Asymmetric supercapacitor devices based on dendritic conducting polymer and activated carbon

Article abstract of DOI:10.1016/j.electacta.2017.01.168

Dendritic conducting polymers(CPs) are a novel class of porous pseudocapacitive electrode materials assembled with the combination of highly reversible redox active triphenylamine(TPA) and thiophene, 3-methylthiophene, selenophene and thieno[3,2-b]thiophen moieties. Due to the unique combination of three dimensional conducting network, fast redox reversible reactions, porous morphology, high thermal and electrochemical stability have fetched these pseudocapacitive polymers to exhibit high specific capacitance and emerged as an ideal candidate for energy storage devices. The electrochemical performance of as-prepared polymers showed specific capacitance of 278, 257, 246 and 315 Fg^?1 for poly tris[4-(2-thienyl)phenyl]amine (P1), poly tris(4-(3-methylthiophene-2-yl)phenyl)amine (P2), poly tris(4-(selenophen-2-yl)phenyl)amine (P3) and poly tris(4-thieno[3,2-b]thiophen-2-yl) phenyl)amine (P4) respectively with low internal resistance. An insertion of selenophene and thieno(3,2-b)thiophene linkers in TPA block showed enhanced electrochemical performance than the thiophene-TPA pair. Furthermore, asymmetric supercapacitors were assembled with the polymer as cathode and activated carbon as an anode and the detailed electrochemical characterizations has been investigated. This research may shed light on designing new redox active psuedocapacitors and other electrochemical devices.

Full text of DOI:10.1016/j.electacta.2017.01.168

Accepted Manuscript
Title: Asymmetric supercapacitor devices based on dendritic
conducting polymer and activated carbon
Authors: Darshna D. Potphode, Sarada P. Mishra, P.
Sivaraman, Manoranjan Patri
PII:
S0013-4686(17)30207-4
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.electacta.2017.01.168
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25-10-2016
24-1-2017
25-1-2017
Please cite this article as: Darshna D.Potphode, Sarada P.Mishra,
P.Sivaraman, Manoranjan Patri, Asymmetric supercapacitor devices based
on dendritic conducting polymer and activated carbon, Electrochimica Acta
http://dx.doi.org/10.1016/j.electacta.2017.01.168
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Asymmetric supercapacitor devices based on dendritic
conducting polymer and activated carbon
Darshna D. Potphode, Sarada P. Mishra, P. Sivaraman, Manoranjan Patri*
Polymer Science and Technology Centre, Naval Materials Research Laboratory (DRDO),
Shill-Badlapur Road, Anand Nagar P.O., Ambernath (E) 421506,Maharastra, India
Corresponding author. Tel.: +91 251 2623201; fax: +91 251 2623004.
E-mail address: mrpatri@gmail.com (M. Patri).
Graphical abstract
1

ABSTRACT
Dendritic conducting polymers(CPs) are a novel class of porous
pseudocapacitive electrode materials assembled with the combination of highly
reversible redox active triphenylamine(TPA) and thiophene, 3-methylthiophene,
selenophene and thieno[3,2-b]thiophen moieties. Due to the unique combination
of three dimensional conducting network, fast redox reversible reactions, porous
morphology, high thermal and electrochemical stability have fetched these
pseudocapacitive polymers to exhibit high specific capacitance and emerged as
an ideal candidate for energy storage devices. The electrochemical performance
of as-prepared polymers showed specific capacitance of 278, 257, 246 and 315
Fg-1 for poly tris[4-(2-thienyl)phenyl]amine (P1), poly tris(4-(3-methylthiophene-2-
yl)phenyl)amine (P2), poly tris(4-(selenophen-2-yl)phenyl)amine (P3) and poly
tris(4-thieno[3,2-b]thiophen-2-yl) phenyl)amine (P4) respectively with low internal
resistance. An insertion of selenophene and thieno(3,2-b)thiophene linkers in
TPA block showed enhanced electrochemical performance than the thiophene-
TPA pair. Furthermore, asymmetric supercapacitors were assembled with the
polymer as cathode and activated carbon as an anode and the detailed
electrochemical characterizations has been investigated. This research may
shed light on designing new redox active psuedocapacitors and other
electrochemical devices.
Keywords: Triphenylamine, dendritic conducting polymers, energy storage,
psuedocapacitors, asymmetric supercapacitors
2

