CN112652778A

CN112652778A - Graphene-loaded nitrogen-doped carbon nanotube composite material and preparation and application thereof

Info

Publication number: CN112652778A
Application number: CN201910957679.6A
Authority: CN
Inventors: 夏宝玉; 徐洋洋
Original assignee: Huazhong University of Science and Technology
Current assignee: Huazhong University of Science and Technology
Priority date: 2019-10-10
Filing date: 2019-10-10
Publication date: 2021-04-13
Anticipated expiration: 2039-10-10
Also published as: CN112652778B

Abstract

The invention belongs to the field of energy storage nano materials, and particularly relates to a graphene loaded nitrogen-doped carbon nanotube composite material and preparation and application thereof. Firstly, growing a metal organic framework material on a graphene oxide sheet layer, promoting protonation of an organic ligand by using a nucleation promoter so as to accelerate nucleation and dispersion of the metal organic framework on the graphene oxide to obtain a precursor of the metal organic framework uniformly loaded by the graphene oxide, and then annealing at high temperature to obtain the graphene nitrogen-loaded carbon nanotube material. When the material is integrated into an air electrode and applied to a zinc-air battery, the concentration is 5mA cm^‑2And the ultrahigh stability performance of 3000h can be realized under the charging and discharging conditions.

Description

Graphene-loaded nitrogen-doped carbon nanotube composite material and preparation and application thereof

Technical Field

The invention belongs to the field of energy storage nano materials, and particularly relates to a graphene loaded nitrogen-doped carbon nanotube composite material and preparation and application thereof.

Background

The rechargeable zinc-air battery has the advantages of low cost, high voltage, high theoretical energy density, environmental protection and the like, and is considered to be one of the most promising batteries. However, their poor energy conversion efficiency and low lifetime are major bottlenecks limiting their widespread use, these drawbacks mainly resulting from the inherently slow kinetics of the oxygen reduction (ORR) and Oxygen Evolution (OER) reactions, as well as their limited stability in harsh alkaline electrolytes. In order to solve the above problems, much effort has been put into exploring a bifunctional oxygen electrocatalyst with high activity and durability. Although noble metal catalysts, such as platinum (Pt), ruthenium (Ru), iridium (Ir) and their alloys, have shown promising electrocatalytic properties, their scarcity, high cost, catalytic dual functionality and poor stability greatly limit their widespread use. Therefore, in order to realize commercialization of a rechargeable zinc-air battery, the development of a non-noble metal catalyst having excellent bifunctional properties and high stability is a key content and important challenge therein.

Carbonaceous materials (e.g., graphene, carbon nanotubes, and porous carbon) have proven to be promising oxygen electrocatalysts for air cathodes due to large surface area, high electrical conductivity, and good electrochemical stability, among others. In particular, doping carbon materials with heteroatoms (e.g., N, S and P) alters the electronic structure and significantly improves ORR and OER activity. The Metal Organic Frameworks (MOFs) are formed by clusters bridged by certain metal ions and organic functional groups, and proved to be excellent precursors for preparing nitrogen-doped nano carbon materials, the MOFs and Graphene Oxide (GO) are combined, and the derived carbon composite has more active sites, specific surface area and the like compared with a powdery material (nano carbon materials derived from pure MOFs), and shows enhanced electrochemical performance. Therefore, these carbon composites exhibit superior performance when used in oxygen electrochemical reactions. However, there are few documents reporting that these materials are used in a rechargeable zinc-air battery as a bifunctional oxygen catalyst, and that these materials do not exhibit superior cycle stability performance despite the use of some materials in a zinc-air battery.

Disclosure of Invention

Aiming at the defects or the improvement requirements of the prior art, the invention provides a graphene-loaded nitrogen-doped carbon nanotube composite material and preparation and application thereof, wherein the graphene and the nitrogen-doped carbon nanotube are connected through in-situ growth to obtain a large-sheet continuous two-dimensional layered multi-level structure composite material, and when the composite material is used as an electrode active material of a rechargeable zinc-air battery, the composite material has an ultra-long (as long as 3000 hours) battery life, so that the technical problem of poor cycling stability when the existing graphene-loaded nitrogen-doped carbon nanotube composite material is used as the electrode active material of the battery is solved.

In order to achieve the above object, according to an aspect of the present invention, there is provided a method for preparing a graphene-supported nitrogen-doped carbon nanotube composite material, including the steps of:

(1) mixing and stirring a graphene oxide solution, an organic solution of transition metal salt and an organic solution of organic ligand for several hours, and carrying out solid-liquid separation to obtain a solid which is a precursor of a graphene oxide loaded metal organic framework material; the organic solution of the organic ligand also comprises a nucleation promoter, and the nucleation promoter is used for promoting crystallization nucleation and uniform dispersion of the metal organic framework material on the surface of the graphene oxide;

(2) annealing the precursor obtained in the step (1) in a reducing atmosphere to convert transition metal ions in the precursor into metal nano-particles and catalyze an organic ligand to convert into a nitrogen-doped carbon nano-tube, so as to obtain annealed solid powder;

(3) and (3) washing the annealed solid powder obtained in the step (2) in acid liquor to remove unstable transition metal nanoparticles, and performing solid-liquid separation and drying to obtain the graphene nitrogen-loaded carbon nanotube composite material.

Preferably, the nucleation promoter is an amine-based substance soluble in an organic solvent.

Preferably, the nucleation promoter is one or more of ethylamine, diethylamine, triethylamine, ethylenediamine and benzyltriethylammonium chloride.

