Non-noble metal heteroatom doped biomass carbon-based oxygen catalyst and preparation method thereof

Abstract

The invention relates to the field of new energy and new materials, and discloses a non-noble metal heteroatom doped biomass carbon-based oxygen catalyst and a preparation method thereof. The method comprises the following steps: breaking walls of phoenix tree fruits into powder, adding deionized water, carrying out ultrasonic cleaning, naturally airing, and calcining at 280-400 DEG C in a nitrogen atmosphere for 2 hours for pre-carbonization; doping NaCl and ZnCl2 into the carbon precursor, carrying out high-energy ball milling for 4 hours, sieving and collecting; and adding FeCl3 and urea into the precursor subjected to molten salt ball milling, calcining for 1-2 hours at 700-1000 DEG C in a nitrogen atmosphere, soaking the calcined product with inorganic strong acid for 12-24 hours, and repeatedly washing with deionized water to obtain the carbon-based catalyst. The heteroatom-doped biomass carbon catalyst (BM-Fe / N-C) prepared by adopting a ball-milling molten salt method has catalytic activity and stability superior to those of Pt / C, and a reference direction is provided for biomass utilization and preparation and application of a high-performance electrocatalyst.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of new energy and new materials, and in particular to a non-noble metal heteroatom-doped biomass carbon-based oxygen catalyst and a preparation method thereof. BACKGROUND

[0002] With the rapid development of social economy and the rapid increase of population, people's demand for energy is becoming larger and larger, but due to the non-renewable nature of fossil energy such as oil, coal and natural gas, and after a long time of exploitation, it gradually cannot meet the needs of economic and social development. As a kind of clean, efficient and green power, metal fuel cell has been widely concerned since its advent. Compared with traditional batteries, lithium ion batteries have the advantages of high working voltage, wide working temperature range, large energy density and no memory effect. In recent years, with the popularization of intelligent electronic devices, lithium ion batteries are widely used in mobile phones, digital cameras, smart watches and other aspects. In addition, the application in the fields of electric vehicles, new energy vehicles, aerospace and military equipment at home and abroad has also carried out active research and application. Therefore, lithium ion battery has become a major research trend, and improving the specific capacity, cycle rate and cycle stability of the battery has become the main concern of lithium ion battery research.

[0003] However, its effectiveness is constrained by two main factors. First, the reaction kinetics of oxygen electrocatalysts is inherently slow, and second, the batch production process is short. Although Pt catalysts have significant ORR electrocatalytic activity, their application in energy devices is limited by a series of challenges, including deactivation, methanol crossover effect, high cost, poor stability and scarcity. At present, the research of ORR catalysts has attracted much attention. A large number of non-noble metal catalysts have been proved to have special characteristics, such as improved stability, enhanced electrochemical activity, and enhanced methanol tolerance. However, the preparation of many precursors requires complex manufacturing processes, which limits the development of catalysts. At present, a large number of studies have found that heteroatom dopants can significantly improve the ORR performance of catalysts. During the ORR process, a proper pore structure is beneficial to mass transfer. By heteroatom doping, the surface electronic structure of porous carbon can be modified to form active sites with high ORR activity.

[0004] Nature produces a large amount of biomass waste every day, such as lignocellulosic biomass, agricultural waste and waste, etc. On the other hand, there is a risk of dangerous pollutant release in the process of manufacturing or burning a large amount of biomass. In addition, since it is not only low in price and renewable, but also converts garbage into valuable resources, therefore, the use of waste biomass as a precursor of carbon has attracted much scientific attention. One of the urgent challenges in this field is to develop a high-activity catalyst that can match the performance of commercial Pt / C. This goal is achieved by using biomass and its derived precursors in industrial production processes.

[0005] In invention patent 2018101405658, a preparation method of biomass carbon electrochemical oxygen reduction agent is proposed. Auricularia is used as biomass raw material, and pre-carbonization and complete carbonization treatment are adopted, and then biomass carbon-based catalyst is obtained after acid cooking, which shows good catalytic performance. However, this patent only uses trace Fe and N elements in auricularia for doping, and the doping amount is small and uncontrollable. In addition, the multi-dimensional pore structure required for oxygen reduction cannot be guaranteed under this process treatment.

