-
Progress in Organic Coatings 59 (2007) 2836
Electrochemical composite formation of thiopoly : Mby ic
k Pace anof Liber 2
Abstract
Electroch d N-7m) was ve thfibers. Carb ylpyrelectrocoate
ctronreflectance s tioninvestigated by elpolythiophe than 2007
Else
Keywords: Carbon fiber microelectrodes; Electrochemical
composites; Polythiophene; Poly-N-methylpyrrole;
Electropolymerization
1. Introduction
Electricmuch attenConductingmaterials ibe controllstructure
[1trolled by tsubstrate o
Carbonmaterials, arials such acarbon fibecement. Cations
wherfatigue chaapplicationhigh damp
CorresponE-mail ad
cal conductivity, thermal conductivity, and low linear
coefficient
0300-9440/$doi:10.1016/jally conducting polymers and their
composites attracttion because of their high charge storage
ability.polymers possess several advantages as electrode
n batteries. First, the electrochemical properties caned at the
molecular level by manipulating monomer] or by doping [2]. Second,
surface area can be con-he conditions used for polymerization
and/or by thento which the polymer is coated.fibers (CF), which are
a new breed of high-strengthre mainly used as reinforcements in
composite mate-s CF-reinforced plastics, carboncarbon composites,r
reinforced materials, and carbon fiber reinforcedrbon fiber
composites are ideally suited to applica-
e strength, stiffness, lower weight, and outstandingracteristics
are critical requirements. They also finds where high temperature,
chemical inertness, anding are important in addition to having good
electri-
ding author. Tel.: +90 212 285 31 53; fax: +90 212 285 63
86.dress: [email protected] (A.S. Sarac).
of thermal expansion [3].There has been much interest in
supercapacitors because of
their practical applications as energy storage devices for
memorybackup of computers and for electric vehicles [46].
Polypyrrole is one of the most extensively studied
conductingpolymers due to the ease of synthesis, good redox
properties,stability in the oxidized form, ability to give high
electricalconductivity and useful electrical and optical properties
[710].Poly-N-methylpyrrole (PNMPy) has attracted attention as a
pos-sible alternative to polypyrrole for technological applications
inspite of its lower conductivity [11].
Polythiophenes (PTh) have shown considerable promise
formaterials applications due to exceptional electrical
propertiesand their environmental stability [1114]. On the other
hand, thehigh oxidation potential of thiophene compared to the
polymercauses some degradation of the polymeric film, which has
beenthe subject of a wide variety of studies.
Polythiophene and polypyrrole (PPy) have good electri-cal
properties, however, PNMPy exhibits poor environmentalstability,
also PTh suffers from brittleness, low elongation,and poor
processibility. In order to solve these problems,methods of
preparing composites and copolymers, reforming
see front matter 2007 Elsevier B.V. All rights
reserved..porgcoat.2007.01.008mers on carbon fiber
microelectrodessurface spectroscopy, and electrochem
A. Sezai Sarac a,, Hulya Geyik a, Elif Altura Department of
Chemistry, Istanbul Technical University, Polymer Scien
b Materials and Surface Science Institute, UniversityReceived 7
June 2006; received in revised form 31 Decem
emical composite thin film formation (0.60.7m) of thiophene
ancarried out by cyclic voltammetry in order to understand and
impro
on fiber microelectrodes were coated with polythiophene and
N-methd carbon fiber surface mophology was characterized by
scanning elepectroscopy for their composition. The effect of
monomer concentra
. The impedance behaviour of composite electrodes was
characterizedne and poly-N-methylpyrrole exhibits better charge
storage propertiesvier B.V. All rights reserved.phene and
N-methylpyrroleorphology, characterization
al impedance spectroscopyrlak a, Marina Serantoni b
d Technology, Maslak 34469, Istanbul, Turkeymerick, Limerick,
Ireland006; accepted 9 January 2007
methylpyrrole on carbon fiber microelectrodes (diametere surface
properties and capacitance behaviour of carbonrole was
electrografted onto the thiophene electrode. Themicroscopy and
atomic force microscopy and by FTIR-
and scan number on electropolymerization has also
beenectrochemical impedance spectroscopy. The composite
ofpolythiophene coated carbon fiber microelectrodes.
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A.S. Sarac et al. / Progress in Organic Coatings 59 (2007) 2836
29
the monomers of these conducting polymers and blendingwith some
commercially available polymer that offers bet-ter mechan[15].
In the pized on caCFME/PThof the meastrograftedto the
flexpolymers otheir biocotrograftingflexibility agroup thatAs
polythimaterial aouter side(PNMPy coties). Durinwith PNMber, and
eleThe compowith cyclicMorphologThe capaciCFMEs watroscopy (E
2. Experim
2.1. Mater
AcetoniHaen Chefrom Riedemethylpyrrpurity of 9320,000 (Cing
320,00electrodes.(diameter =The electrodipping lentape.
2.2. Instru
Polymetonitrile (Aelectrodessition cyclewere measu
2263 potenbines potenpotentiostaA three-ele
electrode, platinum wire as the counter electrode, and Ag/AgClas
the reference electrode (in 3.5 M KCl solution) was used.
lms of homopolymers, composites, and copolymers elec-ted onto
carbon fiber surfaces were analyzed by FTIRance spectrophotometry
(Perkin Elmer, Spectrum One,
Universal ATR attachment with a diamond and ZnSeC70951), SEM
(Jed Cheeh from Uness), and AFMO-TopoMetrix Explorer). EIS
measurements were alsoed with Parstat 2263 using the supplied Power
Sine
re package.
ctropolymerization procedure
ormation of electrochemical composite electrodes
Electrochemical polymerization of thiophene onber base
electrodesele
lectrtem
er w
oces50 mn 0
Elec/PTher poerfolectrions.he pntat
emecenele
n th
syngrafe 2.ME
1. ScE.ical and/or chemical properties have been reported
resent study, thiophene (Th) was electropolymer-rbon fiber
microelectrode (CFME) and yielding a
electrode. This electrode composed the main parturements as a
base electrode. NMPy was then elec-
onto the CFME/PTh under various conditions. Dueibility of the
CFME structure, the study of thesen substrates will ease the
processing and improvempatibility. Since PTh has a rigid structure,
elec-NMPy onto a CFME/PTh electrode will also givend uniformity to
the structure because of the methylis bonded to the aromatic ring
at the nitrogen.
ophene has a high oxidation potential as a basemicron-sized
electrode with PNMPy coating on theof electrode was desired to
improve its properties.ating itself has poor physical and
mechanical proper-g the electrochemical composite electrode
formationPy, the effect of NMPy concentration, scan num-ctrolyte on
electropolymerization were investigated.site electrodes surface
properties were characterized
voltammetry (CV) and FTIRATR spectroscopy.ical studies were
performed with SEM and AFM.tance behavior of these conductive
polymer-modifieds investigated with electrochemical impedance
spec-IS).
ental
ials
trile (ACN) was used as-received from Riedel-demical without
further purification. Pure NaClO4l-de Haen Chemicals was used.
Thiophene and N-ole from Lancester and Aldrich Chemicals with a9%
were used. High strength (HS) carbon fibersA) (Sigri Carbon,
Meitingen, Germany) contain-
0 single filaments in a row were used as workingAll of the
electrodes were prepared by using CF7m) attached to a copper wire
with Teflon tape.de area is kept constant (3.3 cm2) by adjusting
thegth and covering the rest of the fibers with the Teflon
mentation
rizations were performed electrochemically in ace-CN) solution
containing 0.2 M NaClO4 for variouswith different monomer
concentrations and depo-numbers. Cyclic voltammograms of the
polymersred on a Princeton Applied Research (PAR) Parstattiostat,
which is a self-contained unit that com-tiostatic circuitry with
phase-sensitive detection. Thet connected to a Faraday cage, BAS
Cell Stand C3.ctrode system comprised of a CFME as the working
Thin fitrocoareflectwith acrystal(VEECperformsoftwa
3. Ele
3.1. F
3.1.1.carbon
Thethree-eat roommonom
tion prrate ofbetwee
3.1.2.CFME
Aftbeen pbase econditPTh, trepresein Sch
Confor theadditioto 8).
