Electrospun flexible lignin/polyacrylonitrile-based carbon nanofiber and its application in electrode materials for supercapacitors

Abstract

In this study, a lignin/polyacrylonitrile (PAN) composite nanofiber membrane is prepared by electrospinning and used as the precursor to prepare flexible carbon nanofibers (CNFs) through pre-oxidation and carbonization. The micromorphology, crystal structure, pore size distribution and specific surface area of the CNFs are characterized by field emission scanning electron microscopy, X-ray diffraction, Raman spectroscopy and specific surface adsorption analysis, respectively. The electrochemical properties of the CNF membrane are also investigated by cyclic voltammetry, galvanostatic charge–discharge and electrochemical impedance spectroscopy due to its potential application in binder-free electrode materials for supercapacitors. We successfully prepared flexible CNFs with an average diameter of about 539 nm and a specific surface area of 1053.78 m²/g when the mass ratio of lignin to PAN was 9:1 in a solution concentration of 28 wt%. The CNFs are loaded onto nickel foam to prepare the electrode materials for supercapacitors without a binder. When the current density is 0.5 A/g, the specific capacitance could be up to 201.27 F/g and the equivalent series resistance is only 0.57 Ω, which shows an excellent electrochemical performance. This study not only provides a theoretical basis for the high-value utilization of lignin and the preparation of flexible lignin/PAN-based CNFs, but also provides a new type of environmentally friendly raw material for the electrodes of supercapacitors and could be helpful to alleviate the energy crisis and environmental pollution.

Keywords

Electrospinning alkaline lignin polyacrylonitrile carbon nanofiber supercapacitor

With the rapid development of industrialization and the massive consumption of fossil fuels, it is urgent to develop energy storage equipment with renewable and environment-friendly materials. Among the types of energy storage devices, the supercapacitor has the characteristics of high power density, high energy density, superior low-temperature performance and a wide range of operating temperatures, as well as the advantages of long service life, economy and environmental protection. Therefore, it is considered as the most promising energy storage device and has become a research hotspot in the field of new energy.¹,² As an important component of a supercapacitor, the electrode largely determines the specific capacitance and other electrochemical performance of the supercapacitor.

Carbon nanofibers (CNFs) have potential application in the field of supercapacitors because of their superior chemical stability, excellent conductivity, high specific surface area and easy control of pore size distribution.³,⁴ According to the existing literature reports, the precursors used to prepare CNFs mainly include viscose, pitch and polyacrylonitrile (PAN). However, carbon fibers prepared from these precursor materials have corresponding shortcomings, such as the low carbonization yield of viscose-based carbon fibers, the complicated preparation process of pitch-based carbon fibers and the high production cost of PAN-based carbon fibers, which limit the application and development of carbon fibers to a certain extent.⁵,⁶ Therefore, it has become an inevitable development trend to find new renewable and easily degradable raw materials to produce carbon fibers.

Lignin is the second-largest renewable organic resource in nature, next only to cellulose. It is the only biomass polymer with a large number of aromatic rings and a carbon content of up to 60%, which is an ideal raw material for the preparation of CNFs. However, the current utilization rate of lignin is only 10% due to its complex structure; most lignin is discharged or burned as waste, which not only is a waste of resources but also seriously pollutes the environment.⁷,⁸ Therefore, the high-value development and utilization of lignin have been the focus of academic research in recent years.

There are many traditional methods for preparing nanofibers, mainly including chemical vapor deposition, the template method and the solid-state synthesis method, but these methods have some problems, such as high cost, complicated process and low product purity.⁹,¹⁰ Compared with the above methods, electrospinning is an effective method for the preparation of continuous nanofibers due to its low cost, simple operation and controllable process.¹¹,¹²

Due to the high polydispersity index and poor spinnability of lignin solution, pure continuous nanofibers cannot be formed by electrospinning technology, while PAN is the most commonly used polymer to prepare CNFs. Therefore, alkaline lignin was used as the main spinning raw material and PAN as the companion spinning polymer in this study. The lignin content could be increased to 90% by optimizing the solvent ratio, and the surfaces of the obtained nanofibers were smooth without beads. The corresponding lignin/PAN composite CNFs were also successfully prepared through pre-oxidation and carbonization processes. The effects of carbonization temperature and the addition of lignin on the morphology, crystal structure, specific surface area and pore size distribution of CNFs were studied. The prepared CNFs were combined with nickel foam to prepare electrode materials for supercapacitors, and the energy storage mechanism and electrochemical properties were discussed.

