Design and synthesis of novel electroactive 2,2′:5′,2″-terthiophene monomers including oxyethylene chains for solid-state flexible energy storage applications

Article abstract of DOI:10.1016/j.electacta.2021.138662

Here, we present the synthesis of novel poly(2,2′:5′,2″-terthiophene) derivatives containing oxyethylene pendant groups for the fabrication of high performance flexible redox-active electrode materials. The poly(3′,4′-bis(2-methoxyethoxy)-2,2′:5′,2″-terthiophene) (PSEDEN1), poly(3′,4′-bis(2-(2-methoxyethoxy)ethoxy)-2,2′:5′,2″-terthiophene) (PSEDEN2) and poly(3′,4′-bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)-2,2′:5′,2″-terthiophene) (PSEDEN3) have been electrochemically polymerized on flexible stainless steel substrates without any binder and directly employed as redox-active materials. The effect of pendant group chain length on morphological characteristics of conducting polymer films have been systematically evaluated and correlated to the charge storage properties of redox-active electrode materials. Capacitive performance tests reveal that PSEDEN1, PSEDEN2 and PSEDEN3 could reach up to specific capacitances of 135 F g^?1, 212.8 F g^?1 and 403.3 F g^?1, respectively, at constant current density of 2.5 mA cm^?2 in the potential range of 0.4–1.8 V with good rate capability performances. In addition, symmetrical flexible solid-state supercapacitor devices based on polymer gel electrolyte have also been assembled using PSEDEN1, PSEDEN2 and PSEDEN3 coated flexible stainless steel substrates and tested by cyclic voltammetry, galvanostatic charge/discharge and electrochemical impedance spectroscopy techniques in detail. Fabricated devices (Cell 1, Cell 2 and Cell 3) have delivered maximum specific capacitances of C_spec= 29.3 F g^?1, 92.1 F g^?1 and 162.4 F g^?1, energy densities of SE= 6.35 W h kg^?1, 22.9 W h kg^?1 and 41.1 W h kg^?1 and power densities of SP= 929 W kg^?1, 937.7 W kg^?1 and 986.4 W kg^?1 at a current density of 2.5 mA cm^?2 in two-electrode cell configuration. Furthermore, flexible supercapacitor devices have achieved high cycle life performances with good capacitance retention values of 80.2%, 84.7% and 91.4% over 10 000 consecutive galvanic charge/discharge cycles at 2.5 mA cm^?2 constant current density from 0.4 to 1.8 V. Similarly, excellent mechanical stabilities have also been observed with 3.4%, 4.66% and 1.97% capacitance losses under various bending conditions from 0° to 170° for all flexible supercapacitor devices. These results confirm that PSEDEN1, PSEDEN2 and PSEDEN3 redox-active materials with gratifying capacitive performances and excellent flexibilities have a great potential for utilization in innovative flexible or wearable energy storage solutions. Keywords

Full text of DOI:10.1016/j.electacta.2021.138662

Electrochimica Acta 389 (2021) 138662
Contents lists available at ScienceDirect
Electrochimica Acta
journal homepage: www.elsevier.com/locate/electacta
ꢀ
ꢀ
ꢀꢀ
Design and synthesis of novel electroactive 2,2 :5 ,2 -terthiophene
monomers including oxyethylene chains for solid-state flexible energy
storage applications
Deniz Yig˘ita,∗, Mustafa Güllüb
a Department of Basic Pharmaceutical Sciences, Faculty of Pharmacy, Lokman Hekim University, Sög˘ütözü, Ankara 06510, Turkey
b Department of Chemistry, Faculty of Science, Ankara University, Bes¸evler, Ankara 06100, Turkey
a r t i c l e i n f o
a b s t r a c t
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Article history:
Here, we present the synthesis of novel poly(2,2 :5 ,2 -terthiophene) derivatives containing
oxyethylene pendant groups for the fabrication of high performance flexible redox-active elec-
Received 6 January 2021
Revised 7 May 2021
ꢀ
ꢀ
ꢀ
ꢀ
(PSEDEN2)
ꢀꢀ
ꢀ
poly(3 ,4 -bis(2-(2-(2-
ꢀ
trode materials. The poly(3 ,4 -bis(2-methoxyethoxy)-2,2 :5 ,2 -terthiophene) (PSEDEN1), poly(3 ,4 -
Accepted 15 May 2021
Available online 25 May 2021
ꢀ
ꢀ
ꢀꢀ
ꢀ ꢀ
bis(2-(2-methoxyethoxy)ethoxy)-2,2 :5 ,2 -terthiophene)
and
ꢀ
ꢀ
ꢀꢀ
methoxyethoxy)ethoxy)ethoxy)-2,2 :5 ,2 -terthiophene) (PSEDEN3) have been electrochemically poly-
merized on flexible stainless steel substrates without any binder and directly employed as redox-active
materials. The effect of pendant group chain length on morphological characteristics of conducting
polymer films have been systematically evaluated and correlated to the charge storage properties
of redox-active electrode materials. Capacitive performance tests reveal that PSEDEN1, PSEDEN2 and
PSEDEN3 could reach up to specific capacitances of 135 F g−1, 212.8 F g−1 and 403.3 F g−1, respectively,
at constant current density of 2.5 mA cm−2 in the potential range of 0.4–1.8 V with good rate capability
performances. In addition, symmetrical flexible solid-state supercapacitor devices based on polymer gel
electrolyte have also been assembled using PSEDEN1, PSEDEN2 and PSEDEN3 coated flexible stainless
steel substrates and tested by cyclic voltammetry, galvanostatic charge/discharge and electrochemical
impedance spectroscopy techniques in detail. Fabricated devices (Cell 1, Cell 2 and Cell 3) have delivered
maximum specific capacitances of Cspec= 29.3 F g−1, 92.1 F g−1 and 162.4 F g−1, energy densities of SE=
6.35 W h kg−1, 22.9 W h kg−1 and 41.1 W h kg−1 and power densities of SP= 929 W kg−1, 937.7 W kg−1
and 986.4 W kg−1 at a current density of 2.5 mA cm−2 in two-electrode cell configuration. Furthermore,
flexible supercapacitor devices have achieved high cycle life performances with good capacitance reten-
tion values of 80.2%, 84.7% and 91.4% over 10 000 consecutive galvanic charge/discharge cycles at 2.5 mA
cm−2 constant current density from 0.4 to 1.8 V. Similarly, excellent mechanical stabilities have also been
observed with 3.4%, 4.66% and 1.97% capacitance losses under various bending conditions from 0° to
170° for all flexible supercapacitor devices. These results confirm that PSEDEN1, PSEDEN2 and PSEDEN3
redox-active materials with gratifying capacitive performances and excellent flexibilities have a great
potential for utilization in innovative flexible or wearable energy storage solutions.
2,2:´5,2´-terthiophene-based electroactive
´
´
monomers
Poly(2,2:´5,2´-terthiophene)s
´
´
Redox-active electrodes
Symmetric solid-state supercapacitors
Flexible supercapacitor devices
Keywords
© 2021 Elsevier Ltd. All rights reserved.
1. Introduction
even though they have generally lower energy densities. Because
of these more attractive properties, there has been more and more
Supercapacitors are new generation high-performance electro-
chemical energy storage systems that can be used in wide range
from electric hybrid vehicles to portable consumer electronics.
With respect to batteries, supercapacitors posses superior features
such as rapid charging/discharging time, long cycle life, high charge
capacity, enviromentally friendly and satisfying operational safety
intensive interest in supercapacitor applications, recently [1–3].
Different from conventional dielectric capacitors, charge stor-
age behavior of the supercapacitors is directly linked to features of
electrode materials. When electrodes are polarized with a bias po-
tential, opposite charges are stored at the electrode/electrolyte in-
terface. Therefore, the physical and electrochemical features of the
electrode material, such as active surface area, surface morphology
and electrical conductivity, are major parameters that play a criti-
cal role in capacitive performance [4–6].
∗
Corresponding author.
E-mail address: deniz.yigit@lokmanhekim.edu.tr (D. Yig˘it).
https://doi.org/10.1016/j.electacta.2021.138662
0013-4686/© 2021 Elsevier Ltd. All rights reserved.

