Hydrogen gas production device using photocatalysis

Abstract

To provide a hydrogen gas production apparatus by a water decomposition reaction using a photocatalyst system, which is configured such that hydrogen gas and oxygen gas are produced in separate water tanks to reduce the restriction of the arrangement place of the water tanks and to increase the amount of hydrogen gas produced per hour with high energy efficiency.SOLUTION: A hydrogen gas production apparatus 1 comprises a water tank 3H in which a hydrogen-side photocatalyst 4H is immersed in water in which a mediator is dispersed, and hydrogen gas is generated and the mediator is oxidized by light irradiation; a light irradiation means 5H for irradiating the hydrogen-side photocatalyst with light; a water tank 3O in which an oxygen-side photocatalyst 4O is immersed in water in which the mediator is dispersed, and oxygen gas is generated and the mediator is reduced by light irradiation; a light irradiation means 5O for irradiating the oxygen-side photocatalyst with light; and water circulation means 10H, 10O for circulating water in which the mediator is dispersed between the hydrogen-side and oxygen-side water tanks, and light sources of the light irradiation means are light emitting diodes 6H, 6O.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 in which photocatalyst particles are dispersed is circulated within a housing having a light-receiving window, causing a water decomposition reaction due to light to generate hydrogen gas together with oxygen 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 onto the photocatalyst member from an electrically powered light source. The device proposes improving the efficiency of hydrogen gas production by heating the water in the container to be decomposed by the photocatalyst member using exhaust heat from the light source to increase the water temperature.

[0003] Furthermore, with regard to photocatalysts that induce water splitting reactions when irradiated with light as described above, various photocatalytic system (Z scheme) configurations have been proposed that use a photocatalyst (hydrogen-side photocatalyst) that exclusively reduces hydrogen ions (protons) to produce hydrogen gas, a photocatalyst (oxygen-side photocatalyst) that exclusively oxidizes water molecules to produce oxygen gas and protons, and a redox substance (mediator substance) that transfers electrons from the oxygen-side photocatalyst to the hydrogen-side photocatalyst (Patent Document 3, Non-Patent Document 1). Non-Patent Document 2 discloses an apparatus in which a substrate (hydrogen-side photocatalyst plate) with a hydrogen-side photocatalyst attached thereto and a substrate (oxygen-side photocatalyst plate) with an oxygen-side photocatalyst attached thereto are placed in separate water tanks filled with an aqueous solution in which a mediator substance is dispersed, and the two water tanks are arranged side by side so that sunlight passes through the oxygen-side photocatalyst plate and the hydrogen-side photocatalyst plate in turn, and the aqueous solution in the two water tanks is circulated, thereby generating oxygen gas in the water tank of the oxygen-side photocatalyst plate and hydrogen gas in the water tank of the hydrogen-side photocatalyst plate. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2015-218103 [Patent Document 2] Patent Publication No. 2022-63186 [Patent Document 3] International Publication No. 2022 / 045283 [Non-patent literature]

[0005] [Non-Patent Document 1] “Prospects and challenges in designing photocatalytic particle suspension reactors for solar fuel processing”, Swarnava Nandy, Sangram Ashok Savant and Sophia Haussener, Chem. Sci., 2021, 12, 9866-9884 [Non-patent document 2] “Designing a Z-scheme system based on photocatalyst panels towards separated hydrogen and oxygen production from overall water splitting”, Chaoyi Dong, Yue Zhao, Yanpei Luo, Hong Wang, Hefeng Zhang, Xu Zong, Zhaochi Feng and Can Li, Catal. Sci. Technol., 2022, 12, 572-578 Summary of the Invention [Problem to be solved by the invention]

[0006] When a single photocatalyst catalyzes the reduction of protons and the oxidation of water molecules (a one-step photocatalytic system), the band gap between the valence band and conduction band of the photocatalyst must be large enough to span the reduction potential of protons (the redox potential in the redox reaction system between protons and hydrogen molecules) and the oxidation potential of water molecules (the redox potential in the redox reaction system between water and oxygen molecules).On the other hand, in the case of a Z-scheme photocatalytic system such as the one described above, the band gap between the valence band and conduction band of the hydrogen-side photocatalyst must span the reduction potential of protons and the redox potential of the mediator substance, and the band gap between the valence band and conduction band of the oxygen-side photocatalyst must span the redox potential of the mediator substance and the oxidation potential of water molecules. Therefore, the Z-scheme photocatalytic system is advantageous in that a wider variety of substances can be used as photocatalysts. (However, as long as the hydrogen-side photocatalyst contains a co-catalyst that selectively promotes the reduction reaction of protons and the oxygen-side photocatalyst contains a co-catalyst that selectively promotes the oxidation reaction of water, the band gap between the valence band and the conduction band of each photocatalyst may span the proton reduction potential and the water molecule oxidation potential.) Furthermore, in a configuration in which the hydrogen-side photocatalyst and the oxygen-side photocatalyst are placed in separate water tanks, if the respective water tanks are configured to generate substantially only hydrogen gas or substantially only oxygen gas, the generated hydrogen gas and oxygen gas can be recovered separately, which is advantageous in that a separator for separating hydrogen gas from oxygen gas is not required. (Note that a separator may be used to increase the purity of the hydrogen gas in the generated gas.)

