Cathode and carbon dioxide reduction device

Abstract

A cathode (10) comprises: a first layer (11) that contains copper as a main component; and a second layer (12) that is formed in at least a partial region of the surface of the first layer (11) and contains a material which is less soluble in the electrolyte solution than copper. The average thickness of the first layer (11) is 600 nm to 50,000 nm inclusive. The average thickness of the second layer (12) is 500 nm to 50,000 nm inclusive. The second layer (12) is electrically connected to the first layer (11). The first layer (11) performs an electrolytic reduction reaction of carbon dioxide.

Description

Cathode and carbon dioxide reduction apparatus

[0001] This disclosure relates to a cathode and a carbon dioxide reduction apparatus.

[0002] To mitigate the rapid climate change of recent years, there is an urgent need to develop technologies for reducing, utilizing (consuming), and storing greenhouse gases (such as carbon dioxide) that cause global warming. Electrolytic methods, which electrochemically reduce carbon dioxide, not only consume carbon dioxide but also convert it into valuable substances (C2 + valuable substances) such as ethylene and ethanol. Such electrolytic methods can be driven, for example, by renewable energy. Therefore, it is expected that systems for the sustainable generation of valuable substances can be constructed.

[0003] Patent Document 1 describes a cathode electrode for electrically reducing carbon dioxide, comprising: a first layer containing cuprous oxide, copper, and at least one other metallic element selected from the group consisting of silver, gold, zinc, cadmium, and tin; and a second layer formed on at least a portion of the first layer, containing a constituent element consisting of at least one metallic element selected from the group consisting of copper, silver, gold, zinc, cadmium, and tin.

[0004] Japanese Patent Publication No. 2023-102374

[0005] Patent Document 1 does not describe the thickness of the first layer containing copper, but it does describe that the average thickness of the second layer containing gold is 10 nm to 200 nm (0.01 μm to 0.2 μm) (paragraph 0036). However, Patent Document 1 does not consider the elution of the first layer containing copper into the electrolyte, and does not describe setting the average thickness of the first layer to 600 nm to 50,000 nm, or the average thickness of the second layer to 500 nm to 50,000 nm. The problem that this disclosure aims to solve is to provide a cathode and a carbon dioxide reduction apparatus capable of reducing carbon dioxide over a long period of time.

[0006] The cathode of this disclosure comprises a first layer containing copper as its main component, and a second layer formed on at least a portion of the surface of the first layer and containing a material less soluble in the electrolyte than copper, wherein the average thickness of the second layer is 500 nm or more and 50,000 nm or less. Other solutions will be described later in the embodiments for carrying out the invention.

[0007] According to this disclosure, it is possible to provide a cathode capable of reducing carbon dioxide over a long period of time and a carbon dioxide reduction apparatus.

[0008] This figure illustrates the structure of the cathode of the present disclosure. This figure illustrates the phenomena occurring in the cathode of the present disclosure and is a schematic diagram of the cathode after initial use. This figure illustrates the phenomena occurring when only the first layer is present in the cathode and is a schematic diagram of the cathode after initial use. This is a schematic diagram of the carbon dioxide reduction apparatus of the present disclosure. This is a schematic diagram of a carbon dioxide reduction apparatus of another embodiment. This is a graph showing the relationship between the thickness of the second layer and the amount of copper and gold eluted. This is a graph showing the relationship between the thickness of the second layer and the Faraday efficiencies of carbon monoxide and valuable substances. This is a graph showing the relationship between the thickness of the second layer and the Faraday efficiency of carbon monoxide. This is a graph showing the relationship between the thickness of the second layer and the Faraday efficiency of hydrogen. This is a graph showing the relationship between the thickness of the second layer and the value obtained by dividing the Faraday efficiency of carbon monoxide by the Faraday efficiency of hydrogen. This is a graph showing the relationship between the thickness of the first layer and the Faraday efficiency of valuable substances. This figure illustrates the region in the first layer where the electrolytic reduction reaction of carbon dioxide proceeds, using the graph in Figure 11.

[0009] The following describes embodiments for implementing this disclosure, with reference to the drawings. The following is merely an example of how to implement the invention related to this disclosure, and this disclosure is not limited to the following example. Within the description of one embodiment below, other embodiments applicable to that embodiment will also be described as appropriate. This disclosure is not limited to the following embodiment, and different embodiments can be combined or modified as appropriate without significantly impairing the effects of this disclosure. In addition, the same reference numerals will be used for the same components, and redundant explanations will be omitted. Furthermore, components having the same function will be given the same name. The illustrations are schematic, and for illustrative purposes, the actual configuration may be changed or some components may be omitted or modified between drawings without significantly impairing the effects of this disclosure. Also, the same embodiment does not necessarily need to have all the components.

[0010] Figure 1 is a diagram illustrating the structure of the cathode 10 of this disclosure. The cathode 10 is provided in a reduction device 100 that reduces carbon dioxide. Therefore, the cathode 10 is an electrode (cathode electrode) used for the reduction of carbon dioxide. It may also be used for the reduction of carbon monoxide or water in addition to carbon dioxide. By reducing carbon dioxide, valuable substances such as ethylene, ethanol, and ethane are produced as C2 valuable substances; propanol, propylene, propane, and acetone are produced as C3 valuable substances; and methane, methanol, and formic acid are produced as C1 valuable substances. Valuable substances are, for example, carbon compounds (unsaturated hydrocarbons, alcohols, ketones, carboxylic acids, etc.) with a carbon number of, for example, 1 to 5, preferably 2 to 4. Hereinafter, when simply referred to as "valuable substances," it means at least one of the C2 + valuable substances (valuable substances with 2 or more carbon atoms).

[0011] The cathode 10 comprises a first layer 11, a second layer 12, and a substrate 13.

[0012] The first layer 11 contains copper and is a layer on which the electrolytic reduction reaction of carbon dioxide proceeds. In the first layer 11, carbon dioxide that comes into contact with the first layer 11 receives electrons. As a result, carbon dioxide is reduced and the above-mentioned valuable substance is produced. Furthermore, by forming the first layer 11 in a layered manner, it is possible to easily form the first layer 11 on a desired part of the substrate 13 (for example, at least a part (preferably the whole) of the surface of the substrate 13). Moreover, by forming it in layers, it is possible to make the thickness of the first layer 11 uniform.

