A method for synthesizing iron phosphide-terminated nitrogen-doped carbon nanotube interwoven networks for seawater electrolysis

Abstract

A method for synthesizing an iron phosphate-terminated nitrogen-doped carbon nanotube interwoven network for seawater electrolysis includes the following steps: Ferric nitrate nonahydrate (Fe(NO3)3·9H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) are mixed with fumaric acid solution in a ratio of 1:(1-3) and heated and stirred hydrothermally to obtain MIL-88A. MIL-88A is then mixed with dicyandiamine (DCD) (1:(0-5)) and ground (1-60 min), placed in a nitrogen atmosphere and heated to 600-800℃ for 2 h to obtain a nitrogen-doped carbon nanotube interwoven network Fe@N-CNTs / NC with Fe clusters at the end. Fe@N-CNTs / NC is then placed together with sodium dihydrogen phosphate (NaH2PO4) in a nitrogen atmosphere (1:(10-60)) and heated to 300-400℃ for 2 h, where the Fe clusters are phosphated to obtain FeP@N-CNTs / NC. FeP grown at the tips of N-CNTs can fully contact seawater and rapidly expel the generated gas, significantly enhancing the kinetic rates of HER and OER. At the same time, the tight anchoring of FeP to N-CNTs not only prevents FeP nanoparticles from agglomerating during the catalytic process, but also increases conductivity, which is beneficial to improving the stability of HER and OER.

Description

Technical Field

[0001] This invention belongs to the field of functional nanocomposite materials technology. Specifically, it is a method for preparing an iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network by using Fe-metal-organic framework materials as precursors and controlling the introduction of external carbon / nitrogen sources through metal center autocatalysis, and its application in seawater electrolysis. Background Technology

[0002] Hydrogen boasts high energy density and zero carbon emissions, making it a promising alternative to traditional fossil fuels and one of the most likely future energy sources. With the rapid development and support of clean new energy power generation technologies (wind, solar, tidal, or geothermal), water electrolysis for hydrogen production is gradually becoming the most promising sustainable and low-cost industrial method. However, large-scale industrial production consumes enormous amounts of freshwater, undoubtedly increasing water costs and potentially triggering a freshwater crisis. Seawater accounts for 96.5% of the world's total water reserves; developing seawater electrolysis technology can conserve freshwater resources and effectively reduce hydrogen production costs, thus facilitating the rapid development of the water electrolysis industry.

[0003] The oxygen evolution reaction (OER) at the anolyte and the hydrogen evolution reaction (HER) at the cathode in water electrolysis are complex gas-liquid-solid three-phase catalytic reactions, placing high demands on the design of catalysts in terms of active sites, conductivity, and gas escape efficiency. Generally, reducing catalyst size can increase the number of active sites per unit mass and promote rapid electrolyte mass transfer to ensure substrate supply to the catalytic sites. However, reducing catalyst size significantly increases particle surface energy, leading to catalyst aggregation. Simultaneously, the presence of spatial intervals can obstruct electron transfer pathways, thus affecting catalytic performance. Metal-organic frameworks (MOFs) are porous three-dimensional network materials composed of organic ligands and inorganic ions or clusters linked by coordination bonds. Due to their diverse composition, morphology, and structure, high specific surface area, abundant and tunable pore structures, and highly dispersed metal centers, they are widely used in water electrolysis. The pyrolysis technology of MOFs provides a method for preparing small-sized nanomaterials and porous carbon composites. The high surface area of ​​the carbon material can effectively inhibit catalyst aggregation, while the strong interaction between the nanomaterials and carbon materials provides a rapid electron transfer pathway, resulting in excellent HER and OER performance. Summary of the Invention

[0004] This invention proposes a method for synthesizing an iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network for seawater electrolysis. The method uses MIL-88A as a precursor, introducing a carbon / nitrogen source through grinding and mixing with dicyandiamine (DCD). Then, a nitrogen-doped carbon nanotube interwoven network (Fe@N-CNTs / NC) with Fe clusters at the end is grown via an autocatalytic reaction of Fe centers during thermal annealing. Finally, the end-terminated Fe clusters are phosphated through a gas-phase phosphating process to prepare a nitrogen-doped carbon nanotube interwoven network (FeP@N-CNTs / NC) with tip-anchored FeP catalytic clusters. The FeP catalytic centers grown at the tips of N-CNTs can fully contact the solution and facilitate rapid gas venting, significantly enhancing the kinetics of HER and OER. Simultaneously, the tight anchoring of FeP to N-CNTs not only prevents FeP nanoparticle aggregation during catalysis but also increases conductivity, which is beneficial for improving the stability of HER and OER.

