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

Abstract

This electrode 1 for water electrolysis comprises a conductive base material 10 and a layered double hydroxide layer 20. The layered double hydroxide layer is provided on the surface of the conductive base material 10. The extinction coefficient k800 of the layered double hydroxide layer 20 at the wavelength of 800 nm is 0.08 or more.

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) in 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 descriptions in the above documents have room for reexamination from the viewpoint of improving the performance of electrodes for water electrolysis. Therefore, the present disclosure provides a novel electrode for water electrolysis that is advantageous from the viewpoint of achieving high 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 layered double hydroxide layer has an extinction coefficient of 0.08 or more at a wavelength of 800 nm.

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

[0012] FIG. 1 is a cross-sectional view showing an example of a water electrolysis electrode according to the present disclosure. FIG. 2 is a diagram schematically showing an example of the crystal structure of a layered double hydroxide (LDH). FIG. 3 is a diagram schematically showing an example of a mechanism for producing a water electrolysis electrode. FIG. 4 is a cross-sectional view showing an example of a water electrolysis cell according to the present disclosure. FIG. 5 is a cross-sectional view showing an example of a water electrolysis apparatus according to the present disclosure. FIG. 6 is a cross-sectional view showing another example of a water electrolysis cell according to the present disclosure. FIG. 7 is a cross-sectional view showing 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, a method of effectively utilizing surplus electricity by producing and storing hydrogen from surplus electricity has been studied.

[0014] Water electrolysis is one possible method for producing hydrogen from surplus electricity. To produce hydrogen inexpensively and stably, there is a need for the development of a highly efficient, long-life water electrolysis device. In a water electrolysis device, oxygen is generated at the anode, and hydrogen is generated at the cathode. The reaction by which oxygen is generated at the anode is also called the anode reaction, and the reaction by which hydrogen is generated at the cathode is also called the cathode reaction. To provide a highly efficient water electrolysis device, it is desirable that the overvoltage at the anode is low. Additionally, it is also desirable that the overvoltage at the cathode is low. Therefore, there is a need for the development of high-performance electrodes for the anode reaction or cathode reaction in water electrolysis.

[0015] For example, LDH is considered to be a promising material for electrodes for water electrolysis in view of its large specific surface area and diverse combinations of metal ions. For example, according to Patent Literature 1, an electrode substrate containing a specific layered double hydroxide is produced by performing an 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 electrodes produced by the electrodeposition process is insufficient, leaving room for improvement. After extensive research, the present inventors have newly discovered that the performance of water electrolysis electrodes can be improved by forming a catalytic membrane containing LDH so that it can absorb near-infrared light with a wavelength of 800 nm at a predetermined level or higher, and have completed the water electrolysis electrode of the present disclosure.

[0016] 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, and connection configurations 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 concepts 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.

[0017] First Embodiment Fig. 1 is a cross-sectional view showing a water electrolysis electrode according to a first embodiment. As shown in Fig. 1, the water electrolysis electrode 1 comprises 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 anodic reaction or cathodic reaction in water electrolysis. The extinction coefficient k of the LDH layer 20 at a wavelength of 800 nm is 800 is 0.08 or more. With this configuration, the overvoltage at the anode or cathode during water electrolysis is likely to be low. Therefore, the water electrolysis electrode 1 is likely to exhibit high performance. It is believed that a substance that has high absorption of near-infrared light is likely to have high conductivity. On the other hand, since LDH is a hydroxide, it was thought that it would not be easy to increase the conductivity of an LDH-containing layer. After extensive trial and error, the inventors finally succeeded in forming a catalyst film containing LDH such that the extinction coefficient at a wavelength of 800 nm, which is in the near-infrared region, is 0.08 or more.

[0018] Extinction coefficient k of LDH layer 20 800 can be determined, for example, by spectroscopic ellipsometry.

[0019] Extinction coefficient k of LDH layer 20 800 is preferably 0.1 or more, more preferably 0.2 or more, even more preferably 0.3 or more, and particularly preferably 0.4 or more.

[0020] Extinction coefficient k of LDH layer 20 800 The upper limit of the extinction coefficient k of the LDH layer 20 is not limited to a specific value. 800 is, for example, 0.9 or less, and may be 0.8 or less, 0.7 or less, 0.6 or less, 0.5 or less, or 0.493 or less.