[0013] The copper contained in the first layer 11 usually functions as a catalyst in the electrolytic reduction reaction of carbon dioxide. The copper contained in the first layer 11 may be elemental copper or a compound (copper compound). Examples of copper compounds include any compound with copper as the main component, such as inorganic copper compounds including copper alloys containing other metals, copper oxides, copper nitrides, and copper sulfides. Other examples include organic copper compounds and copper complexes. The first layer 11 may also contain metals other than copper, but it is preferable that copper is included as the main component in the first layer 11. The main component means that copper is the most abundant component among the multiple metals contained. Even if the first layer 11 contains only copper, it can still be said that the first layer 11 contains copper as its main component.

[0014] The average thickness of the first layer 11 is 600 nm or more and 50,000 nm or less, preferably 2,000 nm or more and 50,000 nm or less, or 600 nm or more and 2,000 nm or less. By making it 600 nm or more, the first layer 11 can cover most of the reaction field where the electrolytic reduction reaction of carbon dioxide proceeds, and the electrolytic reduction reaction can proceed sufficiently.

[0015] Furthermore, by setting the thickness to 50,000 nm or less, the average thickness of the first layer 11 can be made sufficiently smaller than the maximum pore diameter of the substrate 13 (preferably the diameter of all pores). If the average thickness of the first layer 11 is too thick, all the pores of the substrate 13 will be filled, so it is desirable to have a thickness of 50,000 nm or less, which is sufficiently smaller than the maximum pore diameter of the substrate 13. This prevents the first layer 11 from being filled into the pores (voids) present in the porous substrate 13 (described later). As a result, carbon dioxide (CO2) can be released into the interior of the substrate 13. ２) and an appropriate electrolyte L can be diffused, and the carbon dioxide electrolytic reduction reaction can proceed in the first layer 11.

[0016] Furthermore, the presence of the second layer 12 makes it important for the first layer 11 to be located in the reaction field in order to maintain the Faraday efficiency of the valuable material. Since the first layer 11 has a wavelength of 600 nm to 50,000 nm, the reaction field is located in the first layer 11, thus maintaining the production of valuable material through the electrolytic reduction reaction. This allows the Faraday efficiency of the valuable material produced at the cathode 10 to be maintained at a high level over a long period. The Faraday efficiency will be discussed later. Moreover, since the first layer 11 has a wavelength of 600 nm to 50,000 nm, it is difficult for carbon dioxide supplied from the substrate 13 to permeate the first layer 11 and reach the second layer 12. This suppresses the reduction reaction of carbon dioxide (carbon monoxide production reaction) in the second layer 12, reduces the use of electrons in carbon monoxide production, and improves the electrolytic reduction efficiency.

[0017] The first layer 11 is in contact with the electrolyte L. The electrolyte L is a liquid that carries out the electrolytic reduction reaction of carbon dioxide, and is, for example, an ionic liquid, specifically an aqueous solution containing an electrolyte such as potassium bicarbonate or potassium chloride. Therefore, the first layer 11 carries out the electrolytic reduction reaction (reducing electrolysis reaction) of carbon dioxide. The electrolyte L in contact with (exposed to) the cathode 10 and the electrolyte L in contact with (exposed to) the anode 20 may be of the same type or of different types.

[0018] The electrolytic reduction reaction of carbon dioxide proceeds when carbon dioxide, copper, and electrolyte L come into contact. For this reason, it is preferable that the electrolyte L in contact with the second layer 12 (for example, the electrolyte L flowing through the cathode chamber 42 described later) permeates (wets) the second layer 12 and the first layer 11 in that order, reaching the first layer 11. This allows the electrolytic reduction reaction to proceed in the first layer 11. Therefore, it is preferable that both the first layer 11 and the second layer 12 have a thickness (film thickness) that allows the electrolyte L to permeate, and in particular, by making the average thickness of the first layer 11 50,000 nm or less, the electrolyte L can permeate into the first layer 11.

[0019] The average thickness can be measured as follows. For example, in a cross-sectional microscope image of the first layer 11, a range of the first layer 11 that is considered to have roughly the same thickness (e.g., within ±10%) can be selected, and the thickness can be measured randomly at multiple locations (e.g., three locations) within that range and averaged to calculate the average thickness. For example, foreign matter may adhere to the surface of the first layer 11, but by selecting a range of roughly the same thickness in the cross-sectional microscope image, the influence of such foreign matter can be suppressed. In the following description, unless otherwise specified, the term "thickness" refers to the "average thickness".

[0020] The first layer 11 may maintain its state before the electrolytic reduction reaction (before use), or it may change from its state before the electrolytic reduction reaction. However, in either case, it is preferable that at least a part (preferably all) of the first layer 11 is in contact with the substrate 13. This contact does not necessarily have to be on the surface of the substrate 13; it may also be in contact with the substrate 13 internally. As will be described in detail later, at least a part of the substrate 13 is conductive and hydrophobic. For example, even if a part of the first layer 11 diffuses into the microporous layer 131 of the substrate 13 during the electrolytic reduction reaction, the electrolyte L does not penetrate the substrate 13 excessively because the substrate 13 is hydrophobic. On the other hand, carbon dioxide is efficiently supplied to the microporous layer 131, so that the electrolytic reduction reaction can proceed in the first layer 11 whose form has changed, and valuable substances can be produced.

[0021] The first layer 11 can be formed on the surface of the substrate 13 by any method, such as physical vapor deposition, chemical vapor deposition, electrodeposition, or sputtering. In particular, it is preferable that the first layer 11 be formed reproducibly and uniformly on the surface (surface layer) of the substrate 13. Uniform formation allows the electrolytic reduction reaction to proceed evenly. In addition, local defects and deficiencies in the first layer 11 can be suppressed, maintaining high durability of the first layer 11 and maintaining high Faraday efficiency (described later).

