NIKEL IRON-BASED CATALYST DOPED WITH METAL HAVING ELECTRONEGATIVITY LOWER THAN THAT OF NI AND FE, MANUFACTURING METHOD THEREOF, AND ALKALINE WATER ELECTROLYSIS SYSTEM

Abstract

The present invention relates to a NiFe-based catalyst for OER doped with a metal having lower electronegativity than Ni and Fe, and a method for manufacturing the same. More specifically, the present invention offers the advantage of using nickel, a non-noble metal-based active catalyst, which has high economic value without the need for noble metals. The present invention provides a method for manufacturing a NiFe-based catalyst for OER that exhibits excellent activity in oxygen generation reaction by maximizing the surface area compared to existing noble metal-based catalysts, thereby contributing significantly to the cost reduction of hydrogen production.

Claims

1. A doped NiFe-based catalyst for OER, comprising an alloy comprising Fe; Ni; and a metal having electronegativity lower than that of Ni and Fe, wherein the alloy comprises Ni and Fe at a molar ratio of 9:1 to 3:2.

2. The doped NiFe-based catalyst for OER according to claim 1, wherein the metal having electronegativity lower than that of Ni and Fe comprises Al, Cd, or Zn.

3. The doped NiFe-based catalyst for OER according to claim 2, wherein the alloy comprises Ni and Fe at a molar ratio of 3:1.

4. The doped NiFe-based catalyst for OER according to claim 3, wherein the alloy comprises a metal having electronegativity lower than that of Ni and Fe at a molar ratio of 5% to 10%.

5. The doped NiFe-based catalyst for OER according to claim 4, wherein the alloy comprises a metal having electronegativity lower than that of Ni and Fe at a molar ratio of 5%.

6. The doped NiFe-based catalyst for OER according to claim 1, wherein the metal having electronegativity lower than that of Ni and Fe comprises Al.

7. The doped NiFe-based catalyst for OER according to claim 6, wherein the alloy comprises Ni and Fe at a molar ratio of 3:1; and wherein the alloy comprises Al at a molar ratio of 5% to 10%.

8. The doped NiFe-based catalyst for OER according to claim 6, wherein the alloy comprises Al at a molar ratio of 5%.

9. An alkaline water electrolysis system comprising end plates, current collectors, bipolar plates, porous transport layers, gaskets, an anode, a separator, and a cathode, wherein the anode comprises the doped NiFe-based catalyst for OER according to claim 1.

10. A method for manufacturing a doped NiFe-based catalyst for OER, comprising: material mixing in which powders of NI, Fe, and a metal having electronegativity lower than that of Ni and Fe (e.g., Al, Zn, Cd) are mixed; or a Ni precursor, Fe, and a metal having electronegativity lower than that of Ni and Fe are mixed with a solvent; or Fe and a precursor of a metal having electronegativity lower than that of Ni and F are mixed with a solvent and the resulting mixture is mounted on a Ni support; and alloying in which a mixed powder of NI, Fe, and a metal having electronegativity lower than that of Ni and Fe; a Ni precursor, Fe, and a metal having electronegativity lower than that of Ni and Fe mixed with a solvent; or Fe and a precursor of a metal having electronegativity lower than that of Ni and F mounted on a Ni support are thermally treated in a reducing gas atmosphere to form an alloy, wherein Ni and Fe in the alloy is included at a molar ratio of 9:1 to 3:2.

11. The method for manufacturing a doped NiFe-based catalyst for OER according to claim 10, wherein the metal having electronegativity lower than that of Ni and Fe comprises Al, Cd, or Zn.

12. The method for manufacturing a doped NiFe-based catalyst for OER according to claim 11, wherein the alloy comprises Ni and Fe at a molar ratio of 3:1.

13. The method for manufacturing a doped NiFe-based catalyst for OER according to claim 12, wherein the alloy comprises a metal having electronegativity lower than that of Ni and Fe at a molar ratio of 5% to 10%.

