Lignin-based Mn3O4 composite carbon nanofiber film, and preparation method and application thereof

Abstract

The invention provides a lignin-based Mn3O4 composite carbon nanofiber film and a preparation method and application thereof, and belongs to the technical field of carbon nanofiber materials, and the preparation method comprises the following steps: extracting lignin from rice straw; preparing a spinning solution containing lignin, polyacrylonitrile and manganese acetate; preparing the spinning solution into a composite fiber membrane through electrostatic spinning; and carrying out pre-oxidation and carbonization on the composite fiber film to obtain the lignin-based Mn3O4 composite carbon nanofiber film. Renewable resources of rice straw are used as raw materials, so that the production cost is reduced; the electrostatic spinning process is simple and efficient, and large-scale production can be realized; the lignin-based Mn3O4 composite carbon nanofiber thin film prepared by the method does not need to add a binder, has good flexibility and mechanical strength, can be directly used as an electrode, can be used for constructing a light, bendable and portable supercapacitor device, meets the development requirements of wearable electronic devices, and has wide application prospects. Wide application prospects and commercial values are realized in the field of energy storage.

Description

TECHNICAL FIELD

[0001] The application belongs to the technical field of nanometer carbon fiber materials, and more particularly relates to a lignin-based Mn3O4 composite nanometer carbon fiber film and a preparation method and application thereof. BACKGROUND

[0002] As an electrode material of double-layer capacitors, the electrode material surface reversibly adsorbs electrolyte ions in the electrolyte to realize energy storage. The electrode material has the advantages of fast charging and discharging speed, strong cycle stability and high power density, but the energy density is low compared with secondary batteries, which limits the application in electronic products and other equipment.

[0003] The current research direction for its improvement is to improve the specific capacity and energy density, while retaining the advantages such as cycle stability. The electrode material usually realizes energy storage reaction on the surface and inside. The nanometer carbon fiber material has a unique one-dimensional fiber structure and a network structure of interweaving each other, which can improve the ion channel and high specific surface area. At the same time, the nanometer carbon fiber structure is stable and will not react in acidic or alkaline electrolyte environment. The transition metal oxide is compounded with the nanometer carbon fiber, which not only produces double-layer capacitance, but also produces pseudo-capacitance by Faraday reaction on the surface or inside with the metal oxide, thereby improving the specific capacity.

[0004] MnO is a typical pseudo-capacitive transition metal oxide with a theoretical specific capacitance of up to 1370 Fg, which can provide abundant charge storage sites through reversible oxidation-reduction reactions of multi-valence states (Mn / Mn / Mn); at the same time, its spinel structure is stable, which is conducive to improving the cycle life and rate performance; compared with other metal oxides such as CoO and NiO, MnO is rich in resources, low in toxicity, low in cost, green and environmentally friendly, and suitable for large-scale application; in addition, MnO is easy to form a good interface interaction with carbon materials, which helps to improve the electron transport efficiency and material structure stability, and is an ideal choice for constructing high-performance supercapacitor electrode materials.

[0005] The current research mainly focuses on the compounding of MnO with traditional carbon materials such as graphene and carbon nanotubes, although certain progress has been made in improving the specific capacity, but there are still many technical defects: first, these carbon materials usually have two-dimensional or one-dimensional structures, which are difficult to effectively control the dispersion state of MnO, which is easy to cause its aggregation on the material surface, reducing the pseudo-capacitance reaction activity; second, most of the composite materials need to rely on adhesives to prepare electrodes, the preparation process is complex, the overall flexibility and integration are poor, and it is difficult to meet the development needs of flexible electronic devices; in addition, the synthesis process of graphene and carbon nanotubes is complex, the cost is high, and it depends on non-renewable petrochemical resources, which is not conducive to the construction and popularization and application of green and low-carbon material system. SUMMARY

[0006] The present application aims to provide a lignin-based Mn3O4 composite nanocarbon fiber film, a preparation method and applications, so as to solve at least one aspect of the prior art, the lignin-based Mn3O4 composite nanocarbon fiber film of the present application uses renewable lignin as a carbon source, constructs a three-dimensional cross-linked network nanocarbon fiber structure through electrospinning technology, and grows Mn3O4 nanoparticles on the surface of the carbon fiber in situ. The lignin-based Mn3O4 composite nanocarbon fiber film of the present application has good flexibility, self-supporting property, high electrical conductivity and high specific surface area, can effectively improve the pseudo-capacitance reaction efficiency, is suitable for flexible device applications, and has the advantages of green and low-cost preparation.