[0022] The second layer 12 is a layer that suppresses the dissolution of the first layer 11 into the electrolyte L of the cathode chamber 42, for example. The internal volume of the cathode chamber 42 is much larger than the volume of the substrate 13, so once the first layer 11 dissolves into the cathode chamber 42, the possibility of the first layer 11 re-depositing is low. As a result, the volume of the first layer 11 decreases, and the Faraday efficiency of the valuable material decreases when the reaction surface area becomes insufficient. However, as will be described in detail later with reference to Figure 2, etc., in this disclosure, as the electrolytic reduction reaction of carbon dioxide progresses, the copper in the first layer 11 dissolves into the electrolyte L that seeps into the substrate 13, and then the copper contained in the electrolyte L that seeps into the substrate 13 precipitates, and this process is repeated. The second layer 12 can suppress the complete dissolution of the first layer 11 into the electrolyte L and prevent the first layer 11 from disappearing. Therefore, even as the electrolytic reduction reaction progresses, the first layer 11 can remain for a long period of time, and the decrease in reaction efficiency can be suppressed.

[0023] Furthermore, by forming the second layer 12 in a layered manner, it becomes easier to form the second layer 12 on a desired portion of the first layer 11 (for example, at least a part (preferably the whole) of the first layer 11). Moreover, forming it in layers makes it easier to make the thickness of the second layer 12 uniform.

[0024] The second layer 12 contains a material that is less soluble in the electrolyte L than copper. For example, the second layer 12 is made of a material that is less soluble in the electrolyte L than copper (the entire second layer 12 is made of this material), but a part of it may be made of this material and the remainder made of a material other than this material. However, it is preferable that the second layer 12 contains, as a main component, a material that is less soluble in the electrolyte L than copper (hereinafter referred to as the constituent material of the second layer 12 as appropriate). The meaning of the main component is the same as the matter described above for the first layer 11.

[0025] Materials that are less soluble in the electrolyte L than copper, in other words, the constituent materials of the second layer 12, are materials that do not dissolve into the electrolyte L preferentially over copper, for example, by ionization or complex formation. If the second layer 12 is, for example, an elemental metal, then the materials that are less soluble in the electrolyte L than copper are metals with a smaller ionization tendency than copper. Whether or not the constituent materials of the second layer 12 are less soluble in the electrolyte L than copper can be determined by measuring the solubility in the electrolyte L used, at the temperature (reaction temperature) and composition of the electrolyte L during the electrolytic reduction reaction.

[0026] Examples of the constituent material of the second layer 12 include, for example, a metal having a lower ionization tendency than copper, for example, at least one selected from the group consisting of Ag, Au, Os, Pd, Pt, Rh and Ru. Further, examples of the constituent material of the second layer 12 also include stable compounds (for example, oxides, nitrides, sulfides, etc.) and alloys that do not dissolve in the electrolytic solution L and are stable at 0 V under the pH of the surface of the cathode 10 (for example, 11 or more and 14 or less) when the electrolytic reduction reaction is performed. Specific examples thereof include oxides of Cr, Fe, Gd, Ho, In, Lu, Mn, Nd, Sc, Sm, Sn, Tb, Y and Zr, and high-entropy alloys which are alloys of 5 or more kinds of metals (CoCrFeMnNi-based, CoCrCuFeNi-based, systems containing Fe, Mn, Co, etc., systems containing noble metals), etc.

[0027] The second layer 12 is formed on at least a partial region of the surface of the first layer 11. The second layer 12 is formed on at least a part, preferably the whole, of the surface of the first layer 11 on the side opposite to the base material 13. Accordingly, the second layer 12 is preferably formed so as to cover the first layer 11 over the entire surface of the first layer 11. However, the second layer 12 may be formed so as to cover only a part of the first layer 11 within a range that does not significantly impair the effect provided by the first layer 11.

[0028] The second layer 12 may or may not be electrically connected to the first layer 11. Being electrically connected means that although the first layer 11 and the second layer 12 may have different electrical conductivities, for example, a current flowing through the first layer 11 also flows through the second layer 12. In an example of the present disclosure, the second layer 12 is electrically connected to the first layer 11. When the second layer 12 is a semiconductor, an insulator, or the like, no current flows through the second layer 12, and thus these are not electrically connected.

[0029] The average thickness of the second layer 12 is 500 nm or more and 50000 nm or less, preferably 1000 nm or more and 50000 nm or less, or 500 nm or more and 1000 nm or less. By setting the average thickness of the second layer 12 to 500 nm or more, the protective function of the first layer 11 can be enhanced, and elution of the first layer 11 into the electrolytic solution L present on the side opposite to the first layer 11 as viewed from the second layer 12 (for example, the electrolytic solution L in the cathode chamber 42) can be suppressed. Thereby, the first layer 11 can be retained on the side of the base material 13 as viewed from the second layer 12, the electrolytic reduction reaction can proceed over a long period of time, and the Faraday efficiency of valuables generated at the cathode 10 can be maintained at a high level over a long period of time.

[0030] Further, by setting the thickness to 50000 nm or less, the average thickness of the second layer 12 can be made sufficiently smaller than the maximum pore diameter of the base material 13 (preferably the diameters of all pores). If the average thickness of the second layer 12 is too large, all the pores of the base material 13 will be filled, so 50000 μm or less, which is sufficiently smaller than the maximum pore diameter of the base material 13, for example, is desirable. Thereby, it is possible to suppress the second layer 12 from filling the pores (voids) present in the porous base material 13. As a result, carbon dioxide (CO ２ 2) and the electrolytic solution L can be diffused as appropriate, and the carbon dioxide electrolytic reduction reaction can proceed in the first layer 11.

[0031] The average thickness of the second layer 12 can be measured in the same manner as the average thickness of the first layer 11 described above. Therefore, the average thickness is calculated by measuring the thickness at a plurality of locations (for example, 3 locations) that are considered to have substantially the same thickness (for example, within ±10%) in the first layer 11 or the second layer 12, and averaging the thicknesses at the plurality of locations. Further, the second layer 12 can be formed, for example, by the same method as that for the first layer 11 described above.

[0032] The base material 13 is a porous member that supports (fixes) the first layer 11 and the second layer 12. The first layer 11 is formed on the surface (at least part of the surface) of the base material 13. Along with this, for example, as the carbon dioxide electrolytic reduction reaction proceeds, at least a part of the base material 13 may be filled with the electrolytic solution L (contains the electrolytic solution L). FIG. 1 illustrates a state where the electrolytic solution L in the cathode chamber 42 that has passed through the first layer 11 and the second layer 12 permeates into the base material 13.

