Electrodes, water electrolytic cells, water electrolytic devices, carbon dioxide reduction electrolytic cells, and carbon dioxide electrolytic devices

Abstract

To provide an electrode that shows a low activation voltage in a neutral pH range.SOLUTION: The electrode contains a catalyst including at least one first metal element selected from the group consisting of nickel, iron, and cobalt and at least one second metal element selected from the group consisting of copper, silver, and gold.SELECTED DRAWING: Figure 2

Description

[Technical Field]

[0001] The present invention relates to an electrode exhibiting low activation overpotential in the neutral pH range, a water electrolytic cell, a water electrolytic device, a carbon dioxide reduction electrolytic cell, and a carbon dioxide electrolytic device. [Background technology]

[0002] While the effective use of renewable energy is a pressing social need for realizing a sustainable society, renewable energy is subject to significant regional disparities, as well as temporal, seasonal, and meteorological fluctuations, making stable supply difficult and hindering its large-scale use. From this perspective, a method has been proposed to level out the energy supply by producing hydrogen from electricity derived from renewable energy sources through the electrolysis of water (water electrolysis), storing and transporting that hydrogen, and then using hydrogen power generation equipment or fuel cells to extract energy when and where it is needed.

[0003] Industrial methods for water electrolysis mainly include alkaline water electrolysis, which uses concentrated aqueous solutions of sodium hydroxide or potassium hydroxide, and proton exchange membrane (PEM) water electrolysis, which uses proton exchange membranes. Because these technologies use strongly basic or strongly acidic electrolytes, the materials used for the components of the electrolysis apparatus require high corrosion resistance.

[0004] Therefore, if water electrolysis can be carried out in a neutral pH range from weakly acidic to weakly basic, the water electrolysis device can be constructed from general-purpose materials, which will extend the lifespan of the electrolysis device. For this reason, the development of water electrolysis in the neutral pH range is being considered. However, water electrolysis in the neutral pH range tends to be less energy efficient, meaning the electrolysis voltage is higher, compared to strongly basic or strongly acidic conditions. One of the factors contributing to the low energy efficiency of water electrolysis is the low activity of the oxygen evolution reaction in the neutral pH range.

[0005] Therefore, techniques to improve the activity of oxygen evolution reactions in the neutral pH range, that is, techniques to develop oxygen evolution electrodes with low activation overpotential, are being investigated. For example, Non-Patent Document 1 discloses that by creating an electrode on a titanium substrate by electrodepositing an iridium oxide catalyst and performing water electrolysis in a buffering electrolyte such as a phosphate buffer, a low activation overpotential can be achieved even in the neutral pH range.

[0006] Furthermore, Non-Patent Document 2 discloses that by applying a predetermined potential to a nickel foam and performing oxidation-reduction, a nanostructure is formed on its surface, thereby increasing the electrical double layer capacitance and obtaining an oxygen-evolving electrode with low activation overpotential.

[0007] In addition, Non-Patent Document 3 discloses a carbon dioxide reduction electrolytic cell technology in which a carbon dioxide reduction reaction is carried out at the cathode and an oxygen evolution reaction is carried out at the anode. In a carbon dioxide reduction electrolytic cell, if a strongly acidic electrolyte is used, the hydrogen evolution reaction tends to take precedence over the carbon dioxide reduction reaction at the cathode, and if a strongly basic electrolyte is used, the electrolyte may be neutralized by the acid-base reaction between the base and carbon dioxide. For these reasons, since carbon dioxide reduction electrolytic cells are often operated in the neutral pH range (potassium bicarbonate aqueous solution is used as the electrolyte in Non-Patent Document 3), oxygen-evolving electrodes with low activation overpotential in the neutral pH range are an important technology not only for water electrolysis but also for carbon dioxide reduction electrolysis. [Prior art documents] [Patent Documents]

[0008] [Non-Patent Document 1] T. Nishimoto,et al.,ChemSusChem 2021,14,1554-1564 [Non-Patent Document 2] T. Shinagawa,et al.,Angew. Chem. Int. Ed. 2017,56,5061-5065 [Non-Patent Document 3] R. Krause,et al.,Chem. Ing. Tech. 2020,92,53-61 [Overview of the project] [Problems that the invention aims to solve]

[0009] However, even when water electrolysis or carbon dioxide reduction electrolysis is performed using the technologies described in Non-Patent Documents 1 to 3, the energy efficiency of water electrolysis or carbon dioxide reduction electrolysis in the neutral pH range is still insufficient, and further technological improvements were desired.

