A method for preparing a lignin-based carbon fiber / polyaniline composite electrode and its application in supercapacitors.

Abstract

This invention provides a method for preparing lignin-based carbon fiber / polyaniline composite electrodes and their application in supercapacitors, relating to the field of supercapacitor electrode materials technology. The method prepares LCFs through electrospinning and carbonization processes, and constructs a uniform and continuous PANI coating layer on their surface using in-situ polymerization. This preparation method effectively alleviates PANI agglomeration and fully preserves the three-dimensional network pore structure of the LCFs. Electrochemical tests show that the optimal sample Is-LCFs-PANI-3 exhibits good performance at 0.5 A g. ‑1 The specific capacitance reaches 287.36 F g. ‑1 After 3000 cycles, the capacitance retention rate was 86%. The symmetrical supercapacitor assembled using this technology achieved a capacitance of 0.5 A g. ‑1 The lower specific capacitance is 69.02 F g. ‑1 It possesses good structural stability and electrochemical durability.

Description

Technical Field

[0001] This invention relates to the field of supercapacitor electrode materials technology, and in particular to a method for preparing a lignin-based carbon fiber / polyaniline composite electrode and its application in supercapacitors. Background Technology

[0002] Supercapacitors, as a novel energy storage device situated between traditional capacitors and batteries, possess significant application value in fields such as new energy vehicles, smart grids, and portable electronic devices due to their high power density, ultra-long cycle life, and excellent safety performance. Electrode materials, as the core element determining the energy storage performance of supercapacitors, are seeing their research focus shifting towards low cost, high specific capacitance, and environmental friendliness.

[0003] Currently, while commercially available graphene, carbon nanotubes, and transition metal compounds exhibit certain electrochemical activity, they generally suffer from bottlenecks such as high preparation costs, complex processes, or non-renewable resources, limiting their large-scale industrial application. In contrast, utilizing abundant, renewable, and biodegradable biomass resources to prepare carbon-based electrode materials aligns with the strategic needs of green chemistry and sustainable development. Lignin, a natural polymer formed by the random cross-linking of phenylpropane units through carbon-carbon and ether bonds, boasts advantages such as wide availability, low cost, and high carbon content. Lignin-based carbon fibers (LCFs) prepared through electrospinning and carbonization are considered ideal electrode substrate materials due to their excellent conductivity and interconnected three-dimensional network structure. However, pure LCFs primarily rely on the double-layer mechanism for energy storage, exhibiting strong surface chemical inertness and a lack of active sites, resulting in low specific capacitance and difficulty in meeting the demands of high-performance energy storage.

[0004] To overcome the aforementioned shortcomings, incorporating conductive polymers with high pseudocapacitive properties for composite modification is an effective approach. Polyaniline (PANI) is widely used to modify carbon-based materials due to its high theoretical specific capacitance, simple synthesis, and good environmental stability. However, existing technologies typically employ physical impregnation to load PANI onto the carbon fiber surface, which has significant drawbacks: firstly, the strong π-π conjugation between PANI molecules makes them prone to aggregation and stacking on the substrate surface, blocking ion transport channels and resulting in low utilization of active sites; secondly, the bond between PANI and carbon fibers relies solely on weak physical adsorption, making them susceptible to detachment during long-term charge-discharge processes, leading to electrode structure collapse and rapid capacity decay.

[0005] In view of this, it is necessary to design an improved method for preparing lignin-based carbon fiber / polyaniline composite electrodes and their application in supercapacitors to solve the above problems. Summary of the Invention

[0006] The purpose of this invention is to provide a method for preparing a lignin-based carbon fiber / polyaniline composite electrode and its application in supercapacitors.

[0007] To achieve the above-mentioned objectives, in a first aspect, the present invention provides a method for preparing a lignin-based carbon fiber / polyaniline composite electrode, comprising the following steps:

[0008] S1. A spinning solution is prepared by dissolving lignin and polyacrylonitrile in N,N-dimethylformamide. A continuous fiber membrane is prepared by electrospinning. The membrane is then subjected to pre-oxidation and carbonization treatments to obtain lignin-based carbon fibers.

[0009] S2. Polyaniline is loaded onto the surface of the lignin-based carbon fiber obtained in step S1, thus obtaining the lignin-based carbon fiber / polyaniline composite electrode.

[0010] The loading of polyaniline is carried out in the following steps: in the presence of lignin-based carbon fibers, monomer aniline and oxidant ammonium persulfate are polymerized in situ, and polyaniline is loaded onto lignin-based carbon fibers.

[0011] Preferably, in step S1, the process parameters of the electrospinning process are as follows: the injection speed of the spinning solution is 1-3 mL / h, the receiving distance is 8-12 cm, the electric field voltage is 12-18 kV, and the drum rotation speed is 100-500 r / min; the pre-oxidation treatment is carried out in an oxygen atmosphere at a temperature of 200-220℃ for 1.5-2.5 h, and the heating rate from room temperature to the target temperature is 0.1-0.5℃ / min.

[0012] Preferably, in step S1, the pre-oxidation treatment is carried out in an oxygen atmosphere at a temperature of 200-220°C for 1.5-2.5 hours, and the temperature rises from room temperature to the target temperature at a rate of 0.1-0.5°C / min.

[0013] Preferably, in step S1, the mass ratio of lignin to polyacrylonitrile is 2:1, and the mass percentage of the spinning solution is 20 wt%.

