JP7606138B2 - Oxidation-reduction reactor - Google Patents

Description

The present invention relates to an oxidation-reduction reaction device.

Conventionally, there is known an oxidation-reduction reaction device that causes a water decomposition reaction by promoting an oxidation-reduction reaction in an aqueous solution. Conventional oxidation-reduction reaction devices are generally configured by combining a semiconductor photoelectrode and a photovoltaic element.

The water splitting reaction by a semiconductor photoelectrode consists of a water oxidation reaction and a proton reduction reaction. When light is irradiated onto the photocatalytic material that constitutes the semiconductor photoelectrode, holes (h ⁺ ) and electrons ( ^e- ) are generated and separated within the photocatalytic material. The holes move to the surface of the semiconductor photoelectrode and contribute to the water oxidation reaction as shown in equation (1). Meanwhile, the electrons move via a conductor to a reduction electrode inserted in the same aqueous solution as the semiconductor photoelectrode and contribute to the proton (H ⁺ ) reduction reaction as shown in equation (2).

2H ₂ O+4h ⁺ →O ₂ +4H ⁺ …(1)
4H ⁺ +4e ^- → 2H ₂ … (2)
As the above oxidation and reduction reactions proceed, a water decomposition reaction occurs, producing hydrogen, which is the target product.

The photovoltaic element is placed on the back side of the light-receiving surface of the semiconductor photoelectrode and is connected to a conductor that connects the semiconductor photoelectrode and the reduction electrode. The photovoltaic element receives light that has passed through the semiconductor photoelectrode, generates holes and electrons, and supplies them to the semiconductor photoelectrode and the reduction electrode, respectively. The photovoltaic element also generates electricity from the received light, increasing the voltage between the semiconductor photoelectrode and the reduction electrode, increasing the electromotive force of the redox reaction system and improving the efficiency of the redox reaction.

T. Higashi and 9 others, “Transparent Ta3N5 Photoanodes for Efficient Oxygen Evolution toward the Development of Tandem Cells”, Angew. Chem, 2019, 131, p.2322-p.2326.

In conventional redox reaction devices, the photovoltaic element increases the electromotive force and improves the efficiency of the redox reaction. However, the resistance of the semiconductor photoelectrode, which involves the oxidation reaction, is greater than the resistance of the photovoltaic element. Therefore, if holes generated by the photovoltaic element are supplied to the semiconductor photoelectrode in excess of the amount required for the target water oxidation reaction, this leads to heat generation and accelerated degradation reactions in the semiconductor photoelectrode, causing the semiconductor photoelectrode to deteriorate and hindering the long-term operation of the water decomposition reaction. When a gallium nitride (GaN)-based thin film is used as the semiconductor photoelectrode, the degradation reaction shown in formula (3) occurs.

2GaN+3H ₂ O+6h ⁺ →Ga ₂ O ₃ +6H ⁺ +N ₂ …(3)
On the other hand, if the number of holes generated in the photovoltaic element is less than the amount required for the target reaction, the water splitting reaction may be suppressed, which may lead to heat generation and accelerated deterioration of the photovoltaic element. .

The present invention has been made in consideration of the above circumstances, and the object of the present invention is to provide a technology that suppresses deterioration of semiconductor photoelectrodes and enables an oxidation-reduction reaction using light energy to be driven for a long period of time.

One embodiment of the redox reaction device of the present invention comprises a semiconductor photoelectrode that undergoes an oxidation reaction with an aqueous solution in response to light, a reduction electrode that undergoes a reduction reaction with an aqueous solution or gas using electrons transferred from the semiconductor photoelectrode via a conductor, a photovoltaic element that is arranged on the back side of the light-receiving surface of the semiconductor photoelectrode and connected to the conductor and generates electricity using light that has passed through the semiconductor photoelectrode, and a cut filter that is arranged on the optical path between the semiconductor photoelectrode and the photovoltaic element and removes a predetermined wavelength range of light that has passed through the semiconductor photoelectrode to reach the photovoltaic element or reduces the intensity of the light.

The present invention provides a technology that suppresses the deterioration of semiconductor photoelectrodes and enables oxidation-reduction reactions using light energy to be driven for long periods of time.

FIG. 1 is a diagram showing the configuration of an oxidation-reduction reaction system.

Hereinafter, an embodiment of the present invention 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.

