Electrocatalyst, oxygen generating electrode, and method for electrolyzing water

Abstract

To provide an electrode catalyst that is less likely to cause an increase in overvoltage during water electrolysis and can stably operate for an extended period of time even at high current densities, and also to provide an oxygen generating electrode equipped with the electrode catalyst and a water electrolysis method.SOLUTION: An electrode catalyst according to the present invention is an electrode catalyst with a catalyst on an electrode substrate, the catalyst containing a nickel sulfide and a double hydroxide of nickel and cobalt. An oxygen generating electrode according to the present invention includes the electrode catalyst. A water electrolysis method according to the present invention includes a step of using the oxygen generating electrode to electrolyze water. The electrode catalyst according to the present invention, when used as an electrode for water electrolysis, is less likely to cause an increase in overvoltage during water electrolysis and can stably operate for an extended period of time even at high current densities.SELECTED DRAWING: None

Description

[Technical Field]

[0001] The present invention relates to an electrode catalyst, an oxygen generating electrode including the electrode catalyst, 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. Among these, alkaline water electrolysis and polymer electrolyte membrane (PEM) water electrolysis are at the practical stage. PEM water electrolysis has a high current density, making it possible to reduce the size of the system compared to alkaline water electrolysis.

[0003] On the other hand, alkaline water electrolysis can achieve low costs because it has a simple structure and can use inexpensive materials. In water electrolysis, the overvoltage of the oxygen evolution reaction at the positive electrode is larger than that of the hydrogen evolution reaction at the negative electrode, which determines the rate of the entire water electrolysis reaction. Therefore, there is a need to develop a highly active catalyst for the positive electrode. For example, Patent Document 1 proposes an electrode catalyst that uses rare metals such as ruthenium and iridium hydroxide. [Prior art documents] [Patent documents]

[0004] [Patent Document 1] Patent Publication No. 2021-95618 Summary of the Invention [Problem to be solved by the invention]

[0005] However, electrodes using rare metals, as in the technology described in Patent Document 1, are very expensive. In this regard, it is conceivable to use catalysts containing inexpensive metals in electrodes. However, such catalysts tend to become unstable at high current densities during electrolysis, cause an increase in overvoltage, and are prone to poor durability, posing a problem in terms of long-term operation. From this perspective, there has been a strong demand for the development of an electrode catalyst that is less likely to cause an increase in overvoltage during water electrolysis and that can be operated stably for long periods even at high current densities, even when formed from inexpensive metals.

[0006] The present invention has been made in view of the above, and aims to provide an electrode catalyst that is less likely to cause an increase in overvoltage during water electrolysis and that can be operated stably for a long period of time even at a high current density, an oxygen generating electrode including the electrode catalyst, and a method for electrolyzing water. [Means for solving the problem]

[0007] As a result of extensive research into achieving the above object, the inventors discovered that the above object can be achieved by forming a catalyst from nickel sulfide and a nickel-cobalt double hydroxide, and 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 electrode catalyst comprising a catalyst on an electrode substrate, The catalyst is an electrode catalyst containing nickel sulfide and a double hydroxide of nickel and cobalt. Section 2 Item 1. The electrode catalyst according to Item 1, wherein the electrode substrate is a nickel substrate. Section 3 Item 3. The electrode catalyst according to Item 1 or 2, wherein the double hydroxide covers the surface of the sulfide. Section 4 An oxygen generating electrode comprising the electrode catalyst according to any one of items 1 to 3. Section 5 Item 5. A method for electrolyzing water, comprising the step of electrolyzing water using the oxygen generating electrode according to item 4. [Effects of the Invention]

[0009] When the electrode catalyst according to the present invention is used as an electrode for water electrolysis, an increase in overvoltage during water electrolysis is unlikely to occur, and the electrode catalyst can be operated stably for a long period of time even at a high current density. [Brief explanation of the drawings]

