Oxygen generating electrode, method for electrolyzing water, and method for manufacturing oxygen generating electrode

Abstract

To provide an oxygen generating electrode which can suppress an overvoltage rise during electrolysis and can also reduce a tafel gradient, a method for electrolysis of water using this oxygen generating electrode, and a method for manufacturing the oxygen generating electrode.SOLUTION: An oxygen generating electrode of the present invention is equipped with an electrode substrate and a catalyst, the catalyst being formed on the electrode substrate, the catalyst containing a complex oxide containing both Fe and a transition metal element M other than Fe, the complex oxide being doped with boron. The use of the oxygen generating electrode of the present invention as an electrode for water electrolysis makes it possible to suppress an overvoltage rise during electrolysis and to reduce the tafel gradient.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an oxygen generating electrode. very The present invention relates to a method for electrolyzing water and a method for producing an oxygen generating electrode. [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] In water electrolysis, the overpotential of the oxygen evolution reaction at the positive electrode is larger than that of the hydrogen evolution reaction at the negative electrode, which becomes the rate-limiting factor for the entire water electrolysis reaction. Therefore, there is a demand for the development of a highly active oxygen evolution electrode. For example, Non-Patent Document 1 proposes a core-shell CuOx nanowire@NiMnOx nanosheet electrocatalyst, which is produced by electrochemically depositing highly defective NiMn oxide nanosheets on the surface of Cu(OH)2 nanowires, followed by a heat treatment process. Such an electrocatalyst is believed to be able to suppress the increase in overpotential during water electrolysis and to lower the Tafel slope, thereby providing efficient water electrolysis. [Prior art documents] [Non-patent literature]

[0004] [Non-Patent Document 1] Journal of Materials Chemistry A,2020,8,16463-16476 Summary of the Invention [Problem to be solved by the invention]

[0005] In recent years, there has been a strong demand for an oxygen generating electrode that can further suppress the increase in overpotential during electrolysis and further reduce the Tafel slope in water electrolysis. From this perspective, it can be said that the development of an oxygen generating electrode that can effectively suppress the increase in overpotential during electrolysis and further reduce the Tafel slope is extremely important from the viewpoint of further improving the efficiency of water electrolysis and, ultimately, effective use of renewable energy.

[0006] The present invention has been made in view of the above, and aims to provide an oxygen generating electrode that can suppress an increase in overpotential during electrolysis and can also reduce the Tafel slope. Another aim of the present invention is to provide a method for electrolyzing water using the oxygen generating electrode, and a method for manufacturing the oxygen generating electrode. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above object, the inventors have discovered that the above object can be achieved by using a boron-doped composite oxide as an essential component of the catalyst, and have thus completed the present invention.

[0008] That is, the present invention includes, for example, the subject matter described in the following sections. Item 1 An oxygen generating electrode comprising an electrode substrate and a catalyst, the catalyst is formed on the electrode substrate, The catalyst contains a composite oxide containing both Fe and a transition metal element M other than Fe, The oxygen generating electrode, wherein the composite oxide is doped with boron. Section 2 Item 2. The oxygen generating electrode according to Item 1, wherein the transition metal element M is at least one selected from the group consisting of Mn, Cu, Ni, Co, Zn, and Cr. Section 3 Item 3. The oxygen generating electrode according to Item 2, wherein the transition metal element M includes Mn. Section 4 Item 4. A method for electrolyzing water, comprising a step of electrolyzing water using the oxygen generating electrode according to any one of Items 1 to 3. Section 5 A method for producing the oxygen generating electrode according to any one of items 1 to 3, Step 1 of forming a metal organic framework containing both Fe and a transition metal element M other than Fe on an electrode substrate; a step 2 of calcining the metal organic framework to form a composite oxide; Step 3 of contacting the composite oxide with a boron compound to obtain an oxygen generating electrode; A method for manufacturing an oxygen generating electrode, comprising: [Effects of the Invention]

[0009] When the oxygen generating electrode of the present invention is used as an electrode for water electrolysis, it is possible to suppress an increase in overvoltage during electrolysis and also to reduce the Tafel slope. [Brief explanation of the drawings]

