Preparation method of high-activity porous carbon ORR catalyst without transition metal doping

Abstract

The invention discloses a preparation method of a high-activity porous carbon ORR catalyst without transition metal doping. The preparation method comprises the following steps: adding sodium citrate, urea and ZnCl2 into absolute ethyl alcohol for ultrasonic treatment, and then stirring and drying in a water bath to obtain N-rich gel coated solid-phase sodium citrate; the preparation method comprises the following steps: carrying out stepped temperature-programmed pyrolysis on a porous carbon precursor under the protection of an inert atmosphere, sequentially carrying out acid washing, water washing and alcohol washing on a pyrolysis product, carrying out suction filtration to collect the product, and drying to obtain the porous carbon ORR catalyst without transition metal doping. According to the method, sodium citrate is used as a carbon source, urea is used as a nitrogen source, ZnCl2 is introduced as a pore forming agent and an active agent, and the metal-free porous carbon material with high catalytic activity and stability is successfully constructed through stepped pyrolysis. Experiments show that the catalyst shows ORR catalytic activity and excellent electrochemical stability which are equivalent to those of commercial 20% Pt / C in an alkaline environment.

Description

Technical Field

[0001] This invention relates to the field of redox reaction catalysis technology, specifically to a method for preparing a highly active porous carbon ORR catalyst without transition metal doping. Background Technology

[0002] As the energy crisis worsens, greater emphasis is being placed on the development of renewable energy sources and the exploration of energy-efficient utilization methods, such as fuel cells and metal-air batteries. The oxygen reduction reaction (ORR), due to its slow kinetics, plays a crucial role in energy technologies such as fuel cells and metal-air batteries. To date, platinum-based catalysts are considered the most effective ORR catalysts; however, due to the high cost and poor stability of platinum, continuous efforts are being made to develop low-cost, platinum-free ORR catalysts.

[0003] In past studies, transition metal-doped MNC catalysts have attracted widespread attention due to their superior performance. The introduction of transition metal elements such as Fe and Co has resulted in highly active MNC catalysts. x While these metal sites are readily demetallized during catalysis, leading to decreased structural stability and failing to meet practical application requirements, research on such catalysts indicates that nitrogen doping is crucial, with undoped metal sites exhibiting ORR inertness. However, current NC catalysts without transition metal sites suffer from significantly lower catalytic performance compared to commercial Pt / C catalysts due to difficulties in forming highly active pyridine and graphitic nitrogen and insufficient exposure of active sites. Therefore, constructing carbon-based catalysts with highly active nitrogen sites using simple methods under transition metal-free conditions remains a significant challenge in this field.

[0004] Nitrogen-doped carbon achieves intramolecular and intermolecular charge transfer by generating positive / negative charges on adjacent carbon atoms, thereby altering the chemisorption mode of O2 in carbon-based materials and significantly enhancing their catalytic performance. Previous studies have shown that pyridine nitrogen and graphitic nitrogen species are considered key to promoting ORR (Organic Response Rate). Pyridine nitrogen is beneficial for constructing active sites and improving catalytic activity, while graphitic nitrogen is beneficial for improving the conductivity of the material. Pyrrole nitrogen and nitrogen oxides, however, do not show a strong correlation with ORR activity. Past studies have often used toxic and harmful nitrides as nitrogen sources, such as melamine, KSCN, 2-methylimidazole, 2-2-bipyridine, and 1,10-phenanthroline. Furthermore, these studies typically require doping under acid / alkali or high-pressure environments. These operations are not only extremely dangerous and complex, but the doping sites are also highly random and disorganized, resulting in a large number of active sites distributed within the carbon material after pyrolysis, unable to directly contact the reactants and exhibiting low utilization efficiency. Therefore, even with continuously increasing the concentration of N precursors during doping, it remains difficult to effectively improve the ORR activity of the catalyst. Finding a stable, harmless method that can generate more effective active sites is crucial for the preparation, promotion, and large-scale production of catalysts. Summary of the Invention

[0005] To address the aforementioned limitations of the prior art, the present invention aims to provide a method for preparing a highly active porous carbon ORR catalyst without transition metal doping. This invention uses sodium citrate as the carbon source, urea as the nitrogen source, and ZnCl2 as the activator, to prepare a highly efficient porous carbon ORR catalyst without transition metal doping via stepwise pyrolysis. This catalyst exhibits ORR catalytic activity and stability close to that of Pt / C under alkaline conditions.

