CN113437305A

CN113437305A - 2D-Co @ NC composite material and preparation method and application thereof

Info

Publication number: CN113437305A
Application number: CN202110504044.8A
Authority: CN
Inventors: 刘乔; 石青; 杨为佑
Original assignee: Ningbo University of Technology
Current assignee: Ningbo University of Technology
Priority date: 2021-05-10
Filing date: 2021-05-10
Publication date: 2021-09-24

Abstract

The invention belongs to the technical field of electrocatalysis, and relates to a 2D-Co @ NC composite material, and a preparation method and application thereof. According to the invention, water is used as a solvent to synthesize a 2D-ZIF precursor, and then the 2D-Co @ NC composite material with the thickness of 1-10nm and the metal Co nano particles of 2-6nm distributed on the nitrogen-doped carbon nano sheet is obtained after high-temperature carbonization.

Description

2D-Co @ NC composite material and preparation method and application thereof

Technical Field

The invention belongs to the technical field of electrocatalysis, and relates to a 2D-Co @ NC composite material, and a preparation method and application thereof.

Background

Energy and environmental issues have become the most important issue in the twenty-first century, and the application of sustainable energy conversion and storage technologies, such as fuel cells, metal-air batteries, and water splitting technologies, is expected to alleviate the increasing energy and environment. The Oxygen Reduction Reaction (ORR), Oxygen Evolution Reaction (OER) and Hydrogen Evolution Reaction (HER) are three important catalytic reaction processes that are decisive for their efficiency and cycle life in energy storage and conversion devices. However, three catalytic reactions are very slow in kinetics based on the catalytic process occurring at a heterogeneous interface and through a multi-step electron transfer process. Pt-based catalysts are highly efficient ORR and HER catalysts, and Ru-/Ir-based catalysts are highly efficient OER catalysts. However, the disadvantages of high cost, scarce resources and poor stability limit the widespread use of noble metal catalysts in the field of energy storage and conversion. And the noble metal catalyst has poor bifunctional property and cannot simultaneously and efficiently catalyze two or three catalytic reactions, for example, a rechargeable metal-air battery has an OER reaction in the catalyst during charging and an ORR reaction during discharging. Perhydrolysis requires the HER and OER reactions to occur at the cathode and anode, respectively. Therefore, the development of a low-cost, high-efficiency and stable three-function catalyst is of great significance to rechargeable metal-air batteries and total hydrolysis.

In recent years, metal organic frameworks have attracted considerable attention as a new class of carbon composites. The zeolite imidazole framework is taken as a type of MOF material, has abundant C sources and N sources, and strong adjustability of pore structure, and ZIFs derived carbon-based electrocatalysts are widely concerned. ZIFs can be used as precursors for providing a carbon source and a nitrogen source, and can also be used as a template for catalyst synthesis. For example, N-doped carbon catalysts prepared from ZIF-8 as a precursor, Zn-N-C monatomic catalysts prepared from ZIF-8 as a precursor, Co/Co-N-C catalysts prepared from ZIF-67 as a precursor, Co-supported nitrogen-doped carbon catalysts prepared from Zn/Co-ZIF, and Co-N-C monatomic catalysts prepared from Zn/Co-ZIF as a precursor. The above catalysts are all efficient ORR catalysts. Based on the difference of active sites of ORR, OER and HER catalytic reactions, complex synthesis steps are needed to realize phase-to-phase conversion when preparing the multifunctional catalyst, so that the active species with the multifunctional catalyst is prepared. For example, the core-shell structure of ZIF-8@ ZIF-67 is used for synthesizing the catalyst with ORR/OER bifunctional catalytic performanceThe reagent, based on the N-doped carbon microporous structure core produced by ZIF-8, ZIF-67 produces a Co-modified N-doped mesoporous graphitic carbon shell. Preparing Co nanoparticles and Co by compounding polyaniline and ZIF-67 and then performing pyrolysis₃O₄And an N-doped carbon multifunctional catalyst. The multifunctional Co/CoP catalyst is prepared by a secondary phosphorization process after primary carbonization. However, the above-mentioned MOF directly prepared or templated catalyst has disadvantages in that 1) an organic solvent harmful to the environment, such as methanol and DMF, is used in the synthesis process; 2) the specific surface area of the ZIFs nanoparticles is relatively low, and active sites of the catalyst cannot be fully exposed; 3) simple synthesis methods can only yield single-function catalysts, and complex preparation methods are required to synthesize dual-function or multi-function catalysts.

Chinese patent application document (publication number: 107159297A) discloses a bifunctional oxygen catalyst cobalt/cobaltosic oxide/nitrogen carbon composite material and a preparation method thereof, wherein the prepared composite material shows excellent bifunctional oxygen electrode catalytic activity, and Co @ Co of a core-shell structure in an alkaline solution₃O₄The nano particles encapsulated in the graphitized N-doped porous carbon show good stability, but the precursor is synthesized by adopting an organic solvent methanol, the preparation method is complex, and a preparation process of secondary oxidation after primary carbonization is needed.

