Carbon nanofiber electrode material with high specific surface area and preparation method thereof

Abstract

The invention discloses a high-specific-surface-area carbon nanofiber electrode material and a preparation method thereof, and belongs to the technical field of carbon nanofiber electrode materials. The method comprises the following steps: uniformly mixing nanocellulose and mixed molten salt, placing the mixture in a tubular furnace for one-pot carbonization, and then performing centrifugation-washing and drying to obtain the carbon nanofiber electrode material with the high specific surface area. And by utilizing the barrier and mild medium effects of the mixed molten salt, the bonding strength between the fibers is effectively reduced, a mild carbonization environment is provided, and the carbon nanofiber electrode material with the high specific surface area is obtained. The preparation method is rapid, economical, simple and convenient, the complexity of the existing cellulose-based carbon nanofiber preparation technology is overcome, the material with excellent specific surface area and electrochemical performance is prepared, and the application field and application prospect of the cellulose material are widened.

Description

Technical Field

[0001] The present invention belongs to the technical field of carbon nanofiber electrode materials, and particularly relates to a high specific surface area carbon nanofiber electrode material and a preparation method thereof. Background Art

[0002] Carbon nanofibers (CNFs) have become ideal electrode materials due to their excellent chemical stability, high porosity, and developed graphite structure, showing broad application prospects in fields such as supercapacitors and lithium-ion batteries. However, there are significant bottlenecks in the preparation process of traditional CNFs. It is necessary to go through multiple complex processes such as spinning, thermal stabilization, carbonization, and activation in sequence. The process is not only cumbersome but also consumes a large amount of energy, resulting in high production costs. In contrast, nanocellulose fibers, with their ultra-fine diameter (5 - 50 nm) and natural fiber structure advantages, can eliminate the spinning step, providing a new idea for simplifying the preparation process. However, directly carbonizing nanocellulose poses severe challenges: during the high-temperature pyrolysis process, cellulose fibers will undergo intense thermoplastic melting and agglomeration adhesion phenomena, and the original nanofiber morphology will be damaged, making it impossible to obtain CNFs with an ideal structure. This key problem severely restricts the development of high-performance carbon nanofiber electrode materials prepared using nanocellulose as a precursor, and there is an urgent need to develop new technical means to solve core problems such as fiber morphology control and pyrolysis stability.

[0003] Porous carbon nanofibers, a preparation method thereof, and a lithium-sulfur battery (CN 110158200A) disclose a method of mixing a pore-forming agent, a surface dispersant, a carbon source, and an organic solvent to obtain a spinning solution, and then obtaining carbon fibers through electrospinning. The method of carbonizing the carbon fibers, removing the pore-forming agent, washing, and drying to obtain carbon nanofibers, the specific surface area of the carbon nanofibers is about 150 - 350 m 2 / g. A preparation method of lignin-based carbon nanofibers (CN 104947247A) discloses an electrospinning process, using a copolymer of lignin and acrylonitrile as a spinning solution to prepare fibers, and then making carbon fibers through thermal stabilization treatment and carbonization treatment. Since acrylonitrile chain segments are grafted onto lignin molecules, chemical bonds are formed between lignin and acrylonitrile chain segments, and the good thermal stability of acrylonitrile chain segments is utilized to improve the melt resistance of the fibers during the thermal stabilization treatment process. A carbon nanofiber based on silk fibroin fiber, a preparation method thereof, and an application (CN 119465447A) disclose adding Ti3C2T x MXene to a silk fibroin nanofiber solution, stirring under argon gas and ice bath, freeze-drying the obtained mixed solution to obtain silk fibroin nanofibers, and finally performing carbonization treatment to obtain carbon nanofibers.

[0004] The above research shows that the current methods for preparing carbon nanofibers mostly involve first preparing nanofibers by electrospinning and then obtaining carbon nanofibers through carbonization. Summary of the Invention

[0005] One technical problem solved by the present invention is to provide a method for preparing a high specific surface area carbon nanofiber electrode material. By utilizing the barrier and mild medium effects of the mixed molten salt, the bonding strength between fibers is effectively reduced, and a mild carbonization environment is provided, thereby preparing a high specific surface area carbon nanofiber (CNFs) electrode material. Another technical problem to be solved by the present invention is to provide a high specific surface area carbon nanofiber electrode material with a specific surface area (SSA) as high as 899 m 2 / g.

[0006] Technical Solution: To solve the above technical problems, the technical solution adopted by the present invention is as follows:

[0007] A method for preparing a high specific surface area carbon nanofiber electrode material, which comprises mixing nanocellulose and a mixed molten salt evenly, then subjecting them to one-pot carbonization in a tube furnace, and then performing centrifugation - washing and drying to obtain the high specific surface area carbon nanofiber electrode material; using the mixed molten salt as a barrier and a mild medium to reduce the thermoplastic bonding process during the carbonization of the fibers, thereby obtaining high specific surface area carbon nanofibers.

