A cobalt-iron layered hydroxide@carbon nanotube composite material, its preparation method and application

Abstract

This invention provides a cobalt-iron layered hydroxide@carbon nanotube composite material, its preparation method, and its application. The preparation method involves first modifying and carbonizing polypyrrole nanotubes, then mixing them with cobalt nitrate, CTAB, and 2-methylimidazole in water, centrifuging to obtain the product, and finally performing a one-step ion exchange to achieve the carbonization of the composite material and the in-situ generation of cobalt-iron layered hydroxide nanosheets, ultimately yielding the cobalt-iron layered hydroxide@carbon nanotube composite material. Compared with traditional preparation methods, this invention's method is simple, has mild reaction conditions, high yield, and produces a composite material with uniform morphology, coexisting nanosheets and nanotubes. The carbon nanotubes provide a continuous three-dimensional conductive network, while the ultrathin layered hydroxide nanosheets are tightly attached to the carbon nanotube surface, achieving the dual advantages of high electron transport rate and high specific surface area with exposed active sites, exhibiting excellent electrocatalytic performance for the oxygen evolution reaction in water electrolysis.

Description

Technical Field

[0001] This invention relates to the field of cobalt-based catalyst carbon nanomaterials technology, specifically a cobalt-iron layered hydroxide@carbon nanotube composite material, its preparation method and application. Background Technology

[0002] Currently, developing efficient water electrolysis technology for hydrogen production has become a key pathway to achieving large-scale green hydrogen production. As the rate-controlling step in the water electrolysis process, the oxygen evolution reaction (OER) requires overcoming a high theoretical overpotential (1.23V), leading to increased system energy consumption. While commercially available IrO2 / RuO2 catalysts possess excellent catalytic activity, their low global abundance and high cost severely restrict their large-scale application.

[0003] In recent years, 3d transition metal (such as Fe, Co, and Ni) compounds have attracted widespread attention due to their abundant reserves and low cost. Among them, cobalt-based materials have shown potential for electrocatalytic applications in alkaline media. However, traditional cobalt-based catalysts generally suffer from poor intrinsic conductivity, easy aggregation of active sites, and limited specific surface area, which severely restricts their catalytic activity and stability, making it difficult to meet the needs of industrial applications. Although existing studies have attempted to improve conductivity using conductive polymer-derived carbon materials, their support structures often struggle to achieve high-density and uniform dispersion of active sites. While using metal-organic framework (MOF) precursors is beneficial for constructing highly dispersed metal sites, it faces technical bottlenecks such as poor conductivity of the materials themselves and easy structural collapse and loss of active sites during subsequent conversions. Therefore, how to achieve efficient exposure and stable dispersion of active sites while maintaining high conductivity remains a key problem that urgently needs to be solved in the design of current cobalt-based catalyst materials. Summary of the Invention

[0004] The purpose of this invention is to provide a cobalt-iron layered hydroxide@carbon nanotube composite material, its preparation method and application, to solve the problems of poor intrinsic conductivity and easy aggregation of active sites in existing cobalt-based catalysts.

[0005] This invention is implemented as follows:

[0006] A method for preparing a cobalt-iron layered hydroxide@carbon nanotube composite material includes the following steps:

[0007] (1) Modification of carbon nanotubes: Methyl orange and anhydrous ferric chloride were dissolved in water, then pyrrole solution was added and stirred. After suction filtration and drying, a black product was obtained. The black product was dissolved in methanol, sodium dodecylbenzenesulfonate was dissolved in water, the methanol solution of the black product and the aqueous solution of sodium dodecylbenzenesulfonate were mixed, ultrasonically dispersed, washed and dried, and then calcined at high temperature under an inert atmosphere to obtain modified polypyrrole carbon nanotubes.

