CN107346826B - Preparation method of monatomic iron dispersed oxygen reduction electrocatalyst - Google Patents

Abstract

A preparation method of a monoatomic iron-dispersed oxygen reduction electrocatalyst belongs to the technical field of electrocatalysis. Using cheap and nitrogen-rich g-C ₃ N ₄ as raw material, adding surfactant and iron source, through high temperature pyrolysis, a monoatomic iron-dispersed electrocatalyst was obtained. The catalyst has the characteristics of simple synthesis method, no pollution, cheap and easily available reactants, etc. The prepared catalyst exhibits electrochemical performance equivalent to that of platinum carbon in acidity, and has wide application prospects.

Description

A kind of preparation method of monoatomic iron dispersed oxygen reduction electrocatalyst

technical field

The invention relates to a new preparation method of a monoatomic iron-dispersed oxygen reduction electrocatalyst, which belongs to the technical field of electrocatalysis.

Background technique

With the massive combustion and emission of fossil fuels, human beings are faced with major problems of environmental degradation and energy shortage. Therefore, finding and developing a green, environmentally friendly and sustainable clean energy is the main task of current researchers. As a new type of energy, a fuel cell is a power generation device that directly converts chemical energy into electrical energy through an electrochemical reaction. The oxidation reaction of hydrogen occurs at the anode of the battery, and the reduction reaction of oxygen occurs at the cathode. The entire reaction process of the fuel cell The only product is water, which does not bring any complicated side reactions and by-products. It has the advantages of high conversion efficiency, no pollution, low noise and high reliability. It has received extensive attention in recent years, especially now. Proton exchange membrane fuel cells with potential commercialization. Proton exchange membrane fuel cells will play an important role in the near future, and it is known as the ultimate power source for electric vehicles.

However, the reason why fuel cells have not been commercialized so far is that the chemical reactions that take place at the anode or at the cathode require precious metal platinum as a catalyst, otherwise the reaction proceeds slowly and the performance of the battery is greatly attenuated. According to statistics, a 100kW fuel cell vehicle needs about 100g of Pt, while the reserves of Pt in the earth's crust are only 39,000t. Platinum scarcity makes fuel cell vehicles prohibitively expensive. And the reaction rate of cathode oxygen reduction (ORR) is several orders of magnitude smaller than that of anodic oxidation reaction (HOR), so the oxygen reduction reaction at the cathode is the key to the development of fuel cells. At present, the development of non-precious metal catalysts to replace precious metal platinum is the focus of fuel cell research. The current research is roughly divided into two directions: first, to develop catalysts with lower platinum content, such as platinum metal alloy structures and core-shell structures; second, to use non-precious metals to completely replace precious metal platinum, such as doping carbon materials Some non-metals or non-precious metals. Heteroatom-doped carbon materials have excellent properties such as low cost, good performance, methanol resistance, and carbon monoxide resistance, which provide strong technical support for the commercial development of fuel cells. Since Jasinski discovered that cobalt phthalocyanine can catalyze the reduction of oxygen in alkaline electrolytes in 1964, heteroatom-doped carbon materials have developed rapidly electrochemically. With the continuous progress and in-depth research, the oxygen reduction electrocatalytic performance of heteroatom-doped carbon materials under alkaline conditions can reach a level comparable to that of platinum carbon, and exceeds that of commercial platinum in stability and methanol resistance.

Fuel cells are roughly divided into two types according to different electrolytes: one is alkaline fuel cell and the other is acid fuel cell. Among acid fuel cells, proton exchange membrane fuel cell is considered to be the most promising type of battery. , This is because the energy density provided by the proton exchange membrane fuel cell is about 3-10 times that of the alkaline fuel cell, lithium battery, and methanol fuel cell. This type of battery will eventually hopefully become the first choice for future portable devices. Although the development of non-precious metal oxygen reduction catalysts under acidic conditions is also very rapid, there is still a certain gap with commercial platinum carbon in terms of the performance of electrocatalytic oxygen reduction reaction, especially for metal-free carbon-based catalysts. This type of catalyst can achieve the electrocatalytic ability equivalent to platinum carbon under alkaline conditions, but the performance under acidic conditions is very weak and extremely unstable, so the development of a catalyst whether in acidic or alkaline conditions Both can exhibit excellent electrocatalytic performance, which is the focus of our research.

