JP7325082B2 - Carbon dioxide reduction device and carbon dioxide reduction method - Google Patents

Description

TECHNICAL FIELD The present invention relates to a reduction apparatus and a reduction method for reducing and fixing carbon dioxide at an extremely low overvoltage, and more particularly to a reduction apparatus and a reduction method for selectively converting carbon dioxide into methane at an extremely low overvoltage.

There is concern that the concentration of carbon dioxide in the atmosphere will increase year by year as a greenhouse gas. When carbon dioxide is electrochemically reduced to methane, the theoretical voltage occurs at a more positive potential than that of hydrogen production by electrolysis of water. In other words, when electrical energy from surplus electricity is converted into chemical energy and used for energy storage, methane can be produced from carbon dioxide at a lower voltage than producing hydrogen from water. In addition, the storage and transportation of hydrogen poses a large social burden because new infrastructure needs to be built. On the other hand, since methane is the main component of natural gas, it has the advantage of being able to use the infrastructure of modern city gas storage and transportation as it is.

Considering the above social background, the reduction and fixation of carbon dioxide is positioned as an important technology. In contrast, conventional electrolytic reduction of carbon dioxide has been exclusively developed using copper or gold electrodes. In this case, the overvoltage required to reduce carbon dioxide, that is, the voltage that must be applied above the theoretical potential is about 1 to 2 volts. Considering the current situation where the overvoltage of the electrode reduction reaction that generates hydrogen from water is about 0.1 V at most, it is unlikely that carbon dioxide reduction using copper or gold electrodes can contribute to energy storage. Furthermore, when a copper electrode or a gold electrode is used, a plurality of products are generated (Non-Patent Document 1), and a separation technique is required when using the products, which is not suitable.

On the other hand, although there are reports on the reduction and fixation of carbon dioxide using platinum electrodes, none of them have succeeded in efficiently extracting useful products derived from carbon dioxide at a low overvoltage close to the theoretical potential.

Patent Documents 1 and 2 disclose carbon dioxide reduction technology that uses a platinum electrode as the reducing electrode of a membrane electrode assembly, and it has been confirmed that carbon monoxide and methanol are produced by reducing carbon dioxide. . At this time, the carbon dioxide supplied to the cathode may be mixed with an inert gas to adjust the concentration, even if the concentration is high. is also disclosed.

In patent documents 3 and 4, methane, methanol, ethanol, etc., are produced from carbon dioxide supplied to the cathode by controlling the temperature of the electrolysis/power generation part to 200 ° C or less in the electrolysis / power generation method using a membrane electrode assembly. A technology is disclosed that can be generated and controls the amount of generation and the ratio of each type.

Republished No. 2012/118065 Republished No. 2012/128148 JP 2015-54994 A JP 2015-56315 A

The Electrochemical Society of Japan, Electrochemistry, Vol.87, pp.37-42, (2019)

In the known technology described above, even if the concentration of carbon dioxide supplied is high and the concentration is adjusted by mixing an inert gas, a reduction reaction occurs, but there is a problem that the reduction reaction does not continue continuously. . Furthermore, since it is necessary to apply high heat energy in order to selectively take out the product, it is not practical from the viewpoint of energy.

Thus, no technology has been proposed for continuously, efficiently and selectively reducing and fixing carbon dioxide with low overvoltage, that is, with low energy consumption. A carbon reduction method has been long awaited.

A first object of the present invention is to provide a carbon dioxide reduction apparatus and a carbon dioxide reduction method for efficiently reducing and fixing carbon dioxide with low energy consumption.

A second object of the present invention is to provide a carbon dioxide reduction apparatus and a carbon dioxide reduction method that can selectively take out reaction products.

Another object of the present invention is to provide a carbon dioxide reduction apparatus and a carbon dioxide reduction method that can continuously take out a reaction product by reducing carbon dioxide.

Still another object of the present invention is to provide a carbon dioxide reduction apparatus and a carbon dioxide reduction method capable of extracting electrical energy while reducing carbon dioxide.

As a result of extensive studies, the inventors of the present invention have found that the above-described problems can be solved by providing the following configuration, and have completed the present invention.

That is, the present invention has, for example, the following configurations and features.

(Aspect 1)
a cathode and an anode separated by an electrolyte;
cathode reactant supply means for supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling the concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supply means for supplying an anode reactant to the anode;
voltage control means electrically connected to said cathode and said anode;
and
The cathode reactant supply means has a first gas flow control means for controlling the flow rate of the first gas containing carbon dioxide and a second gas flow control means for controlling the flow rate of the inert gas,
the cathode reactant supply means controls the concentration ratio to vary within the range of 0:100 to 60:40 in carbon dioxide concentration:inert gas concentration ratio;
A carbon dioxide reduction device characterized by:
(Aspect 2)
The voltage control means applies a voltage such that the electrode potential of the cathode is in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode.
The carbon dioxide reduction apparatus according to aspect 1, characterized by:
(Aspect 3)
a cathode and an anode separated by an electrolyte;
cathode reactant supply means for supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling the concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supply means for supplying an anode reactant to the anode;
voltage control means electrically connected to said cathode and said anode;
and
The voltage control means applies a voltage such that the electrode potential of the cathode is in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode, and the cathode reactant supply means controls the concentration ratio to Temporarily set the ratio of carbon concentration:inert gas concentration to 0:100,
A carbon dioxide reduction device characterized by:
(Aspect 4)
providing a cathode and an anode separated by an electrolyte;
a cathode reactant supplying step of supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling the concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supplying step of supplying an anode reactant to the anode;
a voltage control step of electrically connecting the cathode and the anode;
and
The cathode reactant supply step includes a first gas flow rate control step of controlling the flow rate of the first gas containing carbon dioxide and a second gas flow rate control step of controlling the flow rate of the inert gas,
The cathode reactant supply step controls the concentration ratio to vary within a range of 0:100 to 60:40 in the ratio of carbon dioxide concentration:inert gas concentration.
A carbon dioxide reduction method characterized by:
(Aspect 5)
The voltage control step includes the step of applying a voltage in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode,
The carbon dioxide reduction method according to aspect 4, characterized by:
(Aspect 6)
providing a cathode and an anode separated by an electrolyte;
a cathode reactant supply step of supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling a concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supplying step of supplying an anode reactant to the anode;
a voltage control step of electrically connecting the cathode and the anode;
and
The voltage control step applies a voltage such that the electrode potential of the cathode is in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode, and the cathode reactant supply step adjusts the concentration ratio to dioxide Carbon concentration: Temporarily controlled to 0:100 at the inert gas concentration ratio,
A carbon dioxide reduction device characterized by:

