JP7804280B2 - Oxygen generating electrode and manufacturing method thereof, and oxygen generating electrode and method for electrolyzing seawater - Google Patents

Description

The present invention relates to an oxygen generating electrode and a method for manufacturing the same, as well as an oxygen generating electrode and a method for electrolyzing water.

Water electrolysis (the process of electrolyzing 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 require electricity, it is necessary to reduce production costs, and with this in mind, various water electrolysis technologies are being developed.

In order to electrolyze water, "pure water (so-called fresh water)" is required, and pure water can easily become scarce when electrolyzing large amounts. Also, because pure water barely conducts 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 it (i.e., to make electricity pass through the water at a low voltage).

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 standard electrodes (platinum, iridium oxide, etc.), hydrogen gas is produced from 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.

For example, Non-Patent Document 1 proposes a catalyst material whose constituent elements are nickel-iron double hydroxides (NiFe-LDH).

ChemSusChem 2016, 9, 962-972

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 is prone to causing an increase in overvoltage during electrolysis in seawater, resulting in a decrease in performance of the OER (oxygen evolution reaction).

The present invention was made in light of the above, and aims to provide an oxygen generating electrode that exhibits small overvoltage even at high current densities and enables stable operation over long periods of time during seawater electrolysis, a method for manufacturing the same, and a method for using the oxygen generating electrode and electrolysis of water.

As a result of extensive research into achieving the above objective, the inventors discovered that the above objective could be achieved by using a material that combines a specific complex metal oxide and a double hydroxide as a catalyst, leading to the completion of the present invention.

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.
Item 5
Item 5. An oxygen generating electrode comprising the electrode catalyst according to any one of items 1 to 4.
Item 6
Item 6. A method for electrolyzing water, comprising a step of electrolyzing water using the oxygen generating electrode according to Item 5.
Section 7
Item 5. 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
Step 3: heat-treating the electrode substrate on which the composite oxide is formed in a solution of a second metal source to obtain an electrode catalyst;
A method for producing an electrode catalyst comprising:

When used as an electrode for electrolyzing seawater, the oxygen-generating electrode catalyst for seawater splitting of the present invention exhibits small overvoltage even at high current densities, enabling stable operation over long periods of time.

1 is a schematic diagram illustrating an example of a method for producing an electrode catalyst according to the present invention. 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. (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. (a) is a multi-current step chronopotentiometry curve, (b) is the measurement result of linear sweep voltammetry after 2000 cycles of testing, and (c) is the result of a long-term operation test. (a) is a multi-current step chronopotentiometry curve, and (b) is the result of a long-term operation test.

Embodiments of the present invention are described in detail below. Note that, in this specification, the expressions "contain" and "comprise" include the concepts of "contain," "comprise," "consist essentially of," and "consist only of."

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.

When used as an electrode for electrolyzing seawater, the oxygen-generating electrode catalyst for seawater splitting of the present invention exhibits small overvoltage even at high current densities, enabling stable operation over long periods of time.

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

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

Examples of carbon substrates include carbon paper, carbon fiber paper, and carbon rods. Examples of glass substrates include conductive glass. Electrode substrates may also be porous materials such as foam.

The electrode substrate is preferably a metal substrate, more preferably a nickel substrate, and particularly preferably nickel foam.

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

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

The composite metal oxide contains at least two transition metals, and in particular, contains at least Mn. In other words, the composite metal oxide contains Mn and a transition metal other than Mn.

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

The type of transition metal other than Mn is not particularly limited, and examples include Co, Fe, Cr, Ni, Mo, W, Cu, and Zn. Among these, Co is preferred as the transition metal other than Mn, as 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.

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 .

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

In the double hydroxide, examples of the divalent metal (M ²⁺ ) include Ni, Fe, Co, Zn, and Mg. Examples of the trivalent metal (M ³⁺ ) include Fe, Mn, Co, and Al.

In particular, it is preferable that the metals contained in the double hydroxide are at least divalent Ni (Ni 2+ ) and trivalent Fe (Fe ³⁺ ), since this makes it easier to form an electrode that is less likely to cause an increase in overvoltage and allows for stable operation for a long period of time even at high current ^densities .

The Fe and Ni content ratios in the double hydroxide are not particularly limited, and can be, for example, an Fe:Ni mass ratio of 1:10 to 10:1. As described below, when a nickel substrate is used as the electrode substrate, the Fe and Ni content ratios tend to fall within the above range.

