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Open ended nitrogen-doped carbon nanotubes for the electrochemical storage of energy in a supercapacitor electrode
Journal of Power Sources 277 (2015) 387e392
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Open ended nitrogen-doped carbon nanotubes for the electrochemical storage of energy in a supercapacitor electrode Anthuvan Rajesh John, Pandurangan Arumugam* Department of Chemistry, Institute of Catalysis and Petroleum Technology, Anna University, Chennai, 600025 Tamilnadu, India
h i g h l i g h t s Open ended N-doped CNTs were prepared by a simple pyrolysis method followed by acid treatment technique. The acid treatment on N-doped CNTs leading to their ring opening. The open ended N-doped CNTs electrode shows superior electrochemical performance than as-synthesized N-doped CNTs. The electrochemical tests exhibited a maximum specific capacitance of 146 F g1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2014 Received in revised form 15 November 2014 Accepted 25 November 2014 Available online 9 December 2014
Open ended nitrogen-doped carbon nanotubes (N-doped CNTs) are synthesized by pyrolysis of acetylene/ammonia (C2H2/NH3) mixture over lanthanum nickel (LaNi5) alloy catalyst and subsequent 3M, HNO3:H2SO4 acid mixture treatment. Transmission electron microscopy and X-ray photoelectron spectroscopy evaluations of the acid treated N-doped CNTs reveal that the nanotubes possess an open ended morphology and oxidation of pyridiniceN groups, respectively. The resultant open ended N-doped CNTs tested as a supercapacitor electrode material by cyclic voltammetry and exhibits high specific capacitance of 146 F g1. © 2014 Elsevier B.V. All rights reserved.
Keywords: Open ended nitrogen-doped CNTs Chemical vapor deposition Electrochemical properties Supercapacitor Specific capacitance
1. Introduction In the last decade, electrical double layer capacitors or supercapacitors play an important role in energy storage applications due to their high power density and long cycling life [1]. Carbon materials have been widely investigated as an attractive electrode material in supercapacitors owing to their high surface area, excellent electronic conductivity, high reversibility and ecoefriendly [2]. Further, modification of carbon materials with nitrogen doping should be potential to improve electron conductivity [3e7]. It is noteworthy, to consider that the different N doping environments would induce different effects on the supercapacitor performance of CNTs [8]. A recent study suggests that pyrrolicestructure enriched nitrogen is highly efficient for supercapacitor behavior [9]. In some studies, the enhanced electron
* Corresponding author. E-mail address: [email protected] (P. Arumugam). http://dx.doi.org/10.1016/j.jpowsour.2014.11.151 0378-7753/© 2014 Elsevier B.V. All rights reserved.
transfer is attributed to quaternary and pyridiniceNeoxides nitrogen groups [10,11]. The pyridiniceN and pyridiniceNeoxide nitrogen also have enhancing effects on capacitance due to the positive charge [12]. Recently, open ended N-doped CNTs are the new kind of intense research topic, because of their large specific surface area, better electroecatalytic performance and enhanced field emission properties than closed ended N-doped CNTs [13e15]. In addition, open ended N-doped CNTs could form more electron transport pathways in open tips, which resulted in enhanced electron conductivity than closed one. Acid oxidation of CNTs is a basic technique for their ring opening [16]. In addition to CNTs cap opening, oxidation enhances the number of defects in CNTs and increases the both nitrogen and oxygen functionalities [17]. It is well known that supercapacitor performance of CNTs significantly improved via the introduction of N and O atoms [10,11]. Therefore, it should be expected that the combined effect of open ended tips and N/O functionalities improve the electrochemical energy storage performance.
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Fig. 1. (a, b) SEM images and (c, d) TEM images of as-synthesized N-doped CNTs.
