Carbon dioxide reduction device

Abstract

The present invention comprises an oxidation electrode 2 which receives light 13 from the outside, an electrolytic bath 6 which holds an electrolyte 5 in which the oxidation electrode 2 is submerged, an electrolyte film 4 which constitutes a portion of one surface of the electrolytic bath 6 except where the light 13 is incident, a reduction electrode 3 which is connected to the electrolyte film 4, a reduction unit 7 on which the reduction electrode 3 is disposed and to which a gas containing carbon dioxide is supplied from the outside, and a blower 10 which generates a flow of air toward the reduction electrode 3 inside the reduction unit 7, wherein the reduction electrode 3 has a plate-like shape, and one surface of the reduction electrode 3 is in contact with the electrolyte film 4.

Description

[Technical Field]

[0001] The present invention relates to a carbon dioxide reduction device. [Background technology]

[0002] The increase in carbon dioxide concentration in the atmosphere is cited as the main cause of global warming. Reducing carbon dioxide emissions has become a long-term challenge on a global scale. Meanwhile, in the medium to long term, energy issues are forcing us to reconsider our reliance on fossil fuels for energy supplies, and there is a need to create next-generation energy sources.

[0003] 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. However, these power generation technologies are limited to generating electrical energy and are unable to store it. They also cannot create chemical products from fossil fuels.

[0004] As a method for simultaneously solving these problems, a technology that uses light energy to reduce carbon dioxide has attracted attention. Non-Patent Document 1 discloses an apparatus for reducing carbon dioxide by light irradiation. In this reduction apparatus, 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 produced by an oxidation reaction of water. At the reduction electrode, hydrogen is produced by the combination of protons and electrons, causing a reduction reaction. This reduction reaction produces carbon monoxide, formic acid, methane, and other substances that can be used as energy resources. [Prior art documents] [Non-patent literature]

[0005] [Non-Patent Document 1] 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 [Non-patent document 2] Qingxin Jia et al.”Direct Gas-phase CO2reduction 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 Summary of the Invention [Problem to be solved by the invention]

[0006] In the carbon dioxide reduction device disclosed in Non-Patent Document 1, a reduction electrode is immersed in a solution (electrolyte), and carbon dioxide is dissolved in the solution to supply carbon dioxide to the reduction electrode to carry out a reduction reaction. However, this carbon dioxide reduction reaction has a problem in that there are limits to the concentration of carbon dioxide dissolved in the solution and the diffusion coefficient of carbon dioxide in the solution, which limits the amount of carbon dioxide that can be supplied to the reduction electrode.

[0007] In response to this, research is being conducted to increase the amount of carbon dioxide supplied to the reduction electrode by removing the solution on the reduction electrode side and directly supplying carbon dioxide to the reduction electrode (Non-Patent Document 2). Non-Patent Document 2 reports that by using a reaction device having a structure that can supply gaseous carbon dioxide to the reduction electrode, the amount of carbon dioxide supplied to the reduction electrode is increased and the reduction reaction of carbon dioxide is promoted.

[0008] However, as the reduction reaction progresses, reduction products of carbon dioxide are generated on the reaction surface of the reduction electrode, resulting in the production of not only gaseous hydrogen, carbon monoxide, and methane, but also liquids such as formic acid, methanol, and ethanol. Furthermore, the liquid solution gradually seeps into the gas phase through the electrolyte membrane. This causes the reaction surface of the reduction electrode on the gas phase side to become covered with these liquids, preventing the reaction from proceeding.

[0009] The present invention has been made in view of this problem, and has an object to provide a carbon dioxide reduction device that can improve the decrease in reaction efficiency of the carbon dioxide reduction reaction. [Means for solving the problem]

[0010] A carbon dioxide reduction device according to one aspect of the present invention comprises an oxidation electrode that receives light from outside, an oxidation tank that holds an electrolyte solution in which the oxidation electrode is immersed, an electrolyte membrane that forms part of one surface of the oxidation tank excluding the surface on which the light is incident, a reduction electrode connected to the outer surface of the electrolyte membrane, a reduction section in which the reduction electrode is disposed and to which gas containing carbon dioxide is supplied from outside, and a blower that generates an airflow inside the reduction section toward the reduction electrode. [Effects of the Invention]

