KR102419093B1 - Layered double hydroxide composite, preparation method thereof and catalyst for oxygen evolution reaction comprising the same - Google Patents

Abstract

The present invention relates to a layered double hydroxide composite, a preparation method thereof, and a catalyst for an oxygen generation reaction comprising the same. More specifically, in the present invention, a layered double hydroxide composite can be prepared by doping a lanthanum (La) ion as a third transition metal ion into a layered double hydroxide (LDH) comprising Ni and Fe and can be applied as a catalyst for an oxygen generation reaction which can improve the activity of an oxygen generation reaction.

Description

Layered double hydroxide composite, preparation method thereof, and catalyst for oxygen evolution reaction comprising the same

The present invention relates to a layered double hydroxide complex, a method for preparing the same, and a catalyst for an oxygen evolution reaction comprising the same.

Recently, with climate change, international movements to reduce carbon consumption, centering on fossil fuels, are becoming more important. Hydrogen produced by carbon-free electrocatalytic water splitting is considered a promising alternative to petroleum energy in the future. However, there is a problem that the application of electrocatalytic water splitting at the industrial level is hindered by oxygen evolution reaction (OER) at the anodic electrode.

OER involves four electron-proton bonding reactions, resulting in a high overpotential for overall water dissociation. Therefore, studies have been conducted to improve the kinetics of OER using high-efficiency electrocatalysts.

Research to improve OER catalytic performance using earth-abundant and cost-effective transition metal-based catalysts is progressing, showing that it is superior to commercially available noble metal-based catalysts such as RuO ₂ and IrO ₂ .

On the other hand, layered double hydroxides (LDHs) are known as 2D anionic compounds that can be represented by the formula [M ²⁺ _1-x M ³⁺ (OH) ₂ ] ^z+ A ^n- _z/n .mH ₂ O. where M ²⁺ and M ³⁺ are divalent and trivalent metal cations located in the host layer, and A ⁿ⁻ is an exchangeable anion in the interlayer region that compensates for the positive charge of the layer.

Recently, LDH has been widely applied in energy storage and conversion fields, such as lithium batteries, supercapacitors, electrocatalysts, and especially water decomposition. Among all existing LDHs, NiFe LDH was considered as the most promising OER electrocatalyst in alkaline electrolytes. However, due to disadvantages such as low electrical conductivity, less exposed active sites, almost NiFe LDH-based OER catalysts only have small current densities (10-100 mA cm ⁻² ) for OER processes at high potentials of about 1.5 V in alkaline electrolytes. can drive

To apply the electrocatalyst to industrial applications, it must have long-term stability and must also deliver current densities greater than 500 mA cm ^-2 at low overpotentials (<300 mV). Therefore, many studies are being conducted to improve the OER performance of NiFe LDH.

For example, in Non-Patent Document 1, more surface sites were exposed by using the liquid phase exfoliation method, and the current density was improved by 4.5 times at an overpotential of 300 mV. In addition, many other studies have shown that the incorporation of a third transition metal ion (M) into NiFe LDH can further enhance OER activity. In addition, for example, in Non-Patent Document 2, it was found that Mn ²⁺ partially transferred electrons from the hydroxide matrix to surrounding Ni ²⁺ and Fe ³⁺ , and the OER activity was improved due to electron-rich Ni ²⁺ and Fe ³⁺ . . Similarly, in Non-Patent Document 3, the electron density in V is increased by partial electron transfer from Fe and Ni to V through bridging O ² ions, and Ni ₃ Fe _0.5 V _0.5 LDH is the intrinsic OER of NiFe LDH. It has been found that the activity can be further improved.

Through the preceding literature, the third element (M) can be summarized to be responsible for improving OER performance through the following concept: (i) synergistic interaction between Ni, Fe, and M regulated local coordination environments of cations; (ii) more defects and vacancies as active sites are formed by M for Ni and Fe, facilitation of adsorption, desorption and migration of oxygen-containing intermediates to OER by these active sites.

Accordingly, the inventors of the present invention can prepare a layered double hydroxide complex by doping a lanthanum (La) ion as a third transition metal ion to a layered double hydroxide (LDH) containing Ni and Fe, using this to generate oxygen The present invention was completed by focusing on its application as a catalyst for an oxygen generation reaction capable of improving the activity of the reaction.

Song, Fang, and Xile Hu. Nature communications 5.1 (2014): 1-9. Zhou, Daojin, et al. Nanoscale horizons 3.5 (2018): 532-537. Jiang, Jian, et al. Nature communications 9.1 (2018): 1-12.

