WATER ELECTROLYSIS CELL USING IRON PRECIPITATE AND METHOD OF MANUFACTURING ELECTRODE FOR WATER ELECTROLYSIS CELL

Description

CROSS-REFERENCE TO RELATED APPLICATION(S)

This application claims priority under 35 U.S.C. § 119 to Korean Patent Application No. 10-2024-0128582, filed on Sep. 23, 2024, which is hereby incorporated by reference for all purposes as if fully set forth herein.

BACKGROUND

1. Field

Embodiments relate to a water electrolysis cell using iron precipitate and a method of manufacturing an electrode for the water electrolysis cell. More particularly, the embodiments relate to the water electrolysis cell including nickel-iron integrated electrode for water electrolysis cell and the method of manufacturing the electrode for the water electrolysis cell.

2. Description of the Related Art

A water electrolysis technology may play a pivotal role in sustainable production of green hydrogen and may serve as an environmentally friendly technology that plays an important role in global transition. Among water electrolysis technologies, anion exchange membrane water electrolysis, AEMWE, may be a promising technology for achieving fast and cost-efficient hydrogen production. However, when using a general particle-type catalyst in anion exchange membrane water electrolysis technology, a problem may occur such as a contact issue between the catalyst and the membrane or substrate, a harsh operating condition and a performance degradation, and a long-term durability problem.

Recently, extensive research may be conducted in the field of alkaline electrolysis to develop high-performance electrode catalysts for energy storage and conversion applications. An Electrode catalyst may be required to have excellent electrical conductivity, excellent electrochemical activity, long-term durability, and economic feasibility. In particular, research using a NiFe mixed catalyst exhibiting high oxygen evolution reaction activity may be actively in progress. In general, the NiFe mixed catalyst may be utilized in forms such as layered double hydroxide or particle-type alloy. However, the layered double hydroxide have a problem of reduced conductivity due to interlayer spacing, and the particle-type alloy catalyst may have a problem of conductivity caused by thick catalyst layers and poor adhesion caused by the use of ionomers. In addition, catalyst engineering additionally conducted to improve these problems may require complicated steps.

SUMMARY

Embodiments provide a water electrolysis cell implementing an anion exchange membrane water electrolysis with improved performance and operating efficiency.

The embodiment provide a method of manufacturing electrode for the water electrolysis cell.

A water electrolysis cell using an iron precipitate according to an embodiment includes an anode structure, an electrolyte, a cathode structure, and an anion exchange membrane (AEM). The anode structure includes an electrode layer including a nickel (Ni) and having a porosity, and a first catalyst layer including a NiFeO_xH_y and covering a first surface of the electrode layer and a second surface of the electrode layer opposite to the first surface. The electrolyte is provided to the anode structure and includes an alkaline solution and the iron precipitate dispersed in the alkaline solution. The cathode structure includes a carbonaceous gas diffusion layer and a second catalyst layer on a surface of the gas diffusion layer facing the anode structure. The anion exchange membrane (AEM) is between the anode structure and the cathode structure.

In an embodiment, the first catalyst layer may be coated on an entire surface of the electrode layer, and a thickness of the first catalyst layer may be about 1 nm or more and about 5 μm or less.

In an embodiment, the electrode layer may include at least one selected from a group consisting of a nickel foam, a nickel fiber, and a nickel paper.

In an embodiment, the iron precipitate included in the electrolyte may include an iron oxyhydroxide (FeOOH).

A method of manufacturing an electrode for the water electrolysis cell includes generating an electrolyte including an iron precipitate by mixing an alkaline solution and a precursor including an iron, generating an oxygen by reacting the electrolyte and a nickel electrode having a porosity, and forming a catalyst layer including NiFeO_xH_y by bonding the iron precipitate on a surface of the nickel electrode.

In an embodiment, the precursor may include at least one selected from a group consisting of FeCl₂, FeCl₃, Fe(NO₃)₃, and Fe(CH₃CO₂)₂.

In an embodiment, the alkaline solution may include at least one selected from a group consisting of KOH, LiOH, NaOH, RbOH, CsOH, Ca(OH)₂, and Mg(OH)₂.

In an embodiment, a pH of the alkaline solution may be about 12 or more and about 14 or less.

In an embodiment, the generating the oxygen and the forming the catalyst layer may be performed simultaneously.

In an embodiment, in the forming the catalyst layer, the iron precipitate may be bonded to the surface of the nickel electrode using an electrochemical reaction selected from a group consisting of cyclic a voltammetry(CV), a chrono amperometry(CA), and a chronopotentiometry(CP).

