Electrode for water electrolysis, anode for water electrolysis, cathode for water electrolysis, water electrolysis cell, and water electrolysis device

Abstract

An electrode 1 for water electrolysis comprises a conductive substrate 10 and a layered double hydroxide (LDH) layer 20. The layered double hydroxide layer 20 is provided to a surface of the conductive substrate 10. The effective film thickness of the layered double hydroxide layer 20 is 250 nm or more and less than 4,000 nm. The layered double hydroxide layer 20 may contain a layered double hydroxide 20a. The effective film thickness of the layered double hydroxide layer 20 may be 3,470 nm or less.

Description

Water electrolysis electrodes, water electrolysis anodes, water electrolysis cathodes, water electrolysis cells, and water electrolysis devices

[0001] The present disclosure relates to an electrode for water electrolysis, an anode for water electrolysis, a cathode for water electrolysis, a water electrolysis cell, and a water electrolysis device.

[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] Patent Document 2 describes an oxygen generating catalyst that includes a graphene oxide layer and a nickel-iron layered double hydroxide layer supported on the surface of the graphene oxide layer, in which the graphene oxide layer has an average thickness of 0.33 to 4 nm.

[0005] 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).

[0006] 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.

[0007] International Publication No. 2017 / 154134 Japanese Patent Application Laid-Open No. 2018-043193

[0008] 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

[0009] 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 an electrode for water electrolysis that is advantageous from the viewpoint of exhibiting excellent performance.

[0010] The present disclosure provides an electrode for water electrolysis, comprising: a conductive substrate; and a layered double hydroxide layer provided on a surface of the conductive substrate, wherein the effective thickness of the layered double hydroxide layer is 250 nm or more and less than 4000 nm.

[0011] According to the present disclosure, it is possible to provide an electrode for water electrolysis that is advantageous in terms of exhibiting excellent performance.

[0012] FIG. 1 is a cross-sectional view schematically showing an example of a water electrolysis electrode according to the present disclosure. FIG. 2 is a cross-sectional view schematically showing an example of an optical model of a water electrolysis electrode obtained by spectroscopic ellipsometry. FIG. 3 is a diagram schematically showing an example of the crystal structure of a layered double hydroxide (LDH). FIG. 4 is a diagram schematically showing a mechanism for producing a water electrolysis electrode. FIG. 5 is a cross-sectional view of an example of a water electrolysis cell according to the present disclosure. FIG. 6 is a cross-sectional view of an example of a water electrolysis apparatus according to the present disclosure. FIG. 7 is a cross-sectional view of another example of a water electrolysis cell according to the present disclosure. FIG. 8 is a cross-sectional view of another example of a water electrolysis apparatus according to the present disclosure.

[0013] (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.

[0014] 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.

[0015] 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.

[0016] For example, layered double hydroxides (LDHs) are considered promising materials for water electrolysis electrodes due to their large specific surface area and diverse combinations of metal ions. Patent Document 1 describes a method for producing a water electrolysis electrode containing a layered double hydroxide represented by a predetermined compositional formula by performing a pulse electrodeposition process in an aqueous solution containing a compound containing metal M1 and a compound containing metal M2, using a conductive substrate as the anode. However, the performance of the electrode produced by the electrodeposition process was insufficient, leaving room for improvement. After extensive research, the present inventors have newly discovered that, in a water electrolysis electrode comprising an LDH layer formed on a conductive substrate, the LDH layer has a predetermined effective film thickness, thereby improving the performance of the water electrolysis electrode. Based on this new finding, the present inventors have completed the water electrolysis electrode according to the present disclosure.

[0017] 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.

[0018] First Embodiment Fig. 1 is a cross-sectional view schematically illustrating an example of a water electrolysis electrode according to the present disclosure. As shown in Fig. 1, the water electrolysis electrode 1 includes a conductive substrate 10 and a layered double hydroxide (LDH) layer 20. The LDH layer 20 is provided on the surface of the conductive substrate 10. The LDH layer 20 can function as a catalyst for the anode reaction or cathode reaction of water electrolysis. The LDH layer 20 contains LDH as a catalyst for water electrolysis.

[0019] In the water electrolysis electrode 1, the effective film thickness of the LDH layer 20 is 250 nm or more and less than 4000 nm. The effective film thickness correlates with the amount of LDH contained in the LDH layer 20. When the effective film thickness of the LDH layer 20 is 250 nm or more and less than 4000 nm, the amount of LDH contributing to the water electrolysis reaction can be increased in the water electrolysis electrode 1, and an increase in reaction resistance can be suppressed. As a result, it is possible to obtain a water electrolysis electrode 1 with excellent electrode activity, and the water electrolysis electrode 1 can exhibit excellent performance.

[0020] The LDH layer 20 is bonded to, for example, the conductive substrate 10. For example, no adhesive layer containing an organic material such as a polymer is disposed between the LDH layer 20 and the conductive substrate 10, and the LDH layer 20 is bonded directly to the surface of the conductive substrate 10.

[0021] The water electrolysis electrode 1 includes an LDH layer 20, which contains LDH. LDH can contribute to the water electrolysis reaction. Therefore, when the water electrolysis electrode 1 contains LDH, the performance of the water electrolysis electrode 1 can be improved. However, studies by the present inventors have revealed that it is difficult to improve the performance of the water electrolysis electrode 1 simply by increasing the amount of LDH and increasing the thickness of the LDH layer 20. This is presumably because LDH is distributed non-uniformly in the LDH layer 20. When LDH is distributed non-uniformly, the LDH layer 20 may contain LDH that does not contribute to the water electrolysis reaction. Therefore, in order to improve the performance of the water electrolysis electrode 1, it is necessary to consider the distribution of LDH in the LDH layer 20. The thickness of the LDH layer 20 can be measured, for example, using an electron microscope. However, analysis using an electron microscope cannot evaluate the distribution of LDH, including the depth, in the LDH layer 20. As will be described later, the effective film thickness takes into account the distribution of LDH in the LDH layer 20. Thus, the effective film thickness differs from film thickness measured using an electron microscope in that it takes into account the distribution of LDH in the LDH layer 20. The present inventors have found that the performance of the water electrolysis electrode 1 can be improved by appropriately adjusting the effective film thickness.