In our previous work, we reported a new strategy to synthesize N-doped CNTs by catalytic pyrolysis of acetylene/ammonia (C2H2/ NH3) mixture over lanthanum nickel (LaNi5) alloy catalyst [18]. Also, we have shown that the effect of 3M, HNO3:H2SO4 acid mixture treatment on the nitrogen content and electronic structure of N-doped CNTs. In this communication, we continue to study the effect of acid treatment on the morphology of the N-doped CNTs. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that the acid treatment on Ndoped CNTs leading to their ring opening. Besides, these open ended N-doped CNTs tested as a supercapacitor electrode material and it was found that increased specific capacitance in comparison with asesynthesized N-doped CNTs.
a H2 flow atmosphere at 30 mL min1. After 20 min of growth, the sample was cooled to room temperature under N2 ambient. After synthesis N-doped CNTs were heated in air at 400 C for removal of the residual carbon. 2.2. Acid treatment on as-synthesized N-doped CNTs The as-synthesized N-doped CNTs were suspended in 3M, 3:1 ratio of nitric acid (HNO3) and sulfuric acid (H2SO4) mixture and refluxed for 12 h at 80 C. After that the sample was washed with distilled water and filtered for several times and then dried in hoteair oven at a temperature of 100 C. 2.3. Characterization
2. Experimental 2.1. Synthesis of N-doped CNTs The N-doped CNTs were synthesized by catalytic pyrolysis of C2H2/NH3 mixture over LaNi5 alloy catalyst (99.5%, metal basis, 100 mesh, Alfa Aesar) in a conventional chemical vapor deposition (CVD) method, according to our detailed synthesis procedure reported recently [18]. In brief, a quartz boat loaded with 100 mg of LaNi5 alloy catalyst was placed at the centre of a tube furnace. The furnace was heated from room temperature to 550 C at a rate of 5 C/min with a flow of nitrogen (N2) gas at 100 mL min1. On reached to 550 C, N2 was switched to hydrogen (H2) gas at 100 mL min1 for 20 min to reduce the catalyst. Then, the center of the furnace temperature was raised to 900 C, 1:2 gas mixture of 60 mL min1 C2H2 and120 mL min1 NH3 introduced into the flow as the C and N sources, respectively. The pyrolysis was performed in
The morphology of as-synthesized and acid refluxed N-doped CNTs was carried out on a Hitachi S4800 SEM operated at 20 kV. TEM analysis was carried out on JEOL 3010 operated at 200 kV to study the microstructure of the samples. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Omicron Nanotechnology ESCA Probe system with monochromatic Al Ka X-rays (energy of 1486.7 eV). Confocal Raman spectroscopy analysis was done with a WiTec GmbH, AlphaeSNOM CRM 200 confocal Raman microscope having a 514 nm laser as the excitation source. 2.4. Electrochemical measurements The electrochemical performances for the as-synthesized and acid treated N-doped CNTs were performed in a threeeelectrode system equipped with a Pt wire counter electrode, an Ag/AgCl
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Fig. 2. (a, b) SEM images of open ended N-doped CNTs, (c) TEM image of individual open ended N-doped CNTs and (d) HRTEM image taken from the squared region in Fig. 2c, and (e, f) high magnified TEM images of open ended region.
reference electrode and an electrochemical analyzer (CHI660D, CH Instruments Inc.). The working electrode preparation method was similar to that described elsewhere [5]. Briefly, an aqueous 2 mg mL1 suspension was prepared by ultrasonically dispersing as-synthesized and acid treated N-doped CNTs (5 mg) in a 1.25 mL mixture of water, ethanol and Nafion (5 wt.%) with the volume ratio of 1.0:0.2:0.05 for 1 h, then 10 mL inks was pipette out and spread on the glassy carbon electrode. The inks were dried at room temperature. Cyclic voltammetry (CV) measurements were conducted in 1 M KCl electrolyte between 0.3 and 0.5 V (vs. Ag/AgCl) at different scan rates ranging from 10 to 400 mV s1. 3. Results and discussion 3.1. Scanning and transmission electron microscopy analysis The morphology of as-synthesized N-doped CNTs was studied by SEM and TEM analysis. SEM images of the as-grown N-doped
CNTs obtained by pyrolysis of C2H2/NH3 mixture over LaNi5 alloy catalyst at 900 C (Fig. 1a and b) shows the large quantity of high density nanotubes formed with uniform diameter. Furthermore, all the observed nanotube lengths were several micrometers and closed by end caps (Fig. 1b). TEM (Fig. 1c) and high magnification TEM (Fig. 1d) analysis were studied to get detailed information about the as-synthesized N-doped CNTs structure, and show a regular bambooelike morphology with an outer diameter of 80e100 nm. It can be seen from Fig. 1b and c that the metal catalyst particles attached on the N-doped CNT ends, indicating the nanotubes growth initiated by LaNi5 alloy catalyst. Previous work from our group [18] has proven that the LaNi5 alloy catalyst initiate the growth N-doped CNTs. The open ended N-doped CNTs obtained by our approach (acid treatment) were first evidenced on the SEM images as shown in Fig. 2a and b. From the low and high magnification SEM images, we can observe that the N-doped CNTs ends were opened after acid treatment and their lengths were up to several micrometers. The
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Fig. 3. (a) XPS survey scan spectra of as-synthesized N-doped CNTs and acid treated N-doped CNTs, (b) deconvoluted N1s spectrum of the as-synthesized N-doped CNTs, (c) deconvoluted N1s spectrum of the acid treated N-doped CNTs, and (d) Raman spectra of as-synthesized and acid treated N-doped CNTs.
