Methods for producing methane

Abstract

This invention provides a method for producing methane with excellent methane selectivity. [Solution] A method for producing methane using a carbon dioxide electrolytic cell comprising a carbon dioxide reduction electrode and an electrolyte, wherein the carbon dioxide reduction electrode has a cathode gas diffusion layer and a cathode catalyst layer that has been treated with a water-repellent coating in that order, and the method includes step A of supplying carbon dioxide from the cathode gas diffusion layer side and water from the electrolyte side to the cathode catalyst layer of the carbon dioxide reduction electrode in a temperature environment of 0°C to 50°C, and applying electricity.

Description

Technical Field

[0001] The present disclosure relates to a method for producing methane.

Background Art

[0002] Methane (CH4) can be synthesized by applying electricity to carbon dioxide (CO2) and water (H2O) and allowing a reduction reaction of carbon dioxide to proceed on a catalyst. Various reports have been made on techniques for controlling the selection and promotion of reaction products generated by the reduction reaction of carbon dioxide.

[0003] For example, Patent Document 1 discloses a technique for controlling the selection and promotion of reaction products generated by the reduction reaction of carbon dioxide by using a metal-containing cluster catalyst, which is a cluster containing one metal atom (M) selected from gold, silver, copper, platinum, rhodium, palladium, nickel, cobalt, iron, manganese, chromium, iridium, and ruthenium, as a catalyst in the reduction reaction of carbon dioxide.

Prior Art Documents

Patent Documents

[0004]

Patent Document 1

Summary of the Invention

Problems to be Solved by the Invention

[0005] In a method for producing methane by the reduction of carbon dioxide using a carbon dioxide electrolytic cell equipped with a carbon dioxide reduction electrode and an electrolyte, the methane production reaction occurs on the cathode side, but components other than methane, such as hydrogen, carbon monoxide (CO), and ethylene (C2H4), are also produced as byproducts. The production of these byproducts leads to excess energy consumption, reducing the energy conversion efficiency to methane. For example, water present on the catalyst causes a hydrogen production reaction in addition to the carbon dioxide reduction reaction, so if there is excess water on the catalyst, hydrogen production is promoted and the methane production rate may decrease. Therefore, there is a need to develop a method for producing methane by the reduction of carbon dioxide that can suppress hydrogen production and improve methane selectivity at the carbon dioxide reduction electrode.

[0006] This disclosure has been made in light of the circumstances described above. One embodiment of this disclosure aims to solve the problem of providing a method for producing methane with excellent methane selectivity. [Means for solving the problem]

[0007] The following are examples of specific means for solving the problem: <1> A method for producing methane using a carbon dioxide electrolytic cell equipped with a carbon dioxide reduction electrode and an electrolyte, The above carbon dioxide reduction electrode has a cathode gas diffusion layer and a cathode catalyst layer that has been treated with a water-repellent coating, in that order. A method for producing methane, comprising step A, in which carbon dioxide is supplied from the cathode gas diffusion layer side and water is supplied from the electrolyte side to the cathode catalyst layer of the carbon dioxide reduction electrode, in a temperature environment of above 0°C and below 50°C, and electricity is applied. <2> In step A above, carbon dioxide is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode from the cathode gas diffusion layer side and water is supplied from the electrolyte side, under a temperature environment of above 0°C and below 30°C, and electricity is applied. <1> The method for producing methane as described above. <3> The above carbon dioxide electrolysis cell further comprises an anode electrode. <1> or <2> The method for producing methane as described above. [Effects of the Invention]

[0008] According to one embodiment of the present disclosure, a method for producing methane with excellent methane selectivity is provided. [Brief explanation of the drawing]

[0009] [Figure 1] Figure 1 is a schematic diagram of the carbon dioxide electrolytic cell used in the evaluation test of the example. [Figure 2] Figure 2 is a graph showing the composition of the produced gas and the Faraday efficiency of each component in the methane production methods of Examples 1-2 and Comparative Examples 1-2. [Figure 3] Figure 3 is a graph showing the composition of the generated gas and the Faraday efficiency of each component in the methane production methods of Examples 1-6 and Comparative Examples 3-4. [Modes for carrying out the invention]

[0010] The embodiments of this disclosure will be described in detail below. This disclosure is not limited to the embodiments described below and can be implemented with appropriate modifications within the scope of the purposes of this disclosure. The dimensional ratios in the drawings do not necessarily represent the ratios of the actual dimensions.

[0011] In this disclosure, a numerical range represented by "~" means a range that includes the numbers written before and after "~" as the lower and upper limits, respectively.

[0012] In numerical ranges described in stages within this disclosure, the upper limit stated in one numerical range may be replaced by the upper limit of another numerical range described in stages, and the lower limit stated in one numerical range may be replaced by the lower limit of another numerical range described in stages. In numerical ranges described in stages within this disclosure, the upper or lower limit stated in one numerical range may be replaced by the values ​​shown in the examples.

[0013] In this disclosure, a combination of two or more preferred embodiments is a more preferred embodiment.

[0014] In this disclosure, the term "process" includes not only independent processes but also processes that cannot be clearly distinguished from other processes, as long as their intended purpose is achieved.