[0010] [Figure 1] FIG. 1 is a schematic diagram illustrating an outline of a method for producing an electrode catalyst. [Figure 2] 1 is an XRD spectrum of the electrode catalyst obtained in Example 1. [Figure 3] (a) is a precursor obtained in Comparative Example 2, (b) is an SEM image of the electrode catalyst obtained in Comparative Example 3, (c) is an SEM image of the electrode catalyst obtained in Example 1, and (d) to (g) are EDX element mapping images of the electrode catalyst obtained in Example 1. [Figure 4] 1 shows the measurement results of the electrochemically active surface area (ECSA) of the electrode catalyst obtained in Example 1. [Figure 5] (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 6] (a) shows the multi-current step chronopotentiometry curve, (b) shows the linear sweep voltammetry measurement results after 1000 cycles of testing, and (c) shows the results of a long-term operation test. 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.Electrode catalyst The electrode catalyst of the present invention comprises a catalyst on an electrode substrate, and the catalyst contains nickel sulfide and a nickel-cobalt double hydroxide. Such an electrode catalyst can be suitably used as an electrode for water electrolysis, particularly as an oxygen generating electrode. In particular, when the electrode catalyst of the present invention is used as an electrode for water electrolysis, an increase in overvoltage during water electrolysis is unlikely to occur, and stable operation for a long period of time is possible even at high current densities.

[0013] 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.

[0014] 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.

[0015] 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.

[0016] The electrode substrate is more preferably a metal substrate, more preferably a nickel substrate, and most preferably nickel foam.

[0017] 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.

[0018] In the electrode catalyst of the present invention, the catalyst is formed on an electrode substrate as described above, and contains nickel sulfide and nickel-cobalt double hydroxide.

[0019] Nickel sulfide is, for example, a compound represented by Ni3S2. In an electrode catalyst, when the electrode substrate is a nickel substrate (e.g., nickel foam), the Ni in the sulfide can be based on the nickel of the electrode substrate. That is, Ni of the nickel substrate can react with sulfur to form nickel sulfide.

[0020] Nickel-cobalt double hydroxides are, for example, hydroxides containing Ni and Co as constituent elements. Double hydroxides are, for example, layered double hydroxides (LDHs) containing Ni and Co as constituent elements. Layered double hydroxides are characterized by having exchangeable anions between metal hydroxide layers and are known as layered materials. The nickel-cobalt double hydroxides contained in the catalyst can form a layered structure in which layers formed by nickel and cobalt hydroxides form a laminated structure, with anions present between the layers.

[0021] In the double hydroxide of nickel and cobalt, for example, the nickel is divalent or trivalent and the cobalt is divalent or trivalent.

[0022] In the nickel-cobalt double hydroxide, the content ratio of Ni and Co is not particularly limited, and for example, the molar ratio of Ni:Co can be 1:0.1 to 1:10, preferably 1:0.5 to 1:5, and more preferably 1:1 to 1:4.

[0023] The form of the catalyst is not particularly limited as long as it contains the sulfide and the double hydroxide. In one embodiment of the catalyst, the catalyst can have a structure in which the double hydroxide covers the surface of the sulfide. When the catalyst has such a structure, the conductive sulfide is covered with the double hydroxide, which has excellent catalytic activity, and the catalytic activity of the electrode catalyst is therefore more excellent. The catalyst preferably has a structure in which the surface of the sulfide, which is formed in a grass-like (turf-like) or needle-like shape, is covered with the double hydroxide. In this case, the catalytic activity of the electrode catalyst is particularly excellent.

[0024] The catalyst may contain other elements and / or compounds in addition to the sulfide and the double hydroxide. Examples of other elements include oxygen, and examples of other compounds (compounds other than the sulfide) include oxides and hydroxides. When the catalyst contains other elements or compounds in addition to the sulfide, the content of these elements or compounds may be 10% by mass or less, preferably 5% by mass or less, more preferably 1% by mass or less, and even more preferably 0.1% by mass or less, based on the total mass of the catalyst. The catalyst may be formed solely from the sulfide. However, in this case, metal elements or compounds inevitably contained in the catalyst are acceptable.

[0025] The content ratio of the sulfide and the double hydroxide in the catalyst is not particularly limited. For example, the content ratio of the sulfide in the catalyst is preferably 1 to 40 mass%, more preferably 10 to 30 mass%, and even more preferably 15 to 20 mass%.