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low magnification TEM image of the individual N-doped CNTs presented in Fig. 2c shows an openeended bambooelike morphology, which is in agreement with the SEM observations. The open ended N-doped CNTs having an outer diameter of 80e100 nm and many internal compartments formed in the cavity of the nanotube. The HRTEM image (Fig. 2d) was taken from the squared region in Fig. 2c indicates that the nanotube possesses thin walled wavy like structure. High magnification TEM images shown in Fig. 2e and f further confirms that the N-doped CNTs were of openended structure, ie., metal catalyst particles were removed after acid treatment leading to their end caps opening. In addition, the surface of wall and internal compartments were rough, showing the lower degree of graphitization. 3.2. X-ray photoelectron spectroscopy analysis The chemical composition of the as-synthesized and acid treated N-doped CNTs was characterized by XPS. Fig. 3a shows the wide scan XPS spectra of as-synthesized and acid treated N-doped CNTs. An obvious nitrogen peak clearly detected in the both assynthesized and acid treated N-doped CNTs. The main peaks around at 284.0, 400.0 and 532.0 eV correspond to C 1s, N 1s and O 1s, respectively. Compared to as-synthesized sample, the nitrogen and oxygen peaks were significantly increased after acid treatment, indicating the oxidation of nitrogen functional groups during the acid treatment. Fig. 3b shows deconvoluted N 1s XPS spectrum of as synthesized sample displays two main peaks N1 (pyridinicelike N) at 398.2 eV and N2 (graphiticelike N) at 400.3 eV. Deconvoluted N1s XPS spectrum is shown in Fig. 3c indicates the existence of three peaks, which are located around at 398.8 eV (N1), 400.5 eV (N2) and 402.3 eV (N3), respectively. The additional peak in acid treated sample at 402.3 eV (N3) is commonly attributed to Neoxides of pyridiniceN [5,14]. The N concentration, defined as N/ (C þ N) atomic ratio%, was estimated by the area ratio of the N 1s and C 1s peaks and the calculated atomic percentage of N in the asesynthesized and acid treated samples were 6.9 and 7.3 at%, respectively. Confocal Raman spectroscopy demonstrated that the level of crystalline perfection present in the asesynthesized and acid treated N-doped CNTs, as shown in Fig. 3d. For asesynthesized Ndoped CNTs, the calculated ID/IG ratio was 0.95, indicates the low graphitic ordering in asegrown N-doped CNTs. Further, the acid treatment on N-doped CNTs was found to that the ID/IG ratio (0.98) enhanced slightly, due to the increased nitrogen and oxygen functionalities (pyridinic N-oxides).