[0021] 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 10 may be a pure metal such as Ni or Fe, or an alloy such as stainless steel or Inconel. Inconel is a registered trademark.

[0022] The conductive substrate 10 contains, for example, Ni. In this case, the conductive substrate 10 is likely to have advantageous properties from the viewpoint of achieving both corrosion resistance and conductivity in alkaline water electrolysis.

[0023] The conductive substrate 10 has a surface made of, for example, Ni. In this case, the conductive substrate 10 is likely to have high alkali resistance. In this case, the entire conductive substrate 10 may be made of Ni, or the conductive substrate 10 may have a surface layer made of Ni. The surface layer made of Ni is, for example, a sputtered film or a plated film.

[0024] When the conductive substrate 10 has a surface made of Ni, the purity of the Ni constituting the surface is not limited to a specific value. The purity is, for example, 90% by mass or more. In this case, the conductive substrate 10 is more likely to have high alkali resistance. The method for determining the purity of the Ni 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) and energy dispersive X-ray spectroscopy (EDX). The purity of the Ni constituting the surface of the conductive substrate 10 may be determined by analyzing the extract obtained by 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 Ni is high, the purity of the Ni 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 Ni.

[0025] The purity of Ni 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.

[0026] 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 portion of the conductive substrate 10 tends to be large, and gas generated in the water electrolysis reaction tends to diffuse easily.

[0027] The thickness of the conductive substrate 10 is not limited to a specific value. The thickness 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.

[0028] The LDH layer 20 covers, for example, the surface of the conductive substrate 10. The coverage of the LDH layer 20 on 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 high electrode activity. In addition, the water electrolysis electrode 1 is likely to have high durability.

[0029] 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.

[0030] The thickness of the LDH layer 20 is not limited to a specific value. The LDH layer 20 has a thickness of, for example, 35 nm or more. With such a configuration, the water electrolysis electrode 1 is more likely to have high electrode activity. The LDH layer 20 includes a portion having a thickness of, for example, 35 nm or more. The thickness of the LDH layer 20 can be determined, for example, by TEM observation of a cross section of the water electrolysis electrode 1. The extinction coefficient k 800 When determining the thickness of the LDH layer 20, the thickness of the LDH layer 20 may be determined.

[0031] The LDH layer 20 may contain a chelating agent together with LDH. This allows the chelating agent to adjust the crystal growth of LDH during the formation of the LDH layer 20, making it easier for the LDH layer 20 to exist in a desired state on the surface of the conductive substrate 10.

[0032] 2 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 into hydroxides during alkaline water electrolysis.

[0033] The LDH 20a has, for example, a composition represented by the following formula (1): 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 required amount of charge balance. n is an integer. m is an appropriate rational number. [M1 2+ 1-x M2 3+ x (OH)2][yA n- ・mH2O] Formula (1)

[0034] The LDH 20a may contain, 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. The LDH 20a may contain, for example, at least two elements 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 high electrode activity.

[0035] 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 is more likely to have high electrode activity. In addition, the production cost of the water electrolysis electrode 1 is likely to be low.

[0036] The LDH 20a may contain Fe and Ni, and for example, in the composition represented by formula (1), M1 may be Ni and M2 may be Fe. In this case, the water electrolysis electrode 1 is more likely to have high electrode activity.

[0037] 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 SO 4- , CH3 (CH2) n COO - , CH3 (CH2)n P.O. 4- , 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 LDH20a are not limited to a specific value. n- or a plurality of types of A n- may also include:

[0038] As shown in FIG. 2, LDH20a is a 2+ or M2 3+ OH at each vertex of the octahedron centered at - LDH20a contains [M 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 21 of metal hydroxide and an anion A n- The LDH 20a has a sheet-like structure in which M1 contained in the metal hydroxide layer and a guest layer 22 of water molecules are alternately laminated. 2+ Part of M2 3+ It has a structure substituted with

[0039] The LDH layer 20 may contain a chelating agent. The chelating agent may be coordinated to a transition metal 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 LDHs are likely to undergo slow crystal growth. This allows the LDH layer 20 to effectively contribute to the anode reaction or cathode reaction of water electrolysis, and the water electrolysis electrode 1 is likely to have high electrode activity.

