An OER electrocatalyst, its preparation method and application

Abstract

The present invention discloses an OER electrocatalyst, a preparation method thereof and an application thereof, relating to the technical field of water electrolysis. The OER electrocatalyst is a core-shell structured Ni-BDC@CoFe-LDH composite material, wherein the core layer is Ni-BDC nanosheets and the shell layer is CoFe-LDH. Based on the Ni-BDC nanorod array, the present invention further in-situ grows Ni-MOF and performs hydroxylation treatment to obtain a Ni-BDC@CoFe-LDH hierarchical structure composite material with regular morphology and ordered array. This in-situ grown heterostructure catalyst not only enhances the structural integrity, but also helps to improve the oxygen evolution reaction performance. The process of the present invention is simple and easy to implement, and high-performance electrode materials can be prepared in batches.

Description

Technical Field

[0001] The present invention relates to the technical field of electrolyzed water, and particularly to an OER electrocatalyst, a preparation method thereof, and an application thereof. Background Art

[0002] At present, humans are increasingly worried about the energy crisis and environmental problems. Therefore, there is an urgent need to find alternatives to fossil fuels. As a highly efficient, clean and green energy, hydrogen energy has received extensive attention. Electrolyzed water for hydrogen production is one of the common means. However, as a key half-reaction in electrocatalytic water splitting, OER (oxygen evolution reaction) limits the energy utilization efficiency due to its multi-electron transfer and slow kinetic process. Precious metals Ir and Ru and their oxides have excellent electrocatalytic activity in OER, but their high cost hinders their further application. Therefore, designing and developing new and inexpensive catalysts remains a great challenge. Summary of the Invention

[0003] Based on the above, the present invention provides an OER electrocatalyst, a preparation method thereof, and an application thereof. Without adding precious metals, by forming a shell layer of CoFe-LDH on the outer surface of the central core layer of Ni-BDC nanosheets, a core-shell structured Ni-BDC@CoFe-LDH composite material is obtained, which has a low overpotential, a high current density, and high stability.

[0004] To achieve the above object, the present invention provides the following solutions:

[0005] One of the technical solutions of the present invention is an OER electrocatalyst, which is a core-shell structured Ni-BDC@CoFe-LDH composite material, wherein the core layer is Ni-BDC nanosheets and the shell layer is CoFe-LDH; the Ni-BDC@CoFe-LDH composite material is an ordered array structure.

[0006] Another technical solution of the present invention is the preparation method of the above OER electrocatalyst, including the following steps:

[0007] Step 1: Add a growth substrate to a mixed solution of a nickel salt and terephthalic acid (molecular formula: C 8 H 6 O 4 ) for reaction to obtain Ni-BDC nanosheets;

[0008] Step 2: Immerse the Ni-BDC nanosheets in a mixed solution of 2-methylimidazole and a cobalt salt for reaction to obtain Ni-BDC@Co-MOF;

[0009] Step 3: Immerse the Ni-BDC@Co-MOF in an iron salt solution for reaction to obtain the OER electrocatalyst.

[0010] Before adding the growth substrate to the mixed solution, the growth substrate was pretreated by ultrasonic treatment with acetone and ethanol for 10 minutes in sequence to remove contaminants on the surface of the growth substrate.

[0011] Further, in step 1, the growth substrate is selected from one of carbon paper, nickel foam, and nickel sheet; the nickel salt is nickel chloride hexahydrate and / or nickel nitrate hexahydrate; the solvent of the mixed solution is obtained by mixing water, absolute ethanol, and N,N-dimethylformamide in a volume ratio of 1:1:10-16. Preferably, the volume ratio of water, absolute ethanol, and N,N-dimethylformamide is 1:1:16.

[0012] Further, in step 1, the molar ratio of the nickel salt to terephthalic acid is 1:1; the concentration of terephthalic acid in the mixed solution is 35-40 mM.

