Method for manufacturing water electrolysis electrode

Abstract

This method for manufacturing a water electrolysis electrode 2 includes promoting mixing of a solution, which includes ions of a transition metal and chloride ions, in a state in which a conductive substrate 10 is immersed in the solution. In addition, the method for manufacturing the water electrolysis electrode 2 includes forming a layered double hydroxide layer 16 having two or more kinds of transition metals on the surface of the conductive substrate 10. The conductive substrate 10 has a mesh structure including a plurality of metal wires. The intersecting metal wires are joined to each other at intersections 3.

Description

Method for manufacturing electrodes for water electrolysis

[0001] The present disclosure relates to a method for producing an electrode for water electrolysis.

[0002] Electrodes for water electrolysis have been known in the past.

[0003] Patent Document 1 describes a method for producing an electrode for water electrolysis, which includes a step of immersing an electrode substrate containing a predetermined layered double hydroxide in an organic solvent. In this production method, the electrode substrate is produced by electrodeposition treatment in an aqueous solution containing a compound containing metal M1 and a compound containing metal M2, using the conductive substrate as the anode.

[0004] Non-Patent Document 1 investigates the activity of an electrode made of Ni-Fe layered double hydride (Ni-Fe LDH) for the oxygen evolution reaction (OER).

[0005] Non-Patent Document 2 describes that the interface interaction between FeOOH and Ni-Fe LDH adjusts the local electronic structure of Ni-Fe LDH, thereby enhancing the OER electrocatalytic activity.

[0006] Patent No. 6443901

[0007] Seyeong Lee et al., “Operational durability of three-dimensional Ni-Fe layered double hydroxide electrocatalyst for water oxidation,” Electrochimica Acta, 2019, Vol.315, p.94-101Jiande Chen et al., “Interfacial Interaction between FeOOH and Ni-Fe LDH to Modulate the Local Electronic Structure for Enhanced OER Electrocatalysis,” ACS Catalysis, 2018, Vol.8, p.11342-11351

[0008] The inventions described in the above documents need to be reconsidered from the viewpoint of improving the performance of electrodes for water electrolysis. Therefore, the present disclosure provides a method for producing an electrode for water electrolysis that is advantageous from the viewpoint of exhibiting excellent performance.

[0009] The present disclosure provides a method for producing an electrode for water electrolysis, comprising: promoting mixing of a solution containing transition metal ions and chloride ions while immersing a conductive substrate in the solution; and forming a layered double hydroxide layer containing two or more types of transition metals on a surface of the conductive substrate, wherein the conductive substrate has a mesh structure containing a plurality of metal wires, and intersecting metal wires are joined to each other at their intersections.

[0010] According to the present disclosure, there is provided a method for producing an electrode for water electrolysis that is advantageous in terms of exhibiting excellent performance.

[0011] FIG. 1 is a flowchart showing an example of a method for producing an electrode for water electrolysis. FIG. 2A is a perspective view schematically showing a conductive substrate. FIG. 2B is a cross-sectional view schematically showing the conductive substrate before etching. FIG. 3 is a cross-sectional view schematically showing the conductive substrate after etching. FIG. 4 is a cross-sectional view schematically showing an electrode for water electrolysis. FIG. 5 is a diagram schematically showing an example of the crystal structure of a layered double hydroxide (LDH). FIG. 6 is a flowchart showing another example of a method for producing an electrode for water electrolysis. FIG. 7 is a diagram schematically showing the mechanism for producing an electrode for water electrolysis. FIG. 8 is a scanning electron microscope image of a cross section of the Ni mesh sintered compact before etching in Example 1. FIG. 9 is a graph showing measurement results of the OER overpotential of the electrode of Example 1 and the electrode of Comparative Example 1. FIG. 10A is a photograph of the electrode after OER overpotential measurement using the electrode of Example 1. FIG. 10B is an enlarged photograph of the X portion of FIG. 10A. FIG. 11A is a photograph of the electrode after OER overpotential measurement using the electrode of Comparative Example 1. FIG. 11B is an enlarged photograph of part Y in FIG. 11A.

[0012] (Knowledge forming the basis of the present disclosure) The use of renewable energy such as solar and wind power has been attracting attention as a measure against global warming. Power generation using renewable energy has the problem of surplus electricity being wasted. For this reason, the utilization efficiency of renewable energy is not necessarily sufficient. Therefore, methods for effectively utilizing surplus electricity by producing and storing hydrogen from surplus electricity have been studied.

[0013] Electrolysis of water is one possible method for producing hydrogen from surplus electricity. To produce hydrogen cheaply and stably, there is a need to develop a highly efficient, long-life water electrolysis device.

[0014] In a water electrolysis device, oxygen is generated at the anode and hydrogen is generated at the cathode. The reaction in which oxygen is generated at the anode is also called the anode reaction, and the reaction in which hydrogen is generated at the cathode is also called the cathode reaction. In order to provide a highly efficient water electrolysis device, it is desirable that the overvoltage at the anode is low. In addition, it is also desirable that the overvoltage at the cathode is low. Therefore, there is a need to develop high-performance electrodes for the anode reaction or cathode reaction in water electrolysis.