[0008] (2) Preparation of ZIF-67@polypyrrole carbon nanotubes: The modified polypyrrole carbon nanotubes, cobalt nitrate hexahydrate and hexadecyltrimethylammonium bromide were dissolved in water to obtain solution one; 2-methylimidazole was dissolved in water to obtain solution two; solution one and solution two were mixed and stirred, allowed to stand, and then washed and dried to obtain ZIF-67@polypyrrole carbon nanotubes;

[0009] (3) Preparation of cobalt-iron layered hydroxide@carbon nanotube composite material: ZIF-67@polypyrrole carbon nanotubes and ferrous sulfate heptahydrate are dissolved in a mixed solution of water and ethanol, and stirred for 0.5~3 h at room temperature to 50℃ to obtain cobalt-iron layered hydroxide@carbon nanotube composite material.

[0010] Preferably, the water used in step (1) is ultrapure water.

[0011] Preferably, the suction filtration in step (1) specifically involves suction filtration using ultrapure water at 60°C and anhydrous ethanol at room temperature.

[0012] Preferably, the high-temperature calcination in step (1) under an inert atmosphere specifically involves heating the furnace to 800°C at a heating rate of 3°C / min under a nitrogen atmosphere in a tube furnace, and holding the furnace at 800°C for 1 hour.

[0013] Preferably, the water used in step (2) is deionized water.

[0014] Preferably, the washing and drying in step (2) are as follows: wash with deionized water and ethanol in sequence, centrifuge at 9000 rpm during washing, and dry in a vacuum oven at 70°C for 12 h after washing.

[0015] The cobalt-iron layered hydroxide@carbon nanotube composite material prepared according to the above method can be applied to the oxygen evolution reaction of water electrolysis. The specific application method is as follows: the cobalt-iron layered hydroxide@carbon nanotube composite material is dispersed in a dispersant (the dispersant is a naphthol solution) to obtain a dispersion, the dispersion is drop-coated onto the surface of a glassy carbon electrode, and dried to obtain the working electrode.

[0016] This invention is the first to construct a composite structure of cobalt-iron layered hydroxide nanosheets@carbon nanotubes. Carbon nanotubes (CNTs) provide a continuous three-dimensional conductive network, while ultrathin layered hydroxide (LDH) nanosheets are tightly attached to the CNT surface, achieving the dual advantages of high electron transport rate and high specific surface area of ​​active sites.

[0017] Nitrogen-doped CNTs formed by polypyrrole carbonization not only improve the conductivity of the carbon framework, but also serve as coordination centers for Co and Fe, strengthening the electronic coupling between the metal-LDH and the support, and enhancing the reaction activation ability.

[0018] The preparation method provided by this invention adopts a three-step integrated strategy (modified carbon nanotubes → epitaxial MOF growth → in-situ ion exchange). The first step: CNT surface activation and nitrogen doping: using Fe 3+ The process involves: 1) Polymerization of methyl orange / pyrrole followed by high-temperature carbonization to introduce pyrrole nitrogen and surface functional groups, enhancing subsequent bonding with MOFs; 2) ZIF-67 epitaxial growth: Constructing regular cubic MOFs on the CNT surface to ensure highly dispersed Co sites and form a uniform precursor; 3) One-step ion exchange method: Adding Fe... 2+ By introducing and converting ZIF-67 into CoFe-LDH nanosheets, a direct phase transition from MOF to LDH is achieved. The reaction is mild (stirring from room temperature to 50°C, without the need for high pressure or complex equipment) and has no extra calcination steps. This avoids the destruction of carbon framework defects and metal site aggregation caused by high temperature, ensuring uniform distribution and ultrathin nanosheets.

[0019] Compared with existing technologies, the synthesis method of this invention is simple, low-cost, high-yield, and mild. The cobalt-iron layered hydroxide@carbon nanotube composite material prepared by the method of this invention has a uniform morphology, achieving the coexistence of nanosheets and nanotubes, exposing more active sites, and exhibiting excellent electrocatalytic performance for the oxygen evolution reaction in water electrolysis, showing a lower onset potential, higher current density, and smaller Tafel slope. This is due to the synergistic effect of the conductivity of the nitrogen-doped carbon framework and the metal sites, which is superior to commercial IrO2 and control materials.

[0020] The cobalt-iron layered hydroxide@carbon nanotube composite material provided by this invention can be used as a catalyst in the oxygen evolution reaction of water electrolysis, providing a new option for research in energy storage, conversion, fuel cells and other related fields. Attached Figure Description

[0021] Figure 1 This is a schematic diagram of the preparation method of the cobalt-iron layered hydroxide@carbon nanotube composite material of the present invention.