Preferably, the graphene oxide solution is obtained by dissolving a graphene oxide initial solution obtained by oxidation stripping by a Hummer method in an organic solvent, the concentration of the graphene oxide initial solution is 6-15g/L, and the graphene oxide initial solution and the organic solvent are mixed according to a volume ratio of 1:4-1:8 to obtain the graphene oxide solution;

the transition metal salt is iron metal salt, nickel metal salt or cobalt metal salt; it is nitrate, acetate or hydrochloride; the concentration of the transition metal salt in the organic solution of the transition metal salt is 0.03-0.08 mol/L; the organic ligand is trimesic acid and/or 2-methylimidazole; the molar ratio of the organic ligand to the transition metal salt is 1:2 to 1: 32; the volume ratio of the nucleation promoter to the organic solution of the organic ligand is 1:1000-1: 4000.

Preferably, a step of strengthening load is further included between the step (1) and the step (2): and re-dissolving the obtained precursor of the graphene oxide loaded metal organic framework material in an organic solvent, mixing and stirring the precursor with the organic solution of the transition metal salt, the organic solution of the organic ligand and the nucleation promoter for several hours, and performing solid-liquid separation to obtain a solid, wherein the solid is the precursor of the graphene oxide loaded metal organic framework material.

Preferably, the reinforcing loading step is repeated three to five times.

Preferably, the mixing and stirring time of the step (1) is 2 to 6 hours.

Preferably, the annealing treatment in the step (2) adopts a mixed atmosphere of hydrogen and inert gas, wherein the volume fraction of the hydrogen is 2-10%; the annealing temperature is 600-900 ℃, the annealing time is 1-6 hours, and the heating rate is 1-10 ℃/min.

According to another aspect of the invention, the graphene loaded nitrogen doped carbon nanotube composite material prepared by the preparation method is provided.

According to another aspect of the invention, the application of the graphene-supported nitrogen-doped carbon nanotube composite material is provided, and the graphene-supported nitrogen-doped carbon nanotube composite material is used as a positive active material of a zinc-air battery.

According to another aspect of the invention, a zinc-air battery is provided, which comprises a positive electrode material, wherein the positive electrode material comprises the graphene-supported nitrogen-doped carbon nanotube composite material.

In general, compared with the prior art, the above technical solution contemplated by the present invention can achieve the following beneficial effects:

(1) the invention provides a preparation method of a graphene loaded nitrogen-doped carbon nanotube composite material with a two-dimensional layered multi-stage structure. And finally, obtaining a series of graphene loaded nitrogen-doped carbon nanotube composite materials through high-temperature pyrolysis, and realizing the construction of a two-dimensional layered multilevel structure consisting of carbon nanotubes.

(2) According to the invention, the crystallization amount of the metal organic framework material on the surface of the graphene oxide is increased by controlling the appropriate concentration of each raw material solution and repeating the crystallization nucleation and dispersion steps of the metal organic framework material on the surface of the graphene oxide for multiple times, so that the amount of the carbon nano tubes dispersed on the surface of GO during high-temperature annealing is increased.

(3) The prepared graphene loaded nitrogen-doped carbon nanotube composite material with the two-dimensional layered multi-level structure is used as a bifunctional oxygen electrocatalyst and applied to a rechargeable zinc-air battery. The graphene loaded nitrogen-doped carbon nanotube with the two-dimensional layered multi-level structure shows ultrahigh cycle stability when applied to a rechargeable zinc-air battery.

(4) The graphene-loaded nitrogen-doped carbon nanotube composite material has a large specific surface area, and an in-situ composite structure formed by graphene and the nitrogen-doped carbon nanotube can improve the overall conductivity and stability of the catalyst, shows excellent catalytic performance in both oxygen reduction and oxygen precipitation reactions in an alkaline solution, and is a bifunctional oxygen electrocatalyst. The zinc-air battery assembled based on the catalyst shows higher stability than a commercial noble metal (platinum carbon + iridium dioxide) combined catalyst, for example, the charging and discharging times can reach 3000h and 9000 cycles in a preferred embodiment, so that the graphene-supported nitrogen-doped carbon nanotube composite catalyst provided by the invention can be used as a high-efficiency oxygen electrode material to be applied to the zinc-air battery.

Drawings

Fig. 1(a) is a flowchart of a preparation method of a graphene-supported nitrogen-doped carbon nanotube catalyst provided in embodiments 1 to 2 of the present invention.

FIGS. 1(b-c) are SEM images of ZIF-67@ GO-4 as provided in example 1 of the present invention.

FIG. 1(d) is an SEM image of GNCNTs-4 provided in example 2 of the present invention.

FIG. 1(e-g) is a TEM image of GNCNTs-4 provided in example 2 of the present invention.

FIG. 1(h-k) is a mapping representation of GNCNTs-4 provided in example 2 of the present invention.

FIGS. 2(a-d) are SEM and TEM images of GNCNTs-1 provided in comparative example 1 of the present invention.

FIGS. 3(a-d) are SEM and TEM images of GNCNTs-8 provided in comparative example 2 of the present invention.

FIGS. 4(a-d) are SEM and TEM images of ZIF-67 and NCNTs provided in comparative example 3 of the present invention.

Figure 5 is an XRD pattern of GNCNTs-4 provided in example 3 of the present invention.

FIG. 6 is a BET plot of GNCNTs-4 provided in example 3 of the present invention.