[0015] For example, layered double hydroxides (LDHs) are considered promising materials for electrodes for water electrolysis in view of their large specific surface area and diverse combinations of metal ions. Patent Literature 1 describes a method for producing an electrode for water electrolysis, which contains a layered double hydroxide represented by a predetermined composition formula, by performing a pulse electrodeposition treatment in an aqueous solution containing a compound containing a predetermined metal, using a conductive substrate as the anode. However, the method for producing an electrode by pulse electrodeposition described in Patent Literature 1 cannot be said to be simple.

[0016] The present inventors have investigated a method for producing an electrode for water electrolysis by forming an LDH layer on a conductive substrate in a solution. In the method for producing an electrode for water electrolysis discovered by the present inventors, first, the conductive substrate is etched in a solution. Specifically, metal contained in the conductive substrate is eluted into the solution. Next, LDH is synthesized using ions of the metal eluted into the solution and ions of the metal that were previously present in the solution, and an LDH layer is formed on the conductive substrate.

[0017] Candidates for conductive substrates include substrates with a non-porous structure such as a plate or foil, and substrates with a porous structure such as a mesh. Substrates with a non-porous structure such as a plate or foil have the problem that it is difficult to increase the surface area of ​​the substrate. On the other hand, substrates with a porous structure such as a mesh have the advantages of being able to increase the surface area of ​​the substrate and of facilitating the diffusion of gas generated in the water electrolysis reaction.

[0018] The present inventors have found that when a water electrolysis electrode is produced by the above-mentioned production method using a conductive substrate having a mesh structure, the performance of the water electrolysis electrode is difficult to improve. When the conductive substrate is etched in the above-mentioned production method, metal contained in the conductive substrate is eluted into a solution. In a conductive substrate having a mesh structure, metal contained in the metal wires constituting the conductive substrate is eluted into a solution, resulting in a reduction in the wire diameter of the metal wires. Therefore, when a conductive substrate having a mesh structure is etched, voids are formed between the metal wires at their intersections. The voids between the metal wires increase the contact resistance between the intersecting metal wires compared to before etching, making it difficult to improve the performance of the water electrolysis electrode. Even if an LDH layer is formed in the void, the conductivity of the LDH layer is generally lower than the conductivity of the metal wires, thereby increasing the contact resistance between the intersecting metal wires and making it difficult to improve the performance of the water electrolysis electrode. Based on this new finding, the present inventors have completed a method for producing a water electrolysis electrode according to the present disclosure.

[0019] Hereinafter, embodiments of the present disclosure will be described with reference to the drawings. The present disclosure is not limited to the following embodiments. Note that the embodiments described below are all comprehensive or specific examples. Therefore, the numerical values, shapes, materials, components, component placement positions, connection forms, etc. shown in the following embodiments are merely examples and are not intended to limit the present disclosure. Furthermore, among the components in the following embodiments, components that are not recited in the independent claims that represent the highest concept will be described as optional components. Furthermore, in the drawings, descriptions of components with the same reference numerals may be omitted. Furthermore, the drawings schematically illustrate each component to facilitate understanding, and the shapes, dimensional ratios, etc. may not be accurately represented.

[0020] (Embodiment) Figure 1 is a flowchart showing an example of a method for producing an electrode for water electrolysis. The electrode for water electrolysis obtained by the production method according to this embodiment includes a conductive substrate and a layered double hydroxide (LDH) layer containing two or more transition metals. The conductive substrate has a mesh structure including a plurality of metal wires. The LDH layer is formed on the surface of the conductive substrate. The LDH layer can function as a catalyst for the anodic reaction or cathodic reaction in water electrolysis.

[0021] As shown in FIG. 1 , the method for producing an electrode for water electrolysis includes steps S10, S11, and S12. Step S10 is a step of preparing a solution. Step S11 is a step of immersing a conductive substrate in the solution. Step S12 is a step of promoting mixing of the solution. As a result, LDH is synthesized in the solution, and an LDH layer is formed on the conductive substrate. As a result, an electrode for water electrolysis can be produced in the solution.

[0022] In step S10, a solution S is prepared. In step S10, a solution for synthesizing a layered double hydroxide is prepared.

[0023] The solution S contains ions of a first transition metal and chloride ions. The ions of the first transition metal include, for example, ions of a transition metal contained in a layered double hydroxide described below. The ions of the first transition metal include, for example, ions of at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. A layered double hydroxide containing such a transition metal can provide a water electrolysis electrode with superior electrode activity.

[0024] The first transition metal ions contained in the solution S preferably include ions of at least one transition metal selected from the group consisting of Ni and Fe. In this case, a water electrolysis electrode having superior electrode activity can be produced.

[0025] The first transition metal ions preferably include Fe ions, which allows the production of a water electrolysis electrode having superior electrode activity and durability.

[0026] The solvent of the solution S may be water, an organic solvent, or a mixed solvent of water and an organic solvent.