[0022] [Layered Double Hydroxide Layer] The effective thickness of the LDH layer 20 can be calculated by steps S1 to S4.

[0023] Step S1 is a step of measuring the reflection spectrum of the water electrolysis electrode 1 by spectroscopic ellipsometry and obtaining measurement data E of the polarization parameters of the water electrolysis electrode 1. Step S2 is a step of creating an optical model of the water electrolysis electrode 1 and obtaining simulation data M of the polarization parameters based on the optical model. Step S3 is a step of fitting the simulation data M of the polarization parameters to the measurement data E of the polarization parameters. Step S4 is a step of calculating the effective film thickness using the results of the fitting calculation according to the following formula (1):

[0024] Effective film thickness [nm] = Σ(R 28i ×T 28i ) + T 27 + (R 26×T 26 ) Formula (1)

[0025] In formula (1), i is an integer of 1 or more. 28i indicates the volume ratio [%] of LDH in the i-th layer of the surface roughness layer 28. 28i indicates the layer thickness [nm] of the i-th layer of the surface roughness layer 28. 27 indicates the thickness [nm] of the dense layer 27. 26 indicates the volume fraction [%] of LDH in the interface layer 26. 26 indicates the layer thickness [nm] of the interface layer 26.

[0026] In step S1, the reflection spectrum of the water electrolysis electrode 1 is measured by spectroscopic ellipsometry. In step S1, measurement data E of the polarization parameters of the water electrolysis electrode 1 can be obtained.

[0027] In step S2, an optical model of the water electrolysis electrode 1 is created. In step S2, simulation data M of polarization parameters based on the optical model can be obtained.

[0028] 2 is a cross-sectional view schematically illustrating an example of an optical model of a water electrolysis electrode obtained by spectroscopic ellipsometry. As shown in FIG. 2, the optical model 1m includes an LDH layer 20 and a conductive substrate 10. The LDH layer 20 has a predetermined thickness T 20 The LDH layer 20 has a surface roughness layer 28, a dense layer 27, and an interface layer 26. The surface roughness layer 28 has voids 25. The surface roughness layer 28 has a predetermined layer thickness T 28 The dense layer 27 has a predetermined layer thickness T 27 The interface layer 26 has a predetermined thickness T 26 It has.

[0029] The thickness T of the surface roughness layer 28 28 , the thickness T of the dense layer 27 27 , and the thickness T of the interface layer 26 26is determined by fitting, which will be described later. As shown in FIG. 2 , in the optical model 1m, the LDH layer 20 includes an interface layer 26, a dense layer 27, and a surface roughness layer 28, in that order from closest to the conductive substrate 10 in the thickness direction. The interface layer 26 is composed of the material of the conductive substrate 10 and the LDH contained in the LDH layer 20. The dense layer 27 is composed only of the LDH contained in the LDH layer 20. The surface roughness layer 28 is composed of air that may be present in the voids 25 and the LDH contained in the LDH layer 20. The volume proportions of the material of the conductive substrate 10 and the LDH in the interface layer 26 can be predetermined proportions. The volume proportions of the LDH and the air in the surface roughness layer 28 can be predetermined proportions.

[0030] In step S3, a fitting calculation is performed, in which simulation data M of the polarization parameters is fitted to measurement data E of the polarization parameters.

[0031] Based on the optical model 1m, a fitting calculation is performed. 28 , layer thickness T 27 , and layer thickness T 26 In addition, the volume fraction of LDH and the volume fraction of air in the surface roughness layer 28, as well as the volume fraction of the material of the conductive substrate 10 and LDH in the interface layer 26, can be determined. As a result, the distribution of LDH in the LDH layer 20 can be evaluated.

[0032] The fitting calculation can be performed using, for example, predetermined software, and is performed so that the mean square error between the simulation data calculated from the optical model 1m and the measurement data is 10° or less. For example, the fitting calculation may be performed using the New Amorphous equation proposed by Horiba Jobin Yvon and the Tauc-Lorentz equation, and the calculation result with the smaller mean square error may be adopted.

[0033] The number of layers included in the surface roughness layer 28 is determined by fitting calculation. The surface roughness layer 28 may be a single layer, or may be composed of two or more layers. When the surface roughness layer 28 is composed of two or more layers, the surface roughness layer 28 has first to n-th layers, where n is an integer of 2 or more. The surface roughness layer 28 is stacked from the first layer to the n-th layer in the thickness direction in order of proximity to the dense layer 27. The upper limit of n is not limited to a specific value. The upper limit of n is, for example, 5.

[0034] When the surface roughness layer 28 is composed of two or more layers, each layer has a predetermined thickness. The volume fraction of LDH and the volume fraction of air in the n-th layer of the surface roughness layer 28 can be determined by the fitting calculation described above.

[0035] In step S4, the fitting result obtained in step S3 is used to calculate the effective film thickness according to the above-mentioned formula (1).

[0036] The effective film thickness of the LDH layer 20 may be 410 nm or more, or 500 nm or more. This allows the overvoltage in the anode reaction and the cathode reaction of water electrolysis to be further reduced, allowing the water electrolysis electrode 1 to exhibit better performance.

[0037] The effective film thickness of the LDH layer 20 may be 3470 nm or less, 2750 nm or less, 2010 nm or less, 1500 nm or less, or 1200 nm or less, which can further reduce overvoltages in the anode and cathode reactions of water electrolysis, allowing the water electrolysis electrode 1 to exhibit better performance.

[0038] In the LDH layer 20, when the effective film thickness is 250 nm or more and 1200 nm or less, or when the effective film thickness is 2100 nm or more and 2750 nm or less, the layer thickness T 28 is the layer thickness T 27 In this case, the gas generating reaction can be more promoted.

[0039] In the LDH layer 20, when the effective thickness is greater than 1200 nm and less than 2100 nm, or when the effective thickness is greater than 2750 nm and less than 4000 nm, the layer thickness T 28 is the layer thickness T 27 In this case, the durability of the water electrolysis electrode 1 can be further improved.