SUMMARY OF THE INVENTION

The first technical problem to be solved by the present invention is to propose a brand-new preparation method of a monoatomic iron-dispersed electrocatalyst in view of the development status of fuel cells and key electrochemical technical problems. The oxygen reduction performance similar to that of platinum carbon can be achieved under alkaline conditions.

The second technical problem to be solved by the present invention is to add iron salt to the precursor of nitrogen source carbon source (gC ₃ N ₄ ), and obtain nanoporous carbon with a relatively high degree of graphitization through high temperature carbonization.

The third technical problem to be solved by the present invention is to add an anionic surfactant F127 to the composite material of (gC ₃ N ₄ ) and iron salt, so that the metal iron salt will not agglomerate under high temperature pyrolysis, which plays a very good role. The role of the dispersant, and then the preparation of monoatomic iron-dispersed electrocatalysts.

In order to solve the above technical problems, the present invention adopts the technical solution of adding surfactant F127 and a small amount of iron salt to the carbon source nitrogen source (gC ₃ N ₄ ), carbonizing in a temperature-programmed tube furnace, and by exploring metallic iron The difference of the amount of salt added, whether to add surfactant, and the difference of carbonization temperature are the control variables, and the electrocatalyst with excellent performance is further prepared.

The concrete synthesis steps of the present invention are as follows:

(1) put a certain amount of precursors containing C and N into a tube furnace for carbonization to obtain the precursor gC ₃ N ₄ ;

(2) Dispersing a certain amount of gC ₃ N ₄ by ultrasonic, magnetic stirring and dissolving in water;

(3) adding surfactant to step (2) solution, magnetic stirring;

(4) adding water-soluble iron salt to the mixture obtained in step (3), stirring, drying, and denoting as product 1;

(5) The product 1 is put into a tube furnace with temperature-programmed temperature for carbonization at high temperature, pickled, washed with water, and dried to obtain a monoatomic iron-dispersed oxygen reduction electrocatalyst.

A series of characterizations such as scanning electron microscopy, transmission electron microscopy, spherical aberration electron microscopy, synchrotron radiation, Raman spectroscopy, X-ray photoelectron spectroscopy, X-ray diffraction spectroscopy, inductively coupled plasma mass spectrometry, organic elemental analysis, and full analysis have verified the as-prepared The material is a highly dispersed monoatomic iron dispersed metal, nitrogen co-doped porous carbon material.

Further, in step (1), the precursor containing C and N is selected from urea, cyanamide, dicyandiamide, melamine, thiourea, ternary ring compound and the like. The heating rate of carbonization in the tube furnace is 2-10°C/min, the carbonization temperature is 500-600°C, and the carbonization retention time is 1-10 hours. The mass of the above precursor in step (1) is 0.01-20 g in one carbonization.

Further, in step (2), the relationship between the dosage of gC ₃ N ₄ and water is 10-100 ml of water corresponding to every 0.01-5 g of gC ₃ N ₄ , and the ultrasonic stirring time is 0.5-10 hours.

Further, in step (3), a surfactant is added, and the surfactant is an anionic surfactant or a non-ionic surfactant, such as carboxylate, sulfate, sulfonate and phosphate, F127, and the like. The mass ratio of gC ₃ N ₄ to surfactant is 0.01-100:1.

Further, the iron salt added in step (4) can be ferric sulfate, ferrous sulfate, ferric nitrate, ferrous nitrate, ferric chloride, ferrous chloride and the like. _The molar ratio of the added iron salt to _gC3N4 is 0.01-10:1.

Further, before the tube furnace is heated up in step (5), first pass into protective gas as protective gas, hold time 1-3 hour, protective gas can be selected from nitrogen, argon, helium, neon, krypton, Xenon or Radon, etc. The heating rate of the carbonization process is 0.5-30°C/min. The temperature of carbonization is 500-1200° C. (preferably 800-1000° C.), the temperature is kept for 0.5-8 h, and the temperature is naturally cooled to room temperature.