Fig. 7 is a schematic graph of linear sweep voltammetry curves obtained from ORR performance of graphene-supported nitrogen-doped carbon nanotubes in 0.1M KOH alkaline electrolyte according to example 4 of the present invention. .

Fig. 8 is a graph illustrating linear sweep voltammetry curves obtained from the OER performance of graphene-supported nitrogen-doped carbon nanotubes in 1.0M KOH alkaline electrolyte according to example 4 of the present invention. .

FIG. 9 shows GNCNTs-4 and comparative materials (Pt/C and IrO) provided in example 5 of the present invention₂Mixture of (d) as a positive electrode material for liquid zinc-air cells.

FIG. 10 is a graph of GNCNTs-4 provided in example 5 of the present invention as a positive electrode material for liquid zinc-air battery for testing charge and discharge cycle stability, and comparative materials are Pt/C and IrO₂A mixture of (a); the test current density is 5mA cm^-2The charging is 10min, the discharging is 10min, and the arrow points to the charging and discharging cycle curve of each electrode loaded with different materials.

FIG. 11 is a drawing of the present inventionGNCNTs-4 and comparative materials (Pt/C and IrO) provided in example 6₂Mixture of (d) as a positive electrode material for a solid state zinc-air cell.

FIG. 12 is a graph of GNCNTs-4 provided in example 6 of the present invention as a positive electrode material for a solid zinc-air battery, and comparing materials Pt/C and IrO₂A mixture of (a); the test current density is 1mA cm^-2The charging is 10min, the discharging is 10min, and the arrow points to the charging and discharging cycle curve of each electrode loaded with different materials.

Fig. 13 is an assembly process diagram of a liquid zinc-air cell.

Detailed Description

In order to make the objects, technical solutions and advantages of the present invention more apparent, the present invention is described in further detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention. In addition, the technical features involved in the embodiments of the present invention described below may be combined with each other as long as they do not conflict with each other.

The invention provides a preparation method of a graphene loaded nitrogen-doped carbon nanotube composite material, which is characterized by comprising the following steps of:

(3) and (3) washing the annealed solid powder obtained in the step (2) in acid liquor, removing unstable transition metal nanoparticles, carrying out solid-liquid separation and drying to obtain the graphene loaded nitrogen doped carbon nanotube composite material.

The nucleation promoter is used for promoting the crystallization nucleation and uniform dispersion of the metal organic framework material on the surface of the graphene oxide, and plays a significant role in improving the catalytic performance of the finally prepared composite material for the active material of the zinc-air battery electrode. The nucleation promoter must be mixed with the organic ligand prior to mixing with the graphene oxide solution and the transition metal salt solution, because the nucleation promoter promotes the rapid nucleation of the metal organic framework material, i.e., the MOF material, by promoting the protonation of the organic ligand. The nucleation accelerant promotes the metal organic framework material to rapidly nucleate on the surface of the graphene oxide sheet layer, and the metal organic framework material is uniformly dispersed and continuous. Experiments show that if the nucleation accelerant is not added, the metal organic framework material in the prepared precursor has larger grains and discontinuous dispersion. The performance of the finally obtained composite material is not good when the composite material is used as an electrode material.

The nucleation accelerant is an amine substance which can be dissolved in an organic solvent, and preferably one or more of ethylamine, diethylamine, triethylamine, ethylenediamine and benzyltriethylammonium chloride.

The graphene oxide is used as a substrate, and is prepared by an improved hummers method. The present invention is not particularly limited to the specific method for modifying the hummers method, and any method known to those skilled in the art may be used. The graphene oxide solution is obtained by dissolving a graphene oxide initial solution obtained by oxidation stripping by a Hummer method in an organic solvent, wherein the concentration of the graphene oxide initial solution is 6-15g/L, and the graphene oxide initial solution and the organic solvent are mixed according to the volume ratio of 1:4-1:8 to obtain the graphene oxide solution;

the transition metal salt is iron metal salt, nickel metal salt or cobalt metal salt; it is nitrate, acetate or hydrochloride; the concentration of the transition metal salt in the organic solution of the transition metal salt is 0.03-0.08 mol/L; the organic ligand is 2-methylimidazole, trimesic acid or the like; the molar ratio of the organic ligand to the transition metal salt is 1:2 to 1: 32.

The organic solvent can be methanol, ethanol, isopropanol, or a mixed solution of methanol and ethanol in any proportion.

The volume ratio of the organic solution of the organic ligand in the addition amount of the nucleation promoter is 1:1000-1: 4000.

The mixing sequence of the graphene oxide solution, the organic solution of the transition metal salt and the organic solution of the organic ligand is not particularly limited, but the nucleation promoter needs to be added into the organic ligand solution in advance in order to accelerate the nucleation and dispersion of the metal-organic framework on the graphene oxide by promoting the protonation of the organic ligand.

In order to increase the amount of the metal-organic framework material loaded on the surface of the graphene oxide in the precursor, in some embodiments, a reinforced loading step is further included between the step (1) and the step (2): and re-dissolving the obtained precursor of the graphene oxide loaded metal organic framework material in an organic solvent, mixing and stirring the precursor with the organic solution of the transition metal salt, the organic solution of the organic ligand and the nucleation promoter for several hours, and performing solid-liquid separation to obtain a solid, wherein the solid is the precursor of the graphene oxide loaded metal organic framework material.