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than as-synthesized N-doped CNTs, and exhibits comparable capacitance value of the reported results (100e300 F g1) [2]. There are two possible reasons proposed for the increased capacitance for open ended nanotubes. First, the open ended nanotubes will necessarily improve the contact with electrolyte which helps to improve the transport of ions. Second, the residue catalyst will block the diffusion path of electrolyte as well as ions in the assynthesized N-doped CNTs but the open ended nanotube doesn't. Fig. 4b and c shows the effect of different scan rates (10e400 mV s1) on the CV for the as-synthesized N-doped CNTs and open ended N-doped CNTs. It can be seen that the increase of scan rates, the area of CV loops increased and maintain the rectangular shapes even at high scan rate of 400 mV s1. According to previous literature reports the various N doping environments would induce different effects on the supercapacitor performance of CNTs [8]. A recent study suggests that pyrrolicestructure enriched nitrogen is highly efficient for supercapacitor behavior [9]. In some studies, the enhanced electron transfer is attributed to quaternary and pyridiniceNeoxides nitrogen groups [10,11]. The pyridiniceN and pyridiniceNeoxide nitrogen also have enhancing effects on capacitance due to the positive charge [12]. In our studies the higher capacitance of open ended N-doped CNTs attributed to the positive charge on the both graphitic or quaternary N as well as pyridiniceNeoxide groups can improve the electron conductance through carbon network and leads to the increased capacitance. Here we also suggested the combined effect of N- and O-containing functional groups of CNTs is responsible for increased capacitance. Our XPS results confirmed that acid treated N-doped CNTs contain three functional groups (graphiticeN, pyridiniceN, pyridiniceNeoxides nitrogen), while the proportion of positive charge groups were dominant. Therefore, the increased capacitance value was quite acceptable and promising electrode material for use in electrical double layer supercapacitors. 4. Conclusion In summary, the open-ended N-doped CNTs have been obtained by pyrolysis of C2H2/NH3 mixture over LaNi5 alloy catalyst and subsequent 3M, HNO3:H2SO4 acid mixture treatment. TEM and XPS results showed that the morphology and electronic structure of Ndoped CNTs were modified after acid treatment. This novel electrode material was showed an electrical doubleelayer capacitive behavior with high specific capacitance of 146 F g1, possibly due to enhanced electrical conductivity of the material. The results suggested that the open ended N-doped CNTs are promising electrode material for use in high-performance energy storage applications.
3.3. Electrochemical properties Acknowledgment The electrochemical performances of open ended N-doped CNTs were evaluated by CV at scan rates of 10e400 mV s1 within the potential range of 0.3e0.5 V in 1 M KCl aqueous solution. For comparison, we also evaluated the electrochemical performance of electrode made from as-synthesized N-doped CNTs. Fig. 4a compares the CV curves of the as-synthesized N-doped CNTs and acid treated (open ended) N-doped CNTs electrodes at a scan rate of 100 mV s1 between 0.3 and 0.5 V. Both the CV curves display rectangular shape without obvious redox peaks, showing the typical characteristic of electrical doubleelayer capacitance [1]. Compared with the as-synthesized N-doped CNTs, the CV area of open ended N-doped CNTs increased profoundly. It indicated that the specific capacitance is dramatically improved resulted from the enhanced electron conductivity in open ended N-doped CNTs. The calculated capacitances were 42 and 146 F g1 for as-synthesized N-doped CNTs and open ended N-doped CNTs, respectively. The capacitance of the open ended N-doped CNTs was much higher
One of the authors J. Anthuvan Rajesh is thankful to UGCeBRS, New Delhi, India for providing the Research Fellowship in Sciences for meritorious students. References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10]
P. Simon, Y. Gogotsi, Nat. Mater. 7 (2008) 845e854. M. Inagaki, H. Konno, O. Tanaike, J. Power Sources 195 (2010) 7880e7903. Y.S. Yun, H.H. Park, H.-J. Jin, Materials 5 (2012) 1258e1266. N. Xiao, D. Lau, W. Shi, J. Zhu, X. Dong, H.H. Hng, Q. Yan, Carbon 57 (2013) 184e190. B. You, L. Wang, L. Yao, J. Yang, Chem. Commun. 49 (2013) 5016e5018. Y. Fan, Z. Zhao, Q. Zhou, G. Li, X. Wang, J. Qiu, Y. Gogotsi, Carbon 58 (2013) 128e133. C.H. Choi, S.H. Park, M.W. Chung, S.I. Woo, Carbon 55 (2013) 98e107. W. Shen, W. Fan, J. Mater, Chem. A 1 (2013) 999e1013. F.M. Hassan, V. Chabot, J. Li, B.K. Kim, L.R. Sandoval, A. Yu, J. Mater. Chem. A 1 (2013) 2904e2912. D.H. Jurcakova, M. Seredych, G.Q. Lu, T.J. Bandosz, Adv. Funct. Mater. 19 (2009)
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438e447. [11] M. Seredych, D.H. Jurcakova, G.Q. Lu, T.J. Bandosz, Carbon 46 (2008) 1475e1488. [12] J.-I. Kim, S.-J. Park, J. Solid State Chem. 184 (2011) 2184e2189. [13] Y. Shen, J. Bai, Chem. Commun. 46 (2010) 1308e1310. [14] R. Lv, T. Cui, M.-S. Jun, Q. Zhang, A. Cao, D.S. Su, Z. Zhang, S-H. Yoon, J. Miyawaki, I. Mochida, F. Kang, Adv. Funct. Mater. 21 (2011) 999e1006.
