A novel synthetic approach for designing metal-free, redox-active quinoxaline-benzimidazole-based organic polymers with high energy storage capacity

Article abstract of DOI:10.1039/c9nj02877c

New organic framework materials, namely, polyphenylquinoxaline (QOP) and polyphenylquinoxaline-benzimidazole (QOP-BOP) were designed using a high-temperature (>100 °C) polymerization reaction with different monomers, i.e., 2,5-bis-[(4-benzoylcarbonyl)phenyl]-3-4 diphenyl thiophene (BbcPDT), aromatic tetraamines and biphenyl dicarboxylic acid. The QOP-BOP copolymer exhibited specific capacitance (SC) of 305 F g^-1 at the current density of 2 A g^-1 and 88% retention of its initial specific capacitance after 1000 cycles, which resulted in good cyclic stability. This work establishes the first use of thiophene integrated with quinoxaline-benzimidazole units for energy storage applications and provides strategies for further developments in the performance of such conjugated materials. Cyclic voltammetry, charge-discharge and electrochemical impedance techniques were used to evaluate the electrochemical parameters, which demonstrated their potential in future energy storage devices.

Full text of DOI:10.1039/c9nj02877c

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A. Ghanwat, Y. Patil, I. A. Dhole, B. Kalshetti, S. R. Mane and V. Patil, New J. Chem., 2019, DOI:
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Volume 42
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March 2018
Pages 3963-4776
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Chechia Hu, Tzu-Jen Lin et al.
Yellowish and blue luminescent graphene oxide quantum dots
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Novel synthetic approach for designing metal free, redox active quinoxaline-benzimidazole
based organic polymers with high energy storage capacity.
a,c
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Pravin S. Salunkhe, Yuvraj S. Patil, Indrajeet A. Dhole, Basavraj S. Kalshetti, Vikas B.
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Patil, Shivshankar R. Mane, and Anil A. Ghanwat
a. Polymer Research Laboratory, School of Chemical Sciences, Solapur University, Solapur-
13255, (MS), India.
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b. Functional Materials Research Laboratory, School of Physical Sciences, Solapur University,
Solapur-413255, (MS), India.
c. Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune
E-mail : anil_ghanwat@yahoo.com
ph.salunkhe@ncl.res.in
ABSTRACT:
A new organic framework materials polyphenylquinoxaline (QOP) and polyphenylquinoxaline-
benzimidazole (QOP-BOP) has been designed through a high temperature (>100°C)
polymerization reaction using different monomers 2, 5-bis-[(4- benzoylcarbonyl) phenyl] -3-4
diphenyl thiophene (BbcPDT), aromatic tetraamines and biphenyl dicarboxylic acid. QOP-BOP
copolymer exhibited specific capacitance (SC) of 305 F/g current density at 2 A/g and 88 %
retention of its initial specific capacitance after 1000 cycles resulted good cyclic stability.
This work establishes the first use of thiophene integrated with quinoxaline-benzimidazole units
for energy storage applications and delivers strategies for further developments in the performance
of such type conjugated materials. Cyclic voltammogram, charge-discharge and electrochemical
impedance techniques are used to evaluate electrochemical parameters which advised its
upcoming potential in energy storage devices.
KEYWORDS: Thermal stability, BbcPDT, Mesoporous polymer, Supercapacitor, Structure
Property Relations.
INTRODUCTION:
Recently, a higher global environmental consciousness has sparked a widespread interest in the
development of clean and efficient energy storage systems. As a result, organic polymer-based
supercapacitors have been identified as environmentally beneficial energy storage systems due to
[1]
their simple, clean, and efficient operation . The carbon species, metals and conjugated polymer
electrode materials are used for energy storage devices. In which carbon containing electrodes