According to the carbon dioxide reduction apparatus and the carbon dioxide reduction method of the present invention, methane can be selectively obtained from carbon dioxide with a low overvoltage close to the theoretical potential, that is, with low energy consumption. By controlling the concentration of carbon dioxide supplied to the cathode and controlling the potential of the cathode electrode within an appropriate range, this reaction can cause a continuous reduction reaction with high efficiency. Also, under these conditions, the reduction of carbon dioxide occurs at a nobler potential than the reduction of water, that is, the hydrogen evolution reaction. This means that the reduction of carbon dioxide to methane can be performed with less energy than the reduction of water to hydrogen. This means that methane, which is more suitable for energy storage than hydrogen, can be implemented with lower energy consumption. Furthermore, when hydrogen oxidation is combined as an anode reaction with the cathode reaction that reduces carbon dioxide to obtain methane, electricity is generated (fuel cell power generation) while reducing carbon dioxide to obtain methane.

BRIEF DESCRIPTION OF THE DRAWINGS It is a conceptual diagram explaining the carbon dioxide reduction apparatus of this invention. FIG. 3 is a conceptual diagram illustrating an electrolytic cell in the carbon dioxide reduction device of the present invention; It is the figure which showed the test result concerning the Example of this invention. It is the figure which showed the test result concerning the Example of this invention. It is the figure which showed the test result concerning the Example of this invention.

First, the carbon dioxide reduction device and carbon dioxide reduction method according to the present invention will be described with reference to the drawings. The following detailed description is intended to describe the invention in detail and is not intended to limit the invention.

(Carbon dioxide reduction device)
FIG. 1 is a conceptual diagram showing one form of the carbon dioxide reduction device of the present invention. The carbon dioxide reduction device comprises at least an electrolytic section 1 having a cathode and an anode separated by an electrolyte, and supplying a carbon dioxide-containing gas and an inert gas to the cathode and controlling the flow rate ratio of each gas. cathode reactant supply means 40, anode reactant supply means 50 for supplying reactant to the anode, and voltage control means 80 electrically connected to the cathode and the anode.

(Electrolytic section)
In FIG. 1, the electrolytic section 1 has at least an electrolyte 11, a cathode 12 and an anode 13. In FIG. The electrolyte 11 is required to have appropriate ionic conductivity. Specifically, electrolyte-containing agar, glass filters, polyolefin-based resin non-woven fabrics, porous films, and the like are used. However, in this case, it is difficult to sufficiently prevent mixing of the electrolyte, and the life of the diaphragm is short. In order to solve this problem, an ion exchange membrane is preferably used as the electrolyte 11 . Ion exchange membranes include cation exchange membranes and anion exchange membranes, each of which can be used alone. Specifically, Nafion membranes, Dow membranes, Flemion membranes, Selemion membranes, Aciplex membranes, Goreselect membranes (all registered trademarks) and the like are useful. It is also within the scope of the present invention to use these ion-exchange membranes on the cathode side of the electrolyte 11 and a liquid having an ion-conducting function on the anode side. It is desirable that the material of the electrolyte 11 has low electronic conductivity so that a voltage can be applied efficiently between the cathode 12 and the anode 13 .

(cathode)
Cathode 12 is composed of a catalytic material. The catalyst material preferably contains a material having a high affinity for carbon dioxide or carbon monoxide produced by reduction of carbon dioxide. By using such a catalyst material, the reduction reaction at the cathode 12 can proceed efficiently. Such catalysts can be selected as necessary, for example, one or more selected from the group consisting of platinum, gold, palladium, ruthenium, osmium, iridium, rhodium, nickel, silver, iron, copper, and cobalt. is mentioned. Among these, those containing platinum as an active ingredient are preferably used. In particular, when two or more types are used, an alloy or a material having a core-shell structure is preferably used. Moreover, when these are used in the form of fine particles, it is useful and preferable to use a method in which the catalyst material is fixed on a carrier represented by particles of carbon, titanium oxide, or the like.

Although it is not clear why a cathode catalyst material containing platinum as an active ingredient is suitable for use, the coexistence of an adsorbent of a carbon-containing compound derived from carbon dioxide, which is the raw material for methane, and adsorbed hydrogen on the material is important. is thought to play an important role. The surface reaction that occurs between heterogeneous adsorbates adsorbed on the catalyst surface is known as the Langmuir-Hinshelwood mechanism. While carbon monoxide derived from carbon dioxide is known to be well adsorbed on platinum, hydrogen ions are also readily adsorbed on the surface of platinum in the form of adsorbed hydrogen. Platinum is considered to be a good catalyst material that provides a place for both of these to coexist.

