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Journal of Power Sources 247 (2014) 660e666

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Journal of Power Sources

journal homepage: www.elsevier .com/locate/ jpowsour

N- and O-doped carbonaceous nanotubes from polypyrrole forpotential application in high-performance capacitance

Chengxin Guo a, Na Li a, Liangliang Ji a, Yaowen Li a, Xiaoming Yang a, *, Yun Lu b, **,Yingfeng Tu a, *

a Jiangsu Key Laboratory of Advanced Functional Polymer Design and Application, College of Chemistry, Chemical Engineering and Materials Science,Soochow University, Suzhou 215123, PR Chinab Department of Polymer Science and Engineering, State Key Laboratory of Coordination Chemistry, School of Chemistry and Chemical Engineering, NanjingUniversity, Nanjing 210093, PR China

g r a p h i c a l a b s t r a c t

By the reactive self-degraded template-directed synthesis and further carbonization, the obtained hollow nanotubular materials show amorphology changes from capped to uncapped one. The uncapped one possesses a reversible capacitance of 470 F g�1 at the current densityof 0.2 A g�1, and maintains 60% even at the higher current density of 10 A g�1, which is much better than the capped one.

a r t i c l e i n f o

Article history:Received 12 July 2013Received in revised form30 August 2013Accepted 3 September 2013Available online 11 September 2013

Keywords:Reactive self-degraded templatePolypyrroleCarbonaceous nanotubesHeteroatomSupercapacitor

* Corresponding authors. Tel./fax: þ86 512 658821** Corresponding author.

E-mail addresses: yangxiaoming@suda.edu.cn (X.

0378-7753/$ e see front matter � 2013 Published byhttp://dx.doi.org/10.1016/j.jpowsour.2013.09.014

a b s t r a c t

By the reactive self-degraded template-directed synthesis and further carbonization, N- and O-dopedcarbonaceous nanotubes from conducting polymer (polypyrrole) precursor have been prepared. Afterbeing chemically activated by KOH, specially, with the weight ratio of KOH to polypyrrole nanotubes of 2and 4, two samples of activated carbonaceous polypyrrole nanotubes (ACPN-2 and ACPN-4) with anuncapped morphology were obtained. The ACPN-4 possesses high heteroatom content (14.85%) andspecific surface area (1226 m2 g�1), and exhibits a reversible capacitance of 470 F g�1 at the currentdensity of 0.2 A g�1, and 280 F g�1 even at a higher current density of 10 A g�1. Moreover, thesenanotubular structured materials have also shown excellent cycling performance with no capacitanceloss over 10,000 cycles. Furthermore, these carbonaceous nanotubes achieved a high CO2 adsorptionuptake of 3.9 mmol g�1 at 273 K and 2.5 mmol g�1 at 298 K.

� 2013 Published by Elsevier B.V.

30.

Yang), yunlu@nju.edu.cn (Y. Lu), tuyingfeng@suda.edu.cn, tuyingfeng@yahoo.com (Y. Tu).

Elsevier B.V.

C. Guo et al. / Journal of Power Sources 247 (2014) 660e666 661

1. Introduction

Carbonaceous supercapacitor materials possess potential ap-plications in various portable electronic devices [1]. To date, carbonelectrodes have played important roles in the future super-capacitors due to the good electric conductivity, long cycle stabilityand stable physicochemical properties. At present, researches havebeen directed towards the development of carbonaceous materialssuch as activated carbon [2,3], mesoporous carbon [4e6], carbononion [7,8], carbon nanotubes [9e12] and graphene [8,13,14] withhigh performance in a cost effective way.