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with decent conductivity and stability generally have better cycling stability and high-power
density. Though, the energy density of these electrodes for capacitors are low because of
limitation of mechanism of energy storage. The metal related electrodes may have brilliant energy
storage response in supercapacitors because of their good activity but they still have difficulties
like less conductivity and big cost.[2] To overcome the limits of carbon and metal electrodes
synthesis of alternate electrodes such conjugated polymers (CPs) electrodes, like polypyrrole
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Ppy), polyaniline (PANi)[3] and also prepared polymers have significantly attention towards
energy storage/conversion, sensors as well as actuators and or many electrochromic devices. The
C, H, N or S components are also show the high affordability in synthesized polymers. We need to
return and look into organic redox-active materials for next-generation energy storage as active
materials, because they have great advantages in terms of desirable molecular structure, superior
chemical and physical properties, and excellent electrochemical storage performance compared
with new inorganic electrode materials like Robust SnO2−x Nanoparticle , black/red phosphorus
[4]
hybrid , graphite/electrolyte interface etc.[7] Therefore, organic redox-active materials can
provide excellent solutions to further improve existing ESS technologies and a diverse basis from
which to develop advanced ESSs. More precisely, redox-active polymers have been mainly
utilized for ESS applications. rather than organic single molecules with same redox groups due to
the well-known advantages of polymeric materials, such as good stability, excellent processability,
[5]
[6]
[8]
and easy device fabrication. Compared with electrochemical double-layer supercapacitors, the
conjugated polymers-based devices showed improved specific capacitance and has quicker
[4]
reaction kinetics QOP and QOP-BOP are an important class of aromatic heterocyclic, it has the
good thermal oxidative stability, batter thermoplastics properties and that have batter glass
transition temperatures as well as tensile properties[9-12] similarly, these have good proton
conductivity when appropriate doping.[13] Also, QOP are a historical and famous example of a
phenyl-functionalized thermally stable polymer whose bulky phenyl group’s leads to the increased
solubility results higher processability as compared to the without phenyl functionalized
polymer.[14-16] The π-conjugated organic polymers contain extended π-conjugation and have the
inherent electrical conductivity because of extended π-conjugated skeleton andionic conductivity
due to conduction of ions within the polymer backbones as promising candidate for the
supercapacitor as well as sensor materials.[17-19] Furthermore, high processability and
physicochemical properties inherent to polymers are ideal for investigation as supercapacitor
materials.[20] Recently different such types of organic polymers are reported e.g. formation of C-N
and C=N bonds as in the covalent organic frameworks via acid catalyzed condensation
reactions,[21] benzimidazole linked polymers (BILPs)[22] and BILPs and synthesized quinoxaline

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linked polymers (PLPs) have been paid much attention from the science community due to the
easy synthesis of the basic imidazole unit by the condensation of aldehyde and amine or
quinoxaline unit by the diketone and tetraamines.
Recently, we reported azo based polyazomethines containing heterocyclic quinoxaline active
redox unit into polymer backbone, it showed good electrochemical energy storage performance i,
e. 252 F/ g.[23] Xu et al. reported a radical functionalized COF and is used as an electrode material.
But the specific capacitance achieved is only 167 F/ g.[24] For MOF synthesized from carbon-
based electrode material, the specific capacitance 251 F/g remain far low the expected level.[25]
Jiang et al. and co-workers have also applied an azo fused π-conjugated microporous polymer as
supercapacitor electrode material with storage capacity of 378 F/g documented.[26] These materials
have good features for energy storage and conversion due to its mesopores nature for quick ion
mobility which helps interfacial charge accumulation through the mesopores. The classical
method for the synthesis of phenylated polyphenylquinoxaline was first reported by Hergenrother
et al. in 1967 and it involves the cyclopolycondensation of aromatic bis(o-diamines) with
aromatic tetra ketones in m-cresol.[27] Herein, we report a new high temperature synthetic strategy
for designing a metal free, redox active quinoxaline and quinoxaline-benzimidazole based organic
polymer via simple one-step polycondensation. The synthesized polymers have been evaluated as
an electrode material for supercapacitor application and its interesting results highlights the
potential of supercapacitor as a high-performance energy storage system for practical applications.
Experimental
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Synthesis of 2, 5-bis-[(4- benzoylcarbonyl) phenyl] -3-4 diphenyl thiophene (BbcPDT):
BbcPDT monomer was prepared according to our previous reported literature[23]
Yield. 2.90 g (88.95 %), mp.140 °C. 1H-NMR: (400 MHz, CDCl ppm) δ: 7.99 d (4H) J = 8.4
3
HZ, 7.85 d (4H) J = 8.4 Hz, 7.71-7.66 m (2H), 7.53 t (4H) J = 8.4 & 1.6 Hz, 7.38 d (4H) J = 8.8
Hz, 7.21-7.15 m (6H), 6.98 d (4H) J = 8.0 Hz.
Synthesis of polyphenylquinoxaline:
Polyphenylquinoxaline was synthesized by using the following general procedure (Scheme 1):
DAB (0.214 g, 1 mmol) and BbcPDT (0.652 g, 1 mmol) were added in a 100 mL three neck
round bottom flask in a nitrogen atmosphere. Polyphosphoric acid (20 g) was added to the flask
and the mixture was stirred under a slow nitrogen purge. The polymerization temperature for the
QOP homopolymer was 125 °C for 4.5 h. As the reaction continued, the viscosity of the solution
increased, and the colour of the reaction mixture was changed from brown to deep red. The
residual polymer solution in the flask was hydrolysed with distilled water. When hydrolysis of
the solution is completed, then the reaction mixture was heated and neutralized with ammonium

