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Nitrogen-doped hollow activated carbon nanofibers as high performance supercapacitor electrodes
Accepted Manuscript Nitrogen-doped hollow activated carbon nanofibers as high performance supercapacitor electrodes Qiang Xu, Xiaoliang Yu, Qinghua Liang, Yu Bai, Zheng-Hong Huang, Feiyu Kang PII: DOI: Reference:
S1572-6657(14)00565-7 http://dx.doi.org/10.1016/j.jelechem.2014.12.027 JEAC 1942
To appear in:
Journal of Electroanalytical Chemistry
Received Date: Revised Date: Accepted Date:
14 August 2014 1 December 2014 16 December 2014
Please cite this article as: Q. Xu, X. Yu, Q. Liang, Y. Bai, Z-H. Huang, F. Kang, Nitrogen-doped hollow activated carbon nanofibers as high performance supercapacitor electrodes, Journal of Electroanalytical Chemistry (2014), doi: http://dx.doi.org/10.1016/j.jelechem.2014.12.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrogen-doped hollow activated carbon nanofibers as high performance supercapacitor electrodes Qiang Xu a, Xiaoliang Yu a, Qinghua Liang a, c, Yu Bai a, Zheng-Hong Huang b,*, Feiyu Kang b, c
a
State Key Laboratory of New Ceramics and Fine Processing, School of Materials
Science and Engineering, Tsinghua University, Beijing 100084, China b
Key Laboratory of Advanced Materials (MOE), School of Materials Science and
Engineering, Tsinghua University, Beijing 100084, China c
Key Laboratory of Thermal Management Engineering and Materials, Shenzhen and
Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China.
* Corresponding author: Tel: +86 10 62773752 E-mail: [email protected]
1
Abstract: Nitrogen-doped hollow activated carbon nanofibers (HACNFs) have been prepared by the concentric electrospinning and the following NH3 activation. The as-obtained samples were directly used as supercapacitor electrode without binders and conductive additives. Owing to the unique hollow architecture and high N-doping level (8.2%), the HACNFs exhibit a high specific capacitance of 197 F g-1 at 0.2 A g-1, which is 1.33 times than that of the solid electrospun nanofibers activated in the same condition. The samples also possess a superior rate capability of 72.1% (143 F g-1) at 20 A g-1 and long-term cycling stability with a retention of 98.6% after 1000 cycles at 5 A g-1 in 6 M KOH.
Keywords: Hollow activated carbon nanofibers, concentric electrospinning, nitrogen doping, supercapacitor
1. Introduction Owing to their high energy and power density as well as long cycle life, supercapacitors (SCs) have become promising energy storage technologies for applications like hybrid electric vehicles, portable electronic devices, and uninterrupted power supplies [1, 2]. Porous carbon materials are nowadays the most widely used electrode materials for supercapacitors. And great efforts have been made to improve the specific capacitance while maintaining the high rate capability [3-5]. Carbon nanofibers (CNFs) prepared via electrospinning could be promising candidates due to their controllable specific surface area (SSA), unique one 2
dimensional nanostructure, high electrical conductivity [6, 7], and especially the free-standing properties [8]. However, the low SSA and poor wettability in aqueous solution are still the bottlenecks limiting their wide applications [9-11]. To date, vigorous works have been focused on enhancing the porosities of ACNFs for improving the capacitive performance [7, 12]. Concentric electrospinning is an effective approach to obtain unique hollow core/porous shell carbon nanofibers [13]. Compared with conventional electrospun fibers, the extra inner surface and mesoporous tubes can increase the SSA and pore volumes [14, 15], which is very beneficial for enhancing the capacitive performance. Herein, we report an efficient method to prepare N-doped hollow activated carbon nanofibers (HACNFs) via the concentric electrospinning of polyvinylpyrrolidone (PVP) as a core precursor and polyacrylonitrile (PAN) as shell precursor. This method is advantageous in facile removing the recyclable PVP just by water-washing instead of being sacrificed by heat treatment. Furthermore, high content of N-doping can be achieved in a short time under the following NH3 activation [3]. More importantly, the free-standing HACNFs film can be directly used as electrode materials for high-performance SCs without binders and conductive additives. 2. Experimental 2.1. Preparation of the HACNFs As illustrated in Fig. 1a, the precursor nanofibers were prepared by the concentric electrospinning technology with a concentric nozzle, of which the inner and outer diameter are 0.5 and 1.5 mm. 40 wt.% of PVP (Mw = 30,000) and 10 wt.% of PAN 3