In an electrode for the water electrolysis cell using iron precipitate according to embodiments of the present disclosure, a catalyst layer including NiFeO_xH_y may be bonded to a surface of an electrode layer including nickel. Accordingly, an electrode for the water electrolysis cell may have low electron transfer resistance in a high frequency band and may deliver a current with a high current density. In addition, the electrode for the water electrolysis cell may increase the activity of oxygen evolution reaction, and since a large surface area of an interface where the electrode layer and the catalyst layer are bonded is formed, the performance and operating efficiency of the water electrolysis cell including the electrode for the water electrolysis cell may be improved.

In a method of an electrode for the water electrolysis cell using iron precipitate, a process of bonding the catalyst layer to the surface of the electrode layer of the electrode for the water electrolysis cell may be performed through an electrochemical reaction such as CV cycling. By controlling the number of the electrochemical reactions, a thickness of the catalyst layer formed on the surface of the electrode for the water electrolysis cell, a degree of activation of oxygen evolution of the electrode for a water electrolysis cell, a current density, and an electron transfer resistance may be easily controlled. Therefore, efficiency in a manufacturing process of the electrode for a water electrolysis cell and the water electrolysis cell including the electrode for the water electrolysis cell may be improved, and manufacturing time and cost may be reduced.

BRIEF DESCRIPTION OF THE DRAWINGS

Illustrative, non-limiting embodiments will be more clearly understood from the following detailed description in conjunction with the accompanying drawings.

FIG. 1 is a view illustrating a water electrolysis cell according to an embodiment of the present disclosure.

FIG. 2 is a view illustrating an anode structure and an anion exchange membrane included in the water electrolysis cell of FIG. 1.

FIG. 3 is a view illustrating a cathode structure and the anion exchange membrane included in the water electrolysis cell of FIG. 1.

FIG. 4 is a view illustrating SEM images illustrating a surface of a nickel-iron integrated electrode manufactured according to a Preparation Example 3 depending on composition.

FIG. 5 is a view illustrating TEM images illustrating a cross-section of a nickel-iron integrated electrode manufactured according to the Preparation Example 3 depending on composition.

FIG. 6 is a view illustrating graphs analyzing the nickel-iron integrated electrode manufactured according to Preparation Example 3.

FIG. 7 is a view illustrating TEM images depending on the number of times of CV cycling of nickel-iron integrated electrodes according to an Example 1 and an Example 2.

FIG. 8 is a view illustrating a graph for explaining performance of nickel-iron integrated electrodes according to the Example 1, the Example 2, and an Example 3.

FIG. 9 is a view illustrating images of nickel-iron integrated electrodes according to the Example 1, an Example 4, and an Example 5.

FIG. 10 is a view illustrating graphs for comparing performance of nickel-iron integrated electrodes according to the Example 1, the Example 4, and the Example 5.

DETAILED DESCRIPTION OF THE EMBODIMENTS

With respect to embodiments of the present disclosure disclosed in the specification, specific structural or functional descriptions are merely illustrated for the purpose of describing the embodiments of the present disclosure, and the embodiments of the present disclosure may be implemented in various forms and should not be construed as being limited to the embodiments described in the specification.

The present disclosure may be modified in various ways and may have various forms, and specific embodiments are illustrated in the drawings and are described in detail in the specification. However, this is not intended to limit the present disclosure to a specific disclosed form, and it should be understood to include all modifications, equivalents, and substitutes included in the spirit and technical scope of the present disclosure.

Terms such as first, second, and the like may be used to describe various components, but the components should not be limited by the terms. The terms may be used for the purpose of distinguishing one component from another. For example, without departing from the scope of the present disclosure, a first component may be named as a second component, and similarly, the second component may be named as the first component.

When it is stated that a component is “connected” or “coupled” to another component, it should be understood that the component may be directly connected or coupled to the other component, but another component may also exist therebetween. On the other hand, when it is stated that a component is “directly connected” or “directly coupled” to another component, it should be understood that no other component exists therebetween. Other expressions describing relationships between components, that is, “between” and “directly between,” or “adjacent to” and “directly adjacent to,” should be interpreted in the same manner.

The terms used in the present application are only used for the purpose of describing specific embodiments and are not intended to limit the present disclosure. Singular expressions include plural expressions unless the context clearly indicates otherwise. In the present application, terms such as “include” or “have” are intended to specify that features, numbers, steps, operations, components, parts, or combinations thereof described exist, but should be understood not to preclude the possibility that one or more other features, numbers, steps, operations, components, parts, or combinations thereof exist or are added.

