Photocatalytic cell and hydrogen gas generation system

Abstract

A photocatalytic cell of the disclosure is installed in an inclined manner at an angle of 5° or more and 45° or less with respect to a horizontal plane. The photocatalytic cell includes: a translucent member; an electrolytic solution; a photocatalytic sheet including photocatalytic particles; an injection port through which the electrolytic solution is injected into an inside of the photocatalytic cell; a discharge port through which the electrolytic solution is discharged to an outside of the photocatalytic cell; and an exhaust port through which gas inside the photocatalytic cell is discharged, at least a part of the photocatalytic sheet is immersed in the electrolytic solution, a position of the exhaust port is higher than a position of the injection port, a gap between a surface of the translucent member and a surface of the photocatalytic sheet is 5 mm or more and 50 mm or less in width, and the injection port and the discharge port allow the electrolytic solution to flow from an upper part toward a lower part in the gap between the translucent member and the photocatalytic sheet.

Description

INCORPORATION BY REFERENCE

[0001] This application is based upon and claims the benefit of priority from the corresponding Japanese Patent Application No. 2024-133330 filed on Aug. 8, 2024, the entire contents of which are incorporated herein by reference.

[0002] The disclosure relates to a photocatalytic cell and a hydrogen gas generation system.

[0003] Since no CO2 is emitted even when hydrogen gas is used, hydrogen gas is expected to be a next generation energy source. In particular, “green hydrogen”, which is produced by electrolyzing water using electricity generated by solar or wind power, is attracting attention because no CO2 is emitted even during a production process of the hydrogen. However, green hydrogen has a problem of high production cost. An electrolysis voltage of industrially used water electrolytic baths is from 1.5 to 2.1 V, which consumes a lot of power.

[0004] A known device capable of generating hydrogen gas from water at a low electrolysis voltage is one in which a photocatalyst that reduces iron ions in an electrolytic solution is combined with an electrolytic bath that generates hydrogen gas.SUMMARY

[0005] A photocatalytic cell according to an aspect of the disclosure is installed in an inclined manner at an angle of 5° or more and 45° or less with respect to a horizontal plane. The photocatalytic cell includes: a translucent member that takes in light from an upper side or an obliquely upper side; an electrolytic solution housed in the photocatalytic cell; a photocatalytic sheet including photocatalytic particles that receive the light taken in from the translucent member; an injection port through which the electrolytic solution is injected into an inside of the photocatalytic cell; a discharge port through which the electrolytic solution is discharged to an outside of the photocatalytic cell; and an exhaust port through which gas inside the photocatalytic cell is discharged. The photocatalytic cell is characterized in that: at least a part of the photocatalytic sheet is immersed in the electrolytic solution; a position of the exhaust port is higher than a position of the injection port; a gap between a surface of the translucent member and a surface of the photocatalytic sheet is 5 mm or more and 50 mm or less in width; and the injection port and the discharge port allow the electrolytic solution to flow from an upper part toward a lower part in the gap between the translucent member and the photocatalytic sheet.

[0006] Preferably, the electrolytic solution contains a first cation, and the electrolytic solution and the photocatalytic particles allow the first cation to be reduced to a second cation by photocatalytic activity of the photocatalytic particles produced by receiving light.

[0007] The first cation is preferably a trivalent iron ion and the second cation is preferably a divalent iron ion.

[0008] The position of the exhaust port is preferably higher than the position of the injection port by 10 mm or more.

[0009] The photocatalytic particles preferably include tungsten oxide particles.

[0010] The disclosure also provides a hydrogen gas generation system including: the photocatalytic cell of the disclosure; and an electrolyzer including a cathode and an anode, the electrolyzer generating hydrogen gas from water or hydrogen ions at the cathode, and oxidizing a second cation to a first cation at the anode, wherein the photocatalytic cell and the electrolyzer allow an electrolytic solution containing the second cation generated in the photocatalytic cell to be supplied to the electrolyzer, and an electrolytic solution containing the first cation generated at the anode to be supplied to the photocatalytic cell.

[0011] This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description with reference where appropriate to the accompanying drawings. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure.BRIEF DESCRIPTION OF THE DRAWINGS

[0012] FIG. 1 is a schematic cross-sectional view of a photocatalytic cell of an embodiment of the disclosure.

[0013] FIG. 2 is a schematic cross-sectional view of the photocatalytic cell of the embodiment of the disclosure.