[0006] In invention patent 201910092377.7, a preparation method of nitrogen-doped peel-based porous carbon material is proposed. Waste peel and melamine are mixed and ultrasonically treated, and the dried precursor is added with potassium bicarbonate for ball milling, and then high-temperature carbonization and acid washing are carried out to obtain a porous carbon material suitable for supercapacitors, which shows high specific capacitance and high energy density, and good cycle stability. However, the porous carbon prepared by this preparation method is only used for supercapacitors, and it cannot be shown that it has good oxygen reduction catalytic performance. In addition, the single inorganic salt used in the preparation method can prepare carbon materials with controllable pore size, but it does not show the characteristics of heteroatom doping.

[0007] In invention patent 201810811336.4, a preparation method of a new type of iron and nitrogen-doped mesoporous biochar oxygen reduction catalyst is proposed. Chlorella pyrenoidosa powder is mixed with iron salt solution or iron-containing organic solution to obtain a mixture, which is dried, and then high-temperature carbonization is carried out on the dried mixture, and then ball milling and acid washing are carried out to obtain an oxygen reduction catalyst. The catalyst is the product of high-temperature carbonization, and then ball milling is carried out. The disadvantage of this method is that the product with a microstructure has been shaped, which on the one hand causes the collapse of the original pore structure, and on the other hand causes the loss or failure of the doped ions. In addition, the batch production of the product obtained by this method is uncontrollable.

[0008] In invention patent 201210453382.4, a Fe and N co-doped carbon black catalyst and its preparation method are proposed. Black pearl 2000 and melamine are ball milled to obtain a precursor powder, and then high-temperature calcination is carried out to obtain an oxygen reduction catalyst. This method has the defects that the Fe and N doping active sites are few and the pore structure is not obvious.

[0009] In invention patent 201611010659.0, a preparation method of biomass carbon is proposed. After pre-carbonization of crushed rice rods, the product is mixed uniformly with ZnCl2 molten salt activator, and then high-temperature carbonization is carried out in an inert atmosphere furnace to prepare biomass carbon. In this method, the activator is liquid immersion activation, which is fundamentally different from the molten salt activation in this patent. In addition, the carbon material is not ball milled and doped, and is not used for oxygen reduction catalyst.

[0010] A nitrogen-iron co-doped graphite carbon and a preparation method thereof are proposed in invention patent 202010514583.5. The mechanical ball milling method is used to reduce the particle size of biomass. Potassium ferrate and urea are used as Fe source and N source respectively. The mixed solution is mixed and doped according to a certain proportion. The dried product is carbonized at high temperature under nitrogen atmosphere, and the Fe and N co-doped graphite carbon is obtained. The ball milling method used in the process is only used to reduce the particle size of biomass, which is different from the pre-carbonization and molten salt ball milling of the present patent. The Fe and N doping amount obtained by the solution impregnation method is generally low.

[0011] A sulfur-doped biomass hard carbon material and a preparation method thereof are proposed in invention patent 202111105546.X. The biomass raw material is first pretreated and crushed to obtain a precursor powder. Then the precursor powder is mixed and ball milled with inorganic salt and sulfur source, and then calcined by molten salt method to obtain an intermediate product. Finally, the intermediate product is soaked in an acid solution, washed and dried to obtain a biomass hard carbon material. Although the process uses ball milling and molten salt process, the carbon prepared by this method is only used for supercapacitors and does not reflect the oxygen reduction catalytic performance. In the present invention, it is pointed out that the synergistic effect of Fe element doping and ball milling and molten salt can greatly improve the activity of the oxygen reduction catalyst. SUMMARY

[0012] To solve the above problems, the present application provides a biomass carbon-based oxygen reduction catalyst preparation method and application which can improve the air cathode catalytic activity of metal-air batteries, fuel cells and microbial fuel cells.

[0013] To achieve the above purpose, the present application provides the following technical scheme:

[0014] In the first aspect, a preparation method of a biomass carbon-based oxygen reduction catalyst is provided, characterized by comprising the following steps:

[0015] Step one: the biomass is subjected to wall breaking and crushing treatment, ultrasonic cleaning in deionized water, and then vacuum drying at 60-80℃ for 12-24h;

[0016] Step two: the material prepared in step one is subjected to pre-carbonization treatment under nitrogen atmosphere;

[0017] Step three: the material prepared in step two is mixed with inorganic salt and subjected to high-energy ball milling in a planetary ball mill;

[0018] Step four: the product after molten salt ball milling is doped with nitrogen source and iron source and subjected to high-temperature calcination under nitrogen atmosphere. The calcined product is soaked in inorganic acid and then washed with deionized water until neutral to obtain a biomass carbon-based catalyst.