Unless otherwise defined, all terms used herein, including technical or scientific terms, have the same meanings as those generally understood by a person of ordinary skill in the art to which the present disclosure belongs. Terms defined in commonly used dictionaries should be interpreted to have meanings consistent with the contextual meaning in the relevant art, and unless explicitly defined in the present application, they should not be interpreted to have an ideal or excessively formal meaning.

Meanwhile, when an embodiment is otherwise implemented, functions or operations described in a specific block may occur in a different order than that described in the flowchart. For example, two consecutive blocks may actually be executed substantially simultaneously, and depending on related functions or operations, the blocks may be executed in reverse order.

Hereinafter, with reference to the accompanying drawings, embodiments of the present disclosure will be described in more detail. The same reference numerals are used for the same components in the drawings, and repeated descriptions of the same components will be omitted.

FIG. 1 is a view illustrating a water electrolysis cell according to an embodiment of the present disclosure. FIG. 2 is a view illustrating an anode structure and an anion exchange membrane included in the water electrolysis cell of FIG. 1. FIG. 3 is a view illustrating a cathode structure and the anion exchange membrane included in the water electrolysis cell of FIG. 1.

Referring to FIGS. 1, 2, and 3, a water electrolysis cell according to an embodiment of the present disclosure may include an anode structure 10, a cathode structure 20, an anion exchange membrane 30, and an electrolyte. The anode structure 10 may include an electrode 100 for the water electrolysis cell and a first flow field 110. The cathode structure 20 may include a catalyst layer 210, a gas diffusion layer 220, and a second flow field 230.

The electrode 100 for the water electrolysis cell may perform both a role of a catalyst and a role of gas introduction. In an embodiment, the electrode 100 for the water electrolysis cell may include nickel (Ni) and iron (Fe). For example, the electrode 100 for the water electrolysis cell may be a nickel-iron integrated electrode. The electrode 100 for the water electrolysis cell may include an electrode layer and a catalyst layer. The electrode layer may include nickel. For example, the electrode layer may include nickel foam, nickel fiber, nickel paper, and the like. These may be used alone or in combination. The electrode layer may have porosity. In the present disclosure, the catalyst layer may be referred to as a catalyst layer or a first catalyst layer.

The catalyst layer may surround a first surface 100A of the electrode layer and a second surface 100B opposite to the first surface 100A. For example, the first surface 100A of the electrode layer may be a surface facing the anion exchange membrane 30, and the second surface 100B of the electrode layer may be a surface facing the first flow field 110.

In an embodiment, the catalyst layer may surround the entire surface of the electrode layer. In an embodiment, the catalyst layer may be entirely coated on the surface of the electrode layer. In an embodiment, the catalyst layer may include NiFeO_xH_y. For example, the catalyst layer may include NiFeOOH. However, materials included in the catalyst layer according to the embodiments of the present disclosure may not be limited thereto.

In an embodiment, the electrode 100 for the water electrolysis cell may not include a separate catalyst layer disposed between the gas diffusion layer and the anion exchange membrane 30. The catalyst layer of the electrode 100 for the water electrolysis cell may be bonded on the surface of the electrode layer and perform the role of a catalyst. The catalyst layer and the electrode layer may together perform the role of a catalyst and the role of gas diffusion.

The catalyst layer may be generated from an iron precipitate included in the electrolyte. The electrolyte may be an alkaline solution in which the iron precipitate is dispersed in a solution. For example, the electrolyte may be generated by mixing an alkaline solution and a precursor including iron Fe. The alkaline solution may include KOH, LiOH, NaOH, RbOH, CsOH, Ca(OH)₂, Mg(OH)₂, and the like. These may be used alone or in combination. The precursor may include FeCl₂, FeCl₃, Fe(NO₃)₃, Fe(CH₃CO₂)₂, and the like. These may be used alone or in combination. In addition, the iron precipitate may include FeOOH. However, the alkaline solution and the precursor included according to the embodiments of the present disclosure may be merely exemplary and may not be limited thereto.

In an embodiment, a pH of the alkaline solution may be about 12 or more and about 14 or less. Preferably, the pH of the alkaline solution may be about 13 or more and about 14 or less. The alkaline solution may be used to adjust the pH of an electrolyte used for anion exchange membrane water electrolysis (AEMWE) of the water electrolysis cell.

