阅读说明：本技术 一种双功能无金属氮掺杂碳催化剂的制备方法及其应用 (Preparation method and application of bifunctional metal-free nitrogen-doped carbon catalyst ) 是由 温珍海 李建权 贾春光 于 2021-01-18 设计创作，主要内容包括：本发明公开了一种简便的双功能无金属的氮掺杂碳催化剂的制备方法及其应用,制备方法是在合成ZIF-8前驱体的过程中,超声使其分散均匀,磁力搅拌一段时间后,再经过水热反应,离心干燥得到催化剂前驱体,最后通过在氮气气氛高温碳化得到最终的高活性N-C催化剂。所制备的双功能无金属氮掺杂碳催化剂不仅能应用于高活性氧还原催化反应,还能用于高活性二氧化碳还原成一氧化碳的电催化反应中。本发明的优点是：该方法简单高效,廉价可靠,通过ZIF-8作为自牺牲模板经过一个简单的热解步骤合成,无需球磨等高能耗高噪音工艺步骤,便于大规模生产。(The invention discloses a simple preparation method and application of a dual-functional metal-free nitrogen-doped carbon catalyst, wherein the preparation method comprises the steps of ultrasonically dispersing a ZIF-8 precursor uniformly in the process of synthesizing the ZIF-8 precursor, magnetically stirring for a period of time, then carrying out hydrothermal reaction, carrying out centrifugal drying to obtain a catalyst precursor, and finally carrying out high-temperature carbonization in a nitrogen atmosphere to obtain the final high-activity N-C catalyst. The prepared bifunctional metal-free nitrogen-doped carbon catalyst can be applied to not only the high-activity oxygen reduction catalytic reaction, but also the electrocatalytic reaction for reducing high-activity carbon dioxide into carbon monoxide. The invention has the advantages that: the method is simple, efficient, cheap and reliable, is synthesized by taking ZIF-8 as a self-sacrifice template through a simple pyrolysis step, does not need high-energy-consumption and high-noise process steps such as ball milling and the like, and is convenient for large-scale production.)

1. A preparation method of a bifunctional metal-free nitrogen-doped carbon catalyst is characterized by comprising the following preparation steps:

firstly, preparing a precursor of a metal framework compound ZIF-8: ultrasonically dispersing the raw materials uniformly, magnetically stirring for a period of time, then carrying out hydrothermal reaction, and finally carrying out centrifugal drying to obtain a ZIF-8 precursor;

secondly, carbonizing the ZIF-8 precursor at 900-1100 ℃ for 3-5 hours in a nitrogen atmosphere, and finally naturally cooling to obtain the high-activity N-C catalyst.

2. The method for preparing a bifunctional metal-free nitrogen-doped carbon catalyst according to claim 1, wherein the method comprises the following steps: in the first step, a precursor of a metal framework compound ZIF-8 is prepared, and the specific method comprises the following steps: at the temperature of 20-27 ℃, 4.76 g of zinc nitrate hexahydrate and 5.3 g of 2-methylimidazole are respectively dissolved in 67 mL of methanol and 100 mL of methanol, the two are respectively subjected to ultrasonic treatment for 15 min, the two are mixed and then are stirred magnetically for 1h, the mixture reacts in a reaction kettle at the temperature of 120 ℃ for 4 h, the mixture is centrifuged at 9000 rpm/min for 5 min, washed with ethanol for three times, and dried in a vacuum drying oven at the temperature of 70 ℃ for 12 h to obtain the ZIF-8 precursor.

3. The method of claim 1, wherein the bifunctional metal-free nitrogen-doped carbon catalyst comprises: the optimal temperature of the high-temperature carbonization in the second step is 1000 ℃.

4. Use of a bifunctional metal-free nitrogen-doped carbon catalyst prepared by the process according to any one of claims 1 to 3, characterized in that: the bifunctional metal-nitrogen-free carbon-doped catalyst can be applied to not only the high-activity oxygen reduction catalytic reaction, but also the electrocatalytic reaction of reducing high-activity carbon dioxide into carbon monoxide.