[0040] The chelating agent is not limited to a specific chelating agent. For example, the chelating agent is an organic compound capable of coordinating with the 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 hydroxycarboxylic acid 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.

[0041] The chelating agent preferably contains at least one selected from the group consisting of acetylacetone and citrate. In this case, the water electrolysis electrode 1 is more likely to have high electrode activity. An example of the citrate is trisodium citrate.

[0042] 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 an alkaline state. This method allows the LDH layer 20 containing the LDH 20a and the chelating agent to be formed on the surface of the conductive substrate 10 in a simple manner. In addition, the extinction coefficient k of the LDH layer 20 can be adjusted by adjusting the concentration of the transition metal ions in the solution, etc. 800 can be adjusted to a desired range. For example, when the concentration of transition metal ions in the solution is relatively high, the extinction coefficient k 800 can be easily adjusted to a desired range.

[0043] The temperature of the solution when adjusted to be alkaline 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, a water electrolysis electrode 1 having high electrode activity is likely to be obtained.

[0044] The solvent of the solution may be water, an organic solvent, or a mixed solvent of water and an organic solvent.

[0045] The method for producing the water electrolysis electrode 1 preferably includes increasing the pH. This allows the LDHs 20a to be formed on the surface of the conductive substrate 10 in a short period of time, making it easier to obtain a water electrolysis electrode 1 with high electrode activity. In addition, the produced water electrolysis electrode 1 is likely to have high durability.

[0046] 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-elevating agent to the above solution. In this case, the pH-elevating agent is not limited to a specific compound. The pH-elevating agent is, for example, a compound having an epoxy group. Examples of pH-elevating agents are propylene oxide, ethylene oxide, and butylene oxide. When a pH-elevating agent having an epoxy group, such as propylene oxide, is added to the solution, in the presence of a nucleophile such as chloride ion, the pH-elevating agent may capture hydrogen ions present in the solution as a ring-opening reaction of the epoxy group occurs. This increases the pH of the solution, making it 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-elevating agent is added to this solution, the pH of the solution gradually increases, for example, from 1, and the solution may eventually become alkaline. The final pH of the solution is, for example, 8 to 12. The addition of a pH-increasing agent to a solution causes a reaction that captures hydrogen ions in the solution. This gradually increases the pH of the solution. The time from the addition of the pH-increasing 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 may be several days.

[0047] The two or more types of transition metal ions contained in the solution are not limited to specific transition metal ions. For example, the two or more types of transition metal ions contained in the solution are 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 high electrode activity can be more easily produced.

[0048] 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 high electrode activity can be more easily produced.

[0049] The conductive substrate 10 contains, for example, Ni. 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 represented by formula (2) may occur, which may etch the conductive substrate 10. The method for producing the water electrolysis electrode 1 preferably includes promoting mixing of the solution before adjusting the solution to be alkaline while the conductive substrate 10 is immersed. Promotion of solution mixing may be achieved, for example, by vibrating the conductive substrate 10, shaking the container containing the solution and the conductive substrate 10, or stirring the solution using a stirrer piece or a stirrer. Such methods may generate forced convection in the solution, promoting solution mixing. This allows the conductive substrate 10 to be etched in a desired state, and the LDH layer 20 to be formed on the conductive substrate 10 in a desired state. As a result, the water electrolysis electrode 1 is likely to have high durability. Promotion of solution mixing may be achieved while the container containing the solution and the conductive substrate 10 is sealed, or may be achieved in an inert gas atmosphere. 4Ni 2+ Cl - 2 + 2Fe 3+ Cl - 3 + 2Ni → 5Ni 2+ Cl - 2 + 2Fe 2+ Cl - 2 + 1Ni formula (2)

[0050] In the production of the water electrolysis electrode 1, the molar ratio of the Fe ion content to the Ni content in the conductive base material 10 is not limited to a specific value. The molar ratio is, for example, 0.75 or less. In this case, it is possible to prevent the dissolution of Ni in the conductive base material 10 due to the reaction represented by formula (2), which would make it difficult to produce the water electrolysis electrode 1.

[0051] The molar ratio is, for example, 0.05 to 0.25. In this case, the LDH layer 20 is more likely to be formed uniformly on the surface of the conductive substrate 10, and the water electrolysis electrode 1 is more likely to have high electrode activity.

