GB2603717A

GB2603717A - Crop straw-based nitrogen-doped porous carbon material preparation method and application thereof

Info

Publication number: GB2603717A
Application number: GB2205965.3A
Authority: GB
Inventors: Xu Lang; Yang Chuangchuang; Jiang Haishen; Zhang Wendu; Bai Peiyao; Liu Weiqi
Original assignee: China University of Mining and Technology CUMT
Current assignee: China University of Mining and Technology CUMT
Priority date: 2020-09-10
Filing date: 2021-06-11
Publication date: 2022-08-10
Anticipated expiration: 2041-06-11
Also published as: WO2022052539A1; CN112062128B; GB2603717B; GB202205965D0; CN112062128A

Abstract

A crop straw-based nitrogen-doped porous carbon material preparation method and an application, where straw is used as raw material, and straw powder and an alkali metal hydroxide are heated and stirred in an aqueous solution. The processing of lignin, cellulose, and hemicellulose by means of alkaline boiling allows the dense raw material structure to loosen, thereby changing the pore size distribution of the material. Subsequently, an activator potassium bicarbonate and a nitrogen source melamin are sequentially added, and stirring and mixing is performed, then after drying, high temperature pyrolysis, pickling, filtration, and drying steps are undergone, and a catalytic material is obtained. The nitrogen-doped porous carbon material possesses excellent oxygen reduction electrocatalytic performance, high cycling stability, strong methanol tolerance under all pHs, and can be used as a fuel cell cathode oxygen reduction reaction electrocatalytic material. The present invention is low-cost and environmentally friendly, usable raw materials are wide-ranging, the preparation process is simple and highly effective, the invention can be used for large scale production, and the invention has highly valuable practical application.

Description

CROP STRAW-BASED NITROGEN-DOPED POROUS CARBON MATERIAL

PREPARATION METHOD AND APPLICATION THEREOF

Technical field

The present invention relates to the field of carbon materials and catalysis, in particular to a method for preparing a nitrogen-doped porous carbon material based on crop straws and use of the carbon material.

Background Art

The oxygen reduction reaction (ORR) is a crucial reaction applicable to the technical field of energy storage and conversion, and can be used in fields such as fuel cells and metal-air cells, etc. However, the kinetic hysteresis of oxygen reduction reaction greatly limits the development and application of fuel cells and metal-air cells. At present, Pt-based materials have good oxygen reduction catalytic activity. However, the noble metal Pt is expensive and has low reserves, resulting in high manufacturing cost of Pt-based catalysts. Moreover, Pt-based materials may be easily poisoned and inactivated by methanol, CO and sulfides, etc., and have poor stability and tolerance. Consequently, it is difficult to achieve large-scale practical applications of Pt-based materials. Therefore, it is of great importance for the development and large-scale application of fuel cells and metal-air cells to develop a substitute of Pt-based materials, which is stable and efficient, and has excellent oxygen reduction catalytic activity.

Biomass, as an ideal cheap, easily available and renewable raw material with abundant heteroatoms (e.g., N, 0, P and 5), has drawn the attention of many researchers. In addition, there are a variety of biomass raw materials, which contain abundant and diverse original pore structures. Although the oxygen reduction catalysis performance of most biomass-derived materials is lower than that of commercial 20% Pt/C catalyst, they are a kind of promising green precursors.

In order to solve these problems, a lot of research work has been done. Among non-noble metal catalysts, transition metal catalysts (oxides, sulfides and nitrides of transition metals, etc.) are also very promising materials. In nonmetallic materials, nonmetallic heteroatoms (N, P, S and B) doped into the carbon skeletons produce defects, which lead to uneven charge distribution on the carbon skeletons and generate active sites for oxygen reduction reaction. Non-metallic heteroatom-doped materials have large specific surface area, and have diverse pore size distribution, * * morphologies and active sites. Among them, nitrogen-doped porous carbon materials exhibit the most excellent oxygen reduction catalytic activity, poisoning resistance, durability and stability. However, most nonmetallic materials have good catalytic performance under alkaline conditions, and have poor catalytic performance under neutral and acidic conditions, which is the obstacle and difficulty that need to be overcome in order to further improve the application range. Most normal-temperature fuel cells are proton exchange membrane fuel cells, i.e., under the condition of acidic electrolyte; in neutral electrolytes, microbial fuel cells also have wide applications and prospects. In order to meet the needs of market and development, it is not only a challenge but also an important direction of future research and development to develop stable, efficient and cheap catalysts with excellent oxygen reduction electrocatalyt c performance in the entire pH range.

