KR102420669B1 - Method For Preparing NiFeCo-Layered Double Hydroxide - Google Patents

Abstract

A simple, low-cost, and large-area manufacturing method of nickel-iron-cobalt layered double hydroxide, comprising: a first step of immersing a nickel substrate in a solution containing a divalent iron salt and a divalent cobalt salt; and a second step of drying the nickel substrate obtained in the first step at room temperature; wherein the molar ratio of the divalent iron salt and the divalent cobalt salt in the solution of the first step is 1.5:1 to 2.5:1, nickel- The present invention relates to a method for producing iron-cobalt layered double hydroxide and to a layered double hydroxide prepared according to the production method.

Description

Method for preparing nickel-iron-cobalt layered double hydroxide {Method For Preparing NiFeCo-Layered Double Hydroxide}

The present invention relates to a method for preparing a nickel-iron-cobalt layered double hydroxide (LDH), and more particularly, to a method for simply preparing a nickel-iron-cobalt layered double hydroxide at room temperature without the need for additional energy is about Furthermore, it relates to nickel-iron-cobalt layered double hydroxide prepared according to this method and its application as an electrochemical catalyst in oxygen catalyzed reaction.

Interest in renewable energy is increasing. Hydrogen energy by water electrolysis has great advantages in that it is eco-friendly and can easily obtain high-purity hydrogen energy. Electrochemical water electrolysis consists of two half-reactions: the hydrogen evolution reaction (HER) at the cathode and the oxygen evolution reaction (OER) at the anode. The oxygen production reaction requires a lot of overvoltage due to the complex reaction mechanism and slow reaction rate compared to the hydrogen production reaction. Generally iridium oxide (IrO ₂ ) or ruthenium oxide (RuO ₂ ) same Although noble metal oxides exhibit high catalytic activity in oxygen production reactions, their commercial applications are limited due to their high price and scarcity. As an alternative to this, many studies related to this are being conducted because catalysts such as oxides, phosphides, and hydroxides based on transition metals such as nickel, cobalt, and iron have high oxygen production reaction activity.

Among them, layered double hydroxide (LDH) has attracted much attention due to its features such as high specific surface area, catalytically active sites, tunable chemical composition, and high oxygen production reaction activity resulting from the two-dimensional layer structure. In addition, nickel-iron layered double hydroxide (NiFe-LDH) is known to have the highest oxygen production catalytic activity among various transition metal-based layered double hydroxides, but recently cobalt (Co) has been introduced into nickel-iron-cobalt. Ternary metal layered double hydroxide (NiFeCo-LDH) is receiving great attention as it has been found to have higher electrochemical catalytic activity than conventional nickel-iron layered double hydroxide.

It has been reported in several studies that the introduction of cobalt can improve the electrical conductivity of the catalyst and can exhibit improved electrochemical catalytic activity by controlling the electron structure of iron in the nickel-iron layered double hydroxide.

Meanwhile, electrodeposition and hydrothermal are representative methods for uniformly growing layered double hydroxide on a substrate. Both methods have the advantage of uniformly growing the layered double hydroxide on the substrate, but have disadvantages in that the preparation process of the reaction solution is complicated and additional energy is required from the outside such as heat or electricity. In addition, most of the known methods are synthesized using a conductive substrate having a small area of 10 - 20 cm ² , so this catalyst synthesis method is not suitable from the viewpoint of commercialization of hydrogen energy by water electrolysis.

In addition, in the preparation of electrochemical catalysts that have been previously studied in relation to layered double hydroxide, powder of layered double hydroxide is obtained and deposited on electrodes using a binder such as Nafion, or electrodeposition or hydrothermal There was a method of growing the electrode in-situ using a method such as a synthesis method. The former has the advantage of uniformly depositing the catalyst regardless of the size or shape of the substrate, but has poor performance and stability. Conversely, the latter has good performance and stability, but the size or shape of the substrate can become a variable in catalyst manufacturing.