[0014] FIG. 3 is a schematic cross-sectional view of a hydrogen gas generation system of an embodiment of the disclosure.DETAILED DESCRIPTION

[0015] An embodiment of the disclosure will be described below with reference to the drawings. Configurations illustrated in the drawings and the following description are examples, and the scope of the disclosure is not limited to the configurations illustrated in the drawings or the following description.Photocatalytic Cell

[0016] FIGS. 1 and 2 are schematic cross-sectional views of a photocatalytic cell according to the present embodiment.

[0017] A photocatalytic cell 13 of the present embodiment is installed in an inclined manner at an angle of 5° or more and 45° or less with respect to a horizontal plane.

[0018] The photocatalytic cell 13 includes: a translucent member 5 that takes in light from an upper side or an obliquely upper side; an electrolytic solution 2a housed in the photocatalytic cell 13; a photocatalytic sheet 3 including photocatalytic particles that receive the light taken in from the translucent member 5; an injection port 10a through which the electrolytic solution 2a is injected into an inside of the photocatalytic cell 13; a discharge port 11a through which the electrolytic solution 2a is discharged to an outside of the photocatalytic cell 13; and an exhaust port 12 through which gas inside the photocatalytic cell 13 is discharged. The photocatalytic cell is characterized in that: at least a part of the photocatalytic sheet 3 is immersed in the electrolytic solution 2a; a position of the exhaust port 12 is higher than a position of the injection port 10a; a gap between a surface of the translucent member 5 and a surface of the photocatalytic sheet 3 is 5 mm or more and 50 mm or less in width; and the injection port 10a and the discharge port 11a allow the electrolytic solution 2a to flow from an upper part toward a lower part in the gap between the translucent member 5 and the photocatalytic sheet 3.

[0019] The photocatalytic cell 13 is a cell that houses the electrolytic solution 2a and the photocatalytic sheet 3. The photocatalytic cell 13 may be included in a cation reductor that reduces the first cation contained in the electrolytic solution 2a to a second cation by photocatalytic activity. The photocatalytic cell 13 may also be included in a device that generates hydrogen gas or oxygen gas from the electrolytic solution 2a by photocatalytic activity.

[0020] The photocatalytic cell 13 is installed in an inclined manner at an angle of 5° or more and 45° or less with respect to the horizontal plane. By installing the photocatalytic cell 13 in an inclined manner, sunlight is likely to enter the photocatalytic cell 13, and an amount of light received by the photocatalytic particles contained in the photocatalytic sheet 3 can be increased. Therefore, the photocatalytic activity of the photocatalytic particles is increased. The photocatalytic cell 13 may have a flat shape.

[0021] The photocatalytic cell 13 may include a translucent member 5. This allows light transmitted through the translucent member 5 to be irradiated onto the photocatalytic particles contained in the photocatalytic sheet 3, allowing the photocatalytic particles to have photocatalytic activity. A material for the translucent member 5 may be glass such as quartz glass, or may be a resin such as an acrylic resin, polycarbonate, or polyvinyl chloride. The translucent member 5 takes light into the photocatalytic cell 13 from an upper side or an obliquely upper side. The translucent member 5 may have a flat plate shape.

[0022] The photocatalytic cell 13 includes, for example, a container 4 and the translucent member 5 that covers an opening of the container 4, as illustrated in FIGS. 1 and 2. The translucent member 5 is fixed to the container 4 by a cover 8 and bolts 9, as illustrated in FIG. 2. A cushioning material 7 is provided between the translucent member 5 and the cover 8, and a sealing member 6 is provided between the container 4 and the translucent member 5.

[0023] The photocatalytic cell 13 may have a flat shape, and the translucent member 5 that serves as a light-receiving surface may be placed on a wide surface having the flat shape. When the photocatalytic cell 13 is installed in an inclined manner, the translucent member 5 can take light (for example, sunlight) from the obliquely upper side into the photocatalytic cell 13.

[0024] The photocatalytic sheet 3 is a sheet containing photocatalytic particles. The photocatalytic sheet 3 may include a carrier sheet and a plurality of photocatalytic particles. At least a part of the photocatalytic sheet 3 is immersed in the electrolytic solution 2a. The photocatalytic sheet 3 may be placed at a bottom of the container 4.