Contents of the Invention A first object of the present invention is to provide a method for preparing a nitrogen-doped porous carbon material based on crop straws, which utilizes cheap and easily available raw materials, employs a simple preparation scheme, and is green and environment-friendly.

A second object of the present invention is to provide a use of the nitrogen-doped porous carbon material prepared with the above-mentioned preparation method in oxygen reduction catalysis.

To attain the above objects, the present invention employs the following technical scheme: a method for preparing a nitrogen-doped porous carbon material based on crop straws, comprising the following steps: step 1: washing, drying and crushing collected crop straws to obtain straw powder; step 2: mixing the straw powder and an alkali metal hydroxide in proportion, adding the obtained mixture into deionized water, heating at 70°C while stirring for 2 hours to obtain a material A; keeping the temperature and the stirring rate unchanged, adding potassium bicarbonate (for increasing the dosage of an activator and preventing the amino group of melamine from being substituted by hydroxyl groups in a strong alkaline solution) and melamine into the material A sequentially, mixing homogeneously, and drying, to obtain a material B, wherein the mass concentration of the alkali metal hydroxide is 1.5 wt.%, and the mass ratio of the straw powder to the alkali metal hydroxide to the potassium bicarbonate to the melamine is (1-5):(1-3) (1-5) (1-5), step 3: placing the material B into a tube furnace for pyrolysis, heating up to 300°C at a heating rate of 5°C/minute under the protection of inert gas and keeping the temperature constant for 2 hours (to sublimate the melamine so that the melamine contacts with the material more extensively, thereby increase the doping efficiency and raw material utilization ratio), then heating up to 900°C and keeping the temperature constant for 2 hours (to increase the degree of graphitization of the material, thereby improve the conductivity), then cooling to 300°C at a cooling rate of 5°C/minute, and cooling naturally to room temperature, to obtain a material C; step 4: pickling the material C with acid, then filtering, washing with water till the filtrate is neutral, and drying, so that the nitrogen-doped porous carbon material is obtained.

Preferably, the mass ratio of the straw powder to the alkali metal hydroxide to the potassium bicarbonate to the melamine in the step 2 is 3:1:3:3.

Preferably, the stirring rate in the step 2 is 300-500 rpm Preferably, the alkali metal hydroxide is at least one of potassium hydroxide and sodium hydroxide.

Preferably, the crop straws are at least one of wheat straws, corn straws, rice straws and cotton straws.

Preferably, the pickling treatment step in the step 4 is: stirring in 1 molt' dilute hydrochloric acid solution at 40-60°C for 1-3 hours The present invention further provides a use of the nitrogen-doped porous carbon material prepared with the above-mentioned method in oxygen reduction catalysis.

The nitrogen-doped porous carbon material can be used for catalyzing cathodic oxygen reduction reactions in fuel cells. The method is as follows: placing 3 mg nitrogen-doped porous carbon material in a centrifugal tube, and adding 170 RL deionized water, 70 ML isopropanol and 10 pL 5% perfluorosulfonic acid-polytetrafluoroethylene copolymer (Nation) solution; ultrasonically mixing for 1 hour to form a uniform dispersion liquid; taking 10 ML dispersion liquid with a pipette and dropping the dispersion liquid on the glassy carbon surface of a clean rotating disc electrode (RDE), and drying out naturally, so as to obtain a working electrode. The prepared working electrode, a silver/silver chloride reference electrode and a platinum wire counter electrode are used to form a three-electrode system. The three-electrode system is tested in an acidic electrolyte (0.5 M H2SO4), a neutral electrolyte (0.1 NI PBS, pH=7) and an alkaline electrolyte (0.1 Ivl KOH) by means of an electrochemical workstation respectively.

