CN112138697B

CN112138697B - Preparation method and application of manganese-nitrogen co-doped carbon nanosheet electrocatalyst

Info

Publication number: CN112138697B
Application number: CN202010959044.2A
Authority: CN
Inventors: 李楠; 蒋鹏阳; 李辰晨; 潘秋任; 刘兆清
Original assignee: Guangzhou University
Current assignee: Guangzhou University
Priority date: 2020-09-14
Filing date: 2020-09-14
Publication date: 2022-12-20
Anticipated expiration: 2040-09-14
Also published as: CN112138697A

Abstract

The invention discloses a manganese-nitrogen co-doped carbon nanosheet electrocatalyst and a preparation method thereof, wherein the preparation method comprises the following steps: mixing and reacting zinc nitrate hexahydrate with a 2-methylimidazole solution, filtering to obtain a ZIF-8 nanosheet precursor, then respectively dispersing the ZIF-8 nanosheet precursor and manganese chloride into an organic solvent, mixing and stirring, filtering to obtain a solid, and then carrying out heat treatment. The electrocatalyst has a two-dimensional nano flaky structure, is large in specific surface area and beneficial to exposure of active sites, is prepared by taking a metal framework material ZIF-8 as a precursor, and has higher stability and excellent catalytic activity. The manganese metal and nitrogen are doped by adsorption and carbonization, and the preparation method is simple and easy to popularize.

Description

Preparation method and application of manganese-nitrogen co-doped carbon nanosheet electrocatalyst

Technical Field

The invention belongs to the technical field of new energy and new material application, and particularly relates to a preparation method and application of a manganese-nitrogen co-doped carbon nanosheet electrocatalyst.

Background

The ever-increasing demand for energy and the growing environmental concerns have prompted the need for renewable energy conversion and storage systems. A new generation of fuel cells represented by metal-air cells and microbial fuel cells are characterized by being eco-friendly, high in safety, and the like, and are considered as a next-generation clean energy device. The kinetics of the Oxygen Reduction Reaction (ORR) at the cathode of a fuel cell are slow, so the selection of the cathode electrocatalyst is critical to the performance of the new cell.

The noble metal Pt and the alloy thereof are fuel cell cathode ORR catalysts widely used in the early stage, the good catalytic activity of the noble metal Pt and the alloy thereof can reduce the ORR overpotential and improve the current density, but the noble metal Pt and the alloy thereof have the advantages of rare reserves, high price and overhigh cost for large-scale application. Meanwhile, pt-based catalysts are poor in stability and are easily poisoned, so it is necessary to find an electrocatalyst which can replace Pt and has low price and stable performance.

The metal manganese has rich natural resources and low price, thereby becoming an option for replacing Pt catalyst. Under alkaline conditions, the manganese oxide catalyst shows good catalytic activity in the ORR reaction due to large specific surface area and high concentration of active sites, but shows poor reversibility and conductivity, and is difficult to be commercially produced. There are studies to find MnN intercalated into carbon ₄ Has high catalytic activity and stability. However, mn atoms easily form unstable and inactive oxides during heat treatment. Therefore, in order to realize high activity of the Mn-N-C catalyst, a preparation method of increasing the density of active sites without forming oxide agglomerates is intensively studied.

Disclosure of Invention

The first purpose of the present invention is to provide a preparation method of manganese nitrogen co-doped carbon nanosheet electrocatalyst, which is used for solving some problems existing in the prior art.

The second object of the present invention is to provide an electrocatalyst prepared by the above preparation method.

A third object of the present invention is to provide the use of the above electrocatalyst.

The technical scheme adopted by the invention is as follows:

the invention provides a preparation method of a manganese-nitrogen co-doped carbon nanosheet electrocatalyst, which comprises the following steps:

(1) Mixing and reacting a zinc nitrate hexahydrate solution and a 2-methylimidazole solution, and filtering to obtain a ZIF-8 nanosheet precursor;

(2) Respectively dispersing the ZIF-8 nanosheet precursor and manganese chloride in an organic solvent in the step (1), mixing and stirring, and filtering to obtain a solid;

(3) And (3) drying the solid obtained in the step (2) and then carrying out heat treatment to obtain the manganese-nitrogen co-doped carbon nano electro-catalyst.

According to the preparation method of the manganese-nitrogen-codoped carbon nanosheet electrocatalyst according to the first aspect of the present invention, in the step (1), the molar ratio of zinc nitrate hexahydrate to 2-methylimidazole is 1: (4 to 12).