The present invention has been devised in consideration of the above problems, and an object of the present invention is to dope a layered double hydroxide (LDH) containing Ni and Fe with a lanthanum (La) ion as a third transition metal ion. It is intended to prepare a complex and apply it as a catalyst for an oxygen evolution reaction capable of improving the activity of an oxygen evolution reaction using the composite.

The problem to be solved by the present invention is not limited to the problem(s) mentioned above, and another problem(s) not mentioned will be clearly understood by those skilled in the art from the following description.

In order to achieve the above object, nickel foam (foam); Formed on the nickel foam, layered double hydroxide (LDH; layered double hydroxide) containing Ni and Fe; and La ions doped to the layered double hydroxide.

The layered double hydroxide may be represented by the following formula (1).

[Formula 1]

[M ¹ _(1-x) M ² _x (OH) ₂ ][A ^n- ] _(x/n) .zH ₂ O

In Formula 1,

M ¹ is at least one selected from Ni ²⁺ and Fe ²⁺ ,

M ² is at least one selected from Ni ³⁺ and Fe ³⁺ ,

A ^n- is CO ₃ ^2- , NO ₂ ^- , NO ₃ ^- , OH ^- , O ^2- , SO ₄ ^2- , PO ₄ ^3- , HPO ₄ ^2- , H ₂ PO ₄ ^- , F ^- , Cl ^- , Br-, ^{I- ,} and an anion selected from the group consisting of combinations thereof,

0<x<1,

n is an integer from 1 to 3,

z is a positive number from 0.1 to 15;

The thickness of the layered double hydroxide may be 1 to 20 nm.

In addition, the present invention provides a catalyst for oxygen evolution reaction (OER) comprising the layered double hydroxide complex according to the present invention.

In addition, the present invention provides an electrode for water electrolysis comprising the catalyst for oxygen evolution reaction according to the present invention.

In addition, the present invention provides a water electrolysis cell including the catalyst for the oxygen evolution reaction according to the present invention.

In addition, the present invention comprises the steps of (a) obtaining a precursor solution comprising a Ni precursor, an Fe precursor, a La precursor and a hydrolyzing agent; and (b) immersing Ni foam in the precursor solution and then performing heat treatment.

The hydrolyzing agent may include at least one selected from the group consisting of urea, NaOH, KOH, NH ₄ OH, and hexamethylenetetramine.

The precursor solution of step (a) may further include ammonium fluoride (NH ₄ F).

The molar ratio of the Ni precursor, the Fe precursor, and the La precursor may be 1: 0.3 to 0.4: 0.02 to 0.05.

The heat treatment of step (b) may be performed at 100 to 200 ℃.

According to the present invention, a layered double hydroxide complex can be prepared by doping a lanthanum (La) ion as a third transition metal ion to a layered double hydroxide (LDH) containing Ni and Fe, and using this, It can be applied as a catalyst for oxygen evolution reaction capable of improving activity.

It should be understood that the effects of the present invention are not limited to the above-described effects, and include all effects that can be inferred from the configuration of the invention described in the detailed description or claims of the present invention.

1 is an image showing the process of synthesizing NiFe LDH and NiFeLa LDH according to an embodiment of the present invention.
2 is (a) NiFe LDH prepared in Comparative Example 1, NiFeLa LDH-1 prepared in Example 1, NiFeLa LDH-2 and NiFeLa LDH-3 X-ray diffraction pattern graph (XRD), (b) NiFe Scanning electron microscope (SEM) image of LDH, (c) Scanning electron microscope (SEM) image of NiFeLa LDH-2, (d) Transmission electron microscope (TEM) image of NiFeLa LDH-2, (e) NiFeLa LDH-2 High-resolution transmission electron microscope (HRTEM) images, (f) limited-field electron diffraction (SAED) pattern images of NiFeLa LDH-2, and (g) elemental mapping images of NiFeLa LDH-2.
3 is (a) 2 mV s ⁻¹ scan rate and LSV in KOH 1M electrolyte of NiFe LDH prepared from Comparative Example 1, NiFeLa LDH-1, NiFeLa LDH-2 and NiFeLa LDH-3 prepared from Example 1 curves, (b) overpotentials at current densities of 50, 300 and 500 mA cm ⁻² , (c) Tafel slopes, (d) EIS Nyquist plots; Graphs showing (e) TOF curves and (f) cathodic peak current and square root plots of scan rates for NiFe LDH and NiFeLa LDH-2.
Figure 4 shows the LSV curves before and after (a) multi-step chronopotentiometry, (b) morphological stability at 100 mA cm ^-2 and (c) stability test of NiFeLa LDH-2 prepared in Example 1; It is a graph.
5 is a graph showing (a) an LSV curve at a scan rate of 2mV s ⁻¹ and a KOH 1M electrolyte condition, (b) Tafel slope, and (d) EIS Nyquist plots of NiCo and NiCoLa LDHs.
6 is XPS high resolution of (a) Ni 3p, (b) Fe 2p, (c) O 1s and (d) La 3d for NiFe LDH prepared from Comparative Example 1 and NiFeLa LDH-2 prepared from Example 1 is the spectrum