[0026] The shape of the catalyst is not particularly limited, and can be, for example, the same as that of known electrode catalysts. For example, the electrode catalyst of the present invention can be formed in the form of a sheet (e.g., 0.1 to 500 μm) on an electrode substrate. In this case, the catalyst is preferably in the form of a nanosheet. When the catalyst is formed in the form of a nanosheet, its thickness is 200 to 500 nm. For example, as described above, a (nano)sheet-shaped catalyst can have a structure in which the double hydroxide covers the surface of the sulfide. When the catalyst is formed in the form of a sheet, such a sheet can also have a porous structure. Furthermore, the sheet can have not only a single layer but also a laminated structure.

[0027] The catalyst can cover part or all of the electrode substrate, and is preferably disposed in the outermost layer of the electrode catalyst.

[0028] 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).

[0029] Conventional transition metal layered double hydroxides (TM-LDHs), such as [M 2+ 1-x M 3+ x (OH)2](A n- ) x / n mH2O (where M 2+ and M 3+ are divalent and trivalent transition metal cations, respectively), A n-Transition metal layered double hydroxides, such as those containing sulfides and double hydroxides (where the charge-balancing anion is present), have been widely used for water splitting because they are composed of positively charged metal hydroxide layers with anions intercalated. However, their OER performance has been significantly limited by their low electrical conductivity, poor electron and charge transfer ability, and insufficient active edge sites. In contrast, the electrode catalyst of the present invention, which includes the sulfide and double hydroxide, can be suitably used as an electrode for water electrolysis, particularly as an oxygen-evolving electrode. In particular, when used as an electrode for water electrolysis, the electrode catalyst of the present invention is less likely to experience an increase in overpotential during water electrolysis, can reduce the Tafel slope, and can operate stably for long periods even at high current densities.

[0030] Therefore, the electrode catalyst of the present invention is suitable for use in electrodes for various electrolysis, and in particular, when used as an electrode for water electrolysis, it can provide excellent oxygen generation efficiency, and is therefore suitable as an electrode for oxygen generation (oxygen generating electrode).

[0031] 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 comprises at least the following steps 1 and 2. Step 1: A step of obtaining a precursor by immersing an electrode substrate in raw material solution A containing a sulfiding agent and performing a heat treatment. Step 2: A step of immersing the precursor obtained in step 1 in a raw material solution B containing a nickel source and a cobalt source for electrodeposition to obtain an electrode catalyst.

[0032] (Process 1) In step 1, the electrode substrate is immersed in a raw material liquid A containing a sulfiding agent and subjected to a heat treatment. By such a heat treatment, a precursor of the electrode catalyst is obtained.

[0033] 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.

[0034] The raw material solution A used in step 1 contains at least a sulfiding agent. A wide variety of known compounds capable of sulfidation can be used as the sulfiding agent, including, for example, sodium sulfide (NaS), sulfur, thioacetamide (CHCSNH), thiourea (CHNS), sodium thiosulfate, and ammonium sulfide.

[0035] When the electrode substrate used in step 1 is a nickel substrate, the raw material liquid A does not need to contain a Ni source. However, when the electrode substrate used in step 1 is a substrate other than a nickel substrate or a substrate that does not contain nickel, the raw material liquid must contain a Ni source in addition to the sulfiding agent.

[0036] Examples of the Ni source include elemental Ni and compounds containing Ni, and Ni-containing compounds are preferred. Examples of Ni-containing compounds that can be widely used include inorganic acid salts of Ni, organic acid salts of Ni, hydroxides of Ni, and halides of Ni.

[0037] A wide variety of known compounds can be used as inorganic acid salts of Ni, including one or more selected from the group consisting of Ni nitrates, hydrochlorides, sulfates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. A wide variety of known compounds can be used as organic acid salts of Ni, including one or more selected from the group consisting of Ni acetates, oxalates, formates, and succinates.

[0038] The Ni source is preferably an inorganic acid salt of Ni, such as Ni(NO3)2. One Ni source may be used alone, or two or more Ni sources may be used in combination. The Ni-containing compound can be obtained by a known production method, or a commercially available Fe-containing compound can be used.