[0007] While sunlight can be used as light to irradiate a photocatalyst to induce a water splitting reaction, its light density is low and the amount of hydrogen gas produced per unit time is relatively small. Furthermore, in the case of sunlight, the proportion of the light energy irradiated onto the photocatalyst that contributes to the water splitting reaction is not very high. More specifically, components of the irradiated light whose photon energy does not reach the band gap of the photocatalyst are not absorbed by the photocatalyst and do not contribute to the water splitting reaction. Components of the irradiated light whose photon energy reaches the band gap of the photocatalyst are absorbed by the photocatalyst and excite electrons in the valence band to the conduction band. However, the portion of the photon energy that exceeds the band gap of the photocatalyst is converted into heat (thermal relaxation) and does not contribute to the water splitting reaction. In other words, in sunlight, which has a wide wavelength range, only a portion of the energy is absorbed by the photocatalyst and contributes to the water splitting reaction. Furthermore, when sunlight is used as the light to irradiate the photocatalyst, the location of the tank containing the photocatalyst is limited to places where a light path to guide sunlight to the photocatalyst can be installed, and the amount of hydrogen gas produced is greatly affected by weather conditions.

[0008] On the other hand, when light is irradiated onto the photocatalyst using an electrically powered light-emitting device, it is expected that a greater amount of hydrogen gas can be produced per hour than with sunlight, it is less susceptible to weather conditions, and there are fewer restrictions on the location of the water tank in which the photocatalyst is immersed. Also, as mentioned above, the photon energy required for the irradiated light only needs to reach the energy width of the band gap of the photocatalyst, and energy exceeding this is not required in excess. Therefore, if the wavelength of the irradiated light from the light-emitting device is adjusted to be as long as possible and not longer than the wavelength corresponding to the energy width of the band gap of the photocatalyst, it is possible to more efficiently contribute the energy of the irradiated light to the water decomposition reaction. In this regard, in a configuration using a Z-scheme photocatalytic system, the band gap of the photocatalyst does not need to be large enough to span the proton reduction potential and the water molecule oxidation potential. Therefore, by using appropriate materials as the hydrogen-side photocatalyst and the oxygen-side photocatalyst, the photon energy of the light irradiating the photocatalyst can be lowered compared to a one-stage photocatalytic system. This has the advantage of broadening the range of light source options for the light-emitting device that emits the irradiated light. Specifically, light-emitting diodes (LEDs) (including semiconductor lasers using LEDs) can be used as light sources, as they have high luminous energy efficiency, are easy to handle during installation, and emit highly monochromatic light. In particular, in the case of LEDs, as described below, the longer the wavelength of light, the higher the luminous efficiency per unit of power consumption. Therefore, the smaller the band gap of the selected photocatalyst, the longer the luminous efficiency and the longer the wavelength of the irradiated light can be selected, thereby increasing the amount of hydrogen gas produced per unit of power consumption.

[0009] Thus, one object of the present invention is to provide an apparatus for producing hydrogen gas through a water decomposition reaction using a Z-scheme photocatalytic system, in which the hydrogen-side photocatalyst and the oxygen-side photocatalyst are placed in separate water tanks so that hydrogen gas and oxygen gas are produced in separate water tanks, by employing an LED as the light source for the light-emitting device, minimizing restrictions on the placement of the water tanks, and by providing a configuration that maximizes the amount of hydrogen gas produced per hour with as high energy efficiency as possible. [Means for solving the problem]

[0010] According to one aspect of the present invention, the above problem is solved by A hydrogen gas production apparatus, As a means of generating hydrogen gas, a hydrogen-side water tank in which a hydrogen-side photocatalytic member carrying a hydrogen-side photocatalyst is immersed in water in which a mediator substance is dispersed; a hydrogen-side light irradiating means for irradiating the hydrogen-side photocatalytic member with light having a wavelength that is absorbed by the hydrogen-side photocatalyst and excites electrons in the valence band to the conduction band; and a hydrogen gas recovery means for recovering the hydrogen gas generated in the hydrogen-side water tank. a hydrogen gas generating means configured such that, when the hydrogen-side photocatalyst is irradiated with light from the hydrogen-side light irradiating means, protons in the water in the hydrogen-side water tank are reduced to generate the hydrogen gas and the reduced mediator substance is oxidized; As an oxygen gas generating means, an oxygen-side water tank in which an oxygen-side photocatalytic member carrying an oxygen-side photocatalyst is immersed in water in which the mediator substance is dispersed; an oxygen-side light irradiating means for irradiating the oxygen-side photocatalyst member with light of a wavelength that is absorbed by the oxygen-side photocatalyst and excites electrons in the valence band to the conduction band, an oxygen gas generating means configured such that, when the oxygen-side photocatalyst is irradiated with light from the oxygen-side light irradiating means, water molecules in the water in the oxygen-side water tank are oxidized to generate oxygen gas, and the oxidized mediator substance is reduced; a water circulating means for circulating the water in which the mediator substance is dispersed between the hydrogen-side water tank and the oxygen-side water tank; Including, The light source of the light emitting device of the hydrogen-side light irradiating means and the light source of the light emitting device of the oxygen-side light irradiating means are light emitting diodes. This is achieved by:

[0011] In the above configuration, The "hydrogen-side photocatalyst," "oxygen-side photocatalyst," and "mediator substance" are, respectively, the hydrogen-side photocatalyst, oxygen-side photocatalyst, and mediator substance used in the so-called "Z-scheme" photocatalytic system, as already described. The mediator substance must have a redox potential between the reduction potential of protons and the oxidation potential of water; the hydrogen-side photocatalyst must have a band gap spanning the reduction potential of protons and the redox potential of the mediator substance; and the oxygen-side photocatalyst must have a band gap spanning the redox potential of the mediator substance and the oxidation potential of water.

[0012] The "hydrogen-side photocatalytic member" and the "oxygen-side photocatalytic member" may be members obtained by attaching the hydrogen-side photocatalyst or the oxygen-side photocatalyst to the surface of a member of any shape, such as a plate-like member made of glass, resin, etc., or by solidifying the hydrogen-side photocatalyst or the oxygen-side photocatalyst itself into any shape, such as a plate.