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hydroxide. The polymer was finally isolated and dried under vacuum to provide the
polyphenylquinoxaline powder.
Synthesis of polyphenylquinoxaline-benzimidazole copolymer (QOP-BOP).
QOP-BOP copolymers were set across a wide range of QOP concentrations:
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Diaminobenzidine (DAB) (2.52 g, 11.8 mmol), BPDA (0.977 g, 5.90 mmol), and BbcPDT (0.97
g, 5.90 mmol) were charged in a 100 mL three neck round bottom flask in a nitrogen
atmosphere. Polyphosphoric acid (20 gm) was added in flask and the mixture was stirred by
stirrer under a slow nitrogen purge. The reaction mixture temperature was increased stepwise to
dissolve all the monomers in PPA, and then polymerize the QOP-BOP portions of the
copolymer. For QOP-BOP, the polymerization temperature was held at 125 °C for 8 h, followed
by an increase in temperature up to 195 °C for an additional 6 h. Note that the time at 195 °C
varied based on the composition of the QOP-BOP copolymer, but the time necessary for low
temperature QOP formation at 125 °C was constant across the QOP-BOP copolymer series. For
all QOP-BOP copolymers it was needed to first obtain a highly viscous final polymer solution,
followed by the addition of phosphoric acid to reduce solution viscosity in order to obtain a high
inherent viscosity, polymer solution with good processability.
Result and discussion:
A new 2, 5-bis- [(4- benzoylcarbonyl) phenyl] -3-4 diphenyl thiophene (BbcPDT) was synthesized
in 3 steps (Presented in scheme-1). QOP homopolymer and QOP-BOP copolymer were
synthesized by polyconderisation of DAB with BbcPDT by high temperature synthetic strategy
via PPA process (scheme 2 and 3). The composition of monomers yields, and viscosities of
polymers are listed in Table 1. The structure of BcbPDT and BbcPDT was confirmed by
spectroscopic analysis. Infrared spectrum of BcbPDT (Figure 1) showed a strong absorption band
-1
at 1684 cm (>C = O) for aryl alkyl carbonyl structure and spectrum of BbcPDT showed a strong
-1
absorption bands at 1670 cm for dicarbonyl (>C = O).
Scheme 1: Synthetic scheme for BbcPDT.

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Reagents and conditions:
a) Reflux, 240 °C, 3 days. b) PhCH COCl, AlCl , DCM 10-15 °C. c) SeO , Glacial
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Acetic Acid, Reflux 12 hrs.
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1H NMR of BcbPDT could not be taken due to insolubility of the compound BcbPDT even in
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chloroform, dimethyl sulfoxide or trifluoroacetic acid. The H NMR spectrum of BbcPDT
conform the structure assigned as evidenced by the chemical shift (all aromatic protons 8.07 to
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.1); number of NMR signals and expected multiplicity of the peaks with appropriate integration
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and no signals due to -CH, in the range of 1 to 6 ppm (Figure 2). C NMR of BcbPDT (Figure 3)
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also consistence with the H NMR spectroscopy. The two carbonyl carbon appeared at 193 and
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94 ppm confirms formation of tetraketone monomer.
Figure 1. FTIR of BcbPDT and BbcPDT.

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Figure 2: H NMR of BbcPDT.
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Figure 3: C NMR of BbcPDT.

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Scheme 2. Synthetic route for QOP homopolymer.
Scheme 3. Synthetic route for QOP-BOP copolymer.

