Lignin-based MnO / Co composite carbon nanofiber film, and preparation method and application thereof

Abstract

The invention provides a lignin-based MnO / Co 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, manganese acetate and cobalt acetate; preparing the spinning solution into a composite fiber membrane through electrostatic spinning; and pre-oxidizing and carbonizing the composite fiber film to obtain the lignin-based MnO / Co composite carbon nanofiber film. The lignin-based MnO / Co composite carbon nanofiber film prepared by the method does not need to be added with a binder, has good flexibility and mechanical strength, can be directly used as an electrode, and can be suitable for flexible electronic devices. The lignin-based MnO / Co composite carbon nanofiber thin film prepared by the invention shows excellent comprehensive electrochemical performance: the specific capacity reaches up to 816.5 F g under the current density of 0.5 A g, and is obviously superior to that of a traditional MnO or CoO-based composite material; and the material has good rate capability and cycling stability.

Description

TECHNICAL FIELD

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

[0002] As electrode materials of double-layer capacitors, carbon-based materials reversibly adsorb electrolyte ions on the surface of the electrode materials in the electrolyte to realize energy storage. The carbon-based materials have the advantages of fast charging and discharging speed, strong cycle stability and high power density, etc. However, compared with secondary batteries, the energy density is low, which limits the application in electronic products and other equipment.

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

[0004] Mn-based and Co-based metal oxides are widely studied pseudo-capacitance metal materials for supercapacitors, and there have been a large number of related reports. However, traditional MnO or CoO composite carbon materials usually rely on high-cost carbon sources such as graphene and carbon nanotubes, although certain progress has been made in improving the specific capacity, 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 easily leads to agglomeration on the surface of the material, 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 relies on non-renewable petrochemical resources, which is not conducive to the construction and popularization and application of green and low-carbon material system. SUMMARY

[0005] The present invention aims to provide a lignin-based MnO / Co composite nanocarbon fiber film, as well as a preparation method and application thereof, to address at least one of the deficiencies in the prior art. The lignin-based MnO / Co composite nanocarbon fiber film of the present invention utilizes renewable lignin as a carbon source, employs electrospinning technology to form a three-dimensional, multi-level porous composite structure with stable structure and excellent conductivity, and in situ growth of MnO and Co-based metal oxides on the carbon fiber surface. The lignin-based MnO / Co composite nanocarbon fiber film of the present invention combines excellent flexibility, self-supporting properties, high conductivity, and a high specific surface area, effectively improving pseudocapacitive reaction efficiency, making it suitable for flexible device applications, and offering the advantages of environmentally friendly and low-cost preparation.

[0006] To achieve the above object, the first aspect of the present invention provides a method for preparing a lignin-based MnO / Co composite nanocarbon fiber film, comprising the following steps: Extraction of lignin from rice straw; preparing a spinning solution containing the lignin, polyacrylonitrile, manganese acetate and cobalt 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 MnO / Co composite nanocarbon fiber film.

[0007] Furthermore, the spinning solution uses dimethylformamide as a solvent; the mass ratio of the lignin, the manganese acetate, and the cobalt acetate in the spinning solution is 1:1:1.

[0008] Furthermore, the spinning solution is prepared by the following method: a polyacrylonitrile solution with a mass fraction of 10% is prepared using dimethylformamide as a solvent, and the lignin, the manganese acetate and the cobalt acetate are added after stirring until the spinning solution becomes clear and transparent.

[0009] Furthermore, the pre-oxidation conditions are: in air at 0.5°C min -1 The temperature was raised to 250 °C at a heating rate and maintained for 1 h.

[0010] Furthermore, the carbonization conditions are: in N2 atmosphere at 2°C min -1 The temperature was raised to 800 °C at a heating rate and maintained for 2 h.

[0011] Furthermore, 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.

[0012] Furthermore, the lignin is extracted according to 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.

[0013] Further, the rice straw powder is obtained through a 100-mesh screen, and the volume fraction of the ethanol solution is 70%.

[0014] In a second aspect, the application provides a lignin-based MnO / Co composite nanocarbon fiber film prepared by the method.

[0015] In a third aspect, the application provides the use of the lignin-based MnO / Co composite nanocarbon fiber film as an electrode material for supercapacitors.

