Water electrolysis electrode, water electrolysis anode, water electrolysis cathode, water electrolysis cell, and water electrolysis device

Abstract

This water electrolysis electrode comprises an electroconductive substrate and a layered double hydroxide (LDH) layer. The electroconductive substrate contains Ni. The LDH layer is provided on the surface of the electroconductive substrate and contains Ni. A grazing-incidence X-ray diffraction (GIXD) pattern of the water electrolysis electrode shows a ratio r110 / Ni111 of less than 0.0029. The ratio r110 / Ni111 is the ratio of the diffraction peak intensity of the LDH (110) plane to the diffraction peak intensity of the Ni (111) plane.

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] In recent years, there has been a growing expectation for the development of electrodes for water electrolysis to be used in water electrolysis devices.

[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.11342-11351P.WTLu, S.Srinivasan, J.Electrochem.Soc.,125, 1416 (1978)CTBowen, Int.J.Hydrogen Energy,9,59 (1984)

[0009] The descriptions in the above documents need to be reconsidered from the viewpoint of improving the durability of water electrolysis electrodes compared to conventional electrodes. Therefore, the present disclosure provides a novel water electrolysis electrode that is advantageous from the viewpoint of exhibiting higher durability than conventional electrodes.

[0010] The present disclosure provides an electrode for water electrolysis, comprising: a conductive base material containing Ni; and a layered double hydroxide layer containing Ni provided on a surface of the conductive base material, wherein in a diffraction pattern obtained by grazing incidence X-ray diffraction measurement of the electrode for water electrolysis, the ratio of the diffraction peak intensity of the layered double hydroxide (110) plane to the diffraction peak intensity of the Ni (111) plane is less than 0.0029.

[0011] According to the present disclosure, it is possible to provide a novel electrode for water electrolysis that is advantageous in terms of exhibiting higher durability than conventional electrodes.

[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. FIG. 8A is a graph showing a diffraction pattern obtained by grazing incidence X-ray diffraction (GIXD) measurement of the electrode according to Example 1. FIG. 8B is a graph showing a diffraction pattern obtained by GIXD measurement of the electrode according to Example 2. FIG. 8C is a graph showing a diffraction pattern obtained by GIXD measurement of the electrode according to Example 3. FIG. 9A is a graph showing a diffraction pattern obtained by GIXD measurement of the electrode according to Comparative Example 1. FIG. 9B is a graph showing a diffraction pattern obtained by GIXD measurement of the electrode according to Comparative Example 2.

[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] It has been considered to use nickel-based materials, which are stable in highly concentrated alkaline aqueous solutions, as anodes for alkaline water electrolysis. In alkaline water electrolysis using a stable power source, it has been reported that nickel-based anodes have a lifespan of several decades or more (see Non-Patent Documents 3 and 4). However, water electrolysis using renewable energy as a power source is often performed under harsh conditions, such as frequent start-stops and load fluctuations, and deterioration of the performance of nickel-based anodes can be a problem (see Non-Patent Document 3).

[0016] For example, LDHs are considered promising materials for electrodes for water electrolysis in terms of their large specific surface area and diverse combinations of metal ions. According to Patent Document 1, an electrode substrate containing a predetermined LDH 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, no study has been conducted on the deterioration of electrode performance in water electrolysis using renewable energy as a power source.

[0017] The present disclosure has been made in consideration of these problems of the conventional art. An object of the present disclosure is to provide an electrode for water electrolysis that is less susceptible to performance degradation than conventional electrodes and that exhibits higher durability than conventional electrodes, even when water electrolysis is performed using a power source with large output fluctuations, such as renewable energy. As a result of extensive investigations, the present inventors have newly discovered that maintaining a water electrolysis electrode including an LDH layer in a predetermined state is advantageous in terms of exhibiting higher durability than conventional electrodes, and have completed the water electrolysis electrode of the present disclosure.

[0018] 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.