[0006] To achieve the above objectives, the present invention adopts the following technical solution: In a first aspect, the present invention provides a method for preparing a highly active porous carbon ORR catalyst without transition metal doping, the method comprising the following steps: (1) Sodium citrate, urea and ZnCl2 were added to anhydrous ethanol and ultrasonically treated, and then stirred in a water bath to obtain gel-coated solid sodium citrate. (2) The gel-coated solid sodium citrate was deheated under a protective atmosphere. The product after pyrolysis was ultrasonically treated in an acidic solution, then stirred and acid-washed, the product was collected by suction filtration, and then washed with water and alcohol in sequence. After drying, a highly active porous carbon ORR catalyst without transition metal doping was obtained.

[0007] Preferably, in step (1), the mass ratio of sodium citrate, urea and ZnCl2 is 2:4:2~4.

[0008] Preferably, in step (1), the ultrasonic treatment time is 4 hours; the water bath temperature is 70°C; and the stirring time is 7 hours.

[0009] Preferably, in step (2), the heating rate of the pyrolysis is 5℃ / min; the pyrolysis is first held at 550℃ for 3h, and then heated to 800℃ and held for 3h.

[0010] Preferably, in step (2), the acidic solution is a 3M hydrochloric acid solution; the ultrasonic treatment time is 10 min; and the stirring and acid washing time is 6 h.

[0011] In a second aspect, the present invention provides a highly active porous carbon ORR catalyst without transition metal doping obtained by the above preparation method.

[0012] Preferably, the N doping of the highly active porous carbon ORR catalyst without transition metal doping is distributed on the surface of the porous carbon.

[0013] A third aspect of the present invention provides the application of a highly active porous carbon ORR catalyst without transition metal doping in improving the catalytic performance of redox reactions.

[0014] Preferably, the oxygen reduction reaction follows a four-electron pathway.

[0015] Preferably, the catalytic performance includes increasing the number of transferred electrons and the yield of H2O2.

[0016] The beneficial effects of this invention are: This invention utilizes sodium citrate as the carbon source, urea as the nitrogen source, and ZnCl2 as a pore-forming agent to prepare a highly efficient ORR catalyst via stepwise pyrolysis. Experiments show that the synthesized catalyst exhibits ORR stability and half-wave potential close to that of commercial Pt / C catalysts under alkaline conditions. The pore-forming effect of the gases generated by the volatilization of Zn and the pyrolysis of urea and sodium citrate improves the pore structure of the catalyst and enhances its activity. This provides a safe and simple new approach for synthesizing porous carbon-structured ORR catalysts with dispersed active sites. Attached Figure Description

[0017] Figure 1 Schematic diagram of catalyst synthesis method; Figure 2 (a~c) SEM images of NC-0; (d~f) SEM images of NC; Figure 3 LSV curves of each group of samples measured in 0.1M KOH; Figure 4 Tafel plots of each group of samples; Figure 5 Cdl plots of each group of samples; Figure 6(a) Number of electrons transferred in NC and H2O2 yield; (b) Number of electrons transferred in Pt / C and H2O2 yield; Figure 7 Stability of NC and Pt / C catalysts. Detailed Implementation

[0018] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application pertains.