In the development of electric double-layer supercapacitor, aproper control over the specific surface area and the pore size arecrucial to ensure a good performance of the supercapacitor. How-ever, the electric double-layer capacitance cannot be enhancedillimitably because the excessive increase of surface area causes ahuge decrease of the mesoporosity, thus reducing the mass transfercapacity. A simple way in developing high-capacitance super-capacitors is combining the electric double-layer capacitance andpseudocapacitance. In general, the heteroatom-doped structuresare believed to provide pseudocapacitance contributed from theredox faradic reactions of these electrochemically active functionalgroups. Usually, heteroatoms in the supercapacitors are nitrogenand oxygen, because the pseudocapacitive contribution of pyrrolic,pyridinic nitrogen and hydroquinone/quinone groups should betaken into account [15e18]. In most cases, nitrogen-doped carbonmaterials were achieved by introducing nitrogen into carbonframeworks through the post treatment, for example, being treatedwith ammonia gas/air, which results in a lower content of nitrogen[19,20]. The drawback can be overcome by employing N-containingmaterials as precursors [21]. By this method, nitrogen in the carbonmaterials can be preserved at a relatively large content. Bio-materials (such as ginkgo shells [22], gelatin biomolecule [23], silkproteins [24], collagen [21]) and polymers (such as phenolic resin[25,26], polyacrylonitrile [27], poly(vinylpyrrolidone) [28], poly-acrylamide [28]) have been reported as suitable precursors forsynthesizing N- and O-doped carbonaceous materials due to theirinherent high heteroatom content. For example, an N- and O-containing (range from 11.2% to 17.16%) microporous activatedcarbon, with the specific capacitance around 200 F g�1, was ach-ieved from coconut-shell-based activated carbon and treated withmelamine and urea at 950 �C [15]. Recently, a heteroatom (nitrogenand oxygen) content of 13.17 wt % and specific capacitance of390 F g�1 were achieved from a mesoporous carbon derived frombiomass-derived proteins [29]. These suggest that heteroatomdoping can improve the wettability of mesoporous carbons toelectrolyte solution, and thus increase its electrochemical activity.

In addition to the aforementioned precursors, the heteroatom-enriched conducting polymers as precursors could be quite usefulfor enhancement of electrodes’ electrochemical performance [30e32]. Yu and co-workers fabricated a kind of porous nitrogen-dopedcarbon nanofibers by carbonization of carbonaceous nanofiberscoated with polypyrrole (PPy), which exhibit a specific capacitanceof 202.0 F g�1 [30]. Hou et al. used the starting material polyanilinenanowires to prepare 3D network of activated carbon nano-whiskers wrapped-on graphitized electrospun nanofibers, whichshowed an ultrahigh power density of 252.8 kW kg�1 at currentdensity of 800 A g�1 [33]. The results demonstrate that theconducting-polymer-based carbon nanostructure might be apromising candidate for high performance supercapacitors. Inrecent years, Lu et al. have conducted a systematic study of con-ducting polymer (polypyrrole, polyaniline and poly(3,4-ethylenedioxythiophene)) nanotubes [34e36]. These nano-structured materials, containing plenty of nitrogen, oxygen or sul-fur atoms, could be selected as precursors to obtain the

carbonaceous materials. More importantly, high aspect ratiopolymer-based carbonaceous nanotubes with the length over10 mm show the behavior as conductive wires that entangle witheach other to formwell interconnected networks [28,37]. Owing tothe special interconnected network and 1D nanotubular structure,these materials could be used as supercapacitor in the hope ofbecoming a potential electrode material for high performancecapacitance.

In our previous study, we reported a new chemical approach, inwhich a fibrillar complex of the anionic azo dye methyl orange(MO) and the oxidant FeCl3 was used as a reactive self-degradableseed template directing the growth of PPy on its surface and pro-moting the assembly into hollow nanotubular structures [34]. Thepyrolysis of the obtained PPy nanotubes was further processed at900 �C in nitrogen atmosphere. This synthesis method of as-prepared capped carbonaceous polypyrrole nanotubes (CPN) wassimple and would be an easy way to produce more nanotubes on alarge scale. After KOH activation, an interesting uncapped nano-tubular material was achieved, which shows a remarkable differ-ence compared with capped CPN in nanotubular morphology, porestructure and electrochemical properties. Importantly, the acti-vated carbonaceous polypyrrole nanotubes (ACPN) shows anexcellent performance as the electrodematerials of supercapacitors(470 F g�1) because of its large surface area (1226 m2 g�1), highheteroatom content (14.85%) and proper micro/mesopores.

