Method for recycling water electrolysis electrodes and apparatus for recycling water electrolysis electrodes

Abstract

This method for recycling water electrolysis electrodes comprises: removing, using an acidic solution, an at least partially altered LDH layer 20a that is disposed on a conductive base material 10; and forming a layered double hydroxide layer 20 on the conductive base material 10 after the removal of the at least partially altered LDH layer 20a.

Description

Method and apparatus for recycling electrodes for water electrolysis

[0001] The present disclosure relates to a method and an apparatus for recycling electrodes for water electrolysis.

[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] 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).

[0005] International Publication No. 2017 / 154134

[0006] Seyeong Lee et al., “Operational durability of three-dimensional Ni-Fe layered double hydroxide electrocatalyst for water oxidation,” Electrochimica Acta, 2019, Vol.315, p.94-101

[0007] The descriptions in the above documents need to be reconsidered from the perspective of recycling electrodes for water electrolysis containing layered double hydroxides. Therefore, the present disclosure provides a novel method for recycling electrodes for water electrolysis containing layered double hydroxides.

[0008] The present disclosure provides a method for recycling electrodes for water electrolysis, comprising: removing, with an acidic solution, an at least partially altered layered double hydroxide layer provided on a conductive substrate; and forming a layered double hydroxide layer on the conductive substrate after the at least partially altered layered double hydroxide layer has been removed.

[0009] According to the present disclosure, a novel method for recycling a water electrolysis electrode containing a layered double hydroxide can be provided.

[0010] Fig. 1 is a flowchart showing an example of a method for recycling water electrolysis electrodes according to the present disclosure. Fig. 2A is a cross-sectional view showing an example of a water electrolysis electrode to be subjected to the recycling method shown in Fig. 1. Fig. 2B is a cross-sectional view showing an example of a water electrolysis electrode obtained by the recycling method shown in Fig. 1. Fig. 3 is a schematic diagram showing an example of the crystal structure of a layered double hydroxide (LDH). Fig. 4 is a schematic diagram showing an example of the mechanism of LDH layer formation. Fig. 5 is a schematic diagram showing an apparatus for recycling water electrolysis electrodes according to the present disclosure.

[0011] (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.

[0012] 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.

[0013] On the other hand, even if high-performance electrodes for water electrolysis can be manufactured in the initial state, the performance of the electrodes may deteriorate if the electrodes are used for a long period of time. For this reason, it is considered to replace electrodes for water electrolysis whose performance has deteriorated in the water electrolysis device. 2In some cases, electrodes having a large area of ​​1000 kJ / cm2 or more are required. In such cases, if water electrolysis is performed using a scheme involving replacement and disposal of the electrodes for water electrolysis, the cost required for water electrolysis becomes extremely high, due in part to the large size of the electrodes. Therefore, recycling of electrodes for water electrolysis is important from the viewpoints of effective use of materials and reduction of the cost required for water electrolysis.

[0014] The present inventors have conducted extensive research in light of these circumstances and have newly discovered that electrodes for water electrolysis containing layered double hydroxides can be recycled by performing a specific process. Based on this new finding, the present inventors have completed a method for recycling electrodes for water electrolysis according to the present disclosure.

[0015] 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.

[0016] Fig. 1 is a flowchart showing an example of a method for recycling a water electrolysis electrode according to the present disclosure. Fig. 2A is a cross-sectional view showing an example of a water electrolysis electrode 1a to be subjected to this recycling method. As shown in Fig. 2A , in the water electrolysis electrode 1a, an at least partially denatured layered double hydroxide (LDH) layer 20a is provided on a conductive substrate 10. As shown in Fig. 1 , the method for recycling a water electrolysis electrode includes the following steps S11 and S12. Step S11: The at least partially denatured LDH layer 20a provided on the conductive substrate 10 is removed using an acidic solution. Step S12: After the at least partially denatured LDH layer 20a has been removed, an LDH layer 20 is formed on the conductive substrate 10.

[0017] Because the water electrolysis electrode 1a has at least a partially altered LDH layer 20a, the water electrolysis electrode 1a exhibits poor performance in water electrolysis. For this reason, it is desirable to replace the water electrolysis electrode 1a in the water electrolysis apparatus. By subjecting the replaced water electrolysis electrode 1a to the above-described recycling method, a new water electrolysis electrode having an LDH layer 20 formed on the conductive substrate 10 can be produced. The above-described recycling method can also be considered a method for producing a water electrolysis electrode. The LDH layer is a layer containing LDH.