[0052] 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 increases the stability of the dispersion of the complex in the solution, making it easier to form the LDH layer 20 in a desired state in the water electrolysis electrode 1. As a result, the water electrolysis electrode 1 is more likely to have high electrode activity.

[0053] FIG. 3 is a schematic diagram illustrating the mechanism for producing a water electrolysis electrode 1. As shown in FIG. 3 , a conductive substrate 10 is immersed in a solution containing transition metal ions TM1, TM2, and a chelating agent CH. For example, the transition metal ions TM1 are Ni ions, and the transition metal ions TM2 are Fe ions. For example, 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 reacts with the surface of the conductive substrate 10 to form a complex C1 between the transition metal ions TM1 and the chelating agent CH derived from the conductive substrate 10. In addition, when the solution is adjusted to an alkaline pH, a complex C1 derived from the transition metal ions TM1 and the chelating agent CH derived from the solution is formed in the solution, and a complex C2 between the transition metal ions TM2 and the chelating agent CH 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, since the complexes C1 and C2 contain the chelating agent CH, crystal growth of the LDH 20a is suppressed. As a result, an LDH layer 20 containing the LDH 20a and the chelating agent CH is formed on the surface of the conductive substrate 10, and a water electrolysis electrode 1 is obtained.

[0054] 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.

[0055] Second Embodiment Figure 4 is a cross-sectional view schematically illustrating an example of a water electrolysis cell according to a second embodiment. As shown in Figure 4, 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 is likely to be high, and the anode 2a or the cathode 2b is likely to exhibit high performance.

[0056] 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.

[0057] 4 , 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.

[0058] 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.

[0059] 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.

[0060] 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.

[0061] 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.

[0062] 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 high performance.

[0063] Third Embodiment Fig. 5 is a cross-sectional view schematically illustrating an example of a water electrolysis apparatus according to a third embodiment. As shown in Fig. 5, 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.

[0064] 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 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.

[0065] With the above configuration, the water electrolysis device 3 can exhibit high performance.

[0066] Fourth Embodiment Figure 6 is a cross-sectional view schematically illustrating an example of a water electrolysis cell according to a fourth embodiment. As shown in Figure 6, 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 high performance.

[0067] The water electrolysis cell 4 is, for example, an anion exchange membrane (AEM)-type water electrolysis cell. As shown in Fig. 6 , the anode 4a includes, for example, an electrode catalyst layer 4m and a gas diffusion layer 4n. The cathode 4b includes, for example, an electrode catalyst layer 4j and a gas diffusion layer 4k. The electrode catalyst layer 4m of the anode 4a is in contact with one main surface of the anion exchange membrane 4p, and the electrode catalyst layer 4j of the cathode 4b is in contact with the other main surface of the anion exchange membrane 4p.

[0068] 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.

[0069] In the water electrolysis cell 4, when the anode 4a includes the water electrolysis electrode 1, the cathode 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 electrode catalyst 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 electrode catalyst 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.

[0070] 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 high performance.

[0071] Fifth Embodiment Fig. 7 is a cross-sectional view schematically illustrating an example of a water electrolysis apparatus according to a fifth embodiment. As shown in Fig. 7, 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.

[0072] 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 device 5 reaches a predetermined set value.

[0073] With the above configuration, the water electrolysis device 5 can exhibit high performance.