14. The method for manufacturing a doped NiFe-based catalyst for OER according to claim 13, wherein the alloy comprises a metal having electronegativity lower than that of Ni and Fe at a molar ratio of 5%.

15. The method for manufacturing a doped NiFe-based catalyst for OER according to claim 10, wherein the metal having electronegativity lower than that of Ni and Fe comprises Al.

16. The method for manufacturing a doped NiFe-based catalyst for OER according to claim 15, wherein the alloy comprises Ni and Fe at a molar ratio of 3:1; and wherein the alloy comprises Al at a molar ratio of 5% to 10%.

17. The method for manufacturing a doped NiFe-based catalyst for OER according to claim 16, wherein the alloy comprises Al at a molar ratio of 5%.

Description

DESCRIPTION OF THE DRAWINGS

[0021] FIG. 1 is a schematic diagram of an alkaline water electrolysis cell according to an embodiment of the present invention.

[0022] FIG. 2 is a flow chart illustrating a method for manufacturing a NiFe-based catalyst for OER according to an embodiment of the present invention.

[0023] FIG. 3 is a graph displaying the measurements of current density-potential with respect to the oxygen evolution reaction when using a catalyst, while varying the ratio of Ni and Fe.

[0024] FIG. 4 is a graph measuring the overpotential when the current density reaches 10 mA/cm2, corresponding to the ratio (molar ratio) of Fe from FIG. 3.

[0025] FIG. 5 is a current density-voltage graph representing the activity of the NiFe-based catalyst for OER, according to the aluminum doping ratio in an embodiment of the present invention.

[0026] FIG. 6 is a graph illustrating the overpotential of the NiFe-based catalyst for OER, according to the aluminum doping ratio in an embodiment of the present invention.

[0027] FIG. 7 is a graph demonstrating the performance enhancement of an aluminum-doped NiFe-based catalyst compared to a conventional Ni catalyst, or a NiFe-based catalyst not doped with a metal having lower electronegativity than Ni and Fe.

[0028] FIG. 8 is a graph evaluating the durability of a NiFe-based catalyst for OER in a 5M KOH electrolyte, according to the aluminum doping ratio in an embodiment of the present invention.

[0029] FIGS. 9A to 9D is an XPS (X-ray Photoelectron Spectroscopy)-based graph aimed at explaining the effect of aluminum doping in a NiFe electrode in accordance with an example of the present invention.

[0030] FIG. 10 is a current density-voltage graph representing the activity of a NiFe-based catalyst for OER doped with a metal material possessing lower electronegativity than Ni and Fe, in accordance with an embodiment of the present invention.

[0031] FIGS. 11A and 11B is a Raman spectrum graph intended to explain the lattice structures of NiFe and NiFeAl, as well as a graph depicting the EXAFS (Extended X-ray Absorption Fine Structure) measurement results for gauging the change in the d-band center, in accordance with an embodiment of the present invention.

BEST MODE

[0032] Hereinafter, the embodiments of the present application will be explained in greater detail with reference to the accompanying drawings. However, the technology disclosed in this application is not confined to the embodiments discussed herein and can be materialized in other forms. The embodiments introduced here are provided to ensure that the disclosed content is comprehensive and complete, and to sufficiently convey the concept of this application to those skilled in the art. In the drawings, the width or thickness of components, among other dimensions, has been somewhat magnified to clearly depict the components of each device.

[0033] Furthermore, for the sake of convenience in description, only some components have been depicted, however, those skilled in the art should be able to readily understand the remaining components. Generally, when explaining the drawings, it was done so from the observer's perspective, and when it is mentioned that one element is located above or below another, it includes all meanings that the particular element could be directly above or below the other, or there could be additional elements interspersed between them.

[0034] Furthermore, someone with ordinary skill in the art would be able to implement the concept of the present application in various other forms within the scope of the technical principle of the present application. Additionally, the same reference numerals in multiple drawings generally indicate the same or similar elements.