In other preferred embodiments, the strengthening and loading steps are repeated three to five times to obtain a proper loading of the metal-organic framework material, however, since the basic group of the nucleation promoter may react with the hydroxyl or carboxyl group on the graphene oxide, the experiment is not repeated too many times, otherwise the graphene sheet layer is reduced.

According to the method, the amount of the metal organic framework loaded on the graphene oxide is regulated and controlled by repeating the method for multiple times, and a series of graphene loaded nitrogen-doped carbon nanotube composites are obtained by high-temperature annealing, so that the acting force between the graphene and the nitrogen-doped carbon nanotube is strong, and the stability of the structure is facilitated; the nitrogen-doped carbon nanotube (GNCNTs-4) material loaded on the large continuous graphene has better stability than the granular carbon nanotubes (NCNTs) when used for a rechargeable zinc-air battery.

In some embodiments, the mixing and stirring of step (1) is for a period of 2 to 6 hours.

In some embodiments, the annealing treatment in step (2) is performed in a mixed atmosphere of hydrogen and an inert gas, wherein the volume fraction of hydrogen is 2% to 10%; the annealing temperature is 600-900 ℃, the annealing time is 1-6 hours, and the heating rate is 1-10 ℃/min.

In some embodiments, the acid solution in step (3) is a diluted acid solution, and the diluted acid solution is diluted sulfuric acid, diluted hydrochloric acid, diluted nitric acid, or the like, and the concentration of the diluted acid solution is 0.2-2 mol/L. The transition metal nanoparticles capable of being dissolved in the dilute acid solution are the unstable transition metal nanoparticles in the step (3).

The prepared composite material is used as the anode active catalytic material of the zinc-air battery, mainly utilizes the catalytic action of the nitrogen-doped carbon nano tube generated by the catalysis of the metal nano particles formed in the reducing atmosphere, the nitrogen-doped carbon nano tube which is grown in the catalysis mode is crystalline nitrogen-doped carbon, has the catalytic action on the precipitation and the reduction of oxygen, and has better corrosion resistance than an amorphous structure in a crystalline structure.

However, it is found in experiments that many metal nanoparticles formed under a reducing atmosphere are unstable, and the unstable metal nanoparticles increase the loading of the catalyst when catalyzing the precipitation and reduction of oxygen, reduce the current density, and are not favorable for the catalytic performance of the catalyst on oxygen. Taking the metal organic framework material ZIF-67 as an example, after unstable metal nanoparticles are removed, residual metal nanoparticles and carbon doped with nitrogen around can form a Co-N species which can stably exist, and the species is also one of a plurality of efficient active centers and is beneficial to the catalytic performance of the catalyst on oxygen.

In some embodiments of the present invention, the transition metal is cobalt, the organic ligand is 2-methylimidazole, the nucleation promoter is triethylamine, and correspondingly, the metal organic framework material is ZIF-67. The graphene loaded nitrogen-doped carbon nanotube composite material provided by the invention has the advantages that the nucleation of ZIF-67 on GO is accelerated by triethylamine, the ZIF-67 is uniformly deposited on GO, the amount of ZIF-67 on GO is increased by repeating the steps for many times, and finally a series of graphene loaded nitrogen-doped carbon nanotube composite materials are obtained by high-temperature pyrolysis, so that the construction of a two-dimensional layered multilevel structure consisting of carbon nanotubes is realized.

The specific technical scheme for preparing the graphene loaded nitrogen-doped carbon nanotube composite material with the two-dimensional hierarchical structure is as follows:

step (1): preparing a GO solution;

step (2): under the action of triethylamine, uniformly loading ZIF-67 on GO, and increasing the quantity of the loaded ZIF-67 by adopting a method of repeating for many times to obtain ZIF-67 with different quantities of GO loaded;

and (3): pyrolyzing the reaction product obtained in the step (2) at high temperature, and washing and drying with hydrochloric acid to obtain a series of graphene loaded nitrogen-doped carbon nanotubes;

and (4): preparing nitrogen-doped carbon nanotube powder;

the following is a detailed description of the above steps:

in some embodiments, the specific preparation method of the GO solution is: (1) under the condition of ice-water bath, 115mL of concentrated sulfuric acid is added into a 1L round-bottom flask, then 5g of graphite powder and 2.5g of sodium nitrate are added, and finally 15g of potassium permanganate is slowly added into the flask (the graphite powder: sodium nitrate: potassium permanganate is 2:1:6) to react for 2-3 h. (2) Heating to 35 ℃ and reacting for 2-3 h. (3) After adding 230-300mL deionized water, the temperature is raised to 95 ℃ and the reaction is carried out for 0.5-1 h. (4) 700mL of deionized water and 20mL of H were added₂O₂And after precipitation and filtration, washing twice with 5% hydrochloric acid solution, then centrifugally washing with deionized water until the supernatant is neutral, and controlling the amount of added water to obtain the GO solution with the concentration of 7-12 mg/ml.

