Nitrogen-doped bifunctional electrocatalyst, preparation method and application thereof

Abstract

This invention discloses a nitrogen-doped bifunctional electrocatalyst, its preparation method, and its application. First, a cobalt-iron layered double hydroxide (Co-Fe LDH) precursor is prepared via a hydrothermal method. Then, a MIL-88A metal-organic framework is grown in situ on its surface using a hydrothermal method to obtain a composite precursor. Subsequently, nitrogen doping is achieved by high-temperature calcination under an ammonia atmosphere, forming the target catalyst. This method synergistically modulates the electronic structure of the catalyst through MOF composite and ammonia annealing, constructing a synergistic structure of defect-rich nitrogen-doped carbon layers and highly dispersed cobalt-iron alloy nanoparticles, significantly improving electrocatalytic performance. The obtained catalyst exhibits excellent bifunctional activity in an alkaline electrolyte: OER at 10 mA·cm⁻¹. ‑ ² and 100 mA·cm ‑ The overpotentials at the given current densities were 180 mV and 235 mV, respectively, and the HER values ​​were 66 mV and 246 mV, respectively. This invention features a simple and low-cost preparation process, and shows promising application prospects in the field of overall water splitting electrocatalysis.

Description

Technical Field

[0001] This invention belongs to the field of catalyst technology, specifically relating to a nitrogen-doped bifunctional electrocatalyst, its preparation method, and its application. Background Technology

[0002] With the continuous growth of the world's population and rapid economic development, global energy demand will continue to rise in the foreseeable future. The massive consumption of fossil fuels not only leads to increasingly depleted resources but also triggers severe environmental pollution and the greenhouse effect. Therefore, developing green and sustainable clean energy has become an important topic in current scientific research. Electrocatalytic water splitting technology, as one of the key pathways to achieving clean hydrogen energy, has received widespread attention. In the electrochemical water splitting process, the oxygen evolution reaction (OER) and the hydrogen evolution reaction (HER) are two core half-reactions. The OER, involving four-electron transfer, exhibits sluggish kinetics, becoming a bottleneck restricting overall efficiency. Although the HER is a two-electron process, the water dissociation step under alkaline conditions has a high energy barrier, also facing kinetic challenges. Currently, noble metal oxides such as Pt, IrO2, and RuO2 are the mainstream choices for commercial electrocatalysts; however, their high cost and scarce reserves severely restrict large-scale application. Developing highly active, low-cost non-noble metal electrocatalysts has become an urgent need in this field.

[0003] Over the past few decades, transition metal-based materials have been widely used to replace noble metal catalysts due to their abundant resources, low cost, and diverse valence states. Among them, cobalt-iron-based materials have shown good catalytic potential for both OER and HER due to their rich redox active sites and excellent basic stability. However, these materials still face multiple challenges in practical applications: a single metal component cannot simultaneously optimize the adsorption energy of various oxygen-containing intermediates in OER, nor can it meet the complex requirements of water desorption and hydrogen desorption in basic HER; at the same time, these materials generally suffer from poor conductivity, low active site density, and easy aggregation during synthesis, which seriously restricts their bifunctional catalytic performance and long-term stability. Therefore, how to achieve a balance between morphology controllability, component synergy, and electronic structure optimization at the nanoscale to simultaneously accelerate the complex reaction kinetics of OER and HER is a key problem that urgently needs to be solved in the current research on cobalt-iron-based bifunctional electrocatalysts. Summary of the Invention

[0004] To address the aforementioned technical problems, this invention provides a nitrogen-doped bifunctional electrocatalyst, its preparation method, and its application. This catalyst is a dual-function catalyst material capable of performing both the oxygen evolution reaction and hydrogen evolution reaction in water electrolysis; specifically, it is a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology.

[0005] The first aspect of this invention provides a method for preparing a nitrogen-doped bifunctional electrocatalyst, specifically comprising the following steps: Step S1: In a solution containing Co and Fe elements, urea and ammonium fluoride are added as morphology modifiers, and nickel foam (NF) is used as a substrate. Co-Fe LDH / NF precursor is obtained through hydrothermal reaction. Step S2: N,N-dimethylformamide is mixed with fumaric acid, and MIL-88A metal-organic framework is grown in situ on the surface of Co-Fe LDH / NF precursor by hydrothermal method to obtain composite precursor material; Step S3: The composite precursor is calcined at high temperature in an ammonia atmosphere to obtain a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology.

