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Accepted Article
CHEMISTRY
AN ASIAN JOURNAL
A sister journal of Angewandte Chemie
and Chemistry – A European Journal
A Journal of
www.chemasianj.org
Title: Polyaniline Stabilized Intertwined network-like Ferrocene/
Graphene Nano-architecture for Supercapacitor application
Authors: Amrita De Adhikari, Ramesh Oraon, Santosh Kumar Tiwari,
Naresh K Jena, Joong Hee Lee, Nam Hoon Kim, and Ganesh
Nayak
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content of this Accepted Article.
To be cited as: Chem. Asian J. 10.1002/asia.201700124
Link to VoR: http://dx.doi.org/10.1002/asia.201700124
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Polyaniline Stabilized Intertwined network-like Ferrocene/Graphene
Nano-architecture for Supercapacitor application
Amrita De Adhikari,[a] Ramesh Oraon,[a] Santosh Kumar Tiwari,[a] Naresh K. Jena[b], Joong Hee
Lee, [c,d] Nam Hoon Kim,[c,d] , and Ganesh Chandra Nayak*[a]
Abstract: The present work highlights the effective H-π interaction
between metallocene (Ferrocene (Fc)) and graphene and their
stabilization in presence of Polyaniline (PANI) via π-π interaction.
PANI stabilized Fc@graphene nanocomposite (FcGA) resembled an
intertwined network like morphology with high surface area and
porosity can make it a potential candidate for energy storage
application. Further, the relative interactions between the
components were assessed through theoretical (DFT) calculations.
Specific capacitance calculated from galvanostatic charging
discharging presented that PANI stabilized ternary nanocomposite
exhibited maximum specific capacitance of 960 F/g at energy
density 85 Wh/Kg at current density 1 A/g .Furthermore, EIS
analysis also confirmed the low internal resistance of as prepared
nanocomposites depicting improved charge transfer property of
graphene after incorporation of Fc and stabilizing with PANI.
Additionally all electrodes were found to be stable up to 5000 cycles
with a specific capacitance retention of 86% demonstrating good
reversibility and durability of the electrode material.
Introduction
In wake of the global climate change associated with the
declining accessibility of the fossil fuels, the society
necessitates tangible steps towards the development of
sustainable and renewable energy resources to encounter the
future energy crisis [1]. In order to recompensate such energy
demands, renewable energy production from natural sources,
i.e. wind, water etc. as well as hybrid electric vehicles have
attracted immense attention in this 21st century [1]. Over the past
few decades immense studies have been carried out on
secondary batteries such as fuel cells and lithium ion batteries
(LIB) [2]. Nonetheless, in recent years, besides LIB,
supercapacitors (SCs) also known as the ultracapacitors, has
been recognized as a promising system for various emerging
energy storage technologies [3]. Such SCs, compared to the
traditional capacitors have received much attention and is well-
thought-out candidate owing to its intriguing advantages like,
high power density, light weight, fast charge-discharge process,
environmental friendliness and a lifelong stability [3]. In order to
improve the performance further, different surface chemistry
and device configuration have been proposed and investigated.
Several types of SCs can be distinguished depending upon the
charge storage mechanism and the active materials being used
[1]. The electrical double layer capacitors (EDLC’s) store energy
through ion adsorption on the carbonaceous material which
often offers a very high power density due to fast charge
recombination [3]. In contrast to the electrical double layer, the
pseudocapacitors provide higher specific capacitance due to
their fast and reversible redox reaction [3]. Transition metal
oxides, conducting polymers, organometallic compounds are
some of the common examples of the pseudo capacitive
materials [1]. Thus, the electrochemical capacitors currently fill
the gap between the batteries and the conventional capacitors
and hence act as an umbilical cord by storing hundreds and
thousands of more charges. However, they do have less
energy density than batteries which makes it a candidate of
research for increasing their energy performance and widening
the temperature limits where the batteries cannot operate [4].
The key to reach a very high specific capacitance is by
using high specific surface area and electroactive materials.