Compared with the prior art, the present invention attains the following beneficial effects: In the present invention, a wheat straw raw material is treated directly by alkali boiling to change the pore size distribution of the material, and a nitrogen-doped porous carbon material that has excellent oxygen reduction catalysis performance in the entire pH range is prepared; the experimental scheme is simple, convenient, green and pollution-free, meets the aims of environmental protection and resource utilization, and realizes effective reuse of agricultural wastes.

The preparation strategy for changing the pore size distribution of the biomass-derived nitrogen-doped material provided by the present invention is simple and highly inventive, and has a great space of exploration and extension; in addition, the oxygen reduction catalysis performance of the prepared nitrogen-doped porous carbon material is much higher than that of similar materials. In 0.5 mol-L-1 sulfuric acid solution, the initial potential is approximately 0.82 V (relative to a reversible hydrogen electrode RHE), the half-wave potential is approximately 0.66 V (relative to a RHE), and the limiting current density is about -6.18 mA*cm-2; in 0.1 mon' neutral solution of dipotassium hydrogen phosphate and potassium dihydrogen phosphate (PBS solution, pH=7.0), the initial potential is approximately 0.89V (relative to a RHE), the half-wave potential is approximately 0.71 V (relative to a RHE), and the limiting current density is about -6.03 miN * cm--2; in 0.1 moLL-1 potassium hydroxide solution, the initial potential is approximately 1.00 V (relative to a RHE), the half-wave potential is approximately 0.86 V (relative to a RHE), and the limiting current density is about -6.52 mA.cm-2. In acidic, alkaline and neutral electrolytes, the catalyst is comparable or even superior to commercial 20% Pt/C. Besides, the catalyst has excellent cyclic stability and methanol resistance, and has great application prospects.

3. In the present invention, based on the properties of the materials, it is found that the change of the pore size distribution of a material leads to the change of pore volume of the material. The limiting current density in an oxygen reduction reaction can be greatly improved by improving the pore size distribution of the material while other conditions are similar. The preparation strategy provided by the present invention can further adjust the pore size distribution of the prepared material by changing the experimental conditions, and has strong inventiveness and great application value.

Description of Drawings

Fig. 1 is a transmission electron microscopy (TEM) image of the nitrogen-doped porous carbon catalyst prepared in the example 1; Fig. 2 is a high-resolution transmission electron microscopy (BREEN) image of the nitrogen-doped porous carbon catalyst prepared in the example 1; Fig. 3 shows the isothermal (77K) nitrogen adsorption-desorption curves of the nitrogen-doped porous carbon catalysts prepared in the example 1 and comparative

examples 1-2,

Fig. 4 shows the pore size distribution curves of the nitrogen-doped porous carbon catalysts prepared in the example 1 and comparative examples 1-2 as calculated in a density functional theory (DFT) model, Fig. 5 is an X-Ray Photoelectron Spectroscopy (XPS) N-peak spectrogram of the nitrogen-doped porous carbon catalyst prepared in the example 1; Fig. 6 shows the linear sweep voltammetry (LSV) curves of the nitrogen-doped porous carbon catalysts prepared in the example 1 and comparative examples 1-2 and the Pt/C in the comparative example 3 in 0.5 M H2SO4 solution saturated with oxygen at 1600 rpm; Fig. 7 shows the linear sweep voltammetry (LSV) curves of the nitrogen-doped porous carbon catalysts prepared in the example 1 and comparative examples 1-2 and the Pt/C in the comparative example 3 in 0.1 M PBS solution saturated with oxygen at 1600 rpm; and Fig. 8 shows the linear sweep voltammetry (LSV) curves of the nitrogen-doped porous carbon catalysts prepared in the example 1 and comparative examples 1-2 and the Pt/C in the comparative example 3 in 0.1 NI KOH solution saturated with oxygen at 1600 rpm.