Disclosure of Invention

The invention aims to solve the problems in the prior art and provides a simple and environment-friendly 2D-Co @ NC composite material which has good catalytic activity and stability.

The purpose of the invention can be realized by the following technical scheme:

A2D-Co @ NC composite material is of a nanosheet structure, and metal Co nanoparticles are distributed on a nitrogen-doped carbon material of the nanosheet.

In the 2D-Co @ NC composite material, the thickness of the nanosheet layer is 1-10nm, and the particle size of the metal Co nanoparticles is 2-6 nm.

In the 2D-Co @ NC composite material, the composite material comprises the following components in percentage by mass: 1.65-2.1% of Co, 5.5-7.5% of O, 9.8-10.5% of N and the balance of C element.

In the 2D-Co @ NC composite material, the surface area of the composite material is 390-425m²Per g, pore volume of 0.15-0.2cm³G, the pore diameter is 4.5-5.5 nm.

The surface area of the 2D-Co @ NC composite material reaches 420-²The/g and the hierarchical mesoporous structure are beneficial to the full exposure of active sites and the transmission of protons in the reaction process, thereby improving the catalytic activity of the catalyst. The specific surface area and the pore volume of the nanosheet structure in the composite material of the present invention are increased due to the certain agglomeration of the metal Co during the carbonization process, and pores are formed after the metal particles on the surface are washed away in the subsequent acid washing process.

The invention also provides a preparation method of the 2D-Co @ NC composite material, which comprises the following steps:

s1, mixing Co (NO)₃·6H₂Dissolving O in ultrapure water to form a solution A, dissolving dimethyl imidazole in ultrapure water to form a solution B, adding the solution B into the solution A, stirring, performing centrifugal treatment to obtain a purple precipitate, cleaning the purple precipitate, and performing freeze drying to obtain a 2D-ZIF precursor;

s2, carbonizing the 2D-ZIF precursor in an argon atmosphere to obtain black powder, pickling the black powder, filtering, washing with deionized water to be neutral, and finally drying to obtain the catalyst.

In the invention, because the amino nitrogen on the dimethyl imidazole is easy to combine with hydrogen ions to form hydrogen bonds, Co²⁺The precursor is subjected to simple high-temperature carbonization and subsequent acidic treatment to remove unstable metal particles on the surface, and finally the three-functional composite material with the Co nanoparticles uniformly dispersed in the N-doped carbon nanosheet structure is formed.

In the above preparation method of the 2D-Co @ NC composite material, step S1Co (NO)₃·6H₂Mass ratio of O to dimethylimidazoleIs 1: (5.6-11.5).

In the preparation method of the 2D-Co @ NC composite material, the carbonization treatment in the step S2 is to heat up to 650-750 ℃ at the speed of 4-6 ℃/min and then preserve heat for 1.5-2.5 h.

The carbonization temperature has obvious influence on the specific surface area and the aperture size, when the carbonization temperature is lower, the surface of the formed 2D-Co @ NC composite nanosheet is smoother, and when the carbonization temperature is too high, the surface of the formed 2D-Co @ NC composite lamellar structure becomes rough, more carbon nanotubes are liberated from the surface through the catalysis of metal particles, and as seen from the surface appearance, when the carbonization temperature is lower, the graphitization degree is lower, the metal particles begin to agglomerate continuously along with the increase of the carbonization temperature, and part of the metal particles form the carbon nanotubes under the catalysis of the metal particles. Because the higher the carbonization temperature, the more serious the agglomeration of the metal particles, the more mesopores and macropores are formed after the subsequent acid washing treatment, the hierarchical pore structure can improve the utilization efficiency of catalytic active sites, the mesopores and the macropores are favorable for the transmission of protons, and the too narrow pure micropore structure is not favorable for the transmission and the full exposure of the active sites.

In the preparation method of the 2D-Co @ NC composite material, the pickling treatment lotion in the step S2 is sulfuric acid, the pickling temperature is 75-85 ℃, and the pickling time is 5-6 hours. The present invention requires controlling the pickling temperature to 75-85 deg.c, which may cause the metal particles embedded in the carbon layer to be washed away.

Preferably, the concentration of sulfuric acid is 0.4-0.6M. The purpose of adding sulfuric acid is mainly to clean unstable metal particles, so that the sulfuric acid concentration cannot be too high, and the invention controls the sulfuric acid concentration to be 0.4-0.6M to achieve the best cleaning effect.

The invention also provides an electrode, and the 2D-Co @ NC composite material is loaded on the electrode.

