WO2024241478A1 - Oxidation tank - Google Patents

Abstract

An oxidation tank 2 includes: an oxidation electrode 21 and an electrolytic solution 5 provided in the oxidation tank 2; a light receiving window 24 which is installed in the oxidation tank 2 and receives light to the oxidation electrode 21; a conducting wire 6 which electrically connects the oxidation electrode 21 and a reduction electrode 31 in a reduction tank 3; and an electrolyte film 5 which separates the oxidation tank 2 from the reduction tank 3. A protection layer 25 is formed on the inner surface of the oxidation tank 2, among the surfaces of the light receiving window 24.

Description

Oxidation tank

This disclosure relates to an oxidation tank.

There is a device that produces hydrogen through a water decomposition reaction using a semiconductor photoelectrode (Non-Patent Document 1). This device has an oxidation tank and a reduction tank. An aqueous solution and an oxidation electrode are placed in the oxidation tank. An aqueous solution and a reduction electrode are placed in the reduction tank. The oxidation tank and reduction tank are connected via a proton exchange membrane. The oxidation electrode and reduction electrode are electrically connected by a conductor.

The water splitting reaction using photocatalysts consists of a water oxidation reaction and a proton reduction reaction. When light is irradiated onto an n-type photocatalyst material, electrons and holes are generated and separated in the photocatalyst. The holes move to the surface of the photocatalyst material and contribute to the water oxidation reaction. Meanwhile, the electrons move to the reduction electrode and contribute to the proton reduction reaction. Ideally, this type of oxidation-reduction reaction progresses, resulting in a water splitting reaction.

Oxidation reaction: 2H ₂ O + 4h ⁺ → O ₂ + 4H ⁺
Reduction reaction: 4H ⁺ + 4e ^- → 2H ₂

The oxidation electrode is a semiconductor coating. For example, the oxidation electrode is a gallium nitride thin film grown on a sapphire substrate. In the gallium nitride thin film, holes are generated and separated under light irradiation, and are consumed in the etching reaction of the gallium nitride itself at the same time as the water oxidation reaction. The photoelectrode may deteriorate, and the light energy conversion efficiency may decrease with the duration of light irradiation.

In order to prevent this type of deterioration, there have been reports of extending the lifespan by forming a promoter for oxygen generation (nickel oxide) as a protective layer. By extending the lifespan of the oxidation electrode, it has become possible to operate the device for long periods of time.

S. Yotsuhashi, et al., “CO2Conversion with Light and Water by GaN Photoelectrode”, Japanese Journal of Applied Physics, The Japan Society of Applied Physics, 2012, Volume 51, pp. 02BP07-1-02BP07-3

During long-term operation, the aqueous solution reacts with the light-receiving window that receives the light entering the oxidation tank. The surface of the material that composes the light-receiving window becomes rough, and the light transmittance of the window deteriorates (it becomes devitrified). As a result, the light energy that reaches the oxidation electrode is attenuated by the deteriorated light-receiving window. This can result in a decrease in the light energy conversion efficiency at the oxidation electrode.

This disclosure has been made in light of the above circumstances, and the purpose of this disclosure is to provide technology that can protect the light-receiving window of an oxidation tank.

The oxidation tank of one embodiment of the present disclosure has an oxidation electrode and an electrolyte solution provided in the oxidation tank, a light-receiving window installed in the oxidation tank that receives light to the oxidation electrode, a conductor that electrically connects the oxidation electrode and the reduction electrode in the reduction tank, and an electrolyte membrane that separates the oxidation tank from the reduction tank, and a protective layer is formed on the surface of the light-receiving window that faces the inside of the oxidation tank.

The oxidation tank of one embodiment of the present disclosure has an oxidation electrode and an electrolyte solution provided in the oxidation tank, a first light-receiving window installed in the oxidation tank to receive light to the oxidation electrode, a second light-receiving window installed in the oxidation tank to receive light that has passed through the oxidation electrode, a conductor that electrically connects the oxidation electrode to the reduction electrode in the reduction tank, and an electrolyte membrane that separates the oxidation tank from the reduction tank, and a protective layer is formed on the inner surfaces of the first light-receiving window and the second light-receiving window that face the oxidation tank.

This disclosure provides technology that can protect the light receiving window of an oxidation tank.

FIG. 1 is a diagram illustrating an apparatus according to a first embodiment. FIG. 2 is a diagram illustrating an apparatus according to the second embodiment. FIG. 3 is a diagram illustrating a conventional device.