[0027] Solution S may contain a chelating agent. The chelating agent is, for example, an organic compound that coordinates with transition metal ions in the layered double hydroxide. The chelating agent may be at least one selected from the group consisting of bidentate organic ligands and tridentate organic ligands. Examples of chelating agents are β-diketones, β-ketoesters, hydroxycarboxylic acids, and hydroxycarboxylates. Examples of β-diketones are acetylacetone (ACAC), trifluoroacetylacetone, hexafluoroacetylacetone, benzoylacetone, thenoyltrifluoroacetone, dipyrrolylmethane, dibenzoylmethane, and ascorbic acid. Examples of β-ketoesters are methyl acetoacetate, ethyl acetoacetate, allyl acetoacetate, benzyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, n-butyl acetoacetate, isobutyl acetoacetate, tert-butyl acetoacetate, 2-methoxyethyl acetoacetate, and methyl 3-oxopentanoate. Examples of hydroxycarboxylic acids and their salts include tartaric acid, citric acid, malic acid, gluconic acid, ferulic acid, lactic acid, glucuronic acid, and salts thereof. The chelating agent contained in solution S preferably contains at least one selected from the group consisting of acetylacetone and citrate salts. This increases the stability of the dispersion of the complex in solution S, making it easier to form an LDH layer in the desired state in the water electrolysis electrode. As a result, the water electrolysis electrode can have better electrode activity. An example of a citrate salt is trisodium citrate.

[0028] In step S11, the conductive substrate is immersed in the solution S. In step S11, the metal contained in the conductive substrate is dissolved into the solution S.

[0029] In step S11, the solution S reacts with the conductive substrate, etching the conductive substrate. Specifically, the first transition metal ions and chloride ions contained in the solution S react with the conductive substrate. As a result, the conductive substrate is etched by the first transition metal ions and chloride ions. As a result, the metal contained in the conductive substrate is eluted into the solution S.

[0030] Fig. 2A is a perspective view schematically illustrating a conductive substrate 10. The conductive substrate 10 includes a plurality of metal wires. The plurality of metal wires have horizontal lines and vertical lines. As shown in Fig. 2A, the conductive substrate 10 has a plurality of horizontal lines 11 and a plurality of vertical lines 12. The conductive substrate 10 has a mesh structure formed by combining the plurality of horizontal lines 11 and the plurality of vertical lines 12. This allows the conductive substrate 10 to have a porous structure, thereby increasing the surface area of ​​the conductive substrate 10.

[0031] The conductive substrate 10 includes, for example, a plurality of first lines extending in a first direction and a plurality of second lines extending in a second direction. The first direction and the second direction intersect with each other. The angle at which the first lines intersect with the second lines is not particularly limited and may be any angle. The first lines are, for example, horizontal lines 11 shown in FIG. 2A. The second lines are, for example, vertical lines 12 shown in FIG. 2A.

[0032] Each horizontal line 11 intersects multiple vertical lines 12. For example, the horizontal line 11 intersects a first vertical line 12 and a second vertical line 12. The horizontal line 11 may intersect the first vertical line 12 above the first vertical line 12 and may intersect the second vertical line 12 below the second vertical line 12. The first vertical line 12 and the second vertical line 12 may be two adjacent vertical lines.

[0033] Each vertical line 12 intersects multiple horizontal lines 11. A vertical line 12 intersects, for example, a first horizontal line 11 and a second horizontal line 11. A vertical line 12 may intersect the first horizontal line 11 above the first horizontal line 11, and may intersect the second horizontal line 11 below the second horizontal line 11. The first horizontal line 11 and the second horizontal line 11 may be two adjacent horizontal lines.

[0034] The conductive substrate 10 may have a structure in which a plurality of metal wires are woven. The conductive substrate 10 may have a woven structure in which horizontal lines 11 and vertical lines 12 are woven according to a certain rule. This can improve the strength of the conductive substrate 10.

[0035] Examples of the woven structure of the conductive substrate 10 are plain weave wire mesh, twill weave wire mesh, and tatami weave wire mesh, which are described in Japanese Industrial Standards (JIS) G3555: 2004. In particular, when the conductive substrate 10 has a plain weave wire mesh structure, it is possible to further increase the surface area of ​​the conductive substrate 10, and gas generated in the water electrolysis reaction can be easily diffused.

[0036] 2B is a cross-sectional view schematically illustrating the conductive substrate 10 before etching. The conductive substrate 10 has horizontal lines 11 and vertical lines 12. The horizontal lines 11 and vertical lines 12 intersect at intersections 3. At the intersections 3, the horizontal lines 11 and vertical lines 12 are joined to each other by joints 13. This maintains the fixed state of the horizontal lines 11 and vertical lines 12 at the intersections 3. As a result, the mechanical strength of the conductive substrate 10 is improved, and therefore the mechanical strength of the water electrolysis electrodes is also improved.

[0037] The conductive substrate 10 includes a plurality of metal wires. The metal wires include a metal. The metal wires may include a resin. The entire conductive substrate 10 may be made of metal. The conductive substrate 10 may have a configuration in which a surface layer including a metal is formed on a resin member such as polypropylene or polyethylene. In this case, the surface layer including a metal may be a plated film or a sputtered film. The metal included in the conductive substrate 10 may be a pure metal such as nickel or iron, or an alloy such as stainless steel or Inconel. Inconel is a registered trademark.

[0038] The conductive substrate 10 contains, for example, a metal M that reacts with chloride ions in the solution S and is eluted. The metal M contains, for example, a second transition metal. The second transition metal is a transition metal of a different type from the first transition metal. That is, the conductive substrate 10 may contain the second transition metal that reacts with chloride ions in the solution S and is eluted. In this case, it is easy to produce a water electrolysis electrode that has advantageous properties, for example, from the viewpoint of achieving both corrosion resistance and electrical conductivity in alkaline water electrolysis.