Graphitic carbon (especially graphene), the unique material of
[a] A. De Adhikari, R.Oraon, S.K.Tiwari, G.C. Nayak*
Department of Applied Chemistry
IIT(ISM) Dhanbad, 826004
Jharkhand,INDIA
E-mail:nayak.g.ac@ismdhanbad.ac.in
[b] Naresh K.Jena
Condensed Matter Theory, Materials Theory Division,
Department of Physics and Astronomy, Uppsala University,
Box 516, SE-751 20, Uppsala, Sweden
[c] J.H.Lee,N.H.Kim
Department of BIN Fusion Technology
Chonbuk National University,Jeonju
Jeonbuk 571-756, Republic of Korea
[d] J.H.Lee,N.H.Kim
Dept of Polymer & Nano Science and Technology
Chonbuk National University,Jeonju
Jeonbuk 571-756, Republic of Korea
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today’s generation, with a two dimensional single layer
structure satisfies all the requirements for such application.
High conductivity, electrochemical stability and open porosity
make it a suitable candidate [5]. Graphene can act as an
electron donor or an electron acceptor and has a direct
dependence on its electronic structure. The electronic
properties can thus be controlled by means of doping,
molecular charge transfer, functionalization or intercalation [6].
Chemical functionalization can facilitate the hydrogen storage,
spintronics and decoration of the defect sites which leads to
modification of the electronic properties by varying energy [6].
The non-covalent modification of graphene involves the strong
π-π interaction between the graphitic sheet and the guest
moiety (especially the pseudocapacitive materials). Many such
pseudo-active materials are investigated which includes the
electronically conducting polymers (ECPs) (i.e. polypyrrole
(PPY), [7] (PANI) [8] PEDOT etc), transition metal oxides (e.g.
RuO2,MnO2, V2O5 etc) [9] as well as other organometallic
compounds. Among various ECPs, PANI is considered as a
promising electroactive material due to its interesting properties
like variable oxidation state, high electronic conductivity, ease
of processability, environmental stability and flexibility [10].
However, the problem of swelling and shrinkage associated
with PANI makes its use restricted alone [11]. Recently in order
to overcome the restricted application of PANI, graphene-PANI
nanocomposites have drawn immense attention. A vast
research has already been carried out on graphene-PANI
electrode materials and their application in the SCs. Cao et al
have reported the synthesis of graphene-PANI nanocomposite
via in-situ polymerization which offered a specific capacitance
(SC) of 338Fg-1 at 20mV/s in 1(M) H2SO4.[12] Wu et al have
reported the synthesis of graphene-PANI nanofiber which
offered a specific capacitance of 210 Fg-1 [13(a)]. Sun et al and
Xiao et al have reported graphene-PANI based paper electrode
material for the SC’s application [13(b), 13(c)]. But all the above
mentioned synthesis was carried out via electrodeposition
which is less feasible and a bit cumbersome. Moreover the
electrolyte used was 1(M) H2SO4 i.e. an aqueous electrolyte
rather than an organic electrolyte resulting in certain drawbacks
and the specific capacitance reported was also less as
compared to the present specific capacitance. As well as all
these SC values are very less and can contribute to much
lesser power density. Thus in order to enhance the
electrochemical activity, various modifications of the
nanocomposites was carried out.
Here in, we have investigated the electrochemical
performance of the graphene (G) in presence of metallocenes
which also constitutes π electron cloud. Such approach has
been taken as it is believed that a molecular level interaction of
two components at the nanometer scale can render advanced
electrical conductivity[14].Ferrocene (Fc) is one such
metallocene which has a vast application owing to its dynamic
chemistry and versatile electrochemical behavior. This
compound has a sandwich-like molecular structure consisting
of two cyclopentadienyl rings with Fe atom squeezed in
between [14]. Fc is a well-known mediator due to its various
properties, such as reversibility, regeneration at low potential,
and generation of stable redox states which can be easily
controlled. The driving force for the electron transfer process is
the strong H-π interaction between the metallocene molecule
i.e Fc and the layered G which have not been studied so far.
The Fc molecule offers an improved thermal stability arising
from the π-conjugation of the system. RGO based
nanocomposites can feature both the conducting π state of the
sp2 carbon and the σ state of the sp3 bonded carbon atoms [15].
Here, an approach has been done to develop a charge transfer
donor- acceptor composite of the Fc based r-GO/PANI for
energy storage application.
In order to assess the relative interactions between G, Fc
and PANI we have computationally modeled these individual
systems before synthesis process was carried out, as shown in
Figure 1. Density functional theory (DFT) calculations with
M06-2X functional [16] and 631G* basis sets for C and H atoms
and LANL2DZ pseudopotential for Fe using Gaussian09 [17]
program for such study. The overall study revealed the
interaction between individual components were non-covalent
in nature, thus the M06-2X functional developed by Truhlar et
al. was chosen which has been quite successful in describing
non-covalent interactions [16]. Several possible binary complex
structures were considered in order to investigate the modes of
interaction between the individual components viz. G, Fc and
PANI and the optimized geometries of complex structures as
presented in Figure 1. The binding energy/interaction of these
structures presented in Table 1.