[0033] The substrate 13 comprises a microporous layer 131 made of, for example, a porous material, and a fiber layer 132 which is, for example, an aggregate of fibers that supports the microporous layer 131. Pores are formed in the microporous layer 131 and the fiber layer 132, respectively. At least a portion of the substrate 13 is hydrophobic and porous. In addition, the substrate 13 has gas diffusivity that allows carbon dioxide (gas) to be transported from one side (for example, the gas chamber 41 (described later)) to the other side (for example, the interface between the first layer 11 and the substrate 13).

[0034] The surface of the substrate 13 facing the first layer 11 is hydrophobic. This prevents the electrolyte L from excessively penetrating the substrate 13. In other words, the electrolyte L is not completely prevented from penetrating the substrate 13; as described above, it can penetrate the substrate 13 to some extent. However, even when the electrolyte L penetrates the substrate 13, it is preferable that the electrolyte L is retained only in at least a portion of the microporous layer 131 and not penetrate the fiber layer 132. The degree of penetration of the electrolyte L into the substrate 13 can be controlled, for example, by forming the second layer 12 having the above-mentioned average thickness, or by operating control such as intermittent stopping of the electrolytic reduction reaction.

[0035] The size of the pores (micropores, voids, air pockets) in the substrate 13 should be large enough to adequately transport carbon dioxide and retain the electrolyte L in the pores. For example, the average pore diameter can be 1,000 nm or more and 750,000 nm or less, preferably 3,000 nm or more and 500,000 nm or less, and more preferably 5,000 nm or more and 300,000 nm or less. The average pore diameter can be determined, for example, by taking the length of the longest part of the pores in the substrate 13 observed in a cross-sectional microscope photograph, and using the average value of the pore diameters measured for multiple pores, such as 10. If the substrate 13 is composed of materials or structures having multiple different average pore diameters, the average pore diameter of the substrate 13 should be the average value of the average pore diameters in each material or structure.

[0036] The base material 13 can be made of, for example, a hydrophobic and porous material such as carbon material or polytetrafluoroethylene. The base material 13 may also be made by decorating the surface of an inexpensive metal mesh with a material such as a hydrophobic and porous material.

[0037] Figure 2 is a diagram illustrating the phenomena occurring in the cathode 10 of this disclosure, and is a schematic diagram of the cathode 10 after the start of use. Figure 2 is a schematic diagram of the cathode 10 after a predetermined time (e.g., 10 minutes to several tens of minutes, several hours, etc.) has elapsed from immediately before the start of use as shown in Figure 1. As the electrolytic reduction reaction progresses, for example, the layered first layer 11, which is formed somewhat densely, begins to dissolve into the electrolyte L held in the pores of the substrate 13. As a result, the first layer 11 begins to become finer. However, since the first layer 11 is sandwiched between the second layer 12 and the substrate 13, the particles 111, which are the finer first layer 11, do not go beyond the second layer 12 and reach the electrolyte L. Also, if the size of the particles 111 is larger than the pore diameter of the substrate 13, the particles 111 do not penetrate the substrate 13.

[0038] If the particles 111 are further refined to produce particles 112 smaller than the pore size of the substrate 13, the particles 112 can penetrate the substrate 13. The particles 112 are, for example, nanoparticles with an average particle size of 1 nm to 500 nm, based on laser diffraction scattering. As described above, the electrolyte L can penetrate the substrate 13 to some extent. For this reason, the particles 112 are retained in the pores of the substrate 13 (especially the microporous layer 131) together with the electrolyte L. The particles 112 are not dissolved in the electrolyte L and exist in the electrolyte L in a fine form. The particles 112 retained in the substrate 13 can also contribute to the electrolytic reduction reaction.

[0039] Thus, the substrate 13 (especially the microporous layer 131) may contain copper derived from the first layer 11. As described above, the copper is usually contained as particles 112, but may also be contained as particles 111. Carbon dioxide is supplied to the microporous layer 131 via the fiber layer 132. Therefore, in addition to the first layer 11, the electrolytic reduction reaction also proceeds near particles 111 and 112 in the substrate 13. In particular, the presence of the second layer 12 suppresses the elution of particles 111 and 112 into the electrolyte L on the opposite side of the substrate 13 (the electrolyte L in the cathode chamber 42). Therefore, the electrolytic reduction reaction can be maintained for a long period of time. Furthermore, since particles 112 are supported on the substrate 13, the electrolytic reduction reaction in the second layer 12 can be controlled.

[0040] As the electrolytic reduction reaction progresses further, the particles 112 eventually dissolve in the electrolyte L within the substrate 13. However, since the reduction reaction continues at the cathode 10, the cathode 10 (at least the first layer 11) is continuously supplied with the energy to return copper ions to elemental copper (solid). Therefore, the copper dissolved near the first layer 11 easily precipitates.

[0041] Thus, in this disclosure (example), as the electrolytic reduction reaction of carbon dioxide progresses, the copper in the first layer 11 dissolves into the electrolyte L in the substrate 13, and copper precipitates from the electrolyte L in the substrate 13, and this process is repeated.

[0042] On the other hand, Figure 3 is a diagram illustrating the phenomenon that occurs when only the first layer 11 is present in the cathode 10 (i.e., the second layer 12 is absent) (a diagram of a comparative example), and is a schematic diagram of the cathode 10 showing the state after the start of use. Figure 3 is a schematic diagram of the cathode 10 after a predetermined time has elapsed (e.g., 10 minutes to several tens of minutes, several hours, etc.) from just before the start of use. In Figure 3, the second layer 12 is absent, and the first layer 11 is in direct contact with the electrolyte L. Therefore, the first layer 11 is more likely to dissolve into the electrolyte L in the cathode chamber 42 than into the substrate 13. As a result, a portion of the first layer 11 gradually dissolves and diffuses into the electrolyte L in the opposite direction from the substrate 13. Once the first layer 11 (particles 111, 112) has dissolved, the possibility of it returning to the first layer 11 is extremely low.

