Co / N-C (at) CNFs with interconnected framework and graphite packaged Co nanoparticles, and preparation method and application of Co / N-C (at) CNFs

Abstract

The invention belongs to the technical field of electrochemistry, and particularly relates to Co / N-C-aton CNFs with an interconnected framework and graphite packaged Co nanoparticles and a preparation method and application thereof.The preparation method comprises the following steps that S1, a methanol solution of cobalt nitrate hexahydrate and a methanol solution of 2-methylimidazole are prepared; s2, dissolving MFC into a methanol solution of cobalt nitrate hexahydrate, then mixing the solution with a methanol solution of 2-methylimidazole, and carrying out a reaction to obtain a mixed solution; s3, centrifuging the mixed solution, and then washing and drying; and S4, calcining the dried solid in an inert atmosphere to obtain the Co / N-C-aton CNFs. Compared with the prior art, the method has the advantages that the problem of internal collapse of the ZIF in the calcining process in the prior art is solved, the ZIF is supported, calcining collapse is avoided, and the prepared catalyst material has relatively high oxygen reduction performance and relatively good stability.

Description

Technical Field

[0001] The present invention belongs to the field of electrochemistry technology, and particularly relates to Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles, a preparation method thereof, and an application thereof. Background Art

[0002] The energy conversion efficiency and performance of fuel cells are usually limited by the slow kinetics of their cathodes. Therefore, it is of great significance to develop highly active and long-life oxygen reduction electrocatalysts. At present, although the development and wide use of high-performance oxygen reduction catalysts still rely on noble metal Pt and its alloys, they still face many development obstacles, including the expensive cost of noble metals, the limited reserves of noble metal platinum, and various performance instability problems of platinum catalysts for large-scale commercial production. In recent years, the design and synthesis of Co and N co-doped carbon material catalysts have been widely studied. Although catalysts such as Co / N-C have made significant progress in catalytic activity, the long-term stability of Co / N-C catalysts is still a huge challenge they currently face.

[0003] Zeolitic imidazolate frameworks (ZIFs) are considered ideal candidate materials for ORR catalysts because of their relatively high nitrogen content, rich M-N bonds, and high surface area. For example, CN 110975921 B discloses a preparation method and application of a nitrogen-doped cobalt-based carbon material with a magnetic porous structure. However, since the internal collapse of ZIFs during calcination is inevitable, directly calcining discrete ZIFs usually destroys their ordered structure and porous morphology, thereby reducing the performance of the catalyst.

[0004] Therefore, it is necessary to find a method to avoid the internal structure collapse of ZIFs to fully utilize the advantages of ZIFs. Summary of the Invention

[0005] The purpose of the present invention is to provide Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles, a preparation method thereof, and an application thereof to solve at least one of the above problems, so as to solve the problem that ZIFs will undergo internal collapse during the calcination process in the prior art, realize the support of ZIFs, avoid calcination collapse, and the prepared catalyst material has high oxygen reduction performance and good stability.

[0006] The purpose of the present invention is achieved through the following technical solutions:

[0007] The first aspect of the present invention discloses a preparation method of Co / N-C@CNFs with an interconnected framework and graphitized carbon-encapsulated Co nanoparticles, comprising the following steps:

[0008] S1: Prepare methanol solutions of cobalt nitrate hexahydrate and 2-methylimidazole respectively;

[0009] S2: Dissolve MFC (nanofibrillated cellulose) in the methanol solution of cobalt nitrate hexahydrate, then mix it with the methanol solution of 2-methylimidazole and react to obtain a mixture;

[0010] S3: Centrifuge the mixture, then wash and dry it;

[0011] S4: Calcinate the dried solid in an inert atmosphere to obtain Co / N-C@CNFs.

[0012] Preferably, in step S2, the mass ratio of MFC, cobalt nitrate hexahydrate to 2-methylimidazole is 1-10:1-5:3-10.

[0013] Preferably, the reaction temperature is 10-35 °C and the reaction time is 0.1-24 h.

[0014] Preferably, stirring is applied during the reaction, and the stirring rate is 60-300 r / min.

[0015] Preferably, the centrifugation rate is 8000 r / min and the centrifugation time is 5-10 min.

[0016] Preferably, methanol is used as the cleaning agent for washing, and the number of washing times is 3-5 times.

[0017] Preferably, freeze-drying is used for drying, and the drying time is 24-72 h.

[0018] Preferably, the calcination temperature is 500-1000 °C, the heating rate of calcination is 3-15 °C·min -1 , and the calcination time is 2-4 h.

