A method for enhancing the surface conductivity of biochar, a composite material of biochar loaded with transition metals, and applications thereof

Abstract

The present invention relates to the technical field of surface modification of biochar, and discloses a method for enhancing the surface conductivity of biochar, a composite material loaded with transition metals thereof, and applications. Aiming at the problem of poor conductivity of biochar, in the present invention, lignocellulose or cellulose is impregnated in a solvent containing transition metal ions and nitrogen / oxygen / phosphorus heteroatoms, and a metal-heteroatom-cellulose complex precursor is formed through a hydrothermal reaction, and then through high-temperature calcination and pickling treatment, a composite material with a high-conductivity carbon layer and a stable transition metal nanoflower structure is prepared. This method utilizes the strong complexation of transition metals with cellulose and the solvent to form a metal-heteroatom-doped carbon layer on the surface of biochar, which has excellent interfacial electron transfer ability (R ct = 1.671 Ω). The obtained material has both a high specific surface area, a porous structure, excellent oxygen electrocatalytic activity, and acid corrosion resistance stability, and can replace noble metal catalysts and be applied to energy devices such as fuel cells.

Description

Technical Field

[0001] The present invention relates to the technical field of surface modification of cellulose biochar, and specifically to a method for enhancing the surface conductivity of biochar, a composite material of biochar loaded with transition metals, and applications thereof. Background Art

[0002] Developing efficient and sustainable energy conversion technologies is crucial for addressing global energy challenges. Among these technologies, oxygen electrocatalysis plays a key role in fuel cells and metal-air batteries. However, the performance of oxygen electrocatalysts is usually limited by their conductivity and stability.

[0003] Biochar is a material obtained by pyrolyzing biomass and has become a promising material due to its high specific surface area, porosity, and environmental friendliness. Despite these advantages, the intrinsic conductivity of biochar is usually low, which limits its application in electrocatalysis. Therefore, enhancing the conductivity of biochar is a key research focus for oxygen electrocatalysts.

[0004] In recent years, various strategies have been explored to improve the conductivity of biochar, including doping heteroatoms and introducing metal nanoparticles. These modifications not only enhance the conductivity but also introduce active sites that can improve the catalytic performance.

[0005] As an innovative method to enhance the conductivity of biochar and evaluate its application as an oxygen electrocatalyst. By systematically comparing the modified biochar with existing catalysts, its potential advantages in terms of activity and stability are demonstrated. Summary of the Invention

[0006] Aiming at the problem of poor conductivity of biochar, based on cellulose, the present invention prepares a conductive composite material that can maintain the original structure of cellulose by constructing a highly conductive carbon layer on its surface, and prepares an oxygen electrocatalyst with low cost, high activity, and high stability.

[0007] To achieve the above object, in the first aspect of the present invention, a method for enhancing the interfacial conductivity of biochar is provided, and the method includes the following steps:

[0008] (1) Completely dissolve transition metal ions in a solvent containing nitrogen, oxygen, and phosphorus heteroatoms, and immerse lignocellulose or cellulose in the solvent containing transition metal ions to obtain a uniform dispersion;

[0009] (2) Transfer the dispersion to a hydrothermal reaction kettle for hydrothermal reaction. During the hydrothermal process, the transition metal ions act as a bridge and will undergo a complexation reaction with the hydroxyl groups on the surface of cellulose and the heteroatom-containing functional groups in the solvent. After the reaction, centrifuge, wash, and dry to obtain a precursor with a carbon layer loaded with heteroatoms and transition metal ions on the surface;

[0010] (3) Place the precursor in a tube furnace and calcine it at a high temperature under the protection of nitrogen or inert gas to obtain a calcined product.

[0011] (4) Place the calcined product in an acid solution for stirring treatment. After pickling, centrifuge, wash, and dry to obtain a cellulose biochar-based transition metal composite material.

[0012] In step (1), the lignocellulose or cellulose is derived from at least one of soybean straw, peanut straw, corn straw, cotton straw, etc. or cellulose extracted therefrom; the lignocellulose or cellulose passes through a 60-mesh sieve.

[0013] The solid-liquid ratio of the lignocellulose / cellulose to the solvent containing nitrogen, oxygen, and phosphorus heteroatoms is 1:3 - 1:60 g / mL.

