JP7598059B2 - Carbon Dioxide Reduction Device - Google Patents

Description

The present invention relates to a carbon dioxide reduction device.

The increase in carbon dioxide concentrations in the atmosphere is cited as the main cause of global warming. Reducing carbon dioxide emissions is a long-term challenge on a global scale. Meanwhile, in the medium to long term, energy issues call for a reassessment of energy supplies that rely on fossil fuels, and the creation of next-generation energy sources is required.

As a means of obtaining energy while reducing carbon dioxide emissions, technological development is underway to utilize unused energy sources such as exhaust heat, snow and ice heat, vibrations, and electromagnetic waves, as well as renewable energy sources such as sunlight. These power generation technologies are only capable of generating electrical energy and are not able to store energy. Furthermore, they are not able to create chemical products made from fossil fuels.

As a method for simultaneously solving these problems, technology that uses light energy to reduce carbon dioxide has attracted attention. For example, Non-Patent Document 1 discloses a device for reducing carbon dioxide by light irradiation. In this reduction device, when light is irradiated onto an oxidation electrode, electron-hole pairs are generated and separated at the oxidation electrode, and oxygen and protons (H+) are generated by the oxidation reaction of water. Hydrogen is generated by the combination of protons and electrons at the reduction electrode, causing a reduction reaction. This reduction reaction produces carbon monoxide, formic acid, methane, and other substances that can be used as energy resources.

The efficiency of the carbon dioxide reduction reaction can be expressed as the Faraday efficiency. The Faraday efficiency is expressed as the ratio of the number of electrons used in the carbon dioxide reduction reaction to the number of electrons transferred between the oxidation electrode and the reduction electrode.

It is known that the Faraday efficiency is closely related to the voltage of the reduction electrode. Therefore, in order to improve the Faraday efficiency, a method has been disclosed in which a solar cell is incorporated in the reduction device and the voltage of the reduction electrode is changed by changing the number of solar cell elements (for example, Non-Patent Document 2).

Satoshi Yotsuhashi et al.”CO2Conversion with Light and Water by GaN Photo electroade”, Japanese Journal of Applied Physics, 51, 2012, p.02BP07-1-p.02BP07-3 Qingxin Jia et al.”Direct Gas-phase CO2 reduction for Solar Methane Generation Using a Gas Diffusion Electrode with a BiVO4:Mo and a Cu-In-e Photoanode”, Chem .Lett.,47,2018,p.436-p .439

However, in the method disclosed in Non-Patent Document 2, the voltage of the reduction electrode can only be adjusted discretely because the electromotive force per element of a solar cell is 0.5 to 0.6 V. Therefore, the voltage of the reduction electrode is set in steps in units of that electromotive force.

For example, the voltage of a reduction electrode that produces carbon monoxide efficiently is 1.25 V relative to the oxidation electrode. To set this voltage, an external power supply that can be set to any voltage is required.

The present invention has been made in consideration of this problem, and aims to provide a carbon dioxide reduction device that can continuously change the voltage of the reduction electrode without using an external power source.

The carbon dioxide reduction device according to one embodiment of the present invention comprises an oxidation electrode formed on a transparent substrate and receiving light from the outside, an oxidation tank holding an electrolyte in which the oxidation electrode is immersed, a reduction electrode, a reduction tank holding the electrolyte in which the reduction electrode is immersed and into which carbon dioxide is bubbled from the outside, an electrolyte membrane disposed between the oxidation tank and the reduction tank and dividing the electrolyte into an oxidation side and a reduction side, and a variable resistor connected between the oxidation electrode and the reduction electrode.

According to the present invention, it is possible to provide a carbon dioxide reduction device that can continuously change the voltage of the reduction electrode without using an external power source.

FIG. 1 is a schematic diagram showing a configuration example of a carbon dioxide reduction device according to a first embodiment of the present invention. FIG. 2 is a schematic diagram showing a modified example of the carbon dioxide reduction device shown in FIG. 1 . FIG. 4 is a schematic diagram showing a configuration example of a carbon dioxide reduction device according to a second embodiment of the present invention. FIG. 4 is a schematic diagram showing a modified example of the solar cell shown in FIG. 3 . FIG. 2 is a graph showing the relationship between the voltage of the reduction electrode and the Faraday efficiency (when the product is carbon monoxide) in Experimental Example 1. FIG. 2 is a diagram showing the relationship between the voltage of the reduction electrode and the faradaic efficiency (when the product is formic acid) in Experimental Example 1. FIG. 6 is a diagram showing the results of a comparative example corresponding to FIG. 5 . FIG. 7 is a diagram showing the results of a comparative example corresponding to FIG. 6 .