Preferably, according to the preparation method of the manganese-nitrogen-codoped carbon nanosheet electrocatalyst according to the first aspect of the present invention, the molar ratio of the zinc nitrate hexahydrate to the 2-methylimidazole in step (1) is 1:8.

according to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst disclosed by the invention, in the step (1), the mixing is specifically as follows: under the ultrasonic condition, dispersing the zinc nitrate hexahydrate and 2-methylimidazole in deionized water respectively, and mixing after ultrasonic dispersion is uniform.

According to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst disclosed by the invention, in the step (2), the manganese chloride is 0.25-2.0 mmol.

Preferably, according to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst according to the first aspect of the present invention, the manganese chloride in step (2) is 0.75mmol.

According to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst according to the first aspect of the present invention, in the step (1), the molar ratio of the nanosheet precursor to manganese chloride is (1-4): (1-4).

Preferably, according to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst according to the first aspect of the present invention, the molar ratio of the nanosheet precursor to manganese chloride in step (1) is 1:1.

according to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst disclosed by the invention, in the step (2), the organic solvent is an alcohol.

Preferably, according to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst according to the first aspect of the present invention, the organic solvent in step (2) is ethanol.

According to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst, according to the first aspect of the present invention, the heat treatment process in step (3) is as follows: raising the temperature to 800-1100 ℃ at a heating rate of 3-8 ℃/min under the protective atmosphere, and preserving the heat for 1-3 h.

Preferably, according to the preparation method of the manganese and nitrogen co-doped carbon nanosheet electrocatalyst according to the first aspect of the present invention, the heat treatment in step (3) is: raising the temperature to 950 ℃ at a heating rate of 5 ℃/min under a protective atmosphere, and preserving the temperature for 2h.

Further, according to the preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst according to the first aspect of the present invention, the protective atmosphere is nitrogen.

In a second aspect of the invention, the manganese-nitrogen-codoped carbon nanosheet electrocatalyst prepared by the method of the first aspect of the invention is provided.

In a third aspect of the invention, the application of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst according to the second aspect of the invention in electrochemical oxygen reduction reaction is provided.

The invention has the beneficial effects that:

1. the catalyst is prepared by using a metal framework material ZIF-8 as a precursor, and the rich nitrogen content and the pore channel structure of the precursor provide basic guarantee for the excellent catalytic activity of the catalyst. The transition metal Mn is used as the active metal, and has lower cost, higher stability and excellent catalytic activity compared with noble metals.

2. The proper carbonization temperature ensures that inactive manganese oxide particles cannot be formed, is beneficial to forming a single Mn-N-C structure, catalyzes the formation of graphitized carbon, improves the conductive network of the catalyst and increases the transfer rate of electrons.

3. The obtained electrocatalyst has a two-dimensional nano flaky structure, a large specific surface area, high electrochemical activity and excellent methanol poisoning resistance compared with a Pt/C catalyst, and is beneficial to the exposure of active sites.

Drawings

FIG. 1 is a linear voltammogram of the electrocatalysts of examples 1 to 4 and comparative examples 1 to 2.

FIG. 2 is an electrochemical impedance spectrum of the electrocatalysts of examples 1 to 4 and comparative examples 1 to 2.

Figure 3 is a linear voltammogram of the electrocatalysts of examples 1, 5 and 6.

FIG. 4 is a structural morphology characterization of the precursor and electrocatalyst in example 1. Wherein, FIG. 4a is a scanning electron microscope image of a ZIF-8 nanosheet as a precursor in example 1, and FIG. 4b is a scanning electron microscope image of 75-Mn-N-cNS in example 1.

FIG. 5 is an X-ray diffraction pattern (XRD) of the 75-Mn-N-C NS electrocatalyst for example 1.

FIG. 6 is a methanol resistance test of the 75-Mn-N-C NS electrocatalyst with Pt/C in example 1.

Detailed Description

The present invention will be described in more detail with reference to the following examples, which should not be construed as limiting the scope of the present invention.

