JP7809277B2 - Photocatalytic water splitting device and water splitting system equipped with the same - Google Patents

Description

The present invention relates to a water splitting device that uses a photocatalyst and utilizes light energy, as well as a water splitting system equipped with the same.

In recent years, hydrogen has been attracting attention as an energy medium that can efficiently store and transport energy. Hydrogen has superior storage properties compared to electricity, and, unlike fossil fuels, does not emit carbon dioxide when burned. While hydrogen-powered fuel cell vehicles and fuel cell cogeneration systems have already been put to practical use, further expansion of hydrogen use is desired to realize a hydrogen society. Using hydrogen as an alternative to fossil fuels also contributes to reducing carbon dioxide emissions, one of the initiatives related to the Sustainable Development Goals adopted by the United Nations.

However, hydrogen rarely exists in its pure form in nature, so it must be extracted from other compounds before it can be used. Traditionally, much of the hydrogen used in industry has been produced by steam reforming fossil fuels (e.g., methane, propane, etc.). However, when producing hydrogen from fossil fuels, large amounts of carbon dioxide are emitted during the production process.

In response to this, technologies have been developed to split water (i.e., produce hydrogen) using photocatalysts and photoelectrodes that utilize solar energy (Patent Documents 1-6). If hydrogen production using renewable energy such as solar energy becomes practical, it will be possible to obtain energy from hydrogen when needed while suppressing the generation of carbon dioxide during hydrogen production.

International Publication No. 2017/221866 Japanese Patent Application Laid-Open No. 2017-124393 International Publication No. 2012/008838 JP 2018-89589 A Japanese Patent Application Laid-Open No. 2006-89336 Japanese Patent Application Laid-Open No. 2007-117811

However, conventional hydrogen production using photocatalysts has a problem in that the solar energy conversion efficiency (i.e., conversion efficiency to hydrogen energy) is low (e.g., 1% or less). Therefore, conventional technologies are not practical in terms of at least the solar energy conversion efficiency and, ultimately, the hydrogen production cost. For example, the Hydrogen Strategy Basic Scenario by Japan's Ministry of Economy, Trade and Industry predicts that the current cost of hydrogen production is approximately 100 yen per ^Nm³ , and that it needs to be significantly reduced to approximately 30 yen ^{per Nm³} by around 2030. The U.S. Department of Energy has also set a similar goal. Therefore, to realize a hydrogen society, further technological innovation is needed to improve the solar energy conversion efficiency.

The key to improving the conversion efficiency of solar energy lies in expanding the utilization efficiency of the solar spectrum (i.e., extending the usable wavelengths) and improving the utilization efficiency of light (photons) absorbed by the photocatalyst (i.e., improving the quantum yield). For practical use of hydrogen production using photocatalysts, a solar energy conversion efficiency of around 5-10% is required.

When photocatalysts primarily use light in the ultraviolet range (e.g., 200nm to 400nm), the maximum solar energy conversion efficiency is only around 2%. In other words, when using light in the ultraviolet range, even if the photocatalyst absorbs all of the photons and water is split with a 100% quantum yield (the proportion of absorbed photons that contributed to the reaction), high conversion efficiency cannot be expected. In contrast, if the wavelength range used by photocatalysts can be expanded to around 600nm in the visible light range, the number of photons in the solar spectrum will increase significantly, improving maximum conversion efficiency to 16%. In that case, even if the average quantum yield is 30%, a solar energy conversion efficiency of around 5% can be expected.

As a result of extensive research, the inventors have discovered that Z-scheme (two-step excitation) water splitting is an effective hydrogen production technology for expanding the wavelength range used by photocatalysts. In Z-scheme water splitting, two types of photocatalysts (a photocatalyst for hydrogen generation and a photocatalyst for oxygen generation) are used for hydrogen generation and oxygen generation, respectively, and it is easy to separately recover the generated hydrogen and oxygen. However, in order to reduce hydrogen production costs, it is desirable to achieve Z-scheme water splitting with a simpler configuration.

In light of the above background, the present invention aims to provide a photocatalytic water splitting device that achieves Z-scheme water splitting using a photocatalyst with a simple configuration, and a water splitting system equipped with the same.

In order to solve the above-mentioned problems, one aspect of the present invention is a water splitting device (3) that uses a photocatalyst to split water into hydrogen and oxygen using light energy, and includes a photocatalyst panel (7, 207, 307) and a reaction vessel (9, 209) that contains the photocatalyst panel and a reaction solution and performs a water splitting reaction in the reaction solution. The photocatalyst panel includes a light-transmitting substrate (11, 311), a first photocatalyst layer (13, 313) formed to overlap the first surface (11A) of the substrate, and a second surface (11B) that is paired with the first surface of the substrate. and a second photocatalytic layer (14, 314) formed on the photocatalytic panel, one of the first photocatalytic layer and the second photocatalytic layer being a photocatalytic layer for generating oxygen containing a first semiconductor, and the other being a photocatalytic layer for generating hydrogen containing a second semiconductor. The reaction vessel has an internal space partitioned by the photocatalytic panel, and is configured to have an oxygen generation tank (21) formed on the oxygen generation photocatalytic layer side, a hydrogen generation tank (22) formed on the hydrogen generation photocatalytic layer side, and a light incident wall (9A) that transmits light incident on the photocatalytic panel toward the first photocatalytic layer.

In this embodiment, incident light that has passed through the first photocatalytic layer and the substrate can be utilized by the second photocatalytic layer. This eliminates the need to introduce light separate from the light incident on the first photocatalytic layer into the second photocatalytic layer, enabling photocatalytic Z-scheme water splitting to be achieved with a simple configuration.

In the above embodiment, the substrate can be either conductive or non-conductive (non-conductive), but non-conductive substrates are more preferred.

This aspect enables significant cost reductions compared to using generally expensive conductive substrates, and because the visible light transmittance is higher than that of conductive substrates, the light utilization efficiency of the second photocatalytic layer placed on the back surface is improved.

In the above aspect, the optical absorption spectrum of the second semiconductor may be located on the longer wavelength side compared to the optical absorption spectrum of the first semiconductor.

This aspect makes it possible to more effectively utilize incident light (especially light in the visible light range) that has passed through the first photocatalytic layer and the substrate in the second photocatalytic layer.

In the above aspect, the first semiconductor particles may include one or a combination of two or more selected from metal oxides, metal oxynitrides, nitrides, metal oxysulfides, metal oxyhalides, and metal-organic frameworks, and the second semiconductor may include one or a combination of two or more selected from metal oxynitrides, metal nitrides, metal oxysulfides, metal sulfides, metal oxyhalides, metal halides, organometallic halides, metal oxides, metal phosphides, silicon, organic semiconductors, metal-organic frameworks, and covalent organic frameworks.

According to this aspect, a photocatalytic panel can be constructed using first and second semiconductors that can effectively utilize light energy.

In the above aspect, the first photocatalytic layer and the substrate portion of the photocatalytic panel preferably have a transmittance of 40% or more for light incident on the first photocatalytic layer in the visible light range of at least 500 nm or more.

This aspect makes it possible to more effectively utilize the light energy in the visible light range that has passed through the first photocatalytic layer and the substrate in the second photocatalytic layer.

In the above aspect, the reaction solution contains a redox medium, which is an ion pair that stably undergoes repeated oxidation and reduction in an aqueous solution, and may contain one or a combination of two or more ion pairs selected from the group consisting of iron ion pairs, ion pairs containing iodine, ion pairs containing vanadium, complex ion pairs, and polyoxometalate ion pairs.

According to this aspect, photocatalytic Z-scheme water splitting can be effectively performed using an appropriate redox medium.

In the above aspect, a communication section (G) may be formed between the reaction vessel and the photocatalytic panel, allowing the reaction solution in the oxygen generation tank and the hydrogen generation tank to flow through.

This aspect eliminates the need to provide a system for circulating the reaction solutions in the oxygen generation tank and hydrogen generation tank outside the reaction vessel, making it possible to achieve photocatalytic Z-scheme water decomposition with a simpler configuration.

In the above aspect, light may be incident on the reaction vessel only through the light incident wall.

This embodiment makes it easy to control the amount of light incident on the reaction vessel, thereby making it easy to control the water decomposition reaction (i.e., the production of oxygen and hydrogen).

Another aspect of the present invention is a water splitting system comprising the water splitting device and a water storage tank (36, 236) for storing water to be supplied to the water splitting device.

This aspect makes it possible to achieve photocatalytic Z-scheme water splitting with a simple configuration.

The above aspect may also include a first circulation line (5, 105) that circulates a portion of the reaction solution discharged from the oxygen generation tank together with oxygen to the hydrogen generation tank, and a second circulation line (6, 106) that circulates a portion of the reaction solution discharged from the hydrogen generation tank together with hydrogen to the oxygen generation tank.

In this embodiment, the product (redox medium, etc.) generated in either the oxygen generation tank or the hydrogen generation tank is sent to the other tank and used in the water splitting reaction, allowing the photocatalytic Z-scheme water splitting reaction to be carried out continuously in the reaction vessel.

In the above aspect, the first circulation line may be connected to an oxygen separation line (5D) that transports the oxygen separated from the reaction solution, and the second circulation line may be connected to a hydrogen separation line (6D) that transports the hydrogen separated from the reaction solution.

This embodiment makes it easy to obtain the oxygen and hydrogen produced in the reaction vessel.

In the above aspect, the water storage tank may be provided in at least one of the first circulation line and the second circulation line.

This aspect allows water to be supplied to the water decomposition device using a simple configuration.

In the above aspect, a gas-liquid separator (35) may be provided upstream of the water storage tank.

According to this embodiment, a portion of the reaction solution discharged together with oxygen or hydrogen can be circulated to the water decomposition device using a simple configuration.

The above aspects make it possible to achieve photocatalytic Z-scheme water splitting with a simple configuration.