[0025] A gap between a surface of the translucent member 5 and a surface of the photocatalytic sheet 3 is 5 mm or more and 50 mm or less in width, preferably 5 mm or more and 40 mm or less in width, and more preferably 5 mm or more and 30 mm or less in width. This can shorten a distance of the light passing through the electrolytic solution 2a before reaching the photocatalytic particles, and the photocatalytic particles can receive more light. In addition, this can shorten a distance of air bubbles of the oxygen gas generated in the photocatalytic sheet 3 moving to the vicinity of the translucent member 5 due to buoyancy, thereby making it possible to suppress oxidation, by the oxygen gas, of a reduced product (e.g., a divalent iron ion) generated by the photocatalytic activity.

[0026] The electrolytic solution 2a can fill 90% or more of the gap between the translucent member 5 and the photocatalytic sheet 3. This can reduce a contact area between the electrolytic solution 2a and a gas phase, and can suppress oxidation of a reduced product (e.g., a divalent iron ion) contained in the electrolytic solution 2a.

[0027] The photocatalytic particles are particles that become photocatalytically active by receiving light. The photocatalytic particles may be supported or fixed on the carrier sheet. The photocatalytic particles may include tungsten oxide particles (WO3 particles). Tungsten oxide has a wider light absorption band than titanium dioxide, and photocatalytic activity is caused even when tungsten oxide absorbs visible light that does not contain ultraviolet light. Therefore, photocatalytic activity can be caused even when light incident on the photocatalytic cell 13 passes through the electrolytic solution 2a and is then irradiated onto tungsten oxide particles (photocatalytic particles). Further, tungsten oxide has a higher specific gravity than titanium oxide. Therefore, a mass per unit area of the photocatalytic sheet 3 can be increased. This can suppress floating of the photocatalytic sheet 3 in the electrolytic solution 2a, and a change in position of the photocatalytic sheet 3 in the photocatalytic cell 13 can be suppressed.

[0028] The tungsten oxide particles (WO3 particles) included in the photocatalytic particles may be tungsten oxide particles having a composition deviating from a stoichiometric composition as long as the tungsten oxide particles have photocatalytic activity. The tungsten oxide particles may contain impurity atoms or additive atoms within a range in which photocatalytic activity is not lost. The photocatalytic particles may have a catalytic promoter on surfaces thereof. Examples of the catalytic promoter include platinum group metals such as Pt, Pd, Rh, Ru, Os, and Ir.

[0029] An average particle size D50 of the photocatalytic particles (primary particles) contained in the photocatalytic sheet 3 is, for example, 150 nm or more, preferably 200 nm or more and 50 μm or less, and more preferably 500 nm or more and 30 μm or less.

[0030] The carrier sheet is a sheet included in the photocatalytic sheet 3 and supports the photocatalytic particles. The carrier sheet is paper, nonwoven fabric, woven fabric, a resin sheet, a glass sheet, a ceramic sheet, or the like. The photocatalytic particles may be attached to a surface of the carrier sheet. When the carrier sheet is paper or a nonwoven fabric, the photocatalytic particles may be incorporated in the carrier sheet.

[0031] In the photocatalytic cell 13, the photocatalytic sheet 3 and the photocatalytic particles are in contact with the electrolytic solution 2a. This allows the first cation contained in the electrolytic solution 2a to be reduced to a second cation by photocatalytic activity, or the photocatalytic cell 13 can generate hydrogen gas or oxygen gas from the electrolytic solution 2a by photocatalytic activity. For example, when the photocatalytic particles receive light, the first cation in the electrolytic solution 2a is reduced to the second cation, and oxygen gas is generated from the electrolytic solution 2a. This can be explained as follows. Light excites an electron in a valence band of the photocatalytic particle to a conduction band, forming a hole in the valence band. The electron in the conduction band moves to a surface of the photocatalytic particle, and the first cation to which the electron is added is reduced to the second cation (first reaction). Further, the hole in the valence band moves to the surface of the photocatalytic particle and reacts with H2O to generate oxygen gas (second reaction). The generated oxygen gas becomes air bubbles in electrolytic solution 2a, which float along the translucent member 5, move into a gas phase in photocatalytic cell 13, and are discharged from at least one exhaust port 12 to an outside of photocatalytic cell 13.

[0032] For example, when the first cation is a trivalent iron ion (Fe3+) and the second cation is a divalent iron ion (Fe2+), the following reactions proceed.

[0033] First reaction: Fe3++e−→Fe2+

[0034] Second reaction: 2H2O→O2+4H++4e−

[0035] The generated divalent iron ion (Fe2+) and hydrogen ion (H+) can be used in an electrolyzer 30 described later.

[0036] The electrolytic solution 2a may be an aqueous solution containing a first cation. The first cation is reduced to a second cation by photocatalytic activity of the photocatalytic particles produced by receiving light.