[0019] First Embodiment Fig. 1 is a cross-sectional view showing an electrode for water electrolysis according to the first embodiment. As shown in Fig. 1, the electrode for water electrolysis 1 includes a conductive substrate 10 and a layered double hydroxide (LDH) layer 20. The conductive substrate 10 contains Ni. The LDH layer 20 is provided on the surface of the conductive substrate 10 and also contains Ni. The LDH layer 20 is bonded to the conductive substrate 10. The LDH layer 20 is bonded directly to the surface of the conductive substrate 10 without an adhesive containing an organic material such as a polymer. Grazing incidence X-ray diffraction (GIXD) measurement of the electrode for water electrolysis 1 shows a diffraction pattern with a ratio r 110 / Ni111 is less than 0.0029. 110 / Ni111 is the ratio of the diffraction peak intensity of the LDH(110) plane to the diffraction peak intensity of the Ni(111) plane. With this configuration, the water electrolysis electrode 1 is likely to exhibit higher durability than conventional electrodes. Therefore, even when water electrolysis is performed using a power source with large output fluctuations, such as renewable energy, high catalytic activity is likely to be maintained and the water electrolysis performance is less likely to deteriorate than conventional electrodes.

[0020] Ratio 110 / Ni111 The fact that the ρ is less than 0.0029 is considered to indicate that the LDH layer 20 is formed in a state in which the crystal growth of LDH in the direction perpendicular to the LDH (110) plane is suppressed. This is considered to result in an LDH layer 20 in an advantageous state from the viewpoint of higher durability than conventional LDH layers.

[0021] Ratio 110 / Ni111 is preferably 0.0027 or less. In this case, the water electrolysis electrode 1 is more likely to exhibit higher durability than conventional electrodes.

[0022] Ratio 110 / Ni111 is, for example, 0.0011 or more. In this case, the water electrolysis electrode 1 is more likely to exhibit higher durability than conventional electrodes.

[0023] The conductive substrate 10 is not limited to a specific substrate as long as it contains Ni and is conductive. The conductive substrate 10 may contain a metal other than Ni or may contain 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.

[0024] The surface of the conductive substrate 10 is preferably made of Ni. In this case, the conductive substrate 10 is likely to have high alkali resistance. When the surface of the conductive substrate 10 is made of Ni, 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.

[0025] When the surface of the conductive substrate 10 is 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 of the Ni constituting the surface of the conductive substrate 10 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.

[0026] 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.

[0027] The conductive substrate 10 is, for example, in the form of a sheet. The conductive substrate 10 may have a non-porous structure such as a plate or foil, or may have 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.

[0028] 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.

[0029] The LDH layer 20 contains, for example, a chelating agent. 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 conductive substrate 10 and for the water electrolysis electrode 1 to have high electrode activity. The LDH layer 20 does not necessarily need to contain a chelating agent.

[0030] 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 more likely to have high electrode activity.

[0031] The thickness of the LDH layer 20 is not limited to a specific value, and is, for example, 230 nm or more and 690 nm or less.

[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 two or more types of transition metals. The two or more types of transition metals in the LDH 20a are not limited to a specific transition metal. In other words, M1 and M2 in the composition shown in formula (1) are not limited to a specific transition metal.

[0035] The LDH 20a may further contain, in addition to Ni, at least one element selected from the group consisting of V, Cr, Mn, Fe, Co, Cu, W, and Ru. In this case, the water electrolysis electrode 1 is more likely to have high electrode activity.

[0036] The LDH 20a preferably contains Fe in addition to 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. 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 21. 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+ The crystal structure and crystallinity of LDH20a can be qualitatively and quantitatively analyzed by X-ray diffraction (XRD).