[0040] The LDH layer 20 covers, for example, the surface of the conductive substrate 10. The coverage of the LDH layer 20 with respect to the surface of the conductive substrate 10 is not limited to a specific value. The coverage is preferably 99% or more. In this case, the water electrolysis electrode 1 is likely to have excellent electrode activity. In addition, the water electrolysis electrode 1 is likely to have excellent durability.

[0041] [Conductive Substrate] The conductive substrate 10 is not limited to a specific substrate as long as it is conductive. The conductive substrate 10 may contain a metal or a resin. The entire conductive substrate 10 may be made of metal. The conductive substrate 10 may have a configuration in which a metal-containing surface layer is formed on a resin member such as polypropylene or polyethylene. In this case, the metal-containing surface layer may be a plated film or a sputtered film. The metal contained in the conductive substrate may be a pure metal such as nickel or iron, or an alloy such as stainless steel or Inconel. Inconel is a registered trademark.

[0042] The surface of the conductive substrate 10 is preferably made of nickel. In this case, the conductive substrate 10 tends 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.

[0043] When the surface of the conductive substrate 10 is made of nickel, the purity of the nickel constituting the surface is not limited to a specific value. The purity is, for example, 90% by mass or more. This allows the conductive substrate 10 to have superior alkali resistance. The method for determining the purity of the nickel constituting the surface of the conductive substrate 10 is not limited to a specific method. The purity may be determined by elemental analysis such as X-ray fluorescence spectroscopy (XRF) or energy dispersive X-ray spectroscopy (EDX). The purity of the nickel constituting the surface of the conductive substrate 10 may be determined by analyzing an extract obtained by, for example, completely dissolving the conductive substrate 10 in aqua regia using a method such as inductively coupled plasma atomic emission spectroscopy (ICP-AES). When the purity of the nickel is high, the purity of the nickel constituting the surface of the conductive substrate 10 may be determined by comparing the specific gravity of the conductive substrate 10 with the specific gravity of pure nickel.

[0044] The purity of the nickel forming the surface of the conductive substrate 10 is preferably 95% by mass or more, more preferably 97% by mass or more, even more preferably 98% by mass or more, and particularly preferably 99% by mass or more.

[0045] The shape of the conductive substrate 10 is not limited to a specific shape. The conductive substrate 10 is, for example, sheet-shaped. The conductive substrate 10 may have a non-porous structure such as a plate or foil, or a porous structure such as an expanded metal, mesh, foam, or nonwoven fabric. The conductive substrate 10 preferably has a porous structure. In this case, the surface area of ​​the conductive substrate 10 can be increased, and gas generated in the water electrolysis reaction can be easily diffused.

[0046] The thickness of the conductive substrate 10 is not limited to a specific value. The conductive substrate is, for example, 0.02 mm or more. In this case, the conductive substrate 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.

[0047] [Layered double hydroxide] Fig. 3 is a diagram schematically illustrating an example of the crystal structure of an LDH. The LDH 20a contained in the LDH layer 20 is active in a reaction for producing gases such as hydrogen and oxygen at the anode or cathode of water electrolysis. For example, the LDH 20a can be converted to hydroxide during alkaline water electrolysis.

[0048] The LDH 20a 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.

[0049] [M1 2+ 1-x M2 3+ x (OH)2][yA n- ・mH2O] Formula (2)

[0050] The LDH 20a contains, for example, two or more types of transition metals. The two or more types of transition metals in the LDH 20a 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 LDH 20a contains, for example, at least two transition metals selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. In this case, the water electrolysis electrode 1 is likely to have excellent electrode activity.

[0051] The LDH 20a preferably contains at least one transition metal selected from the group consisting of Fe and Ni. In this case, the water electrolysis electrode 1 can have better electrode activity. In addition, the production cost of the water electrolysis electrode 1 can be easily reduced.

[0052] The LDH 20a may contain 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 can have better electrode activity.

[0053] In LDH20a, the interlayer anion A 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 - , and CH3(CH2) n PO4 2- 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 LDH20a are not limited to a specific value. n- or a plurality of types of A n- may also include:

[0054] As shown in FIG. 3, LDH20a is a soluble form of M1 2+ or M2 3+ OH at each vertex of the octahedron centered at - LDH20a 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. In other words, the LDH 20a as a whole is composed of a host layer of metal hydroxide and an anion A n-The LDH 20a has a sheet-like structure in which guest layers of metal hydroxide and water molecules are alternately stacked. 2+ Part of M2 3+ It has a structure substituted with

[0055] The crystal structure and crystallinity of LDH20a can be qualitatively and quantitatively analyzed by X-ray diffraction measurement (XRD).

[0056] [Chelating Agent] The LDH layer 20 may contain a chelating agent. The chelating agent may be coordinated to a transition metal ion contained in the LDH 20a. This allows the LDH 20a to be stably present in the LDH layer 20. In addition, the LDH 20a is likely to have a small particle size. In addition, the LDH layer 20 is likely to be a dense layer containing the LDH 20a with few voids, and is likely to be firmly fixed to the conductive substrate 10 with a desired thickness. This is because, when the LDH layer 20 is formed on the conductive substrate 10, nucleated LDH is likely to undergo slow crystal growth. As a result, the LDH layer 20 is likely to effectively contribute to the anode reaction or cathode reaction of water electrolysis, and the water electrolysis electrode 1 can have superior electrode activity.

[0057] The chelating agent is not limited to a specific chelating agent. The chelating agent is, for example, an organic compound capable of coordinating with a transition metal ion in LDH20a. 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 include β-diketones, β-ketoesters, hydroxycarboxylic acids, and hydroxycarboxylate salts. Examples of β-diketones include acetylacetone (ACAC), trifluoroacetylacetone, hexafluoroacetylacetone, benzoylacetone, thenoyltrifluoroacetone, dipyrrolylmethane, dibenzoylmethane, and ascorbic acid. Examples of β-ketoesters include 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 are tartaric acid, citric acid, malic acid, gluconic acid, ferulic acid, lactic acid, glucuronic acid, and salts thereof.

[0058] The chelating agent preferably contains at least one selected from the group consisting of acetylacetone and citrate, which allows the water electrolysis electrode 1 to have excellent electrode activity. An example of the citrate is trisodium citrate.