[0010] [Figure 1] (a) and (b) are SEM images of the composite oxide on the nickel foam obtained in step 2 of Example 1, and (c) and (d) are SEM images of the composite oxide on the nickel foam obtained in step 3 of Example 1. [Figure 2] FIG. 1 is a diagram illustrating a synthesis procedure for the oxygen generating electrode of Example 1. [Figure 3] (a) shows the results of linear sweep voltammetry measurements, and (b) shows the results of Tafel slope measurements calculated from the linear sweep voltammetry curve shown in (a). [Figure 4] (a) is a graph showing the results of overpotential (mV) measurements at 50 mA / cm2 and 100 mA / cm2, and (b) is the results of electrochemical impedance (EIS) measurements. [Figure 5] (a) is a multi-current step chronopotentiometry curve, and (b) is the result of a long-term operation test. [Figure 6]1 shows the results of linear sweep voltammetry measurements using oxygen generating electrodes of Example 1, Example 5, and Comparative Example 3. DETAILED DESCRIPTION OF THE INVENTION

[0011]

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

[0012] 1. Oxygen Evolving Electrode The oxygen generating electrode of the present invention includes an electrode substrate and a catalyst. The catalyst is formed on the electrode substrate, and the catalyst contains a composite oxide containing both Fe and a transition metal element M other than Fe, and the composite oxide is doped with boron.

[0013] The oxygen generating electrode of the present invention having the above-described configuration can suppress an increase in overpotential during electrolysis and reduce the Tafel slope when used as an electrode for water electrolysis. Therefore, the oxygen generating electrode of the present invention is suitable as an oxygen generating electrode for water electrolysis.

[0014] (electrode base material) 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.

[0015] Examples of metal substrates include substrates of simple metals such as nickel, titanium, iron, and copper, as well as substrates of nickel-phosphorus alloys, nickel-tungsten alloys, and stainless steel alloys, and various metal foams (e.g., nickel foam, copper foam).

[0016] Examples of the carbon substrate include carbon paper, carbon fiber paper, and carbon rods. Examples of the glass substrate include conductive glass. The electrode substrate may be a porous material such as foam.

[0017] The electrode substrate is more preferably a metal substrate, more preferably a nickel substrate, even more preferably nickel foam or copper foam, and particularly preferably nickel foam.

[0018] 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.

[0019] (catalyst) In the oxygen generating electrode of the present invention, the catalyst contains a composite oxide. Such a composite oxide contains both Fe and a transition metal element M other than Fe, and the composite oxide is doped with boron. In the composite oxide, Fe may be, for example, trivalent, and the transition metal element M may be, for example, divalent.

[0020] The transition metal element M is preferably at least one selected from the group consisting of Mn, Cu, Ni, Co, Zn, and Cr, for example. In this case, when the obtained oxygen generating electrode is used as an electrode for water electrolysis, the increase in overpotential during electrolysis can be further suppressed and the Tafel slope can be further reduced.

[0021] The transition metal element M more preferably contains Mn, and even more preferably is Mn. That is, the composite oxide is particularly preferably a composite oxide of Fe and Mn (MnFeO). In this case, when used as an electrode for water electrolysis, the increase in overpotential during electrolysis can be particularly suppressed, and the Tafel slope can also be particularly reduced.

[0022] In the composite oxide, there is no particular limitation on the content ratio of Fe and the transition metal element M. For example, the molar ratio of the transition metal element M to Fe (transition metal element M:Fe) can be 1:10 to 10:10, preferably 1.5:10 to 9.5:10, and more preferably 2:10 to 8:10.

[0023] The composite oxide is doped with boron. Specifically, boron can substitute for some oxygen atoms of the metal oxide in the composite oxide and coordinate to the metal.

[0024] In the composite oxide, the content of boron is not particularly limited, and is preferably 1 to 15 mol %, and more preferably 5 to 10 mol %, based on the total amount of the composite oxide, for example.

[0025] In the oxygen generating electrode of the present invention, the catalyst may contain components other than the boron-doped composite oxide, or may be formed solely from the boron-doped composite oxide, as long as the effects of the present invention are not impaired. The catalyst preferably contains 80 mass % or more of the boron-doped composite oxide, more preferably 90 mass % or more, and particularly preferably 99 mass % or more.