[0007] To achieve the above-mentioned purpose, the first aspect of the present application provides a preparation method of a lignin-based Mn3O4 composite nanocarbon fiber film, comprising the following steps: extracting lignin from rice straw; preparing a spinning solution containing the lignin, polyacrylonitrile and manganese acetate; preparing a composite fiber film from the spinning solution by electrospinning; obtaining a lignin-based Mn3O4 composite nanocarbon fiber film after pre-oxidation and carbonization of the composite fiber film.

[0008] Further, the spinning solution uses dimethylformamide as a solvent; the mass ratio of the polyacrylonitrile to the lignin in the spinning solution is 1:2, and the mass ratio of the lignin to the manganese acetate is 6:1-5.

[0009] Further, the spinning solution is prepared by the following method: dissolving the polyacrylonitrile and the lignin in dimethylformamide according to a mass ratio of 1:2 to prepare a mixed solution, and adding the manganese acetate to the mixed solution.

[0010] Further, the pre-oxidation condition is: heating to 250℃ at a heating rate of 0.5℃ / min in air and keeping for 1h. -1

[0011] Further, the carbonization condition is: heating to 800℃ at a heating rate of 2℃ / min in N2 atmosphere and keeping for 2h. -1

[0012] Further, the electrospinning condition is: a spinning voltage of 10.8kv; using a 5mL syringe; a receiving distance of 15cm; an air humidity of 40%; a spinning temperature of 25℃; and a feeding rate of 0.013mL / min.

[0013] Further, the lignin is extracted by the following method: ​​The crushed rice straw powder is mixed with an ethanol solution in a mass ratio of 1:12, and is reacted at 200 DEG C for 3 hours. Deionized water is added to the reacted black liquor, and the precipitate is separated and dried to obtain the lignin.

[0014] Further, the rice straw powder is obtained through a 100-mesh screen, and the volume fraction of the ethanol solution is 70%.

[0015] In a second aspect, the application provides a lignin-based Mn3O4 composite nanocarbon fiber film prepared by the method.

[0016] In a third aspect, the application provides the use of the lignin-based Mn3O4 composite nanocarbon fiber film in an electrode material of a supercapacitor.

[0017] Compared with the prior art, the application has the following technical effects: The lignin-based Mn3O4 composite nanocarbon fiber film is prepared by using rice straw as a renewable resource, thereby reducing production costs.

[0018] The lignin-based Mn3O4 composite nanocarbon fiber film prepared by the method has a specific capacity of 437.55 F g at a current density of 0.5 A g, and a capacity retention rate of 55.3% when the current density is increased from 0.5 A g to 10 A g. BRIEF DESCRIPTION OF DRAWINGS

[0019] In order to more clearly illustrate the technical solutions in the embodiments of the application, the following will briefly introduce the drawings needed to be used in the embodiments or the prior art description.

[0020] Figure 1 Preparation flow chart of the lignin / Mn3O4 composite nanofiber film with different Mn3O4 contents provided by the embodiment of the present application; Figure 2 Lignin-based Mn3O4 composite nanofiber film prepared by the embodiment 3 of the present application before carbonization (a), after carbonization (b), and before and after 180° folding (c)-(e) figure; Figure 3 SEM images (a)-(c) of the lignin-based Mn3O4 composite nanofiber film with different Mn3O4 contents provided by the embodiments 2-4 of the present application, and (d)-(f) are the corresponding diameter distribution cubic chart; Figure 4 EDS element distribution chart and energy spectrum chart of the CNFs / Mn3O4-3 sample provided by the embodiment 3 of the present application; Figure 5 XRD spectrum of the lignin-based Mn3O4 composite nanofiber film with different Mn3O4 contents provided by the embodiments 2-4 of the present application; Figure 6 XPS spectrum of the CNFs and CNFs / Mn3O4-3 composite material provided by the embodiments 1 and 3 of the present application: (a) XPS total spectrum of CNFs, (b) high-resolution C1s spectrum of CNFs, (c) XPS total spectrum of CNFs / Mn3O4-3, (d) high-resolution C1s spectrum of CNFs / Mn3O4-3, (e) high-resolution Mn2p spectrum of CNFs / Mn3O4-3; Figure 7 Raman spectrum of the CNFs and CNFs / Mn3O4 composite material provided by the embodiments 1-4 of the present application; Figure 8 CV curves of the lignin-based Mn3O4 composite nanofiber film with different Mn3O4 contents under different scan rates provided by the embodiments of the present application under three electrodes: (a) CNFs / Mn3O4-1, (b) CNFs / Mn3O4-3, (c) CNFs / Mn3O4-5, and (d) CV curves under 5 mV s-1 scan rate; Figure 9 GCD curves of the lignin-based Mn3O4 composite nanofiber film with different Mn3O4 contents under different current densities provided by the embodiments of the present application under three electrodes: (a) CNFs / Mn3O4-1, (b) CNFs / Mn3O4-3, (c) CNFs / Mn3O4-5, (d) GCD curves under 0.5 A g-1 current density, (e) rate performance chart, and (f) specific capacity under 0.5 A g-1 current density; -1 ​Figure 10 CV curves of the symmetric supercapacitor provided by the embodiment of the present application: (a) CNFs / Mn3O4-1, (b) CNFs / Mn3O4-3, (c) CNFs / Mn3O4-5; Figure 11 GCD curves of the symmetric supercapacitor provided by the embodiment of the present application: (a) CNFs / Mn3O4-1, (b) CNFs / Mn3O4-3, (c) CNFs / Mn3O4-5 and (d) 0.2-10 A g -1 Lower specific capacitance map; Figure 12 Ragone plot of the symmetric supercapacitor provided by the embodiment of the present application, (b) and (c) are 1000 / 10000 cycle performance plots of CNFs / Mn3O4-3 at 0.5 A g-1 / 5 A g -1 Current density, (d) is the EIS plot of CNFs / Mn3O4-3. DETAILED DESCRIPTION