[0016] Compared with the prior art, the application has the following technical effects: The lignin-based MnO / Co composite nanocarbon fiber film prepared by the method of the application uses renewable rice straw as a raw material, is green and environmentally friendly, and reduces production costs; the flexible and self-supporting nanocarbon fiber film is prepared by combining electrospinning and high-temperature carbonization processes, and MnO and Co-based metal oxides are loaded by in-situ growth technology to form a three-dimensional hierarchical porous composite structure with stable structure and excellent conductivity. The electrospinning process is simple and efficient, and can realize large-scale production. The lignin-based MnO / Co composite nanocarbon fiber film prepared by the method of the application does not need to add a binder, has good flexibility and mechanical strength, and can be directly used as an electrode, and can be applied to flexible electronic devices. The method of the application conforms to the development direction of current sustainable materials and low-carbon manufacturing, and has good industrial application prospects.

[0017] The lignin-based MnO / Co composite nanocarbon fiber film prepared by the method of the application has excellent comprehensive electrochemical performance: the specific capacity is as high as 816.5 F / g at a current density of 0.5 A / g, which is significantly better than traditional MnO or CoO-based composite materials; the specific capacity retention rate is still 40.3% when the current density is increased from 0.5 A / g to 10 A / g; in a symmetric supercapacitor, the specific capacity is 539.2 F / g at 0.2 A / g, and in an asymmetric device, the specific capacity is 210.2 F / g under the same conditions, the energy density is still 11.80 Wh / kg at a power density of 2656.2 W / kg, and the cycle stability is good. BRIEF DESCRIPTION OF DRAWINGS

[0018] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed to be used in the embodiments or prior art description 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 any creative effort on the basis of these drawings.

[0019] Figure 1 The preparation flow chart of the lignin-based composite nanocarbon fiber film provided by the embodiments of the present application is shown in the figure. Figure 2 Figures 2(a) and 2(b) are comparative diagrams of the CNFs-MnO / Co film prepared in Embodiment 3 of the present application before and after carbonization, respectively, and Figures 2(c)-(e) are comparative diagrams of the CNFs-MnO / Co film prepared in Embodiment 3 of the present application before and after folding at 180°. Figure 3 Figures 3(a)-(c) are SEM diagrams of the lignin-based composite nanocarbon fiber films provided by Embodiments 1-3 of the present application, and Figures 3(d)-(f) are the corresponding cubic diagrams of the diameter distribution. Figure 4 The EDS element distribution diagram and the energy spectrum diagram of the CNFs-MnO / Co sample provided by Embodiment 3 of the present application are shown in the figure. Figure 5 The XRD spectrum diagram of the lignin-based composite nanocarbon fiber films provided by Embodiments 1-3 of the present application is shown in the figure. Figure 6 The XPS spectrum of the CNFs-MnO / Co provided by Embodiment 3 of the present application is shown in the figure, in which Figure 6(a) is the XPS total spectrum of the CNFs-MnO / Co, Figure 6(b) is the high-resolution C1s spectrum, Figure 6(c) is the high-resolution Mn2p spectrum, and Figure 6(d) is the high-resolution Co2p spectrum. Figure 7 The Raman spectrum of the lignin-based composite nanocarbon fiber films provided by Embodiments 1-3 of the present application is shown in the figure. Figure 8 Figure 7(a) is the N2 adsorption / desorption isotherm of the lignin-based composite nanocarbon fiber films provided by Embodiments 1-3 of the present application, Figure 8 Figure 7(b) is the pore size distribution diagram of the lignin-based composite nanocarbon fiber films provided by Embodiments 1-3 of the present application. Figure 9 The CV curves of the lignin-based composite nanocarbon fiber film under different scan rates in a three-electrode system provided by the embodiments of the present application are shown in the figure, in which (a) is the CV curve of CNFs-MnO, (b) is the CV curve of CNFs-Co, (c) is the CV curve of CNFs-MnO / Co, and (d) is the CV curve at a scan rate of 20 mV s -1 Figure 10 ​GCD curves of the lignin-based composite nanocarbon fiber film under three electrodes at different current densities provided for the embodiments of the present application: (a) CNFs-MnO, (b) CNFs-Co, (c) CNFs-MnO / Co, (d) GCD curves at 0.5 A / g, (e) specific capacity size at different current densities, and (f) specific capacity size at 0.5 A / g; Figure 11 CV curves of the lignin-based composite nanocarbon fiber film at different scan rates under two electrodes provided for embodiments 1-3 of the present application: (a) CNFs-MnO, (b) CNFs-Co, (c) CNFs-MnO / Co; Figure 12 GCD curves of the lignin-based composite nanocarbon fiber film at different current densities under two electrodes provided for embodiments 1-3 of the present application: (a) CNFs-MnO, (b) CNFs-Co, (c) CNFs-MnO / Co, and (d) specific capacity at different current densities; Figure 13 (a) Ragone plot and (b) EIS plot of the lignin-based composite nanocarbon fiber film under two electrodes provided for embodiments 1-3 of the present application; Figure 14 Test results of CNFs-MnO / Co / / PL-12 provided for embodiment 3 of the present application: (a) CV, (b) GCD, (c) specific capacity, and (d) Ragone plot. DETAILED DESCRIPTION