[0026] (Oxygen generating electrode) In the oxygen generating electrode of the present invention, the catalyst is formed on the electrode substrate. The shape of the catalyst is not particularly limited and may be the same as that of catalysts in known electrode catalysts. For example, the catalyst is formed in the form of nanowires on the electrode substrate. The catalyst may be formed in the form of multiple nanowires on the electrode substrate. When the catalyst is formed in the form of nanowires, for example, the total length of the nanowires is 1 to 10 μm (preferably 2 to 8 μm), and the wire width is 100 to 500 nm.

[0027] The catalyst can cover part or all of the electrode substrate. Preferably, the catalyst is disposed in the outermost layer of the electrode catalyst. For example, the catalyst may be formed directly on the electrode substrate (without any other layer interposed therebetween).

[0028] The oxygen generating electrode 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. Examples of other materials include known materials used in conventional oxygen generating electrodes.

[0029] The electrode catalyst of the present invention, which includes a catalyst containing the boron-doped composite oxide as an essential component, can be suitably used as an electrode for water electrolysis, particularly as an oxygen generating electrode. In particular, when used as an electrode for water electrolysis, the oxygen generating electrode of the present invention is less likely to experience an increase in overvoltage during water electrolysis, can reduce the Tafel slope, and can operate stably for a long period of time.

[0030] 2. Manufacturing method of oxygen generating electrode The oxygen generating electrode of the present invention can be produced by various methods, and is not particularly limited. For example, the oxygen generating electrode of the present invention can be produced by a method including at least the following steps 1, 2, and 3. Step 1: A step of forming a metal organic framework containing both Fe and a transition metal element M other than Fe on an electrode substrate. Step 2: A step of firing the metal organic framework to produce a composite oxide. Step 3: The composite oxide is brought into contact with a boron compound to obtain an oxygen generating electrode.

[0031] (Process 1) Step 1 is a step for forming a metal organic framework (MOF) containing both Fe and a transition metal element other than Fe, M, on an electrode substrate. Metal organic frameworks (MOFs) are known as three-dimensional microporous materials with a porous coordination network structure that has a surface area far exceeding that of activated carbon or zeolite, due to the interaction between metal and organic ligands.

[0032] The type of electrode substrate used in step 1 is not particularly limited and is the same as the electrode substrate used in the oxygen generating electrode described above. 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.

[0033] In step 1, the method for forming the metal organic framework on the electrode substrate is not particularly limited, and for example, a method similar to a known method for forming a metal organic framework can be adopted. For example, by using an Fe source, a transition metal element M source, and an organic ligand as raw materials, the metal organic framework can be formed on the electrode substrate.

[0034] Examples of the Fe source include elemental Fe and compounds containing Fe, and Fe-containing compounds are preferred. Examples of Fe-containing compounds that can be widely used include inorganic acid salts of Fe, organic acid salts of Fe, hydroxides of Fe, and halides of Fe.

[0035] A wide variety of known compounds can be used as inorganic acid salts of Fe, including, for example, one or more selected from the group consisting of Fe nitrates, hydrochlorides, sulfates, carbonates, hydrogencarbonates, phosphates, hydrogenphosphates, etc. A wide variety of known compounds can be used as organic acid salts of Fe, including, for example, one or more selected from the group consisting of Fe acetates, oxalates, formates, succinates, etc.

[0036] The Fe source is preferably an inorganic acid salt of Fe, and more preferably a sulfate of Fe. For example, an example of the Fe source is ferrous sulfate. One Fe source may be used alone, or two or more Fe sources may be used in combination. The Fe-containing compound can be obtained by a known production method, or a commercially available Fe-containing compound can be used.

[0037] Examples of the source of the transition metal element M include the simple substance of the transition metal element M or a compound containing the transition metal element M, and the compound containing the transition metal element M is preferred.

[0038] The transition metal element M is preferably at least one selected from the group consisting of Mn, Cu, Ni, Co, Zn, and Cr, more preferably contains Mn, and further preferably is Mn.

