Coal Liquefaction Residues Based Carbon Nanofibers Film Prepared

Research Article pubs.acs.org/journal/ascecg

Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Coal Liquefaction Residues Based Carbon Nanofibers Film Prepared by Electrospinning: An Effective Approach to Coal Waste Management Xiao Li,†,‡ Xiaodong Tian,† Tao Yang,†,‡ Yiting He,†,‡ Wenhong Liu,† Yan Song,*,† and Zhanjun Liu† †

CAS Key Laboratory of Carbon Materials, Institute of Coal Chemistry, Chinese Academy of Sciences, Taoyuan South Road, 030001 Taiyuan, China ‡ Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, China ACS Sustainable Chem. Eng. Downloaded from pubs.acs.org by MACQUARIE UNIV on 02/27/19. For personal use only.

S Supporting Information *

ABSTRACT: As a major byproduct of direct coal liquefaction, coal liquefaction residue (CLR) waste is environmentally harmful but also valuable. Considering environmental and economic efficiency, exploiting these wastes as electrodes for new generation energy storage devices will increase the economic value and decrease environmental pollution synchronously. In this regard, CLRs are used to prepare carbon nanofibers film for supercapacitors via electrospinning followed by HNO3 preoxidization, air stabilization, and carbonization processes for the first time. The influences of HNO3 preoxidization over the formation of fiber morphology, textural structure, surface chemistry, and electrochemical performance are investigated. Our work demonstrates that HNO3 preoxidization can enhance the content of the heteroatom of the as-spun nanofibers and promote the polymerization of the asphaltene (CLRA) molecular during air stabilization, leading to the changes of the thermal behaviors and thus avoiding the fibers melting successfully. The results show that the obtained sample exhibits a 3D nonwoven network with an average diameter of 200 nm, good flexibility, high content of nitrogen, and large specific surface area. Owing to these merits, the as-obtained sample shows high specific capacitance, excellent rate capability (143 F g−1 at 100 A g−1), and long lifespan (98% of its initial capacitance after 10 000 cycles) as supercapacitor electrode. KEYWORDS: Coal liquefaction residues, Asphaltene, Carbon nanofibers film, Supercapacitors, Electrospinning, HNO3 preoxidization

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INTRODUCTION Energy storage devices play an essential role in tackling the issues of storage of the intermittent energy from solar or wind and alleviating environment and energy crises.1−3 Among modern energy storage technologies, supercapacitors (SCs) have become a fast-growing system due to the distinct characteristics of high power density, safety and long cycling life.4−6 The United States and European Union have reported that carbon materials play an important role in energy devices. However, large amounts of carbon-based electrodes made by conventional slurry-coating method can hardly fulfill the demands of the fabrication of high-performance SCs because the polymer binder not only decreases the conductivity of the active materials but also blocks the contact between active material and electrolyte, which leads to low utilization efficiency and undesirable rate capability.7 Therefore, the preparation of high-quality integrated electrode is of great significance. Moreover, this innovative device also provides a new approach to develop highly flexible and lightweight SCs © XXXX American Chemical Society

used as power supply for next-generation portable and wearable electronic applications. Owing to its advantages of high surface area, light weight, structural controllability, and convenience in fabrication, electrospinning carbon nanofibers (CF) film has drawn great attention as flexible and freestanding electrodes for SCs.8,9 To date, a myriad of precursors, such as coal,10 pitch,11 biomass,12 lignin,13 and so on, has been used to fabricate the CF films. As a major byproduct of direct coal liquefaction, coal liquefaction residues (CLR) waste is environmentally harmful but also valuable.14,15 It would be meaningful if we can extract valuable organic components to fabricate high-value-integrated free-standing products for electronic and energy-related applications rather than combustion or gasification, which can increase the economic value and decrease environmental Received: October 10, 2018 Revised: November 26, 2018

A

DOI: 10.1021/acssuschemeng.8b05210 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Properties Analysis Data of CLRA elemental analysis (%) sample

extraction yield (%)

Mw

C

H

N

S

Oa

H/C

softening point (°C)

CLRA

39.1

530

90.81

5.62

1.44

0.09

2.04

0.73

108

a

By differential.

