Catalyst for oxygen generation reaction, method for manufacturing the same, and method for reusing disposable hand warmers after use.

Abstract

The object of the present invention is to provide an iron-containing catalyst that can be used as a catalyst in oxygen evolution reactions, and is suitable for use in, for example, the electrolysis of water. Another object of the present invention is to reuse the contents of a used disposable hand warmer as a catalyst. [Solution] A catalyst for oxygen evolution reaction having a first layer containing iron oxyhydroxide particles and a second layer provided on the surface of the first layer containing nickel hydroxide or nickel oxyhydroxide.

Description

Technical Field

[0001] The present invention relates to a catalyst for oxygen generation reaction using iron oxyhydroxide particles, a method for producing the catalyst for oxygen generation reaction, and a method for recycling a used disposable warmer using the content of the used disposable warmer after use as iron oxyhydroxide particles.

Background Art

[0002] In recent years, while the global energy demand has been accelerating and increasing, most of it is supplied by fossil fuels, and the associated global warming and depletion of reserves have been regarded as problems. In response to this problem, the acquisition of hydrogen energy utilizing renewable energy has attracted great attention. However, in the electrolysis reaction of water, the overvoltage required for the oxygen generation reaction at the anode is high compared to the hydrogen generation reaction at the cathode, and it is regarded as a problem that it becomes the rate-determining step of the entire reaction, and the development of a more efficient catalyst for oxygen generation reaction at the anode in water electrolysis is required. Originally, catalysts in water electrolysis have used precious metals such as expensive and highly efficient platinum (Pt) and iridium (Ir), but there are concerns about the high cost and depletion of reserves. In recent years, transition metal oxides that are inexpensive and abundant have attracted attention. Among them, several catalysts using iron oxides have been proposed. In Non-Patent Document 1, it has been proposed to use iron oxyhydroxide contained in a used disposable warmer as a catalyst for oxygen generation reaction as an iron oxide source, but the effect as such a catalyst was not sufficient. In addition, in Non-Patent Document 2, it has been proposed to pulverize an iron ore mainly composed of hematite (Fe2O3), coat it with nickel hydroxide (Ni(OH)2), and use it as a catalyst for oxygen generation reaction under alkaline conditions. However, with natural iron ore, it is difficult to maintain stable quality, and it was used under alkaline conditions.

Prior Art Documents

Non-Patent Documents

[0003]

Non-Patent Document 1

[0004] The object of the present invention is to provide an iron-containing catalyst that can be used as a catalyst in oxygen evolution reactions, and is suitable for use in, for example, the electrolysis of water. Another object of the present invention is to reuse the contents of a used disposable hand warmer as a catalyst. [Means for solving the problem]

[0005] The inventors focused on using the contents of used disposable hand warmers in their development of catalysts utilizing iron oxides. Further investigation revealed that the contents of used disposable hand warmers contain iron oxyhydroxide particles, which have potential as a catalyst. However, it was difficult to obtain sufficient catalytic activity using only the contents of used hand warmers. Therefore, further investigation led to the discovery that by forming a layer containing iron oxyhydroxide particles using the contents of used hand warmers and coating this layer with nickel hydroxide, specifically nickel hydroxide or nickel oxyhydroxide, excellent catalytic activity as an oxygen evolution catalyst could be obtained. This finding is applicable not only to the contents of used disposable hand warmers but also to other iron oxyhydroxide particles. Thus, the present invention was completed.