[0015] [Methods for producing methane] The present disclosure is a method for producing methane using a carbon dioxide electrolytic cell comprising a carbon dioxide reduction electrode and an electrolyte, wherein the carbon dioxide reduction electrode comprises a cathode gas diffusion layer and a cathode catalyst layer that has been treated with a water-repellent coating, in that order, and the method includes step A of supplying carbon dioxide from the cathode gas diffusion layer side and water from the electrolyte side to the cathode catalyst layer of the carbon dioxide reduction electrode in a temperature environment greater than 0°C and less than or equal to 50°C, thereby applying electricity. The method for producing methane according to this disclosure exhibits excellent methane selectivity. The reason why the methane production method of this disclosure may produce such effects is not clear, but the inventors speculate as follows. However, the following speculation is not intended to be a restrictive interpretation of the methane production method of this disclosure, but is explained as an example.

[0016] The method for producing methane according to the present disclosure is a method for producing methane using a carbon dioxide electrolysis cell including a carbon dioxide reduction electrode and an electrolyte. The carbon dioxide reduction electrode included in the carbon dioxide electrolysis cell used in the method for producing methane according to the present disclosure has a cathode gas diffusion layer and a cathode catalyst layer in this order, and the cathode catalyst layer is subjected to a water repellent treatment. When excessive water exists on the catalyst, the generation of hydrogen is promoted, so that there is a tendency for extra energy consumption to occur. However, the cathode catalyst layer included in the carbon dioxide reduction electrode used in the method for producing methane according to the present disclosure is subjected to a water repellent treatment, and water is appropriately removed from the catalyst. Therefore, it is considered that the generation of hydrogen is suppressed and the energy consumption required for the generation of methane is suppressed. Further, by performing the reduction reaction of carbon dioxide in an environment at a specific temperature or lower, it is presumed that the generation of hydrogen is further suppressed, the energy conversion efficiency to methane is improved, and the production ratio of methane is increased. Note that in a temperature environment exceeding 0°C, the freezing of the electrolytic solution tends to be suppressed.

[0017] On the other hand, the technique described in Patent Document 1 is different from the technique in the present disclosure in that the catalyst used for the reduction reaction of carbon dioxide is not subjected to a water repellent treatment. Further, the technique described in Patent Document 1 does not pay attention to the reduction reaction temperature of carbon dioxide when controlling the selection and promotion of the reaction product generated by the reduction reaction of carbon dioxide. Further, Patent Document 1 does not mention selectively generating methane.

[0018] Hereinafter, first, each component of the carbon dioxide electrolysis cell used in the method for producing methane according to the present disclosure will be described, and then the steps included in the method for producing methane according to the present disclosure will be described in detail.

[0019] 〔Carbon dioxide electrolysis cell〕 The method for producing methane according to the present disclosure is a method using a carbon dioxide electrolysis cell including a carbon dioxide reduction electrode and an electrolyte. In the carbon dioxide electrolysis cell, carbon dioxide is electrolyzed together with water. In the method for producing methane according to the present disclosure, methane is produced by performing electrolytic reduction of carbon dioxide using the carbon dioxide electrolysis cell. The carbon dioxide electrolytic cell used in the methane production method of this disclosure comprises a carbon dioxide reduction electrode (so-called cathode electrode) and an electrolyte. Preferably, the carbon dioxide electrolytic cell further comprises an anode electrode.

[0020] <Carbon dioxide reduction electrode> The carbon dioxide electrolytic cell used in the methane production method of this disclosure is equipped with a carbon dioxide reduction electrode, the carbon dioxide reduction electrode having a cathode gas diffusion layer and a cathode catalyst layer that has been treated with a water-repellent coating in that order.

[0021] (Cathode gas diffusion layer) The cathode gas diffusion layer can be made of materials used as cathode gas diffusion layers in known carbon dioxide reduction electrodes. For example, the cathode gas diffusion layer can be made of a material that is conductive and allows fluids (e.g., gases and liquids; the same applies hereinafter) to flow through it. Examples of such materials include porous bodies, powder sintered bodies, and fiber sintered bodies made of conductive materials. Conductive fibers are preferred as the conductive material. Specific examples of conductive fibers include carbon fibers and titanium fibers. The carbon fibers and titanium fibers may both be sintered bodies. Carbon fibers are preferred as the conductive fibers. Graphite fibers are preferred as the carbon fibers.

[0022] When the cathode gas diffusion layer is a layer formed of conductive fibers (also called the "conductive fiber layer"), the porosity of the conductive fiber layer is not particularly limited, but is preferably 30% to 80%, and more preferably 30% to 60%.

[0023] The density of the conductive fiber layer is not particularly limited, but for example, 0.10 g / cm³ 3 ~1.00g / cm 3 It is preferable that this be the case.

[0024] The density of a conductive fiber layer is a value determined from the mass per unit area and the thickness.

[0025] The thickness of the conductive fiber layer is not particularly limited, but is preferably 100 μm to 300 μm, and more preferably 100 μm to 250 μm.