[0014] The metal ions include but are not limited to Fe 3+ , Zn 2+ , Mo 5+ , Ni 2+ , Co 3+ or several of the above transition metal ions. Compared with other metals (such as main group metals Na⁺, Ca²⁺, Al³⁺), transition metals have unfilled d orbitals, variable oxidation states, and coordination diversity, and are more likely to undergo complexation with solvents containing heteroatoms (such as nitrogen, oxygen, phosphorus, etc.), connecting cellulose with the heteroatom carbon layer formed by the solvent during the hydrothermal process.

[0015] The solvent containing nitrogen, oxygen, and phosphorus heteroatoms is selected from at least one of solvents such as formamide, citric acid aqueous solution, phytic acid, etc., as a heteroatom source.

[0016] In step (2), the treatment device for the hydrothermal / solvothermal reaction is a polytetrafluoroethylene reaction kettle.

[0017] The temperature of the hydrothermal reaction is 120 - 200 °C, and the time of the hydrothermal reaction is 6 - 24 h.

[0018] The number of times of washing with deionized water is 3 times.

[0019] The drying conditions are freeze-drying or drying at 105 °C.

[0020] In step (3), the inert gas includes one or a mixture of nitrogen, argon, and helium; the flow rate of the nitrogen or inert gas is 100 - 500 mL / min.

[0021] The heating rate is 1 - 10 °C / min.

[0022] The temperature for heat treatment controlled in the present invention is 900 - 1000 °C. A high calcination temperature is conducive to the stable anchoring of metal particles, and the time for heat treatment is controlled to be 1 - 3 h.

[0023] In step (4), the acid solution is an aqueous solution of inorganic acid or organic acid, preferably a sulfuric acid, hydrochloric acid or nitric acid solution with a concentration of 0.5 - 1 M.

[0024] The time for pickling treatment is 24 hours; the temperature for pickling treatment is from room temperature to 80 °C, preferably room temperature.

[0025] After pickling, it is washed with deionized water until neutral, and the drying conditions are freeze-drying or drying at 105 °C.

[0026] The second aspect of the present invention provides a composite material of cellulose biochar supported with transition metals prepared by the above method.

[0027] The third aspect of the present invention provides the application of the above composite material of cellulose biochar supported with transition metals in electrocatalysis, especially as an oxygen electrocatalyst.

[0028] The present invention directly uses lignocellulose or cellulose as raw materials, and prepares a transition metal composite material with a cellulose carbon framework (CF) through two steps of solvothermal treatment and pyrolysis;

[0029] The catalyst of the present invention provides a high specific surface area and a good porous carbon framework structure by cellulose carbon fiber (CF), and a heteroatom-doped carbon layer is loaded on its surface to enhance the conductivity of biochar, which can enhance the transfer of electrons at the interface and on the carbon carrier;

[0030] The preparation steps of the cellulose biochar-based composite material of the present invention are simple, and the raw materials have wide sources and low costs. Compared with traditional noble metal oxygen electrocatalysts, it has greater potential for large-scale production and has broad market prospects in the application fields of proton exchange membrane fuel cells, zinc-air batteries and other energy storage devices. Description of the Drawings

[0031] Figure 1 Specific surface area (a) and pore size distribution (b) of the catalyst precursor n-Fe-NCF prepared in Example 1.

[0032] Figure 2 X-ray photoelectron spectroscopy (XPS) of the catalyst precursor n-Fe-NCF prepared in Example 1.

[0033] Figure 3 Scanning electron microscope (SEM) image of the catalyst precursor n-Fe-NCF prepared in Example 1.

[0034] Figure 4 SEM image of the catalyst Fe-NCF prepared in Example 1.

[0035] Figure 5 Theoretical calculation verification diagram of the surface carbon layer formation mechanism of the catalyst Fe-NCF prepared in Example 1.

[0036] Figure 6 High-resolution scanning transmission electron microscope (TEM) image of the catalyst Fe-NCF prepared in Example 1.

[0037] Figure 7 Fourier transform infrared spectroscopy (FTIR) of the catalyst Fe-NCF prepared in Example 1.

[0038] Figure 8 Element content change of the catalyst Fe-NCF prepared in Example 1 before and after pickling.

[0039] Figure 9 XPS spectrum of the catalyst Fe-NCF prepared in Example 1.

[0040] Figure 10 X-ray diffraction pattern (XRD) of the catalyst Fe-NCF prepared in Example 1.

[0041] Figure 11 Raman spectrum (Roman) of the catalyst Fe-NCF prepared in Example 1.

[0042] Figure 12 Linear sweep voltammetry (LSV) curve of the oxygen reduction reaction (ORR) of the catalyst Fe-NCF prepared in Example 1 under acidic conditions.