[0022] Figure 2 These are scanning electron microscope (SEM) images of the materials prepared in Example 1 of the present invention; wherein, A, B and C are SEM images of the modified polypyrrole carbon nanotubes, ZIF-67@polypyrrole carbon nanotubes and cobalt iron layered hydroxide@carbon nanotube composite materials, respectively.

[0023] Figure 3 This is an elemental mapping image of the cobalt-iron layered hydroxide@carbon nanotube composite material prepared in Example 1 of this invention.

[0024] Figure 4This is an X-ray powder diffraction pattern of the material prepared in Example 1 of the present invention; wherein, a represents modified polypyrrole carbon nanotubes, b represents ZIF-67@polypyrrole carbon nanotubes, and c represents cobalt-iron layered hydroxide@carbon nanotube composite material.

[0025] Figure 5 The image shows the infrared spectrum of the material prepared in Example 1 of this invention; where a represents modified polypyrrole carbon nanotubes, b represents ZIF-67@polypyrrole carbon nanotubes, and c represents cobalt-iron layered hydroxide@carbon nanotube composite material.

[0026] Figure 6 This is the Raman spectrum of the material prepared in Example 1 of the present invention; wherein, a represents modified polypyrrole carbon nanotubes, b represents ZIF-67@polypyrrole carbon nanotubes, and c represents cobalt-iron layered hydroxide@carbon nanotube composite material.

[0027] Figure 7 This is the X-ray photoelectron spectroscopy spectrum of the cobalt-iron layered hydroxide@carbon nanotube composite material prepared in Example 1 of this invention.

[0028] Figure 8 The figures are linear scanning voltammetric polarization curves and corresponding Tafel curves of the materials prepared in the embodiments of the present invention and commercial IrO2 materials. In the figures, A is the linear scanning voltammetric polarization curve, B is the Tafel curve, a represents the modified polypyrrole carbon nanotubes in Example 1, b represents the ZIF-67@polypyrrole carbon nanotubes in Example 1, c, d and e represent the cobalt-iron layered hydroxide@carbon nanotube composite materials prepared in Examples 1, 2 and 3, respectively, and f represents commercial IrO2 materials.

[0029] Figure 9 The material prepared in Example 1 is in the range of 30-180 mV·s -1 Cyclic voltammograms at scan rates and linear fitting plots of current density difference versus scan rate at a given potential; where A represents the modified polypyrrole carbon nanotubes at 30-180 mV·s. -1 Cyclic voltammograms at scan rates, B represents ZIF-67@polypyrrole carbon nanotubes in the range of 30-180 mV·s. -1 Cyclic voltammograms at scan rates, where C represents the cobalt-iron layered hydroxide@carbon nanotube composite material in the range of 30-180 mV·s. -1 Cyclic voltammetry at scan rates: In A, B, and C, different colored lines represent different scan rates. Following the direction of the arrows, the scan rates corresponding to each line are 30, 60, 90, 120, 150, and 180 mV / s, respectively. D represents the current density difference Δj of the material prepared in Example 1 (Δj = j a -j c ja For the anode current, j c The linear fit plot of the scan rate at a given potential (+1.023 V vs. RHE) is shown. a represents modified polypyrrole carbon nanotubes, b represents ZIF-67@polypyrrole carbon nanotubes, and c represents cobalt-iron layered hydroxide@carbon nanotube composite material.

[0030] Figure 10 The image shows the AC impedance spectrum of the materials prepared in the embodiments of the present invention, where a represents the modified polypyrrole carbon nanotubes in Example 1, b represents the ZIF-67@polypyrrole carbon nanotubes in Example 1, and c, d, and e represent the cobalt-iron layered hydroxide@carbon nanotube composite materials prepared in Examples 1, 2, and 3, respectively.

[0031] Figure 11 This is a CV cycle curve of the cobalt-iron layered hydroxide@carbon nanotube composite material prepared in Example 1 of the present invention for the oxygen evolution reaction after 1000 cycles. Detailed Implementation

[0032] The present invention will be further described below with reference to the embodiments. The processes and methods not described in detail in the following embodiments are conventional methods known in the art. Unless otherwise stated, the raw materials or reagents used in the embodiments are commercial products that can be purchased through commercial channels.