[0008] In the above method, the mixed molten salt is a mixture of a low melting point molten salt and a high melting point molten salt, and the mass ratio of nanocellulose, the low melting point molten salt, and the high melting point molten salt is 1:1:10 - 1:10:40; the melting point of the high melting point molten salt differs from that of the low melting point molten salt by 300 - 500 °C; the molten salt component with a lower melting point plays the role of providing a mild carbonization environment, and the molten salt component with a higher melting point plays a barrier role to prevent thermoplastic bonding during the carbonization of the fibers.

[0009] In the above method, the mixed molten salt is a mixture of NaOH and NaCl, and the mass ratio of nanocellulose, NaOH, and NaCl is 1:5:30.

[0010] In the above method, the solid content of the nanocellulose is 2 wt% - 6 wt%, the diameter is 1 - 15 nm, and the length is 1 - 5 μm; preferably, the solid content is 2.5 wt%, the diameter is 10 nm, and the length is 2 μm.

[0011] In the above method, the carbonization temperature of the mixture in the tube furnace is 800 - 1000 °C, the carbonization time is 2 - 4 h, and the heating rate is 10 - 15 °C / min; preferably, the carbonization temperature is 900 °C, the carbonization time is 2 h, and the heating rate is 10 °C / min.

[0012] The method comprises the following steps: the number of centrifugation-washing after carbonization is 3-6 times; the drying method is oven normal pressure drying, the drying time is 6-12 hours, and the drying temperature is 65-100° C.; preferably, the number of centrifugation-washing is 5 times; and the drying is performed at 85° C. for 12 hours.

[0013] The method comprises the following steps:

[0014] (1) slowly heating the nanocellulose at 80° C. and drying it to a solid content of 10 wt %;

[0015] (2) grinding the mixed molten salt into fine powder, passing through a 100-mesh sieve, and mixing evenly with the nanocellulose;

[0016] (3) transferring the mixture into a ceramic crucible, placing it in a tube furnace, heating it, and keeping it warm in a nitrogen atmosphere;

[0017] (4) After the heat preservation is completed, the mixture is naturally cooled to room temperature, washed with deionized water, centrifuged, and dried to obtain a carbon nanofiber electrode material with a high specific surface area.

[0018] The carbon nanofiber electrode material with high specific surface area obtained by the method.

[0019] Beneficial effects: Compared with the prior art, the present invention has the following advantages:

[0020] (1) The present invention utilizes the barrier and mild medium effects of mixed molten salt to prepare carbon nanofibers in one step, and reduces the degree of adhesion during fiber pyrolysis by salt barrier carbonization, thereby achieving the preparation of carbon nanofibers with high specific surface area of ​​all components. The prepared CNFs have excellent electrochemical properties and can be used as electrode materials.

[0021] (2) This method has simple steps, is green, efficient, and sustainable. The raw materials are easily available, the equipment cost is low, the operation is simple, and the time consumption is short. It overcomes the problems of high brittleness and low specific surface area caused by thermoplastic adhesion in the existing CNFs preparation process, opens up a new idea for the preparation of carbon CNFs, broadens the application field and application prospects of cellulose materials, and is very suitable for the promotion of industrial production.

[0022] (3) The carbon nanofibers prepared by the present invention use the mixed molten salt component with a lower melting point as a mild medium to reduce the overall melting point of the molten salt, provide a mild liquid carbonization environment for the fiber, and weaken the damage to the fiber caused by pyrolysis. The component with a higher melting point acts as a barrier, penetrates between the fibers to form a barrier, prevents the adhesion and agglomeration between the fibers during the carbonization process, and keeps the fibers in a complete shape. At the same time, the molten salt can also serve as an activator to increase the specific surface area of ​​the carbon material. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1Microscopic morphology and average pore size diagrams of carbon nanofibers prepared in Example 1 and Comparative Example 1;

[0024] Figure 2 Transmission electron microscope (TEM) and EDS energy spectrum diagrams of carbon nanofibers prepared in Example 1;

[0025] Figure 3 Specific surface area adsorption - desorption curve and pore size distribution diagram of carbon nanofibers prepared in Example 1;

[0026] Figure 4 Electrochemical performance data diagram of carbon nanofibers prepared in Example 1;

[0027] Figure 5 Process flow chart of the preparation method of the present invention. Detailed implementation manners

[0028] The following further clarifies the present invention in conjunction with specific embodiments. The embodiments are implemented on the premise of the technical solution of the present invention. It should be understood that these embodiments are only used to illustrate the present invention and not to limit the scope of the present invention.