[0010] Therefore, the present invention aims to provide electrodes, water electrolytic cells, water electrolytic devices, carbon dioxide reduction electrolytic cells, and carbon dioxide electrolytic devices that exhibit low activation overpotential in the neutral pH range. [Means for solving the problem]

[0011] As a result of their investigations to solve the above problems, the present inventors have found that by incorporating a catalyst containing at least one primary metal element selected from the group consisting of nickel, iron, and cobalt, and at least one secondary metal element selected from the group consisting of copper, silver, and gold, into the electrode, it is possible to exhibit high oxygen evolution activity, i.e., low activation overpotential in the oxygen evolution reaction, in the neutral pH range (pH of 1.5 to 12.6 when not electrolytic), and have completed the present invention.

[0012] This invention is based on the above findings, and its gist is as follows. 1. An electrode characterized by containing a catalyst comprising at least one primary metallic element selected from the group consisting of nickel, iron, and cobalt, and at least one secondary metallic element selected from the group consisting of copper, silver, and gold. 2. The electrode according to claim 1, characterized in that the catalyst is a metal oxide containing the first metal element and the second metal element. 3. The electrode according to claim 1 or 2, characterized in that the catalyst is supported on a conductive substrate. 4. The electrode according to 3, wherein the conductive substrate contains a transition metal. 5. The electrode according to 3 or 4, wherein the conductive substrate is a porous body. 6. The electrode according to any one of 3 to 5, wherein the transition metal is at least one selected from the group consisting of nickel, titanium, and iron. 7. The electrode according to any one of 3 to 6, wherein the conductive substrate is produced by an activation treatment of electrochemically oxidizing and reducing. 8. The conductive substrate is 1.25 mF / cm 2 10. The electrode according to any one of 3 to 7, which has an electric double layer capacitance of 1.25 mF / cm or more. 9. The electrode according to any one of 3 to 8, wherein the catalyst is obtained by immersing the conductive substrate in an electrolytic solution containing a soluble salt of the first metal element and a soluble salt of the second element and performing electrodeposition. 10. The electrode according to any one of 1 to 9, which is an electrode for oxygen generation. 11. A water electrolysis cell, wherein the electrode according to any one of 1 to 10 is used as an oxygen generation electrode. 12. The water electrolysis cell according to 11, wherein a buffer solution having a pH of 1.5 to 12.6 during non-electrolysis is used as the electrolytic solution. 13. A water electrolysis apparatus, comprising the water electrolysis cell according to 11 or 12. 14. A carbon dioxide reduction electrolysis cell, wherein the electrode according to any one of 1 to 10 is used as an oxygen generation electrode. 15. The carbon dioxide reduction electrolysis cell according to 14, wherein a buffer solution having a pH of 1.5 to 12.6 during non-electrolysis is used as the electrolytic solution. 16. A carbon dioxide electrolysis apparatus, comprising the carbon dioxide reduction electrolysis cell according to 14 or 15.

Advantages of the Invention

[0013] According to the present invention, it is possible to provide an electrode, a water electrolysis cell, a water electrolysis apparatus, a carbon dioxide reduction electrolysis cell, and a carbon dioxide electrolysis apparatus that exhibit a low activation overvoltage in the neutral pH range.

Brief Description of the Drawings

[0014] [Figure 1] It is a diagram showing the relationship between the current density and the potential of the working electrode (iR correction) when the electrodes of each example and comparative example are used. [Figure 2] It is a diagram showing the change in potential (iR correction) in an on-off test (on: 400 mA / cm2 for 15 minutes, off: 5 minutes) when the electrodes of each example and comparative example are used as oxygen evolution electrodes. [Figure 3] It is a photograph obtained by observing, with a scanning electron microscope, (a) the activated nickel foam and (b) the state in which iron-copper oxide is supported thereon, for the oxygen evolution electrode of Example 1. [Figure 4] It is a photograph obtained by observing, with a transmission electron microscope, (a) the iron-copper oxide for the oxygen evolution electrode of Example 1 and (b) a magnified photograph of a part thereof.

