Materials Letters 76 (2012) 211–214

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Materials Letters

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Phenolic-based carbon nanofiber webs prepared by electrospinningfor supercapacitors

Chang Ma a,b, Yan Song a,⁎, Jingli Shi a,⁎, Dongqing Zhang a, Ming Zhong a,b, Quangui Guo a, Lang Liu a

a Key Laboratory of Carbon Materials, Institution of Coal Chemistry, Chinese Academy of Sciences, Taiyuan 030001, Chinab Graduate University of Chinese Academy of Sciences, Beijing 100049, China

⁎ Corresponding authors. Tel.: +86 351 4250553; fax: +E-mail addresses: yansong1026@126.com (Y. Song), sh

0167-577X/$ – see front matter © 2012 Elsevier B.V. Aldoi:10.1016/j.matlet.2012.02.100

a b s t r a c t

a r t i c l e i n f o

Article history:Received 13 December 2011Accepted 22 February 2012Available online 3 March 2012

Keywords:ElectrospinningPorous materialsPhenolic-basedCarbon materialsSupercapacitors

Phenolic-based carbon nanofiber webs (PCNFWs) were prepared by electrospinning resole-type phenolicresin/PVA blend solution, followed by curing and carbonization. The PCNFWs presented rich microporesand a BET surface area of 416 m2/g without any activation process. The electrochemical properties of thePCNFWs were investigated in two- or three-electrode cell. In spite of moderate surface area, the PCNFWsshowed excellent capacitance performance. The specific capacitance was up to 171 F/g at 5 mV/s andremained 84% at 100 mV/s. The excellent electrochemical performance was further confirmed through com-parison with commercial activated carbon cloth (1201 m2/g). The encouraging results showed the potentialof the PCNFWs as supercapacitor electrode material.

© 2012 Elsevier B.V. All rights reserved.

1. Introduction

Electrospun porous carbon nanofiber webs have attracted consid-erable attention as a promising electrode material for supercapacitorsdue to high electrical conductivity, high specific surface area and free-standing nature [1]. Several research efforts have attempted to pre-pare electrospun porous carbon nanofiber webs with high capaci-tance [2–4]. Most researchers tried to develop porosity throughactivation treatment, including physical activation and chemical acti-vation [5,6]. However, these activation processes are complicated,cost-consuming and likely to impair the flexibility of the webs. There-fore, it is still of great significance to fabricate electrospun porous car-bon nanofiber webs in one-step carbonization.

Many properties of carbon materials, including the pore structure,depend in large part upon precursor. Among various carbon precur-sors, phenolic resin has been widely used to prepare porous carbonmaterials due to certain features, such as high carbon yield, good di-mensional stability and low cost. Additionally, a prominent advantageis that abundant micropores are produced in the carbonization [7].Consequently, electrospun porous carbon fiber webs could beobtained in one-step carbonization using phenolic resin as precursor.However, electrospun porous carbon nanofiber webs from phenolicresin as electrode material of supercapacitors have not been reported.

Generally, the phenolic resin is classified into two types: novolac-type phenolic resin and resole-type phenolic resin (RPR). Consideringthat the former one involves a complex curing process, which is time-

86 351 4083952.ijingli1963@163.com (J. Shi).

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consuming and might cause pollution [7,8], the latter is considered tobe more worthwhile. Thus, in the present work, the RPR was synthe-sized in polyvinyl alcohol (PVA) solution, yielding a RPR/PVA blend asspinning solution. Addition of the PVA led to good spinnability.Phenolic-based carbon nanofiber webs were obtained by electrospin-ning RPR/PVA blend solution followed by a simple heat curing andone-step carbonization. The obtained carbon webs presented excel-lent capacitance performance.

2. Experimental

2.1. Preparation of PCNFWs

All the chemical reagents in this work were of analytical grade andused without further purification. The spinning solution was synthe-sized by formaldehyde (F) and phenol (P) (molar ratio F/P=1.45)with NaOH (molar ratio NaOH/P=0.2) as catalyst, PVA (mass ratioPVA/(F+P)=2/7) as additive. Firstly, 12 wt.% PVA aqueous solutionwas prepared. The mixture of P, PVA, NaOH and H2O was heated to92 °C under stirring, then 80% of the total formaldehyde was added.Reaction was kept for 60 min at 95 °C and the remaining 20% of form-aldehyde was added. Polymerization was ceased by rapid coolingafter 100 min. Subsequently, the as-synthesized solution was dilutedto 18 wt.% in solid content with distilled water for electrospinning.The electrostatic spinning apparatus consisted of a HV power supply(ES50P-20W/DDPM, Gamma High Voltage Research, USA) equippedwith the positively charged capillary from which the spinning solu-tion was extruded by a syringe pump(KDS100, KD Scientific Inc.,USA) and a negatively charged copper foil for collecting the fibers.Electrospinning was carried out with a spinning distance of 18 cm

212 C. Ma et al. / Materials Letters 76 (2012) 211–214

at 26 kV. The as-electrospun webs were cured under 150 °C for 1 hfollowed by carbonation at 800 °C for 180 min in nitrogen. The result-ing carbon nanofiber webs were denoted as PCNFWs. Activated car-bon cloth ACC-507-15(Kynol Company, Japan) labeled as ACC wasused as a reference.