2. Experimental

2.1. Synthesis of ACPN electrodes

In a typical procedure, 0.243 g (1.5 mmol) of FeCl3 was dissolvedin 30 mL of 5 mM MO (sodium 4-[40-(dimethylamino) phenyl-diazophenylsulfonate ((CH3)2NC6H4eN]NC6H4SO3Na)] deionizedwater solution (0.15 mmol). A flocculent precipitate appearedimmediately. Then 105 mL (1.5 mmol) of pyrrole monomer (Aldrich)was added into it and the mixture was stirred at room temperaturefor 24 h. The formed PPy precipitate was washed with deionizedwater/ethanol several times until filtrate was colorless and neutral,and finally dried under a vacuum atmosphere at room temperaturefor 24 h.

The obtained PPy nanotubes were further carbonized to fabri-cate carbonaceous nanotubes in a quartz tubular furnace undernitrogen atmosphere. The sample was gradually heated up to900 �C at a heating rate of 3 �C min�1, held 900 �C for 5 h and thencooled to room temperature. The obtained CPN were chemicallyactivated by heating a CPNeKOH mixture (KOH/CPN at a weightratio of 2 and 4) under nitrogen atmosphere up to 600 �C for 1 h.

2.2. Material characterization

The as-prepared samples were characterized by scanning elec-tron microscope (SEM, Hitachi s-4800), energy dispersive X-rayspectroscopy (EDX, Hitachi s-4800), transmission electron micro-scope (TEM, FEI Tecani G2 F20 S-TWIN), X-ray photoelectronspectroscopy (XPS, UlVAC-PHI PHI 5000 VersaProbe). The Bru-nauereEmmetteTeller (BET) surface areas and pore size distribu-tions (PSD) of the samples weremeasured at 77 K using NOVA 2000(Micromeritics ASAP 2020). The adsorption of CO2 was measuredusing NOVA 2000 and the samples were degassed at 200 �C forseveral hours prior to perform at 0 �C and 25 �C.

2.3. Electrochemical measurements

The working electrode was prepared by mixing 80 wt % activematerial, 10 wt % carbon black and 10 wt % polytetrafluoroethylene

Table 1Parameters on pore structure and elemental composition.

Sample Textural properties Chemical compositionat %

SBET(m2 g�1)

Poresize (nm)

Vtotal

(cm3 g�1)Vmicro

(cm3 g�1)C O N N þ O

CPN 208 4.4 0.22 0.08 86.33 7.68 5.98 13.66ACPN-2 667 3.5 0.51 0.21 82.31 6.98 10.71 17.69ACPN-4 1226 3.0 0.68 0.24 85.15 4.59 10.26 14.85

C. Guo et al. / Journal of Power Sources 247 (2014) 660e666662

(PTFE) binder in water. The slurry of the mixture were painted onthe nickel foam (1 cm2) and pressed under a pressure of 10 MPa.The mass loading of the active materials is exactly 5 mg. All elec-trochemical measurements except power density and energydensity were done in a three-electrode setup: A Ni foam coatedwith CPN, ACPN-2 and ACPN-4 served as the working electrode, aplatinum gauze electrode and a saturated calomel electrode (SCE)or Hg/HgO electrode served as counter and reference electrodesrespectively. The measurements were carried out on a CHI 660Eelectrochemical workstation in a 6 M KOH and 1 M H2SO4 aqueouselectrolyte at room temperature. The two electrodes were sepa-rated by a filter paper soakedwith electrolyte (1M LiPF6 in ethylenecarbonateedimethyl carbonate, 1:1 in volume), the working elec-trode was prepared by mixing the carbon with polyvinylidenefluoride (PVDF) and carbon black (8:1:1) in N-methyl 2-pyrrolidone(NMP) to form a homogeneous slurry. Thenwrappedwith parafilm,and at last dipped in the electrolyte solution under vacuum toreduce air contamination and improvewettability of the electrodes.