[0006] In other words, the present invention is defined by the following: (1) A catalyst for an oxygen evolution reaction having a first layer containing iron oxyhydroxide particles and a second layer provided on the surface of the first layer containing nickel hydroxide or nickel oxyhydroxide. (2) The catalyst for the oxygen evolution reaction of (1) above, wherein the iron oxyhydroxide particles are β-FeOOH particles. (3) The catalyst for the oxygen evolution reaction in (2) above, wherein the iron oxyhydroxide particles are β-FeOOH particles contained in a disposable hand warmer after use. (4) The catalyst for the oxygen evolution reaction according to (1) above, comprising iron oxyhydroxide particles and activated carbon particles in the first layer. (5) Any of the oxygen evolution catalysts described in (1) to (4) above, wherein the second layer is formed by electrodeposition. (6) A method for producing a catalyst for an oxygen evolution reaction, comprising applying a coating solution containing iron oxyhydroxide, a conductive additive, and a binder to the surface of a substrate to form a first layer containing iron oxyhydroxide particles, and depositing nickel oxyhydroxide on the surface of the first layer by electrodeposition to form a second layer containing nickel oxyhydroxide. (7) A method for reusing a used disposable hand warmer, comprising: removing the contents containing β-FeOOH particles from the used disposable hand warmer; forming a first layer containing the β-FeOOH particles contained in the contents; and forming a second layer containing nickel hydroxide or nickel oxyhydroxide on the surface of the first layer to produce the catalyst for the oxygen generation reaction described in (3). [Effects of the Invention]

[0007] The oxygen evolution catalyst of the present invention is an iron-containing catalyst that can be used as a catalyst in an oxygen evolution reaction, and is suitably used, for example, in the electrolysis of water. Furthermore, the method for reusing used disposable hand warmers of the present invention allows the contents of the used disposable hand warmers to be reused as a catalyst. [Brief explanation of the drawing]

[0008] [Figure 1] Figure 1 shows the results of X-ray absorption fine structure analysis (XAFS) measurements of the hand warmer powder prepared in the example. [Figure 2] Figure 2 shows the results of electrochemical measurements of the samples obtained in Examples 1-4 and Comparative Examples 1-4. [Figure 3] Figure 3 shows the results of elemental mapping by EDX of the sample obtained in Example 4. [Modes for carrying out the invention]

[0009] The oxygen evolution catalyst of the present invention is a catalyst for use in an oxygen evolution reaction, comprising a first layer containing iron oxyhydroxide particles and a second layer provided on the surface of the first layer containing nickel hydroxide or nickel oxyhydroxide. Examples of iron oxyhydroxide in the present invention include α-FeOOH, β-FeOOH, γ-FeOOH, etc. Among these, β-FeOOH is preferred as the iron oxyhydroxide in the present invention. The particle size of the iron oxyhydroxide particles is not particularly limited as long as they can form the first layer and have catalytic activity, but for example, an average particle size of 50 μm or less can be given, and ranges such as 0.1 to 30 μm, 0.1 to 20 μm, 0.2 to 10 μm, 0.2 to 3 μm, 0.2 to 1 μm, etc. The first layer containing iron oxyhydroxide particles may consist only of iron oxyhydroxide particles, or it may contain conductive additives, binders, etc. in addition to iron oxyhydroxide particles. Examples of conductive additives include carbon black, graphene, and carbon nanotubes, while examples of binders include Nafion® and PVDF (polyvinylidene fluoride). When the first layer contains components other than iron oxyhydroxide particles, such as conductive additives and binders, the proportion of iron oxyhydroxide particles in the entire first layer is preferably 60 to 80% by mass.

[0010] The second layer in the present invention contains nickel hydroxide (Ni(OH)2) or nickel oxyhydroxide (NiOOH). Examples of nickel oxyhydroxide include β-NiOOH and γ-NiOOH, with γ-NiOOH being preferred. The second layer may consist only of nickel hydroxide or nickel oxyhydroxide, or it may contain both. It may also contain other components as long as the catalytic effect can be maintained. The second layer in the present invention is provided on the surface of the first layer and is provided to cover the surface of the first layer. Since the first layer in the present invention contains iron oxyhydroxide particles, irregularities are formed on the surface of the first layer. The second layer in the present invention is provided to cover this irregular surface, but it is not necessary to cover the entire surface; it is sufficient to cover to the extent that the effect of providing the second layer is achieved. For example, the second layer may cover 70% or more, 80% or more, or 90% or more of the total surface area of ​​the first layer. In the present invention, the mass ratio of iron (Fe) contained in the first layer to nickel (Ni) contained in the second layer is preferably 0.001 to 0.3, more preferably 0.002 to 0.1, more preferably 0.003 to 0.09, and even more preferably 0.003 to 0.03, with Fe being set to 1.

