C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3

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Highly porous electrospun polyvinylidene fluoride (PVDF)-based carbon fiber

Ying Yang 1, Andrea Centrone 2, Liang Chen, Fritz Simeon, T. Alan Hatton,Gregory C. Rutledge *

Department of Chemical Engineering, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA

A R T I C L E I N F O

Article history:

Received 13 February 2011

Accepted 7 April 2011

Available online 19 April 2011

0008-6223/$ - see front matter � 2011 Elsevidoi:10.1016/j.carbon.2011.04.015

* Corresponding author.E-mail address: rutledge@mit.edu (G.C. R

1 Current address: Department of Electrica2 Current address: Center for Nanoscale Sc

20899-6203, USA.

A B S T R A C T

Porous poly(vinylidene fluoride) fibers were prepared by electrospinning from solutions in

dimethylformamide, poly(ethylene oxide) (PEO) and water. The PVDF fiber mats were then

converted into electrospun carbon fiber paper using a low temperature chemical stabiliza-

tion treatment (‘‘dehydrofluorination’’) followed by carbonization at 1000 �C. The resulting

self-supporting carbon fiber paper exhibits unusually high surface area, in excess of

380 m2/g as measured by the nitrogen adsorption method, and a hierarchical pore struc-

ture. The largest pores are formed by the interstices between fibers; intermediate-sized

pores arise from liquid–liquid phase separation during electrospinning to form polymer-

rich and solvent-rich domains within the fibers; the smallest pores form upon decomposi-

tion of the PEO during carbonization. The electrospun carbon paper performs well as an

electrode for driving the redox chemistry of ferrocene/ferrocenium. This is attributed to

the high surface area of the electrode and the ease of diffusion of the redox-active species

within the porous structure. The ratio of the dehydrofluorination agent (1,8-diazabicy-

clo[5.4.0]undec-7-ene) to vinylidene fluoride during dehydrofluorination was found to be

the key to retaining the as-spun pore morphology during carbonization. The structure

and morphology were further characterized by Scanning Electron Microscopy, Energy Dis-

persive X-ray Spectroscopy, X-ray diffraction, and Raman spectroscopy.

� 2011 Elsevier Ltd. All rights reserved.

1. Introduction

Carbon fiber materials are attracting scientific and technolog-

ical interest for their existing and potential commercial appli-

cations as catalyst supports [1], electrochemical probes [2],

energy storage electrodes in supercapacitors [3] and in redox

flow batteries [4]. In principle, any fibrous material with a car-

bon backbone can potentially be used as a precursor for such

carbon fiber materials. The mechanical properties, as well as

the thermal and electrical conductivities of the resulting

er Ltd. All rights reserved

utledge).

l Engineering, Tsinghua U

ience and Technology, 9 N

carbon fibers depend on the choice of precursor. Rayon and

polyacrylonitrile (PAN) are used as precursors for most of

the commercial carbon fibers [5], although other precursors

such as pitch [6], phenolic resins [7], and poly(vinylidene fluo-

ride) (PVDF) [8] have also been reported.

One of the challenges in using carbon fiber materials as

electrodes for driving electrochemical reactions, as in the

redox flow battery [9], is to increase the number of active sites

on the electrode, since these heterogeneous electron transfer

processes occur only at the interface between the solid

.

niversity, Beijing 100084, China.

ational Institute of Standards and Technology, Gaithersburg, MD

3396 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3

electrode and the redox chemistry-containing phases. For a

given electrode, the density of the active sites is a character-

istic of the material. Increasing the surface area of the elec-

trode with a suitable hierarchical pore structure is one

strategy to improve the electrode performance. Electrospin-

ning is a general method to form continuous polymer fibers

with diameters in the submicron to nanometer range in a

nonwoven mat with substantial surface area [10]. The nonwo-

ven mat can subsequently be converted into electrospun car-

bon fiber paper (ECP). Carbon nanofibers with a solid interior

and smooth surface are usually obtained, with surface areas

generally around 1–50 m2/g [5].