[0074] (Supplementary Notes) The above disclosure discloses the following technologies. (Technology 1) An electrode for water electrolysis comprising: a conductive substrate; and a layered double hydroxide layer provided on the surface of the conductive substrate, wherein the layered double hydroxide layer has an extinction coefficient of 0.08 or more at a wavelength of 800 nm. (Technology 2) The electrode for water electrolysis according to Technology 1, wherein the extinction coefficient is 0.1 or more. (Technology 3) The electrode for water electrolysis according to Technology 2, wherein the extinction coefficient is 0.2 or more. (Technology 4) The electrode for water electrolysis according to Technology 3, wherein the extinction coefficient is 0.3 or more. (Technology 5) The electrode for water electrolysis according to Technology 4, wherein the extinction coefficient is 0.4 or more. (Technology 6) The electrode for water electrolysis according to any one of Technology 1 to 5, wherein the layered double hydroxide layer contains two or more types of transition metals. (Technology 7) The water electrolysis electrode according to Technology 6, wherein the two or more transition metals include at least two selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. (Technology 8) The water electrolysis electrode according to Technology 7, wherein the two or more transition metals include at least one transition metal selected from the group consisting of Fe and Ni. (Technology 9) The water electrolysis electrode according to any one of Technology 1 to 8, wherein the layered double hydroxide layer contains a chelating agent. (Technology 10) The water electrolysis electrode according to Technology 9, wherein the chelating agent contains at least one selected from the group consisting of acetylacetone and citrate. (Technology 11) An anode for water electrolysis, comprising the water electrolysis electrode according to any one of Technology 1 to 10. (Technology 12) A cathode for water electrolysis, comprising the water electrolysis electrode according to any one of Technology 1 to 10. (Technology 13) 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 11 and the cathode being the cathode for water electrolysis according to Technology 12 is satisfied. (Technology 14) 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 11 and the cathode being the cathode for water electrolysis according to Technology 12 is satisfied.(Technology 15) A water electrolysis device comprising: the water electrolysis cell according to Technology 13 or 14; and a voltage applicator that applies a voltage between the cathode and the anode.

[0075] 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.

[0076] Example 1 A solution was prepared by dissolving 0.123 g of nickel chloride hexahydrate and 0.070 g of iron chloride hexahydrate in 3.63 milliliters (mL) of water. Nickel chloride hexahydrate and iron chloride hexahydrate were purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.020 mL of acetylacetone (ACAC) was added to this solution as a chelating agent to obtain a chelating agent-containing solution according to Example 1. ACAC was purchased from Sigma-Aldrich.

[0077] A Ni plate manufactured by Nilaco Corporation was cleaned with acetone for 10 minutes and then with a 1M 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 0.335 g. The Ni plate was then rinsed with water and dried to complete the cleaning process.

[0078] 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. During this time, the outermost surface of the Ni plate was etched according to the above formula (2).

[0079] Next, 0.260 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 way, the electrode according to Example 1 was obtained.

[0080] 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 0.150 g of nickel chloride hexahydrate and 0.040 g of iron chloride hexahydrate in 3.64 mL of water. 0.020 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution according to Example 2. An electrode according to Example 2 was fabricated in the same manner as Example 1, except that the chelating agent-containing solution according to Example 2 was used instead of the chelating agent-containing solution according to Example 1, and 0.250 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent.

[0081] 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 0.170 g of nickel chloride hexahydrate and 0.020 g of iron chloride hexahydrate in 3.64 mL of water. 0.030 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution according to Example 3. An electrode according to Example 3 was fabricated in the same manner as Example 1, except that the chelating agent-containing solution according to Example 3 was used instead of the chelating agent-containing solution according to Example 1, and 0.250 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent.

[0082] Example 4 An electrode according to Example 4 was fabricated in the same manner as Example 1, except for the following points. A mixed solvent was prepared by mixing 1.45 mL of water and 2.18 mL of ethanol. Ethanol was purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.15 g of nickel chloride hexahydrate and 0.043 g of iron chloride hexahydrate were dissolved in this mixed solvent to prepare a solution. 0.020 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution according to Example 4. An electrode according to Example 4 was fabricated in the same manner as Example 1, except that the chelating agent-containing solution according to Example 4 was used instead of the chelating agent-containing solution according to Example 1, and 0.25 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent.

[0083] Example 5 An electrode according to Example 5 was fabricated in the same manner as Example 1, except for the following points. A mixed solvent was prepared by mixing 1.33 mL of water and 2.00 mL of ethanol. Ethanol was purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.28 g of nickel chloride hexahydrate and 0.079 g of iron chloride hexahydrate were dissolved in this mixed solvent to prepare a solution. 0.050 mL of ACAC was added as a chelating agent to this solution to obtain a chelating agent-containing solution according to Example 5. An electrode according to Example 5 was fabricated in the same manner as Example 1, except that the chelating agent-containing solution according to Example 5 was used instead of the chelating agent-containing solution according to Example 1, and 0.47 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent.