The invention also provides a battery, which comprises the electrode.

The stability of the catalyst is a factor which must be considered in practical application, the 2D-Co @ NC composite material has good stability and is detected by a timing current method, and the current value can still keep the initial value after 50000s of test94% of the current value, whereas the commercial Pt/C catalyst can only maintain 71% of the initial current value under the same test conditions. The 2D-Co @ NC load is loaded on carbon cloth to serve as an air electrode, the open-circuit voltage of the assembled zinc-air battery is 1.47V, the open-circuit voltage is close to the theoretical open-circuit voltage of the zinc-air battery of 1.68V, and the power density of the battery can reach 170mW cm^-2The specific capacity can reach 766.35mA h g^-1Corresponding to an energy density of 934.93Whg^-1. And the voltage difference of charging and discharging of the battery is obviously changed after 600 cycles.

The 2D-Co @ NC composite material can also be applied to hydrogen production by electrolyzing water, the 2D-Co @ NC catalyst is loaded on carbon paper to be used as two electrodes, the total hydrolysis performance of the carbon paper is tested, and when the loading capacity is 1mg cm^-2In 1M KOH electrolyte, 10mA cm was obtained during the total hydrolysis^-2The overpotential of the current density is 1.70V, and the stability of the current density in the whole hydrolysis process is tested by adopting a chronoamperometry method, wherein the test current density is 20mA cm^-2The test time is 48h, the potential of the electrode does not change obviously, and the two electrodes which are subjected to full hydrolysis also have good stability.

Compared with the prior art, the invention has the following beneficial effects:

1. according to the preparation method, water is used as a solvent to synthesize a 2D-ZIF precursor, and then the 2D-Co @ NC composite material with the thickness of 1-10nm and the metal Co nano particles of 2-6nm distributed on a nitrogen-doped carbon nano sheet is obtained after high-temperature carbonization, so that the prepared 2D-Co @ NC composite material has the advantages of large specific surface area, hierarchical mesoporous structure, high nitrogen content, small metal particle size and uniform distribution;

2. the nitrogen-doped carbon in the 2D-Co @ NC composite material is used as a main active site of ORR reaction, and the simple substance Co and the nitrogen-doped carbon jointly promote the catalytic performance of OER and HER;

3. the zinc-air battery and the total hydrolysis electrolytic cell assembled by the 2D-Co @ NC composite material have good catalytic activity and stability.

Drawings

FIG. 1 is a schematic diagram of the preparation of 2D-Co @ NC composite material prepared in example 1.

FIG. 2 is a scanning electron microscope image of the precursor of the composite material in the preparation process of examples 1-3 and comparative examples 1-2: (a) comparative example 1; (b) example 2; (c) example 1; (d) example 3; (e) comparative example 2.

FIG. 3 is an X-ray diffraction pattern of a precursor of the composite material of examples 1 to 3 and comparative examples 1 to 2;

FIG. 4 is a structural representation of the 2D-Co @ NC composite prepared in example 1: (a) XRD patterns of example 1 and comparative example 2; (b-c) scanning electron micrographs of different magnifications; (d) atomic force microscopy images; (e) a transmission electron microscope image; (f) a high resolution transmission map; (g-h) distribution diagram of Co, C and N elements; (j) a selected area electron diffraction pattern.

FIG. 5 is an XRD pattern of composites of examples 1-5 and comparative example 2.

FIG. 6 is a scan from low power to high power of the composite material of examples 1-3: (a-c) example 2; (d-f) example 1; (g-i) example 3.

FIG. 7 is a scan from low power to high power of the composites of examples 1 and 4-5: (a-c) example 4; (d-f) example 1; (g-i) example 5.

Fig. 8 is a scan of the comparative example 2 composite from low power to high power.

FIG. 9 is a graph comparison of the composites of example 1 and comparative example 2: (a) XPS full spectrum; (b) a narrow spectrum of C1 s; (c) a narrow spectrum of N1 s; (d) narrow spectrum plot of Co 2 p; (e) a nitrogen adsorption and desorption curve; (f) pore size distribution curve.

FIG. 10 is a graph comparison of composites of examples 1-3: (a) XPS full spectrum; (b) a narrow spectrum of C1 s; (c) a narrow spectrum of N1 s; (d) narrow spectrum of Co 2 p.

FIG. 11 shows the nitrogen adsorption and desorption curves and the pore size distribution curves of the composites of examples 1 to 3.

FIG. 12 shows the nitrogen adsorption/desorption curves and the pore size distribution curves of the composites of examples 1 and 4 to 5.

FIG. 13 is an electrocatalytic performance curve of the composite of example 1 and comparative example 2 with Pt/C: (a) the LSV curve of ORR catalytic performance; (b) tafel plot; (c) ORR stability curve; (d) LSV curve of OER catalytic performance; (e) tafel plot; (f) OER stability curve; (g) an LSV curve of HER catalytic performance; (h) tafel plot; (i) HER stability curve.