Modes for Carrying Out the Invention

[0015] Hereinafter, a mode for carrying out the electrode according to the present invention (hereinafter referred to as "the present embodiment") will be described in detail. Note that the present embodiment is an exemplification for explaining the present invention, and the present invention is not limited only to the embodiments thereof. That is, the present invention can be variously modified without departing from the gist thereof.

[0016] <Electrode> [[ID=]]The electrode of the present embodiment contains a catalyst including at least one first metal element selected from the group consisting of nickel, iron, and cobalt, and at least one second metal element selected from the group consisting of copper, silver, and gold. [[ID=]]By using the catalyst containing the first metal element and the second metal element as the electrode material, a high reaction activity can be obtained, and as a result, a low activation overvoltage can be achieved in the neutral pH range.​

[0017] (catalyst) The catalyst used in the electrode of this embodiment includes the first metal element and the second metal element. The first metal element is at least one selected from the group consisting of nickel, iron, and cobalt, and from the viewpoint of reaction activity, it is preferable to include at least iron. Furthermore, the second metal element is at least one selected from the group consisting of copper, silver, and gold. By including the first metal element and the second metal element in the catalyst, high reaction activity can be obtained.

[0018] The forms in which the first and second metal elements exist are not particularly limited, but from the viewpoint of reaction activity and chemical stability, it is preferable that they exist as elemental metals or metal oxides. In the case of the aforementioned metal oxides, the composition may be stoichiometric, such as FeO, Fe3O4, or Fe2O3, or it may be a non-stoichiometric composition resulting from the absence of oxygen ions or metal ions. Furthermore, the first metal element and the second metal element may be mixed at the atomic level as an alloy or composite metal oxide, for example, in which case iron oxide particles and gold particles are mixed together.

[0019] The elemental ratio (A1 / A2) of the first metal element (A1) to the second metal element (A2) is preferably 0.05 to 10, more preferably 0.1 to 5, and even more preferably 0.15 to 3.

[0020] Among those mentioned above, the catalyst is preferably iron-copper oxide, iron-silver oxide, iron-gold oxide, nickel-iron-copper oxide, nickel-silver-copper oxide, nickel-iron-gold oxide, etc., from the viewpoint of reaction activity and chemical stability.

[0021] Furthermore, from the viewpoint of ensuring higher conductivity, it is preferable that the catalyst is supported on a conductive substrate, which will be described later. The method of supporting the conductive substrate is not particularly limited, but from the viewpoint of simple and uniform support, a method of electrodeposition is preferably used in which the conductive substrate is immersed in an electrolyte containing a soluble salt of the first metal element and a soluble salt of the second element.

[0022] The soluble salts of the first metal element and the second metal element are not particularly limited, but are preferably water-soluble salts. Examples of water-soluble salts of the first metal element and the second metal element include fluorides, chlorides, bromides, iodides, nitrates, sulfates, acetates, and the like. Furthermore, the electrolyte preferably uses water as the solvent, and in order to control electrodeposition, acids, bases, salts, or organic substances can be added in addition to the soluble salt of the first metal element and the soluble salt of the second element.

[0023] (Conductive base material) In this embodiment, it is preferable to support the catalyst on a conductive substrate in order to ensure higher conductivity. The material of the conductive substrate is not particularly limited, but from the viewpoint of conductivity and chemical durability, it is preferably a transition metal. The transition metal is more preferably at least one selected from the group consisting of titanium, vanadium, chromium, manganese, iron, cobalt, nickel, copper, molybdenum, ruthenium, rhodium, palladium, silver, iridium, platinum, and gold; even more preferably at least one selected from the group consisting of titanium, chromium, manganese, iron, cobalt, and nickel; and particularly preferably at least one selected from the group consisting of nickel, titanium, and iron. The conductive substrate may be made of a single transition metal, or an alloy or mixture of two or more transition metals. Furthermore, in addition to transition metals, it may also contain typical elements such as carbon, oxygen, nitrogen, silicon, phosphorus, and sulfur.