[0084] Comparative Example 1 An electrode according to Comparative Example 1 was fabricated in the same manner 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 according to Comparative Example 1. An electrode according to Comparative Example 1 was fabricated in the same manner as in Example 1, except that the chelating agent-containing solution according to Comparative Example 1 was used instead of the chelating agent-containing solution according to Example 1, and 0.13 mL of POX was added to the chelating agent-containing solution as a pH-increasing agent.

[0085] [Evaluation of Extinction Coefficient of Electrode] Measurements were performed on a single point in the center of each electrode using a UVISEL2 spectroscopic ellipsometer manufactured by Horiba, Ltd., under the conditions of an incident angle of 70 degrees, a measurement wavelength range of 0.6 to 5.5 eV, and a spot diameter of 1 mm x 2 mm. The measurement data was analyzed using Delta Psi2 software included with the UVISEL2 spectroscopic ellipsometer, and the extinction coefficient of the LDH layer of each electrode at a wavelength of 800 nm was calculated. The results are shown in Table 1.

[0086] [Electrode Overpotential Evaluation] The oxygen evolution (OER) overpotential of the electrodes according to each Example and Comparative Example 1 was evaluated. For the measurements, a potentiostat VersaSTAT4 manufactured by Princeton Applied Research, an alkali sample vial (200 mL) manufactured by BAS, a Teflon® cap (for 200 mL) manufactured by BAS, and an EC Frontier plate electrode AE-2 were used as the working electrode fixture. The working electrodes of each Example and Comparative Example 1 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. (Measurement conditions) Solution: 1 M 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

[0087] Current density at the fifth cycle: 10 mA / cm 2 The overpotential was determined by subtracting the theoretical potential of 1.229 V required for the oxygen evolution reaction to proceed from the voltage corresponding to the reaction. The results are shown in Table 1.

[0088] As shown in Table 1, the overvoltage when the electrodes according to each Example were used was lower than the overvoltage when the electrode according to Comparative Example 1 was used. Comparing each Example with Comparative Example 1, it is understood that when the extinction coefficient of the LDH layer of the electrode at a wavelength of 800 nm is 0.08 or more, the overvoltage is reduced and high performance can be exhibited in water electrolysis. It was also suggested that the overvoltage decreases in inverse proportion to the extinction coefficient of the LDH layer of the electrode at a wavelength of 800 nm.

[0089]

[0090] 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 in operating conditions, composition, structure, and / or function can be made without departing from the spirit of the present disclosure.

[0091] The water electrolysis electrode of the present disclosure can be used as an anode or a cathode for water electrolysis.

Claims

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 layered double hydroxide layer has an extinction coefficient of 0.08 or more at a wavelength of 800 nm.

2. The water electrolysis electrode according to claim 1, wherein the extinction coefficient is 0.1 or more.

3. The water electrolysis electrode according to claim 2, wherein the extinction coefficient is 0.2 or more.

4. The water electrolysis electrode according to claim 3, wherein the extinction coefficient is 0.3 or more.

5. The water electrolysis electrode according to claim 4, wherein the extinction coefficient is 0.4 or more.

6. The water electrolysis electrode according to any one of claims 1 to 5, wherein the layered double hydroxide layer contains two or more types of transition metals.

7. The water electrolysis electrode according to claim 6, wherein the two or more transition metals include at least two selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru.

8. The water electrolysis electrode according to claim 7, wherein the two or more transition metals include at least one transition metal selected from the group consisting of Fe and Ni.

9. The water electrolysis electrode according to any one of claims 1 to 8, wherein the layered double hydroxide layer contains a chelating agent.

10. The water electrolysis electrode according to claim 9, wherein the chelating agent comprises at least one selected from the group consisting of acetylacetone and citrate salts.

11. An anode for water electrolysis comprising the electrode for water electrolysis according to any one of claims 1 to 10.

12. A cathode for water electrolysis comprising the electrode for water electrolysis according to any one of claims 1 to 10.

13. 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 claim 11 and the cathode being the cathode for water electrolysis according to claim 12 is satisfied.

14. 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 claim 11 and the cathode being the cathode for water electrolysis according to claim 12 is satisfied.

15. A water electrolysis device comprising: the water electrolysis cell according to claim 13 or 14; and a voltage applicator that applies a voltage between the cathode and the anode.