FIG. 14 is a graph of the electrocatalytic performance of the composites of examples 1, 2-3: (a) the LSV curve of ORR catalytic performance; (b) tafel plot; (c) LSV curve of OER catalytic performance; (d) tafel plot; (e) an LSV curve of HER catalytic performance; (f) tafel curve.

FIG. 15 is a graph of the electrocatalytic performance of the composites of examples 1, 4-5: (a) the LSV curve of ORR catalytic performance; (b) tafel plot; (c) LSV curve of OER catalytic performance; (d) tafel plot; (e) an LSV curve of HER catalytic performance; (f) tafel curve.

Fig. 16 is a set of drawings showing the application of the composite material of example 1 in a cell, (a) a macroscopic photograph of an illuminated display panel of an assembled zinc-air cell; (b) the open circuit voltage of the zinc-air battery is assembled by respectively using the composite material and Pt/C of the embodiment 1 as air electrode catalysts; (c) corresponding to the specific capacity curve; (d) corresponding to a power density curve; (e) the zinc-air battery is at 10mA cm^-2A lower charge-discharge curve; (f) co @ NCL// Co @ NCL, Pt/C// RuO₂Making LSV curve corresponding to two electrodes for total hydrolysis and at 20mA cm^-2The lower corresponding stability curve; (g) macroscopic photos of the two electrodes are fully hydrolyzed; (h) the macroscopic picture of the bubbles generated on the two electrodes is fully hydrolyzed.

Detailed Description

The following are specific examples of the present invention and further describe the technical solutions of the present invention, but the present invention is not limited to these examples.

Example 1:

582mg of Co (NO)₃·6H₂Dissolving O in 300mL of ultrapure water to form solution A, dissolving 4290mg of dimethyl imidazole in 30mL of ultrapure water to form solution B, adding the solution B into the solution A, continuously stirring for 24 hours, centrifuging the solution to obtain purple precipitate, cleaning the purple precipitate with deionized water for three times, freeze-drying to obtain a 2D-ZIF precursor, putting the precursor into a crucible, putting the crucible into a tube furnace, and carrying out argon atmosphere at 5 ℃ for min^-1Heating to 700 deg.C, maintaining at 700 deg.C for 2 hr, cooling in furnace to obtain black powder, and mixing with 0.5M H₂SO₄At 80 deg.CAnd (3) treating for 6h, then filtering, repeatedly washing with deionized water until the solution is neutral, and drying the washed sample in an oven at 60 ℃ for 12h to obtain the 2D-Co @ NC composite material.

Example 2:

582mg of Co (NO)₃·6H₂Dissolving O in 300mL of ultrapure water to form solution A, dissolving 3280mg of dimethyl imidazole in 30mL of ultrapure water to form solution B, adding the solution B into the solution A, continuously stirring for 24 hours, centrifuging the solution to obtain purple precipitate, cleaning the purple precipitate with deionized water for three times, freeze-drying to obtain a 2D-ZIF precursor, putting the precursor into a crucible, putting the crucible into a tube furnace, and carrying out argon atmosphere at 5 ℃ for min^-1Heating to 700 deg.C, maintaining at 700 deg.C for 2 hr, cooling in furnace to obtain black powder, and mixing with 0.5M H₂SO₄The mixture is treated at 80 ℃ for 6h, then filtered and repeatedly washed by deionized water until the solution is neutral, and the washed sample is dried in an oven at 60 ℃ for 12h to obtain the 2D-Co @ NC composite material.

Example 3:

582mg of Co (NO)₃·6H₂Dissolving O in 300mL of ultrapure water to form solution A, dissolving 6560mg of dimethyl imidazole in 30mL of ultrapure water to form solution B, adding the solution B into the solution A, continuously stirring for 24 hours, centrifuging the solution to obtain purple precipitate, cleaning the purple precipitate with deionized water for three times, freeze-drying to obtain a 2D-ZIF precursor, putting the precursor into a crucible, putting the crucible into a tube furnace, and carrying out argon atmosphere at 5 ℃ for min^-1Heating to 700 deg.C, maintaining at 700 deg.C for 2 hr, cooling in furnace to obtain black powder, and mixing with 0.5M H₂SO₄The mixture is treated at 80 ℃ for 6h, then filtered and repeatedly washed by deionized water until the solution is neutral, and the washed sample is dried in an oven at 60 ℃ for 12h to obtain the 2D-Co @ NC composite material.

Example 4:

the only difference from example 1 is that the carbonization temperature in the preparation process of example 4 is 600 ℃.

Example 5:

the only difference from example 1 is that the carbonization temperature in the preparation process of example 5 is 800 ℃.