[0035] Moreover, unless the context clearly dictates otherwise, singular expressions should be understood to include plural expressions. Terms such as comprise and have should be understood to specify the presence of features, numbers, steps, operations, components, parts, or combinations thereof, without excluding the possibility of the presence or addition of one or more other features, numbers, steps, operations, components, parts, or combinations thereof.

[0036] Additionally, in the execution of a method or manufacturing method, the individual processes comprising the method may occur in a sequence different from the specified sequence unless a specific order is explicitly stated in the context. Thus, the individual processes may occur in the specified sequence, be performed substantially simultaneously, or be performed in the reverse order.

[0037] Moving forward, a more detailed description of the present invention will be provided.

[0038] FIG. 1 is a schematic diagram illustrating an alkaline water electrolysis system according to an example of the present invention. As shown in FIG. 1, the alkaline water electrolysis system of the present invention includes end plates (110, 170), current collectors (115, 175), bipolar plates (116, 176), porous transport layers (120, 160), gaskets (125, 165), an anode (130), a separator (140), a cathode (150), and cell frames (135, 165).

[0039] End plates (110, 170) ensure that each component can be uniformly compressed and fastened using bolts/nuts during the assembly of the electrolysis system, while also providing protection for the bipolar plates (116, 176) when they are fastened and compressed.

[0040] Current collectors (115, 175) are connected to a DC power source to supply the required current (electrons) for electrolysis to the entire system.

[0041] Bipolar plates (116, 176) have internal fluid flow channels through which an electrolyte solution (e.g., KOH) is supplied. Oxygen generated through the oxidation reaction at the anode and hydrogen generated through the reduction reaction at the cathode are discharged through these fluid flow channels. Bipolar plates (116, 176) can be made of materials such as nickel.

[0042] Porous transport layers (120, 160) perform the function of removing oxygen and hydrogen gas bubbles from the electrode surface under high current density operating conditions. For example, porous transport layers (120, 160) may be made of materials like nickel or nickel foam.

[0043] The separator (140) serves as an electrical insulator and acts as a medium for the transport of hydroxide ions during electrolysis, while also physically separating oxygen and hydrogen. The separator material must possess durability in highly alkaline (30% KOH) environments exceeding 80 C. and have low gas permeability to prevent the mixing of hydrogen and oxygen gases generated from both electrodes. Additionally, it requires high ion conductivity. In the past, asbestos, known for its high durability, was commonly used. However, more recently, materials such as porous composites with dispersed ceramic particles and a polymer binder, polyphenylene sulfide (PPS) reinforced with glass fibers, and nickel porous matrices formed by sintering a nickel oxide layer are being utilized.

[0044] An anode (130) and cathode (150) are typically composed of transition metals such as Ni, Fe, Co, and Mo. They are coated onto stainless steel porous bodies (perforated plates, meshes, expanded metal meshes, etc.). For instance, Raney Ni is employed as a hydrogen-generating electrode due to its high electrode activity.

[0045] In this specific example, Raney Ni was used as the material for the cathode (150). Furthermore, in the Examples of the present invention, the anode (130) employed NiFe alloy doped with a metal of lower electronegativity than Ni and Fe, such as NiFeAl, NiFeCd, or NiFeZn.

[0046] Subsequently, a detailed description will be provided for the NiFe-based catalyst for OER included in the cathode (130) of the alkaline water electrolysis system according to an Example of the present invention.

MODE OF THE INVENTION

NiFe-Based Catalyst for OER According to an Example of the Present Invention

[0047] A NiFe-based catalyst for OER, which is an Example of the present invention, comprises an alloy that includes Fe, Ni, and a metal with lower electronegativity than Ni and Fe. The alloy may consist of Ni and Fe at a molar ratio ranging from 9:1 to 3:2. In the most preferable case, the molar ratio of Ni and Fe is 3:1. The metal with lower electronegativity than Ni and Fe can be, for example, Al, Zn, or Cd, with Al being the most preferable choice.