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Table 1
There have been several propositions by various research
groups which suggests Fc favorably interacts with G[15], [18],[14].
After trying out various orientations, it was found that the
interaction energy for such a system (Fcv- G; where, v for Fc
axis being vertical w.r.t G) turns out to be very low (-0.45
kcal/mol). But when Fc is kept horizontal w.r.t. G i.e., Fch-G
(h=horizontal), it was observed to have higher interaction energy
of -1.78 kcal/mol where the Fc hydrogens interact with π-ring of
G. Sandwiched geometries with Fc placed in between two
graphitic sheets were also considered. In this case also, it was
observed that H- π interaction is more favorable (Fch –G2,
interaction energy of -2.74 kcal/mol) compared to a π- π
stacking (Fcv- G2, interaction energy of -0.93 kcal/mol) mode of
interaction. However, there was no favourable interaction
between PANI and Fc as evidenced from the interaction energy
of 1.72kcal/mol. Interestingly, we have obtained significantly
higher interaction for PANI with G as visible from the interaction
energy of -17.08 kcal/mol which could be attributed to π- π
stacking interactions between two benzenoid systems i.e. PANI
and G. Hence, it can be concluded that interaction between G
and Fc are weak in nature whereas H- π mode of interaction is
more favorable as compared to π- π mode. Meanwhile, PANI
has very strong interaction with G which is primarily governed by
π- π stacking interactions (as in GA). Hence, in order to make a
stable ternary composite of G, Fc and PANI, a binary composite
was made first by dissolving Fc in Tetrahydrofuran (THF)
followed by addition of G and sonication to intercalate Fc
through weak interaction with G. To stabilize this structure
aniline was polymerized over this sandwiched sample. Thus, by
synthesizing ternary composites using all the three components,
a high capacitive electrode material can be obtained and can be
utilized for the future energy storage devices. Abbreviations of
different electrode materials has been tabulated in table 2.
Figure 1: Relaxed structures for G, Fc and PANI and their possible
interactions as modelled by DFT
Table 2: Sample abbreviations
Abbreviation
System
FcG
Ferrocene@graphene
GA
Graphene@polyaniline
FcGA
Ferrocene@graphene@polyanilianline
FcA
Ferrocene@polyaniline
Results and Discussion
The bonding properties of as prepared nanocomposites
were confirmed from FTIR analysis as shown in figure 2a.The
spectra corresponding to GA and FcGA suggests the succesful
incorporation of graphene and PANI in presence of Fc
moeties.The peak around 1580 cm-1, 1480 cm-1,1300 cm-1 and
1129 cm-1 in both the nanocomposites (GA and FcGA) can be
attributed to the C=C stretching of the quinoid ring, C=C
stretching of the benzenoid ring ,C-N stretching of the benzenoid
ring and C-N stretching of the quinoid unit respectively, thus
suggesting the presence of PANI in the nanocomposites.[22]
Again the peak around 1580 cm-1 (C=C streching frequency) is
found to be present in all the nanocomposite which revelas the
presence of graphene. The presence of characteristic Fc peaks
around 1125 cm-1 (C-Fe), 1003 cm-1(C-H out of plane bending
ring breadth), 815 cm-1(C-H ring metal stretching) confirms the
successful incorporation of Fc moieties in FcG and FcGA,
respectively [14] . These observations were in good agreement
with the presence of characteristic peak of Fc around 580 cm-1
observed to be present in all Fc based nanocomposites. The
presence of aforementioned bands gives the clear indication of
successful incorporation of PANI in nanocomposite FcGA. The
System
Interaction energy (kcal/mol)
Fch- G
-1.78
Fcv- G
-0.45
Fch- G2
-2.74
Fcv- G2
-0.93
FcA
1.72
GA
-17.08
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broad band around 3300 cm-1 and 2900cm-1 corresponds to the
N-H stretching and C-H stretching vibrations, respectively, for
both nanocomposite FcGA and GA further gives the idea of
successful polymerization of PANI in presence of graphene and
Fc[23].