Comparative example 1:

582mg of Co (NO)₃·6H₂Dissolving O in 300mL of ultrapure water to form solution A, dissolving 2640mg of dimethylimidazole in 30mL of ultrapure water to form solution B, adding the solution B into the solution A, continuously stirring for 24 hours, centrifuging the solution to obtain purple precipitate, cleaning the purple precipitate with deionized water for three times, freeze-drying to obtain a precursor, putting the precursor into a crucible, putting the crucible into a tube furnace, and keeping the temperature of the crucible for 5 ℃ for min under the argon atmosphere^-1Heating to 700 deg.C, maintaining at 700 deg.C for 2 hr, cooling in furnace to obtain black powder, and mixing with 0.5M H₂SO₄Acid washing at 80 deg.C for 6h, and the black powder was found to be completely dissolved during acid washing, failing to obtain the final product.

Comparative example 2:

582mg of Co (NO)₃·6H₂Dissolving O in 300mL of methanol to form solution A, dissolving 4290mg of dimethylimidazole in 30mL of methanol to form solution B, adding the solution B into the solution A, continuously stirring for 24 hours, centrifuging the solution to obtain purple precipitate, washing with deionized water for three times, freeze-drying to obtain a 3D-ZIF precursor, putting the precursor into a crucible, putting the crucible into a tubular furnace, and performing argon atmosphere at 5 ℃ for 5 min^-1Heating to 700 deg.C, maintaining at 700 deg.C for 2 hr, cooling in furnace to obtain black powder, and mixing with 0.5M H₂SO₄The mixture was treated at 80 ℃ for 6h, then filtered and washed repeatedly with deionized water until the solution was neutral, and dried in an oven at 60 ℃ for 12h to give a 3D-Co/NC composite.

Application example 1:

the composite material prepared in the example 1 is loaded on carbon cloth to be used as an air electrode, a zinc sheet is used as a positive electrode, 6M KOH is used as electrolyte, and a zinc-air battery is assembled to be tested for performance, wherein the test is carried out in a normal atmospheric environment. Wherein the electrolyte of the secondary zinc-air battery is 6M KOH +0.2M zinc acetate.

Application comparative example 1:

a conventional Pt/C catalyst is loaded on carbon cloth to serve as an air electrode, a zinc sheet serves as an anode, 6M KOH serves as electrolyte, and a zinc-air battery is assembled to test the performance and is tested under a normal atmospheric environment.

Table 1: component contents of composite materials obtained in examples 1 to 3 and comparative examples 2 to 4

Examples	O(％)	N(％)	Co(％)	C(％)
					Example 1	7.43	10.06	1.88	Balance of
Example 2	6.72	10.19	2.06	Balance of
					Example 3	5.95	9.85	1.67	Balance of
Example 4	7.79	16.26	2.25	Balance of
					Example 5	4.77	6.11	1.07	Balance of
Comparative example 1	/	/	/	/
					Comparative example 2	5.85	9.62	1.79	Balance of

Table 2: physical Properties of composites of examples 1-3 and comparative examples 2-4

Examples	Surface area (m)²/g)	Pore volume (cm)³/g)	Pore size (nm)
				Example 1	423.04	0.19	5.49
Example 2	392.41	0.15	4.67
				Example 3	422.55	0.18	5.03
Example 4	348.1	0.10	4.85
				Example 5	427.47	0.37	6.17
Comparative example 1	/	/	/
				Comparative example 2	349.39	0.12	4.98

Table 3: results of ORR, OER and HER catalytic performance tests of composite materials of examples 1-5 and comparative example 2 and Pt/C

Table 4: performance test results of zinc-air battery using application example 1 and application comparative example 1

FIG. 1 is a schematic diagram of a process for preparing a 2D-Co @ NC composite material in example 1, and it can be seen that the 2D-Co @ NC composite material with metal Co nanoparticles distributed on N-doped carbon nano-sheets is obtained by synthesizing a 2D-ZIF precursor with water as a solvent and then carbonizing at a high temperature.

FIG. 2 is a scanning electron microscope image of a precursor of a composite material in the preparation processes of examples 1-3 and comparative examples 1-2, (a) comparative example 1; (b) example 2; (c) example 1; (d) example 3; (e) comparative example 2. From the figure, it can be known that the 2D-ZIF synthesized by using water as a solvent has an irregular lamellar structure, and the polyhedral structure of a typical 3D-ZIF synthesized by using methanol as a solvent; and with the increase of the proportion of dimethyl imidazole in the precursor, the color and the appearance of the synthesized precursor are obviously changed: the precursor obtained in comparative example 1 has a polygonal lamellar structure, the morphology of the precursor is greatly different from that of examples 1-3, and examples 1-3 have irregular lamellar structures.

