Electrode catalyst, cathode, ion exchange membrane-electrode assembly, and solid electrolyte electrolysis device

Abstract

Provided is an electrode catalyst comprising a first single-atom catalyst which contains a first carbon carrier and a first metal atom on the first carbon carrier, and a second single-atom catalyst which contains a second carbon carrier and a second metal atom on the second carbon carrier, wherein the first metal atom and the second metal atom are different from each other.

Description

Electrode catalyst, cathode, ion exchange membrane-electrode assembly, and solid electrolyte electrolysis device

[0001] The technology of the present disclosure relates to an electrode catalyst, a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte electrolysis device.

[0002] Carbon dioxide is emitted when energy is extracted from fossil fuels, etc. Rising carbon dioxide concentrations in the atmosphere are said to be one of the causes of global warming. There is a need for technology that can convert carbon dioxide into other substances and reuse them as resources without emitting it. At the same time, new technologies are needed to convert carbon dioxide into other substances and reuse them as resources. For example, carbon dioxide reduction devices that can directly reduce gaseous carbon dioxide are being developed.

[0003] Research into carbon dioxide reduction using electrical energy is being conducted worldwide. Among carbon dioxide reduction devices, polymer electrolyte electrolysis cells with membrane electrode assemblies (MEAs) can directly reduce gaseous carbon dioxide, and the use of thin-film polymer electrolytes can suppress ion migration resistance.

[0004] A cathode for carbon dioxide reduction generally has a structure in which a catalyst layer consisting of a mixture of catalyst particles, a conductive support, and a binder (also called a binder resin, ionomer, or ionomer) is laminated on a substrate. The binder is used to improve the adhesion of the catalyst (the adhesion between the catalyst layer and the substrate). The substrate is often called a gas diffusion layer because it allows gaseous reactants and products to pass through. Relatively inexpensive, highly porous carbon materials (such as carbon fiber paper) are commonly used for the substrate.

[0005] As catalysts, in addition to fine particles of metals such as gold and silver, catalysts made of nitrogen-containing carbonaceous materials in which metals are coordinated to and supported by nitrogen are known to exhibit relatively high activity (see, for example, Non-Patent Document 1). Such nitrogen-containing carbonaceous materials are generally synthesized by high-temperature firing of a mixture of a carbon support, a nitrogen source precursor, and a metal source precursor, accompanied by thermal decomposition of the precursor. For example, Non-Patent Document 1 reports that a reduction catalyst capable of highly selectively generating carbon monoxide can be synthesized by dispersing graphene oxide, pentaethylenehexamine, and nickel chloride in ethanol, evaporating the solvent, and then firing at 900°C for several tens of seconds.

[0006] Furthermore, Non-Patent Document 2 reports a monoatomic catalyst using iron (Fe) as the metal species in a nitrogen-containing carbon-based material. Furthermore, Non-Patent Document 3 reports that a catalyst having a monoatomic catalyst (nickel; Ni) site and a monoatomic site of a different metal species (Fe) is constructed on a carbon support prepared by pyrolysis of a zeolite-imidazolate structure, and that the monoatomic metal species are supported adjacent to each other by indirect bonding (non-bonding), thereby accelerating the carbon dioxide reduction performance. Patent Document 1 also develops a catalyst supported by indirect bonding (non-bonding), characterized in that a first transition metal (M1) and a second transition metal (M2) are linked to each other in the form of M1-Nx-Nx-M2.

[0007] JP 2023-172910 A

[0008] Panpan Su, Kazuyuki Iwase, Shuji Nakanishi, Kazuhito Hashimoto,and Kazuhide Kamiya,small 2016, 12, 44, 6083-6089Jun Gu1, Chia-Shuo Hsu, Lichen Bai, Hao Ming Chen, Xile Hu1, Science 2019, 364, 1091-1094Long Jiao, Juntong Zhu, Yan Zhang, Weijie Yang, Siyuan Zhou, Aowen Li, Chenfan Xie, Xusheng Zheng,Wu Zhou, Shu-Hong Yu, and Hai-Long Jiang, J. Am. Chem. Soc. 2021, 143, 19417-19424

[0009] On the other hand, further improvement of the performance of catalysts used in electrodes requires a reduction in the electrical energy required for the reduction (conversion) of carbon dioxide to carbon monoxide, and therefore requires a reaction under low overpotential conditions. In this regard, the single-atom catalyst described in Non-Patent Document 2 has succeeded in producing carbon monoxide at a low potential of -0.19 V versus RHE. However, when Fe is used as the metal species, although carbon dioxide can be reduced to carbon monoxide under low overpotential conditions, there are issues with low current density and the tendency for hydrogen generation as a side reaction to proceed.

[0010] Furthermore, the technologies described in Non-Patent Document 3 and Patent Document 1 report catalysts in which two monatomic metal species are introduced onto the same conductive material. However, when different metal species are introduced onto the same conductive support, precise synthesis techniques are required to control the distance and ratio of the two metal species in order to obtain heterometallic interactions, making this unsuitable for mass synthesis. Furthermore, the coordination environment for the different metal species is limited, making it difficult to create an optimal coordination environment for each metal species.

[0011] The technology of the present disclosure has been made in consideration of the above-mentioned circumstances, and an object of the present disclosure is to provide an electrode catalyst that is excellent in catalytic activity and ease of production, as well as a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte electrolysis device that use the same.

[0012] <1> An electrocatalyst comprising: a first carbon support; a first monoatomic catalyst containing a first metal atom on the first carbon support; and a second carbon support; a second monoatomic catalyst containing a second metal atom on the second carbon support, wherein the first metal atom and the second metal atom are different metal atoms. <2> The electrocatalyst according to <1>, wherein at least one of the first monoatomic catalyst and the second monoatomic catalyst contains a heteroatom on the carbon support. <3> The electrocatalyst according to <2>, wherein the heteroatom is at least one selected from the group consisting of a nitrogen atom, an oxygen atom, a halogen atom, a sulfur atom, and a phosphorus atom. <4> The electrocatalyst according to any one of <1> to <3>, wherein the first monoatomic catalyst and the second monoatomic catalyst contain a nitrogen atom on the carbon support. <5> The electrocatalyst according to any one of <1> to <4>, wherein the first metal atom and the second metal atom are each a metal atom of Groups 4 to 14. <6> The electrode catalyst according to any one of <1> to <5>, wherein the first metal atom and the second metal atom are titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, molybdenum, ruthenium, rhodium, palladium, silver, indium, tin, tungsten, rhenium, iridium, platinum, gold, mercury, lead, or aluminum, respectively. <7> The electrode catalyst according to any one of <1> to <6>, wherein the electrode catalyst is a mixture of the first single-atom catalyst and the second single-atom catalyst. <8> The electrode catalyst according to any one of <1> to <7>, wherein the content [y] of the second single-atom catalyst is 1 mass % to 99 mass % relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y]. <9> The electrode catalyst according to any one of <1> to <8>, wherein the content [y] of the second single-atom catalyst is 5% by mass to 70% by mass relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y]. <10> The electrode catalyst according to any one of <1> to <9>, wherein the ratio of the content of the second metal atoms to the content of the first metal atoms is 0.1 to 25.0. <11> The electrode catalyst according to any one of <1> to <10>, which is used for carbon dioxide electrolysis.<12> A cathode having a catalyst layer containing the electrode catalyst according to any one of <1> to <10> and a gas diffusion layer. <13> An ion exchange membrane-electrode assembly having the cathode according to <12>, a solid electrolyte, and an anode. <14> The ion exchange membrane-electrode assembly according to <13>, in which the solid electrolyte is an anion exchange membrane. <15> A solid electrolyte electrolysis device comprising: the cathode according to <12>, an anode constituting a pair of electrodes with the cathode, a solid electrolyte interposed in contact between the cathode and the anode, and a voltage application unit that applies a voltage between the cathode and the anode.