[0048] Moreover, Ni and a metal with lower electronegativity than Ni and Fe (e.g., Al, Zn, Cd) can be included in the alloy at a molar ratio of 5% to 10%. In the optimal scenario, Ni and a metal with lower electronegativity than Ni and Fe (e.g., Al, Zn, Cd) are included at a molar ratio of 5%.

[0049] Thus, the present invention provides an excellent doped NiFe-based catalyst for OER, demonstrating outstanding activity and durability in the oxygen generation reaction. The catalyst contributes significantly to reducing the cost of hydrogen production by utilizing a non-noble metal-based catalyst.

A Method for Manufacturing a NiFe-Based Catalyst for OER According to an Example of the Present Invention.

[0050] According to FIG. 2, another aspect of the present invention is a method for manufacturing a NiFe-based catalyst for OER. The method includes a material mixing step (S100) where powders of Ni, Fe, and a metal with lower electronegativity than Ni and Fe (e.g., Al, Zn, Cd) are mixed to prepare a mixed powder or a step where a precursor of Ni, Fe, and a metal with lower electronegativity than Ni and Fe is dissolved in a solvent, such as ultrapure water (DI water), and the resulting solution is applied onto a Ni support. The method further includes an alloy formation step (S200) where the mixed powder, a Ni support with a precursor of Ni, Fe, and a metal with lower electronegativity than Ni and Fe mounted on it, or a dissolved precursor of Ni, Fe, and a metal with lower electronegativity than Ni and Fe is subjected to heat treatment in a reducing gas atmosphere to form an alloy.

[0051] FIG. 2 is a flow chart illustrating a method for manufacturing a NiFe-based catalyst for OER according to an Example of the present invention.

[0052] Referring to FIG. 2, the method for manufacturing a NiFe-based catalyst for OER in the present invention includes a material mixing step (S100) and an alloying formation step (S200).

Material Mixing

[0053] The material mixing step (S100) is performed to mix Fe, Ni, and Al in a desired composition to form an Al-doped NiFe-based catalyst for OER as intended in the present invention. While Al is used for the convenience of explanation in this Example, it is understood that other metals with lower electronegativity than Ni and Fe, such as Cd and Zn, can be mixed in a similar manner to Al.

[0054] Among these, Fe can be, for example, Fe powder or an Fe oxide. Alternatively, in another specific example of the present invention, an Fe precursor selected from the group consisting of iron sulfate (FeSO.sub.4, Fe.sub.2 (SO.sub.4).sub.3), iron acetate (Fe(CO.sub.2CH.sub.3).sub.2), iron chloride (FeCl.sub.2, FeCl.sub.3), iron chloride hydrate (FeCl.sub.3.Math.nH.sub.2O), iron nitrate hydrate (Fe(NO.sub.3).sub.3.Math.9H.sub.2O), and combinations thereof can be used. However, the type of Fe precursor is not limited as long as it is used to achieve the objectives of the present invention.

[0055] Nickel in the alloy acts as a transition metal that serves as a non-noble metal-based active catalyst for achieving the objectives of the present invention. In this case, nickel can be prepared as a powder. Alternatively, it can be prepared as a precursor. Examples of nickel precursors include NiCl2.Math.xH2O, (CH.sub.3COO).sub.2Ni.Math.xH.sub.2O, nickel (II) carbonate hydroxide, nickel (II) acetylacetonate, nickel (II) hydroxide, Ni(NO.sub.3).sub.2.Math.xH.sub.2O, NiSO.sub.4.Math.XH.sub.2O, NiI.sub.2, and NiF.sub.2, or any combination thereof. However, the type of nickel precursor is not limited as long as it is used to achieve the objectives of the present invention.

[0056] In this case, a desirable molar ratio for combining Ni and Fe is preferably 9:1 to 3:2, and most preferably 3:1.