To investigate the possible interaction among the various
components in the nanocomposites UV analysis was further
performed and presented in Figure 2b. Fc shows three distinct
peaks of the ferrocenyl moieties around 263 nm which could be
assigned to π-π* transition of the C=C bonds present in the Fc
moieties [14],[10]. Other two peaks around 288 nm and 368 nm
gives the signature of d-d electronic transition which are the
characteristic of the ferrocenyl moieties [18]. Appearances of all
such bands are in good agreement with previous works which
further gives the clear idea of successful incorporation and
interaction of Fc with graphene even after polymerization. When
the Fc moieties are sandwiched between the graphitic sheets (in
nanocomposite FcG), implicitly there occurs an interaction
between Fc and the planar graphitic sheets as evident from the
UV spectral characteristics of FcG. It should be noted that Fc
peaks are gradually broadened and such peak broadening
phenomena suggests delocalization of π-electron in immobilized
Fc molecule with acceptor graphitic basal plane [15]. These
observations were also well documented by DFT studies. The
UV study also appendages two different synergism of polyaniline
in as prepared nanocomposite. Results revealed no interaction
of PANI with Fc molecule, but have higher compatibility and
interaction with graphitic planes which is nicely depicted through
DFT study. It is observed that native PANI has three distinct
peaks around 236 nm, 273 nm and 370 nm could be attributed
to the π-π*, polaron-π* and n-π* transition in quinoid and
benzenoid units of PANI, respectively [10], [14], [23]. When these
PANI interacts with the graphitic sheets (in GA) the peaks gets
shifted to higher wavelength. The band near 270 nm was also
appeared to be red shifted to 280 nm giving the idea of
improved conjugation of GA. Similar peak shifting phenomena
were also notified in case of FcGA which corroborates possible
establishment of delocalized π-electron environment aroused by
well-defined redox behavior of Fc with sp2 bonded carbon atoms
of graphene in presence of PANI in compact sandwiched
structure of FcGA [14].
Figure 2. (a) FTIR spectra; (b) UV visible spectra and (c) RAMAN spectra of
the nanocomposites
The interaction of Fc with G and PANI was further
determined by the Raman analysis as shown in Figure 2c. This
study reveals the electronic structure and the vibrational
properties of the synthesized nanocomposites. As can be seen
all raman spectra exhibits bands at 1570 cm-1 and 1353 cm-1
which are the characteristic Raman peak corresponding to the D
and G band of the graphitic materials, respectively. The G band
is related to the Eg vibration mode of the sp2 carbon and D band
is associated to the structural defects and partially disordered
structures of sp2 domains. The band at 1576 cm-1 is blue shifted,
thus indicating the electron accepting feature of the Fc moiety,
as the electron acceptors cause blue shift in the composites and
suggests that the Fc moieties on the surface of the graphene
sheets acts as buffers between the graphene layers, and thus
reduces their deformation and lowers the defect density [14]. In
presence of PANI (as in GA and FcGA), two peaks are obtained
in the same region of D and G band as that of graphene which
can be assigned to the C-C benzenoid units and stretching
mode of delocalized polaronic charge carriers of conjugated
PANI, respectively. It should be noted that a slight shift in the
band from 1345cm-1 [24] to 1353 cm-1 after composite fabrication
with graphene suggesting possible ᴨ-ᴨ interaction between the
sp2 carbons of PANI and graphene.[19] Additionally, bands
appeared around 603 cm-1 and 830 cm-1 also indicates the
presence of quinone form of PANI ring and C-H out of plane
bending vibration [24] , [25]. In case of FcG and FcGA, appearance
of two prominent bands observed around 580 cm-1 indicates that
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Fc moieties remains as sandwiched structure and not distorted
even after composite fabrication with PANI. Thus Raman
spectra can disclose the structural defects of G and its
interaction with the intercalated Fc and PANI moieties and
furthermore, the XPS study and DFT calculations also reveals
the interaction of Fc with the graphitic sheets.
The attachment of the ferrocenyl moieties on the graphitic
sheets can be manifested by the XPS study as in Figure 3. It
was observed that in case of FcG, there were two additional
peaks as compared to Fc at around 284.5eV and 532.2eV along
with the Fe 2P1/2 and 2P3/2 peaks at around 753 eV and 773 eV
respectively [14] The peaks at 284.5 eV and 532.2 eV
corresponds to C1s and O1s atoms, respectively, which suggests
the presence of G.The C1s peak can be decomposed into three
peaks which corresponds to the C-C, C-O and C=O and the O1s
peaks can be decomposed into –O- and –OH. However, the
binding energy peak corresponding to Fe in case of pure Fc is
higher as compared to that in the nanocomposite which reveals
the impact of π-π confinement effect between graphene and Fc
[15].