[0019] The second aspect of the present invention discloses a Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles, which is prepared by the method described in any one of the above.

[0020] The third aspect of the present invention discloses the application of the above-mentioned Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles in fuel cells.

[0021] The working principle of the present invention is:

[0022] ZIF67 is composed of Co 2+The porous dodecahedral crystal structure formed by coordination complexation with 2-methylimidazole has N and Co elements that are beneficial to ORR activity. In addition, the porous structure of ZIF67 gives it a large specific surface area, which is more conducive to the exposure of active sites. However, during the calcination process, the Co metal on the ZIF67 crystal framework is prone to agglomeration, causing the collapse of the ZIF67 structure, thus affecting its activity and long-term stability. Therefore, we grow ZIF67 in situ in the dispersion solution of MFC, enabling MFC to support and "rivet" ZIF67, avoiding the agglomeration and collapse of ZIF67. Moreover, due to the presence of MFC during the calcination process, it can promote the graphitization of the Co-catalyzed carbon matrix and form a graphitized encapsulation on the surface of Co metal particles. Thus, a novel hierarchical-structured Co-N co-doped carbon catalyst (Co / N-C@CNFs) coexisting with Co nanoparticles with a nano-interconnected 3D framework and graphitic encapsulation can be prepared, which has excellent catalytic activity and long-term stability.

[0023] Compared with the prior art, the present invention has the following beneficial effects:

[0024] The preparation method of the present invention is simple, the raw materials are cheap, and it is environmentally friendly; the prepared catalyst has high oxygen reduction performance and good stability:

[0025] (1) The preparation method is simple: through room-temperature reaction and calcination carbonization, a composite carbon material catalyst with an interconnected three-dimensional framework structure can be obtained.

[0026] (2) The nanofibrillated fibers used are of biomass origin and are rich in resources.

[0027] (3) The nanostructure of Co / N-C@CNFs is a hierarchical structure coexisting with a 3D framework formed by interconnected ZIF67 derivatives and graphitically encapsulated Co nanoparticles. The CNFs formed after calcination of MFC play a role in supporting and anchoring ZIFs during the calcination process, and the interconnected and anchored structure prevents the aggregation of ZIFs during the calcination process. The graphitized encapsulation structure protects the metal Co nanoparticles.

[0028] (4) The abundant micropores and mesopores in the prepared Co / N-C@CNFs catalyst endow it with a high surface area. The N-C and Co-N active sites and the graphitic layer-encapsulated cobalt metal particles are uniformly dispersed in the carbon matrix, enabling the catalyst to have much higher oxygen reduction activity and stability than commercial Pt / C catalysts under alkaline conditions. Description of the Drawings

[0029] Figure 1 It is a scanning electron microscope image of MFC used for preparing Co / N-C@CNFs in Example 1;

[0030] Figure 2Scanning electron microscope image (1μm) of Co / N-C@CNFs prepared in Example 1;

[0031] Figure 3 Scanning electron microscope image (500nm) of Co / N-C@CNFs prepared in Example 1;

[0032] Figure 4 Transmission electron microscope image (200nm) of Co / N-C@CNFs prepared in Example 1;

[0033] Figure 5 Transmission electron microscope image (5nm) of Co / N-C@CNFs prepared in Example 1;

[0034] Figure 6 Nitrogen adsorption - desorption isotherm of Co / N-C@CNFs prepared in Example 1;

[0035] Figure 7 X-ray photoelectron spectroscopy of Co / N-C@CNFs prepared in Example 1;

[0036] Figure 8 Chronoamperometry curves of Co / N-C@CNFs prepared in Example 1 and Co / N-C prepared in the comparative example (comparative sample);

[0037] Figure 9 Linear sweep voltammetry curves of Co / N-C@CNFs prepared in Example 1, Co / N-C prepared in the comparative example (comparative sample), and commercial Pt / C catalyst. Detailed implementation mode

[0038] The present invention will be described in detail below with reference to the accompanying drawings and specific examples.

[0039] In the following description, unless otherwise specified, the reagents used are commercially available products, and the methods used are well-known means in the art.

[0040] In the present invention, cobalt nitrate hexahydrate and 2-methylimidazole are respectively dissolved in methanol. Under continuous stirring, MFC is added to the methanol solution of cobalt nitrate and maintained for at least a certain time, and then it is mixed with the methanol solution in which 2-methylimidazole is dissolved. After reacting for a certain time, the product obtained by centrifugal separation is washed with methanol and then freeze-dried in a freeze dryer. Then, the obtained sample is pyrolyzed in an inert gas atmosphere to obtain a cobalt-nitrogen co-doped carbon composite oxygen reduction catalyst with a 3D interconnected network structure.