[0057] Meanwhile, aluminum (Al) can be prepared as a powder or as a precursor. Examples of Al precursors include aluminum sulfate (Al.sub.2 (SO.sub.4).sub.3), aluminum chloride (AlCl.sub.3), aluminum nitrate (Al(NO.sub.3).sub.3), aluminum nitrate hydrate (Al(NO.sub.3).sub.3.Math.nH.sub.2O), or any combination thereof. However, the type of Al precursor is not limited as long as it is used to achieve the objectives of the present invention.

[0058] In this case, it is desirable for Al to be included in the alloy comprising Fe, Al, and Ni at a molar ratio of 2% to 10%. Most preferably, Al is included at a molar ratio of 5%.

[0059] The solvent used can be a polar solvent, such as water or an alcohol-based solvent. The alcohol-based solvent may include, for example, methanol, ethanol, propanol, butanol, pentanol, or a combination thereof. Water may preferably be ultrapure water (DI Water). However, the type of solvent is not limited as long as it is used to achieve the objectives of the present invention.

Alloying

[0060] Alloying (S200) is performed with the purpose of forming an alloy by subjecting a mixture of Fe, Al, and Ni powders, a precursor of Ni, Fe, and Al dissolved in a solvent, or a Ni support with a dissolved precursor of Fe and Alto thermal treatment in a reducing gas atmosphere. The reducing gas can include gases such as hydrogen and argon, with a preferred composition of 5% hydrogen and 95% argon. It should be noted that while this Example explains the process with Al as the focus for convenience, it is understood that metals with lower electronegativity than Ni and Fe, such as Cd and Zn, can be applied in the same manner as Al.

[0061] The thermal treatment can be carried out at temperatures ranging from 1000 to 1500 C. for a duration of 50 to 100 seconds in a reducing gas atmosphere. Preferably, the thermal treatment can be conducted at temperatures ranging from 1200 to 1300 C. for 70 to 80 seconds in a reducing gas atmosphere. If the conditions fall below the mentioned ranges, a composite of Fe, Ni, and Al may not be formed, whereas exceeding the ranges may result in a reduction of the specific surface area of the formed composite due to sintering.

[0062] A NiFe-based catalyst for OER, manufactured using the manufacturing method of the present invention, offers high economic value by utilizing nickel as a non-noble metal-based active catalyst instead of noble metal materials. It demonstrates excellent characteristics, particularly in terms of exhibiting outstanding activity in the oxygen generation reaction and contributing significantly to the reduction of hydrogen production costs.

[0063] Subsequently, a more detailed explanation of the configuration and operation of the present invention will be provided through preferred Examples. However, it should be noted that these Examples are presented as preferred instances of the present invention and should not be interpreted as limiting the scope of the invention in any way.

[0064] Any additional information not described here can be readily inferred by those skilled in the art, and therefore, its explanation is omitted.

Experimental Example

Determination of Optimal Ratios of NiFe-Based Catalysts

[0065] According to the desired compositions (varying the ratio of Ni to Fe to 0:10, 2.5:7.5, 5:5, 7.5:2.5, and 10:0), a Fe precursor was dissolved in ultra-pure water (DI water) and mounted on a Ni support. Subsequently, an alloy in the form of a thin film was formed by thermally treating it at 1100 C. for 70 seconds in a 5% hydrogen and 95% argon atmosphere.

[0066] FIG. 3 is a graph illustrating the measurements of current density-potential according to the oxygen generation reaction when using Ni and Fe as catalysts with varying ratios of 0:10, 2.5:7.5, 5:5, 7.5:2.5, and 10:0, respectively. FIG. 4 is a graph showing the measurements of overvoltage at a current density of 10 mA/cm.sup.2 corresponding to the ratio (molar ratio) of Fe in FIG. 3.

[0067] Referring to FIGS. 3 and 4, it can be observed that the maximum efficiency in the oxygen generation reaction is achieved when the Ni:Fe ratio is preferably 9:1 to 3:2. However, the most preferable ratio for maximum efficiency (lowest overvoltage) is Ni:Fe at 3:1. In other words, the potential at which the current density increases sharply is minimized at the ratio of Ni3Fe1. Therefore, the experiment was conducted by fixing the Ni:Fe ratio at 3:1 and doping Al at different ratios.