[0020] In order to make the technical problems to be solved by the present application, the technical solutions and beneficial effects more clearly and clearly, the following embodiments are combined to further detail the present application. It should be understood that the specific embodiments described herein are only used to explain the present application, and are not used to limit the present application.

[0021] 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.

[0022] The embodiment of the present application provides a kind of rice stalk as raw material, with manganese acetate and cobalt acetate as manganese source and cobalt source respectively, lignin-based MnO / Co composite nanocarbon fiber film is prepared using electrospinning technology, and it is applied to the electrode material of supercapacitor: first, rice stalk material is pretreated, such as crushing, impurity removal etc., to extract lignin, then according to certain proportion, add certain proportion of lignin, manganese acetate and cobalt acetate in PAN solution with DMF as solvent.Secondly, under suitable process conditions, fiber structure is formed by electrospinning technology, and carbonization is carried out in subsequent process, to obtain lignin-based MnO / Co composite nanocarbon fiber film.Finally, the prepared composite nanocarbon fiber film is applied to the preparation of supercapacitor, as electrode material, to improve the performance of supercapacitor.

[0023] To achieve the above object, the embodiment of the present application adopts the following technical solution: A preparation method of electrospinning preparation of lignin-based MnO / Co composite nanocarbon fiber film, comprising the following steps: (1) electrospinning method is used to prepare lignin-based composite film containing manganese and cobalt; (2) the composite film obtained in step (1) is calcined to obtain lignin-based MnO / Co composite nanocarbon fiber film.

[0024] In step (1), the following steps are included: Lignin separation: after the rice stalk is crushed and passed through 100 mesh screen, the fine powder is mixed with 70% ethanol solution at a mass ratio of 1:12, and the mixture is placed in an iron fluorine dragon lined stainless steel autoclave, and reacted at 200℃ for 3 hours.

[0025] Lignin extraction: the mixture after reaction in the reaction kettle is suction filtered, 2 times volume of deionized water is added to the black liquor after suction filtration, and the precipitate is centrifuged and dried to obtain lignin.

[0026] Spinning solution configuration: dimethylformamide (DMF) is used as solvent to configure polyacrylonitrile (PAN) solution with mass fraction of 10%, and the spinning solution is stirred at room temperature until it is clear and transparent, and then a viscous polymer mixture solution with mass fraction of 27.8% is configured according to the mass ratio of lignin:manganese acetate: cobalt acetate = 1:1:1, and the electrospinning solution is continued to be stirred at high speed for 12h at room temperature.

[0027] Electrospinning method is used to prepare lignin / PAN / manganese acetate / cobalt acetate composite fiber film: the spinning solution is electrospun to obtain lignin / PAN / manganese acetate / cobalt acetate composite fiber film. 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.

[0028] Step (2) specifically includes the following steps: Pre-oxidation: The lignin-based film peeled off from the aluminum foil 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.

[0029] 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 MnO / Co composite nanocarbon fiber film.

[0030] The embodiment of the present invention uses manganese acetate as a manganese source, cobalt acetate as a cobalt source, and rice straw as a carbon source. By electrospinning technology and high-temperature carbonization, a lignin-based MnO / Co composite carbon fiber film containing MnO / Co is successfully prepared. By structural regulation, an optimized combination of Mn-based and Co-based materials can be achieved, and nanocomposite structures, core-shell structures or porous structures can be designed to achieve effective contact and interaction between the two materials, thereby improving electrochemical performance. The lignin-based MnO / Co composite carbon fiber film prepared by the embodiment of the present invention has good flexibility. After folding at 180 °, the morphology before and after the carbon fiber film is unfolded does not change, and good flexibility makes it possible to have a strong application prospect in the field of flexible energy storage materials.