In some embodiments, the specific preparation method of the graphene oxide loaded with different amounts of ZIF-67 comprises: firstly, ultrasonically dispersing 2ml of graphene oxide in 10-15ml of solvent, wherein the solvent can be: methanol, ethanol, mixed solution of methanol and ethanol in any proportion, isopropanol, and then respectively adding 291mg of Co (NO)₃)₂·6H₂O and 657mg of 2-methylimidazole in 20ml of this solvent in 2-methylimidazole solutionAfter addition of 10. mu.l of triethylamine, Co (NO) was added₃)₂·6H₂And sequentially pouring the O and the 2-methylimidazole solution into the graphene oxide solution, magnetically stirring for 2-6h at room temperature, and centrifugally separating and drying to obtain a graphene oxide loaded ZIF-67(ZIF-67@ GO-1) precursor. In order to increase the amount of ZIF-67 loaded on GO, a repeated method is adopted, a precursor ZIF-67@ GO-1 is re-dispersed in an organic solvent instead of GO, mixed and stirred with another two organic solutions, and repeated for four times and eight times to respectively obtain a precursor ZIF-67@ GO-4 and a precursor ZIF-67@ GO-8.

In some embodiments, the specific preparation method of the series of graphene-supported nitrogen-doped carbon nanotube composites is as follows: the precursor material (ZIF-67@ GO-1, ZIF-67@ GO-4, ZIF-67@ GO-8) is added into 5% H₂Annealing at 600-900 deg.C for 1-6h with a heating rate of 1-10 deg.C/min, and placing at 0.2-2M H₂SO₄Reacting in the solution at 40-80 ℃ for 12-72h, centrifuging, and drying to obtain the graphene loaded nitrogen doped carbon nanotube composite material (GNCNTs), wherein the GNCNTs-1, GNCNTs-4 and GNCNTs-8 are catalyst materials obtained by repeating once, four times and eight times correspondingly.

In some embodiments, the method for preparing the nitrogen-doped carbon nanotube powder comprises: 291mg of Co (NO) respectively₃)₂·6H₂O and 657mg of 2-methylimidazole in 20ml of a solvent, 10. mu.l of triethylamine was added to the 2-methylimidazole solution, and Co (NO) was added₃)₂·6H₂Mixing O and 2-methylimidazole solution, magnetically stirring at room temperature for 2-6H, centrifuging to obtain powder, dispersing in 10-15ml metal salt solution, reacting with the 2-methylimidazole solution, repeating the step for four times, centrifuging, drying, and adding 5% H to a certain amount of dried powder₂Annealing at 600-900 deg.C for 1-6h with a heating rate of 1-10 deg.C/min, and placing at 0.2-2M H₂SO₄Reacting in the solution at 40-80 ℃ for 12-72h, centrifugally separating and drying to obtain nitrogen-doped carbon nanotube powder;

the invention also provides the graphene loaded nitrogen doped carbon nanotube composite material prepared by the preparation method.

As shown in fig. 1(a) of the accompanying drawings, a proper amount of a nucleation promoter is added to uniformly distribute a metal-organic framework on both sides of graphene oxide, and then high-temperature annealing is performed to form metal nanoparticles under a reducing atmosphere and catalyze organic ligands to convert into nitrogen-doped carbon nanotubes, so that the nitrogen-doped carbon nanotube material with a layered multi-level structure continuously on both sides of a reduced graphene sheet is finally obtained.

The invention also provides application of the graphene loaded nitrogen-doped carbon nanotube composite material, and the graphene loaded nitrogen-doped carbon nanotube composite material can be used as a positive active material of a zinc-air battery and can also be regarded as a non-noble metal bifunctional oxygen electrocatalyst. The zinc-air cell can be a primary discharge, rechargeable, liquid or solid state rechargeable cell.

The invention also provides a zinc-air battery which comprises the positive electrode material, wherein the positive electrode material comprises the graphene loaded nitrogen-doped carbon nanotube composite material.

In some embodiments, the zinc-air battery is a liquid zinc-air battery and comprises a positive electrode, a negative electrode, an electrolyte and a battery shell, wherein the positive electrode is a zinc-air battery positive electrode comprising the graphene-loaded nitrogen-doped carbon nanotube composite material, the negative electrode is a polished zinc sheet, and the electrolyte is KOH and zn (ac)₂The mixed solution, in the preferred embodiment, is a solution of 6mol/L KOH and 0.2mol/L Zn (Ac)₂Mixing the solution, wherein the battery shell is made of PMMA material, and the specific structure is shown in FIG. 13.

In some embodiments of the present invention, a solid zinc-air battery is further provided, including a positive electrode, a negative electrode, a solid electrolyte and a battery case, where the positive electrode is a zinc-air battery positive electrode including the graphene-supported nitrogen-doped carbon nanotube composite material of the present invention, the negative electrode is a polished zinc sheet, and the solid electrolyte specifically includes the steps of: 11.25M KOH and 0.25M ZnO were dissolved in 5ml deionized water, mixed with 0.5g acrylic acid and 0.075g N, N-methylenebisacrylamide, filtered, and saturated with 75. mu.l K₂S₂O₄As an initiator, a gel polymer electrolyte was prepared. The battery shell isAcrylic adhesive tape.

In some embodiments of the present invention, a positive electrode of a zinc-air battery is further provided, where the positive electrode of the zinc-air battery includes an active material, a flexible current collector and an air diffusion layer, the active material is the above graphene loaded nitrogen-doped carbon nanotube composite, the flexible current collector may be nickel foam, copper foam, carbon cloth, and the like, and the air diffusion layer is prepared by: 0.1g of acetylene black and 0.3g of activated carbon were mixed uniformly, followed by addition of 0.1g of PTFE solution and isopropanol, stirring of the mixture to a dough-like state, and roll-pressing to a 300nm thick film in a roll press. And respectively attaching the 2 rolled air diffusion layers to two sides of the flexible current collector, rolling again to enable the air diffusion layers to completely cover and press the air diffusion layers into gaps of the flexible current collector, and drying in an oven to obtain the air diffusion layers. The preparation process of the zinc-air battery anode comprises the following steps: ultrasonically dispersing 5mg of the graphene-loaded nitrogen-doped carbon nanotube composite in 0.49ml of N, N-dimethyl methylamine (isopropanol, ethanol and the like) and 0.01ml of naphthol solution to prepare an electrode active material dispersion liquid. And (3) dropwise coating the electrode active material dispersion liquid on the air diffusion layer, and drying at 25-80 ℃ to prepare the zinc-air battery anode.

