A core-shell structure graphene material with alkaline overall water splitting performance and a preparation method thereof

Abstract

The application discloses a core-shell structure graphene material with alkaline full water splitting performance and a preparation method thereof, and realizes phase control of a metal core in the core-shell structure graphene material through a hetero-element doping strategy for the first time, so that the water splitting performance of a graphene shell layer is optimized, the experimental operation is simple, the sample yield is large, and the problems of low catalytic activity and poor stability of current graphene materials are effectively solved, which is beneficial to realize large-scale industrial production. Compared with traditional graphene materials, the core-shell structure graphene material synthesized by the application has the characteristics of large catalytic performance adjustability and high catalytic stability. The phase control method provided by the application can be realized in the sample synthesis process, and does not need additional sample preparation steps.

Description

TECHNICAL FIELD

[0001] The application belongs to the field of graphene material modification, and particularly relates to a core-shell structure graphene material with alkaline overall water splitting performance and a preparation method thereof. BACKGROUND

[0002] Electrocatalytic water splitting technology is the key to realizing the requirements of storable and sustainable energy development. In order to promote the practical application of new energy conversion technology, it is necessary to improve the reaction activity of the catalyst and reduce the industrial cost. Among them, carbon-based materials are considered as potential multifunctional electrocatalysts due to their high conductivity, strong corrosion resistance and low economic cost. However, the low activity of the planar C-C bond of carbon materials leads to weak hydrogen and oxygen production activity, which hinders its practical application in the field of electrocatalytic water splitting. So far, people have tried various strategies to optimize the performance of carbon-based materials, such as defect engineering, heteroatom doping, morphology control, etc. However, these control methods still face great challenges in improving the catalytic activity of carbon-based materials, such as limited solubility of doped atoms in carbon materials and uncontrollability of defect engineering. Therefore, how to realize the controllable optimization of the catalytic performance of carbon-based materials is very important. SUMMARY

[0003] In view of the problems in the prior art, the application provides a core-shell structure graphene material with alkaline overall water splitting performance and a preparation method thereof.

[0004] To achieve the above-mentioned purpose, the technical scheme adopted by the application is as follows:

[0005] The first object of the application is to provide a preparation method of a core-shell structure graphene material with alkaline overall water splitting performance, comprising the following steps:

[0006] (1) dissolving a cobalt salt in water to obtain a reaction liquid A; dissolving potassium cobalt cyanide and a surfactant in water to obtain a reaction liquid B; adding the reaction liquid A drop by drop into the reaction liquid B, and fully mixing to obtain an organic-inorganic hybrid material precursor solution; the cobalt salt is cobalt acetate, cobalt sulfate or cobalt nitrate; the surfactant is polyvinylpyrrolidone. The role of potassium cobalt cyanide in the above reaction is to provide cobalt cyanide ions for the precursor molecules, and the role of the surfactant is to promote the formation and stability of the precursor precipitate;

[0007] (2) adding rhodium salt into the organic-inorganic hybrid material precursor solution, uniformly mixing, and then placing in a temperature environment of 60-120℃ for constant temperature reaction for 1-12h, and then separating the obtained solid product and washing to obtain the doped precursor powder; preferably, the rhodium salt is rhodium chloride, rhodium acetate or rhodium nitrate; the constant temperature reaction can promote the diffusion of the doped element in the precipitation to achieve uniform distribution; it should be noted that the constant temperature reaction temperature can be 60℃, 80℃, 100℃ or 120℃, and the constant temperature reaction time can be 1h, 3h, 5h, 8h, 10h or 12h, which can be appropriately selected according to the actual situation, and all of them can achieve the purpose of the present application.

[0008] (3) placing the doped precursor powder in a protective gas atmosphere and annealing at a temperature of 500-800℃ for 3-5h to obtain the core-shell structure graphene material with alkaline overall water splitting performance. Preferably, the protective gas atmosphere is a nitrogen atmosphere or an inert gas atmosphere; the main reaction occurring during the annealing process is that the cobalt cyanide cobalt precursor molecules lose water molecules and carbon dioxide to generate the graphene material wrapping the metal cobalt core.

[0009] The second object of the present application is to provide a core-shell structure graphene material with alkaline overall water splitting performance, which is prepared by the preparation method as described in the first object; the core-shell structure graphene material comprises a metal cobalt core and a graphene shell layer wrapping the surface of the metal cobalt core; the metal cobalt core is doped with rhodium elements.

