Lycium barbarum derived porous carbon catalyst as well as preparation method and application thereof in oxygen reduction reaction

Abstract

The invention discloses a wolfberry-derived porous carbon catalyst, a preparation method thereof and an application of the wolfberry-derived porous carbon catalyst in an oxygen reduction reaction, and relates to the field of energy storage materials and technologies. According to the invention, wolfberry is used as a carbon source, melamine is used as a nitrogen source, and the ORR catalytic effect of the catalyst is realized by increasing the specific surface area of the carbon material and adjusting asymmetric electrons on the surface of the carbon material. The preparation process is simple, compared with a noble metal catalyst material, the raw material has the advantages of low price and abundant reserves, the source is wide and very easy to obtain, and the obtained wolfberry derived porous carbon catalyst material can be used in an electrocatalytic oxygen reduction reaction and has excellent electrocatalytic activity.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of energy storage materials and technologies, and particularly relates to a wolfberry derived porous carbon catalyst, a preparation method thereof and application thereof in oxygen reduction reaction. BACKGROUND

[0002] Proton exchange membrane fuel cells (PEMFCs) provide a zero-emission energy conversion solution, which is essential for achieving full decarbonization of the automotive and other energy industries. However, the platinum catalyst used on the traditional fuel cell electrode is not only scarce in raw materials and expensive, but also has poor stability. After a long time of work, platinum particles will dissolve, migrate, agglomerate or fall off from the carrier, reducing the active area and thus affecting the catalytic performance.

[0003] In view of the problems of high cost of catalyst and poor stability of platinum particles, there are two main solutions at present: one is to reduce the platinum loading and improve the use efficiency of the catalyst; the other is to develop high-activity non-precious metal catalysts (NPMCs) and non-metal catalysts to replace expensive platinum catalysts.

[0004] Due to the factors such as low cost, abundant resources and unique structure characteristics of carbon materials, a large amount of work has been invested in the research of metal-free carbon electrocatalysts. The development of heteroatom-doped carbon and related defect structures is considered to activate adjacent carbon atoms, thereby improving the adsorption and dissociation of oxygen, which is crucial for accelerating the catalytic kinetics of the oxygen reduction reaction (ORR). Therefore, many researchers have begun to explore different heteroatom (such as N, P or S) doping methods to destroy the local pi electron cloud on the surface of carbon materials, so as to redistribute the electric charge on the carbon surface to enhance the ORR activity. However, many research results show that the modification of carbon materials by using the doping method to improve the ORR performance does not have high effect. SUMMARY

[0005] The purpose of the present application is to provide a wolfberry derived porous carbon catalyst, a preparation method thereof and application thereof in oxygen reduction reaction, so as to solve the problems existing in the prior art. The wolfberry is used as a carbon source, and melamine is used as a nitrogen source. The specific surface area of the carbon material is increased, and the asymmetric electrons on the surface of the carbon material are adjusted, so as to realize the ORR catalytic effect of the catalyst.

[0006] To achieve the above purpose, the present application provides the following solutions.

[0007] One of the technical solutions of the present application provides a preparation method of a wolfberry derived porous carbon catalyst, which comprises the following steps:

[0008] The wolfberry residue and melamine are mixed and heat treated to prepare the wolfberry derived porous carbon catalyst.

[0009] As a further preferred embodiment of the present application, the temperature of the heat treatment is 800-1300℃, and / or, the time of the heat treatment is 2h.

[0010] As a further preferred embodiment of the present application, the heat treatment is performed in a protective atmosphere.

[0011] As a further preferred embodiment of the present application, the mass ratio of the wolfberry residue and melamine is (0.5-8):2.

[0012] As a further preferred embodiment of the present application, the wolfberry residue is obtained by freeze-drying of the softened and seed-removed wolfberry.

[0013] As a further preferred embodiment of the present application, the preparation method of the wolfberry-derived porous carbon catalyst comprises the following steps:

[0014] The wolfberry residue and melamine are mixed in a mass ratio, then added into water, and dispersed uniformly by ultrasonic treatment. The obtained mixture is dried to remove the solvent, and then the dried solid is ground into a uniform powder. The mixed powder is placed in an argon protective atmosphere for high-temperature calcination to carbonize and activate the raw materials, thereby generating a porous carbon material. The calcined carbon material is dispersed in deionized water, and the residual impurities are removed by washing. Finally, the target product, a wolfberry-derived porous carbon catalyst material, is obtained after drying.

