Accepted Manuscript Title: Bamboo-like carbon nanotubes containing sulfur for high performance supercapacitors Author: Yincong Yang Lang Liu Yakun Tang Yang Zhang Dianzeng Jia Lingbing Kong PII: DOI: Reference:
S0013-4686(16)30151-7 http://dx.doi.org/doi:10.1016/j.electacta.2016.01.149 EA 26531
To appear in:
Received date: Revised date: Accepted date:
5-12-2015 20-1-2016 20-1-2016
Please cite this article as: Yincong Yang, Lang Liu, Yakun Tang, Yang Zhang, Dianzeng Jia, Lingbing Kong, Bamboo-like carbon nanotubes containing sulfur for high performance supercapacitors, Electrochimica Acta http://dx.doi.org/10.1016/j.electacta.2016.01.149 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.
Bamboo-like carbon nanotubes containing sulfur for high performance supercapacitors
Yincong Yanga,b, Lang Liua,b,*, Yakun Tanga,b, Yang Zhanga,b, Dianzeng Jiab, Lingbing Kongc
School of Chemistry and Chemical Engineering, Xinjiang University, Urumqi 830046,
Xinjiang, China. b
Key Laboratory of Energy Materials Chemistry, Ministry of Education, Institute of
Applied Chemistry, Xinjiang University, Urumqi 830046, Xinjiang, China. c
School of Materials Science and Engineering, Nanyang Technological University, 50
Nanyang Avenue, Singapore 639798.
Graphical Abstracts S-doped bamboo-like carbon nanotubes have been synthesized through the pyrolysis of polymer nanotubes, which is cost-effective, easy and scalable. As supercapacitor materials, which show excellent specific capacitance and cycling stability.
Highlights 1. The method of preparing S-doped bamboo-like carbon nanotubes is simple, cost-effective and easy to scale up. 2. S-doped bamboo-like carbon nanotubes exhibit a large specific surface area and multimodal porosity. 3. As the supercapacitor electrode, S-doped bamboo-like carbon nanotubes show excellent specific capacitance and cycling stability.
ABSTRACT Bamboo-like carbon nanotubes containing sulfur (BCNT-S) were prepared through the carbonization and the activation of sulfonated polymer nanotubes in CO2. The BCNT-S showed a remarkably high specific capacitance of 259 F g-1 at the current density of 1 A g-1, while 97.7% of its initial capacitance was retained after 1000 cycles at the current density of 5 A g-1. In contrast, amorphous carbon without S had a specific capacitance of 129 F g-1 at 1 A g-1, which was only about half of that of the BCNT-S. Therefore, the introduction of a trace amount of sulfur (1.66%) could greatly enhance the specific capacitance of carbon nanotubes (CNTs). In addition, the high specific capacitance and excellent cycling stability of the BCNT-S were also benefited from its high specific surface area and bamboo-like tubular morphology.
Keywords: Supercapacitor; Carbon nanotubes; S-doped; Porosity; Polymers
1. Introduction Supercapacitors can rapidly store and release energy with long cycle-lives and high power density. They have been widely applied in portable electronic devices, hybrid electric vehicles, digital communication devices, pulse laser techniques, uninterruptible power supplies and industrial power management [1, 2]. Based on the mechanism of energy storage, supercapacitors are mainly divided into electrochemical double layer capacitors (EDLC) and pseudo capacitors [3, 4], in which EDLC are the most developed form and received widespread attention . The capacitance of EDLC mainly depends on the characteristics of electrode materials, such as the surface area and the pore-size distribution. Most of electrode materials in EDLC were focused on carbon materials, such as activated carbons , carbon nanofibers , carbon aerogels , carbon nanotubes  and graphene . Among them, carbon nanotubes (CNTs) are considered to be prospective electrode materials for EDLC, owing to their unique tubular structure, high porosity, superior electrical conductivity, favorable cycle stability . In general, important factors influencing the performance of supercapacitors include specific surface area, pore-size distribution, pore shape and structure, electrical conductivity and surface functionality . Unfortunately, CNTs exhibit low specific capacitances due to their relatively small specific surface area. Many efforts have been devoted to increase the specific surface area and porosity of CNTs through physical and chemical activations . However, pure EDLC can only deliver a limited energy density . Besides EDLC, some of the compounds such as metal oxides, sulfides, or 5