[0043] In the example of this disclosure, the amount of carbon dioxide consumed per unit time by the reduction reaction of carbon dioxide in the first layer 11 is greater than the amount of carbon dioxide consumed per unit time by the reduction reaction of carbon dioxide in the second layer 12. This can be achieved by the average thickness of the first layer 11 being within the above range, and the second layer 12 being located on the electrolyte side rather than the substrate 13 side when viewed from the first layer 11. That is, by setting the average thickness of the first layer 11 to 600 nm or more and 50,000 nm or less, the supply of carbon dioxide to the second layer 12 via the first layer 11 can be suppressed. This suppresses the consumption of carbon dioxide in the second layer 12.

[0044] Here, the amount of a predetermined component produced or decomposed per unit electron is defined as the Faraday efficiency of the predetermined component. In this case, the value obtained by dividing the Faraday efficiency of carbon monoxide produced by the electrolytic reduction reaction of carbon dioxide by the Faraday efficiency of hydrogen produced by the electrolytic reduction reaction of carbon dioxide is 0.5 or more and 1.5 or less. Therefore, even if the reduction reaction of carbon dioxide proceeds, the value does not increase or decrease because only one of carbon monoxide or hydrogen increases or decreases. In the example of this disclosure, the electrolytic reduction reaction of carbon dioxide mainly proceeds in the first layer 11 and hardly proceeds in the second layer 12. Also, since the hydrogen and carbon monoxide that may be produced as by-products are not converted into valuable substances, the value is maintained at 0.5 or more and 1.5 or less.

[0045] Figure 4 is a schematic diagram of the carbon dioxide reduction apparatus 100 of this disclosure. The reduction apparatus 100 shown in Figure 4 is a flow-type electrochemical cell that reduces carbon dioxide while flowing an electrolyte L and carbon dioxide.

[0046] The reduction device 100 comprises a cathode 10, an anode 20, an ion exchange membrane 30, a housing 40, and an electrolyte L. The cathode 10, anode 20, and ion exchange membrane 30 are housed in the housing 40. The anode 20 is placed between the cathode 10 and the ion exchange membrane 30 (an example of a diaphragm). Thus, the ion exchange membrane 30 is placed between the cathode 10 and the anode 20. Inside the housing 40, an anode chamber 43 is formed between the anode 20 and the ion exchange membrane 30. Inside the housing 40, a cathode chamber 42 is formed between the cathode 10 (particularly the second layer 12) and the ion exchange membrane 30. The second layer 12 is located between the first layer 11 and the electrolyte L (for example, the electrolyte L in the cathode chamber 42).

[0047] A control unit 52, equipped with, for example, a liquid transfer pump, is connected to the cathode chamber 42 and the anode chamber 43. The control unit 52 is a device that delivers electrolyte L to the cathode chamber 42 and the anode chamber 43 at arbitrary timing, flow rate, and pressure. The control unit 52 supplies electrolyte L to the cathode chamber 42 and the anode chamber 43. As a result, the electrolyte L comes into contact with the cathode 10 and the anode 20. The supplied electrolyte L is discharged from the cathode chamber 42 and the anode chamber 43, respectively, on a side other than the supply side (for example, the opposite side). At this time, if a liquid product L1 (ethanol, etc.) is generated at the cathode 10, the liquid product L1 is discharged from the cathode chamber 42 along with the electrolyte L.

[0048] On the other hand, inside the housing 40, a gas chamber 41 is formed on the side opposite to the cathode chamber 42 as viewed from the cathode 10. The base material 13 of the cathode 10 is exposed in the gas chamber 41. Therefore, the base material 13 faces the gas chamber 41, and carbon dioxide is supplied to the cathode 10. A control unit 51, which includes, for example, an air pump and a mass flow controller, is connected to the gas chamber 41. The control unit 51 is a device that supplies carbon dioxide to the gas chamber 41 at any timing, flow rate, and pressure. Carbon dioxide is supplied to the gas chamber 41 by the control unit 51. It is preferable that the control unit 51 supplies carbon dioxide to the gas chamber 41 at a constant pressure so that the gaseous product G1 does not flow back into the control unit 51.

[0049] In the gas chamber 41, as described above, carbon dioxide enters the cathode 10 from the substrate 13, and the electrolytic reduction reaction of carbon dioxide proceeds at the cathode 10. At this time, if gaseous product G1 is produced at the cathode 10, the gaseous product G1 (for example, ethylene) is discharged from the gas chamber 41 along with unreacted carbon dioxide.

[0050] The ion exchange membrane 30 is provided as a diaphragm to allow only specific ions to pass through and to enable a sustained reaction. However, the ion exchange membrane 30 is not necessarily required. Furthermore, in order to control the potential of the cathode 10, the reduction device 100 preferably includes a reference electrode (not shown). That is, in order to control the electrolytic reduction reaction with good reproducibility, it is preferable to adopt a three-electrode structure using a reference electrode.

[0051] A control unit 50 is connected to the cathode 10 and the anode 20. The control unit 50 is a device that controls the current and voltage applied to the cathode 10 and the anode 20. Therefore, the anode 20 is energized together with the cathode 10. The control unit 50 includes, for example, a potentiostat.

[0052] At the cathode 10, carbon dioxide is mainly reduced by accepting electrons, and valuable substances such as ethanol are produced in the cathode chamber 42. Meanwhile, at the anode 20, water in the electrolyte L releases electrons, generating hydrogen ions and oxygen molecules in the anode chamber 43. The released electrons reach the cathode 10. The hydrogen ions permeate the ion exchange membrane 30 and reach the cathode chamber 42.

[0053] Figure 5 is a schematic diagram of a carbon dioxide reduction device 100 according to another embodiment. The reduction device 100 shown in Figure 5 is an electrode-integrated electrochemical cell in which the cathode 10 and anode 20 are integrated. By integrating the electrodes consisting of the cathode 10 and anode 20, the distance between the cathode 10 and anode 20 can be made as close to zero as possible, and the electrolysis voltage can be reduced.