In some embodiments, the steps for assembling the rechargeable zinc-air battery are as follows:

step A: and preparing the battery cathode. Polishing a zinc sheet with a certain size and a thickness of 0.01-0.5mm by using sand paper until the surface presents metallic luster, washing away the polished zinc oxide powder by using ethanol or water, and drying for later use.

And B: an air diffusion layer was prepared. 0.1g of acetylene black and 0.3g of activated carbon were first mixed homogeneously, then 0.1g of PTFE solution and isopropanol were added, the mixture was stirred to a dough-like state and rolled to a 300nm thick film in a roll press. And respectively attaching the rolled 2 air diffusion layers to two sides of the foamed nickel, rolling again to enable the air diffusion layers to completely cover the nickel foam and press the nickel foam into gaps of the foamed nickel framework, and drying in an oven to obtain the air diffusion layers.

And C: and preparing the battery anode. Ultrasonically dispersing 5mg of the graphene-loaded nitrogen-doped carbon nanotube composite in 500 microliters of solvent to prepare a positive active material dispersion liquid, wherein the solvent can be0.49ml of N, N-dimethylformamide (isopropanol, ethanol, etc.) and 0.01ml of a binder (naphthol) were selected. 100 microliter of the dispersion was drop-coated onto a 1 cm square air diffusion layer at a concentration of 1mg cm^-1And drying at 25-80 ℃ to prepare the zinc-air battery anode.

Step D: and assembling the liquid zinc-air battery. And D, assembling the zinc sheet in the step A and the battery positive electrode in the step C into a self-made zinc-air battery mould, and adding liquid electrolyte into a gap between the positive electrode and the negative electrode.

Step E: a solid electrolyte is prepared. 11.25M KOH and 0.25M ZnO were dissolved in 5ml deionized water, mixed with 0.5g acrylic acid and 0.075g N, N-methylenebisacrylamide, filtered, and saturated with 75. mu.l K₂S₂O₄As an initiator, a gel polymer electrolyte was prepared.

Step F: and assembling the solid zinc-air battery. And (4) filling a gel electrolyte between the zinc sheet in the step A and the positive electrode in the step C, and packaging by using an acrylic adhesive tape to prepare the solid zinc-air battery.

The invention relates to a composite material with a two-dimensional layered multilevel structure, which is composed of graphene-loaded nitrogen-doped carbon nanotubes, and a preparation method thereof. In some embodiments, a method repeated for multiple times is first selected to load a sufficient amount of MOF material on graphene oxide, and a series of graphene-loaded nitrogen-doped carbon nanotube materials are obtained through high-temperature annealing. The composite material obtained by repeating for four times has larger graphene sheets and more crystallized nitrogen-doped carbon nanotubes, shows better oxygen reduction and oxygen precipitation performances, and is applied to a zinc-air battery at 5mA cm when used as an air electrode^-2And the ultrahigh stability performance of 3000h can be realized under the charging and discharging conditions.

In order to further understand the present invention, the following examples are provided to illustrate the non-noble metal bifunctional oxygen electrocatalyst, the preparation method and the zinc-air battery provided by the present invention, and the detailed embodiments and specific operation procedures are given, but the following examples should not be construed as limiting the scope of the present invention.

The following are examples:

example 1: preparation of ZIF-67@ GO-4

Firstly, ultrasonically dispersing 2ml of graphene oxide in 10ml of methanol, and then respectively dispersing 291mg of Co (NO)₃)₂·6H₂O and 657mg of 2-methylimidazole in 20ml of methanol, 10. mu.l of triethylamine was added to the 2-methylimidazole solution, and Co (NO) was added₃)₂·6H₂Sequentially pouring O and 2-methylimidazole solution containing triethylamine into the GO solution, magnetically stirring at room temperature for 3h, centrifugally separating and drying to obtain a product, re-dispersing the product into 10ml of methanol solution, and adding Co (NO) again₃)₂·6H₂And (3) magnetically stirring O and a 2-methylimidazole solution containing triethylamine for 3 hours, and repeating for four times to obtain a precursor ZIF-67@ GO-4. FIGS. 1(b) and 1(c) are SEM pictures of ZIF-67@ GO-4, showing that ZIF-67 is uniformly distributed on both sides of GO.

Example 2: preparation of GNCNTs-4

ZIF-67@ GO-4 from example 1 was placed in a tube furnace and H was fed into the furnace₂Mixed gas of/Ar (H)₂5 percent of the mixed gas by volume), raising the temperature to 700 ℃ at the speed of 2 ℃/min, keeping the temperature for 3 hours, naturally cooling, and placing at 0.5M H₂SO₄And (5) neutralizing for 48h, and centrifugally drying to obtain the GNCNTs-4 material. FIG. 1(d) is a scanning electron micrograph of GNCNTs-4, from which it can be seen that CNTs are distributed on both sides of graphene, and have a layered multi-level structure. FIGS. 1(e), 1(f) and 1(g) are TEM photographs of GNCNTs-4, showing the successful preparation of CNTs. FIGS. 1(h), 1(i), 1(j), and 1(k) are mapping representations of GNCNTs-4 showing the presence of C, N, Co.

