Carbon Dioxide Reduction Device

TECHNICAL FIELD

The present invention relates to a carbon dioxide reduction device.

BACKGROUND ART

An increase in the concentration of carbon dioxide in the atmosphere is mentioned as a main cause of global warming. Reduction of carbon dioxide emissions has become a long-term challenge on a global scale. On the other hand, as an energy issue, there is a need to review energy supply relying on fossil fuels in the medium to long term, and creation of a next-generation energy supply source is required.

As a way of suppressing emission of carbon dioxide and obtaining energy, techniques have been developed for utilizing unused energy such as exhaust heat, snow and ice heat, vibration, and electromagnetic waves and renewable energy such as sunlight. These power generation techniques enable only generation of electrical energy, and storage of energy is impossible with these techniques. Production of chemical products using fossil fuels as raw materials is also impossible.

As a method of simultaneously solving these problems, a technique of reducing carbon dioxide using light energy has attracted attention. Non Patent Literature 1 discloses a carbon dioxide reduction device by light irradiation. In the reduction device, in a case where the oxidation electrode is irradiated with light, electron-hole pairs are generated and separated at the oxidation electrode, and oxygen and protons (H+) are generated by an oxidation reaction of water. At the reduction electrode, a proton and an electron are bounded to each other, and thus hydrogen is generated. This leads to a reduction reaction. By the reduction reaction, carbon monoxide, formic acid, methane, and the like that can be used as energy resources are generated.

CITATION LIST
Non Patent Literature

Non Patent Literature 1: Satoshi Yotsuhashi et al., “CO₂Conversion with Light and Water by GaN Photoelectrode”, Japanese Journal of Applied Physics, 51, 2012, p. 02BP07-1-p. 02BP07-3

Non Patent Literature 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—Se Photoanode”, Chem. Lett., 47, 2018, p. 436-p. 439

SUMMARY OF INVENTION
Technical Problem

In the carbon dioxide reduction device disclosed in Non Patent Literature 1, a reduction electrode is immersed in a solution (electrolytic solution), and carbon dioxide is dissolved in the solution. Thereby, the carbon dioxide is supplied to the reduction electrode, and a reduction reaction is performed. However, in the reduction reaction of carbon dioxide, there is a limit to a concentration of carbon dioxide dissolved in the solution and a diffusion coefficient of carbon dioxide in the solution. As a result, there is a problem that an amount of carbon dioxide supplied to the reduction electrode is limited.

On the other hand, in order to increase the amount of carbon dioxide supplied to the reduction electrode, studies have been conducted to eliminate the solution on the reduction electrode side and directly supply carbon dioxide to the reduction electrode (Non Patent Literature 2). In Non Patent Literature 2, it is reported that the amount of carbon dioxide supplied to the reduction electrode is increased and a reduction reaction of carbon dioxide is promoted by using a reaction device having a structure capable of supplying carbon dioxide in a gas phase to the reduction electrode.

However, in a case where the reduction reaction proceeds, a reduction product of carbon dioxide is generated on a reaction surface of the reduction electrode. In this case, not only gases such as hydrogen, carbon monoxide, and methane but also liquids such as formic acid, methanol, and ethanol are generated. In addition, the solution on a liquid phase side gradually exudes to a gas phase side through an electrolyte membrane. For this reason, there is a problem that the reaction surface of the reduction electrode on a gas phase side is covered with these liquids and the reaction does not proceed.

The present invention has been made in view of the above problems, and an object of the present invention is to provide a carbon dioxide reduction device capable of improving a decrease in reaction efficiency of a carbon dioxide reduction reaction.

Solution to Problem

According to an aspect of the present invention, there is provided a carbon dioxide reduction device including: an oxidation electrode that receives light from the outside; an oxidation bath that holds an electrolytic solution in which the oxidation electrode is immersed; an electrolyte membrane that constitutes a part of one surface of the oxidation bath excluding a surface on which the light is incident; a reduction electrode that is connected to an outer surface of the electrolyte membrane; a reduction unit in which the reduction electrode is disposed and to which a gas containing carbon dioxide is supplied from the outside; and a blower that generates an airflow toward the reduction electrode inside the reduction unit.

Advantageous Effects of Invention

According to the present invention, it is possible to provide a carbon dioxide reduction device capable of improving a decrease in reaction efficiency of a carbon dioxide reduction reaction.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic diagram illustrating a configuration example of a carbon dioxide reduction device according to an embodiment of the present invention.

FIG. 2 is a schematic diagram illustrating a modification example of the carbon dioxide reduction device illustrated in FIG. 1.