Therefore, in order to develop an electrochemical catalyst for an improved oxygen production reaction, a method for preparing a simple and low-cost nickel-iron-cobalt layered double hydroxide that can be applied over a large area is required.

Accordingly, an object of the present inventors is to provide a simple, low-cost, and large-area manufacturing method of nickel-iron-cobalt layered double hydroxide by solving the above problems. In addition, an object of the present invention is to provide a nickel-iron-cobalt layered double hydroxide prepared by this method, and to apply the layered double hydroxide as an electrochemical catalyst in an oxygen generation reaction of water electrolysis.

In order to achieve the above object, the present invention provides a first step of immersing a nickel substrate in a solution containing a divalent iron salt and a divalent cobalt salt; and a second step of drying the nickel substrate obtained in the first step at room temperature; A method for preparing a phosphorus, nickel-iron-cobalt layered double hydroxide is provided.

In addition, the present invention includes the nickel-iron-cobalt layered double hydroxide prepared according to the above production method, its application as an electrochemical catalyst in the oxygen production reaction of water electrolysis, and the nickel-iron-cobalt layered double hydroxide An electrochemical catalyst is provided.

According to the present invention, nickel-iron-cobalt layered double hydroxide can be uniformly grown on a nickel substrate regardless of the size or shape of the substrate. A nickel-iron-cobalt layered double hydroxide having excellent performance and stability can be prepared.

1 is a process diagram schematically showing the manufacturing process of the nickel-iron-cobalt layered double hydroxide of Example 1 of the present invention.
2 is a photograph of each step in the manufacturing process of the nickel-iron-cobalt layered double hydroxide of Example 1 of the present invention.
3 is a graph showing the analysis results by XPS (X-ray Photoelectron Spectroscopy) before and after self-oxidation during the manufacturing process of nickel-iron-cobalt layered double hydroxide.
FIG. 4 is a field-emission scanning electron microscopy (FE-SEM) image of a) low magnification and b) high magnification of the nickel-iron-cobalt layered double hydroxide prepared in Example 1 of the present invention.
5 is a graph showing the measurement results of HRTEM-EDS (High-Resolution Transmission Electron Microscopy and Energy Dispersive Spectroscopy) elemental analysis mapping of the nickel-iron-cobalt layered double hydroxide prepared in Example 1 of the present invention. .
6 is a graph showing the measurement result of XRD (X-Ray Diffraction) of the nickel-iron-cobalt layered double hydroxide prepared in Example 1 of the present invention.
7 is a graph showing the measurement results of FT-IR (Fourier-transform infrared spectroscopy) of the nickel-iron-cobalt layered double hydroxide prepared in Example 1 of the present invention.
8 is a graph showing the analysis results of the XPS survey of the nickel-iron-cobalt layered double hydroxide prepared in Example 1 of the present invention.
9 is a graph showing high-resolution XPS analysis results of a) Fe 2p, b) Co 2p, c) Ni 2p, d) O 1s of the nickel-iron-cobalt layered double hydroxide prepared in Example 1 of the present invention; to be.
10 is an electrochemical catalyst activity analysis result of the oxygen production reaction of nickel-iron-cobalt layered double hydroxide according to Example 2 of the present invention, a) Linear sweep voltammetry (LSV) measurement result, b) Tafel plot, c) Chronopotentiometry results, d) 1000 cycles of Cyclic voltammetry (CV) measurements before and after a graph showing the measurement results of the linear scanning potential method
11 is an image showing the surface analysis of the nickel-iron-cobalt layered double hydroxide using FE-SEM after the time potentiometric measurement performed in FIG. 10c.
12 is a photograph of nickel-iron-cobalt layered double hydroxide grown on various types of nickel foam such as a) square, b) star, c) triangle, and d) circle according to Example 3 of the present invention.
13 is a graph showing an FE-SEM image of a nickel-iron-cobalt layered double hydroxide prepared using a reaction solution containing Co ²⁺ and Fe ²⁺ in various molar ratios according to Comparative Example 1 of the present invention ((( a) Co ²⁺ :Fe ²⁺ = 3:1, (b) Co ²⁺ :Fe ²⁺ =2:1, (c) Co ²⁺ :Fe ²⁺ =1:1, (d) Co ²⁺ :Fe ²⁺ =1:3).
14 is a voltage-current using a linear scanning potential method of a nickel-iron-cobalt layered double hydroxide prepared using a reaction solution containing Co ²⁺ and Fe ²⁺ in various molar ratios according to Comparative Example 1 of the present invention; It is a graph showing the measurement result.
15 is a graph showing the voltage-current measurement results using the linear scanning potential method of a catalyst made of an iron (III) nitrate solution and an iron (III) nitrate + cobalt nitrate (II) reaction solution according to Comparative Example 2 of the present invention. .
16 is a synthesis result of nickel-iron-cobalt layered double hydroxide using an FTO transparent electrode and carbon cloth according to Comparative Example 3 of the present invention, (a) a photograph before and after the reaction of the FTO transparent electrode and the carbon cloth electrode, ( b) FE-SEM image of FTO transparent electrode after reaction, (c) FE-SEM image of carbon cloth electrode after reaction, (d) voltage-current using linear scanning potential method before and after reaction between FTO transparent electrode and carbon cloth It is a graph showing the measurement result.