Summary of the Invention
In the present invention, a cut filter is provided on the optical path between the semiconductor photoelectrode and the photovoltaic element to remove a specific wavelength range of light transmitted through the semiconductor photoelectrode or to weaken the intensity of the light. By providing a cut filter in front of the photovoltaic element, the photocurrent generated by the photovoltaic element is controlled to be the same as the photocurrent generated by the semiconductor photoelectrode. In other words, the cut filter maximizes the efficiency of the water splitting reaction by combining the semiconductor photoelectrode and the photovoltaic element, while controlling the number of holes generated in the photovoltaic element to be approximately the same as the number of electrons generated in the semiconductor photoelectrode. This suppresses heat generation and deterioration of the semiconductor photoelectrode, and realizes long-term operation of the water splitting reaction by oxidation-reduction using light energy and a long life for the oxidation-reduction reaction device.

The water decomposition reaction is one example of a target reaction. By changing the type of metal in the reduction electrode or the atmosphere in the tank on the reduction electrode side, the present invention can be applied to reactions other than water decomposition reactions, that is, target products other than hydrogen. In other words, the present invention can be applied to any oxidation-reduction reaction device that advances an oxidation-reduction reaction.

[Configuration of the oxidation-reduction reaction device]
FIG. 1 is a diagram showing the configuration of an oxidation-reduction reaction system according to this embodiment.

The redox reaction system includes an redox reaction device 1 and a light source 2. The light source 2 is, for example, a xenon lamp, a mercury lamp, a halogen lamp, a pseudo-sun light source, sunlight, or a combination of these.

The redox reaction device 1 comprises a tank 11, an aqueous solution 12, a semiconductor photoelectrode 13, a reduction electrode 14, a photovoltaic element 15, a cut filter 16, and a conducting wire 17.

The aqueous solution 12 is, for example, a sodium hydroxide aqueous solution, a potassium hydroxide aqueous solution, a sodium chloride aqueous solution, a potassium chloride aqueous solution, or hydrochloric acid.

The semiconductor photoelectrode 13 is, for example, a gallium nitride or nitride semiconductor. It may also be a metal oxide having a photocatalytic function, such as titanium oxide, tungsten oxide, or gallium oxide, or a compound semiconductor, such as cadmium sulfide. The semiconductor photoelectrode 13 is inserted into the aqueous solution 12 in the tank 11, and is connected to the reduction electrode 14 in the same aqueous solution 12 through the photovoltaic element 15 and the conductor 17. When the photocatalytic material constituting the semiconductor photoelectrode 13 is irradiated with light from the light source 2, holes and electrons are generated and separated in the photocatalytic material. The holes move to the surface of the semiconductor photoelectrode 13, where the oxidation reaction of water proceeds. Therefore, the semiconductor photoelectrode 13 functions as an oxidation electrode. On the other hand, the electrons move to the reduction electrode 14 via the photovoltaic element 15 and the conductor 17.

The reduction electrode 14 is a metal or a metal compound, such as nickel, iron, gold, platinum, silver, copper, indium, or titanium. The shape of the reduction electrode 14 is, for example, a wire, a plate, a wire mesh, or an electrode substrate with metal particles applied onto a conductive substrate. The reduction electrode 14 causes a proton reduction reaction on its surface by electrons transferred from the semiconductor photoelectrode 13 via the photovoltaic element 15 and the conductor 17. This produces the target product, hydrogen.

The photovoltaic element 15 is, for example, a silicon-based element, a copper indium gallium selenide-based element, a III-V-based element, a cadmium telluride-based element, a dye-sensitized element, or an organic semiconductor-based element. The photovoltaic element 15 is disposed behind the light-receiving surface of the semiconductor photoelectrode 13 and behind the cut filter 16, and is connected to the conductor 17. The photovoltaic element 15 receives light that has passed through the semiconductor photoelectrode 13 and the cut filter 16, generates holes and electrons, and supplies them to the semiconductor photoelectrode 13 and the reduction electrode 14, respectively. The photovoltaic element 15 also generates electricity using the received light, boosting the voltage between the semiconductor photoelectrode 13 and the reduction electrode 14, increasing the electromotive force of the redox reaction system, and improving the efficiency of the redox reaction.

The cut filter 16 is, for example, a removal filter that removes a predetermined wavelength range, or a dimming filter that uniformly attenuates light intensity. Filters available on the market may be used. The cut filter 16 is disposed on the optical path between the semiconductor photoelectrode 13 and the photovoltaic element 15, and removes a predetermined wavelength range of light that passes through the semiconductor photoelectrode 13 and reaches the photovoltaic element 15, or weakens the intensity of the light. For example, the cut filter 16 removes a predetermined wavelength range of light that passes through the semiconductor photoelectrode 13 and reaches the photovoltaic element 15, or weakens the intensity of the light, so that the current value (the number of holes generated in the photovoltaic element) generated in the photovoltaic element 15 by the light of the light source 2 (the number of electrons generated in the semiconductor photoelectrode) generated in the semiconductor photoelectrode 13 by the light transmitted through the semiconductor photoelectrode 13 and itself is the same as the current value (the number of electrons generated in the semiconductor photoelectrode) generated in the semiconductor photoelectrode 13 by the light of the light source 2.