[0013] The "hydrogen-side water tank" and the "oxygen-side water tank" may each be a water tank configured to store water in which a mediator substance is dispersed and to hold the hydrogen-side photocatalytic member and the oxygen-side photocatalytic member immersed in water in any manner.

[0014] The "hydrogen-side light irradiation means" and "oxygen-side light irradiation means" may each be any type of device including a light source device and an optical system that generates light that excites electrons in the valence band of the hydrogen-side photocatalyst and the oxygen-side photocatalyst to the conduction band and irradiates the hydrogen-side photocatalytic member and the oxygen-side photocatalytic member with the light. In particular, in the case of the present invention, a light-emitting diode that is driven by electricity and has high monochromaticity is used as the light source that generates the irradiated light. Note that the light-emitting diode may include not only a diode element that emits light by spontaneous emission, but also a semiconductor laser that emits light by laser oscillation.

[0015] The "water circulation means" is a means for circulating water so as to exchange the water stored in the hydrogen-side water tank and the oxygen-side water tank in an arbitrary manner. Specifically, the "water circulation means" may be a device configured to circulate water between the hydrogen-side water tank and the oxygen-side water tank using a pump or the like through a water supply pipe connected between the two tanks, but is not limited to this.

[0016] In the operation of the above-described device, first, in the "hydrogen gas generating means" including the "hydrogen-side photocatalytic member," "hydrogen-side water tank," "hydrogen-side light irradiating means," and "hydrogen gas recovery means," light is irradiated from the hydrogen-side light irradiating means onto the hydrogen-side photocatalyst, causing two reactions: one in which protons in the water are reduced by excited electrons in the hydrogen-side photocatalyst to produce hydrogen gas, and the other in which the reduced mediator substance is oxidized by holes (positive holes) generated in the hydrogen-side photocatalyst. Meanwhile, in the "oxygen gas generating means" including the "oxygen-side photocatalytic member," "oxygen-side water tank," and "oxygen-side light irradiating means" (means for recovering oxygen gas may or may not be provided), light is irradiated from the oxygen-side light irradiating means onto the oxygen-side photocatalytic member, causing two reactions: one in which water molecules in the water are oxidized by holes generated in the oxygen-side photocatalyst to produce oxygen gas (and protons), and the other in which the oxidized mediator substance is reduced by excited electrons in the oxygen-side photocatalyst. Furthermore, by circulating water between the hydrogen-side water tank and the oxygen-side water tank using the water circulation means, the oxidized form of the mediator substance is sent from the hydrogen-side water tank to the oxygen-side water tank, and the reduced form of the mediator substance and protons are sent from the oxygen-side water tank to the hydrogen-side water tank. Therefore, the hydrogen-side photocatalyst, oxygen-side photocatalyst, mediator substance, and the irradiated light from the hydrogen-side light irradiating means and the irradiated light from the oxygen-side light irradiating means are each selected so that the above series of reactions are established. If necessary, the hydrogen-side photocatalyst may use a co-catalyst to promote the reduction reaction of protons (over the oxidation reaction of water molecules and the reduction reaction of the oxidized form of the mediator substance), and this case is also within the scope of the present invention. If necessary, the oxygen-side photocatalyst may also use a co-catalyst to promote the oxidation reaction of water (over the reduction reaction of protons and the oxidation reaction of the reduced form of the mediator substance), and this case is also within the scope of the present invention.

[0017] The device of the present invention uses a so-called "Z-scheme" photocatalytic system for the water splitting reaction. The hydrogen-side photocatalyst, which induces the reaction that generates hydrogen gas, and the oxygen-side photocatalyst, which induces the reaction that generates oxygen gas, are placed in separate water tanks. This separates the water tank that primarily produces hydrogen gas from the water tank that primarily produces oxygen gas, making selective recovery of hydrogen gas easier. In this configuration, a highly monochromatic light-emitting diode is used as the light source that emits light to irradiate the hydrogen-side and oxygen-side photocatalysts. As mentioned above, unlike sunlight, this configuration allows the light-emitting diode's emission wavelength to be selected to be as long as possible, but not exceeding the bandgap of the photocatalyst used. This facilitates improving the energy utilization efficiency of the irradiated light in hydrogen gas generation. Furthermore, when the light source is a light-emitting diode, the intensity of the irradiated light can be easily increased at a wavelength that contributes to the water splitting reaction or at a wavelength selected to increase energy utilization efficiency, which is expected to facilitate increasing the amount of hydrogen gas produced per hour. Furthermore, since the light source is a light-emitting diode, the device of the present invention can be installed anywhere as long as an aquarium can be placed, which means there are fewer restrictions on where it can be installed, and it is also advantageous in that the amount of hydrogen gas produced is less affected by weather conditions.