Figure 3. XPS (a) survey plot of FcG;(b) C1s; (c) O1s and (d) Fe2p
Figure 4(a-c) shows the FESEM morphologies of the Fc,
FcG and FcGA nanocomposites. Figure 4a shows the FESEM
image of Fc depicting the nanostructured rod like structure of Fc
which is observed to be agglomerated and clumsy. On
fabrication with graphene as in FcG, uniform growth of Fc can
be notified this could be ascribed to templating effect of
graphene.the Fc moeties were found to be distributed over the
graphitic sheets and the agglomeration was reduced as
observed from image 4b.The yellow encircled area clearly
depicts the presence of graphitic sheets over which the Fc
nanorod structures are distributed. But, on incorporation of PANI
in FcG to form ternary nanocomposite FcGA (as in figure 4c),
interwined network like morphology (encircled with yellow) was
obtained suggesting the better porosity and surface area of the
nanocomposite. Such enhanced surface area and network like
morphlogy can lead to better electrode material which was
further well consistent the CV analysis. Thus on incorporation of
PANI the entire system gets stabilized owing to its reduced
interlayer stacking, high surface area, better mesoporous nature
and mesh-like network morphology.
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Figure 4. FESEM images of (a) Fc; (b) FcG; (c) FcGA
To further confirm the morphology of the Fc nanostructures,
HRTEM anaysis was carried out. The images as shown in figure
S8, which resembles the nanorod like structure of the Fc
moeties. Figure S8 (c) depicts the TEM image of the binary
composite FcG. The red encircled rod-like structure are
attributed to the Fc moeties and these structures are
sandwiched within the graphene sheets. TEM images of FcG, as
shown in figure S8 (a) and (b), shows the presence of
particulated PANI(yellow encircled), rod shaped Fc moeties(red
encircled) and the transparent graphene sheet. This further
supporrts the FESEM analsysi of developement of a sandwich
like morphology.
BET isotherm of prepared nanocomposites was performed
by N2 adsorption desorption method and presented in Figure 5a.
Cleary, presence of hysteresis loop within relative pressure
(P/P0~1) confirms the capillary condensation of N2 gas within
nanocomposite so prepared demonstrating porous character of
electrode material. These results were also in good agreement
with pore size distribution analysis calculated from NLDFT
method (shown in Figure 5 b) which indicates maximum pores
lay the region of mesoporous behavior. Surface area of samples
were calculated from DFT method which revealed that FcGA
exhibited maximum surface area of 231 m2/g relatively higher
than GA (137 m2/g), FcG (59 m2/g). Higher surface area of FcGA
could be ascribed to formation of intertwined structure
constituted by Fc, graphene and PANI. The influences of these
structural evolutions were further confirmed from
electrochemical performance.
Figure 5. (a) BET surface area and (b) Pore size distribution of the
nanocomposites
From the fundamental point of view, there is no room for
the formation of the electrical double layer between the solid-
electrolyte interfaces. Thus, in order to address this issue, a
three electrode configuration was employed which
discriminates between anion and cation adsorption [1]. 1M NEt-
4BF4-acetonitrile was used as an electrolyte in order to address
this issue. Cyclic voltammetry tests show the voltammogram of
Fc, FcG, GA and FcGA possesses almost a non-rectangular
shape, which exhibits capacitive behavior from both EDLCs
and pseudocapacitance. The cyclic voltammetry experiments
were carried out at scan rates of 1 mV/s. It is notable that
distinct redox peaks were obtained at the lowest scan rate i.e.
at 1 mV/s as depicted in Figure 6a. A potential window was
selected from 0 to 0.8 V so as to avoid the PANI over oxidation
by reversing the anodic potential scan immediately before the
onset of the over oxidation peak [27]. In addition to the double-
layer capacitance, manifestations of hetero-atoms and other
functional groups also contributed pseudo- capacitance to the
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whole system [28], [29]. The shape of the CV loop should have a
larger surface area with low contact resistance. However, a
larger resistance contorts the loop resulting in a narrower loop
with an oblique angle [10]. In a slow voltage scan the flux to the
electrode surface is considerably smaller as compared to faster
rates and as the current is proportional to the flux towards
electrode, the magnitude of the current will be lower at slow
scan rates and higher at high rates [10]. In case of the Fc, an
oxidation and reduction peak is obtained at about 0.18 V and
0.3 V, respectively. These redox peaks are due to the Fc
moieties which generates an Fc/Fc+ couple and the presence
of such peaks also suggests that the Fc molecules remains
intact in the nanocomposites. Further, on incorporation of
graphene the current density gradually increases followed by
the increase in the enclosed area of the CV curve. However,
cyclic integrated areas under CV curves were further increased
after PANI decoration indicating higher charge accumulation
near electrode surface during electrochemical process. This
could be ascribed to higher surface area of intertwined
morphology of FcGA as evident from surface area analysis.