[0043] Figure 13 Impedance curve of the catalyst Fe-NCF prepared in Example 1.

[0044] Figure 14 Current-time curve (i-t) of the catalyst Fe-NCF prepared in Example 1.

[0045] Figure 15 LSV curve in the ORR test of the catalyst Fe-NCF-1 prepared in Example 2.

[0046] Figure 16 Impedance test curve of the catalyst Fe-NCF-1 prepared in Example 2.

[0047] Figure 17 SEM image of the catalyst Mo-NCF prepared in Example 5.

[0048] Figure 18 SEM image of the catalyst Mo, Fe-NCF prepared in Example 6.

[0049] Figure 19 SEM image of the catalyst NCF prepared as Comparative Example 1.

[0050] Figure 20 SEM image of the catalyst @NCF prepared as Comparative Example 2.

[0051] Figure 21 SEM image of the catalyst CFC prepared as Comparative Example 3. Detailed implementation manners

[0052] Next, in combination with the embodiments of the present invention, the technical solutions in the embodiments of the present invention will be clearly and completely described. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative efforts shall fall within the protection scope of the present invention.

[0053] The present invention provides a technical solution: a method for enhancing the surface conductivity of biochar, a composite material of biochar loaded with transition metals, and applications. Example 1

[0054] A cellulose biochar-based transition metal composite material in Example 1 was prepared by the following method, and the steps are as follows:

[0055] Step (1): Completely dissolve ferric chloride and zinc chloride in formamide, and immerse 0.5 g of cellulose in 30 mL of this solution to obtain a uniform cellulose dispersion;

[0056] Step (2): Transfer the cellulose dispersion obtained in step (1) to a polytetrafluoro hydrothermal reaction kettle, control the temperature of the hydrothermal reaction to 180 °C, and the time of the hydrothermal reaction to 12 h. After the reaction is completed, perform centrifugation, then wash 3 times with deionized water, and freeze-dry at -48 °C to obtain a precursor;

[0057] Step (3): Place the precursor obtained in step (2) in a tubular furnace, under a nitrogen atmosphere of 200 mL / min, heat it to 900 °C at a heating rate of 5 °C / min and calcine for 1 h to obtain a black solid, which is the precursor of the biochar-based transition metal composite material, denoted as catalyst precursor n-Fe-NCF.

[0058] Figure 1 a shows that the specific surface area of this composite material is 510.27 m 2 / g, and the pore size distribution ( Figure 1 b) It can be seen that n-Fe-NCF has a porous structure. Through Figure 2It can be seen that elements C, N, O, and Fe exist on the surface of this material. The presence of C–N bonds in the C 1s spectrum and the N 1s spectrum both confirm the successful doping of nitrogen into the carbon matrix, and the Fe 2p spectrum proves the successful introduction of Fe. Figure 3 From the SEM image of Figure 3 , it can be seen that nanoflowers cover the entire surface of the cellulose, obscuring the fiber structure.

[0059] Step (4): The catalyst precursor n-Fe-NCF was further stirred in 0.5 M H2SO4 for 24 h, and then washed with deionized water to obtain Fe-NCF. After acid washing, the biochar-based transition metal composite catalyst Fe-NCF was obtained. This step corresponds to method step (4). By acid washing, unstable metal active sites were removed, and the transition metal nanoflower structure stably loaded on the cellulose interface was retained. This structure is stable and resistant to acid corrosion.