Hereinafter, embodiments of the present invention will be described in more detail with reference to the accompanying drawings. Embodiments of the present invention may be modified in various forms, and the scope of the present invention should not be construed as being limited to the following embodiments. This example is provided to more completely explain the present invention to those of ordinary skill in the art. Accordingly, the shapes of elements in the drawings are exaggerated to emphasize a clearer description.

Electrochemical energy conversion, such as reducing water to H ₂ and reducing CO ₂ to valuable carbon products, is attracting great attention as a potential green energy technology to replace fossil fuels. In this regard, a NiFe LDH-based OER catalyst has been widely used as an electrocatalyst for a conventional oxygen evolution reaction (OER). However, due to disadvantages such as low electrical conductivity, less exposed active sites, NiFe LDH-based OER catalysts only drive low current densities (10-100 mA cm ⁻² ) for OER processes at high potentials of about 1.5 V in alkaline electrolytes. There were downsides to being able to do it.

To apply the electrocatalyst to industrial applications, it must have long-term stability and must also deliver current densities greater than 500 mA cm ^-2 at low overpotentials (<300 mV).

In order to solve the above problems, in the present invention, NiFe LDH is doped with La ions to prepare a layered double hydroxide complex, and by using it, it is intended to be applied as a catalyst for an oxygen generation reaction that can improve the activity of an oxygen generation reaction. .

Layered double hydroxide complex

According to an embodiment of the present invention, in order to solve the above technical problem, nickel foam (foam); Formed on the nickel foam, layered double hydroxide (LDH; layered double hydroxide) containing Ni and Fe; And there is provided a layered double hydroxide complex comprising La ions doped in the layered double hydroxide.

The nickel foam may have a porous structure. Specifically, the nickel foam may be a three-dimensional porous nickel 　 foam, but is not limited thereto.

The layered double hydroxide may be formed on the nickel foam, specifically, grown vertically on the nickel foam, but is not limited thereto.

The La ions may be uniformly distributed along with Ni and Fe in the structure of the layered double hydroxide.

According to an embodiment of the present invention, the layered double hydroxide may be represented by the following formula (1).

[Formula 1]

[M ¹ _(1-x) M ² _x (OH) ₂ ][A ^n- ] _(x/n) .zH ₂ O

In Formula 1,

M ¹ is at least one selected from Ni ²⁺ and Fe ²⁺ ,

M ² is at least one selected from Ni ³⁺ and Fe ³⁺ ,

A ^n- is CO ₃ ^2- , NO ₂ ^- , NO ₃ ^- , OH ^- , O ^2- , SO ₄ ^2- , PO ₄ ^3- , HPO ₄ ^2- , H ₂ PO ₄ ^- , F ^- , Cl ^- , Br-, ^{I- ,} and an anion selected from the group consisting of combinations thereof,

0<x<1,

n is an integer from 1 to 3,

z is a positive number from 0.1 to 15;

Specifically, the layered double hydroxide may be represented by the following Chemical Formula 1-1.

[Formula 1-1]

NiFe(CO ₃ )(OH) ₂ ^. H ₂ O

More specifically, the layered double hydroxide including doped La ions may be represented by the following formula (2).

[Formula 2]

NiFeLa(CO ₃ )(OH) ₂ ^. H ₂ O

The thickness of the layered double hydroxide may be 1 to 20 nm. Specifically, the thickness of the layered double hydroxide may be 1 to 10 nm, 1 to 5 nm, or 2 to 3 nm. When it has a thickness within the above range, when applied as an OER catalyst, long-term shape preservation ability and operational stability are excellent.

In addition, the present invention provides a catalyst for oxygen evolution reaction (OER) comprising the layered double hydroxide complex according to the present invention.