[0031] In the embodiment of the present invention, the lignin-based MnO / Co composite nano-carbon fiber film is used as the electrode material of the symmetrical supercapacitor and the asymmetrical supercapacitor, which has high specific capacity and good stability.

[0032] The preparation method of the embodiment of the present invention utilizes renewable resources such as rice straw, reducing production costs; the electrospinning process is simple and efficient, and can achieve large-scale production; the Co-based material and the Mn-based material each have unique electrochemical properties and structural characteristics. Through compounding, the electron transmission rate and electrode reaction rate of the capacitor are effectively improved. Therefore, the lignin-based MnO / Co composite nanocarbon fiber film prepared by the embodiment of the present invention has broad application prospects and commercial value in the field of energy storage.

[0033] The lignin-based MnO / Co composite nanocarbon fiber film prepared in the embodiment of the present application has significant advantages in material structure design, electrochemical performance, flexible integration and green sustainability. In the embodiment of the present application, a flexible self-supporting nanocarbon fiber film is prepared by using renewable lignin as a carbon source, combining electrospinning and carbonization process, and then MnO and Co-based metal oxides are grown in situ to construct a three-dimensional hierarchical porous composite structure, which effectively improves the electronic transmission capacity and active site exposure of the material. In terms of performance, the specific capacity of the lignin-based MnO / Co composite nanocarbon fiber film prepared in the embodiment of the present application reaches 816.5 F g at 0.5 A g, which is superior to the reported similar materials; the specific capacity retention rate is still 40.3% when the current density is increased from 0.5 A g to 10 A g; in the symmetric supercapacitor, the specific capacity reaches 539.2 F g at 0.2 A g, and the specific capacity of the asymmetric device is 210.2 F g under the same conditions, the energy density is still maintained at 11.80 Wh kg when the power density is as high as 2656.2 W kg, and the cycle stability is good. The lignin-based MnO / Co composite nanocarbon fiber film prepared in the embodiment of the present application has good rate performance and strong cycle stability, and is suitable for high-performance supercapacitors. At the same time, the material does not need a binder, the preparation process is green and environmentally friendly, has low cost and good flexibility, is suitable for practical application of flexible energy storage devices, and has significant technological progress and promotion prospects.

[0034] The following illustrates a lignin-based MnO / Co composite nanocarbon fiber film, a preparation method and application of the embodiment of the present application through a plurality of specific embodiments.

[0035] Embodiment 1 Rice straw is used as raw material to extract lignin. A 10% PAN solution is prepared with DMF as solvent, and the spinning solution is fully stirred at room temperature until it is clear and transparent. A viscous polymer mixed solution with a mass fraction of 22.6% is prepared according to the mass ratio of lignin:manganese acetate=1:1, and the solution is continuously stirred at high speed for 12 h at room temperature, and then electrospinning process is carried out. The film peeled off from the aluminum foil is heated in air at a heating rate of 0.5 ℃ / min -1 to 250 ℃ and kept for 1 h, and then heated to 800 ℃ at a heating rate of 2 ℃ / min -1 under N2 atmosphere in a tube furnace and kept for 2 h, and the obtained nanofiber film is named as CNFs-MnO.

[0036] Embodiment 2 Rice straw was used as raw material to extract lignin. PAN solution of 10% was prepared with DMF as solvent, and the spinning solution was stirred at room temperature until it was clear and transparent. The viscous polymer mixed solution with a mass fraction of 22.6% was prepared according to the mass ratio of lignin: cobalt acetate = 1:1, and the electrospinning process was carried out after the solution was continuously stirred at room temperature for 12 hours. The film peeled off from the aluminum foil was heated to 250℃ at a heating rate of 0.5℃ / min -1 in air and kept for 1h, and then heated to 800℃ at a heating rate of 2℃ / min -1 in N2 atmosphere in a tube furnace and kept for 2h, and the obtained nanofiber film was named as CNFs-Co.