The acid used for pickling in step (5) can be selected from nitric acid, sulfuric acid, hydrochloric acid, hydrofluoric acid and the like.

Preferably, in step (1), urea is selected as the precursor for preparing gC ₃ N ₄ , the temperature is increased at 3° C. per minute under nitrogen protection gas, and kept at 550° C. for 2 hours. In step (2), preferably every 0.3 g of gC ₃ N ₄ , add 30ml water and ultrasonically stir for 2 hours, step (3) anionic surfactant preferably uses polyether F127, and the mass ratio of gC ₃ N ₄ preferably uses 1:1, and preferably uses FeCl in step (4) ₃ As the iron salt, the molar ratio to gC ₃ N ₄ is preferably 0.1:1. In the step (5), nitrogen is preferably used as the protective gas, the holding time is 2 hours, the rate is increased at a rate of 2°C per minute, and the temperature is maintained at 800°C for 2 hours. For pickling, hydrochloric acid is preferably used.

The beneficial effects of the present invention are as follows:

1) Using cheap gC ₃ N ₄ as the precursor, adding surfactant and a small amount of metal iron salt, and one-step pyrolysis, a highly dispersed single-atom iron-doped porous carbon catalyst was obtained. Much lower than that of noble metal platinum carbon, the oxygen reduction electrocatalytic performance of this catalyst can reach a level comparable to that of commercial platinum carbon in both acidic and basic conditions, and its stability is comparable to methanol resistance. Commercial platinum has good performance, and this catalyst will hopefully replace commercial platinum-carbon, thereby making it possible to commercialize acidic proton exchange membrane batteries, and provide some basic research for the development of electrocatalysis, which is widely used in electrocatalytic oxygen reduction. application prospects.

2) Compared with the prior art, the method has simple preparation process and convenient operation. After the carbonization of the present invention, gC ₃ N ₄ becomes a structure similar to graphene flakes, and iron is uniformly dispersed in the porous carbon material in the form of single atoms; the porous carbon material has excellent electrical conductivity, high specific surface area and microscopic The hierarchical pore structure with coexistence of pores, mesopores and macropores is more conducive to mass and charge transport. The transition metal iron-doped porous carbon provides a large number of active sites and promotes the electrocatalytic performance of the catalyst.

3) The preparation process of the present invention avoids the use of toxic reagents and complex synthesis processes, the preparation process is simple, the operation is convenient, and large-scale production is easily realized.

4) The present invention uses urea and iron salts as precursors, which are convenient and easy to obtain and greatly reduce the cost of catalysts.

Description of drawings

FIG. 1 is a schematic diagram of a method for preparing a monoatomic iron-dispersed electrocatalyst using gC ₃ N ₄ as a raw material in Example 1. FIG.

FIG. 2 is a scanning electron microscope photograph of the monoatomic iron-dispersed catalyst in Example 1. FIG.

3 is a transmission electron microscope photograph of the monoatomic iron-dispersed catalyst in Example 1. FIG.

FIG. 4 is an element distribution diagram of the monoatomic iron dispersed catalyst in Example 1. FIG.

FIG. 5 is the X-ray diffraction spectrum of the monoatomic iron dispersed catalyst in Example 1. FIG.

Figure 6 is a graph comparing the oxygen reduction catalytic LSV of the monoatomic iron dispersed catalyst in Example 1 and commercial Pt/C (in alkaline 0.1 M potassium hydroxide).

Figure 7 is a graph comparing the oxygen reduction catalytic LSV of the monoatomic iron dispersed catalyst in Example 1 and commercial Pt/C (in acidic 0.5M sulfuric acid).

FIG. 8 is a transmission electron microscope photograph of the iron nanoparticle-doped porous carbon catalyst in Comparative Example 2. FIG.

FIG. 9 is a transmission electron microscope photograph of the iron nanoparticle-doped porous carbon catalyst in Example 3. FIG.

FIG. 10 is a comparison diagram of the oxygen reduction catalyzed LSV of iron nanoparticle-doped porous carbon and commercial Pt/C in Comparative Example 2 (in alkaline 0.1 M potassium hydroxide).