[0026] The thickness of the conductive fiber layer refers to the average thickness of the conductive fiber layer. The average thickness of the conductive fiber layer is determined by the following method. The cross-section of the conductive fiber layer is observed using a scanning electron microscope (SEM). The thickness is measured at six arbitrarily selected locations in the thickness direction of the conductive fiber layer. The arithmetic mean of the measured values ​​is calculated, and the resulting value is taken as the average thickness of the conductive fiber layer.

[0027] The cathode gas diffusion layer preferably comprises a conductive fiber layer and a microporous layer (MPL) provided on the conductive fiber layer and formed of conductive particles. In an embodiment where the cathode gas diffusion layer has MPL on a conductive fiber layer, the number of contact points between CO2 gas, cathode catalyst, and electrolyte increases, and the number of three-phase interfaces that form the reaction field increases. As a result, the reduction reaction of carbon dioxide proceeds more efficiently, and methane tends to be produced more selectively and efficiently. The MPL is preferably provided on one surface of the conductive fiber layer.

[0028] The conductive particles are not particularly limited as long as they contain a conductive material. Examples of conductive particles include carbon particles and conductive metal particles. Examples of conductive metal particles include titanium particles, copper particles, silver particles, and platinum particles. Carbon particles are preferred as the conductive particles. Graphite particles are preferred as the carbon particles.

[0029] The shape of the conductive particles is not particularly limited. The shape of the conductive particles may be, for example, spherical (e.g., perfectly spherical and ellipsoidal), plate-like, or irregular in shape.

[0030] The size of the conductive particles is not particularly limited. The particle size of the conductive particles is preferably 50 nm or larger, for example, from the viewpoint of forming pores of a size that allows fluid to flow smoothly. In one embodiment, the particle size of the conductive particles may be 50 nm to 500 nm, or 50 nm to 200 nm. Here, the particle diameter of the conductive particles refers to the average primary particle diameter of the conductive particles. The average primary particle diameter of the conductive particles is a value obtained by image analysis of the primary particles of the conductive particles, which is acquired by observing the surface of the MPL using a scanning electron microscope (SEM).

[0031] The shape of the pores in the porous material MPL is not particularly limited. The shape of the hole may be, for example, circular (e.g., perfectly circular and elliptical), rectangular, or irregular.

[0032] The size of the pores in a porous material like MPL (also called the "pore diameter of MPL") is not particularly limited, as long as fluid flow is possible. The pore size of the MPL is preferably 0.01 μm or larger, and more preferably 0.1 μm or larger. Furthermore, the pore size of the MPL is preferably 3.0 μm or smaller, and more preferably 1.0 μm or smaller. In one embodiment, the pore size of the MPL may be 0.01 μm to 3.0 μm, or 0.1 μm to 1.0 μm. Here, the pore diameter of the MPL refers to the average pore diameter of the MPL. The average pore diameter of the MPL is a value obtained by image analysis of the pores in the MPL, which is acquired by observing the cross-section in the thickness direction of the MPL using a scanning electron microscope (SEM).

[0033] The pore size of the MPL can be controlled, for example, by the size of the conductive particles.

[0034] The thickness of the MPL is not particularly limited, but is preferably 150 μm or less, more preferably 50 μm to 150 μm, and even more preferably 50 μm to 100 μm.

[0035] The thickness of the MPL refers to the average thickness of the MPL. The average thickness of MPL is a value that can be determined by the following method. The cross-section of the MPL is observed using a scanning electron microscope (SEM). The thickness is measured at six arbitrarily selected points in the thickness direction of the MPL. The arithmetic mean of the measured values ​​is calculated, and the resulting value is taken as the average thickness of the MPL.

[0036] Commercially available carbon paper may be used for the cathode gas diffusion layer. An example of a commercially available carbon paper that can function as a cathode gas diffusion layer is SIGRACET 28BC (trade name, carbon paper with a microporous layer (MPL) made of graphite particles on a sintered body of graphite fibers, thickness: 235 μm, density: 0.45 g / cm³). 3 [Manufactured by SGL CARBON], and Toray Paper TGP-H-060 [product name, thickness: 190 μm, density: 0.44 g / cm³]. 3 Examples include products manufactured by Toray Industries, Inc.

[0037] (Water-repellent treated cathode catalyst layer) The cathode catalyst layer is a layer containing a cathode catalyst and is treated with a water-repellent coating. A cathode catalyst accelerates the reduction reaction of carbon dioxide. The type of cathode catalyst is not particularly limited, as long as it can promote the reduction reaction of carbon dioxide. Examples of cathode catalysts include metals or alloys such as Cu, Ag, Zn, Sn, and Al. Preferably, the cathode catalyst is in the form of particles of these metals or alloys. The cathode catalyst layer is preferably a layer containing Cu particles, for example, from the viewpoint of methane production efficiency. In this disclosure, the layer containing Cu particles is also referred to as the "Cu particle layer". The Cu particle layer is preferably a layer made up of Cu particles.