[0013] Further, the conditions of the reaction in step 1 are specifically: reacting at 100-130 °C for 3-5 hours. Preferably, reacting at 120 °C for 4 hours.

[0014] Further, in step 2, the molar ratio of 2-methylimidazole to the cobalt salt is 16:3-5; the concentration of the cobalt salt in the mixed solution in step 2 is 9-13 mM. Preferably, the concentration of the cobalt salt is 9.375 mM; the conditions of the reaction in step 2 are specifically: reacting at 5-10 °C for 3-4 h under the stirring condition of 350-400 revolutions per minute. Preferably, reacting at 5 °C for 3.5 h under the stirring condition of 360 revolutions per minute.

[0015] The solvent of the mixed solution in step 2 is selected from methanol and / or water; the selection of the solvent is based on whether it is beneficial to the formation of Co-MOF.

[0016] 2-Methylimidazole is one of the raw materials of Co-MOF in the technical solution of the present invention.

[0017] Further, in step 2, the cobalt salt is cobalt nitrate hexahydrate and / or cobalt chloride hexahydrate.

[0018] Further, the conditions of the reaction in step 3 are specifically: reacting at 60-80 °C for 5-10 min. Preferably, reacting at 65 °C for 5 min.

[0019] If the reaction temperature is too high, the Ni-BDC structure will be damaged; if the reaction temperature is too low, the outer layer Co-MOF cannot be derived into CoFe-LDH; if the reaction time is too long, it is not conducive to the integrity of the CoFe-LDH morphology; if the reaction time is too short, all of the Co-MOF cannot be derived. Therefore, the conditions of the reaction in the present invention are specifically: reacting at 60-80 °C for 5-10 min. Preferably, reacting at 65 °C for 5 min.

[0020] Further, the solvent of the mixed solution in Step 3 is ethanol and / or water; the concentration of the iron salt in the iron salt solution in Step 3 is 0.05 - 0.5 mg / mL; the iron salt in Step 3 is ferric nitrate nonahydrate and / or ferric chloride hexahydrate.

[0021] In the preparation method of the present invention, in Step 1, when synthesizing Ni-BDC nanosheets, different solvent ratios will affect the sparsity of the growth of Ni-BDC nanosheets; in Step 2, different ratios of ligands to metal salts for synthesizing Ni-BDC@Co-MOF will affect the growth density of the Ni-BDC@Co-MOF; in Step 3, the adjustment of the reaction temperature and time will affect the derivatization degree of CoFe-LDH on Ni-BDC@CoFe-LDH (OER electrocatalyst).

[0022] The third technical solution of the present invention is the application of the above OER electrocatalyst in electrolyzing water.

[0023] The technical concept of the present invention:

[0024] Metal-organic framework materials (MOFs) are reticular porous structures constructed by metal ions or ion clusters and organic ligands through coordination. The high specific surface area, porosity, adjustable pore size, and pore surface functions of MOFs themselves make them have potential application values in the fields of adsorption separation, catalysis, energy storage, etc. In recent years, it has been reported that some MOFs can be effectively used as electrode materials in the electrocatalytic water splitting system. However, due to the poor conductivity and water stability of MOFs themselves, the loss of active sites and the decline of catalytic activity occur.

[0025] The present invention utilizes the in-situ growth mechanism of MOF-on-MOF to construct a derivative Ni-BDC@CoFe-LDH composite material with a MOFs heterostructure to improve the conductivity and water stability of MOFs as electrode materials, increase the current density of the reaction, and achieve efficient electrocatalytic water splitting.

[0026] The present invention discloses the following technical effects:

[0027] Based on the Ni-BDC nanorarray, the present invention further in-situ grows Ni-MOF and performs hydroxylation treatment to obtain a Ni-BDC@CoFe-LDH multi-level structure composite material with a regular morphology and ordered array derived from MOF. This in-situ grown heterostructure catalyst not only enhances the structural integrity but also helps to improve the oxygen evolution reaction performance.