[0018] The at least partially altered LDH layer 20a is generated by alteration of LDH contained in the water electrolysis electrode. For example, the at least partially altered LDH layer 20a can be formed by using a water electrolysis electrode, which includes an LDH layer provided on the conductive substrate 10, for water electrolysis for a predetermined period of time.

[0019] The at least partially denatured LDH layer 20a is not limited to a specific layer as long as it is formed by the denaturation of LDH. The at least partially denatured LDH layer 20a may contain, for example, oxyhydroxide formed by the denaturation of LDH. Even in such a case, the water electrolysis electrode 1a can be recycled into a new water electrolysis electrode by the above-described recycling method.

[0020] In step S11, for example, the water electrolysis electrode 1a is brought into contact with an acidic solution, thereby removing the at least partially denatured LDH layer 20a and exposing the surface of the conductive substrate 10 that was covered with the at least partially denatured LDH layer 20a. For example, the water electrolysis electrode 1a is immersed in an acidic solution. The at least partially denatured LDH layer 20a may be only a partially denatured LDH layer or an entirely denatured LDH layer.

[0021] The pH of the acidic solution used in step S11 is not limited to a specific value as long as it can remove at least a partially denatured LDH layer 20a. The pH of the acidic solution is, for example, 4 or less. In this case, at least a partially denatured LDH layer 20a can be removed. In particular, when the at least partially denatured LDH layer 20a contains oxyhydroxides formed by denaturation of LDH, the at least partially denatured LDH layer 20a can be easily removed.

[0022] The pH of the acidic solution is preferably 2 or more and 3 or less. In this case, the LDH layer 20a, at least a part of which has been altered, is easily selectively removed from the water electrolysis electrode 1a, and the conductive substrate 10 is less susceptible to the effects of the acidic solution.

[0023] The pH of the acidic solution may be equal to or lower than 2. In this case, since the acidic solution exhibits strong acidity, the time required to remove at least a portion of the altered LDH layer 20a tends to be shortened.

[0024] The solute contained in the acidic solution is not limited to a specific solute. The acidic solution may be, for example, at least one solution selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, citric acid, oxalic acid, chloride salts, sulfate salts, acetate salts, citrate salts, and oxalate salts. In this case, the pH of the acidic solution can be easily adjusted to a desired range.

[0025] When the acidic solution is a citric acid solution, the LDH layer 20a, at least a part of which has been altered, is easily removed selectively in the water electrolysis electrode 1a, and the conductive substrate 10 is less susceptible to the effects of the acidic solution.

[0026] The temperature of the acidic solution is not limited to a specific temperature. For example, the temperature of the acidic solution is room temperature, 20° C.±15° C. In this case, a mechanism for adjusting the temperature of the acidic solution is not required, and the cost required for recycling the water electrolysis electrodes tends to be low.

[0027] In step S11, the acidic solution may be stirred. In this case, the acidic solution can be uniformly brought into contact with the surface of the water electrolysis electrode 1a, which tends to shorten the time required to remove at least a partially altered LDH layer 20a. The water electrolysis electrode 1a may be subjected to ultrasonic treatment while immersed in the acidic solution.

[0028] In step S11, for example, when the LDH layer 20a at least partially altered by the acidic solution is removed, metal ions in the LDH layer 20a at least partially altered are dissolved in the acidic solution.

[0029] 2B is a cross-sectional view showing an example of a water electrolysis electrode obtained by the above-described recycling method. As described above, in step S12, the LDH layer 20 is formed on the conductive substrate 10 after removing the at least partially altered LDH layer 20a, thereby obtaining the water electrolysis electrode 1.

[0030] As shown in FIG. 2B , the water electrolysis electrode 1 includes a conductive substrate 10 and an LDH layer 20 , and the LDH layer 20 is provided on the conductive substrate 10 .

[0031] 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 nickel or iron, or an alloy such as stainless steel or Inconel. Inconel is a registered trademark.

[0032] The conductive substrate 10 preferably has a surface made of nickel. In this case, the water electrolysis electrode 1 is likely to have the desired properties in terms of high alkali resistance and electrical conductivity. When the surface of the conductive substrate 10 is made of nickel, the entire conductive substrate 10 may be made of nickel, or the conductive substrate 10 may have a surface layer made of nickel. The surface layer made of nickel is a sputtered film or a plated film. When the conductive substrate 10 has a surface made of nickel, the cost of the water electrolysis electrode 1 tends to be relatively high compared to when other materials are used. Therefore, the above-mentioned recycling method is particularly useful in such cases.

