Raw cotton derived N-doped carbon fiber aerogel as efficient

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Raw cotton derived N-doped carbon fiber aerogel as efficient electrode for electrochemical capacitors Juan Du, Lei Liu, Zepeng Hu, Yifeng Yu, Yue Zhang, Senlin Hou, and Aibing Chen ACS Sustainable Chem. Eng., Just Accepted Manuscript • DOI: 10.1021/ acssuschemeng.7b04396 • Publication Date (Web): 30 Jan 2018 Downloaded from http://pubs.acs.org on February 5, 2018

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ACS Sustainable Chemistry & Engineering

Raw cotton derived N-doped carbon fiber aerogel as efficient electrode for electrochemical capacitors Juan Du†, Lei Liu†, Zepeng Hu†, Yifeng Yu†, Yue Zhang†, Senlin Hou*,‡, Aibing Chen†,* †

College of Chemical and Pharmaceutical Engineering, Hebei University of Science

and Technology, 70 Yuhua Road, Shijiazhuang 050018, China. ‡

The Second Hospital of Hebei Medical University, 215 Heping Road, Shijiazhuang,

050000, China. *Corresponding Authors: E-mail: (A.C.) [email protected], (S.H.) [email protected].

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ABSTRACT: The carbon fiber aerogel with high surface area, as one of promising ideal electrodes for electrochemical capacitors (ECs), has attracted tremendous attentions because they can offer fast charge-discharge rate and exceptionally rate capability. However, there is still a big challenge to acquire highly porous fiber aerogel with elastic properties and good flexibility. Herein, N-doped porous carbon fiber aerogel (N-CFA) has been successfully fabricated using raw cotton as a fibrous template to obtain shaped aerogels. ZIF-8, in situ fabricated on the surface of cotton, played the role of an active agent and nitrogen source to create rich porous structure with certain nitrogen content. Benefiting from the macro monolith morphology with good flexibility, favorable mechanical durability, nitrogen doping and high specific surface area, the N-CFA exhibits excellent electrochemical performance with high capacity of 365 F g-1 at current 0.5 A g-1 and a remarkable rate capability of 65.8 % from 0.5 A g-1 to 100 A g-1 as a binder free electrode in ECs. Additionally, N-CFA also displays a high capacitance retention of 93.6 % after 10000 consecutive cycles at 5 A g-1. The strategy may offer a low-cost and scalable method to produce high-performance N-doped carbon aerogels for electrode from biomass.

KEYWORDS: Electrochemical capacitors, Binder free electrode, N-doped porous carbon fiber aerogel, Raw cotton.

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INTRODUCTION The rapid increase in world population and economic expansion around the world have led to increasing use of energy based appliances, which eventually results in high energy consumption[1]. Owing to their high power density and superior cyclability relative to batteries, electrochemical capacitors (ECs) have emerged as an important electrical energy storage technology that will play a critical role in the large-scale deployment of intermittent renewable energy sources, smart power grids, and electrical vehicles[2-5]. Generally, carbon materials have been considered as promising electrode materials for ECs owing to their large specific surface area, rich porous structure, tunable pore sizes and stable chemical properties[3, 6]. However, the conventional powder carbon materials largely hinder their utilization owing to the dustability and tedious operation process when they are assembled as electrode in ECs. The monolith carbon or carbon aerogels, such as porous carbon monolith, graphene aerogels and carbon nanotube (CNT) aerogels, can solve these problems well. As one of ultralight materials with many interesting properties, carbon fiber aerogel (CFA) has been extensively investigated due to its 3D inter-connected networks and outstanding properties, such as low density, highly continuous porosity, good electrical conductivity and chemical inertness[7]. Moreover, the operation is simpler when CFA is used as electrode because no binder and conducting additive are needed. Generally, there are three kinds of CFA, such as organic-derived CFA[8-9], graphene or CNT-based aerogels[10-11] and biomass-derived CFA[7, 12-13]. Conforming to green economy and sustainable development of the society, biomass-derived CFA has 3