[0028] The process of the present invention is simple and easy to implement, and high-performance electrode materials can be prepared in batches. Description of the Drawings

[0029] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the following will briefly introduce the accompanying drawings required for the embodiments. Obviously, the accompanying drawings in the following description are only some embodiments of the present invention. For those of ordinary skill in the art, without creative efforts, other accompanying drawings can be obtained based on these drawings.

[0030] Figure 1 Scanning electron microscope image of Ni-BDC prepared in Example 1 of the present invention;

[0031] Figure 2 Scanning electron microscope image of Ni-BDC@Co-MOF prepared in Example 1 of the present invention;

[0032] Figure 3 Scanning electron microscope image of Ni-BDC@CoFe-LDH prepared in Example 1 of the present invention;

[0033] Figure 4 Voltammetric linear polarization curve of Ni-BDC@CoFe-LDH prepared in Example 1 of the present invention in 1M potassium hydroxide solution;

[0034] Figure 5 Overpotential diagram of Ni-BDC@CoFe-LDH prepared in Example 1 of the present invention at a current density of 20 mA cm -2 Current density;

[0035] Figure 6 Transmission electron microscope image and elemental distribution map of Ni-BDC@CoFe-LDH prepared in Example 1 of the present invention;

[0036] Figure 7 Chronopotentiogram of Ni-BDC@CoFe-LDH prepared in Example 1 of the present invention. Detailed implementation manners

[0037] Now, the various exemplary embodiments of the present invention will be described in detail. This detailed description should not be considered as a limitation of the present invention, but rather as a more detailed description of certain aspects, characteristics, and implementation manners of the present invention.

[0038] It should be understood that the terms used in the present invention are only for describing specific implementation manners and are not used to limit the present invention. Additionally, for the numerical ranges in the present invention, it should be understood that each intermediate value between the upper and lower limits of the range is also specifically disclosed. Each intermediate value within any stated value or stated range, as well as each smaller range between any other stated value or intermediate value within the stated range, is also included in the present invention. The upper and lower limits of these smaller ranges can be independently included or excluded from the range.

[0039] Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. Although this invention only describes preferred methods and materials, any methods and materials similar or equivalent to those described herein can also be used in the practice or testing of this invention. All documents mentioned in this specification are incorporated by reference to disclose and describe the methods and / or materials related to the said documents. In case of conflict with any incorporated document, the content of this specification shall prevail.

[0040] Without departing from the scope or spirit of this invention, various modifications and variations can be made to the specific embodiments of the specification of this invention, which are obvious to those skilled in the art. Other embodiments obtained from the specification of this invention are obvious to those skilled in the art. The specification and examples of this invention are merely exemplary.

[0041] Regarding the use of "comprising", "including", "having", "containing", etc. herein, they are all open-ended terms, meaning including but not limited to.

[0042] The "parts" mentioned in this invention are by mass parts unless otherwise specified.

[0043] The Ir / C described in this invention contains 20 wt% Ir.

[0044] The preparation method of CP / Ir / C described in this invention is as follows:

[0045] Disperse 0.1 g of Ir / C (20 wt% Ir) in a mixed solution of 2 mL of ethanol and 0.2 g of 5 wt% nafion solution. Ultrasonically treat the mixture for 30 minutes. Subsequently, immerse a clean carbon paper (10×10×1 mm) into the above solution for 10 minutes, and then let the solvent evaporate in air for 12 h to obtain CP / Ir / C.

[0046] The preparation method of CoFe-LDH described in this invention is as follows: First, grow Co-MOF in situ on carbon paper, and then dissolve it together with 5 mg of iron(III) nitrate nonahydrate in 20 mL of an ethanol mixed solution for reaction. The reaction temperature is 65 °C and the reaction time is 5 minutes to obtain CoFe-LDH grown on carbon paper.