Embodiments The innovative features and contents of the present invention will be further illustrated in the following examples, but the examples should not be construed as constituting any limitation to the present invention. In accordance with the principles and content of the present invention, modifications made to methods and steps and other conditions of the present invention all belong to the scope of protection of the present invention

Example 1

Collected wheat straws were washed and dried, and then crushed in a crusher into powder. According to a mass ratio, three parts of wheat straw powder and one part of potassium hydroxide were put into a beaker, deionized water was added, the mass concentration of potassium hydroxide was 1.5 wt.%, and the mixture was heated at 70°C for 2 hours while it was stirred at a stirring rate of 400 rpm. The temperature and the stirring rate were kept unchanged, three parts of potassium bicarbonate were added into the beaker, and the mixture was further heated for 10 minutes while it was stirred; finally, three parts of melamine were added, and the mixture was mixed for 10 minutes while it was stirred, and then was transferred to an oven and dried at 80°C The dried product was placed in a tube furnace for pyrolysis; in a N2 atmosphere, at a heating rate of 5°C/minute, the product was kept at 300°C and 900°C for 2 hours respectively for doping and activation treatment; then the product was cooled to 300°C at a cooling rate of 5°C/minute, and then naturally cooled to room temperature, so that an unpickled nitrogen-doped porous carbon material was obtained. The unpickled nitrogen-doped porous carbon material was placed into 1 mo1*14-1 hydrochloric acid and pickled at 50°C at 400 rpm for 2 hours, then filtered, washed with water till the filtrate is neutral, dried and stored, so that a nitrogen-doped porous carbon material was obtained.

3 mg nitrogen-doped porous carbon material of the example 1 was placed in a centrifugal tube, and 170 [IL deionized water, 70 p.14 isopropanol and 10 L 5% perfluorosulfonic acid-polytetrafluoroethylene copolymer (Nafion) solution were added. The mixture was ultrasonically mixed for 1 hour to form a uniform dispersion liquid; 10 1,11_, dispersion liquid was taken with a pipette and dropped on the glassy carbon surface of a clean rotating disc electrode (RDE), and dried out naturally, so that a working electrode was obtained. The prepared working electrode, a silver/silver chloride reference electrode and a platinum wire counter electrode were used to form a three-electrode system. The three-electrode system was tested in an acidic electrolyte (0.5 NI H2SO4), a neutral electrolyte (0.1 M PBS, pH=7) and an alkaline electrolyte (0.1 Ni KOH) by means of an electrochemical workstation respectively. A cyclic voltammetry curve and a linear sweep voltammetry curve were swept respectively in an electrolyte saturated with oxygen or nitrogen, at sweeping rates of 5 mV*s-1 and 10 mV*s-1 respectively.

Comparative Example 1 Collected wheat straws were washed and dried, and then crushed in a crusher into powder. According to a mass ratio, three parts of wheat straw powder were placed into a beaker, the same amount of deionized water as in the example 1 was added, and the mixture was heated for 2 hours at 70°C while it was stirred at a stirring rate of 400 rpm. Then three parts of potassium bicarbonate were added, the mixture was further heated for 10 minutes while it was stirred, finally three parts of melamine were added, and the mixture was mixed for 10 minutes while it was stirred, and then transferred to an oven and dried at 80°C. The dried product was pyrolyzed in a tube furnace; in a N2 atmosphere, at a heating rate of 5°C/minute, the product was kept at 300°C and 900°C for 2 hours respectively for doping and activation treatment; then the product was cooled to 300°C at a cooling rate of 5°C/minute, and then naturally cooled to room temperature, so that an unpickled nitrogen-doped porous carbon material was obtained. The unpickled nitrogen-doped porous carbon material was added into 1 mol*L-1 hydrochloric acid and pickled at 50°C at 400 rpm for 2 hours; then the material was filtered, washed with water till the filtrate was neutral, dried and stored, so that a nitrogen-doped porous carbon material was obtained.

The method of preparing working electrode with the nitrogen-doped porous carbon material of the above-mentioned comparative example 1 as an oxygen reduction catalyst for fuel cells and the electrochemical test method are the same as those of the example I. Comparative Example 2 Collected wheat straws were washed and dried, and then crushed in a crusher into powder. According to a mass ratio, three parts of wheat straw powder were placed into a beaker, the same amount of deionized water as in the example 1 was added, and the mixture was heated for 2 hours at 70°C while it was stirred at a stirring rate of 400 rpm. One part of potassium hydroxide was added, and the mixture was heated for 10 minutes while it was stirred; then three parts of potassium bicarbonate were added, and the mixture was further heated for 10 minutes while it was stirred; finally, three parts of melamine were added, and the mixture was mixed for 10 minutes while it was stirred, and then was transferred to an oven and dried at 80°C. The dried product was pyrolyzed in a tube furnace; in a N2 atmosphere, at a heating rate of 5°C/minute, the product was kept at 300°C and 900°C for 2 hours respectively for doping and activation treatment; then the product was cooled to 300°C at a cooling rate of 5°C/minute, and then naturally cooled to room temperature, so that an unpickled nitrogen-doped porous carbon material was obtained. The unpickled nitrogen-doped porous carbon material was added into 1 molt' hydrochloric acid and pickled at 50°C at 400 rpm for 2 hours; then the material was filtered, washed with water till the filtrate was neutral, dried out and stored, so that a nitrogen-doped porous carbon material was obtained.