[0018] In the above-described configuration of the device of the present invention, it is preferable that the light emitted from the hydrogen-side light irradiating means and the light emitted from the oxygen-side light irradiating means are directly irradiated onto the hydrogen-side photocatalyst and the oxygen-side photocatalyst, respectively, so that the above-described reactions at each photocatalyst occur more reliably and efficiently. In this regard, the light sources of the hydrogen-side light irradiating means and the oxygen-side light irradiating means are light-emitting diodes, which can be relatively easily positioned and can be easily installed at any location. Therefore, the device of the present invention is also advantageous in that the light paths from the light sources of the hydrogen-side light irradiating means and the oxygen-side photocatalyst members to the hydrogen-side photocatalyst member and the oxygen-side photocatalyst member can be easily designed so that the light directly hits the hydrogen-side photocatalyst and the oxygen-side photocatalyst. Furthermore, if the emission wavelength of the light source is equal to or shorter than the wavelength corresponding to the larger energy width of the band gap of the hydrogen-side photocatalyst and the band gap of the oxygen-side photocatalyst, both photocatalysts can be excited by light from the same light source. Therefore, the light source of the light-emitting device of the hydrogen-side light irradiation means and the light source of the light-emitting device of the oxygen-side light irradiation means may be the same as long as the condition that the wavelength is equal to or shorter than the larger energy width of the band gap of the hydrogen-side photocatalyst and the band gap of the oxygen-side photocatalyst is satisfied. In this case, it is advantageous because there is no need to prepare two types of light source. In this case, the light source of the hydrogen-side light irradiation means and the light source of the oxygen-side light irradiation means may be a single common light source (or they may be separate light sources of the same type).

[0019] As already mentioned, in the case of the Z-scheme photocatalytic system employed in the device of the present invention, the band gap of the hydrogen-side photocatalyst needs only to span the reduction potential of protons and the redox potential of the mediator substance, and is preferably narrower than the energy gap between the reduction potential of protons and the oxidation potential of water molecules. In this configuration, the potential of the valence band peak (the boundary between the band gap and the valence band) of the hydrogen-side photocatalyst is more negative than the oxidation potential of water molecules, so the oxidation reaction of water molecules does not substantially occur, and the gas generated in the hydrogen-side water tank is essentially hydrogen gas alone. This means that high-concentration hydrogen gas can be recovered without passing the generated gas from the hydrogen-side water tank through a separator that separates hydrogen gas and oxygen gas. Note that the gas recovered from the water tank may be passed through a hydrogen gas separator to remove water vapor and other air components. Even in this case, the high purity of hydrogen gas in the gas recovered from the water tank is advantageous in that the purity of the separation process is further increased or the requirements for separation capacity are reduced.

[0020] Furthermore, if the band gap of the hydrogen-side photocatalyst is narrower than the energy difference between the reduction potential of protons and the oxidation potential of water molecules, the emission wavelength of the light-emitting diode of the hydrogen-side light irradiation means may be equal to or shorter than the light wavelength corresponding to the energy difference between the band gap of the hydrogen-side photocatalyst and longer than the light wavelength corresponding to the energy difference between the reduction potential of protons and the oxidation potential of water molecules. This configuration allows the photon energy of the light irradiating the hydrogen-side photocatalyst to be lower than the energy difference between the reduction potential of protons and the oxidation potential of water molecules, which is advantageous in that it increases the number of light-emitting diodes that can be used. In this regard, as already mentioned, the luminous efficiency per watt of a light-emitting diode generally increases with the emission wavelength. Therefore, the use of light-emitting diodes with longer wavelengths increases the light intensity per watt, thereby improving the energy efficiency of hydrogen gas production in the photocatalyst.

[0021] In the above configuration, as with the hydrogen-side photocatalyst, the band gap of the oxygen-side photocatalyst needs only to span the oxidation potential of water molecules and the redox potential of the mediator substance, and is preferably narrower than the energy gap between the reduction potential of protons and the oxidation potential of water molecules. This makes it possible to make the potential of the bottom of the conduction band of the oxygen-side photocatalyst (the boundary between the conduction band and the band gap) more positive than the reduction potential of protons, thereby preventing substantial hydrogen gas generation in the oxygen-side water tank. Furthermore, the light-emitting diode of the oxygen-side light irradiation means may have an emission wavelength equal to or shorter than the light wavelength corresponding to the energy gap of the band gap of the oxygen-side photocatalyst and longer than the light wavelength corresponding to the energy gap between the reduction potential of protons and the oxidation potential of water molecules. This increases the variety of light-emitting diodes available, increases the light intensity per unit of power, and reduces the energy consumed for hydrogen gas generation in the photocatalyst.

[0022] Furthermore, in the above-described configuration of the present invention, it is preferable that the potential of the top of the valence band of the hydrogen-side photocatalyst is more negative than the oxidation potential of water molecules, and the potential of the bottom of the conduction band of the oxygen-side photocatalyst is more positive than the reduction potential of protons. With such a configuration, the gas produced by the water decomposition reaction can theoretically be only hydrogen gas in the hydrogen-side water tank and only oxygen gas in the oxygen-side water tank. This is advantageous in that it is expected to increase the yield of hydrogen gas in the hydrogen-side water tank and to recover high-purity hydrogen gas without using a separator. The gas produced in the hydrogen-side water tank may be passed through a separator to remove impurities other than hydrogen gas. In this case, the purity of the hydrogen gas before passing through the separator is expected to be high, which is advantageous in that the purity of the separation process is further increased or the requirements for separation capacity are reduced.

[0023] As noted in Patent Document 1, it has been found that an increase in water temperature promotes the water decomposition reaction and increases the yield of hydrogen gas. Therefore, the above-described configuration of the present invention may be provided with a means for heating the water in each tank using exhaust heat (heat emitted by the light-emitting diodes when they emit light) from the light-emitting devices of the hydrogen-side light irradiation means and the oxygen-side light irradiation means. This configuration is expected to promote the water decomposition reaction and recover hydrogen gas with higher energy efficiency without using a separate means for heating water. [Effects of the Invention]

[0024] Thus, the above-described apparatus for producing hydrogen gas by the water decomposition reaction induced by light irradiation of the present invention employs a Z-scheme photocatalytic system, uses light-emitting diodes as light sources for irradiating the hydrogen-side photocatalyst and the oxygen-side photocatalyst, respectively, and places the hydrogen-side photocatalyst and the oxygen-side photocatalyst in separate water tanks, separating the water tank that primarily produces hydrogen gas from the water tank that primarily produces oxygen gas. This facilitates selective recovery of hydrogen gas, facilitates selection of the type of material usable as a photocatalyst and the type of light-emitting diode usable as the light source for irradiation, improves energy utilization efficiency, and minimizes restrictions on the location of the apparatus, allowing it to be installed anywhere. This apparatus configuration of the present invention is expected to make it easier to use hydrogen gas production apparatuses that use photocatalytic water decomposition reactions. Furthermore, using a solar power generation system or other renewable energy-based power generation system as the power source for the light-emitting diodes is expected to enable more efficient hydrogen gas production while reducing CO2 emissions.