[0011] In this invention, β-FeOOH particles contained in used disposable hand warmers can be used as iron oxyhydroxide particles. A disposable hand warmer is made by putting iron powder in a bag made of nonwoven fabric or the like, and it utilizes the heat generated when the iron powder reacts with oxygen in the air and oxidizes. In addition to iron powder, disposable hand warmers may contain water to accelerate the oxidation rate of iron, a water-retaining agent such as vermiculite to retain this water, activated carbon to take in air into the micropores on the surface and promote oxygen supply, and salts to accelerate the oxidation rate of iron. In this invention, a hand warmer that contains iron powder and utilizes the heat generated by the oxidation of iron powder is called a disposable hand warmer. The proportion of iron powder in the contents of a disposable hand warmer is usually 50 to 60% by mass of the total contents. Also, for example, the proportion of water to the total contents is 15 to 25% by mass, the proportion of water-retaining agent is 5 to 10% by mass, the proportion of activated carbon is 15 to 25% by mass, and the proportion of salts is 4 to 6% by mass. Disposable hand warmers after use contain β-FeOOH particles, which are iron oxyhydroxide produced by the oxidation of iron powder. In the present invention, β-FeOOH particles contained in disposable hand warmers after use can be suitably used as β-FeOOH particles.

[0012] In the present invention, when using β-FeOOH particles contained in a used disposable hand warmer as the iron oxyhydroxide particles, it is possible to use only the β-FeOOH particles extracted from the contents of the disposable hand warmer, or to use a mixture in which β-FeOOH particles are mixed with other components such as activated carbon and water-retaining agents. When using a mixture of β-FeOOH particles and other components, it is preferable to remove unwanted components such as salts by washing with water. The activated carbon contained in the mixture is expected to have the effect of improving conductivity. When using a mixture containing β-FeOOH particles and activated carbon, it is preferable that the mass ratio of iron to carbon in the activated carbon in the mixture be iron:carbon = 1:0.25~0.5. Furthermore, while conventional catalysts using iron oxide are used in alkaline environments such as the electrolysis of water using an alkaline aqueous solution with a pH of 11 or higher, the oxygen evolution reaction catalyst of the present invention can be used in neutral to alkaline environments such as the electrolysis of water using an aqueous solution with a pH of 7~11.

[0013] The method for producing the oxygen evolution reaction catalyst of the present invention is not particularly limited, but examples include the following. For the first layer, a coating solution is prepared by dispersing iron oxyhydroxide particles, a binder such as Nafion®, and optionally a conductive additive such as carbon black in a solvent such as tetrahydrofuran. This solution is then applied to the surface of a substrate, such as a metal plate, which will serve as an electrode or a current collector attached to an electrode, and dried to form the first layer. The second layer can be formed by electrodepositing nickel oxyhydroxide onto the surface of the formed first layer. Electrodeposition can be performed, for example, by immersing the substrate on which the first layer is formed in a potassium carbonate electrolyte in which a nickel salt such as nickel nitrate is dissolved, and applying a positive potential. A positive potential of 1.6V or higher is preferred. The pH of the potassium carbonate electrolyte in which the nickel salt is dissolved is preferably 7 to 11. Other examples of nickel salts include nickel chloride, nickel acetate, and nickel sulfate, as well as hydrates such as nickel nitrate hexahydrate (Ni(NO3)2·6H2O), nickel chloride hexahydrate (NiCl2·6H2O), nickel acetate tetrahydrate ((CH3COO)2Ni·4H2O), and nickel sulfate hexahydrate (NiSO4·6H2O). By electrodeposition, the amount of nickel deposited can be adjusted by controlling the concentration of the nickel salt, the potential, the electrodeposition time, etc., and a second layer can be uniformly formed on the surface of the first layer. Nickel oxyhydroxide deposited on the surface of the first layer becomes nickel hydroxide when the potential is released and time passes. However, the oxygen evolution reaction catalyst of the present invention changes to nickel oxyhydroxide at high potentials (e.g., 1.4V or higher) and to nickel hydroxide at low potentials (e.g., 1.4V or lower) when used in the electrolysis of water, etc., so in the present invention, the second layer may be nickel oxyhydroxide or nickel hydroxide.