FIG. 3 is an X-ray diffraction pattern of a precursor of the composite materials of examples 1 to 3 and comparative examples 1 to 2; as can be seen from the figure, the composite material of comparative example 1 has the same obvious difference with the composite materials of examples 1-3, and only Co (OH) appears in the composite material of comparative example 1₂Peaks of (2 θ) ═ 19.5 ° and 37.91 °, respectively corresponding to Co (OH)₂The (001) and (101) crystal planes of (A) indicate that Co is used in comparative example 1²⁺And does not form a stable structure with dimethylimidazole. While the diffraction peaks of XRD of example 1, example 2 and example 3 are the same, the XRD diffraction peak of 3D-ZIF of comparative example 2 is substantially identical to the diffraction fitted to ZIF-67, confirming that the synthesized 3D-ZIF is typical of ZIF-67.

FIG. 4 is a structural representation of the 2D-Co/NC composite prepared in example 1, (a) XRD pattern; (b-c) scanning electron micrographs of different magnifications; (d) atomic force microscopy images; (e) a transmission electron microscope image; (f) a high resolution transmission map; (g-h) distribution diagram of Co, C and N elements; (j) a selected area electron diffraction pattern. As can be seen from the graph a, XRD derivative peaks of the carbonized material correspond to a metallic cobalt simple substance, the morphology graphs of b-c show that 2D-Co @ NC is a typical lamellar structure, and the thickness of the material is about 3nm as can be seen from a D atomic force microscope graph; transmission electron microscopy images from e show that the metal particles are uniformly distributed on the carbon nanoplatelets and that the average size of the nanoparticles is about 4.57 nm; from the high-resolution transmission diagram of the graph f, the interplanar spacing of the metal particles is 0.204nm, and the energy spectrum analysis of a transmission electron microscope shows that C, N, O and Co four elements exist in 2D-Co @ NC, which corresponds to the (111) crystal plane of the metal Co atom; from the graphs g-i, it can be seen that the metal element is not only distributed in the metal particle region, but the distribution of the metal Co element exists in the whole region, and the N element is also distributed in the whole region, so that Co can be combined with N to form the electrocatalytic active sites of Co-Nx.

FIG. 5 is an XRD pattern of the composite materials of examples 1 to 5 and comparative example 2, and it can be seen from FIG. a that there is a significant difference in XRD between 3D-ZIF and 2D-ZIF before carbonization, and phases of 3D-Co @ NC and 2D-Co @ NC formed after carbonization are the same. The 2 θ ═ 26.3 ° corresponds to a graphite carbon diffraction peak, and the 2 θ ═ 44.2 °, 51.5 °, and 75.8 ° correspond to the (111), (200), and (002) crystal planes of the simple metal Co. The XRD test result is consistent with the high resolution and electron diffraction result of a transmission electron microscope, and compared with the composite material of the example 2 and the composite material of the example 1, the XRD diffraction peak of the composite material of the comparative example 2 is relatively weaker and is equivalent to that of the example 3. The carbonization temperature required for the lamellar state was lower than that for the granular material, and the crystallinity of metallic Co of the composite material of comparative example 2 was lower at the same carbonization temperature. The carbonization temperature has a great influence on the size and crystallinity of the metallic Co particles. As can be seen from fig. b, the diffraction peaks corresponding to carbon and metallic cobalt gradually increase as the temperature changes from 600 c to 800 c. The carbonization temperature is increased, the crystallinity of the material is enhanced, the electrical conductivity is increased, and the electrocatalytic reaction is promoted, but the content of metal Co and nitrogen is reduced due to the increase of the carbonization temperature.

FIG. 6 is a scan from low power to high power of the composite material of examples 1-3, (a-c) example 2; (d-f) example 1; (g-i) example 3. It can be seen from the figure that the SEM morphology of 2D-Co @ NC before and after carbonization has not changed significantly, and is also a typical lamellar structure.

FIG. 7 is a scan from low power to high power of the composite of examples 1, 4-5, (a-c) example 4; (d-f) example 1; (g-i) example 5. As can be seen from the figure, the carbonization temperature has a remarkable influence on the appearance of the lamellar structure, when the carbonization temperature is lower, the surface of the formed 2D-Co @ NC nanosheet is smoother, and when the carbonization temperature is increased, the surface of the formed 2D-Co @ NC lamellar structure becomes rough, and the more carbon nanotubes are catalytically dissociated from the surface of the 2D-Co @ NC lamellar structure by metal particles. As seen from the surface appearance, when the carbonization temperature is lower, the graphitization degree is lower, and with the increase of the carbonization temperature, the metal particles start to agglomerate continuously, and part of the metal particles form the carbon nano tube under the catalytic action.