[0060] Figure 4 Figure 9 is the SEM image of Fe-NCF. It can be seen that after acid washing, the loosely bound nanoflowers were removed, and the nanoflowers that can be stably loaded are evenly distributed on the surface of Fe-NCF. From the cross-section and plan view of the cellulose, it can be seen that a thick carbon layer wraps around the fiber, and the nanoflowers grow from the carbon layer. Through quantum mechanics theory calculation ( Figure 5 ) it was verified that the formation of the carbon layer is due to the following reasons: Cellulose and formamide have abundant hydroxyl and carbonyl groups, which can provide sites for binding with iron ions ( Figure 5 a); During the hydrothermal process, the highest occupied molecular orbital (HOMO) energies of cellulose and formamide are relatively high, indicating that their electrons are more likely to transfer to iron ions during adsorption. Therefore, cellulose and formamide are more likely to coordinate with iron ions in this environment ( Figure 5 b); Further analysis of the formamide and cellulose system ( Figure 5 c), the binding energy of formamide-Fe is -233.8 kcal / mol, and the binding force of cellulose-Fe is -334.2 kcal / mol. This indicates that the structure formed by the binding of iron with cellulose and formamide can exist stably. Subsequent pyrolysis can induce the movement of metal particles and promote the formation of nanoflower active sites on the surface of the carbon layer. Figure 6 Figure 19 is the TEM image of Fe-NCF, showing that metal active sites are encapsulated within the carbon layer. Figure 7 In the Fourier transform infrared spectrum of Figure 7 , the intensity of the aromatic C-H peak gradually increases at 900 - 650 cm −1 −1, indicating the enhancement of the aromatic structure, proving that an ordered carbon layer tends to form on the surface of the biochar carrier. This, together with the theoretical calculation, SEM, and TEM results, proves that a carbon layer containing metals and heteroatoms has been formed on the surface of the cellulose-derived carbon. Figure 8The comparison of the element contents before and after pickling shows that approximately one-third of the iron elements on the surface are removed, indicating that the unstable active sites are washed away under acidic conditions. According to Figure 9 XPS, we can see that in Fe-NCF, the peak-fitting results of the C 1s, N 1s, and Fe 2p orbitals are consistent with those of n-Fe-NCF, indicating that pickling does not change the type of active sites but only washes away the unstable loaded active sites. From Figure 10 XRD, it can be seen that it can also prove that most of the unstable active metal components are washed away by pickling without changing the catalyst structure. From Figure 11 Raman spectroscopy proves that Fe-NCF retains the stably loaded active components.

[0061] The ORR performance test was carried out using a typical three-electrode system. The catalyst Fe-NCF was used as the working electrode, a carbon rod as the counter electrode, and a saturated calomel electrode as the reference electrode. The ORR performance test was carried out in a 0.5 M H2SO4 electrolyte. Before the test, O2 was introduced into the electrolyte to reach a saturated state to reduce the influence of oxygen consumption in the solution during the ORR process. The linear sweep voltammetry curve (LSV) was scanned at a rate of 10.0 mV s ‒1 , and the voltage range was 0 - 1.0 V vs. SCE.

[0062] From Figure 12 , it can be seen that Fe-NCF has a good half-wave potential (0.743 V vs. RHE), showing good catalytic activity and a half-wave potential comparable to that of commercial Pt / C. From Figure 13 The impedance test curve shows that Fe-NCF has a low interfacial impedance of only 1.671 Ω, proving that the catalyst interface has good electron transfer ability and enhanced interfacial conductivity of the catalyst.

[0063] In addition, from Figure 14 , it can be seen that the current of Fe-NCF hardly decays in nearly 100 h, showing excellent stability, and also proving that Fe-NCF can resist acid corrosion. Example 2

[0064] The difference between Example 2 and Example 1 is that: only the addition amount of cellulose in step (1) is adjusted to 1.0 g. 1.0 g of cellulose is impregnated in 30 mL of formamide solution dissolved with iron ions and zinc ions to obtain a uniform cellulose dispersion. The remaining steps (2), (3), and (4) of Example 2 are the same as those of Example 1, and the catalyst Fe-NCF-1 is obtained.

[0065] The ORR performance of Fe-NCF-1 in acidic electrolyte was tested according to the method of Example 1. The LSV curve of ORR is as shown in Figure 15 . It can be seen from the LSV curve that it has good onset potential (0.764 V) and half-wave potential (0.677 V). The performance decay is due to the increase in cellulose content, resulting in less carbon layer loaded on the cellulose surface, which proves that the surface carbon layer is the reason affecting the catalyst performance. The impedance test is as shown in Figure 16 . It shows an interfacial transfer impedance of 4.327 Ω. Although the decrease in the carbon layer on the cellulose surface leads to an increase in the interfacial impedance, it still shows a high charge transfer ability, which further proves that the carbon layer on the cellulose surface will significantly affect the conductive ability of the biochar interface. Example 3

[0066] The difference between Example 3 and Example 1 is that only the type of solvent in step (1) is adjusted to aqueous citric acid solution, and the remaining steps (2), (3), and (4) are the same as those in Example 1. Example 4

[0067] The difference between Example 4 and Example 1 is that only the type of solvent in step (1) is adjusted to phytic acid, and the remaining steps (2), (3), and (4) are the same as those in Example 1. Example 5

[0068] The difference between Example 5 and Example 1 is that only the type of metal ions in step (1) is adjusted. In Example 5, zinc chloride and molybdenum chloride are added, and the remaining steps (2), (3), and (4) are the same as those in Example 1, obtaining the catalyst Mo-NCF. It can be seen from the SEM image in Figure 17 that there are rough substances on the surface, which are the carbon layers formed by the metal and formamide. Example 6