The cathode 12 may contain a conductive material in addition to the catalytic material. The inclusion of a conductive material is preferable because it improves the efficiency of the reduction and immobilization reaction. Materials having conductivity are not particularly limited, for example platinum, gold, silver, palladium, ruthenium, iridium, rhodium, rhenium, osmium, tin, iron, chromium, copper, nickel, cobalt, titanium, zirconium, stainless steel, carbon, At least one selected from the group consisting of tin-doped indium oxide and fluorine-doped indium oxide, or a mixture thereof, or an alloy or composite of these metals can be used.

Further, the cathode 12 preferably contains a solid electrolyte capable of transporting cations. This is because the inclusion of the solid electrolyte can increase the interface (three-phase interface) between the electrolyte and the electrode, which is considered to be the reaction point of the electrochemical reaction.

The shape of the cathode 12 is not particularly limited, and any shape used for ordinary cathodes such as wire, sheet, plate, rod, and mesh can be selected. Among these shapes, a structure with a large surface area is preferable from the viewpoint that the reduction reaction of carbon dioxide proceeds efficiently. In addition, it is preferred that the cathode 12 have voids so that the reactant carbon dioxide can easily permeate the entire cathode 12 . The term "void" as used herein means a channel through which the reactants are effectively supplied to the reaction field at the three-phase interface in terms of electrochemistry. Also, in order to efficiently transfer reactants to and from the electrolyte 11, it is preferable that the electrolyte 11 and the cathode 12 have a large junction area. As such a structure, the shape of the cathode 12 is preferably a sheet shape.

When the cathode 12 is composed only of a catalyst, either a single crystal, polycrystal, or amorphous catalyst material may be used. A structure with a large surface area is preferable in terms of efficiently advancing the reduction reaction of carbon dioxide. As such a structure, a structure in which fine particles of the catalyst material are carried on the bulk of the catalyst material, or an aggregate of only the fine particles of the catalyst material is preferable. The particle diameter of the fine catalyst particles contained therein is preferably 1 nm or more and 10 μm or less. If it is less than 1 nm, the crystallinity of the catalyst material may not be sufficient and sufficient performance may not be exhibited, and if it exceeds 10 μm, the surface area may be small and sufficient performance may not be exhibited. For example, platinum black may be used as catalyst fine particles having such a structure.

(anode)
Anode 13 is composed of a catalytic material. The catalyst material is not particularly limited. However, if a material with high activity for the reaction selected as the anode reaction is selected, the reaction efficiency can be increased, which is preferable. As an anode reaction, as described in the present embodiment, if oxidation of hydrogen, water, and hydroxide ions is selected as an example, catalyst materials include platinum, gold, palladium, ruthenium, and osmium. , iridium, rhodium, nickel, silver, iron, copper, and cobalt. The above catalyst material may be single crystal, polycrystal or amorphous.

As with the cathode 12, the anode 13 may contain a conductive material in addition to the catalytic material. Also in this case, the effect of improving the efficiency of the reductive immobilization reaction can be obtained. As the conductive material, the same material as that of the cathode 12 can be used. Also, the anode 13 may contain a solid electrolyte capable of transporting ions. The inclusion of such a solid electrolyte is preferable because the interface (three-phase interface) between the electrolyte and the electrode, which is considered to be the reaction point of the electrochemical reaction, can be increased.

The shape of the anode 13 is not particularly limited, and a configuration similar to that of the cathode 12 can be adopted. A sheet form is particularly preferred. In order to efficiently promote the reduction reaction of carbon dioxide, a structure having a large surface area is preferable, and like the cathode 12, a structure using a particulate catalyst material such as platinum black is preferable. Furthermore, when the anode 13 is composed of a catalyst material and a material containing at least one of the conductive material and the solid electrolyte, the particle size, the content of the conductive material, and the content of the solid electrolyte are also A configuration similar to that of the cathode 12 can be employed.

(anode/cathode gas diffusion layer)
The cathode gas diffusion layer 14 and the anode gas diffusion layer 15 are not particularly limited as long as they are conductive and can supply gas to the cathode 12 and the anode 13, respectively. In order to increase the reaction efficiency, the reactants supplied from the cathode reactant supply means 40 and the anode reactant supply means 50 are uniformly diffused to the cathode 12 and the anode 13 through the cathode gas diffusion layer 14 and the anode gas diffusion layer 15. It is preferable to be able to For example, known materials such as carbon paper and stainless steel mesh can be used as such materials.

The cathode gas diffusion layer 14 and the anode gas diffusion layer 15 may be formed separately from the cathode 12 and the anode 13, or may be integrally formed. When integrally formed, for example, the cathode gas diffusion layer 14 and the anode gas diffusion layer 15 can be integrated by applying the constituent material of the cathode 12 and the anode 13 .

(Flow path forming member)
FIG. 2 shows an electrolysis cell in a carbon dioxide reduction apparatus, where flow path forming members 21 and 22 supply a gas containing carbon dioxide and an inert gas, and control the flow rate ratio of each gas. Fluids containing reactants supplied via reactant supply means 40 and anode reactant supply means 50 for supplying reactants to the anode are respectively supplied from the cathode reactive fluid introduction part 25 to the cathode gas diffusion layer 14 through the channel 23 . , the function of supplying from the anode reactive fluid introduction part 27 to the anode gas diffusion layer 15 in the same way, and the function of discharging the reaction products and the like generated at the cathode 12 and the anode 13 to the outside through the fluid discharge parts 26 and 28. have. The flow path forming members 21 and 22 are made of a current collecting material and have the function of electrically connecting the cathode gas diffusion layer 14, the anode gas diffusion layer 15 and the external power supply 80. FIG. Examples of the current collecting material include known materials such as stainless steel, titanium, carbon, and the like, and surface-treated materials thereof.