[0038] When the cathode catalyst is made of Cu particles, the particle size of the Cu particles is not particularly limited and may be, for example, 1 nm to 100 nm, 5 nm to 50 nm, or 5 nm to 10 nm. Here, the particle diameter of the Cu particles refers to the average primary particle diameter of the Cu particles. The average primary particle diameter of the Cu particles is a value obtained by image analysis of the primary particles of Cu particles, which is obtained by observing the surface of the cathode catalyst layer using a scanning transmission electron microscope (STEM).

[0039] The water-repellent treatment applied to the cathode catalyst layer is not particularly limited. For the water-repellent treatment, known water-repellent treatments can be applied. One example of a water-repellent treatment is the formation of a water-repellent film. Examples of water-repellent films include fluororesin films such as polytetrafluoroethylene (PTFE). The water-repellent treatment applied to the cathode catalyst layer may be applied to the entire cathode catalyst layer or to a part of it, but it is preferable that it be applied to the entire layer.

[0040] In the case where the cathode gas diffusion layer has an MPL on a conductive fiber layer, it is preferable that the cathode catalyst layer is provided on the MPL-side surface of the cathode gas diffusion layer. When the cathode catalyst layer is located on the MPL-side surface of the cathode gas diffusion layer, the number of contact points between CO2 gas, the cathode catalyst, and the electrolyte increases, and the number of three-phase interfaces that form the reaction field increases. As a result, the reduction reaction of carbon dioxide proceeds more efficiently, and methane tends to be produced more selectively and efficiently.

[0041] The thickness of the cathode catalyst layer is not particularly limited, but is preferably, for example, 200 nm to 350 nm.

[0042] The thickness of the cathode catalyst layer refers to the average thickness of the cathode catalyst layer. The average thickness of the cathode catalyst layer is determined by the following method. The cross-section of the cathode catalyst layer is observed using a scanning electron microscope (SEM). The thickness is measured at six arbitrarily selected locations in the thickness direction of the cathode catalyst layer. The arithmetic mean of the measured values ​​is calculated, and the resulting value is taken as the average thickness of the cathode catalyst layer.

[0043] <<Manufacturing Method for Carbon Dioxide Reduction Electrodes>> The method for manufacturing a carbon dioxide reduction electrode is not particularly limited. Carbon dioxide reduction electrodes can be manufactured by known methods. A preferred method for manufacturing a carbon dioxide reduction electrode (also referred to as "manufacturing method X") will be explained using as an example a case in which the cathode gas diffusion layer has MPL on a conductive fiber layer and the cathode catalyst layer is a layer containing Cu particles as a cathode catalyst. In the explanation of manufacturing method X, explanations of matters that are common to those already explained in the section on carbon dioxide reduction electrodes will be omitted.

[0044] The manufacturing method X includes a step a of preparing a cathode gas diffusion layer, a step b of forming a cathode catalyst layer P by depositing Cu particles on the MPL-side surface of the prepared cathode gas diffusion layer using an arc plasma method, and a step c of obtaining a water-repellent cathode catalyst layer Q by applying a water-repellent treatment to the formed cathode catalyst layer P. Manufacturing method X may include processes other than processes a, b, and c (so-called other processes).

[0045] -Process a- Step a is the step of preparing the cathode gas diffusion layer. "Preparing the cathode gas diffusion layer" means making the cathode gas diffusion layer usable, and unless otherwise specified, includes manufacturing the cathode gas diffusion layer. That is, step a may be a step of preparing a pre-fabricated cathode gas diffusion layer, or it may be a step of manufacturing the cathode gas diffusion layer.

[0046] The method for fabricating the cathode gas diffusion layer is not particularly limited. The cathode gas diffusion layer can be fabricated by known methods. Commercially available carbon paper may be used for the cathode gas diffusion layer.

[0047] -Process b- Step b is a step in which Cu particles are attached to the MPL-side surface of the cathode gas diffusion layer prepared in step a by the arc plasma method to form a cathode catalyst layer P. According to step b, a cathode catalyst layer P, which is a layer of Cu particles, is formed. By forming a Cu particle layer using the arc plasma method, it is possible to obtain a carbon dioxide reduction electrode that can produce methane more efficiently. The arc plasma method is a gas-phase method that generates metal nanoparticles by vaporizing a metal through a high-temperature arc discharge between two electrodes. In the arc plasma method, the metal evaporates and vaporizes due to the plasma generated between the two electrodes, and the resulting metal vapor cools while reacting with the ambient gas, growing into nano-sized particles. Therefore, the arc plasma method allows for the formation of a Cu particle layer with nano-sized Cu particles attached to the MPL-side surface of the cathode gas diffusion layer. The smaller the particle size of the Cu particles forming the Cu particle layer, the more contact points there are with the CO2 gas. Therefore, when a Cu particle layer with nano-sized Cu particles attached is provided on the MPL-side surface of the cathode gas diffusion layer, the number of contact points between the CO2 gas, the Cu particles (which act as the cathode catalyst), and the electrolyte increases, and the number of three-phase interfaces that form the reaction field increases. This allows the reduction reaction of carbon dioxide to proceed more efficiently, and tends to enable more selective and efficient methane production.