[0068] The catalyst was evaluated using the following method.

[0069] A three-electrode experiment was conducted using the catalyst (Ni.sub.3Fe.sub.1Al.sub.x) fabricated as the working electrode, along with a reference electrode and a counter electrode. The experiment was set up using a Rotating Disk Electrode (RDE) configuration, and the OER polarization curve was measured under ambient temperature and pressure conditions. Subsequently, the potential at the corresponding current density was measured to compare the overvoltage at 10 mA/cm.sup.2.

Conditions for all Experiments

[0070] a. working electrode: The manufactured catalyst was used as a Rotating Disk Electrode (RDE), which was rotated at 1600 rpm. [0071] Reference electrode: Ag/AgCl [0072] Counter electrode: Pt Wire [0073] b. Electrolyte: 1M KOH (pH 14) [0074] c. Temperature: Room temperature (25 C.) [0075] OER activity evaluation experiment [0076] a. The electrolyte was purged with argon for 30 minutes to establish an argon atmosphere. [0077] b. The working electrode was rotated at 1600 rpm to remove oxygen generated from the electrode due to OER. [0078] c. Scan rate: 1 mV/s [0079] d. Scan range: 0.05 V (vs RHE) to 2.00 V (vs RHE)

[0080] A catalyst property evaluation experiment was conducted as described above, and the experimental results are shown in FIGS. 5 and 6.

[0081] FIG. 5 is a current density-voltage graph illustrating the activity of a Ni.sub.3Fe.sub.1-based catalyst for OER based on the aluminum doping ratio according to an Example of the present invention. FIG. 6 is a graph showing the overvoltage of a Ni.sub.3Fe.sub.1-based catalyst for OER based on the aluminum doping ratio according to an Example of the present invention.

[0082] Referring to FIGS. 5 and 6, it can be observed that when the aluminum doping ratio, which refers to the molar ratio of Al in the overall Fe, Al, and Ni alloy, was within the range of 2% to 10%, the current density increased rapidly at significantly lower voltages compared to the NiFe-based catalyst without aluminum. Moreover, the most favorable activity was achieved when the aluminum doping ratio was 5%. More specifically, it was observed that at an aluminum doping ratio of 5%, the potential at 10 mA/cm.sup.2 was approximately 1.509 V, indicating a difference of about 37 mV in overvoltage compared to the NiFe-based catalyst without aluminum doping.

[0083] FIG. 7 is a graph illustrating the performance improvement of an aluminum-doped NiFe-based catalyst compared to a conventional Ni catalyst and an aluminum-undoped NiFe-based catalyst.

[0084] Referring to FIG. 7, it can be observed that the aluminum-doped NiFe-based catalyst according to the present invention showed a 236% increase in the oxygen generation reaction compared to the Ni catalyst (nickel foam (NF)) and a 142% increase compared to the aluminum-undoped NiFe-based catalyst (based on 1.8V).

[0085] Furthermore, FIG. 8 is a graph showing the durability evaluation conducted in a 5M KOH electrolyte. More specifically, the durability was tested under extreme conditions of 5M KOH used in an actual alkaline water electrolysis cell, rather than 1M KOH. As indicated in FIG. 8, it was confirmed that the aluminum-doped NiFe-based catalyst according to the present invention exhibited excellent durability without performance degradation for over 100 hours even under the extreme conditions of 5M KOH.

