How methane is produced

Abstract

A method for producing methane with excellent methane selectivity is provided. [Solution] A method for producing methane using a carbon dioxide electrolysis cell including a carbon dioxide reduction electrode and an electrolyte, wherein the carbon dioxide reduction electrode has, in that order, a cathode gas diffusion layer and a cathode catalyst layer that has been treated to be water repellent, 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 exceeding 0°C and not exceeding 50°C, and applying electricity.

Description

[Technical Field]

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

[0002] Methane (CH4) can be synthesized by applying electricity to carbon dioxide (CO2) and water (H2O) to promote the reduction of carbon dioxide over a catalyst. Various techniques have been reported for controlling the selection and promotion of reaction products produced by the reduction of carbon dioxide.

[0003] For example, Patent Document 1 discloses a technology for controlling the selection and promotion of reaction products produced 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] International Publication No. 2017 / 169682 Summary of the Invention [Problem to be solved by the invention]

[0005] In a method for producing methane by the reduction of carbon dioxide using a carbon dioxide electrolysis cell equipped with a carbon dioxide reduction electrode and an electrolyte, the methane production reaction occurs on the cathode side. However, in addition to methane, components such as hydrogen, carbon monoxide (CO), and ethylene (C2H4) are also produced as by-products. The production of these by-products results in excess energy consumption, reducing the efficiency of energy conversion to methane. For example, water present on the catalyst induces a hydrogen production reaction in addition to the carbon dioxide reduction reaction. Therefore, the presence of excess water on the catalyst can promote hydrogen production and reduce the rate of methane production. Therefore, there is a need for a method for producing methane by the reduction of carbon dioxide that can suppress hydrogen production at the carbon dioxide reduction electrode and improve methane selectivity.

[0006] The present disclosure has been made in consideration of the above circumstances. An object of one embodiment of the present disclosure is to provide a method for producing methane with excellent methane selectivity. [Means for solving the problem]

[0007] Specific means for solving the problems include the following aspects. <1> 1. A method for producing methane using a carbon dioxide electrolysis cell comprising a carbon dioxide reduction electrode and an electrolyte, the carbon dioxide reduction electrode has, in this order, a cathode gas diffusion layer and a cathode catalyst layer that has been subjected to a water-repellent treatment; a 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 exceeding 0°C and not exceeding 50°C, and applying electricity. <2> In the 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 to the cathode catalyst layer from the electrolyte side in a temperature environment of more than 0°C and not more than 30°C, and electricity is applied. <1> The method for producing methane according to claim 1. <3> the carbon dioxide electrolysis cell further comprises an anode electrode; <1> or <2> The method for producing methane according to claim 1. [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 drawings]

[0009] [Figure 1] FIG. 1 is a schematic diagram of a carbon dioxide electrolysis cell used in the evaluation tests of the examples. [Figure 2] FIG. 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 and 2 and Comparative Examples 1 and 2. [Figure 3] FIG. 3 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 to 6 and Comparative Examples 3 and 4. DETAILED DESCRIPTION OF THE INVENTION

[0010] Hereinafter, embodiments of the present disclosure will be described in detail. The present disclosure is not limited to the following embodiments, and can be implemented by making appropriate modifications within the scope of the object of the present disclosure. The dimensional ratios in the drawings do not necessarily represent the actual dimensional ratios.

[0011] In the present disclosure, a numerical range expressed using "to" means a range that includes the numerical values ​​before and after "to" as the lower and upper limits.

[0012] In the numerical ranges described in stages in the present disclosure, the upper limit value described in one numerical range may be replaced with the upper limit value of another numerical range described in stages, and the lower limit value described in one numerical range may be replaced with the lower limit value of another numerical range described in stages. In the numerical ranges described in stages in the present disclosure, the upper or lower limit value described in one numerical range may be replaced with a value shown in the examples.