[0048] The arc plasma conditions are not particularly limited. The applied voltage is preferably 80V to 150V, and more preferably 110V to 120V. For example, a discharge cycle of 50 to 1000 times is preferable. For example, a capacitance of 300μF to 1500μF is preferred. A pulse frequency of, for example, 0.2 Hz to 10 Hz is preferred. A preferred example of arc plasma conditions is an applied voltage of 120V, a discharge cycle of 500 times, a capacitor capacitance of 1080μF, and a pulse frequency of 1Hz. As the arc plasma apparatus, the arc plasma nanoparticle formation apparatus (model: APD-1S-C) manufactured by Advance Engineering Co., Ltd. can be suitably used. However, the arc plasma apparatus is not limited to this.

[0049] -Process c- Step c is a step in which a water-repellent treatment is applied to the cathode catalyst layer P formed in step b to obtain a water-repellent treated cathode catalyst layer Q. The method for applying the water-repellent treatment to the cathode catalyst layer P is not particularly limited. A known water-repellent treatment can be applied to the cathode catalyst layer P. One method for applying a water-repellent treatment to the cathode catalyst layer P is to form a water-repellent film of a fluororesin such as polytetrafluoroethylene (PTFE) on all or part of the cathode catalyst layer P formed in step b. A water-repellent film of fluororesin can be formed on the cathode catalyst layer by, for example, applying a liquid containing fluororesin to the cathode catalyst layer and then drying it. The method for applying a liquid containing fluororesin to the cathode catalyst layer is not particularly limited, but one example is the spray coating method. As a specific method for applying a water-repellent treatment to the cathode catalyst layer, the method described in the Examples section below is preferred. Other methods for applying a water-repellent treatment to the cathode catalyst layer include, for example, using particles with a water-repellent treatment applied to their surface as the cathode catalyst.

[0050] <Electrolytes> The electrolyte can be selected from known ion-exchange membrane type electrolytes used in carbon dioxide electrolytic cells. The ion-exchange membrane type electrolyte may have properties that selectively permeate cations, or it may have properties that selectively permeate anions. Examples of electrolytes include polymer electrolyte membranes (PEMs). The electrolyte may be, for example, a cation-conducting membrane or an anion-conducting membrane.

[0051] Commercially available electrolytes can be used. Examples of commercially available electrolytes include "Nafion" (registered trademark) [manufactured by Chemours K.K.], "Flemion" (registered trademark) [manufactured by AGC Inc.], "Neosepta" (registered trademark) [manufactured by Atoms Co., Ltd.], "Ceremion" (registered trademark) [manufactured by AGC Inc.], and "Sustainion" (registered trademark) [manufactured by Dioxide Materials Inc.].

[0052] <Anode electrode> The anode electrode is not particularly limited as long as it is made of a material capable of oxidizing water to produce oxygen and hydrogen ions, and can be selected from known anode electrodes used in carbon dioxide electrolytic cells, for example.

[0053] Examples of materials for the anode electrode include metals such as iridium, platinum, palladium, and nickel; alloys containing these metals; intermetallic compounds containing these metals; binary metal oxides such as iridium oxide, manganese oxide, nickel oxide, cobalt oxide, iron oxide, tin oxide, indium oxide, and ruthenium oxide; ternary metal oxides such as Ni-Co-O, Ni-Fe-O, La-Co-O, Ni-La-O, and Sr-Fe-O; quaternary metal oxides such as Pb-Ru-Ir-O and La-Sr-Co-O; and metal complexes such as Ru complexes and Fe complexes. The anode electrode may be, for example, a composite electrode formed by laminating these materials on a substrate. Various shapes can be applied to the anode electrode, such as mesh, wire, particulate, porous, thin film, and island shapes.

[0054] <<Methods for producing methane>> The method for producing methane according to the present disclosure includes step A, in which carbon dioxide is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode described above from the cathode gas diffusion layer side and water is supplied from the electrolyte side, under a temperature environment of above 0°C and below 50°C, and electricity is applied. The methane production method of this disclosure may include steps other than step A (so-called other steps).

[0055] -Process A- Step A is a process in which, under a temperature environment between 0°C and 50°C, carbon dioxide is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode described above from the cathode gas diffusion layer side and water is supplied from the electrolyte side, and electricity is applied. In process A, at least methane is produced. Other products besides methane in step A include, for example, hydrogen, carbon monoxide, and ethylene.

[0056] When electricity is applied to the cathode catalyst layer of the carbon dioxide reduction electrode, carbon dioxide is supplied from the cathode gas diffusion layer side and water is supplied from the electrolyte side, and the ambient temperature is above 0°C and below 50°C. When the ambient temperature exceeds 0°C, freezing of the electrolyte can be suppressed. When the ambient temperature is below 50°C, the rate of methane production tends to increase. This is presumably because the suppression of hydrogen production improves the energy conversion efficiency to methane. In step A, it is preferable to supply carbon dioxide from the cathode gas diffusion layer side and water from the electrolyte side to the cathode catalyst layer of the carbon dioxide reduction electrode in a temperature environment of 0°C to 40°C and to apply electricity; more preferably to supply carbon dioxide from the cathode gas diffusion layer side and water from the electrolyte side to the carbon dioxide reduction electrode in a temperature environment of 0°C to 35°C and to apply electricity; even more preferably to supply carbon dioxide from the cathode gas diffusion layer side and water from the electrolyte side to the carbon dioxide reduction electrode in a temperature environment of 0°C to 30°C and to apply electricity; and particularly preferably to supply carbon dioxide from the cathode gas diffusion layer side and water from the electrolyte side to the carbon dioxide reduction electrode in a temperature environment of 0°C to 25°C and to apply electricity.