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Carbonaceous meterials, π-conjugated electrically conducting
polymers and transition metal oxides are the most commonly
used electrode materials for supercapacitor applications. Carbona-
ceous meterials, including CNT, CNF and graphene, store and re-
lease charges at the interface between a porous electrode material
and the electrolyte through electrostatic interactions (adsorption-
desorption) so-called as non-Faradaic processes. Even though
carbon-based substrates are attracting considerable interest as
suitable electrode materials for electrochemical energy storage ap-
plications because of their high specific surface area, highly porous
morphology, better rate capability and excellent operation life,
their solely physical charge-storage mechanism (electrical double-
layer capacitance) leads to limited energy density and capacitive
performance [5–14]. As being another class of capacitive electrode
materials, metal oxides (RuO2, V2O5, MnO2, TiO2,SnO2 and NiO)
[15–32] and electrically conducting polymers (ECPs) [33–35] use
a Faradaic process based on fast and reversible redox reactions
for charge storage as well as non-Faradaic processes. Metal oxides
and ECPs, also known as redox-active pseudocapacitive materials,
achieve a greater amount of charge storage and deliver higher en-
ergy denstiy than carbonaceous meterials due to the fact that a
Faradaic redox process occurs not only on the surface but also in
the matrix of electroactive materials. On the other hand, conduct-
ing polymer-based electroactive materials often suffer poor cycle
life caused by physical degradation while many transition metal
oxides have low electrical conductivity and limited electron trans-
fer ability [35–40]. Notwithstanding the common disadvantage of
poor cycle life, ECPs as redox-active pseudocapacitive materials
have attracted tremendous attention for high performance super-
capacitor applications in recent years owing to their higher redox
capacitance, rapid Faradaic charge transfer ability, excellent flexi-
ble nature, low fabrication cost and unlimited structural modifi-
cation possibilities [41–43]. Particularly, with rapid and competi-
tive developments in the field of portable, flexible and wearable
consumer electronics, many researchers have focused on desinging
novel conducting polymer derivatives that can meet requirements
of new generation energy storage solutions.
allows to form a homogeneous polymeric film covering the whole
electrode material and to prepare thin and flexible redox-active
surfaces with low internal resistance including interfacial contact
resistance between the current collector and electroactive mate-
rial [59,71]. The electropolymerization method also offers obtain-
ing of different morphological structures with the same electroac-
tive material since electropolymerization conditions such as poly-
merization method and type of dopant ion directly affect the mor-
phological characteristics of the conducting polymer film [72]. In
terms of storing electrical charges and reversible ion transport in
the polymer matrix, the morphology is considered to be a pri-
mary parameter [73]. On the other hand, substituent changes on
the polymer chain have a pronounced effect on morphology as
much as the electropolymerization method. Our recent studies
have shown that the altering of pendant groups on the conducting
polymer backbone created remarkable differences in morphologi-
cal structure of the polymeric networks. Redox-active films based
on poly(terthiophene) and poly(3,6-dithienylcarbazole) containing
butyl–, hexyl- and octyl- pendant groups exhibited different mor-
phological features with increase in the alkyl chain length. De-
pending on the morphological structure, these polymeric electrode
materials have reached a wide range of specific capacitance values
(F g−1) with an equal amount of redox-active substrate under the
same conditions [74,75].
In the present study, novel conducting polymer films based on
poly(terthiophene) containing oxyethylene pendant groups were
synthesized by one-step electrochemically deposition method
on stainless steel substrates without any binder and the effect
of substituent chain length on capacitive performances of the
conducting polymer films was systematically evaluated. In the
light of our previous reports, it is known that it is possible
to create morphological diversity for redox-active materials by
changing the length of the substituents on conducting polymer
structure [74,75]. With a similar approach, it was aimed to obtain
different energy storage performances for poly(terthiophene)-
based pseudocapacitive redox-active materials having the same
π-conjugated polymer backbones, only depending on the mor-
phological structures. Furthermore, poly(terthiophene) derivatives
were electrochemically coated on stainless steel mesh current
collector to create a polymeric network with highly accessible
electroactive surface area. Thus, it was expected to increase
dopant ion movements on redox-active matrix and enhance ca-
pacitive performance of electrode materials by creating efficient
diffusion pathways or channels. Pseudocapacitive characteristics
Polyaniline (PANI) [36,44–51], polypyyrole (PPy) [42,52–
58],
polythiophene
(PT)
[35,37,59–65]
and
poly(3,4-
ethylenedioxythiophene) (PEDOT) [39,66–68] and their derivatives
have been widely studied for both fundamental energy storage and
supercapacitor device applications. These redox-active electrode
materials based on PPy, PANI, PT and PEDOT have exhibited redox
capacitance values between 22 and 950 F g−1 under the different
measurement conditions (2- and 3-electrode cell configuration).
Recently, novel poly(3,6-dithienylcarbazole)-based conducting
polymer films with high capacitive performances have been syn-
thesized in our research group and better mechanical strength
properties have been observed in micro-supercapacitor devices.
The N-substituted poly(3,6-dithienylcarbazole) redox-active mate-
rials have delivered high specific capacitances (Cspec= 554–640 F
ꢀ
ꢀ
ꢀ
ꢀ ꢀꢀ
and performances of poly(3 ,4 –bis(2-methoxyethoxy)−2,2 :5 ,2 -
ꢀ ꢀ
poly(3 ,4 –bis(2-(2-methoxyethoxy)
terthiophene)
ꢀ
(PSEDEN1),
ꢀ
ꢀꢀ
ꢀ ꢀ
ethoxy)−2,2 :5 ,2 -terthiophene) (PSEDEN2) and poly(3 ,4 –bis(2-
ꢀ
ꢀ ꢀꢀ
(2-(2-methoxyethoxy)ethoxy)ethoxy)−2,2 :5 ,2 -terthiophene)
(PSEDEN3) were evaluated by cyclic voltammetry and galvanos-
tatic charge/discharge techniques with a standard three-electrode
cell configuration. Furthermore, p-p type symmetrical flexible
supercapacitor devices based on a polymer gel electrolyte were
also assembled using PSEDEN1, PSEDEN2 and PSEDEN3 coated
redox-active electrode materials. Two-electrode supercapacitor
device performances of flexible supercapacitors were examined
under various bending angles by cyclic voltammetry, galvanostatic
charge/discharge and electrochemical impedance spectroscopy
measurements.
g
−1). Moreover, poly(3,6-dithienylcarbazole)-based electrode mate-
rials exhibited outstanding long-term cycle life stabilities with up
to 93% capacitance retention values over 10,000 charge/discharge
cycles [69].
ECPs-based electrode materials are generally prepared by using
two different procedures [70]. In one of these processes, a con-
ducting polymer derivative is physically mixed with a polymeric
binder and other additives, and then a slurry mixture is applied
to the current collector by a typically solution-blending method.
Even though this process seems like a simple method for large-
scale fabrications, it leads to a considerable increase in the in-
ternal resistance of electroactive materials due to insulator poly-
meric binders. In the other and most common preparation method,
CP films are electrochemically synthesized directly onto any sub-
strate without using a polymeric binder. This one-step technique
2. Experimental
2.1. Materials and instrumentation
ꢀ
ꢀ
all
ꢀ
ꢀ ꢀꢀ
2-
For the synthesis of 3 ,4 -bis(2-methoxyethoxy)−2,2 :5 ,2 -
terthiophene
monomers,
chemicals
including
(tributylstannyl)thiophene,
lithium
perchlorate,
and
2