1. INTRODUCTION
Supercapacitors are energy storage devices having capacity of
bridging the gap between high energy density batteries and high power density
conventional capacitors.[1] It can be used for harvesting energy and delivering
high pulse power for short periods.[2,3] Depending upon the level and duration of
the storage the electrode materials can be modified and used in many
applications like hybrid vehicles, telecommunications, military weapons,
spaceships etc.[4-8] For large scale practical application supercapacitors still
suffer for low energy density as a major bottleneck. To overcome this, an
asymmetric hybridization is considered as one of the most promising tact with
psuedocapacitive material as the energy source and electric double layer
capacitance material as the power source. This combination can offer
outstanding energy density for whole system.[9]
The rapid growth in energy storage system demands advances in
electrode materials for high specific capacitance and adequate stability. Carbon
based electrode materials limits their use due to the non faradiac charge storage
at the electrode-electrolyte interface which exhibits lower specific capacitance
(200 Fg-1) than pseudocapacitive materials like metal oxides and CPs.[10-11]
Pseudocapacitive materials are most promising materials since the charge
storage in those can be induced by reversible redox reactions taking place at
surface and bulk of the electrode.[12,13] Among pseudocapacitive materials,
3

metal oxides are potential candidates for higher specific capacitance (upto1200
Fg-1) and high stability than the CPs but polymeric materials can compete them
interms of low cost, easy route of synthesis, light weight etc as long as
commercialization concern.[14-16] Further, the electrochemical properties of the
conducting polymers can be tuned by controlling the chemical structure of the
polymer by inexpensive ways than the metal oxides. The CPs, such as
polyaniline, polypyrrole, polythiophene and their derivatives are widely studied for
electrochemical energy storage in both aqueous and non aqueous
systems.[13,17] Polyaniline, being most used polymer in electrochemical
strorage, showed highest specific capacitance upto 815 Fg-1 with good
environmental stability than the polypyrrole and polythiophenes.[16]
Polythiophenes and its various derivatives have been well explored as electrode
materials in supercapacitors with specific capacitance range of 180-250 Fg-1 in
non-aqueous system.[18-19] Three dimentional structured TPA based materials
are known for fast charge transfer and reversibly redox activity and have been
utilized in various applications viz. organic field effect transistors, electrochromic
devices, solar cells etc.[20,21] Moreover combination of TPA core with electron
rich thiophene moiety to form a conducting polymer has been proved a potential
electrode material for electrochemical storage.[22] Robert et al have reported
electrochemically polymerized poly tris(4-(thiophene-2-yl) phenyl)amine based
electrode material for energy storage. Nevertheless these kind of materials have
not been fully explored for energy storage application. As polymeric structure and
morphology plays an important role in energy storage,[23] incorporation of such
4

kind of materials in device fabrication has been receiving more attention due to
their inherent redox properties and unusual high specific capacitance.[22]
In this work, the newer supercapacitor electrode materials have been
developed by synthesizing of structurally homologous dendritic conducting
polymers differing in the density of electron rich linkers (3-methyl thiophene,
selenophene and thieno(3,2-b)thiophene) in direct conjugation with TPA core. As
per aromaticity of the selenophene and thieno(3,2-b)thiophene concern, these
polymer have been prepared following the idea that an aromatic system similar to
the thiophene-TPA pair to exhibit superiority in contributing more quinoidal
character and inter-ring -electron delocalization in polymer backbone to
enhance the electrochemical performance. The dendritic nature of the polymer
plays an important role enhancing electrochemical storage to achieve highly
porous morphology in the solid state by preventing close packing unlike linear
chains. Moreover, our approach to creating the dendritic polymers through easy
synthetic route by oxidative chemical polymerization has general applicability for
large scale production at a very low cost and easy processible to use as
electrode material in energy storage application.
2. Experimental
2.1 Materials
Thiophene, 3-methyl thiophene, selenophene, 2-isopropoxy-4,4,5,5-
tetramethyl-1,3,2-dioxaborolane,
nbutyl
lithium,
tetrakis(triphenyl
phosphine)palladium(0) (Pd(PPh3)4), unhydrous ferric chloride (anh.FeCl3),
potassium carbonate (K2CO3), acetonitrile (ACN) and tetraethylammonium
5