[0025] 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]

[0026] [Figure 1] FIG. 1 is a schematic diagram (cross-sectional view) of a hydrogen gas production device according to this embodiment. [Figure 2] FIG. 2 is a diagram illustrating the reaction occurring in the photocatalyst in the hydrogen gas production device according to this embodiment. [Figure 3] Figure 3 is a diagram illustrating the relationship between the potentials of the bottom CBM of the conduction band and the top VBM of the valence band of the hydrogen-side photocatalyst HEP and oxygen-side photocatalyst OEP, which can be selected in the hydrogen gas production device according to this embodiment, and the reduction potential VH of protons, the oxidation potential VO of water molecules, and the redox potential VM of the mediator substance. [Figure 4] Figure 4 is a graph showing the conversion efficiency from input electrical energy to light energy in various light-emitting diodes. The notations in the figure marked with a black circle indicate the type and efficiency of the LED. [Figure 5] FIG. 5 is a schematic diagram (cross-sectional view) of the hydrogen gas production device according to this embodiment, in which a common light source is used on the hydrogen side and the oxygen side. [Explanation of symbols]

[0027] 1...hydrogen gas production device, 2H,O...hydrogen side device, oxygen side device, 3H,O...hydrogen side water tank, oxygen side water tank, 4H,O...hydrogen side photocatalytic member, oxygen side photocatalytic member, 4Ha,Oa...hydrogen side co-catalyst, oxygen side co-catalyst, 5H,O...hydrogen side light-emitting device, oxygen side light-emitting device, 6H,O...hydrogen side LED, oxygen side LED, 7H,O...hydrogen side heat exchanger, oxygen side heat exchanger, 8H,O...hydrogen side heat exchanger pump, oxygen side heat exchanger pump, 9H,O...hydrogen side produced gas recovery pipe, oxygen side produced gas recovery pipe, 10H,O...hydrogen side water circulation device, oxygen side water circulation device, 11H,O...hydrogen side power transmission line, oxygen side power transmission line, 12...battery, 13...solar power generation system, w...water, Lh,Lo...hydrogen side irradiation light, oxygen side irradiation light BEST MODE FOR CARRYING OUT THE INVENTION

[0028] Hydrogen gas production equipment configuration As shown in Fig. 1, in one aspect, the hydrogen gas production apparatus 1 of this embodiment is composed of a hydrogen-side device 2H that exclusively or mainly produces hydrogen gas and an oxygen-side device 2O that exclusively or mainly produces oxygen gas. The hydrogen-side device 2H and the oxygen-side device 2O each have a hydrogen-side water tank 3H and an oxygen-side water tank 3O of any shape, respectively, and a hydrogen-side photocatalytic member 4H and an oxygen-side photocatalytic member 4O are disposed in the water tanks 3H and 3O, respectively, immersed in water (liquid) w stored therein. As described below, a reversibly oxidized or reduced mediator substance is dispersed or dissolved in the water w to transfer electrons from the oxygen-side water tank 3O to the hydrogen-side water tank 3H during the hydrogen gas production process. The hydrogen-side photocatalytic member 4H and the oxygen-side photocatalytic member 4O may be formed by adhering or coating the hydrogen-side photocatalyst and oxygen-side photocatalyst, which will be described later, onto the surface of a member of any shape made of glass, resin, or the like, or by solidifying the hydrogen-side photocatalyst and oxygen-side photocatalyst themselves into any shape (the hydrogen-side photocatalyst member 4H and the oxygen-side photocatalyst member 4O are plate-shaped members extending in a direction perpendicular to the paper surface, but they may also have any other shape or be fixed to the inner wall of the water tank.) Then, to irradiate the hydrogen-side photocatalytic member 4H and the oxygen-side photocatalyst member 4O with light Lh and Lo, a hydrogen-side light-emitting device 5H and an oxygen-side light-emitting device 5O, each of which is composed of a plurality of LEDs 6H and 6O as light-emitting sources, are arranged outside or inside the water tanks 3H and 3O. Furthermore, as will be explained later, the water tanks 3H, 3O are provided with mechanisms 10H, 10O for circulating the water w in the water tanks 3H, 3O to transfer the mediator substance oxidized on the hydrogen-side photocatalytic member 4H to the oxygen-side photocatalytic member 4O and to transfer the mediator substance reduced on the oxygen-side photocatalytic member 4O and protons generated by the water oxidation reaction to the hydrogen-side photocatalytic member 4H. Specifically, such mechanisms may be achieved, for example, by a configuration in which the water tanks 3H, 3O are connected to each other near their bottoms by a pipe, and water from one tank is forcibly sent to the other tank through the connecting pipe by a pump P or the like.