Figure 6. (a) Cyclic voltammograms of the nanocomposites at 1mV/s; (b)
Galvanostatic charging-discharging plot of different nanocomposites; (c)
Galvanostatic charging-discharging plot of FcGA at different current densities;
(d) Variation of specific capacitance at different current densities of the
nanocomposites.
Galvanostatic charging-discharging (GCD) analysis was
further performed to investigate the practical applicability of as
prepared electrode material. Charging discharging was
performed within the potential window ranging from 0-0.8V
using 1 M NEt4BF4-acetonitrile solution as electrolyte at current
density of 1A/g. As can be seen from the Figure 6b, all loops
are identical, symmetrical to their corresponding charging
discharging counter parts and are of linear nature which
depicts the presence of double layer capacitance. Apart from
this, deviation is also observed from conceptual ideality. This
non-ideality can be attributed to the pseudocapacitance
contribution from the PANI, Fc and PANI coated graphene
towards total capacitance. Discharging time is the dominant
parameter for electrode performance. Here, larger discharging
time duration of any electrode material corresponds to higher
specific capacitance of electrode material. Here in our case, the
discharging time of FcGA was higher as compared to FcG and
GA. The reason for this enhanced behavior could be attributed
to the presence of highly conducting graphene sheet with PANI
and sandwiched Fc which facilitate the charge transfer and
shorten the diffusion path for electrolyte accessibility.
To justify these observations and to investigate practical
application of prepared sample specific capacitance (SC),
energy density (ED) and energy density (PD) was calculated
from the GCD curves using the formulas mentioned before. At 1
A/g, results revealed that nanocomposite FcGA offered a very
high SC of about 960 F/g as compared to GA and FcG which
offers a SC value of 553 F/g and 241.25F/g, respectively . This
thus suggests the successful electron transfer properties
between FcG and PANI leading to better synergistic effect and
enhanced capacitive behavior.The galvanostatic charging-
discharging of FcGA was also carried out at different current
densities of 2, 3,5A/g and it was observed that the specific
capacitance gradually decreases with the increasing current
density owing to the incremental voltage drop and insufficient
active material involved in redox reaction at higher current
densities (as shown in figure 6c and 6d).Further, the
nanocomposite FcGA was also tested in two electrode system
and the specific capacitance was found to be 770F/g at a current
density 1A/g. The relevant plots related to two electrode system
has been provided in the supplementary information (figure S5
and S6). Figure S5 demonstrates the CV curves of the
nanocomposite FcGA in two electrode system ranging from 10
to 200mV/sec.A quasi rectangualr shape of the CV curves has
been observed with redox peaks. Thus suggestin the
combination of both pseudocapacitance and electrical double
layer properties at all scan rates.Even at higher scan rates the
shape and peaks of the CV curves remains well preserved
which confimed the rate capability of the hybrid material for the
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SCs application. It has also been observed that in figure S6 the
discharge time is less as compared to the three electrode
system.Our data demonstrates that a three electrode system is
useful to investigate the electrochemical behaviour of the single
electode. The driving force for the synergistic effect is the strong
π- π interaction between layered G sheets and conducting
polymer, PANI and the H- π interaction between the metallocene
molecule i.e. Fc and the G sheets [30,31].Owing to the high
surface area, high π- conjugation, G can show various non-
covalent interactions with Fc moieties and PANI. Zhang et al.