Below, an embodiment of the present disclosure will be described with reference to the drawings. In the description of the drawings, the same parts are given the same reference numerals and the description will be omitted.

First, referring to FIG. 3, the device 100 used in the oxidation-reduction reaction test will be described. The device 100 includes a light source 101, an oxidation tank 102, a reduction tank 103, a proton exchange membrane 104, and a lead wire 106. The oxidation tank 102 includes an oxidation electrode 121 and an aqueous solution 122. The oxidation tank 102 is provided with a light receiving window 124. The light receiving window 124 receives light from the light source 101 to the oxidation electrode 121. The reduction tank 103 includes a reduction electrode 131 and an aqueous solution 132.

In the oxidation tank 102, the oxidation electrode 121 is in contact with the aqueous solution 122. The oxidation electrode 121 is a nitride semiconductor, titanium oxide, amorphous silicon, or the like. The aqueous solution 122 is, for example, an aqueous sodium hydroxide solution, an aqueous potassium hydroxide solution, or hydrochloric acid.

In the reduction tank 103, the reduction electrode 131 is in contact with the aqueous solution 132. The reduction electrode 131 is a metal or a metal compound, such as nickel, iron, gold, platinum, silver, copper, indium, or titanium. The aqueous solution 132 is, for example, an aqueous solution of sodium bicarbonate, an aqueous solution of potassium chloride, or an aqueous solution of sodium chloride.

The proton exchange membrane 104 is sandwiched between the oxidation chamber 102 and the reduction chamber 103. Protons are generated in the oxidation chamber 102. The generated protons diffuse into the reduction chamber 103 via the proton exchange membrane 104. The proton exchange membrane 104 is, for example, a perfluorocarbon material. Purple carbon material is composed of a hydrophobic Teflon (registered trademark) skeleton made of carbon and fluorine, and perfluoro side chains with sulfonic acid groups. The proton exchange membrane 104 is, for example, Nafion (registered trademark).

The oxidation electrode 121 and the reduction electrode 131 are electrically connected by a conductor 106. Electrons move from the oxidation electrode 121 to the reduction electrode 131 via the conductor 106.

The light source 101 is, for example, a xenon lamp, a mercury lamp, a halogen lamp, a pseudo-sun light source, sunlight, or a combination of these. The light source 101 irradiates light with a wavelength that can be absorbed by the material that constitutes the oxide electrode 121. For example, in the case of the oxide electrode 121 made of gallium nitride, the wavelength that can be absorbed by the oxide electrode 121 is 365 nm or less.

In such an apparatus 100, the lifespan of the oxidation electrode has been extended, making it possible to operate for long periods of time. However, during long-term operation, the aqueous solution 122 reacts with the light-receiving window 124, which receives light into the oxidation tank 102. The surface of the material that constitutes the light-receiving window 124 becomes rough, and the light transmittance of the light-receiving window 124 deteriorates (it becomes devitrified). As a result, the light energy that reaches the oxidation electrode 121 is attenuated by the deteriorated light-receiving window 124. This may result in a decrease in the light energy conversion efficiency in the oxidation electrode 121.

The present disclosure prevents deterioration of the light transmittance of the light receiving window 124 due to deterioration of the light receiving window. The present disclosure enables long-term operation of a device in which the oxidation reaction of water and the reduction reaction of protons proceed.

Specifically, in the device disclosed herein, a protective layer is formed on the surface of the light-receiving window that faces the inside of the oxidation tank. The protective layer is formed so that the light-receiving window does not come into contact with the electrolyte (aqueous solution). As the light-receiving window does not come into contact with the electrolyte, deterioration of the light-receiving window can be prevented.

The protective layer is made of a light-transmitting resin that transmits light. By making the protective layer out of a light-transmitting resin, the light transmittance of the light-receiving window is guaranteed, and light from the light source can irradiate the oxidation electrode.

The protective layer is made of a material that does not react with the electrolyte. Because the protective layer does not react when it comes into contact with the electrolyte, the protective effect of the protective layer on the light receiving window can be maintained.

The light receiving window is coated with a protective layer so that it does not come into contact with the electrolyte. This prevents the light receiving window from losing transparency and improves the stability of the light energy conversion efficiency.

(First embodiment)
An apparatus 1 according to a first embodiment will be described with reference to Fig. 1. The apparatus 1 is used for an oxidation-reduction reaction test, similar to the apparatus 100 shown in Fig. 3. In the oxidation-reduction reaction test, an oxidation reaction of water and a reduction reaction of protons proceed.