[0014] Preferably, in step S1, the carbonization process is carried out under a nitrogen protective atmosphere at a temperature of 800°C for 1 hour.

[0015] Preferably, in step S2, the molar ratio of aniline to ammonium persulfate is 1:1.

[0016] Preferably, the heating rate during the carbonization process is 3°C / min.

[0017] Secondly, the present invention provides a lignin-based carbon fiber / polyaniline composite electrode, wherein the lignin-based carbon fiber / polyaniline composite electrode has a three-dimensional network structure with a pore size of 3.8 nm and a specific surface area of ​​186.5 m². 2 / g, with a porosity of 58%.

[0018] Thirdly, the present invention provides an application of a lignin-based carbon fiber / polyaniline composite electrode in a supercapacitor.

[0019] Fourthly, the present invention provides a supercapacitor comprising a positive electrode, a negative electrode, a separator, and an electrolyte. The positive and negative electrodes are both lignin-based carbon fiber / polyaniline composite electrodes, the separator is a hydrophilic polypropylene microporous separator, and the electrolyte is 1MH2SO4.

[0020] The beneficial effects of this invention are:

[0021] The preparation method provided by this invention employs in-situ polymerization to construct a uniform and continuous PANI coating layer on the surface of an LCF substrate prepared by electrospinning and carbonization. This approach not only effectively alleviates PANI aggregation and ensures efficient connectivity of conductive pathways, but also fully preserves the rapid ion transport channels provided by the three-dimensional network pore structure of LCFs. Electrochemical tests show that the optimal sample Is-LCFs-PANI-3 exhibits a high efficiency of 287.36 F g in a three-electrode system. -1 The specific capacitance, and at 5 A g -1 After 3000 cycles at high current density, the capacitance retention rate still reaches 86%; the symmetrical supercapacitor assembled with it can withstand 0.5 A g -1 The lower specific capacitance is 69.02 Fg. -1 After 3000 cycles, the retention rate was 78%, which indicates that the composite electrode prepared by the present invention has both high specific capacitance, excellent rate performance and long-term cycling stability. Attached Figure Description

[0022] Figure 1 This invention provides a method for preparing a lignin-based carbon fiber / polyaniline composite electrode.

[0023] Figure 2 The FTIR spectra and thermogravimetric analysis curves of lignin and PAN used in this invention are shown.

[0024] Figure 3 The results of three-dimensional TG-FTIR analysis of the gases released from the pyrolysis of lignin and PAN in this invention are shown below.

[0025] Figure 4 The SEM morphology and water contact angle results of the composite electrodes prepared in Examples 3, 7, and 11 of this invention are shown.

[0026] Figure 5 These are SEM-EDS elemental surface scan images of the composite electrodes prepared in Examples 3, 7, and 11 of this invention;

[0027] Figure 6 The crystal structure and surface chemical composition characterization results of the composite electrodes prepared by different polyaniline loading methods in Examples 1 to 12 of this invention;

[0028] Figure 7 The above are high-resolution XPS images of C 1s, N 1s and O 1s of the composite electrodes prepared in Examples 3, 7 and 11 of this invention.

[0029] Figure 8 The electrochemical performance results of the composite electrodes prepared under different conditions according to the present invention are shown.

[0030] Figure 9 The electrochemical performance characterization results of Is-LCFs-PANI-3 and its assembled symmetrical supercapacitor are presented in this invention. Detailed Implementation

[0031] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be described in detail below with reference to the accompanying drawings and specific embodiments.

[0032] It should also be noted that, in order to avoid obscuring the present invention with unnecessary details, only the structures and / or processing steps closely related to the present invention are shown in the accompanying drawings, while other details that are not closely related to the present invention are omitted.

[0033] Additionally, it should be noted that the terms “comprising,” “including,” or any other variations thereof are intended to cover non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such process, method, article, or apparatus.

[0034] Please see Figure 1 As shown, the present invention provides a method for preparing a lignin-based carbon fiber / polyaniline composite electrode, comprising the following steps:

[0035] S1. A spinning solution is prepared by dissolving lignin and polyacrylonitrile (PAN) in N,N-dimethylformamide (DMF). A continuous fiber membrane is prepared by electrospinning. The membrane is then subjected to pre-oxidation and carbonization treatments to obtain lignin-based carbon fibers (LCFs).

[0036] S2. Polyaniline (PANI) is loaded onto the surface of the lignin-based carbon fibers obtained in step S1 to obtain a lignin-based carbon fiber / polyaniline composite electrode, which has a three-dimensional network structure with a pore size of 3.8 nm and a specific surface area of ​​186.5 m². 2 / g, with a porosity of 58%.

[0037] In some embodiments, in step S1, the mass ratio of lignin to polyacrylonitrile is 2:1, and the mass percentage of the spinning solution is 20 wt%.

[0038] In some embodiments, the electrospinning process for preparing the fiber membrane in step S1 is as follows: The prepared spinning solution is drawn into a 5 mL syringe, clamped to the electrospinning machine's injection pump, and a 20 G needle is used to extrude the solution at a flow rate of 1-3 mL / h. Under a 12-18 kV high-voltage electric field, the jet is sprayed through a receiving distance of 8-12 cm onto a grounded roller collector coated with aluminum foil. By adjusting the roller speed to 100-500 r / min, the jet is stretched and oriented, and the fibers are uniformly deposited, thus completing the continuous preparation of the fiber membrane. The pre-oxidation treatment is carried out in an oxygen atmosphere at a temperature of 200-220℃ for 1.5-2.5 h, with a heating rate of 0.1-0.5℃ / min from room temperature to the target temperature. The carbonization treatment is carried out in a nitrogen protective atmosphere at a temperature of 800℃ for 1 h, with a heating rate of 3℃ / min during this process.