The oxygen evolution catalyst is a catalyst that generates oxygen by decomposing water, and the active materials used as catalysts for the conventional oxygen evolution reaction include ruthenium (Ru), ruthenium oxide (RuO ₂ ) and iridium oxide (IrO ₂ ). The above materials were not suitable for long-term mass production of hydrogen through water electrolysis due to their high price, and also were vulnerable to the harsh acidic atmosphere, which is the operating condition of the water electrolysis cell, and thus catalytic activity and stability were deteriorated.

On the other hand, when the layered double hydroxide composite according to the present invention is applied as a catalyst for OER reaction, it exhibits excellent OER catalytic activity by showing low overpotential under ultra-high current density conditions despite using Ni, Fe and La instead of noble metals. However, it has the effect of indicating OER operation stability over a long period of time. For example, at a high current density of 500 mA cm ^-2 , a low Tafel slope of 42 mV dec ^-1 , a low overpotential of 309 mV, and long-term stability of OER operation over 140 hours at 100 mA cm ^-2 . was confirmed.

Previous studies on OER catalytic activity confirmed overpotentials at low current densities (eg 10 ~ 100 mA cm ^-2 ), but as described above, for application of electrocatalysts to industrial applications, low overpotentials It must deliver a current density of at least 500 mA cm ^-2 at (<300 mV). That is, the layered double hydroxide composite according to the present invention achieved a low overpotential under ultra-high current density conditions, which could not be achieved conventionally.

In addition, the present invention provides an electrode for water electrolysis comprising the catalyst for oxygen evolution reaction according to the present invention.

In addition, the present invention provides a water electrolysis cell including the catalyst for the oxygen evolution reaction according to the present invention.

As such, the layered double hydroxide complex may be useful in an electrochemical reaction system, in particular, a hydrogen energy source device such as an electrode for water electrolysis that requires hydrogen production through electrolysis of water, a water electrolysis cell, and the like.

Manufacturing method of layered double hydroxide complex

The present invention comprises the steps of (a) obtaining a precursor solution comprising a Ni precursor, an Fe precursor, a La precursor and a hydrolyzing agent; and (b) immersing Ni foam in the precursor solution and then performing heat treatment.

The Ni precursor, Fe precursor and La precursor are each independently, pure metal powder, nitrate (nitrate), chloride (chloride), bromide (bromide), iodide (iodide), nitrite (nitrite), sulfate (sulfate), It may be selected from the group consisting of acetate, sulfite, acetylacetoante, and hydroxide, but is not limited thereto. Specifically, each of the precursors may be a nitrate hydrate. More specifically, the Ni precursor may be Ni(NO ₃ ) ₂ .6H ₂ O, the Fe precursor may be Fe(NO ₃ ) ₃ .6H ₂ O, and the La precursor may be La(NO ₃ ) ₃ . 6H ₂ O.

The precursor solution may further include a solvent. The solvent is deionized water, glycerol, ethylene glycol, polyethylene glycol, diethylene glycol, triethylene glycol, tetraethylene glycol ( tetraethylene glycol), formamide, N-methylformamide, dimethyl formamide, dimethyl acetylamide, dimethylsulfoxide, methanol, aceto Nitrile (acetonitrile), 2-pyrrolidinone (2-pyrrolidinone), acetamide (acetamide), acrylamide (acrylamide), N- methylurea (N-methylurea), urea (urea) and the group consisting of combinations thereof It may be selected from, but is not limited thereto, and the solvent may be a material of a water-soluble liquid phase.

The hydrolysis agent may serve to help the hydroxylation reaction. The hydrolyzing agent may be at least one selected from the group consisting of urea, hexamethylenetetramine (HMTA), NaOH, KOH, and NH ₄ OH, but is not limited thereto. Specifically, the hydrolyzing agent may use urea, and in this case, by performing a reversible hydrolysis reaction at an appropriate reaction rate, it is possible to prevent local aggregation of the La element and induce a uniform distribution on the layered double hydroxide structure. and, thus, has the effect of further improving the OER catalytic activity.

According to an embodiment of the present invention, the precursor solution of step (a) may further include ammonium fluoride (NH ₄ F). The ammonium fluoride may serve to activate the nickel foam. Specifically, when performing the heat treatment in step (b) using the precursor solution further containing ammonium fluoride, ammonium fluoride activates the nickel form through the generation of more nucleation sites to form Ni foam and the layered double hydroxide (LDH) causes a tight bond, and the ammonium fluoride causes the growth of layered double hydroxide (LDH) on the surface of the activated nickel foam, and the β-LDH phase with excellent electrochemical activity ) can play a role in promoting the growth of

According to an embodiment of the present invention, the molar ratio of the Ni precursor, the Fe precursor, and the La precursor may be 1: 0.3 to 0.4: 0.02 to 0.05. Specifically, the molar ratio may be 1: 0.3 to 0.4: 0.03 to 0.04. Specifically, within the molar ratio range, through strong electron interaction between the host metals (Ni and Fe) of the layered double hydroxide and La, the number of active sites is greatly increased, and the charge transfer resistance is reduced as well as oxygen By increasing the formation of vacancies, better OER activity can be exhibited.

