that are totally reflected at a light-transmitting surface when this embodiment is not applied. [Explanation of symbols]

[0014] 1...Hydrogen gas production equipment 2...Container part 3…Light source device 4...Light source 5...Substrate 6...Printed circuit board 7…Photocatalyst layer 7a...Photocatalyst (semiconductor particle) layer 7b…promoter layer W…Water L...ray BEST MODE FOR CARRYING OUT THE INVENTION

[0015] Hydrogen gas production equipment configuration 1(A), in one aspect, the hydrogen gas production device 1 of this embodiment includes a container 2 of any shape that holds water (liquid) W, a light source device 3 that is placed in the water in the container 2 and emits light, and an air supply pipe 2a that sends the generated hydrogen gas and oxygen gas to a separator. Referring to FIG. 1(B), the light source device 3 specifically includes a light-emitting light source 4 mounted on a substrate 5, the periphery of which is covered with a translucent sealant or outer jacket member 6, and the outer surface of the translucent sealant or outer jacket member 6 is further covered with a photocatalytic layer 7 made of a photocatalytic substance.

[0016] In the above configuration, the light source 4 may be any light-emitting element that emits light that is absorbed by the photocatalytic substance (described in detail below) to initiate a water decomposition reaction. Because the light absorption rate and quantum yield of the photocatalytic substance have wavelength characteristics that increase when irradiated with light shorter than a certain wavelength, the light source 4 is selected from a light-emitting element or light-emitting element that emits light in a wavelength range where the light absorption rate and quantum yield of the photocatalytic substance increase. Specifically, various light-emitting diodes (LEDs) using indium gallium nitride (InGaN), diamond (ultraviolet), gallium nitride (GaN) / aluminum gallium nitride (AlGaN) (ultraviolet, blue), zinc selenide (blue), zinc oxide (near-ultraviolet, purple, blue), etc. may be used as the light-emitting element or light-emitting element. Note that the light source 4 may also be an organic electroluminescent (EL) element, an incandescent lamp, a xenon lamp, a mercury lamp, etc., as long as it can be placed underwater. When SrTiO3 is used as the photocatalytic substance, for example, the absorptivity and quantum yield increase when the wavelength of the irradiated light is below 380 nm, so an InGaN-based LED having a peak emission wavelength at 360 to 370 nm can be advantageously used as the light emitter of the light source device 4. The substrate 5 supporting the light source 4 may be made of any material used in this field. In particular, to facilitate the conduction of the exhaust heat 4 from the light source to water, a metal material, glass epoxy resin, ceramic, or the like with high thermal conductivity may be used.

[0017] The sealing material or outer covering member 6 that covers the light source 4 in the light source device 3 may be any material that is used to seal a light source that is hard and light-transmitting. Specific examples of the material for the sealing material or outer covering member 6 include silicone resin, epoxy resin, glass, and quartz glass.

[0018] As described above, the outer surface of the sealing material or outer jacket member 6 is coated with a layer (photocatalytic layer) 7 of a photocatalytic material that, when irradiated with light, absorbs photons to generate excited electrons and holes, thereby inducing a water decomposition reaction and reducing water to generate hydrogen gas. Examples of photocatalytic materials that can be used include SrTiO3 (strontium titanate), La2Ti2O7 (lanthanum titanate), Ga2O3 (gallium oxide), GaN (gallium nitride), NaTaO3 (sodium tantalate), TiO2 (titanium oxide), gallium carbide, and cerium carbide. These photocatalytic materials are typically prepared by adding a co-catalyst (e.g., Rh (rhodium), Cr (chromium), or Co (cobalt)) to the surface of powdered semiconductor particles (see, for example, Non-Patent Document 1). Furthermore, research by the inventors of the present invention has revealed that such photocatalytic materials have the property of being adsorbed and fixed to the surface of the sealing material or outer jacket member 6. Therefore, the photocatalyst layer 7 can be formed by dispersing a particulate photocatalytic substance in an organic solvent or the like, applying the solution to the surface of the sealing material or outer jacket member 6, drying it, and removing the solvent. Alternatively, the photocatalyst layer 7 may be formed by dispersing a photocatalytic substance in any resin (preferably one with a higher refractive index than the sealing material or outer jacket member 6), applying the dispersed photocatalytic substance to the surface of the sealing material or outer jacket member 6, and curing the resin. As shown in FIG. 1(C), a layer 7a of semiconductor particles serving as the photocatalytic substance may be formed on the surface of the sealing material or outer jacket member 6, and a layer 7b of a co-catalyst may be further formed thereon. The co-catalyst may be added to the semiconductor particles by photoelectrodeposition, as described in Non-Patent Document 1, for example. Alternatively, the photocatalyst may be dispersed in water, so that it can be excited by light transmitted through the water to initiate a water decomposition reaction and generate hydrogen gas.

