Electrochimica Acta 50 (2004) 883–887

Supercapacitor performances of activated carbon fiber websprepared by electrospinning of PMDA-ODA

poly(amic acid) solutions

Chan Kima, Yeong-Og Choib, Wan-Jin Leea, Kap-Seung Yanga,∗a Faculty of Applied Chemical Engineering, Chonnam National University, 300 Youngbong-dong, Buk-gu, Gwangju 500-757, Korea

b Technical Textile Research Team, Korea institute of Industrial Technology, ChonAn 330-825, Korea

Received 2 June 2003; received in revised form 10 January 2004; accepted 24 February 2004Available online 7 August 2004

Abstract

The poly(amic acid) (PAA) solution was successfully electrospun at voltages between 13 and 15 kV, forming yellow webs of fibers withd eraturer eb witht l fort re range of6 or thep rent densityo©

K

1

pmiclfitetafm

ndndThe0and

car-rous

ereide-

0d

iameters of 2–3�m. The PAA web was then imidized with yield of about 81%. The imidized webs were carbonized in the tempange from 700 to 1000◦C under a nitrogen atmosphere with yields greater than or equal to 53%. The flexible carbonized fiber whe amorphous structure and the relatively high electrical conductivity of 2.5 S/cm at 1000◦C appears to be a good candidate materiahe electrode of electrical double layer capacitors (EDLCs). The carbonized webs were activated under steam in the temperatu50–850◦C resulting surface specific surface area of 940–2100 m2/g. The activated carbonized webs were tested electrochemically ferformances as electrodes of EDLC in 30 wt.% KOH aqueous solution. The specific capacitance was 175 F/g even at a high curf 1000 mA/g.2004 Elsevier Ltd. All rights reserved.

eywords:Polyimide; Electrospinning; Ultra-fine fiber; Activated carbon fiber; Supercapacitor

. Introduction

The main advantage of electrospinning is the ability toroduce ultra-fine fibers ranging from nanometer to sub-icron in diameter[1,2]. Also, the electrospinning process

s fast, simple, and relatively inexpensive. The processan be used to form fibers from a wide range of polymeriquids (solution or melt). The process can also producebers from very small quantities of polymer. Because ofhe extremely small diameters and excellent uniformity oflectrostatically spun fibers, non-woven fabrics produced by

his technique have high porosities and the large surface-to-rea ratios[3]. These characteristics make the non-woven

abrics attractive for many applications, such as filters andembranes[4,5], wound dressings and vascular grafts[6,7],

∗ Corresponding author. Tel.: +82 62 530 1774; fax: +82 62 530 1779.E-mail address:ksyang@chonnam.ac.kr (K.-S. Yang).

composite reinforcements[8], and nanoelectronics[9]. Inthe previous work, pyromellitic dianhydride (PMDA) a4,4′-oxydianiline (ODA) copolymer was synthesized athe solution was electrospun into submicron fibers.submicron fibers were successfully carbonized at 100◦Cto be carbon fiber webs with a tensile strength 5.0 MPaan electrical conductivity of 2.5 S/cm[10].

In recent work, these submicron webs have also beenbonized or activated. The goal is to create a highly pomaterial for electrical double layer capacitors (EDLCs)[11].

2. Experimental

2.1. Sample preparation

Poly(amic acid) (PAA) precursors for spinning wprepared by copolymerizing of pyromellitic dianhydr(Aldrich) and 4,4′-oxydianiline (Aldrich) in a tetrahydrofu

013-4686/$ – see front matter © 2004 Elsevier Ltd. All rights reserved.oi:10.1016/j.electacta.2004.02.072

884 C. Kim et al. / Electrochimica Acta 50 (2004) 883–887

Fig. 1. The overall procedures of experiment.

rane/methanol (THF/MeOH, 8/2 by weight) mixed solvent.The PAA solution was spun into fiber web using an elec-trostatic spinning apparatus. The apparatus consisted of a15 kV d.c. power supply (HYP-303D, Han Young Co., Korea)equipped with the positively charged capillary from whichthe polymer solution was extruded and a negatively chargeddrum for collecting the fibers.

Solvent removal and imidization from PAA were per-formed concurrently by stepwise heat treatments under airflow at 40◦C for 12 h, 100◦C for 1 h, 250◦C for 2 h, and350◦C for 1 h. The thermally cured polyimide (PI) websamples were carbonized at 1000◦C and subsequently ac-tivated for 30 min in the temperature range of 650–800◦C in40 vol.% steam in the nitrogen (Fig. 1).

The electrical conductivities in the winding direction ofthe webs were measured by the four-point probe method(Model 3387-11, Kotronix, Japan).

Specific surface areas and pore size distributions of the ac-tivated carbon fiber (ACF) webs were evaluated by using theBrunauer–Emmett–Teller (BET) equation, after preheatingthe ACF at 150◦C for 2 h to eliminate water adsorbed.

Two-electrode supercapacitors cells were fabricated withtwo 4 cm2 ACF electrodes, polypropylene separator (Cell-gard 3501, Scimat Co., UK), and Ni foil of 50�m thick asa current collector. Aqueous solution of 30 wt.% KOH wasu

TT

S re volu

PCAAAA

Cyclic voltammetry (CV) of the unit cell was performed inthe potential range of 0–0.9 V at a scan rate of 10 mV/s. Thedischarge capacitance (C) of the electrodes in EDLC werecalculated on the basis of theEq. (1),

C = 4(i × �t)

W × �V(1)

wherei is the current,�t the discharging time from 0.54 to0.45 V (about 60–50% of the initial voltage),�V the voltagevariation in the time range measured, andW is the weight ofthe two electrodes.

The a.c. impedance measurement of the unit cell in thefrequency range from 100 to 10 mHz was performed by usingan electrochemical impedance analyzer (Jahner Electrik IM6,Germany).

3. Results and discussion

The diameters of the PAA fibers decreased slightlythrough imidization and carbonisation. The diameter contin-ued to decrease when heat treatment temperature increasedfrom 700 to 1000◦C reaching to minimum value of 1–2�m,as shown inFig. 2(a–d).

The conductivity of the PI web after being carbonizeda ant t thes edt sed to2

-c reas-i ath ea ior toct cifics de-d teama m-p roughu orem per-a 0 to1

sed as electrolyte.

able 1he BET results of the PI webs activated at various temperatures

ample Burn-off (%) BET SSA (m2/g) Micro po

I – 17 0.0008F 1000 – 622 0.245CF 650 45 941 0.370CF 700 50 1127 0.425CF 750 58 1453 0.563CF 800 67 1411 0.553

me (cc/g) External SSA (m2/g) Average pore diameter (A)

6.27 19.2411.55 16.958.80 16.575.22 15.83

12.91 16.387.95 16.18

t 1000◦C was 2.5 S/cm. This is significantly higher thhe 1.96 S/cm measured for a PAN web prepared aame conditions[12]. The measured conductivity increaso 5.26 S/cm as the heat treatment temperature increa200◦C (Fig. 3).

The BET results were summarized inTable 1. The speific surface area had a tendency to increase with incng activation temperature to 750◦C, on the other hand,ighest activation temperature of 800◦C, the specific surfacrea was decreased. This result is an opposite behavonventional activated carbon fibers with ca. 10�m diame-er. Because the ACF fiber is submicron scale, the speurface area of the fiber itself should be very high anduced to rapid increase in specific surface area by sctivation. The further activation at higher activation teerature would decrease the specific surface area thnifications of the micropores rather than creations of micropores at fiber surface at elevated activation temture. The specific surface area was ranged from 90450 m2/g.

C. Kim et al. / Electrochimica Acta 50 (2004) 883–887 885

Fig. 2. SEM microphotographs of PI webs carbonized at (a) 700, (b) 800, (c) 900, and (d) 1000◦C.

The maximum capacitance was obtained as 175 F/g fromthe electrode with the largest specific surface area (Fig. 4).Higher the specific surface area, higher the specific capaci-tances were resulted. The electrode activated at higher tem-perature would have a higher specific capacitance at highercurrent density, possibly originated both from elevated elec-trical conductivity and enlarged pore size. The high capac-itance at high current density would be a practical impor-tance for capacitor applications. The specific capacitance ofPI-based ACF electrodes activated at both 750 and 800◦C

F ebs.

showed similar performance at the whole discharge currentdensity, the electrode activated at 750◦C exceeded than theone activated at 800◦C. The larger drop in the capacitance atlower activated temperatures would indicate that more inter-nal resistance was occurred. The behaviors can be explainedby the accessibility of the ions on the pore surface. The elec-trodes activated at 750 and 800◦C exhibited specific sur-face area above 1400 m2/g. Consequently, the electrode with

ig. 3. The electrical conductivities of carbonized and graphitized PI w

Fig. 4. Capacitance dependence on the current density.

886 C. Kim et al. / Electrochimica Acta 50 (2004) 883–887

Fig. 5. Cyclic voltammogram of the electrode at 10 mV/s.

larger BET surface area and lower resistivity would intro-duce higher specific capacitance at higher discharge currentdensity.

The electrochemical properties of nanofiber web elec-trodes were studied by cyclic voltammetry in 30 wt.% KOHaqueous solution. The typical cyclic voltammograms of thecapacitor cells at 10 mV/s scan rate are shown inFig. 5.The figures demonstrate that the electrodes are stable in the30 wt.% KOH aqueous solution within the potential rangeemployed, and the peaks from faradic current were not ob-served for the capacitor cells. The voltammograms also ex-hibit that the induced current is an increasing function withthe activation temperature. When the activation temperatureincreased from 650 to 800◦C, the CV curve approachedto a rectangular shape, indicating not only the reductionin equivalent series resistance (ESR) of the electrode butalso the reduction in hindrance of motion of the ions in thepores.

The electrochemical behavior of the electrodes could bemore clearly understood by a.c. impedance measurement. Ingeneral, the resistance in an electrochemical capacitor haselectronic and ionic contributions. The ionic contributionsinclude the separator resistance and that due to ion conduc-tion in electrolyte in the electrode pores[11]. The electroniccontributions include the bulk resistivity of the electrode ma-t t col-l ires.F sam-p Thei ightl se-r g thed hee odec g in

Fig. 6. Nyquist impedance plots of samples in 30 wt.% KOH aqueous elec-trolyte with frequency ranging from 100 kHz to 10 mHz.

activation temperature. With increasing activation tempera-ture, the slope of the impedance, representing mass transferapproached to an ideally straight line (Fig. 6). An increasein activation temperature from 650 to 800◦C reduced theresistance of the sample. The formation of abundant micro-pores and mesopores may also enhance the diffusivity of thehydrated ions in the pores, and consequently reduces the re-sistance of the PI-based ACF electrode. The enhanced con-ductivity and enlarged pore openings would contribute to re-duction of the cell resistance representing the sum of the bulkand electrode–solution interfacial resistances from separator,electrode itself, the contact among the component fibers, andion transfer in the pores. Lowering the resistance would in-crease current density on the surface of electrode leadingto enhancement of the diffusing rate of the ions toward theelectrode. This, in turn, results in high specific capacitanceparticularly at high current density.

4. Conclusions

Ultra-fine carbon fiber webs were prepared through elec-trospinning of PAA solutions. The ACFs through stem acti-vation resulted high specific surface of 1450 m2/g and spe-cific capacitance of 175 F/g. The specific capacitance wasr rodesa i-n sede ngtho s. Itw onglyd a thatw rede

erial such as particle–particle contacts, particle–currenector contacts, the bulk current collector and external wig. 6shows the complex impedance spectra for variousles. The frequency is swept from 10 mHz to 100 kHz.

deally polarizable capacitance will give rise to a straine along the imaginary axis. In a real capacitor with aies resistance, this line has a finite slope, representiniffusive resistivity of the electrolyte within the pore of tlectrode. The diffusive line of the nanofiber web electromes closer to an ideally straight line with an increasin

etained even at elevated current density for the electctivated 750 and 800◦C. The behavior would be origated from the reduced resistivity not only the increalectrical conductivity but also due to the short path lef the ions from enlarged pore of the PI-based ACFas proposed that the capacitor performances were strepended on the charge resistance and surface areould be a parameter for delivering the majority of stonergy.

C. Kim et al. / Electrochimica Acta 50 (2004) 883–887 887

Acknowledgements

Support for this work was provided by KOSEF underGrant number R01-2003-000-10100-0. One of us (Kim) ac-knowledges the support by Korea Research Foundation Grant(KRF-2003-037-D00008).

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