[0015] The present application utilizes the strategy of mixing melamine as an activator with wolfberry residue for carbonization to prepare a porous carbon material. Preferably, the wolfberry is soaked in clean tap water for 2-3 days to allow the wolfberry to swell and soften, and then the seeds are removed. The softened wolfberry skin is placed in a petri dish for freeze-drying treatment. The pretreated wolfberry is crushed and sieved with a 100-mesh sieve to obtain wolfberry residue.

[0016] As a further preferred embodiment of the present application, the mixed powder of wolfberry residue and melamine is ground thoroughly, and then pyrolyzed in an inert atmosphere at 800-1300℃. After pyrolysis, the obtained powder is ground thoroughly, washed with a large amount of deionized water to remove soluble impurities, and then dried to obtain a wolfberry-derived porous carbon catalyst.

[0017] The present application utilizes the freeze-drying process to treat the fully soaked and swollen wolfberry, which significantly optimizes the physical structure and active ingredient retention effect. During freeze-drying, water is directly removed from the material through sublimation, avoiding the pulling effect of liquid water surface tension on molecular chains. The loose network structure formed in the pre-swollen state is "frozen" in the dried skeleton, allowing polysaccharides, proteins, and other biological macromolecules to maintain an extended conformation, which is beneficial for maintaining their functional activity (such as the immune regulation ability of wolfberry polysaccharides).

[0018] In the complex carbonization process of wolfberry residue and melamine, melamine mainly plays a key role in nitrogen doping and pore regulation. Melamine (C3H6N6) has a high nitrogen content of 66.7%, and when carbonized at high temperature (usually > 500℃), it decomposes to generate NH3, HCN and other nitrogen-containing gases, while releasing active nitrogen species (such as amino, cyano). These nitrogen atoms form doped forms such as pyridine nitrogen (N-6), pyrrole nitrogen (N-5), and graphite nitrogen (N-Q) by replacing sp 2 Hybrid sites in the carbon skeleton, significantly improving the electronic conductivity and surface chemical activity of the carbon material. At the same time, the gaseous products (such as NH3) produced by the decomposition of melamine during carbonization will escape from the carbon matrix, forming gas escape channels, directly inducing the formation of mesopores (2-50nm) and macropores (> 50nm), effectively increasing the specific surface area of the carbon material and increasing the active sites, thereby improving the catalytic performance of the carbon material.

[0019] The second technical scheme of the present application provides a wolfberry-derived porous carbon catalyst prepared by the preparation method.

[0020] The wolfberry-derived porous carbon catalyst prepared by the present application is a defective carbon catalyst material with an asymmetric surface charge.

[0021] The third technical scheme of the present application provides the application of the above-mentioned wolfberry-derived porous carbon catalyst in oxygen reduction reaction.

[0022] The fourth technical scheme of the present application provides the application of the above-mentioned wolfberry-derived porous carbon catalyst as an electrode catalyst for proton exchange membrane fuel cells.

[0023] The present application selects wolfberry as the carbon source, which contains rich polysaccharides and amino acids in its composition. During carbonization, carbon atoms rearrange to form carbon materials with graphite structure. Such structure is beneficial for N atom doping and the formation of more high-activity sites on the carbon configuration after high-temperature denitrification. At the same time, melamine with high nitrogen content is selected as the nitrogen source to provide nitrogen for nitrogen doping to carbon (N→C) to form point defects. Melamine can effectively produce meso / macropores in wolfberry carbon during pyrolysis, which plays an important role in deriving a carbon skeleton with ultra-high surface area and sufficient total pore volume. Carbon defects produced after denitrification can activate irregularly twisted periodic structures, thereby asymmetrically rearranging the surrounding electrons, leading to the rearrangement of local electron distribution and coordination environment of active sites, thereby enhancing the surface adsorption of ORR intermediates (*O2, *OOH, *O and *OH) in oxygen reduction reaction (ORR) and improving the ORR performance.