The method of preparing working electrode with the nitrogen-doped porous carbon material of the above-mentioned comparative example 2 as an oxygen reduction catalyst for fuel cells and the electrochemical test method are the same as those of the example 1.

Comparative Example 3 1 mg commercial 20% Pt/C was taken, and 170 tL deionized water, 70 [IL isopropanol and 10 [IL 5% perfluorosulfonic acid-polytetrafluoroethylene copolymer (Nation) solution were added. The mixture was ultrasonically mixed for 1 hour to form a uniform dispersion liquid; 6 fa, dispersion liquid was taken with a pipette and dropped on the glassy carbon surface of a clean rotating disc electrode (RDE), and dried out naturally, so that a working electrode was obtained. The prepared working electrode, a silver/silver chloride reference electrode and a platinum wire counter electrode were used to form a three-electrode system. The three-electrode system was tested in an acidic electrolyte (0.5 NI H2SO4), a neutral electrolyte (0.1 NI PBS, pH=7) and an alkaline electrolyte (0.1 M KOH) by means of an electrochemical workstation respectively. A cyclic voltammetry curve and a linear sweep voltammetry curve were * 7 * swept respectively in an electrolyte saturated with oxygen or nitrogen, at sweeping rates of 5 mV s-1 and 10 mV s-1 respectively.

(1) Transmission electron microscopy (TEM) test and high-resolution transmission electron microscopy (HRTEM) test The prepared metal-free nitrogen-doped porous carbon material (example 1) was placed under a transmission electron microscope (TEM) (Fig. 1) and high-resolution transmission electron microscope (HRTENI) (Fig. 2) for observation. Generally speaking, KOH alkali boiling treatment of lignocellulose biomass has the following effects: 1) destroying the hydrogen bonds and making the cellulose swell; 2) dissolving the hemicellulose and causing an alkaline degradation reaction; 3) degrading the lignin, decreasing the strength of the material, so that the chemical reagents can treat the material more fully; 4) causing a reaction with the acidic functional groups and improving hydrophilicity, etc. As shown in Fig. 1, the edges of the carbon material exhibit a large quantity of disordered, porous and thin cotton yam-like structures, with large specific surface area and pore volume. This is due to the above-mentioned effects of alkali boiling treatment on the lignocellulose material, resulting in the dissolution of a large quantity of small molecular substances from the material, the compactness of the original material is destroyed, resulting in a looser material structure.. After the mixed material is dried out in the oven, the dissolved small molecules will adhere to the surface of the material; after the pyrolysis and activation, the small molecules are bonded again, forming porous cotton yarn-like structures. As shown in Fig. 2, the material has obvious lattice fringes, owing to the influence of the pores, the lattice fringes are distorted around the pores, which not only demonstrates that the material has a high degree of disorder and porous channel structures, but also demonstrates that the carbon skeleton material has a high degree of graphitization. The developed pore structures and high degree of graphitization are conducive to the formation of a two-way efficient synergistic effect of mass transfer and electron transfer, and greatly enhance the catalytic activity for oxygen reduction reaction.