[0037] When the electrolytic solution 2a contains iron sulfates (FeSO4 and Fe2(SO4)3), the first cation is a trivalent iron ion and the second cation is a divalent iron ion.

[0038] When the electrolytic solution 2a contains iron perchlorates (Fe(ClO4)3 and Fe(ClO4)2), the first cation is a trivalent iron ion and the second cation is a divalent iron ion.

[0039] The first and second cations may be metal complex ions. A metal contained in the metal complex ions is, for example, iron or cobalt.

[0040] A pH of the electrolytic solution 2a may be within a pH range in which a zeta potential of the photocatalytic particles is 0 V or higher. Electrostatic attraction force produced by this can increase a probability of contact between the photocatalytic particle and the first cation, thereby increasing a probability of reduction of the first cation to the second cation by photocatalytic activity. Thus, an electrolytic solution containing a larger amount of second cations can be produced. Further, by using this electrolytic solution to generate hydrogen gas in a hydrogen gas generation system described later, an efficiency of generating hydrogen gas can be improved.

[0041] When the photocatalytic particles include the tungsten oxide particles, the pH of the electrolytic solution 2a can be made smaller than 2 (to be on an acidic side). This allows the zeta potential of the photocatalytic particles to be made 0 V or higher, and also allows divalent iron ions and trivalent iron ions to exist stably in the electrolytic solution 2a. This suppresses oxidation of divalent iron ions to trivalent iron ions due to dissolved oxygen in the electrolytic solution 2a, oxygen gas in the gas phase, oxygen gas generated by photocatalytic activity, or the like.

[0042] For example, the pH of the electrolytic solution 2a may be adjusted by adjusting an iron sulfate concentration, an iron perchlorate concentration, or the like of the electrolytic solution 2a, or the pH of the electrolytic solution 2a may be adjusted by adding an acidic material such as sulfuric or perchloric acid to the electrolytic solution 2a.

[0043] When an electrolytic solution is prepared by dissolving about 50 g of iron perchlorate n-hydrate (manufactured by FUJIFILM Wako Pure Chemical Cooperation: n is about 8) in 10 L of water, the electrolytic solution has a pH of about 2.

[0044] An iron ion concentration of the electrolytic solution 2a is preferably, for example, from 10 mmol / L to 1 mol / L. This is a concentration at which the iron ions can stably maintain respective valence states thereof. More preferably, the iron ion concentration of the electrolytic solution 2a is from 10 mmol / L to 100 mmol / L. The lower the iron ion concentration, the smaller an effect of coloring of the electrolytic solution caused by the iron ions, and the more decrease in amount of light received by the photocatalyst can be suppressed.

[0045] For example, as illustrated in FIG. 1, the photocatalytic cell 13 may include an injection port 10a through which the electrolytic solution 2a containing the first cation is supplied into the photocatalytic cell 13 and a discharge port 11a through which the electrolytic solution 2a containing the second cation is discharged from the photocatalytic cell 13. The injection port 10a and the discharge port 11a allow the electrolytic solution 2a to flow from an upper part (from an upper part of the photocatalytic cell 13) toward a lower part (a lower part of the photocatalytic cell 13) in the gap between the translucent member 5 and the photocatalytic sheet 3. Since the electrolytic solution 2a flows from the upper portion to the lower portion due to its own weight, a pump or the like that flows the electrolytic solution 2a can be omitted or power consumption of the pump can be reduced.

[0046] This also allows the first cation contained in the electrolytic solution 2a injected into the photocatalytic cell 13 from the injection port 10a to come into contact with the photocatalytic particle, and the first cation can be reduced by photocatalytic activity and converted into the second cation. Further, the electrolytic solution 2a containing the second cation generated by photocatalytic activity can be taken out from the photocatalytic cell 13, and the electrolytic solution 2a containing the second cation can be used.

[0047] As described above, the air bubbles of the oxide gas produced by the photocatalytic activity float along the translucent member 5, and thus a direction in which the electrolytic solution 2a flows can be opposite to a direction in which the air bubbles rise. Therefore, the probability that the reduced product (for example, divalent iron ion) contained in the electrolytic solution 2a flowing downward comes into contact with the air bubbles of the oxide gas can be reduced. Thus, the oxidation of the reduced product by the oxygen gas can be suppressed.