[0039] The surface of the conductive substrate 10 is preferably made of nickel. In this case, the conductive substrate 10 is likely to have excellent alkali resistance. When the surface of the conductive substrate 10 is made of nickel, the entire conductive substrate 10 may be made of nickel, or the conductive substrate 10 may have a surface layer made of nickel. The surface layer made of nickel is a sputtered film or a plated film. When the surface of the conductive substrate 10 is made of nickel, it is easy to produce a water electrolysis electrode that has advantageous properties, for example, from the viewpoint of achieving both corrosion resistance and conductivity in alkaline water electrolysis.

[0040] Methods for forming the joint 13 include welding, sintering, and bonding with a conductive paste material. The horizontal wire 11 and the vertical wire 12 may be welded using a filler material containing at least one metal selected from the group consisting of the metals contained in the horizontal wire 11 and the metals contained in the vertical wire 12. The conductive paste material may contain at least one metal selected from the group consisting of the metals contained in the horizontal wire 11 and the metals contained in the vertical wire 12.

[0041] In the conductive substrate 10, the horizontal wires 11 and the vertical wires 12 may be joined to each other by sintering. That is, the joints 13 may be formed by sintering. With this configuration, even when the horizontal wires 11 and the vertical wires 12 are etched in the solution S, the joints between the metal wires at the intersections are likely to be maintained. That is, even when the horizontal wires 11 and the vertical wires 12 are etched, gaps are unlikely to be formed between the horizontal wires 11 and the vertical wires 12, and the contact resistance between the intersecting metal wires is unlikely to increase. This prevents loss of conductivity between the horizontal wires 11 and the vertical wires 12, and allows for the production of a water electrolysis electrode with excellent electrode activity.

[0042] In the conductive substrate 10, not all of the intersections of the horizontal lines 11 and the vertical lines 12 need to be joined. It is sufficient that at least one of the intersections of the horizontal lines 11 and the vertical lines 12 in the conductive substrate 10 is joined.

[0043] The thickness of the conductive substrate 10 is not limited to a specific value. The thickness of the conductive substrate 10 is, for example, 0.02 mm or more. In this case, the conductive substrate 10 tends to be easy to handle. The thickness of the conductive substrate 10 is, for example, 10 mm or less, and preferably 1 mm or less.

[0044] FIG. 3 is a cross-sectional view schematically illustrating the conductive substrate 10A after etching. As described above, when the conductive substrate 10 is immersed in the solution S, the horizontal lines 11 and the vertical lines 12 are etched. When the conductive substrate 10 is immersed in the solution S, the metal contained in the horizontal lines 11 and the metal contained in the vertical lines 12 are dissolved into the solution S. As a result, in the etched conductive substrate 10A, the diameters of the horizontal lines 11a and the vertical lines 12a are reduced compared to the diameters of the horizontal lines 11 and the vertical lines 12, respectively, in the conductive substrate 10 before etching. In this embodiment, the horizontal lines 11 and the vertical lines 12 are joined to each other by joints 13 at intersections 3 where the horizontal lines 11 and the vertical lines 12 intersect. Therefore, even when the horizontal lines 11 and the vertical lines 12 are etched, the conductive substrate 10A still has joints 13a. As a result, the joints between the horizontal lines 11a and the vertical lines 12a are maintained even when the conductive substrate 10 is etched. This prevents gaps from forming between the horizontal wires 11 a and the vertical wires 12 a, reduces the contact resistance between the intersecting metal wires, and prevents loss of electrical continuity between the horizontal wires 11 a and the vertical wires 12 a, further improving the mechanical strength of the conductive substrate 10A.

[0045] Next, in step S12, mixing of the solution S is promoted. In step S12, an LDH layer is formed on the conductive substrate to obtain an electrode for water electrolysis.

[0046] By promoting the mixing of the solution S, the conductive substrate 10 is etched in the solution S, and an LDH layer is formed on the conductive substrate.

[0047] Methods for promoting the mixing of the solution S include vibrating the conductive substrate 10, shaking a container in which the solution S and the conductive substrate 10 are sealed, and stirring the solution S using a stirrer piece and a stirrer. Such methods cause forced convection of the solution S, promoting the mixing of the solution S. The promotion of the mixing of the solution S may be carried out in a state in which the container containing the solution S and the conductive substrate 10 is sealed, or may be carried out in an inert gas atmosphere.

[0048] "Promoting the mixing of the solution S" means promoting the diffusion of ions of the metal M eluted from the conductive substrate 10 into the solution S by the above-mentioned method.

[0049] The solution S preferably contains Fe ions as the first transition metal ions. The conductive substrate 10 preferably contains nickel as the metal M. In this case, the reaction shown in formula (1) occurs in step S12. As a result, the conductive substrate 10 is etched and an LDH layer is formed on the etched conductive substrate 10A. As a result, the water electrolysis electrode is likely to have excellent durability.