(2) Nitrogen adsorption and desorption test The specific surface area and pore structures of the samples in the example 1 and comparative examples 1 and 2 were tested with a nitrogen physical adsorption and desorption apparatus. The results show that the specific surface areas in the example 1, comparative example 1 and comparative example 2 are 1,430.18 m2.g-1, 1,311.45 m2-g1 and 1,409.16 m2-g-1 respectively; the total pore volumes of the catalysts are 1.230 0.713 cm''gl and 0.903 cm".g-' respectively; the micropore volumes of the catalysts are 0.375 cm3-g-1, 0.371 cm3-g1 and 0.418 cm3.g-1 respectively; the mesopore volumes of the catalysts are 0.856 0.342 cm".g-I and 0.485 cm3-g-1 respectively. It can be seen from the above data and the nitrogen adsorption-desorption curves (Fig. 3) that the three samples all have large specific * * surface area, and the difference in the specific surface area among the samples is not large. Based on the pore volume data and the pore size distribution curves in a DFT model (Fig. 4), through the comparison between the example 1 and the comparative example 2, although the amount of activator is the same and the specific surface areas are essentially the same, there is a great difference in the pore volume between the example 1 and the comparative example 2. The sample in the example 1 has a larger pore volume after 2 hours of alkaline boiling treatment, especially, the mesoporous pore volume has been greatly improved, which well proves that the alkaline boiling treatment has changed the pore size distribution of the material. However, the pore size distribution and pore volume changes of comparative example 1 and comparative example 2 without alkali boiling treatment were similar, which indicates that although the content and type of activators were different, the activator could not significantly change the pore size distribution of the material at the same pyrolysis temperature.

(3) X-ray photoelectron spectroscopy (XPS) test X-ray photoelectron spectroscopy (XPS) tests were carried out for the examples in the example 1, comparative example 1 and comparative example 2. The results show that the atomic percentages of total nitrogen content in the example 1, comparative example 1 and comparative example 2 are 2.39%, 2.49% and 2.21%, respectively. Fig. 5 is a narrow sweep peak fitting curve of the nitrogen peaks of the example 1, the nitrogen peaks can be mainly divided into pyridine nitrogen, pyrrole nitrogen, graphite nitrogen and nitric oxide, wherein, pyridine nitrogen and graphite nitrogen are the main contributors to the catalytic activity for oxygen reduction reaction. The total relative contents of pyridine nitrogen and graphite nitrogen in the example 1, comparative example 1 and comparative example 2 are 55.69%, 60.55% and 51.25% respectively. According to the XPS test results, at the same pyrolysis temperature, the total nitrogen contents of example 1, comparative example 1 and comparative example 2 are similar, and the total relative contents of pyridine nitrogen and graphite nitrogen are also very close.

(4) Oxygen reduction electrocatalysis performance test Working electrodes were prepared from the samples in the example 1, comparative example 1, comparative example 2 and comparative example 3 with the above method, and were used together with a silver/silver chloride reference electrode and a platinum wire counter electrode to form a three-electrode system respectively. A linear sweep voltammetry (LSV) curve test was carried out in an acidic electrolyte (0.5 NI H2SO4), a neutral electrolyte (0.1 M PBS, pH=7) and an alkaline electrolyte (0.1 M KOH) respectively, at a sweep rate of 10 mV*s-l. Figs. 6, 7 and 8 show the linear sweep voltammetry (LSV) curve tests at 1600 rpm in the acidic electrolyte, the neutral electrolyte and the alkaline electrolyte respectively. Under an acidic condition, the initial potentials (relative to a reversible hydrogen electrode RHE) of the example 1, comparative example 1, comparative example 2 and comparative example 3 were all 0.82 V, and the half-wave potentials (relative to a reversible hydrogen electrode RHE) were 0.66 V, 0.63 V, 0.67 V and 0.60 V respectively; the limiting current densities were -6.18 mA-cm-2, -4.61 mA-cm-2, -5.00 mA-cm-2 and -5.50 mA*cm-2 respectively. Under a neutral condition, the initial potentials (relative to a reversible hydrogen electrode RHE) of the example 1, comparative example 1, comparative example 2 and comparative example 3 were 0.89 V, 0.88 V, 0.89 V and 0.88 V respectively, and the half-wave potentials (relative to a reversible hydrogen electrode RUE) were 0.71 V, 0.67 V, 0.68 V and 0.58 V respectively, the limiting current densities were -6.03 mA*cm-2, -4.90 mA*cm-2, -5.64 mA*cm-2 and -5.41 mA*cm-2 respectively. Under an alkaline condition, the initial potentials (relative to a reversible hydrogen electrode RIM) of the example 1, comparative example 1, comparative example 2 and comparative example 3 were 1.00 V, 0.99 V, 1.00 V and 0.98 V respectively, and the half-wave potentials (relative to a reversible hydrogen electrode RHE) were 0.86 V, 0.85 V, 0.87 V and 0.82 V respectively; the limiting current densities are -6.52 mA*cm-2, -4.28 mA*cm-2, -5.00 mA-cm-2 and -5.38 mA*cm-2 respectively. It is seen from the comparison between the example 1 and the comparative example 3 that the oxygen reduction catalytic activity of example 1 is much better than that of comparative example 3 in acidic, neutral and alkaline electrolytes, which indicates that the catalyst prepared according to the present invention has a great potential to be a substitute for platinum-based catalytic materials.