[0039] The raw material liquid A contains a solvent. The solvent is, for example, water, and may also contain a lower alcohol compound. The solvent may be water alone.

[0040] The raw material liquid A is, for example, a solution or a dispersion, preferably a solution, and more preferably an aqueous solution.

[0041] The raw material solution A may contain other additives, such as a pH adjuster. The type of pH adjuster is not particularly limited, and a wide variety of known pH adjusters can be used.

[0042] In raw material solution A, the concentration of the sulfiding agent is not particularly limited and is, for example, 0.1 to 10 M, preferably 0.5 to 5 M, more preferably 0.8 to 3 M, and particularly preferably 1 to 2 M. When raw material solution A contains a Ni source, the total concentration of the Ni source 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.

[0043] The heat treatment method in step 1 can be, for example, a hydrothermal synthesis method in which the electrode substrate is immersed in raw material solution A in a container, the container is sealed, and the container is heated. This hydrothermal synthesis method causes a sulfiding reaction by the sulfiding agent, and nickel sulfide is formed on the electrode substrate. This sulfide is NiS.

[0044] When the electrode base material is a nickel base material, the nickel of the nickel base material is involved in the reaction by the heat treatment (hydrothermal synthesis), and Ni derived from the nickel base material reacts with the sulfiding agent to produce nickel sulfide on the electrode base material.

[0045] In step 1, the temperature inside the container during the heat treatment is not particularly limited and can be, for example, 50 to 250°C, preferably 80 to 200°C, more preferably 100 to 180°C, and even more preferably 120 to 160°C. The heating time is also not particularly limited and can be determined appropriately depending on the heating temperature, for example, 30 minutes to 24 hours, preferably 5 to 12 hours, and more preferably 6 to 10 hours. The pressure inside the container during the heat treatment can also be set appropriately.

[0046] A precursor of the electrode catalyst is obtained by the heat treatment (hydrothermal synthesis) in step 1. This precursor is formed by forming a double hydroxide containing Ni and Fe on an electrode substrate.

[0047] The heat treatment in step 1 sulfides nickel to obtain a precursor of the electrode catalyst. This precursor is formed by forming nickel sulfide on the electrode substrate. The nickel sulfide has, for example, a glass-shaped or needle-shaped nanosheet structure.

[0048] (Process 2) Step 2 is a step for electrodeposition treatment by immersing the precursor obtained in step 1 in a raw material solution B containing a nickel source (Ni source) and a cobalt source (Co source). By such electrodeposition treatment, the electrode catalyst of the present invention can be obtained.

[0049] Examples of the Ni source include elemental Ni and compounds containing Ni, and Ni-containing compounds are preferred. Examples of Ni-containing compounds that can be widely used include inorganic acid salts of Ni, organic acid salts of Ni, hydroxides of Ni, and halides of Ni.

[0050] A wide variety of known compounds can be used as inorganic acid salts of Ni, including one or more selected from the group consisting of Ni nitrates, hydrochlorides, sulfates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. A wide variety of known compounds can be used as organic acid salts of Ni, including one or more selected from the group consisting of Ni acetates, oxalates, formates, and succinates.

[0051] The Ni source is preferably an inorganic acid salt of Ni, such as Ni(NO3)2. One Ni source may be used alone, or two or more Ni sources may be used in combination. The Ni-containing compound may be obtained by a known production method, or a commercially available Ni-containing compound may be used.

[0052] Examples of the Co source include Co alone or a compound containing Co, and a compound containing Co is preferred. As the Co-containing compound, a wide range of compounds can be used, including inorganic acid salts of Co, organic acid salts of Co, hydroxides of Co, and halides of Co.

[0053] A wide variety of known compounds can be used as inorganic acid salts of Co, including at least one selected from the group consisting of Co nitrates, hydrochlorides, sulfates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. A wide variety of known compounds can be used as organic acid salts of Co, including at least one selected from the group consisting of Co acetates, oxalates, formates, and succinates.

[0054] The Co source is preferably an inorganic acid salt of Co, such as Co(NO3)2. The Ni source may be used alone or in combination of two or more. The Co-containing compound can be obtained by a known production method, or a commercially available Co-containing compound can be used.