[0057] Examples of water supplied from the electrolyte side include electrolyte solution. The electrolyte is not particularly limited, and any known electrolyte can be used. Specific examples of electrolytes include aqueous solutions of potassium hydroxide (KOH), potassium bicarbonate (KHCO3), and potassium sulfate (K2SO4). The concentration of the electrolyte is not particularly limited, and can range from, for example, 0.1 mol / L (liters; the same applies hereafter) to 5 mol / L.

[0058] The flow rate of carbon dioxide (CO2 gas) supplied to the carbon dioxide reduction electrode is not particularly limited, but is preferably, for example, 1 mL / min to 10 mL / min.

[0059] The method of applying electricity to the carbon dioxide reduction electrode is not particularly limited, but it is preferable to apply a voltage to the cathode gas diffusion layer side such that the potential between the working electrode and the reference electrode is -0.9V to -3.0V. [Examples]

[0060] The present disclosure will be described in detail below with reference to examples. However, the present disclosure is not limited to the following examples. The matters shown in the following examples may be modified as appropriate without departing from the spirit of the present disclosure.

[0061] [Fabrication of carbon dioxide reduction electrodes] [Manufacturing example 1A] Carbon paper (CP) [Product name: SIGRACET® 28BC, Density: 0.45 g / cm³] 3 A CP (carbon fiber composite) with a thickness of 235 μm, manufactured by SGL CARBON, was cut into 5 cm squares and used as a cathode gas diffusion layer. The CP has a structure in which a microporous layer (MPL), which is a porous body formed of graphite particles, is provided on a conductive fiber layer made of a sintered carbon fiber body. Next, Cu particles were deposited on the MPL side surface of the cut-out CP using an arc plasma device (product name: Arc Plasma Nanoparticle Forming Device, model: APD-1S-C, manufactured by Advance Riko Co., Ltd.) by the arc plasma method. Specifically, the cut-out CP was placed in the arc plasma device using imide tape so that the MPL side surface was exposed, and then Cu particles were generated by evaporating and vaporizing Cu in a vacuum, and the generated Cu particles were deposited on the MPL side surface of the CP. The arc plasma conditions were set to a voltage of 120 V, a capacitor capacity of 1080 μF, and a discharge cycle of 500 times. As described above, a laminate having a layer structure of a cathode gas diffusion layer (conductive fiber layer / MPL) and a cathode catalyst layer (Cu particle layer) was first fabricated.

[0062] Next, the laminate was cut into 2.2 cm squares. Then, polytetrafluoroethylene (PTFE) [product name: Fluoro-Placoat, product number: FC-115, manufactured by Fine Chemical Japan Co., Ltd.] was spray-coated onto both sides of the cut laminate, and then dried at room temperature (25°C) for 15 minutes. In this manner, the carbon dioxide reduction electrode of Production Example 1A was fabricated. The carbon dioxide reduction electrode of Production Example 1A has a layer structure consisting of a cathode gas diffusion layer (conductive fiber layer / MPL) and a cathode catalyst layer (Cu particle layer), and both sides of the cathode catalyst layer are treated with a water-repellent coating using PTFE.

[0063] [Production example 2A] A laminate was fabricated using the same procedure as in Manufacturing Example 1A. The fabricated laminate was cut into 2.2 cm squares. These cut-out laminates were used as the carbon dioxide reduction electrode in manufacturing example 2A.

[0064] [Fabrication of electrode bodies] [Manufacturing example 1B] An electrode body was fabricated using the carbon dioxide reduction electrode and electrolyte from Manufacturing Example 1A. First, the following pretreatment was performed on the electrolytes. An electrolyte (product name: Sustainion® X37-50 Grade 60 Membrane, anion exchange membrane, thickness: 50 μm, manufactured by Dioxide Materials Inc.) was immersed in an electrolyte solution (product name: 1 mol / L KOH solution, manufactured by Fujifilm Wako Pure Chemical Industries, Ltd.) for 24 hours. The electrolyte was removed from the electrolyte solution and its surface was washed with pure water.

[0065] Next, the pre-treated electrolyte and the carbon dioxide reduction electrode of Production Example 1A were superimposed so that the electrolyte and the Cu particle layer side of the cathode catalyst layer of the carbon dioxide reduction electrode of Production Example 1A were in contact. Then, the electrodes were press-molded at room temperature (25°C) using a press machine [product name: small hot press machine, model: H300-05, manufactured by AS ONE Corporation; the same applies hereinafter]. In this manner, the electrode body of Production Example 1B was manufactured. The electrode body of Production Example 1B has a layer structure of carbon dioxide reduction electrode of Production Example 1A [cathode gas diffusion layer (conductive fiber layer / MPL) / water-repellent treated cathode catalyst layer] / electrolyte.