(Mw = 130,000) were separately dissolved in N, N-dimethylformamide (DMF) by magnetic stirring at 65 °C for 12 h. The condition of electrospinning was set up as follows: the applied voltage was 22 kV, the tip-to-collector distance was 20 cm, and the flow rates of inner (PVP) and outer (PAN) polymer solution were 0.6 and 1 mL h-1. The obtained precursor nanofibers were washed with deionized water at 50 °C for three times to completely remove PVP. After being dried under vacuum, the fibers were stabilized at 250 °C in air for 2 h. Subsequently, the stabilized fibers were transferred to a quartz tubeand heated to 800 °C at a rate of 5 °C min-1 in N2. Once the temperature reached 800 °C, the gas was switched to NH3 for activation and N-doping. During the period of cool-down, the gas was switched to N2 again. To study the influence of the NH3 activation on the product, the activation time was set at 2 h and 1 h, and the corresponding products were respectively named as HACNF-2h and HACNF-1h. For comparison, a sample was carbonized for 2 h at 800 °C in N2 and was denoted as HCNF. In addition, the common solid PAN electrospun nanofibers activated in NH3 at 800 °C for 2 h (named as ACNF-2h) were also prepared for comparison. 2.2. Materials characterization and electrochemical evaluation The morphologies of the samples were observed by scanning electron microscopy (SEM, LEO1530) and transmission electron microscopy (TEM, Tecnai G20, 200 kV). N2 adsorption/desorption isotherms were measured by a volume adsorption apparatus (autosorb-1) at 77 K. BET method, density functional theory (DFT) and t-plot method were used to determine the SSA, the pore size distributions (PSD) and the micropore 4
volumes, respectively. The surface chemistry was investigated by X-ray photoelectron spectroscopy (XPS, PHI Quantera Imaging). The electrochemical measurement was carried out using a three-electrode cell with a reference electrode of Hg/HgO and a counter electrode of Pt wire in 6 M KOH. The working electrodes were prepared by direct pressing the sample (2-3 mg) between two nickel foams without binders and conductive additives. Cyclic voltammetry (CV) tests and galvanostatic charge/discharge cycling (GCD) tests were separately carried out on a VSP300 electrochemical workstation and an Arbin-BT2000 test station. 3. Results and discussions The typical SEM and TEM images of the prepared samples are shown in Fig. 1.All concentric electrospun fibers exhibit very similar continuous fibrous morphologies (Fig. 1b-d)with an outer and a inner fiber diameter of ~300 nm and ~150 nm, respectively (Fig. 1e). We can also observe plenty of swells randomly distributed in the fibrous network, which could be generated by the instability of electrospinning (Fig. 1f). These open swells can serve as effective entrances where electrolyte ions can quickly transfer to inner surface of the fibers at high charge/discharge rates. In contrast, the ACNF-2h sample shows a solid nanofibrous morphology with a similar fiber diameter of ~300 nm (Fig. 1g). N2 adsorption measurements have been taken to estimate the texture properties of the prepared samples. As shown in Fig. 2a, the hollow CNFs exhibit typical pseudo-type I isotherm with obvious hysteresis loops at high relative pressure, which should be caused by the hollow structure. The inset in Fig. 2a shows the 5
corresponding PSD curves. All samples show developed micropores and the hollow fibers also show obvious mesopore peaks at 3 and 4.5 nm. The detailed porosity parameters are summarized in Table 1. From HCNF to HACNF-1h, enormous increase of SSA values from 380 m2 g-1 to 655 m2 g-1 can be observed, whereas the value of HACNF-2h just increases 50 m2 g-1 compared with HACNF-1h.This suggests that over long time of NH3 activation at 800 °C would not effectively enhance the microporosity. Meanwhile, with the activation time increasing, the total pore volumes of the hollow fibers gradually increase from 0.284 to 0.497 cm3 g-1. For comparison, the ACNF-2h sample shows lower SSA value of 603 m2 g-1 and total volume of 0.309 cm3 g-1. Since the surface functionalities play an important role in determining the electrochemical performance, XPS tests have been taken. Figs. 2b-e show the N1s spectra of HCNF, HACNF-1h, HACNF-2h and ACNF-2h. According to previous reports, the N1s spectra can be divided into four different types of N-doping: quaternary-N (401.0 eV), pyridinic-N (398.5 eV), oxidized pyridinic-N (403.2 eV) and pyrrolic-N (399.9 eV) [16, 17]. The peak intensity of pyridinic-N has a remarkable increase relative to quaternary-N’s peak intensity when the activation time increases. Nitrogen-doping has been widely used to optimize the electrochemical properties of carbonaceous electrode materials. The nitrogen-doping not only can contribute some pseudocapacitance through a redox but also improve the electric conductivity and the wettability with the electrolyte of carbon materials. Although the detailed mechanism 6
