Oxygen generating electrode and manufacturing method thereof, and oxygen generating electrode and method for electrolyzing seawater

Abstract

To provide an oxygen generating electrode that hardly raises an over-voltage and enables a stable operation for a long period of time even at high current density in electrolyzing sea water and a method for producing the same, and an oxygen generating electrode and a method for electrolyzing water.SOLUTION: The oxygen generating electrode for use in electrolyzing seawater comprises a catalyst on an electrode substrate, wherein the catalyst contains a composite metal oxide including at least two species of transition metal, and a double hydroxide including at least two species of metal, the transition metal included in the composite metal oxide is at least Mn, and the metal included in the double hydroxide contains at least both of divalent metal and trivalent metal.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an oxygen generating electrode and a method for producing the same, as well as an oxygen generating electrode and a method for electrolyzing water. [Background technology]

[0002] Water electrolysis (the electrolysis of water) is a promising method for producing hydrogen from water using renewable energy electricity, as it aims to solve environmental and energy resource problems. Because hydrogen production methods using water electrolysis use electricity, it is necessary to reduce production costs, and from this perspective, various water electrolysis technologies are being developed.

[0003] Electrolysis of water requires "pure water (so-called fresh water)," which can easily become scarce when electrolysis is performed in large quantities. Furthermore, because pure water conducts almost no electricity, it is sometimes necessary to dissolve highly toxic substances such as sodium hydroxide or sulfuric acid in the water to make it easier for electricity to pass through (i.e., to conduct electricity through water at a low voltage).

[0004] From this perspective, there is great promise for using seawater, which covers over 70% of the Earth's surface and is virtually inexhaustible, for electrolysis. However, when seawater is electrolyzed using conventional electrodes (platinum, iridium oxide, etc.), hydrogen gas is produced at the negative electrode (cathode), but at the positive electrode (anode), the chlorine evolution reaction (CIER) caused by the oxidation of chloride ions occurs before the oxygen evolution reaction caused by the oxidation of water. This makes it easy for chlorine gas, which is highly toxic and corrosive, to be produced. For this reason, active development is underway to develop electrode catalysts suitable for use in seawater electrolysis.

[0005] For example, Non-Patent Document 1 proposes a catalyst material containing nickel-iron double hydroxide (NiFe-LDH) as a constituent element. [Prior art documents] [Non-patent literature]

[0006] [Non-Patent Document 1] ChemSusChem 2016, 9, 962-972 Summary of the Invention [Problem to be solved by the invention]

[0007] However, the electrode catalyst composed of the nickel-iron double hydroxide disclosed in Non-Patent Document 1 is unstable in chlorine-containing solutions such as seawater, and therefore tends to cause an increase in overvoltage during electrolysis in seawater, resulting in a decrease in the performance of the OER (oxygen evolution reaction).

[0008] The present invention has been made in view of the above, and aims to provide an oxygen generating electrode that has a small overvoltage even at high current densities and can be operated stably for a long period of time during seawater electrolysis, a method for manufacturing the same, and an oxygen generating electrode and a method for electrolyzing water. [Means for solving the problem]

[0009] As a result of extensive research into achieving the above object, the inventors discovered that the above object can be achieved by using a composite material of a specific composite metal oxide and a double hydroxide as a catalyst, and thus completed the present invention.

[0010] That is, the present invention includes, for example, the subject matter described in the following sections. Item 1 An oxygen generating electrode used for seawater decomposition, A catalyst is provided on an electrode substrate, The catalyst contains a composite metal oxide containing at least two transition metals and a double hydroxide containing at least two metals, the transition metal contained in the composite metal oxide is at least Mn, The oxygen generating electrode catalyst for seawater splitting, wherein the metal contained in the double hydroxide contains at least both a divalent metal and a trivalent metal. Section 2 Item 2. The oxygen generating electrode catalyst for seawater splitting according to Item 1, wherein the metal contained in the double hydroxide is at least divalent Ni and trivalent Fe. Section 3 Item 3. The oxygen generating electrode catalyst for seawater splitting according to Item 1 or 2, wherein the catalyst has a structure in which the surface of the composite metal oxide is coated with the double hydroxide. Section 4 4. The oxygen generating electrode catalyst for seawater splitting according to claim 1, wherein the catalyst is formed in a sheet shape on the electrode substrate. Section 5 Item 5. An oxygen generating electrode comprising the electrode catalyst according to any one of items 1 to 4. Section 6 Item 6. A method for electrolyzing water, comprising the step of electrolyzing water using the oxygen generating electrode according to item 5. Section 7 A method for producing the electrode catalyst according to any one of items 1 to 4, Step 1: forming a precursor on an electrode substrate by heat-treating the electrode substrate in a solution of a first transition metal source containing at least Mn; Step 2: calcining the electrode base material on which the precursor is formed to obtain an electrode base material on which a composite oxide is formed; and a step 3 of heat-treating the electrode substrate on which the composite oxide has been formed in a solution of a second metal source to obtain an electrode catalyst; A method for producing an electrode catalyst comprising: [Effects of the Invention]