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

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

[0015] [How methane is produced] The methane production method disclosed herein is a methane production method using a carbon dioxide electrolysis cell including a carbon dioxide reduction electrode and an electrolyte, wherein the carbon dioxide reduction electrode has, in this order, a cathode gas diffusion layer and a cathode catalyst layer that has been subjected to a water-repellent treatment, and includes a 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 exceeding 0°C and not exceeding 50°C, and applying electricity. The methane production method of the present disclosure has excellent methane selectivity. Although it is unclear why the methane production method of the present disclosure can achieve such effects, the inventors speculate as follows: However, the following speculation is not intended to limit the methane production method of the present disclosure, but is provided as an example.

[0016] The methane production method disclosed herein uses a carbon dioxide electrolysis cell equipped with a carbon dioxide reduction electrode and an electrolyte. The carbon dioxide reduction electrode included in the carbon dioxide electrolysis cell used in the methane production method disclosed herein has a cathode gas diffusion layer and a cathode catalyst layer, in this order, with the cathode catalyst layer treated to be water-repellent. The presence of excess water on the catalyst promotes hydrogen production, which tends to result in excessive energy consumption. However, the cathode catalyst layer included in the carbon dioxide reduction electrode used in the methane production method disclosed herein is treated to be water-repellent, allowing water to be appropriately removed from the catalyst. This is thought to suppress hydrogen production and reduce the energy consumption required for methane production. Furthermore, it is believed that performing the carbon dioxide reduction reaction in an environment below a specific temperature further suppresses hydrogen production, thereby improving the efficiency of energy conversion to methane and increasing the methane production rate. Note that temperatures above 0°C tend to suppress freezing of the electrolyte.

[0017] On the other hand, the technology described in Patent Document 1 differs from the technology of the present disclosure in that the catalyst used in the carbon dioxide reduction reaction is not subjected to a water-repellent treatment. Furthermore, the technology described in Patent Document 1 does not pay attention to the carbon dioxide reduction reaction temperature when controlling the selection and promotion of reaction products produced by the carbon dioxide reduction reaction. Furthermore, Patent Document 1 does not mention selective production of methane.

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

[0019] [Carbon dioxide electrolysis cell] The method for producing methane disclosed herein uses 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 disclosed herein, methane is produced by electrolytic reduction of carbon dioxide using the carbon dioxide electrolysis cell. The carbon dioxide electrolysis cell used in the methane production method of the present disclosure includes a carbon dioxide reduction electrode (so-called cathode electrode) and an electrolyte. The carbon dioxide electrolysis cell preferably further includes an anode electrode.

[0020] <Carbon dioxide reduction electrode> The carbon dioxide electrolysis cell used in the methane production method of the present disclosure includes a carbon dioxide reduction electrode, which has, in this order, a cathode gas diffusion layer and a cathode catalyst layer that has been subjected to a water-repellent treatment.

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

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

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

[0024] The density of the 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, for example.

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

[0027] The cathode gas diffusion layer preferably has a conductive fiber layer and a microporous layer (MPL) that is provided on the conductive fiber layer and is a porous body formed from conductive particles. When the cathode gas diffusion layer has an MPL on the conductive fiber layer, the number of contact points between the CO gas, the cathode catalyst, and the electrolyte increases, and the number of three-phase interfaces that serve as reaction sites increases, which allows the carbon dioxide reduction reaction to proceed more efficiently, and tends to enable more selective and efficient production of methane. The MPL is preferably provided on one side of the conductive fibrous layer.

[0028] The conductive particles are not particularly limited as long as they contain a material having conductivity. 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. The conductive particles are preferably carbon particles. As the carbon particles, graphite particles are preferred.

[0029] The shape of the conductive particles is not particularly limited. The shape of the conductive particles may be, for example, spherical (eg, perfect sphere or oval sphere), plate-like, or irregular.

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

[0031] The shape of the pores in the porous MPL is not particularly limited. The shape of the holes may be, for example, circular (for example, perfect circle or ellipse), rectangular, or irregular.