The catalyst layer may be formed by the iron precipitate being bonded to the surface of the nickel electrode. The nickel electrode may have porosity. The iron precipitate may diffuse through pores of the nickel electrode to the entire surface of the nickel electrode. Accordingly, the iron precipitate may be evenly bonded to the entire surface of the nickel electrode. In addition, the iron precipitate may diffuse through the pores of the nickel electrode, thereby manufacturing the electrode 100 for the water electrolysis cell including nickel and iron.

The electrode layer of the electrode 100 for the water electrolysis cell may correspond to the nickel electrode. Accordingly, the porous structure of the electrode layer may have a structure substantially the same as or similar to the porous structure of the nickel electrode.

In an embodiment, a thickness of the nickel electrode may be greater than a thickness of the catalyst layer. In an embodiment, the thickness of the catalyst layer may be about 1 nm or more and about 5 μm or less. Preferably, the thickness of the catalyst layer may be about 10 nm or more and about 1 μm or less. More preferably, the thickness of the catalyst layer may be about 300 nm or more and about 500 nm or less. In an embodiment, the thickness of the nickel electrode may be about 300 μm or more and about 400 μm or less. Preferably, the thickness of the nickel electrode may be about 300 μm or more and about 320 μm or less.

Fluid may be introduced into the first flow field 110. For example, the fluid may move toward the anion exchange membrane 30 from the first flow field 110. Specifically, the fluid may pass through pores of the electrode layer included in the electrode 100 for the water electrolysis cell from the first flow field 110 and reach the anion exchange membrane 30. Accordingly, an oxygen generation reaction may be performed in the anode structure 10.

In an embodiment, the fluid may be the electrolyte including the iron precipitate. The electrolyte and the nickel electrode having porosity may react to generate oxygen. In addition, as the iron precipitate is bonded to the surface of the nickel electrode, the electrode 100 for the water electrolysis cell may be manufactured. In an embodiment, the process in which the electrolyte and the nickel electrode having porosity react to generate oxygen and the process in which the iron precipitate is bonded to the surface of the nickel electrode to form the catalyst layer of the electrode 100 for the water electrolysis cell may be simultaneously performed. Specifically, while the nickel electrode is oxidized by the electrolyte to generate oxygen, the iron precipitate may be bonded to the surface of the nickel electrode.

In an embodiment, in the process in which the iron precipitate is bonded to the surface of the nickel electrode to form the catalyst layer of the electrode 100 for a water electrolysis cell, the iron precipitate may be bonded to the surface of the nickel electrode through an electrochemical reaction. For example, the electrochemical reaction may include a cyclic voltammetry(CV), a chrono amperometry(CA), a chronopotentiometry(CP), and the like. These may be used alone or in combination.

The anode structure 10 may include a first end plate providing a first inlet of fluid (e.g., the electrolyte including the iron precipitate) toward an outside, and a gasket disposed between the first flow field 110 and the electrode 100 for a water electrolysis cell. The first end plate may perform a role of supplementing rigidity of the anode structure 10 or forming an external shape. In addition, the first inlet of the first end plate may introduce the electrolyte into the first flow field 110 and deliver the electrolyte to the anode structure 10. The gasket may perform a role of preventing leakage of fluid inside the anode structure 10 or supplementing rigidity of the anode structure 10 while oxygen is generated in the anode structure 10. However, a structure of the anode structure 10 according to the present disclosure may not be limited thereto.

In the cathode structure 20, a hydrogen generation reaction through electrolysis of water may be performed. The cathode structure 20 is illustrated as including a particle-type catalyst layer 210 disposed between the anion exchange membrane 30 and the gas diffusion layer 220, such a structure is exemplary, and the cathode structure 20 according to the embodiments of the present disclosure may not be limited thereto and may have various structures. For example, the cathode structure 20 may have a structure substantially the same as the anode structure 10, or may have a structure with a catalyst of layered double hydroxide.

The catalyst layer 210 may perform a catalytic role in the cathode structure 20. For example, the catalyst layer 210 may be a catalyst in the form of particles. In an embodiment, the catalyst layer 210 may include carbon (c). For example, the catalyst layer 210 may include carbon fiber, carbon nanotube, and the like. However, types of the catalyst layer 210 according to the embodiments of the present disclosure may be merely exemplary and may not be limited thereto.