[0011] When the oxygen generating electrode catalyst for seawater decomposition of the present invention is used as an electrode for electrolyzing seawater, the overvoltage is small even at high current densities, enabling stable operation for a long period of time. [Brief explanation of the drawings]

[0012] [Figure 1] 1 is a schematic diagram illustrating an example of a method for producing an electrode catalyst according to the present invention. [Figure 2] The results of X-ray diffraction measurement (XRD) of the negative electrode active materials obtained in each of the Examples and Comparative Examples are shown. [Figure 3](a) shows the results of linear sweep voltammetry measurements, (b) shows the Tafel slope calculated from the linear sweep voltammetry curve shown in (a), and (c) shows the results of electrochemical impedance (EIS) measurements. [Figure 4] (a) shows the multi-current step chronopotentiometry curve, (b) shows the linear sweep voltammetry measurement results after 2000 cycles of testing, and (c) shows the results of a long-term operation test. [Figure 5] (a) is a multi-current step chronopotentiometry curve, and (b) is the result of a long-term operation test. DETAILED DESCRIPTION OF THE INVENTION

[0013]

[0023] In the present specification, the terms "contain" and "comprise" include the concepts of "contain," "comprise," "consist essentially of," and "consist only of."

[0014] 1.Electrode catalyst The oxygen generating electrode catalyst for seawater splitting of the present invention (hereinafter simply referred to as "the electrode catalyst of the present invention") is an oxygen generating electrode used for seawater splitting, comprising a catalyst on an electrode substrate. This catalyst contains a composite metal oxide containing at least two transition metals and a double hydroxide containing at least two metals, the transition metal contained in the composite metal oxide is at least Mn, and the metal contained in the double hydroxide contains at least both a divalent metal and a trivalent metal.

[0015] When the oxygen generating electrode catalyst for seawater decomposition of the present invention is used as an electrode for electrolyzing seawater, the overvoltage is small even at high current densities, enabling stable operation for a long period of time.

[0016] The type of electrode substrate is not particularly limited, and for example, a wide variety of known conductive substrates can be used. Examples of electrode substrates include substrates used as electrodes for water electrolysis, and specific examples include metal substrates, carbon substrates, and glass substrates.

[0017] Examples of metal substrates include substrates of simple metals such as nickel, titanium, iron, and copper, substrates of nickel-phosphorus alloys, nickel-tungsten alloys, and stainless steel alloys, and various metal foams (e.g., nickel foam and copper foam). Of these, nickel foam is preferred as the metal substrate. In this case, the catalyst can be formed from nickel derived from the substrate.

[0018] Examples of carbon substrates include carbon paper, carbon fiber paper, and carbon rods. Examples of glass substrates include conductive glass. The electrode substrate may be a porous material such as foam.

[0019] The electrode substrate is more preferably a metal substrate, more preferably a nickel substrate, and most preferably nickel foam.

[0020] The electrode substrate can be obtained, for example, by a known manufacturing method, or can be obtained from a commercial product. The shape and size of the electrode substrate are not particularly limited and can be appropriately selected depending on the intended use and required performance. For example, the shape of the electrode substrate can be foam, sheet, plate, rod, mesh, etc., and a foam shape is preferred.

[0021] As described above, the electrode catalyst of the present invention is formed on an electrode substrate and contains a composite metal oxide containing at least two transition metals and a double hydroxide containing at least two transition metals.

[0022] The composite metal oxide contains at least two transition metals, and in particular, contains at least Mn. That is, the composite metal oxide contains Mn and a transition metal other than Mn.

[0023] The composite metal oxide may contain two or more transition metals including Mn, or the composite metal oxide may contain only two transition metals.