[0039] As the compound containing the transition metal element M, a wide variety of salts of inorganic acid salts of the transition metal element M, salts of organic acid salts of the transition metal element M, hydroxides of the transition metal element M, halides of the transition metal element M, etc. can be used.

[0040] A wide variety of known compounds can be used as the inorganic acid salt of the transition metal element M, and examples thereof include one or more selected from the group consisting of nitrates, hydrochlorides, sulfates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates of the transition metal element M. A wide variety of known compounds can be used as the organic acid salt of the transition metal element M, and examples thereof include one or more selected from the group consisting of acetates, oxalates, formates, succinates, and the like of the transition metal element M.

[0041] The source of the transition metal element M is preferably an inorganic acid salt of the transition metal element M, and more preferably a sulfate of the transition metal element M. For example, an example of the source of the transition metal element M is manganese(II) sulfate. The source of the transition metal element M may be used alone or in combination of two or more. The compound containing the transition metal element M can be obtained by a known production method, or a commercially available compound containing the transition metal element M can also be used.

[0042] The type of organic ligand is not particularly limited, and a wide variety of organic ligands that can be coordinated to transition metals can be used, including, for example, aromatic carboxylic acid compounds, imidazole compounds, and amino compounds that are known to function as ligands.

[0043] Specific organic ligands include p-benzenedicarboxylic acid (H2BDC), o-benzenedicarboxylic acid, m-benzenedicarboxylic acid, 2,5-dihydroxyterephthalic acid (H4DOBDC), 1,3,5-benzenetricarboxylic acid (H3BTC), 1,4-benzenedicarboxylate, 2,4,6-tris(4-carboxyphenyl)-1,3,5-triazine (H3TATB), 2-aminoterephthalic acid (NH2BDC), 2-methylimidazole (2-MIM), 1- Examples include methylimidazole (1-MIM), 1,4-bis(imidazol-1-yl)benzene (1,4-BIB), 4-(imidazol-1-yl)phthalic acid (H2IPC), 4,4'-dimethyl-2,2'-bipyridyl, 4,4'-oxybisbenzoic acid, fumaric acid, oxalic acid, succinic acid, biphenyl-3,4',5-tricarboxylic acid (BPTC), 4,4'-biphenyldicarboxylate (BPDC), and 2,5-dioxide terephthalate (DOT).

[0044] There are no particular limitations on the method for forming a metal-organic framework on an electrode substrate using an Fe source, a transition metal element M source, and an organic ligand as raw materials. A method for forming a metal-organic framework by hydrothermal synthesis is preferred because it allows for easy formation of a metal-organic framework. Specifically, the electrode substrate can be immersed in a raw material solution containing an Fe source, a transition metal element M source, an organic ligand, and a solvent, followed by heat treatment, to form a metal-organic framework on the electrode substrate.

[0045] The heat treatment can be carried out, for example, by immersing the electrode substrate in a container in the raw material solution, sealing the container, and then heating the container. Examples of the solvent contained in the raw material solution include dimethylformamide, water, and dimethyl sulfoxide.

[0046] 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, even more preferably 90 to 150°C, and particularly preferably 130 to 160°C. The heating time is also not particularly limited and can be appropriately determined depending on the heating temperature, for example, 1 to 25 hours. The pressure inside the container during the heat treatment can also be appropriately set.

[0047] The concentrations of the raw materials in the raw material solution are not particularly limited. For example, the concentrations of the Fe source and the transition metal element M source in the raw material solution can be 0.01 to 100 mM (preferably 0.05 to 50 mM, more preferably 0.1 to 30 mM). The concentration of the organic ligand in the raw material solution can be 10 to 2000 mM (preferably 20 to 1500 mM, more preferably 50 to 1000 mM).

[0048] The heat treatment causes the Fe source, the transition metal element M source, and the organic ligand to react with each other, forming a metal organic framework on the electrode substrate.

[0049] The metal organic framework formed in step 1 can be, for example, a wide variety of known metal organic frameworks, as long as it contains Fe and a transition metal element M. A specific example of such a metal organic framework is MOF-74, which contains Mn and Fe.