[0021] In order to make the technical problems to be solved by the present application, the technical solutions and the beneficial effects clearer and more apparent, the present application will be further described in detail below in conjunction with the embodiments. It should be understood that the specific embodiments described herein are only used to explain the present application and not intended to limit the present application.

[0022] The terms used in the embodiments of the present application are only for the purpose of describing specific embodiments and are not intended to limit the present application. The singular forms "a", "said" and "the" used in the embodiments of the present application and the appended claims are also intended to include the plural forms, unless the context clearly indicates otherwise.

[0023] The embodiments of the present application provide a lignin-based Mn3O4 composite nanocarbon fiber film containing Mn3O4 prepared by using electrospinning technology, and the lignin-based Mn3O4 composite nanocarbon fiber film is applied to electrode materials of supercapacitors: first, the rice straw material is pretreated, such as crushing, impurity removal, etc., to obtain a cellulose solution containing abundant lignin. Then, manganese acetate is added to the obtained lignin solution, and a fibrous structure is formed under suitable process conditions by electrospinning technology, and pre-oxidation and carbonization are carried out in the subsequent process to obtain the lignin-based Mn3O4 composite nanocarbon fiber film. Finally, the prepared composite nanocarbon fiber film is applied to the preparation of supercapacitors as electrode materials to improve the performance of supercapacitors.

[0024] To achieve the above object, the embodiments of the present application adopt the following technical solutions: A preparation method of a lignin-based Mn3O4 composite nanocarbon fiber film prepared by electrospinning, comprising the following steps: (1) Preparation of manganese-doped lignin-based composite membranes by electrospinning; (2) calcining the composite film obtained in step (1) to obtain a lignin-based Mn3O4 composite nanocarbon fiber film.

[0025] Wherein, step (1) specifically includes the following steps: Lignin separation: The fine powder obtained by crushing rice straw and passing it through a 100-mesh sieve was mixed with 70% ethanol solution at a mass ratio of 1:12. The mixture was placed in a Teflon-lined stainless steel autoclave and reacted at 200°C for 3 hours.

[0026] Lignin extraction: The mixture after the reaction in the reactor was filtered, and 2 volumes of deionized water were added to the filtered black liquor, and the mixture was allowed to stand for precipitation. The precipitate was centrifuged and dried to obtain lignin.

[0027] Spinning solution preparation: Polyacrylonitrile (PAN) and lignin were dissolved in dimethylformamide (DMF) in a mass ratio of 1:2, and stirred at room temperature (20-30°C) for more than 6 hours to obtain a polymer mixed solution with a mass fraction of 21.8%. Lignin and manganese acetate were then added and stirred at room temperature for more than 12 hours to form an electrospinning solution.

[0028] Preparation of lignin / PAN / manganese acetate composite fiber membrane by electrospinning: The above spinning solution was electrospun to obtain a lignin / PAN / manganese acetate composite fiber membrane. The technical parameters of electrospinning are as follows: The spinning voltage was 10.8 kV; a 5 mL syringe was used; the receiving distance was 15 cm; the air humidity was 40%; the spinning temperature was 25°C; and the feed rate was 0.013 mL / min.