of the pseudocapacitance from the N-doping is still not clear, it is widely accepted that pyridinic-N, oxidized pyridinic-N and pyrrolic-N can be mutually converted during electrochemical process [2, 18], and stable quaternary-N do not transform to other forms under mild electrochemical condition [19]. Therefore, pyridinic-N, oxidized pyridinic-N and pyrrolic-N are supposed to be the main N-doping configurations contributing to the pseudocapacitance. As shown in Fig. 2g, with the increasing time of NH3 activation, the N content increases from 5.0% to 8.2% and the percentage of quaternary-N decreases from 43.1% to 38.5%, suggesting an increase of electrochemical active N-doping sites. Based on above results, it can be found that NH3 activation results in the N-doping of HCNFs. During the activation process, NH3 decomposes to free radicals like NH2 and NH which attack the surface of carbon at high temperature, leading to its gasification, increase of the surface area and formation of nitrogen functional groups [20]. In the beginning, many micropores formed due to the etching of NH3, resulting in the increase of both surface area and pore volume. With an increase of activation time, more micropores would be produced accompanying with the transformation of the previous micropores to mesopores that have little contribution to the surface area. This may be the reason that the SSA of HACNF-2h just increases 50 m2 g-1 compared with the HACNF-1h. However, a longer activation time results in a larger pore volume and more N content. Electrochemical performance of the materials has been investigated by CV and GCD measurements in 6 M KOH aqueous solution with the potential window ranging 7
from -0.9 to 0.0 V (vs. Hg/HgO). All CV curves at 5 mV s-1 are close to rectangle shape without obvious redox peak, and the distorted rectangle shape may be caused by the redox pseudocapacitive behavior (Fig. 3a). A little deviation from the linear GCD curves at 0.2 A g-1 (Fig. 3b) also confirms the presence of pseudocapacitance. The specific capacitances are calculated from the GCD curves. HACNF-2h exhibits the highest capacitance of 197 F g-1 at 0.2 A g-1 among the four samples. Although the SSA and nitrogen content of HACNF-2h are close to those of ACNF-2h, the specific capacitance value of HACNF-2h is 1.33 times than that of the ACNF-2h (148 F g-1), suggesting the hollow structure is good for enhancing the capacitance. In addition, the HACNF delivers a very high areal capacitance (Cs) of 27-44 µF cm-2. The value is much larger than the theoretical EDL capacitance (15–25 µF cm-2) [9, 21], which may be ascribed to the pseducapacitance caused by the high N-doping level in the HACNF. Rate capability must be taken into consideration for the potential application of SCs. All samples have been tested under different current densities from 0.2 to 20 A g-1 (Fig. 3c). When the current density reaches as high as 20 A g-1, the specific capacitance of HACNF-2h remains 72.1% of the initial value. The capacitance retention is much higher than that of HACNF-1h (68.5%) and HCNF (58.3%). These results reveal that the increase of N-doping level by NH3 activation can significantly enhance the rate capability since N-doping results in the increase of mesopore volume (Table 1), as well as improving the conductivity and the wettability of the interface. Furthermore, the hollow tubular structure also contributes to the good rate 8
performance since they can acts as “ion-buffering channel” for quick ion transport. The cycling test was also performed at a current density of 5 A g-1 (Fig. 3d).It still remains 98.6% of the original capacitance during 1000 cycles, indicating the excellent long-term cycling stability. 4. Conclusions In summary, through facile concentric electrospinning and subsequent NH3 activation, free-standing N-doped HACNFs with a hierarchical porous structure and high N-doping level were successfully prepared. The HACNF materials show a large specific capacitance, an excellent rate capability and a long cycling stability, thus it can be a promising free-standing electrode material for high performance SCs. Acknowledgment The authors gratefully thank the National Natural Science Foundation of China (Grant No. 51232005) and the 973 program of China (No.2014CB932401).
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References [1] Y. Wang, Y. Xia, Recent progress in supercapacitors: from materials design to system construction, Adv. Mater. 25 (2013) 5336-5342. [2] L. Hao, X. Li, L. Zhi, Carbonaceous electrode materials for supercapacitors, Adv. Mater. 25 (2013) 3899-3904. [3] N.D. Kim, S.J. Kim, G.-P. Kim, I. Nam, H.J. Yun, P. Kim, J. Yi, NH3-activated polyaniline for use as a high performance electrode material in supercapacitors, Electrochim. Acta 78 (2012) 340-346. [4] B. Xu, S. Hou, F. Zhang, G. Cao, M. Chu, Y. Yang, Nitrogen-doped mesoporous carbon derived from biopolymer as electrode material for supercapacitors, J. Electroanal. Chem. 712 (2014) 146-150. [5] S. Yoon, S.M. Oh, C.W. Lee, J.H. Ryu, Pore structure tuning of mesoporous carbon prepared by direct templating method for application to high rate supercapacitor electrodes, J. Electroanal. Chem. 650 (2011) 187-195. [6] Z. Dong, S.J. Kennedy, Y. Wu, Electrospinning materials for energy-related applications and devices, J. Power Sources 196 (2011) 4886-4904. [7] S. Cavaliere, S. Subianto, I. Savych, D.J. Jones, J. Rozière, Electrospinning: designed architectures for energy conversion and storage devices, Energy Environ. Sci. 4 (2011) 4761. [8] J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, Coaxial carbon nanofibers/MnO2 nanocomposites as freestanding electrodes for high-performance electrochemical capacitors, Electrochim. Acta 56 (2011) 9240-9247. [9] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 10
38 (2009) 2520-2531. [10] C. Kim, K.-S. Yang, W.-J. Lee, The Use of Carbon Nanofiber Electrodes Prepared by Electrospinning for Electrochemical Supercapacitors, Electrochem. Solid-State Lett. 7 (2004) A397. [11] L. Zhao, Y. Qiu, J. Yu, X. Deng, C. Dai, X. Bai, Carbon nanofibers with radially grown graphene sheets derived from electrospinning for aqueous supercapacitors with high working voltage and energy density, Nanoscale 5 (2013) 4902-4909. [12] D. Liu, X. Zhang, Z. Sun, T. You, Free-standing nitrogen-doped carbon nanofiber films as highly efficient electrocatalysts for oxygen reduction, Nanoscale 5 (2013) 9528-9531. [13] S.-H. Park, B.-K. Kim, W.-J. Lee, Electrospun activated carbon nanofibers with hollow core/highly mesoporous shell structure as counter electrodes for dye-sensitized solar cells, J. Power Sources 239 (2013) 122-127. [14] B.-S. Lee, K.-M. Park, W.-R. Yu, J.H. Youk, An effective method for manufacturing hollow carbon nanofibers and microstructural analysis, Macromol. Res. 20 (2012) 605-613. [15] B.S. Lee, S.B. Son, K.M. Park, G. Lee, K.H. Oh, S.H. Lee, W.R. Yu, Effect of pores in hollow carbon nanofibers on their negative electrode properties for a lithium rechargeable battery, ACS Appl. Mater. Interfaces 4 (2012) 6702-6710. [16] J. Han, G. Xu, B. Ding, J. Pan, H. Dou, D.R. MacFarlane, Porous nitrogen-doped hollow carbon spheres derived from polyaniline for high performance supercapacitors, J. Mater. Chem. A 2 (2014) 5352. [17] K.-S. Kim, S.-J. Park, Morphology control and high electrochemical performance of flower-like N-enriched porous carbons for supercapacitor, J. Electroanal. Chem. 687 (2012) 11
18-24. [18] X. Wang, C.-G. Liu, D. Neff, P.F. Fulvio, R.T. Mayes, A. Zhamu, Q. Fang, G. Chen, H.M. Meyer, B.Z. Jang, S. Dai, Nitrogen-enriched ordered mesoporous carbons through direct pyrolysis in ammonia with enhanced capacitive performance, J. Mater. Chem. A 1 (2013) 7920. [19] D.W. Wang, F. Li, L.C. Yin, X. Lu, Z.G. Chen, I.R. Gentle, G.Q. Lu, H.M. Cheng, Nitrogen-doped carbon monolith for alkaline supercapacitors and understanding nitrogen-induced redox transitions, Chem. Eur. J. 18 (2012) 5345-5351. [20] C.L. Mangun, K.R. Benak, J. Economy, K.L. Foster, Surface chemistry, pore sizes and adsorption properties of activated carbon fibers and precursors treated with ammonia, Carbon, 39 (2001) 1809-1820. [21] X. Yu, J.-g. Wang, Z.-H. Huang, W. Shen, F. Kang, Ordered mesoporous carbon nanospheres as electrode materials for high-performance supercapacitors, Electrochem. Commun. 36 (2013) 66-70.
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Figure and table captions Fig. 1. (a) Schematic illustration of the fabricating process, (b-d,g) SEM images of HCNF, HACNF-1h, HACNF-2h, and ACNF-2h, (e-f) TEM images of HACNF-2h. Fig. 2. (a) N2 adsorption/desorption isotherms of the samples. The inset in (a) is the PSD curves, (b-e) N1s spectra of HCNF, HACNF-1h, HACNF-2h and ACNF-2h, (f) XPS survey spectra, and (g) N-doping content and the different N-doping configurations. Fig. 3. (a) CV curves at 5 mV s-1, (b) GCD curves at 0.2 A g-1, (c) rate performances, and (d) cycling stability of HACNF-2h at 5 A g-1.
Table 1. Physicochemical properties of all samples.
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Table 1. Physicochemical properties of all samples. a
HCNF HACNF-1h HACNF-2h ACNF-2h
b
c
d
e
SBET (m2 g-1)
VT (cm3 g-1)
Vmes (cm3 g-1)
N content (%)
Cg (F g-1 )
Cs (µF cm-2)
380 655 701 603
0.284 0.391 0.497 0.309
0.162 0.173 0.254 0.090
5.0 5.5 8.2 6.9
168 181 197 148
44.2 27.6 28.1 24.5
a
Specific surface area calculated by BET method Total pore volume c The mesopore volume d Gravimetric capacitance at 0.2 A g-1 e Areal capacitance at 0.2 A g-1 b
14
15
16
17
• Novel continuous hollow carbon nanofibers with randomly distributed open swells • NH3 activation leads to high content of N-doping and superior areal capacitance • The HACNFs can be directly used as the electrodes for supercapacitor • The HACNFs electrode shows excellent rate capability and cycling stability
18
S1572-6657(14)00565-7 http://dx.doi.org/10.1016/j.jelechem.2014.12.027 JEAC 1942
To appear in:
Journal of Electroanalytical Chemistry
Received Date: Revised Date: Accepted Date:
14 August 2014 1 December 2014 16 December 2014
Please cite this article as: Q. Xu, X. Yu, Q. Liang, Y. Bai, Z-H. Huang, F. Kang, Nitrogen-doped hollow activated carbon nanofibers as high performance supercapacitor electrodes, Journal of Electroanalytical Chemistry (2014), doi: http://dx.doi.org/10.1016/j.jelechem.2014.12.027
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
Nitrogen-doped hollow activated carbon nanofibers as high performance supercapacitor electrodes Qiang Xu a, Xiaoliang Yu a, Qinghua Liang a, c, Yu Bai a, Zheng-Hong Huang b,*, Feiyu Kang b, c
a
State Key Laboratory of New Ceramics and Fine Processing, School of Materials
Science and Engineering, Tsinghua University, Beijing 100084, China b
Key Laboratory of Advanced Materials (MOE), School of Materials Science and
Engineering, Tsinghua University, Beijing 100084, China c
Key Laboratory of Thermal Management Engineering and Materials, Shenzhen and
Advanced Materials Institute, Graduate School at Shenzhen, Tsinghua University, Shenzhen 518055, China.
* Corresponding author: Tel: +86 10 62773752 E-mail: [email protected]
1
Abstract: Nitrogen-doped hollow activated carbon nanofibers (HACNFs) have been prepared by the concentric electrospinning and the following NH3 activation. The as-obtained samples were directly used as supercapacitor electrode without binders and conductive additives. Owing to the unique hollow architecture and high N-doping level (8.2%), the HACNFs exhibit a high specific capacitance of 197 F g-1 at 0.2 A g-1, which is 1.33 times than that of the solid electrospun nanofibers activated in the same condition. The samples also possess a superior rate capability of 72.1% (143 F g-1) at 20 A g-1 and long-term cycling stability with a retention of 98.6% after 1000 cycles at 5 A g-1 in 6 M KOH.
Keywords: Hollow activated carbon nanofibers, concentric electrospinning, nitrogen doping, supercapacitor
1. Introduction Owing to their high energy and power density as well as long cycle life, supercapacitors (SCs) have become promising energy storage technologies for applications like hybrid electric vehicles, portable electronic devices, and uninterrupted power supplies [1, 2]. Porous carbon materials are nowadays the most widely used electrode materials for supercapacitors. And great efforts have been made to improve the specific capacitance while maintaining the high rate capability [3-5]. Carbon nanofibers (CNFs) prepared via electrospinning could be promising candidates due to their controllable specific surface area (SSA), unique one 2
dimensional nanostructure, high electrical conductivity [6, 7], and especially the free-standing properties [8]. However, the low SSA and poor wettability in aqueous solution are still the bottlenecks limiting their wide applications [9-11]. To date, vigorous works have been focused on enhancing the porosities of ACNFs for improving the capacitive performance [7, 12]. Concentric electrospinning is an effective approach to obtain unique hollow core/porous shell carbon nanofibers [13]. Compared with conventional electrospun fibers, the extra inner surface and mesoporous tubes can increase the SSA and pore volumes [14, 15], which is very beneficial for enhancing the capacitive performance. Herein, we report an efficient method to prepare N-doped hollow activated carbon nanofibers (HACNFs) via the concentric electrospinning of polyvinylpyrrolidone (PVP) as a core precursor and polyacrylonitrile (PAN) as shell precursor. This method is advantageous in facile removing the recyclable PVP just by water-washing instead of being sacrificed by heat treatment. Furthermore, high content of N-doping can be achieved in a short time under the following NH3 activation [3]. More importantly, the free-standing HACNFs film can be directly used as electrode materials for high-performance SCs without binders and conductive additives. 2. Experimental 2.1. Preparation of the HACNFs As illustrated in Fig. 1a, the precursor nanofibers were prepared by the concentric electrospinning technology with a concentric nozzle, of which the inner and outer diameter are 0.5 and 1.5 mm. 40 wt.% of PVP (Mw = 30,000) and 10 wt.% of PAN 3
(Mw = 130,000) were separately dissolved in N, N-dimethylformamide (DMF) by magnetic stirring at 65 °C for 12 h. The condition of electrospinning was set up as follows: the applied voltage was 22 kV, the tip-to-collector distance was 20 cm, and the flow rates of inner (PVP) and outer (PAN) polymer solution were 0.6 and 1 mL h-1. The obtained precursor nanofibers were washed with deionized water at 50 °C for three times to completely remove PVP. After being dried under vacuum, the fibers were stabilized at 250 °C in air for 2 h. Subsequently, the stabilized fibers were transferred to a quartz tubeand heated to 800 °C at a rate of 5 °C min-1 in N2. Once the temperature reached 800 °C, the gas was switched to NH3 for activation and N-doping. During the period of cool-down, the gas was switched to N2 again. To study the influence of the NH3 activation on the product, the activation time was set at 2 h and 1 h, and the corresponding products were respectively named as HACNF-2h and HACNF-1h. For comparison, a sample was carbonized for 2 h at 800 °C in N2 and was denoted as HCNF. In addition, the common solid PAN electrospun nanofibers activated in NH3 at 800 °C for 2 h (named as ACNF-2h) were also prepared for comparison. 2.2. Materials characterization and electrochemical evaluation The morphologies of the samples were observed by scanning electron microscopy (SEM, LEO1530) and transmission electron microscopy (TEM, Tecnai G20, 200 kV). N2 adsorption/desorption isotherms were measured by a volume adsorption apparatus (autosorb-1) at 77 K. BET method, density functional theory (DFT) and t-plot method were used to determine the SSA, the pore size distributions (PSD) and the micropore 4
volumes, respectively. The surface chemistry was investigated by X-ray photoelectron spectroscopy (XPS, PHI Quantera Imaging). The electrochemical measurement was carried out using a three-electrode cell with a reference electrode of Hg/HgO and a counter electrode of Pt wire in 6 M KOH. The working electrodes were prepared by direct pressing the sample (2-3 mg) between two nickel foams without binders and conductive additives. Cyclic voltammetry (CV) tests and galvanostatic charge/discharge cycling (GCD) tests were separately carried out on a VSP300 electrochemical workstation and an Arbin-BT2000 test station. 3. Results and discussions The typical SEM and TEM images of the prepared samples are shown in Fig. 1.All concentric electrospun fibers exhibit very similar continuous fibrous morphologies (Fig. 1b-d)with an outer and a inner fiber diameter of ~300 nm and ~150 nm, respectively (Fig. 1e). We can also observe plenty of swells randomly distributed in the fibrous network, which could be generated by the instability of electrospinning (Fig. 1f). These open swells can serve as effective entrances where electrolyte ions can quickly transfer to inner surface of the fibers at high charge/discharge rates. In contrast, the ACNF-2h sample shows a solid nanofibrous morphology with a similar fiber diameter of ~300 nm (Fig. 1g). N2 adsorption measurements have been taken to estimate the texture properties of the prepared samples. As shown in Fig. 2a, the hollow CNFs exhibit typical pseudo-type I isotherm with obvious hysteresis loops at high relative pressure, which should be caused by the hollow structure. The inset in Fig. 2a shows the 5
corresponding PSD curves. All samples show developed micropores and the hollow fibers also show obvious mesopore peaks at 3 and 4.5 nm. The detailed porosity parameters are summarized in Table 1. From HCNF to HACNF-1h, enormous increase of SSA values from 380 m2 g-1 to 655 m2 g-1 can be observed, whereas the value of HACNF-2h just increases 50 m2 g-1 compared with HACNF-1h.This suggests that over long time of NH3 activation at 800 °C would not effectively enhance the microporosity. Meanwhile, with the activation time increasing, the total pore volumes of the hollow fibers gradually increase from 0.284 to 0.497 cm3 g-1. For comparison, the ACNF-2h sample shows lower SSA value of 603 m2 g-1 and total volume of 0.309 cm3 g-1. Since the surface functionalities play an important role in determining the electrochemical performance, XPS tests have been taken. Figs. 2b-e show the N1s spectra of HCNF, HACNF-1h, HACNF-2h and ACNF-2h. According to previous reports, the N1s spectra can be divided into four different types of N-doping: quaternary-N (401.0 eV), pyridinic-N (398.5 eV), oxidized pyridinic-N (403.2 eV) and pyrrolic-N (399.9 eV) [16, 17]. The peak intensity of pyridinic-N has a remarkable increase relative to quaternary-N’s peak intensity when the activation time increases. Nitrogen-doping has been widely used to optimize the electrochemical properties of carbonaceous electrode materials. The nitrogen-doping not only can contribute some pseudocapacitance through a redox but also improve the electric conductivity and the wettability with the electrolyte of carbon materials. Although the detailed mechanism 6