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The same reference symbols are used for the same parts in the drawings, and the description will not be repeated.

First Embodiment
1 is a schematic diagram showing a configuration example of a carbon dioxide reduction device according to a first embodiment of the present invention. In FIG. 1, the left and right are defined as the X direction, the depth of the drawing is defined as the Y direction, and the top of the drawing is defined as the Z direction.

The carbon dioxide reduction device 100 shown in Figure 1 includes an oxidation electrode 2, an oxidation tank 6, a reduction electrode 3, a reduction tank 7, an electrolyte membrane 4, and a variable resistor 8. The carbon dioxide reduction device 100 generates carbon monoxide, formic acid, methane, and the like that can be used as energy resources through an oxidation-reduction reaction.

The oxidation electrode 2 is formed on the transparent substrate 1 and receives light 9 from the outside. The transparent substrate 1 is, for example, sapphire, and has a predetermined area in a plane in the XY direction. A compound containing at least one selected from the group consisting of, for example, a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, or a rhenium complex is formed on the plane of the transparent substrate 1 to form the oxidation electrode 2. These compounds exhibit photoactivity and redox activity.

The light 9 is, for example, sunlight. Note that the light 9 does not have to be sunlight. For example, the light may be light from a xenon lamp, a pseudo-sun light source, a halogen lamp, a mercury lamp, or a combination of these light sources.

The oxidation tank 6 holds the electrolyte 5 in which the oxidation electrode 2 is immersed. The electrolyte 5 contains at least one selected from the group consisting of an aqueous potassium bicarbonate solution, an aqueous sodium bicarbonate solution, an aqueous potassium chloride solution, an aqueous sodium chloride solution, an aqueous potassium hydroxide solution, an aqueous rubidium hydroxide solution, and an aqueous cesium hydroxide solution. Figure 1 shows an example in which light 9 is irradiated in the Z direction from the bottom of the oxidation tank 6.

The reduction electrode 3 may be, for example, any of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, and porous alloys thereof. The reduction electrode 3 may also be a compound such as silver oxide, copper oxide, copper (II) oxide, nickel oxide, indium oxide, tin oxide, tungsten oxide, tungsten (VI) oxide, or copper oxide, or a porous metal complex having metal ions and anionic ligands. The reduction electrode 3 has a predetermined area in a plane in the XY direction, similar to the oxidation electrode 2. The reduction electrode 3 may be arranged to form a plane in the Y direction, similar to the electrolyte membrane 4 described later.

The reduction tank 7 holds the electrolyte 5 in which the reduction electrode 3 is immersed and into which carbon dioxide is bubbled from the outside. The electrolyte 5 is the same as that in the oxidation tank 6.

The electrolyte membrane 4 is disposed between the oxidation tank 6 and the reduction tank 7, and divides the electrolyte solution 5 into an oxidation side and a reduction side. The electrolyte membrane 4 may be, for example, any of Nafion (registered trademark), ForeBlue, and Aquivion (registered trademark), which are electrolyte membranes having a carbon-fluorine skeleton, or Selemion or Neocepta, which are electrolyte membranes having a hydrocarbon skeleton.

The variable resistor 8 is connected between the oxidation electrode 2 and the reduction electrode 3. The maximum resistance value of the variable resistor 8 is set to, for example, the same value as the resistance value between the oxidation electrode 2 and the reduction electrode 3. When the resistance value of the variable resistor 8 is set to the minimum, the voltage of the reduction electrode 3 relative to the oxidation electrode 2 can be set to the highest. When the resistance value of the variable resistor 8 is set to the maximum, the voltage of the reduction electrode 3 can be set to the lowest.

In the case of a typical variable resistor 8 that rotates a slider, the movable angle of the rotation axis is approximately 310 degrees, and can be adjusted to any angle. Therefore, the voltage of the reduction electrode 3 can be set to any voltage, optimizing the Faraday efficiency.