tetrafluoroborate (TEABF4) were purchased from Sigma Aldrich and used as
received. All solvents were dried by conventional methods prior to use. Activated
carbon (AC) (surface area: 1990 m2 g−1) used in the work was procured from
PICA, France, grade:BP10. Conducting carbon (CC) powder (Vulcan XC-72) and
carbon paper were procured from Cabot Corp., Japan and Toray, Japan
respectively. Teflon (PTFE) suspension was supplied by Hindustan Flurocarbon
Corp., India.
2.2 Instrumentation
The 1H NMR spectra were recorded in a Brucker 400 FT-NMR
spectrometer with chloroform-d as solvent. Chemical shifts were reported in ppm
units with tetramethylsilane as an internal standard. Fourier transform infrared
(FTIR) spectra of polymers were recorded in JASCO FT/IR–4200 over a
scanning range from 500 to 4000 cm-1 using KBr pallets. Raman spectroscopy of
the polymers was performed using Jobin Yovon spectrometer (HR 800 confocal
micro Raman) on polymer powder samples with incident laser excitation
wavelength of 514.5 nm. Thermogravimetric analysis (TGA; Hi Res TGA 2950,
TA Instruments) of the polymers was carried out at a heating rate of 10 ̊
C min-1
under nitrogen atmosphere. Field emission gun-scanning electron microscopic
(FEG-SEM) analysis of polymers was conducted on a Carl Zeiss Supra-55VP
field emission electron microscope. Electrochemical characterization was
performed with an Ecochemie Autolab PGSTAT30 potentiostat.
6

2.3 Synthesis of monomers & polymers
Monomers were synthesized via Suzuki coupling reaction by following
procedure reported elsewhere.[24] The general synthetic route for the monomers
and polymers is shown in scheme 1.
Tris[4-(2-thienyl)phenyl]amine (monomer 1):
The tris (4-iodophenyl)amine (1.86g, 3mmol), 4,4,5,5-tetramethyl-2-
(thiophen-2-yl)-1,3,2-dioxaborolane (2.2g, 10.5mmol) and Pd(PPh3)4 (0.34g,
0.3mmol) were dissolved in 200 mL of toluene. The solution was degased with
N2 for 30 min and then degasified solution of K2CO3 ( 6.73g, 44.9mmol) in 60 mL
of H2O was added. The mixture was refluxed for 24 h under inert atmosphere
and cooled to rt. The compound was extracted in dichloromethane, washed with
water, dried over MgSO4 (anhd.) and evaporated. The purification was carried
out using column chromatography (silica gel, petroleum ether as eluent) to obtain
1
a yellow solid. Yield: 1 g (68.3%), Mp=138 ºC, H NMR (400 MHz, CDCl3) δ:
7.064 (d, J=2.4 Hz, 6H), 7.078 (d, J=2.4 Hz, 6H), 7.144 (d, J=5.6 Hz, 3H), 7.255
(d, J=7.6 Hz, 3H), 7.516 (d, J=5.6 Hz, 3H),
Tris(4-(3-methylthiophene-2-yl)phenyl)amine (monomer 2), Tris(4-(selenophen-2-
yl)phenyl)amine (monomer 3) and Tris(4-thieno[3,2-b]thiophen-2-yl) phenyl)
amine (monomer 4) were synthesized by following similar procedure.
7

Tris(4-(3-methylthiophene-2-yl)phenyl)amine (monomer 2): Yellow solid,
Yield: 65%, Mp= 151 ºC, 1H NMR (400 MHz, CDCl3) δ: 2.36 (s, 9H), 6.94 (d, J=4
6H), 7.16 (d, J=6.8, 6H), 7.39 (d, J=6.4, 3H), 7.57 (d, J=6.8, 3H).
Tris(4-(selenophen-2-yl)phenyl)amine (monomer 3): Mustard-green solid,
Yield: 43%, Mp=141 ºC, 1H NMR (400 MHz, CDCl3) δ: 7.16 (d, J=6.4 Hz, 6H), 7.3
(t, J=7.6 Hz, 3H), 7.45 (d, J=5.6 Hz, 3H), 7.56 (d, J=7.2, 6H), 7.91 (d, J=6.8, 3H).
Tris(4-thieno[3,2-b]thiophen-2-yl) phenyl) amine (monomer 4): Yellowish
1
brown solid, Yield: 69%, Mp=179 ºC, H NMR (400 MHz, CDCl3) δ:7.186 (d,
J=6.8Hz, 6H), 7.252(d, J=4Hz, 6H), 7.349 (d, J=4Hz, 3H ), 7.477 (d, J=4.8Hz,
3H), 7.558 (d, J=7.2 Hz, 3H).
Representative example of polymerization (P1)
To a CHCl3 (6 mL) sloution of Tris[4-(2-thienyl)phenyl]amine (0.3g,
0.61mmol), suspension of anhydrous FeCl3 (0.495g, 3.05 mmol) in CHCl3(4 mL)
was added drop wise over 30 min. The reaction was continued at rt for addional
48h and then quenched with methanol. The residue was stirred in 5% methanolic
solution of hydrazine hydrate (0.08 mmol) for 24h to dedope the polymer.
Polymer was filtered, washed several times with methanol and soxhlet exctracted
with methanol and acetone.
Poly tris[4-(2-thienyl)phenyl]amine (P1) Yield: 87%, IR (KBr, cm-1) max :3024,
1596, 1491, 1318, 1264, 1179, 1108, 966, 825, 730, 691
Polymers P2-P4 were synthesized and purified by similar procedure.
8