The results show that the specific capacitance is 201.27 F/g (current density is 0.5 A/g) and the internal resistance is only 0.57 Ω, which has good energy storage properties and is expected to be used as electrode material for supercapacitors. This study not only realizes the high-value utilization of lignin to the greatest extent, but also solves the problems of brittleness of CNFs obtained through existing technology, and the complex process of assembling energy storage devices. This study provides a theoretical basis for preparation of flexible lignin/PAN-based CNFs (PAN-CNFs), and also provides a new type of environmental raw material for electrodes of supercapacitors. Therefore, our work is helpful to alleviate the energy crisis and environmental pollution.

Experimental details

Materials

Alkaline lignin (M_w = 505.01) was purchased from Shanghai Macklin Biochemical Co., Ltd. PAN (M_w = 100,000) was provided by Shanxi Hengtian Textile New Fiber Technology Co., Ltd. N,N-dimethylformamide (DMF) and dimethyl sulfoxide (DMSO) were purchased from Tianjin Fuyu Fine Chemical Co., Ltd and Sinopharm Chemical Reagent Co., Ltd, respectively. Potassium hydroxide (KOH) was purchased from Tianjin Fengchuan Chemical Co. Ltd. All chemicals were used directly without any purification.

Preparation of lignin/PAN-based CNFs

Preparation of the spinning solution

At room temperature, PAN was dissolved in a mixed solvent of DMF and DMSO with the ratio of 60/40. After it was completely dissolved, lignin (the mass ratio of the lignin and PAN was 90/10) was added to prepare the spinning solution by mechanical stirring (rotating speed 200 rpm) until the solution was evenly mixed. Spinning solutions with concentrations of 25, 28, 30 and 35 wt% were prepared respectively.

Electrospinning process

The prepared spinning solution was used to fill a syringe, which was controlled by a microsyringe pump (customized) to supply the solution for electrospinning. During the electrospinning, the positive pole and negative pole of the high-voltage power supply (DW-P503-1ACDF, China) were connected with the syringe needle and the collecting roller. The surface of the collecting roller is covered with a layer of aluminum foil to collect the composite fiber membrane. The polymer solution was electrospun in a custom-built apparatus using a 0.8 mL/h feed rate, 12 cm tip-to-collector distance, 20 kV potential and 300 rpm collector rotation rate. The composite fiber membrane was removed from the aluminum foil and placed in a vacuum drying oven at 80°C for 2 h, in order to remove the residual solvent in the electrospun fiber. During spinning, the ambient temperature and relative humidity were 33 ± 2°C and 31 ± 3%RH, respectively.

Stabilization and carbonization

The lignin/PAN composite fiber membrane as the precursor was cut into a rectangle with the length of 5.5 cm and the width of 4.0 cm, which was heated from room temperature to 240°C with the heating rate of 1°C/min, and pre-oxidized for 1 h in a muffle furnace with an air atmosphere. The pre-oxidized fibers were carbonized in the quartz boat of a tubular furnace. The carbonization process was carried out in a nitrogen atmosphere with the heat treatment conditions as follows: the temperature was raised from room temperature at 5°C/min to 800°C, 1000°C, 1200°C and 1400°C, respectively, and kept for 1 h, then cooled to room temperature at 10°C/min to prepare lignin/PAN-CNFs, which were expressed as LP-CNFs800, LP-CNFs1000, LP-CNFs1200 and LP-CNFs1400, respectively (LP-CNFs800, LP-CNFs1000, LP-CNFs1200 and LP-CNFs1400 were all prepared from the precursor solution with the concentration of 30 wt%). The effect of carbonization temperature on properties of lignin/PAN-CNFs was investigated. Besides, PAN-CNFs obtained from the solution concentration of 10 wt% were prepared as a reference at the calcination temperature of 1200°C at 5°C/min, which was also adopted by the CNFs obtained from 28 wt% lignin/PAN solution in order to explore the effect of solution concentration (28 and 30 wt%) on the properties of lignin/PAN-CNFs.