[0011] According to the present invention, it is possible to provide a carbon dioxide reduction device that can improve the decrease in the reaction efficiency of the carbon dioxide reduction reaction. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram showing a configuration example of a carbon dioxide reduction device according to an embodiment of the present invention. FIG. [Figure 2] FIG. 2 is a schematic diagram showing a modified example of the carbon dioxide reduction device shown in FIG. [Figure 3] FIG. 3 is a schematic diagram showing an example of the relationship between the reduction electrode and the blower shown in FIG. 2. [Figure 4] FIG. 1 is a diagram showing the experimental results of Experiment 1. [Figure 5]FIG. 4 is a schematic diagram showing another example of the relationship shown in FIG. 3. [Figure 6] FIG. 4 is a schematic diagram showing another example of the relationship shown in FIG. 3. [Figure 7] FIG. 10 is a diagram showing the experimental results of Experiment 2. [Figure 8] FIG. 10 is a diagram showing the experimental results of Experiment 3. DETAILED DESCRIPTION OF THE INVENTION

[0013] Hereinafter, embodiments of the present invention will be described with reference to the drawings. In the drawings, the same reference numerals are used to denote the same parts, and the description thereof will not be repeated.

[0014] 1 is a schematic diagram showing an example of the configuration of a carbon dioxide reduction device according to an embodiment of the present invention. In FIG. 1, the left and right are defined as the X direction, the depth of the drawing as the Y direction, and the top of the drawing as the Z direction.

[0015] 1 includes an oxidation electrode 2, an oxidation tank 6, an electrolyte membrane 4, a reduction electrode 3, a reduction unit 7, and a blower 10. The carbon dioxide reduction device 100 generates both gaseous and liquid reduction products through an oxidation-reduction reaction.

[0016] Carbon dioxide to be reduced using light energy is supplied into the reduction unit 7 through a supply port 8 provided on the top surface of the reduction unit 7 and a supply port 9 provided on the side surface thereof. The supply port 8 is connected to, for example, a cylinder filled with carbon dioxide, and constantly supplies carbon dioxide decompressed to a predetermined pressure. The supply port 9 supplies the same carbon dioxide as that supplied from the supply port 8 from the side surface of the reduction unit 7. It is sufficient to provide either one of the supply ports 8 or 9.

[0017] Furthermore, when supply ports 8 and 9 are provided, a gas containing carbon dioxide may be supplied from supply port 8, and air, for example, may be supplied from supply port 9. The gas supplied from supply port 9 may be nitrogen, argon, helium, or the like.

[0018] The blower 10 is disposed in front of the supply port 9 and inside the reduction unit 7. The blower 10 generates an airflow toward the reduction electrode 3 inside the reduction unit 7.

[0019] A gas recovery port 11 for recovering gaseous reduction products is provided on the upper surface of the reduction unit 7. Furthermore, a liquid recovery port 12 for recovering liquid reduction products is provided on the lower surface of the reduction unit 7.

[0020] The oxidation electrode 2 is formed on a substrate 1 and receives external light 13. The substrate 1 is made of, for example, sapphire and has a predetermined area on 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 substrate 1 to form the oxidation electrode 2. These compounds exhibit photoactivity and redox activity.

[0021] The substrate 1 does not have to be made of a light-transmitting material such as sapphire, but may be made of a material that does not transmit light, such as glass epoxy resin.

[0022] The light 13 is, for example, sunlight. However, the light 13 does not have to be sunlight. For example, the light 13 may be light from a xenon lamp, a solar simulant light source, a halogen lamp, a mercury lamp, or a combination of these light sources.

[0023] The oxidation bath 6 holds an electrolyte 5 in which the oxidation electrode 2 is immersed. The electrolyte 5 contains, for example, 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. Fig. 1 shows an example in which light 13 is irradiated from the bottom of the oxidation bath 6 in the Z direction.

[0024] The electrolyte membrane 4 constitutes a part of one surface of the oxidation basin 6 excluding the surface in the direction in which light 13 is incident. FIG. 1 shows an example in which the electrolyte membrane 4 is provided on the surface of the oxidation basin 6 parallel to the irradiation direction of light 13. The electrolyte membrane 4 may be provided on any one of the four surfaces (side surfaces) of the oxidation basin 6 excluding the surface in which light 13 is irradiated. Furthermore, if the oxidation basin 6 has an upper surface (if the oxidation basin 6 is covered with a lid), the electrolyte membrane 4 may be provided on the upper surface. If the electrolyte membrane 4 is provided on the upper surface of the oxidation basin 6, the reduction unit 7 is disposed above the electrolyte membrane 4.

[0025] The electrolyte membrane 4 is, for example, an electrolyte membrane such as Nafion (registered trademark), ForeBlue, or Aquivion, which has a skeleton made of carbon and fluorine, or Selemion or Neosepta, which has a hydrocarbon-based skeleton.