[0029] In the above configuration, the power driving the LEDs 6H, 6O of the hydrogen-side light-emitting device 5H and the oxygen-side light-emitting device 5O may be power obtained in any manner. This power may preferably be provided by solar energy generated by a solar panel or other renewable energy. To this end, the light-emitting devices 5H, 5O may be configured to receive power via power transmission lines 11H, 11O from a battery 12 that is charged with power generated by a renewable energy power generation source such as a solar panel 13 (the power transmission lines 11H, 11O may also be directly connected to the power generation source 13 to supply power to the light-emitting devices). In this regard, by using LEDs 6H and 6O to irradiate the photocatalytic members 4H and 4O, the wavelength of the irradiated light can be selected so that it is absorbable by the photocatalyst and minimizes the energy loss due to thermal relaxation after light absorption. This improves the energy utilization efficiency of the irradiated light in the water decomposition reaction. Increasing the light intensity at such wavelengths increases the amount of hydrogen gas generated per unit time. Furthermore, the design of the LEDs 6H and 6O placement is easier. Furthermore, unlike when sunlight is used as the irradiated light, using LEDs 6H and 6O to irradiate the photocatalytic members 4H and 4O has the advantage that there are fewer restrictions on the placement of the hydrogen gas production device 1 (it can be placed in a location that is not exposed to sunlight), and the amount of hydrogen gas produced is not affected by weather conditions. The water tanks 3H and 3O do not need to be placed side by side as shown in the figure, and they may be placed anywhere, indoors or outdoors.

[0030] The photocatalytic water decomposition reaction is accelerated by an increase in water temperature (see Patent Document 1). Therefore, heat exchangers 7H, 7O may be provided to exchange heat between the water and the light-emitting devices 5H, 5O, so that the water in the water tanks 3H, 3O can be heated using exhaust heat from the hydrogen-side light-emitting devices 5H and oxygen-side light-emitting devices 5O. In the heat exchangers 7H, 7O, pumps 8H, 8O may be used to draw the water in the water tanks 3H, 3O near the hydrogen-side light-emitting devices 5H and oxygen-side light-emitting devices 5O, and the exhaust heat from the LEDs 6H, 6O may be conducted to the water. This configuration eliminates the need for a separate heat source to heat the water in the water tanks, and the exhaust heat from the LEDs 6H, 6O contributes to the water decomposition reaction, thereby improving energy efficiency.

[0031] The gas generated in the hydrogen-side water tank 3H and the gas generated in the oxygen-side water tank 3O are recovered through the produced gas recovery pipes 9H and 9O, respectively. In the apparatus of this embodiment, the gas generated in the hydrogen-side water tank 3H is basically hydrogen gas H2, so if the purity of the hydrogen gas H2 in the gas recovered in the recovery pipe 9H is sufficient, a separator for separating the hydrogen gas is not required. However, since impurities such as water vapor, oxygen gas, and other gases may be contained, any separation process may be performed to increase the purity of the hydrogen gas.

[0032] Hydrogen gas production reaction process in this embodiment As already mentioned, the hydrogen gas production device of this embodiment is configured to generate hydrogen gas by inducing a water decomposition reaction using a so-called "Z scheme" photocatalytic system, and is configured with separate water tanks for inducing a proton reduction reaction and a water tank for inducing a water oxidation reaction.

[0033] Specifically, as shown in Fig. 2, in this embodiment, first, the hydrogen-side photocatalytic member 4H is placed in the hydrogen-side water tank 3H, and the oxygen-side photocatalytic member 4O is placed in the oxygen-side water tank 3O. In addition, a mediator substance (M / M) for transporting electrons from the oxygen-side photocatalytic member 4O to the hydrogen-side photocatalytic member 4H is placed in the water. +As the mediator substance, a redox compound that reversibly accepts or releases electrons to become a reduced form or an oxidized form, as listed below, is used.

[0034] In the reaction that occurs, first, when a photon h / λ is absorbed on the hydrogen-side photocatalyst 4H, an electron e in the valence band VBM is excited to the conduction band CBM. Since the potential of the bottom CBM of the conduction band is more negative than the reduction potential of the proton, the electron in the conduction band is converted into a proton H according to the following half-reaction: + is donated to produce hydrogen molecules, H2 (hydrogen gas). 2H + +2e - →H2…(1) In addition, the potential of the valence band peak VBM of the hydrogen-side photocatalyst 4H is more positive than the redox potential of the mediator substance M, so that the hole p generated in the valence band VBM receives an electron e from the reduced mediator substance M, resulting in the oxidized form M of the mediator substance M. + Then, the oxidized form of the mediator substance M + As already mentioned, the water is transported to the oxygen-side water tank 30 by the water circulation mechanism 10H.

[0035] On the other hand, when a photon h / λ is absorbed on the oxygen-side photocatalyst 4O, an electron e in the valence band VBM is excited to the conduction band CBM, generating a hole p in the valence band VBM. Since the potential at the top of the valence band VBM is more positive than the oxidation potential of water, an electron e is donated from the water molecule HO to the hole p, forming an oxygen molecule O and a proton H according to the following half-reaction: + and are generated. 2H2O→O2+4H + +4e - …(2) In addition, since the potential of the bottom CBM of the conduction band of the oxygen-side photocatalyst 4O is more negative than the redox potential of the mediator substance M, electrons e excited to the conduction band CBM are donated to the oxidized form of the mediator substance M, and the reduced form M of the mediator substance M is generated. Then, the reduced form M of the mediator substance and protons H + As already mentioned, the water is transported to the hydrogen-side water tank 3H by the water circulation mechanism 10O.