[0010] The reaction principle involved in the present application is as follows:

[0011] The present application selects rhodium elements as the doping source, and adjusts the annealing temperature to prepare the core-shell structure graphene material, so that the electrolytic water bifunctional activity is optimized, and the principle is that: because the hexagonal phase cobalt nanoparticles have a high electron density near the Fermi level, the graphene shell layer wrapping the hexagonal phase cobalt nanoparticles has a strong electronic coupling effect, which is conducive to the improvement of the catalytic activity of the graphene material. However, when the size of the cobalt metal particles is reduced to the nanoscale, the phase transition of cobalt from hexagonal phase to face-centered cubic phase will occur. The introduction of rhodium elements as the dopant, which has a larger atomic radius than cobalt but the same number of outer electrons, can realize the induced lattice distortion effect regulation of the cobalt core.

[0012] The beneficial effects of the present application are as follows:

[0013] The application first utilizes the strategy of hetero-element doping to regulate the phase of the metal cobalt core in the core-shell structure graphene material, thereby regulating the electrocatalytic hydrolysis performance of the graphene shell material, and the experimental operation is simple, the yield is high, and the problems of poor electrocatalytic performance and unstable regulation of the graphene material are effectively solved. Through the preparation method in the application, the catalytic activity of the graphene material can be effectively improved, and the dual-function electrocatalytic hydrolysis performance is realized. Compared with the traditional regulation modes such as defect engineering and chemical doping, the application realizes the direct construction of the core-shell structure graphene, and improves the corrosion resistance of the catalyst through the regulation of the metal core phase. BRIEF DESCRIPTION OF DRAWINGS

[0014] Figure 1 XRD patterns of products prepared for the comparative examples and the examples;

[0015] Figure 2 Morphology diagrams of products prepared for the comparative examples and the examples;

[0016] Figure 3 Electrocatalytic hydrogen production performance diagrams of products prepared for the comparative examples and the examples;

[0017] Figure 4 Electrocatalytic oxygen production performance diagrams of products prepared for the comparative examples and the examples. DETAILED DESCRIPTION

[0018] The application will be further described below in combination with the drawings and examples.

[0019] Example 1

[0020] A preparation method of a core-shell structure graphene material with alkaline overall water splitting performance, comprising the following steps:

[0021] (1) 37.4 mg of cobalt acetate is dissolved in 20 ml of water to obtain a reaction liquid A; 33.2 mg of potassium cobalt cyanide and 600 mg of polyvinylpyrrolidone are dissolved in 20 ml of water to obtain a reaction liquid B; under room temperature conditions, the reaction liquid A is added dropwise into the reaction liquid B, and after sufficient stirring until the reaction is complete, an organic-inorganic hybrid material precursor solution is obtained.

[0022] (2) 0.44 ml of rhodium chloride aqueous solution (0.01 g / ml) is added to the organic-inorganic hybrid material precursor solution, and after uniform stirring, the reaction kettle is continuously kept at 100 DEG C for 5 h, and after the reaction is completed, the obtained solid product is separated by suction filtration, and is washed with deionized water and alcohol solution for multiple times, and then is dried in an oven kept at 60 DEG C to obtain a doped precursor powder;

[0023] (3) The doped precursor powder is annealed at 500°C for 4 hours under a nitrogen atmosphere to obtain a final product, which is a graphene material wrapping a hexagonal phase cobalt core, marked as hcp-Co@NC.

[0024] Example 2

[0025] A method for preparing a core-shell structure graphene material with alkaline overall water splitting performance, comprising the following steps:

[0026] (1) 37.4 mg of cobalt acetate is dissolved in 20 ml of water to obtain a reaction liquid A; 33.2 mg of potassium cobaltate and 600 mg of polyvinylpyrrolidone are dissolved in 20 ml of water to obtain a reaction liquid B; the reaction liquid A is added dropwise into the reaction liquid B under room temperature, and the organic-inorganic hybrid material precursor solution is obtained after fully stirring until the reaction is complete.

[0027] (2) 0.44 ml of rhodium chloride aqueous solution (0.01 g / ml) is added to the organic-inorganic hybrid material precursor solution, and after uniform stirring, the reaction kettle is continuously kept at 100°C for 5 hours. After the reaction is completed, the obtained solid product is separated by suction filtration, and is washed with deionized water and alcohol solution for multiple times, and then is dried in an oven kept at 60°C to obtain a doped precursor powder;

[0028] (3) The doped precursor powder is annealed at 800°C for 4 hours under a nitrogen atmosphere to obtain a final product, which is a graphene material wrapping a face-centered cubic phase cobalt core, marked as fcc-Co@NC.