Example 1

The preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst comprises the following steps:

(1) Weighing 0.0674g (2.04 mmol) of zinc nitrate hexahydrate and 1.3g (16 mmol) of 2-methylimidazole, respectively placing the zinc nitrate hexahydrate and the 2-methylimidazole in two beakers, respectively adding 20mL of deionized water solvent, carrying out ultrasonic treatment for 15min, mixing the zinc nitrate hexahydrate and the 2-methylimidazole, completely dissolving the zinc nitrate hexahydrate and the 2-methylimidazole, uniformly mixing the two solvents, stirring for 8h by using a magnetic stirrer, filtering out a white solid by vacuum filtration, and repeatedly washing by using deionized water to prepare a ZIF-8 nanosheet precursor for subsequent use;

(2) And dissolving 0.75mmol of manganese chloride in 20mL of ethanol solution to obtain a Mn precursor solution. Weighing 1g of ZIF-8, adding into 30mL of ethanol, and uniformly dispersing to obtain a ZIF-8 turbid solution. And mixing the prepared ZIF-8 turbid solution with the Mn precursor solution, performing ultrasonic treatment for 15min, and stirring for 4h by using a magnetic stirrer to prepare Mn-ZIF-8.

(3) Grinding the solid sample, then putting the ground solid sample into a porcelain boat, moving the porcelain boat into a tube furnace, introducing nitrogen, setting the heating rate to be 5 ℃/min, heating to 950 ℃, keeping the temperature for 2h, and collecting the sample after cooling to room temperature. The sample was labeled 75-Mn-N-C NS.

Example 2

The preparation method of the manganese and nitrogen co-doped carbon nanosheet electrocatalyst comprises the following steps of:

(1) Weighing 0.0674g (2.04 mmol) of zinc nitrate hexahydrate and 1.3g (16 mmol) of 2-methylimidazole, respectively placing the zinc nitrate hexahydrate and the 2-methylimidazole into two beakers, respectively adding 20mL of deionized water solvent, carrying out ultrasonic treatment for 15min, mixing, completely dissolving the zinc nitrate hexahydrate and the 2-methylimidazole, uniformly mixing, stirring for 8h by using a magnetic stirrer, filtering out a white solid by vacuum filtration, and repeatedly washing by using deionized water to prepare a ZIF-8 nanosheet precursor for subsequent use;

(2) And dissolving 0.25mmol of manganese chloride in 20mL of ethanol solution to obtain a Mn precursor solution. Weighing 1g of ZIF-8, adding into 30mL of ethanol, and uniformly dispersing to obtain a ZIF-8 turbid solution. And mixing the prepared ZIF-8 turbid solution with a Mn precursor solution, performing ultrasonic treatment for 15min, and stirring for 4h by using a magnetic stirrer to prepare Mn-ZIF-8.

(3) Grinding the solid sample, then putting the ground solid sample into a porcelain boat, moving the porcelain boat into a tube furnace, introducing nitrogen, setting the heating rate to be 5 ℃/min, heating to 950 ℃, keeping the temperature for 2 hours, and collecting the sample after the temperature is reduced to room temperature. The sample was labeled 25-Mn-N-C NS.

Example 3

The procedure in example 3 was substantially the same as in example 1 above, except that: the Mn content of the Mn precursor solution prepared in example 3 was 1.25mmol. The sample was labeled 125-Mn-N-C NS.

Example 4

The procedure in example 4 was substantially the same as in example 1 above, except that: the Mn content of the Mn precursor solution prepared in example 4 was 2.0mmol. The sample was labeled 200-Mn-N-C NS.

Example 5

The procedure in example 5 was substantially the same as in example 1 above, except that: example 5 the temperature was raised to 800 ℃. The sample was labeled 75-Mn-N-C NS 800.

Example 6

The procedure in example 6 was substantially the same as in example 1 above, except that: example 6 the temperature was increased to 1100 ℃. The sample was labeled 75-Mn-N-C NS 1100.

Comparative example 1

The preparation process in comparative example 1 is substantially the same as in example 1 above, except that: the Mn content of the Mn precursor solution prepared in comparative example 1 was 0mmol. The sample was labeled N-C NS.

Comparative example 2

Pt/C platinum carbon catalyst is commonly used on the market.

Example 7 electrochemical Performance characterization test

(1) ORR performance research of nano sheets with different manganese loading amounts.

The samples in examples 1-4 and comparative examples 1-2 were subjected to redox performance analysis, and ORR performance of nanosheets with different manganese loadings was studied.

Preparing a working electrode: weighing 3mg of a carbon material sample, dissolving the carbon material sample in 0.5mL of deionized water, 0.5mL of isopropanol and 20 mu L of Nafion, carrying out ultrasonic treatment for 15min to uniformly disperse the carbon material sample to prepare catalyst ink, using a liquid transfer gun to transfer 5 mu L of catalyst ink, dropwise adding the catalyst ink on the surface of a working electrode, and drying.