showed that the non-covalent interactions with aromatic
molecules can tune the band gap of G sheets [32].The π-system
of G and the H-atoms as well as the π-electrons of the Fc
moieties interact making the band gap energy somewhat lower
resulting in increased conductivity [33]. After PANI coating over
graphene sheets with enhanced surface activity serves as an
efficient conducting path for the charge transfer from ferrocene
moiety and leads to improved specific capacitance as depicted
in Figure 7a. Similar trends were also followed for energy
density (ED) and power density (PD) as shown in Figure 7b. On
coating FcG with PANI, the exposed G sheets offer strong
interlayer interaction with the π electron of the PANI polymer
chains. The uniform coating of PANI over the FcG
nanocomposite reduces the diffusion and migration length of the
electrolyte ions during the fast charge-discharge process and
thus leading to the enhancement of the electrochemical
utilization of PANI [34]. Additionally, it has been noted that the
discharge time gradually becomes high in case of the ternary
nanocomposite, FcGA. This indicates that the energy consumed
by the internal resistance is reduced and the effective energy
stored is amended [34]. Thus, the combination of EDLC of G and
faradic capacitance of Fc and PANI, is responsible for the longer
charge-discharge resulting in faradic charge-transfer
accompanied by double layer charge-discharge process.Hence,
PANI particles on G sheets can interconnect with each other
leading to improved electrochemical performance [34,35].
Figure 7. Bar plots depicting (a) Specific capacitance and (b) Power density,
Energy density of the nanocomposites
Since long durability of electrode material is highly desirable for
supercapacitors application, cyclic stability test was further
performed for consecutive 5000 cycles at 1 A/g. Figure 8b
revealed that all samples were stable up to 5000 cycles with
86 % capacitance retention of initial value demonstrating good
reversibility and cyclic stability of electrode material.
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Figure 8. (a) Nyquist plot for EIS study and (b) Cyclic stability of the
nanocomposites
The Nyquist plot, real part (Z’) Vs imaginary part (-Z”) explains
the frequency dependence of electrode material as depicted in
Figure 8a and all the plots were fitted ( see supporting
information Figure S3,S4 and table S1 ) for better understanding
of the phenomenon. The intercept of real component gives the
equivalent series resistance (ESR) which determines the rate of
charging and discharging. The plot constitutes the high
frequency semi-circular region followed by a straight line in low
frequency region. The high frequency semicircle is attributed to
the combined charge transfer and double layer capacitance. On
the other hand low frequency region depicts the mass transfer
related to the Warburg impedance . The low frequency region is
associated with the adsorption processes, microscopic charge
transfer and surface roughness [18]. The interaction point of these
two frequency region is known as knee frequency, at which
capacitive behavior of electrode material is dominant. It should
be noted that the nanocomposite FcGA has a very low Rs and
Rct which is in well accordance with the CV analysis. The
reason for low Rs value could be due to the presence of
graphene and PANI which is highly conducting in nature.
Moreover, the higher value of the knee frequency also suggests
that the ternary nanocomposite has a better electrochemical
behavior as compared to others. Thus all such property reveals
that FcGA can be an efficient and attractive alternative electrode
material for supercapacitor application.
Conclusions
In summary, all Fc based nanocomposites were synthesized by
in-situ chemical oxidative polymerization. Computational
calculations with DFT based methods rendered insightful
details about the relative interactions between the components
(Fc, G, PANI). Current study revealed that Fc can act as a well-
known mediator due to its various properties, such as
reversibility, regeneration at low potential, and generation of
stable redox states. Due to its ability to display the high redox
behavior of the ferrocene/ferricinium (Fc/Fc+) couple in organic
electrolyte Fc can be easily oxidized and reduced, reversibly.
Fc with graphene through strong H-π stacking interaction
exhibited an enhanced electrochemical property. PANI also
possesses huge open π electron system thus resulting in
formation of nano hybrid and stabilizing the FcG moiety by π-π
stacking interaction leading to the successful synthesis of
hybrid supercapacitor electrode material. All electrochemical
measurements revealed the superior performance of FcGA
with maximum specific capacitance and energy density of 960
F/g and 76.44 Wh/Kg, respectively, at 1 A/g. These
observations were well consistent with CV and EIS analysis.
Cyclic stability test for consecutive 5000 cycles also confirmed
the long durability and reversibility of as prepared electrode
material with 86% capacitance retention.
.Experimental Section
Materials
Graphite powder was obtained from S.D Fine Chemicals Limited,
Mumbai (India) (99.9% particle size and 100 micron). Aniline was
obtained from SRL Pvt. Limited, nitric acid and potassium permanganate
was procured from RFCL limited, New Delhi (India). Sulphuric acid (98%
pure), ortho-phosphoric acid 88%, methanol, sodium hydroxide,
Tetrahydrofuran (THF) and hydrogen peroxide were obtained from Merck
Specialist Pvt. Limited, Mumbai. Ammonium persulphate was procured
from CDH Pvt. Limited, New Delhi (India). Ferrocene was obtained from
Loba chemie.