[0059] The method for producing the water electrolysis electrode 1 is not limited to a specific method. The water electrolysis electrode 1 is produced, for example, by immersing the conductive substrate 10 in a solution containing a chelating agent and two or more types of transition metal ions and adjusting the solution to alkaline. This method allows the LDH layer 20 containing LDH and a chelating agent to be formed on the conductive substrate 10 in a simple manner.

[0060] The temperature of the solution when adjusted to an alkaline state is not limited to a specific temperature. The temperature of the solution is, for example, room temperature of 20° C.±15° C. In this case, the water electrolysis electrode 1 can be obtained with better electrode activity.

[0061] The solvent of the solution may be water, an organic solvent, or a mixed solvent of water and an organic solvent.

[0062] The method for producing the water electrolysis electrode 1 preferably includes increasing the pH. This allows the LDHs 20a to be formed in the conductive substrate 10 in a short period of time, and enables the production of a water electrolysis electrode 1 with excellent electrode activity. In addition, the produced water electrolysis electrode 1 has excellent durability.

[0063] The method for adjusting the solution to alkaline is not limited to a specific method. For example, the solution may be adjusted to alkaline by mixing the above solution with an alkaline solution. Alternatively, the solution may be adjusted to alkaline by adding a pH-increasing agent to the above solution. In this case, the pH-increasing agent is not limited to a specific compound. The pH-increasing agent is, for example, a compound having an epoxy group. Examples of pH-increasing agents are propylene oxide (POX), ethylene oxide, and butylene oxide.

[0064] When a pH-raising agent having an epoxy group, such as propylene oxide, is added to a solution, the ring-opening reaction of the epoxy group occurs in the presence of a nucleophile, such as chloride ion, and the pH-raising agent captures hydrogen ions present in the solution. This increases the pH of the solution, making the solution alkaline. The pH of a solution containing a chelating agent and two or more types of transition metal ions is, for example, 1. When a pH-raising agent is added to this solution, the pH of the solution gradually increases from, for example, 1, and the solution may eventually become alkaline. The final pH of the solution is, for example, 8 or more and 12 or less. The addition of the pH-raising agent to the solution causes a reaction to capture hydrogen ions in the solution. This gradually increases the pH of the solution. The time from the addition of the pH-raising agent to the solution until the pH of the solution reaches a steady state is not limited to a specific time. This time may be, for example, 24 hours or more, or several days.

[0065] The two or more types of transition metal ions contained in the solution are not limited to specific transition metal ions. The two or more types of transition metal ions contained in the solution are, for example, ions of at least two transition metals selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. In this case, a water electrolysis electrode 1 having superior electrode activity can be produced.

[0066] The two or more types of transition metal ions contained in the solution 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 1 having superior electrode activity can be produced.

[0067] The surface of the conductive substrate 10 is preferably made of nickel. In this case, a water electrolysis electrode having advantageous properties, for example, from the viewpoint of achieving both corrosion resistance and electrical conductivity in alkaline water electrolysis, can be easily produced.

[0068] The conductive substrate 10 preferably contains nickel. The two or more types of transition metal ions contained in the solution preferably contain Fe ions. The solution preferably contains chloride ions. In this case, the reaction shown in formula (3) may occur, which may result in etching of the conductive substrate 10. The method for producing the water electrolysis electrode 1 preferably includes promoting mixing of the solution while the conductive substrate 10 is immersed in the solution before adjusting the solution to be alkaline.

[0069] 4Ni 2+ Cl - 2 + 2Fe 3+ Cl - 3 + 2Ni → 5Ni 2+ Cl - 2 + 2Fe 2+ Cl - 2 + Ni Equation (3)

[0070] Methods for promoting the mixing of the solutions include vibrating the conductive substrate 10, shaking a container in which the solution and the conductive substrate 10 are sealed, and stirring the solution using a stirrer piece or a stirrer. Such methods cause forced convection of the solution, promoting the mixing of the solution. This allows the conductive substrate 10 to be etched, and the LDH layer 20 to be formed on the conductive substrate 10. The mixing of the solutions may be promoted in a state in which the container containing the solution and the conductive substrate 10 is sealed, or in an inert gas atmosphere.

[0071] "Promoting mixing of the solution" means promoting the diffusion of metal ions eluted from the conductive substrate 10 into the solution by the above-mentioned method.

[0072] In the production of the water electrolysis electrode 1, the ratio of the amount of substances of Fe ions to the amount of substances of Ni contained in the conductive base material 10 is not limited to a specific value. This ratio is, for example, 0.100 or less. In this case, it is possible to prevent the nickel contained in the conductive base material 10 from dissolving due to the reaction represented by formula (3), which would make it difficult to produce the water electrolysis electrode 1.

[0073] The ratio may be 0.020 or more and 0.080 or less, or 0.020 or more and 0.050 or less. In this case, the LDH layer 20 is more likely to be formed uniformly on the conductive substrate 10, and a water electrolysis electrode 1 having superior electrode activity can be produced.

[0074] The chelating agent contained in the solution may be selected with reference to the above examples of the chelating agent contained in the LDH layer 20. The chelating agent contained in the solution preferably contains at least one selected from the group consisting of acetylacetone and citrate. This tends to improve the stability of the dispersion of the complex in the solution, and facilitates the formation of the LDH layer 20 in the water electrolysis electrode 1. As a result, the water electrolysis electrode 1 can have superior electrode activity.

[0075] FIG. 4 is a schematic diagram illustrating the mechanism of manufacturing the water electrolysis electrode 1. As shown in FIG. 4 , a conductive substrate 10 is immersed in a solution containing transition metal ions TM1, TM2, and a chelating agent 30. For example, the transition metal ions TM1 are Ni ions, and the transition metal ions TM2 are Fe ions. In addition, Ni is present on the surface of the conductive substrate 10. Some of the transition metal ions TM2 etch and dissolve the Ni present on the surface of the conductive substrate 10. Some of the chelating agent 30 reacts with the surface of the conductive substrate 10 to form a complex C1 between the transition metal ions TM1 derived from the conductive substrate 10 and the chelating agent 30. In addition, when the solution is adjusted to an alkaline pH, a complex C1 derived from the transition metal ions TM1 derived from the solution and the chelating agent 30 is formed in the solution, and a complex C2 between the transition metal ions TM2 and the chelating agent 30 is formed. Next, the complexes C1 and C2 react on the surface of the conductive substrate 10 to synthesize LDH 20a along the surface of the conductive substrate 10. In addition, the crystal growth of the LDH 20a is suppressed because the complexes C1 and C2 contain the chelating agent 30. As a result, an LDH layer 20 containing the LDH 20a and the chelating agent 30 is formed on the conductive substrate 10, and a water electrolysis electrode 1 is obtained.