[0006] Preferably, in step S1, a solution containing Co and Fe elements is prepared using cobalt nitrate hexahydrate and ferric nitrate nonahydrate; the molar ratio of Co to Fe is 2:1.

[0007] Preferably, step S1 is performed as follows: 1.6 mmol of cobalt nitrate hexahydrate and 0.8 mmol of ferric nitrate nonahydrate, 5 mmol of urea, and 1.5 mmol of ammonium fluoride are weighed and dissolved in 40 mL of deionized water. After magnetic stirring for 30 minutes, the solution is transferred to the inner liner of a polytetrafluoroethylene (PTFE) reactor and nickel foam is added. The reactor is then placed in an oven at 120°C and reacted for 12 hours. After washing with ethanol and deionized water, it is dried in a vacuum oven for 6 hours to obtain the Co-Fe-LDH / NF precursor. The nickel foam is obtained by pre-treating 2cm*4cm nickel foam, which involves sequentially ultrasonically cleaning with acetone, 2mol / L hydrochloric acid, ethanol, and deionized water for 15 minutes, followed by vacuum drying.

[0008] Preferably, step S2 is specifically performed as follows: the Co-Fe-LDH / NF precursor is placed in the inner liner of a polytetrafluoroethylene reactor containing 40 ml of a dimethylformamide solution containing 3 mmol fumaric acid. The reactor is placed in an oven at 120°C and reacted for 12 h. After that, it is washed with ethanol and deionized water, and then placed in a vacuum oven to dry for 6 h. Finally, the Co-Fe-LDH-MIL88A / NF precursor is obtained.

[0009] Preferably, step S3 specifically involves placing the obtained Co-Fe-LDH-MIL88A / NF precursor in a tube furnace filled with ammonia gas and heating it at 10 °C for 1 minute. -1 The temperature was increased to 400℃ at a certain rate, held for 1.5 h, and then naturally cooled to room temperature to obtain a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology, named Co-Fe-N@MIL88A / NF.

[0010] The second aspect of the present invention provides a nitrogen-doped bifunctional electrocatalyst prepared by the preparation method of the first aspect described above.

[0011] Preferably, the catalyst has a nanoflower morphology formed by the self-assembly of ultrathin nanosheets, and has a porous and open three-dimensional structure.

[0012] The third aspect of this invention provides the application of the nitrogen-doped bifunctional electrocatalyst of the second aspect above in the alkaline oxygen evolution and hydrogen evolution reactions of water electrolysis.

[0013] Preferably, in a 1 M KOH solution, the OER is at 10 mA·cm⁻¹ - ² and 100 mA·cm - The overpotentials at the current densities were 180 mV and 235 mV, respectively, and the HERs were 66 mV and 246 mV, respectively.

[0014] Preferably, the catalyst is at 10 mA cm⁻¹ - ² and 100 mA cm - After operating at current density, the performance degradation rates of OER were 3.3% and 2.8%, respectively, and the performance degradation rates of HER were 2.6% and 4.06%, respectively.

[0015] This invention provides a method for preparing a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology. In the synthesis process, N,N-dimethylformamide is used as the reaction solvent to react organic ligands with transition metal ions, resulting in the in-situ growth of a metal-organic framework (MOF) structure on a Co-Fe LDH substrate. By adjusting the pH and ligand concentration of the reaction system, the precursor grows orderly on the substrate, forming a layered stacked nanosheet structure, thereby constructing a composite precursor with a three-dimensional open structure and a nanoflower morphology. Subsequently, the obtained composite precursor is subjected to high-temperature calcination under an ammonia atmosphere, causing the MOF structure to carbonize and introduce nitrogen, forming a defect-rich nitrogen-doped carbon layer. Simultaneously, the transition metal component is transformed into highly dispersed cobalt-iron alloy nanoparticles under reducing conditions. Finally, an electrocatalytic material is obtained, consisting of cobalt-iron alloy nanoparticles coated with a nitrogen-doped carbon layer while maintaining a nanoflower-like hierarchical structure.

[0016] This invention selects the transition metals Co and Fe, which exhibit excellent catalytic performance in the oxygen evolution reaction and hydrogen evolution reaction, as active centers. The metal nanoparticles are effectively coated and anchored by a MOF-derived carbon layer, which not only inhibits nanoparticle aggregation and improves material structural stability, but also significantly increases the specific surface area and exposes more active sites through the nanoflower-like three-dimensional porous structure. Furthermore, the electronic interactions between the bimetals and the synergistic regulatory effect of the nitrogen-doped carbon layer further optimize the electronic structure of the catalyst, thereby significantly improving the overall electrocatalytic performance.