[0041] The technical solution of the present invention is specifically introduced as follows:

[0042] Preparation method of Co nanoparticles encapsulated by graphite layers and Co / N-codoped carbon oxygen reduction reaction electrocatalyst with interconnected three-dimensional framework structure, the specific steps are as follows:

[0043] 1) Dissolve cobalt nitrate hexahydrate and 2-methylimidazole in methanol respectively, and continuously stir magnetically at room temperature for several minutes to ensure complete dissolution.

[0044] 2) Add MFC to the methanol solution of cobalt nitrate under continuous magnetic stirring and keep it for at least 30 min, then mix it with the methanol solution dissolved with 2-methylimidazole and react for a certain time.

[0045] 3) First, the product obtained by centrifuging the mixture obtained in step 2) is washed several times with methanol and then freeze-dried in a freeze dryer.

[0046] 4) First, put the product obtained in step 3) into a tubular furnace, and then pyrolyze it at 800 °C for 2 h at a heating rate of 5 °C·min -1 in an inert nitrogen atmosphere. The carbonized sample is ground thoroughly to obtain the Co / N-codoped carbon material oxygen reduction catalyst (Co / N-C@CNFs).

[0047] In the above step 1), the mass ratio of MFC, cobalt nitrate hexahydrate, and 2-methylimidazole is (1-10):(1-5):(3-10).

[0048] In the above step 1), the mass ratio of cobalt nitrate hexahydrate and 2-methylimidazole is 0.714:1.63.

[0049] In the above step 2), the temperature condition is 10-35 °C, the reaction time is 0.1-24 h, and the stirring rate is 60-300 rpm.

[0050] In the above step 3), the centrifugation rate is 8000 rpm (8000 rpm, 5 minutes), the number of methanol washing times is 3-5 times, and the sample freeze-drying time is 24-72 hours.

[0051] In the above step 4), the calcination temperature is 500-1000 °C, the heating rate is 3-15 °C·min -1 , and the pyrolysis duration is 2-4 h.

[0052] The present invention also provides a Co nanoparticle encapsulated by graphite layers and a Co / N-codoped carbon oxygen reduction reaction electrocatalyst with interconnected three-dimensional framework structure prepared by the above preparation method.

[0053] The present invention further provides an application of the above Co nanoparticles encapsulated with a graphite layer and a Co / N-codoped carbon material oxygen reduction catalyst with an interconnected three-dimensional framework structure.

[0054] Unless otherwise specified, the methods described in the embodiments of the present invention are all conventional methods; unless otherwise specified, the raw materials can all be obtained from publicly available commercial sources.

[0055] The information on the models and manufacturers of the instruments or equipment used in the embodiments of the present invention is as follows:

[0056] Blast drying oven, model DHG-9920A, manufacturer: Shanghai Yiheng Scientific Instruments Co., Ltd.;

[0057] Scanning electron microscope (SEM), model Nova NanoSEM 450, manufacturer: FEI Company, USA;

[0058] Transmission electron microscope (TEM), model: Talos F200X, manufacturer: Thermo Fisher Scientific, USA;

[0059] X-ray photoelectron spectrometer, model: AXIS UltraDLD, manufacturer: Shimadzu Corporation, Japan;

[0060] Electrochemical workstation, model: Autolab PGSTAT302, manufacturer: Metrohm, Switzerland.

[0061] In the embodiments of the present invention, the cyclic voltammetry curve of the sample was measured in an Autolab PGSTAT302 electrochemical workstation. The specific test conditions were as follows: all electrochemical tests were carried out on an Autolab PGSTAT302 (Metrohm, Netherlands) electrochemical workstation with a standard three-electrode system. A glassy carbon electrode, an Ag / AgCl electrode, and a Pt electrode were used as the working electrode, reference electrode, and counter electrode, respectively. The catalyst slurry (Ink) was composed of 1 mg of catalyst sample, 10 μL of 5% Nafion 117 solution, and 200 μL of a water-alcohol solution (volume ratio of water to ethanol = 4:1). After ultrasonic homogenization, the Ink was coated on the glassy carbon electrode used as the working electrode, and the catalyst loading of all working electrodes was about 0.3 mg / cm 2 。In 0.1 M KOH solution at 10 mV·s -1 The linear sweep voltammetry (LSV) curve was measured. Under an oxygen atmosphere, the linear sweep voltammetry curves of the test electrode were measured at different rotation speeds (400 - 2400 rpm). Before each measurement, the KOH solution was purged with pure N2 or pure O2 for at least 30 minutes to ensure a N2-saturated or O2-saturated KOH solution was obtained.