Comparative example 1: preparation of GNCNTs-1

Firstly, ultrasonically dispersing 2ml of graphene oxide in 10ml of methanol, and then respectively dispersing 291mg of Co (NO)₃)₂·6H₂O and 657mg of 2-methylimidazole in 20ml of methanol, 10. mu.l of triethylamine was added to the 2-methylimidazole solution, and Co (NO) was added₃)₂·6H₂Sequentially pouring O and triethylamine-containing 2-methylimidazole solution into graphite oxideMagnetically stirring the alkene solution for 3 hours at room temperature, centrifugally separating and drying to obtain ZIF-67@ GO-1, placing the ZIF-67@ GO-1 in a tubular furnace, and introducing H₂Mixed gas of/Ar (H)₂5 percent of the mixed gas by volume), raising the temperature to 700 ℃ at the speed of 2 ℃/min, keeping the temperature for 3 hours, naturally cooling, and placing at 0.5M H₂SO₄And (5) neutralizing for 48h, and centrifugally drying to obtain the GNCNTs-1 material. FIGS. 2(a), 2(b), 2(c) and 2(d) are scanning and transmission electron microscope images of GNCNTs-1, and it can be seen that a layered multi-level structure composed of NCNTs is not formed.

Comparative example 2: preparation of GNCNTs-8

Firstly, ultrasonically dispersing 2ml of graphene oxide in 10ml of methanol, and then respectively dispersing 291mg of Co (NO)₃)₂·6H₂O and 657mg of 2-methylimidazole in 20ml of methanol, 10. mu.l of triethylamine was added to the 2-methylimidazole solution, and Co (NO) was added₃)₂·6H₂Sequentially pouring O and 2-methylimidazole solution containing triethylamine into the graphene oxide solution, magnetically stirring for 3h at room temperature, centrifugally separating and drying to obtain a product, re-dispersing the product into 10ml of methanol solution, and adding Co (NO) again₃)₂·6H₂And (3) magnetically stirring O and a 2-methylimidazole solution containing triethylamine for 3h, and repeating the stirring for eight times to obtain the precursor ZIF-67@ GO-8. Placing the mixture in a tube furnace, and introducing H₂Mixed gas of/Ar (H)₂5 percent of the mixed gas by volume), raising the temperature to 700 ℃ at the speed of 2 ℃/min, keeping the temperature for 3 hours, naturally cooling, and placing at 0.5M H₂SO₄And (5) neutralizing for 48h, and centrifugally drying to obtain the GNCNTs-8 material. Fig. 3(a), 3(b), 3(c) and 3(d) are scanning and transmission electron microscopy images of GNCNTs-8, which can be seen to form NCNTs, but due to repeated operations, it is likely that the basic groups of triethylamine and the functional groups on GO react, and the size of the graphene sheets decreases.

Comparative example 3: preparation of NCNTs

291mg of Co (NO) respectively₃)₂·6H₂O and 657mg of 2-methylimidazole in 20ml of methanol, 10. mu.l of triethylamine was added to the 2-methylimidazole solution, and Co (NO) was added₃)₂·6H₂Mixing O and 2-methylimidazole solution, magnetically stirring at room temperature for 3H, centrifuging to obtain powder, dispersing in 20ml cobalt nitrate solution, reacting with the above 2-methylimidazole solution, repeating the above step for four times, centrifuging, drying, and collecting a certain amount of dried powder in 5% H₂Annealing at 700 deg.C for 3h with a heating rate of 2 deg.C/min, and placing at 0.5M H₂SO₄And (c) centrifuging and drying for 48h to obtain NCNTs powder, wherein the images of FIG. 4(a), FIG. 4(b), FIG. 4(c) and FIG. 4(d) are scanning and transmission electron microscope images of ZIF-67 and NCNTs, and the formation of CNTs can be seen from the images.

Example 3: XRD and BET characterization of GNCNTs-4

The dried products of examples 1 and 2 were taken out, ground in a mortar, and then a part of the dried products was taken out and analyzed for phase structure by an X-ray diffractometer, and ZIF-67@ GO-4 showed a relatively clear diffraction peak, matching with a standard card of ZIF-67. Due to H₂SO₄Most of the unstable metallic cobalt is removed, and GNCNTs-4 mainly shows diffraction peaks of carbon. The BET test result of GNCNTs-4 prepared in this example, namely its specific surface area was 638.5m² g^-1. See fig. 5 and 6 for specific results.

Example 4: electrocatalytic performance test of GNCNTs-4

The product GNCNTs-4 annealed in example 2 was used for electrocatalytic performance testing, and 5mg was weighed into a centrifuge tube, 0.49mL of isopropanol and 0.01mL of nafion membrane solution were added, and ultrasonic dispersion was performed for 20min to form a black mixture. And transferring 10 microliter of mixture to drop on the surface of a glassy carbon electrode, drying, placing the electrode in 0.1mol/L KOH solution, adopting a three-electrode system, taking the glassy carbon electrode as a working electrode, taking Ag/AgCl as a reference electrode, taking a platinum sheet as a counter electrode, and testing the ORR and OER performances of the material by using an auto lab workstation. In the ORR test, the sweep rate was 5mV/s and the voltage ranged from 0.5 to 1.2V (FIG. 7). In the OER test, the sweep rate was 5mV/s and the voltage range was 1.2-1.7V (FIG. 8). Pt/C and IrO₂The slurry preparation process and test process methods of (a) are the same as described above. GNCNTs-1, GNCNTs-8 and NCNTs obtained in comparative examples 1 to 3 were also tested in the manner described above. See fig. 7 and 8 for results.