[0029] Step (2) specifically includes the following steps: Pre-oxidation: The dried lignin-based film was heated in air at 0.5 °C min -1 The temperature was raised to 250 °C at a heating rate and maintained for 1 h.

[0030] Carbonization: The pre-oxidized film was heated in a tube furnace under N2 atmosphere at 2°C min -1 The temperature was raised to 800 °C at a heating rate of 100 °C and maintained for 2 h to obtain a lignin-based Mn3O4 composite nanocarbon fiber film.

[0031] The present invention uses manganese acetate as a manganese source and rice straw as a carbon source, and successfully produces a lignin-based Mn3O4 composite nanocarbon fiber film containing Mn3O4 through electrospinning technology and high-temperature carbonization. The prepared lignin-based Mn3O4 composite nanocarbon fiber film is formed by stacking multiple layers of fibers, and its color is similar to that of lignin. The macroscopic morphology of the fiber film after carbonization does not change, and its size is uniformly reduced.

[0032] In some embodiments, the content of manganese acetate was adjusted to explore the effect of the content of Mn3O4 in nanocarbon fibers and the structural morphology of nanocarbon fibers on their performance in supercapacitors. When the mass ratio of lignin to manganese acetate was 6:3, the sample was at 0.5Ag -1 The specific capacity is highest at a current density of 0.2Ag. After the lignin-based Mn3O4 composite nano-carbon fiber film of the embodiment of the present invention is assembled into a supercapacitor, the sample -1 The specific capacity is the highest at the current density and has good stability.

[0033] The following uses a number of specific embodiments to illustrate a lignin-based Mn3O4 composite nano-carbon fiber film, a preparation method, and an application of the present invention.

[0034] Example 1 Lignin was extracted from rice straw. PAN and lignin were dissolved in DMF at a mass ratio of 1:2 and stirred for more than 20 hours at room temperature to form a uniform electrospinning solution as a control. The electrospinning process was carried out. The film peeled off from the aluminum foil was placed in air at 0.5℃min -1 The temperature was raised to 250 °C at a rate of 1 °C min-1 and maintained for 1 h, and then heated in a tube furnace at a rate of 2 °C min-1 under N2 atmosphere. -1 The temperature was raised to 800 °C at a heating rate of 100 °C and maintained for 2 h, and the obtained nanofiber membrane was named CNFs.

[0035] Example 2 Lignin was extracted from rice straw. PAN and lignin were dissolved in DMF at a mass ratio of 1:2 and stirred for 6 hours at room temperature to obtain a polymer mixed solution with a mass fraction of 21.8%. Lignin and manganese acetate were then added to control the total mass ratio of lignin to manganese acetate to be 6:1 to form a uniform electrospinning solution for the electrospinning process. The film peeled off from the aluminum foil was placed in air at 0.5°C min -1 The temperature was raised to 250 °C at a rate of 1 °C min-1 and maintained for 1 h, and then heated in a tube furnace at a rate of 2 °C min-1 under N2 atmosphere. -1 The temperature was raised to 800 °C at a heating rate of 100 °C and maintained for 2 h. The obtained nanofiber membrane was named CNFs / Mn3O4-1.

[0036] Example 3 Lignin was extracted from rice straw. PAN and lignin were dissolved in DMF at a mass ratio of 1:2 and stirred for 6 hours at room temperature to obtain a polymer mixed solution with a mass fraction of 21.8%. Lignin and manganese acetate were then added to control the total mass ratio of lignin to manganese acetate to be 6:3 to form a uniform electrospinning solution for the electrospinning process. The film peeled off from the aluminum foil was placed in air at 0.5°C min -1 The temperature was raised to 250 °C at a rate of 1 °C min-1 and maintained for 1 h, and then heated in a tube furnace at a rate of 2 °C min-1 under N2 atmosphere. -1 The temperature was raised to 800 °C at a heating rate of 100 °C and maintained for 2 h. The obtained nanofiber membrane was named CNFs / Mn3O4-3.

[0037] Example 4 Lignin was extracted from rice straw. PAN and lignin were dissolved in DMF at a mass ratio of 1:2 and stirred for 6 hours at room temperature to obtain a polymer mixed solution with a mass fraction of 21.8%. Lignin and manganese acetate were then added to control the total mass ratio of lignin to manganese acetate to be 6:5 to form a uniform electrospinning solution for the electrospinning process. The film peeled off from the aluminum foil was placed in air at 0.5°C min -1 The temperature was raised to 250 °C at a rate of 1 °C min-1 and maintained for 1 h, and then heated in a tube furnace at a rate of 2 °C min-1 under N2 atmosphere. -1 The temperature was raised to 800 °C at a heating rate and maintained for 2 h, and the obtained nanofiber membrane was named CNFs / Mn3O4-5.