[0019] As introduced in the background section, for the active sites of the ORR reaction, pyridine nitrogen and transition metal coordinated nitrogen are considered key to promoting ORR. Pyridine nitrogen is beneficial for constructing active sites and improving catalytic activity, while graphitic nitrogen can improve the conductivity of the material to a certain extent. However, pyrrole nitrogen and nitrogen oxides do not have a strong correlation with the ORR reaction. If the carbon material surface contains more active sites, the catalytic performance can be greatly improved. However, although current nitrogen-doped carbon materials all incorporate nitrogen into the carbon material, it is not possible to achieve nitrogen doping only on the surface of the carbon material, resulting in insufficient exposure of active sites and hindering further improvement of catalytic activity.

[0020] Based on the above research bottlenecks, the purpose of this invention is to provide a method for preparing a highly active porous carbon ORR catalyst without transition metal doping. This invention cleverly utilizes the differences in solubility of substances, uniformly encapsulating an insoluble solid-phase C precursor with a soluble N source in a gel phase, thereby achieving efficient anchoring of N sites on the surface of the carbon material. Simultaneously, a pore-forming agent is used to etch the carbon source surface to form numerous pores, which not only increases the catalyst's specific surface area and active site density but also facilitates gas-liquid transport during the ORR process. Specifically, this invention utilizes the differences in solubility of urea, ZnCl2, and sodium citrate in anhydrous ethanol. The three are mixed and stirred uniformly. The soluble urea (N source) and ZnCl2 (pore-forming agent) form a gel state, uniformly encapsulating the insoluble sodium citrate (C source). After heating and stirring, the free ethanol is evaporated, forming a mixed product of a N-rich gel-coated solid-phase carbon source. After pyrolysis, a large number of N-containing sites are doped onto the catalyst surface, and simultaneously benefiting from Zn... 2+ The pore-forming function can greatly increase the doping amount and exposure of N sites, thereby effectively improving the N doping amount and ORR catalytic activity of the catalyst. The entire process is safe and non-toxic, simple to operate, and extremely low in cost. The resulting catalyst has excellent ORR activity and has great potential for widespread application and large-scale production.

[0021] Sodium citrate, as a carbon source, plays a decisive role in the catalyst yield and structure. To ensure uniform doping of other elements in the catalyst, alcohol, which can dissolve ZnCl2 and urea but not sodium citrate, was selected as the solvent during gel preparation, based on the solubility of each substance. To further ensure uniform dispersion of the substances in the gel, an ultrasonic treatment step was added before heating and stirring. Considering that the boiling point of alcohol is close to 80℃, a water bath temperature of 70℃ was chosen during heating and stirring, ensuring that the solution would not boil while increasing the evaporation rate. After stirring for 7 hours, a gel composed of sodium citrate, ZnCl2, urea, and alcohol was obtained. The alcohol did not evaporate completely because of the metal cation Zn in the solution. 2+ It forms a complex with alcohol, which directly results in a gel rather than a powder after heating and stirring. However, this does not affect the uniformity of the distribution of each element in the gel; after heating and stirring, each element remains uniformly coated on sodium citrate.

[0022] To enable those skilled in the art to better understand the technical solution of this application, the technical solution of this application will be described in detail below with reference to specific embodiments.

[0023] Note: The sodium citrate used in this invention has been dehydrated and ground.

[0024] The test materials used in the embodiments of this invention are all conventional test materials in the art and can be purchased through commercial channels.

[0025] Example 1: Preparation method of highly active porous carbon ORR catalyst without transition metal doping (1) such as Figure 1 As shown, 2g of sodium citrate, 4g of urea and 4g of ZnCl2 were taken and sonicated in 100mL of anhydrous ethanol for 4 hours, and then magnetically stirred in a 70℃ water bath until a gel was obtained.

[0026] (2) The gel was placed in a tube furnace and heated at 550°C for 3 hours under Ar atmosphere. Then, the temperature was increased to 800°C and held for 3 hours. After that, it was naturally cooled to room temperature to obtain the pyrolysis product. The pyrolysis product was sonicated in 3M hydrochloric acid solution for 10 minutes. Then, it was stirred and acid washed on a magnetic stirring table for 6 hours. The product was collected by filtration, washed with water 5 times, washed with alcohol 2 times, and then dried in an oven to obtain a metal-free N-doped porous carbon catalyst, denoted as NC.