[0076] The water electrolysis electrode 1 according to this embodiment can be used, for example, as an electrode in a water electrolysis cell of an alkaline water electrolysis apparatus or an anion exchange membrane water electrolysis apparatus. The water electrolysis electrode 1 is used, for example, in at least one selected from the group consisting of an anode and a cathode in these water electrolysis apparatuses. In other words, at least one selected from the group consisting of a water electrolysis anode including the water electrolysis electrode 1 and a water electrolysis cathode including the water electrolysis electrode 1 can be provided. This tends to increase the activity of the anode reaction or the cathode reaction in water electrolysis.

[0077] Second Embodiment Figure 5 is a cross-sectional view schematically illustrating an example of a water electrolysis cell according to a second embodiment. As shown in Figure 5, the water electrolysis cell 2 includes an anode 2a, a cathode 2b, and a diaphragm 2p. At least one selected from the group consisting of the anode 2a and the cathode 2b includes, for example, the water electrolysis electrode 1 according to the first embodiment. In this case, the activity of the anode reaction or the cathode reaction in the water electrolysis cell 2 is likely to be improved, and the anode 2a or the cathode 2b is likely to exhibit excellent performance.

[0078] The water electrolysis cell 2 is, for example, an alkaline water electrolysis cell that uses an alkaline aqueous solution. The alkaline aqueous solution used in the water electrolysis cell 2 is not limited to a specific alkaline aqueous solution. Examples of alkaline aqueous solutions include a potassium hydroxide aqueous solution and a sodium hydroxide aqueous solution.

[0079] 5 , the water electrolysis cell 2 includes, for example, an electrolytic cell 2s, a first chamber 2m, and a second chamber 2n. A diaphragm 2p is disposed inside the electrolytic cell 2s, separating the interior of the electrolytic cell 2s into the first chamber 2m and the second chamber 2n. An anode 2a is disposed in the first chamber 2m, and a cathode 2b is disposed in the second chamber 2n.

[0080] The diaphragm 2p is, for example, a diaphragm for alkaline water electrolysis. The diaphragm 2p is, for example, a sheet-like porous membrane. The diaphragm 2p has a thickness of, for example, 100 μm to 500 μm and has pores that serve as passages for ions or the electrolyte. The material of the diaphragm 2p is not limited to a specific material. Examples of materials for the diaphragm 2p include asbestos, polymer-reinforced asbestos, potassium titanate bound with polytetrafluoroethylene (PTFE), zirconia bound with PTFE, and antimonic acid and antimony oxide bound with polysulfone. Other examples of materials for the diaphragm 2p include sintered nickel, nickel coated with ceramics and nickel oxide, and polysulfone. The diaphragm 2p may be Zirfon Perl UTP 500 manufactured by AGFA.

[0081] The anode 2a may be disposed in a zero-gap state, i.e., in contact with the diaphragm 2p, or may be disposed with a gap between it and the diaphragm 2p. The cathode 2b may be disposed in a contact state with the diaphragm 2p, or may be disposed with a gap between it and the diaphragm 2p.

[0082] The water electrolysis cell 2 electrolyzes an alkaline aqueous solution to produce hydrogen and oxygen. An aqueous solution containing a hydroxide of an alkali metal or alkaline earth metal is supplied to the first chamber 2m. In addition, an alkaline aqueous solution can be supplied to the second chamber 2n. Electrolysis is performed while an alkaline aqueous solution of a predetermined concentration is discharged from the first chamber 2m and the second chamber 2n, and hydrogen and oxygen are produced.

[0083] When the anode 2a includes the water electrolysis electrode 1, the cathode 2b may include, for example, a known electrode material for the cathode of an alkaline water electrolysis cell. When the cathode 2b includes the water electrolysis electrode 1, the anode 2a may include a known electrode material for the anode of an alkaline water electrolysis cell. In the water electrolysis cell 2, both the anode 2a and the cathode 2b may include the water electrolysis electrode 1.

[0084] According to the above configuration, at least one selected from the group consisting of the anode 2a and the cathode 2b includes the water electrolysis electrode 1, and therefore the water electrolysis cell 2 can exhibit excellent performance.

[0085] Third Embodiment Fig. 6 is a cross-sectional view schematically illustrating an example of a water electrolysis apparatus according to a third embodiment. As shown in Fig. 6, the water electrolysis apparatus 3 includes the water electrolysis cell 2 according to the second embodiment and a voltage applicator 40. The voltage applicator 40 applies a voltage between the cathode 2b and the anode 2a. The water electrolysis apparatus 3 is an alkaline water electrolysis apparatus that uses an alkaline aqueous solution.

[0086] The voltage applicator 40 is electrically connected to the anode 2a and the cathode 2b. The voltage applicator 40 makes the potential of the anode 2a higher than the potential of the cathode 2b. The voltage applicator 40 is not limited to a specific type of voltage applicator as long as it can apply a voltage between the anode 2a and the cathode 2b. The voltage applicator 40 may be a device that adjusts the voltage applied between the anode 2a and the cathode 2b. When the voltage applicator 40 is connected to a DC power source such as a battery, a solar cell, or a fuel cell, the voltage applicator 40 includes, for example, a DC / DC converter. When the voltage applicator 40 is connected to an AC power source such as a commercial power source, the voltage applicator 40 includes, for example, an AC / DC converter. The voltage applicator 40 may be, for example, a power-type power supply. In the power-type power supply, the voltage applied between the anode 2a and the cathode 2b and the current flowing between the anode 2a and the cathode 2b are adjusted so that the power supplied to the water electrolysis apparatus 3 reaches a predetermined set value.