Fig. 8 is a scan of the comparative example 2 composite from low power to high power. It is understood from the figure that the 3D-ZIF of comparative example 2 becomes a typical dodecahedron structure after high temperature carbonization and subsequent acid washing treatment.

In FIG. 9, a-d are X-ray photoelectron spectra of the composite material of example 1 and comparative example 2, (a) XPS survey spectrum; (b) a narrow spectrum of C1 s; (c) a narrow spectrum of N1 s; (d) narrow spectrum plot of Co 2 p; as can be seen from the figure, 2D-Co @ NC and 3D-Co @ NC both contain four elements of Co, C, N and O. There is no significant difference in the narrow spectrum of C1s and N1 s. While the narrow spectrum of Co 2p shows that Co is present in 2D-Co @ NC²⁺Is significantly higher than 3D-Co @ NC, the upper metal particles originating from the lamellar structure are more easily oxidized. As can be seen from FIG. 9(e) pore size distribution curve and (f) nitrogen adsorption/desorption curve, 2D-Co @ NC and 3D-Co @ NC are typical mesoporous materials, and 2D-Co @ NC is a typical mesoporous materialThe specific surface area is greater than 3D-Co @ NC.

FIG. 10 is a spectrum comparison of composites of examples 1-3, (a) XPS survey spectrum; (b) a narrow spectrum of C1 s; (c) a narrow spectrum of N1 s; (d) narrow spectrum of Co 2 p. It can be seen from the figure that the elemental compositions of examples 1-3 are the same and are C, N, O, Co, but the elemental contents and the peaks in the fine spectra of the elements are different.

FIG. 11 shows the nitrogen adsorption and desorption curves and the pore size distribution curves of the composites of examples 1 to 3. It can be seen from the figures that the composite materials of examples 1-3 are typical mesoporous materials.

FIG. 12 shows the nitrogen adsorption/desorption curves and the pore size distribution curves of the composites of examples 1 and 4 to 5. It can be seen from the figure that the composite materials of examples 1 and 4-5 are typical mesoporous materials and have large specific surface areas.

FIG. 13 is a graph of electrocatalytic performance of example 1, comparative example 2 and Pt/C, (a) LSV curve of ORR catalytic performance; (b) tafel plot; (c) ORR stability curve; (d) LSV curve of OER catalytic performance; (e) tafel plot; (f) OER stability curve; (g) an LSV curve of HER catalytic performance; (h) tafel plot; (i) HER stability curve. It can be seen from the figure that the ORR and OER catalytic activities and stabilities of the catalysts obtained in example 1 are superior to those of commercial Pt/C and RuO, respectively₂Its HER catalytic activity was inferior to commercial Pt/C, but superior to the composite of comparative example 2.

FIG. 14 is a graph of the electrocatalytic performance of the composites of examples 1, 2-3, (a) LSV curve of ORR catalytic performance; (b) tafel plot; (c) LSV curve of OER catalytic performance; (d) tafel plot; (e) an LSV curve of HER catalytic performance; (f) tafel curve. As can be seen from the figure, Co in the precursor²⁺Different proportions of the precursor and the dimethyl imidazole have certain influence on ORR, OER and HER catalytic performances, the ORR and OER catalytic performances are reduced along with the increase of the content of the dimethyl imidazole in the precursor, and HER performance rules show differences. At the same carbonization temperature, the content of dimethyl imidazole is increased, the content of cobalt and nitrogen in the composite material of the example 3 is reduced, and ORR and OER catalytic performance is reduced, however, compared with the composite material of the example 2, HER catalytic performance is increased,due to its high pyridine nitrogen content.

FIG. 15 is a graph of electrocatalytic performance of examples 1, 4-5, (a) LSV curve of ORR catalytic performance; (b) tafel plot; (c) LSV curve of OER catalytic performance; (d) tafel plot; (e) an LSV curve of HER catalytic performance; (f) tafel curve. It can be seen from the figure that the catalytic performance of the catalyst is poor when the carbonization temperature is low, and the performance of the catalyst is not significantly increased but rather decreased when the carbonization temperature is changed from 700 ℃ to 800 ℃, because the electrical conductivity of the prepared composite material is poor when the carbonization temperature is low, and the content of active sites in the catalyst is reduced when the carbonization temperature is too high, so that the carbonization temperature is too low or too high, which is not good for the catalytic performance of the catalyst.