The flow path forming members 21 and 22 are not particularly limited as long as they have at least the above fluid transport function. However, in order to promote the reduction and fixation reaction of carbon dioxide, it is preferable that the flow path forming members 21 and 22 have a flow path structure 23 that allows the gas to spread uniformly over the cathode 12 and the anode 13. . As such a channel structure 23, for example, known channels such as a serpentine channel and a straight channel are used.

Further, it is preferable that the flow path forming members 21 and 22 have a high gas barrier property. This is because when the flow path forming members 21 and 22 have high gas barrier properties, the reaction efficiency at the cathode 12 and the anode 13 and the recovery efficiency of the generated gas are improved.

(Electrolytic cell)
In order to configure the electrolysis cell 70 by incorporating the electrolysis unit 1 into the flow path forming members 21 and 22, it is preferable that the entire system be hermetically sealed. However, when the flow path forming members 21 and 22 are in direct contact with each other outside the electrolytic section 1, the ions generated at the cathode 12 or the anode 13 may move through the flow path forming members 21 and 22 instead of the electrolyte 11. be. Such movement of ions is not preferable because the generated ions do not contribute to the electrode reaction and the reaction efficiency of the generated gas deteriorates. Therefore, as shown in FIG. 2, it is preferable to bring the opening end of the flow path forming member 21 and the opening end of the flow path forming member 22 into close contact with each other with an insulating material 60 interposed therebetween.

The insulating material 60 is not particularly limited as long as it can insulate between the flow path forming members 21 and 22 . When a material having an adhesive function is used as the insulating material 60, it becomes possible to easily seal the reduction fixation system. A seal made of Teflon (registered trademark), for example, has such an adhesive function. If the insulating material 60 does not contain an adhesive, a separate adhesive may be used to bond the open ends of the flow path forming members 21 and 22 together. As the adhesive in this case, it is preferable to use an insulating one in order to prevent leakage of ions and electrons to the outside of the system.

(Voltage control means)
The voltage control means 80 is not particularly limited as long as it can apply a voltage necessary for reducing carbon dioxide. A voltage-controlled power supply (for example, a potentiostat) or a current-controlled power supply (for example, a galvanostat) is preferably used for the voltage control means 80 . In order to control the electrode potential of the cathode with respect to the reversible hydrogen electrode (hereinafter sometimes abbreviated as RHE), it is necessary to install it on or inside the electrolyte 11 at a site where the cathode 12 or anode 13 is not provided. (not shown in FIGS. 1 and 2). Specifically, a reversible hydrogen electrode, a silver-silver chloride electrode, a silver-silver sulfate electrode, etc. can be installed by a known method described in References 1 and 2 below. (Reference 1: Journal of The Electrochemical Society, 166, F208-F213, 2019, Reference 2: Journal of Power Sources, 195, 5986-5989, 2010) In addition, hydrogen is supplied as a reactant to the anode to simulate It is also effective to use it as a reversible hydrogen electrode to control the electrode potential.

An electric signal consisting of a DC voltage, an AC voltage, or a pulse voltage, or a combination of two or more of these electric signals, is applied by the voltage control means 80 in an appropriate voltage range and frequency at a potential at which carbon dioxide is reduced. , and it is known that methane can be continuously generated by combining it with the application of a high-potential DC (for example, reference 3: 2018 Electrochemistry Autumn Conference Proceedings 2M05). The production was intermittent and it was not possible to sustain the continuous reduction reaction of carbon dioxide.

In order to sustain continuous generation of methane only by applying a DC voltage, the concentration of carbon dioxide supplied by the cathode reactant supply means 40 is controlled while accurately controlling the voltage range at the potential at which carbon dioxide is reduced. As a result, the continuous reduction reaction of carbon dioxide can be sustained without applying an AC voltage or a pulse voltage.

Specifically, in order to control the cathode potential with high precision using the voltage control means 80, the reversible hydrogen electrode is controlled between 0.11 and 0.19 V to effectively and continuously reduce carbon dioxide. Between 0.14 and 0.18 V, carbon dioxide reduction is more effective and continuous. Although the reason why this electrode potential is preferable is not clear, it can be explained as follows. In other words, as described above, it is thought that the carbon-containing compound adsorbent derived from carbon dioxide and the adsorbed hydrogen derived from hydrogen ions react on the surface of the catalyst material to produce methane. The electrode potentials present correspond to the potentials mentioned above. Furthermore, at the same potential, when methane is produced by the Langmuir-Hinshelwood mechanism, the active sites on the catalyst material become vacant, but there are newly formed carbon-containing compound adsorbents derived from carbon dioxide and adsorbed hydrogen derived from hydrogen ions. , the reaction turns over and proceeds continuously. The continuous reduction reaction mechanism of carbon dioxide is speculated in this way.

Effective methods for controlling this potential include a function generator, a function synthesizer, or a computer-controlled method, or a method of manually driving the voltage control means 80 .

(power generation)
In the carbon dioxide reduction apparatus and the carbon dioxide reduction method according to the present invention, voltage (electric power) may be extracted. Specifically, if carbon dioxide is reduced at the cathode at a potential that is more positive than the hydrogen evolution potential, and if the reaction at the anode is at a negative potential than the reduction potential of carbon dioxide, the principle of the cell will cause fuel cell power generation. . More specifically, this is the case where a hydrogen oxidation reaction occurs at the anode. Under this condition, the voltage control means 80 is preferably a device that controls the voltage by using an electronic load or the like and utilizes the generated power.