According to an embodiment of the present invention, the molar ratio of the metal precursor (Ni precursor, Fe precursor, and La precursor) and the hydrolyzing agent may be 1:3.0 to 4.0. Specifically, the molar ratio may be 1: 3.30 to 3.80 or 1: 3.50 to 3.70, and within the molar ratio range, there is an effect of controlling the number of active sites and oxygen vacancies of the catalyst.

According to an embodiment of the present invention, the nickel foam may be washed with an organic solvent and an acid solution.

The organic solvent may be at least one selected from the group consisting of ethanol, acetone, pyridine, tetrahydrofuran and dimethyl sulfoxide, but is not limited thereto. Specifically, the organic solvent may be ethanol.

The acid solution may be at least one selected from the group consisting of hydrochloric acid, sulfuric acid, hydrofluoric acid and halogen acid, but is not limited thereto. Specifically, the acid solution may be a hydrochloric acid solution.

The washing may be performed through ultrasonic treatment, but is not limited thereto.

According to one embodiment of the present invention, the heat treatment of step (b) may be performed at 100 to 200 ℃. Specifically, the heat treatment may be performed at 100 to 150° C. or 110 to 130° C., and within the heat treatment range, it is possible to prevent local aggregation of the La element and induce a uniform distribution on the layered double hydroxide structure. and, accordingly, there is an effect of further improving the OER catalytic activity.

More specifically, if the hydrolyzing agent is used as urea as an example, urea may be decomposed by the reaction of Scheme 1 below in the above-described heat treatment temperature range. Thereafter, a reversible hydrolysis reaction of Ni, Fe, and La ions may occur slowly to form a layered double hydroxide structure including doped La ions.

[Scheme 1]

(NH ₂ ) ₂ CO + H ₂ O → 2NH ₃ + CO ₂

CO ₂ + H ₂ O → CO ₃ ^2- + 2H ⁺

NH ₃ + H ₂ O → NH ₄ ⁺ + OH ^-

After step (b), (c) may further include washing and drying the layered double hydroxide composite produced after the heat treatment. The washing may be performed several times using deionized water, but is not limited thereto. In addition, the drying may be performed at 50 to 150 ℃, 60 to 100 ℃, or 65 to 75 ℃, but is not limited thereto .

The matters mentioned in the layered double hydroxide composite of the present invention, and the method for preparing the same are equally applied as long as they do not contradict each other.

The above description describes the technical idea of the present invention using one embodiment, and various modifications and variations are possible without departing from the essential characteristics of the present invention by those of ordinary skill in the art to which the present invention pertains. Accordingly, the embodiments described in the present invention are for explanation rather than limiting the technical spirit of the present invention, and the scope of the technical spirit of the present invention is not limited by these embodiments. The protection scope of the present invention should be construed by the claims, and all technical ideas within the scope equivalent thereto should be construed as being included in the scope of the present invention.

Hereinafter, the present invention will be described in more detail through examples.

<Example>

ingredient

Ni(NO ₃ ) ₂ .6H ₂ O, Fe(NO ₃ ) ₃ .9H ₂ O, NH ₄ F and urea were purchased from Samcheonsoonyak Co., Ltd. La(NO ₃ ) ₃ .6H ₂ O was purchased from Sigma-Aldrich. All reagents were used without further purification, and deionized water (DI water) was used in all experiments.

analysis

The properties of the synthesized material were analyzed by X-ray diffraction (XRD, D8 Advance equipped with Cu Kα X-ray source), field emission scanning electron microscopy (FE-SEM, LEO SUPRA 55), and high-resolution transmission electron microscopy (TEM, HRTEM, JEOL JEM-2100F) , 200 kV) and X-ray photoelectron spectroscopy (XPS; K-Alpha spectrometer-thermoelectron).