[15] T. Cui, R. Lv, F. Kang, Q. Hu, J. Gu, K. Wang, D. Wu, Nanoscale Res. Lett. 5 (2010) 941e948. [16] S.C. Tsang, Y.K. Chen, P.J.F. Harris, M.L.H. Green, Nature 372 (1994) 159e162. [17] H. Liu, Y. Zhang, R. Li, X. Sun, H.A. Rachid, J. Nanopart. Res. 14 (2012), 1016 (1e8). [18] J.A. Rajesh, A. Pandurangan, RSC Adv. 4 (2014) 20554e20566.
Contents lists available at ScienceDirect
Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour
Open ended nitrogen-doped carbon nanotubes for the electrochemical storage of energy in a supercapacitor electrode Anthuvan Rajesh John, Pandurangan Arumugam* Department of Chemistry, Institute of Catalysis and Petroleum Technology, Anna University, Chennai, 600025 Tamilnadu, India
h i g h l i g h t s Open ended N-doped CNTs were prepared by a simple pyrolysis method followed by acid treatment technique. The acid treatment on N-doped CNTs leading to their ring opening. The open ended N-doped CNTs electrode shows superior electrochemical performance than as-synthesized N-doped CNTs. The electrochemical tests exhibited a maximum specific capacitance of 146 F g1.
a r t i c l e i n f o
a b s t r a c t
Article history: Received 18 September 2014 Received in revised form 15 November 2014 Accepted 25 November 2014 Available online 9 December 2014
Open ended nitrogen-doped carbon nanotubes (N-doped CNTs) are synthesized by pyrolysis of acetylene/ammonia (C2H2/NH3) mixture over lanthanum nickel (LaNi5) alloy catalyst and subsequent 3M, HNO3:H2SO4 acid mixture treatment. Transmission electron microscopy and X-ray photoelectron spectroscopy evaluations of the acid treated N-doped CNTs reveal that the nanotubes possess an open ended morphology and oxidation of pyridiniceN groups, respectively. The resultant open ended N-doped CNTs tested as a supercapacitor electrode material by cyclic voltammetry and exhibits high specific capacitance of 146 F g1. © 2014 Elsevier B.V. All rights reserved.
Keywords: Open ended nitrogen-doped CNTs Chemical vapor deposition Electrochemical properties Supercapacitor Specific capacitance
1. Introduction In the last decade, electrical double layer capacitors or supercapacitors play an important role in energy storage applications due to their high power density and long cycling life [1]. Carbon materials have been widely investigated as an attractive electrode material in supercapacitors owing to their high surface area, excellent electronic conductivity, high reversibility and ecoefriendly [2]. Further, modification of carbon materials with nitrogen doping should be potential to improve electron conductivity [3e7]. It is noteworthy, to consider that the different N doping environments would induce different effects on the supercapacitor performance of CNTs [8]. A recent study suggests that pyrrolicestructure enriched nitrogen is highly efficient for supercapacitor behavior [9]. In some studies, the enhanced electron
* Corresponding author. E-mail address: [email protected] (P. Arumugam). http://dx.doi.org/10.1016/j.jpowsour.2014.11.151 0378-7753/© 2014 Elsevier B.V. All rights reserved.
transfer is attributed to quaternary and pyridiniceNeoxides nitrogen groups [10,11]. The pyridiniceN and pyridiniceNeoxide nitrogen also have enhancing effects on capacitance due to the positive charge [12]. Recently, open ended N-doped CNTs are the new kind of intense research topic, because of their large specific surface area, better electroecatalytic performance and enhanced field emission properties than closed ended N-doped CNTs [13e15]. In addition, open ended N-doped CNTs could form more electron transport pathways in open tips, which resulted in enhanced electron conductivity than closed one. Acid oxidation of CNTs is a basic technique for their ring opening [16]. In addition to CNTs cap opening, oxidation enhances the number of defects in CNTs and increases the both nitrogen and oxygen functionalities [17]. It is well known that supercapacitor performance of CNTs significantly improved via the introduction of N and O atoms [10,11]. Therefore, it should be expected that the combined effect of open ended tips and N/O functionalities improve the electrochemical energy storage performance.