The voltage control means 80 may be connected to the cathode 12, the anode 13, the gas diffusion layers 14, 15, or the passage forming members 21, 22, but is connected to the end plates 31, 32 from the viewpoint of improving the electronic conductivity. It is preferable to let

(Cathode reactant supply means)
A cathode reactant supply means 40 for supplying a gas containing carbon dioxide (hereinafter also referred to as a "first gas") and an inert gas to the cathode and controlling the flow rate ratio of each gas is provided to the cathode 12. A gas donor 41 containing carbon dioxide, a gas flow rate control means 45 for controlling its flow rate, an inert gas donor 42, and a gas flow rate control means for controlling its flow rate. 46. The flow rate ratio of the gas containing carbon dioxide and the inert gas can be adjusted by the respective supply speed, pressure, flow resistance, and the like. The purpose of controlling the flow ratio is to control the concentration of carbon dioxide supplied to the cathode. The carbon dioxide concentration is the same as the carbon dioxide partial pressure in the mixed gas of the first gas and the inert gas.

The first gas supplied to the cathode 12 by the cathode reactant supply means 40 may consist solely of carbon dioxide, or the first gas and the inert gas may optionally contain water vapor. . (The water vapor supply means is not shown in Figure 1.) As an example of containing water vapor, a bubbling mechanism for containing water vapor in the atmosphere (gas containing carbon dioxide) and a mechanism for sending air containing water vapor are used. It can be configured to include a gas transport mechanism for The case where it is necessary to contain water vapor is, for example, the case where the electrolyte 11 must be kept in a wet state, or the case where water is used as a hydrogen source to obtain hydrocarbons, alcohols, etc. from carbon dioxide.

The case where the electrolyte 11 must be kept in a wet state is the case where the water content of the electrolyte 11 affects the ionic conductivity of the membrane, the gas permeation rate, the membrane strength when the electrolyte 11 is used in a solid state, etc. is.

The flow rate of gas supplied to the cathode 12 is not particularly limited. However, it is preferable that the flow rate is higher than that capable of quickly supplying the carbon dioxide consumed by the reduction, that is, capable of supplying the required carbon dioxide content (concentration). Suitable carbon dioxide is supplied to the cathode 12 at an appropriate concentration and rate from a cathode reactant supply means 40 that supplies a gas containing carbon dioxide and an inert gas and controls the flow rate ratio of each. Reduction can be performed.

(Inert gas supplier)
Gases such as nitrogen, helium, and argon are used as effective components for the inert gas donor 42 .

(Gas supplier containing carbon dioxide)
The gas donor 41 containing carbon dioxide contains carbon dioxide and gases of other components in an arbitrary ratio. The gas of other components referred to here may be any gas, but it is desirable that it does not contain a substance that inhibits the reduction reaction of carbon dioxide.

Further, the gas donor 41 containing carbon dioxide may be provided with a device (carbon dioxide concentrator) capable of concentrating carbon dioxide. Here, to concentrate carbon dioxide means to remove substances that cause reactions that inhibit the reduction reaction of carbon dioxide from the gas supplied to the cathode 12, or to reduce substances other than carbon dioxide. It means increasing the concentration of carbon dioxide.

A reaction that inhibits the reduction reaction of carbon dioxide includes, for example, a reduction reaction of oxygen. Therefore, the operation of removing oxygen from the gas supplied to the cathode 12 corresponds to the operation of concentrating carbon dioxide.

Such a carbon dioxide enrichment device can be arbitrarily selected, and examples thereof include a molten carbonate fuel cell (hereinafter sometimes abbreviated as MCFC). The MCFC is preferred because it can efficiently concentrate carbon dioxide and does not require external energy, unlike other physical or chemical carbon dioxide concentration devices. Moreover, the electric power generated by the MCFC may be directly or indirectly used for the reduction and fixation reaction of carbon dioxide in the electrolytic cell 70 .

(Cathode gas flow control means)
As the gas flow control means 45 and 46, known devices such as mass flow controllers and gas flow meters can be used. By this use, each gas derived from the gas donor 41 containing carbon dioxide and the inert gas donor 42 is supplied to the cathode 12 at an arbitrary flow rate ratio of carbon dioxide and inert gas.

Conventionally, when adjusting the partial pressure of carbon dioxide by mixing gases such as water vapor, N ₂ , He, and Ar with respect to the gas supplied to the cathode, the partial pressure of carbon dioxide is preferably 1% to 70%, and more preferably 1%. Up to 50% has been considered preferable, but in order to reduce and fixate continuously, efficiently and selectively under a low overvoltage close to the theoretical potential, the state where the carbon dioxide partial pressure temporarily becomes 0% It is important to create

When the concentration ratio of carbon dioxide and inert gas is supplied to the cathode to vary with time within the range of 0:100 to 60:40, methane is produced continuously with high yield at low overvoltage. As mentioned above, it is thought that the carbon-containing compound adsorbent derived from carbon dioxide and the adsorbed hydrogen derived from hydrogen ions react on the surface of the catalyst material to produce methane. The amount of carbon-containing compound adsorbent derived from carbon dioxide increases, while the amount of adsorbed hydrogen derived from hydrogen ions increases when the carbon dioxide concentration is low. In other words, as explained above, if methane is produced by the Langmuir-Hinshelwood mechanism, the presence of an appropriate ratio of carbon-containing compound adsorbents derived from carbon dioxide and adsorbed hydrogen derived from hydrogen ions on the catalyst surface produces methane. progresses. The reason why methane is produced continuously at a high yield at a low overvoltage when the concentration ratio of carbon dioxide and inert gas is varied over time within the range of 0:100 to 60:40 is explained in this way. .