The device 1 includes an oxidation tank 2 and a reduction tank 3. The oxidation tank 2 and the reduction tank 3 are separated by an electrolyte membrane 4. In the device 1, the oxidation tank 2 and the reduction tank 3 are described as being adjacent to each other in the horizontal direction, but this is not limited thereto, and the oxidation tank 2 and the reduction tank 3 may be adjacent to each other in the vertical direction.

An electrolyte solution 5 is injected into the oxidation tank 2 and the reduction tank 3. The electrolyte solution 5 is used in the oxidation-reduction reaction. The electrolyte solution 5 may be an aqueous solution in which an electrolyte that moves ions is dissolved. The electrolyte solution 5 may be, for example, an aqueous solution of potassium hydroxide, an aqueous solution of rubidium hydroxide, or an aqueous solution of cesium hydroxide, in addition to sodium hydroxide.

The oxidation tank 2 is provided with an oxidation electrode 21, a gas inlet line 22, a gas exhaust line 23, and a light receiving window 24.

The oxidation electrode 21 is provided so as to be in contact with the electrolyte 5. The oxidation electrode 21 is provided so that light passing through a light receiving window 24 provided in the housing of the oxidation tank 2 is irradiated onto the oxidation electrode 21. The oxidation electrode 21 is a nitride semiconductor such as a GaN (gallium nitride)-based semiconductor. The oxidation electrode 21 may be an oxide semiconductor other than a nitride semiconductor.

The gas inlet line 22 and the gas exhaust line 23 have a hollow pipe shape. One end of the gas inlet line 22 contacts the air above the electrolyte 5, and the other end contacts the electrolyte 5. Both ends of the gas exhaust line 23 contact the air above the electrolyte 5. Nitrogen gas is introduced into the electrolyte 5 by the gas inlet line 22. The gas exhaust line 23 collects oxygen gas exhausted from the electrolyte 5.

The light receiving window 24 is installed in the housing of the oxidation tank 2. The light receiving window 24 receives light from a light source outside the oxidation tank 2 and is directed to the oxidation electrode 21. The light receiving window 24 transmits light from the light source. The transmitted light irradiates the oxidation electrode 21.

The reduction tank 3 is provided with a reduction electrode 31, a gas inlet line 32, and a gas exhaust line 33.

The reduction electrode 31 is provided so as to be in contact with the electrolyte 5.

The gas inlet line 32 and the gas exhaust line 33 are similar to the gas inlet line 22 and the gas exhaust line 33, respectively, provided in the oxidation tank 2. In the reduction tank 3, nitrogen gas is introduced into the electrolyte 5 by the gas inlet line 32. The gas exhaust line 33 collects hydrogen gas exhausted from the electrolyte 5.

The oxidation electrode 21 and the reduction electrode 31 are electrically connected by a conductor 6. Electrons move from the oxidation electrode 21 to the reduction electrode 31 via the conductor 6.

In the device 1 shown in FIG. 1, a protective layer 25 is formed on the surface of the light-receiving window 24 on the inner surface of the oxidation tank 2. The protective layer 25 is formed on the surface of the light-receiving window 24 on a portion that may come into contact with the electrolyte 5. The protective layer 25 prevents the light-receiving window 24 from coming into contact with the electrolyte 5. The light-receiving window 24 is protected. The light transmittance of the light-receiving window 24 does not deteriorate even during long-term oxidation-reduction reaction tests. As a result, the light energy that passes through the light-receiving window 24 reaches the oxidation electrode 21 without attenuation. The light energy conversion efficiency in the oxidation electrode 21 is maintained.

The protective layer 25 is preferably formed from a light-transmitting resin that transmits light. This allows the light receiving window 24 to maintain light transmittance in the same way as if the protective layer 25 were not present. The protective layer 25 is formed from, for example, an acrylic resin. In addition to acrylic resin, the protective layer 25 may also be formed from a polyethylene terephthalate-based, polyvinyl chloride-based, polystyrene-based, or other material.

The protective layer 25 is preferably made of a material that does not react with the electrolyte 5. This allows the performance of the protective layer 25 to be maintained even during long-term oxidation-reduction reaction tests.

Second Embodiment
An apparatus 1 according to a second embodiment will be described with reference to Fig. 2. The apparatus 1a is used for an oxidation-reduction reaction test, similar to the apparatus 100 shown in Fig. 3. In the oxidation-reduction reaction test, an oxidation reaction of water and a reduction reaction of protons proceed.