[0026] The reduction electrode 3 is connected to the electrolyte membrane 4. The reduction electrode 3 is plate-shaped, and FIG. 1 shows an example in which one surface of the reduction electrode 3 is in contact with the outer surface (YZ surface) of the electrolyte membrane 4 (the reduction section 7 side). The reduction electrode 3 is electrically connected to the oxidation electrode 2 via a lead wire, the reference number of which is omitted.

[0027] The reduction electrode 3 can be, for example, any of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, and a porous body of an alloy 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 may be disposed so as to form a plane in the X direction, similar to the electrolyte membrane 4 described below.

[0028] The surface of the reduction electrode 3 is covered with carbon dioxide supplied from the supply ports 8 and 9. Then, an oxidation-reduction reaction occurs on the surface of the reduction electrode 3, and gases such as hydrogen, carbon monoxide, and methane, and liquid reduction products such as formic acid, methanol, and ethanol are produced.

[0029] Reduction products such as hydrogen, carbon monoxide, and methane have smaller molecular weights than carbon dioxide and are therefore lighter, and are discharged to the outside through a gas recovery port 11 provided at the top of the reduction unit 7. On the other hand, liquid reduction products are discharged to the outside through a liquid recovery port 12 provided at the top of the reduction unit 7. The absence of the gas recovery port 11 and the liquid recovery port 12 does not affect the carbon dioxide reduction reaction. Therefore, the gas recovery port 11 and the liquid recovery port 12 are not essential components in this embodiment.

[0030] The blower 10 generates an airflow inside the reduction unit 7 directed toward the reduction electrode 3, thereby removing the liquid on the surface of the reduction electrode 3. In this way, the surface of the reduction electrode 3 is always covered with fresh carbon dioxide, which can improve the decrease in the reaction efficiency of the reduction reaction.

[0031] The blower 10 may generate an airflow constantly or intermittently. When an airflow is generated intermittently, the gas supplied from the supply port 9 may also be supplied intermittently in accordance with the operation of the blower 10. In other words, the blower 10 may be operated intermittently. This allows for a reduction in power consumption compared to when the blower 10 is operated constantly.

[0032] The blower 10 may also change the flow rate of the airflow, which can effectively promote the reduction reaction and remove the reduction products.

[0033] As described above, the carbon dioxide reduction device 100 according to this embodiment includes the oxidation electrode 2 that receives external light 13, the oxidation tank 6 that holds the electrolyte solution 5 in which the oxidation electrode 2 is immersed, the electrolyte membrane 4 that forms part of one surface of the oxidation tank 6 excluding the surface on which light 13 is incident, the reduction electrode 3 that is connected to the electrolyte membrane 4, the reduction section 7 in which the reduction electrode 3 is disposed and to which gas containing carbon dioxide is supplied from the outside, and the blower 10 that generates an airflow inside the reduction section 7 toward the reduction electrode 3. This makes it possible to provide a carbon dioxide reduction device that can improve the decrease in the reaction efficiency of the reduction reaction.

[0034] The reduction electrode 3 is plate-shaped, and one surface of the reduction electrode 3 is in contact with the electrolyte membrane 4. This allows the current flowing between the oxidation electrode 2 and the reduction electrode 3 to be increased, thereby improving the reaction efficiency of the reduction reaction.

[0035] 1, the liquid (reduction product) generated on the surface of the reduction electrode 3 moves downward due to gravity, thereby improving the reduction reaction efficiency.

[0036] (Variation) 2 is a schematic diagram showing a modified example of the carbon dioxide reduction device 100. The modified example shown in FIG. 2 differs from the carbon dioxide reduction device 100 (FIG. 1) in that a blower 20 is provided.

[0037] Blower 20 is a mass flow controller provided at the tip of the inner side of supply port 9, through which pressurized carbon dioxide is supplied. A mass flow controller measures the mass flow rate of a fluid and controls the flow rate, and is sometimes called a flow rate variable device.

[0038] The flow rate of carbon dioxide by the blower 20 is controlled by a control signal (not shown). The control signal is given, for example, as a voltage amplitude. For example, when the voltage of the control signal is 0V, the flow rate is 0, when a predetermined voltage value is used, the carbon dioxide is sprayed at a predetermined flow rate, and when the voltage value of the control signal is the maximum, the carbon dioxide is sprayed at the pressure of the pressurized cylinder. Therefore, a flow of carbon dioxide at a predetermined flow rate can be generated by the control signal. It is also possible to spray high-pressure carbon dioxide intermittently by giving a pulsed control signal.

[0039] The flow of carbon dioxide is directed toward the reduction electrode 3, so that the reduction product (liquid) on its surface can be removed. This can improve the decrease in the reaction efficiency of the reduction reaction. Note that the gas to be injected does not have to be carbon dioxide. It can be air, nitrogen, argon, helium, or other gases.