[0050] 4Ni 2+ Cl - 2 + 2Fe 3+ Cl - 3 + 2Ni → 5Ni 2+ Cl - 2 + 2Fe 2+ Cl - 2 + Ni Equation (1)

[0051] In step S12, the temperature of the solution S is not limited to a specific temperature. The temperature of the solution S is, for example, 20° C.±15° C. In this case, a water electrolysis electrode having excellent electrode activity can be obtained.

[0052] FIG. 4 is a cross-sectional view schematically illustrating a water electrolysis electrode 2. The water electrolysis electrode 2 includes an etched conductive substrate 10A and an LDH layer 16. Specifically, the water electrolysis electrode 2 includes horizontal lines 11a, vertical lines 12a, and the LDH layer 16. The LDH layer 16 is formed on the surface of the conductive substrate 10A. Specifically, the LDH layer 16 is formed on the horizontal lines 11a and vertical lines 12a. The LDH layer 16 contains a layered double hydroxide (LDH) as a catalyst for water electrolysis. The horizontal lines 11a and vertical lines 12a are joined to each other at intersections 3a by joints 13a. With this configuration, even when the LDH layer 16 is formed on the metal wire, electrical continuity between the horizontal lines 11a and vertical lines 12a is unlikely to be impaired. That is, contact resistance between the horizontal lines 11a and vertical lines 12a is unlikely to increase. As a result, the electrode activity of the water electrolysis electrode 2 is likely to be improved, and the water electrolysis electrode 2 is likely to exhibit excellent performance.

[0053] The LDH layer 16 can function as a catalyst for the anode reaction or cathode reaction of water electrolysis. The LDH layer 16 is bonded to, for example, the conductive substrate 10A. For example, no adhesive layer containing an organic material such as a polymer is disposed between the LDH layer 16 and the conductive substrate 10A, and the LDH layer 16 is bonded directly to the surface of the conductive substrate 10A.

[0054] 5 is a diagram schematically illustrating an example of the crystal structure of a layered double hydroxide. Layered double hydroxide (LDH) 20 is active in reactions that generate gases such as hydrogen and oxygen at the anode or cathode of a water electrolysis cell. For example, in alkaline water electrolysis, LDH 20 can be converted into hydroxide by the water electrolysis reaction.

[0055] The LDH 20 has, for example, a composition represented by the following formula (2): 2+ is a divalent transition metal ion. 3+ is a trivalent transition metal ion. n- is an anion between layers. x is a rational number satisfying the condition 0<x<1. y is a number corresponding to the amount of charge balance required. n is an integer. m is an appropriate rational number.

[0056] [M1 2+1-x M2 3+ x (OH)2][yA n- ・mH2O] Formula (2)

[0057] The LDH 20 contains two or more types of transition metals. The two or more types of transition metals in the LDH 20 are not limited to specific transition metals. In other words, M1 and M2 in the composition represented by formula (2) are not limited to specific transition metals. The two or more types of transition metals include, for example, at least two selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. In this case, the water electrolysis electrode 2 is more likely to have excellent electrode activity.

[0058] The two or more transition metals in the LDH 20 preferably include at least one selected from the group consisting of Ni and Fe. In this case, the water electrolysis electrodes are more likely to have excellent electrode activity. In addition, the production costs of the water electrolysis electrodes are likely to be low.

[0059] The two or more transition metals in the LDH 20 include Ni and Fe, and for example, in the composition shown in formula (2), M1 may be Ni and M2 may be Fe. In this case, the water electrolysis electrode is more likely to have excellent electrode activity.

[0060] A is an interlayer anion n- may be an inorganic ion or an organic ion. An example of an inorganic ion is CO 2- , NO3 - , Cl - , SO4 2- ,Br - , O.H. - , F - , I - , Si2O5 2- , B4O5(OH)4 2- , and PO 3- An example of an organic ion is CH3(CH2) n SO4 - , CH3 (CH2) n COO - , CH3 (CH2) n PO4 2- , and CH3(CH2) n No. 3- It is. A n- can be intercalated between the layers of the metal hydroxide along with water molecules. n- The charge and ion size of LDH20 are not limited to a specific value. n- or a plurality of types of A n- may also include:

[0061] As shown in FIG. 5, LDH20 is a 2+ or M2 3+ OH at each vertex of the octahedron centered at - LDH20 contains [M1 2+ 1-x M2 3+ x (OH) x+ This metal hydroxide has a layered structure in which hydroxide octahedra are connected two-dimensionally, sharing edges. Between the metal hydroxide layers, anions A n- and water molecules are present. The metal hydroxide layer functions as a host layer 21, and anions A n- and a guest layer 22 containing water molecules is disposed between the host layers 21. In other words, the LDH 20 as a whole is composed of a host layer 21 of metal hydroxide and an anion A n- The LDH 20 has a sheet-like structure in which M1 contained in the metal hydroxide layer and a guest layer 22 of water molecules are alternately stacked. 2+ Part of M2 3+ It has a structure substituted with:

[0062] The LDH layer 16 may contain a chelating agent. The chelating agent is, for example, a chelating agent contained in the solution S. The chelating agent may be coordinated to a transition metal ion contained in the LDH 20. This allows the LDH 20 to be stably present in the LDH layer 16. In addition, the LDH 20 is easily synthesized to have a small particle size. In addition, the LDH 20 nucleated on the conductive substrate is likely to undergo slow crystal growth. Therefore, the dense LDH layer 16 with few voids containing the LDH 20 is likely to have a desired thickness and be firmly fixed to the conductive substrate. This allows the LDH layer 16 to effectively contribute to the anode reaction or the cathode reaction, and the water electrolysis electrode is likely to have excellent electrode activity.