[0019] Preferably, the biomass in step one is selected from the fruits of the Chinese tallow tree;

[0020] Preferably, the pre-carbonization temperature of step one is 350℃, and the carbonization time is 2h;

[0021] Preferably, the mass ratio of the ball-milled carbon precursor and mixed inorganic salt of step two is 3:1; the mass ratio of the mixed salt NaCl and ZnCl2 is 1:2;

[0022] Preferably, the doping mass ratio of the ball-milled molten salt of step four is 4:2:1; the nitrogen source is urea, and the iron source is FeCl3;

[0023] Preferably, the high-temperature carbonization temperature of step five is 800℃, the nitrogen flow rate is 6m / min, and the holding time is 1.5h;

[0024] Preferably, the inorganic acid of step five is dilute hydrochloric acid, the concentration is 1mol, and the water bath oscillation is 8-12h at 60℃;

[0025] In the second aspect, the application provides the application of the BM-Fe / N-C catalyst in the oxygen reduction of the fuel cell cathode.

[0026] The prepared BM-Fe / N-C is added into a binder and a dispersant to prepare an ink, which is dropped onto a platinum electrode to prepare a catalytic film, and the oxygen reduction characterization is performed.

[0027] Preferably, the binder is a 5% Nafion solution;

[0028] Preferably, the catalytic layer ink solvent is ethanol and water, and the ratio is 3:1;

[0029] Preferably, the oxygen reduction test is in a saturated oxygen solution, and the electrolyte is 3mol KOH;

[0030] Compared with the prior art, the application has the following beneficial effects:

[0031] 1.The present application provides a simple and efficient ball milling-molten salt method for synergistically preparing high-performance oxygen reduction catalysts.The process is distinguished from other processes in that mixed low-temperature eutectic salts are incorporated into the precursor biomass carbon material as a grinding aid during ball milling,which on the one hand refines and disperses the material and on the other hand increases the edge defects on the surface of the carbon material;in the high-temperature molten salt process,as a pore-forming agent and dispersant,under high-temperature molten conditions,gases can be precipitated to etch pores on the surface of the carbon material,secondly,the carbon material is uniformly dispersed under molten conditions,which effectively avoids the sintering and agglomeration of carbon materials in other high-temperature carbonization processes,exposing more specific surface area and facilitating heteroatom doping.In summary,the molten salt ball milling method significantly improves the uniformity,particle refinement and interface interaction of the oxygen reduction catalyst through the synergistic effect of high temperature,mechanical force and molten salt medium,thus outperforming the above-mentioned mixed methods in terms of performance.

[0032] 2.The heteroatom-doped biomass carbon catalyst (BM-Fe / N-C) prepared by the present application has extremely high catalytic activity and stability,with an oxygen reduction potential of 0.98 V (vs.RHE) and a half-wave potential of 0.85 V (vs.RHE),and the oxygen reduction reaction is a 4-electron step.

[0033] 3.The catalyst provided by the present application has already exceeded the noble metal Pt on the market in performance,and the catalyst provided by the present application has simple preparation conditions,high conversion rate and can realize batch production,which greatly reduces the production cost of the catalyst.

[0034] 4.The present application uses discarded sycamore fruit as a precursor to synthesize amorphous BM-Fe / N-C biomass carbon catalysts using a green and environmentally friendly method,realizing the resource utilization of solid waste. BRIEF DESCRIPTION OF DRAWINGS

[0035] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the following will briefly introduce the drawings needed to be used in the embodiments.

[0036] Figure 1 High-resolution image and element composition of BM-Fe / N-C catalyst;

[0037] Figure 2 XRD and Raman spectrum of BM-Fe / N-C catalyst;

[0038] Figure 3 Cyclic voltammogram and oxygen reduction polarization curve of BM-Fe / N-C catalyst;

[0039] Figure 4 Cyclic stability of Pt / C and BM-BM-Fe / N-C catalysts;

[0040] Figure 5 Catalyst cycle stability. DETAILED DESCRIPTION

[0041] The application provides a Fe and N cooperatively doped biomass carbon-based oxygen reduction catalyst, a preparation method and application thereof.