The present invention relates to a first step of immersing a nickel substrate in a solution containing a divalent iron salt and a divalent cobalt salt; and a second step of drying the nickel substrate obtained in the first step at room temperature; 1 It relates to a method for preparing a phosphorus, nickel-iron-cobalt layered double hydroxide.

Hereinafter, the manufacturing method of the nickel-iron-cobalt layered double hydroxide of the present invention will be described in detail as follows.

First, a nickel substrate is immersed in a solution containing a divalent iron salt and a divalent cobalt salt. At this time, the molar ratio of the divalent iron salt and the divalent cobalt salt in the solution is 1.5:1 to 2.5:1, and in one embodiment, the molar ratio of the divalent iron salt and the divalent cobalt salt in the solution is 2:1, but limited thereto it's not going to be This is because, if the ratio of the cobalt salt is too high, overgrowth in which a nanosheet is formed on the nanosheet structure proceeds, and if the molar ratio is satisfied, a low overvoltage may be obtained.

The solution of the first step contains a divalent iron salt as an iron salt. This is because the deposition rate of the divalent iron salt is about 1000 times faster than that of the trivalent iron salt.

The anions contained in the divalent iron salt and the divalent cobalt salt in the solution are 1 selected from the group consisting of carbonate anion, hydrogen carbonate anion, sulfate anion, nitrite anion, nitrate anion, phosphate anion, phosphite anion, hydroxide anion and halogen anion, respectively. It may be more than one anion. In one embodiment, the anion included in the divalent iron salt is a sulfate anion, and the anion included in the divalent cobalt salt is a nitrate anion, but is not limited thereto.

In the first step, nickel-iron-cobalt hydroxide having divalent nickel, divalent iron, and divalent cobalt metal cations may be deposited on the nickel substrate.

In one embodiment of the present disclosure, the nickel substrate is a nickel foam. This is because it is inexpensive and has a large surface area due to its porous structure, so that a large amount of nickel-iron-cobalt layered double hydroxide can be deposited.

Next, the method for producing a nickel-iron-cobalt layered double hydroxide of the present invention includes a second step of drying the nickel substrate obtained in the first step at room temperature. Through the second step, the iron oxidation number of the nickel-iron-cobalt layered double hydroxide deposited on the nickel substrate is self-oxidized from 2 to 3. This can be confirmed by changing the color of the nickel substrate from light green to brown, as shown in FIG. 2 .

Compensation ions carbonate ions (CO ₃ ^2- ), sulfate ions (SO ₄ ^2- ), nitrate ions (NO ₃ ^- ), etc. may exist between the layers of the nickel-iron-cobalt layered double hydroxide prepared according to the above preparation method. and is not limited thereto.