1 is a block diagram showing a water decomposition system according to a first embodiment; An explanatory diagram of the water splitting reaction in the water splitting device in FIG. FIG. 2 is a diagram showing a modification of the water decomposition apparatus in FIG. FIG. 1 is a block diagram showing a water decomposition system according to a second embodiment. FIG. 10 is a block diagram showing a water decomposition system according to a third embodiment. Schematic diagram of the evaluation apparatus (water decomposition apparatus) used in each Example, Comparative Example, and Reference Example. Diagram showing the UV-visible diffuse reflectance spectra of various semiconductors Graph showing transmittance to the hydrogen generation catalyst layer side in the photocatalyst panel of Example 1 Graph showing the transmittance of the photocatalyst panel of Comparative Example 1 Graph showing the transmittance of the photocatalyst panel of Comparative Example 2 Graph showing the transmittance of the photocatalyst panel of Comparative Example 3 Graph showing transmittance to the hydrogen generation catalyst layer side in the photocatalyst panel of Comparative Example 8 Graph showing the change over time in the water splitting reaction in Example 1 Graph showing the change over time in the water decomposition reaction in Comparative Example 7 Graph showing the change over time in the water decomposition reaction in Comparative Example 8 Graph showing the change over time in the water splitting reaction in Example 4 Graph showing the change over time in the water splitting reaction in Example 5 Graph showing the change over time in the water splitting reaction in Example 3 Graph showing the change over time in the water decomposition reaction in Reference Example 1 Graph showing the change over time in the water decomposition reaction in Reference Example 2

The following describes a water splitting device using a photocatalyst according to an embodiment and a water splitting system equipped with the same, with reference to the drawings.

(First embodiment)
Fig. 1 is a configuration diagram showing a water splitting system 1 according to a first embodiment. Fig. 2 is an explanatory diagram of a water splitting reaction performed by a water splitting device 3 in Fig. 1.

As shown in FIG. 1, the water splitting system 1 includes a water splitting device 3 that uses solar energy to split water into hydrogen and oxygen, and a first circulation line 5 and a second circulation line 6 that are connected to the water splitting device 3, respectively.

The water splitting device 3 includes a photocatalyst panel 7 that uses a photocatalyst, and a reaction vessel 9 that contains the photocatalyst panel 7 and a reaction solution (aqueous solution) and performs the water splitting reaction in the reaction solution.

The photocatalyst panel 7 includes a light-transmitting substrate 11, an oxygen generating photocatalyst layer 13 (first photocatalyst layer) formed to overlap the front surface 11A (first surface) of the substrate 11, and a hydrogen generating photocatalyst layer 14 (second photocatalyst layer) formed to overlap the rear surface 11B (second surface) of the substrate 11 that forms a pair with the front surface 11A.

The substrate 11 has a rectangular, flat plate shape and is made of a transparent material with high optical transparency. Examples of transparent materials that can be used include glass, acrylic resin, and polycarbonate. The substrate 11 is preferably insulating or non-conductive. Regarding the insulating or non-conductive properties of the substrate 11, it is preferable that the volume resistivity (see JIS C 2139-3-1 Part 3-1) be ¹⁰ Ω·m or more. The shape and size of the substrate 11 can be changed as appropriate.

The oxygen generating photocatalyst layer 13 is formed so as to overlap the front surface 11A of the substrate 11. As will be described in detail later, the oxygen generating photocatalyst layer 13 includes an oxygen generating semiconductor (first semiconductor) that constitutes the oxygen generating photocatalyst.

The hydrogen generating photocatalyst layer 14 is formed so as to overlap the rear surface 11B of the substrate 11. As described in detail below, the hydrogen generating photocatalyst layer 14 includes a hydrogen generating semiconductor (second semiconductor) that constitutes a hydrogen generating photocatalyst.

These oxygen generating semiconductors and hydrogen generating semiconductors are fixed on the substrate 11 as particles (groups) or thin films. The shapes of such particles are not particularly limited, but include spherical, oval, plate-like, fibrous, scaly, and porous shapes. Furthermore, such thin films are not particularly limited, but include dense semiconductor films, porous films formed by the deposition of semiconductor particles, and porous films with a regular structure.

The semiconductor for oxygen generation absorbs visible light by having a band gap of 3.0 eV or less or an energy gap due to impurity levels. The upper end of the valence band of the semiconductor for oxygen generation is more positive than the oxidation potential of water (O ₂ /H ₂ O: +1.23 V). The potential reference is the potential of a standard hydrogen electrode at pH=0.

The oxygen-generating semiconductor is not particularly limited, as long as it can decompose water to generate oxygen. Examples of oxygen-generating semiconductors include metal oxides, metal oxynitrides, metal nitrides, metal oxysulfides, metal oxyhalides, and metal organic frameworks (MOFs). In the water splitting device 3, one or a combination of two or more selected from these materials can be used as the oxygen-generating semiconductor.

Examples of the metal oxide include WO ₃ , BiVO ₄ , and H ₂ WO ₄ .

Examples of the metal oxynitrides include TaON and BaTaO ₂ N. Examples of the metal nitrides include Ta ₃ N ₅ .

Examples of the metal oxysulfides include Sm ₂ Ti ₂ S ₂ O ₅ .

Examples of the acid halide include Bi ₄ NbO ₈ Cl and Ba ₂ Bi ₃ Nb ₂ O ₁₁ Br _1-x I _x (0≦x≦1).

Examples of the metal-organic framework include MOFs whose building blocks are iron(III) oxoclusters and terephthalic acid.

The oxygen generating semiconductor may be a material doped with impurities or a material that is not doped with impurities. By doping the oxygen generating semiconductor with impurities to form an impurity level, electrons can be excited from the impurity level to the conduction band, or from the valence band to the impurity level.

The semiconductor for hydrogen generation absorbs visible light by having a band gap of 3.0 eV or less or an energy gap due to impurity levels. The lower end of the conduction band of the semiconductor for hydrogen generation is more negative than the reduction potential of protons (H ⁺ /H ₂ : 0 V).

The hydrogen-generating semiconductor is not particularly limited, as long as it can decompose water to generate hydrogen. Examples of hydrogen-generating semiconductors include metal oxynitrides, metal nitrides, metal oxysulfides, metal sulfides, metal oxyhalides, metal halides, organometallic halides, metal oxides, metal phosphides, silicon, organic semiconductors, MOFs, and covalent organic frameworks (COFs). In the water splitting device 3, one or a combination of two or more selected from these materials can be used as the hydrogen-generating semiconductor.

Examples of the metal oxynitrides include TaON, BaTaO ₂ N, CaTaO ₂ N, and SrTaO ₂ N. Examples of the metal nitrides include Ta ₃ N ₅ .

Examples of the metal oxysulfides include Sm ₂ Ti ₂ S ₂ O _5. Examples of the metal sulfides include CdS and Cu _x Ag _x In _{2 x} Zn _{2-2 x} S ₂ (0≦x≦1).

Examples of the metal acid halides include Bi ₄ NbO ₈ Cl and Ba ₂ Bi ₃ Nb ₂ O ₁₁ Br _1-x I _x (0≦x≦1). Examples of the metal halides include Cs ₂ AgBiBr _6. Examples of the organometallic halides include CH ₃ NH ₃ PbI ₃ .

_The metal oxides include _Cu2O and _SnNb2O6 .

Examples of the above metal phosphides include GaP.

The organic semiconductors include carbon nitride (gC ₃ N ₄ ) and dibenzothiophene polymers.

Examples of the metal-organic framework include aminobenzenedicarboxylic acid titanium MOF.

Examples of the covalent organic framework include COFs containing 1,3,5-triformylphloroglucinol and 4,4"-diamino-p-terphenyl as building blocks.

The hydrogen generating semiconductor may be a material doped with impurities or a material that is not doped with impurities. By doping the hydrogen generating semiconductor with impurities to form an impurity level, electrons can be excited from the impurity level to the conduction band, or from the valence band to the impurity level.

The oxygen generating photocatalyst may further contain an oxygen generating co-catalyst. The oxygen generating co-catalyst is supported on the oxygen generating semiconductor and promotes the reaction of generating oxygen from water and holes generated in the oxygen generating semiconductor when the oxygen generating semiconductor is irradiated with visible light. When the oxygen generating co-catalyst is contained in the oxygen generating photocatalyst, the oxygen generating activity of the oxygen generating photocatalyst is significantly improved.

The oxygen generation promoter may include metal oxides, metal hydroxides, and metal phosphates. The oxygen generation promoter may include at least one transition metal or noble metal. Transition metals include Mn, Fe, Ni, and Co. Noble metals include Ru and Ir.

The hydrogen generating photocatalyst may further contain a hydrogen generating co-catalyst. The hydrogen generating co-catalyst is supported on the hydrogen generating semiconductor and promotes the reaction of generating hydrogen from excited electrons and protons (or water) generated in the hydrogen generating semiconductor when the hydrogen generating semiconductor is irradiated with visible light. When the hydrogen generating co-catalyst is contained in the hydrogen generating photocatalyst, the hydrogen generating activity of the hydrogen generating photocatalyst is significantly improved.

The hydrogen generation promoter may include metals, metal oxides, and metal sulfides. The hydrogen generation promoter may contain at least one precious metal. The hydrogen generation promoter may also contain Cr in addition to the precious metal. Examples of precious metals include Ru, Rh, Pd, Ag, Os, Ir, Pt, and Au. Of these, Pt and/or Rh are preferred.

The hydrogen generation promoter may include at least one selected from the group consisting of a core-shell material having a noble metal core and a chromium oxide (CrO _x ) shell, and a mixed oxide of a noble metal and Cr. The mixed oxide of a noble metal and Cr is, for example, an oxide containing rhodium (III) and chromium (III) (Rh _x Cr _2-x O ₃ ). These hydrogen generation promoters can promote the reaction of generating hydrogen from excited electrons and protons (or water) even in the presence of an oxidized form of a redox medium, as described below. Therefore, they prevent the loss of excited electrons without being used for hydrogen generation due to the reduction of the oxidized form of the redox medium. As a result, in the presence of a redox medium, the reduction of protons (or water) proceeds selectively and smoothly, achieving high solar energy conversion efficiency.

