ORIGINAL PAPER

Dual template method to prepare hierarchical porous carbonnanofibers for high-power supercapacitors

Qiang Wang & Qi Cao & Xianyou Wang & Bo Jing &

Hao Kuang & Ling Zhou

Received: 23 April 2013 /Revised: 19 June 2013 /Accepted: 23 June 2013# Springer-Verlag Berlin Heidelberg 2013

Abstract Hierarchical porous carbon nanofibers serving aselectrode materials are prepared through carbonization andhydrofluoric acid treatment of polyacrylonitrile-based elec-trospinning involving dual templates. The hierarchical porousstructures are synergistically tailored by varying template con-tents in the spinning solution. The carbon nanofibers preparedfrom the electrospinning of polyacrylonitrile containing15/15 wt.% polymethylmethacrylate/tetraethyl orthosilicateexhibit the largest specific surface area (699 m2 g−1) andmicroporous volume (0.196 cm3 g−1). In 6MKOH electrolyte,a symmetrical supercapacitor equipped with the hierarchicalporous carbon nanofibers demonstrates its high-end specificcapacitance of 170 F g−1, superior rate capability, and high-power density output up to 14.7 kW kg−1. Cycling evolutionindicates capacitance fading is only 5.8% of initial capacitanceat a current density of 1 A g−1 even after 8,000 cycles. Theexcellent electrochemical performances of the carbonnanofiber are mainly ascribed to the optimized pore size dis-tributions of both micropores and mesopores and the uniquehierarchical pore structures possessed by abundantmicropores.

Keywords Supercapacitor . Electrospinning . Carbonnanofiber . Polyacrylonitrile

Introduction

Tremendous growth in the market of portable systems to-gether with development of hybrid electric vehicles haveprovoked an ever-increasing demand for supercapacitorswith a character of high-power density and long cyclic life[1–3]. Based on energy storage mechanism, supercapacitorsare commonly divided into two categories as follows: anelectric double-layer capacitor that arises from pure electro-static attraction between ions and a charged surface of elec-trode materials [3–5] and a pseudocapacitor that relies on fastand reversible redox reactions of electroactive species on thesurface of the electrode [6, 7]. To explore a supercapacitorwith preferable performance, numerous active materials (car-bon, transition metal oxide and conducting polymer) withvarious components and structures have been probed aspotential electrode materials. Among various electrode ma-terials for supercapacitors, porous carbon materials haveattracted intensive attentions because of their high porosity,large surface area, and good electrical conductivity.

Thus far, conventional processes of pore creation mainlyinclude chemical [8–10] or physical activation [11, 12],which involve complex chemical and physical phenomenaoccurring at multiple time and temperature scales. In the caseof chemical activation by ZnCl2 or KOH, carbon yield surelyreduces with increase of activation degree [13], and manyby-products also remain in the treated carbon materials [14].Additionally, physical activation is known to be difficult foreffectively controlling pore sizes and surface properties ofthe obtained carbon materials, as well as it is time consuming[15–17]. In fact, the supercapacitive phenomenon is associ-ated with specific surface area, pore size, functional group of

Electronic supplementary material The online version of this article(doi:10.1007/s10008-013-2166-4) contains supplementary material,which is available to authorized users.

Q. Wang :Q. Cao (*) :X. Wang :B. Jing :H. Kuang : L. ZhouKey Laboratory of Environmentally Friendly Chemistry andApplications of Minister of Education, College of Chemistry,Xiangtan University, Xiangtan 411105, Chinae-mail: wjcaoqi@163.com

J Solid State ElectrochemDOI 10.1007/s10008-013-2166-4

carbon materials, and electrical conductivity. For these rea-sons, extensive efforts have been made to find alternativecontrollable and cost-effective synthetic routes for preparationof porous carbon materials, such as template methods andusage of polymer blends. To date, many template methodshave been developed to precisely control pore sizes rangingfrom nanoscale to macroscale [18–20]. For instance, Wanget al. synthesized 3D aperiodic hierarchical porous graphiticcarbon using the alkaline system Ni(OH)2/NiO as hard tem-plate [21]. In their work, mesoporous diameters of the gra-phitic carbon were almost centralized in the scope of 10–50 nm, and the mesoporous carbon exhibited high energyand power densities in both aqueous and organic electrolytes.Polyacrylonitrile (PAN)-based nanocarbons with continuousmesopore structures were prepared by Yang et al. using silicagel as template [22]. The as-prepared materials possessed amesoporous ratio as high as 94.2 % and showed the highestspecific capacitance of 210 F g−1 at a current density of0.1 A g−1. In spite of the fact that many templated synthesisof nanoporous carbon have been reported in literature, amajority of template methods are based on inorganic tem-plates. The usage of soft materials such as organic polymeras template has been less reported [23–26]. In this paper, wereport controllable preparation of porous carbon nanofibersusing soft–hard dual templates.