[0087] With the above configuration, the water electrolysis device 3 can exhibit excellent performance.

[0088] Fourth Embodiment Figure 7 is a cross-sectional view schematically illustrating an example of a water electrolysis cell according to a fourth embodiment. As shown in Figure 7, the water electrolysis cell 4 includes an anode 4a, a cathode 4b, and an anion exchange membrane 4p. In the water electrolysis cell 4, at least one selected from the group consisting of the anode 4a and the cathode 4b includes, for example, the water electrolysis electrode 1 according to the first embodiment. In this case, the activity of the anode reaction or the cathode reaction in the water electrolysis cell 4 is likely to be high, and the anode 4a or the cathode 4b is likely to exhibit excellent performance.

[0089] The water electrolysis cell 4 is, for example, an anion exchange membrane (AEM)-type water electrolysis cell. As shown in Fig. 7 , the anode 4a includes, for example, a catalyst layer 4m and a gas diffusion layer 4n. The cathode 3b includes, for example, a catalyst layer 4j and a gas diffusion layer 4k. The catalyst layer 4m of the anode 4a is in contact with one main surface of the anion exchange membrane 4p, and the catalyst layer 4j of the cathode 4b is in contact with the other main surface of the anion exchange membrane 4p.

[0090] The anion exchange membrane 4p is not limited to a specific type of anion exchange membrane. The anion exchange membrane 4p has conductivity of anions such as hydroxide ions. The anion exchange membrane 4p can prevent mixing of oxygen gas generated at the anode 4a and hydrogen gas generated at the cathode 4b. The oxygen gas passes through the gas diffusion layer 4n and is guided to the outside of the anode 4a. The hydrogen gas passes through the gas diffusion layer 4k and is guided to the outside of the cathode 4b.

[0091] In the water electrolysis cell 4, when the anode 4a includes the water electrolysis electrode 1, the cathode 4b may be a known cathode for AEM water electrolysis cells. In this case, the LDH layer 20 of the water electrolysis electrode 1 may function as the catalytic layer 4m, and the conductive substrate 10 of the water electrolysis electrode 1 may function as the gas diffusion layer 4n. In the water electrolysis cell 4, when the cathode 4b includes the water electrolysis electrode 1, the anode 4a may be a known anode for AEM water electrolysis cells. In this case, the LDH layer 20 of the water electrolysis electrode 1 may function as the catalytic layer 4j, and the conductive substrate 10 of the water electrolysis electrode 1 may function as the gas diffusion layer 4k. In the water electrolysis cell 4, both the anode 4a and the cathode 4b may include the water electrolysis electrode 1.

[0092] According to the above configuration, at least one selected from the group consisting of the anode 4a and the cathode 4b includes the water electrolysis electrode 1, and therefore the water electrolysis cell 4 can exhibit excellent performance.

[0093] Fifth Embodiment Fig. 8 is a cross-sectional view schematically illustrating an example of a water electrolysis apparatus according to a fifth embodiment. As shown in Fig. 8, the water electrolysis apparatus 5 includes a water electrolysis cell 4 and a voltage applicator 40. The voltage applicator 40 applies a voltage between the cathode 4b and the anode 4a. The water electrolysis apparatus 5 is, for example, an AEM-type water electrolysis apparatus.

[0094] The voltage applicator 40 is electrically connected to the anode 4a and the cathode 4b. The voltage applicator 40 makes the potential of the anode 4a higher than the potential of the cathode 4b. The voltage applicator 40 is not limited to a specific type of voltage applicator as long as it can apply a voltage between the anode 4a and the cathode 4b. The voltage applicator 40 may be a device that adjusts the voltage applied between the anode 4a and the cathode 4b. When the voltage applicator 40 is connected to a DC power source such as a battery, a solar cell, or a fuel cell, the voltage applicator 40 includes, for example, a DC / DC converter. When the voltage applicator 40 is connected to an AC power source such as a commercial power source, the voltage applicator 40 includes, for example, an AC / DC converter. The voltage applicator 40 may be, for example, a power-type power supply. In the power-type power supply, the voltage applied between the anode 4a and the cathode 4b and the current flowing between the anode 4a and the cathode 4b are adjusted so that the power supplied to the water electrolysis apparatus 5 reaches a predetermined set value.

[0095] With the above configuration, the water electrolysis device 5 can exhibit excellent performance.

[0096] (Additional Note) From the above description, the following techniques are disclosed.

[0097] (Technology 1) An electrode for water electrolysis, comprising: a conductive substrate; and a layered double hydroxide layer provided on a surface of the conductive substrate, wherein the effective thickness of the layered double hydroxide layer is 250 nm or more and less than 4000 nm.

[0098] According to technique 1, it is possible to provide an electrode for water electrolysis that is advantageous in terms of exhibiting excellent performance.

[0099] (Technology 2) The electrode for water electrolysis according to Technology 1, wherein the effective thickness of the layered double hydroxide layer is 3470 nm or less.

[0100] (Technology 3) The electrode for water electrolysis according to Technology 1 or 2, wherein the effective thickness of the layered double hydroxide layer is 410 nm or more.

[0101] According to Techniques 2 and 3, the water electrolysis electrode can have better electrode activity.

[0102] (Technology 4) The electrode for water electrolysis according to any one of Technologies 1 to 3, wherein the layered double hydroxide layer contains at least two transition metals selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru.

[0103] (Technology 5) The water electrolysis electrode according to any one of Techniques 1 to 4, wherein the layered double hydroxide layer contains at least one transition metal selected from the group consisting of Fe and Ni.

[0104] According to Techniques 4 and 5, the water electrolysis electrode can have better electrode activity.

[0105] (Technology 6) The electrode for water electrolysis according to any one of Techniques 1 to 5, wherein the layered double hydroxide layer contains a chelating agent.

[0106] (Technology 7) The electrode for water electrolysis according to Technology 6, wherein the chelating agent includes at least one selected from the group consisting of acetylacetone and citrate salts.

[0107] According to Techniques 6 and 7, the water electrolysis electrode can have better electrode activity.

[0108] (Technology 8) An anode for water electrolysis, comprising the electrode for water electrolysis according to any one of Technologies 1 to 7.