[0024] The type of transition metal other than Mn is not particularly limited, and examples thereof include Co, Fe, Cr, Ni, Mo, W, Cu, Zn, etc. Among these, the transition metal other than Mn is preferably Co, since it has a small overvoltage even at high current densities and is easy to form an electrode that enables stable operation for a long period of time.

[0025] That is, the composite metal oxide is preferably a composite oxide of Mn and Co. An example of the composite oxide of Mn and Co is MnCo2O4.

[0026] The double hydroxide is a double hydroxide containing at least both a divalent metal and a trivalent metal. The double hydroxide may contain two or more metals. Alternatively, the double hydroxide may contain only two metals, in which case the double hydroxide contains only a divalent metal and a trivalent metal.

[0027] In the double hydroxide, a divalent metal (M 2+ ) can include Ni, Fe, Co, Zn, Mg, etc. Trivalent metals (M 3+ ) can include Fe, Mn, Co, Al, and the like.

[0028] Among these, the metal contained in the double hydroxide is at least divalent Ni (Ni 2+ ) and trivalent Fe (Fe 3+ ) is preferred.

[0029] In the double hydroxide, the content ratio of Fe and Ni is not particularly limited, and for example, the Fe:Ni mass ratio can be 1:10 to 10:1. In particular, as will be described later, when a nickel base material is used as the electrode base material, the content ratio of Fe and Ni tends to fall within the above range.

[0030] Here, the layered double hydroxide (LDH) is a compound having exchangeable anions between layers of a metal hydroxide, for example, formed in a layered structure. Specifically, the double hydroxide is represented by the general formula [M 2+ 1-x M 3+ x (OH)2](A n- ) x / n It is a hydroxide that can be expressed in mH2O.

[0031] In the electrode catalyst of the present invention, the anion of the double hydroxide (i.e., A n- The type of the gas is not particularly limited, for example, CO3 2- , O.H. - , Cl - , NO3 - The following can be mentioned:

[0032] The catalyst may contain other components as long as it contains the composite metal oxide and the double hydroxide. The catalyst preferably contains 80% by mass or more of the composite metal oxide and the double hydroxide, more preferably 90% by mass or more, and even more preferably 95% by mass or more. The catalyst may consist only of the composite metal oxide and the double hydroxide.

[0033] The content ratio of the composite metal oxide and the double hydroxide in the catalyst is not particularly limited. For example, when the composite metal oxide is contained in an amount of 10 to 90 mass% based on the total mass of the composite metal oxide and the double hydroxide, an electrode can be formed that exhibits small overvoltage even at high current densities and allows stable operation for a long period of time.

[0034] In the catalyst, the form in which the composite metal oxide and the double hydroxide are present is not particularly limited. In terms of making it easier to form an electrode that is less likely to cause an increase in overvoltage and that enables stable operation for a longer period of time even at high current densities, the catalyst preferably has a structure in which the surface of the composite metal oxide is coated with the double hydroxide.

[0035] An example of such a catalyst is one having a core-shell structure in which the composite metal oxide forms a core layer and the double hydroxide forms a shell layer. More specifically, the catalyst has a structure in which a nanowire-shaped composite metal oxide forms a core layer, and the core layer is covered with the double hydroxide to form a shell layer.

[0036] The shape of the catalyst is not particularly limited, and for example, it is preferable that the catalyst is formed in a sheet shape on the electrode base material, which allows the catalyst to adhere firmly to the electrode base material and exhibit a high catalytic effect.

[0037] When the catalyst is in a sheet form, it is preferably in a nanosheet form. When the catalyst is formed in a nanosheet form, its thickness is, for example, 200 to 500 nm. When the catalyst is formed in a sheet form, the sheet may have a porous structure. In addition, the sheet may have not only a single layer structure but also a laminated structure.

[0038] The catalyst can cover a part or the whole of the electrode substrate, and is preferably disposed in the outermost layer of the electrode catalyst.

[0039] The electrode catalyst of the present invention may be formed only from the electrode substrate and the catalyst, or may be combined with other materials as long as the effects of the present invention are not impaired. For example, the electrode catalyst may be formed directly on the electrode substrate (without any other layer or the like).

[0040] The electrode catalyst of the present invention can be suitably used as an electrode for electrolysis in seawater, particularly as an oxygen generating electrode. In particular, when used as an oxygen generating electrode for seawater splitting, the electrode catalyst of the present invention is less likely to cause an increase in overpotential during seawater electrolysis even at high current densities, can reduce the Tafel slope, and can operate stably for a long period of time.