[0024] In addition, from the viewpoint of increasing the surface area capable of supporting the catalyst, the conductive substrate is preferably a porous body. Further, from the same viewpoint, the electric double layer capacitance of the conductive substrate is preferably 1.25 mF / cm 2 or more, more preferably 1.30 mF / cm 2 or more, and even more preferably 1.35 mF / cm 2 or more. Note that the electric double layer capacitance described above means the value shown by the conductive substrate on which the catalyst is not supported. The electric double layer capacitance is measured by the measurement method described in the examples. The method for manufacturing such a conductive substrate is not particularly limited. For example, a method of activating by electrochemically performing oxidation-reduction on nickel foam can be preferably used.

[0025] <Water electrolysis electrolytic cell> The water electrolysis electrolytic cell of the present embodiment uses the above-described electrode of the present invention as an oxygen generation electrode. In the water electrolysis electrolytic cell of the present embodiment, a cathode and an anode are immersed in an electrolytic solution, and the cathode and the anode are connected to a power source. Here, the anode is the oxygen generation electrode, and a diaphragm or an ion exchange membrane (proton exchange membrane or anion exchange membrane) may be sandwiched between the cathode and the anode.

[0026] The electrolytic solution used in the water electrolysis electrolytic cell of the present embodiment has a pH in the neutral region (1.5 to 12.6) when not electrolyzed. In water electrolysis, protons (H + ) or hydroxide ions (OH - ) are supplied to the electrode and the reaction proceeds. However, when the substrate concentration is not sufficient, the current density is limited by the supply rate of the substrate. At this time, the maximum current density that can be operated is called the "diffusion-limited current density". In industrial water electrolysis, from the viewpoint of productivity, it is usually operated at a current density of 100 mA / cm 2 or more. Therefore, in the present embodiment, when the diffusion-limited current density is less than 100 mA / cm 2 , the concentration of H + or OH -Assuming the concentration is insufficient, i.e., in the neutral pH range, electrolytes with a non-electrolytic pH in the range of 1.5 to 12.6 were defined as electrolytes in the neutral pH range. For details on the calculation method of the values, please refer to T. Shinagawa, and K. Takanabe, Phys. Chem. Chem. Phys. 2015, 17, 15111-15114.

[0027] As described above, the electrolyte of this embodiment has a pH in the range of 1.5 to 12.6 when not electrolyzed. However, from the viewpoint of suppressing corrosion to various components of the electrolytic cell, the pH of the electrolyte when not electrolyzed is preferably 4 to 12, and more preferably 5 to 11.5.

[0028] From the viewpoint of maintaining a constant pH of the electrolyte and promoting the supply of protons or hydroxide ions to the electrode, the electrolyte is preferably a buffer solution. The type of buffer solution is not particularly limited. For example, a carbonate-bicarbonate buffer solution combining an alkali metal (i.e., lithium (Li), sodium (Na), potassium (K), cesium (Cs), and rubidium (Rb)) carbonate and an alkali metal bicarbonate can be preferably used.

[0029] In this embodiment, the cathode constituting the water electrolytic cell preferably includes a porous body made of a conductive metal or carbon material, from the viewpoint of increasing the surface area. Examples of the porous metal include plain weave mesh, perforated metal, expanded metal, and metal foam. Examples of the porous carbon material include woven fabric made of carbon fibers, nonwoven fabric made of carbon fibers bound with a binder, and carbon foam.

[0030] The cathode may be the porous metal or porous carbon material mentioned above, or it may be an electrode in which a highly reactive catalyst layer is formed on the surface of a porous metal or porous carbon material used as the electrode substrate. However, from the viewpoint of reducing the electrolysis voltage, it is preferable to form a highly reactive catalyst layer on the surface of the electrode substrate.

[0031] Here, the material of the electrode substrate is not particularly limited, but from the standpoint of chemical and electrochemical durability, it is preferable to use mild steel, stainless steel, nickel, nickel alloy, titanium, titanium alloy, or carbon material.

[0032] Furthermore, the catalyst layer formed on the cathode preferably has high hydrogen generation capacity, and nickel, cobalt, iron, platinum group elements (e.g., ruthenium, rhodium, palladium, osmium, iridium, etc.) can be used. To achieve the desired activity and durability, the catalyst layer can be formed as a single metal, a compound such as an oxide, a composite metal oxide or alloy composed of multiple metal elements, or a mixture thereof. Specifically, examples include Raney nickel, Raney alloys composed of combinations of multiple materials such as nickel and aluminum, or nickel and tin, porous coatings made by plasma spraying using nickel compounds or cobalt compounds as raw materials, alloys or composite compounds of nickel and elements selected from cobalt, iron, molybdenum, silver, copper, etc., metals or oxides of platinum group elements such as platinum and ruthenium with high hydrogen generation capacity, and mixtures of these platinum group element metals or oxides with compounds of other platinum group elements such as iridium and palladium, or compounds of rare earth metals such as lanthanum and cerium, and carbon materials such as graphene. To achieve high catalytic activity and durability, multiple layers of the above materials may be stacked, or multiple layers may be mixed within the catalyst layer. Organic materials such as polymer materials may be included to improve durability and contact with the substrate.

[0033] Methods for forming a catalyst layer on the electrode substrate include plating, thermal spraying methods such as plasma spraying, thermal decomposition methods where a precursor solution is applied to the substrate and then heated, methods in which a catalyst substance is mixed with a binder component and immobilized on the substrate, and vacuum deposition methods such as sputtering.

[0034] In this embodiment, the water electrolytic cell is preferably operated in a temperature range of 60°C to 110°C, more preferably in a temperature range of 65°C to 105°C, and even more preferably in a temperature range of 70°C to 100°C. By operating at a temperature near or above the boiling point of water, the reaction rate of water electrolysis can be significantly improved.

[0035] <Carbon dioxide reduction electrolytic cell> The carbon dioxide reduction electrolytic cell of this embodiment uses the electrode of the present invention described above as the oxygen generation electrode. In the carbon dioxide reduction electrolytic cell of this embodiment, the cathode and anode are immersed in the electrolyte, and the cathode and anode are connected to a power source. Here, the anode is an oxygen generation electrode. Furthermore, the carbon dioxide reduction electrolytic cell of this embodiment may have a structure in which a diaphragm or an ion exchange membrane (proton exchange membrane or anion exchange membrane) is sandwiched between the cathode and the anode, or a gas diffusion electrode may be used as the cathode.

[0036] The carbon dioxide reduction electrolytic cell of this embodiment can preferably use the same electrolyte as the water electrolytic electrolytic cell described above. Furthermore, the cathode of the carbon dioxide reduction electrolytic cell in this embodiment is preferably a porous body made of a conductive metal or carbon material, similar to the water electrolytic cell described above. The materials constituting the porous metal and catalyst layer are preferably those with high carbon dioxide reduction activity. When the target product is carbon monoxide, gold, silver, or zinc are preferred. When the target product is formic acid, indium, tin, or lead are preferred. When the target product is ethylene or ethanol, copper is preferred.

[0037] In this embodiment, the operating conditions for the carbon dioxide electrolytic cell are preferably in the temperature range of 40 to 100°C, more preferably in the temperature range of 50 to 90°C, and even more preferably in the temperature range of 55 to 80°C, based on a balance between the reaction rate and the solubility of carbon dioxide.

[0038] <Electrolyzer> The electrolytic apparatus of this embodiment comprises a water electrolytic cell or a carbon dioxide reduction electrolytic cell using the electrode of the present invention described above as an oxygen generation electrode. With this electrolysis system, even when using an electrolyte in the neutral pH range, the electrolysis voltage can be suppressed during water electrolysis or carbon dioxide reduction, enabling efficient electrolysis.

[0039] The conditions for the electrolytic cell in the electrolytic apparatus of this embodiment are the same as those for the water electrolytic cell or carbon dioxide reduction electrolytic cell of this embodiment described above. [Examples]

[0040] The present invention will be described below with reference to specific examples and comparative examples, but the present invention is not limited to the following examples.

[0041] (Manufacturing of conductive substrates) Two 1cm x 1cm nickel foam sheets (manufactured by Niraco Co., Ltd., pore size 0.5mm, thickness 1.6mm) were used as the working electrode and counter electrode, respectively, with a mercury / mercury chloride electrode as the reference electrode. The foam was immersed in potassium carbonate buffer (prepared by mixing potassium hydroxide aqueous solution and potassium bicarbonate aqueous solution to achieve a total concentration of carbonate anion species of 1.5 mol / kg and a pH of 10.5 at 25°C) and heated to 67°C, and electrochemical treatment was performed using a three-electrode system. First, the open-circuit voltage was maintained for 20 minutes, followed by the application of a 50 mA oxidation current for 25 minutes. This process was repeated a total of three times. Then, the working electrode was maintained at 1.0 V (reverse hydrogen electrode reference) for 10 minutes, followed by the application of a 50 mA oxidation current for 25 minutes. This process was repeated a total of two times to perform the activation treatment and obtain the conductive substrate sample.

[0042] (Measurement of electrical double-layer capacitance) An activated conductive substrate was used as the working electrode, untreated nickel foam as the counter electrode, and a mercury / mercury chloride electrode as the reference electrode. The substrate was immersed in potassium carbonate buffer and heated to 80°C for electrochemical measurement using a three-electrode system. Cyclic voltammetry was measured by sweeping the voltage range of 0.3 to 0.9 V (reverse hydrogen electrode reference) at 200, 100, 50, 25, and 10 mV / s, and the electrical double-layer capacitance was measured from the current at 0.60 V. Here, the value of the electrical double-layer capacitance was the average of the values ​​obtained from the anode scan and cathode scan. The electrical double-layer capacitance of the untreated nickel foam was 1.06 ± 0.15 mF (1.06 ± 0.15 mF / cm² per unit electrode area). 2 The electrical double-layer capacitance of the nickel foam after activation treatment is 1.90 ± 0.17 mF (1.90 ± 0.17 mF / cm² per unit electrode area). 2 ) was.

[0043] [Comparative Example 1: Electrode made of electrodeposited iron oxide] As described above, an activated conductive substrate (nickel foam) was used as the working electrode and a platinum wire as the counter electrode, and the two were immersed in a 5 mM iron nitrate nonahydrate (Fe(NO3)3·9H2O) aqueous solution, with a current of -10 mA / cm². 2 Electrodeposition was performed for 1 hour to create an oxygen-generating electrode. The oxygen evolution reaction was evaluated using a three-electrode system, with the iron oxide-supported oxygen-evolving electrode described above as the working electrode, a platinum wire as the counter electrode, and mercury / mercury chloride as the reference electrode. The system was immersed in potassium carbonate-potassium bicarbonate buffer (1.5 M, pH=10.5) and heated to 80°C. Figure 1 shows the relationship between the obtained current density and the potential of the working electrode (iR corrected), which is 400 mA / cm². 2 Figure 2 shows the change in the potential of the working electrode (iR corrected) when an on / off test (on for 15 minutes, off for 5 minutes) was performed.

[0044] [Comparative Example 2: Oxygen evolution electrode without catalyst support] Except for using the activated conductive substrate (nickel foam) as the oxygen generation electrode, the oxygen generation reaction was carried out under the same conditions as in Comparative Example 1. Figure 1 shows the relationship between current density and the potential of the working electrode. Figure 2 shows the change in potential (iR corrected) when an on-off test was performed under the same conditions as in Comparative Example 1.

[0045] [Comparative Example 3: Oxygen-generating electrode with iridium oxide supported on titanium mesh] A 1 cm × 1 cm iridium oxide-supported titanium mesh was prepared using the method described in T. Naito, et al., ChemSusChem 2020, 13, 5921-5933. The oxygen evolution reaction was then carried out under the same conditions as in Comparative Example 1, except that this iridium oxide-supported titanium mesh was used as the oxygen evolution electrode. Figure 1 shows the relationship between current density and the potential of the working electrode, and Figure 2 shows the change in potential during the on / off test.

[0046] [Example 1: Oxygen-evolving electrode supported with iron-copper oxide] The oxygen evolution reaction was carried out under the same conditions as in Comparative Example 1, except that an aqueous solution containing 5 mM iron nitrate notahydrate (Fe(NO3)3·9H2O) and 5 mM copper nitrate notahydrate (Cu(NO3)2·9H2O) was used for catalyst electrodeposition to obtain an oxygen evolution electrode supported with iron-copper oxide. Figure 1 shows the relationship between current density and the potential of the working electrode, and Figure 2 shows the change in potential during the on / off test. Furthermore, scanning electron microscope images of the oxygen-evolving electrode obtained in Example 1 are shown in Figures 3(a) and (b). From Figure 3, it can be seen that the iron-copper oxide forms a nanostructure. In addition, images of the iron-copper oxide collected from the surface of the oxygen-evolving electrode obtained in Example 1, observed with a transmission electron microscope, are shown in Figures 4(a) and (b). From Figure 4, it can be seen that within the structure of several tens of nanometers, there are even finer black dots of several nanometers, indicating the localized formation of nanoparticles.

[0047] [Example 2: Oxygen-generating electrode supported with iron-silver oxide] The oxygen evolution reaction was carried out under the same conditions as in Comparative Example 1, except that an aqueous solution containing 5 mM iron nitrate notahydrate (Fe(NO3)3·9H2O) and 5 mM silver nitrate (AgNO3) was used for catalyst electrodeposition to obtain an oxygen evolution electrode supported with iron-silver oxide. Figure 1 shows the relationship between current density and the potential of the working electrode.

[0048] [Example 3: Oxygen-generating electrode supported with iron-gold oxide] The oxygen evolution reaction was carried out under the same conditions as in Comparative Example 1, except that an aqueous solution containing 5 mM iron nitrate notahydrate (Fe(NO3)3·9H2O) and 5 mM tetrachloroauric acid (HAuCl4) was used for catalyst electrodeposition to obtain an oxygen evolution electrode supported with iron-gold oxide. Figure 1 shows the relationship between current density and the potential of the working electrode.

[0049] The results shown in Figures 1 and 2 indicate that using a catalyst containing both a first and second metal species in the oxygen evolution electrode reduces the activation overpotential more effectively compared to using an electrode containing only the first metal species or an electrode made of a conductive substrate without a supported catalyst. In Comparative Example 3, an iridium oxide catalyst, known to exhibit high oxygen evolution activity across a wide pH range from strongly acidic to strongly basic, was used as the oxygen evolution electrode. Although the activation overpotential was lower than that of the oxygen evolution electrode in this embodiment at the start of current application, it was unstable in the neutral pH range, and the activation overpotential tended to gradually increase to 400 mA / cm². 2 After energizing for 5 minutes, it can be seen that the oxygen generation electrode in each embodiment exhibits a lower activation overpotential. [Industrial applicability]

[0050] According to the present invention, it is possible to provide electrodes, water electrolytic cells, water electrolytic devices, carbon dioxide reduction electrolytic cells, and carbon dioxide electrolytic devices that exhibit a low activation voltage in the neutral pH range. Since the electrolytic cell of the present invention can be used in the neutral pH range, it is suitable for use in the field of water electrolysis.

Claims

1. The electrode comprises a metal oxide catalyst containing iron as the primary metal element and at least one secondary metal element selected from the group consisting of copper, silver, and gold, which is supported on a conductive substrate, and the conductive substrate has a conductivity of 1.25 mF / cm². ２ An electrode characterized by being a porous body having the above-mentioned electrical double layer capacitance, wherein the conductive substrate contains nickel, and is an electrode for oxygen generation.

2. The electrode according to claim 1, characterized in that the conductive substrate is subjected to an electrochemical oxidation-reduction activation treatment.

3. The electrode according to claim 1 or 2, characterized in that the catalyst is electrodeposited onto the conductive substrate by immersing the conductive substrate in an electrolyte containing a soluble salt of the first metal element and a soluble salt of the second metal element.

4. A water electrolytic cell characterized by using the electrodes described in any one of claims 1 to 3.

5. The water electrolytic cell according to claim 4, characterized in that a buffer solution having a pH of 1.5 to 12.6 during non-electrolytic operation is used as the electrolyte.

6. A water electrolysis apparatus characterized by comprising a water electrolytic cell as described in claim 4 or 5.

7. A carbon dioxide reduction electrolytic cell characterized by using the electrode described in any one of claims 1 to 3.

8. The carbon dioxide reduction electrolytic cell according to claim 7, characterized in that a buffer solution having a pH of 1.5 to 12.6 when not electrolyzed is used as the electrolyte.

9. A carbon dioxide electrolytic apparatus characterized by comprising a carbon dioxide reduction electrolytic cell according to claim 7 or 8.

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