TheelectroSchemon CF
Schemeon CFMctrochemical polymerizations were carried out in
aode system employing CFME as a working electrodeperature. The
concentration of 0.5 M standard Thas chosen during this study. The
electropolymeriza-
s was performed in 0.2 M NaClO4 in ACN at a scanV/s for four
cycles by CV. The sweep range was
and 1.7 V.
trochemical composite formation of NMPy onbase electrodes
lymerization of PTh the NMPy electrodeposition hasrmed to obtain
a thin composite coating. The sameode was used for all of the NMPy
electrograftingSince PNMPy has a lower oxidation potential then
otential was in the range of 01.1 V. The schematicion of
electrochemical composite formation is shown1.trations of NMPy
between 0.008 and 0.5 M were usedctrografting on the CFME/PTh base
electrode. Ine effect of cycle number was investigated (from 2
thesis route and possible mechanism of PNMPyting on a CFME/PTh
base electrode is shown inFirst, electrooxidation of the thiophene
monomer
results in radical cation formation by electron trans-
hematic representation of electrochemical composite
preparation
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30 A.S. Sarac et al. / Progress in Organic Coatings 59 (2007)
2836
fer from mradical catoligomer fprecoated Cnism.
4. Results
4.1. Cyclicelectropoly
SolutionCFME to pScheme 2. Mechanism of electrochemical
composite formation of N-meth
onomer to the electrode. Dimer formation throughion coupling,
deprotonation, and neutral dimer andormations follow. NMPy
electrografting on the PTh
FME proceeds through the same coupling mecha-
and discussion
voltammetric investigations:merization of Th on CFME
of 0.5 M thiophene was electropolymerized onrepare a
homopolymer-coated base electrode which
was used tcycle numbcoatings.
Electrogcan be seeand cathodvalues meataken
fromelectrogrowpotentials o0.48 V, respCurrent dewere 0.58yl
pyrrole on CFME/PTh base electrode.
o analyze the effect of NMPys concentration anders during
electrochemical formation of composite
rowth of thiophene onto CFME have broad peaks asn in Fig. 1.
During oxidation and reduction, anodicic peak potentials were
determined by taking averagesured from five points on scans. The
values werethe fourth oxidation and reduction cycles of theth
process. Anodic and cathodic electrogrowth peakf Th homopolymer
were determined to be 0.52 andectively. Oxidation potential was
found to be 1.43 V.
nsities corresponding to anodic and cathodic peaksand 0.54
mA/cm2, respectively. The current density
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A.S. Sarac et al. / Progress in Organic Coatings 59 (2007) 2836
31
Fig. 1. Electrfour cycles onin monomer-f
ratio of anobe 0.93 (Ta
To comthiophenewith 50 mVCFME on(Table 1). Tbutton platwhich is
adpolymer elethan a hom
4.2. ElectrN-methylpyelectrochem
4.2.1. EffeSome o
tropolymercan be seen3.3 cm2 fvarying mocycling betfour
cycles
For elecpotentials,fourth cycl
Table 1Redox paramCV
[Th]0.1 M on CFM0.5 M on CFM0.5 M on Pt
Mult] = 0.0
idatfor
An ise of
irrefirstPy
ed foasted noic deer, r
08 M. 3 prcon
scan
.08 Mactivtionogrowth of thiophene in 0.2 M NaClO4 in ACN with
50 mV/s forCFME. Inset: scan rate dependence of 0.5 M Th-coated
CFME
ree electrolyte between 50 and 300 mV/s.
dic and cathodic peak potentials was calculated toble 1).pare
CFMEs and button platinum electrodes, 0.5 Mwas electropolymerized
in 0.2 M NaClO4 in ACN
/s for four cycles. Platinum button electrode andset potentials
were 1.55 and 1.43 V, respectively
hese results indicate that using CFMEs rather thaninum
electrodes decreases the oxidation potentialvantageous for better
and easier electron transfer. Thectrodeposited on CFME has a higher
current densityopolymer electrografted onto Pt button
electrodes.
ochemical composite formation ofrrole on CFME/PTh base electrode
andical characterization
ct of NMPy concentration
Fig. 2.([NMPy
The ox0.73 Vtions.decrea
Theon theent NMobservto be fshowea drastHowevfor 0.0
Fig0.08 Mferentwith 0electroseparaf the cyclic voltammograms
obtained from elec-ization of 0.08 M on CFME/PTh base
electrodes
in Fig. 2. The films were grown on CFMEs (areaor approximately
1000 single fibers) in solutions ofnomer concentrations from 0.008
to 0.5 M of NMPyween 0 and 1.1 V (Ag/AgCl scan rate 50 mV s1)
for.trogrowth processes the current densities, anodicand cathodic
potentials were measured from the
es and redox parameters are summarized in Table 2.
eters of electrogrowth of Th on CFME and on Pt obtained from
Ec (V) Ea (V) Eonset (V) Ia/IcE 0.60 0.58 1.45 0.53E 0.52 0.48
1.43 0.93
1.1 0.80 1.55 0.88
with scan rThe inve
tropolymercompositesthe 0.08 Mare presentobtained fo
Table 2The effect oelectrogrowth
ConcentrationM (NMPy)0.0080.0200.0400.0800.500
a These datisweep experiment for the electropolymerization of
NMPy8 M) on CFME/PTh base electrode in 0.2 M NaClO4 in ACN.
ion potentials of the monomers were 0.86, 0.76, andthe fourth
cycle with increasing NMPy concentra-ncrease in the concentration
of NMPy resulted in athe onset potential of the composite
electrodes.
versible oxidation of the monomer appears clearlycycle followed
by a nucleation loop for five differ-
concentrations. A regular and reversible growth wasr all
composites. Polymer deposition rates appeared
r with increasing NMPy concentration. The resultssignificant
differences inEa, but theEc values showedcrease with increasing the
monomer concentration.
eversibility of electrogrowth of composites was betterNMPy
concentration exhibiting a Ia/Ic value of 0.98.
esents the redox behaviour of composite films with acentration
of NMPy in monomer-free solutions at dif-rates. The current
intensity of the composite obtained
NMPy increases with the scan rate suggesting thee film coats the
CFME/Th base electrode. However,
of anodic and cathodic peak potential (E) increasesate which is
due to a decrease in reversibility.stigation of NMPy concentration
effect during elec-
ization indicates that the current intensity of allis higher
than PTh itself and between compositesconcentration of NMPy is the
highest. The results
ed in Fig. 4 which indicate that a linear behaviour isr the
composite obtained at 0.08 M NMPy, i.e. the
f NMPy concentration on redox parameters during polymer
, Eaa (V) Eca (V) Eonset E Ia/Ic
0.60 0.30 0.86 0.30 0.960.60 0.21 0.82 0.39 0.950.63 0.19 0.77
0.44 0.890.67 0.28 0.76 0.39 0.980.68 0.17 0.73 0.51 0.88
a were taken from fifth cycles.
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32 A.S. Sarac et al. / Progress in Organic Coatings 59 (2007)
2836
Fig. 3. Cyclic voltammogram of 0.08 M PNMPy coated on CFME/Th
baseelectrode in monomer-free solution of 0.2 M NaClO4 in ACN at a
scan rateof: (a) 50 mV/s; (b) 100 mV/s; (c) 150 mV/s; (d) 200 mV/s;
(e) 250 mV/s; (f)300 mV/s.
electron transfer rate is faster and also the composite
showedthin layer behavior. The regression coefficient of the
compos-ite is higher for 0.008 M NMPy (R: 0.997) compared to
otherconcentrati
4.3. FTIR
4.3.1. EffeThe elec
respective astructure wand practiccopolymermicron lev
Fig. 4. Comparison of composite current values under different
scan rates inmonomer-free electrolyte.
Fig. 5 shows the spectra between 700 and 4000 cm1 ofCFME/PTh
homopolymer and CFME/PTh/PNMPy compos-ite electrodes with an
increase in NMPy concentration. The
/PTh 1tic
1200n).of pcreaing0 cming ocon
posce o
Fig. 5. ATRconcentrationons.
characterization
ct of NMPy concentrationtrografted composite structure and
doping with thenion of the supporting electrolyte into the
compositeas studied by FTIRATR. This technique is preciseal and is
a good tool for structural characterization ofs having thicknesses
ranging from the nanometer toel.
CFME(aroma1332positiopeakswas instretchat 142stretchNMPythe
compresenFTIR analysis for the effect of NMPy concentration on
composite formation. Thiovaries as: (a) [NMPy] = 0.008 M; (b)
thiophene homopolymer; (c) [NMPy] = 0.08 Melectrode shows
significant bands at 1418 cmstretching of C C bond) and a broad
peak atcm1 (deformation in plane of CH bond at
These are known to be characteristic vibrationalolythiophene
[16]. When the NMPy concentrationsed, characteristic peaks of PPy
asymmetric ringmode at 1515 cm1 [17] and the symmetric mode1
appear. A peak at about 1300 cm1 shows NCf NMPy as well. The peak
intensities increased withcentration confirming the inclusion of
NMPy intoite structure. Since ClO4 absorbs at 1075 cm1 the
f a strong absorption band at this wave number in allphene
concentration was held constant at 0.5 M Th and NMPy; (d) [NMPy] =
0.5 M.
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A.S. Sarac et al. / Progress in Organic Coatings 59 (2007) 2836
33
Fig. 6. IR ana t four cycles and the cycle number was changed
for 0.5 M Th/0.08 MNMPy compo
compositewith this an
4.3.2. EffeFTIRA
ferent cyclof NMPy wof NMPy,characterisLarraz et awith characbe
seen ne1433 cm1of NMPy wtwo cycles.
4.4. Morph
4.4.1. ConThe mor
tigated comfor sample50 mV/s anmonomer c
tion and costructure. FCFME/PThhave an effhomogenouthe
inclusioposites forwas aboutcentrationThe structucentrationslysis
of the cycle number effect. Thiophene cycle number was held
constant asite to: (a) eight cycles; (b) four cycles; (c) two
cycles.
electrodes indicates that these composites are dopedion.
ct of NMPys cycle numberTR spectra of electrochemical composites
with dif-e numbers are shown in Fig. 6. Characteristic peaks
ere between 1200 and 1600 cm1. For two cyclesindistinct peaks
can be seen. The intensity of thetic peaks increased linearly with
the cycle numbers.l. [18] attributed the bands at 1460 cm1 to
NMPyteristic peaks of CN, C C, and C CN, which canar to 1416 cm1
for four cycles of NMPy and atfor eight cycles. No significant
characteristic peaksere observed for the electrografting of NMPy
forological investigation
centration effect of NMPy (by SEM)phology of PNMPy coated base
electrodes was inves-paratively by scanning electron microscopy
(SEM)
s obtained potentiodynamically with a scan rate ofd compared
with samples obtained for changes ofoncentration, cycle number, and
of composite forma-polymer formation in order to understand the
surfaceig. 7 shows electrochemically deposited samples onbase
electrodes. NMPy concentration was found to
ect on the film morphology. PTh has very smooth ands structure,
however, small grains were formed withn of NMPy. The increase in
radius of polymer com-0.04 and 0.08 M NMPy composites (on
CFME/PTh)0.6 and 0.7m, respectively. As the monomer con-of NMPy
increases, the grains get bigger (Fig. 7b).re of NMPy is more
cauliflower-like for higher con-.
Fig. 7. (a) SEM image of 0.04 M NMPy coated on CFME/PTh (radius
thick-ness increase: 0.6m); (b) SEM image of 0.08 M NMPy coated on
CFME/PTh(radius thickness increase: 0.7m).
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34 A.S. Sarac et al. / Progress in Organic Coatings 59 (2007)
2836
Fig. 8. (a) SEwith two cyclewith eight cyc
4.4.2. CycFig. 8 sh
During thewas used othe NMPysteps. As eincreases th
4.4.3. MorFig. 9
ing monom20 mV/s. Tis very sim[20] wherefrom V-sha45 nm
As showtration of NThis indica
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A.S. Sarac et al. / Progress in Organic Coatings 59 (2007) 2836
35
Fig. 10. Nyquist plots of 0.02, 0.04 and 0.08 M NMPy coated on
CFME/PThbase electrode and 0.5 M Th homopolymer on CFME in 0.2 M
NaClO4 in ACNat 50 mV/s.
4.5. Electrochemical impedance spectroscopy (EIS)
Electrocto monitorelectrodes.capacitive lbon fiber/pfast
chargewith NMPyresults shoture increaangle (seesupercapacsure the
chNMPy onpresents theing monomof electrod
Fig. 11. BodeCFME/PTh b
Fig. 12. Nyquon CFME/Pth
Bode phase plots of 0.08 M NMPy coated on CFME/PTh base
electrodefour, and eight cycles in 0.2 M NaClO4 in ACN at 50
mV/s.
a lower charge storage ability and divergence from thetive
line.hemical impedance spectroscopy was performedthe
electrochemical behavior of the composite
All composites show a slight deviation from theine (y-axis),
indicating fast charge transfer at the car-olymer and
polymer/solution interfaces, as well astransport in the polymer
bulk. Composite electrodesand CFME/PTh base electrodes were
compared and
wed that inclusion of NMPy to the electrode struc-ses the charge
storage from 68 to 80 Bode phaseFig. 10), which is very close to 90
as observed initors. Due to that increase a second step to mea-arge
storage from increasing the concentration of
CFME/PTh base electrode was performed. Fig. 11results due to
NMPys cauliflower structure increas-er concentration, which
destroys the homogeneity
e and therefore decreases the electron transfer result-
Fig. 13.for two,
ing incapaciphase plots of 0.008, 0.02, 0.04, 0.08 and 0.5 M
NMPy coated onase electrode in 0.2 M NaClO4 in ACN at 50 mV/s.
The effwith three sFigs. 12 anabout a sligposite film.thinner
comcapacitor)a phase ang
5. Conclu
The effelectrolytepolythiophcompositesurface chatroscopy, aist
plots of 0.08 M NMPy coated with two, four, and eight cyclesbase
electrode in 0.2 M NaClO4 in ACN at 50 mV/s.ect of NMPy cycle
numbers ranging from 2 to 8teps on CFME/PTh base electrode was
investigated.d 13 indicate that the thicker composite film bringsht
decrease in the capacitive behaviour of the com-That result
supports the Bode plot (Fig. 13) since theposite film presents a
phase angle of 80 (near ideal
and the thick film with eight cycles of NMPy showsle of 70
(smaller).
sion
ect of NMPy concentration, scan number, andon
electropolymerization of N-methylpyrrole on
ene coated CFME was investigated. The obtainedelectrode surface
properties were characterized byracterization techniques, i.e. CV,
FTIRATR spec-nd morphological techniques such as SEM and
-
36 A.S. Sarac et al. / Progress in Organic Coatings 59 (2007)
2836
AFM. The capacitance behaviour of these conductive
polymer-modified carbon fiber microelectrodes is investigated with
EIS.
Conducting polymer nano-composite coatings on micron-sized
carbon fiber electrode materials can be used inmicron-sized battery
applications. These studies indicate thatthe electrochemical
properties (especially capacitance) can becontrolled at the
molecular level by manipulating compositestructure and content,
with the thin composite film exhibitingnear ideal capacitor
behavior.
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Electrochemical composite formation of thiophene and
N-methylpyrrole polymers on carbon fiber microelectrodes:
Morphology, characterization by surface spectroscopy, and
electrochemical impedance
spectroscopyIntroductionExperimentalMaterialsInstrumentation
Electropolymerization procedureFormation of electrochemical
composite electrodesElectrochemical polymerization of thiophene on
carbon fiber base electrodesElectrochemical composite formation of
NMPy on CFME/PTh base electrodes
Results and discussionCyclic voltammetric investigations:
electropolymerization of Th on CFMEElectrochemical composite
formation of N-methylpyrrole on CFME/PTh base electrode and
electrochemical characterizationEffect of NMPy concentration
FTIR characterizationEffect of NMPy concentrationEffect of NMPys
cycle number
Morphological investigationConcentration effect of NMPy (by
SEM)Cycle number effect of NMPy by SEMMorphological investigation
by AFM
Electrochemical impedance spectroscopy (EIS)
ConclusionReferences