Preparation of electrode materials for supercapacitors

To investigate the effect of the addition of lignin on the properties of CNFs, the detailed preparation steps are shown in Figures 1(a)–(e). The CNF membrane was cut into a square of 1 cm × 1 cm with a mass of about 1.3–1.5 mg, which was loaded between two nickel foams under the pressure of 8 MPa without adding any binder to produce electrode materials for supercapacitors (Figure 1(f)). Before testing, the assembled carbon electrode materials were immersed in 6 mol/L potassium hydroxide (KOH) solution for 24 h.

Figure 1.

Schematic illustration of the process by using lignin/polyacrylonitrile (PAN) (the solution concentration is 30 wt%) as a precursor to prepare carbon nanofibers (CNFs) as electrode materials for supercapacitors (the raw fibers themselves are brownish, as shown in Figure 1(c); color online only). DMF: N,N-dimethylformamide; DMSO: dimethyl sulfoxide.

Characterization

The morphology of the nanofibers was investigated with a field emission scanning electron microscope (Quanta-450-FEG, USA) operated at 20 kV. The magnification of field emission scanning electron microscopy (FE-SEM) images is 5000. The average fiber diameter of each sample was determined by random measurements of 100 nanofibers with ImageJ software. The crystal structure of CNFs was measured by X-ray diffraction (XRD, Dmax-Rapid II, Japan) equipped with Cu radiation with the scanning rate and range of 5°C/min and 10°–80°, respectively.

The grain size (L_c, nm) and the interplanar spacing (d, nm) were calculated from the XRD spectrum with the following equations¹³,¹⁴

L c = \frac{k λ}{β cosθ}

(1)

d = \frac{λ}{2 s in θ}

(2)

where k represents the Scherrer constant, which is 0.9; λ represents the X-ray wavelength, which is 0.154 nm in this experiment; β (rad) represents the half-height width of the diffraction peak; θ (°) represents the Bragg diffraction angle.

The degree of graphitization of CNFs was measured by a Raman spectrometer (InVia, Renishaw Co., UK) with an excitation wavelength of 514 nm. The D peak at 1340 cm⁻¹ and the G peak at 1590 cm⁻¹ corresponding to the sp² and sp³ hybridization of carbon can be observed with the Raman spectroscopy.¹⁵,¹⁶ Generally, the ratio of the relative intensity of the D peak to the G peak is represented by R, that is, R = I_D/I_G, and the lower the R value, the higher the graphitization degree of the carbon materials.¹⁷,¹⁸ The microcrystalline size (L_a) of CNFs is inversely proportional to the R value with the functional relationship of L_a = 4.4/R.¹⁹

The specific surface area and pore size distribution of the CNFs were carried out by nitrogen adsorption–desorption in a Gemini VII2390 Specific Surface Adsorption Analyzer.

The electrochemical properties of the electrode materials were evaluated by cyclic voltammetry (CV), galvanostatic charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS) through a CHI 660E electrochemical workstation purchased from Shanghai Chenhua Instrument Company. The three electrodes included the working electrode (WE) made of CNFs, the reference electrode (RE) made of saturated calomel and the auxiliary electrode (CE) made of platinum, and the electrolyte was 6 mol/L KOH solution. The CV tests were measured with the potential window from –1 to 0 V at the scanning rates of 5, 10, 20, 75 and 100 mV/s. The GCD tests were measured at the current densities of 0.5, 1, 2, 5, 10 and 20 A/g. The EIS tests were measured with the frequency ranging from 0.01 Hz to 100 kHz.