[0086] FIGS. 9A to 9D is a graph utilizing XPS (X-ray Photoelectron Spectroscopy) to explain the effect of Al doping in a NiFe electrode according to an Example of the present invention. Core level spectra of the NiFe and NiFeAl electrodes were collected at a photon energy of 1150 eV before and after the OER for comparison. FIG. 9A shows the Ni 2p.sub.3/2 spectrum of NiFe and NiFeAl before the OER. It can be observed that the intensity of the NiO peak increased with the addition of Al, indicating charge transfer to Ni due to Al doping. The applicant of the present application clearly confirmed this phenomenon through the Ni 2p.sub.3/2 spectrum of NiFe and NiFeAl after the OER (refer to FIG. 9B). Specifically, in the spectrum of NiFeAl, the peaks attributed to Ni(OH) 2 (855.3-855.1 eV) and NiOOH (857.3-857.1 eV) were shifted in the negative direction compared to NiFe.

[0087] Such changes in the electronic structure were also observed in the Al 3p spectrum (FIG. 9C). Referring to FIG. 9C, clear Al 3p peaks were observed after the OER, indicating that Al was preserved and able to participate in the reaction. Peaks around 72.2 and 73.85 eV corresponded to metallic Al 3p.sub.3/2 and Al.sub.2O.sub.33p.sub.3/2, respectively. However, the Al 3p spectrum of NiFeAl after the OER showed that the peaks of Al 3p.sub.3/2 (72.2-72.55 eV) and Al.sub.2O.sub.33p.sub.3/2 (73.85-74.3 eV) shifted in the positive direction, indicating that Al exhibited greater electron depletion. Moreover, negligible changes were observed in the Fe 2p spectrum of NiFe and NiFeAl, indicating that Fe did not participate in the charge transfer. Therefore, the XPS results demonstrated that charge transfer occurs from Al to Ni.

[0088] Charge transfer from a dopant can influence the bonding strength of adsorbate species on the catalyst surface, thereby impacting the OER activity. In fact, the intrinsic activity of OER catalysts can be optimized by adjusting the adsorption energy of adsorbate species such as *OH H and *O. Generally, Ni-based (oxy) hydroxides demonstrate increased bonding strength to adsorbate species and enhanced OER activity. The electronegativity of neighboring atoms affects the OH adsorption energy at the catalytic site. Thus, an increase in electron population is known to decrease the repulsion of adsorbates and increase the bond strength. Therefore, NiFeAl, where the surface site of Ni (with a Mulliken electronegativity of 4.4 eV) coordinates with Al (3.2 eV) having lower electronegativity, exhibits stronger bonding to OH compared to NiFe.

[0089] The applicant of the present application conducted additional experiments by doping Cd and Zn, metals with low electronegativity, in NiFe-based (oxy) hydroxides to confirm the effect of low-electronegativity dopants. Doping these metals improves the OER activity, demonstrating a similar effect to doping with low-electronegativity metals. Specifically, referring to FIG. 10, when metals with lower electronegativity than Ni and Fe, such as Cd or Zn, were doped, for example, NiFeCd or NiFeZn exhibited superior catalytic performance compared to the existing NiFe catalyst. However, most preferably, the catalyst including NiFeAl showed the best performance.

[0090] Valence band measurements also provide direct information about the changes in the electron structure. As the d-band center approaches the Fermi level, the antibonding state shifts upward, reducing occupancy and increasing the *OH binding energy. FIG. 9D illustrates the atomic orbital band spectrum of NiFe and NiFeAl, from which the d-band center can be calculated. It is clearly observed that the calculated d-band center shifts closer to the Fermi level of NiFeAl, indicating a stronger *OH binding energy.

[0091] Meanwhile, the coexistence of electron-rich Ni species (Ni.sup.2+) in Ni-based (oxy) hydroxides (Ni.sup.3+) can enhance the binding energy of the adsorbate on the catalyst surface. The formation of electron-rich Ni-based (oxy) hydroxides can be confirmed by the appearance of a Ni.sup.2+/3+ redox peak in the OER current-voltage curve. The Ni(OH).sub.2/NiOOH oxidation peak of electron-rich Ni-based (oxy) hydroxides shift in the positive direction because the electron-rich Ni.sup.2+ species inhibits the oxidation of Ni.sup.2+ to Ni.sup.3+/4+. As shown in FIG. 5, the Ni(OH).sub.2/NiOOH oxidation peak of NiFeAl, which has a low Al concentration (10%), exhibits an anodic shift compared to NiFe. Therefore, these results confirm that an Al dopant enhances the OER activity by promoting the formation of electron-rich Ni species, which enhances the *OH binding energy in Ni-based (oxy) hydroxides.