[0042] The preparation method comprises biomass pretreatment, biomass pre-carbonization, biomass carbon ball milling molten salt, heteroatom doping high-temperature carbonization and post-processing.

[0043] In the application, the preparation method of the heteroatom doped biomass carbon-based oxygen reduction catalyst preferably comprises the following steps:

[0044] 1. In the application, the biomass adopts the fruit of a phoenix tree, the waste phoenix tree fruit is crushed by a cell wall breaking machine, and then is cleaned by deionized water, and is vacuum dried at 60 DEG C for 12 hours.

[0045] 2. The pretreated biomass is placed in a tube furnace, heated under a nitrogen protective atmosphere, and then cooled with the furnace. The nitrogen flow rate is preferably 5-10 m / min, the tube furnace heating rate is set to 5 DEG C / min, the heating temperature is set to 350-400 DEG C, and the holding time is preferably 1-2 hours.

[0046] 3. The biomass carbon and inorganic salt are mixed and placed in a ball mill jar for dry ball milling in a planetary ball mill. The inorganic salt is a mixture of ZnCl2 and NaCl, the mass ratio of the ball-milled carbon precursor and the mixed inorganic salt is 3-5:1-3, the mass ratio of the mixed salt NaCl and ZnCl2 is 1-2:2-4, the mass ratio of the grinding ball and the material is 10-20:1-5, the rotation speed of the ball mill is set to 450 rpm / min, and the ball milling time is 4-8 hours. The sieved material after ball milling is 100-300 mesh.

[0047] 4. The ball-milled molten salt material, nitrogen source and iron source are uniformly mixed and placed in a crucible for high-temperature carbonization in a tube furnace. The doping mass ratio of the ball-milled carbon, nitrogen source and iron source is 4-8:2-4:1-2, the nitrogen source is urea, and the iron source is FeCl3. The tube furnace heating rate is set to 5 DEG C / min, the high-temperature carbonization temperature is 750-1000 DEG C, the nitrogen flow rate is 5-10 m / min, and the holding time is 1-2 hours.

[0048] 5. The material after high-temperature carbonization is placed in an inorganic acid bath for acid washing, followed by washing with deionized water and centrifugation until the solution is neutral. The collected material is then dried in a vacuum drying oven to obtain a non-precious metal heteroatom-doped biomass carbon-based oxygen reduction catalyst, denoted as BM-BM-Fe / NC. The inorganic acid used is HCl with a concentration of 1–5 mol; the water bath temperature is set at 60–80℃ for 8–12 hours; and the vacuum drying temperature is 60–80℃ for 12–24 hours.

[0049] 6. To facilitate comparison of the performance of the BM-Fe / NC oxygen reduction catalyst, this invention also prepared a carbon catalyst without iron or nitrogen sources, denoted as BM-C; and a nitrogen-doped carbon-based catalyst doped only with a nitrogen source and without iron sources, denoted as BM-NC. The remaining preparation process and procedures are the same as those for BM-BM-Fe / NC.

[0050] 7. The prepared BM-BM-Fe / NC was characterized by high-resolution and elemental composition analysis, such as... Figure 1 As shown, nanoscale particles are uniformly attached to the carbon layer and exhibit a loose and porous structure. Elemental characterization revealed the presence of Fe and N elements in the material, indicating that Fe and N were successfully doped into the carbon matrix.

[0051] 8. Compare the XRD patterns and Raman spectra of the three components, such as... Figure 2 As shown, all three materials exhibited distinct diffraction peaks at diffraction angles 2θ of 26.5° and 42.8°, corresponding to the (002) and (100) crystal planes of the carbon material, indicating that the three materials are mainly composed of amorphous carbon. BM-Fe / NC showed a sharp peak on the (002) crystal plane, indicating that Fe doping increased the graphitization degree of the carbon matrix material. The ratio of Id to Ig in the Raman spectrum of the carbon material represents the disorder of the material. The ratio for BM-C was 1.13, for BM-NC it was 1.22, and for BM-BM-Fe / NC it was 1.16. This shows that N doping can increase the defects in the carbon matrix, but with the addition of Fe, the graphitization degree of the carbon material increased, and the ratio decreased.