Here, the flow ratio of carbon dioxide and inert gas supplied to the cathode may be fixed or variable, but it is not appropriate to temporarily set the content ratio (concentration) of carbon dioxide to 0. This is important because it is possible to create a state in which the carbon-containing compound adsorbent derived from carbon dioxide and the adsorbed hydrogen derived from hydrogen ions are present on the surface of the catalyst. However, if the content ratio (concentration) of carbon dioxide is always 0, the amount of carbon-containing compound adsorbent derived from carbon dioxide becomes too small, which is not preferable.

(Anode reactant supply means)
The anode reactant supplying means 50 for supplying the reactant to the anode is means for supplying the anode reactant to the anode 13 . It consists of an anode reactant donor 51 and flow control means 55 for controlling its flow rate. It is desired that the anode reactant undergo an oxidation reaction and that the reaction products can be easily removed from the anode. Additionally, the reaction preferably does not destroy or dissolve the anode components. Hydrogen and water are preferably used as anode reactants that meet this requirement. The hydrogen oxidation reaction produces hydrogen ions, which are consumed by the cathode reaction via the electrolyte 11 . The oxidation reaction of water produces oxygen gas and hydrogen ions. The oxygen gas is discharged outside through the fluid discharge section 28, and the hydrogen ions are consumed as described above.

The anode reactant donor 51 is particularly restricted as long as it has a mechanism capable of delivering a substance capable of generating hydrogen ions and electrons through an anodic oxidation reaction in the form of gas (including mist, aerosol, etc.), liquid, or solid. not. The amount of water, hydrogen, and the like involved in the reaction at the anode 13 is not particularly limited. However, it is preferable that the amount be equal to or greater than the amount that allows the oxidation reaction of the anode 13 to proceed so that the supply of hydrogen ions to the cathode 12 is not delayed. In addition, when the gas is supplied from the anode reactant donor 51, nitrogen, helium, argon, etc. are contained within a range in which the main components involved in the reaction, such as water and hydrogen, can be sufficiently supplied. Concentration may be adjusted. Nitrogen, helium, argon, etc. may be separately supplied in order to quickly send the gas generated by the reaction to the fluid discharge part 28 .

If the anode reactant is gaseous, it can be humidified in the same manner as the cathode for the reasons and methods described above.

(Anode gas flow control means)
The flow rate control means 55 can be a known device such as a mass flow controller or a gas flow meter when the reactant delivered from the anode reactant donor 51 is gas, and when the reactant is liquid Known devices can be used, including liquid delivery pumps and liquid flow meters. With this use, the anode reactant donor 51 is delivered to the anode 13 at any flow rate.

(Electrolytic cell)
The electrolysis section 1 of the electrolysis cell 70 may be provided with a temperature control mechanism for controlling the temperature. Such a temperature control mechanism is not particularly limited as long as it can control the temperature of the reaction fields of the cathode 12 and the anode 13 within a range of -70°C or higher and 200°C or lower, and conventionally known heating and cooling devices can be used. can. The above temperature range is the temperature range in which the electrolyte 11 typically used in the electrolytic part 1 has ionic conductivity, and it is preferable to change the control range appropriately according to the type of the electrolyte 11 . The electrolysis part 1 of the electrolysis cell 70 is preferably carried out at 0° C. or higher and 100° C. or lower, more preferably 10° C. or higher and 90° C. or lower.

(power generation)
In the carbon dioxide reduction apparatus and the carbon dioxide reduction method according to the present invention, voltage (electric power) can also be taken out. Specifically, when the reduction of carbon dioxide occurs at the cathode at a potential more positive than the hydrogen evolution potential, and the reaction at the anode is at a potential more negative than the reduction potential of the carbon dioxide, the principle of the cell causes fuel cell power generation. The most straightforward example is the case where a hydrogen oxidation reaction occurs at the anode. If the carbon dioxide reduction reaction at the cathode repeats appropriate turnover, the conditions for fuel cell power generation are satisfied. This means that when the chemical energy of hydrogen is used to reduce carbon dioxide, a portion of the chemical energy of hydrogen remains without being used up. Surplus chemical energy is converted into heat and discarded in normal chemical reactions or catalytic reactions, but is converted into electrical energy in fuel cell power generation. This means that the carbon dioxide reduction apparatus and the carbon dioxide reduction method according to the present invention can effectively recover and utilize the excess energy as electrical energy without discarding it as thermal energy.

Next, the present invention will be described in detail with reference to examples, but the purpose of the examples is to explain the present invention in detail, and it goes without saying that the present invention is not limited by these examples.

[Examples 1 to 4, Comparative Examples 1 to 2]
(Pretreatment of carbon paper)
Water-repellent carbon paper (gas diffusion layer) was cut into 3×3 cm squares and washed thoroughly.

(Pretreatment of Nafion® 117 membrane)
An electrolyte membrane, Nafion (registered trademark) 117, was cut into pieces of 6×6 cm and treated with pure water, hydrogen peroxide solution, and dilute sulfuric acid aqueous solution according to a standard method.

(Preparation of electrode catalyst spray solution)
Using a ball mill, 46.2 wt% platinum-supported carbon (Pt/C), ultrapure water, 2-propanol, methanol, and Nafion (registered trademark) 117 solution were mixed to obtain a Pt/C electrode catalyst spray solution.

(Application of electrode catalyst spray liquid)
The electrode catalyst spray solution prepared above was applied onto the above carbon paper by a spray coating method so that the amount of metal was 1.0 mg·cm ⁻² .

(Fabrication of membrane electrode assembly (MEA))
The above-described Nafion (registered trademark) 117 film was sandwiched between two sheets of carbon paper coated with the Pt/C catalyst prepared above, and hot-press molded to produce an MEA.