[0033] Example 1: Preparation of cobalt-iron layered hydroxide@carbon nanotube composite material.

[0034] Combination Figure 1 The preparation method of the cobalt-iron layered hydroxide@carbon nanotube composite material in this embodiment is as follows:

[0035] (1) Modify carbon nanotubes.

[0036] 0.1964 g of methyl orange and 0.972 g of anhydrous ferric chloride were dissolved in 120 mL of ultrapure water (water with a resistivity of 18 MΩ·cm at 25 °C) and stirred for 30 min (stirring at room temperature at 500 rpm). Then, 420 μL of pyrrole solution was added and stirred for 24 h (stirring at room temperature at 500 rpm). The mixture was then separated by suction filtration using ultrapure water at 60 °C and anhydrous ethanol at room temperature, and finally dried at 80 °C for 5 h to obtain a black product. 1 g of the black product was dissolved in 25 mL of methanol (all methanol used in this example was anhydrous methanol) to obtain a methanol solution of the black product; 2.5 g of sodium dodecylbenzenesulfonate was dissolved in 25 mL of ultrapure water to obtain an aqueous solution of sodium dodecylbenzenesulfonate; the methanol solution of the black product and the aqueous solution of sodium dodecylbenzenesulfonate were mixed and ultrasonically dispersed for 3 h, and finally washed 3 times with methanol, dried at 80 °C for 5 h, and then heated to 800 °C at a heating rate of 3 °C / min in a tube furnace under nitrogen atmosphere, and held at 800 °C for 1 h to obtain modified polypyrrole carbon nanotubes.

[0037] (2) Preparation of ZIF-67@polypyrrole carbon nanotubes.

[0038] 20 mg of the modified polypyrrole carbon nanotubes, 0.584 g of cobalt nitrate hexahydrate, and 10 mg of cetyltrimethylammonium bromide (CTAB) from step (1) were dissolved in 20 mL of deionized water and sonicated for 1 h to obtain solution one; 9.08 g of 2-methylimidazole was dissolved in 140 mL of deionized water to obtain solution two; solution one and solution two were mixed and stirred for 30 min, then allowed to stand for 24 h, and finally washed 3 times each with deionized water and ethanol. The washing was done by centrifugation at 9000 rpm for a total washing time of 5 minutes. After that, the mixture was dried in a vacuum oven at 70 °C for 12 h to obtain ZIF-67@polypyrrole carbon nanotubes.

[0039] (3) Preparation of cobalt-iron layered hydroxide@carbon nanotube composite material.

[0040] Dissolve 50 mg of ZIF-67@polypyrrole carbon nanotubes and 50 mg of ferrous sulfate heptahydrate obtained in step (2) in a mixed solution of 10 mL of water and 40 mL of ethanol, and stir at 50 °C for 1 h to obtain cobalt-iron layered hydroxide@carbon nanotube composite material.

[0041] The material obtained in this embodiment was structurally characterized, and the results are as follows: Figure 2-7 As shown.

[0042] Figure 2Scanning electron microscope (SEM) images of the modified polypyrrole carbon nanotubes, ZIF-67@polypyrrole carbon nanotubes, and cobalt-iron layered hydroxide@carbon nanotube composites prepared in this embodiment are shown. As can be seen from the images, all three materials exhibit good morphology. The cobalt-iron layered hydroxide@carbon nanotube composite has a uniform morphology, exhibiting an overall tubular structure with nanosheets attached to its surface.

[0043] Figure 3 The elemental mapping diagram of the cobalt-iron layered hydroxide@carbon nanotube composite material prepared in this embodiment is shown, wherein C, N, O, Co and Fe are uniformly distributed therein, with contents of 83.74%, 0.60%, 13.04%, 1.38% and 1.24%, respectively.