[0041] Therefore, the electrode catalyst of the present invention is suitable for use in electrodes for various electrolysis, and is particularly suitable as an electrode for oxygen generation (oxygen generating electrode) when used as an electrode for electrolysis of seawater, since it can provide excellent oxygen generation efficiency. The seawater may be natural seawater or may be imitation seawater (e.g., an aqueous solution containing 1 M KOH and 0.5 M NaCl).

[0042] In particular, the electrode catalyst of the present invention can reduce overpotential even at high current densities, resulting in high OER selectivity and reduced hypochlorite formation. This is presumably because the double hydroxide protects the high activity of the composite oxide, preventing the generation of chloride ions. Furthermore, the double hydroxide is presumably capable of effectively increasing surface activity and charge transfer.

[0043] 2. Manufacturing method of electrode catalyst The electrode catalyst of the present invention can be produced by various methods, and is not particularly limited. For example, the method can include at least the following steps 1, 2, and 3. Step 1: A step of forming a precursor on an electrode substrate by heat treating the electrode substrate in a solution of a first transition metal source containing at least Mn. Step 2: A step of calcining the electrode base material on which the precursor is formed to obtain an electrode base material on which a composite oxide is formed. Step 3: A step of heat treating the electrode substrate on which the composite oxide has been formed in a solution of a second metal source to obtain an electrode catalyst.

[0044] (Process 1) In step 1, the electrode substrate is subjected to a heat treatment in a solution of a first transition metal source containing at least Mn, thereby forming a precursor on the electrode substrate.

[0045] The type of electrode substrate used in step 1 is not particularly limited and is the same as the electrode substrate used in the above-mentioned electrode catalyst. Therefore, examples of the electrode substrate used in step 1 include a metal substrate, a carbon substrate, a glass substrate, etc., and is preferably a metal substrate, more preferably a nickel substrate, and particularly preferably nickel foam.

[0046] In step 1, a solution of a first transition metal source containing at least Mn is used as a raw material. The first transition metal source containing Mn contains Mn and transition metal elements other than Mn as transition metals. Examples of transition metals other than Mn include Co, Fe, Cr, Ni, Mo, W, Cu, and Zn, and Co is preferred.

[0047] The first transition metal source may be a transition metal element or a compound containing a transition metal. When the first transition metal source is a compound containing a transition metal, the first transition metal source may include a compound containing Mn and a compound containing a transition metal other than Mn. Examples of compounds that can be used include inorganic acid salts of Mn, organic acid salts of Mn, hydroxides of Mn, and halides of Mn. Similarly, examples of compounds that can be used include inorganic acid salts of transition metals other than Mn, organic acid salts of transition metals other than Mn, hydroxides of transition metals other than Mn, and halides of transition metals other than Mn.

[0048] Examples of the inorganic acid salts include one or more selected from the group consisting of nitrates, hydrochlorides, sulfates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. Examples of the organic acid salts include one or more selected from the group consisting of acetates, oxalates, formates, and succinates. Compounds containing Mn or transition metals other than Mn can be obtained by known production methods, or commercially available products can be used.

[0049] The first transition metal source is preferably a nitrate, and therefore preferably includes a nitrate of Mn and a nitrate of Co.

[0050] In the solution of the first transition metal source, the solvent is, for example, water, and may also contain a lower alcohol compound. The solvent may be water alone. In the solution of the first transition metal source, the concentration of the first transition metal source is not particularly limited, and the total concentration of the transition metal per 100 mL of solvent is preferably 1 to 200 mmol, more preferably 5 to 150 mmol, and even more preferably 10 to 100 mmol.

[0051] The solution of the first transition metal source may contain other additives. Examples of the other additives include a pH adjuster. Examples of the pH adjuster include urea (CO(NH)), NHF, and ammonium hydroxide. The pH adjusters may be used alone or in combination of two or more.

[0052] When the solution of the first transition metal source contains a pH adjuster, it is preferable that 10 to 50 mmol of each pH adjuster is dissolved per 100 mL of the aqueous solvent. In this case, the solution of the first transition metal source tends to have a pH in the alkaline region, which makes it easier for the composite oxide of the transition metals formed in the first step to have a desired shape.

[0053] The solution of the first transition metal source may consist only of a compound containing a transition metal, a pH adjuster, and an aqueous solvent.