Linear voltammetry: a standard three-electrode system is adopted, a graphite electrode is used as a counter electrode, hg/HgO is used as a reference electrode, and the glassy carbon electrode dripped with the catalyst is used as a working electrode. The electrolyte solution is 0.1M KOH, oxygen is introduced before the test, after oxygen saturation is waited, an LSV curve is tested, the scanning speed is 10mV s < -1 >, the rotating speed is 1600rpm, the voltage interval of ORR reaction is-0.8-0.3V vs. s < -Hg >/HgO, data are recorded, and specific results are shown in Table 1 and figure 1.

TABLE 1 electrochemical data of Mn-N-C nanosheets and Pt/C with different manganese doping amounts

As can be seen from Table 1, the ORR performance of all the samples with the manganese source added is better than that of the samples without the manganese source added, so that the manganese metal can be obtained as an active species and plays an important role in the process of catalyzing the ORR. FIG. 1 is a linear voltammogram of nanosheets of different manganese loadings, wherein the limiting current density and the initial potential of 75-Mn-N-cNS are both greater than other manganese source loaded nanosheets, closest to Pt/C, 75-Mn-N-cNS with the best ORR performance. The more positive the oxygen reduction potential, the smaller the overpotential, O ₂ The more readily electrons are received to cause an oxygen reduction reaction, and therefore, when used as a cathode catalyst, the more likely the power generation performance of air fuel is improved.

Electrochemical impedance spectroscopy: under the open circuit voltage, the high frequency is 100000Hz, the low frequency is 0.01Hz, the amplitude is 0.005V, and the data is recorded. The specific results are shown in FIG. 2. Wherein the semi-circle radius, i.e. the charge transfer resistance, is generally related to the activity of the electrode surface. The smaller the semi-circle radius is, the smaller the charge transfer resistance of the material is, and the higher the charge transfer efficiency is.

From FIG. 2, it can be seen that 75-Mn-N-cNS has the lowest charge transfer resistance in all the examples, indicating that it has higher charge transfer efficiency, and the smaller the electron transfer barrier, the better the corresponding kinetic performance is, and providing strong evidence for the better catalytic performance of 75-Mn-N-cNS. It is shown that when the loading amount of manganese is different, the ORR electrochemical performance of the catalyst is influenced, wherein 75-Mn-N-C NS has the maximum limiting current density, the most positive oxygen reduction potential and lower charge transfer resistance, and shows the optimal ORR catalytic activity in all samples.

(2) ORR performance research of Mn-N-C nanosheet electrocatalyst with different calcination temperatures.

The redox performance analysis of the Mn-N-C nanosheet electrocatalyst in examples 1, 5 and 6 was performed to investigate the ORR performance of the nanosheet electrocatalyst at different calcination temperatures.

Preparing a working electrode: weighing 3mg of a carbon material sample, dissolving the carbon material sample in 0.5mL of deionized water, 0.5mL of isopropanol and 20 mu L of Nafion, carrying out ultrasonic treatment for 15min to uniformly disperse the carbon material sample to prepare catalyst ink, transferring 5 mu L of catalyst ink by using a liquid transfer gun, dripping the catalyst ink on the surface of a working electrode, and drying.

Linear voltammetry: a standard three-electrode system is adopted, a graphite electrode is used as a counter electrode, hg/HgO is used as a reference electrode, and the glassy carbon electrode dripped with the catalyst is used as a working electrode. The electrolyte solution is 0.1M KOH, oxygen is introduced before the test, after oxygen saturation, an LSV curve is tested, and the scanning speed is 10 mV.s ^-1 The rotation speed was 1600rpm, the voltage range of ORR reaction was-0.8-0.3V vs. Hg/HgO, and the data was recorded, and the specific results are shown in Table 2 and FIG. 3.

TABLE 2 electrochemical data of Mn-N-C nanosheets of different calcination temperatures

Table 2 and fig. 3 show a comparison of ORR activity of Mn-N-C nanoplates at different calcination temperatures. The limiting current density and the initial potential of the Mn-N-C-950 nanosheet are both greater than those of Mn-N-C-800 and Mn-N-C-1100 nanosheets, and the catalytic effect on the oxygen reduction reaction is the best. Therefore, the catalyst effect is best when the calcination temperature is 950 ℃. The catalytic effect is reduced at the calcining temperature of 1100 ℃, which may be due to skeleton collapse and structural damage of the nanosheets caused by overhigh temperature, or due to loss of heteroatoms such as nitrogen caused by overhigh temperature. The catalytic efficiency of the Mn-N-C nanosheet catalyst calcined at 800 ℃ is slightly lower than that of Mn-N-C-950, which may be due to the fact that the graphitization degree at the temperature is incomplete, so that the conductivity is low, and the charge cannot be effectively transferred, or due to the fact that more Mn-N active sites are not easily generated at the low temperature, so that the catalytic efficiency is lower than that of Mn-N-C-950.