The device 1a includes an oxidation tank 2 and a reduction tank 3, similar to the device 1 shown in FIG. 1. The oxidation tank 2 and the reduction tank 3 are separated by an electrolyte membrane 4. In the device 1a, the oxidation tank 2 and the reduction tank 3 are described as being adjacent to each other in the vertical direction, but this is not limited thereto. The oxidation tank 2 and the reduction tank 3 may be adjacent to each other in the horizontal direction. In the oxidation tank 2 of the device 1a according to the second embodiment, light passes through the first light receiving window 24a, the oxidation electrode 21, and the second light receiving window 24b, so it is preferable to install the oxidation tank 2 and the reduction tank 3 in the vertical direction. This is because the reduction tank 3 is not installed around the oxidation tank 2.

Apparatus 1a has a similar configuration to apparatus 1 shown in FIG. 1. Apparatus 1a differs from apparatus 1 in that it includes a solar power generation element 7 and that the oxidation tank 2 includes a first light receiving window 24a and a second light receiving window 24b.

The solar power generation element 7 emits light when it is irradiated onto the oxidation electrode 21. The solar power generation element 7 is, for example, a Si (silicon)-based element. In addition to Si-based elements, the solar power generation element 7 may be made of various light-emitting elements such as CIS (thin film type: copper indium selenium), CIGS (copper indium gallium selenium), GaAs (gallium arsenide), organic, and perovskite-type light-emitting elements.

The first light receiving window 24a is installed in the oxidation tank 2 and receives light that reaches the oxidation electrode 21. The second light receiving window 24b is installed in the oxidation tank 2 and receives light that has passed through the oxidation electrode 21. In the oxidation tank 2, light irradiated by the solar power generation element 7 passes through the first light receiving window 24a, the oxidation electrode 21, and the second light receiving window 24b.

A first protective layer 25a is formed on the surface of the first light receiving window 24a facing the inside of the oxidation tank 2. Similarly, a second protective layer 25b is formed on the surface of the second light receiving window 24b facing the inside of the oxidation tank 2. The first protective layer 25a in the second embodiment is similar to the protective layer 25 described in the first embodiment.

The first protective layer 25a and the second protective layer 25b are formed of a light-transmitting resin that transmits light. The first protective layer 25a and the second protective layer 25b are formed of a material that does not react with the electrolyte.

The materials of the device 1 shown in the first embodiment and the device 1a shown in the second embodiment are merely examples and are not limited to these. In this disclosure, a case has been described in which the target product is hydrogen. By changing the material of the reduction electrode 31 or the atmosphere within the device, it is possible to generate various target products, such as the generation of carbon compounds through the reduction reaction of carbon dioxide, or the generation of ammonia through the reduction reaction of nitrogen. The reduction electrode 31 may be formed of, for example, Ni (nickel), Fe (iron), Au (gold), Pt (platinum), Ag (silver), Cu (copper), In (indium), Ti (titanium), Co (cobalt), Ru (ruthenium), etc.

Next, the results of an oxidation-reduction reaction test performed using the device according to the present disclosure will be described. In the test, as shown in Table 1, the amount of hydrogen gas generated after a specified time was measured for each of several combinations of electrodes and aqueous solution types, with and without a resin protective layer 25 formed on the light receiving window 24. In this test, the aqueous solution was electrolyte solution 5.

Example 1
In Example 1, the device 1 according to the first embodiment shown in FIG. 1 was used.

The creation of the light receiving window 24 on which the protective layer 25 is formed will now be explained. Quartz was used as the material for the light receiving window 24. Only the surface that comes into contact with the aqueous solution was coated with acrylic resin. The light transmittance of the light receiving window 24 coated with acrylic resin is 90%.

The generation of the oxidation electrode 21 will be explained. A GaN substrate was used as the substrate. A silicon-doped n-GaN semiconductor thin film was epitaxially grown on a 2-inch diameter GaN substrate by MOCVD (Metal Organic Chemical Vapor Deposition). The n-GaN film had a thickness of 2 μm, which is sufficient to absorb light. The carrier density was 3×10 ¹⁸ cm ^3. Then, aluminum gallium nitride AlGaN was grown. The aluminum composition ratio was 10%. The film thickness was 100 nm, which is sufficient to absorb light. Then, the 2-inch semiconductor thin film was cleaved into four equal parts, and one of them was used to fabricate the electrode.