[0069] The difference between Example 6 and Example 1 is that only the type of metal ions in step (1) is adjusted. In Example 6, zinc chloride, iron chloride, and molybdenum chloride are added, and the remaining steps (2), (3), and (4) are the same as those in Example 1, obtaining the catalyst Mo , Fe-NCF. It can be seen from the SEM image in Figure 18 that there is a carbon layer on the sample surface that is significantly different from the cellulose-derived carbon, and nanoparticles are evenly dispersed on it. Example 7

[0070] The difference between Example 7 and Example 1 is that the type of biomass in step (1) is adjusted to lignocellulose, and the remaining steps (2), (3), and (4) are the same as those in Example 1. Comparative Example 1

[0071] The difference between Comparative Example 1 and Example 1 is as follows: In step (1), metal ions are not added, and only 0.5 g of cellulose is uniformly dispersed in 30 mL of formamide solvent. The other steps (2), (3), and (4) are the same as those in Example 1, and the catalyst NCF is prepared; according to Figure 19 From the SEM image of Comparative Example 2

[0072] The difference between Comparative Example 2 and Example 1 is as follows: 0.5 g of the cellulose in step (1) of Example 1 is directly mixed with zinc chloride and pyrolyzed at 900 °C for 1 h to obtain @NCF. According to Figure 20 From the SEM image of Comparative Example 3

[0073] The difference between Comparative Example 3 and Example 1 is as follows: 0.5 g of the cellulose in step (1) of Example 1 is directly pyrolyzed at 900 °C for 1 h to obtain CFC. According to Figure 21 From the SEM image of

Claims

1. A method for enhancing the surface conductivity of biochar, characterized in that, The method comprises the following steps: (1) Completely dissolve transition metal ions in a solvent, and immerse lignocellulose or cellulose in the solvent containing transition metal ions to obtain a uniform dispersion; the solvent contains nitrogen and oxygen heteroatoms, and the solvent is formamide and / or acetamide; (2) Transfer the dispersion to a hydrothermal reaction kettle for solvothermal reaction. After the reaction is completed, centrifuge, wash, and dry to obtain a precursor with a carbon layer loaded with heteroatoms and transition metal ions on the surface; (3) Place the precursor in a tubular furnace, introduce an inert gas as a protective gas, and hold at a high temperature for a period of time to obtain a calcined product; (4) Place the calcined product in an acid solution for stirring treatment. After pickling is completed, centrifuge, wash, and dry to obtain a cellulose biochar-based transition metal composite.

2. The method for enhancing the surface conductivity of biochar according to claim 1, wherein: The transition metal ions are selected from one or more of Fe 3+ , Zn 2+ , Mo 5+ , Ni 2+ , Co 3+ .

3. A method for enhancing the surface conductivity of biochar according to claim 1, characterized in that: The lignocellulose or cellulose is derived from at least one of soybean straw, peanut straw, corn straw, and cotton straw or cellulose extracted therefrom.

4. A method for enhancing the surface conductivity of biochar according to claim 1, characterized in that: The solid-liquid ratio of the lignocellulose / cellulose to the solvent is 1:3 - 1:60 g / mL.

5. The method for enhancing the surface conductivity of biochar according to claim 1, wherein: The treatment device for the solvothermal reaction is a polytetrafluoroethylene reaction kettle, the temperature is controlled between 120 - 200 °C, and the time of the solvothermal reaction is controlled to be 6 - 24 h.

6. A method for enhancing the surface conductivity of biochar according to claim 1, characterized in that: The acid solution is an aqueous solution of inorganic acid or organic acid, and the concentration of the aqueous solution is 0.5 - 1 M sulfuric acid, hydrochloric acid, or nitric acid solution.

7. A method for enhancing the surface conductivity of biochar according to claim 1, characterized in that: The flow rate of the introduced inert gas is 100 - 500 mL / min; the calcination temperature is 700 - 1000 °C, the heating rate is 1 - 10 °C / min, the heat treatment time is controlled to be 1 - 3 h; the washing condition of the precursor is washing with deionized water not less than three times, and the drying condition is freeze-drying or drying at 105 °C.

8. A biochar-based transition metal composite prepared by the method according to any one of claims 1 - 7.

9. Application of the biochar-based transition metal composite according to claim 8 in the electrocatalytic ORR reaction.

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