[0052] Catalytic inks were prepared from the three catalysts. These inks were then drop-coated onto a platinum-carbon electrode, and oxygen reduction catalysis was tested in a three-electrode system. The catalyst inks comprised a catalyst, a binder, and a dispersant. The catalysts were the three prepared materials. The binder was a 5% Nafion solution. The dispersant was a mixture of water and ethanol. The electrolyte in the three-electrode system was a KOH solution.

[0053] 1. Catalyst 3-5 mg is weighed, added to a mixed solution of ethanol and deionized water, ultrasonically dispersed for 20-30 min, then a nafion solution is added, ultrasonically dispersed for 30-60 min, and the temperature should not exceed 30-40℃; the ratio of the ethanol and deionized water is 3:1; the nafion addition amount is 10-50 μL.

[0054] 2. The prepared ink 3-5 μL is weighed and drop-coated on a platinum carbon electrode, and dried at room temperature to form a catalyst film.

[0055] 3. The catalyst-loaded electrode is placed in a three-electrode system for oxygen reduction catalysis testing; the electrolyte is a 1-3 moL / KOH solution; and the three-electrode system is filled with oxygen to saturation.

[0056] 4. By comparing the cyclic voltammetry curves and oxygen reduction polarization curves of three different catalysts, as shown in Figure 3 , the oxygen reduction potential and half-wave potential of the BM-BM-Fe / N-C catalyst are 0.895 V (vs. RHE), close to the Pt / C catalyst, indicating that the Fe and N co-doping synergistically improves the oxygen reduction catalytic activity.

[0057] 5. The cyclic stability curves of the Pt / C catalyst and the BM-BM-Fe / N-C catalyst are compared, as shown in Figure 5 , the half-wave potential of the Pt / C catalyst attenuates by 22 mV after 5000 cycles, and the half-wave potential of the BM-BM-Fe / N-C catalyst attenuates by 11 mV after 5000 cycles, obviously the cyclic stability of the BM-BM-Fe / N-C catalyst is better than that of the Pt / C catalyst.

[0058] In order to further illustrate the present application, the present application will be described in detail below in conjunction with the examples, but they should not be understood as limiting the scope of protection of the present application.

[0059] Example 1

[0060] A method for preparing an Fe and N doped oxygen reduction catalyst by a ball milling method, the specific implementation steps are as follows:

[0061] S1: Biomass pre-carbonization treatment

[0062] The crushed Platanus fruit powder is placed in a tube furnace, heated to 350℃ under nitrogen atmosphere, and taken out after 2h of heat preservation.

[0063] S2: Ball milling treatment

[0064] The pre-carbonized precursor material was mixed with FeCl3and urea and ground uniformly, then poured into a ball mill tank, with a total mass and steel ball ratio of 1:10. After 4 hours of ball milling at 450 rpm using a planetary ball mill, the mixture was removed, sieved, and collected.

[0065] S3: High-temperature carbonization

[0066] The ball-milled carbon catalyst precursor was ground and placed in a crucible, heated to 850°C in a nitrogen atmosphere tube furnace, and held for 2 hours. After cooling to room temperature, the mixture was removed, ground into powder, and collected.

[0067] S4: Post-treatment

[0068] The high-temperature carbonized powder was continuously stirred in 3M HCl solution, heated to 80°C, and held for 8 hours. After deionization and repeated ethanol washing three times, the mixture was collected by centrifugal filtration and dried in a vacuum drying oven for 12 hours to obtain the ball-milled catalyst (labeled as B-Fe / N-C).

[0069] Example 2

[0070] A method for preparing an Fe, N-doped oxygen reduction catalyst by a molten salt method, the specific implementation steps are as follows:

[0071] S1: Same as S1 in Example 1

[0072] S2: Molten salt high-temperature carbonization

[0073] The pre-carbonized precursor was mixed with ZnCl2, NaCl, FeCl3, and urea and ground, then placed in a crucible and heated to 850°C in a nitrogen atmosphere tube furnace, and held for 2 hours. After cooling to room temperature, the mixture was removed, ground into powder, and collected.