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Fig. 1. (a, b) SEM images and (c, d) TEM images of as-synthesized N-doped CNTs.
In our previous work, we reported a new strategy to synthesize N-doped CNTs by catalytic pyrolysis of acetylene/ammonia (C2H2/ NH3) mixture over lanthanum nickel (LaNi5) alloy catalyst [18]. Also, we have shown that the effect of 3M, HNO3:H2SO4 acid mixture treatment on the nitrogen content and electronic structure of N-doped CNTs. In this communication, we continue to study the effect of acid treatment on the morphology of the N-doped CNTs. The scanning electron microscopy (SEM) and transmission electron microscopy (TEM) results showed that the acid treatment on Ndoped CNTs leading to their ring opening. Besides, these open ended N-doped CNTs tested as a supercapacitor electrode material and it was found that increased specific capacitance in comparison with asesynthesized N-doped CNTs.
a H2 flow atmosphere at 30 mL min1. After 20 min of growth, the sample was cooled to room temperature under N2 ambient. After synthesis N-doped CNTs were heated in air at 400 C for removal of the residual carbon. 2.2. Acid treatment on as-synthesized N-doped CNTs The as-synthesized N-doped CNTs were suspended in 3M, 3:1 ratio of nitric acid (HNO3) and sulfuric acid (H2SO4) mixture and refluxed for 12 h at 80 C. After that the sample was washed with distilled water and filtered for several times and then dried in hoteair oven at a temperature of 100 C. 2.3. Characterization
2. Experimental 2.1. Synthesis of N-doped CNTs The N-doped CNTs were synthesized by catalytic pyrolysis of C2H2/NH3 mixture over LaNi5 alloy catalyst (99.5%, metal basis, 100 mesh, Alfa Aesar) in a conventional chemical vapor deposition (CVD) method, according to our detailed synthesis procedure reported recently [18]. In brief, a quartz boat loaded with 100 mg of LaNi5 alloy catalyst was placed at the centre of a tube furnace. The furnace was heated from room temperature to 550 C at a rate of 5 C/min with a flow of nitrogen (N2) gas at 100 mL min1. On reached to 550 C, N2 was switched to hydrogen (H2) gas at 100 mL min1 for 20 min to reduce the catalyst. Then, the center of the furnace temperature was raised to 900 C, 1:2 gas mixture of 60 mL min1 C2H2 and120 mL min1 NH3 introduced into the flow as the C and N sources, respectively. The pyrolysis was performed in
The morphology of as-synthesized and acid refluxed N-doped CNTs was carried out on a Hitachi S4800 SEM operated at 20 kV. TEM analysis was carried out on JEOL 3010 operated at 200 kV to study the microstructure of the samples. X-ray photoelectron spectroscopy (XPS) measurements were performed using an Omicron Nanotechnology ESCA Probe system with monochromatic Al Ka X-rays (energy of 1486.7 eV). Confocal Raman spectroscopy analysis was done with a WiTec GmbH, AlphaeSNOM CRM 200 confocal Raman microscope having a 514 nm laser as the excitation source. 2.4. Electrochemical measurements The electrochemical performances for the as-synthesized and acid treated N-doped CNTs were performed in a threeeelectrode system equipped with a Pt wire counter electrode, an Ag/AgCl
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Fig. 2. (a, b) SEM images of open ended N-doped CNTs, (c) TEM image of individual open ended N-doped CNTs and (d) HRTEM image taken from the squared region in Fig. 2c, and (e, f) high magnified TEM images of open ended region.