[0054] In step 1, the method for immersing the electrode substrate in the solution of the first transition metal source is not particularly limited, and can usually be carried out so that the entire electrode substrate is immersed in the solution of the first transition metal source. The immersion of the electrode substrate can be carried out, for example, in a container capable of hydrothermal synthesis, as described below. An example of such a container is a pressure-resistant autoclave. The inner surface of the autoclave can be coated with a fluororesin such as Teflon (registered trademark).

[0055] In step 1, the electrode substrate is subjected to a heat treatment while immersed in the first raw material. The heat treatment in step 1 can be a hydrothermal synthesis method. The hydrothermal synthesis method referred to here is a method in which the electrode substrate is immersed in a first transition metal source, the container is sealed, and the inside of the container is heated.

[0056] The temperature inside the vessel during hydrothermal synthesis is not particularly limited as long as it satisfies the conditions for forming the hydroxide of the transition metal M, and can be, for example, 110 to 200°C. The time for which the vessel is maintained at this temperature is also not particularly limited, and can be, for example, 2 to 24 hours. The pressure inside the vessel during hydrothermal synthesis can also be set appropriately.

[0057] A precursor is formed on the electrode substrate by the heat treatment (hydrothermal synthesis) in step 1. Such a precursor is a hydroxide of a transition metal.

[0058] (Process 2) Step 2 is a step for firing the electrode base material on which the precursor has been formed in step 1. This makes it possible to obtain an electrode base material on which the precursor has been oxidized to form a composite oxide.

[0059] In step 2, the calcination method is not particularly limited, and for example, a wide variety of known calcination methods can be used. The calcination temperature can be, for example, 300 to 400°C, and preferably 340 to 380°C. The calcination time can be appropriately selected depending on the calcination temperature, and can be, for example, 1.5 to 5 hours. The rate of temperature increase during calcination is also not particularly limited, and can be appropriately set to a level at which the desired oxide is formed.

[0060] The calcination may be carried out in air or in an inert gas atmosphere, preferably in air. For the calcination, a known heating device such as a commercially available heating furnace may be used.

[0061] By the above-mentioned calcination, the precursor (hydroxide) on the electrode substrate is converted into an oxide, and an electrode substrate modified with a composite oxide containing Mn can be obtained.

[0062] (Step 3) Step 3 is a step of heat treating the electrode substrate on which the composite oxide has been formed in step 2 in a solution of the second metal source. In step 3, the target electrode catalyst can be obtained.

[0063] In step 3, a solution of a second metal source is used as a raw material. The second metal source preferably contains a divalent metal and a trivalent metal. However, when the electrode substrate used in step 1 is a nickel substrate, Ni in the substrate serves as the metal source (Ni source). In this case, for example, the second metal source may contain only a trivalent metal. Of course, when the electrode substrate is a nickel substrate, the second metal source preferably contains a divalent metal and a trivalent metal.

[0064] In the production method of the present invention, it is preferred that the electrode substrate is a nickel substrate, and the solution of the second metal source used in step 3 contains a trivalent metal. Examples of the trivalent metal include Fe, Mn, Co, and Al, with Fe being preferred.

[0065] The second metal source may be a metal element or a compound containing a metal, and examples of the second metal source that can be used include inorganic acid salts of Fe, organic acid salts of Fe, hydroxides of Fe, and halides of Fe.

[0066] Examples of inorganic acid salts of Fe include one or more selected from the group consisting of Fe nitrate, hydrochloride, sulfate, carbonate, hydrogencarbonate, phosphate, and hydrogenphosphate. Examples of organic acid salts of Fe include one or more selected from the group consisting of Fe acetate, oxalate, formate, and succinate. The Fe-containing compound can be obtained by a known production method, or a commercially available product can be used.

[0067] The second metal source is preferably a halide of Fe.

[0068] In the solution of the second metal source, the solvent is, for example, water, and may also contain a lower alcohol compound. The solvent may be water alone. In the solution of the second metal source, the concentration of the second metal source is not particularly limited, and the total concentration of the transition metals per 100 mL of solvent is preferably 1 to 200 mmol, more preferably 3 to 150 mmol, and even more preferably 5 to 100 mmol.

[0069] The solution of the second metal source may contain other additives. Examples of other additives include a pH adjuster. Examples of the pH adjuster include NaNO3. When the raw material solution contains a pH adjuster, for example, it can lower the pH value when ferric iron is hydrolyzed. In addition, when a nickel substrate such as nickel foam is used as the electrode substrate, it can lower the pH value when the nickel is hydrolyzed. 2+ It can act as an etchant to generate ions.