Example 8 structural morphology characterization of Mn-N-C nanosheets and precursors thereof

The precursor ZIF-8 nanosheets and carbonized 75-Mn-N-C NS were scanned by electron microscopy (SEM) and the results are shown in FIG. 4. As clearly seen from FIG. 4a, the precursor ZIF-8 nanosheet presents an obvious two-dimensional sheet-like morphology, the nanosheets are different in size and between 1 and 6 microns in size, but the thickness of the nanosheets is similar, and the nanosheets are randomly dispersed and stacked together. FIG. 4b shows the results of scanning electron microscopy characterization of calcined and carbonized 75-Mn-N-C NS nanosheets, which are similar in morphology to the precursor ZIF-8 nanosheets, substantially maintaining a two-dimensional sheet shape, but having a rougher surface with many small particles uniformly distributed thereon, which should be the active metal component exposed by the reduction of the metal during the carbonization process.

FIG. 5 is an XRD diffractogram of 75-Mn-N-C NS nanoplatelets. It can be seen from fig. 5 that 75-Mn-N-C NS nanoplatelets present two diffraction peaks for amorphous carbon at 30 ° as well as 42 °. Corresponding to the (002) and (101) crystal faces of the graphitic carbon. No diffraction peak of manganese oxide was found due to the higher carbonization temperature and the nitrogen atmosphere suppressed the formation of manganese oxide.

Example 9 analysis of methanol resistance

The results of the methanol poisoning resistance test conducted on 75-Mn-N-C NS in example 1 and Pt/C in comparative example 2 are shown in FIG. 6, and it can be seen that, after methanol is injected into the test system, pt/C immediately shows a large polarization phenomenon, and the current density drops suddenly, while 75-Mn-N-C NS can be kept relatively stable, which reflects that 75-Mn-N-C has methanol poisoning resistance significantly better than Pt/C, and highlights its utility in fuel cells.

The above-mentioned embodiments only express several embodiments of the present invention, and the description thereof is specific and detailed, but not to be understood as limiting the scope of the present invention. It should be noted that, for a person skilled in the art, several variations and modifications can be made without departing from the inventive concept, which falls within the scope of the present invention.

Claims

1. The preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst is characterized by comprising the following steps of:

(1) Respectively dispersing a zinc nitrate hexahydrate solution and a 2-methylimidazole solution in deionized water under an ultrasonic condition, performing ultrasonic dispersion to be uniform, then performing mixed reaction, and filtering to obtain a ZIF-8 nanosheet precursor;

(3) Drying the solid obtained in the step (2), and then carrying out heat treatment in a protective atmosphere to obtain the manganese-nitrogen co-doped carbon nanosheet electrocatalyst;

in the step (1), the molar ratio of the zinc nitrate hexahydrate to the 2-methylimidazole is 1: (4-12);

in the step (2), the molar ratio of the ZIF-8 nanosheet precursor to manganese chloride is (1-4): (1-4);

the temperature of the heat treatment in the step (3) is 800-1100 ℃.

2. The preparation method of the manganese-nitrogen co-doped carbon nanosheet electrocatalyst according to claim 1, wherein the organic solvent in step (2) is an alcohol.

3. The method of claim 2, wherein the alcohol is ethanol.

4. The preparation method of the manganese-nitrogen-codoped carbon nanosheet electrocatalyst according to claim 1, wherein the heat treatment in step (3) specifically comprises: raising the temperature to 800-1100 ℃ at the heating rate of 3-8 ℃/min under the protective atmosphere, and preserving the temperature for 1-3 h.

5. The preparation method of the manganese-nitrogen-codoped carbon nanosheet electrocatalyst according to claim 1, wherein the protective atmosphere is nitrogen.

6. A manganese nitrogen co-doped carbon nanosheet electrocatalyst prepared according to the preparation method of any one of claims 1 to 5.

7. The use of the manganese nitrogen co-doped carbon nanosheet electrocatalyst of claim 6 in electrochemical oxygen reduction reactions.