11 is a comparison diagram of the oxygen reduction catalytic LSV of iron nanoparticle-doped porous carbon and commercial Pt/C in Example 3 (in alkaline 0.1 M potassium hydroxide).

Detailed ways

The present invention will be described in further detail below with reference to the embodiments and accompanying drawings, but the present invention is not limited to the following embodiments.

The experimental drugs were obtained from commercial regular sales (Annagy, Sinopharm Group, Beijing Chemical Factory, DuPont, J&K, Alfar), and were not further purified unless otherwise specified.

Example 1: Method for preparing monoatomic iron _- dispersed electrocatalyst using _gC3N4 as raw material

1) Weigh 3g of urea, put it into a tube furnace to heat up at a heating rate of 3°C per minute, keep at 550°C for 2 hours, and automatically cool down.

2) Weigh 0.3 g of the product gC ₃ N ₄ synthesized in step (1), add 30 ml of deionized water, sonicate for half an hour, and stir for 1.5 hours.

3) Add 0.3 g of polyether F127 to the product of step (2), and stir for 2 hours.

4) Add 1 ml of 0.298M FeCl ₃ solution to the solution in step (3), stir for 8 hours, and heat up to 100° C. in a heated stirrer with natural stirring and air-drying.

5) The above-mentioned dried powder is put into a tube furnace for carbonization, and the temperature rises to 800°C (at a rate of 2°C per minute, 800°C is maintained for 2 hours), the carbonized product is taken out, and soaked with 1M HCl 5ml 12 hours, filter, rinse with copious amounts of water until pH=7, denoted SA-Fe/NG.

Comparative Example 2: Preparation of Nanoparticle Iron and Nitrogen Co _- doped Carbon Electrocatalyst Using _gC3N4 as Raw Material

1) Weigh 3g of urea, put it into a tube furnace to heat up at a heating rate of 3°C per minute, keep at 550°C for 2 hours, and automatically cool down.

2) Weigh 0.3 g of the product g-C3N4 synthesized in step (1), add 30 ml of deionized water, sonicate for half an hour, and stir for 1.5 hours.

3) Add 1 ml of 0.298M FeCl3 solution to the solution in step (2), stir for 8 hours, and heat up to 100° C. in a heated stirrer with natural stirring and air-drying.

4) The above-mentioned dried powder is put into a tube furnace for carbonization, and the temperature rises to 800°C (rising at a rate of 2°C per minute, and 800°C is maintained for 2 hours), and the carbonized product is taken out and soaked in 1M HCl 5ml 12 hours, filter, rinse with copious amounts of water until pH=7, denoted Fe/NG.

Example 3: Method for preparing nanoparticle iron and nitrogen co-doped carbon electrocatalyst using gC ₃ N ₄ as raw material

1) Weigh 3g of urea, put it into a tube furnace to heat up at a heating rate of 3°C per minute, keep at 550°C for 2 hours, and automatically cool down.

2) Weigh 0.3 g of the product gC ₃ N ₄ synthesized in step (1), add 30 ml of deionized water, sonicate for half an hour, and stir for 1.5 hours.

3) Add 0.3 g of polyether F127 to the product of step (2), and stir for 2 hours.

4) Add 5 ml of 0.298M FeCl ₃ solution to the solution in step (3), stir for 8 hours, and heat up to 100° C. in a heated stirrer with natural stirring and air-drying.

5) The above-mentioned dried powder is put into a tube furnace for carbonization, and the temperature rises to 800°C (at a rate of 2°C per minute, 800°C is maintained for 2 hours), the carbonized product is taken out, and soaked with 1M HCl 5ml After 12 hours, filter and rinse with copious amounts of water until pH=7, denoted as Fe/NG-1.

The above embodiments are only examples for clearly illustrating the present invention, rather than limiting the implementation of the present invention. For those of ordinary skill in the art, other different forms can also be made on the basis of the above descriptions. Changes or changes, it is impossible to list all the embodiments here, and all obvious changes or changes derived from the technical solutions of the present invention are still within the protection scope of the present invention.

Fig. 2 is a scanning electron microscope photograph of the catalyst dispersed with monoatomic iron after carbonization in Example 1. It can be seen from the figure that gC ₃ N ₄ becomes a structure similar to graphene flakes after carbonization, and no aggregation of iron particles is found. , which is consistent with the TEM picture in Figure 3.