[0029] Example 1

[0030] A method for preparing a high specific surface area carbon nanofiber electrode material by salt - barrier carbonization, the process flow chart is as Figure 5 shown, and it includes the following steps:

[0031] (1) Slowly heat NFC with a solid content of 2.5 wt%, a diameter of 10 nm, and a length of 2 μm on an electric furnace at 80 °C until the solid content is 10 wt%;

[0032] (2) Grind NaOH and NaCl into fine powders, pass through a 100 - mesh sieve, and mix them with NFC, and mix evenly according to the mass ratio of 1:5:30 (NFC:NaOH:NaCl);

[0033] (3) Transfer the mixture to a ceramic crucible, place it in a tube furnace, heat it to 900 °C at a rate of 10 °C / min, and keep it warm for 2 h in a nitrogen atmosphere;

[0034] (4) After natural cooling to room temperature, wash the sample with deionized water, centrifuge it 5 times, and then dry it at 85 °C for 12 h. Finally, carbon nanofibers (N - CNFs) are obtained.

[0035] Figure 2 Transmission electron microscope (TEM) and EDS energy spectrum diagrams of carbon nanofibers prepared in Example 1, Figure 2(a, b, c) indicates that the diameter of the carbon nanofibers prepared by the present invention is between 5 - 15 nm, and shows branched single carbon nanofibers, which confirms the feasibility of the molten salt carbonization method. The molten salt penetrates between the fine fibers, causing a mild pyrolysis reaction of the fibers and successfully preventing the agglomeration between the fibers. From Figure 2 In the EDS spectrum of (d, e, f), it can be observed that the distribution of carbon and oxygen elements in the carbon nanofibers is relatively uniform, where Figure 2 d is the surface of the scanned sample, Figure 2 e is the distribution of carbon elements in the sample, Figure 2 f is the distribution of oxygen elements in the sample.

[0036] Figure 3 Figure (a) shows the specific surface area adsorption - desorption curve and figure (b) shows the pore size distribution diagram of the carbon nanofibers prepared in Example 1. The curve shows a combination of type I and type IV isotherms, that is, it rises sharply at low relative pressure (P / P0 < 0.2) and shows a hysteresis loop at medium relative pressure (0.4 < P / P0 < 0.9), indicating the presence of abundant micropores and high - density mesopores in N - CNFs.

[0037] The specific surface area (SSA) of N - CNFs was calculated to be as high as 899 m 2 / g. The specific surface area is much higher than that of cellulose - based CNFs obtained by carbonizing carbon fibers prepared by electrospinning (the specific surface area is about 150 - 350 m 2 / g), indicating that N - CNFs have an extremely developed pore structure.

[0038] In addition to the pores caused by the pyrolysis behavior of the fibers, more are attributed to the activation effect of NaOH during the carbonization process. The presence of NaOH not only reduces the overall melting point of the molten salt, providing a mild liquid environment for the pyrolysis of the fibers. At the same time, NaOH also plays an activation role, greatly increasing the specific surface area of N - CNFs. Sodium chloride plays a blocking role. Cellulose undergoes a pyrolysis - thermoplastic process before carbonization, and this temperature is lower than the melting temperature of sodium chloride, so it can play a role in blocking and preventing fiber adhesion.

[0039] The pore size distribution diagram shows that the interior of N - CNFs is composed of micropores and mesopores. Because charges mainly accumulate in the micropores and the conversion of electrolyte ions mainly occurs in the mesopores. Therefore, the porous structure of N - CNFs has great application prospects in electrode materials.

[0040] Figure 4 Figure (a, b) shows the electrochemical performance data of the carbon nanofibers prepared in Example 1. The electrochemical behavior of N - CNFs was tested using a symmetric two - electrode system in 0.5 mol / L phosphoric acid solution.

[0041] At a high scan rate of 200 mV s -1 , the curve shape is close to a rectangle, showing excellent charge response speed and low equivalent series resistance. The potential range of the GCD test of the N-CNFs electrode is determined by the CV curve. The GCD curve at a current density of 1-20 A g -1 is a highly symmetric isosceles triangle, indicating that the N-CNFs electrode has good electrochemical reversibility. According to the GCD curve, the specific capacitance of the N-CNFs electrode is 150 Fg -1 at a current density of 1 A g -1 .

[0042] Figure 4 c shows that the Nyquist impedance curve of the N-CNFs electrode consists of a semicircle at high frequencies and a straight line at low frequencies, indicating the performance of the double-layer capacitance. Therefore, when used as a supercapacitor electrode material, N-CNFs has a high electrode conductivity and a good electron transfer rate between the electrolyte and the electrode interface.