These promoters can be supported on semiconductor particles in the form of particles. There are no particular restrictions on the size of the promoter particles. The average particle size of the promoter particles is usually 0.1 nm or more and 100 nm or less.

In the water splitting system 1 according to this embodiment, the oxygen generating semiconductor (oxygen generating photocatalyst) utilizes sunlight incident on the water splitting device 3, and the hydrogen generating semiconductor (hydrogen generating photocatalyst) utilizes light that has passed through the oxygen generating photocatalyst layer 13 and substrate 11. Therefore, the oxygen generating semiconductor and hydrogen generating semiconductor used in the water splitting device 3 preferably have different absorption wavelength ranges for sunlight (particularly light in the visible light range). For example, the optical absorption spectrum of the hydrogen generating semiconductor should be located on the longer wavelength side compared to the optical absorption spectrum of the oxygen generating semiconductor used in combination with it. This allows the oxygen generating semiconductor to primarily absorb light in a wavelength range with a shorter upper limit (e.g., 200-500 nm), while the hydrogen generating semiconductor can primarily absorb light in a wavelength range with a longer upper limit (e.g., 200-650 nm).

In the water decomposition device 3 according to this embodiment, examples of suitable combinations of semiconductors for generating oxygen and semiconductors for generating hydrogen are shown in Table 1.

As shown in Table 1, a combination of an oxygen generating semiconductor with a lower upper limit of available wavelength (i.e., primarily absorbing light in a shorter wavelength region) and a hydrogen generating semiconductor with a higher upper limit of available wavelength (i.e., primarily absorbing light in a longer wavelength region) is preferred. For example, the available wavelength of _WO3 as an oxygen generating semiconductor is approximately 450 nm or less, and the available wavelengths of TaON, _Ta3N5 , _Sm2Ti2S2O5 , and _BaTaO2N as hydrogen generating semiconductors combined with _WO3 are approximately ₅₀₀ nm or less, approximately ₆₀₀ nm or less, approximately 600 nm or less, and approximately 650 nm _or less, respectively. Such a combination of an oxygen generating semiconductor and a hydrogen generating semiconductor enables the energy of incident light to be effectively utilized in both the oxygen generating photocatalytic layer 13 and the hydrogen generating photocatalytic layer 14.

The reaction vessel 9 is box-shaped and holds a reaction solution containing water as a main component in its internal space. The reaction solution contains a redox medium (a combination of an oxidant and a reductant). In this embodiment, an Fe ³⁺ /Fe ²⁺ ion pair is used as the redox medium. At the start-up (at the start of the water decomposition reaction), Fe ²⁺ may be present in both the reaction solutions of the oxygen generation tank 21 and the hydrogen generation tank 22. Alternatively, Fe 3+ may be present in both the reaction solutions of the oxygen generation tank 21 and the hydrogen generation tank 22 at the start-up. Alternatively, Fe ²⁺ and Fe ³⁺ may be present in the reaction solutions of the oxygen generation tank 21 and the hydrogen generation tank 22 at the start-up. The following description is based on the assumption that water containing dissolved FeCl ₂ is injected into the reaction vessel 9 as a redox source at the start-up, and Fe ²⁺ is present in both the reaction solutions of the oxygen generation tank 21 and ^the hydrogen generation tank 22. In FIG. 1, the up-down direction of the drawing corresponds to the approximate vertical direction with respect to the arrangement of the reaction vessel 9.

In the reaction vessel 9, so-called Z-scheme (two-step excitation) water splitting is carried out. In Z-scheme water splitting, two different types of photocatalysts (i.e., photocatalyst layer 13 for oxygen generation and photocatalyst layer 14 for hydrogen generation) are used, and electron transfer between the two photocatalysts occurs via a redox medium in the reaction solution.

The redox reaction of the Fe3 ⁺ /Fe2 ⁺ ion pair is a one-electron process involving one electron. Therefore, when the Fe3 ⁺ /Fe2 ⁺ ion pair is used, electron transfer between the oxygen generation photocatalyst and the hydrogen generation photocatalyst proceeds smoothly. Furthermore, light absorption by the Fe3 ⁺ /Fe2 ⁺ ion pair is also low. Furthermore, since the redox potential of the Fe3 ⁺ /Fe2 ⁺ ion pair is 0.77 V (vs. SHE), the use of the Fe3 ⁺ /Fe2 ⁺ ion pair as a redox medium can smoothly transfer electrons between many hydrogen generation semiconductors and oxygen generation semiconductors and the redox medium.

In the water splitting device 3, iron ions, iodine-containing ions, vanadium-containing ions and complex ions, polyoxometalate ions, and the like can be used as redox mediators. Examples of iron ions include the Fe ³⁺ /Fe ²⁺ ion pair described above. Examples of iodine-containing ions include the IO ₃ ^- /I ^- ion pair and the I ₃ ^- /I ^- ion pair. Examples of vanadium-containing ions include VO ₂ ⁺ /VO ²⁺ . Examples of complex ions include the [Fe(CN) ₆ ] ^3- /[Fe(CN) ₆ ] ^4- ion pair, the [Co(bpy) ₃ ] ³⁺ /[Co(bpy) ₃ ] ²⁺ ion pair, and the [Co(phen) ₃ ] ³⁺ /[Co(phen) ₃ ] ²⁺ ion pair. Examples of polyoxometalate ions include the [SiW ₁₁ O ₃₉ Mn(H ₂ O)] ^5- /[SiW ₁₁ O ₃₉ Mn(H ₂ O)] ^6- ion pair. In the water splitting device 3, one or more ion pairs selected from these can be used as the redox medium.

The oxygen generating photocatalyst described above may further contain a reduction promoter for the redox medium. The reduction promoter for the redox medium is supported on the oxygen generating semiconductor and promotes the reduction reaction of the redox medium by excited electrons generated in the oxygen generating semiconductor when the semiconductor is irradiated with visible light. When the reduction promoter for the redox medium is contained in the oxygen generating photocatalyst, recombination of excited electrons and holes generated in the oxygen generating semiconductor is suppressed, significantly improving the oxygen generation activity of the oxygen generating photocatalyst.

Redox mediator reduction promoters include metals and metal oxides. The redox mediator reduction promoter may contain at least one transition metal or noble metal. Transition metals include Fe. Noble metals include Ru, Ir, and Pt.

The above-mentioned hydrogen generating photocatalyst may further contain an oxidation promoter for the redox medium. The oxidation promoter for the redox medium is supported on the hydrogen generating semiconductor and promotes the oxidation reaction of the redox medium by holes generated in the hydrogen generating semiconductor when the hydrogen generating semiconductor is irradiated with visible light. When the oxidation promoter for the redox medium is contained in the hydrogen generating photocatalyst, recombination of holes generated in the hydrogen generating semiconductor with excited electrons is suppressed, significantly improving the hydrogen generation activity of the hydrogen generating photocatalyst.

Examples of oxidation promoters for redox mediators include oxides or metal cyanometalates containing Ir. Examples of metal cyanometalates include indium(III) hexacyanoferrate.

The reaction vessel 9 has a front wall 9A (light incident wall) arranged to face the oxygen generating photocatalyst layer 13. The front wall 9A is arranged facing diagonally upward so that its outer surface is roughly perpendicular to the direction of sunlight incidence. The internal space of the reaction vessel 9 is partitioned by a photocatalyst panel 7 arranged roughly parallel to the front wall 9A. More specifically, the upper edge of the photocatalyst panel 7 is in close contact with the upper wall 9C of the reaction vessel 9, and the lower edge of the photocatalyst panel 7 is in close contact with the lower wall 9D of the reaction vessel 9.

As a result, an oxygen generation tank 21 is formed in the reaction vessel 9 on the front side of the photocatalyst panel 7 (the oxygen generation photocatalyst layer 13 side). The oxygen generation photocatalyst layer 13 forms part of the inner surface of the oxygen generation tank 21. Furthermore, a hydrogen generation tank 22 is formed in the reaction vessel 9 on the rear side of the photocatalyst panel 7 (the hydrogen generation photocatalyst layer 14 side). The hydrogen generation photocatalyst layer 14 forms part of the inner surface of the hydrogen generation tank 22.

The outer wall of the reaction vessel 9 is formed from a transparent material with high light transmittance, similar to the substrate 11 described above. However, it is sufficient that at least the light incident portion of the outer wall of the reaction vessel 9, where sunlight enters (here, the front wall 9A as the light incident wall), is formed from a transparent material. For example, in a configuration in which only the light incident portion is formed from a transparent material (e.g., portions of the outer wall other than the light incident portion are not transparent, or the transmittance of portions other than the light incident portion is lower than the transmittance of the light incident portion), it becomes easier to control the amount of light incident on the reaction vessel 9 (and therefore the water decomposition reaction).

The shape and size of the reaction vessel 9 can be modified as appropriate. In addition, in the reaction vessel 9 shown in FIG. 1, the oxygen generating photocatalyst layer 13 is disposed on the sunlight incident side (i.e., facing the front wall 9A), but the photocatalyst panel 7 may be inverted and the hydrogen generating photocatalyst layer 14 may be disposed on the sunlight incident side.