Here, a layered double hydroxide (LDH) is a compound that has exchangeable anions between layers of a metal hydroxide, and is formed, for example, in a layered structure. Specifically, a double hydroxide is a hydroxide that can be expressed by the general formula [M ²⁺ _1-x M ³⁺ _x (OH) ₂ ](A ^n- ) _x/n mH ₂ O.

In the electrode catalyst of the present invention, the type of anion (that is, A ⁿ⁻ ) of the double hydroxide is not particularly limited, and examples thereof include CO ₃ ²⁻ , OH ⁻ , Cl ⁻ , and NO ₃ ⁻ .

The catalyst may contain other components as long as it contains the complex metal oxide and the double hydroxide. The catalyst preferably contains 80% by mass or more of the complex 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 solely of the complex metal oxide and the double hydroxide.

The content ratio of the composite metal oxide and the double hydroxide in the catalyst is not particularly limited. For example, if the composite metal oxide is contained in an amount of 10 to 90 mass% relative to 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 enables stable operation for long periods of time.

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

One example of this is a catalyst 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, a structure in which a nanowire-shaped composite metal oxide forms a core layer, and the double hydroxide covers this core layer to form a shell layer can be mentioned.

The shape of the catalyst is not particularly limited, but for example, it is preferable that the catalyst be formed in a sheet shape on the electrode substrate. This allows the catalyst to adhere firmly to the electrode substrate, thereby achieving a high catalytic effect.

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

The catalyst can cover part or all of the electrode substrate. It is also preferable that the catalyst be positioned as the outermost layer of the electrode catalyst.

The electrode catalyst of the present invention may be formed solely 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 layers interposed therebetween).

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 overvoltage during seawater electrolysis even at high current densities, and the Tafel slope can be reduced, allowing for stable operation over long periods of time.

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

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 thought to be because the high activity of the composite oxide is protected by the double hydroxide, preventing the generation of chloride ions. It is also thought that the double hydroxide can effectively increase surface activity and charge transfer.

2. Method for Producing 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.

(Step 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.

The type of electrode substrate used in Step 1 is not particularly limited and is the same as the electrode substrate used in the electrode catalyst described above. Therefore, examples of the electrode substrate used in Step 1 include metal substrates, carbon substrates, and glass substrates. Metal substrates are preferred, nickel substrates are more preferred, and nickel foam is particularly preferred.

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 as a transition metal and a transition metal element other than Mn. Examples of transition metals other than Mn include Co, Fe, Cr, Ni, Mo, W, Cu, and Zn, with Co being preferred.

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. Compounds containing Mn may widely include inorganic acid salts of Mn, organic acid salts of Mn, hydroxides of Mn, and halides of Mn. Similarly, compounds containing a transition metal other than Mn may widely 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.

The inorganic acid salt can be, for example, one or more selected from the group consisting of nitrates, hydrochlorides, sulfates, carbonates, hydrogencarbonates, phosphates, and hydrogenphosphates. The organic acid salt can be, for example, 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.

The first transition metal source is preferably a nitrate. Therefore, the first transition metal source preferably includes a nitrate of Mn and a nitrate of Co.

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.

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 ₂ ) ₂ ), NH ₄ F, and ammonium hydroxide. The pH adjusters may be used alone or in combination of two or more.

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 be dissolved per 100 mL of aqueous solvent. In this case, the solution of the first transition metal source is likely to have a pH in the alkaline range, which makes it easier for the composite oxide of transition metals formed in the first step to be formed in the desired shape.

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

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 typically be carried out so that the entire electrode substrate is immersed in the solution of the first transition metal source. The electrode substrate can be immersed, 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).

In step 1, the electrode substrate is immersed in the first raw material and subjected to a heat treatment. The heat treatment in step 1 can be performed using 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.

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.

The heating treatment (hydrothermal synthesis) in step 1 forms a precursor on the electrode substrate. This precursor is a hydroxide of a transition metal.

(Step 2)
Step 2 is a step for firing the electrode base material on which the precursor has been formed in step 1. This allows for the production of an electrode base material on which the precursor has been oxidized to form a composite oxide.