Fig. 16 is a set of drawings showing the application of the composite of example 1 in a cell, (a) a macroscopic photograph of a lighted display panel of an assembled zinc-air cell; (b) the composite material and Pt/C in the embodiment 1 are respectively used as air electrode catalysts to assemble the open-circuit voltage of the zinc-air battery; (c) corresponding to the specific capacity curve; (d) corresponding to a power density curve; (e) the zinc-air battery is at 10mA cm^-2A lower charge-discharge curve; (f) co @ NCL// Co @ NCL, Pt/C// RuO₂Making LSV curve corresponding to two electrodes for total hydrolysis and at 20mA cm^-2The lower corresponding stability curve; (g) macroscopic photos of the two electrodes are fully hydrolyzed; (h) the macroscopic picture of the bubbles generated on the two electrodes is fully hydrolyzed. It can be seen from the figure that the zinc-air battery assembled with the catalyst using the composite material of example 1 as the air electrode can drive a display screen with about 1.5V, and the primary zinc-air battery assembled with the composite material of example 1 has better performance than the commercial Pt/C catalyst primary zinc-air battery, and the secondary zinc-air battery assembled with the composite material of example 1 has 10mA cm^-2Can maintain the charge-discharge cycle stability of 200h at the current density of (2). Both electrodes were fully hydrolyzed with the composite material of example 1, which was brought to 10mA cm^-2The required potential for the current density of (1.7V) at 20mA cm^-2No significant decay in performance occurred at 48h of the lower test. Two zinc-air batteries connected in series can drive full hydrolysis. The performance test results of the devices show that the composite material in the example 1 has the advantages in the fields of zinc-air batteries and total hydrolysisHas good application prospect.

In conclusion, the 2D-Co @ NC composite material with the thickness of 1-10nm and the metal Co nano particles of 2-6nm distributed on the nitrogen-doped carbon nano sheet is obtained by using water as a solvent to synthesize the 2D-ZIF precursor and then carbonizing at high temperature, and the prepared 2D-Co @ NC composite material has the advantages of large specific surface area, hierarchical mesoporous structure, high nitrogen content, small metal particle size and uniform distribution; the nitrogen-doped carbon in the composite material is used as a main active site of ORR reaction, and the simple substance Co and the nitrogen-doped carbon jointly promote the catalytic performance of OER and HER; the zinc-air battery and the total hydrolysis electrolytic cell assembled by the 2D-Co @ NC composite material have good catalytic activity and stability.

The technical scope of the invention claimed by the embodiments of the present application is not exhaustive, and new technical solutions formed by equivalent replacement of single or multiple technical features in the technical solutions of the embodiments are also within the scope of the invention claimed by the present application; in all the embodiments of the present invention, which are listed or not listed, each parameter in the same embodiment only represents an example (i.e., a feasible embodiment) of the technical solution, and there is no strict matching and limiting relationship between the parameters, wherein the parameters may be replaced with each other without departing from the axiom and the requirements of the present invention, unless otherwise specified.

The technical means disclosed by the scheme of the invention are not limited to the technical means disclosed by the technical means, and the technical scheme also comprises the technical scheme formed by any combination of the technical characteristics. While the foregoing is directed to embodiments of the present invention, it will be appreciated by those skilled in the art that various changes may be made in the embodiments without departing from the principles of the invention, and that such changes and modifications are intended to be included within the scope of the invention.

The specific embodiments described herein are merely illustrative of the spirit of the invention. Various modifications or additions may be made to the described embodiments or alternatives may be employed by those skilled in the art without departing from the spirit or ambit of the invention as defined in the appended claims.

Claims

1. The 2D-Co @ NC composite material is characterized in that the composite material is of a nano-sheet structure, and metal Co nano-particles are distributed on a nitrogen-doped carbon material of a sheet layer.

2. The 2D-Co @ NC composite material according to claim 1, wherein the thickness of the nanosheet layer is 1-10nm, and the metallic Co nanoparticles have a particle size of 2-6 nm.

3. A 2D-Co @ NC composite material as claimed in claim 1 or 2, wherein the composite material comprises the following components in percentage by mass: 1.65-2.1% of Co, 5.5-7.5% of O, 9.8-10.5% of N and the balance of C element.

4. The 2D-Co @ NC composite material as claimed in claim 3, wherein the composite material has a surface area of 390-425m²Per g, pore volume of 0.15-0.2cm³G, the pore diameter is 4.5-5.5 nm.

5. A method of making the 2D-Co @ NC composite of claim 1, comprising the steps of:

6. The method for preparing a 2D-Co @ NC composite material according to claim 5, wherein the step S1Co (NO)₃·6H₂The mass ratio of O to dimethyl imidazole is 1: (5.6-11.5).

7. The method as claimed in claim 5, wherein the carbonization step S2 is performed by raising the temperature to 650-750 ℃ at a rate of 4-6 ℃/min, and then maintaining the temperature for 1.5-2.5 h.

8. The method for preparing a 2D-Co @ NC composite material according to claim 5, wherein the pickling treatment washing liquid of step S2 is sulfuric acid, the pickling temperature is 75-85 ℃, and the pickling time is 5-6 h.

9. An electrode comprising the 2D-Co @ NC composite material of claim 1 supported thereon.

10. A battery comprising the electrode of claim 9.