[0039] In some embodiments, in step S2, the method for loading polyaniline is direct impregnation (corresponding to...). Figure 1 Figure (b) in the middle), LS (sodium lignosulfonate) assisted impregnation method (corresponding to Figure 1 Figure (c) in the middle), in-situ polymerization method (corresponding to) Figure 1 One of the methods is shown in Figure (d). Specifically, the direct impregnation method is carried out according to the following steps: polyaniline is dispersed in a hydrochloric acid solution, and lignin-based carbon fibers are impregnated in the above solution to achieve polyaniline loading. The LS-assisted impregnation method is carried out according to the following steps: polyaniline is dispersed in a hydrochloric acid solution, and lignin sulfonate is added to the dispersion to improve the dispersibility of polyaniline according to the mass ratio of polyaniline to sodium lignin sulfonate of 1:0.5. The lignin-based carbon fibers are then impregnated in the above solution to complete the polyaniline loading.

[0040] The in-situ polymerization method is carried out according to the following steps: lignin-based carbon fibers are impregnated in an aniline (AN) monomer solution. An ammonium persulfate (APS) hydrochloric acid solution is added to the system at a rate of 1 mL / min, and the reaction is carried out at 0-5°C for 6 h to induce in-situ oxidative polymerization of aniline on the fiber surface. In this process, the monomer is directionally enriched by utilizing the electrostatic adsorption between protonated aniline and oxygen-containing functional groups on the LCF surface under acidic conditions. A low-temperature environment and a slow-dropping strategy of the oxidant effectively suppress the polymerization rate and avoid excessively high local concentrations, thereby guiding the uniform growth of polyaniline along the fiber surface and ultimately constructing a continuous and dense core-shell structure coating layer. Preferably, the molar ratio of aniline to ammonium persulfate in this step is 1:1.

[0041] The following specific embodiments further illustrate the preparation method of the lignin-based carbon fiber / polyaniline composite electrode proposed in this invention and its application in supercapacitors:

[0042] Example 1

[0043] This embodiment provides a method for preparing a lignin-based carbon fiber / polyaniline composite electrode, which includes the following steps:

[0044] S1. Lignin and PAN were dissolved in DMF (analytical grade) at a mass ratio of 2:1 to prepare a 20wt% spinning solution. After electrospinning to obtain a fiber membrane, it was vacuum dried at 80℃ for 12 h to completely remove residual solvent. Subsequently, it underwent pre-oxidation treatment in air atmosphere, with the temperature increased to 220℃ at a rate of 0.2℃ / min and held at that temperature for 2 h to stabilize the fiber structure. Next, carbonization was carried out under a nitrogen protective atmosphere, with the temperature increased from 220℃ to 800℃ at a rate of 3℃ / min and held at that temperature for 1 h. After completion, it was naturally cooled to room temperature to obtain lignin-based carbon fibers, denoted as LCFs, which were then cut into 1×1.5 cm pieces. 2 Prepare fragments (approximately 15-30 mg in weight);

[0045] S2. Disperse 5.4 mg of polyaniline in hydrochloric acid solution, and impregnate the lignin-based carbon fiber obtained in step S1 in the above solution to achieve polyaniline loading. The resulting composite electrode is denoted as LCFs-PANI.

[0046] FTIR spectra and thermogravimetric analysis curves of lignin and PAN are as follows: Figure 2 As shown, where Figure 2 Figure (a) shows the FTIR spectrum of lignin. Figure 2 Figure (b) shows the FTIR spectrum of PAN. Figure 2 Figure (c) shows the TG curves of lignin and PAN. Figure 2Figure (d) shows the DTG curves of lignin and PAN. Figures (a)-(b) show that lignin is rich in phenolic and alcoholic hydroxyl groups (3371.9 cm⁻¹). -1 Aliphatic CH bonds (2938.5 cm) -1 ), aromatic skeleton (1601.6 cm) -1 ) and β-O-4 ether bond structure (1123.3 cm) -1 This confirms that it has a typical phenylpropane structure; while PAN is located at 2244.7 cm⁻¹. -1 The peak exhibits a characteristic cyano (C≡N) stretching vibration, accompanied by the CH vibration of the methylene group in the main chain (2940.9 cm⁻¹). -1 The linear polymer configuration of lignin was clearly defined. Figures (c)-(d) show that the pyrolysis of lignin exhibits a multi-stage gradual characteristic. The wide weight loss range of 130-500℃ corresponds to the removal of adsorbed water (130-230℃) and the side chain cleavage and aromatic ring condensation (230-500℃). The DTG curve is broad and gentle, and the char residue at 800℃ reaches 40.1%. In contrast, the thermal weight loss of PAN is concentrated in the 300-450℃ range, and the DTG curve is sharp, corresponding to the rapid conversion process of cyano cyclization and dehydrogenation to form a heat-resistant trapezoidal structure. The char residue is 43.6%. The above results indicate that lignin possesses the basic pyrolysis characteristics as a carbon fiber precursor. Its char residue is slightly lower than that of PAN, so it can be blended with PAN to control the pyrolysis behavior and is suitable for the preparation of carbon fiber precursors.