The water decomposition reaction is an example of a target reaction. The redox reaction device 1 can also be applied to target products other than hydrogen. Specifically, by changing the type of metal of the reduction electrode 14 (e.g., Ni, Fe, Au, Pt, Ag, Cu, In, Ti, Co, Ru), by separating the semiconductor photoelectrode 13 and the reduction electrode 14 tanks and changing the atmosphere in the reduction electrode 14 tank to a gas, or by changing the type of gas or aqueous solution, it is possible to generate carbon compounds by the reduction reaction of carbon dioxide, or ammonia by the reduction reaction of nitrogen.

[Example 1]
[Fabrication of semiconductor photoelectrode]
A semiconductor thin film of n-GaN was epitaxially grown on a GaN substrate by metal organic chemical vapor deposition (MOCVD). The thickness of the n-GaN was 2 μm. The carrier density was 3×10 ¹⁸ cm ^-3 . Then, InGaN with a composition ratio of indium (In) of 5% was grown. The thickness of the InGaN was 100 nm, which is enough to absorb light. Ni with a thickness of about 1 nm was vacuum-deposited on the surface of the InGaN. This semiconductor thin film was heat-treated in air at 300°C for 1 hour to form NiO. When the cross section of the NiO was observed with a transmission electron microscope (TEM), the thickness was 2 nm. When the optical transmittance of NiO was measured, it was found that it absorbed sunlight of about 400 nm or less.

[Oxidation-reduction reaction test]
The surface of the InGaN was scribed, and a conductor was connected to a part of the exposed n-GaN surface, and soldered using In. The In surface was then covered with epoxy resin so that it would not be exposed. This was placed in an aqueous solution 12 as a semiconductor photoelectrode 13. A 1 mol/l aqueous solution of sodium hydroxide was used for the aqueous solution 12. Platinum was used for the reduction electrode 14. A Si-based pn junction semiconductor was used for the photovoltaic element 15.

The photovoltaic element 15 can absorb light up to about 1100 nm out of the light of about 400 nm or more that has passed through the semiconductor photoelectrode 13, and the current it generates is greater than the current generated by the semiconductor photoelectrode 13. The short-circuit current of the photovoltaic element 15 alone was about 10 mA. The respective electromotive forces were 0.6 V for the photovoltaic element 15 and 1.2 V for the semiconductor photoelectrode 13.

A cut filter 16 that cuts light in the wavelength region of 960 nm or less was installed on the optical path between the photovoltaic element 15 and the semiconductor photoelectrode 13. A long-wavelength cut filter such as SIX960 may also be used. By using the cut filter 16, the photocurrent value generated by the photovoltaic element 15 was made equal to the photocurrent value (1.0 mA (when 0.6 V was applied)) between the semiconductor photoelectrode 13 and the reduction electrode 14 that had been measured in advance.

Nitrogen gas was flowed into the tank 11 at 10 ml/min, the light receiving area of the semiconductor photoelectrode 13 was set to ¹ cm2, and the aqueous solution 12 was stirred at the center of the tank bottom at a rotation speed of 250 rpm using a stirrer and a stirrer. After the inside of the tank 11 was sufficiently replaced with nitrogen gas, the light source 2 was fixed so as to face the NiO formation surface of the semiconductor photoelectrode 13 prepared by the above-mentioned procedure. A 300 W high-pressure xenon lamp was used as the light source 2, and light was uniformly irradiated onto the semiconductor photoelectrode 13 and the photovoltaic element 15. 10 hours, 50 hours, 100 hours, 200 hours, and 300 hours after the light irradiation, the gas in the tank 11 was sampled, and the reaction products were analyzed by gas chromatography. As a result, it was confirmed that oxygen and hydrogen were generated.