FIG. 3 is a schematic diagram illustrating a relationship example between a reduction electrode and a blower illustrated in FIG. 2.

FIG. 4 is a diagram illustrating an experimental result of Experiment 1.

FIG. 5 is a schematic diagram illustrating another example of the relationship example illustrated in FIG. 3.

FIG. 6 is a schematic diagram illustrating another example of the relationship example illustrated in FIG. 3.

FIG. 7 is a diagram illustrating an experimental result of Experiment 2.

FIG. 8 is a diagram illustrating an experimental result of Experiment 3.

DESCRIPTION OF EMBODIMENTS

Hereinafter, an embodiment of the present invention will be described with reference to the drawings. The same components in the plurality of drawings will be denoted by the same reference numerals, and description thereof will be omitted.

FIG. 1 is a schematic diagram illustrating a configuration example of a carbon dioxide reduction device according to an embodiment of the present invention. In FIG. 1, a right-left direction is defined as an X direction, a direction toward the back side of the drawing is defined as a Y direction, and a direction toward the upper side of the drawing is defined as a Z direction.

A carbon dioxide reduction device 100 illustrated in FIG. 1 includes an oxidation electrode 2, an oxidation bath 6, an electrolyte membrane 4, a reduction electrode 3, a reduction unit 7, and a blower 10. The carbon dioxide reduction device 100 generates reduction products of both gases and liquids by an oxidation-reduction reaction.

Carbon dioxide that is reduced using light energy is supplied to an inside of the reduction unit 7 from a supply port 8 provided on an upper surface of the reduction unit 7 and a supply port 9 provided on a side surface of the reduction unit 7. The supply port 8 is connected to, for example, a cylinder filled with carbon dioxide, and constantly supplies carbon dioxide that is decompressed to a predetermined pressure. The supply port 9 supplies, from the side surface of the reduction unit 7, carbon dioxide that is the same as the carbon dioxide supplied from the supply port 8. Note that either one of the supply ports 8 and 9 may be provided.

In addition, in a case where the supply ports 8 and 9 are provided, gas containing carbon dioxide may be supplied from the supply port 8, and, for example, air may be supplied from the supply port 9.

The gas supplied from the supply port 9 may be nitrogen, argon, helium, or the like.

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

A gas recovery port 11 for recovering reduction products of gases is provided on the upper surface of the reduction unit 7. In addition, a liquid recovery port 12 for recovering reduction products of liquids is provided on a lower surface of the reduction unit 7.

The oxidation electrode 2 is formed in a film state on a substrate 1 and receives light 13 from the outside. The substrate 1 is, for example, a sapphire having a predetermined area on a plane in the XY direction. On the substrate 1, for example, a compound including at least one selected from a group consisting of a nitride semiconductor, titanium oxide, amorphous silicon, a ruthenium complex, and a rhenium complex, is formed in a film state on a plane, and thus the oxidation electrode 2 is formed. The compound exhibits photoactivity and redox activity.

Note that the substrate 1 may not be a substrate using a material such as sapphire that transmits light. The substrate 1 may be made of, for example, a glass epoxy resin or the like that does not allow light to pass therethrough.

The light 13 is, for example, sunlight. Note that the light 13 is not required to be sunlight. For example, the light source may be a xenon lamp, a pseudo sunlight source, a halogen lamp, a mercury lamp, or a combination of these light sources.

The oxidation bath 6 holds an electrolytic solution 5 in which the oxidation electrode 2 is immersed. The electrolytic solution 5 includes, for example, at least one selected from a group consisting of a potassium hydrogen carbonate aqueous solution, a sodium hydrogen carbonate aqueous solution, a potassium chloride aqueous solution, a sodium chloride aqueous solution, a potassium hydroxide aqueous solution, a rubidium hydroxide aqueous solution, and a cesium hydroxide aqueous solution. FIG. 1 illustrates an example in which a bottom of the oxidation bath 6 is irradiated with the light 13 in the Z direction.

The electrolyte membrane 4 constitutes a part of one surface of the oxidation bath 6 excluding a surface in a direction in which the light 13 is incident. FIG. 1 illustrates an example in which the electrolyte membrane 4 is provided on the surface of the oxidation bath 6 parallel to the irradiation direction of the light 13. The electrolyte membrane 4 may be provided on any one of the four surfaces (side surfaces) of the oxidation bath 6 excluding the surface on which the light 13 is incident. In addition, in a case where the oxidation bath 6 has an upper surface (in a case where the oxidation bath 6 is covered with a lid), the electrolyte membrane 4 may be provided on the upper surface of the oxidation bath 6. In a case where the electrolyte membrane 4 is provided on the upper surface of the oxidation bath 6, the reduction unit 7 is disposed above the electrolyte membrane 4.