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
tetrakis(triphenylphosphine)palladium (0) were purchased from
Sigma-Aldrich and used directly without any purification. The flex-
ible mesh stainless steel sheet (FSS) with a thickness of 0.15 mm
was specially prepared with dimensions of 1 cm width and 1 cm
length to be used as a current collector. Acetonitrile (ACN) was
freshly prepared by fractional destillation in the presence of P2O5
before use. Lithium perchlorate (LiClO4) was kept in an oven at
80 °C for 4 h before each electrochemical experiment (Note: Drying
of LiClO4 is a completely safe procedure). The silver wire (Ag/Ag+)
pseudo-reference electrode was calibrated using ferrocene redox
J = 2 Hz, 2H, Th-H), 7.22 (dd, J = 6.8 Hz and J = 1.2 Hz, 2H,
Th-H), 7.28 (dd, J = 7.8 Hz and J = 3.2 Hz, 2H, Th-H). 13C NMR
(400 MHz, CDCl3, 25 °C, TMS): δ/ ppm= 59 (-CH3), 70.2 (-CH2-),
70.4 (-CH2-), 70.5 (-CH2-), 71.9 (-CH2-), 97.8 (Th-C5), 124.3 (Th-C7),
124.6 (Th-C9), 127 (Th-C8), 141.6 (Th-C6), 147 (Th-C4). IR (ATR) ʋ/
cm−1 3095 (m, aromatic C H strecthing), 2934, 2869 (m, aliphatic
–
–
C H strecthing), 1634 (m, aromatic C=C strecthing), 1445, 1365,
–
1277 (m, aliphatic C H bending), 1200, 1110 (s, -C-O- strecthing).
MS (70 eV): m/z (%): 484.4 (20) [M+, C22H28O6S3, 484.6], 425.3
(30), 403.4 (100), 343.4 (20), 279.4 (60), 225.2 (12), 179.1 (25).
Elemental analysis: anal. calcd. for C22H28O6S3 (484.6): C 54.52, H
5.82; found. C 54.73, H 5.44.
couple (Fe/Fe+) (E
(Fe/Fe+)= 0.3 V) [76].
1/2
CEM Discover S-Class single-mode microwave instrument was
used for microwave-supported synthetic procedures. 1H NMR and
13C NMR spectra were collected on a Varian-Mercury 400 MHz
digital Fourier-transform (FT) NMR spectrometer in deuterated
chloroform with TMS as an internal standard. FTIR and mass spec-
tra were recorded with a Perkin Elmer Spectrum 100 spectrometer
(crystal plate ATR apparatus) and a Waters 2695 Alliance Micro-
mass ZQ LC/MS using a direct inlet probe, respectively. Scanning
electron microscopy (SEM) images of conducting polymer films
were obtained using a Zeiss Ultra Plus FE-SEM and EVO 40 500 V
to 30 kV instrument. Electrochemical characterization studies and
pseudocapacitive performance measurements were conducted with
a Radiometer VoltaLab PST050 potentiostat/galvanostat-high volt-
age booster 100 V HVB100 and a Princeton Applied Research PAR-
2273 potentiostat/galvanostat.
ꢀ
ꢀ
ꢀ
ꢀ ꢀꢀ
3 ,4 -Bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)−2,2 :5 ,2 -
terthiophene) (SEDEN3): Light brown oily product, 79%. 1H NMR
(400 MHz, CDCl3, 25 °C, TMS): δ/ ppm= 3.38 (s, 6H, -CH3), 3.51
– 3,65 (m, 16H, -CH2-), 3.76 (t, J = 4.8 Hz, 4H, -CH2-), 4.28 (t,
J = 4.8 Hz, 4H, -CH2-), 7.01 (dd, J = 7.2 Hz and J = 2 Hz, 2H,
Th-H), 7.22 (d, J = 6.8 Hz, 2H, Th-H), 7.28 (d, J = 7.8 Hz, 2H,
Th-H). 13C NMR (400 MHz, CDCl3, 25 °C, TMS): δ/ ppm= 66.6
(-CH3), 69.77 (-CH2-), 70.47 (-CH2-), 70.54 (-CH2-), 70.57 (-CH2-),
71.89 (-CH2-), 117.84 (Th-C5), 123.44 (Th-C7), 124.64 (Th-C9),
126.98 (Th-C8), 134.24 (Th-C6), 144.56 (Th-C4). IR (ATR) ʋ/ cm−1
–
2943, 2885 (m, aliphatic C H strecthing), 1647 (m, aromatic C=C
–
strecthing), 1438, 1356, 1259 (m, aliphatic C H bending), 1223,
1118 (s, -C-O- strecthing). MS (70 eV): m/z (%): 572.3 (12) [M+,
C26H36O8S3, 572.7], 513 (18), 445.4 (5), 341.3 (12), 291.4 (14), 279.2
(17), 235.2 (18), 215 (100), 193.3 (33). Elemental analysis: anal.
calcd. for C26H36O8S3 (572.7): C 54.52, H 6.34; found. C 54.29, H
6.67.
ꢀ
ꢀ ꢀꢀ
2.2. Synthesis of 2,2 :5 ,2 -terthiophene monomers (SEDEN1, SEDEN2
and SEDEN3)
ꢀ
ꢀ ꢀꢀ
The synthesis of novel electroactive 2,2 :5 ,2 -terthiophene
monomers including oxyethylene chains were performed through
a multi-step process. Firstly, diethyl 3,4-dihydroxythiophene-2,5-
dicarboxylate was derivatized with homologous pendant groups
of oxyethylene chain structure by a typical nucleophilic substi-
2.3. Electrochemical characterization of monomers and conducting
polymer films
ꢀ
ꢀ
ꢀꢀ
The electrochemical properties of 2,2 :5 ,2 -terthiophene
ꢀ
ꢀ
monomers were investigated by cyclic voltammetry (CV).
A
tution reaction under microwave irradiation. Subsequently, 3 ,4 -
bis(2-methoxyethoxy)thiophene derivatives were modified for cou-
pling reactions utilizing hydrolysis, decarboxylation and bromina-
standard three-electrode configuration was set up in a 0.1 M
LiClO4/ACN supporting electrolyte system at 25 °C under a ni-
trogen atmosphere for CV studies. Platinum discs were used as
the working electrode and counter electrode while a silver wire
was employed as a Ag/Ag+ pseudo-reference electrode. The cyclic
voltammograms were recorded at 150 mV s−1 scan rate in the
potential range from 0.0 to 2.0 V. Following the determination of
monomer oxidation potentials, conducting polymer film-coated
platinum disk electrodes were rinsed with ACN. The modified
platinum disk electrodes were thereafter subjected to single scan
cyclic voltammetry in monomer-free 0.1 M LiClO4/ACN solution to
examine redox behaviors and CV responses of conducting polymer
films (PSEDEN1, PSEDEN2 and PSEDEN3). The single scan voltam-
mograms were recorded between 0.0 and 2.0 V potential scale at
150 mV s−1 scan rate using a standard three-electrode system. The
electrochemical reversibility and stability of all conducting poly-
mer films were evaluated by cyclic voltammetry studies performed
at a scan rate of 25 mV s−1 for 100 cycles in monomer-free 0.1 M
LiClO4/ACN solution.
ꢀ
ꢀ
ꢀ
ꢀ ꢀꢀ
tion reactions, respectively. 3 ,4 -Bis(2-methoxyethoxy)−2,2 :5 ,2 -
terthiophene monomers, SEDEN1, SEDEN2 and SEDEN3, were fi-
nally obtained using the palladium-catalyzed Stille cross-coupling
reactions (Scheme 1). The chemical structures of SEDEN1, SEDEN2
and SEDEN3 were confirmed by FTIR, 1H NMR, 13C NMR, mass
spectroscopy and elemental analysis techniques. The synthetic pro-
cedures and spectral data of intermediates and monomers can be
found in the Supporting Information.
ꢀ
ꢀ
ꢀ
ꢀ ꢀꢀ
3 ,4 -Bis(2-methoxyethoxy)−2,2 :5 ,2 -terthiophene)
(SEDEN1):
Pale yellow oily product, 91%. 1H NMR (400 MHz, CDCl3, 25 °C,
TMS): δ/ ppm= 3.42 (s, 6H, -CH3), 3.76 (t, J = 5.2 Hz, 4H, -CH2-),
4.13 (t, J = 4.8 Hz, 4H, -CH2-), 7.0 (dd, J = 7.2 Hz and J = 2 Hz, 2H,
Th-H), 7.23 (d, J = 6.8 Hz, 2H, Th-H), 7.27 (d, J = 7.8 Hz, 2H, Th-H).
13C NMR (400 MHz, CDCl3, 25 °C, TMS): δ/ ppm= 59.14 (-CH3),
69.72 (-CH2-), 70.80 (-CH2-), 97.93 (Th-C5), 123.38 (Th-C7), 124.49
(Th-C9), 126.79 (Th-C8), 141.76 (Th-C6), 147.16 (Th-C4). IR (ATR) ʋ/
cm−1 2946, 2875 (m, aliphatic C H strecthing), 1632 (m, aromatic
–
–
C=C strecthing), 1443, 1352, 1275 (m, aliphatic C H bending),
1218, 1112 (s, -C-O- strecthing). MS (70 eV): m/z (%): 396.7 (3)
[M+, C18 H20O4S3, 396.5], 337.3 (43), 279 (10), 255.1 (100), 242.4
(70), 233.3 (30), 175.1 (11), 142.9 (8). Elemental analysis: anal.
calcd. for C18 H20O4S3 (396.5): C 54.52, H 5.08; found. C 54.85, H
5.37.
2.4. Preparation and characterization of flexible redox-active
electrode materials
PSEDEN1, PSEDEN2 and PSEDEN3 conducting polymer films
were directly deposited on current collectors by a constant po-
tential electrolysis technique. All constant potential electrolyses
were carried out with an 3-electrode setup consisting of a flexible
mesh stainless steel working electrode (FSS), a platinum counter
ꢀ
ꢀ
ꢀ
ꢀ ꢀꢀ
3 ,4 -Bis(2-(2-methoxyethoxy)ethoxy)−2,2 :5 ,2 -terthiophene)
(SEDEN2): Greenish-yellow oily product, 82%. 1H NMR (400 MHz,
CDCl3, 25 °C, TMS): δ/ ppm= 3.36 (s, 6H, -CH3), 3.54 (t, J = 5.4 Hz,
4H, -CH2-), 3.65 (t, J = 5.6 Hz, 4H, -CH2-), 3.81 (t, J = 5.4 Hz,
4H,-CH2-), 4.29 (t, J = 4.8 Hz, 4H, -CH2-), 7.0 (dd, J = 7.2 Hz and
electrode and
a M
Ag/Ag+ pseudo-reference electrode in 0.1
3

