CN108923051A - A kind of nitrogen-doped carbon nanometer pipe composite catalyst of package metals cobalt nano-particle and its application - Google Patents

Abstract

The invention discloses a kind of nitrogen-doped carbon nanometer pipe composite catalyst of package metals cobalt nano-particle and its applications, belong to battery material field.The composite catalyst is with dicyandiamide class cobalt-based metal-organic framework materials（Co(dca)₂pyz）It for presoma, was only carbonized under nitrogen atmosphere by three hours, is used for efficient electric catalytic oxidation-reduction.In 0.1 mol L^‑1Potassium hydroxide electrolyte in, the hydrogen reduction half wave potential of the catalyst is 0.82 V, and limiting diffusion current is 5.3 mA cm^‑2.Pass through the cyclic charging and discharging test of 65 h with the zinc and air cell that prepared catalyst assembles, charging/discharging voltage difference shows that prepared catalyst has very strong stability, have higher practical application value there is no significantly changing.

Description

A nitrogen-doped carbon nanotube composite catalyst encapsulating metal cobalt nanoparticles and its application

technical field

The invention relates to a composite catalyst, belonging to the field of battery materials, in particular to a nitrogen-doped carbon nanotube composite catalyst encapsulating metal cobalt nanoparticles, a preparation method thereof, and an application thereof in electrocatalytic oxygen reduction reactions and zinc-air batteries.

Background technique

With the increasingly severe energy crisis and environmental pollution, electrochemical energy conversion devices that directly convert the chemical energy of fuels into clean electrical energy have become a great challenge in the 21st century. Among them, zinc-air battery technology has attracted extensive attention due to its high energy density, environmental friendliness, safety, and low cost. Since the zinc-air battery directly uses oxygen in the air as the cathode active material, the catalyst for the oxygen reduction reaction of the air cathode is a key factor determining the performance of the battery. But the hysteresis in the rate of the oxygen reduction reaction at the cathode has prompted scientists to develop efficient and stable electrocatalysts to overcome this shortcoming. So far, noble metal materials (such as platinum, palladium, etc.) are the most effective electrocatalytic oxygen reduction catalysts, but their high cost, limited reserves, and poor stability greatly limit the large-scale commercial use of catalysts in clean energy. Therefore, the development of inexpensive, efficient, and durable oxygen reduction electrocatalysts is crucial for the practical application of Zn-air batteries.

Since Yaghi first defined metal-organic frameworks (MOFs) in the 1990s, MOFs have attracted extensive attention from scientists due to their fascinating structures and properties. Recently, MOFs have been widely developed as self-sacrificing templates and precursors for constructing carbon materials due to their high crystallinity, porosity, and adjustable composition. Nanomaterials derived from MOFs generally have the advantages of high specific surface area, porosity, and functional controllability, and show attractive application prospects in many fields such as electrocatalysis, photocatalysis, and fuel cells.

In recent years, hybrid hybrid materials of transition metals and nitrogen-doped carbon nanotubes (NCNTs) are expected to become a class of electrocatalysts to replace noble metal materials due to their high electrical conductivity and stability. Currently, researchers are working on effective strategies to synthesize such hybrid materials, such as arc discharge, laser ablation, and chemical vapor deposition. Due to the harsh synthesis conditions, the preparation of carbon nanotube hybrid materials is a big problem. Therefore, it is very important to develop a mild and efficient synthesis strategy for the practical application of transition metal-carbon nanotube hybrid materials. Among them, the direct pyrolysis of MOFs is an effective method for the synthesis of high-yield and precisely doped carbon nanotubes. There are still relatively few studies on the use of metal framework materials as a single precursor to obtain carbon nanotubes under mild conditions. Therefore, the development of novel MOFs to efficiently prepare transition metal/nitrogen-doped carbon nanotube composite catalysts has important practical significance for promoting the research and development of catalysts in this field.

Contents of the invention

The purpose of the present invention is to provide a nitrogen-doped carbon nanotube composite catalyst (abbreviation: Co@NCNTs) encapsulating metal cobalt nanoparticles, so as to realize its cheap, efficient and durable application in electrocatalytic reactions and zinc-air batteries Effect.

In order to realize the purpose of the present invention, the present invention selects the three-dimensional cobalt-based metal-organic framework material (Co(dca) ₂ pyz) that two kinds of nitrogen-containing ligands and soluble cobalt salts are constructed of cheap and easy-to-obtain dicyandiamide sodium and pyrazine Preparation of composite electrocatalytic oxygen reduction catalysts (Co@NCNTs) as precursors for electrocatalytic reactions and zinc-air batteries.

The nitrogen-doped carbon nanotube composite catalyst is prepared by the following method:

(1) Dissolve sodium dicyandiamide, pyrazine, and soluble cobalt salt in hot water respectively, stir at room temperature, and mix evenly. After the reaction is completed, a cloudy solution is obtained.

(2) Filter the turbid solution obtained in step (1), wash and filter repeatedly with deionized water to obtain a solid powder, and obtain a Co(dca) ₂ pyz precursor after drying.

(3) Calcining the Co(dca) ₂ pyz precursor obtained in step (2) under a nitrogen atmosphere, the resulting powder was ultrasonically washed with dilute sulfuric acid, then repeatedly centrifuged and washed with deionized water and ethanol several times, and dried to obtain the target product .

The soluble cobalt salt is selected from cobalt nitrate, cobalt chloride, cobalt sulfate and the like.

In step (1), the molar ratio of sodium dicyandiamide, pyrazine and soluble cobalt salt is 2-3:1:1.

Step (3) The temperature for calcination of the precursor is 700-1000° C., and the heating rate is 10° C./min.

The composite material is used as a catalyst in electrocatalytic reactions and zinc-air batteries.

The above application method is as follows: 1. Electrocatalytic oxygen reduction reaction The prepared nitrogen-doped carbon nanotube composite material encapsulating metal cobalt nanoparticles is used as a catalyst for the working electrode, and a three-electrode system is used for its catalytic oxygen reduction reaction performance. test. A silver-silver chloride (Ag/AgCl) electrode is used as a reference electrode, a platinum wire is used as an auxiliary electrode, and an aqueous potassium hydroxide solution is used as an electrolyte, preferably 0.1mol L ^-1 .

2. Zinc-air battery test: The prepared nitrogen-doped carbon nanotube composite material encapsulating metal cobalt nanoparticles is used as a catalyst and coated on foamed nickel as the positive electrode of the battery, the polished zinc plate is used as the negative electrode, and the separator is in the middle of the positive and negative electrodes. , the side of the positive electrode in contact with the air is an air diffusion layer, and the electrolyte is 6mol L ^-1 potassium hydroxide aqueous solution and 0.2mol L ^-1 zinc acetate aqueous solution.

The advantage of the present invention is that: the electrocatalytic oxygen reduction material uses three-dimensional cobalt-based metal-organic framework material (Co(dca) ₂ pyz) as a precursor, and forms nitrogen-doped carbon nanotubes coated with metal cobalt nanoparticles through carbonization in one step composite catalyst. Dicyandiamide can be cracked at high temperature to form gC ₃ N ₄ , which is a commonly used precursor of nitrogen-doped graphitic carbon nanomaterials. In addition, the mixture of MOF and dicyandiamide helps to obtain transition metal/carbon nanotube hybrid materials. Based on this, we choose dicyandiamide-based metal-organic framework materials as a single precursor, and nitrogen-doped carbon nanotubes can be prepared at a lower temperature (down to 700 degrees) and in a shorter time (three hours) under a nitrogen atmosphere. Composite material coated with cobalt metal. The carbon nanotube hybrid material has a high specific surface area and multi-level pore characteristics in which micropores, mesopores and macropores coexist. These structural features can expose more active sites while facilitating the transport of reactants and products during electrocatalytic energy conversion. On the other hand, the incorporation of nitrogen heteroatoms in carbon nanotubes can cause electronic modulation, causing a large number of defects in the nanotube wall, thereby generating positive charges on adjacent carbon atoms, providing additional active sites. The two complement each other and further enhance the electrocatalytic activity of the target hybrid material, which can be applied in the field of electrocatalytic oxygen reaction and zinc-air battery research, which is of great value and practical significance. Experimental tests show that the catalyst prepared by the invention has better stability than the platinum carbon catalyst and has a very good ability to resist methanol poisoning. In the electrocatalytic oxygen reduction application, the catalytic effect is similar to platinum carbon and other noble metal catalysts, the half-wave potential is 0.82V, and the limiting diffusion current is 5.3mA cm ^-2 . In the application of zinc-air battery, when the current density is 10 and 100mA cm ^-2 , the battery voltage is 1.20 and 0.82V respectively, and the maximum power density of the battery is 90mW cm ^-2 . Compared with the current noble metal catalysts, it has a better application effect. At the same time, it also provides new ideas for the preparation of other carbon nanotube composite materials.

Description of drawings

Fig. 1 is the powder X-ray diffraction (PXRD) spectrum of the Co(dca) ₂ pyz precursor synthesized in step (1) and the comparison chart of the single crystal simulation PXRD spectrum; Wherein, 1 is the PXRD spectrum simulated by the single crystal data ; 2 is the PXRD spectrum of the precursor Co(dca) ₂ pyz of the present invention. It can be seen that the prepared precursor has high purity and good crystallinity.

Fig. 2 is the powder X-ray diffraction (PXRD) spectrum of the catalyst Co@NCNTs prepared by the present invention, wherein 1 is the PXRD spectrum simulated by the cubic Co standard card; 2 is the PXRD of the catalyst Co@NCNTs prepared by the present invention Spectrum.

Fig. 3 is a scanning electron micrograph (a, b) of different magnifications of the catalyst prepared in the present invention.

Figure 4 is a transmission electron microscope image of the catalyst prepared in the present invention, Figure a is a high-resolution transmission electron microscope image, wherein 1 is a carbon nanotube layer, and 2 is a lattice diffraction fringe of cobalt nanoparticles coated by a graphite carbon layer. Figure b is a selected area electron diffraction pattern, and Figures c-f are element surface transmission (mapping) electron microscope images.

Fig. 5 is the nitrogen adsorption isotherm of the catalyst prepared in the present invention at a temperature of 77K, and the inset is the pore size distribution curve of the catalyst calculated according to the adsorption isotherm.

Fig. 6 is the cyclic voltammetry curve of the catalyst Co@NCNTs prepared in the present invention in an oxygen-saturated 0.1 mol L ^-1 potassium hydroxide solution. In the figure, 1 is a nitrogen atmosphere, and 2 is an oxygen atmosphere.

Figure 7 shows the linear sweep voltammetry curves of the catalyst prepared by the present invention in an oxygen-saturated 0.1 mol L ^-1 potassium hydroxide solution at different calcination temperatures, and the rotational speed of the rotating disk electrode is 1600 rpm. In the figure, 1 is the calcination temperature of 1000°C, 2 is the calcination temperature of 800°C, 3 is the platinum carbon catalyst with a mass percentage of 20%, and 4 is the calcination temperature of 900°C.

Figure 8 is a comparison of the stability test between the catalyst prepared by the present invention and platinum carbon in an oxygen-saturated 0.1mol L ^-1 potassium hydroxide solution, the rotational speed of the rotating disk electrode is 1600 rpm, wherein 1 is prepared by the present invention The catalyst, 2 is 20% platinum carbon catalyst in mass percent.

Fig. 9 is a test of the methanol poisoning resistance of the catalyst prepared in the present invention and platinum carbon in an oxygen-saturated 0.1 mol L ^-1 potassium hydroxide solution, and the rotational speed of the rotating disc electrode is 1600 rpm. Wherein, 1 is the catalyst prepared by the present invention, 2 is the platinum-carbon catalyst with a mass percent content of 20%, and 3 is adding 3 mol L ^-1 methanol into the system.

Figure 10 is a schematic diagram of a zinc-air battery device. In the figure, 1-zinc plate (negative electrode), 2-Co@NCNTs (positive electrode), 3-electrolyte, 4-oxygen, 5-gas diffusion layer.

Fig. 11 is the discharge polarization curve and power density curve of the zinc-air battery assembled with the catalyst prepared by the present invention as the positive electrode material.

Fig. 12 is a cycle stability test of a zinc-air battery assembled with the catalyst prepared by the present invention as the positive electrode material, and the charge and discharge current density is 5 mA cm ^-2 .

Detailed ways

Below by example the present invention will be further described:

Example 1: Synthesis of a nitrogen-doped carbon nanotube composite catalyst encapsulating metallic cobalt nanoparticles.

(1) Dissolve 0.38g of cobalt nitrate in 20mL of 90°C deionized water, 0.096g of sodium dicyandiamide and 0.2g of pyrazine in 20mL of 90°C of deionized water, mix the two evenly, and stir at room temperature for 30min to obtain a pink turbid solution. After filtering, washing with water and vacuum drying, the precursor (Co(dca) ₂ pyz) pink solid powder was obtained.

(2) 500 mg of the pink solid powder obtained in step (1) is placed in a quartz boat, the quartz boat is placed in a tube furnace, and the air in the furnace is exhausted by passing nitrogen gas for 30 minutes in advance, and then the tube furnace is heated for 10 minutes under a nitrogen atmosphere. The temperature was raised to 900°C at a rate of °C/min, and calcined at a constant temperature of 900°C for 3h. Naturally cooled to room temperature, the obtained black loose solid was ultrasonically washed with dilute sulfuric acid, then repeatedly centrifuged and washed several times with deionized water and ethanol, dried and ground to obtain a black solid powder. That is, the target catalyst (Co@NCNTs).

Example 2: The performance test of the nitrogen-doped carbon nanotube composite material encapsulating metal cobalt nanoparticles prepared in the present invention as an electrocatalyst.

Add 2 mg of the Co@NCNTs catalyst of the present invention to a mixed solution of 150 uL isopropanol and 150 uL deionized water, add 20 uL of 5% Nafion solution by mass percent, and ultrasonically disperse for 30 min to obtain a uniformly dispersed black catalyst slurry. 10 uL of the slurry was drop coated on the rotating disk electrode and dried at room temperature. The electrocatalytic performance was tested using a three-electrode system with a silver-silver chloride (Ag/AgCl) electrode as the reference electrode, a platinum wire as the auxiliary electrode, and a 0.1mol L ^-1 potassium hydroxide aqueous solution as the electrolyte. The test instrument is a constant potential/current meter of Wavedriver10 from Pine Company, and an MSR rotating disk electrode device.

As shown in Figure 6, compared with the cyclic voltammogram in nitrogen-saturated 0.1mol L ^-1 potassium hydroxide solution, the catalyst prepared under oxygen-saturated conditions has a very obvious oxygen reduction peak, proving that the material Has excellent oxygen reduction properties.

As shown in Figure 7, in oxygen-saturated 0.1mol L ^-1 potassium hydroxide solution, under the condition of electrode rotation speed of 1600 rpm, the catalysts obtained at different carbonization temperatures have obvious oxygen reduction currents. Among them, the catalyst obtained by carbonization at 900 °C has the best electrocatalytic oxygen reduction properties, which is similar to the catalytic effect of noble metal catalysts (platinum carbon, 20% mass fraction), with a half-wave potential of 0.82 V and a limiting diffusion current of 5.3 mA cm ^-2 .

As shown in Figure 8, in an oxygen-saturated 0.1mol L ^-1 potassium hydroxide solution, the electrode rotation speed is 1600 r/min, the catalyst prepared by the present invention has better stability than the platinum carbon catalyst.

As shown in Figure 9, in an oxygen-saturated 0.1 mol L ^-1 potassium hydroxide solution, the electrode rotation speed is 1600 rpm. When methanol was added to the system, the catalytic current of the platinum-carbon catalyst was greatly reduced, while the catalytic current of the prepared catalyst of the present invention was substantially unchanged except that it was slightly disturbed, indicating that compared with noble metal catalysts, the catalyst prepared by the present invention had Very good resistance to methanol poisoning.

Example 3: Application of the nitrogen-doped carbon nanotube composite catalyst encapsulating metal cobalt nanoparticles prepared in the present invention in zinc-air batteries.

Combined with Figure 10, the schematic diagram of the zinc-air battery device. The negative pole is a zinc plate, and the positive pole is nickel foam loaded with the catalyst prepared in the present invention. The electrolyte is 6mol L ^-1 potassium hydroxide aqueous solution and 0.2mol L ^-1 zinc acetate aqueous solution, the positive and negative electrodes are separated by a diaphragm, and the side of the positive electrode in contact with the air is a gas diffusion layer.

Figure 11 is the polarization discharge curve and the corresponding power density curve of the zinc-air battery device assembled with the prepared catalyst. When the current density is 10 and 100mA cm ^-2 , the battery voltage is 1.20 and 0.82V, and the battery maximum The power density is 90mW cm ^-2 .

As shown in Figure 12, the zinc-air battery assembled with the catalyst prepared by the present invention has undergone a 65-h cycle charge-discharge test, and the charge-discharge voltage difference has not changed significantly, indicating that the prepared catalyst has very strong stability and has relatively high High practical application value.

Claims (5)

1. a kind of nitrogen-doped carbon nanometer pipe composite catalyst of package metals cobalt nano-particle, which is characterized in that by as follows Method is prepared：

（1）Dicyandiamide sodium, pyrazine, soluble cobalt are dissolved in hot water respectively, stirred evenly, is reacted at room temperature, after reaction, Obtain turbid solution；

（2）By step（1）Obtained in turbid solution filtering, washed repeatedly with deionized water, obtain solid powder, after drying It obtains Co (dca)₂Pyz presoma；

（3）Under nitrogen atmosphere by step（2）Resulting Co (dca)₂The calcining of pyz presoma, gained powder are washed through dilute sulfuric acid ultrasound It washs, is then centrifuged repeatedly washing with deionized water and ethyl alcohol, obtains target product Co NCNTs after dry.

2. the nitrogen-doped carbon nanometer pipe composite catalyst of package metals cobalt nano-particle as described in claim 1, feature exist In the soluble cobalt is any one of cobalt nitrate, cobalt chloride, cobaltous sulfate.

3. the nitrogen-doped carbon nanometer pipe composite catalyst of package metals cobalt nano-particle as claimed in claim 1 or 2, feature It is,

Step（1）The molar ratio of middle dicyandiamide sodium, pyrazine and soluble cobalt is 2-3：1：1；

Step（3）The temperature of presoma calcining is 700-1000 DEG C, and heating rate is 10 DEG C/min.

4. the nitrogen-doped carbon nanometer pipe composite catalyst of package metals cobalt nano-particle as described in one of claim 1-3 Application, which is characterized in that it is as follows as elctro-catalyst working electrode application method：

（1）Isopropanol is added in the nitrogen-doped carbon nanometer pipe composite catalyst Co@NCNTs of package metals cobalt nano-particle and is gone In the mixed solution of ionized water, Nafion solution is added, obtains finely dispersed catalyst slurry after ultrasonic disperse；By catalyst Slurry drops are coated in rotating disk electrode (r.d.e) and are dried at room temperature for；（2）Using three-electrode system, with silver-silver chloride (Ag/AgCl) Electrode is reference electrode, platinum filament is auxiliary electrode, and potassium hydroxide aqueous solution is electrolyte.

5. the nitrogen-doped carbon nanometer pipe composite catalyst of package metals cobalt nano-particle as described in one of claim 1-3 Application, which is characterized in that it is as follows as zinc-air battery application method：

The nitrogen-doped carbon nanometer pipe composite catalyst Co@NCNTs of package metals cobalt nano-particle is coated in nickel foam and is made For anode, it is diaphragm among positive and negative anodes that zine plate, which is cathode, and anode is air diffusion layer, electrolyte with air contact side For potassium hydroxide and zinc acetate aqueous solution.