[0014] The catalyst for oxygen generation reaction of the present invention is also effective as a method for recycling used disposable hand warmers. In the present invention, the contents can be taken out from a used disposable hand warmer, and the first layer can be formed with the contents. Regarding the method of forming the first layer, as described above, only β-FeOOH particles may be taken out to form the first layer, or a mixture in a state where β-FeOOH particles are mixed with other components such as activated carbon and water retention agent may be used to form the first layer. When using a mixture, for example, the mixture may be immersed in water and then washed with water by filtration or the like to remove unnecessary components such as salts or components that have a negative impact on the catalytic activity. Also, after filtration, the residue remaining on the filter medium may be sieved to perform sieving. By sieving the residue, a part of the water retention agent can be removed. When using the contents of a used disposable hand warmer in the present invention, the ratio of β-FeOOH particles to the entire mixture used is preferably 55% by mass or more, and more preferably 60% by mass or more. After forming the first layer, the second layer is formed by the method described above to produce the catalyst for oxygen generation reaction of the present invention, whereby the used disposable hand warmer can be recycled.

Examples

[0015] Hereinafter, the present invention will be specifically described with reference to examples of the present invention, but the technical scope of the present invention is not limited to these examples.

[0016] The following powders were prepared. · α-FeOOH powder (purity: ≧95.0%, manufactured by Kanto Chemical Co., Inc.) · β-FeOOH powder (prepared by the method described later) · γ-FeOOH powder (purity: ≦100%, manufactured by Fujifilm Wako Pure Chemical Corporation) · Hand warmer powder (prepared by the method described later)

[0017] (Preparation of β-FeOOH) 250 mL of an aqueous FeCl₃ solution with a concentration of 1 mol / L was placed in a 1 L beaker, and an aqueous NaOH solution with a concentration of 1 mol / L was added while stirring. When the pH reached approximately 10, the addition of the aqueous NaOH solution was terminated, and stirring was continued for another 1 hour. Thereafter, the precipitate was washed about 10 times using ultrapure water, and finally dried using a drying oven set at 40°C. The dried β-FeOOH was gently crushed using an agate mortar to prepare β-FeOOH. When the obtained dried product was finely crushed in a mortar and subjected to XRD measurement, the same values as those in the paper (Ind. Eng. Chem. Res. 2012, 51, 2, 972-977) were obtained, confirming that the obtained dried product was β-FeOOH.

[0018] (Preparation of the powder for the heat pack) The disposable heat pack ("Heat Pack Mini to be pasted on clothes" (equivalent to the current product "Pokapoka Family Pasting Mini PKN-30HM"), manufactured by Iris Fine Products Co., Ltd.) was opened to generate heat and left to cool completely. Thereafter, the powder inside was taken out and put into ultrapure water, and stirred to suspend it. Next, the suspension was filtered, and the residue remaining on the filter medium was dried at 70°C. The dried powder was sieved through a 25 μm sieve, and the material passing through the sieve was used. The material remaining on the sieve was about 5% by mass of the dried powder, and most of it was vermiculite as a water retention agent. By suspending the contents of the disposable heat pack after heat generation in water and filtering, most of the salts contained in the powder of the contents are dissolved in water and removed. Also, a part of the vermiculite as a water retention agent is removed at the stage of sieving.

[0019] (Measurement of the particle size of the prepared powder) Each of the prepared powders was observed using a scanning electron microscope (SEM). Each particle in the SEM image was colored, and the area of ​​each particle was calculated using the image processing software ImageJ. Assuming that the calculated particle area was the area of ​​a circle, the radius was calculated using the formula for the area of ​​a circle, and this radius was doubled to obtain the particle diameter. For each powder, the particle diameter was calculated for 200 to 300 particles as described above, and the average value (average particle diameter) and standard deviation were determined. The average particle diameter of α-FeOOH powder was 0.297 μm (standard deviation: 0.096 μm), the average particle diameter of β-FeOOH powder was 0.400 μm (standard deviation: 0.110 μm), the average particle diameter of γ-FeOOH powder was 0.421 μm (standard deviation: 0.283 μm), and the average particle diameter of Cairo powder was 2.05 μm (standard deviation: 2.03 μm).

