JP7783507B2 - 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 has become a long-term challenge on a global scale. Meanwhile, in the medium to long term, energy issues necessitate a reassessment of energy supplies that rely on fossil fuels, and the creation of next-generation energy sources is required.

Technology is being developed 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, as a means of obtaining energy while reducing carbon dioxide emissions. However, these power generation technologies are only capable of generating electrical energy and are unable to store it. Furthermore, they are unable to create chemical products from fossil fuels.

As a method for simultaneously solving these issues, technology that uses light energy to reduce carbon dioxide is attracting attention. Non-Patent Document 1 discloses a device for reducing carbon dioxide through 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 produced through a water oxidation reaction. At the reduction electrode, protons and electrons combine to produce hydrogen, triggering a reduction reaction. This reduction reaction produces carbon monoxide, formic acid, methane, and other energy resources.

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 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

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 the carbon dioxide to the reduction electrode, thereby carrying out a reduction reaction. However, this carbon dioxide reduction reaction has limitations on 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.

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 supplying carbon dioxide directly to the reduction electrode (Non-Patent Document 2). Non-Patent Document 2 reports that by using a reactor structured to supply gaseous carbon dioxide to the reduction electrode, the amount of carbon dioxide supplied to the reduction electrode is increased, accelerating the carbon dioxide reduction reaction.

However, as the reduction reaction progresses, reduction products of carbon dioxide are produced on the reaction surface of the reduction electrode, producing not only gaseous hydrogen, carbon monoxide, and methane, but also liquids such as formic acid, methanol, and ethanol. Furthermore, the solution on the liquid phase gradually seeps into the gas phase through the electrolyte membrane. This creates the problem that the reaction surface of the reduction electrode on the gas phase side becomes covered with these liquids, preventing the reaction from progressing.

The present invention was made in consideration of this problem and aims to provide a carbon dioxide reduction device that can improve the decline in reaction efficiency of the carbon dioxide reduction reaction.

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.

The present invention provides a carbon dioxide reduction device that can improve the decrease in reaction efficiency of the carbon dioxide reduction reaction.

1 is a schematic diagram showing a configuration example of a carbon dioxide reduction device according to an embodiment of the present invention. FIG. FIG. 2 is a schematic diagram showing a modified example of the carbon dioxide reduction device shown in FIG. 1. FIG. 3 is a schematic diagram showing an example of the relationship between the reduction electrode and the blower shown in FIG. 2 . FIG. 10 is a diagram showing the experimental results of Experiment 1. FIG. 4 is a schematic diagram showing another example of the relationship shown in FIG. 3 . FIG. 4 is a schematic diagram showing another example of the relationship shown in FIG. 3 . FIG. 10 is a diagram showing the experimental results of Experiment 2. FIG. 10 is a diagram showing the experimental results of Experiment 3.

Embodiments of the present invention will be described below with reference to the drawings. The same reference symbols will be used to refer to the same parts in multiple drawings, and descriptions will not be repeated.

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

The carbon dioxide reduction device 100 shown in Figure 1 comprises an oxidation electrode 2, an oxidation tank 6, an electrolyte membrane 4, a reduction electrode 3, a reduction section 7, and a blower 10. The carbon dioxide reduction device 100 generates both gaseous and liquid reduction products through an oxidation-reduction reaction.

The carbon dioxide to be reduced using light energy is supplied to the interior of 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 its side. Supply port 8 is connected to, for example, a cylinder filled with carbon dioxide, and constantly supplies carbon dioxide decompressed to a predetermined pressure. Supply port 9 supplies the same carbon dioxide as that supplied from supply port 8 from the side of the reduction unit 7. It is sufficient to provide either supply port 8 or 9.

Furthermore, if 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, etc.

The blower 10 is positioned inside the reduction section 7, in front of the supply port 9. The blower 10 generates an airflow inside the reduction section 7, directed toward the reduction electrode 3.

A gas recovery port 11 is provided on the upper surface of the reduction unit 7 to recover the gaseous reduction product. Furthermore, a liquid recovery port 12 is provided on the lower surface of the reduction unit 7 to recover the liquid reduction product.