5. Use of the bifunctional metal-free nitrogen-doped carbon catalyst according to claim 4, wherein: double-function gold-freeThe nitrogen-doped carbon catalyst has excellent oxygen reduction activity under an alkaline condition, and the half-wave potential is close to 0.87V, so that the performance of the Pt/C catalyst is achieved; and 1M KHCO₃The catalyst is an electrolyte solution, has extremely high selectivity on reducing carbon monoxide into carbon monoxide at the voltage of-0.27 to-0.97V, and the maximum Faraday efficiency can reach 95 percent.

Technical Field

The invention belongs to the technical field of electrocatalyst preparation, and particularly relates to a preparation method and application of a bifunctional metal-free nitrogen-doped carbon catalyst.

Background

Since the industrial revolution, the excessive use of fossil fuels such as petroleum, coal, etc. has brought about many serious consequences such as exhaustion of fossil fuels and global warming. To achieve sustainable development, the development of a high-efficiency electrocatalyst having excellent oxygen reduction and carbon dioxide reduction properties is an effective approach to solve the above-mentioned problems. At present, the electrolytic catalyst and the performance thereof are one of the key factors for restricting energy conversion. Therefore, the development of inexpensive, durable, high efficiency catalysts has become a key to energy conversion and storage today. In recent years, ZIF and its derivatives have attracted much attention for their unique structures and physicochemical properties for use in electrochemical energy conversion. Most of ZIF materials doped with metals mainly comprising precious metals such as Au, Ag and the like have good carbon dioxide reduction active sites, but the ZIF materials are not suitable for large-scale production due to low abundance and high cost. The development of the field of electrocatalysis is restricted by the defects of few active sites, low tolerance to CO poisoning, low product selectivity and the like of a ZIF material doped with part of other elements. In order to reduce costs, the development of high active site catalysts that can be metal-free has been a focus of this research area. Among them, the metal-free nitrogen-doped carbon catalyst is expected to be one of candidates in this field.

Most of the metal-free nitrogen-doped carbon material catalysts aim at a single catalytic oxidation-reduction reaction process, but the metal-free nitrogen-doped carbon catalyst obtained by final high-temperature carbonization has both electrocatalytic oxygen reduction and electrocatalytic CO₂The reduction into CO has double functions, and the application of the organic framework material is widened.

Disclosure of Invention

The invention aims to provide a preparation method and application of a bifunctional metal-free nitrogen-doped carbon catalyst, which are used for electrocatalytic oxygen reduction and carbon dioxide reduction.

In order to achieve the above purpose, the technical scheme adopted by the invention is as follows.

A preparation method of a bifunctional metal-free nitrogen-doped carbon catalyst is characterized by comprising the following preparation steps:

firstly, preparing a precursor of a metal framework compound ZIF-8: ultrasonically dispersing the raw materials uniformly, magnetically stirring for a period of time, then carrying out hydrothermal reaction, and finally carrying out centrifugal drying to obtain a ZIF-8 precursor;

secondly, carbonizing the ZIF-8 precursor at 900-1100 ℃ for 3-5 hours in a nitrogen atmosphere, and finally naturally cooling to obtain the high-activity N-C catalyst.

Further, the preparation method of the metal framework compound ZIF-8 precursor in the first step comprises the following specific steps: at the temperature of 20-27 ℃, 4.76 g of zinc nitrate hexahydrate and 5.3 g of 2-methylimidazole are respectively dissolved in 67 mL of methanol and 100 mL of methanol, the two are respectively subjected to ultrasonic treatment for 15 min, the two are mixed and then are stirred magnetically for 1h, the mixture reacts in a reaction kettle at the temperature of 120 ℃ for 4 h, the mixture is centrifuged at 9000 rpm/min for 5 min, washed with ethanol for three times, and dried in a vacuum drying oven at the temperature of 70 ℃ for 12 h to obtain the ZIF-8 precursor.

Further, the optimal temperature of the high-temperature carbonization in the second step is 1000 ℃.

The invention also provides the following technical scheme.

The application of the bifunctional metal-free nitrogen-doped carbon catalyst prepared by the method is characterized in that: the bifunctional metal-nitrogen-free carbon-doped catalyst can be applied to not only the high-activity oxygen reduction catalytic reaction, but also the electrocatalytic reaction of reducing high-activity carbon dioxide into carbon monoxide.