[0063] FIG. 6 is a flowchart showing another example of the method for producing electrodes for water electrolysis.

[0064] The method for producing a water electrolysis electrode may include step S13. Step S13 is a step of adjusting the solution S to be alkaline. In step S13, an LDH layer is formed on the conductive substrate to obtain a water electrolysis electrode. Step S13 is performed after step S12.

[0065] An LDH layer containing a layered double hydroxide is formed in step S13. The LDH layer containing a layered double hydroxide is formed on the conductive substrate.

[0066] The method for adjusting the solution S to alkaline is not limited to a specific method. For example, the solution S may be adjusted to alkaline by mixing the solution S with an alkaline solution. Alternatively, a pH-raising agent may be added to the solution to adjust the solution to alkaline. In this case, the pH-raising agent is not limited to a specific compound. The pH-raising agent is, for example, a compound having an epoxy group. Examples of the pH-raising agent are propylene oxide, ethylene oxide, and butylene oxide.

[0067] When a pH-raising agent having an epoxy group, such as propylene oxide, is added to solution S, in the presence of a nucleophile, such as chloride ions, a ring-opening reaction of the epoxy group occurs, and the pH-raising agent captures hydrogen ions present in solution S. This increases the pH of solution S, making solution S alkaline. The pH of solution S is, for example, 1. When a pH-raising agent is added to this solution S, the pH of solution S gradually increases from, for example, 1, and eventually, solution S becomes alkaline. The final pH of solution S is, for example, 8 or more and 12 or less. The addition of the pH-raising agent to the solution S causes a reaction to capture hydrogen ions in solution S. This gradually increases the pH of solution S. The time from the addition of the pH-raising agent to solution S until the pH of solution S reaches a steady state is not limited to a specific time. This time may be, for example, 24 hours or more, or may take several days.

[0068] The temperature of the solution S when the solution S is adjusted to be alkaline is not limited to a specific temperature. The temperature of the solution S is, for example, room temperature of 20° C.±15° C. In this case, a water electrolysis electrode having excellent electrode activity is likely to be obtained.

[0069] Step S13 preferably includes increasing the pH. This allows a layered double hydroxide to be formed on the conductive substrate in a short period of time, making it easier to obtain a water electrolysis electrode with excellent electrode activity. In addition, the produced water electrolysis electrode is likely to have excellent durability.

[0070] Next, the mechanism of producing electrodes for water electrolysis in the solution S will be described.

[0071] FIG. 7 is a schematic diagram illustrating the mechanism of manufacturing a water electrolysis electrode. As shown in FIG. 7 , a conductive substrate 10 is immersed in a solution S containing first transition metal ions TM1 and a chelating agent 30. The solution S may also contain second transition metal ions TM2. For example, the first transition metal ions TM1 are iron ions, and the second transition metal ions TM2 are nickel ions. In addition, for example, nickel is present on the surface of the conductive substrate 10. Some of the first transition metal ions TM1 etch and dissolve nickel present on the surface of the conductive substrate 10. In addition, some of the chelating agent 30 reacts with the conductive substrate 10 to form a complex C2 between the chelating agent 30 and the second transition metal ions TM2 derived from the conductive substrate 10. This produces an etched conductive substrate 10A.

[0072] When the solution S is adjusted to an alkaline state, a complex C1 is formed in the solution S from ions TM1 of a first transition metal derived from the solution S and the chelating agent 30. In addition, when ions TM2 of a second transition metal are contained in the solution S, a complex C2 is also formed from ions TM2 of the second transition metal and the chelating agent 30. Next, the complexes C1 and C2 react on the surface of the conductive substrate 10A, and LDH 20 is synthesized along the surface of the conductive substrate 10A. Because the complexes C1 and C2 contain the chelating agent 30, crystal growth of the LDH 20 is suppressed. As a result, an LDH layer containing the LDH 20 and the chelating agent 30 is formed on the conductive substrate 10A, and an electrode for water electrolysis is obtained.

[0073] The water electrolysis electrode 2 according to this embodiment can be used, for example, as an electrode of a water electrolysis cell in an alkaline water electrolysis apparatus or an anion exchange membrane water electrolysis apparatus. The water electrolysis electrode 2 is used, for example, in at least one selected from the group consisting of an anode and a cathode in these water electrolysis apparatuses. This tends to increase the activity of the anode reaction or the cathode reaction in water electrolysis.

[0074] (Additional Note) From the above description, the following techniques are disclosed.

[0075] (Technology 1) A method for manufacturing an electrode for water electrolysis, comprising: promoting mixing of a solution containing transition metal ions and chloride ions while immersing a conductive substrate in the solution; and forming a layered double hydroxide layer containing two or more types of transition metals on a surface of the conductive substrate, wherein the conductive substrate has a mesh structure including a plurality of metal wires, and intersecting metal wires are joined to each other at their intersections.

[0076] According to Technology 1, it is possible to provide a method for producing an electrode for water electrolysis that is advantageous in terms of exhibiting excellent performance.