[0039] The crystalline structure and crystallinity of the LDH 20a are qualitatively and quantitatively analyzed by X-ray diffraction (XRD). Because a thin LDH layer 20 reduces the signal and makes analysis difficult, grazing incidence X-ray diffraction (GIXD) is more preferably used for qualitative and quantitative analysis. The angle between the incident X-rays and the sample surface is fixed at 3°, and the water electrolysis electrode 1 is rotated horizontally by 360° to measure the intensity of diffraction peaks originating from the (111) plane of Ni contained in the conductive substrate 10, which appear within a diffraction angle 2θ range of 42° to 46°. The diffraction pattern of the water electrolysis electrode 1 is measured in the plane direction of the water electrolysis electrode 1 where the diffraction peak intensity originating from the Ni (111) plane is at its maximum. In this diffraction pattern, for example, diffraction peaks originating from the (003), (012), (015), and (110) planes of LDH20a are obtained in the diffraction angle 2θ ranges of 10° to 12°, 33° to 36°, 38° to 40°, and 59° to 61°, respectively. The peak intensity of each diffraction peak is determined as the height of the diffraction peak from the baseline.

[0040] As described above, the LDH layer 20 contains, for example, a chelating agent. The chelating agent may be coordinated to transition metal ions 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 easily synthesized to have a small particle size. In addition, the LDH 20a nucleated on the conductive substrate 10 is likely to undergo slow crystal growth, and therefore the dense LDH layer 20 with few voids containing the LDH 20a is likely to have a desired thickness and be firmly fixed to the conductive substrate 10. This allows the LDH layer 20 to effectively contribute to the anode reaction or the cathode reaction, and the water electrolysis electrode 1 is likely to have high electrode activity.

[0041] 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.

[0042] 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.

[0043] 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. According to this method, for example, the LDH layer 20 containing the LDH 20a and the chelating agent can be formed on the conductive substrate 10 in a simple manner.

[0044] 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.

[0045] The solvent of the solution may be water, an organic solvent, or a mixed solvent of water and an organic solvent.

[0046] The method for producing the water electrolysis electrode 1 preferably includes increasing the pH. This allows the LDH layer 20 to be formed on 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 more likely to have higher durability than conventional electrodes.

[0047] 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 the pH-increasing agent are propylene oxide, ethylene oxide, and butylene oxide.

[0048] 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.

[0049] The pH-elevating agent may be an organic compound having an epoxy group and a hydroxy group, such as glycidol. In this case, the water electrolysis electrode 1 is also likely to have high electrode activity. In the presence of a nucleophile, such as chloride ions, the pH-elevating agent may capture hydrogen ions present in the solution as a result of a ring-opening reaction of the epoxy group of glycidol. This increases the pH of the solution, making the solution alkaline. The interaction of the ring-opened compound of glycidol with transition metal ions makes it easier for the LDH layer 20 to assume a desired state, and the water electrolysis electrode 1 is likely to have high electrode activity. Even when an organic compound having an epoxy group and a hydroxy group other than glycidol is used, the water electrolysis electrode 1 is also likely to have high electrode activity.

[0050] Organic compounds having an epoxy group and a hydroxy group, such as glycidol, have a higher flash point, are less flammable, and are less expensive than other epoxides. Therefore, the use of such organic compounds as a pH-increasing agent can eliminate the need for explosion-proof specifications in the manufacturing equipment for the water electrolysis electrodes 1, and tends to reduce the manufacturing cost of the water electrolysis electrodes 1.

[0051] The pH increasing agent may be sodium carbonate.

[0052] The pH of a solution containing 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 occur in which hydrogen ions in the solution are captured. This causes the pH of the solution to gradually increase. 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, one hour or more, or several days.

[0053] 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.

[0054] 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.

[0055] As described above, the surface of the conductive base material 10 is preferably made of Ni. In this case, it is easy to produce a water electrolysis electrode that has advantageous properties from the viewpoint of achieving both corrosion resistance and electrical conductivity in alkaline water electrolysis, for example.

[0056] As described above, the conductive substrate 10 contains 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 the following 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 an alkaline state 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 higher durability than conventional electrodes. 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)

[0057] 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.