3. Results and discussion

The schematic model (Fig. 1a) is designed to describe the syn-thetic route andmechanism of the reactive self-degraded template-directed synthesis method. At the initial stage of synthesis, thecomplex of MO and the oxidant FeCl3 was used as a reactive self-degradable seed template directing the growth of PPy on its sur-face and promoting the assembly into hollow nanotubular struc-tures. After further pyrolysis and chemically activated with KOH,the obtained homogeneous hollow nanotubular materials show amorphology changes from capped to uncapped one. The SEM im-ages shown in Fig. 1b, c and Fig. S1 displays the dense entanglednetwork structure. Specially, according to the TEM image in Fig. 1d,the part in the red box (in the web version) clearly shows thecapped structure of CPN, which has a strong impact on their ca-pacity to absorb N2 and CO2 leading to a lower surface area. Com-mon activating agent KOH is utilized to improve the surface areaand tailor the pore texture of the carbonaceous nanotubes. Theobtained CPN solids were activated with KOH under two different

Fig. 1. (a) Schematic model of preparing ACPN. The use of fibrillar complex as a reactive seedpolymerization of pyrrole and automatically degrades during the subsequent polymerizationKOH activation. (b) SEM images of CPN showing randomly entangled nanotubular structure.TEM image of CPN showing resultant capped nanotubes. (e) Typical TEM image of ACPN-4

conditions (mKOH/mCPN ¼ 2:1 and mKOH/mCPN ¼ 4:1) and calcinedby further thermal treatment at 600 �C. Up to now, KOH activationof mesoporous carbon has been reported in many previous studies,which show significant improvement of specific surface area [38]. Itis reported that the activation of carbon with KOH proceeds as6KOH þ C 4 2K þ 3H2 þ 2K2CO3, followed by decomposition ofK2CO3 and/or reactions of K/K2CO3/CO2 with carbon [14,39]. In ourstudy, KOH could etch the carbonwalls at high temperature leavingmicroporous/mesoporous fissures so that high specific surface areais achieved. TEM image clearly (Fig. 1e) shows that the as-preparedACPN-4 are mainly hollow open tubes with an inner diameter of50 nm, and an outer diameter of about 70 nm. Thus, mass pro-duction of uncapped nanotubular structure was achieved using thecost-effective method.

In situ evaluation of the carbon and nitrogen distribution byelemental mapping with SEM revealed that carbon, nitrogen andoxygen atoms exist homogeneously in the nanotubes (Fig. S2).Meanwhile, the nature of the heteroatom functionalities wasdetailedly investigated by means of XPS technique. XPS analysis ofACPN samples has shown high heteroatom content and chemicalstructure variation compared with CPN. The percentage of nitrogenin CPN, ACPN-2 and ACPN-4 was calculated, given in Table 1, to bearound 5.98, 10.71 and 10.26%, respectively. At the same time,abundant oxygen functionalities are preserved, as shown in Table 1.Herein, the MO is shown to be the most possibility that act as ox-ygen resource. It is noteworthy that ACPN-2 has the maximumheterogeneity (N þ O w 17.69 at %), which is higher than the

template that instantaneously forms when adding MO and FeCl3 together prior to the. Finally, ACPN was synthesized through the pyrolysis of the PPy nanotubes followed by(c) SEM images of ACPN-4 showing dense and short nanotubular structure. (d) Typicalshowing resultant uncapped nanotubes.

C. Guo et al. / Journal of Power Sources 247 (2014) 660e666 663

mesoporous carbons using iron-impregnated polypyrrole as pre-cursor reported previously [40]. In addition, the percent of nitrogencan be as high as 12.14% calculated from the XPS survey spectrumusing polypyrrole as precursor [30]. As is known to all, rich het-eroatom contributed to high capacitance a lot. According to Fig. 2a,it can be seen that, for CPN, two intense peaks at 398.1 and 400.7 eVcan be observed. The former one corresponds to pyridinic nitrogen(N-6), and the latter one to quaternary-N (N-Q) [15]. At the sametime, a small peak at 403.1 eV can be distinguished, correspondingto N-oxides of pyridine-N group (N-X) [41,42]. As for ACPN-2 inFig. 2b, the peaks of 400.7 eV and 403.1 eV disappear, and the newpeak at 400 eV represents pyrrolic-/pyridonic-N groups (N-5).Upon further increasing the KOH ratio, the N-5 peak (400 eV,Fig. 2c) becomes weaker than that of CPN due to the carbonization.