Fig. 4 is the spherical aberration electron microscope image of the catalyst dispersed with monoatomic iron after carbonization in Example 1. It can be seen from the overall image that there are no large iron particles, but a large number of small spots are found at a high magnification of 10 nm, which is proved by elemental analysis. The point is iron, which indicates that iron is uniformly dispersed in the porous carbon material in the form of single atoms, so that a large number of active sites of the catalyst are exposed, and the catalytic performance is thus improved.

Claims (7)

1. A preparation method of a monatomic iron-dispersed oxygen-reducing electrocatalyst is characterized by comprising the following steps:

(1) putting a certain amount of precursor containing C, N into a tube furnace for carbonization to obtain precursor g-C₃N₄；

(2) A certain g-C₃N₄Ultrasonic dispersion and magnetic stirring are carried out to dissolve the mixture in water;

(3) adding a surfactant into the solution obtained in the step (2), and magnetically stirring;

(4) adding water-soluble iron salt into the mixture obtained in the step (3), stirring and drying to obtain a product 1;

(5) putting the product 1 into a tubular furnace with programmed temperature rise for high-temperature carbonization, acid washing, water washing and drying to obtain a monoatomic iron dispersed oxygen reduction electrocatalyst; the surfactant in the step (2) is selected from F127;

the precursor containing C, N in the step (1) is selected from urea, cyanamide, dicyandiamide, melamine, thiourea and triazine ring compound; in the step (1), the temperature rise rate of carbonization in the tube furnace is 2-10 ℃/min, the carbonization temperature is 500-; g-C₃N₄The mass ratio of the surfactant to the surfactant is 0.01-100: 1; before the tubular furnace is heated in the step (5), introducing protective gas, and keeping the time for 1-3 hours; the temperature rising speed in the carbonization process is 0.5-30 ℃/min, the carbonization temperature is 500-1200 ℃, the temperature is kept for 0.5-8h at the temperature, and the product is naturally cooled to the room temperature.

2. The method for preparing a monoatomic iron-dispersed oxygen-reducing electrocatalyst according to claim 1, wherein g-C in the step (2)₃N₄The dosage of the water is 0.01-5g of g-C₃N₄The corresponding water is 10-100 ml; the ultrasonic stirring time in the step (2) is 0.5 to 10 hours.

3. A process for preparing a monatomic iron-dispersed oxygen-reducing electrocatalyst according to claim 1, characterized in that the iron salt and g-C are added₃N₄The molar ratio of (A) to (B) is 0.01-10: 1.

4. The method for preparing a monoatomic iron-dispersed oxygen-reducing electrocatalyst according to claim 1, wherein an iron salt selected from iron sulfate, ferrous sulfate, iron nitrate, ferrous nitrate, iron chloride, and ferrous chloride is added in the step (4).

5. The method for preparing a monatomic iron-dispersed oxygen-reducing electrocatalyst according to claim 1, wherein the carbonization temperature in step (5) is 800-1000 ℃.

6. The method for preparing a monoatomic iron-dispersed oxygen-reducing electrocatalyst according to claim 1, wherein urea is selected in the step (1) as the material for preparing g-C₃N₄Under the protection of nitrogen gas, the temperature of the precursor is raised at 3 ℃ per minute, the precursor is kept at 550 ℃ for 2 hours, and each time in the step (2) is 0.3g g-C₃N₄Adding 30ml of water, ultrasonically stirring for 2 hours, and mixing the surfactant and g-C in the step (3)₃N₄The mass ratio of (1) to (1) and FeCl in the step (4)₃As iron salts, FeCl₃And g-C₃N₄In a molar ratio of 0.1: 1; in the step (5), nitrogen is used as protective gas, the holding time is 2 hours, the temperature rises at the speed of 2 ℃ per minute, and the holding time is 2 hours at 800 ℃; hydrochloric acid is selected for pickling.

7. A monoatomic iron-dispersed oxygen-reducing electrocatalyst prepared by the preparation method according to any one of claims 1 to 6.

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