[0024] The present application discloses the following technical effects:

[0025] The application creatively proposes that after N atom doping is performed on a wolfberry-based carbon skeleton, a high-temperature denitrification treatment mode is further performed to cause defects of carbon atom configuration, to adjust and optimize the asymmetric electronic structure of the surface of the carbon material locally, and to create more electrocatalytic active sites to ensure the influence of high-temperature denitrification on the ORR performance of the carbon material.

[0026] The application has the advantages of simple preparation process, low cost of raw materials, rich reserves, low price, wide sources and easy availability, and the obtained wolfberry-derived porous carbon catalyst material can be used in electrocatalytic oxygen reduction reactions and has excellent electrocatalytic activity. BRIEF DESCRIPTION OF DRAWINGS

[0027] In order to more clearly illustrate the technical solutions in the embodiments of the present application or the prior art, the drawings needed in the embodiments will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can also be obtained by those skilled in the art without creative effort on the basis of these drawings.

[0028] Figure 1 In the figure, a is a 10-micron SEM image of PC-900; b is a 2-micron SEM image of PC-1100; and c is a 200-nanometer SEM image of PC-1100.

[0029] Figure 2 In the figure, a is an XPS full spectrum of PC-900, PC-1100 and PC-1300; and b is an N1s spectrum of PC-900, PC-1100 and PC-1300.

[0030] Figure 3 In the figure, a is a CV curve of PC-900 in 0.1M KOH under Ar saturation (black line) / O2 saturation (red line); b is a CV curve of PC-1100 in 0.1M KOH under Ar saturation (black line) / O2 saturation (red line); and c is a CV curve of PC-1300 in 0.1M KOH under Ar saturation (black line) / O2 saturation (red line).

[0031] Figure 4 In the figure, a is a CV curve of PC-900 in 0.1M KOH under different scanning speeds; b is a CV curve of PC-1100 in 0.1M KOH under different scanning speeds; c is a CV curve of PC-1300 in 0.1M KOH under different scanning speeds; and d is a double-layer capacitance (C dl ) value of PC-900, PC-1100 and PC-1300.

[0032] Figure 5In the figure, a is the LSV curve of PC-900, PC-1100 and PC-1300, b is the initial potential E onset and the half-wave potential E 1 / 2 of PC-900, PC-1100 and PC-1300.

[0033] Figure 6 In the figure, a is the chronoamperogram of PC-900, PC-1100 and PC-1300 running at 1600 rpm for 15 h; b is the LSV curve of PC-1100 before and after aging. DETAILED DESCRIPTION

[0034] Various exemplary embodiments of the present application will now be described in detail, which should be considered to be illustrative of the application and not as limiting the application. It will be understood that the detailed description and specific examples, while indicating certain aspects of the application, are given by way of illustration only, since various changes and modifications within the scope of the application will become apparent to those skilled in the art from this detailed description.

[0035] It should be understood that the terms used herein are for the purpose of describing particular embodiments only and are not intended to limit the application. Further, for a range of values, it is intended that every intermediate value between the upper and lower limits of that range is also specifically disclosed. Each smaller range between any stated value or intervening value in a stated range and any other stated or intervening value in that stated range is encompassed within the application. The upper and lower limits of these smaller ranges can independently be included or excluded in the range.

[0036] Unless defined otherwise, 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 belongs. Although preferred methods and materials are described herein, any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the present application. All documents mentioned herein are incorporated by reference to disclose and describe the methods and / or materials in connection with which the documents are cited. In case of conflict, the content of the present specification will control.

[0037] Various modifications and changes can be made to the specific embodiments of the present application described herein without departing from the scope or spirit of the application. Other embodiments of the application will be apparent to those of ordinary skill in the art from the description and examples presented herein. The description and examples are illustrative of the application and are not intended to limit the scope of the application.

[0038] As used herein, the terms "comprise", "comprising", "include", "including", "have", "having" and the like are open-ended and do not exclude additional elements or steps.

[0039] It should be noted that the present application does not describe in detail the conventional operation means in the art, and is not the focus of the present application.