Poly tris(4-(3-methylthiophene-2-yl)phenyl)amine (P2) Yield: 96%, IR (KBr,
cm-1) max :3070, 2921, 2853, 1597, 1482, 1314, 1268, 1180, 1109, 1005, 822,
721
Poly tris(4-(selenophen-2-yl)phenyl)amine (P3) Yield: 97%, IR (KBr, cm-1) max
: 3024, 1589, 1488, 1316, 1277, 1180, 1106, 1004, 821, 727
Poly tris(4-thieno[3,2-b]thiophen-2-yl) phenyl) amine (P4) Yield: 85%, IR
(KBr, cm-1) max :3078, 1596, 1486, 1320, 1283, 1176, 1014, 970, 810, 729, 637
2.4 Preparation of electrodes
Electrodes were fabricated by coating carbon paper with slurry of polymer
or AC with CC in isopropyl alcohol and PTFE suspention(<5% of active material).
The thickness of coating was maintained at 40±5 m. The electrodes were then
dried at 60 °C under vacuuo before using for electrochemical characterization.
The electrodes of size 2 cm  2 cm were prepared in which loading of active
material was 1.25 mg cm-2. The comppstion of electrode for both single electrode
and two electrode characterisation is given in the table 1.
2.5 Device fabrication
Device was fabricated by using two electrodes of size 2 cm  2 cm
separated by capacitor grade paper. Positive electrode (cathode) was prepared
with polymer material while negative electrode (anode) was with AC. Amount of
AC in the negative electrode was kept constant (1.25 mg cm-2) while polymer
quantity was varied according to mass ratio of negative to positive electrode
(max).[25,26] Coating thickness were varied depending upon electrode mass
9

loading. The assembly was kept for drying in vacuum oven at 60°C for 8 h. It was
sealed with plastic coated aluminium foil after addition of electrolyte. All the
fabrication process was carried out in the glove box. Polymer/AC devices were
named as PD1, PD2, PD3 and PD4 for P1/AC, P2/AC, P3/AC and P4/AC based
compositions, respectively.
2.6 Electrochemical characterization
The electrochemical studies of asymmetric hybrid supercapacitors were
studied by two electrode method in non-aqueous electrolyte (1M solution of
TEAFB4 in acetonitrile). In order to estimate the loading mass ratio of active
materials in asymmetric hybrid supercapacitors, their individual specific
capacitance was calculated by using three electrode configuration at initial stage
where platinum mesh and Ag/AgCl were used as the counter electrode and
reference electrode respectively. The electrochemical properties of the
supercapacitor electrodes were studied by cyclic voltammetry (CV), galvanostatic
charge–discharge (GCD) and electrochemical impedance spectroscopy (EIS). All
the electrochemical measurements were carried out at room temperature using
an Ecochemie Autolab PGSTAT30.
3. Results & Discussion
3.1 TGA
Thermogravimetric analysis (TGA) of the polymers indicates that all
polymers are thermally stable upto 300C with maximum 5% weight loss (Fig.1).
At 300 C, P1 showed only 0.7% weight loss. In P2, the incorporation of 3-methyl
thiophene unit in place of thiophene shows typical degradation pattern with
10