[0033] When the surface of the conductive substrate 10 is made of nickel, the purity of the nickel constituting the surface is not limited to a specific value. For example, the purity of the nickel constituting the surface of the conductive substrate 10 is 90% by mass or more. This makes the conductive substrate 10 more likely to have high alkali resistance. The method for determining the purity of the nickel constituting the surface of the conductive substrate 10 is not limited to a specific method. The purity of the nickel 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). For example, the purity of the nickel may be determined by completely dissolving the conductive substrate 10 in aqua regia and analyzing the extract obtained by a method such as inductively coupled plasma atomic emission spectroscopy (ICP-AES). When the purity of the nickel is high, the purity of the nickel may be determined by comparing the specific gravity of the conductive substrate 10 with the specific gravity of pure nickel.

[0034] The purity of the nickel forming the surface of the conductive substrate 10 is preferably 95% by mass or more, more preferably 97% by mass or more, even more preferably 98% by mass or more, and particularly preferably 99% by mass or more.

[0035] 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.

[0036] The thickness of the conductive substrate 10 is not limited to a specific value. The thickness of the conductive substrate 10 is, for example, 0.02 mm or more. In this case, the conductive substrate 10 tends to be easy to handle. The thickness of the conductive substrate 10 is, for example, 10 mm or less, and preferably 1 mm or less.

[0037] FIG. 3 is a diagram schematically illustrating an example of the crystal structure of LDH. LDH 25 may be active in a reaction for producing gases such as hydrogen and oxygen at the anode or cathode of a water electrolysis cell. Therefore, the water electrolysis electrode 1 may be the anode or the cathode of a water electrolysis cell. For example, in alkaline water electrolysis, LDH 25 may be converted into an oxyhydroxide by the water electrolysis reaction. LDH 25 has a composition represented by the following formula (1), for example. In formula (1), M1 2+ is a divalent transition metal ion. 3+ is a trivalent transition metal ion. k- 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. k is an integer. m is an appropriate rational number. [M1 2+ 1-x M2 3+ x (OH)2][yA k- ・mH2O] Formula (1)

[0038] The LDH layer 20 contains two or more types of transition metals. The two or more types of transition metals contained in the LDH layer 20 are not limited to specific transition metals. In other words, M1 and M2 in the composition shown in formula (1) are not limited to specific transition metals. The LDH layer 20 contains, for example, at least two transition metals selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. In this case, the conductive substrate 1 is likely to have the desired electrode activity.

[0039] The LDH layer 20 preferably contains at least one selected from the group consisting of Fe and Ni. In this case, the water electrolysis electrode 1 is likely to have high electrode activity. In addition, the production cost of the water electrolysis electrode 1 is likely to be low. The LDH layer 20 more preferably contains Ni and Fe. In this case, the water electrolysis electrode 1 is more likely to have high electrode activity. For example, in the composition shown in formula (1), M1 may be Ni and M2 may be Fe.

[0040] A is an interlayer anion k- 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- n is an integer of 1 or more. k- can be intercalated between the layers of the metal hydroxide along with water molecules. k- The charge and size of the ion are not limited to a specific value. k- or a plurality of types of Ak- may also include:

[0041] As shown in Figure 3, LDH25 is a soluble form of M1 2+ or M2 3+ OH at each vertex of the octahedron centered at - LDH25 has [M1 2+ 1-x M2 3+ x (OH) x+ This metal hydroxide has a layered structure in which hydroxide octahedra are connected two-dimensionally, sharing edges. Between the metal hydroxide layers, anions A k- and water molecules are present. The metal hydroxide layer functions as a host layer 21, and anions A k- and a guest layer 22 containing water molecules is disposed between the host layers. In other words, the LDH 25 as a whole is composed of a host layer of metal hydroxide and an anion A k- LDH25 has a sheet-like structure in which guest layers of metal hydroxide and water molecules are alternately stacked. 2+ Part of M2 3+ The crystal structure and crystallinity of LDH are qualitatively and quantitatively analyzed by X-ray diffraction measurement (XRD).

[0042] The LDH layer 20 covers, for example, the surface of the conductive substrate 10. The coverage of the LDH layer 20 with respect to the surface of the conductive substrate 10 is not limited to a specific value. The coverage is preferably 99% or more. In this case, the water electrolysis electrode 1 is likely to have high electrode activity. In addition, the water electrolysis electrode 1 is likely to have high durability.

[0043] As described above, the LDH 25 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 the water electrolysis cell. For example, in alkaline water electrolysis, a water electrolysis reaction transforms part or all of the LDH 25, i.e., at least a part of the LDH 25, into oxyhydroxide. For example, an LDH layer 20a in which at least a part is transformed in this manner is formed on the conductive substrate 10.

