Substituent effect on supercapacitive performances of conducting polymer-based redox electrodes: Poly(3′,4′-bis(alkyloxy) 2,2′:5′,2″-terthiophene) derivatives

Article abstract of DOI:10.1002/pola.28927

This work reports the synthesis of novel poly(3′,4′-bis(alkyloxy)terthiophene) derivatives (PTTOBu, PTTOHex, and PTTOOct) and their supercapacitor applications as redox-active electrodes. The terthiophene-based conducting polymers have been derivatized with different alkyl pendant groups (butyl-, hexyl-, and octyl-) to explore the effect of alkyl chain length on the surface morphologies and pseudocapacitive properties. The electrochemical performance tests have revealed that the length of alkyl substituent created a remarkable impact over the surface morphologies and charge storage properties of polymer electrodes. PTTOBu, PTTOHex, and PTTOOct-based electrodes have reached up to specific capacitances of 94.3, 227.3, and 443 F?g^?1 at 2.5 mA?cm^?2 constant current density, respectively, in a three-electrode configuration. Besides, these redox-active electrodes have delivered satisfactory energy densities of 13.5, 29.3, and 60.7 W?h?kg^?1 and power densities of 0.98, 1, and 1.1 kW?kg^?1 with good capacitance retentions after 10,000 charge/discharge cycles in symmetric solid-state micro-supercapacitor devices.

Full text of DOI:10.1002/pola.28927

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Substituent Effect on Supercapacitive Performances of Conducting
Polymer-Based Redox Electrodes: Poly(30,40-bis(alkyloxy)
2,20:50,200-terthiophene) Derivatives
ꢀ
€ €
Deniz Yigit , Melis Aykan, Mustafa Gullu
Department of Chemistry, Ankara University, Besevler 06100, Ankara, Turkey
ꢀ
€ €
Correspondence to: D. Yigit (E-mail: dyigit@science.ankara.edu.tr) or M. Gullu (E-mail: gullu@ankara.edu.tr)
Received 23 October 2017; accepted 29 November 2017; published online 00 Month 2017
DOI: 10.1002/pola.28927
ABSTRACT: This work reports the synthesis of novel poly(30,40-
bis(alkyloxy)terthiophene) derivatives (PTTOBu, PTTOHex, and
PTTOOct) and their supercapacitor applications as redox-active
electrodes. The terthiophene-based conducting polymers have
been derivatized with different alkyl pendant groups (butyl-,
hexyl-, and octyl-) to explore the effect of alkyl chain length on
the surface morphologies and pseudocapacitive properties.
The electrochemical performance tests have revealed that the
length of alkyl substituent created a remarkable impact over
the surface morphologies and charge storage properties of
polymer electrodes. PTTOBu, PTTOHex, and PTTOOct-based
electrodes have reached up to specific capacitances of 94.3,
227.3, and 443 F g21 at 2.5 mA cm22 constant current density,
respectively, in a three-electrode configuration. Besides, these
redox-active electrodes have delivered satisfactory energy den-
sities of 13.5, 29.3, and 60.7 W h kg21 and power densities of
0.98, 1, and 1.1 kW kg21 with good capacitance retentions after
10,000 charge/discharge cycles in symmetric solid-state micro-
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supercapacitor devices. 2017 Wiley Periodicals, Inc. J. Polym.
Sci., Part A: Polym. Chem. 2017, 00, 000–000
KEYWORDS: conducting materials; energy storage; materials
science; polymers; redox-active electrodes; solid-state devices;
supercapacitors; terthiophenes
Therefore, the great majority of recent efforts have been
devoted to designing ideal electrode materials possessing
high capacitance, high energy density, high power density,
and high cycling durability.
INTRODUCTION Over the past 10 years, there has been a
ever-increasing demand for innovative electrical energy stor-
age solutions to fulfill the requirements of advance electronic
applications. Especially, the fast advancement of portable and
flexible consumer electronic products such as roll-up dis-
plays, electronic papers, wearable sensors, smart phones,
and touch screens makes it necessary to develop low-cost,
lightweight, thin and flexible energy storage systems. In this
context, supercapacitors, also commonly known as electro-
chemical capacitors or ultracapacitors, are considered as
new generation energy storage devices in terms of their
energy and power densities.1–3
In electrochemical double-layer capacitors (EDLCs), carbon-
based materials such as carbon nanotubes,8,9 graphenes,10,11
carbon nanofibers,12 and aerogels13 have been widely used
as electrode materials because of their outstanding electronic
properties and excellent mechanical stabilities. These carbo-
naceous materials use a non-Faradaic mechanism based on
physical sorption/desorption to store charges and release
energy, so do conventional dielectric capacitors.14 Hence,
EDLCs usually provide low capacitance and energy density
despite the fact that they have high surface area, highly
porous network, fast ion kinetic, good chemical and thermal
stability, and excellent mechanical strength.15
Compared to primary and secondary batteries, even though
supercapacitors generally deliver lower energy density they
have desirable advantages including long cycle lifes, high
charge capabilities, short charging/discharging times, cost-
effective fabrication, and good operational safety.4–6 The
capacitive performance of a supercapacitor is directly related
to the electroactive materials. In particular, morphological
and electrochemical features like surface area and porosity,
thickness, electrical conductivity, and internal resistance play
critical role on charge storage capacity of electrode material.7
Transition metal oxides (MOx) and p-conjugated conducting
polymers are promising electrode material groups for elec-
trochemical energy storage applications.16–21 In contradis-
tinction to carbon-based structures, MOx- or conducting
polymer-based electrode materials store charge through not
Additional Supporting Information may be found in the online version of this article.