[0048] The at least one exhaust port 12 is a portion through which gas inside the photocatalytic cell 13 is discharged, and is disposed at a position higher than the injection port 10a. The exhaust port 12 can be disposed at a position higher than the injection port 10a by 10 mm or more, for example. This makes it possible to dispose the exhaust port 12 at a position higher than a liquid level of the electrolytic solution 2a, to form a gas phase between the injection port 10a and the exhaust port 12, to prevent the electrolytic solution 2a from being stored near a connection between the inside of the cell and the exhaust port 12 and becoming a resistance to gas discharge, and to efficiently discharge the gas in the gas phase inside the photocatalytic cell 13 from the exhaust port 12 to the outside of the photocatalytic cell 13. When the photocatalytic cell 13 has a plurality of exhaust ports 12, the plurality of exhaust ports 12 can be disposed at substantially the same height. The exhaust port 12 can be disposed with it not overlapping the bolt 9.

[0049] The photocatalytic cell 13 may have an emergency discharge port 14 disposed at a position higher than the injection port 10a and lower than the exhaust port 12. The emergency discharge port 14 can be disposed, for example, in a side portion of the photocatalytic cell 13. The emergency discharge port 14 allows the electrolytic solution 2a inside the photocatalytic cell 13 to be discharged from the emergency discharge port 14 to the outside of the photocatalytic cell 13 when the electrolytic solution 2a is excessively supplied from the injection port 10a to the inside of the photocatalytic cell 13, and the liquid level of the electrolytic solution 2a in the photocatalytic cell 13 becomes higher than the injection port 10a and becomes high to a height of the emergency discharge port 14. Providing such an emergency discharge port 14 makes it possible to suppress the electrolytic solution 2a from flowing into the exhaust port 12.Hydrogen Gas Generation System

[0050] FIG. 3 is a schematic cross-sectional view of a hydrogen gas generation system of the present embodiment.

[0051] A hydrogen gas generation system 40 of the present embodiment includes: a photocatalytic cell 13; and an electrolyzer 30 including a cathode 16 and an anode 17. The electrolyzer 30 generates hydrogen gas from water or hydrogen ions at the cathode 16, and oxidizes a second cation to a first cation at the anode 17. The photocatalytic cell 13 and the electrolyzer 30 allow an electrolytic solution 2a containing the second cation generated in the photocatalytic cell 13 to be supplied to the electrolyzer 30, and an electrolytic solution 2a containing the first cation generated at the anode 17 to be supplied to the photocatalytic cell 13.

[0052] The electrolyzer 30 may include a power supply unit that applies a voltage between the anode 17 and the cathode 16. The electrolyzer 30 may include an anode chamber 22 and a cathode chamber 21 separated by an ion exchange membrane 18.

[0053] The electrolytic solution 2a containing the second cations generated by the photocatalytic cell 13 is supplied to the electrolyzer 30, the anode chamber 22 is filled with the electrolytic solution 2a, and the cathode chamber 21 is filled with an electrolytic solution 2b. Then, when a voltage is applied between the anode 17 and the cathode 16 using the power supply unit, an anodic reaction proceeds on a surface of the anode 17, and a cathodic reaction proceeds on a surface of the cathode 16. With the anodic reaction and the cathodic reaction, hydrogen ions (H+) contained in the electrolytic solution 2a in the anode chamber 22 move to the electrolytic solution 2b in the cathode chamber 21 through the ion exchange membrane 18. The electrolytic solution 2b can be an acidic electrolytic solution.

[0054] In the anode 17, a reaction proceeds in which the second cation contained in the electrolytic solution 2a in the anode chamber 22 transfers electrons to the anode 17 and is oxidized to the first cation (anodic reaction).

[0055] For example, when the first cation is a trivalent iron ion (Fe3+) and the second cation is a divalent iron ion (Fe2+), the following anodic reaction is considered to proceed.

[0056] Anodic reaction: Fe2+→Fe3++e−

[0057] The anode chamber 22 may include an injection port 10b through which the electrolytic solution 2a containing the second cation generated in the photocatalytic cell 13 is supplied into the anode chamber 22 and a discharge port 11b through which the electrolytic solution 2a containing the first cation generated from the second cation at the anodic 17 is discharged from the anode chamber 22. The injection port 10b and the discharge port 11b allow the electrolytic solution 2a injected from the injection port 10b to pass through the anode chamber 22 and then to be discharged from the discharge port 11b. By circulating the electrolytic solution 2a in this way, the anodic reaction can proceed continuously and stably.