[0070] When the solution of the second metal source contains a pH adjuster, it is preferable that 10 to 50 mmol of each pH adjuster is dissolved per 100 mL of the aqueous solvent. From another perspective, when the solution of the second metal source contains a pH adjuster, for example, the molar ratio of Fe to NaNO3 is preferably 1:0.1 to 1:10, and more preferably 1:0.5 to 1:2.

[0071] The solution of the second metal source may consist only of a compound containing a transition metal, a pH adjuster, and an aqueous solvent.

[0072] The heat treatment method performed in step 3 can be, for example, a so-called hydrothermal synthesis method, in which the electrode substrate is immersed in a solution of the second metal source in a container, the container is sealed, and the container is heated. This hydrothermal synthesis method forms a hydrothermal reaction product on the surface of the composite oxide formed on the electrode substrate. This hydrothermal reaction product is a double hydroxide. When a nickel substrate is used as the electrode substrate and the second metal source is Fe, the nickel in the nickel substrate participates in the reaction by hydrothermal synthesis, producing a double hydroxide of Ni derived from the nickel substrate and Fe derived from the Fe source contained in the raw material solution.

[0073] In step 3, a pressure-resistant autoclave can be used, and the inner surface of the autoclave can be coated with a fluororesin such as Teflon (registered trademark).

[0074] In step 3, the temperature inside the container during the heat treatment is not particularly limited and can be, for example, 50 to 250°C, preferably 70 to 200°C, more preferably 80 to 180°C, and even more preferably 90 to 150°C. The heating time is also not particularly limited and can be determined appropriately depending on the heating temperature, for example, 30 minutes to 5 hours. The pressure inside the container during the heat treatment can also be set appropriately.

[0075] A catalyst is formed on the electrode substrate by the heat treatment (hydrothermal synthesis) in step 3. Such a catalyst has a structure in which the surface of a composite metal oxide is coated with a double hydroxide, for example. The catalyst formed can be formed in the form of a sheet on the electrode substrate.

[0076] 3. Oxygen Evolving Electrode The electrode catalyst of the present invention is suitable as an oxygen generating electrode. The oxygen generating electrode may be composed of, for example, only the electrode catalyst of the present invention, or may be formed by combining the electrode catalyst of the present invention with other components as necessary.

[0077] Since the oxygen generating electrode comprises the electrode catalyst of the present invention, an increase in overvoltage during water electrolysis can be suppressed, seawater can be electrolyzed stably, and oxygen can be generated efficiently. Furthermore, since the oxygen generating electrode comprises the electrode catalyst of the present invention, seawater can be electrolyzed stably for a long period of time.

[0078] 4. Seawater electrolysis method The method for electrolyzing seawater of the present invention can include, for example, a step of electrolyzing seawater using the oxygen generating electrode. By such an electrolysis method, oxygen or hydrogen can be produced. In the method for electrolyzing seawater of the present invention, the oxygen generating electrode is used as an anode.

[0079] On the other hand, in the electrolysis method of the present invention, an electrode generally used as a cathode in water electrolysis can be used as the cathode, for example, a carbon rod or a platinum wire can be used, and the electrode catalyst of the present invention can also be used as the cathode.

[0080] In the electrolysis method of the present invention, the seawater may be natural seawater, or imitation seawater (for example, an aqueous solution containing 1 M KOH and 0.5 M NaCl) may be used as seawater. [Example]

[0081] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0082] Example 1 According to the scheme shown in Figure 1, a catalyst in which MnCo2O4 was coated with NiFe double hydroxide was formed on nickel foam. 2The electrode substrate was pretreated by sequentially treating 1M hydrochloric acid, ethanol, and deionized water under ultrasonic conditions for 1 hour and then drying in a vacuum oven at 60°C for 1 hour. This nickel foam was then immersed in 30 ml of distilled water containing 1 mmol of Mn(NO3)2·4H2O, 2 mmol of Co(NO3)2·6H2O, 6 mmol of NHF, and 12 mmol of urea. The nickel foam was then transferred to a Teflon-lined autoclave and heated at 120°C for 4 hours. After allowing it to cool naturally, the electrode substrate was removed, washed with ethanol and deionized water, and dried in a vacuum oven at 60°C for 12 hours (Step 1). The electrode substrate obtained in Step 1 was then calcined in air at 350°C for 2 hours (Step 2). The calcined electrode substrate was sealed in an autoclave containing 2 mmol of FeCl3·6H2O, 2 mmol of NaNO3, and 30 ml of deionized water, and heated at 120 °C for 2 hours (step 3). After this heating treatment, the electrode substrate was washed several times with ethanol and deionized water to obtain an electrode catalyst. This electrode catalyst was named "NiFe-LDH@MnCo2O4 / NF."