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Scheme 1. Synthetic pathway for 2,2:5,2-terthiophene electroactive monomers, SEDEN1, SEDEN2 and SEDEN3.
LiClO4/ACN. PSEDEN1, PSEDEN2 and PSEDEN3-based redox-active
electrode materials were prepared by applying a constant poten-
tial (1.72 V for SEDEN1, 1.80 V for SEDEN2 and 1.83 V for SEDEN3)
to mesh stainless steel electrodes at 225 mC cm−2 charge den-
sity for 115 s in the presence of 0.045 M standard monomer con-
centration. After each deposition, polymer coated FSS electrodes
were electrochemically reduced at −0.40 V for 120 s in monomer-
free supporting electrolyte solution so as to provide the electrical
charge balance by expelling trapped perchlorate dopant ions on the
polymer network. Afterwards, modified FSS electrodes were rinsed
with ACN and they were dried at 50 °C for 2 h under vacuum at-
mosphere. The resulting dried polymeric mass on FSS substrates
was estimated by weight with a microanalytical balance (ꢀm =
0.001 mg). Five parallel weightings were executed for each elec-
trode material and the average of five weightings was accepted as
the redox-active material mass for charge storage measurements.
The average conducting polymer loading on modified electrode
materials was measured to be 0.84 mg cm−2 for PSEDEN1, 0.86 mg
cm−2 for PSEDEN2 and 0.87 mg cm−2 for PSEDEN3.
The chemical structures of PSEDEN1, PSEDEN2 and PSEDEN3
redox-active films on FSS substrates were characterized by FTIR
technique. The morphological properties of PSEDEN1, PSEDEN2 and
PSEDEN3 conducting polymer films were also visualized by scan-
ning electron microscopy (SEM) technique.
ꢀ
ꢀ
ꢀ
ꢀ
ꢀꢀ
Poly(3 ,4 -bis(2-methoxyethoxy)−2,2 :5 ,2 -terthiophene)
(PSEDEN1):
IR (ATR) ʋ/ cm−1 3096 (w, aromatic C H strecthing), 2932, 2868
–
–
(w, aliphatic C H strecthing), 1642 (m, polyconjucated -C=C- strec-
–
thing), 1450, 1365, 1256 (m, aliphatic C H bending), 1220, 1115 (s,
-C-O- strecthing).
ꢀ
ꢀ
ꢀ
ꢀ ꢀꢀ
Poly(3 ,4 -bis(2-(2-methoxyethoxy)ethoxy)−2,2 :5 ,2 -
terthiophene) (SEDEN2):
4

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 1. Schematic illustration for fabrication procedure of symmetric type flexible solid-state supercapacitor devices, Cell1, Cell2 and Cell3.
IR (ATR) ʋ/ cm−1 3092 (w, aromatic C H strecthing), 2941, 2873
CV and GCD techniques, respectively, at bending angles of 0˚, 45°,
90°, 135°, 170° The long-term cycling performances of supercapac-
itor devices were evaluated with a standard GCD test under at
2.5 mA cm−2 constant current density for 10 000 charge/discharge
cycles. The electrochemical impedance spectroscopy (EIS) tests
were conducted in the frequency range of 10 kHz to 0.01 Hz at
0.0 V DC applied voltage using a voltage amplitude of 5 mV rms.
Specific capacitance (Cspec), specific energy (SE) and specific power
(SP) were calculated based on GCD curves using the following
equations:
–
–
(w, aliphatic C H strecthing), 1645 (m, polyconjucated -C=C- strec-
–
thing), 1441, 1382, 1270 (m, aliphatic C H bending), 1210, 1112 (s,
-C-O- strecthing).
ꢀ
ꢀ
ꢀ
ꢀ ꢀꢀ
Poly(3 ,4 -bis(2-(2-(2-methoxyethoxy)ethoxy)ethoxy)−2,2 :5 ,2 -
terthiophene) (SEDEN3):
IR (ATR) ʋ/ cm−1 3089 (w, aromatic C H strecthing), 2945, 2880
–
–
(w, aliphatic C H strecthing), 1646 (m, polyconjucated -C=C- strec-
–
thing), 1432, 1362, 1261 (m, aliphatic C H bending), 1221, 1116 (s,
-C-O- strecthing).
Eq. (1). Cspec = (I x td) / (ꢀV x mac
)
Eq. (2). SE = [(Cspec) x (ꢀV)2] / 7.2
2.5. Symmetric type flexible solid-state supercapacitor cell fabrication
Eq. (3). SP= (3600 x SE) / tdwhere I, td and ꢀV describe the
discharge current (mA), discharge time (s) and potential difference
(V) during discharge process, respectively. mac is the total mass
in both electrodes for two-electrode measurements while it cor-
responds to the redox-active material mass in working electrodes
only for three-electrode measurements.
The p-p type symmetric flexible solid-state supercapacitor cells
were assembled in a packaged sandwich structure by using two
similar redox-active electrodes separated by a gel electrolyte. The
polymer gel electrolyte was prepared by a solution cast method
using poly(methyl methacrylate), propylene carbonate and lithium
perchlorate (Supporting Information). The warm viscous gel elec-
trolyte was precisely spreaded to the conducting polymer coated
surface of modified FSS electrodes. After that, redox-active elec-
trodes were put together by positioning in a 180° angle to each
other, ensuring the overlap active areas. The p-p type symmet-
ric cell configuration was lightly and carefully pressed to increase
the penetration of gel electrolyte layer into conducting polymer
network. The solid state supercapacitor cell was wrapped several
times with a paraffin band in order to seal hermetically (Fig. 1).
3. Results and discussion
ꢀ
ꢀ ꢀꢀ
The novel 2,2 :5 ,2 -terthiophene-based monomers to be used
in preparation of conducting polymer films were synthesized by
a multi-step method. 2,5-Dibromothiophene derivatives contain-
ing oxyethylene chain pendant groups of different lengths were
prepared through microwave-assisted nucleophilic substitution re-
action in the first step of the synthetic procedure. Afterwards,
ꢀ
ꢀ ꢀꢀ
2,2 :5 ,2 -terthiophene-based target monomers were synthesized
by palladium-catalyzed Stille cross-coupling reactions, as seen in
Scheme 1. The monomers were fully characterized using FTIR, 1H
NMR, 13C NMR, mass spectrometry and elemental analysis tech-
niques (Supporting Information).
2.6. Capacitive performance measurements and calculations
Single electrode tests of redox-active electrodes based on
PSEDEN1, PSEDEN2 and PSEDEN3 were performed with a three-
electrode setup in 0.5 M LiClO4/ACN electrolyte solution. In the
three-electrode measurement cell, FSS substrate and silver wire
were used as the counter electrode and the pseudo-reference elec-
trode, respectively. For evaluation of the current-potential response
of PSEDEN1, PSEDEN2 and PSEDEN3 redox-active electrodes, their
cyclic voltammograms were recorded in the potential range 0.4 to
1.8 V using 5, 10, 25, 50, 100, 150, 250 mV s−1 scan rates. PSEDEN1,
PSEDEN2 and PSEDEN3 electrodes were then subjected to galvano-
static charge/discharge (GCD) technique over a potential window
of 1.4 V at current densities of 2.5, 4.5, 6.5, 8.5, 10.5 and 12.5 mA
cm−2 to investigate the charge/discharge profile and the specific
capacitance behavior.
Following synthesis and chemical characterization studies,
ꢀ
ꢀ ꢀꢀ
cyclic voltammograms of 2,2 :5 ,2 -terthiophene-based monomers,
SEDEN1, SEDEN2 and SEDEN3, were recorded to reveal electro-
chemical features of both the monomers and their conducting
polymers. SEDEN1, SEDEN2 and SEDEN3 electroactive monomers
exhibited typical anodic oxidation behaviors in the CV studies.
In the first cycle of voltammograms, irreversible monomer ox-
idation peaks were observed for SEDEN1, SEDEN2 and SEDEN3
at 1.72 V, 1.80 V and 1.83 V, respectively (Fig. 2a–c). After the
first cycle, redox waves of PSEDEN1 (1.27 V/0.90 V), PSEDEN2
(1.48 V/1.29 V) and PSEDEN3 (1.43 V/1.09 V) appeared prominetly
and conducting polymer film layer began to regulary grow on the
surface of platinum disk working electrodes with an increasing
current density. The cyclic voltammograms clearly demonstrated
that there is a slight difference between the monomer oxidation
potentials of SEDEN1, SEDEN2 and SEDEN3. This small difference
arises from steric hindrance effect of oxyethylene pendant groups
on the monomer structures. The increase in the oxyethylene chain
The flexible solid-state supercapacitor cells were characterized
in a two-electrode configuration following a standard charge stor-
age performance test procedure. Different from a three-electrode
setup, reference and counter electrode connections were made di-
rectly via the same redox-active electrode of the supercapacitor
cell. The current-potential and charge/discharge profiles of sym-
metric flexible solid-state supercapacitor cells were examined by
length of substituents at the 3- and 4- positions of terthiophene
´
´
5