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Figure 4: Mode of multi-block formation of QOP-BOP copolymer.
Figure 4 showed multi-block formation of copolymer containing quinoxaline and
benzimidazole unit. The formation of polymer was confirmed by FTIR and NMR
spectroscopy. Figure 5 showed the FTIR spectra of polymers. FTIR spectrum of QOP
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showed absence of absorption band at 1670 cm indicating ketonic functionality of BbcPDT
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Figure 5: FTIR spectra of QOP and QOP-BOP.
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Figure S-1 showed the H NMR spectra of QOP (because of poor solubility of QOP in
DMSO, so CDCl were used). All aromatic protons were well assigned to the supposed
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chemical structure. The integration ratio of the peaks corresponded to the polymer
composition expected from the feed monomer. Aromatic protons were strongly shielded, and
their signals appeared at low frequency (7.0-7.9 ppm) area, while the protons located at
ortho-positions of –C=N– were deshielded due to their strongly electron-withdrawing effects,
and their signals appeared at high frequency area (7.9-8.6 ppm). No characteristic chemical
shift of –NH (around 5 ppm) was observed, which also indicated that a fully cyclization of
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quinoxaline has been attained through the polymerization progress. NMR of QOP-BOP could
not be taken due to insolubility even in chloroform, dimethyl sulfoxide or trifluoroacetic acid.
Thermal properties and crystallinity:
Thermal properties of synthesized polymers were evaluated by thermogravimetry and results
are given in figure 6 and table-1. The polymers showed the initial decomposition temperature
between 205 to 273 °C and T in the range of 433 to 779 °C whereas the residual weight at
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stable.

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Figure 6: TGA curves of QOP and QOP-BOP.
The glass transition temperatures (Тg) of the polymers evaluated from the DSC curves
figure-7). They were in the range of 256-291 °C. QOP showed no evidence of crystalline
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peaks, which proves an amorphous morphology result slightly, lowers the glass transition
temperature. This decrease in Тg is due to the increase in the flexibility of the polymer
backbone due to the pendent phenyl groups present in monomer segment.
Figure 7: DSC curves of QOP and QOP-BOP.
As shown in figure 8 the crystallinity of synthesized polymer films was analysed by wide
angle X-ray diffraction. The WAXD patterns of QOP showed broad peak, which indicated
that it is more amorphous pattern. The QOP-BOP also showed broad peak with some
crystalline peaks was in polymer backbone which indicated that it was less amorphous

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pattern. Some crystalline peaks were observed in QOP-BOP may be due to rigid
benzimidazole unit in polymer backbone. The more amorphous nature of QOP can be mainly
explained by the introduction of bulky pendent groups and then greatly decrease the
crystallinity of polymers.
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Figure 8: WAXD of QOP and QOP-BOP.
Polymer solubility yield and viscosity.
QOP homopolymer was soluble in chloroform, tetrachloroethane, tetrahydrofuran and m-
cresol etc. QOP-BOP copolymer was soluble in tetrahydrofuran and m-cresol etc. However,
they are partly soluble in aprotic polar solvents such as dimethylacetamide (DMAc),
dimethylformamide (DMF) and N-methylpyrrolidone (NMP) and partly soluble in
chlorinated solvents. These polymers were insoluble dimethyl sulfoxide (DMSO). A 3 %
solution was taken as criterion for solubility. On qualitative comparison, it was observed that
the polymers containing higher % of BbcPDT i.e. QOP was more easily soluble than QOP-
BOP.
Table 1: Yield, viscosity and thermal properties of QOP and QOP-BOP from TAB.
Residual
Yield Viscosity Thermal Properties
Glass Transition
Temperature (Tg)
Polymer Code
Weight at
o
o
(%)
(dL/g)
T
T
10 ( C)
i ( C)
o
9
00 C (%)
QOP
98
99
0.63
0.82
245
273
663
779
57
68
256
304
QOP-BOP