[0050] (Process 2) In step 2, the electrode base material having the metal organic framework formed thereon obtained in step 1 is subjected to a firing treatment, whereby the metal organic framework on the electrode base material is oxidized to produce a composite oxide.

[0051] In step 2, the method of calcination is not particularly limited, and a wide variety of known calcination methods can be employed. For example, the calcination temperature can be 180°C or higher. The calcination temperature is preferably 600°C or lower. A preferred calcination temperature is 200 to 400°C, and a more preferred calcination temperature is 220 to 350°C. The calcination time can be appropriately selected depending on the calcination temperature, and can be, for example, 1.5 to 5 hours. The temperature rise rate during calcination in step 2 is also not particularly limited, and can be appropriately set, for example, 1 to 10°C / min.

[0052] The calcination treatment may be carried out in air or in an inert gas atmosphere. Preferably, the calcination treatment is carried out in air. For the calcination treatment, a known heating device such as a commercially available heating furnace can be used.

[0053] The calcination treatment in step 2 oxidizes the metal-organic framework and decomposes the organic ligands, forming a composite oxide of Fe and the transition metal element M.

[0054] (Step 3) Step 3 is a step for contacting the composite oxide produced in step 2 with a boron compound, thereby forming a boron-doped composite oxide.

[0055] The type of boron compound is not particularly limited as long as it can provide boron to the composite oxide. The boron compound preferably has a reducing action, and examples thereof include sodium borohydride (NaBH), KBH, and LiBH, with NaBH being preferred.

[0056] In step 3, the method for contacting the composite oxide with the boron compound is not particularly limited. For example, the composite oxide can be contacted with the boron compound by immersing the electrode substrate on which the composite oxide obtained in step 2 is formed in an aqueous solution of the boron compound. In this case, the concentration of the aqueous solution of the boron compound can be, for example, 0.5 to 2 mol / L. The temperature for the immersion treatment can be, for example, 10 to 40°C, and preferably 20 to 35°C. The time for the immersion treatment is, for example, preferably 10 to 60 minutes, and more preferably 15 to 30 minutes.

[0057] In step 3, the composite oxide is brought into contact with the boron compound to form a boron-doped composite oxide, i.e., a catalyst, on the electrode substrate. The electrode substrate on which such a catalyst is formed can be used as an oxygen generating electrode.

[0058] 3.Water electrolysis method The water electrolysis method of the present invention can include, for example, a step of electrolyzing water using the oxygen generating electrode. By such a water electrolysis method, oxygen or hydrogen can be produced. In the water electrolysis method, the oxygen generating electrode is used as an anode.

[0059] On the other hand, in the water 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.

[0060] In the water electrolysis method of the present invention, the aqueous solution used in the electrolysis can be an aqueous solution containing components commonly used in water electrolysis. The aqueous solution can also contain halogens such as iodine and bromine, sulfate ions, etc. When an aqueous solution containing iodine is used, iodate ions are generated at the anode. The aqueous solution may be in the acidic, neutral, or alkaline range. For example, in the alkaline range, aqueous solutions of KOH, NaOH, etc. can be used; in the acidic range, aqueous solutions of hydrochloric acid, sulfuric acid, etc. can be used; and in the neutral range, PBS (phosphate buffered saline), etc. can be used. [Example]

[0061] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0062] Example 1 The size is 2 x 2 cm 2 The nickel foam (nickel foam) was treated with 1M hydrochloric acid, ethanol, and deionized water, successively, under ultrasonic conditions for 1 hour, and then dried in a vacuum oven at 60°C for 12 hours to pretreat the electrode substrate. Separately, a raw material solution was prepared by dispersing 0.049 g of 2,5-dihydroxyterephthalic acid, 0.03 mmol of manganese(II) sulfate, and 0.1 mmol of ferrous sulfate in 25 mL of DMF solution. The resulting raw material solution and the pretreated nickel foam were then transferred to a Teflon-lined autoclave. The nickel foam was immersed in the raw material solution, the autoclave was sealed, and the reaction was carried out at 140°C for 20 hours. This resulted in the production of a metal-organic framework (hereinafter abbreviated as "MnFe-MOF-74") on the nickel foam (Step 1).