The electrolyte membrane 4 is, for example, an electrolyte membrane such as any one of Nafion (registered trademark), Forblue, and Aquivion having a carbon-fluorine skeleton, or Selemion, Neosepta, or the like having a carbon-hydrogen skeleton.

The reduction electrode 3 is connected to the electrolyte membrane 4. The reduction electrode 3 has a plate shape, and FIG. 1 illustrates an example in which one surface of the reduction electrode 3 is in contact with a surface (YZ plane) of the electrolyte membrane 4 on the outer side (the reduction unit 7 side). The reduction electrode 3 is electrically connected to the oxidation electrode 2 via a lead wire of which the reference numeral is omitted.

As the reduction electrode 3, for example, a porous body of any one of copper, platinum, gold, silver, indium, palladium, gallium, nickel, tin, cadmium, or an alloy thereof can be used. In addition, alternatively, the reduction electrode 3 may 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 a metal ion and an anionic ligand. Note that the reduction electrode 3 may be disposed so as to form a plane in the X direction similarly to the electrolyte membrane 4 to be described below.

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

Since the reduction products such as hydrogen, carbon monoxide, and methane have a smaller molecular weight than carbon dioxide, the reduction products are lighter. Thus, the reduction products are discharged to the outside from the gas recovery port 11 provided at the upper portion of the reduction unit 7. On the other hand, the reduction products of liquids are discharged to the outside from the liquid recovery port 12 provided at the lower portion of the reduction unit 7. Note that the gas recovery port 11 and the liquid recovery port 12 do not affect the carbon dioxide reduction reaction even in a case where the gas recovery port 11 and the liquid recovery port 12 are not provided. Therefore, the gas recovery port 11 and the liquid recovery port 12 are not essential components in the present embodiment.

The blower 10 generates an airflow toward the reduction electrode 3 inside the reduction unit 7 to remove the liquid on the surface of the reduction electrode 3. By the airflow, the surface of the reduction electrode 3 is always covered with fresh carbon dioxide. Thus, a decrease in reaction efficiency of the reduction reaction can be improved.

The blower 10 may constantly or intermittently generate the airflow. In a case where the airflow is intermittently generated, the gas supplied from the supply port 9 may also be intermittently supplied in accordance with the operation of the blower 10. That is, the blower 10 may be intermittently operated. Thus, power consumption can be reduced as compared with a case where the blower 10 is operated at all times.

In addition, the blower 10 may change a flow rate of the airflow. Thus, promotion of the reduction reaction and removal of the reduction products can be effectively performed.

As described above, the carbon dioxide reduction device 100 according to the present embodiment includes the oxidation electrode 2 that receives light 13 from the outside, the oxidation bath 6 that holds the electrolytic solution 5 in which the oxidation electrode 2 is immersed, the electrolyte membrane 4 that constitutes a part of one surface of the oxidation bath 6 excluding a surface on which the light 13 is incident, the reduction electrode 3 connected to the electrolyte membrane 4, the reduction unit 7 in which the reduction electrode 3 is disposed and to which a gas containing carbon dioxide is supplied from the outside, and the blower 10 that generates an airflow toward the reduction electrode 3 inside the reduction unit 7. Thereby, it is possible to provide a carbon dioxide reduction device capable of improving a decrease in reaction efficiency of the reduction reaction.

In addition, the reduction electrode 3 has a plate shape, and one surface of the reduction electrode 3 is in contact with the electrolyte membrane 4. Thereby, current flowing between the oxidation electrode 2 and the reduction electrode 3 can be increased, and thus reaction efficiency of the reduction reaction can be improved.

Further, by disposing the reduction electrode 3 as illustrated in FIG. 1, the liquid (reduction product) generated on the surface of the reduction electrode 3 moves downward by gravity. Therefore, a decrease in reaction efficiency of the reduction reaction can be improved.

Modification Example

FIG. 2 is a schematic diagram illustrating a modification example of the carbon dioxide reduction device 100. The modification example illustrated in FIG. 2 is different from the carbon dioxide reduction device 100 (FIG. 1) in that a blower 20 is provided.

The blower 20 is a mass flow controller provided at a tip portion of an inside of the supply port 9, and pressurized carbon dioxide is supplied to the supply port 9. The mass flow controller measures a mass flow rate of a fluid and controls the flow rate, and may be referred to as a flow rate changing device.