received considerable attentions in recent years owing to their rich resource, low-cost, nontoxicity, and renewability[14-15]. Raw cotton, a biomass consisting of entangled microscale cellulose fibers, is light and flexible and can be converted into CFA with 3D networks by thermal annealing process[16]. Meanwhile, raw cotton can be even spun into long fibers and woven into fabrics, exhibiting excellent processibility[17]. Generally, cotton derived CFA usually needs special activation and modification to achieve functionality and high performance in ECs. Many efforts have been made to enhance the performance through adjusting the pore structure and modifying the surface property[18]. Chemical activation by KOH, ZnCl2 and K2CO3 is an efficient way to create rich porous structure and improve the surface area for carbon fiber. Introducing surface functional groups or doping heteroatoms can significantly modify the surface properties of CFA. Among them, nitrogen doping have been widely studied to modify the surface property of CFA to improve the electronic conductivity. Metal-organic frameworks (MOFs) have gained particular attention, because they can be modularly synthesized by self-assembly of transition-metal clusters and organic molecules, leading to design framework structures and diverse function[6]. Moreover, MOFs can lead to heteroatom doping for carbon framework due to the heteroatom containing substances, for example, 2-Methylimidazole (H-mim) and so on. Motivated by their high surface area, rich porous structure and doped heteroatom, several MOFs, such as ZIF-67 and ZIF-8, have recently been demonstrated as promising precursors to construct microporous carbons[3, 19] with high performance in electrochemistry[1, 20]. But the obtained microporous carbon materials derived 4


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from MOFs are normally powders, limiting their application. In this work, raw cotton was employed as a fibrous template, on which ZIF-8 in-situ growed, to produce N-doped flexible hierarchically porous carbon fiber aerogel (N-CFA). The easy processibility of raw cotton endowed N-CFA with macro monolith morphology and good flexibility, meanwhile, in situ fabricating of ZIF-8 on raw cotton allowed the construction of rich porous structure with large surface area with nitrogen doping. This strategy showed a feasible, low-cost and environmentally friendly method for preparing N-CFA from biomass, which is rich-resourced, inexpensive and renewable. As binder free electrode, the N-CFA exhibited high capacity. Notably, the N-CFA also showed an excellent rate capability and good stability. EXPERIMENTAL SECTION Preparation of N-CFA: ZIF-8/Cotton composite was synthesized by a one-pot method. Briefly, H-mim (263 mg) and Zn(NO3)2·6H2O (258 mg) were dissolved into methanol (20 mL) respectively to form a solutions. Subsequently, 0.1 g raw cotton, which was cut into a small rectangular shape, was added into the solution of H-mim for stirring about 12 h at room temperature. Then, the solution of Zn(NO3)2·6H2O was added into the composite of H-mim and cotton, and stirred for 5 min. The solution was aged at room temperature for 24 hours. The cotton was then taken out carefully and washed very carefully with methanol and dried. Finally, the N-CFA was obtained after carbonization at 950 °C in N2.

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In order to confirm the superiority of N-CFA derived from ZIF-8/Cotton composite, ZIF-8 and raw cotton was also carbonized at 950 °C in N2 (denoted as CFA and PC respectively) and ZIF-8 was prepared as previous reported[21]. Characterization: X-ray diffraction (XRD) patterns were achieved using a Rigaku D/MAX-2500 system with Cu-Kα (λra.15406 nm). The morphology and microstructure of N-CFA were investigated by scanning electron microscopy (SEM, HITACHI S-4800-I). Nitrogen adsorption-desorption isotherms were carried out on a Micromeritics TriStar 3020 instrument at -196 °C. The Brunauer-Emmett-Teller (BET) method was employed to calculate the specific surface area, while the Barrett-Joyner-Halenda (BJH) method was applied to analyze the pore size distribution using the desorption branch of isotherm. The total pore volume was obtained from the amount of N2 adsorbed at the relative pressure (P/P0=0.97). X-ray photoelectron spectroscopy (XPS) was conducted on a Thermo Scientific ESCALab 250Xi system using an Al-Kα radiation under a vacuum of 3×10-10 mbar. Electrochemical Measurements: The as-obtained N-CFA was cut into pieces and used as binder free working electrode directly. Electrochemical measurements were carried out in both three-electrode and two-electrode system using an electrochemical workstation (CHI 760E, Chenhua Instruments, China) with 6 M KOH solution as the electrolyte. For three-electrode system, the N-CFA samples (with the thickness of 2 mm and mass of 5 mg) were connected with Pt plate (1cm×1.5cm) using conductive glue. The Pt wire and Hg/HgO were used as the counter and reference electrodes. For the electrochemical test in the two-electrode system, two slices of N-CFA (with each 6