2, in the oxygen generation tank 21, the oxygen generation semiconductor in the oxygen generation photocatalyst layer 13 absorbs light, generating electrons (e ^- ) in the conduction band and holes (h ⁺ ) in the valence band. As a result, in the oxygen generation photocatalyst layer 13, the holes generate O ₂ from H ₂ O (or OH ^- ), and the electrons generate a reductant (Red) from the oxidant (Ox). Similarly, in the hydrogen generation tank 22, the hydrogen generation semiconductor (semiconductor photocatalyst) in the hydrogen generation photocatalyst layer 14 absorbs light, generating electrons (e ^- ) and holes (h ⁺ ). As a result, in the hydrogen generation photocatalyst layer 14, the electrons generate H ₂ from H ⁺ (or H ₂ O), and the holes generate an oxidant (Ox) from the reductant (Red).

Referring again to Figure 1, the first circulation line 5 is provided with a first gas-liquid separator 25, a first water storage tank 26, and a first circulation pump 27.

The first circulation line 5 includes an oxygen discharge line 5A consisting of piping connecting an upper opening 29 located at the top of the oxygen generation tank 21 and the first gas-liquid separator 25. The oxygen discharged from the upper opening 29 is transported to the first gas-liquid separator 25 through the oxygen discharge line 5A. The fluid transported through the oxygen discharge line 5A contains oxygen as well as a portion of the reaction solution. In the first gas-liquid separator 25, the reaction solution (containing Fe ²⁺ ) is separated from the oxygen flowing through the oxygen discharge line 5A. The oxygen separated in the first gas-liquid separator 25 is supplied to an oxygen storage tank (not shown) through an oxygen separation line 5D connected to its upper portion (for example, a portion corresponding to the gas phase in the drum body of the knockout drum). However, the oxygen separated in the first gas-liquid separator 25 may be released into the atmosphere.

The first circulation line 5 also includes a water discharge line 5B consisting of piping connecting the bottom of the first gas-liquid separator 25 (e.g., the portion corresponding to the liquid phase in the drum body of the knockout drum) to the first water storage tank 26. The reaction solution discharged from the first gas-liquid separator 25 is transported to the first water storage tank 26 via the water discharge line 5B. The reaction solution discharged from the first gas-liquid separator 25 is stored in the first water storage tank 26. The first water storage tank 26 can be replenished with water, a redox medium (or a redox source, which is a raw material for the medium), etc. from the outside as needed.

The first circulation line 5 also includes a first water supply line 5C consisting of piping connecting the first water storage tank 26 and a lower opening 31 located at the bottom of the hydrogen generation tank 22. A first circulation pump 27 is provided on the first water supply line 5C. The reaction solution stored in the first water storage tank 26 is transported to the reaction vessel 9 through the first water supply line 5C by the first circulation pump 27.

The second circulation line 6 is provided with a second gas-liquid separator 35, a second water storage tank 36, and a second circulation pump 37.

The second circulation line 6 includes a hydrogen discharge line 6A composed of piping connecting an upper opening 39 located at the top of the hydrogen generation tank 22 and the second gas-liquid separator 35. The hydrogen discharged from the upper opening 39 is transported to the second gas-liquid separator 35 through the hydrogen discharge line 6A. The fluid transported through the hydrogen discharge line 6A contains hydrogen as well as a portion of the reaction solution. In the second gas-liquid separator 35, the reaction solution (containing Fe ³⁺ ) is separated from the hydrogen flowing through the hydrogen discharge line 6A. The hydrogen separated in the second gas-liquid separator 35 is supplied to a hydrogen storage tank (not shown) through a hydrogen separation line 6D connected to its upper portion.

The second circulation line 6 also includes a second water discharge line 6B consisting of piping connecting the lower part of the second gas-liquid separator 35 and the second water storage tank 36. The reaction solution discharged from the second gas-liquid separator 35 is transported to the second water storage tank 36 via the second water discharge line 6B. The second water storage tank 36 stores the reaction solution discharged from the second gas-liquid separator 35. The second water storage tank 36 can be replenished with water, a redox medium (or a redox source, which is a raw material for the medium), etc. from the outside as needed.

The second circulation line 6 also includes a second water supply line 6C consisting of piping connecting the second water storage tank 36 and a lower opening 41 located at the bottom of the oxygen generation tank 21. A second circulation pump 37 is provided on the second water supply line 6C. Water stored in the second water storage tank 36 is transported to the reaction vessel 9 through the second water supply line 6C by the second circulation pump 37.

Thus, in the water decomposition system 1, the upstream end of the first circulation line 5 is connected to the oxygen generation tank 21, and the downstream end of the first circulation line 5 is connected to the hydrogen generation tank 22. This allows the reductant (here, Fe ²⁺ ) generated in the oxygen generation tank 21 to be supplied to the hydrogen generation tank 22. Furthermore, the upstream end of the second circulation line 6 is connected to the hydrogen generation tank 22, and the downstream end of the second circulation line 6 is connected to the oxygen generation tank 21. This allows the oxidant (here, Fe ³⁺ ) generated in the hydrogen generation tank 22 to be supplied to the oxygen generation tank 21. Therefore, in the water decomposition system 1, even if the oxygen generation tank 21 and the hydrogen generation tank 22 are completely separated by the photocatalyst panel 7, the redox medium required in one tank can be sequentially supplied from the other tank.

In the water splitting system 1, when sunlight is incident on the front wall 9A of the reaction vessel 9, the photocatalytic panel 7 utilizes the light energy to split the water inside the vessel into oxygen and hydrogen. The sunlight that passes through the front wall 9A and enters the oxygen generation photocatalytic layer 13 then passes through the substrate 11 and reaches the hydrogen generation photocatalytic layer 14. This allows the hydrogen generation photocatalytic layer 14 to generate hydrogen using light not absorbed by the oxygen generation photocatalytic layer 13. Therefore, the reaction vessel 9 does not need to receive sunlight for water splitting from the hydrogen generation photocatalytic layer 14 side (i.e., the rear wall 9B side of the reaction vessel 9), achieving a simple configuration. The water splitting system 1 can function as a hydrogen production system whose primary purpose is to generate hydrogen.

Although only Fe ²⁺ exists as a redox medium in the reaction vessel 9 initially, Fe ³⁺ generated together with hydrogen in the hydrogen generation tank 22 is supplied to the oxygen generation tank 21 through the second circulation line 6. As a result, oxygen is generated in the oxygen generation tank 21, and Fe ²⁺ is again generated from Fe ³⁺ by electrons in the oxygen generation photocatalyst layer 13.

(Modification of the first embodiment)
Figure 3 is a diagram showing a modified example of the water splitting apparatus 3 shown in Figure 1. In Figure 3, the same components as those in the water splitting apparatus 3 shown in Figure 1 are denoted by the same reference numerals. Furthermore, with regard to the modified example, matters not specifically mentioned below are the same as those in the first embodiment described above, and therefore detailed description thereof will be omitted.

In the first embodiment, sunlight is directly incident on the reaction vessel 9, but this is not limiting. Alternatively, sunlight may be concentrated by a light collector 45 and incident on the reaction vessel 9, as shown in FIG. 3. The reaction vessel 9 shown in FIG. 3 is cylindrical with a bottom, and light is incident from one side of the peripheral wall (here, the oxygen generating photocatalyst layer 13 side). In this way, by using the light collector 45, sunlight can be used more effectively without increasing the size of the water decomposition device 3.

Second Embodiment
Fig. 4 is a configuration diagram showing a water splitting system 1 according to a second embodiment. In Fig. 4, the same components as those in the water splitting apparatus 3 shown in Fig. 1 are denoted by the same reference numerals. Furthermore, with regard to the second embodiment, matters not specifically mentioned below are the same as those in the first embodiment described above, and therefore detailed description thereof will be omitted.

The water decomposition system 1 according to the second embodiment has a reaction vessel 9 similar to that of the first embodiment. However, in the water decomposition system 1 according to the second embodiment, the configurations of the first circulation line 105 and the second circulation line 106 are different from those of the first circulation line 5 and the second circulation line 6, respectively, in the first embodiment.

The first circulation line 105 includes an oxygen discharge line 5A and a water discharge line 5B. Meanwhile, the first circulation line 105 omits the first water supply line 5C, first water storage tank 26, and first circulation pump 27 from the first embodiment (see Figure 1).

In the first circulation line 105, the first gas-liquid separator 25 is disposed at a higher position than the reaction vessel 9. The downstream end of the oxygen discharge line 5A is connected to the first gas-liquid separator 25. The upstream end of the water discharge line 5B is connected to the bottom of the first gas-liquid separator 25. On the other hand, the downstream end of the water discharge line 5B is connected to the upper opening 39 of the hydrogen generation tank 22. The fluid flowing through the oxygen discharge line 5A is sent to the first gas-liquid separator 25 by being pushed from the oxygen generation tank 21 side by a second circulation pump 37 provided in the second circulation line 106. Furthermore, in the water discharge line 5B, the reaction solution (containing Fe ²⁺ ) separated by the first gas-liquid separator 25 flows from its upstream end to its downstream end under its own weight.

With this configuration, the first circulation line 105 can supply the reaction solution (containing Fe ²⁺ ) separated by the first gas-liquid separator 25 to the hydrogen generation tank 22 without the need for a pump.

In addition, in the first circulation line 105, a gas venting device (e.g., a gas vent valve) having a known configuration can be provided in at least one of the upper part of the reaction vessel 9 and the oxygen discharge line 5A. This allows the first gas-liquid separator 25 to be omitted from the first circulation line 5. In this case, the water discharge line 5B is omitted, and the downstream end of the oxygen discharge line 5A is connected to the upper opening 39 of the hydrogen generation tank 22.

In the second circulation line 106, the upstream portion of the hydrogen discharge line 6A is connected to a lower opening 31 located at the bottom of the hydrogen generation tank 22. However, in the hydrogen generation tank 22, the generated hydrogen may accumulate in the upper part of the tank. Therefore, in order to smoothly discharge hydrogen from the hydrogen generation tank 22, it is recommended that the upstream end of the hydrogen discharge line 6A (hydrogen discharge port) be located at the top of the hydrogen generation tank 22 through the lower opening 31.