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 2. Repetitive cyclic voltammograms for electropolymerization of (a) SEDEN1, (b) SEDEN2 and (c) SEDEN3 at scan rate of 150 mV s−1 and single scan cyclic voltammo-
grams in the anodic region of (d) PSEDEN1, (e) PSEDEN2 and (f) PSEDEN3 at 150 mV s−1 scan rate in monomer-free supporting electrolyte solution.
6

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
skeleton makes steric hindrance more noticeable for electroac-
tive monomers [77–80]. This effect partially complicates the ap-
proach of SEDEN3 to the electrode surface and the subsequent
electron transfer process. Therefore, SEDEN3 was oxidized at a rel-
atively more positive potantial compared to SEDEN1 and SEDEN2.
Contrary to monomer oxidation potentials, the plots of the ca-
thodic peak current densities versus numbers of scan reveal that
SEDEN3 electroactive monomer has a higher electropolymerization
rate than those of SEDEN1 and SEDEN2 (Fig. S11 in the Supporting
Information).
face distribution on the current collector than those of PSEDEN1
and PSEDEN2. Furthermore, as can be understood from Fig. 3a–i,
PSEDEN3 possess more suitable polymeric networks for reversible
ion movements in charge and discharge cycles.
This morphological advantage of PSEDEN3-based redox-active
electrode material will likely enhance its charge storage capac-
ity by means of more and effective ion diffusion pathways in
the structure of a three-dimensional network, as expected. Con-
versely, irregular and agglomerated morphological behaviors were
observed for PSEDEN1 and PSEDEN2 in SEM images, which can
be attributed to the self-aggregation caused by the low solubil-
ity properties of the shorter oxyethylene chain pendant groups
(methoxyethoxy- and 2-[2-methoxyethoxy]ethoxy- chains). The ir-
regular polymeric networks of PSEDEN1 and PSEDEN2 are likely
to create a barrier against ion diffusion and penetration in the
three-dimensional structures. These theoretical predictions were
supported by the plots of log [peak current] versus log [scan
rate] prepared for the conducting polymer films (Fig. S14 in the
Supporting Information). The corresponding slopes for PSEDEN1
(m ≈ 0.71) and PSEDEN2 (m ≈ 0.68) redox-active materials in-
dicate limited ion mobility during charge/discharge processes in
PSEDEN1- and PSEDEN2-based polymeric networks compared to
the matrix of PSEDEN3 (m ≈ 0.60). As a result, SEM images
recorded for PSEDEN1, PSEDEN2 and PSEDEN3-based redox-active
electrodes clearly reveal that the prolongation of oxyethylene pen-
dant groups in conducting polymer chains affecting the remarkable
variety of morphologies of conducting polymer films, as intended.
Before comprehensive capacitive performance evaluations, all
redox-active materials were subjected to CV in potential ranges be-
tween 0.0 and 2.0 V at a constant scan rate of 150 mV s−1 to iden-
tify their ideal working potential scale. In the CV studies, the most
ideal current-potential (I-V) profiles close to rectangular-like shape
were observed in the potential window of 0.4–1.8 V for all ECP-
based electrode materials. Hence, the potential scale of 0.4–1.8 V
was determined as optimal operating potential range for all elec-
trochemical capacitive performance measurements (Fig. S15 in the
Suppoting Information).
To examine p- and n-doping tendencies of PSEDEN1, PSEDEN2
and PSEDEN3 films, single scan voltammograms were recorded in
a monomer-free solution in anodic and cathodic potential regions
[81]. Reversible redox couples demonstrating p-doping characteris-
tic were observed at 1.30 V/1.08 V for PSEDEN1, at 1.44 V/1.32 V for
PSEDEN2 and at 1.47 V/1.02 V for PSEDEN3 in the anodic sweeps of
current-potential curves (Fig. 2d–f). On the other hand, PSEDEN1,
PSEDEN2 and PSEDEN3 conducting polymer derivatives did not ex-
hibit any n-doping property in single scan voltammetry studies
(Fig. S12 in Supporting Information). The electrochemical stability
of all conducting polymer films were also tested by cyclic voltam-
metry studies for 100 charge/discharge cycles. As can be seen
in Fig. S13, slight differences in the voltammetry currents were
observed after 100 charge/discharge cycles, indicating high elec-
trochemical stability and reversibility characteristics for PSEDEN1,
PSEDEN2 and PSEDEN3 polymeric materials.
In the light of results obtained from cyclic voltammetry studies,
SEDEN1, SEDEN2 and SEDEN3 electroactive monomers were elec-
trochemically polymerized on flexible mesh stainless steel work-
ing electrodes (FSS). Constant potential electrolysis method was
especially preferred in the preparation of PSEDEN1, PSEDEN2 and
PSEDEN3 coated electrode materials in order to better control the
mass loading on FSS current collector substrates and to provide
a more homogeneous redox-active layer. Thanks to constant po-
tential electrolyses performed at suitable monomer oxidation po-
tentials, similar amounts of the polymeric electroactive material
were loaded per FSS current collectors, which will ensure that per-
formance comparison among PSEDEN1, PSEDEN2 and PSEDEN3-
based redox-active electrode materials. In the FTIR spectra of
PSEDEN1, PSEDEN2 and PSEDEN3, the strong absorption bands
at around 1640 cm−1 which is originated from polyconjugation
prove that all electroactive monomers have been successfully poly-
merized on the FSS substrates. In addition to FTIR spectra, the
morphological properties and diversity of PSEDEN1, PSEDEN2 and
PSEDEN3 polymeric networks were also examined by SEM ob-
servations. The top-view SEM images at different magnifications
in Fig. 3a–i endorse the formation of conducting polymer lay-
ers across each FSS current collector substrate surface. On the
other hand, it can be clearly seen from Fig. 3 that the PSEDEN3
film grew more regularly on FSS substrate than PSEDEN1 and
PSEDEN2 layers. This obvious variation in morphological proper-
ties is due to the difference in length of pendant oxyethylene
chains on poly(terthiophene)-based polymer backbones, as envis-
aged. Prolongation of oxyethylene chains as pendant groups on
the structure of SEDEN1, SEDEN2 and SEDEN3 monomers causes
an apparent physical phenomenon for electroactive monomers and
their oligomers or polymers. The increase in the length of oxyethy-
The pseudocapacitive behaviors of PSEDEN1, PSEDEN2 and
PSEDEN3-based electrode materials were evaluated with CV and
GCD techniques. To examine current-potential responses and
charge/discharge characteristics of the respective ECP electrodes,
cyclic voltammograms were initially recorded between 0.4–1.8 V
at various scan rates ranging from 5 to 250 mV s−1. The I-
V
curve of non-Faradaic capacitive materials is a rectangular-
like and mirror-image shape which is defined as ideal double-
layer capacitive behavior [14]. However, deviations from ideal
rectangular-like profile are generally observed for pseudocapaci-
tive materials such as conducting polymers due to their inten-
sive Faradaic redox reactions during charge/discharge processes as
well as non-Faradaic double-layer characteristic [33]. As can be
seen in Fig. 4a–c, PSEDEN1, PSEDEN2 and PSEDEN3-based elec-
trode materials exhibited shuttle-like I-V profiles, as an expected
response from redox-active capacitive materials. The efficiency of
charge/discharge of redox-active pseudocapacitive materials gener-
ally decreases as scan rate is gradually increased because of rel-
atively lower ion diffusion rate in the conducting polymer film
network at high rates. This distorts the symmetries of the I-V
curves and causes shape deformations in CV profiles for long-
term cycles. Fig. 4a–c obviously reveal that PSEDEN1, PSEDEN2 and
PSEDEN3-based redox-active electrode materials maintained their
initial CV shapes at even high scan rates (250 mV s−1). These re-
sponses mean that PSEDEN1, PSEDEN2 and PSEDEN3 electrodes
possess fast and highly reversible redox capability characteristics
as a result of their sufficiently fast ion transportion abilities dur-
ing Faradaic charge/discharge proceses [87,88]. On the other hand,
PSEDEN3-based redox-active material exhibited both larger CV area
lene side chains decreases substituent mobility at the 3- and
´
4-positions of terthiophene monomers owing to the intense po-
´
lar interactions between oxyethylene chains [82–86]. This effect
originating from solubility and hydrophobicity in acetonitrile-based
electrolytic media allows the formation of more uniform poly-
meric layers on FSS substrates during the electropolymerization
process since relatively lower mobility of substituents provides the
growth of oligomer/polymer chains in a certain direction [74,75].
Hence, PSEDEN3 exhibited a more regular and homogeneous sur-
7