(Variation 1)
Fig. 2 is a schematic diagram showing a first modified example of the carbon dioxide reduction device 100. The carbon dioxide reduction device 110 shown in Fig. 2 is an example in which the oxidation electrode 2 is configured by stacking two semiconductor layers.

The oxidation electrode 2 is composed of two layers, a first semiconductor film 21 and a second semiconductor film 22. For example, if the semiconductor material of the first semiconductor film 21 is GaN, the second oxidation electrode 22 can be made of CdS or C3N4.

As described above, the carbon dioxide reduction device 100 according to this embodiment includes an oxidation electrode 2 formed on a transparent substrate 1 and receiving light from the outside, an oxidation tank 6 holding an electrolyte 5 in which the oxidation electrode 2 is immersed, a reduction electrode 3, a reduction tank 7 in which the reduction electrode 3 is immersed and holding an electrolyte 5 in which carbon dioxide is bubbled from the outside, an electrolyte membrane 4 disposed between the oxidation tank 6 and the reduction tank 7 and dividing the electrolyte 5 into an oxidation side and a reduction side, and a variable resistor 8 connected between the oxidation electrode 2 and the reduction electrode 3. This makes it possible to provide a carbon dioxide reduction device that can continuously change the voltage of the reduction electrode without using an external power source.

In addition, the oxidation electrode 2 of the first modified example is configured by stacking two semiconductor layers, and the energy level of the conduction band of the second semiconductor film 22 arranged on the reduction electrode 3 side is higher than the energy level of the valence band of the first semiconductor film 21, and the energy level of the valence band of the second semiconductor film 22 is lower than the energy level of the conduction band of the first semiconductor film 21. This makes it possible to increase the voltage of the reduction electrode 3.

{Second embodiment}
3 is a schematic diagram showing a configuration example of a carbon dioxide reduction device according to a second embodiment of the present invention. The carbon dioxide reduction device 200 shown in FIG. 3 differs from the carbon dioxide reduction device 100 (FIG. 1) in that it includes a solar cell 20.

The solar cell 20 is disposed on the transparent substrate 1, and generates a voltage by light 9 transmitted through the oxide electrode 2 and the transparent substrate 1. The solar cell 20 can be any of a crystalline silicon solar cell, a single crystal silicon solar cell, a polycrystalline silicon solar cell, an amorphous silicon solar cell, a compound semiconductor solar cell, and a dye-sensitized solar cell.

The solar cell 20 is constructed by depositing a cathode electrode 20a and an anode electrode 20b on a transparent substrate 20c. The cathode electrode 20a is connected to the oxidation electrode 2, and the anode electrode 20b is connected to the terminal of the variable resistor 8 on the oxidation electrode 2 side.

The band gap of the cathode electrode 20a and the anode electrode 20b is preferably narrower than the band gap of the oxidation electrode 2. The oxidation electrode 2 may have the configuration of the above-mentioned variant 1.

As described above, the carbon dioxide reduction device 200 according to this embodiment includes a solar cell 20 that connects the cathode electrode 20a to the oxidation electrode 2 and the anode electrode 20b to the variable resistor 8. This allows the voltage of the reduction electrode 3 to be further increased by utilizing light energy. The solar cell 20 also converts light of wavelengths other than the light 9 absorbed by the oxidation electrode 2 into electricity. Therefore, the carbon dioxide reduction device 200 can effectively utilize light energy over a wide wavelength range.

The solar cell 20 shown in Figure 3 is an example of one element with one layer of PN junction. The terminal voltage of the solar cell 20 in the case of one element is 0.3V to approximately 0.6V, in the case of two elements it is approximately 0.6V to 1.2V, and in the case of three elements it is approximately 0.9V to 1.8V. In the case of two or three elements, two or three pairs of cathode electrodes 20a and anode electrodes 20b are stacked. In this way, the voltage of the reduction electrode 3 can be changed by changing the number of solar cells 20 stacked.

In addition, the solar cell 20 and the variable resistor 8 may be connected in a reversed manner. That is, the variable resistor 8 may be disposed on the oxidation electrode 2 side, and the solar cell 20 may be disposed on the reduction electrode 3 side.