[0066] The pressing conditions were a pressing pressure of 5 MPa and a pressing time of 5 minutes. In addition, to maintain the electrolyte in a moist state, Kimwipes (registered trademark) moistened with pure water were placed in contact with the side of the electrolyte that was not overlapping with the carbon dioxide reduction electrode during press molding. The following manufacturing example 2B was also manufactured using the same pressing conditions and press molding process.

[0067] [Manufacturing example 2B] Similar to manufacturing example 1B, the electrolyte was first subjected to the above pretreatment. Next, the pre-treated electrolyte and the carbon dioxide reduction electrode of Production Example 2A were superimposed so that the electrolyte and the Cu particle layer side of the cathode catalyst layer of the carbon dioxide reduction electrode of Production Example 2A were in contact, and then press-molded using a press machine at room temperature (25°C). In this manner, the electrode body of Production Example 2B was manufactured. The electrode body of Production Example 2B has a layer structure of carbon dioxide reduction electrode of Production Example 2A [cathode gas diffusion layer (conductive fiber layer / MPL) / cathode catalyst layer without water-repellent treatment] / electrolyte.

[0068] [Evaluation of methane selectivity] 1. Presence or absence of water-repellent treatment <Example 1> The electrode assembly from Manufacturing Example 1B was incorporated into a gas diffusion type half-cell to produce the carbon dioxide electrolytic cell 100 shown in Figure 1. As shown in Figure 1, the gas diffusion type half-cell has a structure that allows the electrolyte 20 to be stored on one side of the electrode assembly (the side with the electrolyte 10) and CO2 gas to be supplied to the other side of the electrode assembly (the side with the cathode gas diffusion layer of the carbon dioxide reduction electrode 30). A Cu electrode was used for the working electrode (WE) 40, an Ag / AgCl electrode for the reference electrode (RE) 50, and a carbon rod for the counter electrode (CE) 60. A 1 mol / L KHCO3 aqueous solution was used for the electrolyte 20. The KHCO3 aqueous solution was pre-treated by bubbling CO2 gas (purity: 99.99%) through it and then adjusting the pH to 7.9 before use.

[0069] Using the carbon dioxide electrolytic cell manufactured as described above, CO2 gas (purity: 99.99%) was flowed at a flow rate of 5 mL / min through the cathode gas diffusion layer of the carbon dioxide reduction electrode at a temperature of 25°C to replace the line with CO2 gas. Next, a voltage was applied to the cathode gas diffusion layer of the carbon dioxide reduction electrode, with the Ag / AgCl electrode acting as the reference electrode, so that the potential between the working electrode and the reference electrode was -2.0V. After starting the application of voltage and confirming that current was flowing, the flow rate of the generated gas was measured for 1.5 to 2 minutes using a flow meter (product name: Defender® 530+, manufactured by Mesa Labs) connected to the exhaust line of a gas chromatograph (product name: Nexis® GC-2030, manufactured by Shimadzu Corporation). After measuring the flow rate of the generated gas, the composition of the generated gas was analyzed using a gas chromatograph in the flow path. The generated gas was continuously circulated through the gas chromatograph's flow path, and the flow path was switched at predetermined intervals to introduce the generated gas into the column (product name: MICROPACKED-ST, manufactured by Shinwa Chemical Co., Ltd.) for analysis.

[0070] Based on the measured flow rate of the generated gases and the analysis of their composition, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the converted electric charge to the actual current flow. The composition of the generated gases and the Faraday efficiency of each component are shown in Table 1 and Figure 2.

[0071] Faraday efficiency indicates the proportion of the total current used for the reaction of the product. A higher Faraday efficiency (in %) for methane indicates a more methane-selective method.

[0072] <Example 2> Except for applying a voltage such that the potential between the working electrode and the reference electrode was -2.5V, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 1. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 1 and Figure 2.

[0073] <Comparative Example 1> A carbon dioxide electrolytic cell was manufactured in the same manner as in Example 1, except that the electrode body incorporated into the gas diffusion type half-cell was changed from the electrode body of Manufacturing Example 1B to the electrode body of Manufacturing Example 2B. The flow rate of the generated gas and the composition of the generated gas were then measured and analyzed. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 1 and Figure 2.

[0074] <Comparative Example 2> Except for changing the electrode body incorporated into the gas diffusion type half cell from the electrode body of Manufacturing Example 1B to the electrode body of Manufacturing Example 2B, and applying a voltage so that the potential between the working electrode and the reference electrode was -2.5V, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 1. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 1 and Figure 2.