enhanced thermal stability of the polymer. As the molecular structure determines
the stability of polymer backbone, herein polymers are composed of highly rigid
aromatic rings which imparts high thermal stability. P3 showed nearly similar
weight loss pattern like P1 with quite high thermal stability while P4 is most stable
among all polymers which could be ascribed to the highly rigid two fused
thiophene rings therby contributing stability in polymer backbone. The carbonised
residue (char yield) of these polymers were found to be 7.3%, 57%, 37.5% and
67% for P1, P2, P3 and P4 respectively at 800C. The polymers P2-P4 showed
higher thermal stability than the previously reported polymer P1, which could be
attributed to the aromaticity of each individual linker.
3.2 Raman analysis
Fig. 2 shows the Raman spectra of the polymers. The highlighted region
of Fig. 2 indicates the Raman active modes of polymers, indexing has been done
according to the report by Hasoon et al. The major Raman active modes present
in these polymers are Ag+B2g for ⸹C-H(C-H in plane stretching of thiophene and
selenophene), 2Ag+B2g for C-C ring stretching and Ag+B2g for C=C
stretching.[27] The polymers P1 and P2 showed C-S bending peaks
corresponding to the thiophene unit at 742 and 740 cm-1 respectively. In case of
P3, C-Se bending is observed at 738 cm-1 while P4 shows two distinct peaks at
704 cm-1 and 733 cm-1 for C-S bending due to two fused thiophene rings. The
peak at 1168 cm-1 in both P1 at P2 corresponds to C-H inplane stretching
(Ag+B2g mode) of thiophene, P3 shows at 1176 cm-1 for C-H stretching of
selenophene and P4 shows at 1136 and 1166 cm-1 respectively for thieno[3,2-
11

b]thiophen ring. The inter ring symmetric stretching of Cα-Cα is observed at 1185,
1182 and 1275 cm-1 in P1, P2 and P3 respectively and whereas for P4, two
peaks observed at 1182 and 1201 cm-1. The peaks at 1332, 1329, 1322 and
1347 cm-1 correspond to N-Ar-N stretching in P1, P2, P3 and P4 respectively.
The symmetric ring stretching Cα-Cβ (2Ag+B2g mode) of thiophene unit in P1 and
P2 is observed at 1457 and 1465 cm-1 respectively, while P3 showed at 1457 cm-
1
for selenophene and thieno[3,2-b]thiophen (P4) showed at 1481 cm-1. The
peaks at 1507, 1509, 1503 and 1520 cm-1 corresponds to C-C and C=C
stretching of quinoid in P1, P2, P3 and P4 respectively. The C=C stretching
(Ag+B2g mode) of benzonoid ring for P1, P2, P3 and P4 appears at 1607, 1602,
1603 and 1595 cm-1 respectively.[26-30] The functional group interpretation of
the polymers from Raman spectra is corroborated with FTIR results which
provided the evidence of polymerization.
3.3 SEM
SEM micrographs (Fig.3) showed the characteristic morphologies of
polymers. The polymers P1, P3 and P4 exhibited porous structure assembled
with small spherical particles of about 400 nm, on the other hand P2 showed
highly porous polymer network. The molecular aggregation of the moieties
causes the different morphologies of polymers which might be due to different
molecular structures of TPA linkers. These well textured morphologies are
expected to lead fast diffusion of ions inside the porous structure so that the
12

innermost active sites of electrode get doped to reduce the charge trapping
during charge-discharge processes.
3.4 Electrochemical studies of Polymers
3.4.1. Cyclic Voltammetry (CV) studies
Fig. 4 shows the CV profile of polymers measured in 1 M TEAFB4/ACN
system in the potential window of 0 to 1.5 V at a scan rate of 25 mVs-1. The
electrochemical properties of the individual polymers were characterized by three
electrode method (vs Ag/Agcl). The CV plots of each polymer electrode at
different scan rates (i.e.10, 25, 50 and 100 mVs-1) are given in Fig.1S. CV curves
of polymers during p-doping exhibit a couple of anodic and cathodic peaks in
quasi-reversible electron transfer pattern. P1-P4 showed anodic peak at 1.2V,
1.15V, 1.12V and 1.23 V while cathodic peak at -0.67V, -0.69V, -0.67V and -
0.77V respectively. The potential difference between anodic and cathodic peak is
0.53V, 0.46V, 0.45V and 0.46V for P1, P2, P3 and P4 respectively. The lesser
the potential difference faster will be the charge-discharge processes because of
low charge transfer resistance at electrode-electrolyte interface and redox
reaction tend to be diffusion controlled.[31] Difference in oxidation potential can
be ascribed to difference in electronic density of thiophene, selenophene and
thieno(3,2-b)thiophene linkers on polymer backbone. According to Cravino and
co-workers, difference in - electron distribution along the polymer backbone is
responsible for the difference in lattice relaxation and vibrational behaviours of
the CPs due to electron-phonon coupling,[32] which can be accountable for the
13