[0020] (Component analysis of hand warmer powder) The components of the hand warmer powder were analyzed using SEM-EDX measurements with a thermal electrolytic emission scanning electron microscope (JEOL Ltd. JSM-7600F). The results showed that the components of the hand warmer powder were 64.5 mass% Fe, 25.6 mass% C, 4.3 mass% Al, 3.4 mass% Si, 1.5 mass% Mg, 0.6 mass% K, 0.04 mass% Na, and 0.06 mass% Cl. Al, Si, and Mg are thought to be components derived from vermiculite, and given the amounts of these, the amount of Fe derived from vermiculite is small, so most of the above amount of Fe is thought to be Fe from β-FeOOH. K, Na, and Cl are thought to be components derived from salts. Furthermore, X-ray absorption fine structure analysis (XAFS) measurements (SPring-8 BL01B1) showed waveforms similar to those of β-FeOOH when compared with each iron oxyhydroxide, indicating that the iron in the hand warmer powder exists as β-FeOOH. Figure 1 shows the XAFS results. Figure 1(a) shows the XANES spectrum, and Figure 1(b) shows the radial distribution function. In addition, although Cl is contained in the Kyro powder, elemental mapping by energy-dispersive X-ray spectroscopy (EDX) measurement (SEM-EDX measurement using a thermal electro-emission scanning electron microscope (JEOL Ltd. JSM-7600F)) showed that the mapping positions of Fe and Cl coincided. Since the structure of β-FeOOH is stabilized by the presence of chloride ions within the structure, the above elemental mapping results showed that almost all of the chloride ions are incorporated into the structure of β-FeOOH.

[0021] [Examples 1-4] In Example 1, α-FeOOH powder was used; in Example 2, β-FeOOH powder was used; in Example 3, γ-FeOOH powder was used; and in Example 4, Cairo powder was used to prepare the catalyst of the present invention as follows.

[0022] (Formation of the FeOOH layer) To 1000 μL of tetrahydrofuran (THF), 10 μL of 0.1 mol / L aqueous KOH solution and 20 μL of Nafion® dispersion (5% by mass) were added and stirred. After stirring, 10 mg of carbon black and 25 mg of the powder used in Examples 1-4 were added and thoroughly stirred to prepare a coating solution for FeOOH layer formation. By dropping 10 μL of this coating solution onto the electrode surface, an FeOOH layer was formed on the electrode surface. A gold electrode (RDE2 AUE) was used as the electrode.

[0023] (electrodeposition) A potassium carbonate aqueous solution with a concentration of 0.1 mol / L was prepared by dissolving 1 mmol / L nickel nitrate hexahydrate. The pH of the prepared potassium carbonate aqueous solution was 9.0. Each electrode, on which the FeOOH layer was formed, was immersed in the prepared potassium carbonate aqueous solution, and electrodeposition was performed for 5 minutes under conditions of 1.70 V vs RHE while rotating the electrodes at 3600 rpm using an RDE-2 rotating disk electrode apparatus (manufactured by BAS Corporation). In this way, catalysts for Examples 1 to 4 were produced.

[0024] (Electrochemical measurement) After electrodeposition, the potassium carbonate aqueous solution used for electrodeposition was replaced with a potassium carbonate aqueous solution with a pH of 9.0 and a concentration of 1.5 mol / L that did not contain dissolved nickel. Electrochemical measurements were then performed under conditions of 1.70 V vs RHE while the electrode was rotated at 3600 rpm without changing the rotation speed. A potentiostat / galvanostat HA-151B (manufactured by Hokuto Denko Co., Ltd.) was used as the measuring instrument, and LabVIEW (manufactured by National Instruments) was used as the software.