[0063] Next, the nickel foam on which MnFe-MOF-74 was formed was calcined at 250°C for 2 hours to form a composite oxide of Fe and Mn on the nickel foam (step 2). The nickel foam on which this composite oxide was formed was immersed in a 1 M NaBH4 solution for 20 minutes, then rinsed with distilled water and dried overnight in an oven to obtain an oxygen generating electrode containing a boron-doped composite oxide (boron content: approximately 6 mol%) as a catalyst. The obtained oxygen generating electrode was designated "3 / 10MnFe oxide B20."

[0064] Example 2 An oxygen generating electrode was obtained in the same manner as in Example 1, except that the amount of manganese (II) sulfate used was changed to 0.01 mmol to prepare the raw material solution. The obtained oxygen generating electrode was referred to as "1 / 10MnFe oxide B20."

[0065] Example 3 An oxygen generating electrode was obtained in the same manner as in Example 1, except that the amount of manganese (II) sulfate used was changed to 0.02 mmol to prepare the raw material solution. The obtained oxygen generating electrode was referred to as "2 / 10MnFe oxide B20."

[0066] Example 4 An oxygen generating electrode was obtained in the same manner as in Example 1, except that the amount of manganese (II) sulfate used was changed to 0.04 mmol to prepare the raw material solution. The obtained oxygen generating electrode was referred to as "4 / 10MnFe oxide B20."

[0067] Example 5 An oxygen generating electrode containing a boron-doped composite oxide (boron content: approximately 9 mol%) as a catalyst was obtained in the same manner as in Example 1, except that the immersion time in the NaBH solution was changed to 40 minutes. The obtained oxygen generating electrode was designated "3 / 10MnFe oxide B40."

[0068] (Comparative Example 1) An oxygen generating electrode was obtained in the same manner as in Example 1, except that the amount of manganese (II) sulfate used was changed to 0 mmol to prepare the raw material solution. The obtained oxygen generating electrode was referred to as "Fe oxide B20."

[0069] (Comparative Example 2) An oxygen generating electrode was obtained in the same manner as in Example 1, except that the amount of Fe sulfate (III) used was changed to 0 mmol to prepare the raw material solution. The obtained oxygen generating electrode was referred to as "Mn oxide B20."

[0070] (Comparative Example 3) An oxygen generating electrode was obtained in the same manner as in Example 1, except that the nickel foam on which the composite oxide was formed was not treated with a 1 M NaBH solution. The obtained oxygen generating electrode was referred to as "3 / 10 MnFe oxide."

[0071] 1(a) and (b) are SEM images of the composite oxide on the nickel foam obtained in step 2 of Example 1 (i.e., before treatment with a boron compound) ((b) is an enlarged image of the dashed line area in (a)), and (c) and (d) are SEM images of the composite oxide on the nickel foam obtained in step 3 of Example 1 (i.e., after treatment with a boron compound) ((d) is an enlarged image of the dashed line area in (c)).

[0072] Figures 1(a) and (b) confirm that the composite oxide on the nickel foam obtained in step 2 formed a uniform nanowire array grown directly on the nickel foam. These nanowires were several micrometers long and approximately 200 nm wide.

[0073] Figures 1(c) and (d) show that the oxygen generating electrode obtained in Example 1 has a uniform nanowire array grown directly on the nickel foam. Abundant nanoflakes grow on the surface of these nanowires, and the boron compound treatment increases the nanowire radius. Therefore, the nanowire-nanoflake hierarchical structure in the oxygen generating electrode obtained in Example 1 can provide a large active surface area and expose more active sites. This promotes ion migration in the electrolyte, which is expected to improve the catalytic performance of the OER process.

[0074] Figure 2 is a diagram illustrating the synthesis procedure for the oxygen evolving electrode of Example 1. Considering the results of the SEM images in Figure 1, it is presumed that nanowire-like composite oxides are formed on the nickel foam in steps 1 and 2, and that nanowires grow further by treatment with a boron compound in step 3.