[0032] The size of the pores in the porous MPL (also referred to as the "pore diameter of the MPL") is not particularly limited as long as it allows fluid to pass through. The pore size of the MPL is, for example, preferably 0.01 μm or more, more preferably 0.1 μm or more, and, for example, preferably 3.0 μm or less, more preferably 1.0 μm or less. In one embodiment, the pore size of the MPL may be 0.01 μm to 3.0 μm, or may be 0.1 μm to 1.0 μm. Here, the pore size of MPL means the average pore size of MPL, which is a value determined by image analysis of the pores in MPL obtained by observing a cross section of MPL in the thickness direction 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 the MPL is a value determined by the following method. The cross section of the MPL is observed using a scanning electron microscope (SEM). The thickness of the MPL is measured at six randomly selected locations in the thickness direction. The arithmetic mean of the measured values ​​is calculated, and this value is taken as the average thickness of the MPL.

[0036] The cathode gas diffusion layer may be made of commercially available carbon paper. 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 fiber, which is carbon fiber; thickness: 235 μm; density: 0.45 g / cm). 3 , manufactured by SGL CARBON Co., Ltd.], and Toray Paper TGP-H-060 [trade name, thickness: 190 μm, density: 0.44 g / cm 3 , manufactured by Toray Industries, Inc.

[0037] (Cathode catalyst layer with water-repellent treatment) The cathode catalyst layer is a layer containing a cathode catalyst and is subjected to a water-repellent treatment. The cathode catalyst promotes 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 the cathode catalyst include metals such as Cu, Ag, Zn, Sn, and Al, or alloys thereof. The cathode catalyst is preferably 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 the present disclosure, a layer containing Cu particles is also referred to as a "Cu particle layer." The Cu particle layer is preferably a layer made of Cu particles.

[0038] When the cathode catalyst is 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 size of the Cu particles refers to the average primary particle size of the Cu particles, which is determined by image analysis of the primary particles of the Cu particles 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. The water-repellent treatment can be a known water-repellent treatment. An example of the water-repellent treatment is the formation of a water-repellent film. The water-repellent film may be, for example, a film made of a fluororesin 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 the cathode catalyst layer, but is preferably applied to the entire cathode catalyst layer.

[0040] In an embodiment in which the cathode gas diffusion layer has an MPL on the conductive fiber layer, the cathode catalyst layer is preferably provided on the surface of the cathode gas diffusion layer facing the MPL. When the cathode catalyst layer is provided on the MPL side of the cathode gas diffusion layer, the number of contact points between the CO2 gas, the cathode catalyst, and the electrolyte increases, and the number of three-phase interfaces that serve as reaction sites increases, which makes the carbon dioxide reduction reaction proceed more efficiently and tends to result in more selective and efficient production of methane.

[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 means the average thickness of the cathode catalyst layer. The average thickness of the cathode catalyst layer is a value determined by the following method. The cross section of the cathode catalyst layer is observed using a scanning electron microscope (SEM). The thickness of the cathode catalyst layer is measured at six randomly selected locations in the thickness direction. The arithmetic mean of the measured values ​​is calculated, and this value is used as the average thickness of the cathode catalyst layer.

[0043] <<Method of manufacturing carbon dioxide reduction electrode>> The method for producing the carbon dioxide reduction electrode is not particularly limited. The carbon dioxide reduction electrode can be produced by a known method. A preferred method for producing a carbon dioxide reduction electrode (also referred to as "production method X") will be described below, taking as an example a case in which the cathode gas diffusion layer has an MPL on a conductive fiber layer and the cathode catalyst layer is a layer containing Cu particles as the cathode catalyst. In the description of Production Method X, the description of the matters common to the matters already described in the section on the carbon dioxide reduction electrode will be omitted.

[0044] The manufacturing method X includes the steps of: a) preparing a cathode gas diffusion layer; b) forming a cathode catalyst layer P by adhering Cu particles to the MPL-side surface of the prepared cathode gas diffusion layer by an arc plasma method; and c) providing a water-repellent cathode catalyst layer Q by subjecting the formed cathode catalyst layer P to a water-repellent treatment. The production method X may include steps other than the steps a, b, and c (so-called other steps).

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

[0046] The method for producing the cathode gas diffusion layer is not particularly limited. The cathode gas diffusion layer can be produced by a known method. The cathode gas diffusion layer may be made of commercially available carbon paper.