[0040] (experiment) Electrochemical measurements were carried out using the modified configuration (Fig. 2). The experimental conditions are as follows.

[0041] The oxidation electrode 2 was constructed by epitaxially growing a thin film of GaN, an n-type semiconductor, and AlGaN in that order on a substrate (sapphire substrate) 1, and then vacuum-depositing Ni on top of that and performing heat treatment to form a NiO promoter thin film. The oxidation electrode 2 was immersed in an electrolyte 5.

[0042] The electrolyte 5 used was a 1.0 mol / L aqueous solution of sodium hydroxide.

[0043] The reduction electrode 3 was made of porous copper.

[0044] The electrolyte membrane 4 was made of Nafion (registered trademark).

[0045] A Kofloc Corporation model (Model EX-250S Series) was used as the blower 20. The blower 20 was connected to a carbon dioxide cylinder via a supply port 9 and was positioned so that the carbon dioxide was sprayed perpendicularly onto the surface of the reduction electrode 3. The flow rate of the carbon dioxide was set to, for example, 5 ml / min and the pressure was set to 0.5 MPa.

[0046] Light 13 was a 300W xenon lamp instead of sunlight. Wavelengths above 450nm were cut off with a filter, and the illuminance was set to 6.6mW / cm. 2 The light receiving surface of the oxidation electrode 2 for the light 13 was set to 2.5 cm 2 It was decided.

[0047] For the purpose of analyzing the reaction products of the reduction reaction, nitrogen was bubbled into the oxidation vessel 6. Carbon dioxide was continuously supplied to the reduction section 7 under the above conditions.

[0048] The current flowing between the oxidation electrode 2 and the reduction electrode 3 due to irradiation with light 13 was measured with a potentiogalvanostat (Model 1287 manufactured by Solartron).

[0049] The gas and liquid produced in the oxidation vessel 6 and the reduction section 7 were sampled and analyzed using a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer.

[0050] The faradaic efficiency of the carbon dioxide reduction reaction was calculated from the results of the experiment conducted under the above conditions. The faradaic 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.

[0051]

number

[0052] Here, the "number of electrons in the reduction reaction" in equation (1) is calculated by converting the measured cumulative amount of carbon dioxide reduction product into the number of electrons required for the production reaction. 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), and the gas model is V. g (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.

[0053]

number

[0054] When the reduction product is liquid, the number of electrons can be calculated using the following formula:

[0055]

number

[0056] where C is the concentration of the reduction reaction product (mol / L), V l is the volume of the liquid sample (L), Z is the number of electrons required for the reduction reaction, and F is the Faraday constant (C / mol).

[0057] (Experiment 1) FIG. 3 is a diagram schematically showing the relationship between the reduction electrode 3 and the blower 20 in Experiment 1. As shown in FIG.

[0058] In Experiment 1, the blower 20 was positioned so that the carbon dioxide jet from the blower 20 hit the reduction electrode 3 perpendicularly. As shown in Figure 3, the distance between the tip of the blower 20 and the reduction electrode 3 was set to 2 cm.

[0059] The carbon dioxide supply pressure was set to 1.0 MPa, and the carbon dioxide was injected for 5 seconds at 1-minute intervals. This injection of carbon dioxide can remove the liquid (droplets) generated on the surface of the reduction electrode 3 by the reduction reaction.

[0060] Figure 4 shows the results of Experiment 1. The horizontal axis of Figure 4 is the test time (reduction time), and the vertical axis is the Faraday efficiency of formic acid (%). □ is a plot when the blower 20 is in operation, and × is a plot when the blower 20 is not in operation (comparative example).

[0061] 4, the Faraday efficiency, which was about 21% after a 6-hour test, decreased to about 18% after a 24-hour test without the blower 20. On the other hand, according to this embodiment, the Faraday efficiency after a 24-hour test was about 20%, which shows that the decrease in Faraday efficiency was improved (-3% → -1%).

[0062] The positional relationship between the blower 20 and the reduction electrode 3 is not limited to the example shown in Fig. 3. For example, the blower 20 may be arranged as shown in Figs.

[0063] When the reduction electrode 3 is arranged upright in the Z direction, it is preferable to place the blower 20 above the upper end of the reduction electrode 3, as shown in Figures 5 and 6. The liquid (reduction product) produced on the surface of the reduction electrode 3 falls downward due to gravity. Therefore, by injecting carbon dioxide from above, the movement of the liquid is promoted and it is possible to prevent the liquid from reattaching to the surface of the reduction electrode 3.