[0027] Comparative Example 1 The gel is not prepared. The difference from step (2) of Example 1 is that sodium citrate is used instead of gel, and the final product is denoted as 800-C.

[0028] Comparative Example 2 The difference from Example 1 is that ZnCl2 is not added, and the final product is denoted as NC-0.

[0029] Comparative Example 3: Catalysts prepared with different raw material ratios In order to investigate the reaction ratio conditions for producing gel, the ratio of ZnCl2 to other raw materials was adjusted, and the rest of the preparation was carried out according to the steps (1) of Example 1. The product states are shown in Table 1.

[0030] Table 1 Product State The products obtained from groups 1 to 2 in Table 1 were prepared according to the method in step (2) of Example 1, and the catalysts finally prepared were denoted as NC-1.5 and NC-1, respectively.

[0031] Example 2: Characterization (1) such as Figure 2 As shown, both NC and NC-0 catalysts have a large number of pore structures, with NC having more macropores, which is more conducive to mass transfer, and the morphology is more regular.

[0032] (2) The ring-disc electrode polarization test was performed on the catalyst prepared in Example 1 and the commercial 20% wt. Pt / C catalyst (Alfa Aesar Chemical Co., Ltd., China) at 1600 rpm. The measured ring current and disk current were calculated using the following formula: (1); (2); Where N=0.25 is the collection efficiency, I D For disk current, I R Let n be the ring current and n be the number of transferred electrons (corresponding to the n electron transfer paths).

[0033] Different reaction pathways result in completely different catalytic products. When the reaction is based on a two-electron reaction, the reduction product is excess H₂O₂. However, when the reaction pathway is a four-electron pathway, the product is water. To clarify the reaction pathway of the catalyst, the number of electrons transferred and the H₂O₂ yield of the NC catalyst in the catalytic oxygen reduction reaction were obtained. Figure 6 As can be seen, the NC catalyst exhibits superior four-electron selectivity compared to commercial Pt / C catalysts.

[0034] Experimental Example 1 10 mg of catalysts NC, NC-1.5, NC-1, NC-0, and 800-C prepared in Examples 1 and 1-3 were taken, with a commercial 20% wt. Pt / C catalyst (Alfa Aesar Chemical Co., Ltd., China) as a control. 635 μL of deionized water, 1270 μL of anhydrous ethanol, and 100 μL of Nafion solution were added to each, respectively, followed by sonication for 10 min to form a homogeneous mixture. 5 μL of the mixture was dropped onto a 4 mm diameter disk electrode and dried at room temperature at 350 rpm. The catalyst loading was 0.198 mg / cm². 2 The working electrode was obtained. The electrolyte was 0.1M KOH solution, and a three-electrode system was used, with a Hg / HgO electrode as the reference electrode and a platinum sheet as the counter electrode. All electrochemical tests were performed on a Koster C52350M workstation. The LSV curves of each group of samples were measured by linear sweep voltammetry in O2-saturated 0.1M KOH solution, and the results are shown in [Figure number missing]. Figure 3 .

[0035] like Figure 3 As shown, the LSV curves of each group of samples exhibit significant differences. Comparing the NC and 800-C samples reveals a significant increase in the half-wave potential and onset potential. This is because the addition of urea as a nitrogen source successfully doped the carbon material with N and various active sites. Simultaneously, the gas-generating effect of urea pyrolysis promotes the formation of pore structures, thereby improving the catalyst's mass transfer capacity and increasing the limiting current density of the ORR reaction. Based on NC-0, the addition of the activator ZnCl2 during preparation resulted in even larger half-wave potentials and limiting current densities in the NC sample's LSV curve. This is because the pore-forming effect of ZnCl2 further improves the pore distribution and structure, while also altering the types and distribution of active sites in the sample. Ultimately, the catalyst's half-wave potential reached 0.85 V, superior to commercial platinum-carbon (0.83 V). Its activity surpasses most reported non-noble metal catalysts and even approaches that of state-of-the-art commercial Pt / C.