The gas diffusion layer 220 may have porosity. In an embodiment, the gas diffusion layer 220 may be substantially the same as the nickel electrode for manufacturing the electrode 100 for a water electrolysis cell. For example, the gas diffusion layer 220 may include nickel foam, nickel fiber, nickel paper, and the like, having porosity. These may be used alone or in combination. However, materials included in the gas diffusion layer 220 according to the embodiments of the present disclosure may be merely exemplary and may not be limited thereto.

Fluid may be introduced into the second flow field 230. For example, the fluid may move toward the anion exchange membrane 30 from the second flow field 230. Specifically, the fluid may pass through pores of the gas diffusion layer 220 from the second flow field 230 and reach the anion exchange membrane 30. Accordingly, a hydrogen generation reaction may be performed in the cathode structure 20.

The cathode structure 20 may include a second end plate providing a second inlet of fluid toward the outside, and a gasket disposed between the second flow field 230 and the gas diffusion layer 220. The second end plate may perform a role of supplementing rigidity of the cathode structure 20 or forming an external shape. In addition, an electrolyte for a hydrogen generation reaction of the cathode structure 20 may be introduced into the second inlet of the second end plate. The type of the electrolyte introduced into the second inlet of the cathode structure 20 may be different from the type of the electrolyte introduced into the first inlet of the anode structure 10. However, materials introduced into the first inlet and the second inlet according to the embodiments of the present disclosure may not be limited thereto. The gasket may perform a role of preventing leakage of fluid inside the cathode structure 20 or supplementing rigidity of the cathode structure 20 while oxygen is generated in the cathode structure 20. However, a structure of the cathode structure 20 according to the present disclosure may not be limited thereto.

As described above, in the electrode 100 for a water electrolysis cell, the catalyst layer including NiFeO_xH_y may be bonded to the surface of the electrode layer including nickel. Accordingly, the electrode 100 for the water electrolysis cell may have low electron transfer resistance in a high frequency band and may deliver a current having a high current density. In addition, the electrode 100 for the water electrolysis cell may increase the activity of an oxygen generation reaction, and since a relatively wide surface area of an interface in which the electrode layer and the catalyst layer are bonded is formed, the performance and operational efficiency of the water electrolysis cell including the electrode 100 for the water electrolysis cell may be improved.

As described above, in the method of the electrode 100 for the water electrolysis cell using iron precipitate, a process of bonding the catalyst layer 210 to the surface of the electrode layer of the electrode 100 for the water electrolysis cell may be performed through an electrochemical reaction such as CV cycling. By controlling the number of the electrochemical reactions, a thickness of the catalyst layer 210 formed on the surface of the electrode 100 for the water electrolysis cell, a degree of activation of oxygen evolution of the electrode 100 for the water electrolysis cell, a current density, and an electron transfer resistance may be easily controlled. Therefore, efficiency in a manufacturing process of the electrode 100 for the water electrolysis cell and the water electrolysis cell including the electrode 100 for the water electrolysis cell may be improved, and manufacturing time and cost may be reduced.

Hereinafter, effects of the electrode for the water electrolysis cell according to exemplary embodiments, the water electrolysis cell including the electrode for the water electrolysis cell, and the method of manufacturing the electrode for the water electrolysis cell will be described in greater detail through specific experimental examples. The experimental examples are provided merely for illustration, and the scope of the present disclosure may not be limited thereto.

Preparation Example 1—an Electrolyte Manufacturing

According to the conditions of a Table 1 below, after generating an iron precipitate by reacting an iron aqueous solution obtained by mixing an FeCl₂ precursor and water distilled water with an alkaline solution of KOH, the electrolyte was prepared by physically shaking the solution including the iron precipitate so that the iron precipitate was uniformly dispersed in the solution including the iron precipitate

	TABLE 1
	Component	Condition
	FeCl2	36 mg
	KOH	0.1M, 1 kg

Preparation Example 2—a Membrane-Electrode Assembly Manufacturing

In order to exchange Br⁻ ions of a PiperION® anion exchange membrane having a thickness of 30 μm with OH⁻ ions, the anion exchange membrane was subjected to pretreatment by immersing the anion exchange membrane in a 1M KOH solution for 1 hour. On one side of the pretreated anion exchange membrane, a nickel foam having a thickness of 300 μm, a porosity of 98%, and a surface area of 4 cm² was disposed, and on the other side opposite to said one side of the anion exchange membrane, a catalyst layer including PtRu was disposed. Thereafter, a pair of gaskets respectively disposed at the anode and the cathode, a titanium block adjacent to the nickel foam, a graphite block adjacent to the catalyst layer, and a flow field of a single serpentine were assembled with the anion exchange membrane, the nickel foam, and the catalyst layer to manufacture a membrane-electrode assembly MEA. An active area of the manufactured membrane-electrode assembly was 4 cm², and the single cell was fastened with eight bolts to which a torque of 20 kgf cm was applied.