[0047] The three-dimensional TG-FTIR analysis results of the gases released from the pyrolysis of lignin and PAN are as follows: Figure 3 As shown, the TG-FTIR test was conducted under a nitrogen atmosphere, with an initial temperature of 50℃, a heating rate of 10℃ / min, and an endpoint temperature of 800℃. Figure 3 Figure (a) shows the PAN three-dimensional TG-FTIR spectrum. Figure 3 Figure (b) shows the two-dimensional FTIR spectrum at the characteristic temperature of PAN. Figure 3 Figure (c) shows the three-dimensional TG-FTIR spectrum of lignin. Figure 3 Figure (d) shows the two-dimensional FTIR spectra of lignin at the characteristic temperature. Figures (a)-(b) show that PAN undergoes major physical changes within the range of 100-300℃, retaining a 2920 cm⁻¹ spectrum. -1 Aliphatic CH vibration at 2245 cm -1 The characteristic peak of the cyano group (C≡N) is observed at 1550-1600 cm⁻¹. When the temperature rises to 300-500℃, the C≡N peak significantly weakens, while the peak at 1550-1600 cm⁻¹ also decreases. -1The appearance and continuous enhancement of a new C=N vibrational peak at 1600 cm⁻¹ signifies the initiation of the cyanocyclization reaction and the formation of a nitrogen-containing heterocyclic structure; in the 500-700℃ range, the C≡N peak essentially disappears, and the functional group characteristics gradually converge with those at 1600 cm⁻¹. -1 The overlapping of aromatic C=C peaks at the location indicates further dehydrogenation and aromatization of the heterocyclic structure, leading to a transformation into a graphite-like conjugated structure. Figures (c)-(d) show that the pyrolysis process of lignin exhibits distinct stages. At 100-200℃, the pyrolysis peaks reach 2920 cm⁻¹. -1 The CH peak is dominant at 200-500℃; the period of severe weight loss occurs between 200-500℃, and the peak is at 2350 cm⁻¹. -1 The CO2 peak at 1700 cm⁻¹ -1 The carbonyl (C=O) peak at the pyrolysis site is significantly enhanced, corresponding to ether bond cleavage, aromatic ring decarboxylation, and the formation of carbonyl compounds, releasing a large amount of small molecule volatiles. At 500-700℃, the CH, -OH, and C=O peaks continuously weaken, but CO2 remains, corresponding to the deep condensation and aromatization of the residual aromatic ring, ultimately forming a carbon structure rich in oxygen-containing functional groups and defects. The pyrolysis behaviors of PAN and lignin are complementary; the rapid cyclization exothermic reaction of PAN promotes the pyrolysis initiation of lignin, while the slow pyrolysis of lignin alleviates the rapid weight loss of PAN, optimizing the thermal stability of the blend system. After carbonization at 800℃, both form a carbon fiber material with both a highly conductive network and abundant surface functional groups.

[0048] Examples 2 to 4

[0049] The only difference between Examples 2 to 4 and Example 1 is that the amount of polyaniline added during the loading of polyaniline in step S2 is different from that in Example 1. All other experimental parameters are the same as in Example 1 and will not be repeated here. The surface composite electrodes prepared in Examples 1 to 4 are respectively named Imp-LCFs-PANI-1 (Example 1), Imp-LCFs-PANI-2 (Example 2), Imp-LCFs-PANI-3 (Example 3), and Imp-LCFs-PANI-4 (Example 4).

[0050] Examples 5 to 8

[0051] The difference between Examples 5 to 8 and Example 1 is that in step S2, the polyaniline loading was carried out using the LS-assisted impregnation method. The amount of polyaniline used was the same as in Examples 1 to 4. The corresponding amount of sodium lignosulfonate was added according to a mass ratio of polyaniline to sodium lignosulfonate of 1:0.5, as shown in Table 1. All other experimental parameters were the same as in Example 1 and will not be repeated here. The composite electrodes obtained in Examples 5 to 8 are sequentially designated as Imp-LCFs-PANI-LS-1 (Example 5), Imp-LCFs-PANI-LS-2 (Example 6), Imp-LCFs-PANI-LS-3 (Example 7), and Imp-LCFs-PANI-LS-4 (Example 8).

[0052] Examples 9 to 12

[0053] The difference between Examples 9 to 12 and Example 1 is that in step S2, the loading of polyaniline is carried out by in-situ polymerization. The specific steps are as follows: the required mass of aniline is added to 1 mol / L hydrochloric acid, and ultrasonication is used to obtain a uniformly dispersed aniline monomer solution; the corresponding mass of APS is added to 1 mol / L hydrochloric acid, and pre-cooled to 0°C to obtain an APS solution. LCFs are immersed in the aniline monomer solution for thorough wetting, and then the APS solution is added dropwise to the aniline monomer solution at a rate of 1 mL / min. After the addition is completed, the reaction is continued at 0-5°C for 6 h to allow aniline to undergo in-situ oxidative polymerization on the fiber surface, thereby achieving the loading of polyaniline.

[0054] All other experimental parameters were the same as in Example 1, and will not be repeated here. The composite electrodes prepared in Examples 9 to 12 are respectively called Is-LCFs-PANI-1 (Example 9), Is-LCFs-PANI-2 (Example 10), Is-LCFs-PANI-3 (Example 11), and Is-LCFs-PANI-4 (Example 12). The amount of each raw material used when loading polyaniline in Examples 1 to 12 is shown in Table 1.