Instruments
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The Raman analysis was carried out using a Nano finder 30 (Tokyo
Instruments Co., Japan). The XPS characterization was carried out in
Theta Probe AR-XPS System, Thermo Fisher Scientific (U.K) with X-ray
source , monochromated Al Ka, hv=1486.6eV and X-ray energy 15kv,
150W and Spot size : 400 µm. UV analysis of Fc based nanocomposite
was performed using SCHIMADZU spectrophotometer. The UV spectra
of all samples were recorded in the wavelength ranging from 200-500 nm
by dispersing the sample in THF. The FTIR spectra of as synthesized
nanocomposite were recorded within the wavelength range of 400 to
3500 cm-1 using NEXUS 870 FTIR (Thermo Nicolet) to investigate the
bonding properties in the resultant nanocomposites.The surface
morphology of the as prepared nanocomposite was characterized by a
field-emission scanning electron microscopy (FESEM) using Supra 55
(Carl Zeiss, Germany). The BET surface area measurement of as
synthesized nanocomposite was characterized by the
QUANTACHROME NOVA WIN at the degassing temperature of 90°C.
HRTEM analysis was carried out with Jeol/JEM 2100 with a LaB6 source
at an acceartion voltage of 200 kV.
The electrochemical properties of Fc composites were characterized by
three electrode system using 1 M NEt4BF4-acetonitrile as electrolyte at
room temperature in CH instrument (Model No. CHI 760D). The Fc
based nanocomposites were used as the working electrode and Ag/AgCl
and platinum wire was used as reference and counter electrode,
respectively. The electrochemical impedance spectrum (EIS) was
analyzed by 5mV of amplitude and in the frequency range of 1 Hz to
100000Hz.
Synthesis of G:
Graphene oxide (GO) was synthesized by improved Hummer
method [19]. G was obtained by reduction of GO using hydrazine
hydrate.[20] Briefly, about 1.5 g of GO was taken in 200 ml DI water in
round bottom flask followed by sonication for 30 mins till complete
dispersion of GO. The round bottom flask was kept in an oil bath and
consequently temperature was maintained around 80˚C while under
constant stirring. To this solution mixture, about 5 drops of hydrazine
hydrate was slowly added and the whole solution mixture was then
stirred for 2 hours around 80˚C. After 2 hours, solution was stirred at
room temperature for 12 hours. The resulting wet chemical product was
then centrifuged, washed with methanol and dried vacuum to obtain G.
Synthesis of Graphene@PANI (GA) nanocomposite:
GA was synthesized via in-situ polymerization of aniline in
presence of graphene. In typical synthesis process, 0.05g of graphene
was dispersed in 50 ml THF under sonication. To this, 1ml of aniline was
added followed by dropwise addition of ammonium per sulphate (APS)
(~2.5g dissolved in 50 ml of water) in order to obtain insitu polymerized
GA nanocomposite.
Synthesis of Ferrocene@Graphene (FcG) and
Ferrocene@Graphene@PANI (FcGA):
In a typical synthesis of FcG, 0.025g Fc (this concentration of Fc
was taken by studying the concentration variation of Fc on the capacitive
behavior, which revealed that as the concentration of Fc was increased
there was a decreased current contribution of the nanocomposite(see
supporting information Figure S1 as well as cyclic stability figure S7 and
table S2)), solution in THF was added to 50 ml G solution (containing
0.05g G) of distilled THF in 100 ml round bottom flask. After the
dissolution the solution mixture was sonication and the mixture was
thoroughly stirred for 24 hrs [14]. The residue obtained was centrifuged,
washed several folds with distilled water and dried under vacuum around
35˚C. The obtained product was marked FcG (in chronological order
Ferrocene@graphene). Similarly, typical in-situ process was employed
for the synthesis of FcGA. Briefly, 1 ml aniline was added with same
quantity of graphene (0.05g) and Fc(0.025g). Polymerization was
initiated by the slow addition of APS while maintaining the temperature
around 5˚C. The wet chemical product was then refrigerated overnight
followed by washing and drying. The as obtained product was marked
as FcGA (in chronological order Ferrocene@graphene@PANI).
Keywords: Supercapacitors • Ferrocene (Fc)• Graphene (G) •
Polyaniline (PANI) • H-π interaction
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A theoretical approach to study
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ferrocene/graphene
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