Scheme 1. Schematic of the Preparation Process for CLRACF Film

hexane and toluene soluble. The properties of the CLR and CLRA are listed in Tables S1 and 1, respectively. The CLRA is mainly composed of C, H, O and N elements and shows higher H/C ratio than CLR. By analyzing the relative data are shown in Figure S1 and Tables S2 and 1, the possible molecular structure of the CLRA could be roughly estimated. As depicted in Figure S2, the CLRA is mainly constructed by aromatic groups, aliphatic groups, and heteroatom, quite similar to the precursors of commercial pitch-based fibers.30 Synthesis of CLR-Based Carbon Nanofibers (CLRACF) Film. CLRA (2 g) was dissolved into tetrahydrofuran (THF) to form a 11 wt % solution; then, 6 wt % PAN (Mw = 1 500 000) in DMF was added into the above solution under magnetic stirring overnight. The mass ratio of CLRA to PAN is 2/3. Thereafter, the solution was transformed into a syringe with a needle diameter of 0.5 mm. The solution was spun into nanofibers through a positively charged needle using an electrospinning apparatus at 15 kV and a negatively charged copper foil to collect the fiber with a spinning distance of 12 cm. HNO3 pretreatment was introduced before traditional air stabilization. The as-spun nanofibers films were soaked in HNO3 (40 wt %) for 1, 2, and 4 h, respectively, at room temperature. The ratio of CLRACF to HNO3 is 1 mg/5 mL. Then, they were washed with distilled water until the filtrate was neutral. The yield of fibers after acid treatment is about 98%. Afterward, air stabilization was carried out in a tube furnace with the step temperature program, i.e., the sample was heated at 5 °C min−1 from 25 to 120 °C and held for 2 h, then the temperature raised to 160, 230, and 280 °C at 2 °C min−1 and kept for 2, 2, and 3 h, respectively. Finally, CLRACF films were obtained by carbonizing at 800 °C for 2 h under Ar. The obtained samples with different HNO3 preoxidation time were named as CLRACF-0 h, CLRACF-1 h, CLRACF-2 h, and CLRACF-4 h, respectively. As comparison, the as-spun nanofibers, HNO 3 preoxidization sample, air stabilization sample, and the sample treated with HNO3 preoxidization and air treatment were denoted as CLRA/ PAN film, L-CLRA/PAN film, G-CLRA/PAN film, and L/G-CLRA/ PAN film, respectively. The whole synthetic process of CLRACF film is schematically illustrated in Scheme 1. Characterizations. The structures and properties of CLRA were characterized by Fourier Transfer infrared spectroscopy (FT-IR, Bruker Tensor 27 spectroscopy), liquid 13C, 1H nuclear magnetic resonance spectroscopy (13C NMR and 1H NMR, AVANCE III), elementary analysis (EA, Vario EL CUBE), and gel chromatography analysis (GPC, PL-GPC50). The SP was obtained by a Fisher-Johns melting point apparatus.

pollution synchronously. However, it is well-accepted that the oxidation process plays an important role in controlling the physical and chemical properties for thermoplastic organic matters. Generally, stabilization in air under a reasonable range of time was commonly used to endow the dimensional stability of the as-obtained materials, especially for the preparation of carbon fibers.16 In addition, it has been confirmed that it is not easy to stabilize in air if the softening point (SP) of the precursor is lower than 200 °C.17 As an important thermoplastic component derived from CLR, asphaltene (CLRA) also faces such issue.18,19 Taking into consideration the SP of the CLRA (108 °C, much lower than 200 °C), how to solve the stabilization of the CLRA properly is thought to be a key role for the preparation CLRA-based carbon nanofibers (CLRACF) film. However, there are yet few reports about this aspect. Herein, inspired by recycling CLR wastes, CLRACF film was prepared by using the mixture of polyacrylonitrile (PAN) and CLRA as precursor through electrospinning technology, HNO3 preoxidization, air stabilization, and carbonization. HNO3 preoxidization prior to traditional air stabilization was chosen to solve the fusion problem faced in low SP precursors. In addition, the effects of the HNO3 preoxidization on the carbon fiber morphology and electrochemical performance for SCs are also investigated. Interestingly, HNO3 treatment can not only maintain the nanofibers morphology but also regulate the textural structure and surface chemistry. When the HNO3 pretreatment is 2 h, the sample shows high capacitance of 258 A g−1 at 100 mA g−1, admirable rate performance (145 F g−1 at 100 A g−1), and long lifespan (98% capacity retention after 10 000 cycles at 2 A g−1). Meanwhile, utilizing these CLR wastes based carbon fibers as binder-free electrodes for energystorage devices not only alleviates the reliance on the fossil fuels but also enhances the high value utilization of wastes.

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EXPERIMENTAL SECTION

Materials. The CLR wastes were obtained from pilot plant for direct liquefaction process in China. All reagents were purchased from the market without further purification. The asphaltene (CLRA) was derived from CLR wastes by Soxhlet extraction method using nB


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ACS Sustainable Chemistry & Engineering The structure characterizations of the samples were performed via field-emission scanning electron microscopy (FESEM, JSM-7001F) and transmission electron microscopy (TEM, JEM-2010), X-ray diffraction employing the Cu Kα radiation (XRD, D8 Advance), Raman spectrum (Raman, Horiba Labram HR800 spectrometer), and physical adsorption of N2 at 77 K using an automatic adsorption system (BET, ASAP 2020, Micromeritics). The composition of the samples was carried out by X-ray photoelectron spectroscopy (XPS, Thermo ESCALAB 250 spectrometer). Electrochemical Characterization. The free-standing electrodes were punched into rotundity with the mass loading of 1.5 mg cm−2. Cyclic voltammetry (CV), galvanostatic charge−discharge (GCD) curves, and electrochemical impedance spectroscopy (EIS) were carried out to confirm the electrochemical performance of the obtained samples, which used electrochemical workstation (CHI660E, Shanghai Chenhua Apparatus Co. Ltd., China). Land battery measurement system (CT2001A, Wuhan LAND Electronics Co., Ltd.) was used to test the cycling performance of the electrodes. For three-electrode system, the electrolyte, counter electrode, and reference electrode are 6 M KOH, a platinum foil, and a Hg/HgO electrode, respectively. For a two-electrode cell, two working electrodes with the same weight were systematically assembled with a polypropylene film as the separator. CV profiles were performed at the different scanning rates from 5 to 500 mV s−1 within the voltage range from −0.9 to 0 V for the three-electrode system and 0−1 V for the two-electrode cell. The results of the electrochemical performances were calculated by the following equations:20

Figure 1. SEM images of CLRACF film derived from as-spun nanofibers film (a, b) with different HNO3 preoxidization timed of (c) 0 h, (d) 1 h, (e) 2 h, and (f) 4 h, and the insets are photograph of them.

For the three-electrode system:

C=

I Δt m ΔV

(1)

For the two-electrode system:

2I Δt C= m ΔV E=

1C 1 ΔV 2 24 3.6

P=

3600E Δt

(Figure 1d−f). It can be deduced that the HNO 3 preoxidization time plays a crucial role to the final morphology and flexibility of the CF. More concretely, the surface of CF becomes rough with partial defects when the HNO 3 preoxidization time increases from 1 h (Figure 1d) to 2 h (Figure 1e). The larger amount of functional groups along with the extension of preoxidization time should be responsible for this phenomenon. After the HNO3 treatment, oxidization degree could be more effective after air stabilization, which leads to the abundant gas release during the carbonization process. In this stage, the resultant CLRACF films also show good flexibility (insert in Figure 1d,e) and can be directly applied in electrode for energy storage devices. However, when the HNO3 preoxidization time was reached up to 4 h, mechanical performance deteriorated due to sufficient pores or defects on the nanofibers (insert in Figure 1f). TEM measurement was also carried out to further get insight into the impact of HNO3 preoxidization time. CLRACF-2 h film (red cycle in Figure 2b) displays more surface roughness than that of CLRACF-1 h film (yellow cycle in Figure 2a), which agreed with the SEM images. The heterogenicity of the fiber would facilitate the electrolyte permeation, which in turn enhance the capacitance. The HRTEM (Figure 2c) image confirms the porous character of CLRACF-2 h, which would provide more contact area between electrode and electrolyte.21 The typical line-scan EDS profile of one single CLRACF-2 h shown in Figure 2d shows that C, N, and O elements are uniformly dispersed in the whole fiber. Taking flexibility and fiber morphology into consideration, herein CLRACF-2 h was selected to investigate the effects of HNO3 preoxidation on the as-spun nanofibers. EA and XPS results show that the obtained samples are mainly constituted

(2) (3) (4) −1

here C (F/g) is the specific galvanostatic capacitance, E (Wh kg ), and P (kW kg−1) are energy density and power density, respectively, I (A) is the current intensity, Δt (s) is the discharge time, m (g) is the total mass of electrode, and ΔV (V) is the voltage window except IR drop.

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RESULTS AND DISCUSSION The SEM images of the as-spun nanofibers film are shown in Figure 1a,b. It is clear that the pristine nanofibers show regular fibrous morphology with diameters mostly in the range of 250−300 nm, which stack randomly to from a threedimensional (3D) nonwoven network. In contrast to the pure PAN as-spun nanofibers film (Figure 1a2), the brown color of CLRACF as-spun film (Figure 1a1) indicates the coexistence of CLRA and PAN. No new absorption peaks are found in the CLRA/PAN film (Figure S3), which further indicates the physical mixture nature of CLRA and PAN. When the conventional air stabilization followed by carbonization process was adopted, the as-obtained CLRACF displays severe melting or deforming, and the film cannot maintain flexibility (insert in Figure 1c). However, with the introduction of HNO3 preoxidization prior to air stabilization, the morphology of resultant CLRACF exhibits the fibrous morphology without significant melting after carbonization C


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ACS Sustainable Chemistry & Engineering

previous works.26,27 Meanwhile, two new bonds at 1529 and 1338 cm−1 are ascribed to the characteristic of −NO2, indicating that HNO3 preoxidization can introduce the NO2 groups, which is consistent with the results of XPS. The peak at 1664 cm−1 ascribing to amide CO stretching of DMF disappeares in L-CLRA/PAN film, suggesting that the HNO3 preoxidization process is also conducive to the removal of DMF.28 The effect of HNO3 preoxidization on the thermal behavior was elucidated by TGA and DSC measurements. As shown in Figure 4b, the exothermic peaks in the DSC curves of the LCLRA/PAN film shift to lower temperature, which might originate from the higher oxygen functional group in L-CLRA/ PAN film.16,29 Meanwhile, the heat release of the cyclization reaction decreases from 536 J g−1 in CLRA/PAN film to 387 J g−1 in L-CLRA/PAN film, which makes cyclization reaction more easy to occur. The differences between G-CLRA/PAN film and L/G-CLRA/PAN film are depicted in Figure 4c,d. As illustrated in Figure 4c, the intensity of peaks at 810, 1370, and 1598 cm−1, assigned to −C−N−H or aromatic −C−H deformation, −C−H− and C−N− or aromatic CC,30 respectively, becomes stronger for L/G-CLRA/PAN. It can be assigned to the enhanced polymerization and polycondensation reactions during the air stabilization after HNO3 preoxidization, which has been proven by previous findings for the catalytic effect of HNO3 and nitrogen oxides on the polymerization of pitches.31,32 As a consequence, the carbon yield of L/G-CLRA/PAN film is largely enhanced compared to G-CLRA/PAN. In addition, the lower decomposion rate at the range of 380−1000 °C shown in L/G-CLRA/PAN film should be beneficial to maintain the fiber structure. From the measurements above, one can know that HNO3 preoxidization prior to air stabilization is essential to improve the stability of fibrous structure through improving the content of heteroatom, thus changing the thermal behaviors during the air stabilization and carbonization. But what about the effects of HNO3 preoxidization time on the carbon structure? In order to elucidate this issue, some other tests were carried out. Two

Figure 2. TEM images of CLRACF-1 h film (a) and CLRACF-2 h film (b); (c) HR-TEM image and (d) EDX mapping of CLRACF-2 h.

of C, N, O, and H. After the HNO3 preoxidization, the contents of N and O increase while that of C and H decrease. For the N 1s spectra of the L-CLRA/PAN film (Figure 3d), a new peak appearing at 404.1 eV corresponding to the −NO2 groups might result from the reduction product of CLRA molecular by HNO3.17 The probable reaction was illustrated in Scheme 2a. The high-resolution spectrum of O 1s (Figure 3e) shows that three peaks at the binding energy of 532.1, 532.7, and 533.8 eV assigned to CO (OI), C−O−H/C−O−C (OII), and O−CO (OIII) were found.22 The augment of OIII group for L-CLRA/PAN film suggests that HNO3 preoxidization mainly contributes to the existence of carboxyl, acid anhydride, and ester group, as illustrated in Scheme 2b. Moreover, an increment of 5.7% for peak of C−C aromatic sp2 bonds at binding energy of 284.5 eV is observed in Figure 3f.23 Figure 4a displays the FT-IR spectra of the obtained samples. As anticipated, after the HNO3 preoxidization, the amount of oxygen groups (3500 cm−1, 1560−1700 cm−1) in LCLRA/PAN film increases, which is in agreement with

Figure 3. (a) Element analysis of C, H, O, and N. (b) XPS spectra and (c) calculated weight percentages of C, O, and N elements by XPS; highresolution XPS spectra of (d) N 1s, (e) O 1s, and (f) C 1s of CLRA/PAN film and L-CLRA/PAN film. D


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ACS Sustainable Chemistry & Engineering Scheme 2. Oxidation Mechanisms for CLRA with HNO3a

a

Adapted from mechanisms in refs 24 and 25. Copyright 2005 and 2016, respectively.

Figure 4. FT-IR spectra of as-obtained samples (a) and (c); TGA and DSC curves (b) of samples in air. (d) TGA curves of the stabilization samples under Ar.

Figure 5. (a) XRD pattern and (b) Raman spectra of CLRACF-1h and CLRACF-2h.

Table 2. Textural of the Samplesa samples

SBET (m2/g)

Smic (m2/g)

Smic/SBET (%)

Vtot (m3/g)

Vmic (m3/g)

OXPS (at %)

NXPS (at %)

CLRACF-1 h CLRACF-2 h

633 760

558 692

88.2 91.1

0.306 0.368

0.259 0.320

4.45 4.83

5.12 6.34

a

SBET is the specific surface area by BET method. Smic is the micropore area achieved by t-Plot method. Vtot is total pore volume at P/P0 = 0.995; Vmic is the micropores volume; O and N content derived from XPS.

spectra for CLRACF-1 h film and CLRACF-2 h film show similar shape with a peak of D band at 1370 cm−1 and G band at 1580 cm−1, corresponding to the defect and disorders at the edges of the graphite layers and the vibration of in-plane C−C bonds of graphitic carbon,36 respectively. In general, the intensity ratio of ID/IG is indicative of the disorder degree of

broad peaks around 23 and 43°, corresponding to the (002) and (001) diffraction modes of carbon, are observed in Figure 5a.33 With the expansion of HNO3 preoxidization time, the d(002) increases from 0.3658 to 0.3705 nm, indicating more defects and less dense microstructure in CLRACF-2 h, which would be beneficial for the storage of the ions.34,35 The Raman E


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Figure 6. High-resolution XPS spectra of (a) O 1s and (b) N 1s of the samples.

Figure 7. (a) N2 adsorption/desorption isotherms and (b) pore size distributions of the samples.

Figure 8. Electrochemical performance of the obtained CF films carried out in a three-electrode system. (a) CV curves at the sweep range of 5−200 mV s−1. (b) Galvanostatic discharge−charge curves. (c) Galvanostatic discharge−charge curves of samples at 0.2 A g−1. (d) Specific capacitance at various density, (e) IR drop, and (f) EIS analysis.

the carbon structure. The ID/IG ratio of CLRACF-2 h film is slightly higher (1.13) than that of CLRACF-1 h film (1.04), which is well in accordance with the XRD result. The XPS analysis (Table 2) shows that the contents of N and O increase along with the increase of HNO 3 preoxidization time. CLRACF-2 h film exhibits higher existence of the heteroatom, which could improve the wettability of the electrode and electrolyte and induce the pseudocapacitance. The O 1s spectra (Figure 6a) can be deconvoluted into four peaks of OI, OII, OIII, and adsorption of water or oxygen at the binding energies of 521.4, 532.7, 533.8, and 535.1 eV, respectively.37 With increasing HNO3

preoxidation time, the content of OIII enhances, while that of the OII content decreases. As shown in the N 1s spectrum in Figure 6b, three peaks at 397.6, 399.6, and 402.4 eV, ascribing to pyridinic N (N-6), pyrollic N (N-5) and oxidic N, respectively,38 are found. In detail, CLRACF-2 h film shows the content of N-Ox (12%) higher than that of CLRACF-1 h film (5.9%), which can be ascribed to the longer time of HNO3 preoxidization. The N2 adsorption/desorption isotherms of CLRACF-1 h film and CLRACF-2 h film were investigated to reveal the porous texture. As shown in Figure 7a, all samples show a typical type I isotherm, indicating the well-defined micropores. F


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Figure 9. Electrochemical performances of CLRACF-2 h film experimented in two-electrode system. (a) CV curves at the scan range of 5−500 mV s−1. (b) Specific capacitance at various current densities from 0.1 to 100 A g−1. (c) Ragone plot measured in 6.0 mol L−1 KOH. (d) Cyclic performance at the current of 2 A g−1.

The improved adsorption quantity of CLRACF-2 h film demonstrates the positive effect of HNO3 preoxidation on porous structure. Besides, Figure 7b exhibits the pore size distributions (PSD) decided by the NLDFT method. The two samples exhibit similar pore size distributions from 0.6 to 1.8 nm, which are suitable for the transportation of the solvated OH− and K+ in KOH solution.39 As listed in Table 2, the SSAs of the samples mainly originate from the micropores, indicating the characteristic of micropores as well. CLRACF2 h film has a SSA of 760 m2 g−1 and is larger than that of CLRACF-1 h film (633 m2 g−1) and other carbon fiber,40,41 resulting from the higher content of heteroatom thus leading to the release of large amount of gas during the carbonization process. It can be found that the pore parameters are relevant to the HNO3 preoxidization time. With the increasing of HNO3 preoxidization time, both the micropores surface area and micropores volume enhanced, suggesting that the introduction of HNO3 would promote the formation of micropores. Apart from flexibility, the high SSA, richness of heteroatoms, and proper PSD make CLRACF-2 h film a promising candidate for SC application. Cyclic voltammetry (CV) curves of CLRACF-2 h film show a typical of quasi-rectangle shape (Figure 8a), which presents slightly changes even at the scan rate of 200 mV s−1, indicating the fast rate response.42 The highly symmetric charge−discharge curves at different current densities shown in Figure 8b suggest that the CLRACF-2 h electrode exhibits high Coulombic efficiency and excellent reversibility. To better elucidate the advantage of CLRACF-2 h film, the GCD curves of CLRACF-1 h and CLRACF-2 h electrodes were compared with each other. As described in Figure 8c, the better capacitive performance of CLRACF-2 h electrode can be evaluated from the longer discharge time at 0.2 A g−1. The distinct arc indicates the existence of pseudocapacitance contributed by heteroatom functional groups. In order to further probe the contribution of pseudocapacitance to the total specific capacitance, the discharge plot can be divided into two parts. The proportion

of pseudocapacitance can be calculated from the difference between the total discharge time and the discharge time of electric double layer. The deflections of the samples are enlarged with the extension of HNO3 preoxidation time due to the increased heteroatom content. The gravimetric capacitances calculated on the basis of discharge curves by eq 1 reveal that the CLRACF-2 h film shows a higher specific capacitance than CLRACF-1 h film at the whole current rates. As the current density enhances to 100 A g−1, the specific capacitance of CLRACF-2 h film still presents 143 F g−1, indicating the good rate performance (55% capacitance retention after current density enlarging for 1000 times). Meanwhile, the capacitances slightly drop from 2 to 100 A g−1 with the capacitance retention of 74 and 75% for CLRACF-1 h and CLRACF-2 h films, respectively. The small discrepancy in rate performance of the samples can be attributed to the similar PSD. CLRACF-1 h and CLRACF-2 h films show better capacitance performance than those of CLRACF-0 h (Figure S5) and PANCF (Figure S6). Figure 8e shows the internal resistance (IR) drops of the samples. It is clearly seen that the CLRACF-2 h film has a smaller IR drop than CLRACF-1 h film at the given current density, which might due to that the higher content of heteroatom results in good electrode wettability. This observation is verified by the comparison of the electrochemical impedance spectroscopy (EIS) measurement. The EIS curves are composed of a semicircle in the high frequency region and a vertical sloping line at low frequency (Figure 8f).43 The diameter of the semicircle for CLRACF-2 h film (0.46 Ω) is smaller than that for CLRACF-1 h film (0.67 Ω), indicating a small charge-transfer resistance. The symmetric SCs were assembled to completely determine the electrochemical performance of CLRACF-2 h film. The CV curves of CLRACF-2 h film at various scan rates exhibit perfect rectangular shapes even at the scan rate of 500 mV s−1 (Figure 9a), indicating the faster and more efficient charge transfer in the aqueous electrolyte.44 Figure 9b shows that the capacitance of CLRACF-2 h film is 103 F g−1 at 100 A g−1, resulting in the capacitance retention of 75% to the G


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ACS Sustainable Chemistry & Engineering capacitance at 1 A g−1. The Ragone plot of CLRACF-2 h film is shown in Figure 9c. It exhibits high energy density of 4.7 Wh kg−1 at a power density of 249 W kg−1, which is higher than that of other carbon materials.45,46 The energy density can remain at 2.0 Wh kg−1 at a power density of 9120 W kg−1. As described in Figure 9d, even after 10 000 cycles, the capacitance retention still remains beyond 98%, suggesting the remarkable stability of the structure. Moreover, the electrochemical performance of this work is even better than that in the previous studies (Table S3). The superior rate capability and cyclic life should be ascribed to the following: (i) 3D conductive network ensures highways for electronic transport; (ii) high SSA and large amount of active sites of the nanofibers provide more accessible surface for energy storage; (iii) freestanding and binder-free nature endows fast ion response. Furthermore, the coexistence of oxygen and nitrogen enhances the wettability between the electrolyte and electrodes.

Notes

The authors declare no competing financial interest.

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CONCLUSIONS CLR-based carbon nanofibers film has been fabricated by electrospinning the mixture of CLRA and PAN into as-spun nanofibers followed by HNO3 preoxidization, air stabilization, and carbonization. HNO3 preoxidization of the as-spun nanofibers films plays a rival role in keeping the nanofibers morphology, which results in changes with respect to functional group and thermal behavior. The carbon texture and electrochemical performance are largely influenced by HNO 3 preoxidization time. The increment of HNO 3 preoxidization time results in higher content of heteroatom functional group, more surface defects and larger SSA, leading to an remarkable rate capability with the capacitance of 143 F g−1 at 100 A g−1 (55% capacitance retention after current density expanding 1000 times). Moreover, CLRACF-2 h also shows a high energy density (4.7 Wh kg−1) and intriguing cycling life (98% after 10 000 cycles at 2 A g−1), which originates from the unique 3D interconnected conductive network and short diffusion path of ions and charge. Furthermore, CLRACF’s combination of low cost, scalable synthesis, heteroatom functional group, flexible nature, and superior electrochemical performance might promote the application for the flexible supercapacitors and/or flexible lithium-ion battery or potassium-ion battery. ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.8b05210.

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ACKNOWLEDGMENTS

This work was supported by the Natural Science Foundation of China (Nos. U1610119, U1610252), the Key Research and Development Program of Shanxi Province (No. 201603D112007), and Youth Innovation Promotion Association, Chinese Academy of Sciences (118800QCH1).

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Properties of CLR, FT-IR spectrum, structure parameters and possible structure model of CLRA, FT-IR spectra of CLRA, PAN and CLRA/PAN film, thermogravimetric-mass spectroscopy (TG-Ms), specific capacitance of the compared samples (PDF)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Xiaodong Tian: 0000-0002-4737-809X Yan Song: 0000-0001-7939-6316 H


Research Article


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Coal Liquefaction Residues Based Carbon Nanofibers Film Prepared

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