(Single cell assembly)
The MEA prepared above was incorporated into the cell shown in FIG. 2 as the electrolysis unit 1 and attached to the mounted electrolysis cell 70 to form a polymer electrolyte cell (single cell). Cathode reactant supply means 40 for supplying carbon dioxide and inert gas to the cathode shown in FIG. Then, a voltage control means 80 connected to the cathode and anode was installed. At this time, a high-purity carbon dioxide gas cylinder was used as the gas donor 41 containing carbon dioxide, a high-purity argon gas cylinder was used as the inert gas donor 42 , and a high-purity hydrogen cylinder was used as the anode reactant donor 51 . Mass flow controllers are used for the gas flow control means 45 and 46 and the flow control means 55, respectively.

(Pretreatment for single cell operation)
A humidified argon gas was supplied to one electrode (cathode) of the single cell prepared above, and a humidified hydrogen gas was supplied to the other electrode (anode), and potential scanning was repeatedly performed using a potentiostat until the potential stabilized.

(Carbon dioxide reduction experiment)
A humidified hydrogen gas was supplied to the anode of the single cell prepared above, and a humidified gas of CO ₂ and Ar was supplied to the cathode at an arbitrary ratio (CO ₂ partial pressure 0, 4, 7, 10, 50, 100%). and a potentiostat and a potential scanning device (potential scanner). The single-cell _CO2 exhaust gas was connected directly to a mass spectrometer (not shown). Table 1 shows the potential at the top of the m/z 15 (methane fragment) peak observed when the cathode potential was swept from 1.0 V vs. RHE to 0.05 V vs. RHE at a sweep rate of 10 mV/s. It can be seen that the potential at which methanogenesis occurs most is 0.1 V vs. RHE regardless of the CO ₂ partial pressure. In addition, the amount of methane produced per unit charge and The duration results are summarized in Table 1. When the CO ₂ partial pressure was 0% or 100% (Comparative Examples 1 or 2), methane production did not occur, but when the CO ₂ partial pressure was 4-50% (Examples 1-4), methane was produced. It can be seen that In addition, although methane production was deactivated, its duration was the longest when the CO ₂ partial pressure was 4% (Example 1).

Table 1 shows the above results.

[Examples 5-9, Comparative Examples 3-5]
Using the same equipment as used in Example 1, the cathode potential was stepped from 0.4 V vs. RHE to 0.2 V vs. RHE at a CO ₂ partial pressure of 4%, held for 5 minutes, and then gas supplied to the cathode. The CO ₂ partial pressure was varied to 0% and held for 1 hour, and then the potential was stepped to 0.1 V vs. RHE. The results of the relationship between time and detected intensity at m/z 15 are shown in FIG. The time when the cathode potential is stepped to 0.1 V vs. RHE is defined as 0 s. From Fig. 3, m/z 15 was detected immediately after the potential was stepped, indicating that methane production based on CO ₂ reduction occurred at 0.1 V vs. RHE. This is referred to as Example 5. Also, the peak area in FIG. 3 indicates the amount corresponding to the amount of methane produced. Table 2 shows the amount of methane produced per unit charge calculated from FIG. 3 and the duration of methane production.

Next, a similar experiment was conducted under the conditions of varying the CO ₂ partial pressure shown in Table 2. Table 2 summarizes the amount of methane produced per unit charge and the duration of methane production for Examples 6, 7, 8, and 9 and Comparative Examples 3, 4, and 5. From Table 2, it can be seen that both the amount of methane produced and the duration of methane production in Examples 5, 6, and 7 were significantly improved in comparison with Examples 1, 2, and 3. It can be said that highly efficient and sustained methane production was achieved by changing the CO ₂ partial pressure of the gas to 0%. Methane production was also confirmed in Example 9 in which the CO ₂ partial pressure was varied from 0% to 4%. Among these examples, Example 5, in which the CO ₂ partial pressure was varied from 4% to 0%, had the greatest methane production amount and methane production duration. On the other hand, in Comparative Example 5 in which the CO ₂ partial pressure of the gas supplied to the cathode was changed from 4% to 100%, the same results as in Comparative Example 2 were obtained.

[Examples 10 to 17]
The same apparatus as used in Example 1 was used and gas was supplied at a CO ₂ partial pressure of 4%. The cathodic potential was stepped from 0.25 V vs. RHE to 0.10 V vs. RHE every 2 min to the negative side in 8 steps. FIG. 4 shows the potential fluctuation and the behavior of m/z 15 at that time. As is clear from FIG. 4, it can be seen that methane production occurs continuously during the period in which the potential is varied between 0.25 V vs. RHE and 0.10 V vs. RHE. From the above, methane production was deactivated in Examples 5, 6, 7, 8, and 9, but the potential varied between 0.25 V vs. RHE and 0.10 V vs. RHE as in Examples 10 to 17. It can be said that the continuous production of methane was achieved by doing so.

The amount of methane produced was calculated based on FIG. It can be seen that the amount of methane produced changes depending on the electrode potential. Here, at 0.20 V vs. RHE, although the relative amount of methane produced was large, the decay rate of methane production was fast. In other words, by varying the cathode potential between 0.12 and 0.18 V vs. RHE, the amount of methane produced increases and is continuously produced.

[Example 18]
The same apparatus as used in Example 1 was used and gas was supplied at a CO ₂ partial pressure of 4%. The cathode potential was stepped from 0.4 V vs. RHE to 0.15 V vs. RHE and fixed for 5 minutes. This is referred to as Example 18. The behavior of m/z 15 at that time is shown in FIG. The time when the potential is stepped to 0.15 V vs. RHE is defined as 0 s. As is clear from Fig. 5, methanation occurred continuously during the period fixed at 0.15 V vs. RHE.