[0013] According to the technology of the present disclosure, it is possible to provide an electrode catalyst that is excellent in catalytic activity and ease of production, as well as a technology relating to a cathode, an ion exchange membrane-electrode assembly, and a solid electrolyte electrolysis device that use the electrode catalyst.

[0014] 1 is a schematic diagram of an ion exchange membrane-electrode assembly preferably used in this embodiment. 2 is a schematic diagram of a solid electrolyte electrolysis device preferably used in this embodiment. 3 is a graph showing the relationship between the ratio of the content of first metal atoms to the content of second metal atoms (second metal atoms / first metal atoms) and the CO generation current density in each example.

[0015] The upper and lower limit values ​​of the numerical ranges described in this specification can be combined in any way. For example, when "A to B" and "C to D" are described as numerical ranges, the numerical ranges "A to D" and "C to B" are also included in the scope of the present disclosure. Furthermore, unless otherwise specified, a numerical range of "lower limit value to upper limit value" described in this specification means that the range is equal to or greater than the lower limit value and equal to or less than the upper limit value.

[0016] <Electrode Catalyst> The electrode catalyst of this embodiment includes a first carbon support and a first monoatomic catalyst containing a first metal atom on the first carbon support, and a second carbon support and a second monoatomic catalyst containing a second metal atom on the second carbon support, where the first metal atom and the second metal atom are different metal atoms. In the technology of this embodiment, a configuration containing metal atoms on a carbon support is referred to as the smallest unit "catalyst," and the first and second monoatomic catalysts correspond to "catalysts." The electrode catalyst of this embodiment includes two types of catalysts. Note that the electrode catalyst of this embodiment may include a catalyst other than the first and second monoatomic catalysts, as long as the effects of the present invention are not impaired.

[0017] The electrode catalyst of this embodiment includes at least a first single-atom catalyst and a second single-atom catalyst. Hereinafter, the first and second single-atom catalysts may be collectively referred to simply as "single-atom catalysts." Herein, "single-atom catalyst" refers to a catalyst containing one type of metal atom on a carbon support, and is a catalyst containing one type of metal atom that serves as a catalyst source (hereinafter, sometimes referred to as "main metal"). Note that a catalyst containing two or more types of main metal atoms on a carbon support may be referred to as a "composite single-atom catalyst."

[0018] In the electrode catalyst of this embodiment, the first metal atom and the second metal atom in the first and second monoatomic catalysts are different metal atoms. "The first metal atom and the second metal atom are different metal atoms" means that the types of the main metal atoms in the first and second monoatomic catalysts are different. For example, when at least one of the first and second monoatomic catalysts contains a trace amount of another metal in addition to the main metal atom, and the main metal atom of one monoatomic catalyst and the trace amount of another metal in the other monoatomic catalyst are the same type, this also falls within the concept of "the first metal atom and the second metal atom are different metal atoms." In this embodiment, the atomic number of the first metal atom is greater than the atomic number of the second metal atom.

[0019] The content of the first metal atom is preferably 0.010 mass% to 0.260 mass%, more preferably 0.035 mass% to 0.200 mass%, and even more preferably 0.050 mass% to 0.150 mass%, relative to the total mass of the electrode catalyst of this embodiment.

[0020] The content of the second metal atom is preferably 0.010 mass% to 0.550 mass%, more preferably 0.040 mass% to 0.450 mass%, and even more preferably 0.080 mass% to 0.350 mass%, relative to the total mass of the electrode catalyst of this embodiment.

[0021] The ratio of the content of the second metal atoms to the content of the first metal atoms (second metal atoms / first metal atoms) is preferably 0.10 to 25.00, more preferably 0.10 to 22.00, even more preferably 0.15 to 10.00, and particularly preferably 0.30 to 2.00.

[0022] The content of each metal atom and the ratio of the content of each metal atom can be measured by the following method. A sample piece is cut from the obtained cathode (negative electrode), and the content (mass %) of each metal atom relative to the total mass of the electrode catalyst is measured by inductively coupled plasma (ICP) analysis. More specifically, the total mass of the electrode catalyst supported on the carbon paper of the sample piece is measured by subtracting the mass of the carbon paper before the catalyst is applied from the total mass of the carbon paper after the catalyst is applied. The content of the first metal atom and the content of the second metal atom are calculated using the metal content measured by ICP analysis as the numerator and the amount of electrode catalyst as the denominator. From the obtained values, the ratio of the content of the second metal atom to the content of the first metal atom (second metal atom / first metal atom) is calculated.

[0023] The contents of the first and second metal atoms are preferably 90 mass% or more, more preferably 95 mass% or more, and even more preferably 100 mass% relative to the total mass of all metal atoms in the first and second single-atom catalysts, respectively.

[0024] The electrode catalyst of this embodiment has excellent catalytic activity and is easy to manufacture, and can exhibit, for example, high catalytic activity and reactant selectivity derived from the metal species. By changing the metal species of the first and second single-atom catalysts, the electrode catalyst of this embodiment can be applied to a wide range of electrolysis fields, including not only carbon dioxide reduction reactions but also hydrogen evolution reactions, oxygen reduction reactions, oxygen evolution reactions, and nitrogen reduction reactions.

[0025] As described above, the use of the electrode catalyst of this embodiment can exhibit excellent catalytic activity and ease of manufacture. For example, the electrode catalyst of this embodiment is less difficult to formulate due to the influence of the combination of metal species than an electrode catalyst made of a composite single-atom catalyst, so it is possible to easily optimize the distance between the metals in each single-atom catalyst and control the blending ratio of the metal atoms. Therefore, the precise control required for the manufacture of a composite single-atom catalyst is not required, and electrode catalysts using different metals and having excellent catalytic effects can be easily mass-produced compared to electrode catalysts made of a composite single-atom catalyst.

[0026] Furthermore, when the electrode catalyst of this embodiment is used as an electrode catalyst for carbon dioxide electrolysis, which produces carbon monoxide through a reduction reaction, for example, it is possible to increase the carbon monoxide production current density (hereinafter, sometimes referred to as "CO production current density") in the carbon dioxide electrolytic reduction, and as a result, it is possible to increase the carbon dioxide electrolytic reduction reaction rate under low overpotential conditions, etc. Furthermore, it is possible to increase the carbon monoxide production selectivity (hereinafter, sometimes referred to as "CO selectivity").