[0043] At the same time, the electrode cycle stability of the carbon nanofibers prepared in Example 1 was detected, and the results are as shown in Figure 4 d. At a current density of 1 A g -1 , the specific capacitance retention rate after 10,000 cycles reaches 96%, proving that N-CNFs has great advantages as a supercapacitor electrode material.

[0044] Comparative Example 1

[0045] The difference from the aforementioned Example 1 is that the diameter of the selected NFC is increased to 20 nm, and the remaining steps remain unchanged. The average diameter of the obtained carbon nanofibers is higher than that of Example 1.

[0046] Figure 1 are the microscopic morphologies and average pore size diagrams of the carbon nanofibers prepared in Example 1 (a) and Comparative Example 1 (b). The average diameter of the fibers in Example 1 is about 5.38 nm, and the average diameter of the fibers obtained in Comparative Example 1 is about 8.43 nm.

[0047] In the molten salt carbonization system, as water evaporates, a large amount of molten salt crystallizes between the fibers, isolating the contact between the fibers and preventing the formation of hydrogen bonds. As carbonization proceeds, pyrolysis reactions occur and the hydroxyl groups are destroyed. The molten salt permeating between the fibers acts as a barrier, preventing the adhesion between the fibers and providing a mild liquid environment for the pyrolysis of the fibers, protecting the fiber structure, and thus obtaining complete carbon nanofibers.

[0048] The carbon nanofibers prepared by the present invention use the component with a lower melting point in the mixed molten salt as a mild medium to reduce the overall melting point of the molten salt, provide a mild liquid carbonization environment for the fibers, and weaken the damage of pyrolysis to the fibers. The component with a higher melting point acts as a barrier, penetrates between the fibers to form a barrier, prevents the adhesion and agglomeration of the fibers during the carbonization process, and keeps the fibers in a complete form. At the same time, the molten salt can also be used as an activator to increase the specific surface area of the carbon material.

[0049] The above are only the preferred embodiments of the present invention. It should be noted that for those of ordinary skill in the art, without departing from the principle of the present invention, several improvements and refinements can be made, and these improvements and refinements should also be regarded as the protection scope of the present invention.

Claims

1. A preparation method of a carbon nanofiber electrode material with high specific surface area, characterized in that After uniformly mixing nanocellulose and a mixed molten salt, it is placed in a tube furnace for one-pot carbonization, and then centrifuged, washed, and dried to obtain a high specific surface area carbon nanofiber electrode material.

2. The preparation method of the high specific surface area carbon nanofiber electrode material according to claim 1, characterized in that, The mixed molten salt is a mixture of a low melting point molten salt and a high melting point molten salt, and the mass ratio of nanocellulose, low melting point molten salt, and high melting point molten salt is 1:1:10 - 1:10:40; the melting point of the high melting point molten salt differs from that of the low melting point molten salt by 300 - 500 °C.

3. The preparation method of the high specific surface area carbon nanofiber electrode material according to claim 1, wherein, The mixed molten salt is a mixture of NaOH and NaCl, and the mass ratio of nanocellulose, NaOH, and NaCl is 1:5:

30.

4. The preparation method of the high specific surface area carbon nanofiber electrode material according to claim 1, characterized in that, The solid content of nanocellulose is 2 wt% - 6 wt%, the diameter is 1 - 15 nm, and the length is 1 - 5 μm.

5. The preparation method of the high specific surface area carbon nanofiber electrode material according to claim 1, wherein, The carbonization temperature of the mixture in the tube furnace is 800 - 1000 °C, the carbonization time is 2 - 4 h, and the heating rate is 10 - 15 °C / min.

6. The preparation method of the high specific surface area carbon nanofiber electrode material according to claim 1, characterized in that, The number of centrifugation-washing times after carbonization is 3 - 6 times; the drying method is oven drying at normal pressure, the drying time is 6 - 12 h, and the drying temperature is 65 - 100 °C.

7. The preparation method of the high specific surface area carbon nanofiber electrode material according to claim 1, characterized in that, It includes the following steps: (1) Under the condition of 80 °C, slowly heat the nanocellulose and dry it to a solid content of 10 wt%; (2) Grind the mixed molten salt into fine powder, pass through a 100-mesh sieve, and mix it uniformly with the nanocellulose; (3) Transfer the mixture to a ceramic crucible, place it in a tube furnace, heat it, and keep it warm in a nitrogen atmosphere; (4) After the heat preservation ends, naturally cool it to room temperature, wash it with deionized water, centrifuge it, and then dry it to obtain a high specific surface area carbon nanofiber electrode material.

8. The high specific surface area carbon nanofiber electrode material prepared by the method according to any one of claims 1 - 7.

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