In the water decomposition system 1 according to the second embodiment, the upstream end of the first circulation line 105 is connected to the oxygen generation tank 21, and the downstream end of the first circulation line 105 is connected to the hydrogen generation tank 22. This allows the reductant (here, Fe ²⁺ ) generated in the oxygen generation tank 21 to be supplied to the hydrogen generation tank 22. Furthermore, the upstream end of the second circulation line 106 is connected to the hydrogen generation tank 22, and the downstream end of the second circulation line 106 is connected to the oxygen generation tank 21. This allows the oxidant (here, Fe ³⁺ ) generated in the hydrogen generation tank 22 to be supplied to the oxygen generation tank 21. Therefore, in the water decomposition system 1, even if the oxygen generation tank 21 and the hydrogen generation tank 22 are completely separated by the photocatalyst panel 7, the redox medium required in one tank can be sequentially supplied from the other tank.

(Third embodiment)
Fig. 5 is a configuration diagram showing a water splitting system 1 according to a third embodiment. In Fig. 5, components similar to those of the water splitting apparatus 3 shown in Fig. 1 are denoted by the same reference numerals. Furthermore, matters not specifically mentioned below regarding the third embodiment are similar to those of the first embodiment, and therefore will not be described in detail.

In the water-splitting system 1 according to the third embodiment, the configurations of the photocatalytic panel 207 and the reaction vessel 209 differ from those of the photocatalytic panel 7 and the reaction vessel 9 in the first embodiment (see FIG. 1). Furthermore, in the water-splitting system 1 according to the third embodiment, the circulation line for circulating the reaction solution between the oxygen generation tank 21 and the hydrogen generation tank 22 is omitted.

In the first and second embodiments described above, the internal space of the reaction vessel 9 can be partitioned by the photocatalytic panel 7, thereby forming an oxygen generation tank 21 and a hydrogen generation tank 22 that do not communicate with each other. In other words, fluid movement between the oxygen generation tank 21 and the hydrogen generation tank 22 within the reaction vessel 9 is not necessary.

On the other hand, the internal space of the reaction vessel 209 is not completely partitioned by the photocatalytic panel 207, and the lower edge 207A of the photocatalytic panel 207 is spaced apart from the bottom wall 209D of the reaction vessel 209. As a result, in the reaction vessel 209, the oxygen generation tank 21 and the hydrogen generation tank 22 are in communication with each other via the space G (communication portion) between the photocatalytic panel 207 and the bottom wall 209D.

The upstream end of the oxygen separation line 205D is connected to the upper opening 29 of the oxygen generation tank 21. Oxygen discharged from the oxygen generation tank 21 through the oxygen separation line 205D is supplied to an oxygen storage tank (not shown) or released into the atmosphere.

The upstream end of the hydrogen separation line 206D is connected to the upper opening 39 of the hydrogen generation tank 22. Hydrogen discharged from the hydrogen generation tank 22 through the hydrogen separation line 206D is supplied to a hydrogen storage tank (not shown).

The water decomposition apparatus 3 is also provided with a water storage tank 236 and a water supply line 206C whose upstream end is connected to the water storage tank 236. A water supply pump 227 is provided on the water supply line 206C. The reaction solution stored in the water storage tank 236 is transported to the reaction vessel 209 through the water supply line 206C by the water supply pump 227. The downstream end of the water supply line 206C is connected to a lower opening 231 provided in the bottom wall 209D of the reaction vessel 209. The lower opening 231 is located opposite the lower end of the photocatalyst panel 207 (i.e., near the communication portion).

As described above, in the water decomposition system 1 according to the third embodiment, the oxygen generation tank 21 and the hydrogen generation tank 22 are configured to be connected within the reaction vessel 9, so that the reaction solution (including the redox medium) within the reaction vessel 209 can move between the two tanks. This allows the reductant (here, Fe ²⁺ ) generated in the oxygen generation tank 21 to move to the hydrogen generation tank 22, and the oxidant (here, Fe ³⁺ ) generated in the hydrogen generation tank 22 to move to the oxygen generation tank 21. Therefore, it is possible to omit the circulation line for circulating the reaction solution within the reaction vessel 209 as in the first and second embodiments.

Specific examples of photocatalyst panels used in the water splitting system 1 and water splitting device 3 are described below based on several examples, comparative examples, and reference examples. However, the present invention is not limited to these examples, and various modifications are possible without departing from the scope of the present invention.

In each example, comparative example, and reference example, the photocatalyst and its photocatalytic activity were evaluated using the following methods.

(Photocatalyst evaluation)
<Powder X-ray diffraction measurement>
The crystal structure of the sample was analyzed using an X-ray diffractometer (MiniFlex II, manufactured by Rigaku Corporation). CuKα radiation generated at 30 kV and 15 mA was monochromatized using a carbon monochromator, and measurements were made over the range of 2θ = 3 to 70 deg at a scanning speed of 10 deg/min.

<UV-visible spectroscopy measurement>
The optical absorption characteristics of the sample were evaluated using an ultraviolet-visible spectrophotometer (V-650, manufactured by JASCO Corporation). Measurements were performed using an integrating sphere with a standard reflector (BaSO ₄ ) as a reference, and the ultraviolet-visible diffuse reflectance spectrum of the sample powder was measured under the conditions of a scanning range of 200 to 800 nm, a scanning speed of 1000 nm/min, and a sampling interval of 0.5 nm.

<Shape observation>
The particle shape of the powder sample was observed using a transmission electron microscope (JEM-2100F, manufactured by JEOL Ltd.) by fixing the sample to a microgrid-attached mesh.

(Evaluation of photocatalytic activity)
<Evaluation device>
6 is a diagram showing the configuration of the evaluation device 303 (water splitting device) used in each example, comparative example, and reference example. The evaluation device 303 includes a photocatalytic panel 307 that uses a photocatalyst, and a downward-irradiated reaction cell 51 (reaction vessel) that contains the photocatalytic panel 307 and a reaction solution and performs a water splitting reaction.

The evaluation device 303 has the same configuration as the water splitting device 3 described above, except for the points specifically mentioned below. The photocatalyst panel 307 includes a light-transmitting substrate 311, an oxygen generation photocatalyst layer 313 formed to overlap the lower surface 311A (first surface) of the substrate 311, and a hydrogen generation photocatalyst layer 314 formed to overlap the upper surface 311B (second surface) of the substrate 311 that forms a pair with the lower surface 311A. The photocatalyst panel 307 is supported by a spacer 53. In the evaluation device 303, light from a xenon lamp was used instead of sunlight.

The evaluation device 303 was prepared as follows: 40 mL of each reaction solution was added to a downward-illuminated reaction cell 51 equipped with a Pyrex (registered trademark) window (circular, 7.0 cm diameter), and then the photocatalyst panel 307 of each Example, Comparative Example, and Reference Example was placed horizontally against the Pyrex window. Spacers 53 made of carbon tape were attached to all four sides of the photocatalyst panel 307 in the areas where no photocatalyst was applied, separating the underside of the photocatalyst panel 307 from the bottom of the cell 51. The reaction cell 51 containing the reaction solution and photocatalyst panel 307 was attached to a closed-circulation evaluation device, and after evacuating, Ar gas was introduced.

<Water decomposition reaction and its evaluation>
The water splitting reaction in the evaluation device 303 was carried out as follows. Visible light was irradiated vertically (from the bottom to the top in FIG. 6 ) from the Pyrex window side of the reaction cell 51 using a 300 W xenon lamp equipped with a CM-1 cold mirror and a cutoff filter (L42, λ > 400 nm). The distance between the xenon lamp (light irradiation port) and the irradiation window was 10 mm or less. The temperature of the reaction solution was controlled using a cooling water circulator to maintain it at 288 K. The amount of generated gas was analyzed and measured using a gas chromatograph analyzer (Shimadzu Corporation, GC-8A, Molecular Sieve 5A column) using Ar gas as the carrier gas.

<Preparation of photocatalyst powder for oxygen generation>
The oxygen generating photocatalyst powder used in the oxygen generating photocatalyst layer 313 was prepared as follows.

<<Synthesis of Fe—H—Cs— _WO3 >>
The _WO3 powder was classified, and coarse particles were collected. 1 g of the classified and collected _WO3 was added to an evaporating dish, and 950 μL of a 0.1 M CsCl aqueous solution was added dropwise. The water was then evaporated to dryness. The amount of CsCl added was adjusted so that the Cs content was 2.1 mol% relative to the _WO3 . The sample, still in the evaporating dish, was placed in an electric furnace and fired in air at 500°C for 30 minutes. 0.5 g of the collected sample was suspended in 50 mL of a _{1 M H2SO4} aqueous solution containing 50 mM _FeSO4 aqueous solution and stirred for 30 minutes. The product was then collected by suction filtration, washed with ultrapure water, and dried at room temperature. This yielded Fe-H-Cs- _WO3 powder.

<Preparation of photocatalyst powder for hydrogen generation>
The photocatalytic powder for hydrogen generation used in the photocatalytic layer for hydrogen generation 314 was prepared as follows.

<<Synthesis of ZrO ₂ /TaON>>
The raw materials used were _{powdered Ta2O5} and ZrO( _NO3 ) _{2.2H2O . The Ta2O5 and ZrO(NO3)2.2H2O were weighed so that} the molar ratio of Zr/Ta was _0.1 . These powder samples were mixed with a small amount of methanol in a mortar to obtain a mixed powder. The mixed powder was filled into an alumina crucible, dried at 70°C for 1 hour, and then fired in air at 800°C for 2 hours. This resulted in TaON modified with _ZrO2 .