[0027] Although the reason why the electrode catalyst of this embodiment can exhibit excellent catalytic activity and ease of production is unclear, it is speculated that the physical mixing of two types of single-atom catalysts (for example, a first single-atom catalyst (M1-N-C) and a second single-atom catalyst (M2-N-C) (M1 and M2 represent metal atoms; N represents a nitrogen atom; and C represents a carbon atom)) brings M2 into proximity with the M1-N bond. This changes the electronic states of M1-N and M2-N, shifting the energy required to generate intermediate products of carbon dioxide reduction in a direction favorable for carbon dioxide electrolysis. Furthermore, the physical mixing of two types of single-atom catalysts (for example, a first single-atom catalyst (M1-N-C) and a second single-atom catalyst (M2-N-C)) brings M2 into proximity with the M1-N bond. The intermediate products of carbon dioxide reduction, —COOH and —CO, generated on M1, move to M2. This allows —COOH to be generated with lower energy, making it easier for CO to be desorbed.

[0028] <Single-atom Catalyst> In this embodiment, the single-atom catalyst comprises a metal atom on a carbon support. That is, in this embodiment, the first single-atom catalyst comprises a first carbon support and a first metal atom on the first carbon support. Furthermore, the second single-atom catalyst comprises a second carbon support and a second metal atom on the second carbon support. In this embodiment, the term "single-atom catalyst" refers to both the first single-atom catalyst and the second single-atom catalyst, and the term "carbon support" refers to both the first carbon support and the second carbon support.

[0029] The particle size of each single-atom catalyst can be selected appropriately depending on the application of the electrode catalyst, but from the viewpoint of increasing the active site density and improving the carbon dioxide reduction reaction rate, the particle size of each single-atom catalyst is preferably 5 nm to 200 nm, more preferably 10 nm to 100 nm, and particularly preferably 10 nm to 50 nm. Furthermore, when preparing a catalyst layer together with an ionomer described below, from the viewpoint of thoroughly mixing the catalyst with the ionomer and preparing a catalyst layer in which the catalyst is uniformly dispersed in the ionomer, the particle size of the catalyst is preferably 20 nm to 40 μm, and more preferably 30 nm to 40 μm. The particle size of the catalyst can be confirmed using a laser diffraction / scattering particle size distribution analyzer.

[0030] (Carbon Support) The carbon support is a support containing carbon. Since carbon is usually conductive, the carbon support in this embodiment may be a conductive support. The carbon support can be appropriately changed depending on the application of the electrode catalyst. Examples of the carbon support include carbon black (furnace black, acetylene black, ketjen black, medium thermal carbon black, etc.), activated carbon, graphite, carbon nanotubes, carbon nanofibers, carbon nanohorns, graphene nanoplatelets, nanoporous carbon, etc., and among these, carbon black is preferred from the viewpoint of improving the active site density.

[0031] The carbon black may be a commercially available product, and examples thereof include Vulcan (registered trademark) XC-72 (manufactured by Cabot Corporation), BLACKPEARL 2000 (manufactured by Cabot Corporation), Mitsubishi (registered trademark) Carbon Black (brand name: #4000B, etc.) (manufactured by Mitsubishi Chemical Corporation), etc. Only one type of carbon support may be used for one single-atom catalyst, or two or more types may be used in combination.

[0032] The carbon support is preferably in the form of particles. When the carbon support is in the form of particles, from the viewpoint of improving the active site density and improving the current density, the primary particle diameter of the carbon support is preferably 5 to 200 nm, more preferably 10 to 100 nm, and even more preferably 10 to 50 nm. The primary particle diameter of the carbon support can be measured using a transmission electron microscope. The primary particle diameter can be measured by measuring the length in the longest direction of the particles revealed by the microscope, taking this as the major diameter, and calculating the average of the obtained major diameters. From the same viewpoint, the secondary particle diameter (particle diameter of the aggregates) of the carbon support is preferably small, and the amount of functional groups is preferably large.

[0033] (Metal Atoms) The first and second monoatomic catalysts each have a first or second metal atom as a catalyst source (the aforementioned "main metal atom"). The first and second metal atoms are supported on first and second carbon supports, respectively. The first and second metal atoms can be appropriately selected depending on the application of the electrode catalyst. For example, metal atoms of Groups 4 to 14 can be used as the first and second metal atoms, respectively, in terms of their electronic state and the adsorptivity and desorption properties of reaction intermediates. Examples of the first and second metal atoms include titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, molybdenum, ruthenium, rhodium, palladium, silver, indium, tin, tungsten, rhenium, iridium, platinum, gold, mercury, lead, and aluminum. When the electrode catalyst is used for carbon dioxide reduction, a metal atom having the effect of producing at least carbon monoxide through a reduction reaction can be used. For example, a metal atom selected from the group consisting of gold, silver, copper, nickel, iron, cobalt, zinc, manganese, molybdenum, and aluminum can be used. From the viewpoints of being an alternative to gold and silver in terms of cost and of the reaction efficiency of the carbon dioxide reduction reaction, a metal atom selected from the group consisting of nickel, cobalt, and iron is preferred.

[0034] From the viewpoint of further improving the active site density, the content of metal atoms in each single-atom catalyst is more preferably 0.1 to 50 mass%, and even more preferably 0.7 to 30 mass%, relative to the total amount of the single-atom catalyst. The content of metal atoms can be measured, for example, by X-ray absorption fine structure analysis (XAFS) or X-ray fluorescence analysis (XRF). Furthermore, as described below, when a portion of the metal atoms is coordinated with a heteroatom (e.g., a nitrogen atom), the content of metal ions coordinated with the nitrogen atom can be calculated from the results of XAFS measurement and XRF measurement.

[0035] (Heteroatom) At least one of the first monoatomic catalyst and the second monoatomic catalyst may contain a heteroatom on a carbon support. Further, each monoatomic catalyst may have a metal atom bonded to the heteroatom.

[0036] The heteroatom is preferably at least one selected from the group consisting of a nitrogen atom, an oxygen atom, a halogen atom, a sulfur atom, and a phosphorus atom, and more preferably a nitrogen atom. The heteroatom contained in each monoatomic catalyst may be one or more types, but is preferably one type. One example is an embodiment in which the first monoatomic catalyst and the second monoatomic catalyst have nitrogen atoms on a carbon support, and further, an embodiment in which some metal atoms are bonded to the nitrogen atoms.

[0037] The heteroatom contained in each single-atom catalyst is, for example, the remainder of a heteroatom-containing compound (e.g., a nitrogen-containing compound) used during the production of the single-atom catalyst. Examples of the nitrogen-containing compound used during the production of the single-atom catalyst are not particularly limited and include, for example, phenanthroline, pentaethylenehexamine, tetraethylenepentamine, triethylenepentamine, diethylenetriamine, ethylenediamine, and diethylamine. From the viewpoint of improving electrolytic activity, pentaethylenehexamine, tetraethylenepentamine, and triethylenepentamine are preferred, and phenanthroline, pentaethylenehexamine, and tetraethylenepentamine are more preferred.

[0038] From the viewpoint of improving the active site density, the heteroatom content in each single-atom catalyst is more preferably 5 to 75 mass %, and even more preferably 10 to 60 mass %, relative to the total amount of the single-atom catalyst. The heteroatom content can be measured, for example, by X-ray absorption fine structure analysis (XAFS) or X-ray photoelectron spectroscopy (XPS).

[0039] (Relationship between the first single-atom catalyst and the second single-atom catalyst) As described above, the electrode catalyst of this embodiment includes at least a first single-atom catalyst and a second single-atom catalyst. From the viewpoint of further enhancing the efficiency of the catalytic effect of the different metals, the electrode catalyst of this embodiment is preferably a mixture of the first single-atom catalyst and the second single-atom catalyst. In other words, the electrode catalyst of this embodiment preferably has one single-atom catalyst dispersed in the other single-atom catalyst.