[0058] The electrolytic solution 2a containing the second cation generated in the photocatalytic cell 13 may be supplied to the anode chamber 22 of the electrolyzer 30 through a liquid feed pipe or a pump. Also, the electrolytic solution 2a containing the second cation generated in the photocatalytic cell 13 may be supplied to the anode chamber 22 of the electrolyzer 30 due to its own weight. The electrolytic solution 2a containing the second cation generated in the photocatalytic cell 13 may be stored in a storage tank. The electrolytic solution 2a may then be transported in a state of being stored in the storage tank, and the electrolytic solution 2a stored in the storage tank may be supplied to the anode chamber 22 of the electrolyzer 30 at a destination.

[0059] The electrolytic solution 2a containing the first cation discharged from the anode chamber 22 of the electrolyzer 30 may be supplied to the photocatalytic cell 13 through a liquid feed pipe or a pump 15. The electrolytic solution 2a containing the first cation discharged from the anode chamber 22 of the electrolyzer 30 may be supplied to the photocatalytic cell 13 due to its own weight. The electrolytic solution 2a containing the first cation discharged from the anode chamber 22 of the electrolyzer 30 may be stored in a storage tank. The electrolytic solution 2a may then be transported in a state of being stored in the storage tank, and the electrolytic solution 2a stored in the storage tank may be supplied to the photocatalytic cell 13 at a destination.

[0060] In such a system in which the electrolytic solution 2a is circulated, water may be added to the circulating electrolytic solution 2a. This allows the water consumed in the second reaction described above to be replenished.

[0061] At the cathode 16, the following cathodic reaction proceeds in which hydrogen ions contained in the electrolytic solution 2b in the cathode chamber 21 receive electrons and hydrogen gas is generated.

[0062] Cathodic reaction: 2H++2e−→H2

[0063] The generated hydrogen gas is discharged to the outside of the cathode chamber 21 through a hydrogen gas discharge hole 19 and stored in a hydrogen storage tank.

[0064] The anodic reaction and the cathodic reaction described above proceed at a lower applied voltage (a voltage applied between the cathode 16 and the anode 17 by the power supply unit) than in known water electrolysis devices. Therefore, the cost of producing hydrogen gas can be reduced.

[0065] It is to be understood that the embodiments herein are illustrative and not restrictive, since the scope of the disclosure is defined by the appended claims rather than by the description preceding them, and all changes that fall within metes and bounds of the claims, or equivalence of such metes and bounds thereof are therefore intended to be embraced by the claims.

Claims

1. A photocatalytic cell installed in an inclined manner at an angle of 5° or more and 45° or less with respect to a horizontal plane,the photocatalytic cell comprising: a translucent member that takes in light from an upper side or an obliquely upper side; an electrolytic solution housed in the photocatalytic cell; a photocatalytic sheet including photocatalytic particles that receive the light taken in from the translucent member; an injection port through which the electrolytic solution is injected into an inside of the photocatalytic cell; a discharge port through which the electrolytic solution is discharged to an outside of the photocatalytic cell; and an exhaust port through which gas inside the photocatalytic cell is discharged, whereinat least a part of the photocatalytic sheet is immersed in the electrolytic solution,a position of the exhaust port is higher than a position of the injection port,a gap between a surface of the translucent member and a surface of the photocatalytic sheet is 5 mm or more and 50 mm or less in width, andthe injection port and the discharge port allow the electrolytic solution to flow from an upper part toward a lower part in the gap between the translucent member and the photocatalytic sheet.

2. The photocatalytic cell according to claim 1, whereinthe electrolytic solution contains a first cation, andthe electrolytic solution and the photocatalytic particles allow the first cation to be reduced to a second cation by photocatalytic activity of the photocatalytic particles produced by receiving light.

3. The photocatalytic cell according to claim 2, whereinthe first cation is a trivalent iron ion, andthe second cation is a divalent iron ion.

4. The photocatalytic cell according to claim 1, wherein the position of the exhaust port is higher than the position of the injection port by 10 mm or more.

5. The photocatalytic cell according to claim 1,wherein the photocatalytic particles include tungsten oxide particles.

6. A hydrogen gas generation system comprising: the photocatalytic cell according to claim 2; and an electrolyzer including a cathode and an anode, the electrolyzer generating hydrogen gas from water or hydrogen ions at the cathode, and oxidizing a second cation to a first cation at the anode,wherein the photocatalytic cell and the electrolyzer allow an electrolytic solution containing the second cation generated in the photocatalytic cell to be supplied to the electrolyzer, and an electrolytic solution containing the first cation generated at the anode to be supplied to the photocatalytic cell.

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