[0047] -Process b- Step b is a step of forming a cathode catalyst layer P by depositing Cu particles by an arc plasma method on the MPL side surface of the cathode gas diffusion layer prepared in step a. According to step b, a cathode catalyst layer P that is a Cu particle layer is formed. Forming a Cu particle layer using the arc plasma method tends to produce a carbon dioxide reduction electrode that can produce methane more efficiently. The arc plasma method is a gas-phase process that generates metal nanoparticles by vaporizing metals through a high-temperature arc discharge between two electrodes. In the arc plasma method, the metal is evaporated by the plasma generated between the two electrodes. The resulting metal vapor then reacts with the ambient gas and cools, growing into nanoparticles. Therefore, the arc plasma method can form a Cu particle layer with nanosized Cu particles attached to the MPL side of the cathode gas diffusion layer. The smaller the particle size of the Cu particles that make up the Cu particle layer, the more contact points there are with CO2 gas. Therefore, when a Cu particle layer with nanosized Cu particles is attached to the MPL side of the cathode gas diffusion layer, the number of contact points between CO2 gas, the cathode catalyst Cu particles, and the electrolyte increases. This increases the number of three-phase interfaces that serve as reaction sites, which allows for more efficient carbon dioxide reduction, leading to more selective and efficient methane production.

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

[0049] -Process c- Step c is a step of subjecting the cathode catalyst layer P formed in step b to a water-repellent treatment, thereby obtaining a cathode catalyst layer Q that has been subjected to the water-repellent treatment. The method for subjecting the cathode catalyst layer P to water repellency treatment is not particularly limited. The cathode catalyst layer P can be subjected to a known water-repellent treatment. As a method for subjecting the cathode catalyst layer P to water repellency, for example, a method of forming a water repellent film of a fluororesin such as polytetrafluoroethylene (PTFE) on the whole or part of the cathode catalyst layer P formed in step b can be mentioned. The water-repellent film of fluororesin can be formed on the cathode catalyst layer, for example, by applying a liquid containing fluororesin to the cathode catalyst layer and then drying it. The method for applying the liquid containing the fluororesin to the cathode catalyst layer is not particularly limited, but may be, for example, a spray coating method. As a specific method for subjecting the cathode catalyst layer to water repellency treatment, the method described in the Examples section below is suitable. Another method for subjecting the cathode catalyst layer to water repellency treatment is, for example, to a method in which particles whose surfaces have been subjected to a water repellency treatment are used as the cathode catalyst.

[0050] <Electrolytes> The electrolyte can be selected from known ion-exchange membrane-type electrolytes used in carbon dioxide electrolysis cells. The ion-exchange membrane-type electrolyte may have the property of selectively permeating cations or anions. The electrolyte may be, for example, a polymer electrolyte membrane (PEM). The electrolyte may be, for example, a cation-conducting membrane or an anion-conducting membrane.

[0051] As the electrolyte, commercially available products can be used. Examples of commercially available electrolytes include "Nafion" (registered trademark) (manufactured by Chemours Corporation), "Flemion" (registered trademark) (manufactured by AGC Corporation), "Neosepta" (registered trademark) (manufactured by Atoms Corporation), "Selemion" (registered trademark) (manufactured by AGC Corporation), and "Sustainion" (registered trademark) (manufactured by Dioxide Materials Corporation).

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

[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 in which these materials are laminated on a substrate. The anode electrode may have various shapes such as mesh, wire, particle, porous, thin film, and island shapes.

[0054] <<How to produce methane>> The methane production method of the present disclosure includes a 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 described above in a temperature environment exceeding 0°C and not exceeding 50°C, and applying electricity. The methane production method of the present disclosure may include a step other than step A (so-called other step).

[0055] -Process A- Step A is a step 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 described above in a temperature environment exceeding 0°C and not exceeding 50°C, and applying electricity. In step A, at least methane is produced. Products other than methane in step A include, for example, hydrogen, carbon monoxide, and ethylene.