In addition, the present invention relates to a nickel-iron-cobalt layered double hydroxide prepared according to the above production method and an electrochemical catalyst comprising the same, wherein the nickel-iron-cobalt layered double hydroxide is an electrochemical reaction in an oxygen catalytic reaction of water electrolysis. It can be applied as a chemical catalyst.

Hereinafter, preferred embodiments according to the present invention will be described in detail with reference to the accompanying drawings. However, the following examples and accompanying drawings are provided so that those of ordinary skill in the art can fully understand the present invention, and can be modified in various other forms, and the scope of the present invention is described below. It is not limited to the embodiment and the accompanying drawings.

Example 1 Preparation of Nickel-Iron-Cobalt Layered Double Hydroxide

Nickel foam (1.5 x 5 cm) with 2 M After immersion in HCl and ultrasonic washing to remove the oxide layer on the surface, it was washed several times with distilled water and ethanol. 30 ml of a mixed metal salt solution of 20 mM cobalt (II) nitric acid and 40 mM iron (II) sulfate was prepared, and the nickel foam was immersed in the mixed metal salt solution for 5 hours (until it turned pale green). After that, the nickel foam was washed several times with distilled water and dried at room temperature for 12 hours. At this time, the molar ratio of Co ²⁺ and Fe ²⁺ in the cobalt (II) nitric acid and iron (II) sulfate mixed metal salt solution is 1:2.

After being supported in a mixed metal salt solution of cobalt (II) nitric acid and iron (II) sulfate, the nickel foam has a pale green color, and is brown after drying at room temperature. This is because the oxidation number of iron in the nickel-iron-cobalt layered double hydroxide before drying is 2, and after drying at room temperature, the oxidation number is 3 and undergoes self-oxidation. Step-by-step photos before and after drying after immersion in nickel foam itself, a mixed metal salt solution of cobalt (II) nitric acid and iron (II) sulfate, are shown in FIG. 2 .

In addition, the results of the iron 2p XPS analysis before and after drying of the nickel-iron-cobalt layered double hydroxide are shown in FIG. 3 . It can be seen that the binding energy of the iron 2p orbital increased by about 0.5 eV after drying, which is due to the self-oxidation process of iron.

The FE-SEM image of the surface analysis of the nickel-iron-cobalt layered double hydroxide synthesized by the method of Example 1 is shown in FIG. 4 . It can be seen that the two-dimensional nanosheet structure with an average size of 200 - 300 nm grows evenly.

HRTEM-EDS element mapping analysis of the nanosheet of Example 1 is shown in FIG. 5 . It can be seen from FIG. 5 that nickel, iron, and cobalt are evenly distributed in the nanosheet structure. At this time, the element ratio of nickel: iron: cobalt in the nanosheet structure is 4: 5: 10.

The XRD analysis of the nickel-iron-cobalt layered double hydroxide is shown in FIG. 6 , and it can be seen from FIG. 6 that the nickel-iron-cobalt layered double hydroxide synthesized according to Example 1 is amorphous.

The FT-IR analysis of the nickel-iron-cobalt layered double hydroxide is shown in FIG. 7, and from FIG. 7, carbonate ions (CO ₃ ^2- ) and sulfate ions (SO It can be seen that ₄ ^2- ) exists.

The XPS survey result of the nickel-iron-cobalt layered double hydroxide is shown in FIG. 8, and it can be seen from FIG. 8 that the constituent elements nickel, iron, cobalt, and oxygen are present. The XPS analysis of each element of the nickel-iron-cobalt layered double hydroxide is shown in FIG. 9, and it can be seen that iron has trivalent oxidation numbers, cobalt has divalent oxidation numbers, and nickel has divalent oxidation numbers. In addition, through the oxygen 1s XPS analysis shown in Fig. 9(d), it can be confirmed that 525.5 eV is a peak related to water adsorbed on the surface, 531.3 eV corresponds to metal oxyhydroxide, and the peak at 529.5 eV is related to metal hydroxide. have.