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of thickness of about 2 mm) as electrodes were separated by a filtrate paper (pore size: 0.4 μm), and immersed in 6 M KOH, then tested by the current collector. Electrochemical performances were evaluated by cyclic voltammetry (CV), galvanostatic charge-discharge (GCD) and electrical impedance spectroscopy (EIS) analysis. For the two-electrode system, the specific capacitances (C, F g-1), energy density (E, Wh kg-1) and power density (P, W kg-1) were calculated by the following equations: C=4 I∆t/∆Vm, E=0.5 C(∆V)2 and P=E/∆t, where I (A), ∆t (s), ∆V (V) and m (g) are GCD current, discharge time, voltage window, and mass of active material, respectively. In the three-electrode system, the specific gravimetric capacitance was according to the GCD measurements: C=I∆t/∆Vm. RESULTS AND DISCUSSION

Figure 1. Schematic representation of the fabrication processes of N-CFA. The N-CFA was prepared by converting raw cotton into shaped carbon fiber aerogels and in situ fabricating ZIF-8 on the surface of cotton to introduce a significant amount of nitrogen and rich pores on the wall, as shown in Figure 1. Raw 7



cotton was first immersed in the H-mim solution for hours. In this process, the group of =N in the H-mim could interact with -OH group on the cotton fiber through hydrogen-bonding and electrostatics attraction. Secondly, Zn(NO3)2 solution was added into the composites and ZIF-8 in situ generated on the surface of cotton. The photograph of ZIF-8/Cotton composite (Figure S1a) showed that macroscopic feature of cotton was not destroyed when incorporated with ZIF-8. The black monolithic N-CFA was fabricated after annealing treatment under 950 °C in N2, as shown in Figure 1. In the process of pyrolysis, the sublimation of Zn species existed in the ZIF-8 can create a large number of pores. In addition, the carbon fiber derived from cotton can be activated by the large amount of gas. The N-CFA contracted approximately 10% due to pyrolysis of precursor (Figure S1b). Remarkably, the N-CFA exhibited good flexibility and favorable mechanical durability, as shown in Supporting Video and Figure 2a, indicating that the N-CFA was robust enough to sustain repeated bending and buckling without destroying and providing a good foundation for its application as flexible electrode. During the annealing process, ZIF-8 could introduce nitrogen into carbon framework and Zn species would sublime which created rich porous in N-CFA. The morphology of Cotton/ZIF-8 composite was investigated by SEM observation as shown in Figure S2. Generally, the fibers of raw cotton wool is smooth[22]. However, the Cotton/ZIF-8 composite showed rough surface (Figure S2a). By zooming in a single fiber, it was worth noting that some substances were seen on the surface of the fibers (Figure S2b), indicating that there was a good affinity between 8


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ZIF-8 and cotton[16]. In order to confirm that ZIF-8 had successfully grown on the surface of cotton, FT-IR spectra of the ZIF-8, raw cotton and Cotton/ZIF-8 composite were analyzed (Fig S3a). As shown in FT-IR spectrum of ZIF-8, the stretching and bending modes of the imidazole ring were in the range of 600-1500 cm-1. The peak at 1584 cm-1 corresponded to the stretching mode of C=N bond in H-mim, and the peaks at 2929 and 3138 cm-1 were attributed to the C-H stretching of the aromatic ring and the aliphatic chain in H-mim, agreeing well with the reported spectrum of ZIF-8[23]. Some main peaks ascribed to ZIF-8 also could be found in the spectrum of Cotton/ZIF-8 composite, indicating that ZIF-8 had grown on the surface of cotton. The wide angle XRD patterns further confirmed the growth of ZIF-8 on the surface of cotton, as shown in Figure S3b. It could be seen that there was only one sharp peak found in XRD patterns of raw cotton. However, the Cotton/ZIF-8 composite showed sharp peaks at 2θ =16.8, 19.5, 23.8 , 26.5 and 27.8° (the red sign in Figure S3b), which can be well indexed to the ZIF-8[24-25].

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(a)

(b)

(c)

(d)

(e)

(f)

Figure 2. Photograph (a) and SEM (b-d) and TEM (e-f) images of N-CFA. SEM were performed to obtain detailed morphology of the as prepared N-CFA as shown in Figure 2b-d. SEM images at low resolution (Figure 2b) showed that the size of cotton fiber was about 7 µm, indicating the flexible fiber of raw cotton was not destroyed during the annealing process. Furthermore, unlike the surface of Cotton/ZIF-8, the surface of the N-CFA fibers abounded with wrinkle and gully inherited from raw cotton[7]. Higher resolution SEM image (Figure 2c and d) showed rich pores with size of 10-50 nm (the red sign) on the surface of N-CFA, indicating ZIF-8 had played positive effect in generating the porous structure of N-CFA and had 10


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introduced richer porous into cotton fiber due to the sublimation of Zn species at high temperature[26]. The TEM images (Figure 2e and f) reveal more details of the porous texture of the HPC-1000. It was clear that the N-CFA possesses fiber morphology and had rich macroporous structure with the size of 50-300 nm. Rich disordered mesopores and micropores could be seen from the high-resolution TEM images (Figure 2e and inset), which further confirm the rich porous structure.

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50 40

(b)

N-CFA CFA PC

Intensity (a.u.)

(a) Volume Adsorbed (cm /g)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


30 20 10 0 0.0

0.2

0.4

0.6

0.8

1.0

20

Relative Pressure (P/P0)

40 60 2 Theta (degrees)

80

Figure 3. Textual characteristic: N2 adsorption-desorption isotherm (a) of N-CFA, CFA and PC and XRD (b) patterns of N-CFA. The highly porous texture of N-CFA was verified by N2 adsorption/desorption measurement. As shown in Figure 3a, N-CFA displayed significantly enhanced N2 uptake and obvious capillary condensation at relative pressure (P/P0) below 0.1, which was ascribed to adsorption of N2 in abundant micropores[27]. There was a pronounced hysteresis in the P/P0 range of 0.4-0.9, implying the presence of a large number of mesoporous[5]. Additionally, the adsorption-desorption isotherms of all the samples were not closed at the relative pressure range of 0.9-1, indicating the presence of macropores. In order to confirm the favorable influences of in-situ grown ZIF-8 on N-CFA, the N2 adsorption/desorption of CFA and PC, obtained by directly 11



carbonized raw cotton and ZIF-8 respectively, were also analyzed. As shown in Figure 3a, the very low N2 uptake of CFA revealed its poor porous structure[17]. In addition, there were no obvious hysteresis loops from the N2 adsorption/desorption of PC, indicating the main microporous structure[6]. The porous structure of N-CFA, CFA and PC was further clarified by the pore size distribution. It was obvious that N-CFA showed a wide peak at 11.2 nm (Figure S4). But the CFA and PC had not obvious pore size distribution, indicating their poor porous structure. The Table S1 showed the detail textural properties of N-CFA, CFA and PC. It could be seen that the specific surface area of N-CFA was far higher than that of CFA and PC. Notably, the specific surface area of N-CFA as high as 2183 m2 g-1, was higher than many CFA derived from raw cotton[7, 17, 27] and other porous carbon derived from bio-mass[22, 28-29]. Meanwhile, the N-CFA possessed richer micropores than CFA, indicating that introduction of ZIF-8 into cotton fiber could improve its specific surface area and create richer pores. The wide angle XRD pattern of N-CFA could be observed in Figure 3b. One wide characteristic peak of the sample was located at around 2θ = ~23o corresponding to the amorphous carbon[30].

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100

96.90% 90.08%

CFAs N-CFAs

C1s

80 60 40

C1s

20

7.95% 1.97% 3.10%

0 C

O

Intensity(a.u.)

Intensity(a.u.)

(b)

120

Content (%)

(a)

N

Elements

O1s N-CFA

N1s

CFA 1200 1000

800

600

400

200

0

292

288 284 Binding Energy(eV)

Binding Energy(eV)

(c)

(d)

280

N1s

Intensity(a.u.)

O1s

Intensity(a.u.)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


540

408

536 532 528 Binding Energy(eV)

404 400 Binding Energy(eV)

Figure 4. XPS spectra of N-CFA and CFA (a), C1s (b), O1s (c) and N1s (d) spectrum of N-CFA. XPS was used to identify the elemental composition of N-CFA and CFA. As shown in Figure 4a, both CFA and N-CFA exhibited two peaks at 284.6 and 533.1 eV, corresponding to C and O elements respectively, suggesting an effective carbonization. In addition, there was no N element observed from the XPS spectra of CFA. In contrast, the peaks at 533.1 eV, corresponding to N 1s, was observed clearly for the N-CFA (Figure 4a). Moreover, contents of carbon, oxygen and nitrogen was shown in inset of Figure 4a. It was obvious that the N-CFA consisted of 90.08 at% carbon, 7.95 at% oxygen and 1.97 at% nitrogen. High-resolution XPS spectra of each element were performed and fitted, and the corresponding ratios in different forms of each element were calculated based on the fitted peaks. The C1s core-level spectrum of both samples were divided into three components centering at about 284.6, 285.5 and 13



289.0 eV, as shown in Figure 4b and Figure S5a, ascribed to the primary C-C/C=C carbon bonds, residual C-O bonds and C=O bonds. In the high resolution oxygen spectrum of N-CFA shown in Figure 4c, there are two peaks assigned to the carboxyl C=O bands centering at 534.1 eV and hydroxyl oxygen in amorphous hydrogenated carbon at 533.1 eV, respectively. The high-resolution O1s XPS of CFA revealed (Figure S5b) three peaks located at 532.2, 532.6 and 533.8 eV, assigned to the carboxyl C=O bands and hydroxyl oxygen. The high-resolution N1s XPS analysis revealed the introduction of nitrogen atoms into the structure of N-CFA (Figure 4d). Accordingly, nitrogen atoms of the N-CFA were found in three different contributions in the carbon matrix: pyridinic N (399.6 eV), pyrrolic-N (400.3 eV), and pryridine-N-oxide (401.2 eV), agreeing with three types doped nitrogen in carbon depending on the bonding environments, including pyrrolic N, pyridinic N, and graphitic N (inset of Figure 4d). The different surface chemical properties between N-CFA and CFA revealed that the ZIF-8 have effectively modified and changed the surface of cotton derived carbon fiber.

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(a)

(c) 20

100

0 -1

10 mV s -1 20 mV s -1 30 mV s -1 40 mV s -1 50 mV s -1 100 mV s

-20

-40 -1.0

-0.8

-0.6

-0.4

-0.2

0.0

Current Density (A g-1)

Current Density (A g-1)

(b)

0

-1

200 mV s -1 300 mV s -1 400 mV s -1 500 mV s

-100

-1.0

0.2

-0.8

-0.6

Potential (V) 0.0

-1

0.5 A g -1 1Ag -1 2Ag -1 3Ag -1 4Ag -1 5Ag -1 10 A g

-0.2 -0.4 -0.6 -0.8

-0.4

-0.2

0.0

0.2

Potential (V)

(e)0.0

-1

20 A g -1 30 A g -1 40 A g -1 50 A g -1 100 A g

-0.2 Potential (V)

(d)

Potential (V)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


-0.4 -0.6 -0.8 -1.0

-1.0 0

400

800

1200

0

5

10

Times (s)

15 20 Times (s)

25

30

Figure 5. The model of porous carbon fiber for ion transport (a), CV curves at different scan rates (b, c), GCD curves at different current densities (d, e) of N-CFA. Porous carbon fiber materials have been regarded as one attractive candidate for supercapacitor electrode materials due to the rich porous structure and high specific surface area. The N-CFA is a suitable electrode material with unique 3D porous structure, nitrogen doping and very large exposed surfaces to electrolyte, as shown in Figure 5a[31-32]. The as-prepared N-CFA was directly applied as a binder-free ECs electrode in 6 M KOH for a three-electrode cell. The performance of N-CFA aerogels as ECs electrode was characterized by CV measurements (Figure 5b and c). In Figure 5b, it could be seen that when the scan rate gradually increased from 10 to 100 mV s-1, 15



a regular rectangular shape was retained, indicating a good capacitance performance at a high scan rate and an ideal EDLC. When the current density increases to 200 mV s-1 (Figure 5c), the CV curve showed a little tilt rectangular-like shape, implying a fast charge/discharge process with high power capability and low equivalent series resistance. Rate capability is another key factor for the practical application of carbon based electrode materials. Figure 5d and e show GCD curves of N-CFA over the range of current densities from 0.5 to 100 A g-1. All GCD curves at various current densities were quasi-triangular and symmetrical, indicating that the electrodes possessed typical EDLC behavior and superior charge-discharge reversibility. The discharge curves only exhibited a rather small voltage drop of 0.20 V (Figure 5e) at 100 A g-1, suggesting a rather low internal resistance. The specific capacities calculated by using discharge branches was 365 F g-1 at 0.5 A g-1, which was much higher than that of many other biomass-derived carbons (details in the Table S2). As well as, the rate performance at the current density ranging from 0.5 to 100 A g-1 demonstrated the capacitance retention 65.8 % (Figure 6a).

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(b)100

300

100

-0.4 358.5 F g

-0.6 -0.8

85.1 F g

-1

168 F g

-1

-1

-Z'' (Ohm)

ZIF-8/Cotton ZIF-8 Cotton

-0.2

60

-Z'' (Ohm)

200

4

80

65.8% 0.0

Potential (V)

Specific Capacitance/Fg

-1

(a)

3 2 1

40

0

20

0

1

-1.0 0

0

0

200

20

400 Times (s)

2 Z' (Ohm)

3

4

600

0

40 60 80 -1 Cuttent Densiyt/Ag

100

(c)100

0

20

40

60 Z' (Ohm)

80

100

(d) -1

Energy Density (Wh kg )

Capacitance Retention(%)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


80

93.6 %

10

60 40 20 0

0

2000

4000 6000 Cycle Number

8000

10000

1

N-CFA cellulose glucose pine-cone fungi peanut shell rice husk pomelo peel kenaf stem coconut shell sugar cane willow catkin mushroom wood sawdust chetrry stones shrimp shells chitosan disposable cashmere bamboo

100 1000 -1 Power Density (W kg )

(e)

Figure 6. Specific capacitances at different GCD (a) and GCD curves of N-CFA, PC and CFA at 1 A g-1 respectively (inset), Nyquist plots with fitting curves (b) and their corresponding high frequency ranges (inset). Cycle stability of the electrode at 5.0 A g-1 (c) of N-CFA and Ragone plots comparison with the reported biomass-based carbons with and without activation of N-CFA (d). Schematic illustration of carbon fiber modified by ZIF-8 for supercapacitor (e). In order to confirm the superiority of N-CFA as supercapacitor, the PC and CFA were also tested by GCD. As shown in inset of Figure 6a, the specific capacities calculated 17