The flow rate of the carbon dioxide by the blower 20 is controlled by a control signal (not illustrated). The control signal is given by, for example, an amplitude of a voltage. For example, carbon dioxide is injected at a flow rate of 0 in a case where a voltage of the control signal is 0 V, at a predetermined flow rate in a case where a voltage of the control signal is a predetermined voltage value, and at a pressure of a pressurized cylinder in a case where a voltage of the control signal is a maximum voltage value. Therefore, a flow of the carbon dioxide at a predetermined flow rate can be generated by the control signal. In addition, in a case where a pulsed control signal is given, it is also possible to intermittently inject high-pressure carbon dioxide.

Since the flow of the carbon dioxide is directed to the reduction electrode 3, reduction products (liquid) on the surface of the reduction electrode 3 can be eliminated. Therefore, a decrease in reaction efficiency of the reduction reaction can be improved. Note that the gas to be injected may not be carbon dioxide. The gas may be a gas such as air, nitrogen, argon, or helium.

(Experiments)

An electrochemical measurement was performed with the configuration of the modification example (FIG. 2). Experimental conditions of the measurement will be described.

An oxidation electrode 2 is configured by performing epitaxial growth of GaN as an n-type semiconductor and epitaxial growth of AlGaN in this order on a substrate (sapphire substrate) 1, vacuum-depositing Ni on the AlGaN, and heat-treating a resulting product to form a promotor thin film of NiO. The oxidation electrode 2 was immersed in the electrolytic solution 5.

As the electrolytic solution 5, a 1.0 mol/L sodium hydroxide aqueous solution was used.

As the reduction electrode 3, a porous copper body was used.

As the electrolyte membrane 4, Nafion (registered trademark) was used.

As the blower 20, MODEL EX-250S SERIES manufactured by Koflock Corporation was used. The blower 20 was connected to a carbon dioxide cylinder via the supply port 9, and was disposed such that the injection direction of the carbon dioxide perpendicularly hits the surface of the reduction electrode 3. The flow rate of the carbon dioxide was set to, for example, 5 ml/min at a pressure of 0.5 MPa.

As the light 13, light from a xenon lamp with 300 W was used instead of sunlight. A wavelength equal to or longer than 450 nm was cut with a filter, and illuminance was set to 6.6 mW/cm². In addition, an area of the light receiving surface of the oxidation electrode 2 for the light 13 was set to 2.5 cm².

For the purpose of analyzing the reaction product of the reduction reaction, nitrogen bubbling was performed on the oxidation bath 6. Carbon dioxide was continuously supplied to the reduction unit 7 under the above conditions.

The current flowing between the oxidation electrode 2 and the reduction electrode 3 by irradiation with the light 13 was measured with a potentiogalvanostat (model 1287 manufactured by Solartron Corporation).

Gases and liquids generated in the oxidation bath 6 and the reduction unit 7 are recovered, and the reaction products were analyzed using a gas chromatograph, a liquid chromatograph, and a gas chromatograph mass spectrometer.

Faraday efficiency of the carbon dioxide reduction reaction was calculated from experimental results obtained by performing an experiment under the above experimental conditions. The Faraday efficiency of carbon dioxide indicates a ratio of the number of electrons used in the carbon dioxide reduction reaction to the number of electrons moved between the oxidation electrode 2 and the reduction electrode 3 by light irradiation or voltage application.

[Math. 1]

$\begin{matrix} Faraday efficiency = \frac{N umber of electrons in reduction reaction}{N umber of electrons moved between electrodes} & (1) \end{matrix}$

Here, the “number of electrons in reduction reaction” in the expression (1) is determined by converting the measured value of the integrated amount of the generated carbon dioxide reduction product into the number of electrons required for the production reaction. In a case where the concentration of the reduction reaction product is represented by A (ppm), the flow rate of the carrier gas is represented by B (L/see), the number of electrons required for the reduction reaction is represented by Z (mol), the Faraday constant is represented by F (C/mol), the volume of the model body of the gas is represented by V_g(L/mol), and the light irradiation or voltage application time is represented by T (sec), the “number of electrons in reduction reaction” can be calculated with following expression.

[Math. 2]

$\begin{matrix} Number (C) of electrons in reduction reaction = \frac{(A \times B \times Z \times F \times T \times 10^{- 6})}{V_{g}} & (2) \end{matrix}$

The number of electrons in a case where the reduction product is a liquid can be calculated by the following expression.

[Math. 3]

$\begin{matrix} Number (C) of electrons in reduction reaction = C \times V_{l} \times Z \times F & (3) \end{matrix}$

Here, C is a concentration (mol/L) of the reduction reaction product, V₁is a volume (L) of the liquid sample, Z is the number of electrons required for the reduction reaction, and F is a Faraday constant (C/mol).