The _ZrO2 -modified TaON was loaded into an alumina boat and placed in a quartz tube. The quartz tube was connected to a tubular furnace, and the _ZrO2 -modified TaON was heated in an ammonia gas flow at 12 mL/min. The alumina boat was heated to 850°C at a rate of 10°C/min and held at that temperature for 15 hours. After holding for 15 hours, the product was allowed to cool naturally to room temperature. The alumina boat was removed from the furnace, and the _ZrO2 -modified TaON powder was recovered. The average particle size of the _ZrO2 -modified TaON particles was in the range of 200 to 500 nm.

In this specification, the term " _ZrO2 /TaON" means TaON modified with _ZrO2 . _ZrO2 suppresses the generation of reduced Ta species ( _TaIV ) during the nitridation process of _Ta2O5 . ^TaIV acts as a recombination center between electrons and holes, reducing photocatalytic activity. By modifying the surface of ^TaON particles with _ZrO2 , the generation of ^TaIV is suppressed, resulting in TaON particles with higher photocatalytic activity.

<<Synthesis of InHCF nanoparticles>>
_InCl3 · _4H2O (30 mmol) was dissolved in ultrapure water to obtain a 30 mL solution. _K4 [Fe(CN) ₆ ]· _3H2O (30 mmol) was dissolved in ultrapure water to obtain a 60 mL solution. These solutions were mixed and vigorously stirred for 5 minutes. The resulting precipitate was washed three times with ultrapure water and once with methanol, and then centrifuged. The precipitate was then dried in a vacuum dryer at 35°C. This yielded indium(III) hexacyanoferrate powder.

Powder X-ray diffraction and infrared spectroscopy of the precipitate confirmed that indium(III) hexacyanoferrate ( _KxIn [Fe(CN) ₆ ] _yn.nH2O , ₀ ≦x≦1, 0.75≦y≦1, 0≦n) was obtained. In this specification, the term "InHCF" means indium(III) hexacyanoferrate.

Observation using a transmission electron microscope confirmed that the InHCF was in the form of nanoparticles with an average particle size of less than 80 nm.

<<Supporting of promoter on ZrO ₂ /TaON>>
Rh _x Cr _2-x O ₃ (reduction promoter) and InHCF (oxidation promoter) were sequentially supported on ZrO ₂ /TaON by the following method.

A dispersion was obtained by dispersing _ZrO2 /TaON powder in _{150 mL of a methanol aqueous solution containing Na3RhCl6.nH2O and K2CrO4 dissolved therein . The} methanol aqueous solution was prepared so that the methanol concentration was 20 vol%, the amount of Rh relative to TaON was 1 mass%, and the amount of Cr relative to TaON was 1.5 mass%.

Before light irradiation, dissolved oxygen in the dispersion was removed by Ar bubbling at a flow rate of 200 mL/min. Ar bubbling was continued at a flow rate of 50 mL/min during light irradiation. The dispersion was irradiated with visible light (λ > 400 nm) for 6 hours using a 300 W xenon lamp equipped with an L-42 cutoff filter and a CM-1 cold mirror. After light irradiation, the powder was recovered by suction filtration, washed with ultrapure water, and vacuum dried overnight at 35°C. Hereinafter, this sample will be referred to as "Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON".

Next, Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was suspended in a small amount of water in which InHCF had been dispersed to obtain a suspension. The amount of InHCF was adjusted so that the amount of Fe in InHCF was 10 mol% relative to TaON. The suspension was uniformly mixed by ultrasonic treatment for 30 seconds. Then, water was evaporated from the suspension to dry the sample. The sample was baked at 100°C for 1 hour in an Ar gas flow with a flow rate of 20 mL/min. This resulted in InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON, the photocatalyst for hydrogen generation described in Example 1 below.

<<Synthesis of SrTiO ₃ :Rh>>
The raw materials _used were powdered reagents of _SrCO3 , _TiO2 , and _Rh2O3 . The powdered reagents were weighed to a molar ratio of Sr:Ti:Rh = 1.07:1.00:0.01. These powdered reagents and a small amount of methanol were mixed in a mortar to obtain a mixed powder. The mixed powder was filled into an alumina crucible and fired in air at 600°C for 2 hours, and then further fired in air at 1000°C for 10 hours. This resulted in Rh-doped _SrTiO3 . Note that in this specification, the term " _SrTiO3 :Rh" means Rh-doped _SrTiO3 .

<<Supporting of Ru Promoter on SrTiO ₃ :Rh>>
The Ru promoter was sequentially supported on ZrO ₂ /TaON by the following method: A dispersion was obtained by dispersing SrTiO ₃ :Rh powder in 150 mL of aqueous methanol solution containing RuCl ₃ ·nH ₂ O. The aqueous methanol solution was prepared so that the methanol concentration was 20 vol % and the amount of Ru was 0.7 mass % relative to the TaON.

Before light irradiation, dissolved oxygen in the dispersion was removed by Ar bubbling at a flow rate of 200 mL/min. Ar bubbling was continued at a flow rate of 50 mL/min during light irradiation. The dispersion was irradiated with visible light (λ>400 nm) for 6 hours using a 300 W xenon lamp equipped with an L-42 cutoff filter and a CM-1 cold mirror. After light irradiation, the powder was recovered by suction filtration, washed with ultrapure water, and vacuum dried overnight at 35°C. Hereinafter, this sample will be referred to as "Ru/SrTiO ₃ :Rh".

Figure 7 shows the UV-visible diffuse reflectance spectra of various synthesized semiconductors ( _ZrO2 /TaON, _SrTiO3 :Rh, _WO3 ). In Figure 7, the vertical axis shows the diffuse reflectance value obtained by Kubelka-Munk transformation. In particular, the absorption edge wavelength of _ZrO2 /TaON is located on the longer wavelength side compared to the other semiconductors.

Next, we will explain the details of Examples 1-5.

[Example 1]
First, _BiVO4 was applied as a photocatalyst for oxygen generation to one side of a quartz glass substrate (3.8 cm x 5 cm x 1.0 mm thick). The application area was 11.4 ^cm2 (3.8 cm x 3 cm). A precursor solution was prepared: a ₁ M acetic acid solution containing Bi( _NO3 ) _3.5H2O and a 0.15 M methanol solution containing VO(acac) ₂ . These two solutions were mixed at a molar ratio of Bi/V = 1 to obtain a precursor solution. 100 μL of this precursor solution was dropped onto the quartz glass substrate and spin-coated at 750 rpm for 2 minutes. The substrate was then baked on a hot plate at 500°C for 30 minutes. The deposition amount of _BiVO4 could be controlled by repeating the cycle of spin-coating the precursor solution and baking on a hot plate. In Example 1, this cycle was repeated four times to deposit 2.5 mg of _BiVO4 on the quartz glass substrate.

8 shows the transmittance of the glass substrate on which _BiVO4 of Example 1 is deposited (i.e., the transmittance toward the hydrogen generation catalyst layer side of the photocatalytic panel). As shown in Fig. 8, the transmittance of the substrate 311 on which the oxygen generation photocatalytic layer 313 is formed is preferably at least 40% or more in the wavelength region of 500 nm or more (at least 500 to 650 nm) of light incident on the oxygen generation photocatalytic layer 313. This allows the light energy in the visible light region that has passed through the oxygen generation photocatalytic layer 313 and the substrate 311 to be more effectively utilized in the hydrogen generation photocatalytic layer 314.

Next, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was applied as a photocatalyst for hydrogen generation to the surface opposite to the BiVO ₄ coating, using the following procedure to achieve a coating area of 11.4 cm ² (3.8 cm × 3 cm) and a deposition amount of 6.6 mg. Using an agate mortar, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was thoroughly dispersed in a small amount of ultrapure water. This dispersion was dropped onto the surface opposite to the BiVO ₄ coating of the glass substrate, and spread evenly with a glass rod. After application, _the substrate was dried at room temperature. Hereinafter, this photocatalytic panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|glass|BiVO ₄ ".

The water splitting activity of the photocatalyst panel of Example 1, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | glass | BiVO ₄ , was evaluated in an FeCl ₂ aqueous solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before the reaction (before light irradiation) was 2.3.

[Example 2]
In Example 2, a photocatalyst panel was prepared by the same method as in Example 1. The water splitting activity of the photocatalyst panel of Example 2 was evaluated by the same method as in Example 1, except that the pH before the reaction in the _FeCl2 aqueous solution was changed to 2.5.

[Example 3]
In Example 3, a photocatalyst panel was prepared by the same method as in Example 2. The water splitting activity of the photocatalyst panel of Example 3 was evaluated by the same method as in Example 1, except that the FeCl ₂ aqueous solution in the reaction solution was changed to an Fe(NO ₃ ) ₃ aqueous solution.

[Example 4]
First, _BiVO4 was applied as a photocatalyst for oxygen generation to a quartz glass substrate by the same method as in Example 1. The application area was 11.4 ^cm2 (3.8 cm x 3 cm). The application amount was 2.7 mg.

Next, Ru/ _SrTiO3 :Rh was applied as a photocatalyst for hydrogen generation to the surface opposite the _BiVO4 -coated surface, using the following procedure to achieve a coating area of 11.4 ^cm2 (3.8 cm x 3 cm) and a deposition amount of 6.0 mg. Using an agate mortar, Ru/ _SrTiO3 :Rh was thoroughly dispersed in a small amount of ultrapure water. This dispersion was then dropped onto the surface opposite the _{BiVO4 -} coated surface of the glass substrate, and spread evenly with a glass rod. After coating, the panel was allowed to dry at room temperature. Hereafter, this photocatalyst panel will be referred to as " _SrTiO3 | glass | _BiVO4 ."

The water splitting activity of the photocatalyst panel of Example 4, SrTiO ₃ |glass|BiVO _4, was evaluated by the same method as in Example 1.

[Example 5]
First, Fe—H—Cs-WO ₃ was applied as a photocatalyst for oxygen generation to a quartz glass substrate in the following manner, so that the application area was 11.4 cm ² (3.8 cm × 3 cm) and the deposition amount was 6.0 mg. Using an agate mortar, Ru/SrTiO ₃ :Rh was thoroughly dispersed in a small amount of ultrapure water. This dispersion was dropped onto the quartz glass substrate and spread evenly with a glass rod. After application, it was dried at room temperature.