Preparation Example 3—a Nickel-Iron Integrated Electrode Manufacturing

The electrolyte manufactured according to Preparation Example 1 was introduced to circulate to the anode of the membrane electrode assembly. CV cycling was performed under the conditions of a Table 2 below so that the iron precipitate included in the electrolyte was bonded to the surface of the nickel foam. Within the pH range of KOH and the voltage range according to Table 2 below, NiFeOOH was spontaneously bonded to the surface of the nickel foam, thereby manufacturing a nickel-iron integrated electrode.

	TABLE 2
	Condition	Value

Unit cell temperature

60°

C.

Voltage range

1.56 V-1.75 V

Scan rate

50

mVs−1

pH of KOH

13-14

	Flow rate in the flow field of KOH	5	ml/min
	Number of times of CV cycling	800	times

FIG. 4 is a view illustrating SEM images illustrating a surface of a nickel-iron integrated electrode manufactured according to a Preparation Example 3 depending on composition. FIG. 5 is a view illustrating TEM images illustrating a cross-section of a nickel-iron integrated electrode manufactured according to the Preparation Example 3 depending on composition. FIG. 6 is a view illustrating graphs analyzing the nickel-iron integrated electrode manufactured according to Preparation Example 3.

For example, FIG. 4(a) is an SEM image illustrating the surface of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, FIG. 4(b) is an SEM image illustrating nickel particles distributed on the surface of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, FIG. 4(c) is an SEM image illustrating iron particles distributed on the surface of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, FIG. 4(d) is an SEM image illustrating oxygen particles distributed on the surface of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, and FIG. 4(e) is an SEM image simultaneously illustrating the nickel particles, the iron particles, and the oxygen particles distributed on the surface of the nickel-iron integrated electrode manufactured according to the Preparation Example 3.

For example, FIG. 5(a) is a TEM image scanned in the arrow direction of a cross-section of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, FIG. 5(b) is a TEM image illustrating nickel particles distributed in the cross-section of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, FIG. 5(c) is a TEM image illustrating iron particles distributed in the cross-section of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, FIG. 5(d) is a TEM image illustrating oxygen particles distributed in the cross-section of the nickel-iron integrated electrode manufactured according to the Preparation Example 3, and FIG. 5(e) is a TEM image illustrating the nickel particles, the iron particles, and the oxygen particles distributed in the cross-section of the nickel-iron integrated electrode manufactured according to the Preparation Example 3.

For example, FIG. 6(a) is a graph showing mass fractions of materials included in the nickel-iron integrated electrode manufactured according to the Preparation Example 3, FIG. 6(b) is a graph analyzing peak intensities of materials included in the nickel-iron integrated electrode manufactured according to the Preparation Example 3 using XRD X-ray diffraction, and FIG. 6(c) is a graph analyzing peak intensities of materials included in the nickel-iron integrated electrode manufactured according to the Preparation Example 3 using Raman spectroscopy.

Referring to FIGS. 4, 5, and 6, the distribution of nickel, iron, and oxygen on the surface of the nickel-iron integrated electrode manufactured according to the Preparation Example 3 was confirmed through an SEM scanning electron microscope. In addition, the cross-section of the inside of the nickel-iron integrated electrode manufactured according to the Preparation Example 3 was observed using an FIB focused ion beam. Accordingly, a state that nickel, iron, and oxygen were distributed in the inside adjacent to the surface of the nickel-iron integrated electrode was confirmed through a TEM transmission electron microscope. Thus, a state that nickel atoms, iron atoms, and oxygen atoms are distributed on the surface and the inside of the nickel-iron integrated electrode manufactured according to the Preparation Example 3 may be confirmed.

In an internal area of the nickel-iron integrated electrode up to a depth of 50 nm from the surface, nickel particles were confirmed to account for the largest mass fraction. In addition, in the internal area of the nickel-iron integrated electrode deeper than 110 nm from the surface, iron particles were confirmed to account for the largest mass fraction, and nickel particles accounted for the smallest mass fraction. The internal area up to the depth of 50 nm from the surface, which is a nickel-rich area, corresponds to an interface area between the iron precipitate and the nickel foam. In addition, the nickel particles in the internal area up to the depth of 50 nm from the surface may be confirmed as particles generated by oxidation of the nickel foam.