(Experiment 1)

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

In Experiment 1, the blower 20 was disposed such that the injection of the carbon dioxide from the blower 20 perpendicularly hits the reduction electrode 3. As illustrated in FIG. 3, a distance between the tip of the blower 20 and the reduction electrode 3 was set to 2 cm.

In addition, the supply pressure of the carbon dioxide was set to 1.0 MPa, and carbon dioxide was injected for 5 seconds with a period of 1 minute. By the injection of carbon dioxide, liquid (droplets) generated on the surface of the reduction electrode 3 by the reduction reaction can be removed.

FIG. 4 illustrates an experimental result of Experiment 1. In FIG. 4, a horizontal axis represents a test time (reduction time), and a vertical axis represents Faraday efficiency (%) of formic acid. □ is a plot in a case where the blower 20 is operated, and x is a plot in a case where the blower 20 is not provided according to a comparative example.

As illustrated in FIG. 4, the Faraday efficiency was approximately 21% at a test time of 6 hours, and the Faraday efficiency drops to approximately 18% at a test time of 24 hours in a case where the blower 20 is not provided. On the other hand, according to the present embodiment, the Faraday efficiency at a test time of 24 hours is approximately 20%. Therefore, it can be seen that a decrease in the Faraday efficiency can be improved (−3%→-1%).

Note that the positional relationship between the blower 20 and the reduction electrode 3 is not limited to the example illustrated in FIG. 3. For example, as illustrated in FIG. 5 and FIG. 6, the blower 20 may be disposed.

In a case where the reduction electrode 3 is disposed upright in the Z direction, the blower 20 may be disposed above the upper end of the reduction electrode 3 as illustrated in FIG. 5 and FIG. 6. The liquid (reduction product) generated on the surface of the reduction electrode 3 drops downward by gravity. Therefore, by injecting carbon dioxide from above, it is possible to promote the movement of the liquid and to prevent the liquid from being reattached to the surface of the reduction electrode 3.

(Experiment 2)

In Experiment 2, the Faraday efficiency of the carbon dioxide reduction reaction was obtained using the blower 10 having the configuration illustrated in FIG. 1.

As the blower 10, a propeller fan (LittleFAN40U manufactured by Timely Corporation) was used. The propeller fan was rotated at 5000 rpm. Therefore, the flow of the carbon dioxide generated by the blower 10 always hits the surface of the reduction electrode 3.

FIG. 7 illustrates an experimental result of Experiment 2. The relationship between the horizontal axis and the vertical axis in FIG. 7 is the same as that in FIG. 4.

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

(Experiment 3)

In Experiment 3, the same blower 20 as in Experiment 1 was used. In addition, the supply pressure of the carbon dioxide was set to 0.5 MPa, and the mass flow controller was controlled so as 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.

FIG. 8 illustrates an experimental result of Experiment 3. The relationship between the horizontal axis and the vertical axis in FIG. 8 is the same as those in FIG. 4 and FIG. 7.

As illustrated in FIG. 8, it can be seen that a decrease in Faraday efficiency can be improved in a case where the blower 20 is provided (□).

As described above, it can be seen that a decrease in reaction efficiency can be improved by providing the blower 10 or 20.

The present invention is not limited to the above embodiment, and modifications can be made within the scope of the gist of the present invention. For example, although the light 13 is generated by a xenon lamp in the embodiment, sunlight may be used.

In addition, the electrolyte membrane 4 and the reduction electrode 3 may be integrally formed. The electrolyte membrane 4 and the reduction electrode 3 may be replaced with a gas diffusion electrode (GDE (registered trademark)) formed of a porous member and a catalyst. The number of components can be reduced. Note that the electrolyte membrane 4 and the reduction electrode 3 may be integrated by press-fitting the electrolyte membrane 4 into a porous copper body.

In addition, the description has been given with an example in which the blower 10 or 20 is disposed in front of the supply port 9. On the other hand, the blower 10 or 20 may be disposed in front of the supply port 8.

As described above, it is needless to say that the present invention includes various embodiments and the like not described herein. Therefore, the technical scope of the present invention is defined only by subject matters to specify the invention according to the valid scope of claims based on the above description.

INDUSTRIAL APPLICABILITY

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

REFERENCE SIGNS LIST

- 1 Substrate
- 2 Oxidation electrode
- 3 Reduction electrode
- 4 Electrolyte membrane
- 5 Electrolytic solution
- 6 Oxidation bath
- 7 Reduction unit
- 8, 9 Supply port
- 10, 20 Blower
- 11 Gas recovery port
- 12 Liquid recovery port
- 13 Light

Carbon Dioxide Reduction Device

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