[Example 2]
[Fabrication of semiconductor photoelectrode]
A semiconductor thin film of n-GaN was epitaxially grown on a GaN substrate by MOCVD. The thickness of the n-GaN was 2 μm. The carrier density was 3×10 ¹⁸ cm ⁻³ . Then, a Ta ₃ N ₅ thin film was formed by sputtering tantalum metal or tantalum oxide and nitriding. The thickness of the Ta ₃ N ₅ was 500 nm, which is sufficient for absorbing light. Ni was vacuum-deposited on the surface of the _{Ta 3} N _5. This semiconductor thin film was heat-treated in air at 300° C. for 1 hour to form NiO. When the cross section of the NiO was observed with a TEM, the thickness was 2 nm. When the light transmittance of NiO was measured, it was found that it absorbed sunlight of about 600 nm or less.

[Oxidation-reduction reaction test]
The surface of _{the Ta3N5} was scratched, and a conductor was connected to a part of the exposed n-GaN surface, and soldered using In. After that, it was covered with epoxy resin so that the In surface was not exposed. This was placed in an aqueous solution 12 as a semiconductor photoelectrode 13.

The photovoltaic element 15 can absorb light up to about 1100 nm out of light of about 600 nm or more that has passed through the semiconductor photoelectrode 13, and the current it generates is greater than the current generated by the semiconductor photoelectrode 13. The short-circuit current of the photovoltaic element 15 alone was about 10 mA. The respective electromotive forces were 0.6 V for the photovoltaic element 15 and 0.8 V for the semiconductor photoelectrode 13. Two of these photovoltaic elements 15 were connected in series between the semiconductor photoelectrode 13 and the reduction electrode 14.

A cut filter 16 that cuts light in the wavelength region of 840 nm or less was installed on the optical path between the photovoltaic element 15 and the semiconductor photoelectrode 13. A long-wavelength cut filter such as SIX840 may also be used. By using the cut filter 16, the photocurrent value generated by the photovoltaic element 15 was made equal to the photocurrent value (2.0 mA (when 1.2 V was applied)) between the semiconductor photoelectrode 13 and the reduction electrode 14 that had been measured in advance.

The rest of the procedure was the same as in Example 1.

[Comparative Example 1]
In Comparative Example 1, the photovoltaic element 15 and the cut filter 16 were not used.

[Comparative Example 2]
In Comparative Example 2, the cut filter 16 was not used. The subsequent steps were the same as in Example 1.

[Comparative Example 3]
In Comparative Example 3, the cut filter 16 was used to control the photocurrent value generated by the photovoltaic element 15 to 0.5 mA so as to be smaller than the photocurrent value (1.0 mA) measured in advance between the semiconductor photoelectrode 13 and the reduction electrode 14. The subsequent steps were carried out in the same manner as in Example 1.

[Comparative Example 4]
In Comparative Example 4, the photovoltaic element 15 and the cut filter 16 were not used.

[Comparative Example 5]
In Comparative Example 5, the cut filter 16 was not used. The subsequent steps were the same as in Example 2.

[Comparative Example 6]
In Comparative Example 6, the cut filter 16 was used to control the photocurrent value generated by the photovoltaic element 15 to 1.0 mA so as to be smaller than the photocurrent value (2.0 mA) measured in advance between the semiconductor photoelectrode 13 and the reduction electrode 14. The subsequent steps were carried out in the same manner as in Example 2.

[Experimental Results]
Table 1 shows the amounts of oxygen and hydrogen gas produced versus the light irradiation time in the examples and comparative examples.

The amount of each gas produced is shown normalized by the surface area of the semiconductor photoelectrode 13. In all cases, it was found that oxygen and hydrogen were produced upon irradiation with light.

Comparing Example 1 with Comparative Examples 1 and 2, it was found that the amount of gas generated was significantly improved by combining the photovoltaic element 15.

In addition, when comparing Example 1 with Comparative Examples 1 and 3, the amount of gas generated in Comparative Example 3 was similar to that of Comparative Example 1, unlike Example 1, despite the incorporation of a photovoltaic element 15 as in Example 1. This is thought to be because the cut filter 16 reduces the amount of current generated from the photovoltaic element 15 to be less than that generated from the semiconductor photoelectrode 13, and therefore the effect of improving the efficiency of the redox reaction by the photovoltaic element 15 was not realized in Comparative Example 3.

In addition, when comparing Example 1 and Comparative Example 2, there was no difference in the amount of gas generated 10 hours after light irradiation. On the other hand, in the case of Example 1, the same characteristics were maintained even after 300 hours, but in Comparative Example 2, the amount of hydrogen generated gradually decreased after 10 hours and was almost inactivated after 100 hours. The amount of oxygen generated was also almost inactivated after 100 hours. The hydrogen to oxygen generation ratio after 50 hours and 100 hours of light irradiation in Comparative Example 2 was not 2:1. This is thought to be because the semiconductor degradation reaction, rather than the oxidation reaction of water, is progressing simultaneously on the surface of the semiconductor photoelectrode 13. From this, it is thought that in Comparative Example 2, an excessive charge was supplied to the semiconductor photoelectrode 13, which promoted the heat generation and degradation reaction of the semiconductor photoelectrode 13, and the reaction field decreased over the course of light irradiation.