[0005] To prepare a high-performance seawater electrolysis catalyst, this invention utilizes the MIL-88A precursor and constructs an interwoven network of nitrogen-doped carbon nanotubes with iron phosphide end caps to obtain a highly stable catalyst exhibiting both HER and OER properties in seawater. The catalyst synthesis can be achieved through the following technical route: 1. A method for synthesizing an iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network for seawater electrolysis, comprising the following steps: (1) Preparation of MIL-88A precursor: Ferric nitrate nonahydrate (Fe(NO3)3·9H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) in a certain molar ratio (1:(1~3)) were dissolved in deionized water, and this solution was added to a rotary osmosis system with a rotation speed of 400. rpm The mixture was added to a fumaric acid solution and stirred for 10 min. The ratio of fumaric acid to the metal compound was 1.2:1.3. After the solution was thoroughly mixed, it was placed in an environment of 110 °C and reacted for 6 h to obtain the target product MIL-88A.

[0006] (2) Preparation of nitrogen-doped carbon nanotube interwoven network (Fe@N-CNTs / NC) with Fe cluster end caps: First, the MIL-88A prepared in (1) was mixed and ground with dicyandiamine (DCD) in a certain ratio (1:(0~5)) for 1-60 min. Then, the mixed sample was heated to 600~800 ℃ in a nitrogen environment and held for 2 h. After the reaction was completed, it was naturally cooled to room temperature to obtain nitrogen-doped carbon nanotube material Fe@N-CNTs / NC with Fe cluster end caps.

[0007] (3) Preparation of iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network (FeP@N-CNTs / NC): First, Fe@N-CNTs / NC prepared in (2) and sodium dihydrogen phosphate (NaH2PO4) are placed together in a nitrogen environment (mass ratio 1:(10~60)). Sodium dihydrogen phosphate is placed upstream of the gas flow and Fe@N-CNTs / NC is placed downstream of the gas flow. Then, the temperature is increased to 300~400 ℃ at a rate of 2 ℃ / min and held for 2 h to obtain the final product, which is the iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network FeP@N-CNTs / NC.

[0008] 2. The preparation method according to technical route 1 is characterized in that: the molar ratio of ferric nitrate nonahydrate (Fe(NO3)3·9H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) in step (1) affects the aspect ratio of the obtained MIL-88A.

[0009] 3. The preparation method according to technical route 1 is characterized in that: in step (2), the mixing of MIL-88A and dicyandiamine (DCD) is carried out by physical grinding, and the ratio of the two can be effectively adjusted to obtain the length and number of N-CNTs. The optimized mass ratio of MIL-88A to dicyandiamine (DCD) is 1:1, and the optimal pyrolysis temperature is 700 ℃.

[0010] 4. The preparation method according to technical route 1 is characterized in that: step (3) adopts a gas-phase phosphating process, and the amount of phosphating agent and the reaction temperature have a significant impact on the phase of the product, thereby affecting its electrocatalytic performance. The optimized ratio is 1:20, and the optimal reaction temperature is 350 ℃.

[0011] 5. The preparation method according to technical route 1 is characterized in that: the FeP@N-CNTs / NC prepared in step (3) can be used as a bifunctional catalyst for electrocatalytic oxygen evolution and hydrogen evolution in seawater electrolyte.

[0012] As a further feature of the present invention: the optimized FeP@N-CNTs / NC, at 10 mA cm⁻¹ -2 At a current density, its OER and HER overpotentials are 280 mV and 206 mV, respectively. After 2000 CV cycles, its properties remain essentially stable, even at 100 mA cm⁻¹. -2 It maintains structural and performance stability after operating at current density for 11 hours.