conductive polymers can store charges by fast surface redox reaction. These reactions are so fast in kinetics that they usually show a capacitive behavior rather than a typical Faradaic behavior. Therefore, composite materials, such as [email protected]
oxide  or [email protected]
polymers , have been developed to improve the supercapacitor performance by introducing Faradaic redox reactions. The drawback of the introduction of these redox-type compounds is hard to keep the shape of charge-discharge curve triangle-like. Numerous works are focused on electrode material with significant redox peaks, leading to a battery-like charge-discharge curve. Though the large specific capacitance, this kind of material, can only deliver a relative mediocre power output, thus making them neither a battery nor a capacitor. In order to enhance the capacitance of the CNTs while introducing redox reaction appropriately, doping with heteroatoms, such as O, N, B, P and S, has been proven to be an effective way of improving the capacitance of carbonaceous electrode materials while maintaining a high power output [17-21]. Among these heteroatoms, nitrogen has been extensively investigated. However, there are a few reports on S-doped carbons for EDLC, especially S-doped CNTs [22-24]. The introduction of groups containing S can tune carbon chemistry in different ways, but depending on their oxidation states [25-28]. Amorphous carbons are more advantageous in supercapacitor electrode since they are easily activated compared with graphitic carbons. Besides, the introduction of heteroatoms is easier for amorphous carbon. Therefore, compared with commercial graphitic CNTs produced by CVD or arc-discharge, an amorphous carbon nanotube is more desirable for supercapacitor application. Fortunately, this kind of CNTs can be 6
large scale produced by pyrolysis of polymer nanotubes. We herein propose a simple approach to prepare porous bamboo-like CNTs doped with S (BCNT-S), which is cost-effective and easy to be scaled up. As illustrated in Fig. 1, the starting polymer nanotubes (PNTs) were large scale synthesized through the cationic polymerization using immiscible initiator nanodroplets of boron trifluoride diethyl etherate complex within minutes at room temperature . Then, the PNTs were sulfonated in sulfonic acid. The sulfonated polymer nanotubes (SPNTs) were finally carbonized and activated in CO2 , leading to porous bamboo-like CNTs containing S (BCNT-S). As electrode materials for EDLC, BCNT-S exhibits the high specific capacitance and the excellent cycling stability. Fig. 1 here 2. Experimental 2.1 Materials Divinyl benzene (80.0%), vinylbenzylchloride (90.0%) and boron trifluoride diethyl etherate complex were purchased from Aladdin Reagent Company. All other reagents were AR grade and purchased from commercial suppliers. They were all used without further purification. 2.2 Preparation of bamboo-like carbon nanotubes To prepare bamboo-like polymer nanotubes (PNTs): boron trifluoride diethyl etherate complex (80.0 mg) was added into the solution of divinyl benzene (2.4 g) and vinylbenzyl chloride (0.8 g) in n-heptane (80.0 g) at room temperature. After reacting for 15 min under the activation of ultrasonic waves, a large quantity of precipitates was 7
produced. Then, the reaction was terminated by dropping ethanol. The white fibers were filtered, washed with ethanol and dried at 40 oC in a vacuum oven overnight . The obtained sample was denoted as PNTs. To synthesize sulfonated bamboo-like polymer nanotubes (SPNTs): 0.2 g of PNTs was immersed in 30 mL of concentrated sulfuric acid, which was stirred at 50 oC for 6, 12 and 24 h, respectively. The mixture was diluted with a large amount of deionized water. Samples were collected with suction filtration and washed with water and ethanol [31, 32]. The obtained samples were denoted as SPNT-6, SPNT-12, SPNT-24, corresponding to the sulfonation times of 6, 12 and 24 h. To produce bamboo-like carbon nanotubes: PNTs, SPNT-6, SPNT-12 and SPNT-24 were heated at 800 oC at a heating rate of 5 oC min-1 in CO2 for 2 h. Accordingly, the obtained samples were denoted as C-S-0, C-S-6, C-S-12 and C-S-24, respectively. 