[0058] The molar ratio is preferably 0.05 to 0.25. In this case, the LDH layer 20 is more likely to be formed uniformly on the conductive substrate 10, and the water electrolysis electrode 1 is more likely to have high electrode activity.

[0059] 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.

[0060] FIG. 3 is a schematic diagram illustrating the mechanism of manufacturing a water electrolysis electrode. 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 nickel ions, and the transition metal ions TM2 are iron ions. In addition, nickel is present on the surface of the conductive substrate 10. Some of the transition metal ions TM2 etch and dissolve the nickel present on the surface of the conductive substrate 10. Some of the chelating agent CH 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, synthesizing 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 conductive substrate 10, and a water electrolysis electrode 1 is obtained.

[0061] The water electrolysis electrode 1 according to this embodiment can be used, for example, as an electrode of a water electrolysis cell in an alkaline water electrolysis apparatus or an anion exchange membrane water electrolysis apparatus. The water electrolysis electrode 1 is used, for example, in at least one selected from the group consisting of an anode and a cathode in these water electrolysis apparatus. This tends to increase the activity of the anode reaction or the cathode reaction in water electrolysis.

[0062] Second Embodiment Figure 4 is a cross-sectional view schematically illustrating an example of a water electrolysis cell according to the 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 2 is likely to be high, and the anode 2a or the cathode 2b is likely to exhibit high performance.

[0063] 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.

[0064] 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.

[0065] 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.

[0066] 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.

[0067] 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.

[0068] 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.

[0069] 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.

[0070] 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.

[0071] 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.

[0072] With the above configuration, the water electrolysis device 3 can exhibit high performance.

[0073] 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.

[0074] 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.

[0075] 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.

[0076] 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 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 the water electrolysis cell 4, both the anode 4a and the cathode 4b may include the water electrolysis electrode 1.

[0077] 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.

[0078] 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.

[0079] 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.

[0080] With the above configuration, the water electrolysis device 5 can exhibit high performance.

[0081] (Additional Notes) The above disclosure discloses the following technologies. (Technology 1) An electrode for water electrolysis, comprising: a conductive base material containing Ni; and a layered double hydroxide layer containing Ni provided on the surface of the conductive base material, wherein in a diffraction pattern obtained by grazing incidence X-ray diffraction measurement of the electrode for water electrolysis, the ratio of the diffraction peak intensity of the layered double hydroxide (110) plane to the diffraction peak intensity of the Ni (111) plane is less than 0.0029. (Technology 2) The electrode for water electrolysis according to Technology 1, wherein the ratio is 0.0027 or less. (Technology 3) The electrode for water electrolysis according to Technology 1 or 2, wherein the ratio is 0.0011 or more. (Technology 4) The electrode for water electrolysis according to any one of Technology 1 to 3, wherein the layered double hydroxide layer further contains at least one element selected from the group consisting of V, Cr, Mn, Fe, Co, Cu, W, and Ru. (Technology 5) The electrode for water electrolysis according to Technology 4, wherein the layered double hydroxide layer further contains Fe. (Technology 6) The electrode for water electrolysis according to any one of Technology 1 to 5, wherein the surface of the conductive substrate is made of Ni. (Technology 7) The electrode for water electrolysis according to Technology 6, wherein the Ni forming the surface of the conductive substrate has a purity of 90 mass% or more. (Technology 8) An anode for water electrolysis comprising the electrode for water electrolysis according to any one of Technology 1 to 7. (Technology 9) A cathode for water electrolysis comprising the electrode for water electrolysis according to any one of Technology 1 to 7. (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. (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. (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.

[0082] 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.

[0083] Example 1 A solution was prepared by dissolving 0.208 g of nickel chloride hexahydrate and 0.118 g of iron chloride hexahydrate in 3.33 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. ACAC was purchased from Fujifilm Wako Pure Chemical Industries, Ltd.