[0109] (Technology 9) A cathode for water electrolysis, comprising the electrode for water electrolysis according to any one of Technologies 1 to 7.

[0110] Techniques 8 and 9 tend to increase the activity of the anode reaction or cathode reaction in water electrolysis.

[0111] (Technology 10) A water electrolysis cell comprising: an anode; a cathode; and a diaphragm, wherein at least one selected from the group consisting of the anode being the anode for water electrolysis according to Technology 8 and the cathode being the cathode for water electrolysis according to Technology 9 is satisfied.

[0112] (Technology 11) A water electrolysis cell comprising: an anode; a cathode; and an anion exchange membrane, wherein at least one selected from the group consisting of the anode being the anode for water electrolysis according to Technology 8 and the cathode being the cathode for water electrolysis according to Technology 9 is satisfied.

[0113] Techniques 10 and 11 tend to improve the activity of the anode reaction or the cathode reaction in a water electrolysis cell.

[0114] (Technology 12) A water electrolysis device comprising: the water electrolysis cell according to Technology 10 or 11; and a voltage applicator that applies a voltage between the cathode and the anode.

[0115] According to Technique 12, the water electrolysis device can exhibit excellent performance.

[0116] 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.

[0117] Example 1: A solution was prepared by dissolving 0.150 g of nickel chloride hexahydrate and 0.043 g of iron chloride hexahydrate in 3.64 milliliters (mL) of water. Nickel chloride hexahydrate and iron chloride hexahydrate were purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.025 mL of acetylacetone (ACAC) was added to this solution as a chelating agent to obtain a chelating agent-containing solution. ACAC was purchased from Sigma-Aldrich.

[0118] A Ni plate manufactured by Nilaco Corporation was cleaned with acetone for 10 minutes and then with a 1 mol / L HCl aqueous solution for 10 minutes to degrease the Ni plate and remove impurities. The Ni plate had a thickness of 0.2 mm and a circular shape with a diameter of 15 mm in plan view. The Ni plate weighed 0.315 g. The Ni plate was then rinsed with water and dried to complete the cleaning process.

[0119] Next, the Ni plate after the cleaning treatment was immersed in the chelating agent-containing solution. In this state, the chelating agent-containing solution containing the Ni plate was shaken and stirred at 25°C for 24 hours. At this time, the outermost surface of the Ni plate was etched according to the above formula (3).

[0120] Next, 0.254 mL of propylene oxide (POX) was added to the chelating agent-containing solution as a pH-raising agent. The resulting mixed solution was shaken and stirred at 25°C for 72 hours. After 72 hours of shaking and stirring, the Ni plate was recovered, washed with water, and dried. In this manner, the electrode according to Example 1 was obtained. In Example 1, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.030.

[0121] Example 2 An electrode according to Example 2 was fabricated in the same manner as Example 1, except for the following points. A solution was prepared by dissolving 0.123 g of nickel chloride hexahydrate and 0.070 g of iron chloride hexahydrate in 3.62 mL of water. 0.0365 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.263 mL of POX was added to the chelating agent-containing solution as a pH-raising agent. In Example 2, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.048.

[0122] Example 3 An electrode according to Example 3 was fabricated in the same manner as Example 1, except for the following points. A solution was prepared by dissolving 0.123 g of nickel chloride hexahydrate and 0.070 g of iron chloride hexahydrate in 3.63 mL of water. 0.0245 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.264 mL of POX was added to the chelating agent-containing solution as a pH-raising agent. In Example 3, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.048.

[0123] Example 4 An electrode according to Example 4 was fabricated in the same manner as Example 1, except for the following points. A solution was prepared by dissolving 0.124 g of nickel chloride hexahydrate and 0.070 g of iron chloride hexahydrate in 3.64 mL of water. 0.0123 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.265 mL of POX was added to the chelating agent-containing solution as a pH-raising agent. In Example 4, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.048.

[0124] Example 5 An electrode according to Example 5 was fabricated in the same manner as Example 1, except for the following points. The weight of the Ni plate was 0.335 g. A solution was prepared by dissolving 0.123 g of nickel chloride hexahydrate and 0.070 g of iron chloride hexahydrate in 3.63 mL of water. 0.02 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.26 mL of POX was added to the chelating agent-containing solution as a pH-raising agent. In Example 5, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.045.

[0125] Example 6 An electrode according to Example 6 was fabricated in the same manner as Example 1, except for the following points. The weight of the Ni plate was 0.315 g. A solution was prepared by dissolving 0.132 g of nickel chloride hexahydrate and 0.075 g of iron chloride hexahydrate in 5.80 mL of water. 0.013 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.28 mL of POX was added to the chelating agent-containing solution as a pH-raising agent. In Example 6, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.05.

[0126] Example 7 An electrode according to Example 7 was fabricated in the same manner as Example 1, except for the following points. The weight of the Ni plate was 0.315 g. A solution was prepared by dissolving 0.200 g of nickel chloride hexahydrate and 0.114 g of iron chloride hexahydrate in 5.89 mL of water. 0.039 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.43 mL of POX was added to the chelating agent-containing solution as a pH-raising agent. In Example 7, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.08.

[0127] Example 8 An electrode according to Example 8 was fabricated in the same manner as Example 1, except for the following points. The weight of the Ni plate was 0.31 g. A solution was prepared by dissolving 0.127 g of nickel chloride hexahydrate and 0.072 g of iron chloride hexahydrate in 5.62 mL of water. 0.0253 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.226 mL of POX was added to the chelating agent-containing solution as a pH-raising agent. In Example 8, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.05.

[0128] Comparative Example 1 An electrode according to Comparative Example 1 was fabricated using the same method as in Example 1, except for the following points. A mixed solvent was prepared by mixing 1.52 mL of water and 2.29 mL of ethanol. Ethanol was purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.079 g of nickel chloride hexahydrate and 0.022 g of iron chloride hexahydrate were dissolved in this mixed solvent to prepare a solution. 0.010 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.13 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent. In Comparative Example 1, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.015.