[0092] In summary, charge transfer to Ni from dopants with electronegativity lower than that of Ni and Fe (e.g., Al, Cd, Zn) has at least three effects on NiFe doped with metals with lower electronegativity: 1) The increased electron density decreases the repulsive force of the adsorbate, 2) induces d-band upshift, and 3) promotes the formation of electron-rich Ni species. As a result, the OER activity is improved by increasing the bonding strength of the adsorbate *OH species.

[0093] Meanwhile, FIGS. 11A and 11B is a Raman spectrum graph explaining that the lattice structure of NiFeAl was changed compared to NiFe by the addition of Al.

[0094] When a material is doped with a heteroatom, internal deformation occurs due to the coexistence of ions with different ionic radii. In this case, lattice strain, either tensile or compressive, modifies the electron structure of the catalyst by adjusting the bond distance and modifying the orbital overlap. In particular, lattice tension can shift the d-band center of transition metals, enhancing their bonding with adsorbates.

[0095] The Raman spectra of free-standing NiFe and NiFeAl layers after the OER, shown in FIG. 11A, were obtained at a wavenumber range of 100-1000 cm.sup.1 (see FIG. 11A). The signal at around 550 cm.sup.1 is attributed to the NiO bonding in the NiFe film. The Raman spectrum in FIG. 11A exhibited a redshift from 563 cm-1 to 554 cm.sup.1, induced by phonon softening due to lattice strain. This lattice strain is caused by the addition of Al, which has a smaller ionic radius (78 pm) compared to Ni (53.5 pm) and Fe (69 pm). These findings align with the upshift of the d-band center shown in FIG. 9D.

[0096] The change in the d-band center due to the tensile strain was further confirmed by conducting Extended X-ray Absorption Fine Structure (EXAFS) measurements (FIG. 11B). The XAFS spectrum was collected using both transmission and fluorescence modes. Since the sample was a catalyst layer grown on a Ni substrate, the analysis was performed at the Fe K edge instead of the Ni K edge to eliminate the influence of the substrate. The prominent peak near 2.2 corresponded to the Fe-M bond, and the radial distance of the Fe-M bond slightly shifted in the positive direction in NiFeAl (2.526 ) compared to NiFe (2.512 ). This observation explains the lattice tension exerted on the structure. Consequently, it can be inferred that the lattice disorder induced by the addition of Al enhanced the O-intermediate adsorption energy. As observed, the physical property evaluation experiments demonstrated that the NiFe-based catalyst for OER, doped with metals such as Al, Cd, or Zn, having lower electronegativity than Ni and Fe, exhibited excellent activity and durability in the oxygen generation reaction. This contributes significantly to the cost reduction of hydrogen production by employing non-noble metal-based catalysts.

INDUSTRIAL APPLICABILITY

[0097] Therefore, the present invention provides a NiFe-based catalyst for OER that exhibits excellent activity and durability compared to conventional NiFe-based catalysts by doping aluminum into the non-noble metal-based catalyst, NiFe.

[0098] Furthermore, the NiFe-based catalyst for OER in the present invention holds economic value by not utilizing noble metals and can significantly contribute to reducing the cost of hydrogen production due to its superior activity compared to noble metal-based catalysts.

[0099] However, the effects of the present invention are not limited to the exemplary content described above, and the present specification includes various other effects.

[0100] Moreover, although the present invention has been described with specific examples and drawings, it is not limited to the described examples, and those skilled in the art would recognize various modifications and variations based on the general knowledge in the field to which the present invention pertains.

[0101] Therefore, the scope of the present invention should not be limited to the described examples but should be determined by the claims set forth herein, as well as their equivalents.