[0056] Carbon dioxide is supplied to the cathode catalyst layer of the carbon dioxide reduction electrode from the cathode gas diffusion layer side, water is supplied from the electrolyte side, and electricity is applied under an ambient temperature of above 0°C and 50°C or lower. When the environmental temperature exceeds 0°C, the freezing of the electrolyte can be suppressed. When the environmental temperature is 50°C or lower, the rate of methane production tends to increase. This is presumably because the suppression of hydrogen production improves the efficiency of energy conversion 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 greater than 0°C and less than 40°C, and apply electricity; it is more preferable 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 greater than 0°C and less than 35°C, and apply electricity; it is even more preferable 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 greater than 0°C and less than 30°C, and apply electricity; and it is particularly preferable 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 greater than 0°C and less than 25°C, and apply electricity.

[0057] The water supplied from the electrolyte side may be, for example, an electrolytic solution. The electrolyte is not particularly limited, and any known electrolyte can be used. Specific examples of the electrolyte include an aqueous solution of potassium hydroxide (KOH), an aqueous solution of potassium bicarbonate (KHCO3), and an aqueous solution of potassium sulfate (K2SO4). The concentration of the electrolyte is not particularly limited, and is, for example, 0.1 mol / L (liter; the same applies hereinafter) 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 for 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 so that the potential between the working electrode and the reference electrode is −0.9V to −3.0V, for example. [Example]

[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 appropriately changed without departing from the spirit of the present disclosure.

[0061] [Preparation of carbon dioxide reduction electrode] [Manufacturing example 1A] Carbon paper (CP) [product name: SIGRACET (registered trademark) 28BC, density: 0.45 g / cm 3 A 5 cm square piece of CP (235 μm thick, manufactured by SGL CARBON) was cut out and used as a cathode gas diffusion layer. The CP consisted of a conductive fiber layer made of sintered carbon fiber and a microporous layer (MPL) made of graphite particles on top of it. Next, Cu particles were deposited on the MPL side of the cut CP by arc plasma deposition using an arc plasma device (product name: Arc Plasma Nanoparticle Formation Device, model: APD-1S-C, manufactured by Advance Riko Co., Ltd.). Specifically, the cut CP was placed in the arc plasma device using imide tape so that the MPL side was exposed. Cu was then evaporated in a vacuum to generate Cu particles, which were then attached to the MPL side of the CP. The arc plasma conditions were set to a voltage of 120 V, a capacitor capacitance of 1080 μF, and 500 discharge cycles. As described above, first, a laminate having a layer structure of cathode gas diffusion layer (conductive fiber layer / MPL) / cathode catalyst layer (Cu particle layer) was produced.

[0062] The laminate was then cut into 2.2 cm square pieces. Polytetrafluoroethylene (PTFE) [product name: Fusso Placoat, product number: FC-115, manufactured by Fine Chemical Japan Co., Ltd.] was then sprayed onto both sides of the cut laminate using a spray coating method, and the laminate was 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 had a layer structure of a cathode gas diffusion layer (conductive fiber layer / MPL) / cathode catalyst layer (Cu particle layer), and both sides of the cathode catalyst layer were treated with a water-repellent PTFE coating.

[0063] [Production example 2A] A laminate was produced in the same manner as in Production Example 1A. The produced laminate was cut into a 2.2 cm square, and this cut laminate was used as the carbon dioxide reduction electrode of Production Example 2A.

[0064] [Preparation of electrode body] [Manufacturing example 1B] An electrode assembly was produced using the carbon dioxide reduction electrode of Production Example 1A and an electrolyte. First, the electrolyte was subjected to the following pretreatment. An electrolyte (trade name: Sustainion (registered trademark) X37-50 Grade 60 Membrane, an anion exchange membrane, thickness: 50 μm, manufactured by Dioxide Materials) was immersed in an electrolyte solution (trade 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 the surface of the electrolyte was washed with pure water.

[0065] Next, the pretreated electrolyte and the carbon dioxide reduction electrode of Production Example 1A were overlapped so that the electrolyte was in contact with the surface of the cathode catalyst layer of the carbon dioxide reduction electrode of Production Example 1A facing the Cu particle layer, and then press-molded at room temperature (25°C) using a press (product name: small heat press, model: H300-05, manufactured by AS ONE Corporation; the same applies hereinafter). In this way, the electrode body of Production Example 1B was produced. The electrode body of Production Example 1B had a layer structure of the carbon dioxide reduction electrode of Production Example 1A [cathode gas diffusion layer (conductive fiber layer / MPL) / cathode catalyst layer subjected to water-repellent treatment] / electrolyte.

[0066] The pressing conditions were a pressure of 5 MPa and a pressing time of 5 minutes. To keep the electrolyte moist, Kimwipes (registered trademark) moistened with pure water was placed in contact with the surface of the electrolyte that was not overlapped with the carbon dioxide reduction electrode during press molding. The following Production Example 2B was also subjected to similar press molding under the same pressing conditions.

[0067] [Manufacturing example 2B] As in Production Example 1B, first, the electrolyte was subjected to the above pretreatment. Next, the pretreated electrolyte and the carbon dioxide reduction electrode of Production Example 2A were overlapped so that the electrolyte was in contact with the Cu particle layer side of the cathode catalyst layer of the carbon dioxide reduction electrode of Production Example 2A, and then press-molded using a press at room temperature (25°C). In this manner, the electrode body of Production Example 2B was produced. The electrode body of Production Example 2B had a layer structure of the carbon dioxide reduction electrode of Production Example 2A [cathode gas diffusion layer (conductive fiber layer / MPL) / cathode catalyst layer not subjected to water-repellent treatment] / electrolyte.

[0068] [Evaluation of methane selectivity] 1. Water-repellent treatment Example 1 The electrode assembly of Production Example 1B was incorporated into a gas diffusion half-cell to produce the carbon dioxide electrolysis cell 100 shown in FIG. 1. As shown in FIG. 1, the gas diffusion half-cell has a structure in which an electrolyte solution 20 is stored on one side of the electrode assembly (the side of the electrolyte 10) and CO2 gas can be supplied to the other side of the electrode assembly (the side of the cathode gas diffusion layer of the carbon dioxide reduction electrode 30). A Cu electrode was used as the working electrode (WE) 40, an Ag / AgCl electrode was used as the reference electrode (RE) 50, and a carbon rod was used as the counter electrode (CE) 60. A 1 mol / L KHCO3 aqueous solution was used as the electrolyte solution 20. The KHCO3 aqueous solution was bubbled with CO2 gas (purity: 99.99%) and then adjusted to a pH of 7.9 before use.

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

[0070] From the measurement results of the product gas flow rate and the analysis results of the product gas 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 of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiency) was calculated from the ratio of the converted electric charge to the actual current value. The composition of the product gas and the Faraday efficiency of each component are shown in Table 1 and Figure 2.

[0071] The faradaic efficiency indicates the proportion of the current used in the reaction to produce the product, and the higher the faradaic efficiency (unit: %) for methane, the better the method's methane selectivity.

[0072] <Example 2> The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 1, except that a voltage was applied so that the potential between the working electrode and the reference electrode was −2.5 V. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 1 and FIG. 2.

[0073] <Comparative Example 1> A carbon dioxide electrolysis cell was produced in the same manner as in Example 1, except that the electrode assembly incorporated into the gas diffusion-type half-cell was changed from the electrode assembly of Production Example 1B to the electrode assembly of Production Example 2B, and the flow rate of the produced gas was measured and the composition of the produced gas was analyzed. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 1 and FIG. 2.

[0074] <Comparative Example 2> The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 1, except that the electrode body incorporated in the gas diffusion half cell was changed from the electrode body of Production Example 1B to the electrode body of Production Example 2B, and a voltage was applied so that the potential between the working electrode and the reference electrode was −2.5 V. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 1 and FIG. 2.

[0075] The methane production methods of Examples 1 and 2 both use a carbon dioxide electrolysis cell equipped with a carbon dioxide reduction electrode and an electrolyte. The carbon dioxide reduction electrode has, in this order, a cathode gas diffusion layer and a cathode catalyst layer that has been treated to be water repellent. The methods include a step (i.e., step A) of supplying carbon dioxide (more specifically, CO gas) 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 25°C, and applying electricity. On the other hand, the methane production methods of Comparative Example 1 and Comparative Example 2 differ from Examples 1 and 2, respectively, in that the cathode catalyst layer was not subjected to a water-repellent treatment. Furthermore, Examples 1 and 2, and Comparative Examples 1 and 2, differ in the potential between the working electrode and the reference electrode. The results shown in Table 1 and FIG. 2 reveal that the methane production methods of Example 1 and Example 2 are superior in methane selectivity compared to the methane production methods of Comparative Example 1 and Comparative Example 2, respectively.

[0076] 2.Temperature dependence Example 3 The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 1, except that CO gas (purity: 99.99%) was flowed into the cathode gas diffusion layer side of the carbon dioxide reduction electrode in a temperature environment of 5°C. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 2 and FIG. 3.

[0077] Example 4 The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 1, except that CO gas (purity: 99.99%) was flowed into the cathode gas diffusion layer side of the carbon dioxide reduction electrode in a temperature environment of 40°C. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 2 and FIG. 3.

[0078] <Comparative Example 3> The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 1, except that CO gas (purity: 99.99%) was flowed into the cathode gas diffusion layer side of the carbon dioxide reduction electrode in a temperature environment of 60°C. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 2 and FIG. 3.

[0079] <Example 5> The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 2, except that CO gas (purity: 99.99%) was flowed into the cathode gas diffusion layer side of the carbon dioxide reduction electrode in a temperature environment of 5°C. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 2 and FIG. 3.

[0080] Example 6 The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 2, except that CO gas (purity: 99.99%) was flowed into the cathode gas diffusion layer side of the carbon dioxide reduction electrode in a temperature environment of 40°C. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 2 and FIG. 3.

[0081] <Comparative Example 4> The flow rate of the produced gas was measured and the composition of the produced gas was analyzed in the same manner as in Example 2, except that CO gas (purity: 99.99%) was flowed into the cathode gas diffusion layer side of the carbon dioxide reduction electrode in a temperature environment of 60°C. As in Example 1, the amounts of hydrogen (H), carbon monoxide (CO), methane (CH), and ethylene (CH) produced were calculated from the measurement results of the product gas flow rate and the analysis results of the product gas composition, and converted into electric charge amounts. The production efficiencies of hydrogen, carbon monoxide, methane, and ethylene (so-called Faraday efficiencies) were calculated from the ratio of the converted electric charge amount to the actual current value. The composition of the product gas and the Faraday efficiencies of each component are shown in Table 2 and FIG. 3.

[0082] [Table 1]

[0083] [Table 2]

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

[0085] The methane production methods of Examples 1 to 6 and Comparative Examples 3 and 4 all use a carbon dioxide electrolysis cell equipped with a carbon dioxide reduction electrode and an electrolyte, and the carbon dioxide reduction electrode has, in this order, a cathode gas diffusion layer and a cathode catalyst layer that has been subjected to a water-repellent treatment. Examples 1, 3, 4, and Comparative Example 3, and Examples 2, 5, 6, and Comparative Example 4 differ in the temperature at 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, and electricity is applied. Also, Examples 1, 3, 4, and Comparative Example 3 differ in the potential between the working electrode and the reference electrode from Examples 2, 5, 6, and Comparative Example 4. The results shown in Table 2 and FIG. 3 reveal that the methane production methods of Examples 1, 3, and 4 are superior in methane selectivity compared to the methane production method of Comparative Example 3, and that the methane production methods of Examples 2, 5, and 6 are superior in 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 electrolysis cell

Claims

1. 1. A method for producing methane using a carbon dioxide electrolysis cell comprising a carbon dioxide reduction electrode and an electrolyte, the carbon dioxide reduction electrode has, in this order, a cathode gas diffusion layer and a cathode catalyst layer that has been subjected to a water-repellent treatment by forming a water-repellent film made of a fluororesin; the cathode gas diffusion layer has a conductive fiber layer and a microporous layer provided on the conductive fiber layer and being a porous body formed of conductive particles; the cathode catalyst layer is provided on a surface of the cathode gas diffusion layer facing the microporous layer, a 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 exceeding 0°C and not exceeding 5°C, and applying electricity; The method for producing methane, wherein the cathode catalyst layer contains Cu particles.

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

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