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Morphology and surface wettability study:
FE-SEM image of QOP figure 9 (a) shows the nanoparticles like morphology which have
some particles aggregations are also occurred. The figure 9 (b) obviously shows that the
nanoparticles which are homogeneous and well defined with particles have a narrow size
distribution with an average size of 100-170 nm.
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Figure 9: FE-SEM images QOP (a) and QOP-BOP (b). Water contact angle
measurements of QOP (c) and QOP-BOP (d).
Figure 10: TEM images QOP (a) and QOP-BOP (b).
TEM images of QOP and QOP-BOP (figure 10) consistence with the FE-SEM results.
Energy dispersive X-ray spectroscopy (supporting information figure S-2, section S-3) shows
the peaks around 0.1 and 2.3 KeV are related to the energies of S, along with the peak of
nitrogen and carbon at 0.4 and 0.3 KeV respectively. Therefore, the EDX confirmed the
presence of sulphur, nitrogen and carbon in QOP-BOP. The atomic percentages obtained
from EDX quantification were 85.93% of C, 12.88% of N and 1.19% of S. To study the
contact at the electrode/electrolyte interface, a wettability test was carried out. The average
contact angle was acquired by measuring for at least five separate drops on polymer film

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surface by delivering deionised water. Measurement of surface water contact angle is
inversely proportional to the wettability and can be determined by Young’s relation. Figure 9
c and d) shows the digital photograph of water contact angle measurement on synthesized
(
polymer electrode surfaces. Interestingly QOP and QOP-BOP electrode surfaces has
exhibited hydrophilic behaviour with 54° and 47° water contact angle values respectively. i.e.
< 90°. Generally, low water contact angle (hydrophilic behaviour) increases the
electrochemical performance, where interfacial contact at electrolyte-electrode is
considerably high.[28, 29] The surface of QOP and QOP-BOP thin films is hydrophilic which is
useful for forming intimate contact between aqueous electrolyte and thin films in
supercapacitors. Due to the mesoporous nature of synthesized polymers, water can easily
intercalate inside the pores. Thus, positive ions in aqueous electrolyte can easily intercalate or
de-intercalate, i.e. ionic conductivity increases and hence the supercapacitance increases.
Surface area and pore size distribution:
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The samples were characterized by nitrogen adsorption and desorption isotherms at 77 K as
shown in figure 11 (a and b). This isotherm shows nitrogen uptake at 1 bar pressure
suggestive of porous nature of the synthesized polymers. In terms of shape, the isotherms of
the QOP and QOP-BOP structures show hysteresis loops that specify the presence of
mesopores with size ranging from 2 to 50 nm.[30] QOP and QOP-BOP samples show BET
surface areas of 45.851 m²/g and 11.056 m²/g respectively. These data indicate that the QOP
have a larger surface area than the QOP-BOP. The measured BET surface area of QOP-BOP
2
(
11.056 m /g) was lower than QOP. It is noticing that the polymer reported here is copolymer
containing only a small amount of comonomer. Speciously, at such low comonomer content,
its impact on surface area is limited.[31] Many polymers reported in the literature are
homopolymers containing large amounts of conformationally rigid monomers whose effect
on surface area is considerable. It is supposed that in such isotherm’s ascorbate-adsorbent
interactions play an important role. The adsorption isotherms show a slow increase in middle
level relative pressures and a steeper increase at higher pressures. Such behaviour has been
reported previous as characteristic of mesoporous.[32] The BJH pore size distributions plots
(
figure 11 c and d) of QOP and QOP-BOP showed a narrow distribution which designated
that the pores size was homogeneous. The average pore sizes calculated using the BJH
equation from the adsorption branch of the isotherms of QOP and QOP-BOP was found to be
2
3.4 and 21.2 Å respectively. Pore size distribution was estimated by fitting the uptake
branches of the nitrogen isotherm with nonlocal density functional theory and half pore
width of QOP and QOP-BOP found to be 10 Å and 13 Å respectively as well as pore

New Journal of Chemistry
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volume is 0.100 and 0.022 cc/g respectively. NLDFT data presented in supporting
information section S-5 and figure S-6.
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Figure 11: N Adsorption-desorption isotherm of QOP (a) and QOP-BOP (b). BJH pore
2
size distribution plots of QOP (c) and QOP-BOP (d).
Electrochemical analysis:
The electrochemical charge storage activity of the prepared QOP and QOP-BOP electrode
materials was analysed by cyclic voltammetry (CV) galvanostatic charge discharge (GCD)
and electrochemical impedance spectroscopy (EIS) in 1 M Na SO electrolyte. For the three-
2
4
electrode systems, saturated calomel electrode (SCE) served as the reference electrode,
2
platinum acted as the counter electrode and the prepared electrode (1 cm ) was used as
working electrode. The working electrode was fabricated by dissolving active material in
2
chloroform, then coated on the stainless-steel substrate (1 cm ) and heated at 50 °C in
vacuum oven for 10 hrs. The mechanism of the polymer QOP-BOP is presented in Figure 12.
Firstly, the nitrogen of benzimidazole as well as quinoxaline in QOP-BOP oxidized at low
applied potential and loses one electron to form the benzimidazole and quinoxaline radical

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cation, which is equivalent to the resonance form II. In II, the electron pair and the single
3
electron are both in sp hybrid orbitals. Owing to the distinct coplanar structure between
quinoxaline, benzimidazole and thiophene, there are three resonance forms (II, II#, and II*)
of this radical, representing the delocalization of the single electron over the quinoxaline,
benzimidazole and thiophene units, the most stable form of which should be II* where the
single electron is on the thiophene unit. The mechanism involved in QOP is same as QOP-
BOP.
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Figure 12. Electrochemical mechanism for oxidation of QOP-BOP.
The CV recorded with prepared QOP electrode over different scan rates, it shows redox
peaks at ~ -0.5 V during forward (anodic) and backward (cathodic) sweep it suggested that its
pseudo capacitive nature[33] while for QOP-BOP material, when a potential is applied to a
pseudocapacitor cell oxidation and reduction takes place on the polymer electrode material,
which involves the passage of charge across the double layer, resulting in faradic current
passing through the supercapacitor cell. The faradic process involved in pseudocapacitor
allows them to improve specific capacitance compared to electrical double layer capacitors.
Pseudocapacitor store charge via faradic process which involves the transfer of charge

New Journal of Chemistry
Page 16 of 25
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between electrode and electrolyte.[34] Further the lengthening of peaks indicates the insertion
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DOI: 10.1039/C9NJ02877C
of ions onto the electrode materials.[35] The specific capacitance of the electrode material is
estimated from following equation,
Csp=
(IdV)/vmV
(1)
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Where I is response current, V is potential window, ν is potential scan rate, and m is mass of
the active materials respectively. The electrochemical properties of polymer films were
investigated in aqueous media viz. 1M Na SO as electrolyte at different scan rates because
2
4
the resistance of the supercapacitor material is independent on the resistivity of the electrolyte
and the size of ions from the electrolyte. The organic electrolytes have a greater resistance,
but the adjunct power reduction is usually offset by the increase in higher cell voltage. This is
usually not a problem for an aqueous electrolyte and aqueous electrolytes are cheaper as well
as it purifies easily.[36] Moreover, the concentration of electrolytes increases, specific
capacitance also increases due to the take off the material from substrate, so greater than 1 M
concentration was avoided.[37] Usually, the CV curve of polymers shows a typical rectangular
shape without redox peaks, and the energy is stored by electrical double-layer (EDL) between
polymer electrode materials and electrolytes (non-faradic mechanism)[38, 39] Typical CV
curves of the synthesized polymers in 1M Na SO measured at various scan rates are given in
2
4
figure 13 (a and b).

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0
0
0
.00024
.00016
.00008
0
0
0
.0004
.0002
.0000
(a)
(b)
0.00000
0.00008
0.00016
-0.0002
-0.0004
-0.0006
-0.0008
-0.0010
-
-
1
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-0.00024
-0.00032
2
mV/s
2
2
5
mV/s
5mV/s
0mV/s
25 mV/s
50 mV/s
100 mV/s
-0.00040
100mV/s
-1.2
-1.0
-0.8
-0.6
-0.4
-0.2
0.0
-1.0
-0.9
-0.8
-0.7
-0.6
-0.5
-0.4
Potantial (V) vs SCE
Potantial (V) vs SCE
Measured
Stimulated
Measured
Stimulated
2000
1500
1000
(
c)
4000 (d)
3000
CdI
CdI
CPE
2000
1000
0
CPE
5
00
Rs
Rs
0
R1
R1
R2
R2
0
250
500
750 1000 1250 1500
0
250 500 750 1000 1250 1500 1750
Z' (ohm)
Z' (ohm)
re
re
Figure 13: comparison of CV curves of QOP(a) and QOP-BOP (b) thin film electrode at
different scan rates of 2, 25, 50 and 100 mV/s in 1 M Na SO electrolyte.
2
4
Electrochemical impedance spectra of QOP(c) and QOP-BOP (d).
The CV curves of synthesized QOP and QOP-BOP shows redox peaks therefore, the
capacitance of synthesized polymer materials is mainly based on redox reactions (faradic
mechanism).[40] The shape of the CVs for these synthesized polymers remains unchanged
from 2 to 100 mV/s indicating good rate properties. Another proof, for good rate ability of
these materials is the current increases with the scan rates, as can be seen in figure 13. It
showed that the specific capacitance changes with the change in scan rate, so that higher
specific capacitance was observed at low scan rate. (The scan rate vs sp. capacitance figure
given in supporting information section S-4). As the scan rate, has increased, the specific
capacitance has decreased, which may be explained by the increase of ions concentration at
the electrode/electrolyte interface while the diffusion rate of electrolyte from
electrolyte/electrode interface remain limited to assure electrochemical reactions.[41] The
QOP and QOP-BOP polymers documented maximum specific capacitance of 36 and 59 F/g
is at 2 mV/s scan rate.

New Journal of Chemistry
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In case of conjugated polymer films, due to the electrochemically active nature of the
polymer itself, a highly adaptable technique such as the electrochemical impedance
spectroscopy (EIS) is ideal to accommodate the complex processes involved. Traditionally,
EIS is applied to study the AC polarography and in determining double layer capacitance at
the electrodes.[42] Electro-active polymers have been widely studied in last two decades
because of their potential applications such as in electrocatalysis, energy storage, and sensors.
In the current article, the electrochemical features of prepared QOP and QOP-BOP electrode
surfaces were subjected to EIS analysis. The Nyquist plots of these polymer films deposited
on stainless steel plate are shown in figure 13 (c and d). An equivalent circuit has been used
to collect valuable information about different electrochemical parameters defining the
process.[43] The equivalent circuit and parameter values by fitting the impedance data of QOP
and QOP-BOP films to equivalent circuit shown in figure 13 and table S-1 (supporting
information section S-3). The model circuit comprised of eight elements, the solution
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resistance (R ) was in series with the electrical double layer capacitance (C ) at the electrode
s
dl
and electrolyte. C was in parallel with R and (R , W, and Q or CPE) is used to evaluate the
dl
1
2
different resistive and capacitive characteristics of QOP and QOP-BOP electrode. The
Warburg impedance was associated with the semi-infinite diffusion of ions in the QOP and
QOP-BOP electrode[44] suggested its good electrocapacitive activity. EIS measurement of
QOP and QOP-BOP electrode measured within the frequency range of 1 MHz to 0.1 Hz in 1
M Na SO electrolyte.
2
4
The charge-discharge measurements of the synthesized electrodes were carried out by using
chronopotentiometry from -1.00 to 0.0 and -1.0 to -0.4 V respectively at different current
densities as shown in figure 14 (a, b). The shape of the measured charge and discharge curves
is not an ideal straight line, indicating that the observed behaviour is a result of the faradic
type of redox reaction[45] The charging process of polymer electrode occurs above -1.0 V in
the Na SO electrolyte because of the oxidation and in the discharging process, no potential
2
4
drop is observed indicates a good interfacial contact between the active material and the
substrate during the charge-discharge process[46] All the GCD profile (figure 12 a, and b)
showed near triangular characteristics resulting ideal charge storage behaviour. It is quite
important to note that the discharge consists of two characteristics region having different
discharge rate. First region is the rapid decay up to -0.4 and -0.5 V and then slow decay over
rest potential window. The rapid decay is responsible for the EDLC formation and slow
decay contributed from charge stored via redox reaction at electrode surface i. e,

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pseudocapacitance.[22] The specific capacitance of prepared QOP and QOP-BOP polymers
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over different current density is estimated from following equation,
Csp = (I. Δt) / (Δv. M)
(2)
Where, ‘I’ represents the applied current, Δv stands for potential range (V) and ∆t denotes the
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discharge time (s) and ‘m’ signifies mass of active material.
Figure 14: Galvanostatic charge–discharge curves of QOP (a) and QOP-BOP (b)
electrodes. Cycling performance of QOP (c) and QOP-BOP (d) electrodes (capacitance
as a function of cycle number).
A maximum specific capacitance of QOP-BOP is 305 F/g documented at 2 A/g current
density. The high specific capacitance attained with the present QOP-BOP electrode is
credited to the presence of redox active imidazolium and quinoxaline moiety. The obtained
energy storage performance of the synthesized polymer material found to be outstanding than
β-ketoenamine-linked covalent organic framework[47a] and conjugated microporous
polymer[47b] (48 at 0.1 A/g and 252 at 1A/g) respectively and radical functionalized COF (167
F/g at 0.1 A/g).[48] Moreover, the specific capacitance of QOP-BOP is comparable with the
reduced graphene oxide hydrogels (252.6 F/g at 1 A/g),[49] N-rich hollow carbon sphere (230

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F/g at 1A/g),[50] the hierarchically porous carbon-AS-ZC-800 (251 F/g at 0.25 A/g),
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interconnected microporous carbon (258 F/g at 0.5 A/g),[52] and nitrogen enriched porous
carbon sphere (388 F/g at 1 A/g).[53] Additionally, the specific capacitance of QOP-BOP
electrode material is analogous to metal oxide and oxide base nano composite materials, like
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[54]
HTiO nanocrystal/N-doped graphene (385.2 at 1A/g), PANI-doped graphene composite
2
(480 F/g at 0.1 A/g)[55] and MnO2/CNT (201 at 1 A/g).[56] Thus, the present metal free QOP-
BOP can be used as a potential electrode material for electrochemical energy storage and the
structural restriction of covalent organic framework limits charge storage, which is
completely absent on QOP-BOP electrode material. The cyclic stability of prepared QOP and
QOP-BOP electrodes has been tested for 1000 cycles at 2 A/g current density and shown in
figure 14 (c and d). The present QOP and QOP-BOP electrodes retain 52 and 88 % of its
initial specific capacitance after 1000 cycles resulting excellent electrode material for
supercapacitor application.
CONCLUSION:
We have designed new high temperature (>100 °C) polymerization method for the
preparation of non-cross linked QOP homopolymer and multi-block QOP-BOP copolymer
via PPA process. The resulting polymers were highly thermally stable and soluble in some
organic solvents. Presence of characteristics bands and expected elemental peaks of monomer
and polymer was confirmed from FTIR and NMR spectroscopy. The XRD patterns showed
that the QOP and QOP-BOP have amorphous nature. Thermogravimetric analysis of
polymers showed 10 % weight loss in the range of 433 to 779 °C indicating good thermal
stability. The SEM picture of the QOP and QOP-BOP powder shows that the spherical
nanoparticles. They are homogeneous with average size of about 100-170nm. The 51 and 47°
water contact angle values of QOP and QOP-BOP was evidenced from the surface wettability
measurement test, which favours for an excess infiltration of electrolyte ions for more redox
reactions through wide range of area coverage. QOP-BOP electrodes have demonstrated a
maximum specific capacitance of 305 F/g at 2 A/g in 1 M Na SO electrolyte. The overall
2
4
improved electrochemical performance of the copolymer QOP-BOP electrode prepared via
high temperature PPA method makes it a promising material for potential applications in high
performance supercapacitors because of its low cost, simple preparation and environmental
friendliness.
ACKNOWLEDGEMENT
The author AAG thanks to University Grant Commission, New Delhi for financial assistance
in the form of major research project. (Scheme No.: -UGC-MAJOR-MRP-CHEM-2013-

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9804). Author would like to thank Dr. Prakash P. Wadgaonkar for BET surface area
DOI: 10.1039/C9NJ02877C
measurement.
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There are no conflicts to declare.
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DOI: 10.1039/C9NJ02877C
Novel synthetic approach for designing metal free, redox active quinoxaline-benzimidazole
based organic polymers with high energy storage capacity.
a,c
a
b
a
Pravin S. Salunkhe, Yuvraj S. Patil, Indrajeet A. Dhole, Basavraj S. Kalshetti, Vikas B.
b
c
a*
Patil, Shivshankar R. Mane, and Anil A. Ghanwat
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a. Polymer Research Laboratory, School of Chemical Sciences, Solapur University, Solapur-
413255, (MS), India.
b. Functional Materials Research Laboratory, School of Physical Sciences, Solapur University,
Solapur-413255, (MS), India.
c. Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Pune
E-mail : anil_ghanwat@yahoo.com
ph.salunkhe@ncl.res.in
Graphical Abstract:
We established the first use of thiophene integrated with quinoxaline-benzimidazole unit for
energy storage applications and delivers strategies for further developments in the performance of
such type materials.