2.2. Characterization

Images of SEM and TEMwere taken by Hitachi S-4800 and FEI Tec-nai G2 T20 microscopes, respectively. Characterization of the poroustexture was conducted by physical adsorption of N2 at 77 K using anautomatic adsorption system (ASAP2020, Micromeritics).

2.3. Electrochemical measurements

For the three-electrode configuration, the PCNFWs were directlyused as work electrode. Hg/HgO and a slice of platinum were usedas reference and counter electrode, respectively. Two-electrode cellwas built by assembling two electrodes separated by a polypropylenepaper. Electrodes were directly obtained by cutting the PCNFWs orACC. Mass of electrodes was kept almost the same for both samples(2.5–3.5 mg). KOH solution (6 M) was employed as electrolyte forboth test systems. All the cyclic voltammetry (CV, scan rate from 5

Fig. 1. a) SEM images of the PCNFWs. The inset picture shows a photograph of the PCNFWssection of the webs at different magnifications. e) TEM image of carbon fiber.

to 100 mV/s), galvanostatic charge/discharge (current density from0.1 to 20 A/g) and electrochemical impedance spectra (EIS, frequencyrange from 0.01 Hz to 100 kHz) were measured using CHI 660 C in-strument (Shanghai Chenhua Apparatus Co. Ltd).

3. Results and discussion

Fig. 1a shows the SEM image of the PCNFWs. Fibers stack random-ly to form a network structure. The morphology of the PCNFWs andas-electrospun webs is shown in the inset picture. The narrow diam-eter distribution of the fibers shows that the fibers have relatively ho-mogeneous diameter (as shown in Fig. 1b). The average diameter ofthe fibers is about 390 nm. The nanometer-scale diameter providesa short ion diffusion distance. Fig. 1c and d displays the cross-section of the webs at different magnifications. The thickness of thewebs is about 160 μm. The surface of fiber is smooth and there areno noticeable pores on the surface or the cross-section. Slight adhe-sion occurs between fibers, which facilitates the electron transport.From the TEM image, numerous interconnected micropores areobserved.

Pore structural properties of the PCNFWs and ACC are summarizedin Table 1. For both samples, most surface area is provided by micro-pores. The result is in good accordance with TEM measurement. The

and as-electrospun webs. b) Diameter distribution of fibers. c, d) SEM images of cross-

Table 1Pore structure properties of samples.

Sample SBET(m2/g)

Smic

(m2/g)Vtot

(cm3/g)Vmicro

(cm3/g)Vmicro/Vtot

(%)

PCNFWs 416 397 0.19 0.18 94.7ACC 1201 1117 0.54 0.51 94.4

SBET is the BET surface area. Smic is the micropore surface area, derived from t-plotmethod. Vtot is the total pore volume, measured at P/P0 =0.995. Vmicro is the microporevolume, obtained by t-plot method.

213C. Ma et al. / Materials Letters 76 (2012) 211–214

PCNFWs present a BET surface area (SBET) of 416 m2/g which isapproaching to that of polyacrylonitrile-based porous carbon nanofi-ber webs activated by ZnCl2 [5].

The electrochemical properties of the PCNFWs were first studiedin three-electrode configuration. The CVs at different scan rates areshown in Fig. 2a. The CVs are slightly-distorted rectangular, showingdeviation from the ideal capacitor behavior. It may be due to pseudo-faradic reactions of functional groups rich on the phenolic-based car-bon [9,10]. According to the CVs, the specific capacitance (SC) of theelectrode is calculated via Cs=(∫ IdV)/(υmΔV), where I is the currentin a given potential (A), V is the potential (V), υ is the potential scanrate (V/s), m is the mass of electrode (g), and ΔV is the potential dif-ference(V). Fig. 2b presents the SCs of the PCNFWs at different scanrates. With the scan rate increasing from 5 to 100 mV/s, the SC de-creases from 171 to 143 F/g, suggesting the good rate performanceof the sample. In spite of the microporous structure of the PCNFWs,

Fig. 2. a) CVs of the PCNFWs. The arrow indicates the increase of the scan rates from 5to 100 mV/s. b) The specific capacitances of the PCNFWs at different scan rates.

the capacitance retention is as high as 84%, even higher than somemesoporous carbon [11].

The excellent capacitance performance was further confirmedthrough comparison with the ACC in two-electrode capacitor. The im-pedance spectrum (Nyquist plot, Fig. 3a) was measured to study im-pedance performance. As seen from the plot, the spike of the PCNFWsis nearly vertical to the real impedance axis, yielding an ideal capaci-tive behavior. The Warburg impedance, representing the diffusion ofions into pores, is unnoticeable for the PCNFWs. It indicates ions

Fig. 3. a) Nyquist plots of the PCNFWs and ACC. b) CVs of the PCNFWs and ACC. Thesolid arrow indicates the scan rates from 100 to 5 mV/s. The pink curve is the CV ofthe ACC at 100 mV/s. c) The specific capacitance of the PCNFWs and ACC at differentcurrent densities.

214 C. Ma et al. / Materials Letters 76 (2012) 211–214

diffuse fast in the pores of the PCNFWs. Considering the same electro-lyte resistance and contact resistance, the semicircular arc at highfrequencies represents the resistance of the electrode (Rf). The Rf

of the PCNFWs is as low as 0.12 Ω, much lower than that of theACC (1.5 Ω).

The low resistance of the PCNFWs was also reflected in CVs(Fig. 3b). Even at high scan rate, the CV presents ideal rectangularshape, indicating low equivalent series resistance. The CV of thePCNFWs at 100 mV/s is in vivid contrast to that of the ACC. The resultsabove demonstrate that the PCNFWs possess high conductivity,which could be explained by high conductivity of phenolic-based car-bon and highly-intermingled nanofiber network.

The galvanostatic charge/discharge measurement was also per-formed, and the SC was obtained by Cg=2it/mΔV, where i is the dis-charge current(A), t is the discharge time(s), m is the mass of singleelectrode(g) and ΔV is the potential difference(V). The SC against cur-rent density is plotted in Fig. 3c. In spite of much lower SBET, thePCNFWs present superior capacitance performance to the ACC. Withthe current density increasing from 0.1 to 10 A/g, the SC of thePCNFWs decreases from 157 to 122 F/g, whereas that of the ACC de-creases from 146 to 56 F/g. The specific surface capacitance (SSA)was calculated by Ca=Cg/SBET. The PCNFWs present a SSA of 0.38 F/m2. This value is higher than that of the ACC (0.12 F/m2) and the con-ventional activated carbon (ca. 0.1–0.3 F/m2) [9]. It is believed thatthe comprehensive effect of interconnected micropores, nanometer-scale ion diffusion distance and surface functional groups causes thehigh surface utilization efficiency.

4. Conclusions

In this work, we reported phenolic-based carbon nanofiber webs(PCNFWs) as electrode material for supercapacitors for the firsttime. The PCNFWs were prepared by electrospinning RPR/PVA blendsolution, followed by curing and one-step carbonization. The fiberwebs were formed by stacking randomly of fibers which were ofnanometer-scale diameter and rich micropores. High specific capaci-tance and rate performance were achieved due to interconnected mi-cropores, high conductivity, short ion diffusion distance and surfacefunctional groups. The results showed the potential of the PCNFWsas supercapacitor electrode material.

References

[1] Dong Z, Kennedy SJ, Wu Y. J Power Sources 2011;196:4886–904.[2] Kim C. J Power Sources 2005;142:382–8.[3] Kim C, Park SH, Lee WJ, Yang KS. Electrochim Acta 2004;50:877–81.[4] Wu Y, Balakrishna R, Reddy MV, Nair AS, Chowdari BVR, Ramakrishna S. J Alloys

Compd 2012;517:69–74.[5] Kim C, Ngoc BTN, Yang KS, Kojima M, Kim YA, Kim YJ, et al. Adv Mater 2007;19:

2341–6.[6] Ra EJ, Raymundo-Piñero E, Lee YH, Béguin F. Carbon 2009;47:2984–92.[7] Liu C-L, Dong W-S, Song J-R, Liu L. Mater Sci Eng. A 2007;459:347–54.[8] Zhang D-Q, Shi J-L, Guo Q-G, Song Y, Liu L, Zhai G-T. J Appl Polym Sci 2007;104:

2108–12.[9] Raymundo-Piñero E, Leroux F, Béguin F. Adv Mater 2006;18:1877–82.[10] Wen ZB, Qu QT, Gao Q, Zheng XW, Hu ZH, Wu YP, et al. Electrochem Commun

2009;11:715–8.[11] Zhao X, Wang A, Yan J, Sun G, Sun L, Zhang T. Chem Mater 2010;22:5463–73.