[0044] The oxyhydroxide produced by the alteration of LDH25 has a composition represented by, for example, MOOH. In MOOH, M may be a divalent transition metal ion, a trivalent transition metal ion, or both a divalent transition metal ion and a trivalent transition metal ion. The oxyhydroxide may also have interlayer water between its layers.

[0045] The oxyhydroxide produced by the alteration of LDH25 may have a different crystal structure from that of the initial LDH25 due to differences in charge state. For example, if the initial LDH25 contains nickel, the nickel exists in an α-nickel hydroxide structure. The structure of α-nickel hydroxide is the same as the crystal structure of the hydroxide in LDH25 described above. If LDH25 is continuously immersed in an alkaline solution without an external voltage being applied, the α-nickel hydroxide structure changes to a stable β-nickel hydroxide crystal structure. This crystal structure has a composition of approximately Ni(OH). If the temperature of the alkaline solution is increased, the change from α-nickel hydroxide to the β-nickel hydroxide crystal structure accelerates. On the other hand, when a water electrolysis reaction is occurring, i.e., when a voltage is applied to LDH25, the valence of nickel increases due to oxidation, and protons (hydrogen ions) from the hydroxide are released, so that nickel exists in the form of nickel oxyhydroxide, i.e., NiOOH. Furthermore, for example, when a high voltage is applied to LDH 25 so that a potential exceeding 1.6 V is applied to LDH 25, the valence of nickel formally exceeds +3, and nickel may exist in a γ-type structure. When a high voltage is applied directly to LDH 25 without taking the path of changing the crystalline structure of β-type nickel hydroxide by continuing to immerse α-type nickel hydroxide in an alkaline solution without applying an external voltage, α-type nickel hydroxide can change directly to a γ-type structure. When the voltage applied to LDH 25 is reduced from the state in which nickel has changed to a γ-type structure, and a state in which no potential is applied to LDH 25, the nickel with a γ-type structure can return to α-type nickel hydroxide.

[0046] The transition metal in the oxyhydroxide contained in the at least partially altered LDH layer 20a is determined depending on the type of transition metal contained in the LDH 25 and is not limited to a specific transition metal. If the initial LDH 25 contains, for example, Ni and Fe, the oxyhydroxide in the at least partially altered LDH layer 20a may exist as NiFeOOH or may exist separately as NiOOH and FeOOH. The crystal structure and crystallinity of the oxyhydroxide are qualitatively and quantitatively analyzed by XRD.

[0047] The LDH layer 20 contains, for example, a chelating agent. The chelating agent may be coordinated to a transition metal ion contained in the LDH 25. This allows the LDH 25 to be stably present in the LDH layer 20. In addition, the LDH 25 is easily synthesized to have a small particle size. In addition, the LDH 25 nucleated on the conductive substrate 10 is likely to undergo slow crystal growth, so that the dense LDH layer 20 with few voids containing the LDH 25 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.

[0048] The chelating agent is not limited to a specific chelating agent. For example, the chelating agent is an organic compound capable of coordinating with a transition metal ion in LDH25. The chelating agent may be at least one selected from the group consisting of bidentate organic ligands and tridentate organic ligands. Examples of chelating agents include β-diketones, β-ketoesters, hydroxycarboxylic acids, and hydroxycarboxylate salts. Examples of β-diketones include acetylacetone (ACAC), trifluoroacetylacetone, hexafluoroacetylacetone, benzoylacetone, thenoyltrifluoroacetone, dipyrrolylmethane, dibenzoylmethane, and ascorbic acid. Examples of β-ketoesters include methyl acetoacetate, ethyl acetoacetate, allyl acetoacetate, benzyl acetoacetate, n-propyl acetoacetate, isopropyl acetoacetate, n-butyl acetoacetate, isobutyl acetoacetate, tert-butyl acetoacetate, 2-methoxyethyl acetoacetate, and methyl 3-oxopentanoate. Examples of hydroxycarboxylic acids and their salts are tartaric acid, citric acid, malic acid, gluconic acid, ferulic acid, lactic acid, glucuronic acid, and salts thereof.

[0049] The chelating agent preferably contains at least one selected from the group consisting of acetylacetone and citrate. In this case, the water electrolysis electrode is more likely to have high electrode activity. An example of the citrate is trisodium citrate.