[0037] Example 3 Rice straw was used as raw material to extract lignin. PAN solution of 10% was prepared with DMF as solvent, and the spinning solution was stirred at room temperature until it was clear and transparent. The viscous polymer mixed solution with a mass fraction of 22.6% was prepared according to the mass ratio of lignin: cobalt acetate = 1:1, and the electrospinning process was carried out after the solution was continuously stirred at room temperature for 12 hours. The film peeled off from the aluminum foil was heated to 250℃ at a heating rate of 0.5℃ / min -1 in air and kept for 1h, and then heated to 800℃ at a heating rate of 2℃ / min -1 in N2 atmosphere in a tube furnace and kept for 2h, and the obtained nanofiber film was named as CNFs-MnO / Co.

[0038] Figure 1 is a preparation flowchart of the lignin-based composite nanocarbon fiber film provided by the embodiments of the present application, which clearly shows the process of preparing the nanofiber film.

[0039] Figure 2 in which a and b are the macroscopic photographs of the lignin-based MnO / Co composite nanocarbon fiber film prepared in Example 3 of the present application before and after carbonization, Figure 2 in which c-e are the macroscopic photographs of the lignin-based MnO / Co composite nanocarbon fiber film prepared in Example 3 of the present application before and after being folded by 180°. As can be seen from Figure 2 a, the lignin-based MnO / Co composite nanocarbon fiber film prepared by electrospinning is formed by superimposing multiple layers of fibers, and the color is similar to that of lignin. As can be seen from Figure 2 b, the carbonized fiber film maintains the original multi-layer fiber structure, and the macroscopic morphology of the carbonized fiber film does not change, and the size uniformly shrinks. As can be seen from Figure 2 c-e, the lignin-based MnO / Co composite nanocarbon fiber film has good flexibility, and after being folded by 180°, the morphology of the carbon fiber film after unfolding does not change, and the good flexibility makes it have strong application prospect in the field of flexible energy storage materials.

[0040] Figure 3 a-c are SEM images of the lignin-based composite nanocarbon fiber films prepared in Examples 1-3 of the present application, respectively, Figure 3 d-f are their corresponding diameter distribution cubic graphs. As can be seen from 3a-c: the fiber diameters of CNFs-MnO, CNFs-Co and CNFs-MnO / Co are relatively uniform, and the fiber network structure helps the rapid diffusion of electrolyte ions, which can help to achieve fast charge and discharge; as can be seen from the diameter distribution cubic graphs: the average diameters of CNFs-MnO, CNFs-Co and CNFs-MnO / Co are 290 nm, 183.9 nm and 803.9 nm, respectively. The diameters of lignin-based composite nanocarbon fibers containing Mn or Co alone are about 200-300 nm, while the diameters of lignin-based composite nanocarbon fibers containing both Mn and Co are about 800 nm. Under the condition that the voltage and other conditions are constant, the mass fraction of the spinning solution increases, and the fiber diameter also increases.

[0041] Figure 4 EDS and energy spectrum of the CNFs-MnO / Co sample prepared in Example 3 of the present application. As can be seen from Figure 4 b, 4c, 4d, the C, Mn and Co element distribution graphs are consistent with Figure 4 the SEM images in a, and the Mn and Co elements are uniformly distributed on the nanocarbon fibers.

[0042] Table 1 gives the EDS energy spectrum analysis results of the CNFs-MnO / Co sample prepared in Example 3 of the present application. From the analysis results in Table 1, it can be concluded that the mass content of C element is 77.38%, the atomic percentage content of C element is 92.53%, the mass content of Mn element is 14.64%, the atomic percentage content of Mn element is 4.09%, the mass content of Co element is 12.98%, and the atomic percentage content of Co element is 3.38%.

[0043] Table 1 ; Figure 5 XRD spectrum of the lignin-based composite nanocarbon fiber film prepared in Examples 1-3 of the present application, from Figure 5It can be seen that the diffraction peaks of the CNFs-MnO sample correspond to MnO (PDF. 78-0424), and the diffraction peaks at 2θ = 34.98°, 40.58°, 58.74°, 70.21° and 73.8° correspond to the (111), (200), (220), (311) and (222) crystal planes of the MnO crystal, respectively. The diffraction peaks of the CNFs-Co sample mainly exist at 2θ = 44.21°, corresponding to the (111) crystal plane of Co, and the diffraction peaks of the CNFs-MnO / Co sample are relatively obvious and correspond to MnO (PDF. 78-0424) and Co (PDF. 15-0806) one by one.