In order to attain electrospun carbon fibers with larger sur-

face areas, several research groups have attempted to pro-

duce porous carbon fibers from different bicomponent blend

electrospun fibers, followed by selective extraction of one of

the blend components in a second process step. For example,

45/55 w/w PAN/poly(vinyl pyrrolidone) (PVP) fibers [11], 95/5

w/w PAN/polystyrene (PS) fibers [12] and 85/15 w/w PAN/

cellulose acetate(CA) fibers [12] have been used to prepare

porous PAN nanofibers upon removal of PVP, PS or CA

domains, respectively, by solvent extraction. The fibers

retained a high density of pores after thermal treatment.

Porous carbon fibers have also been derived by electrospinning

emulsions of poly(methyl methacrylate) (PMMA) in a continu-

ous phase of PAN and DMF, followed by oxidation and carbon-

ization processes to pyrolyze the PMMA component [13].

Rather than using a selective solvent to extract one com-

ponent from a bicomponent electrospun polymer fiber after

the fibers are formed, porous polymer fibers can be produced

from polymer blends or solutions directly during the electros-

pinning process by means of a liquid–liquid phase separation

that occurs when the solution shifts from a stable single

phase to a two-phase region of the phase diagram. Humidity

has been used as a non-solvent to induce phase separation

during electrospinning, with consequent effects on the fiber

surface morphology [14]. Such behavior is typical of polycar-

bonate (PC) [15,16], poly-L-lactic acid (PLLA) [15], PS [17], and

PMMA [18] electrospun from low boiling point solvents. Qi

et al. reported that porous PLLA fibers can be produced from

a mixed solvent of low boiling point dichloromethane (sol-

vent) and high boiling point butanol (nonsolvent) combined

in a 60/40 ratio [19]. Surface pores on electrospun PS, PVC

and PMMA fibers were produced from high boiling point sol-

vent DMF in a high humidity environment [20].

In this work, we demonstrate a general strategy to make

porous fiber electrode materials by electrospinning directly

from a solution composition of Polymer A/Polymer B/Sol-

vent/Non-solvent. Furthermore, if two polymers with differ-

ent thermal stability are chosen, a carbon paper with a

hierarchical pore structure can be created. The largest pores

are formed by the interstices between fibers, while the inter-

mediate sized pores are formed within the fibers themselves

by the phase separation into polymer-rich and solvent-rich

domains that occurs during the electrospinning process. Re-

dox-active species can diffuse easily into the inner pores of

such electrodes. The smallest pores are formed by the ther-

mal decomposition of one of the two polymer components

in the polymer-rich domains during chemical treatment or

carbonization. This paper reports our findings on testing this

concept by electrospinning poly(vinylidene fluoride) (PVDF),

followed by stabilization and carbonization. The morpholo-

gies of the fibers thus obtained are characterized, and the

electrochemical behavior of the ferrocene redox couple is

studied to evaluate the performance of the carbon fiber paper

as an electrode.

2. Experimental

2.1. Preparation of porous PVDF fiber

Solef� 1015 poly(vinylidene fluoride) (PVDF) was kindly pro-

vided by Solvay Solexis Inc. Poly(ethylene oxide) (PEO)

(Mv = 2000 kg/mol) and dimethylformamide (DMF) (ACS

reagent, >99.8%) were purchased from Sigma Aldrich, Inc. De-

ionized water was used for solution preparation in all experi-

ments. All materials were used without further purification.

One gram of PVDF and 0.06 g of PEO were dissolved in 9 g

of a mixture of DMF and water (mass ratio 50:3) under gentle

stirring for at least 8 h at 90 �C; the solutions were cooled

down to room temperature before electrospinning. The solu-

tion was unstable and formed a gel within 48 h, but could be

electrospun into fibers prior to gelation. The parallel-plate

electrospinning setup described by Shin et al. was used in

the experiments [21]. The flow rate, plate-to-plate distance,

and voltage were 0.05 mL/min, 40 cm, and 22.5 kV, respec-

tively. In each case, randomly oriented nonwoven meshes

were collected on a grounded aluminum foil in a controlled

environment with a temperature of 24 ± 1 �C and a relative

humidity of 56 ± 1%. The porous morphology of the as-spun

fibers can be varied by changing the molecular weight and

concentration of PEO relative to PVDF, the amount of water

in the solution and the relative humidity at which the fibers

are formed; details are provided in the Supporting

Information.