Oxidation electrode 2 is formed on substrate 1 and receives external light 13. Substrate 1 is, for example, sapphire, and has a predetermined area in a plane in the XY direction. Oxidation electrode 2 is formed on substrate 1 by forming a film of a compound containing at least one selected from the group consisting of nitride semiconductors, titanium oxide, amorphous silicon, ruthenium complexes, and rhenium complexes on the plane. These compounds exhibit photoactivity and redox activity.

Note that substrate 1 does not have to be made of a light-transmitting material such as sapphire. Substrate 1 may also be made of an opaque material such as glass epoxy resin.

Light 13 is, for example, sunlight. However, light 13 does not have to be sunlight. For example, it 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.

The oxidation tank 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. Figure 1 shows an example in which light 13 is irradiated from the bottom of the oxidation tank 6 in the Z direction.

The electrolyte membrane 4 constitutes part of one surface of the oxidation basin 6 excluding the surface in the direction in which light 13 is incident. Figure 1 shows an example in which it 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 a top surface (if the oxidation basin 6 is covered), the electrolyte membrane 4 may be provided on that top surface. If the electrolyte membrane 4 is provided on the top surface of the oxidation basin 6, the reduction unit 7 is located on top of the electrolyte membrane 4.

The electrolyte membrane 4 is, for example, an electrolyte membrane such as Nafion (registered trademark), ForeBlue, or Aquivion, which have a carbon-fluorine skeleton, or Selemion or Neocepta, which have a hydrocarbon-based skeleton.

The reduction electrode 3 is connected to the electrolyte membrane 4. The reduction electrode 3 is plate-shaped, and Figure 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.

The reduction electrode 3 can be made of, for example, copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or a porous alloy of any of these. The reduction electrode 3 may also be made of compounds 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 arranged to form a flat surface in the X direction, similar to the electrolyte membrane 4 described below.

The surface of the reduction electrode 3 is covered with carbon dioxide supplied from the supply ports 8 and 9. This causes an oxidation-reduction reaction on the surface of the reduction electrode 3, producing gases such as hydrogen, carbon monoxide, and methane, as well as liquid reduction products such as formic acid, methanol, and ethanol.

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 the gas recovery port 11 located at the top of the reduction unit 7. Liquid reduction products, on the other hand, are discharged to the outside through the liquid recovery port 12 located at the top of the reduction unit 7. The absence of the gas recovery port 11 and liquid recovery port 12 does not affect the carbon dioxide reduction reaction. Therefore, the gas recovery port 11 and liquid recovery port 12 are not essential components in this embodiment.

The blower 10 generates an airflow inside the reduction unit 7 directed toward the reduction electrode 3, removing the liquid on the surface of the reduction electrode 3. This ensures that the surface of the reduction electrode 3 is always covered with fresh carbon dioxide, thereby improving the reduction reaction efficiency.

The blower 10 may generate an airflow constantly or intermittently. When generating an airflow 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 reduced power consumption compared to when the blower 10 is operated constantly.

The blower 10 may also vary the airflow rate, which can effectively promote the reduction reaction and remove the reduction products.

As described above, the carbon dioxide reduction device 100 according to this embodiment comprises an oxidation electrode 2 that receives light 13 from outside, an oxidation tank 6 that holds an electrolyte solution 5 in which the oxidation electrode 2 is immersed, an electrolyte membrane 4 that forms part of one surface of the oxidation tank 6 excluding the surface on which light 13 is incident, a reduction electrode 3 connected to the electrolyte membrane 4, a reduction section 7 in which the reduction electrode 3 is disposed and to which gas containing carbon dioxide is supplied from outside, and a 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 reaction efficiency of the reduction reaction.

The reduction electrode 3 is plate-shaped, and one side 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.

In addition, by positioning the reduction electrode 3 as shown in Figure 1, the liquid (reduction product) generated on the surface of the reduction electrode 3 moves downward due to gravity. This can improve the reduction reaction efficiency.

(Modification)
Fig. 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.

The blower 20 is a mass flow controller installed at the tip of the inner side of the 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.

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 0 V, the flow rate is 0, at a predetermined voltage value, carbon dioxide is sprayed at a predetermined flow rate, and at the maximum voltage value of the control signal, 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.

The flow of carbon dioxide is directed toward the reduction electrode 3, eliminating the reduction product (liquid) on its surface. This improves the reduction reaction efficiency. The gas injected does not have to be carbon dioxide. It can be air, nitrogen, argon, helium, or other gases.