[0055] The raw material solution B contains a solvent. The solvent is, for example, water, and may also contain a lower alcohol compound. The solvent may be water alone.

[0056] The raw material solution B may contain other additives, such as a pH adjuster. The type of pH adjuster is not particularly limited, and a wide variety of known pH adjusters can be used. When the raw material solution B contains a pH adjuster, the pH of the solution is stabilized, which makes it easier to suppress hydrolysis of the Co source and the like, and allows the electrodeposition process to be carried out more stably.

[0057] The raw material liquid B is, for example, a solution or a dispersion, preferably a solution, and more preferably an aqueous solution.

[0058] The concentration of raw material solution B is not particularly limited, and the concentrations of the Ni source and Co source are each preferably 1 to 100 mM, more preferably 3 to 50 mM.

[0059] In step 2, the electrodeposition method is not particularly limited, and for example, a wide variety of known electrodeposition methods can be used. For example, the electrodeposition can be performed by immersing the precursor in the raw material solution B.

[0060] The electrodeposition treatment can be performed by various electrodeposition methods. Examples of the electrodeposition method include constant current (GM), constant voltage (PM), cyclic voltammetry (CV), and pulse electrodeposition. The pulse electrodeposition method is an electrodeposition treatment method that can control the electrodeposition rate of metal ions, and examples include pulse voltage (PPM), which applies a high-end voltage and a low-end voltage at a constant cycle, pulse current (PGM), which applies a high-end current and a low-end current at a constant cycle, and unipolar pulse voltage (UPED), which repeatedly applies a high-end voltage and an open circuit state at a constant cycle.

[0061] The conditions for the electrodeposition treatment are not particularly limited. For example, the applied voltage can be -2 to 2 V (preferably -1 to 1.5 V), and the electrodeposition time can be 100 to 2000 seconds (preferably 300 to 1800 seconds). The temperature of the solution used for the electrodeposition treatment is not particularly limited, and can be, for example, about 0 to 50°C, preferably 20 to 30°C.

[0062] The electrodeposition treatment can be performed using the precursor as a cathode (working electrode). In addition to the cathode, a counter electrode, a reference electrode, an electrolysis device, a power supply, control software, and the like can be used in the electrodeposition treatment. The types of these are not particularly limited, and known electrodes can be used depending on the purpose. For example, a silver / silver chloride electrode (Ag / AgCl electrode), a mercury / mercury chloride electrode (Hg / HgCl electrode), a standard hydrogen electrode, and the like can be used as the reference electrode. A platinum wire can be used as the counter electrode.

[0063] A double hydroxide containing nickel and cobalt as constituent elements is produced by the electrodeposition treatment in step 2. The double hydroxide is formed, for example, by covering the sulfide in the precursor.

[0064] The electrode catalyst of the present invention can be obtained, for example, by a production method including the above steps 1 and 2, that is, it can be produced by a simple method.

[0065] 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.

[0066] Since the oxygen generating electrode comprises the electrode catalyst of the present invention, an increase in overvoltage during water electrolysis can be suppressed, water electrolysis can be performed stably, and oxygen can be generated efficiently. Furthermore, since the oxygen generating electrode comprises the electrode catalyst of the present invention, water electrolysis can also be performed stably for a long period of time.

[0067] 4.Water electrolysis method The method for electrolyzing water of the present invention may include, for example, a step of electrolyzing water using the oxygen generating electrode. By such a method for electrolyzing water, oxygen or hydrogen can be produced.

[0068] In the water electrolysis method, the oxygen generating electrode is used as an anode.

[0069] 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.

[0070] 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]

[0071] The present invention will be explained in more detail below with reference to examples, but the present invention is not limited to these examples.

[0072] Example 1 Figure 1 shows a schematic diagram of the manufacturing flow of the electrode catalyst. 2The nickel foam (nickel foam) was pretreated by sequentially treating it with 1M hydrochloric acid, ethanol, and deionized water under ultrasonic conditions for 1 hour, and then drying it in a vacuum oven at 60°C for 1 hour. This nickel foam was then immersed in raw material solution A, which consisted of a 1M aqueous solution of thiourea (CH4N2S) (distilled water), and heated at 150°C for 6 hours (Step 1). The heated nickel foam was then washed several times with ethanol to obtain a precursor.