[0062] In the embodiments of the present invention, the chronoamperometric curve of the sample was measured in an Autolab PGSTAT302N electrochemical workstation under the following test conditions: in a three-electrode system, with Ag / AgCl as the reference electrode, a platinum sheet as the counter electrode, a rotating disk electrode loaded with the catalyst as the working electrode, 0.1 M KOH as the electrolyte solution, and a scanning rate of 10 mV / s; the preparation process of the working electrode was as follows: the catalyst slurry (Ink) consisted of 1 mg of the catalyst sample, 10 μL of 5% Nafion 117 solution, and 200 μL of a water-alcohol solution (volume ratio of water to ethanol = 4:1). After ultrasonic dispersion in the mixed solution, 10 μL was taken and dropped onto the rotating disk electrode, and after drying at room temperature, it became the working electrode. The test conditions for the chronoamperometric curve were: at a constant potential of -0.35 V and a rotation speed of 1600 rpm, the change in current over time was measured within 70000 s.

[0063] Example 1

[0064] 0.714 g of cobalt(II) nitrate hexahydrate and 1.63 g of 2-methylimidazole were separately dissolved in 50 mL of methanol, and magnetic stirring was continued for several minutes to dissolve them thoroughly. Under continuous magnetic stirring, 1.0 g of MFC was added to the methanol solution of cobalt nitrate and maintained for at least 30 min, and then it was mixed with the methanol solution in which 2-methylimidazole was dissolved. After stirring at room temperature for 24 h, the product obtained by centrifugation (8000 rpm, 5 minutes) was washed several times with methanol and then freeze-dried in a freeze dryer for 24 h. Then the obtained sample was named ZIF67@MFC. The prepared sample was heated to 800 °C at a heating rate of 5 °C·min -1 in a nitrogen atmosphere and pyrolyzed for 2 h, and the final product was named Co / N-C@CNFs.

[0065] From Figure 1 the dispersed part, it can be seen that MFC has a large aspect ratio, with its radial size in the range of 20 - 60 nm and the length range between 2 - 30 μm.

[0066] From Figure 2 it can be known that the ZIF67 crystal has a regular dodecahedral shape and is uniformly sized at about 400 nm. MFC penetrates through the ZIF67 crystal particles and connects them together to form a network structure. Figure 3 In Co / N-C@CNFs, the carbon framework maintains a relatively clear polyhedral morphology and good dispersibility, and the MFC fibers penetrate through the ZIF67 crystals and provide sufficient support for the ZIF67 crystals.

[0067] Figure 4In the structural framework of Co / N-C@CNFs, the size of the calcined ZIF67 (Co / N-C) is about 200 - 300 nm. Co / N-C is interwoven by CNFs, and this is in good agreement with the previous SEM characterization results. Figure 5 It shows that the Co nanoparticles in Co / N-C@CNFs are almost completely wrapped by graphite layers, and the graphite layer structure is clearly visible.

[0068] Figure 6 The adsorption curve isotherm is a combination of type II and type IV isotherms, indicating that Co / N-C@CNFs is a porous material with mesoporous and macroporous structures.

[0069] From Figure 7 it can be seen that there are only three elements in the nitrogen-doped carbon spheres, and the contents of carbon, oxygen, nitrogen, and cobalt are 85.3%, 7.2%, 3.2%, and 4.3% respectively.

[0070] Comparative example

[0071] Dissolve 0.714 g of cobalt(II) nitrate hexahydrate and 1.63 g of 2-methylimidazole in 50 mL of methanol respectively, and continuously stir magnetically for several minutes.

[0072] In this example, MFC was not added to be used as a comparison sample for Example 1.

[0073] Mix the two solutions under continuous magnetic stirring. After stirring at room temperature for 24 h, the product obtained by centrifugation (8000 rpm, 5 minutes) was washed several times with methanol and then freeze-dried in a freeze dryer for 24 h. Then the obtained sample was named ZIF67. The prepared sample was heated to 800 °C at a heating rate of 5 °C·min -1 in a nitrogen atmosphere and pyrolyzed for 2 h, and the final product was named Co / N-C.

[0074] From Figure 8 it can be seen that after 70000 s of chronoamperometry test, the current density of Co / N-C@CNFs in Example 1 remained at 94%, while that of the Co / N-C catalyst of the comparison sample was 58.5%, indicating that the catalyst of the example is superior to the comparison sample in durability.