reference electrode and an electrochemical analyzer (CHI660D, CH Instruments Inc.). The working electrode preparation method was similar to that described elsewhere [5]. Briefly, an aqueous 2 mg mL1 suspension was prepared by ultrasonically dispersing as-synthesized and acid treated N-doped CNTs (5 mg) in a 1.25 mL mixture of water, ethanol and Nafion (5 wt.%) with the volume ratio of 1.0:0.2:0.05 for 1 h, then 10 mL inks was pipette out and spread on the glassy carbon electrode. The inks were dried at room temperature. Cyclic voltammetry (CV) measurements were conducted in 1 M KCl electrolyte between 0.3 and 0.5 V (vs. Ag/AgCl) at different scan rates ranging from 10 to 400 mV s1. 3. Results and discussion 3.1. Scanning and transmission electron microscopy analysis The morphology of as-synthesized N-doped CNTs was studied by SEM and TEM analysis. SEM images of the as-grown N-doped
CNTs obtained by pyrolysis of C2H2/NH3 mixture over LaNi5 alloy catalyst at 900 C (Fig. 1a and b) shows the large quantity of high density nanotubes formed with uniform diameter. Furthermore, all the observed nanotube lengths were several micrometers and closed by end caps (Fig. 1b). TEM (Fig. 1c) and high magnification TEM (Fig. 1d) analysis were studied to get detailed information about the as-synthesized N-doped CNTs structure, and show a regular bambooelike morphology with an outer diameter of 80e100 nm. It can be seen from Fig. 1b and c that the metal catalyst particles attached on the N-doped CNT ends, indicating the nanotubes growth initiated by LaNi5 alloy catalyst. Previous work from our group [18] has proven that the LaNi5 alloy catalyst initiate the growth N-doped CNTs. The open ended N-doped CNTs obtained by our approach (acid treatment) were first evidenced on the SEM images as shown in Fig. 2a and b. From the low and high magnification SEM images, we can observe that the N-doped CNTs ends were opened after acid treatment and their lengths were up to several micrometers. The
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Fig. 3. (a) XPS survey scan spectra of as-synthesized N-doped CNTs and acid treated N-doped CNTs, (b) deconvoluted N1s spectrum of the as-synthesized N-doped CNTs, (c) deconvoluted N1s spectrum of the acid treated N-doped CNTs, and (d) Raman spectra of as-synthesized and acid treated N-doped CNTs.
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Fig. 4. (a) CV curves of as-synthesized and acid treated N-doped CNTs electrodes at a scan rate of 100 mV s1, (b) and (c) CV curves for as-synthesized and acid treated N-doped CNTs at different scan rates, respectively.
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low magnification TEM image of the individual N-doped CNTs presented in Fig. 2c shows an openeended bambooelike morphology, which is in agreement with the SEM observations. The open ended N-doped CNTs having an outer diameter of 80e100 nm and many internal compartments formed in the cavity of the nanotube. The HRTEM image (Fig. 2d) was taken from the squared region in Fig. 2c indicates that the nanotube possesses thin walled wavy like structure. High magnification TEM images shown in Fig. 2e and f further confirms that the N-doped CNTs were of openended structure, ie., metal catalyst particles were removed after acid treatment leading to their end caps opening. In addition, the surface of wall and internal compartments were rough, showing the lower degree of graphitization. 3.2. X-ray photoelectron spectroscopy analysis The chemical composition of the as-synthesized and acid treated N-doped CNTs was characterized by XPS. Fig. 3a shows the wide scan XPS spectra of as-synthesized and acid treated N-doped CNTs. An obvious nitrogen peak clearly detected in the both assynthesized and acid treated N-doped CNTs. The main peaks around at 284.0, 400.0 and 532.0 eV correspond to C 1s, N 1s and O 1s, respectively. Compared to as-synthesized sample, the nitrogen and oxygen peaks were significantly increased after acid treatment, indicating the oxidation of nitrogen functional groups during the acid treatment. Fig. 3b shows deconvoluted N 1s XPS spectrum of as synthesized sample displays two main peaks N1 (pyridinicelike N) at 398.2 eV and N2 (graphiticelike N) at 400.3 eV. Deconvoluted N1s XPS spectrum is shown in Fig. 3c indicates the existence of three peaks, which are located around at 398.8 eV (N1), 400.5 eV (N2) and 402.3 eV (N3), respectively. The additional peak in acid treated sample at 402.3 eV (N3) is commonly attributed to Neoxides of pyridiniceN [5,14]. The N concentration, defined as N/ (C þ N) atomic ratio%, was estimated by the area ratio of the N 1s and C 1s peaks and the calculated atomic percentage of N in the asesynthesized and acid treated samples were 6.9 and 7.3 at%, respectively. Confocal Raman spectroscopy demonstrated that the level of crystalline perfection present in the asesynthesized and acid treated N-doped CNTs, as shown in Fig. 3d. For asesynthesized Ndoped CNTs, the calculated ID/IG ratio was 0.95, indicates the low graphitic ordering in asegrown N-doped CNTs. Further, the acid treatment on N-doped CNTs was found to that the ID/IG ratio (0.98) enhanced slightly, due to the increased nitrogen and oxygen functionalities (pyridinic N-oxides).