[0017] Due to their unique electronic structure and metal-like conductivity, transition metal nitrides, through the introduction of nitrogen atoms, can effectively modulate the d-band center of the metal sites, optimize the adsorption and desorption behavior of oxygen- and nitrogen-containing intermediates, and thus simultaneously improve the catalytic performance of OER and HER. Meanwhile, combining nitrides with nitrogen-doped carbon layers can not only effectively suppress nanoparticle aggregation but also construct a highly efficient electron transport network, significantly improving the overall conductivity of the catalyst and the accessibility of active sites. In particular, in-situ nitridation of metal-organic framework-derived precursors under an ammonia atmosphere enables the synergistic construction of nitrogen-doped carbon coating and alloy nitridation, providing a highly promising new strategy for developing bifunctional all-water splitting catalysts with both high activity and high stability. Compared with existing technologies, this invention has at least the following beneficial effects: (1) This invention utilizes a synergistic strategy of morphology engineering and component regulation to significantly increase the electrochemical active surface area by using a nano-flower-like hierarchical structure, which exposes abundant active sites and promotes electrolyte penetration and rapid gas release. At the same time, the electronic synergistic effect between cobalt and iron bimetals optimizes the electronic structure of the active center and regulates the position of the d-band center, thereby enhancing the intrinsic activity of the catalyst.

[0018] (2) Experimental and characterization analysis showed that the unique nanoflower structure effectively alleviated the aggregation phenomenon of metal nanoparticles during synthesis and reaction; while the nitrogen-doped carbon coating layer not only anchored the alloy particles as a skeleton and enhanced the structural stability, but also further improved the conductivity of the overall material and accelerated the reaction kinetics.

[0019] (3) In 1 M KOH electrolyte, the nitrogen-doped bifunctional electrocatalyst with nanoflower morphology prepared in this invention exhibits excellent bifunctional catalytic performance: reaching 10 mA cm⁻¹. - At a given current density, the overpotential for the oxygen evolution reaction (OER) is only 180 mV, and the overpotential for the hydrogen evolution reaction (HER) is as low as 66 mV. Furthermore, the Tafel slope is significantly reduced, demonstrating rapid reaction kinetics for both OER and HER.

[0020] (4) The catalyst exhibits extremely strong durability in long-term electrochemical tests, with almost no activity decay after running for more than 300 hours, overcoming the disadvantage of traditional defective materials being prone to structural collapse. Attached Figure Description

[0021] Figure 1 The image shows a comparison of the LSV polarization curves of the nitrogen-doped bifunctional electrocatalyst with nanoflower morphology obtained in Example 1 for HER and OER, tested in 1 M KOH, which visually reflects the differences in overpotential among the samples. Figure 2 The chronocurrent curve of the nitrogen-doped bifunctional electrocatalyst with nanoflower morphology obtained in Example 1 in 1M KOH; Figure 3 The image shows the XRD pattern of the nitrogen-doped bifunctional electrocatalyst with nanoflower morphology obtained in Example 1. Figure 4 The image shows the In-situ Raman diagram of the nitrogen-doped bifunctional electrocatalyst with nanoflower morphology obtained in Example 1. Figure 5 This is a SEM image of the nitrogen-doped bifunctional electrocatalyst with nanoflower morphology obtained in Example 1. Detailed Implementation

[0022] The technical solution of the present invention will be further described below with reference to the accompanying drawings and specific embodiments. However, the following examples are merely simplified examples of the present invention and do not represent or limit the scope of protection of the present invention. The scope of protection of the present invention is determined by the claims.

[0023] Unless otherwise specified, the raw materials and materials used in the embodiments of the present invention were purchased through general commercial channels.

[0024] The source information of the raw materials, materials, and instruments involved in the following embodiments or comparative examples is as follows: Table 1. Reagents required for the experiment

[0025] Table 2 Experimental Instruments

[0026] <Example 1> A method for preparing a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology specifically includes the following steps: Step S1: Weigh 1.6 mmol of cobalt nitrate hexahydrate and 0.8 mmol of ferric nitrate nonahydrate, 5 mmol of urea, and 1.5 mmol of ammonium fluoride, dissolve them in 40 mL of deionized water, and stir magnetically for 30 minutes. Transfer the solution to the inner liner of a polytetrafluoroethylene (PTFE) reactor and add 2*4 cm of nickel foam (NF). Place the reactor in an oven at 120°C and react for 12 hours. After washing with ethanol and deionized water, dry in a vacuum oven for 6 hours to obtain the Co-Fe-LDH / NF precursor. The 2 cm*4 cm nickel foam was pretreated by ultrasonically cleaning it sequentially with acetone, 2 mol / L hydrochloric acid, ethanol, and deionized water for 15 minutes, followed by vacuum drying.