[0075] Figure 9 This is the linear sweep voltammogram of this catalyst, the comparison sample, and the commercial Pt / C catalyst (20%, brand: Johnmn Matthey). Co / N-C (comparison sample) and Co / N-C@CNFs have onset potentials and more positive half-wave potentials (E0 = 0.95 V vs RHE; E 1 / 2 = 0.825 V vs RHE) that are fully comparable to those of the commercial Pt / C. This indicates that they have excellent oxygen reduction electrocatalytic performance.

[0076] Example 2

[0077] This example is basically the same as Example 1, except that the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 5:3 (while keeping the amount of cobalt nitrate hexahydrate unchanged); the reaction temperature is 10 °C, the reaction time is 24 h; the centrifugation time is 10 min; freeze-drying is carried out for 72 h; the calcination temperature is 1000 °C, and the heating rate is 15 °C·min -1 , and the calcination time is 3 h.

[0078] Example 3

[0079] This example is basically the same as Example 1, except that the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:10 (while keeping the amount of cobalt nitrate hexahydrate unchanged); the reaction temperature is 35 °C, the reaction time is 2 h; the centrifugation time is 8 min; freeze-drying is carried out for 48 h; the calcination temperature is 500 °C, and the heating rate is 3 °C·min -1 , and the calcination time is 4 h.

[0080] Example 4

[0081] This example is basically the same as Example 1, except that the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:3 (while keeping the amount of cobalt nitrate hexahydrate unchanged); the reaction temperature is 18 °C, the reaction time is 22 h; the centrifugation time is 6 min; freeze-drying is carried out for 24 h; the calcination temperature is 750 °C, and the heating rate is 10 °C·min -1 , and the calcination time is 3 h.

[0082] Example 5

[0083] This example is basically the same as Example 1, except that the mass ratio of cobalt nitrate hexahydrate to 2-methylimidazole is 1:5 (while keeping the amount of cobalt nitrate hexahydrate unchanged); the reaction temperature is 30 °C, the reaction time is 12 h; the centrifugation time is 7 min; freeze-drying is carried out for 30 h; the calcination temperature is 700 °C, and the heating rate is 8 °C·min -1 , and the calcination time is 4 h.

[0084] The above description of the embodiments is for the convenience of those of ordinary skill in the art to understand and use the invention. Those who are familiar with the technology in this field can obviously make various modifications to these embodiments easily, and apply the general principles described herein to other embodiments without creative labor. Therefore, the present invention is not limited to the above embodiments, and the improvements and modifications made by those skilled in the art without departing from the scope of the present invention according to the disclosure of the present invention should be within the protection scope of the present invention.

Claims

1. A preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles, characterized in that, It includes the following steps: S1: Prepare a methanol solution of cobalt nitrate hexahydrate and a methanol solution of 2-methylimidazole respectively; S2: Dissolve MFC in the methanol solution of cobalt nitrate hexahydrate, and then mix it with the methanol solution of 2-methylimidazole and react to obtain a mixed solution; S3: Centrifuge the mixed solution, and then wash and dry it; S4: Calcinate the dried solid in an inert atmosphere to obtain Co / N-C@CNFs.

2. The preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles according to claim 1, characterized in that, In step S2, the mass ratio of MFC, cobalt nitrate hexahydrate to 2-methylimidazole is 1-10:1-5:3-10.

3. The preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles according to claim 1, characterized in that, The reaction temperature is 10-35 °C, and the reaction time is 0.1-24 h.

4. The preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles according to claim 3, characterized in that, Stirring is applied during the reaction, and the stirring rate is 60-300 r / min.

5. The preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles according to claim 1, characterized in that, The centrifugation rate is 8000 r / min, and the centrifugation time is 5-10 min.

6. The preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles according to claim 1, characterized in that, Methanol is used as the cleaning agent for washing, and the number of washing times is 3-5 times.

7. The preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles according to claim 1, characterized in that, Freeze drying is used for drying, and the drying time is 24-72 h.

8. The preparation method of Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles according to claim 1, wherein The calcination temperature is 500 to 1000 °C, and the heating rate of calcination is 3 to 15 °C·min -1 , and the calcination time is 2 to 4 h.

9. A Co / N-C@CNFs with an interconnected framework and graphite-encapsulated Co nanoparticles, characterized in that, Prepared by the method according to any one of claims 1-8.

10. Application of Co / N-C@CNFs having an interconnected framework and graphite-encapsulated Co nanoparticles as described in claim 9 in a fuel cell.

Images