[0019] The water stored in the container 2 is preferably ultrapure water from which impurities have been removed as much as possible, in order to prevent the excited electrons and holes generated in the photocatalytic substance from reacting with substances other than water molecules.

[0020] Path of light rays on a light-transmitting surface 2(B), in a configuration in which the light source device 3 is placed underwater W, if the surface of the sealant or outer member 6 through which light passes into the water W (the light-transmitting surface) does not have a photocatalytic layer and the light-transmitting surface is in direct contact with the water layer, the refractive index ne (approximately 1.5 to 1.6) of the light-transmitting surface (sealant or outer member 6) is typically greater than the refractive index nw (1.33) of water. Therefore, at the interface between the light-transmitting surface and the water layer, light ray L with an incident angle θ greater than the critical angle is totally reflected, does not enter the water layer, and is trapped inside the light-transmitting surface, where it does not contribute to the generation of hydrogen gas. In other words, in the configuration of FIG. 2(B), a large amount of light energy is lost without contributing to the generation of hydrogen gas.

[0021] Therefore, in this embodiment, as described with reference to FIG. 1 , the light-transmitting surface is coated with a photocatalytic layer 7. The refractive index nc of the material used for the photocatalytic layer 7 is, for example, approximately 2.4 (in the case of strontium titanate), which is typically higher than the refractive index ne of the inner surface of the light-transmitting surface. Therefore, as shown in FIG. 2(A), the light ray L from the light source 4 is refracted toward the photocatalytic layer 7 at the interface between the light-transmitting surface and the photocatalytic layer 7, and is not totally reflected. Therefore, a large amount of light is transmitted to the photocatalytic layer 7. Furthermore, since the photocatalytic layer 7 is in direct contact with the water layer W, excited electrons and holes generated by irradiation of the light ray L in the photocatalytic layer 7 react with water molecules to generate hydrogen gas and oxygen gas. However, since the amount of light entering the photocatalytic layer 7 is increased as described above, a larger amount of hydrogen gas is expected to be generated. That is, according to the configuration of this embodiment, an increase in the proportion of energy contributing to hydrogen gas generation among the light energy emitted by the light source 4 is expected. Specifically, referring to Figure 3, for example, when the directional angle θd of the light ray emitted from the light source 4 is 120°, if a water layer is in contact with the light-transmitting surface, the critical angle θc is approximately 55°, and therefore light rays in the range of θe (= θd - θc - 90°) cannot enter the water layer due to total reflection.However, in the case of this embodiment, such totally reflected light rays enter the photocatalytic layer 7 and contribute to the generation of hydrogen gas, and it has been found that the energy utilization efficiency is improved by approximately 10%.

[0022] Thus, in this embodiment, in a hydrogen gas production device configuration in which the light source device is submerged in water and the exhaust heat from the light source is released into the water, thereby raising the temperature of the water and improving photocatalytic efficiency, the light-transmitting surface of the light source device is covered with a photocatalytic layer having a higher refractive index, thereby preventing the light rays from the light source from being totally reflected at the light-transmitting surface, increasing the amount of light irradiated onto the photocatalyst and increasing the light energy that contributes to the generation of hydrogen gas.

[0023] The above description has been made in relation to the embodiments of the present invention, but it will be apparent that many modifications and changes will be readily apparent to those skilled in the art, and the present invention is not limited to the above-exemplified embodiments, but can be applied to various devices without departing from the concept of the present invention.

Claims

[Claim 1] A hydrogen gas production apparatus, a container portion for receiving water; a light source device that is placed in the water in the container and emits light into the water to induce a decomposition reaction of the water; Including, A device in which a light-transmitting surface of the light source device through which light passes is coated with a photocatalytic layer made of a photocatalytic material that has a refractive index higher than the refractive index of the light-transmitting surface and that, when irradiated with light, generates excited electrons and holes, causing a water decomposition reaction that decomposes water into hydrogen and oxygen, thereby generating hydrogen gas, and the photocatalytic layer is in contact with the water.

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