by using discharge branches, were 86.1, 168.0 and 358.5 F g-1 at 1 A g-1 for CFA, PC and N-CFA, respectively, indicating that loading ZIF-8 on surface of cotton has improved the electrochemical performance of N-CFA. The facilitated ion and electron transport behavior of N-CFA is confirmed by the EIS test in the open-circuit voltage and the fitting equivalent circuit model by the coupled nonlinear Schrodinger equation method displayed in the inset of Figure 6b. In the low-frequency region of Nyquist plots (Figure 6b), N-CFA electrodes displayed a straight line as an EDLC supercapacitor. The more vertical the line, the more closely the supercapacitor behaves as an ideal capacitor. In addition, The N-CFA displayed a high capacitance retention of 93.6 % after 10000 consecutive cycles at 5 A g-1, as plotted in Figure 6c. In order to evaluate the capacitive performances of N-CFA electrode for real supercapacitor, a symmetric capacitor was built by using N-CFA as the electrode without use of any binder and conductive additive. Figure S6a presented the CV profiles of N-CFA at different scan rates in a potential range of -0.6~0 V. It was interesting that the capacitor remained a good rectangular CV profiles at both low and high scan rate, demonstrating N-CFA electrode could serve as promising electrodes for high-rate supercapacitor. Furthermore, the quasilinear GCD curves at different current densities confirmed its good EDLC feature (Figure S6b). The calculated specific capacitances of the N-CFA was 327.7 F g-1 at a current density of 0.2 A g-1. As illustrated in the Ragone plots (Figure 6d), calculated by GCD in the symmetric supercapacitor, the N-CFA exhibited much better performance than most of 18


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biomass-derived carbons reported previously with higher power output capability at corresponding energy density[33-48]. The energy densities of N-CFA accordingly decreased from 16.1 to 5.5 Wh kg-1 when power densities increased from 0.2 to 3.7 kW kg-1, demonstrating again that the special nanostructure created by ZIF-8 pyrolysis led to the superior performance. In addition, the long cycling is another crucial parameter for practical applications of symmetric capacitor. The cycle stability experiment for N-CFA is carried out with 3000 cycles of GCD at a current density of 1 A g-1 in two-electrode system. As displayed in Figure S7, 86% of initial capacity can be retained after 5000 cyclic tests. The GCD curve of the 5000th cycle shows the same shape as the first cycle (Figure S7 inset) with linearity and symmetry, indicating excellent capacitive property and long term electrochemical stability. This strategy of preparing N-doped carbon fiber aerogel from raw cotton, modified by ZIF-8, was sustainable, environmentally friendly and economic due to the large production and renewable source of the raw cotton, as shown in Figure 6e. At the same time, with excellent electrochemical performance, the N-CFA showed potential for capacitors, providing a new thinking for energy storage, such as in wind power and so on. CONCLUSION In summary, we have demonstrated a feasible and economic strategy for scalable fabrication of N-CFA. The raw cotton was employed as a fibrous template to fabricate shaped aerogels and ZIF-8 was in situ fabricated on the surface of cotton to create rich porous structure and nitrogen doping. The obtained N-CFA exhibited macro monolith morphology, highly elastic properties and good flexibility inherited from raw cotton. 19



The introduction of ZIF-8 endowed the as-prepared N-CFA with high surface area of 2183 m2 g-1, rich porous structure and nitrogen doping. As a binder free electrode, the N-CFA showed remarkable capacity of 365 F g-1 at current 0.5 A g-1. Additionally, the N-CFA exhibited an excellent rate capability of 65.8 % from 0.5 A g-1 to 100 A g-1, which is better than many other porous carbon. A good stability with a high capacitance retention of 93.6 % after 10000 consecutive cycles at 5 A g-1 was obtained. We anticipate such a method using ZIF-8 modified raw cotton to fabricate shaped aerogels will provide a new orientation for preparation of N-CFA which have broad applications in the fields of supercapacitors, lithium ion batteries, sensors and so on. ASSOCIATED CONTENT Supporting Information Information as mentioned in text. This material is available free of charge via the Internet at http://pubs.acs.org. AUTHOR INFORMATION Corresponding Author *E-mail: (A.C.) [email protected], (S.H.) [email protected]. ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (21676070), Hebei Natural Science Foundation (B2015208109), Hebei Training Program for Talent Project (A201500117), Five Platform Open Fund Projects of Hebei University of Science and Technology (2015PT37), Hebei One Hundred-Excellent Innovative Talent Program (III) (SLRC2017034), Hebei Science 20


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For Table of Contents Use Only Table of Contents (TOC) Graphic and Synopsis Synopsis: N-doped porous carbon fiber aerogel with high performance in supercapacitor has been successfully fabricated using raw cotton as a fibrous template and ZIF-8 as active agent and nitrogen source.

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Raw cotton derived N-doped carbon fiber aerogel as efficient

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