[0050] In step S12, the method for forming the LDH layer 20 on the conductive substrate 10 is not limited to a specific method. For example, the LDH layer 20 can be produced by immersing the conductive substrate 10 in a solution containing two or more types of transition metal ions and adjusting the solution to an alkaline state. This method allows the LDH layer 20 to be easily formed on the conductive substrate 10. The solution may contain, for example, a chelating agent. In this case, as described above, the LDH layer 20 may contain a chelating agent. 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.

[0051] The temperature of the solution used to form the LDH layer 20 is not limited to a specific temperature. The temperature is, for example, room temperature (20°C ± 15°C).

[0052] The solvent of the solution used to form the LDH layer 20 may be water, an organic solvent, or a mixed solvent of water and an organic solvent.

[0053] The pH of the solution may be increased during the formation of the LDH layer 20. This allows LDH 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 likely to have high durability.

[0054] The method for adjusting the solution to alkaline in forming the LDH layer 20 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. For example, the pH-elevating agent is 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 result of a ring-opening reaction of the epoxy group. 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 from, for example, 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.

[0055] 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.

[0056] 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.

[0057] As described above, the conductive substrate 10 preferably has a surface made of nickel. In this case, the two or more types of transition metal ions contained in the solution preferably include Fe ions. The solution preferably includes chloride ions. In this case, the reaction represented by formula (2) may occur, which may etch the conductive substrate 10. The method for forming the LDH layer 20 in step S12 preferably includes promoting mixing of the solution before adjusting the solution to alkaline while the conductive substrate 10 is immersed. Promotion of mixing of the solution may be achieved, for example, by vibrating the conductive substrate 10, shaking a container containing the solution and the conductive substrate 10, or stirring the solution using a stirrer piece or a stirrer. Such a method may generate forced convection in the solution, promoting mixing of the solution. 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 mixing of the solution 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)

[0058] In the LDH layer 20, the molar ratio of the Fe ion content to the Ni content in the conductive substrate 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 nickel contained in the conductive substrate 10 from being dissolved by the reaction shown in formula (2), which would make it difficult to manufacture a water electrolysis electrode.

[0059] 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 a water electrolysis electrode 1 having high electrode activity is more likely to be produced.

[0060] FIG. 4 is a schematic diagram illustrating an example of the mechanism of formation of the LDH layer 20. As shown in FIG. 4, the 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 is formed. Next, the complexes C1 and C2 react on the surface of the conductive substrate 10, synthesizing LDH 25 along the surface of the conductive substrate 10. In addition, the complexes C1 and C2 contain a chelating agent, which inhibits crystal growth of the LDH 25. As a result, an LDH layer 20 containing the LDH 25 and the chelating agent CH is formed on the conductive substrate 10, and a water electrolysis electrode 1 is obtained.

[0061] As described above, when the at least partially denatured LDH layer 20a is removed in step S11, metal ions in the at least partially denatured LDH layer 20a are dissolved in the acidic solution. In step S12, these metal ions may be used to form the LDH layer 20. In this case, more effective use of materials can be achieved.

[0062] For example, a solution for forming the LDH layer 20 in step S12 may be prepared by adding a predetermined additive to the acidic solution used in step S11 as needed. In this case, no waste liquid is generated after step S11, or the amount of waste liquid can be reduced, which tends to reduce the cost of recycling the water electrolysis electrodes. The solution for forming the LDH layer 20 in step S12 may be prepared separately from the acidic solution used in step S11.

[0063] As shown in Fig. 1 , the above-described method for recycling a water electrolysis electrode may include, for example, step S13 between step S11 and step S12. In step S13, the LDH layer 20a, at least a portion of which has been altered, is removed, and then the conductive substrate 10 is washed. This makes it easier for the surface of the conductive substrate 10 to be in a state suitable for the formation of the LDH layer 20. As a result, it is easier to produce a water electrolysis electrode 1 having high electrode activity. For example, ultrapure water is used to wash the conductive substrate 10.

[0064] Fig. 5 is a schematic diagram of a recycling apparatus for water electrolysis electrodes. As shown in Fig. 5, the recycling apparatus 3 includes a first container 31, a second container 32, and a device 33. The first container 31 is a container that contains the acidic solution used in step S11. The second container 32 is a container that contains a solution for forming the LDH layer 20, which may be used in step S12. The device 33 is a device for inserting the water electrolysis electrode 1a into the first container 31 and removing the conductive substrate 10 from the first container 31. The conductive substrate 10 removed from the first container 31 by the device 33 is inserted into the second container 32. The recycling apparatus 3 allows the recycling method including the above steps S11 and S12 to be performed efficiently.