[0027] Step S2: The Co-Fe-LDH / NF precursor is placed in the inner liner of a polytetrafluoroethylene reactor containing 40 ml of dimethylformamide solution containing 3 mmol fumaric acid. The reactor is placed in an oven at 120°C and reacted for 12 h. After washing with ethanol and deionized water, it is then placed in a vacuum oven and dried for 6 h to obtain the composite precursor material, namely the Co-Fe-LDH-MIL88A / NF precursor.

[0028] Step S3: The obtained Co-Fe-LDH-MIL88A / NF precursor is placed in a tube furnace filled with ammonia atmosphere and heated at 10 °C for 1 min. -1 The temperature was increased to 4000℃ at a heating rate, held for 1.5 h, and then naturally cooled to room temperature to obtain a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology, named Co-Fe-N@MIL88A / NF.

[0029] The obtained catalyst was subjected to electrochemical testing.

[0030] Figure 1 The figures show the polarization curves of the catalysts obtained in this invention in alkaline electrolytes. Figure a shows the HER polarization curves of different catalysts; Figure b shows the OER polarization curves of different catalysts.

[0031] Figure 1 The test method or conditions are as follows: In the HER test conditions shown in Figure a, the alkaline electrolyte is 1 mol / L KOH solution, the measurement voltage is -0.9 V to -1.3 V, and the scan rate is 5 mV s. -1 The OER test conditions in Figure b are: alkaline electrolyte of 1 mol / L KOH solution, measurement voltage from 0 V to 0.8 V, and scan rate of 5 mV / s. -1 .

[0032] As shown in Figure a, in the hydrogen evolution reaction (HER), a value of 10 mA cm⁻¹ is reached. - ² and 100 mA cm - The required overpotentials for the current densities are 66 mV and 246 mV, respectively (η). 10 =66 mV, η 100 =246 mV); As shown in Figure b, in the oxygen evolution reaction (OER), 10 mA cm⁻¹ is reached. - ² and 100 mA cm - The required overpotentials for the current densities are 180 mV and 235 mV, respectively (η). 10 =180 mV, η 100 =235 mV); This indicates that the catalyst exhibits excellent bifunctional electrocatalytic activity under both low current and high current density, which is superior to conventional water electrolysis catalysts.

[0033] Figure 2 Figure 1 shows the potentiostatic stability test curves. Figure 2a is the potentiostatic stability test curve for HER; Figure 3b is the potentiostatic stability test curve for OER.

[0034] Figure 2 The test methods or conditions are as follows: In Figure a, the HER test conditions are initial voltages of -0.01V and -0.1V, and a test time of 305h; in Figure b, the OER test conditions are initial voltages of 0.01V and 0.1V, and a test time of 305h. from Figure 2 It can be seen that the catalyst is at 10 mA cm⁻¹ - ² and 100 mA cm - After operation at current density², the OER performance degradation rates were 3.3% and 2.8%, respectively, and the HER performance degradation rates were 2.6% and 4.06%, respectively. The extremely low degradation rates demonstrate its excellent structural stability and chemical durability.

[0035] Figure 3 The XRD pattern of the CoFe-N@MIL88A / NF catalyst.

[0036] from Figure 3 It can be seen that the sample showed characteristic peaks that were highly consistent with the Co4N and Fe3N standard cards, indicating that the high-temperature annealing successfully induced the precursor phase transition and formed a bimetallic nitride composite structure, which is beneficial for exposing active sites and improving the interfacial electron transport capability.

[0037] Figure 4 This is the in-situ Raman spectrum.

[0038] from Figure 4 It can be seen that as the applied voltage increases (>1.3 V), the 450~550 cm - ¹ and 1050 cm - The Raman peaks in the ¹ region are significantly enhanced, exhibiting characteristics of high-valence metal oxygen / hydroxyl species, proving that the catalyst surface undergoes phase transformation and "in-situ surface reconstruction" during the OER process, dynamically generating highly active catalytic centers and optimizing reaction kinetics.

[0039] Figure 5 Figure 1 shows the SEM images of the catalyst. Figure 2a is a SEM image of the numerous nanoflowers of the catalyst; Figure 3b is a high-magnification SEM image of an isolated single nanoflower of the catalyst.