[0040] The first and second monoatomic catalysts may contain different types of metal atoms, and the carbon supports and heteroatoms described above may be the same or different. The combination of the first metal (of the first monoatomic catalyst) and the second metal (of the second monoatomic catalyst) can be selected appropriately depending on the application of the electrode catalyst. For example, when used for carbon dioxide reduction, a combination of two metals selected from iron, nickel, and cobalt may be used in terms of the stability of the reaction intermediate. Examples of combinations using different carbon supports as the first and second carbon supports include, in terms of particle size, a combination in which the carbon support in one monoatomic catalyst (e.g., containing nickel as the metal atom) is carbon black having a particle size of 10 to 25 nm, and the carbon support in the other monoatomic catalyst (e.g., containing iron as the metal atom) is carbon black having a particle size of 10 to 50 nm. Furthermore, when producing each single-atom catalyst, if different heteroatom-containing compounds are used for the first and second single-atom catalysts, an example of such a combination is a combination of phenanthroline and pentaethylenehexamine.

[0041] In the electrode catalyst of this embodiment, the mixing ratio [x:y] of the first single-atom catalyst [x] and the second single-atom catalyst [y] can be appropriately selected depending on the application of the electrode catalyst. For example, when used for carbon dioxide reduction, from the viewpoint of the electronic state and the adsorption and desorption properties of the reaction intermediate, a ratio of 1:99 to 99:1 is preferable, a ratio of 5:95 to 95:5 is more preferable, and a ratio of 65:35 to 15:85 is particularly preferable. In composite single-atom catalysts, it is necessary to consider the influence of coordination due to the combination of metals, etc., and therefore it is difficult to control the compounding ratio of the metal atoms in a single composite single-atom catalyst. On the other hand, in the electrode catalyst of this embodiment, for example, when controlling the mixing ratio of the first and second metal atoms within a desired range, it is sufficient to determine the ratio of each single-atom catalyst based on the weight of the metal atom in each single-atom catalyst, and the mixing ratio of the different metals in the electrode catalyst can be easily controlled.

[0042] From the viewpoint of excellent catalytic activity, the content of the first single-atom catalyst [x] is preferably 1% by mass to 99% by mass relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y]. The content of the first single-atom catalyst [x] is more preferably 5% by mass or more, even more preferably 15% by mass or more, and particularly preferably 30% by mass or more, relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y]. The content of the first single-atom catalyst [x] is more preferably 95% by mass or less, even more preferably 85% by mass or less, and particularly preferably 70% by mass or less, relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y].

[0043] From the viewpoint of excellent catalytic activity, the content of the second single-atom catalyst [y] is preferably 1% by mass to 99% by mass, and more preferably 5% by mass to 70% by mass, relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y]. The lower limit of the content of the second single-atom catalyst [y] is more preferably 5% by mass or more, even more preferably 15% by mass or more, and particularly preferably 30% by mass or more, relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y]. The upper limit of the content of the second single-atom catalyst [y] is more preferably 95% by mass or less, even more preferably 85% by mass or less, and particularly preferably 70% by mass or less, relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y].

[0044] (Applications of Electrode Catalyst) As described above, by changing the metal atom, the electrode catalyst of this embodiment can be used in a wide range of electrolysis fields, such as carbon dioxide reduction reactions, hydrogen evolution reactions, oxygen reduction reactions, oxygen evolution reactions, and nitrogen reduction reactions. Therefore, the electrode catalyst of this embodiment can be used, for example, in carbon dioxide electrolysis, as well as in fuel cells, water electrolysis, ammonia electrolysis, ammonia electrolysis, and the like. Furthermore, the electrode catalyst of this embodiment may be used in a cathode (negative electrode) or an anode (positive electrode), which will be described later.

[0045] <Method for producing single-atom catalyst> The method for producing each single-atom catalyst is not particularly limited. For example, a production method including a mixing step of mixing a metal atom, a carbon support, and optionally a heteroatom-containing compound to obtain a mixture, and a calcination step of calcining the obtained mixture to obtain a calcined product, may be mentioned. The method for producing a single-atom catalyst may further include a washing step of washing the calcined product, a crushing step of crushing the calcined product, etc. It is preferable to produce the first and second single-atom catalysts separately.

[0046] (Mixing step) The metal atom, carbon support, and heteroatom-containing compound used in the mixing step are as described above. In the mixing step, the metal atom, carbon support, and heteroatom-containing compound are preferably added to an organic solvent such as ethanol to form a dispersion. The dispersion may be irradiated with ultrasound for degassing.

[0047] The amount of the metal atoms can be selected as appropriate as desired, for example, 0.1 to 100 parts by mass, or 5 to 50 parts by mass, relative to 100 parts by mass of the carbon support. Furthermore, when a heteroatom-containing compound is used, the amount can also be selected as appropriate as desired, for example, 5 to 300 parts by mass, or 20 to 70 parts by mass, relative to 100 parts by mass of the carbon support.

[0048] (Firing Step) In the firing step, the mixture obtained in the mixing step is fired. When firing the mixture, the mixture can be placed in a quartz tube, the inside of the quartz tube is replaced with an inert gas such as argon gas, and the quartz tube is sealed and fired.

[0049] The firing temperature can be appropriately selected depending on the material used, but is, for example, preferably 500 to 1400° C., more preferably 600 to 1000° C., and even more preferably 750 to 950° C. The firing time is preferably 15 seconds to 10 minutes, more preferably 20 seconds to 5 minutes, and even more preferably 30 seconds to 90 seconds.

[0050] (Washing Step) The washing step is a step in which the fired product obtained in the firing step is washed with water, an inorganic acid, or the like. Ion-exchanged water, pure water, or the like is used as the water. It is preferable to use sulfuric acid as the inorganic acid. The inorganic acid is preferably, for example, 1 to 3 mol / L (hereinafter, mol / L may be abbreviated as M), and inorganic acid heated to 60 to 90°C may also be used. In the washing step, it is preferable to wash the fired product with water, then with inorganic acid, and finally with water again.

[0051] (Crushing Step) The crushing step is a step of crushing the calcined product obtained in the calcining step. By performing the crushing step as necessary after the calcining step, the active site density of the catalyst can be improved. The crushing step may be performed after the washing step, but is preferably performed before the washing step. The crushing of the calcined product can be performed using, for example, zirconia balls or alumina balls. The diameter of these balls is preferably 0.05 to 5 mm, and the crushing conditions are preferably 400 to 1100 rpm and 2 minutes to 20 hours.

[0052] <Method for producing an electrode catalyst> The method for producing an electrode catalyst is not particularly limited, but the electrode catalyst can be obtained by mixing the first and second single-atom catalysts obtained above in a desired mixing ratio. The method for producing an electrode catalyst may also include a step of coating the electrode catalyst with an ionomer (coating step).

[0053] [Coating Step] The coating step is a step of coating the electrode catalyst with an ionomer after mixing the first and second single-atom catalysts produced through the calcination step to form an electrode catalyst, or simultaneously with mixing the first and second single-atom catalysts produced through the calcination step. By coating the electrode catalyst with an ionomer, ion-conducting channels are easily formed between the coated catalyst and the solid electrolyte described below, facilitating the movement of ions generated by the reaction and enabling improved electrolysis efficiency. Details of the ionomer will be described later.