[0065] The configuration of the device 33 is not limited to any particular configuration as long as it allows the water electrolysis electrode 1a to be inserted into the first container 31 and the conductive substrate 10 to be removed from the first container 31. The device 33 may be, for example, a robot arm capable of gripping the water electrolysis electrode 1a and the conductive substrate 10. The device 33 may also be a crane.

[0066] As shown in FIG. 5 , in the recycling apparatus 3, the first container 31 may also serve as the second container 32. This is advantageous from the viewpoint of reducing the manufacturing cost of the recycling apparatus 3 and reducing the space required for installing the recycling apparatus 3. The recycling apparatus 3 further includes, for example, a first input unit 36, a disposal unit 37, and a second input unit 38. The first input unit 36 ​​is configured to input the acidic solution used in step S11 into the container that serves as both the first container 31 and the second container 32. The first input unit 36 ​​includes, for example, a flow path 36a, a pump 36b, and a valve 36c. The first input unit 36 ​​inputs the acidic solution prepared outside the first container 31 into the first container 31. In the first input unit 36, by operating the pump 36b with the valve 36c open, the acidic solution outside the first container 31 passes through the flow path 36a and is input into the first container 31. Thereafter, the water electrolysis electrode 1a is inserted into the first container 31 by the device 33, and the process of step S11 is performed. The valve 36c is, for example, a solenoid valve. After the process of step S11 is completed, for example, the LDH layer 20a, at least a portion of which has been altered, is removed, and then the conductive substrate 10 is removed from the first container 31 by the device 33.

[0067] The waste unit 37 is configured to discard the acidic solution used in step S11 from the container that serves as both the first container 31 and the second container 32. As shown in Fig. 5 , the waste unit 37 includes, for example, a flow path 37a and a valve 37b. The flow path 37a is connected to the container that serves as both the first container 31 and the second container 32. When the valve 37b is opened, the acidic solution passes through the flow path 37a and is discharged to the outside of the recycling device 3.

[0068] The second input unit 38 is configured to input a solution for forming an LDH layer 20 on the conductive substrate 10 in step S12 into a container that serves as both the first container 31 and the second container 32. As shown in FIG. 5 , the second input unit 38 includes, for example, a flow path 38a and a pump 38b. The second input unit 38 inputs the solution for forming the LDH layer 20, which has been prepared outside the second container 32, into the second container 32. In the second input unit 38, the pump 38b is operated, causing the solution for forming the LDH layer 20 outside the second container 32 to pass through the flow path 38a and be input into the second container 32. Thereafter, the conductive substrate 10 obtained by the process of step S11 is inserted into the second container 32, and the process of step S12 is performed. A valve 38c may be provided in the flow path 38a of the second input unit 38. The valve 38c is, for example, a solenoid valve.

[0069] The first container 31 and the second container 32 may be separate containers.

[0070] 5 , the recycling device 3 may further include a sensor 35. The sensor 35 optically detects, for example, the surface condition of the water electrolysis electrode 1a or the conductive substrate 10, or the condition of the solution contained in the first container 31 or the second solution 32. The completion of step S11 or step S12 may be determined based on the detection result of the sensor 35. The completion of step S11 or step S12 may also be determined by the lapse of a predetermined time using a timer.