[0029] Comparative Example 1

[0030] A method for preparing a nitrogen-doped graphene material which is inert to conventional electrocatalytic reactions, comprising the following steps:

[0031] (1) 5 mg of polymethyl methacrylate and 0.5 mg of melamine are dissolved in 5 ml of N,N-dimethylformamide.

[0032] (2) 200 g of sodium chloride is dissolved in the above solution, and after uniform mixing, the DMF is evaporated in an oven kept at 70°C.

[0033] (3) The powder obtained in step 2 is annealed at 780°C for 2 hours. After the sample is cooled, the powder is dissolved in a large amount of deionized water to remove the NaCl template, thereby obtaining a nitrogen-doped graphene, marked as NC.

[0034] Comparative Example 2

[0035] Comparative Example 2 is different from Example 1 in that no rhodium chloride aqueous solution is added in step (2), and the other processes are the same as those in Example 1. The product obtained is marked as Co2C@NC.

[0036] Structural characterization

[0037] The products prepared in the above examples and comparative examples were subjected to structural characterization, Figure 1 XRD patterns of the products prepared in the comparative examples and examples, Figure 2 Morphology patterns of the products prepared in the comparative examples and examples. From Figure 1 It can be seen that the comparative examples and examples have different metal core phase structures, which illustrates the phase regulation; from Figure 2 It can be seen that the samples prepared in the comparative examples and examples have good graphene structures;

[0038] Performance detection

[0039] The materials prepared in the above comparative examples and examples were subjected to alkaline electrocatalytic hydrogen and oxygen production performance detection, and the test results are shown in Figure 3 and Figure 4 From Figure 3 and 4 It can be seen that the examples have lower electrocatalytic hydrogen and oxygen production overpotential than the comparative examples, which illustrates that the graphene material realizes the optimization of the electrocatalytic hydrolysis performance after the phase of the metal core is regulated by doping, and the hexagonal phase cobalt core has the highest performance optimization for the graphene material.

[0040] Obviously, the described examples are part of the embodiments of the present application, rather than all the embodiments. Based on the examples in the present application, all other embodiments obtained by those of ordinary skill in the art without creative labor are within the scope of protection of the present application.

Claims

1. A method for preparing a core-shell structured graphitic material having an alkaline overall water-splitting performance, characterized by: The method comprises the following steps: (1) dissolving cobalt salt in water to obtain reaction liquid A; dissolving potassium cobalt cyanide and a surfactant in water to obtain reaction liquid B; adding reaction liquid A into reaction liquid B, and mixing uniformly to obtain an organic-inorganic hybrid material precursor solution; (2) adding rhodium salt into the organic-inorganic hybrid material precursor solution, mixing uniformly, and then placing in a constant temperature reaction at a temperature of 60-120℃; after the reaction is completed, the obtained solid product is separated and washed to obtain a doped precursor powder; (3) placing the doped precursor powder in a protective gas atmosphere and annealing at a temperature of 500-800℃ to obtain a core-shell structure graphene material having basic overall water splitting performance; the core-shell structure graphene material comprises a metal cobalt core and a graphene shell layer coated on the surface of the metal cobalt core; the metal cobalt core is doped with rhodium element.

2. The method of claim 1, wherein the method is characterized by: In step (1), the cobalt salt is cobalt acetate, cobalt sulfate or cobalt nitrate.

3. The method for preparing the core-shell structured graphene material with alkaline water-splitting properties according to claim 1, characterized in that: In step (1), the surfactant is polyvinylpyrrolidone.

4. The method of claim 1, wherein the method is characterized by: In step (2), the rhodium salt is rhodium chloride, rhodium acetate or rhodium nitrate.

5. The method of claim 1, wherein the method is characterized by: In step (2), the constant temperature reaction time is 1-12h.

6. The method of claim 1, wherein the method is characterized by: In step (3), the annealing time is 3-5h.

7. The method of claim 1, wherein the method is characterized by: In step (3), the protective gas atmosphere is a nitrogen atmosphere or an inert gas atmosphere.

8. A core-shell structured graphenic material having an alkaline overall water-splitting performance, characterized by: It is prepared by the preparation method of any one of claims 1 to 7; the core-shell structure graphene material comprises a metal cobalt core and a graphene shell layer coated on the surface of the metal cobalt core; the metal cobalt core is doped with rhodium element.

Images