In short, the solar cell 20 is connected in series to the variable resistor 8. If the variable resistor 8 is placed on the oxidation electrode 2 side, when the voltage of the oxidation electrode 2 is smaller than the generated voltage of the solar cell 20, the voltage adjustment by the variable resistor 8 affects only the oxidation electrode 2, making it possible to adjust the voltage more precisely.

(Variation 2)
Fig. 4 is a schematic diagram showing a modified example of the solar cell 20 described in the second embodiment. As shown in Fig. 4, the solar cell 20 may be formed on the surface of the transparent substrate 1 opposite to the oxidation electrode 2. The solar cell 20 is exposed from the surface of the electrolyte 5.

In this way, in the solar cell 20 of this modified example, the oxidation electrode 2 is formed on the surface of the transparent substrate 1 opposite the electrolyte 5, on which the film is formed, and is exposed from the surface of the electrolyte 5. This makes the transparent substrate 20c unnecessary, and reduces the number of transparent substrates to one, thereby improving the efficiency of using light energy.

(experiment)
Electrochemical measurements were carried out in the second embodiment described above. The experimental conditions will be described.

The oxidation electrode 2 was constructed by epitaxially growing a thin film of n-type semiconductor GaN and AlGaN in that order on sapphire, vacuum-depositing Ni on top of that, and then performing heat treatment to form a NiO promoter thin film. The transparent substrate and the oxidation electrode 2 were immersed in the electrolyte 5.

Electrolyte 5 was a 1.0 mol/L aqueous potassium hydroxide solution.

A copper plate was used as the reduction electrode 3. The reduction reaction of carbon dioxide takes place on the surface of the copper plate.

The electrolyte membrane 4 separating the oxidation tank 6 and the reduction tank 7 is made of Nafion (registered trademark).

The solar cells 20 used were Sphelar Power's (model name: KSP-0C-1830MR-ER-X03) connected in series in any number.

The variable resistor 8 is a Bourns potentiometer (model number: 3386V-1-203LF).

A 300 W xenon lamp was used as the light 9 instead of sunlight. Wavelengths of 450 nm or more were cut off with a filter, and the illuminance was set to 6.6 mW/ ^cm2 . The area of the oxidation electrode 2 irradiated with the light 9 was set to 2.5 ^cm2 .

Helium was bubbled into the oxidation tank 6 and carbon dioxide into the reduction tank 7 at a flow rate of 5 ml/min and a pressure of 0.18 MPa. Helium was bubbled in for the purpose of analyzing the reaction products. After sufficient replacement of the helium with carbon dioxide, the above-mentioned light 9 was irradiated.

The current flowing between the oxidation electrode 2 and the reduction electrode 3 upon irradiation with light 9 was measured using an electrochemical measurement device (Solartron, Model 1287 potentiogalvanostat).

The gases and liquids produced in the oxidation tank 6 and the reduction tank 7 were collected and the reaction products were analyzed using a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer.

The Faraday efficiency of the carbon dioxide reduction reaction was calculated. The Faraday efficiency of carbon dioxide indicates the ratio of the number of electrons used in the carbon dioxide reduction reaction to the number of electrons transferred between the oxidation electrode 2 and the reduction electrode 3 by light irradiation or voltage application.

Here, the "number of electrons in the reduction reaction" in formula (1) is calculated by converting the measured cumulative amount of the reduction product of carbon dioxide into the number of electrons required for the production reaction. If the concentration of the reduction reaction product is A (ppm), the flow rate of the carrier gas is B (L/sec), the number of electrons required for the reduction reaction is Z (mol), the Faraday constant is F (C/mol), the model body of the gas is _Vm (L/mol), and the time of light irradiation or voltage application is T (sec), the "number of electrons in the reduction reaction" can be calculated using the following formula.

(experiment)
In the experiment, the Faraday efficiency of the carbon dioxide reduction reaction was obtained in the configuration of the second embodiment (FIG. 3).

The light 9 was a 300 W high-pressure xenon lamp (wavelengths of 450 nm or more were cut off with a filter) with an illuminance of 6.6 mW/cm ² in order to make it easier to quantify. The oxidation electrode 2 was placed so as to be the irradiated surface.

Two solar cells 20 manufactured by Sphelar Power (model name: KSP-0C-1830MR-ER-X03) were used, connected in series.