[0071] (Additional Notes) The above disclosure discloses the following technologies. (Technology 1) A method for recycling electrodes for water electrolysis, comprising: removing an at least partially altered layered double hydroxide layer provided on a conductive substrate using an acidic solution; and forming a layered double hydroxide layer on the conductive substrate after the at least partially altered layered double hydroxide layer has been removed. (Technology 2) The method for recycling electrodes for water electrolysis according to Technology 1, wherein the at least partially altered layered double hydroxide layer contains an oxyhydroxide produced by alteration of the layered double hydroxide. (Technology 3) The method for recycling electrodes for water electrolysis according to Technology 1 or 2, wherein the pH of the acidic solution is 4 or less. (Technology 4) The method for recycling electrodes for water electrolysis according to Technology 3, wherein the pH of the acidic solution is 2 or more and 3 or less. (Technology 5) The method for recycling electrodes for water electrolysis according to any one of Techniques 1 to 4, wherein the acidic solution is at least one solution selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, citric acid, oxalic acid, chlorides, sulfates, acetates, citrates, and oxalates. (Technology 6) The method for recycling electrodes for water electrolysis according to any one of Techniques 1 to 5, wherein metal ions in the at least partially altered layered double hydroxide layer dissolved in the acidic solution are used to form the layered double hydroxide layer. (Technology 7) The method for recycling electrodes for water electrolysis according to any one of Techniques 1 to 6, wherein the layered double hydroxide layer contains at least two transition metals selected from the group consisting of V, Cr, Mn, Fe, Co, Ni, Cu, W, and Ru. (Technology 8) The method for recycling electrodes for water electrolysis according to Technique 7, wherein the layered double hydroxide layer contains Fe and Ni. (Technology 9) The method for recycling electrodes for water electrolysis according to any one of Techniques 1 to 8, wherein the conductive base material has a surface made of nickel. (Technology 10) The method for recycling electrodes for water electrolysis according to Technique 9, wherein the nickel forming the surface has a purity of 90 mass% or more. (Technology 11) The method for recycling electrodes for water electrolysis according to any one of Techniques 1 to 10, wherein the layered double hydroxide layer contains a chelating agent.(Technology 12) The method for recycling electrodes for water electrolysis according to Technology 11, wherein the chelating agent includes at least one selected from the group consisting of acetylacetone and citric acid. (Technology 13) The method for recycling electrodes for water electrolysis according to any one of Technology 1 to 12, wherein the electrodes for water electrolysis are anodes. (Technology 14) The method for recycling electrodes for water electrolysis according to any one of Technology 1 to 12, wherein the electrodes for water electrolysis are cathodes. (Technology 15) A recycling apparatus for electrodes for water electrolysis, comprising: a first container for containing an acidic solution; equipment for inserting a water electrolysis electrode comprising a conductive substrate and a layered double hydroxide layer provided on the conductive substrate, at least a portion of which has been altered, into the first container and for removing the conductive substrate from the first container; and a second container for containing a solution for forming a layered double hydroxide layer on the conductive substrate, wherein the conductive substrate removed from the first container by the equipment is inserted into the second container. (Technology 16) The recycling device for water electrolysis electrodes according to Technology 15, wherein the first container doubles as the second container, and the recycling device comprises: a disposal unit for disposing of the acidic solution from the container serving as both the first container and the second container; and an introduction unit for introducing a solution for forming a layered double hydroxide layer on the conductive substrate into the container serving as both the first container and the second container. (Technology 17) A method for producing an electrode for water electrolysis, the method comprising: removing, with an acidic solution, an at least partially altered layered double hydroxide layer provided on a conductive substrate; and forming a layered double hydroxide layer on the conductive substrate after the at least partially altered layered double hydroxide layer has been removed.

[0072] 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.

[0073] Example 1: 3.64 milliliters (mL) of water was placed in a perfluoroalkoxyalkane (PFA) minivial OD-98-5MV (manufactured by Taiyo Co., Ltd.), and 0.151 g of nickel chloride hexahydrate and 0.043 g of iron chloride hexahydrate were dissolved to prepare a solution. Nickel chloride hexahydrate and iron chloride hexahydrate were purchased from Fujifilm Wako Pure Chemical Industries, Ltd. 0.025 mL of acetylacetone (ACAC) was added to this solution as a chelating agent to obtain the chelating agent-containing solution of Example 1. ACAC was purchased from Sigma-Aldrich.

[0074] 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 0.315 g. The Ni plate was then rinsed with water and dried to complete the cleaning process.

[0075] Next, the Ni plate after the cleaning treatment was immersed in the chelating agent-containing solution according to Example 1. 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). Next, 0.254 mL of propylene oxide (POX) was added as a pH-raising agent to the chelating agent-containing solution. 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. It is believed that in the electrode according to Example 1, an LDH layer containing LDH containing Ni and Fe and a chelating agent was provided on the Ni plate substrate.

[0076] Next, water electrolysis evaluation was performed under the following conditions using the electrode according to Example 1 as the anode. For this evaluation, a potentiostat VersaSTAT4 manufactured by Princeton Applied Research and a rotating electrode AFE3T050GC manufactured by Pine Research were used. An oxygen generation reaction was performed by water electrolysis using the rotating disk electrode (RDE) method under the following measurement conditions. [Measurement conditions] Cathode: Platinum electrode Solution: 1 M KOH aqueous solution Potential sweep range: 1-1.7 V vs. reversible hydrogen electrode (RHE) Potential sweep rate: 10 mV / s Number of cycles: 10 times

[0077] It is believed that the surface of the electrode changed from yellowish brown to black due to this water electrolysis, and at least a part of the LDH layer was altered.