Furthermore, comparing Example 2 with Comparative Examples 4 and 5, it was found that the amount of gas generated was significantly improved by combining the photovoltaic element 15. Example 2 and Comparative Examples 4 and ₅ show that when a semiconductor having a narrow band gap such as _Ta3N5 and not straddling the water oxidation reaction level and the hydrogen generation level is used for the semiconductor photoelectrode 13, the target reaction will not occur unless the photovoltaic element 15 is combined to increase the pressure.

In addition, when comparing Example 2 and Comparative Example 6, the amount of gas generated up to 200 hours in Comparative Example 6 was about half that of Example 2, despite the incorporation of a photovoltaic element 15 as in Example 2. This is thought to be because the cut filter 16 reduces the amount of current generated by the photovoltaic element 15 below that generated by the photoelectrode, and therefore the effect of improving the efficiency of the redox reaction by the photovoltaic element 15 was not realized in Comparative Example 6.

In addition, when comparing Example 2 and Comparative Example 5, there was no difference in the amount of gas generated 10 hours after light irradiation. On the other hand, in the case of Example 2, similar characteristics were maintained even after 200 hours, but Comparative Example 5 was almost inactivated after 100 hours. The hydrogen to oxygen generation ratio after 50 hours of light irradiation in Comparative Example 5 was not 2:1. This is thought to be because, on the surface of the semiconductor photoelectrode 13, a deterioration reaction of the semiconductor, rather than a water oxidation reaction, was simultaneously progressing. From this, it is thought that in Comparative Example 6, an excessive charge was supplied to the semiconductor photoelectrode 13, which promoted heat generation and deterioration reactions of the semiconductor photoelectrode 13, and the reaction field decreased over the course of light irradiation time.

From the above, by combining a cut filter 16 with a combination of a semiconductor photoelectrode 13 and a photovoltaic element 15, it is possible to achieve a longer life for the amount of hydrogen and oxygen produced by the water decomposition reaction (light energy conversion efficiency).

[effect]
According to this embodiment, a cut filter 16 that removes a predetermined wavelength range of light that passes through the semiconductor photoelectrode 13 and reaches the photovoltaic element 15 or that weakens the intensity of the light is disposed on the optical path between the semiconductor photoelectrode 13 and the photovoltaic element 15, so that the photocurrent generated from the photovoltaic element 15 can be controlled to be the same as the photocurrent generated from the semiconductor photoelectrode 13. This makes it possible to suppress heat generation and deterioration of the semiconductor photoelectrode 13, and to realize long-term operation of the water decomposition reaction by oxidation-reduction using light energy and a long life for the oxidation-reduction reaction device.

1: Oxidation-reduction reaction device 11: Tank 12: Aqueous solution 13: Semiconductor photoelectrode 14: Reduction electrode 15: Photovoltaic element 16: Cut filter 17: Conductor 2: Light source

Claims (2)

a semiconductor photoelectrode that undergoes an oxidation reaction with an aqueous solution by light;
a reduction electrode that performs a reduction reaction with an aqueous solution or a gas by electrons transferred from the semiconductor photoelectrode via a conductor;
a photovoltaic element that is disposed on the back side of the light-receiving surface of the semiconductor photoelectrode, connected to the conductive wire, and generates electricity using light that has passed through the semiconductor photoelectrode;
a cut filter that is disposed on an optical path between the semiconductor photoelectrode and the photovoltaic element and that removes a predetermined wavelength range of light that passes through the semiconductor photoelectrode and reaches the photovoltaic element or reduces the intensity of the light ;
The cut filter is
An oxidation-reduction reaction device that removes a predetermined wavelength range of light that passes through the semiconductor photoelectrode and reaches the photovoltaic element, or attenuates the intensity of the light, so that the current value generated in the photovoltaic element by light transmitted through the semiconductor photoelectrode and itself becomes the same as the current value generated in the semiconductor photoelectrode by the light . The redox reaction device according to claim 1, wherein the wavelength range of light absorbed by the semiconductor photoelectrode is smaller than the wavelength range of light absorbed by the photovoltaic element.

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