Example 2 Measurement of electrochemical catalytic activity of oxygen production reaction for nickel-iron-cobalt layered double hydroxide

The three-electrode system was constructed and an oxygen generation experiment was conducted in 1M KOH to confirm the electrochemical catalytic activity of the oxygen generation reaction of nickel-iron-cobalt layered double hydroxide. Specifically, nickel-iron-cobalt layered double hydroxide and nickel foam prepared in Example 1 were used as working electrodes, a platinum coil as a counter electrode, and a mercury-mercury oxide (Hg/HgO) electrode as a reference electrode. was used. At this time, the area of the working electrode was fixed to 1 x 1 cm.

FIG. 10A shows a current-voltage graph measured by linear sweep voltammetry (LSV). In general, when the current density is 10 mA·cm ^-2 , the required overvoltage ( η = E - 1.23V) is measured to evaluate the electrochemical catalytic activity. It can be seen from FIG. 10a that the nickel-iron-cobalt layered double hydroxide prepared in Example 1 has an overvoltage of 220 mV, and the pure nickel foam has an overvoltage of 404 mV.

Through the Tafel plot shown in FIG. 10b, the Tafel slope of nickel-iron-cobalt layered double hydroxide is 43 mV·dec ^-1 , which is smaller than 90 mV·dec ^-1 , which is the Tafel slope of nickel foam. can be checked This indicates that the nickel-iron-cobalt layered double hydroxide has excellent electrochemical catalytic performance.

In addition, for safety evaluation, a voltage change was observed while applying a current density of 10 mA·cm ⁻² using the time-to-potential method, and this is shown in FIG. 10C . And, as shown in FIG. 10D , after the oxygen generation reaction was applied by 1000 cycles of cyclic voltammetry, the linear scanning voltammetry was measured, and it was confirmed that there was no significant difference from the initial current-voltage graph. This indicates that the prepared nickel-iron-cobalt layered double hydroxide has excellent durability. In particular, as shown in FIG. 11 , after the stability test, surface analysis through FE-SEM was performed to confirm that the nanosheet structure was well maintained without significant change even under oxygen production reaction conditions.

Example 3 Preparation of nickel-iron-cobalt layered double hydroxide on a large-area nickel foam substrate

The nickel foam of Example 1 was carried out in the same manner as in the manufacturing method described in Example 1, except that various types of large-area nickel foam were used. Specifically, the molar concentrations of cobalt (II) nitric acid and iron (II) sulfate in the reaction solution and the ratio of the reaction solution per nickel foam area of the reaction solution were the same as those of Example 1. 12 shows nickel-iron-cobalt layered double hydroxide grown on various types of large-area nickel foam used in Example 3.

From FIG. 12, it can be confirmed that the nickel-iron-cobalt layered double hydroxide can grow evenly on the nickel foam substrate regardless of the area or size of the nickel foam substrate.

Comparative Example 1- Co in the reaction solution ²⁺ with Fe ²⁺ Preparation of nickel-iron-cobalt layered double hydroxide by varying the molar ratio of

In Example 1, except that the molar ratio of Co ²⁺ and Fe ²⁺ in the cobalt (II) nitric acid and iron (II) sulfate mixed metal salt solution was 3:1, 2:1, 1:1, 1:3 It was carried out in the same manner as in the manufacturing method.

FIG. 13 shows FE-SEM of nickel-iron-cobalt layered double hydroxide prepared using reaction solutions containing Co ²⁺ and Fe ²⁺ in various molar ratios. From this, it can be confirmed that the nanosheet structure can be obtained regardless of the molar ratio of Co ²⁺ and Fe ²⁺ in the reaction solution, but when the ratio of Co ²⁺ is high, overgrowth in which the nanosheet is formed on the nanosheet structure proceeds. .

14 shows a voltage-current graph measured by the linear scanning potential method of the prepared nickel-iron-cobalt layered double hydroxide according to the molar ratio of Co ²⁺ and Fe ²⁺ in the reaction solution. From this, it can be confirmed that Co ² :Fe ²⁺ =1:2 has the lowest overvoltage.