The variable resistor 8 is a Bourns potentiometer (model number: 3386V-1-203LF).

The results of experiment 1, in which the voltage of the reduction electrode 3 was changed in increments of 0.1 V by changing the variable resistor 8, are shown in Figures 5 and 6. In each case, the horizontal axis indicates the voltage of the reduction electrode 3 relative to the oxidation electrode 2, and the vertical axis indicates the Faraday efficiency.
Figure 5 shows the case where the product is carbon monoxide. As shown in Figure 5, the voltage of the reduction electrode 3 can be changed continuously by using a variable resistor 8. As a result, it is found that the maximum value of the faradaic efficiency of carbon monoxide is between 0.7 and 0.9 V, which is about 18%.

Figure 6 shows the case where the product is formic acid. As shown in Figure 5, the maximum value of the faradaic efficiency of formic acid is between 0.6 and 0.8 V, which is about 2%.

7 and 8 show the relationship between the voltage of the reduction electrode 3 and the Faraday efficiency of a comparative example. The comparative example is a configuration in which the variable resistor 8 is removed from the configuration of the second embodiment (FIG. 3). The comparative example is not shown.

Figure 7 shows the results of a comparative example corresponding to Figure 5. Figure 8 shows the results of a comparative example corresponding to Figure 6. The relationship between the horizontal and vertical axes in Figure 7 is the same as in Figure 5. The relationship between the horizontal and vertical axes in Figure 8 is the same as in Figure 6.

As shown in Figure 7, the voltage of the reduction electrode 3 is controlled by the number of elements of the solar cell 20, so it can only take discrete values such as 0.6V and 1.2V. Therefore, the maximum value of the Faraday efficiency obtained in this embodiment (about 18%) cannot be obtained in the comparative example. In the example shown in Figure 7, it is about 16% (0.6V) and about 9% (1.2V).

The results are similar when the product shown in Figure 8 is formic acid. Explanation of Figure 8 is omitted.

As described above, the carbon dioxide reduction device 100, 200 according to this embodiment can continuously vary the voltage of the reduction electrode 3 without using electric power. Therefore, it is possible to maximize the faradaic efficiency of the carbon dioxide reduction reaction without using an external power source. Therefore, the efficiency of the entire system using the carbon dioxide reduction device 100, 200 can be improved.

The present invention is not limited to the above embodiment, and modifications are possible within the scope of the invention. For example, in the embodiment, the light 9 is generated by a xenon lamp, but sunlight may also be used.

As such, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is determined only by the invention-specific matters related to the scope of the claims that are appropriate from the above description.

The present invention can be widely used in the field of carbon dioxide recycling.

1: Transparent substrate 2: Oxidation electrode 3: Reduction electrode 4: Electrolyte membrane 5: Electrolyte solution 6: Oxidation tank 7: Reduction tank 8: Variable resistor 9: Light 20: Solar cell 20a: Cathode electrode 20b: Anode electrode

Claims (4)

an oxidation electrode formed on a transparent substrate and configured to receive light from outside;
an oxidation tank for holding an electrolyte in which the oxidation electrode is immersed;
A reduction electrode;
a reduction tank for holding the electrolytic solution in which the reduction electrode is immersed and carbon dioxide is bubbled from the outside;
an electrolyte membrane disposed between the oxidation chamber and the reduction chamber and dividing the electrolyte into an oxidation chamber and a reduction chamber;
and a variable resistor connected between the oxidation electrode and the reduction electrode. The oxidation electrode is
2. The carbon dioxide reduction device according to claim 1, wherein the device is configured by stacking two semiconductor films, and the energy level of the conductor band of the second semiconductor film disposed on the reduction electrode side is higher than the energy level of the valence band of the first semiconductor film, and the energy level of the valence band of the second semiconductor film is lower than the energy level of the conduction band of the first semiconductor film. Equipped with solar cells,
The carbon dioxide reduction device according to claim 1 or 2, wherein the solar cell is connected in series with the variable resistor. The solar cell comprises:
4. The carbon dioxide reduction device according to claim 3, wherein the oxidation electrode is formed on a surface of the transparent substrate opposite to the electrolyte solution on which the film is formed, and is exposed from the surface of the electrolyte solution.

Images