2.3 Characterization Morphology, microstructure and composition of the samples were examined by a field emission scanning electron microscope (FESEM Hitachi S-4800) and a transmission electron microscope (TEM Hitachi H-600). Energy dispersive X-ray spectroscopy (EDS) measurements were performed by a TEAM™ EDS analysis system. X-ray diffraction (XRD) measurements were carried out with a Bruker D8 using filtered Cu Kα radiation. Fourier transform infrared (FT-IR) spectra were recorded by a Bruker VERTEX 70 spectrometer. Nitrogen adsorption-desorption isotherms were obtained by an ASAP 2020 Xtended Pressure Sorption Analyzer at 77 K. Specific surface areas of the samples were calculated by the Brunauer Emmett Teller (BET) method, while pore 8
size distributions were calculated by the Density Functional Theory (DFT) method, X-ray photoelectron spectroscopy (XPS) measurements were performed by a Thermo ESCALAB 250 instrument. 2.4 Electrochemical characterization Electrochemical experiments were conducted by using KOH electrolyte (6 mol/L) in a conventional three-electrode cell on an electrochemical work station (CHI660D, Chenhua, China). Platinum electrode and saturated calomel electrode were used as the counter electrode and reference electrode, respectively. Supercapacitor electrode was prepared by using the porous bamboo-like carbon nanotubes (85.0 wt%) as activated material, acetylene black (10.0 wt%) as a conducting agent and polytetrafluoroethylene (5.0 wt%) as a binder. Each electrode for supercapacitor was 1×1 cm2 in size. They were produced by sandwiching an active material (2.0 mg) between pieces of nickel foam, followed by pressing on a hydraulic press at 10 MPa. 3. Results and Discussion 3.1 Morphology, composition and microstructure of the carbon materials Fig. 2 shows TEM and SEM images of the PNTs and SPNTs. The PNTs synthesized through the cationic polymerization consisted of fibers with the length of 5-10 μm (Fig. 2a), these fibers take on clearly the bamboo-like morphology as shown in Fig. 2b. They have an average exterior diameter of 100-150 nm and interior diameter of 50-80 nm. After the PNTs were sulfonated in concentrated sulfuric acid, their bamboo-like tubular structure was well retained as shown in Fig. 2c and d. Fig. 2 and Fig. 3 here 9
Fig. 3 shows FT-IR spectra of the samples. The two characteristic peaks located at 2900 and 1400 cm-1 observed in all samples can be assigned to the stretching vibration and bending vibration of C-H, respectively. Some new peaks at 1250-1150 and 3300 cm-1 are observed in IR spectra of the sulfonated polymers, which are attributed to the stretching vibration of sulfonic acid groups. According to the peak intensities of the sulfonic acid groups and hydroxyl groups, we found that the longer the sulfonation time was, the more sulfonic acid groups and hydroxyls on the polymer nanotubes were. The XPS results for SPNTs with different sulfonation times also confirmed the conclusion (Table. S1). Fig. S1 shows XRD patterns of the carbonized samples. XRD patterns of the four samples are similar. The two peaks located at 24.3 and 44.4° can be easily indexed to be (002) and (101) diffraction planes of graphitic carbon, which indicates that the carbon materials are amorphous after the carbonization of PNTs and SPNTs at 800 oC in CO2 . It is also found that the peaks become broad with the increase of sulfonation time, implying that crystallinity of the carbon materials was decreased. Fig. 4 here SEM and TEM images of the carbon materials through the carbonization of PNTs and SPNTs are shown in Fig. 4. From Fig. 4a, we can see that the sample C-S-0 by the carbonization of the nonsulfonated PNTs consists of short nanorods or nanofibers. The TEM image (Fig. 4e) further indicates that the serious sintering and excessive melting occurred among nanorods. Obviously, it was very difficult to preserve the bamboo-like nanotube structure if the PNTs were not sulfonated before the carbonization. Also, 10
according to the SEM image in Fig. 4b, the sample C-S-6 still shows a slight melting phenomenon. The corresponding TEM image (Fig. 4f) indicates that the hollow structure has not been entirely preserved, leading to the coexistence of carbon nanorods and bamboo-like nanotubes. Therefore, a short sulfonation time (6 h) cannot guarantee the formation of bamboo-like CNTs during the carbonization of SPNTs. In contrast, the carbon materials (C-S-12 and C-S-24) have an obvious bamboo-like tubular structure (Fig. 4g and 4h), which means that the morphology of the pristine polymer tubes has been well preserved. These nanotubes exhibited an exterior diameter of 100–150 nm and an interior diameter of 50–80 nm. The corresponding SEM images (Fig. 4c and 4d) indicate that C-S-12 and C-S-24 have an average length of 5-10 μm. These results imply that the sulfonation plays an important role in preserving the tubular structure during the carbonization of PNTs. Sulfonation introduces acid sulfonated layer on the surface of PNTs. Moreover, the sulfonic groups mutually crosslink to form a protective layer (preoxidation passivation layer), which can play a catalytic protection during carbonization [34, 35]. The PNTs were sulfonated for 12 and 24 h, which also can make sulfonic acid group into the internal wall of PNTs . Therefore, the tubular structure of SPNTs can be preserved during the carbonization. In addition, EDS analysis results indicated that the sulfur content of C-S-6, C-S-12 and C-S-24 are 0.41%, 0.80% and 1.11%, respectively (Table. S2). Obviously, the S content in carbon materials increased with the increase of sulfonatioin time. The XPS spectra of C-S-0, C-S-6, C-S-12 and C-S-24 samples are displayed in Fig. 5. The survey XPS spectra (Fig. 5a) indicate that all the samples mainly contain carbon 11
and a little oxygen. From the magnified spectra of S2p (Fig. 5b), we can find sulfur contents in C-S-6, C-S-12 and C-S-24 are 0.76%, 0.86%, 1.66%, respectively (Table. S3). The change trend of S is consistent with EDS analysis of samples (Table. S2). The above result further confirmed the existence of the S element in C-S-6, C-S-12 and C-S-24. The asymmetric S2p peak of C-S-24 was fitted to five peaks at binding energies of 163.9, 165.1, 168.4, 169.2, 170.1 eV, respectively (Fig. 5d). The two major peaks corresponded to S2p3/2 (S1) and S2p1/2 (S2) of the C-S-C covalent bond of the thiophene-S arising from spin–orbit coupling . The three minor peaks (S3, S4 and S5) were assigned to oxidized sulfur form of C-SOx-C (x = 2-4) bond . Moreover, the peaks of the C-SOx-C bond of C-S-24 are obviously weak, indicating that the C-SOx-C bond has a low thermal stability at high temperature and that the C-S bond is the predominant bond type after the thermal treatment, which could contribute to the electrochemical performance. Based on sp2 C-C bonding at 284.5 eV , the high resolution spectrum of the C1s for C-S-24 may be fitted to four peaks, corresponding to C-C at 284.8 eV, C=O at 285.5 eV, C-S at 286.9 eV , and O=C-O at 290.1 eV (Fig. 5c). The C1s peak of C-S-24 intensified and shifted to a higher binding energy after thermal treatment. Fig. 5 here N2 adsorption-desorption isotherms and pore size distribution curves of the carbon materials are shown in Fig. 6. It can be found that the curves of four samples are very similar, having the characterization of the type I behavior. The N2 uptake of the four samples at P/P0 < 0.1 shows relative volume of the micropores (0.5-1.7 nm). As shown 12
in Table. 1, the sample C-S-0 exhibits the lowest volume of micropores (0.27 cm3 g-1), but it has the highest ratio of Vmic/Vt (90.0%). In contrast, the sample C-S-24 exhibits the highest volume of micropores (0.41 cm3 g-1) and the lowest ratio of Vmic/Vt (64.1%). Moreover, we can find that the ratio of Vmic/Vt decreases with the increase of sulfonation time, which indicate that the volume of mesopores and macropores gradually increases. Fig. 6 here BET surface areas of C-S-0, C-S-6, C-S-12 and C-S-24 are 759, 870, 931 and 1272 m2 g-1, respectively, showing that all the samples possess a large specific surface area, which is important for EDLC applications. Furthermore, with increasing sulfonation time, the specific surface area of the materials increases. Remarkably, the specific surface area of C-S-24 (sulfonated for 24 h) has the highest value of 1272 m2 g-1. The above results indicate that the deep sulfonation may be an effective method to greatly enhance the specific surface area of as-synthesized carbon nanotubes. 3.2 Electrochemical performance CV responses of the C-S-0, C-S-6, C-S-12 and C-S-24 were studied in 6 M KOH at -1.2 to 0.1 V vs. SCE. Fig. 7 shows electrochemical performances of C-S-0, C-S-6, C-S-12 and C-S-24. The specific capacitances of the electrodes are calculated from the discharge slopes by using the following equation,
In this equation, Cs is specific gravimetric capacitance, I is the discharge current, Δt is the discharge time, ΔV is the scan potential range and m is the mass of the active 13
material. The specific capacitances of all the samples are listed in Table. 2. From Fig. 7a, it is found that all the curves obviously deviate from linear relationship, which is due to the pseudo capacitance characteristic. Based on the data in Table. 2, C-S-24 shows the highest specific capacitance of 259 F g-1 at 1 A g-1, which is higher than the values reported in the open literature [40, 41]. The non-S-doped carbon fiber sample (C-S-0) displays the lowest specific capacitance of 129 F g-1 at 1 A g-1, which is only 49.8% of that of C-S-24 at the same current density. Moreover, the specific capacitance of samples increased with the increase of S content. Reasonable explanations are as follows: S-doping can efficiently increase the active sites for redox reaction on the surface of carbon materials . Thus, with increase of S amount, more redox reaction will take place, leading to the increase of the overall capacitance. Besides, the S-doping can increase the wettability on the surface of material to electrolyte , which is beneficial to improve the performance. Lastly, with the increase of S amount, the specific surface area of samples becomes larger, further leading to larger EDLC of the material. Therefore, doping with S in carbon materials is an effective route to enhance the specific capacitance of supercapacitors. Fig. 7b shows specific capacitances of the four samples as a function of current density. The capacitance of all the four samples rapidly decreased with increasing current density. Particularly, for the non-S-doped carbon fiber sample (C-S-0), its specific capacitance sharply decreased to 1 F g–1 at the current density of 20 A g-1. However, the capacitance retentions for the S-doped CNTs (C-S-6, C-S-12 and C-S-24) are obviously much better than that of the non-S-doped sample. The specific 14
capacitance of the C-S-24 electrode is 100 F g–1 at a high current density of 100 A g-1, which is even higher than the specific capacitance of C-S-0 at low current density. Fig. 7 and Fig. 8 here Electrochemical impedance spectroscopy (EIS) can give some information regarding internal resistance of the electrode material and resistance between the electrode and the electrolyte. It is well-known that a semicircle reflects the electrochemical reaction impedance of the electrode, and a smaller semicircle means smaller charge transfer resistance . It can be clearly seen from Fig. 8 that the charge transfer resistance for C-S-24 was the lowest among all samples. In contrast, the resistance of C-S-24 was the largest. In addition, the plot of C-S-24 showed the steepest slope, which indicates that C-S-24 had the lowest charge-transfer resistance and excellent conductivity. The sample C-S-24 exhibits rich pores, which facilitates rapid ion transport. Moreover, the introduction of S in carbon materials will result in low charge transfer resistance . Fig. 9 here Representative CV curves of C-S-24 in 6 M KOH aqueous solution at scan rates from 5 to 100 mV s-1 are shown in Fig. 9a. All the curves are close to the rectangle feature of EDLC. However, there is an obvious hump in the curve at the scan rate of 100 mV s-1, which suggests the presence of possible pseudo capacitive reactions due to the redox reaction of S on the electrode surface. The presence of S enhanced the hydrophilic characteristic of the electrode material and increased the contact area between the electrolyte and the electrode . Fig. 9b shows current densities (extracted from the C-V plot at -0.6 V) as a function of scan rate, which indicates that the 15
reversibility is very good. The near linear shape shows a high rate speed performance of the material, because of the high conductivity of the carbon nanotubes and the fast ion transmission mainly controlled by the adsorption process. Fig. 9c shows galvanostatic charge-discharge of C-S-24 at different current densities. The quasi-triangular shape of the curves indicates an EDLC response. Smooth classic triangle belongs to the characteristics of EDLC, whereas the slight distortion of the curves is caused by the pseudo capacitance contribution of S, which is consistent with the shape of CV curves. The calculated specific capacitances of the electrode are listed in Table. 3. As the current density is increased from 1 to 50 A g-1, the capacitance gradually decreases from 259 to 129 F g-1. Capacitance retention of the electrode is shown in Fig. 9d. After 1000 cycles of charge and discharge, the specific capacitance of C-S-24 showed a slight loss from 171 to 167 F g-1, corresponding to a capacity retention of 97.7%, which indicates that the S-doped bamboo-like carbon nanotubes possessed excellent cyclic stability. 4. Conclusions S-doped bamboo-like carbon nanotubes have been synthesized through the pyrolysis of sulfonated bamboo-like polymer nanotubes in CO2 atmosphere. The materials exhibited a high specific surface area, which ensured a large contact area between electrodes and electrolyte. Moreover, they displayed a multimodal porous distribution, consisting of micropores, mesopores and macropores, which guaranteed a sufficiently rapid transport of both ions and electrons during the charge–discharge process. In addition, the introduction of sulfur also provided pseudo capacity characteristic for the 16
electrode. As a result, S-doped bamboo-like carbon nanotubes have high specific capacitance and excellent cyclic stability. Specifically, the supercapacitor electrode, based on the bamboo-like carbon nanotubes contain sulfur 1.66% (C-S-24), reached a specific capacitance as high as of 259 F g-1 at the current density of 1 A g-1 in 6 M KOH and 97.7% capacity retention after 1000 cycles at the current density of 5 A g-1, which is an ideal electrode material for supercapacitors. The simple synthesis method and low synthesis cost of the materials are expected to show a better potential application. Acknowledgements This work was supported by the excellent Youth Fund of Xinjiang Uygur Autonomous Region of China (2014721005),the Opened Fund of the Key Laboratory of Xinjiang Uygur Autonomous Region (2015KL010), the National Natural Science Foundation of China (21362037), the Joint Funds of NSFC-Xinjiang of China (U1303391, U1203292), Urumqi Talent Engineering project (P111310010) and the Program for Changjiang Scholars and Innovative Research Team in the University of Ministry of Education of China (IRT1081).
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Fig. 1. Schematic diagram for preparing BCNT-S.
Fig. 2. SEM and TEM images of the PNTs (a, b) and SPNTs (c, d).
Fig. 3. FT-IR spectra of the PNTs, SPNT-6, SPNT-12 and SPNT-24.
Fig. 4. SEM and TEM images of carbon materials through the pyrolysis of SPNTs for different sulfonation times: (a, e) C-S-0, (b, f) C-S-6, (c, g) C-S-12 and (d, h) C-S-24. 25
Fig. 5. XPS spectra (a) and magnified XPS of S 2p (b) of samples; high-resolution XPS of C 1s (c) and S 2p (d) of the C-S-24.
Fig. 6. N2 adsorption–desorption isotherms and pore size distribution curves of samples: (a) C-S-0, (b) C-S-6, (c) C-S-12 and (d) C-S-24.
Fig. 7. Galvanostatic charge–discharge curves of samples at the current density of 1 A g-1 (a) and specific capacitance of samples as a function of current densities from 1 to 100 A g-1 (b). 27
Fig. 8. Nyquist plots for carbon materials.
Fig. 9. Electrochemical performance of C-S-24 in a three-electrode system using 6 M KOH aqueous solution as the electrolyte: (a) C-V curves at scan rates from 5 to 100 mV s-1, (b) plot of current density versus scan rate, (c) galvanostatic charge–discharge curves at current densities ranging from 1 to 50 A g-1 and (d) cycling performance of the C-S-24 electrode at 5 A g-1. 28
Table. 1. BET specific surface areas and porosities of the samples.
BET specific surface area (SBET).
Total pore volume (Vt) at P/P0=0.99.
micropore (Vmic) calculated by using the t-plot method, d Ratio of micropore volume to total volume, e Average pore size (Dap= 4Vt /SBET).
Table. 2. Specific capacitances of the samples at the current density of 1 A g-1. Samples Capacitance (F g-1)
Table. 3. Galvanostatic charge-discharge of C-S-24 at different current densities. Current density (A g-1) Capacitance (F g-1)