[0025] [Comparative Examples 1-4] Using each of the prepared powders, an FeOOH layer was formed on the electrode surface in the same manner as in Examples 1 to 4, and electrochemical measurements were performed in the same manner as in Examples 1 to 4 for samples where electrodeposition was not carried out. Comparative Example 1 used α-FeOOH powder, Comparative Example 2 used β-FeOOH powder, Comparative Example 3 used γ-FeOOH powder, and Comparative Example 4 used Kairo powder.

[0026] The results of the electrochemical measurements are shown in Figure 2. Figure 2(a) shows the results for Example 1 and Comparative Example 1, Figure 2(b) shows the results for Example 2 and Comparative Example 2, Figure 2(c) shows the results for Example 3 and Comparative Example 3, and Figure 2(d) shows the results for Example 4 and Comparative Example 4. In this measurement, catalytic activity was compared from the value of the current density under a constant potential. When comparing the current density values ​​for each iron oxyhydroxide particle with only the first layer of iron oxyhydroxide particles and when nickel oxyhydroxide was electrodeposited to form a second layer, the current density value was significantly higher in all cases of iron oxyhydroxide particles when the second layer was formed. This confirmed an improvement in catalytic activity. Furthermore, when comparing the current density values ​​after the formation of the second layer for each iron oxyhydroxide particle, the catalytic activity was highest in the order of β-FeOOH > γ-FeOOH > α-FeOOH. In addition, catalysts using used disposable hand warmer powder showed even higher catalytic activity. This is thought to be because, during the oxidation process of the iron powder when using the hand warmer, a small amount of carbon is incorporated into the surface or interior of the β-FeOOH particles, resulting in improved conductivity.

[0027] Figure 3 shows the results of elemental mapping by EDX on the electrode surface after electrodeposition in Example 4. The elemental analysis confirms the Ni spectrum, and the elemental mapping results show that Ni is electrodeposited throughout the FeOOH layer. Furthermore, from the mass ratio obtained by EDX measurement, the ratio (mass ratio) of iron to nickel in the catalysts obtained in Examples 1 to 4, with iron set to 1, was 0.004 to 0.022 for nickel in Examples 2 and 4, and 0.007 to 0.087 for nickel in Examples 1 and 3. [Industrial applicability]

[0028] The oxygen evolution catalyst of the present invention can be suitably used as a catalyst for oxygen evolution reactions in processes such as the electrolysis of water. The method for reusing used disposable hand warmers of the present invention allows for the reuse of used disposable hand warmers as a catalyst for oxygen evolution reactions, contributing to the effective utilization of resources.

Claims

1. A catalyst for an oxygen evolution reaction, comprising a first layer containing iron oxyhydroxide particles and a second layer provided on the surface of the first layer containing nickel hydroxide or nickel oxyhydroxide.

2. The catalyst for oxygen evolution reaction according to claim 1, wherein the iron oxyhydroxide particles are β-FeOOH particles.

3. The catalyst for oxygen generation reaction according to claim 2, wherein the iron oxyhydroxide particles are β-FeOOH particles contained in a used disposable hand warmer.

4. The catalyst for oxygen evolution reaction according to claim 1, comprising iron oxyhydroxide particles and activated carbon particles in the first layer.

5. The catalyst for oxygen evolution reaction according to any one of claims 1 to 4, wherein the second layer is formed by electrodeposition.

6. A method for producing a catalyst for an oxygen evolution reaction, comprising applying a coating solution containing iron oxyhydroxide, a conductive additive, and a binder to the surface of a substrate to form a first layer containing iron oxyhydroxide particles, and depositing nickel oxyhydroxide on the surface of the first layer by electrodeposition to form a second layer containing nickel oxyhydroxide.

7. A method for reusing a used disposable hand warmer, comprising: removing the contents containing β-FeOOH particles from the used disposable hand warmer; forming a first layer containing the β-FeOOH particles contained in the contents; and forming a second layer containing nickel hydroxide or nickel oxyhydroxide on the surface of the first layer to produce the catalyst for the oxygen generation reaction described in claim 3.

Images