Comparative Example 1. Synthesis of layered double hydroxide (NiFe LDH)

First, Ni foam (1.5 x 3 cm ² ) was prepared by ultrasonic washing with ethanol and 3M HCl for 30 minutes, respectively, and drying. Then 0.450 mmol of Ni(NO ₃ ) 2.6H ₂ O, 0.150 mmol of Fe(NO ₃ ) _{3.6H 2} O, _2.23 mmol of urea and 1.48 mmol of NH ₄ F were stirred vigorously for 30 min. By dissolving in 25 ml of deionized water, a precursor solution was obtained. Then, the precursor solution was placed in a 30 ml volume Teflon-lined stainless steel autoclave, and the prepared Ni foam was immersed in the precursor solution. After sealing, the autoclave was heated to 120° C. for 10 hours to obtain a NiFe LDH sample, which was washed several times with deionized water and dried in an oven at 70° C.

Example 1. Synthesis of layered double hydroxide composite (NiFeLa LDH)

NiFeLa LDH was synthesized in the same manner as in Comparative Example 1, except that a precursor solution obtained by adding La(NO ₃ ) ₃ .6H ₂ O to the precursor solution of Comparative Example 1 was used. The layered double hydroxide complex synthesized by adjusting the La(NO ₃ ) ₃ .6H ₂ O dose to 0.010 mmol, 0.0150 mmol and 0.020 mmol was named NiFeLa LDH-1, NiFeLa LDH-2 and NiFeLa LDH-3, respectively. (see FIG. 1).

Experimental Example 1. Characterization of NiFe LDH and NiFeLa LDH

Referring to the XRD pattern of Figure 2a, the LDHs prepared from Comparative Example 1 and Example 1 showed characteristic peaks at 44.56 and 51.80 for Ni foam (JCPDS #04-8050), and NiFe(CO ₃ )(OH 11.67, 33.50, 34.43, 39.00, 59.80 corresponding to the (003), (101), (012), (015), (110) and (113) crystal planes for ) H ₂ O (JCPDS #40-0215) And it can be confirmed that six diffraction peaks at 61.25 appeared. On the other hand, compared with NiFe LDH, no shift of the diffraction peak was observed in NiFeLa LDH, suggesting that La ions do not affect the structure of the host NiFe LDH. Through this, it can be confirmed that La ions can be easily co-precipitated into the lattice of LDH due to the hexacoordinate structure that matches well with the crystal structure of hydrotalcite.

Referring to the SEM images of FIGS. 2B and 2C , it can be confirmed that NiFe and NiFeLa LDH-2 nanosheets are grown on Ni foam with a porous network, creating a hierarchical 3D structure.

In addition, referring to the TEM image of FIG. 2D , it can be confirmed that NiFeLa LDH-2 has a thickness of about 2 to 3 nm, which is consistent with that confirmed in the SEM image.

Also, referring to the HRTEM image of FIG. 2E , it can be confirmed that NiFeLa LDH-2 shows a lattice spacing of 0.268 nm corresponding to the (012) crystal plane of NiFe LDH.

In addition, referring to the SAED pattern image of FIG. 2f , the bright diffraction ring of SAED indexed (003), (101) and (015) gratings, which was confirmed to be consistent with the XRD pattern.

In addition, referring to the element mapping image of FIG. 2G , in a typical NiFeLa LDH-2 nanosheet, La elements were uniformly distributed along with Fe and Ni elements in the LDH structure, and local aggregation of La was not observed.

Experimental Example 2. Oxygen evolution reaction (OER) performance check

The OER catalytic performance of the layered double hydroxide (LDH) synthesized from Comparative Example 1 and the layered double hydroxide composite synthesized from Example 1 was obtained using a Pt mesh, Hg/HgO, and 1M NaOH as a counter electrode and a reference electrode. A three-electrode cell was used and evaluated in 1M KOH electrolyte. Polarization curves were performed at a slow scan rate of 2 mV sec ⁻¹ in 1 M KOH electrolyte, and electrochemical impedance spectroscopy (EIS) analysis was performed at a frequency of 10 mHz to 100 kHz with an amplitude potential of 10 mV at 0.55 V (vs Hg/HgO, 1 M NaOH). range was performed.

Referring to FIG. 3A , an iR -corrected linear sweep voltammetry (LSV) curve at a scan rate of 2 mV s ⁻¹ shows that the NiFeLa LDHs of Example 1 have higher current densities than the NiFe LDHs of Comparative Example 1 in the potential range. It can be confirmed that the .

In addition, referring to FIG. 3b, in particular, when the current density is 50 and 300 mA cm ^-2 , the optimal catalyst overpotential (η) of NiFeLa LDH-2 is 228 and 285 mV, respectively, for NiFe LDH (251 and 317 mV). It can be seen that the lower

In addition, Table 1 below shows the overpotentials of NiFeLa LDH-2 of Example 1 compared with some previous reports on NiFe LDH doped with a third metal and transition metal-based catalysts for state-of-the-art OER.