[0038] Figure 1 This is a flow chart for preparing lignin Mn3O4 composite nanofiber films with different Mn3O4 contents provided by an embodiment of the present invention. The figure clearly shows the process of preparing the nanofiber film.

[0039] Figure 2 Herein, a and b are macroscopic photographs of the lignin-based Mn3O4 composite nano-carbon fiber film prepared in Example 3 of the present invention before and after carbonization, respectively. Figure 2 Figures ce and d are macroscopic photographs of the lignin-based Mn3O4 composite nano-carbon fiber film prepared in Example 3 of the present invention before and after folding 180°. Figure 2 From a, we can see that the lignin-based Mn3O4 composite nanocarbon fiber film prepared by electrospinning is formed by multi-layer fiber superposition, and its color is similar to that of lignin. Figure 2 It can be seen from b that the carbonized fiber membrane maintains the original multi-layer fiber structure. The macroscopic morphology of the carbonized fiber membrane has not changed, and the size has been uniformly reduced. At the same time, the lignin-based Mn3O4 composite nanocarbon fiber film has good flexibility. After folding 180°, the morphology of the carbon fiber membrane after unfolding has not changed. Its good flexibility makes it have strong application prospects in the field of flexible energy storage materials.

[0040] Figure 3 SEM images and corresponding particle size distribution of different content of Mn304 nanocarbon fiber films prepared by the present application embodiment 2-4 are shown. As can be seen from 3a and 3b, the morphology of CNFs / Mn304-1, CNFs / Mn304-3 fibers is complete, and the structure is compact. As can be seen from 3c, the fiber structure of CNFs / Mn304-5 is not complete, and cross-linking and stringing phenomenon occurs, because too much manganese acetate is added in the spinning solution, which reduces the viscosity of the spinning solution, and due to the reduction of the viscosity of the spinning solution, the uniform and independent spinning cannot be realized. Moreover, the average diameter can be seen from the diameter distribution histogram, which is 697.9 nm, 789.3 nm and 1062.8 nm respectively. It can be seen that with the increase of the content of manganese acetate in the spinning solution, the diameter of the nanocarbon fiber increases, because the content of manganese acetate increases, the concentration of the spinning solution increases, and in the process of electrospinning, the diameter of the fiber increases.

[0041] Figure 4 EDS image and energy spectrum of CNFs / Mn304-3 sample prepared by the present application embodiment 3 are shown. Figure 4 As can be seen from b and 4c, the C and Mn element distribution maps are consistent with the SEM image in a, and the Mn element is uniformly distributed on the nanocarbon fiber. Figure 4

[0042] Table 1 gives the EDS energy spectrum analysis results of CNFs / Mn304-3 sample prepared by the present application embodiment 3, from which it can be seen that the mass content of C element is 87.06%, the atomic percentage content of C element is 96.85%, the mass content of Mn element is 12.92%, and the atomic percentage content of Mn element is 3.14%.

[0043] Table 1 ; Figure 5 XRD spectrum of different content of lignin-based Mn304 composite nanocarbon fiber films prepared by the present application embodiment 2-4 is shown, from which it can be seen that all the samples correspond to Mn304 (PDF.80-0382), and the diffraction peaks at 2θ=28.9°, 32.38°, 36.49°, 44.4°, 59.9° and 64.6° correspond to the (112), (103), (202), (220), (224) and (400) crystal planes of Mn304 crystal respectively, and with the increase of the content, the diffraction peak is enhanced. Figure 5

[0044] ​​Table 2 is the elemental composition analysis of the lignin-based Mn304 composite nanocarbon fiber films with different Mn304 contents prepared in Examples 2-4 of the present application. As can be seen from Table 2, the content of C element decreases from 69.43% to 49.21% as the content of Mn304 in the sample increases.

[0045] Table 2 ; Figure 6 The XPS spectra of the CNFs prepared in Example 1 of the present application and the CNFs / Mn304-3 composite material prepared in Example 3 are shown. As can be seen from Figure 6 a, in the range of 0-800 eV of binding energy, the peaks from left to right are O1s, N1s and C1s, respectively located at about 532 eV, 400 eV and 285 eV, indicating that the CNFs prepared in Example 1 contain only carbon, nitrogen and oxygen. Figure 6 b is the high-resolution XPS spectrum of C1s in the CNFs sample. Peak fitting can obtain three peaks, with center positions of 284.8 eV, 286.5 eV and 289 eV, respectively corresponding to the C-C / C=C, C-N / C-O and C=O functional group characteristic peaks in the CNFs. The area integrals of the peaks are 75%, 13.8% and 11.2%, respectively. Figure 6 c is the peak situation of the XPS total spectrum of the CNFs / Mn304-3 sample in the range of 0-800 eV of binding energy. From left to right, they are Mn2p, O1s, N1s, C1s, Mn3s and Mn3p, respectively located at about 650 eV, 532 eV, 400 eV, 285 eV, 84 eV and 50 eV, indicating that they contain manganese, carbon, nitrogen and oxygen. Figure 6 d is the high-resolution XPS spectrum of C1s in the CNFs / Mn304-3 sample. After data fitting, three peaks can be obtained, with center positions of 284.8 eV, 286 eV and 288 eV, respectively corresponding to the C-C / C=C, C-N / C-O and C=O functional group characteristic peaks in the CNFs / Mn304-3. The area integrals of the peaks are 82.77%, 12.3% and 4.93%, respectively. Compared with the high-resolution C1s spectrum of the CNFs, the C-C / C=C connection is more and the C=O content is less, which may be related to the formation of Mn-O-Mn / Mn-OH bonds after the addition of Mn304. Figure 6 e is the high-resolution XPS spectrum of Mn2p in the CNFs / Mn304-3 sample. Due to spin-orbit splitting, Mn2p produces two possible doublets of 3 / 2 and 1 / 2, with binding energies of 641.5 eV and 653.3 eV, respectively, and a difference of 11.8 eV. The area integrals of the peaks are 63.5% and 36.5%, respectively. 3 / 2The peaks were fitted and found to be Mn around 641eV. 2+ peak, Mn around 642.5eV 3+ peak and Mn around 646eV 4+ Peak, for Mn2p 1 / 2 The peaks were fitted and found to be Mn around 652.5eV. 2+ peak, Mn around 653.7eV 3+ peak and Mn around 655.5eV 4+ Peak. Mn3O4 is usually a normal spinel structure, and Mn has different oxidation states and can exist in tetrahedral (Mn 2 + ) and octahedron (Mn 3+ ) on the surface, the chemical composition is Mn 2+ [Mn 3+ 2]O 2- 4. For Mn2p 3 / 2 orbital Mn 2+ / Mn 3+ / Mn 4+ The peak areas were integrated, accounting for 30.17%, 60.34% and 9.49% respectively, indicating that there are multiple valence states of Mn in the CNFs / Mn3O4-3 sample.

[0046] Figure 7 The Raman spectra of CNFs and CNFs / Mn3O4 composite materials prepared in Examples 1-4 of the present invention are shown in the figure. -1 There is an obvious scattering peak at 1350cm, which belongs to the radial breathing vibration of Mn2p in the Mn-O bond in Mn3O4. -1 and 1580cm -1 The D-band diffraction peak and G-band diffraction peak of the carbon material correspond to the SP 3 Hybrid disordered carbon structure and SP 2 Hybrid carbon structure.

[0047] Table 3 shows the I of CNFs and CNFs / Mn3O4 composite materials prepared in Examples 1-4 of the present invention. D / I G Statistical value.

[0048] Table 3 ; Figure 7 As can be seen from Table 3, after the introduction of Mn3O4 on the basis of lignin-based nanocarbon fibers, the D peak of the carbon material increases, indicating that the defects of the composite nanocarbon fiber material increase. After the addition of Mn3O4, the I D / I G Keep at about 1.11.

[0049] Figure 8 CV curves of different Mn304 content of lignin-based Mn304 composite nanocarbon fiber films prepared by the present application under different scan speeds, from 8a-c, it can be seen that from high to low 200-5mV s -1 scan speed, CV curves can keep the shape of rectangle, which shows that lignin-based Mn304 composite nanocarbon fiber has good capacitive characteristics. 8d is 5mVs -1 scan speed, CV curves of CNFs, CNFs / Mn304-1, CNFs / Mn304-3, CNFs / Mn304-5, it can be seen that there is obvious oxidation-reduction peak in lignin-based Mn304 composite nanocarbon fiber electrode material at about 0.4V, and with the increase of scan rate, the position of oxidation-reduction peak is offset by irreversible polarization, which reflects the pseudo-capacitive characteristics of Mn304.

[0050] Figure 9 GCD curves of CNFs / Mn304-1, CNFs / Mn304-3, CNFs / Mn304-5 prepared by the present application in examples 2-4 at current density of 0.5-10Ag -1 , all GCD curves keep symmetrical triangle at all rates, which shows that it has higher capacitive energy storage, at current density of 0.5Ag -1 , all sample GCD curves are curved, which shows that Mn304 in the composite material has obvious oxidation-reduction reaction in the charging and discharging process. 9d is the charge-discharge curve of different samples at current density of 0.5Ag -1 , it can be seen that the charge-discharge time of CNFs / Mn304-3 is the longest, which shows that its specific capacity is the highest. According to the GCD curve, the specific capacity of the sample can be calculated, and the results are shown in Figure 9 e. The specific capacity of CNFs / Mn304-1 is 275.25F g -1 at current density of 0.5Ag -1 , the specific capacity is 170F g -1 at current density of 10Ag -1 , the specific capacity retention rate is 61.76%, the specific capacity of CNFs / Mn304-3 at current density of 0.5-10Ag -1 is 437.55F g -1 , 375.7F g -1 , 348.6F g -1 , 297F g -1 and 242F g -1capacity retention rate was 55.3%, the specific capacity of CNFs / Mn304-5 was 309.85 F g at a current density of 0.5 A g -1 -1 the specific capacity was 131 F g at a current density of 10 A g -1 -1 the specific capacity retention rate was 42.28%. It can be seen that with the increase of current density, the specific capacity of the sample decreased, which was because at a higher current density, the Mn304 which provided the pseudo-capacitance was affected by the diffusion dynamics of the electrolyte ions, and it was not enough to have a redox reaction with the electrolyte ions, resulting in a decrease in specific capacity. According to the GCD curve, the specific capacity of the lignin-based nanocarbon fiber with different Mn304 contents at 0.5 A g -1 Figure 9 It was found that CNFs / Mn304-3 had the largest specific capacity of 437.55 F g -1 , and the specific capacity of CNFs / Mn304-5 was 309.85 F g -1 The increase of the content of Mn304 did not improve the specific capacity of the lignin-based Mn304 composite nanocarbon fiber, and the main reason was that the morphology of the nanocarbon fiber changed after too much Mn304 was added.

[0051] Figure 10 CV curves of the symmetric supercapacitor, from 10a-c, it can be seen that the CV curves of the symmetric supercapacitors assembled with CNFs / Mn304-1, CNFs / Mn304-3 and CNFs / Mn304-5 as electrode materials still can keep similar rectangular shape under the voltage window of-1-0 V, and the scanning rate decreased from 200 mV s -1 to 5 mV s -1 , the CV curves still kept rectangular shape, indicating that the symmetric supercapacitors assembled with lignin-based Mn304 composite nanocarbon fiber had good capacitive characteristics and rate performance.

[0052] Figure 11 a-c are the GCD curves of CNFs / Mn304-1, CNFs / Mn304-3 and CNFs / Mn304-5 at a current density of 0.2-5 A g -1 It can be seen directly that there was no obvious voltage drop in the discharge process, and the GCD curve was approximately a symmetrical triangle. According to the GCD curve, the specific capacity of the sample can be calculated, the specific capacity of CNFs / Mn304-1 was 214.8 F g at a current density of 0.2 A g -1 -1 the specific capacity was 150 F g at a current density of 5 A g -1 -1 ​​​​​, the specific capacity retention rate is 69.83%, the specific capacity of CNFs / Mn3O4-3 at 0.2-5 A g -1 is 276 F g -1 , 254 F g -1 , 236 F g -1 , 220 F g -1 and 200 F g -1 , the specific capacity retention rate is 72.46%, the specific capacity of CNFs / Mn3O4-5 at a current density of 0.2 A g -1 is 260.4 F g -1 , the specific capacity at a current density of 5 A g -1 is 150 F g -1 , and the specific capacity retention rate is 57.6%. It can be clearly seen from d that as the current density increases, the specific capacity of the sample decreases. At a higher current density, the Mn3O4 providing pseudocapacitance is affected by the diffusion of electrolyte ions, and it is not enough to undergo redox reaction with electrolyte ions, resulting in a decrease in specific capacity. Figure 11