[0013] By adopting the above technical solution, the present invention has the following beneficial effects: This experiment successfully constructed a novel nitrogen-doped carbon nanotube structure with FeP nanotubes fixed at the top (FeP@N-CNTs / NC) using a DCD-assisted pyrolysis process of MIL-88A and a further gas phosphating method. During pyrolysis, the Fe centers of MIL-88A catalyze the growth of nitrogen-doped carbon nanotubes using DCD as the nitrogen and carbon source, forming an interwoven nanotube network. Fe clusters are covered and fixed at the top of each nanotube, and the FeP species obtained after further phosphating inherit this strong interaction well. The unique interwoven nanotube network and strongly coupled FeP promote electron and electrolyte transport, accelerate bubble overflow, effectively inhibit catalyst aggregation, and provide a rapid electron transfer pathway. In addition, the doped N atoms effectively modulate the electronic structure and surface properties of the carbon matrix, optimizing the adsorption energy of each step in the electrochemical catalysis. Therefore, the N-doped carbon nanotube structure with FeP fixed at the top prepared by the method of this invention exhibits excellent electrochemical performance. Specifically, FeP@N-CNTs / NC achieves excellent electrochemical performance at 10 mA cm⁻¹. -2 It exhibits excellent OER overpotential of 280 mV at 100 mA cm⁻¹. -2 At a voltage of 340 mV, it is significantly superior to commercial RuO2. In the HER process, FeP@N-CNTs / NC exhibits excellent activity and reaction kinetics at 10 mA cm⁻¹. -2 The overpotential is only 206 mV at 100 mA cm⁻¹ -2 The overpotential is 313 mV. Attached Figure Description

[0014] Figure 1 Scanning electron microscope image of MIL-88A obtained in Example 1; Figure 2 X-ray diffraction pattern of MIL-88A obtained in Example 1; Figure 3 Transmission electron microscope image of MIL-88A obtained in Example 1; Figure 4 Scanning electron microscope image of Fe@N-CNTs / NC-700 obtained in Example 1; Figure 5 X-ray diffraction pattern of Fe@N-CNTs / NC-700 obtained in Example 1; Figure 6 X-ray diffraction pattern of FeP@N-CNTs / NC-300 obtained in Example 1; Figure 7 OER curve of FeP@N-CNTs / NC-300 obtained in Example 1; Figure 8HER curve of FeP@N-CNTs / NC-300 obtained in Example 1; Figure 9 Scanning electron microscope image of FeP@N-CNTs / NC-350 obtained in Example 2; Figure 10 Transmission electron microscopy image of FeP@N-CNTs / NC-350 obtained in Example 2; Figure 11 X-ray diffraction pattern of FeP@N-CNTs / NC-350 obtained in Example 2; Figure 12 XPS plot of FeP@N-CNTs / NC-350 obtained in Example 2; Figure 13 OER curve of FeP@N-CNTs / NC-350 obtained in Example 2; Figure 14 HER curve of FeP@N-CNTs / NC-350 obtained in Example 2; Figure 15 X-ray diffraction pattern of FeP@N-CNTs / NC-400 obtained in Example 3; Figure 16 OER curve of FeP@N-CNTs / NC-400 obtained in Example 3; Figure 17 HER curve of FeP@N-CNTs / NC-400 obtained in Example 3; Figure 18 X-ray diffraction pattern of Fe@C obtained in Example 4; Figure 19 X-ray diffraction pattern of FeP@C obtained in Example 4; Figure 20 OER curve of FeP@C obtained in Example 4; Figure 21 HER curve of FeP@C obtained in Example 4; Figure 22 X-ray diffraction pattern of Fe@N-CNTs / NC-600 obtained in Example 5; Figure 23 OER diagram of Fe@N-CNTs / NC-600 obtained in Example 5; Figure 24 HER map of Fe@N-CNTs / NC-600 obtained in Example 5; Figure 25 X-ray diffraction pattern of Fe@N-CNTs / NC-800 obtained in Example 5; Figure 26 OER diagram of Fe@N-CNTs / NC-800 obtained in Example 5; Figure 27 HER map of Fe@N-CNTs / NC-800 obtained in Example 5. Detailed Implementation

[0015] The technical features of this invention are described below with reference to specific experimental schemes and accompanying drawings, but this invention is not limited thereto. Unless otherwise specified, the experimental methods described in the following embodiments are conventional methods; the instruments and materials described, unless otherwise specified, are all commercially available.