[0044] Table 2 is the elemental composition analysis of the lignin-based composite nanocarbon fiber film prepared in Examples 1-3 of the present application. As can be seen from Table 2, the carbon content of the CNFs-Co sample is the highest, being 69.38%, and the carbon content of the CNFs-MnO / Co sample is the lowest, being 48.47%.

[0045] Table 2 ; Figure 6 The XPS spectrum of the CNFs-MnO / Co composite material is shown in Figure 6a, and the high-resolution C1s spectrum, the high-resolution Mn2p spectrum and the high-resolution Co2p spectrum are shown in Figures 6b, 6c and 6d, respectively. Figure 6 As can be seen from the high-resolution C1s spectrum, the high-resolution Mn2p spectrum and the high-resolution Co2p spectrum, in the range of binding energy 0-1000 eV, from left to right, there are Co2p, Mn2p, O1s and C1s, indicating that there are four elements of cobalt, manganese, carbon and oxygen. Figure 6 The high-resolution XPS spectrum of C1s of the CNFs-MnO / Co sample is shown in Figure 6b. After fitting the data, three peaks can be obtained, with center positions of 284.8 eV, 286 eV and 289 eV, respectively, corresponding to the characteristic peaks of C-C / C=C, C-O and C=O functional groups in the CNFs-MnO / Co. The peak area is integrated, and the proportions of the three functional groups are 68.56%, 13.38% and 18.06%, respectively. Figure 6 The high-resolution XPS spectrum of Mn2p in the CNFs-MnO / Co sample is shown in Figure 6c. Due to spin-orbit splitting, Mn2p produces two possible doublets of 3 / 2 and 1 / 2, with binding energies of 641.5 eV and 653.2 eV, respectively, and a difference of 11.7 eV. By fitting the Mn2p 3 / 2 peak, it is found that the Mn2p 3 / 2 peak is composed of two characteristic peaks with binding energies of 641.5 eV and 645.1 eV, corresponding to the binding energies of Mn 2+ and Mn 3+ . By integrating the peak area, it is found that the content of Mn 3+ is 16%, and the presence of trivalent manganese ions in the sample is due to the reduction of high-valence Mn ions to Mn2+ The conversion is incomplete, and in order to maintain the electrical neutrality of the sample, cation vacancies exist in the MnO lattice. Figure 6 d is the high-resolution XPS spectrum of Co2p, with Co2p appearing at 780.4eV and 796.4eV 3 / 2 、Co2p 1 / 2 The two spin-orbit doublets and two satellite vibration peaks at 786.3 eV and 803.7 eV indicate that the Co element in the CNFs-MnO / Co sample is Co 2+ exists in the form of .

[0046] Figure 7 The Raman spectra of CNFs-MnO, CNFs-Co and CNFs-MnO / Co composite materials prepared in Examples 1-3 of the present invention are shown in the figure. -1 There is an obvious scattering peak at 660 cm, which belongs to the radial breathing vibration of Mn2p in the Mn-O bond in MnO. -1 There is a scattering peak at 650 cm- -1 There is an obvious scattering peak at 1350cm -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, from Figure 7 It can be seen that the R value of CNFs-MnO / Co is the smallest, which is 1.02, indicating that the degree of graphitization of the carbon material is high and the charge transfer is rapid during the energy storage process.

[0047] Figure 8 As shown in Figure a, the N2 isothermal adsorption-desorption curves of CNFs-MnO exhibit the characteristics of a Type I curve. At relatively low pressures (P / P0 < 0.05), the N2 adsorption increases rapidly, showing a linear trend of increase. The N2 adsorption amount increases rapidly and reaches a plateau. This is due to the predominance of microporous structures in the sample. Between relative pressures of 0.03 and 0.2, the N2 adsorption amount increases, but the rate slows. After reaching a relative pressure of 0.2, the rate of increase in N2 adsorption increases, and the adsorption is essentially balanced. The N2 isothermal adsorption-desorption curves of CNFs-Co and CNFs-MnO / Co exhibit the characteristics of a Type IV curve, with the presence of an H2 hysteresis loop. This is due to capillary condensation in the mesopores, resulting in a misalignment between the desorption and adsorption isotherms. This indicates the presence of a large number of micropores and a small amount of mesoporous structure in the sample. Figure 8b is the BJH method pore size distribution graph of the test sample, the pore size distribution of the carbon nanofiber is concentrated in 2-5 nm, in the micropore range inside the carbon material, the micropore structure shortens the distance of the ion transmission channel. The pore structure parameters can be calculated according to the isothermal adsorption-desorption curve.