From the above, unlike Examples 5, 6, 7, 8, and 9, it can be said that continuous production of methane was achieved by fixing the cathode potential to 0.15 V vs. RHE. Further, in Example 18, the potential is stepped to 0.05 V vs. RHE after being fixed at 0.15 V vs. RHE for 5 minutes. As a result, as can be seen from Fig. 5, no methane is generated at 0.05 V vs. RHE. On the other hand, hydrogen evolution (detection of m/z 2) was observed at 0.05 V vs. RHE.

[Example 19]
In Example 19, where the cathode potential was stepped from 0.4 V vs. RHE to 0.10 V vs. RHE under the same apparatus and gas supply conditions as in Example 18, methane production was confirmed at 0.10 V vs. RHE. The production of the thing was deactivated in about 10 seconds.

[Examples 20 and 21]
The same apparatus, cathode gas supply conditions, and potential conditions as in Example 18 were used. In supplying the anode, a tank of water was used as the anode reactant donor 51 and a liquid feed pump was used as the flow rate control means 55 to flow water to the anode at 5 mL/min. This is referred to as Example 20. Also, the same apparatus, cathode gas supply conditions, and potential conditions as in Example 18 were used, and a high-purity N ₂ cylinder was used as the anode reactant donor 51 . This is referred to as Example 21. In Examples 20 and 21, the same results as in Example 18 were confirmed.

[Example 22]
Using the same apparatus as used in Example 1, hydrogen gas was supplied to the anode and gas was supplied to the cathode at a CO ₂ partial pressure of 4%. A power density of 0.011 mW/cm ^-2 was obtained at a cell voltage of 0.15 V when CO ₂ reduction was performed under these conditions. From the above, it can be seen that continuous methane production and power generation by CO ₂ reduction occur simultaneously in this system.

According to the carbon dioxide reduction apparatus and the carbon dioxide reduction method of the present invention, methane can be selectively obtained from carbon dioxide with lower energy consumption than in the case of hydrogen production by water electrolysis. Methane, which can be obtained selectively with this low energy consumption, is also superior to hydrogen in energy storage. Furthermore, when hydrogen oxidation is combined as an anode reaction with the cathode reaction that reduces carbon dioxide to obtain methane, electricity is generated (fuel cell power generation) while reducing carbon dioxide to obtain methane.

1. Electrolysis part 11 . electrolyte 12 . cathode 13 . Anode 14 . Cathode gas diffusion layer 15 . Anode gas diffusion layer 21 . Flow path forming member 22 . Flow path forming member 23 . flow path 25 . Cathode reactive fluid inlet 26 . Fluid discharge 27 . Anode reactive fluid inlet 28 . Fluid discharge 31 . end plate 32 . end plate 40 . Cathode reactant supply means 41 . Gas donor containing carbon dioxide 42 . inert gas donor 45 . gas flow control means 46 . gas flow control means 50 . Anode reactant supply means 51 . Anode Reactant Donor 55 . flow control means 60 . insulating material 70 . electrolytic cell 80 . voltage control means

Claims (6)

a cathode and an anode separated by an electrolyte;
cathode reactant supply means for supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling the concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supply means for supplying an anode reactant to the anode;
voltage control means electrically connected to said cathode and said anode;
and
The cathode reactant supply means has a first gas flow control means for controlling the flow rate of the first gas containing carbon dioxide and a second gas flow control means for controlling the flow rate of the inert gas,
the cathode reactant supply means controls the concentration ratio to vary within the range of 0:100 to 60:40 in carbon dioxide concentration:inert gas concentration ratio;
A carbon dioxide reduction device characterized by: The voltage control means applies a voltage such that the electrode potential of the cathode is in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode.
The carbon dioxide reduction device according to claim 1, characterized in that: a cathode and an anode separated by an electrolyte;
cathode reactant supply means for supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling the concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supply means for supplying an anode reactant to the anode;
voltage control means electrically connected to said cathode and said anode;
and
The voltage control means applies a voltage such that the electrode potential of the cathode is in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode, and the cathode reactant supply means controls the concentration ratio to Temporarily set the ratio of carbon concentration:inert gas concentration to 0:100,
A carbon dioxide reduction device characterized by: providing a cathode and an anode separated by an electrolyte;
a cathode reactant supply step of supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling the concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supplying step of supplying an anode reactant to the anode;
a voltage control step of electrically connecting the cathode and the anode;
and
The cathode reactant supply step includes a first gas flow rate control step of controlling the flow rate of the first gas containing carbon dioxide and a second gas flow rate control step of controlling the flow rate of the inert gas,
The cathode reactant supply step controls the concentration ratio to vary within the range of 0:100 to 60:40 in the ratio of carbon dioxide concentration:inert gas concentration.
A carbon dioxide reduction method characterized by: The voltage control step includes a step of applying a voltage in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode,
The carbon dioxide reduction method according to claim 4, characterized in that: providing a cathode and an anode separated by an electrolyte;
a cathode reactant supply step of supplying a first gas containing carbon dioxide and an inert gas to the cathode and controlling the concentration ratio of the carbon dioxide and the inert gas in the first gas;
an anode reactant supplying step of supplying an anode reactant to the anode;
a voltage control step of electrically connecting the cathode and the anode;
and
The voltage control step applies a voltage such that the electrode potential of the cathode is in the range of 0.11 V or more and 0.19 V or less with respect to the reversible hydrogen electrode, and the cathode reactant supply step adjusts the concentration ratio to dioxide Carbon concentration: Temporarily controlled to 0:100 at the ratio of inert gas concentration,
A carbon dioxide reduction device characterized by: