Fabrication of graphitized carbon fibers from fusible lignin and their application in supercapacitors

doi:10.21203/rs.3.rs-2431477/v1

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Fabrication of graphitized carbon fibers from fusible lignin and their application in supercapacitors

https://doi.org/10.21203/rs.3.rs-2431477/v1

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Lignin-based carbon fibers (LCFs) with graphitized structures decorated on their surfaces were successfully prepared using simultaneous catalyst loading and chemical stabilization of melt-spun lignin fibers, followed by quick carbonization functionalized as catalytic graphitization. This technique not only enables surficial graphitized LCF preparation at a relatively low temperature of 1200 °C but also avoids additional treatments used in conventional carbon fiber production. The LCFs were then used as electrode materials in a supercapacitor assembly. Electrochemical measurements confirmed that LCF-0.4, a sample with relatively low specific surface area of 89.9 m² g⁻¹, exhibited the best electrochemical properties. The supercapacitor with LCF-0.4 had a specific capacitance of 50.2 F g⁻¹ at 0.5 A g⁻¹, power density of 644.9 kW kg⁻¹, energy density of 15.7 Wh kg⁻¹, and capacitance retention of 100% after 1500 cycles even without activation.

lignin

carbon fiber

catalytic graphitization

supercapacitor

electrochemical performance

Many studies have explored biomaterial production to address environmental deterioration and diminishing oil reserves (Ubando A. T. et al., 2020; Xu, 2020; Zheng et al., 2019). Lignin, an inexpensive biopolymer with high carbon content and excellent biodegradability, receives considerable interest (Li et al., 2021a; Wang et al., 2022). Despite being the second most common form of biomass on Earth, most lignin is burned for energy recovery, and only 5% of the material is utilized to produce biobased products, causing severe lignin underutilization and wastage (Bengtsson et al., 2020; Lobato-Peralta et al., 2021; Sun et al., 2022; Zhu et al., 2020). Consequently, novel use of lignin must be identified to raise its value.

Carbon fibers (CFs) derived from lignin are promising for the long–term production of high–value commodities used in energy storage and conversion, such as batteries and supercapacitors, environmental cleanup, and gas separation(Meng et al., 2021; Yan et al., 2021). Owing to its high carbon content over 60%, lignin is a potential alternative to polyacrylonitrile for affordable CF production (Cai et al., 2020; Luo et al., 2021; Puziy et al., 2018; Wang et al., 2020a; Xu et al., 2021; Yang et al., 2020). Conventional preparation of lignin-based carbon fibers (LCFs) follows these steps: 1) fabricating lignin fibers (LFs) via spinning technology; 2) stabilizing the fibers using chemical or thermal treatment to obtain lignin-based stabilized fibers (LSFs) as CF precursors; 3) carbonization under thousands Celsius to prepare CFs; 4) functionalizing CFs to afford unique morphological and structural properties, such as large surface area, appropriate pore size distribution, and graphitic carbon structure (Bengtsson et al., 2020; Cao et al., 2019; Chen et al., 2019; Cho et al., 2019; Liu et al., 2022; Maťko et al., 2019; Novák et al., 2019; Park et al., 2022; Pignol et al., 2022; Schierholz et al., 2019; Shokrani Havigh and Mahmoudi Chenari, 2022; Teo et al., 2016; Yang et al., 2016). Among these steps, fiber stabilization process is the most expensive as it takes over 30 hours and accounts for a greater proportion of the entire cost of CF production (Sun et al., 2022). LCFs with graphitic structures are favored as electrode materials in energy storage devices (Dasari et al., 2017; Geng et al., 2021; Gomez-Martin et al., 2021; Gong et al., 2017; Wang et al., 2016). However, realizing such a structure requires energy-intensive graphitization at 2000 ℃–3000 ℃ (Sagues et al., 2019). Therefore, investigating high-performance supercapacitors using inexpensive LCFs with unique structures is crucial.

Herein, fusible polyethylene glycol lignin (PEGL) fibers were first prepared using melt-spinning without solvents. Nickelous catalyst loading and melt-spun LFs chemical stabilization were then conducted, followed by fast simultaneous carbonization and catalytic graphitization under 1200°C. Such a combined LCF preparation process should dramatically decrease cost. The resulting LCFs with a graphitic structure were used as electrode materials in supercapacitors. The electrochemical performance of the devices was investigated.

Materials

Poplar wood chips were purchased from Senyi Wood Company (Shandong, China), while polyethylene glycol 400 (PEG 400), HCl, H₂SO₄, and NaOH were bought from Damao Chemical Reagent Company (Tianjin, China). Nickel (II) acetylacetonate [Ni(acac)₂, 95%] was supplied by Aladdin Biochemical Technology Co. Ltd. (Shanghai, China). Carboxymethyl cellulose sodium (CMC) and carbon black (CB) were purchased from Macklin Biochemical Co. Ltd. (Shanghai, China), while triethylmethylammonium tetrafluoroborate (TEMABF₄) and dimethyl carbonate (PC) were purchased from TCI (Shanghai) Development Co., Ltd. All chemicals were used as received.

Melt-spinning Of Pegl

PEGL was first isolated from poplar wood chips using organosolv pulping with PEG 400 (Takahashi et al., 2020; You et al., 2016; Zhang et al., 2016). Briefly, 200 g of air-dried wood chips were cooked using 0.3% H₂SO₄/PEG 400 at 140°C for 1.5 h. The liquor to wood ratio was 5:1. After the solvolysis reaction, 0.2 M NaOH aqueous solution was used to extract the lignin components. The extractives were then washed with water and dried to obtain PEGL. Then, PEGL was melt-spun into LFs using a high-pressure melt-spinning apparatus (TL-03; Tongli Co., Ltd., Shenzhen, China). Melt-spinning was performed at 150°C–200°C under boost pressure between 0.30 and 1.20 MPa. The syringe needle size was 0.44 mm and collector rolling speed, 10 rpm.

Preparation Of Cfs From Pegl

Different amounts of Ni(acac)₂ were dissolved in 6 M HCl to prepare a homogenous solution. LFs were then immersed in the solution and kept at 105°C for 2 h for chemical stabilization to afford LSFs with Ni(acac)₂ loading. Then, LSFs were carbonized in a horizontal tube furnace (GSL-1600X, Hefei Kejing Material Technology Co, Ltd., China) using programmed heating under N₂ stream protection. The program involved 5°C min^− 1 heating from room temperature to 250°C, 3°C min^− 1 heating from 250°C to 1200°C with 1 h dwell at 1200°C, and cool down to room temperature to obtain LCFs. These fibers were referred to as LSF-x and LCF-x, where x is the concentration of Ni(acac)₂/HCl solution ranging from 0 to 0.6 M.

Supercapacitor Assembly

Powdered LCF samples with CB were suspended in a 2 wt% CMC aqueous solution as electrode material slurry. The LCF/CMC/CB weight ratio was maintained at 85:10:5. The slurry was then coated on Al foil to dry. Circular sheets were cut to obtain the electrodes (diameter = 16 mm). Subsequently, the electrodes and cellulosic paper (separator) were both immersed in 1.2 M TEMABF₄/PC electrolyte solution and then degassed. Finally, the supercapacitors were assembled by sandwiching the electrodes and separator in a split test cell.

Characterization

Morphological analysis

The morphology of the LSF and LCF samples was investigated using field-emission scanning electron microscopy (SEM, S4800, Hitachi, Japan) at an acceleration voltage of 5 kV. The elemental composition at the LCF surfaces was investigated using energy-dispersive X-ray spectroscopy (EDS; Rigaku Corporation, Japan). To quantify the surface area and porosity of the LSFs and LCFs, we performed N₂ adsorption/desorption measurements at − 196°C using a chemisorption analyzer (ASAP2460, Micromeritics, USA). The specific surface area and average pore size were calculated using the Brunauer − Emmett − Teller (BET) model. The internal and external specific surface areas were calculated using the t-plot method in a relative pressure range of 0.2–0.5. The pore size distribution was calculated using the non-local density functional theory model.

Thermal Analysis

Thermogravimetry (TG) and differential scanning calorimetry (DSC) were conducted using a dual system (TG–DSC, STA449F3-1053-M, Netzsch, Germany) from room temperature to 800°C at a heating rate of 10°C min^− 1 under N₂ atmosphere. The extrapolated onset (T_od) and endset (T_ed) decomposition temperatures were obtained from the intersection of the horizontal baseline and tangent at the point of maximum gradient from the TG curves.

Carbonaceous Structure Analysis

Carbon structure identification of the LCFs was conducted using X-ray diffraction (XRD, D8 Advance, Bruker, Germany), with CuKα (λ = 1.5406 Å) radiation at 40 kV and 40 mA. XRD patterns were recorded in a 2θ range of 10°–60° at a scan rate of 6° min^− 1. Equations (1) and (2) were used to compute the spacing between the sheets (d₀₀₂) of LCFs (Li et al., 2021a; Meng et al., 2021; Torres-Canas et al., 2020; Zhengshuai Sun, 2022-06-20 ) and average crystallite size (L_c) (Li et al., 2021a; Meng et al., 2021):

$${\text{d}}_{002}=\frac{\text{λ}}{\text{2}\text{s}\text{i}\text{n}\text{θ}}$$

(1)

$${\text{L}}_{\text{c}}=\frac{\text{Kλ}}{\text{β}\text{cos}\text{θ}}$$

(2)

where K = 0.94, θ is the diffraction angle, and β is the peak width at half maximum intensity.

Raman spectroscopy with a 532 nm excitation laser (Thermo Scientific DXRxi, Thermo Fisher Scientific, USA) was used to characterize the microstructure of the LCF surface. The laser power was set to 0.1 mW to avoid overheating the LCF surface. Each spectrum was recorded for 300 s. The crystalline size (L_a, in nm) of the LCFs is calculated using Eq. (3) (Sun et al., 2021):

$${\text{L}}_{\text{a}}\text{=2.4×}{\text{10}}^{\text{–}\text{10}}{\text{λ}}^{\text{4}}{\text{R}}^{\text{–}\text{1}}$$

(3)

where λ is the laser wavelength (nm) and R is the integrated area ratio of the D-band (1350 cm^− 1) to the G-band (1600 cm^− 1) in the Raman spectra.

Fine carbon structural observations were conducted using transmission electron microscopy (TEM, Talos F200s, FEI, USA) operating at 200 kV with a resolution of 0.14 nm. Before transferring to the TEM chamber, the powdered LCF samples were dispersed in ethanol and a droplet of the supernatant was placed on Cu grids.

Electrochemical Performance Measurements

Electrochemical characterization of the assembled supercapacitor, including cyclic voltammetry (CV), galvanostatic charge/discharge (GCD), and electrochemical impedance spectroscopy (EIS) analyses, were performed on an electrochemical workstation (Donghua DH7000, Jiangsu Donghua Analytical Instrument Co., Ltd., China). EIS experiments were conducted between 10⁶ Hz and 1 Hz with an amplitude of 10 mV.

CV measurements were performed at various scan rates from 0.05 to 0.2 V s^− 1. The gravimetric specific capacitance of the LCF electrodes (C_CV) was computed using Eq. (4) (Kwon et al., 2021; You et al., 2017):

$${\text{C}}_{\text{CV}}=\frac{\text{4}\bullet \int \text{i }\text{dV}}{\text{m}\bullet \text{v}\bullet \text{∆E}}$$

(4)

where m is the material loading of the two electrodes, ΔE is the voltage window, i is the response current, and v is the scan rate. In GCD measurements, a current density of 0.5 A g^− 1 was applied, and the specific capacitance (C_GCD) was calculated using Eq. (5) (You et al., 2017; Zhi et al., 2014):

$${\text{C}}_{\text{GCD}}=\text{4}\frac{\text{i}}{\text{m}\text{∆v}/\text{∆t}}$$

(5)

where i is the applied current and $\varDelta v$/$\varDelta t$ is the slope of the discharge curve within the range of 1.8–1.35 V (45–60% of V_max, where V_max is the maximum value of the working potential window).

The energy density (E) and power density (P) of the assembled supercapacitors were calculated using Equations (6) and (7) (Adam et al., 2020; Han et al., 2017; Kwon et al., 2021; Wang et al., 2020b; Yang et al., 2019; Zhao et al., 2019):

$$\text{E=}\frac{{\text{C}}_{\text{GCD}}\bullet {\text{V}}_{\text{max}}^{\text{2}}}{8}$$

(6)

$$\text{P}=\frac{{({\text{V}}_{\text{max}}\text{–}{\text{V}}_{\text{drop}}\text{)}}^{\text{2}}}{\text{4}{\text{mR}}_{s}}$$

(7)

where V_max and V_drop are the maximum operating potential difference and voltage drop during discharge initiation, respectively, and R_s is the equivalent series resistance obtained from EIS profiles.

Morphological And Thermal Analysis

The morphology and composition of the LFs, LSFs, and LCFs were observed using SEM and EDS. As shown in Fig. S1, LFs had an average diameter of 158.5 ± 15.9 µm and exhibited a smooth surface. After chemical stabilization, the fibrous diameter decreased to 53.9 ± 9.6 µm. Pores and splits were generated on the LSF-0 surface (Fig. 2a). With Ni(acac)₂ addition, the inner and exterior morphologies of the LSFs were clearly altered (Figs. 2b–d). These changes can be explained by material interactions. Firstly, ether bond cleavage occurs between the lignin matrix and PEG side chains under concentrated acidic conditions. These residues at low molecular weight wash out easily, forming defects on the LSF surface (Lin et al., 2012). Secondly, Ni cations, which are also electrophiles in this system, coordinate different nucleophiles and promote ether bond cleavage, yielding a rough fibrous surface (Fig. 3). Additionally, the deposited Ni ions can be a graphitic catalyst in subsequent carbonization(Malik et al., 2021).

LCF-0 with an average diameter of 64.68 ± 4.6 µm was obtained, as shown in Figs. 2e–h. The fiber diameter was reduced to 48.9 ± 5.6 µm after carbonization in LCF-0.4. Notably, a fibrous melt surface was observed from LCF-0.4 and LCF-0.6. With confirmed increased Ni loading from EDS (Fig. S2), the melt surface might be caused by Ni-assisted catalytic graphitization. This graphitization is explained in the following sections.

Table 1

Decomposition temperatures and mass loss analysis for the LSF samples.
Sample	Extrapolated onset decomposition temperature (T_od, °C)	Extrapolated endset decomposition temperature (T_ed, °C)	Mass loss at decomposition (%)	Mass loss at carbonization and graphitization (%)	Total mass loss (%)
LSF-0	312.7	471.7	39.4	14.1	53.5
LSF-0.2	309.1	468.4	41.2	13.3	54.5
LSF-0.4	308.4	467.9	43.9	12.5	56.4
LSF-0.6	289.5	463.5	45.7	12.4	58.1

Simultaneous TG–DSC measurements were performed under N₂ atmosphere to investigate the decomposition and melting behavior of LSFs during carbonization (Fig. 4 and Table 1). All the LSFs exhibited one main weight-loss stage in the TG curve, which corresponds to the decomposition of unstable small-molecule components. The onset temperatures of decomposition decreased from 312.7°C to 289.5°C with increasing Ni content. Such decomposition afforded a mass loss of 39.4% for LSF-0, which increased to 45.7% for LSF-0.6. This increase proves that Ni assists in ether bond cleavage and small-fragment formation. From the DSC curve above the extrapolated T_ed, LSF-0 exhibited a broad exothermic region with an exothermic peak at 582.7°C, a typical feature in biomass carbonization (Yoo et al., 2018). Meanwhile for LSF-0.2, the DSC heat flow changes from an exothermic to endothermic reaction at 660.6°C. This temperature decreased to 491.4°C and 472.0°C for LSF-0.4 and LSF-0.6, respectively, indicating the developing ordered carbon structure (graphitic and turbostatic carbons) (Gutiérrez-Pardo et al., 2015; Inagaki and Kang, 2014; Yoo et al., 2018). According to the proposed mechanism for catalytic graphitization, the endothermic reaction suggests dissolution of sp³ C atoms in Ni and precipitation of sp² C in saturated C/Ni solution, affording the melt surface observed in the SEM images.

Structural Determination

To understand the carbon structure of the LCF samples, we performed Raman spectroscopy on the fibers. As shown in Fig. 5a, all LCF spectra had both D-band at 1350 cm^–1 and G-band at ~ 1600 cm^–1 after deconvolution, where the G-band refers to the in-plane vibration of graphitic carbon atoms, and D-band corresponds to disordered sp²-hybridized graphitic carbon atoms (Zhengshuai Sun, 2022-06-20 ). The integrated area ratio of I_D/I_G (R) can be calculated to gauge the degree of disorder at the surface structure of the LCFs (Gong et al., 2017). As shown in Table 1, LCF-0 exhibited the highest R value of 8.16 among the prepared samples. With increasing Ni loading, R decreased to 2.57 for LCF-0.4. This sample exhibited the largest L_a plane size of 7.48 nm among the LCF samples. When the Ni(acac)₂ amount was further increased, the R and L_a values of LCF-0.6 increased to 3.71 and 5.18 nm, respectively, indicating a smaller graphitic region than LCF-0.4. This difference may be due to Ni being a graphitization catalyst as well as an etching reagent during chemical stabilization, generating a porous surface on the LSFs. Thus, high Ni concentrations yield aggressive etching, causing discontinuous graphitic carbon regions after carbonization. This phenomenon indicates that biomass graphitization could not be improved by simply increasing the amount of Ni ions (Gai et al., 2021; Zhengshuai Sun, 2022-06-20 ).

XRD characterization was conducted to elucidate the carbon structure changes from those of the bulk LCF samples. As shown in Fig. 5b, no diffraction peaks were observed for LCF-0 in the entire scan range, indicating that only amorphous carbon was generated without Ni(acac)₂. Meanwhile, broad XRD peaks from the LCF samples at 24° and 44° were identified, which correspond to the (002) and (101) crystal planes, respectively.(Inagaki and Kang, 2014) This is because Ni ions can only be adsorbed on the surface of the LSFs during chemical stabilization and cannot access the insides. The average L_c values for LCF-0.2, LCF-0.4, and LCF-0.6, based on the (002) peaks, were 0.26, 0.41, and 0.39 nm, respectively (Table 2).

Table 2

Calculated Raman spectroscopy and XRD results for the LCFs.
Samples	Raman		XRD
Samples	R = I_D/I_G	L_a (nm)	L_c (nm)	d₀₀₂ (nm)
LCF-0	8.16	2.36	–	–
LCF-0.2	3.74	5.14	0.26	0.37
LCF-0.4	2.57	7.48	0.41	0.35
LCF-0.6	3.71	5.18	0.39	0.37

High-resolution TEM clearly shows nanostructures in the LCFs (Fig. 5). Consistent with the XRD and Raman spectroscopy results, ordered microstructures were difficult to find in LCF-0 and LCF-0.2 owing to low Ni content on the fibrous surface. For LCF-0.4 and LCF-0.6, distinct lattice fringes embedded in the amorphous regions were identified. LCF-0.4 exhibited large ordered layers. The measured spacing between the crystal layers was 0.34 nm, proving Ni-assisted graphitic structure generation. Thus, the LCFs comprised mainly amorphous carbon in the bulk covered by crystallized carbon layers on the surface. Such a structure would be beneficial for electrolyte ion transport and accumulation in supercapacitors.

Table 3

Textural properties of the LCFs with varying Ni(acac)₂ content.
Samples	S_BET ^a (m² g^–1)	W_p ^b (nm)	V_total ^c (cm³ g^–1)	S_internal ^d (m² g^–1)	S_external ^e (m² g^–1)
LCF-0	350.95	1.62	0.13	316.67	34.28
LCF-0.2	103.52	1.65	0.04	93.96	9.57
LCF-0.4	89.92	1.61	0.03	83.74	6.19
LCF-0.6	269.56	1.67	0.11	234.48	35.08

^a S_BET: Specific surface area computed using the BET model.

^b W_p: Adsorption average pore width.

^c V_total: Total pore volume.

^d S_internal: Surface area of micropores and mesopores using the t–plot method.

^e S_external: Surface area of macropores using the t–plot method.

N₂ adsorption/desorption curves (Fig. S3) exhibit typical type-I behavior. The specific surface area and pore information for the LCFs are listed in Table 3. The specific surface area of LCF-0 was 350.95 m² g^− 1, with 316.67 m² g^− 1 from micro/mesopores and 34.28 m² g^− 1 from macropores, according to the t-plot method. The specific surface area decreased with Ni(acac)₂ content and reached a minimum value of 89.92 m² g^− 1 for LCF-0.4. Moreover, the total volume and internal/external surface areas exhibited a similar trend. This can be explained by Ni-induced melting on the fiber surface, which filled up several pores and decreased the specific surface area. Thus, the total pore volume decreased from 0.13 m³ g^− 1 for LCF-0 to 0.03 m³ g^− 1 for LCF-0.4. Interestingly, the surface area and total pore volume increased to 269.56 m² g^− 1 and 0.11 m³ g^− 1 for LCF-0.6, respectively. These may be due to the influence of Ni ions on etching rather than filling the surface at high Ni concentrations in the Ni-treated LCF samples. This feature is expected to influence supercapacitor performance, as discussed in the next section (Kim et al., 2021; Li et al., 2021b; Xia et al., 2020; Xu et al., 2021).

Electrochemical Performance Of The Lcf-based Supercapacitors

EIS, CV, and GCD experiments using a two-electrode system with 1.2 M TEMABF₄/PC electrolyte were performed to determine the electrochemical performance of the LCF-based electrodes. As shown in Fig. 6a, Nyquist plots at the high-frequency region show the electrode resistance (R_e) of the supercapacitors at the x-axis intercept, which indicates the resistance of the electrode materials and their contacts. The equivalent series resistance (R_s) was obtained from the intersection of the straight line in the low-frequency region and Z_re axis. R_s is the sum of the electrode and electrolyte resistances (Mei et al., 2017; Vicentini et al., 2021). Without graphitic structures on its surface, LCF-0 had very large R_e (39.74 Ω) and R_s (326.39 Ω) even when CB was added in the electrode recipe. When Ni(acac)₂ was used in chemical thermostabilization, R_e and R_s decreased remarkably and achieved the lowest values of 0.9 and 1.2 Ω for LCF-0.4, respectively. The low resistances suggest that a low-resistance network was established on the surface of LCF-0.4 owing to Ni catalytic graphitization. Additionally, the straight line in the low-frequency region for LCF-0.4 exhibited the highest slope among the LCF samples, indicating rapid diffusion of electrolyte ions during the charge–discharge process.

Table 4

Summarized supercapacitor performance using different LCF electrodes.
Sample	C_CV ^a (F g^–1)	C_GCD ^b (F g^–1)	R_s ^c (Ω)	E ^d (Wh kg^–1)	P ^e (kW kg^–1)
LCF-0	1.7	2.30	6.3	0.7	13.4
LCF-0.2	52.6	37.4	2.0	11.7	282.4
LCF-0.4	75.0	50.2	1.2	15.7	644.9
LCF-0.6	66.4	42.5	21.0	13.2	202.6

^a C_CV: Gravimetric specific capacitance calculated from CV profiles.

^b C_GCD: Specific capacitance calculated from GCD profiles.

^c Rs: Equivalent series resistance from Nyquist plots.

^d E: Energy density.

^e P: Power density.

CV curves of the supercapacitors with LCFs exhibited quasi-rectangular shapes within 0–3 V, indicating typical supercapacitor behavior (Fig. 6b). Even though LCF-0 had a large surface area and well-developed porous structure, the capacitance of the assembled supercapacitor was only 1.7 F g^− 1 at a scan rate of 0.05 V s^− 1. This value increased considerably for the Ni-catalyzed LCF electrodes, and a high capacitance of up to 75.0 F g^− 1 was obtained for LCF-0.4.

GCD measurements were conducted to further evaluate the electrochemical performance of the LCF electrodes (Fig. 6c). Very short discharge time of 0.1 s was observed when LCF-0 was used as the electrode. The discharge time extended to 65 s and specific capacitance of 50.2 F g^− 1 was obtained when graphitic LCF electrodes at the same current density (0.5 A g^− 1) were employed. These findings are consistent with the CV results. Moreover, LCF-0.4 exhibited excellent electrochemical stability of 100% after 1500 cycles. Notably, the normalized capacitance of the LCF-0.4 electrode outperformed that of most reported biomass-derived supercapacitors at 0.5 A g^− 1 (Fig. 7a) (Chmiola et al., 2006; Du et al., 2021; Wang et al., 2017). Thus, the assembled supercapacitor using LCF-0.4 exhibited the best performance. It had an energy density of 15.7 Wh kg^− 1 and power density of 644.9 kW kg^− 1, which are comparable to those of previously reported biomass-derived supercapacitors (Fig. 7b) (Dai et al., 2019; Poochai et al., 2021; Schlee et al., 2019; Sevilla et al., 2019; Zheng et al., 2019; Zou et al., 2019). Thus, we infer that the Ni-catalyzed graphitization induced conductive network had more influence on electrolyte ion accumulation than on the surface area and pore size distribution, affording high electrochemical performance.

In summary, we demonstrated a simple preparation procedure for LCFs involving loading the catalyst during chemical stabilization and combining graphitization and carbonization. The resulting LCFs mainly comprise bulk amorphous carbon covered with graphitic carbon layers on the surface. Although LCF-0.4 exhibited a low specific surface area of 89.9 m² g^− 1 and small total pore volume of 0.03 m³ g^− 1, the assembled supercapacitor using this material exhibited excellent electrical properties even without activation. The supercapacitor had a capacitance of 50.2 F g^− 1 at 0.5 A g^− 1, power density of 644.9 kW kg^− 1 at an energy density of 15.7 Wh kg^− 1, and no capacitance loss after 1500 cycles. Therefore, transforming melt-spun LFs into LCFs with graphitic surfaces is an inexpensive and effective strategy to prepare high-performance electrode materials that are promising for energy storage applications.

Acknowledgements

The authors gratefully acknowledge financial support from National Natural Science Foundation of China (31901264 and 22002079) and Research Start–up Fund of the Shaanxi University of Science & Technology (2018–BJ20).

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Graphical abstract: Lignin-based carbon fibers with graphitic structures decorated on their surfaces were successfully prepared and further applied as electrode materials in supercapacitors.

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Fabrication of graphitized carbon fibers from fusible lignin and their application in supercapacitors

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Abstract

Figures

Introduction

Experimental

Materials

Melt-spinning Of Pegl

Preparation Of Cfs From Pegl

Supercapacitor Assembly

Characterization

Morphological analysis

Thermal Analysis

Carbonaceous Structure Analysis

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Electrochemical Performance Measurements

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Results And Discussion

Morphological And Thermal Analysis

Structural Determination

Electrochemical Performance Of The Lcf-based Supercapacitors

Conclusions

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Acknowledgements

References

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