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only electrostatic ion sorption/desorption mechanism, but
also a fast and reversible Faradaic redox processes which
results in much higher specific capacitance and power den-
sity values than EDLCs.22,23 To date, various MOx with vari-
able valence states, TiO2,24 MnO2,25 SnO2,26 RuO2,27 V2O5,28
and NiO29 have been extensively explored as pseudocapaci-
tive materials and shown good charge storage/energy gener-
ation performances. However, redox-active electrode
materials produced from such transition MOx generally suf-
fer from conductivity problem, poor electron transfer ability,
limited surface area, high cost and toxicity.30 Comparingly, p-
conjugated conducting polymers offer considerable advan-
tages such as excellent electronic conductivity, high redox
capacitance, better mechanical flexibility, environmentally
friendly materials, and ease of structural modification in
addition to their fast switching ability between redox states
during doping/dedoping process.31–33 Moreover, many con-
ducting polymer derivatives can be electrochemically grown
on current collector substrates without the use of any poly-
meric binder.
polymer backbone. Futhermore, poly(3,6-dithienylcarbazole)-
based redox-active materials have delivered high specific
capacitances (Cspec 5 554 and 640 F g21) with a broad
potential window of 2.0 V in flexible solid-state symmetric
supercapacitor devices based on organic gel electrolyte. Our
results have endorsed that molecular design approach is
simple and effective tool to prepare pseudocapacitive poly-
meric materials with high long-term stability as well as high
specific capacitance, energy, and power density.
Herein, we report syntheses and evaluation of pseudocapaci-
tive performances of novel poly(30,40-bis(alkyloxy)terthio-
phene) derivatives, poly(30,40-dibutoxy-2,20:50,200-terthiophene)
(PTTOBu),
(PTTOHex),
poly(30,40-bis(hexyloxy)-2,20:50,200-terthiophene)
and
poly(30,40-bis(octyloxy)-2,20:50,200-terthio-
phene) (PTTOOct). A series of three terthiophene-based mono-
mers were derivatized with homologous alkyl pendant groups
such as butyl-, hexyl-, and octyl- and the effect of alkyl chain
length on the surface morphology of polymeric redox-active
network was investigated in detail. It was aimed to influence
the morphological properties of pseudocapacitive layer by
altering pendant groups on conducting polymer backbones
and this was expected to enhance charge storage capacity of
electrode material by creating suitable and efficient diffusion
pathways. PTTOBu, PTTOHex, and PTTOOct-based redox-active
electrodes were electrochemically prepared with minimum
pseudocapacitive material mass loading on stainless steel (SS)
substrates by using binder-free direct deposition method and
characterized by cyclic voltammetry (CV) and galvanostatic
charge/discharge (GCD) technique under a standard three-
electrode cell system in organic supporting electrolyte solu-
tion. Moreover, poly(30,40-bis(alkyloxy)terthiophene) coated SS
substrates were employed as both anode and cathode materi-
als and p-p type solid-state micro-supercapacitor devices were
assembled in symmetrical two-electrode configuration. Super-
capacitive performances of prototype micro-devices were
tested using CV and GCD measurements with a working
potential of 1.6 V. Electrochemical impedance spectroscopy
(EIS) tests were further conducted to determine other elec-
tronic parameters of solid-state supercapacitor devices such
as charge transfer kinetics, ion diffusion rate, and equivalent
series resistance (ESR).
A variety of conducting polymer derivatives including polya-
nilines (PANI), polypyrroles (PPy), polythiophenes (PTh),
poly(carbazole)s, poly(3,4-ethylenedioxythiophene) (PEDOT)
have been extensively employed as redox-active electrode
materials in supercapacitors.34–42 These conducting polymer
derivatives have shown specific capacitance values in the
range of 33.4–640 F g21 in different supercapacitor cell con-
figurations. Among all polymeric pseudocapacitive materials,
PEDOT and its derivatives have attracted tremendous atten-
tion owing to their fast charge/discharge rate in a relatively
wider working potential window, high chemical and electro-
chemical stability, and suitable surface morphology although
they generally have low specific capacitances. Furthermore,
PEDOT can serve as both p-type (anode material) and n-type
(cathode material) pseudocapacitive material for symmetric
and asymmetric supercapacitor applications.43–46 Recently,
Ermis et al. have introduced novel poly(N-alkylthieno[3,4-
b][1,4]oxazine) derivatives as alternatives to PEDOT and
evaluated their pseudocapacitive performances. These mate-
rials delivered better specific capacitances (Cspec 5 313 and
325 F g21) and energy densities (E 5 65.24 and 86.74
W h kg21) than those of PEDOT in organic supporting elec-
trolyte with symmetric cell configurations.47 However, the
common disadvantage for many conducting polymer-based
electrode materials is poor long-term cycling stability caused
by swelling and shrinkage of the conducting polymer back-
bone during the Faradaic charge/discharge processes.43 The
design and synthesis of novel p-conjugated conducting poly-
mers can be considered as an effective method to boost
cycling stability of pseudocapacitive materials. Very recently,
we have reported, for example, reasonable polymer design
strategy to improve mechanical properties of redox-active
materials.37 The N-substituted poly(3,6-dithienylcarbazole)
derivatives have exhibited excellent cycling performances
with close to 93% capacitance retentions after 10,000 suc-
cessive charge/discharge cycles owing to the high mechani-
cal strength and flexibility of carbazole units on conducting
EXPERIMENTAL
Materials and Instrumentation
All chemicals were purchased from Sigma-Aldrich. SS sub-
strates (99.9%, SS, 10 mm width, 10 mm length, and 0.4 mm
thickness) were obtained from a local manufacturer. Before
electrodeposition of conducting polymer films, SS substrates
were immersed in a 2.5 M HCl solution for 5–6 min to clean
potential oxide layer on the surfaces and rinsed with metha-
nol in an ultrasonic bath and directly used. Acetonitrile
(ACN) was refluxed in the presence of P2O5 for 8 h and frac-
tionally distilled. ACN was stored in a brown glass bottle
over activated 4A molecular sieves under a nitrogen atmo-
sphere. Tetrabutylammonium tetrafluoroborate (Bu4NBF4)
was dried at 85 8C for 12 h prior to use in electrochemical
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studies. Fc/Fc1 ferrocene redox couple was used for calibra-
tion of the Ag/Ag1 pseudoreference silver wire (0.3 V vs. sil-
ver wire). The novel terthiophene derivatives (TTOBu,
TTOHex, and TTOOct) were characterized by FTIR, NMR,
mass spectroscopy, and elemental analysis techniques. The
chemical structures of conducting polymer films (PTTOBu,
PTTOHex, and PTTOOct) were confirmed using FTIR and ele-
mental analysis while their morphological properties were
studied with scanning electron microscopy (SEM). Conductiv-
ity measurements for PTTOBu, PTTOHex, and PTTOOct
coated substrates were performed with a standard four-
probe system at room temperature. A CEM Discover S-Class
single-mode microwave instrument was used for nucleophilic
substitution reactions. FTIR spectra were recorded with a
Perkin-Elmer Spectrum 100 spectrometer (ATR apparatus).