[0047] Example 1

[0048] Step 1: The carbon paper was pretreated by ultrasonic treatment with acetone and ethanol for 10 minutes in sequence, and the pretreated carbon paper was placed obliquely into a 20 mL polytetrafluoroethylene reaction kettle containing Solution A for hydrothermal reaction (where Solution A is a mixed solution of nickel chloride hexahydrate and terephthalic acid with the same molar concentration dissolved in water, absolute ethanol, and N,N-dimethylformamide. The concentrations of nickel chloride hexahydrate and terephthalic acid in Solution A are both 35 mM, and the volume ratio of water, absolute ethanol, and N,N-dimethylformamide is 1:1:16). The reaction conditions were 120 °C for 4 hours. After the reaction, it was washed with absolute ethanol to obtain Ni-BDC nanosheets.

[0049] Step 2: The Ni-BDC nanosheets prepared in Step 1 were immersed in Mother Liquid B and reacted in a 20 mL quartz glass container (Mother Liquid B is a mixed solution of an 80 mM methanol solution of 2-methylimidazole and a 25 mM methanol solution of cobalt nitrate hexahydrate in a volume ratio of 5:3). The reaction temperature was maintained at 5 °C, and magnetic stirring was carried out at 360 revolutions per minute for 3.5 hours. After the reaction, it was washed with ethanol to obtain the multi-structured Ni-BDC@Co-MOF.

[0050] Step 3: 5 mg of iron nitrate nonahydrate was dissolved in 20 mL of ethanol to obtain Solution C. The Ni-BDC@Co-MOF prepared in Step 2 was immersed in Solution C for reaction. The reaction temperature was 65 °C, and the reaction time was 5 minutes. After the reaction, it was washed with ethanol and deionized water in sequence to finally obtain the OER electrocatalyst (Ni-BDC@CoFe-LDH).

[0051] The Ni-BDC, Ni-BDC@Co-MOF, and Ni-BDC@CoFe-LDH prepared in this example were characterized by a field emission scanning electron microscope of model SU8020, and the results are as Figures 1-3 shown.

[0052] Figure 1 is the scanning electron microscope image of Ni-BDC prepared in this example; Figure 2 is the scanning electron microscope image of Ni-BDC@Co-MOF prepared in Example 1 of the present invention; Figure 3 is the scanning electron microscope image of Ni-BDC@CoFe-LDH prepared in Example 1 of the present invention. It can be seen from Figures 1-3 that the surface is a tightly bound CoFe-LDH and Ni-BDC, which is an ordered array structure.

[0053] The linear voltammetric polarization curve of Ni-BDC@CoFe-LDH prepared in this example in 1 M potassium hydroxide solution was tested using a German ZAHNER electrochemical workstation, and the results are as Figure 4 shown. It can be seen from Figure 4It can be seen that the Ni-BDC@CoFe-LDH prepared in this example only requires an overpotential of 310 mV to reach a current density of 20 mA cm -2 . It has a lower overpotential and a high current density.

[0054] Figure 5 Figure showing the overpotential of Ni-BDC@CoFe-LDH prepared in Example 1 of the present invention at a current density of 20 mA cm -2 (In the figure, CP / Ni-BDC@CoFe-LDH indicates that the electrocatalyst is Ni-BDC@CoFe-LDH grown on carbon paper, CP / Ir / C indicates that the electrocatalyst is Ir / C grown on carbon paper, CP / CoFe-LDH indicates that the electrocatalyst is CoFe-LDH grown on carbon paper, and CP / Ni-BDC indicates that the electrocatalyst is Ni-BDC grown on carbon paper); It can be Figure 5 seen that the overpotential of Ni-BDC@CoFe-LDH at a current density of 20 mA cm -2 is 310 mV, the overpotential of Ir / C at a current density of 20 mA cm -2 is 375 mV, the overpotential of CoFe-LDH at a current density of 20 mA cm -2 is 473 mV, and the overpotential of Ni-BDC at a current density of 20 mA cm -2 is 490 mV. At the same current density, Ni-BDC@CoFe-LDH exhibits the lowest overpotential, demonstrating that the oxygen evolution reaction uses less energy.