low energy band gap and the enhanced charge carrier capability of the polymers.
The lowest oxidation potential of P3 is attributed to higher electron donating
character of selenium atom than that of sulphur atom. An introduction of methyl
group at thiophene linker also reduces oxidation potential of P2. Polymer P4
showed anodic/cathodic peaks at 1.23/-0.77V which is advantageous for
broadening the electrochemical potential window in practical application. Hence
polymers P2-P4 showed superior potential as electrode materials with fast redox
reversibility than P1. The peak specific capacitance of 477, 520, 439 and 546 Fg-
1
was achieved for P1, P2, P3 and P4 respectively. Whereas the actual specific
capacitance obtained by integrating area under the curve is 271, 254, 243 and
310 Fg-1 for P1, P2, P3 and P4 respectively at scan rate of 25 mVs-1. An
alteration in thiophene linker by 3-methyl thiophene, selenophene and thieno(3,2-
b)thiophene constitutes apparent change in specific capacitance of the polymers.
The high specific capacitance and high redox reversibility during voltammetric
scan are attributed to three dimensional structure of TPA core which helps to
maintain good charge carrier pathway in polymer backbone as well as porous
morphology for effective intercalation of electrolyte ions into the inner most part
of the polymer chains. The prominent enhancement found in P4 can be attributed
to thieno(3,2-b)thiophene linker for contributing more quinoidal character and
therefore the -electron inter-ring delocalization along the whole polymer.
3.4.2. Charge-discharge studies
The galvanostatic charge-discharge behavior of the as-prepared polymers
was investigated at different current densities. The specific capacitance of the
14

polymers was calculated from the linear portion of discharge curve after IR drop
using reported equation.[26] Triangular pseudocapacitive geometry and less IR
drop in charge-discharge profile (Fig. 5) indicate their potential in energy storage.
Polymer material in single electrode shows the specific capacitance of 278, 257,
246 and 315 Fg-1 for P1, P2, P3 and P4 respectively at 2.5 mAcm-2 current
density. These polymers showed high specific capacitance than the reported
polythiophenes due to presence of easily oxidizable electron rich TPA moiety to
form radical cation. The redox activity of the polymer backbone is responsible for
the enhancement in faradic charge storage since these materials are known for
fast electronic transport pathway.[33] The highest specific capacitance value for
P4 is attributed to two fused thiophene ring attached to the TPA core which
contributes more thiophene character to store the charge than the other
polymers. The polymer P3 was expected to exhibit high specific capacitance than
the P1 and P2. Elemental contribution of selenium with higher atomic size, easy
polarization of selenophene unit and intermolecular Se-Se interaction than
sulphur should enforce inter-chain charge transfer in polymer chain.[33]
According to Asit et al. polyselenophenes are able to attain higher doping level
than polythiophenes.[33] However it shows lesser specific capacitance than the
other thiophene based polymers might be as a result of increase in
intermolecular Se-Se distance in three dimensional dendritic structure.The
specific capacitance of P3 obtained in this study is still higher than regular
polythiophenes and polyselenophenes.[28]
15

3.5 Asymmetric device performance evaluation
3.5.1.CV of the asymmetric supercapacitor cell.
The supercapacitor cell capacitance is depends on voltage splitting across
each electrodes which is mainly dependant on mass and specific capacitance of
the active material used in each electrode. In symmetric supercapacitor the
applied voltage can split equally between the two electrodes because of the
same materials used in each electrodes with equal amount of active material.
However in asymmetric supercapacitors the voltage split depends on the
capacitance of the active material in each electrodes. Therefore to maintain the
voltage equally, electrode mass was optimized according to report by Snook et
al.[25] The coupling of electron rich aryl linkers with TPA core makes this
combination more electron rich to give an efficient p-doping than n-doping. As
these polymers are p-dopable, asymmetric supercapacitors have been
assembled with polymers as a positive electrode materials and AC as a negative
electrode. All the unit cells have been prepared by maintaining fixed amount of
activated carbon (1.25 mg cm-2) at negative electrode and amount of positive
electrode active materaial was varied to maintain the mass ratio as per the
reference [25]. Fig. 6 shows the CVs of AC electrode, polymer (P1) electrode and
polymer/AC (PD1) unit cell at 25 mVs-1. The asymmetric unit cell is scanned from
0 to 2.5 V, the potential sweep of negative (AC) electrode is varied from 0 to -1.5
V vs. Ag/Agcl and the positive (polymer) electrode is varied from 0 to 1.5 V vs.
Ag/Agcl reference. The CV profile of AC electrode shows rectangular shape CV
due to prominent electric double layer capacitance with coexitance of small
16