[0044] Figure 4 , Figure 5 and Figure 6 The X-ray powder diffraction pattern, infrared spectrum, and Raman spectrum of the material prepared in this embodiment are shown respectively. Figure 4 In the diffraction pattern, the diffraction peak at 26.6° belongs to the (006) crystal plane of C, the diffraction peaks at 9.5°, 19.16°, and 38.03° belong to the (001), (002), and (102) crystal planes of Co(OH)2, and the diffraction peaks at 19.23°, 31.70°, and 37.12° belong to the (001), (100), and (101) crystal planes of Fe(OH)2. Figure 5 In the infrared spectrum, it is located at 3367 cm⁻¹ -1 and 1628cm -1 The peaks at these locations are attributed to the stretching and tensile vibrations of -OH, respectively, indicating that the surface of the synthesized composite material has a large number of hydroxyl groups. Figure 6 In the Raman spectrum, characteristic peaks of layered hydroxide (LDH) and the D and G bands of the carbon material can be observed. All of these demonstrate the successful synthesis of the cobalt-iron layered hydroxide@carbon nanotube composite material.

[0045] Figure 7 The X-ray photoelectron spectroscopy (XPS) spectrum of the cobalt-iron layered hydroxide@carbon nanotube composite material prepared in this embodiment is shown, confirming the presence of five elements: C, N, O, Co, and Fe. In the C 1s high-resolution XPS spectrum, the peaks at 284.8 eV, 286.5 eV, and 288.6 eV are attributed to CC, CO, and OC=O, respectively. In the N 1s high-resolution XPS spectrum, the peak at 399.9 eV is attributed to pyrrole N. In the O 1s high-resolution XPS spectrum, the peaks at 529.9 eV, 531.5 eV, and 532.9 eV are attributed to metal-oxygen, hydroxyl groups, and surface-adsorbed water, respectively. In the Co 2p high-resolution XPS spectrum, the peaks at 780.9 eV and 783.1 eV are attributed to Co.3+ and Co 2+ In the high-resolution XPS spectrum of Fe 2p, the peaks at 711.3 eV and 714.1 eV are attributed to Fe, respectively. 3+ Fe coordinated with tetrahedrons 3+ Its high-valence metal ions are conducive to the OER reaction.

[0046] Example 2

[0047] Compared with Example 1, the reaction time of ferrous sulfate heptahydrate in this example is reduced to half of the original, that is, "stirring at 50°C for 1 h" in step (3) of Example 1 is changed to "stirring at 50°C for 30 min", and everything else is the same as in Example 1.

[0048] Example 3

[0049] Compared with Example 1, the reaction time of ferrous sulfate heptahydrate in this example is increased to twice the original time. That is, in step (3) of Example 1, "stirring at 50°C for 1 h" is changed to "stirring at 50°C for 2 h". Everything else is the same as in Example 1.

[0050] Electrocatalytic performance test of oxygen evolution reaction in water electrolysis

[0051] The cobalt-iron layered hydroxide@carbon nanotube composite material prepared in Example 1 was applied to the electrocatalysis of the oxygen evolution reaction in water electrolysis, and its electrocatalytic performance was compared with that of the modified polypyrrole carbon nanotubes in Example 1, the ZIF-67@polypyrrole carbon nanotubes in Example 1, the commercial IrO2 material, and the cobalt-iron layered hydroxide@carbon nanotube composite materials prepared in Examples 2 and 3. The specific steps are as follows:

[0052] 1) A three-electrode testing system (CHI760 electrochemical workstation) was used, with silver / silver chloride as the reference electrode, platinum wire as the counter electrode, and glassy carbon electrode modified with the test material as the working electrode. The electrolyte solution was 1 mol / L potassium hydroxide solution.

[0053] 2) Disperse 2 mg of the test material in 1 mL of naphthol solution to make its concentration 2 mg / mL. Take 5 µL of the dispersion and drop it onto the surface of the glassy carbon electrode. Dry it under an infrared lamp (150 W) to obtain the working electrode.

[0054] 3) Place the three electrodes in the electrolytic cell, immerse them in potassium hydroxide solution, select an electrochemical method, set the parameters, and conduct an electrochemical test of the oxygen evolution reaction in water electrolysis.