[0054] The cathode (negative electrode) of this embodiment has a catalyst layer including the electrode catalyst of this embodiment and a gas diffusion layer. The cathode (negative electrode) of this embodiment has high electrolytic activity due to the catalyst layer including the electrode catalyst manufactured by the catalyst manufacturing method according to an embodiment of the present disclosure.

[0055] [Catalyst Layer] The catalyst layer contains at least the electrode catalyst produced by the catalyst production method according to the embodiment of the present disclosure, and may further contain an ionomer. The ionomer functions as a binder resin in the catalyst layer, and is a matrix resin (continuous phase) that can disperse and immobilize the catalyst according to the embodiment of the present disclosure, and also transmits ions generated by electrolysis, allowing CO ２It also has the function of improving the efficiency of electrolysis. Furthermore, from the viewpoint of improving the transfer efficiency of ions generated by electrolysis, the ionomer is preferably conductive, and is more preferably a polymer electrolyte. The polymer electrolyte is further preferably an ion exchange resin. The ion exchange resin may be either a cation exchange resin or an anion exchange resin, but is preferably an anion exchange resin. In particular, when an anion exchange resin is used, the anion exchange resin itself has the ability to adsorb carbon dioxide, which, together with the ease of ion transfer of the ion exchange resin, makes it possible to greatly improve the efficiency of carbon dioxide electrolysis.

[0056] Examples of cation exchange resins include fluororesins having sulfonic groups and styrene-divinylbenzene copolymers having sulfonic groups. Commercially available products can also be used, such as Nafion (manufactured by Chemours), Aquivion (manufactured by Solvay Specialty Polymers), DIAION (manufactured by Mitsubishi Chemical Corporation), and Fumasep (manufactured by FUMATECH). Examples of anion exchange resins include resins having one or more ion exchange groups selected from the group consisting of quaternary ammonium groups, primary amino groups, secondary amino groups, and tertiary amino groups. Commercially available products can also be used, such as Sustainion (manufactured by Dioxide Materials), Fumasep (manufactured by FUMATECH), PENTION (manufactured by Xergy), DURION (manufactured by Xergy), NEOSEPTA (manufactured by Astom), and TOYOPEARL (manufactured by Tosoh Corporation).

[0057] From the viewpoint of improving conductivity, the anion exchange resin has a basic site density of 2.0 to 5.0 mmol / cm in a dry state. ３ and preferably 2.5 mmol / cm ３ Above, 4.5 mmol / cm ３ More preferably, it is less than 2.9 mmol / cm ３ Above, 4.5 mmol / cm ３ The basic site density of the anion exchange resin is more preferably less than １The dryness of an anion exchange resin can be obtained from the integrated value of the signal when H NMR measurement is performed. Furthermore, with respect to an anion exchange resin, the dry state means that the anion exchange resin does not contain free water, and the anion exchange resin can be brought into a dry state by, for example, heating in a vacuum.

[0058] When the cathode (negative electrode) of this embodiment is used in an ion exchange membrane-electrode assembly and a solid electrolyte electrolysis device described below, it is preferable to use the same resin as that of the solid electrolyte (ion exchange membrane) as the ionomer, from the viewpoint of improving conductivity.

[0059] The content of the catalyst according to the embodiment of the present disclosure in the catalyst layer is determined based on the electrolytic activity and CO ２ From the viewpoint of further improving the reduction reaction rate, the content is preferably 50 to 99% by mass, more preferably 75 to 97% by mass, and even more preferably 90 to 95% by mass.

[0060] [Gas Diffusion Layer] The gas diffusion layer includes, for example, carbon paper or nonwoven fabric, or a metal mesh, such as graphite carbon, glassy carbon, titanium, or SUS steel.

[0061] <Ion Exchange Membrane-Electrode Assembly> The ion exchange membrane-electrode assembly of this embodiment has the cathode of this embodiment described above, a solid electrolyte, and an anode. The ion exchange membrane-electrode assembly of this embodiment has a cathode containing the electrode catalyst of this embodiment, and therefore has high electrolytic activity. FIG. 1 is a schematic diagram of an ion exchange membrane-electrode assembly that is preferably used in this embodiment. FIG. 1 shows an ion exchange membrane-electrode assembly 50 that has a gas diffusion layer 10, a catalyst layer 20, a solid electrolyte 30, and an anode 40. The catalyst layer 20 contains a plurality of electrode catalysts 24 of this embodiment and an ionomer 22. The combination of the gas diffusion layer 10 and the catalyst layer 20 constitutes the cathode (negative electrode) of this embodiment. As shown in FIG. 1, carbon dioxide (CO ) is introduced through the gas diffusion layer 10. ２ ) is supplied to the catalyst layer 20, where a reduction reaction produces carbon monoxide (CO). In the following description, the reference numerals in FIG. 1 will be omitted.

[0062] [Solid Electrolyte] The ion exchange membrane-electrode assembly of this embodiment has a solid electrolyte. A polymer membrane can be used as the solid electrolyte. Various ionomers can be used as the polymer, and although it may be a cation exchange resin or an anion exchange resin, it is preferable that it is an anion exchange resin. In other words, the solid electrolyte is preferably an anion exchange membrane. It is more preferable to use the same anion exchange resin as the ionomer used in the above-mentioned catalyst layer.

[0063] The solid electrolyte may be a commercially available product such as a cation exchange membrane or an anion exchange membrane. When an anion exchange membrane is used as the solid electrolyte, the base site density is 0.5 to 5.0 mmol / cm in a dry state. ３ and preferably 2.5 mmol / cm ３ Above, 4.5 mmol / cm ３ More preferably, it is less than 2.9 mmol / cm ３ Above, 4.5 mmol / cm ３ It is more preferable that it is less than 10 ...

[0064] Examples of the cation exchange membrane that can be used include strongly acidic cation exchange membranes in which sulfonic groups have been introduced into a fluororesin matrix, such as Nafion 117, Nafion 115, Nafion 212, and Nafion 350 (manufactured by Chemours Corporation), and strongly acidic cation exchange membranes in which sulfonic groups have been introduced into a styrene-divinylbenzene copolymer matrix, such as Neosepta CSE (manufactured by Astom Corporation).

[0065] Examples of anion exchange membranes include those having one or more ion exchange groups selected from the group consisting of quaternary ammonium groups, primary amino groups, secondary amino groups, and tertiary amino groups. Specific examples include Neosepta (registered trademark) ASE, AHA, ACS, and AFX (manufactured by Astom Corporation), and Selemion (registered trademark) AMVN, DSVN, AAV, ASVN, and AHO (manufactured by Asahi Glass Co., Ltd.).

[0066] The reduction reaction of carbon dioxide at the cathode (negative electrode) in this embodiment differs depending on the type of solid electrolyte. When a cation exchange membrane is used as the solid electrolyte, the reduction reactions shown in the following reaction formulas (1) and (2) occur. When an anion exchange membrane is used as the solid electrolyte, the reduction reactions shown in the following reaction formulas (3) and (4) occur.

[0067] CO ２ +2H++2e － →CO+H ２ O(1) 2H++2e － →H ２ (2) H ２ O+CO ２ +2e － →CO + 2OH － (3) 2H ２ O + 2e － →H ２ +2OH － (4)

[0068] [Anode] The oxidation reaction at the anode varies depending on the type of solid electrolyte. When a cation exchange membrane is used as the solid electrolyte, the oxidation reaction shown in the following reaction formula (5) occurs. When an anion exchange membrane is used as the solid electrolyte, the oxidation reaction shown in the following reaction formula (6) occurs.

[0069] 2H ２ O → O ２ +4H ＋ +4e － (5) 4OH － →O ２ +2H ２ O+4e － (6)

[0070] The anode is a gas diffusion electrode including a gas diffusion layer, such as a metal mesh. The electrode material of the anode is, for example, Ir or IrO. ２ , Ru, RuO ２ , Co, CoOx, Cu, CuOx, Fe, FeOx, FeOOH, FeMn, Ni, NiOx, NiOOH, NiCo, NiCe, NiC, NiFe, NiCeCoCe, NiLa, NiMoFe, NiSn, NiZn, SUS, Au, and Pt.

[0071] <<Solid electrolyte electrolysis device>> The solid electrolyte electrolysis device of this embodiment includes the cathode of this embodiment described above, an anode constituting a pair of electrodes with the cathode, a solid electrolyte interposed between the cathode and the anode in contact with each other, and a voltage application unit that applies a voltage between the cathode and the anode. The solid electrolyte electrolysis device of this embodiment has a cathode (negative electrode) containing the electrode catalyst of this embodiment, and therefore has high electrolytic activity.

[0072] Fig. 2 is a schematic diagram of a solid electrolyte electrolysis device preferably used in this embodiment. Fig. 2 shows a solid electrolyte electrolysis device 800 having a cathode (negative electrode) 200 of this embodiment, an anode (positive electrode) 400 that forms a pair of electrodes with the cathode 200, a solid electrolyte 300 that is interposed in contact between the cathode 200 and the anode 400, and a voltage application unit 700 that applies a voltage between the cathode 200 and the anode 400.

[0073] The solid electrolyte electrolysis device 800 shown in Fig. 2 further includes a cathode current collector 100, an anode current collector 500, and an electrolytic solution 600. The cathode of the present embodiment described above is used as the cathode 200. The solid electrolyte 300 is the same as the solid electrolyte 30 in Fig. 1, and the solid electrolyte 300 is preferably an anion exchange membrane. The anode 400 is the same as the anode 40 in Fig. 1. Details of the cathode 200, the solid electrolyte 300, and the anode 400 are as described above.

[0074] In the following description, the reference numerals of the elements other than the cathode 200, the solid electrolyte 300, and the anode 400 will be omitted.

[0075] [Cathode current collector] Examples of the cathode current collector (negative electrode current collector) include metal materials such as copper (Cu), nickel (Ni), stainless steel (SUS), nickel-plated steel, and brass, among which copper is preferred from the viewpoints of ease of processing and cost. When the cathode current collector is made of a metal material, examples of the shape of the cathode current collector include metal foil, metal plate, metal thin film, expanded metal, punched metal, and foam metal.

[0076] The cathode current collector may be provided with a gas supply hole for supplying a source gas containing carbon dioxide to the cathode and a gas recovery hole for recovering a product gas containing carbon monoxide. The provision of the gas supply hole and the gas recovery hole allows the source gas to be uniformly and efficiently delivered to the cathode and the product gas (including unreacted source gas) to be discharged. The cathode current collector may be provided with only one gas supply hole and one or more gas recovery holes. The shape, location, size, etc. of the gas supply hole and the gas recovery hole are not limited and may be set appropriately. In addition, if the cathode current collector is breathable, the gas supply hole and the gas recovery hole are not necessarily required. Note that if the cathode has the function of transferring electrons, the cathode current collector is not necessarily required.

[0077] [Anode Current Collector Plate] The anode current collector plate (positive electrode current collector plate) preferably has electrical conductivity to receive electrons from the anode and rigidity to support the anode. From this viewpoint, the anode current collector plate can be made of a metal material such as titanium (Ti), copper (Cu), nickel (Ni), stainless steel (SUS), nickel-plated steel, or brass.

[0078] The anode current collector is connected to the anode via a source gas (H ２ A gas flow path may be provided for feeding a source gas (e.g., O). By providing the gas flow path in the anode current collector plate, the source gas can be uniformly and efficiently fed to the anode. The number, shape, location, size, etc. of the gas flow path are not limited and may be set appropriately.

[0079] [Voltage Application Unit] The voltage application unit applies a voltage between the cathode and anode by applying a voltage to the cathode current collector and the anode current collector. Here, since both current collectors are conductors, they supply electrons to the cathode and receive electrons from the anode. In addition, a control unit (not shown) may be electrically connected to the voltage application unit to apply an appropriate voltage.

[0080] [Electrolyte] The electrolyte is preferably an aqueous solution having a pH of 5 or more. Examples of the electrolyte include an aqueous carbonate solution and an aqueous hydrogen carbonate solution (e.g., KHCO３ aqueous solution), sulfate aqueous solution, borate aqueous solution, sodium hydroxide aqueous solution, potassium hydroxide aqueous solution, sodium chloride aqueous solution, etc.

[0081] (Reactant Gas Supply Unit) The solid electrolyte electrolysis device of this embodiment may be provided with a reactant gas supply unit (not shown) outside the solid electrolyte electrolysis device. That is, a reactant gas, CO ２ The reactive gas may be supplied from a reactive gas supply unit to the gas supply hole via a pipe (not shown) or may be sprayed onto the surface of the cathode current collector opposite to the surface that contacts the cathode. From an environmental perspective, it is preferable to use factory exhaust gas as the reactive gas.

[0082] [CO Production Method] Next, a CO production method using the solid electrolyte electrolysis device of this embodiment will be described. First, CO 2 , which is a reactant gas as a raw material, is supplied by a reactant gas supply unit (not shown). ２ is supplied in a gaseous state to the solid electrolyte electrolysis device. ２ is supplied to the cathode through a gas supply hole provided in the cathode current collector plate. ２ When a cation exchange membrane is used as the solid electrolyte, the reduction reactions of the above-mentioned reaction formulas (1) and (2) occur by contacting the catalyst layer of the cathode. When an anion exchange membrane is used as the solid electrolyte, the reduction reactions of the above-mentioned reaction formulas (3) and (4) occur, resulting in the conversion of CO and H. ２ The synthesis gas containing at least CO and H is generated. ２ The synthesis gas containing the above is sent to a gas recovery device (not shown) through gas recovery holes provided in the cathode current collector plate, for example, and is recovered in predetermined gas units.

[0083] Although the present invention has been described above with reference to the embodiments, the present invention is not limited to the above-described embodiments.

[0084] The present embodiment will be specifically described below using examples, but the technology of the present disclosure is not limited to these examples in any way.

[0085] [Synthesis of Single-Atom Catalyst] (Preparation of Ni Single-Atom Catalyst) In a beaker, 0.5 mmol of pentaethylenehexamine (a nitrogen-containing compound; hereinafter, sometimes referred to as "PEHA") and 0.4 mmol of metal chloride (nickel(II) chloride hexahydrate) were mixed in an ethanol solvent to form a nickel-pentaethylenehexamine complex. Then, 200 mg of a carbon black carrier (either "#4000B" manufactured by Mitsubishi Chemical Corporation or VULCAN (XC-72) manufactured by The Fuel Cell Store, Inc. (hereinafter, sometimes referred to as "Vulcan") was used, as shown in the table below) was added, and the resulting ethanol dispersion was subjected to ultrasonic irradiation for 10 minutes. The ethanol dispersion was then heated and dried to evaporate the ethanol, and the resulting mixture was heated and calcined in an inert gas atmosphere at 890°C to 900°C for 10 seconds or longer in a calcining furnace. The product was then washed with an aqueous sulfuric acid solution, and the solid matter was collected using a suction filter and vacuum dried overnight at 60°C to obtain a catalyst powder (intermediate) carrying a metal complex. Hereinafter, a Ni monoatomic catalyst using "#4000B" as the carrier species may be referred to as "Ni monoatomic catalyst (#4000B)," and a Ni monoatomic catalyst using "Vulcan" as the carrier species may be referred to as "Ni monoatomic catalyst (Vulcan)."

[0086] The resulting catalyst powder was placed in a pot together with 10 g of 0.5 mm diameter zirconia balls and 10 mL of water, and milled at 800 rpm for 20 minutes using a planetary ball mill to recover a catalyst slurry. The slurry was washed again with an aqueous sulfuric acid solution, and the solid was recovered using a suction filter. The solid was then vacuum-dried at 60°C overnight to obtain the final catalyst powder (Ni single-atom catalyst).

[0087] (Synthesis of Fe monoatomic catalyst) A catalyst powder (Fe monoatomic catalyst) was obtained in the same manner as in the synthesis of the Ni monoatomic catalyst described above, except that the metal chloride was changed to iron (II) chloride tetrahydrate. Hereinafter, the Fe monoatomic catalyst using "#4000B" as the carrier species may be referred to as the "Fe monoatomic catalyst (#4000B)," and the Fe monoatomic catalyst using "Vulcan" as the carrier species may be referred to as the "Fe monoatomic catalyst (Vulcan)."

[0088] (Synthesis of Composite Single-Atom Catalyst) A composite single-atom catalyst powder was obtained in the same manner as in the synthesis of the Ni single-atom catalyst described above, except that the metal chloride was changed to nickel (II) chloride hexahydrate (0.2 mmol) and iron (II) chloride tetrahydrate (0.2 mmol) (total 0.4 mmol).

[0089] (Synthesis of Co monoatomic catalyst) A catalyst powder (Co monoatomic catalyst) was obtained in the same manner as in the synthesis of the Ni monoatomic catalyst described above, except that the metal chloride was changed to cobalt (II) chloride hexahydrate. Hereinafter, the Co monoatomic catalyst using "Vulcan" as the support species may be referred to as "Co monoatomic catalyst (Vulcan)."

[0090] [Preparation of Electrode Catalysts] (Production of Electrode Catalysts and Cathode in Examples 1 to 7) In Example 1, an electrode catalyst was prepared by mixing a first monoatomic catalyst (Ni monoatomic catalyst (#4000B)) and a second monoatomic catalyst (Fe monoatomic catalyst (Vulcan)) at a mixing ratio (by mass) of 1:1. 11 mg of each of the first and second monoatomic catalysts was weighed out and dispersed in ethanol, and 33 μL of a cation exchange resin (Nafion®) was mixed with the dispersion as a binder. In Examples 2 to 7, electrode catalysts with different mixing ratios and carrier types were prepared by varying the types and amounts of Ni monoatomic catalyst and Fe monoatomic catalyst used according to Table 1. After mixing, each electrode catalyst was subjected to ultrasonic treatment for 15 minutes, and then spray-coated onto carbon paper with a dry loading of 1.5 to 3 mg / cm. ２ A cathode (negative electrode) was produced by coating the solution so as to form a cathode.

[0091] (Production of cathodes in Comparative Examples 1 to 4) In Comparative Example 1, 22 mg of catalyst powder (Fe single-atom catalyst (Vulcan)) was weighed and dispersed in ethanol, and 33 μL of cation exchange resin (Nafion (registered trademark)) was mixed as a binder with the dispersion. After mixing, ultrasonic treatment was performed for 15 minutes, and the mixture was sprayed onto carbon paper using a spray coater to form a coated layer with a dry loading of 1.5 to 3.5 mg / cm. ２ In Comparative Examples 2 to 4, cathodes were produced in the same manner as in Comparative Example 1, except that the Fe single-atom catalyst (Vulcan) was replaced with the electrode catalyst shown in Table 1. In Comparative Example 3, Ni and Fe were supported on the first support.

[0092] <Evaluation> (Electrolysis process: Measurement of CO generation current density) Each of the obtained cathodes was bonded to an anion exchange membrane with a thickness of approximately 30 μm and an anode prepared by supporting iridium oxide on a titanium mesh (manufactured by Taiyo Wire Netting Co., Ltd., aperture ratio 56%) to form an ion exchange membrane-electrode assembly. The anode was placed in an electrolyte (0.5 mol / L KHCO ３ This device was designed to be in contact with the tank containing pure CO ２ was supplied to the cathode, and a potential was applied, and the CO generation current density [mA / cm ] at an applied voltage of −1.8 V relative to the silver / silver chloride reference electrode was measured. ２ ] was measured.

[0093] (CO generation current density) The obtained CO generation current density [mA / cm ２ ] was evaluated according to the following criteria. When the evaluation was B or higher, it can be said that the CO generation current density was excellent. S: CO generation current density [mA / cm ２ A: CO generation current density [mA / cm ２ ] was 200 or more and less than 300. B: CO generation current density [mA / cm ２ ] was 150 or more and less than 200. C: CO generation current density [mA / cm ２ ] was less than 150.

[0094] (CO Selectivity) The synthesis gas obtained by CO current density measurement was analyzed using a gas chromatograph, and the carbon monoxide selectivity (CO selectivity) was calculated using the following formula. The results are shown in the table below. CO Selectivity (%) = [(CO production rate (mol / s) × Faraday constant × 2) / Total current (A)] × 100 (where Faraday constant = 96485.3329 sA / mol).

[0095] The obtained CO selectivity was evaluated according to the following criteria. A rating of B or higher indicates excellent CO selectivity. A: CO selectivity was 90% or higher. B: CO selectivity was 50% or higher but less than 90%. C: CO selectivity was less than 50%.

[0096] (Ease of manufacturing) The ease of manufacturing of each electrode catalyst was evaluated according to the following criteria. Evaluations of A and above indicate excellent ease of manufacturing. A: A single monoatomic catalyst was used, so there was no need to control the compounding ratio of the two metals, or two monoatomic catalysts were simply mixed, so there was no need to specifically control the compounding ratio of the two metals. C: A composite monoatomic catalyst was used, so precision was required in controlling the compounding ratio of the two metals.

[0097] (Measurement of Potential) When measuring the CO generation current density described above, the CO generation current density was 10 mA / cm ２ The potential [V vs. Ag / AgCl] at a CO generation current density of 10 mA / cm was also measured. ２ The more noble the potential at the electrode, the lower the overpotential at which CO can be produced, meaning that CO production efficiency is excellent. Overpotential is the difference between the theoretical potential of a thermodynamically determined reaction and the potential of the electrode when the reaction actually proceeds.

[0098] (Measurement of metal mass ratio) 0.45 cm from the obtained cathode (negative electrode) ２A sample piece was cut out, and the content (mass %) of each metal atom relative to the total mass of the electrode catalyst was measured by inductively coupled plasma (ICP) analysis. More specifically, the total mass of the electrode catalyst supported on the carbon paper of the sample piece was measured by subtracting the mass of the carbon paper before the catalyst was applied from the total mass of the carbon paper after the catalyst was applied. The content of the first metal atoms and the content of the second metal atoms were calculated using the metal content measured by ICP analysis as the numerator and the amount of electrode catalyst as the denominator. From the obtained values, the ratio of the content of the second metal atoms to the content of the first metal atoms (second metal atoms / first metal atoms) was calculated. Figure 3 shows a graph showing the relationship between the ratio of the content of the first metal atoms to the content of the second metal atoms (second metal atoms / first metal atoms) and the CO generation current density for each example.

[0099]

[0100] Examples using an electrode catalyst including a first carbon support and a first monoatomic catalyst containing a first metal atom on the first carbon support, and a second carbon support and a second monoatomic catalyst containing a second metal atom on the second carbon support, where the first metal atom and the second metal atom are different metal atoms, exhibited excellent CO generation current density and CO selectivity, and thus excellent catalytic activity. Furthermore, the examples were also easy to manufacture. The CO generation current density of Example 1, which used a mixed catalyst containing a Ni monoatomic catalyst and an Fe monoatomic catalyst in a 1:1 ratio, was significantly higher than the theoretical CO generation current density calculated using the following formula from Comparative Example 1, which used only an Fe monoatomic catalyst, and Comparative Example 2, which used only an Ni monoatomic catalyst. That is, the CO generation current density of 215 in Example 1 was significantly higher than the theoretical value of 79.5 calculated from Comparative Examples 1 and 2.

[0101]

[0102] ​From the results in Table 1, when comparing Example 1 with Comparative Examples 1 and 2, which used a single Ni or Fe single-atom catalyst (using the same type of carbon support for the same type of single-atom catalyst), the Example using a mixture of single-atom catalysts had superior catalytic activity (CO current density; CO reaction rate). Furthermore, the composite single-atom catalyst (Comparative Example 3: a catalyst in which two types of metal atoms are combined on the same support) was inferior in ease of manufacture. Furthermore, compared with Comparative Example 4, which used a Co single-atom catalyst support, Example 1 had improved catalytic activity (CO current density; CO reaction rate). Furthermore, comparing Examples 1 to 6 (each using the same type of single-atom catalyst for the same type of carbon support) showed that the electrode catalysts containing a mixture of single-atom catalysts had improved current density and 10 mA / cm by changing the mixture ratio. ２ It was found that the potential at 1000 kJ / cm2 varied. Furthermore, a comparison of Examples 1 and 7 with Comparative Example 2 revealed that the catalytic activity (CO current density; CO reaction rate) of the Ni single-atom catalyst varied depending on the type of support. It was also found that the mixed catalyst Example was able to obtain sufficient effects even when the support was changed.

[0103] In FIG. 3 , "S," "A," and "B" correspond to the above-mentioned (CO generation current density) evaluation (S, A, B), and "Ni single-atom catalyst" and "Fe single-atom catalyst" indicate the CO generation current density when each catalyst was used alone (i.e., Comparative Example 2 or 1). As shown in FIG. 3 , the electrode catalysts of the examples were superior in CO generation current density to Comparative Example 2, which used a single-atom catalyst of Ni alone. In particular, Examples 6 and 7, in which the second metal atom / first metal atom ratio was 1.00 or less, were superior in CO generation current density.

[0104] According to this embodiment, for example, CO emitted from a factory is supplied to the solid electrolyte electrolysis device. ２ By using gas as a raw material and utilizing renewable energy such as solar cells for the voltage application section, at least CO and H can be produced in the desired ratio. ２ The synthesis gas thus produced can be used to produce fuel base materials, chemical raw materials, etc., by methods such as Fischer-Tropsch synthesis (FT synthesis) and methanation.

[0105] 10: gas diffusion layer, 20: catalyst layer, 22: ionomer, 24: catalyst, 30: solid electrolyte (ion exchange membrane), 40: anode (positive electrode), 50: ion exchange membrane-electrode assembly, 100: cathode current collector, 200: cathode (negative electrode), 300: solid electrolyte (ion exchange membrane), 400: anode (positive electrode), 500: anode current collector, 600: electrolytic solution, 700: voltage application unit, 800: solid electrolyte electrolysis device

[0106] The disclosure of Japanese Patent Application No. 2024-072625, filed on April 26, 2024, is incorporated herein by reference in its entirety. All documents, patent applications, and technical standards mentioned herein are incorporated herein by reference to the same extent as if each individual document, patent application, and technical standard was specifically and individually indicated to be incorporated by reference.

Claims

1. An electrode catalyst comprising: a first carbon support; a first monoatomic catalyst comprising a first metal atom on the first carbon support; and a second carbon support; a second monoatomic catalyst comprising a second metal atom on the second carbon support, wherein the first metal atom and the second metal atom are different metal atoms.

2. The electrocatalyst of claim 1, wherein at least one of the first single-atom catalyst and the second single-atom catalyst comprises a heteroatom on a carbon support.

3. The electrode catalyst according to claim 2, wherein the heteroatom is at least one selected from the group consisting of a nitrogen atom, an oxygen atom, a halogen atom, a sulfur atom, and a phosphorus atom.

4. The electrode catalyst according to any one of claims 1 to 3, wherein the first single-atom catalyst and the second single-atom catalyst have nitrogen atoms on the carbon support.

5. The electrode catalyst according to any one of claims 1 to 4, wherein the first metal atom and the second metal atom are each a metal atom of Groups 4 to 14.

6. The electrode catalyst according to any one of claims 1 to 5, wherein the first metal atom and the second metal atom are each titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, zinc, gallium, molybdenum, ruthenium, rhodium, palladium, silver, indium, tin, tungsten, rhenium, iridium, platinum, gold, mercury, lead, or aluminum.

7. The electrocatalyst according to any one of claims 1 to 6, which is a mixture of the first single-atom catalyst and the second single-atom catalyst.

8. The electrode catalyst according to any one of claims 1 to 7, wherein the content [y] of the second single-atom catalyst is 1 mass % to 99 mass % relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y].

9. The electrode catalyst according to any one of claims 1 to 8, wherein the content [y] of the second single-atom catalyst is 5 mass % to 70 mass % relative to the total mass of the first single-atom catalyst [x] and the second single-atom catalyst [y].

10. The electrode catalyst according to any one of claims 1 to 9, wherein the ratio of the content of the second metal atoms to the content of the first metal atoms is 0.10 to 25.

00.

11. The electrode catalyst according to any one of claims 1 to 10, which is used in carbon dioxide electrolysis.

12. A cathode comprising a catalyst layer containing the electrode catalyst according to any one of claims 1 to 11 and a gas diffusion layer.

13. An ion exchange membrane-electrode assembly comprising the cathode according to claim 12, a solid electrolyte, and an anode.

14. The ion-exchange membrane-electrode assembly according to claim 13, wherein the solid electrolyte is an anion-exchange membrane.

15. A solid electrolyte electrolytic device comprising: the cathode according to claim 12; an anode forming a pair of electrodes together with said cathode; a solid electrolyte interposed in contact between said cathode and said anode; and a voltage application unit applying a voltage between said cathode and said anode.

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