The samples in the example 1, comparative example 1 and comparative example 2 have similar initial potentials in acidic, neutral and alkaline electrolytes, but there is a great difference in the limiting current density among them The reason is that the three catalysts have very similar nitrogen content and distribution of nitrogen species, and have essentially the same specific surface area. The data indicates that the number and exposure conditions of active sites in the three catalysts are similar, so the catalysts have similar initial potentials in acidic, neutral and alkaline electrolytes An appropriate pore volume is beneficial to the transfer and transportation of substances, thereby can increase the limiting current density of the catalyst The limiting current densities of the samples in the example 1, comparative example 1 and comparative example 2 measured in acid, neutral and alkaline electrolytes have a positive correlation relationship with the corresponding pore volumes The present invention provides a catalyst preparation strategy that can optimize the pore size distribution. In addition, the preparation strategy doesn't cause significant change to other data, has no negative influence on catalytic activity, breaks through the original technical barriers, points out a new direction of catalyst optimization, and has strong inventiveness and great application value * 10 *

Claims

Claims A method for preparing a nitrogen-doped porous carbon material based on crop straws, characterized in that, said method comprises the following steps: step 1 washing, drying and crushing collected crop straws to obtain straw powder; step 2: mixing the straw powder and an alkali metal hydroxide in proportion, adding the obtained mixture into deionized water, heating at 70°C while stirring for 2 hours to obtain a material A; keeping the temperature and the stirring rate unchanged, adding potassium bicarbonate and melamine into the material A sequentially, mixing homogeneously, and drying, to obtain a material B, wherein the mass concentration of the alkali metal hydroxide is 1.5 wt.%, and the mass ratio of the straw powder to the alkali metal hydroxide to the potassium bicarbonate to the melamine is (1-5):(1-3):(1-5):(1-5); step 3. placing the material B into a tube furnace for pyrolysis, heating up to 300°C at a heating rate of 5°C/minute under the protection of inert gas and keeping the temperature constant for 2 hours, then heating up to 900°C and keeping the temperature constant for 2 hours, then cooling to 300°C at a cooling rate of 5°C/minute, and cooling naturally to room temperature, to obtain a material C; step 4. pickling the material C with acid, then filtering, washing with water till the filtrate is neutral, and drying, so that the nitrogen-doped porous carbon material is obtained The method for preparing a nitrogen-doped porous carbon material based on crop straws according to claim 1, wherein the mass ratio of the straw powder to the alkali metal hydroxide to the potassium bicarbonate to the melamine in the step 2 is 3:1:3:3.The method for preparing a nitrogen-doped porous carbon material based on crop straws according to claim 1, wherein the stirring rate in the step 2 is 300-500 rpm The method for preparing a nitrogen-doped porous carbon material based on crop straws according to claim 1, wherein the alkali metal hydroxide is at least one of potassium hydroxide and sodium hydroxide.The method for preparing a nitrogen-doped porous carbon material based on crop straws according to claim 1, wherein the crop straws are at least one of wheat straws, corn straws, rice straws and cotton straws The method for preparing a nitrogen-doped porous carbon material based on crop straws according to claim 1, wherein the pickling treatment step in the step 4 is: stirring in 1 moTL-1 dilute hydrochloric acid solution at 40-60°C for 1-3 hours.Use of the nitrogen-doped porous carbon material prepared with the preparation method according to any one of claims 1-6 as oxygen reduction catalyst.