[0036] Typically, in the hydrogen-side photocatalyst 4H, the reduction reaction of protons produces the oxidized form M of the mediator substance. + A co-catalyst 4Ha is added to the hydrogen-side photocatalyst 4H so that the reduction reaction of the hydrogen molecule (and the oxidation reaction of the water molecule) occurs in preference to the reduction reaction of the hydrogen molecule (and the oxidation reaction of the water molecule). The presence of the co-catalyst 4Ha essentially promotes only the donation of electrons to the protons, and essentially the oxidized form M of the mediator substance + (Furthermore, since there is no co-catalyst that promotes the oxidation reaction of water molecules on the hydrogen-side photocatalyst 4H, the oxidation reaction of water molecules hardly occurs.) Similarly, a co-catalyst 4Oa is added to the oxygen-side photocatalyst 4O so that the oxidation reaction of water molecules occurs preferentially over the oxidation reaction of the reduced form M of the mediator substance (and the reduction reaction of protons) on the oxygen-side photocatalyst 4O. The presence of the co-catalyst 4Oa essentially promotes only the donation of electrons from water molecules to holes p, and substantially, the oxidation of the reduced form M of the mediator substance hardly occurs (Furthermore, since there is no co-catalyst that promotes the reduction reaction of protons on the oxygen-side photocatalyst 4O, the reduction reaction of protons hardly occurs.)

[0037] With the above configuration, hydrogen gas is produced exclusively or mainly in the hydrogen-side water tank 3H, and oxygen gas is produced exclusively or mainly in the oxygen-side water tank 3O.

[0038] Types of photocatalysts that can be used in this embodiment Referring to FIG. 3, as already described, the photocatalyst employed in this embodiment has a band gap (CBM-VBM) that is equal to or smaller than the proton reduction potential V H and the redox potential V of the mediator substance M M At the oxygen side OEP, the band gap (CBM-VBM) is equal to the redox potential V of the mediator substance M. M and the oxidation potential of water V OIn this regard, a material that satisfies the condition of spanning the valence band top potential VBM is selected on the hydrogen side HEP, as shown in Figure 3(i), so as to reliably prevent the oxidation of water molecules on the hydrogen side and the reduction of protons on the oxygen side. O A catalyst that is more negative than the oxygen side is selected, and the potential CBM of the bottom of the conduction band is the proton reduction potential V H However, as already mentioned, by appropriately using a promoter, it is possible to prevent the oxidation of water molecules on the hydrogen side and the reduction of protons on the oxygen side. Therefore, as shown in Figures 3(ii) to (iv), in at least one of the hydrogen-side HEP and the oxygen-side OEP, the band gap (CBM-VBM) is set to be equal to the proton reduction potential V H and the oxidation potential of water V O A material that straddles these two may also be used.

[0039] However, from the perspective of selecting an LED as the irradiating light source, it is preferable that the band gap of the photocatalyst be as narrow as possible, whether it is the hydrogen-side HEP or the oxygen-side OEP. More specifically, the narrower the band gap of the photocatalyst, the longer the light wavelength required to excite electrons in the photocatalyst. Meanwhile, as shown in Figure 4, the efficiency ξ (%) of the luminous intensity Pph relative to the power Pev of an LED generally increases with the emission wavelength λ. Therefore, if an LED with a long emission wavelength λ can be used, higher light intensity can be obtained with less energy, thereby increasing the amount of hydrogen gas produced. Therefore, as described above, by selecting a photocatalyst with as narrow a band gap as possible and correspondingly employing an LED with a long emission wavelength λ, the energy efficiency of hydrogen gas production can be improved. Specifically, when a photocatalyst with a band gap smaller than the energy width between the reduction potential of protons and the oxidation potential of water molecules is selected for the hydrogen side or oxygen side photocatalyst, it is preferable to select an LED for the hydrogen side or oxygen side, respectively, whose emission wavelength is shorter than the light wavelength corresponding to the energy width of the band gap of the photocatalyst and longer than the light wavelength corresponding to the energy width between the reduction potential of protons and the oxidation potential of water molecules.

[0040] As the photocatalyst and co-catalyst in this embodiment, for example, the following catalysts can be used. Hydrogen-side photocatalyst: Rh-doped SrTiO3, CdSe, Ta3N5, La3Ti2CuS5O7, Y2Ti2O5S2, Si, CuO, Cu2O, ZnMn2O4, CuBi2O4, CuNb3O8, CuFeO2, MnV2O5, CuInS2, AgIn5S8, Ir-La-doped BaTa2O6, Ir-La-doped SrTiO3, CuGaS2, CuGaZn2S4 Hydrogen-side promoter: Pd, Rh, Ru, Ni, Au, Fe, NiO, RuO2, Cr-Rh oxide Oxygen-side photocatalysts: BiVO4, WO3, CuWO4, ZnFe2O4, BiOI, Bi2MoO6, Rh-Sb doped TiO2, Rh-Ba doped NaNbO3, Ir doped SrTiO3 Oxygen evolution promoter: IrO2, Co, Fe, C, Ag, CoO, Co3O4

[0041] Types of mediator substances that can be used in this embodiment As a mediator substance, for example, the following substance can be used: [Co(bpy)3] 3+ / 2+ , FeSO4, K4[Fe(CN)6]·3H2O, Fe(NO3)3·5H2O, K3[Fe(CN)6]

[0042] Modified example of hydrogen gas production device In the device of this embodiment, if the wavelength of the light irradiating the hydrogen-side photocatalytic member 4H and the oxygen-side photocatalytic member 4O is equal to or less than the wavelength corresponding to the larger energy width of the band gap of the hydrogen-side photocatalyst and the oxygen-side photocatalyst, both photocatalysts can be excited with light of a single wavelength. Therefore, in the device of this embodiment, a common light-emitting device may be used to irradiate the hydrogen-side photocatalytic member 4H and the oxygen-side photocatalytic member 4O. Specifically, as shown in FIG. 5 , for example, a light-emitting device 5 carrying an LED 6 may be disposed between the hydrogen-side water tank 3H and the oxygen-side water tank 3O, and the LED 6 may be used as a common light source for the irradiated light Lh and Lo. In this case, the light-emitting device 5 may be sandwiched between the water tank 3H and the water tank 3O, so that the heat emitted when the LED 6 is driven by electricity can heat the water in the water tank, and the wall of the water tank may be in contact with the light-emitting device 5.