[0055] Figure 6 Transmission electron microscopy image of Ni-BDC@CoFe-LDH prepared in this example. It can be Figure 6 seen that CoFe-LDH particles and lattice fringes are loaded on the Ni-BDC nanosheets.

[0056] The stability of Ni-BDC@CoFe-LDH was tested using chronopotentiometry, and the results are as Figure 7 shown. It can be Figure 7 seen that at a current density of 20 mA cm -2 , the potential can remain stable for 12 h.

[0057] Example 2

[0058] Step 1: The carbon paper was pretreated by ultrasonicating with acetone and ethanol for 10 minutes in sequence, and the pretreated carbon paper was inclined and placed into a 20 mL polytetrafluoroethylene reaction kettle containing solution A for hydrothermal reaction (where solution A is a mixed solution of nickel chloride hexahydrate and terephthalic acid with the same molar concentration dissolved in water, absolute ethanol and N,N-dimethylformamide, and the concentrations of nickel chloride hexahydrate and terephthalic acid in solution A are both 40 mM, and the volume ratio of water, absolute ethanol and N,N-dimethylformamide is 1:1:12). The reaction conditions were 130 °C for 3 hours. After the reaction, Ni-BDC nanosheets were obtained by washing with absolute ethanol.

[0059] Step 2: The Ni-BDC nanosheets prepared in Step 1 were immersed in mother liquor B and reacted in a 20 mL quartz glass container (mother liquor B is a mixed solution obtained by mixing an 80 mM methanol solution of 2-methylimidazole and a 25 mM methanol solution of cobalt nitrate hexahydrate in a volume ratio of 5:5). The reaction temperature was maintained at 10 °C, with magnetic stirring at 380 revolutions per minute for 3 hours. After the reaction, ethanol washing was carried out to obtain the multi-structured Ni-BDC@Co-MOF.

[0060] Step 3: 10 mg of iron nitrate nonahydrate was dissolved in 20 mL of ethanol to obtain solution C. The Ni-BDC@Co-MOF prepared in Step 2 was immersed in solution C for reaction. The reaction temperature was 60 °C and the reaction time was 10 minutes. After the reaction, washing was carried out with ethanol and deionized water in sequence to finally obtain the OER electrocatalyst (Ni-BDC@CoFe-LDH).

[0061] The structural morphology of the Ni-BDC@CoFe-LDH prepared in this example is similar to that in Example 1, and the attached drawings are not repeated here.

[0062] The Ni-BDC@CoFe-LDH prepared in this example only requires an overpotential of 319 mV to reach a current density of 20 mA cm -2 . It has a lower overpotential and a high current density.

[0063] The same effect verification of the stability of the Ni-BDC@CoFe-LDH prepared in this example as in Example 1 was carried out. The results show that the stability of the Ni-BDC@CoFe-LDH prepared in this example is similar to that in Example 1 and has high stability.

[0064] Example 3

[0065] Step 1: The carbon paper was pretreated by ultrasonicating in acetone and ethanol for 10 minutes successively, and the pretreated carbon paper was placed obliquely into a 20 mL polytetrafluoroethylene reaction kettle containing Solution A for hydrothermal reaction (where Solution A is a mixed solution of nickel chloride hexahydrate and terephthalic acid with the same molar concentration dissolved in water, absolute ethanol, and N,N-dimethylformamide. The concentrations of nickel chloride hexahydrate and terephthalic acid in Solution A are both 35 mM, and the volume ratio of water, absolute ethanol, and N,N-dimethylformamide is 1:1:10). The reaction conditions were 120 °C for 4 hours. After the reaction, it was washed with absolute ethanol to obtain Ni-BDC nanosheets.