of the pseudocapacitance from the N-doping is still not clear, it is widely accepted that pyridinic-N, oxidized pyridinic-N and pyrrolic-N can be mutually converted during electrochemical process [2, 18], and stable quaternary-N do not transform to other forms under mild electrochemical condition [19]. Therefore, pyridinic-N, oxidized pyridinic-N and pyrrolic-N are supposed to be the main N-doping configurations contributing to the pseudocapacitance. As shown in Fig. 2g, with the increasing time of NH3 activation, the N content increases from 5.0% to 8.2% and the percentage of quaternary-N decreases from 43.1% to 38.5%, suggesting an increase of electrochemical active N-doping sites. Based on above results, it can be found that NH3 activation results in the N-doping of HCNFs. During the activation process, NH3 decomposes to free radicals like NH2 and NH which attack the surface of carbon at high temperature, leading to its gasification, increase of the surface area and formation of nitrogen functional groups [20]. In the beginning, many micropores formed due to the etching of NH3, resulting in the increase of both surface area and pore volume. With an increase of activation time, more micropores would be produced accompanying with the transformation of the previous micropores to mesopores that have little contribution to the surface area. This may be the reason that the SSA of HACNF-2h just increases 50 m2 g-1 compared with the HACNF-1h. However, a longer activation time results in a larger pore volume and more N content. Electrochemical performance of the materials has been investigated by CV and GCD measurements in 6 M KOH aqueous solution with the potential window ranging 7
from -0.9 to 0.0 V (vs. Hg/HgO). All CV curves at 5 mV s-1 are close to rectangle shape without obvious redox peak, and the distorted rectangle shape may be caused by the redox pseudocapacitive behavior (Fig. 3a). A little deviation from the linear GCD curves at 0.2 A g-1 (Fig. 3b) also confirms the presence of pseudocapacitance. The specific capacitances are calculated from the GCD curves. HACNF-2h exhibits the highest capacitance of 197 F g-1 at 0.2 A g-1 among the four samples. Although the SSA and nitrogen content of HACNF-2h are close to those of ACNF-2h, the specific capacitance value of HACNF-2h is 1.33 times than that of the ACNF-2h (148 F g-1), suggesting the hollow structure is good for enhancing the capacitance. In addition, the HACNF delivers a very high areal capacitance (Cs) of 27-44 µF cm-2. The value is much larger than the theoretical EDL capacitance (15–25 µF cm-2) [9, 21], which may be ascribed to the pseducapacitance caused by the high N-doping level in the HACNF. Rate capability must be taken into consideration for the potential application of SCs. All samples have been tested under different current densities from 0.2 to 20 A g-1 (Fig. 3c). When the current density reaches as high as 20 A g-1, the specific capacitance of HACNF-2h remains 72.1% of the initial value. The capacitance retention is much higher than that of HACNF-1h (68.5%) and HCNF (58.3%). These results reveal that the increase of N-doping level by NH3 activation can significantly enhance the rate capability since N-doping results in the increase of mesopore volume (Table 1), as well as improving the conductivity and the wettability of the interface. Furthermore, the hollow tubular structure also contributes to the good rate 8
performance since they can acts as “ion-buffering channel” for quick ion transport. The cycling test was also performed at a current density of 5 A g-1 (Fig. 3d).It still remains 98.6% of the original capacitance during 1000 cycles, indicating the excellent long-term cycling stability. 4. Conclusions In summary, through facile concentric electrospinning and subsequent NH3 activation, free-standing N-doped HACNFs with a hierarchical porous structure and high N-doping level were successfully prepared. The HACNF materials show a large specific capacitance, an excellent rate capability and a long cycling stability, thus it can be a promising free-standing electrode material for high performance SCs. Acknowledgment The authors gratefully thank the National Natural Science Foundation of China (Grant No. 51232005) and the 973 program of China (No.2014CB932401).
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References [1] Y. Wang, Y. Xia, Recent progress in supercapacitors: from materials design to system construction, Adv. Mater. 25 (2013) 5336-5342. [2] L. Hao, X. Li, L. Zhi, Carbonaceous electrode materials for supercapacitors, Adv. Mater. 25 (2013) 3899-3904. [3] N.D. Kim, S.J. Kim, G.-P. Kim, I. Nam, H.J. Yun, P. Kim, J. Yi, NH3-activated polyaniline for use as a high performance electrode material in supercapacitors, Electrochim. Acta 78 (2012) 340-346. [4] B. Xu, S. Hou, F. Zhang, G. Cao, M. Chu, Y. Yang, Nitrogen-doped mesoporous carbon derived from biopolymer as electrode material for supercapacitors, J. Electroanal. Chem. 712 (2014) 146-150. [5] S. Yoon, S.M. Oh, C.W. Lee, J.H. Ryu, Pore structure tuning of mesoporous carbon prepared by direct templating method for application to high rate supercapacitor electrodes, J. Electroanal. Chem. 650 (2011) 187-195. [6] Z. Dong, S.J. Kennedy, Y. Wu, Electrospinning materials for energy-related applications and devices, J. Power Sources 196 (2011) 4886-4904. [7] S. Cavaliere, S. Subianto, I. Savych, D.J. Jones, J. Rozière, Electrospinning: designed architectures for energy conversion and storage devices, Energy Environ. Sci. 4 (2011) 4761. [8] J.-G. Wang, Y. Yang, Z.-H. Huang, F. Kang, Coaxial carbon nanofibers/MnO2 nanocomposites as freestanding electrodes for high-performance electrochemical capacitors, Electrochim. Acta 56 (2011) 9240-9247. [9] L.L. Zhang, X.S. Zhao, Carbon-based materials as supercapacitor electrodes, Chem. Soc. Rev. 10
38 (2009) 2520-2531. [10] C. Kim, K.-S. Yang, W.-J. Lee, The Use of Carbon Nanofiber Electrodes Prepared by Electrospinning for Electrochemical Supercapacitors, Electrochem. Solid-State Lett. 7 (2004) A397. [11] L. Zhao, Y. Qiu, J. Yu, X. Deng, C. Dai, X. Bai, Carbon nanofibers with radially grown graphene sheets derived from electrospinning for aqueous supercapacitors with high working voltage and energy density, Nanoscale 5 (2013) 4902-4909. [12] D. Liu, X. Zhang, Z. Sun, T. You, Free-standing nitrogen-doped carbon nanofiber films as highly efficient electrocatalysts for oxygen reduction, Nanoscale 5 (2013) 9528-9531. [13] S.-H. Park, B.-K. Kim, W.-J. Lee, Electrospun activated carbon nanofibers with hollow core/highly mesoporous shell structure as counter electrodes for dye-sensitized solar cells, J. Power Sources 239 (2013) 122-127. [14] B.-S. Lee, K.-M. Park, W.-R. Yu, J.H. Youk, An effective method for manufacturing hollow carbon nanofibers and microstructural analysis, Macromol. Res. 20 (2012) 605-613. [15] B.S. Lee, S.B. Son, K.M. Park, G. Lee, K.H. Oh, S.H. Lee, W.R. Yu, Effect of pores in hollow carbon nanofibers on their negative electrode properties for a lithium rechargeable battery, ACS Appl. Mater. Interfaces 4 (2012) 6702-6710. [16] J. Han, G. Xu, B. Ding, J. Pan, H. Dou, D.R. MacFarlane, Porous nitrogen-doped hollow carbon spheres derived from polyaniline for high performance supercapacitors, J. Mater. Chem. A 2 (2014) 5352. [17] K.-S. Kim, S.-J. Park, Morphology control and high electrochemical performance of flower-like N-enriched porous carbons for supercapacitor, J. Electroanal. Chem. 687 (2012) 11
18-24. [18] X. Wang, C.-G. Liu, D. Neff, P.F. Fulvio, R.T. Mayes, A. Zhamu, Q. Fang, G. Chen, H.M. Meyer, B.Z. Jang, S. Dai, Nitrogen-enriched ordered mesoporous carbons through direct pyrolysis in ammonia with enhanced capacitive performance, J. Mater. Chem. A 1 (2013) 7920. [19] D.W. Wang, F. Li, L.C. Yin, X. Lu, Z.G. Chen, I.R. Gentle, G.Q. Lu, H.M. Cheng, Nitrogen-doped carbon monolith for alkaline supercapacitors and understanding nitrogen-induced redox transitions, Chem. Eur. J. 18 (2012) 5345-5351. [20] C.L. Mangun, K.R. Benak, J. Economy, K.L. Foster, Surface chemistry, pore sizes and adsorption properties of activated carbon fibers and precursors treated with ammonia, Carbon, 39 (2001) 1809-1820. [21] X. Yu, J.-g. Wang, Z.-H. Huang, W. Shen, F. Kang, Ordered mesoporous carbon nanospheres as electrode materials for high-performance supercapacitors, Electrochem. Commun. 36 (2013) 66-70.
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Figure and table captions Fig. 1. (a) Schematic illustration of the fabricating process, (b-d,g) SEM images of HCNF, HACNF-1h, HACNF-2h, and ACNF-2h, (e-f) TEM images of HACNF-2h. Fig. 2. (a) N2 adsorption/desorption isotherms of the samples. The inset in (a) is the PSD curves, (b-e) N1s spectra of HCNF, HACNF-1h, HACNF-2h and ACNF-2h, (f) XPS survey spectra, and (g) N-doping content and the different N-doping configurations. Fig. 3. (a) CV curves at 5 mV s-1, (b) GCD curves at 0.2 A g-1, (c) rate performances, and (d) cycling stability of HACNF-2h at 5 A g-1.
Table 1. Physicochemical properties of all samples.
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Table 1. Physicochemical properties of all samples. a
HCNF HACNF-1h HACNF-2h ACNF-2h
b
c
d
e
SBET (m2 g-1)
VT (cm3 g-1)
Vmes (cm3 g-1)
N content (%)
Cg (F g-1 )
Cs (µF cm-2)
380 655 701 603
0.284 0.391 0.497 0.309
0.162 0.173 0.254 0.090
5.0 5.5 8.2 6.9
168 181 197 148
44.2 27.6 28.1 24.5
a
Specific surface area calculated by BET method Total pore volume c The mesopore volume d Gravimetric capacitance at 0.2 A g-1 e Areal capacitance at 0.2 A g-1 b
14
15
16
17
• Novel continuous hollow carbon nanofibers with randomly distributed open swells • NH3 activation leads to high content of N-doping and superior areal capacitance • The HACNFs can be directly used as the electrodes for supercapacitor • The HACNFs electrode shows excellent rate capability and cycling stability
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