[0016] Example 1 A method for synthesizing iron phosphide-terminated nitrogen-doped carbon nanotube interwoven networks for seawater electrolysis (1) Preparation of MIL-88A precursor: Ferric nitrate nonahydrate (Fe(NO3)3·9H2O) and nickel nitrate hexahydrate (Ni(NO3)2·6H2O) in a certain molar ratio (1:3) were dissolved in deionized water, and this solution was added to a stirring tank with a stirring speed of 400 rpm. rpm The mixture was added to a fumaric acid solution and stirred for 10 min. The ratio of fumaric acid to the metal compound was 1.2:1.3. After the solution was thoroughly mixed, it was placed in an environment of 110 °C for 6 h to obtain the target product MIL-88A. The morphology of MIL-88A is shown in its scanning electron microscope image. Figure 1 The crystallinity of MIL-88A is shown in its powder X-ray diffraction pattern. Figure 2 The internal morphology of MIL-88A is shown in its transmission electron microscope image. Figure 3 SEM and TEM images of MIL-88A revealed highly uniform hexagonal rod-shaped nanostructures with a length of approximately 2 μm and a diameter of approximately 400 nm. (2) Preparation of nitrogen-doped carbon nanotube interwoven network with Fe clusters capped: First, MIL-88A prepared in (1) was mixed and ground with dicyandiamine (DCD) in a certain ratio (1:1). The mixed sample was heated to 700 °C in a nitrogen atmosphere and held for 2 h. After the reaction was completed, it was naturally cooled to room temperature to obtain Fe@N-CNTs / NC-700, an iron-based material with nitrogen-doped carbon nanotubes. The morphology of Fe@N-CNTs / NC-700 is shown in the scanning electron microscope (SEM) image. Figure 4 As shown in the figure, its crystallinity X-ray diffraction pattern is as follows ( Figure 5 As shown in the figure.

[0017] (3) A nitrogen-doped carbon nanotube interwoven network with iron phosphide end caps: First, Fe@N-CNTs / NC-700 prepared in (2) and sodium dihydrogen phosphate were placed together in a nitrogen atmosphere. Sodium dihydrogen phosphate was placed upstream of the gas flow, and Fe@N-CNTs / NC-700 was placed downstream of the gas flow. Then, the temperature was increased to 300 ℃ at a rate of 2 ℃ / min and held for 2 h to obtain the final product, namely, nitrogen-doped carbon nanotubes FeP@N-CNTs / NC-300 encapsulating iron phosphide nanoparticles. The crystallinity of FeP@N-CNTs / NC-300 is shown in its powder X-ray diffraction pattern. Figure 6 The OER (Organic Emission Rate) was measured in a three-electrode system with alkaline seawater as the electrolyte. Figure 7 ) and HER ( Figure 8 The properties of FeP@N-CNTs / NC-300 are shown in the figure. Calculations show that FeP@N-CNTs / NC-300 exhibits good performance at a current density of 10 mA cm⁻¹. -2 At that time, the overpotentials of OER and HER were 337 mV and 241 mV, respectively.

[0018] Example 2 A method for synthesizing iron phosphide-terminated nitrogen-doped carbon nanotube interwoven networks for seawater electrolysis: (1) The preparation of the MIL-88A precursor is the same as in Example 1.

[0019] (2) The preparation of Fe@N-CNTs / NC-700 is the same as in Example 1.