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than as-synthesized N-doped CNTs, and exhibits comparable capacitance value of the reported results (100e300 F g1) [2]. There are two possible reasons proposed for the increased capacitance for open ended nanotubes. First, the open ended nanotubes will necessarily improve the contact with electrolyte which helps to improve the transport of ions. Second, the residue catalyst will block the diffusion path of electrolyte as well as ions in the assynthesized N-doped CNTs but the open ended nanotube doesn't. Fig. 4b and c shows the effect of different scan rates (10e400 mV s1) on the CV for the as-synthesized N-doped CNTs and open ended N-doped CNTs. It can be seen that the increase of scan rates, the area of CV loops increased and maintain the rectangular shapes even at high scan rate of 400 mV s1. According to previous literature reports the various N doping environments would induce different effects on the supercapacitor performance of CNTs [8]. A recent study suggests that pyrrolicestructure enriched nitrogen is highly efficient for supercapacitor behavior [9]. In some studies, the enhanced electron transfer is attributed to quaternary and pyridiniceNeoxides nitrogen groups [10,11]. The pyridiniceN and pyridiniceNeoxide nitrogen also have enhancing effects on capacitance due to the positive charge [12]. In our studies the higher capacitance of open ended N-doped CNTs attributed to the positive charge on the both graphitic or quaternary N as well as pyridiniceNeoxide groups can improve the electron conductance through carbon network and leads to the increased capacitance. Here we also suggested the combined effect of N- and O-containing functional groups of CNTs is responsible for increased capacitance. Our XPS results confirmed that acid treated N-doped CNTs contain three functional groups (graphiticeN, pyridiniceN, pyridiniceNeoxides nitrogen), while the proportion of positive charge groups were dominant. Therefore, the increased capacitance value was quite acceptable and promising electrode material for use in electrical double layer supercapacitors. 4. Conclusion In summary, the open-ended N-doped CNTs have been obtained by pyrolysis of C2H2/NH3 mixture over LaNi5 alloy catalyst and subsequent 3M, HNO3:H2SO4 acid mixture treatment. TEM and XPS results showed that the morphology and electronic structure of Ndoped CNTs were modified after acid treatment. This novel electrode material was showed an electrical doubleelayer capacitive behavior with high specific capacitance of 146 F g1, possibly due to enhanced electrical conductivity of the material. The results suggested that the open ended N-doped CNTs are promising electrode material for use in high-performance energy storage applications.
3.3. Electrochemical properties Acknowledgment The electrochemical performances of open ended N-doped CNTs were evaluated by CV at scan rates of 10e400 mV s1 within the potential range of 0.3e0.5 V in 1 M KCl aqueous solution. For comparison, we also evaluated the electrochemical performance of electrode made from as-synthesized N-doped CNTs. Fig. 4a compares the CV curves of the as-synthesized N-doped CNTs and acid treated (open ended) N-doped CNTs electrodes at a scan rate of 100 mV s1 between 0.3 and 0.5 V. Both the CV curves display rectangular shape without obvious redox peaks, showing the typical characteristic of electrical doubleelayer capacitance [1]. Compared with the as-synthesized N-doped CNTs, the CV area of open ended N-doped CNTs increased profoundly. It indicated that the specific capacitance is dramatically improved resulted from the enhanced electron conductivity in open ended N-doped CNTs. The calculated capacitances were 42 and 146 F g1 for as-synthesized N-doped CNTs and open ended N-doped CNTs, respectively. The capacitance of the open ended N-doped CNTs was much higher
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