Further, the application of the bifunctional metal-free nitrogen-doped carbon catalyst has the advantages that the bifunctional metal-free nitrogen-doped carbon catalyst has excellent oxygen reduction activity under an alkaline condition, the half-wave potential is close to 0.87V, and the Pt/C catalyst performance is achieved; and 1M KHCO₃The catalyst is an electrolyte solution, has extremely high selectivity on reducing carbon monoxide into carbon monoxide at the voltage of-0.27 to-0.97V, and the maximum Faraday efficiency can reach 95 percent.

The invention has the advantages that: the method is simple, efficient, cheap and reliable, is synthesized by taking ZIF-8 as a self-sacrifice template through a simple pyrolysis step, does not need high-energy-consumption and high-noise process steps such as ball milling and the like, and is convenient for large-scale production. The preparation method has the advantages of simple preparation process, easily controlled reaction conditions, high catalytic efficiency of the obtained product and the like. The zinc-air battery built by using ORR performance can generate continuous bioelectricity, and the maximum power density is about 170 mw/m²Even better than 20% wt Pt/C. The bioelectricity is used for CO₂Electrochemically reduced, carbon dioxide can be converted to carbon monoxide with high selectivity without the need for an external power source. Realizes the recycling of greenhouse gases and provides a new idea for constructing the bifunctional electrochemical reduction catalytic material. Therefore, the catalyst has better application prospect in the fields of environmental protection and energy regeneration.

Drawings

FIG. 1 is an X-ray diffraction (XRD) spectrum of N-C-900 deg.C, N-C-1000 deg.C, N-C-1100 deg.C and ZIF-8 precursor.

FIG. 2 is a Scanning Electron Microscope (SEM) image of N-C nano-catalyst, wherein A, B, C is N-C material with carbonization temperature of 900 deg.C, 1000 deg.C, and 1100 deg.C, respectively.

FIG. 3 is a graph of Cyclic Voltammetry (CV) at N-C-900 deg.C, N-C-1000 deg.C, N-C-1100 deg.C and Pt/C during oxygen reduction.

FIG. 4 shows CO₂CO faradaic efficiency plot of atmospheric reduction potential gas products.

FIG. 5 shows CO₂H of atmospheric reduction potential gas product₂Faraday efficiency plot.

Detailed Description

The invention is further illustrated by the following specific examples.

The method comprises the steps of firstly, dispersing raw materials uniformly by ultrasonic waves, carrying out magnetic stirring, then, carrying out centrifugal drying to obtain a certain amount of ZIF-8, and then, carbonizing a product at a high temperature to obtain the high-activity N-C catalyst. The high-activity nitrogen-doped carbon catalyst consists of N, C elements and is formed by doping various pyrrole N, pyridine N and graphite N on a C substrate.

Example 1:

the preparation method of the bifunctional metal-free nitrogen-doped carbon catalyst comprises the following specific steps of:

firstly, respectively dissolving 4.76 g of zinc nitrate hexahydrate and 5.3 g of 2-methylimidazole in 67 mL of methanol and 100 mL of methanol, respectively carrying out ultrasonic treatment on the two solutions for 15 min, mixing, and then carrying out magnetic stirring for 1 h;

and secondly, reacting the final product of the first step in a reaction kettle at 120 ℃ for 4 h, cooling, centrifuging for 5 min at the rotating speed of 9000 rpm/min, washing with ethanol for three times, and drying in a vacuum drying oven at 70 ℃ for 12 h to obtain the ZIF-8 precursor.

And thirdly, heating the ZIF-8 precursor for 4 hours at 900 ℃, 1000 ℃ and 1100 ℃ respectively in a nitrogen atmosphere, and finally naturally cooling to obtain the N-C-900 ℃, N-C-1000 ℃ and N-C-1100 ℃ catalysts.