Next, a layer of Ni with a thickness of approximately 1 nm was deposited on the AlGaN surface using electron beam (EB). This semiconductor thin film was then heat-treated on a hot plate in an air atmosphere at 300°C for 1 hour to form NiO.

Next, the redox reaction test in Example 1 will be explained. In the redox reaction test, parts of the NiO and AlGaN surfaces were scratched to expose the n-GaN surface. A conductor 6 was connected to part of the exposed n-GaN surface and soldered using In. After that, it was covered with epoxy resin so that the In surface would not be exposed. This was installed as the oxidation electrode 21 in Figure 1.

The electrolyte 5 was a 1 mol/L aqueous solution of sodium hydroxide. The reduction electrode 31 was made of platinum (manufactured by Nilaco). The electrolyte membrane 4 was made of Nafion (registered trademark). Nitrogen gas was passed through each reaction tank at 10 mL/min. The light irradiation area of the oxidation electrode 21 was 1 ^cm2 .

After the inside of the reaction tanks had been fully replaced with nitrogen gas, the oxidation electrode 21 prepared by the above procedure was fixed so that the light source was facing the surface on which NiO was formed. The light source was a 300W high-pressure xenon lamp (illuminance 5mW/cm2). The oxidation electrode 21 was uniformly irradiated with light from the light source. During the light irradiation, gas samples were taken from inside each reaction tank at any time, and the reaction products were analyzed by gas chromatography. As a result, it was confirmed that oxygen was produced in the oxidation tank 2, and hydrogen was produced in the reduction tank 3.

Example 2
The electrolyte 5 in Example 1 was changed to a 1 mol/L aqueous potassium hydroxide solution.

Example 3
The electrolyte 5 in Example 1 was changed to a 1 mol/L aqueous solution of rubidium hydroxide.

Example 4
The electrolyte 5 in Example 1 was changed to a 1 mol/L aqueous cesium hydroxide solution.

Example 5
In Example 5, the device 1a according to the second embodiment shown in FIG. 2 was used.

The production of the first light receiving window 24a on which the first protective layer 25a is formed will be explained. The second light receiving window 24b on which the second protective layer 25b is formed is produced in the same manner. Quartz was used as the material for the first light receiving window 24a. Only the surface that comes into contact with the aqueous solution was coated with acrylic resin. The light transmittance of the acrylic resin-coated light receiving window 24 is 90%.

The generation of the oxidation electrode 21 will be described. A GaN substrate was used as the substrate. A silicon-doped n-GaN semiconductor thin film was epitaxially grown on a 2-inch diameter GaN substrate by MOCVD (Metal Organic Chemical Vapor Deposition). The n-GaN film had a thickness of 2 μm, which was sufficient to absorb light. The carrier density was 3×10 ¹⁸ cm ^3. Then, indium gallium nitride InGaN was grown. The indium composition ratio was 10%. The film thickness was 100 nm, which was sufficient to absorb light. Then, the 2-inch semiconductor thin film was cleaved into four equal parts, and one of the pieces was used to fabricate the electrode.

Next, the redox reaction test in Example 5 will be described. As shown in FIG. 2, an oxidation electrode 21 was installed. A silicon-based p-n junction semiconductor element was used as the photovoltaic power generation element 7. The oxidation electrode 21, the photovoltaic power generation element 7, and the reduction electrode 31 were connected in series. The rest of the experiment was the same as in Example 1.

Example 6
The electrolyte 5 in Example 5 was a 1 mol/L aqueous potassium hydroxide solution.

Example 7
The electrolyte 5 in Example 5 was a 1 mol/L aqueous rubidium hydroxide solution.

Example 8
The electrolyte 5 in Example 5 was a 1 mol/L aqueous cesium hydroxide solution.

<Comparative Example 1>
This embodiment was carried out in the same manner as in Example 1, except that the protective layer 25 was not formed.

<Comparative Example 2>
This embodiment was carried out in the same manner as in Example 2, except that the protective layer 25 was not formed.

<Comparative Example 3>
This embodiment was carried out in the same manner as in Example 3, except that the protective layer 25 was not formed.

<Comparative Example 4>
This example was carried out in the same manner as in Example 4, except that the protective layer 25 was not formed.

<Comparative Example 5>
This embodiment was carried out in the same manner as in Example 5, except that the first protective layer 25a and the second protective layer 25b were not formed.