[0020] (3) A nitrogen-doped carbon nanotube interwoven network with iron phosphide end caps: First, Fe@N-CNTs / NC-700 prepared in (2) and sodium dihydrogen phosphate were placed together in a nitrogen atmosphere. Sodium dihydrogen phosphate was placed upstream of the gas flow, and Fe@N-CNTs / NC-700 was placed downstream of the gas flow. Then, the temperature was increased to 350 ℃ at a rate of 2 ℃ / min and held for 2 h to obtain the final product, namely, nitrogen-doped carbon nanotubes FeP@N-CNTs / NC-350 encapsulating iron phosphide nanoparticles. The morphology of FeP@N-CNTs / NC-350 is shown in the scanning electron microscope (SEM) image. Figure 9 ) and transmission electron microscopy (as shown) Figure 10 FeP@N-CNTs / NC-350 exhibits a well-inherited morphology from Fe@N-CNTs / NC-700, with nanotubes covering the surface of the nanorods. Transmission electron microscopy images show that phosphide nanoparticles are located on top of each nanotube. The crystallinity of FeP@N-CNTs / NC-350 is shown in its powder X-ray diffraction pattern. Figure 11 The electronic spectrum of FeP@N-CNTs / NC-350 is shown in its XPS plot. Figure 12The material contains elements such as Fe, Ni, N, P, and O, indicating that the material has been successfully phosphated. The OER of FeP@N-CNTs / NC-350 ( Figure 13 ) and HER ( Figure 14 As shown in the figure, calculations show that at 10 mA cm⁻¹ -2 At current densities of 280 mV and 206 mV, respectively, its overpotentials are 280 mV and 206 mV.

[0021] Example 3 A method for synthesizing iron phosphide-terminated nitrogen-doped carbon nanotube interwoven networks for seawater electrolysis: (1) The preparation of the MIL-88A precursor is the same as in Example 1.

[0022] (2) The preparation of Fe@N-CNTs / NC-700 is the same as in Example 1.

[0023] (3) A nitrogen-doped carbon nanotube interwoven network with iron phosphide end caps: First, Fe@N-CNTs / NC-700 prepared in (2) and sodium dihydrogen phosphate were placed together in a nitrogen atmosphere. Sodium dihydrogen phosphate was placed upstream of the gas flow, and Fe@N-CNTs / NC-700 was placed downstream of the gas flow. Then, the temperature was increased to 400 ℃ at a rate of 2 ℃ / min and held for 2 h to obtain the final product, namely, nitrogen-doped carbon nanotubes FeP@N-CNTs / NC-400 encapsulating iron phosphide nanoparticles. The crystallinity of FeP@N-CNTs / NC-400 is shown in its powder X-ray diffraction pattern. Figure 15 The OER (Organic Emission Rate) was measured in a three-electrode system with alkaline seawater as the electrolyte. Figure 16 ) and HER ( Figure 17 The properties of FeP@N-CNTs / NC-400 are shown in the figure. Calculations show that FeP@N-CNTs / NC-400 exhibits good performance at a current density of 10 mA cm⁻¹. -2 At that time, its overpotentials were 310 mV and 233 mV, respectively.

[0024] Example 4 A method for synthesizing iron phosphide-terminated nitrogen-doped carbon nanotube interwoven networks for seawater electrolysis: (1) The preparation of the MIL-88A precursor is the same as in Example 1.

[0025] (2) Preparation of nitrogen-doped carbon nanotube interwoven network with Fe clusters at the ends: First, MIL-88A prepared in (1) was mixed and ground with dicyandiamine (DCD) in a certain ratio (1:0). The mixed sample was heated to 700 °C in a nitrogen atmosphere and held for 2 h. After the reaction was completed, it was naturally cooled to room temperature to obtain Fe@C. The X-ray diffraction pattern of Fe@C crystallinity is shown in ( Figure 18 As shown in the figure.

[0026] (3) The obtained Fe@C and sodium dihydrogen phosphate were placed together in a nitrogen atmosphere, with sodium dihydrogen phosphate upstream of the gas flow and Fe@C downstream. The temperature was then increased to 400 °C at a rate of 2 °C / min and held for 2 h to obtain the final product FeP@C. The crystallinity of FeP@C is shown in its powder X-ray diffraction pattern. Figure 19 Its OER ( Figure 20 ) and HER ( Figure 21 The properties of FeP@C are shown in the figure. Calculations show that FeP@C at a current density of 10 mA cm⁻¹ -2 At that time, its overpotentials were 320 mV and 236 mV, respectively.