[0055] Table 1. Amounts of each raw material used in Examples 1 to 12

[0056]

[0057] The SEM morphology and water contact angle results of the composite electrodes prepared in Examples 3, 7, and 11 are as follows: Figure 4 As shown, where Figure 4 Figure (a) in the figure corresponds to the LCFs of Example 1. Figure 4 Figure (b) in the figure corresponds to the composite electrode of Example 3. Figure 4 Figure (c) in the figure corresponds to the composite electrode of Example 7. Figure 4Figure (d) corresponds to the composite electrode of Example 11. Figure (a) shows that the pure LCFs substrate exhibits a continuous, complete, and smooth fiber morphology, interwoven to form a loose three-dimensional network structure. Although it possesses excellent electron conduction channels, its inherent hydrophobicity limits the full wetting of the electrolyte. Figure (b) shows that although the Imp-LCFs-PANI-3 prepared by the direct impregnation method improves hydrophilicity to some extent, as the PANI loading increases, it is very easy to form large-sized aggregates on the fiber surface and block the pores, hindering the ion transport efficiency. After introducing LS-assisted impregnation (Figure (c)), the dispersibility of Imp-LCFs-PANI-LS-3 is significantly improved, and the hydrophilicity is significantly enhanced, but local particle accumulation still exists under high loading. In contrast, the in-situ polymerization method shows the best morphology control advantage, such as Figure 4 As shown in (d), Is-PANI-LCFs-3 constructs a uniform and continuous PANI coating layer on the surface of a single fiber, retaining a three-dimensional network with interconnected pores. The PANI loading of this composite electrode is 1.2 mg / cm³. 2 Its pore size is 3.8 nm, and its specific surface area is 186.5 m². 2 / g. PANI coating did not cause significant pore blockage, maintaining the ion transport channels of the three-dimensional network while providing abundant electrochemical reaction sites. Simultaneously, the material surface underwent a hydrophobic-to-hydrophilic transition, allowing the electrolyte to fully and smoothly wet all active sites, which is beneficial for efficient and rapid ion transport.

[0058] SEM-EDS elemental surface scan images of the composite electrodes prepared in Examples 3, 7, and 11 are shown below. Figure 5 As shown, where Figure 5 Figure (a) in the figure corresponds to the LCFs of Example 1. Figure 5 Figure (b) in the figure corresponds to the composite electrode of Example 3. Figure 5 Figure (c) in the figure corresponds to the composite electrode of Example 7. Figure 5 Figure (d) in the table corresponds to the composite electrode of Example 11. The comparison results of C, N, and O contents in the composite electrodes prepared under the corresponding conditions are shown in Table 2. Figure 5As shown in Table 2, the original LCFs exhibited the strongest and most uniform C element signal. The N and O elements were 15.55% and 4.18%, respectively. The N element primarily originated from the nitrogen-containing structures remaining from the cyano groups in the PAN molecular chain during carbonization. The low O element content indicates that the original carbon fiber surface was inert and hydrophobic, with a limited number of functional groups. While the Imp-LCFs-PANI-3 prepared by the direct impregnation method increased the N content to 18.45% and the O content to 5.42%, confirming the successful loading of PANI, the elemental mapping showed that the N signal was patchy, indicating that PANI existed as discontinuous, large-sized aggregates with extremely uneven coating. After introducing the LS dispersant, the N and O contents of Imp-LCFs-PANI-LS-3 (18.27% and 5.55%, respectively) did not show a significant increase. Although the elemental distribution was slightly more diffuse than the former, it still did not achieve the ideal continuous and uniform coating, indicating that physical dispersion can only slightly improve dispersibility and cannot fundamentally solve the aggregation problem. In contrast, the Is-PANI-LCFs-3 prepared by in-situ polymerization exhibited drastically different elemental distribution characteristics: the C content decreased to 70.33% due to polymer coating, while the N and O contents increased significantly to 19.34% and 10.34%, respectively. Elemental surface scans clearly showed that the C, N, and O signals were seamlessly connected and highly uniformly distributed throughout the three-dimensional skeleton. This not only confirmed that in-situ polymerization achieved high PANI loading and dense coating, but also revealed that the process induced abundant oxygen-containing functional groups on the carbon fiber surface. These newly added active sites facilitated the uniform coating of polyaniline on the carbon fiber surface.

[0059] Table 2 Comparison of C, N, and O contents in the composite electrodes prepared in Examples 3, 7, and 11

[0060]