1H NMR and 13C NMR spectra were recorded on a Varian-
Mercury 400 MHz (FT)-NMR spectrometer at 25 8C using
tetramethylsilane internal standard. Chemical shifts were
reported as ppm (d). Mass spectra were collected on a Water
2695 Alliance Micromass ZQ LC/MS using a direct inlet
probe. The morphological structures of polymeric films were
observed by a Zeiss Ultra Plus FE-SEM and Zeiss EVO 40
500 V to 30 kV. Electrochemical studies were performed
using a Princeton Applied Research PAR-2273 potentiostat/
galvanostat and a Radiometer VoltaLab PST050 potentiostat/
galvanostat-high voltage booster 100 HVB100.
Preparation of PTTOBu, PTTOHex, and PTTOOct
Redox-Active Electrode Materials
PTTOBu, PTTOHex, and PTTOOct were directly coated on the
SS substrates by a constant potential electrolysis technique
at suitable monomer oxidation potential (1.76 V for TTOBu,
1.79 V for TTOHex and 1.82 V for TTOOct) to control the
mass loading on the SS current collectors. The constant
potential electrolysis studies were performed using a SS
working electrode, a SS auxiliary electrode and a Ag/Ag1
pseudoreference silver wire electrode with a charge density
of 200 mC cm22 in the presence of 0.04 M monomer in
0.1 M Bu4NBF4/ACN supporting electrolyte solution under a
nitrogen atmosphere. After deposition, PTTOBu, PTTOHex,
and PTTOOct coated SS electrodes were reduced under
20.40 V constant potential for 100 s to expel trapped
charges and dopant ions on the polymer surfaces. The poly-
meric redox-active electrodes were rinsed with ACN to
remove unreacted monomer residues and ions of electrolyte.
Then, the modified electrode materials were put in an oven
at 70 8C for 3 h. The electroactive material mass on each SS
substrate was measured six times in a row using a microan-
alytical balance (Dm 5 6 0.001 mg). The average of six
measurements was used as total mass of redox-active mate-
rial for the calculation of specific capacitance, energy density,
and power density. The average mass loading of PTTOBu,
PTTOHex, and PTTOOct electrodes was ꢀ0.97 mg cm22. The
chemical characterization of polymeric materials were per-
formed by FTIR and elemental analysis techniques while the
morphological properties of PTTOBu, PTTOHex, and PTTOOct
coated SS substrates were probed with SEM.
Syntheses of Monomers
A multi-step reaction pathway was applied for the syntheses
of novel electroactive terthiophene derivatives, 30,40-dibu-
toxy-2,20:50,200-terthiophene (TTOBu), 30,40-bis(hexyloxy)-
2,20:50,200-terthiophene (TTOHex) and 30,40-bis(octyloxy)-
2,20:50,200-terthiophene (TTOOct). The details of synthetic
procedures and spectral data can be seen in the Supporting
Information document.
Poly(30,40-dibutoxy-2,20:50,200-terthiophene) (PTTOBu)
IR (ATR): v 5 3109 (w, aromatic CAH strecthing), 2964,
2943, 2860 (w, aliphatic CAH strecthing), 1634 (s, polycon-
jugation), 1562 (m, aromatic C@C strecthing), 1458, 1375
(m, aliphatic CAH bending), 1224, 1154 cm21 (s, CAOAC
strecthing). Anal. calcd. for (C20H23O2S3)n: C 61.34, H 5.92;
found: C 60.97, H 6.11.
Electrochemical Properties of Monomers and Their
Conducting Polymers
Poly(30,40-bis(hexyloxy)-2,20:50,200terthiophene)
(PTTOHex)
IR (ATR): v 5 3096 (w, aromatic CAH strecthing), 2932,
2853 (w, aliphatic CAH strecthing), 1626 (s, polyconjuga-
tion), 1465, 1412, 1372 (m, aliphatic CAH bending), 1276,
1220 cm21 (s, CAOAC strecthing). Anal. calcd. for
(C24H31O2S3)n: C 61.39, H 6.98; found: C 61.57, H 6.64.
The electrochemical redox behaviors of the novel terthio-
phene monomers, TTOBu, TTOHex, and TTOOct were exam-
ined by CV. CV studies were performed with a standard
three-electrode cell configuration consisting of platinum wire
working and auxiliary electrodes versus a Ag/Ag1 pseudore-
ference silver wire electrode. Cyclic voltammograms were
recorded by scanning the potential between 0.0 and 2.0 V at
150 mV s21 scan rate in 0.1 M Bu4NBF4/ACN supporting
electrolyte solution under a nitrogen atmosphere. Single scan
CV technique was employed to evaluate p- and n-type dop-
ing properties of PTTOBu, PTTOHex, and PTTOOct polymeric
films. PTTOBu, PTTOHex, and PTTOOct coated platinum
wires were used as working electrodes in single scan vol-
tammetry studies. The cyclic voltammograms of PTTOBu,
PTTOHex, and PTTOOct were recorded by scanning potential
between 0.0 and 2.0 V for anodic sweep and 0.0 and 22.5 V
for cathodic sweep versus a Ag/Ag1 pseudoreference silver
wire electrtode in the monomer-free solution at a scan rate
Poly(30,40-bis(octyloxy)-2,20:50,200-terthiophene)
(PTTOOct)
IR (ATR): v 5 3084 (w, aromatic CAH strecthing), 2938,
2864 (w, aliphatic CAH strecthing), 1624 (s, polyconjuga-
tion), 1456, 1404, 1368 (m, aliphatic CAH bending), 1269,
1227 cm21 (s, CAOAC strecthing). Anal. calcd. for
(C28H39O2S3)n: C 66.75, H 7.80; found: C 66.38, H 7.96.