[0054] An ion exchange membrane 30 is placed between the integrated cathode 10 and anode 20. Therefore, the substrate 13, the first layer 11, the second layer 12, the ion exchange membrane 30, and the anode 20 are stacked (in contact) in this order. The resulting stack is placed inside the housing 40, and the inside of the housing 40 is divided into a gas chamber 41 and a liquid chamber 44. Here, when a voltage is applied to the cathode 10 and anode 20, the electrolyte L passes through the ion exchange membrane and reaches the surface of the cathode 10, that is, the second layer 12, where the reaction begins. In Figure 5, as in Figure 4, the electrolyte L enters from the surface side of the second layer during the electrochemical reaction.

[0055] Unlike the example in Figure 4, the reduction apparatus 100 in Figure 5 includes a liquid chamber 44 that serves as both a cathode chamber 42 and an anode chamber 43. The electrolyte L is supplied to the liquid chamber 44 by the control unit 52. When liquid products L1 are generated at the cathode 10, the liquid products L1 are discharged from the liquid chamber 44 along with the electrolyte L. If the liquid products L1 cannot permeate the ion exchange membrane 30, the liquid products L1 are discharged from the gas chamber 41.

[0056] In the cathode 10 of this disclosure, as described above, the average thickness of the first layer 11 is 600 nm or more and 50,000 nm or less. This allows copper to be placed in the reaction field of the electrolytic reduction reaction in the first layer 11 even when the second layer 12 is provided on the surface of the first layer 11, and the Faraday efficiency of the valuable material can be maintained. Furthermore, in the cathode 10 of this disclosure, as described above, the average thickness of the second layer 12 is 500 nm or more and 50,000 nm or less. This suppresses excessive leaching of copper into the electrolyte L on the side of the second layer 12 opposite to the first layer 11 (for example, the electrolyte L of the cathode chamber 42 or the liquid chamber 44). This allows leaching and deposition into the electrolyte L on the side of the first layer 11 relative to the second layer 12 to be repeated, and the leaching becomes reversible. For this reason, the electrolytic reduction reaction with copper can be carried out over a long period of time.

[0057] The present disclosure will be further described below with reference to examples.

[0058] <Example 1> In Example 1, we investigated the effect of carbon monoxide generation on the production of valuable substances (ethylene, ethanol, etc.).

[0059] The reduction apparatus 100 shown in Figure 4 above was fabricated. The materials used are as follows: First layer 11: Pure copper Second layer 12: Either pure gold or pure silver Substrate 13: AvCarb GDS2130 manufactured by AvCarb Material Solutions Anode 20: Pure platinum mesh Reference electrode: Ag / AgCl electrode (3M KCl)

[0060] Using the fabricated reduction apparatus 100, an electrolytic reduction reaction was carried out under the following experimental conditions: Electrolyte L supplied to cathode 10: 1M KCl aqueous solution; Electrolyte L supplied to anode 20: saturated KHCO3 ３ Current value applied to aqueous solution cathode 10 and anode 20: 400 mA / cm² ２ Reaction time (energizing time): 60 minutes. Flow rate of carbon dioxide supplied to cathode 10: 30 sccm (standard mL / min; flow rate is volume at 0°C).

[0061] To investigate the effect of the second layer 12 on suppressing the elution of the first layer 11, the metal ion concentration in the electrolyte L was analyzed. The thickness of the first layer 11 was fixed at 1000 nm, and the thickness of the second layer 12 was varied to t (nm).

[0062] Figure 6 is a graph showing the relationship between the thickness of the second layer 12 (Au, elemental gold) (horizontal axis) and the amount of copper (Cu) and gold (Au) leached (vertical axis). In the graph, black squares represent the amount of copper leached, and black triangles represent the amount of gold leached. For the amount of copper leached, error bars based on the measurement accuracy of the analyzer are shown as solid lines, and a curve (dashed line) smoothly connecting the plots at each thickness is also shown. The amount of leached was determined by calculating the ratio (%) of the mass of the first layer 11 and the second layer 12 before the start of the reaction, using the average of the copper and gold content (mass) in the electrolyte L after the reaction was completed, measured twice. Therefore, an leaching amount of 100% means that all of the formed first layer 11 or second layer 12 has leached into the electrolyte L, and the first layer 11 or second layer 12 has disappeared.

[0063] As shown in Figure 6, gold (Au) did not dissolve into the electrolyte L regardless of the thickness of the second layer 12. On the other hand, when the thickness of the second layer 12 was 0 nm and 100 nm, about 10% of the copper (Cu) dissolved, and clear dissolution was observed. In particular, as mentioned above, Patent Document 1 describes setting the average thickness of the gold-containing second layer to 10 nm or more and 200 nm or less. However, even considering the measurement error shown in the error bar, clear dissolution of the copper-based first layer 11 was observed in the case of 100 nm, which falls within that range. Therefore, it was shown that the gold-based second layer does not suppress the dissolution of the copper-based first layer in the technique described in Patent Document 1.

[0064] On the other hand, by making the thickness (average thickness) of the second layer 12 (Au) 500 nm or more, the amount of copper leached out could be reduced to about half (about 5%) of the amount leached out at 0 nm and 100 nm (about 10%). If it is less than 500 nm, the second layer 12 is thin, and the first layer 11 that dissolves into the electrolyte L through the gaps in the second layer 12 leaches out. On the other hand, there are no properties required of the second layer 12 other than resistance to leaching into the electrolyte L. For this reason, there are no restrictions on interatomic distance, etc., and the same effect can be obtained with substances other than gold. For example, when the thickness of the first layer 11 (Cu) was 1000 nm and the thickness of the second layer 12 (Ag, the elemental silver mentioned above) was 500 nm, the amount of Ag leached out was below the detection limit (0%), and the amount of copper (Cu) leached out was about 5%, which is the same result as when gold (Au) was used. Therefore, it was found that by making the thickness of the second layer 12 500 nm or more, the second layer 12 can function as a protective film that sufficiently suppresses the elution of the first layer 11.