[0084] A Ni plate manufactured by Nilaco Corporation was prepared. The Ni plate had a thickness of 0.2 mm and a shape in plan view of a square A with a side length of 10 mm connected to a square B with a side length of 2 mm. The Ni plate had a mass of 0.195 g. Next, the Ni plate was washed with water and dried to complete the cleaning process of the Ni plate.

[0085] 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 1 hour. At this time, the outermost surface of the Ni plate was etched according to the above formula (2).

[0086] Next, 0.424 mL of glycidol (GL) was added as a pH-raising agent to the chelating agent-containing solution, and the solution was shaken and stirred for another 4 hours at 25° C. After 4 hours of shaking and stirring, the Ni plate was recovered, washed with water, and dried. In this way, an electrode according to Example 1 was obtained, in which an LDH layer was formed on the Ni plate.

[0087] 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 0.646 g of nickel chloride hexahydrate and 0.368 g of iron chloride hexahydrate in 3.36 mL of water. 0.063 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.132 mL of GL was added to the chelating agent-containing solution as a pH-raising agent.

[0088] Example 3 An electrode according to Example 3 was fabricated in the same manner as in Example 1, except for the following points. A solution was prepared by dissolving 0.303 g of nickel chloride hexahydrate and 0.172 g of iron chloride hexahydrate in 3.39 mL of water. 0.029 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.296 mL of GL was added to the chelating agent-containing solution as a pH-raising agent.

[0089] 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 solution was prepared by dissolving 0.557 g of nickel chloride hexahydrate and 0.317 g of iron chloride hexahydrate in 3.35 mL of water. 0.054 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.216 mL of GL was added to the chelating agent-containing solution as a pH-raising agent.

[0090] Comparative Example 2 An electrode according to Comparative Example 2 was fabricated in the same manner as in Example 1, except for the following points. A solution was prepared by dissolving 0.429 g of nickel chloride hexahydrate and 0.244 g of iron chloride hexahydrate in 3.17 mL of water. 0.042 mL of ACAC was added to this solution as a chelating agent to obtain a chelating agent-containing solution. 0.419 mL of GL was added to the chelating agent-containing solution as a pH-raising agent.

[0091] [Evaluation of Electrode Crystallinity] The crystallinity of the electrodes was evaluated by small-angle X-ray diffraction (GIXD) measurement. A Rigaku SmartLab was used for the GIXD measurement, and a Cu anticathode was used for the X-ray generator. Cu-Kα radiation was used as the X-ray source. The parallel beam method was used. The scanning conditions were as follows: the electrodes of each example and comparative example were placed on a sample stage, the angle between the incident X-rays and the sample surface was fixed at 3°, and the sample was rotated 360° horizontally. The diffraction peak intensity derived from the Ni(111) plane, appearing in the diffraction angle 2θ range of 42° to 46°, was measured. GIXD measurement was performed in continuous scanning mode in the direction where the diffraction peak intensity derived from the Ni(111) plane was maximized, over a diffraction angle 2θ range of 5° to 65°, to obtain a diffraction pattern. In the GIXD measurement, the step size was set to 0.04°, and the scanning speed was set to 1.5° / min. In the obtained diffraction pattern, a diffraction peak originating from the (110) plane of LDH was confirmed in the diffraction angle 2θ range of 59° to 61°. A baseline position that did not overlap with the tail of the diffraction peak in the obtained diffraction pattern was specified, and the baseline was determined by interpolating the baseline with a spline function. The peak intensity of each diffraction peak was calculated by determining its height from the baseline. The results are shown in Table 1.