(experiment)
Electrochemical measurements were carried out using the modified configuration (FIG. 2). The experimental conditions will now 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 the substrate (sapphire substrate) 1, 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 the electrolyte 5.

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

The reduction electrode 3 was made of porous copper.

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

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

A 300 W xenon lamp was used as the light 13 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 light-receiving surface of the oxidation electrode 2 for the light 13 was set to 2.5 ^cm² .

To analyze the reaction products of the reduction reaction, nitrogen was bubbled into the oxidation tank 6. Carbon dioxide was continuously supplied to the reduction section 7 under the above conditions.

The current flowing between the oxidation electrode 2 and the reduction electrode 3 upon irradiation with light 13 was measured using a potentiogalvanostat (Solartron Model 1287).

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

The Faraday efficiency of the carbon dioxide reduction reaction was calculated from the results of the experiment conducted under the above experimental conditions. 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 due to 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 carbon dioxide reduction product produced into the number of electrons required for that production reaction. When 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 gas model is _Vg (L/mol), and the light irradiation or voltage application time is T (sec), the "number of electrons in the reduction reaction" can be calculated using the following formula.

When the reduction product is liquid, the number of electrons can be calculated using the following formula.

where C is the concentration of the reduction reaction product (mol/L), _V 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).

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

In Experiment 1, the blower 20 was positioned so that the carbon dioxide spray 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.

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 removes the liquid (droplets) generated on the surface of the reduction electrode 3 during the reduction reaction.

Figure 4 shows the experimental 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 operating, and × is a plot when the blower 20 is not operating as in the comparative example.

As shown in Figure 4, the Faraday efficiency, which was approximately 21% after a 6-hour test, decreased to approximately 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 approximately 20%, demonstrating that the decrease in Faraday efficiency was improved (-3% → -1%).

Note that the positional relationship between the blower 20 and the reduction electrode 3 is not limited to the example shown in Figure 3. For example, the blower 20 may be positioned as shown in Figures 5 and 6.

When the reduction electrode 3 is positioned upright in the Z direction, it is recommended to position 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, spraying carbon dioxide from above promotes the movement of the liquid and prevents it from re-adhering to the surface of the reduction electrode 3.

(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.

A propeller fan (Timely, LittleFAN40U) 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 always hits the surface of the reduction electrode 3.

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.

As shown in Figure 7, when blower 10 is present (□), it can be seen that the decrease in Faraday efficiency is improved.

(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.

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

As shown in Figure 8, when blower 20 is present (□), it can be seen that the decrease in Faraday efficiency is improved.

As explained above, it can be seen that the reduction in reaction efficiency can be improved by providing blowers 10 and 20.

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

The electrolyte membrane 4 and reduction electrode 3 may also be constructed as a single unit. The electrolyte membrane 4 and reduction electrode 3 may also be replaced with a gas diffusion electrode (GDE (registered trademark)) consisting of a porous material and a catalyst. This reduces the number of parts. The electrolyte membrane 4 and reduction electrode 3 may also be integrated by pressing the electrolyte membrane 4 into porous copper.

Furthermore, although an example has been shown in which blowers 10 and 20 are placed in front of supply port 9, blowers 10 and 20 may also be placed in front of supply port 8.

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-specific matters relating to the scope of the claims that are appropriate from the above description.

The present invention can be widely used in fields related to carbon dioxide recycling.

1: Substrate 2: Oxidation electrode 3: Reduction electrode 4: Electrolyte membrane 5: Electrolyte solution 6: Oxidation tank 7: Reduction section 8, 9: Supply ports 10, 20: Fan 11: Gas recovery port 12: Liquid recovery port 13: Light

Claims (7)

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 . The reduction electrode is
The carbon dioxide reduction device according to claim 1 , wherein the reduction electrode is plate-shaped, and one surface of the reduction electrode is in contact with the electrolyte membrane. The blower is
The carbon dioxide reduction device according to claim 1 or 2, comprising a propeller fan provided in front of and inside a supply port through which pressurized gas is supplied. 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 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: The blower is
The carbon dioxide reduction device according to claim 3 , which operates intermittently. The blower is
The carbon dioxide reduction device according to claim 3 or 5 , wherein the flow rate of the airflow is changed. The carbon dioxide reduction device according to claim 1 , wherein the electrolyte membrane and the reduction electrode are integrally formed.