[0129] Comparative Example 2 An electrode according to Comparative Example 2 was fabricated using the same method as in Example 1, except for the following points. The weight of the Ni plate was 0.340 g. A mixed solvent was prepared by mixing 1.52 mL of water and 2.28 mL of ethanol. Ethanol was purchased from Fujifilm Wako Pure Chemical Industries, Ltd. A solution was prepared by dissolving 0.065 g of nickel chloride hexahydrate and 0.037 g of iron chloride hexahydrate in this mixed solvent. 0.010 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.14 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent. In Comparative Example 2, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.024.

[0130] Comparative Example 3 An electrode according to Comparative Example 3 was fabricated using the same method as in Example 1, except for the following points. A mixed solvent was prepared by mixing 1.44 mL of water and 2.16 mL of ethanol. Ethanol was purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.123 g of nickel chloride hexahydrate and 0.070 g of iron chloride hexahydrate were dissolved in this mixed solvent to prepare a solution. 0.0486 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.262 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent. In Comparative Example 3, the ratio of the amount of Fe ions to the amount of Ni contained in the Ni plate (Fe / Ni ratio) was 0.048.

[0131] [Evaluation of Effective Layered Double Hydroxide Layer Thickness] Measurements were performed on a single center point of each electrode using a UVISEL2 spectroscopic ellipsometer manufactured by Horiba, Ltd., under conditions of an incident angle of 70 degrees, a measurement wavelength range of 0.6 eV to 5.5 eV, and a spot diameter of 1 mm x 2 mm. This resulted in measurement data E of the polarization parameters. Next, an optical model such as that shown in Figure 2 was created, and simulation data M of the polarization parameters based on the optical model was obtained. The simulation data M of the polarization parameters was then fitted to the measurement data E of the polarization parameters. Specifically, the measurement data E was analyzed using Delta Psi2 software provided with the UVISEL2, and the thicknesses of the interface layer, dense layer, and surface roughness layer were determined using the optical model shown in Figure 2. Additionally, the volume fraction of LDH (LDH ratio) in the interface layer and surface roughness layer was determined. The fitting calculations in this analysis used the New Amorphous equation proposed by Jobin Yvon of HORIBA and the Tauc-Lorentz equation, and adopted the result that produced the smallest mean square error between the simulation data M calculated from the optical model and the measurement data E. In addition, the fitting calculations were performed so that the mean square error was 10° or less.

[0132] Using the layer thickness and LDH ratio values ​​of each layer obtained by the above-mentioned methods, the effective film thickness of the electrode according to each Example and Comparative Example was calculated based on the above-mentioned formula (1). The results are shown in Tables 1 and 2.

[0133] [Evaluation of Electrode Overpotential] The oxygen evolution (OER) overpotential of the electrodes in each Example and Comparative Example was evaluated. For the measurements, a potentiostat VersaSTAT4 manufactured by Princeton Applied Research, an alkali sample vial (200 mL) manufactured by BAS, and a Teflon cap (for 200 mL) manufactured by BAS were used. An AE-2 plate electrode manufactured by EC Frontier was used as the working electrode fixture. The electrodes in each Example and Comparative Example, which served as the working electrode, were fixed to this fixture. A Metrohm double platinum wire counter electrode D.6.0305.200J was used as the counter electrode. 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 the oxygen evolution reaction.

[0134] (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: 25°C

[0135] Current density at the fifth cycle: 10 mA / cm 2 The overpotential was determined by subtracting the theoretical potential of 1.229 V required to drive the oxygen evolution reaction from the voltage corresponding to the reaction. The results are shown in Tables 1 and 2.

[0136] As shown in Tables 1 and 2, the effective thickness of the LDH layer in the electrodes according to each Example was greater than that in the electrodes according to each Comparative Example. The overpotential in each Example was smaller than that in each Comparative Example. This result indicates that the water electrolysis electrodes used in each Example were superior in terms of electrode activity.

[0137] The difference in effective film thickness between the Examples and Comparative Examples is presumably due to the fact that the chelating agent-containing solution in the Examples contains water but does not contain ethanol, whereas the chelating agent-containing solution in the Comparative Examples contains water but also contains ethanol. From this, it is presumed that the effective film thickness of the LDH layer can be adjusted by changing the solvent composition of the chelating agent-containing solution.

[0138]

[0139]

[0140] 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.

[0141] The water electrolysis electrode of the present disclosure can be used as an anode or a cathode for water electrolysis.

Claims

1. A conductive substrate and The conductive substrate comprises a layered double hydroxide layer provided on its surface, The effective film thickness of the layered double hydroxide layer, as determined by spectroscopic ellipsometry, is 250 nm or more and less than 4000 nm. Electrode for water electrolysis.

2. The effective thickness of the layered double hydroxide layer is 3470 nm or less. The electrode for water electrolysis according to claim 1.

3. The effective film thickness of the layered double hydroxide layer is 410 nm or more. The electrode for water electrolysis according to claim 1.

4. The layered double hydroxide layer contains at least two transition metals selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. The electrode for water electrolysis according to claim 1.

5. The layered double hydroxide layer contains at least one transition metal selected from the group consisting of Fe and Ni. The electrode for water electrolysis according to claim 1.

6. The aforementioned layered double hydroxide layer contains a chelating agent. The electrode for water electrolysis according to claim 1.

7. The chelating agent comprises at least one selected from the group consisting of acetylacetone and citrate. The electrode for water electrolysis according to claim 6.

8. The water electrolysis electrode is provided as described in claim 1. Anode for water electrolysis.

9. The water electrolysis electrode is provided as described in claim 1. Cathode for water electrolysis.

10. A-scatter, Cathode and, Equipped with a diaphragm, At least one selected from the group consisting of the anode being the anode for water electrolysis described in claim 8 and the cathode being the cathode for water electrolysis described in claim 9 is satisfied. water electrolysis cell.

11. A-scatter, Cathode and, It comprises an anion exchange membrane, At least one selected from the group consisting of the anode being the anode for water electrolysis described in claim 8 and the cathode being the cathode for water electrolysis described in claim 9 is satisfied. water electrolysis cell.

12. A water electrolysis cell according to claim 10 or 11, The system includes a voltage injector that applies a voltage between the cathode and the anode. Water electrolysis equipment.