In step 2, the calcination method is not particularly limited, and 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 selected appropriately depending on the calcination temperature, and can be, for example, 1.5 to 5 hours. The rate of temperature rise during calcination is also not particularly limited, and can be set appropriately to a level that results in the formation of the desired oxide.

Firing may be carried out in air or in an inert gas atmosphere. Firing is preferably carried out in air. Firing can be carried out using a known heating device, such as a commercially available heating furnace.

By performing the above calcination, the precursor (hydroxide) on the electrode substrate is converted to an oxide, resulting in an electrode substrate modified with a composite oxide containing Mn.

(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.

In step 3, a solution of the second metal source is used as a raw material. The second metal source preferably contains a divalent metal and a trivalent metal. However, if the electrode substrate used in step 1 is a nickel substrate, the 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, even when the electrode substrate is a nickel substrate, it is preferable that the second metal source contain a divalent metal and a trivalent metal.

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

The second metal source may be a metal element or a compound containing the metal. For example, a wide range of second metal sources can be used, including inorganic acid salts of Fe, organic acid salts of Fe, hydroxides of Fe, and halides of Fe.

Examples of inorganic acid salts of Fe include one or more selected from the group consisting of Fe nitrate, hydrochloride, sulfate, carbonate, bicarbonate, phosphate, and hydrogen phosphate. Examples of organic acid salts of Fe include one or more selected from the group consisting of Fe acetate, oxalate, formate, and succinate. Compounds containing Fe can be obtained by known production methods, or commercially available products can be used.

The second metal source is preferably an Fe halide.

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.

The second metal source solution may contain other additives. Examples of such additives include a pH adjuster, such as _NaNO3 . When the source solution contains a pH adjuster, the pH value at which ferric iron is hydrolyzed can be lowered. Furthermore, when a nickel substrate such as nickel foam is used as the electrode substrate, the pH adjuster can function as an etching agent to generate Ni2 ⁺ ions.

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.

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

The heat treatment method used in step 3 can be, for example, a 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 during 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.

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

In step 3, the temperature inside the container during the heating 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 heating treatment can also be set appropriately.

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. The catalyst formed can be formed in the form of a sheet on the electrode substrate.

3. Oxygen Evolving Electrode The electrode catalyst of the present invention is suitable as an oxygen evolving electrode. The oxygen evolving 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.

Because the oxygen generating electrode is equipped with the electrode catalyst of the present invention, it is possible to suppress an increase in overvoltage during water electrolysis, enabling stable seawater electrolysis and efficient oxygen generation. Furthermore, because the oxygen generating electrode is equipped with the electrode catalyst of the present invention, it is also possible to stably electrolyze seawater over long periods of time.

4. Seawater Electrolysis Method The seawater electrolysis method 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 seawater electrolysis method of the present invention, the oxygen generating electrode is used as an anode.

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 platinum wire can be used, and the electrode catalyst of the present invention can also be used as the cathode.

In the electrolysis method of the present invention, the seawater may be natural seawater, or imitation seawater (e.g., an aqueous solution containing 1 M KOH and 0.5 M NaCl) may be used as seawater.

The present invention will be explained in more detail below using examples, but the present invention is not limited to these examples.

Example 1
According to the scheme shown in Figure 1, a catalyst consisting _{of MnCo2O4} coated with NiFe double hydroxide was formed on nickel foam. First, a 2 x 2 ^cm2 nickel foam was pretreated with 1 M hydrochloric acid, ethanol, and deionized water, successively, under ultrasonic conditions for 1 hour, and then dried in a vacuum oven at 60°C for 1 hour to prepare the electrode substrate. The 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 _NH4F , and 12 mmol of urea. It was then transferred to a Teflon-lined autoclave and heated at 120°C for 4 hours. After allowing to cool, 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). Next, the electrode substrate obtained in step 1 was 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 heat-treated at 120°C for 2 hours (step 3). 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@ _MnCo2O4 / _NF ."

(Comparative Example 1)
A 2 x 2 ^cm2 nickel foam (nickel foam) was treated with 1 M hydrochloric acid, ethanol, and deionized water, successively, under ultrasonic conditions for 1 hour, and then dried in a vacuum oven at 60 °C for 1 hour to pretreat the electrode substrate. This nickel foam was immersed in 30 ml of distilled water containing 1 mmol of Mn(NO ₃ ) ₂ ·4H ₂ O, 2 mmol of Co(NO ₃ ) ₂ ·6H ₂ O, 6 mmol of NH ₄ F, and 12 mmol of urea. Next, it was 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). Next, the electrode substrate obtained in Step 1 was calcined in air at 350 °C for 2 hours (Step 2). The calcined electrode substrate was washed several times with ethanol and deionized water to obtain an electrode catalyst. Such an electrocatalyst was named "MnCo ₂ O ₄ /NF."