[0043] Thus, this embodiment provides an apparatus for producing hydrogen gas through a photocatalytic water decomposition reaction, employing a Z-scheme photocatalytic system to separate a water tank into one that generates hydrogen gas and one that generates oxygen gas, enabling highly pure hydrogen gas to be recovered without passing the produced gas through a separator. The apparatus uses an LED as the light source for the irradiation light that excites the photocatalyst. By using an LED with high monochromaticity driven by electricity as the light source for the irradiation light, the proportion of energy in the irradiation light that contributes to hydrogen gas generation can be increased, improving the amount and efficiency of hydrogen gas production, increasing the flexibility of the device's location, and making hydrogen gas generation less susceptible to weather conditions.

[0044] 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

1. A hydrogen gas production apparatus, comprising: As a means of generating hydrogen gas, a hydrogen-side water tank in which a hydrogen-side photocatalyst member carrying a hydrogen-side photocatalyst is immersed in water in which a mediator substance is dispersed; a hydrogen-side light irradiating means for irradiating the hydrogen-side photocatalyst member with light having a wavelength that is absorbed by the hydrogen-side photocatalyst and excites electrons in the valence band to a conduction band; and a hydrogen gas recovery means for recovering the hydrogen gas generated in the hydrogen-side water tank. a hydrogen gas generating means configured such that, when the hydrogen-side photocatalyst is irradiated with light from the hydrogen-side light irradiating means, protons in the water in the hydrogen-side water tank are reduced to generate the hydrogen gas and the reduced mediator substance is oxidized; As an oxygen gas generating means, an oxygen-side water tank in which an oxygen-side photocatalyst member carrying an oxygen-side photocatalyst is immersed in water in which the mediator substance is dispersed; an oxygen-side light irradiating means for irradiating the oxygen-side photocatalyst member with light having a wavelength that is absorbed by the oxygen-side photocatalyst and excites electrons in the valence band to a conduction band; an oxygen gas generating means configured such that, when the oxygen-side photocatalyst is irradiated with light from the oxygen-side light irradiating means, water molecules in the water in the oxygen-side water tank are oxidized to generate oxygen gas, and the oxidized mediator substance is reduced; a water circulating means for circulating the water in which the mediator material is dispersed between the hydrogen side water tank and the oxygen side water tank; Including, a light source of the light emitting device of the hydrogen-side light emitting means and a light source of the light emitting device of the oxygen-side light emitting means are light emitting diodes, An apparatus in which the band gap of the hydrogen-side photocatalyst is narrower than the energy width between the reduction potential of protons and the oxidation potential of water molecules, and the emission wavelength of the light-emitting diode of the hydrogen-side light irradiation means is shorter than the light wavelength corresponding to the energy width of the band gap of the hydrogen-side photocatalyst and longer than the light wavelength corresponding to the energy width between the reduction potential of protons and the oxidation potential of water molecules. Device.

2. A hydrogen gas production apparatus, comprising: As a means of generating hydrogen gas, a hydrogen-side water tank in which a hydrogen-side photocatalyst member carrying a hydrogen-side photocatalyst is immersed in water in which a mediator substance is dispersed; a hydrogen-side light irradiating means for irradiating the hydrogen-side photocatalyst member with light having a wavelength that is absorbed by the hydrogen-side photocatalyst and excites electrons in the valence band to a conduction band; and a hydrogen gas recovery means for recovering the hydrogen gas generated in the hydrogen-side water tank. a hydrogen gas generating means configured such that, when the hydrogen-side photocatalyst is irradiated with light from the hydrogen-side light irradiating means, protons in the water in the hydrogen-side water tank are reduced to generate the hydrogen gas and the reduced mediator substance is oxidized; As an oxygen gas generating means, an oxygen-side water tank in which an oxygen-side photocatalyst member carrying an oxygen-side photocatalyst is immersed in water in which the mediator substance is dispersed; an oxygen-side light irradiating means for irradiating the oxygen-side photocatalyst member with light having a wavelength that is absorbed by the oxygen-side photocatalyst and excites electrons in the valence band to a conduction band; an oxygen gas generating means configured such that, when the oxygen-side photocatalyst is irradiated with light from the oxygen-side light irradiating means, water molecules in the water in the oxygen-side water tank are oxidized to generate oxygen gas, and the oxidized mediator substance is reduced; a water circulating means for circulating the water in which the mediator material is dispersed between the hydrogen side water tank and the oxygen side water tank; Including, a light source of the light emitting device of the hydrogen-side light emitting means and a light source of the light emitting device of the oxygen-side light emitting means are light emitting diodes, A device in which the potential at the boundary between the valence band and the band gap of the hydrogen-side photocatalyst is more negative than the oxidation potential of water molecules, and the potential at the boundary between the conduction band and the band gap of the oxygen-side photocatalyst is more positive than the reduction potential of protons.

3. 3. The device of claim 1 or 2, wherein the light irradiated from the hydrogen-side light irradiating means and the light irradiated from the oxygen-side light irradiating means are directly irradiated onto the hydrogen-side photocatalyst and the oxygen-side photocatalyst, respectively.

4. 3. The apparatus according to claim 1, wherein the light source of the light emitting device of said hydrogen-side light irradiating means is the same as the light source of the light emitting device of said oxygen-side light irradiating means.

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