Characterization test:

the final material synthesized in the above example 1 was tested by X-ray diffraction, and it can be seen that the precursor has a typical ZIF-8 crystal structure. And it was observed that the N — C catalysts each showed two broad peaks at only 25 ° and 44 °, corresponding to the characteristic carbon (002) and (101) diffraction, respectively. In addition, no diffraction peaks of the impurities were found, indicating that the zinc was evaporated and that the precursor had been completely converted to a metal-free nitrogen-doped carbon structure after heat treatment.

As shown in fig. 2, the images of N-C-900 ℃, N-C-1000 ℃ and N-C-1100 ℃ are respectively shot under a scanning electron microscope, and the microscopic morphology of each material in the synthesis process is recorded, specifically, as shown in fig. 2, A is an N-C material with the carbonization temperature of 900 ℃, no regular morphology can be observed, and small agglomeration occurs; b is an N-C material with the carbonization temperature of 1000 ℃, has a uniform porous structure, and is distributed with dense and hemp pores on the surface, which is beneficial to increasing the specific surface area of the material; c is an N-C material with the carbonization temperature of 1100 ℃, and a flaky structure generated at the edge of the material can be observed.

The prepared bifunctional metal-free nitrogen-doped carbon catalyst is electrochemically tested in the application process:

（1）CO₂electrocatalytic reduction process:

the process adopts a standard three-electrode system, an N-C catalyst is used as a working electrode, a platinum electrode and an Ag/AgCl electrode are respectively used as a counter electrode and a reference electrode, and the test is carried out on an electrochemical workstation.

Carbon dioxide reduction was measured by continuous CO sparging in 1M KHCO3 solution (pH = 7.3)₂（20 cm³ min^-1) The method is carried out. The reactor was divided into two chambers, an anode chamber and a cathode chamber, by a nafion 117 proton exchange membrane. In the course of the test,all potentials were not resistance compensated and were scaled as follows, based on the potential relative to the standard hydrogen electrode (RHE): e (rhe) ═ E (Ag/AgCl) +0.0591 pH + 0.19V. The reduction potential interval of the constant voltage electrolysis test is-0.27V to-0.97V, and the reduction time is 1 h.

In CO₂In the reduction test, gas chromatography was used to detect and analyze the gaseous products. The main reduction product is H₂And CO. Figures 4 and 5 are graphs of faradaic efficiency calculated and collated for gas phase products at different potentials. Respectively electrolyzing for 1h at a potential of-0.27V to-0.97V at the temperature of N-C-900 ℃, N-C-1000 ℃ and N-C-1100 ℃, wherein the CO Faraday efficiency reaches 95% at the temperature of NC-1000 ℃ and is only 80% and 70% at the maximum CO Faraday efficiencies at the temperature of NC-900 ℃ and NC-1100 ℃. In addition, in the full potential range, the Faraday efficiencies of CO generated at NC-1000 ℃ are higher than those of the comparison sample. Confirmation of NC-1000 ℃ to CO₂Electrocatalytic reduction to CO has excellent electrocatalytic activity and the highest selectivity.

(2) And (3) oxygen reduction process:

and respectively taking 5mg of the synthesized N-C catalyst, adding 50 mu L of Nafion, 30 mu L of ethanol and 420 mu L of deionized water, and performing ultrasonic treatment for 20min to uniformly disperse. And respectively taking 6 mu L of ultrasonic dispersion liquid for sample preparation, and testing on an electrochemical workstation.

The N-C-900 ℃, N-C-1000 ℃, N-C-1100 ℃ and Pt/C-V diagram in the oxygen reduction process shown in FIG. 3 show that the N-C-1000 ℃ has the most excellent oxygen reduction activity under the alkaline condition, the half-wave potential is close to 0.87V, the initial potential reaches 0.96V, and the performance of the catalyst can be compared with that of a commercial Pt/C catalyst.

The N-C material synthesized by the invention realizes a high active oxygen reduction process by doping pyrrole N, pyridine N and graphite N on a C substrate, and can be used for CO₂The process of reducing CO with high selectivity provides a new idea for constructing the bifunctional electrochemical reduction catalytic material.

The embodiments described above are described to facilitate an understanding and use of the invention by those skilled in the art. The present invention is not limited to the above-described embodiments, and those skilled in the art should make improvements and modifications within the scope of the present invention based on the disclosure of the present invention.