(Comparative Example 2)
A 2 x 2 ^cm2 nickel foam was treated with 1 M hydrochloric acid, ethanol, and deionized water, successively, under ultrasonic conditions for 1 hour, and then dried in a vacuum oven at 60°C for 1 hour to pretreat the electrode substrate. 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 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."

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 Corporation "SmartLab" with a Cu-Kα (λ = 1.540 Å) radiation source in the 2θ range of 10 to 100°.

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). As a result, it was found that the catalyst of Example 1 contained _MnCo2O4 and _NiFe double hydroxide. It is presumed that a NiFe-LDH layer is 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 .

Figure 3 shows SEM images and EDX elemental mapping images of the electrode catalyst surfaces obtained in the examples and comparative examples. Specifically, (a) in Figure 3 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.

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 , and O) is uniformly present throughout the catalyst, indicating that the catalyst formed on the electrode substrate contains MnCo2O4 and NiFe double hydroxide.

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 conducted 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 and a VersaSTAT 4 potentiostat galvanostat electrochemical workstation (USA) were used to evaluate electrical characteristics such as linear sweep voltammetry curves.

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).

Table 1 shows the overpotential, Tafel slope and charge transfer resistance (R _ct ) of each electrode catalyst at 10 mAcm ⁻² , 100 mAcm ⁻² and 500 mAcm ⁻² , derived based on the results in FIGS. 4(a), (b) and (c).

From the above results, the electrode catalyst obtained in Example 1 exhibited low overpotential even at a high current density (500 mAcm ^-2 ), the best Tafel slope, and the lowest charge transfer resistance. 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.

Figure 5(a) shows a multi-current step chronopotentiometry curve when the electrode catalyst obtained in Example 1 was used as the anode. The current density was measured every 600 seconds at 100 mA/cm² ^intervals over a range from 100 mA/ ^cm² to 1000 mA/ ^cm² (using the imitation seawater as the electrolyte), and the potential-time graph was obtained by returning the current density to 100 mA/ ^cm² . Figure 5(b) shows the results when electrolysis was continued for 50 hours at current densities of 100 mA/ ^cm² and 1000 mA/ ^cm² . The measurement conditions were the same as those used in the tests to obtain the linear sweep voltammetry curves. The measurement was performed using a two-electrode cell and a VersaSTAT4 potentiostat galvanostat electrochemical workstation (USA).

5, the electrode catalyst obtained in Example 1 exhibited excellent stability for 100 hours without catalyst dissolution (peel-off), and there was no significant change in potential at a high current density (1000 mA/ ^cm2 ). It also exhibited excellent stability in simulated seawater for 50 hours without corrosion at high current densities. Therefore, it was demonstrated that the electrode catalyst obtained in Example 1 can operate stably for a long period of time even at high current densities, and is suitable for an oxygen generating electrode capable of efficient seawater electrolysis.

In particular, the low overpotential allows for higher OER selectivity than ClER, and hypochlorite is not formed. This is presumably because the high activity of the _{MnCo2O4 nanowires} is protected by NiFe-LDH, preventing the generation of chloride ions. It is also presumed that NiFe-LDH can effectively increase surface activity and charge transfer.

Claims (6)

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 an oxygen generating electrode catalyst for seawater decomposition, containing _MnCo2O4 and _a double hydroxide containing divalent Ni and trivalent Fe. 2. The oxygen generating electrode catalyst for seawater splitting according to claim 1, wherein the catalyst has a structure _in which the surface of _MnCo2O4 is coated with the double hydroxide. The oxygen generating electrode catalyst for seawater splitting according to claim 1 or 2, wherein the catalyst is formed in a sheet form on the electrode substrate. An oxygen generating electrode comprising the electrode catalyst described in any one of claims 1 to 3. A method for electrolyzing seawater , comprising the step of electrolyzing seawater using the oxygen generating electrode according to claim 4. 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: