CN113410475B

CN113410475B - Graphitized carbon layer coated transition metal nanoparticle catalyst and preparation method thereof

Info

Publication number: CN113410475B
Application number: CN202010188908.5A
Authority: CN
Inventors: 胡劲松; 唐堂; 万立骏
Original assignee: Institute of Chemistry CAS
Current assignee: Institute of Chemistry CAS
Priority date: 2020-03-17
Filing date: 2020-03-17
Publication date: 2022-08-12
Anticipated expiration: 2040-03-17
Also published as: CN113410475A

Abstract

The invention provides a high-loading graphitized carbon layer coated transition metal nanoparticle catalyst and a preparation method thereof. The graphitized carbon layer coated transition metal nanoparticle catalyst is formed by distributing porous carbon network anchored nanoparticles with a core-shell structure on a carbon substrate, wherein the core is the transition metal nanoparticles, the shell is the graphitized carbon layer coated with the transition metal nanoparticles, and the metal nanoparticles are fixed with the core-shell structure on the carbon substrate by the porous carbon network. The catalyst is obtained by using a supported metal hydroxide on a carbon substrate as a metal source, coating a high molecular polymer on the outer layer of a transition metal hydroxide through a polymerization reaction, and then performing heat treatment. The graphitized carbon layer coated transition metal nanoparticle catalyst prepared by the method has the advantages of high metal loading, small particle size and uniform distribution. The catalyst has excellent catalytic performance for oxygen reduction and oxygen evolution reaction, and has great potential application value in a plurality of industrial catalysts or other scientific fields.

Description

Graphitized carbon layer coated transition metal nanoparticle catalyst and preparation method thereof

Technical Field

The invention relates to the field of catalysts and batteries, in particular to a graphitized carbon layer coated transition metal nanoparticle catalyst and a preparation method and application thereof.

Background

Since the introduction of the industrialized society, the rapid increase in the consumption rate of fossil energy has caused the existing storage to be exhausted in centuries, and there is a need for the development of sustainable energy and the development of new clean energy conversion devices. As a branch type of fuel cells, water-based rechargeable zinc-air batteries have attracted attention in various new clean energy conversion devices due to their advantages of high theoretical energy density, abundant raw materials, environmental friendliness, safety, and low cost. The zinc-air battery system utilizes a potassium hydroxide aqueous solution with high ionic conductivity as an electrolyte, has higher safety and faster reaction kinetics compared with a lithium ion battery using an organic electrolyte, and is expected to realize high-power output. Unfortunately, the practical use of rechargeable zinc-air batteries is still hampered by the low practical power density and poor durability. This is mainly due to the slow kinetics of the oxygen reduction that occurs during cathodic discharge and the oxygen evolution reaction that occurs during charging, which is inefficient. Generally, the catalyst which is commercialized usually adopts noble metals such as platinum, iridium, ruthenium and the like as active ingredients of the catalyst, but the cost of the noble metals is high, and the further popularization and application of the zinc-air battery are limited. Although a number of non-noble metal catalysts have been reported that nitrogen-doped carbon-based materials can catalyze oxygen reduction reactions as well as metal hydroxides, metal sulfides, etc. can catalyze oxygen evolution reactions. However, these materials often catalyze only a single reaction and cannot achieve charge-discharge cycling in zinc-air batteries. And the catalytic performance is poor, the stability in high-concentration strong base electrolyte is poor, so that the power density of the battery is low, and the cycle life is short. Therefore, the application of zinc-air battery is urgently needed to make breakthrough in improving power density and prolonging battery life, namely, designing high-density, high-activity and good-durability bifunctional active sites in a proper electrode structure.

The structure of the transition metal nano-particles coated by the graphitized carbon layer shows excellent catalytic activity on oxygen reduction and oxygen evolution reaction, so that the graphitized carbon layer is expected to be applied to zinc-air batteries. It is believed that this enhanced activity results from the electronic manipulation of the doped carbon shell by the metal nanoparticles. Such structures are generally obtained by direct pyrolysis of the corresponding precursors, such as carbon-based materials containing metal salts or metal oxides, and the like. However, since metal nanoparticles are very reactive at high temperatures and are prone to migration, such uncontrolled pyrolysis often leads to inevitable agglomeration and exposure of metal components, resulting in large metal particles with no catalytic activity. The effective active metal loading mass of the catalysts of the presently reported graphitized carbon layer coated transition metal nanoparticle structures is typically less than ten percent. High discharge power density requires a battery catalyst layer to have high-load active components, but simply increasing the coating loading introduces excessive carbon substrates without catalytic activity, which results in excessively thick catalyst layers and causes problems of difficult diffusion of reactive active substances and the like, and the performance cannot be further improved. In addition to output power, cycle durability is another requirement for battery utility. During the charging process of the zinc-air battery, the high voltage of more than 2V can cause any unprotected metal nano-particles or low-graphitization carbon to be subjected to electrochemical oxidation, so that the transition metal nano-particles are dissolved and fall off, and the battery performance is lost.

Therefore, developing a catalyst with high loading, uniform size and well-protected transition metal nanoparticles coated with a graphitized carbon layer uniformly distributed on a carrier is of great significance in promoting the development of zinc-air batteries (and fuel cells) with high rate output and long cycle life.

Disclosure of Invention

In order to solve the problems of low catalyst loading capacity, poor catalytic activity and efficiency and poor cycle durability of zinc-air batteries in the prior art, the invention provides a graphitized carbon layer coated transition metal nanoparticle catalyst, and solves the problem that the zinc-air batteries are difficult to practically use.

One of the purposes of the invention is to provide a high-loading graphitized carbon layer coated transition metal nano-particle catalyst, and the catalytic oxygen reduction performance of the catalyst of the invention reaches or is superior to that of a commercial platinum-carbon catalyst and is superior to that of most non-noble metal catalysts. The catalytic oxygen evolution capacity is close to that of commercial iridium oxide catalysts and is superior to that of most non-noble metal catalysts. When the catalyst is applied to a zinc-air battery, the peak discharge power of the battery is obviously superior to that of a commercial noble metal catalyst, and the cycling stability is good. Therefore, the catalyst is a non-noble metal catalyst which is efficient and stable and can replace commercial noble metal catalysts.

Specifically, the invention provides a graphitized carbon layer coated transition metal nanoparticle catalyst which is composed of a carbon substrate, a porous carbon network and core-shell structure nanoparticles, wherein the core is a transition metal nanoparticle, the shell is a graphitized carbon layer which coats the transition metal nanoparticle to form a core-shell structure, and the core-shell structure nanoparticles are fixed on the carbon substrate through the porous carbon network.

The carbon substrate is at least one of graphene, a carbon nanotube and porous carbon.

The mass percentage of the metal in the catalyst is 30-80wt%, and preferably 40-65 wt%. The nanoparticles are iron, cobalt, nickel, copper, zinc or alloys thereof, preferably alloys such as iron-cobalt alloy, iron-nickel alloy, cobalt-zinc alloy, iron-cobalt-nickel alloy, cobalt-nickel-zinc alloy. Preferably an iron-cobalt alloy, wherein the mass fraction of iron in the alloy is between 10 and 40 wt.%, preferably between 20 and 30 wt.%.

The size of the nano-particles is 5-50nm, preferably 10-20 nm.

The coating thickness of the carbon layer on the outer side of the catalyst particles is 0.5-10nm, and preferably 2-5 nm.

Preferably, the graphitized carbon layer is nitrogen-doped, and the content of nitrogen-doped atoms is 7-15 at.%.

Another object of the present invention is to provide a method for preparing the transition metal nanoparticle catalyst coated with the graphitized carbon layer, which comprises the following steps: preparing a metal hydroxide array which grows on a carbon substrate in advance by using a precipitator and a metal salt solution, using the array as a metal source in a heat treatment process, coating a high molecular polymer on an outer layer of a transition metal hydroxide in the transition metal hydroxide array through a polymerization reaction, then carrying out first high-temperature heat treatment under the protection of an inert atmosphere, and then cooling to prepare the catalyst.

Preferably, after the cooling and temperature reduction, the catalyst and a nitrogen source are mixed and subjected to a second high-temperature heat treatment under the protection of an inert atmosphere to further increase the nitrogen content of nitrogen doping.

In the above method steps, the carbon substrate includes, but is not limited to, at least one of graphene, carbon nanotubes, and porous carbon, and is preferably graphene.

The precipitant is selected from alkaline aqueous solution of ammonia water, sodium hydroxide, potassium hydroxide, sodium carbonate, potassium carbonate and the like, and preferably ammonia water; the metal salt solution is at least one selected from cobalt salt, iron salt, nickel salt, copper salt and zinc salt, specifically may be a halogen salt (metal chloride, metal bromide), sulfate, carbonate, sodium bicarbonate, phosphate, hydrogen phosphate, nitrate and the like, and preferably is a solution of iron salt and cobalt salt in a mass ratio of 1-2: 1-2 mixed metal salts; the carbon substrate is selected from at least one of graphene, carbon nanotubes and porous carbon.

Wherein the mass ratio of the metal salt to the carbon substrate is 50-100: 1, preferably 60-80: 1.

The metal hydroxide array grown on the carbon substrate is obtained by a preparation method comprising the following steps: and dispersing metal salt in water, adding graphene for uniform dispersion, slowly adding a precipitator, stirring, and centrifuging to obtain the metal hydroxide array loaded on the carbon substrate.

The obtained metal hydroxide is determined according to the added metal salt, and specifically is a combination of iron hydroxide, cobalt hydroxide, nickel hydroxide, copper hydroxide, zinc hydroxide and doped products thereof such as binary cobalt iron hydroxide, cobalt nickel hydroxide and iron nickel hydroxide, and ternary products such as iron cobalt zinc hydroxide, cobalt nickel zinc hydroxide and the like. Most preferred is cobalt iron hydroxide.

The preparation method of the graphitized carbon layer coated transition metal nanoparticle catalyst comprises the following steps of:

the high molecular polymer is polydopamine, polyphenolic resin, polypyrrole, polyaniline and the like, and polydopamine is preferred.

The first and second high temperature heat treatments are performed under an inert atmosphere, such as argon and/or nitrogen.

The temperature of the first high-temperature heat treatment and the second high-temperature heat treatment is 600-1200 ℃, preferably 600-900 ℃, and most preferably 700-800 ℃; the first high-temperature heat treatment time is 1 hour to 6 hours, preferably 2 hours to 3 hours, and the second high-temperature heat treatment time is 0.5 hour to 1 hour.

The nitrogen source is at least one selected from melamine, nitrile ammonia, dinitrile ammonia, urea and ammonia gas.

It is a further object of the present invention to provide the use of the above-described graphitized carbon layer-coated transition metal nanoparticle catalyst as a cathode catalyst for a battery, preferably a metal-air battery or a fuel cell, more preferably a zinc-air battery.

The graphitized carbon layer coated transition metal nano-particle catalyst has high metal loading, small particle size and uniform distribution. The preparation method is simple, low in cost and suitable for large-scale production. The catalyst is applied to devices such as zinc-air batteries, fuel batteries and the like, the battery performance is obviously superior to that of a noble metal catalyst, and the catalyst has potential to replace the existing noble metal catalyst.

The preparation method mainly comprises the steps of taking metal hydroxide growing in situ as a metal source and coated polymer as a reducing agent, decomposing the polymer into a porous carbon network under high-temperature treatment, reducing the metal hydroxide in situ to form small-size and uniformly-distributed metal particles, and forming a structure of the graphitized carbon layer coated transition metal nanoparticles in the subsequent cooling process. The nitrogen content can be further improved by means of secondary heat treatment.

Compared with other prior art, the invention has the following characteristics:

1. the raw materials adopted in the invention are transition metals such as iron, cobalt, zinc, copper, nickel and the like, the reserves in the earth crust are rich, the cost is low, the large-scale industrial production can be carried out, the current noble metal catalyst is replaced, the cost of a metal air battery or a fuel battery is facilitated, and the large-scale practicability and industrialization are further facilitated.

2. The invention can obtain higher metal loading under the condition of ensuring small and uniform particle size by using the metal hydroxide as a metal source. Compared with the traditional methods of using the adsorbed metal salt as a metal source and the like, the method has the advantages that the pyrolysis process is more controllable, the prepared particles are small in size, uniform in distribution and high in loading. In the preferred embodiment of the invention, the catalyst has the size of 10-20nm, provides good catalytic activity, is uniformly distributed, has high loading capacity, is used as a cathode reaction catalyst of an air battery or a fuel battery, has high catalytic activity, ensures excellent performance of the battery, has high energy density, and keeps very excellent cycle stability after long-term operation.

3. The porous carbon network generated by the method of coating the polymer and providing in-situ reduction does not influence the contact between the nitrogen-doped carbon coated particles and electrolyte in the electrochemical reaction, and can prevent the particles from falling off in the reaction process. Compared with the traditional method for grinding the mixed reducing agent, the method not only prevents the particles from agglomerating in the heat treatment process, but also can provide better protection for the particles in subsequent application.

4. The catalyst prepared by the invention has excellent catalytic performance, and compared with other non-noble metal catalysts reported in documents, the catalyst is applied to zinc-air batteries, the peak discharge power of the batteries is obviously superior to that of commercial noble metal catalysts, and the cycling stability is good.

5. The method has the advantages of simple process, economy, convenient operation and easy large-scale production, and has huge potential application value in a plurality of industrial catalysts or other scientific fields.

Drawings

FIG. 1 is a scanning electron microscope image of an array of hydroxide precursors supported on graphene obtained in example 1.

FIG. 2 is an X-ray powder diffraction curve of the catalyst obtained in example 1.

FIG. 3 is a TEM image of the catalyst obtained in example 1, wherein (a) is a SEM image and (b) is a high-resolution SEM image.

FIG. 4 is a scanning electron microscope photograph of the catalyst obtained in example 1.

FIG. 5 is a 1s curve of nitrogen element with high resolution in X-ray photoelectron spectrum of the catalyst obtained in example 1.

FIG. 6 is a thermogravimetric analysis curve of the catalyst obtained in example 1 under an air atmosphere.

Fig. 7(a) is a specific surface area test curve of the catalyst obtained in example 1, and fig. 7(b) is a corresponding pore size distribution test curve.

FIG. 8(a) is an experimental polarization curve for oxygen reduction of the catalyst obtained in example 1 and a commercial platinum-carbon catalyst, and FIG. 8(b) is an experimental polarization curve for oxygen evolution of the catalyst obtained in example 1 and a commercial iridium oxide catalyst.

Fig. 9(a) is a discharge and charge polarization curve of the zinc-air battery of the catalyst obtained in example 1 and a commercial platinum-carbon and iridium oxide mixed catalyst, and fig. 9(b) is a power density curve corresponding to the discharge curve.

Figure 10 shows the cycling stability of zinc-air cells with the catalyst obtained in example 1 and a commercial platinum-carbon and iridium oxide mixed catalyst.

FIG. 11 is a TEM photograph of the catalyst obtained in comparative example 1.

Detailed Description

The present invention will be described below with reference to specific examples, but the present invention is not limited thereto.

The experimental methods used in the following examples are all conventional methods unless otherwise specified; reagents, materials and the like used in the following examples are commercially available unless otherwise specified.

Example 1

Firstly, preparing a cobalt ferric hydroxide array loaded on graphene, and dispersing 0.5g of cobalt sulfate and 0.25g of ferrous sulfate in 100 ml of water for uniform dissolution. And then adding 10 ml of graphene water dispersion liquid with 1 mg/ml of uniform dispersion, stirring at a constant speed for 30 minutes, then dropwise adding 5 ml of ammonia water as a precipitator, continuously stirring for 30 minutes, and centrifuging to obtain the cobalt iron hydroxide nanosheet array loaded on the graphene.

And then dispersing the cobalt iron hydroxide array loaded on the graphene substrate in a mixed solution of 40 ml of water and 40 ml of ethanol, uniformly stirring until no obvious precipitate exists, dissolving 80 mg of tris (hydroxymethyl) aminomethane serving as an initiator of a polymerization reaction in 40 ml of water, uniformly stirring, adding the mixed solution, and uniformly stirring for 30 minutes. 60 mg of dopamine hydrochloride was dissolved in 40 ml of water, and the above mixed solution was added dropwise with stirring. The solution turned black after about 1 hour, indicating that polydopamine had successfully coated the hydroxide surface. The solution was centrifuged and the product dried. And then transferring the product into a quartz boat, placing the quartz boat into a tube furnace, purging the quartz boat for half an hour by using argon, then heating the quartz boat to 800 ℃ under the condition of keeping argon purging, carrying out heat treatment for two hours, then cooling the quartz boat, taking out the product, mixing the product with melamine according to the ratio of 1:10, and carrying out second heat treatment, wherein the process is the same as the first heat treatment, and the heat treatment time is 1 hour. And cooling to obtain the nitrogen-doped graphitized carbon layer coated cobalt-iron alloy nanoparticle catalyst uniformly loaded on the graphene substrate.

As shown in fig. 1, the scanning electron microscope of the obtained cobalt iron hydroxide shows that the nanosheets uniformly grow on the surface of the graphene. During the subsequent conversion process, these nanoplatelets undergo shrinkage, decompose into small, individual oxide particles and are eventually reduced to metal particles.

The X-ray powder diffraction curve of the resulting catalyst is shown in fig. 2. From fig. 2, it can be seen that the catalyst can be well matched with a standard diffraction card of cobalt-iron alloy, which indicates that the obtained metal particles are cobalt-iron alloy. The peak ratio of the catalyst is lower, indicating that a smaller size of the catalyst is obtained. The broad peak around 25 degrees comes from the carbon substrate.

The transmission electron micrograph of the resulting catalyst is shown in FIG. 3. It can be seen from the low power state of fig. 3a that the graphene substrate has high density, and the nitrogen-doped graphitized carbon layer-coated cobalt-iron alloy nanoparticles are uniformly distributed, and the size and distribution of the metal particles are very uniform. From the high magnification photograph of fig. 3b, it is known that the size of the obtained catalyst particles is about 15nm, wherein the carbon layer having a thickness of about 3 nm is coated on the outer surface of the metal core, and the carbon layer has good crystallinity and high graphitization degree.

The scanning electron micrograph of the resulting catalyst is shown in FIG. 4. Therefore, the outer layer of the cobalt-iron alloy nano particles coated by the nitrogen-doped graphitized carbon layer is provided with a thin porous carbon network, which is formed by carbonizing the outer layer of polymer and has good protection effect on the formed particles.

The X-ray photoelectron spectrum high-resolution nitrogen element 1s curve of the obtained catalyst is shown in figure 5. As can be seen from fig. 4, the catalyst contains a large amount of nitrogen, and the total content of nitrogen atoms is about 10 at%. Wherein the main component is high-activity pyridine nitrogen, which accounts for 30-40% of the nitrogen atom. Next, the graphitized nitrogen occupies a high proportion, approximately 15 to 25% of the number of nitrogen atoms, indicating that the degree of crystallization of the carbon layer on the outer side of the particles is relatively good.

The thermogravimetric curve of the catalyst is shown in FIG. 6. It can be seen that the catalyst was calcined in an oxygen atmosphere to 800 ℃ with a mass loss of 26.68%, and the product was spinel type cobalt iron oxide, which was calculated to account for 54% of cobalt iron metal in the catalyst.

The specific surface area of the catalyst is shown in fig. 7a, the absorption and desorption curves of the material have obvious hysteresis at the middle section, which shows that the catalyst has rich pore structures, and as shown in fig. 7b, the pore distribution is mainly below 10 nanometers and concentrated at 2 nanometers and then continuously distributed, which shows that various pore structures in the catalyst are rich, not only providing a large number of exposed active sites, but also being beneficial to the material transmission in the electrochemical reaction.

In conclusion, the catalyst is high-loading nitrogen-doped graphitized carbon layer coated cobalt-iron alloy nanoparticles uniformly distributed on graphene at high density, the size of the catalyst particles is small, the activity is high, and the outer side of the catalyst is protected by a porous carbon network.

To verify the electrochemical performance of the resulting catalyst, the following tests were performed:

the oxygen reduction experimental curves of the resulting catalyst and the commercially used platinum-carbon catalyst are shown in fig. 8 a. The specific experimental method comprises the following steps: the oxygen reduction curve was measured with a rotating disk electrode at 1600 rpm in 0.1 mol/l potassium hydroxide solution and the sweep rate of the curve was 10 millivolts per second. The control was purchased from Alfa Aesar (atran) catalyst ltd using a platinum-carbon catalyst.

Comparing the two curves, it can be seen that the prepared high-load nitrogen-doped graphitized carbon layer-coated cobalt-iron alloy nanoparticle catalyst shows equivalent performance to commercial platinum carbon, and the half-wave potential is 0.85V, which is slightly superior to that of the platinum carbon catalyst.

The oxygen evolution experimental curve of the resulting catalyst and the commercially used iridium oxide catalyst is shown in fig. 8 b. The specific experimental method comprises the following steps: the oxygen evolution curve was measured with a rotating disk electrode at 1600 revolutions per minute in 0.1 mol/l potassium hydroxide solution and the curve scan rate was 5 millivolts per second. The iridium oxide catalyst for comparison was purchased from Alfa Aesar (tianjin) catalyst ltd.

Comparing the two curves, it can be seen that the catalyst prepared above requires a potential of about 1.64V to achieve a current density of 10 milliamps per square centimeter in the oxygen evolution experiment, which is slightly worse than the commercial iridium oxide catalyst. But at higher current density intervals the performance of the catalyst has reached that of an iridium oxide catalyst.

The catalyst was further used for the air electrode of an actual zinc-air battery. The resulting properties are shown in fig. 9. The specific experimental method comprises the following steps: 2 mg of catalyst is mixed with 10 microliter of naphthylene solution and dissolved in 1 milliliter of ethanol for uniform ultrasonic treatment to obtain catalyst ink, and the catalyst ink is coated on conductive carbon paper in a spinning way to prepare the air electrode. Then, the air electrode and a thin zinc plate were assembled into a zinc-air battery, and the electrolyte was 6 mol/l potassium hydroxide aqueous solution. The test curve scan speed was 10 millivolts per second. The control platinum carbon and iridium oxide catalysts were purchased from Alfa Aesar (tianjin) catalyst ltd.

As can be seen from fig. 9a, the catalyst exhibited better performance than the platinum carbon and iridium oxide mixed catalyst in both the charge and discharge regions. The catalyst exhibits excellent activity particularly in the discharge region. The maximum discharge current may reach about 650 milliamps per square centimeter. The power curve corresponding to the discharge curve is shown in fig. 9 b. The power density calculated for the catalyst can be 423.7 milliwatts per square centimeter. Much higher than 175.8 milliwatts per square centimeter of platinum carbon and iridium oxide mixed catalyst.

We simultaneously compared the cycling stability of the catalyst and noble metal mixed catalyst in zinc-air cells. The charge and discharge current was 2 milliamps per square centimeter for 20 minutes per cycle. The cycle results are shown in fig. 10, the charge-discharge potential difference of the catalyst is increased by only 7 mv after 750 cycles (250 hours), while the platinum-carbon and iridium oxide mixed catalyst has obviously decayed in about 100 hours. The good stability of the catalyst is derived from the nitrogen-doped carbon layer with good crystallinity and the porous carbon network of the outer layer, so that the metal particles are protected from being oxidized, dissolved and dropped. The excellent activity and stability indicate that the catalyst has strong application potential in batteries.

Example 2

A catalyst of nitrogen-doped graphitized carbon layer-coated transition metal nanoparticles was prepared in the same manner as in example 1. The difference is that cobalt iron hydroxide is changed into cobalt hydroxide with equal mass (namely only 0.75g of cobalt sulfate is added in the process of preparing a hydroxide array), and the obtained catalyst is a high-loading graphitized carbon layer coated cobalt nanoparticle catalyst. The catalyst maintained substantially similar morphology with a metallic cobalt content of 41% and a catalyst particle size of approximately 15 nm.

Example 3

A catalyst of nitrogen-doped graphitized carbon layer-coated transition metal nanoparticles was prepared in the same manner as in example 1. Except that cobalt iron hydroxide is changed into nickel iron hydroxide with equal mass (namely, the feeding metal is changed into 0.5g of nickel salt and 0.25g of iron salt in the process of preparing a hydroxide array), and the obtained catalyst is a high-loading graphitized carbon layer coated nickel-iron alloy nanoparticle catalyst. The catalyst basically maintains similar appearance, wherein the content of metallic ferronickel is 47%, and the particle size of the catalyst is about 15 nm.

Example 3

A catalyst in which a graphitized carbon layer was coated with transition metal nanoparticles was prepared in the same manner as in example 1. Except that the polymer is changed into polymeric phenolic resin with equal mass, and the obtained catalyst is a high-loading graphitized carbon layer coated cobalt-iron alloy nano-particle catalyst. The catalyst maintains substantially similar morphology. Wherein the content of metallic cobalt and iron is 52 percent, and the particle size of the catalyst is about 17 nm.

Example 4

A catalyst in which a graphitized carbon layer was coated with transition metal nanoparticles was prepared in the same manner as in example 1. Except that the calcining temperature in the second step is reduced to 600 ℃, and the obtained catalyst is a high-loading graphitized carbon layer coated cobalt-iron alloy nano-particle catalyst. The catalyst basically maintains similar appearance, and the nitrogen content is greatly reduced. Wherein the content of metallic cobalt and iron is 48%, and the particle size of the catalyst is about 21 nm.

Example 5

A catalyst in which a graphitized carbon layer was coated with transition metal nanoparticles was prepared in the same manner as in example 1. The difference is that urea is used as a nitrogen source for the second heat treatment, and the obtained catalyst is a high-loading nitrogen-doped graphitized carbon layer coated cobalt-iron alloy nanoparticle catalyst. The catalyst maintains substantially similar morphology. Wherein the content of metallic cobalt and iron is 48%, and the particle size of the catalyst is about 21 nm.

Example 6

A catalyst in which a graphitized carbon layer-coated transition metal nanoparticle was prepared in the same manner as in example 1. Except that the second heat treatment is not carried out, the obtained catalyst is a high-loading graphitized carbon layer coated cobalt-iron alloy nano particle catalyst, and the total content of nitrogen atoms is about 3 at%. The catalyst basically maintains similar appearance, the content of metallic cobalt and iron is 49%, and the particle size of the catalyst is about 18 nm.

Example 7

A catalyst of nitrogen-doped graphitized carbon layer-coated transition metal nanoparticles was prepared in the same manner as in example 1. Except that when preparing the metal hydroxide loaded on the graphene, 0.75g of cobalt sulfate is used for replacing 0.5g of cobalt sulfate and 0.25g of ferrous sulfate, so as to obtain the cobalt hydroxide nanosheet array loaded on the graphene. A catalyst of graphitized carbon layer-coated transition metal nanoparticles was then prepared in the same manner as in example, except that the above-described cobalt hydroxide nanosheet array supported on graphene was used in place of the cobalt hydroxide iron nanosheet array supported on graphene in example 1. Finally obtaining the nitrogen-doped graphitized carbon layer coated cobalt alloy nano-particle catalyst, wherein the content of metal cobalt is 45%, and the particle size of the catalyst is about 19 nm.

Example 8

A catalyst of nitrogen-doped graphitized carbon layer-coated transition metal nanoparticles was prepared in the same manner as in example 1. Except that when preparing the metal hydroxide loaded on the graphene, 0.75g of ferrous sulfate is used for replacing 0.5g of cobalt sulfate and 0.25g of ferrous sulfate, so as to obtain the ferric hydroxide nanosheet array loaded on the graphene. A catalyst of graphitized carbon layer-coated transition metal nanoparticles was then prepared in the same manner as in example, except that the above-described iron hydroxide nanosheet array supported on graphene was used in place of the iron cobalt hydroxide nanosheet array supported on graphene in example 1. Finally obtaining the nitrogen-doped graphitized carbon layer coated cobalt alloy nano-particle catalyst, wherein the content of metallic iron is 46%, and the particle size of the catalyst is about 22 nm.

Example 9

A catalyst in which a graphitized carbon layer was coated with transition metal nanoparticles was prepared in the same manner as in example 1. Except that when preparing the metal hydroxide loaded on the graphene, the dosage of the metal salt is 0.25g of cobalt sulfate and 0.5g of ferrous sulfate, and the obtained catalyst is a high-loading graphitized carbon layer coated cobalt-iron alloy nano-particle catalyst. The catalyst maintains substantially similar morphology. Wherein the content of metallic cobalt iron is 51 percent, and the particle size of the catalyst is about 17 nm.

Example 10

A catalyst in which a graphitized carbon layer was coated with transition metal nanoparticles was prepared in the same manner as in example 1. Except that when preparing the metal hydroxide loaded on the graphene, the dosage of the metal salt is 0.15g of cobalt sulfate and 0.6g of ferrous sulfate, and the obtained catalyst is a high-loading graphitized carbon layer coated cobalt-iron alloy nano-particle catalyst. The catalyst maintains substantially similar morphology. Wherein the content of metallic cobalt and iron is 51 percent, and the particle size of the catalyst is about 17 nm.

Comparative example 1

0.5g of cobalt sulfate and 0.25g of ferrous sulfate are dissolved in water and uniformly dissolved. Then 10 ml of graphene water dispersion liquid with 1 mg/ml of uniform dispersion is added, the mixture is stirred for 30 minutes at a constant speed, and then the precipitate is separated and dried by a suction filtration method. Obtaining the metal salt crystal loaded on the graphene substrate. Then, according to the same method and conditions as in example 1, the metal salt crystals supported on the graphene substrate are dispersed in a mixed solution of 40 ml of water and 40 ml of ethanol, and uniformly stirred until no obvious precipitate is formed, and then 80 mg of tris (hydroxymethyl) aminomethane serving as an initiator of polymerization is dissolved in 40 ml of water, uniformly stirred, added to the mixed solution, and uniformly stirred for 30 minutes. 60 mg of dopamine hydrochloride was dissolved in 40 ml of water, and the above mixed solution was added dropwise with stirring. The solution turned black after about 1 hour, indicating that polydopamine had successfully coated the hydroxide surface. The solution was centrifuged and the product dried. And then transferring the product into a quartz boat, placing the quartz boat into a tube furnace, purging the quartz boat for half an hour by using argon, then heating the quartz boat to 800 ℃ under the condition of keeping argon purging, carrying out heat treatment for two hours, then cooling the quartz boat, taking out the product, mixing the product with melamine according to the ratio of 1:10, and carrying out second heat treatment, wherein the process is the same as the first heat treatment, and the heat treatment time is 1 hour. And cooling to obtain the nitrogen-doped graphitized carbon layer-coated cobalt-iron alloy nanoparticle catalyst loaded on the graphene substrate. The electron micrograph of the catalyst obtained in comparative example 1 is shown in FIG. 11, and it can be seen that the size is very uneven and the distribution is 20nm to 200 nm. And large particles are abundant, no carbon layer is coated outside the particles or only a small amount of carbon layer is coated outside the particles, the content of the metal cobalt iron is 65wt%, but most of the metal cobalt iron exists in a state of large size and low activity.

The catalysts obtained in the above examples and comparative examples were tested for their electrochemical properties in the same manner and under the same conditions as described in example 1, and the results are shown in the following table 1:

TABLE 1

As can be seen from the data in table 1, compared with the catalyst obtained by the conventional method, the graphitized carbon layer coated transition metal nanoparticle catalyst provided by the present invention has the characteristics of high loading capacity, small and uniform size, multiple protection, and the like. The catalyst has high catalytic activity, and can obviously improve the discharge power of an actual device and improve the cycle stability of the battery when being applied to a catalyst of a metal-air battery or a fuel battery. The practicability and large-scale industrialization of the metal-air battery or the fuel battery provide further feasibility.

The applicant states that the present invention is illustrated by the above examples to describe the detailed preparation method of the present invention, but the present invention is not limited to the above detailed preparation method, i.e. it does not mean that the present invention must rely on the above detailed preparation method to be carried out. It should be understood by those skilled in the art that any modification of the present invention, equivalent substitutions of the raw materials of the product of the present invention, addition of auxiliary components, selection of specific modes, etc., are within the scope and disclosure of the present invention.

Claims

1. A graphitized carbon layer coated transition metal nanoparticle catalyst is composed of a carbon substrate, a porous carbon network and core-shell structured nanoparticles, wherein the core is transition metal nanoparticles, the shell is a graphitized carbon layer which coats the transition metal nanoparticles to form a core-shell structure, and the core-shell structured nanoparticles are fixed on the carbon substrate through the porous carbon network; the carbon substrate is graphene;

the preparation method of the graphitized carbon layer coated transition metal nanoparticle catalyst comprises the following steps: preparing a metal hydroxide array which grows on a carbon substrate in advance by using a precipitator and a metal salt solution, using the array as a metal source in a heat treatment process, coating a high molecular polymer on an outer layer of a transition metal hydroxide in the transition metal hydroxide array through a polymerization reaction, then carrying out first high-temperature heat treatment under the protection of an inert atmosphere, then cooling, mixing an obtained product with a nitrogen source, and carrying out second high-temperature heat treatment under the protection of the inert atmosphere to prepare the catalyst;

wherein the mass ratio of the metal salt to the carbon substrate is 50-100: 1;

the metal hydroxide array grown on the carbon substrate is obtained by a preparation method comprising the following steps: dispersing metal salt in water, adding graphene for uniform dispersion, slowly adding a precipitator, stirring, and centrifuging to obtain a metal hydroxide array loaded on a carbon substrate;

the precipitator is ammonia water; the metal salt solution is selected from at least one of cobalt salt, iron salt, nickel salt, copper salt and zinc salt; the high molecular polymer is polydopamine;

the temperature of the first high-temperature heat treatment and the second high-temperature heat treatment is 700- ^o C；

The size of the catalyst particles is 10-20nm, and the coating thickness of the carbon layer on the outer side of the catalyst particles is 2-5 nm;

the graphitized carbon layer is doped with nitrogen, and the content of nitrogen-doped atoms is 7-15 at.%.

2. The catalyst according to claim 1, wherein the metal content in the catalyst is 30-80wt%, and the nanoparticles are iron, cobalt, nickel, copper, zinc or alloys thereof.

3. The catalyst of claim 2, wherein the metal content of the catalyst is 40-65wt%, and the nanoparticles are an iron-cobalt alloy, an iron-nickel alloy, a cobalt-zinc alloy, an iron-cobalt-nickel alloy, or a cobalt-nickel-zinc alloy.

4. The catalyst of claim 3, wherein the nanoparticles are an iron-cobalt alloy, wherein the mass fraction of iron in the alloy is between 10 and 40 wt%.

5. The catalyst of claim 4, wherein the mass fraction of iron in the alloy is between 20 and 30 wt%.

6. The catalyst of claim 1, wherein the metal salt is a halide salt, sulfate salt, carbonate salt, sodium bicarbonate salt, phosphate salt, hydrogen phosphate salt, or nitrate salt.

7. The catalyst of claim 6, wherein the halide salt is a metal chloride or a metal bromide.

8. The catalyst of claim 6, wherein the metal salt is a salt of iron and cobalt in a mass ratio of 1-2: 1-2.

9. The catalyst of claim 1, wherein the first and second high temperature heat treatments are performed under an inert atmosphere; and/or the time of the first high-temperature heat treatment is 1 hour to 6 hours, and the time of the second high-temperature heat treatment is 0.5 to 1 hour; and/or the nitrogen source is at least one selected from melamine, nitrile ammonia, bis-nitrile ammonia, urea and ammonia gas.

10. The catalyst of claim 9 wherein the first high temperature heat treatment time is from 2 to 3 hours.

11. Use of a catalyst according to any one of claims 1 to 10 in zinc air cells and fuel cells.