2.2. Dehydrofluorination and carbonization

Pretreatment of the PVDF precursor fiber was required to en-

hance thermal stability before the high temperature carbon-

ization. For many carbon fiber precursors such as PAN,

thermal stabilization is carried out at a low temperature in

air. However, thermal stabilization of PVDF occurs mainly at

temperatures above 400 �C, which is much higher than its

melting point of around 190 �C [22]. For this reason, chemical

dehydrofluorination at lower temperature was used as the

stabilization step, which introduces numerous C–C bonds to

the polymer matrix, as described previously [23]. The electro-

spun PVDF fiber mats were soaked in a mixture of DMF and

methanol with the volume ratio 9:1 at 50 �C for several hours,

with different amounts of 1,8-diazabicyclo[5.4.0]undec-7-ene

(DBU) (Sigma Aldrich, Inc.) added per vinylidene fluoride unit

(DBU/F). After dehydrofluorination, the mats were washed in

methanol for 1 h and dried at room temperature overnight.

The dehydrofluorinated fibers were then heated at a rate of

3 �C/min up to 1000 �C in nitrogen atmosphere and held for

1 h to complete the carbonization process. During the heat-

treatment, two 5 mm thick molybdenum (Mo) plates were

used to make a Mo-fiber mat-Mo sandwich fixed by Mo wires

C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3 3397

in order to keep the mat flat. No additional pressure was ap-

plied to the sample during the heat treatment.

2.3. Characterization

Scanning Electron Microscopy (SEM) (JEOL-6700SEM, JEOL Ltd.,

Japan) was usedfor morphologicalcharacterization of the fibers.

The IMAGEJ image analysis software (National Institutes of

Health) was used to determine the fiber diameter, pore size

and pore number density. Fiber samples were sputter-coated

with a 10 nm layer of gold/palladium using a Desk II cold sput-

ter/etch unit (Denton Vacuum LLC). SEM was used to observe

the surface structure of the fibers at 10 kV acceleration voltage

and 8 mm working distance. The fiber cross sections were pre-

pared by cutting the nonwoven meshes on aluminum foil im-

mersed in a bath of liquid nitrogen. The approximate number

density (i.e. number of pores per unit of surface area) and aver-

age pore diameteron the fiber surfacewere estimated by inspec-

tion of SEMs of roughly 100 fibers in each sample. Given the

magnification required to image multiple fibers using the

JEOL-6700SEM, the detection limit for pore sizewas about 20 nm.

Energy Dispersive X-ray Spectroscopy (EDS) was used for

semi-quantitative analysis of the elemental composition of

each mat (typical penetration depth 1–2 lm).

The surface area of the porous fiber mat was measured by

means of the Brunauer�Emmett�Teller (BET) method

(ASAP2020, Micromeritics) with nitrogen as the adsorbing gas.

XRD diffraction patterns were recorded using a PANalytical

X’Pert Pro Multipurpose Diffractometer with Cu Ka radiation.

The XRD profiles were recorded at a scanning speed of 3�/min

between 5� and 70� in 2h.

A Horiba Jobin Yvon Labram HR800 spectrometer was used

for recording the Raman spectra using different excitation

sources (514.532 nm, 633.0 nm, 784.8 nm) at 3 mW of laser

power. For all samples, three spectra were acquired and aver-

aged, using a 100· objective and a grating with 600 grooves/

mm. The spectral resolution was approximately 4 cm�1,

3 cm�1 and 2 cm�1 using 514.532 nm (Argon-ion laser, Coher-

ent-Innova 90C model), 633.0 nm (He–Ne laser, Melles Griot-

LXH2) and 784.8 nm (diode laser, Sacher-TEC 510) excitation

sources, respectively. The acquisition time for each spectrum

was 120 s, 200 s and 300 s when using the 514.534 nm,

633.0 nm and 784.8 nm excitation lines, respectively. Silicon

was used as a calibration standard.

The XPS measurements were recorded with a Kratos Axis

Ultra instrument equipped with a monochromatic Al Ka

source operated at 150 W. The pressure in the analyzer cham-

ber during acquisition of spectra was 7.0 · 10�9 torr. The ana-

lyzer angle was 90� with respect to the specimen surface.

Wide-scan spectra were recorded over a binding energy range

of 0–1100 eV and pass energy of 160 eV.