[0055] Test results are available Figure 8-11 .Depend on Figure 8-10It can be seen that, compared with the modified polypyrrole carbon nanotubes in Example 1, the ZIF-67@polypyrrole carbon nanotubes in Example 1, and the commercial IrO2 material, the cobalt-iron layered hydroxide@carbon nanotube composite material in Example 1 exhibits higher electrocatalytic performance for the oxygen evolution reaction, characterized by a lower overpotential (321 mV) and a lower Tafel slope (54.44 mVdec). -1 Higher electrochemical active surface area (12.35 cm²) 2 It also exhibits lower charge transfer resistance (302.5 W). The electrocatalytic performance of the cobalt-iron layered hydroxide@carbon nanotube composite materials in Examples 2 and 3 is slightly lower than that in Example 1, but the overall effect is still good.

[0056] Figure 11 This is a CV cycle curve of the cobalt-iron layered hydroxide@carbon nanotube composite material prepared in Example 1 after 1000 cycles of the oxygen evolution reaction. As shown in the figure, after 1000 CV cycles, the LSV curve is basically the same as before the cycle, indicating that the composite material has good stability.

Claims

1. An application of a cobalt-iron layered hydroxide@carbon nanotube composite material, characterized in that, Application of the cobalt-iron layered hydroxide@carbon nanotube composite material as a catalyst in the oxygen evolution reaction of water electrolysis; The preparation method of the cobalt-iron layered hydroxide@carbon nanotube composite material includes the following steps: (1) Modification of carbon nanotubes: Methyl orange and anhydrous ferric chloride were dissolved in water, then pyrrole solution was added and stirred. After suction filtration and drying, a black product was obtained. The black product was dissolved in methanol, sodium dodecylbenzenesulfonate was dissolved in water, the methanol solution of the black product and the aqueous solution of sodium dodecylbenzenesulfonate were mixed, ultrasonically dispersed, washed and dried, and then calcined at high temperature under an inert atmosphere to obtain modified polypyrrole carbon nanotubes. (2) Preparation of ZIF-67@polypyrrole carbon nanotubes: The modified polypyrrole carbon nanotubes, cobalt nitrate hexahydrate and hexadecyltrimethylammonium bromide were dissolved in water to obtain solution one; 2-methylimidazole was dissolved in water to obtain solution two; solution one and solution two were mixed and stirred, allowed to stand, and then washed and dried to obtain ZIF-67@polypyrrole carbon nanotubes; (3) Preparation of cobalt-iron layered hydroxide@carbon nanotube composite material: ZIF-67@polypyrrole carbon nanotube and ferrous sulfate heptahydrate are dissolved in a mixed solution of water and ethanol and stirred for 0.5~3 h at room temperature to 50℃ to obtain cobalt-iron layered hydroxide@carbon nanotube composite material. In step (1), the suction filtration specifically involves using ultrapure water at 60°C and anhydrous ethanol at room temperature for suction filtration in sequence. In step (1), the high-temperature calcination under an inert atmosphere is specifically carried out as follows: in a tube furnace under a nitrogen atmosphere, the temperature is increased to 800℃ at a heating rate of 3℃ / min, and then held at 800℃ for 1 h.

2. The application of the cobalt-iron layered hydroxide@carbon nanotube composite material according to claim 1, characterized in that, The water used in step (1) is all ultrapure water.

3. The application of the cobalt-iron layered hydroxide@carbon nanotube composite material according to claim 1, characterized in that, All water used in step (2) is deionized water.

4. The application of the cobalt-iron layered hydroxide@carbon nanotube composite material according to claim 1, characterized in that, The washing and drying process in step (2) is as follows: wash with deionized water and ethanol in sequence, centrifuge at 9000 rpm during washing, and dry in a vacuum oven at 70°C for 12 h after washing.

5. The application of the cobalt-iron layered hydroxide@carbon nanotube composite material according to claim 1, characterized in that, The application method is as follows: the cobalt-iron layered hydroxide@carbon nanotube composite material is dispersed in a dispersant to obtain a dispersion, the dispersion is drop-coated onto the surface of a glassy carbon electrode, and dried to obtain the working electrode.

6. The application of the cobalt-iron layered hydroxide@carbon nanotube composite material according to claim 5, characterized in that, The dispersant is a naphthol solution.

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