[0061] The crystal structure and surface chemical composition characterization results of the composite electrodes prepared by different polyaniline loading methods in Examples 1 to 12 are as follows: Figure 6 As shown, where Figure 6 Figure (a) in the figure corresponds to the XRD patterns of the composite electrodes prepared in Examples 1 to 4. Figure 6 Figure (b) in the figure corresponds to the XRD patterns of the composite electrodes prepared in Examples 5 to 8. Figure 6 Figure (c) in the figure corresponds to the XRD patterns of the composite electrodes prepared in Examples 9 to 12. Figure 6 Figure (d) in the figure corresponds to the Raman spectra of the composite electrodes prepared in Examples 3, 7, and 11. Figure 6 Figure (e) in the figure corresponds to the XPS full spectrum of the composite electrodes prepared in Examples 3, 7, and 11. Figure 6Figure (f) in the figure corresponds to the elemental radar diagram of the composite electrodes prepared in Examples 3, 7, and 11. The XRD results in Figures (a)-(c) show that LCFs exhibit a broadened diffraction peak of the amorphous carbon (002) crystal plane at 2θ≈23°, which corresponds to the (002) crystal plane of amorphous carbon, indicating that LCFs are mainly composed of amorphous carbon structure and have low crystallinity; while PANI shows characteristic diffraction peaks near 2θ≈20° and 25°, which corresponds to the intrinsic crystal structure of PANI chain segments arranged in an orderly manner, and LS shows sharp crystallization peaks at 2θ≈30° and 45°. Although the Imp-LCFs-PANI series retains the broad peaks of the carbon substrate, the peak positions shift to higher angles and the intensity increases. This is the result of the overlap between PANI and carbon peaks. Moreover, with the increase of loading, the peak intensity in the 2θ≈20-25° region continues to increase, reflecting the improvement of PANI crystallinity. After the introduction of Raman spectroscopy (LS), the Imp-LCFs-PANI-LS series retained the characteristics of the carbon substrate while superimposing the weak, sharp crystallization peaks of LS with the crystallization signal of PANI, without damaging the carbon fiber skeleton, indicating that LS effectively improved the PANI distribution as a dispersant. In contrast, the Is-LCFs-PANI series not only exhibited stronger amorphous carbon envelope peaks, but also higher peak intensities and wider half-maximum widths in the 2θ≈20-25° region, indicating that the in-situ polymerization-induced PANI had higher crystallinity and a tighter bond with the substrate, without introducing new crystalline phases. The Raman spectra in Figure (d) show that all samples reached peak values ​​above 1350 cm⁻¹. -1 (D peak) and 580 cm -1A characteristic peak appears at (G peak), with LCFs exhibiting the highest ID / IG value (1.19), revealing its highly defective and disordered structure. The ID / IG value of Imp-LCFs-PANI-3 (1.15) decreases slightly, indicating that the PANI coating slightly modifies surface defects; the ID / IG value of Imp-LCFs-PANI-LS-3 (1.18) rebounds, confirming that LS did not change the carbon skeleton and had limited dispersing effect; while Is-PANI-LCF-3 shows the lowest ID / IG value (1.04), indicating that the uniform coating layer formed by in-situ polymerization effectively repairs carbon surface defects and significantly improves graphitization order. The XPS full spectrum in Figure (e) and the elemental quantitative analysis in Figure (f) further reveal the evolution of chemical composition: all samples show characteristic peaks at 284.8 eV (C1s), 400 eV (N1s), 532 eV (O1s), 200 eV (Cl2p), and 163 eV (S2p). The results show that after loading PANI, the N 1s and O 1s signals are significantly enhanced. The N atom proportion in Is-LCFs-PANI-3 is as high as 11.95%, directly attributed to the nitrogen-containing conjugated structure of PANI. The O atom proportion in Imp-LCFs-PANI-LS-3 surges to 13.47%, mainly due to the large number of oxygen-containing functional groups introduced by LS and the auxiliary role played by the small number of oxygen-containing groups introduced during the PANI polymerization process. The coexistence of Cl 2p and S 2p peaks strongly confirms the successful loading of PANI and LS dopant ions onto the carbon fiber surface.

[0062] Furthermore, the high-resolution XPS images of C 1s, N 1s, and O 1s of the composite electrodes prepared in Examples 3, 7, and 11 are shown below. Figure 7 As shown, where Figure 7 Figures (a)-(c) in the figure correspond to LCFs. Figure 7 Figures (d)-(f) in the diagram correspond to Imp-LCFs-PANI-3. Figure 7 Figures (g)-(i) in the diagram correspond to Imp-LCFs-PANI-LS-3. Figure 7Figures (j)-(l) in the table correspond to Is-LCFs-PANI-3. The results show that the C 1s high-resolution spectrum reveals that the CN / CO characteristic peak area at 286.3 eV in the PANI-loaded sample is significantly larger than that in pure LCFs, indicating that PANI was successfully grafted onto the LCFs surface through covalent / non-covalent interactions, constructing a continuous conductive network and providing a structural basis for rapid electron transport. The N 1s spectrum, after peak fitting, yielded three nitrogen species: -NH⁺= (~401.8 eV), -NH- (~400.1 eV), and =N- (~398.5 eV), consistent with the typical chemical structure of conductive PANI. The high proportion of -NH⁺= and -NH- can serve as highly efficient active sites, providing abundant pseudocapacitive contributions to the material and significantly enhancing the intrinsic conductivity of the carbon matrix. In the O 1s spectrum, the total peak intensity and the peak area of ​​the CO functional group at ~533.2 eV of Imp-LCFs-PANI-LS-3 are much higher than those of the other samples. This is mainly attributed to the successful introduction of a large number of oxygen-containing functional groups such as sulfonic acid groups, hydroxyl groups, and carboxyl groups in the LS molecule. The O 1s peak intensity of Is-LCFs-PANI-3 and Imp-LCFs-PANI-3 is also higher than that of pure LCFs. This is due to the small number of oxygen-containing groups introduced during the PANI polymerization process. These polar oxygen-containing groups can not only provide additional pseudocapacitive active sites, but also effectively improve the surface wettability of the material and promote the diffusion and transport of electrolyte ions.