[0064] (Experiment 2) In Experiment 2, the faradaic efficiency of the carbon dioxide reduction reaction was determined using the blower 10 having the configuration shown in FIG.

[0065] A propeller fan (LittleFAN40U, manufactured by Timely Corporation) was used as the blower 10. The propeller fan was rotated at 5000 rpm. Therefore, the flow of carbon dioxide generated by the blower 10 was always in contact with the surface of the reduction electrode 3.

[0066] Figure 7 shows the results of Experiment 2. The relationship between the horizontal and vertical axes in Figure 7 is the same as in Figure 4.

[0067] As shown in FIG. 7, it can be seen that in the case where the blower 10 is provided (□), the decrease in the Faraday efficiency can be improved.

[0068] (Experiment 3) Experiment 3 used the same blower 20 as Experiment 1. The carbon dioxide supply pressure was set to 0.5 MPa, and the mass flow controller was controlled to repeat a set of a flow rate of 5 ml / min for 55 seconds and a flow rate of 500 ml / min for 5 seconds.

[0069] Figure 8 shows the results of Experiment 3. The relationship between the horizontal and vertical axes in Figure 8 is the same as in Figures 4 and 7.

[0070] As shown in FIG. 8, it can be seen that when the blower 20 is provided (□), the decrease in the Faraday efficiency can be improved.

[0071] As explained above, it is clear that the provision of the fans 10 and 20 can improve the decrease in reaction efficiency.

[0072] The present invention is not limited to the above-described embodiment, and modifications are possible within the scope of the present invention. For example, although the light 13 is generated by a xenon lamp in the embodiment, sunlight may also be used.

[0073] The electrolyte membrane 4 and the reduction electrode 3 may be integrally configured. The electrolyte membrane 4 and the reduction electrode 3 may be replaced with a gas diffusion electrode (GDE (registered trademark)) composed of a porous member and a catalyst. This allows for a reduction in the number of parts. The electrolyte membrane 4 and the reduction electrode 3 may be integrated by press-fitting the electrolyte membrane 4 into porous copper.

[0074] Furthermore, although an example in which fans 10 and 20 are arranged in front of supply port 9 has been described, fans 10 and 20 may also be arranged in front of supply port 8.

[0075] As such, the present invention naturally includes various embodiments not described herein. Therefore, the technical scope of the present invention is defined only by the invention-specifying matters according to the scope of the claims that are appropriate from the above description. [Industrial Applicability]

[0076] The present invention can be widely used in fields related to the recycling of carbon dioxide. [Explanation of symbols]

[0077] 1: Circuit board 2: Oxidation electrode 3: Reduction electrode 4: Electrolyte membrane 5: Electrolyte 6: Oxidation tank 7: Reduction section 8,9: Supply port 10,20: Blower 11: Gas recovery port 12: Liquid collection port 13:Light

Claims

1. an oxidation electrode that receives light from outside; an oxidation tank for holding an electrolyte in which the oxidation electrode is immersed; an electrolyte membrane that forms a part of one surface of the oxidation vessel excluding the surface onto which the light is incident; a reduction electrode connected to the outer surface of the electrolyte membrane; a reduction section in which the reduction electrode is disposed and to which a gas containing carbon dioxide is supplied from the outside; a blower that generates an airflow toward the reduction electrode inside the reduction unit and removes liquid from the surface of the reduction electrode; A carbon dioxide reduction device comprising:

2. The reduction electrode is The reduction electrode is plate-shaped, and one surface of the reduction electrode is in contact with the electrolyte membrane. The carbon dioxide reduction device according to claim 1 .

3. The blower is It consists of a propeller fan located inside the front of the supply port through which pressurized gas is supplied. The carbon dioxide reduction device according to claim 1 or 2.

4. An oxidation electrode that receives light from an external source; an oxidation tank for holding an electrolyte in which the oxidation electrode is immersed; an electrolyte membrane that forms a part of one surface of the oxidation vessel excluding the surface onto which the light is incident; a reduction electrode connected to the outer surface of the electrolyte membrane; a reduction section in which the reduction electrode is disposed and to which a gas containing carbon dioxide is supplied from the outside; a mass flow controller that is provided at the tip of the inner side of a supply port to which a pressurized gas is supplied, and that sprays the gas toward the reduction electrode and removes liquid from the surface of the reduction electrode; A carbon dioxide reduction device comprising:

5. The blower is Intermittent operation The carbon dioxide reduction device according to claim 3 .

6. The blower is Changing the flow rate of the airflow The carbon dioxide reduction device according to claim 3 or 5.

7. The electrolyte membrane and the reduction electrode are integrally formed. The carbon dioxide reduction device according to any one of claims 1 to 6.

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