Recently, electrospinning technique has been attested tobe a simple but powerful way to prepare nanometer-scaledfibers [27–29]. In the present work, PAN is selected as aprecursor because of its relatively high carbon yield [30],good spinnability in solution [31–33], and flexibility fortailoring the structure of the final carbon nanofibers [34].Hierarchical porous carbon nanofibers are prepared byelectrospinning from PAN solutions containing dual tem-plates, followed by carbonization and subsequent removalof silica template. Concretely, polymethylmethacrylate(PMMA) copolymer is a soft template that completely dis-appears when heated to approximately 400 °C in air and can,therefore, be utilized to control pore size. During the thermaltreatment, tetraethyl orthosilicate (TEOS), which acts as theprecursor of hard template, can be transformed into its oxide(SiOx). Simultaneously, the gas (e.g., CO, CO2, H2, CH4,H2O, etc.) releases in a series of chemical process, whichalso leads to growth of porous structure [35]. Finally, SiOx

can be removed by hydrofluoric (HF) acid etching. Conse-quently, PMMA and TEOS are introduced into the spinningsolution to synergistically tailor hierarchical porous texture.The condition for preparing hierarchical porous carbonnanofibers with optimal physical structures has beendiscussed in this study. Micromorphologies and structuresof carbon nanofibers upon removal of PMMA/TEOS havebeen also analyzed. Additionally, the series of hierarchicalporous carbon nanofibers prepared from the electrospinningof different PAN/PMMA/TEOS blend ratios have been

electrochemically characterized in 6-M KOH electrolyte.The results testify that the obtained carbon nanofibers dem-onstrate good capacitance behavior and high-power densitywhen the blend concentration of PMMA/TEOS is designedas 15/15 wt.%.

Experimental section

Materials

PAN (Mw=150,000), PMMA (Mw=15,000), and TEOSwere purchased from Sigma-Aldrich Chemical CompanyInc. (USA). N,N-dimethylformamide (DMF) and tetrahydro-furan (THF) were supplied by Xi'an Xilong Chemical Re-agent Factory. Hydrofluoric acid (30 wt.%) was obtainedfrom Tianjin Damao Chemical Reagent Factory. Allchemicals were used as received without further purification.

The preparation of hierarchical porous carbon nanofibers

The spinning solutions were the DMF solutions of 7 wt.%PAN involving various blend ratios of PMMA/TEOS (0/0,5/5, 10/10, 15/15, 20/20, and 25/25 wt.%, relative to PAN). Toobtain a homogenous solution, the polymer blend solutionswere dissolved at 50 °C through strong mechanical stirring for24 h. Thereafter, these PAN polymer solutions containingdifferent amounts of PMMA/TEOS were successfullyelectrospun into nanofibers in an typical electrospinning ap-paratus by applying a high positive voltage of 20 kV (DW-P353-3 AC) and tip-to-collector distance of 15 cm. In theprocess of spinning, dry fibers were accumulated on the glassplate and collected as fibrous materials. The carbonization ofas-electrospun nanofibers proceeded in an electric heat-treating furnace (Changcheng Furnaces Factory). Firstly, theelectrospun PAN/PMMA/TEOS composite nanofibers werestabilized at 280 °C for 3 h (heating rate was 2 °C min−1),Subsequently, PMMAwas decomposed totally when thermal-ly treated at 400 °C (heating rate was 1 °C min−1) for 1 h in anair atmosphere. Secondly, samples were further carbonized asthe temperature increased from 400 to 900 °C with a heatingrate of 5 °C min−1 and kept heated for 1.5 h in argon. Finally,the as-prepared carbon/SiO2 composite nanofibers were im-mersed in 30 wt.% HF acid for 24 h. Porous density of carbonnanofibers increased as the SiO2 reacted with HF acid. Toremove the residual HF acid, the obtained porous carbonmaterials were repeatedly washed with distilled water untilthe pH value of the water–carbon mixture was about 7. Theseresultant samples were identified as HPCF0, HPCF5, HPCF10,HPCF15, HPCF20, and HPCF25, indicating blend concentra-tions of 0/0, 5/5, 10/10, 15/15, 20/20, and 25/25 wt.%PMMA/TEOS relative to PAN, respectively.

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Characterization

The nanofiber surface morphologies and diameters were de-termined with a scanning electron microscope (SEM, JEOL,JSM-6360) and a field emission scanning electron microscope(FESEM, Nova NanoSEM 230). The porous structures ofcarbon nanofibers were characterized with a transmissionelectron microscope (TEM, JEOL, JEM-2010 F). Nitrogen(N2) adsorption/desorption isotherms of porous materialswere conducted on a Nova-2100e gas sorption analyzer(Quantachrome Corp.) at 77 K. All samples were preheatedto 200 °C to eliminate gas. The specific surface areas of allsamples were deduced by applying the Brunauer-Emmett-Teller (BET) equation. The pore size distributions were eval-uated from desorption branches of isotherms by using theBarrett-Joyner-Halenda (BJH) model. The micropore vol-umes and micropore surface areas were calculated from thet-plot model. Proximate elemental analysis (C, H, S, N) of allcarbon nanofibers were carried out on an elemental analyzerCHSN Vario EL III (Elementar Analysensysteme, GmbH,Germany). Moreover, Fourier transform infrared (FTIR) spec-trum of carbon nanofibers prepared from pure PAN andPAN/PMMA/TEOS composite (PMMA/TEOS%=15/15 %)were collected by using a PerkinElmer spectrum one spec-trometer at ambient temperature.

Electrochemical performance measurements

In the experiment, all electrochemical properties of carbonnanofibers were performed in 6 M KOH using a symmetricalsandwich-type two-electrode capacitor cell at room tempera-ture. The slurry was prepared by mixing the carbon nanofiberwith polyvinylidenefluoride (PVDF) and acetylene black inN-methyl-2-pyrrolidone (NMP) at a ratio of 80:12:8 and stir-ring the mixture sufficiently at ambient temperature. Afterthat, the homogenous slurry was pressed onto a steel meshwith a size of 1 cm2 (the steel mesh functions as currentcollector), yielding working electrodes with a total active massof 4–7 mg. Before testing, the electrodes were dried at 60 °Cfor 12 h in vacuum and weighed. A kind of sandwich-typesupercapacitor, consisting of two electrodes with similar massand a piece of separator, was assembled. The capacitive be-haviors were measured by means of cyclic voltammetry andelectrochemical impedance experiments using a ZahnerIm6ex electrochemical station (Germany). Within a potentialwindow of −0.5 to 0.5 V, cyclic voltammograms wererecorded at scanning rates of 1, 10, 20, 50, and 100 mV s−1.The electrochemical impedance spectroscopy (EIS) was real-ized on a two-electrode capacitor. Five millivolts of signalamplitude was used to reach the steady-state cell voltage.These impedance data were collected in the frequency rangeof 0.01 to 100 kHz. The equivalent series resistance wasobtained at 100 kHz [3]. Besides, current densities of 0.1,

0.2, 0.5, 1, 2, 5, and 10 A g−1 were chosen for galvanostaticcharge/dischargemeasurements in the potential range scope of0 to 1.0 V using the same electrochemical station. The cyclicperformance of HPCF15 electrode was evaluated by repeatedgalvanostatic charge/discharge cycling in the potential rangeof 0 to 1.0 V at a current density of 1 A g−1. The multicyclecharge/discharge data (in 8,000 cycles) were tested with aNeware battery test system (BTS-XWJ-6.9.27 s) multichannelgenerator.

Results and discussion

Microstructure characteristics

The electrospun organic nanofibers in the form of a whiteweb were obtained, as SEM characterized at low magnifica-tions in Fig. 1a and b. It appears that two kinds ofnanofibrous samples exhibit cylindrical morphologies(Fig. 1c and d), a smooth outer surface and diameter distri-butions in the range of 200 to 400 nm. Accordingly, it isnoted that the surface morphologies of the electrospunnanofibers containing 0/0 wt.% PMMA/TEOS dual tem-plates are similar to that containing 15/15 wt.%. The imagessuggest that the addition of dual templates has a slightinfluence on the surface morphologies of the electrospunnanofibers. This may relate to high spinability of pure PANand its blended solutions. Moreover, Fig. 1c and d shows theFESEM images of the cross-section of HPCF0 and HPCF15carbon nanofibers. In comparison of the two images, severalhollow cores (macropore and mesopore) in the single carbonfiber are visible obviously for the HPCF15 sample.

Meanwhile, the TEM image (in Fig. 2b) indicates thelegible texture of the hierarchical hollow cores. The poretexture determined from TEM measurements agrees wellwith the FESEM analysis. Electrochemically, thesemacropores can form ion-buffering reservoirs, while thesemesopores can provide a short ion-transport pathway [36].However, both Figs. 1c and 2a unsurprisingly demonstrate anonporous carbon nanofiber derived from pure PAN solu-tion. The development of the hierarchical hollow cores main-ly involves the decomposition (or disappearance) of elongat-ed PMMA phase [33, 37, 38] and the removal of SiO2

trapped in the electrospun PAN nanofiber in sequence [35,39].

Pore characteristics of the as-prepared carbon nanofibers

In order to track the pore parameters of the resultant carbonnanofibers accurately, the pore type and size were studiedthrough N2 adsorption/desorption isotherms at 77 K. Asillustrated in Fig. 3a, the adsorption isotherms of samplesHPCF0 and HPCF10 show type I behavior representing the

J Solid State Electrochem

microporous adsorption. The major uptake occurs at a lowrelative pressure (P/P0<0.1). The rest of the four samplesfollow a shape resembling a combination of types I and IIadsorption isotherm and H4 hysteresis loop. A portion of N2

adsorbed in the low relative pressure scope (P/P0<0.2) islinked with micropores, while another adsorption at a highrelative pressure of P/P0>0.4 originates from themacroporosity and mesoporosity developed in the carbonnanofibers. The pore size distribution determined by theBJH model is displayed in Fig. 3b. The HPCF15 samplefeatures two sharp peaks, which reveals the presence ofmicropores andmesopores. Combinedwith FESEM (Fig. 1d)and TEM (Fig. 2b) images, it is further confirmed thatmacropores also exist in the HPCF15 sample. This hierar-chical porosity furnishes a pathway for selective ion adsorp-tion and a short diffusion route for the diffusion of high-speed ion at high current densities [40, 41]. In Table 1, the

porosity parameters of all carbon nanofibers are summarizedin detail. Especially, the HPCF15 sample covers the largestBET micropore surface area of 490 m2 g−1 and possesses thelargest micropore volume value of 0.196 cm3 g−1. Theseabundant micropores of the HPCF15 sample are expectedto efficiently store small-sized ions for energy density in-crease [42, 43]. Through the combined functions of micro-pore and mesopore textures, the capacitance values, ratecapability, and power density are enhanced. In this table,these changes in values designate that the evolution of thehierarchical porous structures in the nanofibers is certainlycaused by the removal of PMMA and TEOS.

Electrochemical performances

The capacitive behaviors of the symmetric capacitor pre-pared from various carbon nanofibers were investigated by

Fig. 2 TEM images of a HPCF0and b HPCH15 carbonnanofibers

Fig. 1 Low-resolution SEMimages of the electrospunnanofibers from PAN solutionscontaining a 0/0, b 15/15 wt.%PMMA/TEOS. Cross-sectionalFESEM images of c HPCF0 andd HPCF15 samples

J Solid State Electrochem

cyclic voltammetry measurements. The typical cyclicvoltammograms measured at 20 mV s−1 scanning rate areshown in Fig. 4a. It is well known that the capacitor with anideal double-layer behavior is characterized by aquasirectangular CV curve [44]. In this case, the CV curveof the HPCF15 electrode matches well with the rectangular-shaped character at a low scanning rate of 20 mV s−1, andapparently, it shows a much larger quasirectangular than

other samples. In the physical characteristics, the idealdouble-layer behavior of the HPCF15 electrode is attributedto a large surface area with hierarchical porosity [45] andgood electrical conductivity [46]. Naturally, the electrolyteions that can move smoothly and swiftly in porous carbonnanofibers are available. To better distinguish the resistanceand capacitance of the devices, the AC impedance spectrawere tested in the range of 0.01 to 100 kHZ. As shown byFig. 4b, only the HPCF15 electrode features two distincttraits as follows: a semicircle in the high-frequency andlow-frequency steep spikes. In the high-frequency region,the X-intercept of the semicircle mainly corresponds to theinterface resistance between the electrode and the currentcollector, and the intrinsic resistance of the porous electrodeitself. In Table 2, the equivalent series resistance (ESR) valueof the HPCF15 sample determined from impedance at highfrequency of 100 kHz is as low as 0.77Ω. In addition, thesemicircle loop validates the faradic charge-transfer resis-tance that is symbolized by Rf. As far as we know, the smallerthe radius of the semicircle, the lower the impedance on theelectrode/electrolyte. From the inset plot, the Rf value of theHPCF15 is the smallest among these samples, which provesits excellent electrical conductivity and consequent electrodechemical properties again. In the low-frequency region, theHPCF15 electrode shows a more vertical line, implyingelectrode-blocking behavior. The almost vertical spike ofan imaginary part suggests that kinetics of ion diffusion insolution and the adsorption of ions onto the electrode surfaceproceed rapidly owing to its high micropore and mesoporedensities [46]. Consequently, the HPCF15 sample possessesan excellent capacitive behavior with a lower diffusion lim-itation. Hence, it may be appropriate for electrode materialsof high-performance supercapacitor.

Next, the electrochemical properties of HPCF15 wereanalyzed individually through measuring the cyclicvoltammograms at various scanning rates and galvanostaticcharge/discharge at different current densities. Typical CVcurves at 1–100 mV s−1 sweep rates are depicted in Fig. 5a.The CV curves slightly deviate from the quasirectangulareven at high voltage scanning rates, which also demonstrate

Fig. 3 a N2 adsorption/desorption isotherms at 77 K of the as-electrospun carbon nanofibers and b their corresponding pore sizedistributions (inset)

Table 1 Textural parameters ofall hierarchical porous carbonnanofibers

Samples BET surfacearea (m2 g−1)

Total pore volume(cm3 g−1)

Microporesurface area(m2 g−1)

Microporevolume (cm3 g−1)

Average porediameter (nm)

HPCF0 59.40 0.014 14.92 0.008 1.765

HPCF5 294.02 0.256 14.82 0.007 3.846

HPCF10 42. 70 0.022 16.12 0.006 3.827

HPCF15 698.85 0.212 490.40 0.196 3.797

HPCF20 233.90 0.106 125.77 0.054 3.778

HPCF25 152.85 0.064 88.86 0.038 3.832

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an excellent capacitive behavior and a minimum mass trans-fer resistance [47]. Theoretically, the slightly distorted CVcurves are caused by the enhancement of distributed capac-itance effects in porous electrodes as the voltage sweep rateincreases [48]. Besides, no obvious humps originating fromthe redox reaction of surface functional groups are observed

within the potential range of −0.5–0.5 V. Some previousworks indicated that the relatively broad feature in the CVcurves of HPCF15 could be the result of a side reactionclosely relating to the intrinsic nitrogen functionalities thatderive from the parent PAN [46, 49]. In this experiment, inspite of the carbonization at high temperature, there areconsiderable amounts of nitrogen in all PAN-based carbonstructures (Table 2). Furthermore, the formation of C=Nbonds (in the form of pyridine) through a cyclization reaction[50] is observed for all samples, evidenced by the peak at1,586 cm−1 in the FTIR spectrum (seen in Fig. S1). However,the contribution of pseudocapacitance originating from in-trinsic nitrogen functionalities is hard to evaluated because itcorrelates to not only the amount of the heteroatoms but alsotheir distribution of the crystalline lattice and the size of thegrapheme planes [49]. In succession, the rate capability ofthe HPCF15 electrode at different current densities and itscycle life are quantified using galvanostatic charge/dischargemeasurement. The typical galvanostatic charge/dischargecurves displayed in Fig. 5b and c for high and low loadingcurrents are isosceles triangles. These curves corroborate thatthe capacitor based on HPCF15 can be loaded to a highregime while still keeping good double capacitive property.Another visible feature of galvanostatic curves is a verysmall voltage drop, which associates with a low equivalentseries resistance of the capacitor cell. This behavior is well inaccordance with the EIS test result (Fig. 4b). Figure 5dshows the gravimetric discharge capacitance of the HPCF15electrode at various current densities. The specific capaci-tance decreases rapidly at low current densities and tends tostabilize above high current density of 1 A g−1. It is note-worthy that nearly 71 % of initial value is retained at acurrent density of 10 A g−1. Therefore, a noticeable featureof the capacitor is its high rate of handing stability. Such highpower stability of PAN-based hierarchical porous nanofiberscan be explained by electron transfer resistance, ion diffusionresistance, and their structural advantage. In this sense, theHPCF15 electrode with appropriate proportion of microporousand mesoporous networks, as well as high specific surface

Fig. 4 a Cyclic voltagmmograms of all resultant carbon nanofibers at ascanning rate of 20 mV s−1, b Nyquist plot of the symmetric cell in thefrequency range of 0.01 to 100 kHz. The inset shows the magnifiedhigh-frequency region of the plot

Table 2 Elemental analyses and equivalent series resistance values of all carbon materials

Samples Relative atomic concentration ESR (Ω)

Carbon Hydrogen Nitrogen Sulfur

HPCF0 74.62 1.212 9.542 0.148 0.42

HPCF5 74.73 1.514 8.687 0.095 0.96

HPCF10 71.25 1.511 9.272 0.114 0.72

HPCF15 67.55 1.790 10.35 0.168 0.77

HPCF20 72.11 1.448 9.713 0.134 0.76

HPCF25 73.39 1.511 8.129 0.124 1.41

ESR equivalent series resistance

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area, provides an easily available electrode/electrolyte interfacefor charging an electric double layer and facilitates ion trans-port at high discharge current. Moreover, the superior perfor-mance of the HPCF15 sample can also be illustrated in theRagone plot presented in Fig. 5e. For the HPCF15 carbonnanofibers presented here, a high power density can beexported with slight degradation. It exhibits the highest powerdensity of 7.5–14.7 kW kg−1 in the energy density range of7.2–5.0 Wh kg−1 for the HPCF15 electrode.

Electrochemical stability of the supercapacitor equippedwith the HPCF15 carbon nanofibers is also found at a currentdensity of 1 A g−1 after repeating 8,000 cycles. The specificcapacitance versus cycle number and the corresponding cou-lombic efficiency are shown in Fig. 6. The specific capaci-tance fades very tardy and gradually stabilizes at 160 F g−1.Only 5.9 % of the initial capacitance is lost after 8,000 cycles.This result announces satisfactory cycle stability substantially.

Fig. 5 Electrochemicalproperties of HPCF15 sampleusing a two-electrode cell in 6 MKOH: a cyclic voltammogramsat different sweep rates;galvanostatic charge/dischargecurves b at high current loadsand c at low current loads; dgravimetric capacitances atdifferent current loads; e Ragonplot

Fig. 6 Variation of capacitance and coulombic efficiency with cyclenumber for HPCF15 electrode measured at 1 A g−1

J Solid State Electrochem

The hierarchical porous carbon with high micropore density,which provides a certain amount of adsorbent sites for thereversible electroadsorption/desorption of K+ and OH−, mayaccount for the particular stability. The coulombic efficiencyof this electrode is always close to 100 %, suggesting highreversibility and electrochemical stability of the PAN-basedhierarchical porous nanofibers.

Conclusions

The carbon nanofibers with hierarchical pore textures havebeen prepared by electrospinning of dual templates in PANsolutions, which involve a high temperature carbonization andsubsequent removal of hard template. The surface area andhierarchical porous structure of carbon nanofibers can besynergistically tailored by changing the blend ratio ofPMMA/TEOS in the spinning solution. Among various pre-pared electrode materials, the carbon nanofiber prepared fromthe electrospinning of PAN solution containing 15/15 wt.%PMMA/TEOS possesses the largest specific surface area(699 m2 g−1), high-end specific capacitance (170 F g−1), andexcellent rate capability. In particular, the HPCF15 electrodeachieves power densities as high as 14.7 kW kg−1 and lowcapacitance fading after 8,000 cycles at a current density of1 A g−1. The unique textures of hierarchical pores can shortenthe pathway of ion diffusion and improve the electrical con-ductivity so as to obtain high power density and goodcycleability. The hierarchical porous carbon nanofibers pre-pared without any activation treatment present promisingcandidates for supercapacitors in the future.

Acknowledgments The workers gratefully appreciate the financialsupports from the Youth Project of the National Nature Science Foun-dation of China (grant nos. 51103124 and 51203131) and Hunanprovince universities innovation platform of Open Fund Project(11 K067).

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