[0075] The methane production methods of Example 1 and Example 2 both use a carbon dioxide electrolytic cell equipped with a carbon dioxide reduction electrode and an electrolyte, wherein the carbon dioxide reduction electrode has a cathode gas diffusion layer and a cathode catalyst layer that has been treated with a water-repellent coating in that order, and the method includes a step (i.e., step A) in which carbon dioxide (specifically CO2 gas) is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode from the cathode gas diffusion layer side and water is supplied from the electrolyte side, and electricity is applied to it, under a temperature environment of 25°C. On the other hand, the methane production methods in Comparative Examples 1 and 2 differ from those in Examples 1 and 2, respectively, in that the cathode catalyst layer is not treated with a water-repellent coating. Furthermore, the potential between the working electrode and the reference electrode differs between Examples 1 and 2, and between Comparative Examples 1 and 2. The results shown in Table 1 and Figure 2 clearly demonstrate that the methane production methods of Example 1 and Example 2 exhibit superior methane selectivity compared to the methane production methods of Comparative Example 1 and Comparative Example 2, respectively.

[0076] 2.Temperature dependence <Example 3> In a temperature environment of 5°C, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 1, except that CO2 gas (purity: 99.99%) was flowed through the cathode gas diffusion layer side of the carbon dioxide reduction electrode. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 2 and Figure 3.

[0077] <Example 4> In a temperature environment of 40°C, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 1, except that CO2 gas (purity: 99.99%) was flowed through the cathode gas diffusion layer side of the carbon dioxide reduction electrode. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 2 and Figure 3.

[0078] <Comparative Example 3> In a 60°C temperature environment, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 1, except that CO2 gas (purity: 99.99%) was flowed through the cathode gas diffusion layer side of the carbon dioxide reduction electrode. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 2 and Figure 3.

[0079] <Example 5> In a temperature environment of 5°C, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 2, except that CO2 gas (purity: 99.99%) was flowed through the cathode gas diffusion layer side of the carbon dioxide reduction electrode. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 2 and Figure 3.

[0080] <Example 6> In a temperature environment of 40°C, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 2, except that CO2 gas (purity: 99.99%) was flowed through the cathode gas diffusion layer side of the carbon dioxide reduction electrode. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 2 and Figure 3.

[0081] <Comparative Example 4> In a temperature environment of 60°C, the flow rate of the generated gas and the composition of the generated gas were measured and analyzed in the same manner as in Example 2, except that CO2 gas (purity: 99.99%) was flowed through the cathode gas diffusion layer side of the carbon dioxide reduction electrode. Similar to Example 1, the amounts of hydrogen (H2), carbon monoxide (CO), methane (CH4), and ethylene (C2H4) produced were calculated from the measured flow rate of the produced gas and the analysis results of the composition of the produced gas, and converted into electric charge. The production efficiency (so-called Faraday efficiency) of hydrogen, carbon monoxide, methane, and ethylene was determined from the ratio of the electric charge obtained by the conversion to the actual current value that flowed. The composition of the produced gas and the Faraday efficiency of each component are shown in Table 2 and Figure 3.

[0082] [Table 1]

[0083] [Table 2]

[0084] Examples 1 and 2 in Table 2 are included for comparison with the other examples listed in Table 2, and are the same examples as Examples 1 and 2 in Table 1.

[0085] The methane production methods of Examples 1 to 6, Comparative Example 3, and Comparative Example 4 all utilize a carbon dioxide electrolytic cell equipped with a carbon dioxide reduction electrode and an electrolyte, wherein the carbon dioxide reduction electrode has a cathode gas diffusion layer and a cathode catalyst layer treated with a water-repellent coating in that order. Examples 1, 3, 4, and Comparative Example 3, and Examples 2, 5, 6, and Comparative Example 4 differ in the temperature at which electricity is applied when carbon dioxide is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode from the cathode gas diffusion layer side and water is supplied from the electrolyte side. Also, the potential between the working electrode and the reference electrode differs between Examples 1, 3, 4, and Comparative Example 3 and Examples 2, 5, 6, and Comparative Example 4. The results shown in Table 2 and Figure 3 clearly demonstrate that the methane production methods of Examples 1, 3, and 4 exhibit superior methane selectivity compared to the methane production method of Comparative Example 3, and that the methane production methods of Examples 2, 5, and 6 exhibit superior methane selectivity compared to the methane production method of Comparative Example 4. [Explanation of Symbols]

[0086] 10: Electrolyte (electrolyte membrane), 20: Electrolyte solution, 30: Carbon dioxide reduction electrode, 40: Working electrode, 50: Reference electrode, 60: Counter electrode, 100: Carbon dioxide electrolytic cell

Claims

1. A method for producing methane using a carbon dioxide electrolytic cell equipped with a carbon dioxide reduction electrode and an electrolyte, The carbon dioxide reduction electrode has a cathode gas diffusion layer and a cathode catalyst layer that has been treated with a water-repellent coating, in that order. A method for producing methane, comprising step A, in which carbon dioxide is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode from the cathode gas diffusion layer side and water is supplied from the electrolyte side, in a temperature environment of above 0°C and below 50°C, and electricity is applied.

2. The method for producing methane according to claim 1, wherein in step A, carbon dioxide is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode from the cathode gas diffusion layer side and water is supplied from the electrolyte side, and electricity is applied in a temperature environment of 0°C to 30°C or less.

3. The method for producing methane according to claim 1 or claim 2, wherein the carbon dioxide electrolytic cell further comprises an anode electrode.

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