Assembly of the Symmetric Solid-State
Micro-Supercapacitor Devices
Three symmetric micro-supercapacitors were constructed by
sandwiching two PTTOBu, PTTOHex, and PTTOOct redox-
of 150 mV s21
.
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FIGURE 1 Fabrication procedure of prototype symmetric solid-state micro-supercapacitor device. [Color figure can be viewed at
wileyonlinelibrary.com]
active electrodes, which were used both as anode and cath-
ode materials in device configurations. First of all, to assem-
ble solid-state supercapacitor devices, PTTOBu, PTTOHex,
and PTTOOct electrode materials were immersed in 0.5 M
Bu4NBF4/ACN supporting solution and kept for 1 h. Thin
separator paper (Whatman Grade 1 filter paper, 15 mm
diameter) soaked to 0.5 M Bu4NBF4/ACN solution was care-
fully placed on the polymer coated surface of one modified
electrode. Then, the second redox-active electrode material
was overlapped and constructed by sandwiching. In the last
stage of fabrication process, the solid-state micro-supercapa-
citor was hermetically sealed by wrapping with a thin paraf-
fin band. The symmetric supercapacitor devices assembled
with PTTOBu, PTTOHex, and PTTOOct polymeric redox-
active materials were symbolized as Device I, Device II, and
Device III, respectively. Figure 1 represents a schematic dia-
gram for preparation procedure of symmetrical solid-state
micro-supercapacitor devices with a sandwich structure.
of micro-supercapacitor devices were tested by recording
typical GCD curves at 2.5 mA cm22 constant current density
between 0.0 and 1.6 V over 10,000 charge/discharge cycles.
EIS measurements were carried out with a frequency range
from 104 to 1022 Hz and voltage amplitude of 5 mV rms at
0.0 V DC voltage.
RESULTS AND DISCUSSION
Monomer Synthesis
In this study, we focused on the design, synthesis, and inves-
tigation of charge storage properties of novel terthiophene-
based conducting polymers as effective redox-active materi-
als. For this purpose, a new class of 2,20:50,200-terthiophene
monomers containing butyl-, hexyl-, and octyl- chains were
initially synthesized according to a multistep synthetic proce-
dure, as briefly illustrated in Scheme 1. The first step of syn-
thetic pathway involves microwave-assisted nucleophilic
substitution reactions of diethyl 3,4-dihydroxythiophene-2,5-
dicarboxylate (1) with alkyl bromide compounds (2,3,4).
Then, hydrolysis, decarboxylation and dibromination of these
3,4-bis(alkyloxy)thiophenes (5,6,7) gave their 2,5-dibromo-
thiophene derivatives (14,15,16) in good yields. The Stille
cross-coupling reactions of the brominated thiophene com-
pounds with 2-(tributylstannyl)thiophene were finally per-
formed in the presence of Pd(PPh3)Cl2 catalyst to obtain
target desired monomers, 30,40-dibutoxy-2,20:50,200-terthio-
phene (TTOBu), 30,40-bis(hexyloxy)-2,20:50,200-terthiophene
Pseudocapacitive Performance Measurements
The capacitive properties of PTTOBu, PTTOHex, and
PTTOOct electrode materials were investigated by CV, GCD,
and EIS techniques in both three-electrode and two-
electrode cell configurations. The three-electrode measure-
ments were carried out using polymer coated SS working
and auxiliary electrodes and a Ag/Ag1 pseudoreference sil-
ver wire electrode in 0.5 M Bu4NBF4/ACN solution under
ambient conditions. However, the capacitive performances of
Device I, Device II, and Device III were tested by connecting
the lead of reference electrode on the side of the auxiliary
electrode. The current-potential (I–V) profiles were evaluated
with CV studies in the potential range of 0.0–1.6 V at various
scan rates (10, 25, 50, 100, 150, and 250 mV s21). GCD per-
formance measurements were performed at constant current
densities of 2.5, 4.5, 6.5, 8.5, 10.5, and 12.5 mA cm22 in 0.0
and 1.6 V potential window. The long-term cycling stabilities
(TTOHex),
and
30,40-bis(octyloxy)-2,20:50,200-terthiophene
(TTOOct; Fig. 2). The chemical structures of novel terthio-
phene monomers were confirmed by FTIR, 1H NMR, 13C
NMR, mass spectroscopy, and elemental analysis techniques.
Electrochemical Characterization of Monomers and
Preparation of Redox-Active Electrode Materials
TTOBu, TTOHex, and TTOOct monomers were initially sub-
jected to CV to explore the electrochemical properties and
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SCHEME 1 Synthetic pathway for TTOBu, TTOHex, and TTOOct.
the effect of alkyl chain lengths on the redox behaviors of
novel terthiophene monomers. Voltammograms were
recorded using a standard three-electrode cell configuration
consisting of platinum working and auxiliary electrodes and
a Ag/Ag1 pseudoreference silver wire electrode in the poten-
tial range of 0.0 and 2.0 V at a scan rate of 150 mV s21. As
can be seen from Figure 3(a–c), TTOBu, TTOHex, and TTOOct
exhibited irreversible monomer oxidation peaks at 1.76,
1.79,and 1.82 V, respectively, in the first cycle of voltammo-
grams. After observing monomer oxidations, characteristic
reversible redox waves were appeared for PTTOBu, PTTO-
Hex, and PTTOOct with an increasing current density of each
successive oxidation/redection cycle and conducting polymer
films were regularly electrodeposited on the platinum surfa-
ces. CV studies clearly reveal that TTOOct electroactive
monomer got oxidized at slightly higher potential when com-
pared with TTOBu and TTOHex. The small difference
between oxidation potentials can be explained by the mono-
mer solubility and steric hindrance of alkyl chains. TTOOct is
better soluble molecule than TTOBu and TTOHex because of
relatively longer alkyl chains. The increase in solubility of
the monomer decreases the mobility of side alkyl chains and
molecule becomes more ordered due to intense van der
Waals interaction between the alkyl chains. This effect gives
the monomer a more planar structure. Thus, monomer is
able to approach each other and electrode surface at a cer-
tain plane.48–51 Because of these reason, TTOBu and TTOHex
were relatively easily oxidized than TTOOct.