[0078] After being subjected to water electrolysis, the electrode was subjected to an immersion treatment under the following conditions: [Immersion treatment conditions] Acid solution: hydrochloric acid pH: 1 Temperature: room temperature Stirring: none

[0079] As a result, a silvery white color thought to be the surface of the Ni plate was confirmed after the 35-minute immersion treatment, and it was confirmed that at least a part of the denatured LDH layer had been removed. Furthermore, it was confirmed that the electrode could be regenerated in the same manner as in the above-mentioned electrode preparation by using a Ni plate after the LDH layer, at least a part of which had been denatured, had been removed by immersion in hydrochloric acid, instead of the Ni plate used in the preparation of the electrode in Example 1.

[0080] Example 2 Electrode formation and an oxygen generation reaction by water electrolysis were carried out under the same conditions as in Example 1.

[0081] After being subjected to water electrolysis, the electrode was subjected to an immersion treatment under the following conditions: [Immersion treatment conditions] Acid solution: hydrochloric acid pH: 2 Temperature: room temperature Stirring: none

[0082] As a result, a silvery white color thought to be the surface of the Ni plate was confirmed after the 210-minute immersion treatment, and it was confirmed that at least a part of the denatured LDH layer had been removed. Furthermore, it was confirmed that the electrode could be regenerated in the same manner as in the above-mentioned electrode preparation by using a Ni plate after the LDH layer, at least a part of which had been denatured, had been removed by immersion in hydrochloric acid, instead of the Ni plate used in the preparation of the electrode in Example 1.

[0083] The method for recycling electrodes for water electrolysis disclosed herein can be used to recycle anodes or cathodes for water electrolysis.

Claims

1. Removing, with an acidic solution, at least a partially altered layered double hydroxide layer provided on a conductive substrate; and forming a layered double hydroxide layer on the conductive substrate after the at least partially altered layered double hydroxide layer has been removed, the recycling method of an electrode for water electrolysis.

2. The recycling method of an electrode for water electrolysis according to claim 1, wherein the at least partially altered layered double hydroxide layer contains an oxyhydroxide formed by alteration of the layered double hydroxide.

3. The recycling method of an electrode for water electrolysis according to claim 1, wherein the pH of the acidic solution is 4 or less.

4. The recycling method of an electrode for water electrolysis according to claim 3, wherein the pH of the acidic solution is 2 or more and 3 or less.

5. The recycling method of an electrode for water electrolysis according to claim 1, wherein the acidic solution is at least one solution selected from the group consisting of hydrochloric acid, sulfuric acid, acetic acid, citric acid, oxalic acid, chloride salts, sulfate salts, acetate salts, citrate salts, and oxalate salts.

6. The recycling method of an electrode for water electrolysis according to claim 1, wherein metal ions in the at least partially altered layered double hydroxide layer dissolved in the acidic solution are used for the formation of the layered double hydroxide layer.

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

8. The recycling method of an electrode for water electrolysis according to claim 7, wherein the layered double hydroxide layer contains Fe and Ni.

9. The recycling method of an electrode for water electrolysis according to claim 1, wherein the conductive substrate has a surface made of nickel.

10. The recycling method of an electrode for water electrolysis according to claim 9, wherein the nickel forming the surface has a purity of 90 mass% or more.

11. The recycling method of an electrode for water electrolysis according to claim 1, wherein the layered double hydroxide layer contains a chelating agent.

12. The recycling method of an electrode for water electrolysis according to claim 11, wherein the chelating agent contains at least one selected from the group consisting of acetylacetone and citric acid.

13. The recycling method of an electrode for water electrolysis according to any one of claims 1 to 12, wherein the electrode for water electrolysis is an anode.

14. The method for recycling the water electrolysis electrode according to any one of claims 1 to 12, wherein the water electrolysis electrode is a cathode.

15. A first container containing an acidic solution, equipment for inserting into the first container and removing from the first container a water electrolysis electrode comprising a conductive substrate and a layered double hydroxide layer in which at least a part has been altered, provided on the conductive substrate, and a second container containing a solution for forming a layered double hydroxide layer on the conductive substrate, wherein the conductive substrate removed from the first container by the equipment is inserted into the second container; a recycling apparatus for a water electrolysis electrode.

16. The recycling apparatus for a water electrolysis electrode according to claim 15, wherein the first container also serves as the second container, and the container that also serves as the first container and the second container is provided with a disposal section for disposing of the acidic solution and an introduction section for introducing a solution for forming a layered double hydroxide layer on the conductive substrate into the container.

17. A method for manufacturing a water electrolysis electrode, including removing, with an acidic solution, a layered double hydroxide layer in which at least a part has been altered, provided on a conductive substrate, and forming a layered double hydroxide layer on the conductive substrate after the at least partially altered layered double hydroxide layer has been removed.

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