Comparative Example 2- Preparation of nickel-iron-cobalt layered double hydroxide using iron (III) nitrate and cobalt (II) nitrate reaction solution

Same as the preparation method of Example 1 except that the mixed solution of Example 1 of iron(II) sulfate and cobalt(II) nitric acid was used as iron(III) nitrate solution and iron(III) nitrate + cobalt(II) nitrate solution. was carried out.

FIG. 15 shows a voltage-current graph of a catalyst made of iron(III) nitrate solution and iron(III) nitrate + cobalt(II) nitrate solution. It can be seen that the performance of the catalyst prepared with the iron(III) nitrate reaction solution is higher than that of the catalyst prepared with the iron(III) nitrate + cobalt(II) nitrate reaction solution.

This is judged to be due to the difference in the solubility product constant of each metal hydroxide. The solubility product constant of iron (III) hydroxide is 3.8·10 ^-38 , and cobalt (II) hydroxide (1.6·10 ^-18 ) and hydroxide It is much smaller than that of nickel ((II) (1.6·10 ^-14 ). This difference in solubility product constant is indicated by the difference in the reaction rate of each metal hydroxide formed on the surface of the nickel foam, and iron(III) nitrate + cobalt(II) It can be confirmed that using the nitric acid reaction solution is not suitable for the synthesis of a uniform nickel-iron-cobalt layered double hydroxide.

Comparative Example 3 - Preparation of layered double hydroxide on a substrate other than the nickel foam substrate

A layered double hydroxide manufacturing experiment was carried out in the same manner as in Example 1 using a fluorine-doped tin oxide (FTO) transparent substrate and carbon cloth instead of nickel foam. Specifically, before the reaction, the FTO transparent substrate was sonicated for 30 minutes using ethanol and distilled water and then washed, and the carbon cloth was treated with oxygen plasma for 1 minute to etch the surface. Other conditions were carried out in the same manner as in the preparation method of Example 1.

Fig. 16 shows the results of an experiment for preparing a layered double hydroxide on the surface of the FTO transparent electrode and carbon cloth. It can be seen from Fig. 16 (a) that there is no color change between the FTO transparent electrode and the carbon cloth before and after the reaction, and from Fig. 16 (b) and (c), a nanosheet structure is formed on the surface of the FTO transparent electrode and the carbon cloth. know what you can't do. In addition, it can be seen from the voltage-current graph shown in Fig. 16(d) that no catalyst was formed on the surface of the FTO transparent electrode and the carbon cloth after the reaction.

Claims (7)

A first step of immersing the nickel substrate in a solution containing only a divalent iron salt and a divalent cobalt salt as a metal salt; and
a second step of drying the nickel substrate obtained in the first step at room temperature;
A method for producing a nickel-iron-cobalt layered double hydroxide comprising:
The molar ratio of the divalent iron salt and the divalent cobalt salt in the solution of the first step is 1.5:1 to 2.5:1,
A method for producing a nickel-iron-cobalt layered double hydroxide, wherein the nickel-iron-cobalt layered double hydroxide is deposited on a nickel substrate in only a first step. The method of claim 1,
The anions contained in the divalent iron salt and the divalent cobalt salt are each selected from the group consisting of a carbonate anion, a hydrogen carbonate anion, a sulfate anion, a nitrite anion, a nitrate anion, a phosphate anion, a phosphite anion, a hydroxide anion, and a halogen anion. Method for producing the above nickel-iron-cobalt layered double hydroxide. The method of claim 1,
A method for producing a nickel-iron-cobalt layered double hydroxide wherein the molar ratio of the divalent iron salt and the divalent cobalt salt in the solution of the first step is 2:1. The method of claim 1,
The second step is a method for producing a nickel-iron-cobalt layered double hydroxide wherein the oxidation number of iron is changed from 2 to 3 by self-oxidation of iron. delete delete delete

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