[0075] Figure 3(a) shows the results of linear sweep voltammetry measurements using the electrode catalysts obtained in the Examples and Comparative Examples. In these measurements, oxygen evolution (OER) tests were performed using the oxygen evolution electrodes prepared in the Examples and Comparative Examples as the cathode, a carbon rod as the anode, and an Ag / AgCl electrode as the reference electrode. The electrolyte used was a 1 M KOH aqueous solution (pH = 14). In these examples, a standard three-electrode cell was used to evaluate electrical characteristics, such as linear sweep voltammetry curves, using a VersaSTAT4 potentiostat galvanostat electrochemical workstation manufactured by the United States.

[0076] FIG. 3(b) shows the Tafel slope calculated from the linear sweep voltammetry curve shown in (a).

[0077] Figure 4(a) shows the current at 50 mA / cm 2 and 100mA / cm 2 The graph shows the overvoltage (mV) at 50 mA / cm 2 , and the right is 100mA / cm2 4(b) shows the electrochemical impedance (EIS) measurement results for the oxygen evolving electrodes obtained in the examples and comparative examples. The measurements were performed in a 1 M KOH solution by electrochemical impedance spectroscopy (EIS) using a three-electrode electrochemical measurement apparatus. The measurement frequency range was 0.01 Hz to 0.1 MHz, and the measurement voltage was -0.35 V vs. Ag / AgCl. The interfacial resistance of the electrode / electrolyte can be determined from FIG. 4(b).

[0078] Table 1 shows the 100 mA / cm of each oxygen generating electrode derived from the results of Figures 3 and 4. 2 Overpotential, Tafel slope and charge transfer resistance (R ct ) results are shown.

[0079] [Table 1]

[0080] From the above results, it can be seen that the electrode catalyst obtained in the examples can be used at high current densities (100 mAcm -2 ), the overpotential was low, the Tafel slope showed good performance, and the charge transfer resistance was also small. In particular, the oxygen evolution electrode of Example 3 showed the best performance among the Examples. Therefore, the electrode catalyst obtained in Example 1 exhibits a good catalytic reaction rate and is advantageous for improving electronic conductivity. In addition, the small charge transfer resistance leads to a high electron transfer rate, which is beneficial for the dynamic behavior of the OER at the electrode catalyst / electrolyte interface.

[0081] FIG. 5(a) shows a multi-current step chronopotentiometry curve when the oxygen generating electrode obtained in Example 1 was used as the anode, and shows the current density of 50 mA / cm 2 ~500mA / cm 2 Up to 50mA / cm 2 Measurements were taken at intervals (the electrolyte was a 1M KOH solution), and finally at 50mA / cm 2Figure 5(b) shows the potential-time graph obtained by returning the 2 The results are shown for electrolysis continued for 7 days 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.

[0082] From the results in FIG. 5, the oxygen generating electrode obtained in Example 1 exhibited excellent stability over a period of 7 days, and was able to withstand high current densities (100 mA / cm 2 ) showed no clear change in potential. Therefore, it was demonstrated that the electrode catalyst obtained in Example 1 can be operated stably for a long period of time even at a high current density, has excellent mechanical robustness, and is suitable for an oxygen generating electrode capable of efficiently performing water electrolysis.

[0083] 6 shows the results of linear sweep voltammetry measurements (measurement conditions were the same as those in FIG. 5) using the oxygen generating electrodes of Example 1, Example 5, and Comparative Example 3. It was found that both Examples 1 and 5 exhibited excellent performance, and it was also found that the amount of boron doping contributed to the electrode performance.

Claims

1. An oxygen generating electrode comprising an electrode substrate and a catalyst, the catalyst is formed on the electrode substrate, The catalyst contains a composite oxide containing both Fe and Mn, The oxygen generating electrode, wherein the composite oxide is doped with boron.

2. A method for electrolyzing water, comprising the step of electrolyzing water using the oxygen generating electrode according to claim 1.

3. 2. A method for producing the oxygen generating electrode according to claim 1, comprising the steps of: Step 1: forming a metal organic framework containing both Fe and Mn on an electrode substrate; Step 2: calcining the metal organic framework to produce a composite oxide; Step 3: contacting the composite oxide with a boron compound to obtain an oxygen generating electrode; A method for manufacturing an oxygen generating electrode, comprising:

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