[0040] from Figure 5As shown in a and b, the sample exhibits a nanoflower morphology formed by the self-assembly of ultrathin nanosheets. This morphology is formed by numerous ultrathin two-dimensional nanosheets as basic building units, which radiate outward from a certain center as the core through nanoscale self-assembly behavior, and finally interweave and stack to form a three-dimensional hierarchical structure.

[0041] The applicant declares that the above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. A method for preparing a nitrogen-doped bifunctional electrocatalyst, characterized in that, Includes the following steps: Step S1: In a solution containing Co and Fe elements, urea and ammonium fluoride are added as morphology modifiers, and nickel foam is used as a substrate. Co-Fe LDH / NF precursor is obtained through hydrothermal reaction. Step S2: N,N-dimethylformamide is mixed with fumaric acid, and MIL-88A metal-organic framework is grown in situ on the surface of the Co-Fe LDH / NF precursor by hydrothermal method to obtain composite precursor material; Step S3: The composite precursor is subjected to high-temperature calcination in an ammonia atmosphere to obtain a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology.

2. The method for preparing the nitrogen-doped bifunctional electrocatalyst as described in claim 1, characterized in that, In step S1, a solution containing Co and Fe elements is prepared using cobalt nitrate hexahydrate and ferric nitrate nonahydrate. The molar ratio of Co to Fe is 2:

1.

3. The method for preparing the nitrogen-doped bifunctional electrocatalyst as described in claim 1, characterized in that, The specific operation of step S1 is as follows: Weigh 1.6 mmol of cobalt nitrate hexahydrate and 0.8 mmol of ferric nitrate nonahydrate, 5 mmol of urea and 1.5 mmol of ammonium fluoride and dissolve them in 40 mL of deionized water. After stirring magnetically for 30 minutes, transfer the solution to the inner liner of a polytetrafluoroethylene reactor and add nickel foam. Place the reactor in an oven at 120°C and react for 12 hours. Then wash with ethanol and deionized water, and then place it in a vacuum oven to dry for 6 hours to finally obtain the Co-Fe-LDH / NF precursor. The nickel foam is obtained by pre-treating 2cm*4cm nickel foam. The treatment process is as follows: ultrasonically cleaned with acetone, 2mol / L hydrochloric acid, ethanol and deionized water for 15 minutes in sequence, and then vacuum dried.

4. The method for preparing the nitrogen-doped bifunctional electrocatalyst as described in claim 1, characterized in that, The specific operation of step S2 is as follows: the Co-Fe-LDH / NF precursor is placed in the inner liner of a polytetrafluoroethylene reactor containing 40 ml of dimethylformamide solution containing 3 mmol fumaric acid. The reactor is placed in an oven at 120°C and reacted for 12 h. After that, it is washed with ethanol and deionized water, and then placed in a vacuum oven to dry for 6 h. Finally, the Co-Fe-LDH-MIL88A / NF precursor is obtained.

5. The method for preparing the nitrogen-doped bifunctional electrocatalyst as described in claim 1, characterized in that, The specific operation of step S3 is as follows: the obtained Co-Fe-LDH-MIL88A / NF precursor is placed in a tube furnace filled with ammonia atmosphere and heated at 10 °C for min. -1 The temperature was increased to 400℃ at a certain rate, held for 1.5 h, and then naturally cooled to room temperature to obtain a nitrogen-doped bifunctional electrocatalyst with a nanoflower morphology, named Co-Fe-N@MIL88A / NF.

6. A nitrogen-doped bifunctional electrocatalyst, characterized in that, It is prepared by the preparation method according to any one of claims 1-5.

7. The nitrogen-doped bifunctional electrocatalyst as described in claim 6, characterized in that, The catalyst exhibits a nanoflower morphology formed by the self-assembly of ultrathin nanosheets, with a porous and open three-dimensional structure.

8. The application of the nitrogen-doped bifunctional electrocatalyst as described in claim 6 or 7 in the alkaline oxygen evolution and hydrogen evolution reactions of water electrolysis.

9. The application as described in claim 8, characterized in that, In 1 M KOH solution, OER at 10 mA·cm - ² and 100mA·cm - The overpotentials at the current densities were 180 mV and 235 mV, respectively, and the HERs were 66 mV and 246 mV, respectively.

10. The application as described in claim 8, characterized in that, Catalyst at 10 mA cm - ² and 100 mA cm - After operating at current density, the performance degradation rates of OER were 3.3% and 2.8%, respectively, and the performance degradation rates of HER were 2.6% and 4.06%, respectively.

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