Example 5: preparation of zinc-air battery

For the air diffusion layer, 0.1g of acetylene black and 0.3g of activated carbon were first mixed uniformly, then 0.1g of PTFE solution and isopropanol were added, the mixture was stirred to a dough-like state, and rolled into a 300nm thick film under a roll press. Respectively attaching 2 rolled air diffusion layers to two sides of the foamed nickel, rolling again to enable the air diffusion layers to completely cover the nickel foam and press the nickel foam into gaps of a foamed nickel framework, drying in an oven, and dropwise adding the black mixture in the embodiment 4, wherein the loading amount is 1mg cm^-2. Finally, the battery is assembled, as shown in fig. 13, and fig. 13(a) to 13(l) are sequentially assembly steps. The battery shell is made of PMMA material, the cathode is made of polished zinc sheets, the anode is the electrode prepared in the process, and the electrolyte is 6mol/L KOH and 0.2mol/L Zn (Ac)₂The solution was mixed.

Similarly, the corresponding commercial Pt/C + IrO load₂A mixed zinc-air cell was also prepared according to the above method, except that GNCNTs-4 catalyst was replaced with the same mass of Pt/C + IrO₂Mixtures (Pt/C and IrO)₂50% of each mass fraction).

Similarly, the preparation of the solid zinc-air battery is similar to that of the liquid zinc-air battery, only the electrolyte is changed into the solid electrolyte, and the battery shell is changed into the flexible acrylic adhesive tape.

Example 6: testing of Zinc air cell Performance

In the process of testing the battery, a two-electrode system is adopted, a working electrode is a positive electrode plate loaded with a catalyst, a counter electrode is a polished zinc plate, and the testing is carried out by utilizing the LAND battery testing system. For the charge and discharge LSV test, the sweep rate was 5mV/s and the voltage range was 0.4-3.0V, as shown in FIG. 9. From the results, GNCNTs-4-loaded cells were compared to commercial Pt/C + IrO-loaded cells₂With a smaller potential difference. For the test of the stability of the charge-discharge cycle, the test current density is 5mA cm^-2Charging for 10min and discharging for 10min as a cycle, compared with loading commercial Pt/C + IrO₂The battery loaded with GNCNTs-4 has greater advantages, and the cycling stability of the battery is up to 3000h, as shown in FIG. 10。

Similarly, the test of the solid zinc-air battery is similar to that of the liquid zinc-air battery, and only the charging and discharging current density is changed to 1mA cm^-2Which also has a small potential difference, as shown in fig. 11, comparable to the loaded commercial Pt/C + IrO₂The stability of the battery of (2) is as shown in fig. 12.

It will be understood by those skilled in the art that the foregoing is only a preferred embodiment of the present invention, and is not intended to limit the invention, and that any modification, equivalent replacement, or improvement made within the spirit and principle of the present invention should be included in the scope of the present invention.

Claims

1. A preparation method of a graphene-loaded nitrogen-doped carbon nanotube composite material is characterized by comprising the following steps:

2. The method of claim 1, wherein the nucleation promoter is an amine soluble in an organic solvent, preferably one or more of ethylamine, diethylamine, triethylamine, ethylenediamine and benzyltriethylammonium chloride.

3. The preparation method according to claim 1 or 2, wherein the graphene oxide solution is obtained by dissolving a graphene oxide initial solution obtained by oxidative exfoliation by a Hummer method in an organic solvent, wherein the concentration of the graphene oxide initial solution is 6-15g/L, and the graphene oxide initial solution is mixed with the organic solvent in a volume ratio of 1:4-1:8 to obtain the graphene oxide solution;

4. The method of claim 1, further comprising, between step (1) and step (2), a step of reinforcing the load: and re-dissolving the obtained precursor of the graphene oxide loaded metal organic framework material in an organic solvent, mixing and stirring the precursor with the organic solution of the transition metal salt, the organic solution of the organic ligand and the nucleation promoter for several hours, and performing solid-liquid separation to obtain a solid, wherein the solid is the precursor of the graphene oxide loaded metal organic framework material.

5. The method of claim 4, wherein the reinforcing loading step is repeated three to five times.

6. The method of claim 1, wherein the mixing and stirring in step (1) is carried out for a period of 2 to 6 hours.

7. The preparation method according to claim 1, wherein the annealing treatment in the step (2) is performed in a mixed atmosphere of hydrogen and an inert gas, wherein the volume fraction of hydrogen is 2% to 10%; the annealing temperature is 600-900 ℃, the annealing time is 1-6 hours, and the heating rate is 1-10 ℃/min.

8. The graphene nitrogen-loaded doped carbon nanotube composite material prepared by the preparation method of any one of claims 1 to 7.

9. The application of the graphene-supported nitrogen-doped carbon nanotube composite material as claimed in claim 8, wherein the graphene-supported nitrogen-doped carbon nanotube composite material is used as a positive active material of a zinc-air battery.

10. A zinc-air battery comprising a positive electrode material comprising the graphene-supported nitrogen-doped carbon nanotube composite of claim 8.