Through peaks according to XRD, peaks of FeOOH were identified in the nickel-iron integrated electrode manufactured according to the Preparation Example 3 including the nickel foam to which NiFeOxHy was bonded on the surface. In addition, through Raman spectroscopy, peaks of NiOOH having peak wavelengths of 480 cm⁻¹ and 557 cm⁻¹ were identified in the nickel-iron integrated electrode manufactured according to the Preparation Example 3 including the nickel foam to which NiFeOxHy was bonded on the surface. Accordingly, bonding between Fe and Ni may be indirectly confirmed, and the presence of NiFeOOH may be identified.

FIG. 7 is a view illustrating TEM images depending on the number of times of CV cycling of nickel-iron integrated electrodes according to an Example 1 and an Example 2. FIG. 8 is a view illustrating a graph for explaining performance of nickel-iron integrated electrodes according to the Example 1, the Example 2, and an Example 3.

For example, FIG. 7(a) is a view illustrating SEM and TEM images of the nickel-iron integrated electrode subjected to 50 times of CV cycling, and FIG. 7(b) is a view illustrating SEM and TEM images of the nickel-iron integrated electrode subjected to 800 times of CV cycling.

For example, FIG. 8(a) is a view illustrating a graph of current density versus voltage of the nickel-iron integrated electrode according to the number of times of CV cycling, FIG. 8(b) is a view illustrating a graph of overpotential increase of the nickel-iron integrated electrode according to the number of times of CV cycling, FIG. 8(c) is a view illustrating a graph of electron transfer resistance of the nickel-iron integrated electrode according to the number of times of CV cycling, and FIG. 8(d) is a view illustrating a graph of current density versus scan rate of the nickel-iron integrated electrode according to the number of times of CV cycling.

Referring to FIGS. 7 and 8, an Example 1 is a nickel-iron integrated electrode manufactured by performing 800 times of CV cycling according to the Preparation Example 3, an Example 2 is a nickel-iron integrated electrode manufactured by substantially the same method as Preparation Example 3 except that 50 times of CV cycling were performed, and an Example 3 is a nickel-iron integrated electrode manufactured by substantially the same method as the Preparation Example 3 except that CV cycling was not performed.

A thickness of the iron precipitate bonded to the surface of the nickel foam of the Example 1 was measured as 275 nm, and a thickness of the iron precipitate bonded to the surface of the nickel foam of the Example 2 was measured as 34 nm. Accordingly, as the number of times of CV cycling increases, increasing of the thickness of the catalyst layer of the nickel-iron integrated electrode was confirmed.

At the same voltage, the current density measured from the nickel-iron integrated electrode of the Example 1 subjected to 800 times of CV cycling may be relatively higher than the current density measured from the nickel-iron integrated electrode of the Example 2 subjected to 50 times of CV cycling. In addition, at the same voltage, the current density measured from the nickel-iron integrated electrode of the Example 2 subjected to 50 times of CV cycling may be relatively greater than the current density measured from the nickel-iron integrated electrode of the Example 3 in which CV cycling was not performed. Accordingly, as the number of times of CV cycling increases, increasing of the current density measured from the nickel-iron integrated electrode under the same voltage was confirmed.

The Tafel slopes of the nickel-iron integrated electrodes of Examples 1, 2, and 3 illustrated in the graph of FIG. 8(b) were measured. The Tafel slope is a value indicating the degree of activation of an oxygen generation reaction, and the lower the Tafel slope, the better the oxygen generation reaction proceeds. The nickel-iron integrated electrode of the Example 1 subjected to 800 times of CV cycling was measured to have a Tafel slope of 69.6 mV dec⁻¹, the nickel-iron integrated electrode of the Example 2 subjected to 50 times of CV cycling was measured to have a Tafel slope of 53.2 mV dec⁻¹, and the nickel-iron integrated electrode of the Example 3 in which CV cycling was not performed was measured to have a Tafel slope of 76.2 m V dec⁻¹. Accordingly, among the examples, the nickel-iron integrated electrode of the Example 2 subjected to 50 times of CV cycling was measured to have the lowest Tafel slope, and thus the nickel-iron integrated electrode of Example 2 had the highest degree of activation of the oxygen generation reaction.