[0077] (Technology 2) The method for producing an electrode for water electrolysis according to Technology 1, wherein the conductive base material contains a metal that reacts with the chloride ions and is eluted from the conductive base material. This configuration facilitates the production of a water electrolysis electrode that has advantageous properties from the viewpoint of achieving both corrosion resistance and electrical conductivity in alkaline water electrolysis.

[0078] (Technology 3) The method for producing an electrode for water electrolysis according to Technology 1 or 2, wherein the metal wires are joined to each other at the intersections by sintering. With this configuration, the joints between the metal wires at the intersections are likely to be maintained even when the conductive base material is etched.

[0079] (Technology 4) The method for producing an electrode for water electrolysis according to any one of Technologies 1 to 3, wherein the plurality of metal wires are woven together. This configuration can improve the strength of the conductive substrate.

[0080] (Technology 5) The method for producing an electrode for water electrolysis according to any one of Techniques 1 to 4, wherein the transition metal ions include ions of at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru.

[0081] (Technology 6) The method for producing an electrode for water electrolysis according to any one of Technologies 1 to 5, wherein the transition metal ions include ions of at least one transition metal selected from the group consisting of Ni and Fe.

[0082] According to Techniques 5 and 6, electrodes for water electrolysis having superior electrode activity can be produced.

[0083] The present disclosure will be described in more detail below with reference to examples. Note that the following examples are examples of the present disclosure, and the present disclosure is not limited to the following examples.

[0084] Example 1 A solution was prepared by dissolving 4.1880 g of nickel chloride hexahydrate and 2.3813 g of iron chloride hexahydrate in 123.175 milliliters (mL) of water. Nickel chloride hexahydrate and iron chloride hexahydrate were purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.831 mL of acetylacetone (ACAC) was added to this solution as a chelating agent to obtain a chelating agent-containing solution. ACAC was purchased from Fujifilm Wako Pure Chemical Industries, Ltd. The amount of ACAC in the chelating agent-containing solution was 1 / 3.25 of the total amount of Ni ions and Fe ions. The chelating agent-containing solution was acidic.

[0085] A sintered Ni mesh manufactured by Fuji Filter Kogyo Co., Ltd. was washed with acetone for 10 minutes and then with a 0.1 mol / L HCl aqueous solution for 10 minutes to degrease the Ni mesh and remove impurities. In the Ni mesh according to Example 1, the wire diameter of the Ni mesh was 0.1 mm. The number of meshes in the Ni mesh was 30. The steel type of the Ni mesh was Ni200. The Ni mesh was rectangular, measuring 120 mm x 50 mm in plan view. The total weight of the Ni mesh was 2.1444 g. Next, the Ni mesh was washed with water and dried to complete the cleaning process of the Ni mesh.

[0086] Fig. 8 is a scanning electron microscope (SEM) image of a cross section of the Ni mesh sintered body before etching according to Example 1. As shown in Fig. 8, the Ni mesh sintered body before etching had horizontal lines 11 and vertical lines 12. In addition, in the Ni mesh sintered body, the horizontal lines 11 and vertical lines 12 intersected at an intersection 3. Furthermore, at this intersection 3, the horizontal lines 11 and vertical lines 12 were joined to each other by a joint 13.

[0087] Next, the Ni mesh after the cleaning treatment was immersed in the chelating agent-containing solution. In this state, the chelating agent-containing solution containing the Ni mesh was shaken and stirred at about 20°C for 24 hours. During this time, the outermost surface of the Ni mesh was etched according to the above formula (1).

[0088] Next, 8.957 mL of propylene oxide (POX) was added to the chelating agent-containing solution as a pH-raising agent. The amount of POX added was adjusted so that the ratio of the amount of POX to the amount of chloride ions in the mixed solution was 2. The resulting mixed solution was shaken and stirred at approximately 20°C for 72 hours. During the shaking and stirring of the mixed solution, the POX gradually captured hydrogen ions in the mixed solution, and the pH of the mixed solution gradually increased and became alkaline. After 72 hours of shaking and stirring, the Ni mesh was recovered, washed with water, and dried. In this way, the electrode according to Example 1 was obtained.

[0089] (Comparative Example 1) An electrode according to Comparative Example 1 was produced in the same manner as in Example 1, except that a Ni mesh manufactured by Kureha Corporation was used. The Ni mesh according to Comparative Example 1 was a non-sintered body. That is, in the Ni mesh according to Comparative Example 1, the intersecting metal wires were not joined to each other at the intersections. In the Ni mesh according to Comparative Example 1, the wire diameter of the Ni mesh was 0.1 mm. The number of meshes in the Ni mesh was 30. The steel type of the Ni mesh was Ni200. The Ni mesh was rectangular, measuring 120 mm x 50 mm in plan view.

[0090] [Electrode Evaluation] The oxygen evolution (OER) overpotential of the electrode according to Example 1 and the electrode according to Comparative Example 1 was evaluated. For the measurements, a potentiostat VersaSTAT4 manufactured by Princeton Applied Research, an alkali sample vial (100 mL) manufactured by BAS, and a Teflon cap (for 100 mL) manufactured by BAS were used. An AE-2 plate electrode manufactured by EC Frontier was used as the working electrode jig. The material of this plate electrode was Ni. The working electrodes, i.e., the electrode according to Example 1 and the electrode according to Comparative Example 1, were fixed to this jig. A 23 cm Pt counter electrode manufactured by BAS was used as the counter electrode. The counter electrode was coil-shaped. The current derived from the anode reaction of the water electrolysis cell was measured using a three-electrode method under the following measurement conditions. The anode reaction was an oxygen evolution reaction. In Example 1 and Comparative Example 1, a sample prepared by cutting the electrode prepared by the above-described method into a circular shape with a diameter of 15 mm was used as the working electrode.

[0091] (Measurement conditions) Solution: 1 mol / L KOH solution Potential to reversible hydrogen electrode (RHE): 1.0 V to 1.7 V Number of cycles: 5 cycles Potential sweep rate: 10 mV / sec Temperature: 20°C Number of measurement samples: 5

[0092] Current density at the fifth cycle: 10 mA / cm 2 The OER overpotential was determined by subtracting the theoretical potential of 1.229 V required to cause the oxygen evolution reaction to proceed from the voltage corresponding to the OER overpotential. The results are shown in Table 1. In Example 1 and Comparative Example 1, the number of measured samples was five. The average overpotential and standard deviation shown in Table 1 are the average value and standard deviation of the OER overpotential for the five measured samples, respectively.

[0093]

[0094] As shown in Table 1, the average overvoltage of the OER in the electrode according to Example 1 was lower than the average overvoltage of the OER in the electrode according to Comparative Example 1. This result indicates that the conductive substrate used in Example 1 is superior in terms of electrode activity. In addition, the standard deviation of the average overvoltage of the OER in the electrode according to Example 1 was lower than the standard deviation of the average overvoltage of the OER in the electrode according to Comparative Example 1. This result indicates that the conductive substrate used in Example 1 is superior in terms of suppressing variation in electrode quality.

[0095] 9 is a graph showing the measurement results of the OER overpotential of the electrode according to Example 1 and the electrode according to Comparative Example 1. In FIG. 9, the vertical axis represents the current density, and the horizontal axis represents the potential relative to the reversible hydrogen electrode. As shown in FIG. 9, when the current density of the electrode according to Example 1 and the current density of the electrode according to Comparative Example 1 were the same value, the electrode according to Example 1 had a lower overpotential than the electrode according to Comparative Example 1. This result indicates that the electrode activity of the electrode according to Example 1 was higher than that of the electrode according to Comparative Example 1.

[0096] FIG. 10A is a photograph of the electrode according to Example 1 after measuring the OER overpotential using the electrode. FIG. 10B is a photograph enlarging the X portion of FIG. 10A . As shown in FIGS. 10A and 10B , the entire surface of the electrode according to Example 1 turned black after measurement. This indicates that the Ni—Fe LDH was converted to hydroxide by the water electrolysis reaction. That is, in Example 1, the intersecting metal wires were joined to each other at their intersections in the conductive substrate. This suggests that, even after the electrode was fabricated, the contact resistance between the intersecting metal wires was unlikely to increase, and electrical continuity between the intersecting metal wires was ensured.

[0097] 11A is a photograph of the electrode according to Comparative Example 1 after measuring the OER overpotential. FIG. 11B is a photograph of a magnified portion Y of FIG. 11A . As shown in FIGS. 11A and 11B , in the electrode according to Comparative Example 1, a portion of the electrode did not turn black after the measurement. This suggests that in the electrode according to Comparative Example 1, the intersecting metal wires were not joined to each other at their intersections in the conductive substrate, and therefore, after the electrode was fabricated, the contact resistance between the intersecting metal wires increased, and electrical continuity between the intersecting metal wires was not ensured.

[0098] It should be noted that many modifications and other embodiments of the present disclosure will be apparent to those skilled in the art from the above description. Accordingly, the above description should be construed as illustrative only and is provided for the purpose of teaching those skilled in the art the best mode of carrying out the present disclosure. Substantial changes can be made in its operating conditions, composition, structure, and / or function without departing from the spirit of the present disclosure.

[0099] The water electrolysis electrode obtained by the method for producing a water electrolysis electrode according to the present disclosure can be used as an anode or cathode for water electrolysis.

Claims

1. promoting mixing of a solution containing transition metal ions and chloride ions while the conductive substrate is immersed in the solution; forming a layered double hydroxide layer having two or more types of transition metals on the surface of the conductive substrate, the conductive substrate has a mesh structure including a plurality of metal wires, and the intersecting metal wires are joined to each other at intersections; the plurality of metal wires are interwoven; A method for manufacturing an electrode for water electrolysis.

2. The method for producing an electrode for water electrolysis according to claim 1 , wherein the conductive base material contains a metal that reacts with the chloride ions and is eluted from the conductive base material.

3. The method for producing an electrode for water electrolysis according to claim 1 , wherein the metal wires are joined to each other at the crossing points by sintering.

4. 2. The method for producing an electrode for water electrolysis according to claim 1, wherein the transition metal ions include ions of at least one transition metal selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru.

5. 2. The method for producing an electrode for water electrolysis according to claim 1, wherein the transition metal ions include ions of at least one transition metal selected from the group consisting of Ni and Fe.