<Comparative Example 6>
This embodiment was carried out in the same manner as in Example 6, except that the first protective layer 25a and the second protective layer 25b were not formed.

<Comparative Example 7>
This embodiment was carried out in the same manner as in Example 7, except that the first protective layer 25a and the second protective layer 25b were not formed.

<Comparative Example 8>
This embodiment was carried out in the same manner as in Example 8, except that the first protective layer 25a and the second protective layer 25b were not formed.

Table 1 shows the amount of hydrogen gas produced for each example 1 hour and 500 hours after light irradiation.

The amount of each gas produced was normalized by the surface area of the semiconductor photoelectrode. It was found that hydrogen was produced in all cases when exposed to light.

In Examples 1 to 4, no significant difference was observed in the amount of hydrogen produced one hour after light irradiation. In addition, when compared to Comparative Example 4, no significant difference was observed in the amount of hydrogen produced one hour after light irradiation.

On the other hand, the amount of hydrogen generated after 500 hours was greater in Example 1 than in Comparative Example 1. The same was true when comparing each of Examples 2 to 4 with each of Comparative Examples 2 to 4.

This is thought to be because the protection provided by the resin prevents the quartz from devitrifying, and reduces the attenuation of light energy to the electrode.

In addition, since the amount of hydrogen generated after 500 hours was greater in Example 4 than in Example 1, it can be said that CsOH is effective in extending the life of the oxidation electrode 21. RbOH and CsOH are more corrosive than NaOH or KOH, and therefore have a greater impact on the devitrification of quartz. However, by protecting the quartz with a resin, it is believed that this drawback can be prevented and the life of the oxidation electrode 21 can be extended.
The same was true for Examples 5 to 8.

In this disclosure, a protective layer 25 is formed on the light receiving window 24 to prevent contact between the light receiving window 24 and the electrolyte 5. The protective layer 25 prevents the light receiving window 24 from losing transparency and improves the stability of the light energy conversion efficiency.

　Conventionally, RbOH or CsOH is expected to extend the life of the oxidation electrode 21, but it was thought that it would be difficult to apply it to the light-receiving window 24 because of its high corrosiveness to quartz. However, as in the device 1 disclosed herein, by forming a protective layer 25 on the light-receiving window 24 made of RbOH or CsOH, it is possible to further improve stability.

In this way, by forming the protective layer 25 on the light-receiving window 24, contact between the light-receiving window 24 and the electrolyte 5 can be prevented, thereby preventing devitrification of the light-receiving window 24 and improving the stability of the light energy conversion efficiency.

Note that this disclosure is not limited to the above-described embodiments, and many variations are possible within the scope of the gist of the disclosure.

Reference Signs List 1, 100 Apparatus 2, 102 Oxidation tank 3, 103 Reduction tank 4 Electrolyte membrane 5 Electrolyte solution 6, 106 Conductor 7 Photovoltaic power generation element 21, 121 Oxidation electrode 22, 32 Gas inlet line 23, 33 Gas exhaust line 24, 124 Light receiving window 25 Protective layer 31, 131 Reduction electrode 101 Light source 122, 132 Aqueous solution

Claims (6)

an oxidation electrode and an electrolyte provided in the oxidation tank;
a light receiving window disposed in the oxidation tank for receiving light onto the oxidation electrode;
A conductor electrically connecting the oxidation electrode and a reduction electrode in a reduction tank;
an electrolyte membrane separating the oxidation chamber and the reduction chamber;
An oxidation tank, wherein a protective layer is formed on the surface of the light receiving window on the inner side of the oxidation tank. The oxidation tank according to claim 1 , wherein the protective layer is formed of a light-transmitting resin that transmits light. The oxidation bath of claim 1 , wherein the protective layer is formed of a material that does not react with the electrolyte. an oxidation electrode and an electrolyte provided in the oxidation tank;
a first light-receiving window disposed in the oxidation tank for receiving light onto the oxidation electrode;
a second light-receiving window disposed in the oxidation tank and receiving light transmitted through the oxidation electrode;
A conductor electrically connecting the oxidation electrode and a reduction electrode in a reduction tank;
an electrolyte membrane separating the oxidation chamber and the reduction chamber;
a protective layer is formed on the inner surfaces of the first light-receiving window and the second light-receiving window of the oxidation bath. The oxidation tank according to claim 4 , wherein the protective layer is formed of a light-transmitting resin that transmits light. The oxidation cell of claim 4 , wherein the protective layer is formed of a material that does not react with the electrolyte.

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