[0083] (Comparative Example 1) The size is 2 x 2 cm 2 The electrode substrate was pretreated by sequentially treating 1M hydrochloric acid, ethanol, and deionized water under ultrasonic conditions for 1 hour and then drying in a vacuum oven at 60 °C for 1 hour. This nickel foam was then immersed in 30 ml of distilled water containing 1 mmol of Mn(NO3)2·4H2O, 2 mmol of Co(NO3)2·6H2O, 6 mmol of NHF, and 12 mmol of urea. The nickel foam was then transferred to a Teflon-lined autoclave and heated at 120 °C for 4 hours. After natural cooling, the electrode substrate was removed, washed with ethanol and deionized water, and dried in a vacuum oven at 60 °C for 12 hours (Step 1). The electrode substrate obtained in Step 1 was then calcined in air at 350 °C for 2 hours (Step 2). The calcined electrode substrate was then washed several times with ethanol and deionized water to obtain an electrocatalyst. This electrocatalyst was named "MnCo2O4 / NF."

[0084] (Comparative Example 2) The size is 2 x 2 cm 2 The electrode substrate was pretreated by treating nickel foam (nickel foam) with 1M hydrochloric acid, ethanol, and deionized water, successively, under ultrasonic conditions for 1 hour, followed by drying in a vacuum oven at 60°C for 1 hour. This nickel foam was sealed in an autoclave containing 2 mmol of FeCl3·6H2O, 2 mmol of NaNO3, and 30 ml of deionized water, and then heat-treated at 120°C for 2 hours. After this heat treatment, the electrode substrate was washed several times with ethanol and deionized water to obtain an electrode catalyst. This electrode catalyst was named "NiFe-LDH / NF."

[0085] Figure 2 shows the results of X-ray diffraction measurements (XRD) of the catalysts obtained in the examples and comparative examples. X-ray diffraction measurements were performed using a Rigaku "SmartLab" with a Cu-Kα (λ = 1.540 Å) radiation source in the 2θ range of 10 to 100°.

[0086] The XRD pattern in Figure 2 confirmed that the catalyst formed in Example 1 contained MnCo2O4 (JCPDS No. 23-1237) and NiFe-LDH (JCPDS No. 38-0715). This indicated that the catalyst in Example 1 contained MnCo2O4 and NiFe double hydroxides. It is presumed that a NiFe-LDH layer was deposited on the surface of the MnCo2O4 nanowires. The peak intensities in Figure 2 suggest that Fe may have a lower degree of crystallinity than MnCo2O4.

[0087] Figure 3 shows SEM images and EDX elemental mapping images of the surfaces of the electrode catalysts obtained in the examples and comparative examples. Specifically, in Figure 3, (a) is an SEM image of the precursor obtained in Comparative Example 1, (b) is an SEM image of the electrode catalyst obtained in Comparative Example 2, and (c) is an SEM image of the electrode catalyst obtained in Example 1. Figures 1(d) to (i) show EDX elemental mapping images of the electrode catalyst obtained in Example 1.

[0088] The SEM image in Figure 3 shows that the electrode catalyst obtained in Example 1 has a core-shell structure and is in the form of a nanosheet, indicating that the catalyst is uniformly formed on the electrode substrate. Furthermore, the mapping image shows that each element (Mn, Co, Fe, Ni, O) is uniformly present throughout the catalyst, indicating that the catalyst formed on the electrode substrate contains MnCo2O4 and NiFe double hydroxides.

[0089] Figure 4(a) shows the results of linear sweep voltammetry measurements using the electrode catalysts obtained in Example 1 and each comparative example. In this measurement, an oxygen evolution (OER) test was performed using the electrode catalysts prepared in Example 1 and each comparative example as the cathode, a carbon rod as the anode, and an Ag / AgCl electrode as the reference electrode. The electrolyte used was an aqueous solution containing 1 M KOH and 0.5 M NaCl (imitated seawater). The scan rate was 2 mV / s. In this example, a standard three-electrode cell was used to evaluate electrical characteristics, such as linear sweep voltammetry curves, using a VersaSTAT4 potentiostat galvanostat electrochemical workstation (USA).