Next, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was applied as a photocatalyst for hydrogen generation to the surface opposite to the surface coated with Fe—H—Cs-WO ₃ using the same method as in Example 1. The coating area was 11.4 cm ² (3.8 cm × 3 cm), and the deposition amount was 6.0 mg. Hereafter, this photocatalyst panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | Glass | Fe—H—Cs-WO ₃ ".

The water splitting activity of the photocatalyst panel of Example 5, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|glass|Fe—H—Cs—WO _3, was evaluated in an FeCl ₂ aqueous solution. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before the reaction (before light irradiation) was 2.5.

Next, we will explain the details of Comparative Examples 1-8 and Reference Examples 1 and 2.

[Comparative Example 1]
Using the same method as in Example 1, _BiVO4 was applied to one side of a glass substrate as a photocatalyst for oxygen generation. The amount of application was 1.2 mg. Nothing was applied to the back side. Hereinafter, this photocatalyst panel will be referred to as "None | Glass | _BiVO4 (1)." Figure 9 shows the transmittance of the obtained None | Glass | _BiVO4 (1) (i.e., the transmittance of the photocatalyst panel).

The oxygen generation activity of the photocatalyst panel of Comparative Example 1, glass-free _BiVO4 (1), was evaluated in an _FeCl3 aqueous solution. The evaluation method was as described above. The Fe2 ⁺ cation concentration in the _FeCl3 aqueous solution was 2 mM, and the pH before the reaction (before light irradiation) was 2.5.

[Comparative Example 2]
Using the same method as in Example 1, _BiVO4 was applied to one side of a glass substrate as a photocatalyst for oxygen generation. The amount applied was 2.4 mg. Nothing was applied to the back side. Hereinafter, this photocatalyst panel will be referred to as "None | Glass | _BiVO4 (2)." Figure 10 shows the transmittance of the obtained None | Glass | _BiVO4 (2) (i.e., the transmittance of the photocatalyst panel).

The oxygen generation activity of the photocatalyst panel of Comparative Example 2, glass-free _BiVO4 (2), was evaluated in an _FeCl3 aqueous solution. The evaluation method was as described above. The Fe2 ⁺ cation concentration in the _FeCl3 aqueous solution was 2 mM, and the pH before the reaction (before light irradiation) was 2.5.

[Comparative Example 3]
Using the same method as in Example 1, _BiVO4 was applied to one side of a glass substrate as a photocatalyst for oxygen generation. The amount of application was 3.6 mg. Nothing was applied to the back side. Hereinafter, this photocatalyst panel will be referred to as "None | Glass | _BiVO4 (3)." Figure 11 shows the transmittance of the obtained No | Glass | _BiVO4 (3) (i.e., the transmittance of the photocatalyst panel).

The oxygen generation activity of the photocatalyst panel of Comparative Example 3, glass-free _BiVO4 (3), was evaluated in an _FeCl3 aqueous solution. The evaluation method was as described above. The Fe2 ⁺ cation concentration in the _FeCl3 aqueous solution was 2 mM, and the pH before the reaction (before light irradiation) was 2.5.

[Comparative Example 4]
Using the same method as in Example 1, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was applied as a photocatalyst for hydrogen generation to one side of a glass substrate. The amount applied was 6.5 mg. Nothing was applied to the other side. Hereafter, this photocatalyst panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | Glass | None."

The hydrogen production activity of the photocatalyst panel of Comparative Example 4, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | Glass | None, was evaluated in an FeCl ₂ aqueous solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before the reaction (before light irradiation) was 2.5.

[Comparative Example 5]
A photocatalyst panel for Comparative Example 5 was prepared by the same method as for the photocatalyst panel for Comparative Example 4. The water splitting activity of the photocatalyst panel for Comparative Example 5 was evaluated by the same method as for Comparative Example 4, except that the pH before the reaction in the _FeCl2 aqueous solution was changed to 2.3.

[Comparative Example 6]
A photocatalyst panel for Comparative Example 6 was prepared by the same method as for the photocatalyst panel for Comparative Example 4. The water splitting activity of the photocatalyst panel for Comparative Example 6 was evaluated by the same method as for Comparative Example 4, except that the pH before the reaction in the _FeCl2 aqueous solution was changed to 2.1.

[Comparative Example 7]
A photocatalyst panel for Comparative Example 7 was prepared by the same method as for the photocatalyst panel of Example 2. The water splitting activity of the photocatalyst panel of Comparative Example 7 was evaluated by the same method as for Example 2, except that _FeCl2 , a redox medium source, was not added to the reaction solution for evaluating the water splitting activity.

[Comparative Example 8]
Using the same procedure as in Example 1, _BiVO4 was deposited on one side of an alumina substrate (3.8 cm x 5 cm x 1.0 mm thick) as a photocatalyst for oxygen generation. The coating area was 11.4 ^cm2 (3.8 cm x 3 cm). The coating amount was 2.9 mg. Figure 12 shows the transmittance of the alumina substrate and glass substrate without a photocatalyst coating. As shown in Figure 12, the alumina substrate has lower light transmittance than the glass substrate.

Next, using the same method as in Example 1, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON was applied as a photocatalyst for hydrogen generation to the surface opposite to the surface coated with BiVO _4. The application area was 15 cm ² (3 cm x 5 cm), and the deposition amount was 5.7 mg. Hereafter, this photocatalyst panel will be referred to as "InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | alumina | BiVO ₄ ".

The hydrogen production activity of the photocatalyst panel of Comparative Example 8, InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON | alumina | BiVO ₄ , was evaluated in an FeCl ₂ aqueous solution. The evaluation method was as described above. The Fe ²⁺ cation concentration in the FeCl ₂ aqueous solution was 2 mM, and the pH before the reaction (before light irradiation) was 2.3.

[Reference example 1]
A photocatalyst panel for Reference Example 1 was prepared by the same method as the photocatalyst panel for Example 2. The water splitting activity of the photocatalyst panel for Reference Example 1 was evaluated by the same method as in Example 2, except that the _FeCl2 aqueous solution in the reaction solution was changed to an _FeSO4 aqueous solution.

[Reference example 2]
A photocatalyst panel for Reference Example 2 was prepared by the same method as that for the photocatalyst panel of Example 2. The water splitting activity of the photocatalyst panel for Reference Example 2 was evaluated by the same method as that of Example ₂ , except that the FeCl2 aqueous solution in the reaction solution was changed to an Fe( _ClO4 ) ₂ aqueous solution.

The configurations and redox sources of the photocatalyst panels used in Examples 1-5, Comparative Examples 1-8, and Reference Examples 1 and 2 are summarized in Table 2. Table 2 provides a simplified description of the oxygen generation photocatalysts and hydrogen generation photocatalysts in each Example, Comparative Example, and Reference Example, but the details are as described above.

The results of the water splitting reactions (hydrogen and oxygen production rates) and the initial pH of the reaction solutions in Examples 1-5, Comparative Examples 1-8, and Reference Examples 1 and 2 described above are summarized in Table 3. In Table 3, the gas production rates (hydrogen production rate, oxygen production rate) for Examples 1-5 are values obtained when the ratio of the hydrogen production rate to the oxygen production rate is approximately 2:1 (i.e., the stoichiometric ratio corresponding to complete water splitting). Furthermore, the gas production rates for Comparative Examples 1-8 and Reference Examples 1 and 2 are based on the amount of gas produced up to one hour after the start of light irradiation.

(1) Necessity of a photocatalyst for hydrogen generation and a photocatalyst for oxygen generation As shown in Table 3, simultaneous generation of hydrogen and oxygen was confirmed in Example 1 (a photocatalyst panel including a photocatalyst for hydrogen generation and a photocatalyst for oxygen generation). Figure 13 is a graph showing the time-dependent changes in the water splitting reaction in Example 1. Figure 13 shows the time-dependent changes in the hydrogen generation rate, oxygen generation rate, and their ratio (oxygen generation rate/hydrogen generation rate).

In Example 1, the amount of hydrogen produced after 6.0 hours of light irradiation exceeded the stoichiometric amount (40 μmol). Furthermore, in Example 1, the ratio of the oxygen production rate to the hydrogen production rate eventually approached 0.5 (the stoichiometric ratio corresponding to complete water decomposition). These results indicate that the hydrogen production photocatalyst in the upper hydrogen production photocatalyst layer 314 oxidized Fe ²⁺ to produce Fe ³⁺ while generating hydrogen, and that the oxygen production photocatalyst in the lower oxygen production photocatalyst layer 313 reduced Fe ³⁺ to produce Fe ²⁺ while generating oxygen. That is, in Example 1, it was demonstrated that photocatalytic water decomposition proceeded by a Z-scheme mechanism.

No hydrogen generation was observed in Comparative Example 1-3 (photocatalytic panel without a photocatalyst for hydrogen generation). Furthermore, no oxygen generation was observed in Comparative Example 4-6 (photocatalytic panel without a photocatalyst for oxygen generation). This demonstrates that both photocatalysts are necessary for Z-scheme water splitting.

(2) Necessity of a redox medium Figure 14 is a graph showing the time course of the water splitting reaction in Comparative Example 7 (a ^{photocatalytic} panel including InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON|glass|BiVO ₄ in the absence of a redox medium (Fe 2+ )). The components of the graph in Figure 14 are the same as those of the graph in Figure 13.