[0040] In the following examples of the present application, the wolfberry residue is prepared by the following steps:

[0041] The wolfberry is soaked in clean tap water for 3 days to fully swell and soften the wolfberry, and then the wolfberry seeds are removed. The softened wolfberry skin is placed in a petri dish for freeze-drying treatment. The pretreated wolfberry is crushed and sieved with a 100-mesh sieve to obtain wolfberry residue.

[0042] Example 1

[0043] A method for preparing a wolfberry-derived porous carbon catalyst, the steps are as follows:

[0044] (1) 1.0 g of wolfberry residue and 2.0 g of melamine are added to a beaker containing 20 ml of water, the mixture is stirred thoroughly and ultrasonicated for 30 min, and then dried in an oven to remove the solvent;

[0045] (2) The dried mixture of wolfberry residue and melamine powder is thoroughly ground, and then pyrolyzed at T = 900℃ for 2 hours under Ar atmosphere at a heating rate of 5℃

[0046] min -1 ;

[0047] (3) After pyrolysis, the obtained powder is thoroughly ground and washed with a large amount of deionized water to remove soluble impurities.

[0048] (4) After deionized water filtration, the collected sample is dried at 60℃ to obtain a wolfberry-derived porous carbon catalyst, labeled as PC-900.

[0049] Example 2

[0050] A method for preparing a wolfberry-derived porous carbon catalyst, the steps are as follows:

[0051] (1) 1.0 g of wolfberry residue and 2.0 g of melamine are added to a beaker containing 20 ml of water, the mixture is stirred thoroughly and ultrasonicated for 30 min, and then dried in an oven to remove the solvent;

[0052] (2) The dried mixture of wolfberry residue and melamine powder is thoroughly ground, and then pyrolyzed at T = 1100℃ for 2 hours under Ar atmosphere at a heating rate of 5℃

[0053] min -1 ;

[0054] (3) After pyrolysis, the obtained powder is thoroughly ground and washed with a large amount of deionized water to remove soluble impurities.

[0055] (4) After being filtered with deionized water, the collected sample is dried at 60°C to obtain the wolfberry-derived porous carbon catalyst, marked as PC-1100.

[0056] Example 3

[0057] A preparation method of a wolfberry-derived porous carbon catalyst, comprising the following steps:

[0058] A preparation method of a wolfberry-derived porous carbon catalyst, comprising the following steps:

[0059] (1) 1.0 g of wolfberry residue and 2.0 g of melamine are added to a beaker containing 20 ml of water, the mixed solution is fully stirred and ultrasonicated for 30 min, and then dried in an oven to remove the solvent;

[0060] (2) The dried mixed powder of wolfberry residue and melamine is fully ground, and then pyrolyzed at T = 1300°C for 2 hours under an Ar atmosphere at a heating rate of 5°C / min;

[0061] min -1 .

[0062] (3) After the pyrolysis is completed, the obtained powder is fully ground and washed with a large amount of deionized water to remove soluble impurities.

[0063] (4) After being filtered with deionized water, the collected sample is dried at 60°C to obtain the wolfberry-derived porous carbon catalyst, marked as PC-1100.

[0064] The samples prepared at different temperatures (PC-900, PC-1100, PC-1300) are microscopically characterized by field emission scanning electron microscopy (SEM), and the results are shown in Figure 1 FIGS. 1a, 1b and 1c, wherein FIG. 1a is a SEM image of PC-900 at 10 μm; FIG. 1b is a SEM image of PC-1100 at 2 μm; and FIG. 1c is a SEM image of PC-1100 at 200 nm.

[0065] As can be seen from Figure 1 FIG. 1a, at a magnification of 10 μm, PC-900 presents a relatively uniform particle distribution, and the particle size is in the micron level. Further magnification to 2 μm Figure 1 (FIG. 1b), it can be observed that some pores and depressions begin to appear on the surface of the particles of PC-1100, and the pore structure on the surface is relatively obvious. This indicates that at a higher carbonization temperature, the pore structure inside the material has been better developed, which will be conducive to the penetration of the electrolyte and the diffusion of oxygen, and thus improve the catalytic performance of the oxygen reduction reaction (ORR). At a higher magnification (200 nm) Figure 1c) The surface details of PC-1100 are more distinct. All samples exhibit abundant nanopores and an irregular surface structure. PC-1100 exhibits a greater number of open pores and a larger specific surface area, consistent with its excellent ORR activity demonstrated in electrochemical performance tests. These nanopores not only increase the material's specific surface area, providing more active sites, but also facilitate the rapid transport of ions in the electrolyte, thereby improving the rate and efficiency of the electrochemical reaction.

[0066] X-ray photoelectron spectroscopy (XPS) is used to analyze the surface chemical composition and elemental valence states of porous carbon materials, which plays a key role in revealing their active sites in the ORR catalytic process. Figure 2 a shows the full XPS spectra of PC-900, PC-1100 and PC-1300. Figure 2 b is their N1s spectra. From the full spectra, we can see that all samples are mainly composed of carbon, while also containing small amounts of heteroatoms such as nitrogen and oxygen.

[0067] Analysis of the N 1s spectrum reveals that nitrogen exists in carbon materials in a variety of chemical states, including pyridinic nitrogen (N-6), pyrrolic nitrogen (N-5), and graphitic nitrogen (NQ). These different types of nitrogen doping have varying effects on the electronic structure and catalytic performance of carbon materials. Pyridinic and pyrrolic nitrogen can introduce local electronic asymmetry, enhancing the adsorption capacity for ORR intermediates; graphitic nitrogen can improve the conductivity of carbon materials. The N1s spectrum of PC-1100 displays a relatively rich variety of nitrogen doping types and a high nitrogen content, which is related to the introduction of a melamine nitrogen source during its preparation and appropriate high-temperature denitrification treatment. Appropriate nitrogen doping not only optimizes the electronic structure of the carbon material and improves its conductivity, but also forms abundant defect sites after high-temperature denitrification, such as non-hexagonal carbon ring structures such as pentagons, heptagons, and octagons. These defect sites destroy the symmetrical electron cloud distribution of the carbon material, causing the local electron density to change, thereby significantly enhancing the adsorption capacity of ORR intermediates (*O2, *OOH, *O and *OH) and improving the catalytic activity.

[0068] Table 1 shows the physical properties of the prepared samples. PC-900 has the highest specific surface area (211.567 m 2 ·g -1 ), the maximum total adsorption pore volume (0.198 cm 3 g -1 ) and micropore advantage (average pore size 3.751nm), and the loose honeycomb structure observed by SEM ( Figure 1 a) is consistent. When the temperature rises to 1100℃ (PC-1100), the specific surface area decreases significantly to 78.401m 2·g -1 , the total pore volume decreased to 0.112 cm 3 g -1 , but the average pore size increased to 5.707 nm, and the mesopore ratio increased (BET analysis), indicating that the decomposition of melamine induced pore reconstruction Figure 1 b, c). Further heating to 1300 °C (PC-1300), the specific surface area continued to decrease (70.680 m 2 ·g -1 ), and the total pore volume slightly increased (0.140 cm 3 g -1 ), but the average pore size further expanded to 7.946 nm. Combined with the sintering of pore walls observed in SEM, it was confirmed that high temperature led to the collapse and merging of part of the mesopores into macropores. This result was echoed by the decrease in the half-height width of the (002) peak in the XRD of PC-1300, indicating that graphitization was enhanced at high temperature but the defect density was reduced.

[0069] Table 1. Physical properties of the prepared samples

[0070]

[0071]

[0072] Elemental content analysis of the prepared samples shown in Table 2 indicated that with the increase of carbonization temperature, the nitrogen content decreased sharply from 13.60 wt% of PC-900 to 1.33 wt% of PC-1300, while the carbon content increased from 75.20 wt% to 88.53 wt%. This trend was consistent with the weakening of the N 1s peak intensity in the XPS survey Figure 2 a), verifying that the gradual loss of nitrogen atoms resulted from high-temperature denitrification. In addition, the oxygen content slightly increased in PC-1100 (11.72 wt% vs. PC-900: 11.20 wt%), which might be attributed to the oxidation of surface oxygen-containing functional groups or the adsorption of residual melamine decomposition products.