[0090] Figure 4(b) shows the Tafel slope calculated from the linear sweep voltammetry curve shown in (a). Figure 4(c) shows the electrochemical impedance (EIS) measurement results for the electrode catalysts obtained in Example 1 and each comparative example. This measurement was performed in the simulated seawater by electrochemical impedance spectroscopy (EIS) using a three-electrode electrochemical measurement device. The measurement frequency range was 0.01 Hz to 0.1 MHz, and the measurement voltage was -0.35 V vs. Ag / AgCl. The electrode / electrolyte interfacial resistance can be determined from Figure 4(c).

[0091] Table 1 shows the 10 mA cm of each electrode catalyst derived based on the results of Figures 4(a), (b), and (c). -2 , 100mAcm -2 and 500mAcm -2Overpotential, Tafel slope and charge transfer resistance (R ct ) results are shown.

[0092] [Table 1]

[0093] From the above results, it can be seen that the electrode catalyst obtained in Example 1 can be used at a high current density (500 mAcm -2 ), the overpotential was low, the Tafel slope was also the best, and the charge transfer resistance was also the lowest. Therefore, the electrode catalyst obtained in Example 1 exhibits a good catalytic reaction rate even in seawater, and is also advantageous in improving electronic conductivity.

[0094] FIG. 5(a) shows a multi-current step chronopotentiometry curve when the electrode catalyst obtained in Example 1 was used as the anode, and shows the results at a current density of 100 mA / cm 2 ~1000mA / cm 2 100mA / cm for 600 seconds 2 Measurements were taken at intervals (using the above-mentioned simulated seawater as the electrolyte), and finally 100 mA / cm 2 Figure 5(b) shows the potential-time graph obtained by returning the 2 and 1000mA / cm 2 The results are shown for electrolysis continued for 50 hours at a current density of 1000 kJ / s. The measurement conditions were the same as those for the tests to obtain the linear sweep voltammetry curves, and the measurement was performed using a VersaSTAT4 potentiostat galvanostat electrochemical workstation (USA) together with a two-electrode cell.

[0095] From the results in FIG. 5, the electrode catalyst obtained in this example exhibited excellent stability for 100 hours without catalyst dissolution (peel-off), and was able to withstand high current densities (1000 mA / cm 2) showed no significant change in potential. Furthermore, the electrode exhibited excellent stability in simulated seawater for 50 hours without corrosion at high current densities. Therefore, the electrode catalyst obtained in Example 1 was shown to be suitable for an oxygen generating electrode capable of stable operation for a long period of time even at high current densities and capable of efficient seawater electrolysis.

[0096] In particular, the low overpotential of the MnCo2O4 nanowires results in higher OER selectivity than ClER, and hypochlorite formation is also suppressed. This is likely due to the NiFe-LDH protecting the high activity of the MnCo2O4 nanowires, preventing the generation of chloride ions. Furthermore, the NiFe-LDH is believed to effectively increase surface activity and charge transfer.

Claims

1. An oxygen generating electrode used for seawater splitting and for suppressing the formation of hypochlorite, A catalyst is provided on an electrode substrate, The catalyst is MnCo ２ O ４ and a double hydroxide containing divalent Ni and trivalent Fe.

2. The catalyst is MnCo ２ O ４ 2. The oxygen generating electrode catalyst for seawater splitting according to claim 1, having a structure in which the surface of said electrode is covered with said double hydroxide.

3. 3. The oxygen generating electrode catalyst for seawater splitting according to claim 1, wherein the catalyst is formed in a sheet shape on the electrode substrate.

4. An oxygen generating electrode comprising the electrode catalyst according to any one of claims 1 to 3.

5. A method for electrolyzing seawater, comprising the step of electrolyzing seawater using the oxygen generating electrode according to claim 4.

6. A method for producing the electrode catalyst according to any one of claims 1 to 3, comprising: Step 1: forming a precursor on a nickel substrate by heat treating the nickel substrate in a solution of a Co source containing at least Mn; Step 2: calcining the nickel base material on which the precursor is formed to obtain a nickel base material on which a composite oxide is formed; and Step 3: heat-treating the nickel base material on which the composite oxide has been formed in a solution of an Fe source to obtain an electrode catalyst; A method for producing an electrode catalyst comprising:

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