[0066] Step 2: The Ni-BDC nanosheets prepared in Step 1 were immersed in Mother Liquid B and reacted in a 20 mL quartz glass container (Mother Liquid B is a mixed solution obtained by mixing a methanol solution of 80 mM 2-methylimidazole and a methanol solution of 25 mM cobalt nitrate hexahydrate in a volume ratio of 5:4). The reaction temperature was maintained at 5 °C, with magnetic stirring at 360 revolutions per minute for 3.5 hours. After the reaction, it was washed with ethanol to obtain the multi-structured Ni-BDC@Co-MOF.

[0067] Step 3: 5 mg of iron nitrate nonahydrate was dissolved in 20 mL of ethanol to obtain Solution C. The Ni-BDC@Co-MOF prepared in Step 2 was immersed in Solution C for reaction. The reaction temperature was 80 °C and the reaction time was 2 minutes. After the reaction, it was washed successively with ethanol and deionized water to finally obtain the OER electrocatalyst (Ni-BDC@CoFe-LDH).

[0068] The above-described embodiments are only descriptions of the preferred embodiments of the present invention, and do not limit the scope of the present invention. Without departing from the design spirit of the present invention, various deformations and improvements made by those of ordinary skill in the art to the technical solutions of the present invention shall fall within the protection scope determined by the claims of the present invention.

Claims

1. An OER electrocatalyst, characterized in that, the OER electrocatalyst is a Ni-BDC@CoFe-LDH composite material with a core-shell structure, where the core layer is Ni-BDC nanosheets and the shell layer is CoFe-LDH; the Ni-BDC@CoFe-LDH composite material is an ordered array structure; the preparation method of the OER electrocatalyst includes the following steps: Step 1, adding a growth substrate into a mixed solution of nickel salt and terephthalic acid for reaction to obtain Ni-BDC nanosheets; Step 2, immersing the Ni-BDC nanosheets into a mixed solution of 2-methylimidazole and cobalt salt for reaction to obtain Ni-BDC@Co-MOF; Step 3, immersing the Ni-BDC@Co-MOF into an iron salt solution for reaction to obtain the OER electrocatalyst; The conditions of the reaction in Step 1 are specifically: reacting at 100-130°C for 3-5 hours; The molar ratio of 2-methylimidazole to cobalt salt in Step 2 is 16:3-5; the concentration of cobalt salt in the mixed solution in Step 2 is 9-13 mM; the conditions of the reaction in Step 2 are specifically: reacting at 5-10°C for 3-4 h under stirring conditions; The conditions of the reaction in Step 3 are specifically: reacting at 60-80°C for 5-10 min.

2. The OER electrocatalyst according to claim 1, characterized in that, the growth substrate in Step 1 is selected from one of carbon paper, nickel foam, and nickel sheet; the nickel salt is nickel chloride hexahydrate and / or nickel nitrate hexahydrate; the solvent of the mixed solution is obtained by mixing water, absolute ethanol, and N,N-dimethylformamide in a volume ratio of 1:1:10-16.

3. The OER electrocatalyst according to claim 1, characterized in that, the molar ratio of the nickel salt to terephthalic acid in Step 1 is 1:1; the concentration of terephthalic acid in the mixed solution is 35-40 mM.

4. The OER electrocatalyst according to claim 1, characterized in that, the cobalt salt in Step 2 is cobalt nitrate hexahydrate and / or cobalt chloride hexahydrate.

5. The OER electrocatalyst according to claim 1, characterized in that, the solvent of the solution in Step 3 is ethanol and / or water; the concentration of the iron salt in the iron salt solution in Step 3 is 0.05-0.5 mg / mL; the iron salt in Step 3 is iron nitrate nonahydrate and / or iron chloride hexahydrate.

6. The application of the OER electrocatalyst as claimed in claim 1 in water electrolysis.

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