The electron transfer resistances of the nickel-iron integrated electrodes of the Examples 1, 2, and 3 were compared using the size of semicircles for EIS electrochemical impedance spectroscopy in the frequency range of 3.16 Hz illustrated in the graph of FIG. 8(c). Accordingly, the electron transfer resistance of the Example 1 subjected to 800 times of CV cycling was the smallest, and the electron transfer resistance of the Example 3 in which CV cycling was not performed was the largest. Therefore, as the number of times of CV cycling performed on the nickel-iron integrated electrode increases, decreasing of the electron transfer resistance was confirmed.

The surface areas of the anodes including the nickel-iron integrated electrodes of Examples 1, 2, and 3 were compared through the slopes of the graphs illustrated in FIG. 8(d). The slope value of the nickel-iron integrated electrode of Example 1 subjected to 800 times of CV cycling was measured as 1.8 F cm⁻², the slope value of the nickel-iron integrated electrode of Example 2 subjected to 50 times of CV cycling was measured as 1.6 F cm⁻², and the slope value of the nickel-iron integrated electrode of the Example 3 in which CV cycling was not performed was measured as 0.2 F cm⁻². Accordingly, as the number of times of CV cycling increases, increasing of the amount of iron precipitate bonded or adsorbed to the surface of the nickel-iron integrated electrode was confirmed, thereby increasing the surface area of the anode. When the surface area of the anode increases, the nickel-rich area forming an interface between the nickel foam and the iron precipitate included in the nickel-iron integrated electrode also increases.

As described above, through the Examples 1, 2, and 3, a formation thickness of the iron precipitate, a degree of activation of the oxygen generation reaction, the current density, and the electron transfer resistance according to the change in the number of times of CV cycling were measured. Accordingly, in the method of manufacturing the electrode 100 for a water electrolysis cell of FIG. 1 according to the embodiments of the present disclosure corresponding to the nickel-iron integrated electrodes of the Examples 1 and 2, the thickness of the catalyst layer bonded to the surface of the electrode layer, the degree of activation of the oxygen generation reaction, the current density, and the electron transfer resistance of the electrode 100 for a water electrolysis cell may be easily adjusted by controlling the number of times of CV cycling. Therefore, the efficiency of the manufacturing process of the electrode 100 for a water electrolysis cell and the water electrolysis cell of FIG. 1 including the electrode 100 for a water electrolysis cell may be improved, and manufacturing time and cost may be reduced.

FIG. 9 is a view illustrating images of nickel-iron integrated electrodes according to the Example 1, an Example 4, and an Example 5. FIG. 10 is a view illustrating graphs for comparing performance of nickel-iron integrated electrodes according to the Example 1, the Example 4, and the Example 5.

For example, FIG. 10(a) is a view illustrating a graph measuring the current densities of the nickel-iron integrated electrodes according to Examples 1, 4, and 5, FIG. 10(b) is a view illustrating a graph measuring the electron transfer resistances of the nickel-iron integrated electrodes according to Examples 1, 4, and 5, and FIG. 10(c) is a view illustrating a graph measuring the peak wavelengths of the nickel-iron integrated electrodes according to Examples 1, 4, and 5.

Referring to FIGS. 9 and 10, an Example 4 is a catalyst electrode manufactured by forming a catalyst layer by dispersing a particle-type NiFe(e.g., NiFeP) alloy on one surface of a nickel foam by a spray method, and an Example 5 is a catalyst electrode manufactured by an SS solid solution method in which the nickel foam was aged in an aqueous solution including NiFe, followed by heat treatment at a temperature of 400° C. for 10 minutes under an air atmosphere.

Through the graph of FIG. 10(a), under the same voltage condition, the current density of Example 1 was measured to be the highest. Through the graph of FIG. 10(b), under both the condition of a current density of 0.04 A·cm⁻² and the condition of 1 A·cm⁻², the electron transfer resistance of Example 1 was measured to be the lowest. Through the graph of FIG. 10(c), among the examples, the peak wavelength measured in Example 1 was the most similar to the peak wavelength for oxygen deficiency, which most greatly contributes to the oxygen generation reaction. Accordingly, the degree of activation of oxygen generation and the performance with respect to current density of the electrode 100 for a water electrolysis cell of FIG. 1 according to the embodiments of the present disclosure corresponding to the Example 1, manufactured by bonding an iron precipitate to the surface of a nickel foam, were measured to be the most excellent.

The present disclosure may be used in electronic devices and electronic apparatuses applied to energy storage, energy conversion, and application fields thereof. While the present disclosure has been described above with reference to exemplary embodiments, it will be understood by those skilled in the art that the present disclosure may be variously modified and changed without departing from the spirit and scope of the present disclosure defined in the following claims.