[0065] Furthermore, the time it takes for the first layer 11 to decrease to the equivalent of 5 nm is defined as the durability time of the cathode 10. It is also assumed that a certain percentage of the total Cu content dissolves in a certain amount of time (10 mass% / h). Based on the results in Figure 6 (especially the dashed curve), the durability time when the second layer 12 is not formed is calculated to be 49 hours, 83 hours when the thickness of the second layer 12 is 500 nm, and 134 hours when it is 1000 nm. Similarly, when calculating for the numerical range described in Patent Document 1 (10 nm to 200 nm), the durability time was calculated to be 50 to 60 hours. Considering measurement errors, a durability time of 50 to 60 hours is almost the same as when the second layer 12 is not formed (49 hours). This is consistent with the fact that, as mentioned above, when the numerical range described in Patent Document 1 is used, the second layer 12 made of gold does not suppress the dissolution of the first layer 11 made of copper. However, by setting the thickness (average thickness) of the second layer 12 to a range larger than the numerical range in Patent Document 1, i.e., between 500 nm and 50,000 nm, the durability can be significantly extended, for example, by more than 1.5 times, and up to approximately 2.6 times, compared to the durability of the technology described in Patent Document 1 (50 to 60 hours).

[0066] Figure 7 is a graph showing the relationship between the thickness of the second layer 12 (Au) (horizontal axis) and the Faraday efficiencies of carbon monoxide and valuable substances (vertical axis). In the graph, the black squares represent the Faraday efficiencies (FE) of valuable substances. Ｃ２＋ ), the black triangle represents the Faraday efficiency (FE) of carbon monoxide ＣＯ ) is FE Ｃ２＋ It fluctuates slightly depending on the thickness of the second layer 12, but generally remains high. On the other hand, FE ＣＯThe same trend is observed, and the levels remain low. Therefore, although the details will be explained in Example 2, if the thickness (average thickness) of the first layer 11 is 600 nm or more, the generation of carbon monoxide can be suppressed, and valuable substances can be efficiently produced. In particular, the production of valuable substances can be mainly carried out in the first layer 11, as will be described later in Example 2. Furthermore, the function of the second layer 12 is not required to be a carbon dioxide reduction catalyst; it is sufficient that the second layer 12 does not dissolve and suppresses the dissolution of the first layer 11. In other words, the constituent material of the second layer 12 does not necessarily have to be gold as described above; the second layer 12 should be composed of a material that is less soluble in the electrolyte L than the material of the first layer 11. Also, since the functions of the first layer 11 and the second layer 12 are independent of each other, as long as the first layer 11 is a catalyst mainly composed of copper, and the second layer 12 is composed of a material that is less soluble in the electrolyte L than the material of the first layer 11, any combination of materials can be selected.

[0067] In Example 1, the thickness of the first layer 11 was fixed at 1000 nm for the sake of simplifying the experiment. However, even if the thickness of the first layer 11 is greater than 1000 nm, as will be explained later with reference to Example 2, only the reaction field for the electrolytic reduction reaction of carbon dioxide will increase, and it is thought that the same trend as in Figure 7 will be obtained. Therefore, by setting the upper limit of the thickness (average thickness) of the first layer 11 to 50000 nm, the same results as in Figure 7 can be applied, and carbon dioxide and electrolyte L can permeate through the first layer 11.

[0068] <Example 2> Let the thickness (average thickness) of the second layer 12 (Au) be t (nm), and the thickness (average thickness) of the first layer 11 (Cu) be 2000-t (nm) (that is, the total thickness of the first layer 11 and the second layer 12 is 2000 nm). The Faraday efficiency of carbon monoxide and hydrogen generated at the cathode 10 was calculated. For carbon monoxide (the same applies to hydrogen), the Faraday efficiency is the ratio of the number of electrons actually used to generate carbon monoxide to the number of electrons actually passed. While measuring the amount of substance of the generated carbon monoxide, the theoretical number of electrons used for generating carbon monoxide can also be calculated from the chemical reaction equation of the electrolytic reduction reaction. In <Example 2>, an experiment was conducted under the same experimental conditions using the reduction apparatus 100 used in <Example 1> above. However, the reaction time was changed to 20 minutes, and the second layer 12 was made of single gold.

[0069] FIG. 8 is a graph showing the relationship between the thickness of the second layer 12 (Au) (horizontal axis) and the Faraday efficiency of carbon monoxide (vertical axis, FE ＣＯ ). FIG. 9 is a graph showing the relationship between the thickness of the second layer 12 (Au) (horizontal axis) and the Faraday efficiency of hydrogen (vertical axis, FE Ｈ２ ). For reference, when the first layer 11 is not provided (that is, when t=2000), FE ＣＯ was 43%, and FE Ｈ２ was 54%. Further, in this case, no valuable material was generated, and the Faraday efficiency of the valuable material (FE Ｃ２＋ ) was 0%.

[0070] When both the first layer 11 and the second layer 12 are formed, that is, when the value of t is other than 0 and 2000, both FE ＣＯ and FE Ｈ２ increased as the thickness of the second layer 12 increased. If the carbon monoxide generated in the second layer 12 is converted into valuable material in the first layer 11, it is considered that carbon monoxide is consumed. Due to the consumption, the amount of carbon monoxide taken out from the gas chamber 41 decreases, so it is considered that the apparent FE ＣＯ also decreases. On the other hand, hydrogen generated in the second layer 12 does not participate in the conversion of carbon monoxide into valuable material. For this reason, FE Ｈ２It is thought that this will increase. As a result, the Faraday efficiency (FE) of carbon monoxide produced by the electrolytic reduction reaction of carbon dioxide will increase. ＣＯ ) is the Faraday efficiency (FE) of hydrogen produced by the electrolytic reduction reaction of carbon dioxide. Ｈ２ The value obtained by dividing by ), i.e., FE ＣＯ / FE Ｈ２ The ratio (hereinafter sometimes simply referred to as "the ratio") should become smaller.

[0071] Figure 10 is a graph showing the relationship between the thickness of the second layer 12 (Au) (horizontal axis) and the value obtained by dividing the Faraday efficiency of carbon monoxide by the Faraday efficiency of hydrogen (vertical axis; "ratio" above). As shown in Figure 10, the ratio generally increases as the thickness of the second layer 12 increases, but decreases when the thickness exceeds 1700 nm. Therefore, although a local decrease was observed, it cannot be said that the ratio decreases regardless of thickness, and it cannot be said that the carbon monoxide produced in the second layer 12 is converted into a valuable substance in the first layer 11. Furthermore, the ratio stays between 0.5 and 1.5, and considering experimental errors, the ratio can be said to be generally constant.

[0072] The results in Figure 10 show that the carbon monoxide generated in the second layer 12 is not converted into a valuable substance, and that the generation of carbon monoxide in the second layer 12 does not have the effect of promoting the generation of valuable substances. In other words, it was found that the generation of carbon monoxide in the second layer 12 does not contribute to the selectivity of valuable substance generation at the cathode 10. Therefore, when the goal is to generate valuable substances, there is no advantage to reducing carbon dioxide in the second layer 12 using the cathode 10, and it is preferable to reduce carbon dioxide as much as possible in the first layer 11, that is, mainly in the first layer 11.

[0073] <Example 3> In order to investigate the reaction site of electrolytic reduction in the first layer 11, the relationship between the thickness of the first layer 11 and the Faraday efficiency of the valuable substance was investigated using the reduction apparatus 100 used in <Example 1> above, under the same experimental conditions. However, unlike the reduction apparatus 100 used in <Example 1> above, the thickness of the first layer 11 was changed to t (nm), and the thickness of the second layer 12 was fixed at 500 nm. Also, the reaction time was changed to 20 minutes, and the second layer 12 was made of elemental gold.

[0074] The experimental results yielded the relationships shown in Table 1 below.

[0075]

[0076] Figure 11 shows the thickness of the first layer (Cu) (horizontal axis, t) and the Faraday efficiency of the valuable material (vertical axis, FE). Ｃ２＋ This is a graph showing the relationship between ). Figure 11 and Figure 12 below are graphs plotting the results from Table 1 above. When t = 0, the first layer 11 is not formed. FE of the first layer 11 and the second layer 12 Ｃ２＋ By evaluating this, it is possible to assess the extent to which carbon dioxide is reduced in the first layer 11. Although not shown in the diagram, in a cathode 10 that has only the first layer 11 and no second layer 12, FE Ｃ２＋ This varied somewhat depending on the thickness of the first layer 11, but was approximately 65% ​​to 70%. However, the durability time was significantly shorter than that of the example in this disclosure.

[0077] When the thickness of the first layer 11 is 70 nm and 300 nm, FE Ｃ２＋ It was less than 60%. However, when the thickness (average thickness) of the first layer 11 was 600 nm, FE Ｃ２＋ It was approximately 65%. And, when the thickness of the first layer 11 was 600 nm to 2000 nm, FE Ｃ２＋ Although the percentage increased to more than 65%, the degree of increase was small. From this, it was found that if the thickness (average thickness) of the first layer 11 is at least 600 nm, carbon dioxide can be efficiently electrolytically reduced mainly in the first layer 11.

[0078] Figure 12 is a diagram illustrating the region in the first layer 11 where the electrolytic reduction reaction of carbon dioxide proceeds, using the graph in Figure 11. Between thicknesses of the first layer 11 from 0 nm to 600 nm, as described above, FE Ｃ２＋ It increases from 0% to approximately 65%. On the other hand, between 600 nm and 2000 nm, the increase is small as described above, and it only increases from approximately 65% ​​to approximately 70%. In that case, the FE obtained between 0 nm and 2000 nm Ｃ２＋ Of the (0% to 70%), it was found that about 90% could be obtained in the 0 nm to 600 nm range. As explained in Figure 12, FE Ｃ２＋If we consider the portion between 0% and 70% as D1 (0% to 65%) and D2 (65% to 70%), then the ratio of the sizes of D1 to D2 is approximately 9:1. Therefore, if the thickness (average thickness) of the first layer 11 is 600 nm or more, the FE will increase. Ｃ２＋ At least 90% of these can be achieved, and high FE Ｃ２＋ It was found that this could be obtained.

[0079] 10 Cathode 100 Reduction device 11 First layer 111 Particles 112 Particles 12 Second layer 13 Substrate 131 Microporous layer 132 Fiber layer 20 Anode 30 Ion exchange membrane 40 Housing 41 Gas chamber 42 Cathode chamber 43 Anode chamber 44 Liquid chamber 51 Control unit 52 Control unit 53 Control unit G1 Gas product L Electrolyte L1 Liquid product

Claims

1. A cathode comprising a first layer containing copper as its main component, and a second layer formed on at least a portion of the surface of the first layer and containing a material less soluble in the electrolyte than copper, wherein the average thickness of the second layer is 500 nm or more and 50,000 nm or less.

2. A cathode comprising a first layer containing copper as its main component, and a second layer formed on at least a portion of the surface of the first layer and containing a material less soluble in the electrolyte than copper, wherein the average thickness of the first layer is 600 nm or more and 50,000 nm or less.

3. A cathode according to claim 1, characterized in that the average thickness of the first layer is 600 nm or more and 50,000 nm or less.

4. A cathode according to any one of claims 1 to 3, characterized in that the second layer is electrically connected to the first layer.

5. A cathode according to any one of claims 1 to 3, wherein the first layer is characterized by performing an electrolytic reduction reaction of carbon dioxide.

6. A cathode according to any one of claims 1 to 3, comprising a porous substrate on which the first layer is formed and which contains an electrolyte, wherein the substrate contains copper derived from the first layer.

7. A cathode according to any one of claims 1 to 3, comprising a porous substrate on which the first layer is formed, characterized in that, as the electrolytic reduction reaction of carbon dioxide progresses, the copper in the first layer dissolves into the electrolyte in the substrate and copper precipitates from the electrolyte in the substrate, and this process is repeated.

8. A cathode according to any one of claims 1 to 3, wherein the average thickness is calculated by measuring the thickness of a plurality of locations in the first layer or the second layer that are considered to be of approximately the same thickness, and averaging the thicknesses of the plurality of locations.

9. A carbon dioxide reduction apparatus comprising a cathode according to any one of claims 1 to 3, an anode that is energized together with the cathode, and an electrolyte that is in contact with the cathode and the anode, wherein carbon dioxide is supplied to the cathode.

10. A carbon dioxide reduction apparatus according to claim 9, further comprising a diaphragm.

11. A carbon dioxide reduction apparatus according to claim 10, characterized in that the second layer is located between the first layer and the electrolyte.

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