[0074] S3: Same as S3 in Example 1, the obtained molten salt catalyst was labeled as M-Fe / N-C.

[0075] Figure 5The cyclic voltammograms of the ball-milling catalyst, the molten salt catalyst and the ball-milling-molten salt synergistic catalyst under nitrogen atmosphere and oxygen atmosphere respectively, from the figures, the oxygen reduction potential of the ball-milling catalyst is about 0.7V (Vs. RHE), the oxygen reduction potential of the molten salt carbonized catalyst is about 0.8V, and the oxygen reduction potential of the ball-milling-molten salt synergistic catalyst is about 0.9V. The reason is that the ball-milling catalyst without adding NaCl and ZnCl2 mixed molten salt, the ball-milling mainly reduces the particle size of the pre-carbonized carbon material and creates defects through mechanical force, and in the calcination process, there is no effect of molten salt, the amount of Fe and N doping is less, and the carbon material is easy to sinter. The molten salt carbonized catalyst without ball-milling, the mixed salt and carbon material are not uniformly dispersed, and the carbon material has less defects, so that the energy barrier of Fe and N doping is larger. Therefore, the ball-milling molten salt method synergistically improves the performance of the non-noble metal heteroatom doped biomass carbon-based oxygen reduction catalyst.

[0076] Although the above embodiments have made a detailed description of the present application, it is only a part of the embodiments of the present application, not all the embodiments, and people can also obtain other embodiments according to the present embodiments without creativity, which all belong to the protection scope of the present application.

Claims

1. A non-noble metal heteroatom-doped biomass carbon-based oxygen catalyst and its preparation method, characterized in that, Includes the following steps: Step 1: The biomass is subjected to cell wall breaking and pulverization, ultrasonically cleaned in deionized water, and then vacuum dried at 60-80℃ for 12-24 hours; Step 2: The precursor obtained in Step 1 is pre-carbonized under a nitrogen atmosphere; Step 3: Add the pre-carbonized material and low-melting-point eutectic mixed inorganic salt into a ball mill jar and perform ball milling. The product after ball milling is doped with nitrogen and iron sources and calcined at high temperature in a nitrogen atmosphere. Step 4: The calcined product is soaked in an inorganic acid and then washed with water until neutral to obtain the biomass carbon-based catalyst.

2. The method for preparing a biomass carbon-based oxygen reduction catalyst according to claim 1, characterized in that, The biomass used is the fruit of the Chinese parasol tree.

3. The method for preparing a biomass carbon-based oxygen reduction catalyst according to claim 1, characterized in that, The pre-carbonization temperature is 350–400℃, and the carbonization time is 2–4 hours.

4. The method for preparing a biomass carbon-based oxygen reduction catalyst according to claim 1, characterized in that, The mass ratio of the ball-milled carbon precursor to the mixed salt is 3-5:1-3; the mass ratio of the mixed salt NaCl to ZnCl2 is 1-2:2-4; the mixed inorganic salt includes, but is not limited to, one or more of NaCl, ZnCl2, and KCl.

5. The method for preparing a biomass carbon-based oxygen reduction catalyst according to claim 1, characterized in that, The mass ratio of carbon, nitrogen, and iron sources in the ball-milled molten salt is 4-8:2-4:1-2; the nitrogen source is urea and the iron source is FeCl3; the preparation of biomass carbon-based materials by doping other nitrogen and iron sources using the ball-milled molten salt method also falls within the scope of this patent.

6. The method for preparing a biomass carbon-based oxygen reduction catalyst according to claim 1, characterized in that, The high-temperature carbonization temperature is 750–1000℃, the nitrogen flow rate is 5–10 m / min, and the holding time is 1–2 h.

7. The method for preparing a biomass carbon-based oxygen reduction catalyst according to claim 1, characterized in that, The inorganic acid used is dilute hydrochloric acid with a concentration of 1-5 mol, which is shaken in a water bath at 60°C for 8-12 hours.

8. A method for preparing a biomass carbon-based oxygen reduction catalyst according to claims 1-7, characterized in that, The carbon-based oxygen reduction catalyst is mainly used as a cathode active material in metal-air batteries, fuel cells, and microbial batteries. The application of this material in other fields also falls within the scope of this patent.

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