Referring to Table 1 below, it can be seen that NiFeLa LDH-2 requires the lowest overpotential of 309 mV to drive an ultra-high current density of 500 mA cm ^-2 to drive an ultra-high current density. This can be very favorable conditions for cost-effective industrial hydrogen production.

In addition, in terms of dynamics, the Tafel slope was calculated from a cathodic curve at a scan rate of 2 mV s-2 to avoid the effect of capacitive current, and the results are shown in FIG. 3C. Referring to FIG. 3c , the Tafel slope of NiFeLa LDH-2 is 42 mV dec ^-1 , which is NiFeLa LDH-1 (44 mV dec ^-1 ), NiFeLa LDH ^- 3 (48 mV dec ^-1 ) and NiFe LDH (52 mV dec -1 ). ¹ ), and it can be seen that the NiFeLa LDHs prepared in Example 1 exhibit faster dynamics than the NiFe LDHs of Comparative Example 1.

In addition, the electrochemical impedance spectrum was performed at a potential of 0.55 V (vs Hg/HgO, 1M NaOH) to evaluate the charge transfer ability of the electrocatalyst. Nyquist plots of NiFeLa LDH samples and NiFe LDH are shown in FIG. 3d . Referring to Fig. 3d, the smaller semicircular radius of the NiFeLa LDH sample suggests a lower charge transfer resistance, which could mean that the intrinsic electrocatalytic activity was enhanced by introducing La into the NiFe LDH.

On the other hand, TOF (turnover frequency) together with EIS is an important parameter to evaluate the intrinsic activity of the catalyst. Referring to FIG. 3E , it can be seen that the TOF value of NiFeLa LDH-2 is higher than the TOF value of NiFe LDH at various potentials. In addition, at an overpotential of 300 mV, NiFeLa LDH-2 showed a TOF value of 0.299 s ⁻¹ , 1.177 times higher than that of NiFe LDH (0.254 s ⁻¹ ), suggesting that element La could intrinsically enhance the catalytic activity. do.

Also, referring to Figure 3f, the higher k value of NiFeLa LDH-2 than NiFe LDH means that OH ⁻ reacts with the active site of NiFeLa LDH-2 faster than NiFe LDH, thereby accelerating the electrode reaction during the OER process. can

In addition, FIG. 4a shows a multi-step chronopotentiometer curve of NiFeLa LDH-2 in a current density range of 20 to 300 mA cm ^-2 within 20000 seconds. Referring to Fig. 4a, a remarkable rapid improvement of the potential response of the NiFeLa LDH-2 electrode was observed with increasing current density. Except for the very high current density, the potential was almost constant for every 2500 seconds, confirming the excellent conductivity and mass transport properties of NiFeLa LDH-2.

In addition, FIG. 4b shows the results of performing the NiFeLa LDH-2 chronopotential test at a current density of 100 mA cm ^-2 to evaluate catalyst stability. Referring to FIG. 4b , there was no noticeable degradation for 140 hours, showing excellent catalyst stability for OER of NiFeLa LDH-2, and potential fluctuations due to adsorption/desorption of O ² bubbles on the electrode surface can also be observed. . Also, referring to the inset in FIG. 4B , it can be confirmed that after 100 hours, the shape of NiFeLa LDH-2 has been changed to a clear hierarchical “nanosheet on nanosheet” structure. This hierarchical structure not only provides an enlarged surface area with more active sites for redox reactions, but also has an open porous structure that can promote rapid mass transport and electrolyte penetration.

In addition, in Fig. 4c, the LSV curve was recorded even after long-term durability. Referring to FIG. 4c , a slight increase in potential of 14 mV was observed at a current density of 500 mA cm ^-2 , and furthermore, excellent stability of the catalyst was confirmed during OER operation under strong basic conditions.

Meanwhile, for comparison with NiFeLa LDH according to the present invention, NiCo LDH was selected as a support for La doping through the same synthesis method at various doses of La precursor, and the electrocatalytic performance of the NiCoLa LDH sample was compared with that of pure NiCo LDH. Thus, it is shown in FIG. 5 . 5a to 5c, it was further confirmed that La plays an important role in improving the OER catalytic performance of LDH (η300 of NiCoLa LDH - η300 of NiCo LDH = 43mV). In addition, the lower activity of optimal NiCoLa LDH (η300 = 386 mV) compared to that of NiFeLa LDH (η300 = 285 mV) may indicate that La doping on NiFe LDH support has better OER performance.