[0027] Example 5 A method for synthesizing iron phosphide-terminated nitrogen-doped carbon nanotube interwoven networks for seawater electrolysis: (1) The preparation of the MIL-88A precursor is the same as in Example 1.

[0028] (2) Preparation of nitrogen-doped carbon nanotube interwoven network with Fe clusters at the ends: First, MIL-88A prepared in (1) was mixed and ground with dicyandiamine (DCD) in a certain ratio (1:1). The mixed sample was heated to 600 °C in a nitrogen atmosphere and held for 2 h. After the reaction was completed, it was naturally cooled to room temperature to obtain Fe@N-CNTs / NC-600. The X-ray diffraction pattern of the crystallinity of Fe@N-CNTs / NC-600 is shown in the figure. Figure 22 As shown in the figure. The OER (October Emission Rate) was measured in a three-electrode system with alkaline seawater as the electrolyte. Figure 23 ) and HER ( Figure 24 The properties of are shown in the figure.

[0029] Example 6 A method for synthesizing iron phosphide-terminated nitrogen-doped carbon nanotube interwoven networks for seawater electrolysis: (1) The preparation of the MIL-88A precursor is the same as in Example 1.

[0030] (2) Preparation of nitrogen-doped carbon nanotube interwoven network with Fe cluster end caps: First, MIL-88A prepared in (1) was mixed and ground with dicyandiamine (DCD) in a certain ratio (1:1). The mixed sample was heated to 800 °C in a nitrogen atmosphere and held for 2 h. After the reaction was completed, it was naturally cooled to room temperature to obtain Fe@N-CNTs / NC-800. The X-ray diffraction pattern of the crystallinity of Fe@N-CNTs / NC-800 is shown in the figure. Figure 25 As shown in the figure. The OER (October Emission Rate) was measured in a three-electrode system with alkaline seawater as the electrolyte. Figure 26 ) and HER ( Figure 27 The properties of are shown in the figure.

[0031] The description of the embodiments disclosed in this invention is not intended to limit the scope of the invention, but rather to describe it. Accordingly, the scope of the invention is not limited to the above embodiments, but is defined by the claims or their equivalents.

Claims

1. A method for synthesizing an iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network for seawater electrolysis, comprising the following steps: (1) Preparation of MIL-88A precursor: Ferric nitrate nonahydrate and nickel nitrate hexahydrate in a certain molar ratio of 1:1~3 were dissolved in deionized water, and this solution was added to a stirring tank with a stirring speed of 400. rpm The mixture was placed in a fumaric acid solution and stirred for 10 min. The ratio of fumaric acid to metal compound was 1.2:1.

3. After mixing evenly, the mixture was placed in an environment of 110 °C and reacted for 6 h to obtain the target product MIL-88A. (2) Preparation of nitrogen-doped carbon nanotube interwoven network with Fe cluster end caps: First, the MIL-88A prepared in (1) is mixed with dicyandiamine at a certain ratio of 1:1~5 and ground for 1~60 min. The mixed sample is heated to 600~800 ℃ in a nitrogen environment and held for 2 h. After the reaction is completed, it is naturally cooled to room temperature to obtain nitrogen-doped carbon nanotube material Fe@N-CNTs / NC with Fe cluster end caps; (3) Preparation of iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network: First, Fe@N-CNTs / NC prepared in (2) and sodium dihydrogen phosphate are placed together in a nitrogen environment at a mass ratio of 1:10~60. Sodium dihydrogen phosphate is placed upstream of the gas flow and Fe@N-CNTs / NC is placed downstream of the gas flow. Then, the temperature is increased to 300~400 ℃ at a rate of 2 ℃ / min and held for 2 h to obtain the final product, which is the iron phosphide-terminated nitrogen-doped carbon nanotube interwoven network FeP@N-CNTs / NC.

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