[0036] Experimental Example 2 Working electrodes were prepared using catalysts NC, commercial Pt / C, and NC-0 according to the method in Example 1. The specific activity of each catalyst was measured by linear sweep voltammetry using the method in Example 1. The three-electrode system of Example 1 was employed. All electrochemical tests were performed on a KOSTER C52350M workstation. The results are shown in [Figure 1]. Figure 4 . Figure 4 The specific activity Tafel plots show that, compared to NC-0 and commercial Pt / C, the NC catalyst has the smallest Tafel slope, at 61.4 mV dec. -1 .

[0037] To investigate the electrochemical active area of ​​the sample, cyclic voltammetry was used to test it in the range of 1.0~1.1V, and the double-layer capacitance Cdl of the sample was calculated. Figure 5 As can be seen, the Cdl size of the NC catalyst is between that of NC-0 and Pt / C. This is because the NC sample is in powder form before high-temperature calcination, while the NC precursor is in gel form. This difference in state leads to the Cdl of the NC sample being larger than that of NC-0. However, the difference in the type of N after calcination results in a significant difference in the catalytic performance of the two catalyst samples.

[0038] Experimental Example 3 Following the method in Example 1, catalyst NC and commercial Pt / C were prepared as the working electrode. The three-electrode system of Example 1 was used, and all electrochemical tests were performed on a KOSTER C52350M workstation. The current on the working electrode was measured while maintaining a constant potential. To verify the stability of the catalyst in an alkaline (0.1M KOH) environment over a long period, the current decay on the working electrode was tested at 1600 rpm under a constant potential of 0.3V and saturated O2. After three hours of continuous testing, the current decayed by 12%, which was less than the 20% decay observed with Pt / C. Figure 7 As shown, NC has better stability than Pt / C.

[0039] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. Preparation of a highly active porous carbon ORR catalyst without transition metal doping, characterized in that, The preparation method includes the following steps: (1) Sodium citrate, urea and ZnCl2 were added to anhydrous ethanol and ultrasonically treated, and then stirred in a water bath to obtain gel-coated solid sodium citrate. (2) The gel-coated solid sodium citrate was deheated under an inert atmosphere. The product after pyrolysis was ultrasonically treated in hydrochloric acid solution, then stirred and acid-washed, the product was collected by suction filtration, and then washed with water and alcohol in sequence. After drying, a highly active porous carbon ORR catalyst without transition metal doping was obtained.

2. The preparation method according to claim 1, characterized in that, In step (1), the mass ratio of sodium citrate, urea and ZnCl2 is 2:4:2~4.

3. The preparation method according to claim 1, characterized in that, In step (1), the ultrasonic treatment time is 4 hours; the water bath temperature is 70°C; and the stirring time is 7 hours.

4. The preparation method according to claim 1, characterized in that, In step (2), the heating rate of the pyrolysis is 5℃ / min; the pyrolysis is to first keep at 550℃ for 3h, and then raise the temperature to 800℃ and keep at 800℃ for 3h.

5. The preparation method according to claim 1, characterized in that, In step (2), the acidic solution is a 3M hydrochloric acid solution; the ultrasonic treatment time is 10 min; and the stirring and acid washing time is 6 h.

6. The highly active porous carbon ORR catalyst without transition metal doping obtained by the preparation method according to any one of claims 1 to 5.

7. The highly active porous carbon ORR catalyst without transition metal doping according to claim 1, characterized in that, The N doping in the highly active porous carbon ORR catalyst without transition metal doping is distributed on the surface of the porous carbon.

8. The application of the high-activity porous carbon ORR catalyst without transition metal doping as described in claim 6 or 7 in improving the catalytic performance of redox reactions.

9. The application according to claim 8, characterized in that, The oxygen reduction reaction follows a four-electron path.

10. The application according to claim 8, characterized in that, The catalytic performance includes increasing the number of transferred electrons and the yield of H2O2.

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