[0073] Table 2. Elemental content analysis of the prepared samples

[0074]

[0075] The effects of high-temperature denitrification treatment on the electrocatalytic ORR performance of the goji-derivative porous carbon (PC) were systematically evaluated by cyclic voltammetry (CV), linear sweep voltammetry (LSV), and stability tests. Cyclic voltammetry (CV) was used to study the electrochemical behavior of electrocatalysts, including their activity, reversibility, and reaction mechanism. In an O2-saturated 0.1 M KOH electrolyte, the CV curve of PC-1100 Figure 3a) shows a significant oxygen reduction peak around 0.83 V (vs. RHE), which disappears under Ar-saturated conditions, indicating its clear ORR catalytic activity. Similarly, PC-900( Figure 3 a) and PC-1300( Figure 3 c) also show more significant oxygen reduction peaks under O2-saturated conditions than under Ar-saturated conditions, indicating their catalytic effects on ORR.

[0076] Further, Figure 4 As can be seen from a to c, with the increase of the scanning speed, the CV curve shape of each sample remains stable, and the peak current is linearly related to the scanning rate; Figure 4 d shows the double-layer capacitance (C dl ) values of PC-900, PC-1100 and PC-1300, which are 17.31 mF cm -2 , 9.42 mF cm -2 and 2.06 mF cm -2 , respectively. Among them, PC-900 has the optimal double-layer capacitance value, indicating that it has more active sites, larger active surface area and better catalytic performance, while PC-1100 is second, which corresponds to the results shown in Table 1.

[0077] Further LSV tests found that the onset potential (E onset = 0.977 V) of PC-1100 is slightly lower than that of PC-900 (E onset = 0.999 V) but much higher than that of PC-1300 (E onset = 0.807 V), and the half-wave potential (E 1 / 2 = 0.854 V) of PC-1100 is significantly higher than that of PC-900 (E 1 / 2 = 0.814 V) and PC-1300 (E 1 / 2 = 0.634 V), and is close to that of commercial Pt / C (E 1 / 2 = 0.85 V) ( Figure 5 a, b). This performance improvement can be attributed to the synergistic effect of defect structures (such as pentagonal / heptagonal topological defects) induced by high-temperature denitrification and high specific surface area, which significantly enhances the adsorption capacity of oxygen intermediates (*O2, *OOH).

[0078] To verify the long-term stability of the catalyst, chronoamperometric tests ( Figure 6a) PC-1100 showed 96.88% of the initial current density after 15 h continuous running, indicating excellent stability and practical application potential, much higher than PC-900 (59.38%) and PC-1300 (66.37%). PC-1100 was subjected to 500 cycles of accelerated degradation test (ADT) in the range of 0.56-1.16 V (vs. RHE). After aging, its E 1 / 2 only negative shift of 14 mV Figure 6 b).

[0079] The above-described embodiments are merely preferred modes of the present application and are not intended to limit the scope of the present application. Various modifications and improvements to the technical solutions of the present application made by those of ordinary skill in the art without departing from the design spirit of the present application shall fall within the scope of protection of the present application as defined by the claims.

Claims

1. A method for preparing a wolfberry-derived porous carbon catalyst, characterized in that: The following steps are involved: The wolfberry residue and melamine are mixed and heat-treated to prepare the wolfberry-derived porous carbon catalyst.

2. The preparation method according to claim 1, characterized in that The heat treatment temperature is 800-1300° C., and / or the heat treatment time is 2 hours.

3. The preparation method according to claim 1, characterized in that The heat treatment is carried out in a protective atmosphere.

4. The preparation method according to claim 1, characterized in that The mass ratio of the wolfberry residue to melamine is (0.5-8):

2.

5. The preparation method according to claim 1, characterized in that The wolfberry residue is obtained by soaking and softening wolfberries, removing seeds and then freeze-drying.

6. The wolfberry-derived porous carbon catalyst prepared by the preparation method according to any one of claims 1 to 5.

7. Use of the wolfberry-derived porous carbon catalyst according to claim 6 in an oxygen reduction reaction.

8. Use of the wolfberry-derived porous carbon catalyst as claimed in claim 6 as an electrode catalyst for a proton exchange membrane fuel cell.

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