[0063] Application Examples

[0064] This application example investigates the electrochemical performance of the composite electrodes prepared in Examples 1 to 12. The specific method is as follows: LCFs and the composite electrode are used as working electrodes, Ag / AgCl electrode is used as reference electrode, platinum sheet is used as auxiliary electrode, and 1 M H2SO4 solution is used as electrolyte.

[0065] The electrochemical performance of the working electrode was evaluated using cyclic voltammetry (CV), galvanostatic charge-discharge (GCD), and cycle performance testing. A three-electrode system and an electrochemical workstation (Autolab PGSTAT204, Metrohm, Switzerland) were used to test the electrochemical performance of all samples at room temperature. A 1×1.5 cm... 2 Carbon fiber samples were weighed and then immersed in the electrolyte for 12 h. The carbon fiber was used directly as the working electrode, with a platinum sheet (1×1 cm) as the electrode. 2 The Is-LCFs-PANI-3 electrode serves as the counter electrode, and the Ag / AgCl electrode serves as the reference electrode. In this two-electrode system, two identical Is-LCFs-PANI-3 electrodes are assembled into a symmetrical capacitor. The direct-use dimensions are 1 cm × 1.5 cm.2 A sample of approximately 1 mg was used without the addition of any binder or conductive agent. Cyclic voltammetry (CV) was performed within a potential window of 0 to 1 V at scan rates of 5, 10, 20, 50, and 100 mV / s. Galvanostatic charge-discharge (GCD) tests were performed at current densities of 0.5, 1, 2, and 5 A g⁻¹. Specific capacitance was calculated based on the GCD curves. , where C(F g) -1 Δt(s) is the mass specific capacitance, I(A) is the discharge current, Δt(s) is the discharge time, m(g) is the weight of the electrochemically active material of the working electrode, and ΔV(V) is the potential window.

[0066] The electrochemical performance results of composite electrodes prepared under different conditions are as follows: Figure 8 As shown, where Figure 8 Figures (a)-(c) show the CV curves at a scan rate of 10 mV / s. Figure 8 Figures (d)-(f) in the figure represent 0.5 A g. -1 GCD curves at current density Figure 8 Figures (g)-(i) show the rate performance curves. Figures (a)-(c) show that all sample curves are approximately rectangular, exhibiting typical ideal capacitance behavior. Compared with LCFs, the area enclosed by the CV curve of the PANI-loaded composite material is significantly increased, indicating a significant improvement in charge transfer kinetics at the electrode / electrolyte interface and a higher specific capacitance. Among them, Is-LCFs-PANI-3 has the largest CV area, followed by Imp-LCFs-PANI-LS-3, indicating that both in-situ polymerization and LS-assisted dispersion of PANI can effectively improve electrochemical activity. Figures (d)-(f) further corroborate this trend. All samples exhibit highly symmetrical approximately isosceles triangles, demonstrating excellent electrochemical reversibility. Compared with pure LCFs, the discharge time of the PANI-loaded samples is significantly prolonged, especially Is-LCFs-PANI-3, whose longest discharge time directly reflects the effectiveness of the in-situ polymerization strategy in leveraging the pseudocapacitive advantage of PANI. The specific capacitance results calculated based on GCD in Figures (g)-(i) show that the performance of all PANI-loaded samples significantly exceeds that of the original LCFs. This is a result of the synergistic effect of the pseudocapacitive contribution of PANI and the conductive network of the carbon fiber. Specifically, in the impregnated samples without LS, Imp-LCFs-PANI-3 at 0.5 A g -1 The specific capacitance is 139.91 F g. -1 After introducing LS-assisted dispersion, the specific capacitance of Imp-LCFs-PANI-LS-3 increased to 259.18 F g. -1This is because the sulfonic acid and hydroxyl groups abundant in LS not only provide additional pseudocapacitive sites but also significantly enhance the hydrophilicity and ion transport rate of the electrode surface. In contrast, the in-situ polymerized series Is-LCFs-PANI-3 showed the best performance at 0.5 A g. -1 It achieved a high of 287.36 F g. -1 Despite the fact that excessive PANI loading can lead to a performance decline due to agglomeration and pore blockage, this sample still highlights the unique advantages of in-situ polymerization technology in constructing high-efficiency composite electrodes due to its uniform and continuous structure, excellent conductivity, and abundant active sites.

[0067] The electrochemical performance characterization results of Is-LCFs-PANI-3 and its assembled symmetric supercapacitor are as follows: Figure 9 As shown, where Figure 9 Figure (a) shows the CV curves of Is-LCFs-PANI-3 at different scan rates. Figure 9 Figure (b) shows the GCD curves under different current densities. Figure 9 Figure (c) in the figure is 5 A g -1 Cyclic stability under the following conditions Figure 9 Figure (d) shows the CV curves of the symmetrical supercapacitor assembled from Is-LCFs-PANI-3 / / Is-LCFs-PANI-3 at different scan rates. Figure 9 Figure (e) shows the GCD curves of the symmetrical supercapacitor assembled from s-LCFs-PANI-3 / / Is-LCFs-PANI-3 at different current densities. Figure 9 Figure (f) shows a symmetrical supercapacitor assembled from s-LCFs-PANI-3 / / Is-LCFs-PANI-3 at 5 A g. -1 Cyclic stability under [condition / condition].