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 3. Top-view SEM images with different magnifications of (a–c) PSEDEN1, (d–f) PSEDEN2 and (g–i) PSEDEN3-based polymeric redox-active layers on FSS substrate.
and higher current density than those of PSEDEN1 and PSEDEN2
under the same conditions. Preliminary results obtained from CV
studies confirm that PSEDEN3 polymeric films have better pseudo-
capacitive behavior and charge storage performance compared to
PSEDEN1 and PSEDEN2.
a limited number of ions since their agglomerated structures cre-
ated a barrier effect for ion diffusions or mobilities in electroac-
tive polymeric network. GCD measurements indicate that PSEDEN1,
PSEDEN2 and PSEDEN3 are promising redox-active electrodes for
practical supercapacitor applications in terms of competing with
many poly(thiophene)-based pseudocapacitive materials previously
reported in the literature [34,63,74,89–91] (Table 1). These results
also reveal that different charge storage capacities could be ob-
tained with conducting polymer derivatives having similar chem-
ical structures only depending on the pendant group diversity (dif-
ferent alkyl- and oxyethylene- chains) [74].
The charge storage properties and performances of PSEDEN1,
PSEDEN2 and PSEDEN3 redox-active electrodes were investigated
in more detail by GCD technique. As presented in Fig. 5a–c, all
redox-active electrode materials demonstrated nearly triangular
charge/discharge profiles with very small ohmic drop values (IR-
drop= 0.07 V for PSEDEN1, 0.05 V for PSEDEN2 and 0.04 V for
PSEDEN3) at 2.5 mA cm−2 constant current density over an op-
timum operating potential window of 1.4 V (0.4–1.8 V).
To enlighten rate capabilities of PSEDEN1, PSEDEN2 and
PSEDEN3-based redox-active electrode materials, GCD studies were
repeated for different current densities (from 4.5 mA cm−2 to
12.5 mA cm−2) over the potential range of 0.4 to 1.8 V. The
specific capacitance value of many pseudocapacitive materials de-
creases considerably depending on the increase in current density
owing to restricted ion duffision at high current densities. Also,
a limited ion diffusion rate causes an increase in IR-drop value
and distortion of symmetry in GCD curves. As is apparent from
Fig. 5a–c, PSEDEN1, PSEDEN2 and PSEDEN3 pseudocapacitive ma-
terials exhibited capacitance retention performances of 56.7% (from
The symmetrical GCD curves mean excellent balanced and
highly reversible Faradaic charge and discharge property for
PSEDEN1, PSEDEN2 and PSEDEN3 redox materials, pointing to an
ideal GCD characteristic. On the other side, negligible ohmic drop
values reveal that PSEDEN1, PSEDEN2 and PSEDEN3-based elec-
trodes have low internal resistances. The single electrode specific
capacitance (Cspec) values for PSEDEN1, PSEDEN2 and PSEDEN3
were also calculated utilizing GCD curves in accordance with Eq.
(1). The results of charge storage capacity measurements show
that the PSEDEN3-based redox active material (403.3 F g−1) deliv-
ered a significantly higher gravimetric capacitance than those of
PSEDEN2 (212.8 F g−1) and PSEDEN1 (135 F g−1) at a constant
current density of 2.5 mA cm−2. Superior charge storage capac-
ity of the PSEDEN3 electrode can be directly associated with its
morphological structure. As discussed earlier, it can be said that
PSEDEN1 and PSEDEN2 redox-active materials were able to store
135 F g−1 to 76.6 F g−1), 72.6% (from 212.8 F g−1 to 154.5 F g−1
)
and 69.3% (from 403.3 F g−1 to 279.5 F g−1), respectively, not
with standing a five-fold increase in current density. Furthermore,
PSEDEN1, PSEDEN2 and PSEDEN3-based electrodes maintained ini-
tial GCD shapes even at high current density values (Fig. 5e). These
charge/discharge behaviors as a function of increasing current den-
sity endorse that all polymeric redox-active materials have satis-
8

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 4. Current-potential profiles (a) PSEDEN1, (b) PSEDEN2 and (c) PSEDEN3 polymeric flexible redox-active electrodes materials at various scan rates (from 5 to 250 mV
−1) in 3-electrode cell configuration.
s
factory charge/discharge rate capabilities resulting from fast charge
transfer features. The high rate capability property is an important
performance indicator showing that the pseudocapacitive electrode
material has the potential to be used in high power energy stroage
applications.
qualitative comparison of capacitive performances. As can be seen
in Fig. 6a–c, Cell 3 not only gives a I-V profile closer to rectangular
shape than those of Cell 1 and Cell 2 but also a remarkably larger
normalized CV area in the 0.4–1.8 V potential range. These CV re-
sponses mean a better pseudocapacitive behavior for Cell 3 com-
pared to Cell 1 and Cell 2 and are in alignment with results ob-
tained from three-electrode configuration measurements. The re-
flection of fast and highly reversible redox charge/discharge charac-
teristics of PSEDEN1, PSEDEN2 and PSEDEN3 redox-active materials
on the flexible device performances is clearly observed in the CV
curves of Cell 1, Cell 2 and Cell 3. No evident distortion was found
in the rectangular-like I-V profile of Cell 3 and quasi-rectangular
CV shapes of Cell 1 and Cell 2 with an increasing scan rate, in-
dicating fast ion transportion ability in the flexible supercapacitor
devices based on polymer gel electrolyte (Fig. 6a–c).
In addition to single electrode performance evaluations, the ca-
pacitive performances of p-p type symmetric flexible supercapac-
itor device assembled by using the three redox-active electrode
materials were also examined in a typically two-electrode cell
system [92]. Unlike a three-electrode setup, potential applied to
the supercapacitor system is almost equally shared by both anode
and cathode materials in a two-electrode measurement technique.
This offers the possibility to calculate charge storage capacity, en-
ergy and power density for the whole supercapacitor device rather
than only the working electrode. Accordingly, the two-electrode
cell configuration allows to obtain more meaningful and realis-
tic results for capacitive performance evaluation. Moreover, elec-
trochemical data acquired by two-electrode measurements provide
valuable insights about the employment of redox-active electrode
materials in actual supercapacitor applications. p-p type flexible
supercapacitor devices Cell 1 (PSEDEN1), Cell 2 (PSEDEN2) and Cell
3 (PSEDEN3) were initially subjected to CV in order to make a
Solid-state, flexibility and mechanical stretching properties of
Cell 1, Cell 2 and Cell 3 were also tested by means of CV studies
at a 250 mV s−1 scan rate for the different bending angles (0, 45,
90, 135 and 170°). The cyclic voltammograms in Fig. 6d–f. depict
that Cell 1, Cell 2 and Cell 3 could maintain their CV loop shapes
with bending angles from 0° to 170° Moreover, almost no change
in normalized CV areas was observed for Cell 1, Cell 2 and Cell 3
9

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 5. GCD curves of (a) PSEDEN1, (b) PSEDEN2 and (c) PSEDEN3 polymeric flexible redox-active electrodes materials at different current densities from 2.5 to 12.5 mA
cm−2, (d) comparative GCD plots of electrode materials at 2.5 mA cm−2 current density and (e) specific capacitance changes of PSEDEN1, PSEDEN2 and PSEDEN3 as a
function of current density.
even at extreme bending condition (170°). The above results reveal
that our p-p type symmetric flexible solid-state supercapacitor de-
vices possess admirable flexibility, mechanical stretching and solid-
ity features.
Fig. 7a–c shows that the galvanic cycling profiles plotted for
Cell 1, Cell 2 and Cell 3 are all similar to trends observed in
three-electrode measurements. Typical triangular charge/discharge
curves with small ohmic drop values (IR-drop= 0.15 V for Cell 1,
10

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Electrochimica Acta 389 (2021) 138662
Table 1
Charge storage performances of polythiophene-based different redox-active materials.
Polymeric redox-active material
Operating potential range
Supporting electrolyte
0.1 M Na2SO4
Specific capacitance (Cspec
103 F g−1 at 0.3 A g−1
)
Ref.
−0.60 – 0.80 V
[89]
1.15 – 2.00V
1 M Et4NBF4 / PC
0.1 M NaClO4
190 F g−1 at 0.3 A g−1
[63]
[90]
−0.50 – 0.90 V
176 F g−1 at 0.75 mA cm−2
0.0 – 1.00 V
0.1 M Bu4NPF6 / MeCN
1 M Et4NBF4 / PC
171 F g−1 at 1 A g−1
[91]
[34]
−0.27 – 1.03 V
190 F g−1
0.0 – 1.60 V
0.5 M Bu4NBF4 / MeCN
94.3 F g−1 at 2.5 mA cm−2
[74]
0.0 – 1.60 V
0.0 – 1.60 V
0.4 – 1.80 V
0.4 – 1.80 V
0.5 M Bu4NBF4 / MeCN
0.5 M Bu4NBF4 / MeCN
0.5 M LiClO4 / MeCN
0.5 M LiClO4 / MeCN
227.3 F g−1 at 2.5 mA cm−2
443 F g−1 at 2.5 mA cm−2
135 F g−1 at 2.5 mA cm−2
212.8 F g−1 at 2.5 mA cm−2
[74]
[74]
This work
This work
0.4 – 1.80 V
0.5 M LiClO4 / MeCN
403.3 F g−1 at 2.5 mA cm−2
This work
0.06 V for Cell 2 and 0.06 V for Cell 3) were observed for each
symmetric flexible solid-state supercapacitor devices at 2.5 mA
cm−2 constant current density in the potential range of 0.4 and
1.8 V. GCD shapes of Cell 1, Cell 2 and Cell 3 reveal that PSEDEN1,
PSEDEN2 and PSEDEN3 electrode materials exhibited highly re-
versible charge and discharge characteristics during charging and
discharging steps in polymeric gel electrolyte medium just as in
the liquid supporting electrolyte media. As illustrated in Fig. 7d,
Cell 3 shows considerably longer charge and discharge duration
than those of Cell 1 and Cell 2, meaning that Cell 3 stored rela-
tively more electrical charges.
The suitable three-dimensional network, more porous structure
and relatively lower ohmic drop value of PSEDEN3-based material
was believed to result in better electrochemical capacitive perfor-
mance and longer discharging duration for Cell 3 compared with
Cell 1 (PSEDEN1) and Cell 2 (PSEDEN2). As seen from Table 2,
Cell 3 reached 162.4 F g−1 specific capacitance, which was approxi-
matly 5.5 and 1.7 times higher than those of Cell 1 (29.3 F g−1) and
Cell 2 (92.1 F g−1). In addition to their charge storage capacities,
Cell 1, Cell 2 and Cell 3 also delivered maximum energy densities
of 6.35 W h kg−1, 22.9 W h kg−1 and 41.1 W h kg−1, respectively.
On the other hand, satisfactory gravimetric power density values
as high as 929, 937.7 and 986.4 W kg−1 were obtained for Cell 1,
Cell2 and Cell 3 in galvanic charge/discharge performance tests. To
determine the change in gravimetric capacitance during galvanos-
tatic cycling, the charge storage performances of flexible solid-state
supercapacitor devices were tested at a current density of 4.5, 6.5,
8.5, 10.5 and 12.5 mA cm−2 over the potential range of 0.4–1.8 V.
When the current density was gradually increased to five times,
48.2%, 31.1% and 33% falls in specific capacitance were observed for
Cell 1 (from 29.3 to 15.2 F g−1), Cell 2 (from 92.1 to 63.5 F g−1) and
This more superior capacitive performance of Cell 3 can be
directly attributed to the appropriate morphological structure of
PSEDEN3-based redox-active electrode material, as expected. On
the other hand, a quantitative assesment of the capacitive perfor-
mances was made by calculating specific capacitance (Cspec, F g−1),
energy density (SE, Wh kg−1) and power density (SP, W kg−1) val-
ues based on the GCD curves at 2.5 mA cm−2 constant current
density for all symmetric flexible solid-state supercapacitor devices
and listed in Table 2.
11

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 6. Cyclic voltammograms of (a) Cell1, (b) Cell2, (c) Cell3 solid-state flexible supercapacitor devices at various scan rates (from 5 to 250 mV s−1) in 2-electrode cell
configuration and current-potential profiles of (d) Cell1, (e) Cell2 and (f) Cell3 at 250 mV s−1 scan rate under different bending conditions.
Table 2
Capacitive performance parameters of flexible solid-state supercapacitor devices (Cell1,
Cell2 and Cell3).
Flexible solid-state supercapacitor device configuration
Cell 1
Cell 2
Cell 3
Specific Capacitance (Cspec
Energy Density (SE)
Power Density (SP)
)
29.3 F g−1
6.35 W h kg−1
929 W kg−1
92.1 F g−1
22.9 W h kg−1
937.7 W kg−1
162.4 F g−1
41.1 W h kg−1
986.4 W kg−1
12

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 7. GCD plots of (a) Cell1, (b) Cell2 and (c) Cell3 solid-state flexible supercapacitor devices at different current densities from 2.5 to 12.5 mA cm−2, (d) comparative GCD
curves of supercapacitor devices at 2.5 mA cm−2 current density and (e) specific capacitance changes of Cell1, Cell2 and Cell3 as a function of current density.
Cell 3 (from 162.4 to 108.8 F g−1), respectively, as seen in Fig. 7e.
The long-term cycle life and high mechanical stability are con-
sidered as essential requirements for practical supercapacitor ap-
plications, as well as charge storage performance parameters (spe-
cific capacitance, energy and power densities). An effecient flex-
ible supercapacitor device should exhibit the features of excel-
lent flexibility and very long operation life without a remarkable
loss of specific capacitance. Our flexible solid-state supercapaci-
The relatively lower rate capability performances in the case of su-
percapacitor device configurations compared to three-electrode cell
measurements resulted from the difficulties in ion movement and
penetration in polymeric gel electrolyte media, as an expected be-
havior.
13