[0073] The precursor was used as the working electrode, a Pt wire as the counter electrode, and an Ag / AgCl reference electrode. The electrodeposition process was carried out in solution B by an electrochemical oxidation process using a three-electrode system. Solution B was a 0.5 M aqueous solution of Ni(NO3)2·6H2O and Co(NO3)2·6H2O, with a Ni:Co (molar ratio) of 1:4. The electrodeposition conditions were room temperature (25 °C) and a potential of -0.9 V for 1200 seconds. The desired electrocatalyst was obtained through this electrodeposition process. The resulting electrocatalyst was named "Ni3S2@NiCo-LDH / NF."

[0074] Example 2 The target electrode catalyst was obtained in the same manner as in Example 1, except that the Ni:Co (molar ratio) in the raw material solution B was changed to 1:1.

[0075] Example 3 The target electrode catalyst was obtained in the same manner as in Example 1, except that the Ni:Co (molar ratio) in the raw material solution B was changed to 4:1.

[0076] (Comparative Example 1) The nickel foam used in Example 1 was prepared without any treatment and designated "BareNF."

[0077] (Comparative Example 2) A precursor was prepared without carrying out the electrodeposition treatment of Example 1. This precursor was named "Ni3S2 / NF."

[0078] (Comparative Example 3) Electrodeposition was performed in solution B by an electrochemical oxidation process using a three-electrode system with nickel foam as the working electrode, a Pt wire as the counter electrode, and an Ag / AgCl reference electrode. Solution B was a 0.5 M aqueous solution of Ni(NO3)2·6H2O and Co(NO3)2·6H2O, with a Ni:Co (molar ratio) of 1:4. The electrodeposition conditions were room temperature (25°C) at -0.9 V for 1200 seconds. This electrodeposition process yielded an electrocatalyst. The resulting electrocatalyst was named "NiCo-LDH / NF."

[0079] Fig. 2(a) shows the X-ray diffraction (XRD) spectrum of the electrode catalyst obtained in Example 1. For comparison, Fig. 2 also shows the X-ray diffraction spectra of the catalysts prepared in Comparative Examples 1 to 3. For the X-ray diffraction measurements, a "SmartLab" manufactured by Rigaku Corporation was used.

[0080] In the XRD spectrum (FIG. 2(a)) of the electrode catalyst obtained in Example 1, the main diffraction peaks observed at 2θ = 21.7°, 31.1°, 38.3°, 44.3°, 55.1°, and 77.6° were found to correspond to the (101), (110), (021), (202), (122), and (223) crystal planes of the Ni3S2 rhombohedron, respectively (see JCPSD No. 44-1418). On the other hand, since the peak intensity corresponding to the NF substrate (nickel foam) was high, it was possible to confirm the peaks corresponding to the nickel and cobalt double hydroxide (NiCo-LDH) on the NF substrate. However, since it was difficult to confirm the formation of the NiCo-LDH structure in the Ni3S2 layer, NiCo-LDH was formed on carbon paper and the XRD spectrum was measured (Fig. 2(b)). From the results in Fig. 2(a) and (b), it was confirmed that the electrode catalyst obtained in Example 1 contained both Ni3S2 and nickel-cobalt double hydroxide. Therefore, as shown schematically in Fig. 1, it is presumed that the electrode catalyst obtained in Example 1 has a catalyst structure in which a glassy Ni3S2 (core layer) is covered with a nanoflake-like NiCo-LDH (shell).

[0081] Figure 3 shows an SEM image and an EDX elemental mapping image of the surface of the electrode catalyst obtained in Example 1, along with SEM images of each substrate prepared in the comparative examples. Specifically, in Figure 1, (a) is an SEM image of the precursor obtained in Comparative Example 2, (b) is an SEM image of the electrode catalyst obtained in Comparative Example 3, and (c) is an SEM image of the electrode catalyst obtained in Example 1. Figures 1(d) to (g) are EDX elemental mapping images of the electrode catalyst obtained in Example 1, with (d) showing the distribution of Ni, Co, and S elements, and (e), (f), and (g) showing the distribution of Ni, Co, and S, respectively.