According to the nitrogen adsorption isotherms and PSD, ACPN-4 registered the largest pore volume, specific surface area and thesmallest average pore size. The nitrogen adsorptionedesorptionisotherms and corresponding PSD curves of CPN, ACPN-2 andACPN-4 are shown in Fig. 3. All of the isotherms (Fig. 3a) are of type-IV curves according to IUPAC classification. The textural parametersof these porous carbons are listed in Table 1. Most importantly, thehigher N2 adsorption capacities imply higher surface areas and porevolumes. It can be seen that the BET surface area and the porevolume increase significantly with the KOH activation from208 m2 g�1 to 1226 m2 g�1 and from 0.22 cm3 g�1 to 0.68 cm3 g�1,respectively. On the other hand, it can be seen that CPN and ACPNpossess pores peaked from 0.55 nm to 5 nm (Fig. 3b), which displaya typical micro/mesoporous structure. The volume increase ofmacropores of ACPN-4 (Fig. S3), compared with those of CPN andACPN-2, is consistent with the result that nanotubular morphologychanges from uncapped to capped one.

Fig. 2. (a) N 1s core-level spectra of the CPN. (b) N 1s core-level spectra of the ACPN-2prepared with KOH/PPy ¼ 2. (c) N 1s core-level spectra of the ACPN-4 prepared withKOH/PPy ¼ 4.

Electrochemical performance from both basic (6 M KOH, Fig. 4)and acidic (1 M H2SO4, Fig. S4) media were characterized in thisstudy. The cyclic voltammetry (CV) testing (Fig. 4a, b) shows rect-angular curves from �1.0 to 0 V at a scan rate of 2 mV s�1 and100 mV s�1, respectively. No matter at a lower scan rate (2 mV s�1,Fig. 4a) or a higher scan rate (100 mV s�1, Fig. 4b) the curves of CPNsample exhibit shapes of ideal nanoporous carbon electrode. ForACPN-2 and ACPN-4, at a lower scan rate, the CV curves exhibitwide reversible humps as well as a large rectangular shape corre-sponding a high capacitance. The rectangular-like shape and theappearance of humps indicate that the capacitive response resultsfrom the combination of electric double-layer capacitance andredox reactions, which are related to the heteroatom functionalitiesof the materials. On the other hand, at a higher scan rate of100 mV s�1, no dramatic distortion of the CV curves was observed.

To gain insight into the kinetic behavior, electrochemicalimpedance spectroscopy (EIS) was studied in three-electrode cellmeasurements in 6 M KOH and 1 M H2SO4 electrolyte range from100 kHz to 0.01 Hz (Fig. 4c and Fig. S5). It is well accepted that theintrinsic resistance of active materials can be calculated from thediameter of the semicircle in the high frequency region of Nyquistplots. On the basis of this point, the equivalent series resistance(ESR) of the ACPN-4 in the 6 M KOH electrolyte is 0.95 U, a signif-icantly smaller value than 1.50 U of the CPN electrode, which isestimated from the value of the real axis at 31.6 Hz. In addition, theESR of the ACPN-4 is also lower than ACPN-2 (1.22 U) indicating a

Fig. 3. (a) Nitrogen adsorptionedesorption isotherms at �196 �C and (b) theadsorption PSD of CPN, ACPN-2 and ACPN-4.

Fig. 4. CV curves of the CPN, ACPN-2 and ACPN-4 electrodes in 6 M KOH electrolyte at different scan rates: (a) 2 mV s�1 and (b) 100 mV s�1. (c) Nyquist plot of the CPN, ACPN-2 andACPN-4 electrodes in 6 M KOH electrolyte over the frequency range from 0.01 Hz to 100 kHz. (d) Galvanostatic chargeedischarge curve of the CPN, ACPN-2 and ACPN-4 electrodes in6 M KOH electrolyte at a constant current density of 0.5 A g�1, respectively. (e) Capacitance versus discharge current density from 0.2 A g�1 from 10 A g�1 comparing CPN, ACPN-2and ACPN-4 electrodes. (f) Specific capacitance of CPN, ACPN-2 and ACPN-4 during chargeedischarge cycle at current densities of first 0.5 A g�1 (5000 cycles) and then 5 A g�1

(5000 cycles).

C. Guo et al. / Journal of Power Sources 247 (2014) 660e666664

better surface property of the porous electrode. At low frequencyregion, the slope reflects the diffusive resistivity of the electrolyteions within the pores. ACPN-2 and ACPN-4 exhibit a near verticallinewhich indicates ideal capacitive behavior. In contrast, CPN has aline that was slightly inclined from the ideal vertical line, whichindicates a higher diffusive resistivity. In summary, this is indicativeof good electrical conductivity of the CPN and ACPN as well as verylow interfacial electrical contact resistance between the activeelectrode material and the Ni foam collectors. The good ESRproperties also imply that the pyridinic-N and pyrrolic-N, beingmore favorable to attract ions in the electrolyte compared to that ofcarbon atoms, play an important role in the energy storage.Considering the PSD described before, the rational PSD maybe alsocontribute to the lower ESR a lot.

The above electrochemical performance results can be furtherconfirmed by the galvanostatic charge/discharge curves of the as-prepared samples in Fig. 4d and Fig. S6. As shown in Fig. 4d, thedischarging time of the ACPN-4 was significantly longercompared with those of other materials, indicating that theACPN-4 offers a much larger capacitor, which agrees well withthose obtained from CV tests. The relatively high capacitance ofACPN-4 as high as 470 F g�1 is due to the pseudocapacitive effectassociated with the larger specific surface area. What’s more, thenanotubular morphology could effectively facilitate the trans-portation of electrolyte ions during the galvanostatic charge/discharge process and, thus, obtain a better supercapacitancethan common micro/meso-porous carbon materials [43,44] andcarbon nanotubes [11,45,46]. Niu et al. have reported a multiwallcarbon nanotube-based supercapacitor electrode showing a highspecific capacitance of 102 F g�1 with surface area of 430 m2 g�1

and a power density of 8 kW kg�1 in an acidic electrolyte [45]. Atypical specific capacitance of 10e200 F g�1 has been obtained[11,46], which is mainly caused by the relatively small specificsurface area of typically hundreds of m2 g�1 without thecontribution from heteroatom. In addition, no obvious ohmic

drop could be noticed when the sign of the current is versed,which reflects a very low value of ESR.

Fig. 4e represents the relationships between specific capaci-tance and charge/discharge current density to study the capaci-tance retention of the materials. In general, the specific capacitancedecreases gradually with increasing discharging current densityowing to large currents may block the entrances of the pores. In ourstudy, when the charge/discharge current density was increasedfrom 0.2 to 10 A g�1, the capacitance retention of all three sampleswas still up to 50%, which demonstrates the as-prepared samplesare very suitable for high current density application. Importantly,it was found that ACPN-4 exhibited a reversible capacitance of470 F g�1 at the current density of 0.2 A g�1, meanwhile main-taining 280 F g�1 even at the current density of 10 A g�1.

For supercapacitor, long cycling life is one of the most importantrequirements in application. In this context, the cycling life test forthree different electrodes were carried out by repeating the charge/discharge test at a current density of 0.5 A g�1 for first 5000 cyclesand 5 A g�1 for last 5000 cycles (Fig. 4f). At low current densitycurves of all three samples, obvious capacitance increase aftermorethan 1000 cycles has been observed, suggesting increased contin-uous impregnation of the electrolyte ions in the nanopores of theelectrode during the chargeedischarge processes [6,47]. Impor-tantly, they exhibited a good electrochemical stability with no anydeterioration of specific capacitance after 10000 cycles. These re-sults again highlight that as-prepared materials have excellentelectrochemical stability no matter at a lower or higher currentdensity.

In addition, the electrochemical behavior of the three electrodematerials in basic media (6 M KOH) and acid media (1 M H2SO4) islisted in Table 2. It can be seen that these carbonaceous nanotubesin 1 M H2SO4 also have high specific capacitance, good capacitanceretention history. However, comparing to the acidic electrolytesolution, the supercapacitor performance in the basic medium isbetter.

Table 2Comparison of the electrochemical performance of carbon materials in the literature.

Electrolyte Samples Capacitance/F g�1 ESR/U

0.2 A g�1 0.5 A g�1 1 A g�1 2 A g�1 4 A g�1 6 A g�1 8 A g�1 10 A g�1

6 M KOH CPN 126 112 102 96 91 86 84 81 1.50ACPN-2 327 247 226 212 194 185 180 174 1.22ACPN-4 470 377 343 324 306 293 287 276 0.95

1 M H2SO4 CPN 174 161 155 146 131 122 115 110 1.50ACPN-2 291 243 240 222 207 193 186 181 1.18ACPN-4 421 360 333 306 270 246 229 210 0.85

Fig. 5. Ragone plots for ACPN-4 compared with various electrical energy storagesystems.

C. Guo et al. / Journal of Power Sources 247 (2014) 660e666 665

In order to completely determine the electrochemical perfor-mance of our materials, energy density and power density wereestimated in a two-electrode cell with a potential range from 0 to3 V, the equations are listed in Supporting Information. The Ragonplots showing the dependence between power output and energydensity displayed in Fig. 5. It reveals that ACPN-4 based super-capacitor exhibits an ultrahigh energy density of 63.1 Wh kg�1,which is higher than those of most commercial carbon materials.The volumetric storage capacity of ACPN-4 was also measured inthis two electrode cell. The value of the volumetric capacitance isabout 56.1 F cm�3 at a current density of 0.2 A g�1. It is consideredthat this good supercapacitor performance is mainly because of therelatively high conductivity as well as the unique 1D nanotubularstructure with proper pore structure, which enhances the kineticsof ion and electron transport in electrodes and at the electrode/electrolyte interface.

In addition to high performance electrode materials of super-capacitors, the carbonaceous materials prepared from nano-structured conducting polymers as precursors show good CO2capture (Fig. S7) ascribed to the unique feature of large surface areaand high heteroatom contents. The CO2 adsorption isotherms of thecarbonaceous nanotubes ACPN-4 with large surface area and highheteroatom content show a maximum capacity of 3.9 mmol g�1 at273 K and 2.5 mmol g�1 at 298 K, which is relatively higher thanthat of commercial activated carbons.

4. Conclusions

Heteroatom-doped uncapped carbonaceous nanotubes from theprecursor polypyrrole nanotubes prepared by using the complex ofFeCl3 and MO as a reactive self-degraded template were synthe-sized. The high heteroatom content (14.85%), large specific surfacearea (1226 m2 g�1) and proper PSD could facilitate the high-

performance energy storage. Thus, the supercapacitor based onACPN-4 exhibits a specific capacitance of 470 F g�1 at 0.2 A g�1 and276 F g�1 at 10 A g�1 in 6M KOH solution. As demonstrated, ACPN-4exhibited excellent performance stability without loss after 10000chargeedischarge cycles. In addition, the reactive self-degradedtemplate could be perfectly adapted to the synthesis of PANI andPEDOT nano/microtubes which can be expanded to other applica-tions including energy storage and gas adsorption.

Acknowledgments

The financial support from the National Natural Science Foun-dation of China (No. 21074079, 21104050, 21274099), China Post-doctoral Science Foundation (2013M541715), a Project Funded bythe Priority Academic Program Development of Jiangsu HigherEducation Institutions, and Natural Science Foundation of JiangsuProvince of China (General Program BK2011356) are gratefullyacknowledged.

Appendix A. Supplementary data

Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.jpowsour.2013.09.014.

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