In addition, in order to better understand the distribution of La ions in the improvement of OER activity, XPS investigations on NiFe LDH and NiFeLa LDH-2 were also performed, and the results are shown in FIG. 6 . All XPS spectra were corrected according to the binding energy of C–C at 284.6 eV before devolution until χ2 reached a minimum value.

Referring to FIG. 6a , since the La 3d peak and the Ni 2p peak overlap, the Ni 3p spectrum was used for deconvolution. The peaks of 67.3 and 68.9 eV and the peaks of 71.1 and 74.1 eV may represent general signals of Ni ²⁺ and Ni ³⁺ oxidation states, respectively.

Referring to the high-resolution spectrum of Fe 2p in Figure 6b, it can be confirmed that the peaks around 708.9 and 722.0 eV and 713.1 and 727.0 eV correspond to Fe ²⁺ and Fe ³⁺ ions, respectively.

In particular, the binding energies for Ni 3p and Fe 2p of NiFeLa LDH-2 shifted by amounts of 0.4 and 0.7 eV, respectively, compared with the values of pure NiFe LDH, indicating that La doping significantly altered the electronic structures of Ni and Fe ions. It can mean that you can control it.

Partial electron transfer from Ni and Fe to La in NiFeLa LDH-2 forms a higher amount of expensive metal sites on the surface, confirming the increase in the number of OER active sites in the catalyst, which is in good agreement with the TOF calculation trend was confirmed.

6c, the O 1s peak is the metal-oxygen (MO) bond (530.0 eV) of the lattice, the hydroxide bond (OH) (530.8 eV) and oxygen vacancies (Ov) (531.9 eV) of the material and adsorbed on the surface. It can be confirmed that it is due to H ₂ O (533.2 eV). In addition, integration and calculation of the peak area for each O species showed that the percentage of Ov was 16.0 and 21.6% for NiFe LDH and NiFeLa LDH-2, respectively, confirming that La doping can promote Ov formation. can It was confirmed that this phenomenon is similar to the previously reported LDH, which can induce fast OH ^- adsorption as mentioned above in the calculation of the OH-diffusion coefficient.

Therefore, the layered double hydroxide composite according to the present invention greatly increases the number of active sites through strong electron interaction between La and the host metal of NiFe LDH, and reduces charge transfer resistance, as well as significant oxygen vacancies. By forming , it was confirmed that it can be applied as an OER electrocatalyst having excellent OER activity.

The above detailed description is illustrative of the present invention. In addition, the above description shows and describes preferred embodiments of the present invention, and the present invention can be used in various other combinations, modifications, and environments. That is, changes or modifications are possible within the scope of the concept of the invention disclosed herein, the scope equivalent to the written disclosure, and/or within the scope of skill or knowledge in the art. The written embodiment describes the best state for implementing the technical idea of the present invention, and various changes required in specific application fields and uses of the present invention are possible. Accordingly, the detailed description of the present invention is not intended to limit the present invention to the disclosed embodiments. Also, the appended claims should be construed to include other embodiments as well.

Claims (15)

delete delete delete delete delete delete (a) obtaining a precursor solution comprising a Ni precursor, an Fe precursor, a La precursor and a hydrolyzing agent; and
(b) immersing Ni foam in the precursor solution and then heat-treating,
The molar ratio of the Ni precursor, the Fe precursor, and the La precursor is 1: 0.3 to 0.4: 0.03 to 0.04,
The heat treatment of step (b) is a method for producing a layered double hydroxide composite is performed at 100 to 200 ℃. 8. The method of claim 7,
The hydrolyzing agent is urea (Urea), NaOH, KOH, NH ₄ OH, and hexamethylenetetramine (Hexamethylenetetramine) method of producing a layered double hydroxide complex comprising at least one selected from the group consisting of. 8. The method of claim 7,
The precursor solution of step (a) may further include ammonium fluoride (NH ₄ F), the method for producing a layered double hydroxide complex. delete delete A layered double hydroxide composite prepared by the method according to claim 7,
nickel foam;
Formed on the nickel foam, layered double hydroxide (LDH; layered double hydroxide) containing Ni and Fe; and
A layered double hydroxide complex comprising La ions doped in the layered double hydroxide. A catalyst for oxygen evolution reaction (OER) comprising the layered double hydroxide complex according to claim 12. An electrode for water electrolysis comprising the catalyst for oxygen evolution reaction according to claim 13. A water electrolysis cell comprising the catalyst for the oxygen evolution reaction according to claim 13.

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