When comparing water splitting activity in the presence or absence of a redox mediator (Fe ²⁺ ), clearer water splitting activity was confirmed in the presence of Fe ²⁺ (Example 1) than in the absence of Fe ²⁺ (Comparative Example 7). This shows that the redox mediator (Fe ³⁺ /Fe ²⁺ ) present in the reaction solution is responsible for electron transfer between the upper (rear) hydrogen generation photocatalyst and the lower (front) oxygen generation photocatalyst, and is essential for promoting water splitting via the Z-scheme mechanism. Furthermore, it was found that water splitting, which proceeds by electron exchange when at least one of the hydrogen generation photocatalyst and the oxygen generation photocatalyst peels off from the substrate and comes into direct contact with each other, hardly progressed here.

(3) Effect of Substrate Material Figure 15 is a graph showing the change over time in the water splitting reaction in Comparative Example 8 (photocatalytic panel using an alumina substrate). It was confirmed that the water splitting activity when an alumina substrate was used was significantly lower than that when a glass substrate was used (Example 1). As shown in Figure 12, the alumina substrate has lower optical transparency than the glass substrate (less than 40% in the visible light range). From this, it is thought that when an alumina substrate was used, almost no light was incident from the lower oxygen generation photocatalyst layer 313 through the substrate 311 (alumina substrate) to the upper hydrogen generation photocatalyst layer 314, and the hydrogen generation photocatalyst was not sufficiently photoexcited. Therefore, the substrate material is required to have sufficient optical transparency to allow the photocatalyst to be photoexcited and exhibit its photocatalytic activity.

(4) Effect of the type of photocatalyst Figure 16 shows the time course of the water splitting reaction in Example 4 (a photocatalytic panel containing Ru/ _SrTiO3 :Rh as a photocatalyst for hydrogen generation). In Example 4, the amount of hydrogen produced after 30 hours of light irradiation exceeded the stoichiometric amount (40 μmol). Furthermore, in Example 4, the ratio of the oxygen production rate to the hydrogen production rate eventually approached 0.5. These results demonstrate that photocatalytic water splitting proceeds via the Z-scheme mechanism even when Ru/ _SrTiO3 :Rh is used as a photocatalyst for hydrogen generation.

Figure 17 shows the time course of the water splitting reaction in Example 5 (a photocatalytic panel containing Fe-H-Cs- _WO3 as an oxygen generating photocatalyst). In Example 5, the amount of hydrogen produced after 6.5 hours of light irradiation exceeded the stoichiometric amount (40 μmol). Furthermore, in Example 5, the ratio of the oxygen production rate to the hydrogen production rate eventually approached 0.5. These results demonstrate that photocatalytic water splitting proceeds via the Z-scheme mechanism even when Fe-H-Cs- _WO3 is used as the oxygen generating photocatalyst.

(5) Effect of the type of counter anion in the redox source Figure 18 is a graph showing the time course of the water splitting reaction of Example 3 (Fe( _NO3 ) ₃ aqueous solution). Figure 19 is a graph showing the time course of the water splitting reaction of Reference Example 1 ( _FeSO4 aqueous solution). Figure 20 is a graph showing the time course of the water splitting reaction of Reference Example 2 (Fe( _ClO4 ) ₂ aqueous solution).

As shown in Figures 8 and 18-20, in Examples 1 and ³ and Reference Examples 1 and 2, water splitting activity varied depending on the type of anion in the redox source ( ^Cl- , _NO3- , _SO42- , ^ClO4- ). Hydrogen and oxygen were generated simultaneously and continuously only when the counter anion ^{was Cl-} or _NO3- (Examples ¹ and ₃ ). These results suggest that photocatalytic activity (water splitting activity) can be optimized not only by optimizing the photocatalytic layer but also by optimizing the type of counter anion in the redox source.

(6) Effect of pH of reaction solution In Example 2, when the pH at the beginning of the reaction was 2.5, a brown precipitate by-product was confirmed on the photocatalyst panel 307 after the reaction. This is thought to be due to a local increase in pH, which caused the Fe source added as a redox medium to precipitate as Fe(OH) ₃ , etc.

As in Comparative Examples 5 and 6, when the initial pH of the reaction was set to 2.3 or 2.1 and a hydrogen generation reaction was carried out using InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON |glass|none, hydrogen generation activity was observed without the formation of a brown precipitate. Furthermore, as in Example 1, when the initial pH of the reaction was set to 2.3 and a water splitting reaction was carried out using InHCF/Rh _x Cr _2-x O ₃ /ZrO ₂ /TaON |glass|BiVO ₄ , water splitting activity was observed without the formation of a brown precipitate. These results demonstrate that, from a practical perspective, appropriate pH control is important for proceeding with Z-scheme water splitting without the formation of by-products.

This concludes the description of the embodiments, including specific examples, but the present invention is not limited to the above-described embodiments and variations, and can be implemented in a wide variety of variations. Not all of the components of the photocatalytic water splitting device and water splitting system equipped therewith shown in the above-described embodiments (including each example) are necessarily essential, and at least those skilled in the art can select appropriate components without departing from the scope of the present invention.

1: Water decomposition system 3: Water decomposition device 5: First circulation line 5A: Oxygen discharge line 5B: Water discharge line 5C: First water supply line 5D: Oxygen separation line 6: Second circulation line 6A: Hydrogen discharge line 6B: Second water discharge line 6C: Second water supply line 6D: Hydrogen separation line 7: Photocatalytic panel 9: Reaction vessel 9A: Front wall (light incident wall)
9B: Rear wall 9C: Upper wall 9D: Lower wall 11: Substrate 11A: Front (first surface)
11B: Rear surface (second surface)
13: Oxygen generating photocatalyst layer (first photocatalyst layer)
14: Photocatalytic layer for hydrogen generation (second photocatalytic layer)
21: Oxygen generation tank 22: Hydrogen generation tank 25: First gas-liquid separator 26: First water storage tank 27: First circulation pump 29: Upper opening 31: Lower opening 35: Second gas-liquid separator 36: Second water storage tank 37: Second circulation pump 39: Upper opening 41: Lower opening 45: Light collector 51: Downward irradiation type reaction cell 53: Spacer 105: First circulation line 106: Second circulation line 205D: Oxygen separation line 206C: Water supply line 206D: Hydrogen separation line 207: Photocatalytic panel 207A: Lower edge 209: Reaction vessel 209D: Lower wall 226: Water storage tank 227: Water supply pump 231: Lower opening 303: Evaluation device 307: Photocatalytic panel 311 :Substrate 311A:Bottom surface 311B:Top surface 313:Photocatalyst layer for oxygen generation 314:Photocatalyst layer for hydrogen generation

Claims (13)

A water splitting device using a photocatalyst that utilizes light energy,
Photocatalytic panels,
a reaction vessel that accommodates the photocatalyst panel and a reaction solution and performs a water decomposition reaction in the reaction solution;
Equipped with
The photocatalytic panel is
a light-transmitting substrate;
a first photocatalytic layer formed so as to overlap the first surface of the substrate;
a second photocatalytic layer formed so as to overlap a second surface of the substrate that is paired with the first surface;
Including,
one of the first photocatalytic layer and the second photocatalytic layer is a photocatalytic layer for generating oxygen containing a first semiconductor, and the other is a photocatalytic layer for generating hydrogen containing a second semiconductor;
The reaction vessel comprises:
The photocatalytic panel divides the interior space,
an oxygen generating tank formed on the oxygen generating photocatalyst layer side;
a hydrogen generation tank formed on the hydrogen generation photocatalyst layer side;
a light incident wall that transmits light incident toward the first photocatalytic layer of the photocatalytic panel. The water splitting apparatus of claim 1, wherein the substrate is non-conductive. The water splitting apparatus of claim 1 or 2, wherein the optical absorption spectrum of the second semiconductor is located on the longer wavelength side compared to the optical absorption spectrum of the first semiconductor. the first semiconductor includes one or a combination of two or more selected from a metal oxide, a metal oxynitride, a nitride, a metal oxysulfide, a metal oxyhalide, and a metal organic framework;
4. The water splitting apparatus according to claim 1, wherein the second semiconductor comprises one or a combination of two or more selected from the group consisting of metal oxynitrides, metal nitrides, metal oxysulfides, metal sulfides, metal oxyhalides, metal halides, organometallic halides, metal oxides, metal phosphides, silicon, organic semiconductors, metal-organic frameworks, and covalent organic frameworks. A water splitting apparatus according to any one of claims 1 to 4, wherein the first photocatalytic layer and the substrate portion of the photocatalytic panel have a transmittance of 40% or more for light incident on the first photocatalytic layer in the visible light range of at least 500 nm or more. The reaction solution includes a redox medium,
6. The water splitting apparatus according to claim 1, wherein the redox medium comprises one or a combination of two or more selected from the group consisting of an iron ion pair, an ion pair containing iodine, an ion pair containing vanadium, a complex ion pair, and a polyoxometalate ion pair. The water decomposition apparatus according to any one of claims 1 to 6, wherein a communication section is formed between the reaction vessel and the photocatalytic panel, allowing the reaction solution in the oxygen generation tank and the hydrogen generation tank to flow through the communication section. A water splitting apparatus according to any one of claims 1 to 7, wherein light is incident on the reaction vessel only through the light incident wall. A water splitting system comprising the water splitting device described in any one of claims 1 to 8 and a water storage tank for storing water to be supplied to the water splitting device. a first circulation line for circulating a portion of the reaction solution discharged together with oxygen from the oxygen generation tank to the hydrogen generation tank; and a second circulation line for circulating a portion of the reaction solution discharged together with hydrogen from the hydrogen generation tank to the oxygen generation tank.
The water splitting system of claim 9, comprising: an oxygen separation line that transports the oxygen separated from the reaction solution is connected to the first circulation line;
The water decomposition system according to claim 10 , wherein the second circulation line is connected to a hydrogen separation line that transports the hydrogen separated from the reaction solution. The water decomposition system described in claim 10 or 11, wherein the water storage tank is provided in at least one of the first circulation line and the second circulation line. The water decomposition system described in claim 12, wherein a gas-liquid separator is provided upstream of the water storage tank.