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 8. Long-term cycling stabilities of (a) Cell1, (b) Cell2, (c) Cell3 solid-state flexible supercapacitor devices at 2.5 mA cm−2 current density under 0° bending angle for
10,000 charge/discharge cycles and (d) specific capacitance retentions of supercapacitor devices over 10,000 charge/discharge cycles at 2.5 mA cm−2 current density.
tor devices were initially subjected to 10 000 consecutive galvanic
charge/discharge cycles at 2.5 mA cm−2 constant current density
from 0.4 to 1.8 V in order to evaluate their long-term cycle lifes
and the change in charge storage capacities. At the end of 10 000
cycles, Cell 1, Cell 2 and Cell 3 reached capacitance retention rates
conditions (bending angle of 170°). These results confirm that our
novel redox-active electrode materials have desirable mechanical
features which can meet the fundamental requirements for the po-
tential applications of high-performance energy storage devices in
flexible electronic technology.
of 80.2% (from 29.3 to 23.5 F g−1), 84.7% (from 92.1 to 78 F g−1
and 91.4% (from 162.4 to 148.4 F g−1), respectively, as presented in
)
The electronic and kinetic properties of flexible solid-state su-
percapacitor devices were tested using EIS. As seen in correspond-
ing Nyquist plots (Fig. 10a–d), Cell 1, Cell 2 and Cell 3 exhibited a
semicircle pattern in the high frequency region and a linear por-
tion in the low frequency region, as expected of an ideal superca-
pacitor. Semicircle parts of Nyquist plots revealed that Cell 1, Cell
2 and Cell 3 not only have low equivalent series resistance (ESR),
but also possess small charge transfer resistance (RCT) (Fig. 10a–c).
The ESR values were calculated to be 7.77 ꢁ for Cell 1, 5.34 ꢁ for
Cell 2 and 3.29 ꢁ for Cell 3 while RCT values of Cell 1, Cell 2 and
Cell 3 were measured as 15.03, 10.64 and 7.38 ꢁ.
Fig. 8a–d.
These acceptable reductions in specific capacitances of flexible
solid-state supercapacitors confirm that the polymeric networks
of PSEDEN1, PSEDEN2 and PSEDEN3 could tolerate structural de-
formation caused by volumetric changes (shrinking and swelling)
on the polymer backbones during repetitive charge/discharge pro-
cesses. Similarly, mechanical stability tests of flexible solid-state
supercapacitor devices were performed with a typically GCD tech-
nique at a current density of 2.5 mA cm−2 in the same potential
range under different bending angles (0°, 45°, 90°, 135° and 170°).
Fig. 9a–d clearly indicate that Cell 1, Cell 2 and Cell 3 still main-
tained a large amount of their initial specific capacitance when
they were bent from 0° to 170° The device gravimetric capaci-
tance performances of Cell 1, Cell 2 and Cell 3 only decreased
by 3.4%, 4.66% and 1.97%, respectively, even at extreme bending
The straight portions of Nyquist plots in the middle and low
frequency region, known as Warburg impedance (W), gave impor-
tant data about ion diffusion kinetics on flexible solid-state super-
capacitor devices during charge storage process. Fig. 10d clearly in-
dicates that Cell 3 drew a steeper and shorter Warburg curve along
the imaginary part of its Nyquist plot than those of Cell 1 and
14

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 9. Typical GCD curves of (a) Cell1, (b) Cell2, (c) Cell3 solid-state flexible supercapacitor devices at 2.5 mA cm−2 current density under various bending angle from 0 to
170° and (d) mechanical stability performance of supercapacitor devices as a function of bending angle at 2.5 mA cm−2 current density.
Cell 2. This impedance characteristic means that a capacitive elec-
trochemical double-layer on the surface of PSEDEN3 redox-active
film formed faster compared to PSEDEN1 and PSEDEN2. Such obvi-
ous difference between ion diffusion kinetics of PSEDEN1, PSEDEN2
and PSEDEN3 redox-active electrode materials explains why Cell 3
has better capacitive performance. In this context, it is apparent
that the results of EIS analysis are consistent with experimental
data obtained from CV and GCD techniques.
ducting polymer films was revealed by SEM observation in de-
tail. Polymeric redox-active layers exhibited more appropriate 3D-
dimensional network structures for easier and rapid ion diffusions
or mobilities with increase in the oxyethylene chain length as
a pendant group of the poly(terthiophene) backbones. In three-
electrode test configuration, PSEDEN1, PSEDEN2 and PSEDEN3-
based redox-active electrode materials reached up to 135 F g−1
,
212.8 F g−1 and 403.3 F g−1 gravimetric capacitance values, re-
spectively, at a constant current density of 2.5 mA cm−2 in the po-
tential range of 0.4–1.8 V with low IR drop and good rate capacil-
ity performances. These differences in charge storage performances
obtained for redox-active materials were correlated to the morpho-
logical properties of PSEDEN1, PSEDEN2 and PSEDEN3 polymeric
films based on the evaluation of SEM images. Furthermore, sym-
metrical flexible solid-state supercapacitor devices with polymer
gel electrolyte were also assembled using PSEDEN1, PSEDEN2 and
PSEDEN3 redox-active electrode materials. These assembled de-
vice (Cell 1, Cell 2 and Cell 3) delivered competitive specific ca-
pacitances (Cspec= 162.4 F g−1, 92.1 F g−1 and 29.3 F g−1), en-
ergy densities (SE= 41.1 W h kg−1, 22.9 W h kg−1 and 6.35 W
h kg−1) and power densities (SP= 986.4 W kg−1, 937.7 W kg−1
and 929 W kg−1) in two-electrode setup. Notably, flexible super-
4. Conclusion
In summary, we have presented a simple and effective strategy
to diversify the morphological features of polymeric redox-active
networks and consequently to enhance the electrochemical charge
storage capacities of electrode materials. For this purpose, novel
poly(terthiophene) derivatives containing oxyethylene substituents,
PSEDEN1, PSEDEN2 and PSEDEN3, have been electrochemically
prepared on the flexible mesh stainless steel current collector sub-
strates and their capacitive performances have been evaluated fol-
lowing a standard test procedure (CV, GCD and EIS techniques).
The effect of prolongation of oxyethylene chains on the morpho-
logical characteristics of PSEDEN1, PSEDEN2 and PSEDEN3 con-
15

D. Yig˘it and M. Güllü
Electrochimica Acta 389 (2021) 138662
Fig. 10. Nyquist plots of (a) Cell1, (b) Cell2, (c) Cell3 solid-state flexible supercapacitor devices and (d) comparative Nyquist impedance plots of supercapacitor devices in
the frequency range of 10,000 to 0.01 Hz at 0.0 V DC applied voltage.
capacitor devices exhibited not only high long-term cycle life per-
formances with good capacitance retentions (close to 92%) over 10
000 consecutive galvanic charge/discharge cycles, but also excellent
mechanical stabilities (1.97% capacitance loss) under various bend-
ing conditions at 2.5 mA cm−2 constant current density from 0.4
to 1.8 V. All results endorse that different charge storage capaci-
ties could be obtained with conducting polymer derivatives having
similar chemical structures only depending on the morphological
diversity. In this context, it is concluded that molecular design ap-
proach can be effectively used to influence morphological charac-
teristics and enhance capacitive performance of electroactive ma-
terials in various conducting polymer-based charge storage appli-
cations.
Acknowledgments
This research was supported by the Scientific and Technological
˙
Research Council of Turkey (TÜBITAK, Grant No: KBAG-114Z167).
˙
We gratefully thank TÜBITAK (KBAG-114Z167) for its generously fi-
nancial support.
Supplementary materials
Supplementary data associated with this article can be found,
in the online version, at 10.1016/j.electacta.2021.138662.
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