pseudocapacitance and the capacitance calculated from the CV profile was
found to be 100 F g-1. As mentioned earlier, polymer CV is potential dependant
due to prominent pseudocapacitance with minor electric double layer
capacitance. As a result, the CV profile of the asymmetric unit cell is a collective
contribution of these two capacitance. Below 1 V of the cell, the charge storage
contribution by polymer electrode is comparatively less and hence the current
response below 1 V is small. However once the oxidation potential of polymer is
reached the current response of the cell increases and attains almost a plateau
above 1V.
Fig. 7 shows the cyclic voltammogram of all hybrid AC/polymer devices at
different scan rates viz. 10, 25, 50 and 100 mVs-1 in 1M TEATFB in acetonitrile.
The specific capacitance of the cells was calculated using the equation 1.
Q1 Q2
Cellsc 
2mV
...........................(1)
where Q1 and Q2 are the sum of anodic and cathodic voltammetric charges
during oxidation and reduction of the scans respectively, m is the mass of active
material in both the electrodes of the cell and V is the potential difference. Q1
and Q2 value obtained by integrating the CV curve between 0.75 to 2.5 (V=1.75
V). The specific capacitance calculated for all polymer devices at various scan
rates are given in the table 2. Even at high scan rate of 100 mVs-1, the cells show
ideal capacitive behaviour, which is desirable condition for fast charge-discharge
of supercapacitor cells. The percentage change in capacitance values, when the
17

sweeping rate is change from 10 to 100 mVs-1 is the range of 13.0 to 16.9 %.
This indicates that supercapacitor cells have high rate capability. The change in
capacitance values between scan rates can be attributed to the following
reasons. At lower scan rate, the high specific capacitance of unit cell is attributed
to the lower concentration gradient at electrode/electrolyte interface which
provides balanced doping and de-doping of active materials at respective
electrodes. Hence the efficient redox reaction get active participation to cause
enhancement in specific capacitance at lower scan rate. However at higher scan
rate the concentration gradients at electrode/electrolyte interface increases and
results an inadequate charge storage and causes lesser specific capacitance
than lower scan rate.
3.5.2 Charge-discharge of unit cells
Since the unit cell performance was found to be appropriate in the voltage
range of 1V to 2.5V in CV profile, charge-discharge profile was expected to show
superior recital in the same range. Fig. 8 displays galvanostatic charge-discharge
of the asymmetric cells carried out at current density of 0.5 Ag-1 in the potential
range between 1V to 2.5V. The symmetry of the charge-discharge profile shows
good capacitive behavior. The specific capacitance has been evaluated from the
charge-discharge curves by following equation 1. The specific capacitance for
asymmetric devices PD1, PD2, PD3 and PD4 was found to be 37.8, 37, 36.7 and
38.9 Fg-1. All devices showed very negligible IR drop (~0.044 V) which attributes
18

to the fast redox reversibility of devices with remarkably low internal resistance.
These results are corroborated with that obtained by cyclic voltammetric studies.
3.5.3 Impedance analysis of unit cells
To further investigate the charge transport and ion diffusion in the
asymmetric supercapacitors based on dendritic conducting polymers we
measured electrochemical impedance by EIS, from which their Nyquist plots
were generated, as shown in Fig. 9. The x-intercept of the Nyquist plots
represents the equivalent series resistance (ESR) for two-electrode
supercapacitor and the charge transport resistance which is collective
contribution of (i) discontinuity in the charge transfer process at the electrode-
electrolyte interface (ii) intrinsic resistance of the active material (iii) contact
resistance between the active material and current collector and (iv) resistance
due to the faradic process.[35,36] Absence of semicircle in impedance plot (inset
of Fig.9) is the evidence for the low internal resistance at the interface of
electrode and current collector.[36] The slope of the Nyquist plots, known as the
Warburg resistance, is a result of the frequency dependence of ion diffusion in
the electrolyte to the electrode interface. As shown in Fig. 9, both the ESR and
Warburg resistance of the all asymmetric supercapacitor are lower. The
electrolytic resistance is almost same for all devices, indicating there is no much
change in ionic conductivity since the morphology of the polymers is almost
same. Three dimentional structures and the porous morphology of the polymers
provides higher specifc electrochemical surface area which enhances the
electrolyte penetration during redox reaction providing low Warburg impedance
19