To investigate the p- and n-doping characteristics of polymer
films, single scan voltammograms were also recorded for
PTTOBu, PTTOHex, and PTTOOct in a monomer free solution
as applied in the CV procedure for monomers. Figure 3(d–f)
show that all conducting polymers exhibited only p-doping
property with reversible redox couples at 1.29/1.18 V for
PTTOBu, at 1.54/0.85 V for PTTOHex and at 1.38/1.19 V for
PTTOOct versus Ag wire pseudoreference electrode. The
cathodic sweeps of CV curves for PTTOBu, PTTOHex, and
PTTOOct can be seen in Fig. S1 (see Supporting
Information).
After electrochemical redox characterization of all electroac-
tive monomers (TTOBu, TTOHex, and TTOOct) and their cor-
responding polymers, conducting polymer film coated SS
electrode materials were electrochemically prepared via
potentiostatic method at a suitable constant potential (1.76
V for TTOBu, 1.79 V for TTOHex, and 1.82 V for TTOOCt).
PTTOBu, PTTOHex, and PTTOOct polymer films deposited on
SS substrates were characterized by FTIR and elemental
analysis techniques and found to be satisfactory. The stan-
dard four-probe technique was employed to the electrical
conductivities of PTTOBu, PTTOHex, and PTTOOct electroac-
tive layers. The conductivities of PTTOBu, PTTOHex, and
PTTOOct homopolymer samples with convenient thick films
(ꢀ50 mm) on SS substrates were measured to be 1.7, 2.6,
and 3.5 S cm21, respectively, (doped with BF–4) at room tem-
perature. However, the morphological observations of con-
ducting polymer films were performed by SEM. The top-view
SEM images shown in Figure 4 confirm the homogeneous
FIGURE 2 Chemical structures of the novel terthiophene mono-
mers, TTOBu, TTOHex, and TTOOct.
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FIGURE 3 Repetitive cyclic voltammograms of (a) TTOBu, (b) TTOHex, and (c) TTOOct and single scan cyclic voltammograms of
(d) PTTOBu, (e) PTTOHex, and (f) PTTOOct in a monomer free 0.1 M Bu4NBF4/ACN solution. [Color figure can be viewed at
wileyonlinelibrary.com]
formation of PTTOBu, PTTOHex, and PTTOOct polymer layer
across the SS substrate surfaces. Before electropolymeriza-
tion, the bare SS substrate has an ordered surface morphol-
ogy [Fig. 4(a)], but after electrodeposition, Figure 4(b–k)
indicate that each SS substrate was regularly coated with
PTTOBu, PTTOHex, and PTTOOct.
mobility of substituents decreases due to high van der Waal
interaction between alkyl groups caused by better solubility
and hydrophobicity. This effect allows more regular and lon-
ger growth of polymer chains on substrate surfaces during
electropolymerization process. However, as can be seen from
Figure 4(b–d), conducting polymer films having shorter alkyl
chains exhibit irregular surface morphologies because of
self-aggregation. The regular morphological property of
PTTOOct is possible to create more effective ion diffusion
pathways and enhance charge storage capability of redox-
active electrode material. Conversely, the agglomerated
The effect of alkyl chain on the surface morphologies of con-
ducting polymer films can also be seen from these SEM
images [Fig. 4(b,e,h)]. Prolongation of alkyl chains provides a
more regular surface morphology to PTTOOct-based con-
ducting polymer film. As the alkyl chain length increases, the
6
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FIGURE 4 Top-view SEM images with various magnificants of (a) bare SS substrate, (b–d) PTTOBu coated SS electrode materials,
(e–g) PTTOHex coated SS electrode materials, and (h–k) PTTOOct coated SS electrode materials.
surface morphology of PTTOBu may cause a barrier effect
for ion movements. SEM observations endorse that the alkyl
chain difference on conducting polymer backbone created a
significant change over the surface morphologies, as
expected.
Electrochemical Performance Measurements of the
Redox-Active Electrode Materials
The capacitive behaviors of PTTOBu, PTTOHex, and PTTOOct
redox-active electrode materials were investigated by means of
CV, GCD, and EIS techniques. The electrochemical
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FIGURE 5 Cyclic voltammograms of (a) PTTOBu, (b) PTTOHex, and (c) PTTOOct-based polymeric redox-active materials as positive
electrode in different potential windows. [Color figure can be viewed at wileyonlinelibrary.com]
measurements were performed both in a typical three-
electrode cell configuration and a real supercapacitor device in
two-electrode system with conducting polymer coated SS
anode and cathode materials. Prior to supercapacitor perfor-
mance test, the ideal working potential window was deter-
mined by preliminary CV studies for each redox-active
electrode material in a symmetric three-electrode cell configu-
ration. Figure 5(a–c) show the CV curves of PTTOBu, PTTOHex,
and PTTOOct in various potential windows (range from 0.0 to
1.8 V) at 100 mV s21 constant scan rate in 0.5 M Bu4NBF4/ACN
organic supporting electrolyte. Especially, it can be seen from
Figure 5(b), the CV current of PTTOHex redox-active electrode
material began to decrease after 1.6 V. Therefore, the potential
range between 0.0 and 1.6 V was determined as optimal voltage
scale for all redox-active electrode materials so as to make an
objective capacitive performance comparison.
materials. As shown Figure 6(a–c), PTTOBu, PTTOHex, and
PTTOOct-based electrodes exhibited deviations from a typi-
cal rectangular-like shape in their I–V profiles due to Fara-
daic redox reactions as well as a pure capacitive electrical
double layer. Relative to PTTOBu and PTTOHex redox-active
materials, PTTOOct possess the larger normalized CV area
and higher redox current density. This indicates that
PTTOOct electrode material displayed better pseudocapaci-
tive behavior than PTTOBu and PTTOHex. To investigate the
effect of scan rate on CV profiles of redox materials, the
cyclic voltammograms for PTTOBu, PTTOHex, and PTTOOct
electrodes were also recorded at different scan rates ranging
from 10 to 250 mV s21. Figure 6(a–c) display PTTOBu,
PTTOHex, and PTTOOct redox-active layers still kept their I–
V profiles compared to their initial shapes and the capacitive
current densities increased as the sweep rate gradually
increased. In addition, the oxidation/reduction redox couples
for PTTOBu, PTTOHex, and PTTOOct were clearly observed
even at high scan rates. These characteristic CV responses
mean that our polymeric redox-active materials demon-
strated high-rate reversible redox capabilities and electronic
CV studies were initially carried out in 0.0 and 1.6 V poten-
tial range in order to evaluate current-potential (I–V)
responses and charge/discharge characteristics of PTTOBu,
PTTOHex, and PTTOOct polymeric films as redox electrode
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FIGURE 6 Cyclic voltammograms of (a) PTTOBu, (b) PTTOHex, and (c) PTTOOct redox-active electrode materials at various scan
rates (from 10 to 250 mV s21) in three-electrode cell configurations. [Color figure can be viewed at wileyonlinelibrary.com]
transportation abilities on the interface between conducting
polymer surface and electrolyte during the Faradaic charge/
discharge processes. The CV studies prove PTTOBu, PTTO-
Hex, and PTTOOct electrodes have good electrochemical
properties and excellent redox behaviors.
where I is the discharge current (mA), td is the discharge
time (s), DV is the potential change during the charge/dis-
charge process (Vmax-IR drop) (V), and mac is the mass of
redox-active material in a single working electrode (mg).
The GCD measurements show the PTTOOct coated electrode
material (443 F g21) delivered greatly higher specific capaci-
Further examination of the supercapacitive performances of
PTTOBu, PTTOHex, and PTTOOct electrode were investigated
by GCD measurements performed in a potential window of
1.6 V. It can be clearly seen from Figure 7(a–c) that poly-
meric redox-active electrodes exhibited linear and nearly
symmetric GCD profiles with negligible ohmic drop (IR-drop)
values at 2.5 mA cm22 constant current density. These char-
acteristic triangular GCD shapes reveal an excellent capaci-
tive behavior and highly reversible Faradaic redox property
as well as good balanced charge storage ability.
tance than PTTOBu (94.3 F g21) and PTTOHex (227.3 F g21
)
electrodes at a current density of 2.5 mA cm22. Obviously,
PTTOOct redox-active layer was able to remarkably enhance
its charge storage capacity owing to its more suitable poly-
meric network for ion movements. As expected, this morpho-
logical property most likely promoted more charge storage
by facilitating faster ion diffusion during Faradaic redox pro-
cesses. However, Faradaic redox reactions (doping/dedoping
mechanism) occurred on PTTOBu polymer chains were more
limited compared with PTTOHex and PTTOOct polymeric
films since the barrier effect caused by self-aggregation. This
effect restricted ion diffusions required for the storage of
electric charges. Thus, PTTOBu redox-active electrode mate-
rial has less storage capacity than PTTOHex and PTTOOct.
On the basis of specific capacitance values of PTTOBu,
The specific capacitances (Cspec, F g21) of PTTOBu, PTTOHex,
and PTTOOct-based electrodes in a three-electrode configura-
tion were calculated using the GCD curves according to eq 1:
Cspec5 ðI x tdÞ=ðDV x macÞ
(1)
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FIGURE 7 Typical GCD curves of (a) PTTOBu, (b) PTTOHex, (c) PTTOOct redox-active electrode materials at different current densi-
ties in a three-electrode cell configuration, and (d) specific capacitance values of PTTOBu, PTTOHex, and PTTOOct at various cur-
rent densities. [Color figure can be viewed at wileyonlinelibrary.com]
TABLE 1 Capacitive Performances of PTh Derivatives as Redox-Active Materials
Operating
Voltage
Supporting
Electrolyte
Specific
Redox-Active Material
Capacitance (Cspec
)
Ref
17
39
40
41
42
47
47
52
53
54
Poly(4,40-dipentoxy-2,20:50,200-terthiophene)
PEDOT
20.27–1.03 V 1 M Et4NBF4/PC
190 F g21
176 F g21 at 0.75 mA cm22
20.5–0.9 V
0.0–1.0V
0.1 M NaClO4
0.1 M Bu4NPF6/ACN 171 F g21 at 1 A g21
Poly(EDOT-bithiophen-EDOT)
Poly(2-(thiophene-2-yl)furan)
PEDOT/PSS film
0.1–1.1 V
20.8–0.8 V
0.1 M LiClO4/ACN
392 F g21 at 5 A g21
334 F g21 at 0.5 A g21
0.5 M Na2SO4
Poly(4-methyl-3,4-dihydro-thieno[3,4b][1,4]oxazine) 0.0–1.0 V
0.1 M Bu4NBF4/ACN 312.3 F g21 at 1 mA cm22
0.1 M Bu4NBF4/ACN 325.1 F g21 at 1 mA cm22
Poly(4-ethyl-3,4-dihydro-thieno[3,4b][1,4]oxazine)
0.0–0.85 V
20.6–0.8 V
1.15–2.0 V
20.2–0.5 V
0.0–1.6 V
0.0–1.6 V
0.0–1.6 V
Poly(thiophene)
1 M Na2SO4
1 M Et4NBF4/PC
1 M H2SO4
0.5 M Bu4NBF4/CAN 94.3 F g21 at 2.5 mA cm22
0.5 M Bu4NBF4/ACN 273.3 F g21 at 2.5 mA cm22 This work
0.5 M Bu4NBF4/ACN 443 F g21 at 2.5 mA cm22
This work
103 F g21 at 0.3 A g21
220 F g21 at 0.3 A g21
71 F g21 at 1.5 mA cm22
Poly(3-methylthiophene)
Poly(30,40-ethylenedioxy-2,20:50,200-terthiophene)
PTTOBu
PTTOHex
PTTOOct
This work
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FIGURE 8 Cyclic voltammograms of (a) Device I, (b) Device II, and (c) Device III symmetric solid-state supercapacitor cells at vari-
ous scan rates. [Color figure can be viewed at wileyonlinelibrary.com]
PTTOHex, and PTTOOct redox-active electrodes, it can be
said that the GCD measurements are in alignment with both
the CV results. The charge storage performances obtained
for PTTOBu, PTTOHex, and PTTOOct-based electrodes are
very competitive compared with many other poly(thio-
phene)-based capacitive electrode materials in the literature
(Table 1).
performances demonstrate our redox-active electrode materi-
als exhibited good charge/discharge rate capabilities even at
high current density. The high rate capability is very favor-
able feature in terms of fabricating electrode materials with
high power density for electrochemical energy storage
devices.
In addition to a three-electrode cell configuration, the capaci-
tive performances of PTTOBu, PTTOHex, and PTTOOct elec-
trodes were also examined by assembling p-p type
symmetrical solid-state supercapacitor devices with 0.5 M
Bu4NBF4/ACN organic supporting electrolyte. The PTTOBu,
PTTOHex, and PTTOOct coated SS electrodes were employed
both as anode and cathode materials for micro-
supercapacitor devices in two-electrode systems. Compared
with three-electode cell configuration, the standard two-
electrode setup makes it possible to obtain more realistic
results, for example, the specific capacitance, energy density
and power density values can be calculated for the entire
supercapacitor device in all electrochemical performance
The GCD studies were also conducted over the potential
range of 0.0 to 1.6 V at various current densities from 4.5 to
12.5 mA cm22 to assess the rate capability performances of
electrode materials. Generally, the specific capacitance of a
conducting polymer-based electrode sharply decreases when
the current density is increased owing to the limited ion dif-
fusion rates and redox reactions at high discharge currents.
But, as depicted in Figure 7(d), PTTOBu, PTTOHex, and
PTTOOct electrode materials still managed to maintain
51.3% (from 94.3 to 48.4 F g21), 50.6% (from 227.3 to 115
F g21), and 46.3% (from 443 to 205 F g21) of their initial
specific capacitance values, respectively. These rate
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FIGURE 9 GCD curves of (a) Device I, (b) Device II, (c) Device III micro-supercapacitor cells at different current densities and (d)
specific capacitance values of Device I, Device II, and Device III at various current densities. [Color figure can be viewed at wileyon-
linelibrary.com]
measurements. More importantly, the supercapacitor devices
with a two-electrode configuration can be easily adapted for
practical electrochemical energy storage applications.
shape changes were observed for Device I, Device II and
Device III micro-supercapacitors with an increasing scan rate
from 10 to 250 mV s21. These electrochemical behaviors can
be directly correlated with good rate capabilities and low
contact resistances of PTTOBu, PTTOHex, and PTTOOct poly-
meric films.
The charge storage behaviors of symmetric solid-state super-
capacitor devices (Device I for PTTOBu, Device II for PTTO-
Hex, and Device III for PTTOOct) were investigated by CV,
GCD, and EIS techniques under the same conditions as in
three-electode cell measurements. Figure 8(a–c) show the CV
curves of micro-supercapacitor devices at scan rates of 10,
25, 50, 100, 150, and 250 mV s21 within the applied poten-
tial range 0.0–1.6 V. As expected from a two-electrode cell
configuration, Device I, Device II, and Device III exhibited I–V
profiles closer to rectangular shape form compared with the
CV curves in three-electrode configuration.
GCD curves of Device I, Device II, and Device III in the
potential range between 0.0 and 1.6 V are given in Figure
9(a–c). The effect of fast ion transport kinetics and highly
reversible Faradaic redox reaction behaviors of PTTOBu,
PTTOHex, and PTTOOct-based electrode materials openly
appeared in the GCD cycle characteristics of micro-
supercapacitor devices. Device I, Device II, and Device III dis-
played a typical triangular GCD profiles at a constant current
density of 2.5 mA cm22. The small ohmic drops (IR-drop)
were also determined to be 0.08, 0.06, and 0.05 V for Device
I, Device II, and Device III, respectively. This result confirms
that PTTOBu, PTTOHex, and PTTOOct redox-active electrode
materials have low internal resistances.
From Figure 8(a–c), it is clearly seen that both the CV cur-
rent and area under the voltammograms of Device III are
significantly higher than those of Device I and Device II, indi-
cating better capacitive nature of PTTOOct. Moreover, no
12
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TABLE 2 Specific Capacitance, Energy Density, and Power Density Values of Solid-State Micro-Supercapacitor Devices
Micro-supercapacitor
device
Specific capacitance
(Cspec,) F g21
Energy density
(E) W h kg21
Power density
(P) kW kg21
Device I
Device II
Device III
42
13.5
29.3
60.7
0.98
1
89
182
1.1
The values of specific capacitance (Cspec, F g21), energy den-
sity (E, W h kg21), and power density (P, kW kg21) of
micro-supercapacitor devices were calculated using GCD
curves at 2.5 mA cm22 from the following eqs (2–4):
P5 ð3600 x EÞ=td
(4)
where I is the discharge current (mA), td is the discharge
time (s), DV is the potential change during the discharge
cycle (Vmax-IR drop) (V), and mac is the total mass of redox-
active material in two electrodes (mg).
C
spec5 ðI x tdÞ=ðDV macÞ
(2)
(3)
h
i
2
E5 Cspecx ðDVÞ =7:2
As presented in Table 2, Device I, Device II, and Device III
reached up to 42, 89, and 182 F g21 specific capacitance
FIGURE 10 Long-term cycling stabilities of (a) Device I, (b) Device II, and (c) Device III at 2.5 mA cm22 constant current density for
10,000 charge/discharge cycles. [Color figure can be viewed at wileyonlinelibrary.com]
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FIGURE 11 (a) Comparative Nyquist plots of supercapacitor devices with a frequency range from 104 to 1022 Hz and voltage ampli-
tude of 5 mV rms at 0.0 V DC voltage and (b) comparative Bode-phase angle plots of Device I, Device II, and Device III micro-
supercapacitors. [Color figure can be viewed at wileyonlinelibrary.com]
values. On the other side, Device I, Device II, and Device III Finally, EIS technique was employed to analyze electronic
delivered energy densities of 13.5, 29.3, and 60.7 W h kg21 properties of Device I, Device II, and Device III such as inter-
and power densities of 0.98, 1, and 1.1 kW kg21, respectively. nal resistance (ESR), charge transfer resistance (RCT), and
Figure 9(a–c) indicates that Device III had the longest dis- ion diffusion kinetics and rates. EIS spectra were recorded
charging duration than Device I and Device II, which implies using a perturbation amplitude of 5 mV rms with a fre-
more suitable morphology of PTTOOct redox-active layer. The quency range from 104 to 1022 Hz at an applied DC voltage
rate capabilities of micro-supercapacitor devices were of 0.0 V. As displayed in Figure 11(a), all Nyquist plots are
explored by recording the GCD curves at various current densi- composed of a semi-circle pattern in the high-frequency
ties of 4.5, 6.5, 8.5, 10.5, and 12.5 mA cm22. Device I, Device II, region and a straight line portion in the low-frequency
and Device III retained 50.7, 50.9, and 41% of their gravimetric region, which is characteristic impedance response for super-
capacitances, respectively, when current density was capacitors based on conducting polymers.
increased, as seen from Figure 9(d). The electrochemical char-
acterization data collected for Device I, Device II, and Device III
agree well with the CV and GCD results of redox-active electro-
des obtained in a three-electrode cell configurations.
In the high-frequency region, the ESR of Device I, Device II,
and Device III were determined to be 5.9, 7.1, and 6.5 X,
respectively, from the intercept point of Zꢁ-axis and the start-
ing point of semi-circle. ESR is described as the combined
For practical supercapacitor applications, long-term cycling resistance involving the ionic resistance of electrolyte, the
stability is another important performance parameter as well electronic resistance of the electroactive material and the
as specific capacitance, energy and power density and rate contact resistance between electrode material and current
capability. The cycling stability performance of our symmet- collector. In this context, these low ESR values of micro-
rical micro-supercapacitor devices were evaluated using a supercapacitor devices stand for PTTOBu, PTTOHex, and
typical GCD measurement at a constant current density of PTTOOct polymeric networks possess low internal resis-
2.5 mA cm22 between 0.0 and 1.6 V for 10 000 charge/dis- tance, indicating reasonable electronic property for solid-
charge cycles. The variations of specific capacitances in state practical supercapacitor applications. In addition, the
Device I, Device II, and Device III as a function of GCD cycles charge transfer resistance (RCT) values were directly
are depicted in Figure 10(a–c). At the end of 10,000 cycles, obtained by the diameter of semi-circle in the high-
Device I, Device II, and Device III exhibited 78, 68, and 63% frequency region. Figure 11(a) demonstrate that Device III
capacitance retentions, repectively. The cycling stability tests has a smaller radius semi-circle pattern (RCT 5 1.6 X) than
clearly reveal that PTTOOct redox-active surface (Device III) those of Device I (RCT 5 20 X) and Device II (RCT 5 6.3 X).
could tolerate the volumetric surface degradation less than This means that PTTOOct polymer film was uniformly
PTOOBu (Device I) and PTTOHex (Device II). This situation coated on the SS current collector and the interaction
is probably due to the more Faradaic reactions occurring on between PTTOOct redox-active network and SS interface
PTTOOct polymeric network during long-term charge/dis- was stronger compared with PTTOBu and PTTOHex poly-
charge cycling.
meric structures.
14
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In the low-frequency region, each Nyquist plot exhibited a
straight line, Warburg portion, toward the Z0 real axis with
an increase in the imaginary part (–Z00-axis). As can be seen
from Figure 11(a), the Warburg regions of Device I and
Device II are longer than that of Device III. This difference
between impedance characteristics is thought to be a result
of relatively slower ion diffusion rates and longer diffusion
pathways in the polymeric networks of PTTOBu and PTTO-
Hex. The more suitable 3D-dimensional morphology of
PTTOOct conducting film helped to improve ion movements
in Device III by offering sufficient and shorter diffusion path
lengths compared with the morphologies of PTTOBu (Device
I) and PTTOHex (Device II). Moreover, the Warburg region of
Device III has also a steeper slope than those of Device I and
Device II. This means that an electrochemical double-layer
was formed faster on PTTOOct surface and it had a better
capacitive performance under the same condition. In addi-
tion to Nyquist plots, the Bode-phase angle plots were also
recorded in the frequency range of 104 to 1022 Hz [Fig.
11(b)]. In the Bode plots, the phase angles of Device I,
Device II, and Device III corresponded to 273.88, 277.28,
and 280.48 at 1022 Hz. According to Bode-phase angle anal-
yses, it can be said that Device III has a better pseudocapaci-
tive performance than Device I and Device II since the
capacitive behavior in the low-frequency region is usually
determined with the phase angle approaching 2908. These
experimental data also prove that PTTOBu, PTTOHex, and
PTTOOct exhibit good electrochemical capacitive properties
and claim potential application as redox-active electrode
materials in energy storage devices.
retentions upon long-term charge/discharge cycling after
10,000 cycles. In this context, the capacitive performances of
PTTOBu, PTTOHex, and PTTOOct were found to be compara-
ble to various polymeric redox electrode materials such as
PTh,
PPy,
and
PANI
derivatives,
previously
reported.17,39–45,47,52–57 The results reveal that molecular
design strategy could be beneficial for the preparation of
novel psuedocapacitive electrode materials with controllable
morphology and improved performance.
ACKNOWLEDGMENTS
The authors thank the Scientific and Technological Research
€ _
Council of Turkey (TUBITAK, Grant No: KBAG-114Z167) for
ꢀ
financial support. D. Yigit is also generously supported by
€ _
TUBITAK KBAG-114Z167 with a postdoctoral scholarship.
€
They also would like to thank Elif Akhuseyin Yıldız for her
assistance with the SEM observations.
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