[0092] Fig. 8A is a graph showing a diffraction pattern obtained by GIXD measurement of an electrode according to Example 1. Fig. 8B is a graph showing a diffraction pattern obtained by GIXD measurement of an electrode according to Example 2. Fig. 8C is a graph showing a diffraction pattern obtained by GIXD measurement of an electrode according to Example 3. Fig. 9A is a graph showing a diffraction pattern obtained by GIXD measurement of an electrode according to Comparative Example 1. Fig. 9B is a graph showing a diffraction pattern obtained by GIXD measurement of an electrode according to Comparative Example 2. In these graphs, the vertical axis represents diffraction intensity [counts per second (cps)], and the horizontal axis represents diffraction angle 2θ [°].

[0093] [Electrode Overvoltage Evaluation] The oxygen evolution (OER) overvoltage of the electrodes according to each example and comparative example was evaluated. For the measurements, a potentiostat (ivium-n-stat) manufactured by Ivium, a custom-made cell manufactured by EC Frontier, and working electrodes fabricated by welding nickel wire to each electrode were used. Additionally, a nickel coil was used as the counter electrode. A HydroFlex electrode manufactured by Gaskatal was used as the reference electrode. The current resulting from the anode reaction of the water electrolysis cell was measured using the three-electrode method under the following measurement conditions. The anode reaction was the oxygen evolution reaction. (Measurement Conditions) Solution: 6 M KOH solution Potential vs. reversible hydrogen electrode (RHE): 0.5 V to 1.7 V Number of cycles: 35 cycles Potential sweep rate: 0.5 V / sec Temperature: 80°C

[0094] To evaluate the durability of the electrodes, the ratio of the current density at the 35th cycle to the initial current density at a voltage of 1.7 V was determined. The results are shown in Table 1. As shown in Table 1, the ratios of the current density at the 35th cycle to the initial current density were high in the electrodes of Examples 1 to 3, indicating that these electrodes have high durability. On the other hand, the ratios of the current density at the 35th cycle to the initial current density in the electrodes of Comparative Examples 1 and 2 were not high. A comparison between the Examples and Comparative Examples suggested that when the ratio of the diffraction peak intensity derived from the LDH(110) plane to the diffraction peak intensity derived from the Ni(111) plane in the diffraction pattern obtained by GIXD measurement of a water electrolysis electrode is less than 0.0029, the durability of the water electrolysis electrode is likely to be higher than conventional electrodes.

[0095] 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.

[0096]

[0097] 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 base material containing Ni; and a layered double hydroxide layer containing Ni provided on the surface of the conductive base material, wherein in a diffraction pattern obtained by grazing incidence X-ray diffraction measurement of the electrode for water electrolysis, the ratio of the diffraction peak intensity of the layered double hydroxide (110) plane to the diffraction peak intensity of the Ni (111) plane is less than 0.0029.

2. The electrode for water electrolysis according to claim 1, wherein the ratio is 0.0027 or less.

3. The electrode for water electrolysis according to claim 1 or 2, wherein the ratio is 0.0011 or more.

4. The water electrolysis electrode according to any one of claims 1 to 3, wherein the layered double hydroxide layer further contains at least one element selected from the group consisting of V, Cr, Mn, Fe, Co, Cu, W, and Ru.

5. The water electrolysis electrode according to claim 4, wherein the layered double hydroxide layer further contains Fe.

6. The water electrolysis electrode according to any one of claims 1 to 5, wherein the surface of the conductive substrate is made of Ni.

7. The water electrolysis electrode according to claim 6, wherein the Ni forming the surface of the conductive substrate has a purity of 90 mass % or more.

8. An anode for water electrolysis, comprising the electrode for water electrolysis according to any one of claims 1 to 7.

9. A cathode for water electrolysis comprising the electrode for water electrolysis according to any one of claims 1 to 7.

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 claim 8 and the cathode being the cathode for water electrolysis according to claim 9 is satisfied.

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 claim 8 and the cathode being the cathode for water electrolysis according to claim 9 is satisfied.

12. A water electrolysis device comprising: the water electrolysis cell according to claim 10 or 11; and a voltage applicator that applies a voltage between the cathode and the anode.

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