since electrolyte diffusion get enhanced and contributes to reduce overall cell
resistance. This also consistent with the results obtained from CV and Charge-
discharge analysis which supports the diffusion control and fast redox reversible
reactions at electrode-electrolyte interface.
5.4 Cycling studies
Further the supercapacitor devices were subjected to continuous charge-
discharge cycles at constant current density of 0.5 Ag-1 to find out the
electrochemical stability of the cells and the results are shown in Fig.10. As the
electrochemical stability of AC is renowned up to 5,00,000 cycles [37], the
charge–discharge stability of asymmetric devices is significantly depends on
polymer stability for at least up to number of cycles reported in this study. In
particular, the cycling stability of PD2 and PD3 exhibits high initial degradation in
specific capacitance while PD1 and PD2 are relatively stable. The values of
specific capacitance loss during continuous charge-discharge for PD1, PD2, PD3
and PD4 are 24.6%, 33.2%, 32.9% and 21% respectively. Hence the
electrochemical charge-discharge stability of device is greatly influenced by the
stability of the polymer rather than AC. The highly porous morphology of the
polymers which provides effective diffusion of electrolyte ions at electrode with
less mechanical degradation. In practical, polymer stability can be explained on
some major issues caused during charging-discharging processes, when
polymers are subjected to continuous charge-discharge process they tends to
acquire charge trapping because of compactness or re-aggregation caused due
20

to looser stacking micro-structure of polymer matrix which affects the degradation
of polymer backbone. [13,34] These polymers showed higher electrochemical
stability than conventional polythiophenes based electrode materials reported
earlier.[38]
4.Conclusions
In summary, we synthesized a series of polymers containing TPA core
and thiophene, 3-methyl thiophene, selenophene and thieno(3,2-b)thiophene
linkers as a pseudocapacitive electrode materials for high-performance
supercapacitor applications. TGA showed high thermal stability of the polymers.
The microstuctured morphology of the polymers provided evidence to improved
intercalation of electrolyte in electrode sites. The electrochemical studies of the
polymers reveals that P4 (thieno(3,2-b)thiophene-TPA) showed highest specific
capacitance and good electrochemical stability than the other polymers. Further,
polymer/AC based asymmetric supercapacitors has been fabricated and
electrochemically explored with CV, charge-discharge and impedance analysis.
CV study showed specific capacitance of 38.5, 37.8, 37.6 and 39.0 Fg-1 for PD1-
PD4 respectively which matched with theoretical and charge-discharge studies
based calculations. FRA showed low internal resistance for all polymer devices
with evidence for diffusion control and fast redox reversible reactions at
electrode. These dentritic conducting polymer based electrode materials are
proved to be promising entrant for energy storage applications.
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Figure captions
1. TGA thermograms for polymers P1-P4 in N2 at a scan rate of 10 C min-1.
2. Raman spectra of polymers P1-P4 measured at 514.5 nm.
3. SEM micrographs of polymers P1-P4.
4. CV curves normalized to specific capacitance for polymers P1-P4 (scan
rate is 25 mVs-1).
5. Galvanostatic charge-discharge curves for polymers P1-P4.
6. CV of polymer electrode, AC electrode and unit cell of Polymer-AC
supercapacitor.
7. CV plots for polymer/AC devices at 10, 25, 50 and 100 mVs-1.
8. Charge-discharge profile of polymer/AC devices at current density 0.5 Ag-1
9. EIS of the polymer/AC based supercapacitor devices at different
frequency. The inset shows EIS at high frequency end.
10.Charge-discharge cycling stability of polymer/AC devices
27

FIGURES
Figure 1
28

Figure 2
29