[0048] Table 3 shows the specific surface area and pore size of the lignin-based composite carbon nanofiber film prepared in Examples 1-3 of the present application, it can be seen that the specific surface area of CNFs-MnO / Co is the largest, which is 348.97 m 2 g -1 , and the total pore volume is 3.76 cm 3 g -1 .

[0049] Table 3 ; Figure 9 a-9c are respectively the CV curves of CNFs-MnO, CNFs-Co and CNFs-MnO / Co at a scan rate of 20-100 mV s -1 , it can be seen that the CV curves of CNFs-MnO and CNFs-Co at a scan rate of 100 to 20 mV s -1 , all show pseudo-capacitive energy storage behavior, while the oxidation-reduction peak position of CNFs-MnO / Co is asymmetric, showing a battery-type pseudo-capacitive energy storage behavior, and with the increase of the scan rate, the oxidation-reduction peak position is offset due to the irreversible polarization.

[0050] Figure 10 a-10c are respectively the GCD curves of CNFs-MnO, CNFs-Co and CNFs-MnO / Co at a current density of 0.5-5 Ag -1 , the GCD curve of the CNFs-MnO sample is slightly curved, which indicates that there is a redox reaction during the charging and discharging process. The GCD curve of the CNFs-MnO / Co sample has a platform, which further indicates that it is a battery-type electrode material.

[0051] Figure 10 d is the charge-discharge curve of different samples at a current density of 0.5 Ag -1 , CNFs-MnO / Co appears a peak in the charging process, indicating that it has a redox reaction during the charging process, it can be seen that the charge-discharge time of CNFs-MnO / Co is the longest, indicating that its specific capacity is the highest. According to the GCD curve, the specific capacity of the sample can be calculated, the specific capacity of CNFs-MnO is 301.85 F g -1 at a current density of 0.5 A g -1 , and the specific capacity is 201 F g -1 at a current density of 5 A g-1 The specific capacity retention rate was 66.59%, the specific capacity of CNFs-Co was 217.55 F g -1 at a current density of 0.5 A g -1 -5 A g -1 The specific capacity was 120 F g -1 at a current density of 5 A g -1 The specific capacities of CNFs-MnO / Co were 816.5 F g -1 , 729.7 F g -1 , 664.6 F g -1 , 501.6 F g -1 and 329 F g -1 at 0.5-5 A g -1 , respectively, and the specific capacity retention rate was 40.29%. It can be obviously seen that the specific capacity of the sample decreases with the increase of the current density, because the transition metal oxide providing pseudo-capacitance is affected by the diffusion dynamics of electrolyte ions at a higher current density, and cannot timely undergo oxidation-reduction reaction with electrolyte ions, resulting in the decrease of the specific capacity.

[0052] Figure 10 As can be seen from e-f, CNFs-MnO / Co has the largest specific capacity, which shows that the lignin composite nanocarbon fiber material doped with Mn-based and Co-based metal oxides at the same time indeed greatly improves the specific capacity, from Figure 3 c and Figure 4 It can be seen that the particles of MnO and Co are uniformly distributed on the surface of the carbon fiber, and in combination with the BET test results, the specific surface area of CNFs-MnO / Co is the largest, which is 348.97 m 2 g -1 During the charging and discharging process, the redox reaction occurs more completely at a smaller current density, while the kinetics of the redox reaction is slower at a larger current density, resulting in the decrease of the rate performance.

[0053] Figure 11 As can be seen from a-c, with CNFs-MnO, CNFs-Co and CNFs-MnO / Co as the electrode material, the CV curve can still maintain a similar rectangle at a voltage window of -1-0 V, and the CV curve still maintains a rectangle when the scanning rate is from 200 mV s -1 to 5 mV s -1 , which shows that the symmetric supercapacitor assembled with the lignin-based MnO / Co composite nanocarbon fiber film has good capacitive characteristics and rate performance.