[0068] The CV curves maintained a typical rectangular profile and distinct redox peaks across a wide scan rate range of 5–100 mV / s, with the enclosing area expanding synchronously with the scan rate. This confirms that the energy storage mechanism originates from the synergistic effect of the electric double-layer capacitance and PANI pseudocapacitance, and possesses rapid charge transport kinetics. The GCD curves showed a similar pattern across the range of 0.5–5 A g. -1 Within the current density range, it exhibits a highly symmetrical isosceles triangle and a clear charge / discharge plateau, demonstrating excellent rate performance and electrochemical reversibility. Although the specific capacitance decreases regularly with increasing current density (attributed to the ion diffusion rate lagging behind the electron transfer rate at high currents, leading to incomplete bulk redox reactions), the overall decrease is controllable. At 5 A g... -1In 3000 cycle tests, the electrode exhibited excellent long-term stability. The initial slight increase in specific capacitance was due to the gradual wetting of the electrolyte, which activated the deep active sites. Subsequently, the retention rate stabilized at 86%, highlighting the anchoring effect of the in-situ coating structure on the active material. This fully demonstrates that Is-LCFs-PANI-3, as a supercapacitor electrode material, has significant structural stability and electrochemical durability. To further verify its practical application potential, a symmetrical supercapacitor was assembled using Is-LCFs-PANI-3 as the positive and negative electrodes. The specific assembly process is as follows: two Is-LCFs-PANI-3 electrodes of the same size were used as the positive and negative electrodes, with a hydrophilic polypropylene microporous membrane (purchased from Shanghai Yanwen Technology Co., Ltd., model XS100) sandwiched in between. Then, a 1mol / L sulfuric acid aqueous solution was injected as the electrolyte, and the encapsulation was completed in a standard CR2032 button cell casing. The device-level CV and GCD curves (Figures (d)-(f)) are at 100 mV / s and 5 A g, respectively. -1 It maintained good rectangular-like characteristics and charge / discharge symmetry at 0.5 A g. -1 The following was achieved: 69.02 F g -1 The specific capacitance is high; after 3000 cycles, the capacitance retention rate of the device still reaches 78%, which fully demonstrates that the symmetrical supercapacitor has good structural stability and electrochemical durability, providing support for its application in energy storage devices.

[0069] The above embodiments are only used to illustrate the technical solutions of the present invention and are not intended to limit it. Although the present invention has been described in detail with reference to preferred embodiments, those skilled in the art should understand that modifications or equivalent substitutions can be made to the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention.

Claims

1. A method for preparing a lignin-based carbon fiber / polyaniline composite electrode, characterized in that, Includes the following steps: S1. A spinning solution is prepared by dissolving lignin and polyacrylonitrile in N,N-dimethylformamide. A continuous fiber membrane is prepared by electrospinning. The membrane is then subjected to pre-oxidation and carbonization treatments to obtain lignin-based carbon fibers. S2. Polyaniline is loaded onto the surface of the lignin-based carbon fiber obtained in step S1, thus obtaining the lignin-based carbon fiber / polyaniline composite electrode. The loading of polyaniline is carried out in the following steps: in the presence of lignin-based carbon fibers, monomer aniline and oxidant ammonium persulfate are polymerized in situ, and polyaniline is loaded onto lignin-based carbon fibers.

2. The preparation method according to claim 1, characterized in that, In step S1, the process parameters of the electrospinning process are as follows: the injection speed of the spinning solution is 1-3 mL / h, the receiving distance is 8-12 cm, the electric field voltage is 12-18 kV, and the roller speed is 100-500 r / min.

3. The preparation method according to claim 1, characterized in that, In step S1, the pre-oxidation treatment is carried out in an oxygen atmosphere at a temperature of 200-220°C for 1.5-2.5 hours. During this process, the temperature rises from room temperature to the target temperature at a rate of 0.1-0.5°C / min.

4. The preparation method according to claim 1, characterized in that, In step S1, the mass ratio of lignin to polyacrylonitrile is 2:1, and the mass percentage of the spinning solution is 20 wt%.

5. The preparation method according to claim 1, characterized in that, In step S1, the carbonization process is carried out under a nitrogen protective atmosphere at a temperature of 800°C for 1 hour.

6. The preparation method according to claim 1, characterized in that, In step S2, the molar ratio of aniline to ammonium persulfate is 1:

1.

7. The preparation method according to claim 5, characterized in that, The heating rate during the carbonization process is 3°C / min.

8. A lignin-based carbon fiber / polyaniline composite electrode prepared by the method according to any one of claims 1-7, characterized in that, The lignin-based carbon fiber / polyaniline composite electrode has a three-dimensional network structure with a pore size of 3.8 nm and a specific surface area of ​​186.5 m². 2 / g, with a porosity of 58%.

9. The application of a lignin-based carbon fiber / polyaniline composite electrode prepared by any one of claims 1-7 or the lignin-based carbon fiber / polyaniline composite electrode of claim 8 in a supercapacitor.

10. A supercapacitor, characterized in that, It includes a positive electrode, a negative electrode, a separator, and an electrolyte. The positive electrode and the negative electrode are lignin-based carbon fiber / polyaniline composite electrodes prepared by the preparation method of any one of claims 1-7, or lignin-based carbon fiber / polyaniline composite electrodes as described in claim 8. The separator is a hydrophilic polypropylene microporous separator, and the electrolyte is 1M H2SO4.

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