[0053] Figure 12 a is the Ragone curve of CNFs / Mn3O4-1, CNFs / Mn3O4-3 and CNFs / Mn3O4-5. It can be seen that the energy density of the supercapacitor decreases with the increase of the power density. The energy density of the supercapacitor is 9.58 Wh kg -1 when the power density of the supercapacitor is 49.99 W kg -1 , and the energy density is increased by 3.12 Wh kg -1 compared with CNFs without adding Mn3O4. The power density of the supercapacitor is 1249.99 W kg -1 , and the energy density is 6.9 Wh kg -1 . At a higher power density, the current density and discharge time of the supercapacitor are relatively short, and the corresponding energy density is relatively low. 12b is the specific capacity change diagram of the supercapacitor assembled by the CNFs / Mn3O4-3 sample at a current density of 0.5 A g -1 , and it can be seen from the figure that after 1000 constant current charge and discharge, the capacity retention rate of the supercapacitor is 88.33%. Figure 12 c is the specific capacity change diagram of the supercapacitor assembled by the CNFs / Mn3O4-3 sample at a current density of 5 A g -1 ​The capacity retention rate of the supercapacitor is 94.44% after 10,000 times of constant current charge and discharge, which shows good cycle stability. The Nyquist curve and equivalent circuit diagram of the supercapacitor assembled by the CNFs / Mn3O4-3 sample are shown in the figure. The slope of the straight line in the low frequency region is large, which shows that the capacitive performance is good, and the 45° inclined line in the medium frequency region is short, which shows that the diffusion impedance of the ions in the electrolyte is small. Figure 12 The R s of the CNFs / Mn3O4-3 sample in the two-electrode system is about 0.71Ω, the R ct is about 1.40Ω.

[0054] The above examples only express several embodiments of the present application, and the description is more specific and detailed, but it cannot be understood as the limitation of the patent scope of the present application. It should be pointed out that for ordinary skilled in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which belong to the protection scope of the present application. Therefore, the protection scope of the patent of the present application should be subject to the appended claims.

Claims

1. A method for preparing a lignin-based Mn3O4 composite nanocarbon fiber film, characterized in that: The following steps are involved: Extraction of lignin from rice straw; preparing a spinning solution containing the lignin, polyacrylonitrile and manganese acetate; preparing a composite fiber membrane from the spinning solution by electrospinning; The composite fiber membrane is pre-oxidized and carbonized to obtain a lignin-based Mn3O4 composite nano-carbon fiber film.

2. The method for preparing a lignin-based Mn3O4 composite nanocarbon fiber film according to claim 1, wherein: The spinning solution uses dimethylformamide as a solvent; the mass ratio of the polyacrylonitrile to the lignin in the spinning solution is 1:2, and the mass ratio of the lignin to the manganese acetate is 6:1-5.

3. The method for preparing a lignin-based Mn3O4 composite nanocarbon fiber film according to claim 2, wherein: The spinning solution is prepared by the following method: the polyacrylonitrile and the lignin are dissolved in dimethylformamide at a mass ratio of 1:2 to prepare a mixed solution, and the manganese acetate is added to the mixed solution.

4. The method for preparing a lignin-based Mn3O4 composite nanocarbon fiber film according to claim 1, wherein: The pre-oxidation conditions are: 0.5°C min -1 The temperature was raised to 250 °C at a heating rate and maintained for 1 h.

5. The method for preparing a lignin-based Mn3O4 composite nanocarbon fiber film according to claim 1, wherein: The carbonization conditions are: 2°C min-1 under N2 atmosphere. -1 The temperature was raised to 800 °C at a heating rate and maintained for 2 h.

6. The method for preparing a lignin-based Mn3O4 composite nano-carbon fiber film according to claim 1, wherein: The electrospinning conditions are as follows: spinning voltage 10.8 kV; using a 5 mL syringe; receiving distance 15 cm; air humidity 40%; spinning temperature 25° C.; and feed rate 0.013 mL / min.

7. The method for preparing a lignin-based Mn3O4 composite nanocarbon fiber film according to any one of claims 1 to 6, characterized in that: The lignin is extracted by the following method: The crushed rice straw powder was mixed with ethanol solution at a mass ratio of 1:12 and reacted at 200°C for 3 h; Deionized water is added to the black liquor after the reaction, and the mixture is allowed to stand for precipitation. The precipitate is separated and dried to obtain the lignin.

8. The method for preparing a lignin-based Mn3O4 composite carbon nanofiber film according to claim 7, wherein: The rice straw powder is obtained by passing through a 100-mesh sieve, and the volume fraction of the ethanol solution is 70%.

9. A lignin-based Mn3O4 composite nanocarbon fiber film, characterized in that: The method is described in any one of claims 1 to 8.

10. Use of the lignin-based Mn3O4 composite nano-carbon fiber film as claimed in claim 9 in an electrode material for a supercapacitor.

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