[0054] Figure 12 a-c are CNFs-MnO, CNFs-Co and CNFs-MnO / Co at 0.2-5 A g-1 The GCD curve under current density can be seen intuitively. There is no obvious voltage drop during the discharge process. The specific capacity of the sample can be calculated based on the GCD curve. The specific capacity of CNFs-MnO at 0.2A g -1 The specific capacity is 246F g at a current density of -1 , at 5A g -1 The specific capacity is 160Fg at a current density of -1 The specific capacity retention rate is 69.83%, and CNFs-Co at 0.2A g -1 The specific capacity is 141.6F g at a current density of -1 , in 5Ag -1 The specific capacity is 100F g at a current density of -1 The specific capacity retention rate is 70.62%. CNFs-MnO / Co at 0.2-5A g -1 The specific capacities under the conditions of -1 、422F g -1 、136F g -1 、116F g -1 、100Fg -1 , the specific capacity retention rate is 18.54%. Figure 12 It can be clearly seen from Figure d that with the increase of current density, the specific capacity of the CNFs-MnO / Co sample decreases significantly. This is because at a higher current density, the movement speed of the electrolyte ions is slow, and the metal oxide has no time to respond and cannot generate pseudocapacitance, resulting in a decrease in specific capacity.

[0055] Figure 13 a is the Ragone curve of CNFs-MnO, CNFs-Co and CNFs-MnO / Co. It can be seen that when the power density of the supercapacitor increases, its energy density decreases. The power density of the supercapacitor assembled with CNFs-MnO / Co as the electrode material is 49.999 W kg -1 When the energy density is 18.722Wh kg -1 , the power density of the supercapacitor is 1249.999W kg -1 , with an energy density of 3.47Whkg -1 At higher power density, the current density and discharge time of supercapacitors are relatively short, and the corresponding energy density is low. Figure 13b is the electrochemical impedance spectroscopy of the assembled symmetric supercapacitor of CNFs-MnO, CNFs-Co and CNFs-MnO / Co, the Nyquist curve and equivalent circuit diagram are shown in the figure. The slope of the straight line in the low frequency region is large, which indicates that the capacitance is good, and the 45° slope in the medium frequency region is short, which indicates that the diffusion impedance of the ions in the electrolyte is small. As can be seen from the figure, the semicircle part of the Nyquist curve of CNFs-MnO / Co is the smallest, which indicates that R s is about 0.89Ω, R ct is about 0.22Ω.

[0056] Figure 14 The test results of the asymmetric supercapacitor assembled with CNFs-MnO / Co as the positive electrode material, Figure 14 a is the CV curve of the asymmetric supercapacitor, from the CV curve, it can be seen that the CV curve of the asymmetric supercapacitor can still maintain a similar rectangle under the voltage window of-1-0V, and the scanning rate is from 200mV s -1 to 5mV s -1 , the CV curve still maintains a rectangle, which indicates that the symmetric supercapacitor assembled with the lignin-based MnO / Co composite nanocarbon fiber film has good capacitance characteristics. Under the current density of 0.2-5A g -1 , the specific capacity is 210.2F g -1 , 174.5F g -1 , 145F g -1 , 116F g -1 and 85F g -1 . The asymmetric supercapacitor has an energy density of 11.80Wh kg -1 when the power density is 2656.2W kg -1 .

[0057] 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 a limitation on the scope of the patent of the present application. It should be noted that for ordinary skilled persons in the art, without departing from the concept of the present application, a number of modifications and improvements can be made, which are within the scope of protection 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 MnO / Co 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, manganese acetate and cobalt 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 MnO / Co composite nanocarbon fiber film.

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

1.

3. The method for preparing a lignin-based MnO / Co composite nanocarbon fiber film according to claim 1, wherein: The spinning solution is prepared as follows: a polyacrylonitrile solution with a mass fraction of 10% is prepared using dimethylformamide as a solvent, and the lignin, the manganese acetate and the cobalt acetate are added after stirring until the spinning solution becomes clear and transparent.

4. The method for preparing a lignin-based MnO / Co 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 MnO / Co 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 MnO / Co composite nanocarbon 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 MnO / Co 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 MnO / Co composite nanocarbon fiber 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 MnO / Co 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 MnO / Co composite nano-carbon fiber film according to claim 9 in an electrode material for a supercapacitor.

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