The electrical conductivity of the ECP was measured by the

impedance method (Solartron 1260 impedance analyzer) with

a 40 mV amplitude signal over the frequency range from

0.1 Hz to 10 MHz at 25 �C and 25% RH. The cross-sectional

area, A, of the ECP was calculated by multiplying the mea-

sured width by the measured thickness of the sample. The

electrical conductivity r was calculated using r = l/AR, where

R is the measured electrical resistance and l is the distance

between the electrodes.

2.4. Electrochemical measurements

Cyclic voltammetry measurements were carried out with a

standard three-electrode cell using a VersaSTAT-3 potentio-

stat with V3Studio software (Princeton Applied Research,

Ametek, Inc., Tennessee, USA). Electrochemical measure-

ments were conducted in a glass cell, thermostated at

25 ± 1 �C. The working electrode was the PVDF-based ECP.

Copper tape was used to make the electrical contacts between

the ECP and the copper wires. The counter and reference elec-

trodes were platinum and silver wires, respectively. All poten-

tials are referred to the silver wire. The electrodes were rinsed

with acetone before each experiment. DMF containing 0.1 M

tetra-n-butylammonium hexafluorophosphate (TBAPF6) was

used as an electrolyte. 3.8 ml of DMF/TBAPF6 was carefully

deoxygenated with nitrogen with gentle stirring for 30 min.

A nitrogen atmosphere was maintained over the solutions

during the experiments. The behavior of the electrode was

first studied in a blank solution over a selected potential range

and for a scan rate of 0.1 V/s. After that, 100 lL of 250 mmol

ferrocene in DMF was added to the blanketed solution for fur-

ther testing in the selected potential range and for scan rates

ranging from 0.005 to 0.1 V/s. In each case, the last cycle was

recorded, although reproducible signals were obtained after

the first cycle. After each experiment, the reference electrode

was re-calibrated against a ferrocene solution.

3. Results and discussion

3.1. As-spun and dehydrofluorinated PVDF fibers

SEM micrographs showing the typical size and morphology of

the as-spun PVDF fibers are presented in Fig. 1. In combina-

tion with the PEO in solution, a high relative humidity pro-

motes the formation of pores on the PVDF fiber surface,

while water in the solution itself promotes the formation of

connected pores throughout the interior of the fiber; this

behavior is described in more detail in the Supporting Infor-

mation. A similar effect for PEO and non-solvent concentra-

tion on porosity and pore size in thin films has been

reported by Xi et al. [24]. The average diameter of the fibers

was 2.5 ± 1.8 lm, the average pore diameter on the surface

was 350 ± 200 nm and the average number density of pores

on the fiber surface was 9 ± 3 pores/lm2. The BET surface area

for the as-spun PVDF fibers, which includes both superficial

and internal pore surfaces, was 20.6 ± 0.3 m2/g; this is about

15 times the nominal specific surface area that would be esti-

mated from the diameters of the fibers alone.

Upon dehydrofluorination using DBU/F ratios from 0.1 to

10, for periods of either 5 h or 10 h, the color of the PVDF mats

turned progressively from white to dark brown. The mass loss

relative to the mass of the starting PVDF mat (DMchem./MPVDF)

and the elemental composition obtained from EDS were used

to monitor the degree of dehydrofluorination. The results

showed that under mild dehydrofluorination (DBU/F = 0.1

for 5 h) the mass-loss was 1.5% and the C/F mole ratio was

1.1; under dehydrofluorination at DBU/F = 1 for 5 h, the mass

lost was 4.6% and the C/F mole ratio was 1.27; under dehydro-

fluorination at DBU/F = 10 for 5 h, the mass lost was 15% and

Fig. 1 – SEM images of as-spun porous PVDF fibers (a) at low

magnification; (b) fiber surface at high magnification; (c)

cross-section of one fiber.

Fig. 2 – C/F mole ratios and mass loss (DMchem./MPVDF) from

EDS as functions of DBU/F (all data at 5 h, with exception of

highest datum at DBU/F = 10, at 10 h).

3398 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3

the C/F mole ratio was 1.35; under dehydrofluorination at

DBU/F = 10 for 10 h, the mass lost was 20% and the C/F mole

ratio was 1.61. There is a good correlation between DMchem./

MPVDF and the C/F mole ratios (Fig. 2) as the dehydrofluorina-

tion treatment progressed. All the fibers retained their porous

morphology after the dehydrofluorination treatment, as

shown by Fig. 3(a.1), (a.2), (b.1) and (b.2). The average fiber

diameter was substantially unchanged after the dehydrofluo-

rination process (2.3 ± 2 lm).

3.2. PVDF-based carbon fibers

Different fiber morphologies were obtained upon carboniza-

tion, depending on the dehydrofluorination pretreatment, as

illustrated in Fig. 3. PVDF fibers with no pretreatment melted

upon heating to elevated temperature, and only carbon char

was obtained. Under mild dehydrofluorination at DBU/

F = 0.1 for 5 h, the fiber morphology was partially retained.

Nevertheless, some of the fibers melted during the process,

as is evident in Fig. 3(a.3) and (a.4), and the microporous

structure of the fiber was totally lost. Under dehydrofluorina-

tion at DBU/F = 1 for 5 h, however, the fiber morphology, as re-

flected in the local curvature and roughness of the carbon

fiber surface, was retained, as shown in Fig. 3(b.3). However,

the cross-sectional SEM in Fig. 3(b.4) shows that the interior

porosity of the fibers was lost under these conditions. Under

dehydrofluorination at DBU/F = 10 for 5 h, the porous fiber

morphology was again partially retained, as shown in

Fig. 3(c.3) and (c.4). The pore diameter and the pore density

on the surface of the fibers were 100 ± 30 nm and

4.3 ± 3 pores/lm2, respectively. These pores were much smal-

ler than the pores on the as-spun PVDF fiber, and the pore

number density was low compared to the number density

in the as-spun fibers. Part of the porous surface melted and

finally formed highly rough fibers. Finally, under dehydrofluo-

rination at DBU/F = 10 for 10 h, the porous fiber morphology

was almost completely retained, as shown in Fig. 3(d.3) and

(d.4). The pore diameter and the number density of pores on

the surface of the fiber were 200 ± 60 nm and 8 ± 5 pores/

lm2, respectively.

Elemental composition (by mass) of the electrospun

PVDF-based carbon fibers was obtained by EDS to analyze

the elements present on the fiber surface. The EDS results

show distinct carbon and oxygen peaks, representing the

major constituents of the carbon fibers investigated. The per-

centage of carbon increased with increasing DBU/F, as shown

in Table 1. The EDS results also show that the fluorine content

was substantially eliminated during carbonization.

The crystalline morphologies of the PVDF-based carbon fi-

bers obtained after different dehydrofluorination treatments

were investigated by XRD and compared with those of a com-

mercial carbon paper (Toray, EC TP1 060) (CCP), as shown in

Fig. 4(a). Two broad diffraction peaks are observed, in the

vicinity of 2h = 22–24� and 43�. The first peak is assigned to

the (0 0 2) reflection of graphitic carbon [25,26], and is notice-

ably shifted towards lower 2h in the ECPs compared to the

Fig. 3 – SEM images of porous fibers after various dehydrofluorination treatments and carbonization. (a) DBU/F = 0.1 for 5 h; (b)

DBU/F = 1 for 5 h; (c) DBU/F = 10 for 5 h; (d) DBU/F = 10 for 10 h. (x.1) PVDF fibers after the dehydrofluorination treatment; (x.2)

cross-sectional view of PVDF fibers after the dehydrofluorination treatment; (x.3) PVDF-based carbon fibers; (x.4) cross-

sectional view of PVDF-based carbon fibers.

Table 1 – Elemental mass composition of the carbon paperby EDS.

DBU/F C (%) O (%) F (%)

0.1 (5 h) 93.24 6.76 0.001 (5 h) 95.61 4.39 0.0010 (5 h) 96.32 3.68 0.0010 (10 h) 96.37 3.63 0.00

C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3 3399

CCP, indicative of larger inter-layer spacing. The second peak

is significantly asymmetric, typical of the (1 0) reflection in

Fig. 4 – XRD patterns of the electrospun fibers. (a) (i) Commercial

PVDF-based ECP (DBU/F = 1, 5 h); (iv) PVDF-based ECP (DBU/F = 1

before and after dehydrofluorination. (i) As-spun PVDF; (ii) dehy

dehydrofluorinated PVDF (DBU/F = 1, 5 h); (iv) dehydrofluorinate

the 2-dimensional lattice of turbostratic carbon; this peak is

absent in the CCP. Both peaks are quite broad in the ECPs,

but apparently shift towards larger 2h with more intensive

dehydrofluorination treatment. The full width at half maxi-

mum also decreases with increasing dehydrofluorination for

both peaks. These observations indicate that the ECPs are

far more turbostratic in content than the CCP, but that the

structure becomes more compact and regular with more in-

tense dehydrofluorination pretreatment.

By comparison, the XRD data for the PVDF fibers before

and after dehydrofluorination (but before carbonization) are

carbon paper (CCP); (ii) PVDF-based ECP (DBU/F = 0.1, 5 h); (iii)

0, 5 h); (v) PVDF-based ECP (DBU/F = 10, 10 h). (b) PVDF fibers

drofluorinated PVDF (DBU/F = 0.1, 5 h); (iii)

d PVDF (DBU/F = 10, 10 h).

3400 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3

shown in Fig. 4(b). The peak observed around 2h = 21� in all of

these samples is typical of the (1 1 0) and (2 0 0) reflections of

the b crystal phase of PVDF. This is as expected, since it is well

known that crystallization from solution as well as the appli-

cation of extensional force such as that present during elec-

trospinning favor the formation of the b-phase [27]. Smaller

peaks at 28� and 40� indicate only a small amount of the a

crystal modification of PVDF. There is no evidence of the crys-

tallographic peaks of PVDF after carbonization (Fig 4(a)).

Raman spectroscopy is widely used as a structural probe of

carbonaceous materials [28–31]. The G band around 1580 cm�1

is characteristic of highly ordered graphite, while the D band

around 1350 cm�1 is indicative of defects and edges in the gra-

phitic domains [32]. In the Raman spectra of carbonaceous

materials, the frequency of the D peak [33] and the ID/IG area

ratio [32] depend strongly on the laser excitation energy. Given

a collection of graphitic domains of various sizes (i.e. with dif-

ferent band gaps), the Raman spectrum is dominated by the

domains whose band gap matches the excitation energy.

Thus, Raman spectroscopy with lower excitation energy is

more sensitive to larger domains, whose band gap is relatively

small, whereas higher excitation energy is more sensitive to

smaller domains. Thus, multiple spectra using different exci-

tation energies can reveal important information about the

population of domain sizes in the mixture [29,30,33].

The results of the Raman analysis are presented in Table 2;

the original Raman spectra can be found in the Supporting

Information. Because the D band shifts as a function of the

excitation line for all the samples, we can conclude that these

materials comprise a collection of graphitic domains of vari-

ous sizes. The increase in the D band frequency for spectra

acquired with 514 nm and 633 nm excitation lines indicates

that, qualitatively, the average dimensions of small and med-

ium size graphitic domains increase as a function of DBU/F.

The average dimension of the large size graphitic domains

appears to be relatively insensitive to, or to slightly decrease

with, DBU/F (i.e., the D band frequency decreases with DBU/

F for spectra acquired with a 785 nm excitation line). For all

excitation lines, the D band full width at half maximum

(FWHM) decreases as a function of DBU/F, indicating that

the domains become more nearly monodisperse as the treat-

ment progresses. Qualitatively, the relative number of defects

in the graphitic domains can be estimated by the ID/IG ratio.

The number of defects in the smaller and larger graphitic do-

mains is reduced with the stronger dehydrofluorination treat-

ment (ID/IG decreases with increasing DBU/F), whereas the

medium domains are relatively unchanged [32].

Table 2 – D band frequency (mD), D band full width at half maximtreatment and exciting lines.

514 nm 2.41 eV (small domains) 633 nm 1.96 eV

DBU/F mD (cm�1) DFWHM (cm�1) ID/IG mD (cm�1) D

0.1 (5 h) 1349.0 214.3 4.69 1322.5 271 (5 h) 1353.4 188.2 4.53 1327.6 2210 (5 h) 1353.7 175.7 4.51 1328.1 2010(10 h) 1357.0 158.9 3.48 1330.0 20

3.3. Electrochemical electrode performance

The PVDF-based ECP produced under the 10 h dehydrofluori-

nation treatment with DBU/F = 10 was characterized electro-

chemically by cyclic voltammetry to test its performance as

an electrode in driving a redox reaction. The surface area of

this material was 382.3 ± 7.7 m2/g based on the BET result,

and the carbonized fiber diameter was 1.6 ± 1.2 lm. If no

new pores were formed and the pore shrinkage in the carbon-

ization process was commensurate with the reduction in fiber

diameter, the calculated surface area of the electrospun car-

bon paper should be less than 100 m2/g, as the BET surface

area for as-spun PVDF fibers was 20.6 ± 0.3 m2/g with an aver-

age fiber diameter of 2.5 ± 1.8 lm. We attribute the unexpect-

edly large BET surface area measured for these carbon fibers

to either, or both, the decomposition of PEO in the fibers dur-

ing the carbonization, which may generate additional fine

pores, or the dehydrofluorination treatment. It has been re-

ported that 1–2 nm diameter nanopores are produced in PVDF

fibers when the weight loss by dehydrofluorination is about

18% under appropriate treatment conditions [22]. In our case,

the weight loss was about 18–20%, and thus is consistent with

these observations. The electrical conductivity of the PVDF-

based ECP was 11.4 ± 1.5 S/cm, while the electrical conductiv-

ity of the Toray CCP was 80 ± 0.3 S/cm. The wettability of the

electrode in DMF was assessed by placing drops of DMF on

the surfaces of both ECP and CCP. The DMF with TBAPF6 elec-

trolyte spread very quickly, resulting in contact angles of 0�for both cases, indicating very good wettability of the ECP

and CCP in DMF/TBAPF6.

Cyclic voltammetry (CV) is the experimental technique

most commonly used in the electrochemical characterization

of redox-active compounds and electrodes. While most redox

reactions are very sensitive to the surface chemistry, this is

not the case for the relatively insensitive ferrocene/ferroce-

nium redox reaction used here. Nevertheless, the surface

chemistry for this electrode is reported in the Supporting

Information for future reference with other redox reactions.

Fig. 5(a) shows the potential windows for the CCP and ECP

electrodes tested in DMF/TBAPF6 under a typical scan rate

of 0.1 V/s. In this study, the working potential window is de-

fined as the difference between the potentials at which the

anodic and cathodic current densities reach 1 mA/g. The

ECP electrode has a larger background current, around 0.4 A/

g, which is attributed to the double layer capacitance associ-

ated with the larger effective surface area, and a wide poten-

tial window ranging from �2.7 V to 1.9 V. The CCP electrode

um (DFWHM) and ID/IG ratio as a function dehydrofluorination

(medium domains) 785 nm 1.58 eV (large domains)FWHM (cm�1) ID/IG mD (cm�1) DFWHM (cm�1) ID/IG

1.0 9.48 1310.4 225.10 19.205.5 8.29 1309.8 231.47 17.127.0 10.25 1307.8 208.83 14.065.2 9.34 1307.7 193.50 13.00

Fig. 5 – (a) Cyclic voltamograms (CV) obtained for CCP and ECP electrodes in DMF (scan rate: 0.1 V/ s); (b) CVs of CCP and ECP

electrodes in 100 lL of 250 mmol ferrocene in DMF containing 0.1 M TBAPF6 3.8 ml at a scan rate of 0.1 V/s; (c) CVs of ECP

electrode at different scan rates in 100 lL of 250 mmol ferrocene in DMF containing 0.1 M TBAPF6 3.8 ml (scan rates increase

from innermost curve to outermost curve) at a scan rate of 0.1 V/s; (d) linear relationship of the peak potential separation to

the square root of scan rate; (e) working curve showing variation of peak potential separation with w [31]; (f) linear

relationship of peak current to the square root of scan rate.

C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3 3401

has a much smaller background current, less than 0.01 A/g,

and a narrow potential window ranging from �1.7 V to 1.5 V.

The ECP electrode thus has the capability to serve as an elec-

trode for redox reactions in the range of �1.7 V to �2.7 V that

cannot be accommodated by the CCP.

To further understand the performances of the electrodes,

cyclic voltamograms obtained with ECP and CCP at a series of

scan rates ranging from 0.005 to 0.1 V/s were measured, as

shown in Fig. 5(b) and (c). Ferrocene was used here to demon-

strate the working properties of the electrode, as ferrocene is

easily oxidized to the ferrocenium cation, producing a single

electron; both ferrocene and ferrocenium are chemically in-

ert. The cyclic voltamograms obtained were normalized by

the weight of the electrode. If the redox process is reversible,

the peak potential separation DEp (=Epc � Epa) should be 59 mV

at 25 oC for single electron transfer redox systems. Fig. 5(d)

shows that DEp increases progressively with increasing scan

rate, and lies in the range from 146 mV to 383 mV for the

ECP and from 77 mV to 187 mV for the CCP. DEp is linearly pro-

portional to the square root of the scan rate. This is attributed

to ‘‘slow’’ electrode kinetics, in which electrochemical

‘‘Nernst’’ equilibrium could not be attained on the electrode

surface at these scan rates. The processes for both the ECP

and the CCP are quasi-reversible.

3402 C A R B O N 4 9 ( 2 0 1 1 ) 3 3 9 5 – 3 4 0 3

Based on the Nicholson model, the standard rate constant

at the standard potential, ks, is used to characterize the pro-

pensity for electron transfer [34]. According to the Nicholson

model, w = caks(paD0)�1/2, where w is a dimensionless charge

transfer parameter, a = nFv/RT, a = 0.5, c = (D0/DR)1/2 and D0

and DR are the diffusion coefficients of the oxidized and re-

duced species, D0 = DR = 1.1 · 10�5 cm2/s in this case [35]. v is

the scan rate, n is the stoichiometric number of electrons con-

sumed in the electrode reaction (one, in this case), and F is the

Faraday constant. Fig. 5(e) shows the dependence of the peak

potential separation on w, based on the data reported in Ta-

ble 1 of Nicholson [34]. The value of w can be inferred from

the observed peak potential separation in Fig. 5(d). In this

way, we obtain a value of ks = 0.84 · 10�3 cm/s for the ECP

electrode, and ks = 5.5 · 10�3 cm/s for the CCP electrode, at a

scan rate of v = 0.01 V/s. According to Ferro and Battisti, ks

for the Pt electrode is on the order of magnitude of 10�3 cm/

s, while ks for the diamond electrode is on the order of mag-

nitude of 10�5 cm/s [36]. The ks results for the ECP and CCP

electrodes are thus intermediate between those for the metal

and sp3 electrodes.

The peak current, ip, depends on analyte concentration

and the stability of the electro-generated species. The anodic

current and the cathodic peak current ratios were unity for all

scan rates. As shown in Fig. 5(f), the average current was pro-

portional to the square root of the scan rate, v. The larger val-

ues of ip/v1/2 correspond to larger production rates of the

oxidized or reduced species. The current per unit mass of

the ECP electrode was approximately four times larger than

that of the CCP electrode.

4. Conclusions

Phase separation and electrospinning were employed to pro-

duce porous PVDF-based carbon fibers using PEO as an addi-

tive and water as non-solvent. The morphology of the

carbon fibers was varied by changing DBU/F. After dehydroflu-

orination with DBU/F = 10 for 10 h and subsequent carboniza-

tion, the fibers were smaller in diameter and retained pores

with diameter of 200 ± 60 nm and surface pore density of

8 ± 5 pores/lm2. XRD and Raman data show that the PVDF-

based carbon fibers, derived at 1000 �C, were turbostratic

and disordered in nature; this structure, combined with the

high surface area of the fibers, could be beneficial for subse-

quent functionalization in some applications, such as catalyst

supports, sensors, or redox flow battery electrodes. The elec-

trochemical test indicates that a porous PVDF-based ECP with

high specific surface area performs well in driving redox reac-

tions. Further efforts to graphitize these porous PVDF-based

ECPs are expected to improve the performance of these mate-

rials as electrodes.

Acknowledgment

This work was supported by the MIT-Dupont Alliance. We

would like to acknowledge Prof. Michael Strano of MIT for

use of the furnace, and Dr. Richa Sharma and Dr. Jae-Hee

Han of MIT, for helpful discussions on the carbonization pro-

cess. This work made use of the Shared Experimental Facili-

ties supported by the MRSEC Program of the National

Science Foundation under award number DMR-0819762. Sup-

porting Information is available online from Wiley Inter-

Science or from the author.

Appendix A. Supplementary data

Supplementary data associated with this article can be found,

in the online version, at doi:10.1016/j.carbon.2011.04.015.

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