The specific capacitance (C, F/g) was calculated from the CV curves and GCD curves according to the following equations²⁰,²¹

C = \frac{S}{2 m V Δ V}

(3)

C = \frac{I Δ t}{m Δ V}

(4)

where m (g) is the mass of the carbon electrode, V (mV/s) is the scanning rate and ΔV (V), I (A) and Δt (s) designate the voltage window, discharge current and discharge time, respectively.

Results and discussion

Effect of solution concentration on the morphology of precursor fibers

The morphologies of the precursor fibers prepared under the conditions of mass ratio of lignin/PAN = 90/10, V_DMSO/V_DMF = 60/40 and different solution concentrations are shown in Figure 2. It can be seen that there are beads in the obtained fiber when the solution concentration is 25 wt%. This is mainly because the entanglement between the PAN and the lignin molecular chain is not enough for the small solution concentration, and the spinning jet cannot effectively resist the stretching effect of the electric field force, resulting in the spinning jet breaking and the aggregation of the molecular chain, thus forming the bead fiber. When the solution concentration is increased to 30 wt%, good fiber morphology with no beads can be obtained due to the stabilization of the spinning jet. With further increase of the solution concentration to 35 wt%, the solution will easily condense at the needle tip due to the small amount of solvent and the high viscosity, which are not conducive to a smooth spinning process.

Figure 2.

Field emission scanning electron microscopy images of composite fiber membranes prepared under different solution concentrations: (a) 25 wt%; (b) 30 wt%; (c) 35 wt%.

Preparation of pre-oxidized lignin/PAN fibers

The macro and micromorphology of the raw lignin/PAN composite fiber membrane and the pre-oxidized composite fiber membrane obtained by the spinning solution with the concentration of 30 wt% are shown in Figure 3. It can be seen that the raw composite fiber membrane shows a bead-free and smooth morphology with a uniform diameter distribution. Pre-oxidation is an essential step in the preparation process of carbon fibers, and the performance of pre-oxidized fibers determines the comprehensive performance of carbon fibers. At the heating rate of 1°C/min from room temperature to 240°C and holding time of 1 h, pre-oxidized fibers do not melt and their diameter distribution is relatively uniform, maintaining the good fiber morphology of the original filament. The color of the fiber membrane gradually changes from light brown to dark brown because the C≡N in the molecular chain was transformed into C=N and a conjugated system was formed between the chromogenic groups during the pre-oxidation.²²

Figure 3.

Macro and micromorphology: (a) raw lignin/polyacrylonitrile (PAN) composite fiber membrane; (b) pre-oxidized lignin/PAN composite fiber membrane.

Structural characteristics of CNFs

Fiber morphology

FE-SEM images of CNFs prepared at different carbonization temperatures are shown in Figure 4. It can be seen that the surface of the obtained CNFs is smooth and free of adhesion at different carbonization temperatures, which maintains structure of the pre-oxidized fibers. During the pre-oxidation process, macromolecular chains crosslink with each other, forming a heat-resistant and stable trapezoidal structure, which avoids the phenomenon of fiber melting and adhesion in the carbonization process. When the carbonization temperature increased from 800°C to 1200°C, the average diameter of CNFs decreased from 896 to 730 nm. When the temperature exceeded 1200°C, some carbon fibers broke due to the destruction of the structure of macromolecular chains in the carbon fibers. Therefore, in terms of fiber morphology and preparation cost, the optimum carbonization temperature is 1200°C. Compared with PAN-CNFs1200, the fiber diameter of LP-CNFs1200 decreased slightly, which indicates that the addition of lignin could reduce the diameter of carbon fibers. In addition, the carbon fibers prepared with different temperatures in this experiment have good flexibility (see Figure 4(f)), and no binder is needed when they are used as electrodes for supercapacitors. It can simplify the fabrication process of the electrode and also avoid the blockage of the pore structure, which helps to reduce the equivalent series resistance (ESR) of the electrode. It lays a foundation for the subsequent practical application of CNFs.²³

Figure 4.