[0082] Comparing the SEM images of Figures 3(a) and 3(b) with those of Figure 3(c), it was found that the electrode catalyst obtained in Example 1 had a Ni3S2 nanoglass core layer grown directly on the NF substrate, and NiCo-LDH nanoflakes deposited on this Ni3S2. Furthermore, the mapping image confirmed that each element was uniformly present throughout the catalyst.

[0083] FIG. 4 shows the measurement results of the electrochemically active surface area (ECSA) of the electrode catalyst obtained in Example 1 (capacitive current density as a function of scan rate).

[0084] The electrochemically active surface area (ECSA) was estimated from the double layer capacitance (Cdl), and the maximum Cdl value of the electrode catalyst obtained in Example 1 was 91.3 mFcm -2 This was approximately seven times that of the precursor of Comparative Example 2.

[0085] Figure 5(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 a 1 M KOH aqueous solution (pH = 14). In this example, a VersaSTAT4 potentiostat galvanostat electrochemical workstation (USA) was used with a standard three-electrode cell to evaluate electrical characteristics such as linear sweep voltammetry curves.

[0086] Figure 5(b) shows the Tafel slope calculated from the linear sweep voltammetry curve shown in (a). Figure 5(c) shows the electrochemical impedance (EIS) measurement results for the electrode catalysts obtained in Example 1 and each comparative example. The measurements were performed in a 1 M KOH solution 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 5(c).

[0087] Table 1 shows the 10 mA cm of each electrode catalyst derived based on the results of Figures 5(a), (b), and (c). -2 and 100mAcm -2 Overpotential, Tafel slope and charge transfer resistance (R ct ) results are shown.

[0088] [Table 1]

[0089] From the above results, it can be seen that the electrode catalyst obtained in Example 1 can be used at a low current density (10 mAcm -2 ), as well as high current density (100mAcm -2), the overpotential was low, the Tafel slope showed the best performance, and the charge transfer resistance was also the smallest. Therefore, the electrode catalyst obtained in Example 1 can be said to exhibit a good catalytic reaction rate and to be advantageous in improving electronic conductivity.

[0090] FIG. 6(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 10 mA / cm 2 ~400mA / cm 2 Up to 50mA / cm 2 Measurements were taken at intervals (the electrolyte was a 1M KOH solution), and finally at 10 mA / cm 2 Figure 6(b) shows the potential-time graph obtained by returning the sample to 1000 cycles. Figure 6(c) shows the linear sweep voltammetry results after 1000 cycles. 2 The results are shown for a period of 24 hours at a current density of 1 M KOH, after which the solution was replaced with a new 1 M KOH solution and electrolysis continued. The measurement conditions were the same as those used 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.

[0091] From the results in FIG. 6, the electrode catalyst obtained in this example exhibited excellent stability for 24 hours without catalyst dissolution (peel-off), and was able to withstand high current densities (100 mA / cm 2 ) showed no significant change in potential. Furthermore, even after 1000 cycles of cyclic voltammetry in an alkaline medium solution, the LSV curve showed almost no change. Therefore, it was demonstrated that the electrode catalyst obtained in Example 1 can be operated stably for a long period of time even at high current densities, and is suitable for an oxygen generating electrode capable of efficient water electrolysis.

Claims

1. An electrode catalyst for alkaline water electrolysis comprising a catalyst on an electrode substrate, The catalyst consists only of glassy Ni3S2 and nanoflake-like nickel and cobalt double hydroxide; The glassy Ni 3 S 2 is covered with the nanoflake-like nickel-cobalt double hydroxide, forming an electrode catalyst for alkaline water electrolysis.

2. The electrode catalyst for alkaline water electrolysis according to claim 1 , wherein the electrode substrate is a nickel substrate.

3. An oxygen generating electrode comprising the electrode catalyst for alkaline water electrolysis according to claim 1 or 2.

4. A method for electrolyzing alkaline water, comprising the step of electrolyzing alkaline water using the oxygen generating electrode according to claim 3.

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