Field emission scanning electron microscopy images of carbon nanofibers (CNFs) prepared at different carbonization temperatures: (a) LP-CNFs800-800°C; (b) LP-CNFs1000-1000°C; (c) LP-CNFs1200-1200°C; (d) LP-CNFs1400-1400°C; (e) PAN-CNFs1200-1200°C; (f) flexible display of LP-CNFs1200-1200°C.

Crystal structure

In order to further understand the crystal structure changes of the prepared lignin/PAN-CNFs, XRD and Raman tests were carried out. As shown in Figure 5, the XRD spectrum of the samples is basically similar with the same diffraction peaks at 23°, 44° and 51° corresponding to the crystallographic planes of (002), (100) and (004) in graphite crystals, respectively.²⁴ According to the Scherrer formula and the Bragg formula, the grain size (L_c) and the interplanar spacing (d₀₀₂) corresponding to the diffraction peak near 23° are quantitatively analyzed to examine the existence of crystalline carbon. The results are given in Table 1. It is shown that L_C is increasing with the increase of carbonization temperature, which indicates that the graphite microcrystals in the CNFs are growing gradually and the degree of graphitization is also increasing. The decrease of d₀₀₂ is mainly because of the decrease of non-carbon elements in the fiber and the increase of the proportion of crystalline carbon with the increase of carbonization temperature, which makes the disordered graphite layer more closely arranged and gradually generates the two-dimensional graphite structure. Its interplanar spacing is closer and closer to that of the graphite crystal plane (0.3354 nm), which means that the increase of carbonization temperature is helpful for improving the order of the graphite crystallite arrangement. In addition, the d₀₀₂ of PAN-CNFs1200 is smaller compared with LP-CNFs1200, indicating that some ordered graphitized structures are transformed into disordered carbon structures due to the existence of lignin.

Figure 5.

Spectrum of carbon nanofibers (CNFs) prepared at different carbonization temperatures: (a) X-ray diffraction spectrum; (b) Raman spectrum.

Table 1.

Structural parameters of the X-ray diffraction spectrum and Raman spectrum of carbon nanofibers (CNFs) prepared at different carbonization temperatures

Samples	L_c/nm	d₀₀₂/nm	R value	L_a/nm
LP-CNFs800	0.0576	0.3815	0.916	4.803
LP-CNFs1000	0.0583	0.3802	0.896	4.911
LP-CNFs1200	0.0619	0.3801	0.893	4.927
LP-CNFs1400	0.0639	0.3765	0.880	5.000
PAN-CNFs1200	0.0590	0.3794	0.883	4.983

PAN: polyacrylonitrile.

From Table 1, it can be seen that L_a increases from 4.803 to 5.000 nm with the increase of carbonization temperature, which means that the graphite microcrystalline structure is gradually forming, and this is consistent with the XRD analysis results. Therefore, the decrease of d₀₀₂ plus the increase of L_c and L_a of LP-CNFs through XRD and Raman test characterization analysis indicate that the order degree of LP-CNFs is improved with the increasing temperature from 800°C to 1400°C. In order to further explore the effect of lignin addition on the graphitization degree of CNFs, the R value of PAN-CNFs1200 and LP-CNFs1200 were compared. The results show that the graphitization degree of PAN-CNFs1200 was higher, which may be because lignin is a kind of heterogeneous macromolecule compounded with complex thermal decomposition behavior. With the addition of lignin, more amorphous carbon was formed, which reduced the ordered degree of graphite lamellar. Although the higher degree of graphitization can promote the efficient and rapid transmission of electrolyte ions, the existence of amorphous carbon can increase the specific surface area of the fiber and store more ions. Therefore, only a good combination of the degree of graphitization and content of amorphous carbon can maximize the improvement of electrochemical performance.²⁵

Specific surface area and pore size distribution

In order to understand the porosity of lignin/PAN-CNFs, CNFs prepared under different carbonization temperatures were tested with nitrogen adsorption and desorption. Figure 6(a) shows the nitrogen adsorption/desorption isotherms of the samples. According to the IUPAC (International Union of Pure and Applied Chemistry) standard, the rapid increase in the nitrogen adsorption capacity of LP-CNFs1000, LP-CNFs1200 and LP-CNFs1400 in the low relative pressure zone (P/P₀ < 0.1) indicates that there are more micropores in the CNFs. The hysteresis loops (capillary condensation) generated in the high relative pressure region (P/P₀ > 0.8) prove that there is a large number of mesopores in the microstructure of CNFs, which belong to the type-IV isotherm.²⁶ LP-CNFs800, LP-CNFs1000, LP-CNFs1200, LP-CNFs1400 and PAN-CNFs1200 possess pore sizes of 2.72, 1.85, 1.91, 1.86 and 1.79 nm, respectively (Figure 6(b)). The above experimental phenomena show the existence of micropores, mesopores and macropores in CNFs. They can provide more channels for the rapid transport of electrolyte ions and make more ions enter the electrode, which can produce reversible ion adsorption at the interface between the electrode and the electrolyte.²⁷ The energy storage mechanism of double-layer capacitors is to generate specific capacitance by adsorption and desorption of electrolyte ions at the interface. Therefore, the CNFs prepared in this experiment are suitable for use as electrode materials for supercapacitors. The specific surface areas of LP-CNFs800, LP-CNFs1000, LP-CNFs1200, LP-CNFs1400 and PAN-CNFs1200 were 100.71, 732.09, 849.89, 641.94 and 50.49 m²/g, respectively, which were calculated according to the nitrogen adsorption/desorption isotherm and the BET (Brunauer–Emmett–Teller) theory. Results show that more pores can be produced on the fiber surface by pyrolysis with the addition of lignin, and LP-CNFs have the largest specific surface area (849.89 m²/g) at 1200°C. When the carbonization temperature further increased to 1400°C, the specific surface area decreased to 641.94 m²/g, which may be because of the change of more micropores to mesopores or macropores due to the destruction of the pore structure.

Figure 6.

Carbon nanofibers (CNFs) prepared at different carbonization temperatures: (a) nitrogen adsorption/desorption isotherms; (b) pore size distribution. (The pore size distribution was evaluated by the Barret–Joyner–Halenda method.)

Electrochemical performance

Effect of carbonization temperature on electrochemical performance

CV tests were carried out with CNF electrode material to evaluate its energy storage mechanism. The CV curves of CNF electrodes prepared at different carbonization temperatures are shown in Figure 7. It can be seen that the CV curves of all the samples at scanning rates from 5 to 100 mV/s show a nearly symmetrical shape of an approximate rectangle, which shows that the CNF electrode has good reversibility. The CV curves have no redox peaks, indicating that the charge storage mechanism of the CNF electrode material is double-layer capacitance. The specific capacitance of CNF electrode material under different carbonization temperatures is shown in Figure 7(f). It can be found that the specific capacitance of the CNF electrode increases with the increase of temperature at the same scanning rate, and the specific capacitance of LP-CNFs1200 reaches the maximum of 164.9 F/g at the scanning rate of 5 mV/s. When the carbonization temperature is further increased to 1400°C, the number of ions entering the internal pores of the CNFs reduces due to the collapse of internal pores of the CNFs, leading to the decreased migration and adsorption of ions in the CNFs, which makes the specific capacitance slightly reduce (128 F/g). In addition, the specific capacitance of the lignin/PAN-CNF electrode is significantly higher than that of PAN-CNFs at different scanning rates, which shows that the addition of lignin can significantly improve the capacitance performance of the CNF electrode, and lignin/PAN-CNFs are more suitable as electrode materials. This excellent performance can be attributed to the pyrolysis of lignin, which introduced more pore structures than the PAN-based surface, and increased the specific surface area of the carbon electrode materials for the storage location of a large number of charges, which helps it to adsorb more charges on the surface of the CNFs so that the specific capacitance performance is improved.

Figure 7.

Cyclic voltammetry curves and specific capacitances of the carbon nanofiber (CNF) electrode prepared at different carbonization temperatures at different scanning rates: (a) LP-CNFs800; (b) LP-CNFs1000; (c) LP-CNFs1200; (d) LP-CNFs1400; (e) PAN-CNFs1200; (f) specific capacitance.

The GCD tests of the CNF electrode were carried out at the current density of 0.5 A/g. The results are shown in Figure 8(a). At the beginning of the discharge, there is no significant resistance voltage drop (IR) in the GCD curves, indicating that the ESR of the CNF electrode is small. LP-CNFs1200 has a longer charge and discharge time with the largest specific capacitance of 156.3 F/g, which is consistent with the test results of CV. In addition, the GCD curves of the electrode material at different current densities almost form an isosceles triangle, which means that the electrode has good electrochemical reversibility and stability (see Figure 8(b)).

Figure 8.

(a) Galvanostatic charge–discharge (GCD) curves of carbon nanofiber (CNF) electrodes prepared at different carbonization temperatures at a current density of 0.5A/g. (b) GCD curves of LP-CNFs1200 electrodes at different current densities. (c) Nyquist curves of CNF electrodes for all samples.

The EIS of the CNF electrode prepared at different carbonization temperatures was measured at the frequencies of 0.01 Hz–100 KHz. The Nyquist plots of all the samples are presented in Figure 8(c). The intersection of the Nyquist curve with the Z'-axis is the ESR, which represents the migration of ions between the electrode and the electrolyte interface.²⁸ The ESR of LP-CNFs800, LP-CNFs1000, LP-CNFs1200, LP-CNFs1400 and PAN-CNFs1200 are 1.24, 0.96, 0.63, 0.72 and 1.40 Ω, respectively. Among them, LP-CNFs1200 has the smallest internal resistance, which is advantageous to improve the ion transfer at the interface of the double layers, so it is in good agreement with the results of the CV and GCD curves. The Nyquist curve of all the samples tends to a straight line in the low-frequency zone, which means that charge transfer resistance is minimal. It meets the requirement of the ideal capacitor slope of 1 and is also the reason for the better specific capacitance performance.

Effect of specific surface area on electrochemical performance

CNFs were prepared from the lignin/PAN solution with concentrations of 28 and 30 wt% to explore the effect of specific surface area on electrochemical performance. The same pre-oxidation and carbonization conditions were adopted for the two solution concentrations. The prepared CNFs were assembled into electrode materials, and a series of electrochemical performance tests were performed to compare the electrochemical performance of the CNFs from the different solution concentrations, as shown in Figure 9.

Figure 9.

(a) Carbon nanofiber (CNF) morphology when the precursor solution has the concentration of 30 wt%. (b) CNF morphology when the precursor solution has the concentration of 28 wt%. (c) Nitrogen absorption/desorption curves. (d) Cyclic voltammetry curves when the scanning rate is 5 mV/s. (e) Galvanostatic charge–discharge curves when the current density is 0.5 A/g. (f) Nyquist curves.

With lower solution concentration a finer CNF diameter (539 nm) was obtained, as shown in Figure 9(b). Figure 9(c) shows that the adsorption capacity of CNFs prepared with the solution concentration of 28 wt% increases rapidly in the low pressure zone, indicating that there are more micropores in the CNFs. In the region of high pressure, it shows a type-IV isotherm, indicating that mesopores exist in the microstructure.²⁶ In addition, compared with the concentration of 30 wt%, CNFs prepared with the solution concentration of 28 wt% have a larger specific surface area (1053.78 m²/g).

The CV curves of CNF electrode material at the scanning rate of 5 mV/s, as shown in Figure 9(d), present a roughly rectangular shape and the rectangle is symmetrical, indicating that the CNF electrode material has the characteristics of double-layer capacitors. The above phenomena show that the solution concentration does not affect the energy storage mechanism of the supercapacitor. However, the specific capacitance of the CNF electrode material calculated according to the GCD curves shows a larger difference: at a current density of 0.5 A/g, the specific capacitance of the CNF electrode from the solution concentration of 28 wt% has a greater value (201.27 F/g) compared to that (156.3 F/g) for 30 wt%. The ESR of the CNF electrode for the solution concentration of 28 wt% is 0.57 Ω, which is also less than that (0.63 Ω) for CNFs prepared with a concentration of 30% according to the Nyquist curves, which means that CNFs prepared with low solution concentration are more suitable for electrode materials. The reasons are as follows: the electrode material prepared with the solution concentration of 28 wt% has more micropores, which can increase the specific surface area of CNFs and provide more storage locations for electrolyte ions and charges. Moreover, smaller ESR can achieve more efficient diffusion and rapid exchange of ions and store more electric charges to improve the specific capacitance performance of the CNF electrode material.

The results of electrochemical performance for the electrode materials in this study were compared with those reported in the literature, as shown in Table 2. The results show that the CNF electrode material prepared when the precursor solution concentration is 28 wt% has a relatively higher specific capacitance, which can be applied to electrode materials.

Table 2.

Comparison of electrochemical of polyacrylonitrile (PAN)/lignin-based carbon nanofiber electrode materials prepared in this study and similar materials reported in the literature

Carbon fiber precursor	Carbonizationtemperature (°C)	Current density (A/g)	Specific capacitance (F/g)	Reference
Alkali lignin/PAN/DMF	900	0.5	129.2	[29]
PAN/lignin/NiCo₂O₄/N-rGO	1000	1	134.3	[30]
AAL/Fe(acac)₃/PEO/DMF	900	0.5	72.1	[31]
PEO/Kraft lignin/NaNO₃/NaOH	800	0.1	192.0	[32]
PAN/EHL/DMF	800	1	216.8	[33]
Lignin/PAN/DMF/DMSO	1200	0.5	201.3	In this study

DMF: N,N-dimethylformamide; DMSO: dimethyl sulfoxide; N-rGO: N-doped reduced graphene oxide ; AAL: acetic acid lignin; PEO: polyethylene oxide; EHL: enzymatic hydrolysis lignin.

Conclusion

In summary, lignin/PAN CNFs with good flexibility were successfully prepared by electrospinning, pre-oxidation and carbonization with PAN and renewable lignin containing a large number of aromatic hydrocarbons. The CNFs prepared from lignin/PAN (90/10) have no adhesion between the fibers with good morphology and flexibility. Compared with the CNFs without lignin, the addition of lignin increases the specific surface area of CNFs. In addition, lignin/PAN-CNFs exhibit excellent electrochemical properties when used as binder-free electrode materials for supercapacitors. The charge storage mechanism is not affected by the concentration of the spinning solution, and it is still a double-layer capacitance. Low concentration (28 wt%) is beneficial to obtain CNFs with a smaller diameter (539 nm) and larger specific surface area (1053.78 m²/g), which can provide more storage sites for electrolyte ions and is helpful to improve the specific capacitance of the electrode materials. When the current density is 0.5 A/g, the specific capacitance can be up to 201.27 F/g. Compared with the CNF electrodes reported in the literature, the prepared CNF electrode materials have relatively higher specific capacitance. These results indicate that the CNF electrode prepared from lignin has a great application prospect in electrode materials for supercapacitors, which provides a new idea for the development of electrode materials to make high-performance capacitors.

Footnotes

Declaration of conflicting interests

The authors declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The authors disclosed receipt of the following financial support for the research, authorship and/or publication of this article: This work was supported by the National Natural Science Foundation of China (51503168), the Innovation Capability Support Plan of Shaanxi, China (2020PT-043), the Innovation Talent Promotion Program of